Colorants doi: 10.3390/colorants5020018

Authors: Peter Zugenmaier

Hydroxygallium phthalocyanine type V (HOGaPc V) is an excellent photo generator and is applied in xerography. The material is only accessible as polycrystalline substances, and the crystal structure for an evaluation of the structure–property relationship cannot be determined from the few X-ray reflections available by powder X-ray pattern. A new method for crystal structure determination is introduced, utilizing molecular interactions. This proposed structure appears to be superior to the published one by the classical application of the Rietveld analysis. Hydrogen bonds are detected and explain the thermal stability, combined with high photosensitivity, and point towards favorable application in electrophotography. A triclinic two-molecule unit cell P-1 with a = 11.63 Å, b = 12.60 Å, c = 8.88 Å, α = 95.7°, β = 95.2°, γ = 69.1° was established close to the one verified by the Rietveld analysis. The structure obtained was successfully tested by a comparison of the observed contacts and the packing energy of known Pc single-crystal structures and by a similar X-ray residual R factor of the mostly overlapping reflections with other materials. The packing contacts of the crystal determined by the Rietveld analysis show too short contacts and too high packing energies. The molecular and crystal structure of HOGaPc V is represented and discussed.

Colorants doi: 10.3390/colorants5020017

Authors: Zianya Gómez-Soto Carlos Ángel-Jijón Otilio Arturo Acevedo-Sandoval Eduardo Cornejo-Velázquez María Aurora Veloz-Rodríguez Rosa Angeles Vázquez-García

This study examines the colorimetric characteristics and possible biological effects of the quinoline-derived dye M1 1,4-bis((E)-2-(quinolin-2-yl)vinyl)benzene. The pi-conjugated structure and high planarity of M1 give it important semiconductor and optical properties, such as the modulation of its fluorescence properties depending on the synthesis method. These optical properties make this dye a candidate for use as a sensor for biological targets. Predictions were made on molecular targets such as receptors and enzymes involved in neurological and inflammatory processes. Docking studies indicated moderate to strong binding affinities (−10.3 to −7.1 kcal mol−1), supported mainly by hydrophobic pi interactions and low RMSD conformations. The predicted biological activity, together with the luminescent properties documented above, position M1 as a versatile dye with potential uses in biomedical applications and as a luminescent detection sensor. These results lay the groundwork for future experimental confirmation combining photophysical analysis and biological testing.

Colorants doi: 10.3390/colorants5020016

Authors: Vanessa M. Fontenele Sampaio Vladimir Lavayen João Henrique Zimnoch dos Santos

Melanin is a biological pigment known for its broadband UV-Vis absorption and structural disorder, which collectively determine its chromatic response. In this study, melanin–silica hybrid materials were synthesized via sol–gel processing under different catalytic conditions (acid, base, sequential acid–base) and using distinct silica precursors (TEOS and sodium metasilicate) to investigate how the inorganic matrix modulates the optical properties of the synthetic melanin. Colorimetric analysis revealed significant variations in lightness among the hybrids, while the chromatic coordinates remained characteristic of eumelanin. The material obtained via the sequential acid–base route exhibited the lowest lightness and highest absorption, indicating more efficient pigment dispersion, whereas the system derived from silicate precursors showed the highest lightness and lowest absorption, suggesting increased light scattering and/or pigment aggregation. Materials prepared under purely acidic and basic conditions displayed intermediate optical behavior. Electron Microscopy revealed distinct morphologies associated with each synthesis pathway, ranging from dense lamellar structures to compact aggregates and hierarchical assemblies, directly influencing light scattering mechanisms. Thermal analysis confirmed effective stabilization of melanin within the silica networks, with significantly reduced mass loss compared to the pure pigment. Confocal microscopy revealed detectable fluorescence only in selected systems, indicating variations in pigment dispersion and aggregation state. Overall, the results demonstrate that the chromatic response of melanin–silica hybrids is governed not only by the intrinsic properties of the pigment, but also by the structural organization of the inorganic matrix. Sol–gel processing thus provides a versatile strategy for tuning the optical behavior of melanin-based materials through controlled interfacial interactions.

Colorants doi: 10.3390/colorants5020015

Authors: Sandra Vega-Maturino Luz Araceli Ochoa-Martínez Silvia Marina González-Herrera Olga Miriam Rutiaga-Quiñones Juliana Morales-Castro José Alberto Gallegos-Infante Miriam Estevez

In recent years, increasing consumer demand for healthier and more natural foods has driven the food industry to replace artificial additives. Among these, colorants play a crucial role, as they influence the sensory perception and acceptance of food products. However, the widespread use of synthetic colorants has raised growing concerns due to their potential association with adverse health effects. In addition, several regulatory agencies have restricted or banned the use of certain synthetic colorants, requiring their replacement with natural alternatives. In this context, anthocyanins have emerged as a promising substitute for artificial colorants, owing to their similar color properties. Despite their potential, their use as a food colorant still faces several challenges, particularly regarding stability, incorporation into food matrices, and regulatory constraints. Therefore, this review examines the challenges and current trends in natural colorants, highlighting the potential of anthocyanins as substitutes for synthetic red colorants in food products.

Colorants doi: 10.3390/colorants5020014

Authors: Masafumi Yano Kengo Sakai Minami Ueda Koichi Mitsudo Yukiyasu Kashiwagi

We report a π-extended N-phenylphenothiazine dye bearing thiophene substituents, designed to address the practical compromise between long-wavelength near-infrared (NIR) absorption and the isolability of a stable radical cation state. The target compound was synthesized via Suzuki–Miyaura cross-coupling and exhibited good solubility in common organic solvents. Cyclic voltammetry in dichloromethane showed a reversible one-electron oxidation at E0 = 0.19 V vs. Fc/Fc+. Chemical oxidation afforded the corresponding radical cation, which showed an intense NIR absorption maximum at 910 nm. DFT calculations support thiophene-induced narrowing of the HOMO–SOMO gap and predict a pronounced bathochromic shift of the main absorption band. The radical cation was isolated as a stable PF6− salt and readily processed into spin-coated films, which retained strong NIR absorption and remained stable for months under ambient conditions.

Colorants doi: 10.3390/colorants5020013

Authors: Jéssica Antunes Agata Nolasco Beatriz Marques Marisa Lopes Philippe Sarra-Bournet Augusta Silva Helena Vilaça Carla J. Silva

The development of bio-based inks represents a promising strategy to reduce the environmental impact of digital printing technologies. This study investigates the formulation and performance of water-based inks incorporating two renewable pigments: a fermentation-derived indigo pigment and a plant-extracted yellow pigment. Special attention was given to dispersion optimization of the poorly water-soluble indigo pigment. Extended mechanical dispersion (115 h in a ball mill) proved critical to achieve colloidal stability, enabling the preparation of inks that met standard rheological and physicochemical criteria for inkjet printing with piezoelectric printheads. Both inks were applied on a variety of substrates, including cotton, polyester, leather, and kraft paper, pre-treated, in the case of the textiles, with either a cationic biopolymer or a synthetic polyurethane-based binder. Colorimetric evaluation confirmed effective deposition and uniformity, with the indigo ink producing deep blue hues and superior overall fastness than the yellow ink, particularly in washing and rubbing tests. The yellow pigment ink showed good stability but once applied to the fabric, the resulting print exhibited poor fastness, particularly against light exposure, indicating limited durability of the coloration on the textile. Shelf-life analysis of the indigo ink revealed a decline in viscosity and surface tension over time, though the colour and particle size remained stable, particularly under room temperature conditions. These findings confirm the potential of fermentation-derived indigo as a robust bio-based alternative to synthetic dyes and its superior performance in relation to other nature extracted pigments, which, although facilitating ink preparation due to their higher water solubility, result in lower-fastness prints.

Colorants doi: 10.3390/colorants5020012

Authors: Magdalena Souto Marcello Picollo Giovanni Bartolozzi Frédéric Jamme Márcia Vieira Paula Nabais Maria J. Melo

By examining historical recipes from the medieval treatise The Montpellier Liber Diversarum Arcium, the creation of bottle green and verdigris pigments involved various types of tempera, such as parchment glue and gum arabic. Malachite was also prepared. These references and paints were analysed using infrared spectroscopy and visible spectroscopy techniques, such as micro-Infrared Spectroscopy (microFTIR) and Fibre Optic Reflectance Spectroscopy, (FORS) over the 350–1000 nm range. This research provided new insights into the pigments used in monastic manuscripts and Books of Hours, supported by valuable data from the Soleil synchrotron. Producing historically accurate reproductions and applying spectroscopy to analyse them promotes sustainable cultural heritage preservation by maintaining ancient artefacts, detecting early signs of degradation, and enabling the development of compatible restoration materials.

Colorants doi: 10.3390/colorants5020011

Authors: Rocío Magdalena Sánchez-Albores Clara López-Aguilar Odín Reyes-Vallejo Francisco Javier Cano Johana De la Cruz-Ascencio J. Escorcia-García A. Cruz-Salomón A. Ashok

Water contamination by synthetic dyes such as malachite green (MG) remains a significant environmental and public health challenge due to their high toxicity, chemical stability, and resistance to biodegradation. In this study, a CaO-CuFe2O4 composite was synthesized through a sustainable route using eggshells and orange peel as agro-industrial waste precursors. Comprehensive structural, spectroscopic and microscopic analyses confirmed the coexistence of a predominant CaO-based phase with spinel CuFe2O4, together with nanometric features, satisfactory elemental dispersion and practical magnetic recoverability. Under the experimental conditions employed, the composite exhibited high adsorption performance towards MG, reaching an equilibrium capacity of 2288.4 mg g−1 and 99.98% decolorization within 60 min. The kinetics were better described by the pseudo-second-order model, while the equilibrium behavior was more satisfactorily fitted by the Langmuir isotherm than by the Freundlich model. Thermodynamic analysis indicated that the adsorption process was favorable over the temperature range studied and became more pronounced at higher temperature. The results suggest that the adsorption behavior arises from the combined influence of surface chemistry, calcium-derived basic sites, ferrite-associated metal centers and interfacial accessibility, rather than from surface area alone. In addition, the material could be readily separated from aqueous solution using an external magnetic field, highlighting its practical post-treatment recoverability. Overall, this work demonstrates a viable waste valorization strategy for the development of a magnetically recoverable CaO-CuFe2O4 adsorbent for cationic dye removal. Beyond the specific case of MG, the study underscores the potential of agro-waste-derived hybrid oxides as application-relevant materials for water remediation.

Colorants doi: 10.3390/colorants5020010

Authors: Panangattukara Prabhakaran Praveen Kumar

Light-based diagnostic and therapeutic approaches are increasingly important in modern biomedicine, with organic dyes emerging as versatile optical agents due to their tunable photophysical properties. Precise control over absorption and emission characteristics has enabled their application in fluorescence, photoacoustic, and Raman imaging, as well as in photodynamic and photothermal therapies. However, challenges related to biocompatibility, aqueous stability, and in vivo performance remain critical for clinical translation. Organic dyes that absorb in the near-infrared region are particularly attractive because of their deeper tissue penetration and reduced background interference. This review highlights key structure property relationships of organic dyes and summarizes current design strategies, including chromophore modification, peripheral functionalization for water solubility, and self-assembled nanotheranostic systems. Recent biomedical applications in cancer diagnosis and therapy, bacterial detection, and imaging-guided treatment are discussed, along with future directions for advancing dye-based technologies in healthcare.

Colorants doi: 10.3390/colorants5010009

Authors: Marina Barros Cavalcanti Ziani Santana Bandeira de Souza Nickolly Bukkyo Vieira Serafim Caroline Maria Bezerra de Araujo Jorge Vinicius Fernandes Lima Cavalcanti Maurício Alves da Motta Sobrinho

Advanced oxidation processes (AOPs) have been widely applied to treat textile wastewater, in which synthetic dyes are among the main pollutants. Some of these processes, such as the Fenton reaction, exhibit enhanced efficiency when coupled with radiation sources, particularly when combined with a compound parabolic concentrator (CPC). In this study, a UV-A photo-Fenton process assisted by CPC, constructed using reused fluorescent lamps as reaction tubes and operating with recirculation was applied to treat real textile wastewater. A preliminary factorial design was employed to optimize reagent concentrations, identifying optimal conditions of 2647.8 g·L−1 of H2O2 and 15 mg·L−1 of Fe2+. Overall, the use of the CPC led to an increase in photon availability, resulting in COD degradation efficiencies of 83%, corresponding to an ~19% relative increase in treatment efficiency, compared to the system without the CPC, as well as 79% removal efficiency for apparent color and 57% for turbidity. Results demonstrate that the CPC-assisted UV-A photo-Fenton process is an efficient and robust approach for treating real textile wastewater. Meanwhile, the reuse of fluorescent lamps represents a low-cost, environmentally sustainable alternative that contributes to waste valorization and process intensification.

Colorants doi: 10.3390/colorants5010008

Authors: Lin Jiang Katie Herbert Bo Wang

Dye-sensitized solar cells (DSSCs) represent a promising low-cost photovoltaic technology with relatively high conversion efficiency and a simple fabrication process. Natural dyes have drawn growing interest compared to ruthenium-based dyes since they are greener. However, the power conversion efficiency (PCE) of natural dyes is generally low. In this study, we investigated novel approaches to improve the PCE of DSSCs using Delonix regia extracts by polarity-based separation using preparative thin-layer chromatography (PTLC). Our study indicated that polarity-based separation can significantly enhance the PCE, with one fraction achieving a PCE of 1.13%, which is high compared to most natural dye-based DSSCs, and is also 1.85 times that of the crude methanol extract. The major compounds in the highest-efficiency layer were flavanol-based dyes. Our study demonstrates the potential antagonistic effects within Delonix regia extracts in DSSC applications, which play a critical role in improving PCE. The study is expected to support future efforts to enhance the PCE of natural compound-based DSSCs, especially those using flavanol-based natural dyes.

Colorants doi: 10.3390/colorants5010007

Authors: Maria I. Myers Armas Andrea M. Fletes Thomas M. Eubanks Arnulfo Mar Jason G. Parsons Helia M. Morales

Water pollution from industrial dyes is a critical challenge due to the resistance of these types of compounds to degradation and potentially harmful effects on living organisms and human health. In this study, the electrochemical degradation of methylene blue (MB) was investigated using ink-based copper foam electrodes with reduced graphene oxide (rGO), antimony trioxide (Sb2O3), and rGO/Sb2O3 composites. The materials used to synthesize the electrodes were characterized by X-ray diffraction (XRD), which showed the successful synthesis of GO, rGO, and the Sb2O3-rGO composite. Additionally, the synthesized electrodes were examined using SEM. The MB degradation was studied using kinetic behavior and removal efficiency at pH levels from 3 through 6, monitored using UV-Vis spectroscopy. The electrocatalytic degradation was studied using sodium sulfate as the electrolyte across a pH range of 3 to 8. All electrodes investigated were determined to follow first-order kinetics. The Sb2O3-rGO composite showed the highest rate constants of MB degradation at pH 7 and 8, with rate constants of 0.0160 and 0.0159 min−1, respectively. At the same time, the rGO ink-based electrode worked fastest at pH 3 and pH 4 with rate constants of 0.0178 and 0.0158 min−1, respectively. The Sb2O3 also works best at pH 3 and 4 with rate constants of 0.0151 and 0.0152 min−1. SEM analysis shows the composite electrode was more resilient to degradation than other materials.

Colorants doi: 10.3390/colorants5010006

Authors: Louisa Campbell Margaret Smith Sarah Dugmore

Heritage materials science techniques, including pXRF, FTIR-ATR, XRD, microphotography, and microsampling, have peeled back concealed layers of polychromy on a Roman Mithraic altar near Hadrian’s Wall. The results break new ground by exposing the interplay between light and dark and the transformative impact of colour cast onto cultic carved stone. A powerful pigment palette is revealed, including vibrant cinnabar/vermilion letters overlying an unprecedented purple inscription panel created from a compound of Egyptian blue, ultramarine, cinnabar/vermilion, red ochre, red lead, and realgar framed with purple, Egyptian blue, ultramarine, and orpiment. The panel was covered in a golden surface crafted from a previously unknown recipe of pyrite (fool’s gold) mixed with beeswax. Tantalising traces are also detected on some iconographic features, but conservator intervention and degradation processes combined to leach colour from the sculpted relief. These are paradigm-shifting results. They force a reinterpretation of the symbolism and performance of altars as personifications of dedicators, and we identify, for the first time, high-ranking Mithraic initiates by name and grade. Critically, we present a digital reconstruction of the altar with original polychromy that revolutionises our understanding of relief-sculpted Roman inscriptions—a category that has, until now, remained underexplored by the burgeoning polychromy research community.

Colorants doi: 10.3390/colorants5010005

Authors: Dineo A. Bopape Mmbulaheni Ramulondi

The trend toward developing sustainable nanotechnology has driven researchers to explore environmentally friendly techniques for nanomaterial fabrication. This review examines the utilisation of Commelina benghalensis plant extracts as an effective biological tool for the green synthesis of various nanomaterials. The procedures involve reducing metal salt precursors with aqueous or polar solvent extracts rich in phytochemicals such as flavonoids and polyphenols, followed by a calcination step that yields crystalline products. The findings show that the properties of ZnO, TiO2, Ag, NiO, and their composites are directly influenced by synthesis factors, including solvent, plant component, and extract concentration. This directly influenced their specific sizes, morphologies, and phases. Furthermore, these C. benghalensis-mediated nanomaterials showed high efficiency in the photocatalytic degradation of textile dyes and pharmaceuticals, as well as potential antibacterial and antioxidant properties. The Commelina benghalensis plant is flexible and renewable for efficient nanomaterial synthesis; nevertheless, issues with standardisation and scalability must be overcome to fully realise its promise for commercial and industrial uses.

Colorants doi: 10.3390/colorants5010004

Authors: Letícia Cristina Gonçalves Matheus Mantuanelli Roberto Maria Aparecida Marin-Morales

Hair dyes are widely used across all socioeconomic groups and regions worldwide. However, some studies indicate that these products contain substances known to be toxic to a wide variety of organisms. Moreover, dyeing practices generate effluents that may carry the toxicity of hair dyes into the environment. Due to these facts, there is great concern about the impacts these products may have on the environment, as well as on the health of their users and professionals in the field of cosmetology. This scoping review analyzed 184 publications from major databases (PubMed, SciELO, Scopus, Google Scholar, and MEDLINE). Ultimately, 126 scientific studies published between 1981 and 2024 were included based on methodological rigor and their relevance to the One Health framework. According to the literature, the components of hair dyes can induce adverse responses in biological systems, ranging from reversible topical irritations to severe systemic effects. Among the studies evaluated, more than half reported significant toxicological or genotoxic associations related to oxidative dye components such as p-phenylenediamine and its derivatives. These compounds are frequently associated with various types of human cancers, including breast, prostate, bladder, skin, ocular cancers, and brain tumors. In addition to their effects on humans, hair dyes exhibit ecotoxicity, which may threaten the maintenance of ecosystems exposed to their residues. The reported environmental impacts result from effluent emissions after successive hair washes that release unreacted dye residues. Due to the low biodegradability of these compounds, conventional wastewater treatment methods are often ineffective, leading to environmental accumulation and changes in aquatic ecosystems, soil fertility, and trophic balance. Data on the toxicity of hair dye effluents remain scarce and sometimes contradictory, particularly regarding the effects of their transformation products and metabolites. Overall, the evidence underscores the need for continuous monitoring, updated risk assessments, and the adoption of advanced treatment technologies specific to beauty salon effluents. The information presented in this work may support further studies and guide public management agencies in developing policies for mitigating the impacts of hair dye pollutants within the One Health perspective.

Colorants doi: 10.3390/colorants5010003

Authors: Louisah Mmabaki Mahlaule-Glory Nomso Charmaine Hintsho-Mbita

Water polluted by dye colorants has been on the rise in the last decade. This is due to the over reliance on the textile industry, and it is holding a high economic value in most countries. This industry is the highest consumer of fresh water whilst also discharging several natural and synthetic pollutants to the environment. Several methods have been used for the removal of these pollutants and one of the most efficient technologies to be developed includes the photocatalysis method, via advanced oxidation processes. This review highlights the developments of green iron oxide nanoparticles as photocatalysts in the last decade. It was noted that tuning and controlling the phytochemical concentration and synthesis conditions, can assist with forming uniform and non-agglomerated materials, as this has limited the vast usage of these materials in major applications. Also, upon controlling the synthesis conditions, improved surface area and charge separation efficiency was noted. Their limitations and need for modification through forming composites are highlighted. Moreover, future perspectives are given on the use of green IONPs as photocatalysts.

Colorants doi: 10.3390/colorants5010002

Authors: Luigina Vezzoli

The correct identification of historical artists’ earth pigments is mandatory for cultural, scholarly, and historical applications. This paper focuses on the definition of the distinctive mineralogical, geological, and geochemical properties and the discussion of the geological genesis and place of origin of the natural Fe-Mn-based earth pigment named terra d’ombra (umber). It one of the dark-brown earth pigment most widely used by Italian and European painters from the Renaissance to the nineteenth century. The terra d’ombra earth pigment is a primary chemical sediment mainly composed of Fe (oxy)hydroxide and Mn oxide, produced by the authigenic precipitation from oceanic or lacustrine waters rich in metal solutes of volcanic hydrothermal origin. The principal areas of provenance are the island of Cyprus and the Monte Amiata volcano (southern Tuscany, Italy). Its peculiar properties in painting derive from this specific mineralogical composition and genetic process, which also exclude its definition as a particular type of ochre and as a clay pigment. Further misinterpretations include confusion with pigments composed of organic materials and the erroneous attribution of the name and area of origin to the Italian region of Umbria.

Colorants doi: 10.3390/colorants5010001

Authors: Anthony Harriman

This Perspective sets out to raise awareness about the chemical and photophysical properties of an assortment of blue colorants; it is generally regarded that blue is the most popular color worldwide and is recognized for its serenity and calming effect. In fact, blue colorants have a long and rich history, perhaps starting with Egyptian Blue, and have found colossal usage in the dyeing of uniforms and popular clothing. Other blue colorants have made major contributions to our understanding of the fields of molecular spectroscopy and photophysics and continue to underpin contemporary opto-electronic devices. This is in addition to their socio-cultural, economic, and ecological benefits to society. Originally, blue colorants were extracted from minerals by tedious and ineffectual grinding to give a product carrying an exorbitant price. Later, these materials were supplemented by synthetic analogues, such as copper phthalocyanine, more affordable to the general public. It is stressed that the journal Colorants would welcome submissions that describe the chemistry and/or spectroscopy of other archetypal colorants.

Colorants doi: 10.3390/colorants4040037

Authors: Domenico Rongai Maria Gabriella Di Serio Daniele Schiavi Cecilia Miccoli Giorgio Mariano Balestra

Scientific research, within the framework of a circular and sustainable economy, has increasingly focused on wild plants and agricultural by-products as valuable sources of bioactive compounds for innovative applications. In this study, the plant species selected for extract preparation and evaluation of their dyeing properties included Isatis tinctoria L., Castanea sativa, Juglans regia L., Rumex crispus L., Arbutus unedo L., and Punica granatum L. Each extract was analyzed to assess its dyeing performance on cotton and other cellulosic fabrics. Cellulose pads dyed with peels of P. granatum extract (PPGE) proved to be versatile and effective, showing potential for use in extending the shelf life of various fruits such as strawberries, apples, cherries, and persimmons. The optimum dyeing condition for cellulose and cotton fabrics was found to be 6% w/v dye. These findings support the development of natural, multifunctional materials suitable for food packaging applications aimed at preserving fresh produce, as well as for sustainable textile dyeing.

Colorants doi: 10.3390/colorants4040036

Authors: Felipe Vázquez-Dávila Miguel Ángel López-Álvarez Pedro Ortega-Gudiño Cristina Neri-Cortés

Water contamination by synthetic dyes is a pressing environmental and social issue, particularly in the textile industry, which is among the largest consumers of freshwater and sources of wastewater pollutants. Malachite green (MG), a synthetic triphenylmethane dye, was selected as a model contaminant due to its persistence, toxicity, and international regulatory restrictions. In this study, a spinel-type cobalt–manganese oxide (CoMn2O4) photocatalyst was synthesized by calcination at 500 °C and characterized through X-ray diffraction (XRD) and transmission electron microscopy (TEM), confirming the formation of the spinel phase with nanoscale morphology. Photocatalytic activity was evaluated under visible-light irradiation using UV-Vis spectrophotometry to monitor MG degradation. Control experiments and scavenger assays identified hydroxyl radicals (•OH), superoxide anions (O2−), and photogenerated holes (h+) as the main reactive species. The catalyst achieved over 90% dye removal within 300 min and retained high activity over four consecutive reuse cycles, with only a slight decrease in efficiency from 97% to 94%. These results confirm both the efficiency and stability of CoMn2O4 under visible light. The study underscores the potential of this material as a sustainable option for wastewater treatment while contributing to environmental governance and supporting the fulfillment of the Human Right to Water.

Colorants doi: 10.3390/colorants4040035

Authors: Seiji Mimida Kazuki Yamaguchi Shota Kato Toshiyuki Masui

(Zn1−xCox)3Mo2O9 (0 ≤ x ≤ 0.15) samples were synthesized as novel inorganic violet pigments by a conventional solid-state reaction method, and the obtained powders were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), ultraviolet and visible (UV-Vis) diffuse reflectance spectroscopy, and L*a*b*Ch° chromatic coordinates. The Co2+-doped samples showed optical absorption at wavelengths of 500−560 and 680 nm, which were attributed to the d–d transitions of Co2+, resulting in a violet color. The absorption intensity increased with increasing the Co2+ concentration. The most vibrant violet color was obtained with a composition of (Zn0.90Co0.10)3Mo2O9.

Colorants doi: 10.3390/colorants4040034

Authors: Edwin Januschewski Greta Bischof Laurenz Küchner Katharina Schmidt Binh Nguyen Thanh Gerold Jerz Volker Heinz Peter Winterhalter Andreas Juadjur

Regulatory alerts regarding unauthorized use of colorants in food are frequently issued, often involving excessive concentration, improper declaration, or prohibited dyes. The illegal use of reactive textile dyes in food-related coloring preparations remains largely unrecognized, but confirmed cases have been reported in 2016 and 2020 and most recently in May 2025. This work presents the qualitative analyses’ results of a blue coloring powder for food use with regard to reactive dyes. The sample was labeled as a natural product, although it exhibited the color stability typical of artificial dyes. Prompting further investigation, combined spectroscopic, chromatographic and mass spectrometric methods were applied for the characterization and comparison with reference samples. In agreement with all analyses performed, strong evidence was found that the blue coloring powder contained several identical constituents with a reference sample of Reactive Blue 21. The overall composition suggests that both are complex mixtures of different phthalocyanines, suspected byproducts of synthesis, and various unknown compounds, rendering the powder unsuitable for human consumption. These findings emphasize the importance of tightened analytical controls regarding the unauthorized addition of textile dyes to food in order to maintain consumer safety.

Colorants doi: 10.3390/colorants4040033

Authors: Samantha Lizbeth Pérez-Jiménez Francisco Javier Aranda-Valdés Gabriela Elizabeth Quintanilla-Villanueva Donato Luna-Moreno José Manuel Rodríguez-Delgado Iris Cristina Arvizu-De León Alma Gómez-Loredo Edgar Allan Blanco-Gámez Juan Francisco Villarreal-Chiu Melissa Marlene Rodríguez-Delgado

This research investigates the biosynthesis optimization of the red pigment prodigiosin produced by Serratia marcescens 11E through submerged fermentation utilizing an alternative cheese whey-based medium, focusing on process parameters and antimicrobial properties. Four types of whey sourced from a local dairy industry were characterized, and the fermentation conditions were optimized using Plackett–Burman and central composite design methodologies, yielding up to 1.43 g/L of prodigiosin under optimal conditions, 25 °C, 200 rpm, pH 7, and 48 h of dark incubation, with whey serving as the sole carbon source. Normalization to biomass yielded 110 mg of prodigiosin per gram of dried cell weight (post-optimization), enabling meaningful comparison with prior studies. Pigment extraction was performed with acidic methanol, and identity was confirmed by UV–Vis spectrophotometry and Fourier transform infrared spectroscopy (FTIR). The antimicrobial activity of the purified pigment was also evaluated. Although cheese whey has significant nutritional value, nearly half of the global production is discarded due to high treatment costs. This study demonstrates that whey can be repurposed as a sustainable and economical fermentation medium for pigment production, which is compatible with dairy plants. This makes it a promising solution to address the underutilization of whey by cheese local producers in Mexico. Prodigiosin has diverse industrial applications, including antimicrobial, insecticidal, and antioxidant properties. These findings highlight the potential for dairy waste valorization in a circular bioeconomy, reducing environmental impacts and promoting the creation of valuable bioproducts.

Colorants doi: 10.3390/colorants4040032

Authors: Víctor Gerardo Ibarra-García Alejandro Téllez-Jurado Juan Antonio Azpeitia-Vera Rosa Angeles Vázquez-García Victor M. Castano

Water hyacinth (Eichhornia crassipes) is one of the most invasive plants around the world. In the state of Hidalgo, Mexico it has invaded several water bodies. Nevertheless, its management is an ongoing challenge because of its rapid growth and the expensiveness of its removal. Therefore, alternatives to valorize its biomass are needed. One of them is the production of optical materials from it. Past reports have demonstrated the viability to obtain fluorophores from lignin and that it is present in E. crassipes biomass. Nevertheless, most works focus on its extraction using harsh process conditions and strong acids or alkalis. No reports about the use of E. crassipes in such processes exist. As the demand for more environmentally friendly processes increases, avoidance of such chemicals is needed. Therefore, in this work the extraction of fluorophores directly from biomass of E. crassipes via a hydrothermal process using water as the sole solvent and catalyzer was studied. The liquid to solid ratio (LSR) varied from 25 to 50 and time from 8 to 16 h. Biomass was almost completely dissolved. Fluorophores with different photoluminescent emissions were obtained. Their extraction was confirmed by photoluminescence spectroscopy. The emission of the obtained materials could be tuned by changing processing time and LSR.

Colorants doi: 10.3390/colorants4040031

Authors: Thiago Guimarães Costa Karen Kremer Fábio Andreas Richter Feik Amil de Campos Júnior Leonardo Negri Furini

In this work, molecular and elemental spectroscopic analyses were carried out on the preparation base, the paintings, the repaintings, and the gilding of an 18th century sacred sculpture of Our Lady found on Anhatomirim Island, where the Santa Cruz fortress was built in 1739 in the state of Santa Catarina, southern Brazil. The preparation base of the sculpture was characterized as gypsum (calcium sulfate dihydrate, (CaSO4.2H2O) [µ-Raman, SEM-EDS], applied directly to the wooden support. The blue paint comprised a mixture of Prussian blue (Fe4[Fe(CN)6]3) and ultramarine (NaxAl6Si6O24Sx) [µ-Raman, FTIR, SEM-EDS]; hematite (Fe2O3) was identified in the brown paint [µ-Raman, SEM-EDS]; and the white paint consisted of lead white (2 PbCO3·Pb(OH)2) [µ-Raman, FTIR, SEM-EDS]. Repainted areas were identified by the presence of lithopone (ZnS + BaSO4) [µ-Raman, SEM-EDS, FTIR], likely resulting from later interventions. In the gilded areas, gold was identified along with traces of iron [SEM-EDS], indicating a lower-quality gilding compared to, for example, silver alloys. Lead white was also identified in the polychrome areas, where it served to produce different tones in the painting. FTIR analyses revealed traces of aged oil used as a binder in the older layers. Mineral oil was detected in some samples, which may indicate that wax was used as a protective layer on the sculpture. The results will assist professionals in the iconographic characterization of the sacred image of Our Lady and in the conservation and restoration processes based on the identified constituent materials.

Colorants doi: 10.3390/colorants4040030

Authors: Yoshiumi Kohno Rika Fukagawa Masashi Shibata Yasumasa Tomita

This study investigates effective antioxidants to stabilize chlorophyll, a valuable and most abundant but unstable natural green pigment, adsorbed on a silica surface. Although fixing chlorophyll on silica offers some protection, significant photo-induced oxidative degradation still occurs. To enhance photostability, the prepared chlorophyll–silica composites were combined with various well-known antioxidants. The stability of these samples was evaluated by the deterioration ratio of the chlorophyll under visible light irradiation. The results showed that gallic acid provided the most significant stabilization effect. This was attributed to its moderate hydrophilicity, allowing it to be positioned near the chromophore part of the chlorophyll molecule adsorbed on the silica surface. Further tests with the derivatives of gallic acid revealed that smaller molecular size and less steric hindrance were also crucial for effectiveness as an antioxidative stabilizer. Pyrogallol and gallic acid, being the smallest molecules, performed best. It was concluded that the ability of an antioxidant to approach a chlorophyll molecule is essential for stabilization. This requires an appropriate balance of hydrophilicity and a small molecular size. Considering the nontoxicity together, gallic acid is recommended as a superior stabilizer for chlorophyll on silica surfaces.

Colorants doi: 10.3390/colorants4040029

Authors: Panangattukara Prabhakaran Praveen Kumar

Excessive copper ions in the human body can cause a variety of diseases, such as gastrointestinal disorders, cirrhosis, and Alzheimer’s disease. Techniques like Inductively Coupled Plasma–Mass Spectroscopy and Atomic Absorption Spectroscopy are available for copper detection, but the associated cost issues for sample preparation and labor limit their application for on-site detection. Herein, we are reporting a versatile method for detecting copper ions using a peptide-functionalized gold nanoparticle sensor in combination with various optical spectroscopic techniques. The peptide (CW) exhibited selective sensing ability for Cu(II) with visual colorimetric and optical spectroscopic changes compared to other metal ions tested. CW showed a visual colorimetric response from colorless to light brown color after interaction with Cu(II). Converting CW to a gold nanoparticle appended (CW-AuNPs) nanoplatform enabled a multimodal detection platform for Cu (II), which utilizes colorimetric and optical spectrum changes and surface-enhanced Raman spectroscopy (SERS) to enable highly sensitive sensing of Cu(II), even at extremely low concentrations (76 nms.). CW-AuNPs exhibit a controlled aggregation property in the presence of Cu(II), resulting in the creation of hot spots for SERS-based detection. Moreover, the peptide unit attached to the gold nanoparticles serves both as a binding motif for Cu(II) and as a Raman reporter for Cu(II) sensing. Our comprehensive analysis, including solution-state and dry-mapping Raman spectroscopic studies, demonstrates remarkable picomolar sensitivity of the peptide–gold nanoparticle system for Cu(II) detection. Moreover, we prepared a paper test strip from CW-AuNPs and used it as a visual colorimetric platform for sensitive detection of copper ions.

Colorants doi: 10.3390/colorants4030028

Authors: Mariana Consiglio Kasemodel Valéria Guimarães Silvestre Rodrigues João Marcos Ribeiro Farah Silva Bruna Soares Campelo Vallim Érica Leonor Romão

This research investigates the adsorption potential of mango and pitanga tree pruning waste biochar produced at 300 °C and 500 °C for the uptake of Methylene Blue (MB) dye. The particle size of biochar, initial MB concentration, adsorbent mass and pH of the solution were varied. Equilibrium data were modeled using Langmuir, Freundlich and Temkin equations. Increasing the temperature of the treatment resulted in a slight increase in the efficiency and adsorption capacity of the material. Finer particles (<0.25 mm) and pH (>6) were more efficient in adsorbing MB. Both materials presented similar modeled parameters for Langmuir, Freundlich and Temkin isotherm equations. The adsorption at equilibrium of MB is best described by Langmuir and Freundlich models, and the modeled maximum adsorption capacity values are 20.53 ± 5.47 mg g−1 for BTP-300 and 23.40 ± 6.41 mg g−1 for BTP-500, proving the biochar’s efficiency in the adsorption of MB and that the temperature of the thermochemical process did not affect qm.

Colorants doi: 10.3390/colorants4030027

Authors: Athanasia Tsatsarou Agathi Anthoula Kaminari Athina Georgia Alexopoulou Nadia Macha Bizoumi Anna Karatzani

This paper focuses on the study of the famous royal purple dye. It aims to present a holistic approach by researching historical evidence, both for its use and its production, to highlight the importance of the dye within the Greek area. As a substantial part of information concerning the dyeing procedure of purple dye has been lost during the ages, it is crucial to establish points of documentation and identification. The latter can be achieved through chemical analysis, but as this dye is found on precious’s cultural heritage items, which cannot always be sampled, a non-destructive approach should be considered as more appropriate. At first, the history of the dye purple is presented within the Greek area. Then, samples of purple dye are created based on traditional recipes from the Greek area, in order to compose a profile with the characteristics of purple using non-destructive and imaging techniques, thus emphasizing the importance of applying these techniques for the study of dyes on textiles. The results of the experiments show differences in behavior between the pure gland and the dyed samples, as well as the intensity of the color depending on the dyeing procedure.

Colorants doi: 10.3390/colorants4030026

Authors: Louise de Sousa Barbosa Sophia Helena Alves Brenno Henrique Silva Felipe Carlos Rafael Silva de Oliveira Catia Rosana Lange de Aguiar

This study evaluated dried eucalyptus leaf extract and annatto seed extract as natural dyes for cotton, polyamide, and polyester knit fabrics. The eucalyptus leaf extract was obtained by aqueous boiling extraction, while the annatto seed extract was prepared in an alcoholic medium at 60 °C. Dyeing was carried out on fabrics mordanted with lemon juice and soy milk, using a cup dyeing machine with infrared (IR) heating at 98 °C for 30 min. SEM and FTIR analyses assessed the results regarding color intensity and color fastness. The findings indicate that both extracts can serve as sustainable alternatives for textile dyeing.

Colorants doi: 10.3390/colorants4030025

Authors: Maria Vitória França Corrêa Gideã Taques Tractz Guilherme Arielo Rodrigues Maia Hagata Emmanuely Slusarski Fonseca Larissa Oliveira Berbel Lucas José de Almeida Everson do Prado Banczek

The present study investigates dye-sensitized solar cells (DSSCs) incorporating natural extracts from the microalgae Spirulina and Chlorella as photosensitizers. TiO2-based electrodes were prepared and immersed in methanolic algae extracts for 24 and 48 h. UV–Vis spectroscopy revealed absorption peaks near 400 nm and 650 nm, characteristic of chlorophyll. Electrochemical analyses, including photochronoamperometry and open-circuit potential, confirmed the photosensitivity and charge transfer capabilities of all systems. The cell sensitized with Chlorella after 48 h of immersion exhibited the highest energy conversion efficiency (0.0184% ± 0.0015), while Spirulina achieved 0.0105% ± 0.0349 after 24 h. Chlorella’s superior performance is attributed to its higher chlorophyll content and enhanced light absorption, facilitating more efficient electron injection and interaction with the TiO2 surface. Although the efficiency remains lower than that of conventional silicon-based solar cells, the results highlight the potential of natural colorants as sustainable and low-cost alternatives for photovoltaic applications. Nonetheless, further, improvements are required, particularly in dye stability and anchorage, to improve device performance. This research reinforces the viability of natural photosensitizers in DSSC technology and supports continued efforts to optimize their application.

Colorants doi: 10.3390/colorants4030024

Authors: Jéssica Antunes Marisa Lopes Beatriz Marques Augusta Silva Helena Vilaça Carla J. Silva

The textile industry is progressively shifting towards more sustainable solutions, particularly in the field of printing technologies. This study reports the development and evaluation of water-based pigment inks formulated with bio-based pigments derived from intermediates produced via bacterial fermentation. Two pigments—indigo (blue) and quinacridone (red)—were incorporated into ink formulations and applied on cotton and polyester fabrics through valve-jet inkjet printing (ChromoJet). The physical properties of the inks were analyzed to ensure compatibility with the equipment, and printed fabrics were assessed as to their color fastness to washing, rubbing, artificial weathering, and artificial light. The results highlight the good performance of the bio-based inks, with excellent light and weathering fastness and satisfactory wash and rub resistance. The effect of different pre-treatments, including a biopolymer and a synthetic binder, was also investigated. Notably, the biopolymer pre-treatment enhanced pigment fixation on cotton, while the synthetic binder improved wash fastness on polyester. These findings support the integration of biotechnologically sourced pigments into eco-friendly textile digital printing workflows.

Colorants doi: 10.3390/colorants4030023

Authors: Taiane L. Dlugoviet Andressa dos Santos Julia de Oliveira Primo Fauze Jacó Anaissi

Annually, more than 10,000 synthetic dyes are produced worldwide, generating around 280,000 tons of waste, posing risks to human and aquatic life, and potentially creating even more toxic products than the dyes themselves. This study aims to immobilize organic dyes, forming hybrid pigments using ZnO as support obtained through starch combustion. ZnO was obtained by starch (sago) combustion and characterized by XRD, SEM and the BET method. It was then used for the adsorption of orange and green textile dyes, evaluating the adsorbent dosage, initial dye concentration, contact time, and selectivity with copper ions. The removal studies indicated up to 100% removal of both dyes at low concentrations. The co-adsorption system showed excellent performance, with removal percentages exceeding 90% for both textile dyes and Cu (II) ions. Hybrid pigments were assessed for solvent resistance and durability under extended white light exposure. ZnO immobilized the dyes, showing resistance to organic solvents and good stability under prolonged white light exposure.

Colorants doi: 10.3390/colorants4030022

Authors: Panangattukara Prabhakaran Praveen Kumar

Recent advancements in phototherapy have underscored the need for effective cellular imaging agents that can enhance therapeutic efficacy and precision. Perylene diimide (PDI) dyes, known for their unique optical properties and biocompatibility, have emerged as promising candidates in this domain. This review paper provides a comprehensive analysis of the potential applications of PDI dyes in cellular imaging, specifically within the context of phototherapies. We explore the synthesis of these dyes, their photophysical characteristics, and mechanisms of cellular uptake. Moreover, this review highlights recent studies that demonstrate the effectiveness of PDI dyes in the real-time imaging of cellular processes and their synergistic effects in photodynamic therapy (PDT) and photothermal therapy (PTT). By evaluating various experimental approaches and their outcomes, we aim to elucidate the advantages of employing PDI dyes in clinical settings. The findings of this review suggest that perylene diimide dyes are not only capable of enhancing imaging contrast but also optimizing the therapeutic response in targeted phototherapy applications. Ultimately, this paper advocates for further research into the integration of PDI dyes in clinical practice, emphasizing their potential to significantly improve patient outcomes in cancer and other diseases requiring photoactive treatment modalities.

Colorants doi: 10.3390/colorants4020021

Authors: Mariana Consiglio Kasemodel Leandro Gonçalves de Aguiar Valéria Guimarães Silvestre Rodrigues Érica Leonor Romão

This research investigates the adsorption potential of four types of adsorbents produced from agro-industrial waste (grape pomace—GP, tree pruning—TP, sugarcane bagasse—SB, and eucalyptus sawdust—ES) for the uptake of thiazine dye methylene blue (MB) from aqueous solution. A kinetic model based on a hybrid-order rate equation was fitted to experimental data. The result showed that BGP-300 presented the highest mass yield (58.84%) and energy yield (69.56%), followed by BTP-300 > BES-300 > BSB-300. Adsorption studies showed that BGP-300 had a better performance in the uptake of MB, with a removal efficiency (Re) of 96.5% and adsorption capacity at equilibrium (qe) of 9.3 mg g−1, followed by tree pruning biochar (BTP-300), with an Re of 65.0% and qe of 5.3 mg g−1. Meanwhile, eucalyptus sawdust (BES-300) and sugarcane bagasse (BSB-300) biochar did not facilitate any significant removal of MB. Adsorption kinetics is best described by a second-order rate with R2 varying from 0.75 to 0.96. Desorption studies show a low concentration released to the solution, indicating that adsorption may occur physically and chemically. Therefore, this research provides comprehensive insights into the adsorption characteristics of different biochars, emphasizing the potential of torrefied materials BGP-300 and BTP-300 as effective for MB uptake from aqueous solution.

Colorants doi: 10.3390/colorants4020020

Authors: Derek Ho Julie Masura

The increasing prevalence of microplastic (MP) pollution and the labor-intensive nature of existing identification methods necessitate improved large-scale detection approaches. Nile Red (NR) fluorescence, which varies with polarity, offers a potential classification method, but standardization of carrier solvents and fluorescence differentiation techniques remains lacking. This study evaluated eight NR-carrier solvents (n-hexane, chloroform, acetone, methanol, ethanol, acetone/hexane, acetone/ethanol, and acetone/water) across ten common MP polymers (HDPE, LDPE, PP, EPS, PS, PC, ABS, PVC, PET, and PA). Fluorescence intensity, Stokes shift, and solvent-induced polymer degradation were analyzed. The study also assessed HSV (Hue/Saturation/Value) color spaces for Stokes shift representation and MP differentiation. Fenton oxidation effectively quenched fluorescence in natural organic matter (e.g., eggshells, fingernails, wood, cotton) while preserving NR-stained MPs. Acetone/water [25% (v/v)] emerged as the optimal solvent, balancing fluorescence performance and minimal degradation.

Colorants doi: 10.3390/colorants4020019

Authors: Chen Li Seri C. Robinson

Spalted wood is a natural material characterized by distinctive colors and patterns from wood decay fungi as they digest their substrate and leave behind colored secretions. As an art form, spalted wood was used heavily in western Europe from the 1400s–1600s; however, its use in other parts of the world remains deeply understudied, even in cultures where wood played a dominant social role. The use of spalted wood in China, in particular, is unknown, despite a growing interest by Chinese researchers in modern spalting practices and their potential commercial value. This study systematically reviews the potential historic use, current artistic value, environmental significance, and future application prospects of spalted wood for a Chinese market. By integrating historical records, modern scientific research, and insights from traditional Chinese woodworking, the study provides a comprehensive analysis of the aesthetic and functional value of spalted wood for Chinese markets. The findings indicate that the random and non-reproducible nature of spalted wood imbues it with exceptional artistic appeal and collectability, which has a strong potential to appeal to Chinese furniture design, decorative arts, and high-end interior applications. Furthermore, spalted wood demonstrates considerable potential for resource recycling by turning otherwise non-commercial, pale, white woods into higher value options—a phenomenon that has been studied across Europe and North America. In China, this has the potential to reduce wood waste and advance ecological design. However, challenges remain in fungal infection control, processing techniques, and market adoption. With ongoing advancements in biotechnology and manufacturing processes, spalted wood is poised to gain greater recognition in Chinese art, design, and cultural innovation while also contributing to green manufacturing and sustainable development.

Colorants doi: 10.3390/colorants4020018

Authors: Gabriela Elizabeth Quintanilla-Villanueva Esther Emilia Ríos-Del Toro Iris Cristina Arvizu-De León Donato Luna-Moreno Melissa Marlene Rodríguez-Delgado Juan Francisco Villarreal-Chiu

Globally, insect pests adversely affect approximately 75% of the most important crops. However, the widespread use of chemical insecticides has significant drawbacks, including non-specific biological activity, toxicity to humans, detrimental effects on beneficial insects, and the rapid development of resistance. In this context, prodigiosin—a tripyrrolic secondary metabolite produced by various microorganisms—emerges as a promising alternative due to its favourable properties, such as being non-toxic, environmentally safe, non-irritant, and non-allergenic, and having non-carcinogenic potential. Prodigiosin has demonstrated insecticidal efficiency against pests at various developmental stages. Studies suggest that prodigiosin inhibits enzymes like acetylcholine esterase, protease, and acid phosphatase and induces oxidative stress. This review explores the potential of prodigiosin as an eco-friendly insecticide, discussing its production, extraction, and purification processes and its advantages, disadvantages, and mechanism of action, and future perspectives. Special emphasis is given to using non-pathogenic strains to mitigate biosafety concerns.

Colorants doi: 10.3390/colorants4020017

Authors: Luiz Castro Larissa Matheus Giane Lenzi Mabel Arantes Lariana Almeida Rodrigo Brackmann Leda Colpini

This study explores the photocatalytic efficiency of Zn/Fe/TiO2 catalysts, synthesized via the wet impregnation method, for degrading the food colorings Allura Red and Tartrazine Yellow. A 22 factorial design with a central point replication guided the catalyst synthesis. Characterization involved BET surface area analysis, SEM-EDX, XRD, and PZC determination. Photocatalytic tests were conducted in batch mode under natural sunlight with 10 mg L−1 food coloring solutions. Kinetic modeling and statistical analysis were performed, and catalyst reuse was evaluated under artificial light. Results showed that low calcination temperatures (200–273 °C) and Zn loadings of 2–10% led to nearly 99% discoloration and degradation efficiency. The Behnajady–Modirshahla–Ghanbery kinetic model best described the discoloration data, confirming the significant impact of both variables. The optimal catalyst for Allura Red degradation was 2%Zn/2%Fe/TiO2 calcined at 200 °C, while for Tartrazine Yellow, 6%Zn/2%Fe/TiO2 calcined at 300 °C was most effective. Both catalysts exhibited excellent stability, maintaining efficiency over four reuse cycles. These findings demonstrate the potential of Zn/Fe/TiO2 catalysts for sustainable wastewater treatment.

Colorants doi: 10.3390/colorants4020016

Authors: Shipra Gupta

The world has recently witnessed the dire consequences of microbial infections in the form of the spread of COVID 19. Like viruses, bacterial infections too are a serious global health concern, especially because of the evolution of multidrug-resistant (MDR) bacterial populations. MDR bacteria are a result of the mindless use and misuse of antibiotics all over the world. Hence, there arises a need to find alternative strategies to effectively combat bacterial infections; all the more so for MDR bacterial infections. A lot of research has been conducted to find alternative antibacterial strategies such as phage therapy, the synthesis of antimicrobial peptides, the development of CRISPR-Cas systems, the incorporation of pro- and pre-biotics into our food or as supplements, and the development of bactericidal nanotechnology and antibacterial materials. Of these many strategies, this review focusses on the last one—the development of antibacterial materials. This article explores the potential of plant-derived natural colorants to serve as effective antibiotic materials to be used in various industries ranging from food, textile, paper, and leather to the pharmaceutical industry. Some major advantages of developing plant-derived natural colorants into antibacterial materials is that many of them possess inherent medicinal properties, they are biocompatible, non-toxic for humans, and biodegradable, and hence environment friendly. Many plant-derived natural colorants, like curcumin, indigo, lawsone, emodin, etc., have already been well studied for their antimicrobial properties. This review article aims at integrating some relevant studies to offer a cohesive overview of the current state of knowledge on the antibacterial properties of plant-derived natural colorants.

Colorants doi: 10.3390/colorants4020015

Authors: Susanne Machill Maximilian Voigtmann David Nescholta Horst Hartmann

Aldehyde green, a dye first obtained by reacting fuchsine with acetaldehyde in 1862, consists of, according to analytical investigations carried out on a sample of this dye deposited in the Historical Dyestuff Collection of the Technical University Dresden and performed with liquid chromatography and high-resolution mass spectrometry, a mixture of various compounds in which the aniline groups of fuchsine are converted into quinaldine and dihydroquinaldine moieties. The dye owns its green color by two absorption bands in the visible range at 435 and 616 nm.

Colorants doi: 10.3390/colorants4020014

Authors: José Balena G. Filho Clóvis G. Vieira Daniel B. de Jesus Henrique F. V. Victória Edmar A. Soares Klaus Krambrock Márcio César Pereira Luiz Carlos A. Oliveira

The use of dyes as sensitizing agents to increase semiconductor activity is a strategy already adopted in the field of heterogeneous photocatalysis, but the compounds applied are noble metal-based and sometimes difficult to synthesize, which make it more expensive. In this work, it was discovered that methylene blue can perform such an effect on an iron molybdate functionalized with peroxo groups on the surface. This material, called MoOxoFe, was tested together with its analogue MoFe (produced without H2O2 in the synthesis) in the degradation of methylene blue. The rapid degradation of the dye led to the hypothesis of sensitization, which was investigated and proven by additional photocatalytic tests with sensitized material, MoOxoFe-MB, and spectroscopies, such as EPR and XPS.

Colorants doi: 10.3390/colorants4020013

Authors: Panangattukara Prabhakaran Praveen Kumar Shivanjali Saxena Rakesh Joshi

BODIPY (Boron-Dipyrromethene) dyes have emerged as versatile fluorescent probes in cellular imaging and therapeutic applications owing to their unique chemical properties, including high fluorescence quantum yield, strong extinction coefficients, and remarkable photostability. This review synthesizes the recent advancements in BODIPY dyes, focusing on their deployment in biological imaging and therapy. The exceptional ability of BODIPY dyes to selectively stain cellular structures enables precise visualization of lipids, proteins, and nucleic acids within live and tumor cells, thereby facilitating enhanced understanding of biochemical processes. Moreover, BODIPY derivatives are increasingly utilized in Photodynamic therapy (PDT) and Photothermal therapies (PTT) for targeting cancer cells, where their capability to generate cytotoxic reactive oxygen species upon light activation offers a promising approach to tumor treatment. Recently, BODIPY derivatives have been used for Boron Neutron Capture Therapy (BNCT) for various tumors, and it is a growing research field. Advancements in nanotechnology have allowed the fabrication of BODIPY dye-based nanomedicines, either alone or with the use of metallic nanoparticles as a matrix offering the development of a new class of bioimaging and theragnostic agents. This review also discusses innovative BODIPY-based formulations and strategies that amplify therapeutic efficacy while minimizing adverse effects, underscoring the potential of these dyes as integral components in next-generation diagnostic and therapeutic modalities. By summarizing current research and future perspectives, this review highlights the critical importance of BODIPY dyes in advancing the fields of cellular imaging and treatment methodologies.

Colorants doi: 10.3390/colorants4020012

Authors: Anthony Harriman

This perspective sets out to raise awareness about the chemical and photophysical properties of indigo, a highly distinguished colorant with an extraordinary history. Indigo, like many other dyes, was first extracted from plants at an inordinately low yield and at great ecological expense. Such was its popularity that indigo was among the first natural colorants to be synthesized in a laboratory before refinement and cost reduction resulted in its economical industrial-scale production. The color of indigo is highly characteristic but difficult to describe, since it falls at the blue/violet interface. It is a small, planar molecule with an exceptionally high degree of π-electron conjugation that pushes the absorption maximum to above 600 nm. Its structure helps explain the high level of photostability enjoyed by indigo, while recent spectroscopic studies have added to our understanding of the longevity of this emblematic colorant. The reversible formation of leuco-indigo increases the ways in which indigo can be used to add color to objects while helping to circumvent the effects of attack by free radicals. It is stressed that the journal Colorants would welcome submissions that describe the chemistry and/or spectroscopy of other representative colorants.

Colorants doi: 10.3390/colorants4020011

Authors: Edilson Locks Selene Maria de Arruda Guelli Ulson de Souza Afonso Henrique da Silva Júnior Carlos Rafael Silva de Oliveira Catia Rosana Lange de Aguiar

Dyeing is a major contributor to pollution, with high concentrations of hydrolyzed dyes and electrolytes in its effluents. Recent studies suggest the possibility of dyeing cotton substrates with reactive dyes in an oil medium, reducing the need for electrolytes. This study evaluated the dyeing of cotton yarns with reactive Red 195 dye in an oil medium using crude and refined soybean oil. The method employed 75% oil and 25% water, with the oil recovered for reuse, significantly reducing water consumption and effluent generation. Dyeing with crude soybean oil showed higher color intensity than the conventional method and the use of refined soybean oil. Additionally, reducing electrolyte concentration from 75 to 18.75 g/L did not affect color intensity, yielding similar results to conventional aqueous dyeing. The dyed substrates were tested for washing, rubbing, and lightfastness, showing comparable performance to conventional methods. The dyeing followed pseudo-second-order kinetics, and the Freundlich isotherm model better fit the oil medium process. FTIR analysis revealed no changes in the functional groups on the yarn surface, and tensile strength tests showed similar results across methods. These findings indicate that oil medium dyeing can reduce electrolyte use, conserve water, and allow for oil reuse, demonstrating potential for industrial-scale application.

Colorants doi: 10.3390/colorants4010010

Authors: Ana P. Carvalho José Costa Angela Martins António M. Fonseca Isabel C. Neves Nelson Nunes

Textile dyes often prove resistant to conventional wastewater treatment processes because of their complex molecular structures. Advanced oxidation methods, such as the Fenton reaction, have thus been recognized as a promising approach for environmental remediation by decomposing these pollutants. This work aimed to study the efficacy of modified zeolites as catalysts in the Fenton reaction for dye degradation, with a particular emphasis on techniques for modifying zeolites and incorporating iron. The zeolite ZSM-5 was selected as the parent structure and underwent desilication and acid treatment procedures. Iron was introduced into the zeolite structure via two distinct methods: ion exchange and mechanochemistry. The modified zeolites with incorporated iron were evaluated in terms of their crystallinity, textural properties, and iron content before being used to degrade methylene blue solutions through the Fenton reaction. The reaction was monitored using UV-Vis spectroscopy, while the experimental outcomes were analyzed using pseudo-first-order and pseudo-second-order kinetic models. The research findings indicate that different treatment methods led to varying impacts on the zeolite properties, which in turn influenced the kinetic results. Moreover, it was observed that an enhancement in the degradation process can be achieved through the harmonious balance between a high iron content, increased mesoporosity (to facilitate diffusion), and adequate crystallinity (essential for maintaining structural integrity).

Colorants doi: 10.3390/colorants4010009

Authors: Saori Gontani Sakiko Takeshima Shinya Matsumoto Kazuo Kabashima

A series of compounds with both urea and urethane moieties have been developed as color developers for high-performance thermo-sensitive paper. The compounds have lower environmental loads than conventional phenolic developers. They were also found to greatly improve the speed of the printed images. In this study, we studied the coloring mechanism of the compounds when used as developers for a fluoran dye, and we investigated the stability of the colored solid state. The urea–urethane compounds were found to form black amorphous solids with the fluoran dye. Infrared (IR) measurements of the black solids, based on six urea–urethane derivatives, revealed that the colored dye has a ring-opened structure in a carboxylic acid form and that the urea group works as a proton donor for the ring-opening reaction. The stability of the black amorphous solids was also evaluated using thermal analysis and molecular orbital calculations in addition to IR data. The results indicate that the number of urea–urethane units and the planarity of the urea moiety are important parameters for the stability of the colored solid state.

Colorants doi: 10.3390/colorants4010008

Authors: Bibiana Bitelo Daniela E. Schneider Cesar L. Petzhold Douglas Gamba Tales S. Daitx

The use of indigo carmine dye in the textile industry, particularly in denim production, presents a significant sustainability challenge due to the large amounts of wastewater generated by this process, since this fabric is one of the most produced around the world. In order to face challenges like this, effluent treatment using polymeric materials has become an area of intense research. In this study, we developed an eco-friendly hydrogel based on oligoglycerol-malic acid polyester crosslinked with citric acid, which was applied to adsorb indigo carmine. The properties of the hydrogel and its precursors were analyzed using spectroscopic, thermal, and morphologic techniques. The hydrogel demonstrated water uptake capacity up to 187% of its own mass and adsorbed approximately 73% of the dye after 24 h of contact. Tests were conducted in the presence of sodium chloride and indicated that the presence of salt impairs the adsorption process. Additionally, the adsorption kinetics and isotherms were evaluated and demonstrated that the adsorption followed a pseudo-second-order model, indicating a chemisorption process, and a Langmuir isotherm, consistent with a monolayer adsorption. These results emphasize the potential of this hydrogel for removing dye and its application in textile industry wastewater treatment, aiming to minimize environmental impacts.

Colorants doi: 10.3390/colorants4010007

Authors: Itumeleng Seete Dineo A. Bopape Louisah M. Mahlaule-Glory Zikhona Tetana Nomso C. Hintsho-Mbita

Water contamination caused by dyes from increased human activities, in particular usage in the textile sector, has led to high rates of disposal of both natural and synthetic dyes in the water stream, affecting the color and the ability of the light to penetrate through the water system. Several methods have been used for the removal of these organic pollutants. However, due to the complex nature of these dyes, researchers have geared toward advanced oxidation processes (AOPs). This method allows for the degradation of these pollutants into more environmentally friendly pollutants. Green synthesis of known catalysts has been on the rise, in particular nickel oxide (NiO) NPs. This material has been shown to have the ability to degrade several pollutants. However, due to the high recombination rate and large bandgap, their limitation has also been highlighted along with the importance of modification. Thus, it is important to understand the work and progress made on green NiO as a photocatalyst for the degradation of dyes and the latest advancements in the field.

Colorants doi: 10.3390/colorants4010006

Authors: Laurenz Küchner Binh Nguyen Thanh Lina Diers Chantal Tautz Gerold Jerz Peter Winterhalter

The detection of reactive dyes in food matrices is crucial for food safety and compliance with regulations, especially since the use of such in food products is not approved. This study investigates the potential of using tin(II)chloride and laccase to cleave anthraquinone reactive dyes and to detect their characteristic degradation products as markers for the presence of dye in food. Nine reactive blue anthraquinone dyes and one green anthraquinone dye were cleaved using tin(II)chloride and laccase. Reactions with reactive dyes bound to maize starch were also carried out to evaluate the suitability of these methods for detecting matrix-bound dyes. Model food matrices, including gummy candy, hard candy, and maize chips, were spiked with the reactive dyes, and the presence of degradation products was analysed using LC-ESI-MS/MS. Two common cleavage products were formed from each sample, namely 1,4-diaminoanthrahydroquinone-2-sulphonic acid (DAHS) and 1-aminoanthraquinone-2-sulphonic acid (AAS). In all examined cases, at least one of the characteristic cleavage products could be detected. Laccase showed lower effectiveness with matrix-bound dyes, whereas treatment with acidic tin(II)chloride was effective even in complex food matrices. These findings suggest that the analysis of cleavage products could be a valuable tool for the detection of reactive dyes in food matrices.

Colorants doi: 10.3390/colorants4010005

Authors: Igor Alves Endrice Sandy Aline Forastieri Gerarduzzi Mariana Carla de Oliveira Marcos Luciano Bruschi Jéssica Bassi da Silva

Photodynamic therapy (PDT) is a light-activated chemical reaction used for the selective destruction of tissue. For this, various colorants may be applied, such as erythrosine (ERI), a dye already approved by the Food and Drug Administration (FDA) for various purposes. Although promising for PDT, ERI has a high hydrophilic profile that impacts its activity. To solve this, the combination of ERI with thermoresponsive and bioadhesive polymers may prove effective. Bio/mucoadhesive and thermoresponsive systems have attracted increasing interest in the development of novel pharmaceutical formulations for topical applications due to their ability to improve adhesion to the mucosa and prolong the residence time at the application site. In this study, systems based on poloxamer 407 (P407) in combination with cellulose derivatives (HPMC and NaCMC) were optimized, aiming at the topical release of ERI for PDT. The results demonstrated that the formulations containing low concentrations of cellulose derivatives exhibited greater adhesiveness and consistency at physiological temperature (37 °C), favoring the maintenance of the system at the application site. Regarding the gelation temperature (Tsol/gel), the formulations displayed values close to body temperature. The formulations with NaCMC showed a slightly higher Tsol/gel compared to HPMC ones, but it was adjustable by the polymer concentration. The addition of ERI influenced the mechanical and adhesive properties of the systems. In formulations containing HPMC, high concentrations of ERI increased bio/mucoadhesiveness, while in systems with NaCMC, the presence of ERI reduced this property. In both cases, the formulations maintained high consistency at 37 °C, contributing to the control of the active release at the application site. Rheological analysis revealed non-Newtonian behavior in all formulations, with greater consistency and elasticity at high temperatures. P407 was mainly responsible for the thermoresponsive transition from sol to gel, conferring desirable characteristics for topical application. Photodynamic activity was relevant in both formulations containing NaCMC and HPMC, which demonstrated greater capacity for degrading uric acid under light exposure. These systems are promising for the controlled release of drugs in photodynamic therapy, providing prolonged retention in the target tissue and maximizing the therapeutic efficacy of ERI.

Colorants doi: 10.3390/colorants4010004

Authors: Gabriela Maria Dyrda Tomasz Pędziński

The effects of photochemical reactions induced by UV radiation in solutions of metal phthalocyanines were carried out to determine the factors that might influence the photostability of photosensitized phthalocyanines. Three different indium phthalocyanines, including the diindium triple-decker phthalocyanine, In2Pc3 (1), sandwich indium phthalocyanine, InPc2 (2) and iodoindium phthalocyanine, InPcI (3) in benzene, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dichloromethane (DCM) and 1-chloronaphtalene, were studied. The rate of decay of absorption is explained by a decomposition reaction that is of first-order kinetics with respect to the phthalocyanine concentration. In general, the presence of ligand I− in phthalocyanine InPcI enhances the rate of decomposition. The kinetics of the degradation process proved to depend on the molecular structure of the complex and seems to be controlled by interactions of the macrocycle bridging nitrogen atoms with the solvent molecules. The indium phthalocyanines in benzene displayed the capacity for singlet oxygen generation.

Colorants doi: 10.3390/colorants4010003

Authors: Danielle Tapia Bueno Amanda Fonseca Leitzke Juliana Porciúncula da Silva Daisa Hakbart Bonemann Gabrielly Quartieri Sejanes Bruno Nunes da Rosa Taís Poletti Guilherme Kurz Maron Bruno Vasconcellos Lopes Matheus de Paula Goularte Darci Alberto Gatto André Luiz Missio Neftali Lenin Villarreal Carreno Claudio Martin Pereira de Pereira

In this study, hybrid materials were synthesized incorporating curcumin, Cu2+ or Fe3+, and Kappa-carrageenan as a reducing agent to improve stability, considering that curcumin has low thermal and solution stability, which limits its applications. Colorimetric analysis showed color changes in the hybrids, ultraviolet–visible spectroscopy revealed band shifts in the hybrids, and infrared analysis indicated shifts in wavenumbers, suggesting changes in the vibrational state of curcumin after bonding with metal ions. These techniques confirmed the formation of hybrid materials. Thermogravimetric and chromatographic analyses demonstrated greater thermal and solution stability for the hybrids compared to curcumin. Additionally, the hybrid composites effectively developed natural and sebaceous latent fingerprints with good clarity and contrast on glass surfaces. Both composites performed similarly to commercial Gold® powder. When applied to surfaces representative of forensic scenarios, the composites were versatile, revealing sufficient fingerprint details for human identification on both porous and non-porous surfaces. Scanning electron microscopy images showed greater clarity in sebaceous and natural fingerprints developed with the Fe composite compared to the Cu composite.

Colorants doi: 10.3390/colorants4010002

Authors: Nemeshwaree Behary Nicolas Volle

Indigo leaves from various plant species are sources of dyes/pigments, not fully exploited for making sustainable textiles. Blue indigo vat dye extracted from indigo leaves yields high wash color fastness but fades slowly with light, and is not easily used for direct printing. Indigo leaves can be used to produce textiles of various color shades, while light-resistant Mayan-inspired hybrid pigments have not yet been used for textile coloring. Using blue indigo dyes from three plant species, with exhaustion dyeing, intense wash-resistant blue-colored textiles are produced, and in the case of Indigofera Persicaria tinctoria, textiles have antibacterial activity against S. epidermis and E. coli. A 100% natural Mayan-inspired blue indigo pigment, made from sepiolite clay and natural indigo dye, was used both in powdered and paste forms to perform pigment textile dyeing by pad cure process, and direct screen printing on textiles. A water-based bio-binder was used efficiently for both padding and printing. Bio-based Na Alginate thickener allowed to produce prints with good color-fastness on both polyester and cotton fabrics, while bio-based glycerin produced excellent print color fastness on polyester only: wash fastness (5/5), dry and wet rub fastness (5/5) and light fastness (7/8).

Colorants doi: 10.3390/colorants4010001

Authors: Maria Elisa Pailliè-Jiménez Jamile Queiroz Pereira Eliseu Rodrigues Adriano Brandelli

Aryl polyene (APE) are bacterial pigments which show great biotechnological potential because of their biological activities. In this study, the presence of gene clusters associated with APE synthesis was investigated in the genome of Chryseobacterium sp. kr6 and Lysobacter sp. A03. The pigments extracted from strains kr6 and A03 were further characterized by liquid chromatography coupled to a high-resolution mass spectrometer (LC-DAD-MS). These bacteria harbor the relevant genes for APE biosynthesis; while kr6 may produce flexirubin pigments and have a 75% similarity with the flexirubin cluster from Flavobacterium johnsoniae UW101, Lysobacter sp. A03 showed a 50% similarity with the xanthomonadin I gene cluster from Xanthomonas oryzae pv. oryzae. A comparison with the gene clusters of APE-producing bacteria revealed that kr6 and A03 harbor genes for key proteins that participate in APE biosynthesis, such as acyl carrier proteins, acyl dehydratases and acyl reductases. The LC-DAD-MS analysis revealed that kr6 produces a possible mixture of flexirubins, whereas the yellow pigment from A03 is proposed to be a xanthomonadin-like pigment. Although the fine molecular structure of these pigments are not yet fully elucidated, strains kr6 and A03 present great potential for the production of natural bioactive pigments.

Colorants doi: 10.3390/colorants3040025

Authors: Payal M. Oak Akash S. Mali

Advancements in cellular imaging have significantly enhanced our understanding of membrane potential and Ca2⁺ dynamics, which are crucial for various cellular processes. Voltage-sensitive dyes (VSDs) are pivotal in this field, enabling non-invasive, high-resolution visualization of electrical activity in cells. This review discusses the various types of VSDs, including electrochromic, Förster Resonance Energy Transfer (FRET)-based, and Photoinduced Electron Transfer (PeT)-based dyes. VSDs are essential tools for studying mitochondrial activity and neuronal function and are frequently used in conjunction with Ca2⁺ indicators to elucidate the complex relationship between membrane potential and Ca2⁺ fluxes. The development of novel dyes with improved photostability and reduced toxicity continues to expand the potential of VSDs in biomedical research. This review underscores the importance of VSDs in advancing our understanding of cellular bioenergetics, signaling, and disease mechanisms.

Colorants doi: 10.3390/colorants3040024

Authors: Masafumi Yano Minami Ueda Tatsuo Yajima Koichi Mitsudo Yukiyasu Kashiwagi

N-Phenylphenothiazine derivatives extended with various aryl groups were designed and synthesized. These derivatives have bent conformation in crystal and exhibit high solubility. Radical cations obtained by one-electron oxidation of these derivatives gave stable radical cations in solution and showed absorption in the near-infrared region. A radical cation was isolated as a stable salt, which exhibited heat resistance up to around 200 °C. A design strategy for radical cation-based near-infrared absorbing dyes, which are easily oxidized and stable not only as a solution but in solid form, is described.

Colorants doi: 10.3390/colorants3040023

Authors: Higor Oliveira da Cunha Arcano Matheus Bragança Leite Rajendran Suresh Babu Hamilton Santos Gama Filho Ramon Silva dos Santos Marcelino José dos Anjos Ana Lucia Ferreira de Barros

This study investigates the effects of varying CuO doping concentrations on the performance of titanium dioxide (TiO2)-based or zinc oxide (ZnO)-based dye-sensitized solar cells (DSSCs). TiO2 or ZnO mixed with CuO at different weight percentages (0–50 wt %) was employed as photoanodes in DSSCs, prepared via mechanical mixing. X-ray diffraction analysis revealed the structural changes, showing that as the CuO content increased in the hybrid, the CuO peaks (notably at 35.5° and 38.7°) became more prominent. Morphological and elemental characterizations were conducted using SEM and XRF, respectively. The solar cells were photosensitized by Vitis lasbrusca (V.L.) extract and N3 dye. The presence of anthocyanin molecules in the extracted V.L. was confirmed using UV-VIS and FTIR spectroscopy. The electrochemical characterization demonstrated optimal solar conversion efficiencies at a 20% doping level for both photosensitizers. Specifically, in the V.L. dye, TiO2-CuO achieved a conversion efficiency of 7.18%, and ZnO-CuO reached 5.77%. In the N3 dye, TiO2-CuO showed an efficiency of 11.34%, and ZnO-CuO, 9.55%. Notably, undoped photoanodes displayed a significantly lower photovoltaic performance: for V.L. dye, TiO2 showed 1.12% and ZnO 0.87%; for N3 dye, TiO2 showed 6.02% and ZnO 4.39%. Doping was therefore effective, yielding up to a seven-fold increase in performance in the case of V.L. with TiO2, compared to undoped DSSCs. The results demonstrate that using the hybrid photoanode led to a considerable increase in performance compared to using only TiO2 or ZnO photoanodes, highlighting the potential of DSSCs as sustainable energy sources.

Colorants doi: 10.3390/colorants3040022

Authors: Arcano Matheus Bragança Leite Higor Oliveira da Cunha Paula Romanhi Leandro Ferreira Pedrosa Rajendran Suresh Babu Ana Lucia Ferreira de Barros

This work addresses the implementation of the co-sensitization technique to increase the energy efficiency of organic dye-sensitized solar cells (DSSCs). Fluorescent dyes derived from boron complexes— (BORANIL) and (BODIPY)— were successfully synthesized and used as co-sensitizers in different volume percentage ratios to verify the most effective concentration for photon capture through these sensitizers. The dyes were optically characterized using ultraviolet–visible spectroscopy (UV-VIS) and Fourier transform infrared (FTIR), analyzing them through the optical performance of each hybrid combination of dyes, an optimization of the photon collection capacity in the tests performed in a volume percentage ratio of 25:75 or 1:3. The morphology and surface roughness of the electrodes were analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. Through electrochemical characterizations, it was found that the highest photovoltaic conversion efficiency was obtained with the ATH1005 (D) dye mixed with ATH032 (G) in the proportion of 25%:75% or DG 1:3, with efficiency (η) of 3.45%, against 2.43% and 1.90% for DG 1:1 and DG 3:1 cells, respectively. Cells with BODIPY dyes also present higher conversion efficiencies compared to BORANIL cells. The results corroborate the presentation of organic solar cells as a viable option for electricity generation.

Colorants doi: 10.3390/colorants3040021

Authors: Julia de Oliveira Primo Raphaella Wolf Fleck Dienifer F. L. Horsth Andressa dos Santos Taiane L. Dlugoviet Fauze J. Anaissi

Hybrid pigments were obtained by combining zinc oxide with the anionic dye Congo red (CR), a breakthrough with significant environmental implications. By adjusting the ratio of solid mass to dye concentration, it is possible to obtain pigments with pink hues from a white solid (ZnO) through its adsorption of CR. The process involved using ZnO, prepared at 800 °C using cassava starch suspension as a suitable fuel. The oxide was characterized using XRD, SEM, and BET, and the results showed that the textural properties are typical of nanoparticles, with a size of 50.5 nm, a pore size of 3.48 nm, and a surface area of 3.03 nm, making it suitable for molecular dye removal. Controlling the adsorbent mass (in grams) and dye concentration (in mg L−1) makes it possible to consistently produce hybrid pigments in various shades of pink that exhibit good thermal resistance. When dispersed in white waterborne paint, they are chemically stable in different solvents, have excellent painted surface coverage, and resist photochemical degradation. The results demonstrate technical feasibility and compatibility with the Sustainable Development Goals, particularly Goals 6, 11, 12, 14, 15, and 17, offering a promising solution for a more sustainable future.

Colorants doi: 10.3390/colorants3040020

Authors: Patrícia Acosta Caetano Pricila Pinheiro Nass Mariany Costa Deprá Tatiele Casagrande do Nascimento Eduardo Jacob-Lopes Leila Queiroz Zepka

With the increasing awareness of socio-environmental issues, a global trend has emerged emphasizing the valorization of natural ingredients that promote health and well-being within sustainable production systems, such as microalgae-based carotenoids. Currently, little is understood about the correlation between biomass productivity and carotenoid content, which is a fundamental parameter for facilitating the immediate expansion of microalgae bioprocesses and ensuring the availability and industrial viability of these compounds. In this context, this study aims to investigate the carotenoid profile of Chlorella vulgaris through growth curve experiments conducted under photoautotrophic and heterotrophic regimes. Additionally, a trade-off analysis was performed for the production of carotenoids from microalgae. Carotenoids were quantified using high-performance liquid chromatography coupled with diode array and mass spectrometry detectors (HPLC-PDA-MS/MS). The performance of kinetic phases and energy demands across growth regimes was assessed to provide insights into production trade-offs. The results indicated that a total of 22 different carotenoids were identified in all the extracts. The all-trans-lutein and all-trans-β-carotene were the majority compounds. The total carotenoid content of Chlorella vulgaris revealed significant differences in the kinetic phases of carotenoid production, indicating that carotenoid volumetric production is only viable if the cultures are conducted until the log and stationary phases, based on the function of the biomass volumetric production (weight.volume−1). Therefore, the best trade-off for the process was to provide photoautotrophic growth until the exponential phase (log). Under this condition, the maximum carotenoid and lutein content was 2921.70 µg.L−1, reaching a maximum cell biomass of 1.46 g.L−1. From an environmental/economic point of view, the energy demand was 7.74 kWh.L−1. Finally, the scientific advances achieved in this study provide a holistic view of the influence of the main cultivation methods on the production of microalgae carotenoids, suggesting a viable initial direction for different industrial applications.

Colorants doi: 10.3390/colorants3040019

Authors: Sthanley R. De Lima Thiago V. Costa Tácio T. S. Santos Dora G. Felipe Teófanes B. Serna Acácio A. Andrade Viviane Pilla

(1) Background: This work evaluated the optical characterization of aqueous fluorescent chitosan-based carbon dots (or carbon nanoparticles CNPs) embedded in natural dye for potential functional packaging applications. Chitosan-based materials are nontoxic, biodegradable, biocompatible, bactericidal, and produced from renewable polymer sources. Anthocyanins are pigments of different colors with a large range of potential applications, such as in bioindicators and biomonitoring; (2) Methods: The CNPs were synthetized in aqueous solutions using chitosan as a carbon source. The natural dye was extracted from the leaves of Tradescantia pallida Purpurea in aqueous solutions. The fluorescence quantum efficiency (η) and fluorescence lifetime (τ) were determined using the mode-mismatched pump–probe thermal lens (TL) technique and time-resolved fluorescence lifetimes (TRFL) measurements, respectively; (3) Results: The η and τ were measured for CNPs embedded in natural dye solution at different concentrations (5.2, 12.09, and 21.57 mass percentage composition). The η and τ photophysical parameters obtained for CNPs embedded in natural dye were compared with those of other CNPs synthesized using different carbon sources, such as leaves, seeds, and protein; (4) Conclusions: Fluorescence spectra and time-resolved fluorescence measurements corroborate the TL results, and relatively high values of η were obtained for the CNP synthesized and embedded in natural dye.

Colorants doi: 10.3390/colorants3040018

Authors: Anthony Harriman

This perspective presents an account of the underlying features associated with the photofading of organic colorants. Photofading is commonly known to the scientific community as photodegradation or photooxidation, while in earlier times the more grandiose term “light fastness” was commonplace. This is a subject of immense diversity and significance, but there are many challenges to be faced when attempting mechanistic reasoning. The text is illustrated by descriptions of several systems taken from the scientific literature, together with anecdotes related to the principal researchers. The chemical challenges to be overcome in order to design photostable materials are outlined and reference is made to the natural world. It is stressed that the journal Colorants would welcome submissions in this field.

Colorants doi: 10.3390/colorants3040017

Authors: Dienifer F. L. Horsth Julia de O. Primo Fauze J. Anaissi Polona Umek Carla Bittencourt

Inorganic cool pigments are widely used as cooling agents in residential coatings due to their ability to achieve near-infrared reflectance. These coatings can be designed to exhibit a variety of colors independent of their reflectivity and absorption properties. Recent studies have highlighted the development of novel near-infrared (NIR) blue pigments, with an increasing emphasis on environmentally sustainable options that demonstrate high NIR reflectivity. This trend highlights the importance of creating novel and eco-friendly NIR reflective blue pigments. This study presents the synthesis of cobalt aluminates with varying concentrations of coloring ions (Co2+), achieved through the recycling of aluminum can seals via chemical precipitation. The formation of the spinel phase was confirmed through X-ray diffraction (XRD), and a colorimetric analysis was performed in the CIEL*a*b* color space. The synthesized pigments exhibited high near-infrared solar reflectance, with R% values ranging from 34 to 54%, indicating their potential as energy-efficient color pigments for use in coatings.

Colorants doi: 10.3390/colorants3030016

Authors: Dalete Araujo de Souza Pedro Hyug de Almeida da Silva Francisco Paulino da Silva Yonny Romaguera-Barcelay Robson Dantas Ferreira Edgar Alves Araujo Junior José Fábio de Lima Nascimento Fagner Ferreira da Costa Litiko Lopes Takeno Yurimiler Leyet Ruiz Lianet Aguilera Dominguez Walter Ricardo Brito Francisco Xavier Nobre

This study presents a simple, low-cost, and efficient route to obtain zinc oxide by adopting the thermal decomposition method of zinc acetate at 300 (Gr@ZnO_300), 400 (Gr@ZnO_400), 500 (Gr@ZnO_500), and 600 °C (Gr@ZnO_600) for 1 h. The diffraction patterns collected for the samples indicated the majority formation of the hexagonal phase (P63mc) for zinc oxide and residual amounts for graphitic carbon, which has a hexagonal structure of space group P63/mmc. The images collected by scanning electron microscopy (SEM) revealed the formation of sub-microcrystals with elongated rod-shaped morphology, with dimensions between 0.223 and 1.09 μm. The optical and colourimetric properties of the obtained materials indicate the presence of graphitic carbon in the samples, corroborating the analysis by XRD and Raman spectroscopy, with an optical bandgap close to 3.21 eV, and energies of the valence (EVB) and conduction (ECB) bands of 2.89 eV and −0.31 eV, respectively. The photocatalytic performance at 20 min of exposure time under UV light of all prepared samples in the decolourisation of rhodamine B (RhB) dye solutions follows the order Gr@ZnO_300 (95.6%) > Gr@ZnO_600 (92.8%) > Gr@ZnO_400 (84.0%) > Gr@ZnO_500 (78.1%), where the photocatalytic performance of Gr@ZnO_300 sample was 16.5 times more effective than the photolysis test. Moreover, the results confirmed that the best performance was archived at pH = 10, and the holes (h+) and superoxide (O2•−) radicals are the main species involved in the discolouration of RhB dye molecules in an aqueous medium. Finally, the reusability experiment shows high stability of the Gr@ZnO_300 sample as a solid photocatalyst and cycling capability, which obtained total discolouration of RhB of a solution under five cycling experiments of 60 min of exposure to UV light at room temperature.

Colorants doi: 10.3390/colorants3030015

Authors: Panangattukara Prabhakaran Praveen Kumar

This study explored the synthesis and application of BODIPY-functionalized gold nanoparticles (AuNPs) for the sensitive detection of biothiols via an indicator displacement assay coupled with surface-enhanced Raman scattering (SERS) techniques, alongside their efficacy for in vitro cancer cell imaging. Moreover, the assay allowed for the visible colorimetric detection of biothiols under normal and ultraviolet light conditions. The BODIPY (boron-dipyrromethene) fluorophores were strategically conjugated to the surface of gold nanoparticles, forming a robust nanohybrid that leverages the plasmonic properties of AuNPs for enhanced spectroscopic sensitivity. The detection mechanism exploited the displacement of the BODIPY indicator upon interaction with biothiols, triggering a measurable change in fluorescence and SERS signals. This dual-mode sensing approach provides high selectivity and sensitivity for biothiol detection, with detection limits reaching nanomolar concentrations using fluorescence and femtomolar concentration for cysteine using SERS. Furthermore, the BODIPY-AuNP complexes demonstrated excellent biocompatibility and photostability, facilitating their use in the fluorescence imaging of biothiol presence within cellular environments and highlighting their potential for diagnostic and therapeutic applications in biomedical research.

Colorants doi: 10.3390/colorants3030014

Authors: Kota Mori Yuki Kurokawa Shyam S. Pandey

Novel far-red sensitive symmetric squaraine (SQ) dyes with terminal alkyl chain modifications were designed, synthesized, and characterized, aiming towards imparting multifunctionalities such as photosensitization, dye aggregation prevention, and source of electrolyte components. The dye sensitizer SQ-80 with alkyl chain terminal modifications consisting of 1-methylimidazolium iodide was designed and synthesized as a new dye sensitizer for DSSCs based on symmetric SQ-4 without any terminal modification used as reference. Upon adsorption on the mesoporous TiO2 surface, SQ-80 demonstrated reduced dye aggregation and stronger binding to the TiO2 surface, leading to enhanced durability of DSSCs. Apart from the most common photosensitization behavior, the newly designed dye demonstrated multifunctionalities such as aggregation prevention and electrolyte functionality, utilizing iodine-based redox electrolytes in the presence and absence of I2 and LiI additives. In the absence of LiI and I2, a mixture of SQ-77 with alkyl chain terminal modifications consisting of iodide and SQ-80 demonstrated a photoconversion efficiency of 1.54% under simulated solar irradiation, which was about six times higher compared with the reference dye SQ-4 (0.24%) (having no alkyl chain terminal modification).

Colorants doi: 10.3390/colorants3030013

Authors: Hye Jin Kim

To protect skin from harmful ultraviolet (UV) radiation, there has been a resurgence in the use of natural dyes with metal mordants to reduce contamination by advanced chemicals. This study achieved natural dyeing in violet and yellow colors from Gromwell red roots and Cape jasmine seeds for UV-protective materials. The dyed fabrics were subjected to zinc oxide (ZnO) and polyphenol treatments, as well as copper post-mordanting. The SEM, TEM, and XRD tests showed that the ZnO nanoparticles, with hexagonal crystal structures, stuck to the fiber surfaces, and twisted strands resulted in the K/S reduction. First, this study found that the untreated cotton in violet, despite the highest K/S, faded the most intensely when exposed to UV. The color variation of untreated polyester was narrow, with little change in L, a*, and K/S. The color change of yellow-dyed samples treated with ZnO/polyphenol was not considerable in yellowness (b*: 28.838), while the violet fabrics displayed a significant decrease in K/S and an increase in b*. The combination of ZnO and polyphenol treatment improved UV absorption at 350 to 250 nm. Among the Cu-mordanted fabrics after ZnO/phenols treatment, the violet cotton turned reddish from blueish (negative to positive b*), with a hue change of 316° to 59° and the highest ΔE (25.90 ± 4.34) after UV exposure. In this study, the combination of ZnO/polyphenol with Cu-mordants allowed the Cape jasmine-dyed polyester to achieve a minimum ΔE as well as to keep its chroma and hue after UV exposure.

Colorants doi: 10.3390/colorants3020012

Authors: Alexandra Coia Jackson Ruddick Olivia Kuang Li-Qiong Wang

Transition metal complexes have historically played a pivotal role in creating vibrant pigments utilized across artistic mediums such as ceramics, paintings, and glass mosaics. Despite their extensive historical use, our understanding of the mechanisms governing transition metal complex behavior has predominantly emerged in recent times, leaving numerous aspects of this process ripe for exploration. These complexes exhibit striking color variations under diverse conditions when employed in pigment formulations. This review utilizes a bottom-up scientific approach, spanning from microscopic to macroscopic scales, to unravel the molecular origins of the colors generated by transition metal complexes in pigments and ceramic glazes. Advanced spectroscopy techniques and computational chemistry play pivotal roles in this endeavor, highlighting the significance of understanding and utilizing analytical data effectively, with careful consideration of each technique’s specific application. Furthermore, this review investigates the influence of processing conditions on color variations, providing valuable insights for artists and manufacturers aiming to enhance the precision and quality of their creations while mitigating environmental impact.

Colorants doi: 10.3390/colorants3020011

Authors: Cássio Siqueira Aline B. Schons Patricia Appelt Weslei D. Silva Nayara Balaba Mário A. A. Cunha Fauze J. Anaissi

Magnesium oxide is typically white and can be colorized with transition metal insertion by doping. We present the preparation of a green-colored hydroxide by the exchange of Mg2+ on the crystalline lattice with Ni2+ in MgO, using three nickel salts. MgOst was prepared by the colloidal starch suspension method, using cassava starch. The oxides and hydroxides, before and after, were characterized by X-ray diffraction (XRD), and show that a phase change occurs: a transition from periclase (MgO) to brucite (Mg(OH)2) due to the incorporation of nickel ions from different salts (acetate, chloride, and nitrate), resulting in the solid solution [NixMg1−x(OH)2]. The FTIR spectrum corroborates the crystallographic structure identified through XRD patterns, confirming the formation of a crystal structure resembling brucite. The new samples present a green color, indicative of the incorporation of the Ni2+ ions. The antimicrobial activity of products resulting from the doping of magnesium oxide with nickel and the precursor MgOst was assessed through the minimum inhibitory concentration (MIC) test. The evaluation included three bacterial strains: Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922), Salmonella gallinarum (ATCC 9184), and a yeast strain, Candida albicans (ATCC 10231). The obtained results were promising; the tested samples exhibited antimicrobial activity, with a MIC ranging from 0.312 to 0.625 μg.μL−1. The nickel compound, derived from the precursor chloride salt, demonstrated superior MIC activity. Notably, all tested samples displayed bactericidal activity against the S. aureus strain and exhibited a broad spectrum of inhibition, encompassing both Gram-positive and Gram-negative strains. Only the nickel compounds derived from precursors with acetate and nitrate anions demonstrated antimicrobial activity against C. albicans, exhibiting a fungistatic behavior. Based on the conducted studies, [NixMg1−x(OH)2] has emerged as a promising antimicrobial agent, suitable for applications requiring the delay or inhibition of bacterial growth.

Colorants doi: 10.3390/colorants3020010

Authors: Anthony Harriman

As Editor-in-Chief of Colorants [ISSN: 2079-6447], I have watched the progressive growth of this journal and witnessed the expansion of topics way beyond the initial consideration [...]

Colorants doi: 10.3390/colorants3020009

Authors: Magali Hernández Carlos Felipe Ariel Guzmán-Vargas José Luis Rivera Enrique Lima

Structural hydroxide groups in layered magnesium–aluminum double hydroxides were partially replaced by fluoride ions. Fluorinated and fluorine-free materials were used as hosts for two dyes, carminic acid and hydroxyl naphthol blue, resulting in a hybrid pigment color palette. The pigments were produced by two ways, either incorporating chromophore during the synthesis of the layered double hydroxide or in a post-synthesis step through the memory effect of the LDHs. Additionally, the pigments were protected with a magnesium hydroxide phase to prevent the color from fading over time. The pigments were stable for periods as long as 10 years. The color properties of the pigments were significantly influenced by the host of dye since the presence of fluorine directly influences the acid–base properties of the layered double hydroxides. The pigments conferred their color to white cream in the preparation of colored creams. The colored creams acquired the color of the layered pigment.

Colorants doi: 10.3390/colorants3020008

Authors: Luis Filipe Vieira Ferreira Ana Olaio Manuel Francisco Costa Pereira Isabel Luisa Ferreira Machado

The ceramic pastes of ca. 31 samples recovered from the Almaraz archaeological site, located in the south bench of Tagus River, were studied in detail using XRF, micro-Raman and GSDR spectroscopies, as well as the XRD technique. The ceramic sherds could be grouped into six categories, red slip tableware, decorated tableware, yellow slip tableware, grey tableware, common tableware, and handmade pottery. Our studies of the mineralogic composition of the sherds’ body indicate all ceramics were produced locally, using siliceous clays in most cases and calcareous clays in a few ones. Micro-Raman and ground state diffuse reflectance absorption spectroscopy provided useful information regarding the materials used to produce the coloured ceramics: hematite and brookite for the red slip and decorated ceramics, jacobsite or carbon black for the black decoration or grey ceramics. For the yellow slip tableware, a simple engobe rich in yellow clay was used. XRF spectroscopic studies provided the elemental composition of all samples, and biplots of the potassium (K) versus calcium (Ca) contents, normalized to the silicon content of each ceramic paste, clearly show Pliocene and Miocene local clays sources were used to produce most ceramics. Only one sherd can be considered a Lisbon production.

Colorants doi: 10.3390/colorants3020007

Authors: Emerson Colonetti Lilian C. da Luz Fabiano S. Rodembusch

Herein, we present the preparation of solid-state photoactive starches with a large Stokes shift, along with the resulting materials. In this investigation, 2-(2′-hydroxyphenyl)benzazole derivatives responsive to intramolecular proton transfer in the excited state (ESIPT) were covalently bonded to the polymeric structure of starch through a reaction involving an isothiocyanate group and the hydroxyl groups of starch. These compounds exhibit absorption at approximately 350 nm, which is related to fully spin- and symmetry-allowed π → π* electronic transitions, and solid-state fluorescence at approximately 500 nm, which features a significant separation between the absorption and emission maxima (~9000 cm−1). Due to the minimal use of fluorophores in functionalized starch preparation, this modification does not affect the original properties of the starch. Finally, photoactive starch-based films with significantly high transparency were successfully produced.

Colorants doi: 10.3390/colorants3020006

Authors: Bence Kontra Zoltán Mucsi László Vanyorek Miklós Nagy

Smart solvatochromic isocyano-aminoarenes (ICAArs) have been gaining attention owing to their unique photophysical, antifungal and anticancer properties. Using a simple dehydration reaction with in situ-generated dichlorocarbene, we prepared 2-amino-7-isocyanofluorene (2,7-ICAF). We studied the effect of the longer polarization axis provided by the fluorene core on the spectral properties and we also compared it to those of the starting diamine. 2,7-ICAF shows a clear solvatochromic behavior close to the blue part (370–420 nm) of the visible spectrum. Quantum chemical calculations show internal charge transfer (ICT) between the donor amino and the electron-withdrawing isocyano groups. 2,7-ICAF has high molar absorptivity (ε = 15–18·103 M−1cm−1) and excellent quantum yield (Φf = 70–95%) in most solvents; however, its fluorescence is completely quenched in water. The high brightness (ε·Φf) and close to zero quantum yield in water may be favorable in biolabeling applications, where background fluorescence should be kept minimal. Overall, 2,7-ICAF shows enhanced photophysical properties compared to its previously investigated relative 4-amino-4′-isocyano-1,1′-biphenyl (4,4′-ICAB).

Colorants doi: 10.3390/colorants3010005

Authors: Kelly Rodrigues Alana M. X. de Sousa Andreza D. O. dos Santos Bárbara C. A. Barbosa A. Rita Silva Luciana Pereira Glória M. M. Silva

Effluents from the textile industry are an active problem in the sector and one of the world’s main environmental problems. The conventional treatments applied are not always efficient in terms of compliance with legislation, and, in many cases, the efficiency of treatment is guaranteed by the enormous energy expenditure involved, camouflaging the momentary problem and not effectively treating it. In this work, batch reactors with immobilized biomass of Aspergillus niger AN400 were arranged in series for the treatment of real textile wastewater containing approximately 20 mg/L of indigo carmine. Sucrose was added as a co-substrate in concentrations of 1 g/L and 0.5 g/L, in the first and second reactors, respectively, over 19 cycles of 48 h. The highest decolorization rate in the system was (93 ± 4) %, with the largest amount removed in the first reactor (90 ± 6) %, occurring mainly by biological means. The production of aromatic by-products from the initial degradation of the dye molecule was reflected in the lower removal efficiency of dissolved organic matter: 52% in the first reactor, and 25% in the second reactor. The number of colonies of fungi was higher than that of bacteria, 2.24:1 and 2.44:1 in the first and second reactors, respectively. The treated effluent in the system showed less toxicity than the raw effluent, and this demonstrates the potential of this technology in the treatment of textile effluents containing indigo carmine.

Colorants doi: 10.3390/colorants3010004

Authors: Estelle Leonard China Takeda Takashiro Akitsu

Azo-Schiff bases contain an azo photochrome showing isomerization accompanying with color change, and an imine moiety (which can contribute to the metal complexation capability). The syntheses of these molecules will be described, and their dyes applications will be discussed, such as for fuel cells, as photometric or colorimetric sensors. In addition, liquid crystals and their antibacterial efficiencies will also be discussed.

Colorants doi: 10.3390/colorants3010003

Authors: Ana N. Nunes Joana Monte Soraya Rodríguez-Rojo Isabel D. Nogueira Luís F. Gouveia Carla Brazinha Ana A. Matias

Dunaliella salina has been recognized as an excellent biomass source of carotenoid, which can be used as a natural orange coloring agent for food products. The most eco-friendly approach for extracting carotenoid is through supercritical carbon dioxide extraction, as it yields highly concentrated extracts while preventing pigment thermal degradation. However, there are limitations when a lipophilic extract is considered a food ingredient, in particular very difficult handling and low solubility in water-based products. The aim of this study was to develop a hydrosoluble form of a natural carotenoid-rich extract recovered from algae biomass within a biorefinery concept to be incorporated in aqueous-based food products. A two-step process was developed, starting with the emulsification of the supercritical extract into a mixture of maltodextrin and gum arabic, using soy lecithin as an emulsifier. The emulsification was followed by a spray-drying step. The impact of process variables on the encapsulation yield, efficiency, emulsion properties, and particle characteristics was studied. The resulting particles exhibited an intense orange color and good water dispersibility, facilitating uniform yellow coloring when incorporated into an aqueous-based product. Overall, spray-drying emulsions containing carotenoids derived from Dunaliella salina prove to be a promising strategy for the global market demand for natural colorants.

Colorants doi: 10.3390/colorants3010002

Authors: Olga Kirkilessi Christina Arapatzi Heribert Reis Vassiliki Kostourou Kyriakos C. Prousis Theodora Calogeropoulou

A library of seven novel 1,3-benzothiazole-substituted BODIPY derivatives with tunable optical properties was synthesized. The new fluorescent dyes exhibited bathochromically shifted absorptions (up to 670 nm) and emissions centered in the red and near-infrared spectral region (up to 677 nm) in comparison to the parent compound 8-phenyl BODIPY (λabs: 499 nm, λemi: 508 nm). (TD)DFT calculations were performed to rationalize the spectroscopic properties of the new dyes. The cellular biodistribution of the new BODIPY dyes, their fluorescence stability and toxicity were investigated in both living and fixed fibroblasts using time-lapse fluorescent imaging and confocal microscopy. Six of the seven new dyes were photostable and non-toxic in vitro at 10 μM concentration. In addition, they efficiently stained the cell membrane, showing diffuse and dotty localization within the cell at low concentrations (1.0 and 0.1 μM). Specifically, dye TC498 was localized in vesicular structures in both live and fixed cells and could be used as a suitable marker in co-staining studies with other commonly used fluorescent probes.

Colorants doi: 10.3390/colorants3010001

Authors: Lydia R. Milam Roy P. Planalp

Remediation of organic dyes in natural waters is a significant environmental need under active study. This review analyzes bimetallic catalytic degradation systems that are based on the Fenton chemistry concept and that generate reactive oxygen species (ROS) as the agent of dye breakdown. Recently developed advanced oxidation processes (AOPs) take advantage of bimetallic heterogeneous catalysts to facilitate rapid rates and full degradation. Catalysts based on two metals including iron, copper, molybdenum, cobalt and magnesium are discussed mechanistically as examples of effective radical ROS producers. The reactive oxygen species hydroxyl radical, superoxide radical, sulfate radical and singlet oxygen are discussed. System conditions for the best degradation are compared, with implementation techniques mentioned. The outlook for further studies of dye degradation is presented.

Colorants doi: 10.3390/colorants2040036

Authors: Kazuki Yamaguchi Satoru Mochizuki Yudai Nagato Takuro Morimoto Toshiyuki Masui

Y(Mn0.95M0.05)2O5 (M = Al, Fe, Ga, Ti, and Zr) samples were synthesized via a sol–gel method using citric acid to find a new near-infrared (NIR) reflective black pigment. Among these samples, the optical reflectance of Y(Mn0.95Fe0.05)2O5 and Y(Mn0.95Ga0.05)2O5 in the near-infrared region was found to be larger than that of YMn2O5. Then, the concentration of the dopant (Fe or Ga) was changed between 0 and 15%, and the resulting UV–Vis–NIR reflectance spectra were measured. As a result, the optical reflectance of the Fe-doped samples decreased in the near-infrared region, while that of the Ga-doped samples increased. Accordingly, Y(Mn1−xGax)2O5 (0 ≤ x ≤ 0.20) samples were synthesized, and the crystal structure, particle size, optical properties, and color of the samples were characterized. The single-phase samples were obtained in the composition range of 0 ≤ x ≤ 0.15, and the lattice volume decreased with increasing Ga3+ concentration. Optical absorption below 850 nm was attributed to the charge transfer transition between O2p and Mn3d orbitals, and the absorption wavelength of Y(Mn1−xGax)2O5 shifted to the shorter wavelength side as the Ga3+ content increased, because of the decrease in the Mn3+ concentration. Although the sample color became slightly reddish black by the Ga3+ doping, the solar reflectance in the near-infrared region reached 47.6% at the composition of Y(Mn0.85Ga0.15)2O5. Furthermore, this NIR reflectance value was higher than those of the commercially available products (R < 45%).

Colorants doi: 10.3390/colorants2040035

Authors: Jose Manuel Barrera-Andrade Natali de la Fuente-Maldonado Ricardo Lopez-Medina Ana Marisela Maubert-Franco Elizabeth Rojas-Garcia

Due to the high stability of azo-type dyes, conventional treatment processes such as adsorption, flocculation, and activated sludge are not efficient for decolorizing wastewater effluents. An alternative to traditional wastewater treatment is photocatalysis, which has gained significant interest because research has shown it to be a viable and cost-effective process that uses sunlight as an inexhaustible energy source. In heterogeneous photocatalysis, a photocatalyst is required, such as TiO2, ZnO, composite materials, and, more recently, metal–organic frameworks (MOFs). MOFs, also known as “coordination polymers”, exhibit photocatalytic properties and have been proven to be promising materials in the photocatalytic degradation of dyes. This study presents recent advances in using MOFs as photocatalysts to degrade recalcitrant contaminants like azo-type dyes. Recent advancements in developing photocatalysts based on MOFs are focused on two strategies. Firstly, the development of new MOFs composed of complex ligands or a mixed ligand system, and secondly, the synthesis of composite materials based on MOFs and metal oxides, metals, sulfides, nitrides, etc. Both strategies have significantly contributed to the search for new semiconductors to degrade some recalcitrate contaminants in wastewater.

Colorants doi: 10.3390/colorants2040034

Authors: Safalmani Pradhan Yuki Kurokawa Suraya Shaban Shyam S. Pandey

The design and development of sensitizing dyes possessing wide-wavelength photon harvesting encompassing visible to near-infrared (NIR) wavelength regions are unavoidable for increasing the overall efficiency of dye-sensitized solar cells (DSSCs). In this study, three far-red-sensitive squaraine sensitizers were designed computationally, synthesized, and characterized, aiming towards their suitability as a potential sensitizer for DSSCs. It has been found that the incorporation of an electron acceptor moiety in the central squaraine core brought about a red shift in the absorption maximum (λmax) and the emergence of a secondary absorption band in the blue region, thus broadening the photon-harvesting window. In addition, it also lowered the dye’s HOMO energy level enabling a facile regeneration of the photo-excited dye, which improved the photovoltaic performance of SQ-223, exhibiting a photoconversion efficiency (PCE) of 4.67%. Thereafter, to address the issue of wide-wavelength photon harvesting, DSSCs were fabricated by co-adsorbing two complementary dyes SQ-223 and D-131 in various molar ratios. The DSSC fabricated with D-131 and SQ-223 in 9:1 molar ratio displayed the best photovoltaic performance with a PCE of 5.81%, a significantly higher PCE when compared to corresponding individual dye-based DSSCs containing D-131 (3.94%) and SQ-223 (4.67%).

Colorants doi: 10.3390/colorants2040033

Authors: Abigail Trujillo-Vazquez Fereshteh Abedini Alina Pranovich Carinna Parraman Susanne Klein

Two of the most significant cases of extant 16th-century featherwork from Mexico are the so-called Moctezuma’s headdress and the Ahuizotl shield. While the feathers used in these artworks exhibit lightfast colors, their assembly comprises mainly organic materials, which makes them extremely fragile. Printed media, including books, catalogs, educational materials, and fine copies, offer an accessible means for audiences to document and disseminate visual aspects of delicate cultural artifacts without risking their integrity. Nevertheless, the singular brightness and iridescent colors of feathers are difficult to communicate to the viewer in printed reproductions when traditional pigments are used. This research explores the use of effect pigments (multilayered reflective structures) and improved halftoning techniques for additive printing, with the objective of enhancing the reproduction of featherwork by capturing its changing color and improving texture representation via a screen printing process. The reproduced images of featherwork exhibit significant perceptual resemblances to the originals, primarily owing to the shared presence of structural coloration. We applied structure-aware halftoning to better represent the textural qualities of feathers without compromising the performance of effect pigments in the screen printing method. Our prints show angle-dependent color, although their gamut is reduced. The novelty of this work lies in the refinement of techniques for printing full-color images by additive printing, which can enhance the 2D representation of the appearance of culturally significant artifacts.

Colorants doi: 10.3390/colorants2040032

Authors: Leonardo Ricardo Bernardes da Conceição Higor Oliveira da Cunha Arcano Matheus Bragança Leite Rajendran Suresh Babu Sebastian Raja Caue Ribeiro Ana Lucia Ferreira de Barros

Herein, we evaluate the conversion efficiency of dye-sensitized solar cells (DSSCs) photosensitized using two different natural dyes extracted from Alpinia purpurata and Alstroemeria flower petals. The appreciable absorption capacity of the extracts in the visible light region was examined through absorption spectroscopy. The functional groups of the corresponding pigments were identified through Fourier transform spectroscopy (FTIR) technique thus indicating the presence of cyanidin 3-glycosides and piperine in the flowers of Alstroemeria and Alpinia purpurata. The extracted dyes were immobilized on TiO2 on transparent conducting FTO glass, which were used as photoanode. The dye-coated TiO2 photoanode, pt photocathode and iodide/triiodide redox electrolyte assembled into a cell module was illuminated by a light source intensity 100 mW/cm2 to measure the photovoltaic conversion efficiency of DSSCs. The TiO2 anode and Pt counter electrode surface roughness and morphological studies were evaluated using atomic force microscope (AFM) and field emission scanning electron microscopy (FESEM), respectively. Through the photoelectric characterizations, it was promising to verify that the solar conversion efficiency was calculated with the photovoltaic cell sensitized by Alstroemeria and Alpinia purpurata. This was achieved with a yield (η) of 1.74% and 0.65%, with an open-circuit voltage (Voc) of 0.39 and 0.53 V, short-circuit current density (Jsc) of 2.04 and 0.49 mA/cm2, fill factor (FF) of 0.35 and 0.40, and Pmax of 0.280 and 0.100 mW/cm2, respectively. The results are promising and demonstrate the importance of the search for new natural dyes to be used in organic solar cells for the development of devices that generate electricity in a sustainable way.

Colorants doi: 10.3390/colorants2040031

Authors: Leonel C. Silva Vanessa Otero Maria J. Melo Eurico J. Cabrita Luís Mafra

This work provides significant insight into the molecular structure of alizarin lake pigments used by artists in the past. To characterize two red powders, lakes 1 and 2, obtained by complexation of 1,2-dihydroxy anthraquinone (alizarin) with Al3+, a multi-analytical approach was designed based on solid and liquid state Nuclear Magnetic Resonance Spectroscopy (NMR), Fourier-Transform Infrared Spectroscopy (FTIR), Mass Spectrometry (MS) and Density Functional Theory (DFT) calculations. Lake 1 was synthesized according to literature and compared with lake 2, a reproduction of an artist’s pigment. FTIR showed Al3+ coordinated to oxygens in C1 and C9, and that in lake 2 the -OH groups in C2 are protonated, being responsible for its low solubility. 1H-NMR proved that lake 2 is formed by two tautomers [Al(Aliz-2-H-)2(OH)(H2O)] and [Al(Aliz-10-H-)2(OH)(H2O)], the latter being the major species. SS-NMR was the only technique that got insight into the Al3+ coordination, octahedral for both lakes. It confirmed the existence of two species in lake 2, in a 5:1 ratio. Both are amorphous “open structures”, resulting in fewer constraints for the ligands and in a large variety of geometries. SS-NMR allowed the analysis of the red pigments without preparation, which is a unique advantage for their study in artworks.

Colorants doi: 10.3390/colorants2040030

Authors: Almudena Martí Pau Arroyo Pablo Gaviña Salvador Gil Margarita Parra José A. Sáez

Derived from malachite green, new triaryl-carbonium-ion-functionalized gold nanoparticles have been synthesized for detecting anions. The detection process, and concomitant colour change, is based on charge compensation on the surface of nanoparticles, which triggers their aggregation, resulting in a bathochromic shift of the plasmon resonance band. The difference in electrophilicity of the malachite green triaryl ions in solution or on gold nanoparticles makes it possible to distinguish different anions related to their nucleophilic character.

Colorants doi: 10.3390/colorants2030029

Authors: Luca M. Sihn Erick L. Bastos Marcelo Nakamura Mayara K. Uchiyama Henrique E. Toma

Blue dyes are relatively uncommon in nature, and a novel dithiophene dye (RanB) is reported in this paper. This dye is derived from an old anti-osteoporotic drug and is a metal ion complexing agent, displaying a planar molecular structure, with two sets of carboxylate, isonitrile, thiophene, and iminodiacetate groups. The blue color originates from a strong absorption peak at 648 nm, accompanied by an unusual fluorescence at 555 nm, with higher energy compared to the main absorption band. RanB forms complexes with lanthanoid ions through the iminodiacetate groups and serves as an effective sensitizer for Tb3+ ions, heightening their emission and improving their use as luminescent agents. Its photo-physical properties and the interaction with Tb3+ have been investigated using absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, along with computational methods (ZINDO/S and DFT). The RanB toxicity in human umbilical vein endothelial cells has also been tested, showing a lack of toxicity, holding promising prospects for application as a luminescent and coloring agent in pharmaceuticals and food.

Colorants doi: 10.3390/colorants2030028

Authors: Daiane Amaral de Ramos Nogueira Tânia Marina Palhano Zanela Monielly Viomar Machado Carlos Alberto Policiano Almeida Rafael Marangoni

Zinc hydroxide nitrate (ZHN) was used as an anionic adsorbent for the removal of methyl orange (MO) dye from aqueous solutions. ZHN was characterized via X-ray diffraction (XRD) and infrared spectroscopy (FTIR) techniques. Investigations were carried out to see how the adsorption of MO was affected by factors such as initial MO concentration, contact time and temperature. Adsorption isotherms were analyzed using the Langmuir and Freundlich equations, with the first one being the better result for the equilibrium data. Adsorption kinetics was studied through applying pseudo-first and pseudo-second-order kinetic models, and the experimental data were better fitted to the pseudo-second-order model. The activation energy was determined using the Arrhenius equation to be 105.45 kJ mol−1, revealing the chemical nature of the adsorption process. The thermodynamic parameters were also determined, showing the adsorption of MO onto ZHN to be a non-spontaneous and exothermic process. The experimental results showed ZHN as a potential adsorbent with adsorption capacity for removing anionic dyes from water medium.

Colorants doi: 10.3390/colorants2030027

Authors: Masanari Namie Jae-Ho Kim Susumu Yonezawa

Surface fluorination with pure F2 gas can easily make the surface on PP (polypropylene) hydrophobic, and it causes limited dyeability, as reported in a previous paper. In this study, to produce a more hydrophilic surface, surface fluorination of PP was performed at 25 °C, total gas pressure of 13.3 kPa, and reaction time of 1 h using F2 and O2 mixtures with different proportions of F2 gas. The surface roughness of the fluorinated PP samples was about 1.5 times higher than that of the untreated sample (5 nm). Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that the PP-derived bonds (-C-C- and -CHx) decreased because they were converted into polar groups (-C–O, -CHF-, and -CFx), which increased the surface electronegativity of the PP. The variation in the F2 gas proportion in the gas mixture significantly affected the hydrophilicity and surface composition of the PP. At F2 gas proportions of <70%, the hydrophilicity of the fluorinated PP samples was increased. Notably, the hydrophilic and negatively charged PP surface enhanced the dyeing of the polymer with basic methylene blue (MB). In contrast, at F2 gas proportions of >90%, the PP surface became hydrophobic owing to increased numbers of hydrophobic -CF3 bonds. Thus, enhanced PP dyeing can be controlled based on the composition of the F2 and O2 gas mixture.

Colorants doi: 10.3390/colorants2030026

Authors: Eleonora Balliana Eugénie Marie Claudine Caveri Laura Falchi Elisabetta Zendri

The 18th century roof tiles from the “Casa delle vigne”, located in the Aosta region (north-east Italy), were investigated as an example of a peculiar historical roof covering: ceramic tiles with a lead-based glaze finishing to waterproof them are used to create colourful patterns. A conservation project proposed the integration of the original tiles with new ones, produced according to traditional methods. Ancient and new tiles were analysed with Fiber Optics Reflectance Spectroscopy, micro-Raman, Fourier Transform Infrared Spectroscopy and X-ray Fluorescence Spectrometry, Thermogravimetry and Differential Scanning Calorimetry for understanding the composition and the production technology of this manufacture. Their resistance to freezing and thawing cycles was then tested, considering their exposure in the severe alpine climate of Aosta. The use of pure clays with low calcium contents, high firing temperature and lead-rich glazes was found in ancient tiles, able to outstand several freezing-thawing cycles without damages. Iron and copper pigments were used in old yellow and green glazes. Zinc-based pigment, low lead and calcium-rich glazes are used in the new ones, which remained mainly coherent to the ceramic body during the freeze-thaw test.

Colorants doi: 10.3390/colorants2030025

Authors: Eva Miguel Guillermo Paulo-Redondo Juan Bautista Carda Castelló Isaac Nebot-Díaz

In this study, a new red ceramic pigment has been developed within a perovskite structure, and microwave heat treatments have been applied. Those red ceramic pigments within the YAlO3 system doped with chromium with the nominal composition Y0.98Al0.98Cr0.04O3 were synthesized by traditional routes and alternative methods like coprecipitation. Also, heat treatment has been studied comparing a traditional electric and microwave kiln. Different flux agents have been incorporated to improve the synthesis reaction. Prepared pigments have been characterized by X-ray diffraction (XRD) as having a predominant phase of perovskite structure, which is responsible for the red shade, and a minority garnet phase that causes more brown colorations. Studies by Ultraviolet-Visible spectroscopy gave rise to a series of absorption bands that indicate the presence of Cr(III) in the octahedral position corresponding to perovskite and Cr(IV) corresponding to garnet in both the octahedral and tetrahedral positions. The perovskite phase is favored with the use of flux mix, corroborating the UV-visible results and being more pronounced in traditional high temperature thermal treatments. The coprecipitation route has been studied to increase the reactivity of the particles given their nanometric size; however, this reactivity favors a greater appearance of undesirable garnet phases with both types of flux. Scanning Electron Microscopy (SEM) micrographs offer information obtained from the secondary electrons of predominantly cubic crystalline phases with sizes between 1 µm and 2 µm in pigments synthesized via the traditional method and sizes less than 1µm together with the glassy phase in pigments synthesized via coprecipitation. Microwave thermal treatments have been studied, obtaining pigments with a majority structure of perovskite and garnet at lower temperatures and relatively short synthesis times. The feasibility of use in porous single-fired ceramic glazes has been studied, whose chromatic coordinates have been collected using an Ultraviolet-Visible Spectrophotometer based on the CIEL*a*b* system.

Colorants doi: 10.3390/colorants2030024

Authors: Sergey V. Shekhovtsov Iryna V. Omelchenko Svitlana V. Shishkina Andrey O. Doroshenko Kateryna O. Vus Hanna S. Vlasenko Nikolay O. Mchedlov-Petrossyan

Contrary to the 4′- and 5′-nitro- and aminofluoresceins, the corresponding 3′-derivatives are practically unexplored. In this paper, we describe the synthesis and spectral properties of 3′-nitrofluorescein and 3′-aminofluorescein, as well as their methyl esters. Among other methods, X-ray analysis, 13C NMR spectroscopy, and ESI mass spectrometry made it possible to establish the molecular structure of the target compounds as well as intermediates and by-products. Some unexpected products, though in small amounts, were revealed within the course of study. Whereas the fluorescence of the double-charged R2− ion of 3′-nitrofluorescein in both aqueous and organic solvents is weak, the R2− anion of 3′-aminofluorescein in a non-hydrogen bonding donor solvent, but not in water, exhibits intensive fluorescence, analogous to the case of 4′- and 5′-aminofluoresceins. Interestingly, the λmax values in water of the R2− ions bearing an NO2 group in the 3′- and 6′-positions are 7 to 10 nm higher than those of the 4′- and 5′-nitro derivatives. The difference was also observed in dimethyl sulfoxide. This correlates with the angles between the xanthene and phthalic planes of the dyes. The dye 3′-aminofluorescein could be used as a fluorescent indicator sensitive to hydrogen bonding ability of the solvent. It could also serve as a platform for synthesizing fluorescent molecular probes for biochemical research, analogous to the very popular application of 4′- and 5′-amino derivatives.

Colorants doi: 10.3390/colorants2030023

Authors: Lariana N. B. Almeida Tatiana G. Josué Maria Eduarda K. Fuziki Yuri B. Fávaro Laura S. Ribas Angelo M. Tusset Onélia A. A. Santos Giane G. Lenzi

In the present work, rock dust was evaluated as an adsorbent and heterogeneous photocatalyst in the discoloration of Basazol Yellow 46 L dye, which is widely used in the dyeing of molded pulp packages. Although rock dust is produced in large quantities in quarries as a byproduct of rock exploration, little is known about its application as a photocatalyst. Rock dust was characterized by XRD, SEM/EDS, photoacoustic spectroscopy, and N2 physisorption and had its photocatalytic activity assessed through phenol and salicylic acid degradation tests. The characterization results showed that the rock dust is mainly composed of silica and alumina in a triclinic structure, has a bandgap energy of 2.36 eV, and has a specific area of 1.5 m2/g. Rock dust was proven to be photocatalytically active in phenol and salicylic acid degradation tests and also presented the adsorptive and photocatalytic capacity for the discoloration of effluent containing Basazol Yellow 46 L dye.

Colorants doi: 10.3390/colorants2030022

Authors: Anabelle Kriznar

Several medieval mural cycles in Austria were studied from the material and technical point of view, aiming to confirm (or reject) the art-historical hypothesis of their stylistic and workshop connection. These paintings can be found in the churches of Rust (“Fischerkirche”), Marz (Virgin’s Coronation parish church), Kobenz (St. Ruprecht parish church), Ofenbach (St. Veid parish church), and St. Johann am Steinfelde (St. John parish church). They were carried out around 1400 in the International Gothic style. Their workshop connections based on the style are doubtful, therefore, a material and technical study was carried out. Results showed different plaster composition, similar pigment palette, and diverse painting procedures. The murals in Marz, St. Johann, and the older register in Rust reveal important similarities such as plaster composition, predominant a fresco painting technique, the use of natural inorganic pigments, as well as many aspects of the painting procedure and modeling. On the contrary, those in Kobenz, Ofenbach, and the younger register in Rust differ considerably. The second group reveals lower quality in plaster composition, larger a secco parts, addition of synthetic pigments (Kobenz), and a rougher color modeling, indicating less skilled artists. The lower quality also results in a worse conservation state of these murals. The obtained results confirm the same workshop, but different artists in the first group, while in the second group no clear workshop/artist connection could be established.

Colorants doi: 10.3390/colorants2020021

Authors: Adamantia P. Panagopoulou Joanita Vroom Anno Hein Vassilis Kilikoglou

This study attempts to take aspects of pottery technology into account while concentrating on the blue pigment and glaze recipes of various kinds of glazed pottery types; that is to say, Iznik ware, Kütahya ware, Miletus ware, glazed fritware, porcelain, polychrome glazed ware, and monochrome glazed ware were collected from a rescue excavation site within the Castle of Mytilene in Lesvos Island, Greece. The decoration, surface treatment, and production technology were investigated on the basis of 23 ceramic fragments that can be dated to the Turkish/Venetian period. The present study concerns the manufacture of glazed pottery, and in particular, colour recipes and issues of glaze technology. This study endeavoured to look into the specifics of the medieval colour recipes used on the glazed ceramics from Mytilene. This was accomplished by using an analytical process that took into account the compositional information of blue pigments, glazes, and slip coatings. The chemical analysis was conducted using scanning electron microscopy (SEM-EDS), Raman spectroscopy provided information about the compositional variation, and the optical examination via optical microscopy (OM) and scanning electron microscopy (SEM-EDS) yielded information about the sample stratigraphy of the examined ceramic sections. This investigation into glazed ceramics was able to define and reflect the key aspects of each society’s perception of colour through a large variety of colour and glaze recipes.

Colorants doi: 10.3390/colorants2020020

Authors: Isolda Duerto Clara Carrera Daniel Barrios Ana M. Benito Wolfgang K. Maser Belén Villacampa Enrique García-Bordejé María-Jesús Blesa

The importance of counter electrodes in Dye Sensitized Solar Cells (DSSCs) cannot be neglected as they enable the transfer of electrons across the outer circuit, thereby facilitating the reduction reaction of the I3−/I− redox electrolyte. However, the dissolution and deposition of the usual platinum layer on the counter electrode has resulted in contamination concerns. To address this issue, metal-free counter electrodes made of reduced graphene oxide (rGO) aerogels were developed and their catalytic performance towards I3− reduction was evaluated. The reduced graphene materials were characterized, and the fitting analysis of XPS revealed the presence of various nitrogen species, with the primary peaks attributed to pyridinic and pyrrolic nitrogen. The hydrothermal treatment of graphene oxide (GO) resulted in a higher graphitic character and the intensification of the contacts between graphene nanosheets, which should entail higher electrical conductivity, both in-plane and between rGO sheets. Additionally, the presence of nitrogen-provided active sites promoted the catalytic reduction of the electrolyte. Encouragingly, good charge transfer rates were observed between the counter electrode and the electrolyte in the assembled DSSCs, resulting in good photocurrents and exceptional stability over the course of nearly 1200 h after cell assembly. The results obtained suggest that these GO-based systems are promising candidates for developing metal-free counter electrodes for DSSC, supporting the interest of further study.

Colorants doi: 10.3390/colorants2020019

Authors: Anuradha Chowdhury Sridharan Balu Kuo-Wei Lan Louis Wei-Chih Lee Thomas C.-K. Yang

Environmental remediation in the presence of robust semiconductor photocatalysts by utilizing renewable energy sources is of keen interest among researchers. In this study, we synthesize a BiVO4/P-g-C3N4 semiconductor heterojunction photocatalytic system through a hydrothermal route followed by utilizing a total-solvent evaporation method. The optical and electronic properties of the as-prepared heterojunction are characterized via various spectroscopic techniques. Rhodamine B (RhB) and Congo Red (CR) are used as synthetic colorants to evaluate the photocatalytic performances of BiVO4/P-g-C3N4. In addition, the chemical environment of the photocatalyst and its mechanistic pathways are confirmed through X-ray photoelectron spectroscopy and electrochemical Mott–Schottky analysis. The BiVO4/P-g-C3N4 photocatalyst shows higher photodegradation (96.94%) of the mixed synthetic dyes under simulated solar-light irradiation. The as-synthesized BiVO4/P-g-C3N4 heterojunction significantly promotes the quick separation of photoexcited carriers due to the excellent synergetic properties, the extended light absorption, and the photoelectrochemical response. Furthermore, a possible type-II charge transfer mechanism is adopted for the BiVO4/P-g-C3N4 system after investigating the band potentials, active species, and charge carrier migration over the heterojunction interface.

Colorants doi: 10.3390/colorants2020018

Authors: David O. Oluwole Nikoletta B. Báthori

Phthalocyanines (Pcs) are 18-electron π-conjugated macrocyclic ring systems with proven activities in diverse fields, including pharmaceuticals and catalysis. These demonstrated activities are often alluded to as their fascinating photophysical and photochemical dispositions, which are usually dependent on their molecular structures. However, many of these molecules suffer from aggregation due to π–π stacking and have limited insolubility in hydrophilic media, which limits their extensive utilisation in pharmaceutical applications. This review will explore the possibility of fine-tuning the physicochemical properties of phthalocyanines when designed as multicomponent crystals. Among the proven and SMART approaches that have been shown to enhance drug solubility without altering the molecular structure is co-crystallisation. This protocol involves the design and formation of non-covalent interactions between two or more molecular entities to create a supramolecular assembly and subsequently afford multicomponent crystals (MCCs). A systematic review of the Cambridge Structural Database repository yielded several single and multicomponent crystals of Pcs; however, most of them were either salts or solvates, with only a few reports on their co-crystals.

Colorants doi: 10.3390/colorants2020017

Authors: Andrey A. Khodonov Nikolay E. Belikov Alexey Yu. Lukin Alexey V. Laptev Valery A. Barachevsky Sergey D. Varfolomeev Olga V. Demina

Methods for preparation of 5′-substituted spiropyrans, their chemical properties, and the effects of various factors on the relative stabilities of the spiropyrans and their isomeric merocyanine forms are examined, reviewed, and discussed.