Colorants

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article