Colorants

This study presents a PRISMA-guided bibliometric and statistical analysis of MOF-based adsorbents for organic dye removal, covering publications from 2014 to 2026. A total of 59 studies were analyzed to evaluate research trends, material classification, and adsorption performance under diverse experimental conditions. The results revealed clear differences in adsorption behavior depending on dye type, molecular properties, and operational conditions, highlighting the key role of electrostatic interactions and solution chemistry. From a materials perspective, pristine MOFs dominated the literature; however, a transition toward functionalized and composite materials was identified, driven by the need to improve adsorption efficiency, selectivity, and stability. MIL-, UiO-, and ZIF-type frameworks were the most frequently investigated due to their balance between structural stability and adsorption performance. The analysis also identified critical limitations, particularly the inconsistent reporting of key experimental parameters, which limits comparability and hinders the identification of reliable structure–property relationships. Overall, this study provides a benchmarking framework supporting the rational design of MOF-based adsorbents for wastewater treatment applications. Full article

Ochres, natural pigments based on iron oxide and (oxy)hydroxide minerals, range in color from yellow to red to violet to brown. Ochres have been used and manipulated by humans for more than 100,000 years, and they continue to be valued because they are intensely colored, stable to light and heat, and cheap. Ochres offer critical evidence for the evolution of human cognition and symbolic behavior. Interpretation of the archeological evidence requires an understanding of ancient processing of ochre. Of particular importance in the Paleolithic is the heating of yellowish-brown goethite to convert it to red hematite. We heat natural ochres containing goethite in a furnace and characterize the colors of the resulting hematites. Within the temperatures achievable by early humans in simple wood fires (300 to 800 °C), the color is almost entirely independent of temperature and time of heating with λm ≥ 580 nm and reflectance at 780 nm varying by less than 5%. At higher temperatures (800 to 1100 °C) natural ochres may become a brighter red (increasing reflectance by 15%), purplish red, or various shades of brown depending on the accessory minerals in the ochre. Full article

Textile dyes are considered emerging pollutants due to their recalcitrant and xenobiotic nature, making them toxic and mutagenic, leading to various environmental impacts. Aspergillus fungi, known for their metabolic diversity and high environmental adaptability, emerge as an alternative for remediating these contaminants. The evolution and trends in research on the application of Aspergillus in the bioremediation of textile dyes were assessed through a scientometric analysis of articles indexed in Web of Science, PubMed, and Scopus, using the Bibliometrix tool. A total of 283 documents were identified over 28 years since the first publication, indicating that although the topic is established, research output remains limited. Publications originated from 43 countries, with India as the leading contributor; however, the low rate of international collaboration (12.37%) highlights the need for stronger global partnerships. Research primarily focused on dye decolorization via biosorption and biodegradation, with Aspergillus niger and Aspergillus flavus as the most frequently studied species. Recent trends emphasize lignolytic enzymes, especially laccase, and integrative approaches combining biological and physicochemical processes. The results also reveal the urgent need for comprehensive toxicological assessments beyond phytotoxicity, considering increasing concerns about textile effluent impacts on ecosystems and human health. Full article

The textile industry contributes significantly to environmental pollution through massive water usage and toxic synthetic dye effluents. Bioremediation offers a sustainable solution by using microorganisms, such as bacteria, to transform complex contaminants into simpler substances. This study evaluated the bioremediation potential of fifteen halotolerant endophytic bacteria isolated from black beans (Phaseolus vulgaris L.) against various textile dyes. The strains included Bacillus cereus, Bacillus amyloliquefaciens, Priestia megaterium, and Staphylococcus warneri. Initial screenings across different TSA (Tryptic Soy Agar) medium concentrations (10%, 50%, 100%) revealed that bacterial growth and discoloration—assessed via halo formation—were most pronounced in 50% medium. While several dyes showed no reaction, Malachite Green and Congo Red were successfully decolorized. In liquid medium assays TSB (Tryptic Soy Broth) (50%) quantitative analysis via spectrophotometry showed that strains PV57, PV107, and PV112 achieved approximately 45% discoloration for Congo Red. Most notably, PV18 and PV114 achieved discoloration efficiencies of 91.69% and 88.72%, respectively, for Malachite Green after 72 h. These findings indicate that salt-tolerant endophytic bacteria are promising candidates for the decolorization of textile dyes. However, further studies are required to determine whether the observed discoloration results from biodegradation, biotransformation, or biosorption. This study underscores the potential of agricultural endophytes in managing industrial waste effectively. Full article

Natural food pigments such as curcumin and β-carotene are valued for their coloring properties and health-promoting bioactivities, including antioxidant and anticancer effects. However, both compounds are susceptible to light-induced degradation, which can compromise their stability and functional integrity. This study investigated changes in the photostability and bioactivity of curcumin and β-carotene under fluorescent light irradiation, focusing on their interactions with the porphyrin photosensitizers zinc protoporphyrin (ZnPP) and protoporphyrin IX (PPIX). Spectral and HPLC analyses revealed significant pigment degradation, with β-carotene exhibiting greater instability than curcumin. ZnPP and PPIX accelerated the bleaching of both pigments in a concentration-dependent manner, with ZnPP consistently showing stronger destabilizing effects. Functional assays further demonstrated that the antioxidant activity of curcumin was altered by light exposure. Co-irradiation with ZnPP or PPIX significantly reduced the scavenging activity of curcumin against DPPH, ABTS, and AAPH radicals. In HCT-116 colon cancer cells, irradiation alone slightly enhanced the cytotoxic activity of curcumin, whereas co-irradiation with ZnPP or PPIX significantly attenuated this effect. These findings indicate that porphyrin photosensitizers accelerate pigment degradation and impair functional properties under light exposure, highlighting the importance of pigment–photosensitizer interactions during food storage and processing. Full article

Fungi in the Chlorociboria genus produce a blue pigment called xylindein—a pigment with high UV stability, photovoltaic capacity, and which is used as a historic wood colorant. The pigment has not yet been synthesized; however, recent studies have shown that optimizing sucrose and glucose levels, as well as nitrogen levels during Chlorociboria growth, affects the amount of pigment produced. Ideal for laboratory and commercial production, most users of xylindein are hobbyist woodworkers for whom this level of control is not feasible. This study sought to determine easily-accessible sugar sources capable of stimulating xylindein production specifically for “at home” cultivation. Four ‘common’ sugar sources were tested for their ability to stimulate pigment production of four Chlorociboria species: honey, maple syrup, coconut cream, and high fructose corn syrup. All fungi produced significantly more xylindein in honey-based media. Across the different types of honey, the three fungi produced the most xylindein on raw honey over the more “processed” honey, although both raw and processed honey induced pigmentation. The use of honey as a media sugar source for Chlorociboria spp. offers a reliable, cheap, and effective method to optimize sucrose for hobbyists interested in maximizing their xylindein output, and may offer a cheaper pigment stimulant for larger-scale, industrial production. Full article

This paper reports the controlled synthesis of ZrO₂ nanocrystals via a peroxide-assisted hydrothermal (HT) route at 120 °C, with processing times ranging from 12 to 72 h, and investigates the correlation between structural evolution, defect chemistry, and functional properties. X-ray diffraction (XRD) combined with Rietveld refinement confirmed the formation of a monophasic monoclinic structure with high structural reliability. Microstructural analysis revealed progressive crystallite growth and lattice ordering with increasing reaction time, accompanied by subtle distortions in local coordination environments. Micro-Raman spectroscopy indicated improved medium-range structural organization at longer synthesis durations, while transmission electron microscopy showed quasi-spherical and nanorod-like aggregates formed through oriented attachment, with particle sizes of 6–9 nm. Optical investigations using diffuse reflectance spectroscopy revealed band gap energies of 3.45–3.65 eV, attributed to defect-induced intermediate electronic states associated primarily with oxygen vacancies. A comprehensive photoluminescence (PL) analysis suggests that the observed emission arises from defect-mediated recombination pathways involving localized states within the band gap, modulated by the interplay between structural order and residual defects. The role of hydrogen peroxide is discussed in terms of regulating oxygen vacancy concentration, promoting structural stabilization while preserving functional defect states. The results demonstrate that precise control of HT processing time enables tuning of structural disorder, defect density, optical response, and the enhanced photocatalytic performance of ZrO₂ toward RhB dye degradation, highlighting its potential for optoelectronic applications. Full article

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. Full article

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