Colorants, Volume 4, Issue 2 (June 2025) – 11 articles

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 (R_e) of 96.5% and adsorption capacity at equilibrium (q_e) of 9.3 mg g⁻¹, followed by tree pruning biochar (BTP-300), with an R_e of 65.0% and q_e 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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article