Abstract:

Objective The development of bio-based dyes is of great importance for sustainable fabrics. However, natural dyes often suffer from low yield, poor stability, and a limited color range. Microbial pigments offer functionality but are costly and difficult to purify. Existing polyphenol-based dyes mostly produce dull yellow-brown tones, limiting their application in multicolor fabrics. Additionally, conventional dyeing methods are energy-intensive and environmentally unfriendly. Therefore, it is essential to develop a sustainable, multicolor, and functional dyeing system using natural resources, suitable for cotton fabrics under mild and eco-friendly conditions.

Method Three natural polyphenols with different structures, such as caffeic acid, protocatechuic acid, and chlorogenic acid, were complexed with ferrous ions to prepare polyphenol-Fe²⁺dyes. The influences of reaction conditions on dye color were investigated. Polyethyleneimine (PEI) grafting modification was applied to enhance dye affinity for cotton fibers. Optimal dyeing parameters were determined, and the dyed fabrics were evaluated for pH-responsive color change and UV protection. Characterization was performed using UV-visible spectroscopy, Fourier fransform infrared spectroscopy(FT-IR), scanning electron microscopy(SEM), and spectrophotometric color measurement.

Results The study demonstrated that three natural polyphenols, i.e. caffeic acid, protocatechuic acid, and chlorogenic acid, formed effective complexes with ferrous ions to produce polyphenol-Fe²⁺ dyes with distinct colors. Optimal molar ratios were 1∶1 for caffeic and protocatechuic acids, and 2∶1 for chlorogenic acid. The complexation was pH-dependent, with pH7 yielding the highest dye concentration and brightest colors by virtue of enhanced coordination from phenolic hydroxyl deprotonation. The dyes showed significant pH-responsive color changes, suitable for pH monitoring. Cotton fabrics modified with polyethyleneimine (PEI) overcame electrostatic repulsion and improved dye uptake. PEI with a molecular weight of 3 000 at 5%(o.w.f) dosage provided the best modification effect. Optimal dyeing conditions were 10 mmol/L dye concentration and 60 ℃ temperature, maximizing color strength (K/S value) without degrading the dye complex. Characterization via elemental mapping and FT-IR confirmed uniform dye adsorption and coordination bonds between polyphenols and ferrous ions. Furthermore, the level dyeing propertyies (S_r values) of the three dyed fabrics were all smaller than 0.05, indicating that under the optimal process, all three dyes exhibited good uniformity in dyeing the fabrics. The dyed fabrics exhibited excellent color fastness, exceeding standard requirements for rubbing and washing. The fabrics also displayed clear color changes in alkaline solutions, confirming their application potential in alkaline environment sensing. Additionally, the polyphenol-Fe²⁺ dyes significantly enhanced UV protection, reducing UV transmittance to about 0.05% through extended conjugation in the complexes.

Conclusion Caffeic acid (CA), protocatechuic acid (PCA), chlorogenic acid (CGA) were complexed with ferrous ions to prepare natural dyes, which were applied to cationized cotton fabrics, producing black, purple, and brown colors. The dyed fabrics exhibited excellent rubbing fastness, uniform coloration, and clear color changes in alkaline environments (pH>9). Additionally, all dyed fabrics showed high ultraviolet resistance (UPF>50), confirming their effective UV-shielding performance. This work offers a sustainable method to produce multifunctional cotton fabrics with pH-responsive and UV-shielding properties via natural polyphenol-metal complexation and fiber surface modification.

Key words: bio-based dye, plant polyphenol, ferrous ion, cotton fabric, multi-color, UV protection performance, functional fabric