In situ generation of azo dyes on silk fibroin through three-step chemical modification

Abstract

To enhance the efficiency and reduce the environmental footprint of silk dyeing, this study introduces a novel method that synthesizes azo dyes directly on silk fibroin (SF) using a three-step chemical modification at room temperature. Initially, SF undergoes modification with isatoic anhydride to yield aromatic primary amino-modified silk fibroin (SF–NH₂). Subsequently, SF-NH₂ is treated with nitrous acid to produce diazotized silk fibroin (SF–N₂⁺). In the final step, SF-N₂⁺ is combined with coupling components to directly form azo chromophores on the SF, culminating in the creation of coloured silk fibroin (SF–NN-CC). The efficacy of the three-step dyeing method was validated using infrared spectroscopy and colour development analysis. The modification sites on SF-NH₂ were identified as the hydroxyl and amino groups of amino acid residues, confirmed through simulated reactions. SF-N₂⁺ demonstrated excellent storage and thermal stability under dry conditions; however, its stability decreased in wet environments. Various coupling components were then reacted with SF-N₂⁺ to synthesize in situ yellow, red, crimson, and green azo dyes on SF. The formation of covalent bonds between the dyes and SF was verified using DMF stripping methods. The dyed SF-NN-CC fabrics underwent testing for fastness, revealing wet fastness rating above grade 4. These findings demonstrate that the three-step modification process represents an innovative, energy-efficient, and fast-reacting dyeing method for SF.