Synthesis of carboxylic acid N-hydroxysuccinimidyl ester dyes and their reactive dyeing on silk fibroin

Abstract

N-hydroxysuccinimide esters (NHS esters) are widely recognized as amine-reactive compounds due to their mild reaction conditions and high efficiency. In this study, NHS ester group was introduced into the dye chromophores to design and synthesize carboxylic acid succinimidyl ester dyes (CSEDs) in red, yellow and blue colors. The CSEDs were characterized using high-resolution mass spectrometry, proton nuclear magnetic resonance, and infrared spectroscopy. When the NHS ester group was introduced, UV–vis absorption spectroscopy only slightly affected the molar extinction coefficient and maximal absorption wavelength. Hydrolysis studies demonstrated that CSEDs hydrolyzed slowly in acidic environments but quickly in weakly alkaline ones, with near-complete hydrolysis taking place at pH 8.5 after half an hour. It was discovered that CSEDs could successfully dye silk fibroin at room temperature in weakly alkaline conditions, producing a high color depth and more than 95 % color fixing. The binding of CSEDs to silk fibroin was confirmed through covalent bonds using DMF stripping method and dialysis method. Furthermore, the color fastness of dyed silk fibroin to washing and rubbing was rated above grade 4–5. Compared to commercial dyes, the milder dyeing conditions of CSEDs can save energy and did not require a significant amount of salt to promote the dyeing process.