A volatile basic nitrogens-responsive tag based on aggregation-induced emission luminogen for real-time monitoring and in situ visualization of salmon freshness

Given the ever-growing food safety issues, the establishment of efficient approaches for monitoring food freshness attracts increasing attention. Volatile basic nitrogens (VBNs), including biogenic amines and ammonia, serve as an important biomarker for monitoring food freshness. In this study, a novel VBNs-responsive tag using glutathione capped copper nanoclusters (GSH-CuNCs) aggregates was developed as a fluorescent probe for in situ and real-time visual monitoring of salmon freshness, and the prepared GSH-CuNCs aggregates were characterized and their sensitivity for detecting VBNs using biogenic amines and ammonium hydroxide as the model targets was evaluated. Based on their remarkable response in liquid status, the GSH-CuNCs aggregates-based tag as a gas indicator was then fabricated, which exhibited visible colour changes under UV light as a function of ammonia vapour concentrations. More importantly, through exploring the sensing mechanism of GSH-CuNCs aggregates in VBNs detection, the existence of ligand exchange between the GSH-CuNCs and VBNs was observed and verified for the first time, confirming the effect of hydrogen bonding reported in the literature. Moreover, the GSH-CuNCs aggregates-based tag was applied for quantitative analysis of salmon freshness during different storage periods, which was validated by the standard method for detection of total VBNs in salmon. In addition, a colour card was developed and its feasibility for application in monitoring salmon freshness was validated, which could be used for consumers to obtain the freshness level directly with the naked eye, demonstrating the feasibility of the fabricated tag for real-time and visual monitoring of salmon freshness, thus showing great potentials for its practical applications in the food industry.