Dual-ligand engineering of copper nanoclusters enables "turn-on" fluorescence label for pH-responsive freshness monitoring of shrimp.

Abstract

With increasing consumer demand for food safety, conventional freshness detection methods remain limited by subjectivity and operational complexity. Herein, highly emissive green fluorescent copper nanoclusters (CuNCs) were developed as intelligent packaging labels via dual-ligand engineering of kojic acid (KA)-stabilized CuNCs with D-penicillamine (DPA). DPA markedly improved the fluorescence quantum yield from 1.2% to 11.4% by promoting ultrasmall cluster formation (∼2 nm), enhancing quantum confinement, and passivating surface defects. The obtained DPA-KA-CuNCs exhibited a pH-responsive fluorescence "turn-on" behaviour toward volatile basic nitrogen compounds (VBNs). Fluorescence quenched under acidic conditions was efficiently restored upon exposure to ammonia, dimethylamine, and trimethylamine through ligand deprotonation and enhanced ligand-to-metal charge transfer. Detection limits reached 0.23, 0.19, and 0.18 ppm, respectively. A paper-based fluorescent label further enabled visual monitoring of shrimp freshness, showing excellent correlation with TVB-N values (R2 = 0.9835). This work provides a sensitive and visual strategy for non-destructive aquatic product freshness evaluation.