Assembly-Induced Emission of Copper Nanoclusters: Revealing the Sensing Mechanism for Detection of Volatile Basic Nitrogen in Seafood Freshness On-Site Monitoring

Total volatile basic nitrogen (TVB-N) is a vital indicator for assessing seafood freshness and edibility. Rapid on-site detection of volatile basic nitrogen (VBN) is of significant importance for food safety monitoring. In this study, highly luminescent self-assembled copper nanoclusters (Cu NCs@<i>p</i>-MBA), synthesized using <i>p</i>-mercaptobenzoic acid (<i>p</i>-MBA) as the ligand, were utilized for the sensitive detection of VBNs. Under acidic conditions, Cu NCs@<i>p</i>-MBA formed compact and well-organized nanosheets through noncovalent interactions, accompanied by intense orange fluorescence emission (651 nm). The benzene carboxylic acid part of Cu NCs@<i>p</i>-MBA provided the driving force for supramolecular assembly and exhibited a strong affinity for amines, particularly low-molecular-weight amines such as ammonia (NH₃) and trimethylamine (TMA). The quantitative determination of NH₃ and TMA showed the detection limits as low as 0.33 and 0.81 ppm, respectively. Cu NCs@<i>p</i>-MBA also demonstrated good responsiveness to putrescine and histamine. Through density functional theory (DFT) calculations and molecular dynamics (MD) simulations, the precise atomic structure, assembly structure, luminescent properties, and reaction processes of Cu NCs@<i>p</i>-MBA were studied, revealing the sensing mechanism of Cu NCs@<i>p</i>-MBA for highly sensitive detection of VBNs. Based on the self-assembled Cu NCs@<i>p</i>-MBA nanosheets, portable fluorescent labels were developed for semiquantitative, visual, and real-time monitoring of seafood freshness. Therefore, this study exemplified the high sensitivity of self-assembly induced emission (SAIE)-type Cu NCs@<i>p</i>-MBA for VBNs sensing, offering an efficient solution for on-site monitoring of seafood freshness.