On-site food monitoring: a smartphone-readable carbon quantum dots sensor for ethanol and dimethylamine.

Carbon quantum dots were synthesized via microwave pyrolysis using o-phenylenediamine and dimethyl 2,2'-azobis(2-methylpropionate) as precursors. Through a solvent screening strategy, two types of functional CQDs were obtained and are denoted as DIW-CQDs and FA-CQDs, corresponding to their dispersion solvents, deionized water and formic acid, respectively. DIW-CQDs enable sensitive ethanol detection, whereas FA-CQDs exhibit a reversible fluorescence response to a dimethylamine-HCl system. Based on these properties, we developed portable dual-mode test strips. Combined with smartphone-based colorimetric analysis, this approach allows for the on-site quantification of ethanol in beverages and DMA in food samples. Furthermore, a multifunctional FA-CQDs@PVA composite film was fabricated by incorporating FA-CQDs into a polyvinyl alcohol matrix. The film simultaneously maintains fruit freshness and visually monitors seafood spoilage through distinct color changes. Finally, a unified analytical framework was established using a random forest machine learning model, which enables the automatic classification of CQDs and the simultaneous quantification of multiple targets.