A Paper-Based Sensor Based on Dual-Mode pH-Responsive Carbon Dots With Colorimetric and Fluorescent Responses for...

Food spoilage threatens human health and causes major economic losses. This study reports the development of pH-responsive carbon dots (Met-CDs) with dual colorimetric and fluorescent sensing capabilities, synthesized from 4-chloro-1,2-phenylenediamine and L-methionine. The Met-CDs exhibit a broad pH response range (pH 1-12). Their sensing mechanism is based on protonation and deprotonation of surface functional groups, which induces aggregation under alkaline conditions and leads to visible color and fluorescence changes. A paper-based sensor incorporating Met-CDs was fabricated and successfully applied to ammonia detection, achieving limits of 8.98 ppm (colorimetric) and 0.77 ppm (fluorescent). In chilled lamb storage experiments, the sensor effectively tracked freshness changes and showed strong linear correlations with key freshness indicators, including pH, total volatile basic nitrogen (TVB-N), and total bacterial count. Specifically, the correlation coefficients (R²) between TVB-N and the sensor signals were 0.9886 (colorimetric) and 0.9919 (fluorescent). For total bacterial count, the R² values were 0.9443 and 0.898, respectively. For pH, the R² values were 0.9441 (colorimetric) and 0.9448 (fluorescent). These results demonstrate that the paper-based sensor enables real-time, visual monitoring of lamb freshness and provides high sensitivity and reliability in fluorescence detection. Overall, it shows strong potential as a low-cost, convenient, and effective tool for food freshness monitoring.