Dual-mode colorimetric and fluorescent pH sensor based on carbon dots for real-time monitoring of shrimp freshness

Abstract

Conventional pH sensors relying on a single signal output are often compromised by external interferences, such as fluctuations in excitation intensity, environmental background noise, and subjective visual interpretation. To address this limitation, we developed fluorescence-colorimetric dual-responsive carbon dots (CDs) through a one-step hydrothermal synthesis using citric acid, o-phenylenediamine, and neutral red. These CDs serve as a robust functional material for non-destructive and rapid assessment of food freshness. As the pH increased, the CDs exhibited a distinct color shift from yellow to red under visible light, accompanied by a synchronous fluorescence enhancement showing linear correlation to ammonia in the range of 1–100 μmol/L (R² > 0.99). Integrated into filter paper-based indicator labels, the CDs retained excellent pH-responsive property, with clear visible and fluorescent color changes upon exposure to ammonia vapor. During shrimp storage at 4 ℃, the chromatic transitions of the labels under daylight correlated linearly with the total volatile basic nitrogen (TVB-N, R² = 0.99), enabling quantitative estimation. Spoilage was indicated on day 3 when TVB-N exceeded 30 mg/100 g, total viable count (TVC) reached 6.34 log₁₀ CFU/g, and indicator labels simultaneously turned red (ΔE = 7.29). The developed sensing system enabled real-time, convenient, and non-destructive visualization of pH changes, providing a reliable approach for monitoring shrimp freshness. This work offers a solid theoretical foundation and technical framework for the design of CD-based intelligent packaging systems in food quality surveillance.