Synergistic indicator labels containing bromothymol blue/red carbon dots for visual real-time fish freshness sensing via colorimetric/fluorescent dual-modal signaling

Food freshness indicator labels are designed to assist consumers in identifying food spoilage. This study developed a colorimetric and fluorescent dual-channel indicator label for real-time, visual, and synergistic monitoring of fish freshness, using bromothymol blue (BTB) and red carbon dots (RCDs) as response signals and carboxymethyl cellulose (CMC) as the matrix (CMC-BTB/RCDs). It exhibits significant color and fluorescence responses in the pH range of 5–9 and volatile amine concentrations of 0.05–20 mg/mL. Under daylight, the label transitions from yellow-brown to blue-green (color difference ΔE = 60.47); under 365 nm ultraviolet light, its red fluorescence gradually decreases while the Blue/Red fluorescence (BRF) increases to 5.64. These changes originate from volatile amines released during fish spoilage, which alter the storage environment's pH and changes BTB's molecular structure. This structural change not only triggers the label's color shift but also adjusts its fluorescence intensity via the inner filter effect between BTB and RCDs. In practical use, CMC-BTB/RCDs enables precise, real-time, and non-destructive monitoring of fish freshness at 25 °C and 4 °C, with the label's ΔE and BRF showing a strong correlation with fish total volatile basic nitrogen. Therefore, CMC-BTB/RCDs showed great potential for application as smart packaging in monitoring fish freshness.