Visualized time-temperature monitoring by triplet-sensitized ratiometric fluorescent nanosensors

Time-temperature indicators (TTIs) with the capability to visually reflect the accumulated time and temperature effect for real time are of substantial significance for securing the safety and quality of perishable products, such as foods and medical products. Herein, we design a novel TTI with high accuracy and low cost based on a ratiometric fluorescent nanosensor capable of self-calibration. The nanosensor was fabricated via the co-assembly of amphiphilic block copolymer and a responsive triplet-triplet annihilation upconversion (TTA-UC) system. The pH responsiveness of the triplet-sensitized nanomicelle was well investigated experimentally and theoretically. A linear relationship between the intensity ratio (of the integrated upconversion emission to the integrated phosphorescence emission) and pH was revealed for the ratiometric fluorescent nanomicelle. The TTA-UC functionalized nanomicelle turned out to be an extraordinary nanosensor with excellent sensitivity, reliability, and anti-reference. Subsequently, the nanomicelle was firstly applied for enzymatic time-temperature indicators (TTIs). Comprehensive kinetic study of the TTI prototype and the released total volatile basic nitrogen during meat storage was conducted under isothermal and non-isothermal conditions, which demonstrated remarkable correlation between the TTI response and the food spoilage. The activation energy of the TTI prototype could be readily modified to satisfy diverse food products including meat, diary, and aquatic products. The TTI based on triplet-sensitized ratiometric fluorescent nanosensor could be employed for visual monitoring food quality with low cost, high accuracy, and good reliability.