Pyrene derivative based fluorometric smart tags for shrimp freshness monitoring via a smartphone

With increasing emphasis on food safety, there is a growing need for simple, effective, and instrument-free methods to monitor food freshness. To meet this demand, we report a hydrogel-based tag for shrimp freshness monitoring that employs a pyrene-derivative fluorometric indicator, 4-(2-(pyren-2-yl)vinyl)pyridine (PY-Py). PY-Py exhibits large spectral red-shifts (>100 nm) and readily distinguishable fluorescence color changes upon protonation in both the solid state and in solution, making it a pH-responsive indicator with significant fluorescence ratiometric properties. The protonated form (PY-Py-H⁺) was doped into a hydrogel matrix to produce a smart tag. Benefiting from the nanometer-scale pore of the hydrogel, PY-Py's strong photostability, and its hydrophobic conjugated architecture, the dye-doped tag retained stable fluorescence signals after one month of storage, demonstrating excellent practical applicability. The smart tag detects volatile amines through changes in fluorescence ratio, showing a linear response to ammonia in the 10-70 ppm range and a limit of detection (LOD) of 3.28 ppm. Based on the tag's response to volatile biogenic amines and the standard total volatile basic nitrogen (TVB-N) method, freshness evaluation of shrimp can be performed either by direct visual inspection or smartphone-based analysis. This work provides a simple and convenient assay for volatile amines and for assessing shrimp freshness.