Benzothiazole regulated ESIPT fluorescent probes: amine sensing and food freshness monitoring

Ensuring food safety requires straightforward methods for assessing food quality to protect public health. The decomposition of meat/fish proteins produces ammonia and biogenic amines (BAs), which can be sensitively detected and are essential for evaluating food safety during storage, transport, and consumption. We have designed and synthesized three ESIPT-exhibiting luminescent probes [3,3″-bis(benzo[d]thiazol-2-yl)-[1,1':4',1″-terphenyl]-4,4″-diol (B1), 3-(benzo[d]thiazol-2-yl)-[1,1'-biphenyl]-4-ol (B2), and 3,5-bis(benzo[d]thiazol-2-yl)-[1,1'-biphenyl]-4-ol (B3)] derived from 2-(2'-hydroxyphenyl)benzothiazole (HBT), simply by varying the number and position of benzothiazolate units on the hydroxy-phenyl backbone. The probability of intramolecular proton-transfer of the phenolic O-H group may be increased in the presence of two benzothiazolate units occupying both ortho positions of the central O-H (designated as B3) due to the possibility of increased intramolecular hydrogen bonding. Such disposition of the functional groups facilitates excited-state intramolecular proton transfer (ESIPT) and thus results in a higher pK_a. In contrast, the presence of only one benzothiazolate unit at the ortho position of the O-H (B1 and B2) relatively imparts a lesser extent of intramolecular hydrogen bonding, leading to a lower pK_a value. The lower pK_a values of B1 and B2 make them particularly suitable for pH sensing, enabling multiple fluorescence color changes at different pH levels. Moreover, these probes have been tested for amine sensing, achieving limits of detection (LOD) of 1-4.5 µM and 0.5-1.5 µM for ammonia and hydrazine, respectively, with ratiometric response. The strip-based detection of fish spoilage has been demonstrated for real field applications.