Biomolecule-functionalized polydiacetylene assembly for visual and on-site sensing of (E)-2-hexenal in fruits

(E)-2-Hexenal (E2H) is an important volatile organic compound (VOC) that can serve as a marker for fruit quality sensing and shelf-life evaluation. However, visual and portable sensors for E2H have not been reported mainly because of the difficulty in selective response to E2H while avoiding interference from other VOC, especially isomers. Herein, we developed a novel colorimetric sensor based on thiol-functionalized polydiacetylene assembly (PDA-SH/PDA) for the quantitative and selectivity of E2H. The mechanism was that E2H underwent a Michael addition reaction with sulfhydryl groups in PDA-SH/PDA. The Michael addition reaction destroyed the hydrogen bond and distorted the conjugated molecular system, resulting in colorimetric responses. Under optimal conditions, a good linear relationship was observed between E2H concentration and red colorimetric shift (R² = 0.9679). The E2H sensor yields a high analytical sensitivity of 1.5203% and a limit of detection of 0.015 g L^-1. Besides, the PDA-SH/PDA sensor can be used to visually distinguish E2H and its isomer (Z)-3-hexenal. The PDA-SH/PDA sensor yielded a statistically significant difference in red colorimetric shift (p < 0.01) between the pairs of isomers in the ripeness of fruits. At last, the sensor was utilized for detecting E2H in grapes from different shelf lives. We found that the E2H concentration increased from 66.16 ± 1.54 to 67.56 ± 1.30 µg L^-1 as storage time was prolonged. The results of this research not only demonstrated the feasibility of visual and on-site detection of E2H but also provided potential promise for portable fruit quality sensing and shelf-life evaluation.