Covalent Michael-type recognition-based dual-mode sensing platform using thiol-functionalized nitrogen-doped carbon...

Patulin is a toxic mycotoxin commonly found in apple and fruit products, requiring sensitive and reliable detection methods. Herein, a novel dual-mode sensing platform integrating turn-off fluorescence and colorimetric detection was developed for rapid patulin determination in apple juice. The strategy exploits patulin's unique electrophilic reactivity toward thiols across two independent signal channels. Thiol-functionalized nitrogen-doped carbon dots (SH-N-CDs) serve as both recognition element and signal transducer, where direct covalent Michael-type addition of patulin onto the CD surface induces static fluorescence quenching confirmed by time-resolved lifetime measurements and XPS analysis. Simultaneously, patulin consumes free cysteine through PAT-cysteine adduct formation, relieving cysteine-mediated inhibition of Fe-MOF nanozyme peroxidase-like activity and restoring TMB oxidation to generate a proportional colorimetric signal. Under optimized conditions, the fluorometric and colorimetric methods exhibited linear ranges of 0.1-50.0 and 5.0-50.0 µg/L with limits of detection of 0.026 and 2.11 µg/L, respectively. Satisfactory recoveries of 97.0-100.4% with RSDs below 3.8% were obtained in spiked apple juice samples, with results showing no statistically significant difference from reference HPLC analysis. Successful determination of patulin in ten commercial apple juice samples further confirms the practical applicability of the proposed platform for routine food safety monitoring.