PDA/Au@Pt-mediated "on-off-on" model for colorimetric and smartphone-assisted visual sequential detection of cysteine...

Accurate and rapid detection of environmental and foodborne small molecules is essential for safeguarding public health. Herein, we prepared a polydopamine-supported gold-platinum bimetallic core-shell nanoparticles (PDA/Au@Pt) with exceptional peroxidase-mimicking activity in the presence of hydrogen peroxide (H₂O₂), which catalyze the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to its blue-colored oxidized form (ox-TMB), accompanied by a pronounced increase in absorbance. The peroxidase-mimicking activity of PDA/Au@Pt is selectively suppressed by cysteine (Cys), resulting in diminished ox-TMB. In contrast, subsequent addition of norfloxacin (NOR) restores the catalytic activity of PDA/Au@Pt, leading to increased ox-TMB production and a correspondingly intensified blue color. Therefore, leveraging the specific inhibition of PDA/Au@Pt's peroxidase-mimicking activity by Cys and the restoration of its catalytic activity upon competitive binding of NOR, an "on-off-on" colorimetric detection model is established for sequential detection of Cys and NOR. The fabricated sensor demonstrates high sensitivity with the limits of detection as low as 0.024 µM for Cys and 0.036 µM for NOR, respectively. Owing to its excellent selectivity and operational stability, the colorimetric sensing platform reliably discriminates Cys and NOR from various interfering substances. Moreover, integration with a smartphone-based optical readout platform enables portable and on-site visual detection of the target analytes. This study not only provides an efficient platform for the rapid and accurate monitoring of Cys and NOR in complex matrices but also establishes a generalizable framework for developing intelligent and multi-analyte colorimetric sensing strategies.