Post-synthetic modification strategy to a ratiometric fluorescent probe Fe-CAU-10-OH@Cou for trimodal detection of zineb in food samples.

A post-modification strategy was employed to fabricate Fe-CAU-10-OH@Cou for the trimodal detection of Zineb. Fe-CAU-10-OH@Cou was fabricated by post-modifying fluorescent CAU-10-OH with Fe3+ and coumarin aldehyde (Cou). In this composite, Fe-doped coordination centers provided peroxidase-like activity, while Cou enabled a ratiometric fluorescence response. In the presence of Zineb, the fluorescence emission at 360 nm remained stable, whereas that at 475 nm was quenched, providing ratiometric fluorescence detection with a detection limit of 0.36 µM. Fe-CAU-10-OH@Cou catalyzed the oxidation of colorless TMB to blue oxidized TMB (ox-TMB). Zineb significantly inhibited this oxidation process, leading to a marked decrease in UV-vis absorption at 650 nm, and concomitantly suppressed the photothermal conversion ability. Based on these phenomena, colorimetric and photothermal detection methods were developed, with detection limits of 15.0 µM and 1.03 µM, respectively. Furthermore, a smartphone-assisted visual detection platform was developed based on the colorimetric signal variation, enabling precise quantitative determination of Zineb (LOD = 10.9 µM). Spike recovery experiments on actual fruit and vegetable samples demonstrated recoveries ranging from 96.0% to 106.0% across the three detection modes, with relative standard deviations (RSDs) below 5%, confirming the practicality and reliability of the proposed method. Moreover, mechanistic studies revealed that the inner filter effect (IFE) constituted the primary mechanism for fluorescence quenching, whereas the suppressive effect of Zineb on the enzymatic activity of Metal-Organic Frameworks (MOFs) accounted for the colorimetric and photothermal detection modes.