Ratiometric fluorescence and photothermal dual-mode immunosensor based on MnO2 nanosheets for the detection of isocarbophos

Isocarbophos (ICP) is a typical highly toxic organophosphorus pesticide, and its rapid and accurate detection is essential for food safety. In this study, we presented a ratiometric fluorescence and photothermal dual-mode immunoassay for the quantification of ICP in food samples, leveraging the oxidase-like activity and photothermal effect of manganese dioxide nanosheets (MnO2NSs). The immunoassay utilized alkaline phosphatase (ALP) as a labeling enzyme, which efficiently catalyzes the dephosphorylation of ascorbic acid 2-phosphate (AA2P) to produce ascorbic acid (AA). The generated AA subsequently initiated the decomposition of MnO2NSs, resulting in a reduction of both the photothermal signal from MnO2NSs and the fluorescence signal from oxidized Amplex Red (oxAR), while moderately restoring the fluorescence emission from blue-emissive carbon dots (b-CDs). The limits of detection (LOD) were as low as 2.62 pg mL- 1 in photothermal mode and 0.21 pg mL- 1 in ratiometric fluorescence mode. Recovery rates from spiked food samples were 94.8-119.8 % in photothermal mode and 97.9-105.5 % in ratiometric fluorescence mode. This study creates promising new avenues for pesticide detection and establishes an effective platform for dual-mode immunosensors.