Colorimetric swab and turbidimetric sensing for dual-mode point-of-use testing of demeton pesticide in foods based on oxidase-like Cu2O@Ag nanoprobe

Dual-mode sensing can enhance detection reliability by cross-validating results from distinct signals. Herein, a colorimetric and turbidimetric dual-mode sensing platform based on Cu 2 O@Ag nanozyme with robust oxidase-like activity is developed for point-of-use (POU) detection of demeton. The Cu 2 O@Ag efficiently catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB), producing concurrent absorbance and turbidity signals. Demeton blocks active sites and suppresses catalytic activity in a concentration-dependent manner. Based on the principle, two modes for POU testing of demeton are developed. The first mode is based on a smartphone-assisted colorimetric swab and shows a linear range of 50 - 4000 ng/mL with the detection limit of 0.190 ng/mL. The other is based on a portable turbidimeter and exhibits a linear detection range of 50 to 3000 ng/mL with a detection limit of 2.997 ng/mL. The approaches are successfully validated in real food samples including baby cabbage, lettuce, cilantro, and tea leaves. This work presents a rapid, low-cost, and instrument-flexible dual-mode sensing strategy for on-site analysis of demeton residue. Also, the dual-mode detection based on colorimetric and turbidimetric signals for the first time offers a novel insight for dual-mode sensing.