Analyte-inhibited in-situ formation of Au nanoparticles suppressing their Tyndall effect for simple, ultrasensitive, and portable quantitative colorimetric detection of nanomolar malathion

• A portable colorimetric malathion assay is designed with Tyndall effect (TE). • It is based on analyte-inhibited in-situ formation of Au nanoparticles (AuNPs). • Only a laser pointer pen and a smartphone are used to realize quantification. • The TE signaling of AuNPs proves superior in concentration discrimination compared with their surface plasmon resonance responses. This work initially reports a simple, instrument-free, quantitative colorimetric assay of organophosphorus pesticides based on the analyte-inhibited in-situ formation of Au nanoparticles (AuNPs) suppressing their Tyndall effect (TE). Malathion was tested as a model analyte of organophosphorus pesticides. L-ascorbic acid-2-phosphate can be dephosphorylated by alkaline phosphatase to generate L-ascorbic acid, which can subsequently reduce chloroauric acid to produce colloidal AuNPs that display a pronounced TE response under irradiation with a laser pointer. In the presence of malathion, the analyte would effectively inhibit catalytic activity of the alkaline phosphatase, preventing the AuNPs’ formation to switch off their TE. The TE is inversely related to the malathion level and can be quantitatively measured by a smartphone. The results showed that this new method enabled a linear concentration range spanning four orders of magnitude (50 nM-15 µM) for malathion quantification. The limit of detection was estimated to be as low as 1.29 nM (3σ). When it was applied to detect real samples (i.e., pond water and tap water), recoveries of malathion ranged from 94.0 to 107.3%. The developed TE-based colorimetric approach offers many advantages including simplicity, low cost, and portable smartphone readout, thus holding great promise for on-site analysis and/or household self-testing of organophosphorus pesticides like malathion.