Etching of Single-MnO₂-Coated Gold Nanoparticles for the Colorimetric Detection of Organophosphorus Pesticides

In this work, we demonstrate an enzyme-correlated catalytic reaction at the single-particle level for the quantitative analysis of organophosphorus pesticides (OPs) by using MnO2-coated gold nanoparticle (GNP) supraparticles (defined as GNP@MnO2 SPs) as the probes. Without OPs, in the presence of acetylcholinesterase (AChE) and acetylthiocholine iodide (ATCh), ATCh can be hydrolyzed to thiocholine (TCh), which is used as a reducing agent to decompose the MnO2 shell from GNP@MnO2 SPs. On the basis of localized surface plasmon resonance of the GNPs, variation of the particle color and scattering intensity can be observed at the single-particle level with dark-field optical microscopy. However, in the presence of OPs, they can act as inhibitors toward AChE, which restrains production of TCh and decomposition of the MnO2 shell. By calculation of the ratio of the scattering intensity from individual GNP@MnO2 SPs in the green and red channels (Rg/r), the content of OPs can be precisely quantified. A linear range from 0.02 to 15 pg/mL and a limit of detection as low as 0.006 pg/mL are obtained, which are much lower than those of other methods. As a consequence, the proposed method for ultrasensitive detection of OPs will have a broader application in the future, providing a viable alternative for water quality and food safety assays.