Self-reduced anchoring of dispersed silver nanoparticles on Wurster COFs combined with graphene quantum dots: signal amplification for a femtomolar chlorpyrifos dual-recognition electrochemical aptasensor.

Accurate detection of the widely used organophosphorus pesticide chlorpyrifos (CPF) is of great significance for ecological systems and human health. In this study, a dual-recognition electrochemical sensing platform was constructed by assembling graphene quantum dots (GQDs) with highly dispersed Ag nanoparticles (AgNPs) anchored on Wurster-type covalent organic frameworks (COFs) via self-reduction for the accurate, selective and sensitive detection of CPF. AgNPs were confined and grown inside the pores and on the surface of the COFs through in situ reduction by the reductive Wurster-type tetraamine monomer, with the latter being oxidized to a radical cation. Owing to the synergistic enhancement of the formed AgNPs and the radical cation, the conductivity of the COFs was significantly improved. The introduction of GQDs not only maintains the activity of the recognition unit and extends the service life of the sensor but also further synergistically enhances the electron transfer at the interface and improves the response signal. Moreover, small-sized GQDs can fill the pores of the COFs, increase the exposure of active sites, and at the same time prevent agglomeration of AgNPs, fully leveraging the advantages of nanomaterials. Based on the COFs@AgNPs/GQDs composites, as well as the dual recognition system constructed by molecular imprinting polymers and aptamers, the as-prepared CPF sensor exhibited excellent electrochemical performance. Under optimal conditions, CPF could be accurately detected in the range of 10 fmol L-1 to 0.1 nmol L-1 with an ultra-low detection limit of 8.53 fmol L-1. The present dual-mode aptasensor also exhibited excellent selectivity, stability, and reproducibility and was proven to be highly suitable for the reliable detection of CPF in complex food samples. This work has expanded the application scope of COF-based composites in dual-model or multi-model electrochemical sensors for the field-ready detection of organophosphorus pesticides in various scenarios.