One pot synthesis of cobalt and nitrogen co-doped graphene quantum dots based fluorescence ‘On-Off-On’ probe for dimethoate sensing: A proof of concept

Abstract

Dimethoate, which is widely used in agriculture, has harmful effects on organs. To address this issue, advanced sensors are crucial for detecting it in real samples. Notably, graphene quantum dots (GQDs)-centered fluorescent sensors offer numerous advantages such as high sensitivity, selectivity, and good stability. Therefore, in this work, we designed a new cobalt/nitrogen co-doped GQD (Co/N-GQD) fluorescence switch “On-Off-On” sensor to achieve selective dimethoate detection. In brief, the synthesis of Co/N-GQDs, achieved through the hydrothermal method, involves Pithecellobium dulce fruit peel as a green precursor, along with cobalt (Co) and urea as dopant sources. Extensive spectral characterizations, including FTIR, UV–Vis spectroscopy, zeta potential analysis, particle size analysis, HR-TEM, Raman, PXRD, and fluorescence studies, were conducted to validate the fabrication of Co/N-GQDs. The resulting nanosized Co/N-GQDs exhibited an enhanced quantum yield of 49.78 %. In terms of sensing, the fluorescence intensity of Co/N-GQDs is selectively quenched (“Turned Off”) by Cu2+ through a dynamic quenching mechanism. When dimethoate is included in the quenching system, a proportional correlation is observed between dimethoate concentration and fluorescence reactivation. This phenomenon is attributed to the potential of dimethoate, which contains amide and phosphorodithioate functionalities, to displace Cu2+ from the electrostatic complex through chelation. Remarkably, the sensor achieves a limit detection limit (LOD) of 64.08 ng/mL, offering a broad linear range spanning from 10 to 800 ng/mL. In addition, it exhibited real-time applicability, good stability, and reproducibility. In conclusion, the design of Cu2+–Co/N-GQDs can be used as a proof of concept for dimethoate sensing within various sample contexts.