Designed fluorescence “on–off–on” probe based on cobalt, zinc, and nitrogen co‐doped graphene quantum dots: A case of quinalphos sensing

Abstract

In this study, we developed a new fluorescence “on–off–on” sensor utilizing water‐soluble cobalt/zinc–nitrogen co‐doped graphene quantum dots (Co/Zn‐N‐GQDs) to recognize quinalphos pesticide in vegetable and fruit samples. Primarily, the synthesis method employed a one‐pot hydrothermal approach, using betel leaves as a natural precursor and cobalt (“Co”), zinc (“Zn”), and urea (“N”) as dopant sources. The Co/Zn‐N‐GQDs probes underwent comprehensive analytical characterization. The Co/Zn‐N‐GQDs were synthesized with a remarkable luminescence yield of 31.49%, exhibiting excitation at 320 nm and emission peak at 393 nm. Interestingly, the luminescence of Co/Zn‐N‐GQDs was selectively “Turned Off” by Cu2+ via a static quenching setup. Remarkably, quenched fluorescence was surprisingly reactivated upon adding quinalphos to the quench setup, indicating a direct correlation between luminescence reactivation and quinalphos concentration. Briefly, this phenomenon is ascribed to the functional groups in quinalphos, such as quinoxalinyl and phosphorothioate, which chelate with Cu2+ ions, disrupting the nonfluorescent Cu2+‐Co/Zn‐N‐GQDs complex. The design sensor demonstrated a limit of detection (LOD) of 0.11 μM and a broad linear span of 0.5 to 200 μM. In conclusion, Cu2+‐Co/Zn‐N‐GQDs sensor showed immediate applicability, stability, and reproducibility, making it highly effective for quinalphos sensing in various samples.