Fluorescence Sensing Mechanisms of Versatile Graphene Quantum Dots toward Commonly Encountered Heavy Metal Ions

Graphene quantum dots (GQDs) have received tremendous attention as fluorescent probes for detection of diverse heavy metal ions (HMIs). Nevertheless, the fluorescence sensing mechanisms of versatile GQDs with respect to different HMIs remain elusive. Herein, the fluorescence sensing behaviors and mechanisms of GQDs with amino and carboxyl groups toward commonly encountered Cr⁶⁺, Fe³⁺, Cu²⁺, Cr³⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, and Hg²⁺ under different pH conditions are systemically explored. The results show that the fluorescence of GQDs can be enhanced by Zn²⁺/Cd²⁺ and quenched by other HMIs at pH 5.8, while it can be enhanced by HMIs except Cr⁶⁺/Fe³⁺/Cu²⁺ at pH 2.0. Systematic studies verify that the fluorescence quenching/enhancing is mediated by the synergistic effect of the inner filter effect (IFE) and the photoinduced electron transfer (PET) or metal orbital-controlled chelation-quenched/enhanced fluorescence (CHQF/CHEF) effect. The strong and weak IFEs of Cr⁶⁺/Fe³⁺ and Cr³⁺/Cu²⁺, respectively, are one of the reasons for the fluorescence quenching, while other HMIs have no IFE. Moreover, the PET effect caused by the interaction of GQDs with Hg²⁺ at pH 5.8 and the CHQF/CHEF effect caused by the interaction of GQDs with other HMIs are also crucial for fluorescence quenching/enhancing. The findings suggest that the pH condition, the existing forms of functional groups on GQDs, and the complexation states of HMIs in aqueous systems dominate the PET and CHQF/CHEF effects. The elucidating of the fluorescence sensing mechanisms of GQDs toward different HMIs paves the way for developing versatile sensing platforms for monitoring of HMI contamination.