Π-Electron and pH-Regulated Photochemical Detection of Trinitrophenol Using Nanoconfined Graphene Quantum Dots

Fluorescence sensors derived from inorganic fluorescent molecules have attracted enormous attention due to their easy processing and the tunability of their chemical properties. However, the fluorescence quenching mechanism derived from the π-electron density and solution pH has been less explored. Covalently tuned graphene quantum dots (GQDs) with quantum-confined electrons efficiently detect 2,4,6-trinitrophenol (TNP) explosives. Herein, GQDs of various sizes were designed by using a bottom-up synthesis approach. The synthesis mechanism of the GQDs was established by chemical analysis and density functional theory (DFT) calculations. During the fluorescence sensing, smaller blue fluorescent-GQDs with photoluminescence (PL) emission at 475 nm upon 410 nm excitation were effectively quenched by TNP, showing sensitivity and selectivity in the 14.56 μM range. In contrast, the GQDs with higher π-electron density, i.e., G-GQDs, Y-GQDs, and O-GQDs, did not show significant quenching phenomena. DFT studies showed that due to the accessible band gap of TNP and B-GQDs, excited LUMO electrons got transferred to the LUMO of TNP by the thermodynamically driven force, and the highest quenching was observed at neutral pH 7.