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The thermally decomposed citric acid (TDCA) possesses either excitation-dependent or excitation-independent fluorescence as well as different quantum yields with varying synthesis conditions (i.e. temperature and reaction duration). These photoluminescent (PL) properties were found to be mainly determined by the quantitative competition between the graphene quantum dots (GQDs, average size in the range 0.7–1 nm) and the large-inhomogeneously-sized particles. Thermal induced reduction of oxygen containing functionalities leads to an enhancing effect to the PL of GQDs. The study reveals the structural evolution of the GQDs upon thermal treatment and attempts to establish their relationship to the PL property. The GQDs synthesized in this study are excellent sensing materials for trivalent iron cation with both notable selectivity and sensitivity.