Enhanced intracellular calcium detection using dopamine-modified graphene quantum dots with dual emission mechanisms

The tracking of calcium ions (Ca²⁺) is of great significance in clinical medicine. Dopamine-functionalized graphene quantum dots (DA-GQDs) have been developed as a novel fluorescence sensor for detecting and imaging Ca²⁺ with high selectivity and sensitivity. Upon the addition of various concentrations of Ca²⁺, the fluorescence intensity of DA-GQDs notably enhanced and exhibited a redshift. This behavior was regulated by the photoinduced electron transfer and internal charge transfer mechanisms, as elucidated by fluorescence titration and density functional theory calculations. The coordination of Ca²⁺ with the oxygen groups on DA-GQDs enhanced its sensitivity and selectivity. The detection limit was determined to be 0.05 µM, with a robust linear response range of 4.93-10.61 µM. Even in the presence of chelating agents such as gluconate, the DA-GQDs accurately quantified Ca²⁺. Competition experiments demonstrated that the DA-GQDs exhibited a high fluorescence response specifically for Ca²⁺, with negligible selectivity toward other ions and biomolecules. The CCK8 assay confirmed that the DA-GQDs are nontoxic and biocompatible. Moreover, DA-GQDs have been successfully employed for imaging intracellular Ca²⁺. This study provides new insights into the design of Ca²⁺ fluorescence sensors using biocompatible molecular components.