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

The tracking of calcium ions (Ca) 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 Ca with high selectivity and sensitivity. Upon the addition of various concentrations of Ca, 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 Ca 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 Ca. Competition experiments demonstrated that the DA-GQDs exhibited a high fluorescence response specifically for Ca, 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 Ca. This study provides new insights into the design of Ca fluorescence sensors using biocompatible molecular components.