Graphene quantum dot based "switch-on" nanosensors for intracellular cytokine monitoring

The detection of cytokines in body fluids, cells, tissues and organisms continues to attract considerable attention due to the importance of these key cell signalling molecules in biology and medicine. We report a graphene quantum dot (GQD) based aptasensor able to specifically detect ultra-small amounts of cytokine molecules intracellularly. Graphene quantum dots modified with cytokine aptamers (Ap-GQDs) and epitope modified GQDs (Ep-GQDs) were prepared; both are normally fluorescent at sufficient dilution. However, the fluorescence of the conjugates of Ap-GQDs and Ep-GQDs is quenched due to aggregation between Ap-GQDs and Ep-GQDs. After incubation of the cytokine-secreting cells with the conjugates of Ap-GQDs and Ep-GQDs, the cytokines secreted in cells compete for binding with the epitope which is then displaced. The ensuing binding of cytokines with the aptamers results in the disaggregation of Ap-GQDs and Ep-GQDs, and the recovery of fluorescence. The conjugates of Ap-GQDs and Ep-GQDs were used as nanosensors for monitoring intracellular cytokine IFN-γ secretion with very high sensitivity (2 pg mL^-1). The disaggregation based sensing strategy in this nanosensor design is simple and universal; similar nanosensors can be used for the detection of a broad spectrum of cell-secreted molecules. Such nanosensors will serve as potential biomaterials for in vivo devices to monitor a variety of biological phenomena, in particular to understand cytokine secretion pathways in live cells.