Due to their low toxicity, excellent biocompatibility and attractive optical properties, graphene quantum dots (GQDs) have attracted significant attention in the analytical and biological fields. The controllable intrinsic nature of graphene nano-sheets, e.g., size, layer, edge state or shape, offer the chance to fabricate GQDs with tunable photoluminescence behaviors. Carboxylic moieties on the surface and edges of GQDs can provide efficient reactive sites for interaction with various polymeric, organic, inorganic and biological species. This not only provides the chance to tune the optical properties of GQDs via facile chemical approaches such as surface modulation and doping, but also makes GQDs eco-friendly alternatives to replace the traditional fluorescent probes in biological assays. This review highlights new insights into the various strategies for tuning the optical features of GQDs, and their employment as attractive and powerful probes for bio-sensing/imaging. The challenges and future perspectives in the related fields are discussed.

Graphene quantum dots prepared by a one-step hydrothermal procedure in a microwave exhibit an unusual emission transformation in strong acidic media and at high concentration, induced by self-assembled J-type aggregation under restrained π–π interactions.

Organelles play crucial roles in cellular activities and the functions of organelles are related greatly to the pH values, therefore, the bio-imaging of targeted organelles and their related pH sensing is of great importance in biological assays. Herein we report the fluorescence imaging of specific organelles, i.e., lysosomes and endoplasmic reticulum, and their pH sensing with surface regulated carbon dots (CDs). Carbon dots functionalized with amine groups (ACDs) are first prepared by hydrothermal treatment of citric acid and urea, and then laurylamine functionalized CDs (LCDs) are obtained via the conjugation of laurylamine with ACDs. The as-prepared ACDs and LCDs provide clear and bright imaging results for the lysosome and endoplasmic reticulum, respectively. The subcellular targeting features of the two CDs are attributed to their surface chemistries and cellular uptake pathways. Moreover, both the CDs are pH responsive within a certain pH range, i.e., 4.0–5.4 for ACDs and 6.2–7.2 for LCDs. The ACDs and LCDs are thus successfully applied to visualize the pH fluctuations of the lysosome and endoplasmic reticulum in MCF-7 cells.