The photoluminescence of graphene quantum dots (GQDs) is rigorously investigated due to their potential applications. However, GQDs from graphene oxide are inherently embedded with non-negligible defects and oxygen grafted onto the edge and basal plane, which induce a change of innate electronic structure in the GQDs. Thus, graphene oxide based GQDs can misrepresent the characteristic properties of primitive GQDs. Here we report the size and pH dependent photophysical properties of GQDs that minimize the content of oxygen and defects. From auger electron spectroscopy, the oxygen content of the GQDs with two different lateral sizes (∼2 nm and ∼18 nm) was probed and found to be ∼5% and ∼8%, respectively. Two common photoluminescence (PL) peaks were observed at 436 nm (the intrinsic bandgap) and 487 nm (the extrinsic bandgap) for both GQDs. The characteristic PL properties in extrinsic and intrinsic bandgaps examined by optical spectroscopic methods show that the emission peak was red-shifted and that the peak width was widened as the size increased. Moreover, the PL lifetime and intensity were not only reversibly changed by pH but also depended on the excitation wavelength. This is in line with our previous report and is ascribed to the size variation of sp² subdomains and edge functionalization.

Hexagonal boron nitride nanoplatelets (BNNPs) can serve as two-dimensional (2D) fillers for elastomer nanocomposites due to their excellent and intriguing mechanical and thermal properties.

Stem cell therapy is one of the novel and prospective fields. The ability of stem cells to differentiate into different lineages makes them attractive candidates for several therapies. It is essential to understand the cell fate, distribution, and function of transplanted cells in the local microenvironment before their applications. Therefore, it is necessary to develop an accurate and reliable labeling method of stem cells for imaging techniques to track their translocation after transplantation. The graphitic quantum dots (GQDs) are selected among various stem cell labeling and tracking strategies which have high photoluminescence ability, photostability, relatively low cytotoxicity, tunable surface functional groups, and delivering capacity. Since GQDs interact easily with the cell and interfere with cell behavior through surface functional groups, an appropriate surface modification needs to be considered to get close to the ideal labeling nanoprobes. In this study, polyethylene glycol (PEG) is used to improve biocompatibility while simultaneously maintaining the photoluminescent potentials of GQDs. The biochemically inert PEG successfully covered the surface of GQDs. The PEG-GQDs composites show adequate bioimaging capabilities when internalized into neural stem/progenitor cells (NSPCs). Furthermore, the bio-inertness of the PEG-GQDs is confirmed. Herein, we introduce the PEG-GQDs as a valuable tool for stem cell labeling and tracking for biomedical therapies in the field of neural regeneration.

The need for highly efficient devices with longer operating lives in the blue wavelength region is increasing.