Supercritical water in top-down formation of tunable-sized graphene quantum dots applicable in effective photothermal treatments of tissues

Abstract Supercritical water was used for simultaneous fragmentation and reduction of graphene oxide (GO) sheets into water-dispersible graphene quantum dots (GQDs) with tunable sizes. Transmission electron microscopy (TEM) demonstrated that by increasing the temperature above the critical point of water, the average size and thickness of the GQDs were decreased and the size uniformity and production yield were increased. The results of thermal conductivity measurement of GQD nanofluids with different weight fractions indicated that the GQDs prepared at supercritical condition could enhance the thermal conductivity of water by 65% as compared to 35% for the GQDs synthesized at sub-critical conditions. Near-infrared (NIR) photothermal induction of a tissue-equivalent model (agarose gel) treated by the graphene nanomaterials showed that using the GQDs significantly increased the thermal conductivity (∼9.6 times), absorption coefficient (25%) and temperature uniformity (∼5.4-fold lower temperature difference) of the tissue in comparison with GO sheets. These results support the promising application of these GQDs in more efficient photothermal therapeutic purposes requiring lower thermal stresses.