Bridge between Temperature and Light: Bottom-Up Synthetic Route to Structure-Defined Graphene Quantum Dots as a Temperature Probe In Vitro and in Cells

Owing to their unique superiorities in chemical and photoluminescence (PL) stability, low toxicity, biocompatibility, and easy functionalization, graphene quantum dots (GQDs) are widely used in cell imaging, probes, and sensors. However, further development and deeper research of GQDs are restricted by their imprecise and complex structure and accompanying controversial PL mechanism. In this work, two kinds of structure-defined water-soluble GQDs, with different oxidation degrees, are synthesized from molecules using bottom-up syntheses methods. After being studied by a series of characterizations, their optical properties, functional groups, molecular weight, and structural information were obtained. The optical properties of GQDs could be optimized by controlling their oxidation degree. The PL mechanism of GQDs was investigated by comparing their structure and properties. Furthermore, robust, stable, and precise temperature probes were designed using the GQDs, which exhibited an excellent wide response range, covering the whole physiology temperature range, from 0 to 60 °C in water. Moreover, the GQDs were successfully applied as temperature-responsive fluorescence probes in the HeLa cell line. These works laid a solid foundation for further applications of GQDs as biological thermoprobes and selectively temperature detectors in vitro cellular and in vivo.