Graphene Quantum Dots from <i>Mangifera indica</i>: Application in Near-Infrared Bioimaging and Intracellular Nanothermometry

We report a simple one-pot microwave-assisted green-synthesis route for the fabrication of bright red-luminescent graphene quantum dots (GQDs) using ethanolic extracts of <i>Mangifera indica</i> (mango) leaves, hence addressing them as mGQDs. The mGQDs were quantum-sized ranging from 2 to 8 nm and exhibited excitation-independent fluorescence emission in the near-infrared (NIR) region between 650 and 750 nm. The mGQDs showed defects in their structure and were highly crystalline in nature as confirmed by Raman spectroscopy and powdered X-ray diffraction analysis, respectively. These mGQDs showed 100% cellular uptake and excellent biocompatibility on L929 cells even at high concentration (0.1 mg/mL) 24 h post-treatment. Cell cycle analysis showed increased proliferation in L929 cells upon mGQDs treatment. Furthermore, the mGQDs were demonstrated as NIR-responsive fluorescent bioimaging probes, self-localizing themselves selectively in the cell cytoplasm. Also, the temperature-dependent fluorescence intensity of these GQDs proved them as a very competent temperature sensing probe (at 10–80 °C). The temperature sensing stability analysis showed that the temperature signal remains stable even after multiple cycles of temperature switching between 30–80 °C. Furthermore, we analyzed intracellular temperature (25–45 °C) of live L929 cells based on the fluorescence intensity of the mGQDs. It was observed that with an increasing temperature there was a decrease in the fluorescence intensity of the mGQDs making it a suitable probe for temperature sensing. In sum, a biocompatible, scalable, photostable, green synthesis based mGQDs were prepared for NIR imaging and nanothermometry applications which can play a pivotal role in biomedical nanotechnology.