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 Mangifera indica (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.

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We report a simple one-pot microwave assisted “green synthesis” of Graphene Quantum Dots (GQDs) using grape seed extract as a green therapeutic carbon source. These GQDs readily self-assemble, hereafter referred to as “self-assembled” GQDs (sGQDs) in the aqueous medium. The sGQDs enter via caveolae and clathrin-mediated endocytosis and target themselves into cell nucleus within 6–8 h without additional assistance of external capping/targeting agent. The tendency to self-localize themselves into cell nucleus also remains consistent in different cell lines such as L929, HT-1080, MIA PaCa-2, HeLa, and MG-63 cells, thereby serving as a nucleus labelling agent. Furthermore, the sGQDs are highly biocompatible and act as an enhancer in cell proliferation in mouse fibroblasts as confirmed by in vitro wound scratch assay and cell cycle analysis. Also, photoluminescence property of sGQDs (lifetime circa (ca.) 10 ns) was used for optical pH sensing application. The sGQDs show linear, cyclic and reversible trend in its fluorescence intensity between pH 3 and pH 10 (response time: ~1 min, sensitivity −49.96 ± 3.5 mV/pH) thereby serving as a good pH sensing agent. A simple, cost-effective, scalable and green synthetic approach based sGQDs can be used to develop selective organelle labelling, nucleus targeting in theranostics, and optical sensing probes.