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Carbon quantum dots (CQDs) have gained significant growing attention in the recent past due to their peculiar characteristics including smaller size, high surface area, photoluminescence, chemical stability, facile synthesis and functionalization possibilities. They are carbon nanostructures having less than 10 nm size with fluorescent properties. In recent years, the scientific community is curiously adopting biomass precursors for the preparation of CQDs over the chemical compounds. These biomass sources are sustainable, eco-friendly, inexpensive, widely available and convert waste into valuable materials. Hence in our work the fundamental understating of diverse fabrication methodologies of CQDs, and the types of raw materials employed in recent times, are all examined and correlated comprehensively. Their unique combination of remarkable properties, together with the ease with which they can be fabricated, makes CQDs as promising materials for applications in diverse biomedical fields, in particular for bio-imaging, targeted drug delivery and phototherapy for cancer treatment. The mechanism for luminescence is of considerable significance for leading the synthesis of CQDs with tunable fluorescence emission. Therefore, it is aimed to explore and provide an updated review on (i) the recent progress on the different synthesis methods of biomass-derived CQDs, (ii) the contribution of surface states or functional groups on the luminescence origin and (iii) its potential application for cancer theranostics, concentrating on their fluorescence properties. Finally, we explored the challenges in modification for the synthesis of CQDs from biomass derivatives and the future scope of CQDs in phototherapy for cancer theranostics.

Herein, we demonstrated a sustainable green approach for the preparation of fluorescent biocompatible carbon quantum dots by microwave-assisted reflux synthesis from Aloe barbadensis Miller (Aloe vera) extract. The Transmission Electron Microscopic images reveal that the as-prepared CQDs are spherical with less than 5 nm in size. The CQDs are amorphous, showed an excitation-independent behaviour, emitted blue fluorescence and have a fluorescence quantum yield of 31%. The presence of –OH groups contributed to the blue emission and helped CQDs to disperse uniformly in an aqueous solution. The prepared CQDs were employed as a photocatalyst for the environmental remediation to degrade the anionic dye, eosin yellow under visible light irradiation. The results showed that the CQDs exhibited excellent photocatalytic efficiency of 98.55% within 80 min and a 100% efficiency within 100 min. Further, the cytotoxic properties of as-prepared CQDs are investigated in the MCF-7 breast cancer cell line using MTT assay. The results demonstrated a notable reduction in cell viability in a dose-subjected manner, and the cell viability decreased to 50% (IC50) at a concentration of 52.2 ± 1.35 μg/mL. Furthermore, cellular internalization of CQDs in breast cancer cells is studied. As expected, CQDs are found to internalize by the cancer cells with blue emission as revealed by fluorescence microscope. In the end, CQDs in human breast cancer cells demonstrate the anti-proliferative effect and are found to be an impressive fluorescent probe for live-cell imaging, paving a path for its potential biomedical applications.