Carbon quantum dots: An overview of their synthesis from natural plant sources, and their potential use as antimicrobial agents

The serendipitous discovery of carbon quantum dots (CQDs), while purifying electrophoretically, single-walled carbon nanotubes (SWNTs) obtained from the soot of an arc-discharge, ignited a global interest in these carbon nanoparticles, due to their potential uses in a broad spectrum of studies like drug delivery, fluorescence, and catalysis. Their low toxicity and high stability endow them with potential medical uses, which hints at the need for CQDs in large quantities. Thus, a sustainable synthesis of CQDs has to be worked out to ensure the minimum use of energy and toxic chemical substances. A ‘top-down’ route to their synthesis involves the disintegration of relatively larger carbon structures like carbon nanotubes, nanodiamonds, and graphite, using energy-intensive processes, such as photoablation, arc discharge, and electrochemical techniques. An alternative to this is a ‘bottom-up’ method, which minimizes energy's utility, and can be achieved through the green synthesis of CQDs, from various plant sources, like herbs and the nightshades (family Solanaceae). Some of these naturally-derived CQDS have exhibited striking antimicrobial activity and cytotoxicity, which makes them potentially useful in therapeutics for bacterial infections and carcinomas. This review aims to discuss the synthesis and applications of such CQDs derived from herbal medicine (HM-CDs).