Synthesis of carbon quantum dots from cabbage with down- and up-conversion photoluminescence properties: Excellent imaging agent for biomedical applications

Carbon quantum dots (CQD) with down and up-conversion photoluminescence (PL) properties have been synthesized through low-temperature carbonization in a facile one step green method from cabbage as the natural source of carbon. The physiochemical and optical properties of the resultant CQD were performed using transmission electron microscopy, confocal laser scanning microscopy and various spectroscopic methods. The CQD with a quantum yield of 16.5% demonstrated excellent solubility and stability in aqueous media, superior resistance to photo bleaching, consistent PL within a biological pH range, excitation-dependent down conversion and excitation-independent up-conversion PL along with large stock shift behaviour. The purified CQD exhibited low cytotoxicity at higher concentration (500 μg ml-1) during the cell viability experiment against HaCaT cell, an immortalized non-tumerogenic human keratinocyte cell. Subsequently, CQD treated cells displayed three distinguished blue, green and red colours under a confocal microscope during in vitro imaging. Due to the advantages of green synthesis, high biocompatibility, excellent optical properties, low cytotoxicity and good cellular imaging outcome, the cabbage derived CQD showed considerable promise in biomedical applications.