Green chemistry route to realize, high quantum yield carbon quantum dots for cellular imaging applications

Carbon quantum dots (CQDs) with exceptionally high photoluminescence quantum yield (PLQY) of 77% are synthesised in a single step from multiwalled carbon nanotubes (MWCNTs) through ecofriendly synthetic route using a greener solvent, lemon juice and this novel approach is the highlight of the present work. The multiple roles played by the high citric acid content in lemon as oxidizing, stabilizing as well as capping agent are the prime factors effecting the generation of CQDs of high PLQY. Transmission electron microscopy (TEM) images show well seperated spherical CQDs of average size 5 nmand the high-resolutionTEM(HRTEM) image reveals that the nanoparticles are highly crystalline with lattice spacing of 0.33 nm, strictly matching with the (002) plane of graphitic carbon. The Fourier transform infrared (FTIR) spectroscopic studies confirm the presence of abundant carboxyl functionalities on the CQDs surfaces. The photoluminescence emission spectra of the CQDs exhibit excitation dependent emission and intensity variations, which are the unique features of the CQDs. Near infra-red (NIR) excited NIR emission and up-conversion photoluminescence (UCPL) emission properties exhibited by these CQDs offer opportunities in many biological applications. Cellular uptake studies using fluorescence imaging and cytotoxicity studies establish the potentials of the CQDs as high performance yet nontoxic fluorescence agents for bio-imaging applications.