Carbon Quantum Dots from Coconut Husk: Evaluation for Antioxidant and Cytotoxic Activity

In this paper we report green synthesis of water soluble, monodisperse carbon quantum dots (CQDs) from coconut husk, a natural biomass as a carbon precursor using a single step hydrothermal carbonization. Structural and morphological characterization of these CQDs were carried out using TEM, XRD, Raman and FTIR. As synthesized CQDs were in the size range of 1 to 8 nm as observed in the TEM, while XRD results confirmed their crystalline nature. Raman spectroscopic measurements exhibited the characteristic "D" (1352 cm(-1)) and "G" (1585 cm(-1)) bands confirming the formation of low dimensional carbon nanostructures. Optical properties of these CQDs were probed using UV-visible spectrometry and room temperature Photoluminescence measurements. The UV-vis spectrum showed a strong absorption at 278 nm. These CQDs exhibited fluorescence constantly at 440 nm when excited at different wavelengths. The fluorescence of CQDs was found to be sensitive to pH linearly over the pH range of 4-12 which can be exploited for pH sensor applications. FTIR spectroscopy confirmed the presence of hydroxyl and carboxyl functional moieties on the surface of the CQDs, which play a crucial role in surface passivation leading to more stable nanoparticle dispersions. The antioxidant activity of CQDs was estimated and EC50 was found to be 60 mu g/mL using DPPH assay. Hemolysis test proved that CQDs derived from coconut husk are biocompatible. The favorable optical properties, antioxidant and non-cytotoxic properties of these CQDs can be exploited very well for bio imaging and targeted drug delivery applications.