SYNTHESIS AND CHARACTERISATION OF CARBON DOTS FROM LEMON JUICE

Abstract

Luminescent carbon dots have gained intensive interests in recent years due to their unique optical properties and widespread applications. In this work, luminescent carbon dots were synthesised from lemon juice by hydrothermal method. The as prepared carbon dot solution changes from brown colour to yellow after excitation by UV light. The synthesised carbon dots were then conjugated with 0.5M boric acid solution. Upon capping with boric acid, luminescence intensity increased which is attributed to the fact that capping provides surface passivation and minimise electronic trapping capabilities of surface defects resulting in higher photoluminescence intensity. This good photoluminescence intensity of boric acid-conjugated carbon dots was exploited for sensing glucose. Upon glucose addition the luminescence intensity increases due to the formation of complexes by boric acid-conjugated carbon dots with glucose. The optical properties of the carbon dots were characterised using UV-Visible absorption spectroscopy and photoluminescence spectroscopy. pH of synthesised carbon dots were determined using a digital pH meter. Morphological characterisations were done using high resolution transmission electron microscopy and dynamic light scattering. Antibacterial properties of carbon dots were also studied by agar well diffusion method. From the UV-Visible spectrum, an absorption peak was observed at 400nm which originates from n-π* transition of C=O bond. From photoluminescence spectra, highest PL intensity was observed at an excitation wavelength of 240 nm. pH of carbon dot solution was found to 5.94 and pH increased to 6.18 when few drops of NaOH solution was added. Zeta potential of bare carbon dots and boric acid-conjugated carbon dots was found to be 0.3 and 0.7eV respectively which indicates positive surface charge of carbon dots. Hydrodynamic diameter of carbon dots and boric acid-conjugated carbon dots was around 100nm. From high resolution transmission electron microscopy, particle size was found to be 10nm.From the antibacterial studies, it was clear that carbon dots exhibit significant antibacterial activity against gram-positive bacteria Staphylococcus aureus. The low cost, eco-friendly synthesis method and favourable optical properties of the synthesised carbon dots make them promising candidates for various optical and biomedical applications.