Properties and molecular structure of carbon quantum dots derived from empty fruit bunch biochar using a facile microwave-assisted method for the detection of Cu2+ ions

Abstract Carbon quantum dots (CQD) have received considerable attention due to its advantageous characteristics of highly fluorescent nature and good biocompatibility. In this study, CQD were successfully prepared from empty fruit bunch (EFB) biochar, a renewable and sustainable resource using a facile microwave-assisted method in just minutes. The structures, morphology, and functional groups of CQD were analysed by HRTEM, FTIR and XPS analysis. CQD displayed bright blue emission upon exposure to UV irradiation. The maximum fluorescence emission of CQD was detected at 420 nm throughout the excitation wavelength 280 nm. To obtain optimum molecular structure of CQD, density functional theory (DFT) calculations were carried out using 6-31G(d) basis set which also includes a comparison of three exchange correlation functions such as B3LYP, CAM-B3LYP and wB97XD. These functionals were used to calculate the ground state molecular structure of CQD. The HOMO-LUMO gap obtained from 6 to 31G(d)/B3LYP was around 6.4 eV and is closer to the experimental band gap of 4.4 eV compared to CAM-B3LYP and wB97XD. The fluorescence quenching effect was observed when Cu2+ ions were added into CQD solution. The experiment results showed that the as-prepared CQD have a sensitive response to Cu2+ ions in the concentration range of 0–400 μM. This work not only provides insight into the properties and molecular structure of CQD obtained via a facile and scalable method, but also shows potential of a fluorescence sensor for metal ions detection.