Solvothermal production of tea residue derived carbon dots by the pretreatment of choline chloride/urea and its application for cadmium detection

Abstract

Cadmium contamination is a serious environmental issue and its detection is important. In this study, tea residue after pretreatment by the deep eutectic solvent of choline chloride/urea was used for one-pot solvothermal synthesis of carbon dots (CDs) applied in cadmium detection for the first time. The effect of choline chloride/urea molar ratio, reaction period, and reaction temperature on the CDs synthesis was learned and a promising synthesis system (choline chloride/urea molar ratio of 1:2, reaction at 200 °C for 8 h) was built. Under this reaction condition, the CDs yield and the quantum yield were 2.25% and 16.99%, respectively. The optical properties and structure of CDs were learned by fluorescence spectrophotometer, UV–vis spectrophotometer, transmission electron microscope (TEM), field emission scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD), and X-ray photoelectron spectrometer (XPS). According to the characterization, the CDs had a strong peak at 410 nm with the corresponding excitation wavelength of 330 nm and contained various functional groups including hydroxyl, amino, and carbonyl. The CDs were suitable for cadmium detection by a linear standard curve with a high correlation coefficient (R2 =0.99). Overall, this study offers a promising method for utilization of waste biomass (tea residue) for fluorescent materials (CDs) production.