Carbon Dots from Pilosocereus gounellei for Fluorimetric Determination of Tannin in Tea Using a Flow-Batch System

In this study, xiquexique (Pilosocereus gounellei) native endemic cactus from the Brazilian semiarid was used for the first time as a natural carbon source (“green precursor”) for the synthesis of highly fluorescent carbon dots (CDs). These CDs were successfully used to develop a fast, low-cost, eco-friendly fluorescence method for the determination of tannins in teas. This method was automatized employing a flow-batch system coupled to an inexpensive ultraviolet light-emitting diode (UV-LED) used as an excitation source and a cheap handheld spectrometer used as a detector. CDs were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV-Vis, and fluorescence spectroscopy. The proposed method presented a linear range from 2.0-30.0 mg L−1, relative standard deviation (RSD) < 3.0%, limit of detection (LOD) = 0.102 mg L–1, and recovery rates from 90.5-109.1%. A paired t-test at a 95% confidence level indicated no statistically significant difference between the proposed and reference methods, revealing that it is a useful alternative for the determination of tannins.