Low Cu(II) Concentration Detection Based on Fluorescent Detector Made from Citric Acid and Urea

Carbon quantum dots (CQD) are an advanced fluorescent material, which has attracted more and more attention in theoretical research and practical applications. To obtain stable CQDs with high fluorescence characteristics for detecting trace metal ions in water, nitrogen-doped carbon quantum dots (N-CQDs) based fluorescent sensors were synthesized by the hydrothermal method, using citric acid and urea as source. Transmission electron microscopy (TEM) images showed that the synthesized N-CQDs maintained a narrow particle size distribution bellow 10 nm, and its average size was 3.07 nm. Fourier transform infrared spectroscopy (FT-IR) indicated that abundant hydroxyl and carboxyl functional groups existed on N-CQDs surface, which helped N-CQDs highly disperse in water. In addition, UV-vis spectroscopy and photoluminescence demonstrated that the N-CQDs obtained a 10.27% of quantum yield (QY) with relatively high and stable fluorescence performance. As a fluorescent sensor, the N-CQDs showed a fluorescence "ON-OFF" mechanism during the Cu²⁺ detection, which was induced from the electrons transition in surface functional groups. The final N-CQDs exhibited a wide linear relationship between fluorescence response and concentration of Cu²⁺ in range of 0.3-0.7 μM with a detection limit of 0.071 μM. Furthermore, the detection of Cu²⁺ in the simulating surface water (by adding interfering metal ions in purified water) and the tap water (from municipal water in Beijing) were used to verify N-CQDs practical application.