Research progress of synthesis of carbon dots and fluorescent sensor based on carbon dots for mercury detection

Abstract: Mercury ion (Hg2+) is one of the most frequently occurring and most toxic heavy metal pollutants, which may seriously damage the growth environment of crops and further threaten the quality and safety of agricultural products and human health. Fluorescence analysis is a simple method of Hg2+ detection with high sensitivity. The analysis performance of fluorescence sensor may be seriously affected by fluorescence probes. Carbon dots (CDs) have the advantages of simple preparation, high fluorescence efficiency, and good biocompatibility. Therefore, fluorescence sensors based on CDs have attracted wide attention. This study introduced the synthesis status of CDs prepared from biomasses such as crops, agricultural products and their wastes, and pointed out that the synthesis of CDs was rich in natural biomass materials, and CDs synthesized by ultrasonic, microwave, and hydrothermal methods have the advantages of low cost, simple preparation process, and environmental protection. In addition, the application research of fluorescence sensor based on CDs for the detection of antibiotics, pesticides, and food additives in the field of agricultural sensing was briefly introduced. Furthermore, the research progress of fluorescence sensor based on CDs in Hg2+ detection was systematically introduced from the perspectives of fluorescence quenching single-signal analysis mode, fluorescence-enhanced single-signal analysis mode and ratio-based signal analysis mode. The fluorescent materials, analytical performance and advantages and disadvantages of each method used in different sensing modes were summarized and compared. Among the single-signal sensors, fluorescence quenching sensors could optimize the detection performance of CDs by doping, surface modification, or "on-off-on" strategies; the fluorescence intensity of fluorescence-enhanced sensors ranged from weak to strong, and fluorescence colors were created from scratch, which could reduce the occurrence of false positive signals, but few fluorescence-enhanced sensors constructed by CDs as probes may occur, which needs to be further developed; The CDs with different properties could be used as reference signals , response signals as well as dual emission matrix in ratio fluorescence sensors. Besides, ratio fluorescence sensor could establish an internal standard to weaken the interference of external factors, the analysis result was more reliable and accurate, and it was easier to realize sensitive visual analysis. Different models of fluorescence sensors were all widely used in the detection of Hg2+ in lake water, soil, and agricultural products. The bottleneck and development trend of the fluorescence sensor based on CDs using biomass as the synthesis source in the field of agricultural sensing were analyzed: 1) the luminescence mechanism of CDs remained unclear due to the complex composition of precursors in the preparation of CDs from crops and agricultural products. Moreover, the fluorescence quantum yield of synthesized CDs was uneven, and it needs to be separated, purified, doped or modified in a targeted manner; 2) most of the CDs based fluorescent sensors could detect only one single target, while the CDs based fluorescent sensors detecting multiple targets at the same time were little reported, and most of the sensing systems were still in the experimental stage, so it was difficult to perform the on-site detection of agricultural environmental quality and crop quality.