Construction Of Biological Mass Quantum Dot Fluorescent Probe And Its Application In The Detection Of Water Pollutants

Carbon quantum dots（CQDs）have the advantages of high fluorescence intensity,strong photostability,low toxicity,good water solubility and biocompatibility.It has been widely used in ion detection,fluorescence sensing,food analysis,biological imaging,disease diagnosis,photoelectric catalysis and other fields.The fluorescence properties and structures of CQDs synthesized by different carbon sources and preparation methods are different.At present,the search for inexpensive and green carbon sources and simple and easy preparation methods to improve the performance of CQDs,such as fluorescence quantum yield,is still a key scientific issue that restricts the application of CQDs in the detection of environmental pollutants.Natural biomass is rich in C,N,S,P and other elements,and is cheap and easy to obtain,green and environmentally friendly,and is often used as a carbon source for the preparation of CQDs,which also broadens its economic and application value.Therefore,in this paper,carbon quantum dots（CQDs）and nitrogen-doped carbon quantum dots（N-CQDs）with excellent performance were synthesized by hydrothermal method using natural loquat pulp and Aloe vera leaves as carbon source and urea as nitrogen source.Their optimal preparation conditions were investigated and used as fluorescent nanoprobes for the detection of water pollutants.The specific research contents are as follows.（1）Using loquat pulp as carbon source,carbon quantum dots（CQDs）with strong blue fluorescence were synthesized by hydrothermal method.The CQDs prepared under the optimal synthesis conditions are spherical,with an average particle size of4.46 nm,a quantum yield of 10.36%,and the surface is rich in hydroxyl（-OH）and carboxyl（-COOH）groups.After adding MnO₄^-,the fluorescence of loquat-based CQDs was quenched,indicating that it is feasible to construct sensitive"ON-OFF"fluorescent nanoprobes.The quenching mechanism of loquat-based CQDs by MnO₄^-was attributed to the combined effect of inner filter effect and static quenching by analyzing the changes of ultraviolet-visible light absorption（UV-Vis）,fluorescence lifetime,Stern-Volmer constant and zeta potential.Under the optimal detection conditions,the fluorescence quenching efficiency of CQDs showed a good linear relationship with the concentration of MnO₄^-in the range of 0.2-150μM,and the limit of detection（LOD）was 0.06μM.The fluorescence system was applied to the detection of MnO₄^-in actual water samples,the recovery rate of standard addition was between 96.0%-106.0%,and the relative standard deviation（RSD）was 1.5%-2.7%.The"on-off"fluorescent probe was successfully applied to the detection of MnO₄^-in actual water samples.（2）Using aloe leaf as biomass carbon source and urea as nitrogen source,nitrogen-doped carbon quantum dots（N-CQDs）were synthesized by hydrothermal method.The effects of synthesis conditions such as the type of biomass carbon source,the amount of biomass carbon source and the amount of nitrogen source added on the fluorescence intensity of N-CQDs were investigated.The N-CQDs prepared under the optimal synthesis conditions are spherical,dispersed uniformly,without obvious agglomeration,with an average particle size of 5.64 nm,lattice spacing of 0.23 nm,and a graphitic carbon-like structure.The fluorescence quantum yield was improved to 21.4%compared to undoped CQDs.The surface of N-CQDs is rich in amino（-NH）,hydroxyl（-OH）,carbonyl（-C=O）and other groups,and has good water solubility.The N-CQDs and glutathione-terminated copper nanoclusters（GSH-CuNCs）constitute a ratiometric fluorescent probe,which exhibits two emission peaks at 440 nm and 660 nm at an excitation wavelength of 350 nm.After the addition of nitenpyram,the fluorescence intensity（FIs）at the emission wavelength of440 nm decreased,and the FIs at the emission wavelength of 660 nm was almost constant.Under the optimal detection conditions,the fluorescence intensity ratio of the two(F₆₆₀/F₄₄₀)and the concentration of nitenpyram had a good linear relationship between 5 and 200μM,and the limit of detection（LOD）was 1.67μM.The spiked recoveries of the ratiometric fluorescent probe for nitenpyram in actual water samples ranged from 95.0%to 107.0%,and its relative standard deviation（RSD）was 1.4%to 2.5%.The N-CQDs/CuNCs ratio probe established by this method can be well applied to the detection of nitenpyram in environmental water.