Preparation Of Alanine Diacetic Acid Carbon Quantum Dots And Its Application In Ion Detection

Currently,the problem of massive emissions and exceedance of various ions in the environment seriously threatens the normal life of human beings and the security of the ecosystem,therefore,the detection and analysis methods for various ions have received wide attention.Traditional mass spectrometry,spectroscopy and electrochemical analysis methods require complex pretreatment schemes and sophisticated instrumentation requirements,and the testing cost is high,which limits the development of ion detection in the conventional environment.Carbon Quantum Dots（CQDs）,as an emerging fluorescent carbon nanomaterial with excellent photoluminescence properties and selective response to a certain ion,have received wide attention as they can be used for rapid quantitative ion detection.In this paper,two fluorescent nitrogen-doped CQDs were prepared from the environmentally friendly chelator Trisodium Dicarboxymethyl Alaninate（also known as Methyl Glycine Diacetic acid Trisodium Salt,MGDA）with two common nitrogen-containing materials using a one-step hydrothermal technique and applied to the quantitative detection of ClO^-,Cr（Ⅵ）and Hg²⁺.The details are as follows:（1）The blue fluorescent nitrogen-doped carbon quantum dots（N1-CQDs）were synthesized by hydrothermal method using MGDA and ethylenediamine as raw materials.The fluorescence quantum yield of Ni-CQDs was calculated to be 22.14%,the optimal excitation wavelength was 350 nm,and the optimal emission wavelength was 410 nm.The transmission electron microscopy（TEM）results show that Ni-CQDs are spherical in shape and can be uniformly dispersed in aqueous solution with an average size of 5.39 nm,and Fourier Transform infrared spectroscopy（FT-IR）and X-ray photoelectron spectroscopy（XPS）characterization shows that the surface of Ni-CQDs is rich in oxygen-containing functional groups and nitrogencontaining functional groups,and nitrogen elements are successfully doped into the structure of carbon quantum dots.These hydrophilic groups make N1-CQDs have good water solubility.The fluorescence intensity of N1-CQDs can be quenched quantitatively by ClO^-and Cr（Ⅵ）ions,so the quantitative detection system of two kinds of ions is established.The linear ranges were 0-300 μmol/L and 0-100 μmol/L for ClO^-and Cr（Ⅵ）ions,respectively,and the detection limits were 5.0 μmol/L and 2.1 μmol/L.The Ni-CQDs was applied to the recovery and detection of ClO^-and Cr（VI）ions in actual water samples,and the recovery rate was between 96%and 107%.The fish acute toxicity test showed that N1-CQDs did not kill fish within 96 h when the dosage was 200 mg/L,and had little harm to the environment.（2）The carbon and nitrogen sources used were MGDA and m-phenylenediamine,followed by the synthesis of nitrogen-doped carbon quantum dots（N₂-CQDs）with green fluorescence by a hydrothermal method.The fluorescence quantum yield of N₂-CQDs is 63.8%.The optimal excitation wavelength was 440 nm and the optimal emission wavelength was 510 nm.In addition,the characterization results show that N₂-CQDs have spherical structure and good water dispersion.The size of the N₂-CQDs ranged from 1 nm to 3 nm,with an average particle size of 1.84 nm.The surface of N₂-CQDs contains abundant nitrogen-containing groups,indicating that nitrogen has been successfully doped into carbon quantum dots.The N₂-CQDs have a high selectivity for mercury ions,and can be used to quantitatively detect Hg2+ ions in aqueous solution.The detection range of this method was between 0 and 100 μmol/L with a detection limit of 0.9 μmol/L.The recoveries of the actual water samples obtained from the experiments were 99%-104%.The acute toxicity test of fish showed that when the dosage of N₂-CQDs was 200 mg/L,the fish would not die within 96 h,which is less harmful to the environment and is a simple,safe and economical fluorescence detection method.