Preparation Of Doped Carbon Dots And Its Experimental Study On Fe³⁺ And Hg²⁺ Detection

With the rapid development of industry,more and more heavy metal pollution caused by human activities is a serious threat to human health.Therefore,the development of a rapid,sensitive and effective detection method is of great significance to prevent such threats.Carbon quantum dot fluorescence probe method can realize the rapid detection of heavy metal ions,and has the advantages of excellent water solubility,chemical stability,low toxicity and photobleaching resistance,so it is a hot research field in recent years.In this paper,N and S doped carbon quantum dots were prepared,the morphology and structural characteristics of the synthesized carbon dots were characterized,their fluorescence properties and the ability to detect metal ions were analyzed,the fluorescence quenching mechanism was explored,and a portable paper sensor was prepared.The main research contents are as follows:（1）N and S co-doped carbon quantum dots were synthesized by one-step hydrothermal method using citric acid and thiourea as precursors.The morphology and fluorescence properties of the synthesized carbon spots were analyzed by transmission electron microscope,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,fluorescence spectrophotometer and ultraviolet-visible absorption spectroscopy.The results show that the average particle size of N-S-CDs is 8nm and its distribution is uniform.The surface is rich in various functional groups and has good water solubility.Blue fluorescence was emitted under the irradiation of 365nm lamp.With the increase of excitation wavelength,the emission fluorescence intensity of N-S-CDs increased at first and then decreased,and the fluorescence intensity reached the highest at 440nm,and the emission peak was obviously red-shifted with the excitation wavelength,and the fluorescence quantum yield was as high as 36.8%.N-S-CDs has high selectivity for Fe³⁺and Hg²⁺without interference from other metal ions,and the linear relationship between Fe³⁺concentration and N-S-CDs fluorescence intensity in the range of 40-130μM is F₀/F=0.02132[Fe³⁺]+0.34945,and the lowest detection limit is 1.4μM.The linear relationship between the concentration of Hg²⁺and the fluorescence intensity of N-S-CDs in the range of 0-80μM is as follows:F₀/F=0.18611[Hg²⁺]-6.27114,and the lowest detection limit is 0.16μM.（2）A yellow fluorescent carbon quantum dot（Y-CDs）was synthesized by carbonization of o-phenylenediamine and p-aminobenzoic acid with hydrogen peroxide.Through structural characterization and fluorescence performance analysis,the results show that the distribution of Y-CDs is uniform,there is no obvious agglomeration,has the lattice structure of graphite,and the surface is rich in various functional groups and has good water solubility.Y-CDs has high selectivity for Hg²⁺.The linear relationship between Hg²⁺concentration and fluorescence intensity in the range of 20μM to 150μM is F₀/F=0.02316[Hg²⁺]+0.52638,and the lowest detection limit is 0.12μM.Y-CDs@ZIF-8 composites were synthesized by combining Y-CDs with ZIF-8,and the fluorescence quenching mechanism of Hg2+on Y-CDs was analyzed.The results showed that the fluorescence of Y-CDs@ZIF-8 composites could not be quenched by Hg²⁺,and the fluorescence lifetime of Y-CDs did not change after adding Hg²⁺,but the UV-vis absorption spectrum changed obviously.It is confirmed that the mechanism of fluorescence quenching is static quenching.（3）T N-S-CDs and Y-CDs were applied to the standard addition and recovery detection of metal ions in two real water samples.Results the fluorescence intensity of the two kinds of carbon dots remained stable within one hour.The actual amount of metal ions added in the standard recovery experiment is very close to the calculated results.The recovery rate of N-S-CDs for Fe³⁺in the two water samples is between98.28%and 103.62%,and the recovery rate for Hg²⁺is between 93.58%and 101.22%.The recovery rate of Hg²⁺by Y-CDs in two kinds of water samples is between 90.11%and 115.52%.Blue and yellow fluorescent portable paper sensors were made and applied to the detection of Hg²⁺.The results show that there are obvious changes in the fluorescence of the two sensors,and both of them have the possibility of visual detection of mercury ions with naked eyes,and the effect of Hg²⁺detection with yellow fluorescent strip is better than that with blue fluorescent strip.