Research And Design Of Raman-enhanced Substrate For Water Nitride Detection

Nitrate nitrogen is an important component of Total Nitrogen and one of the most important indicators of the pollution and eutrophication of water bodies,so the detection of nitrate nitrogen is of great importance.Spectrophotometric and ion chromatographic methods are the traditional methods for the determination of nitrate nitrogen,but these methods have the disadvantages of high detection cost and long detection period,so there is an urgent need to study a rapid and accurate method for the determination of nitrate nitrogen.Raman spectroscopy is an analytical method that uses Raman spectroscopy to study the rotation and vibration of molecules,and is suitable for water quality testing because the Raman signal of water is extremely weak.Surface-enhanced Raman Spectroscopy（SERS）method is a detection method that the Raman signal is greatly enhanced when the sample molecule to be tested is in the range of several nanometers on the surface of rough noble metals,and the affinity of nitrate for noble metals is low,so the sensitivity of the detection of nitrate is low,and the detection limit in similar studies cannot reach the national standard at present.In order to improve the sensitivity of nitrate detection,the thesis uses positively charged cysteamine modified gold nanoparticles to improve the affinity of gold particles with nitrate,reduce the detection limit and improve the detection sensitivity to meet the national environmental quality testing standards for groundwater.The thesis focuses on the simulation design,parameter optimization,fabrication process,performance testing and application experiments of gold nanoparticles（AuNPs）-cysteamine composite SERS substrates,as follows:1.The AuNPs-cysteamine composite SERS substrate structure was designed and optimized by simulation.Peripheral and embedded AuNPs-cysteamine composite SERS substrate structures were designed respectively,and the effects of parameters such as gold nanoparticle shape,gold nanoparticle particle size,cysteamine geometry,and the number of cysteamine and gold nanoparticle composite arrays on AuNPs-cysteamine composite SERS substrates were simulated and analyzed using COMSOL Multiphysics software.It was shown that for the peripheral AuNPs-cysteamine composite SERS substrate,the maximum local field enhancement of 51.2 V/m was obtained for the circular Au nanoparticle size of 50 nm,a cysteamine width of 100 nm,a height of 500nm and a composite array of gold nanoparticles and cysteamine of one;For the embedded AuNPs-cysteamine composite SERS substrate,the maximum local field enhancement of 58.4 V/m was obtained for a round gold nanoparticle size of 50 nm,a cysteamine width of 300 nm,a height of 100 nm,and a composite array of gold nanoparticles and cysteamine of one.2.The preparation process of AuNPs-cysteamine composite SERS substrates was studied and characterized.Firstly,the gold sol was prepared by chemical reduction method,and the characterization results showed that the gold sol with a particle size of about 50 nm could be prepared by a redox reaction of 20 minutes with a volume ratio of500:3 between chlorogold acid and trisodium citrate,meanwhile the mass concentration ratio was 100:1.secondly,the cysteamine was modified on the gold nanoparticles by self-assembly method.The AuNPs-cysteamine composite SERS substrates made of different concentrations of cysteamine,different distribution densities of gold nanoparticles and parameters such as cysteamine soaked in water and not soaked in water were characterized,and the characterization results showed that the AuNPs-cysteamine composite SERS substrates were prepared by soaking cysteamine in deionized water at a concentration of 10^-3mol/L and modifying 20 ml of gold sols,with a relatively uniform distribution of cysteamine and a moderate distribution of gold nanoparticles,and the cysteamine and gold nanoparticles were tightly and uniformly bound.3.Performance testing and analysis of AuNPs-cysteamine composite SERS substrates.Firstly,the effects of different parameters such as trisodium citrate content and reaction time on the performance of gold sol,and the experimental test and analysis of the gold sol detection limit were conducted.The experimental results showed that the best performance of gold sol was produced when 0.6 ml of trisodium citrate was heated and reacted with chloroauric acid for 20 min,and the detection limit of rhodamine 6G was 10^-8mol/L.Secondly,the effects of cysteamine concentration,content of gold sol,soaking time of cysteamine,soaking time of deionized water and soaking time of gold sol on the output Raman signal of nitrate anion were investigated,and the key performance indexes such as detection limit,enhancement factor and repeatability of AuNPs-cysteamine composite SERS substrate were tested and analyzed,and the experimental results showed that the first step of soaking the coverslip in the concentration of 10^-3mol/L.In the second step,the coverslips were immersed in deionized water for 3 hours and dried;in the third step,the AuNPs-cysteamine composite SERS substrate was immersed in 20 ml of gold sol for 4 hours and dried,and the enhancement factor was calculated to be 1.39×10⁵.The RSD was 10.36%,and the detection limit of standard nitrate solution was 0.01 mg/L,which was about 3 orders of magnitude higher than that of similar studies,and reached the detection standard of Class I water（≤0.2 mg/L）in the national groundwater environmental quality standard.4.Based on the AuNPs-cysteamine composite SERS substrate,the actual water sample testing study was carried out.Three types of water samples,Pan Long Xi water,Jialing River water and Baiyun Reservoir water,were tested,and the nitrate content of the three types of water samples was predicted based on the relationship between the concentration of potassium nitrate standard solution and the intensity of Raman spectral peaks.The experimental results showed that the nitrate content of Pan Long Xi water sample was about 5 mg/L;the nitrate content of Jialing River water sample was about0.01 mg/L,while the nitrate content of Baiyun reservoir water sample was lower than0.01 mg/L,which was basically consistent with the comparison of the chemical method test results.