Real-time Sers Detection Research On 4-nitrothiophenol Reduction And O-phenylenediamine Oxidation

Raman scattering occurs during the inelastic collision of photons and molecules.In this process,the energy of scattered photons will change,that is,photons will gain energy or lose energy.The energy change of scattered photons inevitably causes its frequency to change,and the frequency change of scattered photons contains the characteristic vibration information of the molecules,so the Raman spectrum corresponds to the characteristic spectrum of the molecules,which is the "fingerprint spectrum".Under good conditions,the scattering cross-section of conventional Raman scattering is between 10-30-10-25 cm2,this signal is too weak in most cases.So Raman scattering is a relatively weak scattering effect.Under the circumstances,the practical application requirements of Raman spectroscopy cannot be satisfied.In order to increase the Raman signal,researchers have used a variety of methods,such as Electron Resonance Enhancement(ERE),Stimulated Raman Process(SRP)and Surface Enhanced Raman Scattering(SERS).Among these methods,the more effective method to enhance Raman signal is SERS.The basic principle of the SERS technique is that due to the coupling of the surface plasmons,the spectral signals in the nanostructures on the surface of the metal will undergo large changes,and the Raman scattering signals of the analyte molecules near the nanostructures will be greatly enhanced.The high-enhanced SERS technique can obtain single-molecule level Raman spectra.SERS technology is an excellent vibrational spectroscopy technique that can quickly obtain molecular structure information.In addition,SERS technology has the advantages of high sensitivity,simple testing process and simple sample requirements.Based on these advantages,SERS is widely used in the fields of surface science,medicine,and sensing.SERS technology exhibits a highly localized chemical sensitivity,making it an ideal technology for studying chemical reactions,especially catalytic reactions.Using SERS technology,the structure of the catalyst,the adsorbate,and the intermediate products of the reaction process can all be real-time observed,so the chemical reaction process and reaction mechanism can be well studied.In addition,under the right conditions,SERS technology can also be used to trigger the chemical reaction.In this work,the real-time monitoring of the reduction and ’oxidation processes was studied,and we constructed an ultra-sensitive and catalytically active bifunctional metal SERS substrate:Au/Ag nanoparticle-decorated silicon nanowire array(Au/Ag@SiNWAs).In this paper,the morphological structure,SERS performance and reusability of the substrate were tested and characterized,and the finite difference time domain(FDTD)simulation of the substrate were performed.In the real-time online monitoring of chemical reactions,the 4-Nitrothiophenol(4-NTP)reduction and O-Phenylenediamine(OPD)oxidation were selected as the research object.Au/Ag@SiNWAs substrate was selected to study the 4-NTP reduction process,and gold sol was used to study the oxidation process of OPD.There are 4 parts of this paper:First,synthesis of Au/Ag@SiNWAs substrate via a chemical reduction method.The preparation process was divided into two steps:in the first step,silicon nanowires were prepared;in the second step,gold nanoparticles and silver nanoparticles were modified on the silicon nanowires.Silicon nanowire was obtained on an N-type silicon wafer by a chemical etching method,and gold nanoparticles and silver nanoparticles were then loaded on the silicon nanowire by in-situ reduction.The morphology of the Au/Ag@SiNWAs substrate and the distribution of two kinds of nanoparticles were characterized by scanning electron microscopy、transmission electron microscopy and X-ray energy spectroscopy.The results showed that Au and Ag nanoparticles were successfully loaded on the silicon nano wires,and the particle sizes were 80 nm and 120 nm,respectively.This two types of nanoparticles were uniformly distributed on the silicon nanowires..Second,characterization of the SERS enhancement performance,recycling performance.Rhodamine 6G(R6G)was used as a signal molecule to configure SERS enhancement performance with different R6G concentrations.The detection limit of R6G solution was 10-16 M using Au/Ag@SiNWAs substrate,and a quantitative relationship curve of signal molecules was established.Crystal violet(CV),Congo red(CR),Methylene blue(MB)and 4-Aminothiophenol(4-ATP)were used to analyze the reuse of the substrate.The test results showed that the Au/Ag@SiNWAs substrate can be repeatedly tested for these dyes,and the result of each test did not interfere with each other.Third,SERS mechanism research.According to the Au/Ag@SiNWAs substrate’s morphology,the substrate was modeled and FDTD simulated.The simulation results of the surface local electric field intensity of the substrate showed that the maximum value of the electric field was 19.7 V/m,and the enhancement factor was～1.5×105,which was consistent with the experimentally measured result of～3.3×105.Fourth,real-time monitoring of chemical reactions based on SERS technology.Since Au/Ag@SiNWAs substrate has excellent SERS enhancement and catalytic properties,it can also be used to monitor 4-NTP reduction.During the 4-NTP reduction,the Au nanoparticles act as a catalyst,and the Au nanoparticles together with the Ag nanoparticles act to enhance theRaman signal,so that the weak Raman signal during the reaction can be detected.Finally,the entire reduction process of 4-NTP to 4-ATP was successfully monitored,and the intermediate products of the reaction process were also detected.In the real-time monitoring of OPD oxidation,gold sol was used as a SERS substrate.Because Cu2+ is catalyzed during the monitoring of OPD oxidation,a liquid SERS substrate is required to enhance the Raman signal.Since the excellent binding of OPD to gold particles,gold sol was selected for the experiment and OPD oxidation was also successfully monitored.The real-time SERS detection device constructed in this work can be used to monitor chemical reactions,it is of great significance for studying the reaction process and revealing the reaction mechanism.