Raman Enhanced Properties Of Metal Nanoparticles/Optical Fiber Composite Structures

Surface-enhanced Raman scattering has been widely used in the detection of ultra-sensitive substances since it was discovered in 1974.The local surface plasmon resonance generated by the metal nanoparticles and the excitation light can greatly enhance the Raman signal.This advantage makes it widely used in the fields of biomedicine,food safety,detection of explosives and harmful substances.On the one hand,compared with the traditional Raman test,the detection sensitivity of the probe molecule can be greatly improved by the SERS test with metal nanoparticles.To obtain good enhancement effect,it is necessary to optimize the process of the metal nanostructures.In particular,combining the structure of the metal nanoparticle and its corresponding excitation light wavelength in order to obtain a better enhancement effect.The type,size,and morphology of metal nanoparticles all have effects on the SERS enhancement and absorption peak positions.In order to get the relationship between SERS enhancement and absorption peaks,the study of metal nanoparticles with double resonance absorption peaks was carried out.By changing the process parameters during the preparation of gold nanoparticles,a double resonance absorption peak gold nanoparticle SERS substrate with strong resonance absorption peaks near 532 nm and785 nm was obtained.The substrate showed a good Raman enhancement effect on the analyte rhodamine 6G.On the other hand,another way to increase the intensity of Raman light is to use waveguide.In the thesis,the study of waveguide-coupled with metal nanoparticles was also carried out.The fiber SERS probes not only have the SERS enhancement caused by excitation light and metal nanoparticles,but also have the waveguide enhancement caused by the fiber waveguide.The combination of the two enhancement is expected to further improve the detection sensitivity of SERS detection.In this thesis,an Ag/fiber composite structure SERS probe was prepared to achieve double enhancement of Raman signal of analyte.Main works are as follows:(1)Theoretical numerical analysis:The optical characteristics of metal nanoparticles were analyzed by Mie theory,furthermore,local surface plasmon resonance peak and electromagnetic field simulation models of metal nanoparticles with different types and morphologies were established.Through FDTD Solutions,the influence of the main parameters,such as the diameter of gold nanoparticles,the length diameter ratio and distribution of gold nanorods,the diameter and distribution of silver nanoparticles,on the resonance absorption peak and electromagnetic enhancement performance.When the wavelength of the excitation light is 532 nm,the wavelength is closest to the resonance absorption peak of gold nanoparticles and silver nanoparticles.With the increase of the diameter and the decrease of the spacing of silver nanoparticles,the electromagnetic enhancement can be increased,and for gold nanoparticles.When the diameter is 50 nm,the enhancement effect is the largest.When the wavelength of the excitation light is 785nm,the wavelength is closest to the resonance absorption peak of the gold nanorods.When the gold nanorods with an aspect ratio of 4:1,the enhancement effect is the largest.(2)Preparation and SERS experiment of gold nanoparticles with double resonance absorption peaks:The gold nanoparticles with strong absorption near 532 nm and around785 nm were prepared by seed growth of experimentally change the three important parameters:Ag NO₃ content,HCL content,and growth time.The main components of gold nanoparticles are gold nanoparticles with diameters of about 50 nm and gold nanorods with aspect ratios of about 2.5?4.5.Using this sample as the substrate and rhodamine 6G as the probe molecule,the Raman signal of rhodamine 6G was tested under excitation lights of 532 nm,633 nm,and 785 nm,respectively.The experiments showed that the minimum detection concentration can reached at?10^-7 mol/L of the three excitation lights,and the enhancement factor is?10⁵.(3)Experiment and analysis of waveguide-SERS coupling:Ag-sol solutions with a diameter of about 50 nm were prepared by a chemical reduction method.To obtain Ag nanoparticles(Ag NPs)modified optical fiber SERS probes,a mixed solution obtained by mixing the Ag-sol solution and the rhodamine 6G solution at a certain volume ratio was injected into a double-hole microstructured hollow fiber.By comparing the SERS characterization results of pure R6G solution in a beaker,pure R6G solution in an optical fiber,mixed solution in a beaker,and mixed solution in an optical fiber,it was found that the the fiber SERS probe has both fiber waveguide enhancement and SERS enhancement,which greatly increases the SERS enhancement factor.The minimum detection limit of rhodamine 6G solution reached at?10^-15 mol/L.When the fiber length increases,the SERS enhancement factor gradually increases by studying the effect of fiber length on waveguide enhancement.Finally,the effect of Ag-sol solutions with different densities on the performance of SERS was found.When the volume ratio of Ag-sol solutions to rhodamine 6G solutions was 1:6,the Raman enhancement was the largest.The Raman peaks of the gas in the hollow fiber were detected,and the waveguide enhancement factors for O₂ and N₂were 6.57 and 4.91,respectively.