Surface-Enhanced Raman Scattering For Quantitative Detection

The noble metal(Au-Ag)nanomaterials have excellent enhancement performances and irreplaceable importances in the field of surface enhanced Raman scattering(SERS).The morphology,size,structure,element composition and arrangement of Au-Ag nanomaterials will have significant impacts on their SERS performance.Therefore,the design and preparation of SERS substrates based on Au-Ag nanomaterials are of great significance for realizing their quantitative detectionBecause of its highest plasma efficiency and electromagnetic field enhancement performance,Ag is currently the best known SERS substrate materials.As the sizes of Ag nanoparticles(Ag NPs)increase,their scattering efficiency factors will gradually increase,and their performance as SERS substrates will be significantly improved However,the current methods for preparing large-size quasi-spherical Ag NPs have disadvantages such as cumbersome steps,limited size range,poor quality(ellipticity of morphology,deviation of size),and low result repetition rate,which greatly limit their applications for quantitative detection as SERS substrates.Therefore,it is of great significance to achieve the controllable preparation of quasi-spherical Ag NPs with a wide range of size,good quality and high result repetition rate through a simple method for realizing their SERS quantitative detection.At present,the SERS performance of monodisperse quasi-spherical noble metal(Au,Ag)nanoparticles cannot meet the test requirements.Generally,they are prepared as ordered substrates.They can not only improve the performance as SERS substrates,but also achieve uniform and repeatable detection signals,which are the bases for realizing SERS quantitative detection.Generally,the small size of Au NPs synthesized in the oil phase and the high density of organic ligands on the surface of Au NPs limit their subsequent SERS applications.However,due to the influence of surface electrostatic repulsion of Au NPs with weak ligands synthesized in the water phase,it is difficult to obtain macroscopically monolayer films(MMF-OA-Au NPs)of ordered arrays of Au NPs.Therefore,the preparation of water-soluble Au NPs into high-quality MMF-OA-Au NPs is expected to realize the quantitative detection of trace analytes in aqueous solutions.In addition,the SERS quantitative detection of ultra-low concentration analytes is of great significance in the fields of environmental monitoring and food safety.However,the limit of detection of MMF-OA-Au NPs as SERS substrates can only reach 10-10?10-11 M.In order to improve the detection sensitivity of the films of Au-based nanomaterials as SERS substrates,a certain thickness of Ag layer can be grown on the surface of Au NPs to further improve their detection sensitivity,so as to achieve highly sensitive SERS quantitative detection of ultra-low concentration analytes in aqueous solutions.Therefore,the controllable synthesis of core-shell Au@Ag NPs with different thicknesses of Ag layer and the preparation of their macroscopically monolayer films of ordered arrays(MMF-OA-Au@Ag NPs)are expected to realize the SERS quantitative detection of the analytes in aqueous solutions.Based on the above problems,this paper takes the quantitative detection of SERS substrates in aqueous solutions as the research goal.Firstly,quasi-spherical Ag NPs with the size range of 31?300 nm are successfully prepared by the advanced one-step seeded growth method.On the basis of the controllable preparation of large-sized Ag NPs,their performance as flexible SERS substrates in aqueous solutions is explored.Next,using the quasi-spherical Au NPs obtained into toluene phase containing oleylamine by phase transfer as the structural units,MMF-OA-Au NPs are prepared by the improved interfacial self-assembly method.And then their performance as SERS substrates for in-situ quantitative detection of the analytes in aqueous solutions is explored.Finally,the corresponding MMF-OA-Au@Ag NPs are also prepared using core-shell Au@Ag NPs as the structural units,and applied to the highly sensitive SERS quantitative detection of the analytes in aqueous solutions.The main contents of this paper are as follows(Chapter 1 is the introduction):In Chapter 2,firstly,the influences of sodium citrate in the seeds,the concentration of AA and ammonia in the growth solution and pH value during the growth of large-sized Ag NPs are investigated by using 23 nm Ag NPs as seeds and AA as a reducing agent for silver-ammonia complex.Through the advanced one-step seeded growth method,the controllable preparation of quasi-spherical Ag NPs with a size range of 31?300 nm is achieved,and their ellipticities of morphology and deviations of size are both less than 10%.Then,using the as-prepared large-sized quasi-spherical Ag NPs as SERS substrates in aqueous solutions,the SERS performances of Ag NPs with different sizes for the analytes in aqueous solutions(taking rhodamine B and crystal violet as examples)are explored.Under the excitation wavelength of 633 nm laser,the quasi-spherical Ag NPs with an average size of 125 nm show the best SERS performance.As SERS substrates in aqueous solutions,the relative standard deviations of the intensity of their detection signals on the space and time scales are less than 5%,and it can achieve good spectral uniformity and reproducibility.In Chapter 3,firstly,based on the quasi-spherical Au NPs obtained into toluene phase containing oleylamine by phase transfer as the structural units,diethylene glycol(DEG)containing 10%water is used as the water phase to realize the self-assembly at the oil-water interface for preparation of the high-quality MMF-OA-Au NPs.Then,MMF-OA-Au NPs are transferred on PDMS as SERS substrates(MMF-OA-Au NP@PDMS),and the SERS performance for in-situ detection of the analytes(taking crystal violet and malachite green as examples)in aqueous solutions are explored.Their SERS signals have good reproducibility and high sensitivity,which can achieve linear quantitative detection of trace analyte concentrations in aqueous solutions.Finally,MMF-OA-Au NP@PDMS functionalized with 4-mercaptobenzoic acid(4-MBA)molecules are used as SERS-based pH sensors to explore the performance.The pH value of the aqueous solution in the range of 3 to 10 can be directly determined by the well-defined linear relationship with the intensity of the peak of vCOO-without any calibration,instead of the intensity ratio of the Raman peaks of vCOO-to v8a with further calculation.In addition,the as-prepared pH sensors still have excellent stability after long-term storage.In Chapter 4,firstly,quasi-spherical core-shell Au@Ag NPs with different thicknesses of Ag layer are obtained by using 5.5 nm Au NPs as seeds and AA as a reducing agent for silver-ammonia complex.Then,through phase transfer technology and improved self-assembly method,a series of MMF-OA-Au@Ag NPs are prepared.Then,the performances of different films as SERS substrates are systematically compared.All of the SERS substrates exhibit spectral uniformity and reproducibility in SERS detection.Among them,MMF-OA-Au@Ag NPs based on 16 nm core-shell Au@Ag NPs with Ag thickness of 5.25 nm exhibit the best SERS performance.Finally,their quantitative manner and limit of detection for ultra-low concentration analytes in aqueous solutions are explored.It can realize the linear quantitative detection of crystal violet(10-12?10-7 M)and 4-aminothiophenol(10-13?10-6 M)in aqueous solutions.By extending the adsorption time,the reproducible limit of detection of crystal violet in aqueous solutions can reach as low as 10-13 M.In Chapter 5,we have summarized and forecasted the above works.In summary,in this paper,the controllable synthesis and assembly of the noble metal Au-Ag NPs(Au NPs,Ag NPs and core-shell Au@Ag NPs)are realized.The corresponding SERS substrates are further prepared,which can expand the application of the Au-Ag substrates in the field of SERS quantitative detection.