Study On The Fabrication And The Application Of High-performance Magnetic SERS Substrates

Surface-enhanced Raman Scattering?SERS?is a powerful fingerprint-based vibrational spectroscopy technique.Due to its integration of high sensitivity,informative characteristic spectra,and non-destructive data acquisition,SERS has been widely used in various fields such as biochemical assays,analytical chemistry,disease diagnosis and environment monitoring.All these applications are based on proper utilization of high-performed SERS substrates.Recently,Au-/Ag-coated magnetic microspheres have attracted much attention,due to the combined functions of SERS and magnetic properties from the two component materials.These magnetic SERS substrates can effectively concentrate the target analyte and conveniently recover them from the sample solution before immobilization on the Si wafer for in situ SERS detection.However,fabrication of high-performance magnetic SERS substrates is rather complicated and difficult.The absence of high quality SERS substrates seriously limits their wide application.Based on the actual needs,we proposed some facile and effective methods for fabrication of high-performance magnetic SERS substrates.The nanostructure design,morphology optimation,electromagnetic field enhancement simulation,SERS property characterization and practical applications of the fabricated magnetic SERS substrates were studied in this study.The main research contents are as follows:1.A universal synthesis route is reported for fabricating Ag or Au-coated magnetic core–shell microspheres with high performance by using polyethyleneimine?PEI?as an interlayer.Cationic PEI rapidly self-assembled on the Fe₃O₄ microspheres through sonication to form a thin multifunctional interlayer,which can adsorb 3-5nm Au seeds densely and uniformly and stabilize the entire structure.Ag or Au shell formation was completed within 2 min,and generated Ag or Au-coated magnetic microspheres were highly uniform in size and shape with nanoscale roughness.The as-obtained products possess good dispersity,strong magnetic responsiveness,and excellent SERS ability at the same time.The detailed nanostructures of the Ag or Au-coated magnetic microspheres were characterized by TEM,SEM,XRD and EDS spectrum.The SERS activities of as-prepared magnetic SERS substrates have been tested by using PATP as a probe molecule.2.In this chapter,we reported a novel approach for fabricating core–shell–satellite 3D magnetic microspheres,that easily form a porous PEI interlayer to accommodate molecules.High-sensitive detection of adsorbed pesticide thiram and non-adsorbed pesticide paraquat were achieved using the fabricated 3D magnetic SERS substrate.The obtained CSSM comprised three parts:a 500 nm Fe₃O₄@Ag core to provide enough magnetic response property and strong SERS activity,a 1.5 nm PEI interlayer to build a porous nanogap,and dense 50 nm Au@Ag satellites to create additional hot spots.Experiments and FDTD simulation demonstrated that the enhancement factor?EF?was about 2.03×10⁸ and 6.25×10⁶,respectively.Moreover,the micro-scale magnetic based CSSM could be separated from the sample solution quickly,which enriched the target molecules and shortened the detection time.Given these outstanding features,the CSSM is expected to be a versatile SERS substrate,which was verified by the detection of adsorbed pesticide thiram and non-adsorbed pesticide paraquat with a detection limit as low as 5×10^-12 M and 1×10^-10M,respectively.3.A sonochemically assisted seeding growth method,for the preparation of high-performance Fe₃O₄@SiO₂@Ag microflowers is reported for the first time.The highly branched Fe₃O₄@SiO₂@Ag microflower as a wholly new nanostructure possesses good dispersity,high magnetic responsiveness,and a highly reproducible structure.These advantages make the microflowers powerful SERS substrates for chemical and biological analyses.The size and morphology of the silver petal shell of these microflowers can be easily controlled by varying the experimental parameters.The highly branched petals of microflowers provide a large surface area to effectively capture analytes,and the sharp tips generate plasmonic hot spots.The obtained Fe₃O₄@SiO₂@Ag microflowers possess good dispersity,high magnetic responsiveness,and abundant hot spots.Experiments and finite-difference time-domain simulation demonstrated that the enhancement factor values are approximately 5.26×10⁸ and 2.56×10⁶,respectively.The SERS ability test results showed that the detection limits for R6G molecules and the adsorbed pesticide thiram of the optimized Fe₃O₄@SiO₂@Ag microflowers approached 1×10^-14 M and 1×10^-11 M,respectively.4.A rapid,sensitive,and label-free SERS detection method for bacteria pathogens is reported for the first time.This method,named the capture–enrichment–enhancement three-step method,combines the capture and enrichment ability of high-performance Fe₃O₄@Au@PEI microspheres and enhancement ability of magnetic and reinforced SERS particles,thereby achieving rapid bacterial detection within 10 minutes.The negatively charged bacteria were quickly captured and enriched by the positively charged Fe₃O₄@Au@PEI microspheres,and the bacteria SERS signal was synergistically enhanced by using Fe₃O₄@Au@PEI microspheres and Au@Ag NPs in conjunction.Under the optimized incubation time and particle concentration,the presented method can detect different bacteria effectively,as verified by its detection of Gram-positive bacterium E.coli and Gram-positive bacterium S.aureus at a detection limit of as low as 10³ cells per mL.Moreover,the spiked tests show that this method is still valid in tap water and milk.5.In this chapter,we presented a type of efficient SERS biosensor based on the combination of vancomycin-modifed Fe₃O₄@Ag MNPs and Au@Ag NPs for rapid enrichment,sensitive detection,and accurate differentiation of bacteria in the complex solution.High-performance Fe₃O₄@Ag MNPs were functionalized with vancomycin to effectively capture and enrich bacteria in solution samples.In addition,the plasmonic Au@Ag NPs were used as the secondary enhanced particles to improve the detection sensitivity.Given these outstanding features,the combined system is expected to be a sensitive SERS biosensor in detecting a wide range of pathogenic bacteria,as verifed by its detection of the Gram-positive bacterium E.coli,Gram-positive bacterium S.aureus,and drug-resistant bacteria MRSA at a detection limit as low as 5×10²cells/mL.Moreover,the strong and reproducible SERS spectra of bacteria allow accurate differentiation of different bacteria using PCA method.