Preparation And Application Of Multifunctional Magnetic/Noble Metal Composite SERS Substrates

Surface-enhanced Raman spectroscopy(SERS)technology has the advantage of high sensitivity,simple pretreatment,non-invasive injury detection,fast analysis,low cost and in-situ detection.So it was widely used in chemical,environmental,biomedical,food health,biomedical,public safety and other fields.With the development of nanotechnology,magnetic nanomaterials have been applied to scientific research in tumor therapy,drug targeted delivery and cell separation due to their unique properties.When the classical noble metal nanomaterials with excellent SERS activity are combined with the magnetic nanomaterials to form multi-functional magnetic/noble metal composite nanomaterials,they have both the enrichment,targeting and separation properties of magnetic nanomaterials and excellent SERS,catalytic properties of noble metal nanomaterials,which better meets the application needs of various fields.Therefore,it is of great significance to control the preparation of a multifunctional magnetic/noble metal composite SERS substrate with high sensitivity,stability and reproducibility.In this paper,we use different methods to prepare multi-functional magnetic/noble metal composite nanomaterials,then study their SERS properties and their applications.The main contents of this paper are as follows:1.Uniform Ag-Fe3O4 composite nanoparticles have been obtained via a simple one step hydrothermal route and their properties were studied.AgNO3 and Fe(N03)3·9H20 were chosen as the reaction precursors.By adjusting the molar ration of these two precursors,we can obtain a series of Ag-Fe3O4 composite nanoparticles with different core/shell composition ratios.The multi-functional composite nanoparticle is a core-shell structure formed by silver nanoparticles as a core and Fe3O4 magnetic nanoparticles loosely deposited on the outer layer.The effects of the molar ratio of the two precursors on the structure,morphology,size,magnetic and optical properties of the resulting products were systematically investigated.The formation mechanism of Ag-Fe3O4 composite nanoparticles was also discussed from thermodynamics and kinetics respectively,and corresponding rational explanations were given.As shown in the UV-visible absorption spectra of the products,the surface plasmon resonance(SPR)absorption band gradually blue shifts as the Ag/Fe ration increases.Rhodamine 6G(R6G)and crystal violet(CV)were selected as Raman-active probes.In the SERS performance study,the products exhibited very high SERS sensitivity,and their SERS signal gradually enhanced as the Ag/Fe molar ratio increases.This "hot spot" controllable SERS substrate has broad application prospects in pesticide residue detection and biochemical analysis.2.The Fe3O4-Ag composite SERS substrate was prepared by magnetron sputtering silver nanoparticles on the surface of Fe3O4 nanoparticles.A series of composites with different silver coverage can be obtained by adjusting the magnetron sputtering time(0,40,70,100,130,160 and 190 s).The SERS properties of these composites were studied by using p-aminobenzene sulfide(p-ATP)and R6G as Raman active probes.The SERS signal gradually increased with the sputtering time prolonged,and reached the maximum at the sputtering time of 130 s.The SERS substrates have good stability and reproducibility by collecting the random spectra and the intensity distribution of the characteristic peaks in the selected area.The substrate with the strongest SERS effect is used for trace detection of p-ATP with a minimum concentration of 1.0×10-10 M.Finally,the SERS substrate was used for the trace detection of the pesticide thirau,and the detection limit reached 5×10-7 M.The SERS substrate is fixed on a silicon wafer and is easy to carry,so it can be applied to on-site rapid detection of pesticide residues in complex environments.3.Two multi-functional Fe3O4@mTiO2@Ag and Fe3O4@mTiO2@Au NR SERS substrates were prepared by indirect synthesis and layer-by-layer assembly.Fe3O4 nanoparticles act as the inner core and magnetic source;the mesoporous TiO2 in the interlayer has a very large specific surface area,which can adsorb a large amount of molecules and ensure the physical and chemical stability of the entire composite;the outer Ag nanoparticles and gold nanorods impart excellent optical properties to the composites.Fe3O4@mTiO2@Ag was prepared by the in-situ reduction process and Fe3O4@mTiO2@Au NR by electrostatic interaction with polyelectrolyte.The SERS properties of the two composite substrates were studied by using CV,p-ATP and p-mercaptobenzoic acid(MBA)as Raman active molecules respectively.The detection limits of Fe3O4@mTiO2@Ag composite nanoparticles for CV and p-ATP are 1.0×10 9 M and 1×10-12 M,and Fe3O4@mTiO2@Au NR for p-ATP and MBA are 1.0×10-10 M and 1×10-9 M,which proved that these two substrates have very high SERS sensitivity.The SERS spectra of 20 points randomly collected in the selected regions of the two substrates were used to calculate the relative standard deviations of the characteristic peak intensities,which proved that the two substrates have high reproducibility and stability.Finally,the Fe3O4@mTiO2@Ag composite substrate was used for the thiram trace detection and the detection limit can reach to 5×10-8 M(about 0.05 ppm),which is lower than the maximal residue limit of 7 ppm in fruit prescribed by the U.S.Environmental Protection Agency.These versatile SERS substrates with high sensitivity,stability and reproducibility have potential applications in the fields of catalysis,tumor therapy,drug targeted delivery and cell separation.4.The Au-Fe3O4 hybrid composite hollow spheres were prepared by a simple and efficient one-step hydrothermal method,then their SERS properties and catalytic properties were studied.The amount of Au nanoparticles located in the hybrid hollow spheres can be tuned by changing the molar ratio of Au/Fe precursors.A possible synthetic mechanism of the Au-Fe3O4 hybrid hollow spheres has been proposed.The obtained hybrids exhibit not only a superior surface-enhanced Raman scattering(SERS)sensitivity,but also an excellent catalytic activity.The obtained hybrid composite hollow sphere has a very large specific surface area and many SERS active sites,so they exhibit very high SERS activity of R6G adsorbed on the surface.The product with Ag/Fe of 0.2 has the strongest SERS activity,and the detection limit for R6G can reach up to 1.0×10-10 M.Furthermore,the catalytic experiments of the Au-Fe3O4-0.2 hybrid hollow spheres demonstrate that the model of 4-nitrophenol(4-NP)molecules can be degraded within 3 min and the catalytic activity can be recovered without sharp activity loss in six runs,which indicates their superior catalytic degradation activity.The reason may be due to the highly efficient partial charge transfer between Au and Fe3O4 at the nanoscale interface.The results indicate that the bifunctional Au-Fe3O4 hybrid hollow spheres can serve as promising materials in trace detection and industrial waste water treatment.