Study On The Preparation And Performance Of Noble Metal-based Composite SERS Substrates

Raman spectrum is a kind of scattering spectrum produced by inelastic collision of incident light.It shows fingerprint vibrational spectra of molecules,thus can achieve specific biochemical detection.Surface enhanced Raman spectroscopy(SERS)combines Raman spectroscopy with nanotechnology,which greatly enhances the signal intensity of Raman spectroscopy.Due to its advantages of high sensitivity,simple operation,non-destructive detection for rapid detection on site,SERS has attracted tremendous attention in various fields including environmental monitoring,food safety,disease diagnosis,exhibiting promising application prospects.The preparation and development of SERS substrate with high performance are regarded as bottleneck for SERS technology.Nanostructured noble metal materials,especially Au and Ag,are generally considered as the most SERS-active substrates for their local surface plasmon resonances(LSPR).However,the sensitivity and stability of noble metals need further improve.Based on the electromagnetic enhancement and chemical enhancement of SERS mechanism,this paper focus on the preparation and application of noble metal-based composite SERS substrate.The main work are listed as follows:1.The rGO/Ag composite was prepared by in-situ reduction of AgNPs on GO nanosheets.The microstructure and composition of rGO/Ag were characterized by a series of measurements including SEM,TEM and XRD,etc.R6G solutions were used to investigate SERS performance of rGO/Ag.The detection limit was 10^-10 M,a logarithmic linear dependence between Raman intensity and concentration was determined.The RSD of intensities from 20 different spectra was 4.57%,and a high sensitivity can be maintained after 60 days of storage.These results indicate excellent sensitivity,uniformity and stability of rGO/Ag.Then rGO/Ag was utilized for detection of malachite green,melamine and chloramphenicol.The characteristic peak assignment of their SERS spectra and linear fitting equations were obtained.The SERS enhancement mechanism of rGO/Ag was explored by FDTD simulation.The results suggest that strong electromagnetic enhancement exists around rGO/Ag.Combined with the chemical enhancement of rGO,rGO/Ag exhibits excellent SERS performance.2.The integration of membrane separation technology and SERS technology facilitate enrichment and detection of organic compounds.And the introduction of semiconductor materials into substrate composition would endow the ability of photocatalytic degradation simultaneously,then organic compounds can be degraded to achieve recyclable SERS detection.Therefore,the GO/g-C₃N₄/Ag composite membrane was designed and fabricated by a simple filtration-assisted assembly method.R6G solutions were used to investigate its membrane performance,SERS performance and photocatalytic activity.The GO/g-C₃N₄/Ag membrane exhibited a water flux of 230.64L?m^-2?h^-1with a rejection rate of 89.27%.The detection limit was 10^-12 M with an EF value of 6.33×10⁷.It took about 120 min to reach a photocatalytic degradation rate of97.05%.When used for detection of paraoxon-ethyl,the detection limit was 10^-9 M and the degradation rate was 92.8%after irradiation of 150 min.And it remains effective still even the process was recycled for five times with a decrease of about 18%.The photocatalytic mechanism was studied by radical scavengers and EPR measurement,and was finally proposed as the production of active radicals(?OH)and the improved separation efficiency of photogenerated electron-hole.3.SERS detections of bacteria were conducted by four different SERS substrates including AgNPs,GO/Ag membrane,CNT/Ag membrane and Fe₃O₄@Ag magnetic composite.E.coli was used as representative bacterium to investigate their SERS performance.When AgNPs were directly mixed with E.coli,AgNPs could be adsorbed on the outer surface,and the detection limit was 10⁴ CFU/ml.The combination of membrane separation process forGO/Ag and CNT/Ag composite membranes facilitate enrichment of bacteria,especially in low-concentration and large-volume samples,which would improve the detection sensitivity.The detection limit of GO/Ag membrane for E.coli was 10⁴ CFU/ml,while it was 10³ CFU/ml for CNT/Ag membrane.The CNT/Ag exhibits higher detection sensitivity because the accumulated AgNPs on the surface,which produced stronger electromagnetic enhancement.For Fe₃O₄@Ag magnetic composite,the combination of magnetic separation process plays an important role in enrichment of bacteria.The enrichment efficiency for E.coli was about 68.6%,and the detection limit was 10³ CFU/ml.Moreover,three-dimensional principal component analysis was used to identify six different bacteria by SERS spectra.4.Au nanomaterials with special microstructure were constructed as SERS substrates.Au NS and Au NF were prepared by chemical reduction and electrodeposition,respectively.For both of them,there are lots of tips distribute on the surface,which can generate hot spots at the end and gap of tips.The core size of Au NS is about 50 nm,and the length of branch is about 20 nm.The detection limit of R6G was 10^-11 M,and the maximum electric field intensity was 91.2 V/m by FDTD simulation.Au NF was electrodeposited on the surface of CNT film with a diameter of about 1.1?m and showed face centered cubic crystal structure.The detection limit of R6G by CNTM/Au NF was10^-9 M,and the maximum electric field intensity simulated by FDTD was 13.5 V/m.The results indicate that the size of SERS substrate has an important effect on the SERS performance.