Fabrication of noble-metal nanostructures and applications to surface-enhanced Raman scattering

Abstract

Surface enhanced Raman scattering (SERS) can provide important information of surface molecular structure and kinetic process directly. SERS technique is becoming a strong hand in surface science and widely used in trace analysis and even single molecule detecting, medical chemistry, environment sciences, biological and medical systems, nanomaterials and sensors. SERS enhancement is related to the wavelength of the exciting laser, the size and morphology of the substrate, the quantities of the absorbate and so on. The evolution of SERS technique and the broadening the application are depended on the development of SERS substrate. Thus, the preparation and properties of SERS substrates become very important to develop SERS as a usual analytical tool. On other hand, food safety issues caused by contamination of melamine have raised a great deal of concern in China in last few years. Conventional chromatography-based methods for detection of chemical contaminants are time-consuming and laborintensive. On the basis of the above facts, we focus our study on the fabrication of SERS substrates (e.g. Pd nanostructres, Ag-Pd alloy nanostructures, Au nanoparticles decorated ZnO (ZnO/AuNps) nanoarrays, Ag nanoparticles modified carbon (C/AgNps) nanospheres and Ag nanoparticles decorated Ag (Ag/AgNps) nanowires) and exploring the feasibility of using SERS technique coupled with as-fabricated substrates for detection of melamine in food such as egg white and raw milk solution. The main results are summaried as follows: Firstly, Pd nanostructures with different shapes such as urchin-like, hemispherical, flower-like and plate-like were successfully deposited on Si substrate in aqueous hydrogen fluoride (HF) solution at room temperature. The morphology and size of the nanostructures can be tuned by controlling the metal precursor concentration, reaction time and the organic additive concentration. This simple method was extended to preparing other noble metal nanoparticles with various morphologies and sizes. The morphology-dependent SERS activity of the as-synthesized nanostructures was demonstrated. It was found that urchin-like nanoparticles had a higher SERS activity than hemispherical and flower-like Pd nanostructures for Rhodamine 6G (R6G) probe molecules due to its special morphology. Secondly, dendritic and porous Ag-Pd alloy nanostructures were prepared through the coreduction of AgNO3 and Pd(NO3)2 in aqueous HF solution and galvanic replacement reaction using Ag nanoparticles as template, respectively. The morphology and composition of Ag-Pd alloy nanostructures could be modulated by controlling the molar ratios between metal precursors in the solutions or reaction time. In addition, the morphology and composition-dependent activity of the as-synthesized Ag-Pd alloy nanostructures was investigated. The enhancement factor was estimated to be on the order of 106 using R6G as test probe molecules. For the Ag-Pd alloy nanoparticles with similar morphology, the SERS signal intensity decreases with increasing the Pd content in the Ag-Pd alloy. SERS mappings show that the porous Ag-Pd substrate exhibits better reproducibility compared with the dendritic Ag-Pd substrate. Thirdly, ZnO/AuNps nanoarrays were prepared by a simple solution method using well-aligned ZnO nanoneedle arrays as templates. The coverage of Au nanoparticles on the surface of ZnO nanoneedle can be tuned by varying the concentration of Au precursor. High coverage leads to the formation of ZnO/AuNps nanoneedle bundles. The SERS performances of ZnO/AuNps composite nanoarrays toward detection of R6G were evaluated. The SERS enhancement factor is mainly influenced by Au coverage and morphologies of the ZnO/AuNps nanoarrays. Enhancement factor for the optimized ZnO/AuNps nanoneedle arrays was estimated to be of the order of 107. Fourthly, silver/carbon (Ag/C) core-shell nanospheres synthesized by a hydrothermal method were used as templates for fabricating Ag/C/AgNps nanospheres. The particle size of Ag nanoparticles increased with increasing the concentration of Ag precursor. Detection of melamine molecules at concentrations as low as 1.0×10-8 M shows that the Ag/C/AgNps nanosphere is an excellent SERS-active substrate. The effect of heavy metal ions on the detection of melamine is also investigated. It was found that the SERS spectrum profile of melamine is very sensitive to the presence of heavy metal ions: the peak positions of the SERS bands exhibit some apparent change with the kind of metal ion, showing a blue or red shift compared with those in the SERS spectrum of melamine; the SERS signal intensity decrease with increasing the concentration of metal ion. Fifthly, silver nanowires synthesized by a solvothermal method were used as templates for fabricating Ag/AgNps nanowires. The number density and particle size of Ag nanoparticles change with the concentration of AgNO3. Individual Ag/AgNps nanowire exhibited strong SERS effect. Detecting of melamine molecule at concentrations as low as 1.0×10-8 M was used as example to show the possible applications of such Ag/AgNps nanowires. Finally, the application of in rapid detections of melamine in egg white and raw milk solution based on the SERS technique is demonstrated. It was found that melamine in raw milk and egg white solution at low concentration (~10-8 M) can be easily detected with little sample pretreatment.