Fabrication Of New Substrates For Surface Enhanced Raman Spectroscopy And The Related Theory Study

Surface-enhanced Raman scattering (SERS) is the enhancement of Raman scattering intensity of molecules on a nanostructured metallic surface. SERS is an excellent analytical tool with high sensitivity and is capable of obtaining fingerprint information of species by detecting the vibrational bands. The substrate with SERS activity is the prerequisite for enhancement and is very important for the application of SERS. With the development of nanotechnology, the fabrication methods of SERS substrate have made great progress. These methods include roughening of metal surface, lithography, template method, galvanic displacement reaction and synthesis of nanoparticles. Galvanic displacement reaction and synthesis of nanoparticles are two simple and efficient methods and have been widely used. Their applications on the fabrication of SERS substrate have always been the research focus. There is still the space of improvement for SERS sensitivity of noble metal colloids in water. When transporting nanoparticles on a substrate, the uniformity of the final SERS substrate should be improved also. For the galvanic displacement reaction, the sensitivity of substrate needs to be enhanced.The purpose of this paper is to expand the application of synthesis of nanoparticles and galvanic displacement reaction on the fabrication of SERS substrate and the main contents can be summarized as follows.1. Porous ZnO nanoplates were in situ fabricated on a zinc foil. The precursor hydrozincite were synthesized by the reaction of zinc foil and urea solution in a teflon-lined stainless steel autoclave. After calcination the hydrozincite was converted to porous ZnO. A possible formation process of porous ZnO nanoplates on Zinc foil is raised. The precursor hydrozincite nanoplates assemble to microspheres at primary stage and grow with the reaction time. After reaching the maximum dimension at2h the nanoplates begin to dissolve. The prepared porous ZnO showed single-crystalline nature. Ag nanoparticles were deposited on porous ZnO nanoplates to form a surface enhanced Raman scattering (SERS) substrate. The limit of detection for PATP is 1×10-8M. The substrate show good uniformity and temporal stability under laser irradiation. The results indicate that it is a promising SERS substrate.2. The SERS activity of Ag colloids was enhanced by concentration. Ag colloids reduced by hydroxylamine were incubated with PATP and were precipitated by centrifugation. This wet and pre-incubated precipitation was directly used as a SERS substrate. The limit detection of Ag colloids precipitation for PATP is1×10-9M and the SERS activity of Ag colloids was enhanced for about three orders by this method. Pre-incubation of Ag colloids with analyte before centrifugation and using the wet precipitation directly as the SERS substrate are two key factors to enhance the SERS activity of Ag colloids. Pre-incubated Ag colloids aggregated more closely on the substrate than that without the pre-incubation. The wet precipitation is more active than the dry precipitation of Ag colloids. Before the evaporation of water, highly concentrated Ag colloids were irradiated in a three dimension region along the laser and the analyte molecule may be in the active region of Ag colloids. Without pre-incubation or water, the Ag nanoparticle precipitation cannot reach the SERS intensity as high as wet and pre-incubated precipitaion. Pre-incubation with analytes and concentration by centrifugation is a simple and efficient method to enhance the SERS activity of Ag colloids.3. We found that the different contact modes of zinc foil with the solution of AgNO3-CTAB in galvanic displacement reaction result two substrates with different SERS activity. The substrate synthesized by dropping AgNO3-CTAB solution on a zinc foil is more active than the substrate fabricated by immersing a zinc foil in the AgNO3-CTAB solution. The limit of detection of Dropping-substrate for PATP is1×10-11M, while the limit detection of Immersion-substrate is1×10-7M. Nanospheres with the same crystal structure were obtained in the dropping mode and immersion mode reactions. In the immersion situation, these nanospheres attached together to a bulk Ag with the increase of reaction time and the plasma resonance effect was weakened. In the dropping situation, the close-packed nanospheres with smaller size were obtained. The couple of plasma resonance was generated by the close-packed nanosphers and the SERS activity of the substrate was enhanced. The reason for the generation of Ag nanospheres with different sizes is the different contents of AgNO3and CTAB involved in the reaction in two reaction modes. Compared with the traditional galvanic displacement reaction with immersion mode, the dropping mode reaction can control the growth rate of nanospheres and avoid the generation of bulk Ag. The close-packed Ag nanospheres are highly sensitive SERS substrate. The dropping mode reaction makes a progress in the application of galvanic displacement reaction on the fabrication of SERS substrate.