Fabricating SERS-active Substrates Using Anodic Aluminum Oxide Templates

Surface-enhanced Raman scattering (SERS) spectroscopy is a powerful analytical tool for chemical and biological sensing applications due to its advantages of super-sensitivity and wealth of structural information on characteristic"fingerprint"signatures of analytes. But there are still some problems that block SERS to be a universal analytical tool, especially on quantitative analysis. One of the most important problems is the poor performance of SERS substrates. SERS is an optical phenomenon which is closely related with the substrates absorbing molecules and SERS signals are strongly dependent on the feature of substrates. Since the discovery of SERS, kinds of SERS substrates were prepared, including metal colloids, metal island films, ordered arrays, metal-metal or metal-semiconductor composite materials, and so on. Some of these substrates have shown great potential, but there is still some way to go, compared with ideal SERS substrates. Usually, an ideal SERS substrate suitable for quantitative analysis should possess advantages of high sensitivity, homogeneity, stability and purity. Making SERS substrates to satisfy these conditions as much as possible is the key of promoting SERS to be a reliable analytical tool. In the thesis, porous Anodic Aluminum Oxide (AAO) films with regular structure were used as templates to fabricate two-dimensional (2D) and there dimensional (3D) SERS substrates, and then the properties of these substrates were tested. The major contributions of this work are as follows: (1) Preparation of AAO templates and detail characterization on microstructureAAO templates with regular structure were prepared using a two-step anodization approach. During the electrochemical polishing process, changes of current with time were monitored. Electrochemical polishing was performed under 15V, 17V and 19V, respectively. AFM was performed to characterize polishing effect and results indicate that 17V is the most suitable for our preparation process. Changes of current with time were also monitored during anodization approach and step-step voltage decrement process. Anodization approach could be divided into three stages, including the formation of barrier, pore formation and stable growth process. In the step-step voltage decrement process, the thinning rate of barrier layer in each voltage range was different. Microstructure of anodic aluminum oxide templates before and after widening pores in phosphoric acid was investigated by AFM and SEM in detail. The top face of AAO templates takes on regularly convex structure, which is resulted from the uneven distribution of surface electric field. But after widening pores, the convex structure was disappeared due to corrosion.(2) Preparation of ultrastable 2D SERS substrates using AAO templatesThe aluminum substrate remained after the process of oxidation acted as the template to prepare SERS substrate. A layer of silver film was formed on aluminum substrate using magnetron sputtering method. Aluminum substrate was then peeled off and a layer of silver was obtained. The morphology of SERS substrate and aluminum substrates were studied in detail by AFM and SEM. Results show that the silver films replicated the structure of aluminum substrates perfectly, which took on ordered array of Ag spherical caps. Throughout the preparation process, the aluminum substrate not only acted as the patterned matrix, but also served as a coating that protected Ag from being oxidized. UV-vis reflection measurement was performed to monitor the adsorption process of probing molecules. Taking 4-mercaptopyridine as probing molecules, SERS spectra were investigated. Comparing with the ordinary Ag film, the patterned Ag layer exhibited better SERS activity. The ultrastability and regular nanostructure endow these substrates with great potential in SERS applications.(3) Preparation of 3D SERS substrates using AAO templatesHighly ordered porous AAO templates were employed to construct SERS substrates. An alternating current (AC) electrochemical deposition was used to fill AAO templates with Ag nanoparticles. SEM characterization indicated that Ag nanoparticles were 10-20 nm in size and there were some short Ag nanowires. Taking 4-mercaptopyridine as probing molecules, SERS measurement was performed on substrates deposited for 30, 60, 120 and 180 s, respectively. High-quality SERS spectra were observed. The UV-vis mirror reflection spectra were measured to investigate the localized surface plasma resonance (LSPR) absorbance. An interesting phenomenon of LSPR-affected thin film interference was observed. SERS mapping was performed to characterize the homogeneity of as-prepared substrates. Good homogeneity and stability make these substrates good candidates for SERS spectroscopy.(4) Preparation and optical properties of ZnO/Ag nanocompositesAt present, neither electromagnetic enhancement mechanism nor chemical enhancement mechanism could make a full explanation of SERS, which is closely related to the complex experimental conditions and also related to theoretical models and theoretical methods. Until now, there is not yet a unified theory to explain the underlying physical and chemical nature of SERS. Electromagnetic mechanism has long been considered to play the most important role in SERS enhancement, but in recent years, research on chemical enhancement mechanism proved that the chemical charge transfer mechanism play a major role in SERS enhancement of semiconductors. An enhancement of 104 was observed. These experiments conducted in semiconductors exclude interference of electromagnetic field and have great significance for understanding the nature of SERS. However, in most system, electromagnetic mechanism cannot be completely ruled out, so the further study of metal-semiconductor composite is very helpful for investigating the relationship between electromagnetic mechanism and chemical mechanism in complex systems. We prepare ZnO/Ag nanocomposites by trisodium citrate-reduction in light conditions. SEM characterization indicated that silver nanoparticles were found only in parts of the surface sites of ZnO and there were no independent silver particles. These indicated that ZnO induced sodium citrate to reduce silver nitrate. In UV-vis absorption spectra, the plasma band of Ag in ZnO/Ag nanocomposites red shifted and widened compared with the silver colloid. This is due to the electron transfer from Ag to ZnO. In photoluminescence spectra, UV emission of ZnO due to exciton recombination increased compared to pure ZnO under the same concentration. This is also resulted from the electron transfer from Ag to ZnO. In SERS spectra, enhancement of the individual b2 vibration mode is due to electron transfer.