Self-assembled Monolayers: Analysis By In Situ SERS And Raman Mapping

Over the past two decades, self-assembled monolayers (SAMs) technique has witnessed tremendous growth in theory and application. The most attractive SAMs systems involve monolayers of fatty acids, of organosilicon derivatives and of alkanethiolates due to their controllably orderly uniformity with compact array, which provides versatile potentials to be employed in the fields of biologic chemistry, medicine, preparation of sensor, nano science, material science and inhibition of metal from corrosion, etc. However, it should be pointed out that the developing pace of SAMs technique has been still on the way. Recently, some key issues are drawing increasing attentions as follows: (1) Exploration of novel SAMs systems, in particular, the self-assembling molecules with dual or multiple adsorption sites; (2) Investigation of the fundamental relationships between the monolayers structures and properties as a basic need to extend their further applications since the self-assembly process is completely spontaneous and adsorption mode dependent with interaction of molecules and subtract. The self-assembled film at a monolayer level calls for the analytical methods with high sensitivity. The routine investigations of monolayers are conducted by using ellipsometry, contacting angle, electrochemistry, quartz crystal microbalance(QCM), infrared reflectance absorption spectroscopy (IRRAS), Raman spectroscopy, second harmonic generation(SHG), sum frequency generation(SFG), static second ion mass spectroscopy(SSIMS), X-ray photoelectron spectroscopy (XPS), high or low energy electron diffraction (HEED or LEED), X-Ray diffraction (XD) , near-edge extended X-ray adsorption fine structure (NEXAFS), scanning tunnel microscopy (STM), Auger electron spectroscopy(AES) and atomic force microscopy (AFM), etc. According to the above observations, insights into the self-assembling mechanism and film structure have been reached. However, some of aforementioned methods require high vacuum such as XPS, HEED or LEED and STM or AFM can provide image of monolayers in two dimensions, but the information is not for particular molecular species. Normal Raman spectroscopy and FTIR present molecular vibration information and are seldom used to study on the molecules with dual or multiple adsorption sites in two dimensions. The advent of confocal laser Raman scattering analytical technique since 1990's has enabled the spatial resolution and sensitivity of instrument to be increased. Aided with a XY stage strictly controlled by program and PC, Raman mapping has been extensively and successfully employed in analysis for stress distribution of material surface in two dimensions owing to its rich molecular vibration information. In present thesis, we wish to extend the Raman mapping technique together with the super sensitive surface enhanced Raman scattering method to examine the SAMs of molecules with multiple adsorption sites such as nicotinamide adenine dinucleotide (NAD), 6-mercaptopurine(6MP) and inositol hexphosphates (IP6), which are formed at various metallic surfaces under different media. Based on the obtained spectral information, the features of SAMs including the orderly uniformity and the impact factors from chemical environment have been explored. Furthermore, in light of the calculation results for vibrational spectra by quantum chemistry methods along with the SERS mechanism, the structure of the monolayers could be deduced. And also, the electrochemical and in situ spectroelectrochemical methods are used to observe the relationships of structure-properties and examine the stability of SAMs. The main contents of this thesis include following parts: (1) The mechanism of NAD monolayers self-assembled at the silver electrode as well as the effect of surface morphology on the process have been studied by the use of SERS mapping technique(Chapter 2). Two kinds of silver surfaces were obtained with the chemical and electrochemical methods and their average roughness degree are 40 ± 25 nm and 110 ± 50 nm as estimated from their STM image. At more even surface, formation of NAD SAMs was a much rapid process. In the case of a relatively rough silver surface, an intermediate state of adsorption of NAD molecules was found and the stable structure of NAD SAMs could be reached only after a long dynamic self-organized process. According to the SERS-based mechanism, NAD molecules should be vertically adsorbed at the silver surface to form the monolayers via N7 and NH2. The proposed orientation of NAD monolyers is in agreement with the result of minimized energy calculation of molecular configuration. (2) The impacts of potentials on the structure of NAD SAMs at the silver surface have been investigated by using in situ SERS electrochemical methods (Chapter 3). NAD molecule is composed of two 5`-nucleotide bases, AMP and nicotinamide ribose 5`-phosphate, which are linked together with a pyrophosphate bridge. The flexibility of this linkage provides a freedom to some extent for the adenine and nicotinamide moieties to change their conformation in dependence onthe chemical or physical micro-environment nearby the surface. However, question regarding the effect of the charged surface on the adsorption mode of NAD is still open. As the SAMs can provide a simple and good physical mode for understanding of interface phenomena, and the SERS signal is free from the interference of molecules in bulk solution. An NAD modified silver electrode was prepared. In situ SERS spectra indicate the change of NAD adsorption mode at the silver with the shift of potential and desorption of NAD monolayers from the surface occurring around the potential at -0.1 Vvs SCE. And the electrolyte in buffer solution might be a key influencing factor on the adsorption behavior of NAD at the silver surface. (3)This part is the first report and at molecular level investigating the relationship between the structure and properties of monolyers from phytic acid at the silver surface by using SERS mapping, electrochemistry and in situ SERS spectroelectrochemistry (Chapter 4). Phytic acid is an environmentally friendly anti-corrosive agent. Six phosphate groups of IP6 molecule possess the chelating capability with metal and the configuration of IP6 in solution is dependent of the pH value. On the basis of results of SERS mapping and quantum chemistry calculation, one concludes that in a solution of pH 1.27, IP6 tends to anchor at the silver surface via one phosphate group to assemble the monolayers and under pH=13, it adsorb chemically at the surface through four co-planar phosphate groups after the change of configuration. The former orientation is considered as much compact and the later more stable, leading to the different but the excellent inhibition efficiency. (4) The self-assembling process of IP6 at the roughened copper surface from the sodium salt of phytic acid solution has been observed on line with the time dependent SERS spectroscopy (Chapter 5). In domestic water, copper corrosion could be inhibited in the presence of Ca and Mg-salts with the inhibition efficiency around 90 %, and in the case of Na-salt, the maximum inhibition just reached 65.3 %, although it is completely soluble and would promote the formation of a passive film according to results of electrochemical experiment. In situ SERS spectroscopic investigations revealed that the self-assembling of IP6 at the roughened copper surface in a Na-salt phytic acid solution took two steps involving firstly self-cleaning the surface and then forming IP6 monolayers. Based on the calculation results for vibration spectrum of IP6 molecule with PM3 method and SERS mapping, a model for IP6 molecule adsorbed chemically on the roughened copper via two co-planar phosphate moieties was suggested. The copper surface with the IP6 monolayers presents the inhibiting action in a 0.1 mol L-1 KCl solution but the relative inhibition efficiency is of 41.2 %because of the water co-adsorption. (5) SERS mapping was extended to study the SAMs of IP6 at the iron electrode (Chapter 6). After roughening the iron surface by a special oxidation-reduction cycle into SERS active substrate and IP6 self-assembled onto the surface, spectroscopic measurements were carried out. Based on the recorded SERS spectra and quantum chemistry calculation for vibrational modes of IP6 molecule with PM3 method, the adsorption ways of IP6 SAMs formed at the roughened iron surface from bulk solutions with various pH conditions were deduced. In the case of pH 5, the IP6 molecules locate at the surface via four co-planar phosphates, while the pH value of IP6 solution was 11.27,it is assumed that only one phosphate group was adsorbed on the iron surface. The results of electrochemical polarization measurements indicate that inhibition efficiency of IP6 SAMs formed at pH 5 due to the stronger interaction with the iron surface was higher than that formed at pH11.27. (6) A protocol of the combination of SERS mapping technique and DFT calculation method has been applied to look into the monolyers of 6-mercaptopurine of an electron transferring promoter formed at the gold surface (chapter 7). We obtained the normal acid solution Raman spectrum of 6MP for the first time. The SERS spectroscopic results suggest both of the resulted 6MP SAMs from acid and alkaline media finally adopt the same adsorption mode of the S atom of pyrimidine moiety and N7 atom of imidazale moiety anchoring at the gold surface in a vertical way. It was found that the detaching process of the 6MP SAMs from the surface involves one electron reduction and the potential at about 0.7 V vs SCE through in-situ SERS spectroelectrochemical studies. (7)The pH dependent SERS studies have been conducted on 6-mercaptopurine monolayers at the silver surface (Chapter 8). It is concluded that 6MP adsorbs on the silver electrode with a tilted orientation via S, N1, N7 atoms in the acid medium and in the case of alkaline medium it adopts the head-on adsorption modes with S and N1 atoms anchoring the silver surface; however, 6MP molecule turns to the same standing up orientation on the electrode through the S and N7 atoms after the acid or basic solution removed.