Surface Plasmon Resonance And Its Applications To The Probing Of Macromolecules On Gold Surfaces

Surface plasmon resonance (SPR) occurs at the interface of metal and dielectric, whose local field enhancement effect makes it very sensitive to the properties of the surface. This SPR technique is used to study the self-assembling of macromolecules on gold surfaces in this thesis.The research work reported here started from improving the performance of SPR sensors. Some theoretical and experimental works were firstly carried out to analyze the differences between experimental results and theoretical calculations. The SPR configuration is modeled by a multilayer system, its reflection is treated by Fresnel's theory. The topology of gold layer surface is measured by atomic force microscopy (AFM), and the influences of the surface roughness is modeled by effective medium theories. It is found that the differences are not mainly caused by the surface roughness of gold layer, but rather by the inhomogeneity of gold layer. The process of gold deposition is improved to prepare gold films of higher compactness. Higher deposition speed are experimentally proved to result in more compact gold layer and flatter gold surfaces, therefore more favorable for making ideal SPR sensors.In order to control the adsorption of macromolecules on gold surface, special self-assembed monolayers (SAMs) need to be prepared on gold surface. Several kinds of SAMs are prepared successfully, and their properties are studied by electrochemical impedance spectroscopy (EIS). In addition, the dynamics of SAM formation are also characterized in real time by SPR microscopy. Double layer SAMs of 3-mercaptopropyl-trimethoxysilan (MPTS) and 3-Aminopropyl-triethoxysilan (APTES) are proved to be feasible but less effective for DNA adsorption. Since the MUO 11-Mercaptoundecanol (MUO) molecules are uncharged and 11-Amino-1-undecanethiol (AUT) may be positively charged in buffers, the surface charge of mixed MUO/AUO SAMs can be varied by changing the proportion of MUO/AUO molecules and the buffer conditions. The properties of mixed SAMs of various MUO/AUO proportion and their dependence on solution ionic strength and pH value are studied by EIS and SPR microscopy. Both measurements show that the MUO/AUT SAMs have good charge carrier blocker performance. In addition, their responses versus the electric potential of the gold film are also investigated. The SAMs loose charge carrier blocking behavior in a finite range of potential typically [-100 mV, 0 mV] versus Ag/AgCl standard reference. This method of sweeping in potential is very suitable to detect the defects in SAMs, which are less evident in general conditions. EIS measurements also indicate that defects in mixed SAMs may be more serious in pure SAMs.Arming at the preparation of SPR based DNA sensors, our first study concentrated on the preparation of DNA layers on MUO/AUT mixed SAMs. The dynamics of DNA adsorption on these mixed SAMs was followed in real time by SPR microscopy. Two kinds of DNAs were investigated, namely circular DNA plasmids of 3000 base pair (bp), and straight DNAs of 200 bp. The dependence of DNA adsorption on DNA concentration in solution, surface charge, and solution ionic strength are reported here. The topography of adsorbed DNAs are observed by atomic force microscopy ex-situ. Many information can be drawn from the combination of these two kinds of measurements.We propose to use the local field enhancement of SPR to construct a new sensitive surface second harmonic generation (SSHG) microscopy. Numerical simulations show that the SPR enhanced SSHG microscopy is suitable for detecting the components of the second order nonlinear susceptibility of the surface. It also opens the possibility of imaging the surface with higher resolution, and detecting the orientations of surface molecules.