Study Of Porphyryin Self-assembled Monolayers On Gold Electrode By Scanning Electrochemical Microscopy(SECM)

Scanning electrochemical microscopy (SECM) developed is an electrochemical tool that can be used to study ET reactions at various interfaces with high spatial resolution, and can obtain rate constants of homogeneous and heterogeneous redox reactions precisely. An advantage of this technique is that the SECM measurements are carried out under steady-state conditions, so that the complication caused by ohmic potential drop and charging current can be overcome, therefore, the reliability of the data was strengthened. Using the ultramicroelectrode (UME) as the tip, SECM can also be a powerful tool to obtain images of different substrates immersed in electrolyte solution, to distinguish regions of different conductivity or electrochemical activity. In addition, SECM has been used widely for biological systems, such as images of cells, studies of enzymatic reactions, oxygen evolution on leaf surfaces and investigation of the transport of species through skin. Remarkable progress in the field of self-assembled monolayers of thiol has enabled us to design and graft the electrode surfaces. However, due to the difficulty related to synthesis and separation of porphyrins, the reports about porphyrin SAMs are limited. The combination of the trational methods and SECM can provide macroscopic and microscopic information of thiol- porphyrin monolayer on a gold electrode.There are three parts in this paper, main contents are as follows:1. Because our work are completed on the basis of the new technique, SECM, review was given on the scanning electrochemical microscopy. It includes that (1) The introduction of SECM instrument (2) eight work pricinple was intrduced (3) the theory of quantivative analysis about SECM (4) the development of SECM in various field in these year.2. Scanning electrochemical microscopy (SECM) was introduced to characterize the modification of the thiol-porphyrin film on the gold electrode. The application of image not only shows clearly the formation process of self-assembled monolayer at different periods but also provides many electrochemical informations. The consistent results were gained from the conventional cyclic voltammetry and the new SECM and correponding theory was used to explain the results, which showed that the electron transfer also could be affected by substrate potentail, the structure of the probe molecules and the property of the solution. What'more, through the comparision with the effect of substrate potential on thiolβ-cyclodextrin adsorbed gold electrode, it was proved that the porphyrins rings could affect the models of the ET at different substrated potential. The main model of electron transfer would be changed from the ET through the direct tunneling, pinholes and defects to a similar bimolecular reaction when the substrate potential was increased to 900 mV and the porphyrins ring was oxidized. The correponding electron transfer rates calculated verity the accuracy of the experiments.3. Due to the advantage of SECM on studing electron transfer, we studied the difference of many parameter about porphyrins self-assembled monolayer with different chain length and different number of methylene, and the corresponding explanation and analysis were given.