Electrochemical Characterization And Application Studies Of Porphyrin On Self-assemblied Monolayers

Self-assembled monolayers (SAMs) is the ideal model to study the surface and interfacial phenomena, which has been widely applied in the electronic and optical devices, chemical sensors, biological sensors, catalysis, and other research fields and has been the focus of considerable research activity. Owning to porphyrins'favorable optoelectronic properties and biologically activity, the porphyrin and their metal complexes play an important role in the biological electron transport in biomembranes and other investigations, known as the "life of pigment." In this paper, porphyrins are effectively combined with self-assembled monolayer technique to construct functional ordered ultrathin films—monolayer and multiplayer films. The physical and chemical characteristics of porphyrin SAMs, the sensing properties of H2PO4- on porphyrin SAMs have been investigated by a variety of electrochemical methods, such as electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM).Most anion recognition and sensing phenomena reported thus far have been investigated in bulk solution, especially in organic media. However, most anions exist in aqueous medium, such as pollutants, organisms, and so on. Specially, it is of particular significance to detect the dihydrogenphosphate anion in aqueous solution due to its biological importance, such as biological systems, chemical processes, medicine, food chemistry, material sciences and environmental sciences. In order to make anion sensing possible in aqueous solution, the directed assembly of organized arrays of molecules covalently bound to substrate surfaces offers an attractive, versatile method to design and construct a wide range of thin films by controlling the structure and material properties. SAMs of thiol-derivatized tetraphenylporphyrin on gold are able to bind anions reversible from aqueous solutions. Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) were employed to investigate the binding of anions by porphyrin SAMs and proved to be efficient and convenient techniques for detecting anions in aqueous solutions using the redox couple Fe(CN)63-/4- as"reporter ion". SAMs of porphyrins were shown to bind dihydrogenphosphate selectively over various other anions, including Cl-, Br-, NO2- or NO3-. Gold electrodes modified with porphyrins can detect H2PO4- even in the presence of a 10-fold excess of other anions studied.We have studied the electrochemical response of the organic-inorganic multilayers film using CV, SECM, and EIS, and have determined the electrochemical parameters. Multilayers film of nanostructured gold particles (AuNPs) has been fabricated based on the layer-by-layer (LBL) technique using a self-assembled monolayer of 5,15-di-[p-(6-mercaptohexyl)-phenyl] -10,20-diphenyl porphyrin (trans-PPS2). AuNPs act as physical cross-link points in the multilayers. Using the high sensitivity of the electron transfer of ferricyanide to the modification of the gold surface with multilayers film, we selected this reaction as a probe to study the different modification stages at this modified electrode. SECM images obtained from bare and the different modification stages showed very good resolution with different topographies. Based on a comparison with the results of the experiments, a reasonable agreement can be obtained, which means a conjunction of these techniques.Electrochemical impedance spectroscopy (EIS) was employed to explore the permeation of electrolyte ions in thioctic acid SAMs to structural change induced by interaction with copper ions. EIS of thioctic acid SAMs is simulated with the equivalent circuit and challenged by spectra with thioctic acid SAMs submersed in KNO3 solution and subjected to various potential differences. The ion permeation was estimated in the relatively low range of frequencies, 1000 Hz to 0.2 Hz, and quantified by a film resistance in the equivalent circuit (RSAM). We have investigated the effect of structural change of SAM on the extent of ion permeation-induced response affected by the electrode potentials and the electrolyte concentration. The experimental results verified that RSAM decreased～70% by interaction with copper ions and that RSAM increased～2-3 times when the electrolyte concentration was decreased by 10 times. This analysis can be performed without addition of redox species.