Electrochemistry Of Porohyrin On Self-assembled Monolayers

Self-assembled monolayer (SAM) is an important embranchment of supramolecular chemistry, its emergence make the development of modern chemistry come to a new level.Well-ordered, directional, dense, intact self-assembled monolayer with controllable electrode surface microstructure can be designed on a molecule level. The self-assembled monolayer technique afford a convenient way of preparing well-ordered, stable, various properties, pre-designed structure, compared to the conventional modified electrode methods, especially its unique distinct microstructure, which supply an important experiment plat for electrochemical study. Thus the relationship between microstructure and electrochemical respond, the reactivity of functional group and molecular recognition can be explored. Self-assembled monolayer modified electrode is the topmost form of modified electrode and the self-assembled method is considered to be an ideal mode for study many complex phenomenon on surface and interface.Owning to porphyrins' favorable photo- and electro-chemical activity, porphyrins and their metal chelates have been widely applied in catalysis, molecular recognition, photo-electron conversions, memorial material and medicinal fields. In present paper, porphyrins are effectively combined with self-assembled monolayer technique based on the self-assembled monolayer idea as core, electron transfer as masterstroke, various electrochemical methods as instrument, Natural Science Foundation as support. The properties of porphyrin, electron transfer kinetics between self-assembled monolayers of porphyrin and the adsorption kinetics of thiol-derivatized porphyrin on the surface of gold electrode, electrocatalytic dioxygen reduction by metalloporphyrin self-assembled monolayer and the biologic sensor based on the redox protein enzyme containing porphyrin had been investigated by electrochemical method. Found on this work, we hope to develop the theory on electron transfer of porphyrin, and make contribute to the application of porphyrin derivatives in catalysis, bionics and so on.Study and application of porphyrin on self-assembled monolayers In this section, we summarized across-the-board the progress of self-assembled technique, the characteristic of self-assembled monolayers, the adsorption kinetics of self-assembled process, long-distance electron transfer kinetics on self-assembled monolayers, the progress in application of self-assembled monolayers, the study of synthesis and actuality of porphyrins and metal-porphyrins, the preparation of porphyrins self-assembled monolayers, the progress of porphyrins self-assembled monolayers in photo-electron conversion, information storage, electron transfer, electro-catalysis and molecule recognise. In addition, based immobilized horseradish peroxidase on nano-Au and self-assembled monolayers were also expatiated by us.Study on synthesis and spectral and electrochemical properties of a series of porphyrins A series of hydroxylphenyl porphyrins and thiol-derivatized porphyrins were synthesized and the relation between the structure and property were also investigated by UV-Vis spectrum, fluorescence spectrum and electrochemical method.Study of the adsorption kinetics of thiol-derivatized porphyrin on the surface of gold electrode The adsorption kinetics of thiol-derivatized porphyrins (abbreviated as H₂TPPO(CH₂)_nSH, where n = 3, 4, 6, 9,10 and 12) on gold electrode have been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and the intention was to study the effect of alkyl chain length and terminal group on the process of forming SAMs. The time dependence on surface coverage supported that the adsorption of H₂TPPO(CH₂)_nSH molecules typically processes with a two-step adsorption consisting of a fast initial adsorption and a following slow reorganization. The equilibrium constant (K) for the adsorption and the Gibbs free energy (ΔG_ads) of SAMs were evaluated based on the Frumkin isotherm and the interaction factor between adsorbate-adsorbate molecules was also discussed. The adsorption processes at the initial step could be satisfactorily described by simple Langmuir adsorption kinetics, irrespective of alkyl chain length of adsorbate molecule. Based on Langmuir kinetics, the adsorption rate constant was determined at the initial step for the formation of all H₂TPPO(CH₂)_nSH-SAMs studied in this work. The rate constant value was found to decrease with increasing alkyl chain length and the unique structure of porphyrin moiety as a large terminal group had a great influence on the initial adsorption kinetics compared with that of n-alkanethiol on gold.Electron transfer kinetics between self-assembled monolayers of porphyrin and the surface of gold electrode The electron transfer kinetics of porphyrin self-assembled monolayers have been investigated by two methods. One was cyclic voltammetry (CV) by which the redox group porphyrin was linked covalently to self-assembled monolayers and the measurement was taken in organic media (0.1 M n-Bu₄NPF₆/CH₂Cl₂). The other was redox couple Fe(CN)₆^3- which was exposed to solution outside the films with scanning electrochemical microscopy (SECM). The electron tunneling constantβfor the electron transfer in the two systems were estimated to be 0.21 (?)^-1 and 0.095 (?)^-1 respectively, and the electron transfer rate constant k to the underlying electrode were found to fall down exponentially with the length of alkyl chain spacers increasing. The position of the redox couple and the chemical composition, structure and the tilt-angles of the chain and the solvents had possibly played important roles in the electrochemical kinetics.Study of orientation mode of cobalt-porphyrin on the surface of gold electrode by electrocatalytic dioxygen reduction Three orientation modes of self-assembled monolayers (SAMs) of cobalt-porphyrins on the surface of gold electrode have been prepared and tested in the electroreduction of dioxygen, such as cobalt-5-(4-aminophenyl)-10,15,20-triphenylporphyrin binding covalently to 3-mercaptopropionic acid pre-assembled on gold (Co-ATP-MPA); cobalt-tetra-[p-(3-mercapto-propyloxy)-phenyl] porphyrin (Co-TMPP) and cobalt-tetraphenyl-porphyrin axially attached with 4-mercaptopyridine SAMs pre-assembled on gold (Co-TPP-4MPY). A cyclic voltammetric investigation was carried out in an attempt to elucidate the net reaction to the dioxygen reduction. These modes were all active in electrocatalytic reduction of dioxygen through a two-electron mechanism with hydrogen peroxide produced. By combining imidazole axial with CoTPP and the above cobalt porphyrin SAMs and together with by testing the multilayer cobalt-porphyrin films by reduction dioxygen, the study confirmed that the electrocatalytic properties of metalloporphyrin was directly related with the structure or orientation of porphyrin ring on the surface of the electrodes.Study of H₂O₂ biosensor based immobilized horseradish peroxidase on nano-Au and self-assembled monolayers Enzyme modified electrode was prepared based immobilized horseradish peroxidase (HRP) on nano-Au and self-assembled monolayers. The presence of nano-Au in film can efficiently retain HRP activity since the formation of electrostatic complex of nano-Au-HRP. Hydrogen peroxide was determined in the presence of hydroquinone as a mediator to transfer electrons between the electrode and HRP. It was experimentally proved that HRP embedded on nano-Au demonstrated excellently electrocatalytical ability to the reduce of H₂O₂. The influences of experimental parameters on the response of H₂O₂ biosensor were optimized by using an cyclic voltammograms method. The linear range of biosensor for the detection of H₂O₂ was 5.0×10^-6 mol/L to 1.0×10^-3 mol/L and a detective limit of 2.5μmol/L. The equation wasΔI = 0.34765+4.05553C_H2O2 (mM) . The apparent Michaelis-Menten constant (K_m^app) for the sensor was determination to be 0.0675 mmol/L. The characteristics of our sensor include well-retained enzyme activity, long lifetime and high stability for the detection of H₂O₂.