Zhenbin Jia

Fractional coverage of defects in self-assembled thiol monolayers on gold

Au electrodes are alkylated by self-assembled organic monolayers of octadecanethiol from alcohol solution. The electron tunnelling resistance of a monolayer-coated gold electrode has been investigated by ac impedance. The relation between the fractional coverage of different defects and the corresponding film thickness at these ‘collapsed’ sites has been deduced from electron tunnelling theory. By using the concepts of average film thickness at defect (da) and average fractional coverage of defect (θa), we have obtained the θa∼da plot. The influence of the apparent standard rate constant on the shape of the θa∼da plot has been discussed. In our experiments, Fe(CN)63−/4− is used as a redox probe to study the θa∼da plot of an octadecanethiol monolayer. The θa versus da plot indicates that the defects with da<6 methylene groups and θa<0.1 can increase the apparent standard rate constant from 1.9×10−10 cm s−1, which is the theoretical value calculated from electron tunnelling theory, to 2.9×10−7 cm s−1. The average thickness of the whole monolayer (ATWM), which is obtained from the θa versus da plot and which can indicate the blocking property of the monolayer, is 11 methylene groups.

Electrochemical behavior of the self-assembled membrane formed by calmodulin (CaM) on a Au Substrate

Abstract A membrane of calmodulin (CaM) was successfully fabricated by the self-assembly technique on a Au substrate. The electrochemical response of the membrane was investigated systematically with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods, where a redox couple of Fe(CN)63−/4− was employed as the probe. The results showed that a combination of CaM with Ca2+ could lead to a decrease of the reaction resistance (Rct). The most interesting point is that the plots, which were recorded using the above two methods for the Au electrode modified with the CaM membrane (CaM/Au), showed a regular dependence on the pH of the electrolyte studied. After serial experiments, we concluded that the results we obtained could be attributed partly to the conversion process of the spatial structure of the CaM.

Photoinduced electron transfer across a gold supported octadecanethiol/phosphatidylcholine hybrid bilayer membrane mediated by C60 in different redox species solution

Abstract Electron transfer across the biomembrane is central to photosynthesis, to mitochondrial respiration and to the design of molecular systems for solar energy conversion. We report here that gold supported octadecanethiol/phosphatidylcholine hybrid bilayer membrane mediated by C60 (abbreviated C60 HBM) can act as both a photosensitizer for electron transfer from a donor molecule and a mediator for electron transport across a hybrid bilayer membrane. The steady-state photocurrent behaviors in different concentration of ascorbate, Co(bpy)32+/3+ or Fe(CN)64−/3− solution have been studied. The rate-limiting step of the whole photoinduced electron transfer depends on the applied potential and the redox concentration in solution. The C60 HBM electrode was fabricated for the first time and there is no investigation on C60 incorporated octadecanethiol/phosphatidylcholine hybrid bilayer membrane in literature. The sigmoidal-shaped steady-state photocurrent versus the applied potential curve is first observed and it is different from the linear-shaped iph/E curve of C60 modified bilayer lipid membrane reported by Bensasson et al. [R.V. Bensasson, J.-L. Garaud, S. Leach, G. Miquel, P. Seta, Chem. Phys. Lett. 210 (1993) 141].

Electrochemical studies for the formation of sodium lauryl sulfate monolayer on an octadecanethiol-coated gold electrode

Abstract The formation process of sodium lauryl sulfate monolayers on the surface of octadecanethiol self-assembled monolayers was investigated by the method of cyclic voltammetry and ac impedance spectroscopy. The pinhole in an octadecanethiol-coated gold electrode can be patched when adding sodium lauryl sulfate to solution. The charge transfer resistance increased but the membrane capacitance decreased with the addition of sodium lauryl sulfate or other amphiphilic substance, indicating that the adsorption of sodium lauryl sulfate on the electrode surface from solution.

A.c. impedance studies of the mechanism of electron transfer across TCNQ modified Au/thiol/lipid bilayer

Abstract Au supported octadecanethiol/phosphatidylcholine (Au/ODT/PC) bilayers modified with 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) were studied by a.c. impedance. Three kinds of Au/ODT/PC bilayer were prepared: (i) unmodified bilayer (UMB), (ii) bilayer with only the PC monolayer containing TCNQ (ODT/PC-TCNQ) and (iii) bilayer with both the ODT and PC monolayers containing TCNQ (ODT-TCNQ/PC-TCNQ). The Faraday resistances of UMB, ODT/PC-TCNQ and ODT-TCNQ/PC-TCNQ in 1 mmol/l Fe(CN) 6 3−/4− +0.1 mol/l KCl solution were 3.9×10 7 , 5.1×10 6 and 2.2×10 5 Ωcm 2 , respectively. The embedded TCNQ molecules were believed acting as electron mediators and electron transfer by hopping between TCNQ molecules was proposed as the mechanism of electron transfer across ODT-TCNQ/PC-TCNQ.