Ruting Tong

Studies of Structural Disorder of Self-Assembled Thiol Monolayers on Gold by Cyclic Voltammetry and Ac Impedance

Self-assembly of octadecyl mercaptan on gold has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that well-assembled thiol monolayers on gold are essentially free of pinhole defects. Support for the existence of ‘collapsed’ sites in thiol monolayers is obtained by comparing the apparent electron transfer rate constant, which is obtained from CV experiments at a film covered Au electrode, with the theoretical one, which is calculated by assuming electrons tunnel across the collapsed-site-free monolayer. The empirical relationship Z Rs 1:[s(jv) a ] is used to describe the film coated Au in inert electrolyte and the degree of disorder of the monolayer structure is estimated from the a value. It is demonstrated that the admittance plane plot of an ideally polarizable electrode with constant phase element behavior is a circular arc which can be used to determine a. Results of EIS experiments show that a increases sharply after the initial adsorption time and approaches its final value slowly. This is attributed to the rapid adsorption step followed by a slow crystallization process in the kinetics of alkanethiol adsorption onto the Au electrode. For a well-assembled thiol monolayer on gold, a can approach 0.99, which means that the number of collapsed sites is not large and the surface of the monolayer Au electrode is rather smooth.

Characterization of defects in the formation process of self-assembled thiol monolayers by electrochemical impedance spectroscopy

Abstract Self-assembled monolayers of octadecanethiol (ODT) on gold have been studied by electrochemical impedance spectroscopy (EIS). The fractional coverage, pinhole size and separation have been examined as a function of immersion time of Au in the ODT deposition solution. The fractional coverage of ODT monolayer increases sharply from zero to more than 99% of its maximum within the first minute. However, it takes hours or a day for the fractional coverage to approach its final value. The pinhole separation increases from ca. 1.8 μm to 53 μm as the assembling time increases from 5 s to 24 h, indicating that the number of pinholes decreases. The pinhole radius increases slowly from ca. 0.3 μm to 1.6 μm in the same time interval. The slight trend of increasing pinhole size with adsorption time is attributed to the increasing influence of collapsed sites in the ODT monolayer.

Assessing the apparent effective thickness of alkanethiol self-assembled monolayers in different concentrations of Fe(CN)63-/Fe(CN)64- by ac impedance spectroscopy

Abstract The concept of an apparent effective thickness of a self-assembled monolayer is established and assessed with the measurements of ac impedance spectroscopy in different concentrations of Fe(CN) 6 3− /Fe(CN) 6 4− by alkanethiol self-assembled monolayers on a gold electrode. The complex-plane impedance plots for SAMs in solution of various concentrations of Fe(CN) 6 3− /Fe(CN) 6 4− exhibit arc shapes. The apparent resistance ( R 2 ), obtained from the simulation, decreased with increasing concentration of the redox couples. It is determined that the apparent effective thickness of the SAMs is lower than the length of chain of the alkanethiol. Defect sites may exist at the alkanethiol monolayers.

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.

Cyclic voltammetry and a.c. impedance studies of Ca2+-induced ion channels on Pt-BLM

Abstract Self-assembled phosphatidylcholine/cholesterol bilayer lipid membrane (BLM) was fabricated on Pt electrode, which had been cleaned by high power ultrasonic horn. The influence of Ca2+ ions on the electron transfer of Fe(CN)63−/4− couple on Pt supported BLM (Pt-BLM) was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments showed directly that Ca2+ ions could induce electron transfer on Pt-BLM. Three possibilities of the cause of redox reaction induced by Ca2+ ions were discussed: (1) electronic tunneling across bilayer, (2) electronic tunneling across monolayer, (3) direct electron transfer between Fe(CN)63−/4− ions and Pt electrode through Ca2+ induced ion-channels. Results of calculations based on Marcuss theory indicate that electronic tunneling across bilayer and monolayer contributes little to the redox current. The theory of partially blocked electrode was employed to analyze the sigmoidal CV curves, and it was concluded that ion-channels generated by Ca2+ ions were the main cause of electron transfer reaction on Pt-BLM. EIS measurement showed that Pt-BLM became porous after Ca2+ ions were added to the electrolyte. This was consistent with the conclusion drawn from CV experiments. Since ion-channels play the role similar to micro defects, it is concluded from CV and EIS experiments that sigmoidal CV curves and depressed circular arcs are evidence for the existence of micro defects on unmodified solid-supported-BLM.

Unmodified supported thiol/lipid bilayers: studies of structural disorder and conducting mechanism by cyclic voltammetry and AC impedance

Supported thiol/lipid bilayer assembly, one of the most spectacular bilayer systems in recent years, has provided a good model to study biomembranes because of its high mechanical stability. In this work, the structural and conducting property of unmodified Au supported octadecanethiol/phosphatidylcholine bilayers were investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The forming process of bilayer was monitored by capacitance plane plot. The normalized membrane capacitance of supported bilayer is 0.52 microF cm(-2). Kinetically controlled voltammograms determined by Butler-Volmer equation were obtained for both thiol monolayer and thiol/lipid bilayer in linear sweep voltammetry. Results of EIS experiment indicate that collapsed sites and pinhole defects exist in thiol monolayer and lipid monolayer, respectively. The difference between the values of experimental and theoretical standard electron transfer rate constant indicates that the conducting mechanism of Au supported thiol monolayer is electron tunneling at collapsed sites. The conducting mechanism of Au supported thiol/lipid bilayer is attributed as the following: the electroactive species could diffuse through pinholes in the lipid monolayer and reach collapsed sites in thiol monolayer, where electron transfer occurs via a tunneling process. The fractional coverage of the lipid monolayer measure by EIS experiments is about 0.98 or higher.

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.

Ca2+ induced Fe(CN)63−/4− electron transfer at Pt supported BLM electrode

The effect of Ca2+ on the electron transfer of Fe(CN)63−/4− couple at Pt supported lecithin (PC)/cholesterol (CH) bilayer lipid membrane (BLM) electrode has been studied by voltammetry and ac impedance spectroscopy. Experimental results suggest that the interaction of Ca2+ with the BLM can open some kind of channel for Fe(CN)63−/4− ions, allowing increased access of the redox couple to the electrode surface. This phenomenon may be related to the Ca2+ regulation or activation effect of ion channel in some biomembranes.

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.