Surface-bound membrane electrode

Figure 8. (a) Proposed equivalent circuit for surface-bound membrane electrode interface, (b) Simplified equivalent circuit valid at higher frequency region. 

In this simplified form, the membrane capacitance, Cm, is in series with Cox. Thus, Cm = CoxCt/(Ct - Cox), where Ct is the total capacitance of the surface-bound membrane electrode and Cox is the measured capacitance of the electrode before membrane formation. The membrane capacitance, Cm, can thus be estimated at a single frequency. Further, the capacitance of the tightly packed bilayer, Cbl, can be calculated from Cm if the coverage factor and the double-layer capacitance are known Cbl = (Cm — (1 — 0)Cdl)/0. 

By using the imaginary component of the measured impedance data for PtO-OTS and PtO-OTS-Rh electrodes (Table II) at a frequency of 1000 Hz (after subtracting Ru), the calculated Cm is 867 nF/cm2 and Cbl is thus 584 nF/cm2 using = 10 jiF/cm2 and 0 = 0.97, which are close to the theoretical values derived from the best curve fit simulation. We conclude that the simplified equivalent circuit may be adequate for the surface-bound membrane electrode. The thickness of the tightly packed membrane bilayer, d, can be calculated from d = e0e/Cbl, where e is the dielectric constant of... 

To assemble the surface-bound membrane structure, we first form a hydrophobic monolayer by using alkylsilanization to covalently attach long-chained hydrocarbon chains to hydroxyl groups in the oxide layer on the electrode surface. Such a hydrophobic surface can be thought of as one leaflet of a membrane bilayer. Alkylsilane-modified surfaces have been widely used as substrates for lipid monolayers deposited by Langmuir-Blodgett techniques. The lipids in these monolayers have mobilities like those of lipids... 

Model of Surface-Bound Membrane on Pt Electrode. From the foregoing discussion of experimental results we can develop a physical model of the surface-bound membrane that consists of two layers, as schematically depicted in Figure 7. The porous, hydrophobic OTS layer provides a structure to anchor the reconstituted membrane layer. Protein molecules with bound lipid may insert into the pores in the OTS layer. The... 

Figure 7. Model of a single surface-bound membrane formed by detergent dialysis on an alkylsilanated electrode surface.

An equivalent circuit can be derived for the surface-bound membrane formed in this work similar in a manner to the approach taken for porous anodic films and porous electrodes (41-46). An equivalent circuit network, proposed in Figure 8a, corresponds to the model in Figure 7. This network has three RC subnetworks that represent the oxide layer, the surface-bound membrane layer, and the double layer. Cox and Rox are the capacitance and resistance of oxide. and Rdl are the double-layer capacitance and the polarization resistance, known as the charge transfer resistance at the membrane-water interface. For the subnetwork of the surface-bound membrane layer, one branch represents a tightly packed alkylsilane and lipid bilayer in series, and the other branch represents the pores and defects through the bilayer. Calk, Clip and Ralk, Rhp are the capacitances and resistances of... 

The simulation spectra of the PtO electrodes with and without the surface-bound membrane are shown in Figure 9 for comparison with the experimental data of Figure 6. The parameters used in the simulation are listed in Table I. The first column lists the values used for curve fitting the experimental spectra, and the second column gives the corresponding values normalized for unit area. 

Surface-bound membranes formed on PtO electrodes were chemically and mechanically stable. The PtO-OTS-Rh electrodes were monitored by measuring the capacitance while the electrodes were kept in buffer at 4 °C for 11 days. Any dissolution of the surface-bound membrane would result in an increase in capacitance. Little change in capacitance was observed, which indicates that the membranes are stable. 

The simplified equivalent circuit in Figure 8b was used to evaluate surface-bound membranes on Si02, TiOa, and ITO electrodes. Figures 10 and 11 present the capacitance curves for n-Si-Si02 and TiOa electrodes with and without OTS- and rhodopsin-containing lipid membranes in KCl buffer. As with the PtO electrodes, the capacitance decreases upon formation of an OTS layer and the membrane on the oxide surface. Table II lists the... 

Electrochemical impedance spectroscopy provides a sensitive means for characterizing the structure and electrical properties of the surface-bound membranes. The results from impedance analysis are consistent with a single biomembrane-mimetic structure being assembled on metal and semiconductor electrode surfaces. The structures formed by detergent dialysis may consist of a hydrophobic alkyl layer as one leaflet of a bilayer and the lipid deposited by dialysis as the other. Proteins surrounded by a bound lipid layer may simultaneously incorporate into pores in the alkylsilane layer by hydrophobic interactions during deposition of the lipid layer. This model is further supported by the composition of the surface-bound membranes and by Fourier transform infrared analyses (9). 

For application in enzyme electrodes COD has been mainly immobilized by surface fixation (Table 7). As early as 1977, Clark had patented the polarographic analysis of free and esterified cholesterol by means of free as well as immobilized COD and CEH with anodic H2O2 indication. Cholesterol in food and serum samples has been determined by using COD bound to a collagen membrane via glutaraldehyde and coupled to a Pt electrode (Clark, 1978). A similar probe has been devised by Bertrand et al. (1979). In this sensor the enzyme membrane was not protected by a semipermeable membrane. Interferences were compensated for by difference measurements between an enzyme sensor and an enzyme-free membrane electrode. The lower detection limit was 0.05... 

We simultaneously incorporate both lipid and protein by using dialysis to remove detergent from a solubilized lipid-protein mixture in the presence of the alkylsilanated substrate. Under our conditions, from the evidence in this paper and elsewhere (9), the surface structures appear to be single bilayer membranes. Our hypothesis is that the hydrocarbon chains attached to the surface serve as initiation sites for a lipid bilayer membrane to form as the detergent is slowly removed. The model is of a membrane that is anchored to the surface by hydrophobic interactions with the surface-bound hydrocarbon layer. Integral membrane proteins are retained in these structures by their interaction with the hydrophobic core of the membrane without being directly attached to the electrode surface. 

Non-labelled immunosensors rely on various principles (Fig. 3.27.A). Either the antibody or the antigen is immobilized on the solid matrix to form a sensing device. The solid matrix should be sensitive enough at the surface to detect immunocomplex formation. Electrode, membrane, piezoelectric and optically active surfaces may in principle be used to construct non-labelled immunosensors. The antigen or antibody to be determined is dissolved in a solution and reacted with the complementary matrix-bound antibody or antigen to form an immunocomplex that alters the physical e.g. the electrode potential or intrinsic piezofrequency) or optical properties of the... 

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