PtO-OTS electrodes

Electrochemical Analysis of PtO-OTS Electrodes. OTS forms a neutral and low dielectric silane layer and is electrochemically inert. The OTS layer formed on Pt electrodes acts as an insulating layer to block the active Pt surface. Electrochemical examination of the PtO-OTS electrode provides information on the structure of the OTS layer on which the lipid-protein membrane is to be formed. We examined the PtO-OTS... 

The porous structure of the OTS layer on PtO was confirmed by the electrochemical impedance spectra of TCNE. Figure 2 shows the complex plane plots of TCNE reduction at PtO and PtO-OTS electrodes at —0.4 V, near the TCNE reversible potential, where Z and Z" are the real and... 

Figure 1. Cyclic voltammetry of tetracyanoethylene (TCNE) on the PtO and PtO-OTS electrodes at 40 mV/s. 5-mM TCNE in acetonitrile containing 0.1-M...

The effective area of the OTS-coated PtO electrode can be derived if the charge transfer resistance (K ) is known. Rct can be obtained from impedance data measured at a potential near the reversal potential (37, 33) Rct = RT/(nFAI0), where R is the universal gas constant, T is absolute temperature, n is the number of electrons transferred per molecule of TONE, F is Faraday s constant, I0 is the exchange current density, and A is the effective surface area. Because the impedance spectra of the PtO and PtO-OTS electrodes were measured under the same conditions, the value of Rct may be assumed to be affected only by the effective surface area. In Figure 3, the impedance data are replotted as 2 versus 1 /a)1 2, where a) is the angular frequency (2 tt/). Rct is estimated from the intercept on the Z axis by extrapolation. The Rct values are 95 and 980 fl for PtO and PtO-OTS, respectively. An OTS coverage factor, 0, can then be estimated from (1 — 0) = ct(Pto)/ ct(Pto-OTS> In is case 0 = 0.9. 

The electrochemical stability of PtO-OTS in aqueous saline solution was tested by measuring cyclic voltammograms of PtO and PtO-OTS electrodes in 0.1-M KCl, pH 6.8 (Figure 4). The electrode capacitances that correspond to the cyclic voltammograms are shown in Figure 5. These capacitances were measured at 1000 Hz. The OTS layer can be described as a planar capacitor in series with the PtO and double-layer capacitances. Formation of the OTS layer decreased the capacitance however, the PtO beneath the OTS layer was still reduced when the potential was swept below 0.3 V. This observation again is evidence that the OTS layer on PtO is somewhat porous. The onset potential of PtO reduction in aqueous solution is pH-dependent. 

Figure 5. Capacitances of the PtOy PtO-OTS, and PtO-OTS-Rh electrodes measured at 1000 Hz in O.I-M KCl containing 20-mM HEPES, pH 6.8.

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... 

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 OTS layer was chemically unstable at pH > 9. At this pH, the capacitance immediately increased to the value of the bare PtO electrode, which suggests that the OTS layer dissolves in basic solutions. The hydrolysis of silane on Si02 was previously observed in 0.1-M NaOH (24). The silane-PtO is reported to be resistant to most solvents, including dilute aqueous acid (for a few minutes) (30). Our measurements confirm the stability on PtO at neutral pH. 

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... 

---