PtO electrodes

The transfer of the rSi-N02 group to an electrode interface offered the possibility of a series of comparisons in reactivity between interface and solution sites and the appealing possibility that immobilization might inhibit oxidation of the nitro group because of the bimoleeulap nature of the reaction (Scheme 3). The complex [Ru(bpy)2(N02)(4-pyC02H)]+ was successfully attached to a silanized Pt/PtO electrode using the procedure described in eq. 3... 

Subsequently we found that iridium reacts easily with silanes even without preanodization, giving persistent films of monolayer coverage with reproducible electrochemical properties (6). By contrast, the more widely studied Pt/PtO electrode requires a lengthy anodization pretreatment before derivatization, and gives films of variable and unpredictable coverage (7,.8,9) ... 

The cyclic voltammograms of underivatized Ir and Pt/PtO electrodes in 0.2 M TEAP /acetonitrile are shown in Figure 1. A comparison of the two electrodes shows that Ir has a 200 mv. wider potential window between the points where electrolyte breakdown occurs, and 30 percent less residual (capacitive) current than Pt/PtO. The cyclic voltammogram of an underivatized AIROF electrode is shown in Figure 2, It differs from "clean" iridium in having a slightly smaller potential window and considerably greater (and thickness dependent) capacitive current (for example, a 125 nm. film had approximately six times more residual current than bare Ir). 

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 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. 

To characterize the membrane-coated PtO electrodes we have chosen a potential window from 0.3 to 0.6 V where PtO is electrochemically stable and passive according to the cyclic voltammograms in Figure 4. The pH was 6.8. 

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. 

Capacitance values of the PtO, Cox, from the best curve fit is lower for the membrane-coated electrode than for the bare PtO electrode. Also the resistance, Rox, is higher for the membrane-coated electrode than for the... 

At frequencies below 63 Hz, the double-layer capacitance began to dominate the overall impedance of the membrane electrode. The electric potential profile of a bilayer membrane consists of a hydrocarbon core layer and an electrical double layer (49). The dipolar potential, which originates from the lipid bilayer head-group zone and the incorporated protein, partially controls transmembrane ion transport. The model equivalent circuit presented here accounts for the response as a function of frequency of both the hydrocarbon core layer and the double layer at the membrane-water interface. The value of Cdl from the best curve fit for the membrane-coated electrode is lower than that for the bare PtO interface. For the membrane-coated electrode, the model gives a polarization resistance, of 80 kfl compared with 5 kfl for the bare PtO electrode. Formation of the lipid membrane creates a dipolar potential at the interface that results in higher Rdl. The incorporated rhodopsin may also extend the double layer, which makes the layer more diffuse and, therefore, decreases C. ... 

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... 

The important fact found was the relatively high PtO solubility, therefore, platinum gas oxygen electrode, sometimes considered as metal-oxide one, cannot be used for measurement of pO in strongly acidic media because of complete dissolution of the oxide film over its surface. Pt PtO electrode was attempted for pO index measurements in buffer solutions NP/NiO. ... 

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