Poly coated electrode responses

FIGURE 4-18 Permselective coatings flow injection response of a poly(l,2-diaminoben-zene)-coated electrode to the following a, hydrogen peroxide (1 mM) b, ascorbic acid (1 mM) c, uric acid (1 mM) d, L-cysteine (1 mM) and e, control human serum. (Reproduced with permission from reference 63.)... 

A cell with a small Pt disk working electrode coated with a polyelectrolyte multilayer made of poly(allylamine)-poly(vinyl sulfate), a Pt wire counterelectrode and a reference SCSE may be used for selective amperometric determination of H2O2, in the presence of ascorbic acid (22), uric acid (29) and acetaminophen (148). The latter three compounds show a significant response with the bare working electrode at +0.6 V while a practically nil one with the coated electrode. The reason for this selectivity may be an exclusion effect by the coating. ... 

Figure 3. Current response of a poly(l,2-diaminobenzene)-coated electrode to hydrogen peroxide (a), ascorbic acid (b), uric acid (c), cysteine (d), and control human serum (e). Flow injection amperometric operation. (Reproduced from ref. 18. Copyright 1990 American Chemical Society.)...

Fig. 2.17. Plots of the current at +0.1 V for a poly(aniline)/poly(vinylsulfonate)-coated glassy carbon electrode (deposition charge 150 mC, geometric area 0.38 cm2) rotated at 9 Hz in 0.1 mol dm- 1 citrate/phosphate buffer at pH 7 as a function of the NADH concentration showing the stability of the electrode response. Four replicate calibration curves recorded in succession over 4h using the same electrode are shown ( ) run 1 ( ) run 2 (A) run 3 and (O) run 4. The solid line is drawn as a guide for the eye.

NADH oxidation to function equally well for the oxidation of NADPH. Figure 2.22 shows a direct comparison of the responses of a poly(aniline)-coated electrode to NADH and NADPH. At low concentrations, the currents are identical within experimental error but at higher concentration (in this case above 0.6 mmol dm-3), the currents for NADPH fall below those for NADH. These results clearly show some saturation of the current at high NADPH concentrations. 

Much more effectual and very often applied are polymer-coated electrodes. Especially electrochemical polymerization is an attractive method for the immobilization of redox enzymes at electrode surfaces, and/or accumulation of electroactive reactants. An approximative analytical treatment of the response of an amperometric enzymatic electrode leading to plots of fluxes and concentration profiles has been made in [14]. The electron transport through poly-4-vinylpyridine and polystyrene-sulfonate films (widely used for immobilization of redox centers on electrodes) has been studied in [15]. 

TH" attached to poly(V-methylolacrylamide-co-acrylic acid), etc., was coated on a Pt electrode. Its cyclic voltammogram showed the formation of a complex between the coated dye and ferric/ferrous cyanide present in the solution. When the electrode is illuminated in a Fe aqueous solution, cathodic polarization at the coated electrode is observed, in contrast to the bare electrode dipped in a mixture of TH and Fe ", which gave an anodic response at the electrode" . It was proposed that, for the polymer-coated electrode, the excited states of TH and Fe " form a complex, which is stabilized by the polymer network and accepts an electron from the electrode. A flash photolysis study showed the formation of such a complex... 

The pH response of this sensor has been compared to that of bare glassy carbon and polyphenol-coated electrodes (see Figure 3), both of which show a nonlinear change in potential with pH over the pH 2 to 12 range. The response of such films has been attributed to proton diffusion to the underlying electrode surface, as evidenced by their slow response times. Since the poly[Co(p-OH)TPP] electrodes respond linearly to pH in a much faster manner (response times less than 10 s), a selective interaction between the protons and porphyrin is suspected. In such a case. 

A poly(aniline boronic acid)-based conductimetric sensor for dopamine consisting of an interdigitated microarray electrode coated with poly(aniline boronic acid) has also been developed by the Fabre team. The sensor was found to show a reversible chemoresistive response to dopamine without interference by ascorbic acid from their mixtures.42... 

Solid-state ISEs with conducting polymers are also promising for low-concentration measurements [60,63,74], even below nanomolar concentrations [60,74], which gives rise to optimism concerning future applications of such electrodes. In principle, the detection limit can be improved by reducing the flux of primary ions from the ion-selective membrane (or conducting polymer) to the sample solution, e.g., via com-plexation of primary ions in the solid-contact material. For example, a solid-state Pb2+-ISEs with poly(3-octylthiophene) as ion-to-electron transducer coated with an ion-selective membrane based on poly(methyl methacrylate)/poly(decyl methacrylate) was found to show detection limits in the subnanomolar range and a faster response at low concentrations than the liquid-contact ISE [74]. 

Coated-wire electrodes (CWEs), introduced by Freiser in the mid-1970s, are prepared by coating an appropriate polymeric film directly onto a conductor (Fig. 5.18). The ion-responsive membrane is commonly based on poly(vinyl... 

Khalil and El-Aliem constructed a coated-wire benazepril-selective electrode based on incorporation of the benazepril-tetraphenyl borate ion pair in a poly (vinyl chloride) coating membrane [13]. The influence of membrane composition, temperature, pH of the test solution, and foreign ions on the electrode performance was investigated. At 25° C, the electrode showed a Nernstian response over a benazepril concentration... 

Coated-wire electrode — A polymer film containing an ion-responsive material and a binder, e.g., poly(vinyl)chloride, is coated onto a conductor (e.g., a metal wire or graphite). They show useful response to solution concentrations of measured species in the range 10-5 < c < 0.1 mol dm-3. The processes at the metal polymer interface are still not understood. 

N. Oyama and F.C. Anson, Factors affecting the electrochemical responses of metal complex at pyrolytic graphite electrodes coated with films of poly(4-vinylpyridine), J. Electrochem. Soc., 1980, 127, 640-647. 

Factors affecting the electrochemical responses of metal complexes at pyrolytic graphite electrodes coated with films of poly(4-vinylpyridine). 

Dubois et al. (21,22) have showed that the electrochemical oxidation of phenol euid its derivatives, on metal surfaces, produced hydrophobic, adherent, euid insulating polymer films of uniform thickness. Both Yacynych and Mark (17). and Helneman et al. (13) showed the oxidation of 1,2-diaminobenzene to be irreversible, and with successive cyclic voltammetric scans formed an insulating polymer film completely covering the electrode surface. Heineman et al. ( 22) further showed that 1,2-diaminobenzene forms a polymeric film over a pH range of 4 to 10, and that platinum electrodes coated with the poly(1,2-diaminobenzene) provided a nearly Nernstian response to pH. Cheek et al. (J[6) studied the pH response of platinum and vitreous carbon with polymer films of either 1,2-diaminobenzene or phenol. These polymer films are selective enough to allow the permeation of protons, while limiting access to larger molecules, which could be potential interferents. 

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