Permselective electrode layers

CA films by using the phase inversion process. These CA films were cast from solvent/nonsolvent solutions to yield size exclusion membranes consisting of a thin permselective outer layer and a more porous sublayer. These membranes permitted the rapid permeation of a 1500-dalton poly (ethylene glycol) ester of ferrocene however the reproducibility of results presents a problem with these CA mem-branes. Christie et demonstrated that thin films of plasticized polyvinylchloride (PVC), normally used for potentiometric ion-selective electrode applications, applied to electrodes over a polycarbonate dialysis membrane offered improved selectivity ratios for the amperometric detection of phenolic compounds and H2O2 in the presence of the common biological interferents, ascorbic acid and uric acid, over those observed at the dialysis membrane alone or at a composite dialysis/membrane. 

A more simplified model was presented in Ref. 10, where the membrane was assumed to be perfectly permselective toward the counter-ion, and the salt concentration in the macropores of the electrode assumed to be unvarying in time. A basic element in the modeling of the membranes in MCDI is that in the membrane the cation concentration is different from the anion concentration, with the difference compensated by the fixed membrane charge density, X. Except for this difference, the same ion transport model can be used as in free solution (Nemst-Planck equation), thus, with ions moving under the influence of a concentration gradient and because of an electrical field (electromigration). At the edges of the membranes, a Donnan potential difference develops between the outside solution and inside the membrane. For more information on MCDI, see Section 15.4.3, where a porous electrode is modeled which has an ideally permselective membrane layer in front. 

In potentiometric biosensors the biological recognition reaction causes a modulation of a redox potential, a transmembrane potential, or the activity of an ion. So the potentiometric biosensors utilize the measurement of a potential at an electrode in reference to another electrode (Bard and Faulkner, 1980). Mostly, it is comprised of a permselective outer layer and membrane or sensitive surface to a desired species (a bioactive material), usually an enzyme. The enzyme-catalyzed reaction generates or consumes a species, which is detected by an ion-selective electrode. Usually a high-impedance voltmeter is used to measure the electrical potential difference or electromotive force (EMF) between two electrodes at near-zero current. The basis of this type of biosensor is the Nemst equation, which relates the electrode potential (E) to the concentration of the oxidized and reduced species. For the reaction aA + ne bB, the Nemst equation can be described as the following. 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

Additional improvements can be achieved through the use of multilayers (based on different overlaid films). Such combination of the properties of different films has been documented with bilayers of Nation/CA (14) and Nafion/collagen (29). The former allows selective measurements of the neurotransmitter dopamine in the presence of the slightly larger epinephrine and the anionic ascorbic acid (Figure 5). In addition to bilayers, mixed (composite) films, such as PVP/CA (75) or polypyrrole/Eastman Kodak AQ (30) layers can offer additional permselectivity advantages, such composites exhibit properties superior to those of their individual components. Also promising are sensor arrays, based on electrodes coated with... 

Fig. 2. Schematic structure of a Clark-type enzyme electrode A, electrode body B, working electrode (Pt) C, reference electrode (Ag/AgCl) D, inner gas-permselective membrane (polypropylene) E, enzyme layer F, outer membrane.

The direct fixation of the biocatalyst to the sensitive surface of the transducer permits the omission of the inactive semipermeable membranes. However, the advantages of the membrane technology are also lost, such as the specificity of permselective layers and the possibility of affecting the dynamic range by variation of the diffusion resistance. Furthermore, the membrane technology has proved to be useful for reloading reusable sensors with enzyme. In contrast, direct enzyme fixation is mainly suited to disposable sensors. This is especially valid for carbon-based electrodes, metal thin layer electrodes printed on ceramic supports, and mass-produced optoelectronic sensors. Field effect transistors may also be envisaged as basic elements of disposable biosensors. 

Cyclic voltammograms of luminol at different electrodes in 0.1 mol/L phosphate buffer (PBS, pH=7.5) were obtained (Fig. 3A). Similarly, an enhancement of redox current from the analyte was observed at the PDDA-chitosan modified GCE, compared with the response at the bare GCE. This is due to the good permselectivity and highly positive charge density of the PDDA-chitosan composite layer. The negatively charged luminol could be easily absorbed on the surface of modified GCE through electrostatic interaction, which was supported by the linear increase of oxidation current vs. scan rates. 

Electrolyte selection is very similar to what is described for the 3-electrode cell. Since the catalyst layer is separated from the aqueous electrolyte solution by a permselective ionomer membrane, only water and protons can transport through the membrane to reach the catalyst layer. This helps minimize or eliminate the adverse impact of anion adsorption (onto the catalyst surface) on the reaction kinetics. 

Fujiwara et al. (2011) showed the possibility of preparing reversible air electrodes that can be used in metal-air storage batteries or unitized regenerative fuel cells. To reduce the impact of atmospheric carbon dioxide the reversible air electrodes were integrated with a polymer anion-exchange membrane which was placed between the cathode s catalytic layer (with Pt and Pt-Ir catalysts) and the alkaline solution. The membrane with anion permselectivity presumably inhibited the permeation of COj cations to the air electrode and thus suppressed precipitation of carbonates in pores of the air electrode. 

Composites can also be formed by sequential, layer-by-layer construction of mnl-tilayered materials. The silicate or an Ormosil thin film is used to bridge the different layers and immobilize the enzyme. In many cases (e.g., [100,154,155]), a mediator layer is first deposited on the electrode, and then layers containing the enzyme, the silicate, and sometimes another layer of permselective Nafion are deposited to minimize anionic interferences. 

To obtain the total impedance of the electrode/film/solution system, it is necessary to take into account the double-layer capacitances at each interface. Therefore, the impedance of thin permselective films depends on both the electron and the ion transfer elementary impedances because the bulk film impedance, Z2 (co), is negligible. 

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