Composite enzyme electrodes

A. Guzman-Vazquez de Prada, N. Pena, C. Parrado, A.J. Reviejo and J.M. Pingarron, Amperometric multidetection with composite enzyme electrodes, Talanta, 62(5) (2003) 896-903. 

Potentiometric electrodes also can be designed to respond to molecules by incorporating a reaction producing an ion whose concentration can be determined using a traditional ion-selective electrode. Gas-sensing electrodes, for example, include a gas-permeable membrane that isolates the ion-selective electrode from the solution containing the analyte. Diffusion of a dissolved gas across the membrane alters the composition of the inner solution in a manner that can be followed with an ion-selective electrode. Enzyme electrodes operate in the same way. 

Good accuracy and precision were obtained in connection to 100 pL blood samples. A wide range of amperometric enzyme electrodes, differing in the electrode design or material, membrane composition, or immobilization approach have since been described. 

B. Wang and S.J. Dong, Organic-phase enzyme electrode for phenolic determination based on a functionalized sol-gel composite. J. Electroanal. Chem. 487, 45-50 (2000). 

A model of such structures has been proposed that captures transport phenomena of both substrates and redox cosubstrate species within a composite biocatalytic electrode.The model is based on macrohomo-geneous and thin-film theories for porous electrodes and accounts for Michaelis—Menton enzyme kinetics and one-dimensional diffusion of multiple species through a porous structure defined as a mesh of tubular fibers. In addition to the solid and aqueous phases, the model also allows for the presence of a gas phase (of uniformly contiguous morphology), as shown in Figure 11, allowing the treatment of high-rate gas-phase reactant transport into the electrode. 

Composite potentiometric sensors involve systems based on ion-selective electrodes separated from the test solution by another membrane that either selectively separates a certain component of the analyte or modifies this component by a suitable reaction. This group includes gas probes, enzyme electrodes and other biosensors. Gas probes are discussed in this section and chapter 8 is devoted to potentiometric biosensors. 

Because each enzyme sensor has its own unique response, it is necessary to construct the calibration curve for each sensor separately. In other words, there is no general theoretical response relationship, in the same sense as the Nernst equation is. As always, the best way to reduce interferences is to use two sensors and measure them differentially. Thus, it is possible to prepare two identical enzyme sensors and either omit or deactivate the enzyme in one of them. This sensor then acts as a reference. If the calibration curve is constructed by plotting the difference of the two outputs as the function of concentration of the substrate, the effects of variations in the composition of the sample as well as temperature and light variations can be substantially reduced. Examples of potentiometric enzyme electrodes are listed in Table 6.5. 

Sulfite modified enzyme electrode. (2) L-Lactate/L-malate/ sulfite multibiosensor L-lactate dehydrogenase/L-malate dehydrogenase/ sulfite oxidase surface-modified enzyme electrodes/enzymes were deposited on the composite electrodes and covered with a dialysis membrane ... 

Myler S, Eaton S, Higson S. Poly(o-phenylenediamine ultra-thin polymer-film composite membranes for enzyme electrodes. Analytica Chimica Acta 1997, 357, 55-61. 

Redox enzymes are the active component in many electrochemical enzyme electrode biosensor devices.1821 The integration of two different redox enzymes with an electrode support, in which one of the biocatalysts is photoswitchable between ON and OFF states, can establish a composite multisensor array. The biomaterial interface that includes the photoswitchable enzyme in the OFF state electrochemi-cally transduces the sensing event of the substrate corresponding to the nonphoto-switchable enzyme. Photochemical activation of the light-active enzyme leads to the full electrochemical response, corresponding to the analysis of the substrates of the two enzymes. As a result, the processing of the signals transduced by the composite biomaterial interface in the presence of the two substrates permits the assay of the... 

Binyamin, Chen and Heller reported that wired enzyme electrodes constituted of glassy carbon electrodes coated with poly(4-vinylpyridine) complexed with [Os(bpy)2Cl] and quarternized with 2-bromoethylamine or poly[(iV-vinylimidazole) complexed with [Os(4,4 -dimethyl-2,2 -bypyridine)2Cl] or poly(vinylpyridine) complexed with [Os(4,4 -dimethoxy-2,2 -bypyridine)2Cl] quaternized with methyl groups lost their electrocatalytic activity more rapidly in serum or saline phosphate buffer (pH 7.2) in the presence of urate and transitional metal ions such as Zn and Fe " " than in plain saline phosphate buffer (pH 7.2). It was reported that as much as two-thirds of the current is lost in 2 h in some anodes. However, when a composite membrane of cellulose acetate, Nafion, and the polyaziridine-cross-linked co-polymer of poly(4-vinyl pyridine) quaternized with bromoacetic acid was applied, the glucose sensor stability in serum was improved and maintained for at least 3 days [27,50]. 

Another important category of composite electrode sensors is that of enzyme electrodes. In these systems, the analyte is brought into contact with an enzyme immobilized on the surface of the sensor. The analyte then undergoes a catalytic reaction to yield a species for which an ISE is sensitive, for example, ammonia, carbon dioxide, or H" ions. 

The best-known enzyme electrode is that used to analyze for urea in blood. The enzyme urease is immobilized in a polyacrylamide hydrophilic gel and fixed at the bottom of a glass electrode whose characteristics make it an NH4 ISE. Alternatively, the ISE can be a composite system designed to detect NH3. In the presence of the enzyme, urea is hydrolyzed according to the reaction... 

Wang B, Zhang J, Cheng G et al. Amperometric enzyme electrode for the determination of hydrogen peroxide based on sol-gel/hydrogel composite film. Analytica Chimica Acta 2000 407 111-118. 

The search for enzyme electrodes whose modification matrices exhibit better mechanical and chemical properties, such as cohesion, adhesion, air stability, flexibility, selectivity, and sensitivity, has led to the design and application of composite enzyme films. Composite polymer films used in biosensors have two principal profiles, irrespective of whether the film formation is exclusively electro-... 

---