Cellulose acetate membranes enzyme stability

A three-enzyme electrode system, such as needed for creatinine measurement, poses a more difficult enzyme-immobilisation problem, in that different enzymes have different immobilisation requirements and their microenvironmental interrelationships need to be optimised. For one creatine sensor, the requisite creatine amidinohydrolase and sarcosine oxidase were immobihsed in polyurethane pre-polymer and PEG-hnked creatinine amidohydrolase was attached via diisocyanate pre-polymer to create a polyurethane adduct [14]. The likelihood of enzyme inactivation with chemical immobih-sation is high, but provided an enzyme preparation survives this, long-term stability is feasible. In the case of these three particular enzymes, a loss of activity resulted from silver ions diffusing from the reference electrode the material solution was to protect the enzyme layer with a diffusion-resisting cellulose acetate membrane. 

D-glucose and the three-enzyme system GOx, mutarotase and invertase for sucrose estimation. A common format was adopted to facihtate design and operation, in this case immobilization method, the fact that all enzymes used were oxidases and that a common detection principle, reoxidation of H2O2 generated product, was chosen (except for ascorbic acid which was estimated directly). Pectin, a natural polysaccharide present in plant cells, was used as a novel matrix to enhance enzyme entrapment and stabilization in the sensors. Interferences related to electrochemi-caUy active compounds present in fruits under study were significantly reduced by inclusion of a suitable cellulose acetate membrane diffusional barrier or by enzymatic inactivation with ascorbate oxidase. Enzyme sensors demonstrated expected response with respect to their substrates, on analyte average concentration of 5 mM. 

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]. 

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