Enzyme electrodes uric acid

Biosensors fabricated on the Nafion and polyion-modified palladium strips are reported by C.-J. Yuan [193], They found that Nafion membrane is capable of eliminating the electrochemical interferences of oxidative species (ascorbic acid and uric acid) on the enzyme electrode. Furthermore, it can restricting the oxidized anionic interferent to adhere on its surface, thereby the fouling of the electrode was avoided. Notably, the stability of the proposed PVA-SbQ/GOD planar electrode is superior to the most commercially available membrane-covered electrodes which have a use life of about ten days only. Compared to the conventional three-dimensional electrodes the proposed planar electrode exhibits a similar... 

How analytical methods deal with interferences is one of the more ad hoc aspects of method validation. There is a variety of approaches to studying interference, from adding arbitrary amounts of a single interferent in the absence of the analyte to establish the response of the instrument to that species, to multivariate methods in which several interferents are added in a statistical protocol to reveal both main and interaction effects. The first question that needs to be answered is to what extent interferences are expected and how likely they are to affect the measurement. In testing blood for glucose by an enzyme electrode, other electroactive species that may be present are ascorbic acid (vitamin C), uric acid, and paracetamol (if this drug has been taken). However, electroactive metals (e.g., copper and silver) are unlikely to be present in blood in great quantities. Potentiometric membrane electrode sensors (ion selective electrodes), of which the pH electrode is the... 

Such interference falls into two classes competitive substrates and substances that either aaivate or inhibit the enzyme. With some enzymes, such as urease, the only substrate that reacts at reasonable rate is urease hence, the urease-coated electrode is specific for use (59, 165). Likewise, uricase acts almost specifically on uric acid (167), and aspartase on aspartic acid (8, 168). Others, such as penicillinase and amino oxidase, are less specific (63,169,170). Alcohol oxidase responds to methanol, ethanol, and allyl alcohol (171, 172). Hence, in using electrodes of these enzymes, the analyte must be separated if two or more are present (172). Assaying L-amino acids by using either the decarboxylative or the deaminating enzymes, each of which acts specifically on a different amino... 

The enzyme electrodes for determining uric acid monitor reactants or products of the uricase catalyzed reaction ... 

Using platinum electrodes (167, 238) requires +0.6 V versus SCE to oxidize H2O2. However, this potential precludes selective measurements of uric acid because it is also oxidized at the electrode surface (167). Thus, to improve the selectivity, bienzyme amperometric devices using a redox mediator (hexa-cyanoferrate) have been constructed (239). The enzymes uricase and peroxid ise are immobilized together and the hexacyanoferrate(III) is measured at 0.0 V versus Ag/AgCl. Alternatively, a carbon dioxide selective electrode is used for the detection of the enzymatically liberated CO2 (240, 241). 

An amperometric uric acid biosensor based on functionalized MWCNTs with Sn02 nanoparticles has been developed by Zhang et al. This MWCNTs-Sn02 electrode acts as an efficient promoter, and the system exhibits a linear dependence for the uric acid concentration over the range from 1.0 x 10"7 to 5.0 x 10 4 mol L 1. This biosensor exhibits high sensitivity of the MWCNTs-Sn02 modified enzyme electrode. This electrode has been used to monitor trace levels of uric acid in dialysate samples in rat striatum [104],... 

The enzymes have been both physically entrapped in polyacrylamide on nylon netting and chemically bound to polyacrylic acid derivatives both preparations exhibited large measuring times. Improvement of the system in favour of the response time diminished the sensitivity of the sensor. The authors reported a response time between 77 and 235 s and a sensitivity of 40 mV per concentration decade. Besides the low selectivity of the iodide sensitive electrode (thiocyanate, sulfide, cyanide, and silver(I) ions interfere), disturbances by other HRP substrates, e.g. uric acid, ascorbic acid, and Fe(II) ions, restrict the applicability of the method. 

The fluoride formed was indicated at a fluoride sensitive electrode. The substrate, 4-fluorophenol, has a high reaction rate and a favorable diffusion behavior. The authors observed no interferences by ascorbic acid or uric acid glutathione interfered above 0.87 mg/ml. When immobilized by glutaraldehyde crosslinking with BSA, the enzymes were stable for 30 days. 

Laccase also catalyzes the 02-dependent oxidation of ascorbic acid, ferrocyanide, iodide, and uric acid. These reactions have been utilized to eliminate electrochemical interferences in amperometric hydrogen peroxide detection at membrane-covered enzyme electrodes (Wollen-berger et al., 1986). The capacity of the laccase membrane to oxidize ferrocyanide has been characterized by anodic oxidation of ferrocyanide at +0.4 V (Fig. 62). When a fresh enzyme membrane is used, a current signal appears only at substrate concentrations above 5 mmolA the current increases linearly with increasing concentration. This threshold concentration decreases with increasing membrane age until the remaining enzyme activity is too low for complete substrate oxidation. 

Fig. 128. Manual glucose analyzer Glukometer GKM (Zentrum ftir Wissenschaftlichen Gerfltebau, Academy of Sciences of the GDR) consisting of a thermostat furnished with measuring cell and enzyme electrode (left), sample dispenser (center), and electronics (adapted to glucose, uric acid, and lactate measurement, right).

The ExAn enzyme electrode-based analyzer developed by Kulys et al. (1983) is also appropriate for the measurement of uric acid. The characteristics of the device are presented in Table 8 (Section 3.1.11). 

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