Uric acid amperometric detection

Amperometric detection can be then applied for the determination of analytes such as glucose [131-137], ascorbic acid [138-141], uric acid [142,143], amino acids 1144—1491, peptides or DNA [150,151]. Con-ductimetric detection has been also employed in combination with CE microchip for amino acids [152,153], peptides/proteins [152,154,155], carbohydrates [153] or DNA [152],... 

Electrochemical transducers work based on either an amperometric, potentio-metric, or conductometric principle. Further, chemically sensitive semiconductors are under development. Commercially available today are sensors for carbohydrates, such as glucose, sucrose, lactose, maltose, galactose, the artificial sweetener NutraSweet, for urea, creatinine, uric acid, lactate, ascorbate, aspirin, alcohol, amino acids and aspartate. The determinations are mainly based on the detection of simple co-substrates and products such as 02, H202, NH3, or C02 [142]. 

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). 

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. 

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. 

Creatine and creatinine, along with p-aminohipp-uric acid and uric acid, can be determined using CE on a glass microchip with amperometric detection via screen-printed electrodes connected to an electrochemical analyzer. Creatinase, creatininase, and sarco-sine oxidase are mixed with the sample, so that the actual separation is of peroxide, p-aminohippuric acid, and uric acid. This allows total (creatine plus creatinine) to be determined. The concentration of creatine can be determined in the absence of creatininase. 

Immobilization of enzymes in the detector flow cell is of paramount importance in the performance of electrodes. The fabrication and performance of a reticulated vitreous carbon-based tyrosinase flowthrough electrode, in which the enzyme was covalently immobilized, was tested as an amperometric detector for phenolic compounds. The measurement of uric acid in plasma can be carried out through closed-loop FIA using a co-immobilized enzyme flow cell and chemiluminescence detection. 

New developments in this area include uric acid sensors based on the mediation of urate oxidase by a novel redox polymer, poly(N-methyl-o-phenylenediamine) [145], and by the freely diffusing mediator 1-methoxy-5-methylphenazinium [146], continued research on the direct amperometric detection of NADH [147] and the use of redox mediators [148] for dehydrogenase enzymes, to allow practical sensors that exploit this large class of enzymes, and the use of cytochrome P450-modified glassy carbon electrodes as drug metabolism biosensors [149]. 

Amperometric detection of hydrogen peroxide suffers also from interferences from other oxidizable substances present in biological fluids (ascorbic acid, glutathione, cysteine, uric acid, bilirubin). These problems are eliminated by hydrogen peroxide-permselective membranes [197, 198], by simultaneous use of a non-enzymatic membrane electrode... 

Uric acid represents the major catabolite of purine breakdown in humans. Therefore, it remains an important marker molecule for disorders associated with alterations of the plasma urea concentration such as hyperuricemia (gout), renal impairment, leukemia, ketoacidosis, Lesch-Nyhan syndrome and lactate excess. Uric acid may also act as an antioxidant in human body. Consequently its measurement for diagnosis and treatment of some disorders is routinely required. Zhang et developed a reagentless amperometric uric acid biosensor based on carboxyl modified, conductive zinc sulfide (ZnS) Qdots. The biosensor could detect uric acid without the presence of an electron mediator. The fabricated uricase/ZnS Qdot/l-cys biosensor exhibited higher amperometric response compared to the one without Qdots (uricase/l-cys biosensor). They were able to demonstrate a linear dependence on the uric acid concentration ranging from 5.0 x 10 to 2.0 x 10 mol with a detection limit of 2.0 x 10 mol at 3cr. 

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