The response of glucose oxidase (GOx) modified poly(& phenylenediamine) coated P t disk electrodes to glucose was well-behaved with a rapid response time and displaying Michaelis-Menten kinetics. However, the glucose response was lowered in a concentration-dependent manner by ascorbic acid when the glucose calibrations were carried out in solutions containing this reducing agent. The possibility of a homogeneous redox reaction in which the H202 generated by the enzymatic oxidation of glucose at the GOx/polymer surface is consumed by ascorbate was investigated. Similar "negative" interference at GOx-modified carbon powder electrodes not involving membranes and for H202 calibrations at bare Pt electrodes supported the hypothesis. The observation that this interference could be blocked by the chelating agent EDTA suggests that the homogeneous reaction is catalyzed by trace metal ion impurities in solution. A model for the homogeneous reaction based on these experimental findings is proposed and tested by comparing quiescent and stirred solutions. No homogeneous interference by uric acid was observed. The electrodes were found to be free from lipid fouling in vitro, and experiments monitoring brain glucose levels in vivo indicate the absence of the homogeneous reaction in this environment. The results highlight the need to test each individual assay procedure involving HzO2 under relevant conditions for both positive and negative interference by ascorbic acid.The search for the ideal glucose sensor continues to be one of the main focuses of biosensor research despite 3 decades of intense investigation since the development of the first electrochemical glucose sensor by Clark and Lyons in 1962.' This is primarily due to the important role of such a glucose monitoring sensor in industrial2t3 and clinicalw applications, ranging from the analysis of fermentation media, to the development of the artificial @-cell or pancreas for the treatment of the metabolic disease diabetes mellitus.'J'Because of the complexity and serious specificity problems associated with direct electrooxidation of glucose9J0 the (11) where FAD is the oxidized form of the prosthetic group, flavin adenine dinucleotide. The development over the last 30 years of enzyme-based amperometric devices for glucose determination can essentially be divided into three categories. The first involves "classical" devices which monitored either the consumption of oxygen' or the formation of hydrogen peroxide.12 Such devices were originally affected by the ambient concentration of oxygen in the sample and required a large overpotential. In order to avoid these problems "second generation" systems were developed in which the natural dioxygen in reaction I1 is replaced by a mediat0r3-13.1~ resulting in electrodes which are relatively insensitive to changes in dioxygen tension and which, depending on the choice of mediator, can be operated at lower applied potentials. The successful application of these mediated systems relies on the appropriate choice of mediator, b...

Monitoring glucose using biosensors in biological systems is complicated by the presence of reducing agents such as ascorbic acid (AA). This is particularly so in brain extracellular fluid (ECF), where glucose concentrations may be as low as 1 mmol l-1 and AA levels are approximately 500 pmol 1-1. Since glucose oxidase-modified poly(o-pheny1enediamine)-coated Pt (Pt/PPD/GOx) electrodes show good stability in vivo, glucose sensitivity and AA-blocking properties, attempts were made to improve the latter characteristic further by two distinct strategies: incorporating non-enzyme protein into the polymer film and underlaying the polymer with a lipid coat. Both tactics significantly decreased interference by AA without changing the sensitivity to glucose, the lipid modification being the more effective. The current ratio ZGluc/ZAA for 1 mmol 1-1 glucose and 500 pmol 1-l AA for the best 50% of the lipid-modified Pt/PPD/GOx electrodes was approximately 30 : 1, indicating that these sensors are well suited for monitoring brain glucose in vivo.

No abstract

The finding that a 'first generation' glucose oxidase modified poly( o-phenylenediamine) coated Pt electrode, designed t o detect H202' responded t o glucose in N2-saturated solutions with a sensitivity similar to that of air-saturated media is of considerable significance for the application of biosensors in biological systems where O2 availability is severely restricted.Ortho-Phenylenediamine (PD , 1,2-diaminobenzene) forms a self-sealing, highly insulating thin (ca. 10 nm) membrane containing trapped enzyme molecules when electropolymerised onto a Pt anode in enzyme-electrolyte solution.*,* When glucose oxidase (GOx) is used, the GOx-modified polyphenylenediamine (PPD) coated Pt (Pt/PPD/GOx) electrodes are considered 'first generation' sensors as they detect glucose by oxidising H202 formed in the presence of the natural co-substrate for GOx, dioxygen [reactions.(1)-(3)].G O X F A D H~ + 0 2 -+ GOx/FAD + HzO?A number of different laboratories have demonstrated that Pt/PPD/GOx electrodes possess a variety of properties indicating potential suitability for monitoring glucose levels in biomedical applications. 1-10 These properties include fast response time, linearity over the relevant range of concentration, effective elimination of interference by reducing agents such as ascorbic acid, freedom from protein and lipid fouling, stability in vivo, and ease of miniaturisation. However, since the mechanism of electrochemical signal generation involves oxidation of H202 [reaction (3)] formed from the reaction of O2 with reduced enzyme [reaction (2)], changes in ambient oxygen tension may mimic changes in glucose concentration, undermining the reliability of the sensor to monitor glucose unambiguously. We therefore investigated the sensitivity of Pt/PPD/GOx electrodes to glucose for different concentrations of O2 in solution.GOx was immobilised in poly(o-phenylenediamine) films by potentiostatic electropolymerisation of the monomer on the bare disk end of a freshly cut Teflon-insulated Pt wire (125-250 pm diameter) as described in detail recently.8Briefly, a deoxygenated solution of the monomer (300 mmol dm-3) and GOx ( 5 mg cm-3) was prepared in phosphate buffered saline (PBS, pH 7.4). The working electrode potential was maintained at +0.65 V vs. SCE during the electropolymerisation for 15 min using a large Pt wire as auxiliary electrode. All experiments using these Pt/PPD/GOx electrodes were performed in a standard three-electrode glass electrochemical cell containing 20 ml PBS thermostated at 25 5 1 "C. To attain effective anaerobic conditions, all solutions were vigorously purged with 02-free N2 (average O2 content 2 ppm, maximum O2 content 5 ppm) for at least 30 min before recording began and a N2 atmosphere maintained over the cell thereafter. In experiments involving solution O,, either atmospheric air or pure O2 from a gas cylinder was bubbled through the PBS. The mean k standard error is reported with n = number of electrodes or number of electrodes times determinations. Background current recorded in PBS wi...

No abstract