No abstract

A single-use, reagentless, screen-printed biosensor strip for the measurement of free cholesterol was fabricated and its performance characteristics were evaluated by cyclic voltammetry and under steady-state conditions. Cholesterol oxidase was adsorbed on the surface of an H202-sensing cobalt phthalocyanine screen-printed carbon electrode and retained at the sensory interface with a solvent-cast cellulose acetate membrane. The device, operated in the amperometric mode, provides an inexpensive, reliable and rapid means of quantifying this clinically significant sterol. Its analytical utility is further augmented by its ease of fabrication and favourable storage properties. The biosensor exhibits a wide functional range (6 X 10-5-5.0 x 10-3 mol dm-3) and the sensitivities of the devices (slopes of the calibration graphs) were used to derive optimum solution conditions. Several analytical parameters were also investigated to optimize and enhance the response time and sensitivity of the sensor and these are also discussed.

Systematic voltammetric and amperometric studies have been undertaken to examine the electrochemical behaviour of uric acid at bare and surface-modified screen-printed electrodes. The precision of the electrode manufacture was determined by cyclic voltammetry with a 1 .O x 10-4 rnol dm-3 uric acid solution and was calculated to be 6.0% ( n = 5 ) . Several strategies were investigated in an attempt to eliminate interference from ascorbic acid. These involved coating the electrode surface with Nafion, or the enzyme L-ascorbic acid oxidase. The latter was immobilized using one of two methods: either by a simple adsorption process, or by cross-linking with bovine serum albumin and glutaraldehyde. The amperometric response at the surfaceadsorbed enzyme electrode for uric acid was linear over the concentration range from 5.08 x 10-6 to 1.51 x 10-4 rnol dm-3; the limit of detection was 2.54 x 10-6 rnol dm-3 using a full-scale deflection of 0.5 pA. This modified electrode exhibited no response to ascorbic acid at levels up t o 0.53 mmol dm-3. The electrode modified by cross-linking the enzyme to the surface showed no response to ascorbic acid concentrations of up to 0.093 mmol dm-3.

Amperometry in stirred solution has been used for the systematic evaluation of modified screen-printed carbon electrodes (SPCEs) with a view to developing a reagentless biosensor for uric acid. The developed system consists of a base cobalt phthalocyanine (CoPC) electrode tailored to the electrocatalytic oxidation of H2O2 by means of a cellulose acetate (CA)-uricase bilayer. Uricase was immobilized by drop-coating the enzyme onto the CA membrane covering the CoPC-SPCE. The device exploits the near-universal H2O2-generating propensity of oxidases, the permselectivity of the CA film towards H2O2 and the electrocatalytic oxidation of this product at the CoPC-SPCE. The electrochemical oxidation of the resulting Co+ species was used as the analytical signal, facilitating the application of a greatly reduced operating potential when compared with that required for direct oxidation of H2O2 at unmodified electrodes. The time required to achieve 95% of the steady-state current (t95i(ss)) was 44 s [relative standard deviation = 7.5% (n = 10)]. Amperometric calibrations were linear over the range from 13 x 10(-6) to 1 x 10(-3) mol dm-3, with the former representing the limit of detection. The CA membrane extended the linear range of the biosensor by over two orders of magnitude, when apparent Michaelis-Menten constants (Km') of immobilized and free enzymes are compared. This suggests that the process is diffusion-controlled and not governed by the kinetics of the enzyme. The precision of electrode fabrication was determined by cyclic voltammetry to be 4.9% (n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

A method for the rapid and simple determination of paracetamol in urine is described. The transducer is a reagentless, amperometric sensor that was developed through the combined use of screen-printing and permselective membrane technologies. The sensor operates selectively by means of an anti-interference barrier, cellulose acetate, which is drop-coated directly on to the screen-printed carbon electrodes. The disposable, amperometric sensor require no further modification procedures to enhance selectivity, i.e., no enzymes are involved, and hence their fabrication is simple and extremely economical. Given the rapid response times, 13 X 10-6 mol dm-3 limit of detection and wide functional range, linear up to 2 x 10-3 mol dm-3, they represent an attractive alternative for paracetamol monitoring. The surface-modified strips were applied to the detection of paracetamol in urine, correlating well with a standard enzymwolour reagent kit.