Henrik Egesborg Hansen

Study of the new electron transfer mediators in glucose oxidase catalysis

The steady-state oxidation of glucose oxidase from Aspergillus niger by phenothiazines, phenoxazines, Wursters salts, dithia- and tetrathiaaromatic compounds, and nickelocene was investigated spectrophotometrically and electrochemically. At pH 7.0 the determined oxidation rate constants (TN/Km) vary in the range 103 to 108 M−1·s−1. For phenothiazines, phenoxazine and Wursters salts oxidation constants depend on the redox potential of the electron acceptors, and results were interpreted in the framework of the outer sphere electron transfer theory (Marcus and Sutin, Biochim. Biophys. Acta, 811 (1985) 265). The interpretation of the kinetic results concerning thiaaromatic compounds and metallocenes are complicated due to complex formation with the enzyme active center and aggregation of their oxidized form in buffer solution.

Carbon-paste biosensors array for long-term glucose measurement

Abstract An amperometric glucose biosensor based on a new mediator and carbon paste acts at 0-08 V vs. an Ag/AgCl electrode in situ and exhibits a linear response up to 16 mM of glucose. The measurements are performed on freshly renovated electrode surfaces, and every sensor can be used for 100 measurements. The long-term stable biosensors were obtained using stabilized glucose oxidase and an array of biosensors. The application of a chemometric method permits exploitation of the biosensor during a period of more than 166 days at 35°C and relative humidity 75%. The relative error of chemometric prediction was less than 5·4%, and the response changes less than 0·15% per week.

Glucose biosensor based on the incorporation of Meldola Blue

Abstract The phenoxazine compound Meldola Blue (MB) is shown to efficiently mediate the electron transfer from reduced glucose oxidase to a conventional carbon paste electrode. The mediation process is exploited for developing an amperometric biosensor for glucose, which yields a linear response to 0–25 mM glucose at an operating potential of 50 mV (vs. SCE), where interfering reactions do not occur. Experimental variables such as enzyme loading or operating potential are explored. A stable response is observed over several months.

Long-term response of an integrated carbon paste based glucose biosensor

Abstract A biosensor containing integrated reference, auxiliary and two carbon paste based bioelectrodes shows linear calibration graphs for glucose concentrations > 16 mM. The biosensor response is performed on a renovated surface at an electrode potential of 60–80 mV; the capacity of the biosensor is 100 measurements. The long-term response of the biosensor relies on a glucose oxidase stability that depends on the structure of oil, water activity and stabilizer. The response of the biosensors containing silicone oil and stabilized glucose oxidase hardly changes during a period of three months at 35 °C and a relative humidity of 75%.

Personality characteristics and epilepsy.

ABSTRACT— Patients with a long history of temporal lobe epilepsy or primary generalized epilepsy entered a questionnaire study of personality characteristics, based on a modification of the Bear‐Fedio inventory for temporal lobe behavioural syndrome. Psoriasis patients and healthy volunteers served as controls. Four clinical meaningful dimensions of included personality traits were identified: ixoide, ideational, obsessive‐compulsive and affective features. Analyses based on the Rasch model approved of all dimensions except for affective features. The epilepsy group obtained the highest scores on all 3 dimensions, healthy volunteers the lowest, while the psoriasis group repeatedly held an intermediate position in all sets of assessment (subjects, interviewers and relatives). A logistic regression analysis showed ixoide features being most important when the entire epilepsy group was compared with other study groups, while the dimension ideational features was significant when the temporal lobe epilepsy group was entered as target group and opposed to primary generalized epilepsy. The intermediate position of the psoriasis group, however, suggests that in addition to the presence of a cerebral dysfunction in the epilepsy group, the mere presence of a chronic disorder with potential social stigmatization influences personality.

Glucose Oxidase Electrode Based on Graphite and Methylthio Tetrathiafulvalene as Mediator

Abstract A glucose sensor based on glucose oxidase and a new mediator - 4,5-dimethyl-4′-methylthio-Δ 2,2′-bi-1,3-dithiole (MTTTF) is described. The background for sensor action is the effective MTTTF cation interaction (apparent bimolecular constant (2.0+/-0.5)∗106 M−1 s−1 at 25°C and pH 7.0) with reduced glucose oxidase and the high electrochemical rate of mediator transformation. A glucose sensor was prepared by adsorbing mediator (MTTTF) and glucose oxidase on graphite rods. The sensor responds to glucose at electrode potentials higher than 50 mV vs SCE, but the maximal activity is obtained at a potential of 250 mV. In air saturated solution the electrode shows a non-linear calibration curve with a half-saturation concentration 10.4 mM and Hill coefficient 2.08 at 250 mV. Sensor response changes little at pH 6.5–8.0. The energy of activation of the sensor response calculated from the Arrhenius equation was 64.5 kJ/mol, and the temperature coefficient at 25°C was 9.2%.

Bioelectrode response modulation caused by periodical carbon paste filling process

Abstract Carbon paste electrodes (CPEs) doped with glucose oxidase showed periodical variation of responses when new surfaces of the electrode were exposed to glucose solution. The periodicity of the responses was analyzed by discrete Fourier transformation. At an enzyme activity in carbon paste (CP) of 0.11–0.18 U mg −1 the main harmonic was 3.7–6.5 mm in length (12.4–21.6 slices of the electrode). The periodical change of response correlated with the periodicity of the CPE filling process. The modulation factor, expressed as modulation amplitude divided by the mean value of the parameter, depended on enzyme activity in the CP matrix. It was 15.1% and 2.8% at enzyme activities of 0.16 and 1.28 U mg −1 , respectively. The ohmic resistance of the CPEs measured by mercury electrode also changed periodically. The modulation amplitude of resistance was almost independent of enzyme activity and it was 2.0 Ω when CP contained 0.16 and 1.28 U mg −1 enzyme activity, respectively. The periodicity of the resistance modulation also correlated with the periodicity of the electrode filling process. To explain the periodical changes of the responses of the CPEs and factors influencing the modulation factor, the bioelectrode response dependence on enzyme activity was determined and a macrokinetic model was build. The CPE responses exhibit a saturating dependence on enzyme activity, which was explained by external diffusion limitation of the process. The model of external diffusion predicts that: (i) periodical CPE response variation is determined by biocatalytical activity modulation of the CPE surface; (ii) the modulation factor is a function of enzyme activity in CP matrix. The surface activity variation is related to pinhole formation on the electrode surface and coverage of the electrode by the pasting liquid. The surface coverage depends on the variation of local CP composition caused by periodical CP pressing, during the filling process.