Application of Clustering in the Non-Parametric Estimation of Distribution Density

This paper discusses a multimodal density function estimation problem of a random vector. A comparative accuracy analysis of some popular non-parametric estimators is made by using the Monte-Carlo method. The paper demonstrates that the estimation quality increases significantly if the sample is clustered (i.e., the multimodal density function is approximated by a mixture of unimodal densities), and later on, the density estimation methods are applied separately to each cluster. In this paper, the sample is clustered using the Gaussian distribution mixture model and the EM algorithm. The highest efficiency in the analysed cases was reached by using the iterative procedure proposed by Friedman for estimating a density component corresponding to each cluster after the primary sample clustering mentioned. The Friedman procedure is based on both the projection pursuit of multivariate observations and transformation of the univariate projections into the standard Gaussian random values (using the density function estimates of these projections)

A reagentless amperometric carbon paste based sensor for NADH

Carbon paste electrodes, based on zirconium phosphate with immobilized N-methylphenazonium or Nile blue, were prepared. The electrodes show anodic current response to NADH at an operating potential ranging from - 0.05 to 0.30 V (vs. SCE). The sensitivity of electrodes to NADH was shown to increase with increasing operation potential. It was shown that the concentration dependences for electrodes, obtained at different operating potentials. follow Michaelis-Menten scheme. confirming an electrodcatalytic mechanism for electrooxidation of NADH with a precursor complex formation. The detection limit for NADH was found to be ca. 2 mu M for both types of sensors.12319419

Effect of cysteine mutations on direct electron transfer of horseradish peroxidase on gold

Surface exposed cysteines were genetically engineered in the structure of recombinant horseradish peroxidase (rHRP). Recombinant forms of HRP with either a His-tag or a Strep-tag at the C-terminus were produced, which additionally had cysteines at positions 57, 189 or 309 (C-terminus) of the polypeptide chain. An E coli expression system was exploited. The effect of these mutations on the direct electron transfer (ET) between An and the enzyme was studied in the reaction of the bioelectrocatalytic reduction of H2O2, at -50 mV versus AgAgCl, on rHRP-modified Au electrodes placed in a wall-jet flow-through electrochemical cell. Adsorptive immobilisation of rHRPs on pre-oxidised Au from the protein solution at pH 6.0 provided a high and stable current response to H2O2 due to its bioelectrocatalytic reduction based on direct (mediatoriess) ET between Au and the active site of the rHRPs. Comparative analysis of the direct ET rate constants, estimated from the amperometric data on direct and mediated ET in the presence of catechol at pH 7.4 and 6.0, gave evidence that the introduction of the His-tag or cysteine in the C-terminal area of the enzyme resulted in an increased efficiency of direct ET due to a favourable coupled electron and proton transfer pathway. Due to the high efficiency of direct ET, the sensitivity was independent on the addition of the mediator or change of pH indicating that the response to H2O2 is determined solely by the mass transfer of the analyte to the active site of HRP. The sensitivities obtained for the Au electrodes modified with rHRPs (2.0 +/- 0.1 A M-1 cm(-2)) and the low detection limit for H2O2 (10 nM) paves the way to develop the P-chip (peroxidase chip) - a biosensors system of a microscopic size for a mediatorless detection of H2O2 based on direct ET between Au and the recombinant forms of HRP. (C) 2002 Elsevier Science B.V. All rights reserved

Direct heterogeneous electron transfer reactions of fungal laccases at bare and thiol-modified gold electrodes

Mediatorless (direct) electron transfer between bare and thiol-modified gold electrodes and fungal laccases from different sources has been demonstrated. The electrochemical activity of the enzymes from basidiomycetes Trametes hirstita. Trametes ochracea, and Cerrena maxima under aerobic and anaerobic conditions can clearly be observed using cyclic voltammetry and spectroelectrochemistry. Bioelectroreduction of oxygen by T. hirsuta laccase immobilized on amino-thiophenol-modified gold electrodes., starting at +625 mV vs. NHE, is demonstrated and differences in bioelectrocatalysis of the enzyme immobilized on bare and thiol-modified electrodes are shown. It was found that hydrogen peroxide was one of the products of oxygen electroreduction on gold electrodes modified with fungal laccases, whereas no significant peroxide formation was observed for T. hirsuta laccase immobilised on thiol-modified gold electrodes. Thus, a hypothesis about two different mechanisms of oxygen electroreduction by fungal laccases adsorbed on bare and thiol-modified electrodes is proposed. (c) 2006 Elsevier B.V. All rights reserved

Investigation of the Effect of Different Glassy Carbon Materials on the Performance of Prussian Blue Based Sensors for Hydrogen Peroxide

Three different kinds of glassy carbon (GC-R, GC-K, GC-G) were equally pretreated, further modified with electrochemically deposited Prussian Blue and used as sensors for hydrogen peroxide at an applied potential of -50 mV (vs. Ag|AgCl). Their performance was evaluated with respect to the following parameters: the coverage and electrochemistry of the electrodeposited Prussian Blue, the sensitivity and the lower limit of detection for hydrogen peroxide, and the operational stability of the sensors. GC-R showed the best behavior concerning the surface coverage and the operational stability of the electrodeposited Prussian Blue. For this electrode the sensitivity for hydrogen peroxide (10 M) was 0.25 A/M cm2 and the detection limit was 0.1 M. Scanning electron microscopy was used to study the surfaces of the three electrodes before and after the electrodeposition of Prussian Blue and to search for the reason for the three different behaviors between the different glassy carbon materials. The Prussian Blue modified GC-R was also used for the construction of a glucose biosensor based on immobilizing glucose oxidase in Nafion membranes on top of electrodeposited Prussian Blue layer. The operational stability of the glucose biosensors was studied in the flow injection mode at an applied potential of -50 mV (vs. Ag|AgCl) and alternatively injecting standard solutions of hydrogen peroxide (10 M) and glucose (1 mM) for 3 h. For the GC-R based biosensor a 2.8% decrease of the initial glucose response was observed