Mediatorless electrocatalytic reduction of hydrogen peroxide at graphite electrodes chemically modified with peroxidases

Monofunctional pyrenes at carbon nanotube electrodes for direct electron transfer H2O2 reduction with HRP and HRP-bacterial nanocellulose

The non-covalent modification of carbon nanotube electrodes with pyrene derivatives is a versatile approach to enhance the electrical wiring of enzymes for biosensors and biofuel cells. We report here a comparative study of five pyrene derivatives adsorbed at multi-walled carbon nanotube electrodes to shed light on their ability to promote direct electron transfer with horseradish peroxidase (HRP) for H₂O₂ reduction. In all cases, pyrene-modified electrodes enhanced catalytic reduction compared to the unmodified electrodes. The pyrene N-hydroxysuccinimide (NHS) ester derivative provided access to the highest catalytic current of 1.4 mA cm^-2 at 6 mmol L^-1 H₂O₂, high onset potential of 0.61 V vs. Ag/AgCl, insensitivity to parasitic H₂O₂ oxidation, and a large linear dynamic range that benefits from insensitivity to HRP "suicide inactivation" at 4-6 mmol L^-1 H₂O₂. Pyrene-aliphatic carboxylic acid groups offer better sensor sensitivity and higher catalytic currents at ≤ 1 mmol L^-1 H₂O₂ concentrations. The butyric acid and NHS ester derivatives gave high analytical sensitivities of 5.63 A M^-1 cm^-2 and 2.96 A M^-1 cm^-2, respectively, over a wide range (0.25-4 mmol^-1) compared to existing carbon-based HRP biosensor electrodes. A bacterial nanocellulose pyrene-NHS HRP bioelectrode was subsequently elaborated via "one-pot" and "layer-by-layer" strategies. The optimised bioelectrode exhibited slightly weaker voltage output, further enhanced catalytic currents, and a major enhancement in 1-week stability with 67% activity remaining compared to 39% at the equivalent electrode without nanocellulose, thus offering excellent prospects for biosensing and biofuel cell applications.

Electrochemical Biosensors Employing Natural and Artificial Heme Peroxidases on Semiconductors

Heme peroxidases are widely used as biological recognition elements in electrochemical biosensors for hydrogen peroxide and phenolic compounds. Various nature-derived and fully synthetic heme peroxidase mimics have been designed and their potential for replacing the natural enzymes in biosensors has been investigated. The use of semiconducting materials as transducers can thereby offer new opportunities with respect to catalyst immobilization, reaction stimulation, or read-out. This review focuses on approaches for the construction of electrochemical biosensors employing natural heme peroxidases as well as various mimics immobilized on semiconducting electrode surfaces. It will outline important advances made so far as well as the novel applications resulting thereof.

A Novel Conductive Poly(3,4-ethylenedioxythiophene)-BSA Film for the Construction of a Durable HRP Biosensor Modified with NanoAu Particles

In this study, we have investigated the contribution of bovine serum albumin (BSA) to the durability of the electrochemically synthesized poly(3,4-ethylenedioxythiophene) (PEDOT) film on a platinum (Pt) electrode. The electrode was capable to effectively adsorb the nano Au particles (AuNPs) to form a uniform layout, which was then able to immobilize the horseradish peroxidase (HRP) to construct a functional HRP/AuNPs/PEDOT(BSA)/Pt biosensor. Cyclic voltammetry was employed to evaluate the performance of the biosensor through the measurement of hydrogen peroxide. Our results revealed a satisfied linear correlation between the cathodic current and the concentration of H₂O₂. Furthermore, the addition of oxidized form of nicotinamide adenine dinucleotide, or NAD⁺, as the electron transfer mediator in the detection solution could dramatically enhance the sensitivity of detection by about 35.5%. The main advantages of the current biosensor are its durability, sensitivity, reliability, and biocompatibility.

Molecularly Imprinted Electropolymer for a Hexameric Heme Protein with Direct Electron Transfer and Peroxide Electrocatalysis

For the first time a molecularly imprinted polymer (MIP) with direct electron transfer (DET) and bioelectrocatalytic activity of the target protein is presented. Thin films of MIPs for the recognition of a hexameric tyrosine-coordinated heme protein (HTHP) have been prepared by electropolymerization of scopoletin after oriented assembly of HTHP on a self-assembled monolayer (SAM) of mercaptoundecanoic acid (MUA) on gold electrodes. Cavities which should resemble the shape and size of HTHP were formed by template removal. Rebinding of the target protein sums up the recognition by non-covalent interactions between the protein and the MIP with the electrostatic attraction of the protein by the SAM. HTHP bound to the MIP exhibits quasi-reversible DET which is reflected by a pair of well pronounced redox peaks in the cyclic voltammograms (CVs) with a formal potential of -184.4 ± 13.7 mV vs. Ag/AgCl (1 M KCl) at pH 8.0 and it was able to catalyze the cathodic reduction of peroxide. At saturation the MIP films show a 12-fold higher electroactive surface concentration of HTHP than the non-imprinted polymer (NIP).

Interprotein Coupling Enhances the Electrocatalytic Efficiency of Tobacco Peroxidase Immobilized at a Graphite Electrode

Covalent immobilization of enzymes at electrodes via amide bond formation is usually carried out by a two-step protocol, in which surface carboxylic groups are first activated with the corresponding cross-coupling reagents and then reacted with protein amine groups. Herein, it is shown that a modification of the above protocol, involving the simultaneous incubation of tobacco peroxidase and the pyrolytic graphite electrode with the cross-coupling reagents produces higher and more stable electrocatalytic currents than those obtained with either physically adsorbed enzymes or covalently immobilized enzymes according to the usual immobilization protocol. The remarkably improved electrocatalytic properties of the present peroxidase biosensor that operates in the 0.3 V ≤ E ≤ 0.8 V (vs SHE) potential range can be attributed to both an efficient electronic coupling between tobacco peroxidase and graphite and to the formation of intra- and intermolecular amide bonds that stabilize the protein structure and improve the percentage of anchoring groups that provide an adequate orientation for electron exchange with the electrode. The optimized tobacco peroxidase sensor exhibits a working concentration range of 10-900 μM, a sensitivity of 0.08 A M(-1) cm(-2) (RSD 0.05), a detection limit of 2 μM (RSD 0.09), and a good long-term stability, as long as it operates at low temperature. These parameter values are among the best reported so far for a peroxidase biosensor operating under simple direct electron transfer conditions.

Forchlorfenuron detection based on its inhibitory effect towards catalase immobilized on boron nitride substrate

An enzymatic procedure based on a catalase biosensor for the detection of forchlorfenuron (CPPU) has been reported in this work. Catalase was immobilized on boron nitride (BN) sheets dispersed in chitosan by adsorption. The immobilized catalase exhibited direct electron transfer character and excellent electrocatalytic activity towards H2O2 reduction. After introducing CPPU into the H2O2 containing phosphate buffer solution, the catalase-catalyzed H2O2 reduction current decreased. By measuring the current decrease, CPPU can be determined in the range of 0.5-10.0 µM with the detection limit of 0.07 μM. The non-competitive inhibition behavior of CPPU towards catalase was verified by the Lineweaver-Burk plots. Long stability character has been ascribed to this biosensor. Possible use of this biosensor in flow systems is illustrated. The proposed biosensor has been successfully applied to CPPU determination in fruits samples with satisfactory results.

Peroxide-dependent analyte conversion by the heme prosthetic group, the heme Peptide "microperoxidase-11" and cytochrome C on chitosan capped gold nanoparticles modified electrodes

In view of the role ascribed to the peroxidatic activity of degradation products of cytochrome c (cyt c) in the processes of apoptosis, we investigate the catalytic potential of heme and of the cyt c derived heme peptide MP-11 to catalyse the cathodic reduction of hydrogen peroxide and to oxidize aromatic compounds. In order to check whether cyt c has an enzymatic activity in the native state where the protein matrix should suppress the inherent peroxidatic activity of its heme prosthetic group, we applied a biocompatible immobilization matrix and very low concentrations of the co-substrate H₂O₂. The biocatalysts were entrapped on the surface of a glassy carbon electrode in a biocompatible chitosan layer which contained gold nanoparticles. The electrochemical signal for the peroxide reduction is generated by the redox conversion of the heme group, whilst a reaction product of the substrate oxidation is cathodically reduced in the substrate indication. The catalytic efficiency of microperoxidase-11 is sufficient for sensors indicating HRP substrates, e.g., p-aminophenol, paracetamol and catechol, but also the hydroxylation of aniline and dehalogenation of 4-fluoroaniline. The lower limit of detection for p-aminophenol is comparable to previously published papers with different enzyme systems. The peroxidatic activity of cyt c immobilized in the chitosan layer for catechol was found to be below 1 per mill and for p-aminophenol about 3% as compared with that of heme or MP-11.

Control of protein immobilization: coupling immobilization and site-directed mutagenesis to improve biocatalyst or biosensor performance

Mutagenesis and immobilization are usually considered to be unrelated techniques with potential applications to improve protein properties. However, there are several reports showing that the use of site-directed mutagenesis to improve enzyme properties directly, but also how enzymes are immobilized on a support, can be a powerful tool to improve the properties of immobilized biomolecules for use as biosensors or biocatalysts. Standard immobilizations are not fully random processes, but the protein orientation may be difficult to alter. Initially, most efforts using this idea were addressed towards controlling the orientation of the enzyme on the immobilization support, in many cases to facilitate electron transfer from the support to the enzyme in redox biosensors. Usually, Cys residues are used to directly immobilize the protein on a support that contains disulfide groups or that is made from gold. There are also some examples using His in the target areas of the protein and using supports modified with immobilized metal chelates and other tags (e.g., using immobilized antibodies). Furthermore, site-directed mutagenesis to control immobilization is useful for improving the activity, the stability and even the selectivity of the immobilized protein, for example, via site-directed rigidification of selected areas of the protein. Initially, only Cys and disulfide supports were employed, but other supports with higher potential to give multipoint covalent attachment are being employed (e.g., glyoxyl or epoxy-disulfide supports). The advances in support design and the deeper knowledge of the mechanisms of enzyme-support interactions have permitted exploration of the possibilities of the coupled use of site-directed mutagenesis and immobilization in a new way. This paper intends to review some of the advances and possibilities that these coupled strategies permit.

Determination of glutathione in hemolysed erythrocyte with amperometric sensor based on TTF-TCNQ

BACKGROUND

GSH has a relevant role in human metabolism as an indicator of disease risks. An amperometric sensor for glutathione (GSH) determination is described as an alternative method featuring simple construction procedure and short time analysis.

METHOD

The developed sensor was used to determine glutathione at low potential using a TTF-TCNQ complex.

RESULTS

The sensor exhibits a linear response range from 5 to 340 micromol/l under applied potential of 200 mV vs. SCE. The sensitivity and detection limit were 90.1 microA l/mmol cm(2) and 0.3 micromol/l, respectively.

CONCLUSION

When the sensor was tested in hemolysed erythrocyte samples for GSH determination, a good correlation in results was observed between the sensor and the spectrophotometric method. The sensor showed recovery values between 98% and 102%.

Direct electron transfer reactions of laccases from different origins on carbon electrodes

Electrochemical studies of laccases from basidiomycetes, i.e., Trametes hirsuta, Trametes ochracea, Coriolopsis fulvocinerea, Cerrena maxima, and Cerrena unicolor, have been performed. Direct (mediatorless) electrochemistry of laccases on graphite electrodes has been investigated with cyclic voltammetry, square wave voltammetry as well as potentiometry. For all mentioned high potential laccases direct electron transfer (DET) has been registered at spectrographic graphite and highly ordered pyrolytic graphite electrodes. The characteristics of DET reactions of the enzymes were analysed under aerobic and anaerobic conditions. It is shown that the T1 site of the laccase is the primary electron acceptor, both in solution (homogenous case) and at surface of the graphite electrode (heterogeneous case). A mechanism of ET for the process of the electro-reduction of oxygen at the laccase-modified graphite electrodes is proposed and the similarity of this heterogeneous process to the laccase catalysed oxygen reduction homogeneous reaction is concluded.

Amperometric biosensor based on horseradish peroxidase for biogenic amine determinations in biological samples

An amperometric biosensor for total biogenic amine determinations, using a carbon paste electrode modified with horseradish peroxidase (HRP) enzyme is described. The HRP immobilization on graphite was made using bovine serum albumin, carbodiimide and glutaraldehyde. The biosensor response was optimized using serotonin and it presented the best performance in 0.1 mol l(-1) phosphate buffer (pH=7.0) containing 10 micromol l(-1) of hydrogen peroxide. Under optimized operational conditions at -50 mV versus SCE, a linear response range from 40 to 470 ng ml(-1) was obtained. The detection limit was 17 ng ml(-1) and the response time was 0.5s. The proposed sensor presented a stable response during 4h under continuous monitoring. The difference of the response between six sensor preparations was <2%. The sensor was applied in the determination of total biogenic amines (neurotransmitters) in rat blood samples with success, obtaining a recovery average of 102%.

Bienzyme HRP–GOx-modified gold nanoelectrodes for the sensitive amperometric detection of glucose at low overpotentials

Gold nanotubular electrode ensembles were prepared by using electroless deposition of the metal within the pores of polycarbonate track-etched membranes. Mono-enzyme (GOx) and monolayer/bilayer bienzyme (GOx/HRP) bioelectrodes were prepared by immobilizing the enzymes onto gold nanotubes surfaces modified with mercaptoethylamine. Batch amperometric responses to glucose for the different bioelectrodes were determined and compared. The response of the two geometries (monolayer and bilayer) of the bienzyme electrodes was shown to vary with regard to sensitivity at detection potentials above 0V. On the contrary, at detection potentials below 0V, no noticeable influence of the configuration of the bienzyme on the response intensity was observed. The mono-enzyme (650 microAmM-1 in benzoquinone (BQ) at -0.8 V versus Ag/AgCl) and the two bienzyme bioelectrodes (+/-400 microAmM-1 in hydroquinone (H2Q) at -0.2V versus Ag/AgCl) display remarkable sensitivities compared to a classical GOx-modified gold macroelectrode (13 microAmM-1 in BQ at -0.8 V versus Ag/AgCl). A remarkable feature of the bienzyme electrodes is the possibility to detect glucose at very low applied potentials where the noise level and interferences from other electro-oxidizable compounds are minimal. Another important characteristic of the monolayer bienzyme electrode is the possible existence of a direct electronic communication between HRP and the transducer surface.

Electrocatalytic oxidation of ascorbate by heme-FeIII/heme-FeII redox couple of the HRP and its effect on the electrochemical behaviour of an l-lactate biosensor

The measurements of L-lactate using the carbon paste electrode modified with lactate oxidase (LOD), horseradish peroxidase (HRP) and ferrocene (FcH) operating at low working potential in flow injection mode showed that the intensity as well as the shape of peaks were dependent on the concentration of the reducing species present in samples (e.g. ascorbate) even at low operating potentials (-200 to 0 mV vs. Ag/AgCl). The mechanism of the electrochemical contribution of ascorbate to the L-lactate response was examined by using cyclic voltammetry, hydrodynamic voltammetry and FIA results. Comparative studies showed that HRP was catalytically active for the oxidation of ascorbate leading to a decrease in the cathodic electrochemical signal of L-lactate. The results of our investigation postulated that the direct electron transfer from the HRP-Fe(III)/HRP-Fe(II) redox couple to the electrode surface was involved in the electrocatalytic oxidation of ascorbate at the electrode surface.

Bienzyme biosensors for glucose, ethanol and putrescine built on oxidase and sweet potato peroxidase

Amperometric biosensors for glucose, ethanol, and biogenic amines (putrescine) were constructed using oxidase/peroxidase bienzyme systems. The H(2)O(2) produced by the oxidase in reaction with its substrate is converted into a measurable signal via a novel peroxidase purified from sweet potato peels. All developed biosensors are based on redox hydrogels formed of oxidases (glucose oxidase, alcohol oxidase, or amine oxidase) and the newly purified sweet potato peroxidase (SPP) cross-linked to a redox polymer. The developed electrodes were characterized (sensitivity, stability, and performances in organic medium) and compared with similarly built ones using the 'classical' horseradish peroxidase (HRP). The SPP-based electrodes displayed higher sensitivity and better detection limit for putrescine than those using HRP and were also shown to retain their activity in organic phase much better than the HPR based ones. The importance of attractive or repulsive electrostatic interactions between the peroxidases and oxidases (determined by their isoelectric points) were found to play an important role in the sensitivity of the obtained sensors.

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 Au and the enzyme was studied in the reaction of the bioelectrocatalytic reduction of H(2)O(2), at -50 mV versus Ag/AgCl, 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 H(2)O(2) due to its bioelectrocatalytic reduction based on direct (mediatorless) 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 H(2)O(2) 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 H(2)O(2) (10 nM) paves the way to develop the P-chip (peroxidase chip)--a biosensors system of a microscopic size for a mediatorless detection of H(2)O(2) based on direct ET between Au and the recombinant forms of HRP.

Supercritical fluid extraction of a lignocellulosic hydrolysate of spruce for detoxification and to facilitate analysis of inhibitors

This work describes a novel approach to detoxify lignocellulosic hydrolysates and facilitate the analysis of inhibitory compounds, namely supercritical fluid extraction (SFE). The efficiency of the fermentation of lignocellulosic dilute-acid hydrolysates depends upon the composition of the hydrolysate and the organism used. Furthermore, it has been shown that inhibitors in the hydrolysate reduce the fermentation yield. This knowledge has given rise to the need to identify and remove the inhibiting compounds. Sample clean-up or work-up steps, to provide a clean and concentrated sample for the analytical system, facilitate the characterization of inhibitors, or indeed any compound in the hydrolysates. Removal of inhibitors was performed with countercurrent flow supercritical fluid extraction of liquid hydrolysates. Three different groups of inhibitors (furan derivatives, phenolic compounds, and aliphatic acids) and sugars were subsequently analyzed in the hydrolysate, extracted hydrolysate, and extract. The effect of the SFE treatment was examined with respect to fermentability with Saccharomyces cerevisiae. Not only did the extraction provide a clean and concentrated sample (extract) for analysis, but also a hydrolysate with increased fermentability as well as lower concentrations of inhibitors such as phenolics and furan derivatives.

Direct voltammetry and catalysis with Mycobacterium tuberculosis catalase-peroxidase, peroxidases, and catalase in lipid films

Stable films of dimyristoylphosphatidylcholine and M. tuberculosis catalase-peroxidase (KatG), several peroxidases, myoglobin, and catalase showed reversible FeIII/FeII voltammetry on pyrolytic graphite electrodes and catalytic current for hydrogen peroxide and oxygen. Amperometric responses for these films to H2O2 at 0 V are likely to contain significant contributions from catalytic reduction of oxygen produced during the catalytic cycles. Relative apparent turnover rates at pH 6 based on steady-state currents at 0 V versus SCE in the presence of H2O2 were in the order horseradish peroxidase > cytochrome c peroxidase (CcP) > soybean peroxidase > myoglobin > KatG > catalase. Lower currents for the very efficient peroxide scavengers KatG and catalase may be related to the instability of their compounds I in the presence of H2O2. KatG catalyzed the electrochemical reduction of oxygen more efficiently than catalase and CcP but less efficiently than the other peroxidases. DMPC films incorporating glucose oxidase and peroxidases gave good analytical responses to glucose, demonstrating the feasibility of dual enzyme-lipid films for biosensor fabrication.

Mediatorless biosensor for H(2)O(2) based on recombinant forms of horseradish peroxidase directly adsorbed on polycrystalline gold

Four forms of horseradish peroxidase (HRP) have been used to prepare peroxidase-modified gold electrodes for mediatorless detection of peroxide: native HRP, wild type recombinant HRP, and two recombinant forms containing six-His tag at the C-terminus and at the N-terminus, respectively. The adsorption of the enzyme molecules on gold was studied by direct mass measurements with electrochemical quartz crystal microbalance. All the forms of HRP formed a monolayer coverage of the enzyme on the gold surface. However, only gold electrodes with adsorbed recombinant HRP forms exhibited high and stable current response to H(2)O(2) due to its bioelectrocatalytic reduction based on direct electron transfer between gold and HRP. The sensitivity of the gold electrodes modified with recombinant HRPs was in the range of 1.4-1.5 A M(-1) cm(-2) at -50 mV versus Agmid R:AgCl. The response to H(2)O(2) in the concentration range 0.1-40 microM was not dependent on the presence of a mediator (i.e. catechol) giving strong evidence that the electrode currents are diffusion limited. Lower detection limit for H(2)O(2) detection was 10 nM at the electrodes modified with recombinant HRPs.

Electron transfer from diamond electrodes to heme peptide and peroxidase

Direct electron transfer from boron-doped diamond electrodes to heme undecapeptide and horseradish peroxidase (HRP) was examined and evaluated for the application to H2O2 biosensors. As-grown and oxygen plasma-treated diamond electrodes on which heme peptide is adsorbed exhibited cathodic current responses to H2O2 on the basis of the direct electron transfer. In a comparative study of carbon electrodes on which heme peptide was adsorbed, an oxygen plasma-treated diamond electrode exhibited responses comparable with those of an edge-oriented pyrolytic graphite (EOPG) electrode, despite much smaller roughness. However, electron transfer to compounds I and II of HRP from the diamond electrodes was much slower than that from EOPG or glassy carbon, suggesting that the pi electrons of an sp2 carbon may play an important role in the direct electron transfer to the heme moiety of HRP. To examine the applicability of heme peptide-modified diamond electrodes to oxidase-based biosensors, anodic current responses of the oxygen plasma-treated diamond electrode to possible interfering agents, ascorbic acid and uric acid, were examined and compared with those of EOPG. Since the diamond electrode exhibited much less sensitivity to those interfering agents, the heme peptide-modified diamond electrode should be a promising H2O2 biosensor for the application to oxidase-based biosensors.

Polypyrrole-entrapped quinohemoprotein alcohol dehydrogenase. Evidence for direct electron transfer via conducting-polymer chains

It is reported for the first time that direct electron-transfer processes between a polypyrrole (PPY) entrapped quinohemoprotein alcohol dehydrogenase from Gluconobacter sp. 33 (QH-ADH) and a platinum electrode take place via the conducting-polymer network. The cooperative action of the enzyme-integrated prosthetic groups--pyrroloquinoline-quinone and hemes--is assumed to allow this electron-transfer pathway from the enzyme's active site to the conducting-polymer backbone. A hypothetical model of the electron transfer is proposed which is supported by the influence of various parameters, such as, e.g., ionic strength and nature of the buffer salts. This unusual electron-transfer pathway leads to an accentuated increase of the K M app value (102 mM) and hence to a significantly increased linear detection range of an ethanol sensor based on this enzyme.