NADH and glutamate on-line sensors using Os-gel-HRP/GC electrodes modified with NADH oxidase and glutamate dehydrogenase

Glutamate biosensors based on diamond and graphene platforms

l-Glutamate is one of the most important neurotransmitters in the mammalian central nervous system, playing a vital role in many physiological processes and implicated in several neurological disorders, for which monitoring of dynamic levels of extracellular glutamate in the living brain tissues may contribute to medical understanding and treatments. Electrochemical sensing of glutamate has been developed recently mainly using platinum, carbon fibre and carbon nanotube electrodes. In the present work, we explore the fabrication and properties of electrochemical glutamate sensors fabricated on doped chemical vapour deposition diamond electrodes and graphene nanoplatelet structures. The sensors incorporate platinum nanoparticles to catalyse the electrooxidation of hydrogen peroxide, glutamate oxidase to oxidise glutamate, and a layer of poly-phenylenediamine to impart selectivity. The performance of the devices was compared to a similar sensor fabricated on glassy carbon. Both the diamond and the graphene sensor showed very competitive performance compared to the majority of existing electrochemical sensors. The graphene based sensor showed the best performance of the three investigated in terms of sensitivity, linear dynamic range and long term stability, whereas it was found that the diamond device showed the best limit of detection.

Stabilization of Prussian blue with polyaniline and carbon nanotubes in neutral media for in vivo determination of glucose in rat brains

This study demonstrates a new electrochemical microbiosensor for selectivein vivomonitoring of glucose in rat brains.

High-sensitive glutamate biosensor based on NADH at Lauth's violet/multiwalled carbon nanotubes composite film on gold substrates

A highly sensitive amperometric L-glutamate biosensor based on the electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide has been developed on Lauth's Violet (known as thionine)/multiwalled carbon nanotubes (Th-MWCNTs) composite film, which is used as a mediator and an enzyme immobilization matrix. The glutamate biosensor, which is fabricated by immobilizing glutamate dehydrogenase (GLDH) on the surface of Th-MWCNTs, displayed a precipitous response (ca. 3 s), a low detection limit (15.9 nM), a wide linear dynamic range (0.1 to 500 microM), and high sensitivity of 281.6 microAmM(-1) cm(-2), higher biological affinity, as well as good stability and repeatability. Interferences from other biological compounds were also studied for the fabricated sensor. The Th-MWCNTs system exemplifies a simple and efficient approach to the assimilation of GLDH and electrodes, which can provide analytical access to a large group of enzymes for wide range of bioelectrochemical applications in health care fields.

Low potential detection of glutamate based on the electrocatalytic oxidation of NADH at thionine/single-walled carbon nanotubes composite modified electrode

A glutamate biosensor based on the electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH), which was generated by the enzymatic reaction, was developed via employing a single-walled carbon nanotubes/thionine (Th-SWNTs) nanocomposite as a mediator and an enzyme immobilization matrix. The biosensor, which was fabricated by immobilizing glutamate dehydrogenase (GlDH) on the surface of Th-SWNTs, exhibited a rapid response (ca. 5s), a low detection limit (0.1 microM), a wide and useful linear range (0.5-400 microM), high sensitivity (137.3+/-15.7) microA mM(-1)cm(-2), higher biological affinity, as well as good stability and repeatability. In addition, the common interfering species, such as ascorbic acid, uric acid, and 4-acetamidophenol, did not cause any interference due to the use of a low operating potential (190 mV vs. NHE). The biosensor can be used to quantify the concentration of glutamate in the physiological level. The Th-SWNTs system represents a simple and effective approach to the integration of dehydrogenase and electrodes, which can provide analytical access to a large group of enzymes for wide range of bioelectrochemical applications including biosensors and biofuel cells.

A biosensor based on co-immobilized l-glutamate oxidase and l-glutamate dehydrogenase for analysis of monosodium glutamate in food

A monosodium glutamate (MSG) biosensor made by co-immobilized L-glutamate oxidase (L-GLOD) and L-glutamate dehydrogenase (L-GLDH) as the bio-component based on substrate recycling for highly sensitive MSG or L-glutamate determination, has been developed. Regeneration of MSG by substrate recycling provided an amplification of the sensor response. Higher signal amplification was found in the presence of ammonium ion. The sensor was standardized to determine MSG in the range of 0.02-3.0 mg/L. Linearity was obtained from 0.02 to 1.2 mg/L in presence of ammonium ion (10 mM) and NADPH (reduced nicotinamide adenine dinucleotide phosphate) (2 mM), but in absence of L-GLDH, the detection limit of MSG is confined to 0.1 mg/L. The apparent Km for MSG with L-GLOD-L-GLDH coupled reaction was 0.4451 mM but 1.9222 mM when only L-GLOD was immobilized. Cross linking with glutaraldehyde in the presence of bovine serum albumin (BSA) as a spacer molecule has been used for the method of immobilization. The response time of the sensor was 2 min. The optimum pH and temperature of the biosensor has been determined as 7+/-2 and 25+/-2 degrees C, respectively. The enzyme immobilized on the membrane was used for over 50 measurements. The standard error of the sample measurement was 4-5%. The activity of the enzyme-immobilized membrane was tested over a period of 60 days.

Mediatorless voltammetric oxidation of NADH and sensing of ethanol

A simple, selective and sensitive method for the detection of NADH and ethanol is presented. Self-assembled monolayers (SAMs) of mercaptopyrimidine (MPM) and their derivatives, thiocytosine (TC) and 4,6-diamino-2-mercaptopyrimidine (DMP) on gold (Au) electrode are used for the voltammetric detection of NADH and ethanol in neutral aqueous solution. A decrease of 200-300 mV in the overpotential associated with an observable increase in the peak current was obtained for the oxidation of NADH on MPM and TC monolayer-modified electrodes without any redox mediator. The facilitated electron transfer for the oxidation of NADH at the TC monolayer is ascribed to the existence of stable cationic p-quinonoid form of TC. The electrode modified with DMP monolayer could not exhibit stable response for NADH owing to the fouling of electrode surface. The MPM and TC monolayer-modified electrodes show high selectivity and excellent sensitivity (MPM: 0.633+/-0.005 microA cm(-2) microM(-1); TC: 0.658+/-0.008 microA cm(-2) microM(-1)) towards NADH with detection limit (3sigma) of 2.5 and 0.5 microM, respectively. Presence of large excess of ascorbate (AA) does not interfere the detection of NADH and the monolayer-modified electrode shows individual voltammetric peaks for AA and NADH. Voltammetric sensing of ethanol using alcohol dehydrogenase on MPM and TC monolayer-modified electrode is successfully demonstrated and these electrode can detect as low as 0.5 mM ethanol in neutral pH. The sensitivity of the MPM and TC monolayer-modified electrodes toward ethanol was found to be 3.24+/-0.03 and 3.435+/-0.04 microA cm(-2) mM(-1), respectively.

Purification and characterization of an NADH oxidase from extremely thermophilic anaerobic bacterium Thermotoga hypogea

Thermotoga hypogea is an extremely thermophilic anaerobic bacterium capable of growing at 90 degrees C. It was found to be able to grow in the presence of micromolar molecular oxygen (O2). Activity of NADH oxidase was detected in the cell-free extract of T. hypogea, from which an NADH oxidase was purified to homogeneity. The purified enzyme was a homodimeric flavoprotein with a subunit of 50 kDa, revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It catalyzed the reduction of O2 to hydrogen peroxide (H2O2), specifically using NADH as electron donor. Its catalytic properties showed that the NADH oxidase had an apparent Vmax value of 37 micromol NADH oxidized min(-1) mg(-1) protein. Apparent Km values for NADH and O2 were determined to be 7.5 microM and 85 microM, respectively. The enzyme exhibited a pH optimum of 7.0 and temperature optimum above 85 degrees C. The NADH-dependent peroxidase activity was also present in the cell-free extract, which could reduce H2O2 produced by the NADH oxidase to H2O. It seems possible that O2 can be reduced to H2O by the oxidase and peroxidase, but further investigation is required to conclude firmly if the purified NADH oxidase is part of an enzyme system that protects anaerobic T. hypogea from accidental exposure to O2.

Biosensors for real-time in vivo measurements

The current status of sensors capable of continuous measurement of analytes in biological media is reviewed. This review containing 173 references deals with devices whose use in single cells, tissue slices, animal models and humans has been demonstrated. In addition to sensors specific for glucose, lactate, glutamate, pyruvate, choline and acetylcholine, insights obtained from monitoring nitric oxide, Na(+), K(+), Ca(2+), and dopamine are presented. Performance criteria for sensor performance are described as is the subject of biosensor calibration. Biocompatibility issues are dealt with in some detail as is the status of continuous blood glucose monitoring in humans.

Hydrogels of a conducting conjugated polymer as 3-D enzyme electrode

We have utilized the highly conducting poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) aqueous dispersion (PEDOT/PSS) to build a conducting hydrogel matrix. Together with appropriate biomolecules this constitutes a hydrogel bio-electrode. The open hydrogel structure makes diffusion of analytes surrounding the cells into the matrix electrode easier. If enzymes are utilized, osmium is used as mediator between the prosthetic group of the enzyme and the conducting polymer matrix. Osmium also functions as a crosslink point to poly-4-vinylpyridine, which together with the magnesium crosslinked PEDOT/PSS gives a rigid hydrogel. The enzyme Horseradish peroxidase (HRP) was used as a model enzyme to evaluate the enzyme-enhanced electrode. We evaluated the electrode at pH 7, which is the pH choice for many biological systems. From cyclic voltammetry (CV) measurements we deduced that a very low reduction potential was needed to reduce the prosthetic group. Constant potential amperometry were performed to demonstrate the biosensor capabilities. A differential sensitivity of 0.13 A M(-1) cm(-2) through the 0-30 microM concentration range was achieved. Both the biostability and the influence on conductivity, important aspects when for example making nerve- or cell-electrodes, were investigated.

Molecular characterization of H2O2-forming NADH oxidases from Archaeoglobus fulgidus

Three NADH oxidase encoding genes noxA-1, noxB-1 and noxC were cloned from the genome of Archaeoglobus fulgidus, expressed in Escherichia coli, and the gene products were purified and characterized. Expression of noxA-1 and noxB-1 resulted in active gene products of the expected size. The noxC gene was expressed as well but the protein produced showed no activity in the standard Nox assay. NoxA-1 and NoxB-1 are both FAD-containing enzymes with subunit molecular masses of 48 and 69 kDa, respectively. NoxA-1 exists predominantly as homodimer, NoxB-1 as monomer. NoxA-1 and NoxB-1 showed pH optimum of 8.0 and 6.5, with specific NADH oxidase activities of 5.8 U.mg-1 and 4.1 U.mg-1, respectively. Both enzymes were specific for NADH as electron donor, but with different apparent Km values (NoxA-1, 0.13 mm; NoxB-1, 0.011 mm). The apparent Km values for oxygen differed significantly (NoxA-1, 0.06 mm; NoxB-1, 2.9 mm). In contrast with all mesophilic homologues, both enzymes were found to produce predominantly H2O2 instead of H2O. Despite apparent similarities, NoxB-1 is essentially different from NoxA-1. Whereas NoxA-1 resembles typical H2O-producing Nox enzymes that are expected to have a role in oxidative stress defence, NoxB-1 belongs to a small group of enzymes that is involved in catalysing the reduction of unsaturated acids and aldehydes, suggesting a role in fatty acid oxidation. Moreover, NoxB-1 contains a ferredoxin-like motif, which is absent in NoxA-1.

A glass capillary microelectrode based on capillarity and its application to the detection of L-glutamate release from mouse brain slices

A new glass capillary microelectrode for L-glutamate is described using pulled glass capillaries (tip size, approximately 12.5 microm) with a very small volume (approximately 2 microl) of inner solution containing glutamate oxidase (GluOx) and ascorbate oxidase. The operation of the electrode is based on capillary action that samples L-glutamate into the inner solution. The enzyme reaction by GluOx generates hydrogen peroxide that is detected at an Os-gel-HRP polymer modified Pt electrode in a three-electrode configuration. The amperometric response behavior of the electrode was characterized in terms of the capillarity, response time, sensitivity and selectivity for measurements of L-glutamate. The currents at 0 V vs. Ag/AgCl increased linearly with the L-glutamate concentration from 10 to 150 microM for in vitro and in situ calibrations. The response was highly selective to L-glutamate over ascorbate, dopamine, serotonin and other amino acids. The detection of L-glutamate in the extracellular fluids of different regions of mouse hippocampal slices under stimulation of KCl was demonstrated.