Amperometric Biosensors for On-Line Monitoring of Extracellular Glucose and Glutamate in the Brain

A Novel Amperometric Glutamate Biosensor Based on Glutamate Oxidase Adsorbed on Silicalite

In this work, we developed a new amperometric biosensor for glutamate detection using a typical method of glutamate oxidase (GlOx) immobilization via adsorption on silicalite particles. The disc platinum electrode (d = 0.4 mm) was used as the amperometric sensor. The procedure of biosensor preparation was optimized. The main parameters of modifying amperometric transducers with a silicalite layer were determined along with the procedure of GlOx adsorption on this layer. The biosensors based on GlOx adsorbed on silicalite demonstrated high sensitivity to glutamate. The linear range of detection was from 2.5 to 450 μM, and the limit of glutamate detection was 1 μM. It was shown that the proposed biosensors were characterized by good response reproducibility during hours of continuous work and operational stability for several days. The developed biosensors could be applied for determination of glutamate in real samples.

A novel method for simultaneous glutamate and extracellular activity measurement in brain slices with high temporal resolution

Measurement of neurotransmitters during normal or altered function in cerebral slices could be an important tool to better understand the relationship between biochemical changes and electrophysiological activity. Some attempts of this analysis have been made; however, the current techniques do not have the appropriate time resolution to establish this relationship. The use of electrochemical biosensors has allowed for good time resolution, but problems related to the reduction of signal noise and biofouling of the electrode surface could be an important issue. In this work, we propose a new alternative to simultaneously measure glutamate and electrical activity with a high temporal resolution in brain slices. This approach is based on the use of enzymatic reactors that generate a fluorescent derivative from glutamate that can be measured at high temporal resolution. The results presented here show a reliable measurement of this neurotransmitter in brain slices obtained from intact animals under the effect of a glutamate transporter blocker DL-threo-beta-benzyloxyaspartate as well as the potassium channel blocker 4-aminopyridine. Differences in the levels of glutamate and high frequency and amplitude discharges as an effect of drug administration were found in brain slices obtained from epileptic rats (p<0.05). In conclusion, this method could be used to measure neurotransmitter concentration online at a near physiological temporal resolution, which can then be correlated to the electrical activity that is simultaneously recorded.

A high sensitivity MEA probe for measuring real time rat brain glucose flux

The mammalian central nervous system (CNS) relies on a constant supply of external glucose for its undisturbed operation. This article presents an implantable Multi-Electrode Array (MEA) probe for brain glucose measurement. The MEA was implemented on Silicon-On-Insulator (SOI) wafer using Micro-Electro-Mechanical-Systems (MEMS) methods. There were 16 platinum recording sites on the probe and enzyme glucose oxidase (GOx) was immobilized on them. The glucose sensitivity of the MEA probe was as high as 489 µA mM(-1) cm(-2). 1,3-Phenylenediamine (mPD) was electropolymerized onto the Pt recording surfaces to prevent larger molecules such as ascorbic acid (AA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), and dopamine (DA) from reaching the recording sites surface. The MEA probe was implanted in the anesthetized rat striatum and responded to glucose levels which were altered by intraperitoneal injection of glucose and insulin. After the in vivo experiment, the MEA probe still kept sensitivity to glucose, these suggested that the MEA probe was reliable for glucose monitoring in brain extracellular fluid (ECF).

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.

Simultaneous determination of glucose and L-lactate in rat brain by an electrochemical in vivo flow-injection system with an on-line microdialysis sampling

An electrochemical in vivo flow-injection system with an on-line microdialysis sampling is proposed for the simultaneous monitoring of L-lactate and glucose in rat brain. In the first stage of the operation, the dialysate from the microdialysis probe is delivered to a sample loop of the six-way autoinjector by perfusing Ringer's solution for 80 s at 5 microl min(-1). In the second stage, the dialysate collected in the sample loop is automatically injected for 10 s into the flow-injection line. Injected dialysate is split into two streams and two portions pass through two channels with two different immobilized enzyme reactors (glucose oxidase and lactate oxidase immobilized reactors) to produce hydrogen peroxide from glucose and L-lactate in the dialysate. After a subsequent confluence of the streams, produced hydrogen peroxide can be detected amperometrically at a downstream poly(1,2-diaminobenzene) film-coated platinum electrode, without any interference from oxidizable species and proteins present in the dialysate. Because each channel has a different residence time, two peaks are obtained. The first peak corresponds to L-lactate and the second peak to glucose. The peak current is linearly related to the concentrations of L-lactate between 0.2 and 10 mM and glucose between 0.1 and 20 mM. The present method can be successfully applied to the simultaneous in vivo monitoring of L-lactate and glucose in rat brain. The analytical speed is 45 dialysates h(-1).

Continuous Measurement of Glutamate and Hydrogen Peroxide Using a Microfabricated Biosensor for Studying the Neurotoxicity of Tributyltin

We first measured the effects of trace levels of an endocrine disruptor, tributyltin (TBT), on the secretion response from nerve cells using a microfabricated biosensor designed for the continuous measurement of L-glutamate and hydrogen peroxide. We observed higher and long-lasting glutamate and hydrogen peroxide concentrations from the cells when cultured rat cortical neurons were exposed to TBT. Glutamate and hydrogen peroxide release was induced even when we reduced the TBT concentration to 10 nM. This concentration is about two orders of magnitude lower than the concentration that induced apoptosis-like cell death. We also report on the effects of NMDA and non-NMDA receptor antagonists, which can help us to understand the mechanism of TBT neurotoxicity.

Micro-flow in vivo analysis of L-glutamate with an on-line enzyme amplifier based on substrate recycling

A micro-flow enzyme system with a microdialysis probe is proposed for the amperometric detection of trace amounts of neurotransmitter L-glutamate released from rat brain cells. The L-glutamate oxidase (EC 1.4.3.11)/glutamate dehydrogenase (EC 1.4.1.4) coimmobilized reactor was used to enhance the sensitivity of L-glutamate as an on-line amplifier based on substrate recycling. A poly(1,2-diaminobenzene) film-coated platinum electrode was also used to selectively detect only the hydrogen peroxide generated into a upstream enzyme reactor, without interference from oxidizable species, such as L-ascorbate, the adsorption of low molecular-weight proteins in a dialysate, and NADPH added to the carrier solution to initiate substrate recycling. By the present in vivo system, L-glutamate was selectively assayed with about a 600-fold increase in sensitivity compared with the unamplified responses. The detection limit was 0.08 mumol dm-3. This method was applied to an in vivo assay of L-glutamate in the extracellular space of rat brain; also, monitoring of the L-glutamate level changed after a continuous stimulation of KCl to demonstrate the reliability of the system.

Microdialysis coupled to online enzymatic assays

In vivo sampling of interstitial fluid by using microdialysis fibers has become a standard and accepted procedure. This sampling method is generally coupled to offline analysis of consecutive dialysate samples by high-performance liquid chromatography or capillary electrophoresis, but this combination is not the best approach for some applications, especially those which require high temporal resolution and rapid data collection. The purpose of this review is to provide information on enzyme-based online assays, i.e., continuous analysis of the dialysate as it emerges from the outlet of the sampling device. We have focused on methods developed specifically for the analysis of solutions perfused at a very slow flow rate, i.e., a feature of microdialysis and ultrafiltration techniques. These methods include flow enzyme-fluorescence assays, flow enzyme-amperometric assays, and sequential enzyme-amperometric detection. Each type of assay is discussed in terms of principle, applications, advantages, and limitations. We also comment on implantable biosensors, an obvious next step forward for in vivo monitoring of molecules in neuroscience.

Plastic film carbon electrodes: enzymatic modification for on-line, continuous, and simultaneous measurement of lactate and glucose using microdialysis sampling

Ring and split-disk plastic film carbon electrodes (PFCEs) were fabricated for use in thin-layer radial flow cells which were coupled to a microdialysis sampling system. PFCEs, were initially coated with osmium poly(vinylpyridine) redox polymer horseradish peroxidase (Os-gel-HRP). Then a second coat of oxidase enzyme was applied to produce enzyme bilayer (oxidase/Os-gel-HRP) PFCEs which were subsequently over-coated with cellulose acetate for use in the determination of glucose or lactate at 0 mV (vs Ag/AgCl). Split-disk electrode geometry enabled different oxidase enzymes to be immobilized on each half of a split-disk, Os-gel-HRP-coated, PFCE to facilitate the electrochemically independent yet continuous on-line determination of these two analytes from a single dialysate. In continuous-flow experiments, cellulose acetate overcoated oxidase/Os-gel-HRP cast-coated PFCEs were quick to stabilize background current and displayed linear and sensitive responses to substrates. The effect of ascorbic acid was minimal and cross talk between partner split-disk electrodes was demonstrated to be acceptable for in vivo applications. The utility of this analytical system is demonstrated by the quantitative on-line continuous assay of changes in dialysate striatal extracellular glucose and lactate from a conscious rat during (a) local stimulation of neurons by perfusion with the depolarizing agent, Veratridine, and (b) physical restraint.

Determination of glutamate and aspartate in microdialysis samples by reversed-phase column liquid chromatography with fluorescence and electrochemical detection

Five different systems for fast determination of aspartate and glutamate in microdialysis samples are described: (I) a high-speed HPLC using a gradient pump with a sharp elution profile, (II) a column switching technique, (III) an isocratic pump with a low-pressure switching valve for one-step gradients, (IV) microbore chromatography using injections of acetonitrile as a wash-out step, (V) on-line connection of microdialysis and HPLC/derivatization. In all cases, automated precolumn derivatization with o-phthalaldehyde-2-mercaptoethanol reagent were used. Both fluorescence and electrochemical detection techniques were evaluated in terms of reproducibility, sensitivity, interference, maintenance and troubleshooting. The electrochemical detection method required a second derivatization step with 0.2 M iodoacetamide to remove excess of a thiol moiety and regular recalibrations after each six to ten injections. Under these conditions the correlation coefficients for electrochemical vs. fluorescence detectors were 0.918 for Asp and 0.988 for Glu for 65 microdialysis samples. Coefficients of variation for six analyses between calibrations were below 3% for both detectors. The limits of detection for both amino acids were about 0.4 pmol for electrochemical detection with a thiol scavenger step, 50 fmol for fluorescence detection using conventional columns and about 20-30 fmol for the microbore system. All systems are suitable for detecting basal levels of Asp and Glu in 5-10 microl microdialysis samples from a rat brain where typical concentrations lie around 1-10 pmol or more. It is concluded that a microbore setup with one isocratic pump and an autosampler optimized for injections of washing solvent between samples is the most practical and economical. The system allows analysis of minute sample volumes down to 1-2 microl.

On-line electrochemical sensor for selective continuous measurement of acetylcholine in cultured brain tissue

An on-line acetylcholine (ACh) sensor was developed in order to determine extracellular ACh concentrations without interference from choline (Ch). The sensor is composed of a small-volume enzymatic prereactor (22-microL inner volume) in which choline oxidase and catalase are immobilized in series. Carbon electrodes were modified with an acetylcholine esterase (AChE), choline oxidase (ChOx), and osmium poly(vinylpyridine)-based redox polymer containing horseradish peroxidase (Os-gel-HRP). The sensor sensitivity was 43.7 nA/microM (+/- 0.15, n = 3) for ACh under optimized conditions. Almost no response was seen when 100 microM Ch was continuously injected. The detection limit for ACh with the sensor was comparable to that obtained using liquid chromatography with electrochemical detection combined with an enzymatic reactor. The electrical stimulation of cultured rat hippocampal tissue resulted in an extracellular ACh increase of 20 nM (+/- 11 nM, n = 3). This increase was observed continuously with our online sensor combined with a microcapillary sampling probe located very close to the tissue. The continuous measurement of ACh and Ch using a split disk carbon film dual electrode in which one electrode surface was modified with ChOx/Os-gel-HRP and the other with AChE-ChOx/Os-gel-HRP bilayer film was also demonstrated to improve the response time by eliminating the prereactor.

A flow injection microdialysis sampling chemiluminescence system for in vivo on-line monitoring of glucose in intravenous and subcutaneous tissue fluid microdialysates

A novel flow injection on-line microdialysis system for in vivo monitoring of glucose in subcutaneous tissue fluid and blood is described. An implantable loop-type microdialysis probe was used for subcutaneous sampling, and a flow-through microdialyzer was used for intravenous sampling by pumping of the blood from the tested rabbit through the microdialyzer located outside the living system at a flow rate of 10 microL/min. The perfusion rate of the dialysate was 20 microL/min. The glucose in the dialysate was detected on-line with a flow injection chemiluminescence system after passing through an immobilized glucose oxidase reactor. The calibration of the detector system (including reactor) and monitoring of baseline drifts were performed simultaneously to improve the reliability of the monitoring process. The dialysate sample volume was 20 microL, and the sample throughout was 28 h-1. The variation of glucose level in subcutaneous tissue fluid and blood of the rabbits was monitored after the administration of glucose or insulin to demonstrate the favorable resolution and reliability of the system for in vivo on-line monitoring.

In vivo brain glucose measurements: differential normal pulse voltammetry with enzyme-modified carbon fiber microelectrodes

The enzyme glucose oxidase was immobilized on the surface of carbon fiber microelectrodes (CFMEs) either by cross-linking in glutaraldehyde vapor or by enzyme entrapment in electropolymerized films of m-phenylenediamine or resorcinol. The cross-linked enzymatic layer was, in the given conditions, covered with an additional membrane of Nafion or cellulose acetate. The prepared glucose sensors were tested using differential normal pulse voltammetry (DNPV, in which the scan comprises successive double pulses ("prepulse and pulse"), the prepulses are of increasing amplitude, and the current measured is the differential of the current existing between each prepulse and pulse). With properly chosen DNPV parameters, the response to glucose presented a peak at a potential of about 1 V versus an Ag/AgC1-reference, owing to the oxidation of enzymatically produced hydrogen peroxide. The calibration curves obtained (peak height/glucose concentration) were linear from 0.3-0.5 up to 1.5-6.5 mM and showed a sensitivity ranging from 1.4 up to 34.5 mA M-1 cm-2, depending on the sensor type. The DNPV response to glucose exhibited an essential insensitivity toward easily oxidizable interfering substances such as ascorbic acid and acetaminophen present at physiological concentrations. Peptides, the interfering species typical of the cerebral medium, were effectively retained by the above additional membranes. Concentration values of glucose in plasma and cerebrospinal fluid, determined in vitro from the DNPV peak height, agreed well with those measured by standard procedures. In the anesthetized rat, extracellular brain concentration of glucose was also monitored during administration of either insulin or glucagon. Under such pharmacological conditions, the changes observed in the peak height were in perfect agreement with the known effects induced by both substances.

Physiological release of excitatory amino acids

The contribution of in vivo monitoring to the study of glutamate release is reviewed. Physiological stimulation increases both glutamate and aspartate in the extracellular compartment of the brain and both amino acids show Ca(2+)-dependent K(+)-evoked release. However, the finding that only glutamate is stored in synaptic vesicles implies that glutamate is the excitatory transmitter. Released glutamate is taken up into both neurones and glia by glutamate transporters. Uptake of glutamate, in addition to clearing the synapse, has a number of additional functions. Uptake into glia leads to the release of glutamine, which is involved in the recycling of transmitter glutamate; uptake into both neurones and glia leads to the release of ascorbate; uptake into glia leads to an increase glycolysis and export of lactate, an energy substrate for neuronal metabolism. Reversal of the glutamate transporter accounts for the parallel release of glutamate and aspartate from the cytoplasmic compartment. The basal concentration of extracellular glutamate is in the micromolar range. Such levels could lead to desensitisation of both NMDA and non-NMDA receptors. The functional implications of the level of basal glutamate are difficult to assess at present in view of the existence of multiple glutamate receptor subunits with different functional properties and distributions.

Extracellular glutamate: on-line monitoring using microdialysis coupled to enzyme-amperometric analysis

An enzyme-amperometric detector cell is described for flow analysis of glutamate in dialysate emerging from an implanted microdialysis probe. Its small size allows it to be placed within a few centimetres of the animal preparation, reducing the delay for data acquisition to around 2 min. The selectivity is provided by glutamate oxidase, immobilised with glutaraldehyde on surfaces adjacent to the 3-electrode system. A film of 1,2-diaminobenzene, electropolymerized on the platinum working electrode, eliminates interference from ascorbic acid and other endogenous electroactive compounds. The high sensitivity (< 0.5 mumol/l) and fast response time of the cell (90% of maximum response in 30 s) make it particularly suitable for investigating conditions that produce rapid changes in brain extracellular glutamate. This is illustrated by monitoring changes in extracellular glutamate subsequent to cardiac arrest, and K(+)-induced local depolarization.