Lactate, glutamate and glutamine biosensors based on rhodinised carbon electrodes

Multisensor Systems and Arrays for Medical Applications Employing Naturally-Occurring Compounds and Materials

The significant improvement of quality of life achieved over the last decades has stimulated the development of new approaches in medicine to take into account the personal needs of each patient. Precision medicine, providing healthcare customization, opens new horizons in the diagnosis, treatment and prevention of numerous diseases. As a consequence, there is a growing demand for novel analytical devices and methods capable of addressing the challenges of precision medicine. For example, various types of sensors or their arrays are highly suitable for simultaneous monitoring of multiple analytes in complex biological media in order to obtain more information about the health status of a patient or to follow the treatment process. Besides, the development of sustainable sensors based on natural chemicals allows reducing their environmental impact. This review is concerned with the application of such analytical platforms in various areas of medicine: analysis of body fluids, wearable sensors, drug manufacturing and screening. The importance and role of naturally-occurring compounds in the development of electrochemical multisensor systems and arrays are discussed.

Simple Fabrication of Flexible Biosensor Arrays Using Direct Writing for Multianalyte Measurement from Human Astrocytes

Simultaneous measurements of glucose, lactate, and neurotransmitters (e.g., glutamate) in cell culture over hours and days can provide a more dynamic and longitudinal perspective on ways neural cells respond to various drugs and environmental cues. Compared with conventional microfabrication techniques, direct writing of conductive ink is cheaper, faster, and customizable, which allows rapid iteration for different applications. Using a simple direct writing technique, we printed biosensor arrays onto cell culture dishes, flexible laminate, and glass to enable multianalyte monitoring. The ink was a composite of PEDOT:PSS conductive polymer, silicone, activated carbon, and Pt microparticles. We applied 0.5% Nafion to the biosensors for selectivity and functionalized them with oxidase enzymes. We characterized biosensors in phosphate-buffered saline and in cell culture medium supplemented with fetal bovine serum. The biosensor arrays measured glucose, lactate, and glutamate simultaneously and continued to function after incubation in cell culture at 37 °C for up to 2 days. We cultured primary human astrocytes on top of the biosensor arrays and placed arrays into astrocyte cultures. The biosensors simultaneously measured glucose, glutamate, and lactate from astrocyte cultures. Direct writing can be integrated with microfluidic organ-on-a-chip platforms or as part of a smart culture dish system. Because we print extrudable and flexible components, sensing elements can be printed on any 3D or flexible substrate.

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.

Chip-based amperometric enzyme sensor system for monitoring of bioprocesses by flow-injection analysis

A microfluidic chip integrating amperometric enzyme sensors for the detection of glucose, glutamate and glutamine in cell-culture fermentation processes has been developed. The enzymes glucose oxidase, glutamate oxidase and glutaminase were immobilized by means of cross-linking with glutaraldehyde on platinum thin-film electrodes integrated within a microfluidic channel. The biosensor chip was coupled to a flow-injection analysis system for electrochemical characterization of the sensors. The sensors have been characterized in terms of sensitivity, linear working range and detection limit. The sensitivity evaluated from the respective peak areas was 1.47, 3.68 and 0.28 μAs/mM for the glucose, glutamate and glutamine sensor, respectively. The calibration curves were linear up to a concentration of 20 mM glucose and glutamine and up to 10 mM for glutamate. The lower detection limit amounted to be 0.05 mM for the glucose and glutamate sensor, respectively, and 0.1 mM for the glutamine sensor. Experiments in cell-culture medium have demonstrated a good correlation between the glutamate, glutamine and glucose concentrations measured with the chip-based biosensors in a differential-mode and the commercially available instrumentation. The obtained results demonstrate the feasibility of the realized microfluidic biosensor chip for monitoring of bioprocesses.

Control of the oxygen dependence of an implantable polymer/enzyme composite biosensor for glutamate

Biosensors for glutamate (Glu) were fabricated from Teflon-coated Pt wire (cylinders and disks), modified with the enzyme glutamate oxidase (GluOx) and electrosynthesized polymer PPD, poly(o-phenylenediamine). The polymer/enzyme layer was deposited in two configurations: enzyme before polymer (GluOx/PPD) and enzyme after polymer (PPD/GluOx). These four biosensor designs were characterized in terms of response time, limit of detection, Michaelis-Menten parameters for Glu (J max and K(M)(Glu)), sensitivity to Glu in the linear response region, and dependence on oxygen concentration, K(M)(O2). Analysis showed that the two polymer/enzyme configurations behaved similarly on both cylinders and disks. Although the two geometries showed different behaviors, these differences could be explained in terms of higher enzyme loading density on the disks; in many analyses, the four designs behaved like a single population with a range of GluOx loading. Enzyme loading was the key to controlling the K(M)(O2) values of these first generation biosensors. The counterintuitive, and beneficial, behavior that biosensors with higher GluOx loading displayed a lower oxygen dependence was explained in terms of the effects of enzyme loading on the affinity of GluOx for its anionic substrate. Some differences between the properties of surface immobilized GluOx and glucose oxidase are highlighted.

Electrocatalytical properties of polymethylferrocenyl dendrimers and their applications in biosensing

The electrochemical characterization of polymethylferrocenyl dendrimers deposited onto a platinum electrode and their applications as hydrogen peroxide and glucose sensor are described. The redox dendrimers consist of flexible poly(propileneimine) dendrimer cores functionalised with octamethylferrocenyl units. Amperometric biosensors for glucose were prepared by immobilization of glucose oxidase onto these modified electrodes. The influence of the dendrimer generation and the thickness of the dendrimer layer, the effect of the substrate concentration, and the interferences and reproducibility on the response of the sensors were investigated.

Highly selective microbiosensors for in vivo measurement of glucose, lactate and glutamate

An alternative approach to production of amperometric microbiosensors, which combines electrochemical electrometallization and electropolymerisation of phenylene diamine film with covalent binding enzymes, is presented. In this respect, for a sensitive detection of hydrogen peroxide (HP) at +0.4V versus Ag/AgCl (detection limit of 0.5 microM, s/n=3), carbon fiber microelectrodes (30 microm in diameter and 500 microm long) were covered with ruthenium. To obtain a highly selective detection of HP, in the presence of different interfering compounds (ascorbic acid, uric acid, etc.), an additive semi-permeable polymer film was formed on the top of the ruthenium layer by electropolymerisation of m-phenylene diamine (m-PD). The enzymatic selective layers were formed by covalent cross-linking the enzymes (glucose oxidase, lactate oxidase or glutamate oxidase) with BSA by glutaraldehyde in the presence of ascorbate oxidase. An additional polymeric layer based on polyurethane and Nafion was deposited on the top of the enzymatic membrane (glucose oxidase, lactate oxidase, or glutamate oxidase) in order to extend the dynamic range of biosensors up to 4mM for glucose (R=0.997; Y[nA]=-0.22+9.68x[glucose, mM]), 1.75mM for lactate (R=0.991; Y[nA]=0.43+15.36x[lactate, mM]) and 0.25 mM for glutamate (R=0.999; Y[nA]=0.02+29.14x[glutamate, mM]). The developed microbiosensors exhibited also negligible influences from interfering compounds at their physiological concentrations. Microbiosensors remained stable during 10h in a flow injection system at 36 degrees C and pH 7.4. The microbiosensors developed are now used in vivo and, as an example, we report here the data obtained with the glucose biosensor.

Dependence of the response of an amperometric biosensor formed in a micro flow channel on structural and conditional parameters

Comprehensive analysis of the behavior of an amperometric biosensor incorporated in a micro flow channel was conducted by changing the structural and conditional parameters. The device used in the characterization consisted of a thin-film three-electrode system and a silicone rubber flow channel. An enzyme, glucose oxidase, was immobilized either at the bottom of the silicone rubber flow channel or on the electrode substrate. The flow rate, concentration, position of the immobilized enzyme, and channel height were changed, and the changes in the output current and the conversion efficiency were examined. When the flow rate and/or the channel height decreased, the output current and the conversion efficiency significantly increased. The conversion efficiency also increased by decreasing the concentration. The tendency of the flow dependence was reversed when the position of the immobilized enzyme was changed from the silicone rubber side to the electrode substrate. In addition, the influence of l-ascorbic acid was reduced by placing additional working electrodes in the upper stream. l-Ascorbic acid was eliminated more effectively as the flow rate decreased and the area of the working electrode for elimination increased.

Bienzyme sensors based on novel polymethylferrocenyl dendrimers

Amperometric bienzyme electrodes with horseradish peroxidase (HRP) and glucose oxidase (GOx) co-immobilized on polymethylferrocenyl dendrimers deposited onto platinum electrodes have been used for determination of the hydrogen peroxide produced by the oxidase during the enzymatic reaction. The redox dendrimers consist of flexible poly(propylenimine) dendrimer cores functionalised with octamethylferrocenyl units. The effects of dendrimer generation, the thickness of the dendrimer layer, substrate concentration, interferences, and reproducibility on the response of the sensors were investigated. The new bienzyme biosensors respond to substrate at work potential values between 200 and 50 mV (vs. SCE), have good sensitivity, and are resistant to interferences. Figure.

Coupling the lactate oxidase to electrodes by ionotropic gelation of biopolymer

A direct ionotropic gelation of the polycationic biopolymer chitosan (CHIT) with the polyanionic enzyme lactate oxidase (LOx) was used to form thin biopolymer-enzyme films on the surface of platinum electrodes. The electrochemical assays of such films revealed a well-defined capacity of CHIT for the retention of LOx. The stoichiometry of the CHIT-LOx polyelectrolyte complexes was found to be approximately 1:40, i.e., on average, 1 CHIT chain retained 40 molecules of LOx in the CHIT-LOx films. The enzyme retention was ascribed to strong electrostatic interactions between the LOx and a fraction of the protonated amino groups on the CHIT chains. Although the LOx is inherently unstable outside its natural matrix, it displayed high surface activity of 0.26 units cm(-)(2) in the matrix of CHIT. This correlated with good stability of the biopolymer-enzyme films as demonstrated by a constant response of Pt/CHIT-LOx electrodes to lactate during continuous 24-h testing. When compared to other single-film lactate sensors, the Pt/CHIT-LOx electrodes displayed the best combination of analytical properties in terms of a low detection limit (50 nM), high sensitivity (0.23 A M(-)(1) cm(-)(2)), and fast response time (<1 s). Such a performance validated the CHIT-LOx system as an attractive sensing element for the development of new lactate biosensors.

Enzyme microgels in packed-bed bioreactors with downstream amperometric detection using microfabricated interdigitated microsensor electrode arrays

In this article, we describe the use of pH- responsive hydrogels as matrices for the immobilization of two enzymes, glucose oxidase (GOx) and glutamate oxidase (GlutOx). Spherical hydrogel beads were prepared by inverse suspension polymerization and the enzymes were immobilized by either physical entrapment or covalent immobilization within or on the hydrogel surface. Packed-bed bioreactors were prepared containing the bioactive hydrogels and these incorporated into flow injection (FI) systems for the quantitation of glucose and monosodium glutamate (MSG) respectively. The FI amperometric detector comprised a microfabricated interdigitated array within a thin-layer flow cell. For the FI manifold incorporating immobilized GOx, glucose response curves were found to be linear over the concentration range 1.8-280 mg dL(-1) (0.1-15.5 mM) with a detection limit of 1.4 mg dL(-1) (0.08 mM). Up to 20 samples can be manually analyzed per hour, with the hydrogel-GOx bioreactor exhibiting good within-day (0.19%) precision. The optimized FI manifold for MSG quantitation yielded a linear response range of up to 135 mg dL(-1) (8 mM) with a detection limit of 3.38 mg dL(-1) (0.2 mM) and a throughput of 30 samples h(-1). Analysis of commercially produced soup samples gave a within-day precision of 3.6%. Bioreactors containing these two physically entrapped enzymes retained > 60% of their initial activities after a storage period of up to 1 year.

In vivo voltammetric detection of rat brain lactate with carbon fiber microelectrodes coated with lactate oxidase

To allow rat brain lactate measurement in vivo, a specific sensor based on a carbon fiber (phi = 30 microns) microelectrode coated with lactate oxidase was prepared. Combined with the differential normal pulse voltammetry measurement method, such a sensor, with a sensitivity of 9.15 +/- 0.91 mA.M-1.cm-2, provided a lactate linear response in concentrations ranging from 0.1 to 2.0 mM. The measurements performed appeared to be essentially insensitive to usual interference caused by the electroactive compounds present in the brain (ascorbic acid and peptides). In vivo detection performed in the cortex of the anesthetized rat led to the determination of a lactate concentration of 0.41 +/- 0.02 mM. Moreover, to validate the results obtained in vivo, an ex vivo determination of the lactate level was also performed in samples of brain tissue, plasma, and cerebrospinal fluid, using both voltammetry and a clinical analyzer with colorimetric-based detection. A good correlation was observed between the sets of data established by both methods.

Glutamine biosensors for biotechnology applications, with suppression of the endogenous glutamate signal

Glutamine membranes for amperometric measurements are described. The interference of the endogenous glutamate is greatly diminished by using a supplementary "anti-glutamate" layer consisting of immobilized glutamate oxidase and catalase on top of the glutamine-sensitive layer having co-immobilized glutaminase and glutamate oxidase. The use of polycarbonate membranes with different permeability characteristics for the control of the substrate's access to the enzyme layers is presented, as well as the effect of the density of the enzyme layer on the sensitivity of these membranes. The fabricated membranes have good operational stability (at least 5 days) and very good linearity (up to 10 mM glutamine). Using an appropriate choice of membranes and cross-linking conditions, membranes with good rejection of glutamate have been fabricated (less than 6% RSD for a 5 mM glutamine sample containing 5 mM glutamate as interferent). These membranes are suitable for monitoring of glutamine levels in mammalian cell cultures without the need of a separate measurement for glutamate.

Biosensors based on flow-through systems

When combined with biosensors as the sensing element microdialysis and flow injection analysis (FIA) systems become sophisticated tools for handling analytical processes. In particular a FIA system offers a high degree of automation together with high reproducibility and small sample volumes, whereas the biosensor, allows selective and sensitive measurements of the various analytes. Here we describe first a miniaturised microdialysis flow-through system developed for glucose determination, then we focus on amperometric immunosensors and on microbial sensors. In the former, antibodies against low molecular weight environmental contaminants or against high molecular weight proteins are responsible for analyte detection, whereas the latter use immobilised microorganisms as the recognising element for monitoring water pollutants.

Development of an amperometric flow injection immunoanalysis system for the determination of the herbicide 2,4-dichlorophenoxyacetic acid in water

An amperometric flow injection immunoanalysis (FIIA) system based on an immunoreactor with immobilized biocomponents on a silica surface has been developed for the determination of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). In the antigen coating mode the hapten was immobilized and monoclonal primary antibody against 2,4-D together with alkaline phosphatase (AP)-labelled secondary antibody were used as sensing elements in a titration assay. In the antibody coating mode a biotinylated monoclonal antibody was immobilized on the surface of the immunoreactor and a 2,4-D-AP-conjugate was used for detection. For electrochemical measurements p-aminophenol enzymatically generated from p-aminophenyl phosphate was oxidized at a carbon working electrode at +150 mV versus Ag/AgCl. The system enabled the determination of 2,4-D in drinking water samples in the range from 0.2 to 70 micrograms/l. The whole system was computer controlled with a measuring time of 12 min for one determination.

Determination of glutamine and glutamic acid in mammalian cell cultures using tetrathiafulvalene modified enzyme electrodes

Tetrathiafulvalene (TTF) mediated amperometric enzyme electrodes have been developed for the monitoring of L-glutamine and L-glutamic acid in growing mammalian cell cultures. The detection of glutamine was accomplished by a coupled enzyme system comprised of glutaminase plus glutamate oxidase, while the detection of glutamic acid was carried out by a single enzyme, glutamate oxidase. The appropriate enzyme(s) were immoblized on the Triton-X treated surface of tetrathiafulvalene modified carbon paste electrodes by adsorption, in conjunction with entrapment by an electrochemically deposited copolymer film of 1,3-phenylenediamine and resorcinol. Operating conditions for the glutamine enzyme electrode were optimized with respect to the amount of enzymes immoblized, pH, temperature and mobile phase flow rate for operation in a flow injection (FIA) system. When applied to glutamine and glutamic acid measurements in mammalian cell culture in FIA, the results obtained with enzyme electrodes were in excellent agreement with those determined by enzymatic analysis.

Lactate solid-state biosensor with multilayer of electrodeposited polymers for flow-injection clinical analysis

In the lactate biosensor, electrodeposited poly(o-phenylenediamine) serves as a convenient matrix for the immobilization of lactate oxidase, but does not provide sufficient discrimination from several interfering species present in physiological fluids. Their effect, however, can be eliminated by additional modification of the working Pt electrode with a bilayer of electrodeposited polypyrrole/polyphenol. Despite continued decrease in biosensor sensitivity, the newly developed three-layer solid-state biosensor was successfully applied in flow-injection determination of lactate in both undiluted and diluted human blood serum samples over a 10 day period. For the lactate concentration range 0.2-5.0 mM in several series of measurements the correlation coefficient values for comparison with photometric determination using a DuPont dimension clinical analyzer were between 0.96 and 0.99. The reproducibility measured for 1:10 diluted serum was 0.6%. The detection limit was estimated as 2 microM.

Electrochemical sensors for continuous monitoring during surgery and intensive care

The current state of development of electrochemical sensors and biosensors for continuous use during surgery and intensive care is briefly reviewed with an emphasis on recent developments. The clinical usefulness of invasive and non-invasive sensors is discussed. Recent advances in the design of electrochemical sensors and experience with ex vivo and in vivo applications are described. The importance of developing appropriate fabrication technology is emphasised in order to meet the demand for reliable and reproducible analytical devices.

On-line monitoring of glucose, glutamate and glutamine during mammalian cell cultivations

Amperometric biosensors (based on rhodinised carbon electrodes) for glucose, glutamine and glutamate were constructed. The sensors were incorporated into a three cell parallel FIA system and used to monitor the three analytes on-line during two mammalian cell perfusion cultures. All measurements were made simultaneously from undiluted media sample. Use of the FIA system enabled easy and rapid exchange of the sensors, during cultivation. The inclusion of a calibration step, regularly for all sensors, helped to maintain the accuracy of all measurements. Comparison with off-line measurements indicated that all three biosensors operated successfully, providing accurate information.