Dehydrogenase-modified carbon-fiber microelectrodes for the measurement of neurotransmitter dynamics. 2. Covalent modification utilizing avidin-biotin technology

Controlling Nucleation Sites for Metal Oxide Film Growth on Glassy Carbon via Electrochemical Preoxidation

Coating electrode materials with metal oxide thin films can improve the performance of electrocatalysts and charge storage materials. Atomic layer deposition (ALD) enables the deposition of conformal, uniform films on a wide range of electrodes; however, an even film depends on the availability of nucleation sites directly on the electrode surface. Here, we show that the electrochemical oxidation of glassy carbon electrodes prior to the deposition of alumina thin films by ALD leads to more uniform electrochemically passivating films. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) demonstrate that film uniformity increases with the increasing potential of preoxidation until 2.50 V versus Ag/AgCl, at which point the films are fully passivating and appear continuous by SEM. Further increasing the potential of preoxidation leads to uniform but less consistently passivating alumina films. These findings show that electrochemical preoxidation is a rapid and readily tunable strategy for controlling oxygenic nucleation sites and therefore the growth of thin metal oxide films on glassy carbon electrodes.

Bovine serum albumin-coated titanium dioxide modified electrochemical interface for enantioselective discrimination of D/L-aspartic acid

Due to the similar physicochemical properties, the discrimination of chiral isomers faces huge challenges in drug production and biochemical analysis. Herein, the bovine serum albumin-coated titanium dioxide (bovine serum albumin [BSA]/TiO₂ ) was modified as a novel electrochemical interface for efficient, simple, and enantioselective discrimination of aspartic acid enantiomers (D/L-Asp) based on the electrochemical impedance spectroscopy (EIS). Utilizing the structural characteristics of large cavity and high specific surface area, TiO₂ material provided sufficient space for adequate loading of BSA. The BSA/TiO₂ electrochemical interface was successfully fabricated to support abundant chiral recognition sites. The enantioselective discrimination of D/L-Asp was achieved on the interface with a good linear relationship against the impedance difference in the concentration range from 1 to 1000 nM with the detection limit of 0.37 nM for L-Asp and 0.94 nM for D-Asp, reaching the identification coefficient (I_c  = K_L /K_D ) of 1.85. The proposed interface is easy to form with a stable formation of BSA in TiO₂ microporous architecture, which maintained the desired stability and reproducibility. For the unknown racemic solution, I_c levels of different enantio-ratios of D/L-Asp were effectively obtained to evaluate the chiral percentage of racemic sample. The possible mechanism of chiral recognition by density function theory (DFT) was confirmed with a stronger adsorption to L-Asp in accordance with our experiment results, reinforcing the validity of our presented interface. The BSA/TiO₂ electrochemical interface with robust enantioselective discrimination of D/L-Asp has great potential for the practical application in pharmaceutical surveillance and food security.

Biocatalytic and Bioelectrocatalytic Approaches for the Reduction of Carbon Dioxide using Enzymes

In the recent decade, CO₂ has increasingly been regarded not only as a greenhouse gas but even more as a chemical feedstock for carbon-based materials. Different strategies have evolved to realize CO₂ utilization and conversion into fuels and chemicals. In particular, biological approaches have drawn attention, as natural CO₂ conversion serves as a model for many processes. Microorganisms and enzymes have been studied extensively for redox reactions involving CO₂. In this review, we focus on monitoring nonliving biocatalyzed reactions for the reduction of CO₂ by using enzymes. We depict the opportunities but also challenges associated with utilizing such biocatalysts. Besides the application of enzymes with co-factors, resembling natural processes, and co-factor recovery, we also discuss implementation into photochemical and electrochemical techniques.

Trinuclear ruthenium clusters as bivalent electrochemical probes for ligand-receptor binding interactions

Despite their popularity, electrochemical biosensors often suffer from low sensitivity. One possible approach to overcome low sensitivity in protein biosensors is to utilize multivalent ligand-receptor interactions. Controlling the spatial arrangement of ligands on surfaces is another crucial aspect of electrochemical biosensor design. We have synthesized and characterized five biotinylated trinuclear ruthenium clusters as potential new biosensor platforms: [Ru(3)O(OAc)(6)CO(4-BMP)(py)](0) (3), [Ru(3)O(OAc)(6)CO(4-BMP)(2)](0) (4), [Ru(3)O(OAc)(6)L(4-BMP)(py)](+) (8), [Ru(3)O(OAc)(6)L(4-BMP)(2)](+) (9), and [Ru(3)O(OAc)(6)L(py)(2)](+) (10) (OAc = acetate, 4-BMP = biotin aminomethylpyridine, py = pyridine, L = pyC16SH). HABA/avidin assays and isothermal titration calorimetry were used to evaluate the avidin binding properties of 3 and 4. The binding constants were found to range from (6.5-8.0) × 10(6) M(-1). Intermolecular protein binding of 4 in solution was determined by native gel electrophoresis. QM, MM, and MD calculations show the capability for the bivalent cluster, 4, to intramolecularly bind to avidin. Electrochemical measurements in solution of 3a and 4a show shifts in E(1/2) of -58 and -53 mV in the presence of avidin, respectively. Self-assembled monolayers formed with 8-10 were investigated as a model biosensor system. Diluent/cluster ratio and composition were found to have a significant effect on the ability of avidin to adequately bind to the cluster. Complexes 8 and 10 showed negligible changes in E(1/2), while complex 9 showed a shift in E(1/2) of -43 mV upon avidin addition. These results suggest that multivalent interactions can have a positive impact on the sensitivity of electrochemical protein biosensors.

Mediator-less highly sensitive voltammetric detection of glutamate using glutamate dehydrogenase/vertically aligned CNTs grown on silicon substrate

A sensitive glutamate biosensor is prepared based on glutamate dehydrogenase/vertically aligned carbon nanotubes (GLDH, VACNTs). Vertically aligned carbon nanotubes were grown on a silicon substrate by direct current plasma enhanced chemical vapor deposition (DC-PECVD) method. The electrochemical behavior of the synthesized VACNTs was investigated by cyclic voltammetry and electrochemical impedance spectroscopic methods. Glutamate dehydrogenase covalently attached on tip of VACNTs. The electrochemical performance of the electrode for detection of glutamate was investigated by cyclic and differential pulse voltammetry. Differential pulse voltammetric determinations of glutamate are performed in mediator-less condition and also, in the presence of 1 and 5 μM thionine as electron mediator. The linear calibration curve of the concentration of glutamate versus peak current is investigated in a wide range of 0.1-500 μM. The mediator-less biosensor has a low detection limit of 57 nM and two linear ranges of 0.1-20 μM with a sensitivity of 0.976 mA mM(-1) cm(-2) and 20-300 μM with a sensitivity of 0.182 mA mM(-1) cm(-2). In the presence of 1 μM thionine as an electron mediator, the prepared biosensor shows a low detection limit of 68 nM and two linear ranges of 0.1-20 with a calibration sensitivity of 1.17 mA mM(-1) cm(-2) and 20-500 μM with a sensitivity of 0.153 mA mM(-1) cm(-2). The effects of the other biological compounds on the voltammetric behavior of the prepared biosensor and its response stability are investigated. The results are demonstrated that the GLDH/VACNTs electrode even without electron mediator is a suitable basic electrode for detection of glutamate.

Specific oxygen-containing functional groups on the carbon surface underlie an enhanced sensitivity to dopamine at electrochemically pretreated carbon fiber microelectrodes

The in vivo use of carbon-fiber microelectrodes for neurochemical investigation has proven to be selective and sensitive when coupled with background-subtracted fast-scan cyclic voltammetry (FSCV). Various electrochemical pretreatments have been established to enhance the sensitivity of these sensors; however, the fundamental chemical mechanisms underlying these enhancement strategies remain poorly understood. We have investigated an electrochemical pretreatment in which an extended triangular waveform from -0.5 to 1.8 V is applied to the electrode prior to the voltammetric detection of dopamine using a more standard waveform ranging from -0.4 to 1.3 V. This pretreatment enhances the electron-transfer kinetics and significantly improves sensitivity. To gain insight into the chemical mechanism, the electrodes were studied using common analytical techniques. Contact atomic force microscopy (AFM) was used to demonstrate that the surface roughness was not altered on the nanoscale by electrochemical pretreatment. Raman spectroscopy was utilized to investigate oxide functionalities on the carbon surface and confirmed that carbonyl and hydroxyl functional groups were increased by electrochemical conditioning. Spectra collected after the selective chemical modification of these groups implicate the hydroxyl functionality, rather than the carbonyl, as the major contributor to the enhanced electrochemical signal. Finally, we have demonstrated that this electrochemical pretreatment can be used to create carbon microdisc electrodes with sensitivities comparable to those associated with larger, conventionally treated cylindrical carbon fiber microelectrodes.

Reactive surface coatings based on polysilsesquioxanes: controlled functionalization for specific protein immobilization

The key designing in reliable biosensors is the preparation of thin films in which biomolecular functions may be immobilized and addressed in a controlled and reproducible manner. This requires the controlled preparation of specific binding sites on planar surfaces. Poly(methylsilsesquioxane)-poly(pentafluorophenyl acrylates) (PMSSQ-PFPA) are promising materials to produce stable and adherent thin reactive coatings on various substrates. Those reactive surface coatings could be applied onto various materials, for example, gold, polycarbonate (PC), poly(tetrafluoroethylene) (PTFE), and glass. By dipping those substrates in a solution of a desired amine, specific binding sites for protein adsorption could be immobilized on the surface. The versatile strategy allowed the attachment of various linkers, for example, biotin, l-thyroxine, and folic acid. The adsorption processes of streptavidin, pre-albumin, and folate-binding protein were monitored using surface plasmon resonance (SPR), Fourier transform infrared (FTIR) spectroscopy, fluorescence spectroscopy, and atomic force microscopy (AFM). The presented protein immobilization strategy, consisting of four steps (a) spin-coating of PMSSQ-PFPA hybrid polymer from tetrahydrofuran (THF) solution, (b) annealing at 130 degrees C for 2 h to induce thermal cross-linking of the PMSSQ part, (c) surface analogues reaction with different amino-functionalized specific binding sites for proteins, and (d) controlled assembly of proteins on the surface, may find various applications in future biosensor design.

Selective binding of biotinylated albumin to the lymphoid microvasculature

Chemically modified albumin binds to the surface of microvascular endothelia lining the vessel wall in several tissues. In this paper, we report that following their biotinylation, ovalbumin (bioOVA) and bovine serum albumin (BSA) [biotinyated albumin (bioAlb)] showed heterogeneous binding to distinct vascular subsets in different lymphoid tissues. The binding of bioAlb could be demonstrated both by fluorescent and enzymohistochemical techniques. In the spleen, the reaction was restricted to the red pulp sinuses whereas the white pulp vessels (including the central arteriole) and the marginal sinus were negative for bioAlb binding. In lymph nodes, the strongest labeling was observed in the medullary sinuses. In the thymus, the most prominent labeling of capillaries was restricted to the corticomedullary area where it was found to be less intense compared with the splenic reaction. The splenic reactivity of bioAlb in the mouse was defined using antibodies against endothelial cell subsets in distinct vascular beds in the red pulp and marginal zone, respectively. The bioAlb-binding elements of the splenic red pulp sinus architecture corresponded to the display of hyaluronan receptor stabilin-2 and subset-specific marker IBL-9/2 while they differed from the expression pattern of both the complementary red pulp sinus subset and the marginal sinus-lining cells expressing MAdCAM-1 antigen, respectively. Similar red pulp sinus-restricted reactivity could be demonstrated in the human, rat, and guinea pig. The use of bioAlb may thus offer a reliable probe for the histological identification of select microvascular endothelia in lymphoid tissues.

Dense monolayers of metal-chelating ligands covalently attached to carbon electrodes electrochemically and their useful application in affinity binding of histidine-tagged proteins

In this work, monolayers of metal complexes were covalently attached to the surface of carbon electrodes with the goal of binding monolayers of histidine-tagged proteins with a controlled molecular orientation and a maintained biological activity. In this novel method, which is simple, versatile, and efficient, the covalent attachment was accomplished in a single step by the electrochemical reduction of aryl diazonium ions that were substituted with a nitrilotriacetic (NTA) or an imminodiacetic (IDA) ligand at the para position. The transient aryl radicals that were generated in the reduction were grafted to the surfaces of glassy carbon, highly oriented pyrolitic graphite, and graphite-based screen-printed electrodes, producing dense monolayers of the ligands. The NTA- and IDA-modified electrodes were shown to efficiently chelate Cu(II) and Ni(II) ions. The presence of the metal was established using X-ray photoelectron spectroscopy and electrochemistry. Surface coverages of the ligands were indirectly determined from the electroactivity of the copper(II) complex formed on the electrode surface. Studies on the effect of electrodeposition time and potential showed that, at sufficiently negative potentials, the surface coverage reached a saturating value in less than 2 min of electrodeposition time, which corresponds to the formation of a close-packed monolayer of ligand on the electrode surface. Once loaded with a metal ion, the modified electrode was able to bind specifically to histidine-tagged proteins such as the horseradish peroxidase (His-HRP) or to an enhanced, recombinant green-fluorescent protein via its N-terminal hexahistidine tail. In the case of His-HRP, the amount of active enzyme specifically immobilized by metal-chelating binding was determined from the analysis of electrocatalytic currents using cyclic voltammetry. The electrochemical grafting makes it possible to accurately controlled and electronically address the amount of deposited ligand on the conductive surfaces of carbon electrodes with any size and shape.

Formation and characterization of glutamate dehydrogenase monolayers on silicon supports

In this paper we have tested two different procedures (the "three-step" and the "four-step" procedures) for the covalent immobilization of glutamate dehydrogenase (GDH) onto silicon supports. Atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FT-IR), fluorescence spectroscopy and an enzymatic assay were used to probe the structure and activity of the immobilized enzyme. Our results demonstrate that coupling through the "three-step" procedure does not significantly affect either the fold pattern or the activity of the enzyme, suggesting that this method could be ideally suited to the development of high quality monolayers for use in enzyme-based planar biosensors.

Amperometric ATP biosensor based on polymer entrapped enzymes

A dual enzyme electrode for the detection of adenosine-5'-triphosphate (ATP) at physiologically relevant pH levels was developed by co-immobilization of the enzymes glucose oxidase (GOD) and hexokinase (HEX) using pH-shift induced deposition of enzyme containing polymer films. Application of a simple electrochemical procedure for the co-immobilization of the enzymes at electrode surfaces exhibits a major improvement of sensitivity, response time, reproducibility, and ease of fabrication of ATP biosensors. Competition between glucose oxidase and hexokinase for the substrate glucose involving ATP as a co-substrate allows the determination of ATP concentrations. Notable control on the immobilization process enables fabrication of micro biosensors with a diameter of 25 microm. The presented concept provides the technological basis for a new generation of fast responding, sensitive, and robust biosensors for the detection of ATP at physiological pH values with a detection limit of 10 nmol l(-1).

Redox labelled avidin for enzyme sensor architectures

Conjugates of avidin with ferrocene and with microperoxidase 8 have been used as electrochemically active molecular building blocks. Assemblies of the conjugates with biotinylated glucose oxidase or lactate oxidase on gold electrodes were tested as enzyme sensors for glucose and lactate. The electrochemical detection is based either on ferrocene-mediated oxidation of the substrate in oxygen-free solution, or on microperoxidase-catalysed reduction of H2O2 which is enzymatically produced from the substrate and molecular oxygen. Glucose and lactate were detectable with both detection principles in concentrations down to 1 or 0.1 mM, respectively. The molecular architecture concept allows quick adaptation of the sensors to other analytes, and it provides a platform for arrays of sensors with different selectivity.

Fabrication optimisation of carbon fiber electrode with Taguchi method

In this study, we describe an optimised procedure for fabricating carbon fiber electrodes using Taguchi quality engineering method (TQEM). The preliminary results show a S/N ratio improvement from 22 to 30 db (decibel). The optimised parameter was tested by using a glass micropipette (0.3 mm outer/2.5 mm inner length of carbon fiber) dipped into PBS solution under 2.9 V triangle-wave electrochemical processing for 15 s, followed by coating treatment of micropipette on 2.6 V DC for 45 s in 5% Nafion solution. It is thus shown that Taguchi process optimisation can improve cost, manufacture time and quality of carbon fiber electrodes.

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.

Improved ceramic-based multisite microelectrode for rapid measurements of L-glutamate in the CNS

This paper describes improvements and further characterization of a ceramic-based multisite microelectrode for in vivo measurements of L-glutamate. Improvements include increased recording area, insulation deposition using photolithography for more uniform recording sites and forming the microelectrodes using a diamond saw providing smoother microelectrode edges. The new microelectrodes are triangular in shape, 1 cm in length and taper from 1 mm to a 2-5 microm tip. Details on performing in vivo measurements are given, including microelectrode preparation, pitfalls of the recording method and approaches to enhance reproducibility of the technique. The detection limit for L-glutamate was lowered to approximately 0.5 microM and a self-referencing recording technique was utilized to remove interferents as well as decrease noise. Applications of the microelectrodes to study L-glutamate uptake and release in rat prefrontal cortex, cortex, cerebellum and striatum are included.

Noninvasive measurement of hydrogen and potassium ion flux from single cells and epithelial structures

This review introduces new developments in a technique for measuring the movement of ions across the plasma membrane. With the use of a self-referencing ion-selective (Seris) probe, transport mechanisms can be studied on a variety of preparations ranging from tissues to single cells. In this paper we illustrate this versatility with examples from the vas deferens and inner ear epithelium to large and small single cells represented by mouse single-cell embryos and rat microglia. Potassium and hydrogen ion fluxes are studied and pharmacological manipulation of the signals are reported. The strengths of the self-referencing technique are reviewed with regard to biological applications, and the expansion of self-referencing probes to include electrochemical and enzyme-based sensors is discussed.

Fabricating and imaging carbon-fiber immobilized enzyme ultramicroelectrodes with scanning electrochemical microscopy

The scanning electrochemical microscope (SECM) is used to image the activity of enzymes immobilized on the surfaces of disk-shaped carbon-fiber electrodes. SECM was used to map the concentration of enzymatically produced hydroquinone or hydrogen peroxide at the surface of a 33-microm diameter disk-shaped carbon-fiber electrode modified by an immobilized glucose-oxidase layer. Sub-monolayer coverage of the enzyme at the electrode surface could be detected with micrometer resolution. The SECM was also employed as a surface modification tool to produce microscopic regions of enzyme activity by using a variety of methods. One method is a gold-masking process in which microscopic gold patterns act as mask for producing patterns of chemical modification. The gold masks allow operation in both a positive or negative process for patterning enzyme activity. A second method uses the direct mode of the SECM to produce covalently attached amine groups on the carbon surface. The amine groups are anchors for attachment of glucose oxidase by use of a biotin/avidin process. The effect of non-uniform enzyme activity was investigated by using the SECM tip to temporarily damage an immobilized enzyme surface. SECM imaging can observe the spatial extent and time-course of the enzyme recovery process.

Construction of multicomponent catalytic films based on avidin-biotin technology for the electroenzymatic oxidation of molecular hydrogen

Two methods based on the avidin-biotin technology were developed for the multimonolayer immobilization of Desulfovibrio gigas hydrogenase on glassy carbon or gold electrodes. In both methods the molecular structure of the modified interface was the result of a step-by-step process. The first method alternates monolayers of avidin and biotinylated hydrogenase, the mediator (methyl viologen) being free to diffuse in the structure. In the second method, the avidin monolayers were used to immobilize both the biotinylated enzyme and a long-chain biotinylated viologen derivative. The viologen head of this hydrophilic arm shuttles the electrons between the electrode and the enzyme. The modified electrodes were evaluated for the electroenzymatic oxidation of molecular hydrogen, which has interest for the development of enzymatic fuel cells. The parameters that affect the current density of mediated oxidation of H(2) at the modified electrodes was studied. The second structure, which has given typical catalytic currents of 25 microA per cm(2) for 10 monolayers, was found clearly less efficient than the first structure (500 microA per cm(2) for 10 monolayers). In both methods the catalytic currents increased linearly with the number of monolayers of hydrogenase immobilized, which indicates that the multilayer structures are spatially ordered.

Biomolecule immobilization on electrode surfaces by entrapment or attachment to electrochemically polymerized films. A review

The concept and potentialities of electrochemical procedures of biomolecule immobilization based on electropolymerized films are described. The biomolecule entrapment in conventional electrogenerated polymers such as polypyrrole, polyaniline or polyphenol is compared with an electrochemical procedure involving the adsorption of amphiphilic monomers and biomolecules before the polymerization step. Examples of organic phase enzyme electrode and electrical wiring of immobilized enzymes are presented. Furthermore, the construction of controlled architectures based on spatially segregated multilayers, exhibiting complementary biological activities is described. Then, the use of functionalized polymers bearing functional groups for the covalent binding of biomolecules is reported. Moreover, the attachment of biomolecules to biotinylated polymers through affinity interactions based on avidin-biotin bridge is presented.

Background-subtraction of fast-scan cyclic staircase voltammetry at protein-modified carbon-fiber electrodes

Background-subtraction techniques were applied to the voltammetry of nicotinamide adenine dinucleotide (NADH) at protein-modified carbon-fiber microelectrodes. The background currents at carbon-fiber electrodes were stable and voltammetric scans immediately before or after the analyte were effectively used for background subtraction. Digital step-potential waveforms were used to excite these carbon-fiber electrodes, where the resulting voltammetric analysis assessed the optimal switching and initial potentials and the electrochemical response time was determined. The initial potential was 0.0 V and the switching potential 1.1 V (versus Ag/AgCl) and the response time was approximately 300 ms. Some sensitivity to NADH was lost and voltammetric prescans were required at protein-modified electrodes to obtain a stable baseline. Current versus time was assessed by the average current of the faradaic region from each voltammogram and by differential current; the average current minus the current from a non-faradaic potential range. Differential current assessments discriminated against artifacts caused by pH (as high as 1.0 pH unit) and ionic strength flux (100 mM). These background-subtraction techniques allowed the faradaic information to be obtained quickly and conveniently while maximizing sensitivity and maintaining selectivity.

Electron transfer kinetics at a biotin/avidin patterned glassy carbon electrode

Photolithographic techniques using a laser interference pattern were used to attach photobiotin to micron-sized stripes on the surface of a carbon electrode. Fluorophore-tagged avidin was attached to this spatially-patterned biotin with essentially no loss in spatial resolution. The kinetics of the glassy carbon surface were examined to see if electron transfer sites could indeed be segregated from the attachment sites of photobiotin-immobilized avidin. The ECL of luminol and SECM were used to verify the segregation between underivatized sites (which exhibit normal electron transfer kinetics) and extensively derivatized biotin/avidin surfaces (which presumably exhibit slow electron transfer kinetics). Both techniques were found to be capable of differentiating the protein-covered surface from bare carbon with sufficient resolution to tell whether a significant portion of the carbon surface is still active and available to detect the product of an enzyme generated analyte. These results indicate that extensive biotin/avidin derivatization of the surface does decrease the electron transfer rate of a carbon electrode, and that the photolithographic approach was able to modify specific sections of the electrode surface, while leaving other regions untouched and available for facile electron transfer. This leads to a more general protocol for the construction of enzyme-based biosensors which utilize diffusable mediators.

Application of screen-printed electrodes as transducers in affinity flow-through sensor systems

An affinity flow-through sensor system based on a heterogeneous competitive affinity assay for the determination of low molecular weight compounds is described using the examples of biotin and atrazine determination. The binding proteins, either streptavidin or a biotinylated monoclonal antibody, were immobilized on a biotinylated screen-printed electrode, where the competition between the analyte and an analyte-enzyme-conjugate took place. Determination of the bound enzyme was done through the supply of suitable enzyme substrates and electrochemical determination of an enzyme reaction product. In the assays described here, peroxidase was used as enzyme label. As hydrogen peroxide and hydroquinone were used as enzyme substrates, the amount of enzyme retained at the screen-printed graphite electrode was determined amperometrically at a reducing potential of -600 mV vs a screen-printed platinum electrode. The activation of the electrode by biotinylation was done in a batch procedure outside the system, before the electrode was inserted. All following steps of the assay were performed automatically in an unsegmented flow-through system through an appropriate delivery of required reagents. The system was optimized mainly through the determination of biotin. This assay was based on the competition between biotin and biotinylated peroxidase for the binding sites of streptavidin. The method showed a linear range from 0.045 to 2 micrograms/l (r2 = 0.9997, n = 7) with RSD lower than 3.8%. The system was modified further by using a biotinylated monoclonal antibody against atrazine for analyte recognition and performing a competitive assay between atrazine and a triazine-peroxidase-conjugate. The linear range was from 0.01 to 10 micrograms/l, with IC50 = 0.4 microgram/l and RSD lower than 4.6%. The method was also applied to atrazine spiked water samples. Regeneration of the sensor surface was based on removal of streptavidin in both assays.