Layer-by-layer electrodeposition of redox polymers and enzymes on screen-printed carbon electrodes for the preparation of reagentless biosensors

Layer-by-layer electrodeposition of redox polymer/enzyme composition films on screen-printed carbon electrodes for fabrication of reagentless enzyme biosensors has been proposed and the resulting films were found to be very stable and rigid.     

Electron-transfer mechanisms through biological redox chains in multicenter enzymes

A new approach for studying intramolecular electron transfer in multicenter enzymes is described. Two fumarate reductases, adsorbed on an electrode in a fully active state, have been studied using square-wave voltammetry as a kinetic method to probe the mechanism of the long-range electron transfer to and from the buried active site. Flavocytochrome c(3) (Fcc(3)), the globular fumarate reductase from Shewanella frigidimarina, and the soluble subcomplex of the membrane-bound fumarate reductase of Escherichia coli (FrdAB) each contain an active site FAD that is redox-connected to the surface by a chain of hemes or Fe-S clusters, respectively. Using square-wave voltammetry with large amplitudes, we have measured the electron-transfer kinetics of the FAD cofactor as a function of overpotential. The results were modeled in terms of the FAD group receiving or donating electrons either via a direct mechanism or one involving hopping via the redox chain. The FrdAB kinetics could be described by both models, while the Fcc(3) data could only be fit on the basis of a direct electron-transfer mechanism. This raises the likelihood that electron transfer can occur via a superexchange mechanism utilizing the heme groups to enhance electronic coupling. Finally, the FrdAB data show, in contrast to Fcc(3), that the maximum ET rate at high overpotential is related to the turnover number for FrdAB measured previously so that electron transfer is the limiting step during catalysis.     

Multicomponent redox gradients on photoactive electrode surfaces

Redox gradients have been used to tailor the arrangement of photoactive ITO-electrodes at the molecular level.     

Self-assembled monolayers on electrode surfaces: a probe for redox kinetics

The formation and characterization of self-assembled monolayers of organosulfur compounds like alkanethiols and dialkyl (di)sulfides on metal surfaces such as gold are areas of current research interest. The presence of an aromatic ring in a thiol molecule can enhance the binding between Au and the thiol, resulting in the formation of compact and impervious self-assembled monolayers. The formation of a monolayer of 2-mercaptobenzothiazole (MBT), containing an aromatic group with a fused thiazole ring but no long alkyl chain, is achieved on a gold electrode surface. Voltammetric investigations of ferro/ferricyanide and ferrous/ferric redox systems carried out on this Au|MBT electrode are reported. Further, the possibility of using such an Au|MBT electrode to distinguish between inner and outer sphere electron transfer reactions is indicated. Received: 2 January 1998 / Accepted: 14 May 1998     

Biotinylated polypyrrole films: an easy electrochemical approach for the reagentless immobilization of bacteria on electrode surfaces

Biotinylated bacteria were immobilized onto biotin/avidin modified electrode surfaces. Firstly, an electrospotting deposition method, followed by fluorescence microscopy, showed that bacteria were specifically grafted onto a gold surface. Fluorescence intensity versus the quantity of bacteria deposited on the surface was correlated, allowing determination of the microbial saturation point. Secondly, biotinylated bacteria were immobilized onto a glassy carbon macro-electrode in order to assess immobilized bacterial denitrification activity. During a 7-day trial, the modified electrode completely denitrified 5 mM nitrate, with a rate of 1.66 mM/day over the first 3 days. When the same electrode was placed in fresh nitrate solution, the denitrification rate dropped to 0.80 mM/day. Crucially, the immobilized bacteria did not become detached from the electrode during the study.     

Poly(dicarbazole-N-hydroxysuccinimide) film: a new polymer for the reagentless grafting of enzymes and redox mediators

Upon oxidative electropolymerisation of a new dicarbazole derivative functionalised by a N-hydroxysuccinimide group in acetonitrile, electroactive poly(dicarbazole) films were formed on electrodes. The subsequent chemical functionalisation of the poly(dicarbazole) film was easily performed by successive immersions in aqueous enzyme and mediator solutions. Investigations by cyclic voltammetry showed that thionine and toluidine blue o have been irreversible bound to the poly(dicarbazole) backbone. The amperometric responses of the poly(dicarbazole) films grafted with polyphenol oxidase (PPO) and thionine to catechol were then investigated at −0.2 V vs. SCE. The catechol sensitivity and the maximum current at saturating catechol concentrations were 14.57 mA M⁻¹cm⁻² and 15.43 μA cm⁻², respectively. The comparison of this catechol sensitivity with that of a poly(dicarbazole) film only modified with PPO (3.40 mA M⁻¹cm⁻²) highlighted the improvement of the biosensor performance brought by thionine as immobilised redox mediator.     

Electrochemical biosensors based on enzymes immobilized in electropolymerized films

A review based on 135 references concerns the design and properties of electrochemical biosensors for 13 different substrates of enzymatic reactions. In the sensors discussed the enzymes are immobilized within or on the top of electropolymerized films, mostly of conducting polymers. Amperometric detection is most often used for internal electrochemical sensing.Dedicated to our late colleague Wojciech Matuszewski     

Developmental aspects of amperometric ATP biosensors based on entrapped enzymes

A novel concept for a dual-enzyme-based microbiosensor for the detection of adenosine-5′-triphosphate (ATP) was developed. The employed enzymes pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQ-GDH) and hexokinase were entrapped, using pH-shift-induced precipitation of electrodeposition paint (EDP) at platinum microelectrodes (diameter of 25 µm). PQQ-GDH is known showing a superior activity for glucose conversion at the relevant conditions (low oxygen concentration) for ATP detection in targeted biomedical studies. For immobilizing the two enzymes PQQ-GDH and hexokinase, the deposition conditions of EDP Resydrol AY498w/35WA were adapted to ensure high immobilization rates. Prior to ATP sensing, the conversion of glucose, which is the co-substrate for both enzymatic reactions, was optimized. Optimization was targeted towards ATP measurements in biomedical environments by optimizing the PQQ-GDH sensor for glucose. Therefore, different mediators were tested regarding their electron transfer rate and their compatibility with the enzyme: free-diffusing N-methylphenazonium methyl sulfate (PMS) and ferrocenemethanol, and an immobilized chromium hexacyanoferrate layer at platinum electrode. Free-diffusing ferrocenemethanol reveals high sensitivity towards glucose of 1.5?±?0.4 nA/mM. In a next step, hexokinase was co-entrapped in the polymer film resulting in a sensitivity of up to 290 pA/µM.     

Chemosensors and biosensors based on polyelectrolyte microcapsules containing fluorescent dyes and enzymes

The concept of enzyme-assisted substrate sensing based on use of fluorescent markers to detect the products of enzymatic reaction has been investigated by fabrication of micron-scale polyelectrolyte capsules containing enzymes and dyes in one entity. Microcapsules approximately 5 μm in size entrap glucose oxidase or lactate oxidase, with peroxidase, together with the corresponding markers Tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride (Ru(dpp)) complex and dihydrorhodamine 123 (DHR123), which are sensitive to oxygen and hydrogen peroxide, respectively. These capsules are produced by co-precipitation of calcium carbonate particles with the enzyme followed by layer-by-layer assembly of polyelectrolytes over the surface of the particles and incorporation of the dye in the capsule interior or in the multilayer shell. After dissolution of the calcium carbonate the enzymes and dyes remain in the multilayer capsules. In this study we produced enzyme-containing microcapsules sensitive to glucose and lactate. Calibration curves based on fluorescence intensity of Ru(dpp) and DHR123 were linearly dependent on substrate concentration, enabling reliable sensing in the millimolar range. The main advantages of using these capsules with optical recording is the possibility of building single capsule-based sensors. The response from individual capsules was observed by confocal microscopy as increasing fluorescence intensity of the capsule on addition of lactate at millimolar concentrations. Because internalization of the micron-sized multi-component capsules was feasible, they could be further optimized for in-situ intracellular sensing and metabolite monitoring on the basis of fluorescence reporting.     

Electrochemical sensors and biosensors based on redox polymer/carbon nanotube modified electrodes: A review

The aim of this review is to present the contributions to the development of electrochemical sensors and biosensors based on polyphenazine or polytriphenylmethane redox polymers together with carbon nanotubes (CNT) during recent years. Phenazine polymers have been widely used in analytical applications due to their inherent charge transport properties and electrocatalytic effects. At the same time, since the first report on a CNT-based sensor, their application in the electroanalytical chemistry field has demonstrated that the unique structure and properties of CNT are ideal for the design of electrochemical (bio)sensors. We describe here that the specific combination of phenazine/triphenylmethane polymers with CNT leads to an improved performance of the resulting sensing devices, because of their complementary electrical, electrochemical and mechanical properties, and also due to synergistic effects. The preparation of polymer/CNT modified electrodes will be presented together with their electrochemical and surface characterization, with emphasis on the contribution of each component on the overall properties of the modified electrodes. Their importance in analytical chemistry is demonstrated by the numerous applications based on polymer/CNT-driven electrocatalytic effects, and their analytical performance as (bio) sensors is discussed.     

Tunnel electrode: Part II. Electron-transfer theory at an n-type semiconductor tunnel electrode

The formula for the cathodic tunnel current at an n-type semiconductor tunnel electrode was derived from the double adiabatic perturbation theory considering the vibration in the first coordination sphere. In the low-temperature limiting case, the apparent difference between the normal and the abnormal regions was observed in both the transfer coefficient α and the activation energy E* as a function of potential. It was concluded that the semiconductor tunnel electrode has advantageous characteristics for investigation of the electron-transfer mechanism at high overvoltage.     

Electron-transfer properties of different carbon nanotube materials,and their use in glucose biosensors

Different types of carbon nanotube material (single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) of different internal diameter) have been used for preparation of CNT-modified glassy-carbon electrodes. Redox reactions involving ferricyanide and hydrogen peroxide were examined at the CNT-modified electrodes. Electrodes modified with SWCNTs usually had better electron-transfer properties than MWCNT-modified electrodes. Glucose biosensors were also prepared with electropolymerized polyphenylenediamine films, CNT materials, and glucose oxidase. Amperometric behavior in glucose determination was examined. SWCNT-modified glucose biosensors usually had a wider dynamic range (from 0.1 to 5.5 mmol L⁻¹) and greater sensitivity in glucose determination. The detection limit was estimated to be 0.05 mmol L⁻¹.     

Electrical communication between electrode and dehydrogenase by a ferrocene-labeled high molecular-weight cofactor derivative: application to a reagentless biosensor

A ferrocene-labeled high molecular weight cofactor derivative (PEI-Fc-NAD) was prepared by attaching both ferrocene and nicotinamide adenine dinucleotide (NAD⁺) to a water soluble polyelectrolyte, polyethylenimine (PEI). Approximately 9.8% and 2.9% of all the primary amino groups of PEI were coupled with ferrocene and the bioactive cofactor, respectively. The cyclic voltammograms of PEI-Fc-NAD exhibited a one-electron transfer process, and the difference between the anodic and cathodic peak potential was found to be 80 mV. The PEI-Fc-NAD was used together with NAD-dependent dehydrogenase to construct a reagentless biosensor. An NAD-dependent alcohol dehydrogenase (ADH) was selected as the model enzyme, and both ADH and PEI-Fc-NAD were retained onto the sensing area of gold electrodes by a dialysis membrane or immobilized by layer-by-layer adsorption method. In both cases, the modified electrodes showed current response to ethanol without the addition of native NAD⁺ to the system, which suggested that the electrical communication between ADH and the electrode was achieved through PEI-Fc-NAD. In summary, PEI-Fc-NAD provides a new way for immobilization of mediator and cofactor, and exhibits the potential as a platform for constructing reagentless NAD-dependent dehydrogenase biosensors.     

Theory of electrode reactions of redox couples confined to electrode surfaces at monolayer levels: Part I. Expression of the current-potential relationship for simple redox reactions

A statistical mechanical treatment of redox couples and an activated complex confined to electrode surfaces at monolayer levels is presented in which not only interaction energies but also molecular arrangements are taken into consideration. Expressions for the activity coefficients of the relevant species are derived using a two-dimensional quasi-crystalline lattice model and the quasi-chemical approximation introduced by Guggenheim. An expression of the current-potential relationship is derived for simple one-step surface redox-electrode reactions of the species confined to electrode surfaces at monolayer levels based on the transition-state rate theory. It is shown that the equation derived for the current-potential relationship includes, as a limiting case, the one that has been derived on the assumption of a random distribution of the adsorbed species.     

Amperometric biosensors based on deposition of gold and platinum nanoparticles on polyvinylferrocene modified electrode for xanthine detection

In this study, new xanthine biosensors, XO/Au/PVF/Pt and XO/Pt/PVF/Pt, based on electroless deposition of gold(Au) and platinum(Pt) nanoparticles on polyvinylferrocene(PVF) coated Pt electrode for detection of xanthine were presented. The amperometric responses of the enzyme electrodes were measured at the constant potential, which was due to the electrooxidation of enzymatically produced H₂O₂. Compared with XO/PVF/Pt electrode, XO/Au/PVF/Pt and XO/Pt/PVF/Pt exhibited excellent electrocatalytic activity towards the oxidation of the analyte. Effect of Au and Pt nanoparticles was investigated by monitoring the response currents at the different deposition times and the different concentrations of KAuCl₄ and PtBr₂. Under the optimal conditions, the calibration curves of XO/Au/PVF/Pt and XO/Pt/PVF/Pt were obtained over the range of 2.5 × 10^?3 to 0.56 mM and 2.0 × 10^?3 to 0.66 mM, respectively. The detection limits were 7.5 × 10^?4 mM for XO/Au/PVF/Pt and 6.0 × 10^?4 mM for XO/Pt/PVF/Pt. The effects of interferents, the operational and the storage stabilities of the biosensors and the applicabilities of the proposed biosensors to the drug samples analysis were also evaluated.     

Electrochemical biosensors based on redox carbon nanotubes prepared by noncovalent functionalization with 1,10-phenanthroline-5,6-dione

To improve the electrocatalytic activities of carbon nanotubes (CNT) towards the oxidation of nicotinamide adenine dinucleotide (NADH), we derive them with a redox mediator, 1,10-phenanthroline-5,6-dione (PD), by the noncovalent functionalization method. The redox carbon nanotubes (PD/CNT/GC) show excellent electrocatalytic activities towards the oxidation of NADH (catalytic reaction rate constant, k(h) = 7.26 × 10(3) M(-1) s(-1)), so the determination of NADH can be achieved with a high sensitivity of 8.77 μA mM(-1) under the potential of 0.0 V with minimal interference. We also develop an amperometric ethanol biosensor by integration of alcohol dehydrogenase (ADH) within the redox carbon nanotubes (PD/CNT/GC). The ethanol biosensor exhibits a wide linear range up to 7 mM with a lower detection limit of 0.30 mM as well as a high sensitivity of 10.85 nA mM(-1).     

Theory of electrode reactions of redox couples confined to electrode surfaces at monolayer levels: Part II. Cyclic voltammetry and ac impedance measurements

The expression of the current-potential relationship derived in Part I for simple one-step surface redox-electrode reactions of the species confined to electrode surfaces is applied to cyclic voltammetry and the method of faradaic impedance measurements. A method to obtain cyclic voltammograms is presented for a quasi-reversible or general case and equations for reversible and irreversible cyclic voltammograms are derived. The effect of the interaction parameters, W/RT and ΔW^≠/RT, kinetic parameters, Λ and α, and coordination number z on the waveform of the cyclic voltammograms is discussed. An interesting feature in the voltammograms, i.e. the appearance of two peaks, is predicted when W/RT < −2.19 for z = 6 in spite of the simple one-step redox process. Furthermore, equations for the faradaic resistance and capacitance are derived and it is shown that the faradaic impedance is independent of the frequency of ac perturbation.     

Direct electron transfer between heme-containing enzymes and electrodes as basis for third generation biosensors

Direct electron transfer (DET) between redox enzymes and electrodes found the basis for third generation biosensors. Recent investigations in the authors’ laboratories on the bioelectrochemistry of heme-containing proteins and enzymes, primarily peroxidases, but also cellobiose dehydrogenase, will be reviewed.