Reagentless oxidoreductase sensors

Pyrroloquinoline quinone-dependent glucose dehydrogenase bioelectrodes based on one-step electrochemical entrapment over single-wall carbon nanotubes

Development of effective direct electron transfer is considered an interesting platform to obtain high performance bioelectrodes. Therefore, designing of scalable and cost-effective immobilization routes that promotes correct direct electrical contacting between the electrode material and the redox enzyme is still required. As we present here, electrochemical entrapment of pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQ-GDH) on single-wall carbon nanotube (SWCNT)-modified electrodes was carried out in a single step during electrooxidation of para-aminophenyl phosphonic acid (4-APPA) to obtain active bioelectrodes. The adequate interaction between SWCNTs and the enzyme can be achieved by making use of phosphorus groups introduced during the electrochemical co-deposition of films, improving the electrocatalytic activity towards glucose oxidation. Two different procedures were investigated for electrode fabrication, namely the entrapment of reconstituted holoenzyme (PQQ-GDH) and the entrapment of apoenzyme (apo-GDH) followed by subsequent in situ reconstitution with the redox cofactor PQQ. In both cases, PQQ-GDH preserves its electrocatalytic activity towards glucose oxidation. Moreover, in comparison with a conventional drop-casting method, an important enhancement in sensitivity was obtained for glucose oxidation (981.7 ± 3.5 nA mM^-1) using substantially lower amounts of enzyme and cofactor (PQQ). The single step electrochemical entrapment in presence of 4-APPA provides a simple method for the fabrication of enzymatic bioelectrodes.

Recent advances on developing 3rd generation enzyme electrode for biosensor applications

The electrochemical biosensor with enzyme as biorecognition element is traditionally pursued as an attractive research topic owing to their high commercial perspective in healthcare and environmental sectors. The research interest on the subject is sharply increased since the beginning of 21st century primarily, due to the concomitant increase in knowledge in the field of material science. The remarkable effects of many advance materials such as, conductive polymers and nanomaterials, were acknowledged in the developing efficient 3rd generation enzyme bioelectrodes which offer superior selectivity, sensitivity, reagent less detection, and label free fabrication of biosensors. The present review article compiles the major knowledge surfaced on the subject since its inception incorporating the key review and experimental papers published during the last decade which extensively cover the development on the redox enzyme based 3rd generation electrochemical biosensors. The tenet involved in the function of these direct electrochemistry based enzyme electrodes, their characterizations and various strategies reported so far for their development such as, nanofabrication, polymer based and reconstitution approaches are elucidated. In addition, the possible challenges and the future prospects in the development of efficient biosensors following this direct electrochemistry based principle are discussed. A comparative account on the design strategies and critical performance factors involved in the 3rd generation biosensors among some selected prominent works published on the subject during last decade have also been included in a tabular form for ready reference to the readers.

Layered-double-hydroxide-modified electrodes: electroanalytical applications

Two-dimensional inorganic solids, such as layered double hydroxides (LDHs), also defined as anionic clays, have open structures and unique anion-exchange properties which make them very appropriate materials for the immobilization of anions and biomolecules that often bear an overall negative charge. This review aims to describe the important aspects and new developments of electrochemical sensors and biosensors based on LDHs, evidencing the research from our own laboratory and other groups. It is intended to provide an overview of the various types of chemically modified electrodes that have been developed with these 2D layered materials, along with the significant advances made over the last several years. In particular, we report the main methods used for the deposition of LDH films on different substrates, the conductive properties of these materials, the possibility to use them in the development of membranes for potentiometric anion analysis, the early analytical applications of chemically modified electrodes based on the ability of LDHs to preconcentrate redox-active anions and finally the most recent applications exploiting their electrocatalytic properties. Another promising application field of LDHs, when they are employed as host structures for enzymes, is biosensing, which is described considering glucose as an example.

Stripping Voltammetric Determination of Analgesics in Their Pharmaceuticals Using Nano-Riboflavin-Modified Glassy Carbon Electrode

Cyclic voltammetric behaviors of three analgesics, acetaminophen (AAP), acetylsalicylic acid (ASA), and dipyrone (DP), were studied using nano-riboflavin-modified glassy carbon electrode. One well-defined oxidation peak each for AAP and ASA and three oxidation peaks for DP were observed. The influence of pH, scan rate, and concentration reveals irreversible diffusion controlled reaction. The SEM analysis confirmed good accumulation of the drugs on the electrode surface. Calibration was made under the maximum peak current conditions. The concentration range studied for the determination of drugs was 0.02 to 0.4 μg mL−1for AAP and ASA and 0.025 to 0.4 μg mL−1for DP. The lower limit of detection observed for AAP, ASA, and DP was 0.016, 0.007 μg mL−1, and 0.013 μg mL−1, respectively. The suitability of the method for the determination of these analgesics in pharmaceutical preparations and urine samples was also ascertained.

Biosensors for the determination of phenolic metabolites

Antioxidants are groups of chemical substances, the most abundant being polyphenols, mainly found in plants, fruits and vegetables. They include flavonoids, flavonoid derivatives, polyphenols, carotenoids and anthocyanins. Currently, the nutritional quality of many foodstuffs is guaranteed by the presence of antioxidant compounds. The importance of these chemicals as indicators and preservatives of nutritional quality makes necessary the development of accurate, versatile and rapid analytical tools necessary to detect their presence in many foodstuffs and to assess their antioxidant efficacy. In this chapter, enzyme-based biosensors such as monophenol monooxygenase (tyrosinase), catechol oxidase (laccase) and horseradish peroxidase (HRP) are reviewed. Actually, these biosensors are the most commonly used for the detection of polyphenols and flavonoids content.

Integrated nanoparticle-biomolecule hybrid systems: synthesis, properties, and applications

Nanomaterials, such as metal or semiconductor nanoparticles and nanorods, exhibit similar dimensions to those of biomolecules, such as proteins (enzymes, antigens, antibodies) or DNA. The integration of nanoparticles, which exhibit unique electronic, photonic, and catalytic properties, with biomaterials, which display unique recognition, catalytic, and inhibition properties, yields novel hybrid nanobiomaterials of synergetic properties and functions. This review describes recent advances in the synthesis of biomolecule-nanoparticle/nanorod hybrid systems and the application of such assemblies in the generation of 2D and 3D ordered structures in solutions and on surfaces. Particular emphasis is directed to the use of biomolecule-nanoparticle (metallic or semiconductive) assemblies for bioanalytical applications and for the fabrication of bioelectronic devices.

Visualization of micropatterned complex biosensor sensing chemistries by means of scanning electrochemical microscopy

Redox hydrogel-based micropatterned complex biosensor architectures, used as sensing chemistries in amperometric ethanol or glucose biosensors, were deposited on gold, graphite or glass. Well-localized immobilization of active hydrogels with variable compositions was achieved by dispensing 100 pl droplets of cocktails containing alcohol or glucose dehydrogenase, redox polymer (PVI(13)dmeOs) and crosslinker (PEGDGE) while moving the target surface relative to the position of the nozzle of a piezo-actuated microdispenser. The resulting structures were microscopic patterns of enzyme-containing lines of a redox hydrogel with a line width of about 100 microm. Scanning electrochemical microscopy (SECM) in the amperometric feedback mode was used to visualize the immobilized enzyme microstructures and their localized biochemical activity was observed with high lateral resolution by detecting the enzymatically consumed substrate using K(4)[Fe(CN)(6)] as a free-diffusing electron-transfer mediator.

Electrical communication between NAD-dependent enzyme and metal electrode using ferrocene-labeled NAD derivatives

The target N6-ferrocene substituted NAD derivatives (ferrocene-labeled NAD, Fc-NAD) were synthesized as an electron-relay NAD cofactor. The synthesized NAD derivatives possessed some biological activities as substrates in the alcohol dehydrogenase systems. The oxidation current depended on both addition of ethanol and spacer length of Fc-NAD.

Reagentless biosensors based on co-entrapment of a soluble redox polymer and an enzyme within an electrochemically deposited polymer film

A novel biosensor architecture, which is based on the combination of a manual and a non-manual deposition technique for sensor components on the electrode surface is reported. A water-soluble Os-poly(vinyl-imidazole) redox hydrogel is deposited on a graphite electrode by drop-coating (i.e. manually) followed by the electrochemically-induced deposition of an enzyme-containing non-conducting polymer film. The local polymer deposition is initiated by electrochemical generation of H(3)O(+) exclusively at the electrode surface causing a pH-shift to be established in the diffusion zone around the electrode (i.e. non-manually). This pH-shift leads to the protonation of a dissolved polyanionic polymer which in consequence changes significantly its solubility and is hence precipitating on the electrode surface. In the presence of a suitable enzyme, such as quinohemoprotein alcohol dehydrogenase (QH-ADH), the polymer precipitation leads to an entrapment of the redox enzyme within the polymer film. Simultaneously, the water-soluble Os-poly(vinyl-imidazole) redox hydrogel, which is slowly dissolving from the electrode surface after addition of the electrolyte, is co-entrapped within the precipitating polymer layer. This provides the pre-requisite for an efficient electron-transfer pathway from the redox enzyme via the polymer-bound redox centres to the electrode surface. The sensor preparation protocol has been optimised aiming on a high mediator concentration in the polymer film and an effective electron transfer.

Mediated biosensors

Direct electrode transfer between enzyme and the electrode in biosensors requires high efficiency therefore, synthetic replacement for oxygen led to the development of enzyme mediators and modified electrodes in biosensor fabrication. In this context, a number of electron acceptors and complexes have been used. Present paper gives an overview of various methodologies involved in the mediated systems, their merits and wide applications.

Amperometric enzyme biosensors based on optimised electron-transfer pathways and non-manual immobilisation procedures

Development of reagentless biosensors implies the tight and functional immobilisation of biological recognition elements on transducer surfaces. Specifically, in the case of amperometric enzyme electrodes, electron-transfer pathways between the immobilised redox protein and the electrode surface have to be established allowing a fast electron transfer concomitantly avoiding free-diffusing redox species. Based on the specific nature of different redox proteins and non-manual immobilisation procedures possible biosensor designs are discussed, namely biosensors based on (i) direct electron transfer between redox proteins and electrodes modified with self-assembled monolayers; (ii) anisotropic orientation of redox proteins at monolayer-modified electrodes; (iii) electron-transfer cascades via redox hydrogels; and (iv) electron-transfer via conducting polymers.

Amperometric Determination of Glucose with a Carbon Paste Biosensor

A simple glucose biosensor has been developed by bulk modification of a carbon paste electrode with glucose oxidase as a biocomponent and manganese dioxide as a mediator. The sensor was employed as amperometric detector in a flow-injection system at 21 °C in 0.2 M phosphate buffer (pH 7.5). At an applied working potential of 0.48 V vs Ag/AgCl and a flow rate of 0.2 ml min-1, the sensor exhibited well reproducible amperometric response to glucose. A linear relation between the peak current and the analyte concentration was found between 20 and 500 mg l-1 glucose with a detection limit (3σ) of 10.5 mg l-1 glucose. The sensor can be operated continuously for 12 h without any loss in the signal height and can be used for the determination of glucose in white wine samples.

Chronopotentiometry and Faradaic impedance spectroscopy as signal transduction methods for the biocatalytic precipitation of an insoluble product on electrode supports: routes for enzyme sensors, immunosensors and DNA sensors

The biocatalyzed precipitation of an insoluble product produced on electrode supports is used as an amplification path for biosensing. Enzyme-based electrodes, immunosensors and DNA sensors are developed using this biocatalytic precipitation route. Faradaic impedance spectroscopy and chronopotentiometry are used as transduction methods to follow the precipitation processes. While Faradaic impedance spectroscopy leads to the characterization of the electron-transfer resistance at the electrode, chronopotentiometry provides the total resistance at the interfaces of the modified electrodes. A horseradish peroxidase, HRP, monolayer-functionalized electrode is used to sense H(2)O(2) by the biocatalyzed oxidation of 4-chloro-1-naphthol (1), to the insoluble product benzo-4-chlorohexadienone (2). An antigen monolayer electrode is used to sense the dinitrophenyl antibody, DNP-Ab, applying an anti-antibody-HRP conjugate as a biocatalyst for the oxidative precipitation of 1 by H(2)O(2) to yield the insoluble product 2. An oligonucleotide (3) functionalized monolayer electrode is used to sense the DNA-analyte (4), that is one of the Tay-Sachs genetic disorder mutants. Association of a biotin-labeled oligonucleotide to the sensing interface, followed by the association of the avidin-HRP conjugate and the biocatalyzed precipitation of 2 leads to the amplified sensing of 4. The amount of the precipitate accumulated on the conductive support is controlled by the concentration of the respective analytes and the time intervals employed for the biocatalytic precipitation of 2. The electron-transfer resistances of the electrodes covered by the insoluble product (2) are derived from Faradaic impedance measurements, whereas the total electrode resistances are extracted from chronopotentiometric experiments. A good correlation between the total electrode resistances and the electron-transfer resistances at the conducting supports are found. Chronopotentiometry is suggested as a rapid transduction means (a few seconds). The precautions needed to apply chronopotentiometry in biosensors are discussed.

Electrocatalytic reduction of lipoic acid and electroenzymatic reduction of NAD(P)(+) for integrated dehydrogenase biosensors

Biosensors for the oxidized substrates of NAD(P)(+)-specific dehydrogenases demand the reductive recycling of the coenzymes. So far, suitable catalysts for the corresponding two-electron transfer are not available. In the present paper, this transport has been realized by a combined electrocatalytical and electroenzymatic process. Lipoic acid has been reduced on graphite electrodes functionalized with Fe(II)-phthalocyanine in 95% yield at-1200 mV in phosphate buffer pH 7.0. With the electrocatalytically reduced product, dihydrolipoic acid, lipoamide dehydrogenase could reduce NAD(+) in 20% yield and thioredoxin reductase NADP(+) in 18.4% yield. So far, the combined electrocatalytic/electroenzymatic system has not yet been realized, mainly because at the potential needed for the lipoic acid reduction, a parallel one-electron reduction of NAD(P)(+) was observed, implying the dimerization of the coenzyme.

An integrated NAD+-dependent enzyme-functionalized field-effect transistor (ENFET) system: development of a lactate biosensor

An integrated NAD+-dependent enzyme field-effect transistor (ENFET) device for the biosensing of lactate is described. The aminosiloxane-functionalized gate interface is modified with pyrroloquinoline quinone (PQQ) that acts as a catalyst for the oxidation of NADH. Synthetic amino-derivative of NAD+ is covalently linked to the PQQ monolayer. An affinity complex formed between the NAD+/PQQ-assembly and the NAD+-cofactor-dependent lactate dehydrogenase (LDH) is crosslinked and yields an integrated biosensor ENFET-device for the analysis of lactate. Biocatalyzed oxidation of lactate generates NADH that is oxidized by PQQ in the presence of Ca2+-ions. The reduced catalyst, PQQH2, is oxidized by O2 in a process that constantly regenerates PQQ at the gate interface. The biocatalyzed formation of NADH and the O2-stimulated regeneration of PQQ yield a steady-state pH gradient between the gate interface and the bulk solution. The changes in the pH of the solution near the gate interface and, consequently, the gate potential are controlled by the substrate (lactate) concentration in the solution. The device reveals the detection limit of 1 x 10(-4) M for lactate and the sensitivity of 24+/-2 mV dec(-1). The response time of the device is as low as 15 s.