Enhancing biosensor performance using multienzyme systems

Overview on the Development of Electrochemical Immunosensors by the Signal Amplification of Enzyme- or Nanozyme-Based Catalysis Plus Redox Cycling

Enzyme-linked electrochemical immunosensors have attracted considerable attention for the sensitive and selective detection of various targets in clinical diagnosis, food quality control, and environmental analysis. In order to improve the performances of conventional immunoassays, significant efforts have been made to couple enzyme-linked or nanozyme-based catalysis and redox cycling for signal amplification. The current review summarizes the recent advances in the development of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling for signal amplification. The special features of redox cycling reactions and their synergistic functions in signal amplification are discussed. Additionally, the current challenges and future directions of enzyme- or nanozyme-based electrochemical immunosensors with redox cycling are addressed.

Multienzyme nanoassemblies: from rational design to biomedical applications

Multienzyme nanoassemblies (MENAs) that combine the functions of several enzymes into one entity have attracted widespread research interest due to their improved enzymatic performance and great potential for multiple applications. Considerable progress has been made to design and fabricate MENAs in recent years. This review begins with an introduction of the up-to-date strategies in designing MENAs, mainly including substrate channeling, compartmentalization and control of enzyme stoichiometry. The desirable properties that endow MENAs with important applications are also discussed in detail. Then, the recent advances in utilizing MENAs in the biomedical field are reviewed, with a particular focus on biosensing, tumor therapy, antioxidant and drug delivery. Finally, the challenges and perspectives for development of versatile MENAs are summarized.

Electrochemical biosensors based on multienzyme systems: Main groups, advantages and limitations - A review

In the review, the principles and main purposes of using multienzyme systems in electrochemical biosensors are analyzed. Coupling several enzymes allows an extension of the spectrum of detectable substances, an increase in the biosensor sensitivity (in some cases, by several orders of magnitude), and an improvement of the biosensor selectivity, as showed on the examples of amperometric, potentiometric, and conductometric biosensors. The biosensors based on cascade, cyclic and competitive enzyme systems are described alongside principles of function, advantages, disadvantages and practical use for real sample analyses in various application areas (food production and quality control, clinical diagnostics, environmental monitoring). The complications and restrictions regarding the development of multienzyme biosensors are evaluated. The recommendations on the reasonability of elaboration of novel multienzyme biosensors are given.

Immobilized Enzymes in Biosensor Applications

Enzyme-based biosensing devices have been extensively developed over the last few decades, and have proven to be innovative techniques in the qualitative and quantitative analysis of a variety of target substrates over a wide range of applications. Distinct advantages that enzyme-based biosensors provide, such as high sensitivity and specificity, portability, cost-effectiveness, and the possibilities for miniaturization and point-of-care diagnostic testing make them more and more attractive for research focused on clinical analysis, food safety control, or disease monitoring purposes. Therefore, this review article investigates the operating principle of enzymatic biosensors utilizing electrochemical, optical, thermistor, and piezoelectric measurement techniques and their applications in the literature, as well as approaches in improving the use of enzymes for biosensors.

Sensors Based on Bio and Biomimetic Receptors in Medical Diagnostic, Environment, and Food Analysis

Analytical chemistry is now developing mainly in two areas: automation and the creation of complexes that allow, on the one hand, for simultaneously analyzing a large number of samples without the participation of an operator, and on the other, the development of portable miniature devices for personalized medicine and the monitoring of a human habitat. The sensor devices, the great majority of which are biosensors and chemical sensors, perform the role of the latter. That last line is considered in the proposed review. Attention is paid to transducers, receptors, techniques of immobilization of the receptor layer on the transducer surface, processes of signal generation and detection, and methods for increasing sensitivity and accuracy. The features of sensors based on synthetic receptors and additional components (aptamers, molecular imprinted polymers, biomimetics) are discussed. Examples of bio- and chemical sensors' application are given. Miniaturization paths, new power supply means, and wearable and printed sensors are described. Progress in this area opens a revolutionary era in the development of methods of on-site and in-situ monitoring, that is, paving the way from the "test-tube to the smartphone".

Bioelectronic Circuit on a 3D Electrode Architecture: Enzymatic Catalysis Interconnected with Photosystem I

Artificial light-driven signal chains are particularly important for the development of systems converting light into a current, into chemicals or for light-induced sensing. Here, we report on the construction of an all-protein, light-triggered, catalytic circuit based on photosystem I, cytochrome c (cyt c) and human sulfite oxidase (hSOX). The defined assembly of all components using a modular design results in an artificial biohybrid electrode architecture, combining the photophysical features of PSI with the biocatalytic properties of hSOX for advanced light-controlled bioelectronics. The working principle is based on a competitive switch between electron supply from the electrode or by enzymatic substrate conversion.

Electrochemical signal amplification for immunosensor based on 3D interdigitated array electrodes

We devised an electrochemical redox cycling based on three-dimensional interdigitated array (3D IDA) electrodes for signal amplification to enhance the sensitivity of chip-based immunosensors. The 3D IDA consists of two closely spaced parallel indium tin oxide (ITO) electrodes that are positioned not only on the bottom but also the ceiling, facing each other along a microfluidic channel. We investigated the signal intensities from various geometric configurations: Open-2D IDA, Closed-2D IDA, and 3D IDA through electrochemical experiments and finite-element simulations. The 3D IDA among the four different systems exhibited the greatest signal amplification resulting from efficient redox cycling of electroactive species confined in the microchannel so that the faradaic current was augmented by a factor of ∼100. We exploited the enhanced sensitivity of the 3D IDA to build up a chronocoulometric immunosensing platform based on the sandwich enzyme-linked immunosorbent assay (ELISA) protocol. The mouse IgGs on the 3D IDA showed much lower detection limits than on the Closed-2D IDA. The detection limit for mouse IgG measured using the 3D IDA was ∼10 fg/mL, while it was ∼100 fg/mL for the Closed-2D IDA. Moreover, the proposed immunosensor system with the 3D IDA successfully worked for clinical analysis as shown by the sensitive detection of cardiac troponin I in human serum down to 100 fg/mL.

Cascadic multienzyme reaction-based electrochemical biosensors

: Since the first glucose biosensor was developed by Clark and Lyons, there have been great efforts to develop effective enzyme biosensors for wide applications. Those efforts are closely related to the enhancement of biosensor performance, including sensitivity improvement, elevation of selectivity, and extension of the range of analytes that may be determined. Introduction of a cascadic multienzyme reaction to the electrochemical biosensor is one of those efforts. By employing more than two enzymes to the biosensor, its sensitivity and accuracy can be enhanced. Also, the narrow application range that is a typical limitation of single enzyme-based biosensor can be overcome. This chapter will discuss the fundamental principles for the development of cascadic multienzyme reaction-based electrochemical biosensors and their applications in clinical and environmental fields.

Electrochemical Immunosensor Using p-Aminophenol Redox Cycling by Hydrazine Combined with a Low Background Current

Signal amplification and noise reduction are crucial for obtaining low detection limits in biosensors. Here, we present an electrochemical immunosensor in which the signal amplification is achieved using p-aminophenol (AP) redox cycling by hydrazine, and the noise level is reduced by implementing a low background current. The redox cycling is obtained in a simple one-electrode, one-enzyme format. In a sandwich-type heterogeneous immunosensor for mouse IgG, an alkaline phosphatase label converts p-aminophenyl phosphate into AP for 10 min. This generated AP is electrooxidized at an indium tin oxide (ITO) electrode modified with a partially ferrocenyl-tethered dendrimer (Fc-D). The oxidized product, p-quinone imine (QI), is reduced back to AP by hydrazine, and then AP is electrooxidized again to QI, resulting in redox cycling. Moreover, hydrazine protects AP from oxidation by air, enabling long incubation times. The small amount of ferrocene in a 0.5% Fc-D-modified ITO electrode, where 0.5% represents the ratio of ferrocene groups to dendrimer amines, results in a low background current, and this electrode exhibits high electron-mediating activity for AP oxidation. Moreover, there is insignificant hydrazine electrooxidation on this electrode, which also results in a low background current. The detection limit of the immunosensor using a 0.5% Fc-D-modified electrode is 2 orders of magnitude lower than that of a 20% Fc-D-modified electrode (10 pg/mL vs 1 ng/mL). Furthermore, the presence of hydrazine reduces the detection limit by an additional 2 orders of magnitude (100 fg/mL vs 10 pg/mL). These results indicate that the occurrence of redox cycling combined with a low background current yields an electrochemical immunosensor with a very low detection limit (100 fg/mL). Mouse IgG could be detected at concentrations ranging from 100 fg/mL to 100 microg/mL (i.e., 9 orders of magnitude) in a single assay.

Bienzyme HRP–GOx-modified gold nanoelectrodes for the sensitive amperometric detection of glucose at low overpotentials

Gold nanotubular electrode ensembles were prepared by using electroless deposition of the metal within the pores of polycarbonate track-etched membranes. Mono-enzyme (GOx) and monolayer/bilayer bienzyme (GOx/HRP) bioelectrodes were prepared by immobilizing the enzymes onto gold nanotubes surfaces modified with mercaptoethylamine. Batch amperometric responses to glucose for the different bioelectrodes were determined and compared. The response of the two geometries (monolayer and bilayer) of the bienzyme electrodes was shown to vary with regard to sensitivity at detection potentials above 0V. On the contrary, at detection potentials below 0V, no noticeable influence of the configuration of the bienzyme on the response intensity was observed. The mono-enzyme (650 microAmM-1 in benzoquinone (BQ) at -0.8 V versus Ag/AgCl) and the two bienzyme bioelectrodes (+/-400 microAmM-1 in hydroquinone (H2Q) at -0.2V versus Ag/AgCl) display remarkable sensitivities compared to a classical GOx-modified gold macroelectrode (13 microAmM-1 in BQ at -0.8 V versus Ag/AgCl). A remarkable feature of the bienzyme electrodes is the possibility to detect glucose at very low applied potentials where the noise level and interferences from other electro-oxidizable compounds are minimal. Another important characteristic of the monolayer bienzyme electrode is the possible existence of a direct electronic communication between HRP and the transducer surface.

A screen-printed biosensor using pyruvate oxidase for rapid determination of phosphate in synthetic wastewater

A screen-printed phosphate biosensor based on immobilized pyruvate oxidase (PyOD, E.C. 1.2.3.3) has been developed for monitoring phosphate concentrations in a sequencing batch reactor (SBR) system. The enzyme was immobilized by a nafion matrix and covered a poly(carbamoyl) sulfonate (PCS) hydrogel on a screen-printed electrode. PyOD consumes phosphate in the presence of pyruvate and oxygen and generates hydrogen peroxide (H2O2), carbon dioxide and acetylphosphate. The electroactive H2O2, monitored at +420 mV vs Ag/AgCl, is generated in proportion to the concentration of phosphate. The sensor has a fast response time (2 s) and a short recovery period (2 min). The time required for one measurement using this phosphate biosensor was 4 min, which was faster than the time required using a commercial phosphate testing kit (10 min). The sensor has a linear range from 7.5 microM to 625 microM phosphate with a detection limit of 3.6 microM. There was good agreement (R2=0.9848) between the commercial phosphate testing kit and the phosphate sensor in measurements of synthetic wastewater in a SBR system. This sensor maintained a high working stability (>85%) after 12 h of operation and involved a simple operation procedure. It therefore serves as a useful tool for rapid and accurate phosphate measurements in the SBR system and probably for process control.

Amperometric determination of lactate with novel trienzyme/poly(carbamoyl) sulfonate hydrogel-based sensor

A novel trienzyme sensor for the amperometric determination of lactate was constructed by immobilizing salicylate hydroxylase (SHL, E.C. 1.14.13.1), l-lactate dehydrogenase (LDH, E.C. 1.1.1.27), and pyruvate oxidase (PyOD, E.C. 1.2.3.3) on a Clark-type oxygen electrode. The enzymes were entrapped by a poly(carbamoyl) sulfonate (PCS) hydrogel on a Teflon membrane. LDH catalyzes the specific dehydrogenation of lactate consuming NAD(+). SHL catalyzes the irreversible decarboxylation and the hydroxylation of salicylate in the presence of oxygen and NADH produced by LDH. PyOD decarboxylates pyruvate using oxygen and phosphate. SHL and PyOD force the equilibrium of dehydrogenation of lactate by LDH to the product side by consuming NADH and pyruvate, respectively. Dissolved oxygen acts as an essential material for both PyOD and SHL during their respective enzymatic reactions. Therefore, an amplified signal, caused by the consumptions of dissolved oxygen by the two enzymes, was observed in the measurement of lactate. Regeneration of cofactor was found in the trienzyme system. A Teflon membrane was used to fabricate the sensor in order to avoid interferences. The sensor has a fast response (2s) and short recovery times (2 min). The total test time for a measurement by using this lactate sensor (4 min) was faster than using a commercial lactate testing kit (up to 10 min). The sensor has a linear range between 10 and 400 microM lactate, with a detection limit of 4.3 microM. A good agreement (R2 = 0.9984) with a commercial lactate testing kit was obtained in beverage sample measurements.

Allosteric enzymes as biosensors for molecular diagnosis

Biosensors are hybrid analytical devices that amplify signals generated from the specific interaction between a receptor and the analyte, through a biochemical mechanism. Biosensors use tissues, whole cells, artificial membranes or cell components like proteins or nucleic acids as receptors, coupled to a physicochemical signal transducer. Allosteric enzymes exhibit a catalytic activity that is modulated by specific effectors, through binding to receptor sites that are distinct from the active site. Several enzymes, catalyzing easily measurable reactions, have been engineered to allosterically respond to specific ligands, being themselves the main constituent of new-generation biosensors. The molecular basis, robustness and application of allosteric enzymatic biosensing are revised here.

GDH biosensor based off-line capillary immunoassay for alkylphenols and their ethoxylates

The application of a quinoprotein glucose dehydrogenase modified thick-film sensor as label detector in a capillary immunoassay (CIA) for xenoestrogens is presented. The detection of the alkylphenols and their ethoxylates is based on the competition between the analyte and tracer molecules for the binding sites of anti-alkylphenol ethoxylate antibodies. This assay is performed off-line in small disposable PVC capillaries coated with immobilized antibodies. This format allows the combination of the assay with a small portable device potentially useful for on-site environmental monitoring. Beside high amplification the utilization of beta-galactosidase as enzyme label allows the direct combination with a GDH biosensor at optimal pH conditions. The bioelectrocatalytic properties of this biosensor offer an additional amplification and thus allow a very sensitive quantification of 4-aminophenol, generated by the beta-galactosidase. Detection limits of the analytes in the microg/l range were obtained, while other phenolics and surfactants showed no or very little cross reactivity.

Electrochemical biosensors: recommended definitions and classification

Two Divisions of the International Union of Pure and Applied Chemistry (IUPAC), namely Physical Chemistry (Commission 1.7 on Biophysical Chemistry formerly Steering Committee on Biophysical Chemistry) and Analytical Chemistry (Commission V.5 on Electroanalytical Chemistry) have prepared recommendations on the definition, classification and nomenclature related to electrochemical biosensors: these recommendations could, in the future, be extended to other types of biosensors. An electrochemical biosensor is a self-contained integrated device, which is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor) which is retained in direct spatial contact with an electrochemical transduction element. Because of their ability to be repeatedly calibrated, we recommend that a biosensor should be clearly distinguished from a bioanalytical system, which requires additional processing steps, such as reagent addition. A device that is both disposable after one measurement, i.e. single use, and unable to monitor the analyte concentration continuously or after rapid and reproducible regeneration, should be designated a single use biosensor. Biosensors may be classified according to the biological specificity-conferring mechanism or, alternatively, to the mode of physico-chemical signal transduction. The biological recognition element may be based on a chemical reaction catalysed by, or on an equilibrium reaction with macromolecules that have been isolated, engineered or present in their original biological environment. In the latter cases. equilibrium is generally reached and there is no further, if any, net consumption of analyte(s) by the immobilized biocomplexing agent incorporated into the sensor. Biosensors may be further classified according to the analytes or reactions that they monitor: direct monitoring of analyte concentration or of reactions producing or consuming such analytes; alternatively, an indirect monitoring of inhibitor or activator of the biological recognition element (biochemical receptor) may be achieved. A rapid proliferation of biosensors and their diversity has led to a lack of rigour in defining their performance criteria. Although each biosensor can only truly be evaluated for a particular application, it is still useful to examine how standard protocols for performance criteria may be defined in accordance with standard IUPAC protocols or definitions. These criteria are recommended for authors. referees and educators and include calibration characteristics (sensitivity, operational and linear concentration range, detection and quantitative determination limits), selectivity, steady-state and transient response times, sample throughput, reproducibility, stability and lifetime.

Sustainable development: how can biotechnology contribute?

Sustainable development has become a priority for the world's policy makers. Among the broad range of technologies with the potential to reach the goal of sustainability, biotechnology could take an important place, especially in the fields of food production, renewable raw materials and energy, pollution prevention, and bioremediation. However, technical and economic problems still need to be solved. In some cases, the environmental impact of biotechnological applications has been misjudged; in other cases, expectations cannot yet be matched.

Enzyme-based flow injection analysis system for glutamine and glutamate in mammalian cell culture media

We present the setup of a flow injection analysis system designed for on-line monitoring of glutamate and glutamine. These amino acids represent a major energy source in mammalian cell culture. A cycling assay consisting of glutamate dehydrogenase and aspartate aminotransferase produces NADH proportional to the glutamate concentration in the sample. NADH is then measured spectrophotometrically. Glutamine is determined by conversion to glutamate which is fed into the cycling assay. The conversion of glutamine to glutamate is catalyzed by asparaginase. Asparaginase was used in place of glutaminase due to its relatively high reactivity with glutamine and a pH optimum similar to that of glutamate dehydrogenase. The enzymes were immobilized covalently to activated controlled pore glass beads and integrated into the flow injection analysis system. The application of the immobilized enzymes and the technical setup are presented in this paper.

Automated amplified flow immunoassay for cocaine

An amplified flow immunoassay (AFIA) was developed for cocaine, which combines a noncompetitive immunoenzymometric assay (IEMA) with an on-line detection of the enzyme label alkaline phosphatase (ALP) by a substrate-recycling biosensor. In the IEMA, the analyte cocaine first binds to a labeled polyclonal anti-cocaine antibody. Then, the excess labeled antibody is separated on an affinity column that contains a perfusion chromatography carrier modified by immobilized cocaine. The unbound complexes of the analyte cocaine with the ALP-labeled antibody are detected postcolumn. The detector senses phenol produced by ALP from phenyl phosphate. As detector, an amperometric substrate-recycling biosensor was used, which consists of a Clark-type oxygen electrode covered by tyrosinase and pyrroloquinoline quinone-dependent glucose dehydrogenase. The lower limit of detection is 380 pM (38 fmol) for cocaine. The sampling rate is 26/h. Cocaine could be detected from "real samples" with an imprecision of +/- 10% (n = 3) and with a recovery of 49 +/- 3% for various concentrations. AFIA is generally important as a new approach for the fast detection of picomolar concentrations of haptens.

Ion exchanger modified PVC membranes--selectivity studies and response amplification of oxalate and lactate enzyme electrodes

We report on a novel method of oxidase enzyme electrode response amplification, using unplasticized PVC and plasticized PVC, respectively. The anion exchanger tricaprylylmethylammonium chloride (Aliquat 336s) and hydrophobic isopropylmyristate (IPM) plasticizer have been used together to modify PVC. Resulting structures are anionic substrate selective and hydrogen peroxide impermeable and can be used as outer membranes of a classical dual membrane amperometric enzyme electrode where an oxidase is used to generate H2O2 for electrochemical detection. Their effect on sensor sensitivity and linearity is considered.

Phenol-oxidizing enzymes: mechanisms and applications in biosensors

Phenolic compounds are widely distributed in nature. Enzymes which catalyze their oxidation are monophenol monooxygenases, such as tyrosinases and laccases, and peroxidases. Their metabolic role includes the decomposition of natural complex aromatic polymers as well as polymerization of the oxidation products and the degradation of xenobiotics. Their catalytic properties and broad availability gained impact on the development of biosenors for both environmentally important pollutants and clinically relevant metabolites. Mechanisms for the phenol-oxidizine enzymes tyrosinases, laccases, and peroxidases are reviewed and some examples for their use in the construction of phenol selective biosenors are given.

Commercial biosensors for medical application

Biosensors today are represented on the market worldwide by an increasing number of enzyme electrodes working in various areas of medical diagnostics, and by a few opto-immunosensor-based analytical systems suited for protein research in pharmaceutical chemistry. Enzyme electrodes for metabolites and enzyme activities have been commercially available for about 20 years. Such sensors are successfully applied in laboratory autoanalyzers, point-of-care-systems, patient self-monitoring disposable probes, intensive care analyzers, and for on-line monitoring of diabetics.

Enzymatic substrate recycling electrodes

A weak chemical signal might result in a large response when biochemically amplified. Enzymatic recycling of the analyte is one of the biochemical ways of providing an effective increase in biosensor sensitivity by several orders of magnitude. The enhancement of sensitivity is provided by consecutive consumption and generation of the analyte on the sensor surface. The principle of enzymatic substrate regeneration using bioelectrocatalysis and coupled enzymes is shortly reviewed and illustrated with some recent developments of biosensors for catecholamines, and its potential for electrochemical immunoassays is outlined.

Zeptomole-detecting biosensor for alkaline phosphatase in an electrochemical immunoassay for 2,4-dichlorophenoxyacetic acid

A bienzyme substrate-recycling biosensor in a flow injection analysis system is described for the sensitive measurement of alkaline phosphatase (ALP) and applied to the fast readout of a competitive immunoassay for the widely used pesticide 2,4-dichlorophenoxyacetic acid (2,4-D). The phenol-indicating biosensor consists of a Clark-type electrode covered by a membrane with coentrapped tyrosinase and quinoprotein glucose dehydrogenase. ALP dephosphorylates phenyl phosphate to phenol (K(m) = 36 microM) outside the flow system. Phenol is oxidized in the sensor membrane by the oxygen-consuming tyrosinase via catechol to o-quinone. The quinone is reconverted to catechol by glucose dehydrogenase. This substrate cycling results in a 350-fold amplified sensor response to phenol. The oxygen consumption of the enzyme couple in the presence of phenol is monitored as a decrease in current. A total of 3.2 fM ALP (320 zmol/ 100 microL) has been detected after a 57.5 min incubation with phenyl phosphate. All involved reagents are stable over the time of measurement. The sensor does not produce any measurable blank signals. The immunoassay detects 0.1 microgram/L 2,4-D, the maximum concentration for pesticides allowed in drinking water by European Community regulations. The applicability of this biosensor for fast immunoassay readout is demonstrated by a 2 min incubation. By comparison, a standard photometric method (p-nitrophenyl phosphate) requires overnight incubation.

From neural chip and engineered biomolecules to bioelectronic devices: an overview

At the first C.E.C. Workshop, in Brussels on 28-29 November 1991, attended by over 70 leading European scientists and industrialists, bioelectronics was defined as 'the use of biological materials and biological architectures for information processing systems and new devices'. At the end of the Frankfurt Workshop, bioelectronics, specifically bio-molecular electronics, was described as 'the research and development of bio-inspired (i.e. self-assembly) inorganic and organic materials and of bio-inspired (i.e. massive parallelism) hardware architectures for the implementation of new information processing systems, sensors and actuators, and for molecular manufacturing down to the atomic scale'. The subject of this overview is to summarize some of the most significant progress in bio-molecular electronics from neural VLSI networks and bio-molecular engineering. As an example of one possible route, emphasis is placed on the results recently obtained within this laboratory.