Bienzymatic glucose biosensor based on co-immobilization of peroxidase and glucose oxidase on a carbon nanotubes electrode

Immobilization of enzymes: a literature survey

The term immobilized enzymes refers to "enzymes physically confined or localized in a certain defined region of space with retention of their catalytic activities, and which can be used repeatedly and continuously." Immobilized enzymes are currently the subject of considerable interest because of their advantages over soluble enzymes. In addition to their use in industrial processes, the immobilization techniques are the basis for making a number of biotechnology products with application in diagnostics, bioaffinity chromatography, and biosensors. At the beginning, only immobilized single enzymes were used, after 1970s more complex systems including two-enzyme reactions with cofactor regeneration and living cells were developed. The enzymes can be attached to the support by interactions ranging from reversible physical adsorption and ionic linkages to stable covalent bonds. Although the choice of the most appropriate immobilization technique depends on the nature of the enzyme and the carrier, in the last years the immobilization technology has increasingly become a matter of rational design. As a consequence of enzyme immobilization, some properties such as catalytic activity or thermal stability become altered. These effects have been demonstrated and exploited. The concept of stabilization has been an important driving force for immobilizing enzymes. Moreover, true stabilization at the molecular level has been demonstrated, e.g., proteins immobilized through multipoint covalent binding.

ZnO/Cu nanocomposite: a platform for direct electrochemistry of enzymes and biosensing applications

Unique structured nanomaterials can facilitate the direct electron transfer between redox proteins and the electrodes. Here, in situ directed growth on an electrode of a ZnO/Cu nanocomposite was prepared by a simple corrosion approach, which enables robust mechanical adhesion and electrical contact between the nanostructured ZnO and the electrodes. This is great help to realize the direct electron transfer between the electrode surface and the redox protein. SEM images demonstrate that the morphology of the ZnO/Cu nanocomposite has a large specific surface area, which is favorable to immobilize the biomolecules and construct biosensors. Using glucose oxidase (GOx) as a model, this ZnO/Cu nanocomposite is employed for immobilization of GOx and the construction of the glucose biosensor. Direct electron transfer of GOx is achieved at ZnO/Cu nanocomposite with a high heterogeneous electron transfer rate constant of 0.67 ± 0.06 s(-1). Such ZnO/Cu nanocomposite provides a good matrix for direct electrochemistry of enzymes and mediator-free enzymatic biosensors.

Quantification of prothrombin in human plasma amplified by autocatalytic reaction

By site directed mutagenesis, we have produced recombinant mutants of human and mouse prethrombin-2 which are able to convert themselves autocatalytically into α-thrombin. We also have created a new method to amplify the signal of bioanalytical assays based on the autocatalytic activation of these mutated proenzymes. The activation of the mutants by active α-thrombin triggers an autocatalytic reaction which leads to more active thrombin resulting in the amplification of the readout signal. Addition of mutated mouse prethrombin-2 into the conventional assay for prothrombin level in human plasma, employing ecarin and the fluorogenic substrate, resulted in improvement of the detection limit by 2 orders of magnitude.

Amperometric glucose biosensor utilizing FAD-dependent glucose dehydrogenase immobilized on nanocomposite electrode

Amperometric glucose biosensors utilizing commercially available FAD-dependent glucose dehydrogenases from two strains of Aspergillus species are described. Enzymes were immobilized on nanocomposite electrode consisting of multi-walled carbon nanotubes by entrapment between chitosan layers. Unlike the common glucose oxidase based biosensor, the presented biosensors appeared to be O(2)-independent. The optimal amount of enzymes, working potential and pH value of working media of the glucose biosensors were determined. The biosensor utilizing enzyme isolated from Aspergillus sp. showed linearity over the range from 50 to 960 μM and from 70 to 620 μM for enzyme from Aspergillus oryzae. The detection limits were 4.45 μM and 4.15 μM, respectively. The time of response was found to be 60 s. The biosensors showed excellent operational stability - no loss of sensitivity after 100 consecutive measurements and after the storage for 4 weeks at 4 °C in phosphate buffer solution. When biosensors were held in a dessicator at room temperature without use, they kept the same response ability at least after 6 months. Finally, the results obtained from measurements of beverages and wine samples were compared with those obtained with the enzymatic-spectrophotometric and standard HPLC methods, respectively. Good correlation between results in case of analysis of real samples and good analytical performance of presented glucose biosensor allows to use presented concept for mass production and commercial use.

Co-immobilized coupled enzyme systems in biotechnology

The development of coimmobilized multi-enzymatic systems is increasingly driven by economic and environmental constraints that provide an impetus to develop alternatives to conventional multistep synthetic methods. As in nature, enzyme-based systems work cooperatively to direct the formation of desired products within the defined compartmentalization of a cell. In an attempt to mimic biology, coimmobilization is intended to immobilize a number of sequential or cooperating biocatalysts on the same support to impart stability and enhance reaction kinetics by optimizing catalytic turnover. There are three primary reasons for the utilization of coimmobilized enzymes: to enhance the efficiency of one of the enzymes by the in-situ generation of its substrate, to simplify a process that is conventionally carried out in several steps and/or to eliminate undesired by-products of an enzymatic reaction. As such, coimmobilization provides benefits that span numerous biotechnological applications, from biosensing of molecules to cofactor recycling and to combination of multiple biocatalysts for the synthesis of valuable products.

Enzymatic biosensors based on SWCNT-conducting polymer electrodes

This short review is focused on recent advances in the combination of conducting polymers and SWCNTs for the fabrication of electrochemical biosensors. The different properties of conducting polymers and SWCNTs are discussed in respect of their use in immobilizing and wiring biomolecules on electrode surfaces. We further describe the functionalization techniques used in the fabrication of these devices and their associated biosensing performances.

Direct electron transfer from glucose oxidase immobilized on polyphenanthroline-modified glassy carbon electrode

This study reports direct electron transfer (DET) from immobilized glucose oxidase (GOx) via grafted and electropolymerized 1,10-phenanthroline monohydrate (PMH). The layer of poly-1,10-phenanthroline (PPMH) was gained via electrochemical deposition, which was used to create the PPMH-modified GC-electrode (PPMH/GC-electrode). Further, the GOx was immobilized on the PPMH/GC-electrode. The effect of surface-modification by the PPMH on the electron-transfer between enzyme and electrode-surface and some other electrochemical/analytical-parameters of newly designed enzymatic-electrode were evaluated. The PPMH/GC-electrode showed superior DET to/from flavine adenine dinucleotide cofactor of GOx, while some redox-compounds including ferrocene and K(3)[Fe(CN)(6)] were completely electrochemically inactive on the PPMH/GC-electrode. It was also found that the resulting GOx/PPMH/GC-electrode functioned as a "direct response type" glucose-biosensor. The biosensor showed excellent selectivity towards glucose and demonstrated good operational-stability. According to our best knowledge, this study is the first scientific report on electrochemical-polymerization of PMH on the GC-electrode in non-aqueous media followed by its application in the design of glucose-biosensor.

Stable Flexible Electrodes With Enzyme Cluster Decorated Carbon Nanotubes for Glucose-Driven Power Source in Biosensing Applications

Over the years, implantable sensor technology has found many applications in healthcare. Research projects have aimed at improving power supply lifetime for longevity of an implanted sensor system. Miniature power sources, inspired from the biofuel cell principle, can utilize enzymes (proteins) as catalysts to produce energy from fuel(s) that are perennial in the human body. Bio-nanocatalytic hierarchical structures, clusters made of enzyme molecules, can be covalently linked to the electrode’s surface to provide better enzyme loading and sustained activity. Carbon nanotube base electrodes, with high surface area for direct electron transfer, and enzyme clusters can achieve efficient enzymatic redox reaction. A redox pair of such bioelectrodes can make up a power source with improved performance. In this study, we have investigated high throughput processes for coupling enzyme catalysts with power harvesting mechanisms via a screen printing process and solution processing. The process incorporates enzyme (glucosse oxidase and catalase) micro-/nanocluster immobilization on the surface of carboxylated (functionalized) carbon nanotubes with screen printed electrodes. The 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysulfosuccinimide amide linkage chemistries were used to bind the enzyme molecules to nanotube surface, and bis[sulfosuccinimidyl] suberate (BS3) was used as the cross-linker between enzymes. Optimized enzyme cross-linking was obtained after 25 min at room temperature with 0.07 mmol BS3/nmol of enzymes, with which 44% of enzymes were immobilized onto the surface of the bioelectrode with only 24% enzyme activity lost. A cell, redox pair of bioelectrodes, was tested under continuous operation. It was able to maintain most of the enzyme activity for 7 days before complete deactivation at 16 days. Thus, the power harvesting mechanism was able to produce power continuously for 7 days. The results were also analyzed to identify impeding factors such as competitive inhibition by reaction byproduct and cathode design, and methods to rectify them have been discussed. Coupling this new and improved nanobiopower cell with a product removal mechanism and enzyme mutagenesis should provide enzyme protection and longevity. This would bring the research one step closer to development of compatible implantable battery technology for medical applications.

Advances in carbon nanotube based electrochemical sensors for bioanalytical applications

Electrochemical (EC) sensing approaches have exploited the use of carbon nanotubes (CNTs) as electrode materials owing to their unique structures and properties to provide strong electrocatalytic activity with minimal surface fouling. Nanofabrication and device integration technologies have emerged along with significant advances in the synthesis, purification, conjugation and biofunctionalization of CNTs. Such combined efforts have contributed towards the rapid development of CNT-based sensors for a plethora of important analytes with improved detection sensitivity and selectivity. The use of CNTs opens an opportunity for the direct electron transfer between the enzyme and the active electrode area. Of particular interest are also excellent electrocatalytic activities of CNTs on the redox reaction of hydrogen peroxide and nicotinamide adenine dinucleotide, two major by-products of enzymatic reactions. This excellent electrocatalysis holds a promising future for the simple design and implementation of on-site biosensors for oxidases and dehydrogenases with enhanced selectivity. To date, the use of an anti-interference layer or an artificial electron mediator is critically needed to circumvent unwanted endogenous electroactive species. Such interfering species are effectively suppressed by using CNT based electrodes since the oxidation of NADH, thiols, hydrogen peroxide, etc. by CNTs can be performed at low potentials. Nevertheless, the major future challenges for the development of CNT-EC sensors include miniaturization, optimization and simplification of the procedure for fabricating CNT based electrodes with minimal non-specific binding, high sensitivity and rapid response followed by their extensive validation using "real world" samples. A high resistance to electrode fouling and selectivity are the two key pending issues for the application of CNT-based biosensors in clinical chemistry, food quality and control, waste water treatment and bioprocessing.

Electrochemical glucose sensors--developments using electrostatic assembly and carbon nanotubes for biosensor construction

In 1962, Clark and Lyons proposed incorporating the enzyme glucose oxidase in the construction of an electrochemical sensor for glucose in blood plasma. In their application, Clark and Lyons describe an electrode in which a membrane permeable to glucose traps a small volume of solution containing the enzyme adjacent to a pH electrode, and the presence of glucose is detected by the change in the electrode potential that occurs when glucose reacts with the enzyme in this volume of solution. Although described nearly 50 years ago, this seminal development provides the general structure for constructing electrochemical glucose sensors that is still used today. Despite the maturity of the field, new developments that explore solutions to the fundamental limitations of electrochemical glucose sensors continue to emerge. Here we discuss two developments of the last 15 years; confining the enzyme and a redox mediator to a very thin molecular films at electrode surfaces by electrostatic assembly, and the use of electrodes modified by carbon nanotubes (CNTs) to leverage the electrocatalytic effect of the CNTs to reduce the oxidation overpotential of the electrode reaction or for the direct electron transport to the enzyme.

Direct electrochemistry of glucose oxidase and biosensing for glucose based on graphene

We first reported that polyvinylpyrrolidone-protected graphene was dispersed well in water and had good electrochemical reduction toward O(2) and H(2)O(2). With glucose oxidase (GOD) as an enzyme model, we constructed a novel polyvinylpyrrolidone-protected graphene/polyethylenimine-functionalized ionic liquid/GOD electrochemical biosensor, which achieved the direct electron transfer of GOD, maintained its bioactivity and showed potential application for the fabrication of novel glucose biosensors with linear glucose response up to 14 mM.

Synthesis of gelatin-stabilized gold nanoparticles and assembly of carboxylic single-walled carbon nanotubes/Au composites for cytosensing and drug uptake

Gelatin-stabilized gold nanoparticles (AuNPs-gelatin) with hydrophilic and biocompatible were prepared with a simple and "green" route by reducing in situ tetrachloroauric acid in gelatin. The nanoparticles showed the excellent colloidal stability. UV-vis spectra, transmission electron microscopy (TEM), and atomic force microscopy revealed the formation of well-dispersed AuNPs with different sizes. By combination of the biocompatibility of AuNPs and excellent conductivity of carboxylic single-walled carbon nanotubes (c-SWNTs), a novel nanocomposite was designed for the immobilization and cytosensing of HL-60 cells at electrodes. The immobilized cells showed sensitive voltammetric response, good activity, and increased electron-transfer resistance. It can be used as a highly sensitive impedance sensor for HL-60 cells ranging from 1 x 10(4) to 1 x 10(7) cell mL(-1) with a limit of detection of 5 x 10(3) cell mL(-1). Moreover, the nanocomposite could effectively facilitate the interaction of adriamycin (ADR) with HL-60 cells and remarkably enhance the permeation and drug uptake of anticancer agents in the cancer cells, which could readily lead to the induction of the cell death of leukemia cells.

Review: Carbon nanotube based electrochemical sensors for biomolecules

Carbon nanotubes (CNTs) have been incorporated in electrochemical sensors to decrease overpotential and improve sensitivity. In this review, we focus on recent literature that describes how CNT-based electrochemical sensors are being developed to detect neurotransmitters, proteins, small molecules such as glucose, and DNA. Different types of electrochemical methods are used in these sensors including direct electrochemical detection with amperometry or voltammetry, indirect detection of an oxidation product using enzyme sensors, and detection of conductivity changes using CNT-field effect transistors (FETs). Future challenges for the field include miniaturizing sensors, developing methods to use only a specific nanotube allotrope, and simplifying manufacturing.

Electrochemical sensors based on carbon nanotubes

This review focuses on recent contributions in the development of the electrochemical sensors based on carbon nanotubes (CNTs). CNTs have unique mechanical and electronic properties, combined with chemical stability, and behave electrically as a metal or semiconductor, depending on their structure. For sensing applications, CNTs have many advantages such as small size with larger surface area, excellent electron transfer promoting ability when used as electrodes modifier in electrochemical reactions, and easy protein immobilization with retention of its activity for potential biosensors. CNTs play an important role in the performance of electrochemical biosensors, immunosensors, and DNA biosensors. Various methods have been developed for the design of sensors using CNTs in recent years. Herein we summarize the applications of CNTs in the construction of electrochemical sensors and biosensors along with other nanomaterials and conducting polymers.

A simple strategy for one-step construction of bienzyme biosensor by in-situ formation of biocomposite film through electrodeposition

One-step construction of horseradish peroxidase (HRP) and glucose oxidase (GOD) bienzyme biosensor was established based on a simple and controllable electrodeposition approach by in-situ formation of biocomposite film on electrode. With hydrogen peroxide (H(2)O(2)) instantly generated by GOD-catalyzed oxidation of glucose, quantitative measurement of glucose could be achieved. In the proposed electrodeposition strategy, Concanavalin A (Con A) was applied as bifunctional cross-linker for in-situ incorporating of HRP-GOD in biocompatible chitosan (CS) matrix due to its biospecific affinity interaction with CS and glycoproteins. Scanning electron microscopy showed that the HRP-GOD/Con A/CS biocomposite film possessed highly porous surface. The developed bienzyme biosensor exhibited a fast amperometric response for the determination of glucose. The linear response of the developed biosensor for the determination of glucose ranged from 1.0x10(-6) to 2.2x10(-4) M with a detection limit of 6.7x10(-7) M. The biosensor can be used to determine glucose in serum directly. The biosensor presented high stability owing to the design of the introduction of Con A and instant generation of H(2)O(2) with bienzyme system. The one-step construction of biosensor based on non-manual technique in biocompatible environment was direct and facile.