Amperometric biosensors

Quartz Crystal Microbalance-Based Aptasensors for Medical Diagnosis

Aptamers are important materials for the specific determination of different disease-related biomarkers. Several methods have been enhanced to transform selected target molecule-specific aptamer bindings into measurable signals. A number of specific aptamer-based biosensors have been designed for potential applications in clinical diagnostics. Various methods in combination with a wide variety of nano-scale materials have been employed to develop aptamer-based biosensors to further increase sensitivity and detection limit for related target molecules. In this critical review, we highlight the advantages of aptamers as biorecognition elements in biosensors for target biomolecules. In recent years, it has been demonstrated that electrode material plays an important role in obtaining quick, label-free, simple, stable, and sensitive detection in biological analysis using piezoelectric devices. For this reason, we review the recent progress in growth of aptamer-based QCM biosensors for medical diagnoses, including virus, bacteria, cell, protein, and disease biomarker detection.

Polymer/enzyme-modified HF-etched carbon nanoelectrodes for single-cell analysis

Carbon-based nanoelectrodes fabricated by means of pyrolysis of an alkane precursor gas purged through a glass capillary and subsequently etched with HF were modified with redox polymer/enzyme films for the detection of glucose at the single-cell level. Glucose oxidase (GOx) was immobilized and electrically wired by means of an Os-complex-modified redox polymer in a sequential dip coating process. For the synthesis of the redox polymer matrix, a poly(1-vinylimidazole-co-acrylamide)-based backbone was used that was first modified with the electron transfer mediator [Os(bpy)₂Cl]⁺ (bpy = 2,2'-bipyridine) followed by the conversion of the amide groups within the acrylamide monomer into hydrazide groups in a polymer-analogue reaction. The hydrazide groups react readily with bifunctional epoxide-based crosslinkers ensuring high film stability. Insertion of the nanometre-sized polymer/enzyme modified electrodes into adherently growing single NG108-15 cells resulted in a positive current response correlating with the intracellular glucose concentration. Moreover, the nanosensors showed a stable current output without significant loss in performance after intracellular measurements.

A review on recent developments in optical and electrochemical aptamer-based assays for mycotoxins using advanced nanomaterials

This review (with 163 refs) covers the recent developments of nanomaterial-based optical and electrochemical sensors for mycotoxins. The review starts with a brief discussion on occurrence, distribution, toxicity of mycotoxins and the legislations in monitoring their levels. It further outlines the research methods, various recognition matrices and the strategies involved in the development of highly sensitive and selective sensor systems. It also points out the salient features and importance of aptasensors in the detection of mycotoxins along with the different immobilization methods of aptamers. The review meticulously discusses the performance of different optical and electrochemical sensors fabricated using aptamers coupled with nanomaterials (CNT, graphene, metal nanoparticles and metal oxide nanoparticles). The review addresses the limitations in the current developments as well as the future challenges involved in the successful construction of aptasensors with the functionalized nanomaterials. Graphical abstract Recent developments in nanomaterial based aptasensors for mycotoxins are summarized. Specifically, the efficiency of the nanomaterial coupled aptasensors (such as CNT, graphene, metal nanoparticles and metal oxide nanoparticles) in optical and electrochemical methods are discussed.

Quartz-Crystal Microbalance (QCM) for Public Health: An Overview of Its Applications

Nanobiotechnologies, from the convergence of nanotechnology and molecular biology and postgenomics medicine, play a major role in the field of public health. This overview summarizes the potentiality of piezoelectric sensors, and in particular, of quartz-crystal microbalance (QCM), a physical nanogram-sensitive device. QCM enables the rapid, real time, on-site detection of pathogens with an enormous burden in public health, such as influenza and other respiratory viruses, hepatitis B virus (HBV), and drug-resistant bacteria, among others. Further, it allows to detect food allergens, food-borne pathogens, such as Escherichia coli and Salmonella typhimurium, and food chemical contaminants, as well as water-borne microorganisms and environmental contaminants. Moreover, QCM holds promises in early cancer detection and screening of new antiblastic drugs. Applications for monitoring biohazards, for assuring homeland security, and preventing bioterrorism are also discussed.

Synthesis of carbon nanosheet from barley and its use as non-enzymatic glucose biosensor

In this work, carbon nanosheet (CNS) based electrode was designed for electrochemical biosensing of glucose. CNS has been obtained by the pyrolysis of barley at 600–750 °C in a muffle furnace; it was then purified and functionalized. The CNS has been characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopic techniques. The electrochemical activity of CNS-based electrode was investigated by linear sweep voltammetry (LSV) and square wave voltammetry (SWV), for the oxidation of glucose in 0.001 M H₂SO₄ (pH 6.0). The linear range of the sensor was found to be 10⁻⁴–10⁻⁶ M (1–100 µM) within the response time of 4 s. Interestingly, its sensitivity reached as high as ~26.002±0.01 μA/μM cm². Electrochemical experiments revealed that the proposed electrode offered an excellent electrochemical activity towards the oxidation of glucose and could be applied for the construction of non-enzymatic glucose biosensors.

Small electron-transfer proteins as mediators in enzymatic electrochemical biosensors

Electrochemical mediators transfer redox equivalents between the active sites of enzymes and electrodes and, in this way, initiate bioelectrocatalytic redox processes. This has been very useful in the development of the so-called second-generation biosensors, in which they transduce a catalyzed reaction into an electrical signal. Among other pre-requisites, redox mediators must be readily oxidized and/or reduced at the electrode surface and readily interact with the biorecognition component. Small chemical compounds (e.g. ferrocene derivatives, ruthenium, or osmium complexes and viologens) are frequently used for this purpose but, lately, small redox proteins (e.g. horse heart cytochrome c) have also been used as redox partners in biosensing applications. In general, docking between two complementary proteins introduces a second level of selectivity to the biosensor and enlarges the list of compounds analyzed. Moreover, electrochemical interferences are frequently minimized owing to the small overpotentials achieved. This paper provides an overview of enzyme biosensors that are mediated by electron-transfer proteins. The paper begins with a brief discussion of mediated electrochemistry in biosensing systems and proceeds with a detailed description of relevant work on the cooperative use of redox enzymes and biological electron donors and/or acceptors.

Biosensors - classification, characterization and new trends

Biosensors represent promising analytical tools applicable in areas such as clinical diagnosis, food industry, environment monitoring and in other fields, where rapid and reliable analyses are needed. Some biosensors were successfully implemented in the commercial sphere, but majority needs to be improved in order to overcome some imperfections. This review covers the basic types, principles, constructions and use of biosensors as well as new trends used for their fabrication.

Nanoengineered electrically contacted enzymes on DNA scaffolds: functional assemblies for the selective analysis of Hg2+ ions

A DNA construct consisting of a nucleic acid template, (1), on which a nucleic acid-modified glucose oxidase (GOx), (3), was hybridized by cooperative bridging of the T-Hg(2+)-T units, and a nucleic acid-functionalized ferrocene, (5), was directly hybridized on a Au electrode. The resulting nanostructure revealed bioelectrocatalytic activities, where the ferrocene units mediated electron transfer between the redox center of the enzyme and the electrode. The bioelectrocatalytic functions of the system are regulated by the concentration of Hg(2+) ions, which controls the content of the enzyme associated with the DNA template by means of the T-Hg(2+)-T bridging units. This phenomenon allowed the amperometric detection of Hg(2+) ions at a detection limit 1 x 10(-10) M with impressive selectivity.

Glucose biosensors: an overview of use in clinical practice

Blood glucose monitoring has been established as a valuable tool in the management of diabetes. Since maintaining normal blood glucose levels is recommended, a series of suitable glucose biosensors have been developed. During the last 50 years, glucose biosensor technology including point-of-care devices, continuous glucose monitoring systems and noninvasive glucose monitoring systems has been significantly improved. However, there continues to be several challenges related to the achievement of accurate and reliable glucose monitoring. Further technical improvements in glucose biosensors, standardization of the analytical goals for their performance, and continuously assessing and training lay users are required. This article reviews the brief history, basic principles, analytical performance, and the present status of glucose biosensors in the clinical practice.

Responsive hydrogels for label-free signal transduction within biosensors

Hydrogels have found wide application in biosensors due to their versatile nature. This family of materials is applied in biosensing either to increase the loading capacity compared to two-dimensional surfaces, or to support biospecific hydrogel swelling occurring subsequent to specific recognition of an analyte. This review focuses on various principles underpinning the design of biospecific hydrogels acting through various molecular mechanisms in transducing the recognition event of label-free analytes. Towards this end, we describe several promising hydrogel systems that when combined with the appropriate readout platform and quantitative approach could lead to future real-life applications.

Electrochemical Biosensors - Sensor Principles and Architectures

Quantification of biological or biochemical processes are of utmost importance for medical, biological and biotechnological applications. However, converting the biological information to an easily processed electronic signal is challenging due to the complexity of connecting an electronic device directly to a biological environment. Electrochemical biosensors provide an attractive means to analyze the content of a biological sample due to the direct conversion of a biological event to an electronic signal. Over the past decades several sensing concepts and related devices have been developed. In this review, the most common traditional techniques, such as cyclic voltammetry, chronoamperometry, chronopotentiometry, impedance spectroscopy, and various field-effect transistor based methods are presented along with selected promising novel approaches, such as nanowire or magnetic nanoparticle-based biosensing. Additional measurement techniques, which have been shown useful in combination with electrochemical detection, are also summarized, such as the electrochemical versions of surface plasmon resonance, optical waveguide lightmode spectroscopy, ellipsometry, quartz crystal microbalance, and scanning probe microscopy. The signal transduction and the general performance of electrochemical sensors are often determined by the surface architectures that connect the sensing element to the biological sample at the nanometer scale. The most common surface modification techniques, the various electrochemical transduction mechanisms, and the choice of the recognition receptor molecules all influence the ultimate sensitivity of the sensor. New nanotechnology-based approaches, such as the use of engineered ion-channels in lipid bilayers, the encapsulation of enzymes into vesicles, polymersomes, or polyelectrolyte capsules provide additional possibilities for signal amplification. In particular, this review highlights the importance of the precise control over the delicate interplay between surface nano-architectures, surface functionalization and the chosen sensor transducer principle, as well as the usefulness of complementary characterization tools to interpret and to optimize the sensor response.

Fabrication of a Potentiometric/Amperometric Bifunctional Enzyme Microbiosensor

We report the fabrication and functional characterization of a needle-type bifunctional enzyme microbiosensor that has, as technical novelty, simultaneously integrated a potentiometric and amperometric detection of an enzyme-catalyzed reaction at the tip of a pulled glass micropipet. The construction involved immobilizing an enzyme onto the platinized outer tip surface using the precipitation of electrodeposition paint with direct entrapment of the biocomponent in the slowly growing polymer film. Products of enzyme-substrate reaction could then be targeted in a dual-detection mode on one hand with the covered Pt layer at the tip region as amperometric detector and on the other hand with a proton-selective liquid membrane-based potentiometric sensor inside the open pipet tip. Completing and testing bifunctional glucose microsensors demonstrated the functionality of the proposed strategy. Synchronized amperometric and potentiometric detection of the addition of a glucose standard to a buffer solution became evident by observing stepwise increases in the amperometric H2O2 oxidation current and corresponding increases in the potential of the pH-selective sensor, which translates to a local pH decrease around the tip due to hydrolysis of enzymatically formed gluconic acid.

An amperometric biosensor for glucose based on electrodeposited redox polymer/glucose oxidase film on a gold electrode

In this paper, we described a glucose biosensor based on the co-electrodeposition of a poly(vinylimidazole) complex of [Os(bpy)2Cl](+/2+) (PVI-Os) and glucose oxidase (GOX) on a gold electrode surface. The one-step co-electrodeposition method provided a better control on the sensor construction, especially when it was applied to microsensor construction. The modified electrode exhibited the classical features of a kinetically fast redox couple bound to an electrode surface and the redox potential of the redox polymer/enzyme film was 0.14 V (vs. SCE). For a scan rate of up to 200 mV s(-1), the peak-to-peak potential separation was less than 25 mV. In the presence of glucose, a typical catalytic oxidation current was observed, which reached a plateau at 0.25 V (vs. SCE). Under the optimal experimental conditions, the steady-state electrooxidation current measured at 0.30 V (vs. SCE) was linear to the glucose concentration in the range of 0-30 mM. Successful attempts were made in blood sample analysis.

Biosensors in clinical chemistry

Biosensors are analytical devices composed of a recognition element of biological origin and a physico-chemical transducer. The biological element is capable of sensing the presence, activity or concentration of a chemical analyte in solution. The sensing takes place either as a binding event or a biocatalytical event. These interactions produce a measurable change in a solution property, which the transducer converts into a quantifiable electrical signal. Present-day applications of biosensors to clinical chemistry are reviewed, including basic and applied research, commercial applications and fabrication techniques. Recognition elements include enzymes as biocatalytic recognition elements and immunoagents and DNA segments as affinity ligand recognition elements, coupled to electrochemical and optical modes of transduction. The future will include biosensors based on synthetic recognition elements to allow broad applicability to different classes of analytes and modes of transduction extending lower limits of sensitivity. Microfabrication will permit biosensors to be constructed as arrays and incorporated into lab-on-a-chip devices.

Continuous amperometric monitoring of glucose in a brittle diabetic chimpanzee with a miniature subcutaneous electrode

The performance of an amperometric biosensor, consisting of a subcutaneously implanted miniature (0.29 mm diameter, 5 x 10(-4) cm2 mass transporting area), 90 s 10-90% rise/decay time glucose electrode, and an on-the-skin electrocardiogram Ag/AgCl electrode was tested in an unconstrained, naturally diabetic, brittle, type I, insulin-dependent chimpanzee. The chimpanzee was trained to wear on her wrist a small electronic package and to present her heel for capillary blood samples. In five sets of measurements, averaging 5 h each, 82 capillary blood samples were assayed, their concentrations ranging from 35 to 400 mg/dl. The current readings were translated to blood glucose concentration by assaying, at t = 1 h, one blood sample for each implanted sensor. The rms error in the correlation between the sensor-measured glucose concentration and that in capillary blood was 17.2%, 4.9% above the intrinsic 12.3% rms error of the Accu-Chek II reference, through which the illness of the chimpanzee was routinely managed. Linear regression analysis of the data points taken at t>1 h yielded the relationship (Accu-Chek) = 0. 98 x (implanted sensor) + 4.2 mg/dl, r2 = 0.94. The capillary blood and the subcutaneous glucose concentrations were statistically indistinguishable when the rate of change was less than 1 mg/(dl. min). However, when the rate of decline exceeded 1.8 mg/(dl.min) after insulin injection, the subcutaneous glucose concentration was transiently higher.

Electrochemical glucose and lactate sensors based on "wired" thermostable soybean peroxidase operating continuously and stably at 37 degrees C

Following a recent report from our laboratory on a thermostable amperometric H2O2 sensor based on "wiring" soybean peroxidase, glucose and lactate sensors maintaining stable output under continuous operation at 37 degrees C for 12 and 8 days, respectively, were built. The vitreous carbon base of the sensor was coated with four polymer layers. The first was made by cross-linking thermostable soybean peroxidase and the redox polymer formed through complexing part of the rings of poly-(vinylpyridine) with [Os(bpy)2Cl]+/2+ (bpy = bipyridine) and quaternizing part of the rings with bromoethylamine. The second was an insulating and H2O2 transport controlling cellulose acetate layer. The third was an immobilized glucose oxidase or lactate oxidase layer. The fourth was a substrate transport controlling cellulose acetate layer In the case of the glucose sensor, the current output was independent of potential between -0.2 and +0.3 V (vs SCE), and the response time (t10/90) was < 2 min when the concentration was raised from 0 to 5 mM glucose. The current was independent of the O2 partial pressure above 15 Torr. The sensor was relatively insensitive to motion and to interferants. Changing the rotation speed of the electrode from 50 to 2500 rpm increased the current by < 10%. At a glucose concentration of 4 mM, the addition of 0.1 mM ascorbate decreased the current by < 1%. The operational stability was glucose oxidase loading dependent. Though the current decreased by 85% after 100 h of operation at 37 degrees C when the 3-mm-diameter electrode was loaded with only 1.3 micrograms of glucose oxidase, it decreased by < 1% after such operation when loaded with 52 micrograms of the enzyme. Similar results were obtained for the lactate sensor, with the exception of a more noticeable oxygen concentration dependence of the lactate response at low oxygen concentrations.