A review of experimental aspects of electrochemical immunosensors

Detection of the peanut allergen Ara h 6 in foodstuffs using a voltammetric biosensing approach

A voltammetric biosensor for Ara h 6 (a peanut allergen) detection in food samples was developed. Gold nanoparticle-modified screen-printed carbon electrodes were used to develop a sandwich-type immunoassay using two-monoclonal antibodies. The antibody-antigen interaction was detected through the electrochemical detection of enzymatically deposited silver. The immunosensor presented a linear range between 1 and 100 ng/ml, as well as high precision (inter-day RSD ≤9.8%) and accuracy (recoveries ≥96.7%). The detection and quantification limits were 0.27 and 0.88 ng/ml, respectively. It was possible to detect small levels of Ara h 6 in complex food matrices.

A nanocoaxial-based electrochemical sensor for the detection of cholera toxin

Sensitive, real-time detection of biomarkers is of critical importance for rapid and accurate diagnosis of disease for point of care (POC) technologies. Current methods do not allow for POC applications due to several limitations, including sophisticated instrumentation, high reagent consumption, limited multiplexing capability, and cost. Here, we report a nanocoaxial-based electrochemical sensor for the detection of bacterial toxins using an electrochemical enzyme-linked immunosorbent assay (ELISA) and differential pulse voltammetry (DPV) or square wave voltametry (SWV). The device architecture is composed of vertically-oriented, nanoscale coaxial electrodes in array format (~10(6) coaxes per square millimeter). The coax cores and outer shields serve as integrated working and counter electrodes, respectively, exhibiting a nanoscale separation gap corresponding to ~100 nm. Proof-of-concept was demonstrated for the detection of cholera toxin (CT). The linear dynamic range of detection was 10 ng/ml-1 µg/ml, and the limit of detection (LOD) was found to be 2 ng/ml. This level of sensitivity is comparable to the standard optical ELISA used widely in clinical applications, which exhibited a linear dynamic range of 10 ng/ml-1 µg/ml and a LOD of 1 ng/ml. In addition to matching the detection profile of the standard ELISA, the nanocoaxial array provides a simple electrochemical readout and a miniaturized platform with multiplexing capabilities for the simultaneous detection of multiple biomarkers, giving the nanocoax a desirable advantage over the standard method towards POC applications.

Amperometric bioaffinity sensing platform for avian influenza virus proteins with aptamer modified gold nanoparticles on carbon chips

A sandwich assay platform involving a surface formed aptamer-protein-antibody complex was developed to obtain the highly selective and sensitive amperometric detection of H5N1 viral proteins using a gold nanoparticle (NP) modified electrode. This is the first aptamer-antibody pairing reported for the selective detection of H5N1. Nanoparticle deposited screen-printed carbon electrodes were first functionalized by the covalent immobilization of a DNA aptamer specific to H5N1 followed by the adsorption of H5N1 protein. Alkaline phosphatase (ALP) conjugated monoclonal antibody was then adsorbed to form a surface bound Au NPs-aptamer/H5N1/antiH5N1-ALP sandwich complex which was further reacted with the enzyme substrate, 4-amino phenyl phosphate (APP). The current associated with the electrocatalytic reaction of the surface bound ALP with APP increased as the H5N1 concentration increased. A lowest detectable concentration of 100 fM was obtained with a linear dynamic range of 100 fM to 10 pM using differential pulse voltammetry. As an example, the biosensor was applied to the detection of H5N1 protein in diluted human serum samples spiked with different concentrations of the viral protein target.

Label free sensing of creatinine using a 6 GHz CMOS near-field dielectric immunosensor

In this work we present a CMOS high frequency direct immunosensor operating at 6 GHz (C-band) for label free determination of creatinine. The sensor is fabricated in standard 0.13 μm SiGe:C BiCMOS process. The report also demonstrates the ability to immobilize creatinine molecules on a Si3N4 passivation layer of the standard BiCMOS/CMOS process, therefore, evading any further need of cumbersome post processing of the fabricated sensor chip. The sensor is based on capacitive detection of the amount of non-creatinine bound antibodies binding to an immobilized creatinine layer on the passivated sensor. The chip bound antibody amount in turn corresponds indirectly to the creatinine concentration used in the incubation phase. The determination of creatinine in the concentration range of 0.88-880 μM is successfully demonstrated in this work. A sensitivity of 35 MHz/10 fold increase in creatinine concentration (during incubation) at the centre frequency of 6 GHz is gained by the immunosensor. The results are compared with a standard optical measurement technique and the dynamic range and sensitivity is of the order of the established optical indication technique. The C-band immunosensor chip comprising an area of 0.3 mm(2) reduces the sensing area considerably, therefore, requiring a sample volume as low as 2 μl. The small analyte sample volume and label free approach also reduce the experimental costs in addition to the low fabrication costs offered by the batch fabrication technique of CMOS/BiCMOS process.

Controllable gold nanoparticle deposition on carbon nanotubes and their application in immunosensing

A CNT–AuNPs hybrid nanocomposite platform was prepared from nanodisperse AuNPs in N-[3-(trimethoxysilyl)propyl]ethylenediamine (EDAS) sol–gel matrices with purified MWCNT.

Electrochemical detection of protein by using magnetic graphene-based target enrichment and copper nanoparticles-assisted signal amplification

A new electrochemical method for protein detection has been proposed based on magnetic graphene and duplex DNA-templated copper nanoparticles.

Nanomaterial-Based Electrochemical Immunosensors for Clinically Significant Biomarkers

Nanotechnology has played a crucial role in the development of biosensors over the past decade. The development, testing, optimization, and validation of new biosensors has become a highly interdisciplinary effort involving experts in chemistry, biology, physics, engineering, and medicine. The sensitivity, the specificity and the reproducibility of biosensors have improved tremendously as a result of incorporating nanomaterials in their design. In general, nanomaterials-based electrochemical immunosensors amplify the sensitivity by facilitating greater loading of the larger sensing surface with biorecognition molecules as well as improving the electrochemical properties of the transducer. The most common types of nanomaterials and their properties will be described. In addition, the utilization of nanomaterials in immunosensors for biomarker detection will be discussed since these biosensors have enormous potential for a myriad of clinical uses. Electrochemical immunosensors provide a specific and simple analytical alternative as evidenced by their brief analysis times, inexpensive instrumentation, lower assay cost as well as good portability and amenability to miniaturization. The role nanomaterials play in biosensors, their ability to improve detection capabilities in low concentration analytes yielding clinically useful data and their impact on other biosensor performance properties will be discussed. Finally, the most common types of electroanalytical detection methods will be briefly touched upon.

Direct, reagentless electrochemical detection of the BIR3 domain of X-linked inhibitor of apoptosis protein using a peptide-based conducting polymer sensor

In this work, we report a reagentless electrochemical peptide (AVPFAQKG) sensor to directly detect the BIR3 domain of X-linked inhibitor of apoptosis protein (XIAP-BIR3). The bioreceptor was based on a conducting copolymer film electrosynthesized from juglone and a juglone-peptide conjugate (JP) newly designed. The peptide-protein interactions generated an important increase of steric hindrance at the interface and a current decrease (signal off) of the redox reaction from quinone embedded in the polymer backbone as evidenced by Square Wave Voltammetry. This allowed a specific and sensitive detection of XIAP-BIR3 with a detection limit of 1 nM (13 ng mL(-1)). The peptide-protein complex could be then dissociated by adding the free precursor peptide (AVPFAQKG) into solution, causing a shift-back on the signal, i.e. an increase in the current intensity (signal-on). This "off-on" detection sequence was used in this work as a double verification of the specificity and this approach can be employed as a general way to increase the reliability of the results. In general, the approach described in this work may be inspired to develop other direct and reagentless electrochemical protein assays with high specificity and sensitivity.

Electrochemical sensing of cortisol: a recent update

Psychological stress caused by everyday lifestyle contributes to health disparities experienced by individuals. It affects many biomarkers, but cortisol - "a steroid hormone" - is known as a potential biomarker for psychological stress detection. Abnormal levels of cortisol are indicative of conditions such as Cushing's syndrome Addison's disease, adrenal insufficiencies and more recently post-traumatic stress disorder (PTSD). Chromatographic techniques, which are traditionally used to detect cortisol, are a complex system requiring multistep extraction/purification. This limits its application for point-of-care (POC) detection of cortisol. However, electrochemical immunosensing of cortisol is a recent advancement towards POC application. This review highlights simple, low-cost, and label-free electrochemical immunosensing platforms which have been developed recently for sensitive and selective detection of cortisol in bio-fluids. Electrochemical detection is utilized for the detection of cortisol using Anti-Cortisol antibodies (Anti-Cab) covalently immobilized on nanostructures, such as self-assembled monolayer (SAM) and polymer composite, for POC integration of sensors. The observed information can be used as a prototype to understand behavioral changes in humans such as farmers and firefighters. Keeping the future directions and challenges in mind, the focus of the BioMEMS and Microsystems Research Group at Florida International University is on development of POC devices for immunosensing, integration of these devices with microfluidics, cross validation with existing technologies, and analysis of real sample.

A digital microfluidic electrochemical immunoassay

Digital microfluidics (DMF) has emerged as a popular format for implementing quantitative immunoassays for diagnostic biomarkers. All previous reports of such assays have relied on optical detection; here, we introduce the first digital microfluidic immunoassay relying on electrochemical detection. In this system, an indium tin oxide (ITO) based DMF top plate was modified to include gold sensing electrodes and silver counter/pseudoreference electrodes suitable for in-line amperometric measurements. A thyroid stimulating hormone (TSH) immunoassay procedure was developed relying on magnetic microparticles conjugated with primary antibody (Ab1). Antigen molecules are captured followed by capture of a secondary antibody (Ab2) conjugated with horseradish peroxidase enzyme (HRP). HRP catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) which can be detected amperometrically. The limit of detection of the technique (2.4 μIU mL(-1)) is compatible with clinical applications; moreover, the simplicity and the small size of the detector suggest utility in the future for portable analysis.

Point-of-care platforms

Point-of-care applications are gaining increasing interest in clinical diagnostics and emergency applications. Biosensors are used to monitor the biomolecular interaction process between a disease biomarker and a recognition element such as a reagent. Essential are the quality and selectivity of the recognition elements and assay types used to improve sensitivity and to avoid nonspecific interactions. In addition, quality measures are influenced by the detection principle and the evaluation strategies. For these reasons, this review provides a survey and validation of recognition elements, assays, and various types of detection methods for point-of-care testing (POCT) platforms. Common applications of clinical parameters are discussed and considered. In this ever-changing field, a snapshot of current applications is needed. We provide such a snapshot by way of a table including literature citations and also discuss these applications in more detail throughout.

Electrochemical immunosensor for detection of antibodies against influenza A virus H5N1 in hen serum

This paper describes the development of an immunosensor for detection of anti-hemagglutinin antibodies. Its preparation consists of successive modification steps of glassy carbon electrodes: (i) creation of COOH groups, (ii) covalent immobilization of protein A with EDC/NHS coupling reaction, (iii) covering with anti-His IgG monoclonal antibody, (iv) immobilization of the recombinant His-tagged hemagglutinin (His6-H5 HA), (v) filling free space with BSA. The interactions between two variants of recombinant HA (short and long) from highly pathogenic avian influenza virus H5N1 and the anti-H5 HA monoclonal antibody (Mab 6-9-1) have been explored with electrochemical impedance spectroscopy (EIS). The impedimetric immunosensor displayed a very good detection limit (LOD) of 2.1 pg/mL, the quantification limit (LOQ) of 6.3 pg/mL and a dynamic range from 4 pg/mL to 20 pg/mL. In addition, this analytical device was applied for detection of antibodies against His6-H5 HA in serum of vaccinated hen using serial 10-fold dilutions of serum. The immunosensor proposed was able to detect antibody in hen serum diluted up to 7 × 10(7)-fold. The sensitivity of immunosensor was about four orders of magnitude much better than ELISA.

Aptamers: a promosing tool for ochratoxin A detection in food analysis

The contamination of food and feed by mycotoxins has become an increasingly serious problem. Mycotoxins represent a major risk to human and animal health, as well as economics. Herein, we focus on Ochratoxin A (OTA), which is one of the most common mycotoxins contaminating feed and foodstuffs. OTA is a secondary metabolite produced by various Aspergillus and Penicillium strains. Upon ingestion, OTA has a number of acute and chronic toxic effects. It is nephrotoxic, teratogenic, immunosuppressive, and carcinogenic (group 2B). As a consequence, some regulatory limits have been introduced on the levels of OTA in several commodities. The toxic nature of OTA demands highly sensitive and selective monitoring techniques to protect human and animal health. As alternative to traditional analytical techniques, biochemical methods for OTA analysis have attained great interest in the last few decades. They are mainly based on the integration of antibodies or aptamers as biorecognition elements in sensing platforms. However, aptamers have gained more attention in affinity-based assays because of their high affinity, specificity, stability, and their easy chemical synthesis. In this brief review, we present an overview of aptamer-based assays and their applications in OTA purification and detection, appeared in the literature in the last five years.

Designing label-free electrochemical immunosensors for cytochrome c using nanocomposites functionalized screen printed electrodes

We have designed here a label-free direct electrochemical immunosensor for the detection of cytochrome c (cyt c), a heme containing metalloprotein using its specific monoclonal antibody. Two nanocomposite-based electrochemical immunosensor platforms were evaluated for the detection of cyt c; (i) self-assembled monolayer (SAM) on gold nanoparticles (GNP) in polypyrrole (PPy) grafted screen printed electrodes (SPE) and (ii) carbon nanotubes (CNT) integrated PPy/SPE. The nanotopologies of the modified electrodes were confirmed by scanning electron microscopy. Electrochemical impedance spectroscopy and cyclic voltammetry were employed to monitor the stepwise fabrication of the nanocomposite immunosensor platforms. In the present method, the label-free quantification of cyt c is based on the direct electron transfer between Fe (III)/Fe (II)-heme redox active site of cyt c selectively bound to anti-cyt c nanocomposite modified SPE. GNP/PPy and CNT/PPy nanocomposites promoted the electron transportation through the conductive pore channels. The overall analytical performance of GNP/PPy based immunosensor (detection limit 2 nM; linear range: 2 nM to 150 µM) was better than the anti-cyt c/CNT/PPy (detection limit 10 nM; linear range: 10 nM to 50 µM). Further, the measurement of cyt c release in cell lysates of cardiomyocytes using the GNP/PPy based immunosensor gave an excellent correlation with standard ELISA.

Recent advances in cortisol sensing technologies for point-of-care application

Everyday lifestyle related issues are the main cause of psychological stress, which contributes to health disparities experienced by individuals. Prolonged exposure to stress leads to the activation of signaling pathways from the brain that leads to release of cortisol from the adrenal cortex. Various biomarkers have been affected by psychological stress, but cortisol "a steroid hormone" is known as a potential biomarker for its estimation. Cortisol can also be used as a target analyte marker to determine the effect of exposure such as organophosphates on central nervous system, which alters the endocrine system, leading to imbalance in cortisol secretion. Cortisol secretion of individuals depends on day-night cycle and field environment hence its detection at point-of-care (POC) is deemed essential to provide personalized healthcare. Chromatographic techniques have been traditionally used to detect cortisol. The issues relating to assay formation, system complexity, and multistep extraction/purification limits its application in the field. In order to overcome these issues and to make portable and effective miniaturized platform, various immunoassays sensing strategies are being explored. However, electrochemical immunosensing of cortisol is considered as a recent advancement towards POC application. Highly sensitive, label-free and selective cortisol immunosensor based on microelectrodes are being integrated with the microfluidic system for automated diurnal cortisol monitoring useful for personalized healthcare. Although the reported sensing devices for cortisol detection may have a great scope to improve portability, electronic designing, performance of the integrated sensor, data safety and lifetime for point-of-care applications, This review is an attempt to describe the various cortisol sensing platforms and their potential to be integrated into a wearable system for online and continuous monitoring of cortisol rhythm at POC as a function of one's environment.

Chemiluminescence detector based on a single planar transparent digital microfluidic device

We report on a compact and portable prototype of chemiluminescence detector based on a single planar single polar transparent electrowetting-on-dielectrics (EWOD) device. The coupling ground model was proposed to build the EWOD device, which could be driven under a single polar voltage. Such a design not only simplified the chip construction and control circuit, but also had the potential for the ball-like droplet to focus the fluorescence and enhance the detection sensitivity. Simulations and experiments both confirmed that the greater the contact angle, the stronger the detected optical signal, and thus the higher the sensitivity. The sensitivity of the prototype detector to H2O2 was 5.45 mV (mmol L(-1))(-1) and the detection limit was 0.01 mmol L(-1) when the contact angle of the EWOD surface was 120°. To further increase the sensitivity and decrease the detection limit, the contact angle of the EWOD device could be increased and the dark current of the photomultiplier decreased. The prototype shows potential applications as highly sensitive, cost effective and portable immuno-detectors, especially as a blood glucose monitor.

Design and evaluation of an electrochemical immunosensor for measles serodiagnosis using measles-specific Immunoglobulin G antibodies

The design of electrochemical immunosensors for the detection of measles-specific antibodies is reported. The measles-antigen modified surface was used as an antibody capture surface. The detection of measles-specific IgG antibodies was accomplished using the voltammetric method and horse-radish peroxidase (HRP) labeled secondary antibody (anti-IgG) as a detecting antibody. The potential applications of the designed immunosensor were evaluated in buffer and serum solutions. The immunosensor exhibited good linearity at concentrations less than 100 ng mL(-1) with R(2)=0.997 and the limit of detection of 6.60 ng mL(-1) at 3σ. The potential application of the immunosensor was evaluated in the deliberately infected human and newborn calf serum samples with measles-IgG antibody mimicking real-life samples. The designed electrochemical immunosensor could differentiate between infected and un-infected serum samples as higher catalytic currents were obtained for infected serum samples.