Fabrication of comb interdigitated electrodes array (IDA) for a microbead-based electrochemical assay system

Advances in Bioelectrode Design for Developing Electrochemical Biosensors

The critical performance factors such as selectivity, sensitivity, operational and storage stability, and response time of electrochemical biosensors are governed mainly by the function of their key component, the bioelectrode. Suitable design and fabrication strategies of the bioelectrode interface are essential for realizing the requisite performance of the biosensors for their practical utility. A multifaceted attempt to achieve this goal is visible from the vast literature exploring effective strategies for preparing, immobilizing, and stabilizing biorecognition elements on the electrode surface and efficient transduction of biochemical signals into electrical ones (i.e., current, voltage, and impedance) through the bioelectrode interface with the aid of advanced materials and techniques. The commercial success of biosensors in modern society is also increasingly influenced by their size (and hence portability), multiplexing capability, and coupling in the interface of the wireless communication technology, which facilitates quick data transfer and linked decision-making processes in real-time in different areas such as healthcare, agriculture, food, and environmental applications. Therefore, fabrication of the bioelectrode involves careful selection and control of several parameters, including biorecognition elements, electrode materials, shape and size of the electrode, detection principles, and various fabrication strategies, including microscale and printing technologies. This review discusses recent trends in bioelectrode designs and fabrications for developing electrochemical biosensors. The discussions have been delineated into the types of biorecognition elements and their immobilization strategies, signal transduction approaches, commonly used advanced materials for electrode fabrication and techniques for fabricating the bioelectrodes, and device integration with modern electronic communication technology for developing electrochemical biosensors of commercial interest.

Novel same-metal three electrode system for cyclic voltammetry studies

Conventional three-electrode systems used in electrochemical measurement demand time-consuming and maintenance intensive procedures to enable accurate and repeatable electrochemical measurements. Traditionally, different metal configurations are used to establish the electrochemical gradient required to acquire the redox activity, and vary between different electrochemical measurement platforms. However, in this work, we report using the same metal (gold) for the counter, working and reference electrodes fabricated on a miniaturized printed circuit board (PCB) for a much simpler design. Potassium ferricyanide, widely used as a redox probe for electrochemical characterization, was utilized to acquire cyclic voltametric profiles using both the printed circuit board-based gold-gold-gold three-electrode and conventional three-electrode systems (glassy carbon electrode or graphite foil as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode). The results show that both types of electrode systems generated similar cyclic voltammograms within the same potential window (-0.5 to +0.7 V). However, the novel PCB-based same-metal three-electrode electrochemical cell only required a few activation cycles and exhibited impressive cyclic voltametric repeatability with higher redox sensitivity and detection window, while using only trace amounts of solutions/analytes.

Redox Cycling Realized in Paper-Based Biochemical Sensor for Selective Detection of Reversible Redox Molecules Without Micro/Nano Fabrication Process

This paper describes a paper-based biochemical sensor that realizes redox cycling with close interelectrode distance. Two electrodes, the generator and collector electrodes, can detect steady-state oxidation and reduction currents when suitable potential is held at each electrode. The sensor has two gold plates on both sides of a piece of chromatography paper and defines the interelectrode distance by the thickness of the paper (180 μm) without any micro-fabrication processes. Our proposed sensor geometry has successfully exhibited signatures of redox cycling. As a result, the concentration of ferrocyanide as reversible redox molecules was successfully quantified under the interference by ascorbic acid as a strong irreversible reducing agent. This was possible because the ascorbic acids are completely consumed by the irreversible reaction, while maintaining redox cycling of reversible ferrocyanide. This suggests that a sensor based on the redox cycling method will be suitable for detecting target molecules at low concentration.

Improving sensitivity of a miniaturized label-free electrochemical biosensor using zigzag electrodes

Cardiovascular disease (CVD) is a leading cause of death among chronic diseases worldwide. Therefore, it is important to be able to detect CVD biomarkers early so that patients can be diagnosed properly and begin treatment as soon as possible. To detect biomarkers more conveniently, point-of-care (PoC) biosensors, which are easy to use and are of low cost, are becoming even more necessary. This paper focuses on developing a label-free electrochemical biosensor with high sensitivity for PoC applications to detect CVD biomarkers such as S100 beta proteins and C-reactive proteins (CRP). To meet the requirements of a PoC application and to improve the measurement sensitivity for detection purposes, a three-electrode configuration was miniaturized and fitted onto a biochip. Computer simulation of an electrolyte current density was used to investigate several potential effective possibilities. It was found that an electrolyte current density at an edge tip structure near the working electrode (WE) and counter electrode (CE) was higher than at other locations. A zigzag structure was then designed at the edge near the WE and CE positions. With this design, we can obtain a higher total electrolyte current. This newly-designed biochip was then used to measure the electrochemical feature. It was found that the measurement efficiency was also improved using this newly designed biochip.

An interdigital array microelectrode aptasensor based on multi-walled carbon nanotubes for detection of tetracycline

In this study an impedance aptasensor was designed for sensitive, selective, and fast detection of tetracycline (TET) based on an interdigital array microelectrode (IDAM). The IDAM was integrated with impedance detection to miniaturize the conventional electrodes, enhance the sensitivity, shorten the detection time, and minimize interfering effects of non-target analytes in the solution. Due to their excellent conductivity, good biocompatibility, the multi-walled carbon nanotubes (MWCNTs) were used to modify the IDAM to immobilize TET aptamer effectively. The proposed aptasensor produced a sensitive impedance change which was characterized by the electrochemical impedance spectroscopy (EIS). With the addition of TET, the formation of TET-aptamer complex on the surface of MWCNTs modified electrode resulted in an increase of electron transfer resistance (R _et). The change of R _et depends on the concentration of TET, which is applied for the quantification of TET. A wide linear range was obtained from 10^-9 to 10^-3 M. The linear regression equation was y(ΔR) = 21.310 × x(LogC) (M) + 217.25. It was successfully applied to detect TET in real milk samples.

Integrated dielectrophoretic and surface plasmonic platform for million-fold improvement in the detection of fluorescent events

We present an integrated dielectrophoretic (DEP) and surface plasmonic technique to quantify ∼1 pM of fluorescent molecules in low conductivity buffers. We have established a DEP force on target molecules to bring those molecules and place them on the nanometallic structures (hotspots) for quantification through surface plasmonic effects. Our results show that the DEP is capable of placing the fluorescent molecules on the hotspots, which are depicted as a significant reduction in the fluorescence lifetime of those molecules. To efficiently integrate the DEP and plasmonic effects, we have designed and utilized pearl-shaped interdigitated electrodes (PIDEs) in experiments. These electrodes generate 2-3 times higher DEP force than traditional interdigitated electrodes. Therefore, high-throughput assays can be developed. The nanometallic structures were strategically fabricated in the periphery of PIDEs for smooth integration of DEP and plasmonic detection. With the introduction of DEP, about 10⁶-fold improvement was achieved over existing plasmonic-based detection. Therefore, this simple addition to the existing surface plasmonic-based detection will enable the disease related protein detection.

Concept for E.coli detection using interdigitated microelectrode impedance sensor

This paper presents the concept to detect Escherichia coli O157:H7 based on electrochemical impedance spectroscopy at interdigitated microelectrode. Interdigitated microelectrode structures was designed and fabricated, with glass as substrate material and gold electrodes. The performance of the sensors was studied by measuring the capacitance in air and impedance spectra in DI water. The feasibility of the fabricated sensor for detecting different concentrations of Escherichia coli in water was demonstrated. Electrochemical impedance spectroscopy (EIS) was employed as the detection technique. The impedance based response significant change for different E.coli concentrations in the frequency range between 1 kHz to 100 kHz.

Acute on-chip HIV detection through label-free electrical sensing of viral nano-lysate

Development of portable biosensors has broad applications in environmental monitoring, clinical diagnosis, public health, and homeland security. There is an unmet need for pathogen detection at the point-of-care (POC) using a fast, sensitive, inexpensive, and easy-to-use method that does not require complex infrastructure and well-trained technicians. For instance, detection of Human Immunodeficiency Virus (HIV-1) at acute infection stage has been challenging, since current antibody-based POC technologies are not effective due to low concentration of antibodies. In this study, we demonstrated for the first time a label-free electrical sensing method that can detect lysed viruses, i.e. viral nano-lysate, through impedance analysis, offering an alternative technology to the antibody-based methods such as dipsticks and Enzyme-linked Immunosorbent Assay (ELISA). The presented method is a broadly applicable platform technology that can potentially be adapted to detect multiple pathogens utilizing impedance spectroscopy for other infectious diseases including herpes, influenza, hepatitis, pox, malaria, and tuberculosis. The presented method offers a rapid and portable tool that can be used as a detection technology at the POC in resource-constrained settings, as well as hospital and primary care settings.

Electrochemically deposited iridium oxide reference electrode integrated with an electroenzymatic glutamate sensor on a multi-electrode array microprobe

An implantable micromachined multi-electrode array (MEA) microprobe modified for utilization as a complete electrochemical biosensor for rapid glutamate detection is described. A post-fabrication method for electrochemical deposition of an iridium oxide (IrOx) film onto a designated microelectrode enabled incorporation of an IrOx reference electrode (RE) on the microprobe. The on-probe IrOx RE provides an alternative to the commonly utilized Ag/AgCl wire RE, which has been shown to be unstable and to cause an inflammatory response in living tissue. The IrOx film electrodeposited onto a platinum site was tested as part of a complete chemical sensing system that included a platinum counter electrode and enzymatic glutamate sensing electrodes all on a single silicon-based MEA platform. The thin film IrOx was mechanically robust enough to endure conditions of repeated heating and wetting during the MEA fabrication process. The pH dependence of the IrOx open circuit potential (OCP) was measured at -77±0.4 mV/pH and remained stable over a two-week period. The on-probe IrOx RE was tested in a two- and three-electrode system with glutamate biosensors. The biosensors were shown to detect a physiologically relevant range of glutamate concentrations and to reject the interferents, dopamine and ascorbic acid. By incorporating all of the electrodes onto a single device, baseline noise was reduced by an average of ∼61% in vitro and ∼71% in vivo with reduced tissue damage, since only a single probe needed to be implanted.

Automated flow-through amperometric immunosensor for highly sensitive and on-line detection of okadaic acid in mussel sample

An electrochemical immunosensor for okadaic acid (OA) detection has been developed, and used in an indirect competitive immunoassay format under automated flow conditions. The biosensor was fabricated by injecting OA modified magnetic beads onto screen printed carbon electrode (SPCE) in the flow system. The OA present in the sample competed with the immobilized OA to bind with anti-okadaic acid monoclonal antibody (anti-OA-MAb). The secondary alkaline phosphatase labeled antibody was used to perform electrochemical detection. The current response obtained from the labeled alkaline phosphatase to 1-naphthyl phosphate decreased proportionally to the concentration of free OA in the sample. The calculated limit of detection (LOD) was 0.15 μg/L with a linear range of 0.19-25 μg/L. The good recoveries percentages validated the immunosensor application for real mussel samples. The developed system automatically controlled the incubation, washing and current measurement steps, showing its potential use for OA determination in field analysis.

Nanoparticle-enhanced sensitivity of a nanogap-interdigitated electrode array impedimetric biosensor

Interdigitated electrode (IDE) arrays with nanometer-scale gaps have been utilized to enhance the sensitivity of affinity-based detection. The geometry of nanogap IDEs was first optimized on the basis of simulations of the electric field and current density. It was determined that the gap (G) between the electrodes was the most important geometric parameter in determining the distribution and strength of the electric field and the current density compared to the width (W) and height (H) of the IDEs. Several devices were materialized and analyzed for their sensitivity to the electrochemical environment using faradic electrochemical impedance spectroscopy (EIS) as the detection technique. Nanogap optimized IDEs were then employed as biosensors for the label-free, affinity-based detection of antitissue transglutaminase antibodies (αtTG-Abs), a biomarker for the detection of autoimmune disorder celiac sprue, triggered by ingesting gluten. The label-free biosensor assay was found to be less sensitive compared to on-chip ELISA. Gold nanoparticles (GNPs) were then employed to improve the sensitivity of the nanogap IDE-based biosensor. With GNPs, the transducer sensitivity increased by 350% over that of label-free detection. The suitability of nanogap IDEs as biosensor transducers for EIS in label-free and GNP-labeled formats was established. The immunobiosensor assay detection sensitivity with the GNPs was found comparable to ELISA.

3D nanogap interdigitated electrode array biosensors

Three-dimensional interdigitated electrodes (IDEs) have been investigated as sensing elements for biosensors. Electric field and current density were simulated in the vicinity of these electrodes as a function of the electrode width, gap, and height to determine the optimum geometry. Both the height and the gap between the electrodes were found to have significant effect on the magnitude and distribution of the electric field and current density near the electrode surface, while the width of the electrodes was found to have a smaller effect on field strength and current density. IDEs were fabricated based on these simulations and their performance tested by detecting C-reactive protein (CRP), a stress-related protein and an important biomarker for inflammation, cardiovascular disease risk indicator, and postsurgical recuperation. CRP-specific antibodies were immobilized on the electrode surface and the formation of an immunocomplex (IC) with CRP was monitored. Electrochemical impedance spectroscopy (EIS) was employed as the detection technique. EIS data at various concentrations (1 pg/mL to 10 microg/mL) of CRP spiked in buffer or diluted human serum was collected and fitted into an equivalent electrical circuit model. Change in resistance was found to be the parameter most sensitive to change in CRP concentration. The sensor response was linear from 0.1 ng/mL to 1 microg/mL in both buffer and 5% human serum samples. The CRP samples were validated using a commercially available ELISA for CRP detection. Hence, the viability of IDEs and EIS for the detection of serum biomarkers was established without using labeled or probe molecules.

Signal amplification in a microchannel from redox cycling with varied electroactive configurations of an individually addressable microband electrode array

Amperometric detection at microelectrodes in lab-on-a-chip (LOAC) devices lose advantages in signal-to-background ratio, reduced ohmic iR drop, and steady-state signal when volumes are so small that diffusion fields reach the walls before flux becomes fully radial. Redox cycling of electroactive species between multiple, closely spaced microelectrodes offsets that limitation and provides amplification capabilities. A device that integrates a microchannel with an individually addressable microband electrode array has been used to study effects of signal amplification due to redox cycling in a confined, static solution with different configurations and numbers of active generators and collectors. The microfabricated device consists of a 22 microm high, 600 microm wide microchannel containing an array of 50 microm wide, 600 microm long gold microbands, separated by 25 microm gaps, interspersed with an 800 microm wide counter electrode and 400 microm wide passive conductor, with a distant but on-chip 400 microm wide pseudoreference electrode. Investigations involve solutions of potassium chloride electrolyte containing potassium ferrocyanide. Amplification factors were as high as 7.60, even with these microelectrodes of fairly large dimensions (which are generally less expensive, easier, and more reproducible to fabricate), because of the significant role that passive and active (instrumentally induced) redox cycling plays in confined volumes of enclosed microchannels. The studies are useful in optimizing designs for LOAC devices.

Interdigitated array microelectrodes based impedance biosensors for detection of bacterial cells

Impedance spectroscopy is a sensitive technique to characterize the chemical and physical properties of solid, liquid, and gas phase materials. In recent years this technique has gained widespread use in developing biosensors for monitoring the catalyzed reaction of enzymes; the bio-molecular recognition events of specific proteins, nucleic acids, whole cells, antibodies or antibody-related substances; growth of bacterial cells; or the presence of bacterial cells in the aqueous medium. Interdigitated array microelectrodes (IDAM) have been integrated with impedance detection in order to miniaturize the conventional electrodes, enhance the sensitivity, and use the flexibility of electrode fabrication to suit the conventional electrochemical cell format or microfluidic devices for variety of applications in chemistry and life sciences. This article limits its discussion to IDAM based impedance biosensors for their applications in the detection of bacterial cells. It elaborates on different IDAM geometries their fabrication materials and design parameters, and types of detection techniques. Additionally, the shortcomings of the current techniques and some upcoming trends in this area are also mentioned.

Theory and practice of enzyme bioaffinity electrodes. Chemical, enzymatic, and electrochemical amplification of in situ product detection

The two articles in this series are dedicated to bioaffinity electrodes with in situ detection of the product of the enzyme label after recognition by its conjugate immobilized on the electrode. Part 1 was devoted to direct electrochemical detection, whereas the present contribution deals with homogeneous chemical and enzymatic amplification of the primary electrochemical signal. The theoretical relationships that are established for these modes of amplification are applied to the avidin-biotin recognition in a system that involves alkaline phosphatase as enzyme label and 4-amino-2,6-dichloro-phenyl phosphate as substrate, generating 2,6-dichloro-4-aminophenol as electrochemically active product. Chemical amplification then results from the addition of NADH, which reduces the 2,6-dichloro-quinonimine resulting from the electrochemical oxidation of 2,6-dichloro-4-aminophenol. An increased amplification is obtained when the reduction of 2,6-dichloro-quinonimine involves diaphorase in solution with NADH as substrate. The excellent agreement between theoretical predictions and experimental data required a detailed theoretical analysis and the independent determination of the key kinetic parameters of the system. The theoretical analysis was extended to monolayer and multilayered films of auxiliary enzyme as well as to electrochemical amplification by means of closely spaced dual electrodes so as to offer a rational comparative panorama of the amplification capabilities of the various possible strategies. Confinement of the profile of the product, and/or its oxidized form, in the vicinity the electrode surface appears as a key parameter of amplification.

Sensitive electrochemical enzyme immunoassay microdevice based on architecture of dual ring electrodes with a sensing cavity chamber

A novel electrochemical detection architecture was investigated for enzyme immunoassay sensors. Microchips with dual-ring working and counter electrodes, and a sensing cavity chamber were made on glass slides. The glass surface of the microchip was coated by 3-aminopropyltriethoxysilane (APTES). Goat IgG, as a example, was covalently captured on APTES-modified glass surfaces through glutaraldehyde (GA) as a cross-linker. Enzyme substrate, p-aminophenyl phosphate (PAPP) was prepared by electrolysis. The enzyme conversion from home-synthetic PAPP to p-aminophenol (PAP) was examined by differential pulse voltammetry (DPV). A competitive inhibition enzyme-linked immunosorbant assay (ELISA) was designed to test the system. Experimental results demonstrate that a detection limit of 118 fg/ml of goat IgG and a dynamic range of 118 fg/ml to 1.18 ng/ml, up to five orders of magnitude could be achieved. Due to its novel architecture design and electronic detection scheme, the method can be used to fabricate portable electrochemical ELISA lab-on-chip systems. The technology could have great potential in clinical diagnostic applications.