Biosensing at disk microelectrode arrays. Inter-electrode functionalisation allows formatting into miniaturised sensing platforms of enhanced sensitivity

3D microelectrode arrays, pushing the bounds of sensitivity toward a generic platform for point-of-care diagnostics

The increased sensitivity of microelectrode arrays (MEAs) over macroelectrodes for biosensing is well established, and results from reducing the diffusion gradient of the target species to and from the electrode surfaces. The current study describes the fabrication and characterisation of a polymer-based MEA, which exploits the advantages of three dimensionality (3D). Firstly, the unique 3D formfactor promotes release of the gold tips from an inert layer in a controlled fashion, to form a highly reproducible array of microelectrodes in a single step. The 3D topography of the fabricated MEAs significantly enhances the diffusion profile of the target species to the electrode which results in higher sensitivity. Furthermore, the "sharpness" of the 3D structure induces differential current distribution that is focused at the apices of the individual electrodes, reducing the active area, and thereby overcoming the requirement for the electrodes to be sub-micron in size before true MEA behaviour can be achieved. The electrochemical characteristics of the 3D MEAs shows ideal micro-electrode behaviour, with a level of sensitivity of three orders of magnitude greater than that of enzyme-linked immunosorbent assays (ELISA), as the optical based gold standard. The application of the 3D MEAs uses the combination of enzyme-label and substrate approach employed in ELISAs as a generic basis for biosensing and can hence be applied to the plethora of targets that utilise the ELISA approach. As an example, the 3D MEAs are applied to the detection of RNA and demonstrate a level of detection down to single digit picomolar concentrations.

An Electrochemical Immunosensor Based on Carboxylated Graphene/SPCE for IgG-SARS-CoV-2 Nucleocapsid Determination

The COVID-19 pandemic has emphasized the importance and urgent need for rapid and accurate diagnostic tests for detecting and screening this infection. Our proposal was to develop a biosensor based on an ELISA immunoassay for monitoring antibodies against SARS-CoV-2 in human serum samples. The nucleocapsid protein (N protein) from SARS-CoV-2 was employed as a specific receptor for the detection of SARS-CoV-2 nucleocapsid immunoglobulin G. N protein was immobilized on the surface of a screen-printed carbon electrode (SPCE) modified with carboxylated graphene (CG). The percentage of IgG-SARS-CoV-2 nucleocapsid present was quantified using a secondary antibody labeled with horseradish peroxidase (HRP) (anti-IgG-HRP) catalyzed using 3,3',5,5'-tetramethylbenzidine (TMB) mediator by chronoamperometry. A linear response was obtained in the range of 1:1000-1:200 v/v in phosphate buffer solution (PBS), and the detection limit calculated was 1:4947 v/v. The chronoamperometric method showed electrical signals directly proportional to antibody concentrations due to antigen-antibody (Ag-Ab) specific and stable binding reaction.

An Ultrasensitive and Selective Determination of Cadmium Ions at ppt Level Using an Enzymic Membrane with Colorimetric and Electrochemical Detection

Cadmium ions (Cd²⁺) are extremely toxic heavy metal pollutants found in the environment, and which endanger human health. Therefore, it is critical to develop a sensitive and simple method for rapidly detecting Cd²⁺ in water samples. Herein, an enzymic membrane was developed based on an easy and rapid immobilization method of horseradish peroxidase (HRP), for determination of Cd²⁺ in drinking water. Hence, for the first time, an enzymic membrane was applied for the detection of Cd²⁺ without being pretreated. In the first format, the inhibition of horseradish peroxidase was performed using a colorimetric microplate reader. Under optimal conditions, the achieved limit of detection was 20 ppt. In addition, an electrochemical biosensor was developed, by combining the enzymic membrane with screen printed electrodes, which showed a linear calibration range between 0.02–100 ppb (R² = 0.990) and a detection limit of 50 ppt. The use of this enzymic membrane proved to be advantageous when reversible inhibitors such as the copper ion (Cu²⁺) were present in water samples, as Cu²⁺ can interfere with Cd²⁺ and cause erroneous results. In order to alleviate this problem, a medium exchange procedure was used to eliminate Cu²⁺, by washing and leaving only cadmium ions as an irreversible inhibitor for identification. The use of this membrane proved to be a simple and rapid method of immobilizing HRP with a covalent bond.

Electroanalytical Paper-Based Nucleic Acid Amplification Biosensors with Integrated Thread Electrodes

Nucleic acid amplification tests (NAATs) are very sensitive and specific methods, but they mainly rely on centralized laboratories and therefore are not suitable for point-of-care testing. Here, we present a 3D microfluidic paper-based electrochemical NAAT. These devices use off-the-shelf gold plasma-coated threads to integrate electroanalytical readouts using ex situ self-assembled monolayer formation on the threads prior to assembling into the paper device. They further include a sandwich hybridization assay with sample incubation, rinsing, and detection steps all integrated using movable stacks of filter papers to allow time-sequenced reactions. The devices use glass fiber substrates for storing recombinase polymerase amplification reagents and conducting the isothermal amplification. We used the paper-based device for the detection of the toxic microalgae Ostreopsis cf. ovata. The NAAT, completed in 95 min, attained a limit of detection of 0.06 pM target synthetic DNA and was able to detect 1 ng/μL O. cf. ovata genomic DNA with negligible cross-reactivity from a closely related microalgae species. We think that the integration of thread electrodes within paper-based devices paves the way for digital one-time use NAATs and numerous other advanced electroanalytical paper- or textile-based devices.

Numerical Modeling for Sensitive and Rapid Molecular Detection by Membrane-Based Immunosensors

Rapid, simple, and sensitive point-of-care testing (POCT) has attracted attention in recent years due to its excellent potential for early disease diagnosis and health monitoring. The flow-through biosensor design is a candidate for POCT that utilizes the small-sized pores of a porous membrane as a recognition space where it emits a signal comparable to that of a conventional enzyme-linked immunosorbent assay within 35 min of detection time. In this paper, we present a numerical model for this immunosensing technology to systematically design an improved recognition system. The model considers mass transfer into the pore (convection and diffusion), the kinetics between the immobilized receptor and the target molecule, and the flow conditions, successfully leading to a bottleneck step (capture of secondary antibody) in sandwich-type detection. Our simulation results also show that this problem can be solved by adopting both appropriate receptors and analytical conditions. Eventually, the requirements to achieve the sensitivity required for POCT were fulfilled, which will allow for further development of immunosensing devices for disease detection.

Rapid Detection of Legionella pneumophila in Drinking Water, Based on Filter Immunoassay and Chronoamperometric Measurement

Legionella is a pathogenic bacterium, ubiquitous in freshwater environments and able to colonise man-made water systems from which it can be transmitted to humans during outbreaks. The prevention of such outbreaks requires a fast, low cost, automated and often portable detection system. In this work, we present a combination of sample concentration, immunoassay detection, and measurement by chronoamperometry. A nitrocellulose microfiltration membrane is used as support for both the water sample concentration and the Legionella immunodetection. The horseradish peroxidase enzymatic label of the antibodies permits using the redox substrate 3,3′,5,5′-Tetramethylbenzidine to generate current changes proportional to the bacterial concentration present in drinking water. Carbon screen-printed electrodes are employed in the chronoamperometric measurements. Our system reduces the detection time: from the 10 days required by the conventional culture-based methods, to 2–3 h, which could be crucial to avoid outbreaks. Additionally, the system shows a linear response (R² value of 0.99), being able to detect a range of Legionella concentrations between 10¹ and 10⁴ cfu·mL⁻¹ with a detection limit (LoD) of 4 cfu·mL⁻¹.

Label-free electrochemical biosensors based on 3,3',5,5'-tetramethylbenzidine responsive isoporous silica-micelle membrane

3,3',5,5'-Tetramethylbenzidine (TMB) has been frequently used as an indicator in G-quadruplex/hemin DNAzyme (G4zyme)-based chemical and biochemical analysis, and its oxidation products are usually monitored by electrochemical or optical methods to quantify G4zyme formation-related analytes. Herein we report a simple electrochemical approach based on isoporous silica-micelle membrane (iSMM) to measure TMB, instead of its oxidation products, in G4zyme-based detection of specific analytes. The iSMM was grown on the indium tin oxide (ITO) electrode, which was composed of highly ordered, vertically oriented silica nanochannels and cylindrical micelles of cetyltrimethylammonium. The iSMM-ITO electrode was selectively responsive to neutral TMB but not its oxidation products, thanks to the sieving and pre-concentration capacity of micellar structures in terms of molecular charge and lipophilicity. In other words, only TMB could be extracted and enriched into micelles and subsequently oxidized at the underlying ITO electrode surface (namely the micelle/ITO interface), generating an amplified anodic current. Since the depletion of TMB was catalyzed by G4zymes formed in the presence of specific analyte, the decrease of this anodic current enabled the quantitative detection of this analyte. The current variation relative to its initial value ((j₀-j)/j₀), termed as the current attenuation ratio, showed the obvious dependence on the analyte concentration. As proof-of-concept experiments, four substances, i.e., potassium cation (K⁺), adenosine triphosphate, thrombin and nucleic acid, were detected in aqueous media and the analysis of K⁺ in pre-treated human serum was also performed.

Fuel cell-powered microfluidic platform for lab-on-a-chip applications: Integration into an autonomous amperometric sensing device

The present paper reports for the first time the integration of a microfluidic system, electronics modules, amperometric sensor and display, all powered by a single micro direct methanol fuel cell. In addition to activating the electronic circuitry, the integrated power source also acts as a tuneable micropump. The electronics fulfil several functions. First, they regulate the micro fuel cell output power, which off-gas controls the flow rate of different solutions toward an electrochemical sensor through microfluidic channels. Secondly, as the fuel cell powers a three-electrode electrochemical cell, the electronics compare the working electrode output signal with a set reference value. Thirdly, if the concentration measured by the sensor exceeds this threshold value, the electronics switch on an integrated organic display. This integrated approach pushes forward the development of truly autonomous point-of-care devices relying on electrochemical detection.

Superparamagnetic particle dynamics and mixing in a rotating capillary tube with a stationary magnetic field

The dynamics of superparamagnetic particles subject to competing magnetic and viscous drag forces have been examined with a uniform, stationary, external magnetic field. In this approach, competing drag and magnetic forces were created in a fluid suspension of superparamagnetic particles that was confined in a capillary tube; competing viscous drag and magnetic forces were established by rotating the tube. A critical Mason number was determined for conditions under which the rotation of the capillary prevents the formation of chains from individual particles. The statistics of chain length were investigated by image analysis while varying parameters such as the rotation speed and the viscosity of the liquid. The measurements showed that the rate of particle chain formation was decreased with increased viscosity and rotation speed ; the particle dynamics could be quantified by the same dimensionless Mason number that has been demonstrated for rotating magnetic fields. The potential for enhancement of mixing in a bioassay was assessed using a fast chemical reaction that was diffusion-limited. Reducing the Mason below the critical value, so that chains were formed in the fluid, gave rise to a modest improvement in the time to completion of the reaction.

A thin layer-based amperometric enzyme immunoassay for the rapid and sensitive diagnosis of respiratory syncytial virus infections

A simple electrochemical sandwich immunoassay involving a polystyrene microarray slide coated with monoclonal capture antibodies and carbon screen-printed sensors (SPS) was designed for the rapid diagnosis of respiratory syncytial virus (RSV). The detection of the antibody-antigen complex formation relied on the use of a horseradish peroxidase conjugate. Its chronoamperometric measurement detection was performed by confining a droplet of H(2)O(2)/3,3',5,5'-tetramethylbenzidine enzyme substrate/mediator solution within a thin layer between one spot of the microarray and the surface of one screen-printed electrochemical cell. The accumulation of the enzyme product in the thin film of liquid enhanced the electrochemical response which allowed the development of a rapid (25 min) and sensitive thin layer-based amperometric (TLA) enzyme immunoassay. The method was successfully compared to commercially-available immunofluorescent and real-time PCR assays for RSV testing in respiratory secretion clinical samples. This suggests that owing to its rapidity, convenience, low-cost, portability and ability to provide quantified results, the reported concept could be a promising point-of-care diagnostic tool to screen patients with suspected respiratory infection or other types of infectious diseases.

Specific and targeted detection of viable Escherichia coli O157:H7 using a sensitive and reusable impedance biosensor with dose and time response studies

A gold interdigitated microelectrode (IME) impedance biosensor was fabricated for the detection of viable Escherichia coli O157:H7. This sensor was fabricated using lithography techniques. The surface of the electrode was immobilized with anti-E. coli IgG antibodies. This approach is different from other studies where the change in impedance is measured in terms of growth of bacteria on the electrode, rather then the antibody/antigen bonding. The impedance values were recorded for frequency ranges between 100 Hz and 10 MHz. The working range of the dose response for this device was found to be between 2.5×10(4) CFU ml(-1) and 2.5×10(7) CFU ml(-1). The time response studies indicated that antibody/antigen binding is not a function of time, but can decrease if excess times are allowed for binding. It was observed that the impedance values for 60 min antibody/antigen binding were higher than the impedance values for 120 min binding time. The main advantages of the reported device are that, it provides for both qualitative and quantitative detection in 3h while other impedance sensors reported earlier may take up to 24h for detection. If enrichment steps are required then it may take 3-4 days to infer the results. This sensor can be used to detect different types of bacteria by immobilizing the antigen specific antibody. Most of the sensors are not reusable since they either use enzymes or enrichment steps for detection but this device can be reused, following a cleaning protocol which is easy to follow. Each device was used at least five times. The simplicity of this sensor and the ease of fabrication make this sensor a useful alternate to the microfluidics and enzyme based impedance sensors, which are relatively more difficult to fabricate, need programmable fluidic injection pumps to push the sample through the channel, suffer from limitation of coagulation and are difficult to clean.

Improved bacteria detection by coupling magneto-immunocapture and amperometry at flow-channel microband electrodes

This paper describes the first immunosensing system reported for the detection of bacteria combining immunomagnetic capture and amperometric detection in a one-step sandwich format, and in a microfluidic environment. Detection is based on the electrochemical monitoring of the activity of horseradish peroxidase (HRP), an enzyme label, through its catalysis of hydrogen peroxide (H(2)O(2)) in the presence of the mediator hydroquinone (HQ). The enzymatic reaction takes place in an incubation micro-chamber where the magnetic particles (MPs) are confined, upstream from the working electrode. The enzyme product is then pumped along a microchannel, where it is amperometrically detected by a set of microelectrodes. This design avoids direct contact of the biocomponents with the electrode, which lowers the risk of electrode fouling. The whole assay can be completed in 1h. The experiments performed with Escherichia coli evidenced a linear response for concentrations ranging 10(2)-10(8) cell ml(-1), with a limit of detection of 55 cells ml(-1) in PBS, without pre-enrichment steps. Furthermore, 100 cells ml(-1) could be detected in milk, and with negligible interference by non-target bacteria such as Pseudomonas.

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.