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A highly sensitive, easy-and-rapidly-fabricable microfluidic electrochemical cell with an enhanced three-dimensional electric field
Abstract
An NP-μFEC is a reusable, novel microfluidic electrochemical cell with multiple non-planar interdigitated microelectrode arrays, minimal sample volume, and enhanced electric field penetration for highly sensitive electrochemical analysis. (i) The NP-μFEC features spatial 3-electrode architecture, and a small sample volume (∼4 μL). (ii) Here, [Fe(CN)6]3-/4- redox couple are used as an electrochemical reporter. The effects on the electrochemical properties of NP-μFEC due to the change in the reference electrode (RE) and counter electrode (CE)'s position with respect to the working electrode (WE) position are analyzed. For NP-μFEC, the position of the RE with respect to the WE does not affect the CV, DPV electrochemical profiles. However, the spacing between the CE and WE plays a significant role. (iii) The enhanced three-dimensional electric field penetration in NP-μFEC is validated by finite element analysis simulation using COMSOL Multiphysics. (iv) Without electrode surface modifications, NP-μFEC shows a detection limit (DL) of ∼2.54 × 10⁻⁶ M for aqueous [Fe(CN)6]3-/4- probe. (v) The DL for Cu²⁺, Fe³⁺, and Hg²⁺ are 30.5±9.5 μμg L⁻¹, 181±58.5 μμg L⁻¹, and 12.4±1.95 μμg L⁻¹, respectively, which meets the US Environmental Protection Agency (EPA)'s water contamination level for Cu, Fe, and is close to that for Hg (EPA limits are 1300 μμg L⁻¹, 300 μμg L⁻¹, and 2 μμg L⁻¹, respectively). (vi) Further, using a pressure-sensitive adhesive layer to form the channel and create the NP-μFEC configuration simplifies the manufacturing process, making it cost-effective and allowing for rapid adoption in any research lab. NP-μFEC is used to detect heavy metal ions in water. This demonstrates that cost-effective, easy-to-fabricate NP-μFEC can be a new sensitive electrochemical platform.
... 29 The use of microfluidics for developing electrochemical sensors to analyze liquid samples has been widely adopted. It is used in detecting biological analytes like DNA and proteins, 30 heavy metals like Fe 3+ and Hg 2+ , 31 and contaminants like per-and polyfluoroalkyl substances. 32 However, its application in developing chemical gas sensors must be more utilized. ...
... The developed microfluidic electrochemical sensor has a non-planar microelectrode configuration, which has performed better in previous studies than a planar architecture. 31 Using non-planar architecture allows the penetration of the electric field through the microchannel, thus allowing us to register any perturbation caused by the binding of CO 2 to the RTIL from the entire volume of the channel, which is filled with RTIL. 1-Ethyl-3-methylimidazolium 2cyanopyrolide ([EMIM] ) is used as the functionalized RTIL, as it has been demonstrated previously 37 to have high CO 2 solubility at low CO 2 partial pressures. ...
... In addition, increasing the distance between electrodes increases the ohmic drop, leading to a concomitant decrease in the current produced during any charge transfer process. 31 Detection of CO 2 in the ionic liquid.-The impedance spectrums of a microfluidic platform with the ionic liquid, [EMIM][2-CNpyr], exposed to a CO 2 /N 2 gas mixture containing different concentrations of CO 2 (in the range of 0 to 50000 ppm) are shown in Fig. 5. ...
Integrating transducer/sensing materials into microfluidic platforms has enhanced gas sensors' sensitivity, selectivity, and response time while facilitating miniaturization. In this manuscript, microfluidics has been integrated with non-planar microelectrode array and functionalized ionic liquids (ILs) to develop a novel miniaturized electrochemical gas sensor architecture. The sensor employs the IL 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]) as the electrolyte and capture molecule for detecting carbon dioxide (CO2 ). The three-layer architecture of the sensor consists of a microchannel with the IL sandwiched between glass slides containing microelectrode arrays, forming a non-planar structure. This design facilitates electric field penetration through the IL, capturing CO2 binding perturbations throughout the channel volume to enhance sensitivity. CO 2 binding with [EMIM][2-CNpyr] generates carboxylate ([EMIM]+ -CO2- ]), carbamate ([2-CNpyr]-CO2- ]), and pyrrole-2-carbonitrile (2CNpyrH) species, significantly decreasing the conductivity. The viscosity is also increased, leading to a further decrease in conductivity. These cumulative effects increase charge transfer resistance in the impedance spectrum, allowing a linear calibration curve obtained using Langmuir Isotherm. The sensitivity and reproducibility in CO2 detection are demonstrated by two electrode configurations using the calibration curve. The developed sensor offers a versatile platform for future applications.
... In another approach, microfluidic technology was again coupled with electrochemical detection [100]. The reported device consisted of an array of interdigitated microelectrodes with a small volume (~4 µL) and the authors studied the effect of the spacing between the electrodes. ...
The contamination of air, water and soil by heavy metal ions is one of the most serious problems plaguing the environment. These metal ions are characterized by a low biodegradability and high chemical stability and can affect humans and animals, causing severe diseases. In addition to the typical analysis methods, i.e., liquid chromatography (LC) or spectrometric methods (i.e., atomic absorption spectroscopy, AAS), there is a need for the development of inexpensive, easy-to-use, sensitive and portable devices for the detection of heavy metal ions at the point of interest. To this direction, microfluidic and lab-on-chip (LOC) devices fabricated with novel materials and scalable microfabrication methods have been proposed as a promising approach to realize such systems. This review focuses on the recent advances of such devices used for the detection of the most important toxic metal ions, namely, lead (Pb), mercury (Hg), arsenic (As), cadmium (Cd) and chromium (Cr) ions. Particular emphasis is given to the materials, the fabrication methods and the detection methods proposed for the realization of such devices in order to provide a complete overview of the existing technology advances as well as the limitations and the challenges that should be addressed in order to improve the commercial uptake of microfluidic and LOC devices in environmental monitoring applications.
... Electrochemical preconcentration is one of the most frequently discussed preconcentration techniques at a controlled potential [61][62][63][64]. Hybrid SEC techniques also often involve this method to assist the goal of quantifying ultra-trace aqueous target analytes [47,65]. ...
This review paper presents the recent developments in spectroelectrochemical (SEC) technologies. The coupling of spectroscopy and electrochemistry enables SEC to do a detailed and comprehensive study of the electron transfer kinetics and vibrational spectroscopic fingerprint of analytes during electrochemical reactions. Though SEC is a promising technique, the usage of SEC techniques is still limited. Therefore, enough publicity for SEC is required, considering the promising potential in the analysis fields. Unlike previously published review papers primarily focused on the relatively frequently used SEC techniques (ultraviolet-visible SEC and surface-enhanced Raman spectroscopy SEC), the two not-frequently used but promising techniques (nuclear magnetic resonance SEC and dark-field microscopy SEC) have also been studied in detail. This review paper not only focuses on the applications of each SEC method but also details their primary working mechanism. In short, this paper summarizes each SEC technique’s working principles, current applications, challenges encountered, and future development directions. In addition, each SEC technique’s applicative research directions are detailed and compared in this review work. Furthermore, integrating SEC techniques into microfluidics is becoming a trend in minimized analysis devices. Therefore, the usage of SEC techniques in microfluidics is discussed.
Low‐cost, reliable, and efficient biosensors are crucial in detecting residual heavy metal ions (HMIs) in food products. At present, based on distance‐induced localized surface plasmon resonance of noble metal nanoparticles, enzyme‐mimetic reaction of nanozymes, and chelation reaction of metal chelators, the constructed optical sensors have attracted wide attention in HMIs detection. Besides, based on the enrichment and signal amplification strategy of nanomaterials on HMIs and the construction of electrochemical aptamer sensing platforms, the developed electrochemical biosensors have overcome the plague of low sensitivity, poor selectivity, and the inability of multiplexed detection in the optical strategy. Moreover, along with an in‐depth discussion of these different types of biosensors, a detailed overview of the design and application of innovative devices based on these sensing principles was provided, including microfluidic systems, hydrogel‐based platforms, and test strip technologies. Finally, the challenges that hinder commercial application have also been mentioned. Overall, this review aims to establish a theoretical foundation for developing accurate and reliable sensing technologies and devices for HMIs, thereby promoting the widespread application of biosensors in the detection of HMIs in food.
Top and bottom microelectrode glass slide sandwiches a channel made of adhesive tape and packed with reduced graphene oxide to make a new non-planar interdigitated microfluidic device. Novel device integration allows packing of any transducer material at room temperature with no instrument. Electrical impedance spectroscopy signal is stable over long times, fits an equivalent circuit with well-defined circuit elements. Flow visualization concurs that electric double-layer signal shifts to high frequency due to disruption of the diffusive process from increased convective flow. Charge transfer resistance appears at higher frequencies making the device rapid with high signal to noise ratio.
Electrical impedance spectroscopy (EIS) sensors, though rapid and cost-effective, often suffer from poor sensitivity. EIS sensors modified with carbon-based transducers show a higher conductance, thereby increasing the sensitivity of the sensor toward biomolecules such as DNA. However, the EIS spectra are compromised by the parasitic capacitance of the electric double layer (EDL). Here, a new shear-enhanced, flow-through nonporous, nonplanar interdigitated microelectrode sensor has been fabricated that shifts the EDL capacitor to high frequencies. Enhanced convective transport in this sensor disrupts the diffusion dynamics of the EDL, shifting its EIS spectra to high frequency. Concomitantly, the DNA detection signal shifts to high frequency, making the sensor very sensitive and rapid with a high signal to noise ratio. The device consists of a microfluidic channel sandwiched between two sets of top and bottom interdigitated microelectrodes. One of the sets of microelectrodes is packed with carbon-based transducer material such as carboxylated single-walled carbon nanotube (SWCNT). Multiple parametric studies of three different electrode configurations of the sensor along with different carbon-based transducer materials are undertaken to understand the fundamental physics and electrochemistry. Sensors packed with SWCNT show femtomolar detection sensitivity from all the different electrode configurations for a short target-DNA. A 20-fold jump in the signal is noticed from the unique working electrode configuration in contrast to the other electrode configurations. This demonstrates the potential of the sensor to have a significant increase in detection sensitivity for DNA and other biomolecules.
The current Na⁺ storage performance of carbon‐based materials is still hindered by the sluggish Na⁺ ion transfer kinetics and low capacity. Graphene and its derivatives have been widely investigated as electrode materials in energy storage and conversion systems. However, as anode materials for sodium‐ion batteries (SIBs), the severe π–π restacking of graphene sheets usually results in compact structure with a small interlayer distance and a long ion transfer distance, thus leading to low capacity and poor rate capability. Herein, partially reduced holey graphene oxide is prepared by simple H2O2 treatment and subsequent low temperature reduction of graphene oxide, leading to large interlayer distance (0.434 nm), fast ion transport, and larger Na⁺ storage space. The partially remaining oxygenous groups can also contribute to the capacity by redox reaction. As anode material for SIBs, the optimized electrode delivers high reversible capacity, high rate capability (365 and 131 mAh g⁻¹ at 0.1 and 10 A g⁻¹, respectively), and good cycling performance (163 mAh g⁻¹ after 3000 cycles at a current density of 2 A g⁻¹), which is among the best reported performances for carbon‐based SIB anodes.
A highly-sensitive interdigitated electrode (IDE) with vertically-aligned dense carbon nanotube forests directly grown on conductive supports was demonstrated by combining UV lithography and low temperature chemical vapor deposition process (470 °C). The cyclic voltammetry (CV) measurements of K4[Fe(CN)6] showed the redox current of IDE with CNT forests (CNTF-IDE) reached the steady state much more quickly compared to that of conventional gold IDE (Au-IDE). The performance of the CNTF-IDE largely depended on the geometry of the electrodes (e.g. width and gap). With the optimum three-dimensional electrode structure, the anodic current was amplified by a factor of ~18 and ~67 in the CV and the chronoamperometry measurements, respectively. The collection efficiency, defined as the ratio of the cathodic current to the anodic current at steady state, was improved up to 97.3%. The selective detection of dopamine (DA) under coexistence of L-ascorbic acid with high concentration (100 µM) was achieved with the linear range of 100 nM – 100 µM, the sensitivity of 14.3 mA mol⁻¹ L, and the limit of detection (LOD, S/N = 3) of 42 nM. Compared to the conventional carbon electrodes, the CNTF-IDE showed superior anti-fouling property, which is of significant importance for practical applications, with a negligible shift of half-wave potential (ΔE1/2< 1.4 mV) for repeated CV measurement of DA at high concentration (100 µM).
In electroanalytical chemistry, it is often observed that square wave voltammetry (SWV) and differential pulse voltammetry (DPV) are more sensitive techniques compared to linear sweep voltammetry (LSV), due to their method of sampling which minimises the charging current (non-faradaic processes). In this work, a comparison of the three techniques (LSV, DPV and SWV) is performed for ammonia (NH3) gas oxidation (a chemically and electrochemically irreversible redox process) in an ionic liquid over a concentration range of 10–100 ppm. Four different platinum electrodes are employed: a screen-printed electrode (SPE), a thin-film electrode (TFE), a microarray thin-film electrode (MATFE) and a Pt microdisk electrode (μ-disk). Calibration plots (current vs concentration) for all three different electrochemical techniques on all four surfaces showed excellent linearity with increased concentrations of NH3 gas and relatively low limits of detection (LODs). On the larger mm-sized surfaces (SPE and TFE), the current responses for LSV and SWV were quite similar, but DPV gave the lowest currents. Whereas for the smaller micron sized electrodes (MATFE and μ-disk), currents were of the order LSV>SWV>DPV, with LSV being far superior to the pulse techniques. These findings suggest that the pulse techniques of SWV and DPV may not be the optimum methods, particularly on microelectrodes, for the detection of analytes such as ammonia in RTILs.
Despite the growing popularity of cyclic voltammetry, many students do not receive formalized training in this technique as part of their coursework. Confronted with self-instruction, students can be left wondering where to start. Here, a short introduction to cyclic voltammetry is provided to help the reader with data acquisition and interpretation. Tips and common pitfalls are provided, and the reader is encouraged to apply what is learned in short, simple training modules provided in the Supporting Information. Armed with the basics, the motivated aspiring electrochemist will find existing resources more accessible and will progress much faster in the understanding of cyclic voltammetry.
In this paper we demonstrated a functional on-chip multi-microelectrode array within microfluidic channels as an fully on-chip electrochemical cell. The response of Pt electrodes within the multi-microelectrode array in the microfluidic channel filled with stagnant Fe(III) solution was characterized using cyclic voltammetry. We found that the mass transport is confined by the small channel dimensions. Furthermore, the interaction between closely-spaced working electrode (WE)-counter electrode (CE) pairs induced the so-called redox cycling, as a result of the overlap of their diffusion regions. The redox cycling modified the electrochemical response of the micro system, resulting in a larger peak separation and higher steady-state current plateaus compared to a bulk system. These experimental findings were validated by using the COMSOL simulations, which also visualize the underlying concentration profiles: thin-layer behavior in the absence of redox cycling and a steep concentration profile in the presence redox cycling, where the depletion-layer width is exactly identical to the WE-CE distance. This analysis from a particular geometry provides an example for the analysis of general micro systems. This work indicates that redox cycling at the WE-CE mode can enhance the current response of amperometric biosensors.
The quality of an analytical method developed is always appraised in terms of suitability for its intended purpose, recovery, requirement for standardization, sensitivity, analyte stability, ease of analysis, skill subset required, time and cost in that order. It is highly imperative to establish through a systematic process that the analytical method under question is acceptable for its intended purpose. Limit of detection (LOD) and limit of quantification (LOQ) are two important performance characteristics in method validation. LOD and LOQ are terms used to describe the smallest concentration of an analyte that can be reliably measured by an analytical procedure. There has often been a lack of agreement within the clinical laboratory field as to the terminology best suited to describe this parameter. Likewise, there have been various methods for estimating it. The presented review provides information relating to the calculation of the limit of detection and limit of quantitation. Brief information about differences in various regulatory agencies about these parameters is also presented here.
This work describes the construction and characterization of an electrochemical flow-through microcell with the three electrodes (working, pseudo-reference, and auxiliary) inserted in microchannels with thickness smaller than 20 μm. These microchannels were constructed between two stacked polycarbonate slides using one or more overlapped toner masks as spacer. This strategy allows the construction of microcells with a variable internal volume on the working electrode (0.6 to 2.4 μL). Three different materials were optimized as electrodes: gold film or graphite-epoxy composite as working electrode, silver-epoxy composite as pseudo-reference electrode and, graphite-epoxy composite as auxiliary electrode. The performance of the microfluidic cell was characterized by cyclic voltammetry, potentiometric stripping analysis at constant current, and square wave anodic stripping voltammetry using ferrocyanide and heavy metals (Cu2+, Hg2+, Pb2+, and Cd2+) as model analytes.
Maximum power densities by air-driven microbial fuel cells (MFCs) are considerably influenced by cathode performance. We show here that application of successive polytetrafluoroethylene (PTFE) layers (DLs), on a carbon/PTFE base layer, to the air-side of the cathode in a single chamber MFC significantly improved coulombic efficiencies (CEs), maximum power densities, and reduced water loss (through the cathode). Electrochemical tests using carbon cloth electrodes coated with different numbers of DLs indicated an optimum increase in the cathode potential of 117 mV with four-DLs, compared to a <10 mV increase due to the carbon base layer alone. In MFC tests, four-DLs was also found to be the optimum number of coatings, resulting in a 171% increase in the CE (from 19.1% to 32%), a 42% increase in the maximum power density (from 538 to 766 mW m−2), and measurable water loss was prevented. The increase in CE due is believed to result from the increased power output and the increased operation time (due to a reduction in aerobic degradation of substrate sustained by oxygen diffusion through the cathode).
We report the fabrication of disposable and flexible screen printed microelectrodes which are characterised with microscopy and cyclic voltammetry. These new type of screen printed electrochemical platforms consist of micro-sized graphite typically with radii of 60 to 100 microns are defined by an inert dielectric. The advantage of this type of electrochemical sensing platform is that each microelectrode is disposable and cost effective and thus does not require extensive cleaning or electrode pre-treatment between measurements. Prior to measurements the screen printed microelectrode needs only to be calibrated with a suitable redox probe, as is typically the case with microelectrodes. We show proof of concept that the screen printed microelectrodes are advantageous for electro-analytical measurements with the example of determination of lead via cathodic stripping voltammetry. The use of graphite screen printed microelectrodes allows comparable detection limits to that obtained in the literature at insonated boron doped diamond electrodes, without the need for power ultrasound – which otherwise limits the widespread applicability and ease of measurement.
The meaning and significance of uncompensated resistance are carefully explained. Many factors influence the uncompensated resistance and several of these are explored in this article, using idealized models of an electrochemical cell. Among the factors whose roles have been elucidated are the shape and size of the cell, the location of the reference electrode, the shape of the working electrode, and the size and position of the counter electrode. Worthwhile compensation is shown to be impossible with microelectrodes.
The rapid, sensitive, and selective detection of target analytes using electrochemical sensors is challenging. ESSENCE, a new Electrochemical Sensor that uses a Shear-Enhanced, flow-through Nanoporous Capacitive Electrode, overcomes current electrochemical sensors' response limitations, selectivity, and sensitivity limitations. ESSENCE is a microfluidic channel packed with transducer material sandwiched by a top and bottom microelectrode. The room-temperature instrument less integration process allows the switch of the transducer materials to make up the porous electrode without modifying the electrode architecture or device protocol. ESSENCE can be used to detect both biomolecules and small molecules by simply changing the packed transducer material. Electron microscopy results confirm the high porosity. In conjunction with the non-planar interdigitated electrode, the packed transducer material results in a flow-through porous electrode. Electron microscopy results confirm the high porosity. The enhanced shear forces and increased convective fluxes disrupt the electric double layer's (EDL) diffusive process in ESSENCE. This disruption migrates the EDL to high MHz frequency allowing the capture signal to be measured at around 100 kHz, significantly improving device timing (rapid detection) with a low signal-to-noise ratio. The device’s unique architecture allows us multiple configuration modes for measuring the impedance signal. This allows us to use highly conductive materials like carbon nanotubes. We show that by combining single-walled carbon nanotubes as transducer material with appropriate capture probes, NP-μIDE has high selectivity and sensitivity for DNA (fM sensitivity, selective against non-target DNA), breast cancer biomarker proteins (p53, pg/L sensitivity, selective against non-target HER2).
The growing global concerns to public health from human exposure to perfluorooctanesulfonate (PFOS) require rapid, sensitive, in-situ detection where current, state-of-the-art techniques are yet to adequately meet sensitivity standards of the real-world. This work presents, for the first time, a synergistic approach for the targeted affinity-based capture of PFOS using a porous sorbent probe that enhances detection sensitivity by embedding it on a microfluidic platform. This novel sorbent-containing platform functions as an electrochemical sensor to directly measure PFOS concentration through a proportional change in electrical current (increase in impedance). The extremely high surface area and pore volume of mesoporous metal-organic framework (MOF) Cr-MIL-101 is used as the probe for targeted PFOS capture based on the affinity of the chromium center towards both the fluorine tail groups as well as the sulfonate functionalities as demonstrated by spectroscopic (NMR and XPS) and microscopic (TEM) studies. Answering the need for an ultrasensitive PFOS detection technique, we are embedding the MOF capture probes inside a microfluidic channel, sandwiched between interdigitated microelectrodes (IDµE). The nanoporous geometry, along with interdigitated microelectrodes, increases the signal to noise ratio tremendously. Further, the ability of the capture probes to interact with the PFOS at the molecular level and effectively transduce that response electrochemically has allowed us achieve significant increase in sensitivity. The PFOS detection limit of 0.5 ng/L is unprecedented for in-situ analytical PFOS sensors and comparable to quantification limits achieved using state-of-the-art ex-situ techniques.
Rapid and sensitive detection and quantification of trace and ultra-trace analytes is critical to environmental remediation, analytical chemistry and defense from chemical and biological contaminants. Though affinity based electrochemical sensors have gained immense popularity, they frequently do not meet the requirements of desired sensitivity and detection limits. Here, we demonstrate a complementary luminescence mode that can significantly enhance sensitivity of impedance or voltammetric electrochemical sensors. Our methodology involves using a redox probe, whose luminescence properties change upon changing the oxidation state. By tailoring the system such that these luminescence changes can be correlated with the capture of target analytes, we are able to significantly lower the detection limit and improve the efficiency of detection compared to the electrochemical modes alone. Our proof-of-concept demonstration, using a model system designed for Ca²⁺ capture, illustrated that the luminescent mode allowed us to lower the limits of detection by three-orders of magnitude compared to the impedance or voltammetric modes alone without requiring any modification of electrode design or cell configuration. Further, the linear ranges of detection are 10⁻⁸ to 10⁻³ M in the voltammetry mode, 10⁻⁸ to 10⁻⁵ M in the impedance mode and 2.5 × 10⁻¹¹ to 10⁻⁷ M in the luminescent mode, providing a large range of operational flexibility.
The use of impedimetric biosensors based on interdigitated electrode arrays (IDEAs) for biochemical applications has gained increased interest during the last two decades. Recently, a concept of a three-dimensional interdigitated electrode array (3D-IDEA) with insulating barriers separating electrodes digits was introduced. The functional mechanism of the device is based on registration of changes in conductivity at the surface of the barrier provoked by electrical charge redistribution caused by surface chemical reactions. The specific design of this sensor structure permits to enhance its sensitivity for biochemical reactions taking place at the sensor surface in comparison with planar sensors. Discussion of the sensor properties and parameters is presented stressing the important effect of surface conductivity on IDEA sensors response. Examples of application of 3D-IDEA transducers modified with antibodies, proteins, DNA molecules, peptides and aptamers for label-free biosensing are encountered throughout this paper. Another advantage that presents 3D-IDEA is the possibility to use it as a valuable tool to study surface chemical reactions at the insulator–solution interface which are rather difficult to achieve with conventional experimental methods.
An electrochemical sensor based on graphene oxide decorated with gold nanoparticles has been prepared for the simultaneous quantification of uric acid (UA) and ascorbic acid (AA) in urine samples. The gold interdigitated microelectrodes array (Au-IDA) has been modified using graphene oxide doped with gold nanoparticles (AuNPs-GO/Au-IDA), which was characterized by TEM, FE-SEM, XPS and cyclic voltammetry. Excellent results were obtained for the separate quantification of UA and AA by chronoamperometry. The electrochemical sensor exhibits limits of detection (LODs) of 1.4 μM and 0.62 μM for AA and UA, respectively, limits of quantification (LOQs) of 4.6 μM (AA) and 2.0 μM (UA), and the working ranges obtained were between 4.6 μM and 193 μM for AA and between 2 μM and 1.05 mM for UA. The repeatability was studied at 20 μM providing coefficients of variation of 16% for AA and 13% for UA. Moreover, UA does not interfere in the measurement of AA and viceversa (provided that the concentration of UA is equal to or higher than 450 μM in the latter case). For lower concentrations of UA, an easy and fast strategy to quantify both analytes is presented. The good electrocatalytic activity achieved with this material makes it useful for the quantification of AA and UA in biological fluids. Other analytes like glucose, dopamine and epinephrine have been investigated. The results allow us to conclude that they do not interfere in the quantification of AA and UA in PBS (0.25 M, pH 7.0). Human urine samples have been analyzed using the method proposed, contaning AA and UA concentration levels of (0.588 ± 0.002) mM and (1.43 ± 0.02) mM, respectively, which are in the concentration range of these analytes in urine samples for healthy people.
A label-free electrochemical impedance spectroscopy (EIS) aptasensor for rapid detection (< 35 mins) of interferon-gamma (IFN-γ) was fabricated by immobilizing a RNA aptamer capture probe (ACP), selective to IFN-γ, on a gold interdigitated electrode array (Au IDE). The ACP was modified with a thiol group at the 5’ terminal end and subsequently co-immobilized with 1,6-Hexanedithiol (HDT) and 6-Mecapto-1-hexanolphosphate (MCH) to the gold surface through thiol-gold interactions. This ACP/HDT-MCH ternary surface monolayer facilitates efficient hybridization with IFN-γ and displays high resistance to non-specific adsorption of non-target proteins [i.e., fetal bovine serum (FBS) and bovine serum albumin (BSA)]. The Au IDE functionalized with ACP/HDT-MCH was able to measure IFN-γ in actual FBS solution with a linear sensing range from 22.22 pM to 0.11 nM (1-5 ng/ml) and a detection limit of 11.56 pM. The ability to rapidly sense IFN-γ within this sensing range makes the developed electrochemical platform conducive towards in-field disease detection of a variety of diseases including paratuberculosis (i.e., Johne’s Disease). Furthermore, experimental results were numerically validated with an analytical model that elucidated the effects of the sensing process and the influence of the immobilized ternary monolayer on signal output. This is the first time that ternary surface monolayers have been used to selectively capture/detect IFN-γ on Au IDEs.
Recent studies have demonstrated that single-walled carbon nanotube (SWCNT) films can be doped by a nitric/sulfuric acid treatment to significantly improve its conductivity. However, the sheet resistance is still too high for many applications. Here we report on a simple treatment using a piranha mixture that could further enhance SWCNT film conductivity. The high redox potentials of hydrogen peroxides combined with the acidity of sulfuric acid enabled strong p-doping of the carbon nanotube films. Absorption peak suppression and a G-band shift provided proof of strong p-doping of the SWCNT films.
Rational utilization of nanomaterials to construct electrochemical nucleic acid sensors has attracted large attention in these years. In this work, we systematically interrogate the interaction between gold nanoparticles (GNPs) and single-strand DNA (ssDNA) immobilized on an electrode surface, then take advantage of the ultrahigh charge-transfer efficiency of GNPs to develop a novel DNA sensing method. Specifically, ssDNA modified gold electrode can adsorb GNPs due to the interaction between gold and nitrogen-containing bases; thus the negative electrochemical species [Fe(CN)6]3-/4- may transfer electrons to electrode through adsorbed GNPs. In the presence of target DNA, the formed double-strand DNA (dsDNA) cannot capture GNPs onto the electrode surface and the dsDNA may result in a large charge-transfer resistance owing to the negatively charged phosphate backbones of DNA. So, a simple but sensitive method for the detection of target DNA can be developed by using GNPs without any requirement of modification. Experimental results demonstrate that the electrochemical method we have proposed in this work can detect as low as 1 pM breast cancer gene BRCA1 in 10 μL sample volume without any signal amplification process or the involvement of other synthesized complex, which may provide an alternative for cancer DNA detection. This method may also be generalized for detecting a spectrum of targets using functional DNA (aptamer, metal-specific oligonucleotide or DNAzyme) in the future.
The placement of the reference electrode (RE) in various bioelectrochemical systems is often varied to accommodate different reactor configurations. While the effect of the RE placement is well understood from a strictly electrochemistry perspective, there are impacts on exoelectrogenic biofilms in engineered systems that have not been adequately addressed. Varying distances between the working electrode (WE) and the RE, or the RE and the counter electrode (CE) in microbial fuel cells (MFCs) can alter bioanode characteristics. With well-spaced anode and cathode distances in an MFC, increasing the distance between the RE and anode (WE) altered bioanode cyclic voltammograms (CVs) due to the uncompensated ohmic drop. Electrochemical impedance spectra (EIS) were also changed with RE distances, resulting in a calculated increase in anode resistance that varied between 17 to 31 Ω (–0.2 V). While WE potentials could be corrected with ohmic drop compensation during the CV tests, they could not be automatically corrected by the potentiostat in the EIS tests. The electrochemical characteristics of bioanodes were altered by through their acclimation to different current densities that resulted from varying the distance between the RE and the CE (cathode). These differences were true changes in biofilm characteristics because the CVs were electrochemically independent of changes resulting from changing CE to RE distances. Placing the RE outside of the current path enabled accurate bioanode characterization using CVs and EIS due to negligible ohmic resistances (0.4 Ω). It is therefore concluded for bioelectrochemical systems that when possible, the RE should be placed outside the current path and near the WE, as this will result in more accurate representation of bioanode characteristics. Biotechnol. Bioeng. © 2014 Wiley Periodicals, Inc.
Visible absorption spectroscopy coupled with controlled potential electrolysis at SnO2-covered glass transparant electrodes show that, in the course of oxidation of ferrocyanides to ferricyanides, the latter species undergo, under the influence of the electrode field, decomposition to inhibiting, Prussian blue-like, deposits. Addition of an effective Fe(III,II)-complexing agent, namely of CN− at the 0.01-10 mM level either to LiCl or KCl electrolytes, increases the measured standard rate constants (ks) significantly for electron transfer in the Fe(CN)3−/4−6 system. Protection of a Pt indicator electrode from passivation has also been accomplished by intentional modification of its surface via irreversible chemisorption of monoatomic iodine.Both approaches proposed here yield excellently reproducible rotating disk or cyclic voltammograms which, upon kinetic analysis, produce ks values consistently exceeding 10−1 cm s−1. Taken together, the results provide strong evidence that complications with the Fe(CN)3−/4−6 system could be solved essentially by preventing the electrode surface from being blocked by sparingly soluble iron cyanide species.
Through controlled annealing of intimately mixed blends of the polyfluorene copolymers poly(9,9‘-dioctylfluorene-co-bis(N,N‘-(4,butylphenyl))bis(N,N‘-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9‘-dioctylfluorene-co-benzothiadiazole) (F8BT) we observe the change in charge generation dynamics and photovoltaic performance as the length of nanoscale phase separation is varied from 5 nm or less to greater than 40 nm. We find that device efficiency is optimized for a phase separation of 20 nm, significantly larger than the exciton diffusion length of 5−10 nm. Femtosecond time-resolved transient absorption measurements confirm that the charge generation time is longer and charge generation efficiency is lower in films with a more evolved morphology. Photoluminescence quantum efficiency is also observed to monotonically increase with annealing temperature consistent with a decrease in exciton dissociation resulting from a coarsening of phases. Using a Monte Carlo model of exciton diffusion and dissociation in computer-simulated structures, we infer that the domains have purity of >95% and find good agreement between the observed photoluminescence quenching and measured domain sizes. Charge transport studies of single-carrier devices show that charge transport through the blend does not significantly improve as device performance improves, and photocurrent is observed to scale linearly with light intensity independent of blend morphology and device geometry. We conclude that the recombination of geminate charge pairs is limiting device performance, with the optimum phase separation of 20 nm balancing the efficiency of charge generation and charge separation.
Voltammetric studies in the absence of added supporting electrolyte are presently dominated by the use of near-steady-state microelectrode techniques and millimolar or lower depolarizer concentrations. However, with this methodology, large departures from conventional migration−diffusion theory have been reported for the [Fe(CN)6]3-/4- process at both carbon fiber and platinum microdisk electrodes. In contrast, data obtained in the present study reveal that use of the transient cyclic voltammetric technique at glassy carbon, gold, or platinum macrodisk electrodes and K4[Fe(CN)6] or K3[Fe(CN)6] concentrations of 50 mM or greater provides an approximately reversible response in the absence of added electrolyte. It is suggested that the use of very high [Fe(CN)6]3- and [Fe(CN)6]4- concentrations overcomes problems associated with a diffuse double layer and that large electrode surface areas and faster potential sweep rates minimize electrode blockage and passivating phenomena that can plague voltammetric studies at microelectrodes. The cyclic voltammetry of the [Fe(CN)6]3-/4- couple at a range of concentrations at macroelectrodes in the absence of added inert electrolyte is compared with that obtained in the presence of 1 M KCl. The enhanced influences of uncompensated resistance, migration, and natural convection arising from density gradients under transient conditions at macrodisk electrodes also are considered.
Interdigitated ultramicroelectrode arrays (IDUAs) were fabricated on glass wafers and investigated to obtain optimal oxidation and reduction reactions of potassium ferro/ferrihexacyanide, Fe2+/3+(CN)6, when using a 2-electrode set up. These electrodes will be used as transducers in portable microfluidic-based biosensors in the future for the detection in an aqueous, biocompatible matrix. IDUAs were designed to maximize the signal-to-noise ratio (S/N) investigating electrode height, gap size, finger width, and material. Interesting differences in the electrode materials gold and platinum were found, which were due to the oxidization of platinum and gold during the IDUA fabrication process. It resulted in gold IDUAs being by far superior in respect to signal-to-noise ratio and overall signal magnitude to those made of platinum. The effects of gap size, electrode width and number of electrode fingers were as expected. Optimal electrode heights were in the range of 70 nm–140 nm, much larger and smaller electrodes had lower signal-to-noise ratios due to overall reduced signal or increased background. The optimized IDUA was made out of gold, had 400 fingers with a finger width of 2.7 μm, a finger height between 70 nm and 140 nm and a gap size of 0.9–1 μm. A detection limit of as low as 0.1 μM ferro/ferrihexacyanide measured in a simple 2-electrode set up was obtained with a signal-to-noise ratio of 9.7.
A novel design of an interdigitated electrode array impedimetric sensor is proposed with electrode digits separated by an insulating barrier. This configuration results in that the major part of the electric current between electrodes follows close to the surface of the barrier and not through the solution thus permitting to enhance the sensitivity to possible chemical reactions on its surface. As a model system the effect of electrostatically assembled polyelectrolyte layers deposited using layer-by-layer method on the sensor impedance was studied. The sensitivity of the devices depends on the barrier height and is considerably enhanced comparing to conventional flat sensor structures. Devices may be used as a transducer for direct label-free biosensor development.
True diffusion (D m) and partition (α) coefficients for the transport of potassium ferrocyanide through diaphorase (Dp)– and bovine serum albumin (BSA)–glutaraldehyde (GA) membranes with different cross-linking degrees of 1–8% GA concentrations immobilized on gold electrodes are investigated by using potential-step method and rotating-disk-electrode method. The thickness of dry and hydrated immobilized membranes is accurately measured by the focus-difference method with a reflection microscope. The thickness of hydrated Dp–GA and BSA–GA membranes are about 1.4 and 2.4 times that of dry membranes, respectively. In addition, the actual area of electrode surface is calculated by the charge amount of chemisorbed oxygen on gold electrode. Owing to the increase of swelling degree and net negative charge of the immobilized membranes, the values of D m and α for both of Dp–GA and BSA–GA membranes enlarge and decrease with increase of GA concentration, respectively. Furthermore, BSA–GA membranes possess greater D m and α than those of Dp–GA membranes due to the thinner thickness and the greater swelling degree of BSA–GA membranes.
Spectroscopic investigations commonly involve a measurement of power as a function of some external parameters. It is usual, for improvement of signal‐to‐noise, to modulate the interaction between the radiation and sample and to apply the resulting signal to a synchronous detector followed by an integrating filter and a chart recorder. An extended search for a weak signal may require slow scanning rates and long time constants. Such a combination imposes severe requirements on the stability of the equipment, for any nonstochastic noise can frustrate a single scan by exceeding the dynamic range of the chart recorder. An alternative approach to the problem is to accumulate many relatively rapid scans of the full spectrum so that the signals add coherently whereas the noise adds randomly. The first approach may be termed frequency domain filtering while the latter is termed time domain filtering. Here we report an example of the latter approach employing a multichannel pulse‐height analyzer as a storage device. The voltage from the output terminals of a spectrometer (nuclear magnetic resonance or electron paramagnetic resonance) is converted to pulses, the pulse rate being proportional to the voltage. The channels are opened successively in synchronism with the progress of the magnetic field. Thus each channel corresponds to a finite region of the field while the number of counts in each channel is proportional to the time integral of voltage corresponding to that field. In such an arrangement the signal increases directly as the number of scans while the noise increases as the square root of the number. In effect the average response is computed continuously. The great dynamic range, 10<sup>6</sup> pulses/channel, precludes overloading or saturation effects. Since no long time constants are employed, low‐frequency noise is no longer the limiting factor in equipment performance. We present spectra, resulting from many thousands of scans,-
which demonstrate that signal‐to‐noise ratios may be enhanced by one to two orders of magnitude without sacrifice of bandwidth.
Microelectrode arrays (MEAs) offer numerous benefits over macroelectrodes due to their smaller sample size requirement, small form factor, low-power consumption, and higher sensitivity due to increased rates of mass transport. These features make MEAs well suited for microfluidic lab-on-a-chip applications. This paper presents two implementations of MEAs with and without an on chip potentiostat. We first describe an 8times8 array of 6 mum circular microelectrodes with center to center 37 mum spacing fabricated on silicon using conventional microfabrication techniques. Pads are provided for external connections to a potentiostat for electrochemical analysis. The second implementation is an individually addressable 32times32 array of 7 mum square microelectrodes with 37 mum center to center spacing on a CMOS chip with built-in very-large-scale integration potentiostat for electrochemical analysis. The integrated CMOS MEA is post processed at the die level to coat the exposed Al layers with Au. To verify microelectrode array behavior with individual addressability, cyclic voltammetry was performed using a potassium ferricyanide (K<sub>3</sub>Fe(CN)<sub>6</sub>) solution.
A new transducer for biosensor applications has been developed based on a three-dimensional interdigitated electrode array (IDEA) with electrode digits separated by an insulating barrier. Binding of molecules to a chemically modified surface of the transducer induces important changes in conductivity between the electrodes. Three-dimensional sensor shows considerable improvement compared with a standard planar IDEA design. The potential of the developed device as a sensor transducer to detect immunochemical and enzymatic reactions, as well as DNA hybridization events is demonstrated. The immunosensor allows direct detection of the antibiotic sulfapyridine and shows the IC50 parameter value of 5.6 μg L−1 in a buffer. Immunochemical determination occurs under competitive configurations and without the use of any label. Each modified sensor is of a single use. Nevertheless, biochemical reagents can be easily cleaned off the sensor surface for its reuse. Layer-by-layer method of used to deposit polyethyleneimine and glucose oxidase showed that the sensor is also highly effective for detecting single and multilayered molecular assemblies.
Nanoscaled interdigitated electrode arrays were made with deep UV lithography. Electrode widths and spacings from 500 down to 250 nm were achieved on large active areas (0.5×1 mm). These electrodes allow for the detection of affinity binding of biomolecular structures (e.g. antigens, DNA) by impedimetric measurements. Such a sensor with Pd electrodes on SiO2 is developed and theoretically analysed. It was experimentally characterised in KCl solutions demonstrating its bulk-insensitive behaviour and the immobilisation of glucose oxidase (GOD) could be monitored by measuring the double layer impedance.
Some of the recent topics on the use of microelectrodes for the materials characterization, especially for the materials in battery and sensor applications, were overviewed. Results were presented for conducting polymers and for battery active materials such as a graphitized carbon. The filament-type and the interdigitated-array-type microelectrodes were prepared, and used respectively for the particle-level characterization and for the in situ conductivity measurement. It is demonstrated that the microelectrode-based characterization techniques bring unique information, which will contribute to the design of high performance micro devices.
Complex impedance spectra of ion-conducting lithium borate network glasses are used to study the deformed shape of impedance semicircles which are usually described by a parallel circuit of an ohmic resistor and a phenomenological Constant Phase Element (CPE). Based on the Concept of Mismatch and Relaxation (CMR) of Funke et al. which provides a theoretical treatment of the ion dynamics in disordered materials, a quantitative description of the experimental impedance spectra is presented. It takes into account the contribution of the static glass network by introducing an additional capacitor, and provides both, a physical interpretation of the CPE and an easy-to-handle mathematical formula to calculate the ’real’ capacity of a CPE.
Experiments and computer simulations on Pt/YSZ specimens in various electrode configurations were performed to investigate the effect of reference electrode geometry/position on the accuracy of impedance measurements. The internal, Luggin probe-type, geometry is the preferred reference electrode configuration as it accurately measures both electrolyte and electrode impedances. External, ‘pseudoreference’, electrodes sample an averaged effective potential and can register inaccurate electrolyte resistances, sometimes with distorted electrode arcs. A symmetric configuration can accurately measure the impedance of an electrode; however, the electrolyte resistance will not scale linearly according to sample dimensions, as one might expect. An asymmetric configuration exhibits both non-linear partitioning of electrolyte resistance and distortions in the electrode impedance arc under certain circumstances. The reliability of three-electrode measurements is very sensitive to aspect ratio and electrode configuration.
Recent progress in microelectrode voltammetry in solutions without or with low concentrations of supporting electrolyte is reviewed. The following points are addressed: mathematical treatment of transport, experimental setup, steady state and non steady state transport, migration coupled with homogeneous equilibrium, voltammetry in undiluted redox liquids, studies on the mechanism of the electrode processes, transport of ions in solutions of polyelectrolytes and colloids, and analytical applications.
We report a technique for conducting semi-infinite diffusion spectroelectrochemistry on an aqueous micro-drop as an easy and economic way of investigating spectroelectrochemical behavior of redox active compounds and correlating spectroscopic properties with thermodynamic potentials on a small scale. The chemical systems used to demonstrate the aqueous micro-drop technique were an absorbance based ionic probe [Fe(CN)(6)](3-/4-) and an emission based ionic probe [Re(dmpe)(3)](2+/+). These chemical systems in a micro-drop were evaluated using cyclic voltammetry and UV-visible absorbance and luminescence spectroscopies.
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.
Microfluidics has the potential to revolutionize the way we approach cell biology research. The dimensions of microfluidic channels are well suited to the physical scale of biological cells, and the many advantages of microfluidics make it an attractive platform for new techniques in biology. One of the key benefits of microfluidics for basic biology is the ability to control parameters of the cell microenvironment at relevant length and time scales. Considerable progress has been made in the design and use of novel microfluidic devices for culturing cells and for subsequent treatment and analysis. With the recent pace of scientific discovery, it is becoming increasingly important to evaluate existing tools and techniques, and to synthesize fundamental concepts that would further improve the efficiency of biological research at the microscale. This tutorial review integrates fundamental principles from cell biology and local microenvironments with cell culture techniques and concepts in microfluidics. Culturing cells in microscale environments requires knowledge of multiple disciplines including physics, biochemistry, and engineering. We discuss basic concepts related to the physical and biochemical microenvironments of the cell, physicochemical properties of that microenvironment, cell culture techniques, and practical knowledge of microfluidic device design and operation. We also discuss the most recent advances in microfluidic cell culture and their implications on the future of the field. The goal is to guide new and interested researchers to the important areas and challenges facing the scientific community as we strive toward full integration of microfluidics with biology.
Other than concentrating the target molecules at the sensor location, we demonstrate two distinct new advantages of an open-flow impedance-sensing platform for DNA hybridization on carbon nanotube (CNT) surface in the presence of a high-frequency AC electric field. The shear-enhanced DNA and ion transport rate to the CNT surface decouples the parasitic double-layer AC impedance signal from the charge-transfer signal due to DNA hybridization. The flow field at high AC frequency also amplifies the charge-transfer rate across the hybridized CNT and provides shear-enhanced discrimination between DNA from targeted species and a closely related congeneric species with three nucleotide mismatches out of 26 bases in a targeted attachment region. This allows sensitive detection of hybridization events in less than 20 min with picomolar target DNA concentrations in a label-free CNT-based microfluidic detection platform.
Microfluidic flow cells combined with an interdigitated array (IDA) electrode and/or individually driven interdigitated electrodes were fabricated and characterized for application as detectors for flow injection analysis. The gold electrodes were produced by a process involving heat transfer of a toner mask onto the gold surface of a CD-R and etching of the toner-free gold region by short exposure to iodine-iodide solution. The arrays of electrodes with individual area of 0.01cm(2) (0.10cm of lengthx0.10cm of width and separated by gaps of 0.05 or 0.03cm) were assembled in microfluidic flow cells with 13 or 19mum channel depth. The electrochemical characterization of the cells was made by voltammetry under stationary conditions and the influence of experimental parameters related to geometry of the channels and electrodes were studied by using K(4)Fe(CN)(6) as model system. The obtained results for peaks currents (I(p)) are in excellent agreement with the expected ones for a reversible redox system under stationary thin-layer conditions. Two different configurations of the working electrodes, E(i), auxiliary electrode, A, and reference electrode, R, on the chip were examined: E(i)/R/A and R/E(i)/A, with the first presenting certain uncompensated resistance. This is because the potentiostat actively compensates the iR drop occurring in the electrolyte thin layer between A and R, but not from R to each E(i). This is confirmed by the smaller difference between the cathodic and anodic peak potentials for the second configuration. Evaluation of the microfluidic flow cells combined with (individually driven) interdigitated array electrodes as biamperometric or amperometric detectors for FIA reveals stable and reproducible operation, with peak heights presenting relative standard deviations of less than 2.2%. For electrochemically reversible species, FIA peaks with enhanced current signal were obtained due to redox cycling under flow operation. The versatility of microfluidic flow cells, produced by simple and low-cost technique, associated with the rich information content of electrochemical techniques with arrays of electrodes, opens many future research and application opportunities.
A small-volume voltammetric detection of 4-aminophenol (PAP) has been developed using an interdigitated array (IDA) microelectrode cell in order to apply the IDA to electrochemical enzyme immunoassay. The signal of PAP at the IDA was steady state, and its magnitude was amplified compared with that of the usual single electrode due to redox cycling of PAP between the two finger sets of the IDA. A linear relationship between PAP concentration and cathodic limiting current was obtained from 1 to 1000 microM, reproducibly. The minimum sample volume in the measurement was reduced to 800 nL. High sample throughput of less than 1-min detection time per sample was achieved on 2-10-microL PAP samples. This IDA cell was applied to the electrochemical enzyme immunoassay of mouse IgG. Alkaline phosphatase was used as the enzyme label. The mouse IgG concentration was evaluated by detecting the concentration of PAP, which is the product of enzymatic reaction of the substrate, 4-aminophenyl phosphate (PAPP). Anti-mouse IgG was covalently immobilized on the glass surface of the small-volume immunowells by carbodiimide coupling. The assay range was 10-1000 ng/mL using 10-microL sample and 20-microL substrate solutions.
A finite-element simulation study has been made of the effect upon the uncompensated resistance, U, of the size of a reference electrode positioned at various distances above the center of an inlaid-disk working electrode. The idealized reference electrode is treated as a conducting circular disk embedded in a cylindrical insulator. Two effects are encountered, in addition to the expected diminution in U as the reference electrode approaches the working electrode. A "backwater effect", reducing U, arises from the avoidance of the Luggin probe by the current lines. A "short-circuiting" effect, enhancing U, arises if those current lines pass through the reference electrode on their way to the counter electrode. Two detrimental effects of the intrusion of a finite-sized reference electrode into the vicinity of the working electrode--a reduction in the total current and the perturbation of the current density distribution-are also briefly examined. It is demonstrated that U may be reduced indefinitely by decreasing the working-to-reference gap, but, the Luggin probe must have a diameter of one-fourth or less of that of the working electrode if significant shielding is to be avoided.
Integration of microelectrodes in microfluidic devices has attracted significant attention during the past years, in particular for analytical detections performed by direct or indirect electrochemical techniques. In contrast there is a lack of general theoretical treatments of the difficult diffusion-convection problems which are borne by such devices. In this context, we investigated the influence of the confining effect and hydrodynamic conditions on the steady-state amperometric responses monitored at a microband electrode embedded within a microchannel. Several convective-diffusive mass transport regimes were thus identified under laminar flow on the basis of numerical simulations performed as a function of geometrical and hydrodynamic parameters. A rationalization of these results has been proposed by establishing a zone diagram describing all the limiting and intermediate regimes. Concentration profiles generated by the electrode across the microchannel section were also simulated according to the experimental conditions. Their investigation allowed us to evaluate the thickness of the diffusive-convective layer probed by the electrode as well as the distance downstream from which the solution becomes again homogeneous across the whole microchannel section. Experimental checks of the theoretical principles delineated here have validated the present results. Experiments were performed at microband electrodes integrated in microchannels with aqueous solutions of ferrocene methanol under pressure-driven flow.
Impedance biosensors are a class of electrical biosensors that show promise for point-of-care and other applications due to low cost, ease of miniaturization, and label-free operation. Unlabeled DNA and protein targets can be detected by monitoring changes in surface impedance when a target molecule binds to an immobilized probe. The affinity capture step leads to challenges shared by all label-free affinity biosensors; these challenges are discussed along with others unique to impedance readout. Various possible mechanisms for impedance change upon target binding are discussed. We critically summarize accomplishments of past label-free impedance biosensors and identify areas for future research.
Optimization of interdigitated electrodes in electric field distribution and thermal effect
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