Investigation of interfacial capacitance of Pt,Ti and TiN coated electrodes by electrochemical impedance spectroscopy

Electrochemical processes at the electrode-electrolyte (body fluid) interface are of ultimate importance for stimulating/sensing electrode function. A high electrode surface area is desirable for safe stimulation through double-layer charging and discharging. Pt and Pt-Ir alloys have been the most common electrode materials. The use of TiN coating as the surface layer on the electrode has found increasing interest because of its metal-like conductivity, excellent mechanical and chemical properties, and the fact that it can be deposited with a high surface area. In this work, electrochemical impedance spectroscopy (EIS), which is a sensitive and non-destructive technique and widely used for characterization of electrical properties of electrode-electrolyte interfaces, was applied to investigate pure Pt and Ti, and TiN coated electrodes exposed to a phosphate-buffered-saline (PBS) solution. Platinized Pt and Ti were also studied for comparison. The capacitance value of the electrodes in PBS was obtained through quantitative analysis of the EIS spectra. The results reveal that the capacitance of the TiN coated electrodes with a rough surface is several hundreds times higher than that of a smooth Pt surface. Platinization of Ti can also increase the capacitance to the same extent as platina. EIS has been shown to be a powerful technique for characterization of stimulating/sensing electrodes.     

Nano nickel oxide/nickel incorporated nickel composite coating for sensing and estimation of acetylcholine

Pure nickel electrodes can be used as biosensors especially for sensing and estimating acetylcholine neurotransmitter. In the present work, a good electrochemical sensor was developed by electroplating nano nickel oxide reinforced nickel on graphite substrate. The morphology of the working electrode surface was studied by using a scanning electron microscope (SEM). The electrochemical and biological performance of the modified electrode was characterized by polarization studies in different media. The present modified electrode showed good sensing performance with a response time as low as 8s during sensing and estimation of acetylcholine. The sensitivity of the modified electrode was 34.88 microA/(microM cm(2)).     

Electrochemical activation of electrodes for amperometric detection of nitric oxide

An open question in the literature of nitric oxide detection was investigated: does electrochemical activation account for the enhanced properties of certain presumed chemically-modified electrodes? Uniform electrodes of graphite, iridium, palladium, platinum, and ruthenium were exposed to potential cycling and then tested for amperometric response to nitric oxide to identify principles that govern electrochemical activation of nitric oxide electrodes. These electrodes were compared to similar electrodes that were not cycled. Only cycled graphite and ruthenium showed significantly increased responses. Graphite demonstrated enhanced performance after exposure to cycling potentials at which oxygen, CO₂, and soluble carbonates form, suggesting that erosion of the electrode enhanced its response by increasing the surface area accessible to nitric oxide. This may explain the performance of carbon fibers cycled to the same potentials in solutions containing metalloporphyrins. The response of ruthenium was enhanced after cycling to less extreme potentials at which soluble species do not form and at which a metallic conductive oxide, RuO₂, could lay down a stable, adherent layer on the electrode surface. Cycled ruthenium also exhibited a much greater increase in capacitance after cycling, consistent with the formation of a conductive surface layer.     

Electrochemical sensing of dihydrogen phosphate and adenosine-5′-triphosphate anions by self-assembled monolayers of (ferrocenylmethyl)trialkylammonium cations on gold electrodes

The effective electrochemical sensing of dihydrogen phosphate and adenosine-5′-triphosphate anions could be achieved in organic electrolytes using self-assembled monolayers of a (ferrocenylmethyl)trialkylammonium cation. The electrochemical response of these modified electrodes was found to be similar to that of the receptor in homogeneous solution. This observation showed that the electrochemical recognition properties of the redox active cationic receptor were fully retained after immobilization on the electrode surface. The recognition abilities of this redox assembly are based on strong ion pairing effects, further reinforced upon oxidation of the ferrocene centres to ferrocenium. The complexation events could be detected and amperometrically quantified through the emergence of new differential pulse voltammetric peaks corresponding to the electroactivity of the (ferrocenylmethyl)alkylammonium-anion complexes.     

Frequency dependent and surface characterization of DNA immobilization and hybridization

The hybridization of oligomeric DNA was investigated using the frequency dependent techniques of quartz crystal microbalance (QCM) and electrochemical impedance spectroscopy (EIS). Synthetic 5'-amine-terminated single stranded oligonucleotides (ssDNA) were immobilized on the surface of the oxidized platinum driving electrodes of AT-cut quartz QCM crystals using 3-glycidoxypropyl-trimethoxysilane. Similar ssDNA coupling was accomplished on the exposed glass surface between the metallic digits of microlithographically fabricated interdigitated microsensor electrodes (IMEs). Confirmation of this covalent coupling surface chemistry was achieved using Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR). Substantial changes in resonant frequency values (0.012% decrease) and electrochemical impedance values (both real and imaginary components) (35.4 and 42.1% increase in impedance magnitude at 1.0 Hz in buffer and deionized water, respectively) were observed resulting from hybridization of the attached ssDNA upon exposure to its complement under appropriate hybridization conditions. Non-complementary (random) oligomer sequence demonstrated a modest change in resonant frequency and a non-detectable change in impedance. Microarray glass slide surfaces modified with 3-glycidoxypropyltrimethoxysilane (GPS), shown to be advantageous in the design and use of microarrays of amine-terminated ssDNA, is confirmed to arise from direct covalent coupling of the DNA to the surface with little non-specific adsorption. The possibility to detect the binding state of DNA in the vicinity of an electrode, without a direct connection between the measurement electrode and the DNA is hereby reported. The potential for development of label-free, low-density DNA microarrays is demonstrated and is being pursued.     

Metallic oxide CdIn2O4 films for the label free electrochemical detection of DNA hybridization

DNA functionalised semiconductor metallic oxide electrodes have been developed for the direct electrochemical detection of DNA hybridization, without labelling or the introduction of a redox couple. Conductive CdIn(2)O(4) thin films with controlled properties were deposited on glass substrates using an aerosol pyrolysis technique. The films exhibit a polycrystalline microstructure with a surface roughness of 1.5 nm (r.m.s.) and an electrical resistivity ranging between 1 and 3 x 10(-3) Omega cm. These electrodes were functionalised using hydroxylation and silanisation steps, to allow the binding of DNA probe sequences (20 bases). The electrical detection of DNA hybridization with complementary sequences has been performed using electrochemical impedance spectrometry (EIS) measuring the variation of impedance before and after hybridization. Two set-ups were used, a standard set-up including three electrodes and a set-up including two symmetrical electrodes. In both configurations, a significant increase of the impedance modulus, more particularly of the real part of the impedance (160-225% according to the electrochemical cell used) has been obtained over a frequency range of 10-10(5)Hz. DNA hybridization has also been systematically confirmed using the fluorescence spectrometry. This study emphasizes the high sensitivity of the CdIn(2)O(4) as a working electrode for the detection of biological events occurring at the electrode surface.     

A Facile and Very Effective Method to Enhance the Mechanical Strength and the Cyclability of Si‐Based Electrodes for Li‐Ion Batteries

It is well known that the mechanical properties of lithium‐ion battery electrodes impact their electrochemical performance. This is especially critical for Si‐based negative electrodes, which suffer from large volume changes of the active mass upon cycling. Here, this study presents a postprocessing treatment (called maturation) that improves the mechanical and electrochemical stabilities of silicon‐based anodes made with an acidic aqueous binder. It consists of storing the electrode in a humid atmosphere for a few days before drying and cell assembly. This results in a beneficial in situ reactive modification of the interfaces within the electrode. First, the binder tends to concentrate at the silicon interparticle contacts. As a result, the cohesion of the composite film is strengthened. Second, the corrosion of the copper current collector, inducing the formation of copper carboxylate bonds, improves the adhesion of the composite film. The great improvement of the mechanical stability of the matured electrode is confirmed by in‐operando optical microscopy showing the absence of film delamination. The result is a significant electrochemical performance gain, up to a factor 10, compared to a not‐matured electrode. This maturation procedure can be applied to other types of electrodes for improving their electrochemical performance and also their handling during cell manufacturing.     

AC voltammetric carbon paste-based enzyme immunosensors

Carbon paste electrodes, previously anodised in a basic media, are the basis for the development of a new voltammetric immunosensor device. Passive adsorption of the appropriate immunochemical reagent was performed onto the electrode surface. Alkaline Phosphatase labelled immunoglobulin was the tracer used in this work, 3-indoxyl phosphate being a very suitable enzymatic substrate for the electrochemical detection of the corresponding affinity reaction. The hydrolysis of this molecule generates indigo dimmer. This product was detected by alternating current voltammetry taking advantage of the adsorptive and inherent electrodic properties that it exhibits. The same electrochemical anodisation was used at the end of one assay to remove the entire protein layer attached to the carbon paste surface, allowing the formation of a new sensing phase and the use of the same support in several consecutive experiments. The methodology was applied to the design of two different immunoassays for the determination of human IgG. Good reproducibility of the electrodic signal and a limit of detection around 10⁻¹⁰ M were achieved.     

Nanoscale Elastic Changes in 2D Ti3C2Tx (MXene) Pseudocapacitive Electrodes

Designing sustainable electrodes for next generation energy storage devices relies on the understanding of their fundamental properties at the nanoscale, including the comprehension of ions insertion into the electrode and their interactions with the active material. One consequence of ion storage is the change in the electrode volume resulting in mechanical strain and stress that can strongly affect the cycle life. Therefore, it is important to understand the changes of dimensions and mechanical properties occurring during electrochemical reactions. While the characterization of mechanical properties via macroscopic measurements is well documented, in situ characterization of their evolution has never been achieved at the nanoscale. It is reported here with in situ imaging, combined with density functional theory of the elastic changes of a 2D titanium carbide (Ti₃C₂T_x) based electrode in direction normal to the basal plane (electrode surface) during alkaline cation intercalation/extraction. 2D carbides, known as MXenes, are promising new materials for supercapacitors and various kinds of batteries, and understanding the coupling between their mechanical and electrochemical properties is therefore necessary. The results show a strong correlation between the cations content and the out‐of‐plane elastic modulus. This strategy enables identifying the preferential intercalation pathways within a single particle, which is important for understanding ionic transport in these materials.     

Electrochemical fabrication and evaluation of highly sensitive nanorod-modified electrodes for a biotin/avidin system

Bioaffinity sensors need to be rapid, specific, and highly sensitive. To realize these features, electrodes that can elicit high electrochemical performance are necessary. In this study, we developed nanorod array electrode and performed cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments to study the interfacial properties of the nanorod array electrode with Fe(CN)(6)(3-/4-) as the redox molecules. Results showed that both the CV and EIS measurements captured very well the resistive and capacitive changes due to the adsorption of functionalizing molecules and the coupling between avidin and biotin. The EIS measurements were more sensitive in discriminating small changes caused by the surface adsorption of various molecules. The use of avidin-functionalized gold nanorod modified electrodes had led to much increased detection sensitivity along with a detection-limit as low as 1 ng/mL of biotin.     

Reduction of electrode polarization capacitance in low-frequency impedance spectroscopy by using mesh electrodes

Dielectric measurements of biological samples are obscured by electrode polarization, which at low frequencies dominates over the actual sample response. Reduction of this artifact is especially necessary in studying interactions of electric field with biological systems in the α-dispersion range. We developed a method to reduce the influence of electrode polarization by employing mesh instead of solid electrodes as sensing probes, thereby reducing the area of the double layer. The design decreases the electrode-electrolyte contact area by almost 40% while keeping the bulk sample capacitance the same. Interrogation electric fields away from the electrode surface and sensitivity are unaffected. Electrodes were microfabricated (600μm×50μm, spacing of 100μm) with and without mesh holes 7.5μm×7.5μm in size. Simulations of electric field performed using Comsol Multiphysics showed non-uniformity of the electric field within less than 1.5μm from the electrode surface, which encompasses the double layer region, but at greater distance the solid and mesh electrodes gave the same results. Mesh electrodes reduced capacitance measurements for water and KCl solutions of different concentrations at low frequencies (<10kHz), while higher frequency capacitance remained the same for both electrode types, confirming our hypothesis that this design leaves the electric field mainly unaffected. Impedance measurements at low frequencies for water and mice heart mitochondrial suspension were lower for mesh than for solid electrodes. Comsol simulations confirmed these results by showing that mesh electrodes have a greater charge density than solid electrodes, which affects conductance. These electrodes are being used for mitochondrial membrane potential studies.     

The use of cyclic voltammetry to detect biofilms formed by Pseudomonas fluorescens on platinum electrodes

The development of an electrochemical detector to monitor the in situ formation of biofilms is described. The detector consisted of an electrochemical cell containing three electrodes, whose response to the application of a potential profile to the working electrode was sensitive to the amount of biofilm present on the surface. The electrochemical technique used was repetitive cyclic voltammetry. Differences between the response of the uncolonised electrode and after Pseudomonas fluorescens biofilms of different ages were grown on its surface were determined. The results show that cyclic voltammetry applied to platinum electrodes can be used to detect young biofilms. The development of the shape of the voltammogram as the potential is cycled may constitute a means of providing information on the coverage of the surface. Observation of the platinum electrodes before and after the electrochemical measurements showed that even after 30 min of recycling, most of the cells were still adhered to the surface, although some may have lost viability.     

Cell-compatible array of three-dimensional tip electrodes for the detection of nitric oxide release

An array of electrodes on which cells could be grown directly was fabricated using silicon anisotropic etching and a thick-photoresist process and employed for the detection of nitric oxide (NO) released from a population of adherently growing human umbilical vein endothelial cells (HUVEC). The electrodes are tip-shaped and are 40 microm high of which only the top 15 microm are exposed Pt-tips. After electrochemical induced modification of the exposed Pt tips using Ni phthalocyanine the individual addressable electrode tips were sensitive and selective for the detection of NO at an applied constant potential of 750 mV. The silicon nitride insulation of the lower part of the tip electrodes prevented the death of the cells upon the application of the working potential at which NO was detected. It also helped to avoid the perturbation of the integrity of the sensing chemistry imparted on the electrode surface that could have resulted from the contact of the adherently growing cells with the active electrode surface. The release of nitric oxide from HUVEC was successfully monitored with different numbers of tip electrodes simultaneously connected as combined working electrode.     

A Three‐Dimensionally Interconnected Carbon Nanotube–Conducting Polymer Hydrogel Network for High‐Performance Flexible Battery Electrodes

High‐performance flexible energy‐storage devices have great potential as power sources for wearable electronics. One major limitation to the realization of these applications is the lack of flexible electrodes with excellent mechanical and electrochemical properties. Currently employed batteries and supercapacitors are mainly based on electrodes that are not flexible enough for these purposes. Here, a three‐dimensionally interconnected hybrid hydrogel system based on carbon nanotube (CNT)‐conductive polymer network architecture is reported for high‐performance flexible lithium ion battery electrodes. Unlike previously reported conducting polymers (e.g., polyaniline, polypyrrole, polythiophene), which are mechanically fragile and incompatible with aqueous solution processing, this interpenetrating network of the CNT‐conducting polymer hydrogel exibits good mechanical properties, high conductivity, and facile ion transport, leading to facile electrode kinetics and high strain tolerance during electrode volume change. A high‐rate capability for TiO₂ and high cycling stability for SiNP electrodes are reported. Typically, the flexible TiO₂ electrodes achieved a capacity of 76 mAh g^–1 in 40 s of charge/discharge and a high areal capacity of 2.2 mAh cm^–2 can be obtained for flexible SiNP‐based electrodes at 0.1C rate. This simple yet efficient solution process is promising for the fabrication of a variety of high performance flexible electrodes.     

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

This research is directed towards developing a more sensitive and rapid electrochemical sensor for enzyme labeled immunoassays by coupling redox cycling at interdigitated electrode arrays (IDA) with the enzyme label beta-galactosidase. Coplanar and comb IDA electrodes with a 2.4 microm gap were fabricated and their redox cycling currents were measured. ANSYS was used to model steady state currents for electrodes with different geometries. Comb IDA electrodes enhanced the signal about three times more than the coplanar IDAs, which agreed with the results of the simulation. Magnetic microbead-based enzyme assay, as a typical example of biochemical detection, was done using the comb and coplanar IDAs. The enzymes could be placed close to the sensing electrodes (approximately 10 microm for the comb IDAs) and detection took less than 1 min with a limit of detection of 70 amol of beta-galactosidase. We conclude that faster and more sensitive assays can be achieved with the comb IDA.     

Dynamic measurement of the surface stress induced by the attachment and growth of cells on Au electrode with a quartz crystal microbalance

The processes of adhesion, spreading and proliferation of human mammary cancer cells MCF-7 on two Au electrodes with different surface roughness (R(f) and R(f)=3.2 or 1.1) were monitored and clearly identified with the quartz crystal microbalance (QCM) technique. Analyses of the QCM responses on the resonant frequency shifts (Deltaf(0)) vs. the motional resistance changes (DeltaR(1)) revealed a significant surface-stress effect in the involved courses, in addition to a viscodensity effect and a relatively small mass effect (especially at the smooth electrode). Experiments of fluorescence microscopy, cyclic voltammetry and electrochemical impedance spectroscopy were conducted to investigate the cell population on the electrode vs. the electrode-surface roughness. Simplified equations are deduced to quantitatively evaluate the surface stress, and a novel QCM method for dynamically measuring the surface stress on an electrode in cell-culture course is thus described. It was found that the smoother surface (R(f)=1.1) gave a higher surface stress during cell attachment and less cell population on it than the rougher surface (R(f)=3.2). In addition, real-time QCM monitoring showed on the same electrode the surface stress induced by hepatic normal cells being notably higher than that caused by hepatic cancer cells at cell-attachment stage, suggesting that the surface-stress measurement can exhibit the difference of adhesion-performance between the healthy and ill-behaved cells.     

Electrical frequency dependent characterization of DNA hybridization

The hybridization of oligomeric DNA was investigated using the frequency dependent techniques of electrochemical impedance spectroscopy (EIS) and quartz crystal microgravimetry (QCM). Synthetic 5'-amino terminated single stranded oligonucleotides (ssDNA) were attached to the exposed glass surface between the digits of microlithographically fabricated interdigitated microsensor electrodes using 3-glycidoxypropyl-trimethoxysilane. Similar ssDNA immobilization was achieved to the surface of the gold driving electrodes of AT-cut quartz QCM crystals using 3-mercaptopropyl-trimethoxysilane. Significant changes in electrochemical impedance values (both real and imaginary components) (11% increase in impedance modulus at 120 Hz) and resonant frequency values (0.004% decrease) were detected as a consequence of hybridization of the bound ssDNA upon exposure to its complement under hybridization conditions. Non-complementary (random) sequence sowed a modest decrease in impedance and a non-detectable change in resonant frequency. The possibility to detect the binding state of DNA in the vicinity of an electrode, without a direct connection between the measurement electrode and the DNA, has been demonstrated. The potential for development of label-free, low density DNA microarrays is demonstrated and is being pursued.     

The Use of Cyclic Voltammetry to Detect Biofilms formed by Pseudomonas fluorescens on Platinum Electrodes

The development of an electrochemical detector to monitor the in situ formation of biofilms is described. The detector consisted of an electrochemical cell containing three electrodes, whose response to the application of a potential profile to the working electrode was sensitive to the amount of biofilm present on the surface. The electrochemical technique used was repetitive cyclic voltammetry. Differences between the response of the uncolonised electrode and after Pseudomonas fluorescens biofilms of different ages were grown on its surface were determined. The results show that cyclic voltammetry applied to platinum electrodes can be used to detect young biofilms. The development of the shape of the voltammogram as the potential is cycled may constitute a means of providing information on the coverage of the surface. Observation of the platinum electrodes before and after the electrochemical measurements showed that even after 30 min of recycling, most of the cells were still adhered to the surface, although some may have lost viability.     

Highly sensitive electrochemical detection of cocaine on graphene/AuNP modified electrode via catalytic redox-recycling amplification

We demonstrated a new strategy for highly sensitive electrochemical detection of cocaine by using two engineered aptamers in connection to redox-recycling signal amplification. The graphene/AuNP nanocomposites were electrochemically deposited on a screen printed carbon electrode to enhance the electron transfers. The cocaine primary binding aptamers were self-assembled on the electrode surface through sulfur-Au interactions. The presence of the target cocaine and the biotin-modified secondary binding aptamers leads to the formation of sandwich complexes on the electrode surface. The streptavidin-conjugated alkaline phosphatases (ALPs) were used as labels to generate quantitative signals. The addition of the ALP substrate and the co-reactant NADH results in the formation of a redox cycle between the enzymatic product and the electrochemically oxidized species and the signal is thus significantly amplified. Because of the effective modification of the sensing surface and signal amplification, low nanomolar (1 nM) detection limit for cocaine is achieved. The proposed aptamer-based sandwich sensing approach for amplified detection of cocaine thus opens new opportunities for highly sensitive determination of other small molecules.     

Effect of electrode surface properties on enhanced electron transfer activity in microbial fuel cells

The influence of electrode surface chemistry over biofilm growth was evaluated for photo‐bioelectrocatalytic fuel cell. A consortium of photosynthetic bacteria was grown onto different electrodes designed with polyethylenimine (PEI) and multiwall carbon nanotubes as hydrophilic and hydrophobic modifier, respectively. The designed electrodes were loaded with 0.08, 0.17, and 0.33 μg/cm² of PEI to change the hydrophilicity. However, 0.56, 0.72, and 0.83 mg/cm² of multiwall carbon nanotubes were used to alter the hydrophobicity of the electrodes. The surface chemistry of electrode and bio‐interaction was evaluated as a function of contact angle and biofilm formation. The results were compared with those obtained with a carbon paper electrode. The contact angle on the untreated electrode (carbon paper) was 118°, whereas for hydrophobic and hydrophilic electrodes, the maximum and minimum contact angles were 170° and 0°, respectively. Interestingly, the maximum biofilm growth (0.2275 g, wet basis) was observed on highly hydrophobic surface; however, the maximum electrochemical performance (246 mV) was shown by the most hydrophilic electrode surface. PEI‐based electrode with good biofilm formation showed comparatively higher electrogenic activity.