Impedance Response of a Thin Film on an Electrode: Deciphering the Influence of the Double Layer Capacitance.

The different contributions of the interfacial capacitance are identified in the case of passive materials or thin protective coatings deposited on the electrode surface. The method is based on a graphical analysis of the EIS results to determine both the passive-film capacitance in the high-frequency domain and the double-layer capacitance in the low-frequency domain. The proposed analysis is shown to be independent of the physicochemical origins of the frequency dispersion of the interfacial capacitances which results, from an analysis point of view of the experimental results, in the use of a constant-phase element However, for a correct evaluation of the thin-film properties such as its thickness, the high-frequency data must be corrected for the double-layer contribution. In particular, it is shown that if the double-layer capacitance gives a frequency-dispersed response, it is necessary to correct the high-frequency part for the double-layer constant-phase elements. This is first demonstrated on synthetic data and then used for the determination of the thickness of thin oxide film formed on Al in neutral pH solution.

Imaging of a Thin Oxide Film Formation from the Combination of Surface Reflectivity and Electrochemical Methods.

Electrochemical methods (cyclic voltammetry (CV), potential steps, and electrochemical impedance spectroscopy) were successfully combined with in situ reflectometry measurements for a detailed analysis of the passive layer evolution as a function of the electrode potential. Interestingly, both EIS and surface reflectivity allowed a film thickness in the nanometer range to be readily determined. In addition, transient analyses of the reflectivity simultaneously recorded with CVs show the formation of both Fe2O3 and Fe3O4 oxides. The image analysis showed that the steel surface reactivity is heterogeneous and presents micrometric islands coated with a thicker oxide layer than the surrounding surface. The in situ combination of these techniques thus offers a powerful analytical description of the interface on a local scale and its transient response to a perturbation.

Improved electrochemical detection of a transthyretin synthetic peptide in the nanomolar range with a two-electrode system integrated in a glass/PDMS microchip.

An alternative to a three-electrode set-up for electrochemical detection and analysis in microfluidic chips is described here. The design of the electrochemical sensor consists of the surface of the glass substrate covered with a PDMS block which bears the microfluidic channels. A band microelectrode which acts as a working electrode surrounded by a large counter electrode is obtained at the micrometric level to propose a simple and efficient sensing area for on-a-chip analysis. The counter-electrode with a surface area about 22-fold greater than the working-microelectrode can also be considered as a pseudo reference since its current density is low and thus limits the potential variations around the rest potential. To this purpose, the [Fe(III)(CN)6]³â»/[Fe(II)(CN)6]4⁻ redox couple was used in order to set a reference potential at 0 V since both electrodes were platinum. The electrochemical microchip performance was characterized using differential pulse voltammetric (DPV) detection and quantification of the optically multi-labelled transthyretin synthetic peptide mimicking a tryptic fragment of interest for the diagnosis of familial transthyretin amyloidosis (ATTR). The limit of detection of the peptide by the working microelectrode was 25 nM, a value 100-fold lower than the one reported with conventional capillary electrophoresis coupled with laser-induced fluorescence under the same analytical conditions.

A real time affinity biosensor on an insulated polymer using electric impedance spectroscopy in dielectric microchips.

This paper presents development of real time monitoring of binding events on flexible plastic in microchips. Two planar carbon microelectrodes are integrated into an insulated polyethylene terephthalate microchip without direct electrical contact with the solution in the microchannel. It has been possible to probe the electric impedance changes through the interface constituted by the microelectrode/PET microchannel/solution when a biomolecular interaction takes place on the polymer surface. This new transduction for biosensing was demonstrated for the molecular recognition of BSA immobilized on the polymer microchannel surface using the corresponding rabbit anti-BSA antibodies as an analyte in the flow microchannel at the nanomolar range concentration. The equilibrium association constant was determined for the affinity reaction between both ligands and was obtained equal to 5 × 10(7) M(-1). The promising results obtained with this new device make it a competitive biosensor.

Dynamics of BSA adsorption onto a photoablated polymer surface in a dielectric microchip.

Dielectric impedance spectroscopy in a microchip was used for monitoring the adsorption of biomolecules onto a heterogeneous polymer surface obtained after the photoablation process. The sensor comprises a thin dielectric layer with two parallel carbon microband electrodes on the one side, and the photoablated surface on the other. The biomolecules need not to be labelled, as in an optical biosensor, even if they need to be attached to the polymer surface coupled with the microelectrodes when a biomolecular interaction occurs. Based on this principle, a flow sensor has been developed to record the adsorption dynamics illustrated with BSA coating on the heterogeneous surface in a linear dynamic range from 1 picomolar to 1 nanomolar at a fixed low frequency. Modeling the dielectric interface using an appropriate equivalent circuit permits extraction of the values of the interfacial impedance for ultralow protein concentrations. The promising results obtained with this methodology make it a competing method in comparison with optical or electrochemical transduction for biosensor development.

Combined geochemical and electrochemical methodology to quantify corrosion of carbon steel by bacterial activity.

The availability of respiratory substrates, such as H2 and Fe(II,III) solid corrosion products within nuclear waste repository, will sustain the activities of hydrogen-oxidizing bacteria (HOB) and iron-reducing bacteria (IRB). This may have a direct effect on the rate of carbon steel corrosion. This study investigates the effects of Shewanella oneidensis (an HOB and IRB model organism) on the corrosion rate by looking at carbon steel dissolution in the presence of H2 as the sole electron donor. Bacterial effect is evaluated by means of geochemical and electrochemical techniques. Both showed that the corrosion rate is enhanced by a factor of 2-3 in the presence of bacteria. The geochemical experiments indicated that the composition and crystallinity of the solid corrosion products (magnetite and vivianite) are modified by bacteria. Moreover, the electrochemical experiments evidenced that the bacterial activity can be stimulated when H2 is generated in a small confinement volume. In this case, a higher corrosion rate and mineralization (vivianite) on the carbon steel surface were observed. The results suggest that the mechanism likely to influence the corrosion rate is the bioreduction of Fe(III) from magnetite coupled to the H2 oxidation.

Influence of hydrogen-oxidizing bacteria on the corrosion of low carbon steel: Local electrochemical investigations.

Low carbon steel has been considered a suitable material for component of the multi-barrier system employed on the geological disposal of high-level radioactive waste (HLW). A non negligible amount of dihydrogen (H2) is expected to be produced over the years within the geological repository due to the anoxic corrosion of metallic materials and also to the water radiolysis. The influence of the activity of hydrogen-oxidizing bacteria (HOB) and iron-reducing bacteria (IRB) on carbon steel corrosion is considered in this study because of the high availability of energetic nutriments (H2, iron oxides and hydroxides) produced in anoxic disposal conditions. Local electrochemical techniques were used for investigating the activity of IRB as a promoter of local corrosion in the presence of H2 as electron donor. A local consumption of H2 by the bacteria has been evidenced and impedance measurements indicate the formation of a thick layer of corrosion products.

Effect of protein adsorption on the corrosion behavior of 70Cu-30Ni alloy in artificial seawater.

Copper alloys often used in cooling circuits of industrial plants can be affected by biocorrosion induced by biofilm formation. The objective of this work was to study the influence of protein adsorption, which is the first step in biofilm formation, on the electrochemical behavior of 70Cu-30Ni (wt.%) alloy in static artificial seawater and on the chemical composition of oxide layers. For that purpose, electrochemical measurements performed after 1h of immersion were combined to surface analyses. A model is proposed to analyze impedance data. In the presence of bovine serum albumin (BSA, model protein), the anodic charge transfer resistance deduced from EIS data at Ecorr is slightly higher, corresponding to lower corrosion current. Without BSA, two oxidized layers are shown by XPS and ToF-SIMS: an outer layer mainly composed of copper oxide (Cu2O redeposited layer) and an inner layer mainly composed of oxidized nickel, with a global thickness of ~30nm. The presence of BSA leads to a mixed oxide layer (CuO, Cu2O, Ni(OH)2) with a lower thickness (~10nm). Thus, the protein induces a decrease of the dissolution rate at Ecorr and hence a decrease of the amount of redeposited Cu2O and of the oxide layer thickness.

Rotating disk electrodes to assess river biofilm thickness and elasticity.

The present study examined the relevance of an electrochemical method based on a rotating disk electrode (RDE) to assess river biofilm thickness and elasticity. An in situ colonisation experiment in the River Garonne (France) in August 2009 sought to obtain natural river biofilms exhibiting differentiated architecture. A constricted pipe providing two contrasted flow conditions (about 0.1 and 0.45 m s(-1) in inflow and constricted sections respectively) and containing 24 RDE was immersed in the river for 21 days. Biofilm thickness and elasticity were quantified using an electrochemical assay on 7 and 21 days old RDE-grown biofilms (t(7) and t(21), respectively). Biofilm thickness was affected by colonisation length and flow conditions and ranged from 36 ± 15 µm (mean ± standard deviation, n = 6) in the fast flow section at t(7) to 340 ± 140 µm (n = 3) in the slow flow section at t(21). Comparing the electrochemical signal to stereomicroscopic estimates of biofilms thickness indicated that the method consistently allowed (i) to detect early biofilm colonisation in the river and (ii) to measure biofilm thickness of up to a few hundred µm. Biofilm elasticity, i.e. biofilm squeeze by hydrodynamic constraint, was significantly higher in the slow (1300 ± 480 µm rpm(1/2), n = 8) than in the fast flow sections (790 ± 350 µm rpm(1/2), n = 11). Diatom and bacterial density, and biofilm-covered RDE surface analyses (i) confirmed that microbial accrual resulted in biofilm formation on the RDE surface, and (ii) indicated that thickness and elasticity represent useful integrative parameters of biofilm architecture that could be measured on natural river assemblages using the proposed electrochemical method.

Cavity microelectrode for studying manganese dioxide powder as pH sensor.

The potential-pH response of an electrolytic manganese dioxide is investigated by means of a cavity microelectrode (CME). The potential-pH curves show a complex evolution that could be explained by the disporportionation of MnOOH species, leading to the formation of Mn(2+) ions on the MnO(2) surface. Such a behaviour is not suited for pH sensor application. However when the tip of the electrode is coated by a Nafion membrane, the potential-pH evolution shows a unique slope close to -60 mV pH(-1). In addition, the sensor exhibits short time responses to pH variations, a good selectivity, and it can be easily renewed compared to classical sensors.

Polycarbonate microchannel network with carpet of gold nanowires as SERS-active device.

A polycarbonate (PC) microchannel network supporting gold nanowires was developed to be a SERS-active microchip. Observations of large increases in a Raman cross-section, allowed us to collect vibrational signatures which are not easily detectable by Raman techniques due to the high fluorescence level of bare PC. Compared to other SERS experiments, this study relies on the use of dielectric polymer/metal surfaces which are well defined at microscale and nanoscale levels. This device seems a promising tool for sensing the adsorption of biomolecules.

Study of the dissolution of thin films of cerium oxide by using a GaPO4 crystal microbalance.

In this study, microbalance measurements with a GaPO(4) crystal were performed to determine the dissolution rate of cerium oxide thin films at room temperature after a high temperature treatment of the hydroxide precursor Ce(OH)(4) at 700 degrees C. The properties of the GaPO(4) crystal enables gravimetric measurements to be performed after being heated at high temperatures where the classical quartz crystal microbalance irreversibly loses all its piezoelectric properties. The GaPO(4) resonators were calibrated at room temperature by galvanostatic copper deposition before and after a high-temperature treatment, and the sensitivity coefficients K(S) were found to be identical, which proved the high potential of GaPO(4) at high temperatures. However the accuracy of the gravimetric measurements is lower after a high temperature treatment. The rate determining step of the cerium oxide dissolution seems to be the reduction reaction of Ce(IV) to Ce(III), which is carried out by the hydrogen peroxide present in the dissolution medium.

Nanomosaic network for the detection of proteins without direct electrical contact.

A nanomosaic network of metallic nanoparticles for the detection of ultralow concentrations of proteins is reported, which uses two planar microelectrodes embedded in a microchip that permit generation of capacitive coupling to the nanomosaic system without the need for direct electrical contact with the channel. By tailoring the microchannel surface using a sandwich configuration of polyethylene terephthalate/gold nanoparticles/poly(L-lysine), the surface charge can be modified following biomolecular interactions and monitored using a noncontact admittance technique. This nanodevice system behaves like a tunable capacitor and can be employed for the detection of any kind of molecule. The femtomolar detection of an anionic protein, such as beta-lactoglobulin in phosphate-buffered saline medium, is taken as an example.

Electroacoustic polymer microchip as an alternative to quartz crystal microbalance for biosensor development.

Laser photoablation of poly(ethylene terephthalate) (PET), a flexible dielectric organic polymer, was used to design an acoustic miniaturized DNA biosensor. The microchip device includes a 100-microm-thick PET layer, with two microband electrodes patterned in photoablated microchannels on one side and a depressed photoablated disk decorated by gold sputtered layer on the other side. Upon application of an electric signal between the two electrodes, an electroacoustic resonance phenomenon at approximately 30 MHz was established through the microelectrodes/PET/ gold layer interface. The electroacoustic resonance response was fitted with a series RLC motional arm in parallel with a static Co arm of a Buttlerworth-Van Dyke equivalent circuit: admittance spectra recorded after successive cycles of DNA hybridization on the gold surface showed reproducible changes on R, L, and C parameters. The same hybridizations runs were performed concomitantly on a 27-MHz (9 MHz, third overtone) quartz crystal microbalance in order to validate the PET device developed for bioanalysis applications. The electroacoustic PET device, approximately 100 times smaller than a microbalance quartz crystal, is interesting for the large-scale integration of acoustic sensors in biochips.

Electroacoustic miniaturized DNA-biosensor.

A micrometer-sized electroacoustic DNA-biosensor was developed. The device included a thin semi-crystalline polyethylene terephthalate (PET) dielectric layer with two Ag microband electrodes on one side and a DNA thiol-labeled monolayer adsorbed on a gold surface on the other. A resonance wave was observed at 29 MHz with a network analyzer, upon AC voltage application between the two Ag electrodes, corresponding to electromechanical coupling induced by molecular dipoles of the PET polymer chain in the dielectric layer. It was found that the device size and geometry were well adapted to detect DNA hybridization, by measuring the capacity of the resonance response evolution: hybridization induced polarization of the dielectric material that affected the electromechanical coupling established in the dielectric layer. The 0.2 mm(2) sensor sensitive area allows detection in small volumes and still has higher detection levels for bioanalytical applications, the non-contact configuration adopted avoids electric faradic reactions that may damage biosensor sensitive layers, and finally, PET is a costless raw material, easy to process and well adapted for large scale production. The well-balanced technological and economic advantages of this kind of device make it a good candidate for biochip integration.