Thin-film Platinum Electrodes Research Articles

Culturing Neurons on MEMS Fabricated P(VDF-TrFE) Films for Implantable Artificial Cochlea

In this paper, we report an in vitro study on the biocompatibility of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) films for the implantable artificial cochlea. The implantable artificial cochlea comprises a piezoelectric membrane made of P(VDF-TrFE), platinum (Pt) thin film electrodes, and a silicon substrate which are designed to stimulate neurons in a cochlea and fabricated by microelectromechanical systems (MEMS) and thin film technologies. The biocompatibility of P(VDF-TrFE) film is evaluated by culturing cerebral cortical neurons from rats on it. The fibronectin from human plasma and the collagen from the calf skin are used as the cell adhesion factors. Since neurons extend dendrites and axons from the somata, it is found that the neurons are successfully cultured on the surface of P(VDF-TrFE) films modified both by the fibronectin and by the collagen. Furthermore, it is also found that the neurons are also successfully cultured over the Pt electrode on the P(VDF-TrFE) of the implantable artificial cochlea modified by the fibronectin. Consequently, the biocompatibility and the applicability of the MEMS fabricated P(VDF-TrFE) films and the implantable artificial cochlea are confirmed.

Investigation of the Oxygen Exchange Reaction on Pt/YSZ: The Relation between Three Phase Boundaries and Electrode Performance

In this work the electrochemical oxygen exchange reaction on the system platinum/yttria stabilized zirconia (YSZ) was investigated by means of impedance spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS) on well-defined dense platinum thin film electrodes. From impedance measurements on differently sized electrodes it was possible to show that the resistance caused by the rate determining step scales with the inverse length of the three phase boundary (TPB). To reveal information on the properties of the electrochemically active zone - i.e. the zone where oxygen is incorporated into the electrolyte - 18O tracer experiments combined with ToF-SIMS analysis were carried out. The extension of the active zone was found to depend on the overpotential η. In addition, the ToF-SIMS investigations revealed an enrichment of impurities in the vicinity of the TPB after dc load. The influence of sample pretreatment on the electrode performance is discussed.

Structural, morphological and kinetic properties of model type thin film platinum electrodes on YSZ

By combining structural, morphological, spectroscopic and electrochemical information on the kinetics of epitaxial Pt thin film electrodes we obtain a detailed picture of the Pt/YSZ electrode. A route to tailor-made model type electrodes is discussed.

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO 4 and 0.05 M H 2SO 4 solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25–20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O 2 reduction was higher in HClO 4 and decreased with decreasing film thickness. In H 2SO 4, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec −1 in high current density range and −60 mV dec −1 in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O 2 reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O 2 molecule was close to four for all thin Pt films studied.

Flexible parylene-based multielectrode array technology for high-density neural stimulation and recording

Novel flexible parylene-based high-density electrode arrays have been developed for functional electrical stimulation in retinal and spinal cord prosthetics. These arrays are microfabricated according to a single-metal-layer process and a revolutionary dual-metal-layer process that promises to meet the needs of extremely high-density stimulation applications. While in many cases thin-film platinum electrodes in parylene C would be sufficient, high surface-area platinum electroplating has been shown to extend the lifetime of stimulated electrodes to more than 430 million pulses without failing. Iridium electrode arrays with higher charge delivery capacity have also been fabricated using a new high-temperature stabilized parylene variant, parylene HT. In addition, a new heat molding process has been implemented to conform electrode arrays to approximate the curvature of canine retinas, and a chronic implantation study of the mechanical effects of parylene-based electrode arrays on the retina over a 6-month follow-up period has provided excellent results. Retinal stimulation from these parylene-based electrode arrays in an isolated tiger salamander preparation was shown to be comparable to light stimulation in terms of generation of action potentials in the inner retina. Finally, electrode arrays have also been implanted and tested on the spinal cords of murine models, with the ultimate goal of facilitation of locomotion after spinal cord injury; these arrays provide a higher density and better spatial control of stimulation and recording than is typically possible using traditional fine-wire electrodes. Spinal cord stimulation typically elicited three muscle responses, an early (direct), a middle (monosynaptic), and a late (polysynaptic) response, classified based on latency after stimulation. Stimulation at different rostrocaudal levels of the cord yielded markedly different muscle responses, highlighting the need for such high-density arrays.

Potential-Induced COad Island Formation on a Platinum Thin-Film Electrode in the H-upd Potential Region

In situ electrochemical IR spectroscopy data, obtained on a Pt thin-film electrode in 0.5 M H2SO4 solution in an attenuated total reflection configuration (ATR-FTIRS), show an anomalous, reversible blue-shift of the IR peak frequency of linearly bound adsorbed CO with decreasing potential in the H-upd potential range, together with an increase in band intensity. This phenomenon, which is observed only at coverages ≤0.3 monolayers and becomes more pronounced with decreasing COad coverage, is explained by a potential-induced island formation in the CO adlayer at low potentials. The phase transition is driven either indirectly by coadsorption of H-upd or directly by a potential-induced change in the COad−COad interaction energies.

Variation in 111 d-Space and Generation of Hillocks in Platinum Thin-Film Electrodes by Heat Treatment

The variation in 111 d-space of Pt thin-film electrodes during heat treatment was precisely measured by an X-ray diffraction system with a heating stage. The 111 d-space of Pt films deposited at 200°C linearly increased as the temperature was increased to 450°C and was almost constant over the temperatures range from 450 to 600°C due to stress relaxation. The generation of hillocks was observed in the Pt films which were relaxed in this way. On the other hand, in the case of Pt films deposited at temperatures higher than 500°C, the d-space linearly increased and then linearly decreased to almost the same value in the heat treatment at a maximum temperature of 600°C, and no hillocks were observed on the surface after the heat treatment. Furthermore, Pt films prepared at room temperature (RT) at different deposition rates showed different variations in d-space and different sizes of hillocks. These results suggest that knowing the variation in 111 d-space during heat treatment is beneficial for predicting the generation of hillocks in 111-oriented Pt thin films.

Differential resistance analysis of protonic ceramic fuel cells for measuring bulk conductivity

A technique, called differential resistance analysis, was developed to determine the bulk conductivity of protonic ceramic electrolyte in hydrogen/air fuel cells under load. For these experiments, identical specimens of the protonic ceramic BaCe 0.9Y 0.1O 3−α (BCY10) were prepared with thicknesses of 240, 560, 790, and 1150 μm with thin film platinum electrodes. The current–voltage ( I– V) characteristic curves for each specimen were obtained between 600 and 800 °C, and the slope of each I– V curve was determined in the ohmic region between 10 and 20 mA, giving the total effective area specific resistance (ASR cell) of the cell under load as a function of temperature. The bulk electrolyte resistivity was found by taking the difference in resistance of two cells divided by the difference in electrolyte thickness. The bulk conductivity of the electrolyte measured at 1000 K by this technique was 5 mS/cm or less, depending on the overall electrolyte thickness, much lower than the values obtained by impedance spectroscopy. Also, the activation energy for bulk conduction was found to be higher than expected for pure protonic transport. This paper attempts to correlate the two measurement techniques and explain the apparent discrepancies.

Determination of selenoamino acids by coupling of isotachophoresis/capillary zone electrophoresis on a PMMA microchip

The design, the fabrication, and the analytical performance of a PMMA microchip for isotachophoresis and capillary electrophoresis are described. The microchip offers the possibility of on-line coupling of an isotachophoretic preconcentration step prior to the capillary electrophoretic separation. Conductivity detection with thin film platinum electrodes is used for monitoring of the enrichment and of the separation on the microchip. With the preceding ITP step an enrichment factor of about 80 can be achieved. The seleno amino acids selenomethionine, selenoethionine, and selenocystine can be separated down to a concentration range of just a few micromoles/L. With suitable selection and combination of electrolytes the microchip can be used for a wide field of applications.

A new calcium-sensor based on ion-selective conductometric microsensors--membranes and features.

Based on the concept of ion-selective conductometric microsensors (ISCOM) a new calcium sensor was developed and characterized. ISCOM have a single probe, all-solid-state construction and do not need a reference electrode. These sensors are amenable to miniaturization and integration in the true sense of integrated circuit and microsystem technologies. The detection is accomplished by measurement of the bulk conductance Gm of a thin polymeric membrane containing an ion-complexing agent, where the magnitude of Gm can be related to the content of the primary ion in the analyzed solution. Thin-film platinum electrodes forming an interdigitated electrode are used as the transducer to detect the conductivity of the polymeric membrane. Optimization of the membrane composition was carried out by testing different types of calcium-ionophores, polymers, and plasticizers. The sensor characteristics have been investigated. The limit of detection is about 10(-7) mol L(-1). The dynamic range is 10(-6)-10(-1) mol L(-1) with a response time of less than 5 s. These parameters are comparable to those of corresponding potentiometric calcium selective electrodes (ISE). The Ca(2+)-ISCOM demonstrates good practical relevant selectivities against typical interfering ions for biomedical and environmental applications.

Comparison of different conductivity detector geometries on an isotachophoresis PMMA-microchip

Conductivity detection is one of the most often employed means of detection in isotachophoresis. In microanalytical devices, thin-film platinum electrodes can be used for conductivity detection and for other electrochemical methods of detection. The design and the performance of different electrode geometries for on-column contact conductivity detection with thin-film platinum electrodes integrated on an isotachophoresis PMMA-microchip is described. Three different electrode geometries for direct conductivity detection were used for the investigation of isotachophoretic separations. The influence of the width of the electrodes and their positioning relative to the separation channel was investigated. The performance of the different detectors is compared for the analysis of organic carboxylic acid anions.

A new method to analyze polarization curves — its application for the determination of kinetic parameters for oxygen reduction

A new method has been developed to analyze current–potential curves. The treatment was applied to determine the kinetic parameters of oxygen reduction. The reduction of oxygen was studied on thin-film platinum electrodes in alkaline solution. For the purpose of comparison the kinetic parameters were determined by the traditional method of constructing Tafel plots.

Temperature and voltage dependence in PZT ferroelectric thin film capacitors

Ferroelectric thin films of Pb(Zr,Ti)O3 (PZT) are prepared on platinum (Pt) and SrRuO3 (SRO) thin film electrodes by a laser ablation technique. Temperature and voltage dependence of the PZT thin film capacitors are investigated. As the results, it is confirmed that the distribution of the remanent polarization (Pr) and coercive field (Ec) is strongly affected by homogeneity of the grain size of PZT thin film and is estimated by differentiating Pr curves with respect to an applied electric field (dP/dE).

Recessed microelectrode array for a micro flow-through system allowing on-line multianalyte determination in vivo

Using CMOS-compatible processes, a microelectrode system for use in a micro flow-through cell was manufactured. The electrode was specially designed to enable multianalyte determination with immobilized oxidase enzymes and combines minimal flow dependency with a very small dead volume (< 1 μL) of the cell. This allows biomedical applications like measurements of glucose and lactate in interstitial fluid, which can be collected by ultrafiltration. Besides a 3-electrode system with 4 individually addressable platinum working electrodes, the sensor contains 2 electrodes that measure the conductivity of the sample as well as a Pt thermoresistor to measure the temperature. The temperature dependence in enzyme reactions can thus be controlled during on-line measurements. The 4 working electrodes comprise multielectrode arrays, each comprising 192 micro-holes with a diameter of 3.6 μm. They are arranged symmetrically around the central counter electrode, which is surrounded by a circular Ag/AgCl reference electrode. Between the array and the reference electrode are the loops of the Pt thermoresistor. The thermoresistor is electrically insulated from the measurement solution by a Si3N4 layer. A method for the pretreatment of platinum thin-film electrodes that increases the reversibility of the electrode process is described. The chemical modification of the working electrodes by electropolymerization of a resorcinol/1,3-diaminobenzene mixture enables interference-free measurement in blood and plasma as well as protection against electrode fouling.

Extrinsic conductivity in ferroelectric PZT film capacitors made by laser ablation deposition

Electrical properties of ferroelectric lead–zirconate–titanate (PZT) thin-film devices are affected by the composition and structure of the PZT film itself and also by the electrode material and the interface between the electrode and ceramics. In this paper we consider capacitor structures with platinum thin-film electrodes (Pt/PZT/Pt/substrate) which typically have very high dc resistivity, but suffer from the polarization fatigue due to the high stress and oxygen vacancy build-up in the interface between the electrode and ceramic thin film. Both platinum electrodes and Nd-modified PZT (PNZT) thin films were fabricated by the pulsed laser-ablation deposition and post-annealing heat treatments. The behaviour of the conductivity in the PNZT thin-film capacitors was studied as a function of temperature and electric field. Low-field resistivities of the order of 1012 Ωcm were measured for the films at room temperature. Some differences were found in the current–voltage characteristics between capacitor structures made on MgO(100) and oxidized silicon(100) substrates, respectively. Assuming the Poole–Frenkel emission mechanism for the conduction, the activation energies of conduction were obtained to be between 0.15 and 0.18 eV.

Surface techniques for an electrochemical DNA biosensor

The surface of platinum electrodes was modified by a technique that provides binding sites for covalent site-specific immobilization of DNA. During the course of immobilization certain substances such as thiols, iodine and iodide were brought in contact with the metal. The effect on the electrochemical characteristics of thin film platinum electrodes caused by chemisorption of these substances was investigated. Voltage pulsing was very effective in restoring the electrocatalytical properties of the electrodes. A 20 bp oligonucleotide was immobilized via primary amino groups or terminal phosphate and immobilization efficiency of the different immobilized strands was determined. Mercapto-silane coatings and controlled introduction of charged groups during the course of immobilization prevented unspecific DNA binding caused by adsorption.

Surface modification of platinum thin film electrodes towards a defined roughness and microsporosity

Abstract The roughness and microsporosity of platinum thin-film electrodes, which act as transducers in electrochemical biosensors, have been varied by evaporating titanium or aluminum as top layers and subsequent annealing and etching of these metal films. This defined structural variation is very important in the field of chemo- and biosensors. The electrochemical characteristics of the electrode surface were investigated by cyclic voltammetry in 0.5 M H 2 SO 4 . From this the real electrode area was determined by calculating the charge density in the hydrogen adsorption and desorption regions, and the roughness factor, defined as the ratio between the calculated charge density and the theoretical value of smooth polycrystalline platinum, covered a range from 1.1 to 8.85. The platinum surface was studied by cyclic voltammetry, laser-secondary-neutral-mass-spectroscopy and atomic force microscopy.

Mathematical modeling of a solid-state limiting current carbon monoxide sensor

A solid-state carbon monoxide sensor was fabricated using a 9% yttria-stabilized zirconia (YSZ) disc sandwiched between two platinum thin-film electrodes. One of the electrodes was coated with a thin layer of 7 CuO · 10 ZnO · 3 Al2O3 catalyst. The sensor showed limiting current behavior at an applied voltage between 0.5 and 1.2 V. Linear response was observed with carbon monoxide in a nitrogen–oxygen mixture at high temperature and limiting current conditions. The linear carbon monoxide concentration range increased with increase in the operating temperature and its sensitivity increased from 2.437 mA · atm−1 CO at 1,023 K to 10.771 at 1,093 K. These characteristics were adequately described by the proposed mathematical model relating the response to the rate processes occurring in the catalyst layer and in the electrochemical cell under limiting current conditions. The model showed that effective sensing and high sensitivity are best obtained using a catalyst with high catalytic activity toward the test solute and proper design and fabrication of the sensor to ensure its high diffusivity in the catalyst.

Significance of Microstructure for a MOCVD-Grown YSZ Thin Film Gas Sensor

AbstractWe report the fabrication and characterization of a low temperature (200°C-400°C) thin film gas sensor constructed from a MOCVD-grown yttria-stabilized zirconia (YSZ) layer sandwiched between two platinum thin film electrodes. A reproducible gas-sensing response is produced by applying a cyclic voltage which generates voltammograms with gas-specific current peaks and shapes. Growth conditions are optimized for preparing YSZ films having dense microstructures, low leakage currents, and maximum ion conductivities. In particular, the effect of growth temperature on film morphology and texture is discussed and related to the electrical and gas-sensing properties of the thin film sensor device.

Surface characterization of thin-film platinum electrodes for biosensors by means of cyclic voltammetry and laser SNMS

Abstract The surface of platinum thin-film electrodes, which are used for electrochemical biosensors, was investigated by cyclic voltammetry and laser secondary neutral mass spectroscopy (LSNMS). The platinum electrodes act as transducers and amperometrically detect H 2 O 2 , which is produced by enzyme reactions, for application as glucose and lactate sensors. The electrode surface and its impurities, which are caused by technological processing steps, have a strong influence on the electrochemical performance of these biosensors. Therefore some of the platinum electrodes were protected by a titanium layer, which was etched away after the final technological processing step to reduce impurities. Alternatively, an oxygen plasma treatment of the platinum electrodes was tested. The influences of the different processing methods on the platinum surface were investigated.