Iridium Oxide Films Research Articles

In Situ Dispersive EXAFS in Electrocatalysis: The Investigation of the Local Structure of IrOxin Chronoamperometric Conditions as a Case Study

An in situ study with dispersive EXAFS (Extended X-Ray Absorption Spectroscopy) at the Ir-LIIIedge is performed to characterize Electrodeposited Iridium Oxide Films (EIROF) under chronoamperometric conditions. The technique monitors the local chemical environment and electronic structure of iridium during the oxidation of Ir(III) to Ir(IV) with a time resolution of milliseconds. The study is performed in both acidic and basic media. The Fourier transforms of the time-resolved EXAFS signals clearly show that the short-range structure of Ir is similar to that of rutile-type IrO2and is maintained during the reaction, thus accounting for the flexibility of the structure of the electrode material in accommodating different oxidation states. From a more general point of view, the work demonstrates the capabilities of in situ experiments based on state-of-the-art dispersive EXAFS in clarifying the mechanistic aspects of electrochemical processes.

Signal Amplification in Enzyme-Based Amperometric Biosensors

A unique mode of current amplification was investigated in reagentless biosensors based on the clinically significant enzymes including alcohol dehydrogenase, glucose 6-phosphate dehydrogenase, glycerol 3-phosphate dehydrogenase, and glucose oxidase. The biosensors were designed by sandwiching the enzyme-polymer film between an electrode and Nafion film. In particular, each enzyme and its cofactor were covalently attached to the chains of polysaccharide chitosan and mixed with carbon nanotubes on the electrode surface. The coating of such biosensors with Nafion resulted in the current increase by up to 1000%, depending on the enzyme. The results were analyzed considering the interplay between the enzyme activity-pH profiles and the Nafion-induced pH increase in the underlying chitosan film. The data were collected by using the rapid (<5 min) amperometric enzyme assays and pH-sensitive iridium oxide films. The increase in the biosensor current was attributed to the pH-driven increase in the enzyme activity inside the two-film interface. Such signal amplification should also be feasible in other biosensors based on the polyelectrolytes and sandwiched enzymes providing that a proper match is made between the enzyme activity-pH profiles and the pH of buffer solutions.

Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution

The electrochemistry of hydrous iridium oxide films (HIROF) is revisited. Cyclic voltammograms of HIROFs display two reversible redox couples commonly assigned to the Ir(III)/Ir(IV) and Ir(IV)/Ir(V) transitions, respectively. However, compared to the first, the second redox couple has significantly less charge associated to it. This effect is interpreted as partial oxidation of Ir(IV) as limited by nearest neighbor repulsion of resulting Ir(V) sites. Thus, the redox process is divided into two steps: one preceding and one overlapping the oxygen evolution reaction (OER). Here, the ``super-nernstian'' pH dependence of the redox processes in the HIROF is used to expose how pH controls the overpotential for oxygen evolution, as evidenced by the complementary increased formation of Ir(V) oxide. A recently formulated binuclear mechanism for the OER is employed to illustrate how hydrogen bonding may suppress the OER, thus implicitly favoring Ir(V) oxide formation above the thermodynamic onset potential for the OER at low pH.

Electrochromic iridium oxide films: Compatibility with propionic acid, potassium hydroxide, and lithium perchlorate in propylene carbonate

Porous thin films of Ir oxide were prepared by reactive dc magnetron sputtering onto unheated substrates. The crystallite size was ~5nm, and a small amount of unoxidized Ir was present. The electrochromic performance was studied by optical transmittance measurements and cyclic voltammetry applied to films in aqueous and non-aqueous electrolytes, specifically being 1M propionic acid, 1M potassium hydroxide (KOH), and 1M lithium perchlorate in propylene carbonate (Li–PC). Cyclic voltammetry measurements indicated that the films had a fractal surface structure. Good electrochromism, with mid-luminous transmittance modulation between ~55 and ~90% in ~100-nm-thick films, was documented in all of the electrolytes.

Controlled activation of iridium film for AIROF microelectrodes

The electrochemical activation process of the activated iridium oxide film (AIROF) has been investigated on a microelectrode (the geometric surface area is 7850μm2) with sputtered iridium in physiological saline solution (0.9% NaCl). For one activation cycle, the potential is swept from −1.0 to 1.0V at 0.05Hz. The activation process could be controlled by the activation cycles which are characterized by the current–time data, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). After activation, the charge storage capacity of the AIROF microelectrode is 40.07mC/cm2, which is nearly 24 times more than that of iridium microelectrode. The impedance at 1kHz of the AIROF microelectrode is 4064Ω, about one-tenth of that of iridium microelectrode. The double layer capacitance of the AIROF microelectrode is 1.519μF, about 34.5 times more than that of iridium microelectrode. The relationship between the electrochemical performance of the AIROF microelectrodes and the applied activation cycles is also investigated. In the neutral electrolyte at pH 7, the Ir(IV)/Ir(III) surface redox couple exhibits increasing separation of the oxidation/reduction peaks due to the protons released/consumed during the Ir(IV)/Ir(III) redox activity. Finally, the relationship between the separation of the redox peaks and the applied activation cycles is also studied.

Electrocatalytic Oxygen Evolution Reaction on Iridium Oxide Model Film Catalysts: Influence of Oxide Type and Catalyst Substrate Interactions

Different iridium oxides and their interactions with a titanium substrate were investigated with regard to their electrocatalytic oxygen evolution activities. Smooth iridium oxide thin-film model catalysts were synthesized at different temperatures. Thin-film catalysts minimize mass transport limitations, which tend to impede catalytic studies. Prior to the electrocatalytic investigation, the iridium oxide films were characterized by SEM, EDX, XRD, Raman spectroscopy and cyclic voltammetry. Temperature programmed reduction was applied to identify the oxide species formed as function of the calcination temperature. The TPR study was performed on inert silicon and titanium substrates to study iridium oxide properties alone and their interaction with the titanium substrate. Thus, it was possible to relate electrocatalytic oxygen evolution activity with the formation of the various oxide species.

Excimer laser deinsulation of Parylene-C on iridium for use in an activated iridium oxide film-coated Utah electrode array

Implantable microelectrodes provide a measure to electrically stimulate neurons in the brain and spinal cord and record their electrophysiological activity. A material with a high charge capacity such as activated or sputter-deposited iridium oxide film (AIROF or SIROF) is used as an interface. The Utah electrode array (UEA) uses SIROF for its interface material with neural tissue and oxygen plasma etching (OPE) with an aluminium foil mask to expose the active area, where the interface between the electrode and neural tissue is formed. However, deinsulation of Parylene-C using OPE has limitations, including the lack of uniformity in the exposed area and reproducibility. While the deinsulation of Parylene-C using an excimer laser is proven to be an alternative for overcoming the limitations, the iridium oxide (IrOx) suffers from fracture when high laser fluence (>1000mJ/cm2) is used. Iridium (Ir), which has a much higher fracture resistance than IrOx, can be deposited before excimer laser deinsulation and then the exposed Ir film area can be activated by electrochemical treatment to acquire the AIROF. Characterisation of the laser-ablated Ir film and AIROF by surface analysis (X-ray photoelectron spectroscopy, scanning electron microscope, and atomic force microscope) and electrochemical analysis (electrochemical impedance spectroscopy, and cyclic voltammetry) shows that the damage on the Ir film induced by laser irradiation is significantly less than that on SIROF, and the AIROF has a high charge storage capacity. The results show the potential of the laser deinsulation technique for use in high performance AIROF-coated UEA fabrication.

Influence of Some Preparative Parameters on Microstructure and Electrochromic Behavior of Sprayed Iridium Oxide Films

Electrochromic iridium oxide thin films were deposited by spray pyrolysis technique onto indium-doped tin oxide (ITO) coated glass substrates using iridium chloride as an aqueous solution. The influence of the substrate temperature, the deposition period of time and the solution molarity have been investigated as related to the film's electrochromic (EC) performance. The electrochromic properties of the films were investigated in a (0.5M H 2 SO 4 ) aqueous electrolytic solution using cyclic voltammetry (CV). The transmittance has been measured over the spectral wavelength range between 300 and 2400 nm. The substrate temperature (T sub ), the deposition period of time, the solution molarity and the colouration and bleaching potentials were optimized and found to be 400 o C, 10 min, 0.02 M, ±0.8 V, respectively. The films have acquired a maximum transmittance solar modulation ΔT s and a maximum visible modulation ΔTv at 630 nm optimal optical modulation. The iridium oxide thin films have demonstrated a pronounced anodic electrochromic behavior owing to Ir +4 �n Ir +3 intervalance charge transitions.

Electrochemically deposited iridium oxide reference electrode integrated with an electroenzymatic glutamate sensor on a multi-electrode arraymicroprobe

An implantable micromachined multi-electrode array (MEA) microprobe modified for utilization as a complete electrochemical biosensor for rapid glutamate detection is described. A post-fabrication method for electrochemical deposition of an iridium oxide (IrOx) film onto a designated microelectrode enabled incorporation of an IrOx reference electrode (RE) on the microprobe. The on-probe IrOx RE provides an alternative to the commonly utilized Ag/AgCl wire RE, which has been shown to be unstable and to cause an inflammatory response in living tissue. The IrOx film electrodeposited onto a platinum site was tested as part of a complete chemical sensing system that included a platinum counter electrode and enzymatic glutamate sensing electrodes all on a single silicon-based MEA platform. The thin film IrOx was mechanically robust enough to endure conditions of repeated heating and wetting during the MEA fabrication process. The pH dependence of the IrOx open circuit potential (OCP) was measured at −77±0.4mV/pH and remained stable over a two-week period. The on-probe IrOx RE was tested in a two- and three-electrode system with glutamate biosensors. The biosensors were shown to detect a physiologically relevant range of glutamate concentrations and to reject the interferents, dopamine and ascorbic acid. By incorporating all of the electrodes onto a single device, baseline noise was reduced by an average of ∼61% in vitro and ∼71% in vivo with reduced tissue damage, since only a single probe needed to be implanted.

Iridium oxide pH sensor for biomedical applications. Case urea–urease in real urine samples

This work demonstrates the implementation of iridium oxide films (IROF) grown on silicon-based thin-film platinum microelectrodes, their utilization as a pH sensor, and their successful formatting into a urea pH sensor. In this context, Pt electrodes were fabricated on Silicon by using standard photolithography and lift-off procedures and IROF thin films were growth by a dynamic oxidation electrodeposition method (AEIROF). The AEIROF pH sensor reported showed a super-Nerstian (72.9±0.9mV/pH) response between pH 3 and 11, with residual standard deviation of both repeatability and reproducibility below 5%, and resolution of 0.03 pH units.For their application as urea pH sensors, AEIROF electrodes were reversibly modified with urease-coated magnetic microparticles (MP) using a magnet. The urea pH sensor provided fast detection of urea between 78μM and 20mM in saline solution, in sample volumes of just 50μL. The applicability to urea determination in real urine samples is discussed.

Influence of oxidation state on the pH dependence of hydrous iridium oxide films

Many electrochemical reactions taking place in aqueous solution consume or produce protons. The pH in the diffusion layer can therefore be significantly altered during the reaction and there is a need for in situ pH measurements tracing this near surface pH. In the present paper the rotating ring disc technique was used to measure near surface pH changes during oxygen reduction, utilising hydrous iridium oxide as the pH sensing probe. Before such experiments a good understanding of the pH sensing properties of these films is required and the impact of the oxidation state of the film on the pH sensing properties was investigated as well as the influence of solution redox species. The pH sensitivity (depicted by dE/dpH) was found to depend on the average oxidation state of the film in a manner resembling the cyclic voltammetry response. In all cases the pH response is “supernernstian” with more than one proton per electron. The origin of this behaviour is discussed in the context of acid-base properties of the film and the existence of both hydrous and anhydrous oxide phases. The pH response depends also on the redox properties of the solution but can be optimised for various purposes by conditioning the film at different potentials. This was clearly illustrated by adding hydrogen peroxide, an intermediate in the oxygen reduction reaction, to the solution. It was shown that hydrous iridium oxide can be used as a reliable in situ pH sensor provided that care is taken to optimise the oxidation state of the film.

Excimer-laser deinsulation of Parylene-C coated Utah electrode array tips

Utah electrode arrays (UEAs) are highly effective to measure or stimulate neural action potentials from the central or peripheral nervous system. The measured signals can be used for applications including control of prosthetics (recording) and stimulation of proprioceptive percepts. The UEAs are coated with biocompatible Parylene-C, and the electrode tips are deinsulated to expose the active electrode coated with sputtered iridium oxide films (SIROFs) to transduce neural signals. In conventional UEA technology, the electrode tips are deinsulated by poking the electrodes through aluminum foil followed by an oxygen plasma etch of the exposed areas. However, this method suffers from lack of uniformity and repeatability and it is time consuming. We focus on laser tip-deinsulation technology that can provide a repeatable, uniform, and less time consuming tip exposure for UEAs. The laser deinsulated SIROF area is characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), atomic force microscope (AFM), and by measuring the impedance of the exposed sites. The value of impedance and XPS peaks showed that the Parylene was clearly removed. The damage induced by laser irradiation on the SIROF film was also investigated to understand the selectivity of laser deinsulation. Thicker SIROF films showed better resistance to fracture. The results indicate that laser deinsulation is an effective method to etch Parylene films.

Involvement of nanoparticles in the electrodeposition of hydrous iridium oxide films

The highly dispersed nature of hydrous iridium oxide combined with its electrochemical properties makes it a very interesting material. Possible applications can be found in electrocatalysis, neural stimulation, electrochromic devices and pH sensors. In the present work a commonly used electrodeposition solution, based on IrCl4, oxalic acid, H2O2 and NaCO3, was studied with electrochemical methods as well as UV–vis spectroscopy. The hexachloroiridate (IV) complex was initially observed in both UV–vis and cyclic voltammetry. No oxalato complexes were detected, instead oxalate is proposed to act as a stabilising agent in nanoparticle formation. Initially hydrogen peroxide was found to reduce Ir(IV) complexes to Ir(III). However, after increasing the pH by addition of sodium carbonate it was shown to act as an oxidising agent instead. During development of the solution UV–vis showed the formation of multinuclear complexes and with aging also scattering from solid materials was observed. Transmission electron microscopy confirmed the formation of nanoparticles of iridium oxide with a diameter of ∼3nm. The role of nanoparticles and non-particulate species in the deposition process is discussed.

Morphology and Electrochemical Properties of Activated and Sputtered Iridium Oxide Films for Functional Electrostimulation

Iridium oxide (IrOx) has attracted much attention for neural interface applications due to its ability to transfer between ionic and electronic current and to resist corrosion. The physical, mechanical, chemical, electrical and optical properties of thin films depend on the method and parameters used to deposit the films. In this report, the surface morphology, impedance and charge capacity of activated iridium oxide film (AIROF) and sputtered iridium oxide film (SIROF) were investigated in vitro and compared. The Utah Electrode Array (UEA) having similar electrode area and shape were employed in this study. The electrode coated with AIROF and SIROF were characterized by scanning electron microcopy, cyclic voltammetry, electrochemical impedance spectroscopy and potential transient measurements to measure charge injection capacity (CIC). SIROF and AIROF selectively deposited on electrode tip had dendrite and granular microstructure, respectively. The CIC of unbiased SIROF and AIROF was found to be 2 and 1 mC/cm2, respectively, which is comparable to other published values. The average impedance, at a frequency of 1 kHz was ~65 and ~7 kΩ for the AIROF and SIROF, respectively. Low impedance and high CIC makes SIROF highly recommended stimulation and recording material.

Electrochemical detection of catecholamine release using planar iridium oxide electrodes in nanoliter microfluidic cell culture volumes

Release of neurotransmitters and hormones by calcium regulated exocytosis is a fundamental cellular/molecular process that is disrupted in a variety of psychiatric, neurological, and endocrine disorders. Therefore, this area represents a relevant target for drug and therapeutic development, efforts that will be aided by novel analytical tools and devices that provide mechanistically rich data with increased throughput. Toward this goal, we have electrochemically deposited iridium oxide (IrOx) films onto planar thin film platinum electrodes (20μm×300μm) and utilized these for quantitative detection of catecholamine release from adrenal chromaffin cells trapped in a microfluidic network. The IrOx electrodes show a linear response to norepinephrine in the range of 0–400μM, with a sensitivity of 23.1±0.5mA/Mmm2. The sensitivity of the IrOx electrodes does not change in the presence of ascorbic acid, a substance commonly found in biological samples. A replica molded polydimethylsiloxane (PDMS) microfluidic device with nanoliter sensing volumes was aligned and sealed to a glass substrate with the sensing electrodes. Small populations of chromaffin cells were trapped in the microfluidic device and stimulated by rapid perfusion with high potassium (50mM) containing Tyrode's solution at a flow rate of 1nL/s. Stimulation of the cells produced a rapid increase in current due to oxidation of the released catecholamines, with an estimated maximum concentration in the cell culture volume of ∼52μM. Thus, we demonstrate the utility of an integrated microfluidic network with IrOx electrodes for real-time quantitative detection of catecholamines released from small populations of chromaffin cells.

Reduce impedance of intracortical iridium oxide microelectrodes by hydrogel coatings

The study investigated the influence of poly(vinyl alcohol)/poly(acrylic acid) interpenetrating polymer networks (PVA/PAA IPNs) hydrogel film coatings on interface stability between electrodeposited iridium oxide films (EIROF) microelectrodes and neural tissue by monitoring changes in impedance. The impedance of EIROF and PtIr microelectrodes ( d = 75 μm) with and without the coatings was measured weekly, while the microelectrodes were implanted into the motor cortex of rats ( n = 48) for 28 days. The results show that without PVA/PAA coatings the impedance of EIROF microelectrodes was higher than that of PtIr microelectrodes in vivo, however, the impedance of EIROF microelectrodes with coatings resulted in the lowest impedance compared to the impedance of both PtIr microelectrodes with coatings and EIROF microelectrodes without coatings through the whole test period. Moreover, compared to the EIROF microelectrodes without coatings, the impedance of EIROF microelectrodes with coatings reduced by ∼40% at day 21 after implantation. The PVA/PAA IPNs hydrogel coating acted as a stable ions conductive layer which enabled the EIROF microelectrodes to maintain its electrochemical superiority in vivo and enhanced the mechanical stability of the EIROF microelectrodes. The study demonstrates a concept of neural microelectrode modification, in which the hydrophilic property is crucial.

Photodiode Circuits for Retinal Prostheses

Photodiode circuits show promise for the development of high-resolution retinal prostheses. While several of these systems have been constructed and some even implanted in humans, existing descriptions of the complex optoelectronic interaction between light, photodiode, and the electrode/electrolyte load are limited. This study examines this interaction in depth with theoretical calculations and experimental measurements. Actively biased photoconductive and passive photovoltaic circuits are investigated, with the photovoltaic circuits consisting of one or more diodes connected in series, and the photoconductive circuits consisting of a single diode in series with a pulsed bias voltage. Circuit behavior and charge injection levels were markedly different for platinum and sputtered iridium-oxide film (SIROF) electrodes. Photovoltaic circuits were able to deliver 0.038 mC/cm(2) (0.75 nC/phase) per photodiode with 50- μm platinum electrodes, and 0.54-mC/cm(2) (11 nC/phase) per photodiode with 50-μ m SIROF electrodes driven with 0.5-ms pulses of light at 25 Hz. The same pulses applied to photoconductive circuits with the same electrodes were able to deliver charge injections as high as 0.38 and 7.6 mC/cm(2) (7.5 and 150 nC/phase), respectively. We demonstrate photovoltaic stimulation of rabbit retina in-vitro, with 0.5-ms pulses of 905-nm light using peak irradiance of 1 mW/mm(2). Based on the experimental data, we derive electrochemical and optical safety limits for pixel density and charge injection in various circuits. While photoconductive circuits offer smaller pixels, photovoltaic systems do not require an external bias voltage. Both classes of circuits show promise for the development of high-resolution optoelectronic retinal prostheses.

A Hermetic Wireless Subretinal Neurostimulator for Vision Prostheses

A miniaturized, hermetically encased, wirelessly operated retinal prosthesis has been developed for preclinical studies in the Yucatan minipig, and includes several design improvements over our previously reported device. The prosthesis attaches conformally to the outside of the eye and electrically drives a microfabricated thin-film polyimide array of sputtered iridium oxide film electrodes. This array is implanted into the subretinal space using a customized ab externo surgical technique. The implanted device includes a hermetic titanium case containing a 15-channel stimulator chip and discrete circuit components. Feedthroughs in the case connect the stimulator chip to secondary power and data receiving coils on the eye and to the electrode array under the retina. Long-term in vitro pulse testing of the electrodes projected a lifetime consistent with typical devices in industry. The final assembly was tested in vitro to verify wireless operation of the system in physiological saline using a custom RF transmitter and primary coils. Stimulation pulse strength, duration, and frequency were programmed wirelessly from a Peripheral Component Interconnect eXtensions for Instrumentation (PXI) computer. Operation of the retinal implant has been verified in two pigs for up to five and a half months by detecting stimulus artifacts generated by the implanted device.

Magnetron sputtered iridium oxide as anode catalyst for PEM hydrogen generation

Thin films of iridium oxide are deposited by reactive magnetron sputtering. The influence of oxygen partial pressure in the sputtering plasma on the composition, surface structure and morphology of the films has been studied by XRD, SEM, AFM and XPS analysis. An optimal combination of sputtering parameters yields stable microporous amorphous films with highly extended fractal surface. The electrochemical properties of these films are investigated in view of their application as catalysts for PEM water splitting, using the electrochemical techniques of cyclovoltammetry and steady state polarization. A morphology factor assessing the catalyst active surface for a series of sputtered samples with varying thickness/loading is determined and correlated to the catalytic efficiency. It has been proven that iridium oxide is a very efficient catalyst for oxygen evolution reaction (OER). The best performance with anodic current density of 0.3 A cm–2; at potential of 1.55 V (vs. RHE) shows the 500 nm thick film containing 0.2 mg cm-2; catalyst. These results combined with the established long-term mechanical stability of the sputtered iridium oxide films (SIROFs) prove the advantages of the reactive magnetron sputtering as simple and reliable method for preparation of catalysts with precisely controlled composition, loading, and surface characteristics.

Mesoporous IrO2 Films Templated by PEO-PB-PEO Block-Copolymers: Self-Assembly, Crystallization Behavior, and Electrocatalytic Performance

Synthesis of mesoporous iridium oxide films via soft templating and evaporation-induced self-assembly is demonstrated employing an amphiphilic triblock-copolymer PEO-PB-PEO. Films possess nanocrystalline walls and feature locally ordered pores of about 16 nm diameter. Analysis of the film properties by SEM, TEM, EDX, XPS, SAXS, XRD, and BET along the thermal treatment that succeeds dipcoating shows that the polymer template is removed by calcination between 200 and 300 °C, accompanied by uniaxial shrinkage of film and pore system perpendicular to the substrate. Treating the film in excess of 450 °C leads to further growth of crystallite size and loss of surface area progressing gradually with increasing calcination temperature. Templated IrO2 films conditioned at 450 °C show substantially reduced electrocatalytic overpotentials (efficiency increases) for the oxygen evolution reaction (OER) compared to those of untemplated coatings. Pore templating thus enables direct control over surface catalytic propert...