Microelectrochemical Devices Research Articles

A first principles study on the active adsorbates on the hydrogenated diamond surface

Hydrogenated diamond film exhibits a high surface conductivity, which is very suitable for many in-plane microelectronic and microelectrochemical devices. However, the surface conductivity mechanism of hydrogenated diamond film remains unclear up to now. It inevitably retards its further applications. This work is to elucidate the effects of active adsorbate and water molecule on surface conductivity of hydrogenated diamond film. By the first principles method based on density functional theory, several models corresponding to hydrogenated and oxygenated diamond (100) surfaces physisorbed with various active adsorbates are built up. The adsorbed species include H3O+ ion mixed with H2O molecules with different concentrations. The adsorption energy, equilibrium geometry and density of states corresponding to the adsorption system are investigated. At the same time, the electron populations for different atoms of the physisorbed adsorbates are studied. The results show that the equilibrium geometry of H3O+ ion relaxes significantly after adsorption on hydrogenated diamond (100) surface. In addition, its adsorption energy increases dramatically compared with the system of individual H2O molecule adsorbed on hydrogenated diamond (100) surface. It follows that the strong interactions occur between H3O+ ion and hydrogenated diamond surface. With the concentration of the adsorbed H2O molecules increasing, the adsorption energy between the adsorbate and hydrogenated diamond (100) surface decreases gradually. It indicates that the interactions between H3O+ ion and the substrate weaken as the water molecule concentration increases. Concerning the electronic structure of H3O+ ion adsorbed on hydrogenated diamond (100) surface, shallow acceptors appear near Fermi level, which arises from charge transfer from hydrogenated diamond surface to adsorbed H3O+ ion. Therefore, hydrogenated diamond surface exhibits a p-type conductivity. With regard to the mixed adsorptions of H3O+ ion and H2O molecule, no significant effect on its conductivity is detected, though its surface energy band structure changes. At the same time, the electron transfers from hydrogenated diamond (100) surfaces to the adsorbates are also similar for all the systems with the adsorbates including one H3O+ ion and different H2O molecules. Thus, the adsorbed H2O molecule concentration in this work has no effect on the surface conductivity of hydrogenated diamond surface. However, the adsorbates containing H2O molecules and H3O+ ion physisorbed on oxygenated diamond (100) surfaces do not exist stably. The H3O+ ion will decompose into one H2O molecule and one H atom, which form HO bond with one O atom of oxygenated diamond surface. All the oxygenated diamond surfaces with various adsorbates exhibit an electric insulativity.

Thermal characteristics of temperature-controlled electrochemical microdevices

The development of novel, miniaturized sensing systems is driven by the demand for better and faster chemical measurements with lower power consumption and smaller sample sizes. Emerging miniature sensors, or microsensors, also offer rapid thermal and diffusive transport characteristics. For instance, temperature changes, during both heating and cooling, can be achieved on micrometer-scale surfaces much more rapidly than on bulk, macro-scale surfaces. While these rapid thermal characteristics have been most successfully exploited to date in gas-phase sensing devices, the prospect of developing analogous microfabricated, temperature-controlled microsensors for use in aqueous, or solution-phase, environments has been less explored. In this work, electrodes with underlying microheaters were designed and fabricated, and thermal characterization was performed using temperature imaging, transient temperature measurements, and theoretical modeling to determine temperature distributions and thermal response times in both gas- and solution-phase environments. These results will guide the development of solution-phase electrochemical sensors. Temperature-controlled electrochemical characterization was performed using cyclic voltammetry of a model analyte, hexaamineruthenium(III) chloride, to demonstrate the use of the multilayer, microfabricated devices, which consisted of a gold disk electrode and an underlying microheater. Electrochemical signals were enhanced by up to a factor of three at elevated temperatures (up to 81°C) compared to those measured at room temperature (21°C). This improved signal at elevated temperatures was explained by finite element method calculations that accounted for both temperature-dependent diffusion and thermal convection near the heated electrode surface.

Silicon-based electrochemical microdevices for silicate detection in seawater

This paper describes the electrochemical characterisation of gold and platinum microdevices mass fabricated using silicon technology. Specific attention was paid to allow in situ electrochemical detection of silicate in seawater. Thus, using a silicon nitride (Si3N4) inorganic passivation layer patterned using Inductively Coupled Plasma Chemical Vapor Deposition (ICP-CVD), coupled with a non-aggressive lift-off based process, different electrodes were isolated electrically: one gold or platinum working electrode named “macroelectrode” (2mm of diameter), four gold or platinum working ultramicroelectrodes (UME) (15μm of diameter), one platinum counter electrode and one silver electrode which can be used as a reference electrode after its chlorination. Their small size and mass fabrication make them very promising for oceanographic applications. As some components of microdevices release silicate and contaminate the solution, after being immersed in seawater, these microdevices were inserted in a specific cell that only puts the electrodes in contact with the seawater solution. Gold has been tested as a possible material for working electrodes but its lack of adherence to the passivation layer in seawater solutions led to non-accurate measurements. On the contrary, passivation layer on platinum electrodes resists to the seawater corrosive medium. The analytical performances of the platinum microdevices has been tested through different silicate calibrations and shows an outstanding accuracy and reproducibility when measurements are performed, especially with the macroelectrodes which showed only 2.8% signal variation after four months of use and a limit of quantification of 0.50μmolL−1 suitable for oceanographic applications.

Single nanoparticle collisions at microfluidic microband electrodes: the effect of electrode material and mass transfer.

We report on the effect of convection on electrochemically active collisions between individual Pt nanoparticles (PtNPs) and Hg and Au electrodes. Compared to standard electrochemical cells utilizing Hg and Au ultramicroelectrodes (UMEs) used in previous studies of electrocatalytic amplification, microelectrochemical devices offer two major advantages. First, the PtNP limit of detection (0.084 pM) is ∼8 times lower than the lowest concentration measured using UMEs. Second, convection enhances the mass transfer of PtNPs to the electrode surface, which enhances the collision frequency from ∼0.02 pM(-1) s(-1) on UMEs to ∼0.07 pM(-1) s(-1) in microelectrochemical devices. We also show that the size of PtNPs can be measured in flowing systems using data from collision experiments and then validate this finding using multiphysics simulations.

A Microdot Multilayer Oxide Device: Let Us Tune the Strain-Ionic Transport Interaction

In this paper, we present a strategy to use interfacial strain in multilayer heterostructures to tune their resistive response and ionic transport as active component in an oxide-based multilayer microdot device on chip. For this, fabrication of strained multilayer microdot devices with sideways attached electrodes is reported with the material system Gd0.1Ce0.9O(2-δ)/Er2O3. The fast ionic conducting Gd0.1Ce0.9O(2-δ) single layers are altered in lattice strain by the electrically insulating erbia phases of a microdot. The strain activated volume of the Gd0.1Ce0.9O(2-δ) is investigated by changing the number of individual layers from 1 to 60 while keeping the microdot at a constant thickness; i.e., the proportion of strained volume was systematically varied. Electrical measurements showed that the activation energy of the devices could be altered by Δ0.31 eV by changing the compressive strain of a microdot ceria-based phase by more than 1.16%. The electrical conductivity data is analyzed and interpreted with a strain volume model and defect thermodynamics. Additionally, an equivalent circuit model is presented for sideways contacted multilayer microdots. We give a proof-of-concept for microdot contacting to capture real strain-ionic transport effects and reveal that for classic top-electrode contacting the effect is nil, highlighting the need for sideways electric contacting on a nanoscopic scale. The near order ionic transport interaction is supported by Raman spectroscopy measurements. These were conducted and analyzed together with fully relaxed single thin film samples. Strain states are described relative to the strain activated volumes of Gd0.1Ce0.9O(2-δ) in the microdot multilayer. These findings reveal that strain engineering in microfabricated devices allows altering the ionic conduction over a wide range beyond classic doping strategies for single films. The reported fabrication route and concept of strained multilayer microdots is a promising path for applying strained multilayer oxides as active new building blocks relevant for a broad range of microelectrochemical devices, e.g., resistive switching memory prototypes, resistive or electrochemical sensors, or as active catalytic solid state surface components for microfuel cells or all-solid-state batteries.

Electrochemistry in Hollow-Channel Paper Analytical Devices

In the present article we provide a detailed analysis of fundamental electrochemical processes in a new class of paper-based analytical devices (PADs) having hollow channels (HCs). Voltammetry and amperometry were applied under flow and no flow conditions yielding reproducible electrochemical signals that can be described by classical electrochemical theory as well as finite-element simulations. The results shown here provide new and quantitative insights into the flow within HC-PADs. The interesting new result is that despite their remarkable simplicity these HC-PADs exhibit electrochemical and hydrodynamic behavior similar to that of traditional microelectrochemical devices.

Integration of solid-state dye-sensitized solar cell with metal oxide charge storage material into photoelectrochemical capacitor

A solid-state photo-rechargeable capacitor (photocapacitor) is obtained here by coupling a dye-sensitized solar cell and a ruthenium oxide based electrochemical capacitor. This integrated system permits direct storage of energy generated by sunlight within a single optoelectronic microelectrochemical device. It utilizes three planar electrodes arranged sequentially to include a polymer hole conductor (poly-(3-hexylthiophene-2,5-diyl)), between the titanium oxide photoanode modified with dye (E)-3-(5-(4-(Bis(20,40-dibutoxybiphenyl-4-yl)amino)phenyl)thiophen-2-yl)-2-cyanoacrylic acid (D35) and the intermediate silver electrode as well as two hydrous ruthenium oxide layers (separated by protonically conducting Nafion™ membrane) between the intermediate (silver) and the external (counter) electrode. Upon integration of the capacitor and dye-sensitized solar cell into a single photocapacitor hybrid device, the following parameters were obtained under simulated 100 mW cm−2 solar illumination: specific capacitance, 407 F g−1 (3.26 F cm−2); energy and power densities, 0.17 mWh cm−2 and 0.34 mW cm−2 and coulombic efficiency, 88%. These data together with results of experiments performed at different light intensities (10–100 mW cm−2) are consistent with very good performance of the optoelectronic device under various light conditions.

Electrochemical detection of individual DNA hybridization events

We report on real-time electrochemical detection of individual DNA hybridization events at an electrode surface. The experiment is carried out in a microelectrochemical device configured with a working electrode modified with single-stranded DNA probe molecules. When a complementary DNA strand labelled with a catalyst hybridizes to the probe, an easily detectable electrocatalytic current is observed. In the experiments reported here, the catalyst is a platinum nanoparticle and the current arises from electrocatalytic oxidation of hydrazine. Two types of current transients are observed: short bursts and longer-lived steps. At low concentrations of hydrazine, the average size of the current transients is proportional to the amount of hydrazine present, but at higher concentrations the hydrazine oxidation reaction interferes with hybridization.

Highly reproducible chronoamperometric analysis in microdroplets

Here we report a method for highly reproducible chronoamperometric analysis of the contents of microdroplets. Aqueous microdroplets having volumes on the order of 1 nL and separated by a fluorocarbon solvent are generated within a microfluidic device using a T-shaped junction. The key finding is that stable and reproducible quasi-steady-state currents are observed if the electrochemical measurements are made in a narrowed segment of a microchannel. Under these conditions, the microdroplets are stretched, here by a factor of 10, leading to desirable intradroplet mass transfer characteristics. Microdroplet frequencies up to 0.67 s(-1) are accessible using this method. The quasi-steady-state currents resulting from chronoamperometric analysis of microdroplets containing 1.0 mM Ru(NH3)6(3+) have relative standard deviations of just 1.8% and 2.8% at flow rates of 30 nL min(-1) and 60 nL min(-1), respectively. Importantly, the design of the microelectrochemical device ensures direct contact between intradroplet redox molecules and the electrode surface. That is, the fluorocarbon between microdroplets does interfere with inner-sphere electrocatalytic processes such as the oxygen reduction reaction. Finite-element simulations are presented that are in accord with the experimental findings.

Electrochemical Behaviors of Single Microcrystals of Iron Hexacyanides/NaCl Solid Solution

Solid-state solution with excellent ionic conductivity and redox activity has potential applications in electrochemical microdevices, such as transistors, switches, sensors, and actuators, due to its controllable assembly, integration, and package in microchips. In this paper, we present an electrochemical method to synthesize single microcrystals of an iron hexacyanides/NaCl solid solution and to assemble them into microdevices based on scanning electrochemical cell microscopy. The redox behaviors of the single microcrystals were investigated systematically, especially in the "all-in-solid-state", that is, without exposure to any external liquid environment. The unique metal/solid solution interface has similar electrochemical properties as the conventional metal/liquid solution interface. The apparent concentration of ion hexacyanides (1.05 × 10(-3) mol/L), the diffusion coefficient of the counterion Na(+) (8.05 × 10(-8) cm(2)/s), and the electron transfer rate in the lattice (1.22 × 10(-4) cm/s) were evaluated from the data obtained through all-in-solid-state cyclic voltammetry and electrochemical impedance. All the results are comparable to the conventional metal/solution theory of electrochemistry.

Effect of mass transfer on the oxygen reduction reaction catalyzed by platinum dendrimer encapsulated nanoparticles

Here we report on the effect of the mass transfer rate (k(t)) on the oxygen reduction reaction (ORR) catalyzed by Pt dendrimer-encapsulated nanoparticles (DENs) comprised of 147 and 55 atoms (Pt(147) and Pt(55)). The experiments were carried out using a dual-electrode microelectrochemical device, which enables the study of the ORR under high k(t) conditions with simultaneous detection of H(2)O(2). At low k(t) (0.02 to 0.12cms(-1)) the effective number of electrons involved in ORR, n(eff), is 3.7 for Pt(147) and 3.4 for Pt(55). As k(t) is increased, the mass-transfer-limited current for the ORR becomes significantly lower than the value predicted by the Levich equation for a 4-electron process regardless of catalyst size. However, the percentage of H(2)O(2) detected remains constant, such that n(eff) barely changes over the entire k(t) range explored (0.02cms(-1)). This suggests that mass transfer does not affect n(eff), which has implications for the mechanism of the ORR on Pt nanoparticles. Interestingly, there is a significant difference in n(eff) for the two sizes of Pt DENs (n(eff)=3.7 and 3.5 for Pt(147) and Pt(55), respectively) that cannot be assigned to mass transfer effects and that we therefore attribute to a particle size effect.

Dual-electrode microfluidic cell for characterizing electrocatalysts

In this paper we introduce a microelectrochemical cell configured for generation-collection experiments and designed primarily for examining the kinetics of electrocatalysts. The heart of the device consists of two, closely spaced, pyrolyzed photoresist microband electrodes enclosed within a microchannel. The cell is suitable for evaluating the efficiency of electrocatalysts under an unprecedented range of conditions. Specifically, compared to the gold-standard rotating ring-disk electrode (RRDE), this device offers four major advantages. First, collection efficiencies of 97% are easily achieved, compared to values of 20-37% that are characteristic of RRDEs. Second, mass transfer coefficients of 0.5 cm s(-1) are accessible for typical redox species, which is significantly higher than RRDEs (up to 0.01 cm s(-1)). Third, we show that the device can operate effectively at temperatures up to 70 °C, which is important for measuring electrochemical kinetics that are relevant to fuel cell catalysts. Finally, much less catalyst and much smaller volumes of electrolyte solution are required to make kinetic measurements using the microelectrochemical device compared to the RRDE. Here, we present the simple procedure used to fabricate the device, fundamental electroanalytical characterization, and electrocatalytic measurements relevant to the oxygen reduction reaction.

Electrochemical Microdevices for Point-of-Care Testing

Electrochemical Microdevices for Point-of-Care Testing

Facile synthesis of nanostructured gold for microsystems by the combination of electrodeposition and dealloying

For the successful exploitation of microelectrochemical systems, e.g., micro-sensors and micro-fuel cells, synthesis of porous electrodes on tiny current collectors is of great importance. Here, a facile electrochemical method based on the combination of electrodeposition and dealloying is proposed. AuCu alloy was deposited in a sulfuric acid solution containing cupric ions and tetrachloroaurate ions, and then was dealloyed in the same solution just by applying a more positive potential. The synthesized thin film was 100 at% polycrystalline Au and consisted of spongelike nanostructures with 50–100 nm thick ligaments and wider pores. The crystal lattice of the ligament surface was found to be contracted on the basis of the results of X-ray photoelectron spectroscopy and electron diffractometry. This is probably due to the residual stress caused by dealloying. The electrochemical measurements revealed that the surface area was ca. 25 times larger than its geometrical area (thickness: 780 nm), and that the surface contained well-ordered large Au(111) terraces. Finally, feasibility of the application of this synthesis to microsystems was proved by using microelectrode arrays. Thus, this method is promising for preparing Au electrodes for microelectrochemical devices.

Rapid Diagnostic Device for Subclinical Mastitis Based on Electrochemical Detection of Superoxide Produced from Neutrophils in Fresh Milk

Electrochemical microdevices were fabricated to identify mastitic cows based on the increased number of neutrophils in raw milk. Because neutrophils produce superoxide (O2·-), the amount of O2·- can be used as an early indicator for subclinical mastitis. In the microdevices, O2·- was detected on a gold electrode using superoxide dismutase immobilized via a self-assembled monolayer of cysteine. In a preliminary test using xanthine oxidase to produce O2·-, one of the devices detected the production and rapid extinction of O2·-. When neutrophils obtained from a mastitic cow were concentrated by centrifugation and introduced into the device, a current increase distinctly different from the background was observed. Furthermore, a micropillar structure was fabricated on the gold electrode to trap and collect neutrophils, thereby facilitating the concentration of these cells around the electrode. The measured current clearly depended on the number of neutrophils in raw milk samples, demonstrating the applicability of the device for rapid diagnosis of subclinical mastitis.

Metallization on Three Dimensions Microstructures Using Photoresist Spray Coating for Microdirect Methanol Fuel Cell

High-efficientcy micro-electrochemical devices are realized by increasing electrode surface areas while maintaining keeping chip size. The use of three dimensional (3-D) electrodes instead of planar electrodes is effective for this purpose. To realize a metal pattern on 3-D structures, spray coating was utilized and the optimum condition for achieving a good photoresist step coverage was determined. A good step coverage of the photoresist was obtained on the obtuse corner formed by the anisotropic wet etching of (100) silicon and even on the vertical corner formed by deep reactive ion etching (RIE). We applied this method to fabricate a high-performance micro direct methanol fuel cell (µDMFC). An open circuit voltage of 548 mV and a maximum power density of 0.72 mW/cm2 were obtained at room temperature with 2 M methanol.

All-Diamond Microelectrode Array Device

We report the development of all-diamond microelectrochemical devices, namely, a microelectrode array (MEA), in which a periodic array structure with well-defined diameters, distance, and hexagonal unit cell pattern is micromachined using a combination of state-of-the-art microwave-induced plasma growth and laser ablation shaping techniques to prepare and coat a patterned boron-doped diamond (BDD) substrate with an intrinsic diamond insulating layer. The active BDD element can be tuned to between 10 and 50 microm in diameter with a 10 times diameter center-to-center distance between two adjacent conducting elements, which are exactly coplanar to the dielectric surroundings. This type of device should enable applications in harsh conditions such as high temperature, high pressure, and resistive media under dynamic flow regimes.

A Dual‐Electrode Approach for Highly Selective Detection of Glucose Based on Diffusion Layer Theory: Experiments and Simulation

A dual-electrode configuration for the highly selective detection of glucose in the diffusion layer of the substrate electrode is presented. In this approach, a glassy carbon electrode (GCE, substrate) modified with a conductive layer of glucose oxidase/Nafion/graphite (GNG) was used to create an interference-free region in its diffusion layer by electrochemical depletion of interfering electroactive species. A Pt microelectrode (tip, 5 microm in radius) was located in the diffusion layer of the GNG-modified GCE (GNG-G) with the help of scanning electrochemical microscopy. Consequently, the tip of the electrode could sense glucose selectively by detecting the amount of hydrogen peroxide (H2O2) formed from the oxidization of glucose on the glucose oxidase layer. The influences of parameters, including tip-substrate distance, substrate potential, and electrolyzing time, on the interference-removing efficiency of this dual-electrode approach have been investigated systematically. When the electrolyzing time was 30 s, the tip-substrate distance was 1.8 a (9.0 microm) (where a is the radius of the tip electrode), the potentials of the tip and substrate electrodes were 0.7 V and 0.4 V, respectively, and a mixture of ascorbic acid (0.3 mM), uric acid (0.3 mM), and 4-acetaminophen (0.3 mM) had no influence on the glucose detection. In addition, the current-time responses of the tip electrode at different tip-substrate distances in a solution containing interfering species were numerically simulated. The results from the simulation are in good agreement with the experimental data. This research provides a concept of detection in the diffusion layer of a substrate electrode, as an interference-free region, for developing novel microelectrochemical devices.

Microelectrochemical logic circuits.

We report microelectrochemical devices that mimic some functions of solid-state circuit components such as diodes and transistors. The operation of this new family of devices is enabled by a network of microfluidic electrochemical cells that communicate with one-another via conductive solutions and, in some cases, bipolar electrodes. The output of the devices can be either electrochemical or optical. The approach reported here is a first step toward the development of large-scale integrated electrochemical systems having parallel processing capabilities.

Dense diffusion barrier limiting current oxygen sensors

A new type of miniature amperometric oxygen sensor has been developed. The sensors are high temperature micro-electrochemical devices that use a dense diffusion barrier of a metal oxide that readily transports oxygen and conduct electrons. The diffusion barrier is deposited in thin film form on top of a zirconia-based electrochemical pump. When a voltage is applied to the pump, oxygen is depleted from one side of the diffusion barrier and the ionic current is proportional to the flux of oxygen across the thin film layer. If the pumping voltage reaches a high enough value, the transport of oxygen across the membrane and hence the device’s output current, will be limited by the external, oxygen concentration and the transport characteristics of the diffusion barrier. The sensors can be manufactured in a planar design that offers a faster time response, much simpler design and potentially lower cost than existing limiting current oxygen sensors.