Au Microelectrodes Research Articles

The i–V response of an electrochemical cell comprising two polarizable microelectrodes and the influence of impurities on the cell response

The steady-state i–V behavior of an electrochemical cell comprising two polarizable Au microelectrodes in a mixed redox solution containing reversible and soluble redox species (FcCH2OH and Ru(NH)63+) is reported. Analytical solutions describing the cell i–V response in the absence of a well-poised reference electrode are presented, and these expressions are shown to agree with the experimental i–V results. Depending on the relative areas of the electrodes, the i–V response in the absence of a reference electrode is particularly sensitive to the presence of redox impurities. Specifically, we demonstrate that FcCH2OH+ impurity in the solution yields diffusion limited current plateaus that are equal to the currents for the primary redox species present in nearly 100-fold excess.

Direct Voltammetric Determination of Total Iron with a Gold Microelectrode Ensemble

AbstractThe behavior of the Fe(II)/(III) redox system at a Au microelectrode ensemble (Au‐MEE) based on a solid composite by means of direct and cyclic voltammetric analysis (VA) is reported. With a simple electrode activation and sample preparation, the influence of dissolved organic substances was eliminated, providing highly sensitive results. The analytical signal was based on the maximum cathodic current (I) of the first derivative (dI/dE), and iron determination within the 0.002–0.04 mg L−1 range was studied. A sensitive LOD (3σ) value of 0.7 µg L−1 for total iron concentration was calculated; total iron determination in different waters was shown.

Sinusoidal voltage electrodeposition and characterization of conducting polymers on gold microelectrode arrays

To the best of the authors’ knowledge, this is the first report on the electrochemical polymerization of 3,4-ethylenedioxythiophene on gold (Au) disk microelectrode arrays using sinusoidal voltages. The use of electrochemical impedance spectroscopy (EIS) for the in situ deposition of poly(3,4-ethylenedioxythiophene) (PEDOT) coating onto Au microelectrode arrays allowed the estimation of electrochemical parameters, such as charge transfer resistance and exchange current. The PEDOT coatings deposited on different arrays by sinusoidal voltages and potentiostatic methods were compared. The electrochemical characterization of both naked and PEDOT-modified microelectrode arrays by EIS and cyclic voltammetry (CV) revealed different diffusion regimes occurring at the microelectrodes surfaces. The electron transfer kinetics between the PEDOT coatings and redox probe in solution were also investigated. Results demonstrated the suitability of the new preparation procedure for electrogenerating PEDOT coatings. This novel procedure led to insights to the electrochemical properties and diffusion regimes occurring at both naked and modified microelectrode arrays.

Fabrication of a Large‐Area Al‐Doped ZnO Nanowire Array Photosensor with Enhanced Photoresponse by Straining

AbstractThe photosensing properties of flexible large‐area nanowire (NW)‐based photosensors are enhanced via in situ Al doping and substrate straining. A method for efficiently making nanodevices incorporating laterally doped NWs is developed and the strain‐dependent photoresponse is investigated. Photosensors are fabricated by directly growing horizontal single‐crystalline Al‐doped ZnO NW arrays across Au microelectrodes patterned on a flexible SiO2/steel substrate to enhance the transportation of carriers and the junction between NWs and electrodes. The Raman spectrum of the Al:ZnO NWs, which have an average diameter and maximum length of around 40 nm and 6.8 μm, respectively, shows an Al‐related peak at 651 cm−1. The device shows excellent photosensing properties with a high ultraviolet/visible rejection ratio, as well as extremely high maximum photoresponsivity and sensitivity at a low bias. Increasing the tensile strain from 0 to 5.6% linearly enhances the photoresponsivity from 1.7 to 3.8 AW−1 at a bias of 1 V, which is attributed to a decrease in the Schottky barrier height resulting from a piezo‐photonic effect. The high‐performance flexible NW device presented here has applications in coupling measurements of light and strain in a flexible photoelectronic nanodevice and can aid in the development of better flexible and integrated photoelectronic systems.

Spirally oriented Au microelectrode array sensor for detection of Hg (II)

A simple and reproducible carbon microelectrode array (CMA), designed to eliminate diffusive interference among the microelectrodes, has been fabricated and used as a frame to build a gold (Au) microelectrode array (GMA) sensor. To prepare the CMA initially, rather than use an uncontrollable large number of carbon fibers, only 60 carbon fibers of regular size were used to ensure manageable and reproducible arrangement for array construction. In addition, for efficient spatial arrangement of the microelectrode and easy sensor preparation, carbon fibers were oriented in a spiral fashion by rolling around a Cu wire. The distance between carbon fibers was carefully determined to avoid overlap among individual diffusion layers, one of the important factors governing steady-state current response and sensor-to-sensor reproducibility. After the preparation of a spirally arranged CMA, Au was electrochemically deposited on the surface of individual carbon electrodes to build a final GMA sensor. Then, the GMA sensor was used to measure Hg2+ in a low concentration range. Simultaneously, multiple GMA sensors were independently prepared to examine reproducibility in sensor fabrication as well as electrochemical measurement (sensor-to-sensor reproducibility). Overall, highly sensitive detection of Hg2+ was possible using the proposed GMA sensor due to efficient arrangement of microelectrodes and the sensor-to-sensor reproducibility was superior owing to simplicity in sensor fabrication.

Nanotextured au microelectrodes: Electrodeposition-based fabrication and their cyclic voltammograms study

In this letter, the fabrication of nanotextured Au microelectrodes was demonstrated for the first time, carried out by electrodeposition of Au starting from the bottom of a 500-nanometer pore pre-fabricated by microfabrication technology. It was found both the size and the surface nanotexture can be controlled by the potential used. The electrochemical cyclic voltammograms (CVs) response of electroactive species toward such Au microelectrodes was also studied and it was found that the electroactive species used underwent a diffusion-controlled process.

ELECTROCHEMICAL INVESTIGATION OF CYSTEAMINE AT CARBON FIBER MICRODISK ELECTRODE

The electrooxidation of cysteamine compound was carried out using convolutive cyclic voltammetry, linear sweep voltammetry and chronoamperomtry techniques at a carbon fiber microdisk electrode in 0. 1 M perchloric acid. The electrooxidation potential ( E0'' ) of cysteamine occurs at + 0.921 V. The mechanistic pathway of electrooxidation process at carbon fiber microelectrode is loss of 1 electron per molecule. The electrode process is controlled mainly by diffusion. The chemical and electrochemical parameters of the investigated system were determined experimentally and verified theoretically via digital simulation method. The unique properties of microdisk electrodes offer a number of particular advantages for their use in analytical applications. In the steady-state mass transport to the microdisk is dominated by quasi-hemispherical diffusion. As a consequence, the mass transport to and from the microdisk electrode surface is efficient, reproducible and insensitive to random convection. In analytical applications based on accumulation and stripping techniques this obviates the need for stirring during the preconcentration step, improves the precision of the measurement, and reduces analysis times. In addition, the small electrode surface area of the microdisk means that the double layer capacitance of the electrode is small, thus significantly reducing the charging current. As a consequence, the ratio of the steady-state Faradaic current to the non-Faradaic current is increased and the detection limit is enhanced.1 The small size of the electrodes also means that the currents passed at the microelectrode are small resulting in negligible ohmic drop (iR drop) even in the absence of a supporting electrolyte.2,3 This has dual advantages in analytical applications that sample preparation can be simplified and that the possibility of impurities from the added electrolyte is eliminated. Microelectrodes are thus particularly well-suited for trace analysis. Pt, Au, Hg and carbon microelectrodes have been used in stripping analyses for the determination of trace metals by a number of authors.4-8 In contrast, studies using microelectrodes for the determination of organic species are far less numerous and are restricted to the use of carbon fiber microelectrodes. 9-19

Fabrication of Au Microelectrodes with Photopolymerization of Triazine Dithiol Thin Films

In this article, we report a new lift-off process to fabricate Au microelectrodes with photopolymerization of 6-diallylamino-[1,3,5]triazine-2,4-dithiol (DA) thin films, which is easier, more energy saving, and friendlier to the environment than the past processes. The optimum conditions for fabricating fine Au/poly(DA) microelectrodes are as follows: DA film thickness is 40 nm and UV irradiation time is 20 min. The Au/poly(DA) microelectrodes with over 10 µm spaces were developed and could be well applied to pentacene thin-film transistors with a mobility of 1.3×10-1 cm2 V-1 s-1 and a threshold voltage of -13.4 V.

Fabrication of Au Microelectrodes with Photopolymerization of Triazine Dithiol Thin Films

In this article, we report a new lift-off process to fabricate Au microelectrodes with photopolymerization of 6-diallylamino-[1,3,5]triazine-2,4-dithiol (DA) thin films, which is easier, more energy saving, and friendlier to the environment than the past processes. The optimum conditions for fabricating fine Au/poly(DA) microelectrodes are as follows: DA film thickness is 40 nm and UV irradiation time is 20 min. The Au/poly(DA) microelectrodes with over 10 µm spaces were developed and could be well applied to pentacene thin-film transistors with a mobility of 1.3×10-1 cm2 V-1 s-1 and a threshold voltage of -13.4 V.

Impedance measurements on Au microelectrodes using controlled atmosphere high temperature scanning probe microscope

High temperature impedance measurements on Au microelectrodes deposited on polished yttria stabilized zirconia (YSZ) pellets were demonstrated using a newly designed controlled atmosphere high temperature scanning probe microscope (CAHT-SPM). Probes based on Pt 0.8Ir 0.2 were fabricated and employed in all the investigations. The electrical properties of the Au microelectrodes were investigated using CAHT-SPM at various temperatures in air. The influences of the tip-sample force and of the size of the microelectrode on the electrical properties of the Au microelectrodes were also studied. The impedance spectra of circular Au microelectrodes down to 8 μm in diameter at different temperatures were successfully obtained. The CAHT-SPM presented in this paper is capable of obtaining topography images, conductivity images and impedance spectra in-situ in a temperature range from room temperature to at least 650 °C.

Deposition of Au and Ag nanoparticles on PEDOT

The deposition of Au and Ag, locally and from bulk solution, on poly(3,4-ethylenedioxythiophene) (PEDOT) was studied. Specifically, PEDOT was electrochemically polymerized onto a glassy carbon (GC) electrode and used for bulk deposition of Au and Ag from their respective ions dissolved in the solution as well as for the local deposition of these metals using scanning electrochemical microscopy (SECM). These two sets of experiments were utilized to investigate the difference between Au and Ag electrochemical deposition on PEDOT. In particular, SECM experiments, which were conducted by the controlled anodic dissolution of Au and Ag microelectrodes close to GC/PEDOT, probed the effect of different PEDOT oxidation states on local deposition. The current-time transients recorded during the deposition, combined with scanning electron microscopy and EDX analysis provided insight into the reduction processes. AuCl(4)(-) and Ag(+) ions were electrochemically reduced at a potential equal to and more negative than the ions redox potentials (0.4 and 0.2 V, respectively) and more positive than -0.7 V, where the PEDOT starts transforming into the reduced, i.e. insulating, state. We found that the electroreduction of Ag(+) ions was diffusion-controlled and the PEDOT film served as a simple conductor. On the other hand, the reduction of AuCl(4)(-) ions was enhanced on GC/PEDOT as compared with bare GC, indicating that PEDOT catalyzes the reduction of AuCl(4)(-) to Au.

A polypyrrole-based microchip for controlled drug release

A novel polypyrrole-based microchip has been developed by electrochemical deposition of drug-doped polypyrrole films on the Au microelectrode arrays. The drug release from the microchip is controlled by a potential application. Due to its unique configuration, the microchip exhibits some appealing characteristics, such as being able to supply single or multiple drugs with precise small doses, ability to achieve pulsatile release mode, small in size, and good biocompatibility, and thus has the potential for application as implantable device in the field of controlled drug release.

Electrocatalytic microelectrode detectors for choline and acetylcholine following separation by capillary electrophoresis.

Two electrocatalytic enzyme modified microelectrode systems were employed as end-column amperometric detectors of choline (Ch) and acetylcholine (ACh) following separation by capillary electrophoresis (CE). Horseradish peroxidase cross-linked in an osmium based redox polymer hydrogel (HRP-Os) was physically adsorbed on Au microelectrodes followed by chemical cross-linking of the enzymes acetylcholinesterase (AChE) and choline oxidase (ChO). An alternative approach utilized the deposition of the transition metal catalyst, Prussian Blue (PB), on Pt microelectrodes as the electrocatalyst. Utilizing butyrylcholine (BuCh) as an internal standard, the HRP-Os/AChE-ChO and PB/AChE-ChO electrodes exhibited excellent linear responses from 2-2000 microM and 10-2000 microM, respectively, for both Ch and ACh. Detection limits of 0.1 microM or 38 amol were determined for the HRP-Os/AChE-ChO electrode. The limit of detection for ACh and Ch at the PB/AChE-ChO electrode was 5 microM or 9.5 fmol. The electrodes were operated at potentials of +0.10 and -0.10 V vs Ag/AgCl (3 M NaCl), respectively, and thus minimized the potential response from oxidizable interferences. In addition, both electrocatalytic electrodes showed good operational stability for more than 70 h. The enhanced detection capability of the HRP-Os/AChE-ChO and PB/AChE-ChO electrodes in combination with efficient CE separation of Ch and ACh provides a new sensitive and selective strategy for monitoring and quantifying these cholinergic biomarkers in biological fluids.

Surface Manipulation of Microtubules Using Self-Assembled Monolayers and Electrophoresis

We integrate microtubule (MT)-resistant self-assembled monolayers (SAMs) with lithographically patterned electrodes to control MTs in a cell-free environment. Formed through a facile, one-step assembly method, the poly(ethylene glycol) trimethoxysilane SAM prevents MT adsorption on both silicon substrates and Au microstructures without casein. We characterize the SAM using ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy and compare it with other MT passivation techniques. The SAM retains its passivating ability when used as a substrate for electron beam lithography, a key feature that allows us to pattern microtubules on lithographically defined Au structures. Moreover, by combining the SAM-passivated Au microelectrodes and DC electrophoresis, we demonstrate reversible trapping of MTs as well as capture and alignment of individual MTs.

Facile fabrication of UV photodetectors based on ZnO nanorod networks across trenched electrodes

Using a low temperature hydrothermal synthesis method, ZnO nanorod networks have been directly grown across trenched Au microelectrodes arrays, which were modified with a layer of ZnO seeds. The characteristics of the current–voltage (I–V) and the photoresponse were obtained both in the dark and under ultraviolet illumination. The bridged nanorod network demonstrated a highly sensitive response to UV illumination in atmosphere at room temperature. It can be useful for nanoscale optoelectronic applications, serving as chemical sensors, biological sensors, and switching devices.

Microelectrode Study of Oxygen Uptake and Organic Matter Decomposition in The Sediments of Xiamen Western Bay

Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn2+, and Fe2+ within the sediments. The O2 concentrations decreased sharply from about 200 μmol L−1 in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn2+ was detected below the oxic zones with peak concentrations up to 600 μmol L−1, whereas dissolved Fe2+ had peak concentrations up to 1,000 μmol L−1 in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to shallow O2 penetration depths, high dissolved Mn2+ and Fe2+ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O2 profiles within the sediments. Based on the O2 profiles, O2 fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m−2 day−1, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm−3 a−1. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as sewage discharge.

Electrochemical and In Situ Optical Studies of Supported Iridium Oxide Films in Aqueous Solutions

Certain aspects of the dynamic behavior of electrochemically deposited hydrous Ir oxide films supported on Au microelectrodes during charge and discharge have been investigated by a combination of chronocoulometry and simultaneous in situ normalized reflectance spectroscopy techniques in aqueous solutions. Correlations between the reflectance spectra and the optical properties of the films in its various states of oxidation were sought from in situ transmission measurements for films supported on In-doped tin oxide on glass. The current transient response for microelectrodes following a potential step, within the voltage region in which the films display pseudocapacitive characteristics, was found to exhibit a well-defined peak, as opposed to a monotonic decay reported by other groups. Some features of this behavior can be attributed to changes in the conductivity of the film as a function of its state of charge, as has been proposed for electronically conducting polymers. Also presented in this work are data collected over the pH range 0.3–13, which confirm the much faster charge–discharge dynamics in basic compared to acidic media. A primitive model based on proton conductivity within the hydrated oxide lattice is presented which accounts grossly for this pH-induced effect.

Novel method for the determination of trace amounts of metformin in its pharmaceutical formulation by fast Fourier continuous cyclic voltametric technique at Au microelectrode in flowing solutions

An easy and fast Fourier transform continuous cyclic voltametric technique for monitoring of ultra trace amounts of metformin in a flow-injection system has been introduced in this work. The potential waveform, consisting of the potential steps for cleaning, stripping and potential ramp, was continuously applied on an Au disk microelectrode (12.5 μm in radius). The proposed detection method has some of advantages, the greatest of which are as follows: first, it is no more necessary to remove oxygen from the analyte solution and second, this is a very fast and appropriate technique for determination of the drug compound in a wide variety of chromatographic analysis methods. The detection limit for metformin was 43 pg/ml. The relative standard deviation (RSD) of the proposed technique at 5.0 × 10−7 M was 2.2% for 10 runs. The influences of pH of eluent, accumulation potential, sweep rate, and accumulation time on the determination of the metformin were considered. The proposed method was applied to the determination of metformin in a pharmaceutical preparation.

Sotalol nanolevel detection at an Au microelectrode in flowing solutions

Sotalol detection in tablets is achieved with the help of a newly developed, time-saving, and simple method, called fast Fourier transform with continuous cyclic voltammetry (FFTCV). Noticeably, FFTCV illustrates two advantages, not being demonstrated by earlier reports. First, there is no requirement for the oxygen removal from the test solution. In addition, the speed of the method for determining any compound is high in a broad range of chromatographic methods. This research includes the observation of effects of various parameters (influence of the pH of the eluent, accumulation potential, sweep rate, accumulation time) on the sensitivity of the detection system. Finally, the technique was found to be linear for the concentration range of 3–14900 pg/ml (r = 0.998) with a limit of detection and quantitation of 0.95 pg/ml and 3 pg/ml, respectively.

Nonelectroactive recognition: Monitoring of perphenazine by its subsecond adsorption on an Au microelectrode by the fast fourier transform continuous cyclic voltammetric technique (FFTCCV)

A novel method for determining perphenazine in flow-injection systems has been developed in this work. The method was successfully applied for fast determination of perphenazine in its pharmaceutical formulations. Being very simple, precise, accurate, time-saving, and economical, this method has many advantages compared to all of the previously reported methods. Some investigations were also performed to find the effects of various parameters on the sensitivity of the proposed method. The conditions responsible for the performance were a pH value of 2, a scan rate value of 30 V/s, an accumulation potential of 500 mV, and an accumulation time of 0.8 s. Some of the advantages of the proposed method are as follows: the removal of oxygen from the test solution is not required anymore, the detection limit of the method is subnanomolar, and finally, the method is fast enough to determine such compounds in a wide variety of chromatographic methods. We also introduce a special computer-based numerical method to calculate the analyte signal and noise reduction. After subtraction of the background current from noise, the electrode response was calculated based on partial and total charge exchanges at the electrode surface. The integration range of currents was set for all the potential scan ranges, including oxidation and reduction of the Au surface electrode, to obtain a sensitive determination. The waveform potential was continuously applied on an Au disk microelectrode (a radius of 12.5 µm). The detention limit of the method for perphenazine was 15 pg/ml. For eight runs, the relative standard deviation of the method at 1.1 × 10−8 M was 2.1%.