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Miniature Gigahertz Acoustic Resonator and On-Chip Electrochemical Sensor: An Emerging Combination for Electroanalytical Microsystems
Abstract
Performance of electroanalytical lab-on-a-chip devices is often limited by the mass transfer of electroactive species towards the electrode surface, due to the difficulty in applying external convection. This article describes the powerful signal enhancement attained with a 2.54 GHz miniature acoustic resonator integrated with an electrochemical device in a miniaturized cell. Acoustic resonator and an on-chip gold thin-film three-electrode electrochemical cell were arranged facing each other inside a structured polymethyl methacrylate chamber. Cyclic voltammetric and chronoamperometric responses of 1 mM ferrocene-methanol were recorded under resonator’s actuation at powers ranging from 0 to 1 W. Finite element analysis was carried out to study the sono-electroanalytical process. Acoustic resonator’s actuation greatly enhances the mass transport of electroactive species towards the electrode surface. The diffusion limited cyclic voltammetric and chronoamperometric currents increase around 10 and 20 times respectively with an input power of 1 W compared to those recorded under stagnant condition. The improvement in electroanalytical process is mainly associated with acoustic resonator’s vibration induced fluid streaming. The advantages of miniaturized acoustic resonator, including the submillimeter small size, amenability for mass fabrication, cost effectiveness, low energy consumption, as well as outstanding enhancement of coupled electrochemical processes, will enable the production of highly-sensitive compact electroanalytical devices.
... Its acoustic nature as well as gigahertz working frequency (around ten thousand times higher than commonly used ultrasound waves (26)) enables agitating liquid and amazingly generates extremely strong steady state acoustic streaming but not cavitation ( Fig. 1(D)). Also its submillimeter size makes it extremely compatible with miniature platform (27). ...
... The propagation of vibration from the resonator to the fluid induces directional fluid flow in the chamber and the resultant flow profile is mathematically calculatable. Specific calculation details can be referred to our previous publication (27). Fig. 3(A) schematically draws a two-dimensional (2D) diagram of our assembled sono-electrochemical platform. ...
Developing miniature smart electrochemical sweat sensors to perform continuous and noninvasive health diagnosis at molecule level is in the spotlight. Sensor's biofoulings hinder the targets' approaching, decreasing the detection sensitivity over time and further limiting the accurate analysis. Effective cleaning to remove biofoulings on electrochemical sensors is a main challenge and needs to overcome urgently for such purpose. We propose a miniature sono-electrochemical platform utilizing submillimeter gigahertz acoustic resonator to effectively clean away the biofoulings and release sensors inherent electrochemical activities. Resonator induced super strong fluid streaming can completely and repeatedly remove sweat biofoulings from gold electrode surface, leading to 100% recovery of sensor’s activities for continuous reliable measurements. Compared with common sensor renewal strategies, such as acidic or basic solution cleaning, mechanical polishing, traditional low frequency ultrasound treatment, this electrode surface renewal technique is stable, mathematically predictable, effective and gentle so that it does not cause any destructions to inherent electrode surface, and therefore the sensed electrochemical signals on raw and cleaned electrodes are comparable and improvable with no unexpectable deviation. Together with submillimeter size of the resonator, it shows potential to be integrated within wearable electroanalytical microfluidics for developing smart device to carry out continuous health diagnosis.
... for Lab-on-a-Chip analysis has allowed the rapid analysis of small sample volumes [7,8]. However, little has been done to develop thin film electrodes for high-temperature and highpressure (high-T,p) applications in aqueous systems, despite the impact such electrodes can make when combined with flow cells [9]. ...
This work reports a methodology for the fabrication of Pt thin‐film electrodes for electrochemical studies in hydrothermal systems. The research process was meticulous, with particular attention paid to the multilayer Ti/Pt/Ti/Al2O3 film structure and annealing conditions that are expected to impact the morphology of the films, surface composition and electrochemical response. The findings of this study are significant, as they provide valuable insights into the behaviour of Pt thin‐film electrodes in various media and conditions. Two different approaches were adopted for the preparation of the electrodes: in one case, the Ti/Pt/Ti/Al2O3 films deposited on sapphire wafers were exposed to rapid thermal annealing at 900°C under argon for 5 min, followed by argon ion milling to etch the final electrode pattern (Pt‐RA(900)), while in the other case, uncapped Pt films were annealed, after etching, in a tubular oven under argon at specific temperatures between 200°C and 900°C (Pt‐TO). Rapid annealing at 900°C on capped films resulted in the formation of a Pt3Ti intermetallic alloy with remarkable mechanical and chemical stability even after 10 h of immersion in deionised water, acid (0.1 M H2SO4) and alkaline media (0.1 M KOH) conditions at temperatures up to 150°C, despite the dissolution of the Al2O3 top layer at 150°C and long immersion times (> 10 h). In the case of uncapped Pt films, diffusion and oxidation of Ti through the Pt film at high temperature resulted in the formation of TiO2 on the surface of Pt. The results were confirmed by using a comprehensive suite of ex‐situ characterisation techniques to follow changes in the Pt electrode surface morphology and composition before and after immersion in H2O, 0.1 M H2SO4 and 0.1 M KOH solutions under argon. Ex‐situ electrochemical characterisation studies were also conducted to correlate the changes in the electrode surface properties, including the electrochemical surface area, with different annealing conditions and after various hydrothermal treatments in neutral, alkaline and acidic media.
... 5,6 Their fundamental sensing principles are based on the transmission of electrons from a motion target to the surface of a static microelectrode, and the speed of the target's movement in liquid is one of the most important factors affecting the speed and sensitivity of electrochemical detection. 7,8 However, in the conventional microfluidic systems, the volume of the fluid is in the range of microliters to nanoliters, and the fluid transfer rate is usually slow, which significantly limits the detection rate of electrochemical sensors. [9][10][11] Therefore, it is crucial to enhance the flow and mass transfer rates of the fluid and effectively increase the movement speed of the targeted species, thus improving the performance of the electrochemical sensor. ...
Microelectrode-based electrochemical detection methods have been extensively applied in microfluidic sensors, but there are significant challenges for achieving fast and efficient contact between analytes and the microarray electrodes and, thus, enhancing the sensing performance. In this paper, we develop a technique using asymmetrically aligned focused surface acoustic waves (FSAWs) to enhance sensitivity of microarray electrodes detection. Effects of various focusing angles of the FSAW devices on the values and distributions of acoustic wave amplitudes were analyzed using finite element simulations, and torques, which determine the acoustic streaming velocity, were calculated as a function of values and distributions of amplitude. Based on simulation results, the FSAW device with a focusing angle of 30° was used to investigate sensitivity of microarray electrochemical sensors. The maximum value of instantaneous current was increased up to 11 times, researching a current value of 4.3 μA with the applied FSAWs. This developed electrochemical sensing platform shows great potentials for highly sensitive food quality control and biochemical detections.
... The application of metal or metal oxide pseudo-REs that can be readily fabricated is an important alternative when working with electrochemical transduction approaches such as voltammetry or amperometry (Ribet et al., 2017). Drift of the reference to solution potential upon change of solution composition should be taken into consideration but does not imply any device performance limitation in many practical applications (Zheng et al., 2019;Giménez-Gómez et al., 2019. ...
This work reports on the fabrication and performance of a new on-chip array of gold thin-film electrodes arranged into five individually addressable miniaturized electrochemical cells. Each cell shows a two-electrode configuration comprising a single working electrode and a counter/pseudo-reference electrode that is compartmentalized to be shared among all the cells of the array. Using this configuration, just six contact pads are required, which significantly reduces the chip overall surface area. Electrochemical characterization studies are carried out in solutions containing the two species of reversible redox pairs. The concentration of one redox species can reliably be measured at the working electrode by applying potentiostatic techniques to record the current due to the corresponding electrochemical reaction. The redox counterpart in turn undergoes an electrochemical process at the counter/pseudo-reference electrode, which, under optimized experimental conditions, injects current and keeps the applied potential in the electrochemical cell without limiting the current being recorded at the working electrode. Under these conditions, the electrode array shows an excellent performance in electrochemical detection studies without any chemical or electrical cross-talk between cells. The enzymatic activity of horseradish peroxidase, alkaline phosphatase and myeloperoxidase enzymes is analyzed using different redox mediators. Quasi-simultaneous measurements with the five electrochemical cells of the array are carried out within 1 s time frame. This array layout can be suitable for multiplexed electrochemical immunoassays and immunosensor approaches and implementation in simplified electrochemical ELISA platforms that make use of enzyme labels. Moreover, the array reduced dimensions facilitate the integration into compact fluidic devices.
Small-sized, high-sensitivity, and low-cost sensors are required for gas-sensing applications because of their critical role in environmental monitoring, clinic diagnosis, process control, and anti-terrorism. Given the rapid developments in micro-fabrication and microelectromechanical system (MEMS) technologies, film bulk acoustic resonator (FBAR) gas sensors have received increased research attention because of their improved working frequency and reliability. This chapter discusses the state-of-the-art and recent developments in FBAR gas sensors. The sensing mechanism and limitations of these sensors are summarized. Recent progress in the development of four major aspects of FBAR gas sensors, namely, FBAR gas sensors using different sensing materials, FBAR gas sensors used in electronic noses, system integration of FBAR gas sensors, and FBAR gas sensors used as micro-GC detectors, is reviewed. The potential future of FBAR sensors used in flexible electronics is also discussed.
Synthesis of nanoparticles and particulate nanomaterials with tailored properties is a central step toward many applications ranging from energy conversion and imaging/display to biosensing and nanomedicine. While existing microfluidics‐based synthesis methods offer precise control over the synthesis process, most of them rely on passive, partial mixing of reagents, which limits their applicability and potentially, adversely alter the properties of synthesized products. Here, an acoustofluidic (i.e., the fusion of acoustic and microfluidics) synthesis platform is reported to synthesize nanoparticles and nanomaterials in a controllable, reproducible manner through acoustic‐streaming‐based active mixing of reagents. The acoustofluidic strategy allows for the dynamic control of the reaction conditions simply by adjusting the strength of the acoustic streaming. With this platform, the synthesis of versatile nanoparticles/nanomaterials is demonstrated including the synthesis of polymeric nanoparticles, chitosan nanoparticles, organic–inorganic hybrid nanomaterials, metal–organic framework biocomposites, and lipid‐DNA complexes. The acoustofluidic synthesis platform, when incorporated with varying flow rates, compositions, or concentrations of reagents, will lend itself unprecedented flexibility in establishing various reaction conditions and thus enable the synthesis of versatile nanoparticles and nanomaterials with prescribed properties. An acoustofluidic synthesis device based on the acoustic streaming induced by acoustically oscillating sharp‐edge structures is proposed to synthesize various nanoparticles and nanomaterials. The active‐mixing strategy enabled by the acoustic streaming allows for the independent investigation of the reaction time or the reagent ratio in the synthesis process, which, on the other hand, enables the control over the synthesis conditions.
Acoustic tweezers have recently raised great interest across many fields including biology, chemistry, engineering, and medicine, as they can perform contactless, label-free, biocompatible, and precise manipulation of particles and cells. Here, we present wave number–spiral acoustic tweezers, which are capable of dynamically reshaping surface acoustic wave (SAW) wavefields to various pressure distributions to facilitate dynamic and programmable particle/cell manipulation. SAWs propagating in multiple directions can be simultaneously and independently controlled by simply modulating the multitone excitation signals. This allows for dynamic reshaping of SAW wavefields to desired distributions, thus achieving programmable particle/cell manipulation. We experimentally demonstrated the multiple functions of wave number–spiral acoustic tweezers, among which are multiconfiguration patterning; parallel merging; pattern translation, transformation, and rotation; and dynamic translation of single microparticles along complex paths. This wave number–spiral design has the potential to revolutionize future acoustic tweezers development and advance many applications, including microscale assembly, bioprinting, and cell-cell interaction research.
a r t i c l e i n f o Even as gigahertz (GHz) acoustic streaming has developed into a multi-functional platform technology for biochemical applications, including ultrafast microfluidic mixing, microparticle operations, and cellar or vesicle surgery , its theoretical principles have yet to be established. This is because few studies have been conducted on the use of such high frequency acoustics in microscale fluids. Another difficulty is the lack of velocimetry methods for microscale and nanoscale fluidic streaming. In this work, we focus on the basic aspects of GHz acoustic streaming, including its micro-vortex generation principles, theoretical model, and experimental characterization technologies. We present details of a weak-coupled finite simulation that represents our current understanding of the GHz-acoustic-streaming phenomenon. Both our simulation and experimental results show that the GHz-acoustic-induced interfacial body force plays a determinative role in vortex generation. We carefully studied changes in the formation of GHz acoustic streaming at different acoustic powers and flow rates. In particular, we developed a microfluidic-particle-image velocimetry method that enables the quantification of streaming at the microscale and even nanoscale. This work provides a full map of GHz acoustofluidics and highlights the way to further theoretical study of this topic.
A microfluidic platform for hydrodynamic electrochemical analysis was developed, consisting of a poly(methyl methacrylate) chip and three removable electrodes, each housed in 1/16" OD polyether ether ketone tube which can be removed independently for polishing or replacement. The working electrode was a 100-μm diameter Pt microdisk, located flush with the upper face of a 150 μm × 20 μm × 3 cm microchannel, smaller than previously reported for these types of removable electrodes. A commercial leak-less reference electrode was utilized, and a coiled platinum wire was the counter electrode. The platform was evaluated electrochemically by oxidizing a potassium ferrocyanide solution at the working electrode, and a typical limiting current behavior was observed after running linear sweep voltammetry and chronoamperometry, with flow rates 1-6 μL/min. While microdisk channel electrodes have been simulated before using a finite difference method in an ideal 3D geometry, here we predict the limiting current using finite elements in COMSOL Multiphysics 5.3a, which allowed us to easily explore variations in the microchannel geometry that have not previously been considered in the literature. Experimental and simulated currents showed the same trend but differed by 41% in simulations of the ideal geometry, which improved when channel and electrode imperfections were included.
Most of the observations seen in the application of power ultrasound in electrochemistry or also known as sonoelectrochemistry are due to enhanced mass-transport of electroactive species from the bulk solution to the electrode surface caused by efficient stirring, acoustic streaming and cavitation. However, fundamental studies on the effect of ultrasound on electrode kinetics i.e. on the electron-transfer are scarce. The main question still remains to be answered:
Does power ultrasound affect heterogeneous electron transfer kinetics?
This opinion paper discusses the effect of ultrasonic frequency and intensity upon the electrode kinetic parameters such as the half-wave potential (E1/2) and the apparent heterogeneous rate constant (ko). A few sonoelectrochemical studies have highlighted changes in half-wave potential and in apparent heterogeneous rate constant for both quasi-reversible and irreversible systems when the data were compared to silent conditions. These observations are thought to be due to the contribution of mass-transport and macroscopic temperature effects, as well as the continuous cleaning of the electrode surface caused by the collapse of high-energy cavitation bubbles and the production of high velocity jets of liquid. However, there still remains mechanistic controversy in assigning whether these findings could also be due to localised temperature increases, the contribution of sonolysis products or solely due to enhanced mass-transport at the electrode surface. Thus, the effect of stirring, macroscopic temperature and sonication time upon these electrode kinetic parameters is also shown to be important factors in comparing the validity of any sonoelectrochemical effects.
In this article, a transparent integrated microfluidic device composed of a 3D-printed thin-layer flow cell (3D-PTLFC) and an S-shaped screen-printed electrode (SPE) has been designed and fabricated for heavy metal ion stripping analysis. A finite element modeling (FEM) simulation is employed to optimize the shape of the electrode, the direction of the inlet pipeline, the thin-layer channel height and the sample flow rate to enhance the electron-enrichment efficiency for stripping analysis. The results demonstrate that the S-shaped SPE configuration matches the channel in 3D-PTLFC perfectly for the anodic stripping behavior of the heavy metal ions. Under optimized conditions, a wide linear range of 1-80 μg l⁻¹ is achieved for Pb²⁺ detection with a limit of 0.3 μg l⁻¹ for the microfluidic device. Thus, the obtained integrated microfluidic device proves to be a promising approach for heavy metal ions stripping analysis with low cost and high performance.
In single-cell analysis, cellular activity and parameters are assayed on an individual, rather than population-average basis. Essential to observing the activity of these cells over time is the ability to trap, pattern and retain them, for which previous single-cell-patterning work has principally made use of mechanical methods. While successful as a long-term cell-patterning strategy, these devices remain essentially single use. Here we introduce a new method for the patterning of multiple spatially separated single particles and cells using high-frequency acoustic fields with one cell per acoustic well. We characterize and demonstrate patterning for both a range of particle sizes and the capture and patterning of cells, including human lymphocytes and red blood cells infected by the malarial parasite Plasmodium falciparum. This ability is made possible by a hitherto unexplored regime where the acoustic wavelength is on the same order as the cell dimensions.
Using a newly developed multiple scattering scheme, we calculate band structure and transmission properties for acoustic waves propagating in bubbly water. We prove that the multiple scattering effects are responsible for the creation of wide gaps in the transmission even in the presence of strong positional and size disorder.
Acoustic tweezers powerfully enable the contactless collective or selective manipulation of microscopic objects. Trapping is achieved without pretagging, with forces several orders of magnitude larger than optical tweezers at the same input power, limiting spurious heating and enabling damage-free displacement and orientation of biological samples. In addition, the availability of acoustical coherent sources from kilo- to gigahertz frequencies enables the manipulation of a wide spectrum of particle sizes. After an introduction of the key physical concepts behind fluid and particle manipulation with acoustic radiation pressure and acoustic streaming, we highlight the emergence of specific wave fields, called acoustical vortices, as a means to manipulate particles selectively and in three dimensions with one-sided tweezers. These acoustic vortices can also be used to generate hydrodynamic vortices whose topology is controlled by the topology of the wave. We conclude with an outlook on the field's future directions.
Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 52 is January 5, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Due to their easy, fast, and noninvasive natures, acoustic wave methods have been widely used in the physical property measurements of liquids, such as shear waves for viscosity and ultrasound for density. We investigate the physical interaction between approximately GHz longitudinal acoustic waves and simple Newtonian fluids in micro- and nanoscales, and report a longitudinal-acoustic-wave microsensor for microfluidic bulk-modulus measurement. We also verify that the microsensor is insensitive to viscosity, in contrast to shear-wave sensors commonly used for viscosity measurements. With its small size and nanometer wavelengths, the microsensor in nonreflective regimes can be integrated into microfluidics and can make fast measurements even under harsh conditions, such as in limited (15∼200 µm) channel spaces and sample volumes (approximately µl). The longitudinal-acoustic-waves measuring theory is demonstrated by the Mason model, a finite element analytical model, and experiments on glycerol-water mixture samples. The microsensor is originally designed for microfluidic use, nevertheless, it can be used under diverse conditions in which miniaturization matters, namely, in biomedical, chemistry, industry, environmental, healthcare, energy applications, and so on.
In article number 1800068, Xuexin Duan, Wei Pang and co‐workers develop hypersonic‐induced hydrodynamic tweezers (HSHTs) and demonstrate the capability for multifunctional manipulations of micro/nanoparticles including online trapping, on‐situ rotation, queuing, and continuous sorting. Benefitting from the advantages of the hydrodynamic approach, the HSHTs own an excellent biocompatibility and enable wide applications of bio‐objects operations ranging from vesicles, cells to various kinds of microorganisms.
We report the nonlinear acoustic streaming effect and the fast manipulation of microparticles by microelectromechanical Lamb-wave resonators in a microliter droplet. The device, consisting of four Lamb-wave resonators on a silicon die, generates cylindrical traveling waves in a liquid and efficiently drives nine horizontal vortices within a 1−μl droplet; the performance of the device coincides with the numerical model prediction. Experimentally, the particles are enriched at the stagnation center of the main vortex on the free surface of the droplet in open space without microfluidic channels. In addition, the trajectories of the particles in the droplet can be controlled by the excitation power.
We developed a highly sensitive electrochemical detector with a micro-stirrer driven by surface acoustic waves (SAWs) for micro total analysis systems. The detector, fabricated on a LiNbO3 wafer by microfabrication technology, consisted of a three-electrode system for electrochemical measurement and interdigital transducers (IDTs) for stirring the aqueous solution by SAWs. In an amperometry experiment with ferrocenecarboxylic acid and potassium phosphate buffer solution (total volume: 50 μL), the oxidation current was measured while the solution was stirred by SAWs generated by applying 1.4 W electric power to the IDTs. Under this condition, the oxidation current was about 13 times larger than that without SAW-stirring. Furthermore, the diffusion rate of analyte molecules evaluated by the Cottrell relation from amperometry measurements increased 12-fold by using the SAW micro-stirrer.
Carbonaceous materials are extensively applied to the development of electrochemical devices for electroanalysis, energy conversion, and storage or electrocatalysis because of their rather unique features that include high chemical stability, low cost, and wide potential window. Furthermore, they are processed into thick or thin-film structures onto different substrates for producing electrode devices whose widespread use, however, is hampered by the film composition and the poor mechanical stability. Here the application of carbon/silica composites is shown to produce miniaturized thin-film electrodes, which stand out for their robustness and can thus be used repeatedly for electrochemical analysis. Resorcinol-formaldehyde/(3-aminopropyl)triethoxysilane sol–gel films on silicon substrates are subjected to several processes that combine lithography, etching, and pyrolysis steps to yield carbon/silica thin-film electrodes, at wafer scale. The presence of silica in the electrode material promotes strong interfacial adhesion to the substrate, dramatically increasing the number of measurements a single electrode can withstand without loss of performance. The thorough electroanalytical characterization by cyclic voltammetry in solutions containing representative redox species reveals an electrode performance comparable to that of glassy carbon for outer-sphere redox processes. In addition, the modification of the electrode surface with metal nanoparticles allows for the sensitive detection of heavy metal ion pollutants.
On-chip integrating several functional components for developing integrated lab-on-a-chip microsystem remains as a challenge. In this work, by employing multiple microelectromechanical resonators both as actuators and sensors, on-chip heating, mixing and chemical reaction monitoring are successfully demonstrated. Mechanism studies using COMSOL simulations indicate that the local heating and mixing are induced by the acoustic wave attenuation during its transmission in liquid. On-line chemical reaction monitoring is realized by viscosity sensing using the same resonator through impedance analysis. Classic Diels-Alder reaction in a single droplet was performed to verify the feasibility of using such microsystem for mixing, heating and online reaction monitoring at microscale.
Electrochemistry, biosensors and microfluidics are popular research topics that have attracted widespread attention from chemists, biologists, physicists, and engineers. Here, we introduce the basic concepts and recent histories of electrochemistry, biosensors, and microfluidics, and describe how they are combining to form new application-areas, including so-called "point-of-care" systems in which measurements traditionally performed in a laboratory are moved into the field. We propose that this review can serve both as a useful starting-point for researchers who are new to these topics, as well as being a compendium of the current state-of-the art for experts in these evolving areas.
Fluid manipulations at the microscale and beyond are powerfully enabled through the use of 10-1,000-MHz acoustic waves. A superior alternative in many cases to other microfluidic actuation techniques, such high-frequency acoustics is almost universally produced by surface acoustic wave devices that employ electromechanical transduction in wafer-scale or thin-film piezoelectric media to generate the kinetic energy needed to transport and manipulate fluids placed in adjacent microfluidic structures. These waves are responsible for a diverse range of complex fluid transport phenomena - from interfacial fluid vibration and drop and confined fluid transport to jetting and atomization - underlying a flourishing research literature spanning fundamental fluid physics to chip-scale engineering applications. We highlight some of this literature to provide the reader with a historical basis, routes for more detailed study, and an impression of the field's future directions.
Theories for calculating steady streaming associated with sound fields are reviewed, comparing the methods and approximations of various authors. Two illustrative problems are worked out, both for rectilinear flow due to irrotational sound fields. The first deals with a single attenuated plane wave traveling down a tube, as in Cady's quartz wind experiments. In the second, a pair of crossed plane waves is treated, giving rise to a quite different kind of streaming. In obtaining solutions, attention is given to boundary conditions; here, gradients of She excess static pressure, another second‐order quantity, come into consideration. Significantly, streaming speeds depend critically upon α, the attenuation constant, where α may be due to any common cause, such as heat conduction, scattering, thermal relaxation, etc. From these results it appears that streaming measurements cannot be used to distinguish between absorption mechanisms. Numerical values are given for a few cases; high flow speeds may be expected in a bubbly medium.
The diffusion of ferrocene methanol in super-cooled aqueous solutions containing sucrose has been studied, using disk and cylindrical microelectrodes, over a wide viscosity range. The solution viscosity and the reduced temperature T/Tg (Tg being the glass transition temperature) were varied by changing the sucrose concentration and the temperature of the system. The voltammetric limiting current obtained with a disk microelectrode and the i(t) response on a cylindrical microelectrode after a potential step were used to determine diffusion coefficients from 7×10−6cm2s−1 down to 2×10−11cm2s−1. The electrochemical procedure described in this work allows a simple and accurate measurement of the dynamics of electroactive solutes in glass-forming liquids.
Microelectrodes (Pt, Au 25 μm, 10 μm diameter and C 9 μm diameter) have been employed to record individual cavitation events produced as a consequence of high intensity ultrasonic irradiation of a solution containing a redox probe. Cavitation events are detected at a microelectrode as a series of current-time transients. The potential dependence of the observed events in relation to the redox chemistry of the particular probe under investigation is demonstrated. Ferrocene and hexaamine ruthenium(III) chloride have been employed as model redox systems. The effects of temperature, solvent, electrode/ultrasonic source separation, position of the electrode with respect to the ultrasonic source and ultrasonic intensity on the frequency and magnitude of the observed cavitation events are demonstrated. The shapes of the current-time transients recorded are analysed with comparison to conventional chronoamperometry at microelectrodes. Mass transport coefficients of up to 1.6 cm s−1 are reported.
Investigations show that continuous ultrasound produces modulated mass transport in sonovoltammetry. At mass‐transport‐limited
potentials, voltammetry in the presence of ultrasound shows near‐steady‐state behavior with large current output that oscillates
about a stable, average value. The current signal consists of a time‐independent component and a time‐dependent component.
As expected for hydrodynamically modulated mass transport both components are proportional to bulk analyte concentration.
We report chronoamperometric determinations of ferrocene during ultrasonic irradiation that have been analyzed using both
the time‐independent and time‐dependent signal components. In both case, the limit of detection was
. This paper contains detailed investigations of the time‐dependent current signal. The effect of electrolyte viscosity on
sonoelectrochemical measurements demonstrates that both current signals arise from convective‐mass‐transport effects. Third‐order,
high‐frequency cutoff filtering of the chronoamperometric signal shows significant attenuation and smoothing of the time‐dependent
signal. To explain our results we propose a qualitative model of convective mass transport in sonovoltammetry, where the time‐independent
current arises primarily from acoustic streaming and the time‐dependent component comes from a combination of field‐induced
fluid motion and cavitational effects.
The surface tension and surface potential of aqueous solutions of inorganic electrolytes are calculated. The new model includes image forces at liquid interfaces, ions of finite radii, the hydrophobic effect, and the Parsons−Zobel effect. For the first time, a good correlation between theory and experimental data both qualitatively and quantitatively is obtained for different inorganic salts over a wide range of concentrations.
Voltammetric and amperometric experiments conducted during continuous sonication with a Ti immersion macrotip (1 cm2 area) positioned parallel to Pt working electrode surfaces are described. The effects of concentration, electrode area, temperature, kinematic viscosity, amplitude of vibration of the Ti sonicator tip, cell geometry and voltage scan rate on limiting currents are considered and compared to analogous measurements made at a rotating disk electrode. It is found that steady-state voltammograms may be obtained during sonication at scan rates up to 25 V/s. Hydrodynamic conditions are described in terms of the effective rotation rate which would be needed in rotating disk voltammetry to achieve similar transport rates. A procedure for performing sonovoltammetry in a cell geometry-independent manner to probe the chemical effects of acoustic cavitation is discussed.
An attempt is made to define theoretically the conditions for the appearance of cavitation in liquids subjected to alternating pressure changes. It is found that cavitation is restricted to a definite range of variations of the following parameters: (i) alternating pressure amplitude, (ii) frequency of the pressure wave, (iii) radius of the bubble nucleus, (iv) hydrostatic pressure. Under certain conditions, the change from non-cavitating to cavitating conditions is found to be exceedingly sharp, and in these cases it is shown that the threshold for cavitation can be accurately expressed in very simple terms.
An overview of possible mechanisms by which sonication can influence electrochemical processes is given. Four mechanisms are discussed: – acoustic streaming; – microstreaming and turbulence due to cavitation; – formation of microjets in the course of collapse of cavitation bubble; – shock waves; and possible effects are illustrated on several examples. The most effective process is formation of microjets,which can not only decrease diffusion layer thickness under 1 lm, but also activate (depassivate) electrode surface. Design of experimental arrangement with maximum participation of microjets is proposed. Two approaches are proposed: – focusing of ultrasound on the working electrode and reduction of energy losses by over-pressure; – ‘‘tuning” the reactor to obtain resonance, i.e. formation of stationary waves by activating reactor in itsresonant mode.
Here we demonstrate the use of microstereolithography (MSL), a 3D direct manufacturing technique, as a viable method to produce small-scale microfluidic components for electrochemical flow detection. The flow cell is assembled simply by resting the microfabricated component on the electrode of interest and securing with thread! This configuration allows the use of a wide range of electrode materials. Furthermore, our approach eliminates the need for additional sealing methods, such as adhesives, waxes, and screws, which have previously been deployed. In addition, it removes any issues associated with compression of the cell chamber. MSL allows a reduction of the dimensions of the channel geometry (and the resultant component) and, compared to most previously produced devices, it offers a high degree of flexibility in the design, reduced manufacture time, and high reliability. Importantly, the polymer utilized does not distort so that the cell maintains well-defined geometrical dimensions after assembly. For the studies herein the channel dimensions were 3 mm wide, 3.5 mm long, and 192 or 250 mum high. The channel flow cell dimensions were chosen to ensure that the substrate electrodes experienced laminar flow conditions, even with volume flow rates of up to 64 mL min(-1) (the limit of our pumping system). The steady-state transport-limited current response, for the oxidation of ferrocenylmethyl trimethylammonium hexaflorophosphate (FcTMA(+)), at gold and polycrystalline boron doped diamond (pBDD) band electrodes was in agreement with the Levich equation and/or finite element simulations of mass transport. We believe that this method of creating and using channel flow electrodes offers a wide range of new applications from electroanalysis to electrocatalysis.
Surface Acoustic Wave Enhanced Electroanalytical Sensors
- E Kaplan