Microelectrode Surface Research Articles

Impact of microelectrode geometry and surface finish on enzymatic biosensor performance

Food production, environmental protection, biotechnology, and medicine, all depend on biosensors for measuring specific biological molecules. Among biosensors, enzymatic electrochemical biosensors have received particular attention due to their simple operation and enzymes’ inherent specificity. Researchers have extensively explored novel materials and biological designs to improve performance (i.e., sensitivity, linearity, limit of detection, and response time). However, in comparison, physical and geometrical design has not received the same attention. To this end, we compared platinum (Pt) microelectrodes with circular or fractal geometry with the same surface area (2D geometry). We also studied the effect of 3D geometry by nanostructuring both circular and fractal microelectrodes via electroplating Pt black. Fractal Pt black microelectrodes displayed the highest current density, charge storage capacity, and sensitivity towards H2O2. Next, we immobilized glucose oxidase onto various microelectrode geometries by microcontact stamping. Fractal Pt and Pt black glucose biosensors were 91.7% and 83.3% more sensitive than circular counterparts. Circular and fractal Pt black glucose biosensors were 63.0% and 55.9% more sensitive than Pt counterparts. Fractal geometry also provided better linearity and limit of detection. We modified enzyme layer thickness through multiple layer stamping and found a trade-off whereby increasing thickness increased sensitivity but also increased response time. Lastly, we developed a COMSOL Multiphysics numerical model to interpret the amperometric data and the impact of physical design on critical parameters. The work here will serve as a guideline for improving enzymatic electrochemical and other biosensors via physical design, which is simpler to modify than material or biological design.

Utilizing diffusion tensor imaging as an image biomarker in exploring the therapeutic efficacy of forniceal deep brain stimulation in a mice model of Alzheimer’s disease

Objective.With prolonged life expectancy, the incidence of memory deficits, especially in Alzheimer's disease (AD), has increased. Although multiple treatments have been evaluated, no promising treatment has been found to date. Deep brain stimulation (DBS) of the fornix area was explored as a possible treatment because the fornix is intimately connected to memory-related areas that are vulnerable in AD; however, a proper imaging biomarker for assessing the therapeutic efficiency of forniceal DBS in AD has not been established.Approach.This study assessed the efficacy and safety of DBS by estimating the optimal intersection volume between the volume of tissue activated and the fornix. Utilizing a gold-electroplating process, the microelectrode's surface area on the neural probe was increased, enhancing charge transfer performance within potential water window limits. Bilateral fornix implantation was conducted in triple-transgenic AD mice (3 × Tg-AD) and wild-type mice (strain: B6129SF1/J), with forniceal DBS administered exclusively to 3 × Tg-AD mice in the DBS-on group. Behavioral tasks, diffusion tensor imaging (DTI), and immunohistochemistry (IHC) were performed in all mice to assess the therapeutic efficacy of forniceal DBS.Main results.The results illustrated that memory deficits and increased anxiety-like behavior in 3 × Tg-AD mice were rescued by forniceal DBS. Furthermore, forniceal DBS positively altered DTI indices, such as increasing fractional anisotropy (FA) and decreasing mean diffusivity (MD), together with reducing microglial cell and astrocyte counts, suggesting a potential causal relationship between revised FA/MD and reduced cell counts in the anterior cingulate cortex, hippocampus, fornix, amygdala, and entorhinal cortex of 3 × Tg-AD mice following forniceal DBS.Significance.The efficacy of forniceal DBS in AD can be indicated by alterations in DTI-based biomarkers reflecting the decreased activation of glial cells, suggesting reduced neural inflammation as evidenced by improvements in memory and anxiety-like behavior.

Indirect Voltammetry Detection of Non-Electroactive Neurotransmitters Using Glassy Carbon Microelectrodes: The Case of Glutamate

Glassy carbon (GC) microelectrodes have been successfully used for the detection of electroactive neurotransmitters such as dopamine and serotonin through voltammetry. However, non-electroactive neurotransmitters such as glutamate, lactate, and gamma-aminobutyric acid (GABA) are inherently unsuitable for detection through voltammetry techniques without functionalizing the surface of the microelectrodes. To this end, we present here the immobilization of the L-glutamate oxidase (GluOx) enzyme on the surface of GC microelectrodes to enable the catalysis of a chemical reaction between L-glutamate, oxygen, and water to produce H2O2, an electroactive byproduct that is readily detectable through voltammetry. This immobilization of GluOx on the surface of bare GC microelectrodes and the subsequent catalytic reduction in H2O2 through fast-scan cyclic voltammetry (FSCV) helped demonstrate the indirect in vitro detection of glutamate, a non-electroactive molecule, at concentrations as low as 10 nM. The functionalized microelectrodes formed part of a four-channel array of microelectrodes (30 μm × 60 μm) on a 1.6 cm long neural probe that was supported on a flexible polymer, with potential for in vivo applications. The types and strengths of the bond between the GC microelectrode surface and its functional groups, on one hand, and glutamate and the immobilized functionalization matrix, on the other hand, were investigated through molecular dynamic (MD) modeling and Fourier transform infrared spectroscopy (FTIR). Both MD modeling and FTIR demonstrated the presence of several covalent bonds in the form of C-O (carbon–oxygen polar covalent bond), C=O (carbonyl), C-H (alkenyl), N-H (hydrogen bond), C-N (carbon–nitrogen single bond), and C≡N (triple carbon–nitrogen bond). Further, penetration tests on an agarose hydrogel model confirmed that the probes are mechanically robust, with their penetrating forces being much lower than the fracture force of the probe material.

Insight into the electrolyte concentration impact on single hydrogen bubble evolution dynamics at a microelectrode during electrochemical water splitting

Bubbles generated during electrochemical water splitting could adhere to the electrode surface and therefore impede the reaction. Thus, understanding and manipulating the evolution dynamics of bubbles is crucial for enhancing electrolysis efficiency. In this study, we investigated the evolution mechanism and forces acting on individual hydrogen bubble on Pt microelectrode surface by employing different H2SO4 electrolyte concentrations (0.2–1.0 M) under various applied voltages [−2 to −6 V vs saturated calomel electrode (SCE)]. We focused on bubble detachment diameter, average current, and bubble lifetime and subsequently established relational equations over these variables. At −6 V vs SCE, the growth coefficient has a maximum value of 14.42 × 10−4 m/s0.333 when the concentration of electrolyte is around 0.6 M. Gas production at 0.6 M increased by 63.4% compared to 0.2 M and by 11.2% compared to 1.0 M. Therefore, choosing the appropriate electrolyte concentration can maximize gas production and bubble detachment efficiency. Additionally, a force balance model incorporating the Marangoni force for single bubbles on the microelectrode surface was established across varying concentrations of H2SO4 electrolyte. At −4 V vs SCE, the solutal Marangoni force starts to dominate when electrolyte concentrations above 0.4 M. The results demonstrate the critical role of the solutal Marangoni force beyond a certain value of electrolyte concentration.

Hydrogen bubble evolution and gas transport mechanism on a microelectrode determined by cathodic potential and temperature

Enhancing the efficiency of hydrogen production by optimizing gas product transfer within water electrolysis systems is essential. Employing high-speed photography and electrochemical techniques, the entire process of single hydrogen bubble evolution on a Pt microelectrode surface was measured. Results reveal a notable reduction in both bubble detachment radius and growth time with decreasing absolute potential (from −7 to −3 V) and increasing reaction temperature (from 30 °C to 50 °C). Additionally, a comprehensive model estimating bubble coverage on the microelectrode is presented, incorporating bubble radius and current as key influencing factors. This enables an accurate evaluation of mass transfer coefficients during bubble evolution in the absence of forced flow. Furthermore, findings reveal the dominance of bubble-induced micro-convection as the primary mass-transfer mechanism for gas products at high current densities [O (105–106 A/m2)]. The results also indicate that the mass transfer coefficient increases during the inertia-controlled growth stage of bubbles and decreases during the stage controlled by chemical reactions.

Scanning Bubble Electrochemical Microscopy: Mapping of Electrocatalytic Activity with Low-Solubility Reactants.

Determining electrocatalytic activity for reactions that involve reactants with limited solubility presents a significant challenge due to the reduced mass-transport to the electrocatalyst surface. This limitation is seen in important reactions such as the oxygen reduction reaction, nitrogen reduction reaction, and carbon dioxide reduction reaction. We introduce a new approach, which we call scanning bubble electrochemical microscopy, to enable the detection and high-resolution mapping of electrocatalytic activity with low-solubility reactants at high mass-transport rates. Using a scanning probe approach, a dual-barreled nanopipette is used to precisely deliver the gas-phase reactant within micrometers of an electrocatalyst surface, which results in high mass-transport rates at the electrocatalyst surface directly under the probe. We demonstrate the scanning bubble electrochemical microscopy approach by mapping the oxygen reduction reaction on model platinum microelectrode surfaces. We anticipate that scanning bubble electrochemical microscopy will provide an effective tool for measuring the electrocatalytic activity of reactants that have limited solubility.

Real-time monitoring abscisic acid release from single rice protoplast by amperometry at microelectrodes modified with abscisic acid receptor PYL2

It was previously reported that stress induces a cellular production of abscisic acid in plants, but no direct method shows the evidence. Here, an electrochemical microsensor involving an abscisic acid receptor PYL2 modified carbon fiber microelectrode was fabricated by self-assembly method, where the Cu2+ combined with the histidine tag of PYL2 on the surface of microelectrode was used as the detection probe, the mediated reaction between Cu+ and ferricyanide realized the amplification responses and provided the microsensor with a high sensitivity for detection of abscisic acid with a detection limit of 0.8 nM. With use of this microsensor, an increase of extracellular abscisic acid from single rice protoplast induced by sulfate, osmotic and salinity stress was real-time monitored. Direct measurement of free extracellular abscisic acid in single plant cells might offer important new insights into its role in plants challenged by abiotic stresses.

Monolayer of microgel particles based on poly(acrylic acid) modified with dopamine for electrochemical pH sensing

A novel electroactive and pH-responsive microgel was successfully synthesized. The obtained microgel was based on acrylic acid, dopamine methacrylamide and N,N′-bis(acryloyl)cystamine as the crosslinker. Using the distillation precipitation polymerization method, spherical, smooth-surface microgel particles of ca. 400 nm in diameter were obtained. The presence of dopamine groups in the polymer network was confirmed with cyclic voltammetry and NMR spectroscopy. The dependence of microgel hydrodynamic diameter on pH was investigated using the DLS technique. The presence of -S–S- groups in the microgel particle was used to modify the Au electrode surface by chemisorption. The attachment was monitored with the QCM-D technique. The obtained microgel was successfully anchored to the Au regular- and microelectrode surfaces. The electrochemical properties of the modified electrodes were promising. It was found that modified electrodes exhibited potentially useful pH-sensing properties. Modification of Au microelectrode surface with the environmentally sensitive and electroactive microgel monolayer is a novel, innovative approach.

One-step prepared of e-Gelatin-CNTs nano-composite fiber modified carbon fiber microelectrode for continuous dopamine monitoring: Characterization, biocompatibility evaluation and in vivo experiment

Fabricating electrochemical sensor capable of stable sensing in complex organism's environment will undoubtedly underpin key in the development of implanted electrochemical sensor. However, nonspecific protein adsorption can greatly reduce sensing sensitivity of electrode due to the abundance of protein in vivo. Herein, we aim to fabricating a modified layer of good protein adsorption resistance. The nano-composite fibers from gelatin with good hydrophily and multiwall carbon nanotubes (CNTs) for constructing on the surface of carbon fiber microelectrode using one-step electrospinning technology (e-Gelatin-CNTs/CFME). A comprehensive discussing on the protein adsorption resistance of e-Gelatin-CNTs/CFME that are, through the use of SEM, AFM, water contact angle, UV–vis, cyclic voltammetry and I-T. The resulting electrochemical sensor revealed stable sensing sensitivity in protein solution, a slightly decrease (0.02nA/μM). Furthermore, the sensor was successfully used for the detection of DA in human blood. More remarkable, e-Gelatin-CNTs nano-composite fibers presented excellent biocompatibility, which could meet the biosafety as implantation materials in vivo. This work provided a platform for the development of implantable electrochemical sensor.

Measuring Liquid-into-Liquid Diffusion Coefficients by Dissolving Microdroplet Electroanalysis.

Diffusion is a fundamental process in various domains, such as pollution control, drug delivery, and isotope separation. Accurately measuring the diffusion coefficients (D) of one liquid into another often encounters challenges stemming from intermolecular interactions, precise observations at the liquid interface, convection, etc. Here, we present an innovative electrochemical methodology for determining the diffusion coefficient of a liquid into another liquid. The method involves precisely tracking the lifetime of a nonaqueous droplet. An organic droplet is placed on an ultramicroelectrode surrounded by an aqueous solution of potassium hexacyanoferrate(II/III). The droplet initially blocks the reduction or oxidation of the redox species. As the droplet dissolves, giving access to the conductive microelectrode surface, a continuously increasing current is observed in voltammetry and the amperometric i-t response. The electrochemical response thus directly reports on the flux of redox species on the electrode surface, allowing us to precisely determine the lifetime of the droplet. D values are directly determined through a combination of electrochemical analysis and the principles of droplet dissolution. We demonstrate the quantification of 1,2-dichloroethane and nitrobenzene into water, yielding diffusion coefficients of (11.3 ± 1.2) × 10-6 cm2/s and (5.2 ± 1.1) × 10-6 cm2/s, respectively. This work establishes a reliable electrochemical approach for quantifying diffusion coefficients based on droplet lifetime analysis.

Rational design of Au NPs satellite nanostructure modified microelectrode for dual-mode detection of mercury ions in algal solution

Developing a microelectrode with in-situ multiple sensing modes will greatly expand the applicability and reliability of the analytical platform. Hence, a functional substrate with a satellite-like nanostructure was constructed on a gold microelectrode (Au ME). This structure not only amplifies the surface-enhanced Raman scattering (SERS) signal of the labeled molecule (4-nitrobenzenthiol, NBT) on the functionalized Au NPs, but also ensures the good electrical properties of the proposed functionalized microelectrode (Au ME/Au-AuNDNA). Based on this feature, the highly toxic heavy metal mercury ions (Hg2+) were used as a model to verify this dual modal analytical platform. Significantly, the proposed microelectrode has a satisfactory sensitivity to Hg2+and allows the simultaneous presentation of analytical results by dual-mode of SERS signal turn-off (LOD 2.7 × 10−11 M) and current signal turn-on (LOD 6.4 × 10−9 M), the detected concentration of Hg2+ is below the dangerous limit of World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA). This relies mainly on the formation of thymine-Hg2+-thymine (T-Hg2+-T) by the modified probe B ssDNA on the microelectrode to disrupt the satellite structure with AuNDNA, while the applied potential reduces the Hg2+ captured by T-Hg2+-T on the microelectrode surface. Such a fascinating analytical strategy was further evaluated in an environmental water sample simulated with algal solution and still showed a reliable analytical capability. Thus, the proposed dual-mode sensing concept not only opens new avenues for the coordinated application of novel electrochemical and SERS sensing techniques, but also provides a foundation for in-situ detection based on micro/nanoscale electrodes.

Self-assembly method of glucose oxidase in a fully packaged microfluidic glucose biosensor

In this work, a self-assembly approach is proposed to bio-functionalize the fully packaged microfluidic biosensors for catalyzing glucose based on the designed and fabricated microfluidic chip with suspending structure by micro-electro-mechanical system technology. Glucose oxidase is successfully immobilized on the surface of microelectrodes to build composite layers by the proposed self-assembly approach, with cysteamine applied as a functional linker. The biological activity and stability of glucose oxidase is optimized by the electrodeposition of chitosan membrane. Experiments results suggest that the fully packaged microfluidic biosensor integrated with self-assembled glucose oxidase can effectively detect the existence of glucose in sample solution. The detection range of glucose concentration is 0–10 mM. With SNR of 3, the detection limit is 51.2 µM and sensitivity is 23.47 ± 1.36 µA cm−2 mM−1 within low concentration range of 0–0.1 mM, while the detection limit is 497 µM and the sensitivity is 10.63 ± 1.28 µA cm−2 mM−1 among high concentration range of 0.1–10 mM. The proposed study provides a straightforward method to construct a microfluidic biosensor, which is promising in miniaturized clinical medical device.

Surface integrity of tungsten carbide microelectrodes fabricated using reverse micro-EDM by varying electrode materials

Recently, there has been a notable trend towards miniaturizing components across diverse industries. The reverse micro electrical discharge machining (reverse micro-EDM) process has gained prominence for microelectrodes array fabrication. However, this process often induces undesirable surface attributes on microelectrodes, including rough topography, elevated residual tensile stresses, increased surface roughness, microcracks, recast layers, and heat-affected zones (HAZ). Consequently, assessing the influence of tool materials on microelectrode surface integrity becomes crucial. This study systematically investigates the effect of distinct tool materials on the surface roughness of microelectrodes fabricated using reverse micro-EDM. Three tool materials were considered: copper, brass C38500, and aluminum 3003. The results robustly establish copper as the most efficient tool material for reverse micro-EDM applications, showcasing superior productivity and performance. Employing copper as the tool material under specific conditions yielded a remarkable achievement, a minimal surface roughness of 0.38 µm on tungsten carbide microelectrodes at 90 V, 10nF, and 5 µm/sec. These findings underscore the potential of copper as a promising tool material for fabricating microelectrodes via reverse micro-EDM.

Electrode surfaces based on multiwall carbon nanotubes-chitosan composites validated in the detection of homocysteine biomarkers for cardiovascular disease risk monitoring

Abstract This study aimed to modify screen-printed carbon micro-electrode surfaces by coating them with multiwall carbon-based nanotubes conjugated with chitosan and then validated the formed multiwall carbon-based nanotubes-chitosan coated screen printed carbon micro-electrode for the detection of homocysteine, a biomarker analyte known as a risk indicator in cardiovascular disease. The microstructure surface and crystallographic structure stability of the formed multiwall carbon-based nanotubes-chitosan obtained at formed multiwall carbon-based nanotubes per chitosan ratios of 1:1, 2:1, 3:1, and 4:1 were examined via field emission scanning electron microscopy, X-ray radiation, Raman spectroscopy, surface area and pore size, and thermogravimetric analyses. Homocysteine solutions at 30–100 µM were measured by cyclic voltammetry using the different formed multiwall carbon-based nanotubes-chitosan compositions as sensor electrodes. That with an optimal formed multiwall carbon-based nanotubes per chitosan ratio of 4:1 showed the highest crystallinity and electrical conductivity and gave a high coefficient of determination (R2 = 0.9036) between the homocysteine concentration and the oxidation current detection over an operating range of 30–100 µM. This new composite microelectrode for detecting homocysteine concentration makes it a promising candidate for clinical applications.

Preparation of boron-doped diamond microelectrodes to determine the distribution size of platinum nanoparticles using current transient method

Boron-doped diamond (BDD) microelectrodes were prepared to investigate the correlation of hydrazine oxidation current responses with Pt nanoparticle (Pt NP) size distribution. The BDD film was grown on the surface of a tungsten needle with a diameter of 25 µm. An average particle size of around 5 µm BDD crystalline was successfully synthesized using a microwave plasma-assisted chemical vapor deposition technique. The Raman spectrum confirmed the presence of diamond formation as indicated by peaks corresponding to C-C sp3 bonds, while X-ray photoelectron spectroscopy spectrum showed the presence of C-H and C-OH bonds on the surface of the BDD microelectrode. Meanwhile the Pt nanoparticles was synthesized through reduction reaction of PtCl62- solution using NaBH4 with citric acid as the capping agent. Particles size between 4.46 to 4.78 nm were observed by using TEM measurements. The BDD microelectrodes were utilized to investigate the relationship between Pt nanoparticle size distribution and the current generated from the oxidation reaction of 15 mM hydrazine in a 50 mM phosphate buffer solution pH 7.4 in the presence of 1.0 mL nanoparticle solutions. A current range of 5 and 6 nA with a noise level of 0.15 nA was observed showing a good correlation with the particle sizes of Pt NPs. Comparison was also performed with the measurements using Au microelectrodes, indicated that the prepared BDD microelectrodes is promising for the measurements of nanoparticle sizes distribution, especially Pt NPs.

Double deposition/accumulation and double stripping steps system as a new way of decreasing the detection limit and elimination of interferences in stripping voltammetry of inorganic ions: A review

AbstractThe article reviews an application of double deposition/accumulation and stripping steps as a new and beneficial manner of decreasing a detection limit and/or interferences elimination in stripping voltammetric determination of inorganic ions: Pb(II), Cd(II), Tl(I), U(VI), Au(III), As(III), Se(IV), In(III), Co(II), Cu(II), Bi(III) and Ga(III). A simple design of four‐electrodes system, containing two working electrodes differing in their surface area, allows for conducting two preconcentration and two stripping steps in one measurement cycle and in one voltammetric cell. Considerable analytes preconcentration and, consequently, an increase of sensitivity of determinations was obtained by conducting the first stripping step in a small volume of a solution near the microelectrode surface. An appropriate selection of the conditions of the first deposition/accumulation and the first stripping steps allowed for a minimization/elimination of interference effects. The proposed procedures allows for ultra‐trace levels determinations of mentioned inorganic ions (often impossible to determine by means of traditional three‐electrodes systems) without additional preconcentration outside the electrochemical cell and with the use of mercury‐free electrodes.

Label free impedance based acetylcholinesterase enzymatic biosensors for the detection of acetylcholine

Realtime monitoring of neurotransmitters is of great interest for understanding their fundamental role in a wide range of biological processes in the central and peripheral nervous system, as well as their role, in several degenerative brain diseases. The measurement of acetylcholine in the brain is particularly challenging due to the complex environment of the brain and the low concentration and short lifetime of acetylcholine. In this paper, we demonstrated a novel, label-free biosensor for the detection of Ach using a single enzyme, acetylcholinesterase (ACHE), and electrochemical impedance spectroscopy (EIS). Acetylcholinesterase was covalently immobilized onto the surface of gold microelectrodes through an amine-reactive crosslinker dithiobis(succinimidyl propionate) (DSP). Passivation of the gold electrode with SuperBlock eliminated or reduced any non-specific response to other major interfering neurotransmitter molecules such as dopamine (DA), norepinephrine (NE) and epinephrine (EH). The sensors were able to detect acetylcholine over a wide concentration range (5.5–550 μM) in sample volumes as small as 300 μL by applying a 10 mV AC voltage at a frequency of 500 Hz. The sensors showed a linear relationship between Ach concentration and ΔZmod(R2 = 0.99) in PBS. The sensor responded to acetylcholine not only when evaluated in a simple buffer (PBS buffer) but in several more complex environments such as rat brain slurry and rat whole blood. The sensor remained responsive to acetylcholine after being implanted ex vivo in rat brain tissue. These results bode well for the future application of these novel sensors for real time in vivo monitoring of acetylcholine.

Advanced Biomimetic Nanostructured Microelectrode Arrays for Enhanced Extracellular Recordings of Enteric Neurons

AbstractMicroelectrode surfaces covered with nanostructures derived from components of extracellular matrix, such as collagen fibers, have shown immense beneficial effects in promoting neuronal growth and cellular signaling. Synthetic nanostructures mimicking the features of biological nanostructures with durable conductive materials could promote the cell adhesion on microelectrode surfaces by providing topographical cues and simultaneously improve the charge transfer properties by reducing its global impedance. Therefore, an advanced nanostructuring method mimicking the structural and organizational features of natural collagen fibers onto metallic microelectrode surfaces has been presented here, which is adapted from previous technological achievements of the group and is based on nanoimprint lithography and gold electroplating. Surface characterization methods reveal an increase in surface area between 20% and 68% on the microelectrodes fabricated with two different nanostructure heights. Impedance spectroscopy measurements reveal reduction in impedance magnitude (at 1 kHz) between 22% and 41%, depending upon the nanostructure height and density on the microelectrode, which should subsequently modulate its charge transfer properties for biosensing application. Cell adhesion analysis performed with seal impedance measurements reveals a tighter coupling of enteric neurons on the nanostructured microelectrodes. Finally, extracellular recordings from enteric neurons exhibit a significant improvement in spike detection properties of the nanostructured microelectrodes.

Iridium Oxides Based Potentiometric Sensor for pH Monitoring in Biological Samples

The fabrication and long-term application of a pH Au microelectrode based on an iridium oxide film are reported. A uniform iridium oxide film with a typical thickness of around 1 µm was coated on the microelectrode surface through a 2-step procedure involving electrodeposition at constant potential and further continuous voltammetric scans. A super-Nernstian slope of around 77 mV per pH unit was found from open circuit potential measurements in a broad pH range (2 to 10) in 0.01 mol L-1 phosphate buffer. It was demonstrated experimentally that the short-term pH precision of the IrOx sensor is ± 0.1 pH. The response stability was maintained in the physiological pH range, and the sensor exhibited excellent reproducibility, long-term stability, and a short response time of < 10 s. The results reported in this work confirmed that iridium oxide showed very promising pH sensing performance and can serve as an electrode material for detecting local pH changes in samples of increased complexity, such as juice fruits, culture medium, synthetic urine, and blood.

Dynamics of growth and detachment of single hydrogen bubble on horizontal and vertical microelectrode surfaces considering liquid microlayer structure in water electrolysis

The dynamics of the growth and detachment of a single hydrogen bubble on both the horizontal and vertical microelectrode surfaces in water electrolysis were synthetically investigated by combining the numerical simulation, force balance analysis, and available experimental data. Approximately, multiple steady simulation cases with different bubble diameters for different growth instances were conducted to state the actual unsteady bubble growth and detachment behavior. The numerical simulations of the temperature distribution considering the heat transfer caused by the liquid microlayer and induced Marangoni convection effects were performed. Then, a force balance model for predicting the bubble detachment diameter was developed by fully utilizing the simulated multi-physical field parameters and the experimental results of some key bubble geometric parameters. The presented numerical model and the force balance model were validated by comparing them with previous experimental data on the potential and the bubble detachment diameter, respectively. The simulation results indicate a significantly larger potential value occurs within the microlayer, and hence, the Joule heat of the electrolyte is mainly generated in the microlayer and then transferred to the bulk flow region. Obviously, the temperature gradient distribution is formed at the bubble interface, causing unstable Marangoni convection structure. The distribution patterns and evolutions of the electrolyte temperature, Marangoni convection velocity, and microlayer thickness for the horizontal and vertical microelectrode systems are significantly different. The present force balance model presents higher prediction accuracy for the bubble detachment diameters. Moreover, the in-depth force analysis results reveal that some dominant forces influence the bubble growth and detachment.