Ion concentration polarization near microchannel-nanochannel interfaces: effect of pH value

This study investigates the effect of the pH value on the ion concentration polarization phenomenon and the nonlinear current-voltage characteristics of a hybrid soda-lime glass micro/nanochannel for a constant KCl salt concentration of about 1 mM. The experimental results show that the electrical conductance of the nanochannel in the Ohmic regime and the critical threshold voltage of the limiting current are both dependent on the pH value of the salt solution when the electrical double layer thickness is considerable in the nanochannel. Specifically, the nanochannel conductance increases and the critical threshold voltage for the limiting current decreases as the pH value is increased. It also suggests that a higher pH value induces a higher surface charge density on the nanochannel walls, and therefore increases both the ionic conductance and the counter-ion flux within the nanochannel.     

Theory of electrode polarization: application to parallel plate cell dielectric spectroscopy experiments

An extension of the model for electrode polarization of Cirkel et al. [Physica A 235 (1997) 269] is given. The problem is solved using both classical boundary conditions and the new boundary conditions using excess densities presented in a previous paper [J. Phys. Chem. B 105 (2001) 11743]. In the present paper, the electrodes are supposed to be ideal, meaning that charge transfer or adsorption are not considered. The advantage of the new boundary conditions lies in the possibility to extend to more complicated situations including for instance specific ion adsorption. We prove that the new boundary conditions and classical ones give the same results. A comparison of the model predictions, involving no adjustable parameters, experimental dielectric spectroscopy data is performed and fairly good agreement is found.     

Cathodic polarization behavior of self-supporting polyaniline film electrode

The preparation method of a self-supporting doped-polyaniline film electrode and its open-circuit potential (OCP) in NaClO₄ and Na₂SO₄ solutions with different pH value as well as cathodic polarization behavior have been investigated for the purpose of discussing the corrosion electrochemical behavior of polyaniline (PANI) in the acid solution. X-ray photoelectron spectroscopy (XPS) reveals that the lower pH corresponds to higher doping level of H⁺ in the film and a more positive OCP of PANI film electrode. OCP of the PANI film reached 0.35 V vs. SCE in 1M H₂SO₄, which is more positive than that of most metals, suggests that PANI would act as cathode when it couples with these metals. The cathodic polarization experiments indicate that the dominating cathodic polarization process of PANI is reversible doping and dedoping reaction and the reduction of dissolve oxygen has very little contribution to it. The potentiostatic current-time curves exhibit a large transient current density at initial stage of polarization, which should be attributed to the charge stored in the film and a relative less steady state current density at the subsequent stage of polarization, which is provided by its doping/dedoping equilibrium activity. Such a current characteristic of PANI electrode might be the force of PANI to provide the passivation protection for some active-passive metals. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 10, pp. 1205–1212. The text was submitted by the authors in English.     

Effects of microchannel geometry on preconcentration intensity in microfluidic chips with straight or convergent-divergent microchannels

Preconcentration microfluidic devices are fabricated incorporating straight or convergent-divergent microchannels and hydrogel or Nafion membranes. Sample preconcentration is achieved utilizing concentration-polarization effects. The effects of the microchannel geometry on the preconcentration intensity are systematically examined. It is shown that for the preconcentrator with the straight microchannel, the time required to achieve a satisfactory preconcentration intensity increases with an increasing channel depth. For the convergent-divergent microchannel, the preconcentration intensity increases with a reducing convergent channel width. Comparing the preconcentration performance of the two different microchannel configurations, it is found that for an equivalent width of the main microchannel, the concentration effect in the convergent-divergent microchannel is faster than that in the straight microchannel.     

Characteristics of morphology,structure and composition of titanium surface under its modification by electrochemical polarization in phosphate‐alkaline solutions

The modification of titanium surface under electrochemical polarization (EP) in the phosphate‐alkaline solutions has been studied using the methods of X‐ray diffraction, electron probe microanalysis, atomic force microscopy, X‐ray photoelectron spectroscopy (XPS), cyclic voltammetry and spectroscopic ellipsometry. It is shown that the morphological parameters of the surface, e.g. roughness and stringiness, as well as its structural‐chemical characteristics, e.g. preferred orientation, size and habit of crystallites, titanium chemical forms, thickness and phase composition of oxide film are generally dependent on the polarization potential. The characteristics of titanium surface modified at low anodic potentials 500, 750 and 1000 mV and 10‐min polarization time have been measured. The processes of Ti surface dissolution and etching along grain boundaries are found to be most intensive at 750 mV. Under 500 mV, these processes are poorly developed yet, while at 1000 mV, the surface passivating film formation limits the previous processes. Despite relatively low polarization potentials (1 V), the surficial oxide films have sufficient thickness (up to ~20 nm) and a specific multilayer structure of variable composition and oxidation state of titanium. The data obtained allow to assert that EP represents an effective tool for morphological and a structural chemical modification of a titanium surface. Copyright © 2015 John Wiley & Sons, Ltd.     

A simple approach for fabrication of dual-disk electrodes with a nanometer-radius electrode and a micrometer-radius electrode

We developed a new simple approach to fabricate dual-disk electrodes with a nanometer-radius electrode and a micrometer-radius electrode. First, nanometer-sized electrodes and micrometer-sized electrodes were constructed using 10-μm-radius metal wires, respectively. To fabricate the nanometer-sized electrode, after the apex of the 10-μm-radius metal wire was electrochemically etched to an ultrafine point with a nanometer-radius, the metal wire was electrochemically coated with a phenol-allyphenol copolymer film. The micrometer-sized electrode was fabricated by directly electrochemical coating the metal wire with an extremely thin phenol-allyphenol copolymer film. Then, the nanometer-radius electrode (the first electrode) and the 10-μm-radius electrode (the second electrode) were inserted into two sides of a thick-septum borosilicate theta (θ) tubing, respectively. The second electrode protruded from the top of the θ tubing. The top of the θ tubing was sealed with insulating ethyl α-cyanoacrylate. The top of the θ tubing with both electrodes was ground flat and polished successively with fine sandpaper and aluminum oxide powder until the tip of the first electrode was exposed. Since the second electrode protruded from the top of the θ tubing, its 10-μm-radius tip was naturally formed during polishing. The dual-disk electrodes were characterized by scanning electron microscopy and cyclic voltammetry. The success rate for fabrication of the dual-disk electrodes is ∼80% due to double insurance from two coating layers of different polymers.     

Influence of the cathodic polarization on the stability of Ti surface in concentrated KOH solutions

The stability of spontaneous thin layers and thin layers formed upon cathodical polarization of Ti in KOH solutions have been studied by potentiostatic and ellipsometric methods. At open circuit potential (OCP) the strongly adherent films, whose thickness depends on the concentration of the KOH solution, were formed. During the cathodic polarization the transformation of these films to weakly adsorbed precipitated layers on the electrode surface was observed. Comparing the theoretically computed curves with the experimental Ψ vs Δ loci measured ellipsometrically, the complex indices of refraction and the thickness of the generated films, from 3.6 to 60 nm in 1 M KOH and from 36 to 105 nm in 5 M KOH (adherent to the electrode surface), were determined. At OCP the rate of film growth increases with increasing the concentration of KOH solution. Cathodic polarizations change the chemical composition and retard the rate of film growth. Based on the ellipsometric and electrochemical data the chemical compositions of the formed films consisted of TiO₂, Ti₂O₃, TiO₂·H₂O, Ti(OH)₃ and TiOOH·nH₂O.     

Effect of annealing on electrical responses of electrode and surface layer in giant-permittivity CuO ceramic

The effect of heat treatments on the electrical responses of the electrode and surface layer in a giant-permittivity CuO ceramic is investigated. It is found that the giant low-frequency relative permittivity of the CuO ceramic can be tuned by annealing in Ar and O₂—it can be reduced by annealing in Ar, and then it can be enhanced up to the initial value by annealing in O₂. The results indicate to the effect of oxygen vacancy concentration on the giant dielectric properties of the CuO ceramic. Interestingly, three sets of dielectric relaxations are observed in the O₂–annealed sample, which can be assigned as the effects of outmost surface layer, electrode, and grain boundary. Our results reveal that the giant low-frequency dielectric response in the CuO ceramic is associated with both of the interfacial polarization at the sample–electrode interface resulted from a non-Ohmic electrode contact and the outmost surface layer-inner part interface.     

Effects of surface interactions on peptide aggregate morphology

The formation of peptide aggregates mediated by an attractive surface is investigated using replica exchange molecular dynamics simulations with a coarse-grained peptide representation. In the absence of a surface, the peptides exhibit a range of aggregate morphologies, including amorphous aggregates, β-barrels and multi-layered fibrils, depending on the chiral stiffness of the chain (a measure of its β-sheet propensity). In contrast, aggregate morphology in the presence of an attractive surface depends more on surface attraction than on peptide chain stiffness, with the surface favoring fibrillar structures. Peptide-peptide interactions couple to peptide-surface interactions cooperatively to affect the assembly process both qualitatively (in terms of aggregate morphology) and quantitatively (in terms of transition temperature and transition sharpness). The frequency of ordered fibrillar aggregates, the surface binding transition temperature, and the sharpness of the binding transition all increase with both surface attraction and chain stiffness.     

Microfluidic flow focusing: drop size and scaling in pressure versus flow-rate-driven pumping

We experimentally study the production of micrometer-sized droplets using microfluidic technology and a flow-focusing geometry. Two distinct methods of flow control are compared: (i) control of the flow rates of the two phases and (ii) control of the inlet pressures of the two phases. In each type of experiment, the drop size l, velocity U and production frequency f are measured and compared as either functions of the flow-rate ratio or the inlet pressure ratio. The minimum drop size in each experiment is on the order of the flow focusing contraction width a. The variation in drop size as the flow control parameters are varied is significantly different between the flow-rate and inlet pressure controlled experiments.     

Electron transfer of quinone self-assembled monolayers on a gold electrode

Dialkyl disulfide-linked naphthoquinone, (NQ-C_n-S)₂, and anthraquinone, (AQ-C_n-S)₂, derivatives with different spacer alkyl chains (C_n: n = 2, 6, 12) were synthesized and these quinone derivatives were self-assembled on a gold electrode. The formation of self-assembled monolayers (SAMs) of these derivatives on a gold electrode was confirmed by infrared reflection-absorption spectroscopy (IR-RAS). Electron transfer between the derivatives and the gold electrode was studied by cyclic voltammetry. On the cyclic voltammogram a reversible redox reaction between quinone (Q) and hydroquinone (QH₂) was clearly observed under an aqueous condition. The formal potentials for NQ and AQ derivatives were −0.48 and −0.58 V, respectively, that did not depend on the spacer length. The oxidation and reduction peak currents were strongly dependent on the spacer alkyl chain length. The redox behavior of quinone derivatives depended on the pH condition of the buffer solution. The pH dependence was in agreement with a theoretical value of E_1/2 (mV) = E′ − 59pH for 2H⁺/2e⁻ process in the pH range 3–11. In the range higher than pH 11, the value was estimated with E_1/2 (mV) = E′ − 30pH , which may correspond to H⁺/2e⁻ process. The tunneling barrier coefficients (β) for NQ and AQ SAMs were determined to be 0.12 and 0.73 per methylene group (CH₂), respectively. Comparison of the structures and the alkyl chain length of quinones derivatives on these electron transfers on the electrode is made.     

Physicochemical properties,surface morphology and particle size distribution of precipitated silicas

The results presented are from a study on the production of highly dispersed silicas by their precipitation from solutions of sodium metasilicate and ammonium salts. The applied ammonium salts were NH₄HCO₃ and (NH₄)₂CO₃. The precipitation process was conducted in the absence or presence of 1 wt.% non‐ionic surfactant (Rokanol K‐7, Rokafenol N‐6 or Rokafenol N‐9). The highly dispersed silicas obtained were subjected to a physicochemical analysis typical for fillers. The analysis included estimation of the bulk density and the capacities to absorb water, dibutyl phthalate and paraffin oil. Particle size and morphology were evaluated using scanning electron microscopy. In addition, particle size distribution for the precipitated silicas was examined by dynamic light scattering. Silicas with the best physicochemical properties were obtained by precipitation using ammonium hydrogen carbonate. In particular, the silica precipitated in the presence of 1 wt.% Rokafenol N‐9 demonstrated a magnificent capacity to absorb paraffin oil. Copyright © 2003 John Wiley & Sons, Ltd.     

Effect of acidity of the medium and polarization of the surface on albumin adsorption by carbon fibers

Adsorption of bovine serum albumin (BSA) was studied at different acidities of the medium on the non-charged and titanium hydroxide-modified surfaces of the carbon fibers (ACF and ACF-B (Ti)). The fiber covered with titanium hydroxide was shown to have the highest adsorption capacity at pH = 4.7, which is close to the isoelectric point of BSA. The cathodic polarization has the greatest effect on the BSA desorption ability, which enabled the determination of conditions for recovering the adsorption capacity of the fibrous sorbent.     

Fine-tuning in size and surface morphology of rod-shaped polypyrrole using porous silicon as template

We investigated electropolymerization of polypyrrole into porous silicon template. We used three types of porous silicon templates, i.e., porous silicon with ordered macro pores, medium-sized pores and meso pores. Polypyrrole was electropolymerized from pore bottoms to pore tops in all the porous silicon templates. After removal of the templates we obtained rod-shaped polypyrrole arrays. Changing the porous silicon templates easily controlled the size of the rod-shaped polypyrrole. We also used dendritic medium-sized pores for template, and then polypyrrole rod having a roughened surface was obtained. Here we demonstrated that the size and the surface morphology of polypyrrole rod were easily tuned using porous silicon.     

The fabrication and performance of an Ag/AgCl reference electrode in human serum

A simple set of electric circuits was used to assemble a pulse generator. With pulse potentials and under galvanostatical control, a clean silver wire was anodized electrochemically for 0.2–0.5 min in 1.0 mol l⁻¹ HCl with a pulse current density of 20 mA cm⁻², and the pulse wave parameters of t_a/t_c = 1 and a cycle of 4 s forming an Ag/AgCl reference electrode. Even though the AgCl layer was consumed during the working period when the Ag/AgCl electrode was used as a cathode, the AgCl layer could be in situ recovered electrochemically in serum used when a reversed potential was applied to the electrode system immediately after the measuring program was finished. The current response curve of the anode indicated that an AgCl layer in high density was basically accomplished during the first 6 pulse cycles in human serum. In order to keep a stable and uniform AgCl layer on the reference electrode after each measuring cycle, the ratio of the recovery time (t_r) to the working time (t_w) was measured and the smallest value was obtained at 0.03. The open-circuit potential of the Ag/AgCl electrode with respect to a SCE in 0.1 mol l⁻¹ KCl was monitored over a period of 14 days and the mean value was 40.09 mV vs SCE with a standard deviation of 2.55 mV. The potential of the Ag/AgCl reference electrode did remain constant when the measurements were repeated more than 600 times in undiluted human serum with a standard deviation of 1.89 mV. This study indicated that the Ag/AgCl reference electrode could been rapidly fabricated with a pulse potential and could be used as a reference electrode with long-term stable properties in human serum samples.     

Simulation and experiment of asymmetric electrode placement for electrophoretic exclusion in a microdevice

Electrophoretic exclusion (EE) is a counterflow gradient technique that exploits hydrodynamic flow and electrophoretic forces to exclude, enrich, and separate analytes. Resolution for this technique has been theoretically examined and the smallest difference in electrophoretic mobilities that can be completely separated is estimated to be 10^?13 cm²/Vs. Traditional and mesoscale systems have been used, whereas microfluidics offers a greater range of geometries and configurations towards approaching this theoretical limit. To begin to understand the impact of seemingly subtle changes to the entrance flow and the electric field configurations, three closely related microfluidic interfaces were modeled, fabricated, and tested. These interfaces consisted of systematically varying placement of an asymmetric electrode relative to a channel entrance: leading electrode placed outside the channel entrance, leading electrode aligned with the channel, and leading electrode placed within the channel. A charged fluorescent dye is used as a sensitive and accurate probe for the model and to test the concentration variation at these interfaces. Models and experiments focused on visualizing the concentration profile in areas of high temporal dynamics, thus providing a severe test of the models. Experimental data and simulation results showed strong qualitative agreement. The complexity of the electric and flow fields about this interface and the agreement between models and testing suggests the theoretical assessment capabilities can be used to faithfully design novel and more efficient interfaces.     

Amperometric detection of mono- and diphenols at Cerrena unicolor laccase-modified graphite electrode: correlation between sensitivity and substrate structure

Graphite electrode modified with laccase from Cerrena unicolor served as a biosensor for detection of 30 phenolic compounds with different structures. Some correlations of the sensor response to the structures of substrates are discussed. This biosensor responded to: (i) nanomolar concentrations of some of the selected phenolic compounds, e.g., 2,6-dimethoxyphenol, coniferyl alcohol, caffeic acid, DOPAC and hydroquinone, (ii) micromolar concentrations, e.g., ferulic acid, syringic acid, dopamine, 3,4-dihydroxybenzoic acid and dl-noradrenaline, and (iii) millimolar concentrations in the case of phenol and 4-hydroxybenzaldehyde. Among the ortho- or para-substituted phenols, the sensitivity of the C. unicolor laccase-modified electrode increased in the following order H, CH(3), OH, OCH(3) and NH(3)(+) but in the case of para-substituted phenols, the K(m)(app) values were lower. The sensitivity of the laccase electrode increased with an additional OH group in para-substituted phenols. In the case of the selected compounds, kinetic data from electrochemical flow injection system were compared with those obtained from experiments in solution.     

Effects of dielectrophoresis on thrombogenesis in human whole blood

Thrombogenesis (blood clot formation) is a major barrier to the development of biomedical devices that interface with blood. Although state‐of‐the‐art chemically and pharmacologically mediated clot mitigation strategies are effective, some limitations of such approaches include depletion of active agents, or adverse reactions in patients. Increased clotting protein adsorption and platelet adhesion, which occur when artificial surfaces are exposed to blood result in enhanced clot formation on artificial surfaces. It is hypothesized that repelling proteins and platelets using dielectrophoresis (DEP), a contact‐free particle manipulation technique, will reduce clot formation in biomedical devices. In this paper, the effect of DEP on thrombogenesis in human blood is investigated. Undiluted whole blood from human donors is pumped through microchannels at a physiological shear rate (400 s ⁻¹). Experiments are performed by applying 0 V, 0.5 V_rms , 2 V_rms , and 3 V_rms to electrodes in the channel. Clot formation is observed to decrease in experiments in which DEP electrodes are active (average of 6% coverage @ 0V reduced to 0.08% coverage @ 3 V_rms ). Repulsion is more effective at higher voltages. DEP causes a quantifiable reduction in microscopic and macroscopic clot formation in PDMS microchannels.     

Characterization of electrode alignment for optimal droplet charging and actuation in droplet‐based microfluidic system

The actuation method using electric force as a driving force is utilized widely in droplet‐based microfluidic systems. In this work, the effects of charging electrode alignment on direct charging of a droplet on electrified electrodes and a subsequent electrophoretic control of the droplet are investigated. The charging characteristics of a droplet according to different electrode alignments are quantitatively examined through experiments and systematic numerical simulations with varying distances and angles between the two electrodes. The droplet charge acquired from the electrified electrode is directly proportional to the distance and barely affected by the angle between the two electrodes. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of angle provides a great degree of freedom in designing such devices. Not only the droplet charge acquired from the electrode but also the force exerted on the droplet is analyzed. Finally, the implications and design guidance for microfluidic systems are discussed with an electrophoresis of a charged droplet method‐based digital microfluidic device.     

Cathode erosion in inert gases: The importance of electrode contamination

Experimental results are presented for electrode erosion on copper electrodes in magnetically rotated arcs in argon and helium. Measurements were also made of the arc voltage and velocity. The effects due to the contamination of the electrode surface by either a native contaminant layer (copper oxide and carbon traces) or the continuous injection of very small amounts of various diatomic gases (nitrogen, oxygen, chlorine, and carbon monoxide) into the inert plasma gases were determined. The erosion rates for pure argon were significantly higher than those for pure helium (13.5 g/C for argon and 1 g/C for helium) and with both gases, very high arc velocities were measured initially (>60 m/s for argon and >160 m/s for helium) when a natural contaminant layer was still present on the cathode. The removal of this layer resulted in lower velocities (2m/s for argon and 20m/s for helium) and higher erosion rates. The removal of the layer was much faster with argon, due possibly to higher electrode surface current densities for argon arcs.