Positive Potential Step Research Articles

Continuous glucose monitoring using wearable non-enzymatic sensors in a physiological environment

A novel non-enzymatic sensor has been developed for continuous glucose measurement in physiological body fluids using a Step-wise; amperometric method. Unlike; traditional nickel-based catalysts, this sensor overcomes challenges in biological and neutral pH environments. It utilizes a carbon fiber microelectrode modified with gold and nickel nanoparticles, reinforced by a biopolymer layer derived from quince seed mucilage (QSM). By applying a negative pretreatment potential step, hydroxide ions are locally generated, creating a partially alkaline environment that activates the nickel nanoparticles. Glucose concentration is determined by measuring the current at the electrocatalytic potential of nickel, which directly oxidizes glucose. To clean and reactivate the sensor, a positive pulse potential step is applied at the end of each cycle. The sensor exhibits high sensitivity (13.8 μA mM−1.mm−2) and a low limit of detection (11.3 μM) in neutral pH (7.4). These results demonstrate the promising performance of the sensor for continuous glucose measurement in physiological body fluids. Using eco-friendly and biocompatible QSM as a reinforcing layer enhances its potential for biomedical applications. Additionally, a wearable compact electronic module was designed and fabricated to apply the potential, read the sensor's output currents, and monitor and record the results. The module's performance in measuring glucose in blood plasma is comparable to that of a commercial glucometer.

Перенос электронов через границу полупроводник-вакуум с отрицательным и положительным электронным сродством: влияние скачка массы

The effect of jump in electron mass at the crystal-vacuum interface on photoemission from semiconductors is considered. In the effective mass approximation, the angular and energy dependences of the electron transmission coefficient through interfaces with a jump in mass and potential steps of different signs, corresponding to negative and positive electron affinities, are considered. It is shown that due to the jump in mass, there are a critical energy and a critical angle of incidence of electrons, which separate qualitatively different angular and energy dependences of the transmission coefficient, respectively. The jump in mass makes it possible for electrons to transfer (up to complete transmission) through a positive potential step with a normal component of kinetic energy below the height of the step. The calculated dependences of the emission quantum yield of thermalized electrons on the affinity value are compared with the experimental data on photoemission from p-GaAs(Cs,O). Possible reasons for the significant differences between the experiment and the calculation are analyzed: a complex potential profile, including the near-surface band bending in the semiconductor and the image charge potential in vacuum, scattering in the (Cs,O) layer, and the need to go beyond the effective mass approximation and to take into account full Bloch nature of electron wave functions in semiconductors.

Interfacial viscosity and ionic reorientation probed using electrochemical surface plasmon resonance at the gold electrode interface of ionic liquids

• Interfacial dielectric relaxation of ionic liquids has been studied by ESPR. • Interfacial viscosity of ionic liquids is demonstrated to be extremely high. • Smaller ions tend to form the electric double layer with more solid-like structure. • Imidazolium ring has weak reorientation response to the potential steps. The interfacial dielectric relaxation of ionic liquids (ILs) at the gold electrode interface has been investigated using electrochemical surface plasmon resonance (SPR), by analyzing the SPR response to the potential step for four ILs with the two cations, trioctylmethylammonium and 1-butyl-3-methylimidazolium, and three amide anions, bis(fluorosulfonyl)amide, bis(trifluoromethanesulfonyl)amide, and bis(nonafluorobutanesulfonyl)amide. For all the four ILs, the SPR response to the potential step exhibits a fast relaxation process mainly ascribable to the ionic reorientation, followed by an ultraslow relaxation process ascribable to the ionic translation, with the opposite directions of SPR shift to each other. The ultraslow relaxations by the positive potential steps are always significantly slower than those by the negative steps. The average time constants of the ultraslow relaxation process and the amplitudes of the fast relaxation process are evaluated by the fitting with a multiple exponential function and are compared with the measured bulk viscosity and the dipole moments of each ion. The interfacial viscosity, estimated from the time constants of the ultraslow relaxation process, is several orders of magnitude higher than the bulk viscosity, and the viscosity increase is larger for the ILs composed of smaller ions. The amplitude of the fast relaxation, reflecting the ionic reorientation, is moderately correlated with the dipole moments of counter-ions on the electrode at the pre-step potential.

Photoemission and photon-enhanced thermionic emission: Effect of jump in electron mass

The effect of jump in electron mass on the electron transfer through interfaces with positive and negative potential steps is considered in relation with practically important photoemission and photon-enhanced thermionic emission from semiconductors with positive and negative effective electron affinities. For a zero potential step, a direct similarity is established between the expressions for electron reflection and transmission and Fresnel equations for p-polarized light, with the ratio of masses replacing the ratio of dielectric constants. For non-zero potential steps of both signs, due to the jump in mass, there are critical electron energies that separate the energy regions with qualitatively different angular dependences of the transmission coefficient. The influence of the jump in mass on the spectra of photoemission quantum yield, which are measured experimentally under transition from negative to positive electron affinity at the p-GaAs(Cs,O)-vacuum interface, is considered. It is shown that, for a decisive proof of the jump-in-mass contribution to the electron transfer through semiconductor-vacuum interface, measuring the energy and angular distributions at the state of positive electron affinity is more advantageous, as compared to the state of negative electron affinity.

Nonenzymatic Wearable Sensor for Electrochemical Analysis of Perspiration Glucose.

We report a nonenzymatic wearable sensor for electrochemical analysis of perspiration glucose. Multipotential steps are applied on a Au electrode, including a high negative pretreatment potential step for proton reduction which produces a localized alkaline condition, a moderate potential step for electrocatalytic oxidation of glucose under the alkaline condition, and a positive potential step to clean and reactivate the electrode surface for the next detection. Fluorocarbon-based materials were coated on the Au electrode for improving the selectivity and robustness of the sensor. A fully integrated wristband is developed for continuous real-time monitoring of perspiration glucose during physical activities, and uploading the test result to a smartphone app via Bluetooth.

High-Speed Surface X-Ray Diffraction for Monitoring Structural Changes of Electrode Surfaces: An Application to Methanol Oxidation on Pt(111)

To deeply understand how electrochemical reactions proceed at the electrode/electrolyte interfaces, it is necessary to reveal the interface structures at atomic resolution and in real time. Scanning probe microscopies, synchrotron X-ray scattering and absorption spectroscopy, and surface-sensitive optical methods are often used to observe the interface structures in-situ. Among them, X-ray crystal truncation rod (CTR) scattering, which is one kind of surface X-ray scattering, is a powerful technique to determine the atomic structures across the interface. CTR scattering is widely used to determine static structures, but its application to time-resolved measurements is limited. In the previous time-resolved measurements [e.g. Tamura et al., J. Phys. Chem. B 108, 1992 (2004)], they monitored a specific point in reciprocal space during the electrochemical reaction. Such fixed-single-point measurements can monitor structural changes qualitatively, such as roughening/flattening of the electrode surface and a formation/destruction of a superstructure formed by the lateral periodic array of the adsorbates. However, these measurements can not reveal small positional changes (0.1~0.01 Å) of the interface atoms quantitatively, which often occur with a change of surface adsorbate species or a change of the oxidation state of the adsorbates and/or of the electrode surface. In order to determine the structural change quantitatively, simultaneous measurements of a CTR profile, whose range is wide enough to allow the structural analysis at atomic scale, is needed. In this study, we used an energy-dispersive X-ray CTR scattering technique [T. Matsushita et al., J. Appl. Phys. 110, 102209 (2011)], which enables the simultaneous measurement of a wide range of a CTR profile, to study the electrochemical oxidation of methanol on a Pt(111) electrode. In the methanol oxidation, it is widely accepted that CO molecules are strongly adsorbed on the Pt surface to cause the overpotential. The time-resolved measurement revealed that the height of the topmost atomic layer of Pt was changed by up to about 0.1 Å in the methanol oxidation. The behavior is closely related to the CO adsorption density, referring to the previous infrared spectroscopy measurements [Xia et al., Electrochem. Acta 41, 711 (1996)]. The CO-induced structural change shows a hysteresis with respect to the potential loop (0-1.0V vs. SHE), strongly supporting that the CO-adsorbed surface causes the overpotential. The CTR profile measurements during the potential step chronoamperometry indicates that the formation of a CO-desorbed domain takes several seconds to occur after a positive potential step (0 to 0.8 V), while the CO-rich domain forms within a time resolution of 1 s after a negative potential step (0.95 to 0.6 V).

In-situ infrared study of silicon in KOH electrolyte: Surface hydrogenation and hydrogen penetration

The n-Si(111)/6M KOH electrolyte interface has been investigated by in-situ multiple-internal reflection infrared spectroscopy, at room temperature and at 40°C. The potential of the Si electrode was stepped successively to positive and negative values with respect to open-circuit potential, leading to surface oxidation and oxide dissolution, respectively. Infrared spectra were recorded together with the interfacial current. Analysis of the infrared spectra indicates that, following the positive potential step, the electronic state of the surface changes from accumulation to inversion and the surface termination changes from a hydrogenated state to an oxidised state. The hydrogenated state is recovered after an induction time following the negative potential step. However, hydrogen penetration into the silicon lattice is then found to take place, as indicated by the appearance of a new SiH band and a strong background absorption of electronic origin. This sub-surface hydrogenation is associated with a slow increase of the interfacial current. This process is found to be especially important at higher temperature and is attributed to the formation of microcracks partially decorated with hydrogen. These results indicate that the chemistry and morphology of a silicon electrode are not stable even in the presence of an applied negative potential.

Simultaneous ellipsometric and chronoamperometric study of barrier aluminium oxide growth and dissolution in acetate buffer

In situ ellipsometric and chronoamperometic measurements were applied simultaneously to monitor barrier oxide growth and dissolution on aluminium. The steady-state thickness prior to each potential step was determined by electrochemical impedance spectroscopy (EIS) for calibration of the transient data. The growth of the barrier oxide, following a positive potential step, was consistent with the Cabrera-Mott inverse square logarithmic growth law. About 20 % of the measured current resulted in barrier oxide growth. The oxide dissolution rate, following a negative potential step, was controlled by diffusion of aluminium ions into the test solution. The obtained dissolution rate was thus much smaller than the corresponding oxide growth rates. Oxide solubilities calculated from Fick’s second law, by using literature data for the diffusion coefficient of Al3+, were about two orders of magnitude larger than that obtainable from thermodynamic considerations. The methodology developed provides the kinetic and solubility data needed for improving the existing know-how about the growth and dissolution kinetics of the barrier oxide layer in aqueous solution.

Polyelectrolyte-Mediated Electrochemical Fabrication of a Polyacetylene p−n Junction

The diffusion of dopant counterions has made the formation of conjugated polymer p−n junctions challenging. We demonstrate polyelectrolyte mediated electrochemistry (PMEC) as a three-electrode technique for separately introducing n- and p-type regions in an ion-functionalized polyacetylene structure to form a p−n junction. PMEC uses a polyelectrolyte-based supporting electrolyte and ion-functionalized conjugated polymers to control the ions available for exchange between a solid sample and supporting electrolyte, and in this way, select for oxidative vs reductive electrochemical processes. A bilayer consisting of solid layers of anionically and cationically functionalized polyacetylenes was driven first to −1.5 V vs SCE and then to 0.6 V vs SCE using tetrabutylammonium polystyrene sulfonate/acetonitrile as a supporting electrolyte. The negative potential step n-doped the entire structure, and the positive potential step selectively p-doped the anionically functionalized layer to form a p−n junction as fol...

In situ FTIR studies of the catalytic oxidation of ethanol on Pt(1 1 1) modified by bi-dimensional osmium nanoislands

This paper presents the study of ethanol electrooxidation on Pt(1 1 1) electrode modified by different coverage degrees of a submonolayer of osmium nanoislands, which were obtained by spontaneous deposition. The ethanol oxidation reaction was extensively studied by employing in situ FTIR. Collections of spectra of the ethanol adsorption and oxidation processes were acquired over a series of positive potential steps, in order to determine the intermediate species and the main products that are formed. It was shown that the increase in the catalytic activity of Pt(1 1 1) after osmium deposition for ethanol oxidation is greater than that observed on nonmodified Pt(1 1 1). It was also demonstrated that the mechanistic pathway for this reaction depends directly on the degree of osmium coverage. Thus, for low osmium coverage ( θ Os ≤ 0.28), the formation of CO as an intermediate is favored, and hence the full oxidation of adsorbed ethanol to CO 2 is increased, additionally, the formation of acetaldehyde is also observed in low degrees of osmium coverage. For intermediate osmium coverage (0.28 < θ Os ≤ 0.40), the oxidation of ethanol to acetaldehyde and then to acetic acid is favored, although on Pt(1 1 1) the formation of acetaldehyde is promoted. For higher degrees of osmium coverage ( θ Os > 0.51), the catalytic activity of the electrode for ethanol oxidation decreases. For an almost complete osmium layer ( θ Os = 0.92), obtained by electrodeposition at 50 mV, catalytic activity for ethanol oxidation shows the lowest value. In addition, the direct oxidation of ethanol to acetic acid at lower potentials is observed.

FTIR Study of the Ethanol Electrooxidation on Pt(100) Modified by Osmium Nanodeposits

In the present work, ethanol electrooxidation on a Pt(100) electrode modified by different coverage degrees of osmium nanoislands obtained by spontaneous depositions, was extensively studied employing in situ FTIR spectroscopy. A collection of spectra of the ethanol adsorption and oxidation processes was acquired during the first series of a positive potential step, to determine the intermediate species, as well as the main products formed. The spectroscopic results obtained were correlated with conventional electrochemical results obtained by cyclic voltammetry. It was shown that the catalytic activity of Pt(100) for ethanol oxidation increases significantly after osmium deposition and that the mechanistic pathway for this reaction depends directly on the osmium coverage degree. Thus, for low osmium coverage (theta;( Os) up to 0.15) the formation of CO as an intermediate was favored and hence the full oxidation of adsorbed ethanol to CO(2) was increased. For higher osmium coverages (theta;(Os) up to 0.33), the higher the coverage is, the more the direct ethanol oxidation to acetaldehyde and acetic acid is favored. For osmium coverage degree of 0.40, the catalytic activity of the electrode for ethanol oxidation decreased. On an almost complete osmium layer (theta;(Os) = 0.92) obtained by electrodeposition at 50 mV vs reversible hydrogen electrode, the catalytic activity for ethanol oxidation shows a much lower value.

The Role of Hydrophobic Chains in Self-Assembly at Electrified Interfaces: Observation of Potential-Induced Transformations of Two-Dimensional Crystals of Hexadecane by In-situ Scanning Tunneling Microscopy

Insoluble alkanes have been observed by electrochemical scanning tunneling microscopy (STM) to form ordered monolayers at the Au(111)/0.1 M HClO4 solution interface. Hexadecane molecules self-assemble into ordered layers that are stable over the potential range from 0.15 VSCE to 0.55 VSCE, on both reconstructed and unreconstructed Au(111) surfaces under electrochemical control. The hexadecane molecules appear as 2.2 nm long and 0.45 nm wide rods, suggesting an extended conformation. STM images show that a reversible order−disorder transition can be induced by moving the electrode potential positive or negative of the stable potential region (0.15 VSCE to 0.55 VSCE). The hexadecane molecules aggregate immediately after lifting of the surface reconstruction by a positive potential step to 0.65 VSCE. Reversible disappearance of the ordered alkane structure is also observed when the potential is stepped below 0.15 VSCE. The molecules resume an ordered lamellar structure upon return of the substrate to the sta...

Dynamics of Br electrosorption on single-crystal Ag(100): a computational study

We present dynamic Monte Carlo simulations of a lattice-gas model for bromine electrodeposition on single-crystal silver (100). This system undergoes a continuous phase transition between a disordered phase at low electrode potentials and a commensurate c(2X2) phase at high potentials. The lattice-gas parameters are determined by fitting simulated equilibrium adsorption isotherms to chronocoulometric data, and free-energy barriers for adsorption/desorption and lateral diffusion are estimated from ab initio data in the literature. Cyclic voltammograms in the quasi-static limit are obtained by equilibrium Monte Carlo simulations, while for nonzero potential scan rates we use dynamic Monte Carlo simulation. The butterfly shapes of the simulated voltammograms are in good agreement with experiments. Simulated potential-step experiments give results for the time evolution of the Br coverage, as well as the c(2X2) order parameter and its correlation length. During phase ordering following a positive potential step, the system obeys dynamic scaling. The disordering following a negative potential step is well described by random desorption with diffusion. Both ordering and disordering processes are strongly influenced by the ratio of the time scales for desorption and diffusion. Our results should be testable by experiments, in particular cyclic voltammetry and surface X-ray scattering.

Electrochemical quartz crystal microbalance study of the transient response of passive Fe25Cr alloy

Abstract The transient response of a passive ironchromium alloy is studied by applying an anodic potential step and monitoring the resulting current transient and mass change. The Fe25Cr model alloy used in this study is sputter deposited on AT cut quartz crystals which are connected to an oscillator circuit. The change in resonance frequency with time resulting from metal dissolution or film growth is monitored and converted into a mass change using the Sauerbrey equation. A formalism is developed for the interpretation of current and mass transients observed with binary alloys in the passive state. Two selectivity ratios are defined which characterize the reactions at the metal–oxide and oxide–solution interface, respectively. Further quantities of interest are the dimensionless mass/charge ratio determined experimentally and the dissolution ratio which characterizes the relative importance of dissolution and film formation for a given species. It is found that in acid solution a potential step applied to Fe25Cr in the passive region leads to a mass loss due to preferential dissolution of iron. As a consequence chromium becomes enriched in the passive film. In 0.1 M NaOH, on the other hand, iron does not dissolve readily and the electrode exhibits a mass gain upon application of a positive potential step. XPS analysis of the passive films qualitatively confirms the conclusions derived from EQCM experiments.

Fast‐activating cation channel in barley mesophyll vacuoles. Inhibition by calcium

In contrast to the vacuolar ion channels which are gated open by an increase of cytosolic Ca2+ the vacuolar ion currents at resting cytosolic Ca2+are poorly explored. Therefore, this study was performed to investigate the properties of the so‐called fast‐activating vacuolar (FV) current which dominates the electrical characteristics of the tonoplast at physiological free Ca2+ concentrations. Patch—clamp measurements were performed on whole barley (Hordeum vulgare) mesophyll vacuoles and on excised tonoplast patches. Single ion channels were identified, which, based on their selectivity, activation kinetics, Ca2+‐ and voltage‐dependence, carry the whole‐vacuole FV current. Reversal potential determinations indicated a K+ overs C− permeability ratio of about 30. Both inward and outward whole‐vacuole currents as well as the activity of single FV channels were inhibited by an increase of cytosolic Ca2+, with a Kd≈ 6 µM. At physiological vacuolar Ca2+ activities, the FV channel is an outward‐rectifying potassium channel. The FV channel was activated in less than a few milliseconds both by negative and positive potential steps, having a minimal activity that is 40 mV negative of the K+ equilibrium potential. It is proposed that transport of K+ through this cation channel controls the electrical potential difference across the tonoplast.

The effect of thermalisation length and work function on epithermal positron emission from solids

Measurements have been made of the yield of slow positrons from Ag(100), polycrystalline Au, Pb and Sn, and 8 Å, SiO 2 Si . The measured dependence of the total yields of re-emitted positrons upon incident positron energy indicates that only epithermal positron emission is observed from these samples. By fitting the data using a procedure more commonly applied to the evaluation of thermal diffusion lengths by slow-positron implantation spectroscopy the length L T, over which positrons are reduced in energy from ∼ 10 1 eV to below the positron work function, has been determined. L T is related to the positron thermalisation length. The largest value of L T of 24 ± 3 A ̊ is for the SiO 2 Si structure, attributed to the absence of a strong energy loss mechanism for positrons with energies below the SiO 2 Si bandgap(s). By assuming that the most significant effect on the positron yield at the lowest incident energies is the size of the positive surface potential step, work-function values have been deduced for the polycrystalline metallic samples of 0.9 ± 0.25 eV for Au and Pb, and 0.8 ± 0.25 eV for Sn.

Effects of applied current density and potential step on the stress generation during anodic oxidation of tungsten in 0.1 M H 2SO 4 solution

Stresses generated during the anodic oxidation of tungsten in 0.1 M H 2SO 4 solution were measured as a function of applied current density and potential step by using a beam deflection technique. The stresses which amounted to a few GPa were developed in the oxide film and were separated into the volume-generated stress and electrostrictive stress by measuring the transients in deflections upon interrupting the applied current densities. The deflection due to electrostriction was compressive and less than 10% of total deflection irrespective of applied current density and anodic oxidation time, indicating that the magnitude and sign of the stresses developed during the oxide growth are critically dependent upon the volume-generated stress. Compressive and tensile deflections due to the volume-generated stress were observed at the applied current density of 0.02 mA cm −2 and 0.65 mA cm −2, respectively. In contrast, the compressive to tensile deflection transition occurred with growing oxide film at various current densities between 0.05 and 0.10 mA cm −2. The results are discussed with respect to the relevant reactions occurring at the metal-oxide interface during the oxide growth. Positive potential steps imposed upon the anodic oxide film on tungsten produce instantaneous tensile deflections followed by deflections in a compressive direction. From the experimental results it is suggested that the formation of oxygen vacancies at the metal-oxide interface and their subsequent annihilation by the inward movement of oxygen ion are closely related with the mechanism of build-up and relief of stresses developed during the anodic oxidation of tungsten.

Block of receptor response in the stretch receptor neuron of the crayfish by gadolinium.

The trivalent lanthanide gadolinium was found to block the mechanotransducer response in the stretch receptor neuron of the crayfish. At normal calcium concentration (13.5 mM) a 50 per cent block of the receptor current was found at 395 +/- 59 (mean +/- SD) microM gadolinium. At a calcium concentration of 1.35 mM a 50 per cent block of the receptor current was obtained at 103 +/- 14 (mean +/- SD) microM gadolinium. The potential activated potassium current was also affected by gadolinium. At 200 microM the amplitude of the peak outward current as a result of a 90 mV positive potential step was decreased by about 40 per cent. The fast inward sodium current was decreased less than 10 per cent by gadolinium. It is concluded that in the crayfish stretch receptor gadolinium blocks the receptor current, reflecting block of stretch-activated channels, but at higher concentrations than have been described for other stretch-activated channels. In addition the outward rectifier potassium current is also blocked reflecting a block of potassium channels.

Current transients caused by potential jumps applied to passivating films on nickel

Positive and negative potential steps between values well within the passivation range are applied to nickel in borate buffer solution passivated for various times and at various temperatures. The transient cathodic current following a negative potential jump leads to the deduction of the potential dependence of the mean valence of nickel ions in the passivating films. The identity of the magnitudes of charge involved in the up-jumps and down-jumps of potential lead to the inference of reversible behavior at the film-electrolyte interface.

Effects of halothane on the transducer and potential activated currents of the crustacean stretch receptor.

Halothane was applied to the stretch receptor neuron of the crayfish (Astacus astacus) and the effects on the transducer properties and the potential activated currents were studied with potential clamp technique using two microelectrodes. Exposure to halothane reduced the frequency of action potentials during stretch. This was shown to be due to effects both on the action potential generating currents and the transducer current. Halothane partially blocked the TTX sensitive fast inward current in a dose-dependent manner (Apparent KD = 3 mM). Halothane also reduced the outward current produced by a positive potential step. Both the fast and the slow component were affected, although the fast outward current seemed to be most sensitive. There was little or no change in the currents resulting from negative potential steps. The peak of the receptor potential and the receptor current were very little affected by halothane. The amplitude of the static phase of the receptor potential was reduced to a greater degree than the static phase of the receptor current (cells treated with TTX). A change in reversal potential of about--13 mV was observed for the peak and the static phase of the receptor current in four cells indicating an increased cord conductance for the transducer channel.