Current-time Response Research Articles

Diffusion to a cube: A 3D implicit finite difference method

We report the diffusional mass-transport to an isolated cube and numerically determine both the steady-state and the chronoamperometric flux, the latter at both short and long timescales. It is found that across a wide range of timescales the total flux is well approximated (to within 5% error) by the response expected for a sphere of equivalent surface area where the equivalent radius is equal to 1.38 times the half side length of the cube. Under steady-state conditions the size of the equivalent sphere is marginally smaller (equivalent radius 1.34). However, the prime difference between the flux to a cube and a sphere is that in the case of a cube the steady-state flux density across the particle is non-uniform. Towards the cubic particle edges the flux density increases asymptotically. This numerical study was undertaken using a graphics processor unit solving the diffusion equation using a 3D finite difference fully backward implicit scheme. The results are of interest to current-time responses seen in single nanoparticle experiments where non spherical shapes are frequently encountered.

Potential step voltammetry: An approach to corrosion rate measurement of reinforcements in concrete

This paper presents a Potentiostatic Step Voltammetry approach to corrosion rate measurement of reinforcements in concrete. We have termed this approach PSV-TE since it is based on the Tafel extrapolation method, but with the added advantage that long and slow potentiodynamic scans are no longer required to obtain the Tafel slopes. In this way, the irreversible polarization of rebars is prevented, so PSV-TE is considered to be a non-destructive method. The Tafel slopes are obtained by fitting the current-time response of the system to a theoretical model which we have outlined and validated previously. In this way, the concrete's electrical resistance and double layer capacity are also obtained. In this study, the optimal PSV-TE design has been established and validated on a range of reinforced concrete specimens. Results show minimal deviation between PSV-TE and reference methods. Therefore, PSV-TE is part of the corrosion monitoring system we have patented.

Fabrication and formation mechanisms of ultra-thick porous anodic oxides film with controllable morphology on type-304 stainless steel

In this work, pore initialization and further growth of metal-oxides-film (MOsF) with nanoporous structure on stainless steel (SS) substrate by electrochemical anodization have been investigated for controlling morphology of the porous MOsF. The competition between electric field assisted dissolution of the oxidized metals and anodization of SS substrate are analyzed by current-time response combined with the thickness variation of the barrier layer. The current density-time (i-t) curves show a drastic increase-a drastic drop-a gradual increase-stable shape while the barrier layer thickness presents a drastic increase-a gradual decrease-stable trend. Furthermore, the morphology (pore size, pore shape and film thickness) of the porous MOsF on SS substrate is strongly dependent on voltage drop across the barrier layer, anodizing duration and chemical constituents of the SS substrate. An electric field-assisted selective chemical dissolution model has been proposed to explain the formation of porous MOsF with cross-connected nanotube-shaped pores. An anodic etching model and an equivalent circuit of anodization system are built to theoretically analyze the formation mechanisms of pore shrinking (pore size varies from 53 ± 7 nm at the top to 34 ± 4 nm at the bottom) across the porous MOsF. Based on the achieved results, we are able to design an anodization scheme by step-potential tailoring to fabricate thick, porous MOsF with uniform nanostructures. By applying the step-potential tailoring anodization to keep a quasi-stable electric field across the barrier layer, ultra-thick (>17 μm), uniform, porous nanostructured MOsF on the SS substrates have been reported.

Sensing characteristics of dopamine using Pt/n-Si structure

Dopamine sensing using a simple Pt/n-Si structure is investigated for the first time. Polycrystalline Pt membrane with a thickness of 3 nm in Pt/n-Si structure is observed by high-resolution transmission electron microscope image. This Pt/n-Si structure shows a good pH sensitivity of 0.58/pH by measuring current-time response at pH values from 2 to 10. Repeatable and stable sensing of dopamine with a low concentration of 10 pM are obtained. Dopamine with long concentration range of 10 pM to 1 μM is measured and the sensing mechanism is due to Schottky barrier height modulation through reduction-oxidation of Pt membrane in contact of dopamine. This study is useful for future early diagnosis of Parkinson's disease.

Mathematical Modeling, Simulation and Experimental Testing of Interior-Mount LSPMSM Under Stator Inter-Turn Fault

Accurate modeling is the first step in developing a model-based diagnostics tool for different types of failures in electric motors. In this paper, a mathematical model for an interior-mount line start permanent magnet synchronous motor (LSPMSM) under the stator inter-turn fault is developed based on the principles of magnetic coupled circuit theory, electromechanical energy conversion and reference frame theory. The developed mathematical model is simulated using MATLAB and experimentally validated using a 1-hp, 4-pole, 400-V LSPMSM. To investigate the performance of the motor under the stator inter-turn fault at different loading levels, experimental and simulation results are compared and analyzed for both transient and steady-state responses of the current and speed. The simulation and experimental results are in very good agreement and show that under the stator inter-turn fault, the current magnitude and time response of the phase that contains the inter-turn are highly affected. In addition, due to asymmetry in the stator windings under this fault, the speed experiences some oscillations at steady state. Hence, the developed model can be used to detect the possibility and location of the stator inter-turn fault in an LSPMSM.

Structural and electrochemical studies of undoped and In3+-doped co-binary Cu2-xTe and Bi2Te3 thin films for aqueous Na–S batteries

Abstract WO3 electrodes coated with co-binary Cu2-xTe and Bi2Te3 thin films were fabricated for sodium-sulfur (Na–S) batteries. Film fabrication was controlled by adjusting the pH of the solution and the indium doping concentration. The phases of orthorhombic CuTe and hexagonal Cu2Te with rhombohedral Bi2Te3 were formed on the WO3 electrode. After In3+ doping, In3+ ions act as Frenkel defects in the Cu2-xTe structure. This indicated that In3+ ions are located at interstitial sites in the Cu2-xTe structure with higher defect creation energy. Furthermore, more interconnected-like nanoparticles and reduced porosity were observed, thereby indicating that indium segregation with grain boundaries presented and contributed to an enhancement of the surface mobility, nucleation density, and a smoother surface. For electrochemical characteristics, a polysulfide solution was used as a redox electrolyte for ion transport. Optimization of the pH and indium concentration attributed to improve the exchange current density (J0) and time responses for the colored and bleached states because of faster movement of Na+ and S2− ions during inter/de-intercalation. Furthermore, optimization of the electrode by adjusting the pH and doping with indium is advantageous for both Na–S and rechargeable batteries because of long life cycle, reasonably high power and energy density of 306 W/kg and 9.35 Wh/kg, respectively. The highest specific capacity (Cs) values of the charge and discharge cycles for In3+-doped electrodes are ∼ 21 and 19 mAh/g, respectively with the coulombic efficiency approximates 100% (average value of ∼96%). This approach may provide a general path for the fabrication of undoped and In3+-doped co-binary Cu2-xTe and Bi2Te3 films on WO3 electrodes and may increase our knowledge regarding Na–S batteries for further performance improvement.

A reliable automatic simulation of singular transients by the adaptive Huber method: The case of homogeneous reactions at planar and spherical electrodes

Current-time responses having a singularity at time t=0 are typical of some transient electroanalytical experiments (such as e.g. the potential step chronoamperometry). A reliable simulation of such singular responses cannot be performed by conventional simulation techniques, as they fail to provide accurate results close to the singularity. A recent extension [L. K. Bieniasz, J. Comput. Appl. Math. 323 (2017) 136] of the adaptive Huber method for solving Volterra integral equations (IEs) is adopted for the simulation of singular responses at planar and spherical electrodes, in cases when one-dimensional, semi-infinite diffusion transport is coupled with (pseudo-)first order homogeneous reactions. Highly accurate approximants are elaborated, for computing certain integrals of the kernel functions occurring in the related IEs, and needed in this simulation approach. Formerly published approximants to the coefficients of the adaptive Huber method are also improved. The resulting algorithm is tested on four examples of IEs describing chronoamperometry for ErevCirr, ECrevE, CrevE and ErevC'irr reaction mechanisms. Singular transients are simulated automatically with a prescribed accuracy, also for t arbitrarily close to the singularity.

The Effects of Sodium Tungstate on the Characteristics of Microarc Oxidation Coating on Ti6Al4V

Microarc oxidized coatings on Ti6Al4V alloy are produced in a basic Na2SiO3-(NaPO3)6-NaAlO2 electrolyte with different concentrations of sodium tungstate. The effects of sodium tungstate on the characteristics of the coating are investigated through current–time response, microstructure, composition, hardness, bonding strength and antifriction measurements. The coating is mainly composed of Al2TiO5, TiO2, Al2O3 and amorphous substances regardless of the concentration of sodium tungstate. The sodium tungstate is transformed into trace amounts of WO3, which significantly promotes the growth of the coatings. The bonding strength and antifriction performance of the coatings are enhanced with the concentration of sodium tungstate less than 4 g/L, whereas excessive sodium tungstate results in a coarse outer layer with more microdefects, which deteriorates the wear resistance of the coating. Based on the results of scratch and wear test with energy-dispersive x-ray spectroscopy of the worn surface and debris, the optimal concentration of sodium tungstate doped into the electrolyte is identified, and this concentration yields superior tribological property. Excellent wear resistance is achieved by combining a high bonding strength and hardness with a less defective outer layer.

Nanopore Opening at Flat and Nanotip Conical Electrodes during Vesicle Impact Electrochemical Cytometry.

The oxidation of catecholamine at a microelectrode, following its release from individual vesicles, allows interrogation of the content of single nanometer vesicles with vesicle impact electrochemical cytometry (VIEC). Previous to this development, there were no methods available to quantify the chemical load of single vesicles. However, accurate quantification of the content is hampered by uncertainty in the proportion of substituent molecules reaching the electrode surface (collection efficiency). In this work, we use quantitative modeling to calculate this collection efficiency. For all vesicles except those at the very edge of the electrode, modeling shows that ∼100% oxidation efficiency is achieved when employing a 33 μm diameter disk microelectrode for VIEC, independent of the location of the vesicle release pore. We use this to experimentally determine a precise distribution of catecholamine in individual vesicles extracted from PC12 cells. In contrast, we calculate that when a nanotip conical electrode (∼4 μm length, ∼1.5 μm diameter at the base) is employed, as in intracellular VIEC (IVIEC), the current-time response depends strongly on the position of the catecholamine-releasing pore in the vesicle membrane. When vesicle release occurs with the pore opening occurring far from the electrode, lower currents and partial oxidation (∼75%) of the catecholamine are predicted, as compared to higher currents and ∼100% oxidation, when the pore is close to/at the electrode surface. As close agreement is observed between the experimentally measured vesicular content in intracellular and extracted vesicles from the same cell line using nanotip and disk electrodes, respectively, we conclude that pores open at the electrode surface. Not only does this suggest that electroporation of the vesicle membrane is the primary driving force for catecholamine release from vesicles at polarized electrodes, but it also indicates that IVIEC with nanotip electrodes can directly assess vesicular content without correction.

Protein-mimicking nanowire-inspired electro-catalytic biosensor for probing acetylcholinesterase activity and its inhibitors

A highly sensitive electrochemical biosensor based on the synthetized L-Cysteine-Ag(I) coordination polymer (L-Cys-Ag(I) CP), which looks like a protein-mimicking nanowire, was constructed to detect acetylcholinesterase (AChE) activity and screen its inhibitors. This sensing strategy involves the reaction of acetylcholine chloride (ACh) with acetylcholinesterase (AChE) to form choline that is in turn catalytically oxidized by choline oxidase (ChOx) to produce hydrogen peroxide (H2O2), thus L-Cys-Ag(I) CP possesses the electro-catalytic property to H2O2 reduction. Herein, the protein-mimicking nanowire-based platform was capable of investigating successive of H2O2 effectively by amperometric i-t (current-time) response, and was further applied for the turn-on electrochemical detection of AChE activity. The proposed sensor is highly sensitive (limit of detection is 0.0006 U/L) and is feasible for screening inhibitors of AChE. The model for AChE inhibition was further established and two traditional AChE inhibitors (donepezil and tacrine) were employed to verify the feasibility of the system. The IC50 of donepezil and tacrine were estimated to be 1.4 nM and 3.5 nM, respectively. The developed protocol provides a new and promising platform for probing AChE activity and screening its inhibitors with low cost, high sensitivity and selectivity.

A reliable automatic simulation of singular electroanalytical transients, by the adaptive Huber method for Volterra integral equations

In a number of transient electroanalytical experiments (such as, in particular, the potential step chronoamperometry) one expects current-time responses having a singularity at time t=0. A reliable simulation of such singular responses is a difficult task. Conventional simulation techniques fail to provide accurate results close to the singularity. A recent extension [L. K. Bieniasz, J. Comput. Appl. Math. 323 (2017) 136] of the adaptive Huber method for solving Volterra integral equations (IEs) is shown to overcome this difficulty in cases when the current behaves like t−1/2 for t → 0. The method requires an availability of highly accurate approximants for computing certain integrals of the kernel functions occurring in the IEs. Relevant approximants are elaborated for several most important kernel functions specific of one-dimensional diffusion, and one kernel function for two-dimensional diffusion. The resulting algorithm is tested on two examples of single IEs describing chronoamperometry and cyclic voltammetry for a single reversible charge transfer, and one example of an IE system describing chronoamperometry for an ErevErev mechanism. Singular transients are simulated automatically with a prescribed accuracy, even for t arbitrarily close to the singularity.

Characteristics of MAO coating obtained on ZK60 Mg alloy under two and three steps voltage-increasing modes in dual electrolyte

The micro-arc oxidation process was conducted on ZK60 Mg alloy under two and three steps voltage-increasing modes by DC pulse electrical source. The effect of each mode on current-time responses during MAO process and the coating characteristic were analysed and discussed systematically. The microstructure, thickness and corrosion resistance of MAO coatings were evaluated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), microscope with super-depth of field and electrochemical impedance spectroscopy (EIS). The results indicate that two and three steps voltage-increasing modes can improve weak spark discharges with insufficient breakdown strength in later period during the MAO process. Due to higher value of voltage and voltage increment, the coating with maximum thickness of about 20.20μm formed under two steps voltage-increasing mode shows the best corrosion resistance. In addition, the coating fabricated under three steps voltage-increasing mode shows a smoother coating with better corrosion resistance due to the lower amplitude of voltage-increasing.

Merging Ion Concentration Polarization between Juxtaposed Ion Exchange Membranes to Block the Propagation of the Polarization Zone.

The ion concentration polarization (ICP) phenomenon is one of the most prevailing methods to preconcentrate low-abundance biological samples. The ICP induces a noninvasive region for charged biomolecules (i.e., the ion depletion zone), and targets can be preconcentrated on this region boundary. Despite the high preconcentration performances with ICP, it is difficult to find the operating conditions of non-propagating ion depletion zones. To overcome this narrow operating window, we recently developed a new platform for spatiotemporally fixed preconcentration. Unlike preceding methods that only use ion depletion, this platform also uses the opposite polarity of the ICP (i.e., ion enrichment) to stop the propagation of the ion depletion zone. By confronting the enrichment zone with the depletion zone, the two zones merge together and stop. In this paper, we describe a detailed experimental protocol to build this spatiotemporally defined ICP platform and characterize the preconcentration dynamics of the new platform by comparing them with those of the conventional device. Qualitative ion concentration profiles and current-time responses successfully capture the different dynamics between the merged ICP and the stand-alone ICP. In contrast to the conventional one that can fix the preconcentration location at only ~5 V, the new platform can produce a target-condensed plug at a specific location in the broad ranges of operating conditions: voltage (0.5-100 V), ionic strength (1-100 mM), and pH (3.7-10.3).

Amperometric Response Characteristics of Rabeprazole at N-Doped CNTs-Chitosan Nanosensor in Solubilized System

This work reports potential application of nitrogen doped carbon nanotubes and chitosan nanocomposite modified glassy carbon electrode (N-CNTs-CHIT/GCE) nanosensor for amperometric characterization of antiulcer drug, rabeprazole (RB). Synthesized N-CNTs were characterized using microscopic and spectroscopic techniques like scanning electron microscopy (SEM), transmission electron microscopy (TEM), raman spectroscopy, X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDAX). The fabricated N-CNTs-CHIT/GCE sensor showed superior electrocatalytic performance toward oxidation of RB. With highly conductive network, large electrocatalytic surface area and low charge transfer resistance, N-CNTs-CHIT/GCE sensor exhibited remarkable amperometric current-time response curves in 0.1% sodium dodecylbenzenesulfonate (SDBS) solubilized system minimizing the use of hazardous organic solvents during electroanalysis. The nanosensor was found to exert linear relationship between current and increasing concentration of RB with correlation coefficient (r2) of 0.9922 in wide concentration range of 100.0 ng mL−1–1.4 μg mL−1 exhibiting high reproducibility and good storage stability. The high recovery (89.33%–95.0%) of RB in commercial tablets suggests great applicability of N-CNTs-CHIT/GCE sensor for analytical purposes.

Single Fusion Events at Polarized Liquid-Liquid Interfaces.

A new electrochemical framework for tracking individual soft particles in solution and monitoring their fusion with polarized liquid-liquid interfaces is reported. The physicochemical principle lies in the interfacial transfer of an ionic probe confined in the particles dispersed in solution and that is released upon their collision and fusion with the fluid interface. As a proof-of-concept, spike-like transients of a stochastic nature are reported in the current-time response of 1,2-dichloroethane(DCE)|water(W) submilli-interfaces after injection of DCE-in-W emulsions. The sign and potential dependence of the spikes reflect the charge and lipophilicity of the ionic load of the droplets. A comparison with dynamic light scattering measurements indicates that each spike is associated with the collision of a single sub-picoliter droplet. This opens a new framework for the study of single fusion events at the micro- and nanoscale and of ion transport across biomimetic soft interfaces.

The synthesis of a lanthanum metal–organic framework and its sensitivity electrochemical detection of H2O2

A lanthanum metal–organic framework, [La(BTC)(H2O)(DMF)] (H3BTC = 1, 3, 5-benzenetricarboxylic acid), was synthesized under mild hydrothermal conditions. The synthesized [La(BTC)(H2O)(DMF)] was characterized by scanning electron microscopy in combination with energy dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), and fourier transform infrared spectroscopy (FT-IR). Its electrochemical properties and electrocatalytic activity towards H2O2 reduction in acidic media were studied by cyclic voltammetry (CV) and amperometric current–time response. The [La(BTC)(H2O)(DMF)] modified electrode shows good electrochemical behavior and performs well electrocatalytic activity towards hydrogen peroxide (H2O2) reduction at ca. −0.7 V. The modified electrode displays a linear range from 5 μM to 2.67 mM and a limit of detection of 0.73 μM to H2O2. The [La(BTC)(H2O)(DMF)] modified electrode also possesses good selectivity and stability. Thus, [La(BTC)(H2O)(DMF)] will be a promising material for non-enzymatic H2O2 sensor.

Effect of incorporation of nanoparticles with different composition on wear and corrosion behavior of ceramic coatings developed on pure titanium by micro arc oxidation

The aim of this work is to find a simple mechanism to explain how incorporation of nanoparticles into the prepared ceramic coatings on titanium by micro arc oxidation (MAO) technique can affect the corrosion and wear behavior of MAO coatings. In order to attain this purpose, SiC, Al2O3 and TiO2 nanoparticles were added to aluminate based electrolytes and composite MAO coatings were prepared under constant voltage regime using a pulsed power supply and their corrosion and wear properties were compared with a base MAO sample. Furthermore, the effect of addition of nanoparticles on the surface morphology, thickness and chemical and phase composition of the obtained coatings were examined. The results showed that incorporation of nanoparticles (7.5gL−1) into the coatings had no effect on the current-time responses and coating thickness of the samples, despite the nanoparticles had filled the surface micropores and cracks. However, addition of nanoparticles reduced the corrosion current density, surface roughness, friction coefficient and wear rate and increased the corrosion resistance and hardness of the coatings. Consequently, it was concluded that incorporation of nanoparticles improved the anticorrosion and wear properties of the MAO coatings. In addition, the inclusion of TiO2 nanoparticles in the MAO coating showed the best anticorrosion and wear performance among all the other composite coatings under investigation.

Sn Spheres Embedded in a SiO2 Matrix: Synthesis and Potential Application As Self-Destructing Materials.

We introduce a simple process for the fabrication of SiO2 films embedded with β-Sn-rich nano/microspheres. Sn spheres with maximum and minimum sizes of 10 μm (near the SiO2 surface) and 5 nm (at the Si/SiO2 interface) were grown within a 0.7-5.7 μm-thick SiO2 layer by evaporating SnO powders onto an Si (100) substrate for 1-600 min at 600-900 °C and 0.001-5.0 Torr. A possible growth mechanism of these materials is discussed. The current-voltage characteristics of the as-fabricated samples were investigated to identify potential applications. During these tests, small flashes of light and the presence of damaged areas were observed at the oxide surfaces of the samples using an optical camera and a field emission scanning electron microscope, respectively. The electrical breakdown and shutdown of the devices observed in the current-voltage curves were attributed to the destruction of the SiO2 surface. In addition, the current-time responses show that the size of the damaged regions can be controlled by the voltage and duration of the applied stress, and are independent of the size and shape of the electrodes. The present materials thus possess great potential for applications in self-destructing devices.

Highly sensitive d-alanine electrochemical biosensor based on functionalized multi-walled carbon nanotubes and d-amino acid oxidase

With an excellent electron-transfer ability of 3,4,9,10-perylene tetracarboxylic acid functionalized multi-walled carbon nanotubes (PTCA-MWCNTs), and successful maintenance of d-amino acid oxidase (DAAO) activity by the protection of bovine serum albumin (BSA) and glycerol, a signal amplification biosensor for chiral recognition of d-alanine (d-Ala) has been designed. PTCA worked as redox probe due to its self-derived redox activity. The proposed biosensor was characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). After the biosensor interacting with d-alanine and l-alanine respectively, a larger current response was obtained from d-alanine. The linear range of the biosensor under the optimum working conditions was investigated by current-time response in successive addition of d-Ala from 1.0×10−8 to 1.0×10−3M with a lower detection limit of 3.3×10−9M (S/N=3). Possible explanations for substrate specificity of the biosensor were discussed. This method showed high sensitivity and selectivity for chiral recognition of d-alanine. It also exhibited good stability, repeatability and reproducibility. The proposed biosensor was applied for real sample measurement.

Single Nanotube Voltammetry: Current Fluctuations Are Due to Physical Motion of the Nanotube.

Nanoimpacts of single palladium-coated carbon nanotubes on a gold substrate are studied to elucidate the origins of the fluctuation in the current–time response of the hydrogen oxidation reaction mediated at its surface. The chronoamperometric and cyclic voltammetric responses from a single nanotube immobilized on the gold surface were compared to analogous data on a carbon substrate to determine the possible influence of substrate material on the nanotube–electrode electrical contact. No significant distinction between the gold and carbon was found, indicating in light of the considerable differences in the substrate materials’ intrinsic electronic structures that it is the nanomotion of a nanotube at the electrode surface which is likely responsible for the observed current modulation. This nanomotion creates a varying contact resistance, to which the noise in the current–time signal of the mediated reaction is attributed. In addition, stochastic ex-situ adsorption of single nanotubes onto the gold electrode followed by careful drying of the electrode surface was found to drastically reduce the current fluctuation, again implying that a contact resistance arising from physical motion of the nanotube at the electrode is responsible for the modulation of current.