Scan Rate Dependence Research Articles

Time and potential resolved dissolution analysis of rhodium using a microelectrochemical flow cell coupled to an ICP-MS

The dissolution of polycrystalline rhodium in 0.1M H2SO4 is quantitatively investigated during potential cycling and potential step experiments using a novel setup which consists of a microelectrochemical scanning flow cell (SFC) linked to an inductively coupled plasma-mass spectrometer (ICP-MS). The time-resolved dissolution profile during electrochemical treatment is presented for the first time and used for quantitative determination of area-normalized dissolution rates. A high time resolution and very low detection limits allow distinguishing between anodic and cathodic dissolution at a sufficiently low scan rate. It has been found that the reduction of surface oxides triggers a significantly higher mass loss than the oxide formation (about 4–10 fold), and that this ratio and the overall extent of dissolution are dependent on the upper potential limit. Furthermore, a strong scan rate dependence was observed for both the anodic and cathodic dissolution peaks, with a decrease of the amount of rhodium dissolved per cycle with increasing scan rate and a minimum at potentiostatic step experiments between the respective potential limits. Clear evidence for steady state dissolution of the oxidized surface is presented as well, even though this value ranges around a few femtograms per second and square centimeter at potentials up to 1.4VSHE. The differences in dissolution cover more than two orders of magnitude, potentially providing valuable information for electrode operating conditions and degradation mechanisms of noble metal materials.

Lipid and Detergent Effects on the Kinetic Stability of the GPCR, Rhodopsin

The photoreceptor, rhodopsin is a kinetically stable G-Protein Coupled Receptor that is located in rod outer segment disk membranes. Differential scanning calorimetry (DSC) studies have shown that rhodopsin exhibits an irreversible scan rate dependent endothermic transition (Tm) at approximately 72 oC. The activation energy for thermal denaturation (Eact) calculated from the scan rate dependence of the Tm is sensitive to the integrity of the lipid bilayer. It is also sensitive to proteolytic cleavage of the extramembraneous loops of rhodopsin. Here we investigate the influence of palmitoylation, membrane cholesterol and digitonin solubilization on rhodopsin kinetic stability. DSC experiments were performed using a MicroCal VP-DSC microcalorimeter. Samples were scanned at 15, 30, 60 and 90o/hr. Because the protein transitions are irreversible, a second scan was used to determine the baseline. Rhodopsin palmitoylation at cys 322 and cys 323 anchors the C-terminus to the membrane. The Eact for rhodopsin in disk membranes treated with hydroxylamine to remove these palmitate groups was approximately 35 Kcal/mole less than that of native rhodopsin. The Eact for rhodopsin was also determined at different membrane cholesterol levels. These data indicated a decrease in stability both above and below native membrane cholesterol levels. Finally, unlike solubilization in other detergents, solubilization in digitonin had little effect on rhodopsin kinetic stability.

Bio-electrocatalysis of Acetobacter aceti through direct electron transfer using a template deposited nickel anode

We have developed a novel anode for microbial fuel cells (MFC) which exhibits effective direct electron transfer with Acetobacter aceti. A high surface area nickel electrode was obtained using a simple process of template electrodeposition and it significantly improved the bioelectrocatalytic activity of Acetobacter aceti towards the oxidation of ethanol and glucose. Electrodeposition of Ni on carbon paper was carried out in the absence and presence of a soft template which served as the substrate for direct electron transfer. The soft template consists of a two component mixture of Triton X-100 and water. The structural and morphological characterizations of these modified electrodes were carried out using scanning electron microscopy (SEM). The redox properties of the biofilm formed on Ni coated carbon electrodes were investigated using cyclic voltammetry (CV) and chronoamperometry (CA). The formation of redox peaks during CV studies suggests the presence of a membrane bound redox protein linked to pyrroloquinoline quinone (PQQ) within these microbes on the modified electrodes. Investigation of the scan rate dependence reveals that the proteins are surface confined and anchored. From CA studies, we found that the oxidation current density corresponding to glucose and ethanol was more for the template deposited nickel (TNiCP) when compared to the one without the template. The oxidation current density of ethanol was found to be higher (68 μA cm−2i.e., 680 mA m−2) in comparison to glucose (42 μA cm−2i.e., 420 mA m−2). For comparison, we have also carried out all these studies using a bare carbon paper (CP). Our results clearly demonstrate the direct electron transfer between Acetobacter aceti and high surface area nickel electrodes through the membrane bound redox enzyme which presumably contributes to the higher oxidation current density of ethanol and glucose and could be used as an alternative anode material for MFC applications.

Rapid Water Reduction to H2 Catalyzed by a Cobalt Bis(iminopyridine) Complex

A cobalt bis(iminopyridine) complex is a highly active electrocatalyst for water reduction, with an estimated apparent second order rate constant k(app) ≤ 10(7) M(-1)s(-1) over a range of buffer/salt concentrations. Scan rate dependence data are consistent with freely diffusing electroactive species over pH 4-9 at room temperature for each of two catalytic reduction events, one of which is believed to be ligand based. Faradaic H(2) yields up to 87 ± 10% measured in constant potential electrolyses (-1.4 V vs SCE) confirm high reactivity and high fidelity in a catalyst supported by the noninnocent bis(iminopyridine) ligand. A mechanism involving initial reduction of Co(2+) and subsequent protonation is proposed.

The Bilayer Enhances Rhodopsin Kinetic Stability in Bovine Rod Outer Segment Disk Membranes

Rhodopsin is a kinetically stable protein constituting >90% of rod outer segment disk membrane protein. To investigate the bilayer contribution to rhodopsin kinetic stability, disk membranes were systematically disrupted by octyl- β-D-glucopyranoside. Rhodopsin kinetic stability was examined under subsolubilizing (rhodopsin in a bilayer environment perturbed by octyl- β-D-glucopyranoside) and under fully solubilizing conditions (rhodopsin in a micelle with cosolubilized phospholipids). As determined by DSC, rhodopsin exhibited a scan-rate-dependent irreversible endothermic transition at all stages of solubilization. The transition temperature ( T m) decreased in the subsolubilizing stage. However, once the rhodopsin was in a micelle environment there was little change of the T m as the phospholipid/rhodopsin ratio in the mixed micelles decreased during the fully solubilized stage. Rhodopsin thermal denaturation is consistent with the two-state irreversible model at all stages of solubilization. The activation energy of denaturation ( E act) was calculated from the scan rate dependence of the T m and from the rate of rhodopsin thermal bleaching at all stages of solubilization. The E act as determined by both techniques decreased in the subsolubilizing stage, but remained constant once fully solubilized. These results indicate the bilayer structure increases the E act to rhodopsin denaturation.

Simple and direct interpretation of phase angles or derivation degrees in term of energy conservation vs. dissipation with Formal Graphs

With the help of a new theory using the notion of path in a graph, a physical interpretation and meaning can be given to electroanalytical measurements, without recourse to mathematical treatments. The frequency dependence of impedances measured by ac techniques, or the scan rate dependence of current vs. potential characterizations in large signal techniques (cyclic voltammetry), can be interpreted through this approach as a determination of the proportion of conserved energy vs. total energy involved in a mechanism. This total energy includes the energy which is lost by dissipation, i.e., converted into heat. This new way of thinking sheds an interesting light on the difference between normal and anomalous or fractal diffusion and provides to the analyst a simple and immediate tool for interpreting experimental data.

The Disk Membrane Bilayer Enhances Bovine Rhodopsin Kinetic Stability

The photoreceptor, rhodopsin is a kinetically stable membrane protein. This G-protein coupled receptor consitiutes greater than 90 percent of the membrane proteins of rod outer segment disk membranes. Therefore biophysical studies of rhodopsin can be carried out in its native membrane. The role of the native bilayer in maintaining the kinetic stability rhodopsin was investigated. The disks were systematically disrupted by the detergent, octyl-β-D-glucopyranoside (OG). Rhodopsin kinetic stability was examined under sub-solubilizing conditions in which the bilayer was partially disrupted as well as fully solubilizing conditions in which rhodopsin was largely delipidated. In differential scanning calorimetry (DSC) studies rhodopsin exhibited an irreversible scan rate dependent endothermic transition and a scan rate dependent exothermic transition at all stages of solubilization. The endothermic Tm decreased and the exothermic Tm increased as the OG partitioned into the bilayer. There was little change once the fully solubilized stage was achieved even though phospholipids were present in the mixed micelles. The activation energy of denaturation (Eact) was calculated at each stage of membrane disruption from the scan rate dependence of the Tm. The endothermic Eact decreased rapidly in the sub-solubilizing phase, but not once the membrane was fully solubilized. The Eact determined from the rate of thermal bleaching was in agreement with the DSC data. The degree of solubilization had no effect on the exothermic transition Eact. The calorimetric enthalpy (ΔHcal) was independent of the extent of solubilization. The thermal transition broadened during the sub-solubilizing phase suggesting increased protein motional freedom. There was no further broadening in the fully solubilized stage. These results indicate that the phospholipids present in detergent micelles do not kinetically stabilize the protein. The membrane bilayer enhances the kinetic stability rhodopsin by increasing the energy barrier to denaturation.

Suppression of electrochemical creep by cross-link in polypyrrole soft actuators

In soft actuators, the creep under tensile loads is a serious problem for the positioning. The creep during electrochemical cycle (electrochemical creep) in conducting polymers is remarkable compared with the static creep under tensile loads. Correlation between the electrochemical creep and the actuating strain in polypyrrole films doped with docecylbenzene sulfonic (DBS) anion has been studied in NaCl aqueous electrolyte. It has been found that chemical cross-link in polypyrrole with di-pyrrole alkanes suppressed the creep, however, the actuator strain also decreased. The creep in crossliked PPy saturated after several cycles, keeping the strain of actuation constant. The result indicates the creep can be minimized by idling before use. Scan rate dependences of the electrochemical creep and actuation were also examined to find the ways to depress the creep.

Disparate Impact of the S33V Mutation on Conformational Stability in Rat β-Parvalbumin (Oncomodulin) and Chicken Parvalbumin 3

Rat β-parvalbumin (β-PV) and chicken parvalbumin 3 (CPV3) exhibit diminished Ca(2+) affinity. Their sequences, 70% identical, are unusual in that serine replaces the consensus residue, valine, at position 33. Reasoning that the substitution of a compact, polar hydroxymethyl moiety for a bulky, apolar isopropyl group might contribute to the attenuated Ca(2+) affinities, we have characterized the S33V variants of both proteins. The impact of the mutation in CPV3 differs decidedly from that in rat β. Whereas replacement of S33 by valine in CPV3 causes a substantial increase in the solvent-accessible apolar surface in the Ca(2+)-free protein, the mutation evidently decreases the exposed apolar surface area in rat β. Although the mutation has a minimal effect on divalent ion affinity in both proteins, the ΔΔH and -TΔΔS changes for Ca(2+) binding in CPV3 S33V, but not rat β S33V, are consistent with increased burial of the apolar surface. The influence of the S33V substitution on conformational stability likewise differs for rat β-PV and CPV3. Whereas the stability of the former is virtually unperturbed by the sequence alteration, the latter is destabilized by 0.7 kcal/mol. Moreover, the mutation greatly exacerbates the tendency for CPV3 to aggregate. The concentration and scan rate dependence observed in DSC studies of CPV3 S33V denaturation suggest that unfolding proceeds through an intermediate state that is prone to aggregation. Consistent with this idea, reversible unfolding data, collected at very low protein concentration, likewise indicate that the thermal denaturation is not a two-state process.

Relaxation dynamics of glass transition in PMMA + SWCNT composites by temperature-modulated DSC

The experimental technique offered by temperature-modulated differential scanning calorimeter (TMDSC) used to investigate the thermal relaxation dynamics through the glass transition as a function of frequency was studied for pure PMMA and PMMA-single wall carbon nanotubes (SWCNTs) composites. A strong dependence of the temperature dependence peak in the imaginary part of complex heat capacity (Tmax) is found during the transition from the glass-like to the liquid-like region. The frequency dependence of Tmax of the imaginary part of heat capacity (Cp) is described by Arrhenius law. The activation energy obtained from the fitting shows increases while the characteristic relaxation time decreases with increasing mass fraction (ϕm) of SWCNTs. The dynamics of the composites during glass transition, at slow and high scan rates, are also the main focus of this experimental study. The change in enthalpy during heating and cooling is also reported as a function of scan rate and frequency of temperature modulation. The glass transition temperature (Tg) shows increases with increasing frequency of temperature modulation and ϕm of SWCNTs inside the polymer host. Experimental results show that Tg is higher at higher scan rates but as the frequency of temperature modulation increases, the Tg values of different scan rates coincide with each other and alter the scan rate dependence. From the imaginary part of heat capacity, it is obvious that Tmax is not the actual glass transition temperature of pure polymer but Tmax and Tg values can be superimposed when ϕm increases in the polymer host or when the sample undergoes a transition with a certain frequency of temperature modulation.

Hydrazine oxidation catalyzed by ruthenium hexacyanoferrate-modified glassy carbon electrode

A ruthenium (III) hexacyanoferrate (Ru(HCF)) film coated on a glassy carbon electrode was explored as an electrocatalyst for hydrazine oxidation. Surface cyclic voltammograms of Ru(HCF) film showed four reversible one-electron redox waves. Two, which corresponded to the redox processes of Ru(III)/Ru(IV) and Fe(II)/Fe(III), were identified to be responsible for the catalytic activity of hydrazine oxidation. Kinetic studies using potential scan rate dependency, Tafel plots, and rotating disk electrode technique found that this catalyzed hydrazine oxidation was a complete four-electron/four-proton process producing N2, with the rate determining step possibly a one-electron process with a transfer coefficient (α) of ~0.31–0.36. In addition, based on kinetic analysis and findings in the literature, we propose a possible reaction mechanism for catalyzed hydrazine oxidation in order to facilitate further understanding.

Vortex phase diagram in weakly pinned Rh17S15

A vortex phase diagram of the strongly correlated superconductor Rh17S15 has been constructed via exploration of the anomalous variations in critical current density extracted from ac and dc magnetization measurements. The isofield in-phase ac susceptiblity data reveal the presence of multiple steps at different fields. The dc magnetisation hysteresis loops show the presence of a very broad fishtail commencing deep inside the mixed state and lasting upto Hc2. We have also analysed the scan rate dependence of the hysteresis width in the vibrating sample magnetometer data with a view to distinguish between the different possible order-disorder transformations in the flux line lattice.

Glass transitions in one-, two-, three-, and four-dimensional binary Lennard-Jonessystems

We investigate the calorimetric liquid–glass transition by performingsimulations of a binary Lennard-Jones mixture in one through fourdimensions. Starting at a high temperature, the systems are cooled toT = 0 and heated back to the ergodic liquid state at constant rates. Glass transitions areobserved in two, three and four dimensions as a hysteresis between the cooling andheating curves. This hysteresis appears in the energy and pressure diagrams,and the scanning rate dependence of the area and height of the hysteresiscan be described using power laws. The one-dimensional system does notexperience a glass transition but its specific heat curve resembles the shape of theD≥2 results in the supercooled liquid regime above the glass transition. AsD increases, the radial distribution functions reflect reduced geometricconstraints. Nearest neighbor distances become smaller with increasingD due to interactions between nearest and next-nearest neighbors. Simulation datafor the glasses are compared with crystal and melting data obtained with aLennard-Jones system with only one type of particle and we find that with increasingD crystallization becomes increasingly more difficult.

Eletrodo compósito à base de grafite-araldite®: aplicações didáticas

A composite electrode prepared by mixing a commercial epoxy resin Araldite® and graphite powder is proposed to be used in didactic experiments. The electrode is prepared by the students and applied in simple experiments to demonstrate the effect of the composite composition on the conductivity and the voltammetric response of the resulting electrode, as well as the response in relation to the scan rate dependence on mass transport. The possibility of using the composite electrode in quantitative analysis is also demonstrated.

Templated polypyrrole electro-polymerization: Self-assembled bundles of bilayer membranes of amphiphiles and their actuation behavior

The electrochemical properties of conducting polymers are highly dependent on the microstructure. We report a method to produce specific microstructures of polypyrrole through electro-polymerization in the presence of the amphiphile N-{11-(2-hydroxyethyldimethylammonium)undecanoyl}- N, N′-dioctyl- l-glutamate, bromide, which forms supramolecular hydrogels with pyrrole in aqueous solution. These hydrogels were used as templates during polypyrrole electro-polymerization to give microstructures composed of the bundles of bilayer membranes. The highly porous nature of these films resulted in electrochemical properties superior to polypyrrole deposited under the same condition without use of an amphiphilic template. Analysis of the scan rate dependence on cyclic voltammogram reveals that the porous templated films facilitate fast diffusion of dopant ions. The actuation properties were also investigated in aqueous solutions containing sodium p-toluene sulfonate electrolyte. The strains displayed by the template polypyrrole films were twice those synthesized without the use of a template.

Scan rate dependence of magnetization hysteresis in weakly pinned crystals of 2 H-NbSe 2 and Ca 3Rh 4Sn 13: A vibrating sample magnetometer study

We present experimental data relating to magnetic field ( H) scan rate (d H/d t) dependence in the magnetization ( M) hysteresis width across the field regions of the second magnetization peak (SMP) and the peak effect (PE) in weakly pinned crystals of 2 H-NbSe 2 and Ca 3Rh 4Sn 13. We note that the said dependence is small at the low field end, where individual/small bundle pinning dominates, as well as at the high field end (i.e., above the peak field of the PE), where disordered amorphous state is the stationary state. In between the onset field of the SMP anomaly and the peak field of PE, the scan rate dependence in magnetization data displays a non-monotonic variation. We conjecture that the healing of the transient disordered vortex states injected into a superconducting sample during field ramping process could be responsible for the observed behavior. We believe that the disordering commencing at the onset field of SMP has a window to heal between its peak field ( H smp p ) and the onset field ( H p on ) of the PE. A comparison of the scan rate dependence in two different crystals of 2 H-NbSe 2, one of which displays only the PE phenomenon and the other one that displays both SMP and PE anomalies, show that while enhancement in quenched random pinning invokes the occurrence of SMP anomaly, it slows down the temporal decay of currents across the PE region.

Cyclic voltammetry and spectroelectrochemical study of nickel and cobalt diphenyltetraazaannulene complexes

The absorption spectra. cyclic voltammetry and spectroelectrochemistry of [Ni(II)DPTAA] and [Co(II)DPTAA] (DPTAA = 6,13-diphenyldibenzo[b,i][1,4,8,11] tetraaza[14]annulene) complexes in DMF are reported in detail. The ligand oxidation is observed for [Ni(II)DPTAA] at +0.70 V vs. SCE whereas Ni2(+/+) occurs at - 1.60 V. For [Co(II)DPTAA], a ligand oxidation redox couple is seen at +0.56 V while the Co2+/+ and Co2+/3+ redox couples appear at -1.21 and +0.24 V, respectively. All observed redox couples are assigned to reversible one-electron processes on account of peak separations and scan-rate dependency. These processes were further investigated by spectroelectrochemistry for [Co(II)DPTAA]. For [Co(II)DPTAA], axial ligation of pyridine was found to shift the Co2+/3+ redox couple more negative. while the ligand oxidation was shifted to more positive potentials. From a spectrophotometric titration of [Co(II)DPTAA] with pyridine an equilibrium constant, K-f, was determined for the binding of pyridine to [Co(II)DPTAA]. This was found to be 10.2 dm(3) mol(-1), slightly lower than that of [Co(II)TAA], indicating the influence of the phenyl groups. From this value and shifts in the Co2+/3+ redox couple upon ligation, an equilibrium constant for the binding of pyridine to [Co(III)DPTAA], K'(f), was found to be 5.06 x 10(6) dm(3) mol(-1). (c) 2007 Elsevier B.V. All rights reserved.

Coulomb blockade induced negative differential resistance effect in a self-assembly Si quantum dots array at room temperature

We report a Coulomb blockade induced negative differential resistance (NDR) effect at room temperature in a self-assembly Si quantum dots (Si–QDs) array (Al/SiO 2/Si–QDs/SiO 2/p-Si), which is fabricated in a plasma enhanced chemical vapor deposition system by using layer-by-layer deposition and in-situ plasma oxidation techniques. Obvious NDR effects are directly observed in the current–voltage characteristics, while corresponding capacitance peaks are also identified at the same voltage positions in the capacitance–voltage characteristics. The NDR effect in dot array, arising from the Coulomb blockade effect in the nanometer-sized Si–QDs, exhibits distinctive scan-rate and scan-direction dependences and differs remarkably from that in the quantum well structure in the formation mechanism. Better understanding of the observed NDR effect in Si–QDs array is obtained in a master-equation-based numerical model, where both the scan-rate and scan-direction dependences are well explained.

The electrocatalytic examination of cephalosporins at carbon paste electrode modified with CoSalophen

The electrocatalytic oxidation of cephalexin and cefazolin has been studied at a carbon paste electrode modified with cobalt salophen (CoSal) by cyclic voltammetry. The selectivity of the carbon paste modified with CoSal in detecting cephalexin and cefazolin was examined. To suggest the electrocatalytic mechanism for electro-oxidation of cefazolin, the electrochemical behavior of ceftriaxone was investigated which has a thiol group out of the beta lactam ring. The electrocatalytic oxidation of these antibiotics is shown to be irreversible at the CoSal modified electrode. Scan rate dependence of cefazolin, which is a sulfur-containing compound, has been examined. The results indicated that the electrocatalytic oxidation of the compounds is diffusion controlled. The responses of the modified electrode were compared with those of unmodified electrode and it has shown that the modified electrode has better sensitivity than unmodified electrode to the detection of cefazolin. The overall number of electrons contributed to the oxidation of cefazolin is obtained 1 by chronoamperometry; the number of electron involved in the rate-determining step was 1. The results of differential pulse voltammetry (DPV) using the modified electrode with high sensitivity were applied for the determination of cefazolin in human synthetic serum samples. The linear range was obtained from 1 × 10 −5 to 1 × 10 −3 M for DPV determination of cefazolin in buffered solutions (pH 3.0).

A Calorimetric Method to Estimate Molecular Mobility of Amorphous Solids at Relatively Low Temperatures

To present a calorimetry-based approach for estimating the initial (at the onset of annealing) relaxation time (tau (0)) of organic amorphous solids at relatively low temperatures, and to assess the temperature where molecular mobility of the amorphous drug is reduced to a level comparable with the desired shelf-life of the product. Values of tau (0) for six amorphous pharmaceutical compounds were estimated based on the nonlinear Adam-Gibbs equation. Fragility was determined from the scanning rate-dependence of the glass transition temperature (T (g)). The initial enthalpic and entropic fictive temperatures were obtained from the T (g) and the heat capacities (C (p)) of the amorphous and crystalline forms. At a relatively low temperature ( approximately 40 degrees C or more below T (g)), tau (0) for the different compounds varies by over an order of magnitude. For some materials, the practical storage temperature at T (g) - 50 K was found to be still too high to ensure long-term stability. The estimated tau (0) is highly sensitive to the fragility of the material and the C (p) of the crystalline and amorphous forms. Materials with high fragility or greater C (p) differences between crystalline and amorphous forms tend to have longer tau (0). The proposed method can be used to estimate molecular mobility at relatively low temperatures without having to conduct enthalpy recovery experiments. An accurate tau (0) determination from this method relies on faithful fragility measurements.