Estimation of the kinetics of anion transfer across the liquid/liquid interface,by means of Fourier transformed square-wave voltammetry

A novel method of Fourier transformed square-wave voltammetry (FT-SWV) in combination with thin-film modified electrode was employed to investigate the kinetics of anion transfer across the liquid/liquid interface using a conventional three-electrode arrangement. Other than traditional SWV in which currents are sampled only at the end of each pulse, FT-SWV continuously collects the current response and then transforms it into frequency domain. Even harmonic frequencies, which are derived from the faradaic current response, will emerge in the power spectrum. The profile of the even harmonic power spectrum is parabolic and shows a maximum at a certain frequency. The maximum and the corresponding frequency are equivalent to the well-known “quasireversible maximum” and “critical frequency” (f_max) in traditional SWV, respectively. The rate constant and ion transfer coefficient α can be estimated by the obtained f_max. Compared with traditional SWV, FT-SWV is much simpler and faster in ion transfer kinetics estimation.     

An Experimental Investigation of Quasireversible Maximum of Azobenzene on Mercury Electrode by Fourier Transformed Square‐Wave Voltammetry

An experimental investigation of quasireversible maximum (QRM) of azobenzene on mercury electrode by two methods, i.e., traditional square‐wave voltammetry (SWV) and fast Fourier transformed square‐wave voltammetry (FT‐SWV), was presented, and the influence factors on QRM of FT‐SWV were discussed. The results show that the rate constants derived from FT‐SWV agree with that of derived from traditional SWV with acceptable differences, showing a sound verification that the rate constants derived from FT‐SWV were reliable. In addition, some theoretical predictions on FT‐SWV were experimentally confirmed through the characterization of strongly adsorbed azobenzene on mercury film electrode. As a result, FT‐SWV is further proved to be a powerful technique in kinetic studies of the surface adsorbed processes.     

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.     

Spontaneous and facilitated micelles formation at liquid|liquid interface: towards amperometric detection of redox inactive proteins

Spontaneous micelles formation by ionic surfactants has been detected amperometrically as an appearance of ion transfer across the water–dichloroethane interface noticed from linear dependence between the current and potential (Ohm’s law). At low surfactant concentrations, when its spontaneous aggregation does not occur, the micelles formation facilitated by a potential across the interface has been registered. The transfer of redox inactive proteins through water–dichloroethane interface in the presence of surfactant has been observed voltammetrically. It has been shown, that the presence of protein does not affect thermodynamics of micelles formation, but accelerates kinetics of ion transfer through the interface. The electrochemically controlled transfer of redox inactive proteins through liquid|liquid interface may lead to the development of methods for direct amperometric detection of biomolecules.     

New Aspects of Protein‐film Voltammetry of Redox Enzymes Coupled to Follow‐up Reversible Chemical Reaction in Square‐wave Voltammetry

Protein‐film square‐wave voltammetry of uniformly adsorbed molecules of redox lipophilic enzymes is applied to study their electrochemical properties, when a reversible follow‐up chemical reaction is coupled to the electrochemically generated product of enzyme's electrode reaction. Theoretical consideration of this so‐called “surface ECrev mechanism” under conditions of square‐wave voltammetry has revealed several new aspects, especially by enzymatic electrode reactions featuring fast electron transfer. We show that the rate of chemical removal/resupply of electrochemically generated Red(ads) enzymatic species, shows quite specific features to all current components of calculated square‐wave voltammograms and affects the electrode kinetics. The effects observed are specific for this particular redox mechanism (surface ECrev mechanism), and they got more pronounced at high electrode kinetics of enzymatic reaction. The features of phenomena of “split net‐SWV peak” and “quasireversible maximum”, which are typical for surface redox reactions studied in square‐wave voltammetry, are strongly affected by kinetics and thermodynamics of follow‐up chemical reaction. While we present plenty of relevant voltammetric situations useful for recognizing this particular mechanism in square‐wave voltammetry, we also propose a new approach to get access to kinetics and thermodynamics of follow‐up chemical reaction. Most of the results in this work throw new insight into the features of protein‐film systems that are coupled with chemical reactions.     

Heterogeneous electron transfer kinetics at the ionic liquid/metal interface studied using cyclic voltammetry and scanning electrochemical microscopy

The electrochemical behavior of a redox-active, ferrocene-modified ionic liquid (1-ferrocenylmethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) in acetonitrile and in an ionic liquid electrolyte (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) is reported. Reversible electrochemical behavior was observed in each electrolyte with responses typical of those for unmodified ferrocene observed in each medium. In the ionic liquid electrolyte, the diffusion coefficient of the redox-active ionic liquid increased by a factor of 5 upon increasing the temperature from 27 to 90 degrees C. The kinetics of electron transfer across the ionic liquid/electrode interface were studied using cyclic voltammetry, and the standard heterogeneous electron transfer rate constant, k (0) was determined to be 4.25 x 10 (-3) cm s (-1). Scanning electrochemical microscopy was then also used to probe the heterogeneous kinetics at the interface between the ionic liquid and the solid electrode and conventional kinetic SECM theory was used to determine k (0). The k (0) value obtained using SECM was higher than that determined using cyclic voltammetry. These results indicate that SECM is a very useful technique for studying electron transfer dynamics in ionic liquids.     

紫细菌反应中心色素蛋白复合膜电子转移研究

采用自组装单层膜方法在金电极表面形成单分子层的2,3－双巯基丁二酸（DMSA）,聚二甲基二丙烯氯化铵（PDDA）及紫细菌（Rhodobacter Sphaeroides)反应中心色素蛋白复合体的有序复合膜,使用方波估安法研究了该蛋白复合膜的电化学行为,成功地检测到该蛋白内多电对的可逆或准可逆电子转移过程,探讨了紫细菌反应中心蛋白内容各电子受体的氧化还原电位及外加电位驱动对各受体电位的影响,同时,通过对方波信号的非线形拟合,获得了该蛋白复合膜内主要电对的电子转移速率常数及电子迁移数等相关参数。     

Square-wave voltammetry of two-step diffusional electrode mechanism coupled with a reversible follow-up chemical reaction

Journal of Solid State Electrochemistry - Square-wave voltammetry (SWV) is applied to simulate two-step diffusional electrode mechanism, in which the electrochemically active species generated in...     

Comparison of Voltammetric Techniques for Ammonia Sensing in Ionic Liquids

In electroanalytical chemistry, it is often observed that square wave voltammetry (SWV) and differential pulse voltammetry (DPV) are more sensitive techniques compared to linear sweep voltammetry (LSV), due to their method of sampling which minimises the charging current (non‐faradaic processes). In this work, a comparison of the three techniques (LSV, DPV and SWV) is performed for ammonia (NH₃) gas oxidation (a chemically and electrochemically irreversible redox process) in an ionic liquid over a concentration range of 10–100 ppm. Four different platinum electrodes are employed: a screen‐printed electrode (SPE), a thin‐film electrode (TFE), a microarray thin‐film electrode (MATFE) and a Pt microdisk electrode (μ‐disk). Calibration plots (current vs concentration) for all three different electrochemical techniques on all four surfaces showed excellent linearity with increased concentrations of NH₃ gas and relatively low limits of detection (LODs). On the larger mm‐sized surfaces (SPE and TFE), the current responses for LSV and SWV were quite similar, but DPV gave the lowest currents. Whereas for the smaller micron sized electrodes (MATFE and μ‐disk), currents were of the order LSV>SWV>DPV, with LSV being far superior to the pulse techniques. These findings suggest that the pulse techniques of SWV and DPV may not be the optimum methods, particularly on microelectrodes, for the detection of analytes such as ammonia in RTILs.     

离子液体BMIMPF6修饰碳糊电极电化学测定维生素E

用疏水性离子液体1-丁基-3-甲基咪唑六氟磷酸([BMIM]PF6)作粘合剂制备了离子液体修饰碳糊电极(IL/CPE)。采用循环伏安法(CV)研究了维生素E(vitamin E,VE)的氧化产物生育酚红在IL/CPE和未修饰碳糊电极(CPE)上的电化学行为,结果表明生育酚红在IL/CPE上氧化过程更易于进行,峰电流响应ip也明显增加,表明IL/CPE对生育酚红的氧化还原反应具有良好的电催化作用。同时测定了电极过程的动力学参数:电荷转移系数α=0.8746,扩散系数D=1.65×10-3cm2/s,电极反应速率常数kf=6.64×10-2cm/s。采用方波伏安法(SWV)发现生育酚红氧化峰电流与其浓度在1.53×10-4mol/L～8.39×10-7mol/L范围内呈线性关系,检出限为1.58×10-8mol/L。该法可用于VE实际样品的分析测定。     

Dynamics of a triphenylene discotic molecule,HAT6, in the columnar and isotropic liquid phases

Discotic molecules have planar, disklike polyaromatic cores that can self-assemble into "molecular wires". Highly anisotropic charge transfer along the wires arises when there is sufficient intermolecular overlap of the pi-orbitals of the molecular cores. Discotic materials can be applied in molecular electronics, field-effect transistors, and-recently with record quantum efficiencies-photovoltaics (Schmidt-Mende, L.; Fechtenk?tter, A.; Müllen, K.; Moons, E.; Frien, R. H.; MacKenzie, J. D. Science 2001, 293, 1119). A combination of quasielastic neutron scattering (QENS) measurements with molecular dynamics simulations on the discotic molecule hexakis(n-hexyloxy)triphenylene (HAT6) shows that the dynamics of the cores and tails of discotic molecules are strongly correlated. Core and tail dynamics are not separated, the system being characterized by overall in-plane motion, on a time scale of 0.2 ps, and softer out-of-plane motions at 7 ps. Because charge transfer between the molecules is on similar time scales, these motions are relevant for the conducting properties of the materials. Both types of motion are dominated by van der Waals interactions. Small-amplitude in-plane motions in which the disks move over each other are almost entirely determined by tail/tail interactions, these also playing an important role in the out-of-plane motion. The QENS measurements reveal that these motions are little changed by passing from the columnar phase to the isotropic liquid phase, just above the clearing temperature. The model of four HAT6 molecules in a column reproduces the measured QENS spectrum of the liquid phase, suggesting that correlations persist within the liquid phase over about this number of disks.     

Impact of an ionic surfactant on the ion transfer behaviors at meso-liquid/liquid interface arrays

The influence of ionic surfactants,cetyltrimethylammonium bromide(CTAB),self-assembled within silica-nanochannels of a hybrid mesoporous silica membrane(HMSM) on simple ion transfer(IT)behaviors at the meso-water/1,2-dichloroethane(W/DCE) interface arrays supported by such a HMSM was investigated by voltammetry for the first time.Significantly,it is found that the CTAB in HMSM can dramatically enhance the peak-current responses corresponding to ITs of some anions and even lower their Gibbs transfer energies from W to DCE,which could be ascribed to an anion-exchange process between anions and the bromide of CTAB associated with partial ion-dehydration induced by the CTAB.This work will provide a new strategy to study anion transfer processes and improve the electroanalytical performance for anion detection at the liquid/liquid interface.     

Direct modification of mRNA by ferrocenyl carbodiimide and its application to electrochemical detection of mRNA.

Ferrocenyl carbodiimide (1) could be used for the direct labeling of synthetic RNA and expressed mRNA in vitro with the electrochemically active ferrocene moieties. These RNAs modified by 1 could be detected electrochemically coupled with a DNA probe-immobilized electrode. After hybridization of 1.1 Kb mRNA modified by 1 with the DNA probe-immobilized electrode, the peak charge observed by an Osteryoung square wave voltammetry (SWV) measurement correlated well with the concentration of mRNA, having a detection limit at the sub nanogram level.     

Large amplitude Fourier transformed ac voltammetry at a rotating disc electrode: a versatile technique for covering Levich and flow rate insensitive regimes in a single experiment

The theory for large amplitude Fourier transformed ac voltammetry at a rotating disc electrode is described. Resolution of time domain data into dc and ac harmonic components reveals that the mass transport for the dc component is controlled by convective-diffusion, while the background free higher order harmonic components are flow rate insensitive and mainly governed by linear diffusion. Thus, remarkable versatility is available; Levich behaviour of the dc component limiting current provides diffusion coefficient values and access to higher harmonics allows fast electrode kinetics to be probed. Two series of experiments (dc and ac voltammetry) have been required to extract these parameters; here large amplitude ac voltammetry with RDE methodology is used to demonstrate that kinetics and diffusion coefficient information can be extracted from a single experiment. To demonstrate the power of this approach, theoretical and experimental comparisons of data obtained for the reversible [Ru(NH(3))(6)](3+/2+) and quasi-reversible [Fe(CN)(6)](3-/4-) electron transfer processes are presented over a wide range of electrode rotation rates and with different concentrations and electrode materials. Excellent agreement of experimental and simulated data is achieved, which allows parameters such as electron transfer rate, diffusion coefficient, uncompensated resistance and others to be determined using a strategically applied approach that takes into account the different levels of sensitivity of each parameter to the dc or the ac harmonic.     

Quantification of the chiral recognition in electrochemically driven ion transfer across the interface water/chiral liquid

The electrochemically driven transfer of the chiral anions of d- and l-tryptophan across the interface water/chiral liquid (d- or l-menthol) is stereoselective, and it can be used to determine quantitatively the difference in Gibbs energies for the solvation of chiral ions in chiral liquids. The ion transfer can be achieved in a three-phase arrangement where a droplet of the chiral liquid containing decamethylferrocene as the electroactive redox probe is attached to a graphite electrode immersed in the aqueous solution containing the chiral ions.     

Probing carboxylate Gibbs transfer energies via liquid|liquid transfer at triple phase boundary electrodes: ion-transfer voltammetry versus COSMO-RS predictions

Understanding liquid|liquid ion transfer processes is important in particular for naturally occurring species such as carboxylates. In this study electrochemically driven mono-, di-, and tri-carboxylate anion transfer at the 4-(3-phenylpropyl)pyridine|aqueous electrolyte interface is investigated experimentally for a triple phase boundary system at graphite electrodes. The tetraphenylporphyrinato-Mn(III/II) redox system (Mn(III/II)TPP) dissolved in the water-immiscible organic phase (4-(3-phenylpropyl)pyridine) is employed for the quantitative study of the structure-Gibbs transfer energy correlation and the effects of the solution pH on the carboxylate transfer process. For di- and tri-carboxylates the partially protonated anions are always transferred preferentially even at a pH higher than the corresponding pK(a). COSMO-RS computer simulations are shown to provide a quantitative rationalisation as well as a powerful tool for predicting Gibbs free energy of transfer data for more complex functionalised carboxylate anions. It is shown that the presence of water in the organic phase has a major effect on the calculated Gibbs free energies.     

Control of wettability of molecularly thin liquid films by nanostructures

The patterning of liquid thin films on solid surfaces is very important in various fields of science and engineering related to surfaces and interfaces. A method of nanometer-scale patterning of a molecularly thin liquid film on a silicon substrate using the lyophobicity of the oxide nanostructures has recently been reported (Fukuzawa, K.; Deguchi, T.; Kawamura, J.; Mitsuya, Y.; Muramatsu, T.; Zhang, H. Appl. Phys. Lett. 2005, 87, 203108). However, the origin of the lyophobicity of the nanostructure with a height of around 1 nm, which was fabricated by probe oxidation, has not yet been clarified. In the present study, the change in thickness of the liquid film on mesa-shaped nanostructures and the wettability for the various combinations of the thickness of the liquid films and the height of ridge-shaped nanostructures were investigated. These revealed that lyophobicity is caused by a lowering of the intermolecular interaction between the liquid and silicon surfaces by the nanostructure and enables the patterning of a liquid film along it. The tendency of the wettability for a given liquid film and nanostructure size can be predicted by estimating the contributions of the intermolecular interaction and capillary pressure. In this method, the height of the nanostructure can control the wettability. These results can provide a novel method of nanoscale patterning of liquid thin films, which will be very useful in creating new functional surfaces.     

Fourier transform-EPR studies of photoinduced electron transfer in homogeneous and micellar solutions

Fourier transform (FT) EPR was used to study the pulsed-laser-induced electron transfer from porphyrins to quinones in homogeneous and micellar solutions. By monitoring the EPR signal of the quinone anion radicals as a function of delay time (τ_d) between laser and microwave pulses, with τ_d ranging from nanoseconds to 1 millisecond, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. FT-EPR spectra of electron transfer products generated in micellar solution provide evidence for the generation of long-lived spincorrelated radical pairs.     

Probing non-polarizable liquid/liquid interfaces using scanning ion conductance microscopy

The study of microscopic structure of a liquid/liquid interface is of fundamental importance due to its close relation to the thermodynamics and kinetics of interfacial charge transfer reactions.In this article,the microscopic structure of a non-polarizable water/nitrobenzene(W/NB) interface was evaluated by scanning ion conductance microscope(SICM).Using SICM with a nanometer-sized quartz pipette filled with an electrolyte solution as the probe,the thickness of this type of W/NB interface could be measured at sub-nanometer scale,based on the continuous change of ionic current from one phase to another one.The effects for thicknesses of the non-polarizable W/NB interfaces with different electrolyte concentrations,the Galvani potentials at the interface and the applied potentials on the probe were measured and systematically analyzed.Both experimental setups,that is an organic phase up and an aqueous down,and a reverse version,were employed to acquire the approach curves.These data were compared with those of an ideal polarizable interface under the similar experimental conditions,and several characteristics of non-polarizable interfaces were found.The thickness of a non-polarizable interface increases with the decrease of electrolyte concentration and the increase of applied potential,which is similar to the situation of a polarizable liquid/liquid interface.We also find that the Galvani potential across a non-polarizable interface can also influence the interfacial thickness,this phenomenon is difficult to observe when using polarizable interface.Most importantly,by the comparison of two kinds of liquid/liquid interfaces,we experimentally proved that much more excess ions are gathered in the space charge layer of non-polarizable interfaces than in that of polarizable interfaces.These results are consistent with the predictions of molecular dynamic simulations and X-ray reflectivity measurements.