Dr Oleksiy Klymenko

The electrochemically initiated reaction of p-phenylenediamines with sulfide in aqueous media is well documented. We now report the adaptation of this chemistry into nonaqueous media. This is critically appraised as a means of detecting sulfide. The electrochemically initiated reaction of N,N-diethyl-p-phenylenediamine with sulfide is shown at both macro- and platinum microdisk electrodes with quantitative detection of sulfide produced by means of the enhanced currents observed upon its addition. The linear detection range for sulfide is dependent on the concentration of N,N-diethyl-p-phenylenediamine present with a linear range from 28-3290 ¼M and a limit of detection of 22 ¼M achievable. This represents a large increase compared to that found previously in aqueous media and offers the prospect of more ready applications in high temperature systems.

The theory for homogeneous ECrevE, ECrevCE and ECECE mechanisms at macroscopically large channel electrodes is derived. The photoelectrochemical reductions of para-bromonitrobenzene and 2,4-dibromonitrobenzene in acetonitrile solutions and at macroscopic platinum channel electrodes are studied using irradiation at 330 and 470 nm, corresponding to absorption bands in the corresponding radical anions. The former compound is shown to follow a homogeneous ECrevCE pathway in acetonitrile solutions containing tetrabutylammonium-based supporting electrolytes; by changing the supporting electrolyte to a salt of the tetramethylammonium cation, the mechanism is changed qualitatively and follows an ECEE pathway. The photoelectrochemical reduction of 2,4-dibromonitrobenzene in acetonitrile solution containing supporting electrolytes derived from the tetrabutylammonium cation is shown to follow an overall ECECE mechanism, with both chemical steps being chemically-reversible, and with the loss of the ortho-bromo in the dark. In the presence of chloride supporting electrolytes, it is shown that light-induced rupture of a C-Br bond occurs reversibly with the competing formation of a C-Cl bond. Unoptimised bulk photoelectrosynthesis indicates that some halogen exchange occurs, demonstrating the viability of a novel approach to halex reactions. © 2002 Elsevier Science B.V. All rights reserved.

In this work we describe the theory and 2D simulation of ion separation and focusing in a new concept of microfluidic separation device. The principle of the method of ion focusing is classical in the sense that it consists in opposing a hydrodynamic transport ensured by the solution flow to an electrophoretic driving force so that any ionic sample results poised within the microchannel at the point where the two forces equilibrate. The originality of the concept investigated here relies on the fact that thanks to the use of an ion-conducting membrane of variable thickness in electrical contact with the channel the electrophoretic force is varied continuously all along the channel length. Similarly, changing the geometric shape of the membrane allows a facile optimization of the device separation and focusing properties. © Vilnius University, 2012.

The electrochemical oxidation of DEPD proceeds via an ECrevECE mechanism in dimethylformamide. It has been investigated at elevated temperatures up to 130 °C at both micro and macro platinum and glassy carbon electrodes. Kinetic and thermodynamic parameters for the reaction process have been calculated for each temperature. Further, the voltammetric response of DEPD shows enhanced limiting currents in the presence of sulfide. The analytical utility of the approach has been investigated with linear range from 50 to 850 ¼M sulfide concentration observed and a corresponding limit of detection of 20 ¼M achievable at temperatures of 70 °C.

Breaking of symmetry is often required in biology in order to produce a specific function. In this work we address the problem of protein diffusion over a spherical vesicle surface towards one pole of the vesicle in order to produce ultimately an active protein cluster performing a specific biological function. Such a process is, for example, prerequisite for the assembling of proteins which then cooperatively catalyze the polymerization of actin monomers to sustain the growth of actin tails as occurs in natural vesicles such as those contained in Xenopus eggs. By this process such vesicles may propel themselves within the cell by the principle of action-reaction. In this work the physicochemical treatment of diffusion of large biomolecules within a cellular membrane is extended to encompass the case when proteins may be transiently poised by corral-like structures partitioning the membrane as has been recently documented in the literature. In such case the exchange of proteins between adjacent corrals occurs by energy-gated transitions instead of classical Brownian motion, yet the present analysis shows that long-range movements of the biomolecules may still be described by a classical diffusion law though the diffusion coefficient has then a different physical meaning. Such a model explains why otherwise classical diffusion of proteins may give rise to too small diffusion coefficients compared to predictions based on the protein dimension. This model is implemented to examine the rate of proteins clustering at one pole of a spherical vesicle and its outcome is discussed in relevance to the mechanism of actin comet tails growth.

A theoretical approach for flow profile reconstruction in a rectangular microfluidic channel equipped with one or two microband electrodes working in generator-collector and generator-generator regimes was proposed by us previously (ChemPhysChem 2005, 6, 1581-1589; ChemPhysChem 2006, 7, 482-487). The purpose of the current study is to determine the ranges of dimensionless parameters corresponding to the highest sensitivity of the minimized functional to the shape of the flow profile. By application of a cubic spline to approximate the flow profile and analysis of the least-squares functional, which can then be represented as a function of one variable, we derive the area of optimal method performance. Thus, mathematical confirmation of our previous theoretical physical predictions could be obtained.

Steady-state voltammetry is used to measure the heterogeneous electron-transfer rates for the reduction of quinones to determine the dependence of k0 on molecular size, according to Marcus theory. This dependence is then used to predict the electron-transfer rate constants of related quinones, and the predictions are compared to experimental measurements.

In this manuscript, the two novel numerical methods for stiff ODEs-the Almost Runge-Kutta (ARK) and Aluffi-Pentini (AP)-are applied to the solution of two large stiff ODE systems that model the Belousov-Zhabotinskii reaction and air pollution process. The efficiency and accuracy of the two methods are compared revealing advantages of the ARK method especially for multidimensional ODE systems. © 2007 Springer Science+Business Media, LLC.

The measurement of liquid-liquid kinetics in acoustically emulsified media is shown to be conveniently achieved via the monitoring of bulk concentrations. This is exemplified with reference to the extraction of copper from an aqueous solution by the ligand 5-nonyl-salicylaldoxime in ethyl acetate. The speed of the extraction under ultrasound compares favourably with that seen under conventional stirring and the benefits are illustrated with reference to the extraction of copper from industrial effluent.

This article extends our previous works (Amatore, C.; Oleinick, A.; Svir, I. Anal. Chem. 2008, 80, 7947-7956; 7957-7963.) about the effects of resistive and capacitive distortions in voltammetry at disk microelectrodes. The particular case of voltammetry of a self-assembled monolayer carrying one redox site per molecule is investigated here. In addition, the effect of an uneven distribution of the effective electrochemical potential on the possibility of electron hopping (EH) contributions is examined. An original model of EH has been developed considering both diffusion-type (i.e., related to concentration gradients) and migration-type (i.e., imposed by an uneven distribution of the electrical potential due to an ohmic drop and capacitance charging) contributions. This predicts that as soon as the system performs out of thermodynamic equilibrium and provided that the EH rate constants are not too small the system tends to re-establish its out-of-equilibrium state through EH. Hence, EH somewhat tries to compensate the voltammetric distortions that would be enforced by the uneven distribution of the electrochemical driving force incurred by the system due to an ohmic drop and capacitive charging. However, this rigorous analysis established that, though EH may be effective under specific circumstances particularly near the electrode edge, its overall influence on voltammetric waves remains negligible for any realistic experimental situation.

The High Speed Channel Electrode is used to precisely measure the standard electrochemical rate constants under steady state conditions for the oxidation of substituted ferrocenes in alkyl cyanide solvents. First, the oxidation of ferrocene itself is kinetically analysed in the solvents: acetonitrile, propionitrile, butyronitrile and valeronitrile. Second, the oxidation kinetics of the ferrocene derivatives: ferrocene carboxylic acid, decamethyl ferrocene, bis(diphenylphosphino) ferrocene, ferrocene carboxaldehyde, ±-hydroxyethyl ferrocene, t-butyl ferrocene, dimethyl aminomethyl ferrocene, dimethyl ferrocene and n-butyl ferrocene are measured in acetonitrile for comparison to ferrocene. The rate of electron transfer in the different alkyl cyanides is shown to depend on the longitudinal dielectric relaxation times of the solvents in accordance with solvent dynamic theory, whilst the electron transfer rates of the different ferrocene derivatives in acetonitrile are shown to depend on the hydrodynamic radii of the molecules. In both cases, a measured k0 = 1.0 ± 0.2 cm s-1 for ferrocene oxidation in acetonitrile shows an excellent fit to the theoretical dependences. © 2005 Elsevier B.V. All rights reserved.

In this work, we illustrate two approaches to the simulation of surface diffusion over a sphere coupled with the formation of a cluster by reactive particles as a paradigm of a wide variety of problems occurring in many areas of nanosciences and biology. The problem is treated using a Brownian motion approach and a numerical solution of the corresponding continuous Fick?s laws of diffusion. While being computationally more expensive, the Brownian motion approach allows one to consider a wider range of situations, particularly those corresponding to relatively high concentrations of diffusing particles and the ensuing problem of particle overlap when they are ascribed finite sizes.

The application of fast-scan cyclic voltammetry methods to the high-speed microband channel electrode (Rees et al. J. Phys. Chem. 99 (1995) 7096) is reported. Theory is presented to simulate cyclic voltammograms for a simple electron transfer under high convective flow rates within the high-speed channel. Experiments are reported for the oxidation of 9,10-diphenylanthracene (DPA) in acetonitrile solution containing 0.10 M tetrabutylammonium perchlorate (TBAP) for both 12.5 and 40 ¼m platinum microband electrodes using a range of scan rates from 50 to 3000 V s-1 and centre-line flow velocities from 12 to 25 m s-1. Analysis of the voltammograms yielded values for k0 and ± for DPA which were measured to be 0.80±0.27 and 0.52±0.07 cm s-1, respectively. The range of applicability of this method was also investigated. Experiments are also presented using steady-state linear sweep voltammetry to obtain accurate measurements of the heterogeneous kinetic parameters for DPA at a platinum microband electrode. The measured value of k0 for DPA was found to be 0.94±0.16 cm s-1, with ± = 0.53±0.02 and a formal oxidation potential of 1.40±0.01 V (vs. Ag). © 2002 Elsevier Science B.V. All rights reserved.

A finite element approach for numerical simulation of linear sweep voltammetry at the wall tube electrode is presented. Working curves and surfaces are computed and reported which permit the analysis of reversible, quasi-reversible and irreversible voltammograms for all voltage scan rates. © 2002 Elsevier Science B.V. All rights reserved.

An improved configuration for simultaneous electrochemical ESR is described in which a tubular flow cell occupies a cylindrical TE011 ESR cavity. The minimisation of dielectric loss allowed good cavity Q-values to be achieved, resulting in a high level of sensitivity and allowing good quality spectra to be attained for the N,N,N2, N2-tetramethyl-para- phenylenediamine (TMPD) radical cation and the 1,2,4,5-tetrachlorobenzoquinone (para-chloranil) and nitrobenzene radical anions. The theory describing the convective and diffusive mass transport in the tubular arrangement, along with its influence on the steady state ESR signal intensity for these stable radicals is fully developed and shown to be in good agreement with experimental results. The transient ESR response is also considered and shown to agree well with the theory presented in the literature at flow rates for which the Lévêque approximation holds.

The electrochemical oxidation of N,N-diethyl-p-phenylenediamine in dimethylformamide has been studied at platinum and gold microdisk electrodes of various radii between 6.7 and 66 ¼m. The voltammetric responses revealed two electrochemically reversible waves the second of which becomes larger at higher concentrations and bigger electrode radii. The voltammetric signals have been modelled and the electrochemical oxidation reaction is not inconsistent with an ECrevECE reaction. Kinetic parameters are reported.

ECL reaction mechanisms involved in all experimental approaches are characterized by a sequence of very fast second-order reactions taking place in the diffusion layer between transient species that, according to the exact process of ECL generation, may be generated at a single electrode, a pair of electrodes (anode-cathode), or through electron transfers between activated reactants and co-reactants. These extremely fast second-order reactions generate the extremely short-lived electronically excited state species, S*, which are deactivated through a rapid first-order emissive decay giving rise to the emission of light. These crucial species are therefore always generated and consumed within a narrow layer of solution of very small size compared to diffusion layers. Moreover, they are present within this layer at vanishingly small concentrations. This creates kinetic situations termed ?reaction fronts? exact numerical treatment of which is almost impossible by classical numerical approaches. This chapter presents a series of numerical approaches based on the concepts developed by the authors to circumvent these severe complications and allow a precise and fast simulation of ECL reaction mechanisms. These are illustrated taking advantage of experimental examples featuring each main method of ECL generation.

A novel theoretical and numerical framework for the estimation of Sobol? sensitivity indices for models in which inputs are confined to a non-rectangular domain (e.g., in presence of inequality constraints) is developed. Two numerical methods, namely the quadrature integration method which may be very efficient for problems of low dimensionality and the MC/QMC estimators based on the acceptance-rejection sampling method are proposed for the numerical estimation of Sobol? sensitivity indices. Several model test functions with constraints are considered for which analytical solutions for Sobol? sensitivity indices were found. These solutions were used as benchmarks for verifying numerical estimates. The method is shown to be general and efficient.

Modern renewable energy sources have a great disadvantage of being intermittent. Harvesting solar energy directly using photovoltaic panels is one of the most promising renewable energy technologies. While this allows electricity generation during daytime when the sky is clear, at night there is no production at all and it is greatly diminished in cloudy or rainy conditions. Recently a concept of all-weather solar cells was proposed by Q. Tang et al. (Angew. Chem. Int. Ed. 55(17) (2016) 5243-5246) in which a solar panel was covered with a layer of graphene. This allows collecting energy from falling raindrops containing dissolved salts through charging and discharging of an electrical double layer at the water-graphene interface, which acts as a pseudocapacitor. Although this setup allows harvesting both direct solar radiation and some of the kinetic energy of falling rain drops, the output is low for realistic salt concentrations while the graphene layer diminishes the solar-to-electric conversion rate.
In this work, we propose a different approach to the same problem. Instead of relying on a sufficient concentration of salts in rain water, we propose to convert the mechanical energy delivered by drop impacts directly into electrical energy by supporting a thin-layer solar panel with an array of piezo crystals. The advantage of this setup is that the solar-to-electric performance of such a panel is not affected by the added piezoelectric support. However, only a fraction of the kinetic energy of the falling rain drops can be converted due to the energy dissipation within the material of the thin-layer panel. We have conducted detailed modelling of kinetic energy harvesting process from the drop impact and spreading to the dissipation of mechanical strain through the panel to the generation of piezoelectric potential. The results illustrate the viability of this concept, but they are still to be confirmed experimentally and require an economic feasibility analysis to be performed.

The manufacture of protein-based therapeutics presents unique challenges due to limited control over the biotic phase. This typically gives rise to a wide range of protein structures of varying safety and in vivo efficacy. Herein we propose a computational methodology, enabled by the application of constrained Global Sensitivity Analysis, for efficiently exploring the operating range of process inputs in silico and identifying a design space that meets output constraints. The methodology was applied to an antibody- producing Chinese hamster ovary (CHO) cell culture system: we explored > 80 0 0 feeding strategies to identify a subset of manufacturing conditions that meet constraints on antibody titre and glycan distri- bution as an attribute of product quality. Our computational findings were then verified experimentally, confirming the applicability of this approach to a challenging production system. We envisage that this methodology can significantly expedite bioprocess development and increase operational flexibility.

The variance-based method of global sensitivity analysis based on Sobol' sensitivity indices has become very popular among practitioners due to its ease of interpretation. A novel theoretical and numerical framework for the estimation of Sobol? sensitivity indices for models in which inputs are confined to a non-rectangular domain (e.g., in presence of inequality constraints) is developed. MC/QMC estimators based on the acceptance-rejection sampling method are proposed for the numerical estimation of Sobol? sensitivity indices. Random Sampling - High Dimensional Model Representation metamodeling method is used to approximate models and constraints which significantly reduces the cost of evaluating Sobol? sensitivity indices. Several model test functions with constraints are considered. The method is shown to be general and efficient.

In this work, a novel mathematical model for the description of the osmotic behavior during the cryopreservation of human Mesenchymal Stem Cells (hMSCs) from Umbilical Cord Blood (UCB) is proposed. In cryopreservation, the two-parameter formalism of perfect osmometer behavior is typically adopted and preferred due to its simplicity: cell volume osmotic excursions are described as due only to the passive trans-membrane transport of water and permeant solutes such as cryoprotectant agents (CPAs); intracellular solutes, responsible of the isotonic osmolality, are assumed to be impermeant. The application of the two-parameter model fails to capture the osmotic response of hMSCs whenever a swelling phase is involved, as demonstrated by the authors. To overcome this limitation, the imperfect osmometer
behavior of hMSCs is successfully modelled herein by improving the two-parameter formalism through the coupling of osmosis with cell mechanics and cell membrane Surface Area Regulation (SAR): now the transmembrane permeation of solutes (ions/salt) during the swelling phase through the temporary opening of mechanosensitive channels is allowed. This way cells can reach an equilibrium volume different from the initial isotonic one, when isotonic conditions are re-established after contact with impermeant or permeant solutes, such as sucrose or dimethyl-sulfoxide (DMSO), respectively. The sequential best-fit procedure adopted to adjust model parameters is reported herein along with model validation through full predictions.

Background: Human memory B cells play a vital role in the long-term protection of the host from pathogenic re-challenge. In recent years the importance of a number of different memory B cell subsets that can be formed in response to vaccination or infection has started to become clear. To study memory B cell responses, cells can be cultured ex vivo, allowing for an increase in cell number and activation of these quiescent cells, providing sufficient quantities of each memory subset to enable full investigation of functionality. However, despite numerous papers being published demonstrating bulk memory B cell culture, we could find no literature on optimised conditions for the study of memory B cell subsets, such as IgM+ memory B cells.

Methods: Following a literature review, we carried out a large screen of memory B cell expansion conditions to identify the combination that induced the highest levels of memory B cell expansion. We subsequently used a novel Design of Experiments approach to finely tune the optimal memory B cell expansion and differentiation conditions for human memory B cell subsets. Finally, we characterised the resultant memory B cell subpopulations by IgH sequencing and flow cytometry.

Results: The application of specific optimised conditions induce multiple rounds of memory B cell proliferation equally across Ig isotypes, differentiation of memory B cells to antibody secreting cells, and importantly do not alter the Ig genotype of the stimulated cells.
Conclusions: Overall, our data identify a memory B cell culture system that offers a robust platform for investigating the functionality of rare memory B cell subsets to infection and/or vaccination.