John J. O'Dea

Square wave voltammetry at small disk electrodes: Theory and experiment

Abstract An equation is derived for the current response of a reversible electron transfer reaction for square wave voltammetry at an embedded disk electrode. Peak shape and position are invariant to the dimensionless parameter D τ/ r 2 where D is the diffusion coefficient, τ the square wave period, and r the radius of the disk, whereas peak current density increases without limit with increasing D τ/ r 2 . A simple empirical equation predicts the peak current for any value of D τ/ r 2 for square wave amplitude 50/ n mV and step height 10/ n mV. Experimental results for oxidation of ferrocyanide at small platinum electrodes agree well with the theory and demonstrate the practical utility of the experiment.

Square wave voltammetry for the determination of kinetic parameters: The reduction of zinc(II) at mercury electrodes

Abstract Kinetic parameters for the reduction of zinc(II) at mercury electrodes were determined from normal pulse and square wave voltammograms by employing the simple Butler-Volmer model for charge transfer. A non-linear least squares method equivalent to the method of maximum likelihood was employed to obtain the optimal values of parameters from the data as well as their regions of confidence. For ideal experimental conditions, kinetic parameters determined using this method were the same as those obtained using the more common graphical method of analysis. Normal pulse and square wave voltammetry yield the same kinetic information. However square wave voltammetry has advantages of speed and rejection of currents not accounted for in the model. Good kinetic data were obtained at concentrations of Zn(II) as low as 5 μM. Kinetic parameters for the Zn(II)/Zn(Hg) couple Frumkin corrected for the solution double layer are included in this report. A more complex model incorporating double layer corrections yielded values of kinetic parameters independent of supporting electrolyte concentration (0.3–1.0 M KNO3), pulse width (1–100 ms), and concentration of Zn(II) (0.005–1 mM). These values are k°t = 2.64 (±0.16)× 10−4 cm/s, αt = 0.20 (±0.02), Er 1 2 = 1.000 ± 0.001 V vs. SCE.

Analytical and kinetic investigations of totally irreversible electron transfer reactions by square-wave voltammetry

Abstract The influence of slow electron transfer on the analytical performance of square-wave voltammetry is considered from an experimental viewpoint. The magnitude, shape and position of the square-wave response depends on the degree of reversibility and also the value of the transfer coefficient (α). The dimensionless square-wave peak current of 0.93 (for nE sw = 50 mV, n Δ E s = 10 mV) for a reversible system decreases to 0.39 for totally irreversible slow charge transfer when α = 1.0 and decreases monotonically to zero as α goes to zero. Although a further increase in square-wave step height has only a small effect and an increase in amplitude has a moderate effect on the square-wave response when the system is Nernstian, sensitivities can be dramatically increased, at least fivefold, with the proper choice of these parameters for irreversible systems. To illustrate these points the reduction of two compounds, N -acetylpenicillamine thionitrite (α = 0.46) and tert -butyl hydroperoxide (α = 0.08) were examined.

Anodic stripping voltammetry with flow injection analysis

Abstract A flow cell with a wall-jet electrode design is used for anodic stripping voltammetry of lead at concentrations of about 10 −7 mol dm −3 . Maximum peak heights are obtained for narrow nozzle diameters and short nozzle-to-electrode distances. Linear calibration plots are obtainedfor almost four decades of change in concentration and can be extended by judicious choice of sample volume. Increasing sample throughput rates by increasing the solution flow rate decreases the analytical signal. Square wave voltammetry provides shorter analysis times and better sensitivity than differential pulse voltammetry.

Pulse voltammetry at cylindrical electrodes: oxidation of anthracene

Abstract Linear scan (cyclic), staircase, square wave, and normal and reverse pulse voltammetry are used to characterize the response for anthracene oxidation at a platinum microcylinder electrode. It is shown that such electrodes are well-suited to kinetic studies. It is normally assumed that the oxidation of anthracene follows an ECE mechanism. However, it is shown, particularly by comparison of simulated data for reverse pulse voltammetry with experimental values, that the mechanism is DISPI. The rate constant for the following chemical reaction depends on the voltammetric time-scale, and thus the overall reaction is not yet completely characterized.

Developing an automated robotic factory for novel stem cell therapy production

Advanced therapeutics, specifically cell and gene therapies, provide an opportunity to target previously unmet clinical conditions and offer a potential solution to the social and economic burden associated with an aging population [1,2]. Human mesenchymal stem/stromal cells (hMSCs) are a promising cell therapy candidate for the treatment of numerous clinical indications [3]; however, a transformation is required in the way we isolate, manufacture, characterize and deliver these therapies to ensure they are both efficacious and affordable [2,4]. Manufacture of hMSCs requires in vitro expansion to increase the available number of cells to meet clinical demand; however, progress is impeded by the lack of advanced methods for the isolation and expansion of cells that are scalable, amenable for automation and closed. Many allogeneic processes still require manual intervention which has significant quality and cost implications [5], and developing reproducible, consistent bioprocesses is still a major challenge. Even when a scale-out approach is to be employed (e.g., for autologous therapies), there is a significant practical challenge of manipulating, processing and segregating multiple production batches in an aseptic, closed manner. Such processes demand small units for manufacture in which line segregation is a priority in order to avoid product–patient mismatch and disease transmission. As such, there is an industrial trend toward automated systems as it is increasingly recognized that such systems facilitate consistent manufacture and will play a pivotal role in the translation of cell therapies by improving quality control, process economics, scalability, process capability and provide a platform for understanding process variation and optimization [6]. Through AUTOSTEM (European Commission Horizon 2020 funded research and innovation actions), academic and industrial groups from across the EU are working in collaboration to take a holistic approach to enable large-scale hMSC production, at clinical-grade quality, by implementing a robotic automated pipeline for cell isolation and culture [7]. AUTOSTEM builds on the ground-breaking work of the StemCellFactory project [8] which built and demonstrated an automated robotic pipeline for the production of human induced pluripotent cell lines (Figure 1). AUTOSTEM develops this technology further to enable automated, closed and good manufacturing practice (GMP)ready hMSCs (a ‘StromalCellFactory’) and will focus on the development of:

Kinetics of reduction of absorbed reactants in normal pulse voltammetry

Abstract A simple model is proposed for the totally irreversible reduction of adsorbed material and exemplary calculations are used to display its features. A statistically sound optimization method equivalent to the method of maximum likelihood and implemented by the COOL algorithm is employed to extract kinetic parameters from normal pulse voltammetric data for reduction of midazolam (8-chloro-6-(2-fluoro-phenyl)-1-methyl-4H-imidazo[1,5-a] [1,4]benzodiazepine) at a mercury electrode. The quality of results is contrasted with that obtained from conventional graphical methods.