Kyösti Kontturi

Surface redox catalysis for O2 reduction on quinone-modified glassy carbon electrodes

Abstract The electrochemical reduction of oxygen on bare glassy carbon (GC) and on electrodes grafted with anthraquinone has been studied using the rotating ring-disk electrode (RRDE) technique. The electrode surface was grafted by the electrochemical reduction of the corresponding diazonium salt. The functionalised electrode showed quasi-reversible redox behaviour in oxygen-free 0.1 M KOH and the cyclic voltammetric response of the surface bound anthraquinone was stable on potential cycling. The covalently attached anthraquinone acts as an electrocatalyst for oxygen reduction and a well defined diffusion limited current plateau was observed in the potential range from −0.75 to −1.25 V (SCE) in 0.1 M KOH. The RRDE results show that oxygen reduction on both electrode surfaces studied stops at the hydrogen peroxide stage. The modified carbon electrodes are potential candidates for the electrochemical production of hydrogen peroxide.

Polypyrrole as a model membrane for drug delivery

Abstract The possibility of using the conductive polymer polypyrrole as an ion gate membrane for the controlled release of anionic drugs has been studied using three model substances with therapeutic activity: salicylate, naproxen and nicoside. In addition, the release of tosylate, used for the electrosynthesis of the membrane, was investigated. The electrochemical quartz crystal microbalance was used to determine the mass changes occurring in 150 nm thick membranes loaded with the anion as a result of cathodic polarisation. Thicker membranes, 1–6 μm, were grown on a large gold electrode. The release of drugs from these membranes as a result of negative potential steps was followed using HPLC. It was found that the stability of the membrane doped with the appropriate drug towards chloride exchange in a 0.1 M NaCl solution was very good. Less than 5% of the theoretical amount of anion in the membrane was spontaneously released. Negative potential step experiments showed that salicylate, naproxen and tosylate, but not nicoside, could be released from the membrane in a controllable way. These results show that controlled release of a drug from polypyrrole membranes can be achieved using simple electrochemical step or staircase signals. Thus tailor made parts for iontophoretic devices can be manufactured. The amount of drug which can be released is, however, small and the application may therefore be restricted to drugs with high therapeutic activity.

Ion-exchange fibers and drugs: an equilibrium study

The purpose of this study was to investigate the mechanisms of drug binding into and drug release from cation-exchange fibers in vitro under equilibrium conditions. Ion-exchange groups of the fibers were weakly drug binding carboxylic acid groups (-COOH), strongly drug binding sulphonic acid groups (-SO(3)H), or combinations thereof. Parameters determining the drug absorption and drug release properties of the fibers were: (i) the lipophilicity of the drug (tacrine and propranolol are lipophilic compounds, nadolol is a relatively hydrophilic molecule), (ii) the ion-exchange capacity of the fibers, which was increased by activating the cation-exchange groups with NaOH, (iii) the ionic strength of the extracting salt (NaCl), which was studied in a range of 1.5 mM to 1.5 M, and finally (iv) the effect of divalent calcium ions (CaCl(2)) on the release of the model drugs, which was tested and compared to monovalent sodium ions (NaCl), and combinations thereof. It was found that the lipophilic drugs, tacrine and propranolol, were retained in the fibers more strongly and for longer than the more hydrophilic nadolol. The more hydrophilic nadolol was released to a greater extent from the fibers containing strong ion-exchange groups (-SO(3)H), whereas the lipophilic drugs were attached more strongly to strong ion-exchange groups and released more easily from the weak (-COOH) ion-exchange groups. The salt concentration and the choice of the salt also had an effect: at lower NaCl concentrations more drug was released as a result of the influence of both electrostatic and volume effects (equimolar drug:salt ratio). Incorporation of CaCl(2) in the bathing solution increased drug release considerably as compared to NaCl alone. The equilibrium distribution of the drug species between the fiber and external solution phases was also simulated and it was found that the theoretical modelling proposed describes adequately the basic trends of the behavior of these systems.

Hydrogen evolution across nano-Schottky junctions at carbon supported MoS2 catalysts in biphasic liquid systems

The activities of a series of MoS(2)-based hydrogen evolution catalysts were studied by biphasic reactions monitored by UV/Vis spectroscopy. Carbon supported MoS(2) catalysts performed best due to an abundance of catalytic edge sites and strong electronic coupling of catalyst to support.

Controlled Transdermal Iontophoresis by Ion Exchange Fiber

The objective of this study was to assess the transdermal delivery of drugs using iontophoresis with cation- and anion-exchange fibers as controlled drug delivery vehicles. Complexation of charged model drugs with the ion-exchange fibers was studied as a method to achieve controlled transdermal drug delivery. Drug release from the cation-exchange fiber into a physiological saline was dependent on the lipophilicity of the drug. The release rates of lipophilic tacrine and propranolol were significantly slower than that of hydrophilic nadolol. Permeation of tacrine across the skin was directly related to the iontophoretic current density and drug concentration used. Anion-exchange fiber was tested with anionic sodium salicylate. The iontophoretic flux enhancement of sodium salicylate from the fiber was substantial. As the drug has to be released from the ion-exchange fiber before permeating across the skin, a clear reduction in the drug fluxes from the cationic and anionic fibers were observed compared to the respective fluxes of the drugs in solution. Overall, the ion-exchange fibers act as a drug reservoir, controlling the release and iontophoretic transdermal delivery of the drug.

Electrochemical reduction of oxygen on thin-film Au electrodes in acid solution

The reduction of oxygen has been studied on thin-film gold electrodes using the rotating disk electrode (RDE) technique. Thin films of gold were prepared by vacuum evaporation onto glassy carbon and highly oriented pyrolytic graphite substrates. The surface morphology of the films was examined by atomic force microscopy. The kinetic parameters of O2 reduction have been determined and a Tafel slope of (−112±8)mVdec−1 was observed. The specific activity of the electrodes was almost independent of film thickness.

Interfacial capacitance and ionic association at electrified liquid/liquid interfaces

The capacitance of the interface of immiscible electrolytes has been measured for the Li+, Na+, K+, Rb+ and Cs+ chlorides in 1,2-dichloroethane and nitrobenzene. Evidence for ionic specific adsorption at the interface is presented. The nature of the specifically adsorbed charge is discussed in terms of the formation of interfacial ion pairs between the aqueous and the organic ions. The contribution of specific ionic adsorption to the interfacial capacitance is calculated on the basis of an ionic associated model using the Bjerrum theory of ion-pair formation. A mixed solvent region with varying penetration of the ion pairs into it, dependent on their ionic radii, best represents the structure of these interfaces.

Estimation of the pore size and charge density in human cadaver skin

Abstract Streaming potential and the ction transport number were measured in human cadaver skin. The transport number was used to estimate the charge density in aqueous pores of skin using both Donnan equilibrium and Langmuir adsorption isotherm. A microscopic model was employed to evaluate the pore radius from the streaming potential and charge density data. The capillary pore model was extended to account for any distribution of pore size, and applied particularly to the gaussian distribution. The mean radius of the distribution appeared to be slightly less than that of the single pore model. In both cases the radii were ca. 20 nm.

Electrosynthesis of polyphenylpyrrole coated silver particles at a liquid-liquid interface

This communication reports the production of polyphenylpyrrole coated silver nanoparticles at the liquid/liquid interface by an EC-type mechanism. In the electrochemical step of the reaction N-phenylpyrrole facilitates the transfer of the silver ion from an aqueous to an organic phase. This step is followed by a slow homogeneous electron transfer reaction from the N-phenylpyrrole to the silver ion followed by polymerization and metal cluster growth.

Improved stability and release control of levodopa and metaraminol using ion-exchange fibers and transdermal iontophoresis

Achievement of controlled drug delivery and stability of drugs during storage is a problem also in transdermal drug delivery. The objective of this study was to determine, whether an easily oxidized drug, levodopa, could be stabilized during storage using pH-adjustment and ion-exchange fibers. Controlled transdermal delivery of the zwitterionic levodopa was attempted by iontophoresis and ion-exchange fiber. Ion-exchange kinetics and transdermal permeation of a cationic (presumably more stable) model drug, metaraminol, were compared to the corresponding data of levodopa. Levodopa was rapidly oxidized in the presence of water, especially at basic pH-values. At acidic pH-values the stability was improved significantly. Ion-exchange group and the pH had a clear effect on the release of both the levodopa and metaraminol from the ion-exchange fiber. The adsorption/release kinetics of metaraminol were more easily controllable than the corresponding rate and extent of levodopa adsorption/release. Iontophoretic enhancement of drug permeation across the skin was clearly more significant with the positively charged metaraminol than with the zwitterionic levodopa. Ion-exchange fibers provide a promising alternative to control drug delivery and to store drugs that are degraded easily.