Micropipette as a tool for the determination of the ionic species limiting the potential window at liquid/liquid interfaces

Catalytic Activity of Alkali Metal Cations for the Chemical Oxygen Reduction Reaction in a Biphasic Liquid System Probed by Scanning Electrochemical Microscopy

Chemical reduction of dioxygen in organic solvents for the production of reactive oxygen species or the concomitant oxidation of organic substrates can be enhanced by the separation of products and educts in biphasic liquid systems. Here, the coupled electron and ion transfer processes is studied as well as reagent fluxes across the liquid|liquid interface for the chemical reduction of dioxygen by decamethylferrocene (DMFc) in a dichloroethane-based organic electrolyte forming an interface with an aqueous electrolyte containing alkali metal ions. This interface is stabilized at the orifice of a pipette, across which a Galvani potential difference is externally applied and precisely adjusted to enforce the transfer of different alkali metal ions from the aqueous to the organic electrolyte. The oxygen reduction is followed by H2 O2 detection in the aqueous phase close to the interface by a microelectrode of a scanning electrochemical microscope (SECM). The results prove a strong catalytic effect of hydrated alkali metal ions on the formation rate of H2 O2 , which varies systematically with the acidity of the transferred alkali metal ions in the organic phase.

Microelectrochemical Techniques for Probing Kinetics at Liquid/Liquid Interfaces

We provide an overview of recent advances in microelectrochemical approaches to investigate the kinetics of various physicochemical processes that occur at the interface between two immiscible electrolyte solutions (ITIES). To place the advances in context, background material on the structure of the ITIES, derived from both experimental studies and computer simulation, is also provided. The main focus of the article is micro-ITIES techniques, single droplet measurements, microelectrochemical measurements at expanding droplets (MEMED) and scanning electrochemical microscopy (SECM). Recent developments in a combined SECM-Langmuir trough technique for probing diffusion processes across Langmuir monolayers at the water/air (W/A) interface are also highlighted, by considering an organic monolayer at a water surface as a special case of a liquid/liquid interface.

Simple Ion Transfer at Liquid|Liquid Interfaces

The main aspects related to the charge transfer reactions occurring at the interface between two immiscible electrolyte solutions (ITIES) are described. The particular topics to be discussed involve simple ion transfer. Focus is given on theoretical approaches, numerical simulations, and experimental methodologies. Concerning the theoretical procedures, different computational simulations related to simple ion transfer are reviewed. The main conclusions drawn from the most accepted models are described and analyzed in regard to their relevance for explaining different aspects of ion transfer. We describe numerical simulations implementing different approaches for solving the differential equations associated with the mass transport and charge transfer. These numerical simulations are correlated with selected experimental results; their usefulness in designing new experiments is summarized. Finally, many practical applications can be envisaged regarding the determination of physicochemical properties, electroanalysis, drug lipophilicity, and phase-transfer catalysis.

Voltammetric detection of heparin at polarized blood plasma/1,2-dichloroethane interfaces

Heparin, a highly negatively charged polysaccharide, which has been used widely as an anticoagulant and antithrombotic, was detected by ion-transfer voltammetry at the interface between 1,2-dichloroethane and an aqueous buffer solution or undiluted blood plasma. Quaternary ammoniums with different numbers of methyl and long alkyl groups were examined as positively charged heparin ionophores using pipet electrodes filled with the organic electrolyte solutions of their tetrakis(4-chlorophenyl)borate salts. It was shown that octadecyltrimethylammonium most selectively facilitates interfacial heparin adsorption without interference from potential-dependent ionophore transfer into the aqueous phase. Water-filled pipet electrodes were also used to study the stoichiometry of the interfacial complex between a heparin molecule and multiple ionophore molecules, which is discussed as a counterion condensation effect. Stripping voltammetry based on facilitated heparin adsorption and desorption gives a detection limit of 0.012 unit/mL in 0.12 M NaCl buffered at pH 7.2, which is 1 order of magnitude lower than therapeutic heparin concentrations (>0.2 unit/mL) and is comparable to a detection limit of the most sensitive heparin sensor reported so far. The biomedical utility of ion-transfer voltammetry was demonstrated for the first time in an undiluted blood sample. Despite interferences by Na+, Cl-, and plasma proteins such as serum albumin, a detection limit of 0.13 unit/mL was obtained in sheep blood plasma with the stripping method.

Fabrication and Characterization of Submicrometer- and Nanometer-Sized Double-Barrel Pipets

Submicro- and nanometer-sized glass double-barrel pipets have been fabricated by a laser puller with new pulling programs and have been used to support submicro- and nanometer dual liquid/liquid interfaces. The smallest pipet that can be made by this approach is approximately 20 nm in radius. These pipets have been characterized by cyclic voltammetry and scanning electron microscopy. Generation/collection mode of charge-transfer reaction is demonstrated at the submicro- and nanometer dual-liquid/liquid interfaces. The dependence of collection efficiency upon geometric parameters of the pipets has been discussed. Among the micro-, submicro-, and nanopipets, we have found that the submicro-double-barrel pipets have higher collection efficiencies than that of others and are also very close to the values predicted by the theory. Therefore, in terms of G/C mode applications, the optimal size of double-barrel pipets should be in submicrometer scale. As one of the examples of special application, we have also demonstrated that in the case of no supporting electrolyte, only the nanometer double-barrel pipets can provide reasonably good G/C results.

Facilitated protamine transfer at polarized water/1,2-dichloroethane interfaces studied by cyclic voltammetry and chronoamperometry at micropipet electrodes

Cyclic voltammetry and chronoamperometry at micropipet electrodes were applied to study the phase transfer of polypeptide protamine facilitated by complexation with charged ionophore dinonylnaphthalenesulfonate (DNNS) at polarized water/1,2-dichloroethane (DCE) interfaces, i.e., sDNNS(-) (DCE) + protamine(n+) (aq) right harpoon over left harpoon protamine-DNNS complex (DCE). Well-defined current responses based on the selective protamine transfer were obtained reproducibly even in the presence of 0.12 M NaCl. The selective and reproducible responses make this voltammetric/amperometric approach an attractive alternative to the traditional potentiometric counterpart based on mixed potential responses, for which both protamine and Na(+) need to be transferred simultaneously. Using both organic- and water-filled micropipet electrodes, the reaction mechanism was studied under different mass-transfer conditions controlled by diffusion of protamine, DNNS, and the complex in the outer solution of the pipets. Both charge number of transferred protamine, n, and complexation stoichiometry, s, were determined to be approximately 20 by chronoamperometry. With these parameters, the electrochemically irreversible voltammograms were analyzed by assuming a one-step transfer model to obtain experimental transfer coefficients, which represent apparent dependence of the transfer rate on the interfacial potential. The analysis showed that the transfer coefficients are much larger or smaller than a normal value of approximately 0.5 and strongly depend on the diffusion-limiting species, i.e., 0.088 +/- 0.005, 0.89 +/- 0.01, and 0.065 +/- 0.008 for protamine, DNNS, and the complex, respectively. The apparently anomalous transfer coefficients were explained consistently by a phenomenological model based on adsorption and transfer processes.

Wide Electrochemical Window at the Interface between Water and a Hydrophobic Room-Temperature Ionic Liquid of Tetrakis[3,5-bis(Trifluoromethyl)phenyl]borate

Ionic liquids composed of a hydrophobic anion, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB-), and various aromatic and aliphatic ammonium have been prepared. TFPB salts of N-octadecylisoquinolinium (C18Iq+) and trioctylmethylammonium (TOMA+) show low melting points of 31 and 36 degrees C, respectively, whereas TFPB-based ionic liquids of 1-alkyl-3-methylimidazolium with the alkyl chain length of 5-12 show melting points lower than 90 degrees C. The [C(18)Iq][TFPB]|water (W) and [TOMA][TFPB]|W interfaces have the polarized potential window (ppw) of 0.8 V at 56 degrees C, which is twice as wide as the ppw at the interface between W and room-temperature ionic liquids of bis(perfluoroalkylsulfonyl)imide so far reported. The extension of the ppw by more than 0.4 V to the positive direction enables voltammetric measurements of the ion transfer of relatively hydrophobic anions such as bis(trifluoromethylsulfonyl)imide and bis(pentafluoroethylsulfonyl)imide and of hydrophilic cations such as tetraethylammonium and acetylcholine ions.

Immobilized pH Gradient Gel Cell To Study the pH Dependence of Drug Lipophilicity

An experimental setup to study the pH dependence of standard ion-transfer potentials at the water/NPOE interface is presented. The system is composed of a microhole generated by laser photoablation in a 12-microm polyethylene terephthalate film, the aqueous phase consisting of a commercial immobilized pH gradient gel reswelled in electrolyte solution and a droplet of organic solution. Two electrodes are used, an Ag/AgCl aqueous electrode and an Ag/AgTPBCl organic electrode. This setup is applied to the study of two ionizable compounds (pyridine, 2,4-dinitrophenol). Thermodynamic parameters such as the standard transfer potential, the Gibbs energy of transfer, and the partition coefficients of the ionized forms as well as the neutral forms of these drug compounds are evaluated by differential pulse voltammetry. The data obtained are summarized in ionic partition diagrams, which are a useful tool for predicting and interpreting the transfer behavior of ionized drugs at the liquid/liquid interfaces mimicking the biological membranes.

Studies of Electron-Transfer and Charge-Transfer Coupling Processes at a Liquid/Liquid Interface by Double-Barrel Micropipet Technique

Investigation of a heterogeneous electron-transfer (ET) reaction at the water/1,2-dichloroethane interface employing a double-barrel micropipet technique is reported. The chosen system was the reaction between Fe(CN)63- in the aqueous phase (W) and ferrocene in 1,2-dichloroethane (DCE). According to the generation and the collection currents as well as collection efficiency, the ET-ion-transfer (IT) coupling process at such an interface and competing reactions with the organic supporting electrolyte in the organic phase can be studied. In addition, this technique has been found to be an efficient method to distinguish and measure the charge-transfer coupling reaction between two ions (IT-IT) processes occurring simultaneously at a liquid/liquid interface. On this basis, the formal Gibbs energies of transfer of some ions across the W/DCE interface, such as NO3-, NO2-, Cl-, COO-, TBA+, TPAs+, Cs+, Rb+, K+, Na+, and Li+, for which their direct transfers are usually difficult to obtain because of the IT-IT coupling processes, were quantitatively evaluated.

Studies of Effect of Phase Volume Ratio on Transfer of Ionizable Species across the Water/1,2-Dichloroethane Interface by a Three-Electrode Setup

The electrochemical behavior of pyridine distribution at the water/1,2-dichloroethane interface with variable phase volume ratios (r = Vo/Vw) was investigated by cyclic voltammetry. The system was composed of an aqueous droplet supported on a Ag/AgCl disk electrode covered with an organic solution or an organic droplet supported on a Ag/AgTPBCl disk electrode covered with an aqueous solution. In this way, a conventional three-electrode potentiostat can be used to study an ionizable compound transfer process at a liquid/liquid interface with a wide range of phase volume ratios (from 0.0004 to 1 and from 1 to 2500). Using this special cell we designed, only very small volumes of both phase were needed for r equal to unity, which is very useful for the investigation of the distribution of ionizable species at a biphasic system when the available amount of species is limited. The ionic partition diagrams were obtained for different phase volume ratios.

A new approach to micropatterning: application of potential-assisted ion transfer at the liquid-liquid interface for the local metal deposition

A new approach to micropatterning is demonstrated. The approach is based on driving an electrochemical process at the solid-liquid interface through the formation of a flux of ions from a micropipet that is held in close proximity to the surface. The flux of ions is generated by the so-called potential assisted ion transfer at the interface between two immiscible electrolyte solutions (ITIES). As a model system, the local deposition of silver was examined. Specifically, a constant potential, which was applied to a micropipet filled with an aqueous solution of silver ions, caused the transfer of Ag(+) into the outer nitrobenzene (NB) solution that consisted of an electrolyte, tetrabutylammonium tetrakis[4-chlorophenyl]borate (TBATPBCl). To facilitate the transfer of silver ions a macrocyclic ligand, that is, dibenzo-24-crown-8 (DB24C8), was added to the organic phase. The Faradaic current of this micro-ITIES was used as a means of controlling the tip-surface distance in scanning electrochemical microscopy (SECM) and depositing silver microstructures on a gold substrate.

Dual-pipet techniques for probing ionic reactions

Novel dual-pipet electrodes prepared by pulling borosilicate theta-tubing are described. Three types of electrochemical experiments employing such devices include the following: (1) generation/collection experiments in which ions are ejected from one of two micropipets ("generator") into the external solution and collected at the second pipet ("collector"), (2) measurements of ohmic current-voltage curves, and (3) ion-transfer voltammetry "in the air". The first setup is used for probing ion transfers at the interface between two immiscible liquids and homogeneous reactions in solution involving ionic species. Such experiments are reported for two model processes, i.e., simple and facilitated transfers of potassium between aqueous and organic phases and complexation of potassium with dibenzo-18-crown-6 in organic solution. The second arrangement is used for characterization of theta-pipets. The last arrangement can be useful for preparation of gas sensors. The possibility of measuring the concentration of volatile substances (e.g., ammonia and nitric acid) in the gaseous phase has been demonstrated.