A new way of using the scanning electrochemical microscope (SECM) to probe ion transfer (IT) reactivity of the liquid/liquid and liquid/membrane interfaces is described. A micropipet filled with solvent (e.g., water) immiscible with the outer solution (e.g., organic) serves as an SECM tip. The tip current is due to transfer of an ion between the pipet and the outer solution. When such a tip approaches a macroscopic liquid/liquid or liquid/membrane interface, the IT process depletes the concentration of the transferred ion near the phase boundary. If the second phase (e.g., another aqueous solution) contains the same ion, the depletion results in the IT across the interface. The tip current depends on the rate of the interfacial IT reaction, which can be extracted from the tip current vs distance curves. By scanning the pipet tip over the phase boundary the topographic images and maps of IT reactivity of the interface can be obtained, e.g., high-resolution feedback mode images of micrometer-sized pores in polycarbonate membranes. The feedback-mode SECM theory previously developed for metal tips surrounded by a relatively thick insulating glass sheath is not fully applicable to the pipet tips. A family of theoretical tip current vs distance curves for pipet tips with different insulator thicknesses is obtained by numerical solution of partial differential equations using a commercial adaptivegrid program, PDEase2. The developed theory is presented in the form of two analytical approximations suitable for diffusion-controlled heterogeneous reaction and for a blocking interface. The possibilities of measuring fast IT kinetics are also discussed.

Nanometer-sized glass-sealed metal ultramicroelectrodes (UMEs) have been prepared using a laser-based micropipet puller. The tip was exposed to solution either by etching away a small portion of glass insulator or by micropolishing. The characterization of the UMEs was carried out by a combination of steady-state voltammetry, scanning electron microscopy (SEM), and scanning electrochemical microscopy (SECM). The cyclic voltammograms obtained have a regular shape with very small capacitive and resistive background. The effective electrode radii obtained from voltammetry were between 2 and 500 nm. From the SEM micrographs, the shape of polished tips appears to be close to a microdisk, while the geometry of etched electrodes is closer to conical. Accordingly, the SECM current-distance curves (i T -d) obtained with polished electrodes fit well the theory for a disk-shaped tip, while a 20-nm-radius etched electrode was shown to be a fairly sharp cone with a height-to-radius ratio of about 2.5. The experimental data were compared to the theory developed for disk-shaped, conical, and recessed tips to demonstrate suitability of the produced electrodes for quantitative electrochemical experiments. The prospects of steady-state measurements of the rates of fast heterogeneous reactions are discussed. Submicrometer-sized ion selective electrodes (ISEs) were prepared by coating etched Ag tips with silver iodide. The concentration response of such ISEs remained stable and linear after coating of the ISEs with protective Nafion film.

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The use of micropipet electrodes for quantitative voltammetric measurements of ion-transfer (IT) and electron-transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) requires knowledge of geometry of the liquid interface. The shape of the meniscus formed at the pipet tip was studied in situ by video microscopy under controlled pressure. The shape of the interface can be changed from a complete sphere to a concave spherical cap by varying the pressure applied to the pipet, and the diffusion current to the pipet changes accordingly. With no external pressure applied, the water/organic interface turned out to be flat, and the voltammetric response of a pipet must follow the well-known theory for a microdisk electrode. The large deviations from this theory observed previously can be attributed to a small amount of the filling aqueous solution which escapes from the pipet and forms a thin layer on its outer wall. This effect can be eliminated by making the outer pipet wall hydrophobic. Procedures have been developed for independent silanization of the inner and outer walls of the pipet. Pipets with a silanized inner wall can be filled with an organic solvent (e.g., 1,2-dichloroethane) and be used for voltammetric measurements in aqueous solutions. Another mode of voltammetry is based on trapping of a thin layer of organic solvent in the narrow shaft of a pipet between the filling solution and the aqueous outer phase. This arrangement is potentially useful for electrochemical catalysis and sensor applications.

We have extended the negative side of the potential window by using the organic reference electrode Ag/ AgTPB and a low supporting electrolyte concentration in the aqueous phase. We have observed directly, for the first time, the voltammetric wave of chloride transferring across the water(W)/1,2-dichloroethane (1,2-DCE) and the water/nitrobenzene (NB) interfaces. The standard potentials (and the standard Gibbs free energies of transfer) have been evaluated by consideration of half-wave potentials, the variations of the activity coefficients in both phases, and ion-pair formation. Our best estimate for the free energy of Cl -transfer to water-saturated 1,2-DCE is 45-46 kJ/mol, and for NB 38 ( 2 kJ/mol. The Gibbs free energies of transfer of a homologous series of long-chain dicarboxylates have also been obtained for the first time. They show a similar dependence on carbon number as monocarboxylates. The transfers of the singly and doubly ionized dicarboxylic acids can also be observed simultaneously when the pH of the aqueous phase is controlled in the range of 5.5-6.6. Shorter chain dicarboxylates, such as glutarate, -OOC(CH 2 ) 3 COO -, cannot be transferred across either the W/1,2-DCE or W/NB interface because it is more thermodynamically uphill than the transfer of the aqueous electrolyte anion or the organic electrolyte cation. One way to overcome some of the thermodynamic barrier is to use a receptor in the organic phase that would result in facilitated transfer. Our data show that a receptor for glutarate that had a formation constant of about 10 3 for a receptor-glutarate complex would allow for facilitated transfer. Through use of literature data for the transfer of the neutral dicarboxylic acids and H + , we can estimate the difference in pK a values between the aqueous phase and the water-saturated nonaqueous phases. In 1,2-DCE we estimate the differences for first and second ionizations as 18.5 and 19.5, respectively, while for NB they are 13.8 and 14.4.