Probing Ion Transfer at the Liquid/Liquid Interface by Scanning Electrochemical Microscopy (SECM)

Abstract

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.

Topographical and electrochemical nanoscale imaging of living cells using voltage-switching mode scanning electrochemical microscopy

“…1B) were used to produce theoretical approach curves for the same tip geometric parameters. The current profiles were fitted to established theoretical curves for a simple disk geometry (30)(31)(32). Good correlation was observed between theory and experiment.…”

“…The set point was typically in the range 75-90% of the reference current, I REF , which was the steady-state current measured in bulk solution. The corresponding tip/substrate distance could then be estimated by reference to theoretical approach curves for the characteristic electrode (30)(31)(32). The probe position was controlled by a XY and Z piezoelectric translation stage (Physik Instrumente, 621.2CL and 621.ZCL), using an amplifier module (Physik Instrumente, E-503.00).…”

Electrochemical attosyringe

“…The surface tension effects largely responsible for the liquid flow are not limited to the liquid/liquid interface but also depend on the water/glass and organic solvent/glass boundaries, and a three-phase water/organic/glass boundary. The properties of the inner pipette wall can be changed by modifying its surface (17,19). Rendering the glass surface more hydrophobic, e.g., via silanization, inhibits the ingress of water into the capillary (17).…”

“…The properties of the inner pipette wall can be changed by modifying its surface (17,19). Rendering the glass surface more hydrophobic, e.g., via silanization, inhibits the ingress of water into the capillary (17). In this way, both the extent of control over the solution ingress/egress and the magnitude of the required voltage can be varied.…”

“…Additional control over the cell approach and penetration by the pipette can be achieved by measuring the current flowing across the liquid/liquid interface as a function of the pipette displacement. Because of the blocking effect, the current decreases sharply when the distance between the pipette tip and the cell surface becomes comparable to the pipette radius (16,17). The current vs. pipette displacement curve can be used to estimate the tip/cell distance during the approach and subsequently control the penetration depth (SI Text and SI Fig.…”