Ion-transfer voltammetry at silicon membrane-based arrays of micro-liquid–liquid interfaces

Abstract

Microporous silicon membranes, fabricated by lithographic patterning and wet and dry silicon etching processes, were used to create arrays of micro-scale interfaces between two immiscible electrolyte solutions (muITIES) for ion-transfer voltammetry. These membranes served the dual functions of interface stabilization and enhancement of the rate of mass-transport to the interface. The pore radii were 6.5 microm, 12.8 microm and 26.6 microm; the pore-pore separations were ca. 20- to 40-times the pore radii and the membrane thickness was 100 microm. Deep reactive ion etching (DRIE) was used for pore drilling through the silicon, which had been previously selectively thinned by potassium hydroxide etching. DRIE produces hydrophobic fluorocarbon-coated internal pore walls. The small pore sizes and large pore-pore separations used resulted in steady-state voltammograms for the transfer of tetramethylammonium cation (TMA(+)) from the aqueous to the organic phase, whereas the reverse voltammetric sweeps were peak-shaped. These asymmetric voltammograms are consistent with the location of the ITIES at the aqueous side of the silicon membrane such that the organic phase fills the micropores. Comparison of the experimental currents to calculated currents for an inlaid disc micro-interface revealed that the interfaces were slightly recessed, up to 10 microm (or 10% of the pore length) in one case. Facilitated ion transfer, with an organic-phase ionophore, confirmed the location of the organic phase within the pores. These microporous silicon membranes offer opportunities for various analytical operations, including enhancing the rate of mass transport to ITIES-based sensing devices and stabilization of the ITIES for hydrodynamic applications.

Array of water|room temperature ionic liquid micro-interfaces

“…This was not unexpected, since the micropore walls were coated with a hydrophobic polymer during the fabrication process. Similar voltammetry was reported at water|organic solvent [14] and water|organogel interfaces [22] patterned by these micropore array membranes.…”

“…The ionic liquid chosen in this work ([P 14,6,6,6 . However, in our experiments the viscosity may be slightly lower, since the RTIL phase is pre-saturated with the water phase prior to experiments and the "hydrophobic" RTIL does uptake water to some extent [16,17].…”

“…A two-electrode cell was employed, with a Ag|AgCl wire placed in the aqueous phase (10 -2 M LiCl + 0.5 mM of the appropriate salt, XY) and a second Ag|AgCl wire in an aqueous reference solution of the ionic liquid (10 -2 M LiCl + saturated [P 14,6,6,6 ]Cl). The two aqueous phases were housed in borosilicate glass tubes of 4mm o.d.…”

“…In this work, we report the behaviour of a W|RTIL micro-interface array formed between water and the RTIL trihexyltetradodecylphosphonium tris(pentafluoroethyl)trifluoromethylphosphate ([P 14,6,6,6 ][FAP]) within a silicon micropore membrane (consisting of 30 micropores of ca. 23 µm diameter).…”

Electrochemical behaviour of myoglobin at an array of microscopic liquid–liquid interfaces

“…Methods for development of the micro/nano ITIES vary from pulled glass pipettes [11], laser ablation of a substrate [16] to various chemical etching methods [17]. The results reported here utilised a µITIES array fabricated from a silicon membrane containing an array of micropores [18]. In recent years there have been many reports on the behaviour and detection of biomolecules at the ITIES.…”