Detection of Tl(I) Transport through a Gramicidin−Dioleoylphosphatidylcholine Monolayer Using the Substrate Generation−Tip Collection Mode of Scanning Electrochemical Microscopy

We report the use of scanning electrochemical microscopy (SECM) to control the generation of Tl(I) at a mercury substrate and detect this species at a mercury-coated tip to study the transport of these ions through ion channels. The transport of Tl(I) across gramicidin D half-channels imbedded in a dioleoylphosphatidylcholine (DOPC) monolayer supported on a Tl amalgam hanging mercury drop electrode (HMDE) was studied using the substrate generation−tip collection mode of SECM. A Hg/Pt “submarine” electrode, used as the SECM tip, was made through simple contact of the Pt ultramicroelectrode with the HMDE. The tip transient response for the collection of generated Tl(I) at the amalgam HMDE was recorded for several tip to substrate distances. This collection−generation experiment was repeated with a DOPC-modified Tl/HMDE and a gramicidin−DOPC-modified Tl/HMDE. An apparent heterogeneous rate constant (k het = 2.8 (±0.1) × 10-4 cm/s) for the transport of Tl(I) through the gramicidin to the tip was extracted.