Scanning electrochemical microscopy as a probe of Ag⁺ binding kinetics at Langmuir phospholipid monolayers

A new method has been developed for measuring local adsorption rates of metal ions at interfaces based on scanning electrochemical microscopy (SECM). The technique is illustrated with the example of Ag⁺ binding at Langmuir phospholipid monolayers formed at the water/air interface. Specifically, an inverted 25 μm diameter silver disc ultramicroelectrode (UME) was positioned in the subphase of a Langmuir trough, close to a dipalmitoyl phosphatidic acid (DPPA) monolayer, and used to generate Ag ⁺ via Ag electro-oxidation. The method involved measuring the transient current-time response at the UME when the electrode was switched to a potential to electrogenerate Ag⁺. Since the Ag⁺/Ag couple is reversible, the response is highly sensitive to local mass transfer of Ag⁺ away from the electrode, which, in turn, is governed by the interaction of Ag⁺ with the monolayer. The methodology has been used to determine the influence of surface pressure on the adsorption of Ag ⁺ ions at a phospholipid (dipalmitoyl phosphatidic acid) Langmuir monolayer. It is shown that the capacity for metal ion adsorption at the monolayer increased as the density of surface adsorption sites increased (by increasing the surface pressure). A model for mass transport and adsorption in this geometry has been developed to explain and characterise the adsorption process.