Mass Transfer from Ion-Sensing Component-Loaded Nanoemulsions into Ion-Selective Membranes: An Electrochemical Quartz Crystal Microbalance and Thin-Film Coulometry Study

Recent work has shown that ion-selective components may be transferred from nanoemulsions (NEs) to endow polymeric membranes with ion-selective sensing properties. This approach has also been used for nanopipette electrodes to achieve single-entity electrochemistry, thereby sensing the ion-selective response of single adhered nanospheres. To this date, however, the mechanism and rate of component transfer remain unclear. We study here the transfer of lipophilic ionic compounds from nanoemulsions into thin plasticized poly(vinyl chloride) (PVC-DOS) films by chronoamperometry and quartz crystal microbalance. Thin-film cyclic coulovoltammetry measurements serve to quantify the uptake of lipophilic species into blank PVC-DOS membranes. Electrochemical quartz crystal microbalance data indicate that the transfer of the emulsion components is insignificant, ruling out simple coalescence with the membrane film. Ionophores and ion-exchangers are shown to transfer into the membrane at rates that correlate with their lipophilicity if mass transport is not rate-limiting, which is the case with more lipophilic compounds (calcium and sodium ionophores). On the other hand, with less lipophilic compounds (valinomycin and cation-exchanger salts), transfer rates are limited by mass transport. This is confirmed with rotating disk electrode experiments in which a linear relationship between the diffusion layer thickness and current is observed. The data suggests that once the nanoemulsion container approaches the membrane surface, transfer of components occur by a three-phase partition mechanism where the aqueous phase serves as a kinetic barrier. The results help better understand and quantify the interaction between nanoemulsions and ion-selective membranes and predict membrane doping rates for a range of components.