Interfacial Ion Transfers between a Monolayer Phase of Cationic Au Nanoparticles and Contacting Organic Solvent

Abstract

The highly cationic nanoparticle [Au(225)(TEA-thiolate(+))(22)(SC6Fc)(9)] adsorbs so strongly on Pt electrodes from CH(3)CN/Bu(4)NClO(4) electrolyte solutions that films comprised of 1-2 monolayers of nanoparticles can be transferred to nanoparticle-free electrolyte solutions without desorption and ferrocene voltammetry stably observed. (TEA-thiolate(+) = -S(CH(2))(11)N(CH(2)CH(3))(3)(+); SC6Fc = S(CH(2))(6)-ferrocene; Fc = ferrocene). The Fc(+/0) redox couple's voltammetry is used to detect the adsorption. The apparent formal potential (E(o)'(APP)) of the Fc(+/0) couple depends on the electrolyte--its anion, cation, and concentration--in the contacting nanoparticle-free solution. A 10-fold change in electrolyte concentration shifts the Fc(+/0) E(o)'(APP) by 48-67 mV, depending on the electrolyte. The dependency is interpreted to reflect the energetics of transfer of charge-compensating anions from the electrolyte solution to the monolayer nanoparticle "phase", promoted by the formation of Fc(+) sites in the nanoparticle film. This interpretation is supported by electrochemical quartz crystal microbalance results. Some further aspects of the results suggest adsorption of electrolyte cations at the nanoparticle film/electrolyte solution interface. The interface mimics a liquid/liquid interface between immiscible electrolyte solutions, in which the ion transfer approaches permselective behavior. The experimental results show that even 1-2 monolayers of highly ionic nanoparticles can behave as a polyelectrolyte "phase".