Stepwise Size-Selective Extraction of Carboxylate-Modified Gold Nanoparticles from an Aqueous Suspension into Toluene with Tetraoctylammonium Cations

Ion-pair formation between carboxylate anions on particle surfaces and tetraoctylammonium cations caused the phase transfer of well-characterized, water-soluble gold nanoparticles into an organic phase. Stepwise extraction procedures demonstrated for the first time that the initially fractionated solution included gold nanoparticles smaller than those in the following extractions, indicating that the smaller nanoparticles can transfer into an organic phase more easily than the larger ones. The systematic stepwise extraction is expected to provide a new and effective methodology for precise size selections. The particle size dependence of the transferred gold nanoparticles could be qualitatively explained by a phenomenological model for ion-pair extraction.