Introducing amphiphilicity to noble metal nanoclusters via phase-transfer driven ion-pairing reaction

Amphiphilicity is a surface property that has yet to be explored for the noble metal nanoclusters (NCs). This article shows how amphiphilicity may be added to sub-2-nm metal NCs by patching hydrophilic NCs (e.g., Au25(MHA)18 NCs where MHA is 6-mercaptohexanoic acid) with hydrophobic cations (e.g., cetyltrimethylammonium ion, CTA(+)) to about half of a monolayer coverage. Specifically we demonstrate the preparation of amphiphilic Au25(MHA)18@xCTA NCs (x = 6-9 where x is the number of CTA(+) per NC) by the phase-transfer (PT) driven ion-paring reaction between CTA(+) and -COO(-) (derived from the deprotonation of the terminal carboxyl group of MHA). Due to the coexistence of flexible hydrophilic MHA and hydrophobic MHA···CTA ligands in comparable amounts on the NC surface, the Au25(MHA)18@xCTA NCs (x = 6-9) exhibit good amphiphilicity, which enabled them to dissolve in solvents with distinctly different polarities and to self-assemble like a molecular amphiphile. Consequently, the amphiphilic Au25(MHA)18@xCTA NCs (x = 6-9) could self-organize into stacked bilayers at the air-liquid interface, similar to the formation of lyotropic liquid crystalline phases by common surfactants. The good solubility and molecular-amphiphile-like self-assembly properties can significantly increase the utility of noble metal NCs in basic and applied research.