Synthesis and characterization of Janus gold nanoparticles.

A IO N Ligand molecules on monolayer-protected gold nanoparticles (NPs) determine the majority of their surface properties. It has been established that when a binary mixture of dislike ligands forms a self-assembled monolayer either on fl at surfaces [ 1 ] or on a NP, [ 2 ] separation into distinct domains occurs. For gold NPs between ∼ 3 and ∼ 8 nm in diameter, stripe-like domains form. [ 2b , 3 ] This happens because of a balance between the enthalpy of phase separation and the interfacial entropy that arises due to a size/length mismatch between two ligand molecules. [ 3 , 4 ] It has been predicted that as NPs become smaller, the interfacial entropy contribution becomes less and less relevant, since the increase in the cone angle that delimits the ligands yields an overall increase in conformational entropy for the entire chain ( Figure 1 ). Under this condition, the fi nal morphology is primarily determined by the enthalpy of separation; hence, for small particles, the spontaneous formation of completely separated distributions of ligands, i.e., the formation of Janus particles, has been predicted. [ 4 ] Here, we report that gold NPs with a core diameter smaller than ~1.5 nm form Janus NPs in many ligand combinations. We synthesized four different types of gold NPs with varying ligand combinations and characterized them using 2D NMR, analytical ultracentrifugation (AUC), gel electrophoresis (GEP), and scanning tunneling microscopy (STM). All of these techniques confi rm the presence of a majority of Janus particles and show similar cutoff sizes for the Janus-to-stripe transition. Ligand shell-protected NPs have drawn interest as new building blocks in chemistry; they have a central core, whose size and shape can be readily changed, and a coating that can be composed of multiple patches varying in chemical affi nity. [ 5 ]