Divalent metal nanoparticles

Abstract

Metal nanoparticles hold promise for many scientific and technological applications, such as chemical and biological sensors, vehicles for drug delivery, and subdiffraction limit waveguides. To fabricate such devices, a method to position particles in specific locations relative to each other is necessary. Nanoparticles tend to spontaneously aggregate into ordered two- and three-dimensional assemblies, but achieving one-dimensional structures is less straightforward. Because of their symmetry, nanoparticles lack the ability to bond along specific directions. Thus, the technological potential of nanoparticles would be greatly enhanced by the introduction of a method to break the interaction symmetry of nanoparticles, thus inducing valency and directional interparticle interactions. When a nanoparticle is coated with a mixture of two different ligands, the ligands have been shown to phase-separate into ordered domains encircling or spiraling around the core. Topological constraints inherent in assembling two-dimensional vectors (e.g., ligands) onto a sphere (the core of the nanoparticle) dictate the necessary formation of two diametrically opposed defect points within the ligand shell. The molecules at these points are not optimally stabilized by intermolecular interactions and thus these sites are highly reactive. By functionalizing the polar singularities with a third type of molecule, we generate divalent nanoparticles with "chemical handles" that can be used to direct the assembly of the particles into chains. For example, taking inspiration from the wellknown interfacial polymerization synthesis of nylon, we place carboxylic acid terminated molecules at the polar defect points and join the newly bifunctional nanoparticles into chains by reacting them with 1,6-diaminohexane through an interfacial reaction.

(cont.) Furthermore, we perform a full kinetic and thermodynamic characterization of the molecularly defined polar defect points. We demonstrate that the rate of place-exchange at these points is significantly faster than it is elsewhere in the ligand shell. We also determine the equilibrium constant and standard free energy of the place-exchange reaction at the polar defect sites and demonstrate that the reaction is strongly affected by the molecular environment, i.e. the composition of the ligand shell.