, nanowires 5,6 and nanotubes 7 have recently attracted intensive research interest because of the uniqueness and ease in tailorability of their properties 8 .Similarly, nanostructured materials have shown improved properties (for example,the mechanical behaviour of nanostructured metals) 9 , as well as new ones (for example, the creation of a photonic bandgap in a block copolymer because of domain ordering) 10 . The combination of nanosize and nanostructure can lead to a plethora of unique and complex materials. Indeed,nanowires composed of layers of different materials have shown selective and improved hole mobility 11 , and core-shell metal nanoparticles have tuneable surface plasmon resonance 12 . Here, we present a new class of materials: monolayer-protected metal nanoparticles 1,3,4 (MPMN) with phase-separated ordered domains in their ligand shell. Because of the extremely small size of the domains (∼5 Å) these particles interact with the molecular environment in a novel way; for example, they prevent non-specific adsorption of proteins.Self-assembled monolayers (SAMs) are monomolecular layers on surfaces 13 that provide additional properties such as specific surface energies 14 and opto-electronic behaviour 15 . SAMs composed of a mixture of ligands can be easily produced in either one-step, by absorption from a solution of different molecules, or in two-steps, by placing a preformed monolayer into a solution of a different ligand 13,16 . Scanning tunnelling microscope (STM) images have shown that some mixed SAMs present phase-separated domains, but with no particular order [16][17][18][19][20] . It has been established that the phase separation is a thermodynamically driven process We demonstrate that the formation of ordered domains depends on the curvature of the underlying substrate, and that novel properties result from this nanostructuring.For example, because the size of the domains is much smaller than the typical dimensions of a protein, these materials are extremely effective in avoiding non-specific adsorption of a variety of proteins.

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 onedimensional 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.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.

We perform atomistic and mesoscale simulations to explain the origin of experimentally observed stripelike patterns formed by immiscible ligands coadsorbed on the surfaces of gold and silver nanoparticles. We show that when the conformational entropy gained via this morphology is sufficient, microphase-separated stripelike patterns form. When the entropic gain is not sufficient, we instead predict bulk phase-separated Janus particles. We also show corroborating experimental results that confirm our simulational predictions that stripes form on flat surfaces as well as on curved nanoparticle surfaces.

The ligand shell that coats, protects, and imparts a large number of properties to gold nanoparticles is a 2-D self-assembled monolayer wrapped around a 3-D metallic core. Here we present a study of the molecular packing of ligand shells on gold nanoparticles based on the analysis of scanning tunneling microscopy (STM) images. We discuss methods for optimal nanoparticle sample preparation in relation to STM imaging conditions. We show that the packing of a self-assembled monolayer composed solely of octanethiols on gold nanoparticles depends on the particle's diameter with an average headgroup spacing of 5.4 A, which is different from that of similar monolayers formed on flat Au(111) surfaces (5.0 A). In the case of nanoparticles coated with mixtures of ligands-known to phase separate into randomly shaped and ordered domains on flat surfaces-we find that phase separation leads to the formation of concentric, ribbonlike domains of alternating composition. The spacing of these domains depends on the ligand shell composition. We find that, for a given composition, the spacing increases with diameter in a manner characterized by discontinuous transitions at "critical" particle sizes. We discuss possible interpretations for the observed trends in our data.

Self-assembled monolayer-protected nanoparticles are promising candidates for applications, such as sensing and drug delivery, in which the molecular ligands' interactions with the surrounding environment play a crucial role. We recently showed that, when gold nanoparticles are coated with a binary mixture of immiscible ligands, ordered ribbon-like domains of alternating composition spontaneously form and that their width is comparable with the size of a single solvent molecule. It is usually assumed that nanoparticles' solubility depends solely on the core size and on the molecular composition of the ligand shell. Here, we show that this is not always the case. We find that the ligand shell morphology affects the solubility of these nanoparticles almost as much as the molecular composition. A possible explanation is offered through a molecular dynamics analysis of the surface energy of monolayers differing only in their domain structure. We find that the surface free energy of such model systems can vary significantly as a function of ordering, even at fixed composition. This combined experimental and theoretical study provides a unique insight into wetting phenomena at the nano- and subnanometer scale.