Metal nanoparticles: from "artificial atoms" to "artificial molecules".

Nanostructured materials often exhibit properties which are unusual for bulk materials of the same composition. When the size of nanostructures shrinks below a certain threshold, for example, they may behave as quantum dots, that is, as zero-dimensional systems in which the charge carriers are confined in atomic-like potentials. To harness nanoscale properties for applications, one must be able to control the size of the nanostructures as well as their assembly into macroscopic structures (such as arrays or films). Noble-metal nanoparticles exhibit unique electronic and optical properties that are critically related to their size and shape. Solution-based syntheses of metal nanoparticles exploit the chemical reduction of metal salts in the presence of functional surfactant molecules, which wrap a full monolayer around the nanoparticle core, preventing coalescence. This molecular monolayer has a protecting function and determines the nature of the interactions between the particles and their environment, hence controlling their solubility in different media, for example. Longchain thiols are the most widely used surfactant molecules, particularly for gold nanoparticles. Their self-assembly on metal surfaces is based on the formation of a covalent metal–sulfur bond (ca. 50 kcalmol ) and of attractive van der Waals interactions between the hydrocarbon chains, the strength of which scales with the length of the chain (ca. 1.5 kcalmol 1 per methylene group). On flat metallic surfaces, these interactions lead to the formation of highly ordered self-assembled monolayers (SAMs), which were first observed, and most extensively studied, on gold surfaces (predominantly Au(111)). A very similar architecture is generally presumed for SAMs on gold nanoparticles, although conventional wisdom and experimental observations suggest that molecule–surface interactions, which depend on the crystallographic orientation of the surface facets, will be particularly weak at the vertices of the nanoparticle polyhedron. 11–13] The functionality of the molecules composing the SAM at the nanoparticle surface is the key factor defining almost every application of metal nanoparticles. Although the strongly reductive conditions used for the synthesis of nanoparticles set stringent limitations on the insertion of functional groups in the thiol ligands, practically any functionality can be introduced into the nanoparticle by subsequent ligand-exchange reactions. Covalent and strong noncovalent interactions between molecules attached to nanoparticles have been used to engineer various nanostructures, including monoand multilayers, and nanocomposites with conducting polymers or oligomers. Also, gold nanoparticles have been used as “multivalent” cores to build organic dendritic structures. However, the isotropic character of such binding poses a problem for the bottomup fabrication of more complex nanoparticle-based architectures. Several groups have investigated monofunctionalized nanoparticles linked to multidentate molecules with established directional interactions that control the assembly of the nanoparticles. The required monofunctionalized nanoparticles have been prepared through reaction with a functional thiol bound to a solid support. This approach is commonly used in the application of gold nanoparticles as electrode interfaces in molecular junctions. Using the high-fidelity interaction between DNA strands employed as ligands, linear, cyclic, and discrete branched arrays of monofunctionalized gold nanoparticles have been fabricated. However, the “monovalent” nature of monofunctionalized nanoparticles renders them spectator pendant groups, rather than building blocks in these nanoengineering endeavors. Recent work by Stellacci<s group showing that surface curvature plays a key role in molecular self-assembly opened new perspectives in this field. This group performed detailed scanning tunneling microscopy (STM) investigations, which suggested that, below certain nanoparticle dimensions, surface [*] Prof. D. F. Perepichka Department of Chemistry McGill University 801 Sherbrooke Street West Montr1al, QC H3A2K6 (Canada) Fax: (+1)514-398-3797 E-mail: dmitrii.perepichka@mcgill.ca