Nanometer-sized metal particles (e.g., gold and silver) are certain to be important fundamental building blocks of future nanoscale electronic and optical devices. However, there are numerous challenges and questions which must be addressed before nanoparticle technologies can be implemented successfully. For example, basic capping ligand chemistrysnanoparticle electronic function relationships must be addressed in greater detail. New methods for assembling nanoparticles together into higher-order arrays with more complex electronic functions are also required. This review highlights our recent progress toward characterizing electron transport in gold nanoparticles as a function of capping ligand charge state. These studies have shown that single electron tunneling energies can be manipulated predictably via pH-induced charge changes of surfacebound thiol capping ligands. We also show that rigid phenylacetylene molecules are useful bridges for assembling gold and silver nanoparticles into arrays of two, three, and four particles with psuedo D ∞h , D 3h , and T d symmetries. These nanoparticle "molecules" interact electromagnetically in a manner qualitatively consistent with dipole coupling models.

A method for synthesizing hollow nanoscopic polypyrrole and poly(N-methylpyrrole) capsules is described. The method employs gold nanoparticles as templates for polymer nucleation and growth. Etching the gold leaves a structurally intact hollow polymer capsule with a shell thickness governed by polymerization time (ca. 5 to >100 nm) and a hollow core diameter dictated by the diameter of the template particle (ca. 5−200 nm). Transport rates of gold etchant through the polymer shell to the gold core were found to depend on the oxidation state of the polymer, those rates being a factor of 3 greater for the reduced form of the polymer. We show for the first time that not only is the particle a useful template material but also that it can be employed to deliver guest molecules into the capsule core. For example, ligands attached to the gold surface prior to poly(N-methylpyrrole) formation remained trapped inside the hollow capsule following polymer formation and gold etching.

Studies of inorganic clusters continue to reveal fundamental information regarding the size, shape and mediumdependent optical and electronic behaviors of nanoscopic materials.[1] Much of this research has involved characterization of the collective properties of disordered and crystalline two-dimensional (2D) and three-dimensional (3D) arrangements of clusters.[2] Optical absorptions and electron hopping in these crystals of clusters have proven to be strongly dependent on the distance and medium between clusters. These observations have generated interest in nanoclusters on several more applied fronts; e.g., gold cluster chemiresistive sensors and deoxyribonucleic acid assay methods have been reported recently.[3]The fundamental and applied advances described above vis-à-vis extended cluster networks prompted our group, [4] and others, [5] to examine the properties of more discrete assemblies of nanoclusters (e.g., dimers, trimers, etc.) so that the effects of local symmetry on collective particle properties could be better assessed. Herein the assembly of phenylacetylene-bridged gold nanoparticle dimers and trimers from solution is reported (Scheme 1). Phenylacetylene oligomers I and II (PA I, II) were chosen as basic linker repeat units because: 1) they are conformationally rigid molecules which could be expected to keep coupled nanoparticles at a fixed distance, an important difference from the DNA-linked systems reported previously; 2) they can be coupled to form a variety of geometries (e.g., linear, bent, trigonal planar, tetrahedral); [6] 3) lengths of up to 16 repeat units (ca. 20 nm) are readily synthesized without significant solubility problems; and 4) they have been discussed as potential wire candidates for molecular electronic devices. Given these advantages, we anticipated that the successful synthesis of PA-bridged gold nanoparticles would allow particle array symmetry±optical property relationships to be established. Indeed, in our initial optical studies reported herein, we have found, in accord with theoretical predictions, that array symmetry does influence optical properties. Gold nanoparticles were synthesized using the citrate-reduction method reported previously.[7] The strategy for formation of gold nanoparticle dimers and trimers involves producing a locally high concentration of the gold sol with respect to the PA linkers. This was accomplished by the rapid addition of an aqueous gold nanoparticle solution (5 mL of a 0.1 mM solution) to a solution of linker (20 mL of 10 mM solution) in a mixture of CH 3 CH 2 OH/water (1:3). Ethanol was necessary to ensure solubility of the linkers, however, the addition of higher concentrations of ethanol resulted in rapid flocculation of the gold nanoparticles. Phenylacetylene-bridged gold nanoparticle dimer formation was followed in time by visible spectroscopy (Fig. 1).Prior to the addition of PA linker I, the wellknown plasmon of~12 nm diameter gold particles was observed at l max = 525 nm. [7] Following addition of nanoparticles to a slight exce...

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