Particles in the nanometer size range are attracting increasing attention with the growth of interest in nanotechnological disciplines. Nanoparticles display fascinating electronic and optical properties as a consequence of their dimensions and they may be easily synthesized from a wide range of materials. The dimensions of these particles makes them ideal candidates for the nanoengineering of surfaces and the fabrication of functional nanostructures. In the last five years, much effort has been expended on their organization on surfaces for the construction of functional interfaces. In this review, we address the research that has led to numerous sensing, electronic, optoelectronic, and photoelectronic interfaces, and also take time to cover the synthesis and characterization of nanoparticles and nanoparticle arrays.

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The aggregation of Au nanoparticles in solution is induced and influenced by cationic and oligocationic species. This solution-state aggregation bears similarities to multilayer formation on surfaces but is more facile because of the nanoparticles' intrinsic instability in solution. Aggregation is followed by transmission electron microscopy (TEM) and the appearance of features at λ ) 600-900 nm in the absorbance spectrum. It is found that these features are a function of factors such as the aggregant size, charge, and concentration, and the method of mixing the components, and they can be related to aggregate morphology. It seems that there are two mechanisms that can act to cause aggregation. Multiply charged aggregants can bind nanoparticles together into dense aggregates, displaying a defined absorbance at ca. λ ) 700 nm, whereas singly charged aggregants cause a slower aggregation into string-like aggregates with a less defined absorbance. Whereas multiply charged aggregants can "cross-link" the layers in a multilayer structure on a surface, singly charged aggregants cannot.

The cover picture shows a selection of colloidal “nanoparticles”, composed of metallic or semiconductive materials, which may be assembled onto gold or glassy (ITO) surfaces. These organized, hybrid materials exhibit electrical, optical, and catalytic properties that differ from the bulk materials and find application in an increasing number of situations. In all cases, the nature and dimensions of the nanoparticles, the linkers used to immobilize them, the surface to which they are bound, and the organizational style are critically important. Further details are given in the review by Willner et al. on pages 18–52.

A mechanical switch in a [2]catenane, made up of a cyclobis(paraquat-p-phenylene) tetracation interlocked with a macrocyclic polyether containing a redox-active tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene ring system, can be thrown either chemically or electrochemically. The neutral TTF unit resides "inside" the tetracationic cyclophane in the reduced state and "alongside" it in the oxidized species (TTF / TTF ). Switching between the reduced (I ) and oxidized state (I (I )) is accompanied by a dramatic color change.

Recent advances in the assembly of nanoparticle superstructures on electrodes are addressed here. Methods for the assembly and characterization of these arrays are summarized and their electronic, photoelectrochemical, and sensor applications are discussed. The Figure shows a one‐layer architecture of Au nanoparticles and tetracationic cyclophanes on an amine‐functionalized ITO support.

A new approach to the construction of functional materials is demonstrated. Gold nanoparticles immobilized in a polymer hydrogel allow external control over the conductivity by switching the gel between its swollen and shrunken states. This reversible cycling (see Figure) controls the interparticle distances between the Au nanoparticles, which may lead to new methods in sensing and catalysis.