Metal nanoparticles have extraordinary size-dependent optical properties, not present in the bulk metal. Specifically, nanoparticles of silver, gold, and copper show distinct and well-defined plasmon absorption in the visible spectrum, an absorption characterized by an extremely large molar adsorption coefficient. These unique optical, electrical, and surface chemical characteristics are controllable through both particle size and aggregation,[18,19] and can also be "tuned'' to a degree by the nature of the surface functionalization.[20-22] Interestingly, the effect that the size-dependent electrical characteristics of nanoparticles can have on the properties of surface-confined molecules has recently been resolved in a 13C NMR study.[23]

Although diffraction analyses have shown that nanoparticles adopt the same crystal structure as the bulk metal, this appears to be less the norm as the particle size falls. For example, gold nanoparticles can adopt an icosahedral structure quite different to the bulk face-centered cubic arrangement. The valence and conduction band density of states (DOS) of these particles undergo significant variance with size; specifically, the initially continuous DOS is progressively replaced by discrete energy levels, the spacing of which increases with decreasing particle size (Fig. 1). As this spacing exceeds thermal energy, a "band gap'' effectively opens up in the metal. At smaller sizes still, discrete, energetically separated, quanta are resolvable. These size effects are observable in both metallic and semiconducting particles, although, in the latter, "quantum dot'' behavior can be observed at comparatively large (tens of nanometers) particle size.

From the perspective of developing derived electro-analytical devices, the redox properties of nanoparti-cles have been of some interest. The redox behavior of MPCs has been probed both diffusively in sol-ution[24,25] and in surface-confined mono- or multilayer films.[26-30] Again, as soon as nanoparticles fall below a finite size (< 1 nm), available electronic states become quantized and, electrochemically, this has the effect of the particle behaving, in essence, as a multivalent redox species.[31] The specific electrostatic charging of the metallic core of monolayer protected nanoparticles has also been referred to as quantized double layer charging.[20] The ability of these films to mediate current flow is of obvious application to the development of sensors.

Fig. 1 Schematic layout showing the progression from metallic band structure to quantized electronic structure as nanoparticle dimensions fall.
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