1. INTRODUCTION

Nanoparticles (particles of 1-101 nm in diameter) exhibit unique electronic, optical, photonic and catalytic properties ^1-3 and are an ideal size for use as nanotechnological building blocks.4 They may be composed of any substance, including metals,5 ^,6 semiconductors,7 ^,8 core-shell composite architectures, ^9-11 and organic polymers.12 ^,13 These particles often display properties intermediate between quantum and bulk materials because of their intermediate size14 ^,15 and large surface area-to-volume ratios.16 Nanoparticles of different sizes and shapes exhibit different absorbance and fluorescence features and reveal polarization effects.6 ^,17 ^,18 Microscopy techniques including high-resolution TEM, SEM, and scanning microscopy methods such as AFM, STM and NSOM can be employed to characterize their size, shape, crystal morphology and energy band gaps. ^19-25 Ingenious methods for the preparation of nanoparticles have been reported during the last years. Nanoparticles represent metastable clusters exhibiting the fundamental property to aggregate. Thus, the stabilization of the nanoparticles against aggregation is a fundamental pre-requisite in nanoparticle science and technology. The stabilization of nanoparticles may be accomplished by the capping of the nanoparticles with weakly, electrostatically-bound ions (e.g. citrate stabilized Au or Ag particles),5 ^,26 ^,27 by molecular ligands,28 ^,29 and the protection of the nanoparticles in polymer matrices,30 ^,31 or micellar assemblies.32 ^,33

The association of ligands to growing nanoparticles can control the dimensions34 and shapes of nanocrystals.35 Nanoengineering of nanoparticle surfaces, eg. layer-by-layer deposition...