Forms of Gold at the Nanoscale

However, as the cluster becomes larger there comes a point at which it becomes more accurate to consider it to be a nanoparticle, possibly with an exterior coated with a layer of capping ligands. We consider a diameter of 1 nm to be a reasonable although arbitrary value for differentiating between clusters and nanoparticles.  

The topic of gold nanospheres attracted the interest of several famous nineteenth century scientists such as Michael Faraday, Richard Zsigmondy, and Gustov Mie [43]. Interest diminished in the mid-twentieth century although some excellent contributions were made by Turkevich [42, 44], Frens [45], and Brust [46] in that period regarding the controlled preparation of nearly monodisperse colloidal suspensions. 

From a structural point-of-view the bulk metallic state, that is, fee lattice (with varying densities of defects such as twins and stacking faults) is generally established in gold nanoparticles of about 10 nm diameter and upwards. However, such particles still display many unusual physical properties, primarily as the result of their small size. Shrinking the size of gold particles has an important effect it increases both the relative proportion of surface atoms and of atoms of even lower coordination number, such as edge atoms [49] and these atoms in turn are relatively mobile and reactive. 

There are very many papers in the literature that address some aspect of gold nanospheres. In particular, their plasmon response (see Section 7.3.1.1) has been well studied, as has their agglomeration [50-52] and the manner in which they can be assembled into highly ordered colloidal crystals [50, 53, 54]. The latter are interesting and will be further discussed in Section 7.3.8.2. Conjugation of gold nanospheres with proteins and antibodies, for use as a stain in microscopy [55] or possibly, in medical applications [23], is another rich field. 

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