Classification of Nanosized Materials

The variety of compositions and shapes reported to date makes it quite difficult to outline a univocal classification system for nanosized materials. One possible distinction is based on the dimensionality of the structures, as illustrated in Table 6.1 [33]. 

The classification scheme shown is not definite. For example, the distinction between NPs and clusters cannot be established on the basis of dimensional criteria. Although the term cluster is used for small Au NPs [34], in principle, they are characterized by a well-defined structure [35], while the mobility of the surface atoms in the NPs does not allow one to ascribe them an exact geometrical shape. Similarly, although transmission electron microscopy (TEM) images depict carbon black particles as spherical and they are thus classified as OD nano-objects, they actually consist of disordered graphene sheets. 

Other authors [36] suggest a different classification, suitable to categorize a larger number of mono- and multi-component nanomaterials. In particular, the authors identify 36 classes of nanostructures containing up to three different components. 

In most cases, nanostructures possess an inorganic core of metal, carbon, or semiconductor material. A few examples of nanostructured coatings based on pure conductive polymers [37] have also been reported in the literature, as better described in Sect. 6.5. Most metal nanomaterials consist of noble metals, although Ag and Cu have also been reported [8, 11, 13-15, 17-20, 23, 24, 26-28]. For electroanalytical applications, NPs constitute the most common nanostructures, followed by nanowires [38, 39]. Although the former appear spherical, they are actually a polyhedral shape (Fig. 6.1). 

Carbon nanostructures (Fig. 6.2) comprise a variety of materials [42] based mainly on carbon atoms with sp hybridization. The most widespread in the field of electroanalysis are CNTs [1,25,29, 30,43,44] and graphene [1, 7,9,12,21,22, 45 9], although applications involving fullerene [50] and carbon black [51] have also been reported. Both CNTs and graphene are graphite derivatives, consisting of rolled or planar sheets, respectively. Both single- and multi-wall CNTs have been 

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The classification of nanosized materials is quite challenging, due to the extreme heterogeneity of compositions and shapes reported so far. A tentative classification may be based on dimensionality of the structures, as shown in Table 17.1. ... 

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