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Nanomaterials: Classification and Properties

Francesco Trotta1 and Andrea Mele2

1 University of Torino, Department of Chemistry, Via Pietro Giuria 7, 10125 Torino, Italy

2 Politecnico di Milano, Department of Chemistry, Materials, and Chemical Engineering "Giulio Natta", Piazza Leonardo da Vinci 32, 20133 Milano, Italy

1.1 Nanomaterial Classifications

The word "nano" derives from the latin word "nanus" and Greek word "? ??," both indicating a person of very low height, i.e. a dwarf. The International System () of units considers nano as a prefix to indicate 10-9 part of a unit; thus, for instance, a billion of a meter, a billion of a liter, a billion of a kilogram, etc. Not always the term nano is referred to a very small object. For instance, in astronomy, a nanostar is a star having a mass comparable to our Sun or even less. A first, easy, and practical criterion to define nanomaterials is based on the dimensions "tout court": Nanomaterials are conventionally defined as materials having at least a dimension between 1 and 100?nm. As a consequence, nanoparticles have all the three dimensions in the nanometer range, nanoplates present only one dimension below 100?nm, whereas nanofibers have two dimensions in the range of nano being the remaining remarkably longer. Some common terminologies of the nanorange world are listed in Table 1.1.

Table 1.1 Current definitions of terms with the "nano" suffix.

Source: Adapted from ISO/TS 27687.

Type Description Nanoscale Size range from approximately 1-100?nm Nano-object Material with one, two, or three external dimensions in the nanoscale Nanoparticle Nano-object with all three external dimensions in the nanoscale Nanotemplate Nano-object with one external dimension in the nanoscale and the other two external dimensions significantly larger (at least three times) Nanofiber Nano-object (flexible or rigid) with two external dimensions in the nanoscale and the third dimension significantly larger Nanotube Hollow nanofiber Nanorod Solid nanofiber Nanowire Electrically conducting or semiconducting nanofiber

Nevertheless, 100?nm as an upper limit for a nanomaterial is not always accepted. Many organizations in the world fixed different thresholds for the nanoscale, although 100?nm still remains the most common shared limit. Table 1.2 presents some recommendations suggested by different organizations.

Table 1.2 List of the recommended upper limits suggested by different organizations.

Source: Adapted from Klaessig et al. [1].

Upper limit (nm) Source 100 ISO 100 ASTM 100 Royal Society - SCENIHR 100 ETC group 100 Swiss Re 200 Soil Association 200 Defra 300 Chatham House 300 Friends of Earth 500 Swiss federal Office of Public Health 1000 House of Lords Science Committee

It is immediately clear that adequate techniques to determine the dimensions of the nano-objects are required. Table 1.3 reports the methods till now available to measure the size of the objects in the nanometric range. To avoid incorrect results and classification, particular care and attention should be devoted to (i) prepare a representative sample for analysis, (ii) follow a correct sample preparation, (iii) use the most appropriate mathematical analysis to get size distribution, and (iv) consider the comparability among different laboratories. Detailed guidelines for sample preparation in GMO analysis were reported in the Joint Research Centre () technical report in 2014 [3].

Table 1.3 Techniques to measure particle sizes in the nanometer dimension range.

Source: Adapted from Lisinger et al. [2].

Method name (abbreviation) Measurement range and medium (limiting factors) Types of size distribution of raw data Electron microscopy (EM) 1?nm or higher; dry (dynamic range) Number based Dynamic light scattering (DLS) 5-500?nm; suspension (sedimentation and scattering intensity) (No distribution, or scattering intensity based) Centrifugal liquid sedimentation (CLS) 20?nm and higher; suspension (particle density) Extinction intensity based Small-angle X-ray scattering (SAXS) 5?nm and higher; suspension (dynamic range) Scattering intensity based Field flow fractionation (FFF) 1-200?nm; suspension (dynamic range) (Depends on detector) Particle tracking analysis (PTA) 25?nm and higher; suspension (dynamic range) Number based Atomic force microscopy (AFM) 1?nm and higher; dry (dynamic range) Number based X-ray diffraction 1?nm and higher; dry (only for crystalline materials) (No distribution measured)

A sketch of nano-objects is reported in Figure 1.1.

Figure 1.1 Classification of nanomaterials (a) 0D spheres and clusters; (b) 1D nanofibers, wires, and rods; (c) 2D films, plates, and networks; and (d) 3D nanomaterials.

Source: Adapted from Alagarasi [4].

Nanosponges, the subject of this book, can be considered as porous materials having all of the three external dimensions in the micro- or macrorange and the internal cavities, pores, or voids in the nanometer range. Actually, nanosponges can be either of organic or inorganic origin, natural or synthetic [5]. A simple sketch of a type of nanosponges based on cyclodextrins is reported in Figure 1.2.

Figure 1.2 Possible structure of cyclodextrin nanosponge.

Source: Reproduced under CC license from Singh et al. [6]. Published by The Royal Society of Chemistry.

In other words, nanosponges can be counterintuitively classified as nanomaterials because of the presence of a network of nanometer-sized cavities in the bulk, despite the fact that the dimensions of a given specimen along the x-, y-, and z-axes can be larger than 100?nm. From this viewpoint, nanosponges are characterized by nanometric structural features, but they are generally not nanoparticles. A hierarchical classification of nanomaterials has been proposed based on the particular feature falling in the nanometer size domain. A graphical summary, along with the relevant normation (vide ultra), is shown in Figure 1.3. The identification of the characteristic of a given material belonging to the nanometric range is particularly relevant for safety and health. IUPAC Glossary of Terms used in Toxicology indeed gave the following definition of "nanoparticle" [8]: "Microscopic particle whose size is measured in nanometers, often restricted to the so-called nanosized particles (NSPs; <100?nm in aerodynamic diameter), also called ultrafine particles," whereas no listed definition of nanomaterial has been proposed yet.

Figure 1.3 The ISO definition of nano-objects. Included as nano-objects are nanoparticles (nanoscale in all the three dimensions), nanofibers (nanoscale in two dimensions), and nanoplates or nanolayers (nanoscale only in one dimension).

Source: Krug and Wick 2011 [7]. Reprinted with permission from John Wiley & Sons.

Several times, nanomaterials can also be included in other bulk materials to form nanocomposites with external dimensions larger than 100?nm, but entrapping nanoparticles in the bulk. Although under discussion, even these materials should be seen as nanomaterials. This is the case of materials with antibacterial properties and made of nanoparticles dispersed in several environments such as fabrics, plastics, chitosans, etc. In a similar way, nanoparticles of reducing agents dispersed in a hydrogel provided superior performances in the reduction of organic dyes in comparison with molecular systems [9]. Polymers are particularly a suitable matrix to disperse nanoparticles. PVDF can host TiO2 nanoparticles showing an improved surface hydrophilicity and permeability of PVDF membranes and the...