Biomedical Engineering Reference
In-Depth Information
1.3 CLASSIFICATION OF NANOMATERIALS
The classification of NMs is not simple. It may be appropriate to organize the types of NMs accord-
ing to their chemical and physical properties. However, different types of structures synthesized
using various manufacturing processes, along with different surface coatings, can obscure classifi-
cations. Other approaches to categorization may be based on NM points of origin or whether they
are natural or modified (Nowack and Bucheli 2007). Given the range of NM characteristics, it may
be practical to assign categories based on specific properties, such as the potential for health risks
(Tervonen et al. 2009). Therefore, in order to provide a general overview of NMs, multiple catego-
rization systems should be considered.
For instance, taking the point of origin into consideration, NMs can be generated via either natu-
ral or anthropogenic processes (Figure 1.3 demonstrates classification organization of NMs based
on their origin). Naturally produced NMs can be classified into biogenic, geogenic, atmospheric,
and pyrogenic categories, based on the methods and mechanisms of production by living organ-
isms, the soil, the air, and heat, respectively. Anthropogenic NMs, or those produced as a result of
human activity, can be classified into two categories—unintentionally produced and intentionally
engineered NMs (Nowack and Bucheli 2007). Most often, unintentional NMs are created as a by-
product of combustion processes (Nowack and Bucheli 2007). Intentionally developed NMs can
be further classified into five different categories: carbon-based materials, metal-based materials,
dendrimers, polymeric particles, and composites (Tuominen and Schultz 2010).
As NMs have an enormously small size, 100 nm or less in at least one dimension, descriptions of
their size and shape have been attempted, such that if NMs have a nanometer size in one dimension,
they are referred to as surface films; in two dimensions, fibers or strands; and in three dimensions,
particles. They can be present in single, fused, aggregated, or agglomerated forms, with different
shapes, such as spherical, tubular, and irregular (Figure 1.4 shows examples of NM classification
based on their shape).
NMs are resources that are differentiated by an ultrafine grain size (<50 nm in size) or by a
dimensionality restricted to 50 nm. NMs can be produced with different modulation dimension-
alities as described by Richard W. Siegel: atomic clusters, filaments and cluster assemblies (zero),
multilayers (one), ultrafine-grained over layers or buried layers (two), and nanophase materials con-
sisting of equiaxed nanometer-sized grains (three) as shown in Figure 1.4 (Siegel and Fougere 1995).
Therefore, NMs can also be classified based on structure, morphology, and physicochemical
properties. General classifications, based on the types of the NMs, take into account dendrimers,
nanotubes, fullerenes, and quantum dots (QDs) (Nowack and Bucheli 2007).
Nanomaterials
Anthropogenic
Natural
processes
Unintentional
Engineered
Carbon-based materials,
metal-based materials,
dendrimers, polymeric
particles, and composites
Biogenic, geogenic,
atmospheric, and
pyrogenic
FIGURE 1.3
Classification of nanomaterials based on their origin. Natural process such as proliferation of
living organisms, movement of soil, air, and the heat energy may result in the formation of nanomaterials.
Anthropogenic nanomaterials are produced as a result of human activity regardless whether specific intent to
do so was present or not. Depending on the process, unintentional processes or engineering design may result
in nanomaterials of different shapes, sizes, and different properties.
