As-Produced Intrinsic Physico-Chemical Properties and Appropriate Characterization Tools - Safety of Nanomaterials along Their Lifecycle - page 9

distribution). Special care must be taken during sample preparation and analysis
to prevent artifacts, particularly when the method only provides two-dimensional
images (e.g., SEM) (Figure 1.5).
For measurement in liquid-free states, additional care must be taken to avoid dry-
ing the artifacts.
It is worth noting that the absence of information about the state of dispersion
may lead to misleading interpretations of experimental findings; the aggregation/
agglomeration state may cause significant variations in an effective dose of nano-
objects and modulate their fate and transport in vivo (Albanese and Chan 2011).
1.4 SHAPE
Engineered nano-objects of identical composition can be produced in a variety of
shapes including spheres, fibers, and plates, and each of these shapes may possess
different physical and chemical properties. The shape of a nano-object is mainly
determined by the symmetry of its constituting crystallites, if any, and by the mini-
mization of the bulk and surface free energy.
Many properties of NOAA, such as dissolution rate, aggregation behavior, or
availability of reactive sites among others are strongly linked to the shape of a nano-
object. The characterization of a nano-object shape mainly includes analysis of
SEM, TEM, or SPM images.
However, care must be taken during sample preparation and analysis in order
to prevent misleading results due to orientation effects and artifacts coming from
contaminants and aggregates.
It has been reported by Oberdörster et al. (2005) and Powers et al. (2007) that
differently shaped nanomaterials of identical composition would cause different bio-
logical responses. It has also been shown by Sun et al. (2011) that the toxicity and
biodistribution of NOAAs in vivo are strongly shape dependent.
1.5 SURFACE AREA
The surface area of NOAAs can be described as the quantity of accessible surface of
a sample when exposed to either gaseous or liquid adsorbate phase. Surface area is
expressed as the mass specific surface area (m 2 /g) or as the volume specific surface
area (m 2 /cm 3 ), where the total quantity of the area has been normalized to either the
sample's mass or volume (ISO/TR 13014:2012).
The surface area of a nanomaterial is mostly measured through physical adsorp-
tion of an inert gas (typically nitrogen) using the method of Brunauer, Emmett, and
Teller (1938) (BET). However, BET measurements of the surface area can only be
performed on dry powders, which is the main limitation of this method. Moreover,
the analysis of the surface area of many nanomaterials (e.g., hydrated polymeric
nanoparticle systems, micro- and nanoemulsion, nanoliposomes, or dendrimers)
cannot be sufficiently performed due to artifacts that arise from either sample drying
or difficult sample recovery. In this case, the surface area can be estimated through
DLS, cryo-TEM/SEM, or atomic force microscopy (AFM).