As-Produced Intrinsic Physico-Chemical Properties and Appropriate Characterization Tools - Safety of Nanomaterials along Their Lifecycle - page 5

1.2 PARTICLE SIZE/SIZE DISTRIBUTION
The particle size plays a crucial role in nanomaterial properties, thus determining
the dimension of nano-objects, and their agglomerates and aggregates (NOAAs),
is extremely important for science and technology. It is necessary to estimate not
only particle size but also particle size distribution. Both of these parameters have a
significant effect on different properties like mechanical strength, density, chemical
reactivity, and electrical and thermal properties of NOAA.
The size of particles and the distribution of their size can be determined
using numerous techniques and a variety of commercially available instruments.
Microscopy techniques can be applied to study a wide range of materials with
a broad distribution of sizes. Instruments used for the microscopy techniques
include optical light microscopes, scanning electron microscopes (SEMs), and
transmission electron microscopes (TEMs). The choice of the equipment is
mainly determined by the size range of particles, magnification, and resolu-
tion that is desired. The optical microscopes are easier to operate than electron
microscopes, but are more limited in magnification and resolution. The optical
microscope analyzes particles of all kinds, including fibers, in the size range from
0.2 µm to 5 mm. The SEM enables an analysis at higher magnification and resolu-
tion, and it is suitable for particles in the size range of about 0.01-1000 µm (the
lower limit depends upon the quality of the instrument being used). The TEM
enables examination in the size range of 0.001-10 µm with a very high local reso-
lution and is capable of imaging crystal lattice distances. However, most electron
microscopic techniques do not provide good statistical assessment. In comparison
to the other size measurement techniques, they are slow, expensive, and only a
small amount of particles can be examined at the same time. Nevertheless, if the
analyzed NOAAs can be demonstrated to be representative of a whole sample,
these methods can provide useful information. Moreover, sample preparation for
microscopy analysis (e.g., sample dilution or the way of sample drying) may cause
some artifacts, which consequently affect the results of the measurements (see
Fig u re 1.1).
The other method of defining a particle size/size distribution is based on the rela-
tionship between particle behavior and its size. For nano-objects that can be treated
as spherical, dynamic light scattering (DLS) is mainly applied. The DLS enables
rapid and simple estimation of average particle size and of the width of the size dis-
tribution in the measurement range up to 1000 nm, but it cannot reliably extract the
actual shape of the distribution from the raw data (see Figure 1.2).
DLS characterizes particles previously dispersed in a liquid, such that the depen-
dence on suitable dispersion protocols is the main limitation of this technique.
Moreover, DLS lacks the ability to distinguish between primary particles and
agglomerates, thus the method should be applied with caution for polydisperse nano-
materials (Calzolai et al. 2011).
Another technique to estimate the particle size distribution of a material dis-
persed in a liquid is centrifugal sedimentation. It enables measurement of particles
in the size range of 0.1-5 μm by fractionation. Centrifugal sedimentation methods
are based on the rate of NOAAs velocity under a centrifugal field, and the calculation