Biomedical Engineering Reference
In-Depth Information
The most important tools in the study and characterization of inorganic nanomaterials are elec-
tron microscopy and related methods. Structural and morphological characteristics are not often
accessible through x-ray crystallography methods, due to low scattering intensity of NPs, which
leads to pronounced peak broadening as the NP size decreases. TEM helps to directly image the
lattice structure of NPs in the order of a few nanometers as well as to obtain diffraction data,
amplitudes, and phases of NP structures. TEM can also be used to determine the behavior and self-
assembly of NPs under external influences such as magnetic fields. Further, elemental analysis of
NPs can be made using energy-dispersive x-ray spectroscopy (EDS), and modern TEMs that are
equipped with tools to perform elemental mapping and analysis using incident probe sizes in the
order of a few nanometers in diameter.
From a toxicological point of view, TEM data can offer useful information on the surface prop-
erties of NPs. Since many NPs are being produced as mixed compositions, such as “core-shell”
particles (e.g., silica-coated iron oxide NPs), the surface of the nanostructures and their properties
become of considerable importance. Different structures may grow as NPs exposing different
facets to the exterior depending on the mode of synthesis. For obtaining information on which
facets are more prominent in a specific morphology or synthesis may very often only be attainable
through TEM.
NPs are frequently being considered for diverse and important applications as a result of the
high surface area resulting from the reduction in particle size in comparison to bulk materials of
the same composition. The most typical method to determine the specific surface area of NPs is
through the measurement of a nitrogen adsorption-desorption isotherm, which also gives informa-
tion on the average pore size and pore volume.
When NPs are functionalized or tethered with organic groups, it becomes necessary to quantify
the amount of functional groups that reside at the surface of their particle, their binding strength
(e.g., covalent or ionic) to the particle, and their availability to perform their anticipated function.
Spectroscopic techniques such as infrared spectroscopy, UV-visible spectroscopy, and Raman
spectroscopy are invaluable both for the identification and to locate the presence and position of
functional groups on NPs.
Particle size analysis may be conducted in a variety of ways. It is important to differentiate
between the techniques. Electron microscopy-based methods such as scanning electron micros-
copy and TEM do not reflect the average particle sizes values that may be measured in solutions or
biological media containing the dispersed particles. Therefore, one must distinguish between the
hydrodynamic particle size measurements and those values obtained through electron microscopy
observation.
Dynamic light scattering (DLS) offers a routine approach to measure average particle sizes in
different media. This technique utilizes the time variation of scattered light from suspended par-
ticles to obtain their hydrodynamic size distribution. When the putative cytotoxicity of NPs is deter-
mined, it is prudent to consider whether cells do in fact encounter individual NPs, as opposed to
aggregates or agglomorates of several NPs; this becomes particularly relevant when studying cel-
lular recognition and internalization of NPs, as the actual size of the particle(s) will determine the
route of cellular uptake (endocytosis, phagocytosis, etc.).
Data from some pulmonary toxicity studies in rats demonstrate that exposures to ultrafine/NPs
may produce enhanced toxicity when compared to fine-sized (bulk) particle types of similar chemi-
cal composition [105]. Particle surface area and particle number determinations have been postu-
lated to play significant roles in influencing the development of NP-related lung toxicity.
The assumptions made from these studies were that the only differences (i.e., variables) between
the ultrafine and fine-sized particle types were the particle sizes. However, a closer analysis indi-
cates that a number of other physicochemical characteristics including crystal structure, aggrega-
tion potential, and surface coatings were different in the various particle types that were being
compared. Moreover, findings of other recent studies with nanoquartz and ultrafine titanium diox-
ide particle types demonstrate that the toxicity of some NP types may be related, in large part, to
