High Aspect Ratio Nanomaterials - Aerosols: Measurement, Dosimetry, and Health Effects - page 232

10.2.2.2 Scanning Probe Microscopies
These are techniques that acquire an image by raster scanning an atomically sharp, microscopic
probe capable of sensing height changes as small as 0.1 Å (10 pm) (Sakurai, 2000). The two most
widely used high-resolution scanning probe microscopy (SPM) techniques are atomic force
microscopy (AFM) and scanning tunneling microscopy (STM). In an AFM, it is the cantilever
delection signal versus probe base position that results in the image, whereas in an STM, it is the
variation in current as the probe passes over the surface that is translated into an image. Unlike
electron microscopes, which have the potential to destroy or modify sample structure in the process
due to the use of electron beam, SPM techniques are noninvasive as they are able to create highly
resolved three-dimensional images (providing both in-plane as well as height features) without the
need for an electron source. Again, sample preparation is critical; it is particularly important for the
nanoparticles under analysis to have a greater afinity to the lat substrate surface than the sensor
probe tip. If there is weak adhesion between the nanoparticle and substrate, then the image acquired
either shows a reduced resolution or contains “artifacts” (e.g., streaking) (Bonnell, 2001).
10.2.3 n oniMaging t ecHniques
This is a group of techniques that are “nonimaging” in nature, and most have much lower detection
sensitivity than the aforementioned “imaging”-based techniques.
An essential property of interest is the size of the nanoparticles under analysis, as the association
between particle size and toxicity is well founded (Oberdorster et al., 2005). The “state of aggregation”
is a dificult parameter to quantify but is potentially signiicant for toxicological evaluation. This
parameter can be used to describe the degree to which particles are agglomerated (loosely held
together in groups or clusters by attractive interparticle forces, the most fundamental being Van der
Waals forces); particle agglomerate size may play a crucial role in the uptake of such particles inside
the body by macrophages (Rudt and Muller, 1992).
In addition to size, surface area is becoming of increasing importance in relation to particle
toxicity. In industry, surface area characterization of nanoparticles is needed as they can be cor-
related to surface-related phenomena such as catalyst activity, and electrostatic properties that can
inluence the processing and behavior of nanoparticles. Other surface properties, apart from simply
surface area, are emerging as important from a toxicological point of view, as they may provide
mechanistic details in the uptake, persistence, and biological activity of HARN inside living cells.
Such properties include surface charge (zeta-potential measurements) and surface chemistry (Gill
et al., 2007). The techniques now overviewed can be divided into two groups, based on whether they
characterize chemical or physical properties.
10.2.3.1 Chemical Property Information
The molecular composition and structure of the surface of nanoparticles will ultimately deine
its chemistry. Techniques used for chemical characterization are mainly spectroscopic in nature.
Spectroscopic techniques measure the interaction between a probe and matter, yielding a “spec-
trum,” that is, a response plot as a function of wavelength. Such techniques can be very useful in
identifying the “chemical class” of various components of an analyte under study.
10.2.3.1.1 Vibrational Spectroscopy
This is a tool used to probe the “vibrational states” of a molecule and, hence, for the determination
of molecular structure. This can be achieved in several ways. In infrared (IR) spectroscopy,
the molecule will be exposed to a frequency range of IR light and ultimately the measure of
wavelength-intensity of IR absorption by the molecule; for a vibrational motion to be IR active, the
dipole moment of the molecule must change. Raman spectroscopy is a measurement of wavelength-
intensity of inelastically scattered light from molecules; for a molecule to be Raman active, there
must be a change in the polarizability of the molecule. A Raman spectrum of a molecule gives