High Aspect Ratio Nanomaterials - Aerosols: Measurement, Dosimetry, and Health Effects - page 233

complementary information to its corresponding IR spectrum and both techniques are powerful
tools for nondestructive characterization of nanoparticles. Both are suited for routine analysis; they
can be operated by technicians, have a relatively quick analysis time, and have well-established
frequency standards (Laserna, 1996).
10.2.3.1.2 X-Ray Photoelectron Spectroscopy
This is a surface analysis technique, requiring ultrahigh vacuum conditions, that bombards the
sample with monoenergetic soft x-rays (∼1 to 2 keV), causing electrons to be ejected. The number
and kinetic energy of the ejected “photoelectrons” (from a depth of 1 to 10 nm of the material being
analyzed) are then simultaneously measured. Spectra are capable of showing elemental composition,
empirical formula, chemical state, and electronic state of the elements. XPS has the ability to
analyze nonconducting materials with minimum charging effects (unlike techniques such as SEM),
with excellent inter-element resolution. Some of disadvantages include poor lateral resolution, a
relatively weak signal, and possible nonuniqueness of its chemical shift information (Watts and
Wolstenholme, 2003). XPS and secondary ion mass spectroscopy (Lee et al., 2007), in particular,
have been extensively used for characterizing nanoparticles. These are very powerful surface analysis
techniques and have been used to give information-rich spectra detailing the “chemical state” at the
surface; XPS, in particular, has the advantage of being quite oxidation speciic but this is dependent
on the particular element which is analyzed. XPS has been used to probe information relating to
structural modiication due to chemical interaction with organic compounds or gases adsorption
and sidewall functionalization of HARN. A recent review by Powers and coworkers indicated that
the technique is applicable to correlating biomaterial surface properties to physiological endpoints
(Powers et al., 2007).
10.2.3.2 Physical Property Information
10.2.3.2.1 Surface Area
The BET technique is a traditional method for the measurement of surface area and other char-
acteristics such as pore size and pore distribution of nanoparticles. The technique is based on the
addition of a known volume of gas (the adsorbate) and the subsequent gas adsorption onto the solid
material (at cryogenic temperatures), resulting in a direct relationship between the pressure and the
volume of gas in the sample vessel. By measuring the reduced pressure due to adsorption, the ideal
gas law can then be used to determine the volume of gas adsorbed by the sample and, subsequently,
the surface area of the sample, which is reported as the speciic surface area (i.e., surface area per
unit mass, usually m 2 /g) (Lowell et al., 2004).
10.2.3.2.2 Zeta-Potential Measurements
This is a method to probe “surface charge” information of nanoparticles in a liquid suspension.
Theoretically, zeta potential is the electric potential in the interfacial double layer, that is, layer
at the location of the “slipping plane” versus a point in the bulk luid away from the interface. It
has been recognized that the zeta potential is a very good index of the magnitude of the interac-
tion between nanoparticles, and the measurements of zeta potential are commonly used to assess
the stability of colloidal systems. One way to determine zeta-potential measurement is to obtain
the electrophoretic mobility of the particle and this can be done through the combination of laser
Doppler velocimetry and phase analysis light scattering of the sample, under the inluence of an
applied electric ield (Hunter, 1981).
10.2.3.2.3 Photon Correlation Spectroscopy
A common method used to probe size distribution of nanoparticles in the submicron range is
based on photon correlation spectroscopy (PCS) (more commonly called dynamic light scattering).
This nondestructive method determines particle size by measuring the rate of luctuations in laser