Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 225

intensity weighted. The advantage of NTA compared with DLS is that the intensity
of the scattered light should not bias the result. Consequently, some aggregates or
larger particles are tracked and counted in a similar way as the small particles
without skewing the results. The detection limit of the method is determined by the
laser, optics and relative refractive index, and particle size.
Currently available commercial instruments can detect nanoparticles as small as

∼ 10 nm for high refractive index nanoparticles such as silver but only down to

∼ 25 nm particles for titanium dioxide. Improving the optics with a lower noise CCD

camera or lower laser wavelength can improve these lower limits of the detectable
size window. The detection limits in terms of concentration (number of particles
ml − 1 ) are diffi cult to provide absolute numbers for, since the data acquisition (track-
ing) can be carried out for varying amount of time (typically 15- 60 s) to compensate
for low particle concentration. The optimum sample concentration is typically
10 7 - 10 9 particles ml − 1 .

6.2.4 Other Electromagnetic Scattering Methods

To overcome some of the limitations of light scattering, analogous methods with
different wavelengths of electromagnetic radiation can be used. These generally are
limited to large scale facilities to obtain the high photon fl uxes needed for both
X-ray and neutron experiments. In this chapter only a few of these techniques are
mentioned.

6.2.4.1 Small Angle X - ray Scattering

Small angle X-ray scattering (SAXS) uses similar principles to static or dynamic
light scattering but the particle sizes that can be probed are much lower ( 1 - 50 nm)

than for these due to the lower wavelength of X-ray light (Waychunas et al. , 2005 ;
Megens et al. , 1997 ; Mori et al. , 2006). There are laboratory versions available but
these generally provide too low X-ray intensity for characterization of nanoparti-
cles. Generally, to obtain enough intensity of a narrow focused X-ray, it is necessary
to perform the experiment at a synchrotron source (Megens et al. , 1997 ).

∼

6.2.4.2 Small Angle Neutron Scattering

In small angle neutron scattering experiments (SANS) the neutron beam (typical
wavelength 0.01-3 nm) is excellent to interact with matter at the nanometer
scale. While light and X-rays are scattered by the electrons, neutrons are scattered
by the atom nucleus. In contrast to X-ray scattering, there is no sensitivity trend
with increasing atomic number, and the neutron refractive index may differ even
between isotopes of the same atom. The difference is most remarkable for hydro-
gen and deuterium, and this is often used in experiments where contrast matching
is used to selectively probe the interactions between NPs, or between NPs and
surfactants or other coatings (King, 1999). SANS has been used for environmental
studies to some extent but there is great potential for future work (Diallo et al. ,
2005). One application where SANS may be important is to study NP surfactant
interactions.