Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 245

less experience of AFM in water due to diffi culties of scanning over the sample
without moving around the particles when probing them, and diffi culties to be
overcome since organics will adhere to tip and change behaviour.
Sample preparation is also important in AFM, although not as critical as in elec-
tron microscopy, since less perturbing preparations can be used (e.g. only drying to
100% humidity). Nanoparticle deposition on the atomically fl at substrate (e.g. mica
or silicon wafer) has been studied for AFM analysis in air (Balnois and Wilkinson,
2002 ; Lead et al. , 2005 ; Balnois et al. , 2007 ).
The particle height measurement has a lower size limit around 0.1 nm and the
accuracy is generally good, but the lateral dimension (x-y) measurements are not
accurate for NPs in AFM. This problem results from the tip having a radius of
curvature of about 10 nm. For NPs of this size or smaller, the AFM cannot probe
the distances close to the particle edge, but rather starts measuring the defl ections
from the particle before the tip centre is actually over the particle (Balnois et al. ,
2007), and thereby overestimates the lateral dimension. AFM can, in addition to
image sample topography, measure forces between the tip and the sample. By
varying the tip material, nanoparticle interactions with other materials can be
quantitatively measured.
There are other scanning probe microscopy techniques that can be used to
measure magnetic or electron images of the particles. For example, electron tun-
nelling properties of a bacterial surface have been measured by Wigginton et al.
(2007b) .

6.2.6.3 Image Analysis and Automation

Due to the inherent single particle counting nature of microscopic measurements
the statistical representativeness is often low. To obtain enough counting statistics
to derive a number-based average from a distribution of particles it is necessary to
count and measure many thousands of particles (Vigneau et al. , 2000 ). Therefore,
further development of automation of data acquisition and computerized image
analysis algorithms will be important for the application of microscopy in the char-
acterization of NPs in environmental or other complex samples (Bowen, 2002; Li
et al. , 2006 ; Bao et al. , 2004 ; Jose et al. , 2005 ; Reetz et al. , 2000 ).

6.2.7 Spectroscopic Methods

Spectroscopic methods can be used to probe chemical entities on whole samples
and on the nanoparticle ensembles in fractionated samples.

6.2.7.1 Optical Spectroscopy

UV-Vis spectroscopy can, of course, be used for quantitative analysis of light
absorption in NPs according to the Beer-Lambert law. However, NPs are more
complicated than molecules, since the light scattering contribution may contribute
signifi cantly to the extinction coeffi cient. The scattering contribution is especially
important for larger NPs, aggregates and particles with a high refractive index.
Since there is no simple relationship between the scattering contribution to the