Analysis and Characterization of Manufactured Nanoparticles in Aquatic Environments - Environmental and Human Health Impacts of Nanotechnology - page 246

extinction and concentration, the method is less suitable for complex nanoparticle
mixtures of different compositions and sizes. Zattoni et al. (2003) have developed
algorithms to quantitatively convert the extinction coeffi cient (actually turbidity)
to concentration. Nevertheless, due to the minimum perturbation, simple operation
and, most importantly, the ability to measure certain desired properties, optical
spectroscopy has found use in nanoparticle characterization (Yu et al. , 2003 )
In addition to the scattering contributions, small metal NPs possess the special
features of surface plasmon effects (Noguez, 2005), which are special absorption
bands that do not show up in the larger metal particle dispersions nor in metal salt
solutions. The surface plasmon effects come from oscillations in the electronic
clouds at the metal-water interface, and UV/Vis absorption and sometimes fl uo-
rescence due to quantum confi nement effects have been shown for these small
particles. The position of the absorption bands is dependent on both the particle
size and shape and can be used in characterization of the metal nanoparticle disper-
sion (Noguez, 2005; Haiss et al. , 2007 ; Akthakul et al. , 2005 ; Panacek et al. , 2006 ;
Link and El-Sayed, 1999). It can also be used to study the fi rst steps in agglomera-
tion, since agglomeration from primary particles to dimers increases the aspect
ratio of the particle (Aryal et al. , 2006). Since the plasmon absorption bands for
silver and gold are in the 400-550 nm range, and thus are separated from the absorp-
tion maxima for humic substances (250-350 nm), this is a good way to study their
interactions (Diegoli et al. , 2008 ).

6.2.7.2 X - ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is based on X-ray irradiation of the par-
ticulate substrate (in ultra high vaccum) and measure the electron kinetic energy
and electron numbers that dissipate from the surface at an angle. From the differ-
ence between the energy of the X-rays used and the kinetic energy the electron
binding energy can be calculated. An XPS spectrum is then plotted as the number
of electrons detected as a function of binding energy. XPS is a surface analysis
technique that can be used to study the surface (top 1- 10 nm) composition and
redox state of NPs. XPS is hence very suitable for studying surface chemical com-
position that is different from the bulk chemical composition. A study comparing
the surface redox state speciation for ceria NPs found that XPS measured a more
reduced surface than another X-ray spectroscopy method (X-ray absorption near
edges spectroscopy) (Feng et al. , 2004). XPS can also be used to study the adsorp-
tion of capping agents on metal nanoparticle surfaces. However, XPS is sensitive
for contamination of the particle surfaces. Important aspects that need to be further
investigated are sample preparation procedures for nanoparticles.

6.2.7.3 Fluorescence

Fluorescence spectroscopy is of course limited to particles that are either self
fl uorescent based on their chemical structure and surrounding, or to particles that
have been labelled with a fl uorescing molecules. The best example of NPs with
native fl uorescence are semiconductor quantum dots, for example CdSe, CdS or
CdTe NPs (Yu et al. , 2003 ). The fl uorescence spectra are selective to nanoparticle