Page 158 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

144 Principles and Methods

to determine particle surface charge. First the charge can be assessed
using a proton as an atomic probe, by titrating a suspension of particles.
The charge can also be determined indirectly by determining the zeta
potential ( ) of the particles. The zeta potential is the electrical potential
that exists at the “shear plane” at some distance from the particle sur-
face. It is derived from measuring the electrophoretic mobility distribu-
tion of a dispersion of charged particles as they are subjected to an electric
field. The electrophoretic mobility is defined as the velocity of a particle
per electric field unit and is measured by applying an electric field to the
dispersion of particles and measuring their average velocity. Depending
on the concentration of ions in the solution, either the Smoluchowski (for
high ionic strengths) or Huckel (for low ionic strengths) equations are used
to calculate the zeta potential from the measured mobilities. For very
small particles the mobility is generally determined using laser Doppler
velicometry.
It is essential to determine the modifications of the surface charge
(or zeta potential) as a function of pH and ionic strength. By doing so,
it is possible to determine the point of zero charge (PZC) or isoelectric
) for a sample. This is significant because nanoparticle sus-
point (pH iep
pensions are generally stable above and below the pH point of zero
charge <pH > while the relationship with ionic strength is somewhat
pzc
similar.
Application in the particular case of nanoparticles. For biological tests, knowl-
edge of the particle PZC is crucial as it relates to the stability or propen-
sity to aggregate of the sample. The importance of this concept may be
illustrated by considering cytotoxicology experiments conducted with
fibroblasts cells and nano-maghemite. Here, the PZC of the nano-
maghemites is around pH 7, which is close to the solution pH used
during experiments looking at nanoparticle-cell interactions (DMEM, for
example). Adding 8 nm nano-maghemite to a DMEM nutritive medium
results in aggregation of the nano-maghemite, with the particle size
approaching several tens of microns (Figure 4.24).
In order to increase the stability of nanoparticles, their surface
chemistry is often modified through functionalization or polymer
encapsulation to modify their surface charge. In the case of nano-
maghemites, the adsorption of DMSA (dimercaptosuccinic acid) at the
maghemite surface strongly modified the surface charge leading to a
stable dispersion in the nutritive medium. The DMSA is fixed through
the SH group at the maghemite surface. Therefore, the external part
of the DMSA-covered maghemite corresponds to the carboxylic group
of the DMSA. At pH 7, the carboxylic groups are negatively charged
resulting in repulsive electrostatic forces between the various parti-
cles (Figure 4.25).