in the previous section clearly leads to the conclusion, that in
the precipitation process nanoparticles represent either a transition
stage or they are building blocks in the formation of colloids.
In view of this observation, a general comment is in order. Due
to the preponderant present interest in nanomaterials, it is noted
that "nano" is often unjustifiably used or emphasized. Larger particles,
built of small subunits, should not be considered as nanosystems,
because the properties of the latter are lost. Instead, the designation
must be reserved for fully dispersed nanometer particles. Nanocomposites
should represent systems in which unaggregated nanoparticles are
incorporated in a medium, such as in a polymer film, so that the
properties based on their smallness can be utilized.
of the aggregation mechanism can help in the design of methods for
the preparation of nanoparticles, which may proceed in two directions.
The first is to prevent the aggregation stage in the precipitation
process, commonly accomplished by the addition of stabilizers, such
as surfactants or macromolecules. The second, a more rarely described
case, is to peptize the colloidal particles built from a nano precursor.
Matijevi'c's group has followed both routes. Figure
5 illustrates recently
obtained silver particles, prepared in collaboration with Dr. Sondi,
using highly concentrated silver nitrate solutions in the presence
of a reducing agent and a polymeric sulfonate stabilizer. In contrast,
nanosized indium hydroxide was produced by peptization of internally
aggregated colloidal particles of the same composition.
Films of fully
dispersed nanoparticles are of interest in many applications, such
as in the production of color filters, of conducting layers, or
in photolithography. However, the preparation of such thin films
is not a trivial project, especially since hydrophilic particles
need frequently to be incorporated in nonpolar media, without causing
coagulation of the former. The task can be accomplished by modifying
the nanoparticle surfaces by thin coatings, making them compatible
with the carriers, as was done with 8-11 nm silica for the use in
the encapsulated inorganic resist technology.
feature of such films is their transparency, which was documented
with highly conductive polymer layers containing nanosized indium-tin
oxide particles. In another example, fully dispersed nano-pigments
in polymers have resulted in transparent films of very high color
purity. Figure 6 shows the structure of such a color filter containing
60% of the pigment, yet still being 100% transparent (Figure
systems represent just a few examples of the many projects that
are being investigated by Professor Matijevic' and his collaborators.
They may also help one to understand why a lifetime can be spent
in the field of fine particles. The latter certainly offer exciting
challenges, but also cause a sense of satisfaction because of their
usefulness in numerous applications that are beneficial to our well-being.
5. TEM of nanosized silver particles.
6. SEM of a fully dispersed red pigment in a polymer matrix.
The pigment was prepared by interacting D&C red dye with nanosized
7. Transmittance spectra of the film prepared with the pigment
shown in Figure 6.
information about Professor Matijevic' and his research, you may
call him at 315-268-2392 or send e-mail to firstname.lastname@example.org.
Some relevant literature
Fine Particles: Synthesis, Characterization and Mechanisms of
Growth, T. Sugimoto, Ed., Marcel Dekker, New York, (2000).
E. Matijevic': "Uniform Colloid Dispersions - Achievements
and Challenges". Langmuir, 10, 8-16 (1994).
E. Matijevic': "Preparation and Properties of Uniform Size
Colloids". Chem. Mater., 5, 412-426 (1993).