For their integration into a desired matrix, nanoparticles often require a
surface treatment after or during their synthesis to make them compatible with
the surrounding matrix material. Colloidal particles are per definition a
suspension of hard or soft particles suspended in a solution. For a successful
synthesis and colloidal stability, a surface stabilization is necessary, which
can either be electrostatic or steric.
Silica (SiO2) and polystyrene (PS) latex nanoparticles probably
are the most used particles, because of the ease to synthesize them at a large
(gram) scale for various fundamental and applied research applications.
Polystyrene particles are generally made using emulsion or dispersion
polymerization of styrene monomer. Both, the initiator and the presence of a
surfactant, can contribute to the electrostatic stabilisation of the particles,
which prevents them to form aggregates. However this stabilization is often
limited to one charge per surfactant, which led us to develop alternative
Dr. Dietsch and Professor Schurtenberger at the Adolphe Merkle Institute can now
increase the electrostatic barrier by using amphiphilic block copolymers as
stabilizer for the polymerization1. The PS-PSS
(PolyStyrene-PolyStyrene Sulfonate) core-shell particles thus obtained have an
active charge density more than 100 times higher than regular PS latex
particles1. Another advantage of controlling the
electrostatic repulsion is the control of the structure formed via self assembly
mechanisms as illustrated in the following transmission electron microscopy
image (left-hand side presents a double layer of particles, right hand side
presents a monolayer).
Silica particles have the advantage that it is possible to tailor their
properties using silane coupling agents which graft covalently to the surface.
Silica particles are generally made at a lab scale using sol gel process based
on the original work of Stöber et. al.2. Using the
approach of surface modification, Dr. Dietsch and Professor Schurtenberger could show that it is possible to
synthesize aggregation free nanocomposite materials with integrated silica
particles in a polymethylmethacrylate matrix (PMMA)3.
Because of the ease to modify surface silanol groups of silica particles,
thin silica coatings on nanoparticles can open-up the way for a tailored surface
chemistry of a wide variety of different nanomaterials.
The general method of Graf et. al.4 can be
adapted to almost any kind of particles to enhance the colloidal stability of
the particles. This involves in a first step the adsorption of
polyvinylpyrrolidone (PVP) polymer at the particle surface. This increases the
stability of a suspension by a factor 6. This method can be adapted for hematite
nanoparticles5, which are used as model particles,
because their morphology can easily be controlled through the synthesis
For other than oxide nanoparticles the surface can be modified using
adsorption of phosphate agents. As an example, Dr. Dietsch and Professor Schurtenberger have developed a click chemistry
surface agent in collaboration with Professot Mingdi Yan at Portland State University and
proved the covalent immobilization of magnetic particles onto the surface of
Dr. Dietsch and Professor Schurtenberger would like to point out that it often
does not matter how the surface modification is done, it can be covalent
chemistry, click chemistry, adsorption of a phosphate, a surfactant, a
polyelectrolyte7 or simply modification directly
during the synthesis. Surface modification remains, in most cases a key factor
to control the interactions of suspended particles in a desired matrix in order
to achieve stable suspensions or aggregate free nanocomposite materials.
1. Mohanty P.S., Dietsch H. Rubatat L., Stradner A., Matsumoto
K., Matsuoka H. and Schurtenberger P., Langmuir 25 (4), 1940, 2009.
2. Stöber W., Fink, A. and Bohn E., Journal of Colloid and Interface
Science 26 (1), 62, 1968.
3. M. Saric, H. Dietsch and P.
Schurtenberger, Colloids and Surfaces A, Physicochemical and Engineering
Aspects, (2006), 291, 110-116.
4. Graf C. Vossen D.L.J, Imhof
A. and van Blaaderen A., Langmuir 19 (17), 6693, 2003.
Dietsch H., Malik V., Reufer M., Dagallier C., Shalkevich A., Saric M., Gibaud
T., Cardinaux F., Scheffold F., Stradner A. and P. Schurtenberger, Chimia, 62
(10), 805, 2008.
6. Liu L.H., Dietsch H., Schurtenberger P. and
Yan M., Bioconjugate Chemistry, 20 (7), 1349-55, 2009.
Radice S., Kern P., Dietsch H., Mischler S. and Michler J., Journal of Colloid
and Interface Science 318 (2) 264, 2008.
Copyright AZoNano.com, Dr. Hervé Dietsch
(Adolphe Merkle Institute and Fribourg Center for Nanomaterials)