Functionalizing the Surface of Nanoparticles

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 approaches.

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 parameters.

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 E-Coli bacteria⁶.

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.

References

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.
5. 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.
7. Radice S., Kern P., Dietsch H., Mischler S. and Michler J., Journal of Colloid and Interface Science 318 (2) 264, 2008.