Polymer nano and sub-micron particles

or many years we have had an interested in the synthesis and roperties of polymer particles. In the early stages we concentrated on polymer particles produced by emulsion polymerization. We were the first to release the importance of polymer colloidal particles containing a stimulus responsive shell and we were early pioneers of polymer particles with outer layers functionalized with specific shaped cavities for molecular recognition (core-shell molecular imprinted particles (CS-MIPs). We have expertise for the synthesis of polymer particles that range in sizes from circa 10 nm to 10 µm with many different surface functionalities. Our work in this area has attracted industrial funding for many years and is of interest in biotechnology, surface coatings, adhesives, drug delivery and other controlled release applications and analytical/immuno diagnostics. Two examples of the work are given below.

i) Core-shell molecular imprinted particles (CS-MIPs)

imprinted_particlesMolecular imprinted polymers (MIPs) are produced by polymerization around a target molecule that can also bind, either covalently or electrostaticly, to the polymer. When the template is removed a polymer is produced that retains a shape specific memory of the template. These feaures can then be used to rebind the template from complex mixtures. However, prior to our work there were several issues with these materials: they did not work well in aqueous media; it was diffuicult to remove all of the template and their production required a laborious grinding and sieving procedure.

Above right: Coating a particle with a medium in which large template molecules make an imprint by a mix of electrostatic and hydrophobic interactions.

All of these problems were solved by producing the MIPs in a 1-5 nm shell at the surface of polymer nanoparticles (typically 50-30nm in diameter) in water. Emulsion polymerization allows us to produce particles, which do not require sieving, directly. The template can easily removed because it is located in a thin nano-shell at the surface of the particle. In order to use MIPs in life sciences applications they must be usable in aqueous media. We realised that the binding recognition process was not necessarily weak in water but the use of non-aqueous media for the polymerization would not be optimum for binding water.

Our CS-MIPs on the other hand are formed in water so that as the polymerization progresses a set of hydrophobic, hydrophilic and electrostatic interactions are formed, which together constitute the recognition event. Using this approach we were able to produce nano-particles that selectively bind various low molecular weight compounds and some peptides. The work is now focused on CS-MIPs that can bind growth factors and we hope to be able to use these materials as aids to cell growth and tissue formation.

ii) Stimulus responsive particles

polymer_floculation

Above: Schematic representation of the control of flocculation of stimulus responsive polymers.

particles_in_fibroblasts In 1996 we were among the first to release the potential of stimulus responsive core-shell sub-micron particles. At this time the work concentrated on the synthesis of particles with non-responsive (hydrophobic) cores that were functionalized by grafting stimulus responsive brushes. Recently the work has centred on controlling the flocculation of highly branched stimulus responsive polymers above the Tc. The process is shown schematically in the figure above.

Above right: Micrograph showing fluorescent particles inside dermal fibroblast cells (blue = particles, red = F-actin).