Diffusion in nano-scale polymeric systems

The EU-funded study 'Transport studies on polymer based nanodevices and assemblies for delivery and sensing' (TRASNADE) has supported the rational design of improved systems for applications in ionic fluid environments such as those found in cells and tissues. To do so, the team synthesised polyelectrolyte multilayers on planar surfaces and colloidal particles (those in which insoluble particles are suspended). Polymer brushes are assemblies of macromolecules tethered to a surface. Polymerisation was done on both planar and colloidal surfaces. Polymer vesicles were prepared with poly(styrene sulphonate), applying the layer-by-layer technique on planar and colloidal particles.

Researchers analysed the z-potential of spherical brushes relative to the ionic strength of the solution in which they were placed. The z-potential is related to electrochemical equilibrium at interfaces and depends on both liquid and surface properties. Given the ionic nature of the fluids in the body, z-potential is an important indicator of diffusion factors. Interestingly, scientists noted a weak dependence of the z-potential relative to ionic strength.

Diffusion through polymer vesicles was studied using a novel fluorescence quenching approach developed by a project partner. The team assessed the role of layer thickness, salt concentration and the position of the labelled polyelectrolytes in the polymer electrolyte membrane. Deviations from expected results support that the diffusion cannot be considered as a completely random process.

The team went on to conduct electrochemical measurements of transport through the multilayers. Total current was reduced with increasing number of layers. In a study of the effect of grafting density on molecular transport through brushes, scientists demonstrated seven orders of magnitude less diffusion for higher grafting densities than that typically observed in aqueous solutions. The effects of temperature on diffusion were dependent on the salts present in the solution.

Finally, the researchers developed a complete simulation toolbox based on the experimental data to enable a systematic analysis of anomalous diffusion processes in polyelectrolytes at the nano scale.

TRASNADE has significantly enhanced understanding of transport mechanisms related to nanodevices and nanoassemblies in ionic liquid environments. Results will benefit knowledge-based design of new devices for drug delivery and sensing.