Nanoreactors or drug delivery agents could be made by modifying the surface of polymer vesicles with bio-active molecules, say Dutch researchers. 

Jan van Hest and colleagues at the Radboud University Nijmegen, have attached large molecules, such as proteins, to the surface of polymer vesicles, called polymersomes, making them more versatile biomimetic structures. 

'In nature, many biological processes are compartmentalised, for example in vesicular capsules, such as organelles,' said van Hest. 'Our aim is to create bio-inspired materials and processes...and sturdy polymersomes are a logical choice.' 

The polymersomes made by van Hest and his colleagues are hollow capsules with shells composed of the block copolymer, polystyrene-block-poly(acrylic acid). The surface of the capsules is covered with reactive azide functions; these are used to attach other moieties, including fluorescent dye molecules, biotin and proteins to the periphery of the polymersome. 

Polymersomes are known to be exceptionally stable compared with similar lower molecular weight structures. This high stability means that the surface has reduced reactivity. The Nijmegen team overcome this reactivity problem by using a very efficient coupling method at the surface - a click chemistry reaction. 'Since this is a generic coupling strategy, many different types of surface modification can be foreseen which opens the way to tailor-made nanocontainers,' said van Hest. 

The click reaction, copper-catalysed Huisgen cycloaddition, results in the coupling of between 40 and 50 percent of the surface azide groups with alkyne groups on the molecule being attached. By varying the reaction conditions, this degree of functionalisation could not be increased, which leads the team to believe that this is the maximum considering the dense packing of the polymer chains in the vesicles. 

In the future, the researchers hope to incorporate multiple functionalities at distinct locations of the polymersomes (e.g. exterior, interior and within the bilayer). 'This may allow us to perform cascade reactions utilizing these multifunctional vesicular aggregates,' said van Hest.  

Alison Stoddart