Angew. Chem. Int. Ed. (2014)

For inorganic nanoparticles, the relationship between morphology and properties is relatively well established. This is not the case for polymeric nanoparticles, where there is a limited ability to even control the shape of such materials. Craig Hawker and colleagues have now reported a block copolymer nanoparticle that can reversibly change its shape in response to an external stimulus.

The researchers — who are based at the University of California, Santa Barbara, the University of Melbourne and US Army Edgewood Chemical Biological Center — first synthesized spherical polymeric nanoparticles made of the block copolymer polystyrene-b-poly(2-vinlypyridine) (PS-b-P2VP). The two phases of the copolymer separate to form concentric layers and it is possible to select the outside layer by adding a surfactant that favours one phase over the other. An intermediate condition is reached when a mixture of surfactants is used: the nanoparticle becomes ellipsoidal and the phases separate in a lamellar fashion.

To introduce stimuli responsiveness to the system, Hawker and colleagues took advantage of the acid/base properties of the 2VP phase. At low pH, 2VP gets protonated and swells to more than twice its initial volume, significantly changing both the morphology and aspect ratio of the entire nanoparticle. To ensure that swelling does not break the nanoparticle, the researchers added a crosslinker. The shape change is reversible and the initial morphology is restored at higher pH values.