Enzyme Catalysis of the Surface

research highlights

Defect-less melting Enzyme catalysis of the surface. The researchers examine the Science 338, 87–90 (2012) Angew. Chem. Int. Ed. http://doi.org/fz679p (2012) physics of ice formation in supercooled water droplets on different surfaces, and show that The use of surfactant molecules to stabilize their evaporation — which starts explosively aqueous–organic biphasic systems, including as a result of the latent heat released on emulsions, is a dynamic process involving impact — generates a condensation halo the interchange of surfactant molecules at the around each droplet, which in turn crystallizes interface. If an emulsion is stabilized using into ice. Poulikakos and co-workers establish colloidal nanoparticles, namely a Pickering that the size of the frost halo depends, through emulsion, however, the nanoparticles are a delicate balance between heat diffusion absorbed in a permanent manner at the and vapour transport, on the thermal interface. As a result of their high stability, conductivity of the substrate: the droplets Pickering emulsions have several practical forming on a polymer surface have larger advantages in enzymatic biphasic reactions. halos than those on titanium and copper, Now, Jan van Hest and colleagues have shown suggesting that good thermal conductors may that polymersomes — vesicles formed from be effective in minimizing ice formation and amphiphilic block copolymers — can be used frost propagation. AT © 2012 AAAS to form Pickering emulsions in which enzymes It has been commonly assumed that a can be either located in the aqueous phase or Particle perfection crystal heated above the melting point starts within the lumen of the polymersomes, and Nature Commun. 3, 1088 (2012) to melt through the formation of defects, then used to catalyse esterification reactions. which diffuse and coalesce to form liquid The enzyme’s specific activity was highest nuclei (nuclei that reach a critical size grow when encapsulated within the polymersomes spontaneously until the whole crystal is and was maintained at 89% following eight melted). But this is not what Ziren Wang and reaction cycles, when using toluene as collaborators see in their experiments with the organic solvent. In future studies, the crystals of thermoresponsive colloidal gel many distinct compartments within this microspheres, which shrink with increasing polymersome-based Pickering emulsion could temperature. After slowly heating the be exploited to load different enzymes and interior of face-centred cubic crystals with perform a cascade of organic reactions. AS 420 nm a focused laser, the researchers found with © 2012 NPG video microscopy that the precursors to the Frosty reaction liquid nuclei are instead coordinated circular Proc. Natl Acad. Sci. USA 109, 16073–16078 (2012) The structure and optical properties of motions of spheres within regions where π-conjugated polymers can be widely tuned the spheres are highly mobile, as had been The formation of frost on solid surfaces though the choice of appropriate monomers predicted from recent computer simulations. limits the performance and safety of many for their synthesis. This versatility and easy Interestingly, they also observed that stronger technologies, most notably in aviation. processability make these polymers useful base superheating (which caused the colloids Significant efforts have therefore focused on materials for thin-film optoelectronic devices. to shrink faster) leads to the coalescence developing so-called icephobic surfaces that However, preparation routes for conjugated of liquid nuclei, which quickly acquire a slow down or even suppress the formation of polymer nanoparticles, which could be used spherical shape to minimize the liquid– ice. However, their performance is strongly in photonics or as biocompatible alternatives crystal surface tension. The researchers’ affected by environmental conditions such to inorganic nanoparticles for biomedical set-up should lead to a better understanding as humidity and shearing gas flow and, applications, have generally suffered from a of melting dynamics and further challenge as Dimos Poulikakos and colleagues now large distribution of particle sizes, commonly classical nucleation theory. PP demonstrate, also the thermal conductivity referred to as polydispersity. Joris Sprakel and co-workers now report a synthesis scheme for monodisperse nanoparticles made from Rust shines in new light Nature Commun. 3, 1035 (2012) a range of conjugated polymers. They have adapted the Suzuki–Miyaura cross-coupling Electrical cars, wind machines, computer hard drives and most electrical motors require reaction — one of the most important routes permanent magnets to operate. These are typically made of neodymium–iron–boron for the synthesis of such compounds — to alloys, because of their large magnetocrystalline anisotropy and saturation magnetization. a dispersion polymerization process that However, the magnets use considerable amounts of increasingly expensive rare-earth enables the preparation of particles with compounds such as neodymium and dysprosium, thus alternatives are strongly sought well-defined sizes, tunable fluorescence after, but with little success so far. Shin-ichi Ohkoshi and colleagues now demonstrate that wavelengths and controllable shape. The a relative of common rust could solve this problem. Doped with a few per cent of rhodium, researchers demonstrate their self-assembly the ferrite RhxFe2−xO3 is shown to have a very large coercive field of up to 31 kOe, and shows into a photonic crystal and show that the good microwave absorption as well as magnetic rotation properties that are of interest, particles can be functionalized with carboxylic for example, in wireless communication applications. Furthermore, the high coercive field acid groups that allow for further coupling could be useful for information storage applications, where it may allow the reduction of the reactions for bioimaging. CM area necessary for a stored bit. Although rhodium may not be a low-cost material either, the relatively small quantities required still promise a new future for magnetic ferrites. JH Written by Joerg Heber, Christian Martin, Pep Pàmies, Alison Stoddart and Andrea Taroni.

910 NATURE MATERIALS | VOL 11 | NOVEMBER 2012 | www.nature.com/naturematerials