Ice-Templating of Core/Shell Microgel Fibers through `Bricks-and-Mortar' Assembly[ast][ast]

Nanoparticles with unique catalytic, electronic, magnetic, optical and optoelectronic properties provide functional building blocks for organization into spatially well-defined assemblies, allowing for the possible extension of desirable properties of these nanoscale entities to the macroscopic level and the realization of complex devices. In the past decade, ‘bricksand-mortar’ assembly strategies based on non-covalent interactions have been shown to be successful for the controlled assembly of nanoscale objects into various composite materials. Nanoparticle building blocks bearing molecular recognition elements in their surface monolayer can be readily incorporated into macroscopic assemblies via assembly with a suitable mediator of complementary functionality (e.g., DNA, nanoparticles, polymers). However, a general challenge of the above nanoscale ‘bricks-and-mortar’ approaches is their ability to selectively define morphology at a larger length scale. For example, the irregular morphology of the aggregates can limit their applications in complex devices. In this Communication, we report a sub-micrometer scale ‘bricks-and-mortar’ assembly approach to create fibers with alternating organic-inorganic arrangement through a simple ice-templating strategy. The fibers are constructed by closely packed monodisperse inorganic nanoparticle (INP)@poly(N-isopropylacrylamide) (PNIPAm) core/shell microgels. The well-defined core/shell structure of the particles is critical in the formation of ‘bricks-and-mortar’ fibers: the INP cores are bricks while the mortar is made out of PNIPAm layer on the INPs which serves as an organic glue to hold the bricks together. Synthetic methods using ice crystals as templates have attracted considerable attention because the ice-templating method is a highly biocompatible, economical and environmentally benign method for the generation of highly pure materials with unique structures. In pioneering studies, Mahler and Bechtold observed that unidirectional freezing of aqueous silicic acid can produce silica fibers with polygonal cross-sections, a mixture of flakes and honeycombs, or ribbed flakes. More recently, a directional freezing approach has been used in combination with sol-gel chemistry to produce inorganic fibers, aligned porous inorganic structures, aligned porous structures of polymers and polymer/inorganic nanoparticle composites. In the process of directional freezing, the unidirectional submission of hydrosols, hydrogels or aqueous slurries of metal oxides or polymers to liquid nitrogen induces rapid ice formation (hexagonal form) that exhibits strong anisotropic growth kinetics. Hexagonal ice crystals expel the solutes, which are originally homogeneously dispersed in the aqueous gel, from the forming ice phase and entrap them within the directed channels between the ice crystals. Therefore, ice crystals grown in the form of platelets with very high aspect ratios can provide a novel template to microscopically define fibers. In Scheme 1, we show the assembly of PNIPAm and INP@PNIPAm core/shell microgels into one-dimensional fibrous structures through a convenient freezing method. The produced rounded fibers are on the micrometer scale in diameter and can be as long as several millimeters. Given that polymeric and inorganic fibers with diameters in the range of several micrometers down to tens of nanometers are of considerable interest for various applications, we believe that the C O M M U N IC A IO N