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Molding Porous Nanoparticles

Recent Research from Professor Andreas Stein and his research group.

Within the last few years, nanoparticles have transitioned from scientific curiosities to becoming essential components in applications such as sensing, nanoelectronics, catalysis and other uses. Their unique optical, electronic, magnetic and reactive properties compared to bulk solids rely on control over both the size of the particles in the nanometer size-regime and the particle shapes. But how can the synthetic chemist control shapes on such a small scale? Graduate student Fan Li in the Stein group recently developed a novel approach for preparing monodisperse, discrete nanoparticles with specific shapes (e.g., cubes, tetrapods, spheres). These shaped particles can be porous, thereby significantly enhancing surface areas of the particles and opening up possibilities for host-guest applications (e.g., sensors). The method is based on a combination of self-assembly and templating methods in tandem with controlled disassembly of porous solids into their structural building blocks (Figure 1).

Figure 1. Porous silica nanocubes and tetrapods are produced when a bottom-up synthesis based on natural assembly/templating methods is combined with a top-down synthesis involving structural disassembly. Uniform polymer nanospheres are assembled into close packed arrays like marbles in a box. The space between spheres is filled with precursors. During heating the polymer spheres are removed and a solid spongy skeleton forms. Under the appropriate reaction conditions, the skeleton breaks down into uniform building blocks.

Shaped silica nanoparticles are prepared by controlled disassembly of a solid skeleton that is built around a scaffold of close-packed nanospheres. The skeletal structure consists of two basic units, cubes and tetrapods, which correspond to the space between the packed nanospheres and are connected like tinker-toys. Fan Li discovered that these building blocks can be disconnected and isolated under specific synthesis conditions (Figure 2). The disassembly occurs first by thinning of the necks between the two units, followed by complete disconnection of the skeleton at the narrowest connection points. Discrete nanoparticles are obtained in high yield. Their sizes are easily controlled by the diameters of the templating nanospheres. The sponge-like silica nanocubes and tetrapods can themselves be used as molds to produce similar particles made of carbon or polymers. This general method of preparing uniform porous nanoparticles with specific shapes is described in an upcoming issue of Angewandte Chemie, International Edition and is featured on the journal cover.

Figure 2. (a) Illustration of the disassembly of a porous skeleton structure into its building blocks. (b) Transmission electron microscopy image of carbon replicas of the porous silica cubes and tetrapods.

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