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Inorganic precipitation reactions are known to self-organize a variety of macroscopic structures, including hollow tubes. We discuss recent advances in this field with an emphasis on experiments similar to 'silica gardens'. These reactions involve metal salts and sodium silicate solution. Reactions triggered from reagent-loaded microbeads can produce tubes with inner radii of down to 3 mm. Distinct wall morphologies are reported. For pump-driven injection, three qualitatively different growth regimes exist. In one of these regimes, tubes assemble around a buoyant jet of reactant solution, which allows the quantitative prediction of the tube radius. Additional topics include relaxation oscillations and the templating of tube growth with pinned gas bubble and mechanical devices. The tube materials and their nano-to-micro architectures are discussed for the cases of silica/Cu(OH) 2 and silica/Zn(OH) 2 /ZnO tubes. The latter case shows photocatalytic activity and photoluminescence.

The combination of top-down and bottom-up approaches offers great opportunities for the production of complex materials and devices. We demonstrate this approach by incorporating luminescent CdSe-ZnS nanoparticles into macroscopic tube structures that form as the result of externally controlled self-organization. The 1-2 mm wide hollow tubes consist of silica-supported zinc oxide/hydroxide and are formed by controlled injection of aqueous zinc sulfate into a sodium silicate solution. The primary growth region at the top of the tube is pinned to a robotic arm that moves upward at constant speed. Dispersed within the injected zinc solution are 3.4 nm CdSe-ZnS quantum dots (QDs) capped by DHLA-PEG-OCH3 ligands. Fluorescence measurements of the washed and dried tubes reveal the presence of trapped QDs at an estimated number density of 10(10) QDs per millimeter of tube length. The successful inclusion of the nanoparticles is further supported by electron microscopy and energy dispersive X-ray spectroscopy, with the latter suggesting a nearly homogeneous QD distribution across the tube wall. Exposure of the samples to copper sulfate solution induces quenching of about 90% of the tubes' fluorescence intensity. This quenching shows that the large majority of the QDs is chemically accessible within the microporous, about 15-μm-wide tube wall. We suggest possible applications of such QD-hosting tube systems as convenient sensors in microfluidic and related applications.

b S Supporting Information ' INTRODUCTIONExternally controlled self-assembly and self-organization can produce a broad spectrum of hierarchically ordered structures and materials. 1 Examples include three-dimensional objects such as nanoparticle assemblies, 2,3 solid and hollow spheres, 4 helices, 5 fibers, 6 rods, 7 and tubes of large aspect ratio. 8À10 Because molecular and supramolecular self-assembly occurs close to thermodynamic equilibrium, its approximate outcome is typically easier to anticipate than the results of self-organizing processes that require conditions far from equilibrium. 11À17 This design advantage of self-assembly comes at the price of comparably low temporal and spatial complexity which for self-organization can be tremendously large. 18,19 The exploration of these (nonexclusive) processes is of great interest to materials science. Its key challenges, however, are centered in the realm of physical chemistry.The formation of macroscopic, inorganic tubes is an important example for this unconventional approach toward materials synthesis and device production. Several groups have used template-directed solÀgel methods to control the shape of the forming tubes. 20 Nakamura and Matsui prepared rhomboidal nanotubes by hydrolyzing tetraethyl orthosilicate in a mixture of ethanol, water, ammonium hydroxide, and racemic tartaric acid. 21,22 The underlying mechanism involves the deposition of amorphous silica around crystals of ammonium DL-tartrate that grow in reacting solutions of tartaric acid and ammonium hydroxide. 23,24 Another approach has been pursued by Cronin and co-workers 25 who reported the spontaneous formation of hollow tubes in an aqueous system containing an organic cation and polyoxometalate crystals. The diameter of these tubes varies in the range of several micrometers, and their length can reach up to a few millimeters. Moreover, directional control was achieved during synthesis by applying external concentration gradients and electric fields. 26

Mit eigenem Antrieb: Wenn man mit Salzlösungen befüllte Agarose‐Mikrokügelchen in eine Natriumsilicatlösung einbringt, so entstehen Röhren, die an einer anorganischen Schale anhaften (siehe Bild). Diese Röhren haben Innenradien von 3 μm aufwärts, können 0.5 mm lang werden und wachsen mit Geschwindigkeiten bis 50 μm s−1. An Blasen hängende Röhren können eine gerichtete Bewegung der Kügelchen induzieren.