Complexity from precipitation reactions

A key challenge for modern molecular sciences is to bridge the gap between the nanoscopic world and macroscopic devices. The underlying question is whether one can control chemical reactions to produce directly macroscopic complexity, hierarchical order, and ultimately entirely new types of materials. If viewed as a form of complicated chemistry, biology unambiguously answers this question with a resounding “yes” and, furthermore, demonstrates the powerful potential of this approach. However, this fundamental reassurance does not provide significant help in developing nonbiological model systems and leaves us with seemingly insurmountable hurdles. The PNAS article by Haudin et al. (1) is an important step to overcome some of these hurdles, as it provides just such an experimental model. The work by Haudin et al. (1) follows up on hints that can be found in some of chemistry’s earliest literature. In 1664, Johann Glauber described reactions producing “philosophical trees, both pleasant to the eye and of good use” (2). Today, these structures are known as chemical or silica “gardens” and are common demonstration experiments in school and introductory college classes. A chemical garden is typically grown by placing a macroscopic salt particle into a sodium silicate solution (3). The dissolution of the “seed” particle causes the formation of insoluble metal hydroxide that forms colloidal particles, and subsequently surrounds the seed with an inorganic membrane. The system—now compartmentalized by this thin membrane—is subject to osmotic pressure, which drives an inflow of water and subsequently ruptures the membrane (4). From this site, a jet of buoyant salt solution, sustained by the osmotic pump action near the base, rises upwards and templates the growth of a hollow inorganic tube. These precipitation tubes can be several centimeters long and have …