There has been considerable debate regarding whether epitaxy plays a major role in the oriented nucleation of biomineral crystals (with molecular surfaces as the proposed templates). The only direct way to settle this matter is to observe the organic and inorganic interfacial structures in situ during the process of nucleation, using a structural probe such as X-ray scattering. This has not yet been done in vivo, but we have studied biomimetic nucleation using molecular monolayers (Langmuir films) floating on aqueous solutions.  Reversing the more familiar process of thiol monolayer self-assembly on gold (111) surfaces, we have used floating thiol monolayers to grow (111)–oriented gold nanocrystals. Calcium carbonate---a more common biomineral---will not grow oriented crystals under pure acid-terminated monolayers, but it will do so when the monolayers are mixtures of anionic and cationic molecules. Whether this is a result of charge balance, epitaxy or stereochemistry (e.g. molecular tilt) is revealed by the X-ray data.  Our results illustrate the importance of in situ scattering studies that look at organic and inorganic structures simultaneously, and they show how soft (compliant) molecular templates are particularly effective at nucleating oriented crystals.

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