Promotion of CaCO₃ Nucleation by Carboxyl- and Amine-Terminated Peptoid Nanotubes

The use of surfaces to promote heterogeneous nucleation of CaCO₃ has been extensively studied to better understand natural processes of biomineralization, to develop bioinspired approaches for synthesizing composite materials with diverse functionalities, and as a route toward accelerating CO₂ mineralization. Peptoids have emerged as a material of particular interest, because they offer tunable side chain chemistry while maintaining a constant long-range self-assembled architecture. Here, we investigate the ability of films of self-assembled -NH₂ and -COOH terminated peptoid nanotubes to promote the nucleation of CaCO₃. Using a combination of atomic force, scanning electron, and optical microscopy, we find that interwoven films of nanotubes formed from peptoids with -NH₂ side chains promote the preferential formation of vaterite with the (001) plane parallel to the film and exhibit a lower interfacial energy than that reported for inorganic surfaces. In contrast, the film of -COOH-functionalized peptoid nanotubes is a much stronger promoter, giving a particle density 5 orders of magnitude larger than for the -NH₂ terminated peptoid nanotubes and enabling complete coverage of individual peptoid tubes with CaCO₃ nanoparticles that results in a composite material with a 50-fold increase in Young's modulus. Thus, interwoven films of peptoid nanotubes provide a useful platform for investigating the impact of specific chemical groups on nucleation and offer potential application to accelerated mineralization of CO₂ for removal from the environment or modulation of mineralogy and properties of carbonate-based building materials.