Self-Assembled Crystalline Peptoid Assemblies as Tunable Scaffolds for Directed Calcium Carbonate Mineralization

In this study, we present an innovative utilization of assembled peptoid crystalline nanomaterials to form surface-independent coatings with tunable chemistries for calcium carbonate (CaCO₃) mineralization. CaCO₃, one of the most abundant biominerals, forms extensive deposits through the activity of marine organisms, serving as the planet’s largest and oldest reservoir of carbon dioxide (CO₂). In nature, biomolecules precisely control nucleation, crystal growth, phase transitions, and morphology, resulting in biominerals with diverse functionalities. As a result, there is significant interest in understanding bio-regulated crystallization processes and developing biomimetic strategies to promote carbonate mineral formation. One class of materials that is gaining significant attention in biomineralization studies are peptoids due to their versatile chemical modifiability and exceptional stability, while retaining properties akin to biological materials. We hypothesized that through systematically modifying peptoid side-chain chemistries, we can advance the research on bio-inspired mineralization and aid to the design of materials that mimic natural processes.

Our findings demonstrate that we can fine-tune the interaction between these crystalline materials and Ca²⁺ ions, thereby promoting more efficient and organized nucleation events. We employed a combination of ex situ and in situ techniques, including SEM, AFM, and optical microscopy, to comprehensively investigate the crystallization process, accounting for variations in number densities, nucleation rates, and particle sizes across different peptoid tubular film samples. The peptoid tubular films containing -COOH hydrophilic domains exhibited the highest CaCO₃ particle number density, reaching an impressive 3,635 particles/mm²—approximately 45 times higher than the control sample (unmodified silicon substrate, 80 particles/mm²). Remarkably, these particles, identified as vaterite, have the tendency to grow perpendicular to the film substrate through heterogeneous nucleation accounting to closely 82% of the whole population of particles. In contrast, films with basic -NH₂ groups showed only a minimal increase in nucleation density (103 particles/mm^{<span style="font-size:10.8333px">2</span>}). However, these films promoted the formation of larger hexagonal vaterite particles that lay flat on the film substrate. To elucidate more mechanistic details, we are also working on obtaining quantitative parameters such as the effective interfacial energy (αeff) of the film-crystal system by obtaining the nucleation rate (Jn) at different solution supersaturations, given by the equation,
lnJ_n=lnA-Bα_eff³/σ²
where, A and B are the kinetic factor and shape-dependent factor, respectively.
The implications of these findings are substantial for the field of materials science, particularly in the realm of crystalline scaffold materials. We expect that further exploration of various side chain group chemistries and adjusting their spatial distributions can provide a robust framework for designing materials pivotal in enhancing the nucleation rate of calcium carbonate and in controlling the orientation of the resulting crystals, not only for CO₂ sequestration but also the formation of hierarchical carbonate materials with specific mechanical, optical, or thermal characteristics desirable in high-performance applications in various industries, including construction, bioengineering, and environmental remediation.