Insights into the Biomimetic Synthesis of 2D ZnO Nanomaterials through Peptoid Engineering

Achieving predictable biomimetic crystallization using sequence-defined synthetic molecules in mild conditions represents a long-standing challenge in materials synthesis. Herein we report a peptoid-based approach for biomimetic control over the formation of nanostructured ZnO materials in ambient aqueous conditions. A series of two-dimensional (2D) ZnO nanomaterials have been successfully obtained using amphiphilic peptoids with different numbers, ratios, and patterns of various hydrophilic and hydrophobic side chains. By investigating the relationship between peptoid hydrophobicity and the thickness of the resultant ZnO nanomaterials, we found the critical role of peptoid hydrophobicity in the peptoid-controlled ZnO formation. Our results suggest that tuning the hydrophobicity of peptoids can be used to moderate peptoid-ZnO surface interactions, thus controlling the formation of ultrathin (<2.5 nm) 2D ZnO nanomaterials. The peptoid-controlled formation of ZnO nanomaterials was further investigated using ultrasmall-angle X-ray scattering (USAXS). Our work suggests a new approach to synthesizing 2D metal oxide nanomaterials using sequence-defined synthetic molecules.

Multi-Step Nucleation of A Crystalline Silicate Framework via A Structurally Precise Prenucleation Cluster

Hierarchical nucleation pathways are ubiquitous in the synthesis of minerals and materials. In the case of open framework lattices such as zeolites and metal-organic frameworks, pre-organized multi-ion "secondary building units" (SBUs) have long been proposed as fundamental building blocks of the forming crystals. However, detailing the progress of multi-step reaction mechanisms in going from monomeric species to stable crystals and defining the structures of the intermediate SBUs remains an unmet challenge. Combining in situ nuclear magnetic resonance, small-angle X-ray scattering, and atomic force microscopy, we show that crystallization of the framework silicate, cyclosilicate hydrate, occurs through an assembly of cubic octameric Q38 polyanions formed through cross-linking and polymerization of smaller silicate monomers and other oligomers. These Q38 are stabilized by hydrogen bonds with surrounding H2O and tetramethylammonium ions (TMA+). When Q38 levels reach a threshold of ~32% of the total silicate species nucleation within these clathrates occurs. Further growth proceeds through the incorporation of [(TMA)x(Q38)·nH2O](x-8) clathrate complexes into step edges on the crystals. These findings provide a clear picture of the multi-step nucleation process by which SBUs build a framework silicate lattice with implications for the synthesis of both functional materials and natural minerals.

Hierarchical Self-Assembly Pathways of Peptoid Helices and Sheets

Peptoids (N-substituted glycines) are a class of tailorable synthetic peptidomic polymers. Amphiphilic diblock peptoids have been engineered to assemble 2D crystalline lattices with applications in catalysis and molecular separations. Assembly is induced in an organic solvent/water mixture by evaporating the organic phase, but the assembly pathways remain uncharacterized. We conduct all-atom molecular dynamics simulations of Nbrpe6Nc6 as a prototypical amphiphilic diblock peptoid comprising an NH₂-capped block of six hydrophobic N-((4-bromophenyl)ethyl)glycine residues conjugated to a polar NH₃(CH₂)₅CO tail. We identify a thermodynamically controlled assembly mechanism by which monomers assemble into disordered aggregates that self-order into 1D chiral helical rods then 2D achiral crystalline sheets. We support our computational predictions with experimental observations of 1D rods using small-angle X-ray scattering, circular dichroism, and atomic force microscopy and 2D crystalline sheets using X-ray diffraction and atomic force microscopy. This work establishes a new understanding of hierarchical peptoid assembly and principles for the design of peptoid-based nanomaterials.