Session C13: Assembly and Behavior of Hierarchical Materials

Prescribed Self-Assembly of Nanoscale Architectures

The formation of nanoscale structures by means of self-assembly relies on the interplay of interactions, entropic effects and kinetic factors. The resulting phase behavior can be often rationalized and even manipulated, however, a deliberate fabrication of designed targeted structures via self-assembling is challenging. An incorporation of DNA molecules into a nano-object allows establishing highly selective and programmable, yet local, interactions between the components of nano-systems. That, however, might not be sufficient for self-assembly of specifically targeted structures. Is it possible to use a molecular addressability to prescribe a structural organization of the entire system at the multiple length scales? We have explored several strategies for creating targeted nano-architectures, including three-dimensional lattices of various prescribed symmetries, two-dimensional periodic arrays and arbitrarily designed clusters from the multiple types of the nano-components. The developed approaches also allow for controlling dynamic behavior of assemblies, for example, a selective triggering system transformations and cascade events. Application of these self-assembly methods for novel optical and mechanical nanomaterials will be also discussed.   

Self-Assembly and Formation of Hierarchically Structured Nanocrystals with Photocatalytic Activity

Design and engineering of the size, shape, and chemistry of photoactive building blocks enable fabrication of functional nanocrystals for applications in light harvesting, photocatalytic synthesis, and water splitting. Here we report the synthesis of such nanocrystals through a surfactant-assisted self-assembly process using optically active porphyrin as a functional building block. The self-assembly process relies on specific interactions such as π–π stacking and ligand coordination between porphyrin building blocks. The nanocrystals exhibit well-defined 1-3D morphologies and hierarchically ordered internal architectures. At the molecular level, porphyrins with well-defined size and chemistry possess unique optical and photocatalytic properties. On the nanoscale, controlled assembly of porphyrins leads to formation of ordered nanostructures with precisely defined size, shape, and spatial monomer arrangement to facilitate intermolecular charge and energy transfer.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.


  

Interfacial Fluctuations of a Nanoparticle Superlattice

We use liquid-phase transmission electron microscopy to image and understand interfacial fluctuations of a nanoparticle superlattice. With single particle resolution and hundreds of nanoscale building blocks in view, we are able to identify a phase boundary between self-assembled ordered lattice and disordered structure and visualize the kinetics of lattice growth front. By applying the capillary wave theory towards this nanoscale interface, we found that this interface is roughened by thermal energy and we can extract the surface stiffness, fluctuation width and other quantities from real-space imaging. Our experimental results demonstrate the potential of extending model study on collective systems to nanoscale with liquid-phase TEM and also provide insight into formation process self-assembled nanoscale architectures.   

Many Body Effects in Nanoparticle Superlattices

Materials consisting of periodic or quasiperiodic arrangements of nanocrystals, instead of atoms or molcelues have unique fundamental properties. In this talk I discuss the binding patterns (equilvalent to atomic orbitals in the binding or atoms or molecules) that enable the organiztion of nanocrystals into periodic structures. I calculate the free energy explicitly and provide a detailed quantification of the many body effects not described by pair interactions. the results are fully consistent with the recently proposed Orbifold Topological Model (OTM).   

The Impact of Molecular Sequence on Hierarchical Assembly of Biomimetic Polymers

Many macromolecules assemble into ordered structures along hierarchical pathways that begin with formation of amorphous or dense-liquid precursors. Whether these multi-step pathways are general features of macromolecules or a consequence of system-specific structural details is unknown. Moreover, the dynamics of such pathways are poorly understood. Using AFM to observe surface-directed assembly of sequence-defined polymers, we show that pathways are sequence dependent. While some sequences exhibit classical pathways with direct appearance of ordered structures, slight changes in sequence involving addition of hydrophobic groups lead to hierarchical pathways: disordered clusters 10-20 molecules in size first appear and eventually transform into ordered nuclei that then enter the growth phase. Predictions from kinetic models that consider competing rates of cluster formation, deposition and transformation agree with the observations and bound the magnitude of kinetic terms controlling assembly. Molecular dynamics combined with force spectroscopy reveals the role of hydrophobic groups and background electrolytes in driving multi-step pathways and altering nucleation kinetics. The results provide insights into structural and energetic controls on macromolecular assembly pathways.   

Electrostatics-Driven Self-Assembly of Virus-like Particles into Arrays Mediated by Dendrimers

Hierarchical self-assembly common in natural materials provides inspiration for designing synthetic materials with tailored structure and functionalities. Recent experiments have generated bulk materials including FCC superlattices using the P22 Virus-like Particle (VLP), a 60-nm protein cage, as a building block. This assembly into superlattices was mediated by oppositely charged, smaller dendrimers and controlled by tuning the ionic strength. In this study, coarse-grained molecular dynamics simulations are employed to explore charge and size complementarity of P22 VLPs and dendrimers in driving the hierarchical assembly in vitro. Equilibrium structures ranging from isolated VLPs to arrays of VLPs as a function of the ionic strength, VLP charge, and dendrimer permeability are identified and correlated with experiments. Structural properties including cluster size and pair correlation functions are evaluated. Results reveal the electrostatics-driven mechanism for dendrimer condensation on VLP surface and subsequent ordered agglomeration of VLPs. New avenues for exploiting long-range interactions in designing hierarchical, self-assembled materials are proposed.   

Surface assisted self-assembly of designed helix repeated protein

Surface assisted self-assembly is a promising way to synthesize hierarchical biomaterials in nature. For instance, membrane proteins can form channels on cell-membrane to transport ions, small molecules and macromolecules. In another example, S-layer protein self-assembles on cell envelop of bacteria to mechanically and osmotically stabilize cells. In this work, we used in-situ high-resolution and high-speed atomic force microscopy (AFM) to study the dynamic behaviors and self-assembly structures of designed helix repeated protein[1] on mineral surface in electrolyte. We discussed the roles of surface charge, hydration layer, cations, and entropy to protein dynamics at solid-liquid interface. The work is helpful to summarize the general model to describe surface assisted self-assembly of hierarchical biomaterials. It also inspires strategies to create artificial bio-mimetic materials with various applications.
1. Brunette, T.J., et al., Exploring the repeat protein universe through computational protein design. Nature, 2015. 528(7583): p. 580-584.
  

Self-Assembly of Elastic Capsomeres into a Virus Capsid: Experimentally-Informed Molecular Dynamics Simulations

In many viruses, capsomeres self-assemble spontaneously to form symmetric icosahedral capsids. The nature of this self-assembly and the formation of intermediate structures is important for antiviral drug development as well as the design of nanoparticle-based drug-delivery carriers. However, this self-assembly process is not completely understood in part due to vastly different timescales between experimental and computational approaches that make correlated investigations arduous. Coarse-grained molecular dynamics simulations are used to investigate the dynamics of capsomeres of Hepatitis B virus in non-assembly and self-assembly conditions. Capsomeres are modeled as flexible structures rather than rigid bodies to accurately capture the elastic nature of the in vitro protein while retaining computational efficiency. The simulation results are correlated to data from charge detection mass spectrometry, which characterizes stable intermediate structures via the simultaneous detection of particle charge and mass-to-charge ratio. Solutions comprising of oligomers with 2-10 capsomeres observed in non-assembly conditions are computationally investigated. Under assembly conditions, the mechanism of formation of an intermediate structure that is 75% of the capsid mass is explored.   

Alternate Path to the Formation of Co-aligned Hierarchical Mesocrystals

A diverse class of materials occur as hierarchical mesocrystals, which are single crystal structures consisting of atomically-aligned domains — a characteristic attributed to crystallization through oriented attachment (OA). However, many questions about the alignment mechanisms remain unanswered. Here we focus on crystallization of spindle-shaped hematite (Fe₂O₃) from poorly ordered ferrihydrite (5Fe₂O₃ ●9H2O, fh). Spindle structure, crystallization, and dissolution were probed by high resolution transmission electron microscopy (HR-TEM), cryo-TEM and liquid cell TEM. In particular, we developed a method for tracking regions by HR-TEM during the crystallization. Our results show the spindles consist of atomically aligned domains organized into a hierarchical, rod-like structure penetrated by nm-scale pores. This structure forms as follows: First, the fh-to-hematite transformation is highly localized, with the initial hematite nucleating on fh surfaces. Second, spindles grow by addition of new hematite particles that nucleate from the solution. This process can lead to formation of half-spindles with the growth direction pointing into the bulk solution. Consequently, OA occurs via an autocatalytic process in which the hematite-solution interface drives the nucleation and growth.   

Microstructural Control via Acoustic Assembly for Complex Functionalities in Composites

Control of the microscale distribution of phases in composite materials enables spatial modulation and amplification of functional properties within a component. A promising method for dynamically ordering microstructure is acoustic focusing, in which microparticles suspended in a fluid are manipulated with pressure fields generated by piezoelectric actuation within a microfluidic device. With this method we assemble functional particles into structures with tunable microstructural features, which we then freeze within a second phase or deposit onto a substrate as a curable ink for printing components with complex functionalities. The process is largely material agnostic, allowing the patterning of a wide variety of functional microparticles within various fluid media. We use confocal microscopy to pinpoint assembly regimes for structures with controlled particle distributions, particle alignment, and anisotropic packing density modulated spatially throughout components. We demonstrate the feasibility of this method by patterning networks of conductive particles cured in resins to measure thermal and electrical conductivity of the resulting composites.