Session A58: Self-Assembly I

Bundling of Rod-Like Colloids via Depletion Forces

Self assembly of rod-like colloids in to bundles can be driven by entropic processes. We use Coarse Grained Molecular Dynamics simulations to investigate using depletion forces to induce bundling in a suspension of rod-like colloids. The colloids are approximated as bead-spring polymers, with persistence lengths much larger than the length of the polymer. Depletion forces are an effective attractive force between colloids due to the presence of particles significantly smaller than the colloids, called depletants. Depletion forces are modelled as an implicit entropic force between colloids. Bundling begins to occur when the occupied volume fraction occupied by depletants is above a critical value. This volume fraction is dependent on the length and stiffness of the rod-like colloids. We investigate using this property to induce bundling only in rods above a certain length or certain persistence length.
  

Self Assembly of Colloidal Polymers Using Isotropic Potentials

Self-assembly of spherical colloids into fluidic chains with no directional bonding or external perturbation promises to be a facile approach to engineer “designer” materials for enhanced surface transport, catalysis, optics, etc. Employing an Inverse Design technique (Relative Entropy optimization), we discern radially-symmetric pair potentials that result in the formation of fluidic, single-stranded, polydisperse colloidal strings (“colloidomers”). Comparing these potentials with those that form size-selective, compact, isotropic colloidal clusters, we find that the ranges of attraction (δ_A) and repulsion (δ_B) between the two are distinctly different. The energetics combined with the ranges determine the resulting structural motifs. A simple universal potential form is proposed whose morphological phase diagram predicts a gamut of microstructures as a function of δ_A and δ_B at fixed energetics and packing fraction. Four main broad classes of structures are observed - (i) dispersed monomeric fluid, (ii) ergodic, short chains as well as porous, space-spanning, one-monomer diameter thick, percolated strings, (iii) clusters and (iv) thick cylindrical structures including the tri-helical Bernal spirals.   

Encapsulation of circular DNA and non-organic particles inside virus-Like DNA origami icosahedrons.

The principle of quasi-equivalence, annunciated by Caspar and Klug in 1962, articulates how icosahedral viruses can be constructed using a minimal number of distinct sub-units. Following this scheme, we used DNA origami to construct 5MDa monomers of 3-fold symmetry, which interact through base stacking and shape complementarity and self-assemble into icosahedrons. We encapsulated cargo by functionalizing the interior facing portion of the monomers with single stranded DNA that was complementary to strands on cargo. Two categories of cargo were utilized. The first type of cargo were gold colloids, which were functionalized with complementary ssDNA to the monomers. These colloids were smaller than a monomer and did not bridge between monomers. The second type of cargo was 2.5 MDa circular ssDNA, which could bridge between monomers. The assembly yield was studied as a function of the monomer concentration, interaction strength, and amount of encapsulated cargo.
  

Nucleation rate in a heterogeneous quartic system

The nucleation of a stable phase from a metastable phase is studied using the string method applied to a model Ginzburg-Landau system with a quartic potential. A temperature-like parameter is varied in order to move the system between its binodal and spinodal points. A heterogeneity is added to the system in the form of a wedge-shaped substrate. The effect of the heterogeneity on the nucleation barrier height is obtained from the string method. The results are compared with those of the classical nucleation theory. Our results highlight the value of the string method, as well as the validity and limitations of the classical theory.   

Light controlled crystallization of DNA coated particles

DNA coatings have been proposed and successfully applied as a versatile tool for programming the self-assembly of micrometer size particles into numerous crystalline structures. The DNA-mediated interaction is highly temperature dependent. For given buffer conditions, a set of complementary DNA-coated particles present a melting temperature T_melt that marks the transition between aggregated and melted states. Without a change of environment, one has no further control over T_melt and a local control of the interaction is challenging.
Here we present a strategy that uses the combination of DNA coatings and an azobenzene photo-switch to access fine control of the interaction between particles, independently from temperature. Azobenzenes reversibly switch between trans and cis conformations, an exposure to UV or blue light shifts the population towards the cis or trans respectively. Once attached to the backbone of a DNA sticky-end, the flat trans form can intercalate in the double helix and stabilize the duplex. On the contrary the kinked cis azobenzene disrupts the hybridization. We harvest this molecular behavior to modulate the melting temperature of DNA-coated particles, pattern their melting or aggregation, correct defects on the crystals and switch between crystal structures.   

Exploring nucleation pathways and solid-solid transitions in two-dimensional binary colloidal crystallization

Crystals are prevalent in many natural and manmade systems, including metals, minerals, proteins, and colloids. Although the crystal structures themselves are often well understood, the microscopic pathways by which crystals form are difficult to observe or predict. We use a combination of simulations and microscopy experiments to investigate the crystallization pathways of DNA-coated colloids. We observe a rich diversity of behaviors, including one-step and two-step nucleation pathways, as well as a spontaneous solid-solid phase transition during the crystallization of two-dimensional binary mixtures. In this talk, I will present the results of free energy calculations which indicate that the two-step nucleation transition arises from a competition between the free-energy landscapes of two different structures. I will also explore the possibility that the critical size governing this transition can be tuned by changing the relative strengths of interactions. These results may help shed light on fundamental aspects of nucleation, as well as provide new methods for controlling the self-assembly of materials made from colloids.   

Entropy Driven Assembly of Truncated Colloidal Tetrahedra into Diamond Lattice

Building blocks with non-trivial shape are thought to bear invaluable information that largely determines both pathway and final product in self-assembly system. Packing of polyhedra leads to a large diversity of hierarchical structures, among which tetrahedra with various degrees of truncation have been of particular interest due to its unique symmetry. However, the experimental realization of colloidal tetrahedra remains challenging. Here for the first time, surface tension assisted by geometric constraints is used to synthesize truncated colloidal tetrahedra via colloidal fusion, with high size and morphology regularity. The entropy driven assembly of truncated tetrahedral particles was carefully studied via depletion interaction and sedimentation. In both cases, particles exhibit strong face-to-face contact tendency induced by directional entropic force, leading to hexagonal and cubic diamond structures. This study provides information about shape-structure relationship in the packing of truncated tetrahedra, and may further enhance the understanding of crystallization of diamond and its stacking hybrid.
  

Stochastic Yield Catastrophes and Robustness in Self-Assembly

A guiding principle in macromolecular self-assembly is that, for high production yield, nucleation of structures must be significantly slower than their growth. However, details of the mechanism that impedes nucleation are broadly considered irrelevant. Here, we analyze a generic stochastic model for self-assembly into finite-sized target structures, and investigate two key scenarios to delay nucleation: (i) by introducing a slow activation step for the assembling constituents and, (ii) by decreasing the dimerization rate. These scenarios have widely different characteristics. While the dimerization scenario exhibits robust behavior even in the limit of small particle numbers, the activation scenario is highly sensitive to stochastic effects, which can completely suppress yield. Our results reveal that stochasticity is an important limiting factor for self-assembling systems and, in general, details of the nucleation process play a significant role for the final yield.   

Measuring crystal nucleation and growth of DNA-grafted colloidal particles

Grafting DNA onto microscopic colloidal particles can `program' them with information that tells them how to self-assemble into a variety of interesting crystal structures. However, the dynamic pathways by which these crystals self-assemble are largely unknown. In this talk I will present progress on an experimental study of the nucleation and growth of colloidal crystals due to DNA hybridization. Specifically, I will describe a microfluidics-based approach in which we produce hundreds of monodisperse, isolated droplets filled with colloidal particles and then track the formation of crystals within each drop as a function of time. We find that the initial nucleation of crystals from a supersaturated solution involves overcoming a free-energy barrier, and that the height of this barrier decreases dramatically with decreasing temperature. We also find that once nucleated, the crystals grow at a rate that is limited by the diffusive flux of colloidal particles to the growing crystal surface. These findings may help us to devise strategies to tune the nucleation rates and crystal growth kinetics independently, which will be helpful as we try to engineer higher quality or more complex self-assembled structures.   

Nanoparticle superlattice self-assembly via interpolymer complexation

Controlled self-assembly of nanoparticles into ordered structures is an auspicious strategy for fabricating novel devices in nanotechnology. As the number of successful strategies for synthesis increases, emphasis shifts towards those that are general, versatile, inexpensive and scalable. To this end, nanoparticle superlattices formed via interpolymer complexation offer a promising route towards a stable, usable class of materials. Understanding the interactions that drive these systems to equilibrium is a primary goal of the theoretical and computational community, as we develop models that capture the configurational characteristics of the system and ultimately predict experimental results. Making use of molecular dynamics simulations, we present our efforts towards understanding nanoparticle superlattices created via interpolymer complexation.   

Aqueous Electrostatic Colloidal Crystallization

The complementarity demonstrated by atomic ions is an effective strategy to form a range of complex structures from relatively simple building blocks. Due to van der Waals forces, however, mixtures of oppositely charged colloids in water only form disordered aggregates. Consequently, formulating sets of complementary particles has required intensive surface chemistry, despite the availability of these naturally charged starting materials. Here we employ a straightforward technique to stabilize and tune the bonds between oppositely charged colloids to promote ordered self-assembly. After introducing surfactants that can reliably hold particles physically separated, the long-range electrostatic attraction is tuned via charge screening until particles spontaneously crystallize. Single crystals on the centimeter scale are produced in solution that become permanently fixed when introduced to deionized conditions, after which they become robust enough to survive drying for SEM analysis and further processing.   

Understand Janus particle assembly assisted by surface active molecules

Janus particles assemble into remarkable superstructures due to different chemistry on the two sides of a single particle. The directional interactions between particles can lead to clusters, chains and stripes, which present new structures and physics of colloidal systems. However, how Janus particles assemble in the presence of surface active molecules has not been systematically studied. On the contrary, most of the previous studies have focused on purified Janus particles in the absence of surface active species. In our experiment, surface active molecules have drastically changed the assembly behavior of Janus particles. Using real time imaging and particle tracking algorithm, we studied the dynamics the Janus particles. We also observed intriguing new crystal structures with highly correlated orientation formed by amphiphilic Janus particles assembled with ionic liquid molecules.   

Phase behavior of hard colloidal bipyramid family

Colloidal particles with bipyramidal shape are experimentally obtainable today, and some of them are known to self-assemble into highly complex crystals such as clathrates and quasicrystals by maximizing face-to-face alignment, e.g. via DNA-programmable assembly or via entropy maximization. We present a study of the effect of shape variation on the self-assembly of hard bipyramids using Monte Carlo simulation. We report the phase behavior as a function of the number of vertices in the base polygon and the aspect ratio. From geometric and thermodynamic analysis, we identified over 10 different crystalline structures with varying complexity and symmetry. Our results offer guidance to experiments by providing information about entropically driven self-assembly in the bipyramidal shape family.   

Crosslinking 2D gold nanoparticle assembly at the air-water interface

We report the results of crosslinking of two-dimensional gold nanoparticle (Au-NP) assemblies at the air-water interface in situ. We introduce an aqueous soluble ruthenium benzylidene catalyst into the water subphase to generate a robust, elastic two-dimensional network of nanoparticles containing cyclic olefins in their ligand framework. The most striking feature of the crosslinked Au-NP assemblies is that the extended connectivity of the nanoparticles enables the film to preserve much of its integrity under compression and expansion, features that are absent in its non-crosslinked counterparts. The crosslinking process appears to “stitch” the nanoparticle crystalline domains together, allowing the crosslinked monolayers to behave like a piece of fabric under lateral compression.   

Linker-mediated binding of DNA-grafted colloids: how competition affects phase behavior

DNA is a promising tool for programming the self-assembly of new materials: its interactions are chemically specific, tunable, and predictable. In principle, DNA can be grafted to colloids to favor the formation of a predetermined aperiodic structure. In this talk, I promote linker-mediated binding of DNA-coated colloids as an experimental system that could create these hundreds of specific interactions in practice. Here, DNA-coated particles interact through DNA linker strands dissolved in solution. Using optical microscopy, we study the melting transition of our linker system as a function of linker concentration, grafting density, and DNA sequences. We find a phase diagram different from that of directly hybridizing DNA-coated colloids, featuring a reentrant melting transition at high linker concentrations and a region of stable coexistence between solid and fluid. We also examine how the introduction of hundreds of competing linker strands might affect linker-mediated interactions and show that the system can accommodate many interacting linkers simultaneously. These results show the tunability and capacity of our linker-mediated system, and demonstrate how it might expand the design space for aperiodic and adaptive structures.