A single molecule visualization of DNA diffusion and partitioning in model porous materials.

We developed an experimental approach that enables molecule visualizations of macromolecular diffusion and partitioning within well-defined pores. By colloidal templating, two-dimensional arrays of open, submicron cavities interconnected by small holes were created in dense polyacrylamide gels. Cavity size of the arrays varied from 600 to 1400 nm, with the corresponding holes about 4–5 times smaller. DNA molecules of sizes from 2.69 to 48.5 kbp were inserted into the cavity arrays and monitored by fluorescent microscopy. In video sequences, individual chain positions identified as the chains diffused under Brownian motion over a period of seconds to tens of minutes. For larger chains, dynamic configurations were resolved during the motion. Over full range of molecular and pore sizes, we found that chain dynamics could be understood through the entropic barriers transport mechanism. At high confinement (large molecules in small cavities), this mechanism produces unexpected trends, for example, independence of diffusion coefficient on molecular size or faster diffusion of molecules in smaller pores. These trends reflect segmental excluded volume. Complicated dynamics akin to motion of an inchworm characterized the largest DNA chains, those with radius of gyration larger than the cavity radius. Diffusion of linear and circular DNA molecules was compared for different molecular sizes, and the resulting differences in diffusion coefficient explained by differences in diffusion mechanism; linear molecules translocate through holes by forming loops, while linear chains predominantly translocate by threading one chain end. A similar colloidal templating approach was also employed to create isolated cavity pair interconnected by a small hole. When templated by bidisperse colloid, the two cavities have unequal diameters. A DNA chain trapped inside such pair partitions unevenly, preferring the larger cavity, which afford greater configurational freedom. This sort of partitioning underlies many separation technologies but had not been visualized previously. The partition coefficient between cavities was measured visually for many combinations of cavity and molecular sizes, trends in this coefficient were then compared to existing theories for polymer partitioning. Good agreement over a two orders-of-magnitude variation of partition coefficient was obtained when effect of excluded volume on confinement free energy was introduced in a mean-field manner

Valence-programmable nanoparticle architectures.

Nanoparticle-based clusters permit the harvesting of collective and emergent properties, with applications ranging from optics and sensing to information processing and catalysis. However, existing approaches to create such architectures are typically system-specific, which limits designability and fabrication. Our work addresses this challenge by demonstrating that cluster architectures can be rationally formed using components with programmable valence. We realize cluster assemblies by employing a three-dimensional (3D) DNA meshframe with high spatial symmetry as a site-programmable scaffold, which can be prescribed with desired valence modes and affinity types. Thus, this meshframe serves as a versatile platform for coordination of nanoparticles into desired cluster architectures. Using the same underlying frame, we show the realization of a variety of preprogrammed designed valence modes, which allows for assembling 3D clusters with complex architectures. The structures of assembled 3D clusters are verified by electron microcopy imaging, cryo-EM tomography and in-situ X-ray scattering methods. We also find a close agreement between structural and optical properties of designed chiral architectures

Large-area alginate/PEO-PPO-PEO hydrogels with thermoreversible rheology at physiological temperatures

Alginate hydrogels have shown great promise for applications in wound dressings, drug delivery, and tissue engineering. Here, we report the fabrication, rheological properties, and dynamics of a multicomponent hydrogel consisting of alginate and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers, and the achievement of thick, castable gels with tunable, thermoreversible behavior at physiological temperatures (Figure 1). PEO-PPO-PEO triblock copolymers can form temperature-sensitive hydrogels that exist as liquids at low temperatures and soft solids at high temperatures. In this work, we have employed PEO-PPO-PEO triblock copolymers to impart thermoresponsive properties to alginate hydrogels in the form of a multicomponent hydrogel. These systems can transition between a weak gel and a stiff gel, with a corresponding increase in the viscoelastic moduli of approximately two orders of magnitude as temperature is increased. The temperatures corresponding to the upper and lower boundaries of the stiff gel region, as well as the storage modulus at physiological temperatures (e.g., 36 – 40 C), can be controlled through the PEO-PPO- PEO concentration. Additionally, we explore the properties of these materials under compression and large deformations, and describe how alginate and F127 concentration can be used to control the fracture stress and strain. Finally, we compare the results from bulk rheology to the structure and dynamics of the gels measured via small-angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS) experiments. Please click Additional Files below to see the full abstract

Mesoporous Polymer Frameworks from End-Reactive Bottlebrush Copolymers

Reticulated nanoporous materials generated by versatile molecular framework approaches are limited to pore dimensions on the scale of the utilized rigid molecular building blocks (<5 nm). The inherent flexibility of linear polymers precludes their utilization as long framework connectors for the extension of this strategy to larger length scales. We report a method for the fabrication of mesoporous frameworks by using bottlebrush copolymers with reactive end blocks serving as rigid macromolecular interconnectors with directional reactivity. End-reactive bottlebrush copolymers with pendant alkene functionalities were synthesized by a combination of controlled radical polymerization and polymer modification protocols. Ru-catalyzed cross-metathesis cross-linking of bottlebrush copolymers with two reactive end blocks resulted in the formation of polymer frameworks where isolated cross-linked domains were interconnected with bottlebrush copolymer bridges. The resulting materials were characterized by a continuous network pore structure with average pore sizes of 9–50 nm, conveniently tunable by the length of the utilized bottlebrush copolymer building blocks. The materials fabrication strategy described in this work expands the length scale of molecular framework materials and provides access to mesoporous polymers with a molecularly tunable reticulated pore structure without the need for templating, sacrificial component etching, or supercritical fluid drying

Tunable Nanoparticle Arrays at Charged Interfaces

Structurally tunable two-dimensional (2D) arrays of nanoscale objects are important for modulating functional responses of thin films. We demonstrate that such tunable and ordered nanoparticles (NP) arrays can be assembled at charged air-water interfaces from nanoparticles coated with polyelectrolyte chains, DNA. The electrostatic attraction between the negatively charged nonhybridizing DNA-coated gold NPs and a positively charged lipid layer at the interface facilitates the formation of a 2D hexagonally closed packed (HCP) nanoparticle lattice. We observed about 4-fold change of the monolayer nanoparticle density by varying the ionic strength of the subphase. The tunable NP arrays retain their structure reasonably well when transferred to a solid support. The influence of particle’s DNA corona and lipid layer composition on the salt-induced in-plane and normal structural evolution of NP arrays was studied in detail using a combination of synchrotron-based <i>in situ</i> surface scattering methods, grazing incidence X-ray scattering (GISAXS), and X-ray reflectivity (XRR). Comparative analysis of the interparticle distances as a function of ionic strength reveals the difference between the studied 2D nanoparticle arrays and analogous bulk polyelectrolyte star polymers systems, typically described by Daoud–Cotton model and power law scaling. The observed behavior of the 2D nanoparticle array manifests a nonuniform deformation of the nanoparticle DNA corona due to its electrostatically induced confinement at the lipid interface. The present study provides insight on the interfacial properties of the NPs coated with charged soft shells

One-Shot Synthesis and Melt Self-Assembly of Bottlebrush Copolymers with a Gradient Compositional Profile

Morphological control plays a central role in soft materials design. Herein, we report the synthesis of a gradient bottlebrush architecture and its role in directing molecular packing in the solid state. Bottlebrush copolymers with gradient interfaces were prepared via one-shot ring-opening metathesis polymerization of <i>exo</i>- and <i>endo</i>-norbornene-capped macromonomers. Kinetic studies revealed a gradient compositional profile separating the two blocks along the backbone. Side-chain symmetric gradient bottlebrush copolymers exhibited a strong tendency to assemble into cylindrical microstructures, in contrast to their block copolymer analogs with sharp interfaces. Such exquisite architectural control of the interfacial composition affords a delicate handle to direct macromolecular assembly

Valence-programmable nanoparticle architectures.

Nanoparticle-based clusters permit the harvesting of collective and emergent properties, with applications ranging from optics and sensing to information processing and catalysis. However, existing approaches to create such architectures are typically system-specific, which limits designability and fabrication. Our work addresses this challenge by demonstrating that cluster architectures can be rationally formed using components with programmable valence. We realize cluster assemblies by employing a three-dimensional (3D) DNA meshframe with high spatial symmetry as a site-programmable scaffold, which can be prescribed with desired valence modes and affinity types. Thus, this meshframe serves as a versatile platform for coordination of nanoparticles into desired cluster architectures. Using the same underlying frame, we show the realization of a variety of preprogrammed designed valence modes, which allows for assembling 3D clusters with complex architectures. The structures of assembled 3D clusters are verified by electron microcopy imaging, cryo-EM tomography and in-situ X-ray scattering methods. We also find a close agreement between structural and optical properties of designed chiral architectures

Linear Mesostructures in DNA–Nanorod Self-Assembly

The assembly of molecules and nanoscale objects into one-dimensional (1D) structures, such as fibers, tubules, and ribbons, typically results from anisotropic interactions of the constituents. Conversely, we found that a 1D structure can emerge <i>via</i> a very different mechanism, viz, the spontaneous symmetry breaking of underlying interparticle interactions during structure formation. For systems containing DNA-decorated nanoscale rods, this mechanism, driven by flexible DNA chains, results in the formation of 1D ladderlike mesoscale ribbons with a side-by-side rod arrangement. Detailed structural studies using electron microscopy and <i>in situ</i> small-angle X-ray scattering (SAXS), as well as analysis of assembly kinetics, reveal the role of collective DNA interactions in the formation of the linear structures. Moreover, the reversibility of DNA binding facilitates the development of hierarchical assemblies with time. We also observed similar linear structures of alternating rods and spheres, which implies that the discovered mechanism is generic for nanoscale objects interacting <i>via</i> flexible multiple linkers

Advancing Reversible Shape Memory by Tuning the Polymer Network Architecture

Because of counteraction of a chemical network and a crystalline scaffold, semicrystalline polymer networks exhibit a peculiar behaviorreversible shape memory (RSM), which occurs naturally without applying any external force and particular structural design. There are three RSM properties: (i) range of reversible strain, (ii) rate of strain recovery, and (iii) decay of reversibility with time, which can be improved by tuning the architecture of the polymer network. Different types of poly­(octylene adipate) networks were synthesized, allowing for control of cross-link density and network topology, including randomly cross-linked network by free-radical polymerization, thiol–ene clicked network with enhanced mesh uniformity, and loose network with deliberately incorporated dangling chains. It is shown that the RSM properties are controlled by average cross-link density and crystal size, whereas topology of a network greatly affects its extensibility. We have achieved 80% maximum reversible range, 15% minimal decrease in reversibility, and fast strain recovery rate up to 0.05 K^–1, i.e., ca. 5% per 10 s at a cooling rate of 5 K/min