Transport Diffusion of Linear Alkanes (C5-C16) through Thin Films of ZIF-8 as Assessed by Quartz Crystal Microgravimetry.

We report uptake capacities and transport diffusivities, D, for each of eight linear alkanes (ranging from C5 to C16) in quartz crystal-supported films of solvent-evacuated ZIF-8. Analyses of the alkane uptake profiles revealed that the transport dynamics are governed by guest diffusion through metal-organic framework (MOF) (ZIF-8) crystallites rather than by rates of entry into films at the MOF/vapor interface. The obtained diffusivities range from just over 10-18 m2/s to just under 10-14 m2/s. Notably, minimum cross-sectional widths for all guests exceed the crystallographically measured width of ZIF-8's largest apertures and imply consistently with previous experimental and computational studies that apertures expand to accommodate guest uptake. On average, each additional carbon decreases the transport diffusivity of an alkane by twofold. Closer examination, however, reveals an odd-even effect such that linear alkanes having even numbers of carbons diffuse more rapidly than alkanes featuring one more or one less carbon atom. Thus, ZIF-8's differentiation of transport diffusivities for pairs of alkanes differing in length by only one carbon atom can be significantly greater than the aforementioned factor of 2. Elucidation of the microscopic basis for the odd-even behavior, however, awaits the outcome of molecular dynamics calculations that are beyond the scope of the present study. For compact, solvothermally prepared films, guest transport is dominated by 1D diffusion from the film/vapor interface and toward the underlying quartz crystal. For much lower density, electrophoretically deposited (EPD) films, crystallites behave nearly independently, and guest transport can be adequately modeled by assuming rapid permeation of macroscopic voids between crystallites, followed by entry and rate-limiting radial diffusion into isolated crystallites. One consequence is that EPD films can be much more rapidly infiltrated by molecular guests than can compact, solvothermally grown films. The combined results have potentially favorable implications for the development of kinetic separation schemes for closely related analytes.

Assembly of Short-Chain Amphiphilic Homopolymers into Well-Defined Particles.

Linear homopolymers of norbornene (NBE) derivatives equipped with short-chain alcohol pendant groups were prepared by ring-opening metathesis polymerization (ROMP) and subsequently assembled into well-defined structures in alcohol solvents. The ratios of hydrophobic carbons and hydrophilic alcohol groups at the repeating monomeric unit in these short-chain amphiphilic polymers were found to play an important role in determining the size and distribution of the final globular structures. Unlike the assembly of other linear homo- and copolymers possessing long-chain amphiphilicity, NBE-based linear polymers were readily transformed into spherical particles with a layered conformation, whose sizes range from a few hundred nanometers to micrometers with narrow distributions, simply by controlling the concentration and molecular weights of the linear homopolymers without using any surfactants. In addition, the degree of the intermolecular forces with solvents (e.g., solvation) possessing different surface tensions and polarities highly affected the final diameter and distribution of the polymer particles, implying the importance of the selection of a proper solvent to regulate their structural features. As such, understanding the assembly of these types of short-chain homopolymers into uniform particles can allow for regulating the transformation of diverse linear amphiphilic polymers into precisely controlled structures for various applications.

Stiffening of graphene oxide films by soft porous sheets.

Graphene oxide (GO) sheets have been used as a model system to study how the mechanical properties of two-dimensional building blocks scale to their bulk form, such as paper-like, lamellar-structured thin films. Here, we report that the modulus of multilayer GO films can be significantly enhanced if some of the sheets are drastically weakened by introducing in-plane porosity. Nanometer-sized pores are introduced in GO sheets by chemical etching. Membrane-deflection measurements at the single-layer level show that the sheets are drastically weakened as the in-plane porosity increases. However, the mechanical properties of the corresponding multilayer films are much less sensitive to porosity. Surprisingly, the co-assembly of pristine and etched GO sheets yields even stiffer films than those made from pristine sheets alone. This is attributed to the more compliant nature of the soft porous sheets, which act as a binder to improve interlayer packing and load transfer in the multilayer films.

Nanoscale toughening of ultrathin graphene oxide-polymer composites: mechanochemical insights into hydrogen-bonding/van der Waals interactions, polymer chain alignment, and steric parameters.

This paper describes a systematic study on the nanoscale toughening of monolayer graphene oxide (GO) by an ultra-thin polymer adlayer, which impedes the propagation of cracks during intraplanar fracture. Using molecular dynamics simulations, the crack-bridging capabilities of a library of five hydrogen-bonding-capable polymers are explored against an epoxide-rich GO substrate. The best crack-bridging effect is found in polymers with functional groups that can both donate/accept hydrogen atoms and have better capability to form cooperative hydrogen bonds. Aligning the chains of poly(acrylic acid) orthogonally to the crack propagation direction significantly enhances the fracture toughness of monolayer GO (by 310%) in comparison to that for an adlayer with randomly arranged chains (180% enhancement). Notably, van der Waals interactions, which are seldom highlighted in the fabrication of strong GO-polymer interfaces, are found to also provide significant crack-bridging capabilities when the polymers possess large side groups. These results pave the way for a set of design criteria that can help in remediating the intrinsically brittle mechanical behavior of two-dimensional materials, a barrier that currently restricts their potential applications.

Supramolecular Assembly of High-Density Lipoprotein Mimetic Nanoparticles Using Lipid-Conjugated Core Scaffolds.

Synthetic high-density lipoprotein (HDL) mimics have emerged as promising therapeutic agents. However, approaches to date have been unable to reproduce key features of spherical HDLs, which are the most abundant human HDL species. Here, we report the synthesis and characterization of spherical HDL mimics using lipid-conjugated organic core scaffolds. The core design motif constrains and orients phospholipid geometry to facilitate the assembly of soft-core nanoparticles that are approximately 10 nm in diameter and resemble human HDLs in their size, shape, surface chemistry, composition, and protein secondary structure. These particles execute salient HDL functions, including efflux of cholesterol from macrophages, cholesterol delivery to hepatocytes, support lecithin:cholesterol acyltransferase activity, and suppress inflammation. These results represent a significant step toward a genuine functional mimic of human HDLs.

The Role of Water in Mediating Interfacial Adhesion and Shear Strength in Graphene Oxide.

Graphene oxide (GO), whose highly tunable surface chemistry enables the formation of strong interfacial hydrogen-bond networks, has garnered increasing interest in the design of devices that operate in the presence of water. For instance, previous studies have suggested that controlling GO's surface chemistry leads to enhancements in interfacial shear strength, allowing engineers to manage deformation pathways and control failure mechanisms. However, these previous reports have not explored the role of ambient humidity and only offer extensive chemical modifications to GO's surface as the main pathway to control GO's interfacial properties. Herein, through atomic force microscopy experiments on GO-GO interfaces, the adhesion energy and interfacial shear strength of GO were measured as a function of ambient humidity. Experimental evidence shows that adhesion energy and interfacial shear strength can be improved by a factor of 2-3 when GO is exposed to moderate (∼30% water weight) water content. Furthermore, complementary molecular dynamics simulations uncovered the mechanisms by which these nanomaterial interfaces achieve their properties. They reveal that the strengthening mechanism arises from the formation of strongly interacting hydrogen-bond networks, driven by the chemistry of the GO basal plane and intercalated water molecules between two GO surfaces. In summary, the methodology and findings here reported provide pathways to simultaneously optimize GO's interfacial and in-plane mechanical properties, by tailoring the chemistry of GO and accounting for water content, in engineering applications such as sensors, filtration membranes, wearable electronics, and structural materials.

Highly Stable, Ultrasmall Polymer-Grafted Nanobins (usPGNs) with Stimuli-Responsive Capability.

Highly stable and stimuli/pH-responsive ultrasmall polymer-grafted nanobins (usPGNs) have been developed by grafting a small amount (10 mol %) of short (4.3 kDa) cholesterol-terminated poly(acrylic acid) (Chol-PAA) into an ultrasmall unilamellar vesicle (uSUV). The usPGNs are stable against fusion and aggregation over several weeks, exhibiting over 10-fold enhanced cargo retention in biologically relevant media at pH 7.4 in comparison with the parent uSUV template. Coarse-grained molecular dynamics (CGMD) simulations confirm that the presence of the cholesterol moiety can greatly stabilize the lipid bilayer. They also show extended PAA chain conformations that can be interpreted as causing repulsion between colloidal particles, thus stabilizing them against fusion. Notably, CGMD predicted a clustering of the Chol-PAA chains on the lipid bilayer under acidic conditions due to intra- and interchain hydrogen bonding, leading to the destabilization of local membrane areas. This explains the experimental observation that usPGNs can be triggered to release a significant amount of cargo upon acidification to pH 5. These developments put the lipid-bilayer-embedded Chol-PAA in stark contrast with traditional poly(acrylic acid) systems where the molar mass (Mn) of the polymer chains must exceed 16.5 kDa to achieve stimuli-responsive changes in conformation. They also distinguish the small usPGNs from the much-larger polymer-caged nanobin platform where the Chol-PAA chains must be covalently cross-linked to engender stimuli-responsive behaviors.

Enhancing the Stability and Immunomodulatory Activity of Liposomal Spherical Nucleic Acids through Lipid-Tail DNA Modifications.

Liposomal spherical nucleic acids (LSNAs) are an attractive therapeutic platform for gene regulation and immunomodulation due to their biocompatibility, chemically tunable structures, and ability to enter cells rapidly without the need for ancillary transfection agents. Such structures consist of small (<100 nm) liposomal cores functionalized with a dense, highly oriented nucleic acid shell, both of which are key components in facilitating their biological activity. Here, the properties of LSNAs synthesized using conventional methods, anchoring cholesterol terminated oligonucleotides into a liposomal core, are compared to LSNAs made by directly modifying the surface of a liposomal core containing azide-functionalized lipids with dibenzocyclooctyl-terminated oligonucleotides. The surface densities of the oligonucleotides are measured for both types of LSNAs, with the lipid-modified structures having approximately twice the oligonucleotide surface coverage. The stabilities and cellular uptake properties of these structures are also evaluated. The higher density, lipid-functionalized structures are markedly more stable than conventional cholesterol-based structures in the presence of other unmodified liposomes and serum proteins as evidenced by fluorescence assays. Significantly, this new form of LSNA exhibits more rapid cellular uptake and increased sequence-specific toll-like receptor activation in immune reporter cell lines, making it a promising candidate for immunotherapy.

Triblock peptide-oligonucleotide chimeras (POCs): programmable biomolecules for the assembly of morphologically tunable and responsive hybrid materials.

Triblock peptide-oligonucleotide chimeras (POCs) consisting of peptides and oligonucleotides interlinked by an organic core are presented and their assembly behaviour is investigated. Several factors influence POC assembly, resulting in the formation of either vesicles or fibres. Design rules are introduced and used to predict and alter POC assembly morphology.

Thermal Conductivity of ZIF-8 Thin-Film under Ambient Gas Pressure.

Thermal conductivity is a crucial parameter for managing exothermal gas adsorption in metal organic frameworks (MOFs), but experimental studies have been limited. In this work, the thermal conductivity of a zeolitic imidazolate framework ZIF-8 was experimentally determined on thin films using the 3ω technique at different partial pressures in perfluorohexane, nitrogen, air, and vacuum ambients at 300 K. In contrast to theoretical prediction, the thermal conductivity κ = 0.326 ± 0.002 W/m K was approximately independent of ambient gas species and pressure from atmospheric pressure to vacuum. This work introduces a useful approach for probing MOF thermal conductivity under gas adsorption.

The competing effects of core rigidity and linker flexibility in the nanoassembly of trivalent small molecule-DNA hybrids (SMDH3s)-a synergistic experimental-modeling study.

The nanoassembly behavior of trivalent small molecule-DNA hybrids (SMDH3s) was investigated as a function of core geometry and supramolecular flexibility through a synergistic experimental-modeling study. While complementary SMDH3s possessing a highly flexible tetrahedral trivalent core primarily assemble into nanoscale caged dimers, the nanoassemblies of SMDH3 comonomers with rigid pyramidal and trigonal cores yield fewer caged dimers and more large-oligomer networks. Specifically, the rigid pyramidal SMDH3 comonomers tend to form smaller nanosized aggregates (dimers, tetramers, and hexamers) upon assembly, attributable to the small (<109°) branch-core-branch angle of the pyramidal core. In contrast, the more-rigid trigonal planar SMDH3 comonomers have a larger (∼120°) branch-core-branch angle, which spaces their DNA arms farther apart, facilitating the formation of larger nanoassemblies (≥nonamers). The population distributions of these nanoassemblies were successfully captured by coarse-grained molecular dynamics (CGMD) simulations over a broad range of DNA concentrations. CGMD simulations can also forecast the effect of incorporating Tn spacer units between the hydridizing DNA arms and the rigid organic cores to increase the overall flexibility of the SMDH3 comonomers. Such "decoupling" of the DNA arms from the organic core was found to result in preferential formation of nanoscale dimers up to an optimal spacer length, beyond which network formation takes over due to entropic factors. This excellent agreement between the simulation and experimental results confirms the versatility of the CGMD model as a useful and reliable tool for elucidating the nanoassembly of SMDH-based building blocks.

Rendering High Surface Area, Mesoporous Metal-Organic Frameworks Electronically Conductive.

We report the design and synthesis of a metal-organic framework (MOF)-polythiophene composite that has comparable electronic conductivity to reported conductive 3-D MOFs, but with display and retention of high porosity, including mesoporosity. A robust zirconium MOF, NU-1000, was rendered electronically conductive by first incorporating, via solvent-assisted ligand incorporation (SALI), a carefully designed pentathiophene derivative at a density of one pentamer per hexa-zirconium node. Using a cast film of the intermediate composite (termed pentaSALI) on conductive glass, the incorporated oligothiophene was electrochemically polymerized to yield the conductive composite, Epoly. Depending on the doping level of the polythiophene in the composite, it can be tuned from an insulating state to a semiconduting state with conductivity of 1.3 × 10-7 (S cm-1), which is comparable to values reported for 3-D conductive MOFs. The porosity of the thin-film MOF-polythiophene composite was assessed using decane vapor uptake as determined by quartz crystal microgravimetry (QCM). The results indicate a porosity (pore volume) for Epoly essentially identical to that of bulk pentaSALI, and ∼74% of that of unmodified NU-1000. PentaSALI, and by inference Epoly, displays both micro- and mesoporosity, and features a BET surface area of nearly 1,600 m2·g-1, i.e., substantially larger than yet reported for any other electronically conductive MOF.

Cross-Linked Micellar Spherical Nucleic Acids from Thermoresponsive Templates.

A one-pot synthesis of micellar spherical nucleic acid (SNA) nanostructures using Pluronic F127 as a thermoresponsive template is reported. These novel constructs are synthesized in a chemically straightforward process that involves intercalation of the lipid tails of DNA amphiphiles (CpG motifs for TLR-9 stimulation) into the hydrophobic regions of Pluronic F127 micelles, followed by chemical cross-linking and subsequent removal of non-cross-linked structures. The dense nucleic acid shell of the resulting cross-linked micellar SNA enhances their stability in physiological media and facilitates their rapid cellular internalization, making them effective TLR-9 immunomodulatory agents. These constructs underscore the potential of SNAs in regulating immune response and address the relative lack of stability of noncovalent constructs.

Drug-Loaded Polymeric Spherical Nucleic Acids: Enhancing Colloidal Stability and Cellular Uptake of Polymeric Nanoparticles through DNA Surface-Functionalization.

Small-sized (∼65 nm) doxorubicin (Dox)-loaded polymeric nanoparticles (PNPs) were modified with oligonucleotides to form colloidally stable Dox-loaded polymeric spherical nucleic acid (Dox-PSNA) nanostructures in biological media. The nucleic acid shell facilitates the cellular uptake of Dox-PSNA, which results in in vitro cytotoxicity against SKOV3 cancer cells.

The Significance of Multivalent Bonding Motifs and "Bond Order" in DNA-Directed Nanoparticle Crystallization.

Multivalent oligonucleotide-based bonding elements have been synthesized and studied for the assembly and crystallization of gold nanoparticles. Through the use of organic branching points, divalent and trivalent DNA linkers were readily incorporated into the oligonucleotide shells that define DNA-nanoparticles and compared to monovalent linker systems. These multivalent bonding motifs enable the change of "bond strength" between particles and therefore modulate the effective "bond order." In addition, the improved accessibility of strands between neighboring particles, either due to multivalency or modifications to increase strand flexibility, gives rise to superlattices with less strain in the crystallites compared to traditional designs. Furthermore, the increased availability and number of binding modes also provide a new variable that allows previously unobserved crystal structures to be synthesized, as evidenced by the formation of a thorium phosphide superlattice.

The dual capture of AsV and AsIII by UiO-66 and analogues.

UiO-66 and analogues were successfully tailored to chemoselectively capture AsV oxyanions at the hydroxylated node and neutral AsIII species with the thiolated organic linkers. More efficient and faster uptake can be achieved with increasing defect densities, increasing pore aperture sizes, and decreasing particle sizes.

Enhancing DNA-Mediated Assemblies of Supramolecular Cage Dimers through Tuning Core Flexibility and DNA Length--A Combined Experimental-Modeling Study.

Two complementary small-molecule-DNA hybrid (SMDH) building blocks have been combined to form well-defined supramolecular cage dimers at DNA concentrations as high as 102 µM. This was made possible by combining a flexible small-molecule core and three DNA arms of moderate lengths (<20 base pairs). These results were successfully modeled by coarse-grained molecular dynamics simulations, which also revealed that the formation of ill-defined networks in the case of longer DNA arms can be significantly biased by the presence of deep kinetic traps. Notably, melting point studies revealed that cooperative melting behavior can be used as a means to distinguish the relative propensities for dimer versus network formation from complementary flexible three-DNA-arm SMDH (fSMDH3) components: sharp, enhanced melting transitions were observed for assemblies that result mostly in cage dimers, while no cooperative melting behavior was observed for assemblies that form ill-defined networks.

Plasticity and ductility in graphene oxide through a mechanochemically induced damage tolerance mechanism.

The ability to bias chemical reaction pathways is a fundamental goal for chemists and material scientists to produce innovative materials. Recently, two-dimensional materials have emerged as potential platforms for exploring novel mechanically activated chemical reactions. Here we report a mechanochemical phenomenon in graphene oxide membranes, covalent epoxide-to-ether functional group transformations that deviate from epoxide ring-opening reactions, discovered through nanomechanical experiments and density functional-based tight binding calculations. These mechanochemical transformations in a two-dimensional system are directionally dependent, and confer pronounced plasticity and damage tolerance to graphene oxide monolayers. Additional experiments on chemically modified graphene oxide membranes, with ring-opened epoxide groups, verify this unique deformation mechanism. These studies establish graphene oxide as a two-dimensional building block with highly tuneable mechanical properties for the design of high-performance nanocomposites, and stimulate the discovery of new bond-selective chemical transformations in two-dimensional materials.

Entropy-Driven Crystallization Behavior in DNA-Mediated Nanoparticle Assembly.

Herein, we report an example of entropy-driven crystallization behavior in DNA-nanoparticle superlattice assembly, marking a divergence from the well-established enthalpic driving force of maximizing nearest-neighbor hybridization connections. Such behavior is manifested in the observation of a non-close-packed, body-centered cubic (bcc) superlattice when using a system with self-complementary DNA linkers that would be predicted to form a close-packed, face-centered cubic (fcc) structure based solely on enthalpic considerations and previous design rules for DNA-linked particle assembly. Notably, this unexpected phase behavior is only observed when employing long DNA linkers with unpaired "flexor" bases positioned along the length of the DNA linker that increase the number of microstates available to the DNA ligands. A range of design conditions are tested showing sudden onsets of this behavior, and these experiments are coupled with coarse-grained molecular dynamics simulations to show that this entropy-driven crystallization behavior is due to the accessibility of additional microstates afforded by using long and flexible linkers.