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.

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.

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.

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.

Importance of the DNA "bond" in programmable nanoparticle crystallization.

If a solution of DNA-coated nanoparticles is allowed to crystallize, the thermodynamic structure can be predicted by a set of structural design rules analogous to Pauling's rules for ionic crystallization. The details of the crystallization process, however, have proved more difficult to characterize as they depend on a complex interplay of many factors. Here, we report that this crystallization process is dictated by the individual DNA bonds and that the effect of changing structural or environmental conditions can be understood by considering the effect of these parameters on free oligonucleotides. Specifically, we observed the reorganization of nanoparticle superlattices using time-resolved synchrotron small-angle X-ray scattering in systems with different DNA sequences, salt concentrations, and densities of DNA linkers on the surface of the nanoparticles. The agreement between bulk crystallization and the behavior of free oligonucleotides may bear important consequences for constructing novel classes of crystals and incorporating new interparticle bonds in a rational manner.

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.

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.

Molecular-Level Engineering of Adhesion in Carbon Nanomaterial Interfaces.

Weak interfilament van der Waals interactions are potentially a significant roadblock in the development of carbon nanotube- (CNT-) and graphene-based nanocomposites. Chemical functionalization is envisioned as a means of introducing stronger intermolecular interactions at nanoscale interfaces, which in turn could enhance composite strength. This paper reports measurements of the adhesive energy of CNT-graphite interfaces functionalized with various coverages of arylpropionic acid. Peeling experiments conducted in situ in a scanning electron microscope show significantly larger adhesive energies compared to previously obtained measurements for unfunctionalized surfaces (Roenbeck et al. ACS Nano 2014, 8 (1), 124-138). Surprisingly, however, the adhesive energies are significantly higher when both surfaces have intermediate coverages than when one surface is densely functionalized. Atomistic simulations reveal a novel functional group interdiffusion mechanism, which arises for intermediate coverages in the presence of water. This interdiffusion is not observed when one surface is densely functionalized, resulting in energy trends that correlate with those observed in experiments. This unique intermolecular interaction mechanism, revealed through the integrated experimental-computational approach presented here, provides significant insights for use in the development of next-generation nanocomposites.

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.

Directed Assembly of Nucleic Acid-Based Polymeric Nanoparticles from Molecular Tetravalent Cores.

Complementary tetrahedral small molecule-DNA hybrid (SMDH) building blocks have been combined to form nucleic acid-based polymeric nanoparticles without the need for an underlying template or scaffold. The sizes of these particles can be tailored in a facile fashion by adjusting assembly conditions such as SMDH concentration, assembly time, and NaCl concentration. Notably, these novel particles can be stabilized and transformed into functionalized spherical nucleic acid (SNA) structures through the incorporation of capping DNA strands conjugated with functional groups. These results demonstrate a systematic, efficient strategy for the construction and surface functionalization of well-defined, size-tunable nucleic acid particles from readily accessible molecular building blocks. Furthermore, because these nucleic acid-based polymeric nanoparticles exhibited enhanced cellular internalization and resistance to DNase I compared to free synthetic nucleic acids, they should have a plethora of applications in diagnostics and therapeutics.

Liposomal spherical nucleic acids.

A novel class of metal-free spherical nucleic acid nanostructures was synthesized from readily available starting components. These particles consist of 30 nm liposomal cores, composed of an FDA-approved 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid monomer. The surface of the liposomes was functionalized with DNA strands modified with a tocopherol tail that intercalates into the phospholipid layer of the liposomal core via hydrophobic interactions. The spherical nucleic acid architecture not only stabilizes these constructs but also facilitates cellular internalization and gene regulation in SKOV-3 cells.

Simple and compelling biomimetic metal-organic framework catalyst for the degradation of nerve agent simulants.

Inspired by biology, in which a bimetallic hydroxide-bridged zinc(II)-containing enzyme is utilized to catalytically hydrolyze phosphate ester bonds, the utility of a zirconium(IV)-cluster-containing metal-organic framework as a catalyst for the methanolysis and hydrolysis of phosphate-based nerve agent simulants was examined. The combination of the strong Lewis-acidic Zr(IV) and bridging hydroxide anions led to ultrafast half-lives for these solvolysis reactions. This is especially remarkable considering that the actual catalyst loading was a mere 0.045 % as a result of the surface-only catalysis observed.

Discovery of highly selective alkyne semihydrogenation catalysts based on first-row transition-metallated porous organic polymers.

Five different first-row transition metal precursors (V(III), Cr(III), Mn(II), Co(II), Ni(II)) were successfully incorporated into a catechol porous organic polymer (POP) and characterized using ATR-IR and XAS analysis. The resulting metallated POPs were then evaluated for catalytic alkyne hydrogenation using high-throughput screening techniques. All POPs were unexpectedly found to be active and selective catalysts for alkyne semihydrogenation. Three of the metallated POPs (V, Cr, Mn) are the first of their kind to be active single-site hydrogenation catalysts. These results highlight the advantages of using a POP platform to develop new catalysts which are otherwise difficult to achieve through traditional heterogeneous and homogeneous routes.

Acid-degradable polymer-caged lipoplex (PCL) platform for siRNA delivery: facile cellular triggered release of siRNA.

An acid-degradable polymer-caged lipoplex (PCL) platform consisting of a cationic lipoplex core and a biocompatible, pH-responsive polymer shell has been developed for the effective delivery of small interfering RNA (siRNA) through a combination of facile loading, rapid acid-triggered release, cellular internalization, and effective endosomal escape. In vitro testing of this degradable PCL delivery platform reveals ∼45- and ∼2.5-fold enhancement of enhanced green fluorescent protein knockdown in cancer cells in comparison to either free siRNA or siRNA-loaded non-acid-degradable lipoplex formulations, respectively.

Enhanced catalytic activity through the tuning of micropore environment and supercritical CO2 processing: Al(porphyrin)-based porous organic polymers for the degradation of a nerve agent simulant.

An Al(porphyrin) functionalized with a large axial ligand was incorporated into a porous organic polymer (POP) using a cobalt-catalyzed acetylene trimerization strategy. Removal of the axial ligand afforded a microporous POP that is catalytically active in the methanolysis of a nerve agent simulant. Supercritical CO2 processing of the POP dramatically increased the pore size and volume, allowing for significantly higher catalytic activities.

Vapor-phase metalation by atomic layer deposition in a metal-organic framework.

Metal-organic frameworks (MOFs) have received attention for a myriad of potential applications including catalysis, gas storage, and gas separation. Coordinatively unsaturated metal ions often enable key functional behavior of these materials. Most commonly, MOFs have been metalated from the condensed phase (i.e., from solution). Here we introduce a new synthetic strategy capable of metallating MOFs from the gas phase: atomic layer deposition (ALD). Key to enabling metalation by ALD In MOFs (AIM) was the synthesis of NU-1000, a new, thermally stable, Zr-based MOF with spatially oriented -OH groups and large 1D mesopores and apertures.

Preparation and characterization of graphene oxide paper.

Free-standing paper-like or foil-like materials are an integral part of our technological society. Their uses include protective layers, chemical filters, components of electrical batteries or supercapacitors, adhesive layers, electronic or optoelectronic components, and molecular storage. Inorganic 'paper-like' materials based on nanoscale components such as exfoliated vermiculite or mica platelets have been intensively studied and commercialized as protective coatings, high-temperature binders, dielectric barriers and gas-impermeable membranes. Carbon-based flexible graphite foils composed of stacked platelets of expanded graphite have long been used in packing and gasketing applications because of their chemical resistivity against most media, superior sealability over a wide temperature range, and impermeability to fluids. The discovery of carbon nanotubes brought about bucky paper, which displays excellent mechanical and electrical properties that make it potentially suitable for fuel cell and structural composite applications. Here we report the preparation and characterization of graphene oxide paper, a free-standing carbon-based membrane material made by flow-directed assembly of individual graphene oxide sheets. This new material outperforms many other paper-like materials in stiffness and strength. Its combination of macroscopic flexibility and stiffness is a result of a unique interlocking-tile arrangement of the nanoscale graphene oxide sheets.

Experimental-computational study of shear interactions within double-walled carbon nanotube bundles.

The mechanical behavior of carbon nanotube (CNT)-based fibers and nanocomposites depends intimately on the shear interactions between adjacent tubes. We have applied an experimental-computational approach to investigate the shear interactions between adjacent CNTs within individual double-walled nanotube (DWNT) bundles. The force required to pull out an inner bundle of DWNTs from an outer shell of DWNTs was measured using in situ scanning electron microscopy methods. The normalized force per CNT-CNT interaction (1.7 ± 1.0 nN) was found to be considerably higher than molecular mechanics (MM)-based predictions for bare CNTs (0.3 nN). This MM result is similar to the force that results from exposure of newly formed CNT surfaces, indicating that the observed pullout force arises from factors beyond what arise from potential energy effects associated with bare CNTs. Through further theoretical considerations we show that the experimentally measured pullout force may include small contributions from carbonyl functional groups terminating the free ends of the CNTs, corrugation of the CNT-CNT interactions, and polygonization of the nanotubes due to their mutual interactions. In addition, surface functional groups, such as hydroxyl groups, that may exist between the nanotubes are found to play an unimportant role. All of these potential energy effects account for less than half of the ~1.7 nN force. However, partially pulled-out inner bundles are found not to pull back into the outer shell after the outer shell is broken, suggesting that dissipation is responsible for more than half of the pullout force. The sum of force contributions from potential energy and dissipation effects are found to agree with the experimental pullout force within the experimental error.