Continuous Ligand-Free Catalysis Using a Hybrid Polymer Network Support

Although the pharmaceutical and fine chemical industries primarily utilize batch homogeneous reactions to carry out chemical transformations, emerging platforms seek to improve existing shortcomings by designing effective heterogeneous catalysis systems in continuous flow reactors. In this work, we present a versatile network-supported palladium (Pd) catalyst using a hybrid polymer of poly(methylvinylether-alt-maleic anhydride) and branched polyethyleneimine for intensified continuous flow synthesis of complex organic compounds via heterogeneous Suzuki-Miyaura cross-coupling and nitroarene hydrogenation reactions. The hydrophilicity of the hybrid polymer network facilitates the reagent mass transfer throughout the bulk of the catalyst particles. Through rapid automated exploration of the continuous and discrete parameters, as well as substrate scope screening, we identified optimal hybrid network-supported Pd catalyst composition and process parameters for Suzuki-Miyaura cross-coupling reactions of aryl bromides with steady-state yields up to 92% with a nominal residence time of 20 min. The developed heterogeneous catalytic system exhibits high activity and mechanical stability with no detectable Pd leaching at reaction temperatures up to 95 °C. Additionally, the versatility of the hybrid network-supported Pd catalyst is demonstrated by successfully performing continuous nitroarene hydrogenation with short residence times (<5 min) at room temperature. Room temperature hydrogenation yields of >99% were achieved in under 2 min nominal residence times with no leaching and catalyst deactivation for more than 20 h continuous time on stream. This catalytic system shows its industrial utility with significantly improved reaction yields of challenging substrates and its utility of environmentally-friendly solvent mixtures, high reusability, scalable and cost-effective synthesis, and multi-reaction successes.

PDMS networks meet barnacles: a complex and often toxic relationship

The biological impact of chemical formulations used in various coating applications is essential in guiding the development of new materials that directly contact living organisms. To illustrate this point, an investigation addressing the impact of chemical compositions of polydimethylsiloxane networks on a common platform for foul-release biofouling management coatings was conducted. The acute toxicity of network components to barnacle larvae, the impacts of aqueous extracts of crosslinker, silicones and organometallic catalyst on trypsin enzymatic activity, and the impact of assembled networks on barnacle adhesion was evaluated. The outcomes of the study indicate that all components used in the formulation of the silicone network alter trypsin enzymatic activity and have a range of acute toxicity to barnacle larvae. Also, the adhesion strength of barnacles attached to PDMS networks correlates to the network formulation protocol. This information can be used to assess action mechanisms and risk-benefit analysis of PDMS networks.

DFT Analysis of Organotin Catalytic Mechanisms in Dehydration Esterification Reactions for Terephthalic Acid and 2,2,4,4-Tetramethyl-1,3-cyclobutanediol

Polyesters synthesized from 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) and terephthalic acid (TPA) are improved alternatives to toxic polycarbonates based on bisphenol A. In this work, we use ωB97X-D/LANL2DZdp calculations, in the presence of a benzaldehyde polarizable continuum model solvent, to show that esterification of TMCD and TPA will reduce and subsequently dehydrate a dimethyl tin oxide catalyst, becoming ligands on the now four-coordinate complex. This reaction then proceeds most plausibly by an intramolecular acyl-transfer mechanism from the tin complex, aided by a coordinated proton donor such as hydronium. These findings are a key first step in understanding polyester synthesis and avoiding undesirable side reactions during production.

Network-supported, metal-mediated catalysis: progress and perspective

This minireview focuses on recent developments of network-supported catalysts to improve the performance of a wide range of metal-mediated catalytic reactions.

Thermally driven directional free-radical polymerization in confined channels

We report on the formation of poly(acrylamide) (PAAm) with a relatively-narrow molecular weight distribution (MWD) by means of thermally-driven directional free-radical polymerization carried out in polymerization chambers featuring two parallel glass walls separated by various distances, ranging from sub-millimeter to a few millimeters.

Amidation of Polyesters Is Slow in Nonaqueous Solvents: Efficient Amidation of Poly(ethylene terephthalate) with 3-Aminopropyltriethoxysilane in Water for Generating Multifunctional Surfaces

This paper describes surface functionalization of poly(ethylene terephthalate) (PET) films by transamidation of the ester groups with primary amines. The use of water as a solvent improves tremendously the reaction rate and yield compared to conventionally used alcohols. In this study, PET films were exposed to an aqueous solution of 3-aminopropyltriethoxysilane (APTES), which resulted in ester-to-amide reactions on the surface of the film. Hydrolysis of the resulting ethoxy moieties in APTES creates hydroxyl groups that can be used as anchoring points for further modification of PET films. This scheme offers an alternative approach to modify polyesters using water as the solvent.

Targeted Mutagenesis and Combinatorial Library Screening Enables Control of Protein Orientation on Surfaces and Increased Activity of Adsorbed Proteins

While nonspecific adsorption is widely used for immobilizing proteins on solid surfaces, the random nature of protein adsorption may reduce the activity of immobilized proteins due to occlusion of the active site. We hypothesized that the orientation a protein assumes on a given surface can be controlled by systematically introducing mutations into a region distant from its active site, thereby retaining activity of the immobilized protein. To test this hypothesis, we generated a combinatorial protein library by randomizing six targeted residues in a binding protein derived from highly stable, nonimmunoglobulin Sso7d scaffold; mutations were targeted in a region that is distant from the binding site. This library was screened to isolate binders that retain binding to its cognate target (chicken immunoglobulin Y, cIgY) as well as exhibit adsorption on unmodified silica at pH 7.4 and high ionic strength conditions. A single mutant, Sso7d-2B5, was selected for further characterization. Sso7d-2B5 retained binding to cIgY with an apparent dissociation constant similar to that of the parent protein; both mutant and parent proteins saturated the surface of silica with similar densities. Strikingly, however, silica beads coated with Sso7d-2B5 could achieve up to 7-fold higher capture of cIgY than beads coated with the parent protein. These results strongly suggest that mutations introduced in Sso7d-2B5 alter its orientation relative to the parent protein, when adsorbed on silica surfaces. Our approach also provides a generalizable strategy for introducing mutations in proteins so as to improve their activity upon immobilization, and has direct relevance to development of protein-based biosensors and biocatalysts.

Multipurpose Polymeric Coating for Functionalizing Inert Polymer Surfaces

In this work, we report on the development of a highly functionalizable polymer coating prepared by the chemical coupling of trichlorosilane (TCS) to the vinyl groups of poly(vinylmethyl siloxane) (PVMS). The resultant PVMS-TCS copolymer can be coated as a functional organic primer layer on a variety of polymeric substrates, ranging from hydrophilic to hydrophobic. Several case studies demonstrating the remarkable and versatile properties of PVMS-TCS coatings are presented. In particular, PVMS-TCS is found to serve as a convenient precursor for the deposition of organosilanes and the subsequent growth of polymer brushes, even on hydrophobic surfaces, such as poly(ethylene terephthalate) and polypropylene. In this study, the physical and chemical characteristics of these versatile PVMS-TCS coatings are interrogated by an arsenal of experimental probes, including scanning electron microscopy, water contact-angle measurements, ellipsometry, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy.

Elastomeric microparticles for acoustic mediated bioseparations

BACKGROUND

Acoustophoresis has been utilized successfully in applications including cell trapping, focusing, and purification. One current limitation of acoustophoresis for cell sorting is the reliance on the inherent physical properties of cells (e.g., compressibility, density) instead of selecting cells based upon biologically relevant surface-presenting antigens. Introducing an acoustophoretic cell sorting approach that allows biochemical specificity may overcome this limitation, thus advancing the value of acoustophoresis approaches for both the basic research and clinical fields.

RESULTS

The results presented herein demonstrate the ability for negative acoustic contrast particles (NACPs) to specifically capture and transport positive acoustic contrast particles (PACPs) to the antinode of an ultrasound standing wave. Emulsification and post curing of pre-polymers, either polydimethylsiloxane (PDMS) or polyvinylmethylsiloxane (PVMS), within aqueous surfactant solution results in the formation of stable NACPs that focus onto pressure antinodes. We used either photochemical reactions with biotin-tetrafluorophenyl azide (biotin-TFPA) or end-functionalization of Pluronic F108 surfactant to biofunctionalize NACPs. These biotinylated NACPs bind specifically to streptavidin polystyrene microparticles (as cell surrogates) and transport them to the pressure antinode within an acoustofluidic chip.

CONCLUSION

To the best of our knowledge, this is the first demonstration of using NACPs as carriers for transport of PACPs in an ultrasound standing wave. By using different silicones (i.e., PDMS, PVMS) and curing chemistries, we demonstrate versatility of silicone materials for NACPs and advance the understanding of useful approaches for preparing NACPs. This bioseparation scheme holds potential for applications requiring rapid, continuous separations such as sorting and analysis of cells and biomolecules.

Oligomer orientation in vapor-molecular-layer-deposited alkyl-aromatic polyamide films

The surface-limited molecular-layer deposition of alkyl-aromatic polyamide films using sequential doses of 1,4-butane diamine (BDA) and terephthaloyl dichloride (TDC) is characterized using in situ quartz crystal microbalance and ex situ spectroscopy analysis. For the first time, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to offer insight into molecular orientation in films deposited via molecular-layer deposition (MLD). The results show that the oligomer units are lying nearly parallel to the surface, which differs from the linear vertical growth mode often used to illustrate film growth.

Three-dimensional electrospun alginate nanofiber mats via tailored charge repulsions

The formation of 3D electrospun mat structures from alginate-polyethylene oxide (PEO) solution blends is reported. These unique architectures expand the capabilities of traditional electrospun mats for applications such as regenerative medicine, where a scaffold can help to promote tissue growth in three dimensions. The mat structures extend off the surface of the flat collector plate without the need of any modifications in the electrospinning apparatus, are self-supported when the electric field is removed, and are composed of bundles of nanofibers. A mechanism for the unique formations is proposed, based on the fiber-fiber repulsions from surface charges on the negatively charged alginate. Furthermore, the role of the electric field in the distribution of alginate within the nanofibers is discussed. X-ray photoelectron spectroscopy is used to analyze the surface composition of the electrospun nanofiber mats and the data is related to cast films made in the absence of the electric field. Further techniques to tailor the 3D architecture and nanofiber morphology by changing the surface tension and relative humidity are also discussed.

Time dependence of lysozyme adsorption on end-grafted polymer layers of variable grafting density and length

A combined experimental and theoretical approach establishes the long-lived nature of protein adsorption on surfaces coated with chemically grafted macromolecules. Specifically, we monitor the time dependence of adsorption of lysozyme on surfaces comprising polymer assemblies made of poly(2-hydroxyethyl methacrylate) brushes grafted onto flat silica surfaces such that they produce patterns featuring orthogonal and gradual variation of the chain length (N) and grafting density (σ). We show that in the kinetically controlled regime, the amount of adsorbed protein scales universally with the product σN, while at equilibrium the amount of adsorbed protein is governed solely by σ. Surprisingly, for moderate concentrations of protein in solution, adsorption takes more than 72 h to reach an equilibrium, or steady state. Our experimental findings are corroborated with predictions using molecular theory that provides further insight into the protein adsorption phenomenon. The theory predicts that the universal behavior observed experimentally should be applicable to polymers in poor and theta solvents and to a limited extent also to good solvent conditions. Our combined experimental and theoretical findings reveal that protein adsorption is a long-lived phenomenon, much longer than generally assumed. Our studies confirm the previously predicted important differences in behavior for the kinetic versus thermodynamic control of protein adsorption.

Development and testing of hierarchically wrinkled coatings for marine antifouling

We report on the formation and testing of novel marine coatings comprising hierarchically wrinkled surface topographies (HWTS) having wrinkles of different length scales (generations) ranging from tens of nanometers to a fraction of a millimeter. The individual wrinkle generations are arranged in nested patterns, where each larger wrinkle resides underneath and represents a scaled-up version of the smaller wrinkle. We present and discuss results from field tests in seawater and laboratory experiments. The results of our field tests reveal that while coatings with flat topographies foul after relatively short time periods (4-15 weeks), the HWST coatings with the same chemistries as flat coatings remain relatively free of biofouling even after prolonged exposure to seawater (18 months). In contrast to flat coatings, the HWST substrates are not colonized by barnacles. These observations suggest that surface topography plays a dominant role in governing the coating defense against barnacle fouling even without fine-tuning the chemical composition of the overcoat. Laboratory experiments indicate that settlement of zoospores of the green alga Ulva and the strength of attachment of sporelings (young plants) depend on the chemical composition of the coating as well as surface topography.

Propagating waves of self-assembly in organosilane monolayers

Wavefronts associated with reaction-diffusion and self-assembly processes are ubiquitous in the natural world. For example, propagating fronts arise in crystallization and diverse other thermodynamic ordering processes, in polymerization fronts involved in cell movement and division, as well as in the competitive social interactions and population dynamics of animals at much larger scales. Although it is often claimed that self-sustaining or autocatalytic front propagation is well described by mean-field "reaction-diffusion" or "phase field" ordering models, it has recently become appreciated from simulations and theoretical arguments that fluctuation effects in lower spatial dimensions can lead to appreciable deviations from the classical mean-field theory (MFT) of this type of front propagation. The present work explores these fluctuation effects in a real physical system. In particular, we consider a high-resolution near-edge x-ray absorption fine structure spectroscopy (NEXAFS) study of the spontaneous frontal self-assembly of organosilane (OS) molecules into self-assembled monolayer (SAM) surface-energy gradients on oxidized silicon wafers. We find that these layers organize from the wafer edge as propagating wavefronts having well defined velocities. In accordance with two-dimensional simulations of this type of front propagation that take fluctuation effects into account, we find that the interfacial widths w(t) of these SAM self-assembly fronts exhibit a power-law broadening in time, w(t) approximately t(beta), rather than the constant width predicted by MFT. Moreover, the observed exponent values accord rather well with previous simulation and theoretical estimates. These observations have significant implications for diverse types of ordering fronts that occur under confinement conditions in biological or materials-processing contexts.

Study of the packing density and molecular orientation of bimolecular self-assembled monolayers of aromatic and aliphatic organosilanes on silica

Bimolecular self-assembled monolayers (SAMs) of aromatic and aliphatic chlorosilanes were self-assembled onto silica, and their characteristics were established by contact angle measurement, near-edge X-ray absorption fine structure spectroscopy, and Fourier transform infrared spectroscopy. Three aromatic constituents (phenyltrichlorosilane, benzyltrichlorosilane, and phenethyltrichlorosilane) were studied in combination with four aliphatic coadsorbates (butyltrichlorosilane, butyldimethylchlorosilane, octadecyltrichlorosilane, and octadecyldimethylchlorosilane). Our results demonstrate that whereas SAMs made of trichlorinated organosilanes are densely packed, SAMs prepared from monochlorinated species are less dense and poorly ordered. In mixed systems, trichlorinated aromatics and trichlorinated aliphatics formed SAMs with highly tunable compositions; their surfaces were compositionally homogeneous with no large-scale domain separation. The homogeneous nature of the resulting SAM was a consequence of the formation of in-plane siloxane linkages among neighboring molecules. In contrast, when mixing monochlorinated aliphatics with trichlorinated aromatics, molecular segregation occurred. Although the two shortest aromatic species did not display significant changes in orientation upon mixing with aliphatics, the aromatic species with the longest polymethylene spacer, phenethyltrichlorosilane, displayed markedly different orientation behavior in mixtures of short- and long-chain aliphatics.

Formation mechanisms and properties of semifluorinated molecular gradients on silica surfaces

The goal of this study is to elucidate the formation of molecular gradients made of semifluorinated organosilanes (SFOs) on flat substrates by using a methodology developed by Chaudhury and Whitesides (Science 1992, 256, 1539). We use surface-sensitive combinatorial near-edge X-ray absorption fine structure (combi-NEXAFS) spectroscopy to measure the position-dependent concentration and orientation of SFO molecules in SFO molecular gradients on flat silica surfaces. Using the combi-NEXAFS data, we establish the correlation between the fraction of the F(CF(2))(8)(CH(2))(2)- species on the substrate and the average tilt angle of the -(CF(2))(8)F group in the SFO as a function of the deposition gas medium (air vs nitrogen) and the end group around the silicon atom (monofunctional vs trifunctional). In addition, we utilize the gradient geometry to comprehend the mechanism of formation of SFO self-assembled monolayers (SAMs). Specifically, we provide evidence that depending on the nature of the end group in the SFO and the vapor phase the SFO molecules add themselves into the existing SAMs either as individual molecules or as multimolecular complexes.

Recent developments in superhydrophobic surfaces and their relevance to marine fouling: a review

In this review, a brief synopsis of superhydrophobicity (i.e. extreme non-wettability) and its implications on marine fouling are presented. A short overview of wettability and recent experimental developments aimed at fabricating superhydrophobic surfaces by tailoring their chemical nature and physical appearance (i.e. substratum texture) are reviewed. The formation of responsive/"smart" surfaces, which adjust their physico-chemical properties to variations in some outside physical stimulus, including light, temperature, electric field, or solvent, is also described. Finally, implications of tailoring the surface chemistry, texture, and responsiveness of surfaces on the design of effective marine fouling coatings are considered and discussed.

Nested self-similar wrinkling patterns in skins

Stiff thin films on soft substrates are both ancient and commonplace in nature; for instance, animal skin comprises a stiff epidermis attached to a soft dermis. Although more recent and rare, artificial skins are increasingly used in a broad range of applications, including flexible electronics, tunable diffraction gratings, force spectroscopy in cells, modern metrology methods, and other devices. Here we show that model elastomeric artificial skins wrinkle in a hierarchical pattern consisting of self-similar buckles extending over five orders of magnitude in length scale, ranging from a few nanometres to a few millimetres. We provide a mechanism for the formation of this hierarchical wrinkling pattern, and quantify our experimental findings with both computations and a simple scaling theory. This allows us to harness the substrates for applications. In particular, we show how to use the multigeneration-wrinkled substrate for separating particles based on their size, while simultaneously forming linear chains of monodisperse particles.

Surface modification of Sylgard-184 poly(dimethyl siloxane) networks by ultraviolet and ultraviolet/ozone treatment

We report on the surface modification of Sylgard-184 poly(dimethyl siloxane) (PDMS) networks by ultraviolet (UV) radiation and ultraviolet/ozone (UVO) treatment. The effects of the UV light wavelength and ambient conditions on the surface properties of Sylgard-184 are probed using a battery of experimental probes, including static contact angle measurements, Fourier transform infrared spectroscopy, near-edge X-ray absorption fine structure, and X-ray reflectivity. Our results reveal that when exposed to UV, the PDMS macromolecules in the surface region of Sylgard-184 undergo chain scission, involving both the main chain backbone and the side groups. The radicals formed during this process recombine and form a network whose wetting properties are similar to those of a UV-modified model PDMS. In contrast to the UV radiation, the UVO treatment causes very significant changes in the surface and near-surface structure of Sylgard-184. Specifically, the molecular oxygen and ozone created during the UVO process interact with the UV-modified specimen. As a result of these interactions, the surface of the sample contains a large number of hydrophilic (mainly -OH) groups. In addition, the material density within the first approximately 5 nm reaches about 50% of that of pure silica. A major conclusion that can be drawn from the results and analysis described in this work is that the presence of the silica fillers in Sylgard-184 does not alter the surface properties of the UVO- and UV-modified Sylgard-184.

Combinatorial study of the mushroom-to-brush crossover in surface anchored polyacrylamide

We present a method for fabricating anchored polymers with a gradual variation of grafting densities on solid substrates. The technique for generating such structures comprises (i) formation of a molecular gradient of polymerization initiator on the solid substrate and (ii) polymerization from the substrate-bound initiator centers ("grafting from"). We measure the mushroom-to-brush transition in grafted polyacrylamides and show that the mushroom and brush behavior can be described using existing scaling theories.

Creating long-lived superhydrophobic polymer surfaces through mechanically assembled monolayers

We show that elastomeric surfaces can be tailored using "mechanically assembled monolayers" (MAMs), structures that are fabricated by combining self-assembly of surface grafting molecules with mechanical manipulation of the grafting points in the underlying elastic surface. The versatility of this surface modification method is demonstrated by fabricating MAMs with semifluorinated (SF) molecules. These SF-MAMs have superior nonwetting and barrier properties in that they are "superhydrophobic" and nonpermeable. We also establish that these material characteristics do not deteriorate even after prolonged exposure to water, which usually causes surface reconstruction in conventionally prepared SF self-assembled monolayers.