Solvent-Free Direct PEGylation of Collagen Fibers.

The addition of poly(ethylene glycol) (PEG) to biomolecules and biomaterials is a well-established approach to modify their properties for therapeutic applications. For biomaterials, the approach is typically to blend or electrospray the synthetic polymer with the biomaterial. Effective surface modification approaches such as surface-initiated polymer brushes are challenging since the harsh solvents required for brush synthesis may destroy the biomaterial. Herein, we describe the PEGylation of collagen fibers by surface-initiated PEG brushes using a living anionic grafting-from mechanism. This brush synthesis is done in the absence of solvents to minimize the degradation of the native collagen structure. We quantify the effect the brush synthesis has on the native structure of the collagen fiber using differential scanning calorimetry (DSC) and find that even at long reaction times a significant fraction of the native structure remains. Dynamic mechanical analysis indicates the collagen undergoes only modest structural degradation, while adhesion studies find a significant improvement of antifouling properties. Further, our approach opens the way for further chemistry, as the growing polymer chain is a potassium alkoxy group that can be functionalized by termination or by subsequent reaction by a wide variety of molecules.

Chromatographic Approach to Isolate Exfoliated Graphene.

A chromatographic approach for separating exfoliated graphene from natural flake graphite is presented. Graphene is an extremely strong, electrically and thermally conductive two-dimensional hexagonal array of carbon atoms with the potential to transform applications such as supercapacitors, composites, biosensors, ultra-thin touchscreens, and solar cells. However, many of these applications require the use of exfoliated graphene, and the current cost of this material can be prohibitive. The most cost-effective source of graphene is exfoliated graphite, and numerous approaches have been proposed for exfoliating graphite to graphene. Solution approaches are the most common, with graphite often exfoliated by extended sonication treatment followed by separation of graphene from graphite using centrifugation. This time-consuming approach results in low concentrations of small lateral dimension graphene, often in high-boiling-point organic solvents or containing stabilizers. In this study, a chromatographic approach is used in combination with a solvent interface trapping method of graphite exfoliation to isolate graphene. The interface trapping exfoliation approach uses a hydrophobic/hydrophilic solvent interface to spontaneously exfoliate graphite and form a graphene-stabilized water-in-oil emulsion. This emulsion contains both graphene and graphite, and when added to water-wet glass beads, graphene adsorbs onto the glass surface, leaving graphite in the hydrophobic mobile phase, where it is removed by washing with an additional oil phase. The efficiency of this scalable approach to separation is demonstrated by Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and Tyndall effect scattering.

Controlled radical polymerization of hydrophilic and zwitterionic brush-like polymers from silk fibroin surfaces.

Bombyx mori silk fibroin is a fibrous protein whose tunable properties and biocompatibility have resulted in its utility in a wide-variety of applications, including as drug delivery vehicles, wound dressings, and tissue engineering scaffolds. Control of protein and cell attachment is vital to the performance of biomaterials, but silk fibroin is mostly hydrophobic and interacts nonspecifically with cells and proteins. Silk functionalised with hydrophilic polymers reduces attachment, but the low number of reactive sites makes achieving a uniform conjugation a persistent challenge. This work presents a new approach to grow brush-like polymers from the surface of degradable silk films, where the films were enriched with hydroxyl groups, functionalised with an initiator, and finally reacted with acrylate monomers using atom transfer radical polymerisation. Two different routes to hydroxyl enrichment were investigated, one involving reaction with ethylene oxide (EO) and the other using a two-step photo-catalysed oxidation reaction. Both routes increased surface hydrophilicity, and hydrophilic monomers containing either uncharged (poly(ethylene glycol), PEG) pendant groups or zwitterionic pendant groups were polymerised from the surfaces. The initial processing of the films to induce beta sheet structures was found to impact the success of the polymerizations. Compared to the EO modified or unmodified silk surfaces, the oxidation reaction resulted in more polymer conjugation and the surfaces appear more uniform. Mesenchymal stem cell and protein attachment were the lowest on polymers grown from oxidised surfaces. PEG-containing brush-like polymers displayed lower protein attachment than surfaces conjugated with PEG using a previously reported "grafting to" method, but polymers containing zwitterionic side chains displayed both the lowest contact angles and the lowest cell and protein attachment. This finding may arise from the interactions of the zwitterionic pendant groups through their permanent dipoles and is an important finding because PEG is susceptible to oxidative damage that can reduce efficacy over time. These modified silk materials with lower cell and protein attachments are envisioned to find utility when enhanced diffusion around surfaces is required, such as in drug delivery implants.

Interface-exfoliated graphene-based conductive screen-printing inks: low-loading, low-cost, and additive-free.

Paper diagnostics are of growing interest due to their low cost and easy accessibility. Conductive inks, necessary for manufacturing the next generation diagnostic devices, currently face challenges such as high cost, high sintering temperatures, or harsh conditions required to remove stabilizers. Here we report an effective, inexpensive, and environmentally friendly approach to graphene ink that is suitable for screen printing onto paper substrates. The ink formulation contains only pristine graphite, water, and non-toxic alkanes formed by an interfacial trapping method in which graphite spontaneously exfoliates to graphene. The result is a viscous graphene stabilized water-in-oil emulsion-based ink. This ink does not require sintering, but drying at 90 °C or brief microwaving can improve the conductivity. The production requires only 40 s of shaking to form the emulsion. The sheet resistance of the ink is approximately 600 Ω/sq at a thickness of less than 6 µm, and the ink can be stabilized by as little as 1 wt% graphite.

Effect of Aqueous Anions on Graphene Exfoliation.

Ion partitioning behavior in electrolyte solutions plays an important role in drug delivery and therapeutics, protein folding, materials science, filtration, and energy applications such as supercapacitors. Here, we show that the segregation of ions in solutions also plays an important role in the exfoliation of natural flake graphite to pristine graphene. Polarizable anions such as iodide and acetate segregate to the interfacial region of the aqueous phase during solvent interfacial trapping exfoliation of graphene. Ordered water layers and accumulated charges near the graphene surface aid in separating graphene sheets from bulk graphite, and, more importantly, reduce the reversibility of the exfoliation event. The observed phenomenon results not only in the improved stability of graphene-stabilized emulsions but also in a low-cost and environmentally friendly way of enhancing the production of graphene.

PolyHIPE foams from pristine graphene: Strong, porous, and electrically conductive materials templated by a 2D surfactant.

Graphene is attractive as a functional 2D surfactant for polymerized high internal phase emulsions (polyHIPEs) due to its remarkable mechanical and electrical properties. We have developed polyHIPEs stabilized by pristine, unoxidized graphene via the spontaneous exfoliation of graphite at high-energy aqueous/organic interfaces. The exfoliated graphene self-assembles into a percolating network and incorporates into the polyHIPE cell walls as verified by TEM. The resulting composites showed compressive strengths of 7.0 MPa at densities of 0.22 g/cm3 and conductivities up to 0.36 S/m. Systematically reducing the concentration of monomer in the oil phase by dilution with a porogenic-acting solvent increased the porosity and lowered the density of the polyHIPEs. Characterization of these composites indicated that graphene's high compressive strength and modulus was transferred to the polyHIPEs and provided mechanical reinforcement even at low polymer content. SEM showed that the morphology of the polymer changed with decreasing monomer content while the graphene lined cells retained their shape. Moreover, we show that the polyHIPEs contain a continuous graphene percolating network resulting in electrically conductive materials at low graphene loading.

Self-Assembled Graphene Composites for Flow-Through Filtration.

Spontaneously exfoliated pristine graphene is used as a surfactant to template the formation of electrically conductive filters for the adsorption of an organic dye from water. In contrast to other reported graphene-based adsorption materials, our system provides a continuous approach to water treatment rather than a batch approach, and uses pristine graphene instead of the more costly and environmentally challenging graphene oxide. The use of self-assembled graphene also results in our filters being electrically conductive, providing a convenient route to clean the filters by resistive heating. An investigation of the mechanism of formation and filtration by these filters, templated by self-assembled two-dimensional pristine graphene, is presented. The thermodynamically driven exfoliation of natural flake graphite at a high-energy monomer/water interface produces water-in-oil emulsions stabilized by a thin layer of overlapping graphene sheets. Subsequent polymerization of the continuous monomer phase produces polymer foams with cells lined by graphene. With a combination of acoustic spectroscopy and electron microscopy, the effects of graphite concentration and temperature are studied, as is the correlation between droplet size and the size of the cells in the final polymer foam.

Pristine Graphene Microspheres by the Spreading and Trapping of Graphene at an Interface.

The interfacial spreading and exfoliation of graphene was used to create low-density, hollow microspheres defined by a thin shell of graphene. The spheres were templated by a thermodynamically driven self-assembly process in which graphite spontaneously exfoliated and spread at the high-energy interfaces of a water-in-oil emulsion. Graphene thus acted as a 2D surfactant to stabilize the dispersed water droplets utilized as polymerization templates. Using a mixture of organic solvent and monomer as the emulsion oil phase, polystyrene-coated hollow graphene microspheres were created. These spheres were characterized by optical and electron microscopy, thermo-gravimetric analysis, nanoindentation, and particle sizing. The mechanism leading to the microsphere surface morphology and shape is discussed, with the oil phase composition shown to play a critical role.

Graphene and Poly(3,4-ethylene dioxythiophene):Poly(4-styrenesulfonate) on Nonwoven Fabric as a Room Temperature Metal and Its Application as Dry Electrodes for Electrocardiography.

Highly conductive, metal-like poly(ethylene terephthalate) (PET) nonwoven fabric was prepared by coating poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) containing dimethyl sulfoxide (DMSO) onto PET nonwoven fabric previously coated with graphene/graphite. The sheet resistance of the original nonwoven fabric decreases from >80 MΩ□-1 to 1.1 Ω□-1 after coating with 10.7 wt % graphene and 5.48 wt % PEDOT:PSS with a maximum current at breakdown of 4 A. This sheet resistance is lower than previously reported sheet resistances of fabrics coated with graphene films, PEDOT:PSS films, or PEDOT:PSS coated fabrics from the literature. The effect of temperature on the resistance of graphene/PEDOT:PSS coated fabric has revealed that the resistance decreases with increasing temperature, analogous to semiconductors, with a clear semiconductor-metal transition occurring at 290 K. Finally, a coating of 18 wt % graphene/graphite and 2.5 wt % PEDOT:PSS (Rs = 5.5 Ω□-1) screen printed on the nonwoven fabric was shown to function as an electrode for electrocardiography without any hydrogel and with dry skin conditions. This composite coating finds application in wearable electronics for military and consumer sectors.

High-throughput optical thickness and size characterization of 2D materials.

We describe a method using simple optical microscopy and image processing that simultaneously characterizes thousands of nanosheets in a sample area on the order of 1 mm2. Including data acquisition and processing, both the number of atomic layers and the lateral sizes of all sheets can be obtained within a few hours-approximately 100 times faster than with previous methods, such as atomic force microscopy. This is achieved by normalizing the optical image based on substrate brightness, which eliminates inhomogeneities usually limiting optical techniques. Ultimately, the method enables robust statistical analysis of populations of nanosheet materials. We demonstrate the utility of this method by examining fractions made from a sample of graphene oxide (GO) made using an emulsion-based method. Beyond providing the morphological composition of the samples, the reported method is sensitive enough to provide information about the oxidation level of a population of GO sheets and, correspondingly, optical constants of the material.

Formulation of long-wavelength indocyanine green nanocarriers.

Indocyanine green (ICG), a Food and Drug Administration (FDA)-approved fluorophore with excitation and emission wavelengths inside the "optical imaging window," has been incorporated into nanocarriers (NCs) to achieve enhanced circulation time, targeting, and real-time tracking in vivo. While previous studies transferred ICG exogenously into NCs, here, a one-step rapid precipitation process [flash nanoprecipitation (FNP)] creates ICG-loaded NCs with tunable, narrow size distributions from 30 to 180 nm. A hydrophobic ion pair of ICG-tetraoctylammonium or tetradodecylammonium chloride is formed either in situ during FNP or preformed then introduced into the FNP feed stream. The NCs are formulated with cores comprising either vitamin E (VE) or polystyrene (PS). ICG core loadings of 30 wt. % for VE and 10 wt. % for PS are achieved. However, due to a combination of molecular aggregation and Förster quenching, maximum fluorescence (FL) occurs at 10 wt. % core loading. The FL-per-particle scales with core diameter to the third power, showing that FNP enables uniform volume encapsulation. By varying the ICG counter-ion ratio, encapsulation efficiencies above 80% are achieved even in the absence of ion pairing, which rises to 100% with 1∶1 ion pairing. Finally, while ICG ion pairs are shown to be stable in buffer, they partition out of NC cores in under 30 min in the presence of physiological albumin concentrations.

Controlled 3D Assembly of Graphene Sheets to Build Conductive, Chemically Selective and Shape-Responsive Materials.

Driven by the surface activity of graphene, electrically conductive elastomeric foams have been synthesized by the controlled reassembly of graphene sheets; from their initial stacked morphology, as found in graphite, to a percolating network of exfoliated sheets, defining hollow spheres. This network creates a template for the formation of composite foams, whose swelling behavior is sensitive to the composition of the solvent, and whose electrical resistance is sensitive to physical deformation. The self-assembly of graphene sheets is driven thermodynamically, as graphite is found to act as a 2D surfactant and is spread at high-energy interfaces. This spreading, or exfoliation, of graphite at an oil/water interface stabilizes water-in-oil emulsions, without the need for added surfactants or chemical modification of the graphene. Using a monomer such as butyl acrylate for the emulsion's oil phase, elastomeric foams are created by polymerizing the continuous oil phase. Removal of the aqueous phase then results in robust, conductive, porous, and inexpensive composites, with potential applications in energy storage, filtration, and sensing.

Altering and investigating the surfactant properties of graphene oxide.

Graphene oxide (GO) is a two-dimensional material with a hydrophobic carbon lattice functionalized with hydrophilic oxygen groups on the edges and basal plane. Its hydrophobic/hydrophilic nature allows it to behave as a surfactant, stabilizing emulsions of oil in water. In the investigation described here, we study GO's emulsifying ability by using it to template the polymerization of polystyrene while tuning the hydrophobicity and degree of oxidation of the GO sheets. SEM, TGA, optical microscopy, acoustic spectroscopy, and digital image analysis are used to characterize the system. The size of the GO templated PS spheres is found to be a function of the amount of GO in the system, with too little GO resulting in PS spheres with no GO shell. Increasing the hydrophobicity of the sheets is shown to lead to the formation of inverse emulsions, while increasing the graphitic character of the GO results in distorted styrene droplets as the GO sheets become more planar and less able to conform to a smoothly curving interface.

Boron Nitride Surface Activity as Route to Composite Dielectric Films.

The propensity of boron nitride sheets to stack creates obstacles for their application as multifunctional materials despite their unique thermal, mechanical, and electrical properties. To address this challenge, we use a combination of molecular dynamics simulations and experimental techniques to demonstrate surfactant-like properties of BN sheets at the interface between immiscible solvents. The spreading of two-dimensional BN sheets at a high-energy oil/water interface lowers the free energy of the system, creating films of overlapping BN sheets that are more thermodynamically favorable than stacked sheets. Coating such films onto polymers results in composite materials with exceptional barrier and dielectric properties.

PMMA functionalized boron nitride sheets as nanofillers.

We report the functionalization of hexagonal boron nitride (hBN) with polymer chains. These chains are grown by atom transfer radical polymerization (ATRP) from hBN following thermal treatment. When used as a nanofiller, the material shows improved dispersion resulting in significantly improved toughness as compared to pristine hBN without polymer functionalization. The polymer functionalized BN is also characterized by TGA, FTIR and FESEM.

Composite fluorescent nanoparticles for biomedical imaging.

PURPOSE: In the rapidly expanding field of biomedical imaging, there is a need for nontoxic, photostable, and nonquenching fluorophores for fluorescent imaging. We have successfully encapsulated a new, extremely hydrophobic, pentacene-based fluorescent dye within polymeric nanoparticles (NPs) or nanocarriers (NCs) via the Flash NanoPrecipitation (FNP) process. PROCEDURES: Nanoparticles and dye-loaded micelles were formulated by FNP and characterized by dynamic light scattering, fluorescence spectroscopy, UV-VIS absorbance spectroscopy, and confocal microscopy. RESULTS: These fluorescent particles were loaded from less than 1% to 78% by weight core loading and the fluorescence maximum was found to be at 2.3 wt.%. The particles were also stably formed at 2.3% core loading from 20 up to 250 nm in diameter with per-particle fluorescence scaling linearly with the NC core volume. The major absorption peaks are at 458, 575, and 625 nm, and the major emission peaks at 635 and 695 nm. In solution, the Et-TP5 dye displays a strong concentration-dependent ratio of the emission intensities of the first two emission peaks, whereas in the nanoparticle core the spectrum is independent of concentration over the entire concentration range. A model of the fluorescence quenching was consistent with Förster resonant energy transfer as the cause of the quenching observed for Et-TP5. The Förster radius calculated from the absorption and emission spectra of Et-TP5 is 4.1 nm, whereas the average dye spacing in the particles at the maximum fluorescence is 3.9 nm. CONCLUSIONS: We have successfully encapsulated Et-TP5, a pentacene derivative dye previously only used in light-emitting diode applications, within NCs via the FNP process. The extreme hydrophobicity of the dye keeps it encapsulated in the NC core, its extended pentacene structure gives it relatively long wavelength emission at 695 nm, and the pentacene structure, without oxygen or nitrogen atoms in its core, makes it highly resistant to photobleaching. Its bulky side groups minimize self-quenching and localization within the nanoparticle core prevents interaction of the dye with biological surfaces, or molecules in diagnostic assays. Loading of dye in the NP core allows 25 times more dye to be delivered than if it were conjugated onto the nanocarrier surface. The utility of the dye for quantifying nanoparticle binding is demonstrated. Studies to extend the wavelength range of these pentacene dyes into the near infra-red are underway.

Conductive thin films of pristine graphene by solvent interface trapping.

Graphite's insolubility in conventional solvents is a major obstacle to its utilization. This challenge is typically addressed by chemical modification such as oxidation, followed by reduction. However, pristine graphene possesses superior properties as oxidation and reduction lead to degradation of the graphene. Here we demonstrate the use of an interfacial trapping technique to assemble laterally macroscopic films of pristine graphene that are up to 95% transparent. This is accomplished by modest sonication of natural flake graphite in a water/heptane mixture to form continuous films at the interface between two immiscible liquids. Furthermore, the graphene sheets readily climb hydrophilic solid substrates, forming a homogeneous thin film one to four layers thick. These films are composed of a network of overlapping graphene sheets and shown to have long-range structure with conductivities on the order of 400 S/cm.

Optimization of cell receptor-specific targeting through multivalent surface decoration of polymeric nanocarriers.

Treatment of tuberculosis is impaired by poor drug bioavailability, systemic side effects, patient non-compliance, and pathogen resistance to existing therapies. The mannose receptor (MR) is known to be involved in the recognition and internalization of Mycobacterium tuberculosis. We present a new assembly process to produce nanocarriers with variable surface densities of mannose targeting ligands in a single step, using kinetically-controlled, block copolymer-directed assembly. Nanocarrier association with murine macrophage J774 cells expressing the MR is examined as a function of incubation time and temperature, nanocarrier size, dose, and PEG corona properties. Amphiphilic diblock copolymers are prepared with terminal hydroxyl, methoxy, or mannoside functionality and incorporated into nanocarrier formulations at specific ratios by Flash NanoPrecipitation. Association of nanocarriers protected by a hydroxyl-terminated PEG corona with J774 cells is size dependent, while nanocarriers with methoxy-terminated PEG coronas do not associate with cells, regardless of size. Specific targeting of the MR is investigated using nanocarriers having 0-75% mannoside-terminated PEG chains in the PEG corona. This is a wider range of mannose densities than has been previously studied. Maximum nanocarrier association is attained with 9% mannoside-terminated PEG chains, increasing uptake more than 3-fold compared to non-targeted nanocarriers with a 5kgmol(-1) methoxy-terminated PEG corona. While a 5kgmol(-1) methoxy-terminated PEG corona prevents non-specific uptake, a 1.8kgmol(-1) methoxy-terminated PEG corona does not sufficiently protect the nanocarriers from nonspecific association. There is continuous uptake of MR-targeted nanocarriers at 37°C, but a saturation of association at 4°C. The majority of targeted nanocarriers associated with J774E cells are internalized at 37°C and uptake is receptor-dependent, diminishing with competitive inhibition by dextran. This characterization of nanocarrier uptake and targeting provides promise for optimizing drug delivery to macrophages for TB treatment and establishes a general route for optimizing targeted formulations of nanocarriers for specific delivery at targeted sites.

Effervescent redispersion of lyophilized polymeric nanoparticles.

BACKGROUND: Freeze-drying is an attractive method for converting nanoparticulate pharmaceutical dispersions into a stable form with a long shelf life. However, practical challenges in translating laboratory practice to the clinic, such as high protectant osmolarity and infeasible reconstitution methods, currently limit lyophilized formulation development of nanoparticle therapeutics. RESULTS: We demonstrate the use of effervescent redispersion for the reconstitution of lyophilized polymeric nanoparticles and we show that a 3:1 mass ratio of effervescent salt produced the optimum redispersibility. With only low-energy hand agitation, reconstitution to sizes less than 600 nm was achieved. Second, the effect of nanoparticle formulation parameters (dispersion concentration, molecular weight of the stabilizing polymer, and physical state of the nanoparticle core) on particle redispersibility were examined. CONCLUSION: This novel freeze-drying and reconstitution method offers a route to producing redispersible dry powders of nanoparticle therapeutics.