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Biomedical subjects

Paul V Braun

Publications and source records attributed to Paul V Braun.

At least 19 recordsLinked to original sources
PubMed2006

Transformation of hydrogel-based inverse opal photonic sensors from FCC to L1(1) during swelling.

The structural evolution of Bragg diffracting inverse opal hydrogel sensors during swelling is directly observed by two-photon laser scanning fluorescence microscopy and compared to predictions from finite element analysis. A fluorescently labeled pH-sensitive hydrogel is UV-polymerized in a dried polystyrene colloidal crystal template, which is etched to yield an inverse opal. Fluorescence imaging of the hydrogel at different pH values reveals an inhomogeneous deformation of the FCC array of aqueous pores. The pores elongate along the sample normal direction and collapse along the sample parallel directions, consistent with the Bragg response, which indicates a 1-D increase in the interlayer distance. Interconnects between the pores serve as anchor points during hydrogel expansion into the pores. Pinning of the hydrogel to the substrate causes a change of the hydrogel lattice symmetry during deformation, from FCC (ABC stacking) to L1(1) (ABCA'B'C' stacking). Reconstructed cross-sections confirm that a 1-D increase in the interlayer distance along the substrate normal direction is responsible for the diffraction response of an inverse opal hydrogel sensor. Comparison with predictions from finite element analysis shows qualitative agreement, although the experimental mesostructure is significantly more deformed than the calculated data, due to buckling in the experimental system that is not captured by the model.

Biosensing Techniques↗
PubMed2006

Slow vertical deposition of colloidal crystals: a Langmuir-Blodgett process?

For an evaporating colloidal suspension in which the evaporation velocity exceeds the sedimentation velocity, particles will accumulate at the solvent-air interface. If neither diffusion nor convection can disperse this accumulation, it is expected to grow into a colloidal multilayer several microns thick. We observe that the thickness of colloidal crystals vertically deposited from 1 mum diameter polystyrene latex suspensions of 0.002 < or = phi < or = 0.008 increases linearly with distance in the growth direction and that these thickness profiles are consistent with their growth from a horizontal colloidal layer accumulated beneath the solvent-air interface. We describe a means for performing vertical deposition at growth rates slower than the evaporation rate by adding solvent to the bottom of the colloidal suspension and observe that halving the growth rate of vertical deposition increases both the thickness and the reflectivity of the resulting colloidal crystals, effects indistinguishable from those of doubling the concentration of the colloidal suspension, data also consistent with the colloidal crystals' growth from a horizontal layer of particles beneath the interface. If sufficiently little reorganization is involved as particles move from this horizontal layer to the vertically deposited colloidal crystal, slow vertical deposition of polymer microspheres might be thought of as the Langmuir-Blodgett transfer of a horizontal colloidal crystal onto a vertical substrate. Colloidal crystals deposited using both high concentration and slowed growth can have peak IR reflectance in excess of 80%, exceeding most published values. These observations provide a conceptual framework for engineering vertically deposited colloidal crystals that combine thickness with good optical performance.

Journal Article↗
PubMed2006

Thermal conductance of hydrophilic and hydrophobic interfaces.

Using time-domain thermoreflectance, we have measured the transport of thermally excited vibrational energy across planar interfaces between water and solids that have been chemically functionalized with a self-assembled monolayer (SAM). The Kapitza length--i.e., the thermal conductivity of water divided by the thermal conductance per unit area of the interface--is analogous to the "slip length" for water flowing tangentially past a solid surface. We find that the Kapitza length at hydrophobic interfaces (10-12 nm) is a factor of 2-3 larger than the Kapitza length at hydrophilic interfaces (3-6 nm). If a vapor layer is present at the hydrophobic interface, and this vapor layer has a thermal conductivity that is comparable to bulk water vapor, then our experimental results constrain the thickness of the vapor layer to be less than 0.25 nm.

Journal Article↗
PubMed2006

Patterned colloid assembly by grafted photochromic polymer layers.

Quartz surfaces and colloidal silica particles were derivatized with a poly(methyl methacrylate) copolymer containing spirobenzopyran (SP) photochromic molecules in the pendant groups at a concentration of 20 mol %. Two-photon near-IR excitation (approximately 780 nm) was then used to create chemically distinct patterns on the modified surfaces through a photochromic process of SP transformation to the zwitterionic merocyanine (MC) isomer. The derivatized colloids were approximately 10 times more likely to adsorb onto the photoswitched, MC regions. Surface coverage and adsorption kinetics have been compared to the mean-field model of irreversible monolayer adsorption.

Journal Article↗
PubMed2006

Length scale heterogeneity in lateral gradients of poly(N-isopropylacrylamide) polymer brushes prepared by surface-initiated atom transfer radical polymerization coupled with in-plane electrochemical potential gradients.

We report the preparation and characterization of poly(N-isopropylacrylamide) (PNIPAAm) polymer brushes exhibiting controlled lateral variations in the patchiness of polymer chains. These gradients were achieved through an atom transfer radical polymerization (ATRP) grafting-from approach utilizing surfaces on which the spatial profile of the initiator density was carefully controlled. Initiator density gradients were formed on Au by first preparing a hexadecanethiol (HDT) density gradient, by reductive desorption using a laterally anisotropic electrochemical gradient. The bare areas in the original HDT gradient were then back-filled with a disulfide initiator, (BrC(CH3)2COO(CH2)11S)2. The initiator coverage was characterized by X-ray photoelectron spectroscopy (XPS). Then, surface-initiated ATRP was utilized to transfer the initiator density gradient into gradients of PNIPAAm chain density. Ellipsometry, surface plasmon resonance (SPR), and atomic force microscopy (AFM) were used to characterize these PNIPAAm density gradients. The defining characteristic of the PNIPAAm gradients is the evolution of the morphology from discontinuous mushroom structures at extremely low grafting densities to heterogeneous patchy structures at intermediate grafting densities. The size of the patchy domains gradually increases, until at a high grafting density region, the morphology evolves to a smoother, presumably more extended, structure.

Journal Article↗
PubMed2005

Double direct templating of periodically nanostructured ZnS hollow microspheres.

We introduce a "double direct templating" method for obtaining hollow microspheres with periodically nanostructured walls. Silica or polystyrene colloids are dispersed in a self-assembled hexagonal lyotropic liquid crystal containing precursors for ZnS. The semiconductor ZnS mineralizes on the surface of the colloid, expressing a pore morphology that is a copy of the structure of the liquid crystal. After etching of the sacrificial colloids, hollow capsules with templated and uniform mesoporous shells are obtained.

Journal Article↗
PubMed2005

Optical characterization of ultrasmall Si nanoparticles prepared through electrochemical dispersion of bulk Si.

Studying the properties and stability of silicon nanoparticles (Si-np) in aqueous environments may lead to novel applications in biological systems. In this work, we use absorption and photoluminescence (PL) spectroscopy to characterize ultrasmall Si-np prepared through anodic etching and ultrasonic fractionation of a crystalline Si wafer. Their behavior is studied over time in 2-propanol and during treatments with water, NaOH, HCl, and H(2)O(2). The observed population is divided into two types of material: bright species consisting of well-etched Si-np, approximately 1 nm in diameter, and dark species derived from partially etched or aggregated Si structures. The dark material is seen by its scattering in the 2-propanol and water solutions and is largely removed via precipitation with the NaOH or HCl treatment. The bright material includes three distinct species with their respective emissions in the UV-B, UV-A, and hard-blue regions of the spectrum. The hard-blue PL is shown to have a simple pH dependence with a pK(a) approximately 3, providing an important insight into its chemical origin and signaling for possible application of Si-np as environmental probes. Our results offer some potential for tailoring the PL properties of ultrasmall Si-np through control of their surface chemistry.

Absorption↗
PubMed2005

Diffraction response of colloidal crystals: effect of numerical aperture.

We present a quantitative experimental and theoretical study of the effect of numerical aperture (NA) on the Bragg diffraction from a dry polystyrene colloidal crystal. The diffraction peak parameters changed noticeably as the NA was increased from 0.017 to 0.5. The diffraction wavelength blueshifted 1.4% from 584 to 576 nm, and the normalized full width at half-maximum increased from 6.2% to 7.0%. These shifts occurred primarily for NA > 0.3 and agreed qualitatively with results predicted by a layered Korringa-Kohn-Rostoker method. Thus, by using focusing optics with NAs below 0.3, the diffraction response of low-photonic-strength mesostructures may be compared with normal-incidence experimental and theoretical data.

Journal Article↗
PubMed2005

Thermal transport in au-core polymer-shell nanoparticles.

Thermal transport in aqueous suspensions of Au-core polymer-shell nanoparticles is investigated by time-resolved measurements of optical absorption. The addition of an organic cosolvent to the suspension causes the polystyrene component of the polymer shell to swell, and this change in the microstructure of the shell increases the effective thermal conductivity of the shell by a factor of approximately 2. The corresponding time scale for the cooling of the nanoparticle decreases from 200 ps to approximately 100 ps. The threshold concentration of cosolvent that creates the changes in thermal conductivity, 5 vol % tetrahydrofuran in water or 40 vol % N,N-dimethylformamide in water, is identical to the threshold concentrations for producing small shifts in the frequency of the plasmon resonance. Because the maximum fraction of solvent in the polymer shell is less than 20 vol %, the increase in the effective thermal conductivity of the shell cannot be easily explained by contributions to heat transport by the solvent or enhanced alignment of the polystyrene backbone along the radial direction.

Gold↗
PubMed2004

Roles of anionic and cationic template components in biomineralization of CdS nanorods using self-assembled DNA-membrane complexes.

Complexes of anionic DNA and cationic liposomes self-assemble into a multilamellar structure where two-dimensional lipid sheets confine a periodic one-dimensional lattice of parallel DNA chains, between which Cd(2+) ions can condense, and be subsequently reacted with H(2)S to form CdS nanorods. In this work, we identify the synergistic roles of the anionic and cationic components within the DNA-membrane template; DNA is highly anionic and condenses the Cd(2+) ions, while the cationic membrane modulates the concentration of condensed Cd(2+) ions to control the final CdS nanorod dimensions. Due to the strong electrostatic interactions between the DNA sugar-phosphate backbone and the Cd(2+) ions, crystallographic control of CdS nanostructures is possible using these simple DNA-membrane templates, which we demonstrate using nanobeam electron diffraction experiments on individual templated CdS nanorods.

Animals↗
PubMed2004

Patterned poly(N-isopropylacrylamide) brushes on silica surfaces by microcontact printing followed by surface-initiated polymerization.

Patterned poly(N-isopropylacrylamide) (PNIPAAm) brushes were fabricated on oxidized silicon wafers by surface-initiated atom transfer radical polymerization of N-isopropylacrylamide from a micropatterned initiator. The patterned surface initiator was prepared by microcontact-printing octadecyltrichlorosilane and backfilling with 3-(aminopropyl)triethoxysilane followed by amidization with 2-bromo-2-methylpropionic acid. XPS and FTIR confirmed the chemical structure of the surface initiator and the PNIPAAm brushes. Surface analysis techniques, including ellipsometry, contact angle goniometry, and X-ray reflectometry (XRR), were used to characterize the thickness, roughness, hydrophilicity, and density of the polymer brushes. Tapping-mode AFM imaging confirmed the successful patterning of the PNIPAAm brushes on the oxidized silicon substrates. Variable temperature ellipsometry indicated that the lower critical solution temperature of the hydrated PNIPAAm brush was broad, occurring over the range of 20-35 degrees C. A solvatochromic fluorophore, 6-propionyl-2-dimethylaminonaphthalene (Prodan), in the PNIPAAm brush layers yielded a very similar emission to that in DMF, which can be attributed to the similarity of their chemical structures. Fluorescence microscopy further proved the successful patterning of the polymer brushes and suggested that the Prodan is localized in the patterned PNIPAAm brushes and excluded from the surrounding octadecyltrichlorosilane regions.

Acrylic Resins↗
PubMed2004

Structural evolution of colloidal crystals with increasing ionic strength.

We have directly observed the structural evolution of colloidal crystals as a function of increasing ionic strength using confocal scanning laser microscopy. Silica colloids were sedimented onto a glass substrate in deionized water to create large, single domain crystals. The solution ionic strength was then increased by one of three methods of controlled electrolyte addition: (1) direct injection of electrolyte solutions, (2) single step diffusion of electrolyte solutions through a dialysis membrane, and (3) multiple step diffusion of electrolyte solutions of increasing ionic strength through a dialysis membrane. During direct injection of electrolyte solutions, initially large, single domain colloidal crystals were shear melted and then evolved into polycrystalline structures at low ionic strengths and gels at higher ionic strengths. Diffusion of electrolyte solutions though dialysis membranes in a single step produced gradient-driven transport that also melted initial single domain crystals to yield polycrystalline and gel structures similar to the injection approach. Interestingly, the multistep diffusion of several electrolyte solutions through dialysis membranes facilitated retention of large, single domain crystals even as particles came into adhesive contact. This was achieved by reducing the contraction rate of the crystalline lattice to allow sufficient time for diffusion-limited configurational rearrangements to occur within the evolving structure. These mechanically robust, single domain colloidal crystals may find important applications as templates for photonic materials and sensors.

Journal Article↗
PubMed2004

Fabricating complex three-dimensional nanostructures with high-resolution conformable phase masks.

High-resolution, conformable phase masks provide a means to fabricate, in an experimentally simple manner, classes of 3D nanostructures that are technologically important but difficult to generate in other ways. In this approach, light passing through a phase mask that has features of relief comparable in dimension to the wavelength generates a 3D distribution of intensity that exposes a photopolymer film throughout its thickness. Developing this polymer yields a structure in the geometry of the intensity distribution, with feature sizes as small as 50 nm. Rigorous coupled-wave analysis reveals the fundamental aspects of the optics associated with this method; a broad-range 3D nanostructures patterned with it demonstrates its technical capabilities. A nanoporous filter element built inside a microfluidic channel represents one example of the many types of functional devices that can be constructed.

Journal Article↗
PubMed2004

Nanoparticle-mediated epitaxial assembly of colloidal crystals on patterned substrates.

We have studied the assembly of 3-D colloidal crystals from binary mixtures of colloidal microspheres and highly charged nanoparticles on flat and epitaxially patterned substrates created by focused ion beam milling. The microspheres were settled onto these substrates from dilute binary mixtures. Laser scanning confocal microscopy was used to directly observe microsphere structural evolution during sedimentation, nanoparticle gelation, and subsequent drying. After microsphere settling, the nanoparticle solution surrounding the colloidal crystal was gelled in situ by introducing ammonia vapor, which increased the pH and enabled drying with minimal microsphere rearrangement. By infilling the dried colloidal crystals with an index-matched fluorescent dye solution, we generated full 3-D reconstructions of their structure including defects as a function of initial suspension composition and pitch of the patterned features. Through proper control over these important parameters, 3-D colloidal crystals were created with low defect densities suitable for use as templates for photonic crystals and photonic band gap materials.

Journal Article↗
PubMed2004

Polymer size and concentration effects on the size of gold nanoparticles capped by polymeric thiols.

Gold nanoparticles stabilized by thiol-terminated poly(ethylene glycol) monomethyl ethers with molecular weights ranging from 350 to 2000 have been prepared at thiol-to-gold molar ratios ranging from 3:1 to 1:8. Particle size distributions have been constructed for these particles from transmission electron microscopy images of hundreds of particles for each variation in synthetic conditions. The mean diameters of these particles range from 1.5 to 3.2 nm, with a slight increase in particle size with decreasing thiol content; these particles are smaller than those prepared using alkanethiols at similar thiol-to-gold ratios. Particles prepared under thiol-poor conditions exhibit much greater polydispersity than those prepared under thiol-rich conditions and include numerically rare large-particle outliers that contain much of the gold in the sample. The mean diameters of the gold nanoparticles decrease slightly with increasing polymer weight, especially under thiol-rich conditions. A simple model is developed to predict the trends in nanoparticle diameter that would result were the polymer's steric bulk protecting the nanoparticles from additional growth the principal factor controlling nanoparticle size in this system. This model predicts a much stronger dependence on thiol concentration than has been experimentally observed and a dependence on polymer molecular weight opposite to that experimentally observed. This suggests that the polymers' steric bulk is not the principal reason that these polymers yield smaller nanoparticles than alkanethiols at similar thiol-to-gold ratios. It is instead proposed that polar polymers may yield small nanoparticles by accelerating particle nucleation via coordination between functional groups in the polymer and atomic gold.

Gold↗
PubMed2004

Glucose-sensitive inverse opal hydrogels: analysis of optical diffraction response.

A glucose-sensitive inverse opal hydrogel was synthesized through photopolymerization of 2-hydroxyethyl methacrylate and 3-acrylamidophenylboronic acid within the interstitial space of a dried poly(styrene) colloidal crystal template, followed by template removal. Charged complex formation between the phenylboronic acid functional group and the 1,2-cis-diol glucose resulted in reversible swelling of the inverse opal hydrogel, which was observed through shifts in the optical diffraction wavelength. The hydrogel was sensitive to glucose at physiological concentrations and ionic strength. The effects of phenylboronic acid concentration, ionic strength, and buffer pH on the equilibrium hydrogel swelling were also studied. The kinetics of hydrogel swelling was also examined, and it was found that the rate of diffraction shift matched well with diffusion-limited kinetics. Additionally, the diffraction response was compared with simulations using the scalar wave approximation and transfer matrix method.

Boronic Acids↗