Thomas P. Beebe

Infrared spectroscopic observations of surface bonding in physical adsorption: The physical adsorption of CO on SiO2 surfaces

Abstract The physical adsorption of CO onto SiO2 surfaces containing isolated OH groups results in the formation of two types of physisorbed CO. The first, CO species A, exhibits vCO = 2158 cm−1 and is bound by hydrogen bonding to the SiOHδ+ groups to form a surface complex SiOHδ+ … CO. The strength of this interaction is about 2.7 kcal mol−. The second form of physisorbed CO, CO species B, exhibits vCO = 2140 cm−1, and has pronounced rotational wings. The degree of rotational freedom is determined by the extent of shielding by CO of the polar groups on the surface. At high coverages of physisorbed CO species B, solvent effects due to CO species B on both the OH and CO stretching modes of the SiOH … CO complex are observed to cause small downward shifts in stretching frequency.

Quantification of Protein Incorporated into Electrospun Polycaprolactone Tissue Engineering Scaffolds

The surface modification of synthetic tissue engineering scaffolds is essential to improving their hydrophilicity and cellular compatibility. Plasma treatment is an effective way to increase the hydrophilicity of a surface, but the incorporation of biomolecules is also important to control cellular adhesion and differentiation, among many other outcomes. In this work, oriented polycaprolactone (PCL) electrospun fibers were modified by air-plasma treatment, followed by the covalent attachment of laminin. The amount of protein incorporated onto the fiber surface was controlled by varying the reaction time and the protein solution concentration. The protein concentration and coverage were quantified using X-ray photoelectron spectroscopy (XPS), solid-state ultraviolet-visible spectroscopy (UV-vis) and two fluorescence-based assays. XPS results showed a nearly linear increase in protein coverage with increasing protein soaking solution concentration until a monolayer was formed. Results from XPS and the NanoOrange fluorescence assay revealed multilayer protein coverage at protein solution concentrations between 25 and 50 μg/mL, whereas the UV-vis assay demonstrated multilayer coverage at lower protein solution concentrations. The effect of protein concentration on the neurite outgrowth of neuron-like PC12 cells was evaluated, and outgrowth rates were found to be positively correlated to increasing protein concentration.

Infrared spectroscopic observation and characterization of surface ethylidyne on supported palladium on alumina

Abstract The first detailed characterization of the surface ethylidyne species adsorbed on a supported transition metal surface , namely, Pd Al 2 O 3 (10%) is reported. Upon addition of C 2 H 4 or C 2 H 2 to Pd Al 2 O 3 at 300 K, there is an immediate irreversible formation of surface ethylidyne ( CCH 3 ), as evidenced by the development of three characteristic bands in the infrared spectrum [ δ s ( CH 3 ) = 1333 cm -1 , ν ( C  C ) = 1088 cm -1 , and ν s ( CH 3 ) = 2867 cm -1 ]. This assignment is further supported by HD exchange experiments involving chemisorbed ethylidyne which correlate with analogous experiments on Rh(111). Thermal conversion from ethylene adsorbed at low temperatures to surface ethylidyne begins at = 240 K; the surface ethylidyne species is stable up to about 400 K. These vibrational data, as well as the thermal stability data, are in excellent agreement with previous results on the Pd(111) and Rh(111) single-crystal surfaces.

Improvements to atomic force microscopy cantilevers for increased stability

A modification of commercially manufactured atomic force microscopy cantilevers which reduces the bending of the V‐shaped legs due to changes in temperature is described. Gold‐coated silicon nitride cantilevers are a bimorph system in which the different thermal expansion coefficients of the materials comprising the system can produce a temperature‐dependent change in curvature. Other stress‐related effects might also be responsible for the observed bending. By removing the gold film and redepositing gold only at the end of the V‐shaped legs, a reduction in the bending of the cantilever is accomplished while the required optical reflectivity for the laser deflection system is retained. Imaging x‐ray photoelectron spectroscopy, scanning electron microscopy, and changes in the detector photodiode signal related to bending of the cantilever are shown for modified and unmodified tips.

Surface-modified nanofibrous biomaterial bridge for the enhancement and control of neurite outgrowth

Biomaterial bridges constructed from electrospun fibers offer a promising alternative to traditional nerve tissue regeneration substrates. Aligned and unaligned polycaprolactone (PCL) electrospun fibers were prepared and functionalized with the extracellular matrix proteins collagen and laminin using covalent and physical adsorption attachment chemistries. The effect of the protein modified and native PCL nanofiber scaffolds on cell proliferation, neurite outgrowth rate, and orientation was examined with neuronlike PC12 cells. All protein modified scaffolds showed enhanced cellular adhesion and neurite outgrowth compared to unmodified PCL scaffolds. Neurite orientation was found to be in near perfect alignment with the fiber axis for cells grown on aligned fibers, with difference angles of less than 7o from the fiber axis, regardless of the surface chemistry. The bioavailability of PCL fibers with covalently attached laminin was found to be identical to that of PCL fibers with physically adsorbed laminin, indicating that the covalent chemistry did not change the protein conformation into a less active form and the covalent attachment of protein is a suitable method for enhancing the biocompatibility of tissue engineering scaffolds. a) Electronic mail: nicole.zander@arl.army.mil

Coaxial Electrospun Poly(methyl methacrylate)–Polyacrylonitrile Nanofibers: Atomic Force Microscopy and Compositional Characterization

Poly(methyl methacrylate) (PMMA)-polyacrylonitrile (PAN) fibers were prepared using a conventional single-nozzle electrospinning technique. The as-spun fibers exhibited core-shell morphology as verified by transmission electron microscopy (TEM) and atomic force microscopy (AFM). AFM-phase and modulus mapping images of the fiber cross-section and X-ray photoelectron spectroscopy (XPS) analysis indicated that PAN formed the shell and PMMA formed the core material. XPS, thermogravimetric analysis (TGA), and elemental analysis were used to determine fiber compositional information. Soaking the fibers in solvent demonstrated removal of the core material, generating hollow PAN fibers.

Infrared spectroscopic observations of hydrogen bonding and Fermi resonance of adsorbed methyl chloride on alumina surfaces

The physical adsorption of methyl chloride onto Al2O3 surfaces containing surface OH groups has been studied using transmission infrared spectroscopy. Methyl chloride reversibly bonds via hydrogen bonding to the surface hydroxyl groups with a spectroscopically measured heat of adsorption of −3.37±0.38 kcal mol−1. Physisorption of methyl chloride results in a significant reduction in the intensity of the rotational wings of the methyl chloride absorption bands relative to the gas phase, a small downward frequency shift in their band centers, and substantial effects on the ν(OH) region of the alumina spectrum due to hydrogen bonding of surface OH groups with methyl chloride. Fermi resonances are observed for adsorbed methyl chloride and result in the observation of the overtones of both the symmetric and asymmetric methyl deformation modes. It is postulated that the hydrogen‐bonded CH3Cl species possess rotational (or other) degrees of freedom which lead to a high entropy in the adsorbed layer, compared to ...

Physical vapor deposition synthesis of tungsten monocarbide (WC) thin films on different carbon substrates

The synthesis of tungsten monocarbide (WC) thin films has been performed by physical vapor deposition on various substrates including glassy carbon, carbon fiber sheet, carbon foam, and carbon cloth. The WC and W2C phase contents of these films have been evaluated with bulk and surface analysis techniques such as x-ray diffraction, x-ray photoelectron spectroscopy, and scanning electron microscopy. These characterization techniques were also used to determine the effects of synthesis by nonreactive and reactive sputtering. The synthesis of WC particles supported on the carbon fiber substrate has also been accomplished using the temperature programmed reaction method. Overall, the results demonstrate that the phase purity of tungsten carbides can be controlled by the deposition environment and annealing temperatures.

Non-equilibrium cation distribution and enhanced spin disorder in hollow CoFe2O4 nanoparticles

We present magnetic properties of hollow and solid CoFe(2)O(4) nanoparticles that were obtained by annealing of Co(33)Fe(67)/CoFe(2)O(4) (core/shell) nanoparticles. Hollow nanoparticles were polycrystalline whereas the solid nanoparticles were mostly single crystal. Electronic structure studies were performed by photoemission which revealed that particles with hollow morphology have a higher degree of inversion compared to solid nanoparticles and the bulk counterpart. Electronic structure and the magnetic measurements show that particles have uncompensated spins. Quantitative comparison of saturation magnetization (M(S )), assuming bulk Néel type spin structure with cationic distribution, calculated from quantitative XPS analysis, is presented. The thickness of uncompensated spins is calculated to be significantly large for particles with hollow morphology compared to solid nanoparticles. Both morphologies show a lack of saturation up to 7 T. Moreover magnetic irreversibility exists up to 7 T of cooling fields for the entire temperature range (10-300 K). These effects are due to the large bulk anisotropy constant of CoFe(2)O(4) which is the highest among the cubic spinel ferrites. The effect of the uncompensated spins for hollow nanoparticles was investigated by cooling the sample in large fields of up to 9 T. The magnitude of horizontal shift resulting from the unidirectional anisotropy was more than three times larger than that of solid nanoparticles. As an indication signature of uncompensated spin structure, 11% vertical shift for hollow nanoparticles is observed, whereas solid nanoparticles do not show a similar shift. Deconvolution of the hysteresis response recorded at 300 K reveals the presence of a significant paramagnetic component for particles with hollow morphology which further confirms enhanced spin disorder.

DC magnetron sputtered polyaniline-HCl thin films for chemical sensing applications.

Thin films of conducting polymers exhibit unique chemical and physical properties that render them integral parts in microelectronics, energy storage devices, and chemical sensors. Overall, polyaniline (PAni) doped in acidic media has shown metal-like electronic conductivity, though exact physical and chemical properties are dependent on the polymer structure and dopant type. Difficulties arising from poor processability render production of doped PAni thin films particularly challenging. In this contribution, DC magnetron sputtering, a physical vapor deposition technique, is applied to the preparation of conductive thin films of PAni doped with hydrochloric acid (PAni-HCl) in an effort to circumvent issues associated with conventional thin film preparation methods. Samples manufactured by the sputtering method are analyzed along with samples prepared by conventional drop-casting. Physical characterization (atomic force microscopy, AFM) confirm the presence of PAni-HCl and show that films exhibit a reduced roughness and potentially pinhole-free coverage of the substrate. Spectroscopic evidence (UV-vis, FT-IR, and X-ray photoelectron spectroscopy (XPS)) suggests that structural changes and loss of conductivity, not uncommon during PAni processing, does occur during the preparation process. Finally, the applicability of sputtered films to gas-phase sensing of NH(3) was investigated with surface plasmon resonance (SPR) spectroscopy and compared to previous contributions. In summary, sputtered PAni-HCl films exhibit quantifiable, reversible behavior upon exposure to NH(3) with a calculated LOD (by method) approaching 0.4 ppm NH(3) in dry air.