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Charles T Campbell

Publications and source records attributed to Charles T Campbell.

At least 19 recordsLinked to original sources
PubMed2006

n-alkanes on Pt(111) and on C(0001)Pt(111): chain length dependence of kinetic desorption parameters.

We have measured the desorption of seven small n-alkanes (C(N)H(2N+2), N=1-4,6,8,10) from the Pt(111) and C(0001) surfaces by temperature programed desorption. We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J. Chem. Phys. 122, 164708 (2005)]. There we showed an increase in the desorption preexponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value. We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface, but is a general effect for desorption of the small n-alkanes. This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption. In this work, we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface. We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here. On each surface, the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value. Prior results of other groups have yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values. We demonstrate that by allowing the prefactor to increase according to our model, a reanalysis of their data resolves this y-intercept problem to some degree.

Journal Article↗
PubMed2006

Interactions of O(2) with Pd nanoparticles on alpha-Al(2)O(3)(0001) at low and high O(2) pressures.

The interaction of O(2) with small Pd particles (2-10 nm) supported on an alpha-Al(2)O(3)(0001) single crystal under both ultrahigh vacuum (UHV) and high-pressure conditions has been studied by temperature-programmed desorption (TPD), temperature-programmed low-energy ion scattering (TP-LEIS), and X-ray photoelectron spectroscopy (XPS). A low O(2) exposure (30 L) at 500 K leads to surface oxygen adatoms on the Pd nanoparticles, which desorb in TPD as O(2) in a peak at approximately 880 K. Surface O adatoms on the smallest Pd particles move to subsurface sites starting at 400 K, and they almost all move subsurface by approximately 750 K, desorbing mainly at considerably higher temperature. The dominant oxygen species above 700 K is subsurface, implying that it is more stable than oxygen adatoms on Pd. Exposures of the Pd nanoparticles to 25 Torr O(2) at 373-473 K readily convert the Pd to a species whose Pd XPS peak shifts by the same amount as the binding energy difference between bulk Pd and bulk PdO. We attribute this to PdO nanoparticles (or a thin film of PdO on or under the Pd for the larger particles). The decomposition of the PdO on these nanoparticles to Pd in an equilibrium O(2) pressure of 10-7 Torr does not occur until approximately 750 K, or approximately 200 K higher than the equilibrium decomposition of bulk PdO. This is attributed to the higher energy of Pd nanoparticles compared to bulk Pd and, for the larger particles, to the adhesion energy of the PdO film to the Pd, both of which stabilize the PdO on these Pd nanoparticles relative to bulk PdO. This PdO-like film on the larger particles may be similar to the ordered oxide thin film previously reported to form on Pd(111) but may also reside at the alpha-Al(2)O(3) interface and be partially stabilized by adhesion to this interface.

Journal Article↗
PubMed2006

Heat of adsorption of naphthalene on Pt(111) measured by adsorption calorimetry.

The heat of adsorption of naphthalene on Pt(111) at 300 K was measured with single-crystal adsorption calorimetry. The heat of adsorption on the ideal, defect-free surface is estimated to be (300 - 34 - 199(2)) kJ/mol. From this, a C-Pt bond energy for aromatic hydrocarbons on Pt(111) of approximately 30 kJ/mol is estimated, consistent with earlier results for benzene on Pt(111). There is higher heat of adsorption at very low coverage, attributed to step sites where the adsorption heat is >/=330 kJ/mol. Saturation coverage, = 1 ML, corresponds to 1.55 x 10(14) molecules/cm(2). Sticking probability measurements of naphthalene on Pt(111) give a high initial value of 1.0 and a Kisliuk-type coverage dependence that implies precursor-mediated sticking. The ratio of the hopping rate to the desorption rate of this precursor is approximately 51. Naphthalene adsorbs transiently on top of chemisorbed naphthalene molecules with a heat of adsorption of 83-87 kJ/mol.

Journal Article↗
PubMed2006

Thermodynamics of statherin adsorption onto hydroxyapatite.

Statherin is a salivary protein that inhibits the nucleation and growth of hydroxyapatite crystals in the supersaturated environment of the oral cavity. The thermodynamics of adsorption of statherin onto hydroxyapatite crystals have been characterized here by isothermal titration calorimetry and equilibrium adsorption isotherm analysis. At 25 degrees C, statherin adsorption is characterized by an exothermic enthalpy of approximately 3 kcal/mol that diminishes to zero at approximately 25% surface coverage. The initial heat of statherin adsorption increases with temperature, displaying a positive heat capacity change of 194 +/- 7 cal K(-)(1) mol(-)(1) at 25 degrees C. The heat of adsorption during this initial phase is strongly dependent on the buffer species, and from the differential heats of buffer ionization, it can be calculated that approximately one proton is taken up by the crystal or protein upon adsorption. The free energy of adsorption is dominated at all coverages by a large positive entropy (>or=23 cal K(-)(1) mol(-)(1)), which may be partially due to the loss of organized water that hydrates the protein and the mineral surface prior to adsorption. These results are interpreted using a two-site model for adsorption of statherin onto the hydroxyapatite crystals.

Adsorption↗
PubMed2006

Transition metal oxides: extra thermodynamic stability as thin films.

The oxides of many transition metals wet their own metal surface. The adhesion energy at this interface (E(adh,ox/m)) provides extra stabilization, which lowers the O2 pressure required for oxide stability as a thin film below that required for bulk-oxide stability by the factor exp[(2gamma(g/ox) - E(adh,ox/m))/(tN(ox)RT)], where gamma(g/ox) is the surface energy of the oxide, t is the oxide film thickness, and N(ox) is the oxygen concentration in the bulk oxide (moles O2 per volume). For oxide films only approximately 1 nm thick, this correction can be many orders of magnitude. This may extend to other compounds.

Journal Article↗
PubMed2005

Small Pd Clusters, up to the tetramer at least, are highly mobile on the MgO(100) surface.

Density functional theory calculations predict that small clusters of Pd atoms, containing up to at least four atoms, are highly mobile on the MgO(100) surface with the tetramer having the largest diffusion rate at room temperature--larger than the monomer. Surface vacancies are found, however, to bind the larger clusters strongly enough to trap them. These are important considerations when analyzing the growth and sintering of metal islands on oxide surfaces, in particular, the role of point defects.

Journal Article↗
PubMed2005

n-alkanes on MgO(100). I. Coverage-dependent desorption kinetics of n-butane.

High-quality temperature-programmed desorption (TPD) measurements of n-butane from MgO(100) have been made for a large number of initial butane coverages (0-3.70 ML, ML-monolayers) and a wide range of heating ramp rates (0.3-10 K/s). We present a TPD analysis technique which allows the coverage-dependent desorption energy to be accurately determined by mathematical inversion of a TPD spectrum, assuming only that the preexponential factor (prefactor) is coverage independent. A variational method is used to determine the prefactor that minimizes the difference between a set of simulated TPD spectra and corresponding experimental data. The best fit for butane desorption from MgO is obtained with a prefactor of 10(15.7+/-1.6) s(-1). The desorption energy is 34.9+/-3.4 kJ/mol at 0.5-ML coverage, and varies with coverage approximately as Ed(theta)=34.5+0.566theta+8.37 exp(-theta/0.101). Simulations based on these results can accurately reproduce TPD experiments for submonolayer initial coverages over a wide range of heating ramp rates (0.3-10 K/s). Advantages and limitations of this method are discussed.

Journal Article↗
PubMed2005

n-alkanes on MgO(100). II. Chain length dependence of kinetic desorption parameters for small n-alkanes.

Coverage-dependent desorption-kinetics parameters are obtained from high-quality temperature-programmed desorption data for seven small n-alkane molecules on MgO(100). The molecules, CNH2N+2 (N=1-4,6,8,10), were each studied for a set of 29 initial coverages at a heating ramp rate of 0.6 K/s as well as at a set of nine ramp rates in the range of 0.3-10.0 K/s. The inversion analysis method with its least-squares preexponential factor (prefactor) optimization discussed in the accompanying article is applied to these data. This method allows for accurate determination of prefactors and coverage-dependent desorption energies. The prefactor for desorption increases dramatically with chain length from 10(13.1) to 10(19.1) s(-1) over the range of N=1-10. We show that this increase can be physically justified by considering the increase in rotational entropy available to the molecules in the gaslike transition state for desorption. The desorption energy increases with chain length as Ed(N)=6.5+7.1N, which implies an incremental increase of 7.1+/-0.2 kJ/mol per CH2.

Journal Article↗
PubMed2005

Growth and sintering of Pd clusters on alpha-Al2O3(0001).

The growth and sintering of Pd nanoparticles on alpha-Al(2)O(3)(0001) have been studied by noncontact atomic force microscopy (NC-AFM), low-energy ion scattering spectroscopy (LEIS), temperature-programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). This is the first study of metal nanoparticles on a well-defined oxide surface where both NC-AFM and LEIS are used for characterization. These prove to be a powerful combination in assessing particle dimensions. The clean alumina surface showed atomically flat, 200-700 nm wide terraces. The sharp step edges are straight (within our resolution) for lengths of >300 nm and have heights in multiples of 0.2 nm. The Pd grows initially as two-dimensional (2D) islands at 300 K, with the transition to 3D particle growth at 0.25 ML (ML=monolayers). Upon heating at 1 K/s, the Pd starts to sinter below 400 K, and sinters at a nearly constant rate with increasing temperature, covering approximately 50% less of the alumina surface by approximately 1000 K, with a doubling in particle diameter and an eightfold decrease in particle number density. By approximately 1000 K, the number density was approximately 9 x 10(11)cm(2) for 0.8 ML of Pd, with an average diameter of 5 nm and an average thickness of 1 nm.

Journal Article↗
PubMed2005

Monocyte activation on polyelectrolyte multilayers.

The adherence and activation of primary human monocytes was investigated on a polyelectrolyte multilayer film containing hyaluronic acid (HA) and poly-L-lysine (PLL). The sequential layer-by-layer deposition of the multilayer film was characterized by surface plasmon resonance. Eight alternating bilayers displayed an effective thickness of 16.15 nm with a total polymer coverage of 2.10 microg/cm2. For cell studies, HA-PLL multilayers were constructed on tissue culture polystyrene (TCPS) substrates and characterized by time of flight second ion mass spectrometry (ToF-SIMS) analysis. Principal component analysis of the ToF-SIMS spectra resolved no significant difference in surface chemistry between PLL-terminated and HA-terminated multilayer surfaces. Monocyte adhesion on PLL- and HA-terminated surfaces was measured by the lactate dehydrogenase assay and showed a significant decrease in cell adhesion after 24 h incubation. Cell viability measured by Live/Dead fluorescent staining showed significant cell death in the adherent cell population over these 24 h. Tumor necrosis factor-alpha (TNF-alpha) production, a measure of monocyte activation, was quantified by ELISA and normalized to the number of adherent monocytes. The activation of monocytes on PLL-terminated and HA-terminated surfaces was nearly identical, and both surfaces had TNF-alpha levels that were 8-fold higher than TCPS. These results demonstrate that sufficient PLL had diffused into the surface layer to direct monocyte adherence and to induce cytokine activation and cell death on the HA-terminated multilayer films. The diffusion of the second multilayer component to the coating surface should, thus, be taken into account in the design of polyelectrolyte-based biomaterial coating strategies.

Cell Adhesion↗
PubMed2004

A streptavidin linker layer that functions after drying.

The ability of streptavidin (SA) to simultaneously bind four biotins is often used in linker layers, where a biotinylated molecule is linked to a biotin-functionalized surface via SA. For biosensor and array applications, it is desirable that the SA linker layer be stable to drying and rehydration. In this study it was observed that a significant decrease in binding capacity of a SA layer occurred when that layer was dried. For this study a SA linker layer was constructed by binding SA to a biotin-containing alkylthiolate monolayer (BAT/OEG) self-assembled onto gold. Its stability after drying was investigated using surface plasmon resonance (SPR). Approximately a quarter of the SA layer was removed from the BAT/OEG surface upon drying and rehydration, suggesting disruption of SA-biotin binding when dry. This resulted in the dried SA layer losing approximately 40% of its biotinylated ferritin (BF) binding capacity. Coating the layer with trehalose before drying was found to inhibit the loss of SA from the BAT/OEG surface. SPR showed that the trehalose-protected SA linker layer retained approximately 91% of its original BF binding capacity after drying and rehydration. Atomic force microscopy, which was used to image individual surface-bound SA and BF molecules, qualitatively confirmed these observations.

Biotin↗
PubMed2004

Parallel, quantitative measurement of protein binding to a 120-element double-stranded DNA array in real time using surface plasmon resonance microscopy.

Quantitative, real-time measurement of kinetics of sequence-specific binding of DNA-binding proteins to double-stranded DNA (dsDNA) immobilized in a 10 x 12 array on a planar gold surface is demonstrated using surface plasmon resonance (SPR) microscopy. This binding of the yeast transcription factor Gal4 to a 120-spot dsDNA array made with alternating 200-microm spots of its dsDNA operator sequence and an unrelated DNA sequence proves that this method could be used to simultaneously monitor the kinetics of binding of proteins to 120 different dsDNA sequences with a sensitivity to approximately 0.5 pg (<2 x 10(7) molecules) of bound protein in each array spot at a time resolution of 1 s. The method is label free and also allows absolute quantitative determination of the binding stoichiometry (i.e., the number of proteins bound per dsDNA) at each time. The dsDNA array was fabricated using a robotic microspotting system to deliver nanoliter droplets of biotinylated dsDNA solutions onto a streptavidin linker layer immobilized with biotinylated alkylthiols on a thin gold film. Simultaneous monitoring of binding to the many array elements allows the use of reference spots (i.e., array elements with unrelated dsDNA sequences) to correct the signal for nonspecific protein-DNA binding and changes in bulk refractive index of the solutions in the SPR microscope's flow cell. This allows high-throughput analyses of the kinetics and equilibrium of protein-dsDNA binding.

Animals↗
PubMed2004

Quantitative methods for spatially resolved adsorption/desorption measurements in real time by surface plasmon resonance microscopy.

A simple method for converting local reflectivity changes measured in surface plasmon resonance (SPR) microscopy to effective adlayer thicknesses and absolute surface coverages of adsorbed species is presented. For a range of high-contrast angles near the SPR resonance where the local metal surface's reflectivity changes linearly with angle, the change in reflectivity at fixed angle is proportional to the change in effective refractive index (eta(eff)) near the surface. This change in eta(eff) can be converted to absolute adsorbate coverage using methods developed for quantitative SPR spectroscopy. A measurement of the change in reflectivity due to changes in refractive index of bulk solutions, i.e., percent reflectivity change per refractive index unit (RIU), is the only calibration required. Application of this method is demonstrated for protein adsorption onto protein/DNA arrays on gold from aqueous solution using an SPR microscope operating at 633 nm. A detection limit of 0.072% change in absolute reflectivity is found for simultaneous measurements of all 200 microm x 200 microm areas within the 24-mm(2) light beam with 1-s time averaging. This corresponds to a change in effective refractive index of 1.8 x 10(-5) and a detection limit for protein adsorption of 1.2 ng/cm(2) (approximately 0.5 pg in a 200-microm spot). The linear dynamic range is Deltaeta(eff) = approximately 0.011 RIU or approximately 720 ng/cm(2) of adsorbed protein. Using a nearby spot as a reference channel, one can correct for instrumental drift and changes in refractive index of the solutions in the flow cell.

Adsorption↗
PubMed2004

Microspotting streptavidin and double-stranded DNA arrays on gold for high-throughput studies of protein-DNA interactions by surface plasmon resonance microscopy.

We present two strategies for microspotting 10 x 12 arrays of double-stranded DNAs (dsDNAs) onto a gold-coated glass slide for high-throughput studies of protein-DNA interactions by surface plasmon resonance (SPR) microscopy. Both methods use streptavidin (SA) as a linker layer between a biotin-containing mixed self-assembled monolayer (SAM) and biotinylated dsDNAs to produce arrays with high packing density. The primary mixed SAM is produced from biotin- and oligo(ethylene glycol)-terminated thiols bonded as thiolates onto the gold surface. In the first method, a robotic microspotter is used to deliver nanoliter droplets of dsDNA solution onto a uniform layer of this SA ( approximately 2 x 10(12) SA/cm(2)). SPR microscopy shows a density of (5-6) x 10(11) dsDNA/cm(2) (0.2-0.3 dsDNA/SA) in the array elements. The second method uses instead a microspotted array of this SA linker layer, onto which the microspots of dsDNA are added with spatial registry. SPR microscopy before addition of the dsDNA shows a SA coverage of 2 x 10(12) SA/cm(2) within the spots and a dsDNA density of 8.5 +/- 3.5 x 10(11) dsDNA/cm(2) (0.3-0.7 dsDNA/SA, depending on the length of dsDNA) after dsDNA spotting. We demonstrate the ability to simultaneously monitor protein binding with the SPR microscope in many 200-microm spots with 1-s time resolution and sensitivity to <1 pg of protein.

Adsorption↗