John R. Morris

The dynamics of gas-surface energy exchange in collisions of Ar atoms with ω-functionalized self-assembled monolayers

Atomic-beam scattering experiments using n-alkanethiol and ω-functionalized alkanethiol self-assembled monolayers (SAMs) on gold are employed to explore the dynamics of gas-surface energy exchange in collisions with model organic surfaces. The studies are performed by directing a nearly monoenergetic beam of 80 kJ/mol Ar atoms onto a particular SAM at an incident angle of 30° with respect to the surface normal and recording the time-of-flight distributions for the atoms as they scatter from the surface at a final angle of 30°. Among the monolayers studied, long-chain CH3-terminated SAMs are found to be the most effective at dissipating the translational energy of impinging atoms. For alkanethiols with greater than seven total carbon atoms (HS(CH2)n>6CH3), we find that, for specular scattering conditions, over 80% of the incident energy is transferred to the surface and that over 60% of the impinging atoms approach thermal equilibrium with the surface before scattering back into the gas phase. In contrast ...

Packing density and structure effects on energy-transfer dynamics in argon collisions with organic monolayers

A combined experimental and molecular-dynamics simulation study has been used to investigate energy-transfer dynamics of argon atoms when they collide with n-alkanethiols adsorbed to gold and silver substrates. These surfaces provide the opportunity to explore how surface structure and packing density of alkane chains affect energy transfer in gas-surface collisions while maintaining the chemical nature of the surface. The chains pack standing up with 12 degrees and 30 degrees tilt angles relative to the surface normal and number densities of 18.9 and 21.5 A(2)molecule on the silver and gold substrates, respectively. For 7-kJmol argon scattering, the two surfaces behave equivalently, fully thermalizing all impinging argon atoms. In contrast, these self-assembled monolayers (SAMs) are not equally efficient at absorbing the excess translational energy from high-energy, 35 and 80 kJmol, argon collisions. When high-energy argon atoms are scattered from a SAM on silver, the fraction of atoms that reach thermal equilibrium with the surface and the average energy transferred to the surface are lower than for analogous SAMs on gold. In the case of argon atoms with 80 kJmol of translational energy scattering from long-chain SAMs, 60% and 45% of the atoms detected have reached thermal equilibrium with the monolayers on gold and silver surfaces, respectively. The differences in the scattering characteristics are attributed to excitation efficiencies of different types of surface modes. The high packing density of alkyl chains on silver restricts certain low-energy degrees of freedom from absorbing energy as efficiently as the lower-density monolayers. In addition, molecular-dynamics simulations reveal that the extent to which argon penetrates into the monolayer is related to packing density. For argon atoms with 80-kJmol incident energy, we find 16% and 7% of the atoms penetrate below the terminal methyl groups of C(10) SAMs on gold and silver, respectively.

Energy transfer in rare gas collisions with hydroxyl- and methyl-terminated self-assembled monolayers

Gas–surface scattering experiments are used to probe the dynamics of energy transfer and thermalization in collisions of 80 kJ/mol Ar with self-assembled monolayers composed of SH–(CH2)11–CH3 and SH–(CH2)11–OH alkanethiols on gold. We find that the extent of thermalization and the fractional energy transferred to the surface during the collision are significantly greater for the CH3-terminated surface than for the OH-terminated surface. Since the two monolayers are similar in structure, packing density, and mass, the differences in scattering dynamics are likely due to a combination of factors that may include differences in the available energy modes between the two terminal groups and the hydrogen-bonding nature of the OH-terminated alkanethiol self-assembled monolayer.

Classical trajectory study of collisions of Ar with alkanethiolate self-assembled monolayers: potential-energy surface effects on dynamics.

We have investigated collisions between Ar and alkanethiolate self-assembled monolayers (SAMs) using classical trajectory calculations with several potential-energy surfaces. The legitimacy of the potential-energy surfaces is established through comparison with molecular-beam data and ab initio calculations. Potential-energy surfaces used in previous work overestimate the binding of Ar to the SAM, leading to larger energy transfer than found in the experiments. New calculations, based on empirical force fields that better reproduce ab initio calculations, exhibit improved agreement with the experiments. In particular, polar-angle-dependent average energies calculated with explicit-atom potential-energy surfaces are in excellent agreement with the experiments. Polar- and azimuthal-angle-dependent product translational energies are examined to gain deeper insight into the dynamics of Ar+SAM collisions.

Ultrathin chitin films for nanocomposites and biosensors.

Chitin is the second most abundant biopolymer and insight into its natural synthesis, enzymatic degradation, and chemical interactions with other biopolymers is important for bioengineering with this renewable resource. This work is the first report of smooth, homogeneous, ultrathin chitin films, opening the door to surface studies of binding interactions, adsorption kinetics, and enzymatic degradation. The chitin films were formed by spincoating trimethylsilyl chitin onto gold or silica substrates, followed by regeneration to a chitin film. Infrared and X-ray photoelectron spectroscopy, X-ray diffraction, ellipsometry, and atomic force microscopy were used to confirm the formation of smooth, homogeneous, and amorphous chitin thin films. Quartz crystal microbalance with dissipation monitoring (QCM-D) solvent exchange experiments showed these films swelled with 49% water by mass. The utility of these chitin films as biosensors was evident from QCM-D and surface plasmon resonance studies that revealed the adsorption of a bovine serum albumin monolayer.

Surface plasmon resonance studies of pullulan and pullulan cinnamate adsorption onto cellulose.

Surface plasmon resonance studies showed pullulan cinnamates (PCs) with varying degrees of substitution (DS) adsorbed onto regenerated cellulose surfaces from aqueous solutions below their critical aggregation concentrations. Results on cellulose were compared to PC adsorption onto hydrophilic and hydrophobic self-assembled thiol monolayers (SAMs) on gold to probe how different interactions affected PC adsorption. PC adsorbed onto methyl-terminated SAMs (SAM-CH(3)) > cellulose > hydroxyl-terminated SAMs (SAM-OH) for high DS and increased with DS for each surface. Data for PC adsorption onto cellulose and SAM-OH surfaces were effectively fit by Langmuir isotherms; however, Freundlich isotherms were required to fit PC adsorption isotherms for SAM-CH(3) surfaces. Atomic force microscopy images from the solid/liquid interfaces revealed PC coatings were uniform with surface roughnesses <2 nm for all surfaces. This study revealed hydrogen bonding alone could not explain PC adsorption onto cellulose and hydrophobic modification of water-soluble polysaccharides was a facile strategy for their conversion into surface modifying agents.

The effects of solution structure on the surface conformation and orientation of a cysteine-terminated antimicrobial peptide cecropin P1

The surface structure of an antimicrobial peptide, cecropin P1, immobilized to a gold surface via a terminal cysteine residue was investigated. Using reflection-absorption infrared spectroscopy, surface plasmon resonance, and X-ray photoelectron spectroscopy, the effects of pH, solution conformation, and concentration on the immobilized peptide conformation, average orientation, and surface density were determined. Under all conditions investigated, the immobilized peptides were alpha-helical in a predominately flat, random orientation. The addition of the reducing agent Tris(2-carboxyethyl) phosphine hydrochloride to the buffer resulted in a twofold increase in immobilized peptide surface density.

Thiolated Dendrimers as Multi-Point Binding Headgroups for DNA Immobilization on Gold

The synthesis of multithiolated DNA molecules that can be used to produce self-assembled monolayers of single-stranded DNA oligonucleotides on gold substrates is described. Generation 3 polyamidoamine (PAMAM) dendrimers were conjugated to DNA oligomers and functionalized with ~30 protected thiol groups. The protected thiol groups-thioacetate groups-allowed the dendrimer-DNA constructs to be stored in a buffer solution for at least 2 months before deprotection without any observable decrease in their ability to assemble into functional layers. The monolayers formed using these multithiolated DNA probe strands demonstrate target capture efficiencies comparable to those of analogous monolayers assembled with DNA functionalized with single thiol groups. A functional advantage of using dendrimer headgroups is the resistance to probe strand loss in prolonged exposure to buffer solutions at a high temperature (95 °C).

In Situ Probes of Capture and Decomposition of Chemical Warfare Agent Simulants by Zr-Based Metal Organic Frameworks

Zr-based metal organic frameworks (MOFs) have been recently shown to be among the fastest catalysts of nerve-agent hydrolysis in solution. We report a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808, and NU-1000 using synchrotron-based X-ray powder diffraction, X-ray absorption, and infrared spectroscopy, which reveals key aspects of the reaction mechanism. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr6 cores of all MOFs, which ultimately leads to decomposition to phosphonate products. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities in rational design of new and superior decontamination materials.

Gas-surface energy exchange and thermal accommodation of CO2 and Ar in collisions with methyl, hydroxyl, and perfluorinated self-assembled monolayers

Molecular beams of CO(2) and Ar were scattered from long-chain methyl (CH(3)-), hydroxyl (OH-), and perfluoro ((CF(2))(7)CF(3)-) functionalized alkanethiol self-assembled monolayers (SAMs) on gold to study the dynamics of energy exchange and thermal accommodation on model organic surfaces. Ar collisions, for incident energies ranging from 25 to 150 kJ mol(-1), exhibit final energy distributions that depend significantly on the terminal functional group of the SAM. The long-chain CH(3)-terminated monolayers serve as an excellent energy sink for dissipating the incident translational energy. For example, at 150 kJ mol(-1), greater than 90% of the collision energy is transferred to the CH(3)-SAM surface for specularly-scattered atoms (θ(i) = θ(f) = 30° from normal). However, the OH-SAM is a more rigid collision partner due to the formation of an intra-monolayer hydrogen bonding network and the (CF(2))(7)CF(3)-SAM (F-SAM) provides a high degree of rigidity due to the massive CF(3) groups. The final energies for the triatomic, CO(2), scattering from the three surfaces are remarkably similar to the results for Ar scattering. The only significant difference in the translational energy transfer dynamics for these two gases appears in collisions with the OH-SAM. Strong gas-surface attractive forces between CO(2) and the OH-SAM surface appear to counter the rigidity of the hydrogen-bonding network to help bring the majority of the molecules to thermal equilibrium at all incident energies up to 150 kJ mol(-1), resulting in increased energy transfer in comparison to Ar. The similarities in energy transfer for Ar and CO(2) final energy distributions in scattering from the CH(3)- and F-SAMs suggest that the internal degrees of freedom in the triatomic play only a small role in determining the outcome of the gas-surface collision under the scattering conditions employed in this work.