Publications

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In this paper we demonstrate the potential for novel nanoporous framework materials (NFM) such as metal-organic frameworks (MOFs) to provide selectivity and sensitivity to a broad range of analytes including explosives, nerve agents, and volatile organic compounds (VOCs). NFM are highly ordered, crystalline materials with considerable synthetic flexibility resulting from the presence of both organic and inorganic components within their structure. Detection of chemical weapons of mass destruction (CWMD), explosives, toxic industrial chemicals (TICs), and volatile organic compounds (VOCs) using micro-electro-mechanical-systems (MEMS) devices, such as microcantilevers and surface acoustic wave sensors, requires the use of recognition layers to impart selectivity. Traditional organic polymers are dense, impeding analyte uptake and slowing sensor response. The nanoporosity and ultrahigh surface areas of NFM enhance transport into and out of the NFM layer, improving response times, and their ordered structure enables structural tuning to impart selectivity. Here we describe experiments and modeling aimed at creating NFM layers tailored to the detection of water vapor, explosives, CWMD, and VOCs, and their integration with the surfaces of MEMS devices. Force field models show that a high degree of chemical selectivity is feasible. For example, using a suite of MOFs it should be possible to select for explosives vs. CWMD, VM vs. GA (nerve agents), and anthracene vs. naphthalene (VOCs). We will also demonstrate the integration of various NFM with the surfaces of MEMS devices and describe new synthetic methods developed to improve the quality of VFM coatings. Finally, MOF-coated MEMS devices show how temperature changes can be tuned to improve response times, selectivity, and sensitivity. © 2010 Copyright SPIE - The International Society for Optical Engineering.

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The behavior of water confined in porous materials influences macroscopic phenomena such as solute and water mobility, ion exchange, and adsorption. While the properties of bulk water are generally understood, that of nanoconfined water remains an active area of research. We used molecular simulation and inelastic neutron scattering (INS) to investigate the effect of local structure on the vibrational behavior of nanoconfined water. We focus specifically on the nanosized pores found in the 2:1 phyllosilicate clay minerals palygorskite and sepiolite. These are charge neutral, Mg-rich trioctahedral clays with idealized formulas Mg{sub 5}Si{sub 8}O{sub 20} (OH){sub 2} {center_dot} 8H{sub 2}O and Mg{sub 8}Si{sub 12}O{sub 30} (OH){sub 2} {center_dot} 12H{sub 2}O for palygorskite and sepiolite, respectively. The regular pattern of inverted phyllosilicate layers results in narrow channels with effective van der Waals dimensions of 3.61 {angstrom} x 8.59 {angstrom} (palygorskite) and 4.67 {angstrom} x 12.29 {angstrom} (sepiolite). These clay minerals represent a unique opportunity to study water adsorbed at 'inner edge' sites of uncoordinated Mg{sup 2+}. INS spectra taken at 90 K reveal a large shift in the water librational edge between palygorskite (358 cm{sup -1}) and sepiolite (536 cm{sup -1}), indicating less restricted water motion in the smaller-pore palygorskite. The librational edge of the reference sample (ice I{sub h}) is similar to sepiolite, which confirms the unique water behavior in palygorskite. We used both classical molecular dynamics (CMD) simulations and more rigorous density functional theory (DFT) calculations to investigate the hydrogen bonding environment and vibrational behavior of structural water, defined as those water molecules coordinated to Mg{sup 2+} along the pore walls. These waters remain coordinated throughout the 1-ns timescale of the CMD simulations, and the resulting vibrational spectra indicate a similar shift in the water librational edges seen in the INS spectra. The DFT-optimized structures indicate differences in hydrogen bonding between palygorskite and sepiolite, which could explain the librational shift. Corner-sharing silicate tetrahedra in palygorskite are tilted with respect to the crystallographic a-axis due to the induced strain of layer inversion. As a result, only two short (1.9 {angstrom}) hydrogen bonds form between each water and the framework. In contrast, the relatively unstrained sepiolite structure, each water forms three hydrogen bonds with the framework, and at greater distances (2.0 {angstrom} - 2.5 {angstrom}).

Grand canonical Monte Carlo simulations were performed to investigate trends in low-pressure adsorption of a broad range of organic molecules by a set of metal-organic frameworks (MOFs). The organic analytes considered here are relevant to applications in chemical detection: small aromatics (o-, m-, and p-xylene), polycyclic aromatic hydrocarbons (naphthalene, anthracene, phenanthrene), explosives (TNT and RDX), and chemical warfare agents (GA and VM). The framework materials included several Zn-MOFs (IRMOFs 1-3, 7, 8), a Cr-MOF (CrMIL-53lp), and a Cu-MOF (HKUST-1). Many of the larger organics were significantly adsorbed by the target MOFs at low pressure, which is consistent with the exceptionally high isosteric heats of adsorption (25 kcal/mol - 60 kcal/mol) for this range of analyte. At a higher loading pressure of 101 kPa, the Zn-MOFs show a much higher volumetric uptake than either CrMIL-53-lp or HKUST-1 for all types of analyte. Within the Zn-MOF series, analyte loading is proportional to free volume, and loading decreases with increasing analyte size due to molecular packing effects. CrMIL-53lp showed the highest adsorption energy for all analytes, suggesting that this material may be suitable for low-level detection of organics.

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Inelastic neutron scattering, density functional theory, ab initio molecular dynamics, and classical molecular dynamics were used to examine the behavior of nanoconfined water in palygorskite and sepiolite. These complementary methods provide a strong basis to illustrate and correlate the significant differences observed in the spectroscopic signatures of water in two unique clay minerals. Distortions of silicate tetrahedra in the smaller-pore palygorskite exhibit a limited number of hydrogen bonds having relatively short bond lengths. In contrast, without the distorted silicate tetrahedra, an increased number of hydrogen bonds are observed in the larger-pore sepiolite with corresponding longer bond distances. Because there is more hydrogen bonding at the pore interface in sepiolite than in palygorskite, we expect librational modes to have higher overall frequencies (i.e., more restricted rotational motions); experimental neutron scattering data clearly illustrates this shift in spectroscopic signatures. Distortions of the silicate tetrahedra in these minerals effectively disrupts hydrogen bonding patterns at the silicate-water interface, and this has a greater impact on the dynamical behavior of nanoconfined water than the actual size of the pore or the presence of coordinatively-unsaturated magnesium edge sites.

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Inelastic neutron scattering, density functional theory, ab initio molecular dynamics, and classical molecular dynamics were used to examine the behavior of nanoconfined water in palygorskite and sepiolite. These complementary methods provide a strong basis to illustrate and correlate the significant differences observed in the spectroscopic signatures of water in two unique clay minerals. Distortions of silicate tetrahedra in the smaller-pore palygorskite exhibit a limited number of hydrogen bonds having relatively short bond lengths. However, without the distorted silicate tetrahedra, an increased number of hydrogen bonds are observed in the larger-pore sepiolite with corresponding longer bond distances. Because there is more hydrogen bonding at the pore interface in sepiolite than in palygorskite, we expect librational modes to have higher overall frequencies (i.e., more restricted rotational motions); experimental neutron scattering data clearly illustrates this shift in spectroscopic signatures. It follows that distortions of the silicate tetrahedra in these minerals effectively disrupt hydrogen-bonding patterns at the silicate?water interface, and this has a greater impact on the dynamical behavior of nanoconfined water than the actual size of the pore or the presence of coordinatively unsaturated magnesium edge sites.