Anandram Venkatasubramanian

Direct synthesis of single-walled aminoaluminosilicate nanotubes with enhanced molecular adsorption selectivity

Internal functionalization of single-walled nanotubes is an attractive, yet difficult challenge in nanotube materials chemistry. Here we report single-walled metal oxide nanotubes with covalently bonded primary amine moieties on their inner wall, synthesized through a one-step approach. Conclusive molecular-level structural information on the amine-functionalized nanotubes is obtained through multiple solid-state techniques. The amine-functionalized nanotubes maintain a high carbon dioxide adsorption capacity while significantly suppressing the adsorption of methane and nitrogen, thereby leading to a large enhancement in adsorption selectivity over unfunctionalized nanotubes (up to four-fold for carbon dioxide/methane and ten-fold for carbon dioxide/nitrogen). The successful synthesis of single-walled nanotubes with functional, covalently-bound organic moieties may open up possibilities for new nanotube-based applications that are currently inaccessible to carbon nanotubes and other related materials.

Characterization of HKUST-1 Crystals and Their Application to MEMS Microcantilever Array Sensors

Metal organic frameworks (MOFs) are a new class of crystalline nanoporous materials with salient features such as tailorable nanoporosity, high surface area and analyte specific adsorption. Greater part of the research till now has focused on determining the adsorption isotherms of MOFs in a bid to use them in gas separation and chemical sensor applications. However there is modest data available on the thermodynamic properties of MOFs which are essential for these applications. In this paper, thermodynamic characterization of the well known MOF, HKUST-1 has been conducted using Quartz Crystal Microbalance (QCM). MOF coated stress induced microcantilever sensors have been successfully demonstrated for sensor application. HKUST-1 has the structure of formula Cu3(BTC)2(H2O) comprises a binuclear Cu2 paddlewheel [1]. Its structure consists of two types of “cages” and two types of “windows” separating these cages. Large cages (13.2 and 11.1 A° in diameter) are interconnected by 9 A° windows of square cross section. The large cages are also connected to tetrahedral shaped side pockets of roughly 6 A° through triangular shaped windows of about 4.6 A° (3.5 A° in the hydrated form). The thermodynamic properties of HKUST-1 such as heat of adsorption, adsorption capacity, adsorption affinity constant, diffusion coefficient and diffusion activation energy are determined using the QCM technique. The shift in resonant frequency of the crystal was used to calculate the adsorption isotherms from which the thermodynamic properties were determined. The resonant frequency shift was expressed in terms of mass change due to analyte adsorption using the saurbrey’s equation. In this paper, the MOFs were deposited on the QCM by dropcasting a suspension of the MOF crystals in aprotic solvent like acetone and characterization was carried out for CO2 and other gases. The experimental set up of the QCM based measurement cell is illustrated in Figure 1. Prior to the measurement, the sample was heated to 170°C under vacuum in the cell to remove moisture from the pores. N-doped piezoresistive microcantilever array sensors were fabricated using microfabrication technologies with dimensions 230 µm in length and 100 µm in width [2]. The response of the sensor for H2O and CO2 was measured in a custom designed gas test cell. Dry nitrogen was used as carrier gas and H2O was regulated using hydrator. The stress response was obtained by the subtraction of the uncoated reference microcantilever response from the MOF-coated cantilever response using a Wheatstone bridge. The microcantilever array chip was mounted in a stereolithography package for testing. Figure 2 shows typical response to CO2. The substantial resistance changes with MOFs exhibited a strong and completely reversible response for adsorption and desorption.

Selective stress-based microcantilever sensors for enhanced surveillance.

Assessment of component aging and degradation in weapon systems remains a considerable challenge for the Integrated Stockpile Evaluation program. Analysis of weapon atmospheres can provide degradation signatures and indicate the presence of corrosive vapors. However, a critical need exists for compatible in-situ sensors to detect moisture and other gases over stockpile lifetimes. This inhibits development of both “self-aware weapons” and fully instrumented weapon test platforms that could provide in-situ data to validate high-fidelity models for gases within weapons. We developed platforms for on-demand weapon atmosphere surveillance based on static microcantilevers (SMC) and surface accoustic wave (SAW) devices coated with nanoporous metal organic frameworks (MOFs) to provide selectivity. SMC detect analytes via adsorbate-induced stress and are up to 100X more sensitive than resonant