Alex Robinson

Ultrasensitive Humidity Detection Using Metal–Organic Framework-Coated Microsensors

The use of metal-organic framework (MOF) thin films to detect water vapor across a wide concentration range is demonstrated using MOF-functionalized quartz surface acoustic wave (SAW) sensors. A range of 3-14,800 ppmv was obtained with thin films of the MOF Cu(3)(benzenetricarboxylate)(2) (Cu-BTC) deposited by an automated layer-by-layer method. Devices coated by a manual technique demonstrated sensitivity from 0.28 to 14,800 ppmv, the limit of our test system. This exceeds the sensitivity of many commercially available sensors. Cu-BTC layers were covalently bonded directly to the silicon oxide surface, allowing devices to be heated beyond 100 °C to desorb water adsorbed in the pores without decomposition, thereby regenerating the sensors. Sensor response as a function of coating thickness was evaluated, showing that the SAW sensor response is bounded by maximum and minimum layer thicknesses. Computer simulation of H(2)O uptake shows a multistep adsorption isotherm defined by initial adsorption at open Cu-sites, followed by pore-filling and finally full saturation. Modeling and experimental results are consistent. Calculated uptake values suggest an efficient adsorption of H(2)O by Cu-BTC. These results provide the first convincing evidence that MOF functionalization of compact sensing technologies such as SAW devices and microcantilevers can compete with state-of-the art devices.

An Embeddable SOI Radiation Sensor

The feasibility of developing an embeddable silicon-on-insulator (SOI) buried oxide MOS dosimeter (RadFET) has been demonstrated. This dosimeter takes advantage of the inherent properties for radiation-induced charge buildup in the buried oxides of commercial SOI wafers. Discrete SOI buried oxide RadFETs and fully-functional read-out circuitry have been fabricated in Sandias CMOS7 radiation-hardened SOI technology. Discrete RadFETs have been irradiated under various radiation conditions and subjected to post-irradiation anneals. Data show only a small dose rate dependence and less than a 10% annealing or fade of the dosimeters output characteristics when irradiated with all pins shorted. These results show less fade than dual-dielectric RadFETs irradiated under the same bias conditions and support the use of SOI buried oxide RadFETs for low dose rate applications. Read-out circuitry has also been designed and fabricated to monitor changes in the ¿off¿ state leakage current induced by radiation-induced charge buildup in the buried oxide dosimeter. The analog-to-digital output from the read-out circuit changes linearly with the ¿off¿ state leakage current. Preliminary radiation characterizations of the read-out circuitry show no spurious effects of radiation-induced charge buildup in the read-out circuitry on the dosimeter output. These results indicate it is feasible to develop an embeddable SOI buried oxide RadFET as an attractive choice for many low power, low dose rate applications requiring real-time knowledge of total ionizing dose radiation levels.

Two-Dimensional Modeling and Simulation of Mass Transport in Microfabricated Preconcentrators

The adsorption and desorption behavior of a planar microfabricated preconcentrator (PC) has been modeled and simulated using the computational fluid dynamics (CFD) package CFDRC-ACE+trade. By comparison with the results of a designed experiment, model parameters were determined. Assuming a first-order reaction for the adsorption of a light hydrocarbon chemical analyte onto the PC adsorbent and a unity-value sticking coefficient, a rate constant of 36 500 s-1 was obtained. This compares favorably with the value of 25 300 s-1 obtained by application of the Modified-Wheeler equation. The modeled rate constant depends on the concentration of adsorbent sites, estimated to be 6.94 ldr 10-8 kmol/m2 for the Carboxen 1000 adsorbent used. Using the integral method, desorption was found to be first order with an Arrhenius temperature dependence and an activation energy of 30.1 kj/mol. Validation of this model is reported herein, including the use of Aris-Taylor dispersion to predict the influence of fluidics surrounding the PC. A maximum in desorption peak area with flow rate, predicted from a quadratic fit to the results of the designed experiment, was not observed in the 2-D simulation. Either approximations in the simulated model or the nonphysical nature of the quadratic fit are responsible. Despite the apparent simplicity of the model, the simulation is internally self consistent and capable of predicting performance of new device designs. To apply the method to other analytes and other adsorbent materials, only a limited number of comparisons to experiment are required to obtain the necessary rate constants.

Investigation of microcantilever array with ordered nanoporous coatings for selective chemical detection

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.

Micro-Flame Ionization Detection Using a Catalytic Micro-combuster

A microflame-based detector has been developed for sensing a broad range of chemical analytes. This detector combines calorimetry and flame ionization detection (FID) to produce unique analyte signatures. The microcombustor consists of a micromachined microhotplate with a catalyst on its surface, such as platinum/alumina, to rapidly initiate the ionization event. The low power microcombustor design permits quick, efficient heating of the deposited film. To perform calorimetric detection of analytes, the change in power required to maintain the resistive microhotplate heater at a constant temperature is measured. For FID, electrodes are placed around the microcombustor flame zone with an electrometer circuit measuring the production of ions. The calorimetric and FID modes respond generally to all hydrocarbons. Importantly these detection modes can be established on one convenient simultaneous microcombustor platform. The performance of the microFID mode is emphasized herein

Micro-analytical systems for national security applications

Sandia National Laboratories has a long tradition of technology development for national security applications. In recent years, significant effort has been focused on micro-analytical systems - handheld, miniature, or portable instruments built around microfabricated components. Many of these systems include microsensor concepts and target detection and analysis of chemical and biological agents. The ultimate development goal for these instruments is to produce fully integrated sensored microsystems. Described here are a few new components and systems being explored: (1) A new microcalibrator chip, consisting of a thermally labile solid matrix on an array of suspended-membrane microhotplates, that when actuated delivers controlled quantities of chemical vapors. (2) New chemical vapor detectors, based on a suspended-membrane micro-hotplate design, which are amenable to array configurations. (3) Micron-scale cylindrical ion traps, fabricated using a molded tungsten process, which form the critical elements for a micro-mass analyzer. (4) Monolithically integrated micro-chemical analysis systems fabricated in silicon that incorporate chemical preconcentrators, gas chromatography columns, detector arrays, and MEMS valves.

Real-time discriminatory sensors for water contamination events :LDRD 52595 final report.

The gas-phase {mu}ChemLab{trademark} developed by Sandia can detect volatile organics and semi-volatiles organics via gas phase sampling . The goal of this three year Laboratory Directed Research and Development (LDRD) project was to adapt the components and concepts used by the {mu}ChemLab{trademark} system towards the analysis of water-borne chemicals of current concern. In essence, interfacing the gas-phase {mu}ChemLab{trademark} with water to bring the significant prior investment of Sandia and the advantages of microfabrication and portable analysis to a whole new world of important analytes. These include both chemical weapons agents and their hydrolysis products and disinfection by-products such as Trihalomethanes (THMs) and haloacetic acids (HAAs). THMs and HAAs are currently regulated by EPA due to health issues, yet water utilities do not have rapid on-site methods of detection that would allow them to adjust their processes quickly; protecting consumers, meeting water quality standards, and obeying regulations more easily and with greater confidence. This report documents the results, unique hardware and devices, and methods designed during the project toward the goal stated above. It also presents and discusses the portable field system to measure THMs developed in the course of this project.

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

Fiber-optic current sensors based on polarization coherence and power scattering in magneto-optical films

Fiber-optic sensors for sensing electrical current are attractive due to their inherent immunity to electromagnetic interference. Several groups have shown the use of Faraday rotation in magneto-optical materials as a function of current-induced magnetic field. In this work, fiber-optic sensors based on different mechanisms such as magnetic-fielddependent polarization coherence and power scattering effects in magneto-optical materials are demonstrated. These novel sensor configurations can have advantages in that they exhibit power-independent or polarization-independent operation which can ultimately lead to fewer components and relaxed light source requirements compared to fiber-optic current sensor systems based on Faraday rotation.

Microfabricated Chip for Calibration of Field Instruments

Accurate, periodic calibration is required to operate IMS, GC, and portal security systems with maximum efficiency, surety, and operator confidence. To this end, we are presently developing a microfabricated device for on-demand calibration of fieldable contraband detection instruments. Using robotically-assisted picoliter dispensing methods, precise nanogram or larger amounts of calibration compound(s) are placed on micron-sized bridge structures. Resistively heating these structures delivers precisely quantified low-levels of calibration compounds to the detector systems. This provides reliable calibration for identification and quantification of explosives, narcotics, and other contraband.