ENHANCED TOXIC GAS DETECTION USING A MEMS PRECONCENTRATOR COATED WITH THE METAL ORGANIC FRAMEWORK ABSORBER
J. Yeom1, I. Oh2, C. Field3, A. Radadia2, Z. Ni4, B. Bae1, J. Han1, R.I. Masel2,4, M.A. Shannon1,4* 1 Department of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois, USA 2 Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana, Illinois, USA 3 Department of Chemistry, University of Illinois, Urbana, Illinois, USA 4 Cbana Inc., Urbana, Illinois, USA *
[email protected] ABSTRACT Widespread and timely sensing of explosives, toxic chemicals and industrial compounds needs fast, sensitive detection technology that is affordable and portable. Many gas detectors developed for portable applications are based on sensing a change in resistivity or other non-selective material properties, often leading to low sensitivity and selectivity. In this paper, we report a new generation of the UIUC MEMS gas preconcentrator (µGPC) and its integration into a microfluidic M-8 (µM8) detector to demonstrate an enhanced overall detection limit and selectivity in detecting a toxic gas simulant. The integration creates a portable sensor to sniff an analyte of interest at concentration of 10 ppb or below.
Tian et al. developed a thick Si microheater and packed it with porous carbon particles [5, 7]. While their pack-bed PC assumed a very large SA/V and thus high preconcentrator factor, its tortuous fluidic path and large thermal mass led to a higher pressure drop and a slower desorption rate. To address these issues, we developed a micropost-filled preconcentrator that has a high SA/V but experiences a low pressure drop [8] and demonstrated a preconcentration gain of 260 [9]. (a) A-A'
Buried oxide Handle layer
Microposts-filled preconcentrator chamber
Doped Si heater
Device layer
1. INTRODUCTION A recent increase in the number of illicit activities involving hazardous materials such as organophosphate toxins [1], explosives [2], and other toxic industrial chemicals has generated a great interest in developing a gas detector that can detect these materials with high sensitivity, selectivity, and portability. Gas chromatography/mass spectroscopy (GC/MS) is typically utilized in detecting gasphase hazardous materials, but its miniaturization for portable application need more technological breakthrough. Many gas sensors such as thermal conductivity detectors (TCD) and metal-oxide semiconductor sensors have been developed for portable applications. However, the underlying mechanism of these sensors essentially relies on a change in the physical properties (thermal conductivity or electrical resistivity) upon exposure to the target vapor, which intrinsically has a low chemical selectivity. Most of the sensors for liquid-phase analytes, on the other hand, are based on the highly selective liquid-phase chemistry, which is desired in the case of hazardous materials sensors. We reported a portable electrochemical gas sensor based on oxime chemistry to detect the organophosphate toxins at concentration of 10 ppb [3]. However, a higher sensitivity of the gas sensor is often desired. Alternatively, the toxic gas detection can be enhanced with the help of a MEMS gas preconcentrator (µGPC). Several microscale PCs have previously been built as a key component of a micro gas chromatography system [4, 5] or as a front-end device of the detector [6]. Frye-Mason et al. utilized a thin SiNx microhotplate with a Ti/Pt serpentine heater as a µPC [4]. Low thermal mass associated with the microhotplate structure and a thin adsorbent layer enabled rapid heating of the system, but its low surface-area-tovolume-ratio (SA/V) produced a low preconcentrator factor.
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Figure 1. (a) A schematic diagram of the (a) cross-sectional and (b) top view of the µGPC. (c) Photographs of individual layers.
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2. EXPERIMENTAL
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Figure 2. Device fabrication sequence of the PC layer. In this paper, we report a new generation of the UIUC MEMS gas preconcentrator (µGPC) and its integration into a microfluidic M-8 detector (µM8D) to demonstrate an enhanced overall detection limit and selectivity in detecting a toxic gas simulant. The integration creates a portable sensor to sniff an analyte of interest at concentration of 10 ppb or below.
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Figure 3. Optical and SEM (inset) images of the MOFs coated on the microposts in the µGPC.
Design and Fabrication of the µGPC Figure 1a shows a schematic illustration of the cross sectional view of the PC layer and other Si dies that supply fluidic seals and via channels. A relatively thick Au layer was sputtered on heater legs to reduce their electrical resistances, concentrating power deposition at the heater site (see Fig. 1b). Air pockets are etched in the ‘via’ layer to reduce thermal mass and also provide thermal isolation. Photographs of each layer are shown in Fig. 1c. The PC layer was fabricated using traditional MEMS fabrication process from a silicon-on-insulator (SOI) wafer. A 2000 Å Au layer with a 100 Å Cr adhesion layer was deposited and patterned with a liftoff process to reduce the resistance across the heater legs (see Fig. 2a-b). Doublesided photolithography was employed to form a mask for etching of the silicon on both sides of the wafer. The device layer was etched about 40 µm deep using deep reactive ion etching to form the PC chamber filled with an array of microposts (see Fig. 2c). To prevent an applied heating current from flowing the entire device layer and concentrate only on the microheaters, the isolation trenches needed to be etched further around the heater area. The shadow mask whose size is slightly larger then the PC chamber was used to protect the chamber from getting etched in DRIE (see Fig. 2d) during etching of the isolation trench. The handle side of the wafer was subsequently etched using the photoresist (AZ 4620) being a mask until the buried oxide layer was exposed. The buried oxide layer in the electrical contact regions was selectively etched in the buffered oxide etchant. To form the ohmic contacts on four electrical pads (two for excitation and two for sensing), a 2000 Å Au layer with a 100 Å Cr adhesion layer was sputtered followed by annealing at 375°C for 10 minutes. The Via and Channel layer wafers were patterned using the standard double-sided lithography and etched in DRIE. After thorough cleaning, a 3000 Å thick thermal oxide was grown on the etched via layer and channel layer wafers for electrical isolation. A thin, thermally curable epoxy-based adhesive was used to bond a PC layer, a Via layer, and a Channel layer die to form a stack of µGPC. Metal-Organic Framework (MOF) Adsorbents Metal organic frameworks (MOFs), a new class of molecular networks, have high surface areas, tailorable polarity and pore sizes, and high thermal stability. Capitalizing these superior properties, we proposed to use them as novel adsorbents for preconcentartion of trace impurities and showed the preconcentration gain of more than 5000 for DMMP [10]. Here, we have coated IRMOF1 into an array of microposts of the PC chamber as an absorbent of a toxic gas simulant, isopropylsulfonylchloride (IPSC). Figure 3 shows the small size (3~5 µm) MOF crystals coated on an array of microposts. Principle and Fabrication of the µM8 Detector The mechanism of the reaction of keto-oxime with acid anhydride compounds, a stimulant of many toxic gases, was reported previously [11]. The oxime reacts with acid anhydride such as IPSC and produces the cyanide, which
polycarbonate microchannel oxime solution Au electrode Figure 4. A schematic diagram of the µM8 detector. The gas microchannel and the liquid microchannel are aligned to each other and are separated by a nanoporous membrane.
Test Setup The Si microheater in the µGPC was tested with a custom-built testing system as shown in Fig. 5. The National Instruments system can not only supply a fixed voltage to the heater but also measure the current drawn and the voltage drop across the heater. Measured current and voltage were recorded via LabView 8.0 software, which calculated the resistance of the heater. Employing the offline-calibrated resistance-temperature relation, the temperature of the heater could be estimated. A diluted IPSC vapor was prepared by first extracting a concentrated vapor from the head space in the bubbler and subsequently diluting it with ambient air. A syringe pump was used to deliver the diluted IPSC vapor into the µGPC and then µM8 detector. As the steady stream of IPSC was introduced into the µGPC, the organic vapor was adsorbed onto the MOF adsorbents. A single pulse of a fixed voltage was applied to the microheater in the µGPC. The duration of heating was between 2 and 5 sec and induced rapid heating of µGPC for fast desorption. A potential change of the Au electrode in the µM8 detector was monitored using a precision voltmeter (Agilent).
NI data acquisition system exhaust Si µGPC housed in Vespel fixture µM8 detector
Syringe pump with diluted IPSC
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Figure 5. Test setup for characterizing the Si microheater in the µGPC and for preconcentration of IPSC.
3. RESULTS AND DISCUSSIONS Two types of the Si microheaters, one with more thermal isolation and the other with less, are tested with a fixed applied voltage. Thermal isolation was achieved by reducing the contact area between the microheater in the PC layer and the Via layer. Figure 6 shows the thermal responses of these two micro heaters over a range of applied power. The temperature measurements were taken only after 2 sec of heating. Relatively higher magnitudes of input power were applied to the microheater compared to other microheaters in the literature, but a much faster heating rate can be achieved with a high power and doesn’t allow a significant convective heat loss to take place. For example, for an average input power of ~12 W, the microheater with more thermal isolation could reach 250°C within 0.1 seconds, only consuming ~ 1.2 J of energy. In order to verify the preconcentration of IPSC, the µGPC coated with IRMOF1 was placed on-line with the sample stream. An air stream containing 200 ppb of IPSC was fed to the µGPC and subsequently to the µM8 detector with a flow rate of 5 mL/min. After a potential signal became stabilized, a slow increase in the potential was observed as a baseline signal. A 3-sec pulse of heating was applied to the microheater in the µGPC to generate a concentrated plug of IPSC. 350
more thermal isolation
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can be detected by electrochemical potentiometry with a cyanide selective electrode. By fabricating a micro-scale gas-liquid interface on the polymer-based microfluidic platform, we also reported a miniaturized oxime-based electrochemical sensor , called microfluidic M8 (µM8) detector, and demonstrated that the µM8 exhibits the miniaturization advantages, such as faster response, reduced reagent consumption, and enhanced portability [3]. A schematic diagram of the µM8 detector is shown in Fig. 4. A microscale gas-liquid interface is created at a hydrophobic nanoporous membrane (10~50 nm pore diameter) which is sandwiched between the liquid microchannel and gas microchennel. The PDMS microchannels with widths of 500 µm and depths of 100 µm were fabricated using a SU-8 molding process. As a cyanide selective electrode, a thin gold layer was deposited on the liquid side of the nanoporous membrane. nanoporous Air + IPSC membrane
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Figure 6. Thermal responses of two different types of the microheaters in the µGPC, showing the effect of the thermal isolation.
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6. REFERENCES
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Figure 7. A potential response from the µM8 detector when concentrated IPSC is injected from the µGPC. A blue peak (broader) represents a potential response from the µM8 detector while a red peak (narrow) from the microheater. A red, narrow peak in Fig. 7 indicates a temperature profile of the microheater upon heating. The µGPC reached approximately 250°C in 3 seconds. The concentrated injection plug upon heating created a sharp increase in the electrochemical potential from the µM8 detector as seen in Fig. 7 (blue and broad peak). A signal-to-noise-ratio of 20 was estimated, suggesting that a 10 ppb of IPSC can be detected with this integration.
4. CONCLUSIONS We developed a MEMS gas preconcentrator using a SOI wafer and demonstrated a fast heating rate with low energy consumption. With an input power of 12 W, the µGPC could reach 250°C within 100 ms, consuming only 1.2 J of energy. The µGPC coated with high-surface-area metal organic framework (MOF) molecules was integrated into the microfluidic M-8 detector to enhance the overall detection limit of a trace concentration of a toxic vapor simulant. A sharp increase in the electrochemical potential from the µM8 detector was observed upon heating of the µGPC. Further study includes the quantification of a preconcentrator gain, the optimization of the MOF adsorbent coating, and a monolithic integration of the µGPC and µM8 detector.
5. ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Defense Advanced Research Projects Agency (DARPA) under U.S. Air Force grant FA8650-04-1-7121. Any opinions, findings, and conclusions or recommendations expressed in this manuscript are those of the authors and do not necessarily reflect the views of the Defense Advanced Projects Research Agency, or the U.S. Air Force.
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