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On-Chip Monolithic Integrated Multimode Carbon Nanotube Sensor for a Gas Chromatography Detector

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Abstract

Carbon nanotube (CNT)-based chemiresistors are promising gas detectors for gas chromatography (GC) due to their intrinsic nanoscale porosity and excellent electrical conductivity. However, fabrication reproducibility, long desorption time, limited sensitivity, and low dynamic range limit their usage in real applications. This paper reports a novel on-chip monolithic integrated multimode CNT sensor, where a micro-electro-mechanical system-based bulk acoustic wave (BAW) resonator is embedded underneath a CNT chemiresistor. The device fabrication repeatability was improved by on-site monitoring of CNT deposition using BAW. We found that the acoustic stimulation can accelerate the gas desorption rate from the CNT surface, which solves the slow desorption issue. Due to the different sensing mechanisms, the multimode CNT sensor provides complementary responses to targets with improved sensitivity and dynamic range compared to a single mode detector. A prototype of a chromatographic system using the multimode CNT sensor was prepared by dedicated design of the connection between the device and the separation column. Such a GC system is used for the quantitative identification of a gas mixture at different GC conditions, which proves the feasibility of the multimode CNT detector for chromatographic analysis. The as-developed CMOS compatible multimode CNT sensor offers high sensing performance, miniaturized size, and low power consumption, which are critical for developing portable GC.

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The nerve agents Sarin (GB) and Soman (GD) were detected at the picogram level with the use of a field-portable gas chromatograph with a surface-acoustic-wave detector (SAW). The instrument, although primarily a vapor detector, can be modified with a simple front-end thermostat attachment, which allows for small-volume liquid injections for the detection of analytes in the vapor-pressure range of 0.1–5.0 torr. A field analytical method was developed with the use of the analyte surrogates dimethyl methyl phosphonate (DMMP) and triethyl phosphate (TEP). These compounds were used as internal standards by which agents GB and GD can be detected, with the use of this liquid injection method coupled to the vapor detector. Actual tests on GB and GD were performed under the same conditions, but in a more closely monitored but less field-representative laboratory environment. GB was quantitated from a calibration plot that relates signal to mass via a conversion factor in less than 10 s. The turnaround time for the instrument between data sets was 2 min, depending on the exact parameters of the method. ©1999 John Wiley & Sons, Inc. Field Analyt Chem Technol 3:45–53, 1999
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