Available online at www.sciencedirect.com
Procedia Engineering 47 (2012) 1370 – 1373
www.elsevier.com/locate/procedia
Proc. Eurosensors XXVI, September 9-12, 2012, Kraków, Poland
Detection of explosives based on the work function read-out of Molecularly Imprinted Polymers R. Pohlea*, P. Jeanty, S. Stegmeiera, J. Hürttlenb, M. Fleischeraa a
Siemens AG, Corporate Technology, Otto-Hahn-Ring , Munich, Germany b Fraunhofer Institute for Chemical Technology ICT, Pfinztal, Germany
Abstract The feasibility of explosive detection by means of work function readout of Molecularly Imprinted Polymer (MIPs) layers is investigated for the first time. Work function changes of TNT-imprinted MIP layers (TNT-MIP) have been studied using the Kelvin Probe technique. The response to TNT and different interfering substances has been recorded. The TNT concentrations have been verified by means of on-line IMR-MS analysis and subsequent GC-MS analysis. For TNT-MIP, a significant response to TNT concentrations < 50 ppb and an increased selectivity to TNT compared to non imprinted polymer were obtained. © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense © 2012 Published by Elsevier Ltd. Sp. z.o.o. Open access under CC BY-NC-ND license.
Keywords: Explosive detection; molecular imprinted polymer; Kelvin Probe; TNT
1. Introduction Detection of vapor traces of explosives in the atmosphere is a potentially powerful method to reveal the presence of explosive devices and land mines. One of the challenges in trace explosive detection is the required sensitivity in the ppb range due to the low vapor pressures of most explosive substances during concurrent presence of interfering substances at comparably high concentrations. State of the art approaches are mainly using preconcentration methods in order to reach sufficient sensitivity and gas chromatography techniques to establish adequate selectivity. Numerous detection techniques have been developed that are capable of detecting explosive devices [1], but their common limitations are high cost,
* Corresponding author. Tel.: +49 89 636-48934 ; fax: +49 89 636-46881 . E-mail address:
[email protected].
1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o. Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2012.09.411
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rather large sizes and weights and limit reliability. Therefore, there is a need for miniaturized, portable and reliable trace explosive detectors. One of the most commonly used high explosives is 2,4,6-trinitrotoluene (TNT), which not only represents a security threat, but is also of environmental concern because of soil and water contamination. Therefore, TNT is one suitable target analyte for chemical sensing devices. Selective detection of TNT without pre-separation is already demonstrated using mass sensitive readout of molecularly imprinted polymers (MIPs) [2]. Nevertheless, the sensitivity which can be expected from mass sensitive readout is limited due to the low concentrations of TNT in the lower ppb range. The approach followed in this work is the work function readout of MIP for TNT detection. The readout of gas induced work function changes via hybrid suspended gate field effect devices (HSGFET) is accepted as a promising technique for the realization of a versatile, low-cost sensor platform since several years [3]. The freedom in choice of sensing materials is due to the fact that for the hybrid setup of the HSGFET sensing materials can be used, which are produced in independent technological steps not compatible to CMOS standards. Numerous sensing layers for a variety of gases have been developed which use different physical and chemical effects creating a signal in work function [3]. The quantitative measurement of NO2 by work function readout using suspended gate field effect devices was recently demonstrated and validates the ability of this technology for trace gas detection [4]. 2. Experimental TNT-MIP films based on methacrylamide (MAAM) as monomer and ethylene glycol dimethacrylate (EGDMA) as cross linker were prepared on the sputtered gold electrodes. Kelvin probe measurements were performed in a constant gas stream of synthetic air and dry nitrogen in a climate chamber at 50°C. The test setup is schematically depicted in fig. 1. TNT vapor has been obtained streaming N2 through a TNT cell filled with TNT coated glass balls. Up to three Kelvin Probes can be investigated in parallel. Resulting TNT concentrations have been analyzed by gas sampling on TENAX tubes and GC-MS analysis. Transient concentration changes have been monitored using IMR-MS spectroscopy. Additional test gases used for the investigation of cross sensitivities can be applied by a separate gas input.
additional gas input
bubbler furnace N2 TNT cells
KELVIN PROBES 1
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IMRMS
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N2 + TNT
TENAX TUBE
Fig. 1. schematic depiction of the test setup with 3 Kelvin Probes and TNT cells placed in a climate chamber.
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3. Results Fig. 2 shows the transient work function change of a TNT-MIP sample during exposure to ~ 50ppb TNT. After a delay time of several minutes, which is related to the transport of TNT vapor from the TNT cells to the Kelvin Probe, a clear response of the MIP sample to TNT exposure is observed. The response time observed here may also be dominated by transport phenomena of the TNT vapour into the measurement cell. 0.150 delay time
50ppb TNT exposure
0.145
') (V)
response time
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')
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Kelvin measurement with TNT imprinted MIP
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IMR-MS TNT response
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TNT cell flow state 5415
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Fig. 2. Kelvin Probe measurement: transient response of a TNT-MIP sample exposure to 50ppb TNT vapor.
The work function of a TNT-MIP sample exhibited a significantly stronger, concentration dependent response compared to a non imprinted sample (fig. 3a). Since a noise level in the range of 1mV has been achieved in HSGFET sensor devices [3], a detection limit in the range of a view 10ppb for TNT vapor can be estimated. The response time to TNT traces observed in the described test setup is in the range of several minutes (fig. 3b).
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Response time of TNT imprinted and non imprinted MIP Kelvin Probe to TNT exposure
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Response of TNT imprinted and non imprinted MIP Kelvin Probe to TNT
response time (min)
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10 8 6 4
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0 0 ppb
