Automated and remote detection of nitrogen-based explosives is becoming increasingly important for antiterrorism screening, land-mine removal, and remediation of brownfield sites. Detection of common explosives has been difficult because of their low vapor pressure at normal temperatures and the resulting low concentrations in the gas phase. For example, most sensors currently addressing trinitrotoluene (TNT) detection rely on sample preparations like swipe tests to acquire enough material. These techniques are relatively slow and difficult to automate, so that, in practice, most testing is not done systematically: only random tests are possible and human intervention is required to perform them.
In contrast, our new, highly sensitive sensor selectively but reversibly binds TNT molecules on a specially prepared surface and, therefore, only needs to be fed with an air stream to test for the analyte. The sensor measures the baseline resonance frequency of a microring resonator and the shift in resonance frequency caused by selective binding of analyte molecules on the ring surface. The chip design ensures a high degree of miniaturization and cost reduction because of wafer-scale manufacturing.
In operation, light is fed into the ring resonator by evanescent coupling with a nearby, straight waveguide that is connected to a laser diode and detector via single-mode optical fibers. Part of the light in the closed-ring waveguide couples back into the straight waveguide via evanescent coupling because the waveguides are only 1μm apart. After traversing the ring, light that couples back into the straight waveguide is phase shifted by π, leading to destructive interference with light that did not enter the ring resonator.1,2 This results in a frequency dip of the transmitted signal when the input light is tuned over a resonance frequency of the microring resonator. The resonance frequency is determined by the perimeter of the microring and its refractive index, which changes if molecules adsorb on its surface. Therefore, a change in refractive index induced by adsorption of analyte molecules can easily be measured as a shift in resonance frequency.
To selectively accumulate the analyte on the surface, it is coated with specially designed receptor molecules based on triphenylene ketals.3 These receptors selectively bind TNT molecules based on a key-lock principle. The supramolecular interaction between TNT and receptor molecules causes an intense color change and, consequently, a change in refractive index within the ring's cladding layer.
Gas-phase TNT is introduced into the sensor cell at room temperature using a flow of a room-temperature carrier gas containing an equilibrium concentration of TNT of 5.29 parts per billion (ppb) or less. The concentration can be controlled accurately by adjusting the flow of the carrier gas through the TNT source. Figure 1 shows typical transmission spectra of two coated microring resonators as a solid line. (Their resonance frequencies differ because of a slight difference in diameter.) After introduction of TNT-laden carrier gas, both resonance dips are shifted, as can be seen clearly by comparison with the broken line in the figure. The shift in resonance frequency depends linearly on the concentration of TNT. Moreover, the effect is reversible. After flushing with air, TNT desorbs and the resonance dip returns to its original value,4 which means that the sensor is reusable. The detection limit is 0.5ppb for TNT.
Figure 1. Shift of the resonance frequencies upon adsorption of trinitrotoluene (TNT).
In summary, we have demonstrated the feasibility of sensitized microring resonators as sensors for TNT in the gas phase. The chip design enables construction of sensor arrays with different receptor coatings to simultaneously detect different explosives using the same laser diode as light source. As next steps, we intend to test the sensor with other substances to further evaluate possible interference and optimize the receptor coating's thickness.
Financial support from the German Federal Ministry of Education and Research (BMBF) under contract 13N9475 is gratefully acknowledged.
Ulrike Willer, Wolfgang Schade
Clausthal University of Technology
Ulrike Willer received her diploma in physics from the Christian-Albrechts University in Kiel (Germany) and her PhD from Clausthal University of Technology, where she has been a member of the scientific staff since 2001. Her research interests include mid-IR and photoacoustic spectroscopy, evanescent-field sensing, and development of sensor devices.
Wolfgang Schade received his PhD from the Christian-Albrechts University in 1987. He is a full professor at the Institute for Energy Research and Physical Technologies and head of the Department of Fiber Optical Sensor Systems of the Fraunhofer Heinrich Hertz Institute of the University's Energy Campus.
Rozalia Orghici, Peter Lützow
Fraunhofer Heinrich Hertz Institute
Rozalia Orghici received her diploma at the Universitatea de Nord in Baia Mare (Maramures, Romania) in 2005. Her research interests are in laser and evanescent-field spectroscopy, optical-fiber sensors, nanostructures, and integrated ring resonators for sensing applications.
Peter Lützow studied physics at the Freie Universität Berlin (Germany). He pursued work for his diploma thesis at the Fritz Haber Institute of the Max Planck Society and received his diploma in 2007.
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3. M. C. Schopohl, A. Faust, D. Mirk, R. Fröhlich, O. Kataeva, S. R. Waldvogel, Synthesis of rigid receptors based on triphenylene ketals, Eur. J. Org. Chem. 2005, no. 14, pp. 2987-2999, 2005.
4. R. Orghici, P. Lützow, J. Burgmeier, J. Koch, H. Heidrich, W. Schade, N. Welschoff, S. Waldvogel, A microring resonator sensor for sensitive detection of 1,3,5-trinitrotoluene (TNT), Sensors 10, no. 7, pp. 6788-6795, 2010.