Explosives Detection and Identification Using Surface ...

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Explosives Detection and Identification Using Surface PlasmonCoupled Emission Shiou-jyh (Puck) Ja FLIR Systems, 1024 S. Innovation Way, Stillwater, OK 74074, USA ABSTRACT To fight against the explosives-related threats in defense and homeland security applications, a smarter sensing device that not only detects but differentiates multiple true threats from false positives caused by environmental interferents is essential. A new optical detection system is proposed to address these issues by using the temporal and spectroscopic information generated by the surface plasmon coupling emission (SPCE) effect. Innovative SPCE optics have been designed using Zemax software to project the fluorescence signal into clear “rainbow rings” on a CCD with subnanometer wavelength resolution. The spectroscopic change of the fluorescence signal and the time history of such changes due to the presence of a certain explosive analyte are unique and can be used to identify explosives. Thanks to high optical efficiency, reporter depositions as small as 160-μm in diameter can generate a sufficient signal, allowing a dense array of different reporters to be interrogated with wavelength multiplexing and detect a wide range of explosives. We have demonstrated detection and classification of explosives, such as TNT, NT, NM, RDX, PETN, and AN, with two sensing materials in a prototype. Keywords: Fluorescence detection, explosive detection, explosive identification, chemical detection, surface plasmon resonance, surface plasmon-coupled emission.

1. INTRODUCTION Explosive detection using fluorescence polymers has been actively pursued thanks to their extremely high sensitivity [1]. The Amplifying Fluorescence Polymer (AFP) developed by Prof. Tim Swager at MIT has shown great sensitivity to TNT [2]. Based on that polymer material, handheld detectors have demonstrated femtogram detection levels to TNT [3]. We hope to further advance these capable explosive detection platforms to not only have high sensitivity to certain explosives but also detect a wider range of explosives. With the increased number of detectable targets, the capability of identification will certainly become essential to make detection tasks practical and possible without being bogged down by ubiquitous irrelevant chemicals. In addition, fluorescence polymer deposited on a substrate in a solid phase is the common format for such sensors. It is well known that the fluorescence collection from a thin film deposited on a substrate is non-intuitive and often has low efficiency due to the fact that most of the fluorescence signal was trapped inside the substrate and the emission distribution was broad and divergent [4]. Surface plasmon-coupled emission (SPCE) has therefore been proposed as one approach to improve the fluorescence collection efficiency [5]. Since the fluorescence signal funneled out via SPCE has much higher spatial confinement, greater signal collection efficiency can be achieved. More interestingly, the wavelength-dependent emission angle coming from the material dispersion of the involved metal layer and modal selectivity enable the SPCE-based sensing platform to have spectroscopic detection capability, which may provide additional information to improve detection identification accuracy. In this article, an interesting approach to use multiple advantages of SPCE for the fluorescence-based explosives detection application will be reported. The color separation capability allows us not only to acquire changes in the entire fluorescence spectrum when the analyte molecules interact with the optical reporter but also to interrogate multiple optical reporters that have different emission bands. By aggregating multiple advanced fluorescence polymers into a sensor array, a “meta” reporter with scalable and tunable detectability may be formed, which could be interrogated simultaneously with the SPCE technique. The efficient fluorescence spectroscopic collection advantage allows us to capture transient time-domain and frequency-domain fluorescence information altogether. Test data has demonstrated that it is possible to detect and identify multiple explosives with their unique temporal-spectral signatures.

Updated 1 March 2012

In the following subsections, the principle of SPCE phenomenon will first be briefly introduced. The design of SPCE optics to capture SPCE signal is then discussed followed by the concept to improve multiple explosives detection and identification. Then, the spectral extraction approach is explained. Finally, the detection data of multiple explosives and their static and time-dependent spectral signature will be presented.

1.1 Surface Plasmon-Coupled Emission Surface plasmon wave is a phenomenon in which the free electrons in a conductor oscillate collectively. Thin films of noble metals, such as silver and gold, coated on a transparent dielectric substrate are capable of supporting such oscillation at the visible wavelength. Therefore, when an optical emitter such as a fluorescent molecule is deposited onto the metal thin film with a standoff layer on top of a planar substrate, as it is shown at Figure 1(A), most of the optical emission in the transverse magnetic (TM) mode will be first coupled into the surface plasmon wave (wavenumber ksp) and then re-radiated as a propagation wave (wavenumber k 0  2 / 0 ) into the substrate. Such re-radiation is hence focused to a specific emission angle, the SPCE angle spce, governed by the coupling equation:

k 0 n s sin  spce  k sp

(1)

where ns is the refractive index of the substrate. The SPCE angle is always greater than the critical angle of the substrate and ambient interface. Therefore, SPCE belongs to the so-called “forbidden light”, which is free from the ambient light and may have high signal-to-noise ratio (SNR)

Figure 1. (A) The SPCE geometry of an emitter on the multilayer substrate, (B) the SPCE experimental setup, and (C) the comparison of experimentally acquired SPCE maps and theoretical data.

Note that the SPCE angle depends on the wavenumber k0 and hence the wavelength of the emission, λo. Therefore, there is a convenient built-in optical dispersion characteristic in SPCE light, which provides the spectral information without additional instruments such as a spectrometer. Due to the focusing effect, fluorescence collection efficiency with the SPCE technique may be more than two orders of magnitude higher than traditional configuration. The combination of high efficiency and built-in color-dispersion nature of SPCE provides a valuable means to capture high-speed fluorescence spectral data, which is usually not available without using expensive and sophisticated instruments. With the availability of transient fluorescence spectral data, it was later indeed discovered that the transient variation of the fluorescence spectra has distinct a signature among different explosives.

In addition to the emission wavelength, the thickness of the standoff layer, which changes the effective refractive index ns, can also change the SPCE angle. This provides a means to control the spectral range and resolution to be interrogated by the down-stream optics, as it will be discussed later. Since the phase-matching condition described in Equation 1 only dictates the emission polar angle but not the azimuthal angle, the SPCE light projection on the detector will be a ring, which is also termed as “SPCE annulus” thereafter. The spectral information can be extracted by the variation along the radial direction from the inner edge to the outer edge of the SPCE ring. The information along the azimuthal direction will be redundant in an ideal system, which may be used to improve the SNR via averaging. The angular-wavelength relationship has been numerically and experimentally verified. Figure 1(B) shows the diagram of the experimental setup. The SPCE slide coated with AFP and an orange dye material was attached to a half-ball prism via index-matching fluid. The 405-nm LED provides excitation from the front and an Ocean Optic fiber optic spectrometer was used to collect the fluorescence spectra with emission angles of 50° ~ 80°. The AFP emission map peaking at 460 nm and the dye emission map peaking at about 580 nm were carefully overlapped to form a composite wavelength vs. emission angle map, which is shown at the top of Figure 1(C). The theoretical computed SPCE map using the equations in Ref [6] is also shown at the bottom of that figure for comparison. The peak emission of AFP at 460 nm was focused to emit at an angle of about 70° and the peak of orange dye at 580 emitted at about 53°. Good agreement between the experimental and theoretical results can be observed. The data in Figure 1 demonstrated that it is not only possible to acquire the fluorescence spectrum but also interrogate multiple optical reporters simultaneously with the wavelength/angular multiplexing scheme. The color separation capability comes from the large material dispersion of the silver layer in this example. Since there is no light loss due to an entrance slit or grating which is commonly used in a spectrometer, there is further signal intensity advantage in addition to the high fluorescence harvest efficiency of SPCE. Such advantages allow two critical components: (1) very small emitter spot size and (2) high-speed spectra acquisition to be achieved in this project. The emitter dot as small as160 μm in diameter has been used and produced a suitable signal for detection. The smaller emitter spot allows more optical reporters to be deposited in a dense array, for example a 3× 3 array to accommodate 9 reporter spots, to detect a wide range of explosives species, which were not possible to achieve with a single reporter. The responses of different reporters can also be correlated to improve the robustness of the explosive identification and interference rejection. In addition to the spectral data, high-speed fluorescence spectra acquisition will allow us to capture transient responses of the spectral response, which also contains crucial information for target identification and interference rejection purposes. In the vapor mode explosive detection application, the detection time is usually less than a few seconds. We are able to achieve about 20 frames per second with the current design. 1.2 SPCE Interrogation Optics Since the spectral information in SPCE is distributed in the angular space, the angular range and resolution that may be interrogated by the down-stream optics will determine the spectral range and resolution specified by the SPCE optics. Based on our experience, the SPCE in an angular range of 50° ~ 80° contains most of the signal with the fluorescence materials on a silver layer. In order to capture emissions with such angular distribution, we used the combination of a half-ball prism and a parabolic reflector to appropriately collimate the beams. Then, a set of focusing lenses were used to project most of the collimated beams onto the detector. Initially, non-imaging optics consisting of an axicon lens and a focusing lens, as shown in Figure 2, were proposed as the focusing optics. The axicon lens allowed the projection of the collimated SPCE beams to be reduced in a linear fashion so that beam crossing would be excluded and the spectral data extraction would be straightforward. The additional lens was used to speed up the focusing for reduced size. The Zemax model and four SPCE rays with emission angles of 50° ~ 80° from the center of SPCE slide are shown at the left side of Figure 2 to demonstrate the non-imaging concept.

Figure 2. Non-imaging SPCE optics (left) and the ray tracing results with 320-μm dot (right).

However, the further analysis by Zemax simulations revealed a severe limitation to such a design. While the SPCE from an infinitesimally small emitter spot may be imaged as sharp rings, the width of the rings quickly increased as the emitter size grew to a realistic dimension. The ray tracing data at the detector is shown at the right side of Figure 2. The four darker rings were the image of four SPCE beams originated from the center, which are well separated. Other fainter and distorted “rings” were originated from the edges of a 320-μm emitter dot and these “rings” overlapped with each other, which indicated that the angular resolution was so poor (in the range of about 10°) that the proposed spectral acquisition and multiplexing would not be possible even with single emitter dot. In order to address this issue, we have used Zemax to pursue a new design based on some commercially available camera lenses. It was discovered that the combination of parabolic reflector and camera lens can improve focusing performance dramatically.

Figure 3. New SPCE optics and ray tracing results by Zemax.

The ray tracing result of this new imaging-based optics is shown in Figure 3, where the 50° and 75° beams from various locations relative to the center (optical axis) position are plotted. The three upper beams represent the 50° emission from a center, a +0.5-mm up-shifted, and -0.5-mm down-shifted spots. Another three lower beams had 75° emission angle from a center, a +0.5-mm up-shifted and -0.5-mm down-shifted spots respectively. The fanning out of beams was due to the off-center locations of the emitters. Note that the design shown in the figure was based on an existing in-house camera lens (NT59-870, Edmond Optics), which has a minimum working distance of 100 mm. In the future design, another camera lens or custom-designed lens set with shorter working distance may be used to reduce the standing off between parabola mirror and detector. With the additional modifying lens in front of a commercially available camera lens, all the SPCE beams were captured effectively and focused well on the array detector by the new SPCE optics. As it is shown in the figure, all emissions are focused according to its emission angles instead of their spatial position. No matter where the emitter spots are located, all the 50° emissions from the entire array will be focused on the lower focal point on the detector plan and all the 75° emissions will be focused on the higher focal point. Such “angular-oriented” focusing effect is the essential requirement to fulfill the proposed spectral acquisition with an array of reporters. Since all emissions with the same spectral content from any reporter spots in the array will have the same emission angle due to SPCE effect, they will be focused onto the same ring projection by this new SPCE optics. Now, since emission of the same emission angle (same spectral content) is focused onto the same focal point (ring), the angular resolution is simply determined by the focusing performance of the camera lens instead of being limited by the emitter spots size or location. The detailed focal points of the emission beams from a 1-mm spot with 70° (solid line), 69° (dashed line), and 68° (dotted line) are shown at the top-right side of Figure 3. It was clearly demonstrated that the emissions from 70°~68° beams are well separated with angular resolution in the sub-degree range. Conversely, emissions from 55°~57° were relatively poorly resolved (not shown) and they had a resolution of about one degree. Interestingly, the SPCE has already been limited by much lower spectral dispersion at low-angle emissions (50° ~ 60°). Hence, poorer focusing for low angle SPCE beams does not pose further limitation. Still, such angular (spectral) resolution (sub-degree with 1-mm emission spot) is much improved comparing to the non-image optics (10-degree resolution with 0.32-mm spot). The only limitation on the emission spot size and angular resolution is due to the camera lens size and its optical design. With a larger camera lens, the allowed array size can be further expanded, and the angular resolution can be improved with a better lens design. Even with current performance, a 5×5 reporter array with 100-μm spot size may be simultaneously interrogated with the new SPCE optics, and the angular resolution should be sufficient for the spectral resolution provided by SPCE phenomenon. The projection of the SPCE rings on the detector plane with emission angles of 75, 70, 65, and 60 were simulated by Zemax and plotted at the lower right side of Figure 3. Notice that there is no visible distortion or overlapping between rings, which presents a great improvement over the non-imaging optics result shown in Figure 2.

The key advantage of the unique SPCE optics is to provide the “angular-oriented” focusing effect. Traditionally, a camera can image the objects according to their spatial location with great resolution but not the angular information. On the contrary, the new SPCE optics can image the emission according to its emission angle with excellent resolution but omit the spatial information of the emitter. Optically, this angular-oriented focusing effect was introduced by the parabolic mirror, which translates the angular information into the spatial one so that angular information can now be “imaged” by a camera. Therefore, the original fine spatial resolution provided by traditional camera was now translated into excellent angular/spectral resolution by the parabolic mirror. Optically, this angular-oriented focusing effect was introduced by the parabolic reflector, which translates the angular information into the spatial one so that angular information can now be “imaged” by a camera. As it is shown in the Figure 4, the emissions with same incident angle such angle θ1 and θ2 will have a crossing point such as y1 and y2 after the reflection. The initial angular information (θ1 and θ2) is then translated into spatial information (y1 and y2), which then can be recorded by an appropriate optics and an arrayed detector. The translation relation is a one-to-one mapping for both directions. Therefore, the original fine spatial resolution provided by the traditional camera was now translated into excellent angular/spectral resolution by the parabolic reflector.

Figure 4. Conversion of angular information to spatial information (left), the simulated, and experimental SPCE annulus.

With this desired effect, the exact locations of the reporter dots within the array are now disregarded but their wavelength-specific emissions can always be recognized by the emission angles (or the emission wavelengths) as long as there is no significant spectral overlapping or interference by other reporters. This is the main principle of SPCE optics used in this prototype. The image of the SPCE color rings formed by the above mentioned AFP emission on the detector plane was simulated by Zemax and plotted as the middle of Figure 4. The actual SPCE annulus color image acquired by the prototype with a 320-μm AFP spot deposited at the center of sensing slide was shown at the right side. Excellent agreement between the simulation and test data was demonstrated here. The dark line cutting the annulus in half was the shadow of the LED supporting structure, which inevitably blocked the SPCE beam.

2. DATA ACQUISITION AND SPECTRAL SIGNAL EXTRACTION The prototype sensor was designed based on the FLIR Fido XT explosive detector to use the similar sampling front end and control electronics, and hence it is termed SPCE-Fido. The prototype includes a sensor head, Fido body/controller, and an interrogation computer. The sensor head includes the vapor sampling system, sensing flow cell, and optical assembly, which was also the only major hardware design in this project. The Fido body/controller adapted from Fido XT was used to set the operating parameters such as temperatures of fluidic channel, flow rate, and LED intensity. The interrogation computer system was a regular desktop/laptop PC with IEEE-1394 (Firewire) interface and LabView software installed. The fluidic system was temperature controlled and flow stabilized to ensure effective sample delivery to the sensing surface. The sample stream impinges and flows across the SPCE sensing surface before it is drawn out by an air pump. The optical reporter array printed on the SPCE sensing slide is excited by a UV LED from the back of the slide through the half-ball lens, which also focuses the excitation light slightly. The fluorescence signal will be extracted through the half-ball lens, reflected by the parabolic mirror, and then imaged by the camera lens to the CCD detector.

The sensing slide on top of the half-ball lens may be renewed if the sensing surface has been soiled or worn out. The flow cell height was less than 100μm to provide effective interaction between sample molecules and reporter materials. The parabola reflector was fabricated out of an aluminum block with a polished surface. The SPCE beams were first extracted through the half-ball lens and then roughly collimated by the parabolic mirror before they reached the imaging optics. The image of the SPCE emission ring shown in Figure 4 was acquired with this optical system. The half-ball lens, emission filter, modifying lens, camera lens and CCD detector were all off-the-shelf components. Currently a camera lens with a 100-mm minimum working distance was used in the prototype because of the availability of its optical prescription, which allowed us to perform ray tracing analysis using Zemax software. With accurate technical information and modeling capability, we were able to design and fabricate SPCE optics with good efficiency. The excellent agreement between the predicted and experimentally acquired SPCE rainbow rings showed that the fabrication of the parabola mirror was within specification and the SPCE optical design was successful. 2.1 Sample Introduction & Intensity curve: The performance of the SPCE-Fido prototype was first evaluated with TNT detection tests using the AFP reporter. A Fido-XT explosive detector was used as the reference for comparison. An equilibrium adjusted headspace of TNT vapor from a closed container is used as a reference standard. The equilibrium adjusted headspace in the canister provides a consistent amount of TNT vapor, which usually produces a fluorescence quench of about 8~10% with Fido XT detector. The Fido-XT sensorgram was plotted as the solid trace in Figure 5, which shows an about 8% quench at about 13th second. In the SPCE-Fido setup, one 320-μm 2-layer AFP dot was printed at the center of the SPCE sensing slide as the reporter. The sensor temperature and flow settings were similar to the ones used in Fido XT. One snapshot of the SPCE annulus image collected by a monochromic CCD detector is shown at the left side of Figure 5 with false coloring to represent the intensity. During the data acquisition process, the total intensity within the annulus region was integrated and reported at real time, which was effectively the intensity-only sensorgram similar to the data provided by Fido XT. The intensity sensorgram plotted in the right side of Figure 5 as the trace with vertical bars showing about 6% quenching and a much broader dip compared to the Fido XT response. The lesser quenching ratio suggested that the flow cell in the SPCE-Fido has not been fully optimized. It could be due to excessive space in the sensing flow cell, which can be mitigated by reducing the flow cell size. The SNR of Fido XT’s response was slightly better.

Figure 5. The SPCE annulus image (left) and the comparison of TNT detection intensity sensorgrams acquired with Fido XT and SPCE-Fido (right).

The dashed trace in Figure 5 was the intensity sensorgram calculated with 10 radial lines in the SPCE annulus area as those gray radial lines overlaid on the annulus image. The data were calculated by averaging over all points across the line (all emitted wavelengths) and hence still only provided intensity information. Therefore, dashed trace is almost identical to the trace with vertical bar marker but has worse SNR, which was due to much less pixels being used for averaging. The trace with triangular marker was the sensorgram computed by using only one radial coordinate (one wavelength), which corresponded to about 470 nm. The SNR was worse due to even smaller points were included in the calculation, but the quenching ratio was improved to almost exactly the same level as the Fido-XT result. This indicated that the maximum fluorescence quenching was occurred at about 470 nm and reduced quenching ratios at other wavelengths. This was confirmed by the extracted spectral data at later steps.

2.2 Spectral Extraction Procedure During the whole data acquisition process, all acquired SPCE annulus images were stored in the computer disk and then processed by the spectral information extraction codes. A set of TNT raw data shown in Figure 6 is used as an example to demonstrate spectral data extraction process. Initially the video of the raw annulus images (Figure 6a) were retrieved, and the total intensity in the annulus region was replotted in Figure 6(b), which was analyzed to identify the timing of maximum quenching. The last snapshot during the baseline period and the one at maximum quenching were used for further spectral analysis. Therefore, it was termed as the “static spectral signature” since there is no temporal information in this signature. The center and the boundaries of the annulus region in the image were determined by the image processing algorithm. The detected inner/outer boundaries and the ring with peak intensity were plotted as overlays in Figure 6(c). The radial distance has a one-to-one mapping relationship to the emission wavelength: the shorter radius the shorter the wavelength. The actual wavelength can be found via a calibration process. However, since the goal was to identify different explosives instead of being an actual spectrometer, such calibration was not performed. Therefore, a special terminology, Spectral Radius or SpecR in short, was coined to represent the radius in the SPCE annulus image. The spectral intensity profile can be acquired by extracting the intensity profile at any fixed azimuthal angle because of the azimuthal symmetry. The fixed azimuthal angles are termed as Region of Interest (ROI) angles. The ROI angle of 80° and 200° were indicated by two white arrows in Figure 6(c). Since the spectral profiles among multiple ROI angles are supposed to be redundant, they may be averaged to increase the spectrum SNR. Unfortunately, due to the inevitable optical misalignment and asymmetry in the practical optical system, the spectral profiles among the ROI angles always have variations. Therefore, local averaging was used within only neighboring ROI angles instead. In this example shown in the figure, four ROI angle groups were used and they are from 80° to 140°, 140° to 200°, 200° to 260°, and 260° to 320°. The ROI angles of 80° and 200° are labeled on the figure. The intensities along those radial lines across the annulus region were extracted, low-pass filtered, and then grouped together to form four spectral profiles, which are plotted in Figure 6(d). The horizontal axis is SpecR with increasing wavelength from left to right. Good agreement among traces generated with different ROI angles indicates good alignment and installation. One trace apparently has some disagreement with other traces of different ROI groups in the plot. Such discrepancy may be resolved with reinstallation of sensing slide.

Figure 6. The signal post-processing process with TNT detection data: (a) raw image, (b) intensity sensorgram, (c) partition of SPCE annulus, (d) intensity profiles array, (e) quench spectrum array, and (f) averaged quench spectrum

The fluorescence spectral profile of each group at maximum deviation from the baseline value was then normalized by the baseline spectral profile. The resulting spectral response signatures are plotted in Figure 6(e). The final spectral quenching signature was the average of those quenching spectra, which is plotted in Figure 6(f). It has been shown that

the local averaging before spectral signature calculation preserves critical spectral features. The final global averaging smoothed out any small discrepancy among local groups due to optical distortion. 2.3 Static Spectral Signature The first group of explosives, TNT, NT (nitrotoluene), and NM (nitromethane), used in the test were sampled directly from the headspace of a vial. The NT was provided from the headspace of a vial with solid NT crystal immobilized at the bottom of the vial. NM vapor was extracted from the headspace of a vial with a cotton ball soaked with NM at room temperature. The volume of the headspace was adjusted so that these samples provided about 6~10% quenching on the intensity sensorgram. The AFP has limited sensitivity to NT and NM and hence higher dosage needs to be used compare to that of TNT. Since the sample was already in the vapor phase, the sampling process was straightforward. The temperature settings for the sampling nozzle and the following fluidic channel were elevated and controlled to ensure effective sample delivery. The AFP reporter was coated on the SPCE slide to form a circular deposition with a diameter of 320 μm and one to two layers of AFP was coated for each spot. The acquired fluorescence signal was strong and only about 20% of full LED strength was required to maintain suitable reading without saturating the CCD array at a frame rate of about 14 Hz. The excellent fluorescence collection efficiency in SPCE-Fido was clearly demonstrated. The optimized focusing of the SPCE beams for all different wavelengths in theory can be achieved by setting the detector at the correct optical focal plane as it is indicated by the Zemax simulation shown in the left diagram in Figure 7. Unfortunately, locating the optimal focal plane was not trivial. Actually such a plane with perfect focusing for all relevant wavelengths may not exist due to the possible misalignment and abberation of the optical system, but an optimized image distance (the distance between lens and CCD) may be achieved. Based on the simulation, the theoretically optimized focusing occurs at a point closely behind the point where the minimum peak width is observed. Therefore, the first set of data was acquired with the image distance roughly at the spot where the minimum intensity peak occurs. During the data acquisition process (DAQ), the software can provide the real time intensity profiles along several ROI angles groups for the user as the visual feedback to look for the minimum peak width before the actual DAQ begins. With such criteria in mind, we have acquired the static spectral signature of TNT, NT, and NM and plotted in Figure 7. We have observed good agreement between these static spectral signatures and other data sets acquired with a laboratory-grade fiber optic spectrometer, which validated the SPCE optics design and the spectral information extraction procedure. The NM signature shown as the trace with triangular marker can be easily distinguished from TNT and NT because its maximum quenching occurred at a much lower wavelength. The static spectral signatures of TNT and NT were, on the other hand, very similar and difficult to distinguish unless a high level of reproducibility of the alignment of sensing slides and optical components can be maintained. As it is shown in figure, the TNT trace has a quench (spectral dip) at about SpecR=177 position and NT trace with a diamond marker has one at SpecR=181 position. This spectral signature difference was small but reproducible if sufficient amount of alignment effort has been done on the SPCE-Fido prototype. However, if the SPCE image focusing was off and the quenching dips were broadened, then it would be almost impossible to distinguish these two explosives due to insufficient resolution and distortion. Therefore, further effort was invested to investigate a better approach.

Figure 7. The theoretical optimal camera focal plane (left) and static spectral signatures of TNT, NT, and NM.

The focus adjustment on traditional camera optics is straightforward since the clarity and the resolution of the final image can be used as the feedback. However, the focus adjustment of the camera in SPCE optics was rather counter

intuitive because there is no real object with sharp edge to be imaged. Instead, a virtual object (SPCE annulus) formed by the crossing rays with same emission angle was imaged. In the previous data set, the spatial tightness of the major emission peak, such as the 460-nm emission peak from AFP material, was used as the feedback for focus adjustment. However, further investigation has shown a better criterion that may be used to achieve improved spectral resolution. In order to validate that emissions of different colors were imaged at correct SpecR, we used two band-pass filters: one has a pass band at 450~470 nm and the other at 480~500 nm to facilitate focus adjustment. These two pass bands were chosen to match the two major emission bands of AFP material. By using these two filters alternatively, the SpecR of the two major emission peaks of AFP can be identified in the annulus image. The camera focus was adjusted accordingly until those two SpecRs agree with predicted values by the Zemax simulation. For the simulation, the SPCE angles of those two emission bands were acquired by using the SPCE testing setup shown in Figure 1(A). Figure 8 shows the acquired raw annulus images and the extracted fluorescence intensity profiles with two different band-pass filters when the camera lens focus was adjusted correctly. The image acquired through a longer wavelength filter (480~500 nm) has a larger SpecR of about 200 pixels, which are shown in the top row, and the one with a shorter wavelength filter has a shorter SpecR at about 175 pixels shown at bottom row, which agreed with the Zemax prediction. After such adjustment, another round of explosive test was conducted to investigate the possible improvement. The static spectral signatures of multiple explosives are shown in Figure 9 with multiple data plotted for each explosive. The first apparent change was that the spectral signature seems to be zoomed in or expanded especially at the longer wavelength side. This is because the new focal plane was moved away from the “narrowest peak width” position and toward the theoretical optimal focal plane. The dashed beams of longer wavelength in Figure 7 diverge away from the optical axis much faster than other beams so that the spectral signature of the longer wavelength side was stretched out. The new spectral signature of TNT and NT in Figure 9 has “cut-short” longer wavelength wing is another evidence of this argument. With this new focal plane, the new quenching signatures of TNT and NT still share the some resemblance to the previous data, which is expected since the major effect was only the stretching of the SpecR axis and loss of long wavelength due to limited spectral range of the SPCE-Fido. The NM has a much narrower quenching dip and is located at lower wavelength than TNT and NT. The longer wavelength feature of NM was also unique but was close to the edge of spectral range of current SPCE-Fido prototype, and hence its appearance was a hit-or-miss case in the repetitive tests.

Figure 8. The raw image (left column) and processed intensity profiles (right column) acquired with a 480~500-nm band-pass filter (top row) and a 450~470-nm band-pass filter (bottom row).

After the camera focal plane was finalized, additional explosive samples, RDX (cyclotrimethylenetrinitramine), PETN (pentaerythritol tetranitrate), and AN (ammonium nitrate) were prepared by dosing the ACN solution with known explosive concentration on a Teflon swipe. After the solvent has been evaporated, the automatic thermal desorber was used to vaporize the explosive molecule into the sample flow stream. Alternatively, the in-house RDX and AN

reference standards were also used in the test. There was not significant variation in the data acquired using liquid dosing method or reference standard. The RDX signature acquired with narrow-peak-width focus was quite similar to that of TNT in the previous data, but now it is now clearly distinguishable from TNT and NT. At the lower wavelength, RDX has a much reduced quenching response and even a flare response sometimes. The RDX data in Figure 9 did show about 2% flare at SpecR=147, which is quite different than the completely quenching response demonstrated by the TNT static spectral signature. The PETN signature has similar broad quench dip to those of TNT and NT but the dip was at longer wavelength. All these changes coming from the new focus provide improved features for target classification even with the static spectral signature alone. As it will be shown later, the temporal-spectral data of RDX and PETN will have even more unique features in the time-dependent data. In the past, the AFP intensity responses to AN samples were quite variable and may depend on the sensing surface condition or analyte concentration. Similar unpredictable responses have also been observed with the SPCE-Fido platform when only the static spectral signatures were used. Still, the additional spectral information may provide some insight along the spectral axis to explain the variation of AFP responses under different sensing conditions. The static spectral signature of AN shown in Figure 9 was actually quite different from those of other tested explosives shown in the figure. The flare response dominated the entire spectral response with a flare peak at a lower wavelength. The flare peak is similar to the one shown in RDX data but at a slightly longer wavelength. The uranium nitrate and AN spectral response were quite similar.

PETN

Figure 9. The static spectral signatures of the tested explosives.

3. TEMPORAL-SPECTRAL SIGNATURE Although reproducible static spectral signatures have been acquired for the tested explosives, TNT and NT were still not easily distinguishable with the static spectral data alone. In addition, the static spectral signature may be shifted, stretched, or compressed depending on the installation of the SPCE slide or other optical parameters such as the image distance. Therefore, additional effort has been taken to ensure the robustness of explosive identification with the SPCE technology. The new effort has been directed toward the exploration along the temporal axis for additional information and this endeavor has proven fruitful. The time history of the static signatures was extracted and investigated. With the combination of temporal and spectral dependency, the challenge of distinguishing some explosives, such as TNT and NT which have similar static spectral signatures, now has been overcome. In addition, the relative movement of a certain spectral features in time domain is much more resilient to variation of optical alignment than its absolute spectral location. Therefore, it is believed that the temporal-spectral data will be a powerful addition with great potential for the fluorescence-based detection platform.

3.1 Extraction of Temporal-Spectra Data The temporal-spectral signature consisted of multiple static spectral signatures during the entire course of sample introduction. The interrogation process is explained with the plots in Figure 10. The total fluorescence intensity within the SPCE annulus region is plotted in Plot (a) with vertical dashed lines to mark the time stamps where the static spectral signatures were taken. Note that the time axes in the sensorgram have been “zoomed” to the approximate time duration that matches to the temporal data collection. Twenty time stamps were used and they are distributed evenly across the sensing duration. The dotted lines mark the time stamps during the first (onset) stage of the sensing event and the dashed lines for the second (recovering) stages. Currently only a simplified two-stage process was used in the data processing algorithm. The separation point of the stages is usually chosen at the moment where an intensity slope changes its sign. For a rather monotonic sensorgram with no apparent deflection point, the half point during the transition region is chosen. Each time-dependent spectral signature was calculated as the ratio of intensity profiles along the SpecR between one time during baseline and the other at the specified time stamps. Plot (b) of Figure 10 shows the spectral response at the “stage separating moment” or the maximum quenching point in the example, which was the 77th shot indicated in plot (a). There are four traces which were calculated from four groups of ROI angles. They were averaged once the uniformity among them was confirmed. Plot (c1) and (c2) shows the stacked plots of multiple static spectral signatures at the designated time stamps in plot (a). The stacked signatures from the first and second stages are plotted in plot (c2) and (c1) separately. The quenching percentage was shown by the vertical axis and SpecR by the horizontal axis. Only three signatures were acquired in the first stage in (c2) and were shown as traces with increasing quenching levels as time advanced. During the recovering second stage, ten traces were captured due to the slower recovering process. It started with the bottom trace with the deepest quench of 84% at SpecR=184 and then recovered to the topmost trace shown in plot (c1).

Figure 10. Temporal-spectral data interrogation process: (a) intensity sensorgram and time stamps, (b) static spectral signatures of four groups, (c) stack plot of the time history of the spectral signature, and (d) 2D temporal-spectral map.

While the stacked plot shows all the information in one plot, the details of each static spectral signature were covered up by the large dynamic range of the absolute quench level so that the major dip locations at early or later times with shallow absolute depth are difficult to be appreciated. In order to address this issue, each spectral signature was normalized by its own distribution and linear transformed so that the maximum quench ratio was set to zero, the baseline value to unity and flare response to a greater-than-one value. Then such transformed data was shown as a 2D false color map and termed “temporal-spectral map” as they are shown in plot (d1) and (d2). Such a 2D temporal-spectral map was used to easily track the variation of the intensity dip location in the time history. The color designation of this false color map was used to highlight the deepest intensity dip location with dark gray color and any flare event with white to black color depending on its strength. Due to the limitation of gray-scale color map, text labels and arrows are used on the map to indicate the location of quenching and flare event. Plot (d2) shows that there was a flare event right at the beginning of the sensing event for a brief moment, which was never identified in the previous sensing data with AFP material. Such a flare only occurred at SpecR=170 region and the intensities at other

wavelengths were still reduced, and hence it would be hidden by the dominating quench response in the intensity-based system. More interestingly, the maximum quenching ratio was initially established at SpecR=190. However, as more analyte molecules interacted with the sensing material and accumulated, the maximum quenching point was shifted to a shorter (bluer) wavelength. At the ending of sensing event, the maximum quenching occurred at a wavelength region where SpecR=160~180. Such “shifting of quenching dip” behavior was unique and reproducible to TNT samples. It actually provides critical information to robustly distinguish TNT from NT sample with SPCE-Fido detector, which will be discussed further in the following sub-section. 3.2 Temporal-Spectral Signatures of TNT, NT and NM It has been mentioned that the camera focus in the SPCE optics may have a significant impact on the explosive identification using static spectral signature alone. Now with the additional temporal information, the identification can be much less affected by the image distance, even with the most difficult case from our data. The detection data and temporal-spectral signatures of TNT and NT samples acquired with two different image distances are plotted and compared in Figure 11. The snapshots of the raw SPCE annulus images are shown in the first row, and the spectral profiles acquired from four ROI groups are shown in the third row. The difference due to the image distance can be clearly seen from the raw snapshots. The images (TNT1 and NT1) taken with shorter image distance has a narrower annulus width and tighter spectral distribution, and the ones (TNT2 and NT2) with longer image distance have wider distribution. The traditional intensity sensorgrams are shown in the second row, which has great variation due to manual sample introduction process.

Figure 11. The detection data of TNT and NT. The first group, TNT1 and NT1, has shorter image distance and the second group, TNT2 and NT2, has longer image distance.

Note that the TNT1 and NT1 data had a shorter image distance so that the spectral profile has narrower peak but the whole data covered larger spectral range, and TNT2 and NT2 data had a broader peak but covered smaller range. The plots in the fourth row are the static spectral signatures. Good consistency can be observed among the traces of different

ROI groups. However, it would be rather difficult to distinguish TNT from NT with only the static spectral signatures since the dips were approximately at the same location and they were shifted with different camera focus. The temporal-spectral maps shown in the Figure 12, on the other hand, clearly have distinct and recognizable drifting patterns for both TNT and NT samples. The TNT map has a brief flare response earlier at a shorter wavelength and a quenching response at other wavelengths. The quenching response dominated and lasted the entire sensing event with its deepest quenching dip drifting toward the shorter wavelengths (blue shift behavior). When different image distances were used, the initial maximum quenching wavelength may change (SpecR=190 vs. SpecR=220). However, the blue shifting behavior was very consistent. Therefore, much more robust and flexible identification algorithm can be used with the temporal-spectral data. The NT map, on the other hand, has a rather static quenching behavior with its deepest dip fixed at the same wavelength throughout the sensing event. A minor but consistent red shift can be seen close to the end of the sensing event. The dark gray “plumes” indicated by arrows in the temporal-spectral map, which indicates the drifting of deepest dip, may have different width due to different imaging distance. Again, the absolute spectral locations of the dark gray plumes were also different between these two image distances. However, the dynamic shifting behavior in the map was consistent for each explosive, which provides valuable information for explosives identification. The early flare response shown in these two TNT data was actually not always present, although it was often observed with fresh sensing slide. It seems to have some correlation to the surface condition since such a momentary anti-quench occurs at very early stage of sensing events. Sometimes it can also be observed with other explosives. Also, this flare response has smaller amplitude compared to other quenching responses so that the SNR may not be significant enough to support its validity. More tests along this direction will be necessary to have a better understanding on such a phenomenon.

Figure 12. The temporal-spectral maps of TNT and NT derived from the data in Figure 11.

The detection data and the derived temporal-spectral maps of NM are plotted in Figure 13. They were acquired with progressively increasing image distances from the left to the right columns. The effect of various image distances can be verified by the image snapshots in the first row. The longer image distance produced spectral profile with a wider annulus width and “blurry image”. The static spectral signatures shown in the third row also has a wider quenching spectral dip with longer image distance. The deepest quenching dips shown as the dark gray plumes always occurred at the shorter wavelength initially and then drifted to the longer wavelength (red shift behavior). This is the reverse of the drifting behavior seen with the TNT sample. Again, although the static spectral signatures are different due to the different image distance, the temporal-spectral map has a consistent drifting pattern that can be easily recognized. 3.3 Temporal-Spectral Signatures of RDX, PETN, and AN The detection data and temporal-spectral maps of RDX are shown in Figure 14. Again, from left to right column the data were acquired with progressively increasing image distances. In the first row, the intensity sensorgrams of first two columns have an interesting inversion point at the middle of the sensing event, which were often observed with the RDX samples with the intensity-based system. The temporal-spectral maps shown at bottom row now reveal more details about those inversion points. In the beginning of the RDX sensing event, a quenching response was occurring at the middle range of SpecR. However, as more RDX molecules interacted with the sensing surface (AFP), the deepest quenching dip started to shift to a longer wavelength. At the same time, a flare response occurred at a shorter wavelength region. Such a flare response started in the middle of the sensing event caused the total intensity to increase momentarily before further decreasing, and hence an intensity inversion point appeared. The complicated RDX

response would be quite confusing with intensity-only sensorgram. However, it is clearly understandable with the presentation of temporal-spectral map.

Figure 13. The detection data and the temporal-spectral maps of NM with increasing image distance from left to right.

Figure 14. The detection data and the temporal-spectral maps of RDX with increasing image distance from left to right.

Figure 15. The detection data and the temporal-spectral maps of PETN and AN with increasing image distance from left to right.

Even more interestingly, those “inversion points” could be converted into the deepest dip in the intensity sensorgram if the flare response was so significant and overcame the quenching response. Even with such a complex response shown by RDX samples, the temporal-spectral maps still may be used to identify the similar repetitive drifting behavior over many data sets. Therefore, there would be no problem to recognize the RDX sample with the temporal-spectral map. Even some operating parameters have been changed. This characteristic would be desirable for the explosive identification application. The detection data and temporal-spectral signatures of PETN and AN samples acquired with two different focal lengths are plotted and compared in Figure 15. The PETN also has a flare response occurring at the middle of a sensing event similar to the RDX data. However, the deepest quenching dip has different drifting behavior compared to that of RDX. The dip trace of PETN in the 2D map often has a blue shift (moving to a shorter wavelength) during the end time of the sensing event. In addition, there is a less sudden shift of the quenching dip with PETN sample around the inversion point which appeared in the intensity sensorgram. There were fewer PETN tests performed due to the lack of convenient reference standard. Therefore, more test data will need to be collected for the PETN sample in the future effort for a more conclusive result. Still with only a few data sets, the 2D map of PETN appeared to be quite recognizable. As it was mentioned in the sub-section of static spectral signature, the AFP response to AN was quite unpredictable. We have also observed the similar behavior in our intensity sensorgram data shown in the first row of Figure 15. However, with the temporal-spectral data, the variation from quenching to flare response is now understandable. The most distinguishable feature of AN was the early bi-polar response: the flare response at shorter wavelength and the quenching response at the longer wavelength at the same time. Then, the flare response faded as more AN molecules interact with the AFP. However, the AN data at the forth column, which was acquired with longer image distance, may have another flare response with even more AN molecules coming to the sensing surface. We have seen about half of the AN tests with such a reoccurring flare response, but the other half tests did not have such behavior.

4. EXPLOSIVE DETECTION WITH TWO REPORTERS In addition to provide the explosives identification capability, another advantage of the SPCE platform is to interrogate multiple reporters in a dense array format by using wavelength multiplexing. Such capability may be essential to address the “blind spot” of certain well accepted optical reporters such as AFP. We have found another proprietary fluorescence material (RP2) that may be a suitable second reporter to work with AFP to improve the NT and NM sensitivity. The intensity sensorgrams of NM detection with both AFP and RP2 reporters are shown at the right side of Figure 16. There was a roughly eight-fold sensitivity improvement demonstrated in our test data. Although the emission spectrum of RP2 was quite close to that of AFP as it is shown in the middle of figure, the same excitation and collection optics may be used for both reporters, which simplify the system design. The quenching spectra of RP2 to NM and other explosives were rather flat across its emission band (400~500 nm) as it is shown at the right of Figure 16.

Figure 16. NM sensitivity comparison using AFP and RP2 reporter (left), the emission spectra of RP2 and AFP (middle), and the static spectral quench feature of RP2 with NM vapor (right).

SPCE slides with both AFP and RP2 sensing dots deposited side by side with each dot having a diameter of about 320 μm have been used to test the multiplexing capability of SPCE-Fido. The test results are shown in Figure 17 where the intensity sensorgram is in the first row and the temporal-spectral map in the second row. There was little change on the TNT temporal-spectral map until later time of the sensing event, which was because that RP2 has very low response to TNT. On the other hand, the NT signature has been changed significantly because of the high sensitivity of RP2 to NT. The quenching level of NT was greatly enhanced by almost an order of magnitude. The intensity sensorgram has an almost square-wave shape due to the extremely fast response of RP2 to NT. The 2D temporal-spectral map was also dominated solely by the RP2 quenching dip close to 440 nm or SpecR=145. The multiplexing capability of interrogating both AFP and RP2 materials has been demonstrated in this data set.

Figure 17. Two-reporter (AFP and RP2) detection data acquired with TNT (left) and the NT (right) test samples.

Since the emission band of RP2 and AFP still has some overlapping in the wavelength region of 450~500nm, the spectral quench signature in the overlapping region was actually an intensity-weighted average of AFP and RP2 spectral responses. In this specific data set, the AFP response to NT was overwhelmed by RP2 response. Therefore, the original quenching dip at about SpecR=180 with only AFP reporter is now covered up by the much deeper quench produced by RP2 reporter. This issue can be addressed by formulating the AFP and RP2 concentrations so that the emission ratio between AFP and RP2 at the overlapped spectral region can be adjusted. We believed that by lowering the RP2 concentration and emission intensity, the NT spectral response to AFP would re-emerge and may provide richer information for identification purposes.

5. SUMMARY The high fluorescence collection efficiency and built-in spectroscopic nature of SPCE have been explored and combined with advanced fluorescence reporters to provide new capabilities for multiple explosive detection and identification applications. New “angle-oriented” imaging optics has been designed to specifically work with the SPCE technique to produce an SPCE annulus “rainbow” for spectral information extraction with high emission collection efficiency. Image signal processing techniques have been used to dissect the SPCE annulus image and extract the spectral information. A SPCE-Fido prototype built on top of Fido XT platform has been used to demonstrate and validate the new capabilities, such as acquiring static and time-dependent spectroscopic signatures for robust explosives identification and to interrogate multiple reporters with wavelength multiplexing scheme. The SPCE-Fido prototype built out of commercial grade CCD and off-the-shelf components has been extensively tested with several explosives. The DAQ and signal post-processing software has been programmed with LabView software on a PC computer for fast prototyping. Explosives such as TNT, NT, NM, RDX, PETN, AN, and UN have been tested mainly with the AFP sensing material. Their static and temporal-spectral response signatures have also been extracted and collected out of almost 2000 detection tests. Good reproducibility has been observed in the test data. In addition to the AFP material, the RP2 sensing material also has been tested. We have also performed simultaneous two reporter interrogation with AFP and RP2 material. Two-reporter multiplexing in wavelength domain with orthogonal response has been observed. Due to the overlapping of the fluorescence signals from AFP and RP2, the concentration ratio between the two reporters needs to be further optimized in order to maximize the distinguishable features for the analytes that generate responses for both AFP and RP2 materials.

The author gratefully acknowledges the funding provided by Science & Technology Division of the US Army RDECOM CERDEC NVESD under contract W909MY-10-C-0037.

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