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Abstract
The potential of femtosecond filament induced breakdown spectroscopy technique (fs FIBS) towards the detection of explosive/energetic molecules (EMs) at a standoff (ST) distance of ∼6.5 m is demonstrated. A set of six energetic nitroimidazoles were investigated and discriminated with the fs FIBS technique in tandem with principal component analysis (PCA). The fs ST-FIBS spectra of these EMs were dominated with CN emissions with weak C2 emission and devoid of other atomic peaks (H, N, and O). In addition, an enhancement in LIBS/ST FIBS signal of EMs is achieved in the presence of Ag nanoparticles (NPs) using the nanoparticle enhanced LIBS (NELIBS) technique. Furthermore, the potential of fs filaments for detecting explosive traces is also demonstrated by detecting the residue of an explosive molecule CL-20 on a brass target using the fs NEFIBS technique.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Owing to its versatility, robustness and in-situ analyzing capabilities laser induced breakdown spectroscopy (LIBS) technique has evolved as a potential analytical tool in investigating hazardous chemicals, nuclear materials, geological samples and explosives at standoff distances [1-4]. Though portable and hand-held LIBS instruments were recently designed and tested for the robust analysis of several hazardous materials, the open-path configuration of LIBS investigation and progress in field deployable instrumentation i.e. development of rugged instruments (which can be transported from one place to another) has further reinforced the application of LIBS for analyzing several materials including explosives in standoff mode [5]. Detection and identification of explosives (possibly in standoff mode) is a serious concern all over the globe to monitor their transport, counter their usage in anti-national activities and to safeguard the citizens. There are several reports in literature where nanosecond pulses were used to investigate aerosols, materials submerged in water, molten ores, and explosive residues with standoff distances ranging up to ∼100 m [6,7]. Though most of the hand-held or standoff LIBS systems employ nanosecond lasers due to their miniaturized sizes and lower maintenance costs, utilization of fs pulses in the form of long filaments [8,9] for the standoff detection of explosives is challenging and yet to be explored in detail [10]. A first report on the usage of fs pulses, in the form of filaments, for standoff applications was demonstrated by employing a terawatt femtosecond laser by the Teramobile group. Rohwetter et al. [11] reported remote LIBS analyses of Al and Cu at 25 m and simultaneously compared the advantages of pulse duration by using nanosecond, picosecond and femtosecond pulses from the same Teramobile system and found that the LIBS spectra resulting from fs pulses has shown mitigated external interference. Stelmaszczyk et al. [12] have demonstrated the capability of fs filaments to detect Cu and Fe using fs filament LIBS up to a distance of 90 m. Rohwetter et al. [13] again reported the potential of fs filaments for investigation of metallic samples up to 180 m using terawatt laser pulses. Other than investigating metals, fs filaments have been used in remote/standoff analysis of organic materials [14], for atmosphere sensing [15], cultural heritage [16], and saltwater aerosols [17], and explosive molecules [18]. A book chapter by Cremers et al. [19] deliberates the advantages of fs remote LIBS experiments. Though the fs pulses suffer from turbulence and exhibit irregular intensity pattern as they travel in a medium [20], it is possible to control the position and number of filaments by varying the divergence and energy of the pulses [21]. Fisher et al. [22] demonstrated that the onset of filamentation can be controlled by chirping the fs pulses using phase vortex elements. Polynkin et al. [23] have shown that utilization of Bessel beams provide longitudinally extended and stable plasma channels in air. Despite this progress, there are only few reports where fs filaments are used for the standoff investigation of organic molecules. Xu et al. [24] remotely analyzed biological materials such as egg white and yeast from 3 meters. Baudelet et al. [25] remotely analyzed (∼2 m/∼12 m) graphite and polyisobutylene (PIB) as a stimulant of a residual organic material. In our previous studies we had successfully demonstrated the detection and discrimination of explosive molecules in two configurations (i) in standoff mode (∼2 m) and (ii) remote mode (R-LIBS;10 cm/∼8.5 m) using fs pulses [18]. In this letter, we report the investigation of the same energetic molecules using fs FIBS technique at a standoff distance of ∼6.5 m/∼8 m and their classification using principal component analysis (PCA). These results prove the true potential of fs filaments for investigation of explosive molecules in true standoff mode. Further, such standoff detection capability (especially traces) will enhance the capability of discovering buried landmines. Additionally, in both proximal/standoff mode the intensity of LIBS/FIBS signal of energetic molecules was enhanced using Ag NPs (0.02 mg/ml Ag NanoXact NPs, M/s NanoComposix). Finally, ST FIBS spectrum of CL-20 trace (1 mg over 1 cm2) on brass target was recorded through enhancement in the CN band using Ag nanoparticles.
2. Experimental details
Figure 1 depicts the schematic of fs filament induced breakdown spectroscopy (fs ST FIBS) setup (∼6.5 m/∼8 m). Femtosecond pulses with duration of ∼50 fs from an ultrafast Ti: Sapphire amplifier system (Libra, M/s Coherent, 800 nm, 4 mJ total energy, 1 kHz repetition rate) were employed to investigate the energetic molecules in the standoff mode. The details of the samples investigated are listed in Table 1. In the same experimental setup as illustrated in Fig. 1, we have recently reported the detection of bulk explosive molecules such as RDX, HMX and TNT at ∼6.5 m/∼8 m [26]. The complete details of fs ST-FIBS setup (∼6.5 m/∼8 m) are reported in our earlier work where metals, alloys and bimetallic targets were qualitatively investigated using the fs FIBS technique in the standoff mode [27] where a two-lens beam expander was used to focus the pulses at far distances [L1: Plano-concave lens, f = −50 cm, L2: Plano-convex lens f = 100 cm]. The position of fs filament was controlled by changing the distance between the two lenses we have used in this setup. The formation of filaments depends on the repetition rate of the input laser pulses and it has been demonstrated that chirping the laser pulses can control the onset of filament generation. A Schmidt-Cassegrain telescope [(6", f/10), transmission in the range of 370–900 nm] was utilized to collect the plasma emissions resulting from the interaction of filament with the energetic molecules. The fs ST-FIBS spectra of all the HEMs were recorded in accumulation mode (6 accumulations) without any flat field correction with a gate delay of 20 ns, gate width of 1 μs, ICCD gain of 3000, and exposure time of 1.5 s.
Fig. 1. Schematic of fs filament induced breakdown spectroscopic setup (∼6.5 m/ ∼8 m) depicting the two-lens combination system to focus the fs pulses at 6.5 m away from the L2 and a Schmidt Cassegrain telescope to collect the plasma emissions from 8 m. Inset shows a typical fs filament of ∼30 cm length and stage utilized to place the sample.
Table 1. Details of explosive molecules and the configurations employed for their investigated
3. Results and discussions
3.1 Fs ST FIBS spectra of nitroimidazoles at ∼6.5 m
Figure 2(a) depicts the typical fs ST-FIBS spectra (∼6.5 m/∼8 m) of a set of six nitroimidazoles in the spectral region of 320–700 nm. Two CN bands were observed in the spectral region of 386–390 nm, 410–422 nm corresponding to Δν =0 and −1, respectively with the CN violet band (Δν = 0) depicted the maximum intensity. Only the band head of C2 (Δν=0) swan band was observed at 516.42 nm. Since the transmission range of the SCT (used to collect plasma emissions) lies entirely in the visible and near IR region (370 nm- 820 nm) the observation of few essential spectral signature of HEMs (C I 247.8 nm, CN Δν= +1 356–360 nm, NH at 336 nm) was not possible. Figures 3(a)–(c) depict the CN Δν=0, CN Δν=−1 and C2 Δν=0 bands in a typical fs ST-FIBS spectrum of 4-NIm. The molecular bands and atomic peaks were identified and labeled from Gaydon and Pearse molecular database [28] and NIST database [29], respectively. The variation in intensity of CN and C2 molecular bands among the energetic molecules can be attributed to their molecular structure, position of the nitro groups and the complex plasma reactions. Few additional peaks were observed in adjacent to both the molecular bands 379.81 nm, 381.03 nm near CN (Δν = 0) band, 413 nm, 411.13 nm, and 409.49 nm near CN (Δν = −1) band (not shown in here). However, there is very scarce literature discussed on these peaks and further studies are required to identify these transitions. The absence of other atomic transitions (H, N, and O) as observed in Fig. 2(b) could be attributed to the (a) fundamental principle of femtosecond ablation as well as the (b) fs filament interaction.
Fig. 2. Stack plot representing typical fs ST-FIBS spectra of nitroimidazoles in the wavelength region (a) 330–700 nm (b) 700–870 nm obtained at ∼6.5 m in standoff mode. The fs ST-FIBS spectra of all the HEMs were recorded in accumulation mode (6 accumulations) without any flat field correction with a gate delay of 20 ns, gate width of 1 μs, ICCD gain of 3000, and exposure time of 1.5 s
Fig. 3. Molecular bands identified and labelled in a typical fs ST-FIBS spectrum of 4-NIm (a) CN (Δν=0), (b) CN (Δν=−1), and (c) C2 (Δν=0) band heads.
Rohwetter et al. [11] have observed that fs LIBS (focused pulses) as well as fs FIBS (filaments) spectra of copper metal was free from ambient air species in comparison to ns LIBS spectrum. They attributed these features to the fundamental difference in the ablation process associated with each pulse. In case of ns ablation, the surrounding air breaks down due to diffusive mixing of the expanding hot metal vapor with ambient gas. In the case of picosecond ablation, the trailing part of pulse interacts with the surrounding atmosphere and leads to inverse Bremsstrahlung heating of the plasma plume. Especially, in the fs regime, the produced plasma plume is not enough energetic to excite ambient gas and thus minimal or no breakdown of air is observed (as we observed in the case of ablation of triazoles with fs and ns pulses [30]) with fs pulse ablation. Thus, N and O atomic transitions are absent in the spectra. Further, the fs filament ablation results in plasma with low temperatures and thus cannot re-excite the N and O atoms indicating the short life times of atomic transitions [31].
3.2 Discrimination of explosive molecules at (∼6.5 m/ ∼8 m) using principal component analysis (PCA)
Principal component analysis is a simple, un-supervised multivariate analysis technique widely used in the field of LIBS for discrimination of various samples [32]. Femtosecond ST-FIBS spectra of nitroimidazoles in the spectral region of 385–390 nm [CN (Δν=0)] as well as in 375–390 nm were utilized for discrimination studies using principal component analysis (PCA) and a clear discrimination was obtained in both the cases. Figures 4(a) and 4(b) illustrate the PC score plots and first three PCs (depicting essential spectral features contributing to the discrimination) obtained after analyzing through PCA. The first three PCs together accounted for 93% (91%, 1% and 1%) of variance present in the multivariate data. Figure 4(c) and 4(d) show the PC score plot and first three PCs obtained from analyzing the fs FIBS spectra in the 375–390 nm spectral region. First three PCs accounts for 87% (84%, 2% and 1%) of the total variance present in the data set. PCA of normalized FIBS spectra in both the spectral regions has led to an imperfect discrimination. This could be attributed to the weak spectral intensity of few molecules. Therefore, normalization of data may not lead to better classification at all times [33].
Fig. 4. (a) PC score plot and (b) first three PCs of un-normalized ST-FIBS spectra (385–390 nm), (c) PC score plot (d) and first three PCs of un-normalized ST-FIBS spectra (375–390 nm) of nitroimidazoles.
3.3 Enhancement in LIBS/FIBS signal of explosive molecules in the presence of Ag nanoparticles (Ag NPs)
Recently nanoparticle enhanced laser induced breakdown spectroscopy technique (NELIBS) has been investigated by several researchers to increase the sensitivity of LIBS technique [34,35]. Plasmonic metal NPs (Ag, Au, and Cu) have been used as potential substrates in surface enhanced Raman spectroscopy (SERS) owing to superior and tunable plasmonic properties in the visible range especially in detecting explosive traces [36-38]. Earlier we have reported a two factor enhancement in Cu LIBS signal using Ag NPs with fs pulses [39]. Here we have utilized this technique to (i) achieve enhancements in LIBS/FIBS signal and (ii) to investigate trace energetic molecules in standoff mode by drop-casting the NPs onto the surface of the pellet or HEM residue. Standard Ag NPs dispersed in sodium citrate solution (0.02 mg/ml) were utilized to carry out NE-LIBS/NE-FIBS experiments. Figures 5(a) and 5(b) depict the UV-Visible spectrum and morphology of Ag NPs. NELIBS experiments of nitroamino (NHNO2) substituted aryl-tetrazole in the form of a bulk pellet (150 mg, 12 mm diameter, and 2 mm thick) were performed in the proximal setup (setup is reported [30]). The structure, synthesis and energetic parameters of nitroamino (NHNO2) substituted aryl tetrazole are reported by Kommu et al. [40]. In all NELIBS experiments 10 µl of Ag NPs were drop casted onto the pellet of NHNO2 pellet (150 mg). The LIBS (black, solid) and NELIBS (red, dashed) spectra were acquired with gate delay 50 ns, gate width 1 µs, ICCD gain 3000, exposure time 1 s, and 2 accumulations at 2 Hz repetition rate.
Fig. 5. (a) UV-Vis extinction spectra in 300 nm to 750 nm range and (b) TEM micrograph of standard 60 nm Ag nanospheres (NanoXact, 0.02 mg/ml) purchased from nanoComposix. (U.S.A.). (c) and (d) exhibit the two-factor enhancement in spectral intensity of C I 247.8 nm and CN molecular band in proximal setup in the presence of Ag NPs. (e) two factor enhancement in CN molecular band of TNT (bulk pellet, 150 mg) in standoff mode at ∼6.5 m. (f) Detection of CN molecular band head at 338.34 nm from a trace of CL-20 (1 mg/ 1 cm2) in the presence of AgNPs60 nm using NE-FIBS technique in standoff mode (∼6.5 m).
Carbon atomic emission at C I 247.8 nm and CN Δν=0 (only) band were observed in the proximal case owing to few accumulations (4 only). Figures 5(c) and 5(d) depict the enhancement in carbon atomic transition (C I 247.8 nm) and in CN (Δν=0) molecular band respectively of nitroamino (NHNO2) substituted aryl-tetrazole in the presence of Ag NPs. A two factor (2×) enhancement was observed in both carbon atomic transition at 247.8 nm and CN (Δν=0) molecular band. In the same fs ST-FIBS setup [27] the enhancement of FIBS signal from TNT in the form of a bulk pellet (150 mg) was also examined. The ST-FIBS and ST-NEFIBS spectra of TNT in absence and in the presence of standard Ag NPs were acquired with gate delay of 50 ns, 50 ns, gate width 1 µs, ICCD gain 3000, exposure time 1.5 s, and 6 accumulations at 1 kHz repetition rate. Figure 5(e) illustrates the ST-FIBS spectra (black, solid) and ST NE-FIBS (red, dashed) spectra of TNT with a clear two factor enhancement in CN Δν=0 molecular band at 388.34 nm and 387.14 nm. In standoff mode, C I 247.8 nm could be not observed as the effective transmission range of the telescope is in visible and NIR region only. Similarly, in STFIBS setup a residue of CL-20 on brass target was examined for understanding the capability of fs filaments in standoff residue analysis. 22 mg of CL-20 was dissolved in 1 ml of acetone to make 50 mM solution. 50 µl (1 mg) of CL-20 solution was drop casted on brass target over 1 cm2 area. A layer of CL-20 residue was formed after the evaporation of acetone at room temperature. Figure 5(f) illustrates the enhancement in CN 338.34 nm in the presence of standard Ag NPs. However, in the absence of NPs no CN band (signature of organic energetic molecule) was observed. The enhancements achieved in NELIBS/NEFIBS technique could be attributed to the effect of NPs on the laser ablation process as well as to the target properties (such as conducting or insulating). In earlier studies, metal NPs exhibited the reduction in breakdown or ablation threshold by generating the seed electrons leading to more efficient plasma initiation in various metallic targets, a DC discharge and in liquids as well [41,42]. The enhancement in the local electric field in or near the proximity of NPs is due to the excitation of plasmons in noble metallic NPs by an external electric field manifests the intensity of incident laser beam and in turn effectively extract electrons in the form of seed electrons. These seed electrons efficaciously participate in accelerating the breakdown process and result in huge enhancements [43].
3.4 Conclusions and future directions
We have demonstrated the discrimination of energetic materials (nitroimidazoles) using fs ST-FIBS technique coupled with principal component analysis (PCA) at ∼6.5 m. A Schmidt Cassegrain telescope (SCT) was utilized to collect the plasma emissions emanating after the interrogation of fs filament. A two factor (2x) enhancement in the LIBS/FIBS signal of energetic materials is achieved in proximal as well as fs ST-FIBS configurations. The initial results on investigation of explosive traces (CL-20; 1 mg/ 1 cm2) on brass target are promising and prove the potential of fs filaments for trace detection. Thus, these are initial feasibility studies demonstrating the capabilities of NELIBS for near and standoff applications and even for detecting traces of EMs. If successfully implimented, NELIBS studies in the standoff mode can be a potential game changer in landmine detection. The issue of delivering the nanoparticles at standoff distances needs to be investigated further in detail and can possibly be addressed by small drones and/or through a simple spraying technique. Though this idea seems to be slightly far-fetched it is entirely possible to achieve this. For example, recently there has been report demonstrating the fluorescence enhancement while detecting buried landmines and concealed explosive charges in the presence of bacterial sensor cells, which were sprayed on to the site of interest [44]. However further constructive studies are mandated. The reproducibility and the performance of NELIBS technique depends on size, shape, concentration of NPs and their distribution on the target [45].
Funding
Defence Research and Development Organisation (DRDO) (ERIP/ER/1501138/M/01/319/D(R&D)); Board of Research in Nuclear Sciences, India (34/14/48/2014-BRNS/2084).
Acknowledgments
Authors acknowledge Prof. A. K. Sahoo and Dr. K Nagarjuna for providing tetrazole samples.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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