Discrimination of Five Classes of Explosives by a Fluorescence Array

Oct 31, 2017 - In this work, we report a two-member fluorescence array sensor for the effective discrimination of five classes of explosives. This sma...
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Discrimination of Five Classes of Explosives by a Fluorescence Array Sensor Composed of Two Tricarbazole-Nanostructures Qijian Zhu, Wei Xiong, Yanjun Gong, Yingxuan Zheng, Yanke Che, and Jincai Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04083 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Discrimination of Five Classes of Explosives by a Fluorescence Array Sensor Composed of Two Tricarbazole-Nanostructures Qijian Zhu,†,‡,§ Wei Xiong,†,‡,§ Yanjun Gong,†,‡ Yingxuan Zheng,†,‡ Yanke Che,*,†,‡ and Jincai Zhao†,‡ †

Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT: In this work, we report a two-member fluorescence array sensor for the effective discrimination of five classes of explosives. This smallest array sensor is composed of tricarbazole-based nanofibers (sensor member 1) and nanoribbons (sensor member 2) deposited as two film bands in a quartz tube. Based on a simple comparison of the resulting fluorescence quenching ratios between two sensor members and the response reversibility upon exposure to vaporized explosives, five classes of explosives can be sensitively detected and easily discriminated. This array sensor that has only two sensor members and no complex data analysis represents a new design way for discrimination of a broad class of explosives.

Since Swagger and coworkers reported the first work on the fluorescent detection of explosives by π-conjugated pol1,2 ymers, various fluorescence systems have been developed to detect explosives, particularly nitroaromatics (e.g., TNT 3-9 and picric acid). Despite these many advances, precise recognition of the class of explosives, which is significant in 6 chemical and background analysis of terrorism activities, remains largely unexplored. Fluorescence array sensors, which have proven to be a useful tool in discrimination of 10,11 structurally similar components, have been applied in discrimination of explosives. For example, a fluorescent quantum dots (QDs)-based array exhibited variable fluorescence quenching when exposed to various explosives and thereby gave rise to the discrimination of the tested explo12 sives based on the pattern analysis. However, the currently reported array sensors only exhibited responses to a narrow class of explosives (less than three classes of explosives). Furthermore, these array sensors were comprised of multiple sensor members, which would burden the device fabrication and the data analysis (e.g., the requirement of principle 13 component analysis and/or linear discriminant analysis). Therefore, developing the minimum size array sensor (i.e., two-member array) but capable of sufficient discrimination of a broad class of explosives is highly desirable.

zole-based nanofibers and nanoribbons are self-assembled from molecules 1 and 2 with different functional side groups (Figure 1) and were deposited in a quartz tube to form two film bands as the two sensor members. The resulting array sensor can leverage various physicochemical interactions and competitions between the sensor members and explosives to yield distinct fluorescence quenching ratios over the two sensor members and response reversibility, thereby enabling the sensitive detection and easy recognition of the specific type of explosives. Our sensor system containing only two sensor members and no complex data analysis represents an important advance toward sensitive detection and discrimination of a broad class of explosives. For the design of array sensors, applying the smallest number of sensor members is highly favored because it has the advantages of the easy data analysis and the low cost of 13 sensory materials synthesis and device fabrication. To this end, the sensor members are required to have sufficient discrimination ability. Here, we first chose fluorescent nanofibers assembled from molecules 1 (Figure 1a) as one sensor member because these nanofibers exhibit cross-reactive fluo14 rescence responses to various classes of explosives and have good photostability and film-forming properties. In this study, we optimized the self-assembly process to achieve smaller nanofibers and cast them into a film band within a glass tube (Figure S1) that was subsequently applied to a commercial detection device (EF5000, HT-Nova). Some important parameters in the commercial device, such as the deposition position of the sensing materials in the chamber, the pumped air-flow speed, the excitation light intensity, and the detector sensitivity, were well optimized to enhance the sensitivity. Scanning electron microscopy (SEM) measure-

In the present work, we report a two-member fluorescence array sensor for the discrimination of five classes of explosives, i.e., nitroaromatics (trinitrotoluene, TNT, and dinitrotoluene, DNT), black powder (sulfur, S8), aliphatic nitroorganics (2,3-dimethyl-2,3-dinitrobutane, DMNB, and nitromethane, NM), nitramines (1,3,5-Trinitro-1,3,5triazacyclohexane, RDX) or nitro-esters (pentaerythritol tetranitrate, PETN), and ammonium nitrate (AN). Tricarba-

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(c) SEM image of nanoribbons 2 deposited on a silicon substrate. (d) Fluorescence-mode optical microscopic image of nanofibers 1. (e) Fluorescence-mode optical microscopic image of nanoribbons 2. (f) Proposed molecular packing of 1 or 2 within the resulting assemblies.

ments revealed that nanofibers 1 had diameters of 50-80 nm and the length of several micrometers that entangled each other to form a relatively even film, as shown in Figures 1b and S2. Careful examination of the nanofibers by the magnified SEM image showed that the nanofibers actually consisted of the bound small nanofibers with diameters of 10-20 nm (Figure S2). Such tightly bound hierarchical structure can facilitate the emission and thereby amplify the sensing sig15 nal. These nanofibers were highly emissive (Figure 1d), with a fluorescence quantum yield (FQY) of more than 40 % when excited by UV light (350-400 nm). The weak intermolecular π-coupling of 1 should mainly contribute to the high emission efficiency of the resulting nanofibers as evidenced by their optical spectra. Figure S3 showed that the absorption peak of nanofibers 1 red-shifted only 4 nm compared with individual 1 in chloroform solution, indicative of a very weak π-π coupling of 1 in the nanofibers. Likewise, the fluorescence peak of nanofibers 1 red-shifted only ca. 11 nm compared to individual 1 in chloroform solution. X-ray diffraction (XRD) measurements provided more information of the molecular packing of 1 in the nanofibers (Figure S4). The domio nant peak at ca. 1.6 nm (2θ = 5 ) was assigned to the side-byside packing (100) (Figure 1f). The d-spacing value of 0.38 nm at 2θ = 24°, which was assignable to the weak π-stacking (001), was also observed. Based on these information, the molecular packing of 1 in the resulting nanofibers was proposed in Figure 1f.

To achieve the sufficient discriminatory capability of the array sensor, we further synthesized and self-assembled molecule 2 (Figure 1a) into fluorescent nanoribbons as the second member. Compared to molecule 1, the cyano side groups in molecule 2 endowed the resulting nanostructures with distinct dimensions and chemical properties (e.g., the decreased electron-donating ability). SEM image revealed that molecules 2 formed ribbon-like structures with the width of 100-150 nm and length of more than 10 µm (Figure 1c). A new absorption band centered at 424 nm indicates the enhanced π-coupling of 2 in the nanoribbons (Figure S5). The fluorescence spectra also reflect the enhanced πcoupling where the more red-shifted emission (ca. 469 nm) appeared compared to that of the monomer in chloroform solution (ca. 423 nm). The increased π-coupling of molecules 2 slightly disfavored the emission efficiency of the resulting 16,17 nanoribbons where a fluorescence quantum yield of ca. 2o % was observed when excited by UV light (350-400 nm) (Figure 1e). Notably, the well-defined nanoribbons have increased the molecular packing order where more diffraction peaks were observed in XRD measurement. As shown in Figure S6, the dominant peak at ca. 1.9 nm was assigned to the side-by-side packing (100), while the peak at 0.39 nm was assigned to the π-stacking (001). Based on the XRD patterns, molecular packing of 2 is proposed to be similar to that of 1 (Figure 1f). Having successfully obtained the above array sensor members that have distinct dimensions and chemical properties, we then explored their pattern responses to various classes of explosives and evaluated the discriminatory capability. We first fabricated a two-band optical chamber (Figure S1) by casting nanofibers 1 near the tube air-inlet as sensor member 1 and nanoribbons 2 about 30 mm far from the tube air-inlet as sensor member 2. When DNT and TNT with a small o amount were vaporized using a thermal desorber (170 C) o and pumped into the two-band optical chamber (ca. 60 C), the fluorescence responses of two sensor members were simultaneously recorded. As shown in Figure 2a,b, the marked fluorescence quenching was observed over sensor members 1 and 2 when exposed to vaporized sub-nanogram DNT and TNT. Their actually measured limit of detection for TNT and DNT were as low as 0.3 ng and 0.5 ng, respectively. The observed fluorescence quenching should result from an electron transfer from the excited sensing materials to explosives because of the relatively large driving forces (Figure S7). Notably, sensor member 2 exhibited a higher fluorescence quenching than sensor member 1 (Figure 2a,b), particularly considering that less amounts of explosive vapors would reach sensor member 2 that located far from the tube inlet. The ratio of the fluorescence quenching for sensor member 2 to sensor member 1 was calculated to be at the range of 1.1-1.9 when exposed to various amounts of vaporized DNT and TNT (Figure 2d). To demonstrate the effect of the sensor member position in the tube on the sensitivity, we compared the fluorescence quenching of both sensor members that

Figure 1. (a) Molecular structures of molecules 1 and 2. (b) SEM image of nanofibers 1 deposited on a silicon substrate.

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were located at the same position relative to the tube airinlet. As shown in Figure S8, the ratio of the fluorescence quenching for sensor member 2 to sensor member 1 was increased to a range of 1.5-3.5 when exposed to various DNT or TNT vapors. These observations demonstrated that sensor member 2 was indeed more sensitive to DNT and TNT vapors than sensor member 1, which is likely because the increased π-coupling in well-defined nanoribbons 2 favored the exciton diffusion and thereby amplify the signal. The similar result where sensor member 2 exhibited a higher fluorescence quenching than sensor member 1 was also observed in vapor detection of sulfur, one main component of black powder. As shown in Figure 2c, the ratio of the fluorescence quenching for sensor member 2 to sensor member 1 was largely increased to a range of 2.3-3.9 (Figure 2d), which probably results from the larger electron-accepting ability of sulfur compared to DNT and TNT (Figure S7). Importantly, such obvious difference of sulfur vs. DNT and TNT in the ratio of the fluorescence quenching between two sensor members enabled the discrimination of their class, i.e., nitroaromatics vs. black powder (sulfur). In addition, we highlight that the actually measured limit of detection for sulfur was as low as 0.2 ng.

cence quenching when exposed to vaporized RDX and PETN o (170 C), which can still be explained based on an electron transfer mechanism (Figure S7). However, contrary to nitroaromatics and sulfur, RDX and PETN gave rise to higher fluorescence quenching of sensor member 1 over sensor member 2 (Figure 3). The ratio of the fluorescence quenching for sensor member 1 to sensor member 2 was calculated to be at the range of 2.3-3.8 when exposed to various amounts of vaporized RDX and PETN (Figure 3d). The low fluorescence quenching of sensor member 2 should be because of the relatively weak electron-accepting ability of RDX and PETN and the relatively low electron-donating ability of the excited sensor member 2 (Figure S7). In addition, compared to nitroaromatics and sulfur, the very low volatility of RDX and PETN would allow much less vapors to reach sensor member 2 that was located far from the tube inlet and thereby caused a low fluorescence quenching. Indeed, when sensor members 1 and 2 located at the same position relative to the tube airinlet were exposed to the same amount of RDX and PETN vapors, the ratio of the fluorescence quenching of sensor member 1 to sensor member 2 was decreased to a range of 1.12.7 (Figure S9). Similar fluorescence quenching ratio were o also observed in vapor detection of AN (170 C). As shown in Figure 3c, the ratio of the fluorescence quenching of sensor member 1 to sensor member 2 was at a range of 1.2-1.9. The lower volatility of AN should mainly contribute such low ratio of the fluorescence quenching of sensor member 1 to sensor member 2, which was supported by the fact that a much lower ratio at a range of 0.7-1 (Figure S9) was observed for tested sensor members 1 and 2 located at the same tube position. Importantly, such obvious difference of AN vs. RDX and PETN in the ratio of the fluorescence quenching between two sensor members enabled the discrimination of their class.

Figure 2. Fluorescence quenching of sensor members 1 and 2 upon exposure to vaporized explosives with various amounts: TNT (a), DNT (b), and S8 (c). Inset: the Zoomed-in image of the fluorescence quenching. (d) Comparison of fluorescence quenching responses of sensor member 1 vs. sensor member 2 when exposed to explosives with various amounts. We next explored the pattern responses of two sensor 18 members to RDX and PETN that have very low volatility and relatively weak electron-accepting ability (Figure S7). As shown in Figure 3, both sensor members exhibited fluores-

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al detection limit for DMNB was 30 ng, which is also much larger than that for the above-mentioned solid explosives. These should be related to the high volatility of aliphatic nitro-organics and the weak interaction between aliphatic nitro-organics and the sensor members. Notably, the fluorescence quenching of sensor member 1 is more prominent than that of sensor member 2 upon exposure to both DMNB and NM, likely because of the relatively disfavorable electron transfer from the relatively low electron-donating ability of the excited sensor member 2 to aliphatic nitro-organics. However, despite the fluorescence quenching difference in sensor members 1 and 2, the reversible fluorescence responses of sensor members 1 and 2 to aliphatic nitro-organics can be used to differentiate them from other classes of explosives easily.

Figure 3. Fluorescence quenching of sensor members 1 and 2 upon exposure to vaporized explosives with various amounts: PETN (a), RDX (b), and AN (c). Inset: the zoomed-in image of the fluorescence quenching. (d) Comparison of fluorescence quenching responses of sensor member 1 vs. sensor member 2 when exposed to explosives with various amounts. To simply illustrate the discrimination of the classes of above-mentioned explosives, we used the fluorescence quenching ratio of sensor member 1 to member 2 (Q1/Q2) and that of sensor member 2 to member 1 (Q2/Q1) to define the corresponding coordinates as shown in Figure 4a. Obviously, the two-member array sensor enabled the easy discrimination of four classes of explosives based on their distinct coordinates, i.e., nitroaromatics, black powder (sulfur), nitramines or nitro-esters, and ammonium nitrate, with no requirement of the complicated data analysis. In addition, the fluorescence responses of sensor members 1 and 2 to common organic solvents that may act as the interferences were also explored. As shown in Figures 4b and S10, both sensor members 1 and 2 exhibited enhanced fluorescence responses when exposed to various solvents at different concentrations, which are in sharp contrast to the fluorescence quenching by various explosives. This indicates that common organic solvents cannot trap an electron from the excitedstate sensor members to quench the fluorescence. Therefore, the array sensor composed of members 1 and 2 possessed high selectivity for various explosives against common organic solvents.

Figure 5. Fluorescence quenching responses of sensor members 1 and 2 to the vapors of DMNB (a) and NM (b). In conclusion, we have developed a two-member fluorescence array sensor for the effective discrimination of five classes of explosives. This smallest array sensor is composed of tri-carbazole-based nanofibers 1 (sensor member 1) and nanoribbons 2 (sensor member 2) that are deposited into two bands in a quartz tube. The resulting array sensor can leverage various physicochemical interactions and competition between the sensor members and explosives and thereby yield the distinct ratios of the fluorescence quenching and the response reversibility when exposed to various explosives. Based on simple comparison of the fluorescence quenching ratio between two sensor members and the response reversibility, five classes of explosives can be discriminated easily. This work where only two sensor members were used and no complex data analysis was required opens up new ways for the design of simple array sensors for selective detection of vapor analytes.

Figure 4. (a) Discrimination of four classes of explosives. Q1/Q2: fluorescence quenching ratio of sensor member 1 to sensor member 2; Q2/Q1: quenching ratio of sensor member 2 to sensor member 1. More than 20 explosive samples with various amounts were tested here. (b) The fluorescence responses of sensor members 1 and 2 to hexane. Finally, we used the two-member array sensor to investigate the vapor detection of aliphatic nitro-organics (i.e., DMNB and NM) that have relatively high volatility and low electron-accepting ability. Here, the solid DMNB was still o vaporized with a thermal desorber at 170 C, whereas NM, a class of liquid bomb, was directly tested in its vapor phase. As shown in Figure 5, both sensor members 1 and 2 exhibited a reversible fluorescence response, i.e., the initial fluorescence quenching and following recovery, when exposed to DMNB and NM vapors. These observations are different from the case of the above-mentioned four classes of explosives where the fluorescence quenching was irreversible. The actu-

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedure, fabrication of nanofibers 1 and nanoribbons 2, property and sensing characterizations, and other supporting figures and table (PDF)

AUTHOR INFORMATION Corresponding Author

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* Email: [email protected]. Tel: +1186-10-82612075.

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Author Contributions §

Q. Z and W. X contributed to this work equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by NSFC (Nos. 21577147, 21590811, and 21521062), the “Strategic Priority Research Program” of the CAS (No. XDA09030200).

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