Fluorescence Detection of a Broad Class of Explosives with


Oct 24, 2016 - Given a demand for a richer identification of threats, minimized cost, and low detection-limits, developing a fluorescence sensor in re...
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Letter pubs.acs.org/ac

Fluorescence Detection of a Broad Class of Explosives with One Zinc(II)-Coordination Nanofiber Wei Xiong,†,‡,§ Xiaoling Liu,†,‡,§ Tie Wang,†,‡ Yifan Zhang,†,‡ Yanke Che,*,†,‡ and Jincai Zhao†,‡ †

Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China



S Supporting Information *

ABSTRACT: In this work, we report the development of one fluorescent carbazole-based oligomer 1-zinc(II) coordination nanofiber which enabled the detection of five classes of explosives, i.e., nitroaromatics (dinitrotoluene, DNT, and trinitrotoluene, TNT), aliphatic nitro-organics (2,3-dimethyl-2,3-dinitrobutane, DMNB), nitramines (cyclotrimethylenetrinitramine, RDX), nitro-esters (pentaerythritol tetranitrate, PETN), and black powder (sulfur). We demonstrate that the coordination of zinc ion with a carbazole-based oligomer 1 allows the formation of the Lewis acid−base complex between explosives and the nanofiber that enhances the electron-accepting ability of the nitro-based explosives and the binding interactions between the sensing nanofibers and explosives. Furthermore, the resulting nanofiber-based sensor exhibited highly sensitive fluorescence quenching when exposed to trace sulfur, thereby enabling the sensitive detection of black powder. Herein, we present a new fluorescent sensor for five classes of explosives, which represents an important advance toward a richer identification of threats.

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ompared to other detection technologies for explosives,1,2 namely, mass spectrometry,3,4 surface-enhanced Raman spectroscopy,5 surface plasmon resonance,6 and ion mobility spectroscopy (IMS), 7 etc., fluorescence-based detection represents a simple, cost-effective, and highly sensitive method.1,2,8−12 However, the most used fluorescence sensors only respond to one or two classes of explosives,1,8,10,13,14 such as nitroaromatics (e.g., TNT and picric acid) and limited examples have been reported to exhibit response to more than two classes of explosives.15,16 Particularly, there have been no reports of fluorescence sensors for black powder that was easily accessible. Given a demand for a richer identification of threats, minimized cost, and low detection-limits, developing a fluorescence sensor in response to a broad class of explosives is highly desirable.2,16 Nonetheless, there remains a great challenge for the development of such broad-class sensor because of the difficulty to access a sensing material that has broad chemical reactivities to various explosives of distinct physicochemical properties. In the present work, we report the development of one fluorescent carbazole-based oligomer 1-zinc(II) coordination nanofiber which enabled the detection of five classes of © XXXX American Chemical Society

explosives (Scheme 1), i.e., nitroaromatics (dinitrotoluene, DNT, and trinitrotoluene, TNT), aliphatic nitro-organics (2,3Scheme 1. Explosives Used in This Work

dimethyl-2,3-dinitrobutane, DMNB), nitramines (cyclotrimethylenetrinitramine, RDX), nitro-esters (pentaerythritol tetranitrate, PETN), and black powder (sulfur). We demonstrate that the detection of weakly electron-accepting RDX arises from the formation of the Lewis acid−base complex of Zn2+ and NO2 group that allowed an electron trapping from the excited nanofiber and thereby quenched the fluorescence of the nanofibers. Furthermore, because of the enhanced binding of Received: September 13, 2016 Accepted: October 24, 2016 Published: October 24, 2016 A

DOI: 10.1021/acs.analchem.6b03618 Anal. Chem. XXXX, XXX, XXX−XXX

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monodentate coordination.18 Considering that 1-Zn2+ coordination triggered the self-assembly of the nanofibers, we postulate that the dominant metal−ligand bonding creates an end-by-end packing (100) and in turn one-dimensional (1D) coordination polymers where hydrophobic interactions were perpendicular to the nanofiber axis. Such molecular packing is agreement with the X-ray diffraction (XRD) patterns (Figure S3), where the d-spacing values of 1.9 and 1.4 nm are assigned to the end-by-end and side-by-side packing, respectively. A new absorption band at ∼350 nm emerged over the coordination nanofibers (Figure 1d) compared with monomer 1 in DMF (Figure S4), indicative of the existence of π-electron coupling in the 1-Zn2+ coordination nanofibers. However, a dominated absorption band centered at 379 nm assigned to the monomer (Figure S4) indicates that the π-coupling in the coordination nanofibers was relatively weak and the monomer characteristic absorption of molecule 1 remained. Likewise, the characteristic emission of the π-coupled nature (i.e., at 490 nm) and that of the monomer (i.e., at 440 nm) were reflected in the fluorescence spectrum of the 1-Zn2+ coordination nanofibers, as shown in Figure S4. The resulting nanofibers were highly emissive. When excited by UV light (320−375 nm), the 1-Zn2+ nanofibers deposited on a glass slide exhibited a fluorescence quantum yield of ∼13% (as determined from calibrated integrating sphere measurements). The relatively high emission efficiency of the nanofibers and the intrinsic porosity when deposited on a substrate made them suitable as the florescent sensor. We first explored the sensing performance of the above 1Zn2+ coordination nanofibers to RDX and PETN vapors that were difficult to be detected because of their low volatility and weak binding ability to the sensory materials.2,16 We carried out the sensing experiments in a home-built optical chamber (Figure S5) coupled with an Ocean Optics USB4000 fluorometer (using a 385 nm LED lamp through an optical fiber as the light source). Because of the low volatility, RDX and PETN with a small amount were vaporized using a thermal desorber (170 °C) and pumped into the optical chamber (i.e., a quartz tube) covered with the nanofibers inside (Figure S5). As shown in Figure 2a,b, the fluorescence of the 1-Zn 2+ coordination nanofibers was significantly quenched by the vaporized nanogram-scale RDX and PETN and the actually measured detection limit of RDX and PETN could reach as low as 10 ng (Figure 2). Furthermore, the fluorescence quenching response is fast, i.e., ∼5 s. These observations are in sharp contrast to other carbazole-based systems that exhibited no response to RDX and PETN.9,19 Given that RDX has too weak electron-accepting ability (i.e., relatively high LUMO level) to accept electron from common excited-state sensing materials,10,15 we envisaged that the formed Lewis acid−base complex between Zn2+ within the nanofibers and NO2 groups in RDX enhanced the electron-accepting ability that allows the electrontrapping from the excited nanofibers and thus results in fluorescence quenching. To support this hypothesis, we synthesized molecule 2 that had similar molecular structure to molecule 1 and were self-assembled into nanofibers with similar dimensions but without the coordination of Zn2+ (Figure S6). As shown in Figure S7, the nanofibers from molecule 2 exhibited no fluorescence response even upon vaporization of 30 ng of RDX, indicative of the critical role of Zn2+ in the fluorescence detection of RDX. Similar phenomena were observed when the fluorescence quenching experiments were performed in solution. As shown in Figure 2c, the

explosives via the formed Lewis acid−base complex of Zn2+ and NO2 group, the detection sensitivity to TNT, DNT, DMNB, and PETN was remarkably increased in comparison with the nanofibers without the involvement of Zn2+ coordination. More interestingly, the resulting 1-Zn2+ coordination nanofibers exhibited highly sensitive fluorescence quenching to trace sulfur, thereby enabling the sensitive detection of black powder. Herein, our work first provides a new fluorescent sensor to five classes of explosives, which represent an important advance toward a richer identification of threats. Given that Zn2+ in CdSe/ZnS quantum dots can form the Lewis acid−base complex with NO2 groups in explosives14 and that the Lewis acid−base complex may have more positive reduction potential (i.e., higher electron-accepting ability) than the pristine oxidant,17 we expected that the coordination polymer composed of Zn2+ and a carbazole oligomer might enable the detection of RDX and PETN via an electron transfer mechanism. For this consideration, molecule 1 was designed and synthesized as shown in Figure 1a (see details in the

Figure 1. (a) Molecular structures of molecules 1 and 2. (b) SEM image of the resulting 1-Zn2+ coordination nanofibers. Inset: the coordination of Zn2+ with carboxyl group of molecule 1. (c) EDX spectrum of the 1-Zn2+ coordination nanofibers, showing the existence of C, N, O, and Zn. (d) Absorption of the 1-Zn2+ coordination nanofibers dispersion (black) and solid fluorescence spectra (red) of the 1-Zn2+ coordination nanofibers.

Supporting Information). When Zn(OAc)2·2H2O (0.047 mmol, 10.3 mg) in 10 mL of DMF was added dropwise into 40 mL of DMF solution of 1 (0.047 mmol, 50 mg), the solution gradually turned turbid under gentle stirring (300 r/ min) at room temperature, indicative of the formation of 1Zn2+ coordination aggregates. After 2 days, the resulting precipitates were collected by centrifugation and washed three times by ethanol for the characterization and sensing test. Scanning electron microscopy (SEM) revealed the nanofibril morphology of the aggregates, which have the diameter of ∼80 nm and the length up to several micrometers (Figure 1b and Figure S1). The chemical composition of the nanofibers was analyzed with energy-dispersive X-ray (EDX) spectroscopy, which showed that the nanofibers were composed of Zn, O, N, and C (Figure 1c). Infrared (IR) spectrum of the 1-Zn2+ coordination nanofibers showed two carboxylate dominant peaks at 1605 and 1402 cm−1, which were distinct from the carboxylate peaks of molecule 1 and Zn(OAc)2·2H2O (Figure S2), indicating that the carboxylate groups adopted the B

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Figure 2. (a) Fluorescence quenching of 1-Zn2+ coordination nanofibers upon exposure to the vapor of various amounts of RDX. (b) Fluorescence quenching of 1-Zn2+ coordination nanofibers upon exposure to the vapor of various amounts of PETN. (c) Fluorescence quenching of 1-Zn2+ coordination nanofibers in DMF (0.31 μM) by various concentrations of RDX. Inset: Fluorescence spectra of 1-Zn2+ coordination nanofibers in DMF (0.31 μM) in the presence of various concentrations of RDX (0, 0.8, 1.5, 2.3, 3.1, 3.9 mM). (d) Fluorescence quenching of 1-Zn2+ coordination nanofibers in DMF (0.31 μM) by various concentrations of PETN. Inset: Fluorescence spectra of 1-Zn2+ coordination nanofibers in DMF (0.31 μM) in the presence of various concentrations of PETN (0, 0.8, 1.5, 2.3, 3.1, 3.9 mM).

Figure 3. (a) Fluorescence quenching of 1-Zn2+ coordination nanofibers upon exposure to the vapor of various amounts of TNT. Inset: the detection limit to TNT is determined to be 0.5 ng. (b) Fluorescence quenching of the nanofibers assembled from 2 upon exposure to the vapor of various amounts of TNT. (c) Fluorescence quenching of 1-Zn2+ coordination nanofibers upon exposure to the DNT vapor of various amounts. (d) Fluorescence quenching of the nanofibers assembled from 2 upon exposure to the vapor of various amounts of DNT. (e) Fluorescence quenching of 1-Zn2+ coordination nanofibers upon exposure to the DMNB vapor of various amounts. (f) Fluorescence quenching of the nanofibers assembled from 2 upon exposure to the DMNB vapor of various amounts.

fluorescence intensity of molecule 1 in the presence of Zn2+ in DMF continued to linearly decrease with an increase of the added RDX. In sharp contrast, the fluorescence intensity of molecule 2 retained unaltered with an increase of RDX (Figure S8a). These results indicate that the Zn2+-RDX complexation enhanced electron-accepting ability of RDX and thus resulted in the above florescence quenching. Distinct from RDX, PETN could quench the fluorescence of the nanofibers assembled from 2 (Figure S7b) because PETN has lower LUMO energy level comparable to DNT15 and thereby can trap an electron from the excited nanofibers. Interestingly, 1-Zn2+ coordination nanofibers presented more prominent response under the identical conditions, which should arise from the complexation of Zn2+ and PETN that strengthened both the electronaccepting ability and the binding of PETN onto the nanofibers. Again, similar results were observed when the fluorescence quenching experiments were performed in solution. As shown in Figure 2d, the fluorescence intensity of 1 in the presence of Zn2+ in DMF prominently and linearly decreased with an increase in the concentration of PETN. In contrast, the fluorescence intensity of molecule 2 only slightly decreased under the identical conditions (Figure S8b). These results further indicated that the Lewis acid−base complex between Zn2+ and NO2 groups enabled the enhanced sensitivity for vapor detection of explosives. The influence of the complexation of Zn2+ and NO2 groups on the sensing performance of the sensory nanofibers was also reflected in vapor detection of nitroaromatics (e.g., TNT and DNT (heated at 140 °C)) and aliphatic nitro-organics (DMNB (heated at 140 °C)). Compared to the nanofibers assembled from 2 that were able to detect 2 ng of TNT and 10 ng of DNT (Figure 3b and 3d), respectively, the detection limits of 1-Zn2+ coordination nanofibers reached 0.5 ng of TNT and 2 ng of

DNT, respectively, as shown in Figure 3a,c. The effect of the Lewis acid−base complex between Zn2+ and NO2 groups was even more prominent in vapor detection of DMNB. As shown in Figure 3e,f, 1-Zn2+ coordination nanofibers exhibited marked fluorescence quenching when exposed to the vapor of 50 ng of DMNB, whereas the nanofibers assembled from 2 showed no fluorescence response to the vapor of less than 100 ng of DMNB. Furthermore, the quenched fluorescence of the nanofibers from 2 by the vapor of more than 150 ng of DMNB quickly restored when exposed to air flow (Figure 3f), indicative of the weak interactions between DMNB and the nanofibers assembled from 2 and thereby the easy release of DMNB from the nanofiber surface. In contrast, fluorescence quenching over 1-Zn2+ coordination nanofibers was not recoverable when exposed to air flow (Figure 3e), reflecting the relatively strong interactions between the nanofibers and DMNB because of the formed Lewis acid−base complex. The observed fluorescence quenching above should be due to the photoinduced electron transfer from the excited nanofibers to explosives, which is highly favored by the large driving forces (i.e., the gap between the LUMO of explosives and that of the sensing molecules, as shown in Table S1). Finally, we investigated the possibility of 1-Zn2+ coordination nanofibers to detect black powder (BP (heated at 140 °C)), which represented a common explosive of unrestricted availability, low-cost, and flammability.20 Surprisingly, rare investigations on fluorescent detection of BP have been C

DOI: 10.1021/acs.analchem.6b03618 Anal. Chem. XXXX, XXX, XXX−XXX

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demonstrated that the vapor detection of RDX resulted from the formation of the Lewis acid−base complex of 1-Zn2+ coordination nanofiber and RDX that enabled the electron trapping from the excited-state nanofibers and thereby the fluorescence quenching. Compared to the nanofibers without the coordinated Zn2+, 1-Zn2+ coordination nanofiber also exhibited enhanced sensitivity to explosives (e.g., TNT, DNT, DMNB, and PETN) that have relatively low LUMO because the complexity of Zn2+ and NO2 group in explosives increased both the electron-accepting ability and the binding of explosives onto the nanofibers. Furthermore, the achieved 1-Zn 2+ coordination nanofiber was demonstrated to be able to sensitively detect sulfur and thus BP. Herein, the development of a fluorescence sensor for five classes of explosives represents an important advance toward a richer identification of threats, which remains a great challenge.

reported. Given that sulfur, one main component of BP, has certain electron-accepting ability, we first investigated the vapor detection of sulfur using 1-Zn2+ coordination nanofibers above in the same homemade device. Unexpectedly, 1-Zn 2+ coordination nanofibers exhibited very high sensitivity to sulfur. As illustrated in Figure 4a, the actual detection limit of



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b03618. Synthetic procedure, fabrication of nanofibers from 1 and 2, property characterizations, and other supporting figures and table (PDF)

Figure 4. (a) Fluorescence quenching of 1-Zn2+ coordination nanofibers upon exposure to the vapor of various amounts of sulfur. (b) Fluorescence quenching of the nanofibers assembled from 2 upon exposure to the vapor of various amounts of sulfur; (c) fluorescence turn-on response of 1-Zn2+ coordination nanofibers to hexane vapor. (d) Detection limits of 1-Zn2+ coordination nanofibers to TNT, S8, DNT, RDX, PETN, and DMNB.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

the nanofibers for sulfur is as low as 1 ng. When BP (obtained from the fireworks) was tested, the detection limit of the same nanofibers is still very low, i.e., 5 ng. Notably, similar sensing results were also observed over the nanofibers assembled from 2 (Figure 4b), suggesting that sulfur itself was capable of directly trapping electrons from the excited carbazole-based oligomers and thus resulted in the fluorescence quenching. Again, similar results were observed when the fluorescence quenching experiments were performed in solution (Figure S9). These above findings may trigger the development of selective fluorescence sensors for sulfur. In addition, the fluorescence response of 1-Zn2+ coordination nanofibers to the vapor of common organic solvents was explored to evaluate the selectivity to explosives against interferences. Most fluorescence responses of 1-Zn2+ coordination nanofibers toward these solvents were in the turn-on mode at their relatively high vapor concentrations (Figure 4c) and thereby negligible (Figure S10). This is because electron transfer cannot occur from common organic solvents with the relatively high oxidation potentials to the excited 1-Zn2+ coordination nanofibers to quench the fluorescence and vice versa. Therefore, the fluorescent sensory materials based on 1Zn2+ coordination nanofibers exhibited high selectivity to various explosives over interferences (organic solvents) and very high sensitivity (Figure 4d). Note that 1-Zn2+ coordination nanofibers also exhibited the relatively serious photobleaching during the sensing experiments as presented above. The related work of the photostability improvement is under progress in our lab. In conclusion, one fluorescent 1-Zn2+ coordination nanofiber was developed to detect five classes of explosives (i.e., TNT, DNT, DMNB, RDX, PETN, and BP) in high sensitivity. We

Author Contributions §

W.X. and X.L. contributed to this work equally.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the 973 Project (Grant No. 2013CB632405), NSFC (Grant Nos. 21221002, 21322701, 21577147, and 21590811), the “Strategic Priority Research Program” of the CAS (Grant No. XDA09030200), and the “Youth 1000 Talent Plan” Fund.



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DOI: 10.1021/acs.analchem.6b03618 Anal. Chem. XXXX, XXX, XXX−XXX