Interpenetrated Binary Supramolecular Nanofibers for Sensitive

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Interpenetrated Binary Supramolecular Nanofibers for Sensitive Fluorescence Detection of Six Classes of Explosives Wei Xiong, Qijian Zhu, Yanjun Gong, Chen Wang, Yanke Che, and Jincai Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00556 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Analytical Chemistry

Interpenetrated Binary Supramolecular Nanofibers for Sensitive Fluorescence Detection of Six Classes of Explosives Wei Xiong,†,‡ Qijian Zhu,†,‡ Yanjun Gong,†,‡ Chen Wang,§ 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 § HT-NOVA Co., Ltd, Zhuyuan Road, Shunyi District, Beijing 101312, China ABSTRACT: In this work, we develop a sequential self-assembly approach to fabricate interpenetrated binary supramolecular nanofibers consisting of carbazole oligomer 1-cobalt (II) (1-Co2+) coordination nanofibers and oligomer 2 nanofibers for the sensitive detection of six classes of explosives. When exposed to peroxide explosives (e.g., H2O2), Co2+ in 1Co2+ coordination nanofibers can be reduced to Co+ that can transfer an electron to the excited 2 nanofibers and thereby quench their fluorescence. On the other hand, when exposed to other five classes of explosives, the excited 2 nanofibers can transfer an electron to explosives to quench their fluorescence. On the basis of the distinct fluorescence quenching mechanisms, six classes of explosives can be sensitively detected. Herein, we provide a new strategy to design broad-band fluorescence sensors for a rich identification of threats.

Fluorescence detection of various explosives, which represents a simple, quick, cost-effective, and highly sensitive method, has attracted intensive attention given its importance in military operation safety, homeland securi1-18 ty, and environmental and health control. However, very limited examples of fluorescence sensors have reported to be capable of sensing a broad range of explosive classes.15-18 This is mainly because it is challenging to achieve effective responses to various explosives with distinct physicochemical properties over a single sensing material.3,17,19 Particularly, fluorescence sensors for nitrocontaining explosives are generally non-responsive to peroxide explosives that lack nitro and aromatic groups, such as triacetone triperoxide (TATP) and peroxide hydrogen (H2O2).3,20 Therefore, developing fluorescence sensors to detect peroxide explosives sensitively while maintaining the high sensitivity to other classes of explosives is highly desirable, but remains challenging. Recently, we reported the utilization of carbazole-based nanomaterials for the fluorescence detection and discrimination of five classes of explosives, including 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, S8).17,18 Motivated by these results, we explore if another component, which can sense peroxide explosives and is morphologically compatible to the carbazole-based nanofibers, can be introduced to fabricate a binary system toward the detection of six classes of explosives. In the present work, we fabricate interpenetrated binary supramolecular nanofibers via a sequential

self-assembly approach for the simultaneous detection of peroxide explosives (i.e., H2O2 and TATP) along with other five classes of explosives. 1-cobalt (II) (1-Co2+) coordination nanofibers (Figure 1) were first self-assembled in DMF and then transferred and suspended in ethanol, into which molecule 2 was added to aggregate with 1-Co2+ coordination nanofibers to form an interpenetrated nanofiber network. We demonstrate that non-emissive 1-Co2+ coordination nanofibers do not influence the fluorescence of 2 nanofibers, while the generated Co+ reduced by H2O2 can quench the fluorescence of 2 nanofibers. Based on this fluorescence quenching mechanism, peroxide explosives can be sensitively detected. On the other hand, the excited 2 nanofibers in the nanofiber network can sensitively detect other five classes of explosives via direct transferring an electron transfer to explosives. Therefore, the interpenetrated nanofiber network can take full advantage of features of both components for sensitive detection of six classes of explosives. Given that Co2+ have strong interactions with H2O2,21 we envisage that the coordination nanofibers formed from molecule 1 and Co2+ may react with H2O2 and give fluorescence responses. To test this hypothesis, 1-Co2+ coordination nanofibers with the diameter of ∼80 nm and the length up to several micrometers were fabricated (see the Supporting Information), as revealed by scanning electron microscopy (SEM) measurements (Figure 1b). Energydispersive X-ray (EDX) analysis confirmed that the chemical compositions of the above nanofibers were C, O, N and Co (Figure 1c). Infrared (IR) spectrum showed two

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2+

Figure 1. (a) Molecular structures of molecule 2 and 1-Co 2+ coordination polymer. (b) SEM image of 1-Co coordination 2+ nanofibers. (c) EDX spectrum of 1-Co coordination nanofibers. (d) SEM image of the interpenetrated binary nano2+ fibers with the molar ratio of 1-Co coordination nanofibers to 2 nanofibers at 1:1. (e) Bright-field optical microscopic image of the interpenetrated binary nanofibers with the molar 2+ ratio of 1-Co coordination nanofibers to 2 nanofibers at 1:1. (f) Fluorescence-mode optical microscopic image of the same interpenetrated nanofibers as shown in (e). (g) A linear relationship between the fluorescence quantum yield (FQY) of interpenetrated binary nanofibers and the proportion of 2 nanofibers.

dominant peaks at 1605 and 1402 cm-1, indicating that carboxylate groups adopted the monodentate mode in 1-Co2+ coordination nanofibers (Figure S1).22 Unfortunately, the obtained 1-Co2+ coordination nanofibers are non-emissive (Figure S2) and cannot be used as fluorescence sensors directly. To address this problem, fluorescent 2 nanofibers were introduced into 1-Co2+ coordination nanofibers via a sequential self-assembly approach. Typically, molecule 2 (0.25 mg) in chloroform (0.5 mL) was added int0 2.5 mL ethanol, in which a certain amount of prepared 1-Co2+ coordination nanofibers (0.28 mg) were pre-dispersed, then allowed to self-assemble to form a binary nanofiber network. SEM imaging showed that the resulting 2 nanofibers are so similar to 1-Co2+ coordination nanofibers and cannot be distinguished from each other (Figure 1d and

S3). X-ray diffraction (XRD) measurements showed that XRD pattern of the binary nanofiber network was a simple sum of the individual nanofibers, indicating that two nanofibers were self-sorted (Figure S4). Optical microscopic images visualized that non-emissive 1-Co2+ coordination nanofibers and blue-emissive 2 nanofibers were entangled and interpenetrated each other (Figure 1e, f, and Figure S5). Notably, the non-emissive 1-Co2+ coordination nanofibers seemed to have no effect on the emission efficiency of 2 nanofibers. To verify this, we fabricated different interpenetrated nanofibers with the molar ratios of 1-Co2+ coordination nanofibers to 2 nanofibers at 3:1, 2:1, 1:1, and 1:2, respectively, which showed the similar morphology (Figure S3). As shown in Figure 1g, a linear relationship between the fluorescence quantum yield (FQY) of interpenetrated nanofibers and the proportion of 2 nanofibers was observed. This result indicated that the two nanofibers had similar absorption coefficient and that the emission efficiency of 2 nanofibers was not affected by 1-Co2+ coordination nanofibers. No Förster resonance energy transfer (FRET) between 1-Co2+ coordination nanofibers and 2 nanofibers can explain the above phenomenon (Figure S6). For experimental convenience, we used the interpenetrated binary nanofibers with the molar ratio at 1:1 in the following experiments. Having obtained the fluorescent nanofiber network, we then explored if Co+ generated by the reaction between Co2+ and H2O221,23,24 could give rise to the fluorescence quenching of 2 nanofibers as the detection signal of H2O2. We deposited the interpenetrated nanofiber network into a quartz tube and applied this active optical chamber on a commercial detection device (EF5000, HT-Nova) for sensing experiments. As shown in Figure 2a, the interpenetrated nanofiber network exhibited marked fluorescence quenching when exposed to diluted H2O2 vapor. The actual detected concentration can reach as low as 0.5 ppm. Given individual 2 nanofibers exhibited slight fluorescence enhancements when exposed to H2O2 vapors (Figure S7), the possibility of the direct fluorescence quenching of 2 nanofibers by H2O2 was ruled out. Because the reduction potential of the excited 2 (Ered = 0.6 eV) calculated from the HOMO energy level) is higher than the oxidation potential of Co+ (Eox = 0.4 eV),18,23 the excited 2 nanofibers can trap an electron from Co+ to cause the fluorescence quenching (Figure 2b).


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Analytical Chemistry

Figure 2. (a) Fluorescence quenching of the interpenetrated nanofiber network upon exposure to H2O2 vapors at different concentrations. (b) Schematic representation of the fluorescence quenching mechanism of the interpenetrated nanofiber network by H2O2.

We next investigated the possibility of the interpenetrated nanofiber network to detect TATP vapor, which represents one of the most dangerous primary explosives.19,20 Likely because of no reaction between TATP and Co2+, the interpenetrated nanofiber network exhibited no fluorescence quenching but slightly enhanced responses when exposed to TATP vapors (Figure S8), analogous to the case of common organic solvents.25 To address this, we redesigned the optical chamber (Figure 3a) and applied a solid-acid catalyst, amberlyst-15,19 to decompose TATP into H2O2 for the detection. As shown in figure 3b, the interpenetrated nanofibers network in the new optical chamber exhibited marked fluorescence quenching upon exposure to TATP vapor at 0.7 ppm, indicating that the nanofiber network can detect low levels of TATP vapor from its decomposed H2O2. To be compatible with the detection of other classes of explosives where the solid samples are vaporized at relatively high temperature (e.g., 140 oC), solid TATP samples were also tested on the nanofiber network. As shown in Figure 3c, the interpenetrated nanofibers network was very responsive to solid TATP vaporized at 140 oC with a limit of detection of 20 ng. Notably, the redesigned optical chamber that contained the solid-acid catalyst had no effect on the direct detection of H2O2 where very similar sensitivity was observed compared to that obtained using the optical chamber without the solid-acid catalyst. Therefore, by using this redesigned optical chamber, the interpenetrated nanofiber network is capable of sensitive detection of H2O2 and TATP from its decomposed H2O2.

Figure 3. (a) Schematic diagram of the redesigned optical chamber that contained a solid-acid catalyst. (b) Fluorescence quenching of the interpenetrated nanofibers network upon exposure TATP vapors. (c) Fluorescence quenching of the interpenetrated nanofibers network upon exposure to o various amounts of TATP that were vaporized at 140 C.

We finally explored the sensing performance of the interpenetrated nanofiber network in the redesigned optical chamber to other five classes of explosives. As shown in Figure 4, the interpenetrated nanofiber network exhibited high sensitivity via fluorescence quenching to other explosives that were vaporized at 140 oC. Actual limits of detection for S8, TNT, DNT, RDX, PETN, NG, and DMNB were 0.1 ng, 0.1 ng, 0.2 ng, 1 ng, 1ng, 2 ng, and 10 ng, respectively (Figure 4h). Here, the sensitivity of the interpenetrated nanofiber network to various explosives was similar to that of individual 2 nanofibers.18 Given the facts that 1-Co2+ coordination nanofibers did not influence the fluorescence of 2 nanofibers and these explosives cannot reduce Co2+, the detection of these five classes of explosives should result from an electron transfers from excited 2 nanofibers to these explosives, which thereby resulted in the fluorescence quenching of 2 nanofibers. In addition, considering the importance of anti-interference performance of sensors in practical applications, the selectivity of the interpenetrated nanofiber network to explosives against various potential interferents was evaluated. As shown in Figures S9 and S10, all tested interferents (including water) even at relatively high concentrations gave rise to fluorescence enhancement responses because of the solvent effect, indicative of the high selectivity of the nanofibers network to explosives against these interferents. In addition, the fluorescence quenching response of the nanofibers network to various explosives was fast (a couple of seconds, see Figures 2-4), which endowed the utilization of the fluorescence response at 5 s after exposure to explosives as the warning signal of the device (EF5000, HT-Nova) in the real-life test. These above observations allow us to conclude that the interpenetrated nanofiber network can take full advantages of features of the individual nanofibers to work as a broad-band fluorescence sensor for six classes of explosives.



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ASSOCIATED CONTENT Supporting Information 2+

Synthetic procedure, fabrication of 1-Co coordination nanofibers, 2 nanofibers, and the interpenetrated supramolecular nanofibers, property and sensing characterizations, other supporting figures including SEM images, XRD data, UV−vis and fluorescence spectra (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +1186-10-82617315.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the NSFC (Grants 21577147, 21590811, and 21521062), the “Strategic Priority Research Program” of the CAS (Grant XDA09030200) and the “Key Research Program of Frontier Sciences” (No. QYZDY-SSWSLH028) of the Chinese Academy of Sciences.

REFERENCES

Figure 4. Fluorescence quenching of the interpenetrated nanofiber network upon exposure to various amounts of S8 (a), TNT (b), DNT (c), RDX (d), PETN (e), NG (f) and DMNB (g). Inset: zoomed-in images of the fluorescence quenching. (h) Actual limits of detection for explosives by using the interpenetrated nanofiber network.

In conclusion, we have fabricated the interpenetrated binary nanofiber network via a sequential self-assembly approach for the simultaneous detection of peroxide explosives (i.e., H2O2 and TATP) along with other five classes of explosives. The interpenetrated nanofibers network can take full advantage of features of the individual nanofibers to act as a broad-band fluorescence sensor for various explosives. When exposed to peroxide explosives (e.g., H2O2), the formed Co+ in 1-Co2+ coordination nanofibers by H2O2 can transfer an electron to the excited 2 nanofibers and thereby quench their fluorescence. On the other hand, the excited 2 nanofibers in the penetrated nanofiber network can transfer an electron to other five classes of explosives to quench their fluorescence. On the basis of the distinct fluorescence quenching mechanism on the broad-band fluorescence sensor, six classes of explosives can be sensitively and selectively detected. Herein, we provide a new strategy, i.e., the development of the complex nanofiber network as broad-band fluorescence sensors, which may open up many opportunities for a richer identification of threats.

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Analytical Chemistry (19) Lin, H.; Suslick, K. S. J. Am. Chem. Soc. 2010, 132, 1551915521. (20) Germain, M. E.; Knapp, M. J. Inorg. Chem. 2008, 47, 9748-9750. (21) Bohrer, F. I.; Colesniuc, C. N.; Park, J.; Schuller, I. K.; Kummel, A. C.; Trogler, W. C. J. Am. Chem. Soc. 2008, 130, 3712-3713. (22) Zhang, X.; Chen, Z.-K.; Loh, K. P. J. Am. Chem. Soc. 2009, 131, 7210-7211. (23) Gilmartin, M. A.; Ewen, R. J.; Hart, J. P.; Honeybourne, C. L. Electroanalysis 1995, 7, 547-555. (24) We performed X-ray photoelectron spectroscopy (XPS) experiments of the interpenetrated nanofibers before and 3+ after exposure to H2O2 and confirmed that no Co was formed after exposure to H2O2 (Figure S11). These observa3+ tions resembled the case of CoPC where no Co was found in 23 the CoPC film after exposure to H2O2. Based on these re+ sults, we conclude that Co is generated from the reaction 2+ between Co and H2O2 in our system. (25) 2 nanofibers exhibited slight fluorescence enhancements upon exposure to common solvents, see Figure S9.



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Interpenetrated Binary Supramolecular Nanofibers for Sensitive

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