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Sensitive Discrimination of Nerve Agent and Sulfur Mustard Simulants Using Fluorescent Co-Assembled Nanofibers with FRET-Enhanced Photostability and Emission Wei Xiong, Yanjun Gong, Yanke Che, and Jincai Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05225 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019
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Analytical Chemistry
Sensitive Discrimination of Nerve Agent and Sulfur Mustard Simulants Using Fluorescent Co-Assembled Nanofibers with FRETEnhanced Photostability and Emission Wei Xiong,†,‡ Yanjun Gong,†,‡ 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 *Corresponding author, Email:
[email protected]; Tel: +86-10-82612075.
ABSTRACT: In this work, highly sensitive discrimination of nerve agent and sulfur mustard simulants is achieved by using photostable and fluorescent co-assembled nanofibers from molecules 1 and 2. We demonstrate that the introduction of 2 as a FRET acceptor not only enhances the photostability and emission efficiency compared to individual 1 nanofibers, but also induces different binding interactions between analytes and 1-2 co-assembled nanofibers and thereby distinct fluorescence quenching behaviors used for the discrimination of nerve agent and sulfur mustard simulants. Our findings represent an important advance toward sensitive detection and discrimination of CWAs. We demonstrate that Förster resonance energy transfer (FRET) from the non-photostable 1 to photostable 2 can greatly enhance the emission efficiency and the photostability of co-assembled nanofibers thus formed. Importantly, exposure of 1-2 co-assembled nanofibers to trace DCP and CEES gives rise to remarkable fluorescence quenching because the formed complexes between DCP or CEES and 1 can act as exciton traps to compete with FRET, thereby decreasing the emission. Furthermore, 2 within co-assembled nanofibers yields the competitive binding of CEES against 1 to give distinct fluorescence quenching behaviors, which thereby enables the easy recognition of DCP and CEES (Figure 1).
Nerve agents and sulfur mustard, two typical classes of chemical warfare agents (CWAs), are highly toxic chemicals and terroristic use of them severely threatens humankind and homeland security.1-5 Therefore, the sensitive and selective detection of these CWAs has attracted considerable attention. A variety of detection technologies, e.g., ion-mobility spectrometry, flame photometry, electrochemistry, mass spectrometry, photoacoustic infrared spectroscopy, among others, have been developed to sensitively detect CWAs.2,6,7 Compared to these detection technologies that may suffer from one or more disadvantages including high cost, complicated operation, and poor portability, simple and sensitive fluorescence sensing has been widely used in sensitive detection of CWAs.1-3,6-27 However, most of reported fluorescence detection methods have been used for nerve agents.8-27 and very few examples of fluorescent detection of sulfur mustard have been reported.2,3,6,7 However, to the best of our knowledge, fluorescence sensors that are capable of sensitive detection and concurrent discrimination of nerve agents and sulfur mustard do not exist. This is likely because sulfur mustard is a weak electrophile in gas, distinct from the electrophilic nerve agents.1,2 Therefore, development of a fluorescence sensor that can sensitively detect nerve agents and sulfur mustard and concurrently discriminate between them is highly desirable but remains challenging.
Figure 1. Schematic representation of the fluorescence detection and discrimination of DCP and CEES using the coassembled nanofibers from molecules 1 and 2. Based on previously reported work,12 the complex structures as circled were presented.
In this work, we report the sensitive discrimination of nerve agents and sulfur mustard simulants (i.e., diethyl chlorophosphate, DCP and 2-chloroethyl ethyl sulfide, CEES) by utilizing photostable and fluorescence nanofibers co-assembled from molecules 1 and 2 (Figure 1).
Recently, we reported the utilization of 1 nanofibers for ratiometric fluorescence detection of trace DCP vapors
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real FRET efficiency should be much higher than 76%. The emission changes can also be easily observed by optical microscopic images (Figure S6). Notably, the efficient energy transfer from non-stable 1 to 2 effectively limits nonradiative decay pathways and gives a fluorescence quantum yield of 25-30% of co-assembled nanofibers. This value is much larger than the emission efficiency of individual 1 nanofibers (ca. 7.3%).
with high signal amplification,9 However, further inspection of the sensing of 1 nanofibers for CEES vapors showed ratiometric fluorescence responses similar to those caused by DCP vapors (Figure S1). This indicates that the electrophilic attack on two pyridine end groups of 1 by CEES can yield the product with the charge-transfer (CT) emission similar to the pyridine-phosphorylated 1 formed by the DCP attack.9 This further indicates that 1 nanofibers cannot be used to discriminate between nerve agents and sulfur mustard. In addition, we found that 1 nanofibers exhibited the severe photobleaching where the fluorescence quickly decreased ca. 60% under 385 nm UV light irradiation (Figure 3), which greatly constrained their applications in practice. To address these problems, we here synthesized photostable molecule 2 that was incorporated into 1 nanofibers to enhance the photostability and the discriminatory capability. The detailed synthesis and self-assembly of 2 were provided in the supporting information. After the high photostability of nanofibers self-assembled from 2 was confirmed (Figure S2), we performed the co-assembly of 1 and 2 with different molar ratios. Typically, the co-assembly of 1 and 2 with a molar ratio at 10:1 was performed by injecting 0.14 mL dichloromethane solution of 1 (1.75 mM, 0.1 mL) and 2 (0.42 mM, 0.04 mL) into 0.5 mL hexane in a vial and aged for 2 days. Scanning electron microscopy (SEM) reveals that the resulting nanofibers have diameters of 50-100 nm and lengths of several micrometers that entangle each other to form a porous film (Figure 2a, b). The co-assembly of 1 and 2 with different molar ratios resulted in the similar nanofiber morphology (Figure S3). We performed X-ray diffraction (XRD) measurements to verify that 1 and 2 are co-assembled rather than self-sorted. As shown in Figure S4, XRD patterns of co-assembled nanofibers are not a simple sum of nanofibers self-assembled from individual 1 and 2, indicating that 1 and 2 were co-assembled rather than self-sorted into nanofibers. Figure 2c shows the absorption and fluorescence spectra for both 1 nanofibers and 2 molecularly dissolved in solution. The overlap of the emission of 1 nanofibers and the absorption of 2 can give rise to an efficient FRET. Indeed, only emission of 2 was observed for 1-2 co-assembled nanofibers even when the molar ratio of 1 to 2 was increased to 50:1 (Figure 2d). Further increasing of the molar ratio of 1 to 2 resulted in the appearance of the emission of 1 nanofibers (Figure 2d). This indicates that the exciton diffusion length of 1 nanofibers involve more than 50 molecules. Beyond this value, some excitons cannot transport to sites containing 2 molecules and undergo FRET followed by emission. We used co-assembled nanofibers with the molar ratio of 1 to 2 at 100:1 to evaluate the energy transfer efficiency because the emission at 466 nm assigned to 1 nanofibers can still be collected. As shown in Figure S5, the fluorescence lifetime of the co-assembled nanofibers decreased to 23 ps compared to that of 1 nanofibers (96 ps), which gave the energy transfer efficiency (η) of ca. 76%. Given that the collected emission at 466 nm includes a contribution of excitons of 1 nanofibers beyond the diffusion length, the
Figure 2. (a) SEM image of nanofibers co-assembled from 1 and 2 with the molar ratio of 10:1. (b) Magnified SEM image of the co-assembled nanofibers in (a). (c) Normalized absorption spectra (blue) and normalized fluorescence spectra (red) of 2 in dichloromethane (solid) and 1 nanofibers suspended in ethanol (dashes). Inset: zoomed-in image of the absorption spectrum of 2 in dichloromethane at range of 400-500 nm (green area). (d) Fluorescence spectra of individual 1 nanofibers (orange dashes), 2 nanofibers (black dashes), and co-assembled nanofibers with different molar ratios of 1 to 2 (solid) at 10:1 (red), 20:1 (blue), 50:1 (magenta), 100:1 (olive), 200:1 (navy), and 500:1 (violet).
Given that the excitation energy of non-photostable 1 can efficiently transfer to photostable 2, we hypothesized that the energy transfer can diminish the photooxidation of 1 and thereby enhance the photostability of coassembled nanofibers. To support this hypothesis, the fluorescence intensity and spectra of 1-2 co-assembled nanofibers with different molar ratios as a function of the photoirradiation time were monitored. Figure 3 shows that the fluorescence intensity of co-assembled nanofibers decreases much slower than that of 1 nanofibers. For example, the fluorescence intensity of co-assembled nanofibers with a molar ratio of 1 to 2 at 100:1 decreased ca. 25% after 1 h of continuous irradiation, while that of 1 nanofibers decreased ca. 60% under identical conditions. The enhanced photostability of co-assembled nanofibers compared to 1 nanofibers can also be reflected on their time-dependent spectra where the whole spectra of coassembled nanofibers dropped much slower (Figure S7) than that of 1 nanofibers. These results indicate that the efficient energy transfer from 1 to photostable 2 followed
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Analytical Chemistry by emission via FRET can effectively enhance the photostability of co-assembled nanofibers. This thereby provides a simple way to improve the photostability of existing emissive materials that suffer from the severe photobleaching.
Figure 3. (a) Fluorescence intensity of 2 nanofibers (black) and 1 nanofibers (purple), and co-assembled nanofibers with different molar ratios of 1 to 2: 10:1 (red), 20:1 (blue), 50:1 (magenta), 100:1 (olive), 200:1 (dark yellow), and 500:1 (violet), as a function of the irradiation time. (b) Fluorescence quenching of various nanofibers after 1 h of continuous irradiation.
Figure 4. (a) Fluorescence responses of 100:1 co-assembled nanofibers to trace DCP vapors. (b) Fluorescence spectra of the same 1-2 co-assembled nanofibers recorded at time points marked in (a). Inset: zoomed-in image of fluorescence spectra. (c) Fluorescence responses of 100:1 co-assembled nanofibers to CEES vapors at different concentrations. (d) Fluorescence spectra of the same 1-2 co-assembled nanofibers recorded at time points marked in (c). Inset: zoomed-in image of fluorescence spectra.
Having obtained the enhanced photostability and emission, we next explored the detection sensitivity and discriminatory capability of these co-assembled nanofibers for DCP and CEES. Because individual 2 nanofibers (Figure S8) exhibited very weak sensitivity to DCP and CEES compared to individual 1 nanofibers (Figure S9). Coassembled nanofibers with a molar ratio of 1 to 2 at 100:1 (100:1 co-assembled nanofibers), which have a balance of relatively good photostability and sensitivity, were chose for the following sensing experiments. As shown in Figure 4a, exposure of these co-assembled nanofibers to trace DCP vapors resulted in marked irreversible fluorescence quenching. In practice, the measured lowest concentration of DCP vapor was as low as 8 ppb (Figure 4a). Notably, 100:1 co-assembled nanofibers only exhibited irreversible fluorescence quenching (Figure 4b), which is in sharp contrast to individual 1 nanofibers that exhibited ratiometric fluorescence responses.9 This should be a result of the formation of pyridine-phosphorylated 1 complexes that act as exciton traps, thereby competing with the FRET and decreasing the emission (Figure 1). Notably, although the pyridine-phosphorylated 1 complexes may simultaneously give the intramolecular charge transfer (ICT) emission at the range of 500-660 nm, the increase in emission of 1 complexes (a fluorescence quantum yield of ca. 9.0%)9 cannot offset the decrease in FRET emission of co-assembled nanofibers (a fluorescence quantum yield of 25-30 %). Therefore, only fluorescence quenching responses were observed at the range of 500660 nm for the co-assembled nanofibers. Importantly, the fluorescence quenching of co-assembled nanofibers is very fast, i.e., ∼3 s (Figure S10), which facilitates their real-field application.
Intriguingly, exposure of 1-2 co-assembled nanofibers to trace CEES vapors gave rise to partially reversible fluorescence quenching behaviors, i.e., 50 % fluorescence intensity was recovered after exposure (Figure 4c). The actually measured detection limit of CEES was 0.3 ppm (Figure 4c), which is among the lowest values ever reported.2,3,6,7 The partial fluorescence reversibility after exposure to CEES vapors was also reflected on the timedependent spectra changes (Figure 4d). Given irreversible ratiometric fluorescence responses of individual 1 nanofibers for CEES (Figure S1), the 50 % fluorescence reversibility of 100:1 co-assembled nanofibers after exposure to CEES should result from the influence of molecule 2 that facilitates the release of CEES from the resulting 1-CEES complex via competitive interaction with CEES. Notably, CEES is a relatively weak electrophilic agent that would form unstable 1-CEES complex, whereas DCP is a relatively strong electrophilic agent that would yield stable 1-DCP complex resistive to the interaction competition from 2. To confirm the influence of 2 on the binding of CEES, co-assembled nanofibers with molar ratios of 1 to 2 at 10:1 and 50:1 were employed to detection CEES vapors. As shown in Figure S11, the extents of the fluorescence quenching reversibility increase with the 2 proportion within co-assembled nanofibers, indicative of the releasing effect of 2 on trapped CEES. Importantly, such distinct fluorescence quenching behaviors of coassembled nanofibers for DCP and CEES enable the discrimination between them. Finally, we inspected the selectivity of 1-2 co-assembled nanofibers for DCP and CEES against the potential interferents, including diethylcyanophosphonate (DCNP),
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hydrogen chloride (HCl), dimethyl methylphosphonate (DMMP), common organic solvents, and water. Figure S12 shows that exposure of 100:1 co-assembled nanofibers to most of interferents gave a very fast and completely reversible fluorescence quenching responses except HCl. Such fluorescence response patterns are obviously different from the fluorescence quenching behaviors by DCP and CEES and thereby do not interfere with the DCP and CEES detection. Like the case of individual 1 nanofibers,9 HCl-induced responses of 1-2 co-assembled nanofibers are much faster than those by DCP and thus can be discriminated from the DCP detection. These results demonstrate that 1-2 co-assembled nanofibers have high selectivity for CWAs against various interferences, analogous to individual 1 nanofibers. In conclusion, we have fabricated co-assembled nanofibers from molecules 1 and 2 for highly sensitive discrimination of nerve agent and sulfur mustard simulants. The introduction of molecule 2 that acts as a FRET acceptor not only enhances the photostability and emission efficiency of the resulting co-assembled nanofibers, but also results in distinct fluorescence quenching behaviors of co-assemble nanofibers upon exposure to DCP and CEES, thereby enabling the discrimination between them. Our findings represent an important advance toward developing fluorescence sensor for sensitive detection and discrimination of CWAs.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedure, property and sensing characterizations, SEM images, and fluorescence responses. (PDF)
AUTHOR INFORMATION Corresponding Author *Email:
[email protected].
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the NSFC (Nos. 21577147, 21590811, and 21677148) and the “Key Research Program of Frontier Sciences” (No. QYZDY-SSW-SLH028) of the Chinese Academy of Sciences.
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(19) You, L.; Zha, D.; Anslyn, E. V. Recent Advances in Supramolecular Analytical Chemistry Using Optical Sensing. Chem. Rev. 2015, 115, 7840-7892. (20) Pangeni, D.; Nesterov, E. E. “Higher Energy Gap” Control in Fluorescent Conjugated Polymers: Turn-On Amplified Detection of Organophosphorous Agents. Macromolecules 2013, 46, 7266-7273. (21) Kim, Y.; Jang, Y. J.; Lee, D.; Kim, B.-S.; Churchill, D. G. Real nerve agent study assessing pyridyl reactivity: Selective fluorogenic and colorimetric detection of Soman and simulant. Sens. Actuators, B, 2017, 238, 145-149. (22) Kim, Y.; Jang, Y. J.; Mulay, S. V.; Nguyen, T. T. T.; Churchill, D. G. Fluorescent Sensing of a Nerve Agent Simulant with Dual Emission over Wide pH Range in Aqueous Solution. Chem. – Eur. J. 2017, 23, 7785-7790. (23) Sun, X.; Reuther, J. F.; Phillips, S. T.; Anslyn, E. V. Coupling Activity-Based Detection, Target Amplification, Colorimetric and Fluorometric Signal Amplification, for Quantitative Chemosensing of Fluoride Generated from Nerve Agents. Chem. – Eur. J. 2017, 23, 3903-3909.
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