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Ultrasensitive Detection of Sulfur Mustard via Differential Noncovalent Interactions Changkun Qiu, Xiaoling Liu, Chuanqin Cheng, Yanjun Gong, Wei Xiong, Yongxian Guo, Chen Wang, Jincai Zhao, and Yanke Che Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00709 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019
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Analytical Chemistry
Ultrasensitive Detection of Sulfur Mustard via Differential Noncovalent Interactions Changkun Qiu,†,‡ Xiaoling Liu,†,‡ Chuanqin Cheng†,‡ Yanjun Gong,†,‡ Wei Xiong,†,‡ Yongxian Guo,†,‡ Chen Wang,§ Jincai Zhao,†,‡ and Yanke Che*,†,‡ †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 *Corresponding author, Email:
[email protected]; Tel: +86-10-82612075 ABSTRACT: In this work, we fabricate two types of hierarchical microspheres, i.e., one coassembled from two fluorene-based oligomers (1 and 2) and one self-assembled from a fluorene-based oligomer (1), to ultrasensitive and selective detection of trace sulfur mustard (SM) vapor. On the basis of distinct fluorescence responses of 1-2 coassembled and individual 1 hierarchical microspheres that originate from differential noncovalent interactions between analytes and these sensors, SM vapor can be ultrasensitively detected (30 ppb) and easily discriminated from various sulfides and other potential interferents. Our work that utilizes differential noncovalent interactions to give sensitive and selective fluorescence response patterns represents a new detection approach for SM and other hazardous chemicals. As a powerful blistering agent in chemical warfare agents (CWAs), sulfur mustard (SM) can not only lead to the direct damage to the skin, eyes, and lungs, but also cause carcinogenic and mutagenic effects after long-term exposure.1-11 Worse still, toxicity of SM once dispersed remains active in the environment for hours to several weeks depending on the environmental conditions (e.g., temperature and pH of the soil).12 Therefore, development of sensitive and selective sensors for this chemical has attracted considerable attention. However, as a simple, rapid, portable, and highly sensitive detection technology among various detection technologies, fluorescence detection methods of SM or 2-chloroethyl ethyl sulfide (CEES, known as “half mustard”) are very limited,3-7 in contrast to many methods for nerve agents.13-25 The established fluorescence methods for SM or CEES have focused on nucleophilic attack of the probe molecule on the electrophilic site of SM to cause fluorescence signals. For example, Anslyn and co-workers developed an elegant strategy, which involved the analyte-bound receptor and the metalindicator complex or squaraine dye, to selectively detect CEES in the solution and the gas phase.4,5 Kumar and other groups used other fluorescent probes to selectively react with SM to give a turn-on detectable emission.3,7 Despite these recent advances, novel fluorescence detection methods particularly capable of rapid detection of SM below 60 ppb (no adverse effect upon exposure of 10 min11) remains challenging. It is previously reported that nucleophilic attack interactions between probe molecules and SM or CEES required a relatively long duration. However, noncovalent interactions, such as sulfur26-28 and dipole-dipole interactions, may allow rapid and sensitive detection but has not yet been applied to the detection of SM. In this work, we report the fabrication of fluorescent 1-2 coassembled and individual 1 hierarchical microspheres for ultrasensitive and selective detection of trace SM vapor. We demonstrate that differential noncovalent interactions between SM and the two sensing materials, i.e., sulfur- and dipole-dipole interactions, lead to sensitive fluorescence responses, thereby enabling detection of as low as 30 ppb of SM vapor. More
importantly, the resulting fluorescence patterns are distinct from those caused by different sulfides and other potential interferents, thereby giving high detection selectivity of SM. Molecule 1 that bears the 9, 9-dihexyl fluorene units is first designed and synthesized in this work (Figure 1a). The detailed synthesis and characterization methods of 1 are described in the Supporting Information. The self-assembly of 1 was performed by injecting 0.3 mL dichloromethane solution of 1 (4 mM) into 3 mL methanol in a vial and aged for 1 day. Scanning electron microscopy (SEM) showed 1 hierarchical microspheres were formed (Figure 1b). The magnified SEM image further revealed that these hierarchical microspheres were composed of radial nanoribbons with thicknesses of 10-30 nm and widths of 50-80 nm (Figure 1c). A fluorescence microscopic image showed that 1 hierarchical microspheres were blue-emissive (Figure 1d), which had a fluorescence quantum yield of ca. 35%. The weak intermolecular π-coupling of 1 should lead to the high emission efficiency of hierarchical microspheres. To support this, we performed optical characterizations of 1 hierarchical microspheres and free 1 molecules in dichloromethane. Figure 1e showed that the absorption peak of 1 hierarchical microspheres was red-shifted only ca. 8 nm compared with free 1 in dichloromethane, indicative of weak π-coupling of 1 in hierarchical microspheres. The weak π-coupling of 1 was also reflected in the fluorescence spectrum of hierarchical microspheres where the emission was red-shifted ca. 30 nm compared to monomer 1 in dichloromethane (Figure 1e). We used selected area electron diffraction (SAED) (Figure 1f) and X-ray diffraction (XRD) (Figure 1g) to further investigate the molecular packing of these results, the molecular packing of 1 within the nanoribbon can be simulated, as shown in Figures S1 and S2. We then performed sensing experiments for CEES and other sulfides using a home-built optical chamber (Figure 2a) coupled with an Ocean Optics USB4000 fluorometer. Intriguingly, exposure of individual 1 hierarchical microspheres that were cast into a quartz tube to trace CEES vapors gave ratiometric fluorescence
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responses. As shown in Figure 2b, the fluorescence in the range of 410-519 nm decreased while the fluorescence in the range of 519-
Figure 1. (a) Molecular structure of 1. (b, c) SEM images of 1 hierarchical microspheres. (d) Fluorescence-mode optical microscopic image of 1 hierarchical microspheres. (e) Normalized UV-vis absorption spectra (black) and fluorescence spectra (red) of 1 hierarchical microspheres (solid) cast on a glass slide and free 1 in dichloromethane (dashed). (f) SAED pattern of 1 nanoribbon. (g) XRD patterns of 1 hierarchical microspheres. 650 nm increased concurrently. The existence of an isobestic point at 519 nm suggests the formation of a product from 1 and CEES that leads to ratiometric fluorescence changes. The timecourse curve of ratiometric fluorescence responses further revealed the fluorescence quenching behaviors in the range of 420-460 nm and the fluorescence enhancement behaviors in the range of 520-560 nm (Figure 2c). Notably, the actually measured detection limit for CEES reached as low as 10 ppb (Figure 2c) and the ratiometric fluorescence response time were only ca. 5 s (Figure 2d). Notably, although CEES-induced orthogonal fluorescence responses were irreversible, trace CEES can be detected multiple times until the fluorescence intensity dropped to ca. 50%. After this value, the orthogonal fluorescence responses were not proportional to the concentration of CEES. We hypothesized that the morphology of 1 hierarchical microspheres favors the accumulation of CEES and creates the relatively high uptake capacity. To support this, we fabricated the random film by ultrasonication 1 hierarchical microspheres and compared its multiple detection performance to trace CEES. As shown in Figure S3, when the fluorescence intensity of the random film dropped to ca. 15%, the further fluorescence responses were no longer proportional to the concentration of CEES, indicating a lower uptake capacity of the flat nanoribbon film compared to 1 hierarchical microspheres. Given the lack of nucleophilic and hydrogen bond sites in molecule 1, we envisioned that the sulfur- interaction between CEES and 1 generated a complex that resulted in the fluorescence quenching of 1 hierarchical microspheres and concurrently the emission at longer wavelengths (Figure 2b). To support this hypothesis, we monitored and analyzed fluorescence responses of
Figure 2. (a) Schematic representation of the instrument for the sensing experiments. (b) Fluorescence spectra of 1 hierarchical microspheres when exposed to CEES vapors at 0 ppb, 10 ppb, 30 ppb, and 60 ppb. Inset: zoomed-in image of fluorescence spectra in the range of 520-580 nm. (c) Time-course curves of fluorescence responses of 1 hierarchical microspheres monitored at the range of 420-460 nm (blue) and 520-560 nm (red) upon exposure to CEES vapors. Inset: zoomed-in image of the corresponding fluorescence responses. (d) The response time of 1 hierarchical microspheres to 60 ppb CEES recorded in the range of 420-460 nm (blue) and 520-560 nm (red). (e) Time-course curves of fluorescence responses of 1 hierarchical microspheres monitored at the range of 420-460 nm (blue), and 520-560 nm (red) upon exposure to SM vapors. (f) Fluorescence spectra of 1 hierarchical microspheres upon exposed to SM vapors at different concentrations. 1 hierarchical microspheres when exposed to different sulfides that were expected to have similar sulfur- interactions with 1. Indeed, exposure of 1 hierarchical microspheres to different sulfides gave similar orthogonal fluorescence responses (Figure S4). These observations indicate that sulfur- interactions lead to -coupled complexes, which acted as the FRET acceptors to cause the fluorescence quenching of 1 hierarchical microspheres followed by emission at longer wavelengths. Notably, different sulfides actually induced distinct extents of intensity changes in turn-on and turn-off fluorescence responses (Figure S4), suggesting that the substitutes in sulfides have a significant effect on sulfur- interactions. This phenomenon may be even more remarkable because two chlorine substituents in SM can strongly interact with S atom and thereby weaken or prevent sulfur- interactions.4,12 Indeed, as shown in Figure 2e-f, exposure of 1 hierarchical microspheres to SM vapors gave fluorescence enhancement in the whole emission, which is in sharp contrast to fluorescence quenching by sulfur- interactions as aforementioned. Such fluorescence responses by SM should not result from sulfur- interactions but the solvent effect. This hypothesis is supported by the fact that 1,6-dichlrohexane caused similar fluorescence enhancement (Figure S5). To achieve the selective detection of SM against various interferents, we designed molecule 2 (Figure 3a) that has two benzothiadiazole units to yield enhanced dipole-dipole interactions with the electrophilic site of SM. We also expected that enhanced dipole-dipole interactions can restrict the interactions between chlorine substituents and sulfur atoms
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Analytical Chemistry andthus favor sulfur- interactions for fluorescence responses. Encouragingly, exposure of nanoribbons self-assembled from 2 (Figure S6) to SM vapors gave fluorescence quenching responses (Figure S7) likely because of sulfur- interactions. However, the limit of detection (LOD) of SM was relatively high, i.e., 0.8 ppm. Given that 2 nanoribbons also exhibited high LOD of other sulfides (Figure S8), we envisioned that sulfur- interactions formed between 2 and sulfides were compared to the case of 1 and sulfides. These observations further inspired us to fabricate coassembled aggregates from 1 and 2 to increase sulfur- interactions and in turn to improve the de-
Importantly, the LOD was largely decreased to 30 ppb. Furthermore, the fluorescence quenching response was very fast, i.e., ca. 4 s (Figure 3f), which would facilitate the practical application in the real-field detection of SM. Likewise, 1-2 coassembled hierarchical microspheres exhibited enhanced fluorescence quenching responses to other sulfides vapors compared to individual 2 nanoribbons (Figures S8 and S10). Although fluorescence responses of 1-2 coassembled hierarchical microspheres alone cannot be used to selectively detect SM, these results along with distinct responses of individual 1 hierarchical microspheres to SM from other sulfides allow the easy discrimination of SM against other sulfides, as illustrated in Figure 4a. Therefore, a two-member sensor array consisting of 1-2 coassembled and 1 hierarchical microspheres can be fabricated to achieve selective and ultrasensitive detection of SM vapors in practical applications. Finally, we evaluated the selectivity of 1-2 coassembled and 1 hierarchical microspheres for SM against other potential interferents, including diethyl chlorophosphate (DCP), dimethyl methyl phosphonate (DMMP), acids (e.g., HCl), water, and common organic solvents. As shown in Figure S11 and Figure 4b, 1-2 coassembled hierarchical microspheres exhibited no responses to these potential interferents, indicative of high detection selectivity for SM and different sulfides. In contrast, 1 hierarchical microspheres exhibited either modest fluorescence enhancement or fluorescence quenching in the range of 420-460 nm and 520-560 nm when exposed to DMMP, 1,6-dichlorohexane, DCP, HCl, and common organic solvents at relatively high concentrations (Figure S12). These parallel responses can be explained by a swelling mechanism where these molecules swell 1 nanoribbons within hierarchical microspheres and reduce intermolecular π-interactions that decrease the emission. Obviously, these respons es of the two-member (1-2 coassembled and 1 hierarchical microspheres) sensor array to various interferents were distinct from their responses to SM and different sulfides, thereby yielding good selectivity against other various interferents as summarized in Figure 4b.
Figure 3. (a) Molecular structure of 2. (b, c) SEM images of 1-2 coassembled hierarchical microspheres with a molar ratio at 3:1. (d) Fluorescence-mode optical microscopic image of 1-2 coassembled hierarchical microspheres. (e) Normalized UV-vis absorption spectra (black) and fluorescence spectra (red) of 1 hierarchical microspheres (dashed) cast on a glass slide and free 2 in dichloromethane (solid). (f) Time-course curve of fluorescence responses of 1-2 coassembled hierarchical microspheres upon exposure to SM vapors. Inset: The response time of 1-2 coassembled hierarchical microspheres to SM vapor at 80 ppb. (g) Fluorescence spectra of 1-2 coassembled hierarchical microspheres upon exposure to SM vapors at different concentrations. tection sensitivity. Intriguingly, the coassembly of 1 and 2 with different molar ratios resulted in hierarchical microspheres with the morphology similar to 1 hierarchical microspheres (Figure 3b, c and Figure S9). We used coassembled hierarchical microspheres with the molar ratio of 1 to 2 at 3:1 in the following sensing experiments after balancing dipole-dipole and sulfur- interactions that coassembled hierarchical microspheres give. 1-2 coassembled hierarchical microspheres exhibited greenish emission (Figure 3d, g) that was ascribed to the emission of individual 2 (Figure 3e). This is because the good overlap of the absorption of individual 2 and the emission of 1 nanoribbons would allow an effective Förster resonance energy transfer (FRET) from 1 aggregates to 2 followed by emission. Exposure of 1-2 coassembled hierarchical microspheres to trace SM vapors resulted in marked fluorescence quenching (Figure 3f, g).
Figure 4. Columnar comparison of fluorescence responses of 1-2 coassembled and 1 hierarchical microspheres upon exposure to SM and different sulfides (a), as well as other different interferents (b) at various concentrations. ΔI/I0 represents the
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change value of fluorescence intensity. Error bars represent the standard deviation of five measurements. In conclusion, we fabricate 1-2 coassembled and individual 1 hierarchical microspheres and use them as a two-member sensor array for SM vapor. We demonstrate that differential noncovalent interactions between different analytes and sensing materials, such as sulfur- and dipole-dipole interactions, lead to distinct fluorescence responses (Figure 4), thereby enabling ultrasensitive detection of SM vapor (30 ppb) and high detection selectivity against various sulfides and other interferents. Here, our findings, i.e., utilizing differential noncovalent interactions to generate selective fluorescence response patterns, represent a new detection approach for SM that may be extended to the detection of other hazardous chemicals.
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]. Fax: +86-10-82617315
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by the “National Key R&D Program of China” (No. 2018YFA0209302), 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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