Sensitive Detection of a Nerve-Agent Simulant through Retightening

Jan 11, 2018 - Fax: +86-10-82617315 (Y.C.)., *E-mail: [email protected]. Fax: +86-10-82617315 (W.X.). Cite this:Anal. Chem. 90, 3, 1498-150...
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Sensitive Detection of a Nerve-Agent Simulant through Retightening Internanofiber Binding for Fluorescence Enhancement Xiaoling Liu, Yanjun Gong, Yingxuan Zheng, Wei Xiong, Chen Wang, Tie Wang, Yanke Che, and Jincai Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04698 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Analytical Chemistry

Sensitive Detection of a Nerve-Agent Simulant through Retightening Internanofiber Binding for Fluorescence Enhancement Xiaoling Liu,†,‡,§ Yanjun Gong,†,‡ § Yingxuan Zheng,†,‡ Wei Xiong,*,†,‡ Chen Wang,∆ Tie 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

*Corresponding author, Email:[email protected]; [email protected]; Fax: +86-10-82617315.

ABSTRACT: In this work, we develop fluorescent hierarchical nanofiber bundles 1, which involve the internanofiber hydrogen-bonding interactions, for rapid and sensitive detection of DCP vapor. First, the internanofiber hydrogen-bonding strength can be weakened by photoexcitation, which thereby increases the internanofiber spacing and decreases the fluorescence intensity. Then, when exposed to trace DCP vapor, the strong interactions between DCP and hydroxyl groups on the nanofibers can effectively retighten the nanofibers and enhance the fluorescence as the detection signal. By contrast, the interferences, such as common organic solvents, cannot retighten nanofiber bundles 1 because of the lack of strong interactions with the nanofibers. Based on this novel detection mechanism, fluorescence detection of DCP with rapid signal response (ca. 3 s) and high sensitivity (15 ppb) is achieved.

As a simple, portable, cost-effective, and highly sensitive detection technology, fluorescence sensing has been widely 1,2 used in detecting various hazardous chemicals. Among them, fluorescence detection of diethyl chlorophosphate (DCP) has attracted intensive attention because DCP is a simulant of the nerve agent (NA) and also a chemical analogue of cholinesterase inhibiting organophosphate pesti3 cides. Various fluorescence sensors and probes that generally involved the formation of a phosphate ester via nucleophilic reactions and the concomitant fluorescence changes 4-20 have been developed to detect DCP. However, the principal limitations of these existing fluorescence approaches are either low sensitivity (high vapor concentration of DCP is needed) or slow response (more than tens of seconds are 3,6-8,10-15,17 needed). Therefore, the development of fluorescence sensors with rapid response, reliability, high sensitivity, and selectivity remains highly desirable.

cence enhancement as the detection signal (Figure 1). Importantly, this novel detection mechanism allows rapid signal response (ca. 3 s) and high sensitivity (15 ppb) to trace DCP.

In this work, we report the development of fluorescent hierarchical nanofiber bundles 1, which involve internanofiber hydrogen-bonding interactions, for rapid, sensitive, and selective detection of trace DCP vapors. We demonstrate that the internanofiber hydrogen-bonding strength can be weakened by photoexcitation, which results in the increase of the internanofiber spacing and thereby the decrease of the fluorescence intensity (Figure 1). When exposed to trace DCP vapors, the strong interactions between DCP and hydroxyl groups (via hydrogen-bonding or nucleophilic reaction) can effectively retighten the nanofibers and generate the fluores-

Figure 1. Molecular structure of 1 and schematic representation of the loosed nanofiber bundles 1 due to the weakened internanofiber hydrogen-bonding by photoexcitation and of the retightened nanofiber bundles 1 due to the strong interactions between DCP and hydroxyl groups on the nanofiber surface.

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The building block utilized in this study to assemble the suitable fluorescent nanofiber bundles is a perylene diimide (PDI) derivative 1, which bears a 4-(hydroxymethyl)benzyl group as one side chain and a dodecyl group as the other side chain (Figure 1). The detailed synthesis and characterizations of 1 are provided in the supporting information. Upon injection of 0.5 mL of a chloroform solution of 1 (0.074 mM) into 5 mL of ethanol and aging for 3 days, the nanofibers were formed and suspended in solution. Scanning electron microscopy (SEM) revealed that the resulting nanofibers 1 have lengths of more than ten micrometers (Figure 2a). Careful examination by the magnified SEM image shows that nanofibers 1 actually consisted of the bundled small nanofibers with diameters of 15-30 nm (Figure 2b). Given that the polar 4-(hydroxymethyl)benzyl groups should cover the surface of the resulting nanofibers to minimize the surface energy in 21-24 ethanol, internanofiber hydrogen-bonding interactions should play a critical role in the formation of the hierarchical nanofiber bundles. Such tightly bound hierarchical structure can facilitate the emission and thereby amplify the sensing 25 signal. These bundled nanofibers have a higher fluorescence quantum yield (ca. 19 %) than the nanoribbons with 26 larger dimensions (ca. 10%) (Figure S1), suggesting the advantages of the bundled hierarchical structure in emission efficiency. Intriguingly, the fluorescence intensity of nanofiber bundles rapidly decreased ca. 73% in the first 5 min of photoexcitation and then became stable afterward (Figure 2c). When the light was turned off for 2 h, the fluorescence intensity recovered to the original value when photoirradiated again (Figure 2c). These reversible switching can be repeated many times without any performance degradation (Figure 2c). Furthermore, the shape of fluorescence spectra were observed to remain the same when photoirradiated (Figure S2a), indicative of no molecular reorganization within the nanofibers. We hypothesize that the fluorescence decreasing under the photoirradiation is the result of the increased internanofiber spacing that is caused by the reduced internanofiber hydrogen-bonding interactions in the excited state. This hypothesis was supported by theoretical calculations of the electrostatic potential (ESP) of molecule 1 in the ground and excited states. The ESP values of ground and excited states of molecule 1 were obtained using the DFT (B3LYP/6-31G*) and TD-DFT (TD B3LYP/6-31G*) level of theory, respectively. As shown in Figure 2d, the ESP of oxygen atom in the hydroxyl group was increased from -38.97 kcal/mol in the ground state to -34.25 kcal/mol in the excited state. Such a marked ESP increase can greatly reduce the internanofiber hydrogen-bonding interactions and thereby increase the internanofiber spacing. When the photoirradiation was off, the hydrogen-bonding interactions can be restored with the recovery of the ESP of oxygen atom in the hydroxyl group and thereby retighten the nanofiber bundles to restore the emission. The fluorescence recovery kinetics after the photoirradiation was monitored (Figure S2b, c), which reflect the relatively slow but complete retightening of the nanofiber bundles because of the slow recovery of hydrogen-bonding interactions.

nanofiber bundles 2 were fabricated for comparison. Because molecule 2 bears a 4-methoxybenzyl group instead of a 4(hydroxymethyl)benzyl group as the side chain (see the Supporting Information), the assembled nanofibers bundles from 2 (Figure S3) involve no internanofiber hydrogenbonding interactions. Under identical photoirradiation conditions, nanofiber bundles 2 exhibited no fluorescence intensity changes with light-on time and experimental circles (Figure S4), which suggested the critical role of the reversible internanofiber hydrogen-bonding interactions in the breathing adjustment of the internanofiber spacing and the emission.

Figure 2. (a) SEM image of nanofiber bundles 1 deposited onto a silica substrate. (b) Magnified SEM image of nanofiber bundles 1. (c) Fluorescence intensity in response to turning on (5 min) and off the irradiation (120 min). (d) Theoretical calculations of the ESP of molecule 1 in the ground and excited states. We envisaged that the loosed nanofiber bundles 1 formed during photoexcitation may be retightened by the introduction of external molecules that have strong interaction with the nanofibers. Therefore, the nanofiber bundles 1 under photoexcitation would exhibit enhanced fluorescence responses when exposed to such external molecules. Such signal transduction can be used as a novel sensing mechanism

To further support that reversible hydrogen-bonding interactions with light on and off gave rise to the breathing adjustment of the internanofiber spacing and the emission,

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Analytical Chemistry (Figure 1). Given that DCP has strong interactions with hydroxyl groups via hydrogen-bonding or nucleophilic reac2,27,28 tion, we expected that trace DCP molecules enabled the enhancement of the fluorescence of nanofiber bundles 1 and thereby could be sensitively detected. We carried out the detection experiments for DCP vapors in a home-built optical chamber coupled with an Ocean Optics USB4000 fluo25 rometer. Indeed, as shown in Figure 3a, nanofiber bundles 1 exhibited considerable fluorescence enhancement when exposed to trace DCP vapor. The actually measured detection limit of DCP was as low as 15 ppb (Figure 3a). Although the enhanced fluorescence of the nanofiber bundles was not reversible, the multiple detection of trace DCP (e.g., 15 ppb) can be achieved (Figure S5) that was favorable for the practice application. Furthermore, the fluorescence enhancement response is very fast, i.e., ∼3 s (Figure 3b), which is also a critical parameter for the practical application. These observations indicate that trace DCP molecules have strong interactions with the nanofibers and enable retighten the loosed nanofiber bundles quickly; and that the internanofiber retightening can give rise to strong signal output required for the sensitivity.

ed from the direct interactions between 1 and DCP molecules. In addition, the strong interactions between DCP and the hydroxyl groups on the nanofiber surface that generated the fluorescence enhancement was also suggested by control experiments. When the nanofiber bundles assembled from 2 were used to detect DCP vapors, very weak fluorescence responses were observed (Figure S7) because the weak binding of DCP molecules on nanofibers 2 cannot effectively retighten the nanofibers. For sensor applications, selectivity is another significant parameter. For evaluating the selectivity of nanofiber bundles 1 to DCP, various potential interferences at relatively high vapor concentrations were tested, including common solvents, pesticides (e.g., benzene hexachloride (BHC) and chlorothalonil), hydrogen chloride (HCl), diethylcyanophosphonate (DCNP), and water. As shown in Figure 4a and Figure S8, nanofiber bundles 1 exhibited either fluorescence quenching or negligible fluorescence enhancement when exposed to various interferences at even relatively high concentration. This is in sharp contrast to the marked fluorescence enhancement caused by 0.11 ppm DCP vapor (Figure 4b). Obviously, the interferences have no sufficient interactions with the nanofibers to retighten nanofiber bundles 1 for the signal output. Notably, although acetic acid at tens of ppm caused the modest fluorescence enhancement, the fast enhance-decay response behavior enabled the easy discrimination of acetic acid from DCP (Figure S9). Therefore, the above results demonstrate that nanofiber bundles 1 have high selectivity for DCP against various interferences.

Figure 3. (a) Time-dependent fluorescence enhancing profile of nanofiber bundles 1 upon exposure to DCP vapors at different concentrations. (b) The response time of nanofiber bundles 1 to a trace DCP vapor. To further verify that the DCP-induced fluorescence enhancement originated from the internanofiber retightening rather than some interactions with individual molecule 1, we performed the fluorescence quenching experiments in solution. As shown in Figure S6, the fluorescence spectra of molecule 1 in chloroform remained unaltered with the further addition of DCP. These results ruled out the possibility that the above-observed fluorescence responses to DCP originat-

Figure 4. (a) Typical time-dependent fluorescence profile of nanofiber bundles 1 upon exposure to the interferences at

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different concentrations. (b) Fluorescence responses of nanofiber bundles 1 to various potential interferences. Error bars represent the standard deviation of five measurements.

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In conclusion, we report the rapid and sensitive detection of DCP vapor based on fluorescent hierarchical nanofiber bundles 1. We demonstrate that the internanofiber spacing of nanofiber bundles 1 can be reversibly adjusted to generate the detection signals. Upon photoexcitation, the internanofiber hydrogen-bonding strength of nanofiber bundles 1 can be weakened and the internanofiber spacing is increased to reduce the fluorescence. When exposed to trace DCP vapor, the photo-loosed nanofiber bundles 1 can be retightened to greatly enhance the fluorescence because of the strong interactions between DCP and hydroxyl groups on the nanofibers (via hydrogen-bonding or nucleophilic reaction). Based on this novel detection mechanism, we enable the fluorescence detection of DCP with rapid signal response (ca. 3 s) and high sensitivity (15 ppb).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedure, fabrication of nanofiber bundles 1 and 2, property and sensing characterizations, theoretical calculations of the ESP of molecule 1, and other supporting figures (PDF)

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. * Email: [email protected].

Author Contributions §

X. L and Y. G contributed to this work equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by NSFC (Grants 21577147, 21590811, and 21521062) and the “Strategic Priority Research Program” of the CAS (Grant XDA09030200).

REFERENCES (1) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339-1386. (2) Jang, Y. J.; Kim, K.; Tsay, O. G.; Atwood, D. A.; Churchill, D. G. Chem. Rev. 2015, 115, PR1-76. (3) Zhu, R.; Azzarelli, J. M.; Swager, T. M. Angew. Chem. Int. Ed. 2016, 55, 1-6.

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