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Colorimetric and Fluorescent Detecting Phosgene by a Second-Generation Chemosensor Ying Hu, Xin Zhou, Hyeseung Jung, Sang-Jip Nam, Myung Hwa Kim, and Juyoung Yoon Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05011 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018
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Analytical Chemistry
Colorimetric and Fluorescent Detecting Phosgene by a SecondSecondGeneration Chemosensor Ying Hu, ‡a Xin Zhou, ‡*,b Hyeseung Jung,a Sang-Jip Nam, a Myung Hwa Kim*,a and Juyoung Yoon*,a a
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea College of Chemistry and Chemical Engineering, Shandong Sino-Japanese Center for Collaborative Research of Carbon Nanomaterials, Laboratory of Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Shandong 266071, PR China b
ABSTRACT: Because of the current shortage of first-generation phosgene sensors, increased attention has been given to the development of fluorescent and colorimetric based methods for detecting this toxic substance. In an effort focusing on this issue, we designed the new, second-generation phosgene chemosensor 1, and demonstrated that it undergoes a ring-opening reaction with phosgene in association with color and fluorescent changes with a detection limit of 3.2 ppb. Notably, in comparison with the firstgeneration sensor RB-OPD, 1 not only undergoes a much faster response towards phosgene with an overall response time within 2 min, but it also generates no by-products during the sensing process. Finally, sensor 1 embedded nanofibers were successfully fabricated and used for accurate and sensitive detection of phosgene.
Phosgene is a highly toxic gas that is widely used in the chemical industry for the production of isocyanate-based polymers or pharmaceutical compounds.1-2 This substance is also receiving great attention owing to its potential use as a bioterrorism agent. Thus, a strong need exists to develop concise methods for sensing phosgene gas. Fluorescence and color based chemical sensors are of current interest owing to several advantageous characteristics such as high sensitivity and rapid rates of response.3-20 However, in contrast to the wide number of fluorescence and color sensors for nerve gas agents that have been reported,21-42 only a few efforts have focused on developing sensors for phosgene.43-53 Previously, we described the sensor PY-OPD (Scheme 1a), which utilizes an o-phenylenediamine moiety (OPD) as a universal reacting group, that enables discrimination between phosgene and nerve gas mimics.54 Inspired by this strategy, many robust sensors including NBD-OPD, NAP-OPD and RB-OPD (Scheme 1a) were designed for detecting phosgene by using nanofibers and test-papers.47-51 However, these first generation sensors share the common disadvantage that they generate hydrochloric acid (HCl) upon reaction with phosgene (Scheme 1b). This product is a secondary pollutant, which could cause detection errors like, for example, promotion a ring-open reaction of the rhodamine dye moiety RB-OPD.55-56 Thus, to overcome this shortage of the first-generation phosgene sensors, we presented here our effort on design of the second-generation phosgene sensor 1. As showed in Scheme 1c, sensor 1, a benzimidazole-fused rhodamine dye, can undergo a ring-open process with phosgene, resulting in a colorimetic and fluorescent response with a detection limit as low as 3.2 ppb. Furthermore, sensor 1 embedded nanofibers were successfully fabricated and used for concise detecting phosgene via a fluorescent and colorimetric manner.
Scheme 1. a) Chemical structures of the first-generation sensors; b) Reaction between RB-OPD and phosgene; c) Strategy for the design of the second-generation sensor 1.
Experimental Section General Methods. Unless otherwise stated, all materials were obtained from commercial suppliers and used without further purification. Thin layer chromatography (TLC) was carried out using Merck 60 F254 plates with a thickness of 0.25 mm. Preparative TLC was performed using Merck 60 F254 plates with a thickness of 1 mm. Flash chromatography was carried out on silica gel (230-400 mesh). The 1H NMR and 13C NMR spectra were recorded using Bruker 300 MHz or Varian
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Analytical Chemistry
Results and Discussion Synthesis of sensor 1. Sensor 1 was synthesized in a total yield of 67% using a route, which was initiated by preparation of intermediate RBOPD from rhodamine B employing a reported procedure.3e
Then RB-OPD was reacted with lithium aluminum hydride in dry THF to generate 1 as a light pink solid after chromatographic purification. (Detailed procedure, 1H and 13C NMR as well as mass data are given in the ESI). Phosgene Promoted Color and Fluorescence Changes. The fluorescent response of 1 towards phosgene was investigated first by using triphosgene, a nonvolatile precursor of this gaseous substance. Inspection of the spectra displayed in Figure 1a shows that addition of triphosgene to a CHCl3 solution of 1 causes a significant concentration dependent enhancement in the fluorescence intensity at 578 nm accompanied by a small red shift in the emission wavelength maximum. Accordingly, the fluorescence quantum yields (фf) of sensor 1 enhanced from 0.06 to 0.22. The fluorescence increase is due to the unique spirolactam ring-opening process. The detection limit of 1 for phosgene was determined by a fit of the emission titration data, to be 3.2 ppb (Figure S1). Notably, in comparison with the first-generation sensor RB-OPD, 1 undergoes a much faster response upon addition of triphosgene with the overall sensing process being accomplished within 2 min (Figure 1b). This data indicates that the second-generation sensor 1 possesses a higher reactivity that its precursor RBOPD. 400
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500 MHz spectrometer. Chemical shifts were given in ppm and the coupling constants (J) in Hz. The mass spectra were obtained using a JMS-HX 110A/110A Tandem Mass Spectrometer (JEOL). The UV absorption spectra were obtained on a UVIKON 933 Double Beam UV/VIS Spectrometer. The fluorescence emission spectra were obtained using an RF5301/PC Spectrofluorophotometer (Shimadzu). Synthesis of 1. To a solution of RB-OPD (0.9 mmol) in dry THF (10 mL) at ice bath was added lithium aluminium hydride (4.5 mmol). Stirring at room temperature for 24 h under N2 protection. After quenching by add 1-butanol, the mixture was concentrated under vacuum. The residue was diluted with water and extracted with ethyl acetate, the extracts were dried over anhydrous MgSO4 and concentrated under a vacuum to give a residue that was subjected to column chromatography to give sensor 1 as pale pink in 80% yield. 1H NMR (CDCl3, 300MHz) δ (ppm): 8.13 (d, J = 7.2 Hz, 1H); 7.83 (d, J = 8.4 Hz, 1H); 7.51 (t, J = 7.2Hz, 1H); 7.40 (t, J = 7.5Hz, 1H); 7.26 (d, J = 7.5 Hz, 1H); 7.18 (t, J = 8.4 Hz, 1H); 7.04 (t, J = 7.2 Hz, 1H); 6.97 (d, J = 7.2 Hz, 1H); 6.51 (s, 2H); 6.33 (d, J = 9.0 Hz, 2H); 6.18 (d, J = 6.3 Hz, 2H); 3.34 (q, J = 7.2 Hz, 8H); 1.16 (t, J = 7.2Hz, 12H). 13C NMR (CDCl3, 75MHz) δ (ppm): 156.87, 156.53, 153.30, 149.18, 149.06, 131.15, 130.66, 128.84, 128.52, 128.19, 125.08, 122.75, 122.14, 121.59, 120.35, 110.44, 108.37, 106.53, 98.11, 44.57, 12.84. HRMS (ESI) m/z = 515.2823 [M+H]+, calcd for C34H36N4O2 = 515.2733. Fluorescent Study. Stock solutions of sensor 1 (1 mM) were prepared in chloroform. Triphosgene stock solution (10 mM) in chloroform were prepared. The test solutions were prepared by placing 30 µL of the sensor stock solution into a test tube, adding an appropriate aliquot of triphosgene, and diluting these stock solutions to 3 mL with chloroform. For all of the measurements of the fluorescence spectra, excitation was performed at 530 nm with slit widths for excitation and emission are 1.5 nm and 3.0 nm respectively. UV/Vis and fluorescence titration experiments were performed using 10 µM of 1 in chloroform with varying concentrations of triphosgene at room temperature. Preparation of sensor 1 embedded electrospun fibers. To prepare a precusor solution for synthesizing the electrospun fibers, 1.0 mg of sensor 1 were dissolved in 2.5 mL of acetonitrile. In each case, when the solute was completely dissolved, 100.0 mg of poly(ethylene oxide) (PEO, Mw = 600,000) as a matrix polymer was added and the resulting mixture was stirred for 1 h and loaded into stainless steel syringe connected to a needle of gauge 23. The distance between the end of the needle and grounded plate was adjusted to be 15 cm before applying 5.0 kV. The precursor solution was ejected to form fine fibers on the surface of a grounded aluminum plate located 15 cm below the tip of the needle. The flow rate was carefully kept at 0.5 mL/h. After collection, the polymer fibers were dried in a 40 °C vaccum oven for 1 d to prevent agglomeration. The structures and morphologies of the fibers were assessed using electron microscopy (FE-SEM, JEOL JSM6700F).
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Figure 1. (a) Fluorescence spectra of sensor 1 in chloroform (10 µM) upon gradual addition of a solution of triphosgene (0-1.0 equiv.) in chloroform. (λex=530 nm, Slits: 3 nm×1.5 nm). (b) Time dependent spectra of sensor 1 (red) and RB-OPD (black) in chloroform solution (10 µM) following addition of a chloroform solution of triphosgene (0.5 equiv.), respectively (λex=530 nm, λem=578 nm, Slits: 3nm×1.5 nm). (c) Absorbance spectra of sensor 1 in chloroform (10 µM) upon gradual addition of a solution of triphosgene (0-1.0 equiv.) in chloroform. (d) Colorimetric responses of sensor 1 towards different equivalents of triphosgene.
Inspection of the absorption spectrum of a CHCl3 solution of 1 upon treatment with triphosgene (Figure 1c) shows that a new UV band appears at 560 nm. As can be seen by viewing the images displayed in Figure 1d, addition of triphosgene to the CHCl3 solution of 1 causes a distinct color change from colorless to pink, which can be easy observed by using the naked eye. The response of this sensor to the nerve agent mimic, DCP, was investigated next. Unlike in the case of phosgene, addition of even 70 equiv. DCP induces only a slight fluorescence intensity increase (Figure S2). However, sensor 1 does display
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Analytical Chemistry similar color and fluorescence responses to that promoted by phosgene when treated with exceptionally high doses of DCP (200 equiv) (Figure S3-4). Reaction Mechanisms. Based on the UV and fluorescent observations, we propose the plausible mechanism shown in Scheme 2 for the reaction of sensor 1 with triphosgene. The product of this process was separated from the reaction mechanism and identified in order to gain evidence to support this mechanistic proposal. Analysis of the 1H NMR spectrum of the product showed that it possesses the structure represented by the ring opened Nchloroformyl-benzimadazole 2. As can be seen by viewing the spectra of 1 and 2 shown in Figure 2 the signals for H8’, H9’, H10’ and H11’ in 2 are downfield shifted in comparison to their counterparts in 1, as a consequence of the presence of the electron withdrawing formylchloride group.
Figure 3. Partial 1H NMR spectra of 2 and 3 (CDCl3, 300 MHz).
Scheme 2. Mechanism of the reaction of sensor 1 with triphosgene.
Figure 4. Partial 13C NMR spectra of 2 and 3 (CDCl3, 75 MHz).
Figure 2. Partial 1H NMR spectra of sensor 1 (top) and compound 2 (CDCl3, 300 MHz).
To exclude the possibility that the product of the reaction of 1 with phosgene is simply a ring opened protonated species, sensor 1 was reacted with TFA to form the nonchloroformylated-benzimidazole 3. This substance was characterized by using 1H NMR, 13C NMR and ESI-mass spectroscopy. As shown by comparing the 1H NMR spectra displayed in Figure 3, a unique peak at 13.51 ppm exists in the spectrum of 3 that is to the NH proton. Meanwhile, the aromatic protons of 3 resonate at dramatically upfield positions in contrast to those of compound 2. In addition, 13C labelled triphosgene was used in reaction of 1 to gain further evidence for the assignment of 2 as the product of the reaction. As can be seen by viewing the 13C NMR spectra in Figure 4, a remarkablly high intensity peak at 143.36 ppm is present in the spectrum of 2, which is unambiguously assigned to the 13C labelled chloroformyl carbonyl carbon. The results showing that 2 is formed in the reaction of phosgene with 1 also demonstrate that, unlike reactions with the first-generation sensors shown in Scheme 1a, HCl is not generated.
Sensor 1-Embedded Polymer Fibers. To generate a format in which 1 can be employed for sensing phosgene gas, the electrospinning technique was used to fabricate Sensor 1-embedded fibers. For this purpose, a mixture containing 1 and the matrix polymer poly(ethylene oxide) (PEO, Mw = 600,000) was jet ejected from a syringe containing a needle to which a voltage of 5.0 kV is applied. This procedure produced uniformly distributed fibers containing a minimum number of bead structures. The polymer fibers containing 1 display a highly sensitive color as well as fluorescence response upon exposure to phosgene. As shown in Figure 5, the white colored and non-fluorescent (under a UV lamp, 365 nm) sensor 1-embedded fibers upon exposure to phosgene for several seconds change color to dark pink and emit pink fluorescence.
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Color Response
Fluorescent Response
Analytical Chemistry Figure 5. Color / Fluorescent responses of sensor 1 based PEO nanofiber upon exposure to phosgene (0.8 mg/L phosgene gas).
In Figure 6 are shown scanning electron microscope (SEM) images of sensor 1-embedded nanofibers before and after exposure to phosgene. As can be seen, the initial embedded fibers have a uniform structure with a diameter of ca. 1.0 µm. After exposure to phosgene, the surfaces of the fibers become rough and their diameters become irregular. These observations indicate that reaction of 1 with phosgene on the surface of nanofibers causes partial deformation of the in PEO polymer matrix.
ner. We believe that the strategy utilized to design 1 will accelerate the development of new sensors protocols.
ASSOCIATED CONTENT Supporting Information The NMR and MS spectra of sensor 1 and other materials are available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
Sensor 1-Ref
* Fax: (+82) 2-3277-2384 (J. Y.). E-mail:
[email protected] (J. Y.),
[email protected] (M. H. K.) and
[email protected] (X. Z.).
Sensor 1-phosgene
Notes
‡ These authors contribute equally to this work. The authors declare that they have no competing financial interests.
ACKNOWLEDGMENT
Figure 6. Scanning electron microscope images of sensor 1 based PEO nanofibers before and after exposure to phosgene (0.8 mg/L phosgene gas).
Sensor 1-Ref
Confocal microscopy was employed to examine the sensor 1-embedded nanofiber before and after phosgene exposure (Figure 7). The fibers exposed to phosgene were observed to display bright red fluorescence using this imaging technique.
J. Y. acknowledges a grant from the National Creative Research Initiative programs of the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP; No. 2012R1A3A2048814). M.H.K thanks to the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP; No. 2016R1D1A1B03934962). X. Z. acknowledge the Natural Science Foundation of China (NSFC. 21662037, 21762045), the Youth Science Foundation of Jilin Province (20160520003JH), for supporting this work. Mass spectral data were obtained from the Korea Basic Science Institute (Daegu) using a Jeol JMS 700 high-resolution mass spectrometer.
REFERENCES
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Figure 7. Confocal microscope images of sensor 1 based PEO nanofiber upon exposure to phosgene (0.8 mg/L phosgene gas).
Conclusion In conclusion, in the studies described above we developed the new second-generation sensor 1 for color and fluorescence sensing of phosgene. Sensor 1 was found to undergo a spirocyclic ring-open reaction with phosgene, which is associated with both a color change and fluorescence enhancement, and a detection limit as low as 3.2 ppb. Notably, in comparison with that of the first-generation sensor RB-OPD, reaction of 1 with phosgene is much more rapid corresponding to a time for complete reaction of 2 min. More importantly, reaction of sensor 1 with phosgene does not generate HCl. Finally, sensor 1 embedded nanofibers were successfully fabricated and used for detecting phosgene in a fluorescent and colorimetric man-
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