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An ESIPT-based fluorescent probe for the detection of phosgene in the solution and gas phases
Talanta 200 (2019) 78–83
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Talanta journal homepage: www.elsevier.com/locate/talanta
An ESIPT-based fluorescent probe for the detection of phosgene in the solution and gas phases
T
Cuiyan Wua, Hai Xua,1, Yaqian Lia, Ruihua Xiea, Peijuan Lia,b, Xiao Panga, Zile Zhoua, Biao Gub, ⁎ Haitao Lia, , Youyu Zhanga a
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China b Key Laboratory of Functional Organometallic Materials of College of Hunan Province, College of Chemistry and Materials Science, Hengyang Normal University, Hengyang 421008, PR China
ARTICLE INFO
ABSTRACT
Keywords: Fluorescent probe Phosgene Fluorescent test strip Detection Ratiometric
Phosgene is a highly toxic gas that poses a serious threat to public health and safety. Herein, we describe the preparation of a ratiometric fluorescence probe (Pi) bearing hydroxyl and imidazole moieties as recognition sites, and employ it for the excited-state intramolecular proton transfer-based (ESIPT-based) detection of phosgene. It is the first time that hydroxyl and imidazole have been exploited as recognition sites for phosgene. In the presence of phosgene, Pi undergoes sequential nucleophilic substitution and cyclization reactions that facilitate a rapid response, high selectivity, and excellent sensitivity (detection limit = 0.14 μM). The sensing mechanism was verified by 1H NMR spectroscopy and high-resolution mass spectrometry. Furthermore, we fabricated a fluorescent test strip (FTS-Pi) for the detection of phosgene in the gas phase that undergoes a fluorescence color change, from green to blue, under 365 nm UV light in the presence of phosgene, which is easily observed with the naked eye.
1. Introduction
view of its extensive industrial use, lethality, and imperceptibility, phosgene poses a serious threat to public health and safety, which necessitates the development of rapid, facile, and effective methods for its detection at concentrations below the safety threshold. Compared to conventional detection methods, such as gas chromatography [8–10] and electrochemical techniques [11,12], fluorescence-based techniques are advantageous because of the wide instrumentation availability and the possibility of real-time visual detection. However, studies dealing with the synthesis of small-molecule fluorescent probes for the detection of phosgene are rare, and have mainly relied on the electrophilic nature of phosgene to trigger reactions with amines or alcohols [13–20]. Examples of such reactions include two-fold acetylation reactions of probes containing o-phenylenediamine or ethylenediamine units as recognition groups to “turn on” fluorescence by blocking the fluorescence quenching promoted by photoinduced electron transfer [15,16,20], in addition to the ring opening of an amino-containing spiro-(deoxy)lactam to afford a fluor-
Phosgene (COCl2) is a highly reactive and colorless gas that is widely used in the production of isocyanates (as precursors to polyurethanes and polycarbonates), pesticides, rust-removal materials, artificial foaming materials, dyes, and pharmaceuticals [1,2]. However, phosgene is highly toxic [3,4] and has an odor-recognition threshold (1.0 ppm) that is significantly higher than the 24 h-exposure safety margin (~0.6 ppb). Short-term exposure to > 3 ppm phosgene gas causes irritation of the eyes, nose, throat, and skin, in addition to respiratory tract pulmonary edema, pneumonia, respiratory distress syndrome, and sometimes even death [5,6]. Not surprisingly, phosgene was used as a chemical warfare agent during World War I and can potentially be used by terrorists [7]. With the exception of high doses, an asymptomatic incubation period of up to 48 h is typically observed following the inhalation of phosgene, after which the accumulation of phosgene in the lungs leads to life-threatening complications. Hence, in
Abbreviations: ESIPT, Excited-state intramolecular proton transfer; TEA, triethylamine; TLC, thin layer chromatography; HRMS, high-resolution mass spectrometry ⁎ Corresponding author. E-mail address: [email protected] (H. Li). 1 Contributed equally to this work. https://doi.org/10.1016/j.talanta.2019.03.003 Received 19 December 2018; Received in revised form 26 February 2019; Accepted 1 March 2019 Available online 02 March 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
escent rhodamine [17], reactions with cinnamic acids to form fluorescent coumarins [21], and the phosgene-mediated hetero cross-linking of two amine-containing fluorophores to achieve fluorescence resonance electron transfer (FRET) [13]. However, despite the progress achieved in this field, the development of fluorescent probes with optimal performance (i.e., a high sensitivity, response rate, and selectivity) remains challenging. Indeed, the selectivities of some reported probes for phosgene and nerve-gas mimics have not yet been assessed [13,17,19,21,22]. In addition, some previously developed fluorescent probes for phosgene detection rely on the use of recognition reactions that can also be triggered by formaldehyde and/or nitric oxide, while others cannot discriminate between phosgene and triphosgene. Moreover, the sensitivities of some probes do not meet the requirements of actual detection in the environment, as their detection limits are higher than the safety threshold. As is well known, fluorescence detection requires a large fluorescence contrast to detect and monitor trace amounts of analytes, a fast recognition-reaction rate to achieve a rapid response, and high recognition specificity to achieve excellent detection selectivity. The detection sensitivity and selectivity are directly determined by the fluorophore and the recognition group of the fluorescent probe, and specific detection can be realized through the use of special recognition groups. In general, aliphatic amines are more basic than arylamines [15], and anilines are more basic than phenols. The relatively weak alkalinity of phenols results in its weak nucleophilicity and may allow for the selective discrimination of phosgene over triphosgene, as the former is more reactive than the latter [23]. For example, probes that lack catalytic tertiary amine moieties react with phosgene and triphosgene at different rates, which allow these analytes to be distinguished based on differences in the response intensity at a given time [18,20]. However, in a complex environment, the fluorescence intensity signals of fluorescence “turn-on” or “turn-off” probes frequently suffer from interference by factors including the probe distribution, instrumental efficiency, and environmental conditions. In contrast, ratiometric fluorescent probes, which employ the ratio of two emissions at different wavelengths for analysis, can effectively avoid the above interferences and render the results more accurate. Ratiometric fluorescence based on excited-state intramolecular proton transfer (ESIPT) generally results in a large Stokes shift with minimal self-absorption, while effective internal referencing greatly increases the probe's sensitivity and improves quantitation accuracy for the desired analyte [24,25]. However, only a few ratiometric probes based on ESIPT have been reported to date [14,16,18]. Thus, we herein report the synthesis of 2-(1H-imidazol-2-yl)phenol (Pi), which is a new ratiometric fluorescent probe bearing hydroxyl and imidazole moieties that facilitate the ESIPT-based detection of phosgene. Upon photoexcitation, Pi undergoes ESIPT to afford a keto tautomer that in turn undergoes a two-fold carbamylation reaction with phosgene to yield Pio, which is a six-membered ring-containing product (Scheme 1). The fluorescence emission wavelength of Pio differs from that of Pi because of the strongly electron-withdrawing nature of the carbonyl group in the former, which is expected to hamper ESIPT. To investigate the above-mentioned design (Scheme 1), Pi is synthesized in a single step from commercially available phenanthrene-9,10-dione and salicylaldehyde. Moreover, we design and examine a fluorescent test strip (FTS-Pi) for the rapid, sensitive, and selective detection of gaseous phosgene.
2.1. Materials and instrumentation Salicylaldehyde, phenanthrene-9,10-dione, and triphosgene (CCl3OC(O)OCCl3) were purchased from Sigma-Aldrich. Acetic acid, ammonium acetate, triethylamine (TEA), chloroform, petroleum ether, and acetic ether (all of analytical grade) were sourced from the Sinopharm Chemical Reagent Co. All other chemicals were of analytical grade. Deionized water was used in the assay, and 300–400-mesh silica gel (37–54 mm) was used for column chromatography. The reaction progress was monitored by thin layer chromatography (TLC) on silica gel plates (60F-254) that were visualized by UV light. An ATX-224 electronic balance (Shimadzu Corporation, Japan) was used for weighing, and an N-1001 rotary evaporator (Shanghai Ai Lang Instrument Company) was used for solvent removal. 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVB-500 spectrometer in DMSO-d6 containing tetramethylsilane as an internal standard. The probe was characterized using an F-4500 fluorescence spectrophotometer (Japan Hitachi, Ltd.) and a UV-2450 UV spectrophotometer (Japan Shimadzu Corporation). 2.2. Synthesis and characterization of Pi A solution of salicylaldehyde (0.51 g, 4.91 mmol), phenanthrene9,10-dione (1.02 g, 4.91 mmol), and ammonium acetate (2.84 g, 36.77 mmol) in glacial acetic acid (20 mL) was refluxed for 5 h (120 °C), cooled under room temperature, and diluted with a copious amount of water to induce complete precipitation. The resulting precipitate was collected and purified by column chromatography to afford Pi as a white solid (1.18 g, 78%) that was further characterized by 1H/13C NMR spectroscopy and high-resolution mass spectrometry (HRMS). 1H NMR (500 MHz, DMSO) δ 13.71 (s, 1H), 13.15 (s, 1H), 8.90 (s, 2H), 8.55 (d, J = 43.4 Hz, 2H), 8.26 (dd, J = 8.0, 1.1 Hz, 1H), 7.71 (dd, J = 31.4, 24.3 Hz, 4H), 7.39 (dd, J = 11.4, 4.0 Hz, 1H), 7.09 (dd, J = 7.6, 5.3 Hz, 2H). 13C NMR (126 MHz, DMSO) δ 157.87, 149.80, 131.59, 127.96, 127.83, 126.29, 124.44, 122.74, 122.38, 119.58, 117.68, 113.44. HRMS (ESI+) for C21H15N2O+ M+H+: calculated 311.3179, found 311.1177. 2.3. Analytical procedure All analytical experiments were repeated at least three times to ensure a high measurement accuracy. Fluorescence measurements for phosgene detection were carried out as follows. A 0.5 mM stock solution of Pi was prepared in chloroform, while phosgene was generated by the addition of the less-toxic and easier-to-handle triphosgene to a solution of TEA [20,21]. More specifically, triphosgene was dissolved in chloroform containing 0.1 vol% TEA to prepare a 0.01 M stock solution of phosgene (the analyte). To determine the selectivity, stock solutions of potential chlorine-containing interferents were prepared in chloroform. Appropriate amounts of the analyte stock solution were mixed with the probe stock solution (40 μL), and the total volume was made up to 2 mL using chloroform. Fluorescence spectra were recorded at an excitation wavelength of 335 nm with excitation and emission slit widths of 5 and 2.5 nm, respectively. Scheme 1. Synthesis of Pi and the corresponding sensing product, Pio.
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2.4. Detection of phosgene gas by FTS-Pi
3.3. Reactivity of Pi toward (tri)phosgene
FTS-Pi was prepared as follows. One end of a TLC plate (2.55 × 7.35 cm) was immersed in the Pi stock solution (10 µM), then removed and dried under air. A further TLC plate was inscribed with “test” and prepared in the same manner. The prepared FTS-Pi plates were imaged under room light and under a 365 nm UV-light source. A sample of phosgene gas (0.5 ppm) in a sealed container was prepared by sequentially adding a stock solution of triphosgene (50 μL, 150 µM) and TEA (10 μL) in chloroform to a 1 L wide-mouth bottle, after which the lid was quickly closed. The FTS-Pi plate was suspended above the solution (Fig. S1) and imaged with both ambient light and UV light after exposure for 1 min to the phosgene-containing atmosphere. Images of the “test” FTS-Pi were acquired following the same procedure.
To further compare the reactivity of Pi toward phosgene and triphosgene, we recorded the time-dependent Pi fluorescence intensities (at 393 nm) in the presence and absence of 0.1 vol% TEA, as shown in Fig. 3. The fluorescence intensity rapidly increased over 30 s upon the addition of triphosgene to the Pi + 0.1 vol% TEA solution, after which it plateaued (300–330 s) (red line), while a slow and insignificant increase in the fluorescence intensity at 393 nm was observed in the absence of TEA (black line). The low reactivity of Pi toward triphosgene is ascribed to: (1) the absence of tertiary amine moieties that catalyze the decomposition of triphosgene to phosgene in the absence of TEA, and (2) the relatively weak reactivity of triphosgene, which precludes its direct reaction with Pi. Hence, the designed probe clearly discriminates between phosgene and triphosgene.
3. Results and discussion
3.4. Mechanism of the Pi response to phosgene
3.1. Synthesis of Pi and the proposed sensing product
Compared to previously reported phosgene probes that contain two amino groups as recognition sites, Pi features hydroxyl and imidazole moieties as active sites, and these groups undergo intramolecular proton transfer to afford a blue-green fluorescent intermediate (Scheme 2). Upon the addition of phosgene, the amino and hydroxyl groups of Pi react to form Pio, which contains a new six-membered ring featuring a strongly electron-withdrawing carbonyl group. Importantly, Pio does not easily undergo ESIPT and therefore exhibits bluish-violet fluorescence. In support of the proposed mechanism, Pi and purified Pio were characterized by 1H NMR spectroscopy. As shown in Fig. 4 and S5, the 1 H NMR signals of the amino (13.71 ppm) and hydroxyl (13.15 ppm) groups were absent following the reaction with phosgene, which confirms that these groups serve as the phosgene-recognition sites. To further verify the proposed mechanism, the above compounds were also characterized by HRMS (Fig. S6, HRMS (ESI+) for C22H13N2O2+: M +H+: calculated 337.0932, found 337.0983), which revealed that the molecular weight (m/z) of the purified product corresponds with that of Pio. These data provide strong evidence in support of the proposed mechanism.
Scheme 1 shows the synthetic route employed for the preparation of Pi and its rapid reaction with phosgene to afford Pio. The structures of Pi and Pio were confirmed by 1H/13C NMR spectroscopy and HRMS (Figs. S2–S6). 3.2. Spectral response of Pi to phosgene The ability of Pi to serve as a phosgene probe in chloroform solution was assessed by fluorescence spectroscopy. As shown in Fig. 1, a solution of Pi in chloroform (10 μM) displays a 469 nm green emission maximum. The absorption maximum of Pi was observed to shift to 329 nm in the presence of triphosgene/TEA (which generates phosgene) and was accompanied by a decrease in the intensity of the fluorescence emission at 469 nm and a concomitant increase in the fluorescence signal at 393 nm. These changes resulted in a gradual fluorescenceemission color change to blue when irradiated with a 365 nm hand-held UV lamp (Fig. 1B, inset). The fluorescence spectra of Pi solutions (10 µM) containing 0.1 vol % TEA were recorded at various concentrations of triphosgene (0–300 µM), and the ratio of the emission intensities at 469 and 393 nm was found to depend linearly on the phosgene concentration over the 0–4.0 μM range (Fig. 2). The phosgene detection limit was determined (by titration of triphosgene) to be 0.14 ppm (= 3σ/k, where σ is the standard deviation of the blank experiment, and k is the slope of the relationship between the emission-intensity ratio and the phosgene concentration (Fig. 2B)); this detection limit is significantly lower than the threshold level associated with immediate danger to health and life (IDLH, i.e., 2 ppm) [21,26].
0.30
A
For their application in a practical setting, probes should exhibit a high selectivity toward the target analyte. We therefore investigated the selectivity of Pi for the detection of phosgene in the presence of formaldehyde (CH2O), nitric oxide (NO), nerve agent mimics (DCP, DCNP), in addition to several chlorine-containing species as potential interferents, namely acetyl chloride (CH3COCl), thionyl chloride (SOCl2), p-toluenesulfonyl chloride (TosCl), phosphorus oxychloride (POCl3), oxalyl chloride ((CClO)2), sulfuryl chloride (SO2Cl2), tosyl chloride (TsCl), and triphosgene. Pi solutions (1 × 10−5 M) were
Fluorescence Intensity (a.u.)
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3.5. Selectivity for the detection of phosgene
Pi Pi + Phosgene
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280 320 Wavelength (nm)
3000 2500
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Pi
Pi + Phosgene
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450 525 Wavelength (nm)
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Fig. 1. (A) UV–vis and (B) fluorescence-emission spectra of Pi (10 µM) in CHCl3 (0.1 vol% TEA) in the absence (black) and presence (red) of phosgene (200 µM). 80
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Y = 0.011X + 0.038 R2 = 0.9872
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Fig. 2. (A) Emission spectra of Pi (10 µM) recorded in the presence of different concentrations of triphosgene (0–300 µM) in CHCl3 containing 0.1 vol % TEA. (B) Linear correlation between the ratio of emission intensities at 469 and 393 nm (I393 nm/I469 nm) and the phosgene concentration (0–4 µM); λex = 335 nm, λem = 393 nm, slit width = 3/5 nm.
0.03 0.00
0
1 2 3 Cphosgene (µM)
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Fig. 4. 1H NMR spectra of (A) Pi and (B) the purified product.
Fig. 3. Fluorescence intensities at 393 nm as a function of time for solutions of Pi (10 μM) treated with triphosgene (100 µM) at 300 s in the presence (red line) and absence (black line) of 0.1 vol% TEA; λex = 335 nm, λem = 393 nm, slit width = 3/5 nm.
treated with 200 μM phosgene and 200 μM (20 equiv of Pi solution) solutions containing interferents, and the obtained mixtures were examined by fluorescence spectroscopy. Fig. 5 reveals a clear response to phosgene, while no obvious responses to other analytes were detected, which is indicative of the highly specific and selective detection of phosgene. Fig. 5. Responses of Pi (10 μM) to phosgene (200 μM) and potential interferents (200 μM) in TEA-containing chloroform. (1) Blank, (2) CH2O, (3) NO, (4) DCP, (5) DCNP, (6) CH3COCl, (7) TosCl, (8) SOCl2, (9) POCl3, (10) (CClO)2, (11) SO2Cl2, (12) TsCl, (13) triphosgene, and (14) phosgene.
3.6. Detection of phosgene in the gas phase Since phosgene is recognized to be a toxic air pollutant, we established a portable technique for the detection of gaseous phosgene. For this purpose, we soaked a silica-gel-coated plate in Pi/CHCl3 (10 μM) to afford FTS-Pi, which exhibited green fluorescence when irradiated with 365 nm UV light. Importantly, the fluorescence color changed to blue after exposure to phosgene gas (at an estimated concentration of 0.5 ppm) generated by the reaction of triphosgene with TEA (Fig. 6). According to the Matheson Gas Data Book [27], exposure to a phosgene concentration of 20 ppm can cause lung injuries within 2 min, and exposure to a concentration of 25 ppm for as little as 30 min is lifethreatening, while death in 30 min or less is observed at a phosgene concentration of 90 ppm. Herein, phosgene was reliably detected by
FTS-Pi at concentrations equal to or less than dangerous levels. Furthermore, the selectivity of FTS-Pi toward phosgene over related analytes was also investigated (Fig. 7). After exposure to phosgene gas or to the vapor of 5 ppm related analytes (41 μL 5 mM chloroform solutions) for 5 min, the fluorescence color of FTS-Pi changed only in the presence of phosgene. We could therefore conclude that the prepared probe is well suited to the selective and sensitive detection of phosgene in both the solution and gas phases.
Scheme 2. Probable mechanism involved in the response of Pi to phosgene.
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Fig. 6. FTS-Pi fluorescence upon exposure to 0.5 ppm (∼2.2 μg/L) gaseous phosgene. Photographic images of a TLC plate prepared by dipping one end into a Pi/CHCl3 solution followed by air-drying (Sample 1), and those of a plate inscribed with “test” prepared in the same manner (Sample 2), acquired before and after exposure to gaseous phosgene.
Fig. 7. Images showing the fluorescence of FTS-Pi before (upper images) and after (lower images) exposure to phosgene gas and the vapors of various potential interferents (5 ppm). (1) Acetyl chloride, (2) p-toluene sulfonyl chloride, (3) thionyl chloride, (4) phosphorus oxychloride, (5) oxalyl chloride, (6) triphosgene, and (7) phosgene (0.5 ppm).
4. Conclusion
Acknowledgements
Herein, we successfully designed and synthesized a new ratiometric fluorescence probe (Pi) bearing hydroxyl and imidazole moieties as recognition sites, applied it to the detection of phosgene, and revealed that the sensing reaction afforded a new six-membered ring. The above reaction was very fast (competition time = 30 s) and sensitive (detection limit = 0.14 μM). Furthermore, Pi exhibited a high selectivity toward phosgene and could clearly distinguish it from triphosgene and other chlorides. Finally, FTS-Pi, which was fabricated by immobilizing Pi on a silica gel plate, was shown to respond to a gaseous phosgene concentration (0.5 ppm) below the threshold level (2 ppm) associated with immediate danger to health and life. Nonetheless, it should be noted that this probe also has a limitation. The sensitivity of this probe is not sufficient to monitor the phosgene exposure in some special places (such as pesticide, pharmaceutical and other production processes of phosgene trace leakage) because the safety value for 24 hexposure is approximately 0.6 ppb. However, this probe is still expected for reliable detection of phosgene levels in the case of terrorist attack or serious leaks due to its fast reaction time and high selectivity.
This work was supported by the National Natural Science Foundation of China (21874042 and 21675051), and the Foundation of the Hunan Provincial Science and Technology Department (2016SK2020). Notes This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. There are no conflicts of interest to declare. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.talanta.2019.03.003.
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An ESIPT-based fluorescent probe for the detection of phosgene in the solution and gas phases
T
Cuiyan Wua, Hai Xua,1, Yaqian Lia, Ruihua Xiea, Peijuan Lia,b, Xiao Panga, Zile Zhoua, Biao Gub, ⁎ Haitao Lia, , Youyu Zhanga a
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China b Key Laboratory of Functional Organometallic Materials of College of Hunan Province, College of Chemistry and Materials Science, Hengyang Normal University, Hengyang 421008, PR China
ARTICLE INFO
ABSTRACT
Keywords: Fluorescent probe Phosgene Fluorescent test strip Detection Ratiometric
Phosgene is a highly toxic gas that poses a serious threat to public health and safety. Herein, we describe the preparation of a ratiometric fluorescence probe (Pi) bearing hydroxyl and imidazole moieties as recognition sites, and employ it for the excited-state intramolecular proton transfer-based (ESIPT-based) detection of phosgene. It is the first time that hydroxyl and imidazole have been exploited as recognition sites for phosgene. In the presence of phosgene, Pi undergoes sequential nucleophilic substitution and cyclization reactions that facilitate a rapid response, high selectivity, and excellent sensitivity (detection limit = 0.14 μM). The sensing mechanism was verified by 1H NMR spectroscopy and high-resolution mass spectrometry. Furthermore, we fabricated a fluorescent test strip (FTS-Pi) for the detection of phosgene in the gas phase that undergoes a fluorescence color change, from green to blue, under 365 nm UV light in the presence of phosgene, which is easily observed with the naked eye.
1. Introduction
view of its extensive industrial use, lethality, and imperceptibility, phosgene poses a serious threat to public health and safety, which necessitates the development of rapid, facile, and effective methods for its detection at concentrations below the safety threshold. Compared to conventional detection methods, such as gas chromatography [8–10] and electrochemical techniques [11,12], fluorescence-based techniques are advantageous because of the wide instrumentation availability and the possibility of real-time visual detection. However, studies dealing with the synthesis of small-molecule fluorescent probes for the detection of phosgene are rare, and have mainly relied on the electrophilic nature of phosgene to trigger reactions with amines or alcohols [13–20]. Examples of such reactions include two-fold acetylation reactions of probes containing o-phenylenediamine or ethylenediamine units as recognition groups to “turn on” fluorescence by blocking the fluorescence quenching promoted by photoinduced electron transfer [15,16,20], in addition to the ring opening of an amino-containing spiro-(deoxy)lactam to afford a fluor-
Phosgene (COCl2) is a highly reactive and colorless gas that is widely used in the production of isocyanates (as precursors to polyurethanes and polycarbonates), pesticides, rust-removal materials, artificial foaming materials, dyes, and pharmaceuticals [1,2]. However, phosgene is highly toxic [3,4] and has an odor-recognition threshold (1.0 ppm) that is significantly higher than the 24 h-exposure safety margin (~0.6 ppb). Short-term exposure to > 3 ppm phosgene gas causes irritation of the eyes, nose, throat, and skin, in addition to respiratory tract pulmonary edema, pneumonia, respiratory distress syndrome, and sometimes even death [5,6]. Not surprisingly, phosgene was used as a chemical warfare agent during World War I and can potentially be used by terrorists [7]. With the exception of high doses, an asymptomatic incubation period of up to 48 h is typically observed following the inhalation of phosgene, after which the accumulation of phosgene in the lungs leads to life-threatening complications. Hence, in
Abbreviations: ESIPT, Excited-state intramolecular proton transfer; TEA, triethylamine; TLC, thin layer chromatography; HRMS, high-resolution mass spectrometry ⁎ Corresponding author. E-mail address: [email protected] (H. Li). 1 Contributed equally to this work. https://doi.org/10.1016/j.talanta.2019.03.003 Received 19 December 2018; Received in revised form 26 February 2019; Accepted 1 March 2019 Available online 02 March 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental
escent rhodamine [17], reactions with cinnamic acids to form fluorescent coumarins [21], and the phosgene-mediated hetero cross-linking of two amine-containing fluorophores to achieve fluorescence resonance electron transfer (FRET) [13]. However, despite the progress achieved in this field, the development of fluorescent probes with optimal performance (i.e., a high sensitivity, response rate, and selectivity) remains challenging. Indeed, the selectivities of some reported probes for phosgene and nerve-gas mimics have not yet been assessed [13,17,19,21,22]. In addition, some previously developed fluorescent probes for phosgene detection rely on the use of recognition reactions that can also be triggered by formaldehyde and/or nitric oxide, while others cannot discriminate between phosgene and triphosgene. Moreover, the sensitivities of some probes do not meet the requirements of actual detection in the environment, as their detection limits are higher than the safety threshold. As is well known, fluorescence detection requires a large fluorescence contrast to detect and monitor trace amounts of analytes, a fast recognition-reaction rate to achieve a rapid response, and high recognition specificity to achieve excellent detection selectivity. The detection sensitivity and selectivity are directly determined by the fluorophore and the recognition group of the fluorescent probe, and specific detection can be realized through the use of special recognition groups. In general, aliphatic amines are more basic than arylamines [15], and anilines are more basic than phenols. The relatively weak alkalinity of phenols results in its weak nucleophilicity and may allow for the selective discrimination of phosgene over triphosgene, as the former is more reactive than the latter [23]. For example, probes that lack catalytic tertiary amine moieties react with phosgene and triphosgene at different rates, which allow these analytes to be distinguished based on differences in the response intensity at a given time [18,20]. However, in a complex environment, the fluorescence intensity signals of fluorescence “turn-on” or “turn-off” probes frequently suffer from interference by factors including the probe distribution, instrumental efficiency, and environmental conditions. In contrast, ratiometric fluorescent probes, which employ the ratio of two emissions at different wavelengths for analysis, can effectively avoid the above interferences and render the results more accurate. Ratiometric fluorescence based on excited-state intramolecular proton transfer (ESIPT) generally results in a large Stokes shift with minimal self-absorption, while effective internal referencing greatly increases the probe's sensitivity and improves quantitation accuracy for the desired analyte [24,25]. However, only a few ratiometric probes based on ESIPT have been reported to date [14,16,18]. Thus, we herein report the synthesis of 2-(1H-imidazol-2-yl)phenol (Pi), which is a new ratiometric fluorescent probe bearing hydroxyl and imidazole moieties that facilitate the ESIPT-based detection of phosgene. Upon photoexcitation, Pi undergoes ESIPT to afford a keto tautomer that in turn undergoes a two-fold carbamylation reaction with phosgene to yield Pio, which is a six-membered ring-containing product (Scheme 1). The fluorescence emission wavelength of Pio differs from that of Pi because of the strongly electron-withdrawing nature of the carbonyl group in the former, which is expected to hamper ESIPT. To investigate the above-mentioned design (Scheme 1), Pi is synthesized in a single step from commercially available phenanthrene-9,10-dione and salicylaldehyde. Moreover, we design and examine a fluorescent test strip (FTS-Pi) for the rapid, sensitive, and selective detection of gaseous phosgene.
2.1. Materials and instrumentation Salicylaldehyde, phenanthrene-9,10-dione, and triphosgene (CCl3OC(O)OCCl3) were purchased from Sigma-Aldrich. Acetic acid, ammonium acetate, triethylamine (TEA), chloroform, petroleum ether, and acetic ether (all of analytical grade) were sourced from the Sinopharm Chemical Reagent Co. All other chemicals were of analytical grade. Deionized water was used in the assay, and 300–400-mesh silica gel (37–54 mm) was used for column chromatography. The reaction progress was monitored by thin layer chromatography (TLC) on silica gel plates (60F-254) that were visualized by UV light. An ATX-224 electronic balance (Shimadzu Corporation, Japan) was used for weighing, and an N-1001 rotary evaporator (Shanghai Ai Lang Instrument Company) was used for solvent removal. 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AVB-500 spectrometer in DMSO-d6 containing tetramethylsilane as an internal standard. The probe was characterized using an F-4500 fluorescence spectrophotometer (Japan Hitachi, Ltd.) and a UV-2450 UV spectrophotometer (Japan Shimadzu Corporation). 2.2. Synthesis and characterization of Pi A solution of salicylaldehyde (0.51 g, 4.91 mmol), phenanthrene9,10-dione (1.02 g, 4.91 mmol), and ammonium acetate (2.84 g, 36.77 mmol) in glacial acetic acid (20 mL) was refluxed for 5 h (120 °C), cooled under room temperature, and diluted with a copious amount of water to induce complete precipitation. The resulting precipitate was collected and purified by column chromatography to afford Pi as a white solid (1.18 g, 78%) that was further characterized by 1H/13C NMR spectroscopy and high-resolution mass spectrometry (HRMS). 1H NMR (500 MHz, DMSO) δ 13.71 (s, 1H), 13.15 (s, 1H), 8.90 (s, 2H), 8.55 (d, J = 43.4 Hz, 2H), 8.26 (dd, J = 8.0, 1.1 Hz, 1H), 7.71 (dd, J = 31.4, 24.3 Hz, 4H), 7.39 (dd, J = 11.4, 4.0 Hz, 1H), 7.09 (dd, J = 7.6, 5.3 Hz, 2H). 13C NMR (126 MHz, DMSO) δ 157.87, 149.80, 131.59, 127.96, 127.83, 126.29, 124.44, 122.74, 122.38, 119.58, 117.68, 113.44. HRMS (ESI+) for C21H15N2O+ M+H+: calculated 311.3179, found 311.1177. 2.3. Analytical procedure All analytical experiments were repeated at least three times to ensure a high measurement accuracy. Fluorescence measurements for phosgene detection were carried out as follows. A 0.5 mM stock solution of Pi was prepared in chloroform, while phosgene was generated by the addition of the less-toxic and easier-to-handle triphosgene to a solution of TEA [20,21]. More specifically, triphosgene was dissolved in chloroform containing 0.1 vol% TEA to prepare a 0.01 M stock solution of phosgene (the analyte). To determine the selectivity, stock solutions of potential chlorine-containing interferents were prepared in chloroform. Appropriate amounts of the analyte stock solution were mixed with the probe stock solution (40 μL), and the total volume was made up to 2 mL using chloroform. Fluorescence spectra were recorded at an excitation wavelength of 335 nm with excitation and emission slit widths of 5 and 2.5 nm, respectively. Scheme 1. Synthesis of Pi and the corresponding sensing product, Pio.
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2.4. Detection of phosgene gas by FTS-Pi
3.3. Reactivity of Pi toward (tri)phosgene
FTS-Pi was prepared as follows. One end of a TLC plate (2.55 × 7.35 cm) was immersed in the Pi stock solution (10 µM), then removed and dried under air. A further TLC plate was inscribed with “test” and prepared in the same manner. The prepared FTS-Pi plates were imaged under room light and under a 365 nm UV-light source. A sample of phosgene gas (0.5 ppm) in a sealed container was prepared by sequentially adding a stock solution of triphosgene (50 μL, 150 µM) and TEA (10 μL) in chloroform to a 1 L wide-mouth bottle, after which the lid was quickly closed. The FTS-Pi plate was suspended above the solution (Fig. S1) and imaged with both ambient light and UV light after exposure for 1 min to the phosgene-containing atmosphere. Images of the “test” FTS-Pi were acquired following the same procedure.
To further compare the reactivity of Pi toward phosgene and triphosgene, we recorded the time-dependent Pi fluorescence intensities (at 393 nm) in the presence and absence of 0.1 vol% TEA, as shown in Fig. 3. The fluorescence intensity rapidly increased over 30 s upon the addition of triphosgene to the Pi + 0.1 vol% TEA solution, after which it plateaued (300–330 s) (red line), while a slow and insignificant increase in the fluorescence intensity at 393 nm was observed in the absence of TEA (black line). The low reactivity of Pi toward triphosgene is ascribed to: (1) the absence of tertiary amine moieties that catalyze the decomposition of triphosgene to phosgene in the absence of TEA, and (2) the relatively weak reactivity of triphosgene, which precludes its direct reaction with Pi. Hence, the designed probe clearly discriminates between phosgene and triphosgene.
3. Results and discussion
3.4. Mechanism of the Pi response to phosgene
3.1. Synthesis of Pi and the proposed sensing product
Compared to previously reported phosgene probes that contain two amino groups as recognition sites, Pi features hydroxyl and imidazole moieties as active sites, and these groups undergo intramolecular proton transfer to afford a blue-green fluorescent intermediate (Scheme 2). Upon the addition of phosgene, the amino and hydroxyl groups of Pi react to form Pio, which contains a new six-membered ring featuring a strongly electron-withdrawing carbonyl group. Importantly, Pio does not easily undergo ESIPT and therefore exhibits bluish-violet fluorescence. In support of the proposed mechanism, Pi and purified Pio were characterized by 1H NMR spectroscopy. As shown in Fig. 4 and S5, the 1 H NMR signals of the amino (13.71 ppm) and hydroxyl (13.15 ppm) groups were absent following the reaction with phosgene, which confirms that these groups serve as the phosgene-recognition sites. To further verify the proposed mechanism, the above compounds were also characterized by HRMS (Fig. S6, HRMS (ESI+) for C22H13N2O2+: M +H+: calculated 337.0932, found 337.0983), which revealed that the molecular weight (m/z) of the purified product corresponds with that of Pio. These data provide strong evidence in support of the proposed mechanism.
Scheme 1 shows the synthetic route employed for the preparation of Pi and its rapid reaction with phosgene to afford Pio. The structures of Pi and Pio were confirmed by 1H/13C NMR spectroscopy and HRMS (Figs. S2–S6). 3.2. Spectral response of Pi to phosgene The ability of Pi to serve as a phosgene probe in chloroform solution was assessed by fluorescence spectroscopy. As shown in Fig. 1, a solution of Pi in chloroform (10 μM) displays a 469 nm green emission maximum. The absorption maximum of Pi was observed to shift to 329 nm in the presence of triphosgene/TEA (which generates phosgene) and was accompanied by a decrease in the intensity of the fluorescence emission at 469 nm and a concomitant increase in the fluorescence signal at 393 nm. These changes resulted in a gradual fluorescenceemission color change to blue when irradiated with a 365 nm hand-held UV lamp (Fig. 1B, inset). The fluorescence spectra of Pi solutions (10 µM) containing 0.1 vol % TEA were recorded at various concentrations of triphosgene (0–300 µM), and the ratio of the emission intensities at 469 and 393 nm was found to depend linearly on the phosgene concentration over the 0–4.0 μM range (Fig. 2). The phosgene detection limit was determined (by titration of triphosgene) to be 0.14 ppm (= 3σ/k, where σ is the standard deviation of the blank experiment, and k is the slope of the relationship between the emission-intensity ratio and the phosgene concentration (Fig. 2B)); this detection limit is significantly lower than the threshold level associated with immediate danger to health and life (IDLH, i.e., 2 ppm) [21,26].
0.30
A
For their application in a practical setting, probes should exhibit a high selectivity toward the target analyte. We therefore investigated the selectivity of Pi for the detection of phosgene in the presence of formaldehyde (CH2O), nitric oxide (NO), nerve agent mimics (DCP, DCNP), in addition to several chlorine-containing species as potential interferents, namely acetyl chloride (CH3COCl), thionyl chloride (SOCl2), p-toluenesulfonyl chloride (TosCl), phosphorus oxychloride (POCl3), oxalyl chloride ((CClO)2), sulfuryl chloride (SO2Cl2), tosyl chloride (TsCl), and triphosgene. Pi solutions (1 × 10−5 M) were
Fluorescence Intensity (a.u.)
Absorbance (a.u.)
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3.5. Selectivity for the detection of phosgene
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280 320 Wavelength (nm)
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Pi + Phosgene
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Fig. 1. (A) UV–vis and (B) fluorescence-emission spectra of Pi (10 µM) in CHCl3 (0.1 vol% TEA) in the absence (black) and presence (red) of phosgene (200 µM). 80
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Y = 0.011X + 0.038 R2 = 0.9872
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Fig. 2. (A) Emission spectra of Pi (10 µM) recorded in the presence of different concentrations of triphosgene (0–300 µM) in CHCl3 containing 0.1 vol % TEA. (B) Linear correlation between the ratio of emission intensities at 469 and 393 nm (I393 nm/I469 nm) and the phosgene concentration (0–4 µM); λex = 335 nm, λem = 393 nm, slit width = 3/5 nm.
0.03 0.00
0
1 2 3 Cphosgene (µM)
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Fig. 4. 1H NMR spectra of (A) Pi and (B) the purified product.
Fig. 3. Fluorescence intensities at 393 nm as a function of time for solutions of Pi (10 μM) treated with triphosgene (100 µM) at 300 s in the presence (red line) and absence (black line) of 0.1 vol% TEA; λex = 335 nm, λem = 393 nm, slit width = 3/5 nm.
treated with 200 μM phosgene and 200 μM (20 equiv of Pi solution) solutions containing interferents, and the obtained mixtures were examined by fluorescence spectroscopy. Fig. 5 reveals a clear response to phosgene, while no obvious responses to other analytes were detected, which is indicative of the highly specific and selective detection of phosgene. Fig. 5. Responses of Pi (10 μM) to phosgene (200 μM) and potential interferents (200 μM) in TEA-containing chloroform. (1) Blank, (2) CH2O, (3) NO, (4) DCP, (5) DCNP, (6) CH3COCl, (7) TosCl, (8) SOCl2, (9) POCl3, (10) (CClO)2, (11) SO2Cl2, (12) TsCl, (13) triphosgene, and (14) phosgene.
3.6. Detection of phosgene in the gas phase Since phosgene is recognized to be a toxic air pollutant, we established a portable technique for the detection of gaseous phosgene. For this purpose, we soaked a silica-gel-coated plate in Pi/CHCl3 (10 μM) to afford FTS-Pi, which exhibited green fluorescence when irradiated with 365 nm UV light. Importantly, the fluorescence color changed to blue after exposure to phosgene gas (at an estimated concentration of 0.5 ppm) generated by the reaction of triphosgene with TEA (Fig. 6). According to the Matheson Gas Data Book [27], exposure to a phosgene concentration of 20 ppm can cause lung injuries within 2 min, and exposure to a concentration of 25 ppm for as little as 30 min is lifethreatening, while death in 30 min or less is observed at a phosgene concentration of 90 ppm. Herein, phosgene was reliably detected by
FTS-Pi at concentrations equal to or less than dangerous levels. Furthermore, the selectivity of FTS-Pi toward phosgene over related analytes was also investigated (Fig. 7). After exposure to phosgene gas or to the vapor of 5 ppm related analytes (41 μL 5 mM chloroform solutions) for 5 min, the fluorescence color of FTS-Pi changed only in the presence of phosgene. We could therefore conclude that the prepared probe is well suited to the selective and sensitive detection of phosgene in both the solution and gas phases.
Scheme 2. Probable mechanism involved in the response of Pi to phosgene.
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Fig. 6. FTS-Pi fluorescence upon exposure to 0.5 ppm (∼2.2 μg/L) gaseous phosgene. Photographic images of a TLC plate prepared by dipping one end into a Pi/CHCl3 solution followed by air-drying (Sample 1), and those of a plate inscribed with “test” prepared in the same manner (Sample 2), acquired before and after exposure to gaseous phosgene.
Fig. 7. Images showing the fluorescence of FTS-Pi before (upper images) and after (lower images) exposure to phosgene gas and the vapors of various potential interferents (5 ppm). (1) Acetyl chloride, (2) p-toluene sulfonyl chloride, (3) thionyl chloride, (4) phosphorus oxychloride, (5) oxalyl chloride, (6) triphosgene, and (7) phosgene (0.5 ppm).
4. Conclusion
Acknowledgements
Herein, we successfully designed and synthesized a new ratiometric fluorescence probe (Pi) bearing hydroxyl and imidazole moieties as recognition sites, applied it to the detection of phosgene, and revealed that the sensing reaction afforded a new six-membered ring. The above reaction was very fast (competition time = 30 s) and sensitive (detection limit = 0.14 μM). Furthermore, Pi exhibited a high selectivity toward phosgene and could clearly distinguish it from triphosgene and other chlorides. Finally, FTS-Pi, which was fabricated by immobilizing Pi on a silica gel plate, was shown to respond to a gaseous phosgene concentration (0.5 ppm) below the threshold level (2 ppm) associated with immediate danger to health and life. Nonetheless, it should be noted that this probe also has a limitation. The sensitivity of this probe is not sufficient to monitor the phosgene exposure in some special places (such as pesticide, pharmaceutical and other production processes of phosgene trace leakage) because the safety value for 24 hexposure is approximately 0.6 ppb. However, this probe is still expected for reliable detection of phosgene levels in the case of terrorist attack or serious leaks due to its fast reaction time and high selectivity.
This work was supported by the National Natural Science Foundation of China (21874042 and 21675051), and the Foundation of the Hunan Provincial Science and Technology Department (2016SK2020). Notes This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal. There are no conflicts of interest to declare. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.talanta.2019.03.003.
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