BODIPY-Based Fluorescent Sensor for the Recognization of

Mar 2, 2017 - BODIPY-Based Fluorescent Sensor for the Recognization of Phosgene in Solutions and in Gas Phase ... As a highly toxic and widely used ch...
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BODIPY-Based Fluorescent Sensor for the Recognization of Phosgene in Solutions and in Gas Phase Hong-Cheng Xia, Xiang-Hong Xu, and Qin-Hua Song* Hefei National Laboratory for Physical Sciences at Microscale & Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: As a highly toxic and widely used chemical, phosgene has become a serious threat to humankind and public security because of its potential use by terrorists and unexpected release during industrial accidents. For this reason, it is an urgent need to develop facile, fast, and selective detection methods of phosgene. In this Article, we have constructed a highly selective fluorescent sensor o-Pab for phosgene with a BODIPY unit as a fluorophore and o-phenylenediamine as a reactive site. The sensor o-Pab exhibits rapid response (∼15 s) in both colorimetric and turn-on fluorescence modes, high selectivity for phosgene over nerve agent mimics and various acyl chlorides and a low detection limit (2.7 nM) in solutions. In contrast to most undistinguishable sensors reported, o-Pab can react with phosgene but not with its substitutes, triphosgene and biphosgene. The excellent discrimination of o-Pab has been demonstrated to be due to the difference in highly reactive and bifunctional phosgene relative to its substitutes. Furthermore, a facile testing paper has been fabricated with poly(ethylene oxide) immobilizing o-Pab on a filter paper for realtime selective monitoring of phosgene in gaseous phase.

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Scheme 1. Chemical Structures of Reported Fluorescent Sensors for Phosgene7−12

hosgene (COCl2) is a widely used colorless and highly toxic gas, so it was used as a chemical weapon agent (CWA) during World War I.1 Exposure to phosgene will cause noncardiogenic pulmonary edema and pulmonary emphysema and can even lead to individual death.2 On the other hand, phosgene has many important industrial applications as intermediate in productions of insecticides, plastics, isocyanates, resins, and aniline dyes.3 Unlike other CWAs, such as nerve agents (sarin, soman, and tabun), whose production is strictly controlled and prohibited by laws,4 phosgene is a readily available chemical.5 For this reason, phosgene becomes a serious threat to public health and safety because of its potential use by terrorists and unexpected release during industrial accidents. Therefore, it is urgent to develop facile, selective, and reliable methods for phosgene such as cost-effective, highly sensitive and highly selective fluorescent chemosensors. So far, there are only several papers about fluorescent chemosensors for phosgene,6−12 whose design strategies are to utilize electrophilic phosgene triggering the reactions with amines or alcohols (Scheme 1). Specifically, the hetero-cross-linking of two amino-containing fluorophores to achieve a fluorescence resonance transfer (FRET) process,6 ring-opening of the amino-containing spiro-(deoxy)lactam to give a fluorescent rhodamine,7 the reaction with cinnamic acids to form fluorescent coumarins,8 and twice carbamylations of nonfluorescent sensors with o-phenylenediamine (OPD) as a reactive site to give strongly fluorescent products resulting from blocking photoinduced electron transfer (PET) promoted fluorescence quenching.9−11 However, the selectivity of most sensors6−10 for phosgene over other analogues was not reported, except the sensor PY© 2017 American Chemical Society

OPD9 capable of discrimination between triphosgene (phosgene) and a nerve agent mimic, DCP, as well as acetyl chloride. Moreover, these sensors could not discriminate between triphosgene and phosgene.6−10 Triphosgene is a cost-effective substitute for phosgene because it can be handled more safely and more easily than phosgene and is widely used in organic synthesis.13 Hence, a qualified fluorescent sensor must discriminate between triphosgene and phosgene to avoid false alarm. Recently, we reported two fluorescent chemosensors, o-Pac11 and Phos-1, 12 with high selectivity to phosgene over triphosgene, nerve agent mimics, and various acyl chlorides. The two sensors can distinguish phosgene and triphosgene by a Received: January 17, 2017 Accepted: March 2, 2017 Published: March 2, 2017 4192

DOI: 10.1021/acs.analchem.7b00203 Anal. Chem. 2017, 89, 4192−4197

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Analytical Chemistry

spectrometer or MALDI-TOF. UV−vis absorption and fluorescence emission spectra were recorded with a Shimadzu UV-2450 UV/vis spectrometer and a Shimadzu RF-5301PC Luminescence Spectrometer, respectively. Assay Experiments. To avoid handling volatile phosgene, we employed nonvolatile and less toxic counterpart triphosgene (CCl3OC(O)OCCl3), which is a well-known precursor that generates phosgene in the presence of tertiary amines in solutions. Addition of triphosgene solutions into sensor solutions with 0.1% v/v TEA (volume ratio of a measured solution) was used to yield phosgene in situ. Chloroform (CHCl3) was employed as the solvent for all measurements in solutions. Preparation of the Test Paper. o-Pab (2 mg) was dissolved in 40 mL chloroform, and poly(ethylene oxide) (1.5 g) was added in batches. The mixture was stirred until the poly(ethylene oxide) dissolved completely. A filter paper was immersed in the solution and then taken out to dry. Finally, the paper with o-Pab was cut into strips to serve as the test paper for detection of phosgene in the gas phase. Detection of Phosgene Gas in Various Concentrations. Four concentrations of triphosgene solutions 0.14, 0.27, 0.41, and 0.54 M were prepared with chloroform as solvent, and 10μL aliquots of the solutions were deposited into four centrifuge tubes with a HPLC injection needle, followed by the addition of 10 μL of chloroform containing 0.1% TEA to each tube. Finally, the lids were closed. After 5 min, these tubes and a blank tube were photographed under 365 nm light. For the concentration of phosgene gas described in the captions of Figures 6 and S6 were calculated as in the following example: 10 μL of chloroform (0.1% TEA solution) was added into 5 mL-centrifuge tube, followed by the addition of 10 μL (0.54 M) of triphosgene solution; from this, the concentration of phosgene gas was calculated to be 80 ppm. Selective Detection of Phosgene Gas over Vapor of Other Analytes. Solutions of triphosgene (0.54 M) and other analytes (0.54 M) containing DCP, DCNP, (COCl)2, SOCl2, SO2Cl2, CH3COCl, POCl3, and TsCl were prepared in chloroform. Using a HPLC injection needle, 10 μL of each of the above solutions was removed into separate 5 mL-centrifuge tubes. The concentration of analytes was calculated to be 80 ppm if the analyte completely vaporizes. Synthesis and Characterization. Synthesis of N1-(5,5Difluoro-5H-4λ 4 ,5λ 4 -dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-10-yl)benzene-1,2-diamine (o-Pab). Under a nitrogen atmosphere, compound 114 (100 mg, 0.44 mmol), o-phenylenediamine (47.8 mg, 0.44 mmol), and NEt3 (89 mg, 0.88 mmol) were placed in a round-bottom flask with 10 mL of EtOH, and the mixture was stirred at 80 °C for 3 h. After the mixture was cooled to room temperature, the solvent was removed in vacuo. The residue was chromatographed on silica gel (PE/EA, v/v 5:1) to afford product o-Pab (114 mg, 76%) as a yellow solid. Rf = 0.51 (PE/EA 1:1). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 10.90 (s, 1H, NH), 7.68 (br, 1H, Ph-H), 7.46 (s, 2H, pyrrole-H), 7.24 (t, J = 7.6 Hz, 1H, pyrrole-H), 7.08 (d, J = 7.9 Hz, 1H, pyrrole-H), 6.90 (d, J = 8.0 Hz, 1H, pyrrole-H), 6.68 (t, J = 7.6 Hz, 1H, pyrrole-H), 6.46 (br, 1H, Ph-H), 6.22 (br, 1H, Ph-H), 5.86 (br, 1H, Ph-H), 5.43 (s, 2H, NH2) ppm. 13C NMR (100 MHz, DMSO-d6, TMS): δ = 149.5, 145.3, 134.0, 132.0, 130.6, 128.3, 125.9, 123.0, 122.5, 122.4, 118.8, 117.1, 116.5, 114.4, 113.8 ppm. 11B NMR (128.4 MHz, DMSO-d6): δ = 0.13 (t, JB−F = 29.3 Hz) ppm. 19F NMR (376

large difference in their reactivity. Thus, above amine sensors can be classified in two categories: (1) sensors with the tertiary amine that can catalyze triphosgene to decompose into phosgene and not discriminate between triphosgene and phosgene, such as PY-OPD9 and NAP-OPD,10 and (2) sensors without the catalytic tertiary amine, which react with phosgene and triphosgene in different rates, distinguishable by their response intensity, for example, o-Pac11 and Phos-1.12 However, there is still observable and similar response of the sensors to triphosgene in a long determined time, possibly leading to a false alarm. A recognizable fluorescent sensor should be able to react rapidly with phosgene and not react with triphosgene (3).

In this work, we selected an electron-withdrawing BODIPY fluorophore to tune the nucleophilicity of the amines of ophenylenediamine to construct a fluorescent sensor o-Pab (Scheme 2), which exhibited excellent selectivity to phosgene Scheme 2. Chemical Structure of the Probe o-Pab and Proposed Sensing Mechanism

over diphosgene, triphosgene, nerve agent mimics, and various acyl chlorides and gave rapid response (∼15 s) and a low detection limit (2.7 nM) for triphosgene. Moreover, a portable test paper with o-Pab was fabricated for detection of phosgene gas.

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EXPERIMENTAL SECTION Materials and General Methods. All chemicals for synthesis were purchased from commercial suppliers and were used as received without further purification. 1H, 11B, 13 C and 19F NMR spectra were measured in DMSO-d6 with a Bruker AV spectrometer operating at 400, 128, 100 and 376 MHz, respectively and chemical shifts were reported in ppm using tetramethylsilane (TMS) as the internal standard. Mass spectra were obtained with a Thermo LTQ Orbitrap mass 4193

DOI: 10.1021/acs.analchem.7b00203 Anal. Chem. 2017, 89, 4192−4197

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Analytical Chemistry MHz, DMSO-d6): δ = −141.29 (m), −143.21 (m) ppm. HRMS (TOF) m/z: calcd 298.1199 ([M+]); found 298.1191. Synthesis of 1-(5,5-Difluoro-5H-4λ4,5λ4-dipyrrolo[1,2c:2′,1′-f][1,3,2]diazaborinin-10-yl)-1,3-dihydro-2H-benzo[d]imidazol-2-one (2). The solution of o-Pab (107 mg, 0.36 mmol) and NEt3 (72 mg, 0.72 mmol) in 2 mL of DCM was added dropwise into the solution of triphosgene (53 mg, 0.17 mmol) in DCM (2 mL) in an ice bath. Then, the ice bath was removed, and the solution was stirred for 10 min at room temperature. The pH of the reaction mixture was adjusted to neutral with a NaHCO3 solution, and then, the mixture was extracted with DCM (2 × 10 mL). The organic phase was washed with water twice (2 × 10 mL), dried with anhydrous Na2SO4 and filtered, and then, the solvent was removed in vacuo. The residue was chromatographed on silica gel (PE/EA, v/v 3:1) to yield compound 2 (84 mg, 72%) as a dark red solid. Rf = 0.48 (PE/EA 1:1). 1H NMR (400 MHz, DMSO-d6, 25 °C, TMS): δ = 11.58 (s, 1H, NH), 8.17 (s, 2H, pyrrole-H), 7.21 (d, J = 4.2 Hz, 2H, pyrrole-H), 7.13 (dd, J = 4.5 Hz, J = 0.8 Hz, 2H, Ar−H), 6.95−7.03 (m, 2H, Ar−H), 6.66 (d, J = 4.2 Hz, 2H, pyrrole-H) ppm. 13C NMR (100 MHz, DMSO-d6, 25 °C, TMS): δ = 153.2, 146.6, 136.7, 132.9, 131.9, 130.5, 129.5, 123.5, 121.8, 120.0, 110.1, 109.8 ppm. 11B NMR (128.4 MHz, DMSO-d6): δ = 0.16 (t, JB−F = 28.7 Hz) ppm. 19F NMR (376 MHz, DMSO-d6): δ = −141.16 (m), −141.74 (m) ppm. HRMS (TOF) m/z: calcd. 324.0991 ([M+]); found 324.0985.

combination of PET from the OPD to the BODIPY unit and rotational deactivation along the aryl−NH−aryl single bonds. The fluorescence quantum yield for compound 2 was measured to be 0.08 in chloroform, with fluorescein (Φf = 0.90 in 0.1 N NaOH)16 as a reference. The spectral responses of the sensor to phosgene showed that the reaction of o-Pab with phosgene could yield compound 2. Absorption spectra of the o-Pab solutions with 0.1% v/v TEA, obtained upon the addition of triphosgene, were shown in Figure 1a. As shown in Figure 1a, a remarkable absorption

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RESULTS AND DISCUSSION Synthesis of the Sensor o-Pab and Proposed Sensing Product 2. Synthetic procedures for the probe o-Pab and compound 2 were illustrated in Scheme 3. Compound 1 was Scheme 3. Synthetic Procedure of the Sensor o-Pab and Compound 2a

Figure 1. UV−vis absorption (a) and fluorescence spectra (b) of o-Pab (10 μM) in CHCl3 (0.1% TEA) upon addition of various amount of triphosgene (0−5 equiv) recorded after 2 min, excitation at 450 nm. Inset: Linear correlation between the fluorescence increment and the concentration of triphosgene.

Reaction conditions: (a) NEt3, EtOH, 80 °C, 3 h; (b) triphosgene, NEt3, DCM, 10 min.

a

prepared according to the literature.14 Compound 1 and ophenylenediamine were refluxed with triethylamine (TEA) in EtOH for 3 h to afford the sensor o-Pab in a yield of 76%. The sensor o-Pac can react fast with triphosgene in the presence of TEA, which is taken as the method of generation of phosgene in situ,15 giving proposed sensing product 2 in a high yield (72%). The structures of o-Pab and 2 were fully characterized by 1H NMR, 13C NMR, 11B NMR, 19F NMR and highresolution mass spectroscopy (HRMS). Spectral Response of the Probe o-Pab to Phosgene. Photophysical properties of the sensor o-Pab and compound 2 were investigated by determining their UV−vis absorption and fluorescence spectra and comparing them with those of the reaction mixture of o-Pab with phosgene, shown in Figure S1. The absorption maxima are 422 and 514 nm for o-Pac and 2 respectively. This shows a strong conjugation between the carbamylurea and the BODIPY unit in the molecule 2. The sensor o-Pab is nonfluorescent, likely as a result of a

change was observed, that is, a decrease in the short-wavelength band (380−450 nm) and an increase in the long-wavelength band (450−550 nm) with an isosbestic point at 450 nm. This indicates that the sensing reaction of o-Pab to phosgene is a single and effective transformation. Meanwhile, fluorescence spectra of o-Pab solutions exhibit a turn-on response, with a fluorescence peak at 530 nm shown in Figure 1b. The fluorescence intensity increases sharply as the amount of triphosgene increases, until it reaches a 9800-fold increase upon addition of 5 equiv of triphosgene. The fluorescence spectra of o-Pab solutions with 0.1% NEt3 were recorded in various concentrations of triphosgene (0−50 μM). The detection limit can be obtained by titration of triphosgene and was determined to be 2.7 nM in terms of the equation of detection limit = 3σ/k, where σ is the standard deviation of blank measurement and k is the slope between the fluorescence increment (F/F0 ) at 530 nm versus the concentration of triphosgene (inset of Figure 1b). 4194

DOI: 10.1021/acs.analchem.7b00203 Anal. Chem. 2017, 89, 4192−4197

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Analytical Chemistry Sensing Mechanism. To further confirm the sensing mechanism, 1H NMR titration of o-Pab with phosgene was performed. As shown in Figure 2, the chemical shifts at 7.51,

triphosgene, nerve agent mimics (DCP, DCNP), and various acyl chlorides and analogues, such as oxalyl chloride ((COCl)2), thionyl chloride (SOCl2), sulfuryl chloride (SO2Cl2), acetyl chloride (CH3COCl), phosphorus oxychloride (POCl3), and tosyl chloride (TsCl). The fluorescence profiles at 530 nm shows excellent selectivity for phosgene over other analytes. In particular, no fluorescent change of the sensor oPab with triphosgene was observed within a long testing time (vide infra). The sensing behavior can be easily observed by the naked eyes from both the color and fluorescence emission of the solutions. As shown in Figure 4, the o-Pab solutions displayed a

Figure 2. Partial 1H NMR spectra of o-Pab in CD3CN with TEA before (a) and after (b, c) two additions of triphosgene and neat compound 2 (d) in CD3CN.

Figure 4. Photos for color change (upper) and fluorescence (bottom) of o-Pab (10 μM) in CHCl3 in the presence of various analytes (1 mM) except triphosgene (50 μM): 0, blank; 1, triphosgene/TEA; 2, DCP; 3, DCNP; 4, (COCl)2; 5, SOCl2; 6, SO2Cl2; 7, CH3COCl; 8, POCl3; 9, TsCl; 10, only triphosgene.

7.33, 6.82, 7.19, 6.94, 8.98, and 4.54 ppm were assigned to the H1, H2, H3, H4, and H5 protons of o-Pab, respectively. Upon addition of triphosgene, the proton signals of H1, H2, H3, H4, and H5 disappear, and new peaks appear at 7.98, 7.05, 6.62, and 9.15 ppm, which match those of the sensing product 2. In addition, the formation of the sensing product 2 was confirmed by HRMS: a dominant peak at the m/z value of 324.0985 was captured in the assay solution, which corresponded to 2+ (calcd. 324.0991), as shown in Figure S2. Selectivity. To evaluate the selectivity of the sensor o-Pab for phosgene, fluorescence spectra of o-Pab solutions were recorded before and after the addition of various analytes for 2 min (Figure 3). A turn-on fluorescence response of o-Pab solution was observed only for phosgene (triphosgene/TEA), and no change was observed for other analytes, including

color change from light green to orange with green fluorescence under a portable UV lamp (365 nm) only upon exposure to phosgene and showed no obvious color change or fluorescence emission when treated with 100 equiv of other analytes, including DCP, DCNP, (COCl)2, SOCl2, SO2Cl2, CH3COCl, POCl3, TsCl, and 5 equiv. triphosgene. Hence, o-Pab reveals a high selectivity for phosgene over other relevant analytes. Because OPD was employed as the reactive site of fluorescent probe for nitric oxide (NO), forming the sensing product, triazole,17 the reaction of o-Pab with was performed in chloroform or in acetonitrile. A similar spectral response to NO was observed in CHCl3 solutions but a different response was observed in CH3CN solutions, compared to those of phosgene. The color of the CH3CN solution changes to pink and with no fluorescence under 365 nm light for phosgene, shown in Figure S3. In the case of NO, the color of the solution changes a little, and the solution emits strong fluorescence under 365 nm light (Figure S3). In previous papers,6,9,10 triphosgene was employed directly as a phosgene equivalent, that is, the sensors could not discriminate between triphosgene and phosgene. In contrast, the sensor o-Pab exhibits a large difference in the reactivity between phosgene and triphosgene (vide infra). This can clearly distinguish them and avoid false detection. Reactivity of the Sensing Reaction. To further compare the reactivity of o-Pab with phosgene and its substitutes, diphosgene and triphosgene, time-dependent fluorescence intensities at 530 nm of o-Pab with and without 0.1% TEA were recorded, as shown in Figure 5. Upon addition of triphosgene, the fluorescence intensity increases rapidly and reaches to a plateau within 15 s for the solution of o-Pab with TEA, while no change was observed for the solutions without TEA within 35 min. This difference allows clear discrimination

Figure 3. Fluorescence spectra of 10 μM o-Pab in CHCl3 before and after addition of various analytes (1 mM) except triphosgene (50 μM): 0, blank; 1, triphosgene/TEA; 2, DCP; 3, DCNP; 4, (COCl)2; 5, SOCl2; 6, SO2Cl2; 7, CH3COCl; 8, POCl3; 9, TsCl; 10, only triphosgene. Spectra were recorded after 15 min, excitation at 450 nm. Inset: Fluorescence enhancements at 530 nm for the above solutions at 2 and 15 min. 4195

DOI: 10.1021/acs.analchem.7b00203 Anal. Chem. 2017, 89, 4192−4197

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achieving excellent discrimination between phosgene and its substitutes. Scheme 4. Possible Reaction of o-Pac with Acyl Chlorides

Figure 5. Time-dependent fluorescence intensity at 530 nm of 10 μM o-Pab solutions with without 0.1% v/v TEA before and after addition of 40 μM triphosgene and diphosgene, excitation at 450 nm.

In the case of diphosgene, no intramolecular carbamylation with the secondary amine in 3 occurs because the lowly reactive trichloromethoxycarbonyl and the low nucleophilic amine are weakened by electron-withdrawing BODIPY unit. 1H NMR spectra of o-Pab in acetonitrile show the existence of protons at amines before and after addition of acetyl chloride and biphosgene, shown in Figures S4 and S5. Also, HRMS provided additional evidence of the proposed mechanism (Figure S6). Detection of Phosgene Gas with the o-Pab Test Paper. To develop a simple, fast, and reliable method for detection phosgene gas, the test paper with o-Pab was prepared by poly(ethylene oxide) immobilizing o-Pab on a filter paper. The test paper strips were placed into five centrifuge tubes with different amounts of phosgene (0−80 ppm). Upon exposure to various amounts of phosgene for 5 min, the color of the test papers changed from light green to orange (Figure S7 upper). The test papers emit green fluorescence under a portable UV lamp, and fluorescence intensities increase with the concentration of phosgene (Figure S7 bottom). The strips were removed from centrifuge tubes and photographed under indoor light and 365 nm light, respectively (Figure 6).

between phosgene and its substitutes, diphosgene and triphosgene. In previous papers,6,7,9,10 the sensors contained tertiary amine groups, which catalyzed decomposition of triphosgene into phosgene. In general, the basicity of aliphatic amines is stronger than that of arylamines, and an electron-donating group enhances the basicity, while an electron-withdrawing group has an inverse impact. The pKa values of some amines can be used to estimate the basicity/nucleophilicity of amines, listed in Chart S1. The lack of reactivity of o-Pab with triphosgene may originate from (1) o-Pab not catalyzing decomposition of triphosgene into phosgene becuase it is not a tertiary amine and (2) the electron-withdrawing BODIPY unit18 leading to a decrease in the nucleophilicity of the amine groups, thereby, not reacting with triphosgene. Hence, the difference in the reactivity may attributed to the basicity/nucleophilicity of amine groups in the sensors, that is, a strongly nucleophilic tertiary amine acts to catalyze the decomposition of triphosgene into phosgene, whereas a strongly nucleophilic primary or secondary amine acts as a reactant in the carbamylation with phosgene or triphosgene. Hence, sensing triphosgene could undergo two pathways: (1) triphosgene is decomposed into phosgene by tertiary amine groups of a sensor and, subsequently, the phosgene produced by the decomposition reactions with the sensor or (2) triphosgene reacts directly with a highly reactive sensor. The two sensors described belong to the above-mentioned categories I and II, respectively. A comparison of the reactivity of phosgene and its substitutes, diphosgene and triphosgene, has been obtained from the investigation of Pasquato et al.19 The rate constants of pseudo-first-order reactions with 30 equiv of methanol at 25 °C in CDCl3 show that the reactivity of phosgene and diphosgene are 170 and 9.1 times greater than that of triphosgene, respectively. If a sensor can discriminate between phosgene and triphosgene, its reactivity must be between chlorocarbonyl (active) and trichloromethoxycarbonyl (inactive). Thus, the sensor if bifunctional for phosgene, monofunctional for diphosgene, and inactive for triphosgene. The sensor response to bifunctional phosgene is different than that for monofunctional analytes, such as diphosgene and acetyl chloride. The sensor o-Pab can react with bifunctional phosgene via two carbamylations, giving a fluorescent product 2, but cannot react with triphosgene or reacts only once with acyl chlorides at the primary amine, giving a nonfluorescent 3 because of PETpromoted fluorescence quenching (Scheme 4), thereby,

Figure 6. Photos of the color (upper) and fluorescence (bottom) of oPab test papers upon exposure to various amounts (0−80 ppm) of phosgene in centrifuge tubes for 5 min.

According to the Matheson Gas Data Book,20 concentrations of phosgene of 20 ppm can cause lung injuries in 2 min, and exposure to concentrations of 25 ppm for as little as 30 min is very dangerous. A phosgene concentration of 90 ppm is rapidly fatal in 30 min or less. Concentrations of phosgene within the dangerous range can be detected by the o-Pab test papers, implying that the sensor can detect phosgene at or below concentrations shown to lead to a health risk. The selectivity of the test paper with o-Pab for phosgene over related analytes was also investigated. After exposure to phosgene gas or to the vapor of related analytes for 5 min, the test paper displays the color change and green fluorescence only for phosgene; no observable change occurred on the test papers exposed to the vapors of other analytes (DCP, DCNP, 4196

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(COCl)2, SOCl2, SO2Cl2, CH3COCl, POCl3, TsCl) (Figures S8 and 7). These observations are completely in agreement with the results from solutions.

Qin-Hua Song: 0000-0001-6501-1382 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from National Natural Science Foundation of China (Grant No. 21272224), and Anhui Provincial Natural Science Foundation (Grant No. 1708085MB33).

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Figure 7. Color change (upper) and fluorescence (bottom) responses of o-Pab test papers upon exposure in 80 ppm phosgene and various other analytes in 20 μL chloroform solutions: 0, blank; 1, phosgene; 2, DCP; 3, DCNP; 4, (COCl)2; 5, SOCl2; 6, SO2Cl2; 7, CH3COCl; 8, POCl3; 9, TsCl.

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Therefore, above results and observations exhibit that the oPab test paper may be a sensitive and selective method for detecting phosgene gas.

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CONCLUSIONS In summary, a colorimetric and fluorescent sensor, o-Pab, has been developed by the conjunction of o-phenylenediamine with a BODIPY unit in the meso position. The sensing reaction of oPab to phosgene is very fast and sensitive and finishes within 15 s with a low detection limit (2.7 nM). This sensor can not only detect phosgene with high selectivity over nerve-agent mimics and various acyl chlorides and but also discriminate between phosgene and its substitutes, diphosgene and triphosgene, in solutions. The recognition of phosgene has been clarified, that is, the BODIPY fluorophore as an electron-withdrawing group reduces the reactivity of the sensing site o-phenylenediamine, resulting in two carbamylation reactionss for bifunctional phosgene, giving fluorescent product 2, but only one carbamylation reaction to form nonfluorescent 3 for diphosgene and active acyl chlorides, resulting from a PETpromoted fluorescence quenching. These observations would gain some new insights into molecular design of fluorescent sensors for phosgene. Furthermore, the test paper with o-Pab was fabricated for selective detection of phosgene gas by colorimetric and fluorescent sensing modes at or below concentration levels leading to health risk.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b00203. Details of assay experiments, photophysical properties of o-Pab and 2, HRMS for the reaction mixture of o-Pab with phosgene, pKa values of some amines, spectral response of o-Pab to NO, evidence for the formation of 3, photos of o-Pab test papers for detections of gaseous phase, and NMR spectra of o-Pab and 2 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 551 63601592. Tel: +86 551 63607992. 4197

DOI: 10.1021/acs.analchem.7b00203 Anal. Chem. 2017, 89, 4192−4197