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A fluorescent chemosensor for selective detection of phosgene in solutions and in gas phase Hong-Cheng Xia, Xiang-Hong Xu, and Qin-Hua Song ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00723 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016
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A fluorescent chemosensor for selective detection of phosgene in solutions and in gas phase Hong-Cheng Xia, Xiang-Hong Xu, and Qin-Hua Song* Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China. KEYWORDS： Fluorescent chemosensors, Chemical warfare agents, Phosgene, Triphosgene, Coumarin
ABSTRACT: The detection of highly toxic chemicals in a convenient, fast and reliable manner is essential for coping with serious threats to humankind and public security caused by unexpected terrorist attacks and industrial accidents. In this paper, a highly selective fluorescent probe has been constructed through o-phenylenediamine covalently linking to the coumarin (o-Pac), which can respond to phosgene in turn-on fluorescence mode. The response time is less than 0.5 min, and the detection limits is as low as 3 nM, in solutions. More importantly, the sensor exhibits good selectivity to phosgene over triphosgene and various acyl chlorides. Furthermore, a portable test paper has been fabricated with polystyrene membrane containing o-Pac for real-time selective monitoring of phosgene in gas phase.
Phosgene (COCl2) is a widely used colorless and highly toxic gas. Exposure to phosgene may cause noncardiogenic pulmonary edema, pulmonary emphysema, and death.15 As one of the most toxic substances, phosgene was used as a chemical weapon agent (CWA) during the World Wars I. 6 In contrast to never agents (such as sarin, soman, and tabun), whose production is strictly controlled and prohibited by laws, phosgene is widely used industrial material.7-8 For this reason, phosgene becomes a serious threat to public health safety for both its potential use by terrorists and its unexpected release during industrial accidents. However, up to now there are only five papers about fluorescent chemosensors for phosgene,9-13 which reveal excellent design strategies based on electrophilic phosgene triggering the reactions with amines or alcohols. For example, the reactions mediated by phosgene including hetero-cross-linking of two amino-containing fluorophores achieve to a FRET process,9 opening of the aminocontaining spiro-(deoxy)lactam to give a fluorescent rhodamine,10 and converting cinnamic acids into fluorescent coumarins.11 Recently, Yoon and coworkers have developed four sensors based on a novel design strategy. 12, 13 oPhenylenediamine is covalently linked to a fluorophore to construct a non-fluorescent sensor resulting from intramolecular electron transfer, which sense phosgene to generate a fluorescent product ascribing to block the electron transfer. Although these elegant sensors have been reported, there are still some limitations. For example, the most sensors cannot discriminate phosgene and triphosgene in solutions, and the selectivity of all above sensors was not investigated except the sensor PY-OPD12 for discrimination between phosgene and a nerve-gas mimic, DCP.
In this work, we have developed a fluorescent sensor by covalently linking of o-phenylenediamine with the coumarin (o-Pac, shown in Scheme 1), which can detect phosgene, giving turn-on fluorescence response within 1 min, and exhibits good selectivity to phosgene over triphosgene and various acyl chlorides. Moreover, a portable test paper with o-Pac was fabricated for facile detection of phosgene in gas phase. NH2
NH
O NH
N
O
Cl
O
Cl
N
N
O
O
O
o-Pac
Scheme 1. Chemical structure of the probe o-Pac and proposed sensing mechanism.
■ EXPERIMENTAL SECTION Materials and Methods. All the chemicals for synthesis were purchased from commercial suppliers and were used as received without further purification. 1H and 13C NMR spectra were measured in CDCl3 or DMSO-d6 with a Bruker AV spectrometer operating at 400 MHz and 100 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 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.
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Preparation of the test paper. o-Pac (2 mg) was dissolved in 40 mL DCM and polystyrene (1.5 g) was added in batches. The mixture was stirred until polystyrene dissolved completely. A filter paper was immersed in the solution, and then taken it out to dry in air. Finally, the paper with o-Pac was cut into strips as the test paper for detection of phosgene in gas phase.
The measurement of the oxidation potential was carried out in acetonitrile at room temperature with a CHI620D multipurpose equipment interfaced to PC. The working electrode was platinum electrode. The counter electrode was a Pt wire, and the reference electrode was a SCE and 0.1M tetrabutylammonium perchlorate as electrolyte solution. NHBoc Cl + N
O
NHBoc NH2
NH
a
2
c
O
3
O
N
O
O
O
71%
83% N
1
NH
NH
b
48%
O
NH2
N
O
o-Pac
O
N
4
Scheme 2. Synthetic procedure of probe: (a) NEt3, EtOH, 80°C, 24 h; (b) TFA, DCM, r.t., 10 h; (c) Triphosgene, NEt3, DCM, 10 min. Synthesis of tert-butyl (2-((7-(diethylamino)-2-oxoSynthesis of 1-(7-(diethylamino)-2-oxo-2H2H-chromen-4-yl)amino)phenyl) carbamate (3). Unchromen-4-yl)-1,3-dihydro-2H-benzo[d]imidazol-2der a nitrogen atmosphere, a mixture of 4-chloro-7one (4). After o-Pac (100 mg, 0.31 mmol) and NEt3 (62 mg, (diethylamino)-coumarin (1) (200 mg, 0.78 mmol), the 0.62 mmol) were dissolved in 2 mL of DCM, the solution Boc protected o-phenylenediamine (2) (166 mg, 0.78 of triphosgene (46 mg, 0.15 mmol) in DCM (2mL) was mmol) and NEt3 (394 mg, 3.9 mmol) in 10 mL of EtOH added slowly under ice bath. As finishing addition, the ice was stirred at 80°C for 24 h. After cooling to room tembath was removed and the solution was stirred for 10 min perature, the solvent was removed in vacuo. The residue at room temperature. The reaction mixture was adjusted was chromatographed on silica gel (PE/EA, v/v 5:1) to afto neutral with a NaHCO3 solution, and then extracted ford a coupling product (3) (158 mg, 48%) as a white solid. with DCM (2 × 10 mL). The organic phase was washed Rf =0.52 (PE/EA 1:1); 1H NMR (400 MHz, CDCl3, 25°C, with water twice (2 × 10 mL), dried with anhydrous TMS): δ =7.94 (NH, s, 1H), 7.57 (Ar-H, dd, J = 9.0 Hz, J = Na2SO4, filtered and the solvent was removed in vacuo. The residue was chromatographed on silica gel (PE/EA, 1.3 Hz 1H), 7.37-7.44 (Ar-H, m, 2H), 7.14-7.22 (Ar-H, m, 2H), 6.93 (Ar-H, d, J=11.9Hz, 1H), 6.58 (Ar-H, d, J=8.4 Hz, v/v 3:1) to yield compound 4 (77 mg, 71%) as a yellow solid. 1H), 6.47 (N-H, s, 1H), 5.17-5.18 (Ar-H, m, 1H), 3.41 (CH2, q, Rf =0.53 (PE/EA 1:1); 1H NMR (400 MHz, CDCl3, 25°C, J=7.0 Hz, 4H), 1.49 (CH3, s, 9H), 1.20 (CH3, t, J=7.0 Hz, 6H) TMS): δ = 10.45 (NH, s, 1H), 7.16-7.21 (Ar-H, m, 2H), 7.11ppm. 13C NMR (100 MHz, CDCl3, 25 °C, TMS) δ =164.5, 7.15 (Ar-H, m, 2H), 7.07 (Ar-H, dt, J=7.8 Hz, J=1.5 Hz, 1H), 156.2, 154.3, 152.8, 150.6, 132.3, 130.6, 127.4, 127.0, 125.7, 123.6, 6.59 (Ar-H, d, J=2.4 Hz, 1H), 6.53 (Ar-H, dd, J=2.4Hz, 122.2, 108.1, 102.9, 98.1, 82.1, 81.5, 44.7, 28.3, 12.4 ppm. 9.1Hz, 1H), 6.22 (Ar-H, s, 1H), 3.43 (CH2, q, J=7.1 Hz, 4H), HRMS (ESI) m/z calcd for: 424.2506, ([M+Na+]), found: 1.22 (CH3, t, J = 7.1 Hz, 6H) ppm. 13C NMR (100 MHz, 424.2529. CDCl3, 25°C, TMS): δ = 162.3, 157.1, 154.0, 151.5, 146.8, 129.8, Synthesis of 4-((2-aminophenyl)amino)-7128.4, 126.1, 123.1, 122.0, 110.4, 109.8, 108.8, 107.3, 104.9, 97.6, (diethylamino)-2H-chromen-2-one (o-Pac). Com44.9, 12.4 ppm. HRMS (TOF) m/z calcd for: 349.1421, pound 3 (100 mg, 0.24 mmol) was dissolved in 5 mL of ([M+]), found: 349.1412. DCM, and then the solution of trifluoroacetic acid (1 mL) ■ RESULTS AND DISCUSSION in DCM (2 mL) was added slowly under the ice-bath condition. After the addition, the solution was stirred for 10 h Synthesis of Related Compounds. Synthetic proceat room temperature. The solution of NaHCO3 was added dure of the probes was illustrated in scheme 2. The 4to adjust the pH to 7-8, and extracted with DCM (10 mL). chlorocoumarin (1) and the Boc protected oThe organic phase was washed with water twice, dried phenylenediamine (2) were prepared according to methwith anhydrous Na2SO4, filtered and the solvent was reods of literatures.14, 15 Compound 3 was obtained by the moved in vacuo. The residue was chromatographed on coupling reaction of 1 with 2 in the presence of trimethylsilica gel (PE/EA, v/v 3:1) to yield o-Pac (64 mg, 83 %) as a amine (TEA) in the yield of 48%. Using trifluoroacetic white solid. Rf =0.53 (PE/EA 1:2); 1H NMR (400 MHz, acid, the deprotection of Boc in 3 affords the probe o-Pac DMSO-d6, 25°C, TMS): δ = 8.63 (Ar-H, s, 1H), 7.97 (Ar-H, in the yield of 83%. o-Pac can react fast with triphosgene d, J=9.1 Hz, 1H), 7.05-7.09 (Ar-H, m, 1H), 6.99 (Ar-H, d, in the presence of TEA, which is taken as the method of J=7.7 Hz, 1H), 6.81 (Ar-H, d, J=8.0 Hz, 1H), 6.89 (Ar-H, dd, generation of phosgene in situ,16 to generate compound 4 J=9.1 Hz, J=2.4 Hz, 1H), 6.62 (Ar-H, t, J=7.6 Hz, 1H), 6.43 in a high yield (71%). The structures of three compounds, (Ar-H, d, J=2.4 Hz, 1H), 5.00 (NH2, s, 2H), 4.42 (NH, s, 1H), 3, 4 and o-Pac were fully characterized by 1H NMR, 13C 3.42 (CH2, q, J=6.9 Hz, 4H), 1.13 (CH3, t, J=6.9 Hz, 6H) NMR and high-resolution mass spectroscopy. ppm. 13C NMR (100 MHz, DMSO-d6, 25°C, TMS): δ = 162.9, The sensor o-Pac in chloroform is non-fluorescent, and 156.2, 154.3, 150.6, 145.4, 129.0, 128.5, 124.6, 122.5, 116.7, 116.0, compound 4 emits strong blue fluorescence. The fluores108.2, 102.9, 97.4, 80.4, 44.4, 12.8 ppm. HRMS (ESI) m/z cence quantum yield of compound 4 in chloroform was calcd for: 346.1531, ([M+Na+]), found: 346.1529.
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obtained as a high value, 0.87, with quinine sulfate (Φf = 0.546 in 1.0 N H2SO4)17 as the reference. Thus, if the sensing reaction occurs, the system with o-Pac would exhibit a turn-on fluorescence response toward phosgene. Spectral response toward phosgene. As expected, upon addition of triphosgene, the fluorescence spectra of the o-Pac solutions with 0.1% v/v TEA exhibit a turn-on rapid response, with a fluorescence peak at 446 nm shown in Fig. 1a. The fluorescence enhances with increasing amount of triphosgene, reaching 533-fold increments after addition of 5 equiv. of triphosgene. Accordingly, the sensing reaction causes the change in the absorption spectra shown in Fig.1b. A decrease in the shortwavelength band and increase in long-wavelength band with an isosbestic point at 368 nm indicate that the sensing reaction is a single and effective converted process. 600
a
@446 nm
F/F0
Intensity (a.u.)
900
300
5eq
The value is much lower than those of the most sensors except the cinnamic acid sensor (1 nM) reported by Hwang’s group. 11 The sensing mechanism. In the synthesis scale, compound 4 has been obtained from the reaction of o-Pac with phosgene generating in situ from triphosgene in the presence of TEA, and fully characterized by 1H NMR, 13C NMR and high-resolution mass spectroscopy. To further verify the sensing mechanism, 1H NMR titration of o-Pac with phosgene was performed. As shown in Fig. 2, the chemical shifts at 4.99 ppm, 6.49 ppm, 6.59 ppm and 7.45 ppm were assigned to the proton Ha, Hb, Hc and Hd of o-Pac, respectively. Upon addition of triphosgene, the proton signals of Ha, Hb, Hc and Hd disappear and with the concomitant appearance of new peaks at 6.22, 6.59, 6.53, 6.95 and 10.39 ppm, which match those of the sensing product 4. In addition, the formation of the sensing product 4 was confirmed by high-resolution mass spectroscopy, where a dominant peak at an m/z value of 349.1412 (calcd. 349.1426), corresponding to 4+, provided as Supporting Information in Fig. S1.
600
H 2N 0 0
0
20
Hc
Ha
Et2N 400
450
500
550
O
H d HeN
Concentration /µM
300
O
Cl
H d'
Wavelength /nm H
O
O Ha'
Et2N
O
O
H b' P h'
f
N
Hc'
Cl
Hb
600
NHf
Ph'
Ph
40
0
H
d'
H
b'
H
c'
H
a'
d
b
c
0.3
Absorbance
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b 0.2
H
d
Ph
H
c
b
H ,H
e
H
5 equiv.
a 10
0.1 0 0.0 300
350
400
a
450
Wavelength /nm
Figure 1. Fluorescence (a) and UV/vis absorption (b) spectra of o-Pac (10 μM) in CHCl3 (0.1% TEA) in the presence of increasing amount of triphosgene (0-5 equiv.) recorded after 2 min, excitation at 368 nm. Inset: Linear correlation between the fluorescence intensity toward concentrations of triphosgene.
Based on above titration experiment of triphosgene, the detection limit of o-Pac toward triphosgene can be obtained. The fluorescence spectra of o-Pac solutions with 0.1% NEt3 were recorded in various concentrations of triphosgene (0–50 μM). Based on the plot of the fluorescence intensity at 446 nm vs. the concentration of triphosgene, the detection limit of o-Pac was obtained to be 3 nM by fitting straight line and calculated in terms of the formula (3δ/k), shown in the inset of Fig. 1a.
7
ppm
6
5
Figure 2. 1H NMR spectra of o-Pac in CDCl3 (with 0.1% TEA) before (a) and after (b,c) addition of triphosgene and (d) neat compound 4 in CDCl3. To understand the electron-transfer sensing strategy, free energy changes (∆Get) for intramolecular electrontransfer reactions from the o-phenylenediamine or its acylation moiety to the coumarin moiety are estimated in terms of the Rehm-Weller equation,18,19 and calculation details provided in Supporting Information. The calculated values of ∆Get, −0.15 eV for the sensor o-Pac and 0.58 eV for the sensing product 4, show that the photoinduced electron-transfer reaction can occur in o-Pac leading to fluorescence quenching, while cannot in compound 4, giving strong fluorescence. This calculation result supports the electron-transfer mechanism. Selectivity. To evaluate the selectivity of the sensor for phosgene, the fluorescence spectra of the o-Pac solutions were recorded before and after the addition of various analytes for 2 min (Fig. 3). The fluorescence spectrum of o-Pac displays a large change only for phosgene, and no significant change was observed for other analytes includ-
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ACS Sensors ing triphosgene, nerve agent mimics (DCP, DCNP) and active acyl chlorides, oxalyl chloride ((COCl)2), thionyl chloride (SOCl2), sulfuryl chloride (SO2Cl2), acetyl chloride (CH3COCl), phosphorus oxychloride (POCl3) and tosyl chloride (TsCl). The fluorescence profiles at 446 nm of the probe showed a higher selectivity for phosgene over the other analytes including triphosgene. The reaction of o-Pac with triphosgene is slow in the absence of TEA, and only a little change in fluorescence spectrum was observed (Fig. 3). 300
a
100
other analytes
0 400
150
b
450 500 Wavelength /nm
550
600
2 min 15min
100
compound 4, and these observations were provided in Fig. S2 as Supporting Information. In previous papers,9, 10, 12, 13 triphosgene was employed as a phosgene simulant, that is, the amino-containing sensors cannot discriminate between triphosgene and phosgene. The sensor o-Pac in this work can discriminate them observably. To the best of my knowledge, this is the first sensor that can distinguish between triphosgene and phosgene. The time-dependent fluorescence intensities at 446 nm of o-Pac with and without 0.1% v/v TEA were recorded for estimating the response times to phosgene and triphosgene, or their reactivity with o-Pac. Upon addition of triphosgene, the fluorescence intensity of the solution with TEA enhances rapidly, and reaches to a plateau within 20 seconds, while only 1% increment within 20 seconds and
