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Instantaneous Detection of Tri-chlorinated Carbon via Photo-Induced Electron Transfer toward Chemosensor for Toxic Organochlorides Inkyu Lee, Ji Eon Kwon, Yeongkwon Kang, Ki Chul Kim, and Bong-Gi Kim ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00602 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018
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Instantaneous Detection of Tri-chlorinated Carbon via PhotoInduced Electron Transfer toward Chemosensor for Toxic Organochlorides †
‡
†
†
Inkyu Lee, Ji Eon Kwon, Yeongkwon Kang, Ki Chul Kim, and Bong-Gi Kim*
†
†
Division of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea.
‡
Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 08826, Republic of Korea.
KEYWORDS. Organochloride detection, photo-induced electron transfer, instantaneous detection, diphosgene detection, radical reaction ABSTRACT: Despite the usefulness of organochlorides as raw materials for organic synthesis, they cause several issues in human body, such as hepatic dysfunction, tumor, and heavy damage to the central nervous system. Especially, when organochlorides contain three or more chlorinated carbons, they tend to be more toxic to the human body possibly owing to relatively high reactivity. Several electron donors (TPCAs) are designed to devise a novel detection system for toxic organochlorides containing tri-chlorinated carbons, and the detection mechanism of the devised sensor system is systematically identified by EPR measurement and the analysis of solution after the detection of chloroform, which is used as a model compound. Since the detection system simultaneously utilizes the radical-generation capability and the low LUMO level of the tri-chlorinated carbon, it provides high selectivity against most of the common organic compounds including other organochlorides containing mono- or di-chlorinated carbons, and the outstanding selectivity of the designed sensor has been verified with Mirex composed of numerous chlorinated carbons. In addition, the detection system exhibits immediate sensing capability because only electron transfer and radical reaction are involved in the detection process. Finally, when diphosgene is detected with the devised sensing platform, a noticeable change in fluorescence intensities can be identified within 5 s even for a diphosgene concentration of less than 1 ppm.
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TPCA0M
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TPCA6M TPCA0M absorption TPCA0M emission TPCA2M absorption TPCA2M emission TPCA6M absorption TPCA6M emission
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Organochlorides, organic chemicals containing at least one chlorine atom in their molecular structures, are widely used as raw materials for organic synthesis or solvents for device fabrication in industrial or scientific researches owing to their highly activated chemical nature and excellent compatibility with common organic materials.1-2 However, despite their usefulness, organochlorides cause several issues in human body, such as hepatic dysfunction, tumor, and heavy damage to the central nervous system.3-8 Although some organochlorides are considered safe enough for consumption in foods and medicines,9-10 when organochlorides contain three or more chlorinated carbons, they tend to be more toxic to the human body possibly owing to their relatively high reactivity. For example, the reference value of immediately dangerous to life or health (IDLH) defined by the by the US National Institute for Occupational Safety and Health (NIOSH) reduces as the number of chlorine atom attached to same carbon atom increases (e.g., 2300 ppm for CH2Cl2, 500 ppm for CHCl3, and 200 ppm for CCl4). In addition, representative substances, such as diphosgene used as chemical weapon and dichlorodiphenyltrichloroethane (DDT) used as insecticides, reflect the fatal harmfulness of trichloromethyl-containing materials to plants or animals, including humans.11-16 Thus, the development of efficient
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Wavelength (nm) Figure 1. (a) Chemical structures of obtained TPCAs and (b) their absorption and emission spectra in THF solution.
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and selective detection systems for tri-chlorinated carbons among organochlorides is in high demand. Various detection systems combined with analytical processes such as chromatography (gas and liquid), mass spectroscopy, and optical detection have been developed to quantitatively reveal the existence of organochloride.1721 However, the devised detection systems have been limited to accurate specification of analytes in real time because their detection mechanisms are mostly based on the material’s physical properties including the prerequisites for detection, such as complex analysis tools, sample pretreatment, and signal amplification. In particular, rapid and simple screening are crucially required for toxic analytes; however, there have been few studies on sensing systems capable of selectively detecting the overall organochloride containing tri-chlorinated carbons22-23 because its selective reactivity required for the design of probe unit has not been elucidated. Here, we have focused on the lowest unoccupied molecular orbital (LUMO) of the organochlorides containing a tri-chlorinated carbon, which is lowered by the high electron affinity of the attached Cl atoms and the electrophilic reaction of a radical that is possibly generated through the excitation of trichlorinated carbon moiety. Thus, several electron donors were designed for a photo-induced electron transfer (PET) process to utilize the deep LUMO level of the organochlorides containing tri-chlorinated carbon (Figure 1a), and the emission quench rate caused by PET to the organochlorides was quantitatively analyzed. The detection mechanism was identified using chloroform (CF), the smallest molecule containing a tri-chlorinated carbon, as a model compound. Finally, when the newly devised screening system for the tri-chlorinated carbon was applied for diphosgene detection, a typical toxic substance, to confirm the possibility of general application, it was revealed that the detection system worked effectively even at a diphosgene concentration of less than 1 ppm within 5 s.
EXPERIMENTAL SECTION Materials. All starting materials used in this research were purchased from commercial supplier (Aldrich and Tokyo Chemical Industry). TPCA0M and TPCA2M were prepared as previously described manners,24 and detailed synthetic procedure of TPCA6M was illustrated in Scheme S1. The obtained compounds were fully characterized with 1H-NMR, 13C-NMR and high-resolution mass spectroscopy (Figure S1-S4). Compound 4. Compound 4 was obtained via Ullmann coupling reaction. The reaction was carried out in a twonecked round bottom flask under Ar atmosphere. 3,4,5trimethoxy iodobenzene (4.50 g, 15.30 mmol), 4bromoaniline (1.05 g, 6.12 mmol), 1,10-phenanthroline (167 mg, 0.92 mmol), copper (I) iodide (176 mg, 0.92 mmol) and potassium hydroxide (3.43 g, 61.2 mmol) were dissolved in toluene anhydrous (61.2 mL), and the mixture was stirred and refluxed for 48 hours. Then, the organic layer was extracted with methylene chloride and dried with magnesium sulfate (MgSO4) anhydrous. After removing solvent under reduced pressure, compound 4 was obtained through silica column chromatography using n-
hexane/methylene chloride (1/2) in 26.19 % yield. 1H NMR (500 MHz, CDCl3, δ): 7.320 (d, 2H), 6.935 (d, 2H), 6.283 (s, 4H), 3.849 (s, 6H), 3.721 (s, 12H). TPCA6M. The synthesis was performed in a twonecked round bottom flask under Ar atmosphere. 4Bromo-3’,4’,5’,3”,4”,5”-hexamethoxytriphenylamine (1.00 g, 1.98 mmol), 9-phenylcarbazole-3-boronic acid (0.63 g, 2.18 mmol) and Pd(PPh3)4 (115 mg, 0.10 mmol) were added to a solution consisting of tetrahydrofuran (37.5 mL) and potassium carbonate solution (12.5 mL, 2.0M in distillated water). Then, the mixture was stirred and refluxed for 8 hours. After cooling down to room temperature, organic layer was extracted with methylene chloride and dried with MgSO4 anhydrous. After evaporating solvent under a reduced pressure, TPCA6M was finally obtained by silica column chromatography using n-hexane and methylene chloride (3:1) as an eluent (yield: 72.85 %). 1H NMR (500 MHz, (CD3)2CO, δ): 8.3082 (d, 1H), 7.7591-7.6886 (m, 5H), 7.6878-7.6519 (m, 2H), 7.5648 (t, 1H), 7.4922-7.4000 (m, 3H), 7.3881-7.2585 (m, 2H), 7.1453 (d, 2H), 6.4702 (s, 4H), 3.7363 (s, 6H), 3.7155 (s, 12H); 13C NMR (150 MHz, (CD3)2CO, ppm) 153.70, 153.62, 143.59, 134.13, 129.90, 127.70, 127.49, 127.02, 123.59, 120.02, 118.19, 110.02, 109.92, 102.29, 102.21, 77.31, 77.00, 76.68, 61.04, 56.21, 32.98 ; LC-mass (m/z) 667.2 (calcd. 666.8); anal. calcd. for C42H38N2O6: C, 75.66; H, 5.74; N, 4.20; O, 14.40; found: C, 75.62; H, 5.72; N, 4.21; O, 14.45. Optical Analysis. Optical properties of TPCA series were figured out by using UV-Visible spectroscopy (Agilent Technologies Cary 60 UV-Vis) and photoluminescence spectroscopy (Scinco FS-2). All optical analysis was performed in tetrahydrofuran (THF) solution, and photoluminescence spectra were recorded with 0.5 mM of TPCA solution using 400 mV of PMT voltage. Also, the measurements for Stern-Volmer plot were carried out the same solution under 350 mV of PMT voltage. Analysis and Characterization. Liquid chromatography-mass spectroscopy (LC-MS, Thermo Finnigan LTQ) was performed to reveal chemicals generated from radical reaction of TPCAs with acetonitrile on reverse phase column. Electron paramagnetic resonance (EPR) spectroscopy (Bruker EMX Plus CW EPR, microwave frequency: 9.64 GHz, modulation frequency: 100 KHz, microwave power: 1 mW, modulation amplification: 10G, room temperature) was carried out with 10 mM of TPCAs in tetrahydrofuran (10 mL) after UV irradiation for 30 seconds. To generate radical species, chloroform 100 μL was added to the TPCA solutions and DMPO was used as spin-trap agent. Fabrication of membrane-type detection platform. The membrane-type detection platform was fabricated with nylon membrane filters (Whatman, ϕ: 47 mm, pore size: 0.45 μm) by spotting TPCA2M solution (10 μM). After then, solvent was gently removed under normal pressure at room temperature.
RESULTS AND DISCUSSION As shown in Figure 1a and Scheme S1, three triarylamine derivatives (TPCAs: TPCA0M, TPCA2M, and TPCA6M) sharing the same conjugated backbone were designed and different numbers of methoxy units were introduced to
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Figure 2. (a) Energy levels of obtained TPCAs and representative organochlorides including common volatile solvents, and the change of emission behaviors of (b) TPCA0M, (c) TPCA2M and (d) TPCA6M in the representative organochlorides. al theory (t-DFT) revealed that the introduction of methoxy group altered the highest occupied molecular orbital (HOMO) level of TPCAs but caused little change in the LUMO in TPCA2M owing to the electron-donating tendency of the methoxy unit. However, the additional introduction of methoxy group to the meta positions of methoxy-adducted phenyl rings of TPCA2M did not cause a significant change in the molecular energy levels, including the optical band gap (TPCA6M in Figure 2a). To determine the theoretical feasibility of PET from the obtained TPCAs to chlorinated carbons, the energy levels of several chlorinated carbon substances including common volatile organic compounds were estimated with t-
the same conjugated moiety to compare the structural effect on both the electron transfer efficiency in the PET process and the detection sensitivity against analytes. The chemical structures of TPCAs were fully characterized by NMR and high-resolution mass spectroscopy, and the optical properties were characterized using UV-vis. absorption and photoluminescence spectroscopies. A comparison of the optical properties showed that both the absorption and emission spectra in tetrahydrofuran (THF) solution became red-shifted as the number of methoxy units introduced to the conjugated backbone increased (Figure 1b). In addition, the estimation of molecular energy levels by time-dependent density function-
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Figure 3. (a-b) EPR signal depending on the presence of chloroform (CF), and (c) suggested working mechanism of the devised organochloride detection system.
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tives and radicals generated through the pyrolysis of CF.27 LC-MS analysis of the CTC-detected TPCA2M solution
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Table 1. Analytical properties of obtained TPCAs
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Figure 4. CF concentration dependent emission change of TPCAs in THF solution. (a) TPCA0M, (b) TPCA2M, (c) TPCA6M and (d) detection sensitivity of TPCAs against CF. DFT. As illustrated in Figure 2a and Figure S5a, only CF and carbon tetrachloride (CTC) have an electronacceptable LUMO level via the PET from the TPCAs (comparisons of Mirex and diphosgene are discussed later). Each substance (TPCAs/organochlorides = 1) was simultaneously diluted in acetone to a concentration of 30 mM, and fluorescence change was monitored while irradiating with ultraviolet light (365 nm) to confirm the detection characteristics of TPCAs on the applied organochlorides. As depicted in Figure 2b–d and Figure S5b–d, the emission of TPCAs completely disappeared instantaneously only in the solutions containing CF or CTC, which can receive electrons from TPCAs through the PET process, and the solution visibly turned brown. To confirm that the PET process is involved in the detection of organochlorides containing tri-chlorinated carbons, the radical-generation tendency was observed by electron paramagnetic resonance (EPR) spectroscopy using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spintrapping agent and CF as an analyte. As shown in Figure 3, the EPR signal was detected only in the solution containing CF upon photo-excitation with 365 nm, which indicated that the electrons excited in TPCAs are successfully transferred to CF and the resulting radicals were consecutively stabilized by DMPO.25 Since CF has been reported to generate ·CHCl2 by accepting electrons,26 as schematized in Figure 3c, it could be speculated that the excited electrons of TPCAs were consumed by the PET process, generating ·CHCl2. Then, the resulting ·CHCl2 could interact directly with the surrounding TPCAs, which further reacted with Cl, resulting in the emission quenching of TPCAs. To verify the direct coupling between TPCAs and ·CHCl2, we analyzed the TPCA2M solution after detecting CF by high-resolution liquid chromatography-mass spectrometry (LC-MS). As shown in Figure S6a, the TPCA2M having a molecular weight of 546 g/mole almost disappeared while the CHCl2-adducted TPCA2M having a molecular weight of 629 g/mole was present in excess upon UV irradiation for 1 s. Here, it could be speculated that the CHCl2-adducted TPCA2M produces CCl3-substituted TPCA2M (m/z 557.5 in Figure S6a) by a chain reaction with additional chloride radicals similar to the reaction between aromatic amine deriva-
a
all spectra were recorded in THF solution
also revealed that TPCA2M almost disappeared and CCl3substituted TPCA2M-Cl was produced, and EPR measurement supported the generation of radicals, similar to the CF detection (Figure S6b), Therefore, the interpretation of the working mechanism of the devised detection system for organochlorides containing tri-chlorinated carbons seems plausible. In addition, from the suggested detection mechanism, it can be supported that the rapid detection of CF or CTC relates to instantaneous changes, such as electronic transition, electron transfer, and radical coupling; thus, the devised system can be suitable for the screening of toxic substances, which need to be detected in a very short time. In practice, as shown in the CF detection video captured in 1 s (Figure S7), the blue emission disappears as soon as the TPCA2M-coated substrate is exposed to the CF vapor. To quantitatively compare the detection sensitivity according to the chemical structure of the obtained TPCAs, the quench rate constant (kSV) was calculated from the Stern-Volmer equation by measuring the decrease in fluorescence intensity with increasing concentration of the analyte.28 As shown in Figure 4(a-c), the emission intensities of all TPCAs gradually decreased as the concentration of CF increased, and TPCA2M and TPCA6M containing methoxy groups in the conjugated skeleton responded more sensitively to the change in CF concentration than TPCA0M having no methoxy group did. TPCA2M containing two methoxy groups exhibited the highest emission quench rate and was 1.37 and 1.96 times more sensitive than TPCA6M having six methoxy groups and TPCA0M having no methoxy group, respectively (Figure 4d and Table 1). Since the CF detection mechanism of TPCAs involves electronic transition (absorption), electron transfer, radical generation, and radical coupling, the overall quench rate could be determined decisively by both the radical generation tendency and reaction rate of the resulting radicals. As shown in Figure 3b, TPCA6M exhibited a relatively low EPR intensity under the same condition compared with other TPCAs. Because the radical-generation tendency reflects the results of electronic transition and electron transfer through the PET process, the low EPR intensity of TPCA6M implies a low electron transfer efficiency through PET, resulting in poor detec-
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ACS Sensors tion sensitivity. In addition, since the coupling reaction of the resulting radicals with TPCAs follows an electrophilic aromatic substitution route, TPCA2M or TPCA6M containing methoxy groups could be more activated due to the electron donating characteristics of the methoxy groups. Thus, it could be concluded that TPCA2M with a high EPR intensity and a favorable structure for radical coupling have the highest detection sensitivity among the obtained TPCAs.
LUMO level to accept electrons from the TPCAs through PET (Figure 2a). As illustrated in Figure S8a, Mirex consists of numerous chloride atoms, but the change in emission intensity was negligible even under a high concentration (Figure S8b). This could be related to the poor radical-generation tendency of the mono- or di-chlorinated carbons in Mirex, rather than the tri-chlorinated carbon. The absence of a meaningful EPR signal in Mirex also supports its poor radical-generation tendency through PET from TPCAs (Figure S8c). The detection experiment using Mirex indicates that the devised sensing platform could be selectively applied to organochlorides containing tri-chlorinated carbons that exhibit deep LUMO levels through atomic orbital overlap of highly electronegative chloride atoms as well as satisfy the capability of forming radicals easily by the PET process. As aforementioned, diphosgene is one of representative toxic chemicals containing tri-chlorinated carbon. Thus, the emission intensity of TPCA2M was monitored depending on the concentration of diphosgene to demonstrate whether the devised detection system could be effectively applied to the detection of diphosgene. Since TPCA2M exhibited the highest detection sensitivity among the designed TPCAs, TPCA2M was representatively employed for the detection of diphosgene, and the change in emission intensity was analyzed quantitatively both in THF solution (150 ppm of TPCA2M) and with a TPCA2M-coated membrane. As shown in Figure 5a, the emission intensity of TPCA2M solution gradually diminished as the concentration of diphosgene increased, and a noticeable change in the emission intensity was observed at a low concentration of diphosgene, even blow 5 ppm. In addition, the detection sensitivity of TPCA2M solution was quantitatively compared with that of a TPCA2Mcoated membrane that was instantaneously exposed to diphosgene solution (Figure 4c). As illustrated in Figure 4d, the membrane-type detection platform has about 64% higher sensitivity than that achieved by detection in solution state, even though the exposure time of membranetype detection platform (less than 1 s) was considerably short compared to that of direct detection in solution (30 s). The reason for the relatively higher detection sensitivity of the membrane-type detection platform could be
Figure 5. Change in emission intensity of TPCA2M (a) in solution and (b) in TPCA2M-coated membrane depending on diphosgene concentration, (c) sensitivity comparison of membrane-type sensing platform with direct detection in solution, and (d) EPR signal of solution containing TPCA2M and diphosgene As in the suggested working mechanism, the detection of organochlorides containing tri-chlorinated carbons, such as CF and CTC, is determined by both the PET process and the reaction of the resulting radicals. To confirm whether the devised detection system can be selectively applied to organochlorides containing tri-chlorinated carbons, Mirex was adopted owing to its sufficiently deep
Figure 6. Exposure-time-dependent emission change of TPCA2M-coated membrane in diphosgene solution: (a) 1 ppm, (b) 5 ppm, (c) 10 ppm, and (d) 20 ppm.
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ascribed to the screening effect because the detection reaction proceeded on the surface predominantly, resulting in emission-quenching substances at the surface of the TPCA2M-coated membrane. In addition, the emission change using the membrane-type detection platform was monitored as the exposure time against the concentration of diphosgene. As illustrated in Figure 6, the emission intensity tended to decrease with increasing exposure time at all diphosgene concentrations, and significant fluorescence reduction was observed within 5 s even at a very low concentration (1 ppm). Finally, EPR measurement was conducted using a solution containing TPCA2M and diphosgene to confirm that the detection mechanism of diphosgene also followed the PET process and that the emission of TPCA2M disappeared due to the resulting radicals, as described for CF detection. As depicted in Figure 2a, diphosgene had a sufficiently deep LUMO level to which the excited electrons of TPCA2M could migrate, and a noticeable EPR signal was generated in the solution containing TPCA2M and diphosgene when DMPO was used as a spin-trap agent (Figure 5d). This implies that the diphosgene detection mechanism is similar to that of CF.
CONCLUSIONS In summary, a novel detection system for hazardous organochlorides containing tri-chlorinated carbon was designed, and the detection mechanism of the devised sensor system was systematically identified by EPR measurement and the analysis of solution using CF as a model compound. Since the developed detection system simultaneously utilizes the radical-generation tendency of the analyte and the deep LUMO level for PET, it provided high selectivity when most of the common organic compounds including other organochlorides containing mono- or di-chlorinated carbons were subjected to detection experiments. In addition, the detection system exhibited immediate sensing performance against the trichlorinated carbons because only the electron transfer process and radical reaction were involved in the overall detection mechanism. Finally, when diphosgene detection was performed using TPCA2M showing the highest detection sensitivity among the designed electron donors, a noticeable change in fluorescence intensities were observed within 5 s even for a diphosgene concentration of less than 1 ppm. The results of this study systematically evidence that the electron transfer originating from the PET process can be a useful pathway for instantaneous detection of toxic organochlorides containing trichlorinated carbons, which can serve as electron acceptors. In addition, since the designed detection system is based on both the low LUMO level and the radical generation capability of tri-chlorinated carbon, it is expected that the devised detection platform can be widely applied to the detection of overall organochloride containing trichlorinated carbons.
ASSOCIATED CONTENT
Supporting Information Available: The following files are available free of charge. Supporting Information_se-2018-00602w.doc Detailed synthetic procedure; NMR and mass spectra; Detection behavior on common volatile solvents; LC-Mass analysis; Time-dependent detection behavior; Mirex detection characteristics.
AUTHOR INFORMATION Corresponding Author E-mail:
[email protected] ORCID Bong-Gi Kim: 0000-0003-1386-3705 Notes The authors declare no competing financial interest
ACKNOWLEDGMENT This work is supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE, 20174010201490). This research was also supported by the Industrial Fundamental Technology Development Program (10063045) funded by the Ministry of Trade, Industry & Energy (MOTIE) of Korea.
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