Effective Strategy for Colorimetric and Fluorescence Sensing of

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Effective Strategy for Colorimetric and Fluorescence Sensing of Phosgene Based on Small Organic Dyes and Nanofiber Platforms Ying Hu, Liyan Chen, Hyeseung Jung, Yiying Zeng, Songyi Lee, Kunemadihalli Mathada Kotraiah Swamy, Xin Zhou, Myung Hwa Kim, and Juyoung Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07138 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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ACS Applied Materials & Interfaces

Effective Strategy for Colorimetric and Fluorescence Sensing of Phosgene Based on Small Organic Dyes and Nanofiber Platforms Ying Hu,a‡ Liyan Chen,a‡ Hyeseung Jung,a‡ Yiying Zeng,b Songyi Lee,a Kunemadihalli Mathada Kotraiah Swamy,a,c Xin Zhou,b* Myung Hwa Kim,a* and Juyoung Yoona* a

Department of Chemistry and Nano Science, Ewha Womans University, Seoul, 120-750, Korea

b

Key Laboratory of Natural Resources of Changbai Mountain & Functional Molecules, Ministry

of Education, Yanbian University, Yanji 133-002, China c

Department of Pharmaceutical Chemistry, V. L. College of Pharmacy, Raichur-584103, India

KEYWORDS: phosgene sensor, toxic gas sensor, colorimetric sensor, fluorescent chemosensor, nanofiber.

ABSTRACT: Three o-phenylendiamine (OPD) derivatives, containing a 4-chloro-7nitrobenzo[c][1,2,5]oxadiazole (NBD-OPD), rhodamine (RB-OPD) and 1,8-naphthalimide (NAP-OPD) moiety, were prepared and tested as phosgene chemosensors. Unlike previously described methods to sense this toxic agent, which rely on chemical processes that transform alcohols and amines to respective phosphate esters and phosphoramides, the new sensors operate through a benzimidazolone forming reaction between their OPD groups and phosgene. These

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processes promote either naked eye visible color changes and/or a fluorescence intensity enhancements in conjunction with detection limits that range from 0.7 to 2.8 ppb. NBD-OPD and RB-OPD-embedded polymer fibers, prepared by using the electrospinning technique, display distinct color and fluorescence changes upon exposure to phosgene even in the solid state.

1. Introduction Phosgene is highly toxic gas that causes severe damage to the respiratory track and lungs of humans, including noncardiogenic pulmonary edema and pulmonary emphysema, that often leads to death.1-3 In addition, unlike nerve gas agents, such as Sarin, Soman and Tabun, whose production and usage are strictly prohibited by law,4-6 phosgene is a widely used starting material in various chemical processes including those employed in the production of pesticides, pharmaceuticals and isocyanate-based polymers.7, 8 As a result of these properties and potential security threats, a critical need exists for effective methods to detect phosgene. Even though fluorescence or colorimetric chemosensors for nerve gas agents have been actively studied, 9-25 a paucity of publications exist describing techniques to sense phosgene. 26-28 Most common strategies used to design phosgene or nerve gas selective sensors rely on nucleophilic substitution reactions of substances containing hydroxyl and amine groups, that involve respective phosphate and phosphoramide formation. In the sensors, these processes are designed to block photoinduced electron transfer (PET) promoted fluorescence quenching resulting in an enhancement in the intensity of emission. However, this strategy cannot be employed to distinguish between phosgene and nerve gas agents.


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In a recent effort, we devised a unique strategy for sensing of phosgene that takes advantage of the rapid, efficient and selective reaction of the o-phenylenediamine (OPD) group with hths toxic agent. 29 In the current investigation, aimed at further exploring the utilization of this strategy, we prepared three OPD based phosgene chemosensors that contain either 4-chloro-7nitrobenzo[c][1,2,5]oxadiazole (NBD), rhodamine (RB) and naphthalimide (NAP) moieties as fluorophores. We demonstrated that OPD unit in these substances serves as a universal center for reactions with phosgene that promote distinct absorption and fluorescence changes. Specifically, NBD-OPD, NAP-OPD and RB-OPB undergo color changes, and produce blue, green and red emission in the presence of phosgene. Furthermore, electrospinning of these chemosensors can be used to produce diverse forms of fibrous assemblies in a simple, versatile and low cost manner. Included in the types of polymeric assemblies that can be fabricated by utilizing this method are single fibers, nonwoven mats and uniaxially aligned arrays. The exceptionally high porosities and enhanced surface areas of the electrospun fibers are highly attractive properties that are used advantageously to create ultrasensitive sensors for phosgene that are superior to their conventional counterparts that utilize thin film and aqueous solution approaches. We have also shown that NBD-OPD and RB-OPD embedded electrospun fibers experience clear color and fluorescence changes upon exposure to phosgene in the solid state. The results of this effort are described below. 2. Results and Discussion 2.1. Syntheses of NBD-OPD, RB-OPD and NAP-OPD. For preparation of NBD-OPD, NBD-Cl was reacted with o-phenylenediamine (OPD) in methanol at room temperature for 24 h. This process produced NBD-OPD as reddish brown


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solid in 71% yield. For the synthesis of RB-OPD, rhodamine B was first converted to the corresponding acid chloride using phosphorus oxychloride. The acid chloride was immediately reacted with o-diaminobenzene to give, following chromatographic purification, RB-OPD as a white solid in 83% yield.30 Finally, treatment of 4-bromo-1,8-naphthalic anhydride with 4-(2aminoethyl)morpholine generated the corresponding 4-bromo-1,8-naphthalimide, which was then reacted with o-phenylenediamine in dry DMF in the presence of K2CO3 and Pd(OAc)2 to give, after chromatographic purification, NAP-OPD in a 43 % yield.31 1

H and 13C NMR as well as mass spectrometric data for NBD-OPD, RB-OPD and NAP-OPD

are given in Supporting Information.

Figure 1. Structures of NBD-OPD, RB-OPD and NAP-OPD. 2.2 Phospgene Promoted Color and fluorescence changes The colorimertic responses, and UV absorption and fluorescence changes of NBD-OPD, RBOPD and NAP-OPD promoted by phosgene were investigated using triphosgene, a non-volatile precursor of this gaseous substance. The images in Figure 2 display the color changes that take place upon addition of varying molar equivalents of triphosgene to chloroform-methanol


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solutions of NBD-OPD and RB-OPD. In response to phosgene, NBD-OPD, undergoes a clear color transition from orange to pale yellow that can be observed by using the naked eye. In addition, a distinct color change from colorless to pink is observed when phosgene is added to RB-OPD, which is attributed to a process in which spirolactam ring opening occurs in conjunction with benzimidazolone formation (see below). It should be noted that ring opening of rhodamine derivatives bearing spirolactam rings has been utilized earlier as the key element in colorimetric and fluorescence sensors or probes for various analysts.32-47

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Figure 2. Colorimetric responses of NBD-OPD (acetonitrile) (top) and RB-OPD (chloroform /methanol =95/5) (bottom) towards different equivalents of phosgene. Upon addition of various amounts of triphosgene, the intensity of the peak at 475 nm in the absorption spectrum of NBD-OPD in CH3CN decreases concomitant with a large increase in a new band at 270 nm (Figure 3). In contrast, when triphosgene is added to a chloroform solution of RB-OPD, a new UV absorption band appears at 560 nm and large intensity enhancement takes place in the fluorescence band at 575 nm (excitation at 530 nm) (Figure 4). These distinct


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changes are attributed to the unique spirolactam ring opening process. Finally, the UV absorption and fluorescence changes that NAP-OPD undergoes in response to triphosgene are seen by viewing the spectra in Figure 5. Specifically, the absorption peak at 420 nm decreases and an increase in a peak at 480 nm takes place when this agent is added to a chloroform-p-xylene solution of NAP-OPD. In addition, the intensity of the fluorescence peak of NAP-OPD at 480 nm is enhanced when triphosgene is added. Additional studies showed that reaction between NAP-OPD and phosgene is complete within 3 min and the associated fluorescence changes can be detected within 30 s (Figure S4). Also reactions of NBD-OPD and RB-OPD with phosgene take place relatively more rapidly (complete at 2 and 1 min, respectively, Figures S5-S6). Furthermore, the detection limits of NBD-OPD, RB-OPD and NAP-OPD for phosgene using emission intensity changes were determined to be 0.7 ppb, 2.8 ppb, 2.8 ppb, respectively (Figures S1-S3). 0.3

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Figure 3. (A) Absorption spectra of NBD-OPD in CH3CN (10 µM) upon gradual addition of a solution of triphosgene (0 - 2 equiv.) in CH3CN. (B) Fluorescence spectra of NBD-OPD in CH3CN (10 µM) upon gradual addition of a solution of triphosgene (0 - 2 equiv.) in CH3CN. (λex = 270 nm, Slits: 15 ×15 nm).


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Figure 4. (A) Absorption spectra of RB-OPD in chloroform (10 µM) upon gradual addition of a solution of triphosgene (0 - 10 equiv.) in chloroform with 5% methanol as co-solvent. (B) Fluorescence spectra of RB-OPD in chloroform (10 µM) upon gradual addition of a solution of triphosgene (0 - 1.2 equiv.) in chloroform with 5% methanol as co-solvent. (λex = 530 nm, Slits: 1.5×3 nm). 0.6

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Figure 5. (A) Absorption spectra of NAP-OPD (10 µM) upon gradual addition of a solution of triphosgene (0 - 1.0 equiv.) in p-xylene with 1% chloroform as co-solvent. (B) Fluorescence spectra of NAP-OPD in chloroform (10 µM) upon gradual addition of a solution of triphosgene (0 - 1.0 equiv.) in p-xylene. (λex = 340 nm, Slits: 5 × 3 nm).


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The responses of the new sensors to nerve agent mimic, DCP, were examined next. The results demonstrate that even 50 equivalents of DCP causes only a slight, naked eye observable color change of NBD-OPD and a relatively small blue shift in the absorption maximum of this substance (Figure S7). RB-OPD also undergoes a colorless to pink color change only upon addition of 50 equivalents of DCP (Figure S10). These results prove that benzimidazolone ring forming reactions that are key to the operation of these sensors is selective for phosgene. 2.3. Reaction mechanisms As shown in Figure 6, NBD-OPD, RB-OPD and NAP-OPD react with phosgene to form the corresponding benzimidazolone derivatives NBD-BI, RB-BI and NAP-BI. All three products were isolated from the respective reaction mixtures and characterized by using NMR and mass spectrometry (SI). The partial 1H NMR spectra of NBD-OPD (top) and NBD-BI are given in Figure 7. By viewing the spectra, it can be seen that a new resonance at 11.6 ppm arises following reaction of NBD-OPD with triphosgene, which is assigned to the NH proton in NBDBI. Also, the aromatic H5 proton in the benzimidazolone ring in NBD-BI is dramatically downfield shifted to 8.02 ppm in contrast to that in NBD-OPD (Figure 7). In a similar fashion, the new peaks at 11.8 and 9.16 ppm in the respective spectra of RB-BI and NAP-BI are assigned to NH protons (Figures S17 and S18).


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Figure 6. Structures of products of reactions of NBD-OPD, RB-OPD and NAP-OPD with phosgene.


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Figure 7. Partial 1H NMR spectra of NBD-OPD (top) and NBD-BI (DMSO-d6, 300 MHz). 2.4. NBD-OPD, RB-OPD embedded polymer fibers and NAP-OPD containing filter paper Electrospinning is a powerful approach to fabrication of a large number of nano and micro scale architectures. In the current study, NBD-OPD and RB-OPD embedded fibers were prepared by using the electrospinning procedure in which individual solutions containing the NBD derivatives and the matrix polymer poly(ethylene oxide) (PEO) were jet ejected from a syringe, containing a needle to which is applied a high voltage (5.0 kV). During fiber formation, the solvent evaporates and self-assembly takes place as a consequence of the attractive forces between NBD derivatives and the matrix polymer of PEO. These processes produce uniformly distributed fibers containing a minimum number of bead structures. The polymer fibers containing NBD-OPD and RB-OPD display the same types of highly sensitive colorimetric as well as fluorescence responses upon exposure to triphosgene as do solutions of the corresponding sensors (Figure 8 and Figure S19). Specifically, upon exposure to triphosgene, NBD-OPD-embedded nanofibers undergo a color change from ochre to ivory, and a distinct change from non-fluorescent to purple fluorescent. Similarly, triphosgene induces a drastic color change of the RB-OPD-embedded nanofiber from white to dark pink, accompanied by change from non- to pink fluorescent. Finally, inspection of the fluorescence images of NAP-OPD containing filter papers under 365 nm light, before and after exposure to phosgene gas, show that this simple technique can be employed to rapidly detect this toxic gas (Figure S20).


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Figure 8. (A) Visual detection of phosgene (0.8 mg/L phosgene gas) with NBD-OPD absorbed PEO nanofiber. (B) Fluorescent responses of NBD-OPD based PEO nanofiber upon exposure to phosgene (0.8 mg/L phosgene gas). In Figures 9 and S21 are shown scanning electron microscope (SEM) images of NBD-OPD, RB-OPD embedded nanofibers before and after exposure to triphosgene. Before exposure, the embedded fibers have uniform diameters of ca. 1.0 µm with lengths in the millimeter range. The surfaces structures of the electrospun fibers become rough and their diameters become irregular upon exposure to triphosgene (Figures 9(B) and S21 (B). It is expected that reaction of NBDOPD and RB-OPD in PEO polymer matrixes with phosgene form the corresponding benzimidazolone derivatives on the surface of nanofibers, and that this change results in partial deformation of the original structures.

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Figure 9. Scanning electron microscope images of NBD-OPD PEO nanofibers before (A) and after (B) exposure to triphosgene. Lastly, confocal microscope images of the RB-OPD embedded nanofiber before and after triphosgene exposure are displayed in Figure 10. A clear red fluorescent fiber is generated as a consequence of the benzimidazolone forming reaction promoted by addition of triphosgene. The results of this effort demonstrate that the small dimensions and highly sensitive colorimetric and fluorescence responses make NBD-OPD, RB-OPD-embedded nanofibers suitable for application in phosgene sensor devices.

Figure 10. Confocal microscope images of RB-OPD based PEO nanofiber upon exposure to triphosgene. 3. Conclusion In the study described above, we synthesized three o-phenylenediamine (OPD) derivatives that contain 4-chloro-7-nitrobenzo[c][1,2,5]oxadiazole (NBD), rhodamine and naphthalimide


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moieties and explored their use as phosgene sensors. The unique benzimidazolone forming reactions of the OPD group in these substances with phosgene promote distinct color as well as fluorescence changes. In the presence of phosgene, NBD-OPD undergoes a distinct color change from dark orange to pale yellow that can be observed by using the naked eye and a fluorescence enhancement. On the other hand, the spirolactam ring opening process accompanying benzimidazolone formation induces a clear change from colorless to pink as well as turn-on fluorescence when RB-OPD is exposed to this toxic agent. NAP-OPD also displays turn-on fluorescence upon the addition of phosgene with the detection limits of 2.8 ppb. Furthermore, NBD-OPD and RB-OPD-embedded polymeric fibers, prepared by using the electrospinning technique, show distinct color and fluorescence changes upon exposure to phosgene even in the solid state. We believe that strategies using the chemistry of the OPD group and its effect on chromophores and/or fluorophores will be effectively used to design new phosgene sensors. 4. Experimental Section Methods: Unless otherwise noted, materials were obtained from commercial suppliers and used without further purification. Flash chromatography was carried out on silica gel (230-400 mesh). 1

H NMR and 13C NMR spectra were recorded using a 300 MHz NMR or 500 MHz NMR

spectrometer. Chemical shifts were expressed in ppm and coupling constants (J) in Hz. Synthesis of NBD-OPD: To a stirred solution of o-phenylenediamine (0.32 g, 3.0 mmol) in methanol (10 mL) at room temperature was added NBD-Cl (0.20 g, 1.0 mmol) in methanol (10 mL) . The resulting mixture was stirred at room temperature for 24 h and filtered to give a precipitate that was washed several times with methanol. This procedure gave NBD-OPD as a reddish brown solid. Yield: 71%, mp: 220 °C. 1H NMR (DMSO-d6, 300 MHz): δ 10.69 (brs,


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1H), 8.54 (d, J = 8.7 Hz, 1H), 7.09–7.15 (m, 2H), 6.84 (d, J = 7.2 Hz, 1H), 6.64 (t, J = 7.8 Hz, 1H), 5.93 (d, J = 8.7 Hz, 1H), 5.27 (brs, 2H). 13C NMR (DMSO-d6, 75 MHz): δ 145.49, 145.04, 144.96, 144.92, 138.58, 129.44, 128.14, 121.95, 116.77, 116.49, 113.59, 102.34. HRMS (EI) m/z = 272.0792 [M+H]+, calcd for C12H9N5O3 = 272.0705. Synthesis of RB-OPD: To a solution of rhodamine B base (2.3 mmol) in dry 1,2-dichloroethane (8.0 mL) at room temperature was added phosphorus oxychloride (6.9 mmol) over a period of 5 min. After stirring at reflux for 4 h, the mixture was cooled and concentrated under vacuum to give the crude rhodamine B acid chloride. A solution of the acid chloride in dry acetonitrile (10 mL), and added dropwise to a solution of o-diaminobenzene (14 mmol) in dry acetonitrile (6.0 mL) containing triethylamine (8.0 mL). After stirring for 4 h at room temperature, the mixture was concentrated under vacuum and the residue was subjected to column chromatography to give RB-OPD as a white solid in 83% yield. 1H NMR (CDCl3, 300MHz) δ (ppm): 8.03-8.05 (m, 1H); 7.53-7.60 (m, 2H); 7.25-7.27 (m, 1H); 6.94-6.99 (m, 1H); 6.68 (d, J = 8.7 Hz, 2H); 6.58 (dd, J = 7.8 Hz, J = 1,2 Hz, 1H); 6.40-6.46 (m, 1H); 6.31 (d, J = 8.7 Hz, 2H); 6.28 (b, 2H); 6.10 (dd, J = 8.1 Hz, J = 1.5 Hz, 1H); 3.44 (s, 2H); 3.35 (q, J = 6.9 Hz, 8H); 1.16 (t, J = 7.2Hz, 12H). 13

C NMR (CDCl3, 75MHz) δ (ppm): 166.63, 161.22, 154.20, 152.66, 149.15, 144.75, 132.81,

129.03, 128.97, 128.85, 128.55, 124.51, 123.66, 122.46, 118.43, 117.23, 108.21, 98.28, 68.27, 44.60, 31.15, 12.76. HRMS (ESI) m/z = 533.2565 [M+H]+, calcd for C34H36N4O2 = 533.2838. Synthesis of NAP-OPD: To a solution of 4-bromo-1,8-naphthalic anhydride (5.0 mmol) in dry EtOH (20.0 ml) at room temperature was added 4-(2-aminoethyl)morpholine (5.5 mmol). After stirring at reflux for 4 h, the mixture was cooled and filtered to give a precipitate, containing 4bromine-1,8-naphthalimide, To a solution of this naphthalimide (1.0 mmol) and ophenylenediamine (5.0 mmol) in dry DMF (20 mL) under a nitrogen atmosphere were added


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K2CO3 (4 mmol) and Pd(OAc)2 (2 mmol) . Then the mixture was stirred at reflux overnight and concentrated under a vacuum. The residue was diluted with water and extracted with dichloromethane, the extracts were dried over anhydrous MgSO4 and concentrated under a vacuum to give a residue that was subjected to column chromatography to produce 0.18 g (43%) NAP-OPD as a yellow solid (59 %). 1H NMR (CDCl3, 300MHz)δ (ppm) = 8.59 (m, 1H), 8.35 (d, 1H), 8.28 (d, 1H), 7.69 (m, 1H), 7.17 (m, 2H), 6.92 – 6.77 (m, 2H), 6.67 (d, 1H), 6.53 (s, 1H), 4.37 – 4.23 (m, 2H), 3.79 (s, 2H), 3.71 – 3.58 (m, 4H), 2.72 – 2.61 (m, 2H), 2.57 (s, 4H). 13C NMR (75 MHz, CDCl3) δ = 164.51, 163.89, 147.49, 142.82, 133.96, 131.31, 128.32, 127.44, 126.22, 125.32, 124.50, 123.21, 120.74, 119.50, 116.75, 112.30, 107.75, 67.02, 56.23, 53.80, 36.98. HRMS (ESI) m/z = 417.1718 [M+H]+, calcd for C24H25N4O3 = 417.1927. Preparation of NBD-OPD, RB-OPD and NAP-OPD-embedded electrospun fibers: To prepare a precusor solution for synthesizing the electrospun fibers, 1.0 mg of NBD-OPD, RB-OPD and NAP-OPD were independently dissolved in 2.5 mL of acetonitrile. In each case, when the solute was completely dissolved, 100.0 mg of poly(ethylene oxide) (PEO, Mw = 600,000) as a matrix polymer was added and the resulting mixture was stirred for 1 h and loaded into stainless steel syringe connected to a needle of gauge 23. The distance between the end of the needle and grounded plate was adjusted to be 15 cm before applying 5.0 kV. The precursor solution was ejected to form fine fibers on the surface of a grounded aluminum plate located 15 cm below the tip of the needle. The flow rate was carefully kept at 0.5 mL/h. After collection, the polymer fibers were dried in a 40 °C vaccum oven for 1 d to prevent agglomeration. The structures and morphologies of the fibers were assessed using electron microscopy (FE-SEM, JEOL JSM-6700F).


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, characterization data of all key compounds, detection limit spectra for triphosgene; Kinetic profiles of the reaction between triphosgene and sensors, SEM micrographs of nanofiber; DCP-triggered UV-vis and fluorescence emission spectral changes (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was financially supported by a grant from the National Creative Research Initiative programs of the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP; No. 2012R1A3A2048814). M.H.K thanks to the National Research


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Foundation of Korea (NRF) funded by the Korean government (MSIP; No. 2014R1A1A2059791). X. Zhou thanks to the Youth Science Foundation of Jilin Province (20160520003JH). REFERENCES (1)

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Table of Contents Graphic


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Effective Strategy for Colorimetric and Fluorescence Sensing of

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