A Visible Light Responsive On–Off Polymeric Photoswitch for the

Article pubs.acs.org/Macromolecules

A Visible Light Responsive On−Off Polymeric Photoswitch for the Colorimetric Detection of Nerve Agent Mimics in Solution and in the Vapor Phase A. Balamurugan and Hyung-il Lee* Department of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea S Supporting Information *

ABSTRACT: A polymeric probe derived from a visible light responsive donor−acceptor Stenhouse adduct (DASA) was designed for the rapid and selective colorimetric detection of nerve agent mimics. Glycidyl methacrylate (GMA) and dimethylacrylamide (DMA) were copolymerized by reversible addition−fragmentation chain transfer (RAFT) polymerization to yield poly(glycidyl methacrylate-co-dimethylacrylamide) [p(GMA-co-DMA)], herein P1. The epoxide unit of P1 was transformed to 1-((2-(2-hydroxyethoxy)ethyl)amino)-3-methoxypropan-2-ol by the reaction with 2-(2-aminoethoxy)ethanol, leading to P2. The subsequent reaction between the secondary amine of P2 with 5-(furan-2-ylmethylene)-1,3dimethylpyrimidine-2,4,6(1H,3H,5H)-trione yielded P3 with DASA derivatives. P3 exhibited the rapid and selective detection of diethyl cyanophosphate (DCNP), a mimic of the nerve agent, in both solution and the vapor phase. Upon the exposure to DCNP, the color of the P3 solution/film turned from purple to colorless due to the formation of morpholino cations, induced by DCNP-promoted intramolecular N-alkylation. The availability of the electron-rich N-alkyl unit in the triene unit of the DASA chromophore allowed P3 to show excellent sensing behavior toward DCNP. DASA-incorporated P3 has also shown excellent photochromic performance upon irradiation with visible light. Zwitterionic cyclopentenone units formed by irradiation with visible light prevented the DCNP-promoted intramolecular N-alkylation, resulting in no colorimetric responses toward DCNP. Thus, these photocontrollable properties of P3 can offer new insights into the design of new photoresponsive on−off polymeric switches, for colorimetric on−off detection of nerve agent mimics.

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sors,11 and gas chromatography−mass spectrometry.12,13 However, these techniques generally suffered from lots of disadvantages such as poor portability, complicated operation, low sensitivity, and low selectivity. In recent years, molecular probes based on colorimetric or fluorometric detection have been developed for the efficient sensing of nerve agents due to their excellent qualitative or quantitative operational simplicity, portability, and facile reactivity.14−19 Most of these methods are explored to detect nerves agents in solution. However, only a few molecular probes have shown colorimetric responses in thin film form, which can yield real-time detection of trace amounts of nerve agents in the vapor state.20,21 Among many approaches toward

INTRODUCTION Since the Second World War, there has been growing interest in the development of new and advanced methods to detect the most important and dangerous classes of chemical warfare agents (CWAs) because of the possible use of these chemicals in terrorism.1−4 Among the CWAs, volatile organophosphorous nerve agents are regarded as extremely toxic compounds. The common nerve agents such as Sarin, Soman, and Tabun are so neurotoxic that they can easily enter the body through inhalation or the skin.5 For example, these organophosphate chemicals can bind easily with the hydroxyl group of the serine unit of the acetylcholinesterase enzyme (AChE) to inhibit acetylcholinesterase activity, leading to acetylcholine accumulation in the synaptic junctions and finally causing death.1,6 There have been several methods/techniques developed for monitoring these species, including electrochemical sensors,7,8 optical-fiber arrays,9 microcantilevers,10 enzyme-based biosen© XXXX American Chemical Society

Received: February 9, 2016 Revised: March 16, 2016

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DOI: 10.1021/acs.macromol.6b00309 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Schematic representation for a visible light responsive polymeric switch for the on−off colorimetric detection of a nerve agent mimics.

the fabrication of thin film sensors, the covalent attachment or direct incorporation of chromophores into polymers was found to be attractive.21−25 Moreover, for analyte detection the direct incorporation into the polymer backbone of external stimuliresponsive units, as well as the chromophore, offers tunable detection ability upon changes in environmental conditions.22,26−29 Organic photoswitches based on donor−acceptor Stenhouse adducts (DASAs) as photoresponsive units have been recently reported by Read de Alaniz and co-workers.30 These derivatives were found to show photoswitching ability to change from a conjugated, colored to a ring-closed, colorless form upon irradiation with visible light. The incorporation of a nucleophilic hydroxyl group into the N-alkyl position of the chromophore is expected to provide a suitable reactive site for electrophilic phosphorus atoms and the subsequent nerve agent-triggered cyclization to form quaternary ammonium salts, leading to the photoswitchable sensing of a nerve agent.31−35 Herein, we report that a new photoresponsive polymeric probe P3 was developed for the selective and sensitive detection of a nerve agent mimics in both solution and the vapor phase (Figure 1). The on−off sensing of a nerve agent mimics was achieved by photoswitching behavior of the DASA chromophore in the polymeric chain.

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remove inhibitors. Furfural, 1,3-dimethylbarbituric acid, 2-(2-aminoethoxy)ethanol, 2-dodecylsulfanylthiocarbonylsulfanyl-2-methylpropionic acid (CTA), triethyl phosphite (TEP), tributyl phosphate (TBP), diethyl chlorophosphate (DCP), and diethyl cyanophosphate (DCNP) were purchased from Aldrich with the highest purity available and used as received. 2,2′-Azobis(isobutyronitrile) (AIBN, Aldrich, 98%) was recrystallized from ethanol. 5-(Furan-2-ylmethylene)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione was synthesized as previously reported.30 Instrumentation. 1H nuclear magnetic resonance in CDCl3 (NMR, Bruker Avance 400 MHz NMR spectrometer) spectra were collected. The apparent molecular weight and molecular weight distributions were measured by gel permeation chromatography (GPC, Agilent Technologies 1200 series) using a polystyrene standard with DMF as the eluent at 30 °C and a flow rate of 1.00 mL/min. The UV−vis absorption spectra, in dioxane, were recorded using a Varian Cary 100 spectrophotometer. A crystal clear incandescent lamp (40 light bulb, 40 W) was used as the light source for the visible light irradiation. Synthesis of Polymers. Synthesis of [p(GMA-co-DMA)] (P1). Glycidyl methacrylate (GMA) (0.15 g, 1.06 mmol), dimethylacrylamide (DMA) (2.00 g, 20.17 mmol), CTA (0.038 g, 0.11 mmol), and AIBN (0.21 mg, 0.001 mmol) were added to a Schlenk flask with 8 mL of dry DMF. The reaction mixture was purged with argon for 0.5 h and then heated for 12 h at 80 °C. The reaction mixture was precipitated in diethyl ether and filtered. The obtained polymer was further purified by reprecipitation in diethyl ether, filtered, and dried in a vacuum oven for 24 h to yield P1 (1.42 g). 1H NMR (CDCl3, 300 MHz, δ in ppm): 4.37−3.87 (2H, dd, COOCH2), 3.13−0.89 (17H, m, −O−CH2, N−(CH3)2 and aliphatic H). GPC: Mn = 16 800, Mw = 19 900, and PDI = 1.1.

EXPERIMENTAL SECTION

Materials. Glycidyl methacrylate (GMA, 99.0%) and dimethylacrylamide (DMA, 99.0%) were purchased from Aldrich and purified by passing through a column filled with basic alumina, in order to B

DOI: 10.1021/acs.macromol.6b00309 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Scheme for the Preparation of the DASA-Derived Polymeric Probe P3

Synthesis of P2 by the Postmodification of P1. P1 (0.20 g, 0.097 mmol, assuming 6% incorporation of GMA along the backbone) and 2-(2-aminoethoxy)ethanol (0.44 g, 4.22 mmol) were heated in dry acetonitrile (12 mL) at 90 °C for 48 h. The polymer solution was concentrated under vacuum to obtain a viscous solution. The resulting polymer solution was precipitated into diethyl ether; the solid product was washed with cold diethyl ether and dried in vacuo to afford P2 (0.13 g). 1H NMR (300 MHz, δ in ppm) in CDCl3: 3.99 (2H, s, N− CH2), 3.74−3.52 (8H, m, O−CH2−CH2), 3.12−0.94 (17H, m, −O− CH2, N−(CH3)2 and aliphatic H). GPC: Mn = 17 300, Mw = 22 600, and PDI = 1.3. Synthesis of P3 by the Postmodification of P2. To a solution of P2 (0.10 g, 0.047 mmol) in dry THF (10 mL), 5-(furan-2ylmethylene)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione (0.28 g, 1.21 mmol) was added. The reaction mixture was stirred at 23 °C for 5 h, followed by cooling at 0 °C for 20 min. The reaction mixture was then concentrated and precipitated in diethyl ether. The product was washed with cold diethyl ether and dried in vacuo to afford P3 (0.08 g) as a purple solid. 1H NMR (300 MHz, δ in ppm) in CDCl3: 12.35(1H, s, C−OH), 7.50−6.13 (4H, m, CHCH), 4.10 (2H, s, N− CH2), 3.74−3.52 (8H, m, O−CH2−CH2), 3.12−0.94 (17H, m, −O− CH2, N−(CH3)2 and aliphatic H). Mn = 33 600, Mw = 48 100, and PDI = 1.43. Sensing Studies. The stock solution of P3 (2.09 × 10−3 M, assuming 6% incorporation of DASA chromophore moieties along the polymer chain) was prepared in dioxane. The stock solutions of DCP, DCNP, TEP, and TBP (0.1 M) were prepared in dioxane. The P3 stock solution was diluted to be 2.97 × 10−5 M in 0.350 mL of dioxane. To the solution (0.350 mL) of P3 (2.97 × 10−5 M) in the cuvette, a 10 μL aliquot of each, that mimic nerve gases, was added from the stock solutions using a microsyringe.

(RAFT) polymerization, using 2-dodecylsulfanylthiocarbonylsulfanyl-2-methylpropionic acid (DMP) as the chain transfer agent (CTA) and 2,2′-azobis(isobutyronitrile) (AIBN) as the initiator. The initial feed ratio of GMA with respect to DMA was 5:95. The successful formation of P1 was confirmed by 1H NMR spectroscopy (Figure 2). In Figure 2a, the appearance of

Figure 2. 1H NMR spectra of (a) P1, (b) P2, and (c) DASA-derived polymeric probe P3.

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peaks a and a′ at 4.37 and 3.87 ppm corresponding to diastereotopic −OCH2− of GMA and a peak b corresponding to dimethyl unit of DMA confirmed the successful formation of P1.36 The final incorporation ratio of GMA with respect to DMA was calculated by comparing the relative peak intensities between −OCH2− protons (a and a′) of GMA and total peak intensities from 2.3 to 3.3 ppm (b) of DMA protons. The final incorporation ratio of GMA to DMA was calculated to be 6:94.

RESULTS AND DISCUSSION The strategy employed for the synthesis of the DASA-derived polymeric probe, P3, is illustrated in Scheme 1. A random copolymer containing DASA units in the side chains was prepared in three steps. In the first step, glycidyl methacrylate (GMA) and dimethylacrylamide (DMA) were copolymerized to yield P1 by reversible addition−fragmentation chain transfer C

DOI: 10.1021/acs.macromol.6b00309 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. (a) UV−vis absorption spectra of P3 (2.97 × 10−5 M) upon irradiation with visible light with various time intervals in dioxane. (b) UV−vis absorption spectra of P3 (2.97 × 10−5 M) before and after irradiation with visible light in dioxane and their reversibility under dark condition in CHCl3.

Figure 4. (a) UV−vis absorption spectra of P3 (2.97 × 10−5 M) with various amounts of DCNP in dioxane. (b) Selectivity bar diagram of P3 (2.97 × 10−5 M) with various organophorous compounds (6 mM) such as triethyl phosphate (TEP), tributyl phosphate (TBP), diethyl chlorophosphate (DCP), and diethyl cyanophosphate (DCNP) in dioxane before irradiation with visible light. (c) UV−vis absorption spectra of P3-ZCP with various amounts of DNCP in dioxane (after irradiation with visible light). (d) UV−vis absorption spectra of P3-coated thin films before and after the exposure to DCNP vapors.

−OCH2− protons of GMA units of P1 at 4.37 ppm shifted to the upfield region and new peaks a, b, c, and d corresponding to ethylene glycol units appeared at 3.5 ppm (Figure 2b). In the final step, the secondary amine groups of P2 were reacted with 5-(furan-2-ylmethylene)-1,3-dimethylpyrimidine 2,4,6(1H,3H, 5H)-trione in THF to give P3. The incorporation of DASA units in the side chain of P3 was confirmed by the appearance

The final incorporation ratio was almost the same as the initial feed ratio. The number-averaged molecular weight (Mn) of P1 was 16 800 with low polydispersity (Mw/Mn = 1.1) (Figure S1). In the second step, the epoxide unit of the P1 was ring-opened with 2-(2-aminoethoxy)ethanol to produce P2. The successful transformation to P2 was confirmed by 1H NMR spectroscopy in which the proton “a” peak corresponding to diastereotopic D

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Figure 5. (a) Schematic representation of on−off photoswitch for the colorimetric sensing of DCNP. (b) Photographs of P3-coated thin films upon exposure to DCNP vapor before and after irradiation with visible light.

of C−OH (peak a) and −CHCH protons (peaks b, c, and d) at 12.3 and 6.0−7.5 ppm, respectively (Figure 2c). The extent of the postmodification reaction was estimated by comparing the relative peak intensities between protons of the ethylene glycol unit (e, f, g, and h) and vinyl proton (b) of the DASAderived chromophore of P3 by using the deconvolution method. It was found that 85% of secondary amine was reacted with 5-(furan-2-ylmethylene)-1,3-dimethylpyrimidine2,4,6(1H,3H,5H)-trione to give P3. To examine the photoisomerization behavior of the DASA chromophore between purple-colored triene (P3) and colorless zwitterionic cyclopentenone (P3-ZCP), a P3 solution in dioxane (0.350 mL, 2.97 × 10−5 M) was exposed to visible light (crystal clear 40 light bulb, 40 W). UV−vis absorption spectra were measured to monitor the photoisomerization behavior with various time intervals (Figure 3a). As expected,

irradiation with visible light led to a gradual decrease in the absorption band at 550 nm. The decrease in absorbance at 550 nm observed with increasing irradiation time suggested the conversion of triene to zwitterionic cyclopentenone form, which accompanied a notable color change from purple to colorless. Unlike previous results demonstrated in earlier literature,30 the DASA-derived P3 did not show reversibility under dark conditions in dioxane.37 Alternatively, chloroform was employed to achieve the reversibility (Figure 3b). The reverse isomerization of the DASA chromophore from zwitterionic cyclopentenone to triene form is more facile in halogenated solvents under thermal conditions.30 After complete conversion to zwitterionic form with irradiation with visible light, dioxane was removed and P3 was redissolved in chloroform. It was observed that colorless P3 turned to rosy E

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Macromolecules pink at 40 °C, accompanying approximately 75% recovery in the optical density at 550 nm. The colorimetric response of P3 (2.97 × 10−5 M) was tested with mimics of nerve agents, such as diethyl cyanophosphate (DCNP) and diethyl chlorophosphate (DCP) using UV−vis absorption spectra in dioxane. These mimics of nerve agents were chosen for the sensing studies to prevent the health inconveniences during handling real nerve agents while they show relatively similar reactivity and less toxicity. As shown in Figure 4a, the absorbance of P3 at 550 nm decreased upon the gradual addition of DCNP up to 6.0 mM, above which no further decrease was observed. The lowest concentration limit for the detection of DCNP was found to be 1.0 mM. As a consequence, the color of the solution turned from dark purple to colorless, which can be observed easily by the naked eye (photograph in Figure 4a). These behaviors are ascribed to DCNP-promoted intramolecular N-alkylation to yield the morpholino cation (Figure 1).31−35 The selectivity of P3 toward various mimics of nerve agents was also examined (Figures S2−S4), and the results are shown in Figure 4b. While P3 expressed the highest selectivity for DCNP compared to other phosphates, the structurally similar DCP also showed a small decrease in the absorbance at 550 nm with increasing concentration; the other phosphates exhibited no discernible changes. The solution of P3-ZCP with zwitterionic cyclopentanone formed after irradiation with visible light is colorless since zwitterionic cyclopentanone is not an active chromophore. As a result, the solution remained colorless even after exposure to DCNP, leading to no colorimetric sensing (Figure 4c). After the successful demonstration of the sensitive and selective detection of DCNP in dioxane solution, we attempted to detect DCNP in the vapor phase. Vapor phase detection of nerve agent mimics by using DASA-derived polymeric thin films could offer additional benefits for advanced applications such as the fabrication of body suits or wearable protective materials and ultrasensitive CWA devices, etc.22 For this purpose, P3 was deposited directly from a THF solution onto a quartz plate by spin coating to form polymer films. A P3-coated plate was kept hanging inside the glass chamber, at a higher elevation than the bulk of the DCNP. Interestingly, the color of the film turned from purple to colorless when the surface of the film was exposed to the DCNP vapor with a gentle heating of the DCNP. This unique behavior of the film was further monitored by UV−vis absorption spectroscopy. As shown in Figure 4d, the optical density of the absorption band at 550 nm decreased suddenly after the exposure to DCNP vapor in 2 min. The color of the film faded away completely after 5 min. To confirm the DCNP-promoted intramolecular N-alkylation to yield the morpholino cation units, the 1H NMR spectrum of P3 was recorded after the addition of DCNP in CDCl3 (Figure S5). The formation of morpholino cation units could be confirmed by monitoring the shift of the conjugated vinyl protons b, b′, c, and d to the upfield region. The photochromic behavior along with the sensing ability of P3 was integrated to serve as an on−off photoswitch for the phototunable detection of DCNP. The controlled sensing experiments of a P3-coated thin film exposed to DCNP vapor were carried out before and after irradiation with visible light (Figure 5). As discussed above, P3 with purple conjugated triene units (ON state) exhibited an instant colorimetric response by the reaction with electrophilic phosphorus atoms of DCNP, forming six-membered rings of morpholino cations

before irradiation with visible light. On the other hand, no colorimetric response toward DCNP was observed for P3-ZCP with the zwitterionic cyclopentanone formed after irradiation with visible light due to unavailability of active chromophore units and the presence of quarternary N-alkyl groups.

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CONCLUSIONS In conclusion, we have developed an on−off photoswitchable colorimetric method for the real-time detection of DCNP both in solution and in the vapor phase. In our approach, unique visible light responsive DASA chromophores and nucleophilic hydroxyl groups in the N-alkyl positions of the chromophores were incorporated into the polymeric backbone, for achieving photoswitchable detection of DCNP. The direct phosphorylation of hydroxyl groups and the subsequent intramolcular cyclization to form six-membered rings of morpholino cations enabled us to achieve the rapid and selective colorimetric detection of DCNP in solution and in the vapor phase (with the latter detection achieved through the use of a DASAderived spin-coated thin film). These results suggest that the polymeric probe presented here is a potential candidate for exploring advanced applications such as electronic noses, the fabrication of protective cloths, and ultrasensitive devices.

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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.macromol.6b00309. GPC data of P1−P3 and UV−vis absorption spectra of P3 over the addition of various organophosphorous compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Priority Research Center Program (2009-0093818) and the Basic Science Research Program (NRF-2014R1A1A2057197) through the National Research Foundation of Korea, funded by the Ministry of Education, Science, and Technology of Korea.

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