Benzimidazolium-Based Self-Assembled Fluorescent Aggregates for

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Benzimidazolium-Based Self-Assembled Fluorescent Aggregates for Sensing and Catalytic Degradation of Diethylchlorophosphate Amanpreet Singh,† Pushap Raj,† and Narinder Singh*,† †

Department of Chemistry, Indian Institute Technology, Ropar, Punjab 140001, India S Supporting Information *

ABSTRACT: The unregulated use of chemical weapons has aroused researchers to develop sensors for chemical warfare agents (CWA) and likewise to abolish their harmful effects, the degradation through catalysis has great advantage. Chemically, the CWAs are versatile; however, mostly they contain organophosphates that act on inhibition of acetyl cholinesterase. In this work, we have designed and synthesized some novel benzimidazolium based fluorescent cations and their fluorescent aggregates were fabricated using anionic surfactants (SDS and SDBS) in aqueous medium. The prepared fluorescent aggregates have shown aggregation induced emission enhancement, which was further used as detection of chemical warfare agent in aqueous medium. The aggregates (Benz-2/SDBS and Benz-3/SDBS) have shown significant changes in emission profile upon interaction with diethylchlorophosphate. Contrarily, the pure dipodal receptor Benz-4 has not shown any response in emission after interaction with organophosphate, and consequently, it was concluded that benzimidazolium cation plays a decisive role in sensing. The mechanism of sensing was fully validated using 31P NMR spectroscopy as well as GC-MS, which highlights the transformation of diethylchlorophosphate into diethylhydrogen phosphate. The aggregates selectively interact with diethylchlorophosphate over other biological important phosphates. KEYWORDS: fluorescent cations, benzimidazolium, anionic surfactant, aggregates, warfare agents

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cations with anionic surfactant can be explored as sensor.21−25 Such supramolecular assemblies are stable in aqueous medium and widely used in the field of catalysis26 and sensing.27−29 Upon aggregation, the fluorescent intensity of receptor increases significantly, which can be fully explained on the basis of aggregate formation called aggregation induced emission enhancement (AIEE).30−32 First, Tang et al. have explored this phenomenon; this mechanism was used to design OLEDs, sensors, and new fluorescent materials.33−35 Although ionic self-assembled materials have been explored as sensors for metal ions and for some small anions,36−39 the reports of sensing biomolecules and large guest molecules using aggregates are rare. Fang at el. has used bispyrene conjugated with imidazolium ionic liquid for construction of aggregates which were developed for sensing of explosives (nitro aromatics).37 To best of our knowledge, this is the first report in which fluorescent aggregates are used to detect organophosphate based chemical warfare agents. In present work we have chosen benzimidazolium based dipodal receptors for the formation of ionic self-assembled fluorescent aggregates (Scheme 1). Imidazolium and benzimidazolium-based ionic liquids are well-known for their

INTRODUCTION Detection of chemical warfare agents particularly in aqueous medium gains remarkable interest due to their unregulated uses as chemical weapons.1−3 Basically, these are the class of manmade nerve agents that act on inhibition of acetyl cholinesterase enzyme and consequently may cause death due to paralysis of central nerve system.4−7 Due to stringent environmental concern, immediate threat of its misuse and homeland security, detection of CWAs is in exigent need.8 Among various detection methods, the fluorescent technique has advantages such as low detection limit, high specificity, and easy sample preparation.9−11 In the past few years some fluorescence based sensors were developed for analysis of organophosphate based pesticides and chemical warfare agents.12−14 Those are based on metal complexes as well as organic receptor;15−17 although some of these are highly selective and high sensitive, most of them sense the analyte in organic solvent. Therefore, significant work is required to develop novel sensors that pronounce a response in aqueous medium. To achieve detection of analyte in aqueous medium, different methods are known such as organic nanoparticle based receptor, organic−inorganic nanohybrid materials and fluorescent aggregates.18−20 Among these the nanohybrid materials and organic nanoparticles have some serious issues like reduced stability, poor shelf life, and nonuniform size distribution. Whereas self-assembly formed through fluorescent © XXXX American Chemical Society

Received: August 9, 2016 Accepted: October 4, 2016

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DOI: 10.1021/acsami.6b09983 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Structure of Different Ligands Used for Detection of Diethylchlorophosphate

Scheme 2. Cartoon Representation of Aggregates Formed by Organic Cations and Anionic Surfactant

imidazolium moiety is that it can detect analytes by both fluorescent and electrochemical methods.16,46 Earlier, we reported the electrochemical detection of guanine using an imidazolium ionic based copper complex.47 The guanine interacts with copper complex through electrostatic interaction as well as through hydrogen bonding, which results in a change in electrochemical properties. Similarly, in other work we have developed an organic−inorganic hybrid polymer of copper with benzimidazolium ionic liquid based COOH

interaction with carbonyls as well as carboxylic acids through the positively charged heterocyclic ring.40−42 Nowadays, these are widely used in catalysis by activating various substrates such as carbonyls, carboxylic acid, and alcohols. Moreover, these ionic liquids have affinity to interact with various biomolecules through hydrogen bonding and electrostatic interaction.43−45 Therefore, in the past few years, the benzimidazolium cations were gaining interest in molecular recognition of biomolecules. The major benefit of the benzimidazolium group over the B


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ACS Applied Materials & Interfaces

obtained in 86% yield. 1H NMR (400 MHz, DMSO-d6) δ 4.04 (t, J = 8.0 Hz, 4H, CH2), 5.01 (t, J = 8.0 Hz, 4H, CH2), 7.69 (d, 2H, ArH), 8.16 (d, 2H, ArH), 10.07 (s, 1H, CH); 13C NMR (100 MHz, DMSOd6) δ 31.6, 48.7, 114.4, 127.5, 131.3, 143.93. Anal. Calcd for C11H13Br3N2: C, 31.99; H, 3.17; N, 6.78. Found: C, 31.82; H, 3.08; N, 6.59. Synthesis of Compound Benz-2. The prepared Benz-1 (410 mg, 1 mmol) was dissolved in dry acetonitrile (heating was required to completely solubilize the Benz-1). To the saturated solution, 2mercaptobenzimidaole (300 mg, 2 mmol) was added. The resulting mixture was refluxed for 8 h. The white powder was separated; thereafter, product was filtered off and washed with hot chloroform. The resulting white powder constitute pure receptor Benz-2, Yield = 85%. 1H NMR (400 MHz, DMSO-d6:CDCl3) δ 3.99 (t, J = 8.0 Hz, 4H, CH2), 4.96 (t, J = 8.0 Hz, 4H, CH2), 7.03 (dd, 2H, ArH), 7.30 (d, J = 8.0 Hz, 4H, ArH), 7.52 (m, 6H, ArH, NH), 8.04 (dd, J = 8.0 Hz, 2H, ArH), 10.09 (s, 1H, CH). 13C NMR (100 MHz, DMSOd6:CDCl3) δ 149.07, 143.59, 136.17, 131.35, 127.14, 126.64, 123.97, 122.94, 114.87, 114.08, 113.96, 109.94, 46.96, 31.70. Anal. Calcd for C25H23BrN6S2: C, 54.44; H, 4.20; N, 15.24; S, 11.63. Found: C, 54.34; H, 4.11; N, 15.15; S, 11.51. HRMS (ESI, quadrupole): m/z = 471.1257. Synthesis of Compound Benz-3. The compound Benz-3 was synthesized by a similar procedure to Benz-2. The Benz-1 (410 mg, 1 mmol) was dissolved in dry acetonitrile and the solution refluxed until it became clear, and 2-mercaptobenzthiazole (334 mg, 2 mmol) was added to the reaction mixture. The progress of the reaction was monitored using thin layer chromatography, and it was found that after 12 h of refluxing the reaction was completed. The solvent was evaporated and the yellow crude compound obtained was purified with column chromatography. The product was filtered off and washed with hot chloroform, Yield = 82%. 1H NMR (400 MHz, DMSO-d6) δ 3.72 (t, J = 8.0 Hz, 4H, CH2), 4.85 (t, J = 8.0 Hz, 4H, CH2), 7.30 (dd, J = 8.0 Hz, 2H, ArH), 7.37 (d, J = 8.0 Hz, 2H, ArH), 7.48 (d, J = 8.0 Hz, 2H, ArH), 7.71 (dd, J = 8.0 Hz, 2H, ArH), 7.90 (d, J = 8.0 Hz, 2H, ArH), 8.11 (dd, 2H, ArH), 9.88 (s, 1H, CH). 13C NMR (100 MHz, DMSO-d6) δ 32.13, 46.82, 114.26, 121.45, 122.39, 125.22, 126.85, 127.26, 131.57, 135.28, 143.28, 152.55, 165.49. Anal. Calcd for C25H21BrN4S4: C, 51.27; H, 3.61; N, 9.57; S, 21.90. Found C, 51.20; H, 3.53; N, 9.50; S, 21.81, HRMS (ESI, quadrupole): m/z = 504.9936. Synthesis of Compound Benz-4. The compound Benz-4 was synthesized from mesitylene based dibromide, which was prepared using the reported method.48 In a round-bottom flask, dibromide (306 mg, 1 mmol) and 2-mercaptobenzthiazole (334 mg, 2 mmol) was dissolved in dry acetonitrile and refluxed for 5 h. After 5 h of refluxing a white compound was separated out. Yield = 92%. 1H NMR (400 MHz, CDCl3) δ 2.42 (s, 6H, CH3), 2.52 (s, 3H, CH3), 4.69 (s, 4H, CH2), 6.94 (s, 1H, ArH), 7.31 (t, J = 8.0 Hz, 2H, ArH), 7.42 (t, J = 8.0 Hz, 2H, ArH), 7.76 (d, J = 8.0 Hz, 2H, ArH), 7.90 (d, J = 8.0 Hz, 2H, ArH). 13C NMR (100 MHz, CDCl3) δ 167.06, 153.32, 137.83, 137.76, 135.28, 130.72, 129.80, 126.18, 124.38, 121.55, 121.13, 33.59, 20.04, 15.74. Anal. Calcd for C25H22N2S4: C, 62.72; H, 4.63; N, 5.85; S, 26.79. Found: C, 62.61; H, 4.53; N, 5.78; S, 26.70. HRMS (ESI, quadrupole): m/z = 479.0819.

functionalized dipodal ligand, which was explored as a fluorescent sensor for phosphate.16 Encouraged with these results, we were particularly interested in the detection of analyte in aqueous medium. Herein, we have developed a novel dipodal receptor having benzimidazolium moiety in the center and two fluorescent arms as binding units (Scheme 2) as well as a signaling unit that initially interacts with CWAs and captures the hydrolyzed product of organophosphate. The prepared dipodal receptors were solubilized in aqueous medium using anionic surfactants and explored for detection of a CWAs mimic. As it is difficult to procure chemical warfare agents, we have taken a mimic of these CWAs for binding studies because these molecules have similar reactivity but low toxicity.

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EXPERIMENTAL SECTION

Reagent. 2-Mercaptobenzimidazole, 2-mercaptobenzthiazole, benzimidazole, mesitylene, and dibromoethane was purchased from Sigma-Aldrich and used without further purification. The anionic surfactants, i.e., sodium dodecyl sulfate and sodium dodecylbenzenesulfonate, were purchased from Tokyo Chemical Industry (TCI) and were used as received. All the phosphates were procured from SigmaAldrich; as we know, these are highly toxic, therefore precautions were taken while handling these chemicals. Milli-Q water (18.2 Mο at 25 °C) was used to prepare all aqueous solutions. Acetonitrile and deuterated solvents were purchased from Merck and Sigma-Aldrich, respectively. Methods. The 1H NMR, 13C NMR, and 31P NMR spectra were recorded on Joel Instrument working at 400 MHz for 1H NMR and at 100 MHz for 13C NMR. All chemical shifts were recorded ppm relative to trimethylsilane as internal reference. The fluorescence experiments were conducted at room temperature on a PerkinElmer Spectrophotometer with a fixed scanning speed and emission slit width (10 nm). The size of the prepared aggregates was determined with dynamic light scattering using the external probe feature of Metrohm Microtrac Ultra Nanotrac particle size analyzer. The shapes and surface morphology of aggregates were determined with atomic force microscopy. The pH measurements were carried out on an ME/962P instrument. Elemental analyses were carried out using Fisons CHNS analyzers. Mass spectra (HR-MS) were analyzed on Q-TOF high resolution mass spectrometer. The TEM images were recorded on Hitachi (H-7500) instrument worked at 120 kV. Procedure for Photophysical Studies. The solution of each organic cation Benz-1, Benz-2, and Benz-3 was prepared at 1 μM concentration in water and sonicated well before recording the spectra. The compound Benz-4 is not ionic and hence insoluble in water, therefore, binding studies for receptor Benz-4 was performed in THF:water (30:70), whereas for organic cations all binding studies were performed in pure aqueous medium. The stock solutions of surfactants sodium dodecyl sulfate and sodium dodecylbenzenesulfonate were prepared at concentration of 1 mM in water, which was further diluted according to the need of the experiment. The fluorescence aggregates of compound Benz-2 have been prepared by mixing 50 mL of 2 μM solution of Benz-2 into 50 mL of 2.4 μM solution of corresponding surfactant. In a similar way fluorescence aggregates of compound Benz-1 and Benz-3 were prepared by using SDBS and SDS surfactant. The stock solutions of pyrophosphate, diethylchlorophosphate, adenosine triphosphate, adenosine monophosphate, adenosine diphosphate, and nicotinamide adenine dinucleotide were prepared at concentration of 1 mM in DMSO/ H2O (1:1). Synthesis of Compound Benz-1. The compound Benz-1 was resynthesized using the method already reported in the literature.49 In brief, the benzimidazole (1.18 g, 10 mmol) was first dissolved in acetonitrile (50 mL), and upon the addition of bromoethane (8.56 mL, 100 mmol), the mixture was heated to reflux for 24 h. After the completion of the reaction, volatiles were evaporated under high vacuum. The resulting crude mixture was washed with chloroform and then recrystallized in methanol. Colorless crystals (3.58 g) were

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RESULT AND DISCUSSION Synthesis and Characterization of Organic Receptors. Benzimidazolium based dipodal receptors (Benz-2 and Benz3) were designed in such a way that they provide ionic interaction to electron rich species, whereas attached arms will provide selectivity for a particular analyte. To analyze the effect of benzimidazolium cation for binding, another receptor was designed with same functional groups except the benzimidazolium cation was replaced by a neutral mesitylene group. The compound Benz-1 was synthesized by reacting benzimidazole with 10 equiv of dibromoethane in acetonitrile (Scheme S1).49 After 10 h of refluxing the reaction was completed, the volatile liquid was evaporated, and the resultant crude was recrystallized C


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Figure 1. (A) Emission spectra of Benz-2 (1 μM) in water and change in emission spectra upon aggregate formation using SDS and SDBS. (B) Emission spectra of Benz-3 (1 μM) in water and change in emission spectra upon aggregate formation using SDS and SDBS. (C) Effect of gradual addition of SDS (1.2 equiv) on the emission spectrum of Benz-2 in water. (D) Effect of gradual addition of SDBS (1.2 equiv) on the emission spectrum of Benz-2 in water. (E) Effect of gradual addition of SDS (1.2 equiv) on the emission spectrum of Benz-3 in water. (F) Effect of gradual addition of SDBS (1.2 equiv) on the emission spectrum of Benz-3 in water.

in acetonitrile. 1H NMR spectra of compound 1 exhibited two triplets at 4.04 and 5.01, which correspond to methylene protons whereas the signal at 10.07 represents the CH proton

of the benzimidazolium moiety. The organic cations Benz-2/3 were synthesized by reacting benzimidazolium dibromide (1) with 2-mercaptobenzimidazole and 2-mercaptobenzthiazole, D


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Figure 2. (A) AFM measurements of Benz-2/SDBS, showing size distribution from 1.7 to 2.1 μm. (B) Three-dimensional picture of Benz-2/SDBS aggregates. (C) AFM measurements of Benz-3/SDBS, showing size distribution from 1.6 to 2.0 μm. (D) Three-dimensional picture of Benz-3/ SDBS aggregates. (E) Dynamic light scattering histogram showing size of aggregates Benz-2 (1 μM) + SDBS (1.2 μM) and (F) Benz-3 (1 μM) + SDBS (1.2 μM).

signals in the aliphatic region and 11 in the aromatic region of C NMR confirmed the formation of Benz-4. Photophysical Studies of Organic Cations. The compounds Benz-2 and Benz-3 are ionic molecules and have considerable aqueous solubility; therefore, the absorbance spectra were recorded in double-distilled water. The organic cation Benz-2 showed absorbance maxima at 250 and 325 nm, which were signature peaks of π−π* and n−π* transitions. Similarly, receptor Benz-3 has shown absorption maxima at 255 and 315 nm (Figure S1). On excitation at 325 nm, compound Benz-2 showed emission maxima at 405 nm and the compound Benz-3 showed emission maxima at 415 nm on excitation at 315 nm (Figure 1); although with low quantum yield. Furthermore, to achieve a stable sensor system in aqueous

respectively, in dry acetonitrile. The compound Benz-2 was separated out after 8 h of refluxing, whereas compound Benz-3 was purified using column chromatography using hexane:ethyl acetate as eluent. The structures of compounds Benz-2/3 were both fully characterized using NMR and mass spectrometry. The purity of compounds was established from elemental analysis. The compound Benz-4 was synthesized from mesitylene based dibromide. One equivalent of mesitylene based dibromide was reacted with 2 equiv of 2-mercaptobenzthiazole in dry acetonitrile (Scheme S2). Pure light yellowcolored compound was separated out after 5 h of refluxing, the spectra revealed three singlets at 2.42, 2.52, and 4.69 in 1H NMR spectra, corresponding to methyl and CH2 proton. The 3

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E


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ionic interactions. Here, the hydrophobic tail is encapsulated inside the sphere through van der Waals forces of interaction, whereas the anionic head interacts with organic cations through ionic interactions. The critical micelle concentrations (CMC) of anionic surfactants are 8 and 1.5 mM for SDS and SDBS, respectively.39 However, the aggregates Benz-2/SDBS and Benz-3/SDBS were formed far below the CMC. The reason may be the large size of the organic cation as compared to sodium ion that may cause aggregation at lower concentration.52 Similarly, sizes of aggregates were monitored using dynamic light scattering experiments. DLS histograms were recorded at different concentrations of anionic surfactant and it was observed that at concentration 1.2 μM of SDBS, uniform spherical aggregates were formed (Figure 2). However, as we increase the concentration above 1.2 μM, some small aggregates are formed, as can be seen in AFM images as well as DLS histogram. These small nonuniform aggregates did not affect the fluorescence behavior of Benz-2, whereas the aggregates formed using SDS surfactant are not uniformly distributed as observed from DLS histograms as well as AFM (Figure S7). The fluorescence studies also authenticate the observation, as SDS has little effect on emission profile as compared to SDBS. This suggests that SDBS has a greater tendency to form well-organized and uniform aggregates with receptors Benz-2 and Benz-3. This may be due to the presence of aromatic rings in SDBS which helps in perfect packing for formation of aggregates. Using the same method, the aggregates of compound Benz-1 were synthesized and DLS and AFM analysis revealed that these aggregates were not uniformly distributed. Effect of Temperature on Fluorescence Intensity of Aggregates. The effects of temperature on aggregates were studied by recording the emission profile with increase in temperature. It was observed that on increase in temperature from 25 to 100 °C, fluorescence intensity of Benz-2/SDBS decreases which indicates breakage of aggregates at higher temperature (Figure S8). However, as the solution temperature falls, it regains the original fluorescence intensity and similar trends were observed for Benz-3/SDBS aggregates. Both of the aggregates dissociate at higher temperature and reformed their original shape on cooling. The effect of temperature was also studied using differential light scattering experiments. The DLS histograms were recorded at three different temperatures (25, 50, and 80 °C): The size of Benz-2/SDBS was 1.8 μm at room temperature whereas upon heating up to 50 °C, sizes of aggregates decrease and nonuniform distribution was observed. As temperature reached 80 °C, a DLS histogram showed size smaller than 100 nm. Similarly, dissociation of aggregates Benz3/SDBS was observed on heating. Effect of pH on Fluorescence Intensity of Aggregates. In order to evaluate the effect of pH, emission spectra were recorded with change in pH from 4 to 10 as shown in (Figure S9); fluorescence intensity slightly increases with decrease in pH. This indicates the cancellation of PET mechanism upon protonation of organic cations, and these aggregates are quite stable at lower pH also; however, we have done all binding studies at pH 7.5 using HEPES buffer. Dynamic light scattering was also recorded at three different pH values; the results of DLS analysis revealed that size of aggregates did not alter much, at acidic, neutral, and basic pH. Binding Studies of Benz-2 Aggregates with Phosphate. The studies revealed that SDBS is a better candidate for formation of aggregates, and hence for further studies only

medium (for analytical studies), these organic cations were incorporated into surfactant aggregates. As our receptors are cationic in nature; we have used anionic surfactants for construction of aggregates, having the same headgroup with different tails. The basis of using sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate (SDBS) comes from the literature.50 Earlier, Rodenas et al. studied the effect of cationic aggregates on the rate of bimolecular reaction.51 They proposed the pseudophase model of micellar catalysis for large enhancement in the rate of reaction, which suggests that upon aggregate formation, the conformation of receptor changes which will alter its binding affinity toward a particular analyte. Fang et al. have developed bispyrene based fluorescent aggregates using SDS as surfactant and explored them as a sensor for picric acid.37 Similarly Kumar et al. have studied the binding affinity of these anionic surfactants with benzimidazolium based cyclophane.52 The addition of sodium dodecyl sulfate and sodium dodecylbenzenesulfonate (1.2 equiv each) significantly changes the absorbance and emission spectra of receptors, whereas the similar dipodal compound Benz-1 showed little enhancement in emission intensity upon addition of surfactants (Figure S2). Both organic cations (Benz-2 and Benz-3) showed more enhancement in emission profile on addition of SDBS as compared to SDS (Figure 1). Upon addition of SDBS in receptor Benz-2 and Benz-3, emission maxima shifted to higher wavelengths. These conclude the affinity of SDBS for aggregate formation with compounds Benz-2 and Benz-3. To authenticate the binding test, fluorescence spectra of organic cations Benz-2 and Benz-3 were recorded in water upon gradual addition of SDBS and SDS (Figure 1c,d). A linear increase in fluorescence intensity was observed from 0 to 1.2 μM for SDBS as well as SDS (Figure S3). This enhancement in emission intensity is attributed to formation of aggregates. The organic cations in aggregates will have completely different conformation from free molecules in water, because environmental conditions such as pH, solvent polarity, and the presence of specific molecules greatly influence the conformation of the receptor. Therefore, upon aggregate formation, conformation of organic cations changed, resulting in enhancement of fluorescence intensity. Not only photophysical properties, but also their catalytic activity and binding ability of the receptor with other analytes may be altered upon a change in conformation.51 The solvent plays a significant role in the formation of aggregates; a nonaqueous solvent like DMSO may inhibit the formation of aggregates (Figure S4), which may be due to a decrease in ionic interaction between surfactant and organic cation. We have also checked the emission spectra in the solid state; the quantum yield was much less (Benz-2/ SDBS = 0.25, Benz-3/SDBS = 0.22) as compared to the solution state (Benz-2/SDBS = 0.57, Benz-3/SDBS = 0.47); this may be due to the self-quenching of aggregates in the solid state (Table S1). Characterization of Aggregates Using DLS and AFM. AFM studies helped us to calculate the size and analyze the morphology of aggregates. As shown in Figure 2, aggregates Benz-2/SDBS and Benz-3/SDBS are spherical in shape (Figure 2) and have size distribution from 1.6 to 2.1 μM (Figure S5), which was also confirmed from TEM analysis (Figure S6). The uniform distribution of spherical aggregates confirms the formation of micelle-type aggregates far below the critical micelle concentration of anionic surfactants. The aggregation happens due to self-assembly of anionic surfactants and absorption of organic cations on these aggregates through F


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Figure 3. (A) Changes in fluorescence intensity of aggregates Benz-2/SDBS (1.0 μM) upon the addition of particular phosphates (6 equiv) in an aqueous medium (λex = 325 nm). (B) Effect of gradual addition of DCP (6 equiv) on the emission spectrum of Benz-2/SDBS in water. (C) Changes in fluorescence intensity of aggregates Benz-3/SDBS (1.0 μM) upon the addition of particular phosphates (7 equiv) in an aqueous medium (λex = 315 nm). (D) Effect of gradual addition of DCP (7 equiv) on the emission spectrum of Benz-2/SDBS in water.

scan rate 200, and slit widths = 10 nm) was evaluated upon addition of 6 equiv of phosphate in aggregate solution of Benz2/SDBS. It was observed that emission intensity increased significantly upon addition of diethylchlorophosphate, whereas other analytes did not alter the emission profile of aggregates (Figure 3). Similar trends were observed for aggregates Benz3/SDBS, and thus it is concluded that both aggregates Benz-2/ SDBS and Benz-3/SDBS showed high selectivity for diethylchlorophosphate over other phosphates and inorganic phosphates (Figure 3). To confirm the binding, the titration experiments were performed by stepwise addition of diethylchlorophosphate to the solution of Benz-2/SDBS. All emission spectra were recorded after 1 h of mixing. The aggregates Benz2/SDBS show enhancement in fluorescence intensity as concentration of DCP increased from 0 to 6 μM. Similar enhancement in emission spectra was observed upon increment in the concentration of DCP to Benz-3/SDBS solution (Figure 4). After further addition of DCP, fluorescence spectra remain almost the same and the calibration curve was plotted between fluorescence intensity and concentration of DCP. The detection limit was calculated from the 3-σ method from the slope of the nonlinear fitting curve (Figure S10)53 and the detection limit for Benz-2 and Benz-3 was found to be 10 and

SDBS was used to construct aggregates. The phosphate affinity of aggregates Benz-2/SDBS was investigated using fluorescence spectroscopy. Here we have chosen various organic and inorganic phosphates such as pyrophosphate, diethylchlorophosphate, adenodiphosphate, adenotriphosphate, adenomonophosphate, nicotinamide adenine dinucleotide, and nicotinamide adenine dinucleotide phosphate (Scheme S3). Among these, most phosphates are essential for the body and some of these are mimics of chemical warfare agents. Due to the unavailability of hazardous CWAs such as sarin and soman, we used the chemicals that have similar reactivity and size. Also for comparison, some biological important phosphates are included. These are highly poisonous; therefore, precautions need to be taken while preparing solutions as well as for performing binding studies. After binding studies all solutions were disposed carefully. Initially we have studied the interaction of organic cation with these phosphates in aqueous medium using fluorescence spectroscopy. The fluorescence spectrum (λex at 325 nm, scan rate 200, and slit widths = 10 nm) of Benz2 was recorded in water. In aqueous medium, all the organic cations have shown weak emission and even upon addition of 6 equiv of phosphates, less change in emission intensity was observed. Afterward, the fluorescence response (λex at 325 nm, G


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Table 1. Photophysical Properties of Organic Cations (1−3) and Their Aggregates in Water compounds Benz-1 Benz-1 + SDS Benz-1 + SDBS Benz-1 + diethylchlorophosphate Benz-1 + SDS + diethylchlorophosphate Benz-1 + SDBS + diethylchlorophosphate Benz-2 Benz-2 + SDS Benz-2 + SDBS Benz-2 + diethylchlorophosphate Benz-2 + SDS + diethylchlorophosphate Benz-2 + SDBS + diethylchlorophosphate Benz-3

Figure 4. Comparison of fluorescence intensity of organic cations at different conditions (concentration in all cases kept the same).

15 nM, respectively. Thus, our aggregates are quite efficient for detection of DCP in purely aqueous medium with high selectivity and specificity. Addition of DCP to Benz-2/SDBS also altered the shape of the aggregates (Figure S11). To evaluate the efficiency of aggregates and to compare the sensing behavior of aggregates and pure receptors, quantum yields of all compounds were calculated relative to a reference compound (Table 1). QYref is quantum yield of reference compound, η is the refractive index, I is the integrated fluorescence intensity, and A is the absorbance at the excitation wavelength. However, ηref, Aref, and Iref are, respectively, refractive index, fluorescence intensity, and absorbance intensity of reference compound. QY = QYref

Benz-3 + SDS Benz-3 + SDBS Benz-3 + diethylchlorophosphate Benz-3 + SDS + diethylchlorophosphate Benz-3 + SDBS + diethylchlorophosphate Benz-4

η2 I A ref η2 ref A Iref

Benz-4 + DCP

λmax 240 nm (π−π*) and 290 nm (n−π*) 240 nm (π−π*) and 290 nm (n−π*) 240 nm (π−π*) and 290 nm (n−π*) 240 nm (π−π*) and 290 nm (n−π*) 240 nm (π−π*) and 290 nm (n−π*) 240 nm (π−π*) and 290 nm (n−π*) 250 nm (π−π*) and 325 nm (n−π*) 250 nm (π−π*) and 325 nm (n−π*) 250 nm (π−π*) and 325 nm (n−π*) 250 nm (π−π*) and 325 nm (n−π*) 250 nm (π−π*) and 325 nm (n−π*) 250 nm (π−π*) and 325 nm (n−π*) 255 nm (π−π*) and 315 nm (n−π*) 255 nm (π−π*) and 315 nm (n−π*) 255 nm (π−π*) and 315 nm (n−π*) 255 nm (π−π*) and 315 nm (n−π*) 255 nm (π−π*) and 315 nm (n−π*) 255 nm (π−π*) and 315 nm (n−π*) 245 nm (π−π*) and 310 nm (n−π*) 245 nm (π−π*) and 310 nm (n−π*)

emission

% quantum yield

392 nm

0.2497

392 nm

0.2444

395 nm

0.2425

395 nm

0.2135

396 nm

0.2587

396 nm

0.2766

405 nm

0.1570

405 nm

0.3143

411 nm

0.5767

405 nm

0.5559

405 nm

0.4767

411 nm

0.8385

415 nm

0.1665

415 nm

0.2818

420 nm

0.4719

415 nm

0.5331

415 nm

0.5055

420 nm

0.8880

396 nm

0.2679

400 nm

0.2815

the literature that DCP remains stable against hydrolysis over 1 week in H2O/D2O solution.56 Hydrolysis of DCP requires high pH/low pH and detailed studies encouraged us to investigate the fate of Benz-2/SDBS for catalytic degradation of DCP. To confirm the catalytic activity of benzimidazolium cation, we have designed and synthesized a new receptor that has the same functional groups, whereas the benzimidazolium moiety was replaced by the mesitylene group. 31P NMR and fluorescence spectra of receptor Benz-4 were recorded in the presence of DCP; no significance shift in either spectroscopic technique was observed (Figure S13). Nonselectivity of receptor Benz-4 confirmed the role of benzimidazolium moiety in binding with DCP. Similarly, receptor Benz-1 and its aggregates did not alter the fluorescence signal on addition of DCP. Degradation Study. To check the catalytic activity of Benz-2/SDBS aggregates, diethylchlorophosphate was added to the aggregate solution of Benz-2/SDBS (1 μM). After fixed intervals of time, aggregates were removed by centrifugation and the reaction mixture was monitored by gas chromatograph (GC). The GC-MS results revealed the degradation of diethylchlorophosphate (DCP) into diethylhydrogen phosphate (DHP) and tetraethyldiphosphate (TDP). The percen-

As our compounds are showing emissions around 400 nm, we have taken 2-aminopyridine as a reference compound of known quantum yield.54As shown in Table 1, all the receptors show low quantum yield in water. Upon aggregate formation of receptors Benz-2 and Benz-3, a little red shift was observed along with large enhancement in quantum yield. On addition of diethylchlorophosphate to Benz-3/SDBS solution, quantum yield was increased up to 0.88. Similarly, receptor Benz-1 also shows small increment in quantum yield upon aggregate formation that was much less than that of receptor Benz-2 and Benz-3. This suggests the involvement of conformational changes in 2-mercaptobenimidazole for fluorescence enhancement. To determine the mechanism of interaction, 31P NMR spectra of organic cation Benz-2 and Benz-3 were taken in the presence of diethylchlorophosphate. The shift in 31P NMR signals from −0.72 to −12.60 indicates that diethylchlorophosphate has interacted with fluorescent aggregates (Figure S12). In both cases the signal of phosphorus shifted upfield, which indicates the replacement of chlorine with more the electronegative hydroxyl group which is captured by the cavity formed by the cationic part of aggegates.55 It has been documented in H


Research Article


Figure 5. (A) Degradation profile of DCP into diethylhydrogen phosphate as determined from GC-MS. (B) Mass spectra of DHP. (C) Mass spectra of TDP.

Scheme 3. Representation of Possible Mechanism of Sensing and Degradation

tages of these three species at fixed intervals of time were recorded. As shown in Figure 5, percentages of species were plotted against time. Initially, DCP degraded into DHP and TDP; however, after 1 h complete conversion of DCP into nonotoxic DHP had taken place (Figure 5). It was observed that after just 10 min of stirring, DCP was degraded into diethylhydrogen phosphate and tetraethyl pyrophosphate, although the amount of tetraethyl pyrophosphate was much less compared to diethylhydrogen phosphate. Similar results were obtained using aggregates Benz-3/SDBS. As shown in Figure 4, receptors Benz-2 and Benz-3 showed fluorescence enhancement with DCP. However, upon aggregate formation these enhancements are much more significant (Figure S14). Therefore, in the aggregate state, not only can selectivity and sensitivity of sensor be improved, but we can also achieve sensing in purely aqueous medium. The reason for enhancement in fluorescence can be explained on the basis of rigidity induced upon interaction of DCP, which will cancel nonradiative transitions. In the free state, receptor molecules are far away from each other. Due to vibration in the functional groups of the dipodal receptor, a lot of energy is eliminated in the form of nonradiative decay. However, in the aggregate state they come closer and orient in a particular pattern. The rigidity induced upon aggregation reduces the vibrations and nonradiative decay (Scheme 3), as AFM image of aggregates Benz-2/SDBS upon addition of DCP showed the formation of large aggregates which cause rigidity in system.

were prepared and incorporated into aggregates using anionic surfactants (SDBS and SDS). Upon aggregate formation nonfluorescent organic cations show significant increase in emission intensity due to the aggregation induced emission enhancement phenomenon. Due to this conformation change, photophysical properties as well as binding infinity of receptor changes. As independent organic cations did not show any significant change in fluorescence profile; however in aggregate form both of the organic cations Benz-2 and Benz-3 show large enhancement with diethylchlorophosphate. The shift in the signal of 31P NMR confirmed the binding of receptor with analyte. As receptors Benz-1 and Benz-4 do not show any selectivity for organophosphate, which confirms that benzimidazolium (base) and 2-mercaptobenzimidazole (functional group) play an essential role in binding.

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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/acsami.6b09983. Synthesis scheme of receptors, absorption and emission properties, TEM analysis, AFM, DLS, 1H NMR, 13C NMR, HR-MS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-1881242176.

CONCLUSION Benzimidazolium-based receptors containing 2-mercaptobenzimidazole and 2-mercaptobenzthiazole as functional groups

Notes

The authors declare no competing financial interest. I


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Selective Sensing of Phosphates in Water: Applications for Detection of Harmful Organophosphates. Tetrahedron 2015, 71 (36), 6143− 6147. (17) Zhang, K.; Yu, T.; Liu, F.; Sun, M.; Yu, H.; Liu, B.; Zhang, Z.; Jiang, H.; Wang, S. Selective Fluorescence Turn-On and Ratiometric Detection of Organophosphate Using Dual-Emitting Mn-Doped ZnS Nanocrystal Probe. Anal. Chem. 2014, 86 (23), 11727−11733. (18) Kaur, S.; Kaur, A.; Kaur, N.; Singh, N. Development of Chemosensor for Sr 2+ Using Organic Nanoparticles: Application of Sensor in Product Analysis for Oral Care. Org. Biomol. Chem. 2014, 12 (41), 8230−8238. (19) Kaur, G.; Raj, T.; Kaur, N.; Singh, N. Pyrimidine-Based Functional Fluorescent Organic Nanoparticle Probe for Detection of Pseudomonas Aeruginosa. Org. Biomol. Chem. 2015, 13 (16), 4673− 4679. (20) Feng, X.; An, Y.; Yao, Z.; Li, C.; Shi, G. A Turn-on Fluorescent Sensor for Pyrophosphate Based on the Disassembly of Cu2+ Mediated Perylene Diimide Aggregates. ACS Appl. Mater. Interfaces 2012, 4 (2), 614−618. (21) Huang, J.; Peng, A.; Fu, H.; Ma, Y.; Zhai, T.; Yao, J. Temperature-Dependent Ratiometric Fluorescence from an Organic Aggregates System. J. Phys. Chem. A 2006, 110 (29), 9079−9083. (22) Cho, H.; Baker, B. R.; Wachsmann-Hogiu, S.; Pagba, C. V.; Laurence, T. A.; Lane, S. M.; Lee, L. P.; Tok, J. B. H. Aptamer-Based SERRS Sensor for Thrombin Detection. Nano Lett. 2008, 8 (12), 4386−4390. (23) Gonzál ez, D. C.; Savariar, E. N.; Thayumanavan, S. Fluorescence Patterns from Supramolecular Polymer Assembly and Disassembly for Sensing Metallo- and Nonmetalloproteins. J. Am. Chem. Soc. 2009, 131 (22), 7708−7716. (24) Fan, J.; Ding, L.; Bo, Y.; Fang, Y. Fluorescent Ensemble Based on Bispyrene Fluorophore and Surfactant Assemblies: Sensing and Discriminating Proteins in Aqueous Solution. ACS Appl. Mater. Interfaces 2015, 7 (40), 22487−22496. (25) Hu, W.; Ding, L.; Cao, J.; Liu, L.; Wei, Y.; Fang, Y. Protein Binding-Induced Surfactant Aggregation Variation: A New Strategy of Developing Fluorescent Aqueous Sensor for Proteins. ACS Appl. Mater. Interfaces 2015, 7 (8), 4728−4736. (26) Sharma, H.; Kaur, N.; Singh, N.; Jang, D. O. Synergetic Catalytic Effect of Ionic Liquids and ZnO Nanoparticles on the Selective Synthesis of 1,2-Disubstituted Benzimidazoles Using a Ball-Milling Technique. Green Chem. 2015, 17 (8), 4263−4270. (27) Wang, M.; Zhang, M.; Li, J.; Kumar, S.; Walker, G. C.; Scholes, G. D.; Winnik, M. A. Self-Assembly of Colloidal Quantum Dots on the Scaffold of Triblock Copolymer Micelles. ACS Appl. Mater. Interfaces 2010, 2 (11), 3160−3169. (28) Lim, S. K.; Chen, P.; Lee, F. L.; Moochhala, S.; Liedberg, B. Peptide-Assembled Graphene Oxide as a Fluorescent Turn-On Sensor for Lipopolysaccharide (Endotoxin) Detection. Anal. Chem. 2015, 87 (18), 9408−9412. (29) Savariar, E. N.; Ghosh, S.; González, D. C.; Thayumanavan, S. Disassembly of Noncovalent Amphophilic Polymers with Proteins and Utility in Pattern Sensing. J. Am. Chem. Soc. 2008, 130 (16), 5416− 5417. (30) Ma, X.; Sun, R.; Cheng, J.; Liu, J.; Gou, F.; Xiang, H.; Zhou, X. Fluorescence Aggregation-Caused Quenching versus AggregationInduced Emission: A Visual Teaching Technology for Undergraduate Chemistry Students. J. Chem. Educ. 2016, 93 (2), 345−350. (31) Yang, Z.; Qin, W.; Leung, N. L. C.; Arseneault, M.; Lam, J. W. Y.; Liang, G.; Sung, H. H. Y.; Williams, D. A.; Tang, B. Z. Mechanistic Study of AIE Processes of TPE Luminogens: Intramolecular Rotation vs. Configurational Isomerization. J. Mater. Chem. C 2016, 4 (1), 99− 107. (32) Tong, H.; Dong, Y.; Hong, Y.; Haussler, M.; Lam, J. W. Y.; Sung, H. H.; Yu, X.; Sun, J.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. Aggregation-Induced Emission: Effects of Molecular Structure, SolidState Conformation, and Morphological Packing Arrangement on Light-Emitting Behaviors of Diphenyldibenzofulvene Derivatives. J. Phys. Chem. C 2007, 111 (5), 2287−2294.

ACKNOWLEDGMENTS This work was supported with research grant from Interdisciplinary Project: Sensor, sponsored by IIT Ropar. A. S. and P. R. are thankful to CSIR−New Delhi (9/1005(0010)/ 2014-EMR-1), India and UGC (New Delhi) respectively for fellowship.

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REFERENCES

(1) Kirsch, J.; Siltanen, C.; Zhou, Q.; Revzin, A.; Simonian, A. Biosensor Technology: Recent Advances in Threat Agent Detection and Medicine. Chem. Soc. Rev. 2013, 42 (22), 8733−8768. (2) Zhou, X.; Lee, S.; Xu, Z.; Yoon, J. Recent Progress on the Development of Chemosensors for Gases. Chem. Rev. 2015, 115 (15), 7944−8000. (3) Hargrove, A. E.; Nieto, S.; Zhang, T.; Sessler, J. L.; Anslyn, E. V. Artificial Receptors for the Recognition of Phosphorylated Molecules. Chem. Rev. 2011, 111 (11), 6603−6782. (4) Sadik, O.; Land, W. H.; Wanekaya, A. K.; Uematsu, M.; Embrechts, M. J.; Wong, L.; Leibensperger, D.; Volykin, A. Detection and Classification of Organophosphate Nerve Agent Simulants Using Support Vector Machines with Multiarray Sensors. J. Chem. Inf. Comput. Sci. 2004, 44 (2), 499−507. (5) Xie, C.; Li, H.; Li, S.; Wu, J.; Zhang, Z. Surface Molecular SelfAssembly for Organophosphate Pesticide Imprinting in Electropolymerized Poly(p-Aminothiophenol) Membranes on a Gold Nanoparticle Modified Glassy Carbon Electrode. Anal. Chem. 2010, 82 (1), 241−249. (6) Kovarik, Z.; Radić, Z.; Berman, H. A.; Simeon-Rudolf, V.; Reiner, E.; Taylor, P. Acetylcholinesterase Active Centre and Gorge Conformations Analysed by Combinatorial Mutations and Enantiomeric Phosphonates. Biochem. J. 2003, 373 (1), 33−40. (7) Hörnberg, A.; Tunemalm, A. K.; Ekström, F. Crystal Structures of Acetylcholinesterase in Complex with Organophosphorus Compounds Suggest That the Acyl Pocket Modulates the Aging Reaction by Precluding the Formation of the Trigonal Bipyramidal Transition State. Biochemistry 2007, 46 (16), 4815−4825. (8) Kim, K.; Tsay, O. G.; Atwood, D. A.; Churchill, D. G. Destruction and Detection of Chemical Warfare Agents. Chem. Rev. 2011, 111 (9), 5345−5403. (9) Wang, J.; Chatrathi, M. P.; Mulchandani, a; Chen, W. Capillary Electrophoresis Microchips for Separation and Detection of Organophosphate Nerve Agents. Anal. Chem. 2001, 73 (8), 1804−1808. (10) Obare, S. O.; De, C.; Guo, W.; Haywood, T. L.; Samuels, T. A.; Adams, C. P.; Masika, N. O.; Murray, D. H.; Anderson, G. a.; Campbell, K.; Fletcher, K. Fluorescent Chemosensors for Toxic Organophosphorus Pesticides: A Review. Sensors 2010, 10 (7), 7018− 7043. (11) Jurcic, M.; Peveler, W. J.; Savory, C. N.; Scanlon, D. O.; Kenyon, A.; Parkin, I. P. The Vapour Phase Detection of Explosive Markers and Derivatives Using Two Fluorescent Metal-Organic Frameworks. J. Mater. Chem. A 2015, 3 (12), 6351−6359. (12) Yao, G.; Liang, R.; Huang, C.; Wang, Y.; Qiu, J. Surface Plasmon Resonance Sensor Based on Magnetic Molecularly Imprinted Polymers Amplification for Pesticide Recognition. Anal. Chem. 2013, 85 (24), 11944−11951. (13) Jenkins, A. L.; Yin, R.; Jensen, J. L. Molecularly Imprinted Polymer Sensors for Pesticide and Insecticide Detection in Water. Analyst 2001, 126 (6), 798−802. (14) Sambrook, M. R.; Hiscock, J. R.; Cook, A.; Green, A. C.; Holden, I.; Vincent, J. C.; Gale, P. a. Hydrogen Bond-Mediated Recognition of the Chemical Warfare Agent Soman (GD). Chem. Commun. (Cambridge, U. K.) 2012, 48 (45), 5605−5607. (15) Raj, P.; Singh, A.; Kaur, K.; Aree, T.; Singh, A.; Singh, N. Fluorescent Chemosensors for Selective and Sensitive Detection of Phosmet/Chlorpyrifos with Octahedral Ni 2+ Complexes. Inorg. Chem. 2016, 55 (10), 4874−4883. (16) Singh, A.; Singh, J.; Singh, N.; Jang, D. O. A BenzimidazoliumBased Mixed Organic−inorganic Polymer of Cu(II) Ions for Highly J


Research Article

ACS Applied Materials & Interfaces (33) Yuan, W. Z.; Lu, P.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Liu, Y.; Kwok, H. S.; Ma, Y.; Tang, B. Z. Changing the Behavior of Chromophores from Aggregation-Caused Quenching to AggregationInduced Emission: Development of Highly Efficient Light Emitters in the Solid State. Adv. Mater. 2010, 22 (19), 2159−2163. (34) Yuan, W. Z.; Yu, Z.-Q.; Lu, P.; Deng, C.; Lam, J. W. Y.; Wang, Z.; Chen, E.-Q.; Ma, Y.; Tang, B. Z. High Efficiency Luminescent Liquid Crystal: Aggregation-Induced Emission Strategy and Biaxially Oriented Mesomorphic Structure. J. Mater. Chem. 2012, 22 (8), 3323. (35) Sui, B.; Tang, S.; Liu, T.; Kim, B.; Belfield, K. D. Novel BODIPY-Based Fluorescence Turn-on Sensor for Fe3+ and Its Bioimaging Application in Living Cells. ACS Appl. Mater. Interfaces 2014, 6 (21), 18408−18412. (36) Kumari, N.; Dey, N.; Jha, S.; Bhattacharya, S. Ratiometric, Reversible, and Parts per Billion Level Detection of Multiple Toxic Transition Metal Ions Using a Single Probe in Micellar Media. ACS Appl. Mater. Interfaces 2013, 5 (7), 2438−2445. (37) Wang, S.; Ding, L.; Fan, J.; Wang, Z.; Fang, Y. Bispyrene/ surfactant-Assembly-Based Fluorescent Sensor Array for Discriminating Lanthanide Ions in Aqueous Solution. ACS Appl. Mater. Interfaces 2014, 6 (18), 16156−16165. (38) Paek, K.; Yang, H.; Lee, J.; Park, J.; Kim, B. J. Efficent Colorimetric pH Sensor Based on Responsive Polymer À Quantum Dot Integrated Graphene Oxide. ACS Nano 2014, 8 (3), 2848−2856. (39) Zhao, Y.; Zhong, Z. Detection of Hg2+ in Aqueous Solutions with a Foldamer-Based Fluorescent Sensor Modulated by Surfactant Micelles. Org. Lett. 2006, 8 (21), 4715−4717. (40) Lu, Q.-S.; Dong, L.; Zhang, J.; Li, J.; Jiang, L.; Huang, Y.; Qin, S.; Hu, C.-W.; Yu, X.-Q. Imidazolium-Functionalized BINOL as a Multifunctional Receptor for Chromogenic and Chiral Anion Recognition. Org. Lett. 2009, 11 (3), 669−672. (41) Singh, N. J.; Jun, E. J.; Chellappan, K.; Thangadurai, D.; Chandran, R. P.; Hwang, I.-C.; Yoon, J.; Kim, K. S. QuinoxalineImidazolium Receptors for Unique Sensing of Pyrophosphate and Acetate by Charge Transfer. Org. Lett. 2007, 9 (3), 485−488. (42) Wang, S.; Chang, Y.-T. Combinatorial Synthesis of Benzimidazolium Dyes and Its Diversity Directed Application toward GTP-Selective Fluorescent Chemosensors. J. Am. Chem. Soc. 2006, 128 (32), 10380−10381. (43) Zhang, D.; Jiang, X.; Yang, H.; Su, Z.; Gao, E.; Martinez, A.; Gao, G. Novel Benzimidazolium−urea-Based Macrocyclic Fluorescent Sensors: Synthesis, Ratiometric Sensing of H2PO4− and Improvement of the Anion Binding Performance via a Synergistic Binding Strategy. Chem. Commun. 2013, 49 (49), 6149−6151. (44) Sharma, U. K.; Sharma, N.; Kumar, R.; Kumar, R.; Sinha, A. K. Biocatalytic Promiscuity of Lipase in Chemoselective Oxidation of Aryl Alcohols/Acetates: A Unique Synergism of CAL-B and [ Hmim ] Br for the Metal-Free H2O2 Activation. Org. Lett. 2009, 11 (21), 4846−4848. (45) Kim, S. K.; Singh, N. J.; Kwon, J.; Hwang, I.-C.; Park, S. J.; Kim, K. S.; Yoon, J. Fluorescent Imidazolium Receptors for the Recognition of Pyrophosphate. Tetrahedron 2006, 62 (25), 6065−6072. (46) Sandhu, S.; Kumar, R.; Singh, P.; Mahajan, A.; Kaur, M.; Kumar, S. Ultratrace Detection of Nitroaromatics: Picric Acid Responsive Aggregation/Disaggregation of Self-Assembled P -Terphenylbenzimidazolium-Based Molecular Baskets. ACS Appl. Mater. Interfaces 2015, 7 (19), 10491−10500. (47) Singh, A.; Singh, A.; Singh, N. A Cu(II) Complex of an Imidazolium-Based Ionic Liquid: Synthesis, X-Ray Structure and Application in the Selective Electrochemical Sensing of Guanine. Dalton Trans. 2014, 43 (43), 16283−16288. (48) Singh, N.; Jang, D. O. Benzimidazole-Based Tripodal Receptor: Highly Selective Fluorescent Chemosensor for Iodide in Aqueous Solution. Org. Lett. 2007, 9 (10), 1991−1994. (49) Singh, A.; Singh, A.; Singh, N.; Jang, D. O. A BenzimidazoliumBased Organic Trication: A Selective Fluorescent Sensor for Detecting Cysteine in Water. RSC Adv. 2015, 5 (88), 72084−72089.

(50) McGreevy, R. J.; Schechter, R. S. Influence of N-Butanol on the Size of Sodium Dodecyl Sulfate Micelles. J. Colloid Interface Sci. 1989, 127 (1), 209−213. (51) Perez-Benito, E.; Rodenas, E. Influence of Sodium Dodecyl Sulfate Micelles on the Oxidation of Alcohols by Chromic Acid. Langmuir 1991, 7 (2), 232−237. (52) Kumar, S.; Singh, P.; Mahajan, A.; Kumar, S. Aggregation Induced Emission Enhancement in Ionic Self-Assembled Aggregates of Benzimidazolium Based Cyclophane and Sodium Dodecylbenzenesulfonate. Org. Lett. 2013, 15 (13), 3400−3403. (53) Singh, J.; Yadav, M.; Singh, A.; Singh, N. Zinc Metal Complex as a Sensor for Simultaneous Detection of Fluoride and HSO4− Ions. Dalt. Trans. 2015, 44 (28), 12589−12597. (54) Sharma, H.; Singh, A.; Kaur, N.; Singh, N. ZnO-Based ImineLinked Coupled Biocompatible Chemosensor for Nanomolar Detection of Co2+. ACS Sustainable Chem. Eng. 2013, 1 (12), 1600− 1608. (55) Jang, Y. J.; Murale, D. P.; Churchill, D. G. Novel Reversible and Selective Nerve Agent Simulant Detection in Conjunction with Superoxide ‘turn-On’ probing. Analyst 2014, 139 (7), 1614−1617. (56) Kiddle, J. J.; Mezyk, S. P. Reductive Destruction of Chemical Warfare Agent Simulants in Water. J. Phys. Chem. B 2004, 108 (28), 9568−9570.

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