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Research Article pubs.acs.org/journal/ascecg

Lower Rim-Modified Calix[4]arene−Bentonite Hybrid System as a Green, Reversible, and Selective Colorimetric Sensor for Hg2+ Recognition R. J. Maya,†,‡ Athira Krishna,†,‡ P. Sirajunnisa,‡ Cherumuttathu H. Suresh,†,‡ and R. Luxmi Varma*,†,‡ †

Academy of Scientific and Innovative Research (AcSIR), CSIR-NIIST, Thiruvananthapuram 695019, India Organic Chemistry Section, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695019, India

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S Supporting Information *

ABSTRACT: A new quinaldine-functionalized calix[4]arene receptor (QHQC) was synthesized, characterized, and analyzed for its selective sensing properties toward Hg2+ ions, and the binding event with characteristic color change can be followed by the naked eye. The sensing process is reversible, and the detection limit for Hg2+ ions is 2.95 × 10−6 M. A colorimetric solid state Hg2+ sensor was fabricated by intercalating this receptor molecule into the bentonite galleries via the ionexchange method. This organic−inorganic hybrid sensor shows properties analogous to the receptor molecule and effectively detects Hg2+ ions rapidly with visual color change. The solid state Hg2+ sensor is economically affordable, environmentally benign, portable, reversible, and reusable. KEYWORDS: Calix[4]arene, Bentonite, Hg2+ ions, Naked eye detection, Reversible, Environmentally benign

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INTRODUCTION Design and development of reliable and economic organic− inorganic hybrid systems as sensors that can recognize hazardous metal ions by direct visual observation are intriguing endeavors among the scientific community and have received much attention in diverse fields.1−3 Mercury is a well known and extremely poisonous heavy metal that exists in different forms as elemental mercury, ionic mercury, and organic mercury complexes and can cause severe problems to human health and the environment.4,5 Mercury exposure even at very low concentration can lead to serious neurological, kidney, and digestive diseases due to easy passage of Hg2+ through biological membranes.6−8 These adverse effects urge the necessity for the development of new and improved methods to identify Hg2+ ions9 which are cost effective and environmentally benign. Conventional approaches toward Hg2+ determination such as atomic absorption, emission spectroscopy, ICP-MS, electrochemical measurements, gas chromatography, etc. are timeconsuming processes involving multistep sample preparation and sophisticated instrumentation. Even though a large number of fluorescent chemosensors are developed which have several advantages such as selectivity, sensitivity, and low cost,10 syntheses of novel materials containing organic functional molecules with recyclability and environmentally benign nature are of great interest.11,12 Calixarenes,13 the third best receptor molecules, easily synthesizable and readily functionalizable with © 2017 American Chemical Society

suitable binding cores, are considered as excellent molecular scaffolds for the recognition of ions. Albeit a variety of macrocyclic molecular platforms are established, calixarenebased molecular chemosensors are not well exploited for the sensing of Hg2+ ions.14−22 Moreover, calixarene-incorporated receptors would utilize further advantages emerging from its supramolecular behavior. For those reasons, developing suitably functionalized calixarene-based molecular receptors capable of selectively detecting Hg2+ is an interesting area of research. Exploration of solid state colorimetric sensors for hazardous metal ions by naked eye detection is highly desirable, and investigations are currently being pursued by researchers in the area of nanothin films, molecular imprinted polymers, modified sol−gel membranes, etc.23−33 In addition, there is a great demand to develop a simple, rapid responsive, inexpensive, portable, and environmentally benign metal ion sensor material for the selective recognition of Hg2+ ions. To facilitate a stable, efficient, and reliable mercury-sensing system, selection of appropriate supporting materials is crucial. In recent times, a variety of supporting materials are used for the immobilization of organic molecules to fabricate an organic−inorganic hybrid system for the determination of mercury ions.2,34−48 Clays, one of the promising candidates in materials sciences, afford a Received: April 14, 2017 Revised: June 14, 2017 Published: June 19, 2017 6969

DOI: 10.1021/acssuschemeng.7b01158 ACS Sustainable Chem. Eng. 2017, 5, 6969−6977

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthetic Pathway for Preparation of QHQC 7

was added slowly to the above solution and allowed to stir for 72 h at 80 °C. Then, acetonitrile was removed under vacuum, and the crude product obtained was purified by column chromatography [alumina, chloroform: methanol (9:1)]. Product 7 was obtained as a dark violet colored solid (0.260 g) in 73.3% yield. 1 H NMR (500 MHz, CDCl3), δ (ppm): 8.84 (d, 2H, J = 8 Hz), 7.89−7.83 (m, 6H), 7.74 (d, 2H, J = 8 Hz), 7.56 (s, 2H), 7.07 (s, 4H), 6.85 (s, 4H), 4.74 (t, 4H, J = 6.5 Hz), 4.69 (s, 6H), 4.24 (d, 4H, J = 13 Hz), 4.18 (t, 4H, J = 5 Hz), 3.37 (d, 4H, J = 13 Hz), 3.14 (s, 6H), 2.55 (m, 4H), 1.29 (s, 18H), 0.99 (s, 18H). 13C NMR (125 MHz, CDCl3), δ (ppm): 163.1, 161.6, 150.4, 150.3, 150.1, 149.3, 147.4, 142.1, 132.3,131.9, 130.6, 130.2, 127.7, 127.5, 125.8, 125.3, 125.0, 122.9, 111.1, 72.6, 68.6, 46.8, 46.4, 34.9, 34.0, 31.7, 31.0, 25.6. MS (HRMS): Calcd for C72H88I2N2O6, [M-I2+H]+: 1077.6709. Found: 1077.6653. IR νmax: 3371, 3053, 2955, 2895, 2865, 2358, 2327, 1594, 1480, 1359, 1193, 1110, 1049, 860, 830, 747, 657 cm−1. Intercalation of QHQC Salt on Bentonite Clay. Intercalation of QHQC into a clay surface consists of two processes, namely, preparation of Na-Bentonite (Na-Ben) and intercalation of QHQC on Na-Bentonite. Na-Ben was prepared by mixing bentonite with 1 N NaCl using a mechanical stirrer (600 rpm) for 24 h at room temperature. After that, the residue was separated by centrifugation and washed with distilled water until chloride ions were not detected in the supernatant solution upon addition of 0.1 M AgNO3 solution. Subsequently, it was kept at −4 °C and dried in a lyophilizer. About 0.100 g of Na-Ben was dispersed in 50 mL of acetonitrile, and then, 0.112 g of QHQC in 50 mL of acetonitrile was added to it, where it was allowed to stir for 24 h at room temperature. Then, it was allowed to stand for overnight, filtered off, washed with acetonitrile (until there was no unreacted QHQC in supernatant solution), and dried under vacuum. The resultant product was designated as QHQCalBen and characterized using FT-IR spectroscopy, PXRD (powder X-ray diffraction), and TG/DTA.

remarkable potential for the development of hybrid systems due to their low cost, environmentally benign nature, good adsorptive capacity, high stability, large specific surface area, etc.49−54 Moreover, bentonite is a 2:1 clay, which has the capacity to expand its interlayer when treated with water, that is utilized for the intercalation of a variety of organic moieties.55 Herein, we report an economically affordable, green, and reversible lower rim-modified calixarene−bentonite hybrid system (QHQCalBen) as a colorimetric sensor for the selective and rapid monitoring of Hg2+ ions using naked eye detection. Even though several calixarene-based Hg2+sensors were reported in the literature with good sensitivity and selectivity, the newly synthesized quaternary ammonium derivative of the calixarene molecule is not only suitable for reversible mercury sensing by the naked eye but also favors the formation of an organic−inorganic hybrid by the ion exchange method and acts as a solid state Hg2+ sensor. Incorporation of organic molecules into the inorganic host matrices are of great interest as the resulting hybrid material can have high mechanical, thermal, and chemical stability compared to organic molecules. Moreover, calixarene−bentonite hybrid Hg2+ sensors are not exploited in the literature. Here, bentonite facilitates the realization of the sensor as a recyclable one. The key steps which led to the development of this organic− inorganic hybrid system (QHQCalBen) included the synthesis of a lower rim-functionalized 1,3-di(quaternary ammonium salt of 8-hydroxyquinaldine) derivative of p-tert-butylcalix[4]arene (QHQC) as a cone conformer and its intercalation into a sodium−bentonite gallery via the ion-exchange method. The synthesized QHQC itself imparts selectivity and reversibility toward Hg2+ ions in the solution state, and the QHQCalBen hybrid system shows properties analogous to QHQC in acetonitrile as well as in the ACN/H2O (1:9, v/v) system and is reusable.

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RESULTS AND DISCUSSION Synthesis of QHQC (7). The receptor, QHQC 7, was synthesized via three steps starting from p-tert-butylcalix[4]arene as shown in Scheme 1. In brief, precursor 356 was prepared from p-tert-butylcalix[4]arene and 1,3-dibromopropane with K2CO3 as base in refluxing acetonitrile. In the next stage, 3 was treated with 8-hydroxyquinaldine, and K2CO3 in dry acetonitrile at 80 °C fetched HQ-calix[4]arene 5.57 Subsequently, precursor 5 on treatment with excess methyl iodide in refluxing acetonitrile afforded QHQC 7 in 73.3%

EXPERIMENTAL SECTION

Synthesis of Quaternized Salt of 8-HydroxyquinaldineDerived p-tert-Butylcalix[4]arene (QHQC) 7. The synthesis and characterization of precursors 3 and 5 are given in the Supporting Information (see SI). Compound 5 (0.288 g, 0.0003 mol) in dry acetonitrile (10 mL) was refluxed for 5 min. Methyl iodide (excess) 6970

DOI: 10.1021/acssuschemeng.7b01158 ACS Sustainable Chem. Eng. 2017, 5, 6969−6977

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ACS Sustainable Chemistry & Engineering yield. The precursors and the main receptor molecule 7 were characterized by various spectral techniques such as 1H NMR, 13 C NMR, ESI/HRMS, and FT-IR (SI, Figures S11−S18). In the 1H NMR spectrum, two doublets discernible at δ 4.24 and 3.37 ppm for the bridged methylene hydrogens revealed the existence of molecule 7 in cone conformation, and it was further confirmed by the 13C NMR spectrum which showed peaks at δ 31.7 and 31.0 ppm. The peak at a d value of 4.69 ppm in the 1H NMR spectrum and the peak at a d value of 46.6 ppm in the 13C NMR spectrum were attributed to the methyl group attached to a nitrogen atom of the quinaldine ring in consequence of the quaternization of precursor 5. The structure of QHQC 7 was finally disclosed by mass spectral analysis, which showed a molecular ion peak at m/z 1077.6653. The reason behind the synthesis of positively charged arms on the calixarene derivative (compound 7) rather than compound 5 (precursor) which has neutral arms is that compound 7 is a violet-colored compound and furnish naked eye detection with Hg2+ ions. However, compound 5 does not show the same. Moreover, the intercalation of organic moieties in the interlayers of the bentonite by the ion exchange method is possible only if the molecule is positively charged. Synthesis and Characterization of QHQCalBen. The support material, bentonite, used for the development of the hybrid system QHQCalBen is economical and environmentally benign. QHQC-intercalated bentonite clay (QHQCalBen) was synthesized via the cation-exchange method by treating QHQC 7 with Na-Ben in acetonitrile at room temperature for 24 h, and the organic−inorganic hybrid was obtained as a violet-colored solid (Figure 1) which was characterized using FT-IR spectroscopy, powder XRD, TG/DTA techniques, and solid state absorption and emission studies.

Figure 2. XRD patterns of Na-Ben and QHQCalBen.

Na-Ben, and successful intercalation of QHQC moieties into the bentonite gallery has taken place. TG/DTA analysis provided a simple method to measure the content of organic molecules and physisorbed water in the intercalated bentonite. Na-Ben showed two major peaks [SI, Figure S2 (a)] around 87 and 704 °C, corresponding to the loss of physically adsorbed water and dehydroxylation of bentonite, respectively. The decomposition of QHQCalBen exhibited peaks at 64, 313, 625, and 731 °C. For Na-Ben, there is no obvious mass loss in the range from 200 to 600 °C. Thus, the mass losses between 200 and 600 °C are attributed to the organic molecules intercalated in the interlayers of bentonite. Peaks at 313 and 625 °C are assigned as the decomposition of intercalated QHQC from QHQCalBen. Furthermore, the melting point of QHQC was found to be 180 °C. The decomposition of QHQC when intercalated in bentonite galleries takes place at elevated temperature compared to the organic molecule itself. Consequently, confinement of organic molecules into bentonite contributed to its enhanced thermal stability [SI, Figure S2 (b)]. To give more evidence for successful intercalation, FT-IR analysis was performed. FT-IR spectra of QHQC, Na-Ben, and QHQCalBen are shown in Figure 3. QHQC and QHQCalBen showed major peaks at 2955, 2358, and 1480 cm−1. The peak at

Figure 1. Photographs of (a) Na-Ben and (b) QHQCalBen.

The X-ray diffraction (XRD) patterns of Na-Ben and QHQCalBen are shown in Figure 2. Na-Ben shows a diffraction peak which corresponds to the basal spacing (d-spacing) value (d(001)) of 13.4 Å. The QHQCalBen sample exhibited a peak at a d value of 30.7 Å which is greater than the d-spacing of NaBen by 17.3 Å indicating the successful intercalation of QHQC 7 into the bentonite gallery. The peaks present in the hybrid which are the same (or small shift in the position) as those of Na-Ben suggests that Na-Ben is not completely converted to QHQCalBen. The energy minimization studies of molecule 7 using ChemBio3D Ultra 11.0 software showed that this molecule has an approximate length of 17 Å (SI, Figure S1). From the XRD patterns of organoclay, it is evident that the interlamellar space has been increased by 17.3 Å from the original value of

Figure 3. IR spectra of QHQcalBen, Na-Ben and QHQC. 6971

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Figure 4. Solid state absorption spectra of QHQC and QHQCalBen.

Figure 5. Solid state emission spectra of QHQC and QHQCalBen at excitation wavelengths (a) 367 nm and (b) 570 nm.

3621 cm−1 is attributed to the structural hydroxyl stretching vibrations, and the peak at 3440 cm−1 is assigned to the OH stretching vibration of the adsorbed water. Bands observed around 1633 cm−1 are due to the bending vibration of OH group in water molecules. The peak around 1035 cm−1 is assigned to Si−O−Si stretching vibrations. The band at 794 cm−1 is attributed to the presence of silica. In the case of the organo-modified bentonite sample, the profile in the OH stretching region showed clear perturbation, which is due to the displacement of the sodium ions (along with its hydration shell) by the organic moiety. The band at 3621 cm−1 is due to the inner OH groups inaccessibility to QHQC. Additional welldefined absorption bands are observed at approximately 2955, 1480, and 2358 cm−1 and are attributable to sp3−CH stretching, −CH2 bending, and −C−N stretching, respectively. From this information, it is unambiguous that the organic moiety is intercalated into the bentonite gallery. Moreover, we noticed that there were no significant changes in the peaks of QHQCalBen compared to those of pure QHQC, which indicates that the conformation of QHQC is unaffected in the hybrid state. The loading of QHQC into the bentonite gallery for the formation of the organic−inorganic hybrid system was determined by TGA (SI, Figure S3), and it was found to be 19 wt %. The solid state absorption and emission studies proved that the hybrid system, QHQCalBen, exhibited significant absorption and emission which originated from the organic moiety intercalated in the bentonite gallery. The

absorption spectra of QHQC and QHQCalBen in the solid state (Figure 4) show that the peak at 577 nm for receptor 7 is blue-shifted to 563 nm in the hybrid system. However, in the case of solid state emission (Figure 5), upon excitation at 367 nm, the peak around 426 nm of 7 is red-shifted to 482 nm for the hybrid. When the excitation wavelength was changed to 570 nm, the emission band at 702 nm for 7 is blue-shifted to 680 nm in the hybrid system. The construction of a heterogeneous system might have caused changes in the chemical environment due to the interaction of the organic moiety with the inorganic moiety and hence the reported shifting of absorption and emission peaks. These data further confirmed the intercalation of the macrocyclic molecule into the bentonite. To facilitate the aggregation behavior of QHQC when intercalated into galleries of bentonite, we carried out transmission electron microscopic analysis. The images of TEM revealed that QHQC molecules have spherical morphology and are not in the aggregated state. However, the morphology of QHQCalBen showed that QHQC molecules are aggregated as a result of intercalation of QHQC into the bentonite galleries (SI, Figure S20). Metal Ion Binding Studies. In order to dtermine the efficacy of our organic−inorganic hybrid system as a solid state colorimetric sensor, it is necessary to scrutinize the properties of the QHQC alone. Hence, its ion recognition properties were explored using various techniques such as absorption, ESI/ HRMS, and 1H NMR experiments and most importantly by visual detection. The binding affinity of QHQC 7 toward 6972

DOI: 10.1021/acssuschemeng.7b01158 ACS Sustainable Chem. Eng. 2017, 5, 6969−6977

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ACS Sustainable Chemistry & Engineering different metal ions was investigated by UV−visible spectroscopy. The absorption spectrum of QHQC exhibited a strong absorption band at 256 nm and a weak broad band at 567 nm in an acetonitrile medium. In order to obtain an insight into the receptor ability of 7 toward metal ions, such as Mn2+, Fe2+, Al3+, Co2+, Ni2+, Cu2+, Zn2+, Hg2+, Ag+, Ba2+, Cd2+, Na+, Mg2+, Cr3+, and Ca2+, the perchlorates of metals were added to the acetonitrile solution of 7, and UV−vis spectral changes were investigated (Figure 6). QHQC showed a pronounced

Figure 7. Photograph of naked eye detection of Hg2+ with QHQC and its reversibility with iodide solution.

To disclose the nature of the interaction of QHQC with Hg2+, 1H NMR titrations (SI, Figure S7) were carried out. When it was treated with one equivalent of Hg2+, there were only minimal upfield shifts in the δ values of quinaldine ring protons (0.018−0.010 ppm). The −OCH2 protons attached to a quinaldine ring, and −N−CH3 protons also recorded minute upfield shifts (0.008 ppm) indicating the formation of an Hg2+ complex through the oxygen connected to quinaldine moiety. When more equivalents of Hg2+ ions were added, the pattern of the spectrum remained the same, suggesting that only 1 equiv of mercury was coordinated to QHQC. These shifts were unable to differentiate from the NMR spectra at a glance, and expansion of titration spectra (SI, Figures S8 and S9) facilitated understanding of these chemical shifts. The absence of alterations observed in the chemical shift of other protons in the macrocyclic molecule excluded the interaction of other oxygens attached to the arene cavity of calixarene derivative 7. When an iodide solution (3 equiv) was added to the above system, Hg2+ detached from the host molecule, and it instantly regained the original spectrum. From these evidences, it is obvious that Hg2+ binds only weakly to the organic molecule thereby contributing to the reversibility of the system. Reversibility is applicable for more than five cycles with the actual sample. The binding constant of Hg2+ with QHQC was calculated based on 1H NMR titration data by a nonlinear curve fitting procedure for the guest-induced chemical shift for selected peaks using commercially available GraphPad Prism software,58 and the association constant was found to be 1852 M−1 (SI, Figure S19). To substantiate the stoichiometry of the guest− host complex formed, we carried out a modified Job’s plot experiment using 1HNMR titration data, and from the binding isotherm (SI, Figure S20), the maxima were obtained at 0.5 mole fraction and the stoichiometry was confirmed as 1:1. In order to confirm the stoichiometry of the complex, electron spray ionization mass spectroscopic (ESI-MS) titrations of QHQC with Hg2+ were carried out. Upon addition of 1 equiv of Hg2+, a molecular ion peak at m/z of 1274.8008 was obtained which could be assigned to the formation of the 1:1 complex. However, when the titration was performed using more equivalents of Hg2+, additional molecular ion peaks were not observed. Furthermore, the isotopic peak pattern observed for the molecular ion peak (SI, Figure S10) authenticates the presence of mercury(II) ions, and thus, established the 1:1 binding of QHQC with Hg2+. The reversibility of the system

Figure 6. Absorption spectra of QHQC with different metal ions in CH3CN. Inset shows the magnified absorption peak at 567 nm.

selectivity toward Hg2+ ions which could be recognized by the disappearance of the absorption peak at 567 nm along with the enhancement in intensity of the peak at 256 nm upon addition of Hg2+ ions. In the case of Al3+ ions and Fe2+ ions, even if the peak at 567 nm vanished, a visual color change was not observed. Other metal ions have no effect on the absorption spectrum of QHQC. QHQC showed good selectivity toward Hg2+ in the presence of interfering ions like Ba2+, Mn2+, Ni2+, Zn2+, Mg2+, Ca2+, Ag+, Co2+, Na+, etc. The UV−visible absorption spectra of QHQC with Hg(II) titration (SI, Figure S6) showed that no isosbestic points were formed, but the peak at 567 nm disappeared and the intensity of the peak at 256 nm increased. The detection limit of Hg2+ was calculated as three times the standard deviation of the background noise and was found to be 2.95 × 10 −6 M (SI, Figure S5). The response time of the quinaldine-functionalized calix[4]arene receptor (QHQC) is very fast, and within seconds, recognition of Hg2+ takes places. One important aspect of the system is that it could be used for the naked eye detection of Hg2+ ions and its reversibility by the addition of iodide ions. The violet-colored QHQC 7 (9 × 10−6 M in ACN) changed its color to light brown by the addition of Hg2+ (1 × 10−6 M in ACN), and the absorption peak at 567 nm disappeared. However, when iodide (6.8 × 10−6 M in ACN) solution was added to the above system, the violet color reappeared immediately, and the absorption peak at 567 nm reemerged. Further addition of Hg2+ ions to the above solution revealed the repeatability of the system (Figure 7). For a chemical sensor to be widely employed for the detection of specific analytes, reversibility of the system is the most essential aspect, and consequently, QHQC 7 is a good candidate for the naked eye detection of Hg2+ ions. 6973

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Figure 8. Optimized structures of (a) QHQC and (b) QHQC-Hg2+ complexes using DFT method.

was further proved by the addition of an iodide solution and conducting ESI/MS analysis. Consequently, a peak at m/z of 1077.6353 (SI, Figure S11) was obtained that corresponded to the [M + H]+ peak of the receptor molecule 7, confirming the release of Hg2+ from the complex. When Hg2+ ions were again added to the above solution and carried out mass analysis, the peak at m/z of 1274.8263 [QHQC+Hg2+] reappeared which revealed the repeatability of the system to recognize Hg2+ ions. The binding energy of Hg2+ with QHQC was calculated using a density functional theory method and was found to be 32.3 kcal/mol. Also, the reaction is exergonic by 14.0 kcal/mol. The optimized structures of QHQC and the QHQC-Hg2+ complexes using the B3LYP/Gen59,60 level density functional theory method as implemented in Gaussian0961 are shown in Figure 8. Gen indicates a basis set comprising LanL2DZ for (Hg, I) and 6-31G* for (N, C, O, H). The O···Hg···O angle is 158.4°. The geometry adopted by Hg2+ in the complex is distorted octahedra; four oxygens from two perchlorate moieties occupy the equatorial plane, and two oxygens linked with the quinoline moieties occupy the axial positions. The reason for the violet color of compound 7 (QHQC) is presumably due to the electron transfer process (charge transfer)62−66 from the oxygen atom (oxygen attached to quinaldine moiety) to the quinaldine ring of QHQC. When Hg(II) binds with QHQC through its etherial oxygen (oxygen attached to quinaldine moiety), the quenching of the electron transfer process from an oxygen atom to a quinaldine ring occurs as a consequence of the formation of the 1:1 Hg2+− QHQC complex. This leads to a color change from violet to light brown. When an iodide solution was added to the above solution, Hg2+ was easily released from the complex. This may be due to the formation of a more stable HgI2 compared to a weak Hg2+−QHQC complex. The process is reversible, and reusability of the technique was examined by carrying out the experiment several times using Hg2+ and iodide solution. The schematic representation of a plausible mechanism is shown in Scheme 2. On the basis of the stimulating properties of the macrocyclic receptor (QHQC) and its rapid detection of Hg2+ ions, the organic−inorganic hybrid system QHQCalBen was assessed for its credibility as a naked eye sensitive colorimetric sensor for Hg2+ ions. The response of the hybrid system toward the

Scheme 2. Plausible Mechanism of Naked Eye Detection of Hg2+ with QHQC and Its Reversibility with Iodide Solution

selective sensing of hazardous Hg2+ ions was studied by adding Hg2+ ions (1 × 10−6 M) to 2 mg of QHQCalBen. Immediately, the color of the solid sample changed from violet to light brown. When iodide solution was added to the above system, the violet color of the hybrid reappeared (Figure 9) and is found to be proficient to bind Hg2+ ions recurrently. This shows that the hybrid system also displays properties analogous to QHQC in acetonitrile. Additionally, the sensor exhibits naked eye detection in an ACN/H2O (1:9, v/v) system comparable to acetonitrile. Upon complexation, bentonite has

Figure 9. Photographs of naked eye detection of Hg2+ with QHQCalBen and its reversibility with iodide solution. 6974

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ACS Sustainable Chemistry & Engineering no role; to be precise, it is the property of QHQC which is intercalated into the bentonite galleries. Here, bentonite acts only as a support material for the receptor, QHQC, and holds it in its interlayers and imparts characteristic functionality to the hybrid. Furthermore, QHQC was not extracted from QHQCalBen during complexation. Consequently, the mechanism of naked eye detection of Hg2+ and its reversibility with an iodide solution is identical for both QHQC and QHQCalBen. Moreover, we carried out the powder XRD of QHQCalBen after the detection of mercury (SI, Figure S4). The results showed that after the incorporation of mercury into QHQCalBen, the peak of QHQCalBen was shifted toward a higher angle with a d-spacing value of about 20 Å less than the original value of QHQCalBen. This confirmed the binding of Hg2+ in the QHQCalBen. QHQCalBen showed simlar complexation properties as that of QHQC. When the detection limit of QHQCalBen using different concentrations of Hg2+ ions was checked, it was found that it could recognize the Hg2+ ions up to micromolar concentration. It was also observed that the solid state sensor did not show any considerable response when similar experiments were conducted with other metal ions. Accordingly, QHQCalBen can be used as a selective and reversible solid state sensor for Hg2+ recognition. Owing to the inexpensive and environmentally benign supporting material, bentonite, and production of QHQCalBen in a cost-effective fashion, the hybrid sensor is inferred as economically affordable. In addition, this solid state sensor is a violet-colored powder and not sensitive to different environments. As a result, this bentonite-hybrid sensor is deduced as portable. Due to the reversibility of the system with an iodide solution and constancy even after consecutive experiments, QHQCalBen is established as a reusable hybrid sensor devoid of altering the recognition properties. In conclusion, we have successfully synthesized and characterized a new lower rim-functionalized 1,3-di(quaternary ammonium salt of 8-hydroxyquinaldine) derivative of p-tertbutylcalix[4]arene (QHQC) as a cone conformer and analyzed its sensing properties toward Hg2+ ions using various techniques such as absorption, visual color change, ESI/ HRMS, and 1H NMR experiments. Our results revealed that the synthesized molecule is highly capable of recognizing Hg2+ ions selectively through naked eye detection following a marked color change from violet to light brown. The molecule acts as a reversible chemosensor as it regains a violet color in the presence of iodide ions, and the detection limit of Hg2+ was found to be 2.95 × 10 −6 M. In view of these findings, an organic−inorganic hybrid system was synthesized by intercalating QHQC into the bentonite galleries and characterized using FT-IR spectroscopy, powder XRD, TG/DTA techniques, and solid state absorption and emission studies. This hybrid system successfully performed as a colorimetric solid state Hg2+ sensor and showed properties analogous to QHQC. In addition, the sensor is thermally stable, environmentally benign, economically affordable, reversible, portable, and reusable.

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Materials and methods; synthesis of di(bromopropyl)ptert-butylcalix[4]arene, precursor 3; synthesis of 8hydroxyquinaldine derivative of p-tert-butylcalix[4]arene (QHQC), precursor 5; general procedure for the metal ion binding studies; energy minimization diagram of QHQC; TG/DTA curves of Na-Ben and QHQCalBen; calculation of % loading of QHQC in QHQCalBen by TGA; XRD pattern of QHQCalBen after Hg2+ sensing; experiment to determine limit of detection; calculation of LOD; UV−visible absorption titration of compound 7 (QHQC) with Hg2+ ions; 1H NMR titration spectrum of QHQC and its expansions; isotopic peak pattern of molecular ion peak of [QHQC + Hg2+ ]; mass spectrum obtained after reversibility test with iodide solution; NMR spectra; isotherm resulting from the titration data for the calculation of binding constant; modified Job’s plot for the determination of stoichiometry; TEM images; supporting information for DFT method. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: 0471-2515275. E-mail: lux_varma@rediffmail.com. Fax: 0471-2491712. ORCID

Cherumuttathu H. Suresh: 0000-0001-7237-6638 R. Luxmi Varma: 0000-0001-9428-6369 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the CSIR Network project CSC-0135 for financial assistance. R.J.M. thanks UGC, New Delhi, for a research grant. We thank Mrs. Saumini Mathew, Mrs. S. Viji, Mr. Saran P. Raveendran, Mr. Rakesh Gokul, and Ms. Aathira S. of CSIR-NIIST, Trivandrum, for recording NMR and mass spectra. We also thank Mr. Kiran Mohan of CSIR-NIIST, Trivandrum, for transmission electron microscopic analysis.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01158. 6975

DOI: 10.1021/acssuschemeng.7b01158 ACS Sustainable Chem. Eng. 2017, 5, 6969−6977

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