FOCUS REVIEW DOI: 10.1002/asia.201400024

Calixarene-Based Chemosensors by Means of Click Chemistry Miaomiao Song,[a] Zhongyue Sun,[a] Cuiping Han,[a] Demei Tian,[a] Haibing Li,*[a] and Jong Seung Kim*[b]

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Abstract: Click chemistry, a new strategy for organic chemistry, has been widely used in the chemical modification of calixarenes because of its reliability, specificity, biocompatibility, and efficiency. Click-derived triazoles also play a critical role in sensing ions and molecules. This in-depth review provides an overview of calixarene-based chemosensors that incorporate click-derived triazoles, and their three characteristics (chromogenic, fluorescence, and wettability) are reviewed. Keywords: calixarenes · click chemistry · fluorescence · sensors · triazoles

Introduction

groups/moieties to the upper or lower rim of calixarenes. In this study, it is found that the click reaction is not only an effective coupling method, but also the triazole heterocycle formed during the reaction acts as a good recognition site, because it can coordinate to metal cations and also identify anions through hydrogen bonding.[14] We have been committed to the development of calixarene-based chemosensors by click chemistry and have made much progress, which has not yet been summarized. Herein, we provide an overview of calixarene-based chemosensors that incorporate click-derived triazoles. The emphasis here is not the click synthesis of calixarene triazoles, but rather the sensing of ions and molecules. The triazoles play a critical role in the sensing of ions and molecules and cannot be easily replaced by other functional groups. This review is organized into three sections: chromogenic, fluorescence, and wettability of calixarene-based chemosensors.

Calixarene-based chemosensors have been widely studied as a highly selective and sensitive detection technology. Compared to other molecular systems, calixarenes have some advantages such as a hydrophobic cavity and easy functionalization at the upper or lower rims; moreover, the flexible core can be modified for well-defined substrate binding.[1] Recently, we have reviewed calixarene-derived fluorescent probes,[2] the applications of hyperbranched calixarenes,[3] the recognition of amino acids by functionalized calixarenes,[4] and the host–guest sensing by calixarenes on surfaces.[5] These reviews demonstrate the importance of calixarenes in the construction of chemosensors. Efficient coupling methods are the key to constructing calixarene-based chemosensors. For this purpose, many coupling methods have been reported such as 1) Schiff base reactions to form C=N bonds that can bind to metal ions and act as chromophores,[6] 2) nucleophilic substitution reactions to form C O bonds (crown ethers) that can bind to metal ions,[7] and 3) amidation to form C N bonds that can recognize anions.[8] Thus, the development of new coupling reactions is the key to designing calixarene-based chemosensors. Since Sharpless et al. developed click chemistry as a new coupling strategy in 2001,[9] there has been a significant growth in the derivatization of calixarenes owing to its reliability, specificity, biocompatibility, and efficiency. It has been proven to be a promising strategy for the chemical modification of calixarenes. In 2005, Zhao et al. applied click chemistry to the synthesis of water-soluble calixarenes,[10] which laid a solid foundation for the synthesis of calixarene derivatives by means of click chemistry. Click chemistry has also been used to synthesize calixarene conjugates of chromophores and bioactive molecules such as glycosides,[11] sialoclusters,[12] and amino acids.[13] Because of the highly selective nature of the alkyne–azide cycloaddition, the click reaction is a general method to introduce various functional

1. Calixarene-Based Chromogenic Chemosensors Owing to the rapid operation, nondestructive analysis, and multicomponent spectrophotometric analysis, click-derived calixarene-based chromogenic sensors have become increasingly valued. The click reaction has not only been used as an efficient coupling method; the click-derived triazoles can also be used as binding sites for ions to construct selective chemosensors. For example, Chung and co-workers[15] designed and synthesized a triazole- and azo-coupled calixarene 1 as a highly sensitive chromogenic sensor for Ca2 + and Pb2 + ions, in which they coupled p-anisidine into the upper rim by a diazo coupling reaction and 1-(azidomethyl)benzene into the lower rim by means of a click reaction (Scheme 1). In the presence of Ca2 + and Pb2 + ions, the chromogenic sensor showed a bathochromic shift. The intensity of the absorption maximum (lmax) decreased, and a new absorption band appeared at approximately 527 nm. The Job plot experiment showed a 1:1 binding ratio of calixarene 1 with the Ca2 + ion. The complexation mode was characterized by the 1H NMR spectrum, thus indicating that the Ca2 + ion is bound to the two nitrogen atoms of the triazole units and the two hydroxyl (azophenol) groups of 1. Furthermore, the changes in the 1H NMR spectra showed that the Ca2 + ion disrupted the symmetry of the host molecule after the complexation, whereas the Pb2 + ion formed a symmetrical metal complex. The chromogenic sensor showed selective coloration for Ca2 + and Pb2 + ions.

[a] M. Song, Dr. Z. Sun, C. Han, D. Tian, Prof. H. Li Key Laboratory of Pesticide and Chemical Biology (CCNU) Ministry of Education, College of Chemistry Central China Normal University Wuhan, 430079 (P.R. China) E-mail: [email protected] [b] Prof. J. S. Kim Department of Chemistry, Korea University Seoul 136-701 (Korea) E-mail: [email protected]

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On the basis of the above study, Chung and co-workers[16] designed and synthesized another specific and ratiometric chemosensor for Hg2 + ions. A series of o-methoxyphenylazo calixarene derivatives were synthesized. In the presence of metal ions, calixarene 2 with distal bis-azophenol units showed a specific selectivity for Hg2 + ions. The lmax of 2 not only showed a bathochromic shift, but also a hyperchromic

effect. The color of the solution of 2 changed from pale yellow to beige upon adding Hg2 + ions as shown in Figure 1. Thus, calixarene 2 with o-methoxyphenylazo and triazole groups can be used as a naked-eye sensor for Hg2 + ions. The complexation between 2 and Hg2 + ions was further studied by a Job plot experiment, which showed that the binding ratio between 2 and the Hg2 + ion is 1:1. The

Miaomiao Song is working for her masters degree under the supervision of Prof. Haibing Li in the Department of Chemistry at Central China Normal University in Wuhan, China. Her current scientific interest is focused on the design and synthesis of calixarene chemosensors.

Cuiping Han received her Ph.D. from the Department of Chemistry at Central China Normal University in 2012. Since then she has joined the faculty of Xuzhou Medical College. Her research interest focuses on designing supramolecular-assembled nano-interface materials for molecular recognition and imaging.

Zhongyue Sun is currently a Ph.D. candidate at the Department of Chemistry at Central China Normal University. Her current scientific interest is focused on the design and fabrication of calixarene-assembled nanochannels.

Demei Tian received her Ph.D. from the School of Materials Science & Engineering at Wuhan University of Technology in 2008. Currently, she works at the Department of Chemistry at Central China Normal University as an associate professor. Her research interest focuses on the synthesis of calixarene chemosensors by means of click chemistry.

Haibing Li received his Ph.D. from the Department of Chemistry at Wuhan University. After a one-year postdoctoral fellowship at the Universit Joseph Fourier in France, he joined the faculty at Huazhong University of Science and Technology in 2004. In 2006, he then moved to Central China Normal University in Wuhan as a professor. His research interest focuses on supramolecular chemistry on surfaces.

Abstract in Chinese:

Jong Seung Kim received his Ph.D. from the Department of Chemistry and Biochemistry at Texas Tech University. After a one-year postdoctoral fellowship at the University of Houston, he joined the faculty at Konyang University in 1994 and then transferred to Dankook University. In 2007, he moved to the Department of Chemistry at Korea University in Seoul as a professor. To date, his research has been recorded in 310 scientific publications and 50 domestic and international patents.

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3 toward anions was studied. The addition of F , AcO , and H2PO4 anions resulted in a redshift in the UV-visible spectra of 3. Because of the different colors produced when 3 was treated with either the Ca2 + or F ion, the molecular logic gate was possible, as shown in Scheme 2. It is of interest to incorporate both triazole spacers and ester groups into one calixarene because the coordination of alkali-metal ions is promoted by OCH2COOR residues. Thus, Li and co-workers designed and synthesized triazole-modified calixarene 4 by the 1,3-dipolar cycloaddition of azide esters

Scheme 1. Complexation mode between 1 and the Ca2 + ion.

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H NMR spectrum of complex 2·Hg2 + showed that the Hg2 + ion bonded to the two nitrogen atoms of the triazole units to form a symmetrical conformation, and the two distal hydroxyl azophenol groups also participated in the binding of the Hg2 + ion by means of electrostatic interaction as shown in Figure 1. The development of nakedeye chemosensors capable of recognizing and sensing anions and cations is one of the most challenging fields in supramolecular chemistry.[17] Chung and coworkers synthesized a bifunctional chromogenic calixarene 3 that exhibited an INHIBIT logic gate with a YES logic function by using Ca2 + and F ions as the chemical inputs.[18] The metal-ion binding property of 3 was evaluated with perchlorate salts of Li + , Na + , K + , Mg2 + , Ca2 + , Ba2 + , Ag + , Zn2 + , Pb2 + , Ni2 + , Hg2 + , Mn2 + , and Cr3 + ions. From the UV-visible spectra, triazole-azophenol derivative 3, which contains triazoles as the metal-ligating groups, showed remarkable selectivity toward Ca2 + , Pb2 + , and Ba2 + ions. The color of the solution of 3 changed from green to bright yellow upon adding these ions. In addition to the metal-ion binding properties, the sensing properties of

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Figure 1. Chromogenic sensor for the Hg2 + ion based on 2 (reprinted from Ref. [16] with permission).

Scheme 2. Ca2 + - and F -switched INHIBIT logic gate by calixarene 3.

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with alkynylcalixarene.[19] The cooperative complexation ability of the triazoles and ester groups towards alkali-metal ions was investigated. Alkali-metal ions were selectively detected in the presence of picrate ions in the aqueous phase as shown by the UV spectra of 4. The percentage extraction of alkali-metal ions showed that 4 exhibits high selectivity towards Cs + ions.

2. Calixarene-Based Fluorescent Chemosensors Compared to spectrophotometry, fluorometry is considered to be a superior and more sensitive technology for the analysis of ions and molecules. In recent years, researchers have found that click chemistry can easily couple photoelectric groups to calixarenes. Thus, various calixarene-based fluorescent sensors have been designed and synthesized by using click chemistry. Fluorophores such as anthracene, pyrene, pyridine, Schiff base, naphthalene, and quinoline have been efficiently coupled to the upper and lower rims of calixarenes by means of click reactions and showed good performance as fluorescent chemosensors.

Scheme 4. Tentative mode of complexation of Ag + ions by 6.

metal perchlorates, only Ag + ions resulted in a significant fluorescence intensity enhancement of 6. The competitive interaction of 6 with Ag + ions in the presence of other metal ions at 100 mm concentration was further studied. The fluorescence intensity of 6·ACHTUNGRE(Ag+)2 was marginally quenched (30 %) only by adding Cu2 + ions, whereas no significant variation was observed by adding other metal ions. The possible binding mode of 6 with Ag + ions was investigated by 1 H NMR spectroscopic titration. The results indicated that two Ag + ions were chelated by the two distal enaminones and triazoles, and both the complexations were also assisted by the cation–p interactions through the phenoxy rings.

2.1. Anthracene Calixarenes as Fluorescent Chemosensors Anthracene, a common fluorescent compound, has been coupled to the upper or lower rim of calixarenes by means of click reactions. Chung and co-workers synthesized calixarene 5, which contained a crown ether and triazoles as metalion binding sites from 1,3-alternate calixcrown and 9-(azidomethyl)anthracene.[20] The fluorescence of 5 was strongly quenched by Pb2 + ions, whereas the revival of emission from strongly quenched 5·Pb2 + complex could be achieved by the addition of K + ions. The 1H NMR spectra of 5·Pb2 + and 5·K + complexes showed that the K + ion was bound to the crown-5 ring, whereas the Pb2 + ion was bound to the OCH2 triazole unit, as shown in Scheme 3. Thus, 5 acts as a novel fluorescent off/on switchable chemosensor. Chung and co-workers designed and synthesized 1,3-alternate calixarene 6 as a fluorescent chemosensor for Ag + ions.[21] As shown in Scheme 4, in the presence of various

2.2. Pyrene Calixarenes as Fluorescent Chemosensors Pyrene has recently found use in fluorescent chemosensors. Kim and co-workers reported a novel calixarene-based fluorescent chemosensor for the detection of Cd2 + and Zn2 + ions.[22] Calixarene 7 was synthesized from a p-tert-butylcalixarene with terminal alkyne groups and azidopyrenes by means of a click reaction. The addition of Zn2 + or Cd2 + ions to the solution of 7, which bears pyrene–triazoles as the metal-ligating groups, in CH3CN resulted in a remarkable fluorescence enhancement. The 1H NMR spectra of the 7·M2 + (M2 + = Cd2 + or Zn2 + ion) complex indicated that the coordination between the M2 + ion and OCH2 triazole resulted in the configuration change of the pyrene groups, as shown in Scheme 5. Chu-Luo and co-workers independently reported the synthesis of calixarene 7 by the click reaction of 1-(bromomethyl)pyrene, calixarene, and NaN3, and detected the fluorescence response of 7 toward various metal ions.[23] The addition of Cu2 + , Hg2 + , or Pb2 + ions resulted in a remarkable decrease in the fluorescence intensities of both excimer and monomer. In contrast, the addition of

Scheme 3. Fluorescent on/off switchable chemosensor based on 5.

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Scheme 5. Fluorescent chemosensor 7 for sensing Cd2 + and Zn2 + ions. Scheme 7. Fluorescent chemosensor for Pb2 + ions based on 8. 2+

Zn ions resulted in an increase in the fluorescence intensity of the monomer emission and a concomitant decrease in the excimer emission. Because of the different fluorescence responses of 7 toward Cu2 + and Zn2 + ions, the INH (inhibition) and NOR (not or) logic gates were possible. The 1 H NMR spectroscopic chemical shifts of 7·Cu2 + and 7·Zn2 + complexes indicated that the Cu2 + ion was bound to the two triazole moieties and the oxygen atoms on the lower rim of the calixarene, and the two Zn2 + ions were bound to the two triazole moieties as shown in Scheme 6.

Scheme 6. Proposed different complexation behaviors of 7 with Cu2 + and Zn2 +

Yamato and co-workers synthesized a new type of fluorescent chemosensor based on homooxacalixarene 8 (Scheme 7).[24] The fluorescent sensor showed higher selectivity for Pb2 + ions than other metal ions as determined by the enhancement of the monomer emission of the pyrene. The selectivity for Pb2 + ions was further investigated by 1 H NMR spectroscopic titration in a mixture of CDCl3/ CD3CN (10:1 v/v). Only the triazole ring and OCH2 triazole protons of 8 showed significant chemical-shift changes, thus indicating that only the nitrogen atoms in the triazole moieties of 8 participated in the complexation with the Pb2 + ion.

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This symmetric calixarene might extend the applications of fluorescent sensors for sensing heavy-metal ions. Yamato and co-workers also reported that the fluorescence intensity of the excimer emission of 8 gradually decreased, whereas the monomer emission increased with the addition of increasing concentrations of Zn2 + ions.[25] Further, they studied the fluorescence behavior of the 8·Zn2 + complex toward various anions (Figure 2). Only the addition of H2PO4 ions enhanced the fluorescence intensity of the excimer emission relative to that of the 8·Zn2 + complex, whereas a slight change in the quenching of monomer and excimer emission was observed upon the addition of other anions. This system exhibited a novel molecular switching of the excimer emission of the pyrenes from the “on/off” to “off/on” type as shown in Figure 2. Thus, this receptor may be used in the sensing, detection, and recognition of Zn2 + and H2PO4 ions. On the basis of the above ions. study, Yamato and co-workers designed and synthesized novel fluorescent chemosensors 9 and 10 for sensing Ag + ions through pyrene–triazole thiacalixarenes with 1,3-alternate conformation.[26, 27] The addition of Ag + ions enhanced the fluorescence intensity of the monomer emission, whereas the corresponding excimer emission decreased. In contrast, the addition of Hg2 + and Cu2 + ions strongly quenched the excimer and monomer emissions of 9 and 10. Furthermore, the competitive interaction of Ag + ions in the presence of other cations was investigated; no significant interference in the detection of Ag + ions using 9 and 10 was observed. The 1H NMR spectra also showed that the Ag + ion was selectively bound to the nitrogen atoms on

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Figure 2. Molecular switching chemosensor in sensing Zn2 + and H2PO4 ions based on 8 (reprinted from Ref. [27] with permission).

Scheme 8. Proposed complexation mode between 11 and the Zn2 + ion.

the triazole rings. These chemosensors have great potential for use as valuable design tools for self-assembly in supramolecular chemistry and cation-binding sites of receptors.

sensor with a fluorescence off/on switch has been successfully synthesized. On the basis of the above study, Rao and co-workers designed and synthesized a lower-rim triazole-linked Schiff base calixarene 14 and determined its selectivity toward different metal ions (Scheme 9).[29] Receptor 12 exhibited a very weak fluorescence emission. However, the fluorescence intensity was enhanced upon the addition of Zn2 + ions, whereas no significant enhancement was observed in

2.3. Schiff Base Calixarenes as Fluorescent Chemosensors It is of great significance to introduce Schiff bases to calixarenes for ion recognition as well as host–guest chemistry. Schiff base calixarenes have been developed because of their good binding capacity toward metal ions demonstrated by appropriate changes in their absorption and emission properties. Schiff bases can be efficiently conjugated to cal-

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ixarenes through a click reaction. This is a new method for synthesizing calixarene-based fluorescent chemosensors. For example, Rao and co-workers synthesized a series of salicylaldimine-appended triazolelinked calixarene derivatives by using a click reaction and tested their selectivity for metal ions.[28] Calixarene 11 exhibited a weak emission at 440 nm when excited at 320 or 390 nm in methanol. The titration of 11 with Zn2 + ions in methanol showed a progressive enhancement in its fluorescence intensity. The 1H NMR spectroscopic titration of 11 with Zn2 + ions showed that the intensity of the Schiff base OH and CH2OH protons decreased, clearly indicating the binding of the Zn2 + ion inside the core of the Schiff base as shown in Scheme 8. The triazole moieties are not involved in the coordination of the Zn2 + ion; however, it serves as a bridge. Thus, a Zn2 + ion

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Scheme 9. Tentative complexation mode of Zn2 + by 12; fluorescence changes in 12 before and after the addition of Zn2 + ions (reprinted from Ref. [29] with permission).

the presence of other metal ions. Thus, 12 was found to be highly selective toward Zn2 + ions, thus leading to the development of a fluorescence-based chemical sensor for Zn2 + ions. Furthermore, the biological applicability of 12 in sensing Zn2 + ions has been evaluated. Almost no change was observed in the fluorescence intensity of the 12·Zn2 + complex in the presence of proteins or serum. Thus, this study demonstrated the efficiency of 12 in sensing Zn2 + ions and its biological applicability. Zn2 + ions act as the cofactors for a variety of phosphatases. Therefore, it is very important to develop a selective and sensitive chemosensor for Zn2 + ions. Rao and co-workers designed and synthesized a triazole-linked imino–thiophenyl calixarene 13.[30] First, the selectivity of 13 towards Zn2 + ions in the presence of 12 metal ions was studied. In a 1:2 4(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer/ethanol mixture at pH 7.4, only the addition of Zn2 + ions to 13 enhanced the fluorescence intensity, whereas no significant enhancement was observed in the case of other ions as shown in Figure 3. Because of the metal-ligating centers in metallothionein, the secondary sensing of thiols by 13·Zn2 + has been studied. The fluorescence titrations have been carried out by adding cysteine (Cys), dithiothreitol (DTT), glutathione reduced (GSH), mercaptopropionic acid (MPA), homocysteine (Hcy), mercaptoethanol (ME), and cysteamine (Cyst). Considerably high quenching was observed in the presence of Cys, DTT, and GSH, whereas the others exhibited only minimal changes. In the case of Cys or DTT, the Zn2 + ion was freed from the 13·Zn2 + complex. Thus, a sensitive and selective fluorescent receptor for Zn2 + ions has been constructed, and it might be used to mimic some critical steps of the metal detoxification and oxidative stress events in biology. Rao and co-workers also designed and synthesized imino– phenolic–pyridyl calixarene 14 as the primary fluorescence switch-on chemosensor for Zn2 + ions.[31] The addition of Zn2 + ions strongly enhanced the fluorescence intensity of 14. As shown in Figure 4, fluorescence titrations were carried out in the presence of different metal ions to study the

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Figure 3. Sensitive and selective fluorescent chemosensor for Zn2 + ions based on 13; fluorescence changes in 13 after the addition of various metal ions (reprinted from Ref. [30] with permission).

Figure 4. Fluorescent chemosensor for Zn2 + ions based on 14 (reprinted from Ref. [31] with permission).

selectivity of 14 to Zn2 + ions; no significant enhancement in the fluorescence intensity for other ions was observed. Because of the strong affinity of Zn2 + ions towards phosphate ions, the highly fluorescent 14·Zn2 + complex has been studied as a secondary sensor toward phosphate-based anions. The fluorescence intensity of the 14·Zn2 + complex was quenched in the presence of almost all the phosphate ions, and the maximum quenching was observed for inorganic pyrophosphates. Complex 14·Zn2 + showed no significant change in the fluorescence intensity upon the addition of other anions.

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quenched by the addition of Hg2 + ions in contrast to other metal ions. The competitive metal-ion titrations showed the highly selective recognition of 16 for Hg2 + ions. Fluorescence titrations were carried out to further study the selective recognition of 16 for Hg2 + ions. Based on the mole ratio method, the molar ratio between 16 and Hg2 + was determined to be 1:2. 1H NMR spectroscopic titration of 16 with HgACHTUNGRE(NO3)2 was carried out to determine the coordination sites between 16 and Hg2 + ions. The results showed that the Hg2 + ion might be located in the negatively charged cavity formed by the nitrogen-rich triazoles. In further studies, they found that the 16·Hg2 + complex can act as a secondary sensor in the selective recognition of cysteine among the naturally occurring amino acids. In particular, the fluorescence and 1H NMR spectroscopic titrations showed that the coordinated Hg2 + ions were being removed from the complex by cysteine to release free 16; thus, the 16·Hg2 + complex acts as a secondary sensor in the recognition of cysteine. Valeur and co-workers designed and synthesized calixarene 17 and showed the detection of Cd2 + and Zn2 + ions.[34] When a series of different concentrations of Cd2 + and Zn2 + ions were added to the solution of 17, the fluorescence intensity of 17 decreased with the appearance of a new band centered at 480 nm. Furthermore, an isoemissive point at 414 nm was observed. In the second regime that corresponded to the excess amount of cadmium with respect to the ligand, the isoemissive point no longer existed. In this study, the authors designed a novel 2’-pyridinyl-1,2,3-triazole chromophore as the fluorescent chemosensor for Cd2 + and Zn2 + ions.

2.4. Other Types of Calixarenes as Fluorescent Chemosensors Naphthalene, pyridine, and quinoline moieties have been conjugated to calixarenes by a click reaction. Li and coworkers designed and synthesized calixarene 15 from 8-propynoxyquinoline (POQL) and 25,27-bis(4-azidobutanyl)-4tert-butylcalixarene, a selective fluorescent sensor for Hg2 + ions.[32] The binding mode was determined from the 1 H NMR spectra, as shown in Scheme 10. The fluorescence

Scheme 10. Proposed complexation behavior of 15 with Hg2 + ions.

of 15 was found to be quenched at a lower pH. A NOR logic gate was designed based on the fluorescence quenching of 15 toward Hg2 + ions at a lower pH. Li and co-workers also synthesized another Hg2 + fluorescent sensor, a 1,3-alternate thiacalixarene 16 that bore four naphthalene moieties through crown-3 chains.[33] As shown in Figure 5, the fluorescence intensity of 18 was strongly

Chiral recognition plays an important role in many fields of science and technology.[35] Chiral discrimination has been achieved by using various methods such as chiral HPLC,[36] capillary electrophoresis,[37] fluorescence,[38] colorimetric analysis,[39] and electrochemistry.[40] However, the improvement of the sensitivity of chiral recognition remains a challenging task. Li and co-workers designed and synthesized a novel fluorescent calixarene 18 that bore a chiral 1,1’-bi-2naphthol group by means of a click reaction.[41] Calixarene 18 demonstrated highly selective binding toward Cu2 + ions (Figure 6), thereby resulting in remarkable fluorescence quenching. In particular, the 18·Cu2 + complex could be used as a fluorescent sensor for the enantioselective recognition

Figure 5. Highly selective chemosensor for Hg2 + ions based on 16 (reprinted from Ref. [33] with permission).

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showed a binding ratio of 1:1 between them. A more detailed study on the binding of 19 to p-nitroaniline was carried out by 1H NMR spectroscopic titration, which showed that the p-nitroaniline was located inside the hydrophobic cavity. Calcium is the most abundant metallic element in animals, particularly in the human body, and is an important nutrient that is essential for maintaining total body health. Li and coworkers designed and synthesized calixarene 20 that bore an anthraquinone moiety and triazoles by click reaction and used it as a selective fluorescent chemosensor for Ca2 + and F ions.[44] The binding behavior of 20 was evaluated by the addition of different metal cations. As shown in Figure 8a, the fluorescence intensity of 20 at 510 nm was negligible. However, the addition of Ca2 + ions significantly enhanced the fluorescence intensity of 20 at 510 nm. Furthermore, a redshift in the UV-visible spectrum was observed when the Ca2 + ion was added; moreover, the color of the solution changed from colorless to a buff color (Figure 8b). Furthermore, the selective recognition of the 20·Ca2 + complex toward anions was investigated. The addition of F ions quenched the fluorescence intensity of the 20·Ca2 + complex and a blueshift in the UV-visible spectrum was observed when the concentration of F ions reached about 2.2 equiv (Figure 8b). Therefore, the 20·Ca2 + complex might be further used as a chemosensor for F ions. Moreover, an INH logic gate has been operated based on this characteristic of the 20·Ca2 + complex.

Figure 6. Multianalyte chemosensor 18 (reprinted from Ref. [41] with permission).

of mandelic acid (MA) with a fluorescence “turn-on” mode. The addition of (R)- and (S)-MA showed a significant enhancement in the fluorescence intensity; however, the degree of enhancement in the fluorescence intensity by the two enantiomers was different. As shown in Figure 6, the 3. Calixarene-Based Wettability Chemosensors by fluorescence intensity of the 18·Cu2 + complex increased by Means of Click Chemistry 6.35-fold upon the addition of (R)-MA, whereas it increased by only 4.87-fold upon the addition of (S)-MA. This large The design of supramolecular systems provides an original difference in the fluorescence enhancement between the approach to nanoscience and nanochemistry.[3] A series of 2+ enantiomers makes the 18·Cu complex suitable for the calixarene-based chromogenic and fluorescent sensors have been designed and synthesized by click reactions and enantioselective recognition of chiral (R)-MA. showed high sensitivity and selectivity for metal ions. ReIn addition to ion recognition, the design of molecular recognition structures has attracted great interest in supramolecular chemistry.[42] Li and co-workers designed and synthesized a novel naphthyl calixarene 19 by click reaction and showed the high affinity and selectivity of 19 for nitroaniline isomers.[43] The fluorescence response of 19 to anilines was studied. As shown in Figure 7, the fluorescence intensity of 19 was significantly quenched upon the addition of p-nitroaniline. However, other aniline derivatives showed negligible effects on the fluorescence intensity. The fluorescence titration Figure 7. Structure of 19 and fluorescence intensity changes of 19 in CH3CN upon the addition of nitroaniline of 19 with p-nitroaniline isomers (reprinted from Ref. [43] with permission).

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cant changes in the CAs. This result was consistent with the selectivity of the electrochemical impedance. The addition of the methomyl solution increased the charge-transfer resistance (Rct) value from 4.97 to 9.82 KW; however, the Rct values were not affected by the other four carbamates. On the basis of the above study, a novel aldehyde calix[4]arene (C4AH) 23 was synthesized and used to form the SAMS by click reaction to afford a highly selective wettability and impedance sensor for arginine (Arg).[47] The CA images show that the CA of the gold intersurface changed from (135.2  2)8 to (22.3  2)8 after the addition of arginine, whereas the addition of other amino acids did not change the CAs of the gold interface. The success of the C4AH functional interface and Arg-responsive events Figure 8. Novel chemosensor 20 with derived logic gate (reprinted from Ref. [44] with permission). were also characterized by electrochemical impedance spectroscopy (EIS). Furthermore, the C4AH functional interface could be used as wettability cently, calixarenes have been incorporated into gold and siliresponsive cycles for at least eight times. This type of sensor con surfaces by click reactions to synthesize switchable wettis based on the high pKa value (12.48) of arginine. Thereability sensors. For example, Li and co-workers designed and synthesized 25,27-bispropargyloxycalix[4]arene 21 atfore, the arginine was able to selectively form strong hydrotached to a gold surface.[45] First, 3-azido-N-(2-mercaptoegen bonds with aldehyde, thereby resulting in a significant thyl)propanamide was attached to the gold surface through S Au bonds, and then 21 was attached by a click reaction. As shown in Figure 9, the functionalized gold surface selectively recognized paraquat by means of a wettability switch. Furthermore, a cycling experiment for the wettability switching between super-hydrophobicity and super-hydrophilicity was carried out by adding the paraquat alternately. Thus a wettability switch with a good selectivity for paraquat was successfully achieved. Li and co-workers also synthesized a calixarene lipoic acid functionalized gold surface to study its selectivity toward molecules.[46] Calixarene lipoic acid (C4LA) was synthesized from azide-functionalized calixarene and thioctic– propyne by a click reaction. Next, 22 was attached to the gold surface through S Au bonds. The C4LA self-assembled monolayers (C4LA SAMs) were characterized by X-ray photoelectron spectroscopy (XPS), contact angle (CA), and impedance spectroscopy; they showed high sensitivity and selectivity to methomyl. As shown in Figure 10, the addition of a methomyl solution changed the CA of C4LA SAMs Figure 9. Construction of the gold surface modified by calixarene by from (97.2  3)8 to (30.7  3)8, whereas san leafhopper, carmeans of click chemistry, and realizing the distinction of four paraquat bosulfan, carbofuran, and carbaryl did not show any signifianalogues (reprinted from Ref. [45] with permission).

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mine interacted with the two NO2 groups in 24. This system might be developed as a chip or materials for use in the environment, thus enabling significant practical applications. The development of a more direct, convenient, and universal technique has great significance. Li and co-workers designed and synthesized a novel calixazacrown (C4AC) 25 silicon surface as a switchable wettability sensor.[49] C4AC SAMs were constructed by click reaction of Si N3 with the C4AC. This system was characterized by XPS and CA measurements. The C4AC SAMs showed a high sensitivity and selectivity for [C4mim]Cl Figure 10. Functionalization of the gold surface by calixarene, and sensing for methomyl through wettability (C4mim = 1-n-butyl-3-methyli(reprinted from Ref. [46] with permission). midazolium). As shown in Figure 11, after immersing the modified silicon surface in the [C4mim]Cl solution, the CA changed from (154.6  3)8 to (10.1  3)8, whereas the CA of [C4mim]PF6, [C4mim]Br, or a chloride source such as NaCl did not show any significant change in the CA. The treatment of [C4mim]Cl with water for 5 min resulted in a hydrophilic surface, and then the CAs reverted to their original values. A cycling experiment further exhibited a good reversibility. The recognition of Hg2 + ions is very essential because of their extreme toxicity in the environment and food. Li and co-workers reported a novel cysteine (Cys) complex of piperidinecalix[4]arene 26 as a convenient and effective dualchange in the CA. Moreover, the 1H NMR and 2D NOESY signal responsive switch for Hg2 + ions.[50] This system exhibspectra proved that the aldehyde groups of C4AH interacted with the guanidine of the arginine. The proposed wettaited excellent selectivity toward Hg2 + ions in the CA bility sensing device demonstrated remarkable specificity images. The silicon surface changed from (47.3  2.0)8 to and offers an intuitive and convenient method, which should (122.6  2.0)8 after the addition of Hg2 + ions, whereas the be suitable for the production of arginine recognition chips presence of other metal ions did not show a significant that can be used in biology and medical science. change (Scheme 11). Moreover, the 1H NMR spectra and The structural isomers show a small difference in their the atomic force microscopy (AFM) images verified the instructures, and different isomers usually have different functeraction between 26 and Hg2 + ions. Furthermore, the Hg2 + tions. To achieve a good selectivity for the aniline analogues -responsive switch showed an important and potential appliof o-phenylenediamine, Li and co-workers synthesized a nication for a water CA on a functional micro-nano silicon trocalix[4]arene (C4N2) 24 SAMs–gold intersurface.[48] This surface, such as intelligent micro-fluidic and laboratory-onchip devices, controlled release drug-delivery systems, and system showed a significant response for o-phenylenediaself-cleaning surfaces. mine in terms of wettability. The CA of the 24 SAMs–gold intersurface changed from (139.5  1.0)8 to (98.2  1.1)8 after interacting with o-phenylenediamine; however, the addition of other isomers did not show significant changes in the CA Conclusion of 24. Furthermore, the success of o-phenylenediamine-responsive events was also characterized by EIS. The structurHerein, we have reviewed the fabrication of calixareneal features of the complexes formed between 24 and o-phebased chemosensors by means of the click reaction, which nylenediamine were characterized by their 1H NMR and 2D has been widely used as a powerful tool to introduce various chromophores and fluorophores to synthesize chromogenic NOESY spectra. The two NH2 groups in o-phenylenedia-

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action to fabricate wettability chemosensors. In particular, the nitrogen-rich triazole moieties derived by a click reaction can bind to various ions and molecules. We believe that this field of research will be more active in the future because the functionalized calixarenes have great potential as antiviral and anticancer agents. Calixarenes with biocompatibility can be designed and synthesized by a click reaction, and might play an important role in the development of biology as well. Moreover, functionalized calixarenes can be conjugated to nanomaterials to construct light-, pH-, and temperature-responsive materials or sensors by click reactions.

Acknowledgements This study was supported by The Ministry of Science, ICT & Future Planning (MSIP) of the National Research Foundation of Korea (no. 2009-0081566), the National Nature Science Foundation of China (21372092, 21072072, 21102052), the Program for New Century Excellent Talent in University (NCET-10-0428), and self-determined research funds of the CCNU from the colleges basic research and operation of MOE (CCNU11C01002, CCNU13F005).

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Figure 11. a) CA relationship images of a 25-modified micro-/nano-silicon surface with different guests. b) Contact-angle variation histogram for four guests. c) Ion pairs (reprinted from Ref. [49] with permission).

Scheme 11. Constructing a highly selective switch between Hg2 + ions and Cys (reprinted from Ref. [50] with permission).

and fluorescent chemosensors. Further, calixarene can be attached to the gold or silicon surface by means of a click re-

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FOCUS REVIEW I get a click out of you: Click chemistry has been widely used in the chemical modification of calixarenes because of its reliability, specificity, biocompatibility, and efficiency. This in-depth review provides an overview of calixarene-based chemosensors that incorporate click-derived triazoles, and their three characteristics (chromogenic, fluorescence, and wettability) are reviewed (see figure).

Click Chemistry Miaomiao Song, Zhongyue Sun, Cuiping Han, Demei Tian, Haibing Li,* Jong &&&&—&&&& Seung Kim* Calixarene-Based Chemosensors by Means of Click Chemistry

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Calixarene-based chemosensors by means of click chemistry.

Click chemistry, a new strategy for organic chemistry, has been widely used in the chemical modification of calixarenes because of its reliability, sp...
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