Evaluation of a new calix[4]arene based molecular receptorfor sensitive and selective recognition of F� and Cu2þ ions

Har Mohindra Chawla n, Tanu GuptaDepartment of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

a r t i c l e i n f o

Article history:Received 27 December 2013Received in revised form18 March 2014Accepted 3 April 2014Available online 12 April 2014

Keywords:UV–vis spectroscopyFluorescenceChemosensorCopperFluoride

a b s t r a c t

Design and evaluation of a single molecular receptor for multiple analytes reveal that calix[4]arenebased molecular receptor 4 shows a highly selective response towards Cu2þ and F� ions with detectionlimits of 0.5 mM and 0.7 mM respectively when examined through UV–vis, fluorescence and 1H NMRspectroscopy. Simultaneous binding studies on 4 towards metal ion and fluoride reveal that it exhibits anegative allosteric effect towards Cu2þ/F� .

& 2014 Published by Elsevier B.V.

1. Introduction

Design and synthesis of receptors for selective detection of ionsis an important area of current chemical research. Recently therehas been an upsurge in research activity to promote developmentof single chemosensors for multiple analytes as they are endowedwith faster analytical time and potential cost reduction [1–3].Ion recognition studies with such chemosensors require highlysensitive but simple spectroscopic tools. In this regard, opticalsensing methods such as UV–vis and fluorescence spectroscopy haveemerged prominent analytical methods because of their ability toprovide simple, sensitive, selective and precise methods for monitor-ing very low concentrations of target ions without any pre-treatmentof the sample [4,5]. In addition to these methods, 1H NMR spectro-scopy [6,7] is extensively employed for binding studies as it canprovide important information regarding the nature of complexationof the analyte and the synthesized molecular receptor.

Anions are fundamentally useful to a range of biological processesand construction of a potent anion binding receptor is thus significant.Considerable efforts have been dedicated to sensing of fluoride ionsvia UV–vis, fluorescence [8] and other spectroscopic techniques dueto its relevance to environment and human health care [9–11].Likewise, chemosensors for the detection and measurement ofCu2þ [12] have been actively investigated as it is not only a significantenvironment pollutant but also an essential trace element in the

human body. While deficiency of copper can result in anemia, itsaccumulation can lead to gastrointestinal catarrh, Wilson's disease,hypoglycemia, dyslexia and infant liver damage [13,14]. However,designing of selective copper receptors is all the more challenging asmost of the copper receptors suffer cross interference frommetal ionssuch as Zn2þ , Hg2þ , Pb2þ , Fe3þ and Agþ . Thus, designing selectivefluorescent chemosensors for copper and fluoride through onemolecular receptor has recently drawn worldwide attention [15,16].Although, a number of chemosensors have been reported for indivi-dual sensing of F� and Cu2þ [17], the probes which can exhibitobvious fluorescence response to both of them are rather rare [18]and their sensing properties especially selectivity remains unsatisfied.For example, a recently published report [19] on multifunctionalsalicylaldimine based colorimetric and fluorogenic receptors do notshow much selectivity with respect to the metal ion sensing.

Calix[4]arene is one of the most attractive synthetic macrocyclefor the design of fluorescent and dual sensing receptors as itsunique framework allows introduction of appropriate bindingcores suitable for ion recognition [20]. This results in a highlypreorganized architecture for the assembling of converging bind-ing sites. Depending upon the coordination units present, severalcalixarene based receptors for multiple analytes have been synthe-sized. For example, Kim and coworkers have reported a bifunc-tional fluorescent calix[4]arene chemosensor [21] for both Pb2þ

and F� . Xu and coworkers have reported another fluorescencebased chemosensor [22] for Cu2þ and F� .

Keeping the above perspectives in mind and taking forward ourresearch interests that include the synthesis and application ofcalixarene based molecular receptors for UV–vis and fluorescence

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/jlumin

Journal of Luminescence

http://dx.doi.org/10.1016/j.jlumin.2014.04.0040022-2313/& 2014 Published by Elsevier B.V.

n Corresponding author. Tel.: þ91 11 265 91517; fax: þ91 11 265 81102.E-mail addresses: hmchawla@chemistry.iitd.ernet.in,

hmchawla@chemistry.iitd.ac.in (H.M. Chawla).

Journal of Luminescence 154 (2014) 89–94

spectroscopy based detection [23] of metal ions and anions, wedescribe herein a new multifunctional molecular receptor contain-ing 2-oxo-1,2-dihydroquinoline units in the lower rim of calix[4]arene scaffold. The data collected by means of several spectro-scopic techniques like UV–vis, fluorescence and 1H NMR revealedthat receptor 4 is highly selective for F�and Cu2þ recognition.

2. Experimental

2.1. General

All the reagents used in the study were purchased from SigmaAldrich or Merck and were chemically pure. The solvents usedwere dried and distilled prior to use. HPLC grade solvents wereused for UV and fluorescence experiments. In the titration experi-ments, all the anions and metal ions were added in the form oftetrabutylammonium (TBA) salts and perchlorate salts respec-tively. 1H and 13C NMR spectra were recorded in CDCl3 on a300 MHz Bruker DPX 300 instrument using tetramethylsilane(TMS) at 0.00 as an internal standard. Mass spectrum wasrecorded on a Bruker Compass Data Analysis 4.0 Mass spectro-meter. A Perkin Elmer Lambda 35 double beam spectrophotometerwith variable bandwidth was used for the acquisition of theUV–vis molecular absorbance. Melting points were determinedon an electrothermal melting point apparatus obtained fromToshcon–Toshniwal (India) and were uncorrected.

2.2. Synthesis

The starting materials, 2-oxo-1,2-dihydroquinoline-4-carbonylchloride (2) and p-tert-butylcalix[4]arene (3) were synthesized byadopting procedures reported in the literature [24,25].

2.3. Synthesis of receptor 4

A mixture of 3 (1.5 g, 2.3 mmol) and acid chloride 2 (1.0 g,4.8 mmol) was refluxed with potassium carbonate (0.8 g, 5.8 mmol)

in acetonitrile for 12 h. The solution was filtered and solvent wasevaporated under vacuum. An extractive work-up with chloroformafforded the crude product, which was purified by column chromato-graphy (hexane/ethyl acetate, 9.5/0.5, v/v) to obtain 4 in pure form.Yield: 69%; Mp: 180–181 1C; UV (λmax, THF): 281 nm, HRMS (ESI-MS)m/z:calculated 1013.4711, found 1013.4680; 1H NMR (300MHz, CDCl3,δ in ppm): 13.91 (s, 2H, NH, D2O exchangeable), 8.48 (d, 2H), 8.35 (s,2H, OH, D2O exchangeable), 7.18 (m, 6H), 6.93 (dd, 2H), 6.81 (s, 4H),6.65 (d, 2H), 6.01 (s, 1H), 4.11 (d, 4H), 3.55 (d, 4H), 1.34 (s,18H), 0.955 (s,18H); 13C NMR (75MHz, CDCl3, δ in ppm) 30.862, 31.675, 32.052,33.997, 34.047, 77.202, 116.633, 116.846, 122.944, 125.417, 125.589,125.97, 126.64, 130.889, 138.273, 139.377, 141.878, 143.099, 149.268,150.091, 163.852, 164.594.

2.4. Spectrometric procedure

The stock solutions (1 mM) of compound 4 and tetrabutylammonium and perchlorate salts of anions and cations wereprepared in HPLC grade THF. 1.5 mL of the receptor solution wasthen diluted in a 50 mL volumetric flask to obtain 30 μM solutionof 4 to be used for spectroscopic studies. Absorbance and fluores-cence spectra were recorded by gradual addition of increasingamounts of ions to the receptor solution (30 mM).

3. Results and discussion

3.1. Synthesis

The synthesis of desired molecular receptor 4 was achievedthrough the reaction sequence depicted in Scheme 1. The reaction ofcalix[4]arene with 2-oxo-1,2-dihydroquinoline-4-carbonyl chloride (2)in acetonitrile and potassium carbonate gave compound 4 in acone conformation. 2 was obtained from corresponding 2-oxo-1,2-dihydroquinoline-4-carboxylic acid (1) by boiling with thionylchloride. 1, in turn was synthesized by adopting synthetic proce-dures reported in the literature [24].

NH

O

COOH

NH

O

COCl

(i)

1 2

HOOH OHOH

OO OHOH

HNNH

O

O O

3 4

(ii)O

Scheme 1. Reagents and conditions: (i) thionyl chloride, 80 1C; (ii) 2-oxo-1,2-dihydroquinoline-4-carbonyl chloride (2), K2CO3, acetonitrile.

H.M. Chawla, T. Gupta / Journal of Luminescence 154 (2014) 89–9490

The synthesized receptor was characterized by 1H NMR, 13CNMR and HRMS analysis. The 1H NMR spectrum (Supplementarydata, Fig. S1) of 4 exhibited deuterium exchangeable singlet peaksat 13.91 ppm and 8.35 ppm corresponding to dihydroquinoline NHand calixarene phenolic protons and also confirmed the existenceof calix[4]arene unit in a cone conformation, as ArCH2Ar signalsappeared as a pair of doublets at 3.55 and 4.11 ppm. The 13C NMRspectrum of 4 also showed a signal for syn Ar2CH2 around 31 ppm,thereby confirming the existence of 4 in cone conformation. Thestructure was further confirmed by HRMS analysis which showedpeak at 1013.4680 corresponding to 4.

3.2. Anion sensing

The UV–vis spectrum, fluorescence intensity changes and 1HNMR titrations of 4 were carried out to determine its anionbinding abilities.

3.3. UV–vis titration experiment

The absorption spectrum of 2-oxo-1,2-dihydroquinoline appendedcalix[4]arene derivative showed absorption bands at 281 nm and350 nm in THF. When tetrabutyl ammonium salts of F� , Cl� , Br� ,I� , H2PO4

� , HSO4� , CH3COO� were added to the THF solution of 4, only

fluoride (2 equiv.) caused an appreciable modification of the spectralpattern of 4 (30 μM). Addition of 0–2 equiv of fluoride to the THFsolution of 4 induced a shift of bands centered at 281 and 350 nmto 292 and 343 nm respectively with simultaneous formation of a newabsorption band at 319 nm (Fig. 1). The spectral changes could beattributed to the interaction of fluoride with dihydro-quinoline NH inthe lower rim of calixarene.

3.4. Fluorescence studies

When excited at 350 nm in THF, 4 gave a strong fluorescenceemission at 455 nm, which got quenched on addition of 3.5 equiv.of tetrabutylammonium fluoride (TBAF) with the appearance of anew blue shifted band at 435 nm. Fig. 2 shows the fluorescencetitration of 4 upon gradual addition of TBAF from 0–3.5 equiv. Thisspectrum could be attributed to the deprotonated form of 4, whicheventually formed through the intermediate hydrogen bondedspecies. On the other hand, addition of other anions such as Cl� ,Br� , I� , H2PO4

� , HSO4� , CH3COO� (3.5 equiv.), did not lead to any

quenching or shift in the fluorescence intensity of 4.

Fig. S2 (Supplementary data) depicts the relative decrease inthe fluorescence intensity (at λ 455 nm) of 4 upon addition ofvarious anions. It is quite evident that binding of 4 and fluoride isremarkably selective, while other competitive anions causedinsignificant changes in the emission spectrum of 4. The selectivitycoefficients (KX � =F � ¼ FX � =FF � ) measured for all the tested anions(inset, Fig. S2) showed that the interaction of other anions with 4was too miniscule to affect the detection of F� by 4.

The association constant of 4 with fluoride was calculated fromthe emission quenching data by using the Benesi–Hildebrandequation [26]. Plot of 1/F0�F versus 1/[F�] (Supplementary data,Fig. S3) showed a good linear relationship to reveal a 1:1 interac-tion between 4 and F� . The value of association constant has beencalculated to be 9.9�103 M�1. The fluorescence detection limit of4 for fluoride was also calculated on the basis of 3s/K [27]. TheLOD of 4 for fluoride was determined to be 0.7 μM, which is withinthe relevant window of the US Environmental Protection Agency'srecommended F� concentration in water (2 ppm) [28] as well asthe EPA's mandated upper limit (4 ppm). The determined LOD isalso much smaller and hence more sensitive than some of therecently published reports on F� sensing [29].

In order to assess the capability of 4 to resist disturbance andact as a fluoride selective molecular receptor, the competitionexperiments in the presence of potentially competitive anionswere also investigated by fluorescence spectral analysis. It wasdetermined that F� recognition by 4 was barely interfered byequimolar amounts of other competitive anions in the system(Supplementary data, Fig. S4). This high selectivity of 4 towardsfluoride may be attributed to the appropriate acidity of receptorhydrogens to complement F� .

3.5. 1H NMR titrations

The binding mode of F� with 4 was determined by 1H NMRtitrations carried out in CDCl3. The 1H NMR spectrum of 4 (5 mM)showed a singlet at 13.91 ppm corresponding to dihydroquinolineNH protons. It was observed that upon gradual addition of TBAF,the peak for dihydroquinoline NH protons suffered serious broad-ening and finally disappeared upon addition of approximately0.9 equiv. of TBAF (Fig. 3). These observations supported theassumption that F� being basic initially interacts with the dihy-droquinoline NH protons of 4 through hydrogen bonding and then

Fig. 1. Variation in absorption spectrum of 4 (30 mM) upon titration with TBAF(0–2 equiv.) in THF.

wavelength/nm380 400 420 440 460 480 500 520 540

Intensity

0

100

200

300

400

Fig. 2. Changes in the emission spectrum of 4 (30 μM) upon addition of TBAF (0–3.5 equiv.) in THF.

H.M. Chawla, T. Gupta / Journal of Luminescence 154 (2014) 89–94 91

abstracts them in the presence of its excess concentration in thesolution (Fig. 4 ).

3.6. Cation sensing

The cation sensing ability of 4 was examined by absorption andfluorescence spectroscopic analysis on addition of various metalions as their perchlorate salts. The analysis revealed that 4 ishighly selective for Cu2þ sensing.

3.7. UV–vis titrations

Amongst all of the tested metal ions (Mn2þ , Co2þ , Cu2þ , Ni2þ ,Hg2þ , Agþ , Cd2þ , Zn2, Cr3þ , Naþ , and Liþ) as their perchloratesalts, it was determined that only Cu2þ could bring about changesin the absorption spectrum of 4. As shown in Fig. 5, the absorptionband at 350 nm displayed a tiny blueshift (�4 nm) and the wholespectrum presented an ascending trend upon addition of Cu2þ

(0–4 equiv.) to a THF solution of 4 (30 μM).

3.8. Fluorescence studies

In the fluorescence titration experiment of 4 (30 μM) withCu2þ in THF, it was observed that emission of 4 was severelyquenched along with a blueshift of 10 nm. This results in thequenching of its emission upon addition of Cu2þ (0–6 equiv.) ispresented in Fig. 6.

Fig. 3. Partial 1H NMR (300 MHz) spectra of (a) 4 in CDCl3; (b) 4þ0.1 equiv. F�;(c) 4þ0.5 equiv. F�; (d) 4þ0.9 equiv. F� .

4

OO OHOH

NN

O

O O

O

F-OO OH

OH

HNNH

O

O O

O

OO OHOH

HNNH

O

O O

O

F-

Hydrogen bonding between 4 and F- Deprotonated 4

Fig. 4. Binding model for interaction between 4 and F� .

Fig. 5. Variation in absorption spectrum of 4 (30 μM) upon titration with Cu2þ

(0–4 equiv.) in THF.

H.M. Chawla, T. Gupta / Journal of Luminescence 154 (2014) 89–9492

To validate the selectivity of 4 in practice, fluorescence titra-tions of 4 were also carried out with other metal ions. It wasobserved that other tested metal ions showed insignificantchanges on addition to 4. The measured selectivity constant valuesfor other ions are negligibly small to create any problem for copperdetection (Supplementary data, Fig. S5).

The association constant of 4with Cu2þ was obtained by using theBenesi–Hildebrand equation. The linear fit of the curve indicated a 1:1binding stoichiometry between 4 and Cu2þ and determined the valueof binding constant to be 11.6�103 M�1 (Supplementary data, Fig.S6). The formation of 1:1 complex was further confirmed by ESI-MSanalysis. A solution of 4 containing Cu2þ showed a strong peak at m/z1053.3682 (Supplementary data, Fig. S7), which could be assigned to[4.Cu2þ] complex. The detection limit of 4 for Cu2þ as determinedfrom the fluorescence data is 0.5 μM, while U.S. EnvironmentalProtection Agency has set the limited concentration of copper indrinking water at approximately 20 μM [30]. Thus our detectionsystem is well within the EPA limit and is much smaller than someof the recently reported values [31].

The selectivity of 4 for sensing copper was further tested bymeans of competitive experiments. The results of the experiment(Supplementary data, Fig. S8) clearly showed that the emissionspectral response of 4with Cu2þ remains unperturbed, even in thepresence of equimolar amounts of other competitive metal ions inthe solution.

3.9. Simultaneous binding studies of metal ions and anions

The binding ability of one ion in the presence of its competingion was assessed by two sets of simultaneous UV–vis bindingstudies of 4 with F� and Cu2þ (Fig. 7). Upon addition of 5 equiv. ofCu2þ to the solution of 4.TBAF complex, it was observed that theabsorption band at 319 nm obtained due to the interaction offluoride with 4 disappeared and the 343 nm band got shifted to346 nm. Simultaneously, an incremental trend in the absorptionvalues of the entire spectrum was seen. These observationssuggest that as Cu2þ ions come in to the contact of solutioncontaining 4.TBAF complex, interaction of 4 with Cu2þ begins. Inthe reverse experiment, when 4.Cu2þ complex was titrated withTBAF, a similar sequestering process was observed and a newabsorption band started developing around 319 nm, which couldbe assigned to the interaction of 4 with TBAF. Thus, evaluationof sensing in the presence of competing ions indicates that theinteraction of 4 with TBAF triggers the decomplexation of Cu2þ .

4 complex and vice-versa. This negative allosterism exhibited by 4towards F�/Cu2þ binding could be attributed to the weak bindingof ions to the host molecule and strong ion-pairing equilibriabetween two types of ions [32].

4. Conclusions

In conclusion, we have synthesized and evaluated a calix[4]arene based single molecular probe for copper and fluoride ionswhich exhibits impressive detection limits of 0.5 μM and 0.7 μMrespectively. Even though, 4 possesses distinct binding sites forboth metal ion and anion, it did not exhibit positive cooperativebinding for both types of analytes.

Acknowledgment

Tanu Gupta thanks CSIR, India, for fellowship. Financial assistancefrom DST, DBT, MoEF, MoRD and MoFPI, Govt of India is gratefullyacknowledged.

Appendix A. Supplementary information

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.jlumin.2014.04.004.

Wavelength/nm380 400 420 440 460 480 500 520 540

Intensity

0

100

200

300

400

Fig. 6. Changes in the emission spectrum of 4 (30 μM) upon addition of Cu2þ

(0–6 equiv.) in THF.

Wavelength/nm250 300 350 400 450

Absorbance

0.0

0.1

0.2

0.3

0.4

0.5

0.6

wavelength/nm250 300 350 400 450

Absorbance

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Fig. 7. Variation in the absorption spectrum of (A) 4.F� upon addition of Cu2þ (B)4.Cu2þ upon addition of TBAF.

H.M. Chawla, T. Gupta / Journal of Luminescence 154 (2014) 89–94 93

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