New water soluble p-sulphonatocalix[4]arene chemosensor appended
with rhodamine for selective detection of Hg
2
þ
ion
Asif Ali Bhatti
a,c, Mehmet Oguz
a,b, Mustafa Yilmaz
a,* aDepartment of Chemistry, Selcuk University, Konya, 42075, TurkeybDepartment of Advanced Material and Nanotechnology, Selcuk University, Konya, 42031, Turkey cDepartment of Chemistry, Government College University Hyderabad, Hyderabad, 71000, Pakistan
a r t i c l e i n f o
Article history: Received 9 May 2019 Received in revised form 7 October 2019
Accepted 16 November 2019 Available online 21 November 2019 Keywords: Calix[4]arene Metal ions Fluorescence Complex
a b s t r a c t
In this study, we present the synthesis of water soluble calixarene appended with rhodamine (CR) at lower rim asfluorescent moiety. The synthesized water soluble fluorescent derivative was characterized with spectroscopic techniques andfluorescence response was evaluated towards Hg2þion in aqueous environment. Fluorescence studies shows that upon addition of Hg2þ, CR induce OFF-ON behaviour which follow CHEF process. Stoichiometric data show 1:1 coordination of metal and ligand with sensitivity of 3.55 1013molL1concentration range. Selectivity of CR towards Hg2þion was compared in the presence of other metal ions such as Pb2þ, Cu2þ, Zn2þ, Cr3þ, Ni2þ, Co2þ, Al3þ, Cd2þand Fe2þ. Results show that these metal ions have not pronounced effect onfluorescence intensity on CR-Hg2þcomplex.
© 2019 Published by Elsevier B.V.
1. Introduction
Designing of attractivefluorescent sensors by combining large molecular units for selective sensing of ions from environmental and biological samples is an art in sensor technology. Constant ef-forts are being made to develop sensitive, highly specific and cost effective chromogenic material for metal ions in biological media such as enzymes, antibodies and gens [1]. In this regard, calixarene macrocycle has provided much better solution in sensor technol-ogy. Considered as third generation of supramolecular molecular chemistry, calixarenes have proved to be effective molecular units in different fields of application [2e6]. With presence of basic structural requirements, calixarene derivatives are known as fascinating molecular units and well thought-outfirst choice of researchers working in sensor development [7e13]. Structural characteristics of calixarene framework such as rigidity, conformity of cavity size, three-dimensional molecular arrangement and functionalization at either rim with appropriate binding sites such as esters, amides, pyridine, amine, crown ethers, azacrown ethers and carboxylic acids along withflorescent sites generate potential material for recognition and sensing of both cations and anions
[14e21]. Moreover, water solublefluorescent molecules have been preferred in biological application for detection of different ions [22,23]. Number offluorescent molecules have been reported for selective detection of Hg2þ ion, since it is most dangerous and abundant metal ion pollutant in environment. It causes several health problems in living organisms. It can to pass through the skin and gastrointestinal tissues, which leads to the permeant damage to the central nervous system [24,25]. In this study, we have functionalized calix[4]arene with rhodamine according to reported method due to its extensive use influorescent labeling and excel-lent photophysical properties such as long absorption and emission wavelengths elongated to visible region, highfluorescence quan-tum yield, and large absorption coefficient(
2. Material and methods 2.1. General section
All the reagents and solvents were analytical grade and used without further purification. Analytical TLC was performed on pre-coated silica gel plates (SiO2, Merck PF254).1H NMR spectra were
referenced to tetramethylsilane (TMS) at 0.00 ppm as internal standard solution and recorded on a Varian 400 MHz spectrometer at room temperature (25± 1C). Thermo Nicollet AVATAR 5700 IR
spectra were recorded by a Mattson 1000 FT-IR spectrometer as KBr * Corresponding author.
E-mail address:myilmaz42@yahoo.com(M. Yilmaz).
Contents lists available atScienceDirect
Journal of Molecular Structure
j o u r n a l h o m e p a g e : h t t p : / / w w w . e l se v i e r . c o m / l o c a t e / m o l s t r u c
https://doi.org/10.1016/j.molstruc.2019.127436
pellets. Elemental analyses were performed on a Leco CHNS-932 analyzer. Fluorescence emission spectra were recorded on a Per-kinElmer LS 55 spectrometer using standard 1.00 cm quartz cells. All aqueous solutions were prepared with deionized water that had been passed through a Millipore Milli-Q Plus water purification system (ELGA Model CLASSIC UVF, UK).
2.2. Syntheses
Synthesis of rhodamine trisamine and calixarene derivatives (3) was carried out using reported method [26,27].
Synthesis of rhodamine trisamine: Rhodamine B (5 g, 10.4 mmol)
and N(CH2CH2NH2)3(Tren) (3.11 mL 20.8 mmol) were refluxed in
methanol (350 mL) for 36 h. Reaction was monitored with TLC. After complete consumption of rhodamine B reaction was stopped. Methanol was evaporated and residue was poured in acidic water and product was extracted with dichloromethane. Organic layer was dried over MgSO4andfiltered. Volume of filtrate was reduced
under vacuum and remaining product was poured in Petri dish for drying. Light orange powder was obtained in 88% yield (Scheme 1). FTIR:1H NMR (400 MHz, CDCl3). 1.09 (bs, 12H, NCH2CH3), 2.08e1.99
(m, 2H, NCH2CH2N), 2.29e2.27 (m, 2H, NCH2CH2N), 2.59e2.52 (m,
2H, NCH2CH2N), 2.89e2.83 (m, 2H, NCH2CH2N), 3.09 (bs, 2H,
NCH2CH2N), 3.25 (bs, 8H, NCH2CH3), 5.01e4.05 (bs, 4H, NH2), 5.25
Scheme 1. Schematic route for the synthesis of amide derivative of rhodamine B.
(bs, 2H, NCH2CH2N), 6.31e6.21 (m, 6H, ArH), 6.99 (s, 1H, ArH), 7.39
(s, 2H, ArH), 7.83e7.79 (m,1H, ArH) (Fig. S1) Anal. Calculated for C34H46N6O2: C, 71.55; H, 8.12; N, 14.72; Found: C, 70.89; H, 8.55; N,
14.21.
Synthesis of compound4 (CR): Compound 3 (1.0 g, 0.77 mmol) was mixed with equimolar H2SO4and heated for 4 h at 60C. Small
amount was taken out and drawn and poured in water. Reaction was stopped when no insoluble compound was observed. For neutralization, BaCO3was added in aqueous solution. Precipitates
of BaSO4, werefiltered off and washed with hot water and the
combinedfiltrate and washings were evaporated to dryness under reduced pressure. The residue was dissolved in hot water (15 ml) and the solution was adjusted to pH 8 by Na2CO3. Afterfiltration,
methanol was added to the filtrate to afford compound 4 (CR) (Scheme 2). Yield 57%. 1H NMR (400 MHz, D2O). 1.23 (bs, 12H,
NCH2CH3), 2.57 (b, 2H, NCH2CH2N), 3.61 (b, 4H, ArCH2Ar), 3.71 (bs,
12H, NCH2CH3, NCH2CH3, NCH2CH2N), 4.28e4.59 (m, 8H, ArCH2Ar,
NHCH2CH2), 4.86 (s, 4H, OCH2þH2O(solvent)), 6.85 (s, 4H, AreH/
Calix), 7.16 (s, 4H, AreH/Calix) 7.38e8.32 (m, 10H, ArH-Rhodamine) (Fig. 1) Anal. Calculated for C66H70N6O20S4.5H2O: C, 53.36; H, 5.43;
N, 5.66; S, 8.63, Found: C, 53.84; H, 5.93; N, 5.05; S, 8.12.
2.3. General procedure forfluorescence study
Spectrofluorometric studies for the evaluation of fluorescence properties of water soluble CR for its sensing ability toward Hg2þ were carried out using spectrofluorometric [28]. For this stock so-lution CR (1.5 104M) was prepared in 10 mL of water followed by dilution to (1.5 105M) into 100 ml. Emission response of CR
was estimated through titration experiments with metal solution. In 10 ml test tubes, 3 mL of CR (1.5 105M) and 2 ml of Hg2þ
(1.5 104 M) were mixed together. Emission intensities of mixture containing Hg2þwere measured at excitation wavelength 335 nm.
2.4. Stoichiometric ratio of complex
Determination of stoichiometric ratio between the CR and Hg2þ was carried out from continual variation method (Job’s method). Stern-Volmer analysis was utilized to probe the nature of the quenching or enhancement process in the complexation.
Fig. 1.1HNMR (D
2O) spectrum of ligand 4 (CR).
Io
I¼ 1 þ Ksv½Q (1)
Where Iois thefluorescence intensity of CR in the absence of Hg2þ, I
is thefluorescence intensity of CR in presence of Hg2þ, K svstands
for stern-volmer constant.
3. Results and discussion
In supramolecular chemistry, calixarenes have got much atten-tion due to their fascinating structure for complexing metal ions [29,30]. Different calixarene derivatives have been designed for selective detection of different metals at low concentration for their use in variety offields, but few of them are water soluble with potentialfluorescence properties. In this work, new water-soluble calixarene appended with rhodamine signaling moiety has been synthesized successfully (Scheme 2). Firstly, amine derivative of rhodamine B and compound 3 were prepared following reported methods [26]. Acid hydrolysis reaction was performed to convert compound 3 in water soluble calixarene derivative (CR) (Figs. S2 and S3) [31]. As a fluorophore and chromophore probe, rhoda-mine appended on calixarene has attracted chemist for its frequent use in fluorescent labeling agent due to better photophysical character. For example long absorption as well as emission wave-lengths up to visible region and highfluorescence quantum yield with absorption coefficient [32].
3.1. Fluorescence study
Selective complexation ability of CR for Hg2þcan be justified by various parameters like compatible nature of binding sites for guest, ionic radii as well as electronegativity. To gain insight into the role of calixarene as chemosensor for Hg2þ ion,fluorescence experiment performed using excitation wavelength of 335 nm. A distinct increment influorescence intensity of CR (2.5 106M) at ~574 nm was observed upon addition of 10 equivalent Hg2þ(Fig. 2), showing interesting“turn on” fluorescent properties. This effect can be explained on the basis of electron transfer process, whereas CR is functionalized with tren structure at its lower rim that possess nonbonding electrons and PET process prevails due to intermo-lecular oxidation process [33,34]. Upon addition of metal ion, the interamolecular PET fluorescence quenching effect derived from the electron pairs of N donor atoms were fully blocked and relieved by reducing the electronic density of lone pairs through metaledonor binding interaction and consequently increases the probe emission. In addition, reduction potential of receptor increased upon addition of Hg2þion that leads the lower in cor-responding highest occupied molecular orbital (HOMO) energy Fig. 3. Fluorescence response of CR (2.5 106M) upon adding different
concentra-tions of Hg2þ(1e10 equiv.). Inset plot of I/Iovs [Hg2þ].
Table 1
Comparison of different calixarene basedfluorescent materials with respect to limit of detection.
Materials Limit of detection Reference 1 Calix-DANS2 1 107molL1 [35]
2 Naphthlene substituted Thiacalix [4]arene derivative
2.23 10e7molL1 [36]
3 Dansyl substituted calixarene 5.00 10e3molL1 [37]
4 Calixarene capped quantum dots 15.00 10e9molL1 [38]
5 CR 3.55 1013mL1 Present Study
than that of CR; thus, the Photoinduced Electron Transfer route from the receptor to thefluorophore is prohibited and the fluo-rescence enhanced due to chelation-enhancedfluorescence (CHEF). Fromfluorescence titration experiment detection limit was evalu-ated and it was found to be 3.55 1013molL1(Fig. 3), (Table 1)
3.2. Stoichiometry of complex
Binding stoichiometry of CR-Hg2þcomplex was determined by Job’s method and binding constant (Ksv) was calculated from
Stern-Volmer plot (Fig. 4). The plot of absorbance against mole fraction depicted in Fig. S4, indicates maximum absorption values for complex at 0.5, suggesting 1:1 stoichiometric ratio, which infers that CR forms 1:1 coordination with Hg2þ ion. Binding constant (Ksv) from Stern-Volmer equation was found to be 1.22 104M1.
The graph obtained for (Io/I) vs. concentration of Hg2þion shows good linear behaviour in the range of 0.5e3 nM with a coefficient of regression (R2) 0.914. The Ksvvalue suggest the good affinity of CR
for metal ion in aqueous media. The high rate of binding could be due to presence of non-bonding electrons on nitrogen functionality for binding metal ions.
3.3. Interference study
To confirm the potential applicability of CR, a competitive study was carried out. Intensity of CR-Hg2þ complex checked in the presence of various metal perchlorates (Pb2þ, Cu2þ, Zn2þ, Cr3þ, Ni2þ, Co2þ, Al3þ, Cd2þ, Fe2þ) as shown inFig. 5. Upon addition of 10 equivalent of cations to the solution of CR-Hg2þcomplex. The bar graph shows that other metals did not induce any distinct changes influorescence intensity of complex that implies other metal ions had no obvious interference with the detection of Hg2þion and CR has special binding ability towards Hg2þin aqueous environment. 3.4. Stability of complex
Efficiency of chemosensor depends on its stability with respect
to time and considered as zealous area in sensor study. Besides selectivity, it provides important data about stability complex with the passage of time. In order to evaluate stability of complex, fluorescence intensity of complex examined at different time in-tervals of 5, 10, 20, 30, 45, 60, 75, 90 and 105 min (Fig. 6). 4. Conclusion
In this work we explored thefluorescence properties of newly synthesized water soluble fluorescent calixarene appended with rhodamine (CR) towards selective detection of Hg2þ. Photophysical properties of CR show that presence of Hg2þinduce CHEF OFF/ON process. Complex CR-Hg2þ give specific colour change with
Fig. 5. Fluorescence response of CR-Hg2þ(1.0 105M) in the presence of other competing metal ions at 574 nm.
Fig. 6. Time-dependentfluorescence spectra of CR-Hg2þcomplex; (inset) graphs show
increase in fluorescence intensity at 573 nm when excited at 335 nm. Moreover, designing efficient calixarene sensor with low detection limit would help to extend the development of fluores-cent sensors for other toxic metal ion.
Declaration of interest statement
The authors declare no conflict of interest. Acknowledgments
We would like to thank The Research Foundation of Selcuk University (BAP) for theirfinancial support of this work.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.127436. References
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