A novel water-soluble highly selective “switch-on” ionic liquid-based fluorescent chemi-sensor for Ca(II)

Article abstract of DOI:10.1016/j.molliq.2017.06.016

A novel chemi-sensor involve new bis-ionic Schiff base sensor (BISBS), N,N′-bis-[5-((2,4-lutidiniumchloride)methylene)-3-methoxysalicylidene]-R,R-1,2-cyclohexanediimine, has been synthesized and characterized. BISBS chemi-sensor was designed based on internal charge transfer (ICT) fluorescence mechanism. This new water soluble chemi-sensor provides great selectivity fluorescence detection for Ca(II) ions in an important physiological pH range. Moreover, the interaction of Ca(II) with the deprotonated BISBS to produce a metal-ligand complex with a ratio of (1: 1) accompanying with an enhancement in the intensity of emission band located at 502?nm. Fluorescence switching-on during the chemical interaction between BISBS and Ca(II) ions is very easily noticed with naked eye, but other metal cations such as alkali, alkaline earth and transition metal don't give any fluorescence changes. The novel developed BISBS sensor successively offers low limit of detection (LOD) 1.5?nM and fast tracing of Ca(II) in the physiological pH?7.6. Thus BISBS may provide a novel auspicious methodology for detection calcium cations in the environmental and biological samples.

Full text of DOI:10.1016/j.molliq.2017.06.016

Accepted Manuscript
A novel water-soluble highly selective “switch-on” ionic liquid-
based fluorescent chemi-sensor for Ca(II)
Reda F.M. Elshaarawy, Reham Ali, Sayed M. Saleh, Christoph
Janiak
PII:
DOI:
Reference:
S0167-7322(17)31619-7
doi: 10.1016/j.molliq.2017.06.016
MOLLIQ 7462
To appear in:
Journal of Molecular Liquids
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Revised date:
Accepted date:
15 April 2017
31 May 2017
2 June 2017
Please cite this article as: Reda F.M. Elshaarawy, Reham Ali, Sayed M. Saleh, Christoph
Janiak , A novel water-soluble highly selective “switch-on” ionic liquid-based fluorescent
chemi-sensor for Ca(II), Journal of Molecular Liquids (2017), doi: 10.1016/
j.molliq.2017.06.016
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ACCEPTED MANUSCRIPT
A novel water-soluble highly selective “switch-on” ionic liquid-based fluorescent chemi-
sensor for Ca(II).
Reda F.M. Elshaarawya,b,*, Reham Alia,c, Sayed M. Salehc,d, Christoph Janiakb
a Chemistry Department, Faculty of Science, Suez University, 43518 Suez, Egypt.
b
Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine Universität Düsseldorf, 40204 Düsseldorf,
Germany.
c Chemistry Department, Faculty of Science, Qassim University, Buraidah, KSA
dChemistry Branch, Department of Science and Mathematics, Faculty of Petroleum and Mining Engineering, Suez
University, 43721 Suez, Egypt.
*Corresponding author:
Reda_elshaarawi@science.suez.edu.eg
Reda.El-Shaarawy@uni-duesseldorf.de
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A novel water-soluble highly selective “switch-on” ionic liquid-based fluorescent chemi-
sensor for Ca(II).
Graphical Abstract:
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ACCEPTED MANUSCRIPT
A novel water-soluble highly selective “switch-on” ionic liquid-based fluorescent chemi-
sensor for Ca(II).
Abstract
A novel chemi-sensor involve new bis-ionic Schiff base sensor (BISBS), N,N'-Bis-[5-((2,4-
lutidiniumchloride)methylene)-3-methoxysalicylidene)]-R,R-1,2-cyclohexandiimine, has been
synthesized and characterized. BISBS chemi-sensor was designed based on internal charge
transfer (ICT) fluorescence mechanism. This new water soluble chemi-sensor provides great
selectivity fluorescence detection for Ca(II) ions in an important physiological pH range.
Moreover, the interaction of Ca(II) with the deprotonated BISBS to produce a metal-ligand
complex with a ratio of (1 : 1) accompanying with an enhancement in the intensity of emission
band located at 502 nm. Fluorescence switching-on during the chemical interaction between
BISBS and Ca(II) ions is very easily noticed with naked eye, but other metal cations such as alkali,
alkaline earth and transition metal don’t give any fluorescence changes. The novel developed
BISBS sensor successively offers low limit of detection (LOD) 1.5 nM and fast tracing of Ca(II)
in the physiological pH 7.6. Thus BISBS may provide a novel auspicious methodology for
detection calcium cations in the environmental and biological samples.
Keywords: Bis-ionic Schiff base, Structural characterization, Fluorescence, Ca(II) chemi-sensor,
Internal charge transfer, Selectivity.
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1. Introduction
Development and designing of molecular sensing systems, exhibiting significant
spectrophotometric or fluorescence changes upon chelating analytes has attracted much attention.
Design of sensitive and selective molecular systems for tracing different types of metal ions
particularly Ca (II) ion, which takes a crucial systematically role in diverse bio-systems, has been
receiving much interest [1]. Moreover, the Ca(II) ions exhibited potentially high importance in
many fields including food technology, environment [2] and medicine [3]. Many molecular probes
were designed for detection and measuring Ca(II) ions based on different molecular type such as
organic dye [4], protein [5] and nanoparticles [6]. Chemi-sensor probes are predominantly
attractive because they allow simple naked-eye detection of Ca(II) ion. Thus, different types of
colorimetric probes have been synthesized in order to detect calcium ions, but most of these probes
suffer from interfering detection due to the presence of other metal ions such as zinc metal ions
[7].
Concurrent detection of the specified analyte based on the change of fluorescence property is
frequently the most utilized method that accompanied by decreasing in the fluorescence intensity
(quenching) “Turn off” mechanism or enhancing the fluorescence intensity of the sensor molecules
in the presence of analyte or metal ions “Turn on” mechanism [8]. However, the change in the
fluorescence intensities of these chemi-sensor molecules could be attributed to many factors as
system temperature, environmental pH, photobleaching and polarity of solvent. Recently, the
developed optical sensors for Ca(II) ions were developed mainly by Ca(II) ion replace with a
hydrogen proton of the pH-sensitive dye or chelating of Ca(II) ion sensitive dye with neutral ion
carriers [9]. Moreover, the fluorescent Ca(II) chemi-sensor becomes significant technique for
screening Ca(II) ions.
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The extreme advantage properties of fluorescence turn-on sensing mechanism with respect to
turn-off mechanism are the low detection limit of the specified analyte and the relative dark
fluorescence background of the chemi-sensor [10]. These factors decrease the probability of
pseudo signal and increase both sensitivity and selectivity of chemi-sensor. For turn-on and turn-
off mechanisms, ICT internal charge transfer has been extensively utilized in order to interpret the
fluorescence behavior of various sensors [11]. Moreover, the visual sensing based naked eyes
would provide strong tool with high sensitivity and selectivity towards the specified analyte.
Design and synthesis of Schiff base fluorescence chemi-sensor based ICT mechanism has a great
attention and has been used widely as DNA cleavage [12] and nonlinear optical materials [13].
The relentless ease of preparation of Schiff-base derivatives anchoring fluorescent moieties
along with their amazing photophysical properties make them promising and attractive candidates
for building up of novel chemi-sensors for fluorescence detection of numerous types of metal
cations [14] Noteworthy, the isomerization of azomethine (H-C=N) fragment belongs to a Schiff-
base is reasonable for the decay mechanism of the excited-states of Schiff-bases with an un-bridged
C=N skeleton that led to a nonfluoresce profile [15]. Hence, enhancement or restriction of this
isomerization via metal ions binding will play a crucial role in fluctuation of fluorescence intensity
accompanied these compounds. Additionally, Schiff-bases have been recognized as chemi-sensors
for fluorometric and colorimetric detection of Hg2+and Ca2+ cations (20 µM and 0.3 μM,
respectively). The interference of other cations is completely ineffective [16].
The amazing physical and chemical properties of ionic liquids (ILs) [17] along with their
extensive potential in many applications such as COSMO-RS analysis [18], azeotropic separation
process [ 19 ], microbalance sensors and designer solvent in chemical synthesis besides
electrochemistry [ 20 , 21 ] make them attract attention of numerous analytical researchers.
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Recently, they have used to design ionic liquid-based fluorescent nanosensors [22] and Schiff-base
chemi-sensor [Error! Bookmark not defined.b]. Inspired with the aforementioned facts, we
intended herein to developed a new, simple ultrasensitive and selective chemi-sensor for Ca(II)
with efficient dual-solubility in aqueous and organic solvents based on a Schiff-base probe bearing
ionic liquid compartments.
2. Experimental
2.1. Reagents and Materials
See supplementary content.
2.2. Instrumentation
See supplementary content.
2.3. Synthesis of 1-(5-formyl-vanillyl)-2,4-lutidinium chloride (MeO)sal(Lut+Cl–)(2)
To a vigorously stirred solution of 2,4-lutidine(0.52 g, 4.87 mmol) in anhydrous benzene (25
mL), the solution of 5-chloromethyl-3-methoxysalicylaldehyde [16b] (1.96 g, 9.8 mmol) in
anhydrous benzene (50 mL) was added, drop-wise over 30 min, under nitrogen atmosphere at room
temperature. The resulting solution was further stirred at 80 °C for 24 h. After cooling to room
temperature, the isolated product was washed intensively with 5 x 10 mL anhydrous benzene,
several times with ether (5x10 mL), to remove the unreacted materials, and finally dried under
high vacuum to give the lutidinium salt (2) which used for the following preparation without
further purification. Compound 2 was obtained as a faint yellow solid (2.68 g, 89%), mp = 86-87
ºC. FTIR (KBr, cm-1): 3224 (m, br, ν(O-H)), 3031 (m, sh, ν (C-H), Ar), 2982 (m, sh, ν(C-H), Aliph.),
1686 (vs, sh, ν(C=O)), 1643 (vs, sh, ν(C=O)), 1569, 1496, 1390 (s, sh, ν(C=C
Py), 1286 (s, sh, νν(Ar-
+ C=N
Ar
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O)), 1144 (s, sh, ν(H-C=C + H-C=N) , Py).1H NMR (300 MHz, DMSO-d6) δ (ppm) (Fig. S1): 10.32
bend
(s, 1H), 8.06 (s, 1H), 7.85-7.72 (m, 2H), 7.31-7.15 (m, 3H), 5.39 (s, 2H), 3.89 (s, 3H), 3.79 (s, 3H),
2.67 (s, 3H). 13C NMR (75 MHz, DMSO-d6) δ (ppm) (Fig. S2): 191.11, 151.37, 149.30, 144.85,
137.70, 129.25, 128.56, 125.85, 125.67, 122.95, 122.67, 121.43, 119.44, 56.34, 50.51, 35.16 and
21.41. ESI MS: In positive mode peaks atm/z = 272.1 ([C16H18NO3]+, M ‒ Cl) a.m.u.
2.4. Synthesis of (1R,2R)-1,2-diaammoniumcyclohexane L-tartrate (3)
It was prepared according to the method reported by Elshaarawy et al [23].with slight
modifications, in brief, at 70 °C, rac-trans-1,2-diaminocyclohexane (6 ml, 50.0 mmol) was added
dropwise to a solution of L-(+)-tartaric acid (7.5 g, 50.0 mmol) in distilled water (20 ml). Then the
reaction temperature was gradually increased until 90 ºC in order to keep the mixture solubilized.
The resulting slurry was stirred at the same temperature for a further 2 h, and then the isotropic
solution was allowed to spontaneously cool down to RT and then kept at 4 ºC overnight. The
resulting precipitate was collected by vacuum filtration and washed with 5 °C distilled water (2 ×
25 mL) and then methanol (5 × 15 ml). The crude product, 3, was then recrystallized by dissolving
the compound in distilled water at 90 °C and leaving to cool to RT overnight. The recrystallization
process was repeated twice. The purified product was collected by vacuum filtration and dried
under reduced pressure. 1H NMR (200 MHz, D2O) δ (ppm): 4.64 (d, J = 7.1 Hz, 2H), 4.15 (td, J =
10.8, 3.9 Hz, 1H), 3.98 (td, J = 10.6, 4.1 Hz, 1H), 2.71 (3, 6H), 2.38-2.13 (m, 4H), 1.67-1.89 (m,
4H).
2.5. Synthesis of (1R,2R)-1,2-diaminocyclohexane (R,R-DACH) (4)
A NaCl-saturated solution of NaOH (1.91 g, 47.8 mmol) in H2O (25 ml) was added dropwise
to a solution of diammoniumcyclohexane L-tartrate 3 (3.17 g, 12.0 mmol) in H2O (25 ml). After
stirring for 30 min at ambient temperature the mixture was extracted with CHCl3 (5x50 ml). The
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combined extracts were dried (MgSO4). The solvent was evaporated under reduced pressure and
the residue submitted to recrystallization from petroleum ether (40-60) to yield the desired product,
R,R-1,2-diaminocyclohexane (4) as a white crystals which stored at -20 °C.. 1H NMR (200 MHz,
CDCl3) δ (ppm): 4.99 (s, br. 4H), 2.93 (td,, J = 10.7, 4.0 Hz, 2H), 2.01-1.88 (m, 4H), 1.31.1.17
(m, 4H).
2.6. Synthesis of new bis-ionic Schiff base sensor (BISBS) N,N'-Bis-[5-((2,4-lutidinium
chloride)methylene)-3-methoxysalicylidene)]-R,R-1,2-cyclohexanedi-imine (5).
Into a 100 mL Schlenk flask, a methanolic solution (10 mL) of R,R-DACH (0,23 g, 2.0 mmol)
was added dropwise to a solution (20 mL) of methoxysalicylaldehyde-2,4-lutidiniumchloride
(MeO)sal(Lut+Cl–) 2 (1.23 g, 4.0 mmol) in methanol under N2 atmosphere. The reaction mixture
was refluxed with stirring under N2 for 6 h. Then the solvent was partially removed in rotatory
evaporator, and a yellow product of chemi-sensor (5) was departure from the solution by
the addition of ethyl acetate along with keeping in the refrigerator overnight. Then, the solvent was
decanted off and the isolated crude product was sonicated for 15 min in Et2O (3 x 25 mL). Et2O
was also decanted off and the residual solid was washed intensively with MeOH / Et2O mixture
(1:2) to remove unreacted materials and then re-dissolved in MeOH. EtOAc was added slowly
(during ~15 min) to precipitate the product 5 as yellow solid which was collected by filtration and
dried under vacuum. New chemi-sensor, 5, was obtained asyellow-orange powder, (2.30g, 83%),
mp = 181-182 ºC. FTIR (KBr, cm-1): 3436 (m, br, ν(O-H)), 3135 (w, br, νasym(C-H), CH, Py and Ar),
3074 (m, br, νsym(C-H), Py and Ar), 1634 (vs, sh, ν(C=N)Azomethine), 1558, 1460, 1377 (s, sh, ν(C=C
+ C-
Ar
bend)), 1270 (m, sh, ν(Ar-O)), 751 (s, sh, ν(H-C=N)bend, Py). 1H NMR (200 MHz, CDCl3) δ (ppm) (Fig.
H
S3): 10.58 (s, 1H), 10.53 (s, 1H), 8.45 (d, J = 7.4 Hz, 2H), 8.24 (s, 2H), 7.61 (s, 2H), 7.38 (d, J =
1.8 Hz, 2H), 7.30 (s, 2H), 6.97 (s, 2H), 5.43(s, 4H),4.26-4.16 (m, 2H), 4.05 (s, 6H), 3.79 (s, 6H),
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13
2.42 (s, 6H), 2.05-1.88 (m, 4H), 1.34-1.19 (m, 4H). C NMR (125 MHz, CDCl3) δ (ppm) (Fig.
S4): 171.5, 165.26, 152.53, 148.74, 144.74, 124.01, 123.14, 122.88, 121.26, 118.02, 114.92, 70.96,
56.22, 50.70, 39.72, 35.14, 35.10, 32.90 and 24.03. ESI-MS(m/z, amu): calcd for C38H46ClN4O4
and C38H46N4O4: 658.3 [M‒Cl]+ and 622.8 [M‒2Cl]2+; found: 658.2 and 622.7, respectively. Anal.
Calcd.for C38H46Cl2N4O4 (M = 693.70 g/mol): C, 65.79; H, 6.68; N, 8.08; Found: C, 65.68; H,
6.71; N, 7.99.
2.7. Fluorescence and UV−Vis measurements.
Alkali, alkaline and transition metal stock solutions were prepared. The fluorescence titration
of the new BISBS free ligand was achieved in aqueous solution 0.8 μM concentration, 20 mM
HEPES buffer and at pH 7.6. The UV−vis spectra measurements of BISBSand metal complexes
were recorded in an aqueous solution with 0.4 μM. During Absorption titration, the net volume of
BISBS and Ca(II) should be kept 2.0 mL cause the volume of Ca(II) could be neglected with
respect to BISBS.
3. Results and discussion
3.1. Synthesis protocol of new sensor
The synthesis of N,N-bis-(3-methoxysalicylidene)-R.R-1,2-cyclohexanediimine anchoring
lutidinium chloride terminals (5), a new chemi-sensor, is depicted in Scheme 1. At the outset a
new lutidinium ionic liquid (MeO)sal(Lut+Cl–) (2) was prepared, in a highly pure state, starting
from o-vanillin via a combined chloromethylation-quaternization protocol as shown in Scheme 1.
Thereafter, a simple Schiff base condensation between new lutidinium ionic liquid and a chiral
R,R-DACH was the followed strategy for preparation our bis-ionic Schiff base fluorescent chemi-
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sensor (BISBS) (5). This chemi-sensor was isolated in high yield and structurally characterized
using elemental analysis and spectral methods (FTIR, UV-Vis, NMR (1H, 13C), ESI-MS).
Scheme 1 Schematic diagram for the synthesis of lutidinium ionic liquid (MeO)sal(Lut+Cl–) (2)
and bis-ionic Schiff base fluorescent chemi-sensor (BISBS) (5).
3.2. Characterization of BISBS
Notable that the molten salts of melting point (mp) below 100 ºC are recognized as ionic liqiuds
(ILs). Accordingly, our new sensor could not be classified as IL, however, it is considered as a
ionic molten salt as its mp > 100 ºC (181-182 ºC) anchoring vanillyl-2,4-lutidinium chloride ionic
liquid of mp (86-87 ºC).
The obtained data from CHN microanalytical analysis for new bis-ionic Schiff base sensor
(BISBS) was in good agreement with the suggested molecular formula for BISBS (see the
experimental section). Moreover a positive mode electrospray ionization mass spectra (ESI-MS)
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of new sensor (5) exhibited two characteristic peaks at m/z = 658.2 and 622.7 a.m.uassignable to
singly- and doubly-charged cations produced via departure of chloride ions, [M ‒ Cl‒]+ and [M ‒
2Cl‒]2+, respectively.
FTIR offers primary spectral markers for proofing the successful formation of new sensor (5)
as revealed from its FTIR signatures such as: (i) An intense broad humpcentered at 3436 cm−1
along with a maxima at 1270 cm−1 can assigned as a phenolic O-H fragment belongs to an
…
intramolecular H-bonding environment (O–H N). (ii) Displacement of azomethine (-HC=N-)
stretch to a lower-energy region, 1634 cm−1, as compared with that reported for free
azomethinefragent (1657 ± 3 cm−1) [24,25] provides a further evidence for the participation of
azomethinic group in a H-bonded six-membered chelating ring (see Scheme 2). (iii) Anchoring
of bis-Schiff base to the lutidinium compartments can be detected from the observation of three
main IR peaks at 1597, 1158 and 751 cm−1, characteristic for pyridinium moiety.
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Scheme 2 possible tautomeric forms along with intramolecular hydrogen-bonding inBISBS (5).
(v) Noteworthy, missing of any vibrational mode, around 1700 cm–1, attributable for C=O
stretching vibration, offers a plausible evidence for the predominant population of O-protonated
bis-enolimine tautomer in the keto–enol tautomeric equilibrium of solid BISBS (cf. Scheme 2).
1H NMR provides the second and strong spectral markers confirming the successful formation
of new sensor (5) and the predominance of bis-enolimine tautomeric form with little contribution
from imine-zwitterionic form in the deuterated solution of BISBS. This can be easily concluded
1
from the observed H NMR spectral peculiarities for BISBS including: (i) Two characteristic
singlets at 10.58 and 10.53 ppm attributable for the resonance of phenolic O-H, typical for bis-
enolimine tautomer. (ii) A doublet at 8.98 ppm (J = 7.1 Hz) which could be ascribed to resonance
of iminic proton, typical for an ethylenic proton of zwitterionic (phenolate-iminium) fragment.
This splitting pattern is in full agreement with our former suggestion regarding the relative
tautomer, imine/ enamine, population in deuterated solution of BISBS (cf. Scheme 2) [26].
3.3. The optical measurements of the new sensor.
The optical properties of the new synthesized optical sensor BISBS was detected in solution
(Fig. 1). The optical sensor exhibits an emission band located at 502 nm under excitation of 380
nm and the UV-visible absorption spectrum shows three mean peaks located at 250, 325, 380 nm.
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Fig. 1. (a) UV-Vis spectrum and (b) excitation, and emission spectra under 380 nm of the BISBS sensor at
pH 7.6, 20 mM HEPES buffer.
3.4. PH dependence of the fluorescence intensity of ionic Schiff-base.
The detection of metal ions in solution using the fluorescence change of chemi-sensor mainly
produced with transfer of hydrogen proton, this interaction accompanied by alteration the pH of
the environment solution. Therefore, the new designed BISBS was tested utilizing fluorometric
titration by alteration pH of the aqueous medium (see Fig. 2). In low range of pH value, the
fluorescence intensity of the BISBS has a significant decrement within range (1 to 5.6). In contrast,
the fluorescence intensity has a constant value over a wide range of pH (5.6 to 12). The
fluorescence enhancement explanation of the chemi-sensor is mainly attributed to the (ICT)
process [27], The high affinity of BISBS towards water solubility led us to carry out all series of
experiments in physiological pH 7.6 buffered systems with 20 mM HEPES buffer.
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Fig. 2. Influence of the pH-value on emission intensity of the BISBS. Under excitation 380 nm and [BISBS]
= 0.8 μM.
3.5. Fluorescence property of ionic liquid-based Schiff-base sensor (BISBS).
Fluorescence intensity of BISBS and its fluorometric titration with Ca(II) ion solutions of
different concentrations were detected in buffered aqueous medium (20 mM HEPES buffer at pH
7.6) as shown in Fig. 3. The fluorescence emission of the free BISBS shows a broad band with
feeble emission peak centered at 502 nm. The free BISBS in buffered aqueous medium was titrated
with regular adding Ca(II) cations. During the titration, a significant band enhancement centered
at 502 nm was recorded. Simultaneous enhancement of the main BISBS emission peak revealed
the formation of Ca(II)-BISBS complex. The signal intensity is constant with further addition of
Ca(II) ions with respect to BISBS concentration. This indicates that the stoichiometric of metal to
ligand interaction is (1:1). The fluorescence enhancing the behaviour of the emission intensity of
BISBS is referred to the ICT mechanism based the receptor Ca(II) and the donor BISBS. The
response of the BISBS sensor to the Ca(II) cations was measured in buffered solution medium
with concentration limits of 1‒1.6 µM, The limit of detection (LOD) was calculated as 1.5 nM
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with a precision of ±1%. Hence, the precious value of LOD for BISBS enables to detect Ca(II)
ions with high sensitivity and selectivity. Besides, in future we can use this chemi-sensor can be
used efficiently in determination of Ca(II) in environmental samples.
Fig. 3. (a) The effect of [Ca(II)] from 0 to 1.6 µM , on the emission intensity of BISBS sensor. λexc was
380 nm, and λem were at 502 nm. [BSB-IL] was 0.8 µM and (b) Calibration curve I502 as a function of
[Ca(II)]
Interestingly enough, the lutidinium ionic liquid terminals act as a multiple-functions
segments where the amphiphilic nature of these moieties enhances the dual-solubility of the new
sensor in both aqueous and organic solvents allowing the sensing of Ca(II) in biological samples.
Moreover, the positive charges of two lutidinium moieties form assemblies with nitrate ions in
aqueous solution which promote the ICT from the anionic nitrate to cationic and consequently,
fluorescence intensity and sensitivity for Ca(II) ion is significantly increased, thereby, its limit of
detection (LOD) by new sensor decreased.
The excitation spectra of the (0.8 µM) free ligand BISBS sensor and BISBS metal complex
in the range of Ca(II) cations (0-1.6 µM) were determined with optimum aqueous conditions using
20 mM HEPES buffer, pH 7.6. Fig. 4 exhibits the excitation spectra of the free BISBS ligand and
the Ca(II) complex with a considerable enhancement of the excitation peak at 380 nm which could
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be referred to the formation of the Ca(II) metal complex. When the metal-BISBS ratio reaches 1:1
during titration with Ca(II) cations, the excitation intensity of BISBS ligand has a constant value.
Fig. 4. The excitation spectra of the new BISBS sensor in free form and after titration with Ca(II) ions using
Ca(II) within range (0-1.6 µM).
UV-Vis spectra of the BISBS are measured under same optimum aqueous conditions. The
absorption spectra show three main peaks centered at 250, 325 and 380 nm, the two characterized
peaks cantered at 250 and 325 nm were disappeared. On the other hand, the peak centered 380 nm
was enhanced gradually adding Ca(II) ions to aqueous solution of free BISBS chemi-sensor.
Further, the significant changes in the UV-visible spectra of the BISBS during titration with Ca(II)
ions proved the formation of Ca(II)-BISBS complex as shown in Fig. 5. Furthermore, UV-visible
spectra show two isopestic points at which BISBS absorbance doesn’t alter through titration
process. The absorption spectra turn constant without any alteration when the BISBS to Ca(II) ions
ratio is becoming (1:1). Thus, this absorbance behavior proves that the stoichiometry of metal-
ligand ratio of the formed complex is 1:1. Additionally, The red shift of the absorbance spectra is
due to the change of the ICT process which involves the interaction of Ca(II) receptor and BSIBS
as a donor species.
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Fig. 5. UV-Vis spectra of BISBS free ligand and Ca(II)-BISBS complexes (free ligand, formation of Ca
complexes and saturation).
The influence of the other metal ions on the fluorescence intensity of the new BISBS sensor
was studied under optimized conditions. Different types of alkali, alkaline-earth and transition
metal ions were utilized. Particularly, most of the metal ions show a small alteration or quenching
effects on the fluorescence intensity of the free BISBS ligand in aqueous medium containing of 20
µM HEPES buffer, pH 7.6. The BISBS was excited under 380 nm. Fig.6 represents the effect of
different metal ions on the fluorescence maxima of the new chemi-sensor. The concentrations of
the free ligand and the specified metal ions were M 0.8 µM and 1.6 µM respectively. The relation
gives precious prove about the high response of the BISBS to the Ca(II).
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Fig. 6. The influence of different metal ions on the fluorescence of the BISBS new sensor.
Moreover, the sensitivity and selectivity of the free BISBS chemi-sensor to Ca(II) were
examined in presence of other significant metal ions. Fig. 7 show a great potential of the BISBS
ligands towards Ca(II) ions with a highly chelating interaction. The sensing process of 0.8 µM
BISBS with 1.6 µM concentration Ca(II) was carried out in presence of different cations at 1.6
µM, and within mix with different metal cations, respectively. The solution condition proceeded
at pH 7.6 and 20 µM HEPES buffer. Data show that there is no influence or significant alteration
of the fluorescence property of BISBS with Ca (II) with existence or absence of different metal
ions.
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Fig. 7. (a, b) The fluorescence intensity of BISBS in presence of Ca(II) and different metal ions at 1.6 µM.
Under excitation wavelength 380 nm and emission wavelength at 502 nm.
Selective binding of Ca(II) compared to other metal ions could be attributed to: (i) the
compatibility between the ionic size of M2+ and the dimension of hole available by appending
groups that are binded M2+ ions (M2+, ionic radii (Å): Ca2+, 1.00; Cd2+, 0.78; Mg2+, 0.72; Co2+,
0.65; Ni2+, 0.44). (ii) The literature density functional theory (DFT) studies reported for the related
Ca(II)-sensor complex [28] which highlighted that the Ca(II) ion is coordinated with eight oxygen
atoms in the opened rings, while the othe M(II) ions are coordinated in a six-chealtion mode (i.e.
closed), futhermore, electron-withdrawing groups (such as ) are known to stabilize the ring-opened
system and thus favoring Ca(II) ion binding.
3.6. Recycling of senor
The new sensor exhibited a good reversibility as revealed by the restoration of its fluorescence
performance upon the addition of ethylenediaminetetraacetate (EDTA), a very strong Ca(II) ion chelator.
EDTA was interred in competition with our sensor for complexation with Ca(II) ions. By adding an
equivalent amount of EDTA into the sensing system containing the probe and 10 mM of Ca(II) ions,
fluorescence recovery as observed as a result of the BISBS-EDTA exhange to form Ca(II)-EDTA complex
and the liberation of the BISBS probe. Thus, a reduction in fluorescence at 502 nm was observed until they
reached 90% of their original intensity.
3.7. Quantum yield measurement
The quantum yield values were estimated for both free (BISBS) ligand and the metal
complex to be 0.191 and 0.342 correspondingly. The estimated values were calculated by using
quinine sulfate as probe of standard quantum yield [29], standard solution of 5x10-5 M quinine
sulfate in sulfuric acid yields a quantum yield of 55%. The calculation of quantum yield is
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produced as following with limitation error for quantum yield (±7 %) are predicted roughly.
:
AR. IS. η2S
QX = QR
AS. IR. η2R
Where
 R and S are the referencing solutions of BISBS or metal complex
 η is the solvents refractive index at 25 ⁰C
 I is the area that integrated under fluorescence band
 A is the absorption maxima of all cases.
Conclusion
In conclusion, a new BISBS (BISBS), N,N'-Bis-[5-((2,4- lutidiniumchloride)-methylene) -
3-methoxysalicylidene)]-R,R-1,2-cyclohexandi-imine, based fluorescence swich-on chemo-sensor
for calcium metal ions was designed, synthesized and characterized. The novel bis-ionic Schiff
base sensor displayed precious sensitivity and selectivity for sensing Ca(II) ions in comparison to
different types of metal cations in aqueous solution. BISBS is applicable at physiological pH 7.6.
The mechanism of detection process established with emission intensity enhancement which is
depending on the internal charge transfer (ICT) mechanism in the way that Ca(II) cations acts as
a receptor and BISBS Schiff base ligand acts as a donor. The new BISBS chemi-sensor
consecutively provides low LOD 1.5 nM and significant detecting of Ca(II). Thus BISBS may
provide a novel precious methodology for calcium cations detection in environmenal and
biological samples.
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A novel water-soluble highly selective “switch-on” ionic liquid-based fluorescent chemi-
sensor for Ca(II).
Reda F.M. Elshaarawya,b,*, Reham Alia,c, Sayed M. Salehc,d,, Christoph Janiakb
aChemistry Department, Faculty of Science, Suez University, 43518 Suez, Egypt.
b
Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine Universität Düsseldorf, 40204 Düsseldorf,
Germany.
cChemistry Department, Faculty of Science, Qassim University, Buraidah, KSA
dChemistry Branch, Department of Science and Mathematics, Faculty of Petroleum and Mining Engineering, Suez
University, 43721 Suez, Egypt.
*Corresponding authors:
Reda_elshaarawi@science.suez.edu.eg
Reda.El-Shaarawy@uni-duesseldorf.de
Highlights
 Practical synthetic route of new bis-ionic Schiff base sensor (BISBS) has been reported.
 Structural characterization of BISBS was discussed.
 The new fluorescent sensor (BISBS) was applied for sensing of Ca(II) ions in aqueous media.
 BISBS exhibits outstanding Ca(II) sensing performance with higher selectivity.
 Internal charge transfer (ICT) is the proposed fluorescence mechanism for BISBS.
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