A novel terephthalaldehyde based turn-on fluorescent chemosensor for Cu2+ and its application in imaging of living cells

Article abstract of DOI:10.1039/c7pp00182g

A new terephthaldehyde-based chemosensor 1 bearing an aminophenol recognition unit has been synthesized and applied to the fluorescent sensing of metal ions. Molecular system 1 acts as a highly selective and sensitive fluorescence turn-on sensor for Cu

Full text of DOI:10.1039/c7pp00182g

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Torawane, K. Keshav, M. K. Kumawat, R. Srivastava, A. thangaraj, S. K. Sahoo and A. U. Borse, Photochem.
Photobiol. Sci., 2017, DOI: 10.1039/C7PP00182G.
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A novel terephthalaldehyde based turn-on fluorescent chemosensor for Cu2+ and its
application in living cells imaging
Pritam Torawanea, Karunesh Keshavb, Mukesh K Kumawatc, Rohit Srivastavac, Thangaraj
Anandd, Suban Sahood, Amulrao Borsea*, Anil Kuwara*
aSchool of Chemical Sciences, North Maharashtra University, Jalgaonꢀ425001 (Maharashtra),
India.
bDepartment of Chemistry, Indian Institute of Technology, Powai, Mumbai (Maharashtra), India.
cDepartment of Biosciences and Bioengineering, Indian Institute of Technology Bombay, Powai,
Mumbai 400 076, India
dDepartment of Applied Chemistry, SV National Institute Technology, Surat (Gujrat), India.
Abstract
A new terephthaldehydeꢀbased chemosensor 1 bearing aminophenol recognition unit has
been synthesized and applied for the fluorescent sensing of metal ions. The molecular system 1
acts as a highly selective and sensitive fluorescence turnꢀon sensor for Cu2+. The sensing
mechanism has been explored. It is proposed that Cu2+ binds with imine and hydroxyl moiety of
1 in 1:2 binding stoichiometry, thereby enhanced the fluorescence at 386nm. The detection limit
and association constant (Ka) of 1 with Cu2+was found to be 0.62µM and 6.67×104 Mꢀ1,
respectively. The chemosensor 1 has shown excellent specificity towards Cu2+ and has been
successfully applied for the determination of Cu2+ in live L929cells.
Keywords: Chemosensor, Schiff base, turnꢀon fluorescence, Cu2+, intracellular imaging.
*Corresponding authors (Anil Kuwar): Eꢀmail: kuwaras@gmail.com; Tel.: +91ꢀ257ꢀ2257432;
Fax: +91ꢀ257ꢀ2257403, (Amulrao Borse) amulborse@gmail.com
1

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Introduction
Metal ions though present in small amount, play indispensable roles in biology including
structural support, storage, electron transfer, dioxygen (O2) binding, and catalytic function.1ꢀ
4There are increasing evidences that irrespective of whether they are surplus or deficient, it may
induce various disorders. Cu2+ an essential micronutrient for all known life forms, playing the
crucial role of cofactor in nearly 20 enzymes and descending the next place to iron and zinc
among the transition metal in body.2 Equally, Cu2+ is a significant pollutant due to its excessive
use all over the map.3 Long term exposure to high levels of Cu2+ has been reported to induce
liver and kidney damage and can cause oxidative stress and disorders associated with
neurodegenerative diseases such as Menkes syndrome, Wilson disease, Alzheimer’s disease and
prion disease.4 Owing to the continuous attention over Cu2+ in the environment and its diagnostic
role in biological system, the development of Cu2+ selective probe by fluorescence technique
emerged as an axial point among the sensing nation.5ꢀ7 The detection based on simple fluorescent
probes offer innate advantages over traditional techniques such as appreciable detection
sensitivity and selectivity, reliability, operational simplicity and easy signal detection readout in
compared to other traditional analytical methods.8 However, most of the recently reported Cu2+
selective probes offer disadvantages such as multiꢀstep synthesis, cross sensitivity towards other
ions and lack of bioꢀcompatibility.9All these limitations restricted their potential applications in
environment and biology. On the other hand, selective fluorescence sensing of Cu2+over other
paramagnetic cations (Fe3+), heavy atoms (Hg2+ and Pb2+) is still a challenging work because all
of these metal ions can show quenching of fluorescence upon binding of probe via
energy/electron transfer or heavy atom enhanced intersystem crossing/spinꢀorbit coupling.10
2

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Therefore, the development of fluorescence sensors with high selectivity and sensitivity for
sensing of Cu2+ from aqueous media is of current interest.
In this paper, we have successfully employed a novel aminophenolꢀsubstituted Schiff base
ligand embedded for the determination of Cu2+ ion.11 These components make up chemosensor 1,
a terephthaldehyde moiety act as a reporter and aminophenol moiety act as a metal ions
coordination site. Binding of Cu2+ ions to chemosensor 1 causes remarkable fluorescence
enhancement at 386nm. The selectivity of the probe 1 was tested with all metal ions, only 1.Cu2+
shows fluorescence enhancement at 386nm and further the probe was successfully applied into
the intracellular imaging of Cu2+ ions.
Experimental
All reagents and chemicals were purchased from Aldrich Chemicals Ltd and were used
without further purification. The solvents were distilled before use. The reaction was monitored
by thinꢀlayer chromatography (TLC).The FTꢀIR and mass spectra were recorded by using a
Shimadzu FTIRꢀ8400 and Bruker Ultraflex II MALDI/TOF spectrometer respectively.The1H and
13C NMR spectra were obtained on a Bruker AVANCE II 400 MHz spectrometer in DMSOꢀd6
as solvent. Fluorescence measurements were made with a HORIBA JOBIN YVON Fluoromaxꢀ4
spectrofluorometer equipped with a xenon lamp. The excitation wavelength and emission slit
width was 300 nm and 5/5 nm, respectively. UVꢀVis absorption spectra were recorded on a
Shimadzu UVꢀ2450 spectrophotometer. All crystal structures were solved by direct methods.
The program SAINT (version 6.22) was used for integration of the intensity of reflections and
scaling. The program SADABS was used for absorption correction. The crystal structures were
solved and refined using the SHELXTL (version 6.12) package. All hydrogen atoms were
included in idealized positions, and a riding model was used. Nonꢀhydrogen atoms were refined
3

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with anisotropic displacement parameters. The data collection and structure solution parameters
for compound 1 are given in supporting Information (Table S1).
General procedures for spectra acquisition
The stock solution of probe 1 (1×10ꢀ5M) was prepared in acetonitrile. The cationic
solutions (1×10ꢀ4M) such as Na+, K+, Ca2+, Mg2+, Al3+, Co2+, Ni2+, Cu2+, Zn2+, Cr3+, Ba2+, Cs+,
Cd2+, Pb2+, Ag+ and Sr2+ were prepared in aqueous medium. Each time, a 2 mL solution of 1 was
taken to a quartz cell of 1 cm optical path length, and then appropriate volume of the stock
solutions of all cations (1×10ꢀ4M) were added to it by using a micropipette. The binding
stoichiometry of 1 with Cu2+ was determined by using a Job’s plot. For the Job’s plot analysis, a
series of solutions with varying mole fractions of the Cu2+ were prepared by maintaining the total
concentration of 1 and Cu2+ constant. The maximum fluorescence intensity at 386 nm for each
solution was plotted against the mole fraction of the Cu2+ ion.
Synthesis of receptor 1
2ꢀaminophenol (0.325 g, 2.0 mmol) was added to an ethanolic solution of terephthaldehyde
(0.2g, 1.0 mmol) and the mixture was stirred at room temperature for 4 hrs. The golden yellow solid
obtained at room temperature was filtered, dried and further purified by recrystallization (84%
yield). FTꢀIR (cmꢀ1): 3365.90, 3049.56, 1917.31, 1791.93, 1598.08, 1491.99, 1369.50, 1230.63,
1
1149.61, 842.92, 746.48, 589.27. 519.83. H NMR (400 MHz, δ, ppm): 8.91(s, 1H, OH), 8.76(s,
1H, CH=N), 8.13(s, 2H, ArꢀH), 7.24 (dd, 1H, J = 1.2 and 6.6 Hz, ArꢀH), 7.10 (dt, 1H, ArꢀH ), 6.92
13
(dd, 1H, J =7.0 Hz and 0.96 Hz, ArꢀH), 6.84(dt, 1H, J = 7 Hz and 1.0 Hz, ArꢀH); CꢀNMR(100
MHz, δ, ppm):115.93, 118.61, 119.39, 127.71, 128.94, 137.23, 138.46 (Ar), 151.48, 157.89
(CH=N); HRꢀMS calcd forC20H17N2O2(M+H)+m/z317.1290,observed 317.1287.
4

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Synthesis of 1.Cu2+ complex
Copper acetate in water (0.1 g, 1.0 mmol) was added dropwise to the solution of receptor
1(0.316 g, 2.0 mmol) in acetonitrile and then the mixture was reflux for 8hrs. The dark maroon
red colour solid obtained was filtered, dried and further purified by recrystallization (80% yield).
FTꢀIR (cmꢀ1): 3336.00, 3043.77, 1904.77, 1681.02, 1580.72, 1471.74, 1386.86, 1283.67,
1166.97, 949.97, 838.10, 745.51, 595.06; LCMS calcd forC40H30CuN4O4 (M+H) +m/z = 694.24,
observed 693.43
In vitro cell imaging
L929 live cells were seeded over sterilized glass cover slips in wells of 12 well plates
with concentration of 20000 cell/well. Cells were incubated at 37°C in 5% CO2 under humidified
condition. After 24 hours of incubation, the cells were treated with 100µL of compound1
(DMSO+PBS, 1:9 ratio) with concentration of 0.26µM and kept for 30 minutes under incubation
condition. After 30 minutes, cells were washed twice with PBS (pH 7.0) for removing extra
compound. After that 100µL PBS and Cu2+ (0.26µM) was added in to the cell and then again
kept for 20 minutes incubation. After incubation, copper treated cells were washed with PBS to
ensure the complete removal of extra salt. Then, cells present in all wells were fixed by treatment
with 1.0 mL of 3.5% formaldehyde solution for 15 minutes. After careful washing of fixed cells
with PBS solution, it was imaged under CLSM.
Results and discussion
The chemosensor1 was synthesized by condensation reaction of terephthaldehyde with
2.0 equivalent of aminophenol in ethanolic medium by following our previously reported
procedure (Scheme1a).12The structure of the receptor 1 was well characterized by using1H
NMR, 13C NMR, FTꢀIR and HR–MS (Fig. S1-S4).
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Scheme 1. (a) Synthetic scheme of receptor 1and (b) the binding mode of 1 with Cu2+.
X-ray Crystallographic Studies
Yellow needle crystals suitable for an Xꢀray diffraction study were obtained by slowly
evaporating a hexane/dichloromethane solution of 1 at room temperature in period of one week.
Singleꢀcrystal Xꢀray diffraction structure analysis indicates that 1 crystallizes in monoclinic
system, P2(1)/c space group (Table S1,SI).The CIF file for receptor 1 was deposited in the
Cambridge Structure Database with CCDC No 1524229. Compound 1 exists as an Sꢀtrans type
6

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Schiff base chain with a terephthaldehyde group extending out of the aminophenol ring. From
the dihedral angles of 1, all atoms in 1 are located almost in the same plane (Fig.1). The terminal
phenyl ring is in plane with respect to the each other and makes an angle of 10.39(7)° with the
central phenyl ring. This suggests that the whole molecules are almost planar and conjugated.
Moreover, rodꢀlike molecules of compound interacts with neighboring molecule via hydrogen
bonding interactions O1ꢀH1...O1 2.702 Å to form a sheet like structure. Further this sheet like
structure grows up and down via face to face stacking interactions π C(5)–H(10)… π(C1–C3)
with the contact distance of [3.683(1)] Å to forms form a stair like 3D structural arrangement
along bꢀaxis (Fig. 2).
Fig. 1.Xꢀray crystal structure of receptor 1.
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Fig. 2. Two views for the dimer formation via π−π interactions (broken line indicates π−π
interactions).
Cation sensing behavior
The recognition ability of the receptor1(1×10ꢀ5 M, CH3CN)towards various cations, such
as Na+, K+, Ca2+, Mg2+, Al3+, Co2+, Ni2+, Cu2+, Zn2+, Cr3+, Ba2+, Cs+, Cd2+, Pb2+, Ag+ and
Sr2+(1×10ꢀ4 M, H2O) was investigated by fluorescence spectroscopy. In the absence of metal
ions, the receptor1showed a weak fluorescence band at 331 nm when excited at 300nm. Among
the various metal ions examined, the receptor1showed significant fluorescence enhancement at
386 nm in the presence of Cu2+ (Fig. 3). The quantum yield (Φ) of the receptor 1 (Φ = 0.001353) was
increased in the presence of Cu2+ (Φ = 0.026638). However, the fluorescence of 1 remains unchanged
with the addition of the other ions such as Na+, K+, Ca2+, Mg2+, Al3+, Co2+, Ni2+, Zn2+, Cr3+, Ba2+,
Cs+, Cd2+, Pb2+, Ag+ and Sr2+ demonstrating the selectivity of the receptor 1 towards Cu2+(Fig.
S5).
The competitive experiment was conducted to investigate the Cu2+ detection ability of 1
in the presence other interfering metal ions.The competition experiments revealed that the Cu2+(2
equivalents) induced fluorescence enhancement at 386 nm was unaffected even in the presence
of 2 equivalents of environmentally relevant alkali or alkalineꢀearth metals and transition metal
ions such as Na+, K+, Ca2+, Mg2+, Co2+, Ni2+, Zn2+, Ba2+, Cs+, Cd2+, Pb2+, Ag+ and Sr2+except the
trivalent metal ions Al3+and Cr3+(Fig. S6).
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Fig.3.Fluorescencechanges of receptor1 (1×10ꢀ5 M, CH3CN)occurs upon addition of different
metal ions such as Na+, K+, Ca2+, Mg2+, Al3+, Co2+, Ni2+, Zn2+, Cr3+, Ba2+, Cs+, Cd2+, Pb2+, Ag+
and Sr2+(1×10ꢀ4 M, H2O).
To study the nature of emission 1 (1×10ꢀ5 M, CH3CN), the fluorescence titration
experiment was recorded with successive incremental addition of Cu2+ (10ꢀ1000µL, H2O) (Fig.
4). The fluorescence intensity at 331nm was decreased up to the addition of one equivalent of
Cu2+.Upon further addition of Cu2+, the fluorescence intensity was enhanced at 386 nm. The
weak fluorescence from 1 may be due to the combined effects of photoꢀinduced electron transfer
(PET) and the C=N isomerization at the excited state. As described in Scheme 1b, the steady
decrease in the fluorescence intensity of 1 at 331 nm was observed up to the addition of one
equivalent of paramagnetic Cu2+ is presumably due to the presence of an uncomplexed
diminophenol unit of 1that allowed the PET process as well as the conformational C=N
isomerization at the excited state. However, the observed fluorescence enhancement at 386 nm
upon further addition of Cu2+ can be explained from the inhibition of C=N isomerization due to
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the possible coordination of Cu2+ through the hydroxylꢀO, imineꢀN atoms of the receptor 1that
resulted chelationꢀenhanced fluorescence effect (CHEF) and also blocked the PET process.13
Fig.4.Fluorescence emission spectra of 1(1×10ꢀ5 M, CH3CN) upon incremental addition of Cu2+
(1×10ꢀ4 M, H2O).
The binding constant Ka of the complex species formed in solution was evaluated from the
fluorescence titration profile by using the BenesiꢀHidelbrand equation.14 From the fluorescence
titration data, the binding constant of 6.67×10ꢀ4 Mꢀ1was estimated (Fig. S7).Also, the detection
limit of the probe 1 with Cu2+ was calculated to be 0.62µM by applying the standard IUPAC
method of 3σ. The obtained detection limit was comparable/superiour than the recently reported
Cu2+ selective chemosensors (Table S2).15ꢀ17 The binding stoichiometry of 1 with Cu2+ was also
confirmed by Job’s plot method (Fig. S8).18 It clearly shows that the probe 1 forms complex with
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Cu2+ in 2:1 binding stoichiometry. It was further supported by ESIꢀMS, wherein the molecular
ion peaks at m/z = 693.43 correspond to C40H30CuN4O4 (Fig. S9). The IR spectra of 1·Cu+2
complexes shows hydroxyl group peak is still exist as 3336.00 and it revealed the involvement of
only one hydroxyl group in Cu+2 ion binding (Fig S10). The hydroxyl group acts as a good
binding site and plays an effective role in binding of transition metal ion.
UV-Visible studies
Under similar conditions adopted for the fluorescence study, the selectivity of 1 with metal
ions such as Na+, K+, Ca2+, Mg2+, Al3+, Co2+, Ni2+, Cu2+, Zn2+, Cr3+, Ba2+, Cs+, Cd2+, Pb2+, Ag+
and Sr3+ was investigated by using the UVꢀVisible absorption spectroscopy. The absorption
spectrum of probe 1 shows a band at 385 nm in acetonitrile solution can be assigned to the n–π*
transition. Addition of Cr3+, Cu2+ and Al3+ resulted slight decrease in the absorbance of 1 at 385
nm but no noticeable changes in the ground state electronic properties of 1 was observed with
other tested metal ions (Fig. S11). In the presence of Cu2+, a broad absorption between 450 to
600 nm was also observed that cause a nakedꢀeye detectable color change of 1 from light yellow
to maroon red (Fig. 5). These results additionally support the formation of complex between 1
and Cu2+ in the ground state.
The quantum mechanically computed structure of 1 and its complex with Cu2+ was
theoretically obtained by applying the DFT method (B3LYP/6ꢀ31G**/LANL2DZ) in the gas
phase. All calculations were performed by using the computational programmer Gaussian
09W.19 The basis set LANL2DZ was considered only for Cu atom, whereas 6ꢀ31G** for
remaining C, H, N and O atoms. The calculated lowering in the interaction energy
(
Eint = Ecomplex −
Ereceptor − 2ECu2+) for the complexation between 1 and Cu2+ was found to be ꢀ
240.49 kcal/mol, which indicates the formation of a stable complex (Fig. 6). Also, the planarity
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of 1was destroyed upon complexation. The frontier molecular orbitals (FMOs) plots of 1 and its
complex with Cu2+ was analyzed which indicate the uniform distribution of charge density
within the receptor 1surface before complexation. After complexation, the charge density was
mainly observed above the phenolate unit and the Cu2+ that indicate the intramolecular charge
transfer (ICT) occurred between the metal ions and the receptor 1 (Fig. 6). Consequently, the
band gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) of 1 was lowered on complexation with Cu2+ due to the formation of
a stable complex that corroborates well with the appearance of maroon red coloration.
Fig. 5. UVꢀVisible spectra of probe 1 (1×10ꢀ5 M, CH3CN)in the absence and presence of Cu2+.
Inset shows the colorimetric changes of 1 and 1+Cu2+ion.
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Fig. 6.DFT computed (a) optimized structure of receptor 1 and its complex with Cu2+, and the
(b) LUMO and (c) HOMO diagrams of 1 and its 1−Cu2+ complex.
Intracellular imaging
The current research clearly revealed the selective complexation of 1 with Cu2+, which
resulted a significant fluorescence enhancement at 386 nm. The efficient sensing ability of 1with
Cu2+ ions make us to explore its application in live cell imaging of Cu2+ ion in live L929 cells
(Fig. 7). The probe 1 was found to be nonꢀtoxic to the cells under the experimental conditions.
L929 cells were incubated with the probe 1 (0.26 µM) for 30 minutes and subjected to
fluorescence measurement. The fluorescence and bright field images were taken through a
confocal laser scanning microscopy with 300 nm excitation; insignificant fluorescence changes
was observed in probe 1 treated cells. The cells were then washed thrice with HEPES buffer in
order to remove the excess probe present in the cells and then incubated with CuCl2 (0.26 µM)
°
for 30 minutes at 37 C. The Cu2+ treated cells were imaged through confocal fluorescence
microscope. A bright blue fluorescence signal was observed, which is consistent with the
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fluorescence “turnꢀon” profile of the probe 1 with Cu2+ in the solution. Besides, bright field
image measurement clearly shows that the cells are viable throughout the imaging experiments.
In addition, overlay of bright field and fluorescence images of probe and Cu2+ ions treated cells
confirmed that the observed fluorescence signals are from the intracellular region.
Fig.7.Fluorescence intracellular imaging data: (aꢀd) control (eꢀh) L929 living cells with receptor
1, and (iꢀl) cells with probe 1 after incubation with Cu2+ for 30 minutes.
4. Conclusion
In summary, we have synthesized a new terephthaldehydeꢀaminophenol conjugate1 and
applied for the fluorescent sensing of metal ions. The probe1 exhibited a high selectivity and
sensitivity for the 'turnꢀon' fluorimetric detection of Cu2+ with the detection limit down
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to0.67µM. The fluorescence 'turnꢀon' sensing mechanism is proposed on the basis of
complexationꢀinduced inhibition of C=N isomerisation, PET process along with the CHEF. This
probe shows no any significant interference from other tested metal ions, has been successfully
applied in the bio imaging of Cu2+ in living L929cells.
Acknowledgments
AK acknowledges Department of Science and Technology Government of India for funding and
School of Chemical Sciences, North Maharashtra University, Jalgaon, India for providing
infrastructure for the accomplishment of this project.
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DOI: 10.1039/C7PP00182G
Graphical Abstract
A new terephthaldehyde-based chemosensor 1 containing aminophenol recognition unit was
synthesized. This probe exhibits highly selective and sensitive fluorescence turn-on sensor for
Cu2+ and successfully applied to the determination of Cu2+ in L929cells.