A new chromenylium-cyanine chemosensor for switch-ON near-infrared copper (II) sensing

Article abstract of DOI:10.1016/j.molstruc.2019.127640

A highly selective and sensitive near-IR chemosensor having a chromenylium-cyanine backbone for the detection of Cu²⁺ in aqueous samples has been designed and synthesized. The sensor has exhibited linear optical response to copper ions at the range of 0–4.9 × 10^?6 M in the presence of other competing metal ions, such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺, Al³⁺, Cd²⁺, Ag⁺, Sr²⁺, Pb²⁺, Hg²⁺ and Ba²⁺. The limit of detection (LOD) was calculated as 1.28 × 10^?8 M which was lower than that of previously reported analogues. The sensor allows detection of the copper(II) ion through the spirocyclic ring-opening reaction by coordination of Cu²⁺.

Full text of DOI:10.1016/j.molstruc.2019.127640

Journal Pre-proof
A new chromenylium-cyanine chemosensor for switch-ON near-infrared copper (II)
sensing
Kaan Karaoglu
PII:
S0022-2860(19)31749-1
DOI:
https://doi.org/10.1016/j.molstruc.2019.127640
MOLSTR 127640
Reference:
To appear in: Journal of Molecular Structure
Received Date: 24 October 2019
Revised Date: 20 December 2019
Accepted Date: 22 December 2019
Please cite this article as: K. Karaoglu, A new chromenylium-cyanine chemosensor for switch-ON
near-infrared copper (II) sensing, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2019.127640.
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Kaan KARAOGLU: Conceptualization, Methodology, Validation, Investigation.

A new chromenylium-cyanine chemosensor for switch-ON near-infrared copper
II) sensing
Kaan KARAOGLU
(
1
*
1
Department of Chemistry and Chemical Process Technology, Vocational School of
Technical Sciences, Recep Tayyip Erdogan University, Rize, TURKEY
*
Correspondence: kaan.karaoglu@erdogan.edu.tr
ORCID:
Kaan KARAOGLU: https://orcid.org/0000-0003-3265-8328
Abstract: A highly selective and sensitive near-IR chemosensor having a
2
+
chromenylium-cyanine backbone for the detection of Cu in aqueous samples has been
designed and synthesized. The sensor has exhibited linear optical response to copper
-6
ions at the range of 0–4.9×10 M in the presence of other competing metal ions, such as
+
+
2+
2+
3+
2+
3+
2+
2+
2+
3+
2+
+
2+
2+
Na , K , Mg , Ca , Cr , Mn , Fe , Co , Ni , Zn , Al , Cd , Ag , Sr , Pb ,
2
+
2+
-8
Hg and Ba . The limit of detection (LOD) was calculated as 1.28×10 M which was
lower than that of previously reported analogues. The sensor allows detection of the
2
+
copper(II) ion through the spirocyclic ring-opening reaction by coordination of Cu .
Key words: Near-IR chemosensor, copper sensing, chromenylium-cyanine
1
.
Introduction
In the last couple of decades, there has been a growing interest in design of ion-selective
colorimetric sensors owing to their potential applications, such as tissue imagining [1],
online environmental monitoring [2], investigation of cellular events [3] and even in
aerospace technology [4]. Xanthene dyes are one of the most studied classes of
1

compounds thanks to their optical properties, such as high molar absorptivity, high
fluorescence quantum yields and relatively simple preparation procedures. Xanthenes
enable ion sensing by colour change from colourless to pink associated with spirolactam
ring opening through coordination or irreversible reactions [5,6].
The permitted limit of copper in drinking water has been established as 20 ꢀM by the
U.S. Environmental Protection Agency (EPA) [7]. A series of rhodamine B Schiff-base
derivative-sensors have been synthesized, and their copper sensing properties have been
also investigated [8–11]. Although these sensors have effectively detected the ion
content of analysis media, the main disadvantages of rhodamine B Schiff-bases are
lower tissue penetration, photodamage to biomolecules and photoscattering in in vivo
cell imagining. Thus, new copper sensors – having higher selectivity and sensitivity
toward copper ions – have attracted increasing attention due their potential applications
in biomolecular monitoring functions. In the last decade, chromenylium-cyanine
fluorophore (CC-Fluor) sensors have facilitated online monitoring of metal ions lying in
the NIR region (620−760 nm) [12].
In this study, a new chromenylium-cyanine near-IR fluorescent probe was designed and
2
+
characterized for selective and sensitive determination of Cu in aqueous solutions. The
selectivity and sensitivity of the sensor have been investigated in a group of metal ions
+
+
2+
2+
2+
2+
3+
2+
2+
2+
2+
3+
+
containing Na , K , Mg , Ca , Cr , Mn , Fe , Co , Ni , Cu , Zn , Al , Ag ,
2
+
2+
2+
2+
2+
Cd , Sr Hg , Pb and Ba , and analytical data have been compared with studies
that produced similar molecular structures [6,13]. A chromenylium-cyanine derivative
2
+
was found to be highly selective and sensitive to Cu among other metal ions at the
near-IR region.
2

2
.
Experimental
3
-Diethylaminohenol, phthalic anhydride, hydrazine hydrate and (benzotriazol-1-
yloxy)tris(dimethylamino)phosphonium hexafluorophosphate were obtained from
Sigma Aldrich and cyclohexanone were obtained from TEKKIM, a local supplier from
Turkey. The solvents used in chromatography were of reagent grade and used without
further purification – except for ethanol, which was dried over molecular sieve type
+
+
2+
2+
2+
3+
2+
2+
2+
2+
3+
4
A. Metal ion solutions, Na , K , Mg , Ca , Cr , Fe , Co , Ni , Cu , Zn , Al ,
+
2+
2+
2+
2+
2+
Ag , Cd , Sr Hg , Pb and Ba , were prepared freshly from their nitrate salts,
2
+
except for Mn , which was prepared from acetate salt.
2
.1. Synthesis of sensor
4'Z)-2-amino-6'-(diethylamino)-4'-{2-[(2Z)-1,3,3-trimethyl-2,3-dihydro-1H-indol-2-yl
den]ethyliden}-1',2,2',3,3',4'-hexahydrospiro[isoindol-1,9'-xanthene]-3-on was
synthesized by following the synthetic route described in Figure 1 [6,14]. Compound 4
0.573 g, 1 mmol) was dissolved in absolute ethanol and solution purged by argon for
0 min. Then, 2-hydroxy-1-naphthaldehyde (1.1 mmol) was added into the solution
(
(
3
and the mixture was stirred at ambient temperature for 48 h. The solvent was
evaporated by rotary under reduced pressure, and the crude product was purified by
chromatography (Rf: 0.57, EtOH/CH Cl , 1:20, v/v). Colour: Yellow. Yield: 26%.
2
2
+
1
Analytical data for C H N O , ESI-MS (m/z) 726 [M+H] (726.90 calc.). H NMR
4
8
46
4
3
data (500 Mhz, CDCl ) δ, ppm: 1.160 (t, J= 7.0 Hz, 6H), 1.385 (s, 1H), 1.633–1.609
3
(m, 3H), 1.795 (s, 3H), 1.824 (s, 3H), 3.148 (s, 3H), 3.333 (q, J=7.5 Hz, 4H), 5.387 (d
J=12.5 Hz, 1H); 6.286 (d, J=8.5 Hz, 1H), 6.449 (s, 1H), 6.500 (d, J= 9.0 Hz), 6.634 (d,
J=8.0 Hz, 1H), 6.879 (t, J=7.5 Hz, 1H), 7.166 (d, J=6.5 Hz, 1H), 7.147 (d, J=6.0 Hz,
3

1
7
H), 7.210 (t, J=6.5 Hz, 1H), 7.452 (t, J=8.0 Hz, 1H), 7.521 (t, J=7.5 Hz, 1H), 7.587 (t,
.5 Hz, 1H), 7.637 (t, J= 6.0 Hz, 1H), 7.700 (dd, J= 8.5Hz, j= 3.5 Hz, 2H), 7.819 (d,
J=8.0 Hz, 1H), 7.908 (d, J=8.5 Hz, 1H), 8.001 (d, J=8.5 Hz, 1H), 8.370 (d, J=8.5 Hz,
1
3
1
2
1
1
1
1
H), 9.685 (s, 1H), 10.838 (s, 1H). C NMR data (126 Mhz, CDCl ) δ, ppm: 12.252,
3
2.079, 23.010, 28.382, 28.473, 29.122, 44.334, 45.506, 67.849, 92.147, 97.562,
03.170, 104.460, 105.735, 108.701, 108.986, 118.606, 119.194, 119.873, 120.364,
21.542, 123.084, 123.521, 124.513, 127.157, 127.731, 127.989, 128.058, 128.765,
29.141, 129.493, 129.677, 132.443, 132.511, 133.391, 138.837, 139.180, 145.346,
46.522, 149.043, 152.398, 158.851, 164.378.
4

O
OH
OH
OH
O
O
i
+
O
H C
3
N
H C
3
N
O
1
H C
3
H C
3
ii
O
O
HO
iii
HO
O
N
CH3
CH3
O+
N
CH3
ClO -
4
C
H C
N+
CH3
CH3
H
3
3
2
3
iv
OH
O
O
N
N
H N
2
N
v
O
N
CH3
CH3
O
N
CH3
C
H
3
H C
3
CH3
CH3
H C
3
CH3
CH3
N
N
4
5
Figure 1. Synthesis scheme of the sensor. i: Toluene, 3h, 35% NaOH, 10,0 M HCl; ii:
H SO , cyclohexanone, 2h; iii: 2-(1,3,3-trimethylindolin-2-ylden)acetaldehyde; iv:
2
4
hydrazin hydrate, BOP reagent; v: 2-hydroxy-1-naphthaldehyde.
2
.2. Spectrophotometric assays
-3
In typical metal sensing analysis, 1×10 M nitric acid (1.0%) solutions of the metal
+
+
2+
2+
2+
2+
3+
2+
2+
2+
2+
3+
+
2+
2+
ions, Na , K , Mg , Ca , Cr , Mn , Fe , Co , Ni , Cu , Zn , Al , Ag , Cd , Sr
5

2
+
2+
2+
2+
Hg , Pb and Ba , were prepared from their nitrate salts, except for Mn , which was
-5
prepared from acetate. In selectivity experiments, a 1×10 M sensor in 100 mM
tris/nitrate buffer (ethanol/water, 2:1, v/v) and 5 eq. each of metal ion were incubated
for 5 minutes, at which point spectrophotometric measurements were performed at the
visible region. The selectivity of the sensor as a colorimetric chemosensor for the
2
+
detection of Cu was also studied in the presence of various competing metal cations.
In a competition experiment, a series sensor solution was treated with mixtures
2
+
containing equimolar amounts of Cu and competitive ions.
The detection limit of the sensor based on standard deviation of the blank
measurements by The International Union of Pure and Applied Chemistry (IUPAC)
was determined from Eq. (1).
ꢃ×ꢄꢅ
ꢀꢁꢂ =
(1)
ꢄ
Where Sd is the standard deviation of the intercept that is calculated by regression
analysis at the 95% confidence level, and S is the slope of the calibration curve.
3
.
Results and discussion
3
.1. Characterization
The chemical structure of the sensor was characterized by spectroscopic methods, such
1
13
as FTIR, H and C NMR, UV/vis and mass spectrometry. The FT-IR spectrum of
-1
sensor is given in Figure S. 1. Phenolic –OH stretching was observed at 3292 cm , and
3
asymmetric and symmetric C–H stretching modes for sp hybridized carbon atoms were
-1
-1
observed at 2964 and 2926 cm , respectively. Signals detected at 1690 cm and 1620
-1
cm correspond to carbonyl and imine stretching modes of spirocycle, respectively. The
-1
disappearance of the carbonyl band and the shift of the imino stretching to 1616 cm
6

clearly suggest the formation of the ring-opened amide form of the sensor as a result of
2
+
Cu coordination (Figure S. 2 and Figure S. 3). The chemical structure of the sensor
was confirmed by NMR spectra, with the atoms labelled as shown in Figure S. 4 and
Figure S. 5. Signals corresponding to CH protons were detected as singlets at 3.148,
3
1
.824 and 1.795 ppm for H , H and H , respectively. A triplet signal has been
53 54 55
observed at 1.160 ppm (J=7.0 Hz), corresponding to 6H (H and H ). N-substituted
3
8
52
CH signals for H and H have been observed as a quartet at 3.333 ppm (J= 7.5 Hz).
2
37
51
CH signals belonging to the chromenylium ring system gave three sets of signals: H ,
2
20
H and one of the H . The other proton signal of H were observed as a singlet at
2
3
21
21
1
.385 ppm, because of non-planar structure of ring system (for detailed information, see
the theoretical calculations section) [15]. The signals of the imine proton and phenolic –
OH functional were observed at 9.685 and 10.838 ppm, respectively. The characteristic
1
3
C NMR signals of carbonyl (C6) and the C4 corresponding spirocycle closed form
were observed at 164.378 and 67.849 ppm, respectively (Figure S. 6 and Figure S. 7)
16]. Mass spectra of the sensor and its copper complex were also analysed by H-ESI
[
mass spectrometry. The peaks corresponding to the molecular ion and its sodium adduct
were observed at 726.25 and 748.50 m/z, respectively (for isotope distribution, see
Figure S. 8). The molecular ion peaks for copper complex with coordinated methanol
molecule were observed at 821.97 m/z in H-ESI mass spectra (Figure S. 9).
3
.2. Selectivity of the sensor
The selectivity and sensitivity of the similar structure derived from rhodamine B
hydrazide bearing 2-hydroxy-1-naphthaldehyde have been previously investigated, and
2
+
2+
2+
it was found that this derivative showed an optical signal toward Cu , Hg and Mg
7

[
6,13]. The selectivity of the chromenylium-cyanine based sensor was investigated in
+
+
an ethanol/tris-nitrate buffer (2:1, v/v, pH 7.02) with various metal ions such as Na , K ,
2
+
2+
2+
2+
3+
2+
2+
2+
2+
3+
+
2+
2+
2+
2+
Mg , Ca , Cr , Mn , Fe , Co , Ni , Cu , Zn , Al , Ag , Cd , Sr Hg , Pb
2
+
and Ba . While the other metal ions had a negligible response, the solution turned from
2
+
pale yellow to green upon addition of Cu to a solution of the sensor, and the
absorbance was significantly enhanced, with a new peak located at 710 nm (Figure 2).
Figure 2. Changes in absorption spectra of sensor (10 ꢀM) in ethanol/Tris–nitrate buffer
-5
(
2:1, v/v, pH 7.02) solutions – presence of 5.0×10 M metal ions (upper left), with
2
+
-6
various amounts of Cu ions (0-9.0×10 M, upper right), linear working curve derived
2+
2+
from titration with Cu at 710 nm (lower left) and optical response of sensor to Cu
ion among competitive metal ions.
8

Titration experiments showed that the sensor generates an optical signal as a function of
2
+
-6
increasing Cu ion having linearity from 0 to 4.9×10 M. The LOD value for the
2
+
-8
sensor toward the Cu ion by colorimetry was calculated as 1.28×10 M by the linear
working curve. It has previously been observed that a similar Schiff base structure
2
+
2+
having a xanthene ring backbone showed selectivity toward Hg and Cu ions at 558
-7
nm with and a LOD value of 2.0×10 M [6].
In order to investigate the selectivity of the sensor in the presence of competitive mono-,
+
+
2+
2+
2+
2+
3+
2+
2+
di- and trivalent metal ions such as Na , K , Mg , Ca , Cr , Mn , Fe , Co , Ni ,
2
+
3+
+
2+
2+
2+
2+
2+
Zn , Al , Ag , Cd , Sr Hg , Pb and Ba , the experiments were conducted by an
2
+
addition of 1.0 equivalent of Cu to solutions containing 2.0 equivalent of other metal
ions in the same media. The selectivity experiments showed that – as seen in Figure 2 –
2
+
none of the competitive metal ions had obvious interference with the detection of Cu .
2
+
This suggests that the sensor complex could selectively distinguish Cu ions from the
competing metal ions through the naked eye in ethanol/water mixtures (2:1, v/v) at pH
7
.02.
3
.3. Theoretical calculations
The molecular structure of the sensor and its copper(II) complex in the gas phase have
been investigated by density functional theory (DFT) using the Becke-3-Lee-Yang-Parr
(B3LYP) exchange-correlation functional with a 6-31G(d, p) basis set for the H, C, N
and O and LanL2Dz for the Cu atom [17,18]. Geometric structures, vibration
frequencies and vertical excitation energies have been investigated by the GAMESS
2
018 program [19], and computational data were visualized by Gabedit [20]. The
9

positive values of all calculated vibrational wavenumbers show that the optimized
molecular structures are stable at a minimum on the potential energy surface. The
lowest 10 singlet-singlet transitions of the sensor and 5 of its copper complex have been
calculated by time-dependent DFT (TD-DFT) formalism.
The DFT-optimized geometries of the sensor and its copper complex are given in Figure
3
. DFT calculations showed that the nitrogen atom of the N,N-diethyl amino unit
bounded to the chromenylium ring has a trigonal planar structure, and this data confirms
that the group is conjugated with the chromenylium ring system [21]. Because of angle
strain, the cyclic ring fused in chromenylium ring has an eclipsed conformation in
which two axial (C–H20a and C–H ) and three equatorial (C–H , C–H and C–H22b)
22a
20b
21a
bonds lie in almost one side of ring. Since the direction of C–H21b is on the other side of
the ring, the chemical environment of this proton changes dramatically, resulting in the
1
H NMR signal observed as a singlet in the upper field with respect to the adjacent
protons (see Figure S. 10).
Figure 3. DFT optimized structures of sensor (left) and its copper complex (right).
1
0

UV/vis. spectrum of the sensor and its copper complex were recorded at room
temperature (see Figure S. 11). TD-DFT calculations showed that the shoulder located
at 426 nm in experimental UV/vis. spectrum (calc. 443 nm) corresponds to the
HOMO→LUMO transition; and the peak located at 371 nm (calc. 368 nm) corresponds
to the HOMO→LUMO+1 transition (for detailed spectral analysis, see Supplementary
File, Figure S.12). These electronic excitations are mainly contributed by π→π*
transitions from 1,3,5-alkatrien to Schiff base through spirocycle (Figure 4). TD-DFT
calculations also showed that EHOMO and ELUMO of the sensor are -4.53 eV and -1.32 eV,
respectively. The experimental electronic spectrum and the calculated data, as well as
opt
the optical energy gap (Eg ), are in accordance with closed-form of the spirocycle ring
system.
Figure 4. HOMO (left) and LUMO (right) orbitals of the sensor (isovalue set to 0.02
a.u.)
It was suggested that the complex has the ring-opened amide form as a result of the
coordination of copper through the oxygen atom of carbonyl and the nitrogen atom of
1
1

imino moiety, which would be consistent with previously reported findings [6,22]. The
calculated geometry of reported complex by DFT showed that copper centre has square
planar geometry with a D–Cu–D bond angle (D represents donor atoms) of 90.04±6.07º
and a D–Cu–D dihedral angle of 176.2±0.8º. TD-DFT calculations showed that HOMO
was delocalized on Schiff base parts, while the LUMO was delocalized over the
chromenylium unit with a calculated HOMO–LUMO gap of 2.19 eV (Figure 5).
Figure 5. HOMO (left) and LUMO (right) orbitals of the copper(II) complex of sensor
(isovalue set to 0.02 a.u.)
The band observed at 710 nm in experimental UV/vis. spectrum consists of two
transitions, HOMO→LUMO and HOMO→LUMO+1. While the calculated
HOMO→LUMO transition was mainly contributed to by π→π* transitions, the
HOMO→LUMO+1 transition was assigned to a ligand to metal charge transfer
(
LMCT) band (Figure S. 13). The calculated energy gap for the sensor and its copper(II)
complex is also clearly shown in the density of states (DOS) plot (Figure S. 14). The
TD-DFT method has been used to evaluate the electronic structure of the investigated
1
2

compounds, and strong correlations have been noted between the experimental and
calculated data.
4
.
Conclusion
In summary, this study reported the synthesis, characterization and sensing properties of
the chromenylium-cyanine sensor by colorimetric measurements. The
spectrophotometric experiments showed that the sensor enables selective and sensitive
2
+
recognition of the Cu ion in the near-IR region over competitive mono-, di- and
+
+
2+
2+
2+
2+
3+
2+
2+
2+
trivalent metal ions, such as Na , K , Mg , Ca , Cr , Mn , Fe , Co , Ni , Cu ,
2
+
3+
+
2+
2+
2+
2+
2+
Zn , Al , Ag , Cd , Sr Hg , Pb and Ba . The LOD value for the sensor toward
2
+
-8
Cu was calculated as 1.28×10 M based on standard deviation of the blank
2
+
measurements by IUPAC. As a result of the complex formation with the Cu ion,
optical absorption at near-IR region is expected to be generated by a spirolactam ring
opening reaction. The optimized geometries and the electronic structure of the
compounds have been investigated by DFT, and a strong correlation has been noted
between the theoretical and experimental data. Spectrophotometric analysis showed that
there is a significant decrease in the detection limit compared to the xanthene derivative
in existing scholarship, the synthesized sensor showed higher selectivity toward the
copper(II) ion, which makes it possible to perform the analysis in the near-IR region.
Acknowledgment
The numerical calculations reported in this paper were fully performed at TUBITAK
ULAKBIM, High Performance and Grid Computing Center (TRUBA resources).
1
3

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1
7

•
•
•
A new chromenylium-cyanine sensor was designed for coper(II) ion sensing in
aqueous media.
The sensor allows detection of the copper (II) ion through the spirocyclic ring-opening
2
+
reaction by coordination of Cu .
The sensor showed selectivity to copper ion in the presence of competitive mono-, di-
+
+
2+
2+
2+
2+
3+
2+
2+
and trivalent metal ions such as Na , K , Mg , Ca , Cr , Mn , Fe , Co , Ni ,
2
+
3+
+
2+
2+
2+
2+
2+
Zn , Al , Ag , Cd , Sr Hg , Pb and Ba .

Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: