A multi-responsive crown ether-based colorimetric/fluorescent chemosensor for highly selective detection of Al3+, Cu2+ and Mg2+

Article abstract of DOI:10.1016/j.saa.2019.117857

A novel multi-response chemosensor L based on coumarin-chalcone-crown ether was designed and synthesized, which exhibited a high selectivity for the colorimetric detecting Al³⁺ and Cu²⁺ and fluorescent recognizing Al³⁺ and Mg²⁺ in ethanol. L can monitor Al³⁺ and Cu²⁺ via distinct color changes from a slight yellow to pink and to orange, respectively. The sensor L can also monitor Al³⁺ and Mg²⁺ by fluorescence emission responses at 592 nm and 547 nm with low detection limits of 0.31 μM and 0.23 μM, respectively. The selectivity of L toward Al³⁺, Cu²⁺ and Mg²⁺ was not interfered by a large number of coexisting ions and was found to be reversible. By means of spectrometric titration, Job's plot, mass spectrometry, ¹H NMR titration and IR spectroscopy analysis, it was unanimously confirmed that the sensor L had a stoichiometric ratio of 1:1 with Cu²⁺ and Mg²⁺, and 1:2 with Al³⁺. The order of the stability of the complexes formed by L and Al³⁺, Cu²⁺, Mg²⁺ was as follows: L-Al³⁺ > L-Cu²⁺ > L-Mg²⁺. At the same time, some possible bonding modes and sensing mechanisms were further proposed, and the optimized structure of the sensor L and its sensing mechanism for Al³⁺, Cu²⁺ and Mg²⁺ were confirmed by the calculations of DFT/B3LYP and TD-DFT methods in a suite of Gaussian 09 programs.

Full text of DOI:10.1016/j.saa.2019.117857

SAA-117857; No of Pages 13
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A multi-responsive crown ether-based colorimetric/fluorescent
chemosensor for highly selective detection of Al3+, Cu2+ and Mg2+
⁎
Qiang Zhang , Ruifang Ma, Zhuying Li, Zizhong Liu
College of Chemical and Environmental Science, Inner Mongolia Normal University, Hohhot 010022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 April 2019
Received in revised form 21 November 2019
Accepted 23 November 2019
Available online xxxx
A novel multi-response chemosensor L based on coumarin-chalcone-crown ether was designed and synthesized,
which exhibited a high selectivity for the colorimetric detecting Al3+ and Cu2+ and fluorescent recognizing Al3+
and Mg2+ in ethanol. L can monitor Al3+ and Cu2+ via distinct color changes from a slight yellow to pink and to
orange, respectively. The sensor L can also monitor Al3+ and Mg2+ by fluorescence emission responses at 592 nm
and 547 nm with low detection limits of 0.31 μM and 0.23 μM, respectively. The selectivity of L toward Al3+, Cu2+
and Mg2+ was not interfered by a large number of coexisting ions and was found to be reversible. By means of
spectrometric titration, Job's plot, mass spectrometry, 1H NMR titration and IR spectroscopy analysis, it was unan-
imously confirmed that the sensor L had a stoichiometric ratio of 1:1 with Cu2+ and Mg2+, and 1:2 with Al3+. The
order of the stability of the complexes formed by L and Al3+, Cu2+, Mg2+ was as follows: L-Al3+ N L-Cu2+ N L-Mg2
+. At the same time, some possible bonding modes and sensing mechanisms were further proposed, and the op-
timized structure of the sensor L and its sensing mechanism for Al3+, Cu2+ and Mg2+ were confirmed by the cal-
culations of DFT/B3LYP and TD-DFT methods in a suite of Gaussian 09 programs.
Keywords:
Coumarin-chalcone-crown ether
Synthesis
Chemosensor
Colorimetric/fluorescent
Al3+/Cu2+/Mg2+
© 2019 Published by Elsevier B.V.
1. Introduction
and less expensive via exhibiting differential optical responses (colori-
metric and fluorescence) upon selective binding with metal ions [25].
Recently, the development of analytical field has become more and
more important to the evaluation of human survival environment,
which needs to explore the qualitative and quantitative information
about the nature and concentration of pollutants and metal ions [1–8].
In the past few years, many analytical techniques, such as the voltamm-
etry [9–11], the ion selective electrode [12–16], and colorimetric or fluo-
rescent chemosensors [17], have played an important role in the
detection of pollutants and metal ions. Colorimetric and fluorescent
methods in conjunction with suitable chemosensors are attractive ap-
proaches for the measurement of trace metal ions, such as Al3+, Cu2+
and Mg2+, owing to their high selectivity and sensitivity, visual simplic-
ity, instantaneous response, as well as on site and real time monitoring
[18–20]. Recently, the development of colorimetric and fluorescent
chemosensors on a single sensor for multiple metal ions has received in-
creased attention [21]. The concept of single sensor for multiple ions can
be realized mainly by distinguishing different ions with different excita-
tion wavelengths [22], detecting different ions in different solvents [23]
or recognizing different ions by UV–vis and fluorescence spectra [24].
Compared to one-to-one analysis chemosensors, single chemosensors
for multiple analytes have more advantages, including more efficient
Copper is one of the most important metal ions in biological systems.
Because of its redox-active nature, copper serves as an essential cofactor
by taking an active part in a large variety of enzymes, including superox-
ide dismutase, cytochrome oxidase and tyrosinase. Thus, daily ingestion
of copper is indispensable for our good health [26]. Copper deficiency
can lead to many disorders such as anaemia and low white cell count
whereas the uptake of excess amount of copper above a certain limit
is responsible for serious diseases associated with neurodegenerative
ailments, Alzheimer's and Wilson's diseases [27]. Aluminum is the
most abundant metallic element in the earth's crust. Compounds of alu-
minum are widely dispersed in various ways; textile industry, medi-
cines (antacids), bleached flour, paper industry, food additives,
aluminum-based pharmaceuticals, storage/cooking utensils and pro-
duction of light alloys, etc. However, its toxicity can hamper plant per-
formance and kill fish, algae, bacteria and other species in aquatic
ecosystem. Moreover, it is believed that human illness such as
Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclero-
sis have all been induced by aluminum [28,29]. The magnesium ion is
one of the most abundant divalent cations in cells and can play vital
roles in enzyme-driven biochemical reactions, proliferation of cells,
and stabilization of DNA conformation [30–32]. Magnesium found in
macronutrient amount in the bone and plays an active role in bone re-
modeling and skeletal development. Abnormal concentrations of
⁎
Corresponding author.
E-mail address: zhangq@imnu.edu.cn (Q. Zhang).
https://doi.org/10.1016/j.saa.2019.117857
1386-1425/© 2019 Published by Elsevier B.V.
Please cite this article as: Q. Zhang, R. Ma, Z. Li, et al., A multi-responsive crown ether-based colorimetric/fluorescent chemosensor for highly
selective detec..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117857

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Q. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx
magnesium in the cytosol and subcellular regions have relation with
diseases such as diabetes, hypertension, epilepsy and Alzheimer
[33,34]. Therefore, the development of a new approach with high sensi-
tivity and specificity toward Al3+, Cu2+ and Mg2+ is highly desirable. To
date, a large number of colorimetric and fluorescent chemosensors for
Al3+, Cu2+ or Mg2+ have been reported [19,35,36], but most of which
can only detect one target.
2.3. Calculating methods
The limit of detection (LOD) was calculated from the following equa-
tion.
Detection limit ðLODÞ ¼ 3δ=S
ð1Þ
where δ was the standard deviation of blank measurements (by five
times), S was the slope of the fit lines in titration experiment [46–48].
The association constant K was calculated according to the Li's equa-
tion stated below [49,50].
Coumarin moiety has excellent optical properties such as high fluo-
rescence quantum yield, excellent light stability, significant biological
utility, large Stokes shift, visible excitation and emission wavelengths
and less toxicity. Therefore coumarins have been widely used in the
fields of biology, medicine, perfumes, cosmetics, and fluorescent dyes
[37,38]. As a photoactive group, coumarin moiety can also be used as a
signaling unit for a chemosensor or a sophisticated photo-physical sys-
tem [39,40]. On the other hand, the crown ethers are good at selectively
trapping metal ions [41–43]. The positive charge of the ions may cause a
decrease in the charge density on the crown ring when the metal ions
are captured by the azacrown ether with coumarin fluorophore at-
tached, which may change the optical signal of the chemosensor [44].
In addition, the fluorophore and connector moiety in the sensor mole-
cule may also have a significant effect on the sensitivity and selectivity
of the sensor [45]. Based on, we select 7-hydroxy-4-methyl coumarin
as fluorophore and introduce this substance to the N-phenylaza-15-
crown-5 ionophore via a chalcone linker to synthesize a novel multi-
functional chemosensor (L), which displays selective colorimetric sens-
ing as well as fluorescent sensing of multiple metal ions (Al3+, Cu2+ and
Mg2+) in ethanol, for Al3+/Cu2+ by the naked eye and Al3+/Mg2+ by
emission spectra. The fluorescence selectivity of L toward Al3+ and
Mg2+ is found to be strongly dependent on the excitation wavelength,
which allows the efficient simultaneous detection and identification of
Al3+ and Mg2+ ions at different excitation wavelengths. In this contri-
bution we describe the synthesis and characterization of L and its poten-
1
αn
1−α
¼ n K ꢁ
ð2Þ
m
n−1
½Mꢀ ꢁ ½LꢀT
Herein, α = (Imax − I)/(Imax − Imin). Where I was the measured ab-
sorption or fluorescence intensity; Imax and Imin were the signals with
saturated and without metal ions, respectively; [Mn+] was the concen-
tration of metal ions added and K was the association constant. The n
and m were the binding stoichiometry for L and the metal ion,
respectively.
All theoretical calculations of quantum chemistry in this study were
performed using the Gaussian 09 software package [51]. By using DFT/
B3LYP method, the 6-311G(d, p) basis sets were employed by all
atoms except for Cu atom which was calculated by LanL2DZ pseudo-
potential basis set, and the conductor-like polarizable continuum
model (CPCM) was used to consider the solvent effect of ethanol
[50,52,53]. These calculations included, that the sensor L and its com-
plexes were optimized to the energy-minimized geometric structure,
and for all optimized structures, the frequency calculations were carried
out to confirm the absence of imaginary frequencies, the ultraviolet-
visible spectra of L and its complexes were also calculated by applying
the time-dependent density functional theory (TD-DFT) method at the
same level optimized structure, the fractional contribution of various
groups in the molecule to each frontier molecular orbital composition
were calculated and the molecular orbitals were visualized and plotted
by GaussView 5.0 program [54–56].
tial application for the selective detection of Al3+, Cu2+ and Mg2+
.
2. Experimental
2.1. Reagents and apparatus
2.4. Synthesis of intermediates and L
All of the materials for synthesis were obtained from commercial
sources and used without further purification except absolute THF. All
of the solvents used were of analytical reagent grade. NMR spectra
were measured on a Bruker Advance III 500 spectrometer with TMS as
internal standard. Chemical shifts (δ) were reported in ppm. Elemental
analyses were done on Vario instrument Micro Elemental analyzer. UV–
vis spectra were recorded on Hitachi U-2900 UV–vis spectrophotome-
ter, fluorescence spectra were recorded on Hitachi F-4600 Fluorescence
spectrophotometer equipped with quartz cuvettes of 1 cm path length.
ESI-MS spectra were obtained using Thermo LCQ Advantage-MAX LC-
MS spectrometer and SHMADZU LC-MS 2020 spectrometer. MALDI-
TOF-MS spectra were determined on a XEVO G2-XS QTOF spectrometer.
Infrared (IR) spectra were recorded on a Nicolet FTIR-6700 spectrome-
ter (Nicolet Co., USA). Melting points were determined using a Beijing
XT5 microscopic melting point apparatus.
Compounds triethylene glycol di(p-toluenesulfonate) and 7-
hydroxy-4-methyl coumarin were synthesized according to modified
methods reported by literature [57–60]. L was synthesized according
to the route as shown in Scheme 1.
2.4.1. Synthesis of triethylene glycol di(p-toluenesulfonate)
Sodium hydroxide dissolved in water (57 mL, 5 M) and triethylene
glycol (13.4 mL, 0.1 mol) in THF (57 mL) were placed in a flask and
the mixture was cooled on an ice-bath. Under vigorous stirring, a
57 mL THF solution of p-toluenesulfonyl chloride (38 g, 0.2 mol) was
added dropwise to the mixture during 2 h. The solution was reacted
for an additional 3 h, and then poured into ice-water (150 mL). The tos-
ylate, if solidified, was filtered, washed with distilled water, dried, and
then recrystallized from methanol to give a pure specimen, 34.8 g,
76%. m.p. 80.5–81.5 °C.
2.2. Analysis
2.4.2. Synthesis of 7-hydroxy-4-methyl coumarin
A mixture of resorcinol (1.1 g, 10 mmol), ethyl acetoacetate
(1.58 mL, 10 mmol) and NaHSO4·H2O (0.25 g, 1.48 mmol) was taken
in a flask (50 mL), heated to boil and refluxed for 3 h until the solution
turned orange. After completion, the reaction mixture was cooled to
room temperature, poured into ice-water (60 mL), and then a large
amount of yellow solid was precipitated. The precipitated product was
filtered, washed with water, dried, and then recrystallized from ethanol:
H2O (3:2 v/v) mixture to obtain the desired product, 0.7 g, 40%. m.
p. 187.5–188.5 °C. 1H NMR (Fig. S1) (500 MHz, DMSO‑d6), δ (ppm):
Stock solutions of various metal ions (5 mM) were prepared in
ethanol using their corresponding chloride salts. A stock solution of
L (44 μM) was prepared in ethanol. Test solutions were prepared
by placing requisite amounts of the solution of sensor L stock solu-
tion into volumetric flasks, adding an appropriate equivalent of
each ion stock, and diluting the solution to 5 mL with ethanol. For
fluorescence measurements, both the excitation and emission slit
widths were 5 nm.
Please cite this article as: Q. Zhang, R. Ma, Z. Li, et al., A multi-responsive crown ether-based colorimetric/fluorescent chemosensor for highly
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3
Scheme 1. Synthesis of the sensor L.
2.366 (s, 3H), 6.133 (s, 1H), 6.703 (d, J = 2.0 Hz, 1H), 6.793–6.814 (dd,
J1 = 8.5 Hz, J2 = 2.0 Hz, 1H), 7.587 (d, J = 8.5 Hz, 1H), 10.529 (s, 1H).
Anal. Calcd. for C10H8O3: C, 68.18; H, 4.58. Found: C, 68.13; H, 4.52.
mixture was reacted for 10 min at room temperature and filtered. The
residue was recrystallized from absolute ethanol and 7-Acetoxy-4-
methylcoumarin (3) was obtained as a white needle crystal, 288 mg,
44%. m.p. 150–151 °C. 1H NMR (Fig. S4) (500 MHz, CDCl3), δ (ppm):
2.347 (s, 3H), 2.439 (s, 3H), 6.275 (d, J = 1.0 Hz, 1H), 7.076–7.093
(dd, J1 = 8.5 Hz, J2 = 2.0 Hz, 1H), 7.118 (d, J = 2.0 Hz, 1H), 7.605 (d,
J = 8.5 Hz, 1H). Anal. Calcd. for C12H10O4: C, 66.06; H, 4.59. Found: C,
66.27; H, 4.59.
2.4.3. Synthesis of N-phenylaza-15-crown-5 (1)
NaH (0.968 g, 40.3 mmol) was dissolved in absolute THF (300 mL)
under the protection of N2, and the mixture was heated to reflux on
an oil-bath. A solution of N-phenyldiethanolamine (1.82 g, 10 mmol)
in absolute THF (80 mL) and a solution of triethylene glycol di(p-
toluenesulfonate) (4.59 g, 10 mmol) in absolute THF (80 mL) were si-
multaneously added to the mixture drop by drop during 3.5 h. The reac-
tion continued to be refluxed for another 20 h at 81 °C. After completion,
the reaction mixture was cooled and filtered. The filtrate was evapo-
rated under reduced pressure to remove the solvent. The residue was
dissolved in methanol, and a 10-mL methanol solution of sodium per-
chlorate (1.4 g, 10 mmol) was added. Then the mixture was refluxed
for 2 h, evaporated to remove the solvent, and recrystallized from
ethyl acetate to give a white solid. The solid obtained was extracted
with CH2Cl2:H2O (1:1 v/v) mixture to separate the organic phase.
Then filtered, dried over Na2SO4 and evaporated to remove the solvent.
The desired product as a white solid was obtained by recrystallization
from heptane, 0.99 g, 33.5%. m.p. 45.5–47 °C. 1H NMR (Fig. S2)
(500 MHz, CDCl3), δ (ppm): 3.595 (t, J = 6 Hz, 4H), 3.639–3.652 (m,
8H), 3.669–3.684 (m, 4H), 3.756 (t, J = 6 Hz, 4H), 6.644–6.668 (m,
3H), 7.190–7.222 (m, 2H). Anal. Calcd. for C16H25O4N: C, 65.06; H,
8.53; N, 4.74. Found: C, 65.27; H, 8.30; N, 4.76.
2.4.6. Synthesis of 8-acetyl-7-hydroxy-4-methylcoumarin (4)
A mixture of 7-Acetoxy-4-methylcoumarin (3) (0.5 g, 2.3 mmol)
and anhydrous aluminum chloride (2 g, 15 mmol) was heated for 4 h
on an oil-bath (160–165 °C) and cooled on an ice-bath. Then 10% of hy-
drochloric acid was added dropwise. The solution was stirred for
30 min, let stand for 12 h and filtered. The desired product was gained
as a light yellow needle crystal by recrystallized from 60% of ethanol,
200 mg, 40%. m.p. 168–169 °C. 1H NMR (500 MHz, CDCl3) (Fig. S5), δ
(ppm): 2.429 (d, J = 1 Hz, 3H), 2.975 (s, 3H), 6.176 (m, 1H), 6.922 (d,
J = 9 Hz, 1H), 7.677 (d, J = 9 Hz, 1H), 13.602 (s, 1H). Anal. Calcd. for
C12H10O4: C, 66.05; H, 4.62. Found: C, 66.50; H, 4.47.
2.4.7. Synthesis of 7-hydroxyl-4-methyl-8-(4′-phenylaza-15-crown-5-
acryloyl)-coumarin (L)
A
mixture of 8-acetyl-7-hydroxy-4-methylcoumarin (4)
(164.1 mg, 0.75 mmol) and 4-formylphenylaza-15-crown-5 (2)
(243.1 mg, 0.75 mmol), with chloroform (10 mL) as solvent and
diethylamine (0.3 mL) as catalyst, was heated and refluxed for 10 h
on an oil-bath. After completion, the mixture was cooled and evapo-
rated under reduced pressure to give the crude solid. The crudes
were purified by column chromatography and recrystallization
from acetonitrile. m.p. 155.3–157.1 °C. 1H NMR (500 MHz, CDCl3), δ
(ppm): 2.435 (s, 3H), 3.637–3.668 (d, 16H), 3.787 (t, J = 6.0 Hz,
4H), 6.180 (s, 1H), 6.692 (d, J = 8.5 Hz, 2H), 6.929 (d, J = 9.0 Hz,
1H), 7.622–7.655 (d, J = 8.5 Hz, d, J = 9.0 Hz, 3H), 7.999 (d, J =
15.0 Hz, 1H), 8.117 (d, J = 15.0 Hz, 1H), 14.446(s, 1H). 1H NMR
2.4.4. Synthesis of 4-formylphenylaza-15-crown-5 (2)
N-phenylaza-15-crown-5 (1) (1 g, 3.4 mmol) dissolved in DMF
(3 mL, 2.9 g, 40 mmol) was cooled to below 10 °C on an ice-bath.
POCl3 (0.521 g, 0.31 mL, 3.4 mmol) was added to this solution dropwise.
The mixture was firstly stirred for 10 min on an ice-bath, then stirred for
additional 1 h at room temperature, and finally reacted for 4 h at 107 °C.
After reaction, the mixture was cooled, poured into ice (14 g), and neu-
tralized with NaOH 40% aqueous solution on a warm water-bath. The
final product was obtained by filtered, dried, and recrystallized from
heptane, 171.4 mg, 15.6%. m.p. 85–86 °C. 1H NMR (Fig. S3) (500 MHz,
CDCl3), δ (ppm): 3.639–3.714 (m, 16H), 3.788 (t, J = 5.5 Hz, 4H),
6.802 (s, 2H), 7.726 (d, J = 8.5 Hz, 2H), 9.751 (s, 1H). Anal. Calc. for
(500 MHz, CD3OD),
δ (ppm): 2.486 (s, 3H), 3.639 (s, 4H),
3.656–3.696 (m, 12H), 3.778 (t, J = 5.5 Hz, 4H), 6.199 (s, 1H),
6.780 (s, 2H), 6.953 (d, J = 9.0 Hz, 1H), 7.559 (d, J = 9.0 Hz, 2H),
7.577 (d, J = 15.5 Hz, 1H), 7.697 (d, J = 15.5 Hz, 1H), 7.749 (s, 1H),
7.758 (d, J = 9.0 Hz, 1H). 13C NMR (126 MHz, CDCl3), δ (ppm):
192.50, 167.56, 159.71, 154.77, 153.22, 147.52, 131.62 (2C), 130.29
(2C), 120.73, 115.26 (2C), 112.22, 111.94, 110.85 (2C), 110.02,
71.27 (2C), 70.30 (2C), 70.05 (2C), 68.18 (2C), 53.06 (2C), 19.34.
ESI-MS: m/z Calcd for C29H33O8N ([M + H]+): 524.59; Found,
524.44; Calcd for C29H33O8N ([M + Na]+): 546.56; Found, 546.33.
C17H25O5N: C, 63.14; H, 7.79; N, 4.33. Found: C, 63.24; H, 7.56; N, 4.31.
2.4.5. Synthesis of 7-acetoxy-4-methylcoumarin (3)
7-Hydroxy-4-methyl coumarin (0.5 g, 3 mmol) was dissolved in
NaOH 10% aqueous solution (6 mL), crushed ice (6 g) was added, and
then acetic anhydride (1.6 mL, 16 mmol) was added dropwise. The
Please cite this article as: Q. Zhang, R. Ma, Z. Li, et al., A multi-responsive crown ether-based colorimetric/fluorescent chemosensor for highly
selective detec..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117857

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1H NMR, 13C NMR and ESI-MS spectra of L were shown in Figs. S6-1,
S6-2, S7 and S8. Anal. Calcd. for C29H33O8N: C, 66.53; H, 6.35; N, 2.68.
Found: C, 66.45; H, 6.50; N, 2.45%.
Al3+ and Cu2+ induced instant color changes of the solution of L
from a slight yellow to pink and to orange, respectively (Fig. 1b),
which allowed an easy and quick naked-eye detection of Al3+ and
Cu2+. However, no remarkable or detectable changes in the absorp-
tion spectrum and color of L were triggered by other tested metal
ions. These results suggested that L can serve as a potential candidate
of a ‘naked-eye’ chemosensor for Al3+ and Cu2+ in ethanol.
3. Results and discussion
3.1. Synthesis and structural characterization of L
To study the fluorescent response of L toward the stated metal ions
in ethanol, the emission changes were measured when excited at λex
=
A new chemosensor L was obtained by the condensation reaction of
8-acetyl-7-hydroxy-4-methylcoumarin with 4-formylphenylaza-15-
crown-5 (Scheme 1), and characterized by IR, 1H NMR, 13C NMR, ele-
mental analysis and ESI-mass spectrometry. The photophysical proper-
ties of L with several metal ions using their chloride salts in ethanol
were investigated by UV–vis, fluorescence measurements and titration
studies.
470 nm. As shown in Fig. 2, L alone has a very weak fluorescence emis-
sion and a slight red fluorescence of L solution was observable (inset of
Fig. 2). Upon addition of the above metal ions, it was found that L
showed either no or slight change in the emission spectra relative to
the free L except Al3+ and Mg2+. In contrast, the addition of Al3+ and
Mg2+ resulted in strong enhancement of the emission intensity with a
peak at 592 nm for Al3+ (20-folds) and a peak at 547 nm for Mg2+
(15-folds), respectively. Meanwhile, the fluorescent color of L solution
changed from weak light red to bright orange and to bright green re-
spectively which was easily distinguished by the naked eye (Inset of
Fig. 2). These two emissions at different wavelengths indicated that L
could be used as a dual-target fluorescent chemosensor for Al3+ and
3.2. Absorption and fluorescence response of L to different metal ions
First, the UV–vis absorption response of L toward various metal
ions such as K+, Na+, Ca2+, Mn2+, Al3+, Hg2+, Co2+, Cu2+, Ba2+
,
Pb2+, Li+, Cd2+, Zn2+, Mg2+ and Ni2+ was measured in ethanol at
room temperature. The results were shown in Fig. 1. It could be
seen that upon the addition of 20 equiv. of each metal ion, Al3+
and Cu2+ over other metal ions exhibited unique influences on
UV–vis absorption spectra of L which exhibited two absorption
bands at about 295 nm and 456 nm. The addition of Al3+ to the solu-
tion of L resulted in a prominent red-shift of the absorption peak of
about 64 nm from 456 to 520 nm. In the case of Cu2+, the band at
456 nm disappeared and a new peak centered at 504 nm was ob-
served with a large red-shift of 48 nm. (Fig. 1a). Simultaneously,
Mg2+
.
The fluorescent spectra of L were also measured under the same
conditions using the metal ions described above when it was excited
at 520 nm (Fig. 3). Interestingly, only Al3+ caused a drastic enhance-
ment in the emission intensity of L with a prominent peak obtained
at 592 nm. However, other metal ions tested had no obvious effect
on the fluorescence. The results suggested the sensor L, having pre-
dominant recognition and selectivity, was of a good potential in the
detection of Al3+. In addition, the addition of Al3+ led to the
Fig. 1. (a) UV–vis absorption spectra change of L (5 μM) in the presence of 20 equivalents of different metal ions; (b) The color changes of L (5 μM) upon addition of various metal ions
(20 equiv.).
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5
Fig. 4. UV–vis absorption changes of L (5 μM) upon gradual addition of Al3+ (0–9 equiv.).
Fig. 2. Fluorescence spectra of L (5 μM) in the presence of 20 equiv. of various metal ions in
ethanol. Inset showing the fluorescence change of L before and after the addition of Al3+
(20 equiv.) and Mg2+ (20 equiv.). Excitation wavelength was 470 nm.
Inset shows variation of absorbance against the number of equivalents of Al3+. In ethanol.
enhanced slowly. When Cu2+ was added to 18 equiv., the reaction
reached saturation, and the peak position was shifted to 504 nm. The
discontinuous spectral changes shown in Fig. 5 indicate that the molec-
development of the strong bright orange emission that could be ob-
served by the naked eye (Inset of Fig. 3).
ular structure of sensor L was mutated in binding interaction with Cu2+
.
It should be noticed that no isosbestic points was observed, indicating
that more than one equilibrium between L and Cu2+ was involved in so-
lution. Further, by using these UV–vis titration results, the detection
limit for Al3+ and Cu2+ on the basis of 3δ/S was determined to be
3.90 × 10−7 M and 1.68 × 10−7 M, respectively (insets of Figs. 4, 5),
which was sufficiently low to enable the detection of submicromolar
concentrations of Al3+ and Cu2+, in the range found in many chemical
and biological systems.
3.3. Absorption spectra studies of L toward Al3+ and Cu2+
3.3.1. UV–vis titration and detection limits of L with Al3+ and Cu2+
The UV–vis titration experiments were performed to investigate the
binding properties of L with Al3+ in ethanol (Fig. 4). With the increase of
the amounts of Al3+ in L solution, the absorption band at 456 nm was
gradually red shifted and enhanced. When Al3+ increases to
7.2 equiv., the band red shifted to 520 nm and reached the maximum
absorption. And a roughly isosbestic point could be clearly observed at
470 nm. These observations clearly indicate the definite conversion of
free sensor L to the corresponding aluminum complex having equilib-
rium between them. The UV–visible titration experiment of L with Cu2
+ was also carried out (Fig. 5). With the increase of Cu2+ ions, the ab-
sorption band at 456 nm was gradually red shifted and increased in in-
tensity. During the amounts of Cu2+ increased from 2 to 7 equiv., the
intensity of the absorption band which had been red shifted to
475 nm remains basically unchanged, and only the peak position was
still red shifted. When Cu2+ was added from 7 to 18 equiv., the peak po-
sition and intensity of the absorption peak were red shifted and
3.3.2. Competitive selectivity, reversibility and reproducibility of L toward
Al3+ and Cu2+
A high selectivity toward analytes over other potentially competitive
species was an important trait of an available chemosensor. To examine
the selectivity of L for Al3+ and Cu2+, the effect of competing metal ions
in ethanol solvent were investigated. Upon the addition of 20 equiv. of
Al3+ ion to L solution in the presence of equivalent amount of other
metal ions, the absorption enhancement at 520 nm caused by Al3+
was retained and not interfered by other added competitive metal
ions (Fig. 6a). The coexistent metal ions also did not interfere with
Fig. 3. Fluorescence spectra changes of L (5 μM) in the presence of different metal ions
(20 equiv.) in ethanol. Inset showing the fluorescence change of L before and after the
addition of Al3+ (20 equiv.). Excitation wavelength was 520 nm.
Fig. 5. Absorption spectral changes of L (5 μM) upon gradual addition of Cu2+
(0–20 equiv.). Inset shows variation of absorbance against the number of equivalents of
Cu2+. In ethanol.
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Fig. 6. (a) UV–vis absorption (at 520 nm) and (b) color changes of L (5 μM) upon addition of Al3+ (20 equiv.) in the absence and presence of other metal ions (20 equiv.).
naked-eye detection of Al3+ by L (Fig. 6b). Similarly, the competitive
ions showed negligible disturbance to the detection of Cu2+ except for
Al3+ (Fig. 7a). Upon addition of Al3+ to the solution of sensor L and
Cu2+, it was observed that the absorption band at 504 nm was red-
shifted to 520 nm, and its spectrum was basically the same as that of
L + Al3+ system (Fig. 7b), and the color of L + Cu2+ solution changed
from orange to pink, which was easily distinguished by the naked eye
(Fig. 7c). It showed Al3+ could displace Cu2+ in its metal-ligand com-
plex, L-Cu2+, indicating the order of stability of corresponding metal-
ligand complexes (L-Al3+ N L-Cu2+). According to the above results, L
was highly selectivity and sensitive for Al3+ and Cu2+ detection and
could be employed as a colorimetric chemosensor for the recognition
of Al3+ and Cu2+ in the presence of most competing metal ions.
Reversibility was a prerequisite in developing novel chemosensors
for practical application. Thus, the reversibility of the recognition pro-
cess of the sensor L toward Al3+/Cu2+ was confirmed by adding an
Al3+/Cu2+ bonding agent, Na2EDTA (Fig. S9). Upon addition of
20 equiv. of Na2EDTA to a mixture of L with Al3+ (20 equiv.) and L
with Cu2+ (20 equiv.) in ethanol, respectively, the absorbance with yel-
low of sensor L at 456 nm was recovered, which indicated the regener-
ation of the free L. These results indicated that the changes in color and
absorbance of the sensor L induced by recognizable ions were revers-
ible. Therefore the sensor L could be used as a reversible colorimetric
chemosensor for Al3+ and Cu2+ ions in ethanol.
increasing concentrations of Al3+ (Fig. 8, λex = 470 nm). Upon excita-
tion at 520 nm, L exhibited similar titration changes with increasing
concentrations of Al3+, as shown in Fig. S10. When the sensor L was ti-
tration with Mg2+ (Fig. 9), the fluorescence intensity of L at 547 nm
gradually increased and reached a maximum at 5 equiv. of Mg2+ and
then almost no change was observed. From the titration experiments,
the calibration curves were obtained from the plots of fluorescence in-
tensities with the added Al3+ or Mg2+ concentrations, and the linear
ranges of quantitative detection for Al3+ and Mg2+ were determined
to be 0–4.0 × 10−5 M and 0–1.5 × 10−5 M, respectively. The detection
limits for the analysis of Al3+ and Mg2+ were calculated as
3.13 × 10−7 M and 2.30 × 10−7 M using 3δ/S method, respectively (In-
sets of Figs. 8, 9). This observation suggested that L could be employed
as a fluorescence sensor for the detection of Al3+ and Mg2+ of the
submicromolar concentrations.
3.4.2. Competitive selectivity, reversibility and reproducibility of L toward
Al3+ and Mg2+
In order to validate the selectivity of L for Al3+ and Mg2+, the com-
petition experiments were also conducted. As shown in Fig. 10, no influ-
ence was found of all the metal species tested on the detection of Al3+
with fluorescence spectral changes when excited at 470 nm. In Fig. 11
(a), the other background metal ions had little or no obvious interfer-
ence with the detection of Mg2+ ions, except for Al3+, Cu2+, Co2+ and
Ni2+. When adding Al3+ to a solution of sensor L and Mg2+, the maxi-
mum fluorescence emission at 547 nm induced by Mg2+ was rapidly
red shifted to 592 nm, showing an enhanced fluorescence characteristic
peak induced by Al3+ (Fig. 11(b)). It showed Al3+ could displace Mg2+
in its metal-ligand complex, L-Mg2+, indicating the order of stability of
corresponding metal-ligand complexes (L-Al3+ N L-Mg2+). Thus, L
could be a good dual-target selective fluorescent sensor for Al3+ and
Mg2+ in the presence of most competing metal ions. Furthermore, L
was shown to be an efficient Al3+-selective fluorescent chemosensor
with excitation at 520 nm (Fig. S11). Particularly, L could distinguish
Al3+ from Mg2+ or Mg2+ from Al3+ by fluorescence spectrum based
on the very different responses of L to Al3+ and Mg2+ in ethanol. In
3.4. Fluorescence spectra studies of L toward Al3+ and Mg2+
3.4.1. Fluorescence titration and detection limits of L with Al3+ and Mg2+
To get a further insight into the fluorescence chemosensing proper-
ties of sensor L, a quantitative investigation of the binding of sensor L
with Al3+ and Mg2+ was studied by fluorescence titration. Upon addi-
tion of Al3+ ions (0–10 equiv.) to L (5 μM) in ethanol, a significant fluo-
rescence enhancement was observed at 592 nm, where there was a
stable growth, and the fluorescence intensity of sensor L reached ap-
proximately saturation when Al3+ increased to 7.2 equiv., but the fluo-
rescence intensity did not increase significantly with the further
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Fig. 7. (a) UV–vis absorption (at 504 nm) and (c) color changes of L (5 μM) upon addition of Cu2+ (20 equiv.) in the absence and presence of other metal ions (20 equiv.); (b) Al3+
(20 equiv.), Cu2+ (20 equiv.) and their co-existence substance induced absorption spectral changes of L (5 μM).
addition, the relative stability of the complexes L-Mg2+ and L-Cu2+ was
also concerned. It can be seen from Fig. 11(c) that Cu2+ could displace
the Mg2+ in its metal-ligand complex, L-Mg2+, and decrease the inten-
sity of fluorescence peak at 547 nm induced by Mg2+, which indicating
that L could be classified as a reversible fluorescent chemosensor for Al3
+ and Mg2+, and with excellent reproducibility.
3.5. Binding stoichiometries and association constants of sensor L with Mg2
that the stability of the complex L-Cu2+ was higher than that of L-Mg2+
.
+, Al3+ and Cu2+
According to the above experimental results, the order of stability of the
complexes formed by L with Al3+, Cu2+ and Mg2+ could be confirmed
as L-Al3+ N L-Cu2+ N L-Mg2+. Hence, although L could work as a
multi-response chemosensor toward Al3+, Cu2+ and Mg2+, it only se-
The Job's plot for the binding between L and Al3+ showed a 1:2 com-
plexation stoichiometry (Fig. S14), while the binding stoichiometry for
sensor L to Mg2+ or Cu2+ determined by Job's plot was 1:1, respectively
(Figs. S15 and S16). In addition, the binding stoichiometry for sensor L
to Al3+, Mg2+ and Cu2+ was further confirmed by the appearance of a
peak at m/z 764.5715, 546.2110 and 586.30, assignable to [L + 2Al3+
lectively responded to them in an order of Al3+ to Cu2+ to Mg2+
.
Moreover, the fluorescence response of L to Al3+ and Mg2+ was con-
firmed to be reversible by Na2EDTA titration (Figs. S12 and S13). After
addition of 20 equiv. of Na2EDTA to the ethanol solution of L-Al3+ com-
plex and L-Mg2+ complex, respectively, the fluorescent emission inten-
sity of these systems was quenched. The fluorescence quenching was a
result of the high affinity of Na2EDTA toward Al3+ and Mg2+, which re-
sulted in the decomplexation of the complexes. These results indicated
+4Cl−-H++C2H5OH]+ (calcd. 764.41), [L + Mg2+-H+ +
]
(calcd.
(calcd.
546.88) in the MALDI-TOF-MS (Fig. S17) and [L + Cu2+-H+ +
]
586.13) in the ESI-MS (Fig. S18), respectively. According to the UV–vis
titration profiles (Figs. 4 and 5), the association constants (K) were de-
termined as 2.47
×
109
2.22 ×
107 M−2 for L-Al3+ and
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Fig. 8. Fluorescence spectra of L (5 μM) upon addition of increasing concentration of Al3+
(0–10 equiv.), Inset shows variation of fluorescence intensity against the number of
equivalents of Al3+. In ethanol, λex = 470 nm.
Fig. 10. The fluorescence intensity at 592 nm of L (5 μM) with Al3+ (20 equiv.) in the
absence and presence of other metal ions (20 equiv.) under the identical conditions in
ethanol, λex = 470 nm.
7.32 × 104
1.51 × 103 M−1 for L-Cu2+ respectively based on a Li's
equation (Figs. S19 and S20), indicating the order of stability of corre-
sponding metal-ligand complexes (L-Al3+ N L-Cu2+), which was consis-
tent with our previous the result of competitive experiments. By using
the above-mentioned fluorescence titration results (Figs. 8 and 9), asso-
ciation constants (K) of L with Al3+ and Mg2+ were calculated as
5.35 × 109 7.42 × 107 M−2 and 5.77 × 104 7.23 × 102 M−1, respec-
tively (Figs. S21 and S22). The result suggested that L displayed stronger
binding interaction for Al3+ relative to Mg2+, which was in agreement
with our previous experiments.
no significant change were observed, while the H-20 signals of the phe-
nolic hydroxyl group all shifted to the low field region (Δδ =
0.035 ppm–0.053 ppm). These 1H NMR data clearly indicated that the
oxygen atoms of the crown ether ring and the phenolic hydroxyl oxygen
atom of the coumarin moiety in the sensor L were involved in the bind-
ing with Mg2+, Al3+ and Cu2+, respectively.
With increasing concentration of Mg2+ or Al3+ respectively, the H-
17,18 proton signals of the coumarin moiety had a negligible change.
However, the protons (H−11,12,13,14,15,16) signals of the styrene moi-
ety all shifted to the high field as shown in Fig. S23a and b. These shifts
confirmed that the chalcone oxygen atom also participated in the bind-
ing with Mg2+ and Al3+, respectively [19,47,52,64]. Based on these 1H
NMR data, it was suggested that the sensor L could chelate Mg2+ and
Al3+ by the oxygen atom of chalcone group, the oxygen atom of pheno-
lic hydroxyl group and the oxygen atoms of the crown ring moiety. In
addition, maybe because Al3+ has a larger nuclear magnetic moment
and high charges, the 1H NMR peaks of the near neighbor protons
with the atoms bonded to Al3+ were split multiply due to coupling. In
the presence of higher concentration of Al3+, most proton peaks of sen-
sor L became unclear [38,48,50,65].
3.6. 1H NMR titration, IR analysis of the binding modes of sensor L with Mg2
+, Al3+ and Cu2+
To explore the binding modes of L to Mg2+/Al3+/Cu2+ 1H NMR
,
spectroscopic titrations of L with Mg2+/Al3+/Cu2+ ions were performed
respectively in CD3OD/CDCl3 (4/1, v/v)(Fig. S23a, b and c). During the ti-
tration, the concentration of L was kept constant, and the [Mn+]/[L]
ratio were increased. Upon addition of increasing quantities of Mg2+
or Al3+ or Cu2+ to the solution of sensor L respectively, the
\\CH2\\CH2\\ protons signals of the aza-15-crown-5 moiety were
shifted to low field or high field (as shown in Fig. S23a, b and
c) [61–63]. At the same time, the H-19 signals of the coumarin moiety
The partial 1H NMR spectra of L upon the addition of various
amounts of Cu2+ were illustrated in Fig. S23c. Upon the addition of
0.61 equiv. of Cu2+ ions, the H-17, 18 signals of coumarin moiety and
the H-11, 12, 13, 14, 15, 16 signals of styrene moiety were shifted to
high field. Then they were then shifted to the low field after addition
of 1.17 equiv. of Cu2+ ions. At the same time, when various amounts
of Cu2+ were added, the sensor L produced new proton signals corre-
spondingly in the low field region of the original proton peaks. It
could be seen from Fig. S23c, in the presence of 0.61 equiv. Cu2+, the
new peak corresponding to the proton peak of phenolic hydroxyl at
7.798 ppm of sensor L was at 8.087 ppm, the new peak in the presence
of 1.17 equiv. Cu2+ was at 8.090 ppm of low field of the original
7.802 ppm. This suggested that the sensor L binding to Cu2+ made the
structure of its own change. As a result it was possible for sensor L to
partially transform from the ketene structure to the enol structure, the
two hydroxyl proton peaks produced by the two molecular structures
in this system were consistent with the 1H NMR titration data in
Fig. S23c, and was also consistent with the absorption spectra for titra-
tion of sensor L by Cu2+ (in Fig. 5, when the [Cu2+]/[L] increased to
about 2.0 equivalent and continued to titrate, the wavelength and inten-
sity of the spectral absorption peak produced an irregular mutation).
Fig. 9. Fluorescence spectra of L (5 μM) upon addition of increasing concentration of Mg2+
(0–10 equiv.), Inset shows variation of fluorescence intensity against the number of
equivalents of Mg2+. In ethanol, λex = 470 nm.
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collected and compared to the IR spectrum of L (Fig. S24 (a), (b),
(c) and (d)). Comparison with the IR spectra of L before and after the ad-
dition of Mg2+ or Al3+ or Cu2+, the peak position and absorption inten-
sity of some IR peaks have been found to have changed. Compared
Fig. S24 (b) with (a), the O\\H peak of L at 3434.98 cm−1, C_O (ketene)
at 1600.02 cm−1 and C\\O\\C (crown ring) at 1112.49 cm−1 were
shifted or disappeared on adding the Mg2+ ion. Compared Fig. S24 (c),
(d) with Fig. S24 (a), the corresponding IR peaks were also shifted or
weakened on adding Al3+ or Cu2+. These IR spectra data indicated the
participation of the phenol hydroxyl, ketene and crown ring in the com-
plexation of the sensor L with the Mg2+ or Al3+ or Cu2+
.
Based on the Job plot, MALDI-TOF-MS (or ESI-mass), 1H NMR titra-
tion, and the IR analysis results, We proposed a binding model for the
combination of sensor L with Mg2+ or Cu2+ at 1:1 stoichiometry and
with Al3+ at 1:2 stoichiometry, as shown in Scheme 2 [66].
3.7. Theoretical calculation study on the spectral sensing mechanism of sen-
sor L toward Al3+, Cu2+ and Mg2+
To gain an insight into the spectral sensing mechanism of the sensor
L toward Al3+, Cu2+ and Mg2+, and to confirm the correctness of their
binding modes, the density functional theory (DFT) and time-
dependent density functional theory (TD-DFT) were used to optimize
the energy minimum geometry of L and its Al3+, Cu2+ and Mg2+ com-
plexes, the energy levels and compositions of molecular orbitals and the
excited-state electronic spectra in ethanol solvent were calculated. The
spatial distribution, orbital energy, energy gap, and corresponding exci-
tation wavelength and oscillator strength of the most important occu-
pied (HOMOs) and unoccupied (LUMOs) molecular orbitals involved
in the electronic transitions in α and β spin states were presented in
Fig. 12. As shown in Fig. 12, the HOMO–LUMO energy gap (3.024 eV)
of L was significantly higher than that of its complexes L–Al3+
(2.676 eV), L–Cu2+ (2.528 eV) and L–Mg2+ (2.624 eV), which supports
the experimental fact that the absorption band of the sensor L was red
shifted when these metal ions were added. The calculated UV–vis spec-
tra and related data of the sensor L and its metal complexes were given
in Fig. 13 and Table S1, respectively. The theoretical absorption bands
corresponding to HOMO–LUMO for L, L–Al3+, L–Cu2+ and L–Mg2+ in
these calculated UV–vis spectra were predicated at 453.5 nm,
527.1 nm, 519.6 nm and 479.9 nm, respectively. These results were in
good agreement with the experimental absorption bands (456 nm,
520 nm, 504 nm and 465 nm) within the allowable error range, and
the theoretical absorption bands of other excited states were also in
good agreement with the corresponding spectral experimental values,
which further proved the correctness of the proposed binding modes
and optimized structures [55].
For L, L-Cu2+, L-Mg2+ and L-Al3+ compounds, the calculated frac-
tional contribution of each group in the molecule to its frontier molecu-
lar orbitals were respectively listed in Table S2. The data in Table S2
show that the total contribution of nitrogen atom and phenyl group in
the composition of the HOMOs of L, L-Cu2+ and L-Mg2+ compounds
was 70%, 72% and 66% (α-state), respectively, and the total contribution
of vinyl, carbonyl and coumarin group to the composition of their
LUMOs was 81%, 87% and 83% (α-state), respectively. However, the
composition of HOMO and LUMO of L-Al3+ complex was different
from that of L, L-Cu2+ and L-Mg2+, the contribution of coumarin
group in HOMO of L-Al3+ complex was 94%, and the total contribution
of phenyl, vinyl and carbonyl group in LUMO was 68%. We can see
from the results, the strongest absorption bands of the sensor L and its
L-Al3+, L-Cu2+ and L-Mg2+ complexes were attributed to the intramo-
lecular charge transfer (ICT) transition mechanism. From the point of
view of molecular structure, the coumarin-chalcone-azacrown ether L
was a kind of trans styrene derivative with push-pull electron groups
at its molecular both ends. When the L is excited, the superstrong intra-
molecular charge transfer (ICT) process will occur, this ICT will cause
strong changes in molecular polarization and dipole moment, and
Fig. 11. (a) The fluorescence intensity at 547 nm of L (5 μM) with Mg2+ (20 equiv.) in the
absence and presence of other metal ions (20 equiv.) under the identical conditions in
ethanol, λex = 470 nm; (b) Al3+ (20 equiv.) and (c) Cu2+ (20 equiv.) induced
fluorescent spectral changes of L (5 μM) in the presence of 20 equiv. of Mg2+ in ethanol
upon excitation at 470 nm.
Therefore, it can be inferred that the sensor L was binded to Cu2+ by the
oxygen atoms of the crown ether ring moiety, the oxygen atom of
chalcone moiety and the phenol hydroxyl oxygen atom of coumarin
moiety, and the structure of sensor L was converted from ketene to
enol, which was shown in Scheme 2. In addition, Cu2+ also had a larger
nuclear magnetic moment.
To further investigate the binding modes of L with Mg2+/Al3+/Cu2+
the IR spectra of L-Mg2+, L-Al3+ and L-Cu2+ (in solid KBr) were
,
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Scheme 2. Proposed binding modes of L for Al3+, Cu2+ and Mg2+
.
Fig. 12. The energy levels, energy gaps, character and some contours of frontier molecular orbitals of L, L-Al3+, L-Cu2+ and L-Mg2+
.
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11
Fig. 13. The calculated UV–Vis spectra of the L (black line), L–Cu2+ (red line), L–Mg2+ (blue line) and L–Al3+ (green line) compounds.
these changes can significantly improve the fluorescence quantum yield
of L and produce strong fluorescence. However, it has been shown that
the excited state isomerization reaction of the ethylene bond in the mol-
ecule (i.e., the double bond was opened and rotation) is an important
non-radiative deactivation channel [67], so that the free sensor L does
not produce a strong fluorescence, which is in line with the experimen-
tal fact. After Al3+ and Mg2+ bond with L, on the one hand, the isomer-
ization of vinyl double bond in the complex molecule was restricted or
completely blocked, on the other hand, due to the redistribution of elec-
tron cloud density in the complex during excitation, the change of di-
pole moment was greater (Table S2 shows that the calculated values
of dipole moments for L, L-Mg2+ and L-Al3+ ground state molecules
are 3.73, 21.97 and 35.57 Debye, respectively). All of these can effec-
tively improve the probability of radiation decay in the excited state of
complexes. At the same time, the redistribution of the charge in the
complex molecule will cause the change of the ICT energy level [68].
Therefore, compared with the ligand L, the fluorescence emission inten-
sity of L-Al3+ and L-Mg2+ complexes not only increased in varying de-
grees, but also shifted significantly at the fluorescence emission peak
positions as well. Owing to Cu2+ was a paramagnetic cation, which
can always induce fluorescence quenching response.
ethanol solvent were optimized and calculated by DFT/B3LYP and TD-
DFT methods using Gaussian 09 programs. The calculated results show
that the spectral responses of the sensor L to Al3+, Cu2+ and Mg2+
ions should be attributed to the intramolecular charge transfer (ICT)
mechanism, and the experimental results are in good agreement with
the theoretical calculation results.
Author contribution statement
Qiang Zhang: Conceptualization, Writing - Reviewing and Editing,
Methodology, Validation, Software.
Ruifang Ma: Data curation, Writing - Original draft preparation.
Zhuying Li: Formal analysis, Resources.
Zizhong Liu: Software, Validation.
Declaration of conflict interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
4. Conclusions
This work was supported by the Natural Science Foundation of Inner
Mongolia (2013MS0219).
In conclusion, we have developed a new multifunctional sensor L
based on coumarin-chalcone-crown ether. It could colorimetrically de-
tect Al3+ and Cu2+ and fluorimetrically test Al3+ and Mg2+ in ethanol.
The L distinguished Al3+ and Cu2+ ions from other metal ions by color
changes from a slight yellow to pink and to orange, respectively. More-
over, L showed significant fluorescence enhancements in the different
wavelength in the presence of Al3+ and Mg2+ respectively, which
meant that L could be used as a dual-target fluorescent sensor. The L re-
versibly responded to Al3+, Cu2+ and Mg2+, while other common
coexisting metal ions showed almost no obvious interference with the
detection. The detection limits of L for Al3+, Cu2+ and Mg2+ ions were
calculated by the spectral titration, and which were all up to
submicromolar concentrations. The possible binding modes of L with
Al3+, Cu2+ and Mg2+ ions were determined through Job's plot experi-
ment, mass spectra, 1H NMR spectroscopic titration and IR spectroscopy
analysis, which indicated that the phenol hydroxyl, ketene and crown
ring might involve in the complexation and L chelated Cu2+ and Mg2
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.saa.2019.117857.
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