Fluorescent sensor for selective detection of Al3+ based on quinoline-coumarin conjugate

Article abstract of DOI:10.1016/j.saa.2014.05.033

A fluorescence probe, 8-formyl-7-hydroxyl-4-methyl coumarin - (2′-methylquinoline-4-formyl) hydrazone (L) has been synthesized. The chemosensor is found preferential binding to Al³⁺ in presence of other competitive ions with associated changes in its optical and fluorescence spectra behavior. Upon addition of Al³⁺ to a solution of L, it shows 200-fold enhancement of fluorescence intensity which might be attributed to form a 2:1 stoichiometry of the binding mode of LAl(III) and the chelation enhanced fluorescence (CHEF) process at 479 nm in ethanol. The lowest detection limit for Al³⁺ is determined as 8.2 × 10^-7 M.

Full text of DOI:10.1016/j.saa.2014.05.033

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 38–43
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Fluorescent sensor for selective detection of Al3+
based on quinoline–coumarin conjugate
⇑
Jing-can Qin, Tian-rong Li, Bao-dui Wang, Zheng-yin Yang , Long Fan
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
We have designed a new
chemosensor for aluminum ions.
2-Methyl quinoline-4-carboxylic
hydrazide was rarely synthesized.
L shows a large fluorescence
enhancement with turn-on over 200-
fold in ethanol.
ꢀ
It has lower detection limit.
a r t i c l e i n f o
a b s t r a c t
A fluorescence probe, 8-formyl-7-hydroxyl-4-methyl coumarin – (20-methylquinoline-4-formyl) hydra-
zone (L) has been synthesized. The chemosensor is found preferential binding to Al3+ in presence of other
competitive ions with associated changes in its optical and fluorescence spectra behavior. Upon addition
of Al3+ to a solution of L, it shows 200-fold enhancement of fluorescence intensity which might be
attributed to form a 2:1 stoichiometry of the binding mode of LAAl(III) and the chelation enhanced
fluorescence (CHEF) process at 479 nm in ethanol. The lowest detection limit for Al3+ is determined as
8.2 ꢁ 10ꢂ7 M.
Article history:
Received 1 February 2014
Received in revised form 14 April 2014
Accepted 16 May 2014
Available online 28 May 2014
Keywords:
Coumarin
Fluorescence sensor
Aluminum ion
CHEF
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
analytical methods for detecting and controlling the concentration
levels of aluminum in the environment. In recent years, several
As is known, the Al3+ ion existing in natural waters and most
plants can enter the human body through water and foods [1].
WHO has listed aluminum ion as a source of food pollution and
limited its drinking water concentration to 7.41 mM [2–4]. Accu-
mulation of an excessive amount of the metal causes illnesses like
Alzheimer’s disease, Guamanian amyotrophic lateral sclerosis and
Parkinsonism dementia. Therefore, it is crucial to develop some
methods are available for detection of Al3+. In comparison to some
conventional methods, fluorescent chemosensor have attracted
considerable attention due to its easily detectable signals upon rec-
ognition metal ions with high sensitivity and selectivity [5–9].
On the other hand, because of the low coordination ability of
Al3+, the development of Al3+ sensors has been found to be compar-
atively more difficult than other metals. In general, as Al3+ is a
hard-acid, it has been found that Al3+ prefers a coordination sphere
containing N and O as hard-base donor sites. Schiff bases (imines)
are known to be good ligands for metal ions [10–14]. The
⇑
Corresponding author. Tel.: +86 931 8913515; fax: +86 931 8912582.
E-mail address: yangzy@lzu.edu.cn (Z.-y. Yang).
http://dx.doi.org/10.1016/j.saa.2014.05.033
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 38–43
39
Scheme 1. Reagents and conditions: (a) (i) acetone, aq KOH, reflux, 15 h; (ii) conc HCl; (b) conc H2SO4, EtOH, reflux, 18 h; (c) EtOH, N2H4ꢃH2O, 20 h; (d) EtOH, reflux, 10–12 h;
(e) conc H2SO4, CH3COCH2COOC2H5, ꢂ5 °C; (f) HMT.
Fig. 1. Changes in the absorption spectra of
L (10 lM) in ethanol at room
temperature as a function of added Al3+ (0, 5, 10, 15, 20, 25, 30, 35, 40, 45 equiv.).
Fig. 2. Fluorescence spectra of L (10 lM) upon the addition of metal salts (1.0
equiv.) of Na+, K+, Mg2+, Ca2+, Pb2+, Cr3+, Mn2+, Fe2+, Ni2+, Co2+, Fe3+, Ru3+, Cu2+, Ba2+
,
Cd2+, Hg2+, Zn2+and Al3+ in ethanol (kex = 392 nm, slit widths: 5 nm/5 nm).
structures of Schiff bases contain nitrogen–oxygen-rich coordina-
tion environments which provide a hard-base environment for
the hard-acid Al3+ [15]. More importantly, Schiff base derivatives
equipped with a fluorescence moiety are attractive tools for the
detection of metal cations. Keeping this in mind, we have designed
Experimental
a
Schiff-base ligand, which was synthesized by condensing
General
2-methyl quinoline-4-carboxylic hydrazide with 8-formyl-7-
hydroxyl-4-methyl coumarin. Thus far, to the best of our knowl-
edge, there are no reports on the properties of the Schiff base.
Herein, we report a novel coumarin-based ‘‘turn-on’’ fluores-
cent probe for the determination of Al3+ in ethanol, which is attrib-
uted to a chelation-enhanced fluorescence (CHEF) process. It forms
a 2:1 complex and the binding constants Ka is 3.3 ꢁ 106. The free
chemosensor L shows weak fluorescence emission. Upon binding
of Al3+ ions, a significant fluorescence enhancement over 200-fold
is achieved in ethanol.
All chemicals were obtained from commercial suppliers and
used without further purification. 1H NMR spectra were measured
on the Bruker 400 MHz instruments using TMS as an internal stan-
dard. ESI-MS were determined on a Bruker esquire 6000 spectrom-
eter. UV–vis absorption spectra were determined on a Shimadzu
UV-240 spectrophotometer. Fluorescence spectra were recorded
on a Hitachi RF-4500 spectrophotometer equipped with quartz
cuvettes of 1 cm path length.

40
J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 38–43
Synthesis
Synthesis of 8-formyl-7-hydroxyl-4-methyl coumarin [17–19]
Sulfuric acid (98%, 2 mL) was added into the solution of m-dihy-
droxybenzene (0.1 mol, 11.0 g) in tetrahydrofuran (40 mL), and
then acetoacetic ester (13.0 g) was added dropwise. The reaction
mixture was refluxed for 3 h and monitored by TLC until m-dihy-
droxybenzene was completely consumed. After cooling, the solu-
tion was poured into ice water (100 mL), yellow precipitate was
collected by filtration and washed with cold water. Recrystalliza-
tion from ethanol. 7-Hydroxy-4-methyl-coumarin was obtained
(11.3 g, Yield: 64%, m.p.: 185–186 °C). Then 7-hydroxy-4-methyl-
coumarin (0.03 mol, 5.3 g) and hexamine (0.07 mol, 9.8 g) in glacial
acetic acid (50 mL) were refluxed for 6 h, and then 20% HCl (75 mL)
was added and the solution was heated for 1 h. After cooling, the
reaction mixture was extracted with ether, and the combined
organic layers were evaporated under reduced pressure. The resi-
due was poured into ice water and pale yellow solid was obtained.
The product was filtered, dried and recrystallized from ethanol
Ethyl 2-methyl quinoline-4-carboxylate was synthesized
according to the method reported [16]. The Schiff base was synthe-
sized according to the route as shown in Scheme 1.
Synthesis of 2-methyl quinoline-4-carboxylic hydrazide
Hydrazine hydrate (80%, 4 mL) was added dropwise to an etha-
nol solution (40 mL) of ethyl 2-methyl quinoline-4-carboxylate
(5 mmol, 1.015 g). The mixture was refluxed under stirring for
12 h. Then the solvent was concentrated under reduced pressure
and stand overnight in refrigerator. A white needle crystal was
observed. The final product was filtered, dried and recrystallized
from ethanol. Yield: 55%; m.p.: 178–179 °C. 1H NMR (Fig. S1):
(400 MHz; CDCl3): d = 2.71 (s, 3He), 4.25 (s, 2Hh), 7.29 (s, 1Hf),
7.54 (m, 1Hb), 7.59 (s, 1Hg), 7.73 (m, 1Hc), 8.05 (d, J = 8.4 Hz,
1Hd), 8.14 (d, J = 8.1 Hz, 1Ha).
Scheme 2. Proposed mechanism for detection of Al3+ by L.
Fig. 3. Fluorescence spectra of L (10
lM) in ethanol upon the addition of Al(NO3)3 (0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0 lM) with an
excitation of 392 nm. Inset: fluorescence intensity at 479 nm versus the number of equivalents of Al3+ added.

J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 38–43
41
Synthesis of L
An ethanol solution (20 mL) of 2-methyl quinoline-4-carboxylic
hydrazide (1 mmol, 0.203 g) was added to another ethanol (20 mL)
containing 8-formyl-7-hydroxyl-4-methyl coumarin (1 mmol,
0.234 g), then the solution was refluxing for 12 h under stirring
and some white precipitant appeared. Excess solvent was removed
under reduced pressure. After cooling to room temperature, the
mixture was filtered and dried. Recrystallization from DMF. Yield:
63%; m.p.: 283–284 °C. 1H NMR (Fig. S3): (400 MHz; DMSO-d6):
d = 12.67 (s, 1H7), 12.61. (s, 1H13), 8.99 (s, 1H8), 8.18 (d,
J = 8.1 Hz, 1H1), 7.99 (d, J = 8.3 Hz, 1H4), 7.76 (m, 1H3), 7.72
(d, J = 9 Hz, 1H11), 7.69 (s, 1H6), 7.59 (d, J = 8.1 Hz, 1H2), 6.97 (d,
J = 9 Hz, 1H12), 6.22 (d, J = 2 Hz, 1H9), 2.72 (s, 3H5), 2.38 (d,
J = 2 Hz, 1H10). EI-MS (Fig. S4): m/z 388.1.1 [M+H]+, 410.09
[M+Na]+, IR (KBr, cmꢂ1) (Fig. S5): 3438, 3237, 1679, 1586, 863.6.
Results and discussion
General information
Fig. 4. Job’s plot for determining the stoichiometry of L and Al3+ in ethanol (XL = [L]/
([Al3+] + [L]), the total concentration of L and Al3+ was 10
M).
l
Stock solutions of various cations (1 mM) were prepared using
nitrate salts. A stock solution of L (1 mM) was prepared. The solu-
tion of L was then diluted to 10
ments, each time a 2 mL solution of L (10
optical cell of 1 cm optical path length, and the ions stock solution
were added into the quartz optical cell gradually by using a pipette.
l
M in ethanol. In titration experi-
M) was filled in a quartz
(0.612 g, Yield 10%). m.p.: 180–181 °C. 1H NMR (Fig. S2):
(400 MHz; acetone-d6) d = 2.47 (d, J = 2 Hz, 3H6), 6.24 (m, 1H5),
6.92 (d, J = 9 Hz, 1H2), 7.97 (d, J = 9 Hz, 1H1), 10.54 (s, 1H).
l
Fig. 5. The ESI-MS spectra of L and Al3+
.

42
J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 38–43
intermediate aluminum concentration, and at a concentration of
complete interaction, respectively, and where Ka is the binding con-
stant concentration.
The detection limit was calculated based on the fluorescence
titration. To determine the S/N ratio, the emission intensity of
the complex (LAAl3+) without any anion was measured by 1.5
equiv. and the standard deviation of blank measurements was
determined. The detection limit is then calculated with the
following equation: detection limit 3r/m, where r is the standard
deviation of blank measurements, and m is the slope between
intensity versus sample concentration.
UV–vis analysis
The binding ability of L and Al3+ was initially evaluated by
UV–visible spectral analysis in ethanol. As shown in Fig. 1, the
spectrum of L only shows one absorption band centered at
305 nm, Upon addition of Al3+ (0–45 equiv.) to a solution (2 ml)
of L (10 lM), the absorption band at 303 nm gradually decreases
Fig. 6. Fluorescence intensity of L and its complexation with Al3+ in the presence of
while two new absorption bands appear at 330 nm and at
392 nm with increasing Intensity. Moreover, a clear isosbestic
point at 322 nm is observed. This might be attributed to chemical
various metal ions. Black bar: L(10.0
Ba2+, Hg2+, Zn2+, Mg2+, Mn2+, Cd2+, Co2+, Cr3+, Ni2+, Fe2+, Fe3+, Ru3+ and Cu2+, stated.
Red bar: 10.0
M of L with 1.0 equiv. of Al3+; 10.0 M of L and 1.0 equiv. of Al3+ with
l
M); L with 1.0 equiv. of Na+, Pb2+, Ca2+, K+,
interaction of L with Al3+
.
l
l
1.0 equiv. of metal ions stated (kex = 392 nm). (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this
article.)
Fluorescence study
The effect of Al3+on the fluorescence properties of L (10 mM) in
ethanol was investigated. As shown in Fig. 2, the free receptor L
almost not show fluorescence emission at 479 nm when it is
excited at 392 nm in ethanol. Upon the addition of 1.0 equiv. of
Al3+, the fluorescence intensity of L shows a large fluorescence
enhancement (200-fold) at 479 nm. In contrast, addition of other
Spectral data were recorded at 3 min after addition of the ions. In
selectivity experiments, the test samples were prepared by placing
appropriate amounts of ions stock into 2 mL solution of L (10 lM).
For fluorescence measurements, both the excitation and emission
slit widths were 5 nm.
The binding constant value is determined from the emission
intensity data following the modified Benesi–Hildebrand equations
[20] (A):
relevant metal ions including Na+, K+, Mg2+, Ca2+, Pb2+, Cr3+, Mn2+
,
Fe2+, Ni2+, Co2+, Fe3+, Ru3+, Cu2+, Ba2+, Cd2+, Hg2+ and Zn2+ cause
almost no fluorescence increase. The increased emission intensity
of L in the presence of Al3+ is attributed to the chelation of the oxy-
gen atom of AC@O, the nitrogen atom of the AC@N group and the
oxygen of ArAO with Al3+, which results in the efficient inhibition
for the PET process of the AC@N group [21–23] (Scheme 2). To fur-
ther investigate the probe properties of receptor L, we also carried
1
1
1
¼
þ
KðFmax ꢂ FminÞ½Al3þꢄ
F ꢂ Fmin
Fmax ꢂ Fmin
where Fmin, F, and Fmax are the emission intensities of the organic
moiety considered in the absence of aluminum ion, at an
Fig. 7. 1H NMR spectra of L with Al(NO3)3ꢃ9H2O in DMSO-d6: (a) L; (b) L with 0.5 equiv. of Al3+; (c) L with 1 equiv. of Al3+
.

J.-c. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 38–43
43
out fluorescence titration and competition experiment in ethanol.
Acknowledgments
From the fluorescence titration profiles (Fig. 3), the binding con-
stant of L for Al3+ has been estimated using the Benesi–Hildebrand
equation (A). The stability constant is determined as 3.3 ꢁ 106
(Fig. S4), this is within the range 103–109 of those reported for
Al3+-binding sensors [24–27]. The Job’s plot analysis has indicated
that it was possible a 2:1 stoichiometry of the binding mode of L
and Al3+ (Fig. 4). In addition, the formation of 2:1 complex between
L and Al3+ is further confirmed by the appearance of a peak at m/z
799.15, assignable to [L+Al3+ꢂ2H+]+ in the ESI/MS (Fig. 5), The
detection limit of L for Al3+ is found to be 8.2 ꢁ 10ꢂ7 M, which is
sufficiently low to enable the detection of micromolar concentra-
tions of Al3+ in many chemical and biological systems [28,29]. From
the competition experiments, the receptor L was treated with 1.0
equiv of Al3+ in the presence of other metal ions. As shown in
Fig. 6, in the case of Cu(II), Fe(III), Ru(III), Fe(II), although the fluo-
rescence intensity is low but detectable. LAAl(III) complex mixing
with the other metal ions show no significant variation. It means
that the receptor L can be used as a selective fluorescent sensor
for Al3+ in the presence of most competing metal ions.
This work is supported by the National Natural Science
Foundation of China (81171337). Gansu NSF (1308RJZA115).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.05.033.
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DMSO-d6 solution of L (Figs. S5 and S6). The spectral differences
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proton peak (H8) at 8.995 ppm and the signal of ANH (H7) at
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12.942 ppm, respectively. Moreover, addition of Al3+ to L causes
the loss of the AOH proton (H13, d = 12.606 ppm) with subsequent
binding to electron deficient Al3+ which results in the disappear-
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aromatic ring resulting the ortho proton (H12, d = 6.999 ppm) to
the AOH group moving towards the downfield region
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through interactions with carboxylate oxygen, imine nitrogen
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We have successfully developed a novel off–on fluorescence
sensor for Al3+ based on 8-formyl-7-hydroxyl-4-methyl coumarin
with 2-methyl quinoline-4-carboxylic hydrazide. It exhibited high
selectivity for Al3+ over other metal ions with 200-fold fluores-
cence enhancement. More importantly, because of its lower detec-
tion limit, the chemosensor may find potential applications for
detect micromolar concentrations of Al3+ in both biological
systems and the environment.