A novel ratiometric fluorescent probe based on coumarin derivative for the recognition of Al(III) and its application on test strips

Article abstract of DOI:10.1016/j.tetlet.2019.150945

A novel ratiometric probe (L) which was composed of chromone and coumarin moieties has been designed and synthesized for sensing Al³⁺ in EtOH in view of the internal charge transfer (ICT) mechanism. The free probe L exhibited a strong fluorescence emission at 477 nm, and the fluorescence emission here almost disappeared after adding Al³⁺ (10 equiv.) while a new peak appeared at 524 nm. This may be due to the enhancement of intramolecular electron transfer efficiency from donor to acceptor. In addition, this probe L could be form a 1:1 complex with Al³⁺, which could be explained by the ESI-MS spectra, and L had a low detection limit for Al³⁺ with a binding constant of 1.32 × 10⁴ M^?1. More importantly, L could be applied to a solid probe for rapid detection of Al³⁺ with a significant color change.

Full text of DOI:10.1016/j.tetlet.2019.150945

Accepted Manuscript
A novel ratiometric fluorescent probe based on coumarin derivative for the rec-
ognition of Al(III) and its application on test strips
Jia Xue, Limei Tian, Zheng-yin Yang
PII:
S0040-4039(19)30695-1
https://doi.org/10.1016/j.tetlet.2019.150945
150945
DOI:
Article Number:
Reference:
TETL 150945
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
25 May 2019
2 July 2019
15 July 2019
Please cite this article as: Xue, J., Tian, L., Yang, Z-y., A novel ratiometric fluorescent probe based on coumarin
derivative for the recognition of Al(III) and its application on test strips, Tetrahedron Letters (2019), doi: https://
doi.org/10.1016/j.tetlet.2019.150945
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A novel ratiometric fluorescent probe based on coumarin derivative
for the recognition of Al(Ⅲ) and its application on test strips
Jia Xue, Limei Tian, Zheng-yin Yang*
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied
Organic Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
*Corresponding author. Tel: +86 931 8913515; Fax: +86 931 812582; e-mail:
yangzy@lzu.edu.cn (Z.Y. Yang)

Abstract
A novel ratiometric probe (L) which was composed of chromone and coumarin
moieties has been designed and synthesized for sensing Al3+ in EtOH in view of the
internal charge transfer (ICT) mechanism. The free probe L exhibited a strong
fluorescence emission at 477 nm, and the fluorescence emission here almost
disappeared after adding Al3+ (10 equiv.) while a new peak appeared at 524 nm. This
may be due to the enhancement of intramolecular electron transfer efficiency from
donor to acceptor. In addition, this probe L could be form a 1:1 complex with Al3+,
which could be explained by the ESI-MS spectra, and L had a low detection limit for
Al3+ with a binding constant of 1.32 × 104 M-1. More importantly, L could be applied
to a solid probe for rapid detection of Al3+ with a significant color change.
Key words: ratiometric probe; Al3+; ICT; Solid probe
Introduction
Recently, the design of metal ions fluorescent probes with high selectivity has
attracted increasing attention because of its good prospective applications in the
environment and organisms [1-4]. The content of aluminum in the earth’s crust is only
inferior to oxygen and silicon [5-9]. It is extensively used in many fields such as
aviation，architecture，automobile, food additives and other fields [10-16]. Aluminum
is a non-essential element in the human body and is found in very low levels.
However, excessive aluminum has certain harm to the human body and the
environment [17-19]. The wide application of aluminum products in our daily life is
the main source of aluminum pollution, which not only hinders the regular growth of
plants, but also induces many diseases in human body, such as Alzheimer's disease,
Parkinson's disease, and amyotrophic lateral sclerosis [20-27]. Hence, it is meaningful
to develop a fluorescent probe for recognizing Al3+ in the environment and organism
[28, 29]. Nevertheless, compared with others, the number of reported small molecule
probes for Al3+ is limited due to the weak coordination ability of Al3+, and ratiometric

fluorescent probes for Al3+ with high selectivity and excellent sensitivity are less.
Schiff-bases are also acknowledged as a good tool for metal ions [30-32]. The
nitrogen atom in C=N is well coordinated with metal ions. As a hard acid, aluminum
ions have relatively poor coordination ability and tend to coordinate with hard bases
such as Schiff-bases [33]. Coumarin derivatives are widely used in fluorescent probes,
laser dyes, optical materials and other fields because of their excellent photostability,
low cytotoxicity, large Stokes shift and high fluorescence quantum yield [34-36].
Especially, modification of the electron donating group and the electron withdrawing
group at the 7-th and 3-rd positions of the coumarin can constitute an effective
electronic push-pull system to produce an ICT effect [37]. It indicates that Schiff-base
containing coumarin derivatives can be used to design a ratiometric fluorescent probe.
In this work, we have developed a ratiometric fluorescent probe through a simple
condensation reaction using chromone and coumarin derivatives to recognize Al3+ in
EtOH. The free probe L exhibited a significant fluorescence emission at 477 nm in
EtOH. However, the fluorescence emission at 477 nm gradually disappeared upon the
addition of Al3+. At the same time, a new fluorescence emission band appeared at 524
nm increasingly.
Results and discussion
This probe L was synthesized (Scheme 1) according to modify the reporting
method [38-40], and other data were available in the supporting information. The salts
-
of metal ions (NO3 and Cl-) were used to prepare the stock solutions. All kinds of
metal ions stock solutions (10 mM) such as Na+, Al3+, Mg2+, K+, Ag+, Ba2+, Ni2+,
Pb2+, Co2+, Ca2+, Cu2+, Cd2+, Mn2+, Zn2+, Fe3+, Hg2+, Cr3+ and Pd2+ were prepared to
use EtOH as solvent and the stock solution (1 mM) of this probe L was prepared to
use DMF as solvent. In the selective experiments, add the 2 mL of ethanol into the
cuvette in the preparation of the test solution, then adding 20 μL of L and appropriate
metal ions solutions (10 equiv.). In titration experiments, text solution was prepared
by adding 20 μL of this probe L diluting to 10 μM with EtOH (2 mL) into cuvettes,

then adding stock solution of Al3+ (1-10 equiv.). All experiments were performed at
room temperature. For all fluorescence spectroscopic studies, both the excitation and
emission slit widths were 3 nm with an excitation wavelength of 396 nm.
Scheme 1. Synthetic route of L (Reagents and conditions: (a) EtOH, piperidine,
acetic acid glacial, diethylmalonate, reflux, 6 h, (b) EtOH, NH2NH2, stirred at the
room temperature, 20 min, (c) DMF, POCl3, 50 ℃, 2 h. 2’-Hydroxyacetophenone in
DMF, 60 ℃, 2 h, (d) EtOH, reflux, 4 h).
UV–vis analysis
UV-visible spectrophotometer was used to record the absorption spectra of L in
the presence of different amounts of Al3+ in EtOH to explore the bonding ability of L
with Al3+. As shown in Fig. 1, after adding Al3+ into the solution of L, it could be
clearly found that the absorption band at 436 nm disappeared gradually, while two
new absorption bands appeared at 471 and 496 nm increasing in intensity. Meanwhile,
three significant isosbestic points were observed at 342 nm, 373 nm and 450 nm,
respectively. And it indicated that a stable complex was formed between L and Al3+.
The change of the absorption band with the addition of Al3+ may be due to the
bonding of the acceptor (–C=O–) with Al3+ changing the internal charge transfer
(ICT) process, which enhanced the electron withdrawing ability of the acceptor and
resulted in bathochromic of absorption band.

1.0
0.8
0.6
0.4
0.2
0.0
10 equiv.
Al3+
1 equiv.
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. The UV–vis absorption titration spectra of L (10 μM) with Al3+ (1–10 equiv.)
in EtOH.
Fluorescence studies
The selectivity of L to Al3+ was studied in EtOH by fluorescence spectrum and
the result was shown in Fig. 2. The free probe L exhibited obvious fluorescence
emission at 477 nm in ethanol (λex = 396 nm), and it could be found that the
wavelength of L was basically unchanged after adding various other metal ions (10
equiv.). Especially, the fluorescence emission spectra of L were red-shifted by 47 nm
only after the addition of Al3+ (10 equiv.) and a new fluorescence emission peak
appeared at 524 nm, which was possibly because of the internal charge transfer (ICT)
process. In a word, the binding of Al3+ with acceptor (–C=O–) could enhance the
efficiency of electron transfer from the donor (–N(CH2CH3)2) to the acceptor. And
from the Scheme 2, we could clearly see that only the free probe L with Al3+ showed
yellow color under the UV lamp, which was consistent with the fluorescence studies.

L
L+K+
990
880
770
660
550
440
330
220
110
0
L+Al3+
L+Ba2+
L+Ca2+
L+Cd2+
L+Co2+
L+Cu2+
L+Hg2+
L+Zn2+
L+Mg2+
L+ Mn2+
L+Na+
L+Ni2+
L+Pb2+
L+Pd2+
L+Cr3+
L+Fe3+
L+Ag+
440
480
520
560
600
640
Wavelength (nm)
Fig. 2. Fluorescence spectra of L (10 μM) before and after addition various metal ions
(10 equiv.) of K+, Ag+, Mg2+, Ca2+, Ba2+, Co2+, Cu2+, Pb2+, Cd2+, Mn2+, Ni2+, Zn2+,
Cr3+, Al3+, Hg2+, Fe3+ and Pd2+ in EtOH.
Scheme 2. The photographs of free probe L after addition of different metal ions
under illumination using a 365 nm UV lamp.
The competitive experiments were conducted in EtOH to search the selectivity of
L to Al3+ in the presence of other various metal ions. It could be found out from Fig. 3
that the coexistence with other metal ions had little effect on the detection of Al3+

using L except Cu2+, Fe3+ and Ni2+, which may be due to the formation of the
complexes between L and these three metal ions, leading to the electron and energy
transfer from L to Cu2+, Fe3+ and Ni2+. And we also added some appropriate masking
agent (Table S1) to optimize the competitive experiments of this probe L sensing Al3+
in the presence of those three metal ions in order to improve the competitive ability of
this probe L. And it could be seen the Fig. 4 that the ratio of fluorescence intensity at
524 nm to that at 477 nm increased after the addition of the masking agent and Al3+
(10 equiv.). In this case, Al3+ could still be detected under coexistence with these three
metal ions. Therefore, this indicated that L could be used as a high selectively probe
for sensing Al3+ in presence of other metal ions.
L+M
L+M+Al3+
7
6
5
7
7
4
F
/
4
4
2
5
F
3
2
1
0
+
+
+
+
+
+
+
+
2
+
+
+
2
2
+
+
+
3
+
3
+
+
2
2
3l
g
r
+
2
2
2
2
2
2
2i
o
a
g
a
d
g
n
u
e
a
d
b
n
K
N
A
A
C
P
C
M
N
C
F
B
H
P
C
Z
C
M
Cations
Fig. 3. Emission intensity ratio (F524/F477) of L (10 μM) in the presence of Al3+ (10
equiv.) and other metal ions (10 equiv.) such as K+, Ag+, Mg2+, Ca2+, Ba2+, Co2+, Cu2+,
Pb2+, Cd2+, Mn2+, Ni2+, Zn2+, Cr3+, Hg2+，Fe3+ and Pd2+ in EtOH.

L+M+Al3+
L+M+Al3++masking agent
5
4
3
2
1
0
Ni2+
Cu2+
Fe3+
Cations
Fig. 4. Comparison of emission intensity ratio (F524/F477) after adding masking agent
and without masking agent
Fluorescence titration experiments were carried out in order to conduct a
quantitative investigation between L and Al3+ (1–10 equiv.). It could be clearly seen
from Fig. 5 that the fluorescence emission peak at 477 nm decreased to invisible with
the addition of Al3+ continuously to the solution of L, and simultaneously, a new
fluorescence emission peak appeared with increasing intensity. Moreover, the
detection limit of L towards Al3+ was approximated to 1.44 × 10-7 M on the basis of
the equation of LOD = 3σ/k , where σ is the standard deviation of the blank signals
and k is the slope between fluorescence intensity ratio and sample concentration and
the value of σ was calculated to be 0.010. [41, 42] (Fig. 6), and more details were
available in the supporting information. According to a good linear relationship based
on the Benesi–Hildebrand equation (1), the bonding constant was about 1.32 × 104 M-
1 [43] (Fig. 7).
1
1
1


(1)
F  F min
K(F max F min)[M x ]
F
max F min

800
600
400
200
0
10 equiv.
Al3+
1 equiv.
450
500
550
600
650
Wavelength (nm)
Fig. 5. Fluorescence emission spectra of L (10 μM) in EtOH upon gradual addition of
Al3+ (1-10 equiv.)
14
12
y=0.208x-8.56
R2=0.9627
10
8
6
4
2
0
0
20
40
60
80
100
C[Al3+]/M
Fig. 6. The detection limit of L for sensing Al3+ based on the equation of LOD= 3σ/k.

5
4
3
2
1
0
y=1.27210-4x-1.674
R2=0.9868
8000
16000
24000
32000
40000
48000
1/[Al3+] M-1
Fig. 7. Benesi-Hildebrand analysis of the emission changes for the complexation
between L and Al3+.
The proposed mechanism
To further understand the coordination mode of L and Al3+, we have carried out
ESI-MS spectrometry analysis and the 1H NMR titration experiments (Fig. S4). In this
way, the stoichiometric ratio between this probe L and Al3+ could be explained by the
ESI-MS spectra. As depicted in Fig. S5, it could be observed that the peak at m/z
529.9819 attaching to [L + DMF + Al3+ - 2H+]+, which indicated that L chelated with
Al3+ in a ratio of 1:1. From the 1H NMR titration experiments (Fig. 8), the signal of H
from –NH- in this free probe L appeared at 11.979 ppm and H6 exhibited a cluster of
signal from 8.811-8.818 ppm. However, in presence of Al3+, the signals of H in –NH-
and H6 became narrower and decreased in their strength, and two new peaks appeared
at 12.217 ppm and 8.835 ppm, which may be due to the isomerization of carbonyl
group from coumarin moiety during the coordination with Al3+ causing hydrogen
atoms moved to the down-field. In addition, the signal peak of H9 became narrower
and a new peak appeared at the up-field (δ=8.426 ppm) compared with the free
ligand. At the same time, the proton signal of H15 became broadened and the split of
this peak changed obviously. And other proton signals had changed slightly. Thus, it

could be clearly seen that the L coordinated with Al3+ through the carbonyl group of
coumarin moiety, the nitrogen atom in the imine and the carbonyl group in the
chromone moiety and a 1:1 complex was formed. There was the mechanism diagram
of the stable coordination between L and Al3+ (Scheme 3).
Fig. 8. 1H NMR spectra of L in the absence and presence of Al3+ in DMSO-d6
solution.
Scheme 3. The proposed binding mechanism for the responses of L to Al3+.
Comparisons with other reported Al3+ fluorescent probes
The probe L in our work for sensing Al3+ was compared with the previously
reported probes as shown in the Table 1. All the probes listed in the table had lower
detection limit [6, 15, 21, 44-46]. However, most fluorescent probes recognized Al3+
as turn-on probes, and only a few had been reported as ratiometric probes. In contrast,
the probe in this research could be used as a ratiometric probe with lower detection
limit and detection of Al3+ in low toxic solution.

Table 1. Comparison of different properties of L for sensing Al3+ with recently
reported probes.
Reference
Type
Turn-on
Testing media (v/v)
EtOH
LOD/M
6
2.20 × 10−7
5.78 × 10−8
3.66 × 10−6
2.20 × 10−7
2.28 × 10−9
3.18 × 10−6
1.51 × 10−7
15
Turn-on
EtOH/H2O (2/3)
DMSO/H2O (9/1)
CH3OH
21
Turn-on
44
45
Turn-on
Turn-on
DMSO/H2O (0.05/0.95)
EtOH/MeCN/HEPES (6/1/1)
EtOH
46
Ratiometric
Ratiometric
This work
Analysis and application of probe L
In order to investigate the application of L in practice, we carried out the filter
paper experiment. Firstly, the filter paper was immersed in the ethanol solution of L
(100 μM) for a certain time. Then, they was taken out and dried in the air.
Subsequently, various different metal ion solutions (5 mM) were dripped onto the
dried filter paper carefully and then dried in the air. It could be shown in Fig. 9 that
the color of test strips containing Al3+ changed from green to yellow and others
basically have no obvious change in color under the illumination of the UV lamp (365
nm). And we also explored the application of using L as a solid state sensor with
different concentrations (0 μM, 10 μM , 100 μM, 1 mM, 10 mM ) of Al3+. It could be
seen from the Fig. 10 that the color of the filter paper changed significantly with the
increase of the concentration of Al3+. And when the concentration of Al3+ reached 1
mM, a yellow-green didn’t appear very clear but could be observed. Furthermore, it
could be seen from the those two figures that this probe L produced a significant color
change as a solid probe when the concentration of Al3+ reached 5 mM or more, which
could be observed using naked eyes under the irradiation of 365 nm UV lamps.

Fig. 9. The photographs of coated test papers after addition of different metal ions (5
mM) under illumination using a 365 nm UV lamp
Fig. 10. The photographs of test strips coated with L (100 μM) in the presence of
various amounts of Al3+ (0 μM, 10 μM , 100 μM, 1 mM, 10 mM ) in EtOH.
In order to explore the application of this probe L in a real sample, we tested the
concentration Al3+ of the Yellow River sample. First of all, we added different
concentrations of Al3+ (0.050 mM, 0.060 mM, 0.075 mM, 0.100 mM ) to the real
sample, and then detected Al3+ with this probe L more than twice. From the
calculation results in this Table 2, it could be seen that the relative standard deviation
(RSD) values were relatively low, which indicated that there was a lower error
between the measured values and the actual values. In conclusion, this probe L could
be conveniently applied to the detection of Al3+ in a real practical sample.
Table 2. Applicability of using L for detecting of Al3+ in Yellow River water
samples.

Sample
Added/mM
0.0500
Found/mM
Recovery/%
99.20
RSD/%
1.20
Yellow Revie
0.0496 ± 0.0010
0.0593 ± 0.0003
0.0726 ± 0.0002
0.1045 ± 0.0016
0.0600
98.87
0.50
0.0750
96.80
0.37
0.1000
104.50
1.77
Conclusion
In this work, we designed and synthesized a novel Schiff-bases ratiometric
fluorescent probe L based on coumarin derivatives. This probe L could be
successfully used in the recognition of Al3+ in EtOH by the internal charge transfer
(ICT) process. Besides, the free probe L solution showed green color under the UV
lamp, and only the complex (L-Al3+) solution with Al3+ turned yellow. In addition, the
detection limit of L for sensing Al3+ can be as low as 0.144 μM. Furthermore, this
probe L can also be applied to a solid probe for the recognition of Al3+ conveniently
with a significant color change.
Acknowledgments
This work is supported by the National Natural Science Foundation of China
(21761019).
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Highlights

A novel ratiometric probe L was synthesized for sensing Al3+ in EtOH based on
ICT mechanism.


The probe showed a low detection limit.
Importantly, L could be successfully utilized for rapid detection of Al3+ as a solid
probe with a significant color change.