A simple fluorescent-colorimetric probe for selective switch-on detection of Al3+ in ethanol

Article abstract of DOI:10.1016/j.ica.2019.119327

A novel and simple fluorescent probe, 7-methoxychromone-3-carbaldehyde-(3′4′5′-trimethoxybenzoyl) hydrazone (L), was designed and synthesized. The probe L presented outstanding “turn-on” fluorescence response towards Al³⁺ over most other competitive ions in ethanol. The detection limit of L for recognizing Al³⁺ was measured to be 2.14 × 10^?7 M. According to these excellent properties, the detecting of Al³⁺ on filter papers was studied successfully. Photoinduced electron transfer (PET) process, coupled with the C[dbnd]N isomerisation process, is put forward to interpret the obvious fluorescence response.

Full text of DOI:10.1016/j.ica.2019.119327

Journal Pre-proofs
Research paper
A simple fluorescent-colorimetric probe for selective switch-on detection of
3
+
Al in ethanol
Cong Liu, Li-mei Liu, Tian-rong Li, Kui Liu, Zheng-yin Yang
PII:
S0020-1693(19)31362-3
https://doi.org/10.1016/j.ica.2019.119327
ICA 119327
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
6 September 2019
29 November 2019
29 November 2019
Please cite this article as: C. Liu, L-m. Liu, T-r. Li, K. Liu, Z-y. Yang, A simple fluorescent-colorimetric probe for
3
+
selective switch-on detection of Al in ethanol, Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.
019.119327
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A simple fluorescent-colorimetric probe for selective switch-on detection of Al3+
in ethanol
1
1
1
1,2
1
Cong Liu , Li-mei Liu ,Tian-rong Li , Kui Liu* , Zheng-yin Yang*
1College of Chemistry and Chemical Engineering, State Key Laboratory of Applied
Organic Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
2College of Chemical Engineering and Technology, Tianshui Normal
University,TianShui, GanSu 741001, China.
**Corresponding author. Tel: +86 931 8913515; Fax: +86 931 8912582; e-mail:
yangzy@lzu.edu.cn (Z.Y. Yang)，liuk2019@lzu.edu.cn（K.. Liu）
Abstract
A
novel
and
simple
fluorescent
probe,
7
-methoxychromone-3-carbaldehyde-(3’4’5’-trimethoxybenzoyl) hydrazone (L), was
designed and synthesized. The probe L presented outstanding “turn-on” fluorescence
response towards Al3+ over most other competitive ions in ethanol. The detection
3
+
−7
limit of L for recognizing Al was measured to be 2.14 × 10 M. According to these
3
+
excellent properties, the detecting of Al on filter papers was studied successfully.
Photoinduced electron transfer (PET) process, coupled with the C=N isomerisation
process, is put forward to interpret the obvious fluorescence response.
Key words: Fluorescent probe; Aluminum ion; Colorimetric; Test kits.
1

1
. Introduction
It is well known that aluminum is the third most abundant element in the earth's
crust (about 8% by weight) and the essential metal ion in the human body [1-2].
Aluminum plays very important roles in most natural processes and in water resource.
And aluminum is extensively used in numerous fields, from the industrial fields to
daily supplies, which includes aerospace industry, automobiles, computers, various
packaging items, electrical equipment, food additives, building materials, clinical
3
+
drugs , water purification and so on [3-5]. The concentration of Al couldn’t exceed
7
.41µM in drinking water on the basis of World Health Organization (WHO) [6-8].
Currently, a large number of medical studies have shown that the excessive aluminum
level in organism is harmful to human health. It can cause diseases such as
Alzheimer's disease, Parkinson's disease, Osteoporosis and Osteomalacia, etc. [9-13].
Hence, it is very vital to design and synthesize effective chemical probes for the
3
+
selective detection of Al in organisms and environments.
Typical instrumentation methods for identifying Al3+ ions, including atomic
absorption spectroscopy and inductively-coupled plasma mass spectrometry, have
been reported [14-15]. Since these techniques require relatively complex sample
preparation or sample pretreatment schemes and are generally not portable, they can’t
3
+
be used for on-site detections of Al ions. Fluorescent sensors own many advantages
such as simple operation, low detection limit, fast time response and portability ，
2

which have been one of the most favorable methods for detecting metal ions [16-21].
3
+
Up to now, many fluorescent probes for the detection of Al have been reported in
3
+
the literature [22-28]. Generally, Al is a hard-acid and prefers to coordinate with
ligands containing N and O as hard-base donor sites [29-30]. Massive Schiff-base
3
+
compounds and their metal complexes have been used as Al sensors. In recent years,
some simple metal ion sensors have shown their superiority due to their good sensing
performance and clear signal transmission mechanism. Obtaining new Al3+ probes
with simple molecular structure is very appealing and significant.
From what have been discussed above, we synthesized a new fluorescent probe
3
+
L which equipped with good selectivity for detecting Al . The structure of probe L
1
has been confirmed by H NMR and ESI-mass methods and the recognition behaviors
of L were studied by UV–vis and fluorescence measurements. With the coordination
3
+
between L and Al , L presented obvious fluorescence enhancement at 480 nm in
ethanol media. Moreover, L could be used as a practical, visible colorimetric test tool
3
+
for Al .
2
. Experimental
2
.1. Materials and apparatus
All commercially obtained chemical reagents were analytical grade and were
used without further purification. The metal-ions’ solutions used in the whole
experiment were prepared from corresponding nitrate and chloride salts. (NaNO3,
KNO , LiClO , Zn(NO ) ▪ 6H O, Cu(NO ) ▪ 3H O, Mg(NO ) ▪ 6H O, Ni(NO ) ▪
3
4
3 2
2
3 2
2
3 2
2
3 2
3

6
H O, Co(NO ) ▪ 6H O, Cd(NO ) ▪ 4H O, Ca(NO ) , MnCl ▪4H O, BaCl , HgCl ,
2 3 2 2 3 2 2 3 2 2 2 2 2
Al(NO ) ▪ 9H O, Fe(NO ) ▪ 9H O, CrCl ). Deionized water was used throughout the
3
3
2
3 3
2
3
1
entire experiments. H NMR spectra were acquired on the Bruker 400 MHz
1
1
instruments. The H NMR spectrum of ligand was obtained in DMSO-d and H
6
NMR titrations were carried out dissolving L and metal salts in DMSO-d6
respectively at room temperature. Electrospray ionization mass spectra (ESI-MS)
were conducted by a Bruker esquire 6000 spectrometer and we used DMF as solvent.
Fluorescence measurements were recorded on a Hitachi RF-4500 spectrophotometer.
UV-visible measurements were determined on
a
Shimadzu UV-240
spectrophotometer. The melting point of L was acquired by a Beijing XT4-100x
microscopic melting point apparatus.
2
2
.2. Synthesis
.2.1. Synthesis of 7-methoxychromone-3-carbaldehyde
Based
on
the
literature
procedures,
we
synthesized
7
2
-methoxychromone-3-carbaldehyde [31].
.2.2. Synthesis and characterization of L
The
synthetic
route
of
7
-methoxychromone-3-carbaldehyde-(3’4’5’-trimethoxybenzoyl) hydrazone was
shown in Scheme 1. 3’4’5’-trimethoxybenzoyl-hydrazine (0.1131 g, 0.5 mM) in 15
mL ethanol was added to 7-methoxychromone-3-carbaldehyde (0.1020 g, 0.5 mM) in
4

1
5 mL ethanol. Then the above mixed solvent was refluxed and stirred for 6 h. A
yellow precipitate appeared and then the mixture was filtered and washed several
times with ethanol. Finally, the yellow solid was obtained. Yield: 62%, mp: 153-155
1
℃
. H NMR (400MHz, DMSO-d , TMS) (Fig. S1): (ppm): 12.090 (s, 1H, -NH-),
6
6
’
2’
5
8
7
.971 (d, 1H, J=0.8Hz , H ), 8.340 (d,1H, J=0.8Hz, H ), 7.760 (d, 1H, J=8.8Hz, H ),
2
6
8
.459 (s, 2H, -CH=, H ), 6.607~6.582 (m, 2H, H , H ), 3.187 (s, 12H, -OCH );
3
MS(ESI) (Fig. S2): m/z=413.3352 [M+H]+.
2
.3. General information
−
3
−3
Solutions of the probe L (5×10 M) and metal ions (5×10 M) were prepared in
DMF and CH CH OH-H O respectively. We carried out all spectroscopic studies in
3
2
2
ethanol (Fig. S3) and the fluorescence spectra were recorded upon excitation at 370
nm. Both the excitation slit width and emission slit width were 5 nm throughout
fluorescence measurements. The detection limit of L sensing Al3+ was determined
according to the fluorescence titration analysis and the following equation (detection
limit = 3σ/k) [32]. Meanwhile, the association constant of L-Al3+ complex was
calculated by Benesi–Hildebrand equation (E .(1)) [33].
q
1
1
1
min＝
＋
(1)
F－F
K
(
Fmax－Fmin
)
[Al3 +
]
Fmax ― Fmin
3
. Results and discussion
3
.1. Absorption studies
UV–Vis titration experiment was carried out in ethanol at room temperature. The
5

work solution was prepared by diluted probe L (20 µL) to ethanol (2 mL) to make a
3
+
final concentration of 50.0 µM. Then Al (0-1.5 equiv.) was added to the solution of
L (50.0 µM) prepared above and mixed them for a few seconds. The absorption
3
+
spectrum of L with the addition of different concentrations of Al was demonstrated
3
+
in Fig.1. Upon the addition of Al , an obvious absorption band centering at 370 nm
appeared and a clear isosbestic point at 300 nm indicated the formation of complex
3
+
between free probe L and Al [34-35].
3
.2. Emission studies
The selectivity of L was studied by fluorescence analysis and the fluorescence
+
+
+
2+
2+
properties of L were performed with various metal cations (Li , Na , K , Mg , Ba ,
2
+
2+
2+
2+
2+
2+
2+
2+
3+
3+
3+
Mn , Co , Ni , Cu , Zn , Cd , Ca , Hg , Fe , Cr and Al ) in ethanol. As
shown in Fig. 2, L alone exhibited comparatively weak fluorescence intensity. When
3
+
L (50.0 µM) was treated with different metal cations (1.0 equiv.), only Al induced
significant fluorescence enhancement at 480 nm accompanied by a change of the
fluorescence color from colorless to strong yellow and the quantum yield increased to
1
2.36%. The turn-on fluorescence induced by Al3+ could be explained by
photoinduced electron transfer (PET) phenomenon and C=N isomerisation process in
Schiff base structrues [36-37]. The above experimental results indicated that L has a
3
+
good selectivity for identifying Al .
Competitive experiments were carried out to further ascertain the efficient
3
+
selectivity of L (Fig.3) in ethanol. Compared with L-Al solution, the fluorescence
6

+
+
+
2+
2+
2+
2+
intensity hardly change when most metal ions of Li , Na , K , Mg , Ba , Mn , Co ,
Ni , Cu , Zn , Cd , Ca , Hg , Fe and Cr3+ (1 equiv.) were added to L-Al3+
2
+
2+
2+
2+
2+
2+
3+
3
+
solution respectively. With the addition of Cr , the fluorescence emission intensity at
4
80 nm was diminished. However, it still exhibited sufficient turn-on emission
3
+
intensity for the detection of Al . All of these observations indicated L could be a
3
+
promising selective fluorescent probe towards Al over most competing mental ions.
Fluorescence titration experiment was done for the purpose of getting more
insight into the sensing properties of L (Fig.4.). The fluorescence intensity at 480 nm
3
+
was monotonically increased with gradual addition of Al . Moreover, the probe L
showed a commendable linear relationship between the fluorescence emission
3
+
intensity and the concentration of Al within a certain range (Fig. S4). The detection
3
+
-7
limit of L towards Al was calculated to be 2.14 ×10 M according to 3σ/k by means
of the mentioned fluorescence titration. Based on the analysis of the titration profiles
by Benesi–Hildebrand equation, the association constant for L-Al3+ complex was
3
-1
estimated to be 6.07 ×10 M (Fig. S5).
3
.3. Binding studies
The method of Job’s plot analysis was applied to confirm and corroborate the
3
+
stoichiometry between the probe L and Al . Among all experimental processes, the
3
+
total concentration of L and Al was maintained to a constant of 50.0 μM with a
3
+
continuously variable molar fraction ([L]/([L] + [Al ]). As shown in Fig. 5, the
fluorescence emission intensity at 480 nm reached a maximum with a molar fraction
7

of 0.5. The Job’s plot analysis illustrated 1:1 stoichiometric complexation between L
3
+
and Al . Besides, it was found that the appearance of a peak at m/z 491.865 could be
3
+
- +
assigned to [L + Al + H O + 2OH ] in the ESI-MS spectrum, providing further
2
3
+
evidence for the formation of 1:1 complex between L and Al (Fig. S6).
3
.4. Application of probe L
In order to achieve more convenient operating process, the selectivity of L for
3
+
colorimetric detection of Al was investigated under nature light and a 365 nm UV
lamp respectively. Interestingly, only L-Al3+ solution showed the change of color
from colorless to yellow (Fig.6 Left). Inspired by the obvious color change of L-Al3+
3
+
solution, we also performed experiments for a colorimetric detection of Al on filter
papers with different metal ions. Some round filter papers was prepared by
dip-coating solution of L (0.5 mmol), followed by drying them in the air for about 6 h.
Prepared filter papers were dropped by solutions of various metal ions (0.5 mmol)
respectively and then dried in air. It was found that the probe-immersed filter papers
3
+
after interaction with Al exhibited a naked-eye sensitive color change from a dark
blue to a fluorescent yellow under UV light (Fig.6 Right). Consequently, this easily
available test paper could be used to roughly estimate whether the system contains
3
+
Al .
3
.5. The proposed mechanism
3
+
In an endeavor to better understand the binding mode between probe L and Al ,
1
1
H NMR titration experiments were studied. Fig. 7(a) and Fig. 7(b) displayed the H
8

3
+
NMR spectra of the free L and L with the addition of 1.0 equiv. Al , respectively.
Compared with Fig. 7(a) and Fig. 7(b), it was obviously found that the peak of –NH–
was shifted to downfield from 12.090 ppm to 12.169 ppm. Meanwhile, the proton
2
peak of –CH= and H exhibited an up-field shift from 7.459 ppm to 7.441 ppm. Peak
for the proton of H2’ shifted downfield from 8.971 ppm to 9.013 ppm. These
significant spectral changes demonstrated the N atom of the –N=CH– group and two
O atoms of carbonyl groups participated in the coordination processes. The
3
+
coordination between L and Al changed the electron distribution in the chemosensor.
According to some literature, aluminum ion always formed five or six membered
rings when it was involved in the formation of complexes [38-47]. Based on the
analyses of the experimental results above, the binding mode of L with Al3+ was
proposed (Scheme 2).
3
+
3
.6. Comparisons with some of the previously reported Al fluorescent probes
3
+
We compared this designed Al fluorescent probe with some of the previously
reported probes, including solvent, interfering, and detection limit, etc. As shown in
Table.1, each of the listed probes has some good properties, but it is subject to Cu2+
3
+
and some other interfering metals. And during the detection of Al , the solvents used
in these experiments are also environmental unfriendly. The probe proposed in this
paper detects Al3+ in an environmental friendly ethanol solvent, and Cu2+ has no
3
+
effect on the detection of Al . Additionally, L has a lower detection limit compared
with some reported probes. In summary, L is a favorable fluorescent probe with good
9

3
+
selectivity and sensitivity to Al ions.
4
. Conclusion
In conclusion, we have successfully synthesized and characterized
a chromone-based Schiff base derivative as a fluorescent-colorimetric probe for
recognizing Al3+ in ethanol solution. L produced an obvious fluorescence
3
+
enhancement at 480 nm in the presence of Al and owned excellent selectivity and
3
+
sensitivity towards Al over other common metal ions. The detection limit of L for
3
+
detecting Al was 0.214 µM, which is much lower than that suggested by the WHO
(7.41 µM). Moreover, the fluorescent probe L has been successfully applied to
3
+
identify Al as test kits on solid state.
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21761019).
(
1
4

List of Figure captions
3
+
Fig.1. UV–vis spectrum of L (50.0 μM) with increasing concentration of Al (0-1.5
equiv.) in ethanol solution.
Fig.2. Fluorescence spectrum of L (50.0 µM) before and after addition of various
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
metal ions (1.0 equiv.) of Li , Na , K , Mg , Ba , Mn , Co , Ni , Cu , Zn , Cd ,
2
+
2+
3+
3+
3+
Ca , Hg , Fe , Cr and Al in ethanol solution.
3
+
3+
Fig.3. Fluorescence response of L-Al and L-Al solution with the addition of other
various metal ions in ethanol. Black bar: L (50.0 µM) in the presence of 1.0 equiv. of
3
+
various metal ions. Red bar: L (50.0µM) with 1.0 equiv. of Al in the presence of 1.0
equiv. of other various metal ions.
Fig.4. Fluorescence emission titration spectrum of L (50.0 μM) upon addition of Al3+
(0-1.0 equiv.) in ethanol.
Fig.5. Job’s plot for determining the stoichiometry between probe L and Al3+ in
3
+
-5
ethanol solution. The total concentration of L and Al is 5.0×10 mol/L.
3
+
Fig.6. Application of L (Left: L recognized Al by naked-eye under a 365 nm UV
lamp and sunlight respectively. Right: Fluorescence color changes of L (0.5 mM)
loaded filter paper in the presence of various metal ions(0.5 mM) under illumination
using a 365 nm UV lamp.)
1
3+
Fig.7. H NMR spectra of L (a) and L with Al (b) (1.0 equiv.) in DMSO-d .
6
1
5

Scheme 1. The synthetic route of ligand (L).
Scheme 2. The proposed binding mechanism of L for Al .
3
+
3
+
Fig.1. UV–vis spectrum of L (50.0 μM) with increasing concentration of Al (0-1.5
equiv.) in ethanol solution.
Fig.2. Fluorescence spectrum of L (50.0 µM) before and after addition of various
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
metal ions (1.0 equiv.) of Li , Na , K , Mg , Ba , Mn , Co , Ni , Cu , Zn , Cd ,
2
+
2+
3+
3+
3+
Ca , Hg , Fe , Cr and Al in ethanol solution.
1
6

3
+
3+
Fig.3. Fluorescence response of L-Al and L-Al solution with the addition of other
various metal ions in ethanol. Black bar: L (50.0 µM) in the presence of 1.0 equiv. of
3
+
various metal ions. Red bar: L (50.0µM) with 1.0 equiv. of Al in the presence of 1.0
equiv. of other various metal ions.
Fig.4. Fluorescence emission titration spectrum of L (50.0 μM) upon addition of Al3+
(0-1.0 equiv.) in ethanol.
1
7

Fig.5. Job’s plot for determining the stoichiometry between probe L and Al3+ in
3
+
-5
ethanol solution. The total concentration of L and Al is 5.0×10 mol/L.
3
+
Fig.6. Application of L (Left: L recognized Al by naked-eye under a 365 nm UV
lamp and sunlight respectively. Right: Fluorescence color changes of L (0.5 mM)
loaded filter paper in the presence of various metal ions(0.5 mM) under illumination
using a 365 nm UV lamp.)
1
8

1
3+
Fig.7. H NMR spectra of L (a) and L with Al (b) (1.0 equiv.) in DMSO-d .
6
Scheme 1. The synthetic route of ligand (L).
1
9

3
+
Scheme 2. The proposed binding mechanism of L for Al .
Table 1. Comparisons of various properties with previously reported probes used for
detecting Al3+
Solvent
Interferingmetal ions
LOD (M)
5.25×10-6
1.00 × 10-6
1.5 × 10-7
4.32×10-6
3.66×10-6
5.0×10-5
References
[48]
2
+
3+
2+
DMSO
Cu , Fe , Pb
2
+
3+
DMF/HEPES (9:1)
Cu , Fe
[49]
DMF/H O (1:1)
Cu2+
[50]
2
3
+
+
+
DMSO
Cr , Na , K
[51]
DMSO/H O (9:1)
Not mentioned
[52]
2
CH CN/H O (1:1)
Cu2+
[53]
3
2
Ethanol
/
2.14×10-7
This study
We have no conflicts of interest to declare.
Author statement
Manuscript title:
I have made substantial contributions to the conception or design of the work; or the
acquisition, analysis, or interpretation of data for the work; AND
I have drafted the work or revised it critically for important intellectual content; AND
I have approved the final version to be published; AND
I agree to be accountable for all aspects of the work in ensuring that questions
related to the accuracy or integrity of any part of the work are appropriately
investigated and resolved.
2
0

All persons who have made substantial contributions to the work reported in the
manuscript, including those who provided editing and writing assistance but who are
not authors, are named in the Acknowledgments section of the manuscript and have
given their written permission to be named. If the manuscript does not include
Acknowledgments, it is because the authors have not received substantial
contributions from nonauthors.
Attachment: Yes
No
(circle)
_Cong Liu_________________________
Author signature
_
Cong Liu_____________
_2019.11.29_
___
Printed name
Date signed/appended
3
+


We synthesized a simple turn-on fluorescent probe (L) for the detection of Al .
3
+
The probe showed good selectivity and high sensitivity for Al in ethanol, and
-7
the detection limit was as low as 2.14×10 M.
2
+
3+


In this paper, Cu didn’t interfere with the detection of Al .
3
+
The probe has been successfully applied to recognize Al on filter papers.
2
1

2
2