A new off-on fluorescent sensor for the detection of Al(III) based on a chromone-derived Schiff-base

Article abstract of DOI:10.1016/j.ica.2017.09.051

A new Al³⁺ sensor, 6-ethoxychromone-3-carbaldehyde-(3-hydroxy-2-naphthalene acyl) hydrazone (L), was designed and synthesized. The sensor L could exist steadily and detect Al³⁺ in the pH range from 5.0 to 8.0 in ethanol and water (3:1, v/v). This sensor showed good selectivity and high sensitivity towards Al³⁺ in the presence of most metal ions, and a remarkable enhancement in fluorescence emission intensity at 508 nm (λ_ex = 420 nm) was observed with addition of 1equiv Al³⁺, which was attributed to the inhibition of photoinduced electron-transfer (PET) phenomenon and C[dbnd]N isomerization process. With the fluorescence titration experiments and the ESI-MS spectrum data, we reached the conclusion that the binding ratio between L and Al³⁺ was 2:1. Besides, the binding constant (K_a) between L and Al³⁺ was calculated to be 9.24 × 10³ M^?1 and the detection limit of L for Al³⁺ was as low as 1.82 × 10^?7 M.

Full text of DOI:10.1016/j.ica.2017.09.051

Accepted Manuscript
Research paper
A new off-on fluorescent sensor for the detection of Al(III) based on a chro-
mone-derived Schiff-base
Li-mei Liu, Zheng-yin Yang
PII:
S0020-1693(17)31104-0
https://doi.org/10.1016/j.ica.2017.09.051
ICA 17910
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
14 July 2017
19 September 2017
20 September 2017
Please cite this article as: L-m. Liu, Z-y. Yang, A new off-on fluorescent sensor for the detection of Al(III) based
on a chromone-derived Schiff-base, Inorganica Chimica Acta (2017), doi: https://doi.org/10.1016/j.ica.2017.09.051
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A new off-on fluorescent sensor for the detection of Al(III)
based on a chromone-derived Schiff-base
Li-mei Liu, 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
3
+
A new Al sensor, 6-ethoxychromone-3-carbaldehyde-(3-hydroxy-2-naphthalene
acyl) hydrazone (L), was designed and synthesized. The sensor L could exist steadily
3
and detect Al in the pH range from 5.0 to 8.0 in ethanol and water (3:1, v/v). This
+
3
+
sensor showed good selectivity and high sensitivity towards Al in the presence of
most metal ions, and a remarkable enhancement in fluorescence emission intensity at
3
+
5
08nm (λex=420nm) was observed with addition of 1equiv Al , which was attributed
to the inhibition of photoinduced electron-transfer (PET) phenomenon and C=N
isomerization process. With the fluorescence titration experiments and the ESI-MS
3
+
spectrum data, we reached the conclusion that the binding ratio between L and Al
3
+
a
was 2:1. Besides, the binding constant (K ) between L and Al was calculated to be
3
-1
3+
-7
9
.24×10 M and the detection limit of L for Al was as low as 1.82×10 M.
Key Words: Fluorescent sensor; Aluminum ion; PET; C=N isomerization

Introduction
Among metals, aluminum is the most prevalent (8.3% by weight) metallic
element and the third most abundant of all elements (after oxygen and silicon) in the
earth’s crust. It is extensively used in many fields, from industrial fields (water
treatment, food additives, medicines) to our daily life (various packing items,
electrical equipment) [1-8]. Aluminum also plays a great role in biochemical reactions,
such as enzyme-catalyzed reactions, biotechnological transformation [9-10]. But
mounting evidences suggest that excessive absorption of aluminum is harmful to
human health. It sometimes leads to diseases, such as Alzheimer’s disease,
Parkinson’s disease, Osteoporosis and Osteomalacia [11-16]. According to the World
Health Organization (WHO) report, the average daily human intake of aluminum is
3
around 3–10 mg and the concentration of Al in drinking water should not exceed
+
200mg/L [17-18]. Hence, it is of great importance to develop the efficient methods for
the detection of aluminum in organism and environment.
Fluorescent sensors, owing to their visual simplicity, high sensitivity,
inexpensive apparatus, instantaneous response, real-time detection, and
nondestructive properties, have been one of the most convenient methods to detect
metal ions [19-20]. What’s more, fluorescence detection has been widely used in
enormous fields such as analytical chemistry, environment biology, biochemistry and
medical science [21-24]. Nevertheless, because of the poor coordination ability,
3
strong hydration ability, and lack of spectroscopic properties of Al , it is more
+

3
difficult to develop fluorescent sensors for sensing Al compared with recognizing
+
3
+
3+
transition metal ions. In general, as Al is a hard-acid, it has been found that Al
prefers to coordinate with hard-base donor sites like N and O atoms [25-29].
Moreover, Schiff-bases are well known as hard bases which can provide
3
+
nitrogen-oxygen-rich coordination environments for Al . As a consequence, a large
3
number of Schiff-bases can be used as Al probes [30-33].
+
In the present work, we report a new chromone Schiff-base as a fluorescence
3
‘off-on’’ sensor for the detection of Al in ethanol and water (3:1, v/v). Its synthetic
+
‘
1
13
route is shown in Scheme1 and characterized by H NMR, C NMR, ESI-MS and
3
HR-MS. The free sensor shows no fluorescence emission. Upon binding of Al , L
+
exhibits a significant fluorescence emission in ethanol and water (3:1, v/v). Besides,
3
the sensor shows good selectivity and high sensitivity towards Al in the presence of
+
most metal ions.
Experimental
Materials and apparatus
All chemicals were purchased from commercial suppliers and used without
further purification. The solutions of metal ions were performed from their nitrate
1
salts. H NMR spectra were measured on the Bruker 300MHz instruments using TMS
1
3
as an internal standard and DMSO-d
on the Bruker 400MHz instruments using DMSO-d
determined on a Bruker esquire 6000 spectrometer using DMF as solvent. HR-MS
6
as a solvent. C NMR spectrum was measured
6
as a solvent. ESI-MS were

spectrum was obtained on high resolution mass spectrometer (LTQ-Obitrap-ETD) in
DMF. Fluorescence spectra were recorded on a Hitachi RF-4500 spectrophotometer.
UV-Vis absorption spectra were determined on a Shimadzu UV-240
spectrophotometer. FTIR spectra were collected on a VERTEX 70V instrument. The
melting point was determined on a Beijing XT4-100x microscopic melting point
apparatus. All pH measurements were carried out on a FE28 acidometer.
Synthesis
6
-ethoxychromone-3-carbaldehyde was obtained according to the literature
procedures [34]. Synthesis of L was based on the following method (Scheme 1): an
ethanol solution (10mL) of 3-hydroxy-2-naphthoic acid hydrazide (0.1011 g, 0.5mmol)
was added dropwise to a solution of 6-ethoxy chromone-3-carbaldehyde (0.1091g,
0.5mmol) in ethanol (10mL). Then the solution was refluxed for 6 hours under
stirring and some white precipitant appeared. The mixture was filtered and dried
1
under vacuum. Yield: 95.4%, Melting point: 213–214℃. H NMR (DMSO-d
6
,
9
3
1
7
3
1
1
00MHz) δ (ppm): 12.09 (s, 1H, -NH-), 11.27 (s, 1H, -OH-), 8.86 (s, 1H, H ), 8.64 (s,
2
15
14
11
H, H ), 8.45 (s, 1H, H ), 7.91 (d, 1H, J=7.9Hz, H ), 7.76 (d, 1H, J=8.0Hz, H ),
4
.71 (d,1H, J=9.0Hz, H ), 7.54-7.32 (m, 5H, H
5,7,10,12,13
), 4.52 (q, 2H, -CH2-), 1.38 (t,
1
H, -CH3) (Figure.S2). C NMR (DMSO-d
3
6
, 400MHz) δ(ppm): 161.17, 153.82,
45.47, 144.06, 143.95, 140.80, 133.71, 129.19, 124.35, 123.30, 123.02, 121.85,
21.09, 119.76, 119.57, 119.42, 116.62, 116.12, 114.43, 108.96, 105.12, 71.63, 31.97
+
Figure.S3). MS (ESI) 403.1 [M+H] , 425.1 [M+Na] , (Figure.S4). HR-MS 403.13
+
(

+
M+H] , 425.11 [M+Na] , (Figure.S5).
+
[
Scheme 1. The synthetic route of 6-ethoxychromone-3-carbaldehyde-(3-hydroxy-
-naphthalene acyl) hydrazone (L).
2
Analysis
+
+
+
2+
2+
2+
Stock solutions (5.0mM) of the nitrate salts of Na , K , Ag , Mg , Ba , Ca ,
2
+
2+
2+
2+
2+
2+
2+
2+
3+
3+
3+
Cd , Co , Cu , Zn , Mn , Hg , Pb , Ni , Fe , Cr , Al were prepared in
ethanol. Stock solutions of L (5.0mM) were prepared in DMF. The solution of L was
then diluted to 50.0µM in ethanol and water (3:1, v/v). In experiments, each time 2mL
solution of L (50.0µM) was filled in the quartz 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. The excitation and emission slit widths were 3.0nm and 5.0nm respectively.
Results and discussion
Absorption studies
3
+
UV–Vis titration of L with increasing concentrations of Al (Figure 1).

Firstly, UV–Vis titration spectrum of L upon addition of various concentrations of
3
Al was carried out in ethanol and water (3:1, v/v) at room temperature. The L in the
+
3
absence of Al exhibited a relatively weak absorption band at 226nm, which was
+
probably assignable to chromone moiety. The addition of increasing concentrations of
3
Al induced gradual enhancement at 226nm. At the same time, this band had a slight
+
red-shift to 242nm. Besides, the solution of L displayed a broad band around 325nm,
3
+
upon the addition of Al , the absorbance bands at 275nm, 330nm, 435nm enhanced.
3
These results suggested that L participated in the coordination with Al .
+
3
+
Figure 1. The UV-Vis titration spectrum of L (50.0µM) with Al (0.0-1.3equiv) in
ethanol and water (3:1, v/v).
Emission studies
3
Selectivity of L towards Al over other metal ions (Figure 2). A
+
fluorescence study was further used to determine the selectivity and sensitivity of L in
+
the presence of various metal ions (Na , K , Ag , Mg , Ba , Ca , Cd , Co , Cu ,
+
+
2+
2+
2+
2+
2+
2+
2
+
2+
2+
2+
2+
3+
3+
3+
Zn , Mn , Hg , Pb , Ni , Fe , Cr , and Al ). The fluorescence responses were
3
conducted by adding 1.0equiv of Al and 1.0equiv other metal ions to the ethanol and
+
3
+
water (3:1, v/v) solution of L respectively. Fascinatingly, the treatment of L with Al

led to a strong enhancement of the fluorescence intensity in the emission spectrum at
08nm upon excitation at 420nm. What’s more, no such effect was observed when L
5
3
+
was treated with other cations. Therefore, the sensor exhibited high selectivity for Al
over other metal ions.
Figure 2. Fluorescence responses of L (50.0µM) in ethanol and water (3:1, v/v) with
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
equiv Na , K , Ag , Mg , Ba , Ca , Cd , Co , Cu , Zn , Mn , Hg , Pb , Ni ,
1
3
+
3+
3+
Fe , Cr and Al (λex=420 nm).
3
Competition of L towards Al over other metal ions (Figure 3). Competition
+
experiments were carried out to explore the possibility of using L as a practical ion
3
selective fluorescent sensor for Al . For this purpose, L was treated with 1.0equiv
+
3
Al in the presence of 1.0equiv other metal ions. Relatively low interference was
+
3
observed for the detection of Al in the presence of most metal ions. L responding for
+
3
+
3+
2+
Al in the presence of Fe and Ni were relatively low but clearly detectable. In the
2
presence of Cu , the fluorescence of the L-Al system was almost quenched, which
+
3+
might be due to an energy or electron transfer [35-37]. Consequently, L was shown to
3+
be a promising selective fluorescent sensor for Al in the presence of most competing
metal ions.

3
Figure 3. Relative fluorescence of L and its complexation with Al in the presence of
+
various metal ions in ethanol and water (3:1, v/v). Black bar: L (50.0µM) with 1.0
equiv Ag , Ba , Cd , Co , Cu , Fe , Ca , Hg , K , Mg , Mn , Na , Ni , Pb ,
+
2+
2+
2+
2+
3+
2+
2+
+
2+
2+
+
2+
2+
2
+
3+
3+
Zn , Cr stated. Red bar: 50.0µM of L and 1.0equiv Al in the presence of 1.0equiv
other metal ions stated (λex=420 nm).
Fluorescence titration experiments of L in the presence of increasing
3
concentrations of Al (Figure 4). Fluorescence titration experiments were conducted
+
3
upon addition of increasing concentrations of Al . The free L exhibited no emission
+
band at 508nm, and gradual enhancement in fluorescence emission intensity at 475nm
3
+
and 508nm was observed with the addition of Al . These phenomena indicated that
3
the L and Al have successfully combined. The detection limit of L for the analysis of
+
3
+
Al was also calculated based on the fluorescence titration experiments. The
detection limit was calculated by the following equation: detection limit=3σ/k [38],
where σ was the standard deviation of blank measurements, and k was the slope
between fluorescence intensity versus sample concentration. As shown in
-
7
3+
Figure.S9(a), it reached at 1.82×10 M, which was lower than the tolerable Al
concentration in drinking water set by WHO.

3
+
Figure 4. Fluorescence emission titration spectra of L (50.0µM) upon addition of Al
(
0.0-1.8equiv) in ethanol and water (3:1, v/v) (λex=420 nm).
Effect of pH (Figure 5). In view of the importance of pH value adaptation in
practical applications, we investigated the effect of pH on the fluorescence intensity at
08 nm of L. In the pH range of 5.0-8.0, the fluorescence intensity at 508 nm of L
5
3
remained and relatively low interference was observed for the detection of Al .
+
Significantly, on moving beyond the pH range of 5.0-8.0, the fluorescence intensity at
3
08nm of L-Al gradually decreased and even quenched. Therefore, these results
+
5
3
indicated that this sensor could exist steadily and detect Al in the pH range from 5.0
+
to 8.0.
Figure 5. Effect of pH on the fluorescence intensity at 508nm of L (50.0µM) in the
3
+
absence and presence of 1.0equiv Al in ethanol and water (3:1, v/v) (λex = 420 nm).

Binding studies
3
Binding stoichiometry between L and Al (Figure 6). In order to determine the
+
3
stoichiometry between L and Al , the method of continuous variation (Job’s plot) was
+
3
used [39]. During this experimental process, the total concentration of L and Al was
+
3
+
kept constant at 50.0µM, with a continuous variable molar fraction of guest [Al ] /([L]
3
[Al ]). It can be seen that the fluorescence emission intensity at 508 nm reached
+
+
maximum at the molar ratio of 0.36, establishing a 2:1 binding stoichiometry between
3
L and Al . Furthermore, the ESI-MS spectrum of L and Al in DMF showed a
+
3+
3
+
peak at 829.2(m/z) that was assignable to [2L+Al -2H ] (Figure.S7), which further
+ +
3
gave solid evidence for the formation of a 2:1 complex between L and Al .
+
3
Figure 6. Job’s plot of L with Al in ethanol and water (3:1, v/v). The total
+
3
+
concentration of L and Al is 50.0 µM (λex=420 nm).
3
+
Association constant between L and Al . As shown in Figure.S9(b), the
3
-1
association constant K
a
of the complex was calculated to be 9.24×10 M with a
linear relationship by the Benesi–Hildebrand equation (Eq. (1)) [40], where Fmax, Fmin
,
3
and F are fluorescence intensities of L in the presence of Al at saturation, free L, and
+
3
+
any intermediate Al concentration.

ꢀ
ꢀ
ꢀ
= ꢆ(ꢁꢃꢇꢈꢂꢁ
+
(1)
)ꢉꢊꢋꢌꢍ
ꢎ
ꢁ
ꢂꢁꢃꢄꢅ
ꢁꢃꢇꢈꢂꢁꢃꢄꢅ
ꢃꢄꢅ
3+ 1
Binding mode between L and Al . H NMR studies provided additional
3+ 1
evidence of the interaction between L and Al . H NMR spectra of L were recorded
3
+
6
in DMSO-d upon the addition of Al . Spectral changes were observed as shown in
3
+
Figure.S6. The δ 11.27ppm signal for –OH disappeared upon the addition of Al ,
which indicated that the direct involvement of the oxygen atom of the –OH group
3
coordinate with Al . In IR spectra (Figure.S8) ,the C=O of hydrazide frequency
+
-
1
-1
decreased from 1660cm to 1636cm , and the imine C=N vibration at 1602cm was
-1
-
1
absent in the complex. At the same time, a new frequency of 1374cm appeared,
which was assigned to C-N stretching vibration [41-42]. These findings suggested the
carbonyl oxygen atom of hydrazide and the nitrogen atom of C=N group were also in
3
+
for coordinating with Al . Besides, according to some literature, we found that when
aluminum ions were involved in the coordination, the complexes always formed five
or six membered rings, which were relatively stable [43-52].
Based on these results above, the proposed binding mechanism for the
3
+
fluorescence response of L towards Al was discussed. As shown in Scheme 2, the
fluorescence emission of free L was nearly quenched and it was due to the photo
induced electron transfer (PET) phenomenon that was caused by lone pair electron
from Schiff-base nitrogen atom to the chromone moiety and C=N isomerization
3
process. Nevertheless, when Al was treated with L, two nitrogen atoms of the
+
3
azomethine groups participated in the coordination with Al and formed a 2:1
+
complex. As a result, the fluorescence emission of L centered at 508 nm enhanced

3
significantly in the presence of Al .
+
Application of L as solid state sensors
3
Inspired by the conspicuous colorimetric change of L upon addition of Al in
+
3
+
solution, we further tested Al level on filter paper. For this purpose, L was adsorbed
on the filter paper by immersing filter papers in the DMF solution of L (5mM) and
then drying them in the air. Then the ethanol and water (3:1, v/v) solutions of various
metal ions (5mM) were dropped in test strips. As shown in Scheme 3, the color of
coated test paper changed from black to pale yellow under illumination using a 365
3
nm UV lamp with the addition of Al . Such a drastic colorimetric change was
+
3
attributed to the formation of L-Al complex. Therefore, this probe L could be
+
3
applied for the detection of Al in practical samples by the naked eye.
+
Scheme 3: Fluorescence color changes of L(5mM) loaded filter paper in the presence
of various metal ions(5mM ) under illumination using a 365 nm UV lamp.
Conclusion
In conclusions, we designed and synthesized a chromone derived Schiff-base
3
ligand as an Al fluorescent sensor based on the PET mechanism and C=N
+

3
isomerization process. This sensor had good selectivity and high sensitivity for Al in
+
3
the presence of most metal ions, the detection limit of L for sensing Al could reach
+
-
7
3+
.82×10 M. Furthermore, the sensor could exist steadily and detect Al in ethanol
1
and water (3:1, v/v) in the pH range of 5.0–8.0. Thus, we believe that L has the ability
3
+
to serve as a potential sensor for sensing Al . This strategy may provide a general
way for designing new probes to detect other environmentally and biologically
relevant species.
Acknowledgments
This work is supported by the National Natural Science Foundation of China
(
81171337，21761019).

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List of Figure captions
3
+
Figure 1. The UV-Vis titration spectrum of L (50.0µM) with Al (0.0-1.3equiv) in
ethanol and water (3:1, v/v).
Figure 2. Fluorescence responses of L (50.0µM) in ethanol and water (3:1, v/v) with
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
equiv of Na , K , Ag , Mg , Ba , Ca , Cd , Co , Cu , Zn , Mn , Hg , Pb ,
2+
1
2
+
3+
3+
3+
Ni , Fe , Cr and Al (λex=420 nm).
3
Figure 3. Relative fluorescence of L and its complexation with Al in the presence of
+
various metal ions in ethanol and water (3:1, v/v). Black bar: L (50.0µM) with 1.0
+
2+
2+
2+
2+
3+
2+
2+
+
equiv of Ag , Ba , Cd , Co , Cu , Fe , Ca , Hg , K , Mg , Mn , Na , Ni ,
2+
2+
+
2+
2
+
2+
3+
3+
Pb , Zn , Cr stated. Red bar: 50.0µM of L and 1.0equiv of Al in the presence of
.0equiv of other metal ions stated (λex=420 nm).
Figure 4. Fluorescence emission titrations spectrum of L (50.0µM) upon addition of
1
3
+
Al (0.0-1.8equiv) in ethanol and water (3:1, v/v) (λex=420 nm).
Figure 5. Effect of pH on the fluorescence intensity at 508nm of L (50.0µM) in the
3
+
absence and presence of 1.0equiv Al in ethanol and water (3:1, v/v) (λex=420 nm).
3
Figure 6. Job’s plot of L with Al in ethanol and water (3:1, v/v). The total
+
3
+
concentration of L and Al is 50.0µM (λex=420 nm).
Scheme 1. The synthetic route of 6-ethoxychomone-3 carbaldehyde-(3-hydroxy-
2
-naphthalene acyl) hydrazone (L).
3
Scheme 2. The proposed sensing mechanism of L and Al .
+
Scheme 3: Fluorescence color changes of L(5mM) loaded filter paper in the presence
of various metal ions(5mM ) under illumination using a 365 nm UV lamp.

3
+
Figure 1. The UV-Vis titration spectrum of L (50.0µM) with Al (0.0-1.3equiv) in
ethanol and water (3:1, v/v).
Figure 2. Fluorescence responses of L (50.0µM) in ethanol and water (3:1, v/v) with
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
equiv of Na , K , Ag , Mg , Ba , Ca , Cd , Co , Cu , Zn , Mn , Hg , Pb ,
1
2
+
3+
3+
3+
Ni , Fe , Cr and Al (λex=420 nm).

3
Figure 3. Relative fluorescence of L and its complexation with Al in the presence of
+
various metal ions in ethanol and water (3:1, v/v). Black bar: L (50.0µM) with 1.0
equiv of Ag , Ba , Cd , Co , Cu , Fe , Ca , Hg , K , Mg , Mn , Na , Ni ,
+
2+
2+
2+
2+
3+
2+
2+
+
2+
2+
+
2+
2
+
2+
3+
3+
Pb , Zn , Cr stated. Red bar: 50.0µM of L and 1.0equiv of Al in the presence of
.0equiv of other metal ions stated (λex=420nm).
1
Figure 4. Fluorescence emission titrations spectrum of L (50.0µM) upon addition of
3
+
Al (0.0-1.8equiv) in ethanol and water (3:1, v/v) (λex=420 nm).

Figure 5. Effect of pH on the fluorescence intensity at 508nm of L (50.0µM) in the
3
+
absence and presence of 1.0equiv Al in ethanol and water (3:1, v/v) (λex = 420 nm).
3
Figure 6. Job’s plot of L with Al in ethanol and water (3:1, v/v). The total
+
3
+
concentration of L and Al is 50.0 µM (λex=420 nm).

Scheme 1. The synthetic route of 6-ethoxychromone-3-carbaldehyde-(3-hydroxy-
-naphthalene acyl) hydrazone (L).
2
3
Scheme 2. The proposed sensing mechanism of L and Al .
+
Scheme 3: Fluorescence color changes of probe L(5mM) loaded filter paper in the
presence of various metal ions(5mM ) under illumination using a 365 nm UV lamp.

ꢀ
ꢀ
We designed and synthesized a new Off-on fluorescent sensor for the detection of
3
Al , 6-ethoxychromone-3-carbaldehyde-(3-hydroxy-2-naphthalene acyl)
+
hydrazone (L).
3
+
The sensor showed good selectivity and high sensitivity for the detection of Al
3
+
in ethanol and water (3:1, v/v), and the detection limit of L for Al was as low as
-
7
1
.82×10 M.
ꢀ
ꢀ
With the fluorescence titration experiments and the ESI-MS spectrum data, we
3
reached the conclusion that the binding ratio between L and Al was 2:1.
+
The yield of L can be reached 95.4%.