Two Schiff-base fluorescent sensors for selective sensing of aluminum (III): Experimental and computational studies

Article abstract of DOI:10.1016/j.saa.2015.07.095

Two Schiff-base fluorescent sensors have been synthesized, which both can act as fluorescent probes for Al³⁺, upon addition of Al³⁺, they exhibit a large fluorescence enhancement which might be attributed to the formation of 1:1 liga

Full text of DOI:10.1016/j.saa.2015.07.095

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Two Schiff-base fluorescent sensors for selective sensing of aluminum
(
III): Experimental and computational studies
a
a
a
a
a,
⇑
a
b
Jing-Can Qin , Xiao-ying Cheng , Ran Fang , Ming-fang Wang , Zheng-yin Yang , Tian-rong Li , Yong Li
a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
b
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
ꢀ
ꢀ
The sensors show selectively
response toward Al3
+
.
The sensors exhibit a large
fluorescence enhancement in the
3
+
presence of Al
.
ꢀ
The detection limits reached at
ꢁ8
10
M.
a r t i c l e i n f o
a b s t r a c t
Article history:
Two Schiff-base fluorescent sensors have been synthesized, which both can act as fluorescent probes for
Received 3 March 2015
Received in revised form 13 June 2015
Accepted 23 July 2015
3+
3+
Al , upon addition of Al , they exhibit a large fluorescence enhancement which might be attributed to
the formation of 1:1 ligand-Al complexes which inhibit photoinduced electron transfer (PET) progress,
3+
and that the proposed binding modes of the sensors and Al are identified by theoretical calculations.
Ó 2015 Elsevier B.V. All rights reserved.
Keywords:
Fluorescent sensor
3+
Al
PET
Theoretical calculations
1
. Introduction
Following oxygen and silicon, aluminum is the third most abun-
to a WHO report, the average daily human intake of aluminum is
approx. 3–10 mg, the tolerable weekly aluminum intake in the
human body is estimated to be 7 mg/kg of body weight [6,7].
Therefore, it is important to monitor the concentration levels of
aluminum in the environment and many scientific fields.
In the past few years, fluorescent probes have currently
attracted significant interest because of its high sensitivity, selec-
tivity, rapidity and easy operational procedure, therefore, the
design and synthesis of fluorescent chemosensors remains an
important endeavor in chemistry [8–19].
dant of all elements and is the most widely existing metal ion in
the environment because of acidic rain and human activities
[
1,2]. Meanwhile, as a non-essential element for the human body,
the wide use of aluminum made people expose to it and the intake
of aluminum can lead to many diseases, such as microcytic hypo-
chromic anemia, Al-related bone disease (ARBD), encephalopathy,
dementia, myopathy and Alzheimer’s disease [3–5,1]. According
Additionally, compared with other metal cations, the detection
of Al3 has always been problematic because of its poor coordina-
tion ability, strong hydration ability and the lack of spectroscopic
+
⇑
Corresponding author.
E-mail address: yangzy@lzu.edu.cn (Z.-y. Yang).
http://dx.doi.org/10.1016/j.saa.2015.07.095
386-1425/Ó 2015 Elsevier B.V. All rights reserved.
1

J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
353
characteristics [20–22]. In general, as a hard-acid, it has been found
quartz cuvettes of 1 cm path length. Melting points were deter-
mined on a Beijing XT4-100x microscopic melting point apparatus.
3
+
that Al prefers a coordination sphere containing N and O as
hard-base donor sites, on the other hand, the Schiff-base could pro-
vide a nitrogen–oxygen-rich coordination environment for the
2
2
.2. Synthesis
3
+
hard-acid Al [23–26]. Therefore, it can be conceived that the
3
+
Schiff-base would develop a fluorescent probe for detection of Al
.
.2.1. Synthesis of 1-phenyl-3-methyl-4-acetyl-pyrazolone-5 (PMAP)
3
+
Out of consideration of these circumstances, we report two Al
[27]
chemosensors, 1-phenyl-3-methyl-5-hydroxypyrazole-4-aceton
e-(isonicotinoyl)Hydrazone(A),1-phenyl-3-methyl-5-hydroxypyra
zole-4-acetone-(nicotinoyl)Hydrazone (B) which are prepared by
condensing PMAP and the corresponding hydrozones (Scheme 1),
they can form the similar structures with aluminum and are iden-
tified by theoretical calculations. The spectroscopic studies also
1
5 g of 1-phenyl-3-methyl-pyrazolone-5 (PMP) were placed in
a flask equipped with a stirrer, separatory funnel and a reflux con-
denser and dissolved in 60–80 mL dioxane by application heat,
2 g of Ca(OH) were added, and 9.9 mL of acetylchloride were
2
added dropwise within 1 min, the mixture became a thick paste
and the temperature increased during the first few minutes and
reflux for 30 min. The calcium complex in the flask was decom-
posed by pouring the mixture into HCl (200 mL, 2 N) which caused
crystal to separate. The final product was filtered and recrystallized
from methanol and water, yield: 56%. m. p: 58 °C.
1
3
+
demonstrate that they have excellent selectivity toward Al over
3
+
other metal ions, More importantly, the detection limits for Al
are 4 ꢂ 10 , 8 ꢂ 10 M respectively.
ꢁ
8
ꢁ8
2
. Experimental
.1. Reagents and apparatus
Unless otherwise stated, all solvents and reagents were
2.2.2. Synthesis of the sensors
The synthetic route of the sensors is shown in Scheme 1. An
ethanol solution of Isonicotinohydrazide (1.37 g, 0.01 mol) was
added to the ethanol solution which contained PMAP (2.17 g,
0.01 mol). With refluxing at 80–85 °C for 12 h, the final product
2
obtained from commercial suppliers and used without further
purification. The solutions of metal ions were prepared by dissolv-
ing the desired amount of the corresponding metal nitrate in etha-
was collected then washed with ethanol twice. The sensor A: Red
1
precipitates, m. p. 228–229 °C, yield: 89.3%.
(300 MHz in DMSO-d
H NMR of A
1
5
6
) (Fig. S1) d: 12.402 (s, 1H, H ), 11.377 (s,
1
9
14
12
nol. H NMR spectra of A was recorded on a Varian VR 200 MHz
1H, H ), 8.967 (s, 1H, H ), 8.668 (d, J = 3.6 Hz 1H, H ), 8.148 (d,
and the rest 1H NMR spectra were
J = 7.5 Hz, 1H, H ), 7.876 (d, J = 7.8 Hz, 2H, H ), 7.473 (m, 1H,
10
1,5
spectrometer in DMSO-d
6
1
8
1
2,4
3
recorded with a Bruker Avance Drx 300-MHz spectrometer.
ESI–MS were determined on a Bruker esquire 6000 spectrometer.
UV–Vis absorption spectra were measured on a Perkin Elmer
Lambda 35 UV–vis spectrophotometer. Fluorescence spectra were
generated on a Hitachi RF-5301 spectrophotometer equipped with
H
), 7.269 (m, 2H, H ), 7.002 (m, 2H, H ), 2.367 (s, 3H, CH
H ), and 2.219 (s, 3H, CH
1617, 1589. ESI–MS (Fig. S3): [M + 1] : 336.04.
3
,
6
ꢁ1
3
, H ). IR (KBr, cm ) (Fig. S2): 3437,
+
The synthetic method of B is similar to that of A.: Light yellow
1
precipitates, m. p. 228–229 °C yield: 87.5%. H NMR of B (300 MHz
Scheme 1. Synthesis of the sensors.

3
54
J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
1
5
9
in DMSO-d
6
) (Fig. S4) d: 12.516 (s, 1H, H ), 11.589 (s, 1H, H ),
3.2. Fluorescence study
1
1,13
1,5
8
.822 (s, 2H, H
), 7.997 (d, J = 7.5 Hz, 2H, H ), 7.848 (d,
1
0,14
2,4
3
The effect of Al3+ on the fluorescence properties of the sensors
was investigated in ethanol. As shown in Fig. 2, the free sensors
A, B displayed weak fluorescence intensity upon excitation at
J = 6.0 Hz, 2H, H
.461 (s, 3H, CH
), 7.402 (m, 2H, H ), 7.111 (m, 1H, H ),
8
6
ꢁ1
2
(
3
, H ), and 2.377 (s, 3H, CH
Fig. S5): 3427, 1633, 1594. ESI–MS (Fig. S6): [M + 1] : 336.01.
3
, H ). IR (KBr, cm
)
+
+
+
3
69, 375 nm. Upon addition of several metal ions such as Na , K ,
+
2+
3+
2+
2+
3+
2+
3+
2+
2+
2+
2+
Ag , Ca , In , Cd , Co ,Ga , Ni , Fe , Mn , Mg , Pb , Cu
,
Cr , Ba , Zn and Al3+ , the fluorescence of A, B were only signif-
3
+
2+
2+
3
. Results and discussion
.1. UV–Vis analysis
The sensors properties were initially investigated as a function
icantly enhanced with fluorescence emission wavelength change
3
+
3
from 557 to 450 nm in the presence of Al , the changes of fluores-
cence intensity could be better exhibited through fluorescence
titrations. As shown in Fig. 3, with addition of increasing concen-
3+
3
+
of the concentration of Al by UV–Vis analysis in ethanol. In the
tration of Al , the sensor showed maximum fluorescence emission
3
+
3
+
presence of Al , the changes of the absorption spectra were similar
between the sensor A and the sensor B, the the extent of variation
were different as shown in Fig. 1. The maximum absorption wave-
at about 450 nm which are because the addition of Al resulted in
inhibiting photo-induced electron transfer (PET) process
(
Scheme 2). More specifically, the fluorescence bands of the sen-
3+
length was observed at about 380 nm in the absence of Al . Upon
3+
sors without Al at about 450 nm were with low intensity due
to the quenching mechanism by photo-induced electron transfer
3
+
titration of Al , the absorption band at about 380 nm gradually
disappeared and a new absorption bands appeared at 300 nm with
increasing intensity. Moreover, a clear isosbestic points at 350 nm
appeared which clearly indicated the presence of new complex in
equilibrium with the receptor. All of these might be attributed to
the interaction the sensors with Al3 which were further confirmed
by ESI–MS, IR.
(PET) which was induced by lone pair electrons from the nitrogen
3
+
atom of –C@N. After addition of Al , because of the chelation of
+
Fig. 2. (a) Fluorescence spectra of A (10 lM) upon the addition of metal salts (9.0
2+
+
+
+
2+
3+
2+
3+
2+
3+
2+
2+
2+
2+
equiv.) of Na , K , Ag , Ca , In , Cd , Co ,Ga , Ni , Fe , Mn , Mg , Pb , Cu
,
3+
2+
2+
3+
Fig. 1. (a) Changes in the absorption spectra of A (10
temperature as a function of added Al(NO (0,1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Changes in the absorption spectra of B (10 M) in ethanol at room temperature as a
function of added Al(NO (0,1, 2, 3, 4, 5, 6, 7, 8, 9, 10 M).
lM) in ethanol at room
Cr , Ba , Zn , Al in ethanol (slit widths: 3 nm/3 nm). (b) Fluorescence spectra of
+
+
+
2+
3+
)
3 3
lM). (b)
B (10 lM) upon the addition of metal salts (6.0 equiv.) of Na , K , Ag , Ca , In ,
2+ 2+
3+
2+
3+
2+
2+
2+
2+
3+
2+
2+
3+
l
Cd , Co ,Ga , Ni , Fe , Mn , Mg , Pb , Cu , Cr , Ba , Zn , Al in ethanol (slit
widths: 3 nm/3 nm).
)
3 3
l

J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
355
M) in ethanol upon the addition of Al3+
l
0–9.0 equiv.) (b) fluorescence spectra of A (10 lM) in ethanol upon the addition of
Fig. 4. (a) Fluorescence responses of A (10
ethanol. Black bars represent fluorescence intensity of [A–Al] ([A] = 10
[Al] = 90 M). Red bars represent emission intensity of a mixture of A (10
the metal ions written below the bars (90 M) followed by addition of 90
the mixed solutions. (b) Fluorescence responses of B (10 M) to various metal ions
6.0 equiv.) in ethanol solution. Black bars represent fluorescence intensity of [B–Al]
[B] = 10 M and [Al] = 60 M). Red bars represent emission intensity of a mixture
M) with the metal ions written below the bars (60 M) followed by
M Al3 to the mixed solutions. (For interpretation of the references
l
M) to various metal ions (9.0 equiv.) in
M and
M) with
Fig. 3. (a) Fluorescence spectra of A (10
l
l
(
3+
l
Al (0–6.0 equiv.).
3+
l
lM Al to
l
(
(
l
l
l
of B (10
l
+
addition of 60
l
to color in this figure legend, the reader is referred to the web version of this
article.)
the nitrogen atom of –C@N with Al3+ inhibiting the PET process of
the lone pair electrons on N of –C@N– [28–31], as a result, the
fluorescence intensity increased significantly.
To understand the recognition abilities of the sensors toward
Al3+, the competition experiments were carried out. As shown in
3
+
Fig. 4, the systems of other metal ions and Al coexisted were
examined in ethanol, the result suggested that all the coexistent
3
+
metal ions had no obvious interference with the detection of Al
3
+
except in the case of Fe , which were attributed to theirs inherent
to the magnetic property.
In addition, from the fluorescence titration data, the binding
3
+
constants (log K) for the sensors and Al in ethanol were 11.484
Fig. S7) and 8.6234 (Fig. S8) respectively according to the follow-
ing equation.
(
Scheme 2. Proposed mechanism for detection of Al3+ by A/B.

3
56
J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
Table 1
Major IR data of the receptor and its complexes.
Compound
m
(pyrazolone-
m
(benzoyl-
m
cm
(–C@N)
m
cm
(M–O)
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
OH) cm
C@O) cm
A
A–Al
D
B
B–Al
D
3437
3424
13
3437
3427
10
1617
1601
16
1633
1615
18
1589
1556
33
1594
1553
41
–
578
(cm 1
ꢁ
)
)
m
–
577
(cm 1
ꢁ
m
of 0.5, which indicated that it was a 1:1 stoichiometry of the bind-
3
+
ing mode of the sensor A and Al . The result was further confirmed
by the appearance of
a
peak at m/z 423.3 assignable to
a peak at m/z 491.2 assignable to
3
+
ꢁ
+
[
[
HL + Al +NO
3
ꢁ H] and
3
+
ꢁ
+
+
HL + Al +C
2
H
5
OH + NO
3
+ Na ꢁ 2H] in the ESI/MS (Fig. S11). As
shown in Fig. 5b, the maximum point also appeared at a mole frac-
tion of 0.5, the result indicated that it was a 1:1 stoichiometry of the
3
+
binding mode of the sensor B and Al was further confirmed by the
appearance of peak at m/z 423.3 assignable to
a
3
+
ꢁ
+
[
[
HL + Al +NO
3
ꢁ H] and
a peak at m/z 468.2 assignable to
3
+
ꢁ
+
HL + Al + C
2
H
5
OH + NO
3
ꢁ H] in the ESI/MS (Fig. S12).
IR spectra (Figs. S13 and S14) further proved formation of
3
+
complexes between the sensors and Al as shown Table 1, the
large differences of the IR spectra of the sensors in the absence
3
+
3+
and presence of Al indicate that Al indeed directly interact with
nitrogen atom of ꢁC@N, oxygen atom in the –C@O of benzoyl and
–
OH in pyrazolone.
+
Fig. 5. (a) Job’s plot for determining the stoichiometry of
ethanol (XAl = [Al3 ]/([Al3+] + [A]), the total concentration of
A
and Al3 in
+
3+
A
and Al
was
3
+
1
0
l
M). (b) Job’s plot for determining the stoichiometry of B and Al in ethanol
Al = [Al3 ]/([Al ] + [B]), the total concentration of B and Al3+ was 10
+
3+
lM).
(X
F ꢁ Fmin
log
¼ log K þ nlog ½Mꢃ
F
max ꢁ F
The binding constant K was obtained from the plot of linear
regression of log [(F ꢁ Fmin)/(Fmax ꢁ F)] vs log [M] in Eq. (1), where
the intercept was log K. In the equation, Fmin, Fmax, and F are the
3+
fluorescence intensity in the absence of Al , presence of saturated
3
+
Al , and the fluorescence intensity of the [L-Al] complex at time
intervals. [M] was the concentration of free metal ions which could
be assumed equal to its total concentration [32].
More importantly, the lowest detection limits of the sensors for
Al3+ were determined as 4 ꢂ 10 M (Fig. S9) and 8 ꢂ 10
ꢁ8
ꢁ8
M
(
Fig. S10). Thus, the sensors could be used as a selective fluorescent
sensor for the detection of micromolar concentrations of Al in
many chemical and biological systems.
3
+
3
+
3.3. The complexation of the sensors with Al
In order to further validate the stoichiometry of the sensors and
Al3+, we also carried out Job’s plot, the total concentration of the
3
+
3+
3+
sensors and Al
was 10 lM. XAl = ([Al ]/(Al ] + [sensors]. As
Fig. 6. (a) Optimized structure of [Al(A)(C
[Al(B)(C OH)(NO )].
2 5 3
H OH)(NO )]. (b) Optimized structure of
shown in Fig. 5a, the maximum point appeared at a mole fraction
H
2 5
3

J.-C. Qin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 152 (2016) 352–357
357
Table 2
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[
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Lancet 333 (1989) 61–62.
Compound The bond length
Al–O) Å
The bond length
(Al–O=) Å
The bond length
(Al–N) Å
(
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[
[
[
[
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A–Al
B–Al
1.8321
1.8305
1.8784
1.8856
2.1180
2.1170
2412–2416.
[
[
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[
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3
+
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calculations with B3LYP/6-31G(d) method [33–35] According to
the above discussion and the relevant literature reports [36–40],
[
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3
+
we proposed
a
rational coordinated mode that Al
is
[
hexa-coordinated with one tridentate ligand, one bidentate nitrate
and one monodentate ethanol. As depicted in Fig. 6, the sensors
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48 (2012) 416–418.
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3
+
could chelate Al through interactions with carboxylate oxygen
of benzoyl, imine nitrogen and oxygen of pyrazolone group and
forms 1:1 complexes, the optimized configurations of the sensors
[
19] S. Goswami, A.K. Das, K. Aich, A. Manna, H.K. Fun, C.K. Quah, Supramol. Chem.
(2014) 94–104.
2
[20] K. Soroka, R. Vithanage, D.A. Philips, B. Walker, P.K. Dasgupta, Anal. Chem. 59
(1987) 629–636.
3
+
and Al were shown Table 2, the donors of the ligand were almost
[
[
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3+
on the same plane with Al , which was used to explain the sensing
mechanism.
[23] W.H. Hsieh, C.F. Wan, D.J. Liao, A.T. Wu, Tetrahedron Lett. 53 (2012)
848–5851.
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5
[
4
. Conclusion
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(
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In summary, we have successfully developed two simple
[
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3+
chemosensors which exhibited high selectivity for Al over other
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metal ions. Upon addition of Al3 , the sensors showed remarkable
+
3+
fluorescence enhancement in the presence of Al . More impor-
[
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ꢁ8
tantly, the detection limits reached at 10 M. The results sug-
gested that the receptors could serve as an excellent fluorescent
8
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3
+
chemosensor for Al and might accelerate the development of
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5
[
3
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2015.07.095.
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