Bis-Schiff base as a donor-acceptor fluorescent probe: Recognition of Al3+ ions in near 100% aqueous solution

Article abstract of DOI:10.1016/j.jphotochem.2015.02.008

In this paper, a novel bis-Schiff base fluorescent probe (HL) for Al³⁺ has been designed and synthesized on the basis of the mechanism of internal charge transfer (ICT). Compared with the free receptor, the fluorescence intensity of HL shows si

Full text of DOI:10.1016/j.jphotochem.2015.02.008

Accepted Manuscript
Title: Bis-Schiff base as a donor-acceptor fluorescent probe:
3
+
Recognition of Al ions in near 100% aqueous solution
Author: Jing-can Qin Zheng-yin Yang
PII:
S1010-6030(15)00061-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2015.02.008
JPC 9837
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
28-11-2014
23-1-2015
12-2-2015
Please cite this article as: Jing-can Qin, Zheng-yin Yang, Bis-Schiff base as
a donor-acceptor fluorescent probe: Recognition of Al3+ ions in near 100%
aqueous solution, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2015.02.008
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Bis-Schiff base as a donor-acceptor fluorescent probe ：
3
+
Recognition of Al ions in near 100% aqueous solution
Jing-can Qin, 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 8912582; e-mail: Highlights
yangzy@lzu.edu.cn (Z.Y. Yang)
Graphical abstract
3
+
Upon addition of Al , compared with the free receptor, the fluorescence intensity of HL
shows significant fluorescence enhancement accompany with about 40 nm blue shift in the
3
+
presence of Al , the reasons for this phenomenon are attributed to forms 1:1 complex which
inhibit internal charge transfer process.
Highlights
3
+

A novel bis-Schiff base fluorescent probe for Al has been designed and synthesized
3
+

The probe for Al shows significant fluorescence enhancement accompany with
about 40 nm blue shift on the basis of the mechanism of ICT.
3
+
-7

The detection limit for Al reached at 10 M level in 100% aqueous solution.

Abstract
3
+
In this paper, a novel bis-Schiff base fluorescent probe (HL) for Al has been designed and
synthesized on the basis of the mechanism of internal charge transfer (ICT). Compared with
the free receptor, the fluorescence intensity of HL shows significant fluorescence
3
+
enhancement accompany with about 40 nm blue shift in the presence of Al . More
3
+
-7
importantly，the lowest detection limit for Al can reach at 10 M level in near 100%
aqueous solution.
3
+
Keywords: Bis-Schiff base;Fluorescent probe; Al ; Internal charge transfer
. Introduction
The development of fluorescent chemosensors with high selectivity, sensitivity and real-
1
time detection for metal ions such as aluminum ion remains an important endeavor in
chemistry due to their significant importance roles in environmental research and
analytical fields [1-6]. Aluminum is the third most abundant of all elements (after oxygen and
silicon) and is the most widely existing metal ion in the environment because of acidic rain
and human activities [7-8]. Nevertheless, aluminum has severe toxicity and is a non-essential
element for biological systems, high concentration of aluminum ion not only hampers plant
growth but also has been implicated in various neurodegenerative and neurological disorders,
such as Alzheimer’s disease, dialysis encephalopathy, and problems in bone, muscles etc [9-
1
4]. Additionally, According to a WHO report, the average daily human intake of aluminum
is around 3–10 mg. Tolerable weekly aluminum dietary intake in the human body is
estimated to be 7mg / kg body weight [15-16]. In order to protect the environment and human

health, it is crucial to develop an effective fluorescent probe for detection of the concentration
levels of aluminum in the environment [17-20].
3
+
Compared with other metal ions, the development of fluorescent probe for Al has
always been problematic because of its poor coordination ability, strong hydration ability and
3
+
the lack of spectroscopic characteristics [21-23]. Even if there are some of reported Al
sensors, the majority of them worked well only in organic or a mixture of organic solvent and
3
+
water which are difficult to detect the submicromolar concentration of the Al in
environment [24-29]. The design and synthesis of fluorescent sensors in water or in near
1
00% aqueous solution is still a challenge [30].
, 6-diformyl-4-substituent phenol with strong binding ability and excellent optical
2
properties has attracted considerable attention [31-33]. A growing number of fluorescent
probes have been designed and synthesized based on its bis-Schiff base [34-36]. The structure
of bis-Schiff base contains nitrogen–oxygen-rich coordination environments which provide
enough binding sites for metal ions. However, most of the sensors are synthesized by 2, 6-
diformyl-4-methyl phenol, which was performedin non-aqueous solvents, and that the
sensing mechanisms are attributed to a single and simple photo-induced electron transfer
(
PET) process [37-39]. So, it is not only limited in range of application but also their
sensitivity and the built-in correction was apt to be affected by environmental condition.
Out of consideration of these circumstances, we response to these problems through a
possible strategy that if the methyl group is displaced by the ester group at the 4-
position of benzene ring, the bis-Schiff base will probably provide a donor-acceptor system

which is a prerequisite for intramolecular charge transfer (ICT) process [40-42]. And, in
short, herein, we have designed a bis-Schiff base which derives from Methyl 3, 5-diformyl-4-
3
+
hydroxybenzoate and Nicotinohydrazide as shown Scheme 1. Upon binding of Al , the
fluorescence intensity of HL exhibits a large fluorescence enhancement accompany with
about 40 nm blue-shift in water. All of these are attributed to forms 1:1 complex effect on
inhibition of intramolecular charge transfer (ICT) process.
2
. Experimental
2
.1. Apparatus and reagents
All chemicals were obtained from commercial suppliers and used without further
1
purification. H NMR spectra were measured on the JNM-ECS 400MHz instruments using
TMS as an internal standard. Bruker esquire 6000 spectrometer, Perkin Elmer Lambda 35
UV-Vis spectrophotometer, Hitachi RF-5301 spectrophotometer equipped with quartz
cuvettes of 1 cm path length, Beijing XT4-100x microscopic melting point apparatus.
2
.2 Analysis
Stock solutions of various metal ions (5 mM) were prepared using nitrate salts in two-
distilled water. A stock solution of HL (1 mM) was prepared in DMSO, the solution of HL
was then diluted to 10 μM in water. In titration experiments, each time a 2 mL solution of HL
(
10 μM) was filled in quartz optical cell of 1 cm optical path length, and then the ions stock
solution were added into the quartz optical cell. In selectivity experiments, the test samples
were prepared by placing appropriate amounts of ions stock into 2 mL solution of HL (10

μM). For fluorescence measurements, the excitation slit width was 5 nm, the emission slit
width was 3 nm.
2
2
.3. Synthesis
.3.1. Synthesis of Methyl 3, 5-diformyl- 4-hydroxybenzoate [43-44]
Methyl 4-hydroxybenzoate (2.5g, 16.25mmol) and Hexamethylenetetramine
(
9.39g, 66.91 mmol) were dissolved in trifluoroacetic acid (20 mL ) and the yellow solution
was heated at reflux for 4 d. Water (100 mL) was added to the viscous dark-orange solution
and the mixture was heated up to give a homogeneous solution. After cooling, the product
was precipitated slowly from solution. Yield: 65%, M.p.120-122°C.
2
.3.2 Synthesis of HL
An ethanol solution of Nicotinohydrazide (0.27 g, 2 mmol) was added to a solution
containing methyl 3, 5-diformyl 4-hydroxybenzoate (0.21g, 1mmol) in ethanol. The mixture
was refluxed for 12 h. The solution was then cooled to room temperature, and the solvent was
evaporated. The final product was recrystallized from ethanol and DMF. The yield of HL was
1
6
7
1
5%, M. p. 274–276°C. H NMR (400 MHz; DMSO-d
6
), (Fig. S1): δ=3.85 (s, 3H),
.54~7.57 (m, 2H), 8.24~8.26 (m, 2H), 8.33 (s, 2H), 8.73 (s, 2H), 8.75 (s, 2H), 9.06 (s, 2H),
+
2.46 (s, 2H), 13.16 (s, H) ESI-MS (Fig. S2): [M+H] : 447.05. (Calcd: 447.14).
3
. Results and discussion
3
.1 General information

The detection limit was evaluated based on the fluorescence titration. And then it was
calculated with the following equation: detection limit 3σ/K. where σ is the standard
deviation of blank measurements, and K is the slope between intensity versus sample
concentration [45].
The binding constant values were determined from the emission intensity data following
the modified Benesi–Hildebrand equations[46].
Where Fmin, F, and Fmax are the emission intensities of the organic moiety considered in
the absence of aluminum ion, at an intermediate aluminum concentration, and at a
concentration of complete interaction, respectively, and where K
a
is the binding constant
concentration were also used for the characterization of novel compound.
3
1
.2 UV-vis analysis
3
+
The interaction of HL and Al was initially evaluated by UV-visible analysis in near
00% aqueous solution. The changes in the spectra of HL as a function of the concentration
3
+
4
of Al were shown in Fig. 1(εmax=3.9×10 ). The absorption spectrum of the free receptor
3
+
showed three absorption bands at 298 nm, 350nm and 422 nm. Upon addition of Al to a
solution of HL, the former two bands significantly decreased while the third band gradually
3
disappeared (εmax=5.8×10 ), more importantly, a new band appeared at 389 nm with
increasing intensity and a clear isosbestic points at 360 nm was observed which clearly
indicated the presence of new complex in equilibrium with the receptor. In addition, the third
band of HL (at 422 nm) exhibited a blue shift compared to the new band in the spectrum of

[
HL +Al] (at 389nm), perhaps the reasons for this phenomenon was that the aluminium ion
bonding with the donor moiety (-C=N,-OH) changed the photophysical properties of ICT
fluorophore (the acceptor-COOCH
3
) and further influences the efficiency of charge transfer
1
[
47-49]. Of course, all of these were further confirmed by ESI-MS, HNMR titration.
3
+
3
.3 Fluorescence response of HL to Al
+
The selectivity of HL was investigated upon addition of several metal ions such as Li ,
+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
3+
3+
Na , K , Ca , Mg , Cu , Co , Mn , Ni , Zn , Ba , Fe , Cd , Hg , Pb , Fe , Cr ,
3
+
and Al in near 100% aqueous solution. As shown in Fig. 2, the free sensor showed only a
weak fluorescence emission at 506 nm, when it is excited at 389 nm (quantum yield =
5
.13%). On addition of various metal ions to aqueous solution of HL, there is no remarkably
2
+
3+
change in its fluorescence spectrum except in the case of Zn , Al . But the fluorescence
2
+
3+
enhancement which Zn generates is significantly lower than with Al . Thus, the
fluorescence intensity of HL showed a large fluorescence enhancement along with a blue
3
+
shift of 40 nm in the present of Al (quantum yield =15.53%), which was enough to
3
+
3+
selectivity detection of Al . To understand the recognition abilities of HL toward Al , the
3
+
fluorescence titration of HL toward Al was further carried out. As shown in Fig. 3, upon the
3
+
addition of increasing amount of Al into the solution of HL, the emission peak of HL at 460
nm was significantly enhanced. The spectral blue shift should be attributed to a donor-
acceptor system which is a prerequisite for ICT process.More specifically, before addition of
3
+
Al , the receptor displayed an emission band at 506 nm which arose from the ICT involved
electron-donor group (-C=N, -OH) electron-withdrawing group (-COOCH ). Up addition of
3

3
+
3+
Al , the interaction of Al and the electron-donor group of the receptor would be sufficient
to reduce the electron density in electron-donor group, as a result, the efficiency of charge
transfer be weaken from the donor to the acceptor and further occurred a blue shift as shown
in Scheme 2.
3
+
To check the practical applicability of HL as a selective fluorescent sensor for Al , the
3
+
systems of other metal ions and Al coexisted were examined in near 100% aqueous
3
+
solution. As shown in Fig. 4, the receptor HL was treated with 1.0 equiv of Al in the
presence of other metal ions. Relatively low interference was observed for the detection of
3
+
2+
3+
2+
2+
Al in the presence of other metal ions except for Cu , Fe , Ni , Co , which be attributed
2
+
2+
to their inherent to the magnetic property. However, Ni , Co , somewhat inhibited the
3
+
2+
interaction between HL and Al , and Cu did so strongly. In addition, from fluorescence
3
+
-7
titration, the detection limit of HL for Al was found to be 6.9×10 M based on 3σ/K
(
(
σ=2.76) which was is below the WHO acceptable limit (7.4μΜ) in drinking water [50-51]
Fig. S3). Therefore, it means that the receptor HL can be used as a selective fluorescent
3
+
sensor for detection and recognition of Al in environmental analysis
and analytical chemistry.
3
1
.3. Job plot measurements
In order to validate the stoichiometry of HL and Al , we carried out Job’s plot in near
3
+
3
+
3+
00% aqueous solution.. The total concentration of HL and Al was 50 μM. XAl= ([Al ]/
3
+
(
[Al ] + [HL]. As shown in Fig. 5, the maximum point appeared at a mole fraction of 0.5.
3
+
The result has indicated that it was a 1:1 stoichiometry of the binding mode of HL and Al

which is further confirmed by the appearance of a peak at m/z 471.02 assignable to
3
+
+
3+
+
[
HL+Al -2H] and a peak at m/z 517.05 assignable to [HL+CH
3
CH
2
OH+Al -2H] in the
+
ESI/MS (Fig. S4). In addition, a main peak at m/z 447.06 was assigned to [HL+H] . More
3
+
importantly, from titration profile, the binding constant of HL for Al have been estimated as
4
2
1
3
.67×10 (R =0.991) (Fig.S5) based on the modified Benesi–Hildebrand equation.
1
.4 H NMR titration
3
+
1
In order to further validate the conjugation of HL and Al ， HNMR
3
+
titration was desperately needed besides the above experiments. Al (as its nitrate salt) was
added to the DMSO-d
6
solution of HL (Fig. S6), significant spectral changes were observed
1
as shown in Fig. 6, In the H NMR of the neutral HL, the broad peaks at 12.46 ppm and 13.19
ppm should be corresponding to the labile protons of -C-NH and –OH, respectively. Upon
3
+
addition of Al , the proton peak of the phenolic hydroxyl at 13.19 ppm weaken significantly,
3
+
this could be because addition of Al to HL caused the loss of the –OH proton with
3
+
subsequent binding to electron deficient Al . The broad peak also became weak at 12.46
ppm while the new signal was observed at 12.26 ppm which be attributed to the binding
3
+
mode of Al and the oxygen of -CO-NH. the signal for the -CH=N proton at 8.73 ppm was
1
shifted to low-field compared to that in the HNMR spectrum of the parent ligand in the
3
+
presence of Al Furthermore, the new signals for the aryl protons also appeared compared to
1
those in the spectrum of the free receptor. Therefore, the large differences of the H NMR
3
+
spectra of HL in the absence and presence of Al , couple with ESI-MS and Job’s plot

3
+
analysis, suggested that HL could chelate Al through interactions with carboxylate oxygen,
imine nitrogen and oxygen of phenolic hydroxyl group and forms 1:1 complex[52-53].
4
. Conclusion
In summary, we have successfully designed a novel bis-Schiff base chemosensor based
on a donor-acceptor system by introducing ester group. The receptor exhibited high
3
+
selectivity for Al over other metal ions in near 100% aqueous solution (Fig.S7). More
importantly, because of lower detection limit in aqueous solution, the chemosensor may find
3
+
potential applications to detect micromolar concentrations of Al in both biological systems
and the environment.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (81171337),
Gansu NSF (1308RJZA115).
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Figure captions
Scheme1. Reagents and conditions: (a) HMTA, TFA, reflux, 4d; (b) EtOH, reflux, 10-12 h.
3
+
Scheme2. Proposed mechanism for detection of Al by HL.
Fig.1. Absorption spectra of HL (10.0 μM) in near 100% aqueous solution. at room
temperature as a function of added Al(NO
3
3
) (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3, 3.5, 4.0, 4.5, 5.0
equiv.).
Fig.2. Fluorescence spectra of HL (10 μM) upon the addition of metal salts (5.0 equiv.) of
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
3+
Li , Na , K , Ca , Mg , Cu , Co , Mn , Ni , Zn , Ba , Fe , Cd , Hg , Pb , Fe ,
3
+
3+
Cr , and Al in near 100% aqueous solution.. Excitation wavelength was 389 nm.

Fig.3. Fluorescence spectra of HL (10 μM) in near 100% aqueous solution. upon the addition
of Al(NO (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3, 3.5, 4.0, 4.5, 5.0 equiv. ) with an excitation of 389
nm. Inset: color of HL and HL+Al system under UV lamp.
)
3 3
3
+
3
+
Fig.4. Fluorescence intensity of HL and its complexation with Al in the presence of various
metal ions in near 100% aqueous solution.. Black bar: HL (10.0 μM) and HL with 1.0 equiv.
+
2+
2+
2+
2+
+
2+
3+
2+
2+
+
2+
2+
2+
2+
3+
of Li Mg , Mn , Ca , Ni , K , Co , Cr , Ba , Pb , Na , Cd , Hg , Cu , Fe , Fe ,
2
+
3+
Zn , stated. Red bar: 10.0 μM of HL and 1.0 equiv. of Al with 1.0 equiv. of metal ions
stated (λex =389 nm, λem =460 nm).
3
+
Fig.5. Job’s plot for determining the stoichiometry of HL and Al in near 100% aqueous
3
+
3+
3+
solution, (XAl=[Al ]/([Al ] + [HL]), the total concentration of HL and Al was 50 μM).
1
.
3+
Fig.6. H NMR spectra of HL with Al(NO
3
)
3
9H
2
O in DMSO-d : (I) HL; (II) HL and Al
6
Inset: the expansion of chemical shift at11~14ppm

Fig.1
Fig. 2

Fig. 3
Fig. 4

Fig. 5
Fig. 6

Scheme 1
Scheme 2