A simple turn-on Schiff base fluorescence sensor for aluminum ion

Article abstract of DOI:10.1016/j.tetlet.2016.06.112

A simple Schiff base L derived from 2-hydroxy-naphthalene-1-carbaldehyde and benzene-1,2-diamine, was proved to be a turn-on fluorescent probe for the recognition of Al³⁺, based on photoinduced electron transfer (PET) mechanism. It exhibited a

Full text of DOI:10.1016/j.tetlet.2016.06.112

Accepted Manuscript
A Simple Turn-On Schiff base Fluorescence Sensor for Aluminum Ion
Jinli Zhu, Yuhuan Zhang, Lun Wang, Tongming Sun, Miao Wang, Yipu Wang,
Danyang Ma, Qingqing Yang, Yanfeng Tang
PII:
S0040-4039(16)30792-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.06.112
TETL 47839
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
31 May 2016
22 June 2016
24 June 2016
Please cite this article as: Zhu, J., Zhang, Y., Wang, L., Sun, T., Wang, M., Wang, Y., Ma, D., Yang, Q., Tang, Y.,
A Simple Turn-On Schiff base Fluorescence Sensor for Aluminum Ion, Tetrahedron Letters (2016), doi: http://
dx.doi.org/10.1016/j.tetlet.2016.06.112
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A Simple Turn-On Schiff base Fluorescence Sensor for Aluminum Ion
a
a
a
a
Jinli Zhu , Yuhuan Zhangc, Lun Wangb, Tongming Sun , Miao Wang , Yipu Wang ,
Danyang Ma a, Qingqing Yang a and YanfengTang a,
a. School of Chemistry and Chemical Engineering, Nantong University, Nantong 226019, P.R.
China
b. Inspection and Quarantine Center of Shandong Exit & Quarantine Bureau, Qingdao,
266001, P.R. China
c.School of Textiles, Nantong University, Nantong 226019, P.R. China
Abstract: A simple Schiff base L derived from 2-hydroxy-naphthalene-1-carbaldehyde and
benzene-1,2-diamine, was proved to be a turn-on fluorescent probe for the recognition of
Al3+, based on photoinduced electron transfer (PET) mechanism. It exhibited a high
selectivity for Al3+ over other competing ions (e.g., Hg2+, Ag+, Pb2+, Cu2+, Ba2+, Cd2+, Zn2+,
Ni2+, Co2+, Fe2+, Fe3+, Mn2+, Li+, Cr3+, Ca2+, Mg2+, K+ , Na+) in EtOH/HEPES buffer (95:5, v/v,
pH = 7.2). The complex formation of L-Al3+ was determined to be 1:1 for L and Al3+ in molar.
1
This result was based on the Job plot, H NMR titration and ESI-mass. The binding constant
of the complex was 6.53 ×103 M-1 with a detection limit of 1.08×10-7 M. The potential
applications of L to detect Al3+ in live cells and in environmental water samples were also
investigated. The results indicated that L could be a promising probe for Al3+ recognition.
Keywords: Al3+ detection, PET, Schiff base, turn-on fluorescence, live cells
Introduction
Aluminum is the third most abundant metal in the earth’s crust. It is extensively useful in
many fields, from the national defense to our daily lives. These fields include aerospace
industry, automobiles, computers, packing materials, electrical equipment, machinery, food
additives, building construction, clinical drugs and water purification [1,2]. Aluminum also
[3]
plays a great role in biochemical reactions, such as enzyme-catalyzed reactions
,
biotechnological transformation [4], and others. However, nowadays, a large amount of
medical research reveals that excessive absorption of aluminum is harmful to human health. It
[7]
sometimes leads to diseases, such as Alzheimer’s disease [5-6], Parkinson’s disease
,
[8]
osteoporosis
and osteomalacia [9]. Therefore, it is of great importance to develop the
efficient methods for the detection of aluminum, especially in drinking water.
So far, several methods are available for the detection of aluminum, including atomic
absorption spectrometry (AAS) [10], atomic emission spectrometry (AES) [11], voltammetry [12]
 Corresponding author
1

and electrochemical methods [13-15]. However, most of them require expensive instruments and
are time-consuming [16-19]. Compared to these methods, fluorescent chemical sensors have
attracted enormous attention due to their faster-response and lower-cost properties [20-23]. A
few of fluorescent sensors for detecting Al3+ have been developed with good selectivity and
sensitivity [24-28]. However, some of them require complicated synthetic protocols and others
are not sensitive enough to detect Al3+ [29-32]. Thus, for practical applications, it is of great
significance to design readily synthesized Al3+ sensors with simple structures and high
selectivity and sensitivity [33-34]
.
Sun et al synthesized a symmetric Schiff base for Al3+ from benzene-1,2-diamine and
2-hydroxy-1-naphthaldehyde [35]. This sensor showed stable complexation with Al3+ in DMF,
but it also gave an obvious turn-on signal in the presence of Mg2+. Herein, to avoid the
disturbance
of
Mg2+,
we
prepared
an
asymmetric
Schiff
base
L,
(1-[(2-Amino-phenylimino)-methyl]-naphthalen-2-ol), from the above mentioned two
compounds, as shown in Scheme 1. L showed better selectivity to Al3+ without disturbance of
Mg2+ and other metals. Moreover, L has the potential use of detecting Al3+ in living cells and
real water samples.
Scheme 1. The synthesis of Schiff base L.
Results and discussion
The fluorescence responses of receptor L on various metal ions were investigated in
EtOH/HEPES buffer (95:5, v/v, pH 7.2) (Fig. 1). Upon excited at 273 nm, L (10 μM) showed
barely-detectable emissions at 473 nm and 499 nm. And there were no obvious changes in the
spectra after adding other metal ions to the L solution, including Hg2+, Ag+, Pb2+, Cu2+, Ba2+,
Cd2+, Zn2+, Ni2+, Co2+, Fe2+, Fe3+, Mn2+, Li+, Cr3+, Ca2+, Mg2+, K+ and Na+ (100 μM（) Fig. S1）.
However, the addition of Al3+ (100 μM) led to significant fluorescence enhancement (63 fold)
at 473 nm and 499 nm, which indicated that an Al3+-selective “off-on” fluorescent signaling
behavior had occurred. Among these metal ions and L solutions excited by UV at 273 nm,
only the solution mixed with both Al3+ and L (Al3+-L) showed a dramatic color change from
green to fluorescent blue. This change could be easily detected by the naked-eye (Fig. S2).
The observed fluorescence enhancement was attributed to blocking the photoinduced electron
transfer (PET) process when L is bound with Al3+ (Scheme 2) [35-36]
.
2

Fig. 1. Fluorescent selectivity of L (10 μM) upon the addition of various mental ions (100 μM) in
EtOH/HEPES buffer (95:5, v/v, pH 7.2) at 473 nm and 499nm. λex = 273 nm.
Scheme 2. Proposed binding mechanism between the receptor L and Al3+.
The selectivity toward Al3+ was further ascertained by the competitive experiment. As
shown in Fig. 2, when the Al3+-L solution was mixed with one of following metal ions,
including Ag+, Cd2+, Co2+, Li+, Mn2+, Na+, Pb2+, Zn2+, K+ , Ca2+ and Mg2+, the emission
intensities were almost identical to that of Al3+-L solution. In the case of other metal ions,
including Ba2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, Ni2+, the emission intensities diminished to a
different extent, like those obtained in the presence of Al3+ alone, and they still had a
sufficient turn-on ratio for the detection of Al3+. Therefore, L displayed a promising selective
fluorescent sensor for Al3+ in the presence of competing mental ions.
Fig. 2. Fluorescent selectivity of L (10 μM) at 473nm upon addition of various metal ions ((100
μM) Al3+, Ag+, Cd2+, Co2+, Li+, Mn2+, Na+, Pb2+, Zn2+, K+ , Ca2+, Mg2+ , Ba2+, Cr3+, Cu2+, Fe2+,
Fe3+, Hg2+, Ni2+ ) in EtOH/HEPES buffer (95:5, v/v, pH 7.2), λex = 273nm.
3

To further investigate the chemosensing properties of L, a fluorescence titration of L was
done by increasing the concentration of Al3+. As calculated from the fluorescence titration
profile shown in Fig. 3A, the fluorescence quantum yield of L changed from 0.007 to 0.053
after the addition of 20 equivalent of [Al3+] (i.e., 200 μM). Benesi–Hildebrand analysis of the
titration profiles based on 1:1 binding model resulted in a linear relationship of intensity to
concentration (Fig. 3C). This indicated a 1:1 binding stoichiometry of Al3+ and L, and the
binding constant was estimated to be 6.53 ×103 M-1. The 1:1 binding ratio was also supported
by Job’s plot investigation (Fig. 4). In addition, the formation of 1:1 complex between
receptor L and Al3+ was further confirmed by the appearance of a peak at m/z 350.22,
-
assignable to [L+ Al3+ + NO3 ] in the ESI/MS (Fig. 5). By using the above-mentioned
fluorescence titration results, the detection limit for Al3+ was determined to be 1.08×10-7 M
(Fig. 3D). The detection limit was sufficiently low to detect a submicromolar concentration of
Al3+.
Fig. 3. (A) Fluorescence emission spectra of L (10 μM) upon the addition of Al3+ with different
[Al3+] in EtOH/HEPES buffer (95:5, v/v, pH = 7.2), λex = 273nm. (B) A plot of intensity changes
versus the concentrations of Al3+ added. (C) Calculation of detection limit of L for Al3+. The
spectra were recorded in EtOH/HEPES buffer (95:5, v/v, pH 7.2) with excitation at 273 nm and
emission at 499 nm. FL Intensity is the fluorescence intensity of sample L solution (10 μM) upon
addition of different amounts of Al3+. (D) Calculation of binding constant between L and Al3+.
The spectra were recorded in EtOH/HEPES buffer (95:5, v/v, pH = 7.2) with excitation at 273 nm
and emission at 499 nm. I0 is the fluorescence intensity of sample L solution (10 μM) upon
addition of different amounts of Al3+; I is the fluorescence intensity of free L solution.
4

Fig. 4. The Job's plot of fluorescence intensity response of L at 500 nm to the mole fraction of
Al3+ in EtOH/HEPES buffer (95:5, v/v, pH = 7.2), λex=273nm.
-
Fig. 5. ESI Mass spectrum of L-Al3+-NO3 complex.
1
To better understand the complexation of the receptor L with Al3+, H NMR was carried
out. Fig. 6A showed 1H NMR spectrum of L receptor. The peaks at 15.66 and 5.11 ppm were
1
assigned to –OH (Ha) and NH2 (Hc) of L, respectively. Fig. 6B and Fig. 6C showed the H
NMR spectra of the L receptor upon the addition of Al3+ with 1.0 and 2.0 equiv
concentrations, respectively. It was clearly found that the peaks of Ha at 15.66 ppm and Hc at
5.11 ppm disappeared, demonstrating that –OH (Ha) and –NH2 (Hc) were involved in the
complexation of Al3+ and L. The peak of –CH=N– (Hb) was shifted to downfield from 9.61
ppm to 9.96 ppm after complexation. Similarly, the aromatic proton peaks including
naphthalenyl and phenyl rings also underwent a downfield shift from a range of 6.70-8.51
ppm to that of 6.96-8.71 ppm, which further proved the complexation formation of L with
Al3+. Furthermore, there were no obvious differences between the spectra shown in Fig. 6B
and Fig. 6C corresponding to 1:1 of Al3+/L and 2:1 of Al3+/L in molar, respectively. It
revealed that the combination of L and Al3+ was 1:1, which was consistent with the Job’s plot
result.
5

Fig. 6. 1H NMR spectra in DMSO-d6 of (A) L only, (B) L with 1.0 equiv of Al3+, and (C) L with
2.0 equiv of Al3+.
For a practical application of L under different environment and physiological conditions,
it is necessary to verify the pH-stability of L-Al3+. As expected, no obvious fluorescence
emission of L was observed under different pH values. The fluorescence of L-Al3+ complex
was stable from pH 2 to 10, which demonstrated that L and L-Al3+ complex are steady and
work well over this wide pH range (Fig. 7).
Fig. 7. The fluorescence intensity response of L (10 μM) and L with 10 eq of Al3+ at emission of
473 nm to different pH values in Ethanol-H2O (95:5, V/V), λex=273 nm.
Before studying the use of L as a bio-imaging probe, the cytotoxicity of L was assessed.
The HeLa cells were incubated with different concentrations of L (from 20 up to 100 μg/mL)
for 24 h. Subsequently, MTT assay was carried out [37-40]. Fig. 8A showed that over 90% of
cells were alive, even when treated at a high concentration of L at 100 μg/mL. This revealed
that L had very low cytotoxicity and is a good potential for an intracellular imaging probe.
6

Bio-imaging experiments were then conducted to prove the ability of L to detect Al3+ in the
Hela cells. As shown in Fig. 8B, only a very weak green luminescence signal was detected
inside the Hela cells when the cells treated with L without Al3+. However, after the Hela cells
pretreated with L were incubated with Al3+ (0.2 μM, for 15 min), a much brighter green
luminescence was observed inside the cells (Fig. 8C). These results demonstrated that L was
capable of imaging Al3+ in the live cells.
Fig. 8. (A) Cell viability of HeLa cells treated with different concentrations of L. Confocal
fluorescence micrograph of L loaded HeLa cells (B) without and (C) with Al3+.
The potential application of L in the detection of Al3+ in water samples was also
investigated, following the protocol published previously [41]. Three water samples were used
in the experiments including distilled water, lake and river water, which were collected from
our lab, Tongda Lake in Nantong University and Haohe River in Nantong, respectively. Al3+
(0-100 μM) was dissolved in these water samples following the addition of L (1 μM). As
shown in Fig. 9, the fluorescence intensities of all these samples displayed good linear
properties with the concentrations of Al3+. Particularly, the intensities of both the lake and
river samples were only slightly lower than that of distilled water samples. The interesting
result showed that L can be applied for the detection of Al3+ in environmental water systems.
7

Fig. 9. Linear fluorescence intensities of L (10 μM) upon addition of Al3+ (0-100 μM) in the three
natural water samples.
Conclusions
In summary, we synthesized a very simple fluorescence sensor L, which was based on the
blocking PET process from the nitrogen donor to the 2-hydroxy-1-naphthaldehyde
fluorophore. L exhibited high selectivity for Al3+ over other metal ions with 63-fold
fluorescence enhancement, and high sensitivity with the detection limit at 10-7 M level in
EtOH/HEPES buffer (95:5, v/v, pH = 7.2). The predicted configuration of the L-Al3+complex
1
was well-characterized to be 1:1 by the Job plot, H NMR titration and ESI-mass. Moreover,
L showed a good linear property in detecting Al3+ in real water samples. Beyond that, L was
employed to detect the Al3+ in live cells sensitively by emitting visible fluorescence. These
results indicated that L could be served as an excellent fluorescence chemosensor for Al3+ in
water and cells.
Acknowledgements
This work was supported by National Natural Science Foundation of China (No.
21376124, 21476117) and Qing Lan Project 2014.
Supporting information
Detailed the experimental methods and characterizations can be found in the supporting
information.
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Highlights
1. A simple Schiff-based fluorescence sensor for Al3+, was synthesized.
2. This sensor was of good selectivity and sensitivity for Al3+.
3. This sensor can be used in real water and live cells.
11

12