A turn-on Schiff-base fluorescence sensor for Fe3 + ion

Article abstract of DOI:10.1016/j.inoche.2013.10.004

We synthesized a novel Schiff-based fluorescent receptor 2, and investigated its binding properties toward various metal ions. The receptor 2 exhibited a blue shift in emission band upon binding with Fe^{3 +}. The receptor 2 exhibited a high assoc

Full text of DOI:10.1016/j.inoche.2013.10.004

Inorganic Chemistry Communications 38 (2013) 74–77
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A turn-on Schiff-base fluorescence sensor for Fe3+ ion
a
a
b
a,
Chang-Hung Chen , Pei-Ju Hung , Chin-Feng Wan , An-Tai Wu ⁎
a
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
School of Applied Chemistry, Chung Shan Medical University, Taichung City 40201, Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
We synthesized a novel Schiff-based fluorescent receptor 2, and investigated its binding properties toward
Received 13 June 2013
Accepted 8 October 2013
Available online 16 October 2013
3+
various metal ions. The receptor 2 exhibited a blue shift in emission band upon binding with Fe . The receptor
3+
2
exhibited a high association constant with submicromolar detection for Fe in MeOH solution.
Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
Keywords:
Chemosensor
Fluorescence
Turn-on
Schiff-base
In recent years, fluorescent chemosensors have attracted significant
interest because of their potential application in medicinal and
environmental research. Thus, many fluorescent chemosensors specific
Co2+, Ni2+, Cu2+, Pb2+, Cd2+, Zn2+ and Al3+ in MeOH, was
investigated by UV–vis and fluorescence measurements. Fig. S4 shows
significant variations of the absorption spectrum of receptor 2 in the
200–600 nm range for all the added cations. From the fluorescence
spectra of receptor 2 (Fig. 1), receptor 2 alone and other cations all
displayed very weak single fluorescence emission band at 560 nm
for Hg2+, Cu , Zn or other transition metals have been developed
2+
2+
[1–14]. Compared to these transition metal ions, only a few fluorescent
chemosensors have been reported for detection of Fe ion [15–21]. In
addition, fluorescent chemosensors that show enhanced fluorescence
in the presence of Fe ions are even scarcer due to paramagnetism of
3
+
when it was excited at 361 nm except for Fe . Upon addition of
3
+
Fe , receptor 2 exhibited a prominent fluorescence enhancement
with the quantum yield of 0.34% and accompanied by a blue shift of
120 nm from 560 to 440 nm. The fluorescent enhancement efficiency
observed at 440 nm was 222 fold greater than that of the control in
the absence of Fe3 (Fig. S5). The observed fluorescent enhancement
may be attributed to the formation of a rigid system after binding
2
+
3+
Fe
and Fe
causing quenching of the fluorescence [22]. Fe ion is
the most abundant transition-metal ion in humans and other mammals,
and it plays important roles in various biological systems [23]. Fe ion
deficiency leads to anemia, liver and kidney damages, diabetes, and
heart diseases [24]. Therefore, detection of Fe ion is crucial in controlling
its concentration levels in the biosphere and its direct impact on human
health. In addition; the majority of the reported Fe ion has limitations
such as requiring complicated synthesis and is insoluble in polar
solutions. For practical applications, it is necessary to develop Fe ion
sensors that are easily prepared, and possess selective and sensitive
signaling mechanisms.
+
3
+
with Fe
.
Generally, Fe3 usually exists in water solutions; therefore, we also
+
try to confirm the effect of water on the detection. The chemosensor
behavior of receptor 2 with cations was performed in MeOH-H O
2
(1:9 v/v). From the fluorescence spectra of receptor 2 (Fig. S6), not
only Fe3 , other cations all displayed significant fluorescence emission
band at 370 nm. The result indicated that the content of the water
would affect the selectivity of receptor 2 toward cations.
+
Herein, we reported a highly sensitive fluorescence turn-on receptor
+
2
to Fe3 . More importantly, receptor 2 can be readily prepared by two
simple steps, as shown in Scheme 1. The structure of receptor 2 was
confirmed by NMR spectra and Mass data (Figs. S1–S3).
To further investigate the chemosensing properties of receptor 2,
UV/vis and fluorescence titration of receptor 2 with Fe3+ were
performed. As shown in Fig. S7, a Job plot indicated a 1:1 stoichiometric
The chemosensor behavior of receptor 2 with the following 15 metal
+
+
+
2+
2+
2+
2+
3+
3+
ions (as perchlorate salts): Li , Na , K , Ca , Mn , Hg , Fe , Fe
,
complexation of receptor 2 with Fe . In addition, the formation of 1:1
complex between 2 and Fe3 was further confirmed by the appearance
+
3
+
+
of a peak at m/z 308, assignable to [receptor 2+Fe +Na +MeOH] in
the ESI/MS (Fig. 2). From the fluorescence titration profiles (Fig. 3), the
⁎
Corresponding author.
3
+
association constant for 2-Fe
in MeOH was determined as
E-mail addresses: jeanwan02@yahoo.com.tw (C.-F. Wan), antai@cc.ncue.edu.tw
3.93 × 108
M
−1
by a Hill plot (Fig. S8). By using above-mentioned
(A.-T. Wu).
1387-7003/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.10.004

C.-H. Chen et al. / Inorganic Chemistry Communications 38 (2013) 74–77
75
Scheme 1. Synthesis of receptor 2.
3
+
Fe
To check the pH effect, the fluorescence of receptor 2 under the
different pH conditions was examined. It was found that fluorescence
intensity of receptor 2 remained unaffected over a wide pH span of 2–
1
1
60
20
+
+
+
2+
2+
host, Li , Na , K , Mg , Mn
,
10 (Fig. S9).
2+
2+
2+
2+
2+
Fe , Co , Ni , Zn , Ca
,
2+
2+
2+
The selectivity toward Fe3+ was further ascertained by the
Cd , Hg , Pb
competition experiment. As shown in Fig. S10, receptor 2 was treated
with 4.0 equiv of Fe3 in the presence of other metal ions of the same
concentration. Relatively low interference was observed for the
+
8
4
0
0
0
3
+
Al
3+
2+
detection of Fe
Ni , Cu
in the presence of other metal ions except Mn
.
,
2+
2+
and Hg2+
2
+
Cu
Reversibility is a prerequisite in developing novel chemosensors for
practical application. The reversibility of the recognition process of
3
+
400
500
600
700
800
receptor 2 was performed by adding a Fe
2
bonding agent, Na EDTA
(Fig. 4). The binding constant is 1.30 × 1025 between EDTA and Fe
however, the binding constant is 3.93 × 10 between receptor 2 and
EDTA to a mixture of receptor 2
resulted in diminution of the fluorescence intensity at
40 nm. However, its degradation in fluorescence intensity cannot be
3+
;
Wavelength (nm)
8
3
+
Fig. 1. Fluorescence emission spectra (λex. = 361 nm) of receptor 2 (80 μM) in the
presence of 4.0 equiv of various metal ions in MeOH.
Fe . Therefore, the addition of Na
2
3+
and Fe
4
fluorescence titration results, the detection limit for Fe3+ was
restored to the original free receptor 2, indicating a new species was
obtained.
−
10
determined as 8.5 × 10
M. The detection limit was sufficiently low
to detect submicromolar concentration of the Fe3 , which belongs to
+
To better understand the complexation behavior of receptor 2 with
1
the range found in many chemical and biological systems.
Fe3+
,
H NMR titration experiments were carried out in CD
3
OD. The
D:\Exp_data\...\66-Hpyp-Fe3+
3/22/2013 2:07:46 PM
6
6-Hpyp-Fe3+ #1-30 RT: 0.00-0.09 AV: 30 NL: 6.17E5
T: ITMS + c ESI Full ms [150.00-2000.00]
3
08.1
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
3
09.1
1
59.1
74.8 201.0
359.0
1
2
83.7
2
17.0
468.8
3
11.4
1
82.8
247.6
45.1
294.5
343.0
390.8
360.0
396.8
2
2
61.1
446.6
450
540.6
472.5
6
13.4
5
14.7 536.6 541.7 570.7
624.2
650
674.9 695.2
700
1
50
200
250
300
350
400
500
550
600
m/z
Fig. 2. ESI mass spectrum for [receptor 2-Fe3+ + MeOH + Na+] complex.

76
C.-H. Chen et al. / Inorganic Chemistry Communications 38 (2013) 74–77
1
1
20
00
140
1
1
20
00
4.5eq
8
6
4
2
0
0
0
0
0
8
6
4
2
0
0
0
0
0
0eq
0
1
2
3
4
5
3
+
[Fe ]/[sensor]
400
500
600
700
800
Wavelength (nm)
Fig. 3. Fluorescence spectra of receptor 2 (80 μM) in MeOH upon addition of increasing
concentrations Fe ; Inset is a plot of intensity change vs equiv. of Fe3+ added.
3+
spectral differences are depicted in Fig. 5. The imine proton of receptor 2
at around 8.79 ppm was disappeared upon the addition of Fe3 . In
addition, the protons of benzene were all became more symmetrical.
Therefore, we proposed a possible mechanism that Fe3 induced
the cyclization of receptor 2, and at the same time a new species,
+
+
2
-pyridine benzoxazole, with high fluorescence quantum yield was
obtained, as shown in Scheme 2. In addition, we also performed the IR
experiment. The IR spectrum of the receptor 2 was compared with the
3
+
spectrum of Fe
complex. The typical IR spectra were shown in
Fig. S11. The IR spectrum of the receptor 2 indicates a sharp peak at
−
1
1
647 cm , which is assigned to C_N stretching of imine group. The
3
+
−1
Fig. 5. 1H NMR titration plot of receptor 2 with Fe3+ in CD
3
OD.
IR spectrum of Fe
complex exhibits a broad band at 1655 cm . In
addition, the proposed cyclization product is an aryl ether derivative
−
1
that displays an asymmetrical C\O\C stretching band at 1200 cm
−
1
with symmetrical stretching band near 1100 cm . The result showed
3
+
7
9
6
1
4
that the possible of Fe
induced the cyclization of receptor 2. In
HO
N
O
N
2
order to confirm the value state of metal center, the EPR experiment
was also performed. The result indicated that the value state of metal
center was paramagnetic property as shown in Fig. S12.
8
Fe3+
5
N
N
3
Fe
MeOH n
Strong fluorescence
In summary, we prepared a simple receptor 2 for the detection of
selected metal ions. The receptor 2 displayed dramatic enhanced
fluorescence intensity selectively for Fe3 over other ions in MeOH
solution. More importantly, the detection limit was sufficiently low to
2
+
Weak fluorescence
Scheme 2. Proposed binding mode of receptor 2 with Fe3+
.
100
Host
Host+Fe3
detect submicromolar concentration of the Fe3+. Thus, we trust
receptor 2 has an ability to serve as a practical sensor for Fe detection
in biological system and environment.
3
+
3+
+
Host+Fe +EDTA
7
5
2
5
0
5
0
Acknowledgment
We thank the National Science Council of Taiwan for financial
support.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2013.10.004.
400
450
500
550
600
650
700
Wavelength (nm)
References
Fig. 4. Fluorescence emission spectra of receptor 2 in the presence of Fe3+ (4.0 equiv) or
Na EDTA (4.0 equiv) in MeOH solution. λex = 361 nm.
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