A new fluorescence "turn-on" type chemosensor for Fe3+ based on naphthalimide and coumarin

Article abstract of DOI:10.1016/j.dyepig.2013.12.032

A new Fe³⁺-selective "turn-on" chemosensor 1 based on coumarin and naphthalimide was designed and synthesized. The chemosensor exhibits high selectivity for Fe³⁺ over other ions (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Cd ²⁺, Pb²⁺, Ca²⁺, Al³⁺, Ba ²⁺, Hg²⁺, Li⁺, Mg²⁺, Sr ²⁺, Zn²⁺ and K⁺) with fluorescence-enhancement in THF-H₂O solution. The binding ratio of 1-Fe³⁺ complex was determined to be 1:1 according to the Job plot. The association constant K_a of Fe³⁺ binding with sensor 1 was (2.589 ± 0.206) × 10³ M^-1. The detection limit was calculated to be 0.388 μM.

Full text of DOI:10.1016/j.dyepig.2013.12.032

Dyes and Pigments 105 (2014) 7e11
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A new fluorescence “turn-on” type chemosensor for Fe3þ based on
naphthalimide and coumarin
*
Zhengqian Li, Ying Zhou, Kai Yin, Zhu Yu, Yan Li, Jun Ren
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials e College of Chemistry and Chemical Engineering, Hubei University,
Wuhan 430062, PR China
a r t i c l e i n f o
a b s t r a c t
A new Fe3þ-selective “turn-on” chemosensor 1 based on coumarin and naphthalimide was designed and
Article history:
synthesized. The chemosensor exhibits high selectivity for Fe3þ over other ions (Mn2þ, Co2þ, Ni2þ, Cu2þ
,
Received 8 October 2013
Received in revised form
25 December 2013
Accepted 29 December 2013
Available online 8 January 2014
Agþ, Cd2þ, Pb2þ, Ca2þ, Al3þ, Ba2þ, Hg2þ, Liþ, Mg2þ, Sr2þ, Zn2þ and Kþ) with fluorescence-enhancement in
THF-H2O solution. The binding ratio of 1-Fe3þ complex was determined to be 1:1 according to the Job
plot. The association constant Ka of Fe3þ binding with sensor 1 was (2.589 ꢀ 0.206) ꢁ 103
M
ꢂ1. The
detection limit was calculated to be 0.388 mM.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Fluorescent sensor
Turn-on
Chemosensor
Ferric ion
Naphthalimide
Coumarin
1. Introduction
Herein, in this paper we report a new fluorescence “turn-on”
type sensor 1 for Fe3þ based on Schiff base as bridge, which was
synthesized by the condensation of 7-diethylaminocoumarin-3-
As one of the most essential trace elements in biological sys-
tems, ferric ion performs a major role in the growth and develop-
ment of living systems as well as in many biochemical processes at
the cellular level [1,2]. An overload of Fe3þ leads to some
dysfunction of certain organs, such as heart, pancreas, and liver [3e
5]. While, the deficiency of Fe3þ causes anemia, hemochromatosis,
liver damage, diabetes, and Parkinson’s disease [6]. The traditional
techniques for determination of Fe3þ, such as FAAS [7], CE [8], and
ICP-AES [9], are complicated and not suitable for quick and online
monitoring. Therefore, great importance is attached to developing
aldehyde
2
and N-aminonaphthalimide
5
in high yield
(Scheme 1). Coumarin and naphthalimide are commercial material
and they exhibit outstanding chemical, thermal and photochemical
stabilities as well as their fluorescence quantum yield [19e23]. On
the other hand, oxygen heteroatoms of C]O group on coumarin
and naphthalimide could act as chelating sites. Schiff base group
was introduced because C]N bond was easily isomerized in the
excited state and exhibited weak fluorescence. But when they
combined with metal ions and formed metal complexes, the C]N
isomerization was inhibited which leaded to fluorescence changes.
The sensor 1 exhibits high selectivity for Fe3þ over other ions with
fluorescence-enhancement in THF-H2O solution.
analytical methods for the convenient and rapid analysis of Fe3þ
.
Fluorescent detection has become the promising strategy used
for Fe3þ detection [4,5] due to its easy monitoring, low cost and
high selectivity. In recent years, considerable efforts have been
devoted to the development of fluorescent chemosensors for ferric
[10e14]. Generally, it is believed that chemosensors with fluores-
cence enhancement are much more effective when combining or
reacting with the analytes. However, the examples of Fe3þ-selective
“turn-on” chemosensors are still scarce because of the fluorescence
quenching of the paramagnetic nature of Fe3þ [15e18].
2. Materials and methods
2.1. Experimental
All reagents were purchased from commercial suppliers and
were used without further purification except POCl3 and DMF
which were purified according to standard procedures and freshly
distilled prior to use. Double distilled water was used throughout
the experiment. The solutions of metal ions were prepared from
* Corresponding author. Tel.: þ86 27 88662747; fax: þ86 27 88663043.
E-mail addresses: renjun@hubu.edu.cn, rj77lll@hotmail.com (J. Ren).
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.12.032

8
Z. Li et al. / Dyes and Pigments 105 (2014) 7e11
N
POCl3
1) CH2(COOC2H5)2
2) HCl/AcOH
N
OHC
HO
N
OHC
DMF
O
O
O
O
4
3
2
NH2
O
N
O
O
O
O
CHCl3
Reflux 4 h
+ NH2NH2.H2O
6
5
O
N
+
2
5
N N C
H
O
O
O
Scheme 1. The synthetic route of compound 1.
their chloride salts and nitrate salts. Fluorescence detections were
performed on Perkin-Elmer LS 50B fluorescence spectrophotom-
eter. Absorbance detections were carried out on a Shimadzu UV-
1700 spectrophotometer. The 1H NMR spectroscopy study was
conducted with a Varian INOVA-400 spectrometer using tetrame-
ice-water. NaOH solution (20%) was added to adjust the pH of the
mixture to yield large amount of precipitate. The crude product was
filtered, washed with water and recrystallized from absolute ethanol
to give 2 (1.46 g, 5.95 mmol) in 71.8% yield. 1H NMR (400 MHz,
CDCl3):
d (ppm) 10.13 (s,1H, eCHO), 8.27 (s, 1H, eCH]), 7.42 (d,
thylsilane (TMS;
were carried out at room temperature (w298 K).
d
¼ 0 ppm) as internal standard. All measurements
J ¼ 9.0 Hz,1H), 6.65 (dd, J ¼ 9.0 Hz, 2.4 Hz,1H), 6.49 (d, J ¼ 2.4 Hz,1H),
3.48 (q, J ¼ 7.2 Hz, 4H, eCH2CH3), 1.26 (t, J ¼ 7.2 Hz, 6H, eCH2CH3).
2.2. Synthesis
2.2.4. Synthesis of compound 1
Compound 2 (1.16 g, 4.73 mmol) was dissolved in 50 mL of hot
ethanol, then compound 5 (1.0 g, 4.71 mmol) was added to the
solution. The mixture was stirred at room temperature for 30 min,
and the gray solid was precipitated. After filtration, the solid was
collected, washed successively with ethanol and dried in vacuum to
give compound 1 in 91.5% yield (1.89 g). 1H NMR (400 MHz, CDCl3):
2.2.1. Synthesis of N-aminonaphthalimide (5) [24]
5.0 g (25.2 mmol) of 1,8-naphthalic anhydride was dissolved in
160 mL chloroform and stirred at room temperature for 15 min.
Then, 3.16 g of hydrazine hydrate (51 mmol) was added to the above
solution. The reaction mixture was refluxed in a water bath for 4 h
monitoring with thin layer chromatography (TLC). After cooling to
room temperature, yellow solid was collected by filtration, washed
with chloroform (10 mL ꢁ 3) and dried in vacuum (4.79 g, 89.5%
d
(ppm) 8.76 (s, eN]CH, 1H), 8.74 (s, 1H), 8.67 (d, J ¼ 7.2 Hz, 2H),
8.26 (d, J ¼ 7.2 Hz, 2H), 7.79 (t, J ¼ 7.2 Hz, 2H), 7.42 (d, J ¼ 8.8 Hz,1H),
6.63 (dd, J ¼ 8.8 Hz, 2.4 Hz, 1H), 6.50 (s,1H), 3.48 (q, J ¼ 8.0 Hz, 4H),
yield). 1H NMR (400 MHz, DMSO-d6):
8.45 (d, 2H, J ¼ 8.0 Hz), 7.86 (t, 2H, J ¼ 8.0 Hz), 5.79 (s, 2H, eNH2).
d
(ppm) 8.49 (d, 2H, J ¼ 8.0 Hz),
1.26 (t, J ¼ 8.0 Hz, 6H). 13C NMR (100 MHz, CDCl3):
d (ppm) 188.1,
166.4, 161.4, 158.3, 152.6, 145.5, 143.0, 134.5, 132.6, 132.0, 131.7,
131.5, 127.2, 122.8, 111.4, 110.3, 110.0, 108.7, 97.3, 45.3, 12.6. MS (ESI):
m/z ¼ 440.4 [M þ 1]þ. Anal. Calcd. for C26H21N3O4: C 71.06, H 4.82,
N 9.56; Found C 71.12, H 5.01, N 9.45.
2.2.2. Synthesis of 7-diethylaminocoumarin (3) [25]
To the solution of diethylmalonate (6.4 g, 40 mmol) in EtOH
(60 mL), 4-diethylaminosalicylaldehyde (3.86 g, 20 mmol) and
piperidine (2 mL) were added and refluxed for 6 h. After the solvent
was removed, glacial acetic acid (40 mL) and concentrated HCl
(40 mL) were added and stirred for another 6 h. The solution was
cooled to room temperature and poured into 200 mL ice-water. 40%
NaOH solution was added dropwisely to adjust pH of the solution to
about 5, and gray precipitate formed immediately. After stirring for
30 min, the mixture was filtered, washed with water and recrys-
tallized from toluene to give 3 (3.26 g, 75.2% yield). 1H NMR
2.3. Preparation of solutions for absorption and fluorescence
detections
All measurements of spectra were carried out in mixed aqueous
solution of THF-H2O (1/1, v/v). 0.1 mmol of each inorganic salt
(Al(NO3)3, ZnCl2, HgCl2, AgNO3, KCl, NiCl2$6H2O, PbCl2, CaCl2$2H2O,
BaCl2$2H2O,
MgCl2$6H2O,
CdCl2,
CoCl2$6H2O,
LiCl$H2O,
CuCl2$2H2O, MnCl2$4H2O, SrCl2$6H2O and FeCl3) was dissolved in
distilled water (8 mL) to afford 1.25 ꢁ 10ꢂ2 mol/L aqueous solution.
Further dilutions were made to prepare 5.96 ꢁ 10ꢂ4 mol/L solutions
for the experiments. The stock solution of compound 1 was pre-
pared by dissolving 26.2 mg compound 1 in 100 mL dry THF and
subsequently, 10.0 mL of the solution was then transferred to a
100 mL volumetric flask, diluted to the mark with dry THF. Ab-
sorption and fluorescence detections were made using a 5.0 mL
cuvette. For all measurements of fluorescence spectra, excitation
wavelength was 456 nm. The excitation and emission wavelength
bandpasses were both set at 5.0 nm.
(400 MHz, CDCl3)
d
(ppm) 7.55 (d, J ¼ 10.8 Hz, 1H, eCH]), 7.23 (d,
J ¼ 8.8 Hz, 1H), 6.59 (dd, J ¼ 8.8 Hz, 2.4 Hz, 1H), 6.51 (d, J ¼ 2.4 Hz,
1H), 6.06 (d, J ¼ 10.8 Hz, 1H, ]CH), 3.42 (q, J ¼ 7.2 Hz, 4H, e
CH2CH3), 1.21(t, J ¼ 7.2 Hz, 6H,eCH2CH3).
2.2.3. Synthesis of 7-diethylaminocoumarin-3-aldehyde (2) [25]
Anhydrous DMF (2.4 mL) was added dropwise to POCl3 (2.4 mL)
at 30 ꢃC under N2 and stirred for 30 min. To the above solution,
compound 3 (1.8 g, 8.29 mmol) in DMF (12 mL) was added. The
mixture was stirred at 60 ꢃC for 12 h and then poured into 120 mL

Z. Li et al. / Dyes and Pigments 105 (2014) 7e11
9
2.0
1.5
1.0
0.5
0.0
-0.5
30
30
25
20
15
10
5
8 equiv
Fe3+
25
20
15
10
5
0 equiv
0
2
4
6
8
Fe equiv.
Blank and other metal ions
0
300
350
400
450
500
550
480
510
540
570
600
Wavelength (nm)
Wavelength (nm)
Fig. 1. UVeVis absorption of 1 (5.96 ꢁ 10ꢂ5 M) in the presence of different metal ions
Fig. 3. Fluorescent spectra of 1 (5.96 ꢁ 10ꢂ5 M) with the addition of various con-
centration of Fe3þ in THF-H2O (1:1, v/v) (lex ¼ 456 nm). Inset: Changes in the emission
intensity at 504 nm.
(10 equiv. each) in THF-H2O solution (1:1, v/v).
28
25
26
24
22
20
18
16
14
12
10
8
20
15
10
5
25
20
15
10
5
Fe3+
0
1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
+
3+
2+
2+
+
Blank,Ag ,Al ,Ba ,Ca
,
2+ 2+ 2+ 2+
+
Cd ,Co ,Cu ,Hg ,K ,Li
,
6
2+
2+ 2+
2+
2+
2+
Mg ,Mn ,Ni ,Pb ,Sr ,Zn
4
2
0
480
500
520
540
560
580
600
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
[Fe3+]/([1]+[Fe3+])
Fig. 2. Fluorescence spectra of 1 (5.96 ꢁ 10ꢂ5 M) in the presence of 10 eq. different
metal ions in THF-H2O solution (1:1, v/v), lex ¼ 456 nm.
Fig. 4. Job plot for determining the stoichiometry of 1 and Fe3þ ion in THF-H2O so-
lution (1:1, v/v). lex ¼ 456 nm.
3. Results and discussion
The fluorescence spectrum of compound 1 exhibited weak fluo-
rescence emission when excited at 456 nm in THF-H2O solution
(1:1, v/v) (Fig. 2). Upon addition of Fe3þ ions to the solution of
compound 1, remarkable enhancement of emission intensity was
observed at 504 nm. Under the same conditions as used for Fe3þ, no
significant change of the fluorescence spectrum was observed in
the presence of other metal ions, indicating a Fe3þ-selective OFF-
ON fluorescent signaling behavior. The increase of emission in-
tensity may be attributed to the formation of the compound 1-Fe3þ
3.1. Spectral studies
The binding behavior of compound 1 was studied towards
different metal ions (Fe3þ, Mn2þ, Co2þ, Ni2þ, Cu2þ, Agþ, Cd2þ, Pb2þ
,
Ca2þ, Al3þ, Ba2þ, Hg2þ, Liþ, Mg2þ, Sr2þ, Zn2þ and Kþ) as their
chloride or nitrate salts by absorption and fluorescence spectros-
copy. There was no obvious change in UVeVis absorption spectra
of sensors 1 in the presence of any cation as shown in Fig. 1.
λex
456 nm
O
Fe3+
N
N
O
N
O
N
O
N C
N C
H
O
H
O
O
O
λem
504 nm
Fe3+
strong emission
Scheme 2. Possible binding mode of sensor 1 with Fe3þ
weak emission
.

10
Z. Li et al. / Dyes and Pigments 105 (2014) 7e11
1+Fe3+
1+Fe3++ metal ions
25
20
15
10
5
0
Fe3+Ag2+Al3+Ba2+Ca2+Cd2+Co2+Cu2+Hg2+K+ Li+Mg2+Mn2+Ni2+Pb2+Sr2+Zn2+
Fig. 5. Fluorescence response of 1 (1.49 ꢁ 10ꢂ5 M) to various cations in THF-H2O solution (1:1, v/v), lex ¼ 456 nm. The black bars represent the fluorescent intensity of 1 and Fe3þ
(1:1, v/v, 1.49 ꢁ 10ꢂ5 M). The red bars represent the fluorescence changes that occur upon the addition of competing ions to the solution containing 1 and Fe3þ (1:1, 1.49 ꢁ 10ꢂ5 M).
(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
complex via the oxygen atom on C]O group and nitrogen atom on
C]N group as shown in Scheme 2. The coordination of Fe3þ and 1
can enhance the coplanarity of naphthalimide and coumarin,
which could reduce the nonradiative decay of the excited state and
lead to the pronounced fluorescence enhancement [26,27]. To
obtain more insight into the fluorescent properties, the fluores-
cence titration curve of 1 toward Fe3þ was investigated (Fig. 3).
With increasing concentrations of Fe3þ, the fluorescence intensity
of sensor 1 at lmax 504 nm was gradually increased. Fig. 3 (inset)
showed that there was a good linearity between the fluorescence
intensity at 504 nm and concentrations of Fe3þ in the range from
Data were linearly fitted according to Eq. (1) and the Ka value was
(2.589 ꢀ 0.206) ꢁ 103 Mꢂ1
.
3.2. The interference from other metal ions
For an excellent chemosensor, high selectivity is a matter of
necessity. To confirm the selectivity of sensor 1, the competition
experiments were also measured by addition of one equivalent of
other metal ions, such as Mn2þ, Co2þ, Ni2þ, Cu2þ, Agþ, Cd2þ, Pb2þ
,
Ca2þ, Al3þ, Ba2þ, Hg2þ, Liþ, Mg2þ, Sr2þ, Zn2þ and Kþ, to the THF-H2O
solution of 1 in the presence of one equivalent of Fe3þ. As shown in
Fig. 5, we found that all the coexistent metal ions had no obvious
interference with the detection of Fe3þ. These results indicated that
12
quantitatively.
mM to 149 m
M, indicating that sensor 1 could detect Fe3þ ion
In order to determine the stoichiometry of 1-Fe3þ complex, the
method of continuous variation (Job plot) was introduced. The
fluorescence intensity at 504 nm was plotted against the molar
fraction of sensor 1, and the total concentration of the sensor and
Fe3þ ion maintained a constant at 5.96 ꢁ 10ꢂ5 M. The maximum
emission intensity was reached at a molar fraction of 0.5 (Fig. 4),
sensor 1 displayed an excellent selectivity toward Fe3þ
.
4. Conclusion
In summary, we have successfully designed a new chemosensor
based on coumarin and naphthalimide allowing the specific
detection of Fe3þ ion. This sensor displayed an excellent selectivity
for Fe3þ over other metal ions. With increasing concentrations of
Fe3þ, the fluorescence intensity at lmax 504 nm was gradually
enhanced. The binding ratio of 1-Fe3þ complex was determined to
be 1:1 according to the Job plot. This makes the simple and quick
detection of Fe3þ ion possible.
indicating that sensor 1 formed a 1:1 complex with Fe3þ
.
Detection limit (DL) and association constant (Ka) can be also
obtained from the fluorescence titration. The detection limit (DL) of
sensor 1 for Fe3þ was calculated as 0.388
mM by the SterneVolmer
plot [28]:
DL ¼ 3
where
s
=S ¼ 3:88 ꢁ 10ꢂ7M
Acknowledgments
s
is the standard deviation of the blank solution, S is the
This work was supported by Natural Science Foundation of
Hubei Province of China (No. 2013CFB005).
slope of the calibration curve.
The association constant (Ka) of 1-Fe3þ complex was calculated
by the BenesieHildebrand Eq. (1) [27]:
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