A novel coumarin Schiff-base as a Ni(II) ion colorimetric sensor

Article abstract of DOI:10.1016/j.saa.2012.01.017

A novel coumarin Schiff base compound (L) prepared from 7-diethylaminocoumarin-3-aldehyde and 3-amino-7-hydroxycoumarin was synthesized and evaluated as a chemoselective Ni²⁺ sensor. Addition of Ni ²⁺ to CH₃CN solution of L resulted in a rapid color change from yellow to red together with a large red shift from 465 to 516 nm. Moreover, other common alkali-, alkaline earth-, transition- and rare earth metal ions induced no or minimal spectral changes. Experimental results indicated that L could be used as a potential Ni²⁺ colorimetric and naked-eye chemosensor in CH₃CN solution.

Full text of DOI:10.1016/j.saa.2012.01.017

Spectrochimica Acta Part A 90 (2012) 40–44
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A novel coumarin Schiff-base as a Ni(II) ion colorimetric sensor
∗
Lingyun Wang , Decheng Ye, Derong Cao
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel coumarin Schiff base compound (L) prepared from 7-diethylaminocoumarin-3-aldehyde and 3-
Received 11 November 2011
Received in revised form 4 January 2012
Accepted 8 January 2012
2+
amino-7-hydroxycoumarin was synthesized and evaluated as a chemoselective Ni sensor. Addition of
2+
Ni to CH3CN solution of L resulted in a rapid color change from yellow to red together with a large
red shift from 465 to 516 nm. Moreover, other common alkali-, alkaline earth-, transition- and rare earth
metal ions induced no or minimal spectral changes. Experimental results indicated that L could be used
as a potential Ni colorimetric and naked-eye chemosensor in CH3CN solution.
Keywords:
2+
Coumarin Schiff base
Colorimetric and naked-eye chemosensor
Detection
©
2012 Elsevier B.V. All rights reserved.
Nickel(II) ion
UV–vis absorption
1
. Introduction
Because coumarins possess desirable photophysical properties
such as a large Stokes shift and visible excitation and emission
wavelengths, they are ideal platforms for development of colori-
metric chemosensors for specific heavy and transition metal ions
[14,15]. On the other hand, it is well known that C N isomer-
ization is responsible for the predominant decay of the excited
states of compounds containing an unbridged C N structure, which
usually results in weak fluorescence emission of the attached chro-
mophore. If this isomerization is stopped by bonding to metal
ions, high-intensity emission can be achieved. Therefore, there
are already many coumarin Schiff base compounds developed for
The design and synthesis of chemosensors for detection of
metal ions in aqueous and nonaqueous media are active and fas-
cinating areas. Along with other transition metal ions, Ni2 is an
essential trace element in biological systems such as respiration,
biosynthesis, and metabolism. For example, an undersupply of
Ni leads to deficiency and an oversupply results in toxic effects.
However, as an industrial pollutant in a wide variety of metallurgi-
cal processes such as electroplating and alloy production as well
as nickel–cadmium batteries, nickel is a toxic element that can
cause lung injury, allergy and carcinogenesis. Selective monitoring
of Ni2+ in environmental evaluation and clinic analysis is, there-
+
2+
2+
2+
2+
selective detection of different metal ion species (Hg , Cu , Zn
,
2+
Mg , etc.) so far due to their high selectivity, sensitivity and
simplicity [16–20]. However, colorimetric chemosensors based on
coumarin Schiff base for selective detection of Ni2+, are rather rare
[21].
2+
fore, needed [1–4]. Up to date, most of Ni -selective sensors are
based on potentiometric methods [5,6]. Although the fluorescence
quenching nature of paramagnetic Ni2 , several Ni -sensitive flu-
orescent sensors with serious interferences from other transition
metal ions have been reported [7–11,3]. Recently, selective lumi-
nescent probes based on benzothiadiazoyl-triazoyl cyclodextrin
and zinc compound containing symmetrical Schiff base ligand for
+
2+
Herein, we develop a simple, facile and reliable Ni2+ colorimetric
chemosensor based on a new coumarin Schiff base compound (L)
with two different coumarin groups connected via an imine link-
age. Addition of Ni2+ to CH CN solution of L results in a rapid color
3
2+
Ni were also investigated [12,13]. On the other hand, colorimet-
ric sensors are promising due to their simplicity, real-time and
on-line analysis, especially a significantly lower capital cost than
closely related methods, such as fluorescent sensors, for which both
spectrophotometric equipment and a UV light source are required.
Accordingly, development of a novel colorimetric chemosensor for
the rapid and convenient detection of Ni2 is attractive.
change from yellow to red together with a large red shift from 465 to
516 nm, which allows it to selectively detect Ni2+ ion by the naked
eye over a great number of environmental ions including common
alkali-, alkaline earth-, transition- and rare earth metal ions.
2. Experimental
+
2.1. Chemicals and instruments
∗ Corresponding author. Tel.: +86 20 87110245; fax: +86 20 87110245.
E-mail addresses: lingyun@scut.edu.cn, lingyun wr@yahoo.com.cn (L. Wang).
All reactants were commercially available and used without
further purification. All melting points were uncorrected. The IR
1
386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.01.017

L. Wang et al. / Spectrochimica Acta Part A 90 (2012) 40–44
2.0
41
spectra were measured on a Nicolet/Nexus-870 FT-IR spectrome-
−
1
ter with KBr pellets in the range 4000–400 cm . Nuclear magnetic
resonance spectra were recorded on Bruker Avance III 400 MHz and
chemical shifts are expressed in ppm using TMS as an internal stan-
dard. The UV–vis absorption spectra were recorded using a Helios
Alpha UV-Vis scanning spectrophotometer. Fluorescence spectra
were obtained with a Hitachi F-4500 FL spectrophotometer with
quartz cuvette (path length = 1 cm).
1.5
1.0
0.5
0.0
20 equiv.
The recognition between coumarin Schiff base compound (L)
and different metal cations was investigated by UV–Vis and fluo-
rescence spectroscopy in CH CN solution. The stock solution of L
3
0
equiv.
and metal ions was at a concentration of 10.0 mM. All absorption
and emission spectral studies were carried out in freshly purified
CH CN at room temperature.
3
2.2. Synthesis of coumarin Schiff base compound (L)
3
00
400
500
600
Wavelength (nm)
3
-Amino-7-hydroxycoumarin and 7-diethylaminocoumarin-3-
aldehyde were synthesized according to previous methods [18,22],
respectively.
Fig. 1. UV–vis absorption spectra of L (10.0 ␮M) with gradual addition of Ni2+ [0, 4,
2, 20, 32, 40, 80 and 200 ␮M, respectively].
1
A portion of 7-diethylaminocoumarin-3-aldehyde (247 mg,
1
mmol) and 3-amino-7-hydroxycoumarin (195 mg, 1.1 mmol)
were combined in hot absolute ethanol (20 mL) to yield a scarlet
precipitate for a moment. The solution was stirred under reflux
conditions for 6 h, and the precipitate was filtrated, washed with
presence of Ni2+. The free L exhibited main absorption at 465 nm
in CH CN solution, which could be assigned to the charge trans-
3
fer (CT) absorbance, as observed in other compounds with similar
results [16]. As soon as Ni2+ was added at room temperature, the
absorption intensity at ꢀmax of 465 nm decreased and two new
bands at 486 and 516 nm due to ICT appeared. Notably, when the
concentration of Ni2+ increased stepwise, absorption intensity at
hot absolute ethanol three times, then recrystallized with DMF/H O
2
(
v/v, 1/3) to get scarlet crystal L (347 mg, 0.86 mmol) in 86% yield.
−1
IR (KBr, cm ): 2966, 2927, 1718, 1620, 1349, 1506, 1456, 1420,
45, 811, 770, 729, 693. 1H NMR (DMSO-d , 400 MHz, ı): 10.54(s,
8
1
7
6
H, OH), 9.04(s, 1H,
N CH), 8.56(s, 1H, Ar H), 7.81(s, 1H, Ar H),
ꢀmax of 486 and 516 nm gradually increased and finally exceeded
.70(d, 1H, J = 12 Hz, Ar H), 7.58(d, 1H, J = 8 Hz, Ar H), 6.81 (t, 1H,
that of 465 nm. Correspondingly, the color changes of L were also
J = 8 Hz, Ar H), 6.77 (s, 1H, Ar H), 6.61(s, 1H, Ar H), 3.49(q, 4H,
J = 8 Hz, CH CH ), 1.16(t, 6H, J = 8 Hz, CH CH ). Anal. Calcd for
observed by eye simultaneously from yellow to red in presence of
2
3
2
3
2+
Ni (Fig. 2). This result indicated that the color changes were most
probably owing to the formation of a new complex species with
different electronic properties from that of L, and therefore a new
color (red) was observed. The significant color change also sug-
gested that L was a sensitive naked-eye indicator for Ni2+, which
was different from structurally similar bis-coumarin derivative act-
ing as a fluorescence signaling system selectively for Mg(II) [17].
Moreover, well-defined isosbestic points at 363, 394 and 478 nm
further demonstrated that a stable complex was present having
a certain stoichiometric ratio between the L and Ni2+ formed in
C23H20N O5: C 68.31%, H 4.98%, N 6.93%, Found: C 68.55%, H 5.04%,
2
N 6.66%.
3
. Results and discussion
3.1. The design and synthesis of compound L
Diethylamino coumarin and hydroxyl coumarin derivatives
with intramolecular charge transfer (ICT) character are selected
as chromophores, and are linked by bond to form a
C
N
CH CN solution.
3
potential chromogenic-sensing molecule for metal cations. It is
known that chelating groups C N, C O exhibit a high affin-
ity to transition and post-transition metal cations, but less
binding affinity toward alkali metal and alkaline earth metal
cations due to the difference of electronic structures [23]. In this
design, therefore, we constructed compound L as chromogenic
chemosensor. Firstly, by reacting 2,4-dihydroxy benzaldehyde
with N-acetylglycine in acetic anhydride, and subsequently
converted into 3-amino-7-hydroxycoumarin through hydroly-
sis in HCl/ethanol solution. When 4-diethylaminosalicylaldehyde
was allowed to react with diethylmalonate, the correspond-
3.3. Job’s plot analyses
For determination of stoichiometry between L and Ni2+, Job’s
plot analyses were used. The method is that keeping total con-
2+
centration of L and Ni at 10.0 mM, and changing the molar ratio
of Ni2+ (XM; XM = [Ni ]/{[L] + [Ni ]}) from 0.1 to 0.9. From Fig. 3,
2+
2+
2+
when molar fraction of Ni was 0.5, the absorbance at 516 nm got
to maximum, indicating that forming a 1:1 complex between L and
2+
Ni
.
ing intermediate was then treated by POCl /DMF to give
3
7
-diethylaminocoumarin-3-aldehyde. L was readily synthesized
by one-step condensation of 7-diethylaminocoumarin-3-aldehyde
and 3-amino-7-hydroxycoumarin in absolute ethanol as shown in
Scheme 1 and characterized by FT-IR, 1H NMR and elemental anal-
ysis.
3
.2. Selective recognition of Ni2+ with L
Firstly, the metal ion binding and sensing ability of L was
investigated in CH CN. Fig. 1 demonstrates the changes in the
Fig. 2. Photographs of L color changes (10.0 ␮M) with gradual addition of Ni2+ from
3
absorption spectrum of L (10 ␮M) in CH CN in the absence and
left to right: 0, 4, 10, 20 and 50 ␮M, respectively in CH3CN solution.
3

4
2
L. Wang et al. / Spectrochimica Acta Part A 90 (2012) 40–44
CHO
O
CHO
OH
(
1)CH2(COOC2H5)2
POCl3,DMF
(
2)HCl/AcOH
O
N
O
N
N
O
EtOH
refulx
NH
2
CHO
OH
(1) NaOAc/Ac O
2
+
3 2
CH C(O)NHCH COOH
HO
O
HO
(2) HCl/EtOH = 2:1
O
H
C
N
O
HO
O
O
N
O
L
Scheme 1. Synthesis of compound L.
The association constant (Ka) of L with Ni2+ was determined
using the Benesi–Hildebrand equation as follows:
Ni2+ resulted in an obvious change, indicating L had higher binding
affinity toward Ni than other surveyed metal ions.
2+
Furthermore, competition experiment was carried out by
1
1
1
adding 20.0 ␮M Ni2+ to solution of L (10.0 ␮M) in the pres-
=
+
+
2
A − A0
Amax − A0
+
+
2+
2+
2+
Ka(Amax
− A )[Ni
]
0
ence of miscellaneous cations including Na , K , Ca , Mg , Ba ,
2+
2+
2+
2+
3+
3+
Hg , Cu , Zn , Cd , Sb
and Fe
(20.0 ␮M, respectively).
where A and A represent the absorbance of L in the presence and
absence of Ni , respectively, Amax is the saturated absorbance of
L in the presence of excess amount of Ni ; [Ni ] is the concen-
0
2
+
These miscellaneous competitive cations did not induce signifi-
2+
2+
2+
cant absorption changes of L in the absence of Ni . However, upon
addition of 2 equiv. of Ni2+ to the solution, the unique spectral
and color changes were still displayed under the above conditions.
These results revealed that L had a remarkable selectivity toward
2
+
tration of Ni ion added. The measured absorbance [1/(A − A )]
0
2+
varied as a function of 1/[Ni ] in a linear relationship (R = 0.991),
indicating formation of 1:1 stoichiometry between Ni2+ and L. This
2+
Ni over other competitive cations, and more, the detection of
further corroborated 1:1 complex formation based on Job’s plot
analyses. The association constant of L with Ni in CH CN solution
was calculated to be 2.9 × 10 M
2+
2+
Ni by L was hardly affected by these common coexistent metal
3
4
−1
ions.
.
3.5. Influence of metal ions on the fluorescence intensity of L
3.4. Interference from other metal ions
The fluorescence properties of L (1.0 ␮M) with 0.2–10 equiv. of
Selectivity is an important characteristic of an ion-selective
chemosensor. To examine the selectivity of L, the affinity of L for
Ni2+ were studied in CH CN solution, where excitation wavelength
3
+
+
2+
2+
2+
2+
was 465 nm and fluorescence emission intensity was recorded at
498 nm. In CH3CN solution, the fluorescence intensity of L was
weak, which was probably due to the fact that C N isomerization
was the predominant decay process of the excited states for L with
an unbridged C N structure.
other surveyed metal ions, such as Na , K , Ca , Ba , Mg , Hg
,
2+
2+
2+
3+
3+
Cu , Zn , Cd , Sb and Fe was investigated. As shown in Fig. 4,
addition 1 equiv. of these alkali-, alkaline earth-, transition and rare
earth metal ions did not cause significant UV–vis absorption spec-
tral changes of L. However, under the same conditions, addition of
1
1
1
0
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
.0
0
0
0
0
0
0
0
.18
.16
.14
.12
.10
.08
.06
+
+
2+
2+
2+
2+
{
L, K , Na , Mg , Ca , Ba ,Hg
2+ 2+ 2+ 3+ 3+
Cu , Zn , Cd ,Fe ,Sb }
2
+
L+Ni
0
.0
0.2
0.4
0.6
0.8
1.0
300
400
500
600
2
+
2+
[
Ni ]/[Ni ]+[L]
Wavelength (nm)
Fig. 3. Job’s plot of L with Ni2+ in CH3CN solution. Total concentration of [L] + [Ni2+
was kept constant at 10.0 mM. The absorbance at 516 nm was used.
]
Fig. 4. UV–vis absorption spectra of L (10.0 ␮M) upon addition of various metal ions
(10.0 ␮M) in CH3CN solution.

L. Wang et al. / Spectrochimica Acta Part A 90 (2012) 40–44
43
H
C
H
C
N
N
O
Ni2+
Ni2+
HO
O
O
O
HO
O
NEt
2
O
O
O
NEt
2
X
X = ClO - or CH CN
4
3
Scheme 2. Schematic representation of possible binding mode between Ni2+ and L.
When Ni2+ concentration was low or equal to that of L, a fluo-
rescence emission increase of L was observed with slightly changes
in the spectral shapes, as shown in Fig. 5. The maximum increase
of fluorescence emission at 498 nm was found to be 48.3% with
4000
500
3
3000
2500
2
equiv.
2
+
1
equiv. of Ni , which was agreement with the data of 1:1 com-
2+
plex between L and Ni from Job’s plot. The enhancement of
fluorescence was attributed to the introduction of Ni2 and con-
sequently occurrence of the strong complexation with L, resulting
in decreased non-radiative decay of the excited-state and increased
radiative decay. The remarkable increase of the fluorescence inten-
sity is due to the inhibition of the C N isomerization process upon
+
Free L
2
1
1
000
500
000
4 equiv.
10 equiv
2+
Ni binding at the NO2 site. The fourth or higher coordination site
500
0
of Ni2+ may be occupied by the counteranion ClO4 or acetonitrile
−
2+
molecule. The proposed interaction mode between L and Ni was
shown in Scheme 2.
-500
2
+
400
450
500
550
600
650
When Ni concentration was higher than that of L, emission
intensity of L began to gradually decrease as compared with max-
imum emission intensity in presence of 1 equiv. Ni2 (Fig. 6). For
instance, emission intensity of L decreased 8.7% in presence of
Wavelength (nm)
+
Fig. 6. Fluorescence emission spectra of L (1.0 ␮M) with gradual addition of Ni2+
2,4 and 10.0 ␮M, respectively].
[
2
+
2+
2
equiv. of Ni than that of observed in presence of 1 equiv. Ni .
2+
Upon addition of 10 equiv. of Ni , the fluorescence emission of L
was strongly quenched and decrease of fluorescence intensity at
4
98 nm was found to be 74.2% as compared with that of free L.
1
1
.4
.2
Because the maxima and shapes of the fluorescence spectra did
not change and no new peaks appeared, possible mechanism for
the remarkable fluorescence quenching induced by the paramag-
2
+
netic Ni was attributed to an electron or energy transfer between
1.0
0.8
0.6
2
+
excess Ni and L known as the fluorescence quenching mechanism
24,25]. Similar phenomena were also observed in other experi-
ments [26].
[
Addition of 1 equiv. of other surveyed metal ions (Na , K , Ca2+,
+
+
Mg2+, Ba , Hg , Cu , Zn , Cd , Sb and Fe , respectively)
produced different changes in the fluorescence spectra of L, in
terms of maximum emission intensity. As shown in Fig. 7, the
2+
2+
2+
2+
2+
3+
3+
0
0
.4
.2
4
4
3
3
2
2
1
1
500
000
500
000
500
000
500
000
0.0
K+ Ca2+ Mg2+ Ba2+ Hg2+ Cu2+ Zn2+ Cd2+ Sb3+ Fe3+ Ni2+
Fig. 7. Changes in fluorescence emission intensity ratio (I/I0) at 498 nm of L (1.0 ␮M)
in the presence of 1 equiv. of various metal ions in CH3CN solution. I0 is the intensity
of free L, and I is the intensity of L after addition of metal ion.
1.0 equiv.
0
.6equiv.
0.2 equiv.
fluorescence emission was not affected by Na , K , Ca2+, Ba2+ and
+
+
2
+
Mg , and fluorescence intensity ratios of L in the presence and
absence of these surveyed metal ions at 498 nm are almost equal
to one. The fluorescence emission was slightly quenched (no more
than 10%) by Zn2+and Sb . However, the fluorescence emission
3+
Free L
2
+
2+
2+
increased in range of 15–48% with the addition of Hg , Cu , Cd
,
5
00
2+
3+
Ni and Fe
. Conclusions
A colorimetric Ni2+ sensor based on novel coumarin Schiff base
.
0
4
4
00
450
500
550
600
650
Wavelength (nm)
2+
Fig. 5. Fluorescence emission spectra of L (1.0 ␮M) with gradual addition of Ni2+
0,0.2, 0.6 and 1.0 ␮M, respectively].
compound (L) was designed and synthesized. Addition of Ni to
CH3CN solution of L resulted in an obvious color change (from
[

4
4
L. Wang et al. / Spectrochimica Acta Part A 90 (2012) 40–44
yellow to red) in a very short time. This made the real-time,
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