Simple imine linked colorimetric and fluorescent receptor for sensing Zn2+ ions in aqueous medium based on inhibition of ESIPT mechanism

Article abstract of DOI:10.1016/j.tetlet.2011.06.097

Herein, we report the highly selective binding of Zn²⁺ ion by the salicylaldimine based Schiff base chromogenic receptor 1 [(N,N′-bis (salicylidine)-o-phenylenediamine]. Receptor 1 senses Zn²⁺ ion in aqueous medium by colorimetric an

Full text of DOI:10.1016/j.tetlet.2011.06.097

Tetrahedron Letters 52 (2011) 4631–4635
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Simple imine linked colorimetric and fluorescent receptor for sensing Zn2+
ions in aqueous medium based on inhibition of ESIPT mechanism
a
a
b
a,
⇑
D. Udhayakumari , S. Saravanamoorthy , M. Ashok , Sivan Velmathi
a
Department of Chemistry, Organic and Polymer Synthesis Laboratory, National Institute of Technology, Tiruchirappalli 620 015, India
Department of Physics, National Institute of Technology, Tiruchirappalli 620 015, India
b
a r t i c l e i n f o
a b s t r a c t
Herein, we report the highly selective binding of Zn2 ion by the salicylaldimine based Schiff base chro-
+
Article history:
Received 11 May 2011
Revised 23 June 2011
Accepted 26 June 2011
Available online 2 July 2011
0
2+
mogenic receptor 1 [(N,N -bis (salicylidine)-o-phenylenediamine]. Receptor 1 senses Zn ion in aqueous
medium by colorimetric and fluorescent response in the presence of other metal ions like Pb , Hg , Sn
Cd . Receptor 1 on binding with Zn ion exhibits fluorescence enhancement which is due to the inhibi-
tion of the (ESIPT) mechanism.
2+
2+
2+
,
2+
2+
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Schiff base
2+
Zn ion sensors
UV–vis spectroscopy
ESIPT
Fluorescence enhancement
0
The selectivesensing ofcations is one of themost importantareas
of organic chemistry, supramolecular chemistry, drug delivery, and
environmental chemistry because of its potential clinical applica-
fluorescent sensing ability of the salicylaldimine based (N,N -bis
(salicylidine)-o-phenylenediamine) Schiff base as zinc ion sensors
1
2a,b
whose synthesis procedure is well documented.
Though recep-
1
a,b
13
tions.
Zinc ion, which is the second most abundant transition me-
tor 1 has already been reported to act as an adenine sensor and zinc
tal ion in the human body plays an important role in biological
systems such as neuronal signal transmission, regulator of enzymes,
complexes of similar salen ligands and their photo physical proper-
1
4a–c
ties are studied and augmented in the literature,
we are adding a
2
a–c
influencing DNA synthesis, and numerous cellular functions.
As
new dimension to the receptor 1 in that it can be used as a colorimet-
ric and fluorescent sensor for zinc ion.
Zn and Cd have similar properties,3 it is difficult to sense zinc in
a,b
the presence of other metal ions. Usually N or O donor centers and
4
a,b
5a–c
receptors containing imine (CH@N),
hydroxyl (–OH),
thiourea ((NH
C@S),8a–c pyr-
derivatives act as binding sites for cat-
ions. Recently, many reports appeared in the literature dealing with
amide
Colorimetric analysis
6
a–c
7a–d
(
–CONH),
urea ((NH
coumarin
2
)
2
C@O)),
2 2
)
9
a,b
10a,b
role (–NH),
Receptor 1 (Fig. 1) was prepared by a simple condensation reac-
tion between salicylaldehyde and o-phenylene diamine and well
sensing of cations, especially zinc.1
1a–d
These highly effective sen-
1
13
characterized by H, C, NMR, FT IR and UV–vis spectroscopic
techniques (for details see Supplementary data).
sors often require tedious multi-steporganic syntheses for their syn-
thesis, and most of them are fluorescent based sensing of zinc in
acetonitrileor other organic solvents. Sensing of zinc ions in aqueous
medium which is in a physiologically more suitable condition is
really important. Colorimetric sensing, a ‘naked eye’ sensing method
is simple in monitoring the process without using any complicated
spectroscopy methods. Therefore, colorimetric and fluorescent sen-
sors, withimproved performance, capable of sensingzinc in aqueous
medium in the presence of other transition metal ions, are in great
demand. This urge created an interest in studying the colorimetric,
The colorimetric sensing ability of receptor 1 toward different
metal cations like Cd , Hg , Pb , Zn and Sn was studied.
Colorimetric titrations were carried out in different solvents like
2
+
2+
2+
2+
2+
3 3
acetonitrile (CH CN), CH CN/water (v/v) mixture and water. The
N
N
OH HO
⇑
Corresponding author. Tel.: +91 431 2503640; fax: +91 431 2500133; mobile:
91 09486067404.
+
E-mail addresses: velmathis@nitt.edu, svelmathi@hotmail.com (S. Velmathi).
Figure 1. Structure of receptor 1.
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.097

4
632
D. Udhayakumari et al. / Tetrahedron Letters 52 (2011) 4631–4635
color change was visualized by naked eye analysis and optical re-
sponse was studied by UV–vis spectroscopy. In the naked eye
experiments, visual inspection of solution of receptor
1
ꢁ5
(
1 ꢀ 10 M solution in CH
3
CN) showed dramatic color change
from colorless to pale fluorescent green (Fig. 2a) upon the addition
of 200
The change could easily be detected by the naked eye. The addition
+
ꢁ3
l
L (2 equiv) of Sn2 ion (1.5 ꢀ 10 M solution in CH
3
CN).
of other metal ion solutions like Cd2 , Hg , Pb , and Zn did not
show any detectable color changes even on excess addition. The
naked eye analysis was tried in aqueous medium, to suit the phys-
iological condition. Surprisingly Sn2 ions could not give rise to any
color change on interacting with receptor 1. However, Zn2 ions in-
duced enhanced color change of the receptor 1 from colorless to in-
tense fluorescent green (Fig. 2b), whereas other ions did not induce
any color change in aqueous medium. The color intensity increased
gradually corresponding to the increase in the cation concentration
+
2+
2+
2+
ꢁ5
Figure 2b. Color changes of receptor 1 (1 ꢀ 10 M soln in CH
3
CN) before and after
ꢁ3
the addition of 200
l
L (2 equiv) of respective cations (1.5 ꢀ 10 M soln in H
2
O)
(From left to right: R, R+ Cd ,R+ Hg , R+ Pb , R+ Zn and R+ Sn2+).
2+
2+
2+
2+
+
+
(
0–200
mation. Further, we inspected the colorimetric response in aque-
ous CH CN medium (v/v mixture). The color of the receptor 1 in
the presence of 200
with the increase in the water ratio (Fig. 2c). Thus, in pure CH
receptor showed no color change and fluorescent green color
started to appear when the CH CN–H O ratio was 7:3 (v/v) and
lL). The color change can be attributed to the complex for-
3
+
l
L (2 equiv) of Zn2 ion gradually intensified
ꢁ
5
Figure 2c. Color changes of receptor 1 (1 ꢀ 10 M soln in CH
3
CN) before and after
3
CN
L (2 equiv) of Zn ions (1.5 ꢀ 10ꢁ3 M soln in CH
2+
the addition of 200
mixture (v/v)).
l
3 2
CN/H O
3
2
the color was gradually increasing with the increasing amount of
water and reaching its maximum in pure aqueous medium.
3.5
3
2
2
1
.0
.5
.0
.5
UV–vis spectroscopic studies
Rec
In order to have deep understanding of the binding action and
solvent dependent color change noted in naked eye analysis, UV–
vis experiments of receptor 1 with different cations were carried
out. The electronic spectroscopy of the receptor 1 in the absence
of any cation solution revealed three types of transitions. Upon
2+
2+
+
+
2eq Zn
2eq Cd
2+
+2eq Mg
2+
+
+
2eq Pb
1.0
.5
0.0
2+
2eq Sn
L of Zn2 ion (1.5 ꢀ 10 M in H
+
ꢁ3
the addition of 200
l
2
O) to the
0
receptor 1, a new intense absorption band at 380 nm was devel-
oped which led to the color change from colorless to fluorescent
green. The new peak is mainly attributed to the formation of co-
ordination complex of receptor 1 with Zn2 ion. In addition, clear
isosbestic points at 363 nm were observed which proves that
300
400
500
600
+
Wavelength (nm)
ꢁ
5
Figure 3a. UV–vis spectrum of receptor 1 (1 ꢀ 10 M, in CH
3
CN) upon titration
2
+
receptor 1 and Zn bound receptor 1 exist in equilibrium. The
other cations did not give any optical response in aqueous medium
and there was no additional band observed after the addition of
aqueous solutions of cations (Fig. 3a). Thus, incremental addition
with aqueous solution of cations (Zn , Cd , Hg , Pb and Sn2+).
2+
2+
2+
2+
1.3
1.2
1.1
2
+
of Zn ions from 20 to 200 lL to receptor 1 was done. While the
1.0
intensity of the peaks at 265 and 345 nm gradually decreased,
the intensity of the peak at 240 and 295 nm gradually increased
4
3
2
1
0
0.9
0.8
0.7
(Fig. 3b). A new peak at 385 nm in the near visible region was no-
0.6
0.5
2
+
ticed upon addition of 0.8 equiv of the Zn ions to receptor 1
0
.0
0.5
1.0
1.5
2.0
which was gradually increasing in intensity with further addition
No.of equiv
2
+
of Zn ions (up to 200
l
L). Since tin was sensing in acetonitrile
200 µL
2
+
2+
medium, sensing of Sn and Zn in acetonitrile medium was also
2
+
followed by UV–vis titration experiments. Addition of Sn ions
from 20 to 200 L.) to receptor 1, results in the decrease in the
(
l
0
3
00
400
500
Wavelength (nm)
ꢁ
5
Figure 3b. UV–vis spectrum of receptor 1 (1 ꢀ 10 M, in CH
3
CN) upon titration
2+
with aqueous solution of Zn (0–200
addition of Zn2 ion at 400 nm).
lL). (Inset: Changes of absorbance upon
+
intensity of peak at 275, 325 nm and formation of a new peak at
25 nm (Fig. 3c). Similar observation was found for the addition
4
2
+
ꢁ
5
of Zn ion from 20 to 200 lL in acetonitrile solvent to the receptor
1 (Fig. 3d). However, the intensity of the peak was smaller com-
pared to the observed in aqueous medium.
Figure 2a. Color changes of receptor 1 (1 ꢀ 10 M soln in CH
3
CN) before and after
ꢁ
3
the addition of 200
l
L (2 equiv) of respective cations (1.5 ꢀ 10 M soln in CH
3
CN)
2+
2+
2+
2+
2+
(
From left to right: R, R+ Cd , R+ Hg , R+ Zn , R+ Sn and R+ Pb ).

D. Udhayakumari et al. / Tetrahedron Letters 52 (2011) 4631–4635
4633
0
0
0
0
0
0
.30
.25
.20
.15
.10
.05
0
.16
0.14
0.12
0.10
0.08
0.06
0.04
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
0
.0
0.5
1.0
1.5
2.0
No.of equiv
2
00 µL
0.0
0.2
0.4
0.6
0.8
1.0
0
2+
Rec/ [(Rec) + (Zn )]
3
00
400
500
Figure 4. Jobs Plot between receptor 1 and Zn2+ ions.
Wavelength (nm)
ꢁ
5
Figure 3c. UV–vis spectrum of receptor 1 (1 ꢀ 10 M, soln in CH
3
CN) upon
1 showed high selectivity and sensitivity toward Zn2+ ion over other
competing cations in aqueous medium. Generally, salicylaldimines
exist as phenol–imine tautomer in polar aprotic solvents and the
2+
ꢁ3
titration with Sn (1.5 ꢀ 10 M, soln in CH
3
CN). (Inset: Changes of absorbance
upon addition of Sn2 ion at 425 nm).
+
1
6
keto amine form predominates in other solvents. Similarly, recep-
tor 1 has an intra molecular hydrogen bond between the phenolic
O–H and the nitrogen of the imine that undergoes Excited State
Intramolecular Proton Transfer (ESIPT) and yields a normal (phe-
nol–imine form) emission band around 430 nm and a tautomer
emission (Keto amine) band at 520 nm (Scheme 1). In the fluores-
1
1
1
1
.28
.26
.24
.22
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
1.20
1
1
.18
.16
0
.0
0.5
1.0
1.5
2.0
3
cence spectrum of the receptor 1 run in CH CN, the tautomer band
No.of equiv
was not prominent as phenol imine form predominates in the polar
aprotic medium. ( Fig. 5a). Weak fluorescence of free ligand in-
200 µL
2
+
creased notably upon addition of 20
l
L of Zn ion in CH
3
CN.
Quenching as a result of the increased addition of CH
3
CN solution
2
+
of Zn ions to the receptor 1 indicates that the intramolecular pro-
ton transfer might have occurred in receptor 1 leading to a very fast
and efficient non-radiative decay of the excited state of receptor 1.
0
3
00
400
500
2+
A polar aprotic solvent such as acetonitrile tends to solvate the Zn
Wavelength (nm)
2
+
cation and reduce the binding capacity of Zn with the receptor,
resulting in an internal proton transfer. Interestingly, emission
ꢁ
5
Figure 3d. UV–vis spectrum of receptor 1(1 ꢀ 10 M, in CH
3
CN) upon titration
with Zn2 (1.5 ꢀ 10 M, soln in CH
+
ꢁ3
CN). (Inset: Changes of absorbance upon
3
3
due to keto amine form, which is weak in CH CN was substantially
2+
addition of Zn ion at 380 nm).
2+
increased in presence of aqueous solution of Zn ion (Fig. 5b). Emis-
sion intensity at 520 nm band was gradually increasing with the
2
+
increasing Zn concentration (Fig. 5b inset). It appears that the keto
emission at 520 nm was favored in a polar protic solvent. It is
known that ratio of keto and enol emission depends on solvent
From the UV–vis absorption spectroscopy measurements, the
binding constant (Kapp) of the Zn2 (in water and acetonitrile)
+
2
+
and Sn complexes of the receptor 1 were calculated. The Bene-
si–Hildebrand (B–H) plot was used for the binding constant calcu-
lation. Plot can be made with 1/DA as a function of 1/[H] . De can
0
be derived from the intercept while Kapp can be calculated from the
slope based on the linear least square fitting line. The binding con-
stant of the receptor 1 with the corresponding cations is shown in
the Table 1 (for B–H Plot see Fig. S4a,b and c of Supplementary
1
7a–d
10
polarity.
Co-ordination of zinc to receptor 1 which has d con-
figuration (closed shell) utilizes the phenolic proton for complex
formation and inhibits ESIPT and causes emission with an increased
intensity. Thus the aqueous medium functions in two ways, (i) sta-
bilizing the keto form of the receptor and (ii) facilitating the binding
2
+
of Zn ions with the receptor resulting in the colorimetric and fluo-
2
+
2
+
rescent sensing of Zn ions in aqueous medium, which very well
explains the solvent dependent color change observed in the color-
imetric analysis.
data). Receptor 1 shows a higher binding constant value for Zn
4
3
in H
2
O (2.87 ꢀ 10 ) than in CH
3
CN (8.97 ꢀ 10 ) and fairly better va-
2
+
4
lue for Sn in CH
3
CN (1.78 ꢀ 10 ). Jobs plot studies reveal that the
2+
To study the potential of using receptor 1 as ready to use, indi-
cator strips experiments were done using filter paper strips coated
with the solution of receptor 1. When these coated strips were
stoichiometry of the complex formed between receptor 1 and Zn
ions was 1:1 (Fig. 4).
2
+
Generally, Zn selective sensors show fluorescent response and
hence fluorescence spectroscopic analysis was carried out to de-
2
+
introduced into the aqueous solution containing Zn ions, the col-
orless strips were changed to fluorescent color instantaneously.
However, increased concentration of receptor 1 and Zn2+ ions
was required to result in the same color change observed in the
solution study. Whatman filter paper was cut into 3 ꢀ 1 cm size
duce the sensing action of receptor 1 toward Zn2+ ions.
15a,b
Receptor
Table 1
ꢁ
3
Binding constant values for receptor 1 with Zn and Sn2+ ions
2
+
and dipped in a solution of receptor 1 (5 ꢀ 10 M in CH
3
CN).
The strip was dried in a hot air oven set at 45 °C for 30 min. The
Receptor + ions
Binding constant (Kapp
)
2+
ꢁ3
coated strip was dipped in aqueous Zn solutions (5 ꢀ 10 M)
and dried in a similar way. The colorless filter paper is changed into
fluorescent green instantly (Fig. 6). Thus these receptors can be
made into a ready to use strips for detecting the presence of Zn2+
Receptor 1 + Zn2 (in H
+
O)
2.87 ꢀ 10 (±1.17)
4
2
2+
3
Receptor 1 + Zn (in CH
3
CN)
CN)
8.97 ꢀ 10 (±4.65)
Receptor 1 + Sn2 (in CH
+
1.78 ꢀ 10 (±5.06)
4
3

4
634
D. Udhayakumari et al. / Tetrahedron Letters 52 (2011) 4631–4635
ESIPT
ESIPT
N
O
N
O
N
O
N
O
N
O
N
O
Zn2+
Zn2+
H
H
H H
phenolimine
λem=430nm
N
O
N
O
H
H
Ketoamine
λem=520nm
Scheme 1. Proposed binding mechanism between the receptor 1 and Zinc ions.
4
2
0
0
0
0
Figure 6. Filter paper strips coated with receptor 1 changing to fluorescent green.
2
00 µL
inhibition of the ESIPT mechanism. In naked eye analysis, colorless
receptor changed to fluorescent yellow in presence of 200 lL
4
00
450
500
550
600
2+
(
2 equiv) of aqueous solution of Zn ions. It is demonstrated that
Wavelength (nm)
receptor 1 is useful for detecting zinc in physiological conditions
4
(
aqueous and neutral pH). Binding constant (2.87 ꢀ 10 ) shows
Figure 5a. Fluorescence titration spectrum of receptor 1 upon the gradual addition
of (0–200 L) ZnCl in CH CN.
that receptor 1 can sense trace amount of zinc ions in aqueous
medium. On the basis of Jobs plot 1:1 co-ordination model was
proposed for the complex formed. Receptor 1 utilizes Zn2+ co-ordi-
nation to turn off the ESIPT and enhancing the keto emission at
520 nm. Furthermore this receptor can be coated on filter paper
strips for ready to use dip sticks for detecting zinc ions in aqueous
solution. In addition, receptor 1 also senses Sn ions in acetonitrile
medium. Hence further studies are underway to apply receptor 1
as sensors for other transition metal ions.
l
2
3
6
5
4
3
2
1
00
00
00
00
00
00
2
+
6
00
00
00
00
00
00
0
5
0.0
0.5
1.0
1.5
2.0
4
3
2
1
No of equiv
Acknowledgments
2
00 µL
Authors express their thanks to Dr. M. Chidambaram, Former
Director, NIT Tiruchirappalli for his constant support and encour-
agement and DST-FIST and Nanomission Project for financial
assistance.
0
600
Supplementary data
4
00
450
500
550
Supplementary data (general materials, methods, details for
UV–vis and fluorescent titrations, spectroscopic data for receptor
1 and B–H plots used for binding constant calculation) associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2011.06.097.
Wavelength (nm)
Figure 5b. Fluorescence titration spectrum of receptor 1 upon the gradual addition
2 2
of (0–200 lL) ZnCl in H O (Inset: Changes of fluorescence emission upon addition
of Zn2+ ion at 520 nm.) (Excited at 360 nm).
ions in aqueous solution. This makes the receptor more advanta-
geous than the existing Zinc ion sensors.
References and notes
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amine acts both as colorimetric and fluorescent- selective chemo-
2
+
sensor for Zn cation in aqueous medium which operates by

D. Udhayakumari et al. / Tetrahedron Letters 52 (2011) 4631–4635
4635
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