A sensitive schiff-base fluorescent chemosensor for the selective detection of Zn2+

Article abstract of DOI:10.1007/s10895-014-1390-3

A Schiff-base fluorescent probe - N, N^/-bis(salicylidene) trans 1, 2 - diaminocyclohexane (H ₂ L) was synthesized and evaluated as a chemoselective Zn²⁺ sensor. Upon treatment with Zn²⁺, the complexation of H ₂ L with Zn²⁺ resulted in a bathochromic shift with a pronounced enhancement in the fluorescence intensity in ethanol solution. Moreover, other common alkali, alkaline earth and transition metal ions failed to induce response or minimal spectral changes. Notably, this chemosensor could distinguish clearly Zn²⁺ from Cd²⁺. The stoichiometric ratio and association constant were evaluated using Benesi - Hildebrand relation giving 1:1 stoichiometry. This further corroborated 1:1 complex formation based on Job's plot analyses.

Full text of DOI:10.1007/s10895-014-1390-3

J Fluoresc
DOI 10.1007/s10895-014-1390-3
ORIGINAL PAPER
A Sensitive Schiff-Base Fluorescent Chemosensor for the Selective
2+
Detection of Zn
Nayan Roy & Harun A. R. Pramanik & Pradip C. Paul &
Sanjoy T. Singh
Received: 29 January 2014 /Accepted: 7 April 2014
#
Springer Science+Business Media New York 2014
/
Abstract A Schiff-base fluorescent probe – N, N -
submicron visualisation and submillisecond temporal resolu-
tion [3, 4].
bis(salicylidene) trans 1, 2 – diaminocyclohexane (H L) was
2
+
2
synthesized and evaluated as a chemoselective Zn sensor.
Nowadays, among the different analyte, special interest is
devoted to develop chemosensors for transition metal ions:
usually they represent an environmental concern when present
in uncontrolled amounts, but at the same time some of them
such as iron, zinc, copper and cobalt are present as essential
elements in biological systems. Zinc is the second most abun-
dant transition metal ion in the human body after iron, and
plays a myriad of roles in numerous cellular functions such as
regulation of gene expression, apoptosis, co-factors in
metalloenzyme catalysis and neurotransmission in biological
systems [5, 6]. Many server neurological diseases, including
Alzheimer’s disease, cerebral ischemia and epilepsy [7–9] are
2
+
Upon treatment with Zn , the complexation of H L with
2
2+
Zn resulted in a bathochromic shift with a pronounced
enhancement in the fluorescence intensity in ethanol solution.
Moreover, other common alkali, alkaline earth and transition
metal ions failed to induce response or minimal spectral
changes. Notably, this chemosensor could distinguish clearly
2+
2+
Zn from Cd . The stoichiometric ratio and association
constant were evaluated using Benesi – Hildebrand relation
giving 1:1 stoichiometry. This further corroborated 1:1 com-
plex formation based on Job’s plot analyses.
2
+
.
.
.
associated with the disorder of Zn metabolism. Therefore,
Keywords Chemosensor Zn(II) ion Schiff base
2
+
.
.
estimation of Zn is very important in neurobiology. How-
Fluorescence Selectivity Association constant
2
+
ever, there is a great need for developing Zn selective
2
+
sensors that can distinguish Zn from other transition metal
2
+
ions especially Cd , because they have very similar chemical
properties often respond together with similar spectral
changes.
Introduction
In recent years, more and more attentions have been drawn to
devise ingenious fluorescent sensors capable of selective rec-
ognition and effectively detecting the presence of alkali, alka-
line earth and transition metal ions because of their importance
in biological systems as well as to environmental concerned
The nitrogen atom of azomethine C=N double bond in
Schiff base also exhibits a strong affinity for transition metal
ions. Therefore, the Schiff base are known to be good ligand
for metal ions [10, 11] and used to develop chemosensors. A
number of the Schiff base metal complexes have antitumor
properties [12], antioxidative activities [13], and attractive
electronic and photophysical properties [14]. In addition,
Schiff base derivatives incorporating a fluorescent moiety
are appealing tools for optical sensing of metal ions.
Tetradentate ligands such as salen- or pyridine-type symmet-
rical Schiff bases are capable of forming complexes with
certain metal ions which can exhibit unusual coordination,
high thermodynamic stability, good fluorescent properties and
biological activities [15]. However, controllable synthesis of
these compounds is a great challenge because many factors
[1, 2]. However, among different chemosensors, fluorescence-
based ones present many advantages as fluorescence measure-
ments are usually very sensitive, low cost, easily performed
and versatile, offering subnanometer spatial resolution with
:
:
:
N. Roy H. A. R. Pramanik P. C. Paul S. T. Singh (*)
Department of Chemistry, Assam University,
Silchar, Assam 788 011, India
e-mail: takhelsingh@gmail.com
S. T. Singh
e-mail: singhsanjoy2002@yahoo.co.in

J Fluoresc
may affect their self-assembly, in which the coordination
geometry of the complex depends upon the chemical
structure of ligand chosen, the coordination geometry pre-
ferred by metal, pH dependent, metal-to-ligand ratio, reac-
tion temperature, solvent system [16]. Nowadays, design-
ing and synthesis of fluorescent sensors with high selec-
tivity and sensitivity to metal ions is an important and
vibrant field. Having much availability of commercial
metal ions sensors, chemists still continue endeavouring
to design new ones to improve their sensitivity, selectivity
and reliability in order to satisfy various needs that are
due to the wide existence of metal ions in organisms and
its extensive significance. Many excellent metal ions sen-
sors has been contributed significantly based on quinoline,
anthracene, BODIPY, benzoxazole and fluorescein as
fluorophore [17, 18], but some of the reported synthesis
methods are always too complicated. In this paper, we have
Materials and Experimental Method
Materials
Salicylaldehyde and 1, 2 – diaminocyclohexane were
obtained from Aldrich Chemical Company. All the spec-
troscopic grade solvents used were obtained from Sisco
Research Laboratory (SRL) Pvt. Ltd and in some cases,
from Aldrich Chemical Company. Chemical reagents
were obtained from Lancaster as well as S.D. Fine
Chemical Ltd and used without further purification.
All experimental solutions of varying pH were made
with buffer (Qualigen). The analytical grade type – II
water used in the measurements was obtained from
Elix10 water purification system (Millipore India Pvt.
Ltd.). All experiments were carried out at room temper-
ature (293 K) unless mentioned otherwise.
designed and synthesized a Schiff-base fluorescent probe – N,
/
N -bis(salicylidene) trans 1, 2 – diaminocyclohexane (H L)
2
/
by one step condensation of salicylaldehyde and 1, 2 –
Synthesis and Characterization of N, N -bis(salicylidene)
diaminocyclohexane in absolute ethanol (Scheme 1) and char-
acterized by FT-IR, H and C NMR and elemental analysis.
Trans 1,2 – Diaminocyclohexane (H L)
2
1
13
Photophysical behaviors of H L were studied in different
A portion of salicylaldehyde (1.22 g, 10 mmol) and 1, 2 –
diaminocyclohexane (0.58 g, 5 mmol) were separately dis-
solved in absolute ethanol and combined together to get
yellow colour solution. The solution was stirred under reflux
conditions for 3 h in presence of 2–3 drops of acetic acid, and
precipitate was filtrated, washed with cold absolute ethanol
three times, then recrystallized with ethanol/chloroform (1/3,
2
homogeneous solvents and in presence of different metal ions,
focussing the attention on their absorption and emission prop-
erties using steady-state absorption and fluorescence
spectroscopy.
2
+
Till now, a variety of fluorescent sensors for Zn
have been developed with successful applications
19–26]. However, many of these sensors suffer from
interference of some heavy metal ions and transition
[
v/v) to get yellow microcrystal (H L) in 78 % yield. m.p.
2
−1
147 °C; FT-IR (νmax, cm , KBr): 3410(νOH), 2985(ν (C-
as
2
+
metal ions. In addition, some available Zn sensors
H), 2862(ν (C-H),), 1614(νC=N), 1562(νC=C), 1277(ν(C-
s
2
+
2+
1
have difficulty in distinguishing Zn and Cd , because
they have very similar chemical properties often respond
together with similar spectral changes including change
in fluorescence intensity and the wavelength shift. Here-
in, we have observed its prominent fluorescence en-
O)), 1151(ν(C-N)), 758(δ(C-H)); H NMR (400 MHz,
CDCl , TMS, δ, ppm): 13.1 (br s, H-5, H-5 ), 8.6 (s, H-6,
H-6 ), 7.2–7.5 (m, H-1, H-1 , H-3, H-3 , H-7, H-7 , H-8, H-8 ),
7.0 (d, J=8.2 Hz, H-4, H-4 ), 6.8 (t, J=4.0Hz, H-2, H-2 );
/
3
/
/
/
/
/
/
/
13
C
NMR (400 MHz, CDCl , TMS, δ, ppm): 23.81, 28.95, 72.50,
3
2
+
hancement in the presence of Zn , while there was
no enhancement in presence of other metal ions. In
115.89, 122.99, 131.12, 159.86, 162.15; Anal. Calcd. for
C H N O (316.20): C, 75.96 %; H, 5.06 %; N, 8.85 %.
2
0 16 2 2
2
+
2+
particular, it was able to distinguish Zn from Cd .
Found: C, 76.01 %; H, 5.00 %; N, 8.92 %.
Scheme 1 Reaction scheme for
/
the synthesis of N, N -
bis(salicylidene) trans 1,2-
NH
NH
2
2
diaminocyclohexane (H
2
L)
CHO
EtOH
N
N
+
refluxed for 3 h
OH
OH
HO
2:1
H2L

J Fluoresc
Physical Measurements
Elemental analyses were carried out using PE2400 elemental
analyser. The infra-red spectra were recorded on a
PerkinElmer L 120-000A spectrometer with KBr pellets in
−1
1
13
the range 4000–400 cm . H and C Nuclear magnetic
resonance spectra were recorded on Bruker DPX- 400 MHz
spectrometer and chemical shifts are expressed in ppm using
tetramethylsilane as internal standard.
Spectrophotometric and Spectrofluorimetric Measurements
2
50
300
350
Wavelength (nm)
Fig. 1 Steady state absorption (open circles), fluorescence emission
solid squares, λexc=350 nm) and fluorescence excitation (solid triangles,
emm=430 nm) spectra of ligand (H L) in absolute ethanol
400
450
500
Steady-state absorption spectra were recorded on a Shimadzu
UV – 1601PC absorption spectrophotometer. Fluorescence
spectra were obtained in a PerkinElmer LS 45 spectrofluorim-
eter and all the spectra were corrected for the instrument
response function. Quartz cuvettes of 10 mm optical path
length received from PerkinElmer, USA (part no.
B0831009) and Hellma, Germany (type 111-QS) were used
for measuring absorption and fluorescence spectra, respective-
ly. In both fluorescence emission and excitation spectra mea-
surements, 5 nm bandpass was used in the excitation and
emission side. The linearity of the fluorescence emission
versus concentration was checked in the concentration range
(
λ
2
double bond. These absorption spectra were coincident with
the excitation spectra. The fluorescence emission spectra for
H L shows main peak at 430 nm with a shoulder at around
2
3
75 nm upon excitation at 350 nm. The steady state spectral
properties of H L in pure solvent were tabulated in Table 1. It
is interesting to note that emission spectra does not show any
appreciable change in different solvent except very prominent
peak at around 375 nm and little broadening in the less polar
protic solvents like acetonitrile, 1,4-dioxane and toluene. The
fluorescence quantum yield (φ ) values are fairly low and
could be explained by nonradiative decay transitions to the
low-lying nπ states of the nitrogen atoms [28]. The origin of
these fluorescence bands can be attributed to specific species
undergoing different excited state photophysics as mentioned
below. Due to the present of azomethine nitrogen atom in the
free Schiff base ligand, the phenolic OH group make it possi-
ble for intramolecularly hydrogen bonded OH group. So, this
intramolecular hydrogen bond between the azomethine and
phenolic hydrogen can undergo intramolecular proton trans-
location upon excitation. So, the largely Stokes shifted fluo-
rescence is believed to be originated from intramolecular
hydrogen bonded structure and due to isomerisation of the
C=N double bond which usually resulted in weak fluores-
cence emission. The fluorescence in the blue side of the
spectrum (~375 nm) can be attributed due to the spontaneous
emission.
2
used (10− to 10 M). Fluorescence quantum yields (φ ) were
5
−6
f
calculated by comparing the total fluorescence intensity under
the whole fluorescence spectral range with that of a standard
f
(
φ =0.546, quinine sulfate in 1 M sulfuric acid) using the
f
following equation as described before [27].
*
s
ꢀ
ꢁ2
i
−A
ni
ns
F 1−10
i
f
s
f
ϕ ¼ ϕ :
:
:
ð1Þ
s
Ai
1
−10−
F
where F is the total fluorescence intensity under whole fluo-
i
s
rescence spectral curve, A and A is the optical density of the
i
sample and standard, respectively and η is the refractive index
of the solvent at 293 K.
Results and Discussion
Steady-State Spectral Properties of H L in Pure Solvents
2
The absorption, emission and excitation spectra of H L in
Metal Ions Titration
2
absolute ethanol solution at 293 K are reported in Fig. 1.
The absorption spectra of H L were obtained in different
In order to explore metal ion binding and sensing ability of
2
homogeneous solvents giving two different peak positions at
H L towards different metal ions, several titration followed by
2
2
78 nm and 350 nm. The large value of molar extinction
fluorescence emission were performed in ethanol solution at
room temperature. Figure 2 demonstrates the changes in the
5
−1
−1
coefficient (ε ~1.2×10 M cm ) indicates the absorption
to be originated due to ππ transition of the benzene ring.
*
fluorescence emission spectra of H L (2 μM) in ethanol in
2
However, the band at 278 nm is attributed to azomethine C=N
presence of different metal ions. The fluorescence maxima

J Fluoresc
Ta b l e 1 Steady state spectral
properties of ligand (H L) in dif-
ferent homogeneous solvents
a
b
c
−1
d
−3
Solvents
Ethanol
λ
abs (nm)
λfl (nm)
ΔνSS (cm )
φ
F
(10 )
2
268
32
273
50
273
54
276
50
274
56
295
52
273
62
278
55
267
50
268
52
375
432
1742
6972
3.6
2.8
3.4
2.6
2.4
3.2
2.8
5.6
4.4
4.8
3
Methanol
376
430
1904
5315
3
Acetonitrile
388
434
2475
5207
3
1
,4-Dioxane
376
428
1975
5206
3
N,N-dimethylformamide
Toluene
370
415
1062
3993
3
376
428
1813
5044
3
Dimethylsulfoxide
Chloroform
380
414
1308
3469
3
a
Absorption maxima
372
425
440
3792
b
Fluorescence maxima
3
c
Stokes shift (ΔνSS) calculated
Ethyl acetate
376
428
1975
5206
based on low-energy absorption
band
3
Tetrahydrofuran
375
424
1742
4824
d
Fluorescence quantum yield
3
F
(φ ) error limit ±5 %
peak of H L (2 μM) appeared at 430 nm upon excitation at
reported in the histogram (Fig. 3) which clearly indicates high
2
2
+
2+
2+
3
50 nm. As soon as Zn (30 μM) were added at room
selectivity of Zn . Importantly, H L can distinguish Zn
2
2
+
2+
2+
temperature, fluorescence maxima give red shift at 445 nm
with large increased in fluorescence intensity (Fig. 2). How-
from Cd , whereas the discrimination of Zn from Cd is
well known to be a major obstacle.
+
ever, in presence of other different metal ions like alkali (Na ,
Furthermore, a metal ion selectivity study was performed
+
2+
2+
2+
K ), alkaline earth (Ca , Mg , Sr ) and transition metal
for H L under same experimental conditions. The fluores-
2
2
+
2+
2+
2+
2+
2+
3+
ions (Cd , Ni , Co , Cu , Mn , Ba and Fe ) H L
cence intensity of H L (~2 μM) was unaffected upon addition
2
2
showed either no change in the fluorescence peak and very
negligible amount of changes occurs in the fluorescence in-
tensity (Fig. 2). So, there was no appreciable change in the
of an excess amount of different metal ions. Further, tolerance
of the fluorescence intensity due to Zn (30 μM) in the
presence of 50 times an excess of various metal ions like
2
+
fluorescence emission behavior of H L with other metal ions.
2
Again, fluorescence intensity profile changes were also
1
1
1
75
40
05
2
1
1
00
50
00
2
+
Ligand + Zn ion
7
0
5
0
3
5
0
Free ligand
0
400
450
500
550
600
2
Fig. 3 Fluorescence intensity profile changes of H L at λexc=350 nm in
absence and presence of 30 μM concentration of various metal ions at
Wavelength /nm
Fig. 2 Fluorescence spectra of H
concentration of different metal ions at room temperature. Excitation was
2
L in absence and presence of 30 μM
room temperature. Fluorescence intensity changes that occurs upon sub-
sequent addition of Zn (30 μM) in presence of different metal ions were
2+
done at λexc=350 nm
also reported

J Fluoresc
+
+
2+
2+
2+
2+
2+
2+
2+
2+
Na , K , Cd , Ca , Mg , Sr , Ni , Ba , Co , Cu ,
consequently occurrence of the strong complexation with
2
+
3+
Mn and Fe has been successfully examined and verified as
shown in Fig. 3, through competition experiments, with no
significance changes on the fluorescence intensities. In
contrast, the fluorescence emission intensity was found
out to be reduced when it was mixed to the solution
H L, resulting in decreased non-radiative decay of the excited
2
state and increased radiative decay.
However, this effect was not observed in presence of other
+
+
2+
2+
2+
2+
2+
2+
metal ions like Na , K , Cd , Ca , Mg , Sr , Ni , Ba ,
2
+
2+
2+
3+
Co , Cu , Mn and Fe even with very high concentration
(say~60 μM). The fluorescence quantum yields were also
calculated both in the free ligand as well as in presence of
2
+
2+
containing transition metal ions like Co and Ni
which can be explain due to the fact that transition
metal ions containing unoccupied molecular d-orbital
quench the fluorescence of fluorophore [29]. So, all
competitive metal ions had no obvious interference with
the detection of the Zn , which also indicates that,
H L – Zn
2
2
+
Zn . Here, the fluorescence quantum yield increases drasti-
−3
−1
cally from 3.6×10 for H L compared to 1.8×10 for H L
2
2
2
+
– Zn complex.
2
+
The possibility of using fluorescent chemosensor H L for
2
2
+
2+
complex was hardly affected by these
determining Zn in various homogeneous solvents like N,N-
coexistent metal ions, indicating high selectivity of
H L towards Zn .
2
dimethylformamide (DMF), dimethylsulfoxide (DMSO),
methanol, ethanol, ethylacetate, tetrahydrofuran (THF), ace-
tonitrile (ACN), toluene, chloroform and 1, 4-dioxane were
also investigated and reported in Fig. 5. Here, the fluorescence
2
+
2
+
Influence of Zn Concentration
2
+
intensity enhanced largely in addition of Zn to certain sol-
vents like THF, acetonitrile, ethylacetate, toluene and chloro-
form. We also knew that THF, acetonitrile, toluene and chlo-
roform are hard to be coordinated for the lack of easy-
coordinated atom which indicates that no solvent molecule
contributes to the coordination. However this experimental
data may be due to inherent complicated properties of the
solvent based on their different polarity.
2
+
In an attempt to evaluate the influence of Zn concentration,
the fluorescence properties of H L were studied in ethanol
solution with excitation at 350 nm and the fluorescence emis-
sion intensity was monitored at 430 nm. Here, the fluores-
2
cence intensity of H L was very weak which can be explain
2
due to isomerization of C=N double bond. The C=N isomer-
ization is the predominant decay process of the excited states
for compounds with an unbridged C=N structure so that those
compounds were very weak fluorescent or nonfluorescent.
2
+
However, with gradual addition of Zn concentration (satu-
Stoichiometry of Complexation
rated at~45 μM) to that of H L, the fluorescence intensity
2
2
+
yield increases with a red-shift of emission peak from 430 nm
to 445 nm, which can be explain due to prevention of isom-
erization by metal ions binding (Fig. 4). The enhancement of
For determination of stoichiometry between H
L and Zn ,
2
Job’s plot analyses were used. The method is that keeping
total concentration of H L and Zn at 10.0 mM and changing
2
the molar ratio of Zn from 0.1 to 0.9. From Fig. 6 when
molar fraction of Zn was 0.5, the fluorescence maxima at
2
+
2
+
2+
fluorescence was attributed to the introduction of Zn and
2
+
200
150
100
0
0
0
.07
.06
.05
200
150
100
4
5 M
0.04
0
0
0
.03
.02
.01
K = 3.7 x 104M-1
2
R
= 0.998
0
M
0.1
0.2
0.3
Zn2+
0.4
0.5
1
/
[
]
50
50
0
0
400
450
500
550
600
1
2
3
4
5
6
7
8
9
10
Wavelength /nm
Fig. 4 Variation of fluorescence intensity of H L against concentration
of Zn . The concentrations of Zn (μM) are: 0.0 (1), 2.0 (2), 5.0 (3),
0.0 (4), 18.0 (5), 24.0 (6), 30.0 (7), 35.0 (8), 40.0 (9), 45.0 (10). Inset
shows the double reciprocal plot for 1:1 complex formation
Different solvents
Fig. 5 Fluorescence intensity changes of H L in presence of Zn
(30 μM) in different solvents: 1 – methanol; 2 – ethanol; 3 – DMF; 4 –
THF; 5 – 1, 4 – dioxane; 6 – ACN; 7 – ethylacetate; 8 – chloroform; 9 –
DMSO; 10 – toluene at room temperature, respectively
2+
2
2
2+
2+
1

J Fluoresc
2
+
and Zn . The apparent K value obtained from the BH
plot can be used as an initial guess in a non-linear
regression (NLR) analysis to fit the fluorescence data
directly using the following equation [31].
75
60
45
30
15
F0 þ F1K½Zn þŠ
2
F ¼
ð3Þ
2
þ
1
þ K½Zn Š
where, F =aF , and a is the ratio of fluorescence quantum
intensities of H L and H L – Zn . The iterative NLR anal-
1
0
2
+
2
2
0
.0
0.2
0.4
0.6
0.8
+ [Ligand]
2
L with Zn in ethanol solution
1.0
ysis based on Lavenberg – Marquardt algorithm was done
2
+
2+
using ORIGIN 8.0 (Microcal Inc.) software (Fig. 7). The
[
Zn ] / [Zn ]
2
+
association constant of H L with Zn in ethanol solution
2+
2
Fig. 6 Job’s plot of H
4
−1
was calculated to be 3.7×10 M . Moreover, this further
corroborated 1:1 complex formation based on Job’s plot
analyses.
4
45 nm got to maximum, indicating that forming a 1:1 com-
plex between H L and Zn .
2
+
2
The stoichiometric ratio and apparent binding constant of
ligand with Zn was also determined using Benesi –
2
+
Effect of pH
Hildebrand relation as follows [30]
We also measured the fluorescence intensity of H L over a
2
2
+
wide range of pH in absence and presence of Zn . Over a
wide range of pH, there was no obvious change in the fluo-
rescence emission intensity of the ligand alone which indi-
cates insensitivity to pH as shown in Fig. 8. However, in
1
1
1
1
¼
þ
Â
ð2Þ
F−F0
Fα−F0 KðFα−F0Þ ½Zn2þŠ
2
+
presence of Zn , H L had a strong pH dependent even
2
2
+
where, F and F are the fluorescence intensities in the absence
though it had a very weak fluorescence response to Zn in
the acidic environment because of protonation of the phenolic
hydroxyl [32] leading to a weak coordination ability of Zn
[33]. However, satisfactory Zn sensing abilities were exhib-
ited when pH was increased from 8.0 to 11.5 (Fig. 8). Thus,
H L indicates good fluorescence sensing ability to Zn over
0
2
+
and presence of Zn respectively. Fα is the fluores-
2
+
cence intensity in the presence of excess amount of
2
+
2+
Zn . Therefore, for 1:1 complex formation, the double
2
+
reciprocal plot of 1/(F−F ) against 1/[Zn ] should give
0
2
+
a straight line; from the slope and intercept of which,
the equilibrium constant (K) can be calculated. Inset of
Fig. 4 shows the representative linear fitting using
2
a wide range of pH=8.0–11.5, giving almost constant emis-
sion intensity in the physiological conditions. These results
Eq. (2) and confirm 1:1 stoichiometry between H L
indicate that H L can be employed as a selective fluorescent
2
2
1
20
00
150
125
100
1
8
0
75
50
25
60
40
20
0
10
20
30
Zn ] / M
Fig. 7 Variation of fluorescence intensity of H
40
50
4
6
8
10
12
14
2+
[
pH
2
L at λexc=350 nm against
concentration of Zn to determine the association constant
2
Fig. 8 Effect of pH on the fluorescence intensity of H L in absence (○)
and presence (●) of Zn
2
+
2+

J Fluoresc
2
+
probe to recognize and distinguish Zn in presence of other
different metal ions.
conformational rigidity for a 3D supramolecular network, as
well as hydrogen bonds and π….π packing interactions [37].
Effect of Different Substituent on Fluorescence Intensity
of H2L
Conclusions
/
Metal-ligand complexes have been studied showing both the
affect on the fluorescence emission wavelength and intensity
of the ligand through metal coordination [34]. Moreover, the
luminescent properties of Zn(II) complexes were reported to
be determined by the organic ligand because of the electronic
A Schiff-base fluorescent compound–N,N -bis(salicylidene)
trans 1, 2 – diaminocyclohexane (H L) was synthesized and
evaluated as a chemoselective Zn sensor. Addition of Zn
to ethanol solution of H L resulted in a red-shift with a
pronounced enhancement in the fluorescence intensity while
many other common alkali, alkaline earth and transition metal
ions failed to induce response or minimal spectral changes.
Most importantly, H L could differentiate Zn from Cd .
Fluorescence studies on free Schiff base ligand (H L) and
2
2
+
2+
2
2
+
10
0
configuration of Zn (3d 4s ) where the d-shell are
completely filled which causes lack of intrinsic spectroscopic
or magnetic signal [35]. Herein, the fluorescence behaviors of
2
+
2+
2
2
+
H L and complexes with different substituent of Zn were
2
2
2
+
studied in solution phase at room temperature. Here, the
fluorescence emission spectra of complexes are very similar
H L – Zn complex reveal that the quantum yield strongly
2
increases upon coordination. The stoichiometric ratio and
association constant were evaluated using Benesi –
Hildebrand relation giving 1:1 stoichiometry. This further
corroborated 1:1 complex formation based on Job’s plot anal-
with H L except for the fluorescence emission intensity and
2
maximum peak position, indicating that the fluorescence of
complexes is L-based emission. Meanwhile, the fluorescence
emission for the complexes were red-shifted compared to H L
yses. Further, H L exhibits good fluorescence sensing ability
to Zn over a wide range of pH=8.0–11.5, giving almost
2
2
2
+
showing maximum emission wavelength at 430 nm for H L
2
(
(
1), whereas 445 nm, 447 nm and 448 nm for the complexes
2–4), respectively. Comparing the emission spectra of the
constant emission intensity in the physiological conditions.
2
+
The incorporation of Zn effectively increases the conforma-
tional rigidity of the ligand and enhanced fluorescence inten-
sities of the complex, which shows that it is good candidate
material in photochemical applications of these complexes.
corresponding H L varying degrees of red shifts has been
observed in complexes, which are considered to mainly arise
from the coordination of three Zn(II) centres to the ligand. The
2
2+
incorporation of Zn effectively increases the conformational
rigidity of the ligand (1) and enhanced fluorescence intensities
of all three complexes (2–4), which shows that it is good
candidate material in photochemical applications of these
complexes [36]. Moreover, from Fig. 9, the different in fluo-
rescence intensities of the complexes with different substitu-
ent’s like NO , Cl and CH COO, can be explain due to bigger
Acknowledgments Financial support through UGC-BSR Research
Start-Up-Grant project No.F.20-1/2012 (BSR)/20-1(2) (2012(BSR) from
University Grants Commission (UGC) and DST Fast-Track Research
Grant project No. SB/FT/CS-064/2012 from Department of Science and
Technology (DST), Government of India were gratefully acknowledged
by Dr. T. Sanjoy Singh.
2
3
References
1
1
1
50
25
00
4
3
2
+
1
2
3
. Krämer R (1998) Fluorescent chemosensors for Cu ions: fast,
selective and highly sensitive. Angew Chem Int Ed 37:772–773
. Uauy R, Olivares M, Gonzalez M (1998) Essentiality of copper in
humans. Am J Clin Nutr 67:952–959
. De Silva AP, Gunaratne HQN, Gunnlaugsson T, Huxley AJM,
McCoy CP, Rademacher JT, Rice TE (1997) Signaling recognition
events with fluorescent sensors and switches. Chem Rev 97:1515–
1566
2
7
5
2
5
0
5
0
1
4
5
6
. Czarnik AW (1992) Fluorescent chemosensors for ion and molecule
recognition, 1st edn. ACS Washington, Washington DC
. Vallee BL, Falchuk KH (1993) The biochemical basis of zinc phys-
iology. Physiol Rev 73:79–118
. De Silva JJRF, Williams RJP (2001) The biological chemistry of
elements: The inorganic chemistry of life, 2nd edn. Oxford
University Press, New York
4
00
450
500
Wavelength /nm
Fig. 9 Fluorescence emission spectra of H L in complexes with different
metal ions substituents; ligand (1 – free ion) and complexes (2 –
Zn(CH COO) , 3 – ZnCl and 4 – Zn(NO ) in the solution phase at
room temperature, respectively temperature, respectively
550
600
2
7. Bush AI, Pettingell WH, Multhaup G, Paradis M, Vonsattel JP,
Gusella JF, Beyreuther K, Masters CL, Tanzi RE (1994) Rapid
induction of Alzheimer A beta amyloid formation by zinc. Science
265:1464–1467
3
2
2
2 2
)

J Fluoresc
8
9
. Koh JY, Suh SW, Gwag BJ, He YY, Hsu CY, Choi DW (1996) The
role of zinc in selective neuronal death after transient global cerebral
ischemia. Science 272:1013–1016
. Walker CF, Black RE (2004) Zinc and the risk for infectious disease.
Annu Rev Nutr 24:255–275
23. Xu Z, Baek KH, Kim HN, Cui J, Qian X, Spring DR, Shin I, Yoon J
2+
2+
(2010) Zn -triggered amide tautomerization produces a highly Zn -
selective, cell-permeable and ratiometric fluorescent sensor. J Am
Chem Soc 132:601–610
2+
24. Xu Z, Yoon J, Spring DR (2010) Fluorescent chemosensors for Zn .
Chem Soc Rev 391:1996–2006
1
0. Salmon L, Thuéry P, Rivière E, Ephritikhine M (2006) Synthesis,
structure and magnetic behavior of a series of trinuclear schiff base
complexes of 5f (UIV, ThIV) and 3d (CuII, ZnII) ions. Inorg Chem
25. Choi JY, Kim D, Yoon J (2013) A highly selective “turn-on” fluo-
rescent chemosensor based on hydroxy pyrene–hydrazone derivative
2+
4
5:83–93
for Zn . Dyes Pigments 96:176–179
1
1. Epstein DM, Choudhary S, Churchill MR, Keil KM, Eliseev AV,
Morrow JR (2001) Chloroform-soluble schiff-base Zn(II) or Cd(II)
complexes from a dynamic combinatorial library. Inorg Chem 40:
26. Guo Z, Kim GH, Shin I, Yoon J (2012) A cyanine-based fluorescent
sensor for detecting endogenous zinc ions in live cells and organisms.
Biomaterials 33:7818–7827
1
591–1596
27. Singh TS, Mitra S, Chandra AK, Tamai N, Kar S (2008) A combined
experimental and theoretical study on photoinduced intramolecular
charge transfer in trans-ethyl p-(dimethylamino)cinamate. J
Photochem Photobiol A Chem 197:295–305
28. Rettig W, Lapouyade R, Lakowicz JR (2002) Topics in fluorescence
spectroscopy: Probe design and chemical sensing, 3rd edn. Plenum
Press, New York, pp 109–149
29. Gunnlaugsson T, Lee TC, Parkesh R (2004) Highly selective fluo-
rescent chemosensors for cadmium in water. Tetrahedron 60:11239–
11249
30. Benesi HA, Hildebrand JH (1949) A spectrophotometric investiga-
tion of the interaction of iodine with aromatic hydrocarbon. J Am
Chem Soc 71:2703–2707
1
2. Da Silveira VC, Luz JS, Oliveira CC, Graziani I, Ciriolo MR, Ferreira
AM (2008) Double-strand DNA cleavage induced by oxindole-schiff
base copper(II) complexes with potential antitumor activity. J Inorg
Biochem 102:1090–1103
3. Li Y, Yang ZY (2009) DNA binding affinity and antioxidative
activity of copper (II) and zinc (II) complexes with a novel hesperetin
Schiff base ligand. Inorg Chim Acta 362:4823–4831
4. Kasselouri S, Garoufis A, Katehanakis A, Kalkanis G, Perlepes SP,
Hadjiliadis N (1993) 1:1 metal complexes of 2-(2′-pyridyl)
quinoxaline, a ligand unexpectedly formed by the reaction between
2
2
1
1
-acetylpyridine and 1,2-phenylenediamine. Inorg Chim Acta 207:
55–258
1
1
5. Morris GA, Zhou H, Stern CL, Nguyen ST (2001) A general high-
yield route to Bis(salicylaldimine) Zinc(II) complexes: application to
the synthesis of pyridine-modified salen-type zinc(II) complexes.
Inorg Chem 40:3222–3227
6. Chen SC, Zhang ZH, Huang KL, Chen Q, He MY, Cui JA, Li C, Liu
Q, Du M (2008) Solvent-controlled assembly of manganese(II)
tetrachloroterephthalates with 1D chain, 2D layer and 3D coordina-
tion architectures. Cryst Growth Des 8:3437–3445
31. Mitra S, Das R, Mukherjee S (1998) Intramolecular proton transfer in
inclusion complexes of cyclodextrins: role of water and highly polar
nonaqueous media. J Phys Chem B 102:3730–3735
32. Roy P, Dhara K, Manassero M, Ratha J, Banerjee P (2007) Selective
fluorescence zinc ion sensing and binding behavior of 4-Methyl-2,6-
bis(((phenylmethyl)imino)methyl) phenol: biological application.
Inorg Chem 46:6405–6412
33. Liu Y, Zhang N, Chen Y, Wang L (2007) Fluorescence sensing and
binding behavior of aminobenzenesulfonamido- quinolino-β-
1
1
1
2
2
7. Pearce DA, Jotterand N, Carrico IS, Imperiali B (2001) Derivatives of
2
+
8
-Hydroxy-2-methylquinoline are powerful prototypes for zinc sen-
cyclodextrin to Zn . Org Lett 9:315–318
sors in biological systems. J Am Chem Soc 123:5160–5161
8. Zhang Y, Guo XF, Si WX, Jia LH, Qian XH (2008) Ratiometric and
water-soluble fluorescent zinc sensor of carboxamidoquinoline with
an alkoxyethylamino chain as receptor. Org Lett 10:473–476
9. Wong BA, Friedle S, Lippard SJ (2009) Solution and fluorescence
properties of symmetric dipicolylamine-containing dichlorofluorescein-
34. Wu G, Wang XF, Okamura T, Sun WY, Ueyama N (2006) Syntheses,
structures and photoluminescence properties of metal(II) halide com-
plexes with pyridine-containing flexible tripodal ligands. Inorg Chem
45:8523–8532
35. Kotova OV, Eliseeva SV, Averjushkin AS, Lepnev LS, Vaschenko
AA, Rogachev AY, Vitukhnovskii AG, Kuzmina NP (2008) Zinc(II)
complexes with Schiff bases derived from ethylenediamine and
salicylaldehyde: the synthesis and photoluminescent properties.
Russ Chem Bull Int Ed 57:1880–1889
36. Majumder A, Rosair GM, Mallick A, Chattopadhyay N, Mitra S
(2006) Synthesis, structures and fluorescence of nickel, zinc and
cadmium complexes with the N, N, O-tridentate Schiff base N-2-
pyridylmethylidene-2-hydroxy-phenylamine. Polyhedron 25:1753–
1762
2+
based Zn sensors. J Am Chem Soc 131:7142–7152
0. Tomat E, Nolan EM, Jaworski J, Lippard SJ (2008) Organelle-
specific zinc detection using zinpyr-labeled fusion proteins in live
cells. J Am Chem Soc 130:15776–15777
1. Hanaoka K, Kikuchi K, Kojima H, Urano Y, Nagano T (2004)
Development of a zinc ion-selective luminescent lanthanide
chemosensor for biological applications. J Am Chem Soc 126:
1
2470–12476
2
2. Qian F, Zhang C, Zhang Y, He W, Gao X, Hu P, Guo Z (2009) Visible
37. Kang B, Weng L, Liu H, Wu D, Huang L, Lu C, Cai J, Chen X, Lu JX
(1990) Syntheses, structures and properties of vanadium, cobalt, and
nickel compounds with 2-mercaptophenol. Inorg Chem 29:4873–
4877
2+
light excitable Zn fluorescent sensor derived from an intramolecu-
lar charge transfer fluorophore and its in vitro and in vivo application.
J Am Chem Soc 131:1460–1468