A new quinoline-based chemosensor in ratiometric sensing of Hg2+ ions

Article abstract of DOI:10.1080/10610278.2012.733818

A new quinoline-based chemosensor 1 has been designed and synthesised. Its metal ion-binding properties have been documented in organic and aqueous organic solvents. While chemosensor 1 recognises Hg²⁺ ions (K_a = 2.15 × 10⁴ M^-1) by exhibiting ratiometric change in emission in CHCl₃/CH₃OH (1:1, v/v), under similar condition both Zn²⁺ and Cd²⁺ ions are sensed by significant non-ratiometric increase in emission with measurable red shift. In DMSO/H₂O (5:95, v/v), the sensor 1 exhibits a greater selectivity towards Hg²⁺ ions (K_a = 9.20 × 10³ M ^-1) over the other metal ions examined.

Full text of DOI:10.1080/10610278.2012.733818

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A new quinoline-based chemosensor in ratiometric
2+
sensing of Hg ions
a
a
Kumaresh Ghosh & Debojyoti Tarafdar
a
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, India
Published online: 18 Oct 2012.
To cite this article: Kumaresh Ghosh & Debojyoti Tarafdar (2013): A new quinoline-based chemosensor in ratiometric sensing
2+
of Hg ions, Supramolecular Chemistry, 25:3, 127-132
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Supramolecular Chemistry, 2013
Vol. 25, No. 3, 127–132, http://dx.doi.org/10.1080/10610278.2012.733818
21
A new quinoline-based chemosensor in ratiometric sensing of Hg ions
Kumaresh Ghosh* and Debojyoti Tarafdar
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, India
(Received 4 August 2012; final version received 20 September 2012)
A new quinoline-based chemosensor 1 has been designed and synthesised. Its metal ion-binding properties have been
4
2
þ
21
documented in organic and aqueous organic solvents. While chemosensor 1 recognises Hg ions (K ¼ 2.15 £ 10 M
)
by exhibiting ratiometric change in emission in CHCl /CH OH (1:1, v/v), under similar condition both Zn and Cd ions
a
2
þ 2þ
3 3
2
are sensed by significant non-ratiometric increase in emission with measurable red shift. In DMSO/H O (5:95, v/v), the
2
þ
3
21
sensor 1 exhibits a greater selectivity towards Hg ions (K ¼ 9.20 £ 10 M ) over the other metal ions examined.
a
Keywords: mercury recognition; zinc and cadmium sensing; quinoline-based receptor; ratiometric fluorescence;
ratiometric absorbance
2
þ
2þ
ions increased the
emission of 1 greatly in non-ratiometric fashion in
Design and synthesis of simple molecular architectures
that can detect and sense the heavy transition metal ions
are of particular interest in the field of supramolecular
examined, only Zn
and Cd
CHCl
from Hg ions in this study. Furthermore, in DMSO/H
/CH
2þ
OH (1:1, v/v), and thus, they are differentiated
3
3
2
þ
chemistry (1). Of the different metal ions, Hg gains
2
O
(5:95, v/v), the receptor 1 is noted to be only responsive to
special importance due to its biological and environmental
2
Hg
þ
2
þ
ion as observed from a greater quenching of
significances (2). Mercury ion (Hg ) is considered to be
dangerous as it can accumulate in the human body and
affects a wide variety of diseases even in a low
concentration, such as prenatal brain damage, serious
cognitive disorders and Minamata disease (2). Therefore,
emission over the other metal ions studied. In the design 1,
the role of quinoline ring in binding of metal ions was
established by considering the model compound 2.
2
þ
the selective detection of Hg ions in both organic and
semi-aqueous environment is of paramount interest. To
date, a number of receptor modules have been reported,
O
O
N
2
þ
N
which recognise Hg ions by colour and fluorescence
changes (3, 4). Of the different fluorescent probes, the
2
þ
ratiometric probes for the detection of Hg ions are
considerably important, and they are relatively less in
O
O
number (5). Moreover, nearly all ratiometric chemosensors
þ
2þ
X
show interference with Ag and Cu and low sensitivity
5). The ratiometric chemosensors provide advantages
X
(
over the conventional monitoring of fluorescence intensity
at a single wavelength. A dual emission system can reduce
the measurement errors because of the factors such as
photo transformation, receptor concentrations and environ-
mental effects (6).
1
2
: X= N,
: X= CH
Synthesis of 1 was achieved by the reaction sequences
as depicted in Scheme 1. Piperazine was first allowed to
During the course of our work on the fabrication of
receptor modules of different architectures for both cations
and anions (7), we report in this study a simple easy-to-
make structure 1 that is found to be capable of sensing
stir with chloroacetyl chloride in the presence of Et N in
3
dry CH Cl to afford the diamide 3, which on subsequent
2
2
reflux with 8-hydroxyquinoline and anhydrous K CO in
3
2
2
þ
Hg ions in organic solvent (CHCl :CH OH ¼ 1:1, v/v)
dry CH CN gave the compound 1. Use of 1-naphthol
3
3
3
by exhibiting high ratiometric change in both fluorescence
and absorbance. In contrast, among the other metal ions
instead of 8-hydroxyquinoline under similar condition
gave the compound 2 in good yield. Compounds 1 and 2
*Corresponding author. Email: ghosh_k2003@yahoo.co.in
q 2013 Taylor & Francis

1
28
K. Ghosh and D. Tarafdar
O
O
OH
X
N
N
H
N
Cl
Cl
Chloroacetyl chloride, Et N
3
O
O
N
dry CH Cl , stir,rt,10h
O
N
N
H
2
2
dry CH CN, K CO , reflux, 8h
3 2 3
O
3
X
X
1
2
: X= N,
: X= CH
Scheme 1. Syntheses of receptors 1 and 2.
1
13
were characterised by H NMR, C NMR, mass and FTIR
analyses (Supplementary Information, available online).
The optical sensitivity of 1 towards various metal ions
was investigated spectrophotometrically (fluorescence and
UV titration experiments) by adding standard solutions of
Information, available online). However, the distinct
isosbestic point in Figure 1 reveals the formation of a
new species which may remain in equilibrium with the free
receptor in solution.
2
5
In fluorescence, the excitation of 1 (c ¼ 4.0 £ 10 M)
in CHCl /MeOH (1:1, v/v) at 300 nm gave sharp emission
at 395 nm. This is attributed to the monomer emission of
2
þ
2þ
2þ
2þ
2þ
2þ
the cations such as Hg , Cu , Co , Zn , Cd , Pb ,
3
2
Mg , Ni and Ag (taken as their perchlorate salts) to
þ
2þ
þ
2
þ
the fixed concentration of the chemosensor 1 in CHCl /
3
1
MeOH (1:1, v/v). Additionally, H NMR was studied in
the quinoline motif. Upon gradual addition of Hg ions
23
(c ¼ 1.0 £ 10 M) (up to 10 equiv.) to the solution of 1
2
5
d6-DMSO to understand the nature of interaction in the
binding site.
Initially, we investigated the affinity of chemosensor 1
(c ¼ 4.0 £ 10 M) in CHCl
/MeOH (1:1, v/v), a
3
significant decrease in intensity of the emission at
395 nm was observed along with the appearance of a
new emission centred at 482 nm. This resulted in a
ratiometric change in emission and gave an isoemission
point at 435 nm (Figure 2). The new emission at 482 nm
is presumably attributed to the formation of excimer
between the quinoline moieties that are pulled closely due
2
þ
towards Hg ion by monitoring the absorbance changes
2
3
on addition of Hg(ClO4)2 (c ¼ 1.0 £ 10 M) to the
2
5
solution of compound 1 (c ¼ 4.0 £ 10 M) in CHCl3/
MeOH (1:1, v/v). As can be seen in Figure 1, the
absorption spectrum of 1 is characterised by the presence
of a typical absorption at 300 nm for quinoline. With the
2
þ
to strong chelation of Hg in the cavity of 1. This excimer
emission at 482 nm is in accordance with our previous
report on quinoline-based receptor (8). Figure 3 represents
the probable binding mode. Complexation-induced chair-
to-boat conformational switching introduces two quino-
lines at the close distance for the formation of excimer.
2
þ
incremental addition of Hg ions although the absor-
bance at 300 nm decreased, a new red-shifted band at
,
355 nm developed with an isosbestic point at 330 nm.
Such ratiometric change in absorbance was not noted with
the addition of other metal ions studied (Supplementary
1
00
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
5
0
0
350
400
450
500
550
280
300
320
340
360
380
400
Wavelength (nm)
Wavelength (nm)
2
5
25
Figure 2. Change in emission of 3 (c ¼ 4.0 £ 10 M) in
23
Figure 1. Change in absorption of 1 (c ¼ 4.0 £ 10 M) in
CHCl –MeOH (1:1, v/v) upon addition of 10 equiv. amounts of
Hg(ClO ) (c ¼ 1.0 £ 10 M).
3
CHCl –MeOH (1:1, v/v) upon addition of 10 equiv. of
Hg(ClO ) (c ¼ 1.0 £ 10 M).
4 2
3
2
3
4
2

Supramolecular Chemistry
129
8
00
00
O
O
O
N
N
7
N
O
N
O
N
600
O
O
N
O
5
00
00
1
N
N
H
4
1A
H
2+
300
200
=
Hg
2
Figure 3. Probable mode of binding of Hg ion.
þ
100
Such conformational switching in piperazine-based
0
3
2
chemosensor for selective detection of Cu is known in
þ
50
400
450
500
550
the literature (9).
Wavelength (nm)
2
þ
The association constant (10) between 1 and Hg ion
involving 1:1 stoichiometry, confirmed by Job plot (11)
2
5
Figure 5. Change in emission of 1 (c ¼ 4.0 £ 10 M) in
4
21
CHCl –MeOH (1:1, v/v) upon addition of 10 equiv. of
3
(
Figure 4), was estimated as 2.15 £ 10 M . On the other
2
3
2
þ
23
23
Zn(ClO
4
)
2
(c ¼ 1.0 £ 10 M).
hand, upon gradual addition of Zn (c ¼ 1.0 £ 10 M)
2
þ
(
Figure 5) and Cd (c ¼ 1.0 £ 10 M) (Figure 6) ions
3
21
2
5
5.32 £ 10 M , respectively. These values are less than
into the solution of 1 (c ¼ 4.0 £ 10 M), the emission at
2
þ
the binding constant value for Hg ion. To our opinion the
cavity dimension of 1, the ionic size of the metal ion and
also metal–ligand chelation strength are the crucial points
that actually regulate the conformational switching and the
binding features as pointed out in Figure 3. To prove the
pivotal role of quinoline ring in the binding, model
compound 2 was titrated under similar condition with
3
95 nm increased to the greater extent in each case without
showing any other change in the emission spectra. In the
titration with other metal ions, the emission intensity of 1
was reduced, and it was found to be meaningful only in
2
þ
2þ
case of Cu and Ni ions (Supplementary Information,
available online). However, during progression of titration
2
although with Zn ion the emission at 395 nm underwent
þ
2
þ
2þ
2þ
2
þ
Hg , Zn and Cd ions. Interestingly, no significant
change in emission was observed (Supplementary
Information, available online) in the interaction.
a red shift of 20 nm, for Cd it was observed to be
13 nm. The absence of excimer during complexation of
,
2
þ
2þ
Zn and Cd ions is attributed to the non-overlapping of
the quinoline motifs in the p-stacking fashion although
both the cations followed 1:1 stoichiometry in the binding
process. The binding constants (10) as determined for
1
H NMR analysis (Figure 7) further provided evidence
þ
2
that Hg ion is effectively complexed in the cavity of 1
involving quinoline ring nitrogen as well as ether oxygen.
2
þ
2þ
3
21
Zn and Cd ions were found to be 3.43 £ 10 M and
800
8
7
6
5
4
3
2
1
.0x10–6
.0x10–6
.0x10–6
.0x10–6
700
600
500
400
.0x10–6
.0x10–6
300
200
.0x10–6
.0x10–6
100
0
.0
0
–
1.0x10–6
350
400
450
500
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
XG
2
5
Figure 6. Change in emission of 1 (c ¼ 4.0 £ 10 M) in
23
Figure 4. Fluorescence Job plot for receptor
(
1
CHCl –MeOH (1:1, v/v) upon addition of 10 equiv. of
3
2
5
þ2
c ¼ 4.0 £ 10 M) with Hg at 395 nm (lex ¼ 300 nm).
Cd(ClO ) (c ¼ 1.0 £ 10 M).
4 2

1
30
K. Ghosh and D. Tarafdar
Receptor 1 + 2 equiv.amounts of
Cu ,Zn , Pb , Ni ,Cd ,Mg2+, Co2+, Ag+.
2
+
2+
2+
2+
2+
Cd(2+)
1
20
00
Receptor 1
Receptor 1 + 2 equiv.
1
amounts of Cu , Zn2+, Pb2+
2
+
,
Zn(2+)
Hg(2+)
2+
2+
2+
+
Ni , Cd , Mg , Ag ,
2+
80
60
40
Co + 2 equiv. amounts
2+
of Hg
.
Receptor 1
9
20
0
350
400
450
500
550
8
7
6
5
Wavelength (nm)
1
Figure 7. Partial H NMR (400 MHz, d
6
-DMSO) of 1
2
3
25
Figure 9. Change in emission of 1 (c ¼ 4 £ 10 M) upon
(
Hg(ClO ) , Zn(ClO ) and Cd(ClO ) .
c ¼ 1 £ 10 M) and in the presence of equiv. amount of
2
3
addition of 2 equiv. amounts of Hg(ClO
the presence of other metal ions.
4 2
)
(c ¼ 1 £ 10 M) in
4
2
4 2
4 2
In the presence of equiv. amount of Hg(ClO ) , the signals
4
2
for the protons of quinoline ring and also of ZOCH22
group moved downfield to the considerable extents and
thus support our binding proposition as cited in Figure 3.
In comparison, small change in chemical shift in the
in the presence of other metal ions. Figure 9, in this aspect,
demonstrates the change in emission of 1 upon addition of 2
2
þ
equiv. amounts of Hg ions to the solution of 1 containing
other metal ions in 2 equiv. amounts each. The interference
of individual metal ion was also verified (Supplementary
Information, available online), and no metal ion in the study
2
þ
2þ
presence of equiv. amount of Zn
corroborated their weak interaction.
Considering the ratiometric emission characteristics,
and Cd ions
2
þ
perturbed the ratiometric sensing of Hg ion.
The enhancement of fluorescence upon addition of
2
þ
the selectivity of 1 towards Hg ion was realised from the
plot of fluorescence ratio (I482/I395) at the two wavelengths
2
þ
2þ
Zn and Cd ions is explained due to the inhibition of
photoinduced electron transfer (PET) process occurring in
between the excited state of quinoline and binding site.
Similarly, the quenching of emission in the presence of
other metal ions is ascribed due to the activation of PET
(
Figure 8). As can be seen from Figure 8, the greater change
2
þ
in fluorescence ratio for Hg is worth mentioning for its
selective detection from the other metal ions. Such high
ratiometric change in emission remained unperturbed even
2
þ
process. The change in emission in the presence of Hg
ion follows a different mechanism. The heavy atom effect
2
þ
1
2
0
8
6
4
2
0
of Hg
ion (12) quenches the monomer emission
followed by the appearance of a new emission for excimer
that results in from the complexation-induced pulling of
the pendant quinolines.
1
To check the feasibility of the receptor 1 in sensing the
metal ions in aqueous environment, we carried out the
similar interaction study in DMSO:H O (5:95, v/v)
2
considering the solubility of 1 (Supplementary Infor-
mation, available online). In this high-water-content
2
þ
solvent, the receptor 1 responded only to Hg ion by
showing greater quenching of emission. No excimer
emission was noted and to our opinion, this was due to the
2
þ
greater polarity of solvent system that reduced the Hg
3
–
Hg(II) Zn(II) Cd(II) Cu(II) Pb(II) Ni(II) Ag(I) Co(II) Mg(II)
21
ligand interaction effectively (K ¼ 9.20 £ 10 M ).
a
2
þ
Metal ions
Beside Hg
ion, effect of other metal ions on the
emission of 1 was observed to be weak (Figure 10). In the
ground state, the absorption of 1 at 300 nm underwent a
Figure 8. Ratiometric fluorescent response (I482/I395) of 1
2
5
(
c ¼ 4 £ 10 M) in CHCl
0 equiv. amounts of different cations (c ¼ 1 £ 10 M) as their
perchlorate salts.
3
–MeOH (1:1, v/v) upon addition of
2
þ
2
3
considerable red shift in the presence of Hg
(Supplementary Information, available online).
ion
1

Supramolecular Chemistry
131
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þ
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,
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þ
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Cd
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Acknowledgements
D.T. thanks CSIR, Government of India for a fellowship. K.G.
thanks DST, Government of India for providing facilities in the
department under FIST programme.
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