A quinoline-based turn-off fluorescent cation sensor

Article abstract of DOI:10.1039/c2ra22572g

A quinoline-based cation sensor shows turn-off fluorescent behavior in the presence of Hg²⁺, Fe³⁺ and Cu²⁺ over other cations and offers discrimination of these cations from each other on the basis of the extent of quenching. The observed electronic absorption perturbations are in good agreement with theoretical (DFT, TD-DFT) calculations. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2ra22572g

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A quinoline-based turn-off fluorescent cation sensor3
Paramjit Kaur,*a Hardeep Kaurb and Kamaljit Singh*b
Cite this: RSC Advances, 2013, 3, 64
Received 18th October 2012,
Accepted 23rd October 2012
DOI: 10.1039/c2ra22572g
www.rsc.org/advances
A
quinoline-based cation sensor shows turn-off fluorescent
quenching processes in the presence of analytes. Receptor 5 was
synthesized7 by following the steps described in Scheme 1 and was
fully characterized using different spectroscopic techniques (S1,
behavior in the presence of Hg2+, Fe3+ and Cu2+ over other
cations and offers discrimination of these cations from each other
on the basis of the extent of quenching. The observed electronic
absorption perturbations are in good agreement with theoretical
(DFT, TD-DFT) calculations.
ESI ) before use.
3
Our preliminary investigation revealed that the emission
(Fig. 1) as well as absorption spectra (Fig. S1, ESI ) of 5 were
3
perturbed only in the presence of Fe3+, Hg2+ and Cu2+ and not by
the other cations, which led us to quantify the results for these
three ions only. The fluorescence emission spectrum (lex = 330
nm) of 5 (30 mM in CH3OH) exhibits an emission band at 376 nm.
Development of functional chemosensors for the detection and
quantification of important physiological and environmental
analytes such as Hg2+, Cu2+ and related cations has received
increased interest in recent years.1 Effective operational usage of
these chemosensors requires their high sensitivity and selectivity
towards the analytes. Although a large variety of highly selective2
single as well as multianalyte chemosensors3 have been reported,
the development of sensors for multianalyte detection in real time
is still a challenge. With this aim, the emphasis in recent years has
been placed on the development of optically responsive sensors
for the detection of analytes. The use of fluorescence as the signal
transducing method in optical sensors offers distinct advantages
in terms of sensitivity, selectivity and response time.4
Consequently, fluorescent molecular sensors have attracted
considerable recent interest.5 Moreover, the development of
fluorescent sensors for transition metals is of increasing
importance for biological and environmental applications. In
continuation of our interest in the development of chemosensors
and chemodosimeters,6 herein we report a quinoline-based turn-
off fluorescent cation sensor 5 (Scheme 1) which additionally
discriminates between the detected cations by differential
fluorescence emission quenching. The quinoline-based sensing
reported so far is mainly based on dynamic and/or static
The calculated quantum yield is 0.024 (ESI ) which is relatively low
3
compared to the usual values.
The incremental addition of Hg2+, Fe3+ and Cu2+ solutions (0–
1.0 equiv. in distilled H2O) caused 69–94% quenching of the
emission which was stabilized when the addition of 1 equiv. of the
cation was achieved (Fig. 2a, Table 1). No significant change in the
position of the emission maximum was observed up to the
addition of 1 equiv. of Hg2+ as depicted in Fig. 2(a). (For the
fluorescence spectra upon addition of 0–1.5 equiv. see Fig. S2 ).
3
Furthermore, a Job plot suggested 1 : 1 stoichiometry where the
maximum emission change was observed when the mole fraction
of 5 versus the cation was 0.5 in each case (Fig. 2b, Fig. S2 ). The
3
aDepartment of Chemistry, UGC-Centre of Advance Studies-I, UGC-SAP (DRS-I)
Department, Guru Nanak Dev University, Amritsar-143 005, India.
E-mail: paramjit19in@yahoo.co.in
bOrganic Synthesis Laboratory, Department of Applied Chemical Sciences and
Technology, UGC-SAP (DRS-I) Department, Guru Nanak Dev University, Amritsar-143
005, India. E-mail: kamaljit19in@yahoo.co.in
Electronic supplementary information (ESI) available: Synthetic procedures,
3
characterization and spectral data of 5, additional fluorescence, UV-vis and TD-
DFT data, Gaussian optimized structures, calculation of quantum yield and
complete ref. 13. See DOI: 10.1039/c2ra22572g
Scheme 1 Reagents and conditions: (a) pyridinium chlorochromate, CH2Cl2, 24 h;
(b) POCl3, 105 uC, 45 min; (c) THF, K2CO3, r.t., 48 h.
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Table 1 Fluorescence quantum yield (W) and % fluorescence quenching of 5
CompoundFluorescence quantum yielda W% Fluorescence quenchingb,c
5
0.024
0.002
0.003
0.004
—
5:Fe3+
5:Hg2+
5:Cu2+
94 (94)
87 (92)
69 (80)
a
Quinine sulfate was used as standard with a quantum yield of 0.55
b
in 0.1 N H2SO4 at lex = 320 nm. Calculated at 1 equiv. of metal ions.
c
Values in parentheses correspond to 1.5 equiv. of metal ions.
both of these quenching mechanisms operate in combination.
The non-linear nature of the Stern–Volmer plot with an upward
curvature (Fig. 5) indicates the possible involvement of combined
dynamic and static quenching.10 However, it is interesting to note
that the degree of quenching is large for Fe3+ followed by Hg2+ and
lower for Cu2+, which is important for discriminating these cations
from each other (Fig. 5). A similar type of Stern–Volmer plot
behaviour has been reported by Giri et al.11 for fluorescein,
rhodamine 6G and quinine sulfate in the presence of single walled
carbon nanotubes.
Fig. 1 Fluorescence intensities of 5 (30 mM) in CH3OH upon addition of different
metal ions (60 mM) in distilled H2O (lex = 330 nm) at pH 7.4.
detection limits (DL) for Fe3+, Hg2+ and Cu2+ ions using 5 were
determined from the calibration curves of absorbance versus
composition and were found to be 9.24 6 1025 M, 2.94 6 1024 M
and 4.17 6 1024 M, respectively.8 During competition experi-
ments under similar experimental conditions, no interference
The UV-vis absorption spectrum of 5 exhibited bands at 330
nm (emax 15 733 L mol21 cm21) and 255 nm (emax 46 433 L mol21
cm21) (Fig. 6, Fig. S4 ). Addition of aqueous solutions of Hg2+, Fe3+
3
and Cu2+ ions (0–1.5 equiv., as ClO42 salts) to a solution of 5 (30
mM in CH3OH) resulted in the appearance of twin absorption
bands at 330 and 343 nm with increased variable intensities [330
nm: emax 21 633 L mol21 cm21 (Hg2+), 24 566 L mol21 cm21 (Fe3+),
22 433 L mol21 cm21 (Cu2+); 343 nm: emax 20 126 L mol21 cm21
(Hg2+), 24 194 L mol21 cm21 (Fe3+), 19 393 L mol21 cm21 (Cu2+)].
The high energy band at 255 nm was not perturbed significantly
other than some broadening. Fitting the titration data using
HypSpec, a non-linear least squares fitting programme,12 estab-
lished the1 : 1 stoichiometry of the most stable species present in
the solution with binding constant values, log b1,1 = 5.20 (Fe3+),
4.69 (Hg2+) and 4.45 (Cu2+), respectively.
from other metal ions was observed (Fig. 3, Fig. S3 ). Further, the
3
sensing event was reversed upon addition of CN2, which snatches
away the metal ions from 5, forming the metal cyanide complexes
(Fig. 4).9
Stern–Volmer plots were created for the titration of these
cations (Fig. 5). Typically, Stern–Volmer plots are linear for
dynamic (collisional) quenching which occurs when the excited
fluorophore experiences interaction with an atom or molecule
which can facilitate non-radiative transitions to the ground state,
and for static quenching due to the formation of a non-fluorescent
stable complex with a quencher. They deviate from linearity when
In order to understand the nature of these transitions in terms
of the participation of different frontier orbitals in the observed
electronic changes, we carried out TD-DFT calculations for 5 and
5:Mn+ using the Gaussian 09 suite of programs.13 The best
Fig. 2 (a) Fluorescence spectra of 5 (30 mM) in CH3OH upon addition of Hg2+ (0–1.0
equiv.) in distilled H2O (lex = 330 nm) at pH 7.4 (for 0–1.5 equiv. see Fig. S23); (b) Job
plot of Hg2+ complex formation, x = [5]/[5] + [Hg2+] is the mole fraction of 5, F0 is the
fluorescence intensity when x = 1 and F is the fluorescence intensity at respective
values of x (for Fe3+ and Cu2+ titration profiles for 0–1.5 equiv., see Fig. S23).
Fig. 3 Changes in the fluorescence intensity of 5 (30 mM) in CH3OH at 376 nm upon
titration with increasing concentration of Hg2+ (5, 15 and 30 mM) in distilled H2O, in
the presence of other metal ions (30 mM) in distilled H2O at pH 7.4.
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Fig. 6 Changes in the UV-vis absorption spectra of 5 (30 mM) in CH3OH and its
complexes with Fe3+ (32 mM), Hg2+ (35 mM) and Cu2+ (40 mM) in distilled H2O at pH
7.4.
Fig. 4 Changes in the fluorescence spectra of metal complexes (5:Fe3+, 5:Hg2+ and
5:Cu2+) in the presence of CN2 (60 mM) in distilled H2O.
contribution from the H A L + 3 transition (Fig. 7, Table S3 ).
3
Although the HOMO in this case is more stabilized (DE 0.76 eV) as
compared to 5:Hg2+ (Fig. 7), the close values of the appropriate
HOMO–LUMO gap (DE2, DE3 and DE4) corroborate the calculated
binding constant values of the complexes of Hg2+ and Cu2+ with 5.
DFT calculations on the 5:Fe3+ system did not yield reproducible
results.
optimized structures are shown in Fig. S5 . The TD-DFT
3
calculations (Table S1 ) predict that the low energy band of 5
3
has a main contribution from the H A L + 1 transition and these
orbitals are located on the quinoline part of 5 with some
contribution from the H A L transition (Fig. 7). The high energy
band at 255 nm has contributions from the H A L + 3, H 2 2 A L
+ 2, H 2 4 A L + 2, H 2 4 A L, H 2 7 A L + 3 transitions (Table
Conclusions
S1 ). In the case of 5:Hg2+, the twin absorption band has
3
In conclusion, the present study demonstrates that the quinoline-
based chemosensor detects Hg2+, Fe3+ and Cu2+ through
fluorescence quenching to different extents which is helpful in
discriminating these cations from each other. Dynamic and static
quenching mechanisms have been proposed to operate in
combination. The perturbation in the electronic behaviour of 5
contributions from the H A L + 1 and H A L + 2 transitions
(Table S2 ). As shown in Fig. 7, we noticed that on interaction with
3
Hg2+, the HOMO (located on quinoline) was stabilized (DE 0.14 eV)
in comparison to 5 and the L + 1 was raised in energy to a very
small extent (DE 0.03 eV). This suggests that the interaction of
Hg2+ with the quinoline part of the molecule is responsible for the
quenching of the fluorescence emission. On the other hand, in the
case of Cu2+, of the twin absorptions at 330 and 343 nm, the high
energy dominating absorption band is suggested to have a main
Fig. 7 Energy level diagrams of HOMO (H) and LUMO (L) orbitals (isovalue 0.02) of
5, 5:Hg2+ and 5:Cu2+ calculated at the DFT level (for optimized structures of 5,
5:Hg2+ and 5:Cu2+ see Fig. S53).
Fig. 5 Stern–Volmer plots for Mn+, where Mn+ = Fe3+, Hg2+, and Cu2+
.
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Acknowledgements
PK thanks CSIR [01(2265)/08-EMR(II)], New Delhi, KS thanks
UGC, New Delhi for the project 37-188/2009 (SR) and SAP
(DRS-1). HK acknowledges CSIR (F.No. 09/254 (0195)/2009-
EMR-I), New Delhi for a senior research fellowship.
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