Thiacalix[4]crown based optical chemosensor for Fe3+, Li + and cysteine: A Fe3+/Li+ ion synchronized allosteric regulation

Article abstract of DOI:10.1039/c2dt31940c

A heteroditopic chemosensor 2 based on a thiacalix[4]crown with a 1,3-alternate conformation has been synthesized and shows a selective fluorescence turn-off response with Fe³⁺ ions in mixed aqueous media. Furthermore, the 2-Fe³⁺ complex can be used as a chemosensing system for the detection of Li⁺ and cysteine based on two different approaches. Moreover, the sequence addition of Fe³⁺ and Li ⁺ triggers an Fe³⁺/Li⁺ switchable on-off-on fluorescence chemosensor with negative allosteric regulations. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt31940c

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Thiacalix[4]crown based optical chemosensor for Fe3+,
Li+ and cysteine: a Fe3+/Li+ ion synchronized allosteric
regulation†
Cite this: Dalton Trans., 2013, 42, 981
Manoj Kumar,* Naresh Kumar and Vandana Bhalla
A heteroditopic chemosensor 2 based on a thiacalix[4]crown with a 1,3-alternate conformation has been
synthesized and shows a selective fluorescence turn-off response with Fe3+ ions in mixed aqueous media.
Furthermore, the 2–Fe3+ complex can be used as a chemosensing system for the detection of Li+ and
cysteine based on two different approaches. Moreover, the sequence addition of Fe3+ and Li+ triggers an
Fe3+/Li+ switchable on-off-on fluorescence chemosensor with negative allosteric regulations.
Received 24th August 2012,
Accepted 21st September 2012
DOI: 10.1039/c2dt31940c
www.rsc.org/dalton
Our research work involves the design and synthesis of fluo-
rescent chemosensors for the selective sensing of soft metal
Introduction
Iron is a vital element for the activities of numerous physio- ions and anions as well as to further evaluate their switching
logical processes involving electron transfer and oxidation behaviour.9 Recently, we have reported a ditopic chemosensor,
reactions1 because of its redox-active nature and it is an essen- based on the 1,3-alternate conformation of a thiacalix[4]arene
tial trace metal ion that plays a crucial role in chemical, bio- bearing amine groups appended with pyrene moieties and a
logical as well as in environmental systems.2 The deficiency of crown-4 ring, exhibiting selectivity towards Cu2+ ions and
iron throughout the developmental phases is linked with the which can act as a Cu2+/Li+ ion switchable fluorescent
permanent loss of motor skills.3 On the other hand, an excess chemosensor.10 As a continuation of this work, now we have
quantity of iron in the central nervous system is responsible synthesized a new heteroditopic chemosensor 2 based on the
for a number of diseases such as Parkinson’s, Huntington’s 1,3-alternate conformation of thiacalix[4]crown-4 bearing two
and Alzheimer’s disease.4 Keeping in mind the roles played by imino moieties appended with 2-hydroxy-5-nitrophenyl
iron in day to day life, the development of techniques for groups. The presence of the crown ring for metal ion
simple and rapid sensing of iron in biological and environ- complexation as an additional binding site in 1,3 alternate
mental systems is very important. In this respect, fluorescence (thia)calix[4]arene system provides a platform to achieve
signalling,5 owing to its sensitivity, is widely employed to heteroditopic complexation with allosteric regulation.11 Such
quantify trace amounts of iron.6
systems can also be used for mimicking allosteric regulation
Furthermore, the need for miniaturization is encouraging which plays a major role in biological systems and further
the design of single molecule based molecular devices as they opens the possibility of controlling molecular functions by
can perform switching operations analogous to those executed external stimuli. Receptor 2 exhibits selective fluorescence
by their macroscopic analogues.7 These signalling devices quenching with Fe3+ ions in mixed aqueous media, ascribed to
consist of molecular systems which change their optical or the inhibition of the ESIPT phenomenon, whereas Li+
electrical properties upon interaction with external chemical coordinates with the crown ether ring which is accountable
inputs, and the design of such molecular switches is of great for the small fluorescence enhancement in receptor 2.
significance as such organic molecules are expected to open Furthermore, we used the 2–Fe3+ complex as a system for the
new perceptions for the realization of artificial functions at the turn-on detection of Li+ ions and cysteine based on two
molecular level.8
different mechanisms. The addition of cysteine followed
the displacement approach while Li+ exerts
negative
a
allosteric behaviour which is responsible for the turn-on
fluorescence changes. In addition, the metal ion exchange
triggers an Fe3+/Li+ switchable on-off-on fluorescent chemo-
sensor with negative allosteric regulations between the Fe3+
and Li+ ions.
Department of Chemistry, UGC Sponsored Centre for Advanced Studies-1, Guru
Nanak Dev University, Amritsar, Punjab, India. E-mail: mksharmaa@yahoo.co.in;
Fax: +91 (0)183 2258820; Tel: +91 (0)183 2258802 9 ext. 3205
†Electronic supplementary information (ESI) available: 1H, 13C, mass and fluore-
scence spectra. See DOI: 10.1039/c2dt31940c
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300 MHz): δ = 31.31, 31.49, 34.39, 34.44, 56.37, 66.03, 68.66,
69.95, 71.30, 117.22, 118.39, 127.22, 127.69, 12.13, 128.48,
139.52, 146.39, 147.02, 154.78, 158.26, 164.62, and
Experimental
General information
All reagents were purchased from Aldrich and were used 167.25 ppm. MS ES+, m/z: = 1219.5 [M + H]+. C64H74N4O12S4:
without further purification. The experiments were carried out calcd. C 63.03, H 6.12, N 4.59; Found C 63.14, H 6.32, N 4.86.
in a mixture of AR grade THF and HEPES buffer (9 : 1, v/v; pH
7.0). UV-vis spectra were recorded on a SHIMADZU UV-2450
spectrophotometer with a quartz cuvette (path length 1 cm).
The cell holder was thermostated at 25 °C. The fluorescence
Results and discussion
The condensation reaction of diamine 113 with 2-hydroxy-
5-nitrobenzaldehyde furnished compound 2 in a 75% yield
(Scheme 1). The structure of compound 2 was confirmed from
spectra were recorded with a SHIMADZU 5301 PC spectro-
fluorimeter. 1H and 13C spectra were recorded on a JEOL-FT
NMR-AL 300 MHz spectrophotometer using CDCl3 as a solvent
and tetramethylsilane as the internal standards. Data are
reported as follows: chemical shift in ppm (δ), multiplicity (s =
singlet, d = doublet, t = triplet, m = multiplet, br = broad
its spectroscopic and analytical data (see Fig. S5–S8, ESI†). The
binding behaviour of compound 2 towards different cations
(Pb2+, Hg2+, Fe3+, Cu2+, Co2+, Mg2+, Ni2+, Fe2+, Ag+, Zn2+, Cd2+,
Ba2+, Ca2+, K+, Na+, and Li+) as their perchlorate salt was
singlet), coupling constants
interpretation.
J
(Hz), integration and
studied by UV-vis and fluorescence spectroscopy. The absorp-
tion spectrum of receptor 2 (5 μM; Fig. 1) in THF–H2O (9 : 1, v/v)
is characterized by typical absorption bands at 238, 262 and
288 nm. The addition of only Fe3+ ions (0–16 equiv.) results in
a significant absorption enhancement corresponding to these
absorption bands, while titrations with other transition and
alkali metal ions did not alter the absorption spectrum of
receptor 2 (see Fig. S2, ESI†). The fluorescence spectrum of
receptor 2 (5.0 μM; THF–H2O; 9 : 1, v/v) exhibits an emission
band at 356 nm (Fig. 2), attributed to the very fast enol–imine
to keto–amine tautomerism (Scheme 2) involving the pheno-
menon of excited state intramolecular proton transfer (ESIPT).14
UV-vis and fluorescence titrations
UV-vis and fluorescence titrations were performed on a 5.0 μM
solution of the ligand in THF–H2O (9 : 1, v/v). Typically, ali-
quots of freshly prepared M(ClO4)n (M = Pb2+, Hg2+, Fe3+, Cu2+,
Co2+, Ba2+, Mg2+, Ni2+, Ca2+, K+, Na+, Li+, Fe2+, Ag+, Zn2+ and
Cd2+; n = 1 or 2 or 3) standard solutions (10−1 M to 10−3 M)
were added to record the UV-vis and fluorescence spectra.
Quantum yield calculations
Fluorescence quantum yields12 were determined using an opti-
cally matching solution of naphthalene (Φfr = 0.23 in ethanol)
as the standard at an excitation wavelength of 300 nm and the
quantum yield could then be calculated using the equation:
1 ꢁ 10ꢁA L
Ns2 Ds
ꢀ ꢀ
Nr2 Dr
r
r
Φfs ¼ Φfr ꢀ
1 ꢁ 10ꢁA L
r
s
where Φfs and Φfr are the radiative quantum yields of the
sample and reference respectively, As and Ar are the absor-
bance of the sample and reference respectively, Ds and Dr are
the respective areas of emission for the sample and reference.
Ls and Lr are the lengths of the absorption cells of the sample
and reference respectively. Ns and Nr are the refractive indices
of the sample and reference solutions (pure solvents were
assumed respectively).
Scheme 1 Synthesis of compound 2.
Synthesis of 1,3-alternate-25,27-bis[2-(2-hydroxy-5-nitrophe-
nyl)iminoethoxy]thiacalix[4]crown-4 (2). A mixture of com-
pound 1 (0.1 g; 0.10 mmol) and 2-hydroxy-5-nitrobenzaldehyde
(0.03 g; 0.21 mmol) in a mixture of ethanol–dichloromethane
was refluxed for 12 h. After the completion of the reaction, the
solvent was evaporated and the residue left behind was crystal-
lized from CHCl3–CH3OH to give compound 2 in a 75% yield;
m.p. 220 °C. 1H NMR (CDCl3, 300 MHz): δ = 1.28 (s, 18 H,
C(CH3)3), 1.29 (s, 18 H, C(CH3)3), 2.51 (br, 4 H, CH2), 3.24 (t, J =
6 Hz, 4 H, OCH2), 3.50 (br, 4 H, OCH2), 4.03 (br, 4 H, OCH2),
4.15 (t, J = 6 Hz, 4 H, OCH2), 7.01 (d, J = 6 Hz, 2 H, Ar-H), 7.33
(s, 4 H, Ar-H), 7.48 (s, 4 H, Ar-H), 8.18–8.21 (m, 4H, Ar-H), 8.31
Fig. 1 UV-vis spectra of 2 (5.0 μM) with Fe3+ ions (16 equiv.) in THF–H2O; (9 : 1,
(s, 2 H, HCvN), 14.12 (br, OH) ppm. 13C NMR (CDCl3, v/v) buffered with HEPES, pH = 7.0.
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Fig. 2 Fluorescence spectra of 2 (5.0 μM) with Fe3+ ions (20 equiv.) in THF–
H2O; (9 : 1, v/v) buffered with HEPES, pH = 7.0; λex = 300 nm.
Fig. 3 Job’s plot for determining the stoichiometry (1 : 1) of
Fe3+ ions.
2 and the
Scheme 2 Fast enol–imine to keto–amine tautomerism in 2.
However, the gradual addition of Fe3+ ions (0–20 equiv.) to a
solution of compound 2 results in a decrease in the fluorescence
intensity at 356 nm. This decrease in the fluorescence intensity
is ascribed to the participation of imino nitrogens towards the
coordination of Fe3+ ions, which trammels the ESIPT pheno-
menon and hence quenched the fluorescence emission.
Fig. 4 Fluorescence spectra of 2 (1.0 μM) in the presence of various metal ions
(20.0 μM) in THF–H2O; (9 : 1, v/v) buffered with HEPES, pH = 7.0; λex = 300 nm.
Furthermore, by considering the ratio of the fluorescence
intensity of the free receptor at 356 nm (Io) to that of receptor 2
with Fe3+ ions at 356 nm (I), we observed a 13-fold decrease in
the fluorescence emission in the case of the 2–Fe3+ complex.
The fluorescence quantum yield of the 2–Fe3+ complex is 0.02
compared to that of free 2 (0.32), which shows good agreement
with the fluorescence spectra obtained for receptor 2 in the
presence of Fe3+ ions. Fitting the changes in the fluorescence
spectra of compound 2 with Fe3+ ions by using the nonlinear
regression analysis program SPECFIT15 gave a good fit and
demonstrated that a 1 : 1 stoichiometry (host : guest) was the
most stable species in the solution, with a binding constant
Fig. 5 Fluorescence response of 2 (5.0 μM) to various metal ions (20 equiv.
each) in THF–H2O; (9 : 1, v/v) buffered with HEPES, pH = 7.0; λex = 300 nm. The
bars represent the selectivity (Io/I) of 2 upon the addition of different metal
ions. Io and I denote the fluorescence intensity at 356 nm before and after the
addition of metal ions, respectively.
(log β) = 4.78
0.06. The method of continuous variation
(Job’s plot; Fig. 3) was also used to prove the 1 : 1 stoichio-
metry.16 The detection limit17 of 2 for Fe3+ ions was calculated
to be of 30 × 10−9 mol L−1, which is sufficiently low for the
detection of nanomolar concentrations of Fe3+ ions (see
Fig. S3, ESI†). This type of fluorescence behaviour is not
observed upon the addition of any other transition and alkali
metal ions (Fig. 4 and 5). Thus, receptor 2 acts as an efficient
fluorescence ‘turn-off’ chemosensor for the selective detection
of Fe3+ ions in mixed aqueous media. However, the addition of
Li+ ions to a solution of 2 results in a slight enhancement of
the emission at 356 nm (Fig. 6). This enhancement in the
fluorescence intensity is attributed to the binding of Li+ ions
to the crown-4 ring,11a which suppresses the photoinduced
electron transfer (PET) to the photoexcited 2-hydroxy-5-nitro-
phenyl groups. Furthermore, fitting the changes in the fluo-
rescence spectra of receptor 2 with Li+ ions by using the non-
linear regression analysis program SPECFIT gave a good fit
and demonstrates that a 1 : 1 stoichiometry of the host and
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guest was the most stable species in the solution, with a of 20 equiv. of Fe3+ ions mixed with 20 equiv. of Pb2+, Hg2+,
binding constant (log β) of 4.57 0.3. To check the practical Fe3+, Cu2+, Co2+, Mg2+, Ni2+, Fe2+, Ag+, Zn2+, Cd2+, Ba2+, Ca2+,
ability of compound 2 as a Fe3+ ion selective fluorescent K+, Na+, and Li+ ions. As shown in Fig. S4, ESI† no significant
sensor, we carried out competitive experiments in the presence variation in the emission was observed with or without other
metal ions.
Furthermore, we utilized the 2–Fe3+ complex system for the
selective detection of Li+ and biothiols, such as cysteine, based
on different approaches. The presence of the crown-4 moiety
as an additional binding site other than the imino moiety in 2
raises the possibility of discrimination between the Fe3+ and
Li+ ions. When Li+ ions (0–70 equiv.) were gradually added to a
solution of the 2–Fe3+ complex, the fluorescence intensity at
356 nm undergoes a regular increase in the fluorescence emis-
sion (Fig. 7). These results indicate that the decomplexation of
Fe3+ ions taking place upon the addition of Li+ ions, ascribed
to the binding of Li+ ions with the polyether ring (crown-
Fig. 6 Fluorescence spectra of 2 (5.0 μM) with Li+ ions (20 equiv.) in THF–H2O;
(9 : 1, v/v) buffered with HEPES, pH = 7.0; λex = 300 nm.
4 moiety), is responsible for the conformational changes
favouring the release of Fe3+ from the imino binding site and
hence, the ESIPT phenomenon comes into operation and is
answerable for the emission enhancement at 356 nm (Fig. 8).
Thus, the addition of Li+ ions to the 2–Fe3+ complex exerts a
negative heterotropic allosteric effect which means the 2–Fe3+
complex acts as a system for the selective detection of Li+ ions.
The addition of other metal ions, even alkali metal ions such
as Na+ and K+, did not alter the fluorescence emission of the
2–Fe3+ system (Fig. 9A), ascribed to the high selectivity of the
crown-4 ring towards Li+ ions. For a system to be an efficient
chemosensor, it must work in the presence of other analytes.
Thus, we investigated the competitive selectivity of the 2–Fe3+
system in the presence of 70 equiv. of Li+ ions mixed with 70
equiv. of other metal ions. As shown in Fig. 9B, no significant
variation in the emission was observed on comparison with or
without other metal ions.
Fig. 7 Fluorescence spectra of 2 (5.0 μM) upon the addition of Li+ ions (70
equiv.) to the 2–Fe3+ complex in THF–H2O; (9 : 1, v/v) buffered with HEPES, pH =
7.0; λex = 300 nm.
Fig. 8 Schematic representation of the Li+ and Cys induced fluorescence turn-on.
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Fig. 9 Fluorescence response of the 2–Fe3+ complex to various metal ions (70
equiv. each) in THF–H2O; (9 : 1, v/v) buffered with HEPES, pH = 7.0; λex
=
300 nm. The bars represent the selectivity (I/Io) of the 2–Fe3+ complex upon the
addition of different metal ions. (A) The blue bars represent the selectivity (I/Io)
of the 2–Fe3+ complex upon addition of different metal ions; (B) the red bars
represent the competitive selectivity of the receptor 2–Fe3+ complex towards Li+
ions (70 equiv.) in the presence of other metal ions (70 equiv.).
Moreover, we observed an 11-fold fluorescence enhance-
ment in the case of the 2–Fe3+ complex titration with Li+ ions.
The detection limit of the 2–Fe3+ complex was found to be
55 × 10−8 mol L−1 for Li+ ions. The substantial increase in the
quantum yield from 0.2 to 0.26 also reveals the 2–Fe3+ complex
to be a fluorescence turn-on system for Li+ ions. However, in
the reverse of this cation exchange process, the addition of
Fe3+ ions (50 equiv.) to a solution of the 2–Li+ complex results
in fluorescence quenching corresponding to the emission
band at 356 nm (Fig. 10b). This clearly indicates that Fe3+
moves in and Li+ moves out of receptor 2. Thus, negative allo-
steric switching is observed between the Fe3+ and Li+ ions in
mixed aqueous media. Therefore, the metal ion exchange
between Fe3+ and Li+ acts as a chemical input to trigger an on-
off-on switchable fluorescent chemosensor (Fig. 11). However,
the liberation of Fe3+ from the imino binding site of the 2–Fe3+
complex requires excess addition of Li+ which indicates the stron-
ger binding of Fe3+ ions as compared to Li+ binding by the poly-
ether oxygen atoms, as supported by the binding constant data.
In addition to the use of the 2–Fe3+ complex as a chemosen-
sing system for Li+ ions, we further employed this complex for
the detection of cysteine based on the displacement approach.
The addition of cysteine to a solution of the free receptor did
not introduce any evident changes into the emission spectrum
of 2. However, the addition of cysteine to an aqueous solution
of the 2–Fe3+ complex results in the fluorescence enhancement
corresponding to 356 nm (Fig. 12). This increase in the emis-
sion intensity is ascribed to the formation of a Cys–Fe3+
complex and thus, Fe3+ is no longer available to inhibit the
excited state intramolecular proton transfer mechanism
favouring the emission enhancement at 356 nm (Fig. 8). A
Fig. 10 (a) Change in the fluorescence emission spectra of the 2–Fe3+ complex
upon the addition of Li+ ions. (b) Change in the fluorescence emission spectra of
the 2–Li+ complex upon the addition of Fe3+ ions in THF–H2O; (9 : 1, v/v)
buffered with HEPES, pH = 7.0; λex = 300 nm.
Fig. 11 Schematic representation of the ion exchange between metal ions.
9.5-fold fluorescence enhancement is observed in the cysteine complex acts as a chemosensing system for the detection of Li+
treatment with the 2–Fe3+ complex. The considerable increase and cysteine based on two contrasting modes of action. The
in the quantum yield (0.23) and detection limit of 12 × 10−8 detection of Li+ followed the negative allostery route while
mol L−1 of the 2–Fe3+ complex for cysteine suggest this system cysteine detection is associated with the displacement
is a proficient sensing platform for cysteine. Thus, the 2–Fe3+ approach.
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Fig. 12 Fluorescence spectra of 2 (5.0 μM) upon the addition of cysteine (50
equiv.) to the 2–Fe3+ complex in THF–H2O; (9 : 1, v/v) buffered with HEPES, pH =
7.0; λex = 300 nm.
7 (a) V. Balzani, Molecular Devices and Machines: A Journey into
the Nano World, Wiley-VCH, Weinheim, 2003; (b) A. P. de
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Conclusion
In conclusion, we synthesized a heteroditopic fluorescent
chemosensor 2 based on a thiacalix[4]arene with the 1,3-alternate
conformation which possesses
a crown-4 moiety as an
additional binding site other than the imino moieties. Recep-
tor 2 shows a selective fluorescence turn-off response with Fe3+
ions in aqueous media with a detection limit on the nano-
molar level, ascribed to the inhibition of the ESIPT mechanism.
Furthermore, the 2–Fe3+ complex behaves as a chemosensing
system for the turn-on detection of Li+ ions and cysteine based
on the two different mechanisms. The addition of cysteine
follows the displacement approach while Li+ ions exert a nega-
tive allosteric behaviour responsible for the turn-on fluo-
rescence changes attributed to the ESIPT process.
Interestingly, the sequential addition of Fe3+ and Li+ ions trig-
gers a Fe3+/Li+ switchable on–off–on fluorescent chemosensor
with negative allosteric regulations between these ions. The
designs of such molecular based allosteric systems are better
than simple host-guest systems as they open the possibility of
controlling molecular function in biomimetic systems by
chemical inputs.
10 M. Kumar, N. Kumar and V. Bhalla, Dalton Trans., 2012, 41,
10189.
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