A chemosensor built with rhodamine derivatives appended to an aromatic platform via 1,2,3-triazoles: Dual detection of aluminum(III) and fluoride/acetate ions

Article abstract of DOI:10.1021/ic301915s

A triazole-ring-appended rhodamine dye (L) has been synthesized that serves as a chromogenic and fluorogenic sensor for dual sensing of aluminum(III) and fluoride or acetate ions specifically.

Full text of DOI:10.1021/ic301915s

Communication
pubs.acs.org/IC
A Chemosensor Built with Rhodamine Derivatives Appended to an
Aromatic Platform via 1,2,3-Triazoles: Dual Detection of
Aluminum(III) and Fluoride/Acetate Ions
Shubhra B. Maity and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
S
* Supporting Information
synthesis and photophysical behavior of a 1,2,3-triazole-ring-
ABSTRACT: A triazole-ring-appended rhodamine dye
(L) has been synthesized that serves as a chromogenic
and fluorogenic sensor for dual sensing of aluminum(III)
and fluoride or acetate ions specifically.
appended rhodamine dye L for selective colorimetric as well as
fluorimetric detection of the Al3+ ion in a methanolic aqueous
medium. We show here that once the Al3+ ion is anchored in
the dye, it can specifically detect fluoride and acetate ions in the
presence of a host of other anions.
The dye L and a reference rhodamine derivative, L′, were
synthesized in several steps (see the synthetic details in the
Supporting Information, SI). Their molecular structures are
illustrated in Figure 1. Both compounds were characterized by
1H and 13C NMR spectroscopy, electrospray ionization mass
spectrometry, and elemental analysis (see the SI).
luminum is the third most abundant element in the earth’s
A
crust, accounting for almost 8.3% of its mass.1 Regarding
toxicological effects of aluminum, its primary targets are
different from those of heavy metals.2 Aluminum is alleged to
interfere with the functioning of iron−sulfur proteins of the
respiratory chain, and it also raises the risk of Alzheimer’s
disease.3 The widespread use of aluminum in water treatment,
as a food additive, and in many industrial activities including the
manufacturing of cars and computers often exposes people to
this metal.1 Detection of the Al3+ ion in the presence of a host
of other cations is, therefore, of biological as well as
environmental relevance. Graphite furnace atomic absorption
spectroscopy and inductively coupled plasma atomic emission
spectroscopy are commonly employed methods for detection
of aluminum. Both of these techniques are generally time-
consuming and expensive as well. In contrast, optical detection
via fluorescence is an operationally easy technique besides
being highly sensitivity. Detection of anions through emission
readout has attracted a lot of attention4 because of the many
recognized biological roles played by anions. The smallest
anion, fluoride, with high charge density is of particular
importance because of its roles in dental care and other areas.5
The acetate ion, on the other hand, is a critical component of
various metabolic processes and also plays important roles in
several enzymes and antibodies.6 The development of sensors
selective for fluoride and acetate ions is, therefore, of crucial
importance. Few fluorescence chemosensors or chemodosim-
eters for aluminum,7 fluoride,8 and acetate ions9 have been
reported that are operational in purely organic media,
restricting their applications as sensors. When a chemosensor
is operational in an aqueous medium or in a mixed aqueous−
organic medium, its scope increases to a great extent.
Figure 1. Molecular structures of L and reference dye L′.
Because of the lack of solubility of compound L in a purely
aqueous medium, all absorption and emission measurements
were carried out in a mixed 90% methanol−water medium.
Perchlorate salts of metal ions that include Na+, K+, Mg2+, Ca2+,
Al3+ Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Ag+, Pb2+, and
Fe3+ were used in the studies. For Ce3+ and Cr3+ ions, their
nitrate salts were used. Neither metal-free L nor the presence of
any one of the metal ions (except Al3+) listed above show any
absorption band of significant intensity in the visible region. Of
the transition metals used, only Cr3+ and Fe3+ show slight
intensity enhancement because of their higher Lewis acidic
strength. In the presence of the Al3+ ion, however, the color of
the solution changes in 30 min to pink, with a strong band
appearing at 555 nm (Figure 2). This change from colorless to
pink can be observed with the naked eye and is due to breaking
of the spirolactum bond in the presence of the Al3+ ion, leading
to the delocalized xanthane moiety of the rhodamine group.12
Upon excitation at 540 nm, no noticeable emission is
observed with the dye (Φ = 0.002) in a metal-free state or in
Since Czarnik’s pioneering work on the Cu2+-induced ring-
opening reaction of a rhodamine B derivative10 to afford strong
fluorescence, many interesting studies on rhodamine derivatives
have been reported.11 A rhodamine derivative as a dye molecule
is an excellent choice as a chemosensor because the spirolactum
ring does not show any emission, but once the ring is broken by
an ion to afford the corresponding amide, an emission with
high quantum yield is observed. In this paper, we report the
Received: September 2, 2012
Published: January 24, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
was calculated from the absorption titration result and was
found to be 2.55 × 104 (see Figure S12 in the SI).
The selectivity of the dye for the Al3+ ion over other metal
ions was probed by competition experiments in which the dye
was first mixed with 30 equiv of a metal ion, followed by the
addition of 10 equiv of Al3+. Emission spectroscopy was used to
monitor these competition events.
As can be seen from Figure 4, the presence of background
metal ions has either no or a small effect on the emission
Figure 2. Absorption spectra of L (20 μM) and that of L in the
presence of 10 equiv of various metal ions in a mixed aqueous medium
[CH3OH−H2O, 9:1 (v/v)].
the presence of the mono- and divalent metal ions listed above.
Among the trivalent metal ions, both Cr3+ and Fe3+ exhibit a
weak and broad emission band around 570 nm with a slight
increase of the emission quantum yield (Φ = 0.12 and 0.09,
respectively). However, in the presence of the Al3+ ion, a strong
emission band centering at 577 nm (quantum yield, Φ = 0.43)
is observed with almost 200 times fluorescence enhancement
compared to the metal-free L (Figure 3). This is assignable11 to
the delocalized xanthane moiety of the rhodamine group that
results from breaking of the spirolactum ring in the presence of
the Al3+ ion.
Figure 4. Selectivity of the dye L for the Al3+ ion in a methanolic
aqueous medium [CH3OH−H2O, 9:1 (v/v)]. Red bars indicate the
fluorometric response of the dye with 30 equiv of the metal ion of
interest, and orange bars represent the final integrated fluorescence
response after the addition of 10 equiv of Al3+ to each solution
containing other metals over the initial integrated emission. Dye
concentration = 4 μM.
response of the dye to the Al3+ ion. Therefore, L is shown to be
a promising selective fluorescence sensor for Al3+ in the
presence of most competing metal ions.
1H NMR titration of L in the presence of different equivalent
amounts of Al3+ ions was conducted in a CDCl3−CD3OD [1:1
(v/v)] medium. The complexation mode of L toward the Al3+
ion, with spectral differences upon binding of the metal to the
dye, is depicted in Figure S13 in the SI. The signals of Hc and
Hd (Figure 1 shows c and d protons) shifted downfield by 0.62
and 0.11 ppm, respectively, upon the addition of 1.0 equiv of
Al3+ salt. These signals remain unaltered after the addition of up
to 5 equiv of Al3+ salt, indicating 1:1 complexation. In addition,
IR spectra of L and its Al3+ complex were also recorded. It is
observed that, upon addition of the Al3+ ion, the characteristic
carbonyl amide stretching frequency shifts from 1692 cm−1 in L
to 1677 cm−1 in the complex (see Figures S14 and S15 in the
SI).
Figure 3. Emission spectra of L (4 μM) in the presence of 10 equiv of
various metal ions in a mixed aqueous medium [CH3OH−H2O, 9:1
(v/v); excitation at 540 nm; slit, 3 nm/3 nm].
The reference compound L′ bearing a single triazole moiety
was prepared to probe whether the tripodal moiety present in L
is necessary for the selective detection of Al3+. Under identical
conditions of measurements, the reference rhodamine deriva-
tive L′ behaved differently compared to that of L. In the case of
L′, the selectivity for the Al3+ ion is lost. Most of the transition-
metal ions exhibit absorption and emission characteristics
similar to those of Al3+ (see Figures S7 and S8 in the SI). This
suggests that the tripodal platform is necessary for selectivity of
the Al3+ ion.
Interestingly, when the Al3+ complex of the dye is treated
with a sodium salt of either F− or AcO−, the color of the
solution changes from pink to colorless and the emission is
completely quenched within 10 min (Figure 5), while other
anions such as Cl−, Br−, I−, NO3 , SO42−, ClO3 , PF6 , and
−
−
−
−
BF4 do not show any change.
To investigate the binding stoichiometry between L and the
Al3+ ion, both Job’s plot and Benesi−Hildebrand plot
experiments13 (see Figures S9 and S10 in the SI) were carried
out, with the results showing a 1:1 complexation. Such 1:1
complex formation of a tripodal dye molecule with a CuII ion is
available in the literature.14 The 1:1 complexation is also
supported from mass spectral studies, which show a prominent
peak at 589.3210 (60%) due to [L + Al3+ + H2O]3+ (Figure S11
in the SI). The binding constant between L and the Al3+ ion
This observation suggests that the dye, while acting as a
sensor for the Al3+ ion, could serve as a sensor for F− and AcO−
anions as well. Once the AlIII ion interacts with the dye, the
spirolactum ring is broken, giving high fluorescence. In the
presence of F− or AcO−, the metal ion is abstracted and the
spirolactum ring closes (Scheme 1), leading to the absence of
fluorescence.15 We get almost all of the dye L back along with
minute amounts of other products after fluoride/acetate is
added to the aluminum complex.
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Communication
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Figure 5. Emission responses of the aluminum complex of ligand L
(10 μM) toward various anions (100 μM) in a mixed aqueous medium
[CH3OH−H2O, 9:1 (v/v); excitation at 540 nm; slit, 3 nm/3 nm].
Scheme 1. Proposed Mechanistic Pathway for Sensing
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In conclusion, we have developed a tripodal ligand, L,
generated from the click reaction between an alkyne derivative
of rhodamine B and an azide derivative of mesitylene. It serves
as a colorimetric and fluorimetric sensor for Al3+ as well as F−
and AcO− ions without interference from other cations and
anions.
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ASSOCIATED CONTENT
* Supporting Information
Detail experimental procedures, NMR and mass spectra, and
additional UV−visible and fluorescence spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
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(14) Zeng, X.; Dong, L.; Wu, C.; Mu, L.; Xue, S.-F.; Tao, Z. Sens.
Actuators B 2009, 141, 506.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pkb@iitk.ac.in.
Notes
■
(15) Lohani, C. R.; Kim, J.-M.; Chung, S.-Y.; Yoon, J.; Lee, K.-H.
Analyst 2010, 135, 2079.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support received from the
Department of Science and Technology, New Delhi, India (to
P.K.B.), and SRF from the CSIR to S.B.M.
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