Selective fluorescence sensor for Al3+ and Pb2+ in physiological condition by a benzene based tripodal receptor

Article abstract of DOI:10.1016/j.tetlet.2012.11.114

We have synthesized a benzene based tripodal receptor containing naphthalene motifs as signaling unit. The receptor can selectively bind Al ³⁺/Pb²⁺ metal ions with a significant change in its fluorescence and UV/Vis spectra, while the addition of other metal ions (Na ⁺, K⁺, Ca²⁺, Mg²⁺, Cr³⁺, Hg², Cu²⁺, Zn²⁺, Fe²⁺, Co ²⁺, Ni²⁺, Cd²⁺, and Ag⁺) produces insignificant or minor changes. The formation of receptor-metal ion complex is also proved by NMR and mass spectroscopic analysis.

Full text of DOI:10.1016/j.tetlet.2012.11.114

Tetrahedron Letters 54 (2013) 771–774
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective fluorescence sensor for Al3+ and Pb2+ in physiological condition by
a benzene based tripodal receptor
⇑
Barun Kumar Datta, Chirantan Kar, Arghya Basu, Gopal Das
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781 039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We have synthesized a benzene based tripodal receptor containing naphthalene motifs as signaling unit.
The receptor can selectively bind Al3+/Pb2+ metal ions with a significant change in its fluorescence and
Received 18 September 2012
Revised 8 November 2012
Accepted 11 November 2012
Available online 2 December 2012
UV/Vis spectra, while the addition of other metal ions (Na+, K+, Ca2+, Mg2+, Cr3+, Hg2, Cu2+, Zn2+, Fe2+
,
Co2+, Ni2+, Cd2+, and Ag+) produces insignificant or minor changes. The formation of receptor–metal ion
complex is also proved by NMR and mass spectroscopic analysis.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Fluorescence sensing
Benzene tripodal
Aluminum sensing
Lead sensing
The developments of fluorogenic sensors for heavy metal ions,
based on ion-induced changes in fluorescence, are particularly
attractive because of their simplicity, high sensitivity, and instan-
taneous response.1 However, development of ‘turn-on’ fluoregenic
sensor for heavy metal ions is a challenge due to the high fluores-
cence quenching tendency of the heavy metal ions. Lead is the
most abundant among heavy metals and ranks second in the list
of toxic substances.2 Lead poisoning is still an ongoing threat to hu-
man and the environment as most of lead mined to date are still
circulating in soil and groundwater.3 Although both adults and
children suffer from the effects of lead poisoning, lead poisoning
in children is much more frequent. For children, even very low lev-
els of exposure can result in learning disabilities, behavioral prob-
lems, stunted growth, impaired hearing, and kidney damage.4 At
high levels of exposure, a child may become mentally retarded, fall
into coma, and even die from lead poisoning.4 It also affects the
reproductive systems of both males and females. The extensive
pollution of lead in the environment is due to the vast use of it
in storage batteries, gasoline, pigments, building construction,
cable sheathing, bullets as well as alloys and ammunition. On the
other hand Al3+ has crucial roles in biological systems. Aluminum
is the third abundant element in the lithosphere (after oxygen
and silicon) and the most abundant metal in the Earth’s crust.5
However Aluminum in excessive amounts could lead to detrimen-
tal effects. The superfluous ingestion of aluminum influences the
absorption of calcium in the bowel, causing softening of the bone,
atrophy, and even aberrance, and also affects the absorption of iron
in the blood, causing anemia. The toxicity of aluminum causes
damage of the central nervous system, is suspected to be involved
in neurodegenerative diseases such as Alzheimer’s and Parkinson’s,
and is responsible for intoxication in hemodialysis patients.6 To
this end, the development of chemosensors for the facile detection
of Al3+ is of great importance in environmental monitoring and bio-
logical assays.7 However, the detection of Al3+ has always been
problematic due to the lack of spectroscopic characteristics, poor
coordination ability comparing to transition metals,8 and the pH-
dependence solubility in aqueous solution.9 However, compared
to other transition-metal ions, limited examples of Al3+ fluores-
cence sensors based on small molecules have been reported and
most of them worked well only in organic solvents that are difficult
for practical application.7 Therefore, the design of a new probe for
Al3+ which functions in aqueous media with a high selectivity
remains highly desirable for environment and biological studies.
In our continuous effort to design sensors for various analytes,10
here we discuss the synthesis, characterization, and sensing prop-
erties of a novel naphthalene functionalized benzene based tripo-
dal receptor as a selective sensor for Al3+ and Pb2+over other
competitive metal ions in aqueous medium.
According to the literature, ligands consisting of a pair of a hy-
droxyl group and a Schiff base nitrogen atom are well known for
their strong metal chelating properties.11 Transition metal ions
generally require 4–6 binding sites to fulfill the coordination mode,
which results in the formation of 2:1 or 3:1 complexes with chelate
ligands.12 Keeping in mind both these factors we synthesized a
flexible tripodal ligand L1 having N3O3 binding sites with three
Schiff’s base nitrogen atoms and three hydroxyl groups.
⇑
Corresponding author. Tel.: +91 361 258 2313; fax: +91 361 258 2349.
E-mail address: gdas@iitg.ernet.in (G. Das).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.114

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B. K. Datta et al. / Tetrahedron Letters 54 (2013) 771–774
Scheme 1. Synthetic scheme of L1 and L2.
L1 was synthesized in four steps as shown in Scheme 1. Com-
The emission spectrum of a free L1 ligand shows a weak band
with emission maxima positioned at ꢀ447 nm on excitation at
360 nm (Fig. 2). The selectivity of L1 was verified with perchlorate
pound 1 was synthesized by the reported procedure.12 Compound
2 was prepared by a simple nucleophilic reaction between p-nitro
phenol and compound 1 in acetone, and a subsequent reduction of
compound 2 with Pd/C and hydrazine monohydrate in methanol
gives the corresponding amine 3. Simple condensation reaction be-
tween 3 and 2-hydroxy naphthaldehyde in methanol yielded L1 in
73% yield. The structure of L1 was confirmed by its 1H NMR, 13C
NMR, FT-IR and ESI-MS spectra (See Supplementary data).
or nitrate salts of Na+, Mg2+, Al3+, K+, Ca2+, Cr3+, Mn2+, Fe2+, Co2+
,
Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+, and Pb2+ in aqueous HEPES buf-
fer–DMSO (pH 7.4, 99:1, v/v) (Fig. 2). The addition of alkali or alka-
line earth metals did not induce any discernible spectral changes of
the emission spectrum of L1. While transition metal ions Cr3+
,
Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+ slightly quenched
the emission intensity of the ligand. However, the addition of Al3+
or Pb2+ ‘turns on’ the fluorescence intensity of L1 more than 10-fold
in comparison to its initial intensity (Fig. 2).
The spectroscopic properties of the chemosensor L1 were inves-
tigated in an aqueous HEPES buffer–DMSO (99:1, v/v) solution. As
illustrated in Figure 1, free L1 exhibited two broad absorption
bands centered at ꢀ310 and ꢀ395 nm; this spectrum is typical of
a polyaromatic compound, the absorptions arise mainly due to
An enhancement in fluorescence intensity of L1 (kex = 360 nm) at
ꢀ447 nm was observed in presence of either Pb2+ or Al3+ ions. How-
ever, this enhancement was more pronounced for Al3+ (15-fold) as
compared to that for Pb2+ (12-fold) (Fig. 2 and Supplementary data).
Moreover, the addition of Pb2+ to a solution of L1-Al3+ causes no ob-
servable change in fluorescence intensity. However, in the reverse
case, when Al3+ is added to a solution of L1-Pb2+, fluorescence inten-
sity was increased to match the emission spectra of L1 with only
p–
p
⁄ transitions of the signaling subunit. The only change in the
absorption spectra was observed with the addition of Al3+ ion, no
other metal ion causes any change in the absorption spectra of
the free receptor. Complex formation of L1 with Al3+ leads to a
slight increase of both the bands. This may be due to the change
in the orientation of the polyaromatic cheomophoric unit. Two
isosbestic points at ꢀ352 and ꢀ424 nm further confirms the tran-
sition of the receptor into metal complexes.
Al3+
.
The fluorescence titration experiments were performed by
gradually increasing metal ion concentration into an aqueous
HEPES buffer–DMSO (pH 7.4, 99:1, v/v) solution of L1 (10 lM).
Figure 1. (a) Changes of the of UV/Vis absorption of receptor L1 (10
lM) observed
upon addition of metal ion salts (5 equiv) in an aqueous HEPES buffer:DMSO (99:1,
Figure 2. (a) Changes in the fluorescence emission spectra of L1 (10 lM) in HEPES-
buffered solution:DMSO (99:1, v/v) upon addition of various metal ion solution. (b)
v/v, pH = 7.4). (b) UV/Vis titration spectra of L1 (10
l
M) upon addition of Al3+ in an
aqueous HEPES buffer:DMSO (99:1, v/v, pH = 7.4).
Changes observed under UV lamp.

B. K. Datta et al. / Tetrahedron Letters 54 (2013) 771–774
773
The titration experiments reveal that the fluorescence intensity
enhanced gradually with the increase in concentrations of both
Al3+ and Pb2+ ions. The changes in the fluorescence emission of
L1 upon the addition of various concentrations of Al3+ and Pb2+
are shown in Figure 3a and b respectively.
The careful analysis of Job’s plot of the fluorescence titration
experiment shows that the ligand binds both Al3+ and Pb2+ with
1:1 binding stoichiometry (Fig. 4a and c). Binding constants for
two respective metal ions with L1 were evaluated from the emis-
sion changes at 447 nm with the help of Bensei–Hildebrand plots13
(Fig. 4b and d) and found to be KAl3+ = 3.7 Â 105 MÀ1 and
KPb2+ = 2 Â 105 MÀ1 at 25 °C. These high binding constant values
suggest the strong binding affinity of L1 toward both the metal ions
(Al3+ and Pb2+) in aqueous environment.
The selective binding of Al3+ and Pb2+ with the receptor L1 is
reversible in nature. The addition of an excess amount of ethylene-
diamine to the solution of L1-Al3+ or L1-Pb2+ results in the recuper-
ation of fluorescence spectra of the free L1 at 447 nm, which
support the reversible binding of metal ions to L1.
The selectivity of L1 for Al3+ over Fe3+ and Cu2+ is important be-
cause these two metal ions generally interfere in the detection of
Al3+ in mixture. Similarly, the selectivity of L1 for Pb2+ over Ca2+
,
Cd2+, and Hg2+ is particularly important because Pb2+ targets Ca2+
binding sites in vivo and Cd2+/Hg2+ are metal cations that fre-
quently interfere with Pb2+ analysis. Hence, the sensing efficiency
of L1 is further tested in the presence of other cations, which
may interfere in the estimation of Al3+ and Pb2+ (Fig. 5). The recep-
tor L1 performed well in the presence of other ions and sensed the
respective analytes from a competitive environment.
Figure 4. Job’s plot and Benesi–Hildbrand plots. (a) Job’s plot for Al3+. (b) Benesi–
Hildbrand plot for Al3+. (c) Job’s plot for Pb2+. (d) Benesi–Hildbrand plot for Pb2+. For
(b) and (d) X-axis values are in the order of 105.
The signal transduction occurs via reversible chelation en-
hanced fluorescence (CHEF) processes with these metal ions. The
enhancement in fluorescence is probably due to the formation of
a six member chelate ring with the metal ion, which enhances
the conjugation in the ligand framework and in turn results in
the fluorescence enhancement. In the absence of Pb2+ and Al3+ ions,
the extent of intra-molecular charge transfer (ICT) in L1 was suffi-
cient enough to quench its fluorescence. The chelation of L1 with
Pb2+ and Al3+ not only reduces the ICT effect in L1 but also in-
creased the rigidity of the molecular assembly by restricting the
free rotations of the azomethine carbon with respect to the naph-
thalene ring resulting in a significant enhancement of the fluores-
cence intensity which is known as CHEF. The hydroxyl group plays
an important role in the recognition of the metal ions that is, in the
ultimate CHEF process. CHEF does not occur without the coordinat-
ing hydroxyl group, which is confirmed by the non-fluorescent
behavior of the control compounds L2 in the presence of Al3+ and
Pb2+ metal ions. The lack of a phenolic –OH group in L2 restricts
the six membered chelate ring formation with the respective metal
Figure 5. Normalized fluorescence responses of L1 (10 lM) to various metal ions in
HEPES buffer:DMSO (99:1, v/v, pH = 7.4) solution. The red bars represent the
emission intensities of L1 in the presence of metal ions, for (a) Al3+ and (b) Pb2+
(50 lM). The blue bars represent the change of the emission that occurs upon the
subsequent addition of other mentioned metal ions (50 mM) to the above solution.
The intensities were recorded at 448 nm.
ions (See Supplementary data). Hence the free rotation around
C@N is not expected to be stopped in the case of control compound
L2 which was observed in its non-fluorescent nature.
The 1:1 binding stoichiometry of Al3+ and Pb2+ with receptor L1
obtained from Job’s plot (Fig. 4), were further established by ESI-
MS of L1-Al3+ and L1-Pb2+ complexes (see ESI) which shows a peak
at m/z 970.3619 (Calcd. for C63H52AlN3O6 = 973.3437) for the alu-
Figure 3. Changes in the fluorescence emission spectra of L1 (10 lM) in HEPES
minum complex and
a peak at m/z 1152.2701 (Calcd. for
buffer:DMSO (99:1, v/v, pH 7.4) solution upon addition of (a) Pb2+ and (b) Al3+ as
nitrate salts.
C63H52PbN3O6 = 1152.3432) (See Supplementary data) for the lead

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B. K. Datta et al. / Tetrahedron Letters 54 (2013) 771–774
complex. However de-protonation of the receptor was observed
during the complexation with Al3+ as the mass spectra show peaks
at 970.3619 which are three units less than those calculated for
[L1 + Al3+]. On the other hand for Pb2+ two hydroxyl groups of the
receptor L1 are deprotonated.
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Acknowledgments
G.D. gratefully acknowledges the Department of Science and
Technology (DST) and Council of Scientific and Industrial Research
(CSIR), New Delhi, India, for financial support. B.K.D., C.K. and A.B.
acknowledge IIT Guwahati, India, for fellowship.
Supplementary data
Supplementary data (synthesis, experimental details, and addi-
tional spectroscopic data) associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.11.
114.
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