Fluorescent naphthalene-based benzene tripod for selective recognition of fluoride in physiological condition

Article abstract of DOI:10.1007/s12039-015-0779-0

Aluminium complex of a naphthalene-based benzene tripod ligand system has been reported for the selective recognition of fluoride in aqueous medium in physiological condition. The ligand can selectively recognize Al³⁺ through enhancement in the fluorescence intensity and this in situ formed aluminium complex recognizes fluoride through quenching of fluorescence. The receptor system detects fluoride in nanomolar range. The sensing property was extended for practical utility to sense fluoride in tap water, pond water and river water.

Full text of DOI:10.1007/s12039-015-0779-0

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J. Chem. Sci. Vol. 127, No. 2, February 2015, pp. 337–342. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0779-0
Fluorescent naphthalene-based benzene tripod for selective recognition
of fluoride in physiological condition
BARUN KUMAR DATTA, CHIRANTAN KAR and GOPAL DAS∗
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India
e-mail: gdas@iitg.ernet.in
MS received 29 November 2013; accepted 21 July 2014
Abstract. Aluminium complex of a naphthalene-based benzene tripod ligand system has been reported for
the selective recognition of fluoride in aqueous medium in physiological condition. The ligand can selectively
recognize Al3+ through enhancement in the fluorescence intensity and this in situ formed aluminium complex
recognizes fluoride through quenching of fluorescence. The receptor system detects fluoride in nanomolar range.
The sensing property was extended for practical utility to sense fluoride in tap water, pond water and river water.
Keywords. Chemosensor; fluoride anion; fluorescence; environmental.
1. Introduction
turn, encode a series of fluoride binding or transporting
proteins which can reduce the concentration of fluo-
ride, if that is too high.6 Fluoride is also important in
industrial applications and transformations, especially
in steel making and aluminium refining, and it is a
well-established reagent in organic synthesis.7 Novel
applications of fluoride have been discovered in the
fields of ion batteries,8 for enhancing the photo cur-
rent in supramolecular solar cells8b and in 18F-PET
imaging;8c,d and, in the future, it might play a role in the
construction of superconducting8e and hydrogen stor-
age materials.8e Intense fluoride contamination in the
surroundings of industrial areas has been often detected.
As a response to such observations, de-fluoridation
treatments and technologies are drawing wide inte-
rest. More generally, there is a growing need
for systems capable of recognition, binding and/or
sensing of fluoride in a competitive and aque-
ous environment. In view of the pharmacologi-
cal and environmental importance of fluoride, con-
siderable effort has been devoted to the develop-
ment of optical chemosensors for F−.9–11 In the
case of fluoride, the development of potent receptors
which are operational in water is difficult because of the
high solvation enthalpy of F−(ꢀH1 = −504 kJ mol−1).12
Consequently, few of the reported systems tolerate
aqueous conditions10 or work in pure water.11 Suc-
cessful examples of optical chemosensors for the
detection of fluoride in water are based on strong
Lewis acids (e.g., lanthanide, zirconium or organoboron
compounds), or on silicon, which features particularly
stable Si–F bonds.10,11 In some cases, detection of fluo-
ride at concentrations close to or below the maximum
Development of selective optical signalling systems for
anions has received considerable attention in recent
decades due to their important roles in biological and
environmental processes.1 Fluoride, important for its
health effect on humans and nowadays elicited by
mounting evidence, its severe toxicity makes it a contro-
versial subject. While in Anglo-Saxon countries such
as USA, UK and Australia, water fluoridation, i.e., the
addition of fluoride into public water supplies, is a well-
established practice since 1945;2 there is a growing
concern about fluoride contamination, only in part
anthropogenic, over large areas of Asia, Africa and
South America.3 Therefore, while fluoride has a gen-
tle effect up to 1 ppm concentration, it seems to
be toxic at higher doses.4a Over the years, high
concentration of the fluoride anion in the environ-
ment and in drinking water has been related to the
occurrence of several types of pathological condi-
tions in humans such as osteoporosis, neurological
and metabolic dysfunctions, and more recently cancer.4
Some studies have also reported the effects of fluoride
on the biochemistry of organisms. It seems that
DNA synthesis and the activity of some enzymes, as
well as the metabolism of nutrients, can be severely
influenced by the presence of fluoride.5 A recent study
reveals that many bacteria possess RNA structures
(riboswitches) which selectively respond to fluoride by
triggering the expression of specific genes. These, in
∗For correspondence
337

338
Barun Kumar Datta et al.
contaminant level defined by the US Environmental 2. Experimental
Protection Agency (4.0 mg L−1, 211 mM)13 could
be achieved. However, the preparation of these
chemosensors often requires substantial synthetic effort
(scheme 1).
2.1 General information and materials
All the materials used for synthesis were purchased
from commercial suppliers and used without further
purification. The sensor L1 was synthesized according
to the reported procedure.17a The spectroscopic studies
were carried out in a mixed solvent of acetonitrile:
aqueous HEPPES buffer (2:3). To check the sensing
behaviour of the in situ formed Al-complex, L1 and
metal ions were mixed in 1:1 ratio within the cuvette
at pH 7.4 (1 mM pH 7.4 HEPPES). In the sensing
process of fluoride in waste water, experiments were
conducted in the same mixture solvent media. The
buffer was replaced by tap water (TW), pond water
(PW) and river water (RW); and in these mixtures L1,
Al3+ and F− were added, respectively.
An alternative sensing scheme involves the detection
of an anion by sensing of a cation. An ‘umpolung’ of
this kind can be achieved if the anion forms a very
stable complex with the cation. Cyanide, for example,
binds strongly to Cu2+ to yield [Cu(CN)x]n− complexes.
The concentration of cyanide can thus be related to the
concentration of ‘free’ Cu2+, which can be detected by
cation-specific sensors. In a similar fashion, fluoride is
able to complex metal ions such as Al3+,14 Th4+15 and
Zr4+,16 and competitive reactions with metal-binding
dyes can be used to quantify fluoride concentration.
Among the various approaches to sense fluoride, sen-
sors utilizing aluminium–fluoride affinity draw our
special attention. Fluoride is known to react with alumi-
nium ions to form very stable AlF3 species.
2.2 Determination of detection limit
In our study on sensing of different analytes,17 we
have reported a novel naphthalene-based benzene tripod
receptor,17a which can selectively recognize aluminium
and lead ions in the physiological conditions. How-
ever, this sensor displayed a large fluorescence enhance-
ment effect upon the addition of aluminium and lead
ions. In this study, we utilized the aluminium selec-
tive probe for the selective detection of fluoride. In
order to demonstrate the practical utility of the new
assay, we further applied it to sense fluoride in tap
water and waste water, which provide more competitive
media.
The detection limit was calculated using the following
equation.
Detection limit = 3σ/k,
(1)
where σ is the standard deviation of blank measure-
ment, and k is the slope between the ratio of emission
intensity and concentration of analyte.
3. Result and Discussion
When 1 equivalent of aluminium ion was added to sen-
sor L1, large fluorescence enhancement was observed.
Scheme 1. Synthetic scheme of L1.

Fluorescent fluoride sensor in physiological condition
339
Figure 2. Changes in the fluorescence emission spectra of
L1.Al3+ (10 μM) in the mixed solvent upon addition
of increasing concentration of F−. Inset: Visual change in
fluorescence of L1.Al3+ in the presence of fluoride.
Figure 1. Changes in fluorescence emission spectra of
L1.Al3+ (10 μM) in the mixed solvent upon addition
of different anions.
of the pH-dependent response experiments of this probe
to fluoride were also carried out in the pH range of 5.0–
9.0. Results indicate that the system responds to fluoride
within the pH range of 6.0–8.5.
Selectivity of the sensing assay towards fluoride was
tested by performing the experiment in the presence
of potentially interfering analytes. It was found that
the system is unresponsive to most anions, in parti-
cular to heavier halides (Cl−, Br−, I−), pseudohalides
(N3−, SCN−), and that bisanionic species (CO23− (con-
verted to HCO−3 at pH 7.0), SO24−) give weak or
negligible response, respectively. A comparison of
the fluorescence responses obtained for these anions
Among various anions such as SCN−, AcO−, F−,
Cl−, Br−, I−, H2PO4−, HSO4−, NO3− and ClO−4 , only
F− showed a large fluorescence quenching (figure 1),
resulting in ‘On–Off’ type sensing of fluoride. Fluore-
scence emission spectra of the receptor L1.Al3+ (25 μM)
at RT in HEPES buffer (CH3CN/H2O = 2:3, v/v, pH =
7.4) show a dramatic quenching (λex = 360 nm, λem
=
450 nm) on addition of fluoride anion. To learn more
about the properties of the aluminium complex of L1 as
a receptor for fluoride, a titration of the receptor is per-
formed with increasing concentration of fluoride. The
fluorescence intensity of L1.Al3+ complex decreased
as the concentration of tetra-butyl ammonium fluoride
salt increased and gets saturated after the addition of
3 equivalents of fluoride. The colour of the solution
changed from blue to colourless under the UV lamp as
shown in figure 2 (inset). As illustrated in figure 3, the
receptor system exhibited high sensitivity towards fluo-
ride over other anions as the other anions show little
or no interference in the sensing process of fluoride.
From the titration, it was found that our system can
detect fluoride in nanomolar range in the physiological
condition.
Figure 3. Normalized fluorescence responses of L1.Al3+
to various anions in mixed solvent. Yellow bars represent
emission intensities of L1 in the presence of anions of
interest (5 equivalents). Blue bars represent the change
of the emission that occurs upon subsequent addition of
fluoride to the above solution. Intensities were recorded
The detection limit was calculated as 9.71 ppb. We
have also checked the effect of the counter anions of
Al3+ by using chloride, nitrate, perchlorate and sulphate
salts of Al3+. It was seen that these counter anions did
not affect the quenching process. Careful investigation at 450 nm.

340
Barun Kumar Datta et al.
Scheme 2. Proposed sensing mechanism of fluoride by
L1.Al3+
.
(5.0 mM) and for fluoride (1.0 mM) is shown in
figure 3.
As illustrated in scheme 2, the fluorescence signal
transduction occurs via reversible chelation enhanced
fluorescence (CHEF) processes with the metal ion.
Enhancement of fluorescence is probably due to the
formation of a six-member chelate ring with the metal
ion, which enhances conjugation in the ligand frame-
work and in turn results in fluorescence enhancement.
In the absence of Al3+ ions, the extent of intramole-
cular charge transfer (ICT) in L1 was sufficient enough
to quench its fluorescence. Again, the rigidity of the
system increases as the free rotation around the azome-
thine bond with respect to the naphthalene ring was
restricted by bonding with Al3+.17a In presence of
fluoride, as aluminium has more affinity towards
fluoride, the aluminium comes out from the L1.Al3+
complex to form a more stable Al–F complex which
causes the diminution of fluorescence intensity.
Figure 5. Sensing of fluoride by L1.Al3+ in tap water
(TW), pond water (PW) and river water (RW).
an experiment was carried out to check the effect of
Pb2+ on the sensing of fluoride in media which was
mentioned earlier. As the enhancement in fluorescence
intensity of L1 for Al3+ is greater than that of Pb2+,
when fluoride was added in presence of Pb2+ in the
mixture of L1.Al3+ complex, the emission intensity
was reduced but not completely quenched as shown in
figure 4. This may be due to the fact that when fluo-
ride snatches out Al3+ from the L1.Al3+ complex, Pb2+
captures the empty place caused by the release of Al3+
from the L1.Al3+ complex and forms a complex with L1
which prohibits complete quenching of emission inten-
sity. So, it is obvious that Pb2+ interferes in the sensing
process of fluoride by the receptor, L1.Al3+.
3.1 Effect of Pb2+
The effect of Pb2+ in the sensing of fluoride is impor-
tant, as L1 is a sensor for both Al3+ and Pb2+. So,
In order to establish the practical utility of our
sensing system, we have extended this to sense fluoride
in tap water and river water. As shown in figure 5, our
system was able to detect fluoride in tap water, pond
water and river water, where the media were more com-
plicated. Although, the media were more competitive
than the media used for sensing fluoride, our system
can detect fluoride without any interference. From our
results it is evident that the new assay is useful for
practical purposes.
4. Conclusion
We have developed a novel assay using an aluminium
complex of naphthalene-based benzene tripodal for
the detection of fluoride in physiological condition.
The receptor L1 recognizes Al3+ by enhancement of
fluorescence intensity and addition of fluoride to this
Figure 4. Effect of Pb2+on the sensing behaviour of
L1.Al3+ towards fluoride.

Fluorescent fluoride sensor in physiological condition
341
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The new system can detect fluoride in the nanomolar
range. This sensing system was further applied for the
detection of fluoride in more complicated media such
as tap water, pond water and river water. Thus, this new
system can be used for practical purposes.
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Supplementary Information
The supplementary information can be seen in www.
ias.ac.in/chemsci.
Acknowledgements
GD thanks CSIR (01/2727/13/EMR-II) and Science
& Engineering Research Board (SR/S1/OC-62/2011)
New Delhi, India for financial support and CIF of IITG
for providing instrument facilities. BKD and CK thank
IIT Guwahati for fellowship.
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