A water-soluble fluorescent fluoride ion probe based on Alizarin Red S-Al(III) complex

Article abstract of DOI:10.1016/j.saa.2006.03.023

A new water-soluble fluorescent fluoride ion signaling system has been developed based on the ligand exchange mechanism in aqueous medium. This procedure is based on the exchange of two Alizarin Red S (ARS) molecules coordinated to Al(III) by fluoride ion without interference from other common anions. The binary complex of ARS with Al(III) provides a sensitive signaling system for fluoride ion in the concentration range from 5 × 10^-6 to 3 × 10^-4 M. The ligand exchange reaction of ARS-Al(III) complex with fluoride ion has been investigated by UV-vis and fluorescence spectroscopies combined with the AM1 semi-empirical quantum chemical calculations. The pale orange fluorescence (λ_max = 575 nm) exhibited by the complex upon excitation at 435 nm decreases in intensity with fluoride addition with a detection limit of 0.1 mg L^-1.

Full text of DOI:10.1016/j.saa.2006.03.023

Spectrochimica Acta Part A 66 (2007) 457–461
A water-soluble fluorescent fluoride ion probe based
on Alizarin Red S–Al(III) complex
R. Sai Sathish, M. Ravi Kumar, G. Nageswara Rao,
K. Anil Kumar, C. Janardhana∗
Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prashanthi Nilayam, AP 515134, India
Received 15 December 2005; accepted 26 March 2006
Abstract
A new water-soluble fluorescent fluoride ion signaling system has been developed based on the ligand exchange mechanism in aqueous medium.
This procedure is based on the exchange of two Alizarin Red S (ARS) molecules coordinated to Al(III) by fluoride ion without interference
from other common anions. The binary complex of ARS with Al(III) provides a sensitive signaling system for fluoride ion in the concentration
range from 5 × 10−6 to 3 × 10−4 M. The ligand exchange reaction of ARS–Al(III) complex with fluoride ion has been investigated by UV–vis and
fluorescence spectroscopies combined with the AM1 semi-empirical quantum chemical calculations. The pale orange fluorescence (λmax = 575 nm)
exhibited by the complex upon excitation at 435 nm decreases in intensity with fluoride addition with a detection limit of 0.1 mg L−1
© 2006 Elsevier B.V. All rights reserved.
.
Keywords: Alizarin Red S; AlCl3; Ligand exchange; Fluoride sensing; AM1
1. Introduction
have been used as reagents for fluoride ion determination with
the mention of Al(III) interference. Each of these complexes
have their limitations in fluoride binding, requiring either a sup-
port or an organic solvent. ARS is known to form a stable
2:1 fluorescent complex with Al(III) [25–29]. In our previous
paper [30], we had arrived at a fluoride ion sensitive and selec-
tive signaling system using 8-hydroxyquinoline–Zr(IV)–EDTA
complex and in continuation of this research program on design
of novel fluoride sensors, we present the ARS–Al(III) complex,
based on the Lewis acid–base reaction, with no support, a one-
pot, completely water-soluble, inexpensive fluoride ion sensing
system. The high affinity of Al(III) to fluoride ion [31,32] and the
fluorescent characteristics of ARS–Al(III) complex [25] have
been successfully utilized for fluoride detection (Fig. 1).
Fluoride ion quantification in environmental and industrial
wastewaters is important [1] due to its vital health implica-
tions [2]. Currently available methods for fluoride detection
include 19F NMR [3], electrochemical (ion selective electrodes)
[4,5], chromatography [6] and chromogenic sensing systems
[7,8] involving spectrophotometry [9,10]. Chromogenic sen-
sors for anions consist of two parts: anion receptors and chro-
mophores [11]. The different types of chromogenic anion sen-
sors are classified based on their non-covalent interactions as,
electrostatic [12], NH-based hydrogen bonding [13–15], Lewis
acid [1,2,16,17] and combinations of these interactions working
together [18]. Only a few of these receptors function in aque-
ous medium [1,2,17], as water competes with the receptor for
fluoride ion [19]. An impetus for the development of alterna-
tive strategies has been the use of Lewis acid [2,17] receptors
and this led us to explore the known field of ARS and its metal
complexes in search of a new fluorescent fluoride sensor.
ARS (Fig. 1) derivatives [15] and its complexes with zirco-
nium [20], lanthanum [6,21], thorium [22] and cerium [23,24]
2. Experimental
2.1. Reagents
ARS from Sigma was re-crystallized according to the
reported procedure [27]. AlCl3, sodium fluoride (NaF) and
deionized water were used without further purification. All the
other reagents and solvents used in this study were of guaranteed
reagent grade.
∗
Corresponding author. Tel.: +91 8555 287235; fax: +91 8555 287234 260.
E-mail address: chelli janardhana@rediffmail.com (C. Janardhana).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.03.023

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R. Sai Sathish et al. / Spectrochimica Acta Part A 66 (2007) 457–461
Fig. 1. Ligand exchange reaction of complex upon F− addition.
2.2. Instrumentation
tribution by singly excited configurations involved the orbital
criterion set to 10, i.e. the 10 highest occupied molecular orbitals
(HOMO) and the ten lowest unoccupied molecular orbitals
(LUMO) have been considered [39]. Hyperchem (version 7.0)
program was used to perform all the calculations.
The fluorescence spectra were recorded on a Perkin-Elmer
LS5 Luminescence Spectrometer, at a band path of 5 nm for
excitation and emission. Electronic spectra were recorded on a
Hitachi U-2001 spectrophotometer with 1 cm quartz cells.
3. Results and discussion
2.3. Procedures
ARS forms a stable 2:1 complex with Al(III) in aqueous
medium, without hydrolysis of the aluminum ion [25–29]. The
choice of 0.08 M pH 4.38 acetate buffer for the fluoride bind-
ing study with the binary ARS–Al(III) complex was two-fold,
to avoid precipitation of the aluminum-hydroxy species and to
ensure complete complexation between ARS and Al(III) possi-
ble only at pH > 4 [27].
For the fluoride binding study of the ARS–Al(III) complex,
ARS, AlCl3 and NaF were dissolved in deionized water to obtain
a stock solution of 2 × 10−3 M each and 100 mg L−1 fluoride
ion, respectively. For the interference study, stock solutions of
anions were prepared by dissolving NaCl, Na2SO4, NaNO3,
Na2HPO4, KI, KBrindeionizedwaterandeachofthestocksolu-
tions of cations were prepared by dissolving anhydrous FeCl3
and CuCl2. EDTA stock solution was prepared by dissolving
Na2 EDTA in deionized water. In 10 mL standard flasks, 1 mL
of 2 × 10−3 M ARS, 0.5 mL of 2 × 10−3 M AlCl3 solution and
100 ␮L of 0.08 M acetate buffer solution (pH 4.38) were added
to solutions of various fluoride ion concentrations and made up
to the mark with deionized water and were used to record the
absorption and fluorescence spectra. Complexation equilibrium
of each solution was ensured by allowing the solution to stand
for 2 h at room temperature (25 ◦C) and then the electronic and
fluorescence spectra were recorded.
3.1. Detection and binding of fluoride ion
A working concentration of 2 × 10−4 M ARS: 1 × 10−4 M
AlCl3 in water (pH 4.38) gave a red colored chelate (2:1 com-
position) with a characteristic band at λmax = 468 nm, involv-
ing a bathochromic shift of 43 nm when compared to the free
ARS with a λmax = 425 nm. Inference drawn from these spectral
changes is the complexation of ARS to AlCl3. The solution gave
a pale orange fluorescence with a λem-max = 575 nm upon exci-
tation at λex = 435 nm (isosbestic point). The excitation at the
isosbestic point (435 nm) to obtain the fluorescence emission
spectra was carried out instead at the band maxima (468 nm), as
the fluoride ion fluorescence quenching studies of the complex
were monitored better by excitation at 435 nm. For excitation at
468 nm, resulted in spectra where the fluorescence intensity of
the free ARS was relatively high compared to the complex, than
excitation at 435 nm, where the intensity of the free ARS was
very low to that of the complex.
The absorption spectra of a series of solutions having the
red colored chelate (2:1 composition) and different fluoride ion
concentrations ranging from 0 to 6 ppm were recorded. With a
successive increase in the fluoride ion concentration the com-
plex peak at 468 nm showed a decrease in absorbance, while
the free ARS peak at 425 nm showed an increase. Distinct isos-
bestic point at 435 nm (Fig. 2) was obtained from the series of
absorbance spectra. A simultaneous and corresponding decrease
2.4. Calculations
The current study considered the ARS molecule, the cor-
responding dianion and its Al(III) complex theoretically in
vacuo, using the semi-empirical quantum chemical calculations.
Molecular-mechanics [33] MM+ force field [34] was used for
preoptimization of each of the starting geometries, followed
by the Austin Model 1 (AM1) semi-empirical method [35]
within the spin Unrestricted Hartree-Fock (UHF) formalism
[36] to fully optimize the systems geometry. Polak–Ribiere
algorithm [37], a conjugate gradient method, was employed
for the AM1 geometry optimization with the RMS gradi-
˚
ent set to 0.001 kcal/(A mol) and the SCF convergency set to
0.0001 kcal/mol. Configuration interaction treatment [38] was
used to calculate the electronic absorption spectra. The con-

R. Sai Sathish et al. / Spectrochimica Acta Part A 66 (2007) 457–461
459
Table 1
Effect of foreign ions on the sensitivity and selectivity of the complex at a
constant fluoride ion concentration of 1 × 10−5 M
Ions
Concentration (M)
Recovery (%)
None
Cl−
100.0
97.7
100.8
98.5
90.9
98.5
99.1
58.1
84.0
65.5
99.2
1 × 10−3
2−
SO4
NO3
PO4
I−
Br−
1 × 10−3
1 × 10−3
−
3−
1 × 10−3
1 × 10−3
1 × 10−3
Fe(III)
Fe(III) + EDTA
Cu(II)
0.5 × 10−4
0.5 × 10−4 + 0.5 × 10−4
0.5 × 10−4
Cu(II) + EDTA
0.5 × 10−4 + 0.5 × 10−4
are presented in Table 1, in terms of recovery, which indicates
the percentage of the fluorescence signal intensity before and
after addition of foreign ions [1].
Fig. 2. Change in absorption intensity of complex on F− addition in water (pH
4.38).
Common anions like Cl−, SO42−, NO3−, PO43−, I− and
Br− did not interfere even when the concentration of the ions
are increased 100-fold as compared to that of fluoride ion
(0.3 mg L−1) in aqueous medium. Metal ions like Fe(III), Cu(II)
showed interference due to formation of ARS–metal complexes
in the fluorescence signal intensity of the complex (Fig. 3) was
observed in the emission spectra upon fluoride addition. The
quenching of the orange fluorescence of the complex upon fluo-
ride addition was attributed to the replacement of both the ARS
molecules from the metal center by a concerted co-ordination
of the fluoride ion [1] having a high affinity to Al(III) [31,32].
Fluoride ion detection range from 5 × 10−6 to 3 × 10−4 M
with a detection limit as low as 0.1 mg L−1 is offered by this
ARS–Al(III) complex system. Reproducibility with a variation
coefficient of 0.6% was obtained for repeated analysis of five,
1 mg L−1 fluoride samples.
Table 2
˚
Bondlengths(A)calculatedbyAM1methodforARS, ARSdianionandcomplex
Bond length
ARS
ARS dianion
Complex
C1–C2
C2–C3
C3–C4
C4–C14
C14–C10
C14–C13
C10–C11
C11–C5
C11–C12
C5–C6
C6–C7
C7–C8
C8–C12
C12–C9
C9–C13
C13–C1
C1–O1
O1–H1
C2–O2
O2–Na2
C3–S1
1.492
1.434
1.400
1.390
1.456
1.440
1.474
1.400
1.405
1.395
1.395
1.395
1.401
1.481
1.468
1.380
1.361
0.972
1.259
2.576
1.683
1.430
1.442
1.444
2.512
1.126
1.260
1.102
1.100
1.101
1.103
1.246
1.483
1.466
1.436
1.458
1.455
1.394
1.464
1.408
1.416
1.388
1.409
1.379
1.422
1.440
1.458
1.441
1.356
0.974
1.241
1.432
1.396
1.395
1.404
1.467
1.417
1.405
1.401
1.404
1.395
1.396
1.395
1.402
1.477
1.470
1.389
1.407
0.984
1.341
3.2. Interference study
The sensitivity and selectivity of the ARS–Al(III) complex
signaling system is determined by the affinity of this system
to fluoride ion in presence of competing foreign ions as in any
water sample. The influence of the common anions and cations
1.777
1.413
1.420
1.411
1.795
1.398
1.405
1.431
S1–O3
S1–O4
S1–O5
O5–Na5
C4–H4
C10–O10
C5–H5
C6–H6
C7–H7
C8–H8
C9–O9
O1–Al
1.093
1.257
1.101
1.097
1.098
1.101
1.279
1.107
1.241
1.103
1.100
1.100
1.102
1.245
1.836
1.774
Fig. 3. Change in fluorescence spectral intensity of complex on F− addition in
water (pH 4.38); λex = 435 nm.
O2–Al

460
R. Sai Sathish et al. / Spectrochimica Acta Part A 66 (2007) 457–461
Table 3
a planar conformation in the ground state, in complete agree-
ment with [40] and the ARS molecule is stabilized by an
intra-molecular hydrogen-bond: O9–H1–O1, 1.986 A [28]. But
Bond angles (◦) calculated by AM1 method for ARS, ARS dianion and complex
˚
Bond angle
ARS
ARS dianion
Complex
on reaction with AlCl3, the hydrogen-bond in ARS weakens
to: O9–H1–O1, 2.112 A in total agreement with regard to the
C1–C2–C3
C2–C3–C4
116.4
118.2
123.6
119.8
119.1
121.2
117.6
120.2
119.8
120.0
120.1
120.0
120.1
120.3
119.5
121.3
117.7
122.4
119.0
118.6
122.5
126.1
110.7
111.4
118.8
127.3
124.8
123.2
110.2
108.5
103.5
123.6
118.5
122.0
120.4
119.8
122.5
117.9
118.6
119.1
120.8
119.9
120.1
120.1
119.8
120.7
119.1
112.8
111.3
117.3
119.4
117.7
122.8
116.4
119.8
120.4
119.8
121.0
119.6
120.2
121.3
118.2
122.1
117.4
121.4
120.6
118.0
122.6
124.6
109.6
112.8
122.0
120.0
117.3
122.4
119.1
120.3
120.6
117.4
119.0
121.6
119.4
120.5
120.0
119.9
120.3
119.8
121.0
117.2
120.7
122.0
117.3
121.5
126.4
112.3
111.7
116.7
˚
proposed complexation reaction. The main geometrical param-
eters consisting of bond lengths and bond angles of ARS, its
corresponding dianion and the ARS–Al(III) complex attained
with energy minimization using AM1 method, are presented in
Tables 2 and 3. The bond lengths and bond angles for ARS and
its Al(III) complex obtained by the AM1 method are in good
agreement with the data from density functional theory (DFT)
with B3LYP functionals as the basis set [40]. There is not, much
change in the bond lengths and bond angles upon complexation
of the corresponding dianion of ARS with AlCl3. From the bond
C3–C4–C14
C4–C14–C10
C4–C14–C13
C10–C14–C13
C14–C10–C11
C10–C11–C5
C10–C11–C12
C5–C11–C12
C11–C5–C6
C5–C6–C7
C6–C7–C8
C7–C8–C12
C8–C12–C11
C9–C12–C11
C12–C9–C13
C9–C13–C1
C9–C13–C14
C1–C13–C14
C13–C1–C2
C13–C1–O1
C1–O1–H1
C2–C1–O1
C1–C2–O2
C2–O2–Na2
C3–C2–O2
C2–C3–S1
˚
˚
˚
length data of the complex: O1–H1, 0.984 A; O1–Al, 1.836 A;
˚
O2–Al, 1.774 A as compared to that of ARS: O1–H1, 0.972 A,
˚
˚
O2–Na2, 2.576 A; O5–Na5, 2.512 A, the AM1 method clearly
points out that the bonds involving the two sodium atoms of ARS
dissociate first to generate the dianion, which then undergoes a
concerted co-ordination with Al(III).
Simplest excited state theory involving single excitation con-
figuration interaction was used. Good agreement between exper-
imental and theoretical wavelengths was obtained.
125.2
126.1
106.5
103.5
107.4
123.1
118.4
107.3
103.8
101.8
4. Conclusion
C3–S1–O3
C3–S1–O4
C3–S1–O5
S1–O5–Na5
C4–C3–S1
The complexation reaction between ARS and AlCl3 yields a
2:1 stoichiometry composition of the chelate in aqueous medium
and the ligand exchange reaction of this chelate with fluoride
ion, releases both the ARS molecules with a rapid change in the
absorbance and fluorescence intensity, delivering a fluorescent
fluoride ion probe in aqueous medium.
122.6
122.4
121.2
120.6
122.1
121.0
119.7
118.2
120.8
120.6
119.9
119.3
120.5
121.3
117.4
123.8
122.6
120.0
121.2
121.6
118.7
119.0
118.8
120.7
119.9
120.1
120.2
119.9
120.7
119.0
107.6
107.7
129.6
C14–C10–O10
C11–C10–O10
C12–C9–O9
C13–C9–O9
C3–C4–H4
C14–C4–H4
C11–C5–H5
C6–C5–H5
C5–C6–H6
C7–C6–H6
C6–C7–H7
C8–C7–H7
C7–C8–H8
C12–C8–H8
C1–O1–Al
Acknowledgement
The authors are grateful to the Chancellor, Bhagawan Sri
Sathya Sai Baba, Sri Sathya Sai Institute of Higher Learning for
his constant inspiration.
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