An instantaneous and highly selective chromofluorogenic chemodosimeter for fluoride anion detection in pure water

Article abstract of DOI:10.1002/open.201300010

The fast and the fluoride: A pyridine derivative functionalized with ferf-butyl-dimethylsilyl ether has been synthesized and used as a selective chromofluoro-genic fluoride sensor in water/cetyltri-methylammonium bromide solutions. The chromofluorogenic r

Full text of DOI:10.1002/open.201300010

DOI: 10.1002/open.201300010
An Instantaneous and Highly Selective Chromofluorogenic
Chemodosimeter for Fluoride Anion Detection in Pure
Water
Sameh Elsayed, Alessandro Agostini, Luis E. Santos-Figueroa, Ramꢀn Martꢁnez-MꢂÇez,* and Fꢃlix Sancenꢀn[a]
The development of chromofluorogenic sensors for anions is
a well-established and studied field in the supramolecular
chemistry realm.[1] This is due to the important roles of anions
in biological processes, deleterious effects (such as environ-
mental pollutants), and as toxic compounds and carcinogenic
species.[2]
fluoride recognition employ this anion’s well-known ability to
promote the hydrolysis of silyl ethers.[8] In particular, several ex-
amples of selective chemodosimeters for fluoride anion based
on BODIPY derivatives,[9] terphenyl derivatives,[10] diphenylacet-
ylenes,[11] naphthalimides,[12] benzothiazolium hemicyanine de-
rivatives,[13] coumarins[14] and azo dyes,[15] functionalized with
silyl ether moieties, have been recently described. Regardless
of the high selectivity of these probes to the fluoride anion,
most of them work in organic solvents (acetonitrile, acetone,
tetrahydrofuran, dimethyl sulfoxide, dichloromethane) or
organic/water mixtures (acetonitrile/water or ethanol/water).
In fact, as far as we know, there are only two molecular-
based examples that display sensing features for fluoride in
water. One example is based on a N-(3-(benzo[d]thiazol-2-yl)-4-
(hydroxyphenyl) benzamide derivative that is functionalized
with tert-butyldiphenylsilyl ether.[16a] In this case, the chemo-
dosimeter solutions showed an emission band at 418 nm that
was progressively quenched and substituted for new fluores-
cence at 560 nm upon the addition of increasing quantities of
fluoride anion. The observed fluorogenic response was as-
cribed to the hydrolysis of the tert-butyldiphenylsilyl moiety,
which yielded a highly emissive compound. The second is
a very recently published example based on a fluorescein de-
rivative bearing a biocompatible hydrophilic poly(ethylene
glycol) polymer and two tert-butyldiphenylsilyl ether moie-
ties.[16b] Aqueous solutions (buffered at pH 7.4) of this chemo-
dosimeter showed a very weak emission band centred at
526 nm (excitation at 490 nm) that increased gradually upon
addition of fluoride anion. Again, the observed response was
due to the fluoride-induced tert-butyldiphenylsilyl ether hydrol-
ysis. Moreover, it is worthy to mention that reported silyl
ether-based probes for fluoride detection, showing sensing
features in aqueous environments, usually require several mi-
nutes (typically in the 3–50 min range) in order to achieve the
complete hydrolysis of the silyl ether group.
Among inorganic anions, fluoride is widely used in the pre-
vention of dental caries and in osteoporosis treatments.[3] In
spite of these important applications, acute exposure to this
anion can cause various diseases, such as nausea, abdominal
pain, coma, hypocalcaemia, skeletal fluorosis and osteomala-
cia.[4] For all these reasons, the interest in developing chromo-
fluorogenic probes for the selective detection of fluoride has
grown in the last few years.[5]
Most of the examples reported for fluoride sensing are
based on the use of the “binding site–signalling subunit” ap-
proach or on the “chemodosimeter” protocol.[6] In the probes
for which the binding site–signalling subunit approach is used,
an optical signalling unit is covalently bonded to a binding
site, which usually consists of one H-bond donor moiety or
more, such as urea or thiourea groups. The coordination or de-
protonation of the acidic protons of the binding sites by fluo-
ride induces changes in colour or fluorescence, which enables
anion detection.[7] In spite of these important features, the che-
mosensors constructed according to this paradigm present
some drawbacks, such as lack of selectivity (other basic anions
like cyanide, acetate and dihydrogenphosphate usually give
nearly the same optical response) and are unable to display
sensing features in competitive media such as water or mixed
aqueous environments.
In order to overcome these limitations, the chemodosimeter
approach recently has been applied for chromofluorogenic
sensing of fluoride anion.[5b] These probes use selective reac-
tions induced by the target anion, coupled with chromofluoro-
genic changes. Some of the reported chemodosimeters for
Bearing in mind the very few examples that the literature re-
ports of the chromo-fluorogenic sensing of fluoride in pure
water, and after considering our interest in the development
of new optical probes for anions, we designed the novel che-
modosimeter 3 for fluoride sensing, based on a pyridine deriv-
ative functionalized with a tert-butyldimethylsilyl ether group.
Compound 3 was able to sense fluoride instantaneously in
pure water in the presence of a cationic surfactant. The cation-
ic surfactant was selected as a simple method to solubilize the
probe in the inner hydrophobic core of the formed micelles,
thus also favouring the inclusion of fluoride anion due to their
positively charged shell.
[a] S. Elsayed, A. Agostini, L. E. Santos-Figueroa, Prof. R. Martꢀnez-MꢁÇez,
Dr. F. Sancenꢂn
Centro de Reconocimiento Molecular y Desarrollo Tecnolꢂgico (IDM)
Unidad Mixta Universidad Politꢃcnica de Valencia-Universidad de Valencia
Departamento de Quꢀmica, Universidad Politꢃcnica de Valencia
CIBER de Bioingenierꢀa, Biomateriales y Nanotecnologꢀa (CIBER-BNN)
Camino de Vera s/n, 46022 Valencia (Spain)
E-mail: rmaez@qim.upv.es
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This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited and is not used for commercial purposes.
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The synthesis of chemodosimeter 3 was achieved by
a three-step procedure (Scheme 1). In the first step, 2,6-diphen-
ylpyrylium perchlorate was treated consecutively with methyl-
magnesium iodide and triphenylcarbenium tetrafluoroborate
to give 4-methyl-2,6-diphenylpyrylium tetrafluoroborate (1).[17]
Figure 1. UV/Vis spectra of acetonitrile solutions of chemodosimeter 3
(3.0ꢅ10À5 moldmÀ3) in the presence of 10 equivalents of selected anions.
probe 3 changed from colourless to yellow. The other anions
tested induced negligible changes in the UV/Vis profile of 3.
Encouraged by the selective response shown by 3 in aceto-
nitrile, further assays in pure water were performed. UV/Vis
studies in the presence of anions were carried out in this
highly competitive medium. Probe 3 is weakly soluble in water,
but is readily solubilised in water at pH 7.4 containing the cat-
ionic surfactant cetyltrimethylammonium bromide (CTABr) at
a concentration of 5 mm. The improved solubilisation of 3 in
the presence of CTABr is ascribed to the inclusion of 3 into the
hydrophobic core of the surfactant micelles. A cationic surfac-
tant was selected to favour the approach of the anionic fluo-
ride to the positively charged CTABr and enhance diffusion of
anions into the micelles. The aqueous CTABr solutions of 3
(3.0ꢅ10À5 molLÀ1) were also colourless. Moreover, the addition
of fluoride anion induced the appearance of a visible band, to-
gether with a colour change from colourless to yellow. The
other anions tested gave no response (i.e., ClÀ, BrÀ, IÀ, CNÀ,
Scheme 1. Synthesis of probe 3. Reagents and conditions: a) CH3MgI, Et2O,
RT, 10 h; b) (C6H5)3CBF4 (1.06 equiv), RT, 3 h, 66%; c) 4-Hydroxbenzaldehyde
(1.00 equiv), EtOH, reflux, 12 h, 75 %; d) NH4OH, RT, 1 h, 93%; e) TBDMSCl
(1.10 equiv), N-methylimidazole (3.30 equiv), CH3CN, RT, 1 h, 56%.
Then, compound 1 was condensed with 4-hydroxybenzalde-
hyde to give stilbene-pyrylium derivative 2.[18] Finally, 2 was re-
acted with ammonium hydroxide (for transformation of the
pyrylium ring into a pyridine) and with tert-butyldimethylsilyl
chloride (TBDMSCl, for transformation of the hydroxyl moiety
into a silyl ether), to give the final chemodosimeter 3. The
1H NMR (in CDCl3) spectra of 3 showed two singlets centred at
0.25 ppm (6H) and 1.02 ppm (9H) ascribed to the methyl and
tert-butyl groups of the silyl ether group. The most characteris-
tic signals in the aromatic region were one singlet centred at
7.75 ppm, attributed to the two equivalent protons located in
the 2,4,6-trisubstituted pyridine heterocycle, and the two dou-
blets with a 15 Hz coupling constant centred at 7.04 and
7.38 ppm, assignable to the hydrogen atoms of the trans
double bond. HRMS-EI measurements confirmed the structure
of the final product with an m/z value of 464.2403 (464.2331
calculated for C31H34ONSi+).
SCNÀ, AcOÀ, BzOÀ, HCO3À, NO3À, ClO4À, HSO4 and H2PO4 ).
This fluoride-selective response arises from this known abili-
ty of the anion to hydrolyse silyl ether moieties. This hydrolysis
induced the formation of a phenolate anion (see structure 4 in
Scheme 2) with a strong, negatively charged donor oxygen
atom, which is responsible for the appearance of both the red-
À
À
As a first step, the chromogenic behaviour of probe 3 in ace-
tonitrile (3.0ꢅ10À5 molLÀ1) in the presence of 10 equivalents of
selected anions was studied (i.e., FÀ, ClÀ, BrÀ, IÀ, CNÀ, SCNÀ,
AcOÀ, BzOÀ, CO32À, NO3À, ClO4À, HSO4 and H2PO4 ). Only the
addition of fluoride anion induced remarkable optical changes.
In particular, the UV band at 325 nm underwent a hypsochro-
mic shift, while a new band appeared at 445 nm upon fluoride
addition (Figure 1). Due to these changes, the solutions of
À
À
Scheme 2. Mechanism of the chromogenic response of 3 in the presence of
fluoride anion.
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shifted band at 445 nm and the colour change observed.
These results are in agreement with the well-established rule
that an increase in the donor ability of a donor moiety in
a pull–push chromophore results in a bathochromic shift.[19]
Further studies demonstrated that sensing features of 3 in
water containing CTABr and fluoride were observed when the
pH is larger than 6.5, whereas lower pH values resulted in no
colour changes (data not shown).
1
The sensing mechanism was confirmed by H NMR studies
carried out in deuterated acetonitrile. In this solvent, nearly all
the signals of 3 underwent upfield shifts in the presence of
fluoride (Figure 2; see Scheme 2 for proton assignment). More-
over, the most remarkable shifts were observed for protons
He, Ha and Hc. The singlet of the trisubstituted pyridine ring
shifted from 7.80 (He) to 7.73 ppm (He’). In addition, the pro-
tons of the 1,4-disubstituted benzene rings located in an ortho
position of the silyl ether moiety shifted from 6.79 (Ha) to
Figure 3. Emission intensity at 540 nm (lex =445 nm) of chemodosimeter 3
(3.0ꢅ10À5 molLÀ1) in H2O at pH 7.4 containing 5 mm CTABr in the absence
(blank) and in the presence of selected anions (10 equiv).
An additional remarkable feature of probe 3 was
the fact that the hydrolysis reaction in the micellar
medium was instantaneous. In this sense, it is worth
mentioning that other silyl ether-based chemodosim-
eters for fluoride detection, showing sensing features
in aqueous environments (usually mixed organic/
water media), usually require several minutes, typi-
cally in the range of 3–50 min, to achieve the com-
plete hydrolysis of the silyl ether group, whereas
probe 3 displayed changes instantaneously.[14–16]
In summary, we report herein the design of
Figure 2. 1H NMR spectra of probe 3 in CD3CN (bottom) and 3 with one equivalent of
fluoride anion (top).
a chromo-fluorogenic pyridine-based silyl ether-con-
taining probe for the selective recognition of fluoride
anions in water/CTABr solutions. Addition of fluoride
anion induces a change in colour from colourless to
6.37 ppm (Ha’), whereas one of the protons of the double
bond shifted from 7.04 (Hc) to 6.71 ppm (Hc’). These upfield
shifts are in agreement with the formation of phenolate
anion 4 (Scheme 2) by fluoride-induced silyl ether hydrolysis as
a key step in the chromogenic response observed.
yellow and the appearance of a strong emission band. Both
changes are ascribed to a fluoride-induced hydrolysis of the
silyl ether moiety, which yields a phenolate anion. Probe 3 is
Having assessed the selective chromogenic behaviour of
probe 3 in water in the presence of anions, fluorogenic studies
were also carried out. The solutions of chemodosimeter 3
(3.0ꢅ10À5 molLÀ1) at pH 7.4 containing 5 mm CTABr were not
fluorescent upon excitation at 445 nm (F=0.0024). However,
the addition of fluoride induced the appearance of an intense
emission band centred at 540 nm (F=0.042), whereas the
other anions tested (i.e., ClÀ, BrÀ, IÀ, CNÀ, SCNÀ, AcOÀ, BzOÀ,
À
HCO3À, NO3À, ClO4À, HSO4 and H2PO4À) induced negligible
changes in the emission profile of 3 (Figure 3).
To further characterise the fluorogenic behaviour of chemo-
dosimeter 3, changes in the emission intensity band at 540 nm
were studied in the presence of increasing amounts of the
fluoride anion. Figure 4 shows a typical titration profile. From
this calibration curve, a detection limit of 1.4 ppm of fluoride
was calculated using a conventional fluorimeter. This detection
limit is below the recommended value for fluoride content in
drinking water (1.8 ppm).
Figure 4. Calibration curve for fluoride using chemodosimeter 3
(3.0ꢅ10À5 molLÀ1) in water (pH 7.4) containing 5 mm CTABr.
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Synthesis of chemodosimeter 3: Aq NH4OH (32% v/v; 5 mL) was
added to a solution of compound 2 (750 mg, 2.14 mmol) in EtOH
(30 mL), and the solution was stirred at RT for 1 h. The solvent was
evaporated in vacuo to give a blue solid (700 mg, 2 mmol, 93%
yield). This blue solid (700 mg, 2 mmol) was dissolved in anhyd
CH3CN (20 mL), and after tert-butyldimethylsilyl chloride (424 mg,
2.2 mmol) and N-methylimidazole (493 mg, 6.6 mmol) were added,
the mixture was stirred at RT for 1 h. The solvent was evaporated
in vacuo, and the crude was dissolved in Et2O (20 mL), washed
with a concd Na2S2O3 (1ꢅ30 mL), dried with Na2SO4, filtered and
purified by column chromatography (alumina; hexane/acetone
9:1 v/v). The chemodosimeter 3 was obtained as a yellowish brown
highly selective and is one of the very few systems for fluoride
that displays sensing features in pure water. Moreover, the op-
tical changes are instantaneous, and, as far as we know, our
system is the fastest reagent for fluoride signalling in an aque-
ous environment using silyl ether-based probes.
Experimental Section
General: UV/Vis spectra were recorded with a JASCO V-650 spec-
trophotometer (Easton, MD, USA). Fluorescence measurements
were carried out in a JASCO FP-8500 spectrophotometer. 1H and
13C NMR spectra were acquired with a 400 MHz Bruker Avance III,
whereas mass spectra were carried out with a TripleTOF T5600
spectrometer (AB Sciex, Framingham, MA, USA).
1
solid (490.6 mg, 1.12 mmol, 56% yield): H NMR (CDCl3, 400 MHz):
d=0.25 (s, 6H), 1.02 (s, 9H), 6.89 (d, J=6.6 Hz, 2H), 7.04 (d, J=
15.0 Hz, 1H), 7.38 (d, J=15.0 Hz, 1H), 7.48 (m, 8H), 7.75 (s, 2H),
8.19 ppm (d, J=6.5 Hz, 4H); 13C NMR (100 MHz, CDCl3): d=À4.2,
18.4, 25.8, 116.1, 122.9, 128.9, 133.0, 135.0, 135.4, 135.6, 136.3,
141.1, 151.8, 161.2, 176.0, 177.4 ppm; HRMS (EI): m/z [M]+ calcd for
C31H34ONSi: 464.2331; found: 464.2403.
Triethyl orthoformate (98%), CH3I (99%), tert-butyldimethylsilyl
chloride (98%), 4-hydroxybenzaldehyde (98%), acetophenone
(99%), anhyd CH3CN (99.8%), magnesium, tetraphenylphosphoni-
um tetrafluoroborate and hexadecyltrimethylammonium bromide
(CTABr) were purchased from Sigma–Aldrich. Analytical grade sol-
vents, aq NH4OH (32% w/v), perchloric acid (60% v/v) and anhyd
MgSO4 were purchased from Scharlau (Barcelona, Spain).
Acknowledgements
Synthesis of 2,6-diphenylpyrylium perchlorate: Acetophenone
(4 mL, 34.3 mmol) and triethyl orthoformate (10 mL, 60.1 mmol)
were placed in a round-bottomed flask (250 mL) under Ar atmos-
phere at 08C. After 15 min of stirring, perchloric acid (60% v/v;
7.4 mL, 86.5 mmol) was added dropwise over 30 min. The reaction
was allowed to react at RT for 1 h. By addition of Et2O (15 mL), the
final 2,6-diphenylpyrylium perchlorate derivative was precipitated
as a yellow solid (10.1 g, 30.4 mmol, 88.6% yield). NMR data of the
synthesized product agreed with those described in the litera-
ture.[20]
Financial support from the Spanish Government (project
MAT2012–38429-C04–01) and the Generalitat Valencia (project
PROMETEO/2009/016) is gratefully acknowledged. S.E. is grateful
to the Generalitat Valenciana for his Santiago Grisolia fellowship.
Keywords: chemodosimeter
hydrolysis · silyl ether
· colour change · fluoride ·
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Received: February 22, 2013
Published online on April 3, 2013
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ChemistryOpen 2013, 2, 58 – 62 62