A reaction-based colorimetric fluoride probe: Rapid naked-eye detection and large absorption shift

Article abstract of DOI:10.1002/cplu.201200106

Taking advantage of both the well-known azobenzene structure and the special fluoride-promoted cleavage reaction of the SiO bond, a new ratiometric colorimetric probe (F-1) toward the fluoride anion was designed and synthesized by using intramolecular charge transfer (ICT) as a signal mechanism. Upon the addition of F ions, the probe displayed apparent color changes from colorless to deep blue, with a dramatic shift of the maximum absorption wavelength (230 nm). With the aid of UV/Vis measurements, the detection limit could be as low as 15 mm. The probe possessed much higher selectivity for fluoride over other common anions. Excitingly, by virtue of a dipstick approach, F-1 could serve as colorimetric probe for convenient measurements, without the requirement of any additional equipment

Full text of DOI:10.1002/cplu.201200106

DOI: 10.1002/cplu.201200106
A Reaction-Based Colorimetric Fluoride Probe: Rapid
“Naked-Eye” Detection and Large Absorption Shift
Xiaohong Cheng,[a] Shuang Li,[a] Guohua Xu,[b] Conggang Li,[b] Jingui Qin,[a] and Zhen Li*[a]
Taking advantage of both the well-known azobenzene struc-
ture and the special fluoride-promoted cleavage reaction of
the SiÀO bond, a new ratiometric colorimetric probe (F-1)
toward the fluoride anion was designed and synthesized by
using intramolecular charge transfer (ICT) as a signal mecha-
nism. Upon the addition of FÀ ions, the probe displayed appar-
ent color changes from colorless to deep blue, with a dramatic
shift of the maximum absorption wavelength (ꢀ230 nm). With
the aid of UV/Vis measurements, the detection limit could be
as low as 15 mm. The probe possessed much higher selectivity
for fluoride over other common anions. Excitingly, by virtue of
a “dipstick” approach, F-1 could serve as colorimetric probe for
convenient measurements, without the requirement of any ad-
ditional equipment.
Introduction
In recent years, the fluoride anion has emerged as a valuable
target for recognition and sensing resulting from its dual role
as a dangerous pollutant on one hand,[1] and a useful additive
in the human diet and in industrial processes on the other.[2]
For these reasons, an improved method for the detection and
sensing of FÀ that shows high selectivity is of current interest
in the field of chemosensors. Because conventional ion-selec-
tive electrodes and ion chromatography are costly and require
cumbersome equipment and complicated procedures,[3] optical
chemosensors appear to be particularly attractive because of
their simplicity, high sensitivity, high selectivity, and rapid im-
plementation.[4] In particular, colorimetric sensors are more
promising as the color change can be easily observed with the
naked eye, thus requiring less labor and no equipment. Mean-
while, ratiometric colorimetric probes can enable the measure-
ment of absorption intensities at two different wavelengths,
providing a built-in correction for environmental effects and
also increasing the dynamic range of absorption measure-
ment.[5] This method is considered a good approach to over-
come the major limitation of intensity-based probes, in which
variations in the environmental sample and probe distribution
can be problematic for quantitative measurements. In fact, nu-
merous chromogenic sensors have been reported to detect FÀ
selectively,[6] however, in many cases, the probes show relative-
ly small absorption wavelength shifts before and after binding
target ions. Thus, the accurate determination of the ratio of
the two absorption peaks is limited in some degree. Therefore,
the development of chromogenic sensors toward FÀ with
a large absorption shift and good ratiometric response is of
importance.
sors exhibit higher selectivity and stability.[9] Kim and Swager
pioneered a new strategy based on the chemical affinity be-
tween fluoride and silicon.[10] Tert-butyldimethylsilyl (TBDMS)
and tert-butyldiphenylsilyl (TBDPS) were chosen as additional
substituents for the dye molecule, and rendered the dye un-
reactive to potentially interfering compounds and thus only
sensitive to fluoride ions. This approach allowed the develop-
ment of a new class of sensors based on this highly specific
and efficient reaction, and consequently, this facile and reliable
method is set to become one of the emerging strategies in an-
alytical chemistry.
During our research on nonlinear optical materials, we
found that the azobenzene chromophore certainly had merits
such as strong absorption in the visible region.[11] As shown in
Scheme 1 and Figure S1 in the Supporting Information, com-
pound F-0 displays a maximum absorption wavelength cen-
tered at about 380 nm in its UV/Vis spectrum. Interestingly,
under basic conditions the anionic form, F-2, displays a dramat-
ically red-shifted absorption (ꢀ215 nm) corresponding to the
enhanced intramolecular charge transfer (ICT) efficiency de-
rived from its stronger electron-donor ability. Taking advantage
of the chemical affinity between fluoride and silicon, it was ex-
pected that the reaction of chemosensors consisting of
a TBDPS moiety with FÀ would trigger the cleavage of the SiÀ
[a] X. Cheng, S. Li, Prof. J. Qin, Prof. Z. Li
Department of Chemistry, Wuhan University
Wuhan 430072 (P. R. China)
E-mail: lizhen@whu.edu.cn
lichemlab@163.com
Fluoride is a strong hydrogen-bond acceptor, and has a high
affinity to silicon and boron. These unique physical and chemi-
cal properties have been widely exploited in the development
of numerous sensors for fluoride anions.[7] A large number of
reported receptors commit the binding to hydrogen-bonding
interactions.[8] Compared to these sensors, reaction-based sen-
[b] G. Xu, C. Li
State Key Laboratory of Magnetic Resonance and Atomic and Molecular
Physics, Wuhan Institute of Physics and Mathematics
Chinese Academy of Sciences
Wuhan, 430071 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201200106.
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Results and Discussion
Synthesis and structural char-
acterization
The synthetic route to com-
pound F-1
is
shown
in
Scheme S1. The target com-
pound was prepared conven-
iently through a substitution re-
action between F-0 and tert-bu-
tyldiphenyl chlorosilane, accord-
ing to a reported method.[12] The
whole synthetic route was
Scheme 1. The approach in this study for detection of FÀ.
O bond, release the anionic species, and accompanied by a re-
markably red-shifted absorption.
simple and the purification was easy.
Compound F-1 exhibited good solubility in common organic
solvents such as CHCl3, acetone, N,N-dimethylformamide
(DMF), dimethyl sulfoxide (DMSO), CH3CN, and THF, and was
characterized by spectroscopic methods giving satisfactory
spectroscopic data (see the Experimental Section). Com-
pound F-0 was light yellow in color with a maximum absorp-
tion wavelength centered at about 380 nm. After the reaction
with tert-butyldiphenyl chlorosilane, F-1 was colorless with
a maximum absorption wavelength centered at about 365 nm.
Then, it was expected that the fluoride anion could attack the
silicon center in F-1 and trigger the cleavage of the SiÀO bond
to generate the stabilized anionic species, namely, F-2. As
a result, there was stronger electron-donor efficiency and con-
comitant enhanced intramolecular charge transfer (ICT) in the
anionic form. Consequently, the color of the solution became
deep blue with the maximum absorption wavelength shifted
to 595 nm. If the fluoride anion was the only species capable
of inducing these changes, F-1 could act as a fluoride-selective
colorimetric chemosensor.
With these considerations in mind, we conceived that if the
hydroxy functionality in F-0 was protected using a common
silyl group such as tert-butyldiphenylsilyl (TBDPS), the resultant
molecule F-1 would exhibit a pale color similar to that of F-0.
If a fast and quantitative removal of the TBDPS group can be
achieved in the presence of fluoride anions, the molecular
system could be used in fluoride sensing via a ratiometric
mechanism. Thus, we synthesized the TBDPS-protected azo-
benzene compound F-1 (Scheme 2). Upon the addition of FÀ
ions, the probe displayed apparent color changes from color-
FÀ sensing properties
Because the colorimetric signal changes relied on the chemical
reaction between F-1 and a fluoride anion, the reaction rate
might affect the experimental results. First, we investigated the
influence of the reaction time on the probing results, and the
obtained results were shown in Figure S2. There was almost
no time-dependent effects in the colorimetric sensing process,
and the absorption intensity centered at 595 nm reached its
saturation value in just 30 seconds as a result of the rapid de-
protection reaction. Based on its fast response, F-1, as a colori-
metric chemosensor, could provide very rapid detection of FÀ.
Then, FÀ ions were added into a diluted solution of F-1 and
we investigated the response in detail. As shown in Figure 1,
the absorption maximum of F-1 shifted from 365 to 595 nm. It
was noted that the difference in the two absorption wave-
lengths was very large (spectrum shift: Dlmax =230 nm), which
not only contributed to the accurate measurement of the in-
tensities of the two absorption peaks, but also resulted in
a huge ratiometric value. Meanwhile, the presence of the new
band at 595 nm corresponded to a larger molar absorption co-
efficient than the original one. The intensities at different
Scheme 2. The structure of F-1 and the sensing mechanism.
less to deep blue, with a dramatic shift of the maximum ab-
sorption wavelength (ꢀ230 nm). Compound F-1 was sensitive
to fluoride ions with high selectivity, and the detection limit
was determined to be as low as 15 mm. Furthermore, F-1 was
successfully applied to the detection of fluoride ions in test
strips. Herein, we describe the new ratiometric colorimetric
probe for fluoride anion in detail.
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Colorimetric Probes for Anions
pictures of Figure 1), thus validating our design of ratiometric
colorimetric chemosensor toward fluoride, based on the high
affinity of fluoride to silicon.
As mentioned above, the ultimate goal of this research was
to develop a “naked-eye” colorimetric sensor. Compound
F-1 had good sensitivity, thus, potentially, it could be used for
direct naked-eye detection of FÀ. The pictures in Figure 3
showed that although not so sensitive as measured by the UV/
Vis spectrometer, we could easily distinguish the color change
at concentrations as low as 30 mm of FÀ (vialB in Figure 3).
Figure 1. UV/Vis spectra of F-1 (30 mm, in THF) in the presence of increasing
concentration of FÀ. Inset: photograph of F-1 and F-1+FÀ.
Figure 3. Photograph of F-1 (30 mm, in THF) to various concentration of FÀ:
(A) 0, (B) 3, (C) 6, (D) 10, (E) 15, (F) 20, (G) 25, (H) 30, and (I) 40ꢁ10À5 molLÀ1
.
wavelengths of 595 and 365 nm were summarized in Figure 2.
It is easily seen that the intensity change was almost linear to
the concentrations of the fluoride anion. In fact, in the pres-
ence of 1.0 equivalents of FÀ, an approximate 290-fold en-
hancement in the ratiometric value of A595/A365 (from 0.05 to
To assess the specificity of our sensor toward FÀ, various
anions were examined in parallel under the same conditions.
As shown in Figure 4, the reaction of F-1 with FÀ gave appar-
ent absorbance changes; whereas other anions (as their tetra-
À
butylammonium salts) such as ClÀ, BrÀ, IÀ, NO3À, HSO4À, ClO4
,
PF6À, H2PO4À, AcOÀ, did not show any significant changes. The
exception was CNÀ, which gave a slight response. These re-
sults further confirmed that only the addition of fluoride could
induce the apparent color changes (see the inset picture of
Figure 4). Therefore, F-1 had much higher selectivity for FÀ
than other anions, thus enabling the direct detection of fluo-
Figure 2. Ratiometric calibration curve A595 A365 of F-1 (30 mm, in THF) as
a function of the concentration of FÀ.
14.6) was achieved with respect to the fluoride-free solution.
Accordingly, the detection limit could be determined to be
15 mm,[13] which was lower than the enforceable drinking water
standard for fluoride ions (4 mgLÀ1, 210 mm) set by the United
States Environmental Protection Agency (EPA).[14] More impor-
tantly, the colorimetric difference for the solution of F-1 (from
colorless to deep blue) before and after the addition of fluo-
ride could be distinguished by the naked eye (see the inset
Figure 4. UV/Vis spectra of F-1 (30 mm, in THF) in the presence of different
anions. Inset: Photograph of F-1 to various anions. A–L: F-1, F-1+ClÀ,
F-1+BrÀ, F-1+IÀ, F-1+NO3À, F-1+HSO4À, F-1+ClO4À, F-1+PF6À, F-
1+H2PO4À, F-1+AcOÀ, F-1+CNÀ, F-1+FÀ (as their tetrabutylammonium
salts).
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Z. Li et al.
ride ions by the naked eye over other species. As an interest-
ing phenomenon, very few reports of TBAF chemosensors in-
volved the possible influence from [Bu4N]+OHÀ ions.[15] Consid-
ering the alkaline property of [Bu4N]+OHÀ, here, we also inves-
tigated the possible influence of OHÀ ions. As shown in Fig-
ure S3, upon the addition of OHÀ ions, similar phenomena as
in the case of FÀ ions were observed: the absorption maximum
shifted from 365 to 600 nm with a dramatic spectrum shift
(Dlmax =235 nm). The results were unexpected, but it indicated
that in the cleavage reaction of the SiÀO bond, OHÀ could be
an alternative choice to promote the reaction instead of FÀ in
some special cases (i.e., where FÀ ions were prohibited).
Furthermore, the sensing properties of F-1 at different con-
centrations (15 mm) toward FÀ were studied carefully to test
the reproducibility of the sensing behavior of F-1 for fluoride
anions (Figure S4 and S5). At the concentration of 15 mm, the
absorption intensity of F-1 changed even at a concentration of
FÀ as low as 5 mm (Figure S5). This outcome was understanda-
ble: the sensing property of F-1 toward FÀ was based on the
special chemical reaction; only after some needed amount of
F-1 was converted into F-2 with the addition of fluoride could
the absorbance change be monitored. The lower the concen-
tration of F-1, the lower the concentration of fluoride needed
for the conversion from F-1 into the anionic species F-2, and
thus, the relatively higher sensitivity. This might be another ad-
vantage of this kind of chemosensors: the linear range for the
detection of fluoride anion could be adjusted conveniently by
controlling the concentrations of the chemosensors. These re-
sults demonstrated the good response of our probe F-1 for
fluoride, thus realizing our design of new chemosensors
toward FÀ ions based on the ITC mechanism and the fluoride-
promoted removal of the TBDPS group.
Figure 5. Photographs of test strips of F-1 exposed to various concentration
of FÀ: (A) 0, (B) 5, (C) 10, (D) 20, (E) 25, (F) 30, and (G) 40ꢁ10À5 mol LÀ1
.
v/v) as shown in Figure S5. In contrast to the almost instanta-
neous reaction in THF, the reaction of F-1 with FÀ required
more than 5 hours in THF-HEPES buffer depending on the re-
action conditions. This outcome could be attributed to the
strong solvation of FÀ by water, which would undoubtedly in-
crease the activation barrier. Nevertheless, a high selectivity for
FÀ was retained in THF-HEPES buffer, as shown by the little
change in the absorption spectra in the presence of other
anions (Figure S6).
To explore the sensing mechanism of F-1 to fluoride, the re-
action mixture of F-1 with excess FÀ was characterized by
1
1H NMR spectroscopy. The partial H NMR spectra of the resul-
tant complex of F-1 and excess FÀ in [D6]DMSO is shown in
Figure 6b. The aromatic proton (a1’ and a2’) near the TBDPS
group display an apparent upfield shift compared with those
of F-1 (a1 and a2), as a result of the formation of anionic spe-
cies with enhanced electron-donating ability. Meanwhile, the
resonance signal corresponding to protons b’ and c’ under-
went a small upfield shift. Whereas the aromatic proton (d1’
and d2’) near the nitro group did not display much change.
Also, we measured the 29Si NMR spectra before and after the
addition of FÀ. As shown in Figure S7, the silicon signal dis-
played obvious downfield shift (d from À25.2 to 14.9 ppm).
These observations indicate that the fluoride did trigger the
Motivated by the favorable features of this system in solu-
tion, test strips were prepared by immersing a silica gel plate
(3ꢁ1 cm2) into a solution of F-1 in THF (1ꢁ10À3 molLÀ1) and
the plate was then dried in air, to determine the suitability of
a dipstick method for the detection of FÀ, similar to that com-
monly used for the measure-
ment of pH values. When the
test strips coated with F-1 were
immersed into the solution of FÀ
in THF with different concentra-
tions, the obvious color change
from light yellow to blue was
observed (Figure 5). The detec-
tion limit for these dipsticks was
relatively low (10ꢁ10À5 molLÀ1).
The development of such a dip-
sticks approach is extremely at-
tractive because it does not re-
quire any additional equipment
for visualization.
We further explored the prac-
tical application of probe F-1,
and repeated all of the above
experiments in THF-HEPES buffer
solution (10 mm, pH 7.4; 95:5, Figure 6. 1H NMR spectra of F-1 (a) and F-1+FÀ (b) as its tetrabutylammonium salt in [D6]DMSO.
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was warmed to RT, and tert-butyldiphenyl chlorosilane (329 mg,
1.2 mmol) was added. The reaction mixture was stirred for 12 h at
RT. Then the solvent was evaporated, and the crude product was
diluted in CHCl3, washed with H2O and brine, dried over Na2SO4,
and concentrated in vacuo. The resultant residue was purified by
column chromatography on silica gel using petroleum ether/THF
(4:1, v/v) as eluent to afford compound F-1 as a light red solid
(450 mg, 93.5%). 1H NMR (300 MHz, [D6]DMSO): d=8.39–8.36 (d,
2H, J=9.0 Hz, ArH), 7.97–7.94 (d, 2H, J=9.0 Hz, ArH), 7.81–7.80 (d,
2H, J=3.0 Hz, ArH), 7.70–7.67 (m, 5H, ArH), 7.50–7.44 (m, 5H, ArH),
6.94–6.91 (d, 2H, J=9.0 Hz, ArH), 1.05 ppm (s, 9H, CH3); 13C NMR
(75 MHz, [D6]DMSO): d=160.0, 156.2, 148.5, 147.3, 135.7, 135.0,
132.3, 130.5, 129.9, 128.2, 125.5, 124.9, 123.3, 120.7, 26.5, 19.7 ppm;
MS (ESI): m/z calcd for C17H15N3O2: 482.1 [M+H]+; found: 482.2; ele-
mental analysis calcd (%) for C17H15N3O2 (482.1): C 70.26, H 5.68,
N 8.57; found: C 69.83, H 5.65, N 8.72.
cleavage of the SiÀO bond and the formation of the SiÀF
bond. Furthermore, when excess trifluoroacetic acid (TFA) was
added to the mixture of F-1 and FÀ, the color of the solution
gradually faded and the maximum absorption wavelength was
blue-shifted to 380 nm, and the absorption spectra became
closer to that of F-0 (Figure S8). These results suggested the
conversion from the anionic form to the hydroxy form. There-
fore, all these data indicate that the reaction followed the pro-
posed mechanism as shown in Scheme 2.
Conclusion
In summary, naked-eye detection of the fluoride anion was
achieved by using a ratiometric colorimetric probe (F-1) de-
signed by taking advantage of the well-known azobenzene
structure and the special fluoride-promoted cleavage reaction
of the SiÀO bond. Upon the addition of FÀ ions, the probe dis-
played apparent color changes from colorless to deep blue
with a dramatic shift of the maximum absorption wavelength
(ꢀ230 nm). With the aid of the UV/Vis spectrometer, the detec-
tion limit could be as low as 15 mm. Moreover, a practical appli-
cation could be realized, as confirmed by the test-strip experi-
ment. The probe featured advantages such as easy prepara-
tion, a large absorption shift, good ratiometric response, as
well as successful application in test strips based on the fluo-
ride-promoted cleavage of the SiÀO bond of an azobenzene
compound. Further study on the improved design for a probe
for fluoride is in progress.
Preparation of solutions of anions
One millimole of anionic compounds such as [Bu4N]+ClÀ, [Bu4N]+
BrÀ, [Bu4N]+IÀ, [Bu4N]+NO3À, [Bu4N]+HSO4À, [Bu4N]+ClO4À, [Bu4N]+
PF6À, [Bu4N]+H2PO4À, [Bu4N]+AcOÀ, [Bu4N]+CNÀ, [Bu4N]+FÀ (TBAF)
were dissolved in THF (10 mL) to afford 1ꢁ10À1 molLÀ1 solution.
The 25% methanol solution of [Bu4N]+OHÀ was employed and di-
luted to desired concentrations with THF. One millimole of inor-
ganic salt (NaCl, KBr, KI, NaOAc·3H2O, NaNO3, Na2SO3, Na2CO3,
Na2SO4, Na3PO4, KClO3, Na2HPO4·12H2O, NaHSO3, Na2S2O3·5H2O, and
NaF) were dissolved in doubly distilled water (10 mL) to afford 1ꢁ
10À1 molLÀ1 aqueous solution. The stock solutions were diluted to
desired concentrations when needed.
UV absorption changes of F-1 by FÀ
Experimental Section
Compound F-1 (48.2 mg) was dissolved in THF (10 mL) to afford
1ꢁ10À2 molLÀ1 solution. The stock solutions were diluted with THF
to desired concentrations when needed. Then 3.0 mL of the solu-
tion of F-1 was placed in a quartz cell (10.0 mm width) and the ab-
sorption spectrum was recorded. The TBAF solution was intro-
duced in portions and the absorption spectrum were recorded at
room temperature each time.
Materials and instrumentations
Tetrahydrofuran (THF) was dried over and distilled from K/Na alloy
under an atmosphere of dry nitrogen. Anionic salts including
[Bu4N]+ClÀ, [Bu4N]+BrÀ, [Bu4N]+IÀ, [Bu4N]+NO3À, [Bu4N]+HSO4
[Bu4N]+ClO4À, [Bu4N]+PF6À, [Bu4N]+H2PO4À, [Bu4N]+AcOÀ, [Bu4N]+
CNÀ, [Bu4N]+OHÀ, [Bu4N]+FÀ, and the inorganic salt (NaCl, KBr, KI,
NaOAc·3H2O, NaNO3, Na2SO3, Na2CO3, Na2SO4, Na3PO4, KClO3,
Na2HPO4·12H2O, NaHSO3, Na2S2O3·5H2O, and NaF) were of analytical
grade and used as received. Doubly distilled water was used in all
experiments.
À
,
UV absorption changes of F-1 by different anions
Compound F-1 (48.2 mg) was dissolved in THF (10 mL) to afford
1ꢁ10À2 molLÀ1 solution. The stock solutions were diluted with THF
or THF/HEPES (95:5, v/v) to desired concentrations when needed.
Then 3.0 mL of the solution of F-1 was placed in a quartz cell
(10.0 mm width) and the absorption spectrum was recorded. Dif-
ferent anion solutions were introduced and the absorption spectra
were recorded at room temperature each time.
The 1H and 13C NMR spectra were measured on a Varian Mercu-
ry 300 spectrometer. The 29Si NMR spectra were measured on an
INOVA 600 spectrometer using tetramethylsilane (TMS; d=0 ppm)
as the reference standard. The ESI mass spectra were measured on
a Finnigan LCQ advantage mass spectrometer. Elemental analyses
were performed by a CARLOERBA-1106 microelemental analyzer.
The UV/Vis spectra were measured on a Shimadzu UV-2550 spec-
trometer. The pH values were determined by using a DELTA 320
pH meter.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (no. 20974084) for financial support.
Synthesis of compound F-1
Compound F-0 was synthesized according to the literature.[16] A so-
lution of F-0 (243.1 mg, 1 mmol) in anhydrous THF (6 mL) was
added dropwise to an ice-cold solution of NaH (30 mg, 1.25 mmol)
in anhydrous THF (2 mL) . After the addition was completed, the
resultant mixture was stirred at 08C for 5 min. Then the solution
Keywords: azobenzenes
probe · fluorides · sensors
· charge transfer · colorimetric
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Received: May 4, 2012
Revised: June 13, 2012
Published online on && &&, 0000
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ChemPlusChem 2012, 00, 1 – 7
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FULL PAPERS
Seen with the naked eye: A new ratio-
metric colorimetric probe toward fluo-
ride has been designed based on the
fluoride-promoted cleavage of the SiÀO
bond by using intramolecular charge
transfer as a signaling mechanism.
Upon the addition of FÀ, the probe
changes color from colorless to deep
blue and is visible with the naked eye.
This change is accompanied by a dra-
matic shift of the maximum absorption
wavelength (ꢀ230 nm; see scheme).
X. Cheng, S. Li, G. Xu, C. Li, J. Qin, Z. Li*
&& – &&
A Reaction-Based Colorimetric
Fluoride Probe: Rapid “Naked-Eye”
Detection and Large Absorption Shift
ChemPlusChem 2012, 00, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chempluschem.org
&
7
&
ÞÞ
These are not the final page numbers!