A new strategy for highly selective fluorescent sensing of F- and Zn2+ with dual output modes

Article abstract of DOI:10.1039/c2jm16364k

A new strategy for highly selective and sensitive detection of both F ^- and Zn²⁺ with dual output modes was developed, which is based on a combination of F^- induced desilylation reaction and chelation-enhanced fluorescence

Full text of DOI:10.1039/c2jm16364k

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A new strategy for highly selective fluorescent sensing of Fꢀ and Zn2+ with
dual output modes†
*
Yinyin Bao, Bin Liu, Fanfan Du, Jiao Tian, Hu Wang and Ruke Bai
Received 5th December 2011, Accepted 30th January 2012
DOI: 10.1039/c2jm16364k
A new strategy for highly selective and sensitive detection of both Fꢀ
and Zn2+ with dual output modes was developed, which is based on
a combination of Fꢀ induced desilylation reaction and chelation-
enhanced fluorescence effect caused by Zn2+.
8-Hydroxy-2-methylquinoline (Oxn) is a well-known building
block for chelation enhanced fluorescent (CHEF) sensors for tran-
sition metals.12 In our previous papers, the sensors based on
8-hydroxyquinoline were prepared and used to detect Fꢀ (ref. 13) and
Cd2+ (ref. 14) ions, respectively. Herein we report a new strategy for
highly selective and sensitive detection of both Fꢀ and Zn2+ utilizing
fluorescent chemosensor 1, by a combination of Fꢀ induced desily-
lation reaction15 and chelation-enhanced fluorescence (CHEF) effect
caused by Zn2+. The sensor 1 was synthesized through Suzuki
coupling reaction and the synthetic routes are illustrated in Scheme 1.
The commercial 8-hydroxy-2-methylquinoline was reacted with tert-
butyldimethylsilyl chloride (TBDMSCl) in CH2Cl2 (ref. 16) and
brominated with N-bromosuccinimide (NBS) at room temperature
to form 3. Then 3 was borylated by pinacolborane to afford 2 with
a yield of 79%. Finally, 2 was reacted with 4-bromobenzaldehyde
through Suzuki coupling, resulting in the sensor molecule 1 in 70%
Numerous chemosensors and receptors for anions1 as well as metal
ions2 have been reported in recent years due to their biological
importance. Among the anions, fluoride is the smallest and has
unique chemical properties because of the high electronegativity and
strong basicity. Fluoride exists in toothpaste and pharmaceutical
agents and plays an important role in preventing dental caries and
in the treatment of osteoporosis.3 However, excessive ingestion of
Fꢀ may result in fluorosis, cause nephrotoxic changes in both humans
and animals, and lead to urolithiasis.4 On the other hand, zinc, as the
second most abundant transition-metal ion in the human body, plays
vital roles in biological processes. The quantity variance of zinc would
affect the function of the proteins and enzymes concerning neural
signal transmission, DNA reproduction, and other processes.5
Therefore, the accurate determination of the concentration of fluo-
ride and zinc is very important for human health and environment
protection.
yield. The product was characterized by H NMR, 13C NMR, and
1
mass spectra analysis (see ESI†).
The emission spectrum of sensor 1 in CH3CN shows a bright blue
fluorescence with a peak at 458 nm and upon addition of fluoride
ions, a strong quenching phenomenon was observed. As shown in
Fig. 1, the emission peak at 458 nm gradually decreases with
increasing the amount of Fꢀ ions, and the intensity exhibits linear
changes with the concentration of Fꢀ from 0 to 20 mM. It was found
that more than 95% of the fluorescence can be quenched after
interaction with 1.0 equiv. Fꢀ ions, and the emission color changes
under UV lamp can be distinctly observed by the naked eye (Fig. 7).
The quantum yields of the chemosensor 1 before and after interaction
with Fꢀ ions are calculated to be 0.12 and 0.008, respectively
(see ESI†). The detection limit of Fꢀ was determined at least down to
The fluorescent sensing method has been developed as a useful tool
for detecting anions and metal ions due to the simplicity, high
sensitivity, high selectivity, and real-time detection.1d,e Although
a number of Fꢀ (ref. 6–8) and Zn2+ (ref. 9 and 10) fluorescent che-
mosensors have been reported, the probes which can exhibit obvious
fluorescence response to both Fꢀ and Zn2+ are rather rare,11 and the
sensing properties especially selectivity of the probes remain unsat-
isfied. In addition, most of the dual functional fluorescent sensors for
anions and cations are based on the displacement approach,11a,d
which means the sensing output mode is single and the selectivity may
be limited. In order to develop new multifunctional fluorescent
sensors and improve the sensing properties, the design of the sensors
for Fꢀ and Zn2+ with a new sensing mechanism is still a quite
important and intriguing theme.
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer
Science and Engineering, University of Science and Technology of China,
Hefei, P. R. China 230026. E-mail: bairk@ustc.edu.cn; Fax: +86-551-
3631760; Tel: +86-551-3600722
† Electronic supplementary information (ESI) available: Synthetic
procedures, experimental details, additional spectroscopic data. See
DOI: 10.1039/c2jm16364k
Scheme 1 Synthesis of the sensor 1.
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Fig. 1 (a) Fluorescence spectra of sensor 1 + TBAF in CH3CN in
the presence of increasing Fꢀ concentrations. (b) Emission intensity at
458 nm as a function of Fꢀ concentrations. [1] ¼ 20 mM. lex ¼ 340 nm.
1.0 mM by fluorimetric assay (Fig. S4, ESI†). This indicates that
compound 1 is an efficient ON–OFF fluorescent sensor.
Fig. 3 Time course of the fluorescence response of 1 in CH3CN upon
addition of various concentrations of Fꢀ (1–6 equiv.).
To examine the selectivity of 1 toward various anions, the fluo-
ꢀ
rescence responses upon the addition of Clꢀ, Brꢀ, Iꢀ, ClO4ꢀ, HSO4
,
The cation-binding properties of sensor 1 after addition of Fꢀ ions
were studied in CH3CN. It was found that the sensor exhibits high
sensitivity and selectivity for detection of Zn2+. The fluorescence
spectrum of 1 exhibits a weak blue emission band at 458 nm in
CH3CN (F ¼ 0.008, lex ¼ 340 nm) upon interaction with Fꢀ ions.
However, after addition of Zn2+ ions it immediately exhibited strong
yellowish-green fluorescence. As shown in Fig. 4a, the emission
intensity at 458 nm gradually decreased and a new peak with a large
red shift and enhanced intensity appeared at 540 nm with increasing
the concentration of Zn2+, which is due to the ESPT process being
well blocked by the interaction with Zn2+. The complex shows large
Stokes shift of 200 nm, which is much bigger than that of most
traditional dyes such as fluorescein, rhodamine, and polymethine
cyanine.9,10 The fluorescence intensity and the ratio of emission
intensities (F540 nm/F458 nm) change linearly with the concentration of
Zn2+ (Fig. 4b and S8, ESI†). The quantum yield of the Zn2+ complex
was calculated to be 0.05, and the detection limit of Zn2+ was deter-
mined at least down to 120 nM by fluorimetric assay. By direct
fluorimetric titration,18 the association constants (log Ka) of the
complex were calculated to be 10.22. In addition, the fluorescence
detection did not exhibit any appreciable time-dependent effects.
These indicate that the system is an excellent ratiometric fluorescent
sensor as well as a chelation-enhanced fluorescence (CHEF) sensor
for Zn2+ with high sensitivity. Meanwhile, the absorption titration
also showed obvious changes in the spectrum, indicative of the
formation of the Zn2+ complex (Fig. S2, ESI†).
AcOꢀ, H2PO4ꢀ, and CNꢀ were examined. As shown in Fig. 2, upon
addition of 5.0 equiv. of various anions, only Fꢀ ion induced
a dramatic emission quenching effect, and all the other anions did not
cause any obvious intensity decreases. This result indicates that the
new fluorescent chemosensor exhibits a high selectivity for fluoride
anions in CH3CN.
The obvious fluorescence changes of 1 upon interaction with Fꢀ
ions can be attributed to the Fꢀ induced desilylation reaction. Upon
addition of fluoride ions, the sensor 1 is deprotected to give 5-(4-
aldehyde)-8-hydroxy-2-methylquinoline, which is non-fluorescent
owing to the efficient radiationless relaxation to the ground state by
intra and intermolecular excited-state proton transfer (ESPT).17 The
1
detection process was studied by H NMR spectroscopy (Fig. S3,
ESI†). It could be observed that the Si–CH3 signal upfield shifted
from 0.34 to 0.17 ppm, and exhibited a doublet as a result of coupling
with the fluoride, and the C(CH3)3 signal upfield shifted from 1.09 to
0.92 ppm. The evidence above is consistent with the deprotection
reaction and formation of a C–F chemical bond. The desilylation
reaction was also confirmed by the UV-vis titration experiment
(Fig. S1, ESI†).
The effect of the reaction time on the fluorescence emission of the
sensor with various amounts of Fꢀ was examined from 0 to 15 min.
The result shown in Fig. 3 and S7† implicates that the fluorescence
quenching rate depends on Fꢀ concentration, this is because the fact
that the higher the Fꢀ concentration, the faster the desilylation
reaction is. The emission intensity of sensor 1 reaches its saturation
value in about half an hour upon interaction with 1.0 equiv. Fꢀ ions
(Fig. S5 and S6, ESI†).
Upon interaction with 1.0 molar equivalent of various metal ions
of alkali (Na+, K+), alkaline earth (Mg2+, Ca2+), and transition-metal
Fig. 2 (a) Fluorescence spectra of 1 in the presence of various anions
(5 equivalents of Fꢀ, Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ, ClO4ꢀ(TBA+ salts), AcOꢀ,
H2PO4ꢀ, CNꢀ(Na+ salt)) in CH3CN. (b) F0/F of the sensor system at
458 nm in the presence of various anions. [1] ¼ 20 mM. lex ¼ 340 nm.
Fig. 4 (a) Fluorescence spectra of the system [1 + Fꢀ] + Zn2+ in CH3CN
in the presence of increasing Zn2+ concentrations. (b) Ratiometric
calibration curves F540nm/F458nm as a function of Zn2+ concentrations.
[1] ¼ [Fꢀ] ¼ 20 mM. lex ¼ 340 nm.
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ions (Ag+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+), the system shows
excellent selectivity to Zn2+. As shown in Fig. 5, only Zn2+ induces
a dramatic fluorescence enhancement of 32-fold at 540 nm. Upon
addition of 3.0 equivalent Cd2+ ions, a new peak at 525 nm was
observed. However, the intensity is obviously much lower than that
of the Zn2+ complex. And no increase of fluorescence was identified in
the presence of other metal ions. It should be pointed out that Zn2+
metal ions can be distinguished from others by naked eye because of
the bright yellowish-green emission. Thus, the molecule 1 after
addition of Fꢀ ions can be used as a highly selective fluorescent sensor
for distinguishing Zn2+ from other metal ions, especially from Cd2+.
The solvent effect of the novel fluorescent sensor 1 was also
examined. With decreasing solvent polarity from CH3CN, acetone,
to THF, the lem values of sensor 1 shifted significantly to the shorter
wavelength region by 48 nm (from 458 nm in CH3CN, 440 nm in
acetone, to 418 nm in THF) along with decreasing the fluorescence
intensity (Fig. 6). The fluorescent quantum yields of sensor 1 in
acetone and THF were calculated to be 0.023 and 0.006, respectively.
The sensing properties of sensor 1 toward Fꢀ in acetone and THF
were studied. In acetone, the emission peak at 440 nm gradually
decreases upon addition of Fꢀ ions, and the fluorescence quenching is
similar to that in CH3CN (Fig. S9 and S10, ESI†). However, the
sensor 1 in THF exhibits quite different changes when interacting
with Fꢀ ions. As shown in Fig. S20,† after addition of fluoride ions,
sensor 1 immediately changes from blue-violet fluorescence to
yellowish-green fluorescence. The emission intensity at 418 nm
gradually decreases and a new peak at 520 nm appears upon inter-
action with the fluoride anion (Fig. S12 and S13, ESI†). This
phenomenon has been observed in our previous report,13 which can
also be attributed to the combination of desilylation reaction and
excited state proton transfer from the desilylation product to fluoride.
The ratiometric fluorescence can only be observed in THF, which
may be due to the fact that Fꢀ ions have much stronger binding
abilities to the OH group of the desilylation product in THF
compared with that in CH3CN and acetone.
Fig. 6 Fluorescence spectra of 1 in CH3CN, acetone, and THF.
[1] ¼ 20 mM. lex ¼ 340 nm.
are much lower than that in CH3CN. In addition, the system also
exhibits different responses to Cd2+ in different solvents (Fig. S18 and
S19, ESI†). Therefore, the sensing properties of sensor 1 and its
desilylation compound for detection of Fꢀ, Zn2+, and Cd2+ can be
efficiently tuned by changing the solvents. It is necessary to point out
that the sensor 1 and its desilylation compound can be used as a small
sensor array, the naked eye sensing array was shown in Fig. 7.
In conclusion, we have developed a new strategy for highly selec-
tive and sensitive detection of both Fꢀ and Zn2+ utilizing fluorescent
chemosensor 1, which is based on a combination of Fꢀ induced
desilylation reaction and chelation-enhanced fluorescence effect
(CHEF) caused by Zn2+. The sensor 1 shows excellent selectivity for
Fꢀ over other anions, and the deprotection product in situ exhibits
high selectivity to Zn2+ with a nanomolar range detection limit. The
fluorescent sensing process can be obviously observed by the naked
eye under UV illumination, and the sensing properties to Fꢀ and Zn2+
can be efficiently tuned by varying the solvents. Thus, the novel
‘‘naked eyes’’ fluorescent sensor is indeed an excellent candidate probe
for the detection of fluoride and zinc ions, which provides a new idea
for the development of multifunctional sensors for both cations and
anions with dual output modes.
The Zn2+ sensing properties of sensor 1 after addition of Fꢀ ions
were also examined in acetone and THF. We observed that the
system exhibits a dramatic fluorescence enhancement upon addition
of Zn2+ ions both in acetone and THF. The quantum yields of the
complex system in acetone and THF were calculated to be 0.091 and
0.092, respectively, and the values are bigger than the quantum yield
of the complex system in CH3CN (F ¼ 0.05). And the detection limits
of Zn2+ in acetone and THF were determined to be about 20 nM and
28 nM by fluorimetric assay, respectively (Fig. S14–S17, ESI†), which
The financial support from National Natural Science Foundation
of China (no. 20974104 and no. 21074120) and Ministry of Science
and Technology of China (no. 2007CB936401) is gratefully
acknowledged.
Fig. 5 (a) Fluorescence spectra of the system [1 + Fꢀ] in the presence of
various metal ions (30 equiv. of Na+, K+; 3 equiv. of Mg2+, Ca2+, Ag+,
Zn2+, Cd2+, Ni2+, Cu2+, Co2+, Fe3+) in CH3CN. (b) Intensity ratios
F540nm/F458nm of the sensor system in the presence of various metal ions.
[1] ¼ [Fꢀ] ¼ 20 mM. lex ¼ 340 nm.
Fig. 7 Naked eye under UV lamp fluorescent color changes observed
for 1 upon addition of Fꢀ, Zn2+ and Cd2+ in CH3CN, acetone, and THF.
[1] ¼ [Fꢀ] ¼ 20 mM. [Zn2+] ¼ [Cd2+] ¼ 60 mM. lex ¼ 340 nm.
(Full photography can be seen in Fig. S20, ESI†.)
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