A novel fluorescent off-on probe for the sensitive and selective detection of fluoride ions

Article abstract of DOI:10.1039/c9ra06342k

A highly sensitive and selective fluorescent probe for fluoride ions has been developed by incorporating the dimethylphosphinothionyl group as a recognition moiety into the fluorophore of coumarin. The detection mechanism is based on the fluoride ion-triggered cleavage of the dimethylphosphinothionyl group, followed by the release of coumarin, which leads to a large fluorescence enhancement at 455 nm (λ_ex = 385 nm). Under the optimized conditions, the fluorescence enhancement of the probe is directly proportional to the concentration of fluoride ions in the range of 0-30 μM with a detection limit of 0.29 μM, which is much lower than the maximum content of fluoride ions guided by WHO. Notably, satisfying results have been obtained by utilizing the probe to determine fluoride ions in real-water samples and commercially available toothpaste samples. The proposed probe is rather simple and may be useful in the detection of fluoride ions in more real samples.

Full text of DOI:10.1039/c9ra06342k

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A novel fluorescent off–on probe for the sensitive
and selective detection of fluoride ions†
ab
Cite this: RSC Adv., 2019, 9, 32308
Min Zhang,a Kaijing Chang,b Yu Kang,a Guodong Ren,a Xiaoyu Hou,a
*
Lihong Li,
b
b
b
ab
*
Wen Liu, Haojiang Wang, Bin Wang and Haipeng Diao
A highly sensitive and selective fluorescent probe for fluoride ions has been developed by incorporating the
dimethylphosphinothionyl group as a recognition moiety into the fluorophore of coumarin. The detection
mechanism is based on the fluoride ion-triggered cleavage of the dimethylphosphinothionyl group,
followed by the release of coumarin, which leads to a large fluorescence enhancement at 455 nm (lex
¼
385 nm). Under the optimized conditions, the fluorescence enhancement of the probe is directly
proportional to the concentration of fluoride ions in the range of 0–30 mM with a detection limit of 0.29
mM, which is much lower than the maximum content of fluoride ions guided by WHO. Notably, satisfying
results have been obtained by utilizing the probe to determine fluoride ions in real-water samples and
commercially available toothpaste samples. The proposed probe is rather simple and may be useful in
the detection of fluoride ions in more real samples.
Received 14th August 2019
Accepted 4th October 2019
DOI: 10.1039/c9ra06342k
rsc.li/rsc-advances
bonding interaction,16,17 boron–uoride complexion18,19 and
uoride induced chemical reaction.20–32 Among them the rst
two types of probes could only work in non-aqueous media or
Introduction
Fluoride ions (Fꢀ), as the smallest anion with the highest elec-
tronegativity, play a major role in health and environmental
science. Appropriate intake of Fꢀ has proven to be critical for
preventing tooth cavities and treating osteoporosis.1–3 Thus
water uoridation or addition of uoride to toothpaste has been
extensively employed in a great many countries.4,5 However, the
excess ingestion of Fꢀ may lead to uorosis, accompanied with
a series of diseases, such as tooth mottling, kidney and gastric
disorders, metabolic disturbances, neurotoxicity and so on.6–8
Therefore the amount of Fꢀ in water and other uoride
supplements must be strictly limited. As suggested by the World
Health Organization (WHO), the maximum level of Fꢀ in
drinking water should be 1.5 mg Lꢀ1.9 Unfortunately, uoride
contamination in groundwater, which means the Fꢀ concen-
tration is higher than 1.5 mg Lꢀ1, has been regarded as a severe
threat throughout the world.10 Therefore, it is of great impor-
tance to develop a simple approach to monitor uoride ions in
water as well as other samples.
media with high percentage of organic solvent because the
detection signal would be largely hindered in aqueous envi-
ronment.23,24 So the third strategy, that is uoride induced
chemical reaction, has attracted much attention. Owing to the
high affinity of Fꢀ towards silicon, several probes have been
developed relying on the uoride-triggered cleavage of Si–O
bond.20–22,24–30,32 Besides, dimethyl-phosphinothionyl groups,
which have been utilized as the protecting groups for side chain
phenolic OH of tyrosiꢀne in peptide synthesis, could also be
easily removed by F .33,34 Thus dimethylphosphinothionyl
group would be a potential recognition moiety for Fꢀ. However,
uorescent probes employing such a recognition unit are still
rare.23,31 Herein, we report probe 1 as a sensitive and selective
uorescence off–on probe for Fꢀ.
The probe (Scheme 1) is designed by incorporating the
specic recognition moiety (dimethylphosphinothionyl group)
into coumarin. Probe 1 itself is nearly nonuorescent due to the
hydroxyl blocking of coumarin,35–37 which is favourable for
achieving high detection sensitivity. Reaction of 1 with Fꢀ
Among the present analytical methods for Fꢀ, uorescent
probes have caught great attention due to their high sensitivity
and selectivity as well as easy operation.11–15 To date, uorescent
probes based on various sensing mechanisms have been re-
ported for the detection of Fꢀ, including the uoride–hydrogen
aDepartment of Biochemistry and Molecular Biology, Shanxi Medical University,
Taiyuan 030001, PR China. E-mail: lilh@sxmu.edu.cn; diaohp@sxmu.edu.cn
bSchool of Basic Medical Sciences, Shanxi Medical University, Taiyuan 030001, PR
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06342k
Scheme 1 Structure of probe 1 and its reaction with Fꢀ.
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causes the cleavage of the P–O bond between dimethylphos- transferred to a 1 cm quartz cell to monitor uorescence with
phinothionyl group and coumarin, accompanied by the release lex/em ¼ 385/455 nm. Meanwhile, a solution containing no Fꢀ
of coumarin and dramatic uorescence enhancement, which (control) was made and recorded under the same conditions for
results in the establishment of a sensitive and selective method comparison.
for assaying Fꢀ. The probe has been successfully applied to
determine Fꢀ in real-water samples and commercially available
toothpaste samples.
Procedure for determining Fꢀ in real samples
To assess the potential use of probe 1 in determining Fꢀ in real
samples, two water samples were collected from tap water and
Yingze Lake (Taiyuan), respectively. Stock solutions of sodium
uoride (1 mM) were prepared in tap water and Yingze Lake
water which were pretreated by ltering with 0.22 mm cellulose
acetate membranes. The following determination was per-
formed as the aforementioned procedure.
Besides, several commercially available toothpastes were
selected to evaluate the practical application of probe 1. Firstly,
0.2 g of toothpaste was accurately weighed and dissolved in
30.0 mL water. Then, the mixture was heated to 70 ꢂC and
stirred for 3 h. Aer cooling down to room temperature, the
mixture was centrifuged at 13 000 rpm for 10 min. The super-
nate was transferred to a 100 mL volumetric ask and the
residue was washed three times with water. Finally, the stock
solution of toothpaste was prepared by adding water to the
scale.
Experimental
Reagents and apparatus
7-Hydroxyl-coumarin, dimethylthiophosphinoyl chloride and
triethylamine were purchased from Sigma-Aldrich. NaF, NaCl,
KBr, NaI, Na2CO3, Na2SO4, Na2SO3, K2HPO4 and KH2PO4 were
purchased from Beijing Chemicals, Ltd. Sodium acetate and
tris(hydroxymethyl)aminomethane (Tris) were obtained from J
& K Chemical Ltd. Ultrapure water (over 19 MU cm) from
a Smart-N system (Heal Force, China) was used throughout.
1H- and 13C-NMR spectra were performed on a Brucker DMX-
400 spectrometer. High resolution electrospray ionization mass
spectra (HR-ESI-MS) were operated on an APEX IV FTMS
instrument (Bruker Daltonics). UV-Vis Absorption spectra were
measured with a UV-6100 spectrophotometer (Mapada, China)
in 1 cm quartz cells. Fluorescence spectra were collected on
a Varian spectrouorimeter (Cary Eclipse, America) in 1 ꢁ 1 cm
quartz cells.
Results and discussion
Spectroscopic response of probe 1 to Fꢀ
Synthesis of probe 1
Absorption and uorescence spectra of 1 before and aer
reaction with Fꢀ are shown in Fig. 1. As is seen, probe 1 has two
absorption peaks at 285 nm and 315 nm. Besides, 1 itself
exhibits almost no uorescence emission due to the blocking of
hydroxyl in coumarin,35–37 which is rather favourable for the
sensitive detection. Upon treatment with Fꢀ, the maximum
absorption peaks of the reaction system are red-shied and
a new absorption weak appears at 385 nm. Meanwhile, a large
uorescence enhancement is produced at 455 nm, accompa-
nied by a distinct uorescence color change (see the inset of
Fig. 1B). Moreover, both the absorption and uorescence
spectra of the reaction system are similar to those of uo-
rophore coumarin (Fig. S4†), indicating that the reaction of 1
with Fꢀ leads to the release of coumarin. The HR-ESI-MS anal-
ysis (Fig. S5†) further proves the generation of coumarin (m/z
calcd for [M ꢀ 1]ꢀ 161.0244, found 161.0243).
As shown in Scheme S1†, probe 1 was synthesized via a one-
step method. In brief, triethylamine (88 mL, 0.63 mmol) was
added to a solution of 7-hydroxyl-coumarin (40.5 mg, 0.25
mmol) in dry dichloromethane (15 mL), followed by the
addition of dimethylthiophosphinoyl chloride (64.3 mg, 0.5
mmol). Then, the mixture was stirred at ambient temperature
for 12 h. Aer removal of the solvent under reduced pressure,
the residue was puried by silica gel chromatography with
dichloromethane/methanol (20 : 1, v/v) as eluents, affording
probe 1 as a white solid (42 mg, yield 67%). 1H- and 13C-NMR
spectra are shown in Fig. S1 and S2,† respectively. 1H NMR
(400 MHz, 298 K, CDCl3): d 7.68 (d, 1H, J ¼ 9.6 Hz), 7.46 (d, 1H,
J ¼ 8.0 Hz), 7.18 (m, 2H), 6.38 (d, 1H, J ¼ 9.6 Hz), 2.08 (s, 3H),
2.05 (s, 3H). 13C NMR (100 MHz, 298 K, CDCl3): d 160.6, 155.0,
153.3, 143.1, 128.9, 118.8, 116.4, 116.0, 110.5, 24.7, 24.0. HR-
ESI-MS: calcd for 1 (C11H12O3PS, [M + H]+) 255.0245; found,
255.0249 (Fig. S3†).
General procedure for Fꢀ detection
Unless otherwise noted, the uorescence of probe 1 (10 mM)
reacting with Fꢀ was measured in a mixture of DMSO and Tris
buffer solution (pH 8.0) (7 : 3, v/v) as follows. The stock solution
(1 mM) of probe 1 was prepared by dissolving 1 in DMSO. In
a test tube, 3.5 mL of DMSO, 0.5 mL of Tris buffer solution and
50 mL of the stock solution of probe 1 were mixed, followed by
adding Fꢀ solution. The nal volume was adjusted to 5 mL with
Tris buffer solution. Aer incubation at room temperature (25
Fig. 1 (A) Absorption and (B) fluorescence emission spectra of probe 1
(10 mM) before (a) and after (b) reaction with Fꢀ (200 mM) for 30 min in
ꢂC) for 30 min, a 3 mL portion of the reaction solution was a mixture of DMSO and Tris buffer solution (pH 8.0). lex ¼ 385 nm.
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The uorescence kinetics curves of 1 reacting with different 1 (Fig. 3A), indicating that probe 1 demonstrates excellent
concentrations of Fꢀ are shown in Fig. S6.† It can be seen that selectivity for Fꢀ. Furthermore, the corresponding uorescence
higher concentrations of Fꢀ lead to faster reaction velocity and images are clearly showed in Fig. 3B, it can be seen that only Fꢀ
larger uorescence enhancement, while the uorescence causes a distinct uorescence color change and the other
intensity of probe 1 hardly changes during the same period of anions do not signicantly disturb the uorescence of the
time in the absence of Fꢀ, implying the good stability of 1. reaction solution. Besides, the selectivity of the test strips was
Besides, intense uorescence enhancement can be obtained at also evaluated. As depicted in Fig. S9,† the test strip immersing
the reaction time of 30 min, thus, 30 min was chosen as the in Fꢀ solution shows strong uorescence, while the test strips
response time.38,39 Then the effect of pH on the reaction system immersing in solutions containing other anions scarcely uo-
was studied. As shown in Fig. S7†, the uorescence signal can resce. This observation indicates the high selectivity of the test
be effectively turned on in the pH range of 7.0–9.5, and the strips towards Fꢀ.
maximum uorescence enhancement appears at pH 8.0. These
observations suggest that probe 1 is suitable for biological
Application of probe 1 to determining Fꢀ in real samples
applications.
The high sensitivity and selectivity of probe 1 are anticipated to
Under the above optimized conditions, the uorescence
be useful for determining Fꢀ in real samples. As mentioned
response of probe 1 to Fꢀ at varied concentrations (0–200 mM) is
above, uoride contamination in groundwater happens around
depicted in Fig. 2. The uorescence intensity is increased by
increasing the Fꢀ concentration, and a good linearity can be
the world. Hence, water was selected to evaluate the potential
application of probe 1. The real-water samples were collected
obtained in the concentration range of 0–30 mM with the
from tap water and Yingze Lake, respectively and ltered with
equation DF ¼ 1.69 ꢁ c (mM) + 0.02 (R ¼ 0.993), where DF is the
uorescence difference in the presence and absence of Fꢀ. The
0.22 mm cellulose acetate membranes before use. As shown in
Fig. S10† and Table 1, Fꢀ in both of these two blank samples are
not detectable, which may be attributed to the very low
detection limit (3S/m, in which S is the standard deviation of
blank measurements, n ¼ 11, and m is the slope of the linear
concentration of Fꢀ (<0.29 mM, the detection limit of probe 1).
equation) was determined to be 0.29 mM Fꢀ. According to the
WHO standards, the maximum contaminant level for Fꢀ in
The recommended limit for class V surface water in China
(GB3838-2002) is 1.5 mg Lꢀ1 (80 mM).23 Thus both of the two
drinking water is 1.5 mg Lꢀ1 (about 80 mM).20 Therefore, probe 1
can assay Fꢀ in drinking water. Notably, prompted by the high
water samples have not been uoride contaminated. Moreover,
different amounts of Fꢀ were spiked into these two real-water
sensitivity of probe 1, we have tried to utilize 1 to make uo-
samples to minic the contaminated water samples, and the Fꢀ
rescence test strips for Fꢀ. The test strips are prepared by
immersing lter papers (4 ꢁ 1 cm2) in a mixture of DMSO and
contents were determined by probe 1. The results show that the
concentrations calculated from our method are in good agree-
Tris buffer solution containing 1 mM of probe 1 and then drying
in air. Aer immersing in the Fꢀ aqueous solution for several
ment with those added into the samples, and no signicant
difference is found at the 95% condence level using a paired t-
seconds and drying in air, the uorescence test strips displays
test.40,41 This validates the applicability of probe
1 in
an enhancement in uorescence signal with increasing the Fꢀ
concentration. As shown in Fig. S8,† the test strips shows
distinct uorescence when the concentration of Fꢀ is as low as
15 mM. Although the sensitivity of the test strip is lower than
that of the solution made by probe 1, it is fully competent in the
qualitative analysis of Fꢀ.
The specicity of probe 1 towards Fꢀ over other common
2ꢀ
anions, including Clꢀ, Brꢀ, Iꢀ, CO32ꢀ, SO42ꢀ, SO32ꢀ, HPO4
,
H2PO4ꢀ, and AcOꢀ was investigated. At the concentration of 200
mM, only Fꢀ induces intense uorescence enhancement while
other species tested scarcely inuence the uorescence of probe
Fig. 3 (A) Fluorescence response of probe 1 (10 mM) to various anions
(200 mM) in a mixture of DMSO and Tris buffer solution (pH 8.0). The
Fig. 2 (A) Fluorescence responses of probe 1 (10 mM) to Fꢀ at varied results are the mean ꢃ standard deviation (SD) of three separate
concentrations (0, 5, 10, 20, 30, 40, 50, 60, 80, 100, 120, 140, 160, 180, measurements. lex/em ¼ 385/455 nm. (B) Fluorescence images of
and 200 mM). (B) Linear fitting curve of DF toward the concentration (C) probe 1 (10 mM) under UV irradiation (365 nm) in the presence of
of Fꢀ from 0 to 30 mM. lex/em ¼ 385/455 nm.
various anions (200 mM).
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Table
1
Determination of Fꢀ concentration (mM) in real-water
No. Fꢀ added (mM) Fꢀ determined (mM) Recovery (%)
and Development Projects of Shanxi Province (201703D321015-
2), the Youth Science Foundation of Shanxi Province
(201701D221064), the Startup Foundation for Doctors of Shanxi
Medical University (03201601), and the National Students'
Innovation and Entrepreneurship Training Program
(201810114012).
samplesa
Sample
Tap water
1
2
3
1
2
3
0
NDb
19.11 ꢃ 0.71
25.34 ꢃ 0.21
ND
—
20
25
0
20
25
95.6 ꢃ 3.5
101.4 ꢃ 0.8
—
Yingze Lake
Notes and references
21.40 ꢃ 0.65
107.0 ꢃ 3.3
24.38 ꢃ 1.07
97.5 ꢃ 4.3
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a
Mean of three determinations ꢃ standard deviation. b Not detectable.
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Table 2 Determination of Fꢀ content in toothpaste samplesa
Sample
Brand
Fꢀ content (marked)
Fꢀ content (detected)
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0.10%
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0.12% ꢃ 0.01%
0.09% ꢃ 0.02%
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results in water samples, we further analyze Fꢀ contents in
commercially available toothpaste samples with probe 1. The
results (Fig. S10C† and Table 2) show that the Fꢀ contents
determined by our method accord well with those marked on
the labels of toothpaste, and no signicant difference is found
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Conclusions
In summary, we have developed a highly sensitive and selective
uorescent probe for Fꢀ by incorporating the dimethylphos-
phinothionyl group as a recognition moiety into the uo-
rophore of coumarin. The probe exhibits an intense
uorescence off–on response to Fꢀ via the uoride-triggered
cleavage of the dimethylphosphinothionyl group, followed by
the release of uorophore. Compared with the existing uo-
rescent probes for Fꢀ detection, probe 1 displays higher sensi-
tivity even in aqueous environment (Table S1†). The probe has
been applied to determine Fꢀ in real-water samples and
commercially available toothpaste samples, and the results are
satisfying. The simplicity and excellent performance of probe 1
make it useful in detecting Fꢀ in more real-samples.
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Conflicts of interest
There are no conicts to declare.
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