A rapid aqueous fluoride ion sensor with dual output modes

Article abstract of DOI:10.1002/anie.201000790

(Figure Presented) The color purple: A siloxy-functionalized benzamide (see picture) is a highly efficient fluoride ion sensor in water. The sensor, which is activated when the O-Si bond is cleaved by fluoride ions, provides two independent modes for signal recognition. In colorimetric mode, the fluoride ion concentration is transformed into a fluorescence signal that can be observed directly with the naked eye.

Full text of DOI:10.1002/anie.201000790

Angewandte
Chemie
DOI: 10.1002/anie.201000790
Sensors
A Rapid Aqueous Fluoride Ion Sensor with Dual Output Modes**
Rui Hu, Jiao Feng, Dehui Hu, Shuangqing Wang, Shayu Li,* Yi Li,* and Guoqiang Yang*
The development of sensors and receptors for biologically
important anions is currently of great interest because of their
indispensable roles in vital (or physiological) processes.[1]
Among the anions, fluoride ions are one of the most attractive
targets because of their considerable significance for health
and environmental issues. As an essential element of the
body, the U.S. Public Health Service affirmed the optimal
level to be 1 mg of consumed fluoride per day. On the other
hand, unnecessary and inappropriate fluoride ingestion can
result in fluorosis, urolithiasis, or even cancer.[2] The EPA
(United States Environmental Protection Agency) gives an
enforceable drinking water standard for fluoride of 4 mgLÀ1
to prevent osteofluorosis and a secondary fluoride standard of
2 mgLÀ1 to protect against dental fluorosis. Hence, the
accurate determination of the levels of fluoride in drinking
water is necessary. To date, the ion-selective electrode, ion
chromatography, and standard Willard and Winter methods
are generally used for quantitative fluoride analysis.[3] How-
ever, all these methods involve disadvantages, such as
complicated procedures, high costs, or low mobility. There-
fore, it is important to develop highly selective, sensitive,
convenient, and rapid fluoride detection methods.
Fluorescent chemosensors with high specificity and sensi-
tivity, ease, and safety of handling have received considerable
attention, and a number of fluorescence sensors have been
reported that are capable of detecting fluoride ions.[4–6] The
recognition proceeded mostly through hydrogen bonding or
Lewis acid coordination, and the sensors could only be
operated in organic solvents to detect tetrabutylammonium
(TBA+) fluoride rather than inorganic fluoride salts. This
incompatibility with aqueous environments is one of the main
drawbacks that restrict the application of these sensors.
Unavoidable interference from H2PO4À, AcOÀ, or CNÀ ions
is the other disadvantage. To improve the performance of
fluoride sensors, another strategy based on the chemical
affinity between fluoride and silicon was developed. tert-
Butyldimethylsilyl (TBDMS) and tert-butyldiphenylsilyl
(TBDPS) were chosen as additional substituents for the dye
molecule, and rendered the dye unreactive to potentially
interfering compounds and thus sensitive only to fluoride
ions. This approach was pioneered by Kim and Swager, who
developed a fluorescent fluoride receptor in organic sol-
vents.[7] Subsequently, several research groups reported
different chemodosimeters that can probe NaF in mixtures
of organic solvents and water or even pure water.[8] However,
several tens of minutes or even hours are needed to complete
the detection process because low concentrations of these
chemodosimeters severely reduce the reaction rate between
the fluoride ions and the silyl moieties. However, the low
concentration is necessary for general organic dyes to avoid
fluorescence quenching induced by concentration effects,
such as self-absorption and self-quenching.[9]
To date, most fluorescent sensors were based on the
specific fluoride ion dependence of the emission intensity and
could be significantly influenced by the excitation power and
the detector sensitivity. More importantly, fluorescence
intensity changes are not suitable for direct observation
with the naked eye. To develop a more convenient fluores-
cence sensor, it is essential to make use of other fluoride-
dependent measurable signals other than intensity. The
fluorescence color change, which can be measured directly
with a colorimeter or even distinguished easily by eye, is thus
a good choice.
Herein we describe a rapid and portable sensor for
fluoride ions in aqueous solution. The sensor, which has a high
sensitivity, operates through the special affinity between
fluoride ions and silicon, and provides two independent
modes of signal transduction based on fluoride-dependent
changes of fluorescence color (color metric mode) or intensity
(power metric mode), respectively. For the design of the
sensor, we chose N-(3-(benzo[d]thiazol-2-yl)-4-(hydroxy-
phenyl) benzamide (3-BTHPB) as an excited-state intra-
molecular proton transfer (ESIPT) compound. Just like other
ESIPT compounds, 3-BTHPB shows two emission bands,
which originate from the enol and keto forms at 418 and
560 nm, respectively. The ratio of the two bands is determined
by the number of molecules that could undergo ESIPT
reactions.[10] We coupled the tert-butyldiphenylchlorosilane
with the sodium salt of 3-BTHPB to afford a derivative N-(3-
(benzo[d]thiazol-2-yl)-4-(tert-butyldiphenyl silyloxy)phenyl)-
benzamide (BTTPB). As expected, BTTPB shows only blue-
violet fluorescence, which was almost identical to the
emission of the enol form of 3-BTHPB (see the Supporting
[*] R. Hu, J. Feng, D. H. Hu, Dr. S. Q. Wang, Dr. S. Y. Li,
Prof. Dr. G. Q. Yang
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Photochemistry, Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 (China)
Fax: (+86)10-8261-7315
E-mail: shayuli@iccas.ac.cn
gqyang@iccas.ac.cn
Prof. Dr. Y. Li
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing 100190 (China)
Fax: (+86)10-8254-3518
E-mail: yili@mail.ipc.ac.cn
[**] We are grateful for funding from the National Natural Science
Foundation of China (grant nos. 20703049, 20733007, 20873165,
and 50973118) and the National Basic Research Program
(2007CB808004 and 2009CB930802).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000790.
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À
Information). Upon addition of fluoride ions, the Si O bond
of BTTPB is immediately cleaved, and 3-BTHPB, which
exhibits a bright yellow emission in water, is simultaneously
released. Therefore, it is logical to expect that the fluores-
cence color change from blue-violet to yellow upon addition
of fluoride ions reflects the extent of the fluoride-induced
the addition of fluoride ions to the dispersion resulted in
gradual changes in the absorption and emission spectra.
Figure 1b shows the fluorescence spectra of the BTTPB
dispersion 200 seconds after addition of NaF with the
excitation wavelength at the isosbestic point (see Figure 1a).
The dispersion in the absence of fluoride ions exhibits only
one emission band with a maximum at 418 nm, which is
assigned to the BTTPB emission. Upon the addition of
fluoride ions, the blue-violet emission band decreases and a
new emission band located at 560 nm simultaneously appears
and increases gradually. The fluorescence color change of
BTTPB upon addition of one equivalent of fluoride ions (i.e.,
0.38 ppm) is too small to be directly observed with the naked
eye, but can be detected by fluorescence spectroscopy. When
the fluoride ion concentration was further increased, the
change can be seen with the naked eye. As shown in
Figure 1c, 0.95, 1.9, and 3.8 ppm of fluoride ions cause the
dispersion fluorescence color to change from blue-violet to
violet, pink-purple, and to pink-white, respectively. As
mentioned above, the ideal and enforceable standards for
fluoride in drinking water are 1–4 ppm. Therefore we
conclude that the BTTPB/CTAB/water colorimetric sensing
system is sufficiently sensitive for practical fluoride ion
detection.
À
Si O bond cleavage. A schematic of the feasible fluorescence
sensing mechanism in the fluoride ion detection is shown in
Scheme 1.
It is crucial that the absorption wavelength of 3-BTHPB is
slightly longer than that of BTTPB, so that 3-BTHPB is
selectively excited above 380 nm. Figure 1d shows fluoride
ion dependent fluorescence spectra of the BTTPB dispersion
200 seconds after addition of NaF with the excitation
wavelength at 385 nm. No emission was found in the absence
of fluoride ions, and the intensity of the yellow emission
increased drastically with fluoride ion concentration; this
result is indicative of an “off–on” switch triggered by fluoride
ions. As mentioned above, although the intensity change of
the fluorescence alone is not appropriate for detection with
the naked eye, it is still a very sensitive method. The detection
limit was determined as the three-fold standard deviation of
the fluorescence obtained from a blank sample, prepared as
described above but without the addition of BTTPB. The
detection limit of the BTTPB dispersion system for fluoride
ions is measured to be about 100 ppb. This result shows that in
power metric mode the dispersion system has a very good
sensitivity for the detection of aqueous fluoride ions.
Scheme 1. Synthesis and sensing mechanism of chemosensor BTTPB
for the detection of NaF.
Both BTTPB and 3-BTHPB are not soluble in water; to
apply them efficiently in an aqueous environment, cetyltri-
methylammonium bromide (CTAB) was introduced into the
aqueous phase. A stable sensor system was prepared by
rapidly injecting a solution of BTTPB in THF (100 mL,
2.0 mm) into a micellar solution of CTAB in water (10 mL,
2.0 mm) under vigorous stirring for 30 seconds at 208C.
UV/Vis absorption and fluorescence spectra were
recorded with different amounts of NaF to test the sensing
ability of the BTTPB dispersions (Figure 1a,b). As expected,
For further evaluation of the fluoride sensing ability of the
system, the selectivity of the BTTPB dispersion was tested by
addition of 500 equivalents of common anions (in the form of
sodium salts), such as ClÀ, BrÀ, AcOÀ, NO3À, H2PO4
,
À
HSO4À, and FÀ. As shown in Figure 2, only NaF induced an
immediate red shift in the fluorescence maximum from
418 nm to 560 nm. All other anions did not cause any
emission intensity changes at 560 nm until 30 minutes after
the addition of each anion. The slight disturbances at 418 nm
probably originated from the interaction between micelles
and highly concentrated ions. This result indicates that the
system is a very good sensor for recognizing fluoride ions over
other anions.
To confirm the mechanism shown in Scheme 1, ESI-MS
was carried out on agglomerates that resulted from the
demulsification of the dispersion by addition of small
Figure 1. Absorption (a) and fluorescence spectra (b,d) of the BTTPB
dispersions 200 s after addition of NaF, lex =350 nm (b) and 385 nm
(d); pictures of BTTPB dispersions upon addition of NaF (c).
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Fluorescence sensors for fluoride ion detection are usually
solution-based. This approach is inconvenient since the
sensors cannot be used as efficient tools under special
circumstances, such as for in situ on-site detection. To
facilitate the use of our system, we prepared test papers of
BTTPB by immersing a filter paper (2 ꢁ 0.5 cm2) in the
solution of BTTPB in THF (2.0 ꢁ 10À3 m) and then drying it by
exposure to air. For the detection of fluoride ions in water, the
test paper was immersed in a fluoride-containing aqueous
solution (containing 2 mm CTAB) and then exposed to air to
remove water and to avoid further fluorescence color
changes. The Commission Internationale de LꢀEclairage
(CIE) 1931 (x,y) chromaticity diagram of the test papers
after immersion in solutions of NaF with different concen-
trations for three minutes are shown in Figure 3. When the
Figure 2. Luminescence spectra of BTTPB in CTAB micelles (2 mm)
upon addition of 500 equivalents of sodium salts. The inset shows the
selectivity for fluoride ions (monitoring the emission at 566 nm).
amounts of ethanol. The dispersion obtained by addition of
500 equivalents of fluoride ions was composed of three
components, 3-BTHPB, BTTPB, and tert-butylfluorodiphe-
nylsilane, thus confirming the postulated mechanism.
Aggregates of BTTPB and 3-BTHPB were also used as
fluoride ion responsive materials. Two other factors—the
aggregatesꢀ emission intensity and their reactivity with
fluoride ions—are therefore crucial to ensure successful
sensing. In general, most organic dyes have low luminescence
efficiency or are even nonemissive in solutions with high
concentrations or as aggregates, because of concentration-
dependent quenching effects. Only very few compounds with
specific structures maintain or even enhance the lumines-
cence intensity in their aggregated state;[11] This is also the
case for BTTPB as well as 3-BTHPB, as the emission
efficiencies of their aggregates in pure water are 0.032
(BTTPB) and 0.21 (3-BTHPB). The intense emission of the
aggregates makes it possible to employ the compounds as
fluorescent sensors in water.
It is known that CTAB forms spherical micelles about
6 nm in diameter within the concentration range 0.9–100 mm
in water.[12] In our system, the dispersions of BTTPB in 2 mm
CTAB before and after addition of fluoride were transparent,
thus implying that the sizes of the aggregates were smaller
than half of the visible light wavelength. The small aggregate
sizes were also confirmed by means of field emission scanning
electron microscopy (FESEM) (see the Supporting Informa-
tion), which illustrates the morphology of BTTPB aggregates
before and 200 seconds after reaction with fluoride ions. Only
a few 20–30 nm particles appear on the millipore filter surface
(pore size of the filter is about 20 nm), but, by taking the
emission of the filtrates into account, most aggregates should
be below the pore size. In fact, the BTTPB molecules were
concentrated in micelles and the fluoride ions were attracted
to the micellar surface. The combination of small micelle sizes
and the mobility of organic molecules in the micelles made
the interaction between BTTPB and fluoride ions facile.
Figure 3. CIE 1931 (x,y) chromaticity diagram of the test papers for the
detection of NaF at different concentrations derived from fluorescence
spectra (black circles); NaF concentration from left to right: 0, 0.38,
0.95, 1.9, 3.8, 9.5, 19, 38, 95 ppm (top); images of the test papers
after immersion into solutions with different concentrations of NaF
(bottom).
fluoride ion concentration was increased, the color of the test
paper changed from blue-violet to bright yellow. The
immersion time was optimal for the detection of fluoride
ions in the range of the enforced drinking water standard.
According to the diagram, it can be easily seen if the fluoride
ion concentration in drinking water exceeds the standard.
Consequently, the easy-to-prepare test paper can be utilized
to roughly and quantitatively detect and estimate the
concentration of fluoride ions.
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Communications
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In summary, we have demonstrated a selective and
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the specific affinity between fluoride ions and silicon. The
sensor system provides two independent modes of signal
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Received: February 9, 2010
Revised: March 17, 2010
Published online: June 10, 2010
Keywords: fluorescent probes · fluorides · luminescence ·
sensors · silicon
.
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