A colorimetric and fluorescent probe for fluoride ions based on 6-acetyl-2-naphthol

Article abstract of DOI:10.1002/bio.2564

A colorimetric and turn-on fluorescent probe for fluoride ions, tert-butyldimethylsilane 6-acetyl-2-naphtholate, was readily synthesized from 6-acetyl-2-naphthol and tert-butyldimethylchlorosilane (TBSCl). The probe exhibits high sensitivity and good selectivity for fluoride ions in acetonitrile. The inherent mechanism involves the cleavage of the Si-O bond in the probe, which induced yellow color formation and prominent fluorescence enhancement. Copyright

Full text of DOI:10.1002/bio.2564

Research article
Received: 18 March 2013,
Revised: 5 June 2013,
Accepted: 5 June 2013
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2564
A colorimetric and fluorescent probe for
fluoride ions based on 6-acetyl-2-naphthol
Peng Hou,a Song Chena and Xiangzhi Songa,b,c
*
ABSTRACT: A colorimetric and turn-on fluorescent probe for fluoride ions, tert-butyldimethylsilane 6-acetyl-2-naphtholate,
was readily synthesized from 6-acetyl-2-naphthol and tert-butyldimethylchlorosilane (TBSCl). The probe exhibits high sensi-
tivity and good selectivity for fluoride ions in acetonitrile. The inherent mechanism involves the cleavage of the Si–O bond in
the probe, which induced yellow color formation and prominent fluorescence enhancement. Copyright © 2013 John Wiley &
Sons, Ltd.
The following supporting information is available online.
Keywords: colorimetric; fluorescent probe; fluoride ion
Introduction
Experimental
Because of the crucial roles of anions in biological systems and
chemical processes, intensive research has been conducted on
the design of signaling probes for anions (1–5). The fluoride
ion has been widely used as an essential element in toothpaste
and pharmaceutical agents to prevent dental caries and ortho-
dontics (6–8). However, fluoride ions are poisonous to biological
systems when levels exceed normal cellular values, and can
cause fluorosis such as metabolic disorders and immunological
damage in humans (9–13). Thus, it is very important to regulate
the uptake of fluoride ions and the detection of fluoride ions has
attracted considerable attention. Owing to its simplicity and ex-
quisite sensitivity, fluorescent probes are highly attractive for the
selective detection of fluoride ions. A number of fluorescent
probes for fluoride ions have been developed in recent years
(14–22). However, many of the reported probes suffer from com-
plicated synthesis procedures, turn-off fluorescence response,
slow response or low sensitivity. Therefore, there is still a high
demand for probes for fluoride ions with good selectivity and
high sensitivity.
It is well known that tert-butyldimethysilane (TBS) ether is
a useful protective group for the hydroxy group and can be
specifically and efficiently cleaved by fluoride ions. Therefore,
TBS ether has been proven to be an attractive component for
the design of fluoride probes (23–32). 6-Acetyl-2-naphthol,
which has a pull–push electron structure, exhibits outstanding
chemical and photophysical properties such as high fluorescent
quantum yield, good photostability and a large Stokes shift in-
duced by an intramolecular charge transfer process (ICT). There-
fore, derivatives of 6-acetyl-2-naphthol have emerged as very
promising fluorescent probes. Here, we report the design and
synthesis of a TBS ether of 6-acetyl-2-naphthol, 1, as a colorimet-
ric and turn-on fluorescent probe for fluoride ions (shown in
Scheme 1). In probe 1, the ICT process was completely inhibited
due to the protection of phenolate by TBS. However, upon the
addition of fluoride ions, probe 1 can be quickly converted into
6-acetyl-2-naphthol, which leads to a marked color change and
significant fluorescence enhancement.
Materials
Acetonitrile was purchased from J&K Scientific Ltd. (Shanghai,
China) Dry methylene chloride was obtained by distillation after
refluxing with CaH2 for 6 h. Unless otherwise stated, all chemicals
were purchased from commercial suppliers and used as received.
The investigated anions were F-, Cl-, Br-, I-, H2PO-4, HSO4- , NO-3, AcO-
and ClO-4, and tetrabutylammonium (TBA+) was the counter cation.
Synthesis of probe 1
The synthetic route for probe 1 is shown in Scheme 1. Following
to the literature, 6-acetyl-2-naphthol was obtained by refluxing
2-acetyl-6-methoxy-naphthalene with concentration hydrochloric
acid for 2 h (33). To a mixture of 6-acetyl-2-naphthol (0.4mmol,
74.5mg), DMAP (0.16 mmol, 20mg) and triethylamine (2mmol,
202 mg) in 50 mL of dry CH2Cl2 was added tert-butyldimethyl-
chlorosilane (2mmol, 301.4mg). After stirring the reaction mixture
at room temperature for 1 h, 50 mL of water was added. The or-
ganic layer was separated and dried over anhydrous Na2SO4. The
drying agent was removed by filtration and the solvent was re-
moved by distillation. The obtained residue was purified by silica
gel column chromatography (10:1 petroleum:ethyl acetate as
* Correspondence to: X. Song, College of Chemistry & Chemical Engineering,
Central South University, 932 Lushan South Road, Changsha, Hunan Province
410083, People's Republic of China. Email: xzsong@csu.edu.cn
a
College of Chemistry & Chemical Engineering, Central South University, 932
Lushan South Road, Changsha, Hunan Province, P. R. China, 410083
b
Key Laboratory of Resource Chemistry of Nonferrous Metals, Ministry of
Education, 932 Lushan South Road, Changsha, Hunan Province, P. R. China,
410083
c
State Key Laboratory for Powder Metallurgy, 932 Lushan South Road,
Changsha, Hunan Province, P. R. China, 410083
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P. Hou et al.
Scheme 1. Synthesis of probe 1.
Figure 2. UV/vis absorption spectra of probe 1 (100 μM) in CH3CN containing dif-
ferent amounts of TBAF (0, 10, 15, 20, 25, 30, 35, 40, 50, 100, 200, 300 μM).
eluent) to yield the product, 1, as a white solid (102mg, 85%)
(Fig. S1,S2). H NMR (400 MHz, CDCl3): δ ppm = 8.40 (s, 1H), 7.97
1
(t, J = 8.8 Hz, 1H), 7.85 (d, J = 9.2Hz, 1H), 7.72 (d, J = 8.8Hz, 1H),
7.21 (d, J = 2.0Hz, 1H), 7.14 (d, J = 8.8 Hz, 1H), 2.70 (s, 3H), 1.03
(s, 9H), 0.27 (s, 6H). GCMS (m/z): 300.1.
Results and discussion
UV/vis absorption studies with fluoride ion
Prior to the reaction with fluoride ions, probe 1 is colorless in
acetonitrile with an absorption maximum at 304 nm, as shown
in Fig. 1. Upon the addition of 3 eq. of fluoride ions, an apparent
102 nm red-shift (from 304 to 406 nm) instantly appeared. With
increasing addition amounts of fluoride ion, the absorption band
at 304 nm progressively decreased and a new band centered at
406 nm gradually formed with a clear isosbestic point at 329 nm
(Fig. 2). To demonstrate the selectivity of probe 1 as a colorimet-
ric probe, titration experiments were carried out on probe 1 with
other typical anions (Cl-, Br-, I-, H2PO4- , HSO-4, NO3- , AcO- and ClO-4)
by observing the absorption spectral change. As shown in Fig. 3,
only fluoride ions could induce the probe to form a yellow color
with 102 nm a red-shift, whereas other typical anions have no in-
fluence on the absorption spectra. The above results proved that
probe 1 could be used as a highly selective colorimetric probe
for fluoride ion.
Figure 3. Absorption spectra of probe 1 (100 μM) in CH3CN upon addition of var-
ious TBA salts (F-, Cl-, Br-, I-, AcO-, HSO4- , H2PO4- , ClO4- and NO-3; 300 μM each). (Inset)
Color changes of the probe 1 (100 μM) in the presence of 3 eq. of TBA salts (F-, Cl-,
Br-, I-, H2PO-4, HSO4- , NO3- , AcO- and ClO4- ) in CH3CN.
Fluorescence studies with fluoride ion
As shown in Fig. 4, probe 1 in acetonitrile is essentially non-
fluorescent in the absence of fluoride ions. By contrast, a dra-
matic fluorescence enhancement (9663-fold) was observed with
a maximum at 488 nm upon treatment with 10 eq. of fluoride
Figure 1. UV/vis absorption spectra of probe 1 (100 μM) in the absence and pres-
ence of TBAF (300 μM) in CH3CN.
Figure 4. Fluorescence spectra of probe 1 (5 μM) recorded before and after reac-
tion with TBAF (50 μM) in CH3CN.
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A colorimetric and fluorescent probe for fluoride ions
Figure 7. Ratio of emission intensity at 488 nm of probe 1 (5 μM) before (I0) and
after (I) the addition of TBAF (10 μM) and other TBA salts (Cl-, Br-, I-, AcO-, HSO4- ,
H2PO-4, S2-, SCN- and NO-3; 50 μM, each).
Figure 5. Fluorescence spectra of probe 1 (5 μM) in CH3CN with different
amounts of TBAF (0, 2, 3, 4, 5, 6, 8, 10, 20, 30, 40, 50 μM). The excitation wavelength
was set at 406 nm.
Figure 8. Time-resolved fluorescence spectra of probe 1 (5 μM) at 488 nm upon
addition of TBAF (50 μM) in CH3CN.
Figure 6. Fluorescence intensity at 488 nm of probe 1 (5 μM) in CH3CN in the
presence of TBAF. (Inset) Linear portion of the curve at low concentrations of TBAF.
ions. There was a good linear correlation between the fluores-
cence intensity and the concentration of fluoride ion in the
range 0–50 μM (Fig. 5). The linear equation was expressed as
y = -734.4 + 379.8x (R = 0.9915, where y denotes the fluorescence
intensity and x denotes the concentration of fluoride ion). The
detection limit was determined to be 40 nM (Fig. 6).
In order to evaluate the selectivity of the probe for fluoride ions,
we investigated the fluorescence behavior of probe 1 with other
typical anions. The results presented in Fig. 7 show that other typ-
ical anions (Cl-, Br-, I-, H2PO4- , HSO4- , NO3- , AcO-, S2-, SCN- and ClO4- )
have no effect on the fluorescence behavior of probe 1, even at
high concentrations (10 eq. of probe 1). Therefore, it can be con-
cluded that the fluorescent probe 1 displayed excellent selectivity
for fluoride ions over other competing anions.
Time-dependent fluorescence studies of probe 1 with fluoride
ions were also conducted. It can be seen in Fig. 8 that the fluo-
rescence intensity was enhanced with increasing reaction time.
Pronounced fluorescence enhancement was observed within
30 s and leveled off to a constant value after 3 min, which indi-
cated that probe 1 showed rapid detection of fluoride ions.
Scheme 2. TBAF-induced cleavage of the Si–O bond in probe 1.
6-acetyl-2-naphtholate in the presence of fluoride ion (Scheme 2).
In comparison with probe 1, the reaction product, 6-acetyl-2-
naphtholate, has a more electron-donating group and possesses
a push–pull electron structure, which can lead to a red-shift in its
absorption spectrum and a strong fluorescence due to the ICT pro-
cess. Absorption and fluorescence studies of probe 1 with fluoride
ions are consistent with this. In order to further confirm the
reaction mechanism, the resulting mixture of probe 1 with fluoride
ion was subjected to ESI mass spectral analysis. The result in Fig. S3
shows a peak at m/z = 185.3, which clearly indicates the formation
of 6-acetyl-2-naphtholate (calcd. 185.2 for C12H9O2). The absorp-
tion, fluorescence (Fig. S4) and ESI spectral studies strongly
support the proposed mechanism in Scheme 2.
Proposed mechanism for the reaction of the probe with
fluoride ion
It is known that fluoride can effectively cleave the Si–O bond.
In our work, we expected that probe 1 was transferred into
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P. Hou et al.
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SUPPORTING INFORMATION
The following supporting information is available online.
Acknowledgement
This research was supported by NSF of China (No. 21072235)
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