A naphthalene benzimidazole-based chemosensor for the colorimetric and on-off fluorescent detection of fluoride ion

Article abstract of DOI:10.1016/j.saa.2017.05.053

A novel naphthalene benzimidazole (NBI)-based chemosensor (D2) was developed for fluoride ion (F^?) detection. The absorption spectrum of D2 changed dramatically from yellow to blue in the visible region accompanied with a 225?nm red shift of it

Full text of DOI:10.1016/j.saa.2017.05.053

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 185 (2017) 173–178
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A naphthalene benzimidazole-based chemosensor for the colorimetric
and on-off fluorescent detection of fluoride ion
⁎
Dongmei Li, Zhimin Zhong, Gengxiu Zheng, Zhongzhen Tian
Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, No. 336, West Road of Nan Xinzhuang, Jinan
250022, China
a r t i c l e i n f o
a b s t r a c t
A novel naphthalene benzimidazole (NBI)-based chemosensor (D2) was developed for fluoride ion (F−) detec-
tion. The absorption spectrum of D2 changed dramatically from yellow to blue in the visible region accompanied
with a 225 nm red shift of its absorption maximum upon the addition of F− in DMSO. D2 also exhibited a fluo-
rescence turn-off response towards the fluoride ion. The emission intensity of D2 decreased drastically along
the increasing F− concentration and the detection limit for F− was as low as 3.2 × 10−9 mol/L. 1H NMR and
HRMS-ESI results indicated that the formation of NBI-O− through the desilylation reaction of F− with NBI-OSi
was responsible for the spectral changes. Overall, this kind of NBI-type molecules represent a new type
chemosensor for the spectral detection of fluoride ion in solution.
Article history:
Received 11 February 2017
Received in revised form 22 May 2017
Accepted 23 May 2017
Available online 24 May 2017
Keywords:
Fluoride ion
Chemosensor
© 2017 Elsevier B.V. All rights reserved.
Naphthalene benzimidazole
Fluorescence quenching
Desilylation reaction
1. Introduction
[30,31], Fe3+ [32], Cd2+ [33], OH− [34], and Hg2+ [ 35,36]. However,
to the best of our knowledge, there is no report of NBI-based
Anions play an important role in a wide range of chemical and bio-
logical processes [1–4]. Fluoride anion (F−), as the smallest anion,
holds a unique position among all the essential anions [5,6]. F− is best
known for its wide utilization in toothpaste, even though it is also fre-
quently included in various pharmaceutical reagents for the treatment
of osteoporosis and the prevention of dental cares and enamel deminer-
alization [7–10]. However, high dose intake of F− is harmful as it will
cause fluorosis and nephrotoxic changes in both humans and animals
[11–13]. Consequently, it is highly desirable to develop chemosensors
chemosensors for F− detection. Herein, we report a novel fluorescent
sensor of 12-((tert-butyldiphenylsilyl)oxy)-8a,13a-dihydro-7H-
benzo[de]benzo [4,5]imidazo[2,1-a]-isoquinolin-7-one (D2) for F− de-
tection, whose action is triggered by a desilylation and intramolecular
charge-transfer mechanism. Our experimental results demonstrated
that D2 exhibited excellent selectivity and sensitivity for F− detection
in DMSO with a detection limit approaching 3.2 × 10−9 mol/L.
2. Results and Discussion
with excellent selectivity and high sensitivity for the detection of F−
,
preferably in solution phase [14,15]. Up to now, a large number of fluo-
rescent sensors have been reported for F− detection, most of which con-
tain acceptors based on hydrogen bonding interaction or Lewis-acidic
boron moieties, etc. [16]. As one of those most promising sensing strat-
egies, fluorescence dosimetric detection of F− based on irreversible
chemical reaction has attracted increasing attention because of their
high selectivity and stability [17]. Indeed, a series of F− sensors based
on F−-triggered Si\\O bond cleavage and incidental release of
fluorophores have been reported recently [18–26].
2.1. The Synthesis of Chemosensor D2
Our chemosensor D2 was synthesized (Scheme 1) by two steps
starting from 1,8-naphthalic anhydride, 2,3-diaminophenoland tert-
butyl(chloro)diphenylsilane. The 1H NMR spectra of the intermediate
D1 (3-((tert-butyldiphenylsilyl)oxy)benzene- 1,2-diamine) and D2,
and the 13C NMR and HRMS-ESI results of D2 were included in Fig.
S1–S4, respectively.
Naphthalene benzimidazole (NBI), a special class of environmentally
sensitive fluorophore, exhibits excellent light-fastness and robust
chemical and heat stability [27–29]. Several fluorescent sensors based
on NBI have been developed to detect selectively ions, such as Cu2+
2.2. Colorimetric Analysis of D2 in the Presence of F−
Fig. 1 showed the photo images of D2 in DMSO without (blank) and
with various anions such as F−, Cl−, Br−, I−, HSO−4 , AcO−, H2PO−4 , and
ClO−4 (as TBA+ salts). It was visually apparent that a color change
from yellow to blue occurred only in the presence of F− (Fig. 1, top).
⁎
Corresponding author.
E-mail address: chm_tianzz@ujn.edu.cn (Z. Tian).
http://dx.doi.org/10.1016/j.saa.2017.05.053
1386-1425/© 2017 Elsevier B.V. All rights reserved.

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D. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 185 (2017) 173–178
Scheme 1. Synthetic route of D2.
Under a UV lamp, strong fluorescence was observed from D2 with the
response to F− in the presence of other relevant anions. These results in-
dicated that D2 could serve as a chemosensor for F− with high selectiv-
ity and strong anti-interference ability.
addition of other anions but F− (Fig. 1, bottom).
2.3. Spectral Response of D2 Upon the Addition of F−
2.5. The Absorption and Fluorescent Titration Curves of D2 Upon the Addi-
tion of F−
The above color changes could also be verified by spectral methods.
The absorption and fluorescence emission spectra of D2 in the presence
of anions (F−, Cl−, Br−, I−, HSO4−, AcO−, H2PO4−, ClO−4 ) were shown in
Fig. 2. It could be seen from Fig. 2a that the absorption maximum of
D2 in DMSO was 395 nm. When 30 equiv. of F− were added into the so-
lution of D2, a red-shift from 395 to 620 nm (Δλ = 225 nm) was ob-
served. However, other anions of equal concentrations caused
insignificant changes on absorption spectra. The results indicated that
D2 could selectively detect F− in DMSO. On the other hand, free D2 ex-
hibited a strong fluorescence emission centered at 530 nm (Fig. 2b).
When 3 equiv. of anions were added into the DMSO solution of D2 re-
spectively, only F− caused the fluorescence quenching remarkably,
while other anions caused tiny changes. Therefore, according to the
fluorescence and absorption results, D2 was a highly efficient sensor
for F−. Meanwhile, the response of D2 for F− was also very fast (Fig. S5).
In order to investigate the quantitative analysis of D2 for F−, the ab-
sorption titration curves were plotted in Fig. 4a. The absorption maxi-
mum of D2 in DMSO was 395 nm, while the peak shifted to 620 nm
upon addition of F−. As far as we know, such a red shift of 225 nm in ab-
sorption was uncommon as to F− sensors. The ratios of absorption in-
tensities at 620 nm to those at 395 nm (A620 nm/A395 nm) fitted linear
relation to F− concentration in DMSO (Fig. 4b), which indicated that
the D2 could detect F− quantitatively in DMSO with the UV–vis absorp-
tion method.
As mentioned above, D2 also showed fluorescence turn-off property
beside absorption change. To gain an insight into the fluorescence re-
sponse of D2 for F− in DMSO, the fluorescence titration experiments
were conducted and the results were depicted in Fig. 5a. When F−
were added to the DMSO solution of D2, the fluorescence intensi-
ties of D2 at 530 nm decreased gradually until almost completely
quenched when 30 equiv. of F− were added [quenching efficiency
(I0 − I)/I0 × 100% = 98.5%, where I and I0 were the fluorescence
signal in the presence and absence of F−, respectively]. The fluorescence in-
tensities were not only highly sensitive, but also linearly corrected to
2.4. The Control Experiments With Other Common Anions
The selectivity of D2 towards F− was also studied by anion-compet-
itive experiments (Fig. 3). The diagram displayed that the fluorescence
intensities of D2 after adding 3 equiv. F− were almost unaffected by
other common anions. In other words, D2 could maintain its sensing
F
− concentration. We obtained the slope as 411 for D2 from the lin-
ear equation of fluorescence intensity to F− concentration (Fig. 5b).
The limit of detection (LOD) of D2 for F− was 3.2 × 10−9 M based
on LOD = 3σ/s, where σ was the standard deviation of blank
measurements and s was the slope. These data demonstrated that
D2 showed a potential to quantitatively detect F− in DMSO with
the fluorescence method.
2.6. The Sensing Mechanism of D2 for the Detection of F−
Both the changes of absorption and fluorescence indicated that the
structure of D2 might broke and a new species was formed. To elucidate
the mechanism of interaction between D2 and F−, the reaction process
was monitored by 1H NMR and HRMS-ESI. The systematic changes in 1H
NMR signals of D2 upon addition of increasing amounts of F− were
shown in Fig. 6a. When F− selectively attacked the silicon atom of the
silyl ether group, the increased negative charge on the phenolate oxy-
gen atom induced release of NBI-O− (Scheme 2) [37–39]. The change
of chemical structure accounted for the change of color and spectra.
Fig. 1. Color changes induced by adding excess equivalents of various anions (TBA+ salts)
to D2 (1 × 10−4 M) in DMSO under day lighting (top) and UV lamp (bottom).

D. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 185 (2017) 173–178
175
Fig. 2. (a) Absorption spectra of D2 (1 × 10−5 M) in the presence of 30 equiv. of different anions in DMSO. (b) Fluorescent emission spectra of D2 (1 × 10−5 M) in DMSO (λex = 395 nm)
after adding 3 equiv. of different anions.
After addition of F−, the chemical shifts of protons on aromatic ring moi-
ety shifted to up field respectively. On the other hand, to gain insight
into the sensing mechanism, mass spectra were carried out and the
changes in HRMS-ESI signals of sensor D2 upon addition of F− were
shown in Fig. 6b. D2 gave a m/e peak of [M + H]+ at 525.2001. After
adding F−, Si\\O bond broken and promoted deprotection of D2 to
give NBI-O−, and in HRMS-ESI a m/e peak of [M]− at 285.0670 occurred
corresponding to that of NBI-O−. These observations confirmed that the
deprotection proceeded with the addition of F−, and accordingly result-
ed in the spectral changes.
could serve as a chromophore moiety for the design of chemosensors
for the colorimetric detection of F−.
4. Experimental
4.1. Materials
All of the tetrabutylammonium (TBA+) salts and other chemical re-
agents were purchased from Aladdin Shanghai Reagent Company. Or-
ganic solvents were purchased from commercial suppliers and used
without further purification. All reactions were monitored by thin
layer chromatography (TLC) analysis. Flash chromatography separa-
tions were carried out using silica gel (200–300 mesh).
3. Conclusions
An efficient chemosensor D2 was designed for the colorimetric de-
tection of F−. The chemical structure of D2 was confirmed by 1H NMR,
13C NMR, and HRMS-ESI. D2 showed excellent specificity towards F−
based on F−-triggered Si\\O bond cleavage, which gave rise to a visible
color change accompanied by a large red shift (225 nm) in absorption
maximum and fluorescence quenching. Our results suggested that NBI
4.2. Measurements and Instruments
1H NMR and 13C NMR spectra were recorded on a Bruker AV III
400 MHz NMR spectrometer with tetramethysilane (TMS) as the inter-
nal standard. HRMS-ESI data were obtained on an Agilent 6510
equipped with an electrospray source. UV–vis spectra were collected
on a UV-2550 spectrophotometer. Fluorescent spectra were recorded
on an RF-5301PC fluorescence spectrophotometer.
4.3. Calculation of Quantum Yield
The quantum yield was calculated according to the following
Formula (1):
2
FuAsnu
FsAuns
Φu ¼ Φs
ð1Þ
2
where Φ is fluorescence quantum yield; F is the integrated area under
the corrected emission spectra; A is the absorbance at the excitation
wavelength; n is the refractive index of the solution; the subscripts u
and s refer to the unknown and the standard samples, respectively.
The quantum yield is 0.97 when Rhodamine B in ethanol solution was
used as the standard.
4.4. Synthesis of D2
D2 was synthesized from tert-butyl(chloro)diphenylsilane accord-
ing to the reported method [40].
Fig. 3. Anion selectivity of D2 towards F−. Black bars: fluorescence intensities of
D2 (1 × 10−5 M) or D2 and different anions (3 × 10−5 M) in DMSO. Red bars:
fluorescence intensities of D2 (1 × 10−5 M) after addition of F− (3 × 10−5 M)
in the presence of different anions (3 × 10− 5 M). Excitation wavelength was
395 nm. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
4.4.1. Synthesis of 3-((tert-butyldiphenylsilyl)oxy)benzene-1,2-diamine
(D1)
2,3-diaminophenol (0.44 g, 3.6 mmol) and imidazole (0.5 g,
7.3 mmol) were added into 40 mL of dry dichloromethane, and then
tert-butyl(chloro)diphenylsilane (0.96 mL, 3.6 mmol) was added. The

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D. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 185 (2017) 173–178
Fig. 4. Absorption spectra of D2 (1 × 10−5 M) in the presence of F− (0–30 equiv.) in DMSO (left), and the corresponding linear ability of (A620 nm/A395 nm) versus the F− concentration in
DMSO (right).
Fig. 5. (a) Fluorescence titration spectra of D2 (1 × 10−5 M) in the presence of F− (0–3 × 10−5 M) in DMSO (excited at 395 nm). (b) Plot of the emission intensities of D2 versus
concentration of F− in DMSO.
mixture was stirred at room temperature for 48 h. Subsequently, the
mixture was concentrated and purified by preparative thin-layer
chromatography (PTLC) using dichloromethane/methanol (30:1, v/v)
as the eluent to produce compound D1. Yield: 73%. 1H NMR
(400 MHz, CDCl3) δ 7.85 (d, J = 6.5 Hz, 4H), 7.44–7.51 (m, 6H), 6.34–
6.38 (m, 2H), 6.10 (dd, J = 5.8, 3.6 Hz, 1H), 3.64 (s, 4H), 1.23 (s, 9H).
4.4.2. Synthesis of 12-((tert-butyldiphenylsilyl)oxy)-8a,13a-dihydro-7H-
benzo[de]ben-zo[4,5]imidazo[2,1-a]isoquinolin-7-one (D2)
D1 (0.79 g, 2.2 mmol) and 1,8-naphthalic anhydride (0.39 g,
2.0 mmol) were mixed in 10 mL of glacial acetic acid. The mixture
was heated to 118 °C for 10 h. Then the reaction mixture was cooled
to room temperature. The obtained yellow solid precipitation was
Fig. 6. (a) Partial 1H NMR spectra of D2 (1.3 × 10−5 M) in the presence of 0, 0.5, 1, 5 and 10 equiv. of TBAF in DMSO-d6. (b) HRMS-ESI spectra of D2 (top), and D2 in the presence of F−
(bottom) in DMSO.

D. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 185 (2017) 173–178
177
Scheme 2. F− promoted release of NBI-O−
.
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final product D2. Yield: 90%. 1H NMR (400 MHz, CDCl3) δ 8.89 (s, 1H),
8.78 (d, J = 7.3 Hz, 1H), 8.29 (d, J = 8.2 Hz, 1H), 8.15 (d, J = 8.1 Hz,
1H), 8.11 (d, J = 8.0 Hz, 1H), 7.93–7.86 (m, 4H), 7.83 (t, J =
7.6 Hz, 2H), 7.38–7.46 (m, 6H), 7.11 (t, J = 8.1 Hz, 1H), 6.67 (d, J =
8.0 Hz, 1H), 1.27 (d, J = 15.8 Hz, 9H)·13C NMR (100 MHz, CDCl3) δ
160.78, 147.77, 147.51, 136.07, 135.73, 135.08, 133.59, 133.20, 132.17,
131.32, 129.86, 127.83, 127.67, 127.30, 127.11, 126.99, 126.73, 125.86,
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C34H28N2O2Si. [M + H]+ = 525.1998, found: 525.2001.
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This work was financially supported by the National Natural Science
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.saa.2017.05.053.
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