Rational design of a rapid fluorescent approach for detection of inorganic fluoride in MeCN-H2O: A new fluorescence switch based on N-aryl-1,8-naphthalimide

Article abstract of DOI:10.1039/c3nj01389h

A fluoride chemosensor (FCS-1) based on 1,8-naphthalimide bearing a trimethylsilyl ether has been designed and synthesized. FCS-1 displayed high selectivity and sensitivity to fluoride in inorganic forms (KF and NaF) with a short response time in MeCN-H₂O media.

Full text of DOI:10.1039/c3nj01389h

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LETTER
Rational design of a rapid fluorescent
approach for detection of inorganic fluoride
in MeCN–H2O: a new fluorescence switch
based on N-aryl-1,8-naphthalimide†
Cite this: New J. Chem., 2014,
38, 884
Received (in Victoria, Australia)
10th November 2013,
Accepted 23rd December 2013
Angela Bamesberger,a Christopher Schwartz,a Qiao Song,a Weiwei Han,b Zhi Wangb
and Haishi Cao*a
DOI: 10.1039/c3nj01389h
www.rsc.org/njc
A fluoride chemosensor (FCS-1) based on 1,8-naphthalimide bearing sensors have been reported for excellent selectivity and sensitivity
a trimethylsilyl ether has been designed and synthesized. FCS-1 to organic fluoride (tetrabutylammonium fluoride) over other
displayed high selectivity and sensitivity to fluoride in inorganic interfering anions.10 However, detection of inorganic fluoride
forms (KF and NaF) with a short response time in MeCN–H2O media. has still been a challenge, particularly for samples with low
fluoride concentration in water-containing media due to the high
In the past few years, the development of artificial approaches solvation that caused long reaction time.11 Therefore, water
for anion sensing has been an enormously active research field friendly sensors with the ability to detect inorganic fluoride are
throughout several different disciplines of chemistry.1 In particular, quite rare and highly desirable.12
the detection of hazardous anions with a high impact on the
In the present paper, we report the design and synthesis of a
ecosystem and human health has received extremely considerable robust sensor (FCS-1) based on fluoride-triggered desilylation
attention.2 As an essential anion for humans, fluoride (FÀ) is a of trimethylsilyl ether with high affinity and sensitivity to
common additive to drinking water and toothpaste because of rapidly recognize fluoride over other anions in MeCN–H2O
its beneficial effects on human health.3 In the biological media. 4-Methoxy-N-aryl-1,8-naphthalimide and trimethylsilyl
systems, certain levels of fluoride are critical to maintaining (TMS) ether were employed as the fluorophore and the recognition
normal physiological functions (e.g., regulating cellular pH and unit for FCS-1, respectively. As a fluorescent molecule, N-aryl-1,8-
osmosis).4 While the optimal intake of fluoride according to the naphthalimide has been intensively investigated and used as the
data published by the U.S. Public Health Service is 1 mg per day fluorophore for chemosensors due to its unique photophysical
for humans,5 too much will lead to poisoning effects, such as properties.13 Based on previous research work of our group, we
fluorosis and urolithiasis.6 Thus, a rapid, facile and quantitative found that the intramolecular H-bond between the phenolic and
approach for the measurement of fluoride is highly needed.7 naphthalimide moieties in N-aryl-1,8-naphthalimide significantly
Since fluoride is both very basic and electronegative, numerous affected the fluorescence properties of the whole molecule. This
fluorogenic sensors have been designed on the basis of molecules new result explicitly suggested that the aryl moiety may signifi-
containing a Lewis acid boron atom or hydrogen bonds that are cantly affect photophysical properties of N-aryl-1,8-naphthalimide
able to strongly interact with fluoride to trigger the fluorescence that could be used as an optical switch to design fluorescence
change as the detection signal.8 The downside of this type of sensors.14 Inspired by this research work, we developed a new
sensor is that other anions, like CNÀ, OAcÀ and H2PO4À, may sensor (FCS-1) containing a trimethylsilyl ether on the aryl ring
cause significant interference.9 To improve the affinity to fluoride, with a strong fluorescence emission. In the presence of fluoride,
desilylation-based sensors were designed by using an irreversible trimethylsilyl ether was hydrolyzed by cleavage of Si–O bonds that
fluoride-triggered Si–O cleavage reaction widely used as a protection led to significant fluorescence quenching due to photoinduced
method for alcohols in organic synthesis. These reaction-based electron transfer (PET) (Scheme 1).
The FCS-1 and FCS-2 were synthesized by a three-step
reaction as shown in the Scheme 2. The compounds (a) were
a University of Nebraska at Kearney, Department of Chemistry, Kearney, Nebraska,
68849, USA. E-mail: caoh1@unk.edu; Fax: +1-3088658399; Tel: +1-3088658105
prepared from commercially available 4-bromo-1,8-naphthalic
anhydride and ortho/para-aminophenols (ortho for FCS-1 and
para for FCS-2) by a condensation reaction. Then, intermediates
were refluxed with sodium methoxide in methanol to provide
b Key Laboratory for Molecular Enzymology and Engineering, the Ministry of
Education, Jilin University, Changchun 130021, P. R. China
† Electronic supplementary information (ESI) available: Detailed synthetic pro-
cedures, characterization data including 1H NMR, 13C NMR spectra, HRMS data,
and absorption spectra. See DOI: 10.1039/c3nj01389h
methoxy-substituted derivatives (b) that were consequently
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Scheme 1 Sensing process of FCS-1 for fluoride.
Fig. 1 (a) The fluorescence spectra change of FCS-1 (1.0 Â 10À5 M) upon
addition of 0–10 equivalents of TBAF. (b) The fluorescence intensity (at
442 nm) change of FCS-1 (1.0 Â 10À5 M) upon addition of 0–10 equivalents
of TBA salts of FÀ, ClÀ, BrÀ, IÀ, NO3À, HSO4À, CNÀ, SCNÀ, OAcÀ, and
À
H2PO4 (the fluorescence measurements were conducted in MeCN–H2O
(v/v = 7 : 3) at 22 1C, lex = 366 nm).
fluoride and other anions were investigated by using fluorescence
titration in the MeCN–H2O (v/v = 7 : 3) solvent system at room
temperature. As shown in Fig. 1a, FCS-1 displayed a strong
fluorescence with an emission wavelength at 442 nm. Upon addi-
tion of increasing concentration of fluoride (0–10 equivalents of
TBAF), a remarkable fluorescence quenching at 442 nm was
observed, clearly indicating that the fluoride-triggered cleavage
reaction occurred in FCS-1 and the product with a phenolate on
the aryl moiety resulted in significant fluorescence quenching. The
maximum quenching (20% of original fluorescence intensity) was
achieved in the presence of three equivalents of TABF. This strongly
Scheme 2 The synthetic route of FCS-1 and FCS-2.
treated with chlorotrimethylsilane (TMSCl) in pyridine to afford supported the high efficiency of the fluoride-triggered desilylation
FCS-1 (50.7%) or FCS-2 (40.1%). Products were characterized by that allowed FCS-1 to behave as a sensor with rapid response to
1H NMR, 13C NMR and HRMS.
fluoride. The detection limit was calculated to be 41.8 ppb. For
MeOH, MeCN and MeCN–H2O mixture (v/v = 7 : 3) were comparison, fluorescence titrations were also conducÀted for ClÀ,
chosen to investigate the spectral properties of FCS-1 and BrÀ, IÀ, NO3À, HSO4À, CNÀ, SCNÀ, OAcÀ, and H2PO4 under the
FCS-2 (Table 1). FCS-1 exhibited the maximum absorption at same conditions, but none of them led to significant fluorescence
366 nm (e = 13 600 MÀ1 cmÀ1) and emission at 442 nm in quenching as fluoride, revealing the high affinity of FCS-1 to
MeCN–H2O (v/v = 7 : 3). No significant variation was observed fluoride (Fig. 1b).
for absorption and emission spectra in different solvents.
In nature, fluoride widely exists in the ionic forms that
However, the quantum yield of FCS-1 decreased from 0.417 in require the detecting approach possessing a high sensitivity
MeCN to 0.213 in MeCN–H2O and 0.152 in MeOH with increasing to fluoride in inorganic form. For reaction based fluoride
polarity of solvents due to the strong solvation effect. FCS-2 sensors, the responding time determined by the reaction rate
displayed similar absorption at 367 nm and emission at 449 nm, is one of the most crucial factors during the sensing process.
but low quantum yield (0.016) in MeCN–H2O (v/v = 7:3).
Although the desilylation based fluoride sensors significantly
Since FCS-1 was designed on the basis of desilylation of subsided the effects of interfering anions, long reaction time,
trimethylsilyl ether, we believed that FCS-1 would show higher up to several tens of minutes or even hours, has remarkably
sensitivity to fluoride over other anions and the fluoride- hampered their applications for samples containing inorganic
triggered Si–O bond cleavage would increase the electron fluoride with low concentration.15 Based on these considera-
density of aryl moieties that consequently induced significant tions, FCS-1 was designed by using trimethylsilyl ether as the
fluorescence change. If rapid and quantitative deprotection of recognition moiety to react with fluoride. Compared to other
trimethylsilyl ether is achieved by fluoride, FCS-1 will provide a silyl ethers, such as tert-butyldimethylsilyl ether (TBDMS), tert-
sensitive method for the detection of fluoride based on the butyldiphenylsilyl ether (TBDPS), and triisopropylsilyl ether
change of fluorescence. The recognition properties of FCS-1 to (TIPS), trimethylsilyl ether showed lower hydrophobicity and
Table 1 Photophysical properties of FCS-1 and FCS-2
FCS-1
FCS-2
Media
lab (nm)
lem (nm)
e (MÀ1 cmÀ1
)
F (10À2
)
lab (nm)
lem (nm)
e (MÀ1 cmÀ1
)
F (10À2
)
MeOH
MeCN
MeCN–H2O (7 : 3)
366
362
366
446
440
442
13 780
13 470
13 600
15.2
41.7
21.3
367
362
367
445
444
449
14 470
14 740
15 090
1.5
1.6
1.6
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Letter
Fig. 3 Photoinduced electron transfer (PET) between naphthalene and
aryl moieties led to fluorescence quenching in the presence of fluoride.
Fig. 2 The fluorescence intensity change (at 442 nm) of FCS-1 (1.0 Â 10À5 M)
(a) upon addition of 0–10 equivalents of TBAF, KF, and NaF (b) in the presence
of 3 equivalents of TBAF, KF, and NaF within 10 min in the MeCN–H2O
(v/v = 7 : 3) media at 22 1C (lex = 366 nm).
group by formation of phenolate could be another reason to explain
the fluoride-triggered fluorescence quenching. To distinguish these
two possibilities, the UV-Vis spectra of FCS-1 were recorded upon
addition of fluoride (Fig. S2, ESI†). The lack of variation in the
absorption spectra of FCS-1, including the maximum absorption
wavelength shift or intensity change, indicated that ICT was not
considerably disrupted during the desilylation process and
confirmed photoinduced electron transfer is the predominant
mechanism to explain fluorescence quenching.
less steric hindrance that allowed FCS-1 to react with fluoride
quickly in the water-containing media.
The fluoride titrations were conducted by using TBAF and
inorganic fluoride (KF and NaF) in the MeCN–H2O (v/v = 7 : 3)
media (Fig. 2a). After incubating FCS-1 with each fluoride for
5 minutes, fluorescence quenching was observed for all three
sources of fluoride. Although FCS-1 showed high sensitivity to
KF and TBAF in the MeCN–H2O media, significant fluorescence
quenching was also observed with increasing concentration of
NaF. The kinetics of desilylation for FCS-1 was investigated by
using the three fluoride sources (Fig. 2b).18 TABF and KF
displayed a high reaction rate with FCS-1 and reached maximum
fluorescence quenching within 6 minutes. NaF gave a slightly lower
reaction rate, but up to 88% of FCS-1 was desilylated in 10 minutes.
The overall rate constants of FCS-1 (1.0 Â 10À5 M) based on
Moreover, FCS-2 with a para-trimethylsilyl ether on the aryl
moiety was synthesized to investigate the recognition properties
to fluoride as a comparison of FCS-1. FCS-2 also showed specific
recognition to fluoride (TBAF) over the other nine anions as
observed from FCS-1, indicating the high affinity of trimethylsilyl
ether as a recognition unit for detecting fluoride (Fig. 4a).
However, compared to FCS-1, FCS-2 displayed a lower quantum
yield (0.016) in MeCN–H2O (v/v = 7 : 3) and less fluorescence
quenching (51% of original fluorescence intensity) triggered by
fluoride, which may be caused by a weaker electron transfer
effect. Because of less steric hindrance of para-trimethylsilyl
ether, FCS-2 exhibited a high desilylation rate upon addition of
fluoride in the MeCN–H2O media, in particular for TABF and KF
(Fig. 4b). The overall rate constants of FCS-2 (1.0 Â 10À5 M)
pseudo-first-order reaction were calculated to be 7.8 Â 10À3 sÀ1
,
7.1 Â 10À3
s
À1, and 4.2 Â 10À3 sÀ1 for TBAF, KF and NaF (3.0 Â
10À5 M) respectively.
To investigate the quenching mechanism of FCS-1 triggered
by fluoride, the frontier molecular orbitals of FCS-1 were
calculated. Based on the recent investigation by Heagy et al.,
the naphthalene moiety and the aryl moiety of N-aryl-1,8-
naphthalimides could be considered as two separated systems
and the aryl moiety may significantly affect the photophysical
properties of the whole molecule.16 Therefore, compared to
strong fluorescence from FCS-1, the fluoride-triggered desilyla-
tion yielded a phenolate on the aryl moiety showing weak
fluorescence emission. Our hypothesis is that fluorescence
quenching was caused by photoinduced electron transfer
(PET) from the electron-rich aryl moiety to the electron-
deficient naphthalimide ring at the excited state. To verify the
hypothesis, the geometric structure of FCS-1 was optimized by
the B3LYP/6-31G* model and HOMO/LUMO were calculated for
naphthalimide and aryl moieties by using Gaussian 09.17 As
shown in Fig. 3, FCS-1 did not show significant electron
transfer at the excited state because the HOMO of naphthalene
moiety (À6.13 eV) has a similar energy level to the aryl moiety
(À6.11 eV). Upon addition of fluoride, the desilylation of FCS-1
yielded a phenolate, in which the HOMO of the aryl moiety
increased to À5.25 eV that allowed the electron transfer to the
naphthalene moiety at the excited state, resulting in significant
fluorescence quenching. However, disruption of the internal
charge transfer (ICT) between a methoxy group and a carbonyl
were calculated to be 1.9 Â 10À2
s
À1, 1.6 Â 10À2
s
À1, and 2.7 Â
10À3 sÀ1 for TBAF, KF and NaF (3.0 Â 10À5 M) respectively.
In summary, we developed a desilylation based sensor (FCS-1)
by using a trimethylsilyl ether as the recognition unit for the
detection of fluoride. High sensitivity and affinity to fluoride have
been observed for FCS-1 over other interfering anions. Particularly,
FCS-1 displayed high sensitivity to inorganic fluoride including KF
and NaF in the MeCN–H2O (v/v = 7 : 3) media with a short response
time (5 min) that showed potential applications of FCS-1.
Fig. 4 The fluorescence quenching (at 449 nm) of FCS-2 (1.0 Â 10À5 M)
(a) in the presence of difference anions of TBA salts (0–10 equivalents),
(b) upon addition of 3 equivalents of TABF, KF and NaF within 10 min in the
MeCN–H2O (v/v = 7 : 3) media at 22 1C (lex = 367 nm).
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18 Experimental section: (1) fluoride titration: FCS-1 (3.91 mg)
was dissolved in a 10.0 mL MeCN–H2O mixture (v/v = 7 : 3) to
prepare a stock solution (1.0 Â 10À3 M). The fluoride stock
solutions (1.0 Â 10À3 M) were prepared by dissolving TBAFÁ
3H2O (3.15 mg) into a 10 mL MeCN–H2O mixture (v/v =
7 : 3), KF (5.81 mg) into a 100 mL MeCN–H2O mixture (v/v =
7 : 3), and NaF (4.20 mg) into a 100 mL MeCN–H2O mixture
(v/v = 7 : 3). FCS-1 stock solution (10 mL) was incubated with
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0 to 100 mL fluoride stock solutions diluted to 1 mL for
Letter
FCS-1 stock solution (10 mL) was added into a 990 mL MeCN–
H2O mixture (v/v = 7 : 3) containing fluoride stock solution
(30 mL). The fluorescence emission variation at 442 nm was
monitored between 0 and 10 min at 22 1C by using a
FluoroMax-4 Spectrofluorometer (slit = 1 nm, lex = 366 nm).
6 min at room temperature (22 1C). Then, the samples were
transferred into a fluorescence cuvette of 1.4 mL volume to
record fluorescence spectra by using a FluoroMax-4 Spectro-
fluorometer (slit = 1 nm, lex = 366 nm). (2) Kinetics study:
888 | New J. Chem., 2014, 38, 884--888
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