A reaction based chromofluorogenic turn-on probe for specific detection of fluoride over sulfide/thiols

Article abstract of DOI:10.1016/j.tetlet.2014.09.051

A new reaction based probe fluorescein nosylate (R1) has been designed and synthesized for selective recognition of F^-in acetonitrile (CH₃CN) by exploiting both its nucleophilic and basic character. Probe R1 consists of fluorescein dye as a signalling unit while 4-nitrobenzenesulfonyl chloride as the masking unit. The F^-plays the role of de-masking agent to set free the fluorescein moiety in its open form from R1 leading to significant changes in its absorption/emission profile. The detection of F^-amidst of sulfide/thiols through receptors undergoing nucleophilic scission is a tedious job due to similarity in their extent of basicity/nucleophilicity. Here, we present a convenient solution for the same in the form of R1 which detects F^-selectively over sulfide/thiols in CH₃CN with a high detection limit of 4.6 × 10^-7M and 2.75 × 10^-8M determined through UV-visible and fluorescence titration data, respectively.

Full text of DOI:10.1016/j.tetlet.2014.09.051

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A reaction based chromofluorogenic turn-on probe for specific
detection of fluoride over sulfide/thiols
⇑
Sharad Kumar Asthana, Ajit Kumar, Neeraj, K. K. Upadhyay
Department of Chemistry (Centre of Advanced Study), Faculty of Science, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new reaction based probe fluorescein nosylate (R1) has been designed and synthesized for selective
recognition of FÀ in acetonitrile (CH3CN) by exploiting both its nucleophilic and basic character. Probe
R1 consists of fluorescein dye as a signalling unit while 4-nitrobenzenesulfonyl chloride as the masking
unit. The FÀ plays the role of de-masking agent to set free the fluorescein moiety in its open form from R1
leading to significant changes in its absorption/emission profile. The detection of FÀ amidst of sulfide/thi-
ols through receptors undergoing nucleophilic scission is a tedious job due to similarity in their extent of
basicity/nucleophilicity. Here, we present a convenient solution for the same in the form of R1 which
detects FÀ selectively over sulfide/thiols in CH3CN with a high detection limit of 4.6 Â 10À7 M and
2.75 Â 10À8 M determined through UV–visible and fluorescence titration data, respectively.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 11 July 2014
Revised 9 September 2014
Accepted 10 September 2014
Available online xxxx
Keywords:
Fluorescein
Chemodosimeter
Fluoride ion
Nucleophilicity/basicity
Optical sensors
The recognition and sensing of anions have attracted a great
deal of attention because of their biological and environmental
importance.1 Being very small in size and having high charge den-
sity the fluoride ion (FÀ) contributes towards diverse health and
environmental issues.2 The pivotal role of FÀ in dental care as pre-
venting cavities and strengthening teeth is well known.3 However,
a high intake of FÀ may cause most widespread side effects like
fluorosis, urolithiasis, osteoporosis etc. in human beings.4 Fluorides
also enter the environment from various anthropogenic activities
besides the fluoridation of drinking water.5 For these reasons, con-
siderable efforts have been devoted towards the development of
novel receptors for the selective detection of FÀ. The optical probes
for the analysis of different analytes have been in the process of
development for the last few decades owing to their simplicity,
high selectivity and sensitivity.6
Till now, a number of diverse sensing mechanisms have been
employed for optical detection of FÀ,4a,7 including hydrogen bond-
ing interactions, Lewis acid/base coordination, FÀ induced leaving
of the trialkylsilyl group,8 FÀ substitution for p-dimethylaminopy-
ridine,9 FÀ induced B–N bond cleavage10 and de-protection of dini-
tro/nitrobenzenesulfonyl (DNBS/NBS)-protected fluorophores.11
Out of the above the hydrogen bonding interaction based receptors
are generally non-selective and face interferences from a number
of anions of comparable basicity, for example, OHÀ, AcOÀ, H2PO4À
etc.12 To overcome the same, researchers have synthesized
reaction based probes, that is, chemodosimeters7,13 which utilize
chemical affinity between FÀ and silicon/boron involving facile
cleavage of the C–Si, O–Si or B–N/O bond by FÀ.8–10 Moreover fluor-
ide is most electronegative, and has a strong nucleophilic character
which may lead to FÀ induced nucleophilic cleavage of C–S bond
also.11
The basic synthetic approach of these kinds of probes mainly
involves FÀ directed cleavage of the protecting group and release
of fluorophore in the solution which leads to appropriate optical
changes. For this purpose a variety of fluorophores viz., resorufin,
BODIPY, fluorescein, coumarin etc. were used by various workers.13
Fluorescein is an inexpensive, practical and reliable molecular tool
for the construction of optical probes due to its large visible-range
extinction coefficients and high fluorescence quantum yields.14
Although a few FÀ responsive probes incorporating the fluorescein
moiety have been explored previously11,15–19 (Fig. 1), a few com-
mon drawbacks are associated with these probes viz., moderate
detection limit, selectivity, poor off–on response, tedious synthetic
approach etc.
In order to tackle the above limitations of the current FÀ probes,
herein we devised a highly selective reaction based chromofluoro-
genic turn-on probe (R1) for FÀ which incorporates fluorescein as
the fluorophore and 4-nitrobenzenesulfonyl (NBS) as the masking
agent (Fig. 1). Previously reported fluorescein based probes/
chemodosimeters are associated with some limitations. For exam-
ple, Shiraishi and co-workers.15 reported an unmodified fluores-
cein (R2) as fluorescent sensor for FÀ but it experiences
interferences from AcOÀ and H2POÀ4 due to their comparable
⇑
Corresponding author. Tel.: +91 542 670 2488.
E-mail addresses: drkaushalbhu@yahoo.co.in, kku@bhu.ac.in (K.K. Upadhyay).
http://dx.doi.org/10.1016/j.tetlet.2014.09.051
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Asthana, S. K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.09.051

2
S.K. Asthana et al. / Tetrahedron Letters xxx (2014) xxx–xxx
O
HO
TBSO
O
O
OH
OTBS
O
O
OH
S
O
O2N
O
O
O
O
O
O
R1
R2
R3
OH
B
HO
O
O
O
Si
Si
N
O
HO
Cl
O
O
S
O
N
H
Cl
N
H
O
COOH
R4
O
n
R5
NO2
O
O
O
OH
S
O
O
O
N
O
O
S
O2N
O
O
O2N
NO2
O
R6
R7
Figure 1. Chemical structure of probe R1 (present study) along with probes R2–R7 for FÀ reported by other workers.
basicity to FÀ. The probe reported by Yang et al. R3,16 showed
interference only by a less common anion BFÀ4 . A biocompatible
probe reported by Zheng et al. (R4)17 could detect the FÀ selective-
ly with a detection limit of 19 ppb in the aqueous medium but the
synthetic strategy of the same is a tedious one. Swamy et al.
reported yet another fluorescein derivative bearing a boronic acid
group (R5)18 but with a less intense fluorescent response with
the interference of ClÀ.
R1 was prepared following the known synthetic procedure
(Scheme 1).21 The probe R1 was fully characterized by 1H NMR,
13C NMR, IR and ESI-MS spectrometries (ESI; Figs. S1–S4).
The absorption and emission properties of probe R1 towards FÀ
as its tetrabutylammonium (TBA) salt were firstly evaluated in
CH3CN. The UV–visible studies were carried out in 20 lM CH3CN
solution of R1. The UV–visible absorption spectrum of R1 exhibited
two bands at 457 and 490 nm with poor intensity hence giving a
colourless appearance to the same. However, with the addition of
FÀ, a new strong absorption band centred at 517 nm appeared
which can be ascribed to the deprotection of NBS group from R1
hence liberating the parent fluorescein dye in its open form
(Fig. 2). At this stage the colour of the solution became yellow from
colourless instantaneously, indicating that R1 can be used as a
‘naked-eye’ indicator for FÀ (Fig. 2). A considerable number of other
anioÀns viz., S2À, ClÀ, BrÀ, IÀ, CH3COOÀ, C6H5COOÀ, ClO4À, SO32À, NÀ3 ,
HSO4, HSOÀ3 , BFÀ4 , H2PO4À, PF6À and HPO24À did not show any sig-
nificant absorption/colour response towards R1 (ESI; Fig. S5).
Since receptor R1 showed the specificity towards FÀ in colori-
metric measurement as described above, the fluorescence emission
of the same was further investigated to explore the possibility of
dual mode sensing of FÀ by R1. At the same time the fluorescent
methods possess even lower detection limits as compared to the
The probe R1 which we report in the present Letter is synthe-
sized through slight structural modification of the chemodosime-
ter (R6)19 of Yang et al. by utilizing NBS instead of the DNBS
group as the masking agent. Yang’s chemodosimeter (R6) selec-
tively detects sulfide in aqueous acetone solution. We also tried
the same solvent system for sulfide detection but to our surprise
R1 did not respond. However, when we checked ion sensing prop-
erty of R6 in CH3CN, it gave optical responses for sulfide along with
FÀ also. Chang and co-workers developed a resorufin sulfonate (R7)
recently11 which exhibited significant signalling towards FÀ with
remarkable interferences from sulfide ions. Hence, probes based
on de-protection of dinitro/nitrobenzenesulfonyl (DNBS/NBS)-pro-
tected fluorophores responded mainly for thiols20 and only a few of
them could detect FÀ with significant interferences from
sulfide.11,19
Thus, compared to the previously reported reaction based FÀ
probes R2–R711,15–19 (Fig. 1), our results demonstrate that R1 is
more potent towards FÀ in terms of selectivity as it does not
respond to the most commonly interfering analytes like sulfide/
colorimetric ones. The 5 lM CH3CN solution of R1 was colourless
and non-fluorescent upon excitation at 490 nm, indicating the pre-
dominant existence of the closed spirocycle form of fluorescein. As
expected, after esterification with NBS the fluorescein dye lost its
original fluorescence property and turned colourless. However
upon gradual additions of FÀ as its TBA salt to the CH3CN solution
of R1, the same experienced fluorescence switching and exhibited
thiols. The R1 detected FÀ ion with lowest limits of 4.6 Â 10À7
M
and 2.75 Â 10À8 M, determined through UV–visible and fluores-
cence titration data, respectively, which is either comparable/bet-
ter than a number of reaction based fluorescent probes reported
recently by various workers.
a
single emission band at 530 nm with ꢀ60-fold emission
enhancement (Fig. 3). Thus addition of FÀ restored the original
Please cite this article in press as: Asthana, S. K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.09.051

S.K. Asthana et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Cl
S
HO
O
OH
O
O
O
OH
O
O
O
S
+
Pyridine
O
O
Overnight
Stirring
O2N
O
O
NO2
PROBE R1
Scheme 1. Synthesis of probe R1.
between R1 and FÀ, as shown in Figure 4. In the same the emission
maximum was observed at ꢀ0.66 mole fraction [FÀ]/[R1]+[FÀ] sug-
gesting 1:2 binding stoichiometry between R1 and FÀ.
The quantitative aspect of FÀ signalling behaviour of R1 was
investigated by UV–visible and fluorescence titration data (ESI;
Figs. S6 and S7). From the same, the detection limit for the deter-
mination of FÀ in CH3CN was estimated as 4.6 Â 10À7 M and
2.75 Â 10À8 M through UV–visible and fluorescence titration data,
respectively. The detection limit obtained through fluorescence
titration data establishes the high sensitivity of R1 for FÀ, which
is fairly better than that obtained by various workers on similar
type of chemodosimeters.
The possible interferences in the FÀ signalling of R1 from other
coexisting anions were also examined. The emission intensities
were recorded at 530 nm within 2 min after each separate addition
of an anion (10 equiv each) to the CH3CN solution of R1 (5 lM).
Interestingly R1 showed an obvious fluorescence turn-on response
only in the presence of FÀ while no significant changes were
observed in the presence of commonly encountered anions viz.,
S2À, ClÀ, BrÀ, IÀ, CH3COOÀ, C6H5COOÀ, ClOÀ4 , SO32À, N3À, HSOÀ4, HSOÀ3 ,
BFÀ4 , H2PO4À, PF6À and HPO24À as their TBA salts (ESI; Fig. S8). More-
over, NaF is known to influence various cell signalling processes
and functions as a potent G protein activator and Ser/Thr phos-
phatase inhibitor and affects plenty of essential cell signalling ele-
ments.22 Keeping these facts in mind we also tried to detect FÀ
ions by changing its source as NaF instead of TBAF and almost simi-
lar emission profiles were observed in both of the sources of FÀ (ESI;
Fig. S9). Furthermore, as stated above when we resynthesized Yang’s
probe R619 (ESI; Fig. S10) and checked its ion sensing property in
CH3CN than it gave optical responses for sulfide along with FÀ also
(ESI; Fig. S11).
Figure 2. UV–visible titration spectra of 20 lM CH3CN solution of R1 upon
concomitant additions of FÀ ion (0–5 equiv); Inset: corresponding colours of R1
and R1 + FÀ.
fluorescence of the dye as greenish-yellow colour through de-
masking of the NBS and making free the fluorescein dye itself via
cleavage of fluorescein nosylate.
These findings imply that the sensing phenomenon of FÀ by R1
is a two-step process. The first one involves the deprotection of
NBS and liberation of fluorescein dye in its monoanionic form
induced by the nucleophilic nature of FÀ and secondly, the subse-
quent deprotonation of remaining –OH due to the basic character
of FÀ leaving fluorescein in dianionic form which is highly fluores-
cent. This was further confirmed through Job’s plot titration
Nevertheless, previous workers have reported a few thiol/sul-
fide responsive reaction based probes by utilizing similar deigning
strategy as ours.20 It is worth mentioning here the counter effect of
fluoride/sulfide, over the selective detection of thiols was not
2.5x106
2.0x106
1.5x106
1.0x106
5.0x105
0.0
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
[F-]/[[R1]+[F-]
Figure 3. Titration profile of emission spectrum of 5 lM CH3CN solution of R1 upon
gradual additions of FÀ ion (0–250 equiv); Inset: corresponding fluorescent colour
changes of R1 and R1 + FÀ under UV light.
Figure 4. Job’s plot of R1 with FÀ showing 1:2 binding stoichiometry.
Please cite this article in press as: Asthana, S. K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.09.051

4
S.K. Asthana et al. / Tetrahedron Letters xxx (2014) xxx–xxx
checked by the above workers. However we checked the effect of
commonly responsive thiols viz., cysteine (Cys) and homocysteine
(Hcy) towards R1 but to our surprise the same did not affect the
fluoride responsive characteristics of our probe R1 (Fig. 5). Thus,
the selectivity of R1 towards fluoride is unique as the same did
not experience any interferences from a number of anions and thi-
ols viz., Cys and Hcy also. Hence, optical signalling of FÀ by R1 with
high selectivity without the use of any instrument makes the same
as a highly convenient, handy and promising probe. Furthermore,
the time dependence response of R1 towards FÀ was monitored
by means of fluorescence intensity measurement. The results
2 equiv of FÀ was completed within 10 min (ESI; Fig. S12) the cor-
responding colorimetric responses (naked-eye/under UV-light)
were instantaneous.
The proposed mechanism of selective sensing for FÀ ion by R1
was confirmed through 1H NMR, 13C NMR and mass spectral stud-
ies. The unmodified fluorescein shows resonances for two –OH
protons at 9.97 d ppm (ESI; Fig. S13) however protection of one
of the –OH group with NBS in R1 lead upfield shifting of the
remaining –OH at 9.28 d ppm. Other aromatic protons of probe
R1 appeared at their usual positions in the range from 8.45 to
6.88 d ppm. Two additional peaks in the aromatic region also
appeared at 8.42–8.45 d ppm due to incorporation of the NBS
group. Upon addition of TBAF (10 equiv) to R1 in DMSO-d6 for
2 min, the characteristic singlet for the –OH at 9.28 d ppm peak dis-
appeared due to simultaneous deprotection of NBS and deprotona-
tion by the nucleophilic and basic character of FÀ. These lead other
aromatic protons to experience up-field shifting from 8.45–6.88 to
8.19–5.93 d ppm along with their broadening (Fig. 6).
revealed that although the reaction of 5
lM solution of R1 with
The 13C NMR spectrum of R1 showed a peak at 80.52 d ppm due
to quaternary carbon spirocycle-closed form of fluorescein nosy-
late (Fig. 7). The same vanished and a new peak at 101.03 d ppm
appeared due to deprotection of NBS upon the reaction of R1 with
FÀ leading to opening of fluorescein spiro lactam ring and conver-
sion of aliphatic spiro-carbon, that is, the quaternary carbon into
aromatic carbon. Moreover a new peak at 206.92 d ppm was
marked for the C@O of the carboxylic group of fluorescein in its
open form instead of 168.07 d ppm (in closed form) after FÀ addi-
tion (Fig. 7). The deprotection of the NBS was also confirmed by
ESI-MS analysis (ESI; Fig. S14). The MS spectra obtained with
R1 + FÀ in CH3CN showed a peak at m/z 1057.0, which is ascribed
to ([R1-2H] + 3[n-Bu4N])+ ion.
Figure 5. Bar graph showing effect of most competitive species on the emission
spectra of R1.
Thus the above NMR and mass spectral studies clearly indicated
that the sensing of FÀ through R1 involves two steps viz., the lib-
eration of fluorescein dye in monoanionic form, that is, FÀ induced
deprotection of NBS. The same form undergoes subsequent depro-
tonation of the remaining –OH due to basic character of FÀ leaving
fluorescein in dianionic form which is highly fluorescent and is the
basis for selective FÀ sensing (Fig. 8).
The above results were further supplemented by Thin Layer
Chromatographic (TLC) studies. We took the TLC photographs of
the spots of solutions of fluorescein, NBS and R1 with and without
additions of FÀ. As evinced from Figure S15 (see ESI), fluorescein
and probe R1 ran almost equally on the silica coated TLC plate indi-
cating the release of fluorescein dye from R1 upon FÀ addition.
To investigate the practical application of R1, test strips were
prepared by immersing filter papers into the 1 lM CH3CN solution
of R1 and fluorescein and then drying in air. As shown in Fig. 9,
when the dried test strips were dipped in solutions of FÀ for
10 min and dried in air, respectively, the observed colour of test
strips changed from non-fluorescent to green under UV light at
365 nm clearly. Therefore, the test strips demonstrated the poten-
tial application of R1 to detect FÀ.
Figure 6. Partial 1H NMR spectra of R1, R1 + FÀ and fluorescein + FÀ in a DMSO-d6
solution.
Figure 7. Partial 13C NMR spectra of R1 and R1 + FÀ in a DMSO-d6 solution.
Please cite this article in press as: Asthana, S. K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.09.051

S.K. Asthana et al. / Tetrahedron Letters xxx (2014) xxx–xxx
5
O
-O
O
OH
O
O
OH
S
O
HO
O
S
F-
CH3CN (H2O)
+
O
O2N
O
O
NO2
O
O
R1
O-
O
O
O
O
OH
F-
O
O
O-
O-
Dianionic form,
Highly Fluorescent
Monoanionic form,
Resonence promoted Ring Opening,
Fluorescent
Figure 8. Fate of probe R1 in the presence of FÀ in CH3CN solution.
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Figure 9. Fluorescent test strips of R1 (1 l
M) (a) R1, (b) R1 + FÀ, (c) fluorescein + FÀ.
We have synthesized a new fluorescent turn-on green-emitting
FÀ selective probe R1 based upon fluorescein-4-dinitrobenzenesul-
fonyl (NBS) dyads. Selective cleavage of NBS with FÀ, re-establishes
the emissive state of fluorescein by ꢀ60-fold emission enhance-
ment. This sensitive methodology allows for discriminating FÀ
from other anions especially sulfide and thiols. The R1 allows
selective detection of FÀ up to its 4.6 Â 10À7
M
and
2.75 Â 10À8 M concentration determined through UV–visible and
fluorescence titration data, respectively.
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S.K.A. thanks the CSIR, New Delhi for Junior Research Fellow-
ship. Neeraj acknowledges UPE-JRF for research fellowship. AK
acknowledges the UGC, New Delhi for DSK Postdoctoral Fellowship
[F.4-2/2006(BSR)/13-398/2011(BSR)].
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.09.
051.
References and notes
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Chem., Int. Ed. 2001, 40, 486–516; (c) Duke, R. M.; Veale, E. B.; Pfeffer, F. M.;
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Please cite this article in press as: Asthana, S. K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.09.051