A cascade reaction based fluorescent probe for rapid and selective fluoride ion detection

Article abstract of DOI:10.1039/c4cc01665c

A cascade reaction-based colorimetric and fluorescent probe for selective fluoride ion detection is reported. The probe displays a fast response (t _1/2 = 2.41 min) and 550-fold fluorescence enhancement during sensing of fluoride ions. Application of the probe in live cell imaging is demonstrated. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01665c

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A cascade reaction based fluorescent probe for
rapid and selective fluoride ion detection†
Cite this: Chem. Commun., 2014,
50, 5510
Arundhati Roy, Dnyaneshwar Kand, Tanmoy Saha and Pinaki Talukdar*
Received 5th March 2014,
Accepted 19th March 2014
DOI: 10.1039/c4cc01665c
www.rsc.org/chemcomm
A cascade reaction-based colorimetric and fluorescent probe for better selectivity. Generally, the high affinity of Fꢀ ions towards
selective fluoride ion detection is reported. The probe displays a either boron or silicon has been extensively used for the develop-
fast response (t1/2 = 2.41 min) and 550-fold fluorescence enhance- ment of fluoride selective probes. Facile cleavage of either the
ment during sensing of fluoride ions. Application of the probe in live C–Si14–18 or the O–Si19–23 bond by the Fꢀ ion has been the key to
cell imaging is demonstrated.
designing the reaction sites in these probes. Based on these
strategies various chemodosimeters for fluoride sensing were
The development of artificial molecular systems for sensing anions reported. These deprotection strategies were further maneuvered
has received great research interest as anions play a pivotal role in a to develop new probes via the cascade reaction process. Kim and
wide range of biological, metabolic processes and participate in coworkers have reported a probe consisting of the 4-(hydroxy-
various enzymatic reactions.1,2 Fluoride, the smallest anion with methyl)phenol linker between the fluorophore and the silyl group.
high charge density, contributes towards diverse health and envir- Fluoride mediated reaction resulted in the release of the fluoro-
onmental issues.3,4 The presence of fluoride in low levels has proven phore along with the formation of 4-methylenecyclohexa-2,5-dien-
to be critical for the prevention of dental and skeletal fluorosis, 1-one as the by-product.24 Alternately, desilylation followed by
enamel demineralization while wearing orthodontic appliances and cyclization forming either coumarin or iminocoumarin was
during osteoporosis treatment. The optimal level of the ions in the reported for efficient Fꢀ ion detection.25–27 However, most of these
human body is routinely maintained from toothpaste, drinking probes suffered from a common drawback of longer response times
water, foods and other fluoride supplements. However, excess and poor off–on response.
intake of fluoride can result in metabolic disorders, skeletal disease,
Cascade reactions offer advantages such as atom economy as
mottled teeth, inhibition of neuro-transmitter biosynthesis in well as economies of time and waste generation. As such, cascade
fetuses, nephrolithiasis. For these reasons, the controlled consump- reactions can be considered to fall under the category of ‘‘green
tion of fluoride in the human body is a serious concern throughout chemistry’’.28 Herein, we report the design, synthesis, photo-
the world.5,6 Therefore, it is of great necessity to develop techniques physical characterization of a new cascade reaction based highly
for selective detection of fluoride ions. Fluorescent probes offer selective and sensitive colorimetric and fluorescent off–on probe 1
promising approach owing to its simplicity, high selectivity and for Fꢀ ions (Scheme 1). The non-fluorescent species 1 upon
sensitivity.
treatment with fluoride was expected to undergo a cleavage of
A large number of fluorescent probes have been developed for the Si–O bond to release tert-butyldimethylsilyl fluoride 2. A
detection of Fꢀ ions. Synthetic chemosensors recognize the ion subsequent cyclization was proposed to form phthalide 3 with
through either hydrogen bonding interactions (involving functional the release of carboxyfluorescein-based fluorophore 4. The cas-
groups such as phenol, urea, thiourea, amide, pyrrolic and indolic cade reaction is expected to be fast as the process is entropically
N–Hs, etc.)7,8 or Lewis acid/base coordination9,10 or anion-p inter- favorable.
actions.11–13 Chemodosimeters on the other hand, are known to
The synthesis of probe 1 was carried out in two steps (Scheme S1,
undergo irreversible chemical reactions with the ion and provide ESI†). First, 3 was converted to free acid 5 in one-pot three-step
strategy via opening of the lactone using methanolic KOH followed
Department of Chemistry, Indian Institute of Science Education and Research Pune,
by silyl protection of both benzylic alcohol and –COOH groups.
Subsequent selective deprotection of silyl ester provided acid 5 with
an overall 32% yield. Acid 5 was then coupled with the carboxy-
India. E-mail: ptalukdar@iiserpune.ac.in; Fax: +91 20 2589 9790;
Tel: +91 20 2590 8001
† Electronic supplementary information (ESI) available: Experimental proce-
dures, supplemental data, and the 1H-, 13C-NMR spectrum. See DOI: 10.1039/
fluorescein derivative 6 under EDCꢁHCl and DMAP/DIPEA condi-
c4cc01665c
tions to form 1 in 58% yield.
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Scheme 1 Structure and fluoride ion sensing mechanism of probe 1.
The photophysical properties of probe 1 were investigated in
DMSO. Probe 1 did not show any characteristic UV-visible
absorption band between 300 and 700 nm and was non-
fluorescent. Upon addition of tetrabutylammonium fluoride,
TBAF (300 equivalent) to the solution of probe 1 (10 mM), a
fluorescence band centered at lem = 523 nm (lex = 460 nm) was
observed (Fig. 1A). Pseudo first order reaction kinetics was
Fig. 2 Relative fluorescence of 1 (10 mM) in presence of either TBAF
(3 mM) or various analytes (front row) and subsequent addition of TBAF
(back row) to the same sample (A). Colour change for 1 in presence of
various analytes under visible light (B). Emission colour change for 1 in
presence of various analytes (C). Cuvettes were photographed under the
hand-held UV-lamp with lex = 365 nm.
observed with the rate constant k = 0.28 minꢀ1 and t1/2
=
2.41 min (Fig. 1A) and the sensing process was completed in
B7 min. As probe 1 was not capable of Fꢀ detection under
aqueous conditions, to overcome this limitation we prepared
the stock of Fꢀ in water and the assay was carried out in DMSO
solvent.29,30 The sensitivity of 1 towards Fꢀ was comparable
irrespective of the medium of the Fꢀ source (water or THF).
When the response of 1 (10 mM) towards increasing concentra-
tions of TBAF (0–350 equivalents) was monitored, a stepwise
increase in the fluorescence was observed upto 300 equivalents
of the salt (Fig. 1B). When fluorescence intensity measured at
l = 523 nm was plotted against the concentration, a linear
increase was observed within 0–200 equivalents of Fꢀ ions
(Fig. S3, ESI†). Sensing of Fꢀ ions by 1 was independent of
pH when analyzed over the range 3–11 (Fig. S5, ESI†).
application in the detection of Fꢀ in biological and environmental
science as a higher Fꢀ concentration is toxic (0.1 mM for drinking
water standard and >3 mM in biological systems).31 The detection
limit and response time for probe 1 were either comparable or
better than reported probes (Table S1, ESI†).
To evaluate the selectivity of probe 1 towards Fꢀ ion over other
possibly competitive analytes, the fluorescence measurements at
lem = 523 nm (lex = 460 nm) were carried out in the presence of
various interfering ions (Brꢀ, Iꢀ, Clꢀ, ClO4ꢀ, PF6ꢀ, NO3ꢀ, HSO4
,
ꢀ
OAcꢀ and SO42ꢀ) and analytes (H2O2, Cys and GSH). When probe
1 (10 mM) in DMSO was treated with 300 equivalents of TBAF, a
550-fold enhancement in the fluorescence intensity was observed
within 7 minutes of stirring at room temperature (Fig. 2A, blue
bar) but other analytes (3 mM) did not show any change in
emission intensity (Fig. 2A, front row). The probe 1 showed
fluorescence intensity enhancement only after addition of TBAF
into these solutions, confirming the selectivity of the probe
towards Fꢀ ions, even in the presence of competitive analytes
(Fig. 2A, back row). The resulting solution also provided a
quantum yield F = 0.06 (standard: fluorescein, F = 0.85 in 0.1 N
NaOH). The response of 1 towards Fꢀ ions under ambient light
was associated with the colour change from colourless to yellow
and inertness towards other analytes was supported by no change
in color (Fig. 2B). The fluorescence turn-on response upon sensing
of Fꢀ ions was also confirmed by the appearance of green
fluorescence of the reaction mixture when cuvette images were
taken under a hand-held UV-lamp (Fig. 2C). These results con-
firmed that probe 1 is highly selective towards fluoride.
The detection limit of probe 1 toward Fꢀ was evaluated to be
1.03 mM (19.6 ppb) which was well below 4 ppm, the allowed
concentration level of Fꢀ in drinking water specified by the USEPA25
(Fig. S4, ESI†). The low detection limit of 1 shows its potential for
Fig. 1 Fluorescence kinetics of 1 (10 mM) in presence of TBAF (3 mM in
either THF or H2O) in DMSO at l = 523 nm (A). Fluorescence spectra of 1
with increasing concentration of TBAF (0–3.5 mM) in DMSO (B).
The proposed mechanism of Fꢀ ion sensing by probe 1 was
1
confirmed by H-NMR titration studies (Fig. 3). Upon addition of
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1 : 100 DMSO–DNEM v/v, pH = 7.4) at 37 1C for 2 h (Fig. 4B). When
the same cells were again incubated with 20 mM NaF (1 : 100
H2O–DMEM, pH = 7.4) at 37 1C for 30 min, a strong fluorescence
was observed inside the cell (Fig. 4E). These data indicate that 1
can be applied for the selective detection of intracellular Fꢀ ions.
In conclusion, we have successfully designed, synthesized
and investigated the selective Fꢀ ion sensing properties of
probe 1. The non-fluorescent probe undergoes a cascade reac-
tion in the presence of the ion to release the lactone phthalide
and a carboxyfluorescein derivative as the strong fluorescent
species. The fast response of the probe towards the Fꢀ provided
t1/2 = 2.41 min and 550-fold fluorescence enhancement with a
detection limit of 19.6 ppb. Most importantly, the Fꢀ ion
detection ability of the probe in living cells was demonstrated
using confocal microscopy.
We acknowledge the Director, IISER Pune, DAE (Grant No.
2010/20/37C/6/BRNS/2480) and DST-SERB (Grant No. SR/S1/
OC-65/2012) for financial support. A.R. and T.S. thank the
UGC and D.K. thanks CSIR for research fellowships.
Fig. 3 1H-NMR spectral changes for probe 1 in DMSO-d6 with increasing
concentration of TBAF (0–300 equivalent).
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