Reaction-based dual signaling of fluoride ions by resorufin sulfonates

Article abstract of DOI:10.1039/c3ob00040k

We developed new reaction-based probes for the dual signaling of fluoride ions. Selective fluoride-assisted deprotection of resorufin nosylate to generate resorufin fluorochrome was used for signaling. Resorufin nosylate exhibited selective colorimetric a

Full text of DOI:10.1039/c3ob00040k

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Biomolecular Chemistry
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Reaction-based dual signaling of fluoride ions by
resorufin sulfonates†
Cite this: Org. Biomol. Chem., 2013, 11,
2966
Hyun Gyu Im, Hong Yeong Kim, Myung Gil Choi and Suk-Kyu Chang*
We developed new reaction-based probes for the dual signaling of fluoride ions. Selective fluoride-
assisted deprotection of resorufin nosylate to generate resorufin fluorochrome was used for signaling.
Resorufin nosylate exhibited selective colorimetric and fluorogenic signaling of fluoride ions in aceto-
nitrile. The response from sulfide ions was effectively masked by using the TPEN–Cu2+ complex as a
source of Cu2+ ions for masking sulfide. Selective optical signaling of fluoride was possible in the pres-
ence of commonly encountered anions with a detection limit of 1.9 × 10−6 M.
Received 9th January 2013,
Accepted 22nd March 2013
DOI: 10.1039/c3ob00040k
www.rsc.org/obc
Another interesting system reported by Kim and Swager is a
turn-on type dosimeter that uses fluoride-triggered coumarin
Introduction
Anionic species play important roles in many chemical and dye formation by the unique chemical reactivity of fluoride
biological processes. A number of optical chemosensors for with bulky silyl-protecting groups.20
selectively detecting anionic substrates of different shapes and
Resorufin is one of the most attractive fluorochromes used
charges have been reported.1,2 Of the many important anionic to construct advanced optical signaling systems because of its
targets, fluoride ions have attracted much research interest characteristically large fluorescence enhancement and pro-
owing to their importance in many aspects of chemical, bio- nounced colorimetric possibilities.21 The deprotection of sul-
logical, clinical, and environmental sciences.3 In particular, fonic esters has been successfully used to design a variety of
fluoride anions play an essential role in dental care, osteoporo- important chemical species. Typical examples are pentafluoro-
sis treatment, and water fluoridation.4
benzenesulfonyl-fluorescein for hydrogen peroxide,22 bis-
There are a variety of sophisticated fluoride sensors.5 Repre- (dinitrobenzenesulfonyl)-fluorescein for superoxide,23 dinitro-
sentative fluoride sensors operating in organic solvents use benzenesulfonyl-merocyanine and BODIPY for thiols,24
hydrogen bonding between fluoride ions and a variety of dinitrobenzenesulfonyl-fluorescein for sulfide,25 and dinitro-
sensor functional groups, such as phenol, urea, thiourea, benzenesulfonyl-resorufin for acetyl-cholinesterase assay.26
amide, pyrrolic, and indolic NHs. The sensors are made of The deprotection of phenol function in trifluoromethanesulfo-
various molecular skeletons, including unmodified fluor- nate and toluenesulfonate by fluoride ions has been demon-
escein,6 bisurea of anthracene,7 pyrrolylquinoxalines contain- strated in organic synthesis (Scheme 1).27 Using this
ing pyrene antenna moieties,8 calix[4]pyrrole,9 and calix[4]- transformation, we developed a new fluoride-selective probe by
arene.10 Chelating and cationic boranes,11,12 well-known the selective deprotection of resorufin nitrobenzenesulfonate.
boron–dipyrromethene dyes,13 and the axial-substituted sub- The designed compound showed selective and sensitive
phthalocyanine moiety14 have also been successfully devised chromogenic and fluorescence turn-on type signaling for
for selective fluoride signaling. Recently, chemodosimetric fluoride ions in acetonitrile.
probes for fluoride anions have also attracted much research
interest.15 These probes are based on the specific reactivities
of compounds with fluoride ions. Notable examples are dual
chromogenic and fluorescent chemodosimeters based on
Results and discussion
Resorufin nosylate 1, dinitrobenzenesulfonate 2, and tosylate 3
fluoride-induced Si–O bond cleavage of silyloxybenzyl-protected
resorufin and naphthalimide,16,17 and the silyl ether derivative
were prepared by reacting sodium salt of resorufin with
of dicyanomethylene-4H-chromene and fluorescein.18,19
Department of Chemistry, Chung-Ang University, Seoul 156-756, Korea.
E-mail: skchang@cau.ac.kr; Fax: +82-2-825-4736; Tel: +82-2-820-5199
†Electronic supplementary information (ESI) available: Additional chemosignal-
ing behaviors of resorufin sulfonates and 1H and 13C NMR spectra of 1 and 3.
See DOI: 10.1039/c3ob00040k
Scheme 1 Deprotection of aryl sulfonates by fluoride ion.
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using the TPEN–Cu2+ complex as a source of Cu2+ masking
ions, which allowed the desired fluoride-selective signaling of
1 (vide infra).
Fluorescence signaling of fluoride and sulfide ions by 1 was
also pronounced (Fig. 1b). Nosylate 1 showed very weak emis-
sion above 550 nm in acetonitrile, as did other O-derivatized
resorufins. However, upon treatment with 10 equiv. of fluoride
or sulfide ions, intense fluorescence was observed at 591 nm.
The fluorescence intensity ratio at 591 nm in the presence and
absence of added analyte I/Io was 58.9 for fluoride and 62.2 for
sulfide ions (Fig. S2, ESI†). Concomitantly, a deep pink fluo-
rescence developed under illumination with a UV-lamp. Other
anions revealed no significant response and the I/Io at 591 nm
changed within a narrow range between 1.11 for Br− and 1.60
for AcO−. These observations clearly demonstrate the fluoride-
and sulfide-selective chromogenic and fluorescent signaling
behavior of nosylate 1.
Scheme 2 Preparation of resorufin sulfonates for fluoride signaling.
Signaling occurs due to deprotection of the nosylate group
on the phenolic hydroxyl group of resorufin by fluoride ions
(Scheme 3), which is a useful protocol in synthetic organic
chemistry.28 1H NMR, UV–vis, and fluorescence measurements
1
were used to follow the depicted deprotection process. The H
NMR spectrum of probe 1 in the presence of 5 equiv. of fluo-
ride ions transformed to that of resorufin (Fig. 2). The six reson-
ances of 1 in the 6.3–8.0 ppm region were transformed to three
well-defined signals of resorufin at 5.91, 6.29, and 7.21 ppm.
UV–vis and fluorescence spectra of 1 in the presence of 10
equiv. of fluoride ions were also identical to those of resorufin
(Fig. S3 and S4, ESI†). The signaling speed of fluoride by 1 was
Fig. 1 UV–vis spectra (a) and fluorescence spectra (b) of 1 in the presence of
various anions. (a) [1] = 1.0 × 10−5 M, [An−] in TBA salt = 1.0 × 10−4 M. (b) [1] =
5.0 × 10−6 M, [An−] in TBA salt = 5.0 × 10−5 M in CH3CN. λex = 485 nm.
p-nitrobenzenesulfonyl chloride, 2,4-dinitrobenzenesulfonyl
chloride, or p-toluenesulfonyl chloride, respectively, in dichloro-
methane (Scheme 2). The chromogenic and fluorescence sig-
naling of sulfonates toward fluoride ions were investigated in
acetonitrile where the most pronounced signaling selectivity
and efficiency were observed.
The UV–vis absorption spectrum of 1 was characterized by
two moderate intensity bands at 335 and 433 nm (Fig. 1a).
Treating 1 with 10 equiv. of fluoride or sulfide in TBA salt
resulted in a pronounced change in the UV–vis absorption
spectrum. The absorption bands of 1 at 335 and 433 nm dis-
appeared and new strong bands appeared at 550, 569, and
587 nm that are ascribed to resorufin fluorochrome. The solu-
tion color changed markedly from yellow to pink, which
allowed detection of fluoride and sulfide ions with the naked
eye. Other representative anions tested (Cl−, Br−, I−, AcO−,
NO3−, N3−, ClO4−, and HSO4−) showed almost no responses. In
these cases, a large change in the wavelength of the absorption
maximum provides a possibility of ratiometric analysis of fluo-
ride and sulfide signaling. The absorbance ratio at 587 and
433 nm (A587/A433) was used for ratiometry. The change was
very large for fluoride (from 0.01 to 179) and sulfide (from
0.01 to 187), while the other tested anions revealed nearly con-
stant responses between 0.01 for Br− and 0.13 for AcO−
(Fig. S1, ESI†). The response of 1 toward sulfide ions is dis-
appointing for applications in fluoride-selective signaling. As
mentioned earlier, sulfide signaling by the dinitrobenzenesul-
fonate of fluorescein has already been reported.25 However,
the sulfide response of 1 could be effectively suppressed by
Scheme 3 Signaling of fluoride ions by deprotection of resorufin nosylate.
Fig. 2 Partial 1H NMR spectra of 1, 1 + F−, and resorufin + F−. [1] = [Resorufin] =
5.0 × 10−3 M. [F−] in TBA salt = 2.5 × 10−2 M in DMSO-d6. Red starred peaks are
due to the aromatic resonances of the nosyl group.
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Fig. 3 Concentration dependent fluorescence signaling behavior of
1 for
Fig. 4 Signaling of fluoride ions by 1 in the presence of various anions as the
background. [1] = 5.0 × 10−6 M, [F−] = [A−] in TBA salt = 5.0 × 10−5 M in CH3CN.
λex = 485 nm.
fluoride ions. [1] = 5.0 × 10−6 M, [F−] in TBA salt = from 0 to 3.0 × 10−5 M in
CH3CN. Inset: fluorescence intensity at 591 nm as a function of fluoride concen-
tration. λex = 485 nm.
electron donating p-methyl substituent exhibited satisfactory
fluoride selectivity, but the signaling speed and sensitivity for
fluoride ions were much less favorable (Fig. S13 and S14,
ESI†). Complete fluoride signaling with 3 required several
hours.
fast and completed within 5 min after sample preparation
(Fig. S5, ESI†).
The quantitative fluoride signaling behavior of 1 was inves-
tigated by fluorescence titration (Fig. 3). As fluoride concen-
tration increased, the fluorescence intensity at 591 nm
increased steadily and showed a concentration-dependent cali-
bration up to 2.5 × 10−5 M of fluoride ions. From this concen-
tration-dependent signaling profile, the detection limit for the
determination of fluoride ions in acetonitrile was estimated as
1.9 × 10−6 M.29 UV–vis titration also provided a useful cali-
bration plot by ratiometric analysis using the ratio of two
absorbances of 1 at 433 and 587 nm (Fig. S6, ESI†). The detec-
tion limit obtained by UV–vis measurements was 6.2 × 10−6 M,
which is fairly larger than that obtained by fluorescence
measurements.
As mentioned earlier, probe 1 exhibited significant signal-
ing toward sulfide ions comparable to that of fluoride ions
under the same measurement conditions. Compounds 2 and 3
also exhibited substantial responses toward sulfide ions
(Fig. S15, ESI†). To obtain the sole fluoride selectivity of 1,
interference from sulfide ions was suppressed by adding metal
ions that form very insoluble metal sulfides, such as Cu2+ or
Zn2+. The addition of Cu2+ or Zn2+ ions effectively suppressed
signaling by sulfide; however, fluoride signaling was also sig-
nificantly affected by the interaction of added metal ions with
fluoride ions. The undesirable interaction of fluoride with
added metal ions was successfully controlled by using TPEN,
which forms stable complexes with transition metal cations, as
an auxiliary complexing agent (Fig. 5). Because the solubility
product, Ksp, of CuS(s)30 is 8 × 10−37 and the association con-
stant, Kassoc, for the complex between Cu2+ and TPEN31 is 3.47
The possible interferences in the fluoride signaling of 1
from other coexisting anions were checked. Fluoride-selective
fluorescence signaling behavior of 1 was not significantly
affected by the presence of commonly encountered anions
(Fig. 4 and Fig. S7, ESI†). The variation in fluorescence inten-
sity at 591 nm of the 1–F− system in the presence and absence
of coexisting anions (A−) I1+F+A/I1+F under competitive con-
−
ditions fluctuated in a narrow range between 0.93 for NO3
−
and 1.08 for HSO4 ions. The UV–vis spectral behavior also
confirmed that the fluoride-selective signaling was not signifi-
cantly affected by commonly encountered anions as the back-
ground (Fig. S8, ESI†).
The signaling behaviors of resorufin dinitrobenzenesulfo-
nate 2 and tosylate 3 were also tested, expecting more
improved fluoride signaling. For dinitrobenzenesulfonate 2,
the signaling sensitivity for fluoride ions was high and the sig-
naling speed was fast (Fig. S9 and S10, ESI†) due to the
increased electron withdrawing nature of the dinitro substitu-
ent on the sulfonate benzene ring. However, the selectivity
toward fluoride ions was inferior to that of nosylate 1 (Fig. S11
and S12, ESI†). In addition to fluoride (A587/A433 = 25), acetate
(A587/A433 = 3.0) and azide ions (A587/A433 = 15) were also sig-
nificantly responsive. On the other hand, tosylate 3 having an
Fig. 5 Changes in fluorescence intensity I/Io of 1 at 591 nm in the presence of
various anions in CH3CN. The sulfide signaling of 1 was suppressed by the
TPEN–Cu2+ complex. [1] = 5.0 × 10−6 M, [An−] in TBA salt = 5.0 × 10−5 M, [TPEN] =
9.0 × 10−5 M, [Cu2+] = 7.5 × 10−5 M in acetonitrile. λex = 485 nm.
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× 1020, interfering sulfide ions in the presence of the TPEN– sodium salt (0.30 g, 1.3 mmol) and triethylamine (0.54 mL,
Cu2+ complex can be effectively sequestered by forming insolu- 3.9 mmol) in 40 mL dichloromethane. The reaction mixture
ble CuS(s). Conversely, fluoride ions could remain intact from was stirred at room temperature for 12 h, and the resulting
the undesirable interaction of Cu2+ ions which are tightly com- solution was treated with water. The organic phase was separ-
plexed by TPEN. Although the cited constants (Ksp of CuS(s) ated, dried over anhydrous sodium sulfate, and concentrated
and Kassoc of TPEN–Cu2+) are not directly accountable for the under reduced pressure. The crude product was purified by
significantly different situation of the present measuring column chromatography (silica gel, hexane–ethyl acetate =
conditions of acetonitrile, a qualitative interpretation might be 1 : 1) to afford nosylate 1 (0.27 g, 58%) as an orange colored
plausible.
solid. Mp = 199–200 °C. 1H NMR (600 MHz, DMSO-d6) δ
Finally, the practical applicability of 1 for the fluoride-selec- 8.45–8.36 (m, 4H), 7.94 (d, J = 8.7 Hz, 1H), 7.60 (d, J = 2.4 Hz,
tive signaling in mixed aqueous medium was tested. As water 1H), 7.56 (d, J = 9.8 Hz, 1H), 7.46 (dd, J = 8.7 Hz and 2.4 Hz,
content in the aqueous acetonitrile solution increased, the sig- 1H), 6.84 (dd, J = 9.8 Hz and 2.0 Hz, 1H), 6.30 (d, J = 2.0 Hz,
naling became drastically less effective as in other fluoride 1H); 13C NMR (150 MHz, DMSO-d6) δ 186.1, 151.7, 150.7,
probes that operated in organic solutions (Fig. S16, ESI†).32 In 149.7, 149.6, 144.6, 139.5, 135.6, 135.5, 132.6, 131.9, 130.6,
the present resorufin nosylate system, a satisfactory fluoride- 125.6, 119.7, 110.9, 106.7; HRMS (FAB); m/z calcd for
selective signaling is not plausible in acetonitrile solution that C18H11N2O7S [M + H]+: 399.0281, found 399.0287.
contains more than 2% water (Fig. S17, ESI†).
Synthesis of 2
Compound 2 was prepared by reacting resorufin sodium salt
Conclusions
with 2,4-dinitrobenzenesulfonyl chloride following the
reported procedure.26 The crude product was purified by flash
We investigated new reaction-based fluoride signaling system
based on the sulfonates of resorufin fluorochrome. Resorufin
nosylate was smoothly deprotected by fluoride ions in aceto-
nitrile. The deprotection resulted in selective colorimetric and
column chromatography (silica gel, hexane–ethyl acetate =
1 : 1) to yield dinitrobenzenesulfonate 2 (53%) as an orange
colored solid.
fluorescence turn-on type fluoride signaling behavior. Conver-
sely, dinitrobenzenesulfonate of resorufin exhibited low selec-
tivity toward fluoride ions, while tosylate derivative
demonstrated slow fluoride signaling speed. The interference
from sulfide was successfully suppressed by using the TPEN–
Cu2+ complex as a source of Cu2+ masking ions. Signaling was
not affected by the presence of common anions. The designed
resorufin nosylate system could be useful in constructing other
supramolecular systems targeted for fluoride ions.
Synthesis of 3
Resorufin tosylate 3 was prepared by the reaction of resorufin
sodium salt with p-toluenesulfonyl chloride following the pro-
cedure described above. Crude product 3 was purified by flash
column chromatography (silica gel, hexane–ethyl acetate =
1 : 1) to give tosylate 3 (60%) as a yellowish green solid. Mp =
1
195–196 °C. H NMR (600 MHz, DMSO-d6) 7.83 (d, J = 8.7 Hz,
1H), 7.80 (d, J = 8.3 Hz, 2H), 7.52 (d, J = 9.8 Hz, 1H), 7.48 (d, J =
8.0 Hz, 2H), 7.25 (d, J = 2.5 Hz, 1H), 7.07 (dd, J = 8.7 Hz and
2.5 Hz, 1H), 6.82 (dd, J = 9.8 Hz and 2.1 Hz, 1H), 6.28 (d, J =
2.1 Hz, 1H), 2.41 (s, 3H) 13C NMR (150 MHz, DMSO-d6) δ
186.1, 151.2, 149.7, 149.4, 146.7, 144.5, 135.6, 135.4, 132.3,
131.7, 131.4, 130.9, 128.8, 119.7, 110.6, 106.7, 21.7; HRMS
(FAB); m/z calcd for C19H14NO5S [M + H]+: 368.0587, found
368.0591.
Experimental section
General
Resorufin sodium salt, 4-nitrobenzenesulfonyl chloride, 2,4-
dinitrobenzenesulfonyl chloride, p-toluenesulfonyl chloride,
N,N,N′N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), and
tetrabutylammonium (TBA) salts of various anions were pur-
chased from commercial suppliers and used without further
purification. All other chemicals and solvents were used as
received. UV–vis and fluorescence experiments were performed
Preparation of stock solutions
A stock solution of 1 (5.0 × 10−4 M) was prepared in aceto-
nitrile. Stock solutions (1.0 × 10−2 M) of the TBA salt of F−,
Cl−, Br−, I−, AcO−, NO3−, N3−, ClO4−, HSO4−, and S2− were pre-
pared in CH3CN. TBA sulfide was prepared by treating TBA
hydroxide with ammonium sulfide. A stock solution of the
TPEN–Cu2+ complex was prepared by dissolving 25.4 mg
(0.060 mmol) of TPEN and 18.5 mg (0.050 mmol) of copper
perchlorate hexahydrate in CH3CN (5.0 mL). To protect fluo-
ride ions from interaction with added metal ions, 1.2 equiv. of
TPEN with respect to Cu2+ ions was used. The final concen-
1
using HPLC grade acetonitrile. H NMR and 13C NMR spectra
were recorded at 600 MHz and 150 MHz, respectively, and
referenced to the residual solvent signal. Mass spectra were
obtained on
a FAB-Double focusing mass spectrometer.
Column chromatography was performed with silica gel
(240 mesh).
Synthesis of 1
4-Nitrobenzenesulfonyl chloride (0.26 g, 1.2 mmol) in 10 mL trations of TPEN and Cu2+ were 0.012 M and 0.01 M,
dichloromethane was added to a suspension of resorufin respectively.
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Measurement of signaling behaviors
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The UV–vis and fluorescence signaling behaviors of
1
toward analytes in TBA salts were measured in acetonitrile.
Measuring solutions were prepared by successively placing
60 μL (for UV–vis measurements) or 30 μL (for fluorescence
measurements) of stock solution of 1, and 30 μL (for UV–vis
measurements) or 15 μL (for fluorescence measurements) of a
TBA salt solution (1.0 × 10−2 M) in a vial. The resulting solu-
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M for UV–vis, and 5.0 × 10−6 M and 5.0 × 10−5 M for fluo-
rescence measurements, respectively. The excitation wave-
length was 485 nm for fluorescence measurements. The
detection limit was estimated by plotting changes in the
fluorescence intensities of 1 at 591 nm as a function of log[F−]
following the reported procedure.29 A linear regression curve
was fitted to the intermediate values of the sigmoidal plot. The
point at which this line crossed the ordinate axis was taken as
the detection limit.
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Masking of sulfide interference
To remove interference from sulfide ions, the analyte was
treated with the TPEN–Cu2+ complex solution as a masking
agent. To each sample solution containing different anions,
22.5 μL of the TPEN–Cu2+ stock solution was added, followed
by probe 1 under the same conditions. The final concen-
trations of probe 1, anions, TPEN, and Cu2+ for fluorescence
measurements were 5.0 × 10−6 M, 5.0 × 10−5 M, 9.0 × 10−5 M,
and 7.5 × 10−5 M, respectively.
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Acknowledgements
This research was supported by the Chung-Ang University
Research Scholarship Grant in 2012 (HGI).
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