A ratiometric fluorescent probe for rapid, sensitive and selective detection of sulfur dioxide with large Stokes shifts by single wavelength excitation

Article abstract of DOI:10.1039/c5ob00765h

4-(Benzothiazol-2-yl)-3-hydroxybenzaldehyde, probe 1, has been developed as a ratiometric fluorescent probe for the sensitive and selective detection of sulfur dioxide with a fast response time (within seconds). This probe itself exhibited a yellow emissi

Full text of DOI:10.1039/c5ob00765h

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A ratiometric fluorescent probe for rapid, sensitive
and selective detection of sulfur dioxide with large
Stokes shifts by single wavelength excitation†
Cite this: DOI: 10.1039/c5ob00765h
Xingjiang Liu,‡a Qinwei Yang,‡a Wenqiang Chen,a Lingna Mo,a Song Chen,a
Jiang Kangb and Xiangzhi Song*a,c,d
4-(Benzothiazol-2-yl)-3-hydroxybenzaldehyde, probe 1, has been developed as a ratiometric fluorescent
probe for the sensitive and selective detection of sulfur dioxide with a fast response time (within seconds).
This probe itself exhibited a yellow emission (λmaxem = 563 nm) with a 186 nm Stokes shift. Upon treat-
ment with sulfite anions, this probe instantaneously displayed a blue emission (λmaxem = 467 nm) with a
133 nm Stokes shift. The large Stokes shifts and absorption spectral overlap of probe 1 both in the
Received 16th April 2015,
Accepted 1st July 2015
absence and in the presence of SO32−/HSO3 allow the fluorescence detection under single wavelength
−
DOI: 10.1039/c5ob00765h
excitation. Importantly, this ratiometric probe was successfully demonstrated for the imaging of intracellu-
www.rsc.org/obc
lar sulfite anions in living cells.
sulfites in aqueous media and biological samples with good
selectivity and sensitivity.
Introduction
Sulphur dioxide (SO2), produced by massive burning of fossil
Fluorescent sensing has been proved as a powerful tech-
fuel and volcanic activity, is considered as an environmental nique for visualizing anions in living systems.5 While many
pollutant. In addition, sulfite/bisulfite (SO32−/HSO3−), the two fluorescent probes for SO2 and its derivatives were reported in
main forms of SO2 in physiological systems, have been widely the past decade,6 only a few of them exhibit ratiometric
used as food preservative, anti-oxidant and antibacterial sensing properties which can eliminate distortions of data
agents in the food industry.1 Moreover, SO32−/HSO3 can be caused by probe concentration, environmental effects and
−
endogenously generated from sulphur-containing amino acids instrument factors.6a–j,q
in mammals.2 Excessive exposure to SO2 and its derivatives
It is well known that large Stokes shifts can well separate
have broad and complex toxicological effects such as oxidative the excitation and emission lights and improves the sensitivity
damage, DNA damage and inflammation.3 However, recent of fluorescence detection while reducing interference. 2-(Ben-
studies have shown that SO2 and its derivatives at low concen- zothiazol-2-yl)phenol derivatives are a kind of fluorophore pos-
trations (less than 450 µM) have vasodilating, anti-hypertensive sessing favorable optical properties such as good photostability,
and anti-atherogenic effects, and are recognized as novel mes- relatively high fluorescent quantum yield and large Stokes shifts
sengers in the cardiovascular system.4 Due to the dual roles of due to the excited state intramolecular proton transfer (ESIPT)
SO2 and its derivatives, considerable attention has been paid process upon being excited.7 Some ratiometric fluorescent
to developing effective methods for detecting and monitoring probes with blue emission have been developed based on
2-(benzothiazol-2-yl)phenol derivatives.8 Unfortunately, the
favorable large Stokes shift diminished in all these 2-(benzothia-
zol-2-yl)phenol-based ratiometric fluorescent probes due to the
inhibition of the ESIPT process caused by introducing the
sensing group to the hydroxyl group.
aCollege of Chemistry & Chemical Engineering, Central South University, Changsha,
Hunan Province, P. R. China, 410083. E-mail: song@rowland.harvard.edu;
Fax: +86-731-88836954; Tel: +86-731-88836954
bThe Third Xiangya Hospital, Central South University, Changsha, Hunan Province,
Our research group has recently been investigating the
P. R. China, 410013
cState Key Laboratory for Powder Metallurgy, Central South University, Changsha,
photophysical properties of a series of 2-(benzothiazol-2-yl)-
phenol dyes having different substituted groups. We discov-
ered that electron-withdrawing groups at the 3-position in the
part of a phenyl ring could induce a significant red-shift in
their emission spectra due to the enhanced intramolecular
charge transfer (ICT) process. Our finding, coupled with the
Hunan province, P. R. China, 410083
dState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian,
Liaoning Province, P. R. China, 116024
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5ob00765h
‡These authors contributed equally to this work.
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Scheme 1 Synthetic route of probe 1. (a) 2-Hydroxy-4-methylbenzoic
acid, PCl3, toluene, reflux, 5 h, yield 40.3%. (b) Acetyl chloride, Et3N,
CH2Cl2, 0–25 °C, 2 h, yield 80%. (c) NBS, BPO, CCl4, reflux, 24 h, yield
51.3%. (d) (1) Hexamethylenetetramine, HCCl3, reflux, 8 h; (2) CH3COOH,
reflux, 2 h; (3) KOH, CH2Cl2, reflux, 1 h, yield 30%.
Fig. 1 The normalized absorption (dashed lines) and emission spectra
(solid lines) of probe 1 (black lines) and dye 2 (red lines) in PBS buffer
(pH 7.4, 10.0 mM, 1 mM CTAB).
fact that sulfite can selectively adduct to the aldehyde
moiety,6q,9 inspired us to develop a 2-(benzothiazol-2-yl)phenol
derivative containing the aldehyde group, probe 1, as a ratio-
metric fluorescent probe for the detection of SO2 and its
derivatives, shown in Scheme 1. We expected the electron-with-
drawing capacity of the aldehyde moiety in probe 1 can be
strongly changed by the adduction of SO32−/HSO3−, which con-
sequently results in significant spectral changes to realize the
ratiometric measurements. It is noteworthy that the desired
ESIPT process always occurs in probe 1 before and after the
UV-vis and emission spectra of probe 1 in the presence of
2−
SO3
were investigated in PBS buffer (pH 7.4, 10.0 mM,
2−
1.0 mM CTAB). Upon the addition of SO3 to the solution of
probe 1, the absorption exhibited a small blue-shift (shown in
Fig. S3†) and the emission maximum shifted from 563 nm to
467 nm accompanied with a marked fluorescence color
change from yellow to blue (shown in Fig. 2), which are very
similar to the spectral hallmarks of dye 2. On the basis of
absorption and emission studies, the sensing mechanism for
−
addition of SO32−/HSO3 because of the free hydroxyl group,
which allows the large Stokes shift during the whole measur-
ing process. To the best of our knowledge, this is the first ratio-
metric fluorescent probe which displays large Stokes shifts
both in the absence and in the presence of testing species.
Importantly, probe 1 showed a nearly instantaneous response
2−
2−
probe 1 with SO3 is proposed in Scheme 2. SO3 attacked
the aldehyde moiety in probe 1 to form the adduct product,
dye 5, in which the ICT process was inhibited, and thereby
resulted in a significant blue-shift in its emission spectra.
Unlike other 2-(benzothiazol-2-yl)phenol-based fluorescent
probes in which the ESIPT process was inhibited, both probe 1
2−
to SO3 (within seconds), which is much faster than pre-
2−
viously reported SO3 fluorescent probes,6 especially for the
probes with the aldehyde group as a recognition unit (see
Table S1†).6q
2−
and the reaction product with SO3 exhibit an ESIPT process
Probe 1 was synthesized in four steps as shown in
Scheme 1.
upon being excited because of the free hydroxyl group. There-
fore, the favored large Stokes shift resulting from the ESIPT
Results and discussion
2-(Benzothiazol-2-yl)-5-methylphenol, dye 2, exhibited two
absorption bands at 287 nm and 331 nm, and fluoresces a
blue light with a maximum at 464 nm, as shown in Fig. 1. In
probe 1, the strong electron-withdrawing ability of the alde-
hyde group could induce an ICT process upon being excited.
As a result, the solution of probe 1 showed two major absorp-
tion bands centered at 372 nm and 322 nm, and emits in the
green region with a maximum at 563 nm (shown in Fig. 1).
Both probe 1 and dye 2 exhibit large Stokes shifts (133 nm for
dye 2 and 186 nm for probe 1, respectively) due to the ESIPT
process. The fluorescence at shorter wavelength was generated
from the “Enol form” (local excited state) while the fluo-
rescence at longer wavelength is assigned to the emission from
the keto form (ESIPT state) (see Scheme S1, Fig. S1 and S2†).
The keto emission dominated for probe 1 in all solvents due to
the electron-withdrawing effect from the aldehyde group.
Fig. 2 Fluorescence spectra of probe 1 (5.0 µM) upon the addition of
SO32− (0.0–400.0 equiv.) in PBS buffer (pH 7.4, 10.0 mM, 1.0 mM CTAB).
λex = 340 nm. Inset: photograph of probe 1 under the irradiation at
365 nm (left: robe 1 only, right: probe 1 with SO32−).
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2−
Scheme 2 Proposed sensing mechanism of probe 1 for SO3
.
Fig. 4 The linear relationship between the ratio of I467 nm/I563
probe 1 (5.0 µM) upon the addition of SO32− (0.00–0.10 mM).
for
nm
process was always available for probe 1 in the absence and
presence of SO32−. The large Stokes shifts exhibited by probe 1
and the formed dye 5, coupled with their absorption spectral
overlap allowed single wavelength excitation in the fluo-
rescence detection.
linearity (R = 0.99694) (shown in Fig. 4). The detection limit
was calculated to be 3.3 × 10−7 M (based on S/N = 3).
1
To further confirm the reaction mechanism, H NMR spec-
A time-dependent fluorescence experiment was also per-
formed on probe 1 (5.0 μM) with different concentrations of
tral titration analysis was conducted on the mixture of probe 1
with NaHSO3 in DMSO-d6/D2O (v/v, 4 : 1) solution. As illus-
trated in Fig. 3, upon the addition of SO32−, the proton signal
at 9.94 ppm (Ha) corresponding to the aldehyde group shifted
to 5.09 ppm (Hb) which was assigned to the saturated proton
of the formed adduct 5. Moreover, the mass spectral analysis
of probe 1 with Na2SO3 displayed a peak at m/z = 338.3394,
which is very close to the theoretical molecular mass of com-
pound 5 ([M + 1] = 338.015693) (Fig. S29†). These data strongly
support our proposed mechanism for the manner in which
SO32− reacts with probe 1 to generate dye 5.
SO32−. As depicted in Fig. 5, the intensity ratio of I467
/
nm
I563
reached plateau within seconds, which implied that
nm
2−
probe 1 responded to SO3 rapidly. Simultaneously, sulfite-
free probe 1 produced no detectable fluorescence signal
during the same time interval.
Next, we sought to assess the selectivity of probe 1 for the
detection of SO32−. The responses of probe 1 to various
common interfering analytes, including representative anions
(NaF, NaCl, NaBr, NaI, NaNO3, NaNO2, CH3COONa, NaN3,
Na2SO4, Na2S2O3, NaSCN, Na3PO4, Na2S and Na2CO3),
biothiols (Cys, Hcy, and GSH) as well as reactive oxygen
species (H2O2 and NaClO), are illustrated in Fig. 6. The emis-
sion spectra of probe 1 in solution remained unchanged in
response to these potential interfering analytes. Usually, alde-
hyde-based fluorescent probes are subjected to low selectivity
Concentration-dependent experiments indicated that the
2−
incremental addition of SO3 resulted in a gradual decrease
in the fluorescence intensity at 563 nm and the appearance of
a new fluorescence signal at 467 nm (Fig. 2). The ratio between
the fluorescence intensities at 467 nm and 563 nm, I467 nm
/
I563 nm, varied from 0.076 to 3.126 (41-fold enhancement) with
the addition of 1.0 mM Na2SO3 to a 5.0 μM aqueous solution
of probe 1. A plot of ratio (I467 nm/I563 nm) vs. the concen-
2−
trations of SO3 in the range of 0.0–100.0 μM showed good
Fig. 3 1H NMR spectra of probe 1 (a) and the reaction product of probe
1 with NaHSO3 (b) in DMSO-d6/D2O (V/V, 4 : 1) solution.
Fig. 5 Kinetics of I467 nm/I563
for probe 1 (5.0 μM) in the absence/
nm
2−
presence of SO3
.
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Fig. 8 The fluorescence intensity ratio I467 nm/I563
of probe 1
Fig. 6 Fluorescence spectra of probe 1 (5.0 µM) upon the addition of
various species in PBS buffer (pH 7.4, 10.0 mM, 1.0 mM CTAB), including
Na2SO3, NaF, NaCl, NaBr, NaI, NaNO3, NaNO2, CH3COONa, NaN3,
Na2SO4, Na2S2O3, NaSCN, Na3PO4, Na2S, Na2CO3, Cys, Hcy, GSH, H2O2,
and NaClO (200.0 equiv. for each).
nm
(5.0 µM) in the absence and presence of SO32− (200.0 equiv.) at different
pH values from 4.0 to 9.0.
enhancement of I467 nm/I563
was observed when probe 1
nm
was treated with SO32−. These results suggested that probe 1
was suitable as a ratiometric fluorescent probe for the detec-
tion of SO32− under physiological conditions.
2−
for the detection of SO3 in the presence of biothiols. But
2−
probe 1 showed an excellent selectivity to SO3
against
biothiols. Moreover, competitive assays indicated that the co-
existence of all these typical analytes had little impact on the
ability of probe 1 to detect SO3 (see Fig. 7). These results
Finally, we established the ability of probe 1 for the detec-
tion of SO3 in living HNE-2 cells. The cells were incubated
with probe 1 at 37 °C for 30 min in PBS buffer and showed
2−
2−
suggested that probe 1 exhibited an excellent selectivity
strong yellow-green fluorescence (Fig. 9b). In contrast, treat-
2−
2−
toward SO3
.
ment of probe 1-loaded cells with SO3 at 37 °C for another
In order to determine the sensing performance of probe 1
under physiological conditions, we further investigated the
influence of pH on the fluorescence behavior of probe 1 for
the detection of SO32−. It can be seen in Fig. 8 that the ratio of
I467 nm/I563 nm for probe 1 was barely affected within a wide pH
range from 4.0 to 9.0. Within the same pH range, enormous
30 min resulted in strong blue fluorescence (Fig. 9d). Taken
together, these experiments demonstrate that probe 1 is cell-
2−
permeable and can monitor the intracellular SO3 in living
samples.
Fig. 7 Fluorescence intensity ratio (I467 nm/I563 nm) of probe 1 (5.0 µM)
with SO3 (200.0 equiv.) in the coexistence of 200.0 equiv. of various
species in PBS buffer (pH 7.4, 10.0 mM, 1.0 mM CTAB). 1, free; 2, NaF; 3,
NaCl; 4, NaBr; 5, NaI; 6, NaNO3; 7, NaNO2; 8, CH3COONa; 9, NaN3; 10,
Na2SO4; 11, Na2S2O3; 12, NaSCN; 13, Na3PO4; 14, Na2S; 15, Na2CO3; 16,
Cys; 17, Hcy; 18, GSH; 19, H2O2; 20, NaClO.
Fig. 9 Fluorescence images of living HNE-2 cells in PBS buffer (pH 7.4,
10 mM) with 1.0 mM CTAB. (a) Bright-field and (b) fluorescence images
of cells incubated with probe 1 (5.0 μM) for 30 min at 37 °C; (c) bright-
field and (d) fluorescence images of cells firstly incubated with probe 1
(5.0 μM) at 37 °C for 30 min and then treated with SO3 (1.0 mM) at
37 °C for another 30 min.
2−
2−
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ether) afforded pure compound 4 as a white solid (2.3 g, 51.3%
yield). HRMS (EI) m/z: calcd for C16H12BrNO2S [M + H]+:
361.9772; found: 361.9841. H NMR (400 MHz, CDCl3) δH 8.35
Experimental
Materials and instruments
1
All of the chemicals were purchased from commercial suppli- (d, J = 8.1 Hz, 1H), 8.12 (d, J = 7.9 Hz, 1H), 7.96 (d, J = 7.4 Hz,
ers and used without further purification. 1H and 13C NMR 1H), 7.58–7.52 (m, 1H), 7.48–7.42 (m, 2H), 7.33 (s, 1H), 4.54 (s,
spectra were recorded on a Bruker 400 NMR spectrometer. 2H), 2.52 (s, 3H). 13C NMR (100 MHz, CDCl3): δC 168.9, 161.7,
Chemical shifts were reported in parts per million using tetra- 152.9, 148.2, 141.3, 135.3, 130.6, 127.0, 126.4, 126.0, 125.5,
methylsilane (TMS) as the internal standard. Mass spectra 124.3, 123.4, 121.4, 31.7, 21.7.
were recorded on a Shimadzu LC-MS-IT-TOF mass spectro-
meter and an IonSpec4.7 Tesla FTMS-MALDI/DHB mass
spectrometer.
Synthesis of probe 1
All titration experiments except the pH-dependent fluo- A solution of compound 4 (1.5 g, 4.1 mmol) and hexamethy-
rescence measurements were carried out in PBS buffer (pH lenetetramine (1.3 g, 9.1 mmol) in anhydrous chloroform
7.4, 10.0 mM) with 1.0 mM CTAB. All spectral characterizations (40 mL) under an argon atmosphere was refluxed for 8 h. The
were performed in HPLC-grade solvents at 20 °C within a solvent was removed in vacuum and to it was added 50%
10 mm quartz cell. UV-vis absorption spectroscopy was per- aqueous solution of acetic acid (20 mL). The mixture was
formed using a Shimadzu UV-2450 spectrophotometer, and refluxed for 2 h and then 40 mL water was added. The pH of
fluorescence spectroscopy was performed on a Hitachi F-7000 the resulting mixture was adjusted to neutral with 40% NaOH
spectrometer. Fluorescence imaging was performed using an aqueous solution. The organic layer was separated, washed
Olympus IX83 inverted microscope. The pH measurement was with water and dried over anhydrous Na2SO4. The organic
performed on a Leici PHS-3C meter.
solvent was removed in vacuum to give a residue. The residue,
0.3 g KOH and 20 mL dichloromethane were placed into a
flask and refluxed for 1 h. Next, 20 mL water was added to the
Synthesis of compound 2
Compound 2 was synthesized according to the literature reaction mixture and the pH adjusted to neural with dilute
procedure.10
HCl solution. The organic layer was separated and the
aqueous layer was extracted with 20 mL dichloromethane. The
organic layers were combined, washed with water and dried
Synthesis of compound 3
To a solution of compound 2 (5.0 g, 20.7 mmol) in anhydrous over anhydrous Na2SO4. Purification by flash column chrom-
dichloromethane (60 mL) under an argon atmosphere was atography (200–300 mesh silica gel, 1 : 16 ethyl acetate/pet-
added Et3N (2.5 g, 24.9 mmol). After stirring the resulting solu- roleum ether) yielded pure probe 1 as a yellow solid (0.3170 g,
tion for
5 min, a
solution of acetyl chloride (1.95 g, 30% yield). HRMS (EI) m/z: calcd for C14H9NO2S [M + H]+,
24.9 mmol) in dichloromethane (5 mL) was added dropwise at 256.0354; found, 256.0418. 1H NMR (400 MHz, CDCl3): δH
0 °C. The reaction mixture was stirred at 25 °C for 2 h, and 10.04 (s, 1H), 8.07 (d, J = 7.8 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H),
quenched with 40 mL water. The organic layer was separated, 7.87 (d, J = 8.0 Hz, 1H), 7.63–7.55 (m, 2H), 7.50 (t, J = 7.5 Hz,
washed with brine and dried over anhydrous Na2SO4. The 2H). 13C NMR (100 MHz, CDCl3) δC 191.4, 167.9, 158.3, 151.6,
organic solvent was removed in vacuum to give the crude 139.0, 132.9, 128.9, 127.1, 126.3, 122.6, 121.6, 121.2, 119.6,
product. The pure product, compound 3, was obtained by 119.5.
column chromatography (200–300 mesh silica gel, 1 : 16 ethyl
acetate/petroleum ether) (4.0 g, 80% yield). HRMS (EI) m/z:
1
Cell culture and fluorescence imaging
calcd for C16H13NO2S [M + H]+: 284.0667; found: 284.0738. H
NMR (400 MHz, CDCl3): δH 8.23 (d, J = 8.1 Hz, 1H), 8.11 (d, J = HNE-2 cells were provided by Xiangya Third People’s Hospital
8.1 Hz, 1H), 7.94 (d, J = 7.5 Hz, 1H), 7.53 (t, J = 7.7 Hz, 1H), of Central South University (China). Cells were grown in Dul-
7.43 (m, 1H), 7.24 (d, J = 8.1, 1H), 7.09 (s, 1H), 2.51 (s, 3H), becco’s modified Eagle’s medium (DMEM) supplemented with
2.46 (s, 3H). 13C NMR (100 MHz, CDCl3): δC 169.3, 162.7, 10% fetal bovine serum and 1% penicillin at 37 °C in a
153.1, 148.1, 142.5, 135.2, 130.1, 127.4, 126.3, 125.2, 124.2, humidified atmosphere of 5% CO2 and 95% air. Cells were
123.4, 123.2, 121.3, 21.7, 21.4.
seeded in a 6-well plate and adhered for 24 h. Before the
experiments, the cells were washed three times with PSB
buffer. The cells were incubated with probe 1 (5.0 μM) at 37 °C
Synthesis of compound 4
A mixture of compound 3 (3.5 g, 12.4 mmol), NBS (2.4 g, for 30 min in PBS buffer (10.0 mM, pH 7.4) with 1 mM CTAB,
2−
13.5 mmol) and BPO (0.06 g, 0.025 mmol) in dry CCl4 (50 mL) then washed with PBS three times and treated with SO3
under an argon atmosphere was refluxed for 24 h. The reaction (1.0 mM) at 37 °C for another 30 min. For a control experi-
mixture was poured into 50 mL cold water. The organic layer ment, the cells were incubated with probe 1 (5.0 μM) at 37 °C
was separated, washed with water and dried over anhydrous for 30 min in PBS buffer (10.0 mM, pH 7.4) with 1 mM CTAB
Na2SO4. The organic solvent was removed in vacuum to give and washed with PBS buffer three times. Fluorescence
the crude product. Purification by flash column chromato- imaging was performed after the cells were washed three times
graphy (200–300 mesh silica gel, 1 : 14 ethyl acetate/petroleum with PBS buffer solution.
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Conclusions
In conclusion, we have developed a ratiometric fluorescent
probe for imaging SO2 and its derivatives in living cells based
on 2-(benzothiazol-2-yl)phenol. This probe displayed a fast
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selectivity toward SO32−. The addition of SO3 into the solu-
2−
tion of probe 1 resulted in a 96 nm blue-shift in its emission
spectra with a 41-fold ratio enhancement. Notably, this probe
displayed large Stokes shifts before (186 nm) and after
(133 nm) the treatment with SO32−. Furthermore, preliminary
biological experiments have demonstrated the value of this
probe by monitoring the intracellular SO32− in living biological
samples.
Acknowledgements
The research was supported by the State Key Laboratory of
Fine Chemicals (KF1202) and Hunan Nonferrous Metals
Holding Group Co. Ltd.
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