An A-π-A′ structural ratiometric fluorescent probe based on benzo[: E] indolium for bisulfite and its application in sugar samples and living cells

Article abstract of DOI:10.1039/c6ra15605c

An A-π-A′ structural ratiometric fluorescent probe 1 bearing a benzo[e]indolium moiety was developed for the detection of HSO₃^- in PBS buffer solution. HSO₃^- was expected to react with probe 1via a Michael addition reaction, causing dramatic changes in its emission spectrum (from 578 nm to 466 nm) and an obvious color change from orange red to blue which could be observed by the naked eye. The reaction could be complete in 30 seconds in the sensing system, and displayed a high selectivity and sensitivity for HSO₃^- with a detection limit of 15 nM. The practical value of the probe was confirmed for application to detect the level of HSO₃^- in sugar samples with good recovery and in living cells.

Full text of DOI:10.1039/c6ra15605c

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An AꢀπꢀA’ structural ratiometric fluorescent probe based on
benzo [e] indolium for bisulfite and its application in sugar samples
and Living cells
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Yue Sun, Ying Li, Xiaotao Ma, Lian Duan∗,
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School of Chemistry and Chemical Engineering, Taiyuan University of Technology,
Taiyuan 030024, China.
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∗ Corresponding author. Tel: +86 351 6018564; fax: +86 351 6010121.
Eꢀmail: duanlian@tyut.edu.cn (Lian Duan)
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Abstract: An AꢀπꢀA’ structural ratiometric fluorescent probe 1 bearing a benzo [e]
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indolium moiety was developed for the detection of HSO3 in PBS buffer solution.
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HSO3 was expected to react with the probe 1 via a Michael addition reaction, causing
dramatic changes in its emission spectrum (from 578 nm to 466 nm) and an obvious
color change from orange red to blue which could be observed by naked eyes. The
reaction could be complete in 30 seconds in the sensing system, and displayed a high
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selectivity and sensitivity for HSO3 with the detection limit of 15 nM. The practical
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value of the probe was confirmed for application to detect the level of HSO3 in sugar
samples with good recovery and in living cells.
Key words: AꢀπꢀA’ system; Ratiometric fluorescence; Bisulfite; test paper detection;
Cell image
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Bisulfite (HSO3 ), as an effective preservative, has been widely used in foods,
beverages and pharmaceutical products.1 It has been reported that extensive HSO3
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intake would cause asthmatic attacks and allergic reactions, such as difficulty in
breathing, wheezing, hives, and gastrointestinal distress in some individuals.2, 3 Due to
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the above mentioned harmful effects towards people, the threshold levels of HSO3 in
food and medicine have been controlled rigorously in many countries. For instance,
the total content of sulfur (calculated by SO2) in white granulated sugar is strictly
regulated as <30 mg/kg in China.4 Hence, considerable attention has been attracted to
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the development of rapid, facile, and reliable techniques for HSO3 detection because
of the food quality assurance and human healthy.
Fluorescent probes have become powerful tools for sensing and imaging trace
amounts of samples due to their simplicity, sensitivity, lowꢀtoxicity and easy of
operation in recent years.5, 6 To date, some fluorescent probes have been developed for
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monitoring HSO3 on the basis of complexation with amines,7, the selective
deꢀprotection of a levulinate group,9–12 selective nucleophilic reaction with aldehyde
and N=N in metal complexes,21, 22 and Michaelꢀtype additions.23–39 However,
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most of these probes may suffer from the risk of slow response time (2 min to 10 h) or
an organic coꢀsolvent, which limited their utility. Recently, Song and coꢀworkers
reported a fluorescent turnꢀon probe and a ratiometric fluorescent probe,
respectively.31, 32 Both of them were based on a nearꢀinfrared benzopyrylium dye
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which could detect HSO3 in aqueous solution (HEPES, pH = 7.4) with an incredible
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response time (< 15 s) and a satisfactory detection limit. Compared to the
intensityꢀbased fluorescent probes, the ratiometric fluorescent probes enable the
measurement of emission intensities at two different wavelengths, which increase the
dynamic range and provide a builtꢀin correction for environmental effects.40 Thereby,
it has remained a great challenge to construct new sensitive and selective ratiometric
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fluorescent probes for the rapid HSO3 detection in pure aqueous environments and in
real samples.
Herein, we introduce a new ratiometric fluorescent probe 1 which is constructed by
a benzo [e] indolium fluorophore (an electron acceptor A) and a malononitrile group
(another electron acceptor A’), that is, an AꢀπꢀA’ type compound. The AꢀπꢀA’ type
compound could effectively avoid the severe emission quenching which were
frequently encountered in DꢀπꢀA type compounds owing to dipoleꢀinduced
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intermolecular interactions.41 Probe 1 has shown a selective response to HSO3 over
other anions in PBS buffer (pH = 7.4, 10 mM), and a distinguished fluorescent color
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change from orange red to cyan can be observed. HSO3 is expected to undergo a
Michael addition reaction with the C–4 atom in the ethylene group, which is activated
by the electronꢀwithdrawing feature of the positivelyꢀcharged benzo [e] indolium
fragment and the malononitrile group (Scheme 1).
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Scheme 1. Probe 1 and its synthesis route.
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87 2. Experimental
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2.1 Materials and chemicals
All reagents and solvents were purchased from commercial source and used
without further purification, if not stated. All reactions were carried out on the
magnetic stirrers and their reaction process was monitored on thin layer
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chromatography (TLC). Stock solutions (100 mM) of HSO3 , HS–, F–, Cl–, Br–, I–,
AcO–, ClO4 , NO3 , N3 , SO42–, HSO4 , SCN–, PO43–, HPO42–, H2PO4 , Cys, Hcy, and
GSH were prepared by direct dissolution of proper amounts of their sodium salts.
Absorption and fluorescence spectra were taken on a Shimadzu UVꢀ1800
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spectrophotometer and a Hitachi Fꢀ2700 fluorescence spectrometer, respectively. H
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NMR and C NMR measurements were recorded at 600 and 150 MHz on a Brucker
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Avance 600ꢀMHz spectrometer. Dimethyl sulfoxide (DMSOꢀd6) was solvent, and
tetramethylsilane (TMS) was used as internal standard. HRMS (ESI) were taken on a
Fourier transform ion cyclotron resonance mass spectrometry (Varian 7.0T). An
Olympus FVꢀ1000 confocal fluorescence microscope and HepG2 cells were used in
the living cell experiment.
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2.2 Preparation of probe 1
In a 25 mL roundꢀbottom flask, terephthalaldehyde (200 mg, 1.49 mmol) and
malononitrile (98 mg, 1.49 mmol) were dissolved in ethanol (15 mL) and the mixture
was stirred at 80oC for 12 h. After cooling to room temperature, the precipitate was
collected by filtration, washed with cold ethanol to afford the crude product. The pure
product was obtained by further purification using silica gel chromatography as a light
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yellow solid 2 (190 mg, 70%). H NMR (600 MHz, CDCl3) δ 9.87 (s, 1H), 8.65 (s,
1H), 8.48 (d, 2H), 8.45 (d, 2H).
To a solution of 1, 2, 3, 3ꢀTetramethylbenz [e] indolium iodide (70mg, 0.2 mmol) in
ethanol (10 mL) was added the prepared 2 (36 mg, 0.2 mmol) and the solution was
refluxed for 12 h. After cooling to room temperature, the precipitate was collected by
filtration, washed with cold ethanol. The pure product was obtained by
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recrystallization from ethanol as dark red solid (88 mg, 77%). H NMR (600 MHz,
DMSOꢀd6): δ 8.65 (s, 1H), 8.49 (m, 4H), 8.35 (d, 1H), 8.25 (d, 1H), 8.17 (d, 1H), 8.12
(d, 2H), 7.90 (d, 1H), 7.88 (t, 1H), 7.76 (t, 1H), 4.34 (s, 3H), 2.03 (s, 6H). 13C NMR
(150 MHz, DMSOꢀd6): δ186.0, 176.5, 167.2, 152.5, 142.4, 141.9, 137.8, 134.4, 134.0,
131.8, 131.0, 128.9, 127.2, 118.8, 116.9, 57.6, 39.2, 28.0. HRMS (ESI) calcd. for
[M]+ 388.1806, found 388.1808 (Fig. S1ꢀS3).
2.3 Preparation of solutions of probe 1 and analytes
Deionized water was used as the solvent for titration experiment. The titrations
were carried out in 10ꢀmm quartz cuvettes at room temperature. Probe 1 was
dissolved in DMSO (spectroscopic grade) to afford a concentration of 10 mM stock
solution, and then diluted with PBS buffer (pH = 7.4, 10 mM) to a concentration of 20
ꢁM. Anions (as their Na+ salt, 10 mM and 100 mM) in deionized water were added to
the diluted probe solution and used for the sensing behavior experiment. The resulting
solution was shaken well and kept at room temperature for 30 seconds before
measuring the spectra. The excitation wavelength was 400 nm, and the PMT voltage
was 400 V. The excitation and emission slit width were 5 nm and 5 nm, respectively.
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2.4 1H NMR analysis experiments of probe 1
The solution of probe 1 (3 × 10ꢀ3 M) in DMSOꢀd6 (450 ꢁL) and D2O (50 ꢁL) was
placed in the NMR tube, and sodium bisulfite powder was added. All the sodium
bisulfite was soluble.
2.5 Sugar samples test
Granulated sugar, soft sugar and crystal sugar purchased from supermarket were
used in the sample analysis. Sample solution was prepared by dissolving 5.0 g of
sugar in deionized water and diluting to 10 mL. Aliquots of the sugar solution were
added directly to the PBS buffer (pH = 7.4, 10 mM) containing probe 1 (20 ꢁM), and
the emission intensity at 466 nm and 578 nm were recorded.
2.6 Preparation of test papers and detection
The neutral filter papers were dipped in the stock solution of 1 (10 mM) and dried.
Then various solutions of anions (1 mM, 2.5 ꢁL) were seriatim dropped. The
concentrations of sugar samples for test paper experiment were 1, 0.7, 0.4, 0.1 g/mL.
2.7 Cell incubation and fluorescence imaging
The HepG2 cells were provided by of Shanxi Medical University. Cells were
seeded onto the cover slips and cultured in DMEM in an incubator (37oC, 5% CO2
and 20% O2). After 24 hours, the cover slips were washed 3 times with PBS to
remove the media and then cultured in PBS for use. Probe 1 (10 ꢁM) was added to
above cellular samples and incubated for 15 min, and then washed 3 times with PBS.
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In a further experiment, HSO3 (15 ꢁM) was added to the cells and the fluorescence
change was observed under the confocal fluorescence microscope.
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2.8 Cytotoxicity assays.
HepG2 cells were seeded onto the cover slips and cultured in DMEM in an
incubator (37oC, 5% CO2 and 20% O2). Before the experiment, the cells were placed
in a 96ꢀwell plate, followed by addition of various concentrations of probe 1. Then the
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cells were incubated at 37 C in an atmosphere of 5% CO2 and 20% O2 for 12 hours,
followed by MTT assays (n= 5).
160 3. Results and discussion
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3.1 Spectral studies of probe 1
The detection environment was investigated at first. As shown in Fig. S4., the probe
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was stable in a pH range of 1ꢀ8 and displayed the satisfied response for HSO3 in the
physiological pH region. Thus, the UVꢀvis absorption and emission spectra of probe 1
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towards HSO3 were measured in PBS buffer (pH = 7.4, 10 mM). Fig. 1 shows the
absorption (a) and emission (b) responses of probe 1 (20 ꢁM) with different amounts
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of HSO3 (0 – 60 ꢁM). Probe 1 exhibited a yellow color with an obvious band at 380
nm and a shoulder band at 445 nm (Fig. 1a), which was ascribed to the typical
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absorption induced by the πꢀπ conjugation. Upon addition of HSO3 , the absorption
bands were gradually attenuated and a new band at 268 nm was appeared, suggesting
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that the πꢀπ conjugation is interrupted due to the nucleophilic attack of HSO3 . The
addition also induced an obvious color change from yellow to colorless (inset of Fig.
1a), which could be observed by naked eye. Gratifyingly, the free probe 1 showed an
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emission band centered at 578 nm. In contrast, upon addition of HSO3 (Fig. 1b), the
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emission band at 578 nm was decreased and a new band at 466 nm increased
remarkably, which could be attributed to the formation of 2ꢀ methylene benzo [e]
indoline moiety. A wellꢀdefined isoemissive point at 536 nm was also observed. The
ratiometric change in fluorescence intensity indicated that the addition reaction
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between probe 1 and HSO3 interrupted the πꢀconjugation, and the fluorescence of
2ꢀmethylenebenzo [e] indoline was recovered. Accordingly, the fluorescence color
changed from orange red to cyan (inset of Fig. 1b). The distinct emission shift (about
112 nm) was favorable for ratiometric fluorescence detection. Moreover, the ratio of
the emission intensities at 466 and 578 nm (I465/I571) was increased (25ꢀfold, from
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0.10 to 2.5) with the addition of HSO3 and became constant when the amount of
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HSO3 added reached 2.4 equiv. (48 ꢁM) (Fig. S5.). Under similar conditions, the
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ratio changes produced an excellent linear function with HSO3 concentration
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between 0 and 24 ꢁM, and the detection limit for HSO3 was determined to be 0.15
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ꢁM (R2 = 0.9930) based on S/N = 3 (Fig. S6), which is sufficient to probe the HSO3
concentration in cells (ca. 16 ꢁM). 23
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Fig. 1. Absorption (a) and emission spectra (b) of probe 1 (20 ꢁM) in PBS buffer (pH 7.4, 10 mM)
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treated with HSO3 and their photographs in the absence or presence of HSO3− (60 ꢁM) under
sunlight (a) and UV light (b, 365 nm). λex = 400 nm. Slits: 5 nm/5 nm. The concentrations of
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HSO3− were varied in the following order: 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 50 and 60 ꢁM.
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To further evaluate the specific nature of probe 1 towards HSO3 , the influence of
other anions were examined under the same conditions. As shown in Fig. 2 and Fig.
S7, after adding all the other anions into the solution of probe 1 (20 ꢁM), they did not
induce any significant fluorescence change, and the I465/I571 values varied in a limited
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range between 0.12 and 0.25. By contrast, treatment of probe 1 with HSO3 resulted in
an obvious ratiometric fluorescent response. Competition experiments of probe 1 were
also determined by various species mentioned previously in the same condition. The
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signaling of probe 1 toward HSO3 was not affected by the presence of 60 ꢁM of
coexisting species (Fig. S8). The phenomenon suggests that probe 1 could be used as
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an efficient signaling probe for HSO3 detection in aqueous media.
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Fig. 2. Emission ratio I466/ 578 of 1 (20 ꢁM) in PBS buffer (pH 7.4, 10 mM) in the presence of
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various species. λex = 400 nm. Slits: 5 nm/5 nm. (Inset) Corresponding fluorescent color changes
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under a handle UV lamp (365 nm): 1: probe 1 alone; 2–20: probe 1 + HSO3 , HS−, F−, Cl−, Br−,
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I−, AcO−, ClO4 , NO3 , N3 , SO42−, HSO4 , SCN−, PO43−, HPO42−, H2PO4 , Cys, Hcy and GSH
(anions: 60 ꢁM; Cys and Hcy: 200 ꢁM; GSH: 10 mM).
3.2 Sensing mechanism
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A wellꢀknown sensing process takes place between 1 and HSO3 with a Michael
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addition reaction. H NMR analysis was carried out to demonstrate the sensing
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mechanism. As anticipated, after addition of HSO3 into the solution of 1, the
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nucleophilic attack of HSO3 toward C–4 disturbed the πꢀπ conjugation and caused all
the proton signals shifted to upꢀfield (Fig. 3). It was noticed that the proton of the
methyl group (Ha) connected with N+ was dramatically shifted from δ 4.34 to 3.09
ppm. Another irrefutable evidence was the equivalent proton of the two methyl groups
(Hb) at δ 2.03 ppm, then also shifted to upꢀfield and splitted into two signals (Hb’, at
δ 1.89 and 1.56 ppm). This adduct was further confirmed and characterized by HRMS
analysis (Fig. S9), where the peak at m/z 468.1398 (calc. = 468.1387) corresponding
to [MꢀSO3] – was clearly observed.
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Fig. 3. 1H NMR spectral change of 1 (3×10ꢀ3 M) in the absence (A) and presence of 1 equiv. (B),
of NaHSO3 in DMSOꢀd6: D2O = 9: 1 (v/v).
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3.3 Real samples analysis
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To investigate the applicability of the probe to real samples, the levels of HSO3 in
granulated sugar, soft sugar and crystal sugar were examined by using the proposed
method.13 The recoveries were good enough for practical use and determination
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values are listed in Table 1. The HSO3 levels in these samples were 16.96, 10.16, and
14.46 mg/kg, respectively.
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Table 1
Results for the determination of HSO3– in various samples.
ꢀ
HSO3
Added/
Found/
Recovery/
level/
Samples
(ꢁmol/L)
(ꢁmol/L)
(ꢁmol/L)
7.93
%
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4
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4
6
98.26
99.50
102.64
103.79
98.26
98.31
Granulated sugar
Soft sugar
4.07
2.44
3.47
10.02
6.61
8.76
7.34
Crystal sugar
9.31
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3.4 Test papers detection
A preliminary paper test strip system was constructed as shown in Fig. S10. After
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dropping various anions to the prepared test papers, only HSO3 made the color
changed from orange to blue under the UV light (365 nm). Moreover, the test papers
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could also detect the HSO3 in real sugar samples as shown in Fig. 4 and Fig. S11. It
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can be seen that glucose without HSO3 did not cause any color change; while the
sugar solution made the color changing at different concentrations.
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Fig. 4. The photos of the filter paper containing 1 treated with real sugar samples under UV light
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(365 nm, bottom): probe 1, HSO3 , glucose, soft sugar of 1 g/mL, soft sugar of 0.7 g/mL, soft
sugar of 0.4 g/mL, and soft sugar of 0.1 g/mL.
3.5 Cellular imaging
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investigated. Confocal fluorescence imaging in living HepG2 cells was carried out
under selective excitation at 405 nm. First, the MTT assay for probe 1 showed that the
probe with a concentration range between 5ꢀ15 ꢁM has only minimal cytotoxicity
after 12 h (Fig. S12), thus the probe at 10ꢁM was selected for imaging experiments.
The cells incubated with probe 1 (10 ꢁM) for 15 min displayed strong fluorescence in
the red channel and weak fluorescence in the green channel, confirming that probe 1
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is cellꢀpermeable (Fig. 5). Since a certain concentration of HSO3 (15 ꢁM) has been
indicated in cells, the observed strong fluorescence in the green channel is probably a
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result of the fact that probe 1 binds HSO3 in cells. These cell experiments indicated
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that probe 1 could competent for imaging HSO3 in living cells.
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Fig. 5. Confocal Fluorescence images in Hala cells. (A) Bright field imaging of the cells with
adding probe 1 (10 ꢁM) for 15 min; (B) fluorescent imaging of (A) from the green channel; (C)
fluorescent imaging of (A) from the red channel; (D) overlap of (A) and (C). (E) Bright field
imaging of the cells were treated with probe 1 (10 ꢁM) for 15 min and subsequently treated with
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HSO3 (15 ꢁM) for another 15 min; (F) fluorescent imaging of (E) from the green channel; (G)
fluorescent imaging of (E) from the red channel; (H) overlap of (E) and (F). λex = 405 nm.
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In summary, we have developed a new ratiometric fluorescent probe bearing a
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benzo [e] indolium group for HSO3 detection in PBS buffer under physiology
conditions. The probe displayed a dramatic fluorescent change and a color change
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after exposure to HSO3 with a very short response time (within 30 s). Probe 1
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allowed the detection of HSO3 with high sensitivity at a very low concentration.
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These advantages made the probe a potential sensor for HSO3 detection in real sugar
samples and in living cells.
272 Acknowledgements
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This work was supported by the National Natural Science Foundation of china (No.
21301126).
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