A highly selective ratiometric fluorescent probe for the sensitive detection of hypochlorous acid and its bioimaging applications

Article abstract of DOI:10.1039/c6ra15266j

The development of simple techniques for monitoring hypochlorous acid (HOCl) in living cells is urgently needed to disclose the important roles of HOCl in biological systems. Although a number of fluorescent probes have been exploited to detect HOCl, most

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A highly selective ratiometric fluorescent probe for the sensitive detectiDoOnI:o10f.1039/C6RA15266J
hypochlorous acid and its bioimaging applications
Wei Shu, Pan Jia, Xingzhu Chen, Xueyan Li, Yanru Huo, Fang Liu, Zuokai Wang,
Caiyun Liu, Baocun Zhu,* Liangguo Yan,* and Bin Du
School of Resources and Environment, University of Jinan, Shandong Provincial
Engineering Technology Research Center for Ecological Carbon Sink and Capture
Utilization, Jinan 250022, China
*Corresponding author. fax: +86-531-82767617; Tel.: +86-531-82767617
E-mail address: lcyzbc@163.com (B. Zhu) and chm_yanlg@ujn.edu.cn (L. Yan)

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Abstract: The development of simple techniques for monitoring hypochlorous acid
(HOCl) in living cells is urgently needed to disclose the HOCl important roles in the
biological systems. Although a number of fluorescent probes have been exploited to
detect HOCl, most of them are easily influenced by other competing reactive oxygen
species (ROS) and reactive nitrogen species (RNS), and disturbed by many factors
including variabilities in excitation, probe distribution and environmental conditions
due to the recognition of HOCl based on the fluorescence changes in single
wavelength. Herein, in this work, we present a simple fluorescent probe
AETU-HOCl containing a new recognition group (2-amino-ethyl)-thiourea (AETU)
for the ratiometric detection of HOCl in living cells. Probe AETU-HOCl exhibits
outstanding selectivity for HOCl over other ROS/RNS at higher concentrations (0.5
mM, 100 equiv.). More importantly, the rapid response (1 min) and high sensitivity (a
detection limit of 3.9 nM) allow it to successfully visualize the intracellular HOCl
levels by the ratiometric imaging. The present study would offer a simple, ratiometric,
rapid, and ultrasensitive assay for the accurate determination of HOCl in the
biological systems.
Keywords: Hypochlorous acid, fluorescent probe, ratiometric determination,
(2-amino-ethyl)-thiourea, bioimaging.

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1. Introduction
Hypochlorous acid (HOCl) is well-known because of its unique antibacterial
properties in the immune system1. Endogenous HOCl is mainly generated from
hydrogen peroxide and chloride ions with the help of myeloperoxidase (MPO) in
immunocytes.2 However, more and more established evidences show that excessive
production of HOCl is implicated in many diseases including atherosclerosis,3 lung
injury,4 hepatic ischemia-reperfusion injury5 and neuron degeneration.6 Therefore, the
development of simple techniques for monitoring HOCl in living cells is urgently
needed to disclose the HOCl important roles in the biological systems.
Fluorescent probes that can respond directly to the change in concentration of
HOCl would be well used to study the distribution of HOCl in living cells.7 Although
-
various small-molecule probes for HOCl/OCl have been reported for applications in
vitro and in vivo,8 there are still some limitations. For example, most of the available
-
fluorescent probes detect HOCl/OCl only by the fluorescence changes in single
wavelength, which may be disturbed by many factors during the quantitative detection,
such as variabilities in excitation, probe distribution and environmental conditions.9
By contrast, ratiometric fluorescent probes can overcome the limitation via
self-calibration of two emission bands. Almost all of them were restricted by
acid-base conditions,7b,9,10 only a few of them can respond to HOCl across a wide
range of pH.11 What’s more, it is especially notable that most of them are easily
influenced by other competing reactive oxygen species (ROS) and reactive nitrogen
species (RNS).12 Thus, it is still a great challenge to exploit highly selective

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ratiometric fluorescent probes for HOCl.
Extensive research reveals that sulfur atom with strong electron-donating ability
exists special reaction with HOCl.7c,10a,11a So we expected that the
(2-amino-ethyl)-thiourea group may serve as a specific recognition group for HOCl.
Recently, we developed a series of 4-aminonaphthalimide ratiometric fluorescent
probes because of its outstanding internal charge transfer (ICT) structure and desirable
photophysical properties.13 Herein, in this work, we synthesized an ICT-based
ratiometric
fluorescent
probe
AETU-HOCl
with
a
receptor
of
(2-amino-ethyl)-thiourea group and 4-aminonaphthalimide fluorophore (Scheme 1).
Our design strategy is that HOCl-triggered intramolecular cyclization of thiourea
results in the change of emission wavelengths. Experimental results demonstrated that
probe AETU-HOCl has the significant advantages of (1) ratio-imaging, (2) respond
to HOCl at a wide range of pH, (3) highly selectivity over other ROS/RNS at higher
concentrations (0.5 mM, 100 equiv.), (4) the lower detection limit (3.9 nM, based on
3σ/slope), (5) rapid response (about 1 min). Altogether, this study offers a specific
fluorescent probe for the ratiometric detection of HOCl at nanomole level and a
simple, rapid, accurate, ultrasensitive assay that would be widely applied in the
various HOCl-related studies.
2. Experimental
2.1. Materials and Measurements
Unless otherwise stated all chemical materials used in this study were gained
from commercial products and used without further purification. Ultrapure water was

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obtained from Sartorious Arium 611DI system and used throughout the experiment.
-1
-1
HOCl was standardized at pH 12 (ε292 nm = 350 M cm ).14 1H NMR and 13C NMR
were measured on a Bruker AV-400 spectrometer with chemical shifts reported as
ppm (in DMSO-d6, TMS as internal standard). Fluorescence emission spectra were
carried out on a Horiba FluoroMax-4 spectrophotometer with excitation wavelength
of 380 nm. All the fluorescence spectra were not revised. Absorption spectra were
recorded on a UV-3101PC spectrophotometer. All pH were measured with a Sartorius
basic pH-meter PB-10.
2.2. Synthesis of probe AETU-HOCl
As shown in Scheme 1, N-butyl-4-(2-aminoethylamino)-1,8-naphthalimide was
readily synthesized by the reaction between N-butyl-4-bromo-1,8-naphthalimide and
ethylenediamine.15
Benzoylisothiocyanate (163 mg, 1 mmol) was added dropwise to a solution of
N-butyl-4-(2-aminoethylamino)-1,8-naphthalimide (311 mg, 1 mmol) in 20 mL
dichloromethane. The reaction mixture was then stirred at room temperature for 4 h.
The product was purified by chromatography using CH2Cl2 to yield probe
AETU-HOCl (393 mg, 83%).
Melting point: 214-218 °C. FT-IR(KBr) ν: 3423.95, 3354.96, 2955.71, 2864.47,
1687.74, 1649.37, 1587.95, 1542.91, 1433.32, 1395.17, 1364.44, 1299.38, 1241.57,
1
772.47, 755.17, 707.70, 686.76, 663.80 cm-1. H NMR (400 MHz, DMSO-d6 ) δ (×
10-6): 0.92 (t, J = 8.0 Hz, 3H), 1.29-1.38 (m, 2H), 1.55-1.62 (m, 2H), 3.69-3.73 (m,
2H), 3.99-4.06 (m, 4H), 7.01 (d, J = 8.0 Hz, 1H), 7.51 (t, J = 8.0 Hz, 2H), 7.64 (t, J =

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8.0 Hz, 1H), 7.70 (t, J = 8.0 Hz, 1H), 7.87-7.94 (m, 3H), 8.25 (d, J = 8.0 Hz, 1H), 8.43
(d, J = 8.0 Hz, 1H), 8.69 (d, J = 8.0 Hz, 1H), 11.10 (t, J = 8.0 Hz, 1H), 11.42 (s, 1H).
13C NMR (100 MHz, DMSO-d6 ) δ (×10-6): 14.22, 20.31, 30.30, 41.62, 43.62, 104.63,
108.64, 120.68, 122.38, 124.85, 128.85, 128.97,129.97, 131.11, 132.70, 133.40,
134.50, 151.04, 163.36, 164.18, 168.26, 181.24. ESI-MS calcd for C26H27N4O3S
[M+H]+ 475.1804, found 475.1805.
2.3.
Synthesis
of
N-[(N-butyl-1,8-naphthalimide)-4-yl-2,3-dihydro-1H-imidazol-2-yl]benzamide
Probe AETU-HOCl (200 mg, 0.42 mmol) was dissolved in 15 mL ethanol. Then
NaOCl (2 mL) was added and stirred at room temperature for 10 min. The pure
product was obtained by filtering reaction mixture (175 mg, 95%). The structure was
confirmed
to
be
N-[(N-butyl-1,8-naphthalimide)-4-yl-2,3-dihydro-1H-imidazol-2-yl]benzamide.
Melting point: >280 °C. FT-IR(KBr) ν: 3465.14, 2961.64, 1698.08, 1656.00,
1594.86, 1567.91, 1445.79, 1356.94, 1303.49, 1140.65, 1062.41, 995.06, 878.48,
-1
1
848.96, 781.29, 759.50, 644.97, 619.46 cm . H NMR (400 MHz, DMSO-d6 ) δ (×
10-6): 0.85 (t, J = 8.0 Hz, 3H), 1.23-1.29 (m, 2H), 1.44-1.49 (m, 2H), 3.93 (t, J = 8.0
Hz, 2H), 4.06-4.15 (m, 4H), 7.19 (t, J = 8.0 Hz, 2H), 7.34 (t, J = 8.0 Hz, 1H), 7.61 (d,
J = 8.0 Hz, 2H), 7.87 (t, J = 8.0 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H), 8.51-8.59 (m, 3H),
9.44 (s, 1H). ESI-MS calcd for C26H25N4O3 [M+H]+ 441.1927, found 441.1942.
2.4. General procedure for analysis
The parent stock solutions of probe AETU-HOCl (1 mM) were dissolved in the

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ethanol. Parent stock solutions (1 mM) of HOCl and other bioanalytes were prepared
in ultrapure water. Test solutions were prepared by placing 50 μL of the probe stock
solution into a test tube, adding an appropriate aliquot of each stock solution, and then
diluting the solution to 10 mL with the mixture of ethanol and ultrapure water (5:5,
v/v) containing PBS (5 mM, pH 7.4). All spectra were obtained in a quartz cuvette
(path length =1 cm).
2.5. Cell culture
RAW 264.7 cells were cultured in DMEM containing 10% fetal bovine serum,
1% penicillin, and 1% streptomycin at 37 °C (w/v) in a 5% CO2 and 95% air
incubator MCO-15AC (Sanyo, Tokyo, Japan). The concentrations of counted cells
were adjusted to 1 × 106 cells mL-1 for confocal imaging in high-glucose DMEM (4.5
g of glucose/L) supplemented with 10% fetal bovine serum (FBS), NaHCO3 (2.0
ng/L), and 1% antibiotics (penicillin/streptomycin, 100.0 U/mL). Cultures were
maintained at 37 °C under a humidified atmosphere containing 5% CO2.
After 24 h, the control cells were incubated with 10 μM probe AETU-HOCl in
culture media for 30 min at 37 °C, and washed with PBS three times. Then the
fluorescence imaging of cells was carried out. On the other hand, cells pretreated with
PMA (2.0 μg mL-1) for 1 h were incubated with 10 μM probe AETU-HOCl in culture
media for another 30 min at 37 °C, and washed with PBS three times. Then the
fluorescence imaging of cells was carried out. The fluorescence imaging of RAW
264.7 macrophage cells was observed under an Olympus FV1000 confocal
fluorescence microscope.

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2.6. Cytotoxicity assays
RAW 264.7 macrophage cells were cultured in culture media (DMEM) in an
◦
atmosphere of 5% CO2 and 95% air at 37 C. The cells were seeded into 96-well
plates at a density of 3 × 103 cells per well in culture media, then 0, 5, 10, 20 and 30
μM (final concentration) probe AETU-HOCl were added, respectively. Next, the
◦
cells were incubated at 37 C in an atmosphere of 5% CO2 and 95% air for 24 h.
Finally, 20 μL 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT, 5
mg/mL) was added and cultured for another 4 h, respectively.
3. Results and Discussion
3.1. Absorption spectra properties
The absorption spectrum of free probe AETU-HOCl (5 μM) was measured in
the mixture of ethanol and ultrapure water (1:1, v/v) containing (5 mM PBS, pH 7.4).
As shown in Fig. 1, probe AETU-HOCl displayed a major absorption band at 440 nm.
Then, addition of HOCl (5 μM) to the solution of probe AETU-HOCl resulted in a
remarkable decrease of the absorbance at 440 nm, and appearance of a new absorption
band at 376 nm. The blue-shifted absorption spectrum induced by HOCl might be
attributed to the formation of an imidazoline moiety with a much less
electron-donating group (Scheme 1).15
3.2. Fluorescent response of probe AETU-HOCl to HOCl
In the fluorescent spectra, as expected, probe AETU-HOCl exhibited ratiometric
changes upon the addition of increasing concentrations of HOCl under the
above-mentioned analytical conditions. As shown in Fig. 2, the fluorescent intensity

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of the emission band centered at 533 nm was gradually decreased with the concurrent
appearance of a new blue-shifted emission maximum at 473 nm. Notably, there was a
good linearity between the ratio of emission intensities (I473/I533) and concentrations
of HOCl in the range of 0-800 nM (Fig. 3). The limit of detection (LOD) of HOCl
was calculated to be 3.9 nM (3σ/slope). The result makes it sufficiently sensitive for
application in living systems.
Reaction time is a significant factor for reaction-based probes, so the reaction
time of probe AETU-HOCl with HOCl was investigated. As shown in Fig. 4, the
fluorescence intensity increased with reaction time and then almost levels off at
reaction time greater than about 1 min. The result exhibited that probe AETU-HOCl
could provide a fast assay of HOCl.
Consequently, probe AETU-HOCl can be successfully constructed as a
ratiometric fluorescent probe for HOCl, which allows for fast and ultrasensitive
measurements (Table 1).
3.3. Selectivity studies
Then we evaluated the selectivity of probe AETU-HOCl towards HOCl. Firstly,
probe AETU-HOCl was treated with other potentially competing ROS/RNS to
examine the selectivity. As shown in Fig. 5, nearly no fluorescence intensity changes
were observed in the presence of H2O2, tert-butylhydroperoxide (TBHP), tert-butoxy
-
1
-
radical (·OtBu), hydroxyl radical (·OH), peroxynitrite (ONOO-), O2 , O2, NO, NO2 ,
-
NO3 . In addition, the fluorescence responses of probe AETU-HOCl to a variety of
metal ions and bio-reductants were performed. The results showed that K+, Ca2+, Na+,

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Mg2+, Al3+, Zn2+, glutathione (GSH), cysteine (Cys) and ascorbic acid exert no
obvious spectral changes (Fig. 5). It is necessary to point out that 1 equiv. of HOCl
resulted in a considerable fluorescence enhancement, while 100 equiv. (500 μM) of
other bioanalytes only triggered a negligible increase. On the other hand, even though
(2-amino-ethyl)-thiourea group as the receptor of Hg2+ was investigated in the
previous paper,15 the concentration of Hg2+ in biosystems is negligible.7c Therefore,
the above-mentioned results clearly demonstrated that probe AETU-HOCl possesses
the excellent selectivity for HOCl in living systems (Table 1).
3.4. The effect of pH
Probe AETU-HOCl in different pH values ranging from 3 to 8.5 exhibits almost
no distinct variations, suggesting that probe AETU-HOCl is stable under the
measurement conditions. The HOCl-sensing ability of probe AETU-HOCl under the
different pH values was also evaluated (Fig. 6). The experimental results exhibited
that probe AETU-HOCl could detect HOCl in a wide range of pH value, and
importantly, it responded for HOCl rather than OCl- because hypochloric species
almost existed in the only one formation of HOCl under the strong acidic conditions
(pKa of 7.6 for HOCl)16.
3.5. Proposed mechanism for the reaction of probe AETU-HOCl with HOCl
To fully explore the sensing mechanism of probe AETU-HOCl for HOCl, we
carried out the experiment on the reaction of AETU-HOCl and HOCl in ethanol. The
blue fluorescence product was purified
and characterized
to be
by
N-[(N-butyl-1,8-naphthalimide)-4-yl-2,3-dihydro-1H-imidazol-2-yl]benzamide

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1
FT-IR, H-NMR and HRMS. Additionally, the mixture of probe AETU-HOCl and
HOCl under the above-mentioned analytical conditions were subjected to electrospray
ionization mass spectral analysis. The mass spectral peak at m/z 441.1933 [M+H]+
corresponding
to
imidazole
product
N-[(N-butyl-1,8-naphthalimide)-4-yl-2,3-dihydro-1H-imidazol-2-yl]benzamide was
obtained. The fluorescence spectra of AETU-HOCl induced by HOCl also agreed
with
the
fluorescence
spectrum
of
N-[(N-butyl-1,8-naphthalimide)-4-yl-2,3-dihydro-1H-imidazol-2-yl]benzamide (Fig.
1
7). Finally, the titration experiment of AETU-HOCl with HOCl by H-NMR was
further performed (Fig. 8). Therefore, consulted with the previous report15, probe
AETU-HOCl most likely undergoes the proposed mechanism as shown in Scheme 1.
3.6. Biological applications
In consideration of excellent performances of probe AETU-HOCl in vitro, we
then applied it to RAW 264.7 macrophage cells for examining its capability to image
HOCl levels in living cells. As shown in Fig. 9, an intense intracellular fluorescence
of the control cells incubated with probe AETU-HOCl (10 μM) for 30 min
demonstrated that probe AETU-HOCl is cell-permeable (Fig. 9b,c). As expected, the
bigger ratiometric measurement generated from blue channel and green channel in
living cells pretreated with PMA (2.0 μg mL-1) for 1 h was observed. These results
demonstrated that probe AETU-HOCl could be used for the ratiometric fluorescence
imaging of endogenous HOCl in living systems.
Furthermore, to evaluate cytotoxicity of probe AETU-HOCl, we performed

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3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays in RAW
264.7 macrophage cells with 5, 10, 20, and 30 μM probe AETU-HOCl for 24 h. As
can be seen from Fig. 10, cell viabilities were over 90%, implying that probe
AETU-HOCl was of low toxicity to cultured cells under the experimental conditions.
4. Conclusions
In summary, a new ratiometric fluorescent probe AETU-HOCl for HOCl has
been designed and synthesized. The experimental results demonstrated that probe
AETU-HOCl possesses outstanding selectivity toward HOCl over other ROS, RNS,
bio-reductants and metal ions at higher concentrations (0.5 mM). Probe AETU-HOCl
could detect HOCl in the range of 0 to 800 nM with the detection limit of 3.9 nM by
the ratiometric fluorescence spectra. Additionally, probe AETU-HOCl showed a
rapid response (about 1 min) to HOCl at a wide range of pH. Importantly, probe
AETU-HOCl could be successfully used for the ratiometric fluorescence imaging of
endogenous HOCl in living cells. The excellent sensing features of probe
AETU-HOCl make it a promising tool for detecting HOCl in living cells. Our
proposed recognition mechanism would open up new exciting opportunities for
designing highly selective and ultrasensitive fluorescent probes for the ratiometric
determination of HOCl in living systems.
Acknowledgements
We gratefully acknowledge financial support from the National Natural Science
Foundation of China (No. 21107029), Outstanding Young Scientists Award Fund of
Shandong Province (BS2013HZ007), Postdoctoral Science Foundation of China

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(2013M541953, 2016M592127), Graduate Innovation Foundation of University of
Jinan (YCXS15016) for financial support.

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7313.

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Figure captions
Fig. 1. Absorption spectra of probe AETU-HOCl (5 μM) in the absence and presence
of HOCl (5 μM) in the mixture of ethanol and ultrapure water (5:5, v/v)
containing (5 mM PBS, pH 7.4).
Fig. 2. Fluorescence spectra changes of probe AETU-HOCl (5 μM) in the presence
of increasing concentrations of HOCl (0-5 μM) in the mixture of ethanol and
ultrapure water (5:5, v/v) containing (5 mM PBS, pH 7.4). The excitation
wavelength was 380 nm. Excitation and emission slit widths were 3 nm.
Fig. 3. The linearity between the ratio of fluorescence intensities (I473/I533) and
increasing concentrations of HOCl.
Fig. 4. Time-course for probe AETU-HOCl (5 μM) in the presence of HOCl (5 μM).
Fig. 5. The fluorescence responses of probe AETU-HOCl (5 μM) in the presence of
different analytes (0.5 mM except for specific label) in the mixture of ethanol
and ultrapure water (5:5, v/v) containing (5 mM PBS, pH 7.4). a: only probe, b:
-
K+, c: Ca2+, d: Na+, e: Mg2+, f: Al3+, g: Zn2+, h: H2O2, i: O2 , j: TBHP, k: ·OH,
·
-
-
l: OtBu, m: NO, n: 1O2, o: ascorbic acid, p: NO3 , q: NO2 , r: Cys (3 mM), s:
GSH (3 mM), t: ONOO-, u: HOCl (5 μM). Bars represent the ratio of
fluorescence intensities (I473/I533).
Fig. 6. The ratio of fluorescence intensities (I473/I533) of probe AETU-HOCl (5 μM)
in the absence and presence HOCl (5 μM) in different pH values.
Fig. 7. The fluorescence spectra of probe AETU-HOCl, AETU-HOCl with HOCl,
and
imidazole
product

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N-[(N-butyl-1,8-naphthalimide)-4-yl-2,3-dihydro-1H-imidazol-2-yl]benzamide
in the mixture of ethanol and ultrapure water (5:5, v/v) containing (5 mM PBS,
pH 7.4).
1
Fig. 8. The H NMR spectra of probe AETU-HOCl in DMSO-d6 under the absence
and presence of HOCl. (a) free probe AETU-HOCl, (b) probe AETU-HOCl
with 0.5 equiv. HOCl, (c) probe AETU-HOCl with 1.0 equiv. HOCl, (d) probe
AETU-HOCl with 2.0 equiv. HOCl.
Fig. 9. Confocal fluorescence images of living RAW 264.7 macrophage cells: the
control cells were incubated with probe AETU-HOCl (10 μM) for 30 min; (a)
bright-field transmission image, (b) blue channel, (c) green channel, and (d) ratio
image generated from (b) and (c). The cells pretreated with PMA (2.0 μg mL-1)
for 1 h were incubated with probe AETU-HOCl (10 μM) for another 30 min; (e)
bright-field transmission image, (f) blue channel, (g) orange channel, and (h)
ratio image generated from (f) and (g). Incubation was performed at 37 °C under
a humidified atmosphere containing 5% CO2. Scale bar = 10 μm.
Fig. 10. Cytotoxicity assays of probe AETU-HOCl at different concentrations for
RAW 264.7 cells.

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Figures
Fig. 1
Fig. 2

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Fig. 3
Fig. 4

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Fig. 5
Fig. 6
Fig. 7

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Fig. 8
Fig. 9
Fig. 10

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Scheme and scheme caption
Scheme 1. The synthesis and recognition reaction of probe AETU-HOCl for HOCl.
Scheme 1

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Table and table caption
Table 1 Comparison of fluorescent probes for HOCl
Ratiometric Quantitative LOD
Probe
Selectivity References
detection
No
range (μM) (nM)
COOH
O
Cl
0-10
0.33 10 equiv.
[8a]
OH
Cl
O
O
S
O
No
No
0-100
16.6
5 equiv.
[11a]
N
H
OH
OH
2-20
300 2.5 equiv.
[12e]
HO S
SO H
3
3
N
N
B
O
F
F
O
O
B
B
O
O
No
No
0.2-20
0-60
-
5 equiv.
[13a]
[7h]
S
O
Se
N
310 10 equiv.
+
N
N
-
I
O
N
O
HN
Yes
0-0.8
3.9 100 equiv. This work
H
S
N
O
HN

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A highly selective ratiometric fluorescent probe for the sensitive detection of
hypochlorous acid and its bioimaging applications
Wei Shu, Pan Jia, Xingzhu Chen, Xueyan Li, Yanru Huo, Fang Liu, Zuokai Wang,
Caiyun Liu, Baocun Zhu,* Liangguo Yan,* and Bin Du
School of Resources and Environment, University of Jinan, Shandong Provincial
Engineering Technology Research Center for Ecological Carbon Sink and Capture
Utilization, Jinan 250022, China
*Corresponding author. fax: +86-531-82767617; Tel.: +86-531-82767617
E-mail address: lcyzbc@163.com (B. Zhu) and chm_yanlg@ujn.edu.cn (L. Yan)
Graphical abstract
A rapid-response, highly selective and ratiometric fluorescent probe was synthesized
to sensitively detect HOCl in living cells.
1