A coumarin Schiff's base two-photon fluorescent probe for hypochlorite in living cells and zebrafish

Article abstract of DOI:10.1039/c8ra00093j

Selective and sensitive fluorescent probes for ClO^- are desirable due to the importance of ClO^- in biological processes. Here, a coumarin Schiff's base, compound 1, has been developed and successfully used as a one- and two-photon fluorescent probe for ClO^- with high selectivity. This probe can recognize ClO^- with obvious color change from yellow-green to colorless and green to blue fluorescence emission, which can be observed by the naked eye. The properties of low cytotoxicity and good cell permeability allow it to be used for ClO^- detection in living cells and zebrafish by both one- and two-photon microscopy imaging. All these results indicate that the compound is a sensitive probe with potential for analysis of ClO^- in biological samples. The mechanism by which probe 1 recognizes ClO^- is possibly nucleophilic addition followed by hydrolysis.

Full text of DOI:10.1039/c8ra00093j

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PAPER
A coumarin Schiff's base two-photon fluorescent
probe for hypochlorite in living cells and zebrafish†
Cite this: RSC Adv., 2018, 8, 6904
Kangnan Wang, Pengzhen Sun, Xijuan Chao, Duxia Cao, *a Zongwan Mao *b
ab
c
b
d
and Zhiqiang Liu
*
ꢀ
ꢀ
Selective and sensitive fluorescent probes for ClO are desirable due to the importance of ClO in biological
processes. Here, a coumarin Schiff's base, compound 1, has been developed and successfully used as a one-
ꢀ
ꢀ
and two-photon fluorescent probe for ClO with high selectivity. This probe can recognize ClO with obvious
color change from yellow-green to colorless and green to blue fluorescence emission, which can be observed
Received 4th January 2018
Accepted 31st January 2018
ꢀ
by the naked eye. The properties of low cytotoxicity and good cell permeability allow it to be used for ClO
detection in living cells and zebrafish by both one- and two-photon microscopy imaging. All these results
DOI: 10.1039/c8ra00093j
ꢀ
indicate that the compound is a sensitive probe with potential for analysis of ClO in biological samples.
ꢀ
rsc.li/rsc-advances
The mechanism by which probe 1 recognizes ClO is possibly nucleophilic addition followed by hydrolysis.
6,17–24
hypochlorite have been developed,
most of which are based
Introduction
on one-photon microscopy (OPM). The short excitation wave-
ꢀ
Hypochlorite (ClO ) is an important reactive oxygen species length used in OPM may easily damage cells and tissues and
1
ꢀ
ꢀ
(
ROS, mainly including H O , OHc, O , NO
2 2 2 2
, HClO/ClO and lead to photobleaching, thus limiting the applications in tissues
1,2
so on), which is biologically produced from chloride ions and and living cells. Compared with OPM, two-photon uorescence
hydrogen peroxide by the catalytic action of the enzyme mye- microscopy (TPM) normally uses longer excitation wavelength,
3,4
loperoxidase (MPO) in living organisms. Hypochlorite is which enables deep tissue imaging with focused excitation and
2
5–27
a powerful oxidant and plays a critical role in the immune reduces photobleaching and photodamage to biosamples.
ꢀ
5,6
system against inammation and microorganisms. However,
The numbers of selective ClO uorescent probes are still
excessive intake of ClO may lead to tissue damage and limited. And most of the uorescent probes still have some
diseases, such as lung injury, cancers and so on. In addition, limitations, such as time dependence, poor selectivity, or
disinfected water with residual chlorine may cause organisms complicated synthesis and rigorous conditions. Therefore,
to suffer from diseases of the blood circulation and nervous simple, fast, convenient uorescent probes for ClO are still in
ꢀ
7
–9
ꢀ
10,11
system and irritation of sensory organs.
Therefore, detecting high demand. Herein, a coumarin Schiff's base, compound 1,
hypochlorite in drinking water and organisms is especially was developed as a two-photon uorescence probe for ClO . This
ꢀ
signicant and attracts extensive interest.
water-soluble coumarin Schiff's base-based uorescent probe,
Presently, there are a lot of methods available for hypo- which can be obtained by simple synthesis under mild reaction
chlorite determination, such as chemiluminescence and uo- conditions, has high uorescence quantum yield and can detect
ꢀ
12
rescence methods and electroanalysis.
Among them, ClO with high selectivity, rapid response rate and obvious color

uorescence titration is one of the most attractive methods and uorescence change, which can be observed by the naked
owing to its high selectivity and sensitivity, real-time detection eye. The probe is also successfully applied to imaging in living
13–16
and easy processing.
Recently, many uorescent probes for cells and living zebrash by one- and two-photon modes.
a
School of Materials Science and Engineering, University of Jinan, Jinan 250022, PR Results and discussion
China. E-mail: duxiacao@ujn.edu.cn
b
Spectroscopic recognition properties
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry
and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China. Probe 1 was synthesized via the condensation reaction between
E-mail: cesmzw@mail.sysu.edu.cn
7
-diethylaminocoumarin-3-aldehyde and 2-thiophenecarboxylic
c
School of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, PR
28
acid hydrazide according to a previously published method.
Probe 1 exhibits obvious UV-vis absorption and uorescence
recognition for ClO . Fig. 1a shows the change in UV-vis
absorption spectrum of probe 1 to ClO in phosphate buffer/
China
d
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100,
ꢀ
Shandong, PR China. E-mail: zqliu@sdu.edu.cn
ꢀ
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra00093j
1
MeOH (6 : 4, v/v) solution. The absorption spectrum of probe
6904 | RSC Adv., 2018, 8, 6904–6909
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ꢀ
1
1
1
showed a main absorption band at 455 nm (3 ¼ 5.02 ꢁ interfere with the recognition for ClO . The results demonstrate
4
4
ꢀ1
ꢀ1
ꢀ
0
M
cm ) and a minor band at 317 nm (3 ¼ 1.09 ꢁ the high selectivity of the probe for ClO .
ꢀ1
ꢀ1
0 M cm ). Upon titration with NaOCl, which was used as
To further examine the sensing mechanism of probe 1 for
ꢀ
ꢀ
a source of ClO , the absorption band at 450 nm gradually ClO , in situ electrospray ionization-mass spectroscopy (ESI-
decreased and showed a blue-shi to 373 nm with an isosbestic MS) was used to clarify the process of reaction between probe
point at 339 nm. At the same time, the peak at 317 nm also 1 and ClO (Fig. S2†). The mass spectrum peak of probe 1 is at
ꢀ
+
ꢀ
decreased. These spectral changes indicate that
a new 392.43 ((M + Na) ). Aer the addition of ClO , there are two new
compound was generated. The color change from yellow-green mass peaks. A plausible mechanism by which probe 1 reacts
ꢀ
ꢀ
to colorless allows the colorimetric detection of ClO using the with ClO according to ESI-MS data and ref. 14 and 29 is pre-
naked eye, and is invoked by a large blue-shi of ca. 80 nm in sented in Fig. 2. The new mass spectrum peaks at 426.34 and
+
+
the absorption spectrum.
284.30 may correspond to (S-1 + Na) and (P-1 + Na) . The C]N
As expected, the emission proles of probe 1 correlate to bond is blocked, which leads to the decrease of the main
ꢀ
those of the absorption proles in the presence of ClO . In the absorption band at 455 nm and quenching of green uores-
ꢀ
absence of ClO , probe 1 emits strong green uorescence at cence at 518 nm. Blue uorescence emission aer the addition
ꢀ
5
18 nm with uorescence quantum yield of 0.27. Upon the of ClO may be derived from P-1.
ꢀ
addition of increasing concentrations of ClO , the uorescence
intensity decreases signicantly upon excitation at 455 nm. This Cytotoxicity and bioimaging of the probe
ꢀ

uorescence decrease is caused by selective oxidation by ClO .
ꢀ
The need for a probe able to selectively detect ClO in living
As shown in Fig. 1b, a new uorescence emission peak at
95 nm can be readily observed, and the uorescence color
changes from green to blue under UV light can also be clearly
detected by the naked eye (Fig. 1b). This demonstrates that
cells has recently attracted much attention because of the
importance of ClO in biological applications. Therefore, probe
was applied to cells to demonstrate its potential application in
4
ꢀ
1
cellular imaging. The cytotoxicity of probe 1 was evaluated by
methyl thiazolyl tetrazolium (MTT) assay with probe concen-
tration ranging from 0 mM to 50 mM (Fig. S3†). The experimental
results indicate that the viability remains above 80% aer 48 h
incubation with different concentrations (0 mM to 50 mM) of
probe 1, indicating low cytotoxicity of probe 1. Then cellular
uptake of probe 1 in A549 cells was investigated. A549 cells were
incubated with probe 1 at three distinct concentrations (10 mM,
ꢀ
probe 1 is capable of sensing ClO efficiently in aqueous solu-
tion, acting as a ratiometric uorescent probe.
ꢀ
One of the major challenges in ClO detection is to develop
highly selective probes. In order to evaluate the selectivity of
probe 1 toward ClO , possible inuences caused by other ana-
lytes were investigated based on uorescence spectra of solu-
ꢀ
tions. The common anions and biologically related species
ꢀ
3ꢀ
2ꢀ
3ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ
ꢀ
2ꢀ
42ꢀ
+
ClO
4
, C
2
O
4
, NO
2
, F , Cl , I , HCO
3
, H
2
O
2
2ꢀ
, CO
3
4
, NH ,
2
0 mM, 50 mM) for 30 min. Strong green uorescence in cells was
2
O
8
2
, S O
3ꢀ
, SO
3
, SO , GSH,
COO were used together
with ClO to investigate the selectivity and competitiveness. As
shown in Fig. S1,† among the ions tested, there were no obvious
uorescence changes observed except with ClO , and uores-
cence change from green to blue was obviously observed only
with addition of ClO . It is worth noting that other anions (up to
00 equiv.) did not interact with the probe and also did not
HSO , NO , Cys, OH , S
HPO
observed by not only one-photon microscopy imaging (l ¼ 458
ex
2
4
ꢀ
ꢀ
, citric acid, boric acid and CH
3
nm) but also two-photon microscopy imaging (l ¼ 810 nm)
ex
ꢀ
(
Fig. 3). The short incubation time (30 min, Fig. S4†) with cells
indicates quite good cell permeability of the probe. Thus low
cytotoxicity and good cell membrane penetration make it
ꢀ

ꢀ
applicable for detection of ClO in living cells.
ꢀ
Next the mechanism of cellular uptake of the probe was
studied. As known, cells can take up small molecules by different
2
ꢀ
Fig. 1 Changes in UV-vis absorption (a) and fluorescence spectrum (lex ¼ 450 nm, (b) of probe 1 (10 mM)) with various amounts of ClO in
3
potassium phosphate buffer (pH 7.4, containing 40% CH OH as co-solvent). The insets show ratios of absorbance at 455 nm and 373 nm (a) or
ꢀ
ꢀ
fluorescence at 518 nm and 495 nm (b) as a function of ClO concentration, and color changes before and after the addition of ClO under
natural light (a) and UV lamp (b).
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Fig. 2 A plausible mechanism by which probe 1 reacts with ClO .
Fig. 3 One- and two-photon microscopy (OPM, TPM) confocal fluorescence images of A549 cells after incubation with probe 1 (10 mM, 20 mM
and 50 mM) for 30 min and excitation at 458 nm (OPM) or 810 nm (TPM). The emission was collected at 520 ꢃ 30 nm. Overlay-OPM: overlay of
the bright field and OPM columns. Overlay-TPM: overlay of the bright field and TPM columns.
3
0,31
mechanisms.
Some mechanisms such as endocytosis and experiments to demonstrate the potential practical applications
ꢀ
active transport mechanisms are energy-dependent. But some of the probe for uorescence detection of ClO in living cells were
mechanisms such as facilitated diffusion and passive diffusion are performed. A549 cells were rstly incubated with probe 1 (20 mM)
energy-independent. First, temperature-dependence experiments for 30 min; a strong green uorescence was observed by both one-
were conducted. A549 cells (pre-placed at the respective tempera- photon and two-photon excitation (Fig. 5A). Aer the cells were
tures for 30 min) were treated with probe 1 and incubated at further incubated with 120 equiv. ClO , the cell images were
different temperatures (37 C, 25 C and 4 C) for 30 min. As shown recorded for a time series gradient. From Fig. S6,† we can see that
ꢀ
ꢂ
ꢂ
ꢂ
ꢂ
in Fig. 4, at the lowest temperature, 4 C, the images are not green uorescence in living cells was completely quenched aer
obvious and the images gradually become apparent as the 25 min. If the cells were incubated with 10 equiv. (Fig. 5B) or 60
ꢀ
temperature is increased, which indicates that lower temperature equiv. (Fig. 5C) ClO , the one-photon or two-photon uorescence
ꢀ
can reduce cellular uptake efficiency and thus cellular uptake is diminished gradually. When 120 equiv. ClO (Fig. 5D) was added,
energy-dependent. In addition, treatments with two endocytosis both one-photon uorescence and two-photon uorescence were
modulators chloroquine and NH Cl, which can inhibit the acidi- completely quenched. Taken together, these results indicate that
4

cation of endosomes and thus inhibit uptake by endocytosis, have the probe can be successfully applied for one-photon and two-
ꢀ
no effect on the cellular uptake of probe 1, which indicates endo- photon imaging of ClO selectively in living cells.
cytosis is not responsible for the cellular uptake. Taken together,
these results indicate that probe 1 enters A549 cells possibly Fluorescence detection of ClO in zebrash
ꢀ
through an energy-dependent but non-endocytotic pathway.
ꢀ
To further examine whether ClO can be in vivo detected by
probe 1, 2 days-old zebrash larvae were used as a vertebrate
model. As shown in Fig. 6, bright emitted signals were clearly
ꢀ
Fluorescence detection of ClO in living cells
Aer establishing that the probe was capable of uorescence observed in all of the body of the zebrash. When incubated
ꢀ
detection of ClO and had good cellular uptake behavior, with 20 mM probe 1 for 30 min, the zebrash shows strong
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ꢂ
ꢂ
Fig. 4 Confocal images of A549 cells incubated with 1 (20 mM) under different conditions. (A)–(C): cells were incubated with 1 at 37 C (A), 25 C
ꢂ
ꢂ
(
(
B) or 4 C (C) for 30 min. (D) and (E): cells were preincubated with NH Cl (50 mM) or chloroquine (10 mM) for 1 h at 37 C and then incubated with 1
20 mM) for 30 min, respectively. The probe was excited at 458 nm (OPM) or 810 nm (TPM). The emission was collected at 520 ꢃ 30 nm. Overlay-
4
OPM: overlay of the bright field and OPM columns. Overlay-TPM: overlay of the bright field and TPM columns.
Fig. 5 One-photon and two-photon microscopy (OPM, TPM) confocal fluorescence images of A549 cells. Images of cells incubated with probe
ꢀ
1
(20 mM) for 30 min (control, (A)). Images of cells incubated with probe 1 for 30 min and subsequently with ClO (10 equiv., (B); 60 equiv., (C); 120
equiv., (D)). The probe was excited at 458 nm (OPM) or 810 nm (TPM). The emission was collected at 520 ꢃ 30 nm. Overlay-OPM: overlay of
bright field and OPM columns. Overlay-TPM: overlay of bright field and TPM columns.
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Fig. 6 Images of zebrafish larvae treated with 20 mM probe 1 in the absence and presence of 120 equiv. ClO . The probe was excited at 458 nm
OPM) or 810 nm (TPM). The emission was collected at 520 ꢃ 30 nm. Overlay-OPM: overlay of the bright field and OPM columns. Overlay-TPM:
overlay of the bright field and TPM columns.
(

uorescence in the green channels by both one-photon and two- Cell culture conditions
ꢀ
photon modes. Upon further incubation with 120 equiv. ClO
for another 30 min, green uorescence in all of the body of the
zebrash by both one-photon and two-photon imaging was
almost completely quenched. The probe could be suitable for
ClO detection in living zebrash. Therefore, the probe has
potential application of detecting ClO in vivo by uorescence
A549 cells were maintained in Roswell Park Memorial Institute
1
1
640 (RPMI 1640) medium containing 1% antibiotic solution,
ꢀ1
0% FBS (fetal bovine serum), 100 U mL penicillin and 100
ꢀ1
mg mL streptomycin. The A549 cells were cultured in an
ꢀ
incubator with 95% air and 5% CO at a constant temperature
of 37 C. A549 cells were obtained from the Experimental
2
ꢀ
ꢂ
signal changes.
Animal Center of Sun Yat-Sen University (Guangzhou, PR
China).
Conclusions
Cell viability assay
In conclusion, a coumarin Schiff's base-based probe selective
for hypochlorite anion has been developed. The probe shows
ratiometric uorescence and remarkable uorescence intensity
The cytotoxicity of the probe towards A549 cell lines was
determined by methyl thiazolyl tetrazolium (MTT) assay.
34,35
ꢀ
change in response to ClO but not other common anions and
some biologically related species. Moreover, this probe could be
applied to detect ClO in living cells and zebrash by both one-
One- and two-photon cellular imaging
ꢀ
A549 cells were treated with 20 mM probe 1 in RPMI 1640
medium. Aer 30 min, RPMI 1640 medium was removed and
washed with PBS buffer twice to remove excess probe 1. Then
A549 cells were further incubated with NaClO solution in PBS
for 10 min.
and two-photon microscopy imaging. The mechanism of the
quick transition from green uorescence to blue uorescence in
response to hypochlorite is attributed to nucleophilic addition
followed by hydrolysis. With the characteristics of fast response
rate, high selectivity, low cytotoxicity, good cell permeability
and two-photon effect, we believe that the probe could have a lot
of practical application in environmental and biological
systems.
One- and two-photon imaging of zebrash
Zebrash were obtained from the Experimental Animal Centre
of Sun Yat-sen University (Guangzhou, PR China). All the
experiments were approved by the Institutional Animal
Care and Use Committee (IACUC) of Sun Yat-Sen University
according to the requirements of the National guideline
Experimental
Photophysical properties and response to hypochlorite anion
UV-vis absorption and uorescence spectral titrations for the care and use of laboratory animals (PR China).
3
6
were carried out on a Shimadzu UV2550 spectrophotometer The zebrash were incubated in E3 medium for 6 h and
ꢂ
and
a
Horiba Fluoromax-4 uorescence spectrometer, then 20 mM probe was added for another 1 h at 28 C. The
respectively. The solvent for titration experiments was phos- zebrash were anesthetized with 1% chloral hydrate and
phate buffer/MeOH (6 : 4, v/v, pH 7.4). The spectral changes subjected to luminescence imaging by confocal laser scanning
were monitored with the addition of a solution of sodium microscopy. Other zebrash were incubated with 20 mM probe
hypochlorite (available chlorine content 5%) in deionized for 1 h in E3 medium, and then 2.5 mM NaClO was added to
ꢀ
ꢂ
water as ClO source. Fluorescence quantum yield was ob- treat for another 1 h at 28 C. One- and two-photon imaging
tained with coumarin 307 as ref. 32 by a previously published of zebrash was carried out by a previously published
3
3
37
method.
method.
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1
7 K. M. Xiong, F. J. Huo, C. X. Yin, Y. Y. Chu, Y. T. Yang,
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Conflicts of interest
There are no conicts to declare.
18 J. F. Li, F. J. Huo and C. X. Yin, RSC Adv., 2014, 4, 44610–
4
4613.
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1
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