A turn-on fluorescent probe based on quinoline and coumarin for rapid, selective and sensitive detection of hypochlorite in water samples

Article abstract of DOI:10.1002/bio.3882

A fluorescent probe L-Cu²⁺ based on quinoline, coumarin and Cu²⁺ has been synthesized and characterized for hypochlorite determination. After copper ion was added to the solution of ligand L, the fluorescence quenching at 490 nm might result from a ligand–metal charge transfer (LMCT) process and its strong coordination ability for Cu²⁺. In the presence of hypochlorite, the structure of ligand L was destroyed to form 7-(diethylamino)-coumarin-3-carboxylic acid, and the fluorescence was restored at 460 nm. In this case, L-Cu²⁺ complex could be used as a fluorescent probe to detect hypochlorite, with the advantages of rapid, selective, wide linear range and low detection limit.

Full text of DOI:10.1002/bio.3882

Received: 23 December 2019
DOI: 10.1002/bio.3882
Revised: 27 April 2020
Accepted: 16 May 2020
R E S E A R C H A R T I C L E
A turn-on fluorescent probe based on quinoline and coumarin
for rapid, selective and sensitive detection of hypochlorite in
water samples
Ruxin Liu1 | Yuqi Zhao1 | Xiaoqian Cui2 | Xiaoxiao Sun1 | Qiang Fei1
Guodong Feng1 | Hongyan Shan1 | Yanfu Huan1
|
1College of Chemistry, Jilin University,
Abstract
Changchun, People's Republic of China
2The Second Hospital of Jilin University,
Changchun, People's Republic of China
A fluorescent probe L-Cu2+ based on quinoline, coumarin and Cu2+ has been synthe-
sized and characterized for hypochlorite determination. After copper ion was added
to the solution of ligand L, the fluorescence quenching at 490 nm might result from a
ligand–metal charge transfer (LMCT) process and its strong coordination ability for
Cu2+. In the presence of hypochlorite, the structure of ligand L was destroyed to form
7-(diethylamino)-coumarin-3-carboxylic acid, and the fluorescence was restored at
460 nm. In this case, L-Cu2+ complex could be used as a fluorescent probe to detect
hypochlorite, with the advantages of rapid, selective, wide linear range and low
detection limit.
Correspondence
Yanfu Huan, College of Chemistry, Jilin
University, Changchun 130023, People's
Republic of China.
Email: yfhuan@jlu.edu.cn
Funding information
Natural Science Foundation of Jilin Province,
Grant/Award Number: 20200201238JC
K E Y W O R D S
fluorescent, hypochlorite, water samples
1
|
INTRODUCTION
and low detection limit, fluorescence analysis has drawn more
attention in the field of research recently.[14–17] Therefore, it
Hypochlorous acid (HClO)/hypochlorite (ClO−), as a proverbial bio-
logical reactive oxygen species (ROS),[1] plays an indispensable part
in many biochemical processes.[2,3] It is a biological agent and com-
mon disinfectant in daily life that can kill a variety of patho-
gens.[4,5] However, abnormal hypochlorite concentration in the
body could give rise to many illnesses like Alzheimer's disease,
angiocardiopathy, nephropathy and even cancer.[6] In October
2017, the international institution for research on cancer has publi-
shed the list of carcinogens regarding it as a category 3 carcinogen.
Therefore, exploiting a rapid and accurate method to detect hypo-
chlorite is an extremely urgent task.[7]
already has been reported that hypochlorite can be detected by
many fluorescent probes. Nevertheless, most of them have adverse
characters of large self-interference, narrow linear range and long
reaction time etc. To overcome these shortcomings, we report a
sensor for rapid detection of hypochlorite (2 s) with a wide linear
range (1.00 × 10−6 mol L−1 – 8.00 × 10−3 mol L−1).
2
| EXPERIMENTATION
2.1 | Reagents
Up to now, with the development of analytical technology, lots
of methods for hypochlorite determination have been researched,
4-N,N-Diethylaminosalicylicaldehyde,
2-Quinolinecarboxaldehyde
and Diethylmalonate were acquired from Energy Chemical Co., Ltd
(Shanghai, China). All the other reagents and chemicals were
obtained from reagent sales agency. All reagents, without any prior
purification, were analytical reagent grade unless otherwise
specified.
including
iodometry,[8]
phosphorescence
analysis,[9]
electrochemistry,[10] colorimetric method,[11] high performance liq-
uid chromatography (HPLC),[12] and fluorescence analysis.[13]
Among them, because of its merits such as fast, well specificity
Luminescence. 2020;1–8.
wileyonlinelibrary.com/journal/bio
© 2020 John Wiley & Sons, Ltd.
1

2
LIU ET AL.
−
2.2 | Apparatus
HCO3−, NO3−, SO42−, ClO−, CO32−, Fe3+, MnO4 and H2O2. The real
samples were obtained from Changchun Water Supply Company and
the lake water of Peony Park Changchun. The add sequence of solu-
Hitachi F-2700 fluorescence spectrophotometer (Hitachi, Japan) was
used to perform fluorescence measurements. Sartorius PB-10 digital
pH meter was used to make all pH measurements. Varian Mercury
YH-300 spectrometer operated at 400 MHz was used to perform
Nuclear magnetic resonance (NMR) spectra. Agilent 1,290 Infinity LC
System interfaced with the Bruker micrOTOF-Q II mass spectrometer
instrument was used to obtain electrospray ionization (ESI) mass
spectra.
tions was 10 μL probe L (1 mM), 2090 μL methyl alcohol, 10 μL Cu2+
,
890 μL deionized water and 50 μL ClO−. The gross of solutions mar-
ked 3 mL. The emission intensity was recorded at 460 nm with an
excitation wavelength at 401 nm. The excitation and emission slits of
5 nm were applied. A sample cell path length was 1 cm.
3
| RESULTS AND DISCUSSION
2.3 | Synthesis
3.1 | Quenching of fluorescence by Cu2+
The synthetic pathway for G2 and G3 were based on the previous
work.[18,19] Probe L was synthesized conveniently from the reaction
of G3 and 2-Quinolinecarboxaldehyde, which was shown in scheme 1.
It was characterized by 1H NMR (400 MHz, DMSO-d6) δ 12.00 (s, 1H,
N-C=O), 8.78 (s, 1H, quinoline H-3), 8.58 (s, 1H, coumarin H-3), 8.44
(d, J = 8.7 Hz, 1H, quinoline H-5), 8.13 (d, J = 8.6 Hz, 1H, quinoline H-
8), 8.06 (d, J = 8.5 Hz, 1H, C9H6NCH=N), 8.03 (d, J = 8.1 Hz, 1H, quin-
oline H-2), 7.81 (t, J = 7.7 Hz, 1H, quinoline H-7), 7.76 (d, J = 9.0 Hz,
1H, quinoline H-6), 7.66 (t, J = 7.4 Hz, 1H, coumarin H-5), 6.86 (d,
J = 9.1 Hz, 1H, coumarin H-6), 6.68 (s, 1H, coumarin H-8), 3.58–3.47
(m, 4H, NCH2CH3), 1.17 (t, J = 7.0 Hz, 6H, NCH2CH3). ESI-MS: m/z
Calculated for C24H22N4O3 [M] = 414.47, Found [M + H+] = 415.03.
Figure S3 and Figure S4 (a).
With the various concentrations of Cu2+ presenting in the solution
(CH3OH/H2O, 7:3 v/v), the fluorescence intensities of
L were
2.4 | Preparatory work
Stock solution (3 mmol L−1) was gotten by dissolving probe L in
dimethylsulphoxide (DMSO). The corresponding concentration of the
determin and solutions (3 mmol L−1) were achieved by dissolving the
species in redistilled water, including K+, Na+, Ca2+, F−, Cl−, Br−,
FIGURE 1 The effect of various concentrations of Cu2+ on the
fluorescence of L (10 μM), in a mixture solution of water and
methanol (CH3OH/H2O, 7:3 v/v). The concentrations of Cu2+ are
0, 2, 4, 6, 8, 10, 12, 14 μM, respectively
SCHEME 1 Synthesis of the probe L

LIU ET AL.
3
recorded and shown in Figure 1 (The RSD data were shown in
table S1). According to the following consequences, the fluorescence
intensity of L declined accompanied by the increasing concentration
of Cu2+. The fluorescence intensity remained approximately constant
while the concentration of Cu2+ increased to 1 equation (10 μM).
Thus, 10 μM Cu2+ was used as the quenching agent in subsequent
experiments.
intensity changes were recorded in Figure 2. With or without ClO−,
the difference of fluorescence intensity in methanol–water mixed sys-
tem was more significant than that in other organic solvent-water sys-
tems. This means that the reactivity of L-Cu2+ with ClO− and the
solubility of Q1 are both excellent in methanol solvent. Therefore
methanol was chosen as the system solvent for subsequent
experiments.
3.2 | Effect of solvents
3.3 | Effect of water content
Different organic solvents will affect the reactivity of L-Cu2+ and
ClO− and the solubility of Q1, which will affect the detection effect.
In order to survey the fluorescence intensity changes of L-Cu2+ com-
plex in different organic solvents, four commonly used organic sol-
vents were studied (methyl alcohol, ethyl alcohol, methyl cyanide and
DMSO) in the presence or absence of ClO−. The fluorescence
The water content of the system will affect the fluorescence intensity.
In this experiment, the optimal water content was determined by the
double influence factors, one is the reactivity between the L-Cu2+ and
ClO− in different water content solvents and the other is the solubility
FIGURE 4 The fluorescence of L (10 μM) at 460 nm influenced
by various solution pH, in a mixture solution of B.R. buffer solution
and methanol (CH3OH/H2O, 7:3 v/v) with (red line) or without (black
line) ClO- (50 μM)
FIGURE 2 The fluorescence intensities of L-Cu2+ (black bars) and
L-Cu2++ClO- (50 μM) (red bars) at 460 nm in different solvent/water
solutions (7:3, v/v)
FIGURE 3 The effect of various contents of water on the
fluorescence intensity of L-Cu2+ at 460 nm before (red line) and after
(black line) the addition of ClO- (50 μM)
FIGURE 5 The fluorescence of L (10 μM) at 460 nm with reaction
time went on, in a mixture solution of water and methanol
(CH3OH/H2O, 7:3 v/v) with ClO- (50 μM)

4
LIU ET AL.
of the Q1 in solvents. So the fluorescence intensity changes of L-Cu2+
were investigated in a series of water contents with/without ClO−.
The fluorescence intensity before and after the addition of ClO− with
the water contents ranged from 0 to 100% was presented in Figure 3
(The RSD data were shown in table S2). There was the highest fluo-
rescence enhancement when the water content was 30% (ethanol:
water = 7:3). This indicates that the reaction between L-Cu2+ and
ClO− is easier when the water content is 30%, and the solubility of
the Q1 is not affected in significant. Thus, the 30% water was chosen
for subsequent experiments.
3.4 | Effect of pH
It was necessary to explore fluorescence intensity of L-Cu2+ in differ-
ent pH solutions. We recorded the fluorescence intensity of L-Cu2+ in
the existence or inexistence of ClO− within the range of pH 4.0
to11.0 (B.R. buffer of 0.04 mol L−1). As depicted in Figure 4, in the
presence of ClO−, there was no significant change of fluorescence
recovery of L-Cu2+ with different pH (The RSD data were shown in
table S3). Therefore, buffer solution is unnecessary in this system. It is
more convenient to select pure water for experiments.
3.5 | Optimum reaction time
Rapid response is an important advantage of analytical method, so we
investigated the detection time of L-Cu2+ for ClO−. As can be seen
from Figure 5, ClO− was added to L-Cu2+ at 15 s and the reaction
ended about after 2 s. It means that we can test the sample immedi-
ately, which is very fast and saves time.
3.6 | Analysis of ClO− by L-Cu2+
The relationship between fluorescence intensities and ClO− concen-
trations was studied under optimal conditions in this part. As depicted
in Figure 6, with the increase of ClO− concentration, the fluorescence
intensity gradually recovered companied by a blue shift to 460 nm.
There is a linear fitting curve of fluorescence intensities and the con-
centrations of ClO− (1.00 × 10−6 mol L−1 – 7.47 × 10−3 mol L−1
)
FIGURE 6 Fluorescence spectra of L-Cu2+ titrated with a series
of concentrations of ClO- (1.00 × 10–6 – 7.00 × 10–3 Mol L-1) in
CH3OH-water solution (7:3, v/v)
which is shown in Figure 7. Since the error bars are too small to show,
we provided RSD data in table S4. The linear fitting curve is as
FIGURE 7 The linear fitting curve of
fluorescence intensities of L-Cu2+ and a
series of concentrations of ClO-

LIU ET AL.
5
follows:
F
=
11.55
+
0.49 [ClO−] (1.00
×
10−6 mol L−1
–
equilibrium. In other words, the probe has better sensitivity for ClO−
in lower concentrations than higher ones.
7.47 × 10−4 mol L−1) contained the coefficient of association (R) of
0.9992 (R2 0.057 [ClO−]
0.9984) and 334.27
=
F
=
+
(7.47 × 10−4 mol L−1 – 8.00 × 10−3 mol L−1) with the coefficient of
association (R) of 0.9956 (R2 = 0.9889). According to the equation
3.7 | Selectivity and competition
LOD = (3σ/s), LOD for ClO− is calculated to be 5.7 × 10−7 mol L−1
.
There are two different linear relationships here, probably because
that in the lower concentration range of ClO−, the concentration of
probe is much higher for ClO− to achieve equilibrium easily, while in
the higher concentration range of ClO−, the concentration of probe is
insufficient to some extent, so we need much more ClO− to achieve
This probe has a specific response to ClO−. We used 50 μM ClO− or
100 μM other species (K+, Na+, Ca+, F−, Cl−, Br−, HCO3−, CO3
,
2−
NO3−, SO42−, Fe3+, MnO4− and H2O2) for fluorescence recovery of L-
Cu2+, as shown in Figure 8, only ClO− restored fluorescence at
460 nm and other ions restored fluorescence at 490 nm because of
coordination between ions. Meanwhile, the competition experiment
shows that the sensor has satisfactory anti-interference performance,
which is shown in Figure 9.
3.8 | Compare with other hypochlorite fluorescence
probes
We compared this work with several hypochlorite fluorescence pro-
bes which published in recent years and the consequences were listed
in Table 1. There is indistinctive difference between this work and
other hypochlorite probes in detection limit, but it has obvious advan-
tages in linear range and detection time for this work. Therefore, this
work has certain potential application value.
3.9 | Actual sample detection and mechanism
In the interest of studying the feasibility of this procedure when
applied to practical samples, the probe is used in running water and
lake water. In running water the concentration of hypochlorite ions
measured is 3.8 × 10−6 mol L−1 with RSD of 1.49%. It's reliable
according to National standard for water quality of the People's
FIGURE 8 The fluorescence recovery of L-Cu2+ after adding
50 μM ClO- or 100 μM other species (K+, Na+, ca+, F-, cl-, Br-,
HCO3-, CO32-, NO3-, SO42-, Fe3+, MnO4- and H2O2) in CH3OH-
water solution (7:3,v/v)
FIGURE 9 In the presence of other species
(100 μM), the fluorescence intensity
(λem = 460 nm) of L-Cu2+ with ClO- (100 μM,
black line) or without ClO- (red line) in CH3OH-
water solution (7:3, v/v)

6
LIU ET AL.
TABLE 1
Comparison of this work with other hypochlorite fluorescence probes
Probe
Response time
Detection limit
Linear range
Reference
[20]
60 s
0.035 μM
5.0–50.0 μM
[21]
10 s
0.45 μM
0.067 μM
0.37 μM
0–100 μM
0–150 μM
0–60 μM
[22]
60 s
[1]
300 s
[6]
20 s
0.11 μM
60–180 μM
2 s
0.57 μM
1–747 μM
This work
TABLE 2
Results of lake water sample determination
Sample
ClO− added/Mol L−1
5.00 × 10–5
ClO− found/Mol L−1
Recovery/%
113
RSD/%
0.91
Lake water
5.67 × 10–5
6.56 × 10−5
7.22 × 10−5
6.00 × 10−5
109
0.37
7.00 × 10−5
103
0.20
Republic of China. Since hypochlorite ions could not be detected in
the lake water, we carried out labeling recovery experiment on the
lake water samples and the consequences were listed in Table 2. It is
observed that the detection consequences acquired by the sensor are
in well agreement with the amount of hypochlorite ions added, and
the recovery rates are between 103% and 113%, with RSD of 0.20% -
0.91%. Thus, the sensor is applicable for detecting hypochlorite in
actual samples.

LIU ET AL.
7
SCHEME 2 Detection mechanism
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G. Sun, Analyst 2018, 143, 5834.
The credible mechanism obtained by ESI-MS of the interaction
between probe L-Cu2+ and hypochlorite is shown in Scheme 2.[1,23]
As shown in figure S4, the peak of Q1 + Na+ in mass spectra were at
284.0888 and the peak of Q2 + H+ in mass spectra were at 172.0880.
After the addition of hypochlorite, the C-N bond was cut and the cou-
marin was released, which restored the fluorescence intensity.
[8] L. C. Adam, I. Fabian, K. Suzuki, G. Gordon, Inorg. Chem. 1992, 31,
3534.
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H. Wang, L. Ji, H. Chao, Biomaterials 2015, 53, 285.
ꢀ
[10] M. Jovic, F. Cortés-Salazar, A. Lesch, V. Amstutz, H. Bi, H. H. Girault,
J. Electroanal. Chem. 2015, 756, 171.
[11] J. Zhang, X. Wang, X. Yang, Analyst 2012, 137, 2806.
[12] J. M. Boyd, S. E. Hrudey, X.-F. Li, S. D. Richardson, TrAC Trends Anal.
Chem. 2011, 30, 1410.
4
| CONCLUSIONS
[13] Y. Koide, Y. Urano, K. Hanaoka, T. Terai, T. Nagano, J. Am. Chem. Soc.
2011, 133, 5680.
All in all, a highly sensitive probe was synthesized to detect hypochlo-
rite specifically and quickly. When the probe reacted with hypochlo-
rite, it showed a large stokes shift, produced a strong fluorescence
recovery at 460 nm and the response time is as short as two seconds.
There was a linear fitting curve of the intensities of fluorescence
recovery and the concentrations of hypochlorite ranging from
[14] L.-L. Wang, J. Qiao, H.-H. Liu, J. Hao, L. Qi, X.-P. Zhou, D. Li, Z.-
X. Nie, L.-Q. Mao, Anal. Chem. 2014, 86, 9758.
[15] G. Chai, Q. Liu, Q. Fei, J. Zhang, X. Sun, H. Shan, G. Feng, Y. Huan,
J. Biol. Chem. Luminesc. 2018, 33, 153.
[16] J. Zhang, Y. Liu, Q. Fei, H. Shan, F. Chen, Q. Liu, G. Chai, G. Feng,
Y. Huan, Sens. Actuators B 2017, 239, 203.
[17] M. Guan, H. Mi, H. Xu, Q. Fei, H. Shan, Y. Huan, S. Lv, G. Feng,
J. Fluoresc. 2017, 27, 611.
1.00 × 10−6 to 7.47 × 10−4 mol L−1 with a LOD of 5.7 × 10−7 mol L−1
.
[18] Y. Ma, W. Luo, P. J. Quinn, Z. Liu, R. C. Hider, J. Med. Chem. 2004, 47,
6349.
Moreover, we successfully applied this method to environmental
water samples and achieved satisfactory results. Therefore, the probe
is a splendid hypochlorite sensor, which is characterized by rapid,
selective and sensitive features and can meet practical needs.
[19] H. Takechi, Y. Oda, N. Nishizono, K. Oda, M. Machida, Chem. Pharm.
Bull. 2000, 48, 1702.
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[21] Y. Z. HaohanSong, C. Xu, X. Wang, J. Zhang, Y. Wang, X. Liu, M. Guo,
X. Peng, Dyes Pigm. 2019, 162, 160.
ACKNOWLEDGMENTS
Thanks to the Natural Science Foundation of Jilin Province
(20200201238JC) for funding.
[22] L. Pang, Y. Zhou, W. Gao, H. Song, X. Wang, Y. Wang, RSC Adv. 2016,
6, 105795.
[23] D. X. Li, Y. Feng, J. Z. Lin, M. Chen, S. X. Wang, X. Wang, H. T. Sheng,
Z. L. Shao, M. Z. Zhu, X. M. Meng, Sens. Actuators B 2016, 222, 483.
ORCID
Yanfu Huan
https://orcid.org/0000-0002-4977-5179
SUPPORTING INFORMATION
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Additional supporting information may be found online in the
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How to cite this article: Liu R, Zhao Y, Cui X, et al. A turn-on
fluorescent probe based on quinoline and coumarin for rapid,
selective and sensitive detection of hypochlorite in water
samples. Luminescence. 2020;1–8. https://doi.org/10.1002/
bio.3882
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