Rapid and sensitive detection of hypochlorite in ~100% aqueous solution using a bithiophene-based fluorescent sensor: Application to water analysis and live-cell imaging

Article abstract of DOI:10.1016/j.molliq.2020.114396

A new bithiophene-based biocompatible fluorescent and colorimetric sensor 2TD capable of detecting hypochlorite (ClO^?) in enviro/biosystem was successfully synthesized. This sensor 2TD underwent a highly specific and sensitive oxidation reaction with ClO^? and produced a bithiophene aldehyde (2T-CHO) emitting strong blue fluorescence, which was strongly confirmed by ¹H NMR, HRMS, FTIR and DFT calculation. The 2TD for detecting ClO^? displays an ultra-fast response (25 s), great water-solubility (~100% aqueous solution), wide pH working range (7–11), excellent anti-interference capability, and ultra-sensitivity with low detection limit of 8.3 nM. Colorimetric test strips demonstrate that 2TD can be utilized as a cost-effective and efficient solid-state sensor for rapidly and conveniently sensing ClO^? with great sensitivity in practical applications. With the robust applicability, the 2TD was successfully utilized to determine/image ClO^? in environmental water and live-cells. More interestingly, this developed sensor 2TD could be used as an efficient test tool for ClO^? sensing.

Full text of DOI:10.1016/j.molliq.2020.114396

Journal of Molecular Liquids 320 (2020) 114396
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Rapid and sensitive detection of hypochlorite in ~100% aqueous solution
using a bithiophene-based fluorescent sensor: Application to water
analysis and live-cell imaging
⁎
Chunpeng Li, Pengcheng Yin, Tianduo Li, Tao Wei, Tingting Hu, Jianbin Chen, Xuyang Qin, Qingfen Niu
Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Chemical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353,
People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2020
Received in revised form 8 September 2020
Accepted 21 September 2020
Available online 28 September 2020
A new bithiophene-based biocompatible fluorescent and colorimetric sensor 2TD capable of detecting hypochlo-
rite (ClO−) in enviro/biosystem was successfully synthesized. This sensor 2TD underwent a highly specific and
sensitive oxidation reaction with ClO− and produced a bithiophene aldehyde (2T-CHO) emitting strong blue
fluorescence, which was strongly confirmed by 1H NMR, HRMS, FTIR and DFT calculation. The 2TD for detecting
ClO− displays an ultra-fast response (25 s), great water-solubility (~100% aqueous solution), wide pH working
range (7–11), excellent anti-interference capability, and ultra-sensitivity with low detection limit of 8.3 nM. Col-
orimetric test strips demonstrate that 2TD can be utilized as a cost-effective and efficient solid-state sensor for
rapidly and conveniently sensing ClO− with great sensitivity in practical applications. With the robust applicabil-
ity, the 2TD was successfully utilized to determine/image ClO− in environmental water and live-cells. More inter-
estingly, this developed sensor 2TD could be used as an efficient test tool for ClO− sensing.
Keywords:
Fluorescent sensor
Hypochlorite
Water analysis
Bioimaging
Solid-state sensor
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
advantages of high sensitivity, easiness of manipulating, real-time imag-
ing, excellent spatial-temporal sampling capability for nondestructive
Hypochlorite (ClO−), as a vital reactive oxygen species (ROS) in liv-
ing system, plays pivotal roles in various physi/pathological processes
[1–7]. Research has found that biological ClO− is mainly produced by
oxygen during electron transfer process, which could not only act as a
microbicidal mediator to kill many harmful pathogens and bacteria dur-
ing the human immune defense process, but also is an indispensable
disinfection in drinking water and household solutions widely used in
our daily life [8–12]. However, the excessive ClO− in living organisms
can not only bring about oxidative stress and thus induced the oxidative
damage via the oxidation of DNA, lipids as well as proteins, but also re-
sult in a number of serious inflammation-associated diseases such as
atherosclerosis, arthritis, cardiovascular diseases, neuro degeneration
and even cancer [2,13–18]. Presently, the recognizing and detecting
ClO− has become a challenging frontier in the development of chemis-
try. Hence, developing an efficient and convenient method for qualita-
tive, quantitative, and sensitive detection of ClO− in environmental
and biological systems is of great significance.
detection [19–30]. To date, a great deal of ClO−-selective small fluores-
cent sensors have been developed [31–36]. However, to some extent,
some previous sensors for sensing ClO− are limited by fluorescence
quenching, poor selectivity, slow response, and poor aqueous solubility,
hampering their practical application for the real-time detection. Conse-
quently, it is urgently needed to develop efficient fluorescent sensors for
ultraselective, ultrasensitive and quantitative detection of ClO− with ex-
cellent sensing properties and multiple applications.
In continuation to our interest on the development of fluorescent
sensors for various analytes [37–51], in this study, a new bithiophene-
diaminomaleonitrile-derived fluorescent sensor 2TD has been synthe-
sized (Scheme 1), which was used for the specific identification of
ClO− with fluorimetric-colorimetric dual-channel, ultrafast response,
great aqueous solubility, excellent anti-interference capability, and
ultrasensitivity in enviro/biosystem. Colorimetric test strips confirmed
that 2TD could be utilized as a cost-effective solid-state sensor for the
rapid and convenient detection of ClO− with great sensitivity in practi-
cal applications. Moreover, sensor 2TD was not only successfully used to
detect real environmental water sample analysis, but also utilized for
bioimaging in living HeLa cells. Furthermore, the 2TD served as a good
fluorescent display material and the filter paper by letting 2TD could
be developed to prepare an ink-free writable paper by using ClO− as
the sole trigger.
Small fluorescent sensor is essential molecular tool and attracts
much more attention for detecting ClO−, in terms of its prominent
⁎
Corresponding author.
E-mail address: qf_niu1216@qlu.edu.cn (Q. Niu).
https://doi.org/10.1016/j.molliq.2020.114396
0167-7322/© 2020 Elsevier B.V. All rights reserved.

C. Li, P. Yin, T. Li et al.
Journal of Molecular Liquids 320 (2020) 114396
Scheme 1. The design and synthesis of sensor 2TD.
2. Experimental section
(DMSO/H2O, 1:99, v/v). Stock solutions (10 mM) of various anions
(ClO−, Br−, F−, Cl−, ClO4−, HCO3−, CO32−, PO34−, SO42−, NO3−, NO−2
,
2.1. Instruments and chemicals
H2PO4−, HPO24−, S2O32−, AcO−), metal cations (Al3+, Cr3+, Fe3+, Ni2+
,
Pb2+, Cu2+, Hg2+) and various reactive ROS or RNS species (H2O2,
ONOO−, •OH, •NO, ROO•) were dissolved in deionized water and used
freshly. All spectroscopic measurements were measured at room tem-
perature in ~100% aqueous solution (DMSO/H2O, 1/99, v/v) with pH 7.
The spectra of absorption, fluorescence, NMR, FTIR, and HRMS were
completed by the spectrometer of Shimadzu UV-2600, Hitachi F-4600,
Bruker AV-400, Bruker ALPHA FT-IR, and Q-TOF LC/MS Agilent 6510, re-
spectively. The absolute fluorescence quantum yields (ΦF) were ob-
tained by an Edinburgh FLS1000 spectrophotometer. The pH value
was determined by PHS-25 pH-meter. MTT assay was tested by micro-
plate reader (Thermo Scientific, USA), and fluorescence images were
obtained by confocal laser scanning microscope (CLSM) Leica TCS SP8
with a 63× magnification target oil lens. The DFT studies at the B3LYP/
6-311G(d) level were performed via Gaussian 09 package. All the re-
agents involved in the experiments are analytically pure, which are pur-
chased from commercial suppliers (Adamas, China) and were used
directly. 2,2′-Bithiophene (2T) and [2,2′-bithiophene]-5-carbaldehyde
(2T-CHO) were synthesized through the previous method [52,53]. The
ONOO−, ROO•, NO• and •OH were prepared according to the previous
method [5,54]. Deionized water was used throughout the test process.
For fluorescence measurements, the slits for excitation and emission
were set at 5/5 nm, respectively, the voltage was set at 500 V, the scan
rate was set at 2400 nm/min, and the excitation wavelength was set
at 350 nm.
2.4. Cell incubation and fluorescence imaging
The HeLa cells were firstly seeded at a 96-well plate at 37 °C for 24 h,
and then were treated with 10 μM sensor 2TD for 60 min. After the cells
being washed for 3 times with PBS buffer, and further incubated with
30 μM ClO− for 30 min. Finally, the HeLa cell imaging was achieved by
using CLSM under the 430–470 nm blue channel with an excitation at
405 nm.
3. Results and discussion
3.1. Selectivity and anti-interference studies
The sensor 2TD itself is water-insoluble, the effect of solvent (differ-
ent ratios between DMSO and H2O) on the fluorescence spectra was
firstly explored (Fig. S5), and the result indicated that the DMSO/H2O
(1/99, v/v) solution is the best detection media for all the sensing exper-
iments. To evaluate the selectivity of sensor 2TD to ClO−, the absorption
and fluorescence spectra of 10 μM 2TD were performed upon treating
with 20 μM various biological analytes in ~100% aqueous solution
(DMSO/H2O, 1/99, v/v). The anions including ClO−, Br−, F−, Cl−, ClO4−,
2.2. Synthesis of 2-(-([2,2′-bithiophen]-5-ylmethylene)amino)-3-
aminomale-onitrile (2TD)
A mixture of compound 2T-CHO (388 mg, 2.0 mmol) and
diaminomaleonitrile (DAMN) (250 mg, 2.3 mmol) in dry EtOH
(15 mL) was added catalytic amount of acetic acid glacial (2 drops).
The above reaction mixture was stirred for 4 h under reflux. Upon com-
pletion, the orange precipitate was filtered-off, washed with ethanol
and dried in vacuum to produce the final compound 2TD (511 mg,
yield: 90.1%). 1H NMR (400 MHz, DMSO‑d6, ppm): δ = 8.40 (s, 1H),
7.77 (s, 2H), 7.72 (d, J = 4.0 Hz, 1H), 7.64 (d, J = 4.0 Hz, 1H), 7.45 (d,
J = 4.0 Hz, 1H), 7.42 (d, J = 4.0 Hz, 1H), 7.16 (t, J = 4.0 Hz, 1H)
(Fig. S1); 13C NMR (100 MHz, DMSO‑d6, ppm): δ = 149.1, 142.4,
140.5, 135.6, 129.3, 127.7, 126.4, 126.0, 125.5, 115.0, 114.3, 103.4
(Fig. S2); FTIR (KBr, cm−1): v = 3455, 3306 (NH2), 2207 (C`N), 1600
(C=N) (Fig. S3); HRMS (ESI) m/z calcd for C13H8N4S2: 284.0190;
Found: 285.0247, [M+H]+ (Fig. S4).
HCO−3 , CO32−, PO43−, SO24−, NO−3 , NO−2 , H2PO−4 , HPO42−, S2O23−, AcO−
,
metal ions including (Al3+, Cr3+, Fe3+, Ni2+, Pb2+, Cu2+, Hg2+), and re-
active ROS or RNS species including H2O2, ONOO−, •OH, ROO•, and •NO,
were selected for the selectivity and competition tests, because some
oxidizing and reducing species as well as some metal ions may have
some effects on the sensor 2TD through a redox or coordination reac-
tion. As depicted in Fig. 1a, upon adding ClO−, the main absorption
peak at 378 nm disappeared, which was accompanied by a new absorp-
tion peak at 350 nm, with obvious yellow color change to colorless,
while other tested analytes showed negligible response, suggesting
that 2TD had a highly specific selectivity for ClO− over other tested spe-
cies. Under the same conditions, the selectivity of the 2TD towards ClO−
is further evaluated by fluorescence response. As displayed in Fig. 1b,
the free 2TD exhibited almost no fluorescence (ФF = 12.47%) after exci-
tation at 350 nm, only the introduction of ClO− resulted in a notable
fluorescence enhancement at 423 nm (ФF = 20.09%) along with obvi-
ous fluorescence color change, indicating that 2TD can effectively and
selectively detect ClO−.
2.3. Spectral measurements
A stock solution of sensor 2TD (1 mM) was prepared in DMSO, and
then the sensor 2TD was diluted to 10 μM with ~100% aqueous solution
2

C. Li, P. Yin, T. Li et al.
Journal of Molecular Liquids 320 (2020) 114396
competitive experiment was conducted by adding 10 equiv. of ClO− to
sensor 2TD in the presence of 10 equiv. of other competitive active
analytes, including common anions (ClO−, Br−, F−, Cl−, ClO−4 , HCO−3
,
CO23−, PO34−, SO24−, NO3−, NO2−, H2PO4−, HPO24−, S2O32−, AcO−), metal cat-
ions (Al3+, Cr3+, Fe3+, Ni2+, Pb2+, Cu2+, Hg2+) and reactive ROS or
RNS species (H2O2, ONOO−, •OH, ROO• and •NO) [55]. As displayed in
Fig. 1c, a dramatic fluorescence intensity enhancement at 423 nm
0.30
378 nm
350 nm
(a)
0.25
0.20
0.15
0.10
0.05
0.00
2.0 equiv.
-
ClO
0
320
360
400
440
480
Wavelength/nm
1200
900
600
300
0
(b)
2.0 equiv.
-
ClO
0
350
400
450
500
550
Wavelength/nm
1400
1200
1000
800
600
400
200
0
Equation
Weight
Residual Sum of
y = a + b*x
Instrumental
440.91562
Squares
Pearson's r
Adj. R-Square
0.99584
0.99066
Value
Standard Error
18.78763
B
B
Intercept
Slope
-61.9225
132.6753
4.29106
(c)
Fig. 1. (a) UV–Vis and (b) fluorescence spectral change of 10 μM 2TD in ~100%
aqueous media when treated with 20 μM tested analytes, insets: photographs
showing visual color change for various tested analytes under sunlight and 365
illumination (1–28, free 2TD, ClO−, Br−, F−, Cl−, ClO−4 , CO23−, HCO3−, PO43−, SO24−
,
,
NO3−, NO−2 , H2PO−4 , HPO42−, S2O32−, AcO−, H2O2, ONOO−, •OH, •NO, ROO•, Al3+, Cr3+
Fe3+, Ni2+, Pb2+, Cu2+, Hg2+); (c) fluorescence intensity of 2TD (10 μM) exposed
to various analytes (100 μM) and to the mixture of ClO− (100 μM) in ~100%
aqueous solution (λem = 423 nm).
0
2
4
6
8
10
12
14
16
-6
-
CIO (10 mol/L)
Fig. 2. Absorption (a) and fluorescence (b) spectral changes of 10 μM 2TD in ~100%
aqueous solution towards various ClO− concentrations (0–20 μM); (c) the visual
fluorescence variation of 10 μM 2TD in ~100% aqueous solution with an increasing of
ClO− concentration.
To verify whether many other environmentally and physiologically
relevant analytes could cause interference for ClO− detection and further
confirm its specific selectivity in complex physiological environments, the
3

C. Li, P. Yin, T. Li et al.
Journal of Molecular Liquids 320 (2020) 114396
Scheme 2. The proposed mechanism of the sensor 2TD sensing ClO−
.
occurred after treating with ClO− coexisting with other various poten-
tially interfering substances, and almost no remarkable fluorescence
change was observed when other bio-active analytes coexisted. Thus,
these results demonstrated that sensor 2TD has good capable of anti-
interference capability, and could be utilized to detect ClO− in a compet-
itive environment.
noting that the emission intensity increases slowly and gradually as
the continuing ClO− concentration increases, and reaches equilibrium
until 10 μM ClO−, instead of the sharp linear enhancement, implying
that the reaction mole ratio between 2TD and ClO− is 1:1. Besides,
this 1:1 stoichiometric mole ratio is also confirmed by Job's plot analysis
(Fig. S6). Moreover, seen from Fig. 2c, the emission intensity (423 nm)
showed an excellent linearity related to the ClO− concentration
(0–10 μM) (R2 = 0.993), along with a highly sensitive color change,
and the obtained detection limit (DL) was 8.3 nM based on 3σ/k [56],
superior to that of the previously reported [57–61]. These illustrated
that sensor 2TD shows excellent capability for sensitively and quantita-
tively determining ClO− in ~100% aqueous solution.
3.2. ClO− sensing properties
To investigate the quantitative relationship between 2TD and ClO−,
absorption and fluorescence titration experiments were conducted in
~100% aqueous solution (DMSO/H2O, 1/99, v/v) under the conditions
with the concentration of 2TD (10 μM) and variable ClO− concentra-
tions from 0 to 20 μM. As indicated in Fig. 2a, 2TD displayed a decreasing
absorption peak at 378 nm and an emerging peak at 350 nm gradually
enhanced with increasing ClO−, suggesting the occurrence of an oxida-
tion reaction between 2TD and ClO−. Meanwhile, as demonstrated in
Fig. 2b, the fluorescence intensity at 423 nm enhanced greatly with
the continued increase of ClO−, and a 23-fold enhancement is observed
when the amount of ClO− concentration reached around 10 μM, which
accompanied by the ФF increase from 12.47% to 20.09%. It is worth
3.3. Sensing mechanism and DFT studies
The sensor 2TD has no fluorescence ascribing to the isomerization of
C_N, which is a predominant decay process of excited states [12,62].
When treated with the high oxidizing ability of ClO−, it will bring out
the interrupting hydrazone bond in 2TD under the combined action of
H2O molecules, DAMN can be removed and accompanied by the gener-
ation of fluorophore 2T-CHO, thus the blue fluorescence switched on
12
10
8
6
Chemical shift/ppm
Fig. 3. 1H NMR spectra of free 2TD, 2TD treated with ClO− (2TD + ClO−) and free 2T-CHO in DMSO‑d6.
4

C. Li, P. Yin, T. Li et al.
Journal of Molecular Liquids 320 (2020) 114396
(Scheme 2), which is similar to those previously reported sensors
[13,63,64].
(3.024 eV) was lower than that of 2T-CHO (3.697 eV), indicating that
a strong ICT occurred throughout the π-conjugated structure of 2TD.
In addition, the absorption peak at 343 nm of 2T-CHO was well consis-
tent with the experimental data of 354 nm.
The 1H NMR spectra were firstly performed to investigate the reac-
tion mechanism of chemosensor 2TD with ClO−. As presented in
Fig. 3, after treating with ClO−, a new singlet proton peak emerged at
10.31 ppm was assigned to the aldehyde (HC=O) proton signal, imply-
ing the production of cleavage product aldehyde. It was interesting to
3.4. Time response and pH study
note that the 1H NMR spectrum of the sensing system (2TD-ClO−
)
The ClO− is well-known to have a quite short lifetime in biological
applications, therefore, the fast-response time for real-time detection
was highly desired. The influence of time on the fluorescence intensity
(423 nm) change of 2TD (10 μM) in the absence and presence of 2.0
equiv. of ClO− was examined in ~100% aqueous solution (DMSO/H2O,
1/99, v/v). The fluorescence intensity at 423 nm of the 2TD showed no
significant variation with increasing time up to 12 days, along with no
distinct solution color change (Fig. S9), suggesting sensor 2TD in the so-
lution had excellent stability. Fig. 5a revealed that after treated with
ClO−, a significantly enhanced fluorescence was displayed, and the ob-
served fluorescent signals arrived at the maximum and plateaued as
ultra-fast as 25 s, confirmed the sensor 2TD was capable of realizing
its practical application in real-time detection of cellular ClO−.
was identified to that of 2T-CHO. Subsequently, the HRMS (Fig. S7)
showed that after 2TD was treated with ClO−, a distinct mass peak at
m/z 191.9481 (calcd: 191.9460) was observed that is assigned to the ex-
pected product aldehyde [M−2H]−. In addition, this reaction product
was further supported by FTIR spectra (Fig. S8). When 2TD treated
with ClO−, the NH2 and C`N stretching bands at 3455, 3302 cm−1
and 2207 cm−1 completely disappeared, accompanied by a new strong
typical peak emerged at 1650 cm−1 corresponding to an aldehyde
(HC=O) group. These proofs strongly proved the reaction mechanism
that was the ClO−-provoked oxidation-hydrolysis reaction and forma-
tion of bithiophene aldehyde.
Subsequently, to further study the optical properties of 2TD and 2T-
CHO, the TD-DFT studies were used to calculate the frontier molecular
orbitals of 2TD and 2T-CHO, as depicted in Fig. 4. The highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of 2T-CHO are located on the entire molecular skeleton. How-
ever, the HOMO of 2TD was spread over the diaminomaleonitrile and
bithiophene units within the π-conjugated configuration, whereas its
LUMO was located on the DAMN moiety. However, when 2TD was
transformed to 2T-CHO, the ground state energy gap of 2TD
The pH range usually affects the detection accuracy of
a
chemosensor. Thus, it is great essential to study pH effects on identifica-
tion of ClO−. As shown in Fig. 5b, almost no perceptible effects on the
fluorescence intensity at 423 nm of sensor 2TD during a wide pH
range 1–13, implied that no disturbance from the variation of pH.
When treated with ClO−, the fluorescence intensity at 423 nm dramat-
ically enhanced and kept stable within the wide pH 7–11, which may be
explained by that the reaction is completely transformed to the ICT
Fig. 4. The investigated molecular orbitals of 2TD and 2T-CHO.
5

C. Li, P. Yin, T. Li et al.
Journal of Molecular Liquids 320 (2020) 114396
2TD+ClO-
their detection reliabilities was presented in Fig. S10 and Table 1, re-
spectively. The high recoveries for the addition ranged from 98.2% to
101.8% and the RSD values were calculated below 3%, which are favor-
ably compared with those obtained by standard methods in the litera-
ture [66,67]. These satisfactory results demonstrated that sensor 2TD
showed good reliability to sensitively detect and monitor trace levels
of ClO− in practical water samples for practical applications.
1400
(a)
1200
1000
800
3.6. Bio-imaging of ClO− in living cells
600
To investigate its biological application, the fluorescence imaging
of 2TD for detecting ClO− in live cells was performed. Before imag-
ing, the cytotoxic behaviour of 2TD was tested through the MTT
assay with HeLa cells for 24 h (Fig. 6a). The results showed that the
93% HeLa cells can still keep alive at 30 μM, indicated that the sensor
2TD has no obvious cytotoxicity and was relatively safe in bio-
imaging application. The cell imaging test was then performed. The
HeLa cells only incubated with 10 μM 2TD for 60 min caused almost
no observed fluorescence (Fig. 6b, c). By contrast, when the HeLa
cells were treated with ClO− (30 μM) for 10 min, a prominent blue
fluorescence was clearly found (Fig. 6d). And the quantification of
fluorescence intensity was calculated to be 25.56 a.u., which is ca.
21-fold enhancement compared with that of 2TD (Fig. S11). There-
fore, the cell imaging experimental results confirmed that sensor
2TD was capable of imaging ClO− in living HeLa cells.
400
200
0
5
10 15 20 25 30 35 40 45
Time/s
1600
1400
1200
1000
800
600
400
200
0
2TD+ClO-
2TD
(b)
3.7. Application as solid-state sensor for sensing ClO−
To further explore its application for visual, real-time and on-site
monitoring ClO− in environmental system, the usefulness of the 2TD
as a solid-state sensor for sensing ClO− in coated filter papers was tested
(SI). When the 2TD-loaded test strips were immersed into water con-
taining an increasing amount of ClO− for several minutes, rapid and sig-
nificant visual color changes were clearly detected by naked eyes
(Fig. 7), confirming that 2TD could be used as a cost-effective solid-
state sensor for the rapid and convenient sensing ClO− with great sen-
sitivity in practical applications.
0
2
4
6
8
10
12
14
pH
Fig. 5. The influence of time (a) or pH (b) on the fluorescence intensity (423 nm) change of
2TD (10 μM) in the absence and presence of 2.0 equiv. of ClO−
.
3.8. Application as fluorescent display material
fluorescent molecule 2T-CHO. While, with the increase of alkalinity
(pH 11–13), the fluorescence intensity at 423 nm decreased obviously,
which may be attributed to the hydrolysis reaction and the stronger ox-
idation properties of the ClO− under the acidic condition [65]. There-
fore, the investigated results revealed that sensor 2TD can be suitable
for biological and pathological tests (pH 7–11).
Interestingly, due to the above good fluorescent sensing perfor-
mance, sensor 2TD could serve as a rewritable small-molecular smart
fluorescent display material. As displayed in Fig. 8a, when the sensor
2TD (10 μM) was loaded into a pen without any modification and was
used to write text, Chinese characters and patterns on the filter papers,
obvious yellow text, characters (such as “Qilu University of Technol-
ogy”) and patterns on the handwritten filter paper can be easily and
clearly detected under sunlight. As shown in Fig. 8b, when writing on
the 2TD-based filter papers with a pen loaded ClO− aqueous solution
(10 μM), bright blue fluorescent text, characters (such as “Qilu Univer-
sity of Technology”) and fluorescent patterns on the handwritten filter
paper were obviously observed under 365 nm UV light, which showed
significant color change for ClO− and displayed highly distinct from
the background, indicating that 2TD could be used as a conveniently
sensing ClO− test tool and as a good fluorescent display material. There-
fore, developing this type fluorescent display material is more advanta-
geous and convenient for sensing ClO−.
3.5. Real water sample test
To validate the practicality, the sensor 2TD was applied for detecting
and monitoring the levels of ClO− in real water samples using standard
addition method. With the adding an identified amount of standard
ClO− (5.0 and 10.0 μM) to each sample, the fluorescence spectra and
Table 1
Detecting ClO− in four water samples.
Sample
Added
(μM)
Detect
Recovery Relative
RSD
(%)
(x
SD)
(%)
error (%)
(μM)
3.9. Comparison with other sensors
Tap water
5.0
10.0
5.0
10.0
5.0
4.91
10.16
4.95
10.15
4.92
10.17
5.09
0.12
0.15
0.11
0.14
0.11
0.14
0.12
0.15
98.2
101.6
99
101.5
98.4
101.7
101.8
101.4
1.8
1.6
1
1.5
1.6
1.7
1.8
1.4
2.4
1.5
2.2
1.4
2.2
1.4
2.4
1.5
Compared with previous ClO−-specific fluorescent sensors
(Table S1) [68–74], our developed sensor 2TD showed several ad-
vantages: (i) simple structure and worked in ~100% aqueous media
with dual responses; (ii) ultra-sensitive (8.3 nM) and ultra-rapid
(25 s) detecting and monitoring trace levels of ClO−; (iii) multiple
applications in real water sample analysis, test strip, cell imaging,
River water
Distilled water
10.0
5.0
10.0
Lake water of Ji'nan
Garden Expo
10.14
6

C. Li, P. Yin, T. Li et al.
Journal of Molecular Liquids 320 (2020) 114396
Fig. 6. (a) Cell viability of cells treated with varied 2TD for 24 h; Fluorescence images of HeLa cells treated with 10 μM 2TD (b, c), and further treated with 30 μM ClO− for 10 min (d). λex
405 nm, λem = 430–470 nm.
=
and fluorescent display, which provided proofs for good idea of inte-
grating multiple functions into one sensor molecule. Although two
sensors were reported with shorter reaction time, the DL was higher
[71,72]. It indicated that our synthesized sensor could be used as a
efficient solid-state sensor for rapid and convenient detecting ClO−
with great sensitivity in practical applications. Sensor 2TD has excel-
lent practicability of great water solubility, negligible cytotoxicity
and good biocompatibility, and applied to sensitively monitor ClO−
in environmental samples as well as the living HeLa cells. Moreover,
the sensor 2TD could be utilized as a good fluorescent display mate-
rial and an efficient ClO−-sensing test tool.
rapid and sensitive analytical method for detecting ClO−
.
4. Conclusions
CRediT authorship contribution statement
In summary, a new bithiophene-based sensor 2TD was success-
fully designed and synthesized, which showed unique optical selec-
tivity for ClO− in ~100% aqueous solution with colorimetric and
fluorescent turn-on dual responses. Sensor 2TD for ClO− sensing
showed ultra-fast response, wide pH working range, ultra-
sensitivity, as well as strong anti-interference capability. The sensing
mechanism was proposed and was well confirmed by optical spectra,
1H NMR, HRMS, FTIR and TD-DFT studies. Colorimetric test strips
demonstrated that the 2TD can be utilized as a cost-effective and
Chunpeng Li: Conceptualization, Data curation, Software, Investiga-
tion, Writing - original draft. Pengcheng Yin: Conceptualization, Data
curation, Formal analysis. Tianduo Li: Resources, Formal analysis, Writ-
ing - review & editing. Tao Wei: Formal analysis, Writing - review &
editing. Tingting Hu: Writing - review & editing. Jianbin Chen: Writing
- review & editing. Xuyang Qin: Writing - review & editing. Qingfen
Niu: Resources, Writing - review & editing, Supervision, Data curation.
Fig. 7. Photograph of test strips of 2TD with increasing ClO− concentrations.
7

C. Li, P. Yin, T. Li et al.
Journal of Molecular Liquids 320 (2020) 114396
Fig. 8. (a) The photographic images of the colorimetric patterns/text written on a 2TD (10 μM) loaded filter paper under sunlight; (b) the photographic images of the fluorescent patterns/
text written on a 2TD-loaded filter paper using the ClO− aqueous solution (10 μM) under 365 illumination.
[3] Y. Koide, Y. Urano, S. Kenmoku, H. Kojima, T. Nagano, Design and synthesis of fluo-
Declaration of competing interest
rescent probes for selective detection of highly reactive oxygen species in mitochon-
dria of living cells, J. Am. Chem. Soc. 129 (2007) 10324–10325.
The authors declare no competing financial interest.
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(2011) 4783–4804.
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Acknowledgments
We greatly appreciate financial support from the National Natural
Science Foundation of China (21776143, 21801144, 51702178,
21376125), the Jinan Science & Technology Bureau (2019GXRC021),
the Natural Science Foundation of Shandong Province (ZR2017LB009),
the Program for College Students' Innovation and Entrepreneurship
Training of Shandong Province (S201910431035), the Youth Innovative
Talents Recruitment and Cultivation Program of Shandong Higher Edu-
cation, and the Program for Scientific Research Innovation Team in Col-
leges and Universities of Shandong Province.
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cies, J. Clin. Biochem. Nutr. 45 (2009) 111–124.
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Appendix A. Supplementary data
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Godschalk, F.J. Van Schooten, Genotoxic effects of neutrophils and hypochlorous
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Supplementary data to this article can be found online at https://doi.
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