Coumarin Thiourea-Based Fluorescent Turn-on Hg2+ Probe That Can Be Utilized in a Broad pH Range 1–11

Article abstract of DOI:10.1007/s10895-020-02517-y

A novel coumarin-thiourea conjugate was synthesized facilely. It served as a fluorescent turn-on chemosensor for selective detection of Hg²⁺ ion over other common competitive metal ions including Li⁺, Na⁺, K⁺, Ag⁺, Cu²⁺, Fe²⁺, Zn²⁺, Co²⁺, Ni²⁺, Mn²⁺, Sr²⁺, Ca²⁺, Mg²⁺, Al³⁺, Cr³⁺ and Fe³⁺ ions based on the Hg²⁺-promoted desulfurization and cyclization reactions. Addition of Hg²⁺ ion to the sensor solution in 2:8 EtOH/H₂O induced a hypsochromic shift of the UV–Vis absorption band from 360?nm to 340?nm accompanying distinct enhancement in the absorption intensity while addition of other metal ions failed to bring about substantial change in the absorption spectra. Addition of Hg²⁺ to the sensor solution also caused marked increase in the fluorescence emission intensity and most common competitive metal ions did not interfere with the selective sensing of Hg²⁺ ion by the sensor. The detection limit of Hg²⁺ ion by the probe was calculated to be 1.46 × 10^?7?M and the probe could be utilized for selective detection of Hg²⁺ ion by fluorescence turn-on mode over a broad pH range of 1–11.

Full text of DOI:10.1007/s10895-020-02517-y

Journal of Fluorescence
https://doi.org/10.1007/s10895-020-02517-y
ORIGINAL ARTICLE
2+
Coumarin Thiourea-Based Fluorescent Turn-on Hg Probe That Can
Be Utilized in a Broad pH Range 1–11
1
2
3
1
1
1
1
Zhixiu Pan & Zhenxiang Xu & Jie Chen
& Luping Hu & Hongqi Li & Xin Zhang & Xucheng Gao &
1
1
Mengxuan Wang & Jian Zhang
Received: 6 November 2019 /Accepted: 14 February 2020
Springer Science+Business Media, LLC, part of Springer Nature 2020
#
Abstract
A novel coumarin-thiourea conjugate was synthesized facilely. It served as a fluorescent turn-on chemosensor for selective
2
+
+
+
+
+
2+
2+
2+
2+
2+
detection of Hg ion over other common competitive metal ions including Li , Na , K , Ag , Cu , Fe , Zn , Co , Ni ,
2
+
2+
2+
2+
3+
3+
3+
2+
Mn , Sr , Ca , Mg , Al , Cr and Fe ions based on the Hg -promoted desulfurization and cyclization reactions.
2
+
Addition of Hg ion to the sensor solution in 2:8 EtOH/H O induced a hypsochromic shift of the UV–Vis absorption band
2
from 360 nm to 340 nm accompanying distinct enhancement in the absorption intensity while addition of other metal ions failed
2
+
to bring about substantial change in the absorption spectra. Addition of Hg to the sensor solution also caused marked increase in
2
+
the fluorescence emission intensity and most common competitive metal ions did not interfere with the selective sensing of Hg
2
+
−7
ion by the sensor. The detection limit of Hg ion by the probe was calculated to be 1.46 × 10 M and the probe could be utilized
2
+
for selective detection of Hg ion by fluorescence turn-on mode over a broad pH range of 1–11.
2
+
.
.
Keywords Coumarin Fluorescent probe Thiourea; Hg
2
+
Introduction
the use of fluorescent probes. A vast number of Hg ion
fluorescent probes have been constructed in the last two de-
cades from nanomaterials including quantum dots [2–5],
boron-doped graphitic carbon nitride [6], tungsten disulfide
nanosheets [7], noble metal nanoclusters (NCs) [8], and nano-
particles [9], or mostly from organic molecules by utilizing the
It has been well addressed that mercury is one of the most
toxic elements even at low concentrations and is not biode-
gradable. It may bring about serious health and safety prob-
lems to both human beings and environment [1]. Fast, highly
selective and sensitive detection of mercuric ion in various
kind of samples is of great significance in view of the diagno-
sis of diseases related to mercury poisoning and the monitor-
ing of mercury pollution in environment. One of the optimal
2
+
2+
specific reactivity of Hg including Hg -promoted
2
+
deprotection reactions of dithioacetals [10–12], Hg -cata-
lyzed devinylation reactions of vinyl ethers [13–15],
oxymercuration reactions based on the alkynophilicity of
2
+
2+
2+
choices for convenient and selective detection of Hg ion is
Hg ion [16–18], Hg -induced (thiophilic) hydrolysis reac-
2
+
tions [19–25], Hg -mediated desulfurization reactions of
thione compounds [26–28], and complexation with various
heteroatom ligands [29–43]. The majority of these fluorescent
Hg ion probes can only be utilized in approximately neutral
condition or a narrow pH range, not applicable in a broad pH
range especially strongly acidic condition (pH < 2). In this
respect Hg -promoted desulfurization and cyclization reac-
tions of thiourea compounds have been envisaged as an effec-
tive protocol for construction of Hg fluorescent probes with
*
*
Jie Chen
jiechen0922@outlook.com
2
+
Hongqi Li
hongqili@dhu.edu.cn
2
+
1
Key Laboratory of Science and Technology of Eco-Textiles, Ministry
of Education, College of Chemistry, Chemical Engineering &
Biotechnology, Donghua University, 2999 North Renmin Road,
Shanghai 201620, People’s Republic of China
2
+
relatively wide pH ranges, e.g. pH 6.84–9.37 [44], pH 5.81–9.04
2
3
[
45], pH 6.5–9.0 [46], pH 4–11 [47], and pH 5.0–11.0 [48]. A
Penglai Xinguang Pigment Chemical Co., Ltd, Penglai 265601,
People’s Republic of China
coumarin-thiourea conjugate behaves as a fluorescent probe for
Hg(II) with a broad pH range 2–12 [49]. However, the sensing
of Hg is based on the Hg -promoted desulfurization of the
School of Chemical and Material Engineering, Jiangnan University,
Wuxi 214122, People’s Republic of China
2
+
2+

J Fluoresc
probe, leading to a decrease in intramolecular charge transfer
ICT) character and resulting in fluorescence quenching which
hydroxycoumarin) was obtained by a two-step synthetic
protocol consisting of condensation and hydrolysis reac-
t i o n w i t h a c e t y l a m i n o a c e t i c a c i d a n d 2 , 4 -
dihydroxybenzaldehyde as the raw materials. Further re-
action between compound 2 and cyanic methacrylic
thioanhydride formed from potassium thiocyanate and
methacrylic chloride afforded probe D in 88% yield. The
chemical structure of probe D was fully characterized by
(
may be considerably influenced by multiple environmental fac-
tors like pH, medium polarity, temperature, and instrumental
settings [50]. Following our recent endeavors to develop fluo-
rescent chemosensors for selective and sensitive detection of
metal ions [51–55], we synthesize a new coumarin thiourea
derivative, which serves as a fluorescent turn-on probe for
2+
1
13
detection of Hg ion in aqueous solution with a broad pH range
–11. Herein we present the study progress on the new coumarin
H NMR (Fig. S1), C NMR (Fig. S2), FT-IR spectrum
1
(Fig. S3), and mass spectrum (Fig. S4). The data in the
spectra were in good accordance with the structure.
The detailed synthetic procedure of probe D was as fol-
lows: 3-acetylamino-7-acetyloxycoumarin (compound 1)
was prepared by the condensation reaction between 2,4-
dihydroxybenzaldehyde and acetylaminoacetic acid in acetic
anhydride according to the procedure reported in literature
2+
thiourea-based fluorescent probe for Hg ion.
Experimental
Chemicals and Reagents
[52]. Yellowish solid of compound 1 was obtained in 32%
The chemicals, reagents and solvents used in this work
were of analytical grade (99% purity) and were purchased
from Sinopharm Chemical Reagent Co., Ltd. They were
used without further purification unless otherwise ad-
dressed. Water for experiments was deionized prior to
use. Stock solutions of metal ions were prepared from
the salts LiCl, NaNO , KNO , AgNO , Cu(NO ) ,
yield. Mp 230–231 °C (lit. Value: 234–236 °C [56]). To a
three-neck flask were added compound 1 (2.63 g, 10 mmol),
ethanol (13 mL) and concentrated hydrochloric acid (26 mL).
The mixture was heated to 80 °C and stirred for 2 h. The
reaction mixture was poured into ice water (200 mL) and
30% NaOH was added to adjust the solution to neutral. A
large amount of brown solid precipitated and was obtained
by filtration. After drying under reduced pressure, the solid
was recrystallized with anhydrous ethanol to afford 1.41 g of
3-amino-7-hydroxycoumarin (compound 2) as brown powder
in 80% yield. Mp 238–240 °C (lit. Value: 237–238 °C [56]).
3
3
3
3 2
Fe(SO ) ·7H O, Zn(NO ) , CoCl ·6H O, NiCl ·6H O,
4
2
2
3 2
2
2
2
2
MnSO , SrCl ·6H O, Hg(OAc) , CaCl , MgCl ,
4
2
2
2
2
2
Al(NO ) ·9H O, CrCl ·6H O and Fe(NO ) ·9H O.
3
3
2
3
2
3 3
2
1
Instruments and Measurements
H NMR (400 MHz, DMSO-d ): δ 5.26 (s, 2H, NH ), 6.68–
6
2
6
.70 (m, 3H), 7.24 (d, J = 8.4 Hz, 1H), 9.85 (s, 1H, OH).
1
13
H NMR and C NMR spectra were measured on Bruker
Potassium thiocyanate (0.39 g, 4 mmol) and acetone
(20 mL) were added to a flask and was then heated to 60 °C.
After stirring for about 10 min methacrylic chloride (0.21 g,
2 mmol) was added and the mixture was stirred for 2 h. The
color of the solution gradually changed from milky white to
pale yellow. Then solution of compound 2 (0.089 g, 0.5 mmol)
in acetone (10 mL) was added and the reaction was monitored
by thin layer chromatography (TLC). The reaction completed
in 6 h and the mixture was allowed to cool to 25 °C. After
filtration the filtrate was concentrated and the residue was
ACF-500 spectrometer by using TMS as an internal stan-
dard. IR spectra (KBr pellet) were recorded on Bruker
Tensor 27 spectrophotometer. Mass spectra were measured
on a MS Agilent 1100 Series LC/MSD Trap mass spec-
trometer (ESI-MS). UV–Vis spectra were measured on a
Shimadzu UV-1800 spectrophotometer with a specification
of 190–1100 nm. Fluorescence spectra were measured on a
FS5 fluorescence spectrophotometer from Edinburgh
Instruments Ltd. The stock solution of probe D (20 μM)
was prepared by dissolving probe D in ethanol. The stock
solution of metal ions was prepared by dissolving the cor-
responding metal salts in deionized water. The measure-
ments of fluorescence spectra were carried out at an exci-
tation wavelength of 332 nm without use of the polarizer
and magic angle conditions with the quartz cuvettes thick-
ness being 1 cm. The emission spectra were recorded in a
range of 350–650 nm and the slit width was 8 nm.
recrystallized with DMF/H O (1:3, v/v) to give yellowish
2
solids which was further purified by column chromatography
with petroleum ether/ethyl acetate (5:1, v/v) as eluent. The
target probe D was obtained as yellowish powder (0.134 g)
1
in 88% yield. Mp 216–218 °C. H NMR (400 MHz, DMSO-
d ): δ 1.95 (s, 3H, CH ), 5.74 (s, 1H, C=CH ), 6.03 (s, 1H,
6
3
2
C=CH ), 6.90–6.66 (m, 2H), 7.58 (d, J = 8.3 Hz, 1H), 9.40 (s,
2
1H), 10.59 (s, 1H, OH), 11.27 (s, 1H, NH), 13.11 (s, 1H, NH).
1
3
C NMR (101 MHz, DMSO-d ): δ 18.56 (CH ), 102.63,
6
3
Synthesis and Characterization of Probe D
111.07, 114.35, 121.54, 125.33, 128.57, 130.25, 137.91,
52.52, 158.76, 161.02 (C=O), 169.84 (C=S), 177.85
(C=O). IR (KBr): ν 3302.57, 1709.44, 1599.26, 1522.26,
1
The novel coumarin thiourea-derived probe D was synthe-
sized as showed in Scheme 1. Compound 2 (3-amino-7-
−1
+
1339.50, 1180.05, 1104.96 cm . MS: m/z 305 (M + 1).

J Fluoresc
Scheme 1 Synthetic route to
probe D
O
N
H
CO H
H
N
2
CHO
OH
O
HCl/EtOH
0%
Ac O
2
O
8
3
2%
O
O
O
HO
HO
1
O
H
H
N
NH2
N
Cl KSCN
S
O
O
O
88%
HO
O
O
2
D
+
+
+
+
2+
2+
2+
2+
2+
Results and Discussion
ions (Li , Na , K , Ag , Cu , Fe , Zn , Co , Ni ,
2
+
2+
2+
2+
2+
3+
3+
3+
Mn , Sr , Hg , Ca , Mg , Al , Cr and Fe ) were
measured and showed in Fig. 3. It was visible that addition
Sensing Behavior of Probe D
2
+
of Hg ion induced hypsochromic shift of the absorption
band from 360 nm to 340 nm accompanying distinct en-
hancement in the absorption intensity. Addition of other
metal ions did not cause substantial change in the absorp-
tion band at 360 nm. Fluorescence emission spectra of
Probe D is not soluble in pure water and thus UV–Vis spectra
of D in mixed solvent consisting of ethanol and water with
different volume ratio (EtOH/H O = 9:1, 8:2, 7:3, 6:4, 5:5,
2
4
:6, 3:7, 2:8, 1:9) were measured and showed in Fig. 1.
Solution of probe D showed the maximum absorption band
at around 360 nm. The absorption intensity did not change
probe D solution (20 μM in 2:8 EtOH/H O) before and
after addition of different metal ions (Li , Na , K , Ag ,
2
+
+
+
+
2
+
2+
2+
2+
2+
2+
2+
2+
2+
substantially with variation in EtOH/H O ratio unless in 1:9
Cu , Fe , Zn , Co , Ni , Mn , Sr , Hg , Ca ,
2
2
+
3+
3+
3+
EtOH/H O the absorption intensity decreased to a half.
Mg , Al , Cr and Fe ) were measured at an excitation
wavelength of 332 nm as showed in Fig. 4. Addition of
2
Fluorescence spectra of probe D in mixed solvent
2
+
EtOH/H O (9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, v/v) were
Hg ion induced enhancement in the intensity of the fluo-
rescence emission band at 475 nm by 6 fold. Addition of
2
showed in Fig. 2. The maximum fluorescence emission ap-
+
2+
peared at 420 nm in 9:1 EtOH/H O and shifted to 470 nm with
Ag and Cu ion also caused enhancement in the intensity
of the fluorescence emission band at 475 nm by 2 and 1
2
the increase in water ratio to EtOH/H O = 8:2. Further in-
2
+
+
crease in water ratio led to larger shift to about 475 nm and
the maximum emission intensity appeared when the medium
fold, respectively. Addition of other metal ions (Li , Na ,
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
K , Fe , Zn , Co , Ni , Mn , Sr , Ca , Mg , Al ,
3
+
3+
was EtOH/H O = 2:8. Therefore the following sensing test
Cr and Fe ) to probe D did not bring about substantial
change in the fluorescence emission spectra. Thus D can be
used as a fluorescence turn-on sensor for selective detec-
2
experiments were carried out in EtOH/H O (2:8) medium.
2
UV–Vis spectra of probe D solution (20 μM in 2:8
2
+
EtOH/H O) before and after addition of different metal
tion of Hg ion.
2
9
8
7
6
5
4
3
2
:1
:2
:3
:4
:5
:6
:7
:8
200000
150000
100000
0.8
0.6
0.4
0.2
0.0
9:1
:2
7:3
8
9
:1
:9
9:1
6:4
5:5
4:6
3:7
2:8
1:9
8:2
1
1:9
50000
0
350
400
450
500
550
600
650
300
350
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
Fig. 2 Fluorescence emission spectra of probe D solution (20 μM) in
Fig. 1 UV–Vis spectra of probe D solution (20 μM) in different media
EtOH/H O = 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, v/v)
different media (EtOH/H
v/v) excited at 332 nm
2
O = 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9,
(
2

J Fluoresc
Fig. 5 Changes in fluorescence emission intensity at 475 nm (λex
=
2
+
3
32 nm) of probe D solution (20 μM) containing 1 equivalent of Hg
2
Fig. 3 UV–Vis spectra of probe D solution (20 μM in 2:8 EtOH/H O)
ion (red bars) before and after addition of 5 equivalents of different com-
+
+
+
+
2+
+
+
+
+
2+
2+
2+
2+
2+
before and after addition of different metal ions (Li , Na , K , Ag , Cu ,
petitive metal ions (Li , Na , K , Ag , Cu , Fe , Zn , Co , Ni ,
2
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
3+
2+
2+
2+
2+
3+
3+
3+
Fe , Zn , Co , Ni , Mn , Sr , Hg , Ca , Mg , Al , Cr and
Mn , Sr , Ca , Mg , Al , Cr and Fe ) or probe D solution
(20 μM) containing 1 equivalent of different competitive metal ions
3+
Fe , 20 μM) in Tris-HCl buffer (pH = 7.1)
(black bars)
In order to investigate the interference of competitive metal
2
+
2
+
concentrations of Hg (0, 2 μM, 4 μM, 6 μM, 8 μM, 10 μM,
2 μM, 14 μM, 16 μM, 18 μM and 20 μM) were measured
ions on selective sensing of Hg ion by probe D, changes in
fluorescence intensity at 475 nm of probe D upon addition of
1
2
+
and showed in Fig. 6. Gradual enhancement in the fluores-
cence emission intensity at 475 nm was found with increase in
the concentration of added Hg ion from 0 to 20 μM. Based
on the results, a plot was drawn from the variation of the
intensity of the fluorescence emission band at 475 nm with
the concentration of Hg ion (0, 2 μM, 4 μM, 6 μM, 8 μM,
0 μM, 12 μM, 14 μM, 16 μM, 18 μM and 20 μM) added to
probe D (20 μM) to give a line as showed in Fig. 7. The linear
fitting equation can be depicted as Y = 165,294 + 50,908X
with R = 0.9918. According to the formula L = 3S/K, where
L is the detection limit, S denotes the standard deviation of
fluorescence intensity of blank, and K is slope of the
Hg ion (1 equivalent) and/or other competitive metal ions (5
equivalents) were recorded and showed in Fig. 5. It was ob-
2
+
+
served that the coexistence of Ag ion had unfavorable influ-
2
+
ence on the selective detection of Hg ion by probe D. Other
competitive ions did not cause obvious interference with the
2
+
2
+
selective detection of Hg ion. Probe D exhibited good anti-
2
+
1
interference ability for selective sensing of Hg ion over other
common competitive metal ions.
Changes in fluorescence emission intensity at 475 nm
λex = 332 nm) of probe D (20 μM) upon addition of different
2
(
1
200000
0
2
4
6
8
1
1
1
1
µM
µM
µM
1000000
µM
µM
0µM
2µM
4µM
6µM
8
6
4
2
00000
00000
00000
00000
0
18µM
0µM
2
350
400
450
500
550
600
650
Wavelength(nm)
Fig. 4 Fluorescence emission spectra of probe D solution (20 μM in 2:8
+
+
EtOH/H
2
O) before and after addition of different metal ions (Li , Na ,
Fig. 6 Variation of fluorescence intensity at 475 nm of probe D (20 μM)
with increase of Hg concentration (0, 2 μM, 4 μM, 6 μM, 8 μM,
10 μM, 12 μM, 14 μM, 16 μM, 18 μM and 20 μM)
+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
2+
K , Ag , Cu , Fe , Zn , Co , Ni , Mn , Sr , Hg , Ca , Mg ,
3+
3+
3+
Al , Cr and Fe , 20 μM) excited at 332 nm

J Fluoresc
pH=12
pH=1
pH=2
pH=3
pH=4
pH=5
pH=6
pH=7
pH=8
pH=9
pH=10
pH=11
pH=12
pH=13
pH=14
2
000000
000000
0
pH=1
1
pH=2,3
pH=4-11
pH=13
pH=14
350
400
450
500
550
600
650
Wavelength(nm)
Fig. 9 Fluorescence emission intensity of probe D (20 μM) upon addition
of 1 equivalent of Hg ion at different pH (pH = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
Fig. 7 Relationship between the intensity of the fluorescence emission
2+
2+
band at 475 nm and the concentration of Hg ion (0, 2 μM, 4 μM, 6 μM,
μM, 10 μM, 12 μM, 14 μM, 16 μM, 18 μM and 20 μM) added to probe
11, 12, 13, 14)
8
D (20 μM)
2
+
(
Fig. 3). This system consisting of probe D and Hg ion
was stable and exhibited a relatively strong absorption at
40 nm under acidic conditions (pH = 1–6), neutral con-
calibration curve [19], the detection limit is calculated to be
3
.46 × 10− M. Therefore probe D can be used for selective
7
1
dition, and weakly alkaline conditions (pH = 8–10).
When the pH value increased to 11, a bathochromic shift
of the maximum UV-Vis absorption band from 340 nm
to around 385 nm was observed. Further increase of the
pH to 12 or more led to sharp decrease in the absorption
intensity. These results indicated that probe D was stable
under from relatively strong acidic conditions to fairly
strong alkaline conditions and might be utilized for se-
lective detection of Hg ion in a pH range of 1–11.
Fluorescence emission intensity of probe D (20 μM) up-
on addition of 1 equivalent of Hg ion at different pH
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14) showed that
the fluorescence intensity gradually decreased with the
pH value increased from 1 to 4 and maintained constant
in the pH range 5–10. When the pH value increased to
11, the fluorescence intensity increased sharply accompa-
nying a slight shift of the emission band from around
2
+
and sensitive detection of Hg ion in aqueous solution by
fluorescence turn-on mode.
To verify the applicable pH range of probe D for
selective sensing of Hg ion, variation of the maximum
UV-Vis absorption and fluorescence emission intensity of
2
+
2
+
probe D upon addition of 1 equivalent of Hg ion at
different pH (pH = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
1
4) were measured and the results were showed in
2+
Figs. 8 and 9, respectively. In the previous section it
was demonstrated that probe D itself showed the maxi-
mum UV-Vis absorption band at 360 nm and addition of
2+
(
2
+
Hg ion caused a hypsochromic shift of about 20 nm
pH=1
pH=2
pH=3
pH=4
pH=5
pH=6
pH=7
pH=8
pH=9
pH=10
pH=11
pH=12
pH=13
pH=14
0
0
0
0
0
.8
.6
.4
.2
.0
pH=11
4
75 nm to 485 nm. Then the fluorescence intensity de-
creased with further increase of the pH value until the
fluorescence quenched at pH 14. The results implied that
probe D was not stable under strongly alkaline condi-
tions but was stable under relatively strong acidic condi-
tions and weakly alkaline conditions. It might be appli-
cable as a fluorescence turn-on probe for selective detec-
pH=12,13,14
2
+
tion of Hg ion in a broad pH range of 1–11.
Comparison of performance of probe D with other
Hg probes was summarized in Table 1. It can be dis-
2+
3
00
350
400
450
500
550
600
covered that probe D displays two advantages: first, it can
Wavelength(nm)
2
+
detect Hg ions by fluorescence turn-on mode over a
broad pH range of 1–11; and second, it is applicable in
strong acidic condition at pH 1.
Fig. 8 Intensity of the maximum UV-Vis absorption band of probe D
2
+
(
(
20 μM) upon addition of 1 equivalent of Hg ion at different pH
pH = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)

J Fluoresc
2+
Table 1 Comparison of fluorescent probes for Hg ion
Probe
Sensing mode
Applicable
pH range
Limit of
detection
Ref.
[12]
MeO
OH
N
N
S
S
Fluorescence
turn-on
5 9
36 nM
OH
MeO
Fluorescence
turn-on
6 8
8.1 nM
6.5 nM
[20]
[22]
S
O
O
O
O
SH
Fluorescence
turn-on
6 10
O
O
N
I
O
O
H
N
H
N
O
Fluorescence
turn-on
5.81 9.04
6.5 9
0.42 nM [45]
N
S
NEt
2
Et
2
N
O
NEt
2
OMe
O
O
H
N
H
N
Fluorescence
turn-on
300 nM
3.2 nM
[46]
[47]
N
H
S
O
Et
2
N
O
O
O
O
H
O
H
N
N
O
Fluorescence
turn-on
4 11
N
N
S
H
NEt
2
Et
2
N
NEt
2
O
H
H
N
N
N
S
Fluorescence
turn-on
5 11
2 12
9.1 nM
6.5 nM
[48]
[49]
N
O
NEt
2
N
N
MeO
N
Fluorescence
quenching
H
N
H
N
N
O
O
H
S
A plausible sensing mechanism of probe D for fluores-
cent detection of Hg ion is proposed as showed in
with strong fluorescence, similar to the sensing processes
2
+
2+
observed on other thiourea-based Hg fluorescent probes
+
Scheme 2. The weakly fluorescent probe D readily binds
reported previously [44–49]. The coexistence of Ag and
2
+
2+
2+
with Hg ion due to the strong interaction between sulfur
Cu ions may interfere with the selective sensing of Hg
2
+
+
2+
atom and thiophilic Hg ion. Then a desulfurization and
cyclization process occurs to form a hydropyrrolidinone
ion by probe D because Ag and Cu ions also exhibit
thiophilicity like Hg ion [32–34].
2
+

J Fluoresc
Table 1 (continued)
N
SH
Fluorescence
turn-on
3 8
0.35 nM [57]
N
N
S
O
Fluorescence
turn-on
4 7
6 10
3 9
52 nM
6 nM
[58]
[59]
O
N
O
N
S
N
O
N
Et
2
N
O
NEt
2
Fluorescence
turn-on
H N
2
N
C
Fluorescence
turn-on
45.4 nM [60]
2
CO H
N
O
OH
O
O
Fluorescence
turn-on
6 8
2.8 nM
130 nM
[61]
[62]
O
N
SEt
SEt
O
N
O
N
N
CONHEt
Br
Fluorescence
turn-on
3 10
H
2
N
N
NH
2
HO
NC
CN
O
S
N
S
O
Fluorescence
turn-on
2 10
140 nM
[63]
[64]
O
O
NHEt
N
N
O
O
N
N
N
S
NHEt
Fluorescence
turn-on
4 9
26 nM
EtHN
HO
O
NHEt
O
H
N
H
N
O
O
Fluorescence
turn-on
1 11
146 nM
This
work

J Fluoresc
H
N
H
N
H
N
H
N
H
N
Hg2+
HN
-
HgS
S
O
S
O
HO
O
O
HO
O
O
HO
O
O
Hg
O
weakly fluorescent
strongly fluorescent
2+
Scheme 2 Plausible sensing mechanism of probe D towards Hg ion
graphitic carbon nitride as a novel fluorescent probe for
mercury(II) and iron(III): a circuit logic gate mimic. New J
Chem 43:12087–12093
Li X, Liu J, Gong X, Qing T, Zhang P, Feng B (2019) Synthesis of
fluorescent tungsten disulfide by nitrogen atom doping and its ap-
plication for mercury(II) detection. J Mater Chem C 7:4096–4101
Zhou Y, Wang J, Yang G, Ma S, Zhang M, Yang J (2019) Cysteine-
rich protein-templated silver nanoclusters as a fluorometric probe
for mercury(II) detection. Anal Methods 11:733–738
Conclusions
A new fluorescent turn-on chemosensor D for selective
7
8
9
.
2
+
2+
Hg ion detection was developed based on the Hg -pro-
moted desulfurization and cyclization reactions of a
coumarin-thiourea conjugate. UV–Vis spectra measure-
.
.
2
+
ment revealed that addition of Hg ion to the sensor D
solution in 2:8 EtOH/H O induced a hypsochromic shift
of the absorption band from 360 nm to 340 nm accompa-
nying distinct enhancement in the absorption intensity.
2
Jiang Y, Duan Q, Zheng G, Yang L, Zhang J, Wang Y, Zhang H, He
J, Sun H, Ho D (2019) An ultra-sensitive and ratiometric fluores-
2+
cent probe based on the DTBET process for Hg detection and
+
+
+
+
2+
imaging applications. Analyst 144:1353–1360
Addition of other metal ions (Li , Na , K , Ag , Cu ,
2
+
2+
2+
2+
2+
2+
2+
2+
3+
10. Cheng X, Qu S, Xiao L, Li W, He P (2018) Thioacetalized
coumarin-based fluorescent probe for mercury(II): ratiometric re-
sponse, high selectivity and successful bioimaging application. J
Photochem Photobiol A 364:503–509
Fe , Zn , Co , Ni , Mn , Sr , Ca , Mg , Al ,
3
+
3+
Cr and Fe ) did not cause substantial change in the
absorption spectra. Fluorescence spectra measurement re-
2
+
1
1. Ma J, Zhang C, Xiao Y, Zhang M, Wang Q, Zheng W, Zhang S
sults indicated that addition of Hg to the sensor solution
in 2:8 EtOH/H O brought about marked increase in the
(2019) Preparation 2-(anthracen-9-yl)-1,3-dithiolane as a novel
2
dual-channel AIE-active fluorescent probe for mercury (II) ion with
excellent performance. J Photochem Photobiol A 378:142–146
fluorescence emission intensity. Most common competi-
tive metal ions did not interfere with the selective sensing
12. Huang S, Gao T, Bi A, Cao X, Feng B, Liu M, Du T, Feng X, Zeng
W (2020) Revealing aggregation-induced emission effect of
2
+
2+
of Hg ion by the sensor D. The detection limit of Hg
2+
−
7
imidazolium derivatives and application for detection of Hg
Dyes Pigments 172:107830
.
ion by probe D is calculated to be 1.46 × 10 M. The
probe D can be utilized for selective detection of Hg
2
+
1
3. Han Y, Yang C, Wu K, Chen Y, Zhou B, Xia M (2015) A facile
naphthalene-based fluorescent chemodosimeter for mercury ions in
aqueous solution. RSC Adv 5:16723–16726
14. Shang X, Wang N, Cerny R, Niu W, Guo J (2017) Fluorescent
protein-based turn-on probe through a general protection-
ion by fluorescence turn-on mode over a broad pH range
of 1–11. Therefore it may play an important role in vari-
ous samples determination including acidic environmental
waste water samples. Further exploration on this respect
will be performed in the near future.
deprotection design strategy. ACS Sens 2:961–966
2+
1
1
1
5. Wu C, Wang J, Shen J, Bi C, Zhou H (2017) Coumarin-based Hg
fluorescent probe: synthesis and turn-on fluorescence detection in
neat aqueous solution. Sensors Actuators B Chem 243:678–683
6. Lee H, Kim H (2011) Ratiometric fluorescence chemodosimeter for
mercuric ions through the Hg(II)-mediated propargyl amide to
oxazole transformation. Tetrahedron Lett 52:4775–4778
7. Mubarok AZ, Lin S-T, Mani V, Huang C-H, Huang S-T (2016)
Design of controlled multi-probe coupled assay via bioinspired sig-
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58485–58492
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H, Feng F (2019) A novel quick and highly selective “turn-on”
tional claims in published maps and institutional affiliations.