A novel carbonothioate-based benzothiazole fluorescent probe for trace detection of mercury (II) in real water samples

Article abstract of DOI:10.1016/j.ica.2021.120349

In this study, a novel turn-on Hg²⁺ fluorescent probe (2-(1,3-benzothiazol-2-yl)-5-(N,N-diethylamino) phenol)-phenyl carbonothioate (PT) was designed and synthesized. The structure of fluorescent probe PT was characterized by spectral data and

Full text of DOI:10.1016/j.ica.2021.120349

Inorganica Chimica Acta 521 (2021) 120349
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Research paper
A novel carbonothioate-based benzothiazole fluorescent probe for trace
detection of mercury (II) in real water samples
*
Qing-Qin Tian, Zhi-Gang Zhao, Zhi-Chuan Shi
College of Chemistry and Environment, Southwest Minzu University, Chengdu 610041, PR China
A R T I C L E I N F O
A B S T R A C T
In this study, a novel turn-on Hg2 fluorescent probe (2-(1,3-benzothiazol-2-yl)-5-(N,N-diethylamino) phenol)-
+
Keywords:
Fluorescent probe
Mercury ion
phenyl carbonothioate (PT) was designed and synthesized. The structure of fluorescent probe PT was charac-
2+
terized by spectral data and elemental analysis. The probe showed weak fluorescence without Hg ion, and the
Chemo-selectivity
Benzothiazole
Carbonothioate
2+
2+
color of the solution was colorless. Upon the addition of Hg ion, the probe reacted specifically with Hg to
produce a strong fluorescence, and the color of solution turn to yellow, which can be directly inspect with the
naked eye. The limit of detection of the probe PT for Hg2 could reach 0.974 nM. Furthermore, the probe PT can
work in a wide pH range and successfully applied in detection of Hg2 in real water samples.
+
+
1
. Introduction
During the past decades, there are two types of fluorescent probes for
Hg2 detection have reported: coordination-based and reaction-based
+
Mercury, is widely used in industrial production and people’s daily
[17,18]. For reaction-based probes usually exhibits excellent selec-
tivity and high sensitivity towards Hg2 because of the thiophiolic
+
life, play a significant role in chemical and electronic fields [1]. How-
ever, mercury is an important heavy metal ion, and it is one of the most
dangerous heavy metal ions for human body and environmental fields
property of mercury ions, including the Hg2 -promoted desulfurization
+
reaction of thiocarbonyl compounds, derivatives of thiourea, squaraine-
2
+
-
[
2]. There is potential for mercury to enter the food cycle via various
based compounds, thiosemicarbazide compounds, as well as the Hg
foods we need and be absorbed by the human body [3–5], if consumed
promoted deprotection of dithioacetals [19]. In this work, we developed
high levels of mercury in a long term, which can cause kidney damage
a novel fluorescent probe (PT) for the sensitive and rapid detection of
2
+
[
6], genotoxicity [7], abdominal pain [8] and neurotoxicity [9].
Hg ion in water (Scheme 1), using 2-(1,3-benzothiazol-2-yl)-5-(N,N-
diethylamino) phenol as the fluorophore and phenyl thiochloroformate
as the recognition receptor. Probe PT can sensitively and quickly
2
+
Therefore, developing an analytical method for monitoring Hg in
environmental and biological samples quickly and accurately is valuable
monitor Hg2 with a nanomole detection limit in buffer solution. The
+
(
Table 1).
Generally, there are many analytical methods to detect trace levels of
solution of probe PT has visible luminescent intensity changes after the
Hg2 , such as inductively coupled plasma mass spectrum [10], atomic
+
addition of different concentrations of Hg ion. In addition, probe PT
2+
can detect Hg2 in a very short time and a wide pH range.
+
absorption spectrometry [11], surface-enhanced Raman spectroscopy
[
12] and high-performance liquid chromatography [13]. However,
these detective methods have obvious shortcomings, which required the
expensive instrument, professional operations and complicated pro-
cedure of sample preparation, thus there are not suitable for the detec-
tion of mercury ion on-site inspection [14]. In recent years, fluorescent
probes have attracted widespread attentions from researchers because of
their characteristics of high sensitivity, low cost, high selectivity, low
toxicity and easy operation, and have become useful tools for detecting
and monitoring minimal samples [15,16]. Therefore, development of
2. Experimental
2.1. Materials and instruments
All the solvents were purified before use. Melting points were
measured with a WRS-1B digital apparatus and are uncorrected. IR
spectra were recorded on IR200 spectrometer on KBr disks. Proton and
carbon nuclear magnetic resonance spectra were measured on an
2
+
Agilent-NMR-VNMRS 400 (400 MHz for 1H and 100 MHz for C)
13
Hg specific fluorescent probe is still an appealing task.
*
Corresponding author.
E-mail address: szc169@swun.edu.cn (Z.-C. Shi).
https://doi.org/10.1016/j.ica.2021.120349
Received 11 February 2021; Received in revised form 8 March 2021; Accepted 8 March 2021
Available online 16 March 2021
0
020-1693/© 2021 Elsevier B.V. All rights reserved.

Q.-Q. Tian et al.
Inorganica Chimica Acta 521 (2021) 120349
Table 1
2
+
Determination of Hg in real water samples with probe PT.
a
2+
b
2+
c
Sample
pH
Added Hg
µM)
Found Hg
Recovery
(%)
RSD
(%)
(
(µM)
Tap water
6.89
7.18
8.37
0.1
0.107
0.519
1.083
107.0
103.8
108.3
0
1
.5
.0
2.31
2.46
2.49
River water
0.1
0.098
0.512
1.021
98.0
102.4
102.1
0
1
.5
.0
Industrial
sewage
0.1
0.5
0.105
0.523
1.091
105.0
104.6
109.1
1
.0
a
2+
Determined with a pH meter before treating with Hg
Results are based on three measurements.
RSD = Relative standard deviation.
.
b
c
2
+
Fig. 1. Absorption spectra of probe PT (0.7 M) with 10 equiv. of Hg ions.
μ
2
+
Inset: before and after the addition of Hg , the bright yellow change of the
probe PT solution can be observed by the naked eye.
1
4
5
10.18 (CH), 105.09 (CH), 44.05 (CH
2
), 12.39 (CH
3
). ESI-MS: m/z (%):
+
35.76 ([M + H] , 100). Anal. Calcd for C24
22 2 2 2
H N O S : C, 66.33; H,
.10; N, 6.45; S, 14.75. Found: C, 66.14; H, 5.104; N, 6.47; S, 14.79%.
2
.3. Sample preparation and optical measurements
A stock solution of probe 1 was prepared in ethanol/DMSO (2:1, v/v,
Scheme 1. Synthesis route of probe PT.
+ 2+ 2+ +
1
mM), stock solutions (1 mM) of nitrate salts (Ag , Ca , Cd , Na ,
2
+
2+
3+
3+
3+
2+
2+
2+
2+
2+
2+
2+
Co , Cu , Cr , Fe , Hg , Mg , Mn , Ni , Pb and Zn ) , ac-
spectrometer with DMSO‑d
6
as solvent and TMS as internal standard.
etate salts (Al , Ba ) of different metals and tetra butyl ammonium
Elemental analyses (C, H, N and S) were performed with an Elementar
vario MICRO cube auto analyzer. Mass spectra were obtained on a
Finnigan LCQ-DECA instrument. UV–vis spectra were obtained from a
Mapada UV-6100 spectrophotometer (Shanghai, China). Fluorescence
spectra measurements were performed on a HORIBA Dual-FL fluores-
cence spectrophotometer. A METTLER TOLEDO FE28-standard bench-
top pH meter. 2-(1,3-benzothiazol-2-yl)-5-(N,N-diethylamino) phenol
-
-
– – –
-
-
salt (Cl , ClO , CN , CH COO , HSO , NO and SCN ) of different anions
4 3 4 3
were prepared in ultrapure water. The pH was adjusted with dilute HCl
and sodium hydroxide (0.1 M). All absorption and fluorescence emission
spectra were recorded in a quartz optical cell of 1 cm optical path length.
2.4. The calculation of detection limit
(
1) was synthesized by the methods reported in the literature [20].
The detection limit (DL) of probe PT towards Hg2 ion was calcu-
+
lated according to the equation “DL = 3σ/k”. In which, “σ” is the stan-
2
.2. Synthesis of (2-(1,3-benzothiazol-2-yl)-5-(N,N-diethylamino)
dard deviation of seven blank measurements, and “k” is the slope
phenol)-phenyl carbonothioate (PT)
2+
between fluorescence intensity and the concentration of Hg ion.
To
phenol (1) (500 mg, 1.68 mmol) and potassium carbonate (466 mg,
.33 mmol) in 10 mL of acetonitrile, stirred for 10 min at room tem-
a solution of 2-(1,3-benzothiazol-2-yl)-5-(N,N-diethylamino)
3
perature, then Phenyl chlorothionocarbonate (2) (583 mg, 3.38 mmol)
was added. The mixture was heated for reflux for 6 h under an argon
atmosphere. After the time given, the solvent was evaporated under
reduced pressure, and the residue was purified by silica column chro-
matography (Ethyl acetate: Petroleum ether, 1:15, v/v) to give probe PT
◦
as orange-red solid (435 mg, 59.7% yield). Mp 96.6–101.0 C (ethyl
acetate). R
f
= 0.2 (Ethyl acetate:Petroleum ether, 1:5, v/v). IR (KBr,
ꢀ
1
cm ): 3069 (w), 2984 (m), 2940 (w), 1623 (vs), 1558 (m), 1470 (s),
272 (vs), 1210 (vs), 1110 (s), 761 (s), 728 (s), 691 (s). 1H NMR (400
MHz, DMSO‑d
1
6
): δ 8.13 (dd, J = 8.4, 5.7 Hz, 2H, ArH), 7.99 (d, J = 8.1
Hz, 1H, ArH), 7.51 (t, J = 7.9 Hz, 3H, ArH), 7.41 (d, J = 7.8 Hz, 1H,
ArH), 7.36 (dd, J = 11.4, 5.1 Hz, 3H, ArH), 6.79 (dd, J = 9.1, 2.2 Hz, 1H,
ArH), 6.70 (d, J = 2.2 Hz, 1H, ArH), 3.44 (q, J = 6.9 Hz, 4H, CH
2
), 1.14
): δ 193.52
N), 153.29 (C), 153.71 (C), 152.03 (C), 150.37 (C),
1
3
(
(
t, J = 6.9 Hz, 6H, CH
3 6
). C NMR (100 MHz, DMSO‑d
–
–
–
2+
C
–
S), 162.35 (C
Fig. 2. Fluorescence spectra of probe PT (0.7
μ
M) for 10 equiv. of Hg
1
1
34.09 (CH), 130.87 (CH), 129.90 (CH), 126.96 (CH), 126.38 (CH),
24.59 (CH), 121.96 (CH), 121.91 (CH), 121.79 (CH), 111.48 (C),
ex
detection (λ = 386 nm). Inset: Plot of the fluorescence intensity of probe PT
2
+
addition of Hg at 423 nm.
2

Q.-Q. Tian et al.
Inorganica Chimica Acta 521 (2021) 120349
Fig. 3. Fluorescence spectra of probe PT (0.7 M) with difference concentra-
μ
2
+
Fig. 4. Fluorescence responses of probe PT (0.7
metal ions (λex = 386 nm).
μ
M) with 10 equiv. of various
tions of Hg (λex = 386 nm). Inset: plot of the fluorescence intensity of probe
2
+
2+
.
PT-Hg at 423 nm over different concentrations of Hg
. Results and discussion
.1. Absorption spectra study of probe PT
The absorption response of probe PT to mercury ion (10.0 equiv. of
3
3
probe PT) in ethanol/water (1:1, v/v) was investigated, the solution
◦
reacted at room temperature (25 C) for 5 min. As shown in Fig. 1, the
2
þ
maximum UV–vis absorption peaks of probe PT and probe PT-Hg are
located at 360 nm and 366 nm, respectively, which attribute to the - *
π
π
conjugated effect of aromatic conjugated system. A new peak at 450 nm
with larger molar absorption coefficient than the original. The absorp-
tion at 450 nm was responsible for the color change (from colorless to
yellow) and perceptible to the naked eyes.
+
3
3
.2. The properties of probe PT for Hg2 detection
.2.1. Response time study of probe PT
The response time was a significant index for evaluating the fluo-
rescence probe. To evaluate the identification ability of probe PT for
Fig. 5. Fluorescence responses of probe PT (0.7
anions (λex = 386 nm).
μ
M) with 10 equiv. of various
2
+
Hg , the response time of probe PT was investigated. As shown in
2
+
Fig. 2, probe PT displayed weak fluorescence in solution without Hg
.
However, in the presence of Hg2 , the solution showed distinct and fast
changes in fluorescence spectra at 423 nm. The fluorescence intensity
increased drastically at the beginning and gradually stabilized about 5
min, which indicating the detection process of PT for Hg2 is rapid. The
response time was much lower than many other probes previously re-
ported (Table S1).
+
+
3
.2.2. Sensing of probe PT to Hg2 and detection limit calculation
+
To study the sensitivity of probe PT for Hg2 , the relationship be-
+
tween the fluorescence intensity of probe PT and the concentration of
2
+
was investigated. As shown in Fig. 3, the solution was non-
Hg
fluorescent without Hg2 . Upon the continuous addition of Hg ion,
+
2+
the fluorescence intensity of solution was gradually increased. In the
2
+
presence of 7 µM Hg , the fluorescence intensity of the solution reached
to maximum and fluorescence enhancement at 423 nm was detected.
The fluorescence was diminished by the phenyl chlorothionoformate
group due to the quenching effect. Nevertheless, after probe PT treated
with Hg2 , an obviously fluorescence emission peak was emerged at
+
4
23 nm, which indicated an obvious fluorescence “off–on” effect.
In order to assess the sensitivity of probe PT towards Hg2 ion, the
+
detection limit (DL) was calculated according to the acquainted equa-
tion “DL = 3σ/k”. Gratifyingly, the fluorescence intensity at 423 nm
2
Fig. 6. Fluorescence responses of probe PT (0.7
μ
M). Black bar: emission in-
+
showed a great linear correlation with the concentrations of Hg ion
tensity of PT and selected ions. Red bar: emission intensity of PT, selected ions
2
+
from 0 µM to 1 µM (Fig. S8). The detection limit (DL) of probe PT
and Hg (λex = 386 nm).
3

Q.-Q. Tian et al.
Inorganica Chimica Acta 521 (2021) 120349
fluorescence intensity, while other metal ions caused the negligible or
even no enhancement. These data demonstrated that probe PT has a
high specificity for Hg2 detection over other metal ions. Also, another
form of mercury salt with other counter-ion such as sulfate can be
detected by probe PT (Fig. S9).
+
To further evaluate whether probe PT could retain its sensing
2
+
response to Hg in competitive environments, the probe PT was treated
2
+
+
+
2+
3+
2+
2+
2+
with Hg in the presence of various metal ions (Ag , Al , Ba , Ca
,
2
+
2+
2+
3+
3+
2+
2+
2+
Cd , Na , Co , Cu , Cr , Fe , Mg , Mn , Ni , Pb and Zn
)
-
-
4
–
–
-
4
–
-
and anions (Cl , ClO
, CN , CH
3
COO , HSO
, NO
3
and SCN ) in buffer
solution for 5 min. As shown in Fig. 6, no significant change in fluo-
rescence intensity of probe PT was observed whether other metal ions
and anions exist or not. The results revealed that all of the metal ions
+
3+
2+
2+
2+
2+
2+
+
2+
2+
3+
3+
(
such as Ag , Al , Ba , Ca , Cd , Na , Co , Cu , Cr , Fe
,
2
+
2+
2+
-
-
–
–
Mg , Mn , Ni , Pb and Zn ) and anions (Cl , ClO
4
, CN , CH
3
COO ,
HSO-
, NO and SCN ) tested barely had any influence on the fluores-
–
-
4
3
2
+
cence detection of Hg . The above results indicated that probe PT was
able to detect Hg2 ion even in a complex environment.
+
2
+
Fig. 7. Fluorescence spectra of PT (0.7
μ
M) with Hg in different pH.
3
.2.4. Effect of pH on probe PT for Hg2 detection
+
Furthermore, the pH influence on the sensing behavior of probe PT
towards Hg2 ion was calculated to be 0.974 nM, which illustrating the
+
2+
2+
toward Hg was investigated. before and after the addition of Hg , the
fluorescence intensities at 432 nm at different pH value (4.0, 5.0, 6.0,
high sensitivity of probe PT towards Hg2 ion.
+
7
.0, 8.0, 9.0, and 10.0) were recorded, respectively. As shown in Fig. 7,
3
.2.3. Selectivity and competition studies of probe PT
2+
in the pH range of 4.0–10.0, without Hg , probe PT emitted very weak
fluorescence and was stable. Meanwhile, the fluorescence intensity of
Selectivity is an important index to measure the performance of
fluorescent probes. To evaluate the selectivity, probe PT was treated
2+
probe PT showed obvious emission enhancement with Hg . The fluo-
+
3+
2+
2+
2+
+
2+
2+
-
with various metal ions (Ag , Al , Ba , Ca , Cd , Na , Co , Cu
,
rescence intensity of probe PT gradually increased with the change of
pH and reached the maximum at pH = 6.0, and then slightly decreased
with a continuous increment of pH, which might be attributed to the
3
+
3+
2+
2+
2+
2+
2+
2+
Cr , Fe , Hg , Mg , Mn , Ni , Pb and Zn ) and anions (Cl ,
ClO-
– – –
-
-
4 3 4 3
, CN , CH COO , HSO , NO and SCN ) under the same conditions
for 5 min, and the corresponding fluorescence intensities are shown in
Fig. 4 and Fig. 5. The fluorescence spectra shows that the addition of
–
2+
generation of mercuric oxide from the reaction of OH ion with Hg
ion. Even so, the fluorescence intensity of probe PT in aqueous solution
2
+
Hg
ion into solution of probe PT dramatically enhanced the
1
(A), isolated product from reaction of PT + Hg2+ in DMSO‑d
Fig. 8. Partial H NMR spectra comparison of PT in DMSO‑d
6
6 6
(B), and compound 1 in DMSO‑d (C).
4

Q.-Q. Tian et al.
Inorganica Chimica Acta 521 (2021) 120349
2
+
.
Scheme 2. The detection mechanism of probe PT for Hg
with Hg2 ion exhibited an instinct improvement within pH range from
+
4. Conclusion
4
.0 to 9.0. The experimental results indicated that the probe PT has very
2
+
low background fluorescence and could monitor Hg in a wide pH
range.
In conclusion, a simply and easily synthetic procedure was developed
for the preparation of fluorescence chemo-sensor (2-(1,3-benzothiazol-
2
-yl)-5-(N,N-diethylamino) phenol)-phenyl carbonothioate (PT). The
+
2+
3
.2.5. Mechanism study of probe detecting Hg2
After that, the sensing mechanism of probe PT toward Hg2 ion was
studied. In order to conveniently understand this sensing mechanism,
investigation results of probe PT properties for Hg detection showed
that probe PT has satisfactory high sensitivity and the detection limit is
excellent, and its detection limit (0.974 nM) was much lower than many
other fluorescence probes for mercury detection. It also displayed high
+
we carried out the reaction of probe PT with Hg2 in CH
+
3 2
CN-H O mixed
+
solution and monitored the reaction product by thin layer chromatog-
selectivity and competition to Hg2 over other metal ions. A better
detecting performance was revealed when pH of the solution varies from
4.0 to 9.0. Moreover, the probe in ethanol as well as in water shows light
1
1
raphy, whose structure was analyzed by H NMR (Fig. S3). The H NMR
spectra were essentially identical to probe PT, that the peak of hydroxyl
signal at 11.74 ppm (1H, a single peak) in probe PT was disappeared,
while a new signal of hydroxyl over 11.00 ppm (1H, a single peak) in
compound 1 was found. Additionally, the generated production from the
reaction mixture of probe PT with Hg2 ion was confirmed to be com-
pound 1 (Fig. 8). The results supply a solid evidence for the formation of
PT (Scheme 2).
blue emission with Hg2 under a UV lamp, which can be easily detected
+
by the naked eye. The practical applications of probe PT are broadened
for sensing and monitoring Hg2 environmentally and biologically.
+
+
CRediT authorship contribution statement
Qing-Qin Tian: . Zhi-Gang Zhao: . Zhi-Chuan Shi: Writing - review
& editing.
3
.3. Analytical application
For analysis probe PT was used to measure the Hg2 content in actual
water samples, including tap water (from our laboratory), river water
from Jinjiang river Chengdu city) and industrial sewage (from indus-
trial areas in Chengdu city), which were spiked with 0.1, 0.5 and 1.0 µM
+
Declaration of Competing Interest
(
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
2
+
of Hg , and analyzed by proposed. All the water samples were filtered
through a 0.2 mm filter membrane to remove large particular impurities,
followed by removal of remaining organics by extraction processes. The
resulting samples were diluted with ethanol and water (1:2, v/v) in a
Acknowledgement
1
0.00 mL volumetric flask. Table 1 shows the results acquired using
The authors are grateful for financial support from the Fundamental
Research Founds for the Central Universities, Southwest Minzu Uni-
versity (No. 2020YYXS57).
2
+
probe PT with the appropriate concentration gradient of Hg added.
The results indicate that probe PT had good recovery and demonstrated
high accuracy in the analysis of Hg2 . The percentage recovery was
+
found in the range of 98.0–109.1%. Therefore, probe PT can measure
Appendix A. Supplementary data
the concentration of Hg2 in real water samples and has practical value
+
in environmental analysis.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120349.
5

Q.-Q. Tian et al.
Inorganica Chimica Acta 521 (2021) 120349
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