A fluorescent probe based on ICT for selective detection of benzenethiol derivatives

Article abstract of DOI:10.1016/j.saa.2021.120058

This work presented a benzothiazole-based fluorescent probe for the detection of benzenethiol derivatives using 2, 4-dinitrobenzene moiety as a sensing unit. This probe (NCABT) was able to instantaneously respond to 4-methylbenzenethiol (MTP) within 5 min. In detecting MTP, this probe displayed a low limit of detection (49 nM). Furthermore, the probe has been proved to have the potential to detect benzenethiol derivatives with electron-donating group in real water samples.

Full text of DOI:10.1016/j.saa.2021.120058

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120058
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A fluorescent probe based on ICT for selective detection of benzenethiol
derivatives
⇑
Feng Li, Chang-He Tian, Ya-Fei Du, Bao-Xiang Zhao
Institute of Organic Chemistry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
A new ‘‘turn on” fluorescent probe
NCABT was developed to detect
benzenethiol derivatives with
electron-donating group.
ꢀ
ꢀ
NCABT could instantaneously
respond to TMP within 5 min.
NCABT showed good selectivity and
sensitivity toward benzenethiol
derivatives with electron-donating
group.
ꢀ
ꢀ
NCABT was used for the detection of
TMP in real water samples.
NCABT could detect the presence of
TMP by naked eyes.
a r t i c l e i n f o
a b s t r a c t
Article history:
This work presented a benzothiazole-based fluorescent probe for the detection of benzenethiol deriva-
tives using 2, 4-dinitrobenzene moiety as a sensing unit. This probe (NCABT) was able to instantaneously
respond to 4-methylbenzenethiol (MTP) within 5 min. In detecting MTP, this probe displayed a low limit
of detection (49 nM). Furthermore, the probe has been proved to have the potential to detect ben-
zenethiol derivatives with electron-donating group in real water samples.
Received 18 April 2021
Received in revised form 1 June 2021
Accepted 3 June 2021
Available online 6 June 2021
Ó 2021 Elsevier B.V. All rights reserved.
Keywords:
Fluorescent probe
Benzenethiol derivatives
ICT
Real water samples
1. Introduction
ment. The median lethal dose (LD50) value of thiophenol was
reported in a range of 0.01–0.4 mM in fish and around 46.2 mg/
Thiophenol is a kind of compound with a mercapto group cova-
lent bond and an aromatic ring. This kind of compound has a wide
range of applications in the field of organic synthesis, and plays a
very important role in agricultural chemical industry and pharma-
ceutical industry [1]. Though they are very useful, thiophenol
derivatives are highly toxic environmental pollutants, which can
cause a serious impact on organisms and the ecological environ-
kgÀ1 for mouse [2,3]. As
a
thiophenol derivative, 4-
methylbenzenethiol (MTP) was used to antiseptic. The concentra-
tion of MTP has been reported to be 0.52 g/L in a raw sludge from
a wood preservation operation [4]. The median lethal dose (LD50
)
value of MTP was reported in a low range 200 mg/kg for mouse
[5]. Besides, the exposure to high concentration of thiophenol
derivatives can induce a series of health problems, including the
damage of respiration and central nervous systems, muscle atro-
phy, vomiting, and even death [6–8]. Therefore, it is of great signif-
icance to develop a simple and rapid detection technique that can
⇑
Corresponding author.
E-mail address: bxzhao@sdu.edu.cn (B.-X. Zhao).
https://doi.org/10.1016/j.saa.2021.120058
1386-1425/Ó 2021 Elsevier B.V. All rights reserved.

F. Li, C.-H. Tian, Y.-F. Du et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120058
monitor the presence of thiophenol derivatives with high sensitiv-
ity and selectivity. It is urgently needed to differentiate toxic thio-
phenol derivatives over biologically important biothiols.
of the reaction, the reaction mixture was filtered under reduced
pressure. NCABT was obtained as an orange solid (338 mg, 66%
yield), which was characterized by NMR, HRMS and IR (Fig. S1–
To this end, compared with traditional methods, fluorescent
probe has attracted much attention as its excellent properties
including simplicity, easy operation, good sensitivity, good selec-
tivity, on-the-spot detection, low cost detection and non-
destruction [9–14]. Indeed, many fluorescent probes for thiols have
been developed, but it is a challenge to distinguish thiophenol
derivatives and biothiols because of their similar physical and
chemical properties. The first fluorescent probe for selectively
detecting thiophenol derivatives from aliphatic thiols was innova-
tively developed by Wang et al. in 2007 [15]. Because the pKa value
of thiols is around 8.5 and pKa value of thiophenol is 6.5, MTP is
about 6.82 [16], in a neutral reaction medium (pH = 7.4), thiophe-
nol possessing higher dissociation degree results in the predomi-
nant generation of the corresponding thiophenolate [17,18].
Since then, many fluorescent probes for thiophenol derivatives
have been reported [19–41]. However, most of these reported flu-
orescent probes still have some shortcomings such as slow speed
for sensing reaction, low fluorescence intensity, high detection
limit and instability to environmental impact [42]. Herein, a new
fluorescent probe (named NCABT) is synthesized and developed
for selective detection of benzenethiol derivatives (Scheme 1).
4). Melting point: 223–227 °C. IR (KBr)
m
2970, 2226, 1622, 1578,
.
1524, 1411, 1346, 1277, 1194, 1090 cmÀ1
1H NMR (400 MHz,
Chloroform-d) d 8.89 (d, J = 2.7 Hz, 1H), 8.57 (d, J = 9.2 Hz, 1H),
8.34 (dd, J = 9.3, 2.7 Hz, 1H), 8.19 (s, 1H), 8.00 (d, J = 8.0 Hz, 1H),
7.81 (d, J = 7.6 Hz, 1H), 7.48–7.43 (m, 1H), 7.38–7.33 (m, 1H),
7.08 (d, J = 9.3 Hz, 1H), 6.73 (dd, J = 9.3, 2.6 Hz, 1H), 6.31 (d,
J = 2.6 Hz, 1H), 3.43 (q, J = 7.1 Hz, 4H), 1.23 (t, J = 7.1 Hz, 6H).
13C NMR (101 MHz, Chloroform-d) d 163.11, 154.19, 152.53,
151.10, 140.74, 137.60, 130.05, 128.10, 125.65, 124.60, 122.30,
121.19, 120.41, 117.32, 116.35, 110.71, 109.38, 101.58, 99.66,
44.06, 11.50. HRMS (ESI): m/z calculated for [M+H+] C26H22N5O5S+
m/z: 516.1336, found 516.1354.
2.3. Preparation of testing solution
The stock solution of probe NCABT (1 mM) was prepared in
DMF. The stock solutions of thiophenol derivatives were prepared
in DMSO and other analytes were prepared by distilled water.
50 lL stock solution of probe NCABT was transferred into volumet-
ric flask and diluted to 10 mL by the buffer solution (DMSO:
PBS = 5: 5, v/v, pH = 7.4) at room temperature. Then, various equiv-
alents of MTP or other analytes was added.
2. Experiment section
3. Results and discussion
2.1. Methods and materials
3.1. Design of fluorescent probe NCABT
NMR spectra were determined by Bruker Avance 400 spectrom-
eter. Mass spectra were recorded by an Impact II spectrograph
(Bruker). IR spectra were obtained using IR spectrophotometer
Tensor II (Bruker). Fluorescence spectra were obtained on a LS-55
fluorescence spectrophotometer (PerkinElmer) and UV–Vis
absorption spectra were measured by a U-4100 spectrophotometer
(Hitachi). The fluorescence lifetime and quantum yield was per-
formed on an FLS920 Fluorescence Spectrometer (Edinburgh
Instruments). Column chromatography was carried out on silica
gel (200–300 mesh) and the pH of solutions were obtained by a
PHS-3C pH-meter (YouKe). Unless otherwise stated, all reagents
and solvents were purchased from commercial provider. All exper-
iments all used twice-distilled water.
It is known that nitro-substituted phenyl ether is a kind of
potential reaction sites for thiophenol. It is interesting to note a
previous work reported in which a twisted cyclic alkylamino group
was used to construct a probe (Z1) [45]. However, probe Z1 could
respond to H2S by the cleavage of 2, 4-dinitrophenyl ether in abso-
lute HEPES solution (Scheme 3). Therefore, in the present work, the
2, 4-dinitrophenyl moiety was used as the sensing group toward
thiophenol derivatives, because it can quench the probe and turn
on when it removed by analyte. Probe NCABT could quickly
respond to benzenethiol derivatives with a low detection limit. It
is not surprising that the subtle differences in molecular structure
can make a significant difference in the properties of the probe. For
example, two probes with same fluorophore as shown in Fig. 1 can
respond SO2 and HClO respectively [46,47]. As expected, the strong
electron-withdrawing nitro group weaken the intramolecular
charge transfer process (ICT) to cause fluorescence quench. When
probe NCABT interacts with MTP, 2, 4-dinitrophenyl moiety can
be released from the probe and a cyclization reaction of aryl oxy-
gen anion with cyano group take place to produce 2-imino-2H-
chromen derivative that can emit fluorescence (Scheme 1).
2.2. Synthesis of probe NCABT
Compound 1 and compound 2 were prepared according to the
literature [43,44]. As shown in Scheme 2, the mixture of compound
1 (174 mg, 1.0 mmol), compound 2 (359 mg, 1.0 mmol) and p-
methylbenzene sulfonic acid (190 mg, 1.0 mmol) in methanol
(30 mL) was stirred at room temperature for 10 h. After completion
Scheme 1. Sensing mechanism of probe NCABT toward MTP.
2

F. Li, C.-H. Tian, Y.-F. Du et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120058
Scheme 2. Synthesis of probe NCABT.
Scheme 3. Sensing mechanism of probe Z1 toward H2S.
3.3. Selectivity of probe NCABT toward MTP by fluorescence
spectrometry
As the selectivity is an important evaluation indicator, we mea-
sured the fluorescence response of probe NCABT (5
lM) in the
presence of various analytes (10 equivalent each), including reac-
tive species and ions (Cys, Hcy, GSH, S2À, HSÀ, SO32À, CO32À, ClOÀ,
H2O2, NO3À, S2O32À, SCNÀ, FÀ, ClÀ, BrÀ). Although various species
were added, there is no significant fluorescence change. As shown
in Fig. 3 (a), only in the presence of benzenethiol derivatives (p-
CH3OPhSH, MTP), probe NCABT showed obvious fluorescence
changes and the fluorescence intensity of other benzenethiol
derivatives (p-CF3PhSH, PhSH) were significantly weaker than the
former. Even if extend the time to 30 min, the fluorescence
response of probe NCABT toward p-NO2PhSH barely changed. The
results showed that electron-donating group could increase nucle-
ophilicity of MTP and p-CH3OPhSH; however, the electron-
withdrawing group could weaken the nucleophilicity of p-
NO2PhSH and p-CF3PhSH. Besides, the probe displayed excellent
anti-interference performance in a mixture of analyte and MTP
(Fig. 3 (b)). Therefore, NCABT not only possessed high selectivity
for benzenethiol derivatives with electron-donating group, but also
displayed high capability to resist disturbance even in complicated
environments.
Fig. 1. Comparison of probes 1 and CBP.
3.2. Fluorescence and UV–Vis spectra of probe NCABT
After a series of attempts, a buffer solution (DMSO: PBS = 5: 5,
pH = 7.4) was chosen as the suitable solvent for all measurements
of spectral property. Probe NCABT alone in the buffer solution can-
not display fluorescence (
led to the appearance and enhancement of a green fluorescence
centered at 528 nm ( = 77.5%, Fig. S6). It can be observed that flu-
U = 6.41%, Fig. S5). The addition of MTP
U
orescence intensity at 528 nm increased as the dosage of MTP
increased (Fig. 2 (a)). A preferable linearity between the fluores-
cence intensity and the concentrations of MTP from 0 to 3 equiva-
lent can be observed (Fig. 2 (b)). The limit of detection was
calculated to be 49 nM. The fluorescence lifetime was measured
(Fig. S7). Upon the addition of MTP, the fluorescence signal at
528 nm was increased to 15 fold. Furthermore, probe NCABT dis-
played remarkable colorimetric response from colorless to green
without or with the addition of MTP under visible light and a
365 nm UV lamp. These outstanding changes in color and spectra
indicated that NCABT had the potential to detect MTP by naked
eyes even without any test instrument. Similarly, with the increase
of MTP concentration, the absorbance showed a general trend of
increase (Fig. S8).
3.4. Effect of time and pH on the response of NCABT toward MTP
The fluorescence intensity of NCABT (5
lM) as a function of
time in the presence of MTP (0, 1, 2, 5 and 10 equivalent) is dis-
played in Fig. 4. As the reaction proceeded, the fluorescence inten-
sity significantly increased within a few minutes after MTP was
added, and it reached a maximum in 5 min. Such a fluorescence
response indicated that NCABT is suitable for the real-time
3

F. Li, C.-H. Tian, Y.-F. Du et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120058
Fig. 2. (a) Fluorescence titration spectra of NCABT (5
lM) upon addition of MTP (0–9 equiv.) in PBS (pH = 7.4, 10 mM, containing 50% DMSO). (b) The linear relationship
between the fluorescence response at 528 nm against equivalent of MTP (0–3 equiv.). All data were acquired in 5 min of mixing. kex = 465 nm, slit: 15 nm/2.5 nm. Inset: the
color change of NCABT without or with MTP (left: 365 nm UV lamp, right: visible light).
Fig. 3. (a) Fluorescence response of NCABT (5
lM) toward MTP (5 equivalent) and other relevant analytes (5 equivalent for benzenethiol derivatives, 10 equivalent for other
species) in PBS (pH = 7.4, 10 mM, containing 50% DMSO). (b) The fluorescence response of NCABT (5
lM) toward relevant analytes. (1) blank, (2) p-NO2PhSH, (3) p-CF3PhSH,
(4) PhSH, (5) p-CH3OPhSH, (6) MTP, (7) Cys, (8) Hcy, (9) GSH, (10) S2À , (11) HSÀ, (12) SO23À, (13) CO23À, (14) ClOÀ, (15) H2O2, (16) NO3À, (17) S2O23À, (18) SCNÀ, (19) FÀ, (20) ClÀ,
(21) BrÀ. Black bar represents the fluorescence intensity of only a single analyte with NCABT. Red bar represents the fluorescence intensity of a mixture of analyte and MTP
with NCABT. (kex = 465 nm, slit: 15 nm/2.5 nm, kem = 528 nm).
monitoring of MTP. In addition, fluorescence intensity changes of
NCABT at different pH values were researched in the presence or
absence of MTP to measure the influence of pH. As shown in
Fig. S9, an obvious fluorescence intensity change was observed in
the range of pH 6–8. Besides, the solvent system and its pH
strongly influence the properties of fluorescence probes because
different solvent and pH could change the reaction rate and equi-
librium of nucleophilic addition. Moreover, the pKa of thiophenol
is 6.5 but thiol is 8.5, in order to distinguish the two analytes, pH
7.4 was chosen as the condition of all test.
3.5. Recognition mechanism of NCABT toward MTP
To investigate the recognition mechanism of probe NCABT in
sensing MTP, we first examined the change of structure of probe
NCABT before and after reacting with MTP via HRMS. The peak
of probe NCABT appeared at m/z 516.1354 (calculated m/z [M
+H+] 516.1336) as shown in Fig. S4. After reacting with MTP, a
new peak corresponding to the structure of CABT appeared at m/
z
350.1308 (calculated m/z [M+H+] 350.1322) as shown in
Fig. S10. Besides, we also recorded the fluorescence and the UV–
vis absorption spectra of probe NCABT with MTP and compared
them with those of CABT. As shown in Fig. S11 and Fig. S12, the flu-
orescence and UV–vis absorption spectra of CABT were similar to
those of NCABT reacted with MTP. These results demonstrate that
probe NCABT senses MTP by the cleavage of the 2, 4-dinitrophenyl
group which is trigged by MTP and then generates the CABT.
Fig. 4. Time-dependent fluorescence intensity of probe NCABT (5
lM) in the
presence of MTP with different concentration. All data were acquired in PBS
(pH = 7.4, 10 mM, containing 50% DMSO). kex = 465 nm, kem = 528 nm, slit: 15 nm/
2.5 nm.
4

F. Li, C.-H. Tian, Y.-F. Du et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 261 (2021) 120058
Table 1
Determination of MTP concentrations in real water samples.
Sample
MTP spiked (lM)
MTP recovered (
lM)
Standard deviation
Recovery (%)
Tap water
0
2
not detected
1.93
0.010
0.023
0.036
96.3
99.8
101.7
5
10
4.99
10.17
Purified water
Yellow River
0
2
5
10
not detected
2.02
4.97
0.050
0.036
0.043
101.0
99.3
97.8
9.78
0
2
5
10
not detected
2.06
4.97
0.007
0.016
0.028
103.1
99.3
101.4
10.14
3.6. Application of NCABT in real water samples
assistance of Shandong University Structural Constituent and Phys-
ical Property Research Facilities.
Considering the pollution of benzenethiol derivatives to the
environment, we assessed whether NCABT has practical applica-
tion potential. The real water obtained from tap water, purified
water and Yellow River were spiked with MTP at different concen-
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.saa.2021.120058.
trations (2, 5, 10 lM). When probe NCABT was added to the water
sample, there was no obvious fluorescence variation. However,
when NCABT was added to those water samples that contain dif-
ferent concentrations of MTP, it can be seen from Table 1 that good
recoveries (96.3%-103.1%) were obtained. The excellent linear rela-
tionship between the fluorescence intensity at 528 nm and the
concentration of MTP in three water samples were measured
(Fig. S13). These results demonstrated that probe NCABT might
provide a simple method to detect benzenethiol derivatives with
electron-donating group at low concentrations in real water
samples.
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In summary, we developed an ‘off– on’ fluorescent probe
(NCABT) for selective detection of benzenethiol derivatives. The
probe had various remarkable advantages including low detection
limit, high selectivity over aliphatic thiols and other relevant sulfur
species and a fast response towards benzenethiol derivatives with
electron-donating group. Besides, the apparent color change from
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CRediT authorship contribution statement
Feng Li: Conceptualization, Data curation, Formal analysis,
Investigation, Validation, Writing - original draft. Chang-He Tian:
Investigation, Validation, Visualization. Ya-Fei Du: Investigation,
Data curation. Bao-Xiang Zhao: Writing - review & editing,
Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
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