Two novel fluorescent probes based on phenothiazine: synthesis and “naked-eye” colorimetric recognition of Hg2+

Article abstract of DOI:10.1007/s11164-018-03715-x

Two novel colorimetric and fluorescent probes 2-(1,3-dithiolanes)-10-ethyl phenothiazine (PHE–Ed) and 2-(1,3-dithianes)-10-ethyl phenothiazine (PHE–Pd) based on phenothiazine were successfully synthesized, and their structure was confirmed by NMR and high resolution mass spectra. Fluorescence investigations revealed that the synthesized probes could be used for the selective detection of Hg²⁺, which was accompanied with an obvious color change from colorless to light yellow. The applicable ability of the two probes was investigated by a series of competitive experiments, solid colorimetric experiments, and applied?experiments, which proved that these probes showed high sensitivity and great potential to detect Hg²⁺ in environmental analysis systems. Furthermore, the detection mechanism of the probes was investigated by FT-IR spectra and NMR spectra, and the results indicated that the detection of Hg²⁺ was accomplished through the Hg²⁺-promoted deprotection of thioacetal.

Full text of DOI:10.1007/s11164-018-03715-x

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-018-03715-x
Two novel fuorescent probes based on phenothiazine:
synthesis and “naked‑eye” colorimetric recognition of Hg2+
Chuane Wang1 · Lizhen Wang2 · Shimin Fang1 · Dawei Qin1 · Jianhua Zhou1 ·
Geng Yang1 · Shuangshuang Jin1 · Hongdong Duan1
Received: 10 October 2018 / Accepted: 14 December 2018
© Springer Nature B.V. 2019
Abstract
Two novel colorimetric and fuorescent probes 2-(1,3-dithiolanes)-10-ethyl pheno-
thiazine (PHE–Ed) and 2-(1,3-dithianes)-10-ethyl phenothiazine (PHE–Pd) based
on phenothiazine were successfully synthesized, and their structure was confrmed
by NMR and high resolution mass spectra. Fluorescence investigations revealed that
the synthesized probes could be used for the selective detection of Hg2+, which was
accompanied with an obvious color change from colorless to light yellow. The appli-
cable ability of the two probes was investigated by a series of competitive experi-
ments, solid colorimetric experiments, and applied experiments, which proved that
these probes showed high sensitivity and great potential to detect Hg2+ in environ-
mental analysis systems. Furthermore, the detection mechanism of the probes was
investigated by FT-IR spectra and NMR spectra, and the results indicated that the
detection of Hg2+ was accomplished through the Hg2+-promoted deprotection of
thioacetal.
Keywords Phenothiazine · 1,2-ethanedithiol · 1,3-propanedithiol · ICT fuorophore ·
Naked-eye
Introduction
Nowadays, the manufacture and use of mercury has dramatically increased, due
to its wide application in industry [1, 2]. Mercury is hardly biodegradable and can
be biomagnifed in the food chain, thereby causing serious human health prob-
lems, including digestive, cardiac, kidney and DNA damage, mitosis impairment,
*
Hongdong Duan
hdduan67@163.com
1
2
School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology,
(Shandong Academy of Sciences), Jinan 250353, Shandong Province, China
Biology Institute, Qilu University of Technology (Shandong Academy of Sciences),
Jinan 250014, Shandong Province, China
Vol.:(0123456789)
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C. Wang et al.
and permanent damage to the central nervous system [3, 4]. Therefore, the devel-
opment of a highly efcient and novel strategy for detection of Hg2+ ion is impor-
tant for both human health and the environment.
Many fluorescent probes based on fluorescence turn-on or turn-off sensing
mechanisms have been developed for the detection of Hg2+. However, these
strategies have so many limitations, because the fluorescence signal intensity
is very sensitive and susceptible to external factors, such as temperature, exci-
tation power, media properties, etc. [5, 6]. To solve this problem, a new type
of fluorescent probe based on the Hg2+-induced chemical reactions has been
developed. These probes can detect Hg2+ with high selectivity and fluorescence
intensity enhancement, and simultaneously producing obvious color changes
[7]. More importantly, these probes, with high chemoselectivity toward Hg2+
,
are hardly affected by the external factors. Consequently, these reaction-based
probes are superior to other fluorescence detection methods in terms of the sen-
sitivity and selectivity, and have been widely applied for the detection of Hg2+
[8, 9].
Phenothiazine (PHE) derivatives have a long history, with successive periods
of interest in diferent areas of applied chemistry, such as dyes, probes, pharma-
ceuticals, and electrochemistry [10–14]. PHE can provide strong fuorescence,
structural regulation, good hole transport capacity, and low ionization potential.
Therefore, PHE derivatives can be used as raw materials for the development of
various fuorescent probes [15]. This paper reported the synthesis of two col-
orimetric fuorescent probes PHE–Ed and PHE–Pd based on phenothiazine as
shown in Scheme 1. These two probes could be used for the high selective sign-
aling of Hg2+ ions through Hg2+-induced deprotection of dithioacetal, and this
deprotection reaction produced an aldehyde and exhibited pronounced fuores-
cence enhancement [16–18]. Moreover, an obvious color change of the solution
from colorless to light yellow could be observed, indicating its potential applica-
tion for the naked–eye detection of Hg2+ ions [19–25].
Scheme 1 Synthesis of sensors PHE-Ed and PHE-Pd
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Two novel fuorescent probes based on phenothiazine: synthesis…
Experimental
Materials and instrumentation
All solvents and chemicals (analytical grade) were obtained from commercial sup-
pliers and used without further purifcation. The laboratory used TLC to detect the
reaction process. Fourier Transform Infrared (FTIR) spectra were obtained by using
KBr pellets on a TENSOR27 FT–IR spectrometer. The NMR spectra were ana-
lyzed on a Bruker Avance 400 Spectrometer using dimethyl sulfoxide (DMSO–d6)
as the solvent, and chemical changes were recorded in ppm using tetramethylsilane
(TMS) as an internal standard. High resolution mass spectra (HRMS) were carried
out using an Agilent 6510 Accurate–Mass Q–TOF LC/MS system. The fuorescence
data were obtained by a F–4500 (Hitachi) fuorescence spectrometer.
Synthesis and characterization
Synthesis of compound 1
To the mixture of phenothiazine (3.986 g, 20.0 mmol), NaOH (6.40 mL, 79.5 mmol)
and CTAB (0.200 g, 0.5 mmol) in 50 mL of DMSO was slowly added ethyl bro-
mide (2.178 g, 20.0 mmol), and then the reaction was allowed to warm to 35 °C
and stirred at ambient temperature for 6 h. After TLC showed the conversion of the
starting material, the above mixture was cooled to room temperature, and distilled
(150 mL) water was added. The mixture was then extracted three times with ethyl
acetate (150 mL×3), the organic layer was then evaporated and the resultant residue
was purifed by column chromatography (petroleum ether/dichloromethane 10/1) to
provide compound 1 as white crystals (3.520 g, yield 77%); M.p. 103.3–103.7 °C;
IR (KBr, cm–1) 2907 (CH3), 1639 (C=C), 1442 (C=C), 1236 (C–N), 1108 (C–S)
1
cm–1; H NMR (400 MHz, DMSO–d6) δ=7.2–27.17 (m, 2 H), 7.14 (d, J=7.6 Hz,
2 H), 7.01 (d, J=7.4 Hz, 2 H), 6.94 (t, J=7.5 Hz, 2 H), 3.91 (s, 2 H), 1.29 (s, 3
H); 13C NMR (101 MHz, DMSO–d6) δ=144.88, 128.06, 127.47, 123.47, 122.82,
115.92, 13.15; ESI-TOF HRMS (m/z): calcd for C14H13SN, [M+H]+, 227.0918;
found, 227.0912.
Synthesis of compound 2
Phosphorus oxychloride (1.90 mL, 20.0 mmol) was slowly added to 5 mL of dry
DMF at 0 °C, and the mixture was warmed to room temperature and stirred for 1 h.
Then, a solution of 10-ethyl phenothiazine (2.273 g, 10.0 mmol) in 20 mL of DMF
was slowly added to the above mixture, and the reaction was heated to refux for
12 h. After which, distilled water (100 mL) was added, and the reaction was neutral-
ized with saturated NaOH aqueous solution. Then, the mixture was extracted three
times with dichloromethane (150 mL×3), the organic layer was collected and evap-
orated. The resultant residue was purifed by column chromatography (petroleum
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C. Wang et al.
ether/ethyl acetate 4/1) to give compound 2 as yellow crystals; (1.852 g, yield 72%);
M.p. 97.9–98.2 °C; IR (KBr, cm–1) 2907 (CH3), 1671 (CHO), 1609 (C=C), 1462
1
(C=C), 1202 (C–N), 1104 (C–S) cm–1; H NMR (400 MHz, DMSO–d6) δ=9.65
(s, 1 H), 7.65–7.39 (m, 2 H), 7.18–6.84 (m, 5 H), 3.87 (s, 2 H), 1.18 (s, 3 H). 13C
NMR (101 MHz, DMSO–d6) δ=190.99, 149.96, 143.02, 131.22, 130.71, 128.45,
128.03, 127.60, 124.00, 123.27, 122.23, 116.55, 115.54, 13.06; ESI-TOF HRMS
(m/z): calcd for C15H13SNO, [M+H]+, 255.0812; found, 255.0815.
Synthesis of PHE–Ed
To the mixture of 2-aldehyde-10-ethyl phenothiazine (0.205 g, 1.0 mmol) and
1,2-ethanedithiol (0.092 g, 1.0 mmol) in 20 mL of dichloromethane was added
a catalytic amount of boron trifuoride diethyl etherate (0.10 mL, 0.79 mmol) at
0 °C. The reaction was stirred at 0 °C overnight, and 10 mL of methanol was
added. After which, a pale green precipitate was formed, the mixture was then
fltered and the solid was washed with methanol for three times to aford the
desired product as green crystals (0.253 g, yield 61.8%); M.p. 83.4–84.7 °C; IR
(KBr, cm–1) 2906 (CH3), 1637 (C=C), 1455 (C=C), 1211 (C–N), 1107 (C–S)
1
cm–1; H NMR (400 MHz, DMSO–d6) δ = 7.18 (dd, J = 8.4, 2.2 Hz, 1 H), 7.13
(d, J = 2.1 Hz, 1 H), 7.09–7.02 (m, 1 H), 6.99 (dd, J = 7.6, 1.5 Hz, 1 H), 6.87 (d,
J = 8.1 Hz, 1 H), 6.80 (dt, J = 7.5, 3.2 Hz, 2 H), 5.54 (s, 1 H), 3.75 (q, J = 6.8 Hz,
2 H), 3.48–3.27 (m, 2 H), 3.23–3.12 (m, 2 H), 1.14 (t, J= 6.9 Hz, 3 H); 13C NMR
(101 MHz, DMSO–d6) δ = 144.58, 144.40, 135.23, 128.16, 127.66, 127.50,
126.68, 123.30, 122.93, 115.91, 115.57, 54.65, 13.07; ESI-TOF HRMS (m/z):
calcd for C17H17S3NO, [M + H]+, 331.0613; found, 331.0615.
Synthesis of PHE–Pd
To the mixture of 2-aldehyde-10-ethyl phenothiazine (0.205 g, 1.0 mmol) and
1,3-propanedithiol (0.108 g, 1.0 mmol) in 20 mL of ethanol was added a cata-
lytic amount of boron trifuoride diethyl etherate (0.10 mL, 0.79 mmol), and the
reaction was heated to refux for 12 h. After TLC showed the conversion of the
starting material, the reaction was cooled in a refrigerator and a pale pink solid
was formed. The mixture was then fltered and the solid was recrystallized with
acetonitrile to aford the desired product as white crystals (0.223 g, yield 64.5%);
M.p. 129.5–130.1 °C; IR (KBr, cm–1) 2939 (CH3), 1602 (C=C), 1463 (C=C),
1240 (C–N), 1035 (C–S) cm–1; 1H NMR (400 MHz, DMSO–d6) δ = 7.13–7.03 (m,
2 H), 7.03–6.97 (m, 2 H), 6.90–6.77 (m, 3H), 5.17 (s, 1 H), 3.76 (q, J = 6.8 Hz, 2
H), 2.91 (t, J = 12.4 Hz, 2 H), 2.73 (d, J = 14.0 Hz, 2 H), 1.97 (d, J = 14.0 Hz, 1
H), 1.55 (q, J = 12.6, 11.2 Hz, 1 H), 1.15 (t, J = 6.9 Hz, 3H). 13C NMR (101 MHz,
DMSO–d6) δ = 144.74, 144.46, 134.08, 128.21, 127.52, 127.37, 126.31, 123.47,
123.02, 122.82, 115.93, 49.26, 31.51, 25.14, 13.06; ESI-TOF HRMS (m/z): calcd
for C18H19S3NO, [M + H]+, 345.0811; found, 345.0814.
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Two novel fuorescent probes based on phenothiazine: synthesis…
Result and discussion
Selectivity studies
UV–Vis absorption spectra
To investigate the recognition abilities of synthetic probes PHE–Ed and PHE–Pd
toward Hg2+, we carried out a series of host–guest recognition experiments on
UV–Vis spectroscopy. As shown in Fig. 1, the recognition profles of PHE–Ed
Fig. 1 a Absorption spectrum changes of sensor PED–Ed (10−5 M) in the presence of diferent metal
ions (10−5 M) in EtOH/H2O (1/1, v/v) solution; b Absorption spectrum changes of sensor PED–Pd (10−5
M) in the presence of diferent metal ions (10−5 M) in EtOH/H2O (1/1, v/v) solution
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C. Wang et al.
toward various metal ions, including Ag+, Mg2+, Cd2+, Al3+, Co2+, Ni2+, Cu2+,
Zn2+ Cr2+and Hg2+ were investigated in EtOH/H2O (1/1, v/v) solution, and the
results indicated that free probe PHE–Ed in EtOH/H2O (1/1, v/v) solution exhib-
ited an obvious absorption band at 310 nm (Fig. 1a). While addition of Hg2+ to the
above solution containing free probe PHE–Ed resulted in a remarkable red–shift
absorption band at 390 nm, indicating the coordination of probe PHE–Ed with
Hg2+ ion. More importantly, this process was accompanied with an obvious color
change from colorless to light yellow (Fig. 2a). In comparison, addition of other
metal ions showed no signifcant change of the absorption band and color, indi-
cating the high selectivity of probe PHE–Ed toward Hg2+ ion. The recognition
profles of PHE–Pd toward various metal ions were also investigated, and the
similar absorption band shift (Fig. 1b) and color change (Fig. 2b) were observed
after addition of Hg2+ ion to the solution containing free probe PHE–Pd. These
results illustrated the high efcient and selectivity of PHE–Pd toward Hg2+ ion
compared with various other metal ions including Ag+, Mg2+, Cd2+, Al3+, Co2+,
Ni2+, Cu2+, Zn2+ and Cr3+. Next, Job’s plots were used to determine the stoichio-
metries of the complexes formed between PHE–Ed and Hg2+. The results indi-
cated a 1:1 stoichiometry for the binding of probe PHE–Ed with Hg2+ (Fig. 3a).
Furthermore, the association constant Ka for the PHE–Ed with Hg2+ was calcu-
lated to be 9.6 × 103 M−1 using the Benesi-Hildebrand equation. Similarly, the
binding of probe PHE–Pd with Hg2+ was 1:1 and the association constant Ka for
the PHE–Pd with Hg2+ was determined as 1.23 × 104 M−1 (Fig. 3b).
Fig. 2 a Colorimetric performance of sensor PHE–Ed (10−5 M) upon addition of diferent metal ions
(10−5 M) in EtOH/H2O (1/1, v/v) solution; b Colorimetric performance of sensor PHE–Pd (10−5 M)
upon addition of diferent metal ions (10−5 M) in EtOH/H2O (1/1, v/v) solution
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Two novel fuorescent probes based on phenothiazine: synthesis…
Fig. 3 a Job’s plot for determining the stoichiometry of PED–Ed and Hg2+ in EtOH/H2O (1/1, v/v) solu-
tion; b Job’s plot for determining the stoichiometry of PED–Pd and Hg2+ in EtOH/H2O (1/1, v/v) solu-
tion. The total concentration was 10 µM
Fluorescence spectra
The fuorescence spectra of probes PHE–Ed and PHE–Pd toward a series of
metal ions (Ag+, Mg2+, Cd2+, Al3+, Co2+, Ni2+, Cu2+, Zn2+, Cr3+, and Hg2+)
were studied in EtOH/H2O (1/1, v/v) solution, as shown in Fig. 4. When at an
excited wavelength of 390 nm was added Hg2+ ion to the solution containing
free probe PHE–Ed, there resulted distinguished changes with a maximal fuo-
rescence emission band at around 610 nm (Fig. 4a). However, addition of other
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C. Wang et al.
Fig. 4 a Fluorescence spectra response of sensor PHE–Ed (10−5 M) upon addition of diferent metal
ions in EtOH/H2O (1/1, v/v) solution; b Fluorescence spectra response of sensor PHE–Pd (10−5 M)
upon addition of diferent metal ions in EtOH/H2O (1/1, v/v) solution
metal ions to the above solution showed a fuorescence emission band at around
455 nm, which proved the high efciency and selectivity of probe PHE–Ed
toward Hg2+. Similarly, a maximal fuorescence emission band at around 610 nm
(Fig. 4b) was observed after addition of Hg2+ ion to the solution containing free
probe PHE–Pd, and addition of other metal ions showed a fuorescence emission
band at around 455 nm, which illustrated the high efciency and selectivity of
probe PHE–Pd toward Hg2+ ion. Furthermore, the quantum yields (Φ) of probes
PHE–Ed and PHE–Pd were 0.11 and 0.12, respectively, using Rhodamine
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Two novel fuorescent probes based on phenothiazine: synthesis…
B as reference, which proved the potential application of probes PHE–Ed and
PHE–Pd.
Sensitivity studies
The sensitivity of PHE–Ed and PHE–Pd for detection of Hg2+
To ascertain the sensitivity of probes PHE–Ed and PHE–Pd toward Hg2+, the
fuorescence titration experiments were performed in EtOH/H2O (1/1, v/v) solu-
tion. As shown in Figs. 5 and 6, upon addition of an increasing amount of Hg2+
(0–14 µM) to the solution containing free probe PHE–Ed, a gradual increase of the
emission peak could be observed around 610 nm, and the fuorescence intensity
reached the platform when the concentration of Hg2+ was 10 µM, which implied
the 1:1 response of PHE–Ed to Hg2+ (Fig. 5a). In addition, the fuorescence inten-
sity enhancement showed a good linear relationship over Hg2+ (R2 =0.99973)
(Fig. 5b), and the detection limit of PHE–Ed toward Hg2+ was determined to be as
low as 1.87×10–8 mol/L on the basis of 3σ/k, indicating high sensitivity of probe
PHE–Ed for quantitative determination of Hg2+. Similarly, addition of an increas-
ing amount of Hg2+ (0–14 µM) to the solution of PHE–Pd (10 µM) also led to a
gradual increase of the emission peak around 610 nm with about 16–fold of fuo-
rescence intensity enhancement (Fig. 6a), and the fuorescence intensity enhance-
ment was observed with a good linear relationship over the Hg2+ (R2 =0.99764)
(Fig. 6b). Furthermore, the detection limit of PHE–Pd for Hg2+ was 2.07×10–8
mol/L on the basis of 3σ/k. All these results indicated that probe PHE–Pd could be
applied for determination of Hg2+ with high efciency and sensitivity.
Detection mechanism
As shown in Scheme 2, Hg2+–induced deprotection of thioacetals can lead to
the formation of the corresponding aldehydes that other metal ions could not
-induced deprotection reaction of thioacetals, consequently this process is always
applied for the development of some reaction–based probes to detect Hg2+ ion.
In probes PHE–Ed and PHE–Pd, the electron–rich 2-ethyl phenothiazine group
was designed as electron donor, and the dithioacetal group was designed as a
weak electron donor, which formed a donor–donor system to suppress the intra-
molecular charge transfer (ICT) process. When Hg2+ ion was added to the solu-
tion containing free PHE–Ed and PHE–Pd, respectively, an electron–defcient
aldehyde group was formed. This aldehyde group could be served as an electron
acceptor and combined with the electron–rich group (2–ethyl phenothiazine) to
switch on the ICT process, thereby causing a signifcant red shift in the absorp-
tion and emission bands [26–32].
To further confrm the detection mechanism of probes PHE–Ed and PHE–Pd,
the reaction product of probe PHE–Ed and PHE–Pd with Hg2+ has been con-
frmed by the FT-IR spectra. As shown in Fig. 7a, the FT-IR spectra of PHE–Ed
in the presence of Hg2+ (1.0 equiv) clearly showed a new typical and prominent
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C. Wang et al.
Fig. 5 a The fuorescence spectra of sensor PHE–Ed (10 μM) with the increasing concentration of Hg2+
ions (0–14 μM) in EtOH/H2O (1/1, v/v) solution; b The linear ft between sensor PHE–Ed and Hg2+ ions
absorption peak of an aldehyde group at 1665 cm−1, which indicated the successful
deprotection of a dithioacetal group induced by Hg2+. Similarly, the FT-IR spectra
of PHE–Pd in the presence of Hg2+ (1.0 equiv) showed a new typical and promi-
nent absorption peak of an aldehyde group at 1684 cm−1 as shown in Fig. 7b, which
proved the successful deprotection of the dithioacetal group.
Moreover, the 1HNMR titration experiments were implemented to measure
the deprotection process. As shown in Fig. 8a, after addition of Hg2+ ion (10 µM)
to the solution of PHE–Ed (10 µM) in DMSO–d6, the original signal assigned
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Two novel fuorescent probes based on phenothiazine: synthesis…
Fig. 6 a The fuorescence spectra of sensor PHE–Pd (10 uM) with the increasing concentration of Hg2+
ions (0–14 μM) in EtOH/H2O (1/1, v/v) solution; b The linear ft between sensor PHE–Pd and Hg2+ ions
Scheme 2 The proposed sensing mechanism of sensor PHE–Ed and PHE–Pd toward Hg2+
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C. Wang et al.
Fig. 7 a FT-IR spectra of Hg2+ ions (10 µM) to PHE–Ed (10 µM) in EtOH/H2O (1/1, v/v) solution; b
FT-IR spectra of Hg2+ ions (10 µM) to PHE–Pd (10 µM) in EtOH/H2O (1/1, v/v) solution
Fig. 8 a 1H NMR spectra of Hg2+ ions (10 µM) to PHE–Ed (10 µM) in DMSO–d6; b 1H NMR spectra
of Hg2+ ions (10 µM) to PHE–Ed (10 µM) in DMSO–d6
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Two novel fuorescent probes based on phenothiazine: synthesis…
to the methane (CH) proton at 5.54 ppm, and two other signals at 3.48–3.27 and
3.23–3.12 ppm assigned to methylene (CH2CH2) protons of the thioacetal group dis-
appeared completely. Concomitantly, a new peak at 9.65 ppm corresponding to the
aldehyde proton appeared, indicating the successful removal of the thioacetal group.
1
Similarly, the HNMR spectrum of PHE–Pd also dramatically changed after addi-
tion of Hg2+ ions, as shown in Fig. 8b. For example, the original signal ascribed to
the methane (CH) proton (5.17 ppm) and the original signals assigned to the methyl-
ene (CH2–CH2–CH2) protons (2.91, 2.73, 1.97 and 1.55 ppm) all disappeared com-
pletely. At the same time, a new peak at 9.80 ppm corresponding to the aldehyde
Fig. 9 The anti–interference properties of sensor PHE–Ed and PHE–Pd toward Hg2+ upon addition of
diferent metal ions (10−5 M) in EtOH/H2O (1/1, v/v) solution
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C. Wang et al.
1
proton appeared. The two newly formed HNMR spectra were both nearly or the
same as that of compound 2, indicating the successful deprotection of dithioacetal
groups in PHE–Ed and PHE–Pd triggered by Hg2+.
Anti–interference Properties
The selectivity of probes PHE–Ed and PHE–Pd toward Hg2 were investigated by a
series of competitive experiments in EtOH/H2O (1/1, v/v) solution (Fig. 9a). Upon
mixing of probe PHE–Ed (10 µM) with other metal ions (Ag+, Mg2+, Cd2+, Al3+,
Co2+, Ni2+, Cu2+, Zn2+, Cr3+), the fuorescence intensity at 610 nm showed no dis-
tinguishing changes. Subsequent addition of Hg2+ to the above solution led to a
prominent enhancement of the emission band at 610 nm, which demonstrated that
probe PHE–Ed showed high selectivity for detecting Hg2+ ion. A similar phenom-
enon was observed in the competitive experiment of probe PHE–Pd (10 µM) with
various metal ions (Fig. 9b), and the results indicated that probe PHE–Pd exhibited
high selectivity and anti–interference ability toward Hg2+.
The pH Efects and sensing response time
The infuence of pH and response time on fuorescence properties was investigated
in EtOH/H2O (1/1, v/v) solution. Probes PHE–Ed and PHE–Pd were treated with
Hg2+ (10 µM) in a wide pH range of 2.0–12.0 as shown in Fig. 10a, and an obvious
fuorescence enhancement was observed in the pH range of 5.0–9.0, which illus-
trated that the two probes showed high detection abilities in the middle pH value.
Furthermore, the time required for Hg2+-promoted deprotection of PHE–Ed and
PHE–Pd was measured in EtOH/H2O (1/1, v/v) solution as shown in Fig. 10b. Upon
treatment of the two probes with Hg2+ (10 µM), the fuorescence intensity obvi-
ously enhanced with the increasing of time, and reached a platform after 5 min later.
These results revealed that the two colorimetric fuorescent probes can quickly iden-
tify Hg2+ in 5 min and meet the response time requirements for real–time monitor-
ing of Hg2+ with high sensitivity and selectivity in practical samples.
Research on solid colorimetric experimental and practical application
To confrm whether probes PHE–Ed and PHE–Pd can be used as simple and ef-
cient solid–state fuorescent probes for detecting Hg2+, the recognition ability of the
two synthetic probes toward Hg2+ was determined on pretreated flter paper strips,
as shown in Fig. 11. The flter paper strips were placed in solution of PHE–Ed or
PHE–Pd (1.0×10−3 mol/L) in DMSO for several seconds and then removed. The
test strips were dried in air and then treated with Hg2+ solution (1.0×10−5 mol/L)
in water. When the test strips were dipped into the solution of Hg2+, all strips dem-
onstrated distinct color changes. These results proved that the probes PHE–Ed and
PHE–Pd could be applied for the solid–state detection of Hg2+, thus providing a con-
venient, economical and highly efcient method for naked–eye detection of Hg2+.
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Two novel fuorescent probes based on phenothiazine: synthesis…
Fig. 10 a Efect of pH on the fuorescence intensity of PHE–Ed and PHE–Pd (10−5 M) in the presence
of 10 µM Hg2+; b Efect of response time on the fuorescence intensity of PHE–Ed and PHE–Pd (10−5
M) in the presence of 10 µM Hg2+
Furthermore, the practical application of PHE-Ed and PHE-Pd for detection
of Hg2+ was investigated in real samples including drinking water. As shown in
Fig. 12, addition of the synthetic probes to drinking water containing Hg2+ resulted
in color changes from colorless to light yellow, which indicated that the two probes
showed potential applications in environmental samples.
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C. Wang et al.
Fig. 11 The flter paper test strips of PHE–Ed and PHE–Pd in DMSO, treated with Hg2+ solution (10−5
M) in water
DW
DW+
DW+
DW
DW+
DW+
PHE-Ed PHE-Ed+ Hg2+
PHE-Pd PHE-Pd+ Hg2+
Fig. 12 The color change of PHE-Ed and PHE-Pd in real samples of drinking water (DW)
Compared with other probes
Finally, a detailed comparison between probes PHE-Ed and PHE-Pd and some
other Hg2+-selective fuorescent probes reported in the literature are represented in
Table 1. Obviously, probes PHE-Ed and PHE-Pd showed high performances includ-
ing superior DL, similar or even superior response time and pH response range in
most cases compared with the previously reported probes [10, 12, 15, 33–37]. In
addition, these two probes also exhibited excellent detection methods such as colori-
metric, fuorescence in environmentally friendly aqueous EtOH media, which indi-
cated that the two probes could be used to detect trace amounts of hazardous Hg2+
ions in real samples.
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Two novel fuorescent probes based on phenothiazine: synthesis…
Table 1 Comparison of PHE-Ed and PHE-Pd for Hg2+ detection with other Hg2+-selective probes
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C. Wang et al.
Conclusions
Two novel colorimetric fuorescent probes based on phenothiazine were designed
and synthesized, and their detection ability toward Hg2+ was caused by the
Hg2+-promoted deprotection of thioacetal groups. The two probes exhibited high
anti-interference performance, rapid response, high sensitivity and excellent selec-
tivity towards Hg2+ within a wide pH range, and traces of Hg2+ ions could be rap-
idly detected in aqueous solution. Their recognition ability was tested on flter paper
test strips and drinking water, and the results illustrated that the two probes could act
as efcient solid–state colorimetric fuorescent probes for highly selective and sensi-
tive detection of Hg2+. It also illustrated that Hg2+ ions could be rapidly detected in
drinking water by the two probes with high selectivity.
Acknowledgements The work was supported by the Shandong Provincial Natural Science Foundation of
China (ZR2016EMM13, ZR2017LC005, ZR2018PB007), the Major Science and Technology Innovation
Project of Shandong Province (2018CXGC1107).
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