A highly selective dual-channel chemosensor for mercury ions: Utilization of the mechanism of intramolecular charge transfer blocking

Article abstract of DOI:10.1039/c4nj00599f

In this study, we developed a novel chemosensor (ATS) that contains the 5-(4-nitrophenyl)-2-furan group and α-naphthylamine for the dual-channel sensing of mercury ions. Under attack from mercury ions, the probe undergoes a blocked process; moreover, it broke the intramolecular charge transfer (ICT) state of the chemosensor, which can be confirmed by ¹H NMR, IR and MS studies. We designed this probe to produce a significant change in the color and fluorescence intensity that can achieve naked eye recognition. The experimental results proved that the 5-(4-nitrophenyl)-2-furan group is a very good chromophore. In addition, the ICT state inhibited the fluorescence of naphthylamine. The fluorescent intensity changed significantly when naphthylamine was released. Moreover, high selectivity experiments revealed that the fluorescent sensor is specific to mercury ions even with interference by high concentrations of other metal ions. The test strips based on ATS were fabricated, which could act as a convenient and efficient mercury ion test kit. This journal is

Full text of DOI:10.1039/c4nj00599f

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A highly selective dual-channel chemosensor for
mercury ions: utilization of the mechanism of
intramolecular charge transfer blocking†
Cite this: New J. Chem., 2014,
38, 5075
You Ming Zhang, Wen Juan Qu, Guo Ying Gao, Bing Bing Shi, Gui Yuan Wu,
Tai Bao Wei,* Qi Lin and Hong Yao
In this study, we developed a novel chemosensor (ATS) that contains the 5-(4-nitrophenyl)-2-furan
group and a-naphthylamine for the dual-channel sensing of mercury ions. Under attack from mercury
ions, the probe undergoes a blocked process; moreover, it broke the intramolecular charge transfer
(ICT) state of the chemosensor, which can be confirmed by 1H NMR, IR and MS studies. We designed
this probe to produce a significant change in the color and fluorescence intensity that can achieve
naked eye recognition. The experimental results proved that the 5-(4-nitrophenyl)-2-furan group is a
very good chromophore. In addition, the ICT state inhibited the fluorescence of naphthylamine. The
fluorescent intensity changed significantly when naphthylamine was released. Moreover, high selectivity
experiments revealed that the fluorescent sensor is specific to mercury ions even with interference by
high concentrations of other metal ions. The test strips based on ATS were fabricated, which could act
as a convenient and efficient mercury ion test kit.
Received (in Montpellier, France)
16th April 2014,
Accepted 29th July 2014
DOI: 10.1039/c4nj00599f
www.rsc.org/njc
in the supramolecular approach.14 Most of these traditional
chemosensors contain a sulfur moiety and the sensing process
1. Introduction
Mercury pollution is a global problem, and a major source of involves the irreversible coordination of mercury ion to the
human exposure stems from contaminated natural water. As S atom.15 The mechanism for traditional sensors includes an
estimated by the US Environmental Protection Agency (EPA), affinity irreversible organic reaction, driven by the extremely
the annual total global mercury emissions from all sources, strong Hg–S affinity: (1) ring opening of spirocyclic systems
both natural and human-generated, has reached nearly (such as rhodamine and fluorescein), (2) intramolecular cyclic
7500 tons per year.1 Thus, obtaining new methods for detecting guanylation of thiourea derivatives, and (3) conversion of thio-
mercury and other toxic metal ions, which are cost-effective, carbonyl compounds to their carbonyl analogues or a sequential
rapid, facile, and applicable to the environmental and biological desulfurization-lactonization reaction.16 This thiophilic approach
milieus, is an important goal.2–11 Many fluorescent probes for will suffer from potential interference in sulfur-rich environ-
mercury ions have been developed to date.12 A supramolecular ments, where mercury is abundant, and undesired oxidation of
approach has received much attention, in which a binding event the probes containing a sulfur atom by air or oxidizing reagents
between a mercury ion and an appropriate ligand attached to a also inevitably occurs. Other drawbacks may be the requirement
fluorophore leads to change in the fluorescence property. for elevated temperatures or excess quantities of Hg2+ to drive
Recently, a reaction-based approach, which uses a metal-specific these desulfurization reactions to completion. In view of this
chemical conversion to cause fluorescence change, has received requirement and as part of our research efforts devoted to ion
considerable interest.13 Such types of probes, called chemo- recognition,17 an attempt was made to obtain a non-sulfur,
dosimeters or reactive probes, can provide a complete selectivity recyclable and efficient fluorescent sensor, which could sense
toward the target species, which is the most challenging issue mercury ions with specific selectivity and high sensitivity.18
Currently, a wide variety of chemosensors19 recognize metal
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of
Education of China, Key Laboratory of Polymer Materials of Gansu Province,
College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: weitaibao@126.com;
Fax: +86 9317973191; Tel: +86 9317973191
ions mainly by twisted intramolecular charge transfer (TICT)
and the formation of coordination compounds. Our research
group has a longstanding interest in molecular recognition.
Herein, we report a non-sulfur,20 dual-channel (colorimetric and
fluorescent), simple and easy to prepare Schiff base chemosensor
(ATS) for mercury ions, based on the breaking ICT mechanism.
† Electronic supplementary information (ESI) available: Complete experimental
procedures and some of the spectroscopic. See DOI: 10.1039/c4nj00599f
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(TMS, d scale with the solvent resonances as internal standards).
Electrospray ionization mass spectra (ESI-MS) were measured on
an Agilent 1100 LC-MSD-Trap-VL system. UV-visible spectra were
recorded on a Shimadzu UV-2550 spectrometer. The photo-
luminescence spectra were performed on a Shimadzu RF-5301
fluorescence spectrophotometer. The melting points were measured
on an X-4 digital melting-point apparatus (uncorrected). The
infrared spectra were performed on a Digilab FTS-3000 FT-IR
spectrophotometer.
Fig. 1 Single crystal structure of ATS.
2.2. Synthesis of sensor molecule ATS
The compound ATS can be readily prepared by a simple and low-
cost Schiff base reaction of 5-(4-nitrophenyl)-2-furan formalde-
hyde and a-naphthylamine (Scheme 1). 5-(4-Nitrophenyl)-2-furan
formaldehyde (1.085 g, 5 mmol), a-naphthylamine (0.7865 g,
5.5 mmol) and a catalytic amount of acetic acid (AcOH) were
combined in hot absolute EtOH (30 mL). The solution was stirred
under reflux for 4 hours. After cooling to room temperature, the
brown precipitate was filtered, washed three times with hot
absolute ethanol, then recrystallized with DMF/EtOH to obtain
a brown powdered product ATS (4.31 mmol) in 86.2% yield
(m.p. 180 1C). This is an excellent green chemistry method for
the preparation of these kinds of fine chemicals. 1H NMR
(DMSO-d6, 400 MHz) d 8.61 (s 1H, NQCH), 7.28–8.37 (m 14H,
ArH). 13C-NMR (DMSO-d6, 100 MHz) d/ppm 153.60, 153.14,
148.33, 147.97, 146.72, 135.04, 133.58, 128.49, 127.68, 126.55,
126.32, 126.09, 125.98, 125.14, 124.60, 124.60, 123.48, 123.48,
119.79, 119.79, 112.96, 112.56.
We designed this probe to produce a significant change in the color
and fluorescence intensity that can achieve naked eye recognition.
The experimental results prove that the 5-(4-nitrophenyl)-2-furan
group is a very good chromophore. In addition, the ICT state
inhibits the fluorescence of naphthylamine. Moreover, the fluores-
cence intensity can change significantly when naphthylamine
is released.
The sensing was observed due to the coordination of metal
ions with Hg(II) through the imine nitrogen, which thus inter-
fere with the ICT process, and yield CHEF in the presence of
Hg(II) ions. The single crystal structure of ATS also confirmed
the synthesis results (Fig. 1).21 The single crystal structure
photograph indicating that a 5-(4-nitrophenyl)-2-furan group
and a-naphthylamine within the molecule are on a plane and
the ICT state exists at the same time.
The naphthalene groups act as fluorophores, while the pre-
sence of furan groups in the same sensor molecule act as the
functional groups. Sensor ATS showed both colorimetric and
fluorescent selectivity for mercury ions in dimethylsulfoxide
(DMSO) over other common physiologically important metals.
ATS was almost non-luminescent in solution, as a result of the
presence of the ICT state. Upon the addition of mercury ions,
the reaction between mercury ions and HCQN in ATS blocks
the ICT state, and the release of a-naphthylamine leads directly
to strong blue emission. Both the ultraviolet and fluorescence
properties can be seen by naked eyes. Importantly, probe ATS
displays a fast response (o2 h) to mercury ions at room
temperature, and the fluorescence intensity is increased more
than 5 fold with the addition of mercury ions. This is a typical
sensing mechanism between ATS and mercury ions. To the best
of our knowledge, this is a good approach for the selective
recognition of mercury ions.
IR (KBr) v: 1624 (HCQN), 1598 (CQC), 3045 (ArH) cmÀ1
.
Anal. calcd for C21H14N2O3: C, 73.68; H, 4.09; N, 8.19; O, 9.36.
Found C, 73.68; H, 4.09; N, 8.19; O, 9.36. ESI-MS calcd for
C
21H14N2O3 + H+ 343.2. Found 343.2.
2.3. General procedure for spectroscopy
All the UV-vis experiments were carried out in a DMSO solution
on a Shimadzu UV-2550 spectrometer. Any changes in the
UV-vis spectra of the synthesized compound were recorded
upon the addition of perchlorate salts while keeping the ligand
concentration constant (2.0 Â 10À5 M) in all experiments.
The perchlorate salts of Fe3+, Hg2+, Ca2+, Cu2+, Co2+, Ni2+,
Cd2+, Pb2+, Zn2+, Cr3+, and Mg2+ were used for the UV-vis
experiments.
Fluorescence spectroscopy was carried out in a DMSO solution
on a Shimadzu RF-5301 spectrometer. Any changes in the
fluorescence spectra of the synthesized compound were recorded
upon the addition of perchlorate salts while keeping the ligand
concentration constant (2.0 Â 10À5 M) in all experiments.
2. Experimental section
2.1. Materials and physical methods
Fresh doubly distilled water was used throughout the experiment.
All other reagents and solvents were commercially available at
analytical grade and were used without further purification.
1H NMR and 13C NMR spectra were recorded on a Mercury-
400BB spectrometer at 400 MHz for 1H. The chemical shifts
are reported in ppm downfield from tetra spectra were
recorded with a Mercury-400BB spectrometer at 400 MHz Scheme 1 Synthesis of the sensor molecule ATS.
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The perchlorate salts of the ions (Fe3+, Hg2+, Ca2+, Cu2+, Co2+,
Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, and Mg2+) were used for the
fluorescence experiments.
For 1H NMR titrations, the solution of R was prepared in
DMSO-d6 and the appropriate concentrated solution of guest
was prepared in doubly distilled water. Aliquots of the two
solutions were mixed directly in the NMR tubes.
3. Results and discussion
The recognition profiles of the chemosensor ATS toward various
metal cations, Fe3+, Hg2+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+,
Cr3+, and Mg2+, were primarily investigated by UV-vis spectroscopy
and fluorescence spectroscopy in DMSO. When 20 equivalents of
Hg2+ were added to a DMSO solution of sensor ATS (2.0 Â 10À5 M),
there was a significant color change, from yellow to colorless, which
was visible to the naked eye (Fig. 2b).
In the UV-vis spectrum of a solution of ATS in DMSO (2.0 Â
10À5 M), the strong and broad absorption from 405 to 490 nm
disappeared when 20 equivalents of Hg2+ were added (Fig. 2a).
To validate the selectivity of the sensor ATS, the same tests
were carried out using Fe3+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+,
Zn2+, Cr3+ and Mg2+ ions. None of these ions induced any
significant changes in the UV-vis spectrum of the sensor ATS
(ESI,† Fig. S1).
Fig. 3 (a) Fluorescence spectra upon excitation at 345 nm in DMSO of
ATS (2.0 Â 10À5 M) before and after addition of Hg2+ (20 equiv.). Inset:
photographs showing the change in the fluorescence of ATS after addition
of Hg2+ (20 equiv.) in DMSO. (b) Visual fluorescence emissions of probe
ATS (2.0 Â 10À5 M) after the addition of Fe3+, Hg2+, Ca2+, Cu2+, Co2+, Ni2+
,
Cd2+, Pb2+, Zn2+, Cr3+ and Mg2+ (20 equiv.) in DMSO on excitation at
345 nm using UV lamp at room temperature.
sensor ATS (2.0 Â 10À5 M) increased rapidly. The color change
from colorless to blue could be distinguished by the naked eye,
as shown in Fig. 3. To validate the selectivity of sensor ATS, the
same tests were carried out using Fe3+, Ca2+, Cu2+, Co2+, Ni2+,
Cd2+, Pb2+, Zn2+, Cr3+ and Mg2+ metal ions. None of these ions
induced any significant changes in the fluorescence spectrum
of the sensor (ESI,† Fig. S2).
The fluorescence measurements for sensor ATS were per-
formed in the DMSO solutions. As shown in Fig. 3, sensor ATS
emitted very weakly (lem = 428 nm; lex = 345 nm), indicating
that the expected fluorescence of the naphthalene units was
quenched by the formation of the ICT state. Upon the addition
of 20 equivalents of Hg2+ ions, the fluorescence intensity of
To further investigate the interactions between the sensor
ATS and Hg2+, the variation in the UV-vis absorption spectral of
sensor ATS (2.0 Â 10À5 M) in DMSO was recorded during the
titrations with different concentrations of Hg2+ from 0 to
8 equivalents. As shown in Fig. 4, an isosbestic point at 370 nm
was clearly observed with increasing concentration of Hg2+. At the
same time, fluorescence emission spectral variation of sensor
ATS (2.0 Â 10À5 M) in DMSO was monitored during titrations
with different concentrations of Hg2+ from 0 to 4.4 equivalents
(Fig. 5). The selectivity of ATS for Hg2+ over other metal ions was
examined. The results showed that all potentially competitive
metal ions exerted no or little influence on the absorbance and
fluorescence detection of Hg2+ in DMSO.
Time-dependent fluorescence measurements for a 1 : 20
mixture of ATS and Hg(ClO4)2 in DMSO showed that the
UV-vis absorption spectra (lmax = 403 nm) decreased gradually
while another spectrum (lmax = 345 nm) increased slightly. An
isosbestic point at 370 nm was clearly observed with increasing
time (ESI,† Fig. S3a). The absorbance of the complex reached
490% of the maximum after 100 min, which is in good agree-
ment with the results from the time-dependent scatter diagram
(ESI,† Fig. S3b).
Fig. 2 (a) Absorbance spectra of the target compound ATS (2.0 Â 10À5 M)
in DMSO in the presence of Hg2+ (20 equiv.). Inset: photograph showing
the change in color of the solution of ATS in DMSO after the addition of
Hg2+ (20 equiv.). (b) Color changes observed for ATS (2.0 Â 10À5 M) upon
the addition of Fe3+, Hg2+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+
and Mg2+ (20 equiv.) in DMSO solution.
The recognition mechanism of the sensor ATS with Hg2+ was
investigated by IR spectral and 1H NMR titration methods.
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Fig. 5 (a) Fluorescence spectra of ATS (2.0 Â 10À5 M) in the presence of
different concentrations of Hg2+ in DMSO, acquired after 120 min. (b) A
plot of the fluorescence intensity depending on the concentration of Hg2+
in the range from 0 to 4.4 equivalent. Each measurement was performed
after 120 min of mixing for ATS and Hg2+ in DMSO.
Fig. 4 (a) Absorbance spectra of ATS (2.0 Â 10À5 M) in the presence of
different concentrations of Hg2+ (0–8 equiv.) in DMSO, acquired after
120 min. (b) A plot of the variation in absorbance as a function of the
concentration of Hg2+ in the range from 0 to 8 equivalents. Each measure-
ment was performed after mixing ATS and Hg2+ in DMSO for 120 min. The
detection wavelength was 428 nm.
In the IR spectra of ATS, the stretching vibration absorption
peaks for HCQN, and furan CQC appeared at 1624 cmÀ1 and
1598 cmÀ1, respectively. However, when ATS reacted with Hg2+,
the stretching vibration absorption peaks of a-naphthylamine
N–H appeared at 3116 cmÀ1 and 3130 cmÀ1, while the stretch-
ing vibration absorption peaks of HCQN disappeared and a
new peak was observed at 1651 cmÀ1 due to the stretching
vibration absorption of the coordinated imine group, and the
5-(4-nitrophenyl)-2-furan group HCQO appeared at 2928 cmÀ1
and 2959 cmÀ1, which indicated HCQN bond rupture and
the release of the 5-(4-nitrophenyl)-2-furan group, as shown in
Fig. S4 (ESI†).
Fig. 6 Partial 1H NMR spectra of ATS (0.05 M, DMSO-d6) and in the
presence of varying amounts of Hg2+
.
The results of 1H NMR titration experiments also support
this presumption. As shown in Fig. 6, after adding 0.1 equiv. of
Hg2+, the HCQO peak at 9.7 ppm appeared. At the same time, which indicated that H–N–H was involved in the coordination.
the peak at 8.61 ppm decreased gradually, indicating that In addition, the bond breaking mechanism was further con-
charge transfer from the benzene and furan groups is blocked firmed by the appearance of a peak at m/z 144 and m/z 343,
and CQN disappears. We further examined the 1H NMR of assignable to [a-naphthylamine + H+] and [a-naphthylamine +
the 5-(4-nitrophenyl)-2-furan formaldehyde. We found that the Hg2+ + H+] in the ESI/MS (ESI,† Fig. S10).
aldehyde proton signal is the same as the 1H NMR titration,
From the results of the 1H NMR spectroscopic titration,
which suggests for that the signal at 9.6 ppm in 1H NMR titration FT-IR spectra and mass spectra, it can be partially proven that
could be the aldehyde peak (ESI,† Fig. S11). Moreover, the proton the reaction between ATS and Hg2+ had taken place in one
signals of N–H at 6.85 ppm shifted downfield and narrowed, step (Fig. 7).
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Fig. 7 Hg2+ ions prevent the ICT process for ATS facilitating the detection
Fig. 9 Photographs of ATS on the test paper (A) after immersion into
DMSO solutions with 1 Â 10À4 M Hg2+, (B) after immersion into DMSO
solutions with 1 Â 10À5 M Hg2+, (C) after immersion into DMSO solution
with 1 Â 10À6 M Hg2+, (D) only ATS in visible light (E) after immersion into
DMSO solutions with 1 Â 10À4 M Hg2+, (F) after immersion into DMSO
solutions with 1 Â 10À5 M Hg2+, (G) after immersion into DMSO solutions
with 1 Â 10À6 M Hg2+, (H) after immersion into DMSO solutions with
1 Â 10À7 M Hg2+ under irradiation at 365 nm.
of Hg2+ ions.
Upon the attack by mercury ions, the probe undergoes a break
process and blocks the ICT effect. Moreover, with the release of a
5-(4-nitrophenyl)-2-furan group and a-naphthylamine, the solution
turn into colorless and fluorescence enhancement obviously.
To further explore the sensing mechanism between ATS and
Hg2+, the colorimetric and fluorimetric sensing abilities were
investigated by adding Hg2+ ions to the a-naphthylamine. The
colorimetric properties for a-naphthylamine and Hg2+ were
explored in DMSO solutions. As shown in Fig. S5 (ESI†), a strong
and broad absorption at 336 nm was blue shift when 20 equivalents
of Hg2+ were added, which indicates that a-naphthylamine is
coordinated with Hg2+. The same experiment was carried out by
the addition of Hg2+ in ATS solution. After the addition of
20.0 equivalent of Hg2+ ions, a significant decrease in the emis-
sion band at 428 nm was observed, which was the same as that of
a-naphthylamine. This result further confirms that the proposed
mechanism was correct (ESI,† Fig. S5 and S6).
4. Conclusions
By taking advantage of the intramolecular charge transfer
blocking mechanism, we have developed a highly selective dual-
channel chemosensor for mercury(II) ions based on a non-sulfur,
easily prepared Schiff base compound. This work highlights a
typical sensing mechanism between ATS and mercury ions.
Notably, the high selectivity experiments showed that the fluores-
cent sensor is specific for mercury ions even after interference
by high concentrations of other metal ions. The test strips
could conveniently detect Hg2+ in solution. Moreover, this work
may stimulate the further development of new chemosensors
possessing excellent performance.
To investigate the practical application of chemosensor ATS,
test strips were prepared by immersing filter paper into a DMSO
solution of ATS (0.1 M), followed by drying in air. The test strips
containing ATS were utilized to sense Hg2+ and other metal
ions. As shown in Fig. 8, when Hg2+ and the other ions were
added to the test kits, respectively, an obvious color change was
observed only with a Hg2+ solution under a 365 nm UV lamp.
The potentially competitive ions exerted no influence on the
detection of Hg2+ by the test strips.
This work was supported by the National Natural Science
Foundation of China (No. 21064006, 21262032 and 21161018),
the Program for Changjiang Scholars and Innovative Research
Team in University of Ministry of Education of China (No.
IRT1177), the Natural Science Foundation of Gansu Province
(No. 1010RJZA018), the Youth Foundation of Gansu Province
(No. 2011GS04735) and NWNU-LKQN-11-32.
Notes and references
In addition, we also examined the detection limit of the test
strips. As shown in Fig. 9, we prepared the test strips that
contain different concentration of Hg2+. The detection limit of
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