Highly selective and effective mercury(ii) fluorescent sensors

Article abstract of DOI:10.1039/c4nj01147c

A novel mercury non-sulfur and simple fluorescent chemosensor L based on the benzimidazole group and quinoline group as the fluorescence signal group has been designed and synthesized. The receptor could instantly detect the Hg²⁺ cation over other cations such as Fe³⁺, Ag⁺, Ca²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Cr³⁺, and Mg²⁺ by fluorescence spectroscopy changes in H₂O/DMSO (1:9, v/v) solution with specific selectivity and high sensitivity. The fluorescence color of the solution containing sensor L showed a remarkable change from blue to colorless only after the addition of Hg²⁺ in aqueous media while other cations did not cause obvious color changes. Moreover, further study demonstrates that the detection limit on the fluorescence response of the sensor to Hg²⁺ is down to 9.56 × 10^-9 M, which is far lower than the maximum level for mercury of 0.01 M in drinking water, from EPA guidelines. Test strips based on L were fabricated, which could act as convenient and efficient Hg²⁺ test kits. Thus, the probe should have potential applications in an aqueous environment for the monitoring of mercury.

Full text of DOI:10.1039/c4nj01147c

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Highly selective and effective mercury(II)
fluorescent sensors†
Cite this:DOI: 10.1039/c4nj01147c
JingHan Hu,* JianBin Li, Jing Qi and JuanJuan Chen
A novel mercury non-sulfur and simple fluorescent chemosensor L based on the benzimidazole group
and quinoline group as the fluorescence signal group has been designed and synthesized. The receptor
2+
3+
+
2+
2+
2+
2+
could instantly detect the Hg cation over other cations such as Fe , Ag , Ca , Cu , Co , Ni
,
2+
2+
2+
3+
2+
Cd , Pb , Zn , Cr , and Mg
2
by fluorescence spectroscopy changes in H O/DMSO (1 : 9, v/v)
solution with specific selectivity and high sensitivity. The fluorescence color of the solution containing
2+
sensor L showed a remarkable change from blue to colorless only after the addition of Hg in aqueous
Received (in Victoria, Australia)
1
0th July 2014,
media while other cations did not cause obvious color changes. Moreover, further study demonstrates
2
+
ꢁ9
Accepted 28th August 2014
that the detection limit on the fluorescence response of the sensor to Hg is down to 9.56 ꢀ 10 M,
which is far lower than the maximum level for mercury of 0.01 M in drinking water, from EPA guidelines.
DOI: 10.1039/c4nj01147c
2+
Test strips based on L were fabricated, which could act as convenient and efficient Hg test kits. Thus,
www.rsc.org/njc
the probe should have potential applications in an aqueous environment for the monitoring of mercury.
Development of organic molecules as receptors for the
sensing of environmentally hazardous Hg ions is of great
Mercury (Hg ) is considered as one of the most toxic and importance due to their implications in broad areas of chemistry,
dangerous elements for the environment because it is intro- biology, and the environment. Recently, many sensitive fluorescent
1
. Introduction
2
+
2
+
7
8
9
duced into the environment during natural phenomena and probes based on rhodamine, coumarin or squaraine derivatives,
1
0
human activities, including oceanic and volcanic eruptions, as well as other fluorophores, have been developed to detect
wind erosion, water erosion, solid waste incineration, forest mercury ions. Among them, various molecular structures, the
fires, combustion of fossil fuels, electrical apparatus, batteries thioether containing crown ethers/acetals, podands, thioureas,
11
12
13
1
,2
2+
14
15
16
and industrial production. The mercuric ion (Hg ), much amines/amides, spironolactone, heterocycle based moieties,
+
more common than mercurous ion (Hg ), is a caustic and etc. appropriately appended with chromogenic and fluorescent
2+
carcinogenic material with high cellular toxicity, which can be moieties have found applications in developing Hg sensors.
converted into methyl mercury by bacteria in the environment, Numerous analytical and sophisticated techniques have been
and subsequently accumulates in animals and plants and also used for the determination of mercury in real samples. These
3
17
enters into the human body through the food chain. As a include atomic absorption spectrometry, inductively coupled
1
8
19
strong neurotoxin, methylmercury ions can cause human plasma mass spectroscopy, spectrophotometry, neutron
2
0
21
health problems since they can easily pass through the skin activation analysis, anodic stripping voltammetry, X-ray
and respiratory and cell membranes, leading to digestive, fluorescence spectrometry, electrothermal atomic absorption
cardiac, kidney, and DNA damage, mitosis impairment, and spectrometry, atomic fluorescence spectrometry, cold vapor
atomic absorption spectrometry and potentiometric ion-selective
The United States Environmental Protection Agency (EPA) has electrodes. These methods have their own merits for mercury
2
2
2
3
24
4
25
especially permanent damage to the central nervous system.
2
6
2
+
set a maximum Hg contaminant level in food and drinking determination; however, they also have some disadvantages
water at 0.002 mg L (0.01 M). Therefore, it is very important such as being expensive, limited sample adaptability, require-
to detect the level of mercury in water and develop a simple yet ment of well-controlled experimental conditions, some inherent
ꢁ1
5
environmentally friendly mercury sensor with high sensitivity interference and time consuming procedures involving the
6
and selectivity.
use of sophisticated instrumentation, and chemical sensors
based on optical signal measurements are considered to be
advanced techniques because of their operational simplicity,
high selectivity, sensitivity, rapidity, cost effectiveness, direct
visual perception, and applicability to the environmental and
biological milieus. It is distinctly demonstrated that searching
College of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: hujinghan62@163.com
†
Electronic supplementary information (ESI) available: Complete experimental
procedures and some of the spectroscopic data. See DOI: 10.1039/c4nj01147c
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450 mL of distilled water, which produced a white precipitate; it
was quietly placed, filtered, and washed with distilled water
three times, then recrystallized with absolute ethanol to get a
1
white crystal of L (0.98 g, 80%) (m.p. 107–110 1C), H NMR
Scheme 1 Synthesis of sensor L.
(
DMSO-d , 400 MHz) d: 13.21 (s, 1H), 8.56 (d, J = 8.3 Hz, 1H), 8.49
6
(d, J = 8.5 Hz, 1H), 8.17 (d, J = 8.3 Hz, 1H), 8.08 (d, J = 7.9 Hz, 1H),
for production and development of mercury sensors is quite 7.88 (t, J = 7.6 Hz, 1H), 7.77 (d, J = 7.6 Hz, 1H), 7.69 (t, J = 7.3 Hz,
necessary.
1H), 7.62 (d, J = 7.5 Hz, 1H), 7.28 (dd, J = 13.4, 7.4 Hz, 3H); IR
Our research group has a longstanding interest in molecular (KBr, cm ) n: 3482 (–NH), 1659 (CQN–H), 3060 (Ar–H); ESI-MS
ꢁ1
27
recognition. Herein, we have elaborately designed and synthesized calcd for C H N + H 246.10, found 246.07.
1
6
11 3
a non-sulfur, simple and easy to prepare fluorescent chemosensor
2+
(
L, Scheme 1) for the Hg ion, according to the chelation-enhanced 2.3. General procedure
quenching (CHEQ) mechanism, in which the quinoline group acts
as a fluorophore group, benzimidazole groups into the same sensor
molecule as recognition groups, to allow the coordination capacity
required to chelate mercury ions. Sensor L showed fluorescent
All fluorescence spectroscopy measurements were carried
out after the addition of perchlorate metal salts in DMSO/
H O (9 : 1, v/v), while keeping the ligand concentration constant
2
ꢁ5
(
2.0 ꢀ 10 M) on a Shimadzu RF-5301 fluorescence spectro-
2+
2
selectivity for Hg in DMSO/H O (9 : 1, v/v) binary solution over
meter. The excitation wavelength was 352 nm. The solution of
other common physiologically important metal ions. The detection
ꢁ3
+
metal ions (4.0 ꢀ 10 M) was prepared from the perchlorate
2
+
limit on the fluorescence response of the sensor to Hg is down
3+
2+
2+
2+
2+
2+
2+
2+
2+
salts of Fe , Hg , Ag , Ca , Cu , Co , Ni , Cd , Pb , Zn ,
ꢁ9
to 9.56 ꢀ 10 M, which is far lower than the maximum level for
mercury of 0.01 M in drinking water from EPA guidelines, and
indicates that this sensor could potentially be useful as a probe
3+
2+
Cr , and Mg .
1
For H NMR titrations, two stock solutions were prepared in
DMSO-d , one containing only the sensor and the second
6
2
+
for monitoring Hg levels. The mechanism of this process has
containing an appropriate concentration of the metal. Aliquots
of the two solutions were mixed directly in NMR tubes.
Test strips were prepared by immersing filter papers in a
1
been investigated by H NMR and IR spectroscopy and ESI-mass
spectrometry.
2
DMSO–H O binary solution of L (0.01 M) followed by exposure
to air until completely dry. The test strips containing this
sensor L were utilized to detect Hg and other cations.
2. Experimental section
2+
2.1. Materials and physical methods
Fresh double distilled water was used throughout the experiment. The
3
. Results and discussion
inorganic salts Ca(ClO ) ꢂ6H O, Mg(ClO ) ꢂ6H O, Cd(ClO ) ꢂ6H O,
4
2
2
4 2
2
4 2
2
Fe(ClO ) ꢂ6H O, Hg(ClO ) ꢂ6H O, Co(ClO ) ꢂ6H O, Ni(ClO ) ꢂ6H O,
4
3
2
4 3
2
4 2
2
4 2
2
The receptor was found to have limited solubility in water, and
this compelled us to use the sensor in the mixed solvent, such
as H O/DMSO (1 : 9, v/v), for recognition studies of L. The
fluorescence spectral response of chemosensor L was studied
using aqueous solutions of the perchlorate metal salts of all
Cu(ClO ) ꢂ6H O, Zn(ClO ) ꢂ6H O, Pb(ClO ) ꢂ3H O, AgClO ꢂH O,
4
2
2
4 2
2
4 2
2
4
2
0
Cr(ClO
4
)
3
ꢂ6H
2
O and N-2-hydroxyethylpiperazine-N -2-ethanesulfonic
2
acid (HEPES) were purchased from Alfa Aesar Chemical Reagent Co.
Tianjin, China). All solvents and other reagents were of analytical
grade. H NMR and C NMR spectra were recorded on a Mercury-
00BB spectrometer at 400 MHz and 100 MHz. Chemical shifts are
(
1
13
3+
+
2+
2+
2+
common cationic analytes such as (Fe , Ag , Ca , Cu , Co ,
Ni , Cd , Pb , Zn , Cr , and Mg ) as well as Hg . Changes
4
2+
2+
2+
2+
3+
2+
2+
reported in ppm downfield from tetramethylsilane (TMS, d scale
with solvent resonances as internal standards). UV-vis spectra were
recorded on a Shimadzu UV-2550 spectrometer. Photoluminescence
spectra were recorded on a Shimadzu RF-5301 fluorescence spectro-
photometer. Melting points were measured using an X-4 digital
melting-point apparatus purchased from Beijing Tech Instrument
Co. (uncorrected). Infrared spectra were recorded on a Digilab FTS-
in the spectral pattern were observed in the presence of added
2+
10 equivalent of Hg ; the solution of sensor L displayed a
dramatical color change, from blue to colorless, in the fluores-
ꢁ5
cence spectrum recorded on the sensor (2.0 ꢀ 10 M) in a
DMSO/H O (9 : 1, v/v) system (Fig. 1 and 2). To validate the
2
selectivity of sensor L, the same tests were also carried out
using competitive metal ions, and none of these ions induced
any significant changes in the fluorescent spectrum and the
3000 FT-IR spectrophotometer.
ꢁ
4
test strips based on the sensor L (2 ꢀ 10 M) as depicted in
2.2. Synthesis of sensor L
2+
Fig. 3. Thus, compound L shows high selectivity toward Hg .
The synthesis route of sensor L is demonstrated in Scheme 1. An Furthermore, the interference from metal ions can be eliminated
2
+
ethanol solution (25 mL) of 2-quinolinecarbaldehyde (0.786 g, and displayed specific sensitivity to Hg .
5
4
mmol) and NaHSO (0.624 g, 6 mmol) as a catalyst was stirred
h at the room temperature, a DMF (15 mL) of O-phenylenediamine recognition properties of receptor L as a Hg probe (Fig. 4).
Fluorescent titration was performed to gain insight into the
3
2
+
(
0.54 g, 5 mmol) was added to the mixed solution. Then, the The emission band at 410 nm of chemosensor L decreased
2
+
mixed solution was stirred at 353 K for 2 h. After cooling to room remarkably as the Hg (0.02 M) volume increased from 0 to
temperature, the pale white solution was dropwise added to the 24 mL. In the meantime, the minimum concentration of Hg
2
+
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ꢁ
5
Fig. 1 Fluorescence spectra of L (2.0 ꢀ 10
M) in the presence of
1
2
0 equiv. of various cations in H O/DMSO (1 : 9, v/v) binary solution at
room temperature (lex = 352 nm).
Fig. 2 Color changes observed upon the addition of various cations
(
10 equiv.) to solutions of sensor L (2 ꢀ 10ꢁ5 M) in DMSO/H
2
O (9 : 1, v/v)
under a UV-lamp (365 nm).
ꢁ4
Fig. 3 (a) Normalized change in the emission intensity of L (2 ꢀ 10 M)
2
+
ꢁ3
after addition of the Hg ion (4 ꢀ 10 M) in the presence of an excess
ꢁ3
amount of other cations (4 ꢀ 10 M) in the DMSO/H
2
O (9 : 1, v/v) solution.
ꢁ4
that could be observed though one order of magnitude lower (b) Photographs of the test strips based on L (2 ꢀ 10 M) after immersing
2+
ꢁ4
ꢁ
6
the Hg ion (4 ꢀ 10 M) in the presence of an excess amount of other
for fluorescence naked eye detection was 5.0 ꢀ 10 M by using a
ꢁ3
cations (4 ꢀ 10 M) under irradiation at 365 nm.
UV lamp at 365 nm, and the detection limit of the fluorescence
2
8
spectral measurements, as calculated on the basis of 3S
where S is the standard deviation of the blank solution and S is
the slope of the calibration curve; Fig. 5), showed a detection
limit of approximately 9.56 ꢀ 10 M for Hg , which is far lower
than the maximum level for mercury of 0.01 M in drinking water
from EPA guidelines.
B
/S
(
B
ꢁ9
2+
We propose that the reaction mechanism in this system may
proceed through the route depicted in Scheme 2. These results
2
+
reveal that the sensor of L reacts toward Hg and forms an
organometallic compound. It leads to decrease and destruction
of the sensor conjugate rigid plane structure; the probe L shows
fluorescence quenching by chelating effects. Therefore, it can be
clearly seen that the sensor L shows selective and sensitive response
of mercury over other cations in aqueous media (H
: 9, v/v).
The interaction and binding behavior between L and the
2
O/DMSO,
1
2
+
1
Hg ion were investigated with H NMR titration experiments,
as shown in Fig. 6. There was one intramolecular hydrogen
Fig. 4 Fluorescence titration spectra of L (2.0 ꢀ 10ꢁ M) in H
5
O/DMSO
2
2+
bond in the sensor of L: NHꢂ ꢂ ꢂNQC. The formation of this (1 : 9, v/v) solution upon addition of an increasing concentration of Hg
1
(lex = 352 nm).
hydrogen bond led to the H NMR chemical shifts of NH
appearing at a low-field of the probe L at 13.21 ppm, and led
to the sensor L of the conjugate rigid plane structure increasing group at 13.21 ppm gradually disappeared, and increased the
and producing a strong blue fluorescent color. After the addition electronegativity of the whole ring. Thus, there were signal peaks
2
+
of 0.5 equivalent of Hg , the NH peak of the benzimidazole of the benzimidazole group showing a significant upfield shift.
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1
Fig. 6 Partial H NMR spectra of L (0.01 M) in the presence of varying
2+
amounts of Hg (0.5 M).
2+
Fig. 5 (a) Fluorescent changes upon the addition of Hg at the indicated
concentrations. Images were taken under white UV light at 365 nm.
(
(
(
b) Fluorescence detection limit spectra of L (2.0 ꢀ 10ꢁ5 M) in H
2
O/DMSO
2+
1 : 9, v/v) solution upon addition of an increasing concentration of Hg
1.0 ꢀ 10ꢁ M).
4
2+
Fig. 7 Infrared spectra of L (black line) and its complex L + Hg (red line).
obtained mass spectrum confirmed that the sensor L ion peak
was detected at m/z 246.07 (ESI,† Fig. S3), which corresponds
to [L + H] , and the mercury complex ion peak appeared at
Scheme 2 Possible sensing mechanism.
+
2
+
2+
Meanwhile, the Hg coordinate N atom of the quinoline group m/z 481.99, which indicated that probe L reacts with Hg
and electronegativity of the quinoline ring have been reduced; (M = 200.6) and two H
2
O (M = 36) to form a stable complex
O] (ESI,† Fig. S4). In conclusion, the IR and
H NMR titration experiments and mass spectrum experiments
2
+
the signal of the hydrogen atoms in the quinoline ring showed a [L + Hg + 2H
2
2
+
1
significant downfield shift. These results indicated that the Hg
chelates with the N atom of the benzimidazole groups and the suggested the probable binding mode of chemosensor L and
N atom of the quinoline groups, and forms a mercury complex. mercury.
1
Therefore, the results of H NMR titration experiments suggested
The pH dependence of sensor L on the HEPES buffer system
the validity of the submitted mechanism and the cause of the was also checked by Fluorescent spectroscopy. The mercury ion
presented fluorescence quenching. was added to the buffered solution of L at different pH values.
To further investigate the interaction between sensor L and No apparent changes of the fluorescence spectra were observed;
2
+
2+
Hg , the infrared spectra were recorded and are displayed in the results indicated that the binding of L with the Hg can
Fig. 7. Sensor L shows stretching vibration absorption peaks at work well in the range of pH 2.0–11.0 (Fig. 8).
ꢁ1
ꢁ1
ꢁ1
3
482 cm (–NH), 1659 cm (CQN–H), and 3060 cm (Ar–H),
compared with the L + Hg compound whose N–H peak at strips were prepared by immersing filter papers in a DMSO–
To facilitate the use of L for the detection of mercury, test
2
+
ꢁ1
3
482 cm disappeared, at the same time, the peak of (CQN–H)
H
2
O binary solution of L (0.01 M) followed by exposure to air
until completely dry. Intriguingly, the fluorescence color can be
obviously enhanced, which demonstrated that receptor L com- changed immediately from blue to colorless once the test paper
ꢁ
1
ꢁ1
ꢁ1
at 1659 cm shifted to 1619 cm and the peak at 1120 cm
2
+
bined with Hg and formed the new compound. Moreover, the is immersed in an aqueous solution (5 mM) of mercury under
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strips based on L were fabricated, which could serve as a
2
+
practical fluorescent sensor to detect Hg in field measure-
ments or in test kits. Thus, we believe that these characteristics
of L make it attractive for further molecular modifications and
underlying applications as a fluorescence sensor for mercury.
Acknowledgements
This work was supported by the Nature Science Foundation of
China (No. 21467012) and the Science and Technology Bureau
of Lanzhou, Gansu Province of China (No. 2013-4-63).
Notes and references
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ꢁ
5
Fig. 8 Effects of pH on the fluorescence spectra of L (2.0 ꢀ 10 M) and L
2+
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2+
water solutions with Hg , (C) after immersion into water solutions with
2+
other cations, and (D) after immersion into water solutions with Hg and
other cations under irradiation at 365 nm.
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and different cations (Fig. 9). The immersion of these test strips
in the solution mixture of other cations (5 mM) did not cause any
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When these strips were immersed in the solution of mercury
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convenient detection for mercury and cations.
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4
. Conclusions
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2
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2
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2
+
ꢁ9
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0
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