A new fluorescent chemosensor for Hg2+ in aqueous solution

Article abstract of DOI:10.1002/bio.2368

We prepared an aminothiourea-derived Schiff base (DA) as a fluorescent chemosensor for Hg²⁺ ions. Addition of 1 equiv of Hg²⁺ ions to the aqueous solution of DA gave rise to an obvious fluorescence enhancement and the subsequent addition of more Hg²⁺ induced gradual fluorescence quenching. Other competing ions, including Pb²⁺, Cd ²⁺, Cr³⁺, Zn²⁺, Fe²⁺, Co ³⁺, Ni²⁺, Ca²⁺, Mg²⁺, K⁺ and Na⁺, did not induce any distinct fluorescence changes, indicating that DA can selectively detect Hg²⁺ ions in aqueous solution. Copyright

Full text of DOI:10.1002/bio.2368

Research article
Received: 21 December 2011,
Revised: 1 March 2012,
Accepted: 2 March 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2368
A new fluorescent chemosensor for Hg2+ in
aqueous solution
Duong Tuan Quang,a* Nguyen Van Hop,a Nguyen Dinh Luyen,a
Ha Phuong Thu,b Doan Yen Oanh,b Nguyen Khoa Hien,c Nguyen Van Hieu,d
Min Hee Leee and Jong Seung Kime*
ABSTRACT: We prepared an aminothiourea-derived Schiff base (DA) as a fluorescent chemosensor for Hg2+ ions. Addition of 1
equiv of Hg2+ ions to the aqueous solution of DA gave rise to an obvious fluorescence enhancement and the subsequent
addition of more Hg2+ induced gradual fluorescence quenching. Other competing ions, including Pb2+, Cd2+, Cr3+, Zn2+
,
Fe2+, Co3+, Ni2+, Ca2+, Mg2+, K+ and Na+, did not induce any distinct fluorescence changes, indicating that DA can selectively
detect Hg2+ ions in aqueous solution. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information can be found in the online version of this article.
Keywords: Schiff base; fluorescent chemosensor; Hg2+ determination; aqueous solution
systems have shortcomings in practical use such as cross-sensitivity
toward other metal ions, short emission wavelengths, narrow
Introduction
Heavy metals are of great concern not only among the scientific
pH ranges and delayed response. Accordingly, developing new
and practical sensing systems for Hg2+ ions remains a challenge.
community, especially chemists, biologists and environmental-
ists, but also increasingly among the general population who
are aware of some of the dangers associated with them. Among
them, mercury is one of the most hazardous and ubiquitous
As a part of our continuing interest in the design of fluorescent
chemosensors for HTM ions (11,12,22–24), we developed a
dimethylaminocinnamaldehyde-aminothiourea derived Schiff
base (DA) for the selective detection of Hg2+ ions in aqueous
pollutants released through natural events or human activities.
(1–3). Some microorganisms produce methyl mercury, a potent
neurotoxin, from other forms of mercury that poses serious health
problems by damaging central nervous and endocrine systems,
solution (Scheme 1).
leading to many cognitive and motion disorders (4). This is due
to its very high permeability through the skin, respiratory and
Experimental
Apparatus
gastrointestinal systems of the human body. The multiple path-
ways of spreading mercury through air, food, and water is a serious
concern because it persists in the environment and subsequently
bioaccumulates through the food chain (5,6). Concerns over toxic
exposure to mercury provide motivation to explore new methods
for monitoring Hg2+ ions from biological and environment
samples. Several methods, including atomic absorption spectros-
copy, inductively coupled plasma atomic emission spectrometry,
electrochemical sensing and the use of piezoelectric quartz crys-
tals make it possible to detect low concentrations of Hg2+ ions
(7–10). However, these methods require expensive equipment
and involve time-consuming and laborious procedures that can
only be carried out by trained professionals.
All UV–vis spectra and fluorescence spectra were recorded with an
S-3100 spectrophotometer (Scinco Saoul, Korea) and a Hitachi
F-4500 fluorescence spectrometer (Hitachi Hi-Tech, Tokyo, Japan),
respectively. 1H NMR spectra were recorded with a Bruker-400
instrument (Bruker BioSpin, Europe) at 400 MHz. Mass spectra were
recorded with a Finnigan 4021C MS-spectrometer. X-ray analysis
* Correspondence to: Duong Tuan Quang, Department of Chemistry, Hue
University, Hue 8454, Vietnam. E-mail: duongtuanquang@dhsphue.edu.vn
* Jong Seung Kim, Department of Chemistry, Korea University, Seoul
136–701, Korea. E-mail: jongskim@korea.ac.kr
Alternatively, analytical techniques based on fluorescence
detection are very popular because they are easy to perform
and inexpensive and fluorescence measurements are usually
very sensitive (parts per billion/trillion) (11–15). Furthermore,
the photophysical properties of a fluorophore can be easily
tuned using a range of techniques such as: charge-, electron-,
energy-transfer, the influence of the heavy metal ions, as well
as the destabilization of non-emissive n–p* excited states (11).
Consequently, many studies involving on fluorescent chemosensors
have been published (11–13,16–21). However, many of these
a
Department of Chemistry, Hue University, Hue 8454, Vietnam
b
Vietnam Academy of Science and Technology, Hanoi 844, Vietnam
c
Institute of Natural Resources, Environment and Sustainable Development,
Hue 8454, Vietnam
d
International Training Institute for Materials Science (ITIMS), Hanoi University
of Science and Technology (HUST), Hanoi 844, Viet Nam
e
Department of Chemistry, Korea University, Seoul 136-701, Korea
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D. T. Quang et al.
Scheme 1. Synthesis of chemosensor DA.
was measured on a Rigaku R-Axis Rapid IP diffractometer (TST
Trading Service Technology Co., Ltd., Ho Chi Minh City, Vietnam)
Hg2+ ions. However, other competing ions did not lead to any ob-
vious quenching, indicating that DA can be used as a fluorescent
chemosensor for selective detection of Hg2+ in aqueous solution.
Crystals of DA suitable for X-ray diffraction analysis were
grown by the vapour diffusion of n-hexane into an ethanol of
DA for 2 days (Fig. 1). Crystal data and details of the data
collection are provided in Table S1 (Supporting Information).
Diffraction data for ligand DA were collected on a Bruker SMART
D8 goniometer (Bruker AXS, Beijing Office, Beijing, China) with
an APEX CCD detector (Bruker AXS) using MoKa radiation
l = 0.71073 Å (graphite monochromator). The structures were
solved by direct methods (SHELXTL) and refined on F2 by full
matrix least-squares techniques. (26) Hydrogen atoms were
included by using a riding model. The selected bond length
and angle are shown in Table S2 (Supporting Information). The
crystal analysis clearly confirmed the molecular structure of DA
(CCDC number: 857562).
Reagents
4-N,N’-Dimethylaminocinnamaldehyde and aminothiourea were
purchased from Aldrich (Sigma-Aldrich Pte. Ltd. VN R.O., Ho Chi
Minh City, Vietnam) and used without further purification. Pb
(ClO4)2, Cd(ClO4)2, HgCl2, CrCl3, Zn(ClO4)2, FeCl2, CoCl3, NiCl2,
Ca(ClO4)2, Mg(ClO4)2, KCl and NaCl were purchased from Aldrich
and used as received. Ethanol for spectral detection was HPLC
reagent without fluorescent impurity and H2O was deionized
water. All solvents were analytical grade reagents.
Synthesis of dimethylaminocinnamaldehyde-
aminothiourea (DA)
DA showed a characteristic absorption wavelength at 380 nm.
4-N,N’-Dimethylamino-cinnamaldehyde (175 mg, 1.0 mmol) and
aminothiourea (100 mg, 1.1 mmol) were combined in absolute
ethanol (40 mL). The reaction solution was refluxed for 6 h under
a N2 atmosphere and stirred for another 2 h at room tempera-
ture until a solid precipitated. The solid was filtrated, washed
with ethanol three times and the crude product was purified
by recrystallization from absolute ethanol to give 207 mg of
dimethylaminocinnamaldehyde-aminothiourea (DA) in 83.0%
When 5 equiv of various competing metal ions such as Hg2+, Pb2+
,
Cd2+, Cr3+, Zn2+, Fe2+, Co3+, Ni2+, Ca2+ and Na+ were added to the
aqueous solution of DA, no distinct changes in its absorption spec-
tra were observed. However, in the same experimental conditions,
their fluorescence spectra showed different changes upon addi-
tion of these cations. As shown in Fig. 2, DA exhibited an extensive
1
yield as follows: H NMR (DMSO-d6); d 11.23 (s, 1H); 8.03 (s, 1H);
7.86 (d, 1H); 7.47 (s, 1H); 7.38 (d, 2H); 6.70 (d, 1H); 6.63 (d, 2H);
6.59 (s, 1H); 2.96 (s, 6H); FAB-MS (M + H+); m/z = 250.1.
General procedure
A 1.0 x10À3 M stock solution of DA was prepared in EtOH and
diluted to 1.0 x 10À5 M in 1:9 EtOH/H2O solution (v/v). The metal
ion stock solutions were dissolved in deionized water to a
concentration of 1.0 x 10À3 M for the spectral analysis. Each
time, a 2-mL solution of DA was added to a quartz cell of 1 cm
optical path length and different stock solutions of metal ions
were gradually added into the quartz cell using a micropipette.
The volume of ionic stock solution added was < 100 mL to prevent
any changes in the total volume of the testing solution. Excitation
wavelength was at 390 nm and the temperature was ~ 25ꢀC.
Figure 1. ORTEP diagrams of chemosensor DA with displacement atomic ellip-
soids drawn at 30% probability level.
300
DA
Pb2+, Cd2+, Cr3+, Zn2+
Fe2+, Co3+, Ni2+, Ca2+
200
Results and discussion
100
Dimethylaminocinnamaldehyde with a donor-acceptor structure
was selected as a fluorophore and the aminothiourea unit
was used as a strong binding receptor for HTM ions. The
aminothiourea unit could also increase the water compatibility
and biocompatibility (25). As expected, DA gave rise to a strong
green emission at 510 nm in aqueous solution with a quantum
yield of 0.25 and exhibited good stability under a wide pH range
from 5 to 9, covering physiological conditions. The emission could
be increased by~ 30% by adding 1.0 equiv of Hg2+ in aqueous
solution and gradually quenched by subsequent addition of more
Hg2+
0
400
450
500
550
600
650
Wavelength (nm)
Figure 2. Changes of fluorescence spectra of DA (10 mM) in EtOH/H2O solution
(1/9, v/v) at pH 7 upon addition of different metal ions including Hg2+, Pb2+, Cd2+
,
Cr3+, Zn2+, Fe2+, Co3+, Ni2+, Ca2+, and Na+ ions (50 mM each) with an excitation wave-
length at 390 nm.
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Fluorescent chemosensor for Hg2+
400
300
200
100
0
400
a
b
0-1 equiv of Hg2+
1-2 equiv of Hg2+
300
200
100
0
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Figure 3. Fluorescent titration spectra of DA (10 mM) in EtOH/H2O (1/9, v/v) at pH 7 (a) upon addition of 0–1 equiv of Hg2+ ions and (b) upon subsequent addition of 1–2 equiv
of Hg2+ ions with an excitation wavelength at 390nm.
H
N
NH2
2+
Hg
N
H
N
NH2
S
N
2+
N
N
Hg
EtOH/H2O
S
S
N
N
N
H2N
H
Scheme 2. The proposed interaction mechanism between DA and Hg2+
.
400
300
200
100
0
(Fig. 3b). Accordingly, chemosensor DA could be potentially used
to determine Hg2+ in a simple and efficient way.
DA
2+
DA + Hg
To further understand the binding mechanism, MALDI-TOF
mass spectrum of DA in the presence of Hg2+ was performed
in EtOH/H2O (9/1, v/v) solution. In the presence of Hg2+, a peak
at m/z 696.783 corresponding to the adduct (2 DA + Hg)+ was
observed, indicating the formation of a 2:1 complex between
chemosensor DA and Hg2+. Since one metal cation can bind
with two chemosensor molecules, it is proposed that Hg2+ could
locate at the center of the pseudocavity of two DA molecules
and coordinated with two nitrogen atoms of imines and two
sulfur atoms of aminothiourea. The possible interaction mecha-
nism between DA and Hg2+ is shown in Scheme 2.
2
4
6
8
10
12
pH value
For practical application, the proper pH environment of DA for
the selective sensing of Hg2+ was also evaluated. Fig. 4 shows
the fluorescence changes of the free DA (black) and DA + Hg2+
(green) at different pH conditions. It is obvious that the optimum
pH range using DA to determine Hg2+ ions was 5 ~ 9 because in
this region, DA without Hg2+ showed a platform in its fluores-
cence intensity at 510 nm whereas DA with Hg2+ induced the
largest fluorescence quenching. In addition, the sensing process
was very fast and the complexation interaction of DA with metal
cations was completed within 30 seconds, indicating that
chemosensor DA also shows a high sensitivity to Hg2+ in aque-
ous solution.
Figure 4. Fluorescence intensity (510 nm) of DA (10 mM) in the absence and
presence of 5 equiv of Hg2+ ions in EtOH/H2O solution (1/9, v/v) with different
pH conditions.
emission centered at 510nm and its fluorescence quantum yield
was about 0.25 in EtOH/H2O solution (1/9, v/v). We clearly
observed fluorescence quenching (~ 95%) at 510nm in the pres-
ence of 5 equiv of Hg2+ ions. This quenching phenomenon could
be due to the electron transfer of DA into the paramagnetic heavy
metal ion, which is widely used for fluorescence quenching of HTM
ions. (27,28) In contrast, other metal ions including Pb2+, Cd2+, Cr3
+, Zn2+, Fe2+, Co3+, Ni2+, Ca2+ and Na+ did not lead to any distinct
changes in fluorescence. As a result, DA as a fluorescent chemosensor
could selectively detect Hg2+ ions in aqueous solution.
Fluorescence titration spectra were performed to further
investigate the interactions of DA with Hg2+ ions (Fig. 3). As
shown in Fig. 3(a), the fluorescence intensity of DA was
increased~ 30% upon addition of 1.0 equiv of Hg2+ ions, probably
due to the increased ICT effect when Hg2+ was bound with
the aminothiourea of DA. Subsequent addition of Hg2+ ions
(1–2 equiv) to the solution of DA did not result in obviously fluores-
cence quenching due to the well known heavy metal ion effect
Conclusions
In conclusion, we have developed a new aminothiourea-
derived Schiff base (DA) as a fluorescent chemosensor that
showed good selectivity toward Hg2+ over other competing
ions in aqueous solution. The pH range of DA for detection
of Hg2+ was 5 ~ 9 and the sensing process was highly sensi-
tive, which makes it promising as a practical method for deter-
mining Hg2+ ions.
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D. T. Quang et al.
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Acknowledgements
This work was financially supported by the National Foundation
for Science and Technology development of Vietnam – NAFOSTED
(No.104.07.17.09) (DTQ) and CRI project (2011–0000420) of the
National Research Foundation of Korea.
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