An easy-to-synthesize 'turn-on' fluorescent sensor for the selective detection of HgII

Article abstract of DOI:10.1016/j.mencom.2009.09.013

A small molecular fluorescent sensor synthesized through Schiff base reaction can be used to detect Hg²⁺ with an exclusive selectivity, high fluorescent enhancement and reversibility.

Full text of DOI:10.1016/j.mencom.2009.09.013

Mendeleev
Communications
Mendeleev Commun., 2009, 19, 270–271
An easy-to-synthesize ‘turn-on’ fluorescent
sensor for the selective detection of HgII
Ziting Miao, Yanyan Fu, Zhe Xu, Guang Li* and Jianming Jiang
College of Material Science and Engineering, Donghua University, Shanghai 201620, P. R. China.
Fax: +86 21 6779 2798; e-mail: lig@dhu.edu.cn
DOI: 10.1016/j.mencom.2009.09.013
A small molecular fluorescent sensor synthesized through Schiff base reaction can be used to detect Hg2+ with an exclusive
selectivity, high fluorescent enhancement and reversibility.
A variety of natural and anthropogenic environmental contami-
nants cause serious problems for human health and ecology.1,2
1.0
Among them, mercury and mercuric salts are toxic and hazardous
pollutants with recognized accumulative and persistent characters
in the environment and biota, even at very low concentrations.3
0.8
0.6
The body exposure to methylmercury leads to serious risks
for human beings and other organisms,4,5 especially a cause of
prenatal brain damage and notorious Minamata disease.6
0.4
Thus, the development of methods for the detection of mercury
with high efficiency is important. Small molecular fluorescent
chemosensors that selectively respond to the target metal ions
have been recognized powerful technique toward this kind
of purpose. Unfortunately, most of the probes that have been
prepared exhibited fluorescence quenching upon cation com-
plexation.7–9 That is because, like many other heavy metals,
Hg2+ often causes fluorescent quenching via enhanced spin-
orbit coupling associated with the heavy atom effect, which will
facilitate the intersystem crossing process.10,11 Recently, an
example of a highly efficient Hg2+ sensor based on the semi-
naphthofluorescein chromophore and employing a thioether-rich
metal-binding unit can afford both turn-on and single-excitation
dual emission ratiometric Hg2+ detection in aqueous solution.
However, the obtainment of the target compound experienced
a tedious synthesis.12,13 Therefore, the design of sensors that
can be easily synthesized and give fluorescent enhancement upon
Hg2+ binding is a particular challenge.14
On the basis of the fact that amine is an efficient ligand to
mercury ion, we anchored a structure-simple receptor 1, which
contained two kinds of amino substituents. In case of being
exposed to a mercury ion, one of the amine groups would
integrate with the ion and a strong metal-induced intramole-
cular electron transfer (MICT) occurred, simultaneously a push-
pull system formed (the other amine group acted as a donor and
because of MICT this amine would have a very low affinity for
the mercury ion), which could cause a distinct change in the
optical properties (colour change and fluorescence enhancement)
of the receptor.
0.2
0.0
400
500
600
l/nm
Figure 1 UV/Vis titration of 1 (1.0×10–5 M) in MeCN/H2O (9:1 v/v) solution
upon addition of Hg2+ (0–1.5 equiv.).
The synthesis of receptor 1 and model compound 2 is outlined
in Scheme 1. Compound 1 was easily obtained in high yield
through the Schiff base reaction of 4-(diethylamino)benzalde-
hyde and p-phenylenediamine in anhydrous alcohol.
From the absorbance spectra (Figure 1), upon progressive
addition of Hg2+ to a solution of 1 in MeCN–H2O (9:1 v/v,
1.0×10–5 M), the initial absorption band at 394 nm for free
compound 1 gradually decreased with the concomitant forma-
tion of a new absorption band at 474 nm. The absorption
spectrum of 1 exhibited a red shift of 80 nm with the addition
of Hg2+. A well-defined isosbestic point at 425 nm throughout
the titration clearly indicated the existence of a two-state equilib-
rium; namely, one specific complex between compound 1 and
Hg2+ formed, which was responsible for the colour change of the
mixture from pale yellow to salmon pink that could be detected
easily by the naked eyes. A Job’s plot (see Online Supplementary
Materials, Figure S1) showed that receptor 1 formed a 1:1 com-
plex with Hg2+ in the solution and the association constant K
derived from the titration curve was 2.24×104 dm3 mol–1.
Figure 2 shows the emission spectra of sensor 1 at various
Hg2+ concentrations. A gradual increase in the fluorescent
intensity was observed with the addition of Hg2+, which may be
attributed to the formation of a complex between compound 1
and Hg2+, as mentioned above.
R
CHO H2N
NH2
N
To investigate selectivity for Hg2+, the influence of various
metal ions on the fluorescence behaviour of compound 1 was
studied. Upon the addition of 1.5 equiv. of other metal ions
including Fe2+, Co2+, Mg2+, Ni2+, Cu2+, Zn2+, Cd2+, Ba2+
and Ag+, all emission responses except for Hg2+ showed the
fluorescence quenched to a certain degree, even chemically
N
R
R
1 R = 4-(Et2N)C6H4
2 R = Ph
Scheme 1
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© 2009 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2009, 19, 270–271
mentary Materials, Figures S2 and S3), from which it could
be concluded that the binding site in sensor 1 was the aniline
group rather than the imine.
120
100
80
60
40
20
0
In summary, a structure-simple sensor that gives selective
fluorescent enhancement for mercury ion was described. The
Schiff base reaction made the obtaining of the sensor escaping
from a tedious synthesis. Upon binding with Hg2+, a strong
push-pull system formed and the colour change could easily be
detected by naked eyes. Experiments with corresponding model
compound showed the binding site in sensor 1 located in the
aniline group. We believe such design strategies like using
structure-simple, electron-rich compounds and MICT effects to
avoid tedious synthesis will be helpful in the design of other
kinds of sensors for specific ions.
500
550
600
650
700
l/nm
Figure 2 Fluorescence response of 1 (1.0×10–5 M) in MeCN/H2O (9:1 v/v)
solution upon addition of Hg2+ (0–2.0 equiv.), lex = 370 nm.
This work was supported by the China Postdoctoral Science
Foundation (project no. 20080430595) and the National Natural
Science Foundation (grant no. 50673017).
closely related metal ions, such as Cd2+ and Cu2+, did produce
obvious fluorescence quenching (Figure 3).
On the other hand, the reversibility of the binding between
receptor 1 and Hg2+ was also expectant. On addition of thiourea
to the solution of 1·Hg2+, the colour of the solution was found
to recover pale yellow quickly (see Supporting Information).
At the same time, the fluorescence returned to the original
degree, which explained that the adduct 1·Hg2+ decomplexed
immediately against thiourea. The reason is that the complexing
ability between thiourea and Hg2+ is stronger than that between
compound 1 and Hg2+.
As receptor 1 contains both imine and aniline groups, the
compound might integrate Hg2+ on either imine or aniline sites.
In order to understand, which nitrogen combine Hg2+ to form
the complex, model compound 2 was designed and synthesized,
which only contained imine. In the titration of compound 2
with Hg2+, no significant variation could be found in both the
fluorescence intensity and absorbance (see Online Supple-
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2009.09.013.
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Figure 3 Fluorescence response of 1 in MeCN/H2O (9:1 v/v, 1.0×10–5 M)
solution in the presence of 1.5 equiv. of different metal ions, lex = 370 nm.
Received: 2nd March 2009; Com. 09/3292
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