Novel hemicyanine dye as colorimetric and fluorometric dual-modal chemosensor for mercury in water

Article abstract of DOI:10.1039/c0ob01060j

A novel water soluble Hg²⁺-selective chemosensor 1 with hemicyanine as fluorescent reporting group and NO₂Se₂ chelating unit as ion binding site was reported. Chemosensor 1 shows a specific Hg²⁺ selectivity and

Full text of DOI:10.1039/c0ob01060j

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Novel hemicyanine dye as colorimetric and fluorometric dual-modal
chemosensor for mercury in water†
Yuanyuan Li, Song He, Yan Lu* and Xianshun Zeng*
Received 22nd November 2010, Accepted 7th February 2011
DOI: 10.1039/c0ob01060j
A novel water soluble Hg2+-selective chemosensor 1 with
hemicyanine as fluorescent reporting group and NO2Se2
chelating unit as ion binding site was reported. Chemosensor
1 shows a specific Hg2+ selectivity and discrimination between
Hg2+ and chemically similar ions in conjunction with a visible
colorimetric change from red to colorless, potentially leading
to both “naked-eye” and fluorometric detection of Hg2+
cations.
signal detection when receptors interact with analytes; a multi-
modal signal system can minimize the measurement errors because
of factors such as phototransformation, receptor concentrations,
and environmental effects. In particular, some of them can be
easily read with the naked eye. Consequently, the development
of multimodal and naked eye chemosensors for Hg2+ has the
important feature that they can be used to evaluate the analytes
rapidly. In particular, most of the reported colorimetric sensors for
Hg2+ are used in mixed solvents (water/organic solvent);6g,8 only a
few of them are used in neat aqueous solution.5j,9
The development of highly selective chemosensors for metal ions is
particularly important, since some metal ions can have extremely
harmful effects on humans and the environment.1 Mercury is
a particular concern worldwide because of its high toxicity.
Mercury species are considered as prevalent toxic pollutants in
the environment because both elemental and ionic mercury can
be converted by bacteria in the environment to methylmercury,
which subsequently bioaccumulates through the food chain.2
When absorbed in the human body, mercury causes a wide
variety of diseases such as prenatal brain damage, serious cognitive
and motion disorders2b,3 through damage to the central nervous
system, DNA, mitosis, and the endocrine system.4 Thus, concerns
over toxic damage from mercury provide the motivation to explore
the selective monitoring of Hg2+ in biological and environmental
samples. Since chromo- or fluoroionophores are proven to be
highly effective for the determination in terms of ease of handling
and simplicity of equipment, much effort has been paid to the
development of optical chemosensors that selectively respond to
the mercuric ion.5 To date, many chemosensors for mercury ion
based on photoinduced electron transfer (PET), intramolecular
charge transfer (ICT), and fluorescence resonance energy transfer
(FRET) signaling mechanisms have been developed.5 Dosimeters6
and allosteric interaction sensors7 have also been reported.
Significantly, the molecular design of multimodal chemosensors
for mercury is still a challenging but extremely attractive method
because they have important features that permit multimodal
In this regard, we are interested in hemicyanine-based dyes,
known for their excellent spectroscopic properties of large molar
extinction coefficient (e ª 104 M-1 cm-1 in the visible region)
and good fluorescence quantum yield, and which have found
applications in many fields, such as the study of complex biological
systems as molecular probes, photosensitizers in dye-sensitized
solar cells etc, but few as chemosensors.10 Taking into account
the possibility of using positively charged hemicyanine dyes not
only for colorimetric but also for fluorometric mercury sensing,
increasing the water solubility and sensitivity by using fluorescence
emission spectroscopic methods, we report our successful devel-
opment of a novel hemicyanine-based water soluble colorimetric
and fluorometric chemosensor 1 selective for Hg2+ based on the
mechanism of internal charge transfer (ICT).
We report herein a new hemicyanine-based water soluble
chemosensor 1 (Scheme 1), which displayed high Hg2+ selectivity
both in water and in ethanol–water solution. The chemosensor
1 was synthesized from 2 in two steps. As shown in Scheme 1,
aldehyde 3 was prepared by the reaction of 2 with sodium salt
Key Laboratory of Display Materials
&
Photoelectric Devices,
Engi-
Ministry of Education, School of Materials Science
&
neering, Tianjin University of Technology, Tianjin, 300384, China.
E-mail: luyan@tjut.edu.cn, xshzeng@tjut.edu.cn; Fax: +86-22-60215226;
Tel: +86-22-60216748
† Electronic supplementary information (ESI) available: Experimental
procedures and spectral data for all new compounds or other electronic
format. See DOI: 10.1039/c0ob01060j
Scheme 1 Structure and the synthesis of the chemosensor 1.
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of 2-selenolethanol prepared in situ by the reaction of 1,2-(bis-
2-hydroxylethyl)diselenide with NaBH4 in THF–ethanol mixture.
Then, 1 was facilely obtained by the condensation of 1,2,3,3¢-
tetramethylindolium iodide with 3 in 83% yield. The anion
exchange was achieved by refluxing the ethanol solutions 1 with
excess KBF4. The structure of 1 was confirmed by MS, NMR and
elemental analysis. The synthesized chemosensor 1 composed of
an ionophore for selective recognition of metal ions is constituted
of an NSe2O2 chelating unit as cation binding site as well as
a hemicyanine fluorophore unit which is responsible for signal
transduction during spectroscopic studies.
Fig. 2 Photograph of 10 mM 1 in the presence of different nitrate salts:
(10.0 equiv) of Na+, K+, NH4+, Mg2+, Ca2+, Pb2+, Hg2+, Co2+, Cd2+, Zn2+
Ni2+, Cu2+, Cr3+, Fe3+ and Ag+ in H2O.
,
1 in the presence and absence of metal ions in EtOH–H2O (1 : 1,
v/v). As can be seen from Fig. 1a, the response of 1 to Hg2+ leads
to a variation in absorption. When 10 equiv of Hg2+ was added
to the solution of 1 (11.5 mM), the absorption peak at 542 nm
diminished drastically, while a new absorption band appeared at
400 nm (e = 2.0 ¥ 104 M-1 cm-1). There was a large decrease (54-
fold) of absorbance at lmax = 542 nm and a mild enhancement
(42-fold) of absorbance at lmax = 400 nm upon the addition of 10
equiv of Hg2+. Fig. 1b displays the absorbance changes of 1 in
the presence of metal ions in water. When 10 equiv of Hg2+ was
added to the solution of 1 (11.5 mM), the absorption peak at 529
nm diminished drastically, while a new absorption band appeared
at 384 nm (e = 1.7 ¥ 104 M-1 cm-1). There was a large decrease
(74-fold) of absorbance at lmax = 529 nm and a mild enhancement
(27-fold) of absorbance at lmax = 384 nm upon the addition of
10 equiv of Hg2+ (Fig. 1b). As shown in Fig. 1b, under identical
conditions to Hg2+ ion, small increases in absorbance at lmax = 529
nm were observed in the UV/vis spectra of 1 upon addition of 10
Chemosensor 1 is soluble in H2O or in a mixed solvent of
H2O–EtOH. The UV/Vis spectra of 1 both in H2O and in
aqueous ethanol show the characteristic intense charge-transfer
band occurring in the long-wavelength region between 425 and
600 nm, which peaked at 529 nm in water (e = 7.1 ¥ 104 M-1
cm-1) and 542 nm in EtOH–H2O (1 : 1, v/v) (e = 8.3 ¥ 104 M-1
cm-1) (Fig. 1a and 1b). It is interesting to note that the addition
of 10 equiv of Hg2+ into the red solutions of 1 rapidly generated a
colorless solution, while other ions, such as K+, Na+, NH4 , Al3+,
+
Co2+, Fe3+, Pb2+, Ag+, Cu2+, Zn2+, Cd2+, Ni2+, Ca2+, Mg2+ gave no
color change in either H2O or in aqueous ethanol, which means
that the two selenium atoms within 1 bound more efficiently to
with Hg2+. This interesting feature revealed that 1 can serve as a
selective “naked-eye” chemosensor for Hg2+ (Fig. 2).
equiv of K+, Na+, NH4 , Al3+, Co2+, Fe3+, Pb2+, Cu2+, Zn2+, Cd2+,
+
Ni2+, Ca2+, Mg2+ but Ag+ decreases the absorption to some extent
in H2O. From these results, it can be considered that the decrease
of absorption intensity over 500 nm and the increase of absorption
intensity around 400 nm caused by Hg2+ addition are due to the
change in the photophysical properties of the chromophore itself,
and arise via the ICT mechanism.
The absorbance changes of Hg2+ titration of 1 were investigated
both in H2O and in aqueous ethanol (ethanol–water, 1 : 1, v/v)
solutions. As shown in Fig. 3, upon addition of Hg2+ to the solution
of 1, the absorbance at 542 nm in aqueous ethanol and 529 nm in
water decreased sharply, while that at 412 nm in aqueous ethanol
and 404 nm in water increased significantly, which induced a color
change from red to colorless. The isosbestic points at 471 nm
(in EtOH–H2O, 1 : 1, v/v) and 457 nm (in H2O) were observed,
indicating the formation of a new complex between 1 and Hg2+. A
linear dependence of the ratio of absorbance as a function of Hg2+
concentration both in water and in aqueous ethanol was observed.
Next, the nitrate salts of K+, Na+, NH4 , Al3+, Co2+, Fe3+, Pb2+,
+
Ag+, Cu2+, Zn2+, Cd2+, Ni2+, Ca2+, Mg2+ and Hg2+ ions were used
to evaluate the metal ion binding properties and the selectivity
of chemosensor 1 (1 mM) by means of fluorescence spectra in
EtOH–H2O (1 : 1, v/v) and in H2O, respectively. Among the metal
ions examined, 1 showed a selective fluorescence decrease only
with Hg2+ both in EtOH–H2O (1 : 1, v/v) and in H2O, indicating
that 1 displayed a high Hg2+ selectivity (Fig. 4 and Fig. S1,
ESI†). The fluorescence spectra were obtained by excitation of
the hemicyanine fluorophore at 540 nm.
Fig. 1 UV-vis responses of chemosensor 1 (11.5 mM) upon the addition
of the nitrate salts (10.0 equiv) of Na+, K+, NH4+, Mg2+, Ca2+, Pb2+, Hg2+
,
Co2+, Cd2+, Zn2+, Ni2+, Cu2+, Cr3+, Fe3+ and Ag+ (a) in ethanol–H2O (1 : 1,
v/v) and (b) in H2O.
The metal ion binding properties and selectivity of chemosensor
1 were firstly investigated by absorbance changes both in H2O and
in aqueous ethanol (ethanol–water, 1 : 1, v/v) solutions with use
of metal nitrate salts. Fig. 1a displays the absorbance changes of
To achieve a reliable binding constant (K) via fluorescence
titration,11 the titrations were conducted in a very dilute solution
(1.0 ¥ 10-6 M). From the fluorescence titration experiments (Fig. 5),
clear “on–off” fluorescence changes of 1 to Hg2+ were observed.
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ability of 1 in water is much stronger than that in aqueous ethanol.
From the changes in Hg2+-dependent fluorescence intensity (Fig.
S3, ESI), the detection limit was estimated to be 5.0 ¥ 10-8 M,12a
indicating that the limit of detection of 1 to Hg2+ met the
discharge limit for industrial waste water according to the China
SA standard,12b or the US EPA standard.12c
Ion interference experiments were carried out both in H2O and
in aqueous ethanol (ethanol–water, 1 : 1, v/v) solvent systems for
evaluation of the sensing ability of 1 toward Hg2+ in the presence
of 50 equiv of a series of background metal ions, and the results
are shown in Fig. 6. While Ag+ had 5% background quenching
Fig. 3 Absorbance spectra of 1 (10 mM) (a) in EtOH–H2O (1 : 1, v/v) and
(b) in H2O in the presence of different amounts of Hg2+. Inset: the ratio
of absorbance at 560 nm and absorbance at 400 nm as a function of Hg2+
concentration.
Fig. 5 Fluorescent titration spectra: (a) 1 (1 mM) in the presence of
different concentrations of Hg2+ in ethanol–water (1 : 1, v/v), [Hg2+] = 0, 1,
3, 5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 100, 150, 200 equiv. Inset: Fluorescence
intensity Fi as a function of Hg2+ concentration. lex = 540 nm; (b) 1 (1 mM)
in the presence of different concentrations of Hg2+ in water, [Hg2+] = 0, 0.4,
0.8, 1, 1.5, 2, 3.5, 7, 9, 10, 20, 30, 35, 40, 50, 60, 70, 80, 90 equiv. Inset: the
fluorescence at 590 nm of 1 (1 mM) as a function of the Hg2+ concentration.
lex = 530 nm.
Fig. 4 Fluorescence spectra of 1 (1 mM) in ethanol–water (1 : 1, v/v) and
in H2O with 10 equiv of K+, Na+, NH4+, Al3+, Co2+, Fe3+, Pb2+, Ag+, Cu2+
,
Zn2+, Cd2+, Ni2+, Ca2+, Mg2+ and Hg2+. For the entire test, excitation and
emission were performed at 540 and 596 nm in ethanol–water (1 : 1, v/v)
and at 520 and 591 nm in water.
When Hg2+ was added to the solution, a significant decrease of
the fluorescence intensity of 596 nm (in aqueous ethanol)/587 nm
(in water) and with a blue shift to 575 nm was observed. The
quenching is found to follow a Benesi–Hildebrand equation and
may be attributed to the nitrogen lone-pair electrons induced by
the complexation of Hg2+. A linear dependence of the ratio of
fluorescence intensity [1][Hg2+]/(F0 - Fi) at 594 nm (in aqueous
ethanol)/590 nm (in water) as a function of Hg2+ concentration
and the data from Job’s plots from emission spectra (Fig. S2, ESI†)
assumed a 1 : 1 stoichiometry for the 1–Hg2+ complex. The binding
constant was found to be 8.81 ¥ 104 M-1 in aqueous ethanol and
4.30 ¥ 105 M-1 in water, respectively, indicating that the binding
Fig. 6 Quench ratio ((I0 - I)/I0) of fluorescence intensity of 1 (1 mM) in
H2O and in EtOH–H2O (1 : 1, v/v) upon the addition of 10 equiv Hg2+ in
the presence of 50 equiv background metal ions.
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in aqueous ethanol, other metal ions did not cause any effect. No
interference was observed in H2O in the presence of 50 equiv of a
series of metal ions. On the other hand, almost no color change is
observed in the presence of metal ions other than Hg2+. When 10
equiv of Hg2+ is added to these solutions, the red colour disappears
immediately and the spectra are almost identical to that obtained
in the presence of Hg2+ alone.
For practical applicability, the most suitable pH conditions of
chemosensor 1 were evaluated by means of absorption spectra.
The absorption intensities at different pH values of the apo and
Hg2+-bound forms of 1 in EtOH–H2O (1 : 1, v/v) and in water
solutions are shown in Fig. S4 and S5 (ESI†). From pH 2 to pH
10, absorption intensities are essentially insensitive to pH. Such
photophysical properties of 1 in the presence and in the absence
of Hg2+ suggest that this sensor may be very useful in practical
applications.
In summary, we have reported here a simple yet highly selective
colorimetric fluoroionophore (chemosensor 1) for aqueous Hg2+
based on ICT in which the metal ionophore has been incorpo-
rated into the electron donor moiety of the fluorophore. The
chemosensor shows a remarkably high ability to discriminate
between Hg2+ and chemically similar ions in conjunction with
a visible colorimetric change from red to colorless, leading to both
“naked-eye” and fluorometric detection of Hg2+ cations.
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Acknowledgements
This project was partially sponsored by NCET-09-0894, the
National Natural Science Foundation of China (NO: 20972111,
21074093, 21004044), SRF for ROCS, SEM, the Natural Science
Foundation of Tianjin (NO: 08JCYBJC26700) and the State
Key Lab. Elemental-Organic Chemistry at Nankai University
(NO.0908 and 1011).
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