Highly selective and sensitive fluorescent chemosensor for Hg2+ in aqueous solution

Article abstract of DOI:10.1016/j.tetlet.2011.12.110

A new indole-based fluorescent chemosensor 1 was prepared and its metal ion sensing properties were investigated. It exhibits high sensitivity and selectivity toward Hg²⁺ among a series of metal ions in H ₂O-EtOH (7:1, v/v). The association constant of the 1:1 complex formation for 1-Hg²⁺ was calculated to be 9.57 × 10³ M^-1, and the detection limit for Hg²⁺ was found to be 2.25 × 10^-5 M. Computational results revealed that 1 and Hg²⁺ ion formed with a central tetrahedron-coordinated Hg ²⁺.

Full text of DOI:10.1016/j.tetlet.2011.12.110

Tetrahedron Letters 53 (2012) 1169–1172
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
2+
Highly selective and sensitive fluorescent chemosensor for Hg in
aqueous solution
a
a
b
a
a
a
Hsiu-Han Wu , Yao-Lin Sun , Chin-Feng Wan , Shih-Tse Yang , Shau-Jiun Chen , Ching-Han Hu ,
An-Tai Wu
a
a,
⇑
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan
School of Applied Chemistry, Chung Shan Medical University, Taichung City 40201, Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A new indole-based fluorescent chemosensor 1 was prepared and its metal ion sensing properties were
Received 7 November 2011
Revised 23 December 2011
Accepted 26 December 2011
Available online 2 January 2012
2+
2
investigated. It exhibits high sensitivity and selectivity toward Hg among a series of metal ions in H O–
2+
EtOH (7:1, v/v). The association constant of the 1:1 complex formation for 1-Hg was calculated to be
3
À1
2+
À5
9
.57 Â 10 M , and the detection limit for Hg was found to be 2.25 Â 10 M. Computational results
2
+
2
+
revealed that 1 and Hg ion formed with a central tetrahedron-coordinated Hg
.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Indole
Sensor
Mercury
Indole-based receptor systems have recently attracted attention
because an inherent fluorescent property of the indole chromo-
phore is sensitive to change in the local environment.1 Such sys-
tems, including oligoindole foldamers, macrocycles, and molecular
and other groups.
The receptors strongly bind anions because they contain a single
hydrogen bond donor group. To the best of our knowledge, few in-
addition of various cations in aqueous solution (H
2
O: EtOH = 7:1,
v/v) (Fig. 1). After addition of 10 equiv of the following 12 metal
–6
+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
ions: Li , Na , K , Ca , Mg , Hg , Co , Ni , Cu , Pb , Cd , and
2
+
Zn , there was no meaningful absorbance change in the UV–vis
clefts, have been prepared by Jeong7
–11
12,4,13–15
spectrum. However, the fluorescence intensity of 1 was strongly
2
+
quenched only in the presence of the Hg ion. Other ions did not
cause any significant changes under identical conditions. The
dole derivatives have been developed as potential fluorescent
observed quenching efficiency ((IÀI
0
)/I
0
 100%) at 358 nm was
chemosensors for cations, such as Hg2
+ 16
.
Mercury and its deriva-
nearly 100% ( Fig. 2) and the other metal ions caused small
enhancements in fluorescence intensity. To further investigate
the chemosensor properties of 1, we performed fluorescence titra-
tives are significant environmental pollutants with inherent high
toxicity. Therefore, much effort has been focused on designing
2
+
2+
Hg sensors. Although many chemosensors specific for Hg have
tions of 1 (74.0
lM) in the presence of different concentrations of
1
7–21
2+
been developed,
most require complicated synthesis and are
Hg in H O: EtOH (7:1, v/v). Figure 3 shows the gradual reductions
2
aqueous unsolvable. Thus, for practical applications, it is necessary
to develop aqueous soluble Hg2 sensors that are easily prepared,
and possess selective and sensitive signaling mechanisms. Herein,
in fluorescence intensity for 1 upon the addition of increasing con-
+
2+
centrations of Hg . From the fluorescence titration profiles, the
2
+
2
association constant for 1-Hg in H O: EtOH (7:1, v/v) solution
2
2
3
À1
we report the design of a simple and water-soluble sensor 1 that
is composed of two indole fluorophores and a triaminoethylene
linker. The indole-based sensor 1 exhibited selectivity and sensitiv-
was 9.57 Â 10 M , according to the Stern-Volmer plot.
Furthermore, it was found that 1 had a detection limit of
À5
2+
2.25 Â 10 M for Hg . The detection limit was sufficiently low
2
+
2+
ity for Hg in aqueous solution.
to detect submillimolar concentrations of Hg , which is the range
The indole-based sensor 1 was synthesized by a simple one step
reaction of indole-3-acetic acid (IAA) with triaminoethylene, as
shown in Scheme 1. The cation screening studies for receptor 1
were established from changes in the sensor’s fluorescence spectra.
The cation binding and sensing properties of 1 were investi-
gated by UV–vis absorption and fluorescence changes upon
found in many chemical and biological systems. In order to deter-
2
+
mine the stoichiometry of the 1-Hg complex, the Job’s method
was used. In the Job’s plot (Fig. 4), a maximum fluorescence change
was observed with a 0.5 M fraction of ionophore to Hg2+ for 1,
which indicated that only a 1:1 complex was formed.
2
+
To test the practical applicability of 1 as a Hg selective fluores-
cence chemosensor, competition experiments (Fig. 5) were carried
out. We found that the fluorescence intensity of 1 in the presence
⇑
Corresponding author. Fax: +886-4-7211-190.
E-mail address: antai@cc.ncue.edu.tw (A.-T. Wu).
2
+
of 10 equiv of Hg was unaffected by the addition of 10 equiv of
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.110

1
170
H.-H. Wu et al. / Tetrahedron Letters 53 (2012) 1169–1172
OH
O
O
O
NH
NH
N
H
HN
HN
DEPC, TEA
+
H N
2
N
H
NH2
N
H
DMF
IAA
1
Scheme 1. Synthesis of chemosensor 1.
1
00
0
Host
Pb(II)
800
700
600
500
400
300
0
-
100
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
Zn(II)
Cd(II)
Mg(II)
Ni(II)
Cu(II)
Co(II)
Ca(II)
Hg(II)
Li(I)
-200
-300
-400
-
-
-
-
500
600
700
800
0
1
2
3
4
5
7 eq.
2+
[
Hg ]/ [1]
Na(I)
K(I)
200
2
+
100
0
Hg
300
350
400
Wavelength (nm)
450
500
3
00
350
400
Wavelength (nm)
450
500
Figure 3. Fluorescence spectra of
1
(74.0
lM) in the presence of different
concentrations of the Hg ion in H O:EtOH (7/1, v/v) with an excitation at 279 nm.
2+
M) upon the addition of Li , Na , K , Ca2+,
+
+
+
2
Figure 1. Fluorescent spectra of 1 (94.5
Mg , Hg , Co , Ni , Zn , Cu , Cd , and Pb (10 equiv) in H O: EtOH (7/1, v/v)
2
l
2+
2+
2+
2+
2+
2+
2+
2+
(
excitation at 279 nm).
40
35
30
25
20
15
2
0
Pb(II) Zn(II) Cd(II) Mg(II) Ni(II) Cu(II) Co(II) Ca(II) Hg(II) Li(I) Na(I) K(I)
0
-
-
-
-
20
40
60
80
-
100
0.0
0.2
0.4
0.6
0.8
1.0
2
+
X=[1] / [1]+[Hg ]
Figure 2. Fluorescence intensity changes ((IÀI
0
)/I
0
 100%) of 1 (94.5 lM) upon the
2+ 2+ 2+ 2+
+
+
+
2+
2+
2+
2+
2+
addition of Li , Na , K , Ca , Mg , Hg , Co , Ni , Zn , Cu , Cd , and Pb
10 equiv) in H O:EtOH (7/1, v/v) (excitation at 279 nm).
2
Figure 4. Job plot of 1 with Hg(ClO4)2 in H O: EtOH (7/1, v/v).
(
2
À
À
À
À
À1
À1
À1
2
4
for anions (F , Cl , Br , I , HSO , NO3 , and H PO ) using tetrabu-
4
competing metal ions (Li , Na , K , Ca2+, Mg2+, Co2+, Ni2+, Cu2+, Pb2+
+
+
+
,
2
tylammonium as a counteraction in H O: EtOH (7:1, v/v) solution.
2
+
2+
Cd , or Zn ). The results suggested that 1 could potentially be
The fluorescence intensities of the spectra of 1 did not show any
changes upon addition of the above anions, probably because the
conformation of the indole derivative 1 was not suitable for the
recognition of these anions.
2
+
used as a Hg selective fluorescent chemosensor. The coordination
behavior of 1 with Hg was corroborated by 1H NMR titration
experiments carried out in DMSO. It was found that NH signals
underwent continuous upfield shifts with increasing Hg2 concen-
tration from 0 to 2.0 equiv (Fig. 6). These observations further re-
2+
+
2+
To further investigate the Hg accurate coordinating mode of
2
+
2+
the 1-Hg complex, computation for 1-Hg was conducted at
the B3LYP/LanL2DZ theory level with a Gaussian 03 package. On
the basis of these calculations, we found four stationary points that
could be verified as genuine minima via vibrational frequency
analyses. The optimized structure for the most stable isomer of
2
+
vealed that the NH of 1 establishes H-bond interaction with Hg
.
Indole has a hydrogen bond donor group that could play a role as
an anion binding motif. Therefore, in addition to the cation binding
properties, we have also investigated the sensing properties of 1

H.-H. Wu et al. / Tetrahedron Letters 53 (2012) 1169–1172
1171
359 nm
1.0
0.8
0.6
0.4
0.2
0.0
Figure 7. Conformation of 1/Hg2+ optimized by the density functional theory.
2
+
Figure 5. Competitive experiments in the 1+Hg system with interfering metal
ions. [1] = 4.2 Â 10 M, [Hg ] = 4.2 Â 10 M, and [Mn+] = 4.2 Â 10 M in H
À6
2+
À5
À5
O:
2
fluorescence quenching in the presence of the Hg2+ ion (Figs. S8–
S13). These results provided support for the calculated binding
mode of 1 derived in the computational study. The results also
showed that the site of the linker in receptor 1 or 2 does not affect
EtOH (7/1, v/v). kex = 279 nm.
Ar
2
+
the recognition of the Hg ion. The formation of 1:1 complex be-
2
+
tween 2 and Hg was further confirmed by the appearance of a
2
+
HN
peak at m/z 591, assignable to [2+Hg ] in the ESI/MS (Fig. S14).
Hg2+
NH
7
O
O
6
O
O
NH
N
H
HN
5
NH4
NH
H
1
N
NH
HN
3
2
2
In summary, an aqueous indole-based, Hg2+-selective, fluores-
cent sensor 1 was designed and synthesized by coupling indole
and triaminoethylene moieties. Fluorescent sensor 1 showed selec-
2
+
tivity for Hg over other metal ions that was sufficiently low for
2
+
the detection of trace amounts of Hg ion present in many chem-
ical and biological systems. Computational calculations provided
2
+
evidence that a tetrahedron coordinated Hg center was formed
and the triaminoethylene linker spacer acted as a scaffold to bring
functional ligands into a suitable geometry.
Figure 6. 1H NMR spectra of 1 (4.1 mM) in the presence of 0–2.0 equiv of Hg(ClO
in DMSO-d
4 2
)
6
.
Acknowledgment
We thank the National Science Council of Taiwan for financial
support.
1
-Hg2+ is illustrated in Figure 7. It is clear that the Hg2+ occupied
the coordination center of 1, surrounded by two carbonyl groups
and two anti-parallel indole rings. The average of the two bond
Supplementary data
lengths of Hg–O@
those for Hg–C of the indole ring in 1-Hg were 2.47 Å and
.46 Å, respectively. These results indicated that a tetrahedron
C
was estimated at 2.43 Å and the lengths of
2+
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.12.110.
2
2
+
Hg center was nearly formed, where all the neighboring ligands
i.e., two carbonyl oxygen atoms and two indole rings) were in-
(
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2+
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6
) d: 11.00 (s, 1H), 9.38 (s,
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2
011, 346, 978–981.
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6
): 171.91, 136.46, 127.62, 124.38,
121.29, 119.08, 118.71, 111.74, 108.84, 46.63, 35.67, 33.05 HRMS (FAB): Calcd
for C24 (M+H), m/z 418.2243, found m/z 418.2240.
1
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28 5 2
H N O