Phenanthroimidazole-based thiobenzamide as an effective sensor for highly selective detection of mercury(II)

Article abstract of DOI:10.1016/j.bmcl.2013.03.083

Based on highly selective and irreversible Hg²⁺-promoted desulfurization reaction, a new and simple phenanthroimidazole-type sensor was prepared and exhibited high selectivity towards Hg²⁺ ion over other metal ions, accompanied by transformation of a weakly fluorescent thioamide moiety (colorless) to a highly fluorescent amide one (blue), with a 136-fold increase in fluorescent intensity in aqueous solution with a pH span 2.57-9.12.

Full text of DOI:10.1016/j.bmcl.2013.03.083

Bioorganic & Medicinal Chemistry Letters 23 (2013) 3382–3384
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Phenanthroimidazole-based thiobenzamide as an effective sensor
for highly selective detection of mercury(II)
⇑
Yong Guo, Yuanyuan Yan, Xiaoyan Zhi, Chun Yang, Hui Xu
Laboratory of Pharmaceutical Design and Synthesis, College of Sciences, Northwest A&F University, 712100 Yangling, Shaanxi Province, China
a r t i c l e i n f o
a b s t r a c t
Based on highly selective and irreversible Hg2+-promoted desulfurization reaction, a new and simple
phenanthroimidazole-type sensor was prepared and exhibited high selectivity towards Hg2+ ion over
other metal ions, accompanied by transformation of a weakly fluorescent thioamide moiety (colorless)
to a highly fluorescent amide one (blue), with a 136-fold increase in fluorescent intensity in aqueous
solution with a pH span 2.57–9.12.
Article history:
Received 9 January 2013
Revised 13 March 2013
Accepted 22 March 2013
Available online 31 March 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Phenanthroimidazole
Sensor
Fluorescence probe
Turn on
Mercury
Mercury is considered to be a highly toxic and dangerous ele-
ment for human beings because it can be bioaccumulated through
the food chain and cause great damage to the nervous system even
at very low concentrations.1,2 Therefore, it is still very desirable to
explore selective and sensitive chemosensors for detection of Hg2+
ion over other heavy and transition metal (HTM) ions. Recently,
various sensors for recognition of Hg2+ ion over other HTM ions
have been developed with good performance.3 Additionally, many
attractive probes for detection of Hg2+ ion based on the specific
mercury-promoted desulfurization reaction have also been re-
ported.4 In recent years, some excellent fluorescent probes based
on phenanthroimidazole subunit were prepared and showed the
selective recognition of Mg2+,5 Cu2+,6 ClOÀ,7 cysteine and homocys-
compounds 1 and 2 were well characterized by IR spectrometry,
1H and 13C NMR spectroscopy, and high-resolution mass spectrom-
etry (HRMS) (see Supplementary data).
As described in Figure 1, the fluorescent intensity of 1 was a
constant value when pH value was between 2.57 and 10.96.
Figure 2 showed the fluorescent intensity ratio (I/I0) of sensor 1
in the presence of Ca2+, Ni2+, Cs+, Ba2+, Fe3+, Cd2+, Mn2+, Cu2+
,
Pb2+, Mg2+, Ag+, K+, Co2+, Zn2+, and Hg2+ ions in CH3CN–HEPES
(0.02 M, pH = 7.0) (7/3, v/v), respectively. Meanwhile, the fluores-
cent intensity of compound 2 was also tested and used as the con-
trol. The fluorescent behavior of sensor 1 was very weak because
its thioamide group might be a strong intramolecular quenching
entity.13 It is noteworthy that only when Hg2+ ion was added to
the solution of 1, a remarkably increased 136-fold of the fluores-
cent intensity was observed at 445 nm, accompanied with trans-
forming a weak fluorescence (colorless, inset of Fig. 1) to a high
fluorescence (blue). Whereas the addition of other metal ions such
as Ca2+, Ni2+, Cs+, Ba2+, Fe3+, Cd2+, Mn2+, Cu2+, Pb2+, Mg2+, Ag+, K+,
Co2+, and Zn2+, did not lead to a significant changes of the fluores-
cent intensity. The signaling mechanism is caused by the selective
desulfurization of probe 1–2 by reaction with Hg2+ ion (Scheme 2),
and the sulfur atom of 1 is converted to HgS with Hg2+ ion.14 Evi-
dence for the proposed transformation was supported by 1H NMR
spectra (see Supplementary data). Moreover, the reaction product
2 was also isolated from the mixture after 1 reacting with Hg2+
ion. As depicted in the partial 1H NMR spectra of Figure 3, the
NH (amide) proton was obviously shifted from 10.38 (1, (a)) to
8.59–8.61 ppm (2, (c), protons of benzene ring and NH were over-
lapped), and the 1H NMR spectra of (c) was the same as that of (b).
teine,8 Cu2+ and Cd2+ 9 and H2S,10 respectively.
,
As part of our ongoing research program in developing selective
and sensitive chemosensors for detection of Hg2+ ion,11 in this Let-
ter we have prepared a new and simple phenanthroimidazole-type
sensor 1 based on Hg2+-induced transformation of the thioamide
moiety to the amide one (Scheme 1). To the best of our knowledge,
this is the first desulfurized probe based on a phenanthroimidazole
moiety for recognition of Hg2+ ion in aqueous solution.
As shown in Scheme 1 and 9,10-phenanthrenequinone firstly
reacted with N-(n)butyl-4-formylbenzamide to give compound 2
in a 76% yield. Then sensor 1 was obtained in a 73% yield by reac-
tion of 2 with Lawesson’s reagent.12 The chemical structures of
⇑
Corresponding author. Tel./fax: +86 29 87091952.
E-mail address: orgxuhui@nwsuaf.edu.cn (H. Xu).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.03.083

Y. Guo et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3382–3384
3383
CHO
H
N
O
O
O
NH4OAc / AcOH
100 oC, 30 min
76%
+
N
H
N
CONH(CH2)3CH3
2
H
N
S
Lawesson's reagent
N
H
toluene, reflux, 12 h
73%
N
1
Scheme 1. Synthetic approach for probe 1.
The fluorescent intensity of 1 + Hg2+ was almost a constant va-
lue when pH value was between 2.57 and 9.12 (Fig. 1). The titra-
tion reaction curve of probe 1 toward Hg2+ ion was investigated
as shown in Figure 4. The fluorescent intensity of 1 increased in re-
sponse to the increases in the concentration of the added Hg2+ ion,
and the titration reaction curve showed a steady increase until a
plateau was reached when 7.5 equiv of Hg2+ ion was added.
To investigate the utility of 1 as an ion-selective fluorescent
sensor for Hg2+ ion, as shown in Figure 5, the cross-contamination
experiments were conducted in the presence of Hg2+ at 1 Â 10À5
M
,
mixed with other metal ions such as Ca2+, Ni2+, Cs+, Ba2+, Fe3+, Cd2+
Mn2+, Cu2+, Pb2+, Mg2+, Ag+, K+, Co2+, and Zn2+ at 1 Â 10À4 M,
respectively. It demonstrated that the selectivity of 1 towards
Hg2+ ion was almost not affected by other competitive ions.
In conclusion, a new, simple, and highly Hg2+-selective ‘turn on’
phenanthroimidazole-type sensor 1 was prepared. Based on Hg2+
-
induced transformation of a weakly fluorescent thioamide moiety
(colorless) to a highly fluorescent amide one (blue), probe 1 exhib-
ited high selectivity towards Hg2+ over other metal ions with a
136-fold increase in fluorescent intensity in aqueous solution over
a wide-range pH value (2.57–9.12). The Hg2+-promoted selective
desulfurization reaction was demonstrated by the 1H NMR spec-
troscopy. This study may open up new opportunity for develop-
ment of phenanthroimidazole-type sensor for selective detection
of Hg2+ ion in aqueous solution.
Figure 1. Fluorescence characteristics of 1 (1.0 Â 10À6 M) and 1 + Hg2+ at 298 K in
CH3CN–H2O (7/3, v/v) of different pH (pH adjusted by HClO4 or N+(CH3)4OHÀ);
kex = 364 nm and the emission intensity was measured at 445 nm.
Acknowledgments
This work was supported by National Natural Science Founda-
tion of China (No. 31171896), and the Special Funds of Central Col-
leges Basic Scientific Research Operating Expenses (YQ 2013) to Dr.
Xu. We also thank to Professor H.L. Zhang for the NMR
experiments.
Figure 2. Fluorescent intensity ratio (I/I0) of 1 (1.0 Â 10À6 M) in CH3CN–HEPES
Supplementary data
(0.02 M, pH = 7.0) (7/3, v/v) at 298 K in the presence of 10.0 equiv of different metal
ions. Inset: photograph of
1 in the presence of different metal ions. A: 1
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2013.
03.083.
(1.0 Â 10À6 M); Q: 2 (1.0 Â 10À6 M); B–P: 1+Hg2+, Ca2+, Ni2+, Cs+, Ba2+, Fe3+, Cd2+
,
Mn2+, Cu2+, Pb2+, Mg2+, Ag+, K+, Co2+, and Zn2+ (1.0 Â 10À5 M); kex = 364 nm and the
emission intensity was measured at 445 nm.
H
H
S
O
N
N
Hg2+
N
H
N
H
+ HgS
N
N
CH3CN-HEPES
(0.02 M, pH=7.0)
(7/3, v/v)
1
2
Fluorescence OFF
Fluorescence ON
Scheme 2. Selective desulfurization of probe 1–2 by reaction with Hg2+ ion.

3384
Y. Guo et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3382–3384
Figure 3. Partial 1H NMR spectra of 1 (a), 2 (b), and isolated compound 2 from 1 reacting with Hg2+ ion (c).
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Figure 5. Results of the competition experiments of probe 1 (1 Â 10À6 M) between
Hg2+ (1 Â 10À5 M) and selected metal ions (1 Â 10À4 M) in CH3CN–HEPES (0.02 M,
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