A Hg2+-selective chemodosimeter based on desulfurization of coumarin thiosemicarbazide in aqueous media

Article abstract of DOI:10.1016/j.saa.2010.03.028

A fluorescence-enhanced chemodosimeter 1 based on coumarin thiosemicarbazide for Hg²⁺ was developed via a Hg²⁺-induced desulfurization reaction. Spectroscopic results reveal that chemodosimeter 1 exhibits high sensitivity and selectivity for Hg²⁺ in comparison to common interfering metal ions in aqueous media at room temperature.

Full text of DOI:10.1016/j.saa.2010.03.028

Spectrochimica Acta Part A 76 (2010) 248–252
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A Hg2+-selective chemodosimeter based on desulfurization of coumarin
thiosemicarbazide in aqueous media
Wenhui Maa,b, Qun Xub, Jianjun Dua, Bo Songa,b, Xiaojun Penga,∗, Zhuo Wangb, Guodong Lib,
Xifang Wangb
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, PR China
b Key Laboratory of Fine Chemicals, Universities of Heilongjiang Province, Qiqihar University, 42 Wenhua Road, Qiqihar 161006, PR China
a r t i c l e i n f o
a b s t r a c t
A fluorescence-enhanced chemodosimeter 1 based on coumarin thiosemicarbazide for Hg2+ was devel-
oped via a Hg2+-induced desulfurization reaction. Spectroscopic results reveal that chemodosimeter 1
exhibits high sensitivity and selectivity for Hg2+ in comparison to common interfering metal ions in
aqueous media at room temperature.
Article history:
Received 28 January 2010
Received in revised form 9 March 2010
Accepted 16 March 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Coumarin thiosemicarbazide
Chemodosimeter
Mercury ions
Desulfurization
1. Introduction
ones and to improve their sensing properties. Herein, we report
a Hg2+ chemodosimeter 1 (Scheme 1) containing the coumarin
Considering the threat of mercury to the environment and
human health, a great effort has gone into the utilization of flu-
orescent methods for the detection of Hg2+ ions [1–10]. Recently,
a chemodosimetric approach for efficient and selective Hg2+ sig-
naling has been attracting much research interest [11–30]. The
chemodosimeter system can indicate an analyte through a spe-
cific chemical reaction between the dosimeter molecule and target
species, leading to the formation of a fluorescent or colored prod-
uct [15]. A number of Hg2+-selective chemodosimeters have been
successfully developed by virtue of Hg2+-induced desulfurization
and/or transformation of thioamide, thiourea and thione. Unfor-
tunately, several disadvantages, such as poor aqueous solubility,
cross-sensitivities toward other metal ions and strict reaction
condition prevent the application of chemodosimeters. More-
over, Hg2+ often causes a fluorescence-quenching response via
enhanced spin–orbit coupling associated with the heavy atom
effect. Therefore, the exploration of new fluorescence-enhanced
chemodosimeters for monitoring Hg2+ in aqueous media is still very
active and poses a significant challenge for chemical research.
Recently, Tae’s group has reported a few rhodamine thiosemi-
carbazide chemodosimeters for Hg2+ based on Hg2+-induced
desulfurization reaction [13,19,30], which guides us to design new
thiosemicarbazide as a new advance in this field. We choose
coumarin [31–37] as the fluorophore in consideration of its high
photostability and visible excitation and emission. Furthermore,
the diethylamino group in 1 can enhance the quantum yield and
improve solubility of 1 in aqueous media [28]. The thiosemicar-
bazide unit is selected as the dosimetric portion which can be
transformed into the 1,3,4-oxadiazoles unit via a desulfurization
reaction in aqueous media. Thus, chemodosimeter 1 is converted
into compound 4. 1 and 4 will show distinct spectra owing to
the intramolecular charge transfer (ICT) between the electron-
donating diethylamino group at the 7-position of coumarin and
the electron-withdrawing thiosemicarbazide or 1,3,4-oxadiazoles
group at the 3-position of coumarin.
2. Experimental
2.1. Materials and apparatus
Unless otherwise stated, all reagents were obtained from com-
mercial suppliers and were used without further purification.
Solvents used were purified and dried by standard methods
prior to use. Stock solutions of the metal ions were prepared
in twice-distilled water. Reactions were monitored by thin-layer
chromatography on silica-gel plates and column chromatography
was conducted on silica gel (mesh 200–300), both of which were
purchased from Qingdao Ocean Chemicals. 1H NMR and 13C NMR
∗
Corresponding author. Tel.: +86 411 39893899; fax: +86 411 39893906.
E-mail address: pengxj@dlut.edu.cn (X. Peng).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.03.028

W. Ma et al. / Spectrochimica Acta Part A 76 (2010) 248–252
249
Scheme 1. The design concept of fluorescent chemodosimeter 1 for Hg2+
.
spectra were obtained on a Bruker AVANCE-400 spectrometer.
Chemical shifts (ı) were reported in ppm relative to a Me4Si stan-
dard in CDCl3 or DMSO. Mass spectra were measured on a Agilent
6310 MS spectrometer and a Q-TOF MS spectrometer. Absorption
and fluorescence spectra were recorded on a Pgeneral TU-1901
UV–vis spectrophotometer and a Perkin Elmer LS55 fluorescence
spectrophotometer.
ethanol. Hydrazine monohydrate (0.5 mL, 85%) was then added
dropwise with vigorous stirring. After the addition, the stirred mix-
ture was held at room temperature for 1 h under an Ar atmosphere.
The resulting precipitate was filtered and washed 3 times with
anhydrous ethanol (3× 50 mL) and further dried under reduced
pressure to afford 197 mg yellow powder product 3 in 40.5% yield:
1H NMR (400 MHz, CDCl3): ıH 9.73 (s, 1H), 8.68 (s, 1H), 7.44 (d,
J = 9.2 Hz, 1H), 6.66 (q, J = 3.7 Hz, 1H), 6.50 (d, J = 2.4 Hz, 1H), 4.18
(br, 2H), 3.46 (q, J = 7.1 Hz, 4H), 1.25 (t, J = 7.0 Hz, 6H). 13C NMR
(100 MHz, CDCl3): ıC 163.9, 162.1, 157.6, 152.7,148.0, 131.1,110.0,
109.1, 108.2, 96.6, 45.1, 12.4; ESI-MS positive: m/z = 276.2 [M+H]+.
2.2. Synthesis of compounds
Chemodosimeter 1 and compound 4 were prepared according
to the route shown in Scheme 2.
2.2.2. Synthesis of chemodosimeter 1
2.2.1. Synthesis of compound 3
3 (46 mg, 0.17mmol) was dissolved in 10 mL hot ethanol. Phenyl
isothiocyanate (0.08 mL, 0.67 mmol) was then added. The mix-
ture was stirred at room temperature for 10 min under an Ar
atmosphere. The resulting precipitate was filtered and washed 3
The 7-diethylaminocoumarin-3-carboxylic acid ethyl ester (2)
was prepared according to a published procedure [38]. In a 100 mL
flask, 2 (511 mg, 1.77 mmol) was dissolved in 15 mL anhydrous
Scheme 2. Synthetic route of chemodosimeter 1 and compound 4.

250
W. Ma et al. / Spectrochimica Acta Part A 76 (2010) 248–252
Fig. 2. Fluorescence emission spectra of 1 (10 ␮M) upon addition of Hg2+ (10 equiv.)
and common interfering metal ions (10 equiv.) in Tris–HCl (0.02 M) solution
(MeCN/water = 3/7, v/v, pH 7.0). ꢀex = 425 nm. Each spectrum was recorded after
45 min.
Fig. 1. Fluorescence emission spectra of 1 (10 ␮M) in Tris–HCl (0.02 M) solution
(MeCN/water = 3/7, v/v, pH 7.0) in the presence of Hg2+ (0–10 equiv.). Inset: fluores-
cence enhancement at 506 nm as a function of Hg2+ concentration. The photograph
shows the fluorescent color of 1 (10 ␮M) upon addition 10 equiv. Hg2+ in solution
with 365 nm excitation. ꢀex = 425 nm. Each spectrum was recorded after 45 min.
times with anhydrous ethanol (3× 50 mL) and further dried under
reduced pressure to afford 60 mg yellow powder product 1 in 87.5%
yield: 1H NMR (400 MHz, DMSO): ıH 11.37 (br, 1H), 10.18 (br, 1H),
9.82 (br, 1H), 8.72 (s, 1H), 7.72 (d, J = 8.8 Hz, 1H), 7.53 (br, 2H), 7.34 (t,
J = 7.8 Hz, 2H), 7.15 (t, J = 7.4 Hz, 1H), 6.82 (dd, J1 = 2.0 Hz, J2 = 8.8 Hz,
1H), 6.64 (d, J = 2.4 Hz, 1H), 3.49 (q, J = 7.1 Hz, 4H), 1.15 (t, J = 7.0 Hz,
6H); 13C NMR (100 MHz, DMSO): ıC 161.7, 157.8, 153.3, 148.5,
139.6, 132.3,128.7, 125.2, 110.8, 108.1, 96.4, 44.9, 12.9; Q-TOFMS:
calculated for: C21H22N4O3NaS: 433.1310, found: m/z = 433.1322
[M+Na]+.
2.2.3. Synthesis of compound 4
A mixture of chemodosimer 1 (156 mg, 0.38 mmol) in 50 mL
dichloromethane and HgCl2 (833 mg, 3.06 mmol) in 20 mL anhy-
drous ethanol was stirred for 10 min at room temperature. The
solvent was evaporated under reduced pressure and the crude
product was purified by column chromatography on silica gel
using dichloromethane/ethyl acetate (2:1) to afford 140 mg 4 in
97.9% yield: 1H NMR (400 MHz, DMSO): ıH 10.66 (s, 1H), 8.44 (s,
1H), 7.63 (m, 3H), 7.36 (t, J = 8.0 Hz, 2H), 7.01 (t, J = 7.4 Hz, 1H),
6.78 (dd, J1 = 2.4 Hz, J2 = 8.8 Hz, 1H), 6.58 (d, J = 2.4 Hz, 1H), 3.48 (d,
J = 6.9 Hz, 4H), 1.15 (d, J = 7.0 Hz, 6H); 13C NMR (100 MHz, DMSO): ıC
160.2, 157.5, 157.4, 155.7, 152.5, 144.3, 139.2, 131.4, 129.5, 122.2,
117.5, 110.3, 107.8, 103.5, 96.6, 44.8, 12.8; Q-TOFMS: calculated for:
C21H20N4O3Na: 399.1433, found: m/z = 399.1427 [M+Na]+.
Fig. 3. Fluorescence response of 1 (10 ␮M) to Hg2+ (10 equiv.) in Tris–HCl (0.02 M)
solution (MeCN/water = 3/7, v/v, pH 7.0) containing various metal ions (10 equiv.).
ꢀex = 425 nm. Each spectrum was recorded after 45 min.
diethylamino group to electron-withdrawing 1,3,4-oxadiazoles. It
can also be confirmed from the fact that the HOMO–LUMO energy
gap (Fig. S2) undergoes a noticeable decrease on going from 1
(3.48 eV) to 4 (3.36 eV) [40,41].
3.2. Selectivity studies
3. Results and discussion
The fluorescence response of chemodosimeter 1 toward vari-
ous other metal ions including alkali (K+, Na+), alkaline earth (Ca2+
Mg2+), and transition-metal ions (Cd2+, Ag+, Fe3+, Pb2+, Cr3+, Co2+
,
,
3.1. Response of chemodosimeter 1 to Hg2+
Ni2+, Cu2+, Zn2+) was also examined. However, metal ions other
than Hg2+ showed negligible fluorescence response or quenching
effect to some extent under identical conditions (Fig. 2 and Fig. S3).
The competition experiments (Fig. 3) showed that most of the
ions displayed little interference except that Co2+, Ni2+, Cu2+, Zn2+
quenched the fluorescence to some extent. The quenching response
of Co2+, Ni2+, Cu2+ is in accordance with the intrinsic quenching
nature of these ions and always meets in the other chemod-
osimeters [17,31]. The quenching response of Zn2+ might be the
fact that Zn2+ ions exhibit higher affinity for 1 than Hg2+, which
preventing the Hg2+-induced desulfurization reaction. Notably,
the representative thiophilic ions Ag+, Pb2+ and Cd2+ could not
Free 1 showed a relatively weak emission band at 486 nm
(˚0 = 0.019 [39]). Upon addition of Hg2+, a 12.5-fold (˚F = 0.145)
fluorescence enhancement in fluorescence and a 20 nm red-shift
in emission maximum were observed in Tris–HCl (0.02 M) solu-
tion (MeCN/water = 3/7, v/v, pH 7.0) (Fig. 1) with an absorption
peak appeared at ꢀab = 443 nm, and the fluorescence changed from
almost colorless to bluish green. The time-dependent fluorescence
change for 1 was evaluated in the presence of Hg2+ ions. In a typical
experiment, the reaction between 1 and Hg2+ ions could complete
in 45 min (Fig. S1). A noticed red-shift signal in fluorescence spectra
is ascribed to the ICT within compound 4 from electron-donating

W. Ma et al. / Spectrochimica Acta Part A 76 (2010) 248–252
251
Scheme 3. A proposed mechanism for the Hg2+-induced desulfurization reaction of 1.
range of 6.84–9.37, affording the possibility for detection of Hg2+
under neutral and weakly basic conditions.
3.4. Sensing mechanism
The sensing mechanism could be postulated from the design
of molecular structure as shown in Scheme 3. It is certain that
the thiosemicarbazide group in 1 undergoes irreversible mercury-
ion-promoted cyclization to form a 1,3,4-oxadiazole group in 4 in
aqueous media at room temperature. To confirm the sensing mech-
anism of 1 for Hg2+, we provided several lines of evidence. Firstly,
the fluorescence emission spectra of 1 incubated with 10 equiv. of
Hg2+ for 45 min in Tris–HCl (0.02 M) solution (MeCN/water = 3/7,
v/v, pH 7.0) was compared with those of compound 4, and the spec-
tra of 1 + Hg2+ and 4 were found to be almost identical (Fig. S4),
demonstrating that chemodosimeter 1 was transformed into com-
pound 4 induced by Hg2+ ions. Secondly, when 10 equiv. of Hg2+ was
introduced to a 1 solution, the peaks m/z 377.2 and 399.2 appeared
and was assigned to [4+H]+ and [4+Na]+, respectively, instead of
the peak of [1+H]+ (Fig. S5), which confirming the occurrence of
Hg2+-induced desulfurization at room temperature. Finally, treat-
ment by EDTA for 1-Hg2+ system could not change the fluorescence
spectra of 1 (Fig. S6). The result implies an irreversible nature for the
desulfurization, which is also observed in those chemodosimeters
typical for Hg2+ [18,25,28,34].
Fig. 4. Variation of fluorescence intensity at 506 nm of 1 (10 ␮M) in Tris–HCl
(0.02 M) solution (MeCN/water = 3/7, v/v, pH 7.0) in the absence (black) and pres-
ence (red) of Hg2+ (10 equiv.) as a function of pH. ꢀex = 425 nm. (For interpretation
of the references to color in this figure caption, the reader is referred to the web
version of the article.)
promote the desulfurization reaction under identical conditions.
The results indicated that chemodosimeter 1 had good selectivity
toward Hg2+
.
4. Conclusion
3.3. Effect of pH
In summary, we demonstrated a Hg2+ sensitive fluorescent
chemodosimeter 1 based on the coumarin thiosemicarbazide.
Chemodosimeter 1 displays a goog selective fluorescence enhance-
ment for Hg2+, which is attributed to the transformation of
thiosemicarbazide unit to 1,3,4-oxadiazoles via Hg2+-induced
desulfurization reaction in aqueous media at room temperature.
The effects of pH for 1 on the fluorescence response to Hg2+
were investigated and shown in Fig. 4. Chemodosimeter 1 alone
was not sensitive to pH as illustrated by fluorescence intensity. In
the presence of Hg2+, however, the 1-Hg2+ system had a strong pH-
dependence which showed a stable fluorescence response in the pH

252
W. Ma et al. / Spectrochimica Acta Part A 76 (2010) 248–252
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