A highly selective colorimetric sensor for Hg2+ based on nitrophenyl-aminothiourea

Article abstract of DOI:10.1016/j.saa.2012.03.021

A simple and highly selective colorimetric sensor (L1) bearing thiosemicarbazide moiety as binding site and nitrophenyl moiety as signal group were synthesized. Sensor L1 showed great colorimetric single selectivity and high sensitivity for mercury cation in DMSO and DMSO/H₂O binary solutions. When Hg²⁺ was added to the DMSO solution of L1, dramatic color change from brown to colorless was observed. While the cations Ca ²⁺, Mg²⁺, Cd²⁺, Fe³⁺, Co ²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb ²⁺, Ag⁺ and Cr³⁺ could not cause any distinct interferer toward the recognition process for Hg²⁺. The detection limit is allowable to 5.0 × 10^-6 and 1.0 × 10 ^-7 M level of Hg²⁺ according to visual color change and UV-vis change, respectively. The recognition mechanism of the sensor toward mercury cation was evaluated in DMSO solutions by UV-vis and ¹H NMR. The sensor selectively sense Hg²⁺ via the formation of a stable 1:1 complex through C=S and C=O group with Hg²⁺. When these complex bonds formed, the sensor carried out an ICT transition induced color change.

Full text of DOI:10.1016/j.saa.2012.03.021

Spectrochimica Acta Part A 93 (2012) 245–249
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A highly selective colorimetric sensor for Hg2+ based on
nitrophenyl-aminothiourea
Juan Liua,b, Mei Yua, Xi-Cun Wanga,∗, Zhang Zhanga
a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China; Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China
b College of Chemical Engineering, Northwest University for Nationalities, Lanzhou, Gansu 730030, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and highly selective colorimetric sensor (L1) bearing thiosemicarbazide moiety as binding site
and nitrophenyl moiety as signal group were synthesized. Sensor L1 showed great colorimetric single
selectivity and high sensitivity for mercury cation in DMSO and DMSO/H2O binary solutions. When Hg2+
was added to the DMSO solution of L1, dramatic color change from brown to colorless was observed.
While the cations Ca2+, Mg2+, Cd2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Ag+ and Cr3+ could not cause any
distinct interferer toward the recognition process for Hg2+. The detection limit is allowable to 5.0 × 10−6
and 1.0 × 10−7 M level of Hg2+ according to visual color change and UV–vis change, respectively. The
recognition mechanism of the sensor toward mercury cation was evaluated in DMSO solutions by UV–vis
Received 17 August 2011
Received in revised form 20 February 2012
Accepted 12 March 2012
Keywords:
Mercury cation
Colorimetric sensors
Thiourea
Single selectivity
Sensitivity
and 1H NMR. The sensor selectively sense Hg2+ via the formation of a stable 1:1 complex through C
S
and C O group with Hg2+. When these complex bonds formed, the sensor carried out an ICT transition
induced color change.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
sensor L1 bearing thiosemicarbazide groups (Scheme 1). The strate-
gies for design of the sensor are as follows. Firstly, we have
Development of chemosensors for mercury cation (Hg2+) has
received considerable attention because mercury has an extremely
toxic impact on the environment and human health [1–4]. For
example, mercury can lead to the dysfunction of the brain [3], kid-
ney [5], and central nervous systems [6]. Therefore, the rational
design and synthesis of efficient sensors to selectively recognize
mercury cation is an important field of supramolecular chem-
istry [7–9]. Although previous work developed a wide variety
of chemical [10–22] and physical [23,24] sensors for the detec-
tion of Hg2+, so far, it is still a challenge to improvement of
the detection selectivity in the context of interference from
coexisting metal ions. Moreover, most of these methods require
expensive equipment and involve time-consuming and laborious
procedures that can be carried out only by trained professionals
[23–25]. This will significantly restrict the practical applications
of these Hg2+ sensors. For simplicity, convenience and low cost,
the easily prepared Hg2+ colorimetric sensors [26,27] are highly
demanding.
introduced thiourea group as binding site. The C S moiety on
thiourea group possesses high affinity to Hg2+. Secondly, in order
to achieve “naked-eye” recognition, we introduced nitrophenyl
groups as the signal group. Finally, the sensor was designed to be
easy to synthesize. In order to figure out the signal group’s contri-
bution to the Hg2+ sensing abilities of the sensor L1, compound L2
of analogue structure but without containing nitro-group were also
synthesized. Interestingly, the sensor L1 could “naked-eye” recog-
nize Hg2+ with high selectivity and sensitivity, other cations such
as Ca2+, Mg2+, Cd2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Ag+ and Cr3+
could not cause any interference.
2. Experimental
2.1. Materials and physical methods
1H NMR spectra were recorded with a Mercury-400BB spec-
trometer at 400 MHz. 1H chemical shifts are reported in ppm
downfield from tetramethylsilane (TMS, ␦ scale with the solvent
resonances as internal standards). UV–vis spectra were recorded
on a Shimadzu UV-2550 spectrometer. Melting points were mea-
sured on an X-4 digital melting-point apparatus (uncorrected). The
infrared spectra were performed on a Digilab FTS-3000 FT-IR spec-
trophotometer.
In view of these and as an attempt to obtain an efficient Hg2+
colorimetric sensor, we have designed and synthesized a Hg2+
∗
Corresponding author. Tel.: +86 931 7973193.
E-mail address: xicunwang@126.com (X.-C. Wang).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.03.021

246
J. Liu et al. / Spectrochimica Acta Part A 93 (2012) 245–249
Scheme 1. Synthetic procedures for sensors L1, L2.
137.18, 129.89, 122.99, 116.09, 62.16, 14.18; IR (KBr, cm−1) v: 3444
(mb, N H), 3309 (m, N H), 3188 (s, N H), 1736 (s, C O), 1618 (s,
C), 1596 (s, C C), 1556 (s, C C), 1510 (s, C C), 1212 (s, C S);
Anal. Calcd. for C10H11N5O6S: C, 36.47; H, 3.37; N, 21.27; Found: C,
36.51; H, 3.65; N, 21.54.
The inorganic salts Ca(ClO4)2·6H2O, Mg(ClO4)2·6H2O,
Cd(ClO4)2·6H2O,
Ni(ClO4)2·6H2O,
Fe(ClO4)3·6H2O,
Cu(ClO4)2·6H2O,
Co(ClO4)2·6H2O,
Zn(ClO4)2·6H2O,
C
Pb(ClO4)2·3H2O, AgClO4·H2O and Cr(ClO4)3·6H2O were pur-
chased from Alfa Aesar Chemical Reagent Co. (Tianjin, China). All
solvents and other reagents were of analytical grade.
L2: yield: 85.4%; m.p. 149–150 ◦C; 1H NMR (DMSO-d6, 400 MHz)
ı 11.17 (s, 2H, NH), 10.42 (s, 1H, NH), 8.14–8.08 (m, 2H, ArH),
7.38–7.15 (m, 2H, ArH), 6.80–6.72 (m, 1H, ArH), 4.17 (q, J = 7.2,
2H, CH2), 1.21 (t, 3H, CH3); 13C NMR (DMSO-d6, 100 MHz) ı
180.23, 153.01, 147.55, 128.80, 127.90, 122.39, 119.50, 112.84,
61.93, 14.18; IR (KBr, cm−1) v: 3421 (mb, N H), 3280 (s, N–H), 3168
(m, N–H), 1712 (s, C O), 1602 (s, C C), 1533 (s, C C), 1492 (s, C C),
1206 (s, C S); Anal. Calcd. for C10H13N3O2S: C, 50.19; H, 5.48; N,
17.56; Found: C, 50.24; H, 5.25; N, 17.33.
2.2. General procedure for UV–vis experiments
All the UV–vis experiments were carried out in DMSO solution
on a Shimadzu UV-2550 spectrometer. Any changes in the UV–vis
spectra of the synthesized compound were recorded on addition
of perchlorate metal salts while keeping the ligand concentration
constant in all experiments.
2.3. General procedure for 1H NMR experiments
3. Results and discussion
For 1H NMR titrations, two stock solutions were prepared in
DMSO-d6, one of them containing host only and the second one
containing an appropriate concentration of guest. Aliquots of the
two solutions were mixed directly in NMR tubes.
The colorimetric sensing abilities were primarily investigated
by adding various cations Ca2+, Mg2+, Cd2+, Fe3+, Co2+, Ni2+, Cu2+
,
Hg2+, Zn2+, Pb2+, Ag+ and Cr3+ to the DMSO solutions of sen-
sor L1 (Figs. 1 and 2). When adding 5 equiv. of Ca2+, Mg2+
,
Cd2+, Co2+, Ni2+, Zn2+, Pb2+, and Cr3+ to the solutions of sensor
L1 (2.0 × 10−5 M), respectively, no significant color or spectrum
changes were observed. While, when adding 5 equiv. of Hg2+ to the
solution of L1 (2.0 × 10−5 M), the sensor responded with dramatic
color changes from brown to colorless (Fig. 1). In the corresponding
UV–vis spectrum, the absorption at 470 nm disappeared (Fig. 2).
When adding 5 equiv. of Cu2+, the DMSO solution of L1 showed
color changes from brown to green, in the corresponding UV–vis
spectrum, the absorption at 470 nm was shifted to 447 nm and
a weak broad absorption peak appeared at 640 nm. Meanwhile,
the addition of Ag+ and Fe3+ also lead to slight color and UV–vis
spectrum changes. Even though sensors L1 showed colorimetric
response toward Cu2+, Ag+ and Fe3+ in DMSO solutions, owing to
only Hg2+ could bleaches the solution, the sensor L1 exhibited sin-
2.4. Synthesis and characterization of sensors L1, L2
The synthetic procedure is shown in Scheme 1, ethyl chlorofor-
mate (3 mmol), dry and powdered NH4SCN (4 mmol) and 0.06 mL
TMEDA (N, N, Nꢀ, Nꢀ-tetramethylethylenediamine, as catalyst) were
added in dry dichloromethane (15 mL), the reaction mixture was
stirred at room temperature for 2 h, then filtered out the inorganic
salts, the filtrate is the solution of corresponding ethoxylcarbonyl
isothiocyanate, which need not separate, added 2.8 mmol of (2,4-
dinitrophenyl)hydrazine into the filtrate solution and stirred it in
room temperature for 3 h, yield the precipitate of L1. After evapo-
rated the solvent in vacuum, the precipitate was filtrated, washed
with 75% ethanol three times, then recrystallized with ethanol to
get yellow crystal of L1. The other compounds L2 was prepared by
the similar procedure.
gle selectivity for Hg2+
.
The same tests were applied to L2 with various cations. No obvi-
ous color changes were observed when any of the cations were
added to the solutions of L2 in same conditions. These results elu-
cidated that nitrophenyl moiety acted as a signal group and played
a crucial role in the process of colorimetric recognition.
L1: yield: 96.5%; m.p. 217–218 ◦C; 1H NMR (DMSO-d6, 400 MHz)
ı 11.39 (s, 2H, NH), 10.49 (s, 1H, NH), 8.88 (s, 1H, ArH), 8.36–8.33 (m,
1H, ArH), 7.23–7.21 (m, 1H, ArH), 4.21 (q, J = 7.2, 2H, CH2), 1.27 (t,
3H, CH3); 13C NMR (DMSO-d6, 100 MHz) ı 181.02, 152.68, 147.41,
Fig. 1. Color changes observed upon the addition of various cations (5 equiv) to the solutions of sensor L1 (2 × 10−5 M) in DMSO solutions. (For interpretation of references
to color in this figure legend, the reader is referred to the web version of this article.)

J. Liu et al. / Spectrochimica Acta Part A 93 (2012) 245–249
247
Fig. 2. (a) UV–vis absorption spectrum and (b) UV–vis absorption at 470 nm of L1 (2.0 × 10−5 M) in the presence of 5 equiv of various cations in DMSO solution at room
temperature.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
In biological and environmental systems, cation-sensor inter-
actions commonly occur in aqueous solution, therefore, much
attention has been paid to developing cation sensors that work in
the aqueous phase. For these reasons, we investigated the cation
recognition abilities of sensor L1 in DMSO/H2O binary solutions
using UV–vis spectroscopy. We carried out the similar experiments
in 9:1 DMSO/H2O HEPES (10 mM) buffered solution at pH 7.40 for
sensors L1 (2.0 × 10−5 M). When adding 5 equiv. of Hg2+ to the 9:1
DMSO/H2O HEPES buffered solution of L1, the sensor responded
with dramatic color changes from brown to colorless, meanwhile,
in the corresponding UV–vis spectrum, the absorption at 470 nm
disappeared (Fig. 3). While other cations could not cause such
distinct color and spectrum changes, therefore, sensor L1 could
colorimetric recognition Hg2+ in DMSO/H2O binary solutions with
special selectivity.
L1 + other cations: Ca2+, Mg2+, Cd2+, Fe3+,
Co2+, Ni2+, Zn2+, Pb2+ and Cr3+
Cu2+
Hg2+
To gain an insight into the stoichiometry of the sensor L1-Hg2+
complex, the method of continuous variations (Job’s plot) was used
(Fig. 4a). As expected, when the molar fraction of sensor L1 was
0.50, the absorbance value approached a maximum, which demon-
strated the formation of a 1:1 complex between sensor L1 and
250 300 350 400 450 500 550 600 650 700 750
Wavelength/nm
Fig. 3. UV–vis absorption spectrum of L1 (2.0 × 10−5 M) in the presence of 5 equiv
of various cations in 9:1 DMSO/H2O HEPES (10 mM) buffered solution at pH 7.40,
room temperature.
Hg2+
.
The further binding properties of sensor 3 toward Hg2+ were
studied by UV–vis titration experiments (Fig. 4b). It turned out that
in DMSO solution of L1, with increasing of Hg2+, the absorbance
at 470 nm decreases while a new band appears at 401 nm. Such a
blue shift led to the solution color changing from brown to colorless.
Two clear isosbestic points are observed at 435 and 363 nm, which
indicates the formation of L1-Hg2+ complex. By nonlinear least-
squares fitting [28] at ꢀmax = 469 nm, the association constants Ka
To further elucidate the binding mode of sensor L1 with Hg2+, 1H
NMR-titration spectra were measured, which illustrated the char-
acteristic structural changes occurring upon interaction with Hg2+
in DMSO-d6 solution. As shown in Fig. 5, there are two intramolecu-
lar hydrogen bonds in the molecular of L1, the one is N Hb. . .O C,
the other is N Ha. . .O N. The formation of these hydrogen bonds
of the sensor L1 toward Hg2+ was obtained as 6.04 × 104 M−1
.
a
b 0.4
obs
calc
0.4
R=0.995
0.3
0.3
0.2
0.1
0.0
2+] /[L1]
[ Hg
0.2
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
300 350 400 450 500 550 600 650 700 750
Wavelength/nm
2+
2+
[ Hg ] /[ Hg ] +[ L1]
Fig. 4. (a) Job’s plot of L1 and Hg2+, which indicated the stoichiometry of L1-Hg2+ complex is 1:1. (b) UV–vis spectral titration of sensor L1 with Hg2+ in DMSO solution. The
non-linear fitting curve of change in absorbance of L1 with respect to amounts of Hg2+ was shown in the inset.

248
J. Liu et al. / Spectrochimica Acta Part A 93 (2012) 245–249
Fig. 5. The proposed reaction mechanism of the sensor L1 with Hg2+
.
N
Hf shifted downfield and broadened (Fig. 6). With the contin-
uous addition of Hg2+, as shown in Fig. 5, the molecule structure
carried out an intramolecular rotation, which induced the rupture
of N Hb. . .O. . .C and N Ha. . .O N, simultaneously, the interac-
tions between the Hg2+. . .He C and Hg2+. . .Hf C were interrupted
also. As a result, the stable Hg2+-L1 1: 1 complex formed by the
coordinate of Hg2+ with S C and O C groups on L1, however, the
rupture of the hydrogen bonds lead to the signals of N Hb and
N
Ha moved up field (Fig. 6d–f), on the other hand, owing to
the interactions between the Hg2+. . .He C and Hg2+. . .Hf C were
interrupted, the signals of C He and C Hf backed to the original
location (Fig. 6d–f). Along with these processes, the intramolecular
charge-transfer (ICT) transition was interrupted, which induced a
blue-shift in UV–vis spectrum and depigmentation of L1. Namely,
before all the 1 equiv. of Hg2+ was added, the transition-state and
the stable Hg2+-L1 complex coexisted in the solution, therefore,
every proton signals of N Ha, N Hb, N Hc, C He and C Hf dis-
played two signals, for the transition-state and the stable Hg2+-L1
complex state, respectively (Fig. 6d–f). After 1 equiv. of Hg2+ had
been added, the transition-state completely changed into stable
Hg2+-L1 complex, accordingly, in the whole titration process, the
signals of N Hc, N Hb and N Ha shifted to ı 11.22, 10.49 and
10.23 ppm respectively and the proton signals of C He and C Hf
backed to the original position (Fig. 6g). In summary, according to
the 1H NMR-titration experiments, the Hg2+ and L1 formed a stable
1: 1 complex by the coordinate of Hg2+ with S C and O C groups
on L1.
Fig. 6. Partial 1H NMR spectra of sensor L1 (2.5 mM) in DMSO-d6 upon the addition
of Hg2+. (a) Free L1, after adding (b) 0.1, (c) 0.2, (d) 0.5, (e) 0.7, (f) 0.9 and (g) 1 equiv
of Hg2+
.
leads to the 1H NMR chemical shifts of N Ha and N Hb appeared at
low-field. Owing the N Hb. . .O C is a very stronger intramolecular
hydrogen bond, [29–34] as shown in Fig. 6, the 1H NMR chemical
shifts of N Hb appeared at the lowest field of the molecular of L1
at ı 11.39 ppm, which overlap the proton signal of N Hc. While the
1H NMR chemical shifts of N Ha appeared at ı 10.49 ppm. After
the addition of 0.2 equiv. of Hg2+, a transition-state was formed
between L1 and Hg2+(Fig. 5), which induced the 1H NMR chem-
ical shifts of the N Ha, N Hb and N Hc appeared as two very
broad peaks. For the same reason, the proton signals of N He and
Fig. 7. UV–vis absorbance (a) and photographs (b) of sensor L1 (2.0 × 10−5 M) in DMSO solutions in the presence of Hg2+ (5 equiv) and miscellaneous cations including Ca2+
,
Mg2+, Cd2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Ag+ and Cr3 (5equiv, respectively, at 470 nm).

J. Liu et al. / Spectrochimica Acta Part A 93 (2012) 245–249
249
Acknowledgments
This work was supported by the National Nature Science
Foundation of China (Nos. 20902073 and 21062017), the Natu-
ral Science Foundation of Gansu Province (No. 096RJZA116), and
the Fundamental Research Funds for the Central Universities” (No.
zyz2011062).
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In conclusion, we designed and synthesized an easy-to-make
Hg2+ sensor L1 which bearing thiosemicarbazide moiety as recogni-
tion site and nitrophenyl moiety as signal group. Sensor L1 showed
colorimetric single selectivity for Hg2+ in DMSO and DMSO/H2O
binary solutions. Comparison with sensor L2 indicates that the
nitrophenyl moiety acted as a signal group and played a crucial role
in the process of colorimetric recognition. The researches of recog-
nition mechanism indicated that the sensor L1 recognize Hg2+ by
form a stable 1:1 L1-Hg2+ complex. The coexisting of other cations
could not interfere the Hg2+ recognition process, moreover, the
detection limit of the sensor L1 toward Hg2+ is 1.0 × 10−7 M, which
indicated that the sensor L1 could potentially be useful as a probe
for monitoring Hg2+ levels in physiological and environmental sys-
tems.
A 78 (2011)