A simple colorimetric sensor with high selectivity for mercury cation in aqueous solution

Article abstract of DOI:10.1080/10426507.2011.590166

A simple and sensitive mercapto thiadiazole Schiff-base colorimetric chemosensor 2 was synthesized. It exhibits good selectivity and sensitivity for the mercury cation over other metal cations such as Zn²⁺, Pb ²⁺, Cd²⁺, Ni²⁺, Co²⁺, Ag+, Ca ²⁺, Mg²⁺, and Cr³⁺ in both aqueous solution and on paper-made test kits. The change in color is very easily observed by the naked eye for the presence of Hg²⁺ cation, whereas other metal cations do not induce such a change. The chemosensor 2 showed remarkably anti-interference ability toward other metal cations and the detection limit toward Hg²⁺ was 1.5 × 10^-5 M. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426507.2011.590166

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Phosphorus, Sulfur, and Silicon and the
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A Simple Colorimetric Sensor with
High Selectivity for Mercury Cation in
Aqueous Solution
You-Ming Zhang a b , Ping Li a b , Qi Lin a b , Tai-Bao Wei a b & Jun-
Qiang Li a b
a Key Laboratory of Eco-Environment-Related Polymer Materials,
Ministry of Education of China, College of Chemistry and Chemical
Engineering, Northwest Normal University, Gansu, P. R. China
b Key Laboratory of Polymer Materials of Gansu Province, College of
Chemistry and Chemical Engineering, Northwest Normal University,
Gansu, P. R. China
Published online: 31 Oct 2011.
To cite this article: You-Ming Zhang , Ping Li , Qi Lin , Tai-Bao Wei & Jun-Qiang Li (2011) A Simple
Colorimetric Sensor with High Selectivity for Mercury Cation in Aqueous Solution, Phosphorus, Sulfur,
and Silicon and the Related Elements, 186:12, 2286-2294, DOI: 10.1080/10426507.2011.590166
To link to this article: http://dx.doi.org/10.1080/10426507.2011.590166
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Phosphorus, Sulfur, and Silicon, 186:2286–2294, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.590166
A SIMPLE COLORIMETRIC SENSOR WITH HIGH
SELECTIVITY FOR MERCURY CATION
IN AQUEOUS SOLUTION
You-Ming Zhang,1,2 Ping Li,1,2 Qi Lin,1,2 Tai-Bao Wei,1,2
and Jun-Qiang Li1,2
1Key Laboratory of Eco-Environment-Related Polymer Materials,
Ministry of Education of China, College of Chemistry and Chemical Engineering,
Northwest Normal University, Gansu, P. R. China
2Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry
and Chemical Engineering, Northwest Normal University, Gansu, P. R. China
GRAPHICAL ABSTRACT
a
H
N
N
H
S
CH3
CH3
N
N
S
S
N
N
H3C
H3C
Hg(CIO4)2
a
SH
N
Hg2+
N
H
-
H3C
2CIO4
+
S
N
N
H3C
S
H
N
N
a
H
Abstract A simple and sensitive mercapto thiadiazole Schiff-base colorimetric chemosensor
2 was synthesized. It exhibits good selectivity and sensitivity for the mercury cation over other
metal cations such as Zn2+, Pb2+, Cd2+, Ni2+, Co2+, Ag+, Ca2+, Mg2+, and Cr3+ in both
aqueous solution and on paper-made test kits. The change in color is very easily observed by
the naked eye for the presence of Hg2+ cation, whereas other metal cations do not induce such
a change. The chemosensor 2 showed remarkably anti-interference ability toward other metal
cations and the detection limit toward Hg2+ was 1.5 × 10−5 M.
Keywords Mercury cation sensor; mercapto thiadiazole; cation recognition; colorimetic test
kit
INTRODUCTION
In recent years, one of the important research topics in chemistry has been the design
of cheap and easy-to-use molecular receptors for highly toxic element such as mercury.1,2
Received 17 March 2011; accepted 14 May 2011.
This work was supported by the NSFC (No. 21064006) and the Natural Science Foundation of Gansu
(1010RJZA018).
Address correspondence to Tai-Bao Wei, Key Laboratory of Eco-Environment-Related Polymer Materials,
Ministry of Education of China, College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou, Gansu 730070, P. R. China. E-mail: weitaibao@126.com
2286

COLORIMETRIC SENSOR WITH HIGH SELECTIVITY FOR MERCURY CATION
2287
Specifically, Hg2+-sensitive systems have attracted interest in this field given that mercury
is considered as one of the most dangerous and toxic elements.3–6 In view of this, the
rational design and synthesis of efficient chemosensors to selectively and sensitively rec-
ognize Hg2+ are in high demand by organic and analytical chemists.7,8 Recently, a variety
of sensing devices based on fluorescent chemosensors,9,10 organic chromophores,11,12 elec-
trochemical devices,13 and nanomaterials14 have been employed for the selective detection
of Hg2+. However, several significant challenges still exist in these devices for practical
application, such as lower water solubility, cross-sensitivities toward other metal cations,
and especially, expensive and complicated synthesis.15 Therefore, in connection with our
continuing research of sensors for heavy metal cations in supramolecular recognitions,16–18
we attempted to design some easy-to-synthesize, highly selective, and sensitive sensors for
Hg2+ (Scheme 1). We introduced mercapto and thiadiazole groups into the sensor molecu-
lar, so that it can strengthen Hg2+ coordination capacity.19 On the other hand, a Schiff-base
structure, which achieves “naked-eye” recognition upon binding of Hg2+, was used to sig-
nificantly enhance intermolecular charge transfer (ICT).20–22 As a result, the sensor was
synthesized and displayed a highly selective and sensitive response toward Hg2+ in neutral
aqueous solution.
Scheme 1
RESULTS AND DISCUSSION
The Cation Recognition Abilities
The colorimetric sensing abilities were primly investigated by adding various cations
such as Zn2+, Pb2+, Cd2+, Ni2+, Co2+, Hg2+, Ag+, Ca2+, Mg2+, and Cr3+ to dimethyl
sulfoxide (DMSO)/H2O (7.5:2.5, v/v) solutions of sensor 2 (2.0 × 10−5 M). When adding
5 equivalent of Hg2+ to the solution of sensor 2, the sensor responded with dramatic
color changes from yellow to colorless. In the corresponding UV-Vis spectrum, an obvious
change of the UV-Vis spectrum absorption at 438 nm disappeared companying with a blue
shift of 93 nm and a new signal peak at 345 nm appeared (Figure 1a). Meanwhile, when
the same tests were applied to other cations, no significant color changes were observed

2288
Y.-M. ZHANG ET AL.
Figure 1 (a) Absorption of 2 in DMSO/H2O (7.5:2.5, v/v) upon addition of 5 equivalent of various perchlorate
metal salts (4.0 × 10−3 M). (b) Optical density of 2 at 438 nm upon the addition of different cations. (c) Solutions
of 2 (c = 2.0 × 10−5 mol L−1) upon addition of different metal ions.

COLORIMETRIC SENSOR WITH HIGH SELECTIVITY FOR MERCURY CATION
2289
for sensor 2 (Figure 1c). These results showed that 2 is a remarkably selective and sensitive
chemosensor for Hg2+
.
To establish the selective and sensitive utility of sensor 2 for Hg2+ in practice,
competitive experiments of the sensor 2 (2.0 × 10−5 M) toward various metal cations were
investigated by UV-Vis in DMSO/H2O (7.5:2.5, v/v) solutions. As shown in Figure 2(c),
chemosensor 2 (2.0 × 10−5 M) showed an obvious color change from yellow to colorless
Figure 2 (a) Absorption of 2 (c = 2.0 × 10−5 M) in DMSO/H2O (7.5:2.5, v/v) upon addition of 5 equivalent
of Hg2+ and various perchlorate metal salts (4.0 × 10−3 M). (b) Optical density of 2 at 438 nm with addition of
Hg2+ (4.0 × 10−3 M) in the presence of 5 equivalent of other metal ions. (c) Solutions of 2 (c = 2.0 × 10−5 mol
L
−1) upon addition of 5 equivalent of Hg2+ and other different perchlorate metal ions.

2290
Y.-M. ZHANG ET AL.
0.8
0.6
0.4
0.2
0.0
0 equiv.
31.4 equiv.
300
400
500
Wavelength (nm)
Figure 3 Titration curves of chemosensor 2 in DMSO/H2O (7.5:2.5, v/v) (c = 4.0 × 10−5 M) upon addition of
Hg2+ (0–31.4 equivalent).
upon addition of 5 equivalent of Hg2+ (2.0 × 10−5 M) in DMSO/H2O (7.5:2.5, v/v)
solutions; the peak at 438 nm disappeared in the UV-Vis spectrum and a new signal appeared
at 345 nm. On the other hand, a variety of other cations, such as Zn2+, Pb2+, Cd2+, Ni2+
,
Co2+, Ag+, Ca2+, Mg2+, and Cr3+, did not cause such a significant change, indicating
that the system 2-Hg2+ was hardly affected by these coexistent cations. Accordingly, these
results suggested that sensor 2 still displayed an excellent selectivity toward Hg2+
.
To further investigate the interaction between chemosensor 2 and Hg2+, the UV-Vis
titration experiments were conducted using a 4.0 × 10−5 M solution of 2 in DMSO/H2O
(7.5:2.5, v/v) (Figure 3a). Upon the addition of Hg2+ (0.01 M) to the solution, a significant
decrease of the UV-Vis absorbance at 427 nm and an increase of UV-Vis emission band
centered at 344 nm were observed with an isobestic point at 367 nm. These results indicated
that the chemosensor 2 reacted with Hg2+ to form a stable complex.23 The further bind-
1
ing properties of sensors 2 toward Hg2+ were studied by H NMR titration experiments
1
(Figure 4a). The result of H NMR showed that the S–H signal disappeared when 0.5
equivalent of Hg2+ was added. This means that the ratio of [2]/[Hg2+] in the complex is 2
in DMSO/H2O (7.5:2.5, v/v) solutions, and sensors 2 reacted with Hg2+ and formed 2:1
complexes. The binding constant K of the metal complex was determined by Equation (1),
assuming that the concentration of free metal is about equal to its total concentration ([M]
≈ [M]t),24–26
F − F0
Fm − F
[C]
[D]
=
= K[M]n
(1)
where Fo, F, and Fm are the corrected UV-Vis emission intensities of the complex at
initial, interval, and the final states at which the complex was fully formed upon addition
of metal ion, respectively. The binding constant K was determined from the plot of the
linear regression of log[(F – F0)/(Fm – F)] versus log[M] in Equation (2),27 derived from

COLORIMETRIC SENSOR WITH HIGH SELECTIVITY FOR MERCURY CATION
2291
Figure 4 (a) 1H NMR spectra of chemosensor 2 (0.005 mM) upon addition of 0.2 and 1.0 equivalent of Hg2+ in
DMSO-d6. (b) The proposed structure of the sensor 2 with Hg2+
.
Equation (1), to obtain the intercept as log K and the slope as n.
F − F0
log
= log K + n log[M]
(2)
Fm − F
Thus, the association constant K of the chemosensors 2 toward Hg2+ was calculated
as 8.70 × 104 M−1. Furthermore, the lowest detection limit 1.5 × 10−5 M of Hg2+ with
sensor 2 was evaluated from the plot of the titration profile.28 Taken together, these results
illustrate that sensor 2 is a Hg2+-specific chemosensor. Thus, the sensor 2 could potentially
be used as a probe for monitoring Hg2+ in physiological and environmental systems.
To further elucidate the binding mode of sensor 2 with Hg2+, proton NMR titration ex-
periments of sensor 2 were observed, which illustrated the characteristic structural changes
occurring upon interaction with Hg2+ in DMSO-d6 solution. As show in Figure 4(a), the
1H NMR chemical shifts of the S–Ha proton on sensor 2 were at δ = 14.25 ppm before
the addition of the Hg2+. After the addition of 0.2 equivalent of Hg2+, the resonance for
S–Ha proton did not shift, but the intensity of it became weaker. With the addition of 0.5

2292
Y.-M. ZHANG ET AL.
Figure 5 Photographs of the colorimetric test kits with 2 for detecting Hg2+ ion in aqueous solution.
equivalent of Hg2+, the signal of the S–Ha proton disappeared completely and a weak signal
peak appeared at δ 9.67 ppm, which attributed to the NH proton. Thus, the 1H NMR study
tends to confirm the inclusion complex formation, while the 2:1 binding stoichiometry was
confirmed using proton NMR titration spectra. The experimental results suggest a possible
interaction mode between the sensor 2 and Hg2+, in which Hg2+ was coordinated with the
S on mercapto group (Figure 4b).
To investigate the practical application of chemosensor 2, colorimetric test strips were
prepared by immersing filter papers into a DMSO/H2O (7.5:2.5, v/v) solution of 2 (0.01
M) and then drying it via vacuum (Figure 5a). The test strips containing 2 were utilized
to sense different metal ions. After several drops of different metal ions (4.0 × 10−3 M)
were added, the obvious color change from yellow to colorless was observed only with
mercury cation solution (Figure 5b) and no significant color change was observed in the
others (Figure 5a). Accordingly, the colorimetric test strip can easily and rapidly detect
mercury cations.
CONCLUSION
A
simple colorimetric chemosensor was synthesized by coupling p-
dimethylaminobenzaldehyde with 2-amino-5-mercapto-1,3,4-thiadiazole to form
Schiff-base structure with the enhanced intramolecular charge transfer. The selectivity and
sensitivity of 2 to Hg2+ over other metal ions in aqueous solution were demonstrated by
its optical response, which ascribed to the mercapto thiadiazole group. The chemosensor 2
showed remarkably anti-interference ability toward other metal cations and the detection
limit toward Hg2+ was 1.5 × 10−5 M, which indicates that this sensor could potentially be
used as a probe for monitoring Hg2+ in physiological and environmental systems. On the
basis of the colorimetric response of 2 to Hg2+, test strips containing 2 were fabricated,
which also shows a high sensitivity to Hg2+. The qualitative detection of Hg2+ using these
test strips was attempted.

COLORIMETRIC SENSOR WITH HIGH SELECTIVITY FOR MERCURY CATION
2293
EXPERIMENTAL
Materials and Methods
Melting points were measured on an X-4 digital melting point apparatus and were
uncorrected. UV-Vis spectra were recorded on a Shimadzu UV-2550 spectrometer. 1H NMR
spectra were recorded on a Varian Mercury Plus-400 MHz spectrometer with DMSO as a
solvent and tetramethylsilane as an internal reference.
General Procedure for UV-Vis Experiments. All the UV-Vis experiments were
carried out in DMSO or DMSO/H2O binary solution on a Shimadzu UV-2550 spectrometer.
Any changes in the UV-Vis spectra of the synthesized compound were recorded on addition
of perchlorate salt while keeping the ligand concentration constant in all experiments.
Perchlorate metal salts of cations (Zn2+, Pb2+, Cd2+, Ni2+, Co2+, Hg2+, Ag+, Ca2+, Mg2+
and Cr3+) were used for the UV-Vis experiments.
General Procedure for H NMR Experiments. For H 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.
,
1
1
Synthesis and Characterizations
Intermediate 2-amino-5-mercapto-1,3,4-thiadiazole29 was prepared by mixing
thiosemicarbazide (5.52 g, 0.06 mol) and CS2 (7.260 g, 0.096 mol) in dimethylformamide
(15 mL) solution under 80◦C refluxed and stirred for 8–9 h. The residue was purified by
recrystallization from hot anhydrous alcohol and distilled water (1:1, v/v) to get pure mate-
rial 1. Then, using 36% acetic acid (0.15 mL) as a catalyst, p-dimethylaminobenzaldehyde
(1.50 g, 0.01 mol) and material 1 (1.33 g, 0.01 mol) were mixed in absolute methanol
solutions (20 mL). The mixture solution was stirred under refluxed conditions for 5–6 h
at 65◦C, then cooled to room temperature, and the solvent was removed by evaporation.
Finally, the crude product was purified by recrystallistaion from hot anhydrous ethanol to
get pure sensor 2. Yield: 85%, mp 240◦C–241◦C (lit. 242.0◦C–243.4◦C).30
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