Mercapto thiadiazole-based sensor with colorimetric specific selectivity for AcO- in aqueous solution

Article abstract of DOI:10.1016/j.saa.2011.08.015

A novel acetate selective anion sensor 3 based on azophenol and mercapto thiadiazole had been designed and synthesized. Sensor 3 behaves a single selectivity and sensitivity in the recognition for AcO^- anion over other anions such as F^-

Full text of DOI:10.1016/j.saa.2011.08.015

Spectrochimica Acta Part A 83 (2011) 187–193
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Mercapto thiadiazole-based sensor with colorimetric specific
selectivity for AcO− in aqueous solution
Jun-Qiang Li, Tai-Bao Wei, Qi Lin, Ping Li, You-Ming Zhang∗
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
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2011
Received in revised form 1 August 2011
Accepted 10 August 2011
A novel acetate selective anion sensor 3 based on azophenol and mercapto thiadiazole had been designed
and synthesized. Sensor 3 behaves a single selectivity and sensitivity in the recognition for AcO− anion
over other anions such as F−, Cl−, Br−, I−, H2PO4−, HSO4 and ClO4 by naked-eyes and UV–vis spectra
changes in aqueous solution (H2O/DMSO, 5:5, v/v). The color of the solution containing sensor 3 had an
obvious change from colorless to orange only after the addition of AcO− in aqueous solution while other
anions did not cause obvious color change. 1H NMR titration results revealed that the binding process
includes two steps: (i) hydrogen bonding interactions (for small quantities of acetate) and (ii) proton
transfer between the sensor 3 and the coordinated anion (for high quantities of acetate). The association
−
−
Keywords:
Acetate anion sensor
Mercapto thiadiazole
Aqueous solution
Single recognition
Naked-eyes
constant Ka was 7.35 × 103 M−1. The detection limitation of AcO− with the sensor 3 was 1.0 × 10−6 mol L−1
.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
as an indicator of organic decomposition in marine sediments
[4a]. A great effort has been made to the development of sensing
devices for AcO− involving fluorescent chemosensors, chromogenic
chemosensors [20] and sensors based on electrochemical devices
[21]. Although these remarkable achievements, there are still
many disadvantages recognized in many examples of the litera-
tures. For instance, (i) the recognition or/and sensing of anions
could only ever occur in the biochemically noncompetitive organic
solvents [22,23] (e.g. CH2Cl2, DMSO, CH3CN) but never in the bio-
chemically competitive protic solvents (e.g. H2O, CH3CH2OH); (ii)
these acetate chemosensors usually display responses to other
anions such as fluoride and dihydrogenphosphate, especially the
former [14,17b, 24].
In view of this, as one part of our research interesting in ion
and molecular recognitions [25], we attempt to obtain some sen-
sors which are capable of binding AcO− in competitive media with
specific selectivity and high sensitivity in aqueous solutions. Our
strategies for designing of AcO− colorimetric sensors are as fol-
lows: firstly, in order to achieve colorimetric recognition, we have
designed and synthesized a chemosensor 3, in which we introduced
nitrophenyl azobenzol group as the signal group [26]. Secondly,
we have introduced O–H and S–H groups into the same sensor
molecule as the binding sites. The cooperation of these two groups
can increase the anion binding ability of the sensor. Finally, the sen-
sor was designed to be easy to synthesize. The results showed that
sensor 3 displayed a single response toward AcO− accompanied
with the naked-eyes detectable color change in aqueous solution
(H2O/DMSO, 5:5, v/v).
In recent years, the design and synthesis of sensors capable of
binding and sensing anions selectively have been drawn consider-
able attention [1] because anions play a major role in biological,
environmental, medical [2,3] and chemical sciences [4]. Among
various noncovalent interactions, hydrogen-bonding interactions
are particularly effective and useful in this regard. Sensors bear-
ing functional groups such as ureas [5–8], imidazolium [9], amides
[10–12], thioureas [13–15], and positively charged groups [16]
have been widely used to recognize anions via hydrogen-bonding
interactions. To achieve high binding affinity and good selectivity,
hydrogen bonding moieties are arranged in a rigid and conver-
gent manner in space. Moreover, as a subset of the anion sensors,
the chromogenic anion sensors have shown unique merit, because
they can reveal the host–anion binding information through a
change of color [17]. However, it is still a challenge to design the
anion sensors with high selectivity and sensitivity in a competitive
media.
On the other hand, the acetate anion plays an important role
in biochemistry, environment science and pharmaceutical science
[4f,18,19]. For example, acetate production and oxidation rate
have been frequently used because acetate anion is considered
∗
Corresponding author. Tel.: +86 931 7973191.
E-mail address: zhangnwnu@126.com (Y.-M. Zhang).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.08.015

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Scheme 1. The synthese route of sensor 3.
2. Experimental
was performed by Thermo Scientific Flash 2000 organic elemental
analyzer.
2.1. Apparatus
Melting points were measured on an X-4 digital melting-point
apparatus which was 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-d6 as solvent and TMS as an internal reference [27]. The
infrared spectra were performed on a Digilab FTS-3000 FT-IR spec-
trophotometer. Electrospray ionization mass spectra (ESI-MS) was
measured on an Agilent 1100 LC-MSD-Trap-VL system. 13C NMR
was measured on an INOVA-400 MHz system. Elemental analyses
2.2. Reagents
Unless otherwise specified, all reagents used for synthesis
were of analytical grade, and all reagents were obtained com-
mercially and were used without further purification. In the
titration experiments, all the anions were added in the form of
tetra-n-butylammonium (TBA) salts, which were purchased from
Sigma–Aldrich Chemical, stored in a vacuum desiccator and fully
dried before using.
Fig. 1. (a) The UV–vis absorption changes of sensor 3 (20 ␮M) with various anions (50 equiv.) in DMSO; (b) the UV–vis absorption changes of sensor 3 (20 ␮M) with various
anions (50 equiv.) in H2O/DMSO systems (1:9, v/v); (c) the UV–vis absorption changes of sensor 3 (20 ␮M) with various anions (50 equiv.) in H2O/DMSO systems (5:5, v/v).

J.-Q. Li et al. / Spectrochimica Acta Part A 83 (2011) 187–193
189
Fig. 2. The color changes of sensor 3 (20 ␮M) with various anions (50 equiv.) in H2O/DMSO systems (5:5, v/v).
2.3. General method
3. Results and discussion
2.3.1. General procedure for UV–vis experiments
3.1. UV–vis spectroscopy
A solution of the sensor 3 (2.0 × 10−4 M) in DMSO was pre-
pared and stored in the dry atmosphere. The solution was used
for all spectroscopic studies after appropriate dilution. Solutions of
1.0 × 10−2 mol L−1 TBA salts of the respective anions were prepared
in DMSO and were also stored under a dry atmosphere.
All the UV–vis experiments were carried out in DMSO or
H2O/DMSO binary solution on a Shimadzu UV-2550 spectrome-
ter. Any changes in the UV–vis spectra of sensor 3 were recorded
upon the addition of TBA salts while keeping the concentration of
3 constant in all experiments.
Compound 3 was prepared by the Schiff-base condensation
between nitrophenyl azobenzol salicylic aldehyde 1 and thiadia-
zole 2. The colorimetric sensing abilities was primly investigated
by ad−ding various anions such as F−, Cl−, Br−, I−, AcO−, H2PO4
,
−
−
HSO4 and ClO4 (tetrabutylammonium was used as a counterca-
tion) to DMSO solutions of sensor 3 (2 × 10−5 m−ol L−1), respectively.
When adding 50 equiv. of F−, AcO− and H2PO4 to the DMSO solu-
tion of sensor 3, 3 responded with dramatic color changes from
pale yellow to purple. In the corresponding UV–vis spectrum, new
and strong absorption peak appeared at 530–560 nm (Fig. 1(a)). The
same tests were applied to Cl−, Br−, I−, HSO4− and ClO4− no obvious
color changes were observed for the sensor 3. Even though sensor
3 could colorimetrically sense F−, AcO− and H2PO4− in DMSO solu-
tions, owing to the color and UV–vis spectrum changes are very
2.3.2. General procedure for 1H NMR experiments
1H NMR titration experiments were carried out in the DMSO-d6
solution (TMS as the internal standard). Two stock solutions were
prepared in DMSO-d6, one of them containing sensor 3 only and the
second one containing an appropriate concentration of guest AcO−.
Aliquots of the two solutions were mixed directly in NMR tube. The
sensor 3 prepared as 1.0 × 10−2 mol L−1 solution in DMSO-d6 was
titrated by the escalated amount of acetate anion (0.5 mol L−1), and
1H NMR of the 3-guest system was tested too.
similar, sensor 3 could not distinguish F−, AcO− and H2PO4 in
−
pure DMSO.
In DMSO/H2O binary solutions, as it can be anticipated, the
anion sense abilities of the sensor vary with the volume ratio of
water [30]. As shown in Fig. 1(b), in H2O/DMSO (1:9, v/v) solu-
tion, sensor 3 showed an obvious UV–vis spectrum change upon
the addition of F−, AcO−, HSO4 and H2PO4−. Along with the
−
addition of volume ratio of water, sensor 3 showed the color
change from colorless to orange only for AcO− in H2O/DMSO
(5:5, v/v) solution very uniquely (Fig.−1(c)) while other anions
2.4. Syntheses and characterize
2.4.1. Synthese route of 3
such as F−, Cl−, Br−, I−, H2PO4−, HSO4 and ClO4 did not cause
−
The synthese of azophenol and mercapto thiadiazole based sen-
sor 3 was started from the reaction of intermediate 1 [27,28] and 2
[29] to give crude product of compound 3, which was recrystallized
in DMF-EtOH solutions (Scheme 1).
2.4.2. Synthese of 3 and characterize
Intermediate 1 (0.41 g, 1.50 mmol) was dissolved in 20 mL
of absolute alcohol in 100 mL round flask. To the solution
was added 2-amino-5-mercapto-1,3,4-thiadiazole intermediate 2
(0.19 g, 1.50 mmol) and catalyst (catalytic amount of HAc). The
whole solution was stirred at temperature of 80 ◦C for about 8 h.
The resulting reaction mixture was filtered and washed with hot
anhydrous alcohol solution and then evaporated in vacuum, then
the precipitate of compound 3 was filtered out and recrystallized
in DMF-EtOH solutions. Sensor 3 was obtained as a pale yellow-
ish solid. Intermediate 1: m.p.: 186–188 ◦C (lit. m.p. 187–189 ◦C);
intermediate 2: m.p.: 234–236 ◦C (lit. m.p. 234–235 ◦C); sensor 3:
(yield: 83.20%), m.p.: 261–263 ◦C. IR (KBr, cm−1) v: 3441, 3107,
2870, 2443, 1657, 1527, 1332, 1267, 1094, 856, 747 cm−1 1H NMR
.
(DMSO-d6, 400 MHz ppm) ı 11.72 (s, 1H, OH), 10.33 (s, 1H, SH),
8.34 (d, 2H, ArH), 8.07 (d, 2H, ArH), 7.96 (s, 3H, ArH), 7.24 (s,1H,
CH). MS (ESI): m/z: 386.1 (M+H+). 13C NMR (400 MHz, DMSO-d6):
ı 186.75, 182.22, 164.51, 163.94, 163.64, 162.30, 155.23, 148.22,
145.11, 130.08, 125.08, 124.72, 123.31, 120.61, 118.24. Anal. Calc.
for C15H10N6O3S2: C46.62, H2.61, N21.75, S16.60; found: C, 46.23;
H,2.71; N,21.98; S, 15.98.
Fig. 3. Job’s plot analysis for AcO− versus 3 in solution (H2O/DMSO, 5:5, v/v).
measured at 542 nm. [AcO−] + [3] = 20 ␮M at 298.2 0.2 K. A and A0 stand for the
absorbance of the whole system of 3 + AcO− and the absorbance of free sensor 3,
respectively.

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Fig. 4. (a) UV–vis titration of 2 × 10−5 mol L−1 solution of 3 with a standard solution of TBA acetic acid anion in H2O/DMSO (5:5, v/v); (b) the best fitting curves by a 1:1
complexation model of 3 and AcO−
.
Fig. 5. Partial 1H NMR spectra of sensor 3 (10 mM) in DMSO-d6 upon the addition of acetate anion (0–1.0 equiv.).
Scheme 2. The proposed binding mode of 3 with AcO− (small quantities of acetate) in aqueous solution.

J.-Q. Li et al. / Spectrochimica Acta Part A 83 (2011) 187–193
191
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
b
a
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
sensor 3
-
sensor 3
AcO
TBA-OH
250 300 350 400 450 500 550 600 650 700 750 800
Wavelength/nm
250 300 350 400 450 500 550 600 650 700 750 800
Wavelength/nm
0.7
sensor 3
d
1.1
c
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-
AcO
TBA-OH
sensor 3
250 300 350 400 450 500 550 600 650 700
250 300 350 400 450 500 550 600 650 700
Wavelength/nm
Wavelength/nm
Fig. 6. (a) The controlling experiment of sensor 3 with TBA-OH in DMSO; (b) the controlling experiment of sensor 3 with AcO− in DMSO; (c) the controlling experiment of
sensor 3 with TBA-OH in the solutions of DMSO:H2O (1:1, v/v); (d) the controlling experiment of sensor 3 with AcO− in the solutions of DMSO:H2O (1:1, v/v).
2 × 10−5 mol L−1 solution of the sensor 3 gradually changed as
obvious color changes for sensor 3 (Fig. 2). Therefore, in H2O/DMSO
(5:5, v/v) solutions, sensor 3 has the single colorimetric selectiv-
ity to AcO− in H2O/DMSO (5:5, v/v). Maybe it is the function of
water molecular that weaken the interaction between sensor 3
and other anions and then made the different changes of UV–vis
absorbance to different anion while AcO− has a methyl hydropho-
bic group which made the force between AcO− and H2O molecular
weak so as to AcO− could bond with sensor 3 stronger than other
anions and show different absorption. The different wavelengths
for maximum absorption peak for sensor 3 with the excessive AcO−
were at 530 nm in DMSO and 470 nm in DMSO/H2O (1:1, v/v),
respectively.
the concentration of TBA acetate salts was increased. Along with
the increasing of concentration of AcO−, an isosbestic point
was clearly observed at 394 nm in aqueous solutions (Fig. 4(a)),
which indicates that the sensor 3 reacted with AcO− and formed
stable complex. It turned out that in H2O/DMSO (5:5, v/v) solu-
tions, sensors 3 reacted with AcO− and formed 1:1 complex. By
nonlinear leastsquares fitting [31] at ꢀmax = 470 nm for 3, the
association constants Ka of the sensor 3 toward AcO− was obtained
as 7.35 × 103 M−1 (R = 0.9979) (Fig. 4(b)). On the other hand, a new
strong absorption peak appeared at about 470 nm which can be
assigned to deprotonation from sensor 3 to AcO− [32].
3.2. Job’s plot analysis
3.4. 1H NMR titration
The stoichiometry between the sensor 3 and acetate was deter-
mined by Job’s plot with a total concentration of 20 ␮M, which
showed evident 1:1 stoichiometry for small quantities of acetate
in aqueous solutions (H2O/DMSO, 5:5, v/v) (Fig. 3).
1H NMR titration experiments were conducted to further inves-
tigate the interaction of 3 with AcO− in DMSO-d6. It was noticed
in Fig. 5 that original signals of –SHa and –OHb protons, appearing
at 10.3 and 11.7 ppm, were moved to downfield with a change of
about ꢁı = 0.1 and 0.2 ppm, respectively, when 0.1 equiv. AcO− was
added [33]. This suggests that AcO− is combined with sensor 3 by
hydrogen bonding. Above all, these results corroborated that a com-
plex has been formed between sensor 3 and AcO− at the beginning
of titration. On the other hand, when 0.5 equiv. AcO− was added,
signal of –SHa at 10.3 ppm had upfield shift while signal of –OHb
3.3. UV–vis titration
The association between the azophenol and mercapto
thiadiazole-based sensor
3 and acetate was investigated by
UV–vis titration. The absorbtion change of the sensor 3 was
monitored in H2O/DMSO. The intensity of UV–vis spectrum from

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J.-Q. Li et al. / Spectrochimica Acta Part A 83 (2011) 187–193
sensor 3 was at least down to 1.0 × 10−6 mol L−1,which was the
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-
detection limitation.
concentrations of AcO
- 2
- 1
1× 10
mol ·
L
4. Conclusion
- 3
- 4
- 5
- 1
1× 10
1× 10
1× 10
mol ·
mol ·
mol ·
L
L
L
In summary, we have designed and synthesized an easy-to-
synthesis sensor 3 based on azophenol and mercapto thiadiazole
spacer. UV–vis spectra showed that 3 is a good sensor in selec-
tive colorimetric recognition for acetate ov−er other anions such as
- 1
- 1
F−, Cl−, Br−, I−, H2PO4−, HSO4 and ClO4 in aqueous solutions
−
(H2O/DMSO, 5:5, v/v). Naked-eyes detectable color changes and
single colorimetric recognition for AcO− were realized successfully
in aqueous solution, which made it possible to detect AcO− for
practical analysis and application. The detection limitation of AcO−
- 6
- 1
- 1
1× 10
1× 10
mol ·
mol ·
L
L
- 7
Sensor 3
could be at least down to 1.0 × 10−6 mol L−1
.
250
300
350
400
450
500
550
600
650
700
750
Acknowledgements
Wavelength/nm
Fig. 7. The detection limitation of sensor 3 toward AcO− in solution (DMSO:H2O,
1:1, v/v).
This work was supported by the NSFC (No. 21064006) and the
Natural Science Foundation of Gansu (1010RJZA018).
at 11.7 ppm disappeared gradually, which suggested that the –OH
group perhaps underwent a process of deprotonation. The deproto-
nation of –OH group led to an increasing in electronic density of the
phenyl group, so the resonances for the –CH N– proton at 7.24 ppm
showed obvious upfield shifts [32]. In addition, the color change for
the sensor 3 from colorless to orange is due to the deprotonation
of the –OH group.
It is clear that a signal at 11.7 ppm disappeared completely with
the addition of AcO− more than 0.5 equiv., which was ascribed
to the proton transfer of–OH to AcO− dimmer. The formation of
HAc was due to function of the excessive acetate quantities. These
results reveal that the binding process includes two stages [34]: (i)
hydrogen bonding interactions (for small quantities of acetate) and
(ii) proton transfer between the sensor 3 and the coordinated anion
(for high quantities of acetate). Correspondingly, the binding mode
of 3 with AcO− was proposed and is given in Scheme 2.
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