Acylthiourea derivatives as colorimetric sensors for anions: Synthesis, characterization and spectral behaviors

Article abstract of DOI:10.1016/j.saa.2015.09.003

Several acylthioureas have been synthesized to develop colorimetric sensors for detection of biologically important anions. UV-vis titration experiments indicated that the absorbance values have a good linear relationship with concentration of anions when

Full text of DOI:10.1016/j.saa.2015.09.003

Acylthiourea derivatives as colorimetric sensors for anions: Synthesis,
characterization and spectral behaviors
Shuangshuang Liui, Jing Kang, Xiufang Cao, Xiali Yue
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DOI:
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S1386-1425(15)30215-8
doi: 10.1016/j.saa.2015.09.003
SAA 14067
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9 July 2015
1 September 2015
3 September 2015
Please cite this article as: Shuangshuang Liui, Jing Kang, Xiufang Cao, Xiali Yue,
Acylthiourea derivatives as colorimetric sensors for anions: Synthesis, characterization
and spectral behaviors, (2015), doi: 10.1016/j.saa.2015.09.003
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Acylthiourea derivatives as colorimetric sensors for anions: Synthesis,
characterization and spectral behaviors
Shuangshuang Liu†, Jing Kang†, Xiufang Cao*, and Xiali Yue*
College of Science, Huazhong Agricultural University, Wuhan 430070, PR China
†These authors contributed equally to this work.
*Corresponding author. E-Mail address: caoxiufang@mail.hzau.edu.cn (X. Cao) and
yxl@mail.hzau.edu.cn (X. Yue)
ABSTRACT
Several acylthioureas have been synthesized to develop colorimetric sensors for detection of
biologically important anions. UV-Vis titration experiments indicated that the absorbance values
have a good linear relationship with concentration of anions when the anions were added in AR-1,
AR-4 and AR-6 sensor molecules. The detection limit to AcO- and F- is 5×10-6 mol/L when the
concentration of receptors are 2×10-5 mol/L. Especially, compounds AR-1 and AR-4, decorated with
strong electron-withdrawing NO2 substituent, showed augmented anion sensing properties, being
capable of naked-eye detecting of F- and AcO- when the water content is lower than 15%. The
recognition details of anion sensing were also assessed using 1H NMR technique and confirmed that
the basic anions induced deprotonation of N-H.
Keywords: Acylthiourea, Colorimetric sensor, Anion recognition, Naked-eye detection
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Introduction
Anion recognition is one of the popular researches in supramolecular chemistry because of the
important roles that anions play in biological, environmental and chemical industries [1]. For
example, fluoride can help dental health and treat osteoporosis [2], but on the other hand, excessive
fluoride may lead to depression and immune system damaged [3]. As a good hydrogen bond donor,
thiourea is widely used in the design of anion receptors because it can form stable compounds
through the double bond with anions [4,5]. For example, two NH groups can form a chelate ligand of
six with a guest or a chelate ligand of eight with two oxygen atoms. By connecting different
functional groups with thiourea group can design chromogenic sensors [6,7], fluorescent sensors [8,9]
and electrochemical sensors [10,11]. Most of the studies reported are only qualitative detection,
research on the quantitative detection is rare. Moreover, most of the anion recognition is
performed in aprotic solvents (such as acetonitrile, chloroform, dimethyl sulfoxide). However, in
recent years, reports of anion recognition in protic solvent are increasing [12-15]. The effect of
recognition usually depends on the acidity of anion receptors, basicity of anions and stability of the
conjugate base, which all of these have a strong dependence on solvent. Therefore, qualitative and
quantitative detection of anions in aqueous medium is of great significance.
In this paper, a series of acylthiourea compounds have been synthesized and detected for their
properties of anion recognition using UV-Vis absorption spectroscopy. The quantitative
relationships between concentration of anions and absorbance values are measured by UV titration
experiments. The research that conducted in aqueous medium provides a technical basis for
qualitative and quantitative detection of anions rapidly and provides a reference on the design and
synthesis of new receptors.
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Experimental
Reagents and apparatus
The reagents used were of analytical grade commercially, and acetonitrile were confirmed
without water before using. Electronic spectral measurements were carried out using a Shimadzu
UV-2450 spectrophotometer in the range of 200-800 nm. 1H NMR measurements were performed on
a Brucker spectrometer at 400 MHz with DMSO-d6 as the solvent and chemical shifts are recorded
in ppm. Infrared spectra in potassium bromide (KBr) were recorded on an Avatar 330 FT-IR
spectrophotometer. Melting points were measured on an X-4 digital melting point apparatus
(uncorrected).
Synthesis and characterization
The target compounds AR1-6 were conveniently synthesized using a method similar to that
1
reported earlier [16] in 82-90% yields and characterized by melting point, IR and H NMR
(Supplementary data) spectrum. The general synthetic routes for target compounds are described in
the following Scheme 1.
Scheme 1. Synthesis for target compounds AR1-6.
Result and discussion
Spectroscopic studies in organic media
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The anion sensing behaviors of receptors (2×10-5 mol/L) were assessed visually and then
investigated by UV-vis absorption spectra in organic media (DMSO) upon addition of various anions
-
-
such as F-, Cl-, Br-, I-, AcO-, HSO4 and ClO4 (as tetrabutylammonium salts). As shown in Fig. 1A,
the free AR-1 displayed two bands at 290 and 350 nm. Significant changes occurred in the
absorption spectrum of the AR-1 (2×10-5 mol/L) with addition of 10.0 equiv. of F- and AcO-
separately, while the others have no changes. Simultaneously, the almost colorless of the sensor
solution instantly turned to yellow (Fig. 1B), which suggests that AR-1 can distinguish F- and AcO-
from many other anions.
Fig. 1 (A) Changes in the UV-vis absorption spectra of AR-1 (2×10-5 M) with addition of 10.0 equiv.
of different anions in DMSO. (B) Color changes of AR-1 (4×10-5 M) with different anions. up: from
-
-
left to right is AR-1, AR-1-F-, AR-1-Cl-, AR-1-Br-, AR-1-I-, AR-1-AcO-, AR-1-HSO4 , AR-1-ClO4 .
Moreover, similar investigations were performed for the other synthesized receptors. However,
AR-4 and AR-6 can sense F- and AcO- like AR-1, and the others have no ability to identify any one
of the anions. To further explore the binding ability of AR-1 to F- and AcO-, UV-vis spectroscopic
titrations were performed. Maintain a certain concentration of AR-1 (2×10-5 mol/L), to which were
added to different concentrations of F- and AcO-, [F-]/[AR] ([AcO-]/[AR]) were 0, 0.5, 1, 1.5, 2, 2.5,
3, 4, 5, 6, 7, 8, 9, 10. The UV-vis spectrums are shown in Fig. 2, which we can see, within a certain
range the absorbance at 283 nm gradually decreases and the absorbance at 391 nm increases as the
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concentration of anions is increased. The absorbance peak of the new complex at 391 nm will
gradually leveled off when anions reach a certain concentration, indicating the formation of stable
complexes.
Fig. 2 Changes in the UV-vis absorption spectra of AR-1（2×10-5 M）with addition of 0-10 equiv. of
F- (left) or AcO- (right) in DMSO. Inset: the nonlinear curve fitting of AR-1 with F- (left) or AcO-
(right).
In addition, analogous investigations were performed upon the addition of F- and AcO- to the
solutions of AR-4 and AR-6 (Supplementary data).
Furthermore, we set the concentration of anions smaller in UV-vis spectroscopic titrations to
explore whether the receptor can sense anions quantitatively. Keeping the concentration of AR-1 is
still 2×10-5 mol/L, we set the concentration gradient of [F-]/[AR] ([AcO-]/[AR]) were
0, 0.25, 0.5, 0.75, 1. The absorption spectrums were measured at room temperature to see whether
the absorbance values at 391 nm have a certain relationship with the concentration of anions or
not. The results (Fig. 3A and Fig. 3B) show that the linear correlation coefficients are 0.9657
and 0.9801 respectively, which mean that absorbance values of new complexes have a good linear
relationship with the concentration of the F- and AcO-.
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A
B
Fig. 3 (A) Linear relationship of absorbance with [F-]/[AR] at 391 nm of AR-1 (2×10-5 M) with
addition of F-. (B) Linear relationship of absorbance with [AcO-] / [AR] at 391 nm of AR-1 (2×10-5
M) with addition of AcO-.
Based on the linear relationship, we set the concentration of anions lower to measure the
detection limits of AR-1 to F- and AcO-. [F-] or [AcO-] were set to be 0, 10-9, 10-8, 10-7, 10-6 and
5×10-6 mol/L. As evident from Fig. 4, when the anion concentration is 10-9, 10-8, 10-7, and 10-6 mol/L,
the absorbance valuesat 391 nm are ±0.002 compared with that measured without anions. By
examining the model and specification of the UV spectrophotometer, the accuracy is ±0.002 when
the absorbance is in the range of 0-0.5. Therefore, the detection limit of AR-1 to F- and AcO- is
5×10-6 mol/L when the concentration of AR-1 is 2×10-5 mol/L, which makes the application of
quantitative detection of anions quickly possible.
Fig. 4 The detection limit of AR-1 (2×10-5 M) to F- (left) and AcO- (right).
Interference studies of related anions
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From the results above, we can see that the receptors can recognize F- and AcO- individually.
However, considering the complexity of the system, interference studies of related anions is
necessary. AR-4 was set as an example. The absorption spectra of AR-4 in the presence of F- alone
and several other anions with F- are presented in Fig. 5. It can be seen that, there was no interference
in the detection of F- in the presence of other anions in the system. Thus, AR-4 could be used as a
selective sensor for F-.
Fig. 5 Interference testing of compound AR-4.
Studies of the binding mechanism
Both hydrogen-bonding and deprotonation processes are involved in anion interaction, which
depends on the basicity of the anion and acidity of the proton [17,18]. In this paper, we set AR-4 as
an example and explored the mechanism of interaction by IR and 1H NMR titration.
(1) Studies of IR
IR is one of the tools which were usually used to investigate hydrogen bonding interactions [19].
When the hydrogen bond is formed, changes in the force field around the hydrogen atom will cause
X-H stretching force constant changes, which resulting in its stretching frequency moves to lower
wavenumber.
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Using KBr pellets (tetrahydrofuran as solvent), we adjusted the relative amounts of receptor and
anion and measured infrared spectrums. Fig. 6 and Fig. 7 represent infrared spectrums of AR-4 with
different concentrations of AcO-. As can be seen from Fig. 6, there are two sharp peaks at 3414 cm-1
and 3268 cm-1 which represent the -NH stretching vibration of a and b respectively. After addition of
a small amount of acetic acid anion, the peak at 3414 cm-1 gradually disappear while the peak at
3268 cm-1 moves to 3265 cm-1. The results show that when the amount of AcO- is small,
deprotonation first occurs with the -NH at a, and form hydrogen bonds with the -NH at b.
Fig. 6 IR spectrum of AR-4 (Blue) and AR-4 with addition of little AcO- ([AR] / [AcO-] >1).
As shown in Fig. 7, both of the two -NH stretching vibrations disappear upon the addition of
more AcO-, which prove that two -NH protons deprotonated when the concentration of anion is
much higher than the concentration of AR-4.
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Fig. 7 IR spectrum of AR-4 (Green) and AR-4 with addition of excess AcO- ([AR] / [AcO-]<<1).
The same conclusions can be obtained after replace AcO- with F- (Supplementary data) for
AR-4: a small amount of anions first induce the highly reactive -NH deprotonated and form
hydrogen bonds with lower activity -NH, however, when the amount of anions is much larger than
the receptors, the -NH of lower activity is also induced deprotonated.
(2) 1H NMR titration
1H NMR titration was also performed to explore the mechanism of interaction between anions
and receptor in DMSO-d6. As shown in Fig. 8, with 0.5 and 1.0 equiv. AcO- were added, signal of
-NH at 12.76 ppm had completely disappeared, which indicated clearly the -NH had deprotonated.
The signals of aromatic ring were observed to shift upfield with no changes when more equivalents
of AcO- were added, indicating that -NH had deprotonated and not to form hydrogen bonds [20].
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Fig. 8 1H NMR of AR-4 with different equiv. of AcO-.
The interaction of AR-4 with F- was also investigated by 1H NMR spectroscopy. From Fig. 9, it
can be seen that -NH in the lower field disappeared first, and -NH in the higher field disappeared
when adding a large amount of fluoride. With 10.0 equiv. F- were added, we can see the appearance
-
of a characteristic triplet around 16.1 ppm, which means the formation of HF2 .
Fig. 9 1H NMR of AR-4 with different equiv. of F-.
Compare Fig. 8 with Fig. 9, it can be seen that changes of chemical shifts in b are different.
With the concentration of anions increasing from 0 to 0.5 and then to 1.0 equiv., signal of b moves to
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a higher field first and then to a lower field for AcO- while always migrate to a higher field for F-.
Job’s plot (Fig. 10) has been drawn to know the stoichiometry between the guest and host molecules
in DMSO. Differences in the structure and basicity of anions will cause stoichiometric ratio different.
Moreover, the receptors are easy to form intramolecular hydrogen bonds because of the structural
features. The differences in stoichiometric ratio and charge distribution lead to differences in
chemical shift.
Fig. 10 Job-plot curve of AR-4 with F- (left) and AcO- (right).
Spectroscopic studies in aqueous medium
Anions usually exist in aqueous medium, therefore, the detection of anions in aqueous medium
will be a significant progress in anion recognition [21]. We set AR-1 (4×10-5 mol/L) as an example,
add the same concentration of anions (4×10-4 mol/L), but adjust the proportion of water were 0, 5%,
10%, 15%, 20%, 30%, 40% in mixed solvent. The UV spectrums are shown in Fig. 11A and Fig.
11B. The shape of the absorption spectrums had almost no change when the water contents are 5%
and 10%, but the absorbance at 391 nm is reduced slightly with the water content increasing, which
indicates that the water molecules compete with anions. However, the receptor can still recognize F-
and AcO- qualitatively. Furthermore, the UV spectrums were getting closer to the original absorption
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curve of pure receptor without anions with the water content increasing. AR-1 cannot identify F- and
AcO- when the water content is 15% or more.
A
B
Fig. 11 Changes in the UV-vis absorption spectra of AR-1 with addition of F- (A) or AcO- (B) in
aqueous phase.
Consistent with the above, the color of solution changed from almost colorless to pale yellow
by addition of F- (Fig. 12A) or AcO- (Fig. 12B) in DMSO, but faded with the water content
increasing. It became colorless when the proportion of water reaches more than 15%.
Fig. 12 (A) Color changes of AR-1 with F- in aqueous phase. From left to right: AR (0%H2O),
AR+F- (0%H2O, 5%H2O, 10%H2O, 15%H2O, 20%H2O, 30%H2O, 40%H2O). (B) Color changes of
AR-1 with AcO- in aqueous phase. From left to right: AR (0%H2O), AR+AcO- (0%H2O, 5%H2O,
10%H2O, 15%H2O, 20%H2O, 30%H2O, 40%H2O).
Also for compound AR-4, the receptor can recognize F- and AcO- by naked-eye or UV
absorption spectra when the proportion of water is less than 15%.
Conclusion
In summary, six acylthiourea receptors AR1-6 have been synthesized and their spectral
behaviors and anion recognition properties were fully investigated. The results indicated that the
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receptors AR-1, AR-4, and AR-6 can selectively detect AcO- or F- anions. Especially, compounds
AR-1 and AR-4 are capable of naked-eye detecting of F- and AcO- in aqueous media under the water
content is lower than 15%. This research provide a detailed spectral behaviors study on this family
of compounds, and further application and properties for these compounds are well ongoing in our
laboratory.
Acknowledgements
We gratefully acknowledge the support of this work by the scientific research project of
Huazhong Agricultural University (2015026). The authors also gratefully acknowledge the partial
support from the Fundamental Research Funds for the Central Universities (2662015PY034).
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Graphical Abstract
Thioureas have been synthesized to develop colorimetric sensors for detection of biologically
important anions. The recognition details of anion sensing were assessed using UV-Vis titration, IR,
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and 1H NMR techniques.
Highlights

Receptors AR-1, AR-4, and AR-6 can be utilized as a colorimetric chemosensor for detection of
AcO- or F- anions.


Receptors AR-1 and AR-4 can naked-eye detect of F- and AcO-.
Receptors exhibit low detection limit when the concentration of receptors are 2×10-5 mol/L.
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