An acetate sensor based on azo in aqueous media

Article abstract of DOI:10.1016/j.saa.2010.04.042

A colorimetric sensor 1,N,N′-di-(2-hydroxy-5-(phenldiazenyl) benzaldehyde)-1,3-diiminothiourea for acetate in DMSO and 9/1 DMSO/H ₂O (v/v) mixtures was designed and synthesized. The binding ability evaluated by UV-vis experiment reveals that sensor 1 can selectively recognize acetate. In addition, the color changes induced by anions can provide a way of detection by 'naked-eye'. The further insights to the nature of interactions between the sensor 1 and AcO^- were investigated by ¹H NMR titration experiments in 9/1 DMSO-d₆/H₂O (v/v).

Full text of DOI:10.1016/j.saa.2010.04.042

Spectrochimica Acta Part A 77 (2010) 146–149
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
An acetate sensor based on azo in aqueous media
Weiwei Huanga, Hongyan Sua, Jianwei Lia, Hai Linb, Huakuan Lina,∗
a Department of Chemistry, Nankai University, Tianjin 300071, PR China
b Key Laboratory of Functional Polymer Materials of Ministry of Education, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
A colorimetric sensor 1,N,Nꢀ-di-(2-hydroxy-5-(phenldiazenyl)benzaldehyde)-1,3-diiminothiourea for
acetate in DMSO and 9/1 DMSO/H2O (v/v) mixtures was designed and synthesized. The binding abil-
ity evaluated by UV–vis experiment reveals that sensor 1 can selectively recognize acetate. In addition,
the color changes induced by anions can provide a way of detection by ‘naked-eye’. The further insights
to the nature of interactions between the sensor 1 and AcO− were investigated by 1H NMR titration
experiments in 9/1 DMSO-d6/H2O (v/v).
Article history:
Received 2 November 2009
Received in revised form 3 March 2010
Accepted 30 April 2010
Keywords:
Colorimetric
Acetate sensor
Aqueous solution
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
ously accompanied with ‘naked-eyed’ detectable color changes
[9–11].
Over the past few years, the recognition of inorganic and biotic
anions has been a key research area of supramolecular chemistry
[1,2] owing to the fundamental role of anions in the environmen-
tal and biological systems, and in the clinic practices and so on.
For example, the carboxylate anions [3] exhibit specific biochem-
ical functions in the enzymes and antibodies and are also critical
components of numerous metabolic processes. Acetate is one of
the carboxylate anions with the unique trigonal chemical structure,
which can form strong hydrogen-bond interaction with hydrogen-
bond donors.
By exploiting the optical properties of an azo group, we have
recently reported a chromo- and fluorogenic dual responding
H2PO4 sensor in dry DMSO [12]. Considering the limited appli-
cation of this sensor, in this paper, we designed and synthesized a
new and simple azo-based receptor (Scheme 1), which is an organic
colorimetric chemosensor. The sensing of the biologically impor-
tant AcO− is achieved in DMSO solution even in aqueous solution
9/1 DMSO/H2O (v/v).
−
2. Experimental
Very recently, the selective recognition of acetate has attracted
more and more attention. For example, an effective sensor for
acetate ion in dry DMSO was reported by Cheng and co-workers
[4]. Ito et al. reported a novel Fipronil-based receptor can selec-
tively recognize actetate among a group of anions in DMSO [5].
We found that the phenylhydrazone-based indole receptor was an
effective sensor for acetate ion in dry DMSO [6].
Despite these remarkable achievements, there are still many
disadvantages recognized in many examples of literatures. For
example, the recognition and/or sensing of anions could only occur
in biochemically noncompetitive organic solvents [7,8] (e.g. CH3CN,
CH2Cl2, etc.) but unable to sense anions in biochemically com-
petitive protic solvents such as H2O, CH3CH2OH. Consequently,
there is a need to develop receptors capable of binding anion
in competitive media, which is also hopeful to be simultane-
2.1. Apparatus
1H NMR spectra were obtained on a Varian UNITY Plus-400 MHZ
Spectrometer. ESI-MS performed with a MARINER apparatus. C, H,
N elemental analyses were made on an elementar vario EL. UV–vis
spectra were recorded on a Shimadzu UV-2450 Spectrophotometer
(Shimadzu 2.1 Apparatus Corp., Kyoto, Japan) with a quartz cuvette
(path length = 1 cm) at 298.2 0.1 K.
2.2. Reagents
All reagents for synthesis obtained commercially were used
without further purification. In the titration experiments, all the
anions were added in the form of tetrabutylammonium (TBA) salts,
which were purchased from Sigma–Aldrich Chemical, stored in a
vacuum desiccator containing self-indicating silica and dried fully
before using. DMSO was dried with CaH2 and then distilled in
reduced pressure.
∗
Corresponding author. Tel.: +86 2223502624; fax: +86 22 23502458.
E-mail address: hklin@nankai.edu.cn (H. Lin).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.04.042

W. Huang et al. / Spectrochimica Acta Part A 77 (2010) 146–149
147
Scheme 1. General synthetic routes to the target sensor 1.
2.3. General method
All experiments were carried out at 298.2 0.1 K, unless
otherwise mentioned. UV–vis spectra were measured using an
ultraviolet–visible spectrophotometer, UV-2450 (Shimadzu Corp.,
Kyoto, Japan). A 2.0 × 10−4 M solution of the sensor 1 in DMSO was
prepared and stored in the dry atmosphere. This solution was used
for all spectroscopic studies after appropriate dilution. Solutions
of 1.0 × 10−2 M tetrabutyl ammonium (TBA) salts of the respective
anions were prepared in dried and distilled DMSO and were stored
under a dry atmosphere.
1H NMR titration experiments were carried out in 9/1 DMSO-
d6/H2O (v/v) solution (TMS as the internal standard). Certain
amount of the sensor 1 solution was prepared with a concentra-
tion of 0.01 M. 1H NMR of the host–guest system was recorded by
adding increasing amount of acetate into the sensor 1 solution.
2.4. Syntheses and characterization
Fig. 1. UV–vis spectrum changes of sensor 1 (2.0 × 10−5 M) upon addition of acetate
ion (0–4.2 equiv.) in DMSO at 298.2 0.1 K. Inset: Job’s plot for complexation of
sensor 1 with AcO− in DMSO [host] + [guest] = 2.0 × 10−5 M.
N,Nꢀ-di-(2-hydroxy-5-(phenldiazenyl)benzaldehyde)-1,3-
diiminothiourea (1)
ꢀmax of 340 nm, i.e. the ␲–␲* transition bands of the chromophore
(azophenyl). With the addition of more and more doses of acetate
ions, the peak at 338 nm decreased but a new peak appeared at
435 nm, which was ascribed to the charge transfer (CT) between the
anion-bound –NH and –OH units and the electron-deficient effect
from azophenyl. And the color of the sensor 1 solution changed
from light yellow to heavy yellow at the same time (Fig. 3). Obvi-
ously, there was only one well-defined isosbestic points at 380 nm,
which indicated that there existed sole type of 1–AcO− complex.
Similarly, the additions of F− and H2PO4− also led spectral changes.
A
solution of 2-hydroxy-5-(phenyldiazenyl)benzaldehyde
(0.904 g, 4 mmol) in ethanol (20 ml) was added dropwise to a hot
solution of 1,3-diaminothiourea (0.212 g, 2 mmol) in ethanol/water
(20 ml, v/v = 1/1) with stirring at reflux. After being stirred for 2 h,
the solvent was removed by evaporation after recrystallization
from ethanol. This procedure yielded yellow crystals 0.970 g (87%).
ıH (400 MHz, DMSO-d6, Me4Si): 12.26 (s, 2H, O–H), 12.23 (s, 2H,
N–H), 7.87 (dd, 3H, Ar–H), 7.61 (s, 1H, N C–H), 7.52 (dd, 3H, Ar–H).
Elemental analysis: Calc. for C27H22N8O2S: C, 62.04%; H, 4.24%;
N, 21.44%; Found: C, 62.13%; H, 4.02%; N, 21.37%. ESI-mass: m/z
523.58 (M+H)+.
3. Results and discussion
3.1. UV–vis spectroscopy
Firstly, to evaluate the affinity ability to anions, the UV–vis titra-
tion experiments of the sensor 1 were carried out in dry DMSO
solution using standard tetrabutylammonium salts of AcO−, F−,
H2PO4−, Cl−, Br− and I− at 298.2 0.1 K. UV–vis spectrum of the
solution of 1 (1.0 × 10−5 M) recorded upon the addition of AcO− is
shown in Fig. 1. Upon the addition of AcO−, the broad absorption
peaks at 338 nm was decreasing, whereas an absorption peak at
423 nm formed. The resulting titration revealed an isosbestic point
at 390 nm.
Secondly, to explore more about the applicability of the sen-
sor 1 for acetate in minor-water-containing solution, the UV–vis
titrations were performed in the 9/1 DMSO/H2O (v/v) mixtures.
Fig. 2 shows the UV–vis spectral changes of 1 during the titra-
tion with acetate. The original absorbance peaks appeared at the
Fig. 2. UV–vis spectrum changes of sensor
1
(2.0 × 10−5 M) upon addition
of acetate ion (0–23 equiv.) in 9/1 DMSO/H2O (v/v) at 298.2 0.1 K. Inset:
Job’s plot for complexation of sensor
1
with AcO− in 9/1 DMSO/H2O (v/v)
[host] + [guest] = 2.0 × 10−5 M.

148
W. Huang et al. / Spectrochimica Acta Part A 77 (2010) 146–149
Fig. 3. Color changes of the sensor 1 in 9/1 DMSO/H2O (v/v) in absence and presence of anions (from the left to right: 1 only, 1 + AcO−, 1 + F−, 1 + H2PO4−, 1 + Cl−, 1 + Br− and
1 + I−).
Table 1
Association constants for various anions toward receptor 1 in DMSO and 9/1 DMSO/H2O (v/v) at 298.2 0.1 K, respectively.
−
Anionsa
AcO−
H2PO4
F−
Cl−
Br−
I−
b
d
Kass (M−1
Kass (M−1
)
)
3.14( 0.12) × 105
7.14( 0.53) × 104
7.57( 0.62) × 104
2.73( 0.24) × 102
NDc
ND
ND
ND
ND
ND
7.37( 0.54) × 103
1.62( 0.19) × 102
a
The anions were added as their tetrabutylammonium salts.
Kass was determined in dry DMSO.
ND indicated that the spectra showed little or no change with the addition of anion so that the association constants cannot be determined using the spectra.
Kass was determined in 9/1DMSO/H2O (v/v)solution.
b
c
d
However, as adding Cl−, Br− and I− to 1, the spectra responses are
not obvious.
As clearly shown in Table 1, the order of binding
with anions (in DMSO or DMSO/water) is
affinity of
1
Continuous variation method was used to determine the sto-
ichiometric ratio of the receptor to the fluoride anion guest. Job’s
plot [13,14] of sensor 1 and AcO− in DMSO and 9/1 DMSO/H2O (v/v)
(Figs. 1 and 2) shows the maxima are all at a molar fraction of 0.5.
This result indicates that the sensor 1 binds acetate anion guest
with a 1:1 ratio. Moreover, similar results can also be obtained for
other anions (F− and H2PO4−).
For a complex of 1:1 stoichiometry, the relation in Eq. (1) could
be derived easily, where X is the absorption intensity, and CH or CG
is the concentration of the host or the anion guest correspondingly
[15].
AcO− » F− > H2PO4− ∼ Cl− ∼ Br− ∼ I−. The recognition function
of sensor 1 for AcO− is the most remarkable property. The main
reason for preferring AcO− is its triangular shape which deter-
mines the angle of O–C–O is about 120◦ while the angle of O–P–O
in H2PO4 is about 108◦. The distance of two oxygen atoms of
−
AcO− might be more fit to the hydrogen atoms of the recognition
sites than those spherical and tetrahedral anions. That is, the
configuration of AcO− is more matching with 1 than F− and
−
H2PO4
.
Compared with the previous research in our laboratory [12], the
sensor 1 has higher selectivity for acetate than receptor 1, because
the structure of sensor 1 is symmetrical while receptor 1 is unsym-
metrical in the lecture [12]. The advantage of configuration made
sensor 1 has better selectivity than receptor 1. So the association
constant of sensor 1 with AcO− is much bigger than receptor 1 in
the lecture.
2
1/2
X = X0 + (Xlim − X0){CH + CG + 1/Kass − [(CH + CG + 1/Kass
)
− 4CHCG
]
}/2CH
(1)
The affinity constants of receptors 1 for the studied anionic
species are calculated and listed in Table 1 below.
3.2. 1H NMR titration
As a validation to the above qualitative and quantitative analy-
sis, UV–vis changes (Fig. 4) of sensor 1 operated in 9/1 DMSO/H2O
(v/v) (2.0 × 10−5 M) after the additions of 17 equiv. of anions pro-
vided a more convincing evidence that sensor 1 was indeed an
excellent sensor for AcO−.
To further look into the nature of host–guest interactions,
1H NMR titration experiments were conducted in 9/1 DMSO-
d6/H2O (v/v) mixtures. 1H NMR acetate titration spectra of the
sensor 1 is shown in Fig. 5. Upon addition of 0.5 equiv. of
acetate ions, the peaks at 12.23 ppm and 12.26 ppm, which were
Fig. 4. UV–vis spectra spectra of 1 (1 × 10−5 M) in 9/1 DMSO/H2O (v/v) in the pres-
Fig. 5. 1H NMR titration of a 1 × 10−2 M solution of 1 in 9/1 DMSO-d6/H2O (v/v) with
ence of 17 equiv. of F−, or AcO−, H2PO4−, Cl−, Br− and I−
.
[Bu4N]AcO.

W. Huang et al. / Spectrochimica Acta Part A 77 (2010) 146–149
149
Scheme 2. The proposed 1–AcO− binding mode in solution.
assigned to aniline–NH and phenole–OH, respectively, were shifted
Acknowledgement
downfield to 12.43 ppm and 12.57 ppm and the signals on the
phenyl rings changed slightly. This indicated the formation of a
hydrogen-bonding complex was at this stage. With further addi-
tion of acetate ions, upon the addition of 2 equiv. AcO−, the
signals of –NH and –OH were disappeared and the phenyl pro-
tons moved upfield significantly, which indicated the increase of
the electron density on the phenyl ring owing to the through-
bond electronic effects. All the results observed indicate that
there are two stages during the 1H NMR titration: (1) in the
first stage, the acetate ion exhibits a hydrogen-bonding inter-
action with 1, and (2) in the second stage, the excess acetate
ion results the deprotonation of the sensor to take place. Con-
sequently, according to the results of 1H NMR titration, the
proposed binding mode of 1 and AcO− was given below in
Scheme 2.
This project was supported by the National Natural Science
Foundation of China (20371028, 20671052).
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