Colorimeric and fluorescence ON-OFF probe for acetate anion based on thiourea derivative: Theory and experiment

Article abstract of DOI:10.1016/j.saa.2012.11.027

Based on thiourea moiety, three colorimetric and fluorescent anion probes have been synthesized. Results indicated the probe 1 containing p-NO₂ group showed the strongest binding ability for AcO^- among the anions tested (F^-

Full text of DOI:10.1016/j.saa.2012.11.027

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 276–281
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Colorimeric and fluorescence ON–OFF probe for acetate anion based on
thiourea derivative: Theory and experiment
a
b
b
b
b
b
Xuefang Shang a, , Shuanbao Tian , Nankai Xi , Yue Li , Dong liang , Yun Liu , Zhenya Yin ,
⇑
Jinlian Zhang b, Xiufang Xu c
a Department of Chemistry, Xinxiang Medical University, Jinsui Road 601, Xinxiang, Henan 453003, China
b School of Pharmacy, Xinxiang Medical University, Jinsui Road 601, Xinxiang, Henan 453003, China
c Department of Chemistry, Nankai University, Tianjin 300071, China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
The probe containing p-NO2 group showed the strongest binding ability for AcOꢀ among the anions
tested (Fꢀ, AcOꢀ, H2PO4ꢀ, Clꢀ, Brꢀ, Iꢀ), which was not influenced by the existence of other anions. The
interaction of host–guest was accompanied with a dramatic color change from colorless to orange. A vis-
ible decrease in the fluorescence emission intensity of the probe owing to a quenching PET process from
the thiourea NH to –NO2 group.
"
The probe containing p-NO2 group
showed the strong binding ability for
AcOꢀ.
"
The binding ability of 1-AcOꢀ was
not influenced by the existence of
other anions.
"
"
A dramatic color change from
colorless to orange.
The highest LUMO rabital aroused
the red-shift phenomenon in UV–vis
spectra.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2012
Received in revised form 3 November 2012
Accepted 6 November 2012
Available online 28 November 2012
Based on thiourea moiety, three colorimetric and fluorescent anion probes have been synthesized. Results
indicated the probe 1 containing p-NO2 group showed the strongest binding ability for AcOꢀ among the
anions tested (Fꢀ, AcOꢀ, H2POꢀ4 , Clꢀ, Brꢀ, Iꢀ), which was not influenced by the existence of other anions.
The interaction of host–guest was accompanied with a dramatic color change from colorless to orange.
However, the addition of various anions did not cause noticeable spectral response of the probes (2
and 3) containing o, m-NO2 groups. Theoretical investigation demonstrated the electron transition of
the highest LUMO aroused the red-shift phenomenon in UV–vis spectra of 1-anion.
Keywords:
Colorimetric recognition
Fluorescence probe
Thiourea
Ó 2012 Elsevier B.V. All rights reserved.
Theoretical investigation
Interference experiment
Introduction
ical and biological process [1–10]. Among various important anio-
nic analytes, acetate anion is one of the most significant due to its
Anion recognition chemistry has attracted considerable
attention because various anions play fundamental roles in chem-
role in washing blood and softening blood vessel. If there is a high
content of acetate anion in the body, acetate anion can interact
with calcium ions and form calcium acetate which is one of the
constituents of urinary calculi [11]. Therefore, the content of ace-
tate ion during the metabolism of living organisms is important
⇑
Corresponding author. Tel.: +86 373 3029128; fax: +86 373 3029959.
E-mail address: xuefangshang@126.com (X. Shang).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.11.027

X. Shang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 276–281
277
and recently a large number of excellent probes for sensing acetate
anion have been reported [12–14].
the literature. The synthesis method of three compounds (1, 2
and 3) were listed in supporting information.
Colorimetric probes for special anionic species draw increasing
attention for allowing the so-called ‘‘naked-eye’’ detection of the
analyte without using any expensive equipment [15,16]. A probe
usually consists of two parts, the interacted site and signal reporter
which either are linked directly or by a flexible spacer [17,18].
Anion binding in biological systems is very often achieved via
hydrogen bond by highly preorganized proteins with sterically
well-defined complex sites in the interior of proteins [19,20].
Hydrogen-bond donors such as pyrrole [21], urea or thiourea
[22], azophenol [23] and amide [24] usually act as interacted sites.
Among a variety of possible hydrogen bond donors, thiourea deriv-
atives have been shown to be particularly good.
Results and discussion
‘‘Naked-eye’’ observation and UV–vis titration
The recognition and binding properties of compound 1 toward
various anions were firstly investigated by UV–vis titration spectra.
In the absence of anions, compound 1 (4.0 ꢁ 10ꢀ5 mol Lꢀ1 in
DMSO) displayed one obvious absorption peak at 275 nm (Fig. 1).
Significant spectral changes were seen upon the addition of AcOꢀ
to the solution of compound 1. While increasing the concentration
of AcOꢀ, the intensity of absorption peak at 275 nm decreased. At
the same time, the absorption peak at 304 nm was gradually de-
creased and a new absorption peak at 450 nm appeared. In general,
the red-shift phenomenon of absorption peak for compound 1 oc-
curred with the stepwise addition of AcOꢀ. One clear isosbestic
point at 300 nm was seen during the titration process, indicating
compound 1 interacted with AcOꢀ by hydrogen bond and the sta-
ble complex was formed [27]. Noticeably, the colorless solution of
compound 1 changed to orange–red after the interaction with
AcOꢀ, which was attributed to the significant development of
absorption peak at 450 nm (Fig. 1). Interestingly, the addition of
protic solvent (such as H2O or MeOH) to the solution of compound
1 made the orange solution change back to colorless, which sug-
gested the hydrogen bonding donor solvent destroyed the binding
between compound 1 and AcOꢀ. The above observations also dem-
onstrated the interaction between compound 1 and AcOꢀ was
hydrogen bonding interaction in essence [28,29]. The additions of
In this paper, we designed and synthesized three new anion
probes (Scheme 1) containing thiourea, nitro group and naphtha-
line. The thiourea moiety was able to bind with anion guests via
hydrogen bond interaction and the nitro, naphthaline groups
served as chromogenic signaling subunit. The experimental results
indicated probe 1 containing p-nitro group showed the strongest
binding ability for AcOꢀ among the anions tested (Fꢀ, Clꢀ, Brꢀ, Iꢀ,
AcOꢀ and H2POꢀ). The other two probes containing m- or o-nitro
4
group showed very weak binding ability for various anions and
the binding ability could be ignored.
Materials and methods
Most of the starting materials were obtained commercially and
all reagents and solvents used were of analytical grade. All anions,
in the form of tetrabutylammonium salts [such as (n-C4H9)4NF,
(n-C4H9)4NCl, (n-C4H9)4NBr, (n-C4H9)4NI, (n-C4H9)4NOAc, (n-C4-
H9)4NH2PO4], were purchased from Sigma–Aldrich Chemical Co.,
and stored in a desiccator under vacuum, and used without any fur-
ther purification. Tetra-n-butylammonium salts were dried for 24 h
in vacuum with P2O5 at 333 K. Dimethyl sulfoxide (DMSO) was dis-
tilled in vacuum after dried with CaH2. C, H, N elemental analyses
were made on Vanio-EL. 1H NMR spectra were recorded on a Varian
UNITY Plus-400 MHz Spectrometer. ESI-MS was performed with a
MARINER apparatus. UV–vis titration experiments were made on
a Shimadzu UV2550 Spectrophotometer. Fluorometric titration
was performed on a Varian Cary Eclipse Fluorescence Spectropho-
tometer. The affinity constant, Ks, was obtained by non-linear least
squares calculation method for data fitting.
Fꢀ and H2POꢀ induced similar spectral changes compared with
4
AcOꢀ (Fig. 2), which indicated compound 1 also interacted with
Fꢀ and H2POꢀ by hydrogen bond. However, the additions of Clꢀ,
4
Brꢀ and Iꢀ virtually led to very weak spectral response of com-
pound 1 which indicated the interactions of host–guest were very
weak and could be ignored.
The acidity of this kind of probes can be tuned by changing the
electron property of the substituent on the ortho, meta or para po-
sition and the anion binding ability can be tuned according to res-
onance structure [30]. Therefore, 2 (m-NO2) and 3 (o-NO2) were
synthesized to investigate the electron effect of the substituent
on the hydrogen-bond. Unexpected, the additions of Fꢀ, Clꢀ, Brꢀ,
a-Naphthalen-acetyl chloride,
a-naphthalen-ethanoyl isothio-
Iꢀ, AcOꢀ and H2POꢀ did not induce clear spectral response for 2
4
cyanate [25] p-nitro- benzoylhydrazine, m-nitro-benzoylhydrazine
and o-nitro-benzoylhydrazine [26] were synthesized according to
and 3 (Fig. 2) which indicated the interactions between the com-
pounds (2, 3) and anions were very weak and could be ignored.
CH2 CONCS
CH2COOH
CH2 COCl
KSCN
SOCl2, C6H6
O
H
H
N
R
O
R
N
N
H
CONHNH2
S
1: R=p-NO2
2: R=m-NO2
3: R=o-NO2
Scheme 1. The synthesis route of compounds 1, 2 and 3.

278
X. Shang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 276–281
urea through hydrogen bonds. The PET (photoinduced electronic
transfer) mechanism may interpret the fluorescent quenching
[31–33]. Without the addition of AcOꢀ, the oxygen and sulfur
atoms of free probe 1 could form an intramolecular hydrogen bond
with the hydrogen atom of the thiourea group, which led to a pho-
toinduced electron transfer and the strengthening of the fluores-
cence. However, upon addition of the acetate in the solution, the
electron transferred from the electron rich thiourea moiety bonded
with anion to the electron deficient –NO2 moiety became more fea-
sible and the excited state of the fluorophore was quenched by
electron transfer from the probe to the fluorophore. Similar spectra
were observed upon the addition of Fꢀ or H2PO4ꢀ (listed in support-
ing information). On the other hand, no significant spectral
changes were observed upon the titration of 1 with Clꢀ, Brꢀ, Iꢀ, sig-
nifying compound 1 showed insignificant binding affinity toward
these anions.
Calculation of affinity constant
Fig. 1. UV–vis spectral changes of compound 1 (4.0 ꢁ 10ꢀ5 mol Lꢀ1) in DMSO upon
addition of AcOꢀ (0–20 equiv.). The arrow indicates the increase direction of acetate
concentration. Inset graphic is the color changes of compound 1 after addition of
5 equiv. AcOꢀ anion, (A) free 1 and (B) 1 + AcOꢀ.
Job plot curves indicated compound 1 bound anions at 1:1 ratio.
Affinity constants for the compound with anionic species were cal-
culated according to Eq. (1) of 1:1 host–guest complexation [34–36].
2
1=2
Fluorescence response
X ¼ X0 þ 0:5
D
e
cH þ cG þ 1=Ks ꢀ ½ðcH þ cG þ 1=KsÞ ꢀ 4cHcGꢂ
ð1Þ
The photophysical response of probe 1 toward addition of an-
ions tested was also investigated. Just as Fig. 3 showed, probe 1
exhibited a strong emission centered at 340 nm by excitation at
284 nm. Upon the addition of AcOꢀ into the solution of probe 1,
the fluorescence emission was quenched and a weak broad emis-
sion at around 500 nm appeared. This observation was consistent
with the fact that the anions interacted with N–H protons of thio-
where cG and cH were the concentration of guest and host,
respectively. X was the absorbance intensity at certain concentra-
tion of host and guest. X0 was the absorbance intensity of the host
alone. Ks was the affinity constant for the host–guest complexation.
De
was the change in molar extinction coefficient.
According to the UV–vis and fluorescence data, the affinity con-
stants were obtained by the method of non-linear least square cal-
Fig. 2. Changes in the UV–vis spectra of compounds 1, 2 and 3 (4.0 ꢁ 10ꢀ5 mol Lꢀ1) in DMSO after addition of 20 equiv. various anions.

X. Shang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 276–281
279
was added. Whether the binding ability of acetate anion changed
under the condition that all studied anions existed simultaneously,
the UV–vis spectral interference experiment was conducted
(Fig. 4). Results indicated the spectral response of 1 upon the addi-
tion of mixed anions was almost the same as the addition of only
acetate which showed the binding ability of acetate with com-
pound 1 was not influenced by the existence of other anions. This
point is of experimental and practical importance.
1H NMR titration
In order to gain an insight into the interaction details of recep-
tor with guest anions, the 1H NMR titration of probe 1 with AcOꢀ
was conducted as an example. Free compound 1 displayed two
peaks at 10.71, 10.47 ppm in the downfield region and one peak
at 4.37 ppm in the upfield region which were attributed to NHb,
NHc and NHd respectively (Fig. 5). Upon the addition of 0.5 equiv.
of AcOꢀ, the signal of NHb completely disappeared; the signal of
NHc was significantly broadened and shifted to the downfield;
the signal of NHd was gradually weakened. While the addition of
AcOꢀ was increased up to 1.0 equiv., the signal of NHc and NHd
thoroughly disappeared. During the above titration process, a
slight upfield shift was also found for the signal of Ha, which sug-
gested an increase of electron cloud density in phenyl fragment
due to through-bond electronic propagation effect. No detectable
chemical shift changes of CH protons in the aromatic and naphtha-
lene were seen for compound 1. Consequently, all the results de-
clared the anion recognition underwent two steps, firstly, the
acetate anion induced the proton of the NHb deprotoned, then,
the NHc and NHd interacted with anions through hydrogen bonds.
The configuration of compound 1 possibly changed which was in-
duced by anion, then the original intramolecular H-bonding inter-
actions were destroyed and new stronger intermolecular H-
bonding interactions were constructed upon the interaction with
AcOꢀ, simultaneously contributing from thiourea-NH in compound
1. The triangle AcOꢀ may be fixed within the cavity of compound 1
via intermolecular H-bonding interactions, which changed ICT
property of the whole molecule and ultimately resulted in the ob-
served color and spectral changes. The possible binding mode of
host–guest was listed in supporting information.
Fig. 3. Fluorescence titration of probe 1 (4.0 ꢁ 10ꢀ5 mol Lꢀ1) with AcOꢀ in DMSO
excited at 284 nm at 298 K.
culation. The results were summarized in Table 1 (Clꢀ, Brꢀ and Iꢀ
induced weak spectral response with compounds and the affinity
constants cannot be determined). Other compounds (2 and 3)
showed very weak binding ability with various anions which could
be ignored and the affinity constants was not listed.
The binding ability of the anions for probe 1 followed the order
of AcOꢀ > Fꢀ > H2POꢀ ꢃ Clꢀ ꢄ Brꢀ ꢄ Iꢀ. Although a full under-
4
standing of the principles that govern anion recognition had not
yet been achieved, it already became clear early on that the selec-
tivity for special anions can be rationalized on the basis of the
guest basicity and the shape complementarity between the host
and the anionic guests [37]. Actually, it was necessary for the probe
to possess the multiple hydrogen-bonding interaction which could
offer the high-affinity anion binding sites. In particular, the best
selective recognition for AcOꢀ ion was most likely to its best com-
plementarity between the acetate anion and probe 1 among the
anions tested. It was quite clear that the acetate anion was a planar
triangular species and the angle of O–C–O was about 120° (it was
supported by the following theoretical investigation), which fitted
well to the two hydrogen atoms of the recognition site of the
probe. So the acetate anion interacted with probe 1 and formed
effective multiple hydrogen-bonds. On the other hand, by the alka-
lescence of the acetate anion and the basicity of the probe, the ace-
tate should not better fit than those more basic Fꢀ anion. But as for
Fꢀ, its small ball shape could not match the binding site properly.
Then, the tetrahedral shape of dihydrophosphate anion should also
fit the binding site of the probe worse than AcOꢀ to form the multi-
ple hydrogen bonds.
Theoretical investigation
In some cases, intramolecular hydrogen bonding of the receptor
played an important role in the process of anion binding and could
Interference experiment
The binding ability of probe 1 with acetate anion was the stron-
gest among the anions tested according to affinity constant. The
above results were derived from the condition that only one anion
Table 1
Affinity constant of compound 1 with various anions.
Anion
Ks(1) (Mꢀ1
)
Absorption
Emission
Fꢀ
(2.20 0.07) ꢁ 104
(8.63 0.17) ꢁ 104
(1.65 0.04) ꢁ 103
NDa
(3.69 0.23) ꢁ 104
(9.47 0.35) ꢁ 104
(0.98 0.11) ꢁ 103
ND
AcOꢀ
H2POꢀ4
Clꢀ, Brꢀ or Iꢀ
Fig. 4. UV–vis spectral changes of compound
addition of anions, the concentration of anions is 5 equiv. 1.
1
(4 ꢁ 10ꢀ5 mol Lꢀ1
) upon the
a
The spectrum change little and the affinity constant cannot be calculated.

280
X. Shang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 276–281
Fig. 5. Portion of 1H NMR spectra of compound 1 (0.02 mol Lꢀ1) in DMSO-d6 in the presence of increasing amount of AcOꢀ: (A) free 1, (B) 1 + 0.5 equiv., (C) 1 + 1.0 equiv., (D)
1 + 2 equiv. and (E) 1 + 5 equiv.
Fig. 6. Optimized structures of compound 1 using Gaussian03 program at B3LYP/3-21G level (distance is in Å).
enhance the acidity of the hydrogen-bond donors [38]. Molecular
modeling was carried out to shed some light on the intramolecular
hydrogen bonding. The geometry of compound 1 was optimized
(Fig. 6) using density functional theory at B3LYP/3-21G level with
Gaussian03 program [39]. From the viewpoint of relevant atomic
distance and orientation, the protons of thiourea NHs in compound
1 formed intramolecular hydrogen bonding with oxygen and sulfur
atoms of adjacent thiourea group, respectively. Additionally, the
distance of two hydrogen atoms in thiourea-NHs (H1–H2,
2.465 Å) was close to that of two oxygen atoms in AcOꢀ
(2.309 Å). So, the triangle anion, AcOꢀ, well matched for the cavity
of compound 1 and showed the strongest binding ability among
the anions tested.
In addition, the selected frontier orbitals for compound 1 were
also optimized and showed in supporting information. We
introduced molecular frontier orbital in order to explain UV–vis
absorption spectra in the interacted process of host–guest induced
by electron transition of frontier orbital. The highest HOMO den-
sity in compound 1 was mainly localized on the naphthaline and
thiourea moieties. While, the highest LUMO density was mainly
localized on the one phenyl ring moiety. The above results demon-
strated it was the electron transition of the highest LUMO to arouse
the red-shift phenomenon in UV–vis spectra of 1-anion.
Conclusion
In conclusion, a colorimetric and fluorescent ON-OFF anion
probe containing thiourea group, which allowed so-called
‘‘naked-eye’’ detection without using expensive equipment, was
successfully prepared. The addition of AcOꢀ anion elicited color
changes from colorless to orange and a visible decrease in the fluo-
rescence emission intensity of probe 1 owing to a quenching PET
process from the thiourea NH to –NO2 group. Theoretical investiga-
tion indicated the electron transition of the highest LUMO aroused
the red-shift phenomenon in UV–vis spectra of 1-anion. Particu-
larly, the novel design strategy that the thiourea moiety is incorpo-
rated into the fluorescent p-conjugated system would help to

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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
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