A simple and efficient colorimetric anion receptor for H2PO4-

Article abstract of DOI:10.1016/j.saa.2007.08.018

A novel and neutral anion sensor bearing a urea group as binding sites and 2,4-di nitrophenylhydrazine unit as a molecular architecture and a chromophore was synthesized and the visible color changes, the UV-vis and ¹H NMR spectral responses toward anions were assessed.

Full text of DOI:10.1016/j.saa.2007.08.018

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 682–685
−
A simple and efficient colorimetric anion receptor for H2PO4
Jie Shaoa, Hai Linb, Hua-Kuan Lina,∗
a Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
b Key Laboratory of Functional Polymer Materials of Ministry of Education,
Nankai University, Tianjin 300071, People’s Republic of China
Received 6 July 2007; received in revised form 24 August 2007; accepted 27 August 2007
Abstract
A novel and neutral anion sensor bearing a urea group as binding sites and 2,4-di nitrophenylhydrazine unit as a molecular architecture and a
chromophore was synthesized and the visible color changes, the UV–vis and 1H NMR spectral responses toward anions were assessed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Colorimetric; Receptor; Supramolecular chemistry
1. Introduction
larly associated [13]. In the past, a lot of studies [14–15] have
showed that urea is a good H-bond donor and excellent receptor
for tetrahedral anions and Y-shaped anions through the forma-
tion of multitopic H-bonds. Therefore, we attempted to integrate
the urea group as bonding sites into 2,4-dinitrophenylhydrazine
unit (a chromophore) and obtained a simple and efficient col-
The recognition and sensing of anionic analytes is currently
an expanding area within the field of supramolecular chem-
istry [1–3]. Anions play a major function in the environment,
industry and especially in biology where phosphate, carbon-
ate and chloride are the most commonly found. In particular,
organic or inorganic phosphates are key substrates or intermedi-
ates for many biochemical reactions, and are main components
of biomolecules such as DNA and RNA. Accordingly, a sig-
nificant amount of work has been devoted to obtain specific
chemosensors [4–7] that are able to change one or several macro-
scopic properties, upon complexation with the target guests. In
response to the molecular coordination events, the changes in
color, which are used in the development of optical chemosen-
sors, are the most applicable output signals among the changes
in the fluorescence, absorbance, and cyclic voltammetry, etc. In
this regard, the colorimetric anion sensors would allow ‘naked-
eye’ detection of anions without resort to any spectroscopic
instrumentation. In general, such sensor systems are mainly
composed of two parts: one is the anion binding part (recep-
tor), which is typically hydrogen-bonding donors such as pyrrole
[8], urea/thiourea [9], amine [10] or phenol [11] moieties, and
the other is a chromophore, which converts binding induced
changes into an optical signal such as the appearance of color.
These two parts are either directly linked [12] or intramolecu-
−
orimetric H2PO4 sensor, which was synthesized by only one
single-step procedure. Large numbers of receptors [16–18] for
−
H2PO4 have been reported in the literature, but most of them
[14,16,17] cannot act as the colorimetric receptors and require
complicated and rigorous synthesis methods. In this paper, an
easy-to-prepare receptor 1 is proved by the naked-eye experi-
ment, the UV–vis titration and 1H NMR titration methods to be
a highly sensitive to H2PO4− anion.
2. Experimental
2.1. General procedures and materials
All reagents for synthesis were obtained commercially except
CH3CN, which was dried with P2O5 (0.5–1:100, v/v) stirring at
room temperature overnight and then distilled, and were used
1
without further purification. H NMR spectra were obtained
on a Varian UNITY Plus-400 MHz Spectrometer. ESI-MS was
performed with a MARINER apparatus. C, H, N elemental
analyses were made on a elementar vario EL. UV–vis spec-
tra were recorded on a Shimadzu UV2450 Spectrophotometer
with a quartz cuvette (path length = 1 cm) at 298.2 0.1 K. In
the UV–vis titration experiment, all the anions were added in
∗
Corresponding author. Tel.: +86 22 23502624; fax: +86 22 23502458.
E-mail address: hklin@nankai.edu.cn (H.-K. Lin).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.08.018

J. Shao et al. / Spectrochimica Acta Part A 70 (2008) 682–685
683
Scheme 1. General synthetic routes to the target compound 1.
the form of tetra-n-butylammonium (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.
2.2. Synthesis of β-N-(2,4-dinitroanilin-N-yl)-phenylurea
(1)
Fig. 2. UV–vis titration curves of 1 (4 × 10−5 M) in DMSO upon addit−ion of
−
different concentrations of H2PO4 and inset: Job’s plot of 1 vs. H2PO4
.
The synthesis of the receptor 1 was summarized in Scheme 1.
2,4-Dinitrophenyl-hydrazine (0.198 g, 1 mmol) was dissolved
in dry 30 ml CH3CN. To this solution was added phenyliso-
cyanate (0.1119 g, 1 mmol) dropwise slowly and stirred under
inert atmosphere refluxing for 8 h. Then the reaction mixture
was cooled to room temperature. Precipitate formed was filtered
and washed with CH3CN and obtained 0.24 g. Yield = 75%. 1H
NMR (DMSO-d6) δH 10.127 (s, 1H), 8.975 (s, 1H), 8.885 (d,
1H, J = 2.4), 8.741 (s, 1H), 8.389 (m, 1H, J = 12.4), 7.47 (d, 2H,
J = 8), 7.416 (d, 1H, J = 10.4), 7.28 (t, 2H, J = 14), 7.003 (t, 1H).
ESI-mass: m/z (negative ion mode) 316.10 (M − H+). Elemen-
tal analysis calcd. for C13H11N5O5 (M = 317.08): C, 49.22; H,
3.49; N, 22.07, found: C, 49.01; H, 3.62; N, 21.82.
guests for the bonding sites of hosts and therefore, could disturb
hydrogen bonding interactions between hosts and guests. So the
results observed indicated that visible color changes were most
probably due to hydrogen bond interactions between the NH
groups and anions tested.
Secondly, thebindingabilityofthereceptor1withavarietyof
anions in dry DMSO was investigated by UV–vis spectroscopy
at room temperature. The UV–vis titrations were carried out
in DMSO at a concentration level of 4 × 10−5 M upon addi-
−
tion of H2PO4 as tetrabutylammonium and the spectra were
shown in Fig. 2. The receptor 1 exhibited two main bands
centered at 264 and 351 nm and the peak at 264 nm increased
gradually and the peak at 351 nm decreased slowly upon addi-
tions of H2PO4− anion. And simultaneously, a new broad peak
centered at 500 nm, which corroborated well with the color
changes observed in the naked-eye experiment, was observe−d
and increased with the increase of concentration of H2PO4
anion. The big bathochromic shift (from 351 to 500 nm) could
be explained on the basis of internal charge-transfer between
the anion-bond urea unit and the electron-deficient –NO2 moi-
ety. In addition, there were two well-defined isosbestic points at
319 and 407 nm, which indicated that two species were present
3. Results and discussion
The recognition properties of the host molecular 1 toward
different anions were studied by the several methods such as
1
the naked-eye experiment, the UV–vis titration and H NMR
titration. Firstly, in the naked-eye experiments, the receptor 1
(4 × 10−5 M in DMSO) gave birth to dramatic color changes
from light yellow to light purple in the presence of 5 equiv. of
H2PO4−, F− and CH3COO− (see Fig. 1). However, addition
of a large excess of Cl−, Br− and I− could not result in any
color changes of the receptor 1 at all. Furthermore, the color
changes of the host molecular 1 would disappear upon addition
of a small quantity of water. It has been very clear [21] that
the protic solvent such as H2O would compete with the anionic
−
at equilibrium: 1 and 1H2PO4 adduct. And 1:1 stoichiometry
of the complex was also confirmed by the Job’s plot as shown
in Fig. 2 (inset). On the basis of 1:1 stoichiometry, the bind-
ing constant (Kass) was calculated to be 8658 M−1, which was
shown in Table 1, based on the UV–vis titration data. As for
Fig. 1. The visible color changes of the receptor 1 in DMSO (4 × 10−5 M) upon additions of 5 equiv. of different anions (1: host only, 2: host + H2PO4−, 3: host + F−,
4: host + CH3COO−, 5: host + Cl−, 6: host + Br−, 7: host + I−).

684
J. Shao et al. / Spectrochimica Acta Part A 70 (2008) 682–685
Table 1
Association constants for various anions toward receptor 1
Aniona
Kass (M−1
)
−
H2PO4
(8.66 0.31) × 103
F−
(4.49 0.34) × 103
AcO−
Cl−
Br−
I−
(2.33 0.34) × 103
NDb
ND
ND
a
The anions were added as their tetrabutylammonium salts.
The affinity constant cannot be determined by the spectra.
b
the other anions, the additions of F− and CH3COO− induced
similar changes (see Supplementary data) in the UV–vis spec-
trum with H2PO4−, but additions of Cl−, Br− and I− did not
result in significant absorbance spectral changes (see S1 in the
Supplementary data). Obviously shown in Table 1, the selectiv-
ity trends of binding affinities of anions for 1 were determined
to be H2PO4− > F− > AcO− ∼ Cl− ∼ Br− ∼ I−. However, a full
understanding of the principles that govern anion recognition
has not been achieved. It became clear early on that multi-
ple hydrogen-bonding interactions are necessary in high-affinity
anion binding sites. Charge and shape complementarity [19]
between the host and the anionic guests are also extreme−ly
Fig. 3. Plots of 1H NMR spectra of 1 (1 × 10−2 M) on addition of F− in DMSO-
d6.
important. As expected from the basicity of anions, H2PO4
,
F−, and AcO− gave stronger complexes than other anions. How-
ever, H2PO4− anion with four oxygens could form the strongest
1·anion complex via multitopic hydrogen-bonding interactions
with 1 [20] among the rest anions tested (see Scheme 2). That
was why H2PO4− could be recognized selectively from the other
anions.
−
Thirdly, further insights to the nature of 1-H2PO4 inter-
actions were investigated by 1H NMR titration experiment
in DMSO-d6. Fig. 3 displayed the spectra recorded when
a 1 × 10−2 M DMSO-d6 solution of 1 was titrated with
[Bu4N]H2PO4−. UponadditionofH2PO4−, thepeaksat10.127,
8.975 and 8.741 ppm, which were assigned to the N–H protons,
shifted downfield and finally disappear, indicating the forma-
tion of a host-guest hydrogen-bonding complex between 1 and
H2PO4−. In addition, the slight down-field shifts of the aromatic
proton signals (Ha)−were also found, demonstrating that poten-
tial C H· · ·H2PO4 hydrogen bonding −existed in the complex
and cooperated with the N H· · ·H2PO4 hydrogen bonds. 1H
NMR spectra of 1 with F−, which was similar with Fig. 3, was
shown in Fig. 4. Obviously seen from Fig. 4, the proton sig-
nals (Hd and Ha) firstly shifted downfield before addition of the
Fig. 4. Plots of 1H NMR spectra of 1 (1 × 10−2 M) on addition of F− in DMSO-
d6.
first equivalent of fluoride and then shifted upfield upon further
equivalent of fluoride. On the other hand, the N–H signals shifted
downfield and finally disappear. The results observed suggested
that, in the first step, the fluoride established a hydrogen-bonding
interaction with a urea subunit of the receptor. In this situation,
an effective electrostatic effect was exerted on the nearby pro-
tons, including significant deshielding and a downfield shift, by
the H-bond anion. Then, on addition of the second fluoride ion,
Scheme 2. The proposed host-guest binding mode in solution.

J. Shao et al. / Spectrochimica Acta Part A 70 (2008) 682–685
685
an HF molecular was released from the H-bond complex, with
the formation of the deprotonated receptor [22]. As mentioned
above, the proposed binding mode was shown in Scheme 2.
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In summary, a highly sensitive colorimetric sensor bearing
−
urea unit as bonding sites for H2PO4 −has been successfully
presented. A small quantity of H2PO4 anion (only 5 equiv.)
can result in remarkable changes not only in the UV–vis and 1H
NMR spectra but also in the visible colors of the receptor 1 in
−
DMSO solution. The recognition for H2PO4 can be owed to
the formation of multi-hydrogen bonds between the receptor 1
and anionic substrates.
Acknowledgments
This project was supported by the National Natural Science
Foundation of China (20371028, 20671052). The authors also
thank the reviewers for giving advice on the paper.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.saa.2007.08.018.
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