A simple and neutral receptor acting as a sensitive and switch-on fluorescent chemosensor for H2PO4-

Article abstract of DOI:10.1007/s10895-011-0862-y

A novel artificial anion chemosensor 1 based on 2, 2'-di (4-nitrophenylurea-β-N-yl) -1, 1'-binaphthyl is designed and synthesized for sensing anions including halide ions and oxoanions. The fluorescent emission of the binaphthyl of receptor 1, forming the hydrogen bonding with anions as the sensing mechanism, is monitored in DMSO for detecting anions. In brief, while most of the anion chemosensors are switch-off fluorescent chemosensor, or non-fluorescent sensor, receptor 1 exhibits obviously the switch-on emission during the complexation with H₂PO₄^-. Springer Science+Business Media, LLC 2009.

Full text of DOI:10.1007/s10895-011-0862-y

J Fluoresc (2011) 21:1697–1702
DOI 10.1007/s10895-011-0862-y
ORIGINAL PAPER
A Simple and Neutral Receptor Acting as a Sensitive
−
and Switch-on Fluorescent Chemosensor for H2PO4
Weiwei Huang & Hongyan Su & Sikai Yao & Hai Lin &
Zunsheng Cai & Huakuan Lin
Received: 27 December 2010 /Accepted: 13 February 2011 /Published online: 8 March 2011
#
Springer Science+Business Media, LLC 2011
Abstracts A novel artificial anion chemosensor 1 based on
2, 2′-di (4-nitrophenylurea-β-N-yl) -1, 1′-binaphthyl is
designed and synthesized for sensing anions including
halide ions and oxoanions. The fluorescent emission of
the binaphthyl of receptor 1, forming the hydrogen bonding
with anions as the sensing mechanism, is monitored in
DMSO for detecting anions. In brief, while most of the
anion chemosensors are switch-off fluorescent chemosen-
sor, or non-fluorescent sensor, receptor 1 exhibits obviously
the switch-on emission during the complexation with
output signal [5]. In fact, the output signal may include the
changes in either color or fluorescence in the presence of a
certain guest. In addition, the changes in electrochemical
properties such as the oxidation potential of redox active
groups have also been widely used [6–9]. However, it is
noteworthy that fluorescence detection has been widely
used in analytical chemistry, biochemistry, and cell biology
because of its high sensibility and other principal advantage
over other light-based methods [10]. In this article, we have
reported a binaphthyl and urea-based receptor as efficient
fluorescent sensor for selective recognition of dihydrogen-
phosphate [11].
−
H2PO4 .
.
.
Keywords Anion recognition Hydrogen-bonding
The 1, 1 ′-binaphthyl unit is expected to act as
spectroscopic signaling subunit which is able to transduce
the coordination event into changes in fluorescence
behavior. However, as far as we know, few anion receptors
based on 1, 1′-binaphthyl have been reported before. This
urges us to synthesis an anion receptor which composes 1,
1′-binaphthyl and two urea groups at 2- and 2′- positions,
since it is the way that follows the approach of the covalent
attachment of signaling subunits and binding site [12].
Furthermore, in order to increase the number of binding site
and make the bonding stronger, we choose the 4-
nitrophenylisocyanate unit as the component in our exper-
iment. With this information, we have designed and
synthesized receptor 1 containing four binding sites (see
Scheme 1).
.
Fluorescent Switch-on
Introduction
Numerous efforts have been devoted to the development of
chemosensor for anions such as dihydrogenphosphate,
fluoride, acetate, and iodine due to their fundamental role
in a wide range of chemical and biological process [1–4].
From the supramolecular chemistry concept, a chemosensor
molecule is mostly composed by a component selected for
a particular analyte and a signaling unit capable of
translating the changes induced by analyte-binding into an
:
:
:
:
W. Huang H. Su S. Yao Z. Cai H. Lin (*)
Department of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
e-mail: hklin@nankai.edu.cn
Experimental
Materials
H. Lin
Key Laboratory of Functional Polymer Materials of Ministry of
Education, Nankai University,
Tianjin 300071, People’s Republic of China
All reagents for synthesis obtained commercially were
used without further purification. In the titration experi-

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J Fluoresc (2011) 21:1697–1702
Scheme 1 The synthetic
procedures of receptor 1
O
C
N
Cu(AcO)2/Pyridine
N2
NH2
NH2
H2N
1
2
NO2
Hb
c
HN
NH
CH3CN
N2,Reflux
O
O
NH HN
Ha
O2N
NO2
3
ments, 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.
using an ultraviolet–visible spectrophotometer, UV-2450
(Shimadzu 2.1 Apparatus Corp., Kyoto, Japan). Fluores-
cence spectra were recorded on a FP-750 fluorescence
spectrometer with the excitation and emission slits of 10 nm
width at 298.2 0.1 K.
A 2.0×10−3 M solution of the receptor 1 in DMSO was
prepared and stored in the dry atmosphere. Solutions of
1.0×10−2 and 1.0×10−1 M tetrabutylammonium salt of the
respective anion were prepared in dried and distilled
DMSO and were stored under a dry atmosphere.
Apparatus
1H 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 an elementar vario EL. UV–vis spectra were
recorded on a Shimadzu UV-2450 Spectrophotometer with
a quartz cuvette (path length=1 cm) at 298.2 0.1 K and the
width of the slits used is 5 nm. Fluorescent spectra were
recorded on a FP-750 fluorescence spectrometer at 298.2
0.1 K and the width of the slits used is 10 nm.
1H NMR titration experiments were carried out in the
DMSO-d6 solution (TMS as an internal standard). Certain
amount of the receptor 1 solution in the DMSO-d6 was
1
prepared with a concentration of 0.01 M. H NMR of the
host-guest system was recorded by adding increasing
amount of dihydrogenphosphate anions (1.0 M in DMSO-
d6) into the receptor 1 solution.
Synthesis of 2, 2′-Di (4-Nitrophenylurea-β-N-yl) -1,
1′-Binaphthyl
General Method
All experiments were carried out at 298.2 0.1 K, unless
otherwise mentioned. UV–vis spectra were measured
4-nitrophenylisocyanate (0.0952 g, 0.8 mmol) in dry
CH3CN was added dropwise to a refluxing solution of 2,

J Fluoresc (2011) 21:1697–1702
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2′-diamino-1, 1′-binaphthyl (0.1136 g, 0.4 mmol) (2),
which was prepared from 2-aminonaphthalene (3) accord-
ing to the literature [13] (see Scheme 1). Then the reaction
mixture was stirred and refluxed under atmosphere for 8 h.
After cooling to the room temperature, The formed
precipitate was filtered and washed with CH3CN. (yield:
non-fluorescent sensors. This may be interpreted by the
photo-induced electron transfer (PET) quenching mech-
anism [14, 15] or the heavy atom effect of the sulfur atom.
But considering the sensitivity of chemosensing, a switch-
on, rather than a switch-off fluorescent sensor would be
more preferred [16].
1
0.0979 g, 40%). H NMR (DMSO-d6) δH : 9.59 (s, 2 H,
It is evident from Fig. 1, the chemosensor of receptor 1
−
N-H), 8.39 (m, 2 H, N-H), 8.14 (t, 6 H, binaphthyl-H),
8.02 (dd, 2 H, Ph-H), 7.66 (s, 2 H, Ph-H), 7.43 (dd, 6 H,
binaphthyl-H), 7.27 (t, 2 H, Ph-H), 6.81 (t, 2 H, Ph-H).
ESI-mass: m/z (negative ion mode) 611.20 (M-H+)
Elemental analysis calcd for C34H24N6O6 (612.20): C,
66.67; H, 3.92 N, 13.73; found: C, 66.63; H, 4.13; N,
13.88.
displays the switch-on reaction towards H2PO4 , F− and
AcO−, but not towards Cl−, Br− and I−. This phenomenon
may be due to binding-induced rigidity of the host
−
molecule [17, 18]. Upon complexation with H2PO4 ion,
the host molecule 1 was rigidified, which gave birth to a
large increase in emission intensity because of inhibition
vibrational and totational relaxation modes of nonradiative
decay. Similarly, the addition of F− and AcO− resulted in
−
similar fluorescent changes as H2PO4 . On the other hand,
Results and Discussions
receptor 1 was found to be insensitive to the addition of
Cl−, Br− and I−, even in abundance. The fitted affinity
constants of receptor 1 towards anions are listed in Table 1
and the differences on the ability of switch-on the
fluorescence spectrum in Fig. 2. From these results, it is
clearly demonstrated that the cleft of the molecule is more
Fluorescence Spectrum
Fluorescence experiment was performed to investigate
properties of receptor 1 towards various anions in DMSO.
With the aim of evaluating the stoichiometry for the
binding interaction, 20 molar equivalent anions, i.e.
−
preferred by H2PO4 and receptor 1 may be regarded as
−
the anion-selective sensor for H2PO4 .
−
H2PO4 , AcO−, F−, Cl−, Br− and I− as their tetrabutylam-
monium salts (TBA+), were added to the solution of
receptor 1 (2×10−5 M). Then, a strong emission band
centered at 396 nm was observed when excited at 1=
358 nm (see Fig. 1).
UV–Vis Experiments
To gain an insight on the binding and sensing properties of
receptor 1 towards various anions, we carried out UV–vis
experiment in DMSO. As it is shown in Fig. 3, upon
In the literature published before, most of the anion
chemosensors, especially the urea-and thiourea-based
sensors, are switch-off fluorescent chemosensors, or
−
addition 20 equiv of H2PO4 , a bathochromic shift can be
observed passing through the isosbestic point at 358 nm
(form 349 nm to 364 nm). The same behavior can also be
observed in the case of the AcO− and F− . The changes of
absorbance constituted evidence for hydrogen bonding
interaction between receptor 1 and the given anions.
20 equiv
250
0 equiv
200
Determination of the Binding Constant and Stoichiometry
150
100
50
First, the total fluorescence signal intensity F can be
expressed by the following Eq. 1 [19]:
n
ꢁB
n
½Gꢀ Fmax þ 10
10ꢁB þ ½Gꢀ
F
min
F ¼
ð1Þ
In the spectrofluorometry of this work, Fmin, Fmax and F
are the emission intensities of the solution at wavelength
396 nm in the absence of guest, presence of the saturated
guest, and after addition of a given amount of guest to
certain concentration, respectively. [G] is the concentration
of guest, n is the number of G bound per receptor 1. Once
0
360
380
400
420
440
460
480
500
Wavelength/nm
Fig. 1 The fluorescence spectral changes of receptor 1 (2×10−5 M)
observed upon addition of 20 equiv. of H2PO4−, respectively. 1ex
=
358 nm

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Table 1 Affinity constants of receptor 1 with various anions in DMSO
F−
AcO−
Cl−
Br−
I−
−
Anions
H2PO4
NDd
ND
ND
ND
ND
ND
ND
ND
ND
a
lgKass
3.6978 0.11
3.6335 0.10
0.9758
3.0 0.04
3.19 0.06
0.9891
2.58 0.12
2.49 0.03
0.9759
b
lgKass
Rc
a the affinity constants determined by fluorescence in DMSO
b the affinity constants determined by UV–vis in DMSO
c Correlation coefficient (R) determined by non-linear fitting analyses
d ND cannot determined
the fluorescence experiments were done, the sigmoidal
curve was obtained and the total binding constant logKass
was deduced as follows:
Firstly, the equation can be linearised in the form of Hill
polt and the Hill coefficient (n) can be obtained from Eq. 2
[20–23] (see Fig. 4).
At same time, we also treated the data from UV–vis
anion titration in another way as show below.
For a complex of 1:1 stoichiometry, the relation in Eq. 3
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, Kass is the affinity constant of host-
guest complexation [25].
F ꢁ Fmin
log
¼ n log½Gꢀ þ B
ð2Þ
X ¼ X0 þ ðXlim ꢁ X0ÞfCH þ CG þ 1=Kass
Fmax ꢁ F
2
1=2
−
ꢁ ½ðCH þ CG þ 1=KassÞ ꢁ 4CHCGꢀ g=2CH
ð3Þ
Then, the binding constant of compound 1 to H2PO4 ,
logKass is determined by fitting Eq. 2 and is 3.6978. Also,
the Hill coefficient (n) is obtained to be 1.0889, indicating
that receptor 1 binds the H2PO4− anion guest with a ratio of
1:1, which is consistent with the Job’s plot. Job’s plot was
obtained according to the method reported by K.A.
Connors [24]. Job’s plot of receptor 1 and F− in DMSO
shows the maximum at a molar fraction of 0.5, which
indicates that the receptor 1 binds F− guest at a 1:1 ratio.
(see Fig. 3)
The constants of receptor 1 binding to other anions were
also calculated (see Table 1). The results drawn from the
data of fluorescence spectrum are qualitatively matched
with that from the UV–vis spectral titrations, and show that
−
K
ass of receptor 1 toward dihydrogenphosphate (H2PO4 ) is
larger than other anions, which maybe due to the following
−
two reasons: the first, H2PO4 bears four oxygen atoms
which favors the formation of the strongest receptor 1-
1.0
0.45
0 equiv
-
0.40
1+H2PO4
0.35
1+F-
1+AcO-
0.8
0.6
0.4
0.2
0.0
20 equiv
200
0.30
0.25
0.20
0.15
0.10
1;1+Cl -,Br-,I-
150
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
100
50
0
[H]/([H]+[G])
300
400
Wavelength/nm
500
340
360
380
400
420
440
460
480
500
Fig. 3 Evolution of the UV–vis spectra of receptor 1 (2.0×10−5 M)
during the titration with 20 equiv. H2PO4−. Inset: Job’s plot for
Wavelength/nm
−
Fig. 2 The fluorescence titration of receptor 1 (2×10−5 M) with 20
equiv. H2PO4−, F− and AcO−, respectively. 1ex=358 nm
complexation of receptor 1 with H2PO4 in DMSO, [host]+[guest]=
2.0×10−5
M

J Fluoresc (2011) 21:1697–1702
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1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
Y=1.0889X+3.6978
O
NH
HN
O
O
O
P
NH
HN
HO
OH
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
O2N
NO2
log[G]
−
Scheme 2 The proposed mechanism of receptor 1 towards H2PO4
Fig. 4 Fluorescence intensity at 396 nm (F369) of receptor 1 (2×10−5
M) versus increasing concentration of log[G]
previously reported for those receptors based on diaryl urea
and thiourea [26–28], indicating the formation of a host-
guest hydrogen-bonding complex. So we can reasonably
presume that the urea protons (Ha and Hb) function as
anion-binding moieties and participate in the formation of
hydrogen bonds with dihydrogenphosphate ions [29, 30].
The increase of electron density in the aromatic rings
causes a shielding effect and should promote an upfield
shift, which is expected to come through the formed
hydrogen bond [31]. Accordingly, the proposed binding
anion complex via multiple hydrogen-bonding interactions
and the second, the cleft of the complex may be more fitted
for tetrahedral dihydrogenphosphate.
1H NMR Titrations
To further investigate the binding properties of receptor 1
towards these given anions, detailed 1H NMR titrations of 1
in DMSO-d6 were carried out to observe the chemical shift
signal changes in two urea and the aromatic protons upon
the action of recognition of anions. The stack plot of the 1H
−
mode of receptor 1 with H2PO4 in solution can be
demonstrated in Scheme 2.
−
NMR spectra was obtained upon addition of H2PO4 as its
TBA+ salt to the DMSO-d6 solution of receptor 1 (1×10−2
M) (see Fig. 5). Obviously, the proton signals at 9.59 ppm
and 8.39 ppm that have been assigned to Ha and Hb
(marked in Scheme 2) experienced significant downfield
shifts (δ=0.418 ppm and δ=0.331 ppm), which have been
Analytical Application
Experimental result is given to show the performance of
receptor 1: it has high selectivity for the H2PO4 . The
−
changes of fluorescence and UV differential spectra were
observed obviously, upon addition of only 20 equiv of
H2PO4 to the solution of 1 (2.0×10−5 M). However, no
−
detectable spectral responses were observed when adding
other anions. When the concentration of receptor 1 is 2×
16 equiv
12 equiv
8 equiv
4 equiv
10−5 M in DMSO, the limit of [H2PO4 ] detection (LOD) is
−
2×10−5 M, so we hope receptor 1 may be applied in
detection of biologically important anions such as the
−
H2PO4 ion.
Conclusions
Hb
Ha
In conclusion, we synthesized an easily prepared anion
chemosensor based on binaphthyl, which could recognize
H2PO4− among the anions investigated. It is expected to be
0 equiv
ppm
11
10
9
8
7
6
−
Fig. 5 1H NMR titration of receptor 1 (1.0×10−3 M) in DMSO-d6
applied for detection of H2PO4 in analytical chemistry for
with [Bu4N] H2PO4
its easy synthesis and highly selective sensing ability.

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J Fluoresc (2011) 21:1697–1702
Acknowledgement This project was supported by the National
Natural Science Foundation of China (20371028 and 20671052).
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19. Valeur B (2001) Molecular fluorescence: principles and applica-
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