A ditopic fluorescence sensor for saccharides and mercury based on a boronic-acid receptor and desulfurisation reaction

Article abstract of DOI:10.1002/asia.201100635

Two boron-contained fluorescent sensors, 1 and 2, based on coumarin have been prepared. The fluorescence response of the two systems was investigated with addition of saccharide and mercury ions. Sensor 2 behaves as a bifunctional fluorescent switch with chemical inputs of D-fructose and mercury ions.

Full text of DOI:10.1002/asia.201100635

FULL PAPERS
DOI: 10.1002/asia.201100635
A Ditopic Fluorescence Sensor for Saccharides and Mercury Based on a
Boronic-Acid Receptor and Desulfurisation Reaction
Zhitao Xing,[a, b] Hui-Chen Wang,[b] Yixiang Cheng,[a] Tony D. James,*[b] and
Chengjian Zhu*[a]
Abstract: Two boron-contained fluorescent sensors, 1 and 2, based on coumarin
have been prepared. The fluorescence response of the two systems was investigat-
ed with addition of saccharide and mercury ions. Sensor 2 behaves as a bifunction-
al fluorescent switch with chemical inputs of d-fructose and mercury ions.
Keywords: boron · coumarin · fluo-
rescence · mercury · saccharides
Introduction
Mercury ions are potent toxins to humans, since accumu-
lation in the blood–brain barrier results in severe neurologi-
cal disorders.[5] Hence, the design and development of novel
sensors for mercury ions is an important area of research.[6]
Saccharides, on the other hand, are vital to human health
through their use as building blocks and in the production
of metabolic energy.[7]
Although these two species have opposing effects on
human health, they are brought together in industrial food
processing. In particular, food ingredients such as citric acid,
sodium benzoate, and high-fructose corn syrup are produced
using mercury-cell chlor-alkali. High-fructose corn syrup is
extensively used in food products to enhance shelf life.
Therefore, a substantial part of dietary exposure to mercury
may originate from high-fructose corn syrup produced using
mercury cells.[8]
The contamination of fructose corn syrup with mercury
during its manufacture makes the development of a ditopic
fluorescence sensor for mercury ions and fructose an impor-
tant goal. However, there are few systems capable of selec-
tively reporting the presence of saccharides and cationic
guests. Herein, we present a fluorescence signaling system 2
which acts as a ditopic sensor[9] for Hg2+ ions and d-fruc-
tose.
The development of boronic-acid-based saccharide sensors
that rely on the dynamic covalent interaction of boronic
acids with diols has been extensively investigated.[1] Not
only is the pair-wise interaction energy large enough to
allow single-point molecular recognition, but also the pri-
mary interaction involves the reversible formation of a pair
of covalent bonds (rather than non-covalent attractive
forces). This interaction has been widely exploited in the de-
velopment of fluorescent and colorimetric sensors.[2]
The design and construction of chemosensor systems
which respond to chemical and/or photonic inputs by gener-
ating output signals to multiple guests have attracted consid-
erable attention.[3] Most of the successful systems designed
so far are based on photoinduced electron transfer (PET)
mechanisms. Molecular systems combining binding ability
with the initiation or inhibition of PET processes as a mode
of signal transduction are in great demand for developing
molecular-scale information processors.[4]
[a] Z. Xing, Prof. Dr. Y. Cheng, Prof. Dr. C. Zhu
State Key Laboratory of Coordination Chemistry
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing 210093 (PR China)
Fax : (+86)25-83594886
Results and Discussion
E-mail: cjzhu@nju.edu.cn
[b] Z. Xing, H.-C. Wang, Prof. Dr. T. D. James
Department of Chemistry
The synthesis of compounds 1 and 2 are readily achieved in
several steps. The fluorophore ethyl-7-(diethylamino)-2-oxo-
2H-chromene-3-carboxylate (4) is prepared from 4-(diethy-
lamino)-2-hydroxy-benzaldehyde and diethyl malonate in
the presence of a catalytic amount of piperidine and acetic
acid. Without further purification, the crude 4 is reacted
University of Bath
Bath, BA2 7AY (UK)
E-mail: t.d.james@bath.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100635.
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Chem. Asian J. 2011, 6, 3054 – 3058

with hydrazine hydrate to afford 3 at room temperature in
12 minutes. Sensor 2 was then easily prepared by a conden-
sation reaction of 3 with 3-isothiocyanophenyl boronic acid
pinacol ester. Compound 1 was obtained directly from 2
using N,N’-dicyclohexylcarbodiimide (DCC) in 90% yield
(see Scheme 1).
Scheme 1. Synthesis of sensors 1 and 2. Reagents and conditions: a) di-
ethyl malonate, piperidine, acetic acid, reflux, 3 h, 80% yield; b) hydra-
zine monohydrate, EtOH, 12 min, 52% yield; c) 3-isothiocyanophenyl
boronic acid pinacol ester, CH3CN, 2 h, 76% yield; d) DCC, toluene,
reflux, 10 h, 90% yield.
Figure 1. a) and b): Fluorescence spectra of 1 and 2 in the presence of in-
creasing concentrations of d-fructose (lex =430 nm).
The fluorescence titrations of 2 (3.7ꢁ10À7 moldmÀ3) and 1
(8.0ꢁ10À7 moldmÀ3) with different saccharides were carried
out in a pH 8.21 buffer (52.1 wt% MeOH in H2O with KCl,
0.01000 moldmÀ3; KH2PO4, 0.002752 moldmÀ3; Na2HPO4,
0.002757 moldmÀ3). The fluorescence intensity of receptor 1
decreased on the addition of saccharides, the expected be-
havior for directly integrated boronic acid fluorophore sys-
tems.[10] Conversely, we found that the fluorescent intensity
of receptor 2 increased with the addition of saccharide; this
result suggests that a d-PET (donor Photo-induced Electron
Transfer)-type mechanism is operating.[11] The pH profile of
receptor 2 (see the Supporting Information, Figure S1) con-
firms this assumption, where, at low pH, the fluorescence is
switched “off” owing to PET from the fluorophore (as
donor) to the boronic acid receptor unit, whilst at high pH
the fluorescence is “on” and PET is switched “off”. The
fluorescence switch “on” is caused at high pH (or on saccha-
ride binding) because PET from the fluorophore to the neg-
atively charged boronate of the receptor is unfavorable.
The fluorescence spectra of 1 and 2 in the presence of d-
fructose are shown in Figure 1a and Figure 1b; the analo-
gous spectra for the addition of d-glucose and d-mannose
are also available in the Supporting Information. The stabili-
ty constants (K) of fluorescence receptors 1 and 2 with d-
fructose, d-glucose, and d-mannose were calculated by fit-
ting the emission intensity at 478 nm and 492 nm (lex =
430 nm) versus the concentration of saccharide (Figure 2a,
and 2b). Minimal fluorescence response was observed for
other saccharides, such as d-sucrose and d-xylose (see the
Supporting Information, Figures S2 and S3). The stability
constants calculated from these titrations are given in
Table 1.[12] The stability order for receptors 1 and 2, fruc-
tose>mannose>glucose, followed the “inherent stability
order” observed for all simple boronic acids.[13]
Fluorescence chemosensors for the selective detection of
Hg2+ ions are well known.[14] The mercury-desulfurization
reaction has also been used to selectively detect Hg2+ ions
by fluorescence[15] (Scheme 2). Therefore, we decided to in-
vestigate the “On-Off” switching behavior of receptor 2
with d-fructose and Hg2+ ions since it should produce opti-
cal responses towards added saccharides and Hg2+ ions. The
reaction of 2 with Hg2+ ions to produce 1 proceeded to
completion quickly and well within our set time interval
(30 minutes) for our measurements (See the Supporting In-
formation, Figure S4).
Abstract in Chinese:
Chem. Asian J. 2011, 6, 3054 – 3058
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T. D. James, C. Zhu et al.
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Figure 3. Fluorescence spectra of receptor 2 in the absence or presence of
Hg2+ ions or d-fructose. Concentration of 2 was 0.37 mm, [Hg2+]=
0.15 mm, [d-fructose]=0.83m. The solvent was methanol/H2O buffer
(52.1:47.9, 50 mm, pH 8.21) lex =430 nm: a) 2 alone, b) 2+d-fructose,
c) 2+Hg2+, and d) 2+Hg2++d-fructose. All spectra were recorded after
30 min.
buffer (Figure 3, Table 2). Whilst a medium enhancement of
2.94 is obtained for the addition of only Hg2+ ions and a
low enhancement of 1.44 is observed for the addition of d-
fructose alone. We also investigated the effect of other
metal ions: Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Mn2+, Ca2+
,
Cd2+, and Mg2+ with receptor 2. Of the metals investigated,
only Hg2+ ions produced a significant fluorescence enhance-
ment with receptor 2 at 478 nm (see the Supporting Infor-
mation, Figure S10).
Figure 2. a) and b): binding curves for 1 (lem =492 nm) and 2 (lem
=
478 nm) in the presence of increasing concentrations of d-fructose, d-glu-
cose, and d-mannose (lex =430 nm).
Table 2. Fluorescence truth table for receptor 2 with mercury and fruc-
tose as inputs.[a,b]
Table 1. Stability constant K (coefficient of determination, r2) for the
binding of receptors 1 and 2 with monosaccharides.
Input 1
(Hg2+
Input 2
(d-fructose)
Output
(normalized)
Saccharide
K 1 (r2)
K 2 (r2)
G
)
[molÀ1 dm3]
[molÀ1 dm3]
0
1
0
1
0
0
1
1
Low (1.00)
Medium (2.94)
Low (1.44)
d-fructose
d-glucose
d-mannose
478.0Æ14.5 (0.99)
26.9Æ1.2 (0.99)
28.9Æ1.1 (0.99)
587.4Æ89.9 (0.98)
18.6Æ1.8 (0.99)
36.1Æ7.9 (0.97)
High (5.21)
[a] Receptor 2 (0.37 mm), [Hg2+]=0.15 mm, [d-fructose]=0.83m emission
in methanol/H2O buffer (52.1:47.9, 50 mm, pH 8.21). lex =430 nm, and
data of 2 (lem =478 nm) were collected. AND logic requires that the
signal is 1 only when both inputs are 1, and 0 in all other cases. [b] Rela-
tive emission intensities were normalized to the emission intensity of 2
alone at 0.37 mm concentration (lem =478 nm).
Scheme 2. Hg2+-promoted formation of the 1,3,4-oxadiazole moiety.
Conclusions
This system can be compared to an AND gate that re-
ports a HIGH output (1) only when two inputs are simulta-
neously applied. In this paper, the optical output signals of
coumarin-based ditopic receptor 2 in response to saccharide
and metal-ion binding behaves in an AND logic manner.
The fluorescence of receptor 2 (3.7ꢁ10À7 mol dmÀ3) at
478 nm (Output) is significantly enhanced by 5.21 fold in
the presence of both Hg2+ ions and d-fructose in pH 8.21
In conclusion, receptor 2 is a ditopic fluorescent sensor for
d-fructose and Hg2+ ions. This system simultaneously recog-
nizes saccharides and the Hg2+ cation. We believe that our
system will guide the design of new ditopic sensors with
practical utility. We are currently working on developing
saccharide-selective receptors based on the principles out-
lined in this preliminary report.
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Chem. Asian J. 2011, 6, 3054 – 3058

Ditopic Fluorescent Sensor for Saccharides and Hg2+ Ions
(q, J=7.2 Hz, 4H), 6.61 (d, J=2.4 Hz, 1H), 6.80 (dd, J=2.4 9.0 Hz, 1H),
7.33–7.74 (m, 5H), 8.70 (s, 1H), 9.78 ppm (br, 2H); (75 MHz,
[D6]DMSO): dC =12.67, 25.02, 44.76, 84.06, 96.24, 107.97, 110.66, 132.16,
139.17, 148.43, 153.12, 157.75, 161.58 ppm; ESI-MS: m/z 537.2365
([M+H] +, C27H34B1N4O5S1 requires 537.2342).
Experimental Section
General
Infra-red spectra were recorded between 4000 cmÀ1 and 600 cmÀ1. Sam-
ples were either mixed with KBr in a mortar and pressed into a KBr
pellet (KBr), or evaporated to dryness on a NaCl plate (film). All vibra-
tions (n) are given in cmÀ1. Nuclear magnetic resonance spectra were run
Preparation of 1
A solution of 2 (0.27 g, 0.5 mmol) in toluene (5.0 mL) was stirred, and
DCC (0.12 g, 0.6 mmol) was added and the mixture was heated to reflux
for 10 h. After the solvent was evaporated under reduced pressure, the
crude product was purified by column chromatography on silica gel to
give 1 as a yellow solid (0.22 g, 90%); m.p. 249–2508C; IR: n˜ =3260,
1
in either [D]chloroform or [D6]dimethyl sulfoxide. H NMR spectra were
recorded at 300 MHz, 11B {1H} NMR spectra at 96 MHz, and 13C {1H}
NMR spectra at 75 MHz. Chemical shifts (d) are expressed in parts per
million and are reported relative to the residual solvent peak or to tetra-
1
methylsilane as an internal standard in H and 13C {1H} NMR spectra and
1720, 1618, 1589, 1537, 1514, 1356,1236, 1220, 1191, 1135 cmÀ1 1H NMR
;
boron trifluoride diethyl etherate as an external standard in 11B {1H}
NMR spectra. The multiplicities and general assignments of the spectro-
scopic data are denoted as: singlet (s), doublet (d), triplet (t), quartet (q),
quintet (quin), doublet of doublets (dd), doublet of triplets (dt), triplet of
triplets (tt), unresolved multiplet (m), broad (br), and aryl (Ar). Coupling
constants (J) are expressed in Hertz. Chemical shifts were assigned with
consideration of the multiplicities and chemical shifts (1H), 13C {1H} spec-
tra are given as the sign of their pendant spectrum. Data acquisition and
automated processing were controlled using Compass OpenAccess 1.3
software. The observed mass and isotope pattern perfectly matched the
corresponding theoretical values as calculated from the expected elemen-
tal formula. Quartz cuvettes with 10 mm path lengths and four faces pol-
ished. All pH measurements taken during fluorescence experiments were
on pH meter which was calibrated using standard buffer solutions. All
solvents used in fluorescence measurements were HPLC or fluorescent
grade and the water was deionized. All saccharides used in fluorescence
measurements were certified as ꢀ99% pure. The fluorescence studies
were carried out in an aqueous methanolic pH 8.21 buffer. The buffer
was prepared in a 1 L volumetric flask according to a procedure laid out
by Perrin and Dempsey and consisted of: 52.1 wt% HPLC grade metha-
nol, in deionized water with KCl (0.7456 g, 0.01000 moldmÀ3), KH2PO4
(300 MHz, [D6]DMSO): dH =1.12 (t, J=7.2 Hz, 6H), 1.29 (s, 12H), 3.46
(q, J=7.2 Hz, 4H), 6.56 (d, J=2.4 Hz, 1H), 6.76 (dd, J=2.4 9.0 Hz, 1H),
7.31–7.79 (m, 5H), 8.40 (s, 1H), 10.59 ppm (br, 1H); (75 MHz,
[D6]DMSO): dC =12.68, 25.05, 44.66, 84.08, 96.53, 103.37, 107.63, 110.16,
120.41, 123.05, 125.65, 131.29, 144.21, 152.39, 155.65, 157.34, 157.42,
160.00 ppm; ESI-MS: m/z 503.2444 ([M+H]+, C27H32B1N4O5 requires
503.2465).
Acknowledgements
We gratefully acknowledge the National Natural Science Foundation of
China (20832001, 20972065, 21074054) and the National Basic Research
Program of China (2007CB925103, 2010CB923303) for their financial
support. T.D.J. would also like to thank the Royal Society for the award
of an International Joint Project and the Catalysis and Sensing for our
Environment (CASE) consortium for networking opportunities.
(0.3745 g,
0.002752 moldmÀ3),
and
Na2HPO4
(0.3914 g,
0.002757 moldmÀ3).
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A solution of 4-(diethylamino)-2-hydroxybenzaldehyde (1.93 g, 0.01 mol)
and diethyl malonate (1.92 g, 0.012 mol) in glacial acetic acid was treated
with piperidine (0.1 mL) and reflux for 3 h. To the reaction mixture was
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Preparation of 3
Hydrazine monohydrate (1.4 mL, 28 mmol) was added to a solution of 4
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room temperature for 12 min, After cooling in an ice bath for 15 min, the
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(t, J=7.2 Hz, 6H), 3.46 (q, J=7.2 Hz, 4H), 6.50 (d, J=2.4 Hz, 1H), 6.65
(dd, J=2.4 9.0 Hz, 1H), 7.51 (d, J=9.0 Hz, 1H), 8.68 (s, 1H), 9.74 ppm
(s, 1H); TOF-MS: m/z 276.1 ([M+H]+ C15H18N3O3 requires 276.13).
Preparation of 2[14d,17]
3-isothiocyanophenyl boronic acid pinacol ester (0.26 g, 1 mmol) was
added to solution of 3 (0.275 g, 1 mmol) in acetonitrile (30 mL).The reac-
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1416,1372, 1351,1304, 1275,1259, 1231, 1187, 1136 cmÀ1
;
1H NMR
(300 MHz, [D6]DMSO): dH =1.12 (t, J=7.2 Hz, 6H), 1.26 (s, 12H), 3.46
Chem. Asian J. 2011, 6, 3054 – 3058
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Received: July 21, 2011
Revised: August 18, 2011
Published online: September 26, 2011
3058
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Chem. Asian J. 2011, 6, 3054 – 3058