Reversible "off-on" fluorescent probe for anions based on a facile two-component ensemble

Article abstract of DOI:10.1016/j.dyepig.2013.01.004

A specific fluorescent switch for anions was successfully constructed. The novel sensing switch is a two-component ensemble, which was combined using a water-soluble conjugated polyelectrolyte and a boronic acid functionalized pyridine salt. In the ensemble, the polyelectrolyte is used as a fluorescent signal unit, and the pyridine boronic acid acts as receptor and quencher. The two-component ensemble shows a fluorescence reversible "off-on" response toward cyanide and phosphate anions based on different binding ways. Furthermore, the facile design and construction of this ensemble presents a novel opportunity for obtaining an efficient and practical probe.

Full text of DOI:10.1016/j.dyepig.2013.01.004

Dyes and Pigments 97 (2013) 318e323
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Reversible “offeon” fluorescent probe for anions based
on a facile two-component ensemble
,
a,
Xiaoju Wang a, Liheng Feng b , Liwei Zhang
*
*
a Institute of Molecular Science, Chemical Biology and Molecular Engineering, Laboratory of Education Ministry, Shanxi University, Taiyuan 030006, PR China
b School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A specific fluorescent switch for anions was successfully constructed. The novel sensing switch is
a two-component ensemble, which was combined using a water-soluble conjugated polyelectrolyte and
a boronic acid functionalized pyridine salt. In the ensemble, the polyelectrolyte is used as a fluorescent
signal unit, and the pyridine boronic acid acts as receptor and quencher. The two-component ensemble
shows a fluorescence reversible “offeon” response toward cyanide and phosphate anions based on
different binding ways. Furthermore, the facile design and construction of this ensemble presents a novel
opportunity for obtaining an efficient and practical probe.
Received 25 October 2012
Received in revised form
1 January 2013
Accepted 3 January 2013
Available online 17 January 2013
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Polyelectrolyte
Two-component ensemble
Fluorescent probe
Anions
Switch
Quencher
1. Introduction
cyanide and phosphate ions) based on fluorescence spectroscopy
were few and had some drawbacks such as poor water solubility and
Optical molecular probes for anions have attracted extensive
attention because of their important roles in a wide range of
environmental, clinical, chemical and biological applications. Many
research results describe well-designed probes which display high
sensitivity and selectivity to analytes [1e6]. Among various analy-
tes, anions are of concern because they play important roles in
biological processes but are also environmental pollutants [7e10].
Therefore, the development of a multi-functional probe for anion
sensing is desired by chemists and biologists.
To date, molecular probes for anions based on fluorescence
detection are most promising because they offer advantages of high
sensitivity, real-time analysis multiple sensing modes [11,12]. Most
of the fluorescence sensors focus on one-component design that
contains both fluorophore and receptor groups in the same molecule
[13e17]. Combining an analyte with a receptor unit, the light
properties of the fluorophore are changes and further realize rec-
ognition of the analyte. Despite many highly effective one-
component probes have been reported for anions over the past
decade [7e9,18,19], practical probes for anions (especially for
difficult structure modification etc. Therefore, a new mode of probe
design to advance this field is in great demand. Here, we describe
a new strategy and establish a specific reversible fluorescence
ensemble for anions. The probe was a two-component ensemble
comprised of a cationic pyridine salt quencher and an anionic con-
jugated polyelectrolyte (ACP). For the ensemble, the ACP is a signal
reporting group and its fluorescence is modulated by the interaction
between ACP and the quencher. The cationic pyridine salt is both
a quencher and a receptor in the ensemble. It is reasonable believed
that the two-component ensemble has many advantages [20e24].
For example, the chemical structure of the quencher can be modified
to obtain a highly selective and sensitive probe without having to
modify the structure of reporting fluorophore. This sensing
approach allows considerable flexibility in choosing the quencher/
receptor and fluorophore components depending on the particular
requirements of the sensing application.
In this paper, the two-component ensemble is composed of an
anionic water-soluble polyelectrolyte (PBPYRSO3Na) and a pyridine
salt quencher (m-TBPB) with boronic acid (Scheme 1). The anionic
conjugated polyelectrolyte PBPYRSO3Na is a reporting group Cati-
onic m-TBPB is both a quencher and a receptor in the ensemble. It
should be noted that this sensing ensemble has the following
unique features: (1) conjugated polyelectrolytes (CP) can offer
* Corresponding authors. Tel.: þ86 351 7011492; fax: þ86 351 7011688.
E-mail address: lhfeng@sxu.edu.cn (L. Feng).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.004

X. Wang et al. / Dyes and Pigments 97 (2013) 318e323
319
Br
Br
OH
OH
OH
NaO3S
O
Br
Br2
NaOH / H2O
CH3COOH
O
SO3Na
Br
O
O
OH
SO2
/
O
Monomer 1
O
O
O
O
B
B
Br
B
O
O
O
O
B
Pd(dppf)Cl2
KOAc / DMSO
Br
Monomer 2
SO3Na
SO3Na
O
O
B
O
O
Pd(PPh3)4 / N2
DMF/H2O
Br
+
Br
O
O
B
O
O
NaO3S
NaO3S
PBPYRSO3Na
Monomer 1
Monomer 2
HO
OH
B
N
N
HO
N
OH
B
Br
N
OH
B
OH
Br
Br
Br
Br
DMF
Br
OH
B
OH
m-TBPB
Scheme 1. The synthetic route of polyelectrolyte and m-TBPB.
particular properties compared with the normal “neutral” conju-
gated polyelectrolyte due to their ionic side groups attached to the
conjugated main chain. Additionally, the ACP is typically soluble in
water which is an environment-friendly solvent [25e27]; (2)
a boronic acid functionalized pyridine salt with strong quenching
and bonding abilities is used to connect anions; (3) forming
a neutral multidimensional complex can provide a specific micro-
environment with which to complex anions.
Hydroquinone, 1,3-propanesultone, 1,6-dibromopyrene, Pd(PPh3)4,
Pd(dppf)Cl2 and bis-(pinacolato)diboron were purchased from
Aldrich (Steinheim, Germany).1,3,5-Tris(bromomethyl)benzene and
3-boronic acid-pyridine were obtained form Creasyn Finechem
(Tian Jing, China). 1H NMR and 13C NMR were measured on a Bruker
ARX400 spectrometer with chemical shifts reported as ppm (TMS as
an internal standard). 11B NMR spectra were recorded on a Bruker
ARX400 were reported in ppm with respect to BF3$OEt2
(d
¼ 0).
Elemental analyses were performed on a Vario EL elemental analysis
instrument (Elementar Co.). High-resolution mass spectra (HRMS)
were acquired on an Agilent 6510 Q-TOF LC/MS instrument (Agilent
Technologies, Palo Alto, CA) equipped with an electrospray ioniza-
tion (ESI) source. Fluorescence spectra were acquired with a Varian
Cary Eclipse fluorescence spectrophotometer, the excitation and
emission slit widths were both 5 nm. The excitation wavelength was
set at 367 nm according to experimental requirements. All of the
experiments were performed at room temperature.
2. Experimental
2.1. Materials and instruments
Unless otherwise stated, all chemical reagents were obtained
from commercial suppliers and used without further purification.
Solvents used were purified and dried by standard methods prior to
use. All experiments of water were pure water of selling market. p-

320
X. Wang et al. / Dyes and Pigments 97 (2013) 318e323
2.2. The solution preparation and fluorescent measurement of the
ensemble to anions
dioxaborolane)pyrene, dried DMF (60 mL) and 0.224 g (0.18 mmol)
Pd(PPh3)4 under nitrogen. The mixture was stirred under nitrogen
for 30 min, and then 20 mL aqueous solution with 3.816 g
(36.0 mmol) sodium carbonate was generally added by dropping
funnel. The reactionwas heated at 80 ꢂC for 48 h. The reaction turned
black as Pd(0) particles were liberated. The tan-violet filtrate was
collected, precipitated into 1.0 L of acetone, and redissolved in
deionized water. The polymer was dialyzed using a dialysis mem-
brane with a 3.5 kDa molecular weight cutoff for 3 days. The final
product, a violet dark powder, was obtained after dried in vacuo at
The stock solution (1.20 mg/mL) of PBPYRSO3Na was diluted in
1.0 L measuring flask with pure water to afford the working solu-
tion (6.0 ꢀ 10ꢁ3 mg/mL). The stock solution (0.004 M) of m-TBPB
was prepared by 0.29 g m-TBPB in 100 mL measuring flask. The
stock solutions of PO34ꢁ and CNꢁ were 0.1 M in 10 mL measuring
flask, respectively. The standard stock solutions of low concentra-
tions were prepared by suitable dilution of the stock solution with
pure water. The working salts were all sodium salts. All spectra
analysis studies were carried out at pure water solution and the
working solutions were placed in a quartz cuvette with 1 cm path.
The total volume of working solutions was 2 mL. The studies of
fluorescence measurements were used titration experiments and
the volume added did not exceed 3% of the total. After the mixture
solution was shaken for 30 s, the new spectra were measured.
110 ꢂC for24h. (1.53 g, 42.8%).1H NMR (d6-DMSO, 400 MHz)
8.102 (broad, 8H), 7.376 (s, 2H), 4.091 (broad, 4H), 2.285 (broad, 4H),
1.721 (broad, 4H); 13C NMR (d6-DMSO, 100 MHz)
25.73, 48.30,
d 8.434e
d
68.34, 99.99, 117.55, 124.65, 125.07, 126.33, 127.82, 129.01, 129.34,
130.52, 134.66, 150.41; IR (KBr) cmꢁ1: 3012, 2911, 2836, 1628, 1600,
1536, 1441, 1343, 1231, 1254, 1022, 1025, 936 845, 728, 635.
2.3.4. 1,3,5-tris[(30-boronic acid-10-methylene) pyridine]benzene
trisbromide (m-TBPB)
2.3. Synthesis of polyelectrolyte PBPYRSO3Na and quencher m-TBPB
(Scheme 1) [28e31]
To a solution of 1.794 g (5.0 mmol) 1,3,5-tris(bromomethyl)
benzene in 50 mL DMF was added 2.029 g (16.5 mmol) 3-boronic
acid-pyridine, and the reaction mixture was stirred at 70 ꢂC for
72 h under nitrogen. The orange precipitate was collected by fil-
tration, washed with DMF, acetone, then ether and dried under
a stream of nitrogen to yield 1,3,5-tris[(30-boronic acid-10-methyl-
ene) pyridine]benzene trisbromide (m-TBPB) (2.87 g, 75.3%). 1H
2.3.1. 1,4-dibromo-2,5-bis(30-sulfonatopropoxy)benzene (monomer
1)
A solution of 6.35 g (20.0 mmol) 2,5-dibromobenzene-1,4-diol,
2.0 g (50.0 mmol) sodium hydroxide and water (200 mL) in
a Erlenmeyer flask was stirred under nitrogen. Then, a solution of
6.1 g (50.0 mmol) 1,3-propanesultone in 40 mL dioxane was added
to the former solution at once. The resulting mixture was then
stirred at room temperature overnight, during which time a thick
pink slurry formed. The reaction mixture was then stirred at 80e
100 ꢂC for another 30 min and then cooled in a water/ice bath.
The suspension obtained was vacuum filtered, and the retained
solid was washed with cold water followed by acetone. The crude
products were purified by recrystallization twice times from water
to yield product as white powder (7.61 g, yield 68.4%). 1H NMR
NMR (CD3OD, 400 MHz)
d
9.019 (d, J ¼ 18.0 Hz, 6H), 8.717 (s, 3H),
8.019 (dd, J ¼ 8.0 Hz, 14.8 Hz, 3H), 7.804 (s, 3H), 5.962 (s, 6H); 13C
NMR (CD3OD, 100 MHz)
d 62.75, 125.91, 127.81, 130.67, 135.72,
142.59, 143.82, 146.40, 147.94, 149.51; 11B NMR (80 MHz, CD3OD)
18.58. Element Analysis for C24H27B3Br3N3O6 (Mol. Wt.: 725.63)
d
calcd.: C, 39.72; H, 3.75; found: C, 39.42; H, 3.81.
3. Results and discussion
(D2O, 400 MHz)
d
7.16 (s, 2H), 4.03 (dd, J ¼ 6.0, 6.0 Hz, 4H), 3.05 (dd,
3.1. The interaction of the ensemble with cyanide anion
J ¼ 4.4, 5.6 Hz, 4H), 2.21 (m, 4H); 13C NMR (D2O, 100 MHz)
d 24.24,
47.91, 68.97, 111.03, 119.19, 149.42; IR (KBr) cmꢁ1: 2965, 2922, 2871,
1614, 1479, 1444, 1417, 1392, 1353, 1262, 1144, 1051, 1032, 937, 835,
736, 634, 576; Element Analysis for C12H14Br2Na2O8S2 (Mol. Wt.:
556.15) calcd.: C 25.92; H 2.54; found: C 25.89; H 2.57; HRMSeESI
for C12H14Br2Na2O8S2 (m/z) 533 [Mꢁ23].
It is well known that cyanide anion has high nucleophilicity and
can form a stable complex with boronic acid. The introduction of
cyanide anion to a two-component ensemble resulted in a new
intramolecular neutral complex between cyanide anion and the
boronic acids of m-TBPB (Figs. 1 and 4). At the same time the
ground-state intermolecular complex of m-TBPB and PBPYRSO3Na
was destroyed or weakened to some extent, which led to the flu-
orescence recovery of PBPYRSO3Na (Fig. 1). As seen in Fig. 1, a high
signal response of the probe to cyanide anion was observed. The
interaction between the cyanide anion and the boronic acid group
consisted of nucleophilic addition and nucleophilic substitution.
The ability of boronic acid to complex cyanide ion may change from
being electron deficient (R-B(OH)2) in the absence of cyanide to
being electron rich (R-B-(CN)3) upon cyanide complexation.
Therefore, a complex (1:9) can be formed between m-TBPB and the
cyanide ions. The expected bonding proportion had been verified
by the titration of cyanide ions with the ensemble (PBPYRSO3Na:
6.0 ꢀ 10ꢁ3 g/mL, m-TBPB: 1.0 ꢀ 10ꢁ5 mol/L, Fig. S1). In addition, the
detection limit of cyanide anion was depended on the ratio of the
ensemble components.
2.3.2. 1,6-bis(40,40,50,50-tetramethyl-10,30,20-dioxaborolane)pyrene
(monomer 2)
A mixture of 3.6 g (10.0 mmol) 1,6-dibromopyrene, 7.62 g
(30.0 mmol) bis(pinacolato)diboron, 0.58 g (8 mol%) Pd(dppf)Cl2
and 5.88 g (60.0 mol) potassium acetate in DMSO (70 mL) was
stirred at 80 ꢂC for 24 h under a nitrogen atmosphere. The reaction
mixture was cooled to room temperature, poured into the 500 mL
ice water, filtrated and then purified by column chromatography on
silica gel with dichloromethane/petroleum ether (1/4) as the eluant
to afford a light green power (3.98 g, 87.6%). 1H NMR (CDCl3,
400 MHz)
d
9.155 (d, J ¼ 8.8 Hz, 2H), 8.572 (d, J ¼ 7.6 Hz, 2H), 8.229
(d, J ¼ 7.6 Hz, 2H), 8.169 (d, J ¼ 9.2 Hz, 2H), 1.517 (s, 24H); 13C NMR
(CDCl3, 100 MHz)
d 25.08, 83.91, 124.46, 127.90, 129.13, 133.06,
133.69, 136.42; IR (KBr) cmꢁ1: 3002, 2968, 2819, 1654, 1602, 1546,
1484,1391,1385,1205,1160,1058,1052, 933, 834, 728, 684; Element
Analysis for C28H32B2O4 (Mol. Wt.: 454.17) calcd.: C 74.05, H 7.10,
found: C 74.38, H 7.09; HRMSeESI for C28H32B2O4 (m/z): 454 [Mþ].
3.2. The interaction of the ensemble with phosphate anion
To our surprise, while introducing phosphate anion to the
sensing ensemble, the similar reversible fluorescence “offeon”
change was observed (Fig. S2 and S3). Due to the weak nucleo-
philicity and highly delocalized charges of phosphate anion, the
mode of interaction between the sensing ensemble and the
2.3.3. Polymer PBPYRSO3Na
To 100mLthree-neckflask, equipped withmechanicalstirrerwas
added 3.336 g (6.0 mmol) 1,4-dibromo-2,5-bis(3-sulfonatopropoxy)
benzene, 2.724 g (6.0 mmol) 1,6-bis(40,40,50,50- tetramethyl-10,30,20-

X. Wang et al. / Dyes and Pigments 97 (2013) 318e323
321
1000
800
600
400
200
0
7
[m-TBPB]=0.5*10-5 mol/L
[m-TBPB]=1.0*10-5 mol/L
[m-TBPB]=2.0*10-5 mol/L
6
5
4
3
2
1
0
Introduction CN- to
sensing ensemble
Formation of
sensing ensemble
400
450
500
550
600
650
0
1
2
3
4
5
6
7
8
9
10 11
Wavelength / nm
[CN-], 10-5 mol/L
Fig. 1. Characteristic fluorescence response upon introduction of the quencher m-TBPB
followed by CNꢁ to a PBPYRSO3Na solution (6.0 ꢀ 10ꢁ3 mg/mL). The concentration of
m-TBPB was 1.0 ꢀ 10ꢁ5 mol/L and the final CNꢁ concentration was 1.5 ꢀ 10ꢁ4 mol/L.
The red line indicates unquenched fluorescence and the blue line indicates fluo-
rescence after introduction of the quencher m-TBPB. The dark line indicates fluo-
rescence after the introduction of CNꢁ to the ensemble. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this
article.)
Fig. 2. The response to different components ratios of sensing ensemble with the
addition of CNꢁ. The concentration of PBPYRSO3Na was 6.0 ꢀ 10ꢁ3 mg/mL.
3.4. The interactions of the ensemble with other anions
The selectivity of the sensing ensemble for typical nine anions
was determined by titration experiments (Fig. 3). As shown in Fig. 3,
considerable fluorescence enhancements were observed only in the
presence of cyanide and phosphate anions. The enhancement of the
relative fluorescence intensities were 10 times for the cyanide ion
and 6 times for the phosphate ion compared with that of the other
anions. Therefore, the ensemble containing BPYRSO3Na and m-TBPB
was a dual-functional fluorescence ensemble for cyanide and
phosphate anions. This may be understandable by considering that
cyanide has a high nucleophilic ability and that phosphate has
a negative charge delocalized property.
phosphate anion should be not different to that of the cyanide
anion. Given charge delocalized property of the phosphate anion,
another probable bonding way for the ensemble and phosphate
anions may be proposed. There may be a competition action of
phosphate anion and polyelectrolyte sulfonate anions to m-TBPB.
This competition would lead to a part decomposition of the
ground-state complex between PBPYRSO3Na and m-TBPB. The
interaction of hydrogen and phosphate anion was another key
point to form new complex. Based on the two actions of phosphate
anions with m-TBPB, the obvious fluorescent recovery of PBPYR-
SO3Na was observed. In order to give the exactly bonding modes of
the ensemble with cyanide or phosphate anions, many other
quenchers have been designed and these synthesis works are un-
derway. Here, the bonding modes only were speculation analysis.
Other useful results will be reported in the future.
3.5. The color changes of ensemble solution
The reversible “offeon” fluorescence state of the sensing
ensemble toward cyanide and phosphate anions can be observed
3.3. The ratio choice of PBPYRSO3Na and m-TBPB
SO4(II)
PO4(III)
CH3COO(I)
NO3(I)
CN(I)
For the two-component ensemble, another considerable benefit
is the ability to vary the ratio of quencher/reporting group to opti-
mize the magnitude of the sensing response. We took a series of tests
for the cyanide ion sensing response at different quencher/reporting
group ratios (Fig. 2). The experimental results indicated that the high
sensitivity response of the sensing ensemble to the anions can be
realized by varying the ratio of the quencher to the reporting group.
The response sensitivity and the detection limit of the two-
component ensemble for the cyanide or phosphate anions would
change with an increase in the m-TBPB/PBPYRSO3Na ratio. The rel-
ative intensities of fluorescence in the presence of 1.0 ꢀ 10ꢁ3 mol/L
cyanide or phosphate ions were 20e30 times higher than that in the
absence of cyanide or phosphate ions. However, some adverse
interference factors such as the instability of the baseline, bad
repeatability and trace impurity interference emerged in higher
quencher/reporting group ratios. Therefore, based on the titration
experiments, the optimal components of the sensing ensemble for
cyanide and phosphate anions were 6.0 ꢀ 10ꢁ3 g/mL PBPYRSO3Na
and 1.0 ꢀ 10ꢁ5 mol/L m-TBPB.
I(I)
Br(I)
Cl(I)
F(I)
0
4
8
12
16
20
Relative intensities (F/F0)
Fig. 3. Variation of the fluorescence relative intensity at 476 nm for the sensing
ensemble (PBPYRSO3Na: 6.0 ꢀ 10ꢁ3 mg/mL, m-TBPB: 2.0 ꢀ 10ꢁ5 mol/L) after the
addition of 5.0 ꢀ 10ꢁ4 mol/L SO42ꢁ/PO34ꢁ/NOꢁ3 /CNꢁ/CH3COOꢁ/Iꢁ/Brꢁ/Clꢁ/Fꢁ, respec-
tively, in pure water.

322
X. Wang et al. / Dyes and Pigments 97 (2013) 318e323
Fig. 4. The conjectural bonding ways of the ensemble with anions and color change for the PBPRESO3Na (6 ꢀ 10ꢁ3 mg/mL) solution by the introduction of m-TBPB (2.0 ꢀ 10ꢁ5 mol/L)
followed by anions (5.0 ꢀ 10ꢁ4 mol/L). These solutions were irradiated by l365 nm UVeVis light. 1, only PBPRESO3Na; 2, BPRESO3Na þ m-TBPB; 3, 2 þ Clꢁ; 4, 2 þ Brꢁ; 5, 2 þ PO34ꢁ
;
6, 2 þ Fꢁ; 7, 2 þ SO24ꢁ; 8, 2 þ CH3COOꢁ; 9, 2 þ NOꢁ3 ; 10, 2 þ CNꢁ; 11, 2 þ Iꢁ. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of
this article.)
visually under l365 nm UVeVis light (Fig. 4). The original sky-blue
solution of PBPYRSO3Na turned into a dark solution in the pres-
ence of m-TBPB revealing ground-state complex formation (low-
fluorescence). By adding these anions into the dark solution, the
solution containing only cyanide or phosphate anions changes into
a light blue solution, which demonstrated that a new complex had
been formed and the ground-state complex had largely dissociated
associated with this article can be found in the online version, at
http://dx.doi.org/10.1016/j.dyepig.2013.01.004.
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