Water-soluble poly(p-phenylene) incorporating methoxyphenol units: Highly sensitive and selective chemodosimeters for hypochlorite

Article abstract of DOI:10.1016/j.polymer.2012.03.063

A series of water-soluble poly(p-phenylene)s (PPPs), named N-PPPx (x = 10, 25 and 50), were directly synthesized by Suzuki coupling in aqueous solution. The structures of the polymers were characterized by ¹H NMR and elemental analysis. The polymers exhibit similar absorption and emission spectra with three absorption maxima at ca. 205, 290 and 350 nm, and emission maximum at 420 nm in phosphate buffer saline (PBS) solution. Upon addition of hypochlorite, N-PPPx shows a decrease of absorption band at ca. 350 nm and a fluorescent quenching. Compared to their model compound PMOPP, N-PPPx shows a significantly amplified fluorescent quenching. Moreover, the K_sv is decreased with the increasing content of methoxyphenol moieties in N-PPPx. In view of the sensitivity and selectivity, N-PPP10 and N-PPP25 are very promising polymeric fluorescent probes to hypochlorite under the aqueous condition.

Full text of DOI:10.1016/j.polymer.2012.03.063

Polymer 53 (2012) 2356e2360
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Water-soluble poly(p-phenylene) incorporating methoxyphenol units: Highly
sensitive and selective chemodosimeters for hypochlorite
*
Wenjun Zhang, Chen’ge Li, Jingui Qin, Chuluo Yang
Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of water-soluble poly(p-phenylene)s (PPPs), named N-PPPx (x ¼ 10, 25 and 50), were directly
synthesized by Suzuki coupling in aqueous solution. The structures of the polymers were characterized
by 1H NMR and elemental analysis. The polymers exhibit similar absorption and emission spectra with
three absorption maxima at ca. 205, 290 and 350 nm, and emission maximum at 420 nm in phosphate
buffer saline (PBS) solution. Upon addition of hypochlorite, N-PPPx shows a decrease of absorption band
at ca. 350 nm and a fluorescent quenching. Compared to their model compound PMOPP, N-PPPx shows
a significantly amplified fluorescent quenching. Moreover, the Ksv is decreased with the increasing
content of methoxyphenol moieties in N-PPPx. In view of the sensitivity and selectivity, N-PPP10 and
N-PPP25 are very promising polymeric fluorescent probes to hypochlorite under the aqueous condition.
Ó 2012 Published by Elsevier Ltd.
Received 5 December 2011
Received in revised form
28 March 2012
Accepted 29 March 2012
Available online 5 April 2012
Keywords:
Water-soluble conjugated polymer
Hypochlorous acid
Chemodosimeters
1. Introduction
understood, because of the lack of a feasible method for detecting
hypochlorous acid.
Water-soluble conjugated polymers (CPs) can combine the
advantages of conventional polyelectrolytes and signal amplifica-
tion of CPs, and thus offer unique potential in fluorescent probes
[1e4]. Over the past several years, water-soluble CPs have been
widely used in biosensors [5e10]. Recently, chemodosimeters
have attracted considerable attention, however a few of chemo-
dosimeters based on water-soluble conjugated polymers have
been reported [11e15]. For example, Swager’s group reported
a poly(p-phenyleneethynylene) based chemodosimeter for fluoride
ions by using fluoride triggered Si-O bond cleavage [11]; Wang’s
group reported awater-soluble cationic polyfluorenewith boronate-
protected fluorescein covalently linking to the polymer backbone for
the detection of hydrogen peroxide and glucose in serum [14].
As a kind of reactive oxygen species (ROS), hypochlorous acid
(HClO) plays a crucial role in our daily lives and host defense against
invading pathogens [16,17]. In living organisms, hypochlorous acid
is generated by the reaction of hydrogen peroxide with chloride
ions under the catalysis of the heme enzyme myeloperoxidase
(MPO), which is synthesized and secreted by activated phagocytes
[18e21]. However, because of its high reactivity and non-specificity
[22], excessive hypochlorous acid can lead to damage of host tissue
that is implicated in a wide range of human diseases, such as kidney
disease [23,24], atherosclerosis [25e28], and arthritis [29e31].
Unfortunately the detailed pathogenic mechanism is not fully
Recently, a few probes for hypochlorous acid (HClO) have been
reported based on its strong oxidation property [32e39]. For
example, Nagano’s group [32] and Libby’s group [33] reported
rhodamine- and fluorescein-based fluorescent probes for HClO,
respectively, but the fluorescent probes involved in complicated
synthesis; Ma’s group also developed two fluorescent probes for
HClO [34,35], which works in organic co-solvent system. Moreover,
these probes are based on small molecule fluorophores, the
sensitivity is still needed to be improved. Considering the signal
amplification of conjugated polymers, it will be of significant
interest to design a fluorescent polymeric probe for HClO.
Very recently, we have reported a dual-signaling chemo-
dosimeter for HClO based on water-soluble p-methoxyphenol
derivative probe PMOPP [40]. Based on this research, we designed
and synthesized a series of water-soluble CPs by introducing
p-methoxyphenol moieties into the poly(p-phenylene) backbone.
We anticipated that the oxidation of p-methoxyphenol moieties on
the copolymer backbone by HClO would induce an intramolecular
charge transfer (ICT), and result in the changes of absorption and
fluorescence quenching of the copolymer.
2. Experimental part
2.1. Materials
All chemicals were purchased from commercial sources
* Corresponding author.
E-mail address: clyang@whu.edu.cn (C. Yang).
and were used without further purification unless otherwise noted.
0032-3861/$ e see front matter Ó 2012 Published by Elsevier Ltd.
doi:10.1016/j.polymer.2012.03.063

W. Zhang et al. / Polymer 53 (2012) 2356e2360
2357
Scheme 1. Synthetic route of the polymers and structure of model compound PMOPP.
2,5-Dibromo-4-methoxyphenol was synthesized according to
literature procedures [41]. Dimethylformamide (DMF) was purified
2.4. Synthesis of 6,60-(2,5-dibromo-1,4-phenylene)bis(oxy)
bis(N,N,N-trimethylhexan-1-aminium) bromide
by distillation from calcium hydride.
To a solution of 1,4-dibromo-2,5-bis(6-bromohexyloxy)benzene
(5.94 g, 10 mmol) in tetrahydrofuran (THF) (50 mL) was added tri-
methylamine aqueous solution (20 mL, 200 mmol). After stirring for
0.5 h, H2O (10 mL) was added to the mixture. Then additional tri-
methylamine aqueous (20 mL, 200 mmol) solution was added and
the mixture was stirred for 48 h. The solvent was removed by rotary
evaporation, and the crude product was recrystallized from ethanol
to afford the product as a white solid. Yield: 6.62 g, 93%. 1H NMR
2.2. General methods
1H NMR and 13C NMR spectra were obtained in the indicated
solvent with tertramethylsilane as an internal standard on
a MECUYRVX300 spectrometer. Mass spectra were measured on
a ZAB 3F-HF mass spectrophotometer. Liquid chromatography mass
(LC-Mass) spectra were measured on a Waters ZQ-Mass electro-
spray ionizer (ESI). Elemental analyses of carbon, hydrogen, and
nitrogen were performed on a Carlorerba-1106 microanalyzer.
Thermogravimetric analysis (TGA) was performed under nitrogen
with a NETZSCH STA 449C instrument in the temperature range
from 25 to 800 ꢀC at the heating rate of 10 ꢀC minꢁ1. UVevis
absorption measurements were performed on a Shimadzu 2550
spectrophotometer. Fluorescence measurements were performed
on a Hitachi F-4500 fluorescence spectrophotometer. All the tests of
optical response for ClOꢁ were carried out after a few seconds of the
addition of hypochlorite solution. Testing solutions were prepared
(300 MHz, D2O, ppm):
d
7.26 (s, 2H), 3.96 (t, J ¼ 6.3 Hz, 4H), 3.16 (t,
J ¼ 8.4 Hz, 4H), 2.94 (s, 18H), 1.66 (m, 8H), 1.41 (m, 4H), 1.26 (m, 4H).
13C NMR (75 MHz, D2O, ppm):
d 151.3, 118.9, 112.2, 70.3, 66.6, 53.8,
34.5, 33.0, 29.3, 28.4, 25.5. ESI-MS: m/z 275 ((M2þ-2Br)/2). Anal.
Calcd for C24H44Br4N2O2: C, 40.47; H, 6.23; N, 3.93. Found: C, 40.43;
H, 5.74; N, 3.62.
2.5. Synthesis of N-PPP10, N-PPP25 and N-PPP50
General procedure for the copolymerization by Suzuki cross-
coupling reaction: To
a
mixture of 6,60-(2,5-dibromo-1,4-
by adding 2 or 20 mL of the probe stock solution (5 mM) to 2 mL PBS
phenylene)bis(oxy)bis(N,N,N-trimethylhexan-1-aminium)
buffer in a test tube and then adding an appropriate aliquot of each
ROS stock solution. The absorption and fluorescence sensing of ROS
was run immediately after the ROS were added.
2.3. Synthesis of 1,4-dibromo-2,5-bis(6-bromohexyloxy)benzene
A mixture of 2,5-dibromobenzene-1,4-diol (5.36 g, 20 mmol),
1,6-dibromohexane (12.4 mL, 80 mmol), potassium carbonate
(10.00 g, 72 mmol), acetone (10 mL), and water (2.5 mL) was
refluxed for 5 h, after which an additional potassium carbonate was
added and stirring and reflux continued for 3 h. After filtration, the
residue was washed with chloroform and the filtrate was
combined. After evaporation of the solvent, excessive 1,6-
dibromohexane was removed under reduce pressure, and then
the residue was recrystallized from ethanol to afford 1,4-dibromo-
2,5-bis(6-bromohexyloxy)benzene as a white solid. Yield: 7.38 g,
62%. 1H NMR (300 MHz, CDCl3, ppm):
J ¼ 6.3 Hz, 4H), 3.43 (t, J ¼ 6.9 Hz, 4H),1.90 (m, 4H),1.80 (m, 4H),1.53
d 7.08 (s, 2H), 3.96 (t,
(m, 8H). 13C NMR (75 MHz, CDCl3, ppm):
d 150.3, 118.7, 111.4, 70.2,
Fig. 1. Normalized UVevis absorption and fluorescence spectra of the polymers and
the model compound PMOPP in PBS solution. Excitation wavelength: N-PPPx, 350 nm;
PMOPP, 320 nm.
34.1, 32.9, 29.2, 28.1, 25.4. EI-MS: m/z 590 (Mþ). Anal. Calcd for
C18H26Br4O2: C, 36.40; H, 4.41. Found: C, 36.09; H, 4.03.

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W. Zhang et al. / Polymer 53 (2012) 2356e2360
N-PPP25: Yield: 440 mg, 42%. 1H NMR (300 MHz, CD3OD, ppm):
8.0 (br), 7.7e7.4 (br, 4H), 7.2e7.1 (br, 2H), 4.00 (br, 4H), 3.7 (br),
Table 1
Absorption, emission and Stern-Volmer Constants (Ksv) of the N-PPPx and the model
compound PMOPP for hypochlorite. Absorption and emission spectra were tested in
PBS solution.
d
3.24 (br, 4H), 3.04 (br, 18H), 1.73 (br, 4H), 1.49 (br, 4H), 1.36 (br, 4H).
Anal. Calcd for (C30H48Br2N2O2)0.75(C13H10O2)0.25: C, 59.37; H, 7.45;
N, 4.03. Found: C, 61.80; H, 7.94; N, 2.75.
Compound
lmax, abs (nm)
lmax, em (nm)
Ksv (Mꢁ1
)
N-PPP10
N-PPP25
N-PPP50
PMOPP
208, 289, 347
208, 291, 347
204, 290, 348
<200, 267, 314
420
420
420
388
1.35 ꢂ 106
1.18 ꢂ 106
1.40 ꢂ 105
1.00 ꢂ 105
N-PPP50: Yield: 434 mg, 52%. 1H NMR (300 MHz, CD3OD, ppm):
d
8.0 (br), 7.7e7.4 (br, 4H), 7.2e7.1 (br, 2H), 3.99 (br, 4H), 3.7 (br),
3.21 (br, 4H), 3.03 (br, 18H), 1.71 (br, 4H), 1.48 (br, 4H), 1.33 (br, 4H).
Anal. Calcd for (C30H48Br2N2O2)0.50(C13H10O2)0.50: C, 62.47; H, 7.07;
N, 3.39. Found: C, 64.46; H, 7.38; N, 2.38.
bromide, 2,5-dibromo-4-methoxyphenol, 1,4-phenylenediboronic
acid, and 3.0 mol% Pd(dppf)Cl2 was added a degassed mixture of
DMF and 0.2 M sodium carbonate aqueous solution (2:5 in volume).
The mixture was vigorously stirred at 90 ꢀC for 72 h. After the
mixture was cooled down, the solvent was removed under reduce
pressure. The crude product was purified by dialysis against Mill-Q
water using a 3.5 kDa molecular weight cutoff dialysis membrane
for 5 days. After evaporation of the solvent, the resulting polymer
was obtained as deep brown powder.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic route of thewater-soluble CPs is shown in Scheme 1.
The cationic p-dibromobenzene derivative was synthesized from
their corresponding neutral compounds by quaternization with tri-
methylamine aqueous solution. The water-soluble CPs were directly
prepared through Suzuki coupling reaction of the cationic p-dibro-
mobenzene derivative, 2,5-dibromo-4-methoxyphenol, and 1,4-
phenylenediboronic acid. The feed ratios of 2,5-dibromo-4-
methoxyphenol were 10, 25 and 50 mol%, and the corresponding
polymers were named N-PPP10, N-PPP25 and N-PPP50, respectively.
N-PPP10: Yield: 550 mg, 47%. 1H NMR (300 MHz, CD3OD, ppm):
d
8.0 (br), 7.7e7.4 (br, 4H), 7.2e7.1 (br, 2H), 4.04 (br, 4H), 3.7 (br),
3.30 (br, 4H), 3.08 (br, 18H), 1.77 (br, 4H), 1.54 (br, 4H), 1.39 (br, 4H).
Anal. Calcd for (C30H48Br2N2O2)0.9 (C13H10O2)0.1: C, 58.05; H, 7.61;
N, 4.31. Found: C, 59.79; H, 8.71; N, 2.72.
Fig. 2. (a) UVevis absorption ([N-PPP10]
([N-PPP10] ¼ 5 M) spectra of polymer N-PPP10 in PBS buffer (10 mM, pH 7.4) as
a function of hypochlorite. The excitation wavelength is 350 nm.
¼
50 mM) and (b) fluorescence
m
Fig. 3. UVevis absorption spectra of polymer (a) N-PPP25 and (b) N-PPP50 in PBS
buffer (10 mM, pH 7.4) as a function of hypochlorite. [N-PPP25] ¼ [N-PPP50] ¼ 50
mM.

W. Zhang et al. / Polymer 53 (2012) 2356e2360
2359
with pH of 7.4 are shown in Fig. 1. All water-soluble CPs exhibited
almost the same absorption and emission spectra, which are
significantly red shifted compared to model compound PMOPP.
This should be ascribed to that the polymers have a more extended
conjugated length than the model compound PMOPP. The UVevis
absorption spectra of N-PPPx in PBS solution exhibited three
maxima at ca. 205, 290 and 350 nm (Table 1). All the fluorescent
experiments were carried out with an excitation wavelength of
350 nm. The fluorescence spectra of the polymers showed
a maximum at ca. 420 nm. The fluorescence quantum yield of
N-PPP10, N-PPP25 and N-PPP50 in PBS solution was 21%, 26% and
4%, respectively.
3.3. Optical response for hypochlorite
We first examined the sensing properties of the polymer
N-PPP10 to hypochlorite in PBS solution. As shown in Fig. 2, upon
addition of hypochlorite from 0 to 0.25 mM, the absorption band at
ca. 350 nm decreased, whereas the absorption bands at 208 and
290 nm were almost unchanged. In contrast, when 20
mM of
hypochlorite was added, the fluorescence intensity was decreased
dramatically with a 15-fold fluorescence quenching. Then we
investigated the responses of N-PPP25 and N-PPP50 to hypochlo-
rite. Both N-PPP25 and N-PPP50 showed the same absorption
decrease at ca. 350 nm in the presence of hypochlorite (Fig. 3).
Compared to PMOPP, there was no obvious appearance of new
absorption band at long wavelength (>400 nm) because the
absorptions of the polymers are mainly attributed to the backbone
of poly(p-phenylene). Meanwhile, as shown in Fig. 4, upon addition
of hypochlorite, both N-PPP25 and N-PPP50 showed a fluorescence
quenching, though N-PPP50 needed more hypochlorite to quench
the fluorescence completely. Similar to the previous mechanism
based on the small-molecules, the methoxyphenol moieties of the
polymers were oxidized to benzoquinone upon the addition of
hypochlorite, which induced an ICT from the polymer backbone to
benzoquinone moieties, and thus resulted in
quenching (Scheme 2).
The efficiency of fluorescence quenching is quantified through
the Stern-Volmer constant (Ksv), which is calculated according to
the Equation (1).
a fluorescence
Fig. 4. Fluorescence spectra of polymer (a) N-PPP25 and (b) N-PPP50 in PBS buffer
(10 mM, pH 7.4) as a function of hypochlorite. [N-PPP25] ¼ [N-PPP50] ¼ 5
mM. The
excitation wavelength is 350 nm.
The structures of the polymers were characterized by 1H NMR and
elemental analysis. The p-methoxyphenol moieties on the polymer
backbone were confirmed by the broad peaks at around 8.0 and
F0=F ¼ 1 þ Ksv½quencherꢃ
(1)
The Ksv of N-PPPx and PMOPP are listed in Table 1. It was found
that the Ksv decreased with the increasing content of methox-
yphenol moieties in the polymers N-PPPx. Correspondingly, the
limit of detection of N-PPP10, N-PPP25, N-PPP50 and PMOPP is
1
3.7 ppm of H NMR spectra. The polymers are soluble in methanol
and water with a solubility of 8 mg/mL. In the TGA thermograms of
the three polymers (see ESI), a weight loss of 5e10% between 100 and
150 ꢀC was observed for all the polymers, which could be ascribed to
the loss of absorbed and bound water [42e44].
0.02 mM, 0.03 mM, 0.2 mM and 0.8 mM, respectively. This can be
elucidated from the fact that more methoxyphenol moieties
require more hypochlorite to react. In addition, the Stern-Volmer
constants of N-PPPx were obviously higher than that of the
model compound PMOPP. This indicated that the CPs of N-PPPx
amplified the fluorescence quenching due to the facile energy
migration along the polymer backbone.
3.2. Photophysical properties
The absorption and emission spectra of the polymers and the
model compound in 10 mM phosphate buffer saline (PBS) solution
Scheme 2. Mechanism of the reaction of N-PPPx to ClOꢁ
.

2360
W. Zhang et al. / Polymer 53 (2012) 2356e2360
Acknowledgments
We are grateful for financial support from the National Natural
Science Foundation of China (Nos. 20874077), the National Science
Fund for Distinguished Young Scholars of China (No. 51125013), the
program for Changjiang Scholars and Innovative Research Team in
University (IRT1030), and the Fundamental Research Funds for the
Central Universities of China.
Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.polymer.2012.03.063.
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In conclusion, we have developed a series of water-soluble CPs
by incorporating the methoxyphenol moieties into the poly(p-
phenylene) backbone. The polymers exhibit highly sensitive and
selective fluorescent quenching to hypochlorite under the aqueous
condition. The content of methoxyphenol moieties in polymer
N-PPPx plays a crucial role in the behaviors of fluorescence
quenching efficiency. Furthermore, the conjugated polymers show
a significantly amplified quenching efficiency compared to their
model compound. The results reveal that the water-soluble poly(p-
phenylene) derivatives have good potential in detecting hypo-
chlorous acid in biological system.