Water-Soluble hyperbranched polyelectrolytes with high fluorescence quantum yield: Facile synthesis and selective chemosensor for Hg2+and Cu2+ ions

Article abstract of DOI:10.1002/pola.24128

New water-soluble hyperbranched polyfluorenes bearing carboxylate side chains have been synthesized by the simple "A2 + B2 + C3" protocol based on Suzuki coupling polymerization. The linear polyfluorene analogue LPFA was also synthesized for comparative i

Full text of DOI:10.1002/pola.24128

Water-Soluble Hyperbranched Polyelectrolytes with High Fluorescence
Quantum Yield: Facile Synthesis and Selective Chemosensor for
Hg21 and Cu21 Ions
BIQING BAO,1 LIHUI YUWEN,1 XIAOWEI ZHAN,2 LIANHUI WANG1,3
1Laboratory of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, China
2Beijing National Laboratory for Molecular Sciences and CAS Key Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
3Department of Materials Science, Fudan University, 220 Handan Road, Shanghai 200433, China
Received 23 March 2010; accepted 11 May 2010
DOI: 10.1002/pola.24128
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: New water-soluble hyperbranched polyfluorenes
bearing carboxylate side chains have been synthesized by the
simple ‘‘A2 þ B2 þ C3’’ protocol based on Suzuki coupling po-
lymerization. The linear polyfluorene analogue LPFA was also
synthesized for comparative investigation. The optical proper-
ties of the neutral precursory polymers in CHCl3 and final car-
boxylic-anionic conjugated polyelectrolytes in buffer solution
were investigated. The obtained hyperbranched polyelectrolyte
HPFA2 with lower content of branch unit (2%) showed excellent
solubility and high fluorescence quantum yield (UF ¼ 89%) in
aqueous solution. Fluorescence quenching of HPFA2 by differ-
ent metal ions was also investigated, the polyelectrolyte showed
high selectivity for Hg2þ and Cu2þ ions relative to other various
metal ions in buffer solution. The Stern-Volmer constant Ksv
was determined to be 0.80 ꢀ 106 Mꢁ1 for Hg2þ and 3.11 ꢀ 106
Mꢁ1 for Cu2þ, respectively, indicating the potential application
of HPFA2 as a highly selective and sensitive chemosensor for
Hg2þ and Cu2þ ions in aqueous solution. 2010 Wiley Periodicals,
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Inc. J Polym Sci Part A: Polym Chem 48: 3431–3439, 2010
KEYWORDS: fluorescence sensor; hyperbranched; polyelectro-
lytes; synthesis; water-soluble conjugated polymer
INTRODUCTION Water-soluble conjugated polyelectrolytes
(CPEs) are an interesting class of organic macromolecules
with p-conjugated backbones and charged side chains, which
combine the unique optoelectronic properties of traditional
conjugated polymers with aqueous solubility and electrostatic
behaviors of polyelectrolytes.1–3 In recent decades, CPEs have
received remarkable attention because of their potential appli-
cations as sensory1–3 and photovoltaic4–6 materials.
UF of the dendritic-conjugated polymers; the second-genera-
tion dendronized polyfluorenes showed high UF (57%) in
aqueous solution because of suppression of p-stacking aggre-
gation. Bo and coworkers16 have synthesized a series of
water-soluble dendronized poly(fluorene-phenylene)s, and
found that the second-generation dendronized polyfluorene
exhibited excellent UF (94%), which was attributed to the
site isolation of the polyfluorene backbones by the dendritic
wedges. However, synthesis and purification of the dendritic
polyfluorene was somewhat tedious and time-consuming.
Thus, a simple and facile synthetic method is desired to de-
velop CPEs with high UF and good water solubility.
Although CPEs have been studied extensively and have led
to development of various kinds of the fluorescence-based
chemo/biosensors, high water solubility and high fluores-
cence quantum yield (UF) of CPEs in aqueous solution are
difficult to achieve because of the strong polymer aggrega-
tion of their hydrophobic conjugated backbones.7–9 In previ-
ous investigations, relentless synthetic efforts have been
made to avoid the aggregation behaviors of organic materials
in light-emitting devices, examples included the use of poly-
mers bearing bulky substituents,10,11 polymers with dendron
pendants,12,13 and spirofunctionalized polymers.14 Recently,
some of the strategies were further extended to achieve high
UF of water-soluble conjugated polymers. Wang and co-
workers15 attached dendrons to CPE backbones to improve
Recently, hyperbranched polymers have attracted increasing
interest because of their unusual molecular structures and
properties. Considering the highly branched and globular
molecular features of hyperbranched polymers,17 it was
speculated that the hyperbranched polymers could be
employed to reduce the strong aggregation tendency of CPEs
in aqueous media and to develop highly fluorescent CPEs.
Moreover, compared to multigenerational dendrimers which
need stepwise synthesis with a complicated purification pro-
cess at each generation, hyperbranched polymers can easily
Additional Supporting Information may be found in the online version of this article. Correspondence to: L. H. Wang (E-mail: wlhui@fudan.edu.cn)
or X. W. Zhan (E-mail: xwzhan@iccas.ac.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 3431–3439 (2010) 2010 Wiley Periodicals, Inc.
HYPERBRANCHED POLYELECTROLYTES, BAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
be prepared in an one-pot procedure through many strat-
egies including the polycondensation of ABn monomers, A2
þ B3 route, and A2 þ B2 þ C3 approach.
and the reaction mixture was stirred at 50 ꢃC for 15 h. The
resulting mixture was poured into water and extracted with
CH2Cl2 (3 ꢀ 50 mL), the organic extracts were washed with
water (3 ꢀ 50 mL) and dried over anhydrous MgSO4. The
solvent was removed and the residue was purified by col-
umn chromatography (silica gel, 5% ethyl acetate in hexane),
then it was recrystallized from hexane/ethyl acetate to give
M1 (2.95 g, 53%) as white crystals.
1H NMR (400 MHz, CDCl3): d (ppm) 7.54 (d, J ¼ 8.0 Hz, 2H),
7.48 (m, 4H), 4.07 (q, J ¼ 7.2 Hz, 4H), 2.08 (t, J ¼ 7.3 Hz,
4H), 2.00 (m, 4H), 1.21 (t, J ¼ 7.1 Hz, 6H), 0.96 (m, 4H).
Anal. Calcd. for C25H28Br2O4: C, 54.37; H, 5.11. Found: C,
54.35; H, 5.15%.
Heavy metal ions are widespread pollutants in environment,
and some extremely sensitive chemosensors of heavy metal
ions based on amplified fluorescence quenching of conju-
gated polymers have been developed.18–23 However, the
hydrophobicity of most conjugated polymers and their low
fluorescence quantum yield in aqueous solution dramatically
limit their practical application in aqueous environment.
Polyfluorenes and their derivatives are promising polymers
for light-emitting devices and sensory materials because of
their high UF and high chemical stability.24 In this contribu-
tion, we report the design and synthesis of highly fluorescent
carboxylate-based hyperbranched polyfluorenes as selective
and sensitive probe to detect Hg2þ and Cu2þ in aqueous so-
lution. The hyperbranched CPEs were achieved by the simple
‘‘A2 þ B2 þ C3’’ approach based on Suzuki coupling reaction.
The final hyperbranched CPEs show excellent water solubil-
ity as well as extremely high UF in aqueous solution. More-
over, the novel anionic hyperbranched CPEs can provide
optimal interfaces for interactions with oppositely charged
metal ions and biomolecules to improve the detection sensi-
tivity. Therefore, it is worth noting that these new hyper-
branched CPEs could be potential chemo/biosensors for
detecting metal ions and biomolecules in aqueous solution.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
bis(4-ethylbutyrate)fluorene (M2)
A mixture of M1 (1.10 g, 2 mmol), KOAc (0.90 g, 9.2 mmol),
and bis(pinacolato)diboron (1.52 g, 6 mmol) in anhydrous
dimethylformamide (DMF) (10 mL) was charged under nitro-
gen and stirred for 15 minutes, then Pd(PPh3)4 was added
ꢃ
under nitrogen. The reaction mixture was stirred at 100 C for
24 h. After cooling to room temperature, the reaction mixture
was diluted with water and extracted with CH2Cl2 (3 ꢀ 50
mL), the organic layer was washed with water (3 ꢀ 50 mL)
and dried over anhydrous MgSO4. The solvent was evaporated
and the crude solid was purified by column chromatography
(silica gel, 10% ethyl acetate in hexane), then it was recrystal-
lized from acetone to afford M2 (0.97 g, 75%) as a white solid.
EXPERIMENTAL
1H NMR (400 MHz, CDCl3): d (ppm) 7.82 (d, J ¼ 7.7 Hz, 2H),
7.73 (d, J ¼ 7.3 Hz, 4H), 4.02 (q, J ¼ 7.1 Hz, 4H), 2.17 (m,
8H), 1.38 (s, 24H), 1.17 (t, J ¼ 7.2 Hz, 6H), 0.91 (m, 4H).
Anal. Calcd. for C37H52B2O8: C, 68.75; H, 8.11. Found: C,
68.50; H, 8.19%.
Measurements and Materials
Generally all reagents were purchased from Acros, Fluka, or
Aldrich Chemical Co. and were used without further purifica-
tion. All solvents were dried using standard procedures.
Nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker Avance 400 MHz spectrometer with tetramethyl-
silane as the internal standard. The elemental analysis was
performed on a Flash EA1112 elemental analyzer. Gel perme-
ation chromatography results were determined on a Shimadzu
1515 separation module using polystyrene as a standard and
tetrahydrofuran (THF) as the eluant. UV–vis absorption and
photoluminescence (PL) emission spectra were measured
using a Shimadzu UV-3150 spectrophotometer and a Shi-
madzu RF-6301PC spectrophotometer, respectively. The fluo-
rescence quantum yields of the polymers were determined in
aqueous solution using quinine sulfate (ꢂ1 ꢀ 10ꢁ5 M solution
in 1.0 M H2SO4, having UF of 54.6%) as the standard by exci-
tation at 346 and 382 nm, respectively.25 The concentration
was adjusted so that the absorbance of the solution could be
<0.1. All optical measurements were performed at room tem-
perature unless otherwise stated.
1,3,5-Tris(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
benzene (M3)
A mixture of 1,3,5-tribromobenzene (1.57 g, 5 mmol), KOAc
(2.94 g, 30 mmol), and bis(pinacolato)diboron (4.57 g, 18
mmol) in anhydrous DMF (20 mL) was charged under nitro-
gen and stirred for 15 min, then Pd(PPh3)4 was added under
nitrogen. The mixture was stirred at 100 ꢃC for 24 h. After
cooling to room temperature, the resulting mixture was diluted
with water and extracted with CH2Cl2 (3 ꢀ 50 mL), the com-
bined organic layer was washed with water (3 ꢀ 50 mL) and
dried over anhydrous MgSO4. The solvent was evaporated and
the residue was purified by column chromatography (silica gel,
20% CH2Cl2 in hexane), then it was recrystallized from ace-
tone to afford M3 (1.95 g, 86%) as a white solid.
1H NMR (400 MHz, CDCl3): d (ppm) 8.36 (s, 3H), 1.33 (s,
36H). Anal. Calcd. for C24H39B3O6: C, 63.21; H, 8.62. Found:
C, 63.09; H, 8.57%.
Synthesis
2,7-Dibromo-9,9-bis(4-ethylbutyrate)fluorene (M1)
To a solution of 2,7-dibromofluorene (3.24 g, 10 mmol) in
20 mL dimethyl sulfoxide (DMSO) under nitrogen were
added tetrabutylammonium bromide and 50 wt % aqueous
solution of NaOH (0.8 g, 20 mmol). After stirring at 65 ꢃC
for 3 h, ethyl 4-bromobutyrate (4.29 g, 22 mmol) was added
Polymerization
General synthetic procedure for hyperbranched polymers
HPF2, HPF5, and linear polymer LPF: M1, M2, M3, and
Pd(PPh3)4 (2.0 mol %) were added to a 25-mL flask. After
degassed and charged with nitrogen, the degassed toluene
and 2 M K2CO3 aqueous solution were added. The mixture
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TABLE 1 The Proportions of the Monomers Used and GPC Data of the Polymers LPF, HPF2, and HPF5
The Proportions of the
Monomers (%)
Polymer
M1
M2
M3
Reaction Time (h)
Mn (104)
Mw (104)
PDI
LPF
1
1
0
2
5
72
72
48
1.38
0.94
1.21
4.47
1.98
4.83
3.2
2.1
3.3
HPF2
HPF5
50.5
51.25
47.5
43.75
ꢃ
was stirred at 80 C under nitrogen atmosphere. Before gela-
tion, the terminal boronic ester and bromine groups were
end-capped by sequentially adding excess bromobenzene
and phenylboronic acid, respectively. After cooling to room
temperature, the solution was concentrated and precipitated
from methanol twice. The precipitate was filtered and Soxh-
let extracted with methanol for 48 h. The proportions of the
monomers used here were listed in Table 1.
HPFA5
1H NMR (400 MHz, D2O): d (ppm) 7.86–7.37 (br, ArH), 2.02–
1.72 (br, ACH2A).
RESULTS AND DISCUSSION
Synthesis and Structural Characterization
The hyperbranched polyfluorenes were synthesized by the
simple ‘‘A2 þ B2 þ C3’’ protocol based on Suzuki coupling
polymerization. The synthetic routes to the three polymers
are illustrated in Scheme 1.
Linear Polyfluorene (LPF)
M1 (276 mg, 0.500 mmol), M2 (323 mg, 0.500 mmol), toluene
(10 mL), 2 M aqueous K2CO3 solution (5 mL), and Pd(PPh3)4
were used. LPF was obtained as a dark greenish solid.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-bis(4-
ethylbutyrate)fluorene (M2) and 1,3,5-tris(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)benzene (M3) were prepared
according to the published procedures.26 However, the syn-
thesis of 2,7-dibromo-9,9-bis(4-ethylbutyrate)fluorene (M1)
was not well reported. According to the previous literatures,
alkylation of fluorene was achieved by reaction of fluorene
with bromoalkane and excess NaOH in DMSO.27,28 However,
our experiments revealed that the use of NaOH by the same
way gave exclusively carboxylic acid-functionalized fluorene.
Consequently, the preparation of carboxylic ester-containing
fluorene requires more elaborate synthetic procedures.
Based on various conditions tested, it was found that the
reaction products were highly dependent on the NaOH ratio,
and the reaction of 2,7-dibromofluorene with 2 equivalents
of NaOH gave the best yield of carboxylic ester products.
1H NMR (400 MHz, CDCl3): d (ppm) 7.83–7.37 (m, ArH),
4.06 (m, 4H), 2.14 (br, 8H), 1.17 (m, 10H).
Hyperbranched Polyfluorene (HPF2)
M1 (279 mg, 0.505 mmol), M2 (307 mg, 0.475 mmol), M3
(9 mg, 0.020 mmol), toluene (10 mL), 2 M aqueous K2CO3
solution (5 mL), and Pd(PPh3)4 were used. Hyperbranched
polyfluorene (HPF2) was obtained as a dark greenish solid.
1H NMR (400 MHz, CDCl3): d (ppm) 7.83–7.37 (m, ArH),
4.06 (m, 4H), 2.14 (br, 8H), 1.17 (m, 10H).
Hyperbranched Polyfluorene (HPF5)
M1 (283 mg, 0.513 mmol), M2 (283 mg, 0.437 mmol), M3
(23 mg, 0.050 mmol), toluene (10 mL), 2 M aqueous K2CO3
solution (5 mL), and Pd(PPh3)4 were used. HPF5 was
obtained as a dark greenish solid.
The neutral polymers were synthesized by Suzuki coupling
reaction in a mixture of toluene and 2 M K2CO3 solution. Pre-
vious work revealed that the carboxylic ester groups could hy-
drolyze during Suzuki coupling reaction because of the basic
reaction condition. However, in our experiment, it is interesting
to note that carboxylic ester-functionalized polyfluorenes can
be readily obtained without hydrolysis of the carboxylic ester
groups during Suzuki coupling using the basic reagents.
1H NMR (400 MHz, CDCl3): d (ppm) 7.83–7.37 (m, ArH),
4.06 (m, 4H), 2.14 (br, 8H), 1.17 (m, 10H).
Hydrolysis of Neutral Carboxylate-Containing
Polyfluorenes
Neutral carboxylate-containing polymer (LPF, HPF2, and
HPF5) was added to a solution of sodium hydroxide in
methanol; the mixture was refluxed for 12 h. After cooling to
room temperature, the methanol was removed by rotary
evaporation. The corresponding CPE (LPFA, HPFA2, and
HPFA5) was purified by dialyzing against water (Millipore
NanopureTM) using a 7000-Da MWCO cellulose membrane.
The hyperbranched polyfluorenes HPF2 and HPF5 were syn-
thesized by ‘‘A2 þ B2 þ C3’’ method using different propor-
tions of monomers (Table 1). It was found that the solubility
of the hyperbranched polymers was dominantly determined
by the molar ratio of the branching unit M3. The polymers
with 2% and 5% M3 were fully soluble in common organic
solvents such as THF, CH2Cl2 and chloroform. Further
increasing the content of branching unit lead to a lower yield
of soluble polymer due to crosslinking.
LPFA
1H NMR (400 MHz, D2O): d (ppm) 7.86–7.38 (br, ArH), 2.01–
1.71 (br, ACH2A).
HPFA2
1H NMR (400 MHz, D2O): d (ppm) 7.79–7.28 (br, ArH), 2.00–
1.70 (br, ACH2A).
The molecular distributions (PDI) and weights of the polymers
LPF, HPF2, and HPF5, measured by gel permeation
HYPERBRANCHED POLYELECTROLYTES, BAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic routes of
the polymers.
chromatography using THF as eluant are listed in Table 1.
The number-average molecular weight (Mn) of the neutral
polymers ranges from 0.94 ꢀ 104 to 1.38 ꢀ 104 with the
polydispersity indices in the range of 2.1–3.3. The structures
ppm corresponding to ACOOCH2A protons, clearly indicating
complete hydrolysis of ACOOCH2CH3 to ACOONa.
The molar ratio of the branching benzene unit in HPF2 and
HPF5 could not be calculated from the 1H NMR spectra
because the polymers were end-capped by phenyl groups.
However, the integral area in the 1H NMR spectra can
roughly reflect the relative amount of benzene unit. From
the 1H NMR spectra, we can find that the integral area for
the peaks between 7.3 and 8.0 ppm increases with the molar
ratio of monomer M3, indicating the higher content of
branch unit in HPF5.
1
of the polymers were characterized by H NMR spectra. The
neutral polymers (LPF, HPF2, and HPF5) show two peaks
approximately 4.05 and 1.26 ppm in their 1H NMR spectra
(data not shown). These two peaks are assigned to the sig-
nals from ACOOCH2A and ACH3 protons, respectively, indi-
1
cating the existence of the carboxylic ester groups. In the H
NMR spectra of the polyelectrolytes LPFA, HPFA2, and
HPFA5, no residual signal is observed approximately 4.05
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ARTICLE
HPFA2, and HPFA5 are redshifted compared to the corre-
sponding neutral polymers. This is probably due to the
aggregation of the polymer chains through hydrophobic p-
stacking interactions and hydrogen bonding between ACOOH
groups (see Discussion later).19
The fluorescence quantum yield (UF) for LPFA, HPFA2, and
HPFA5 in water were measured to be 53%, 89%, and 55%,
respectively (Table 2). Although the UF of the hyperbranched
polyelectrolytes is not as high as that of their neutral coun-
terparts in organic solvents, the hyperbranched polyelectro-
lyte HPFA2 has remarkably high UF of 89% in aqueous solu-
tion relative to its linear polymer LPFA. These results
indicate that the incorporation of hyperbranched structures
into the conjugated polymers could efficiently reduce the
intra- and intermolecular interactions of CPEs and signifi-
cantly improve the fluorescence efficiency of the polymers.
The relatively low quantum yield of HPFA5 might be associ-
ated with imperfect structures present in the hyperbranched
polymer with higher content of branch unit.
pH-Dependent Aggregations and Optical Properties
The effect of pH on the optical properties of the hyper-
branched polyelectrolyte HPFA2 was studied in a range of
pH from 3 to 11. As shown in Figure 2, at a basic pH level
(pH ꢄ 7.4), HPFA2 exhibits similar absorption and emission
behavior with the absorption maximum located approxi-
mately 382 nm and emission maximum centered at 414 nm,
nearly identical to its neutral polymer HPF2 in CHCl3. This
result indicates that the polymer isolates well in buffer solu-
tion under basic condition and no or little polymer aggrega-
tion forms. As the pH decreases from 7.4 to 7, a break point
is observed where the absorption maximum shifts from 382
to 386 nm and the corresponding emission maximum red
shifts about 18 nm, indicating the formation of aggregates. A
further decrease in the pH results in significant reduction of
the emission intensity of the polyelectrolyte.
FIGURE 1 UV–vis absorption and PL spectra of (a) neutral poly-
mers LPF, HPF2, and HPF5 in CHCl3 and (b) hydrolyzed poly-
mers LPFA, HPFA2, and HPFA5 at pH 7.0 in 10 mM NaH2PO4–
Na2HPO4 buffer solution.
Optical Properties
The UV–vis absorption and PL spectra of the neutral poly-
mers in dilute CHCl3 solution and the polyelectrolytes in
buffer solution are shown in Figure 1. The corresponding op-
tical data are summarized in Table 2.
The pH-dependent optical properties indicate the polymer
may exist two distinct forms at different pH conditions. At
higher pH, the majority of the carboxylic acid groups are
The maximum absorption peaks of the neutral polymers LPF,
HPF2, and HPF5 in CHCl3 locate at 384, 382, and 378 nm,
respectively. The absorption maxima slightly blue shift with
increasing mole percent of branching unit, and similar
results have been observed in earlier studies.29–33 The possi-
ble reason is that the introduction of benzene branching unit
interrupts the linear p-system and leads to reduction in the
p-conjugation length. On the other hand, the PL peaks of the
neutral polymers LPF, HPF2, and HPF5 locate approximately
415 nm with a shoulder at about 440 nm, showing no
obvious change with increasing content of branching unit in
the hyperbranched polymers. According to previous litera-
tures, the similar emission maxima for LPF, HPF2, and HPF5
should be ascribed to efficient excitation energy transfer
from the shorter conjugation chain segments to the longer
ones.29,30 The absorption and emission properties of the
polyelectrolytes LPFA, HPFA2, and HPFA5 in buffer solution
are similar to those of their neutral counterparts in CHCl3.
However, the absorption and emission maxima of LPFA,
TABLE 2 Optical Properties of Neutral Polymersa and
Hydrolyzed Polymersb
Polymers
kmax,abs (nm)
kmax,em (nm)
UF (%)c
LPF
384
382
378
390
388
385
417, 440
415, 440
415, 440
432, 457
432, 457
433, 455
71%
98%
70%
53%
89%
55%
HPF2
HPF5
LPFA
HPFA2
HPFA5
a
Determined in CHCl3 solution.
Determined at pH 7.0 in 10 mM NaH2PO4–Na2HPO4 buffer solution,
b
considering the pH effect on the optical properties of the hydrolyzed
polymers.
c
Fluorescence quantum yield, estimated using quinine sulfate as
a
standard (the neutral polymers were dissolved in CHCl3 solution; the
hydrolyzed polymers were dissolved in deionized water solution).
HYPERBRANCHED POLYELECTROLYTES, BAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
than other metal ions (excluding Cu2þ). Cu2þ showed similar
behavior with a quenching efficiency 39 times higher than
other metal ions (excluding Hg2þ).
Figure 4 shows the changes in fluorescence spectra of
HPFA2 upon titration with Hg2þ ion ([Hg2þ] ¼ 0.2–20 ꢀ
10ꢁ6 M) and Cu2þ ([Cu2þ] ¼ 0.1–10 ꢀ 10ꢁ6 M). The addi-
tion of Hg2þ or Cu2þ both leads to effective quenching of the
HPFA2 emission, and the corresponding Stern-Volmer con-
stants (Ksv) were calculated from the Stern-Volmer relation-
ships (eq. 1), where I0 is the fluorescence intensity without
addition of quencher, I is the fluorescence intensity at a
given quencher concentration [Q],
I0=I ¼ 1 þ Ksv½Qꢅ
(1)
Over the entire titration process (0–20 ꢀ 10ꢁ6 M), a linear
Stern-Volmer plot was obtained for Hg2þ with a Ksv value of
0.80 ꢀ 106
M
ꢁ1. When the concentration of Hg2þ reached
20 ꢀ 10ꢁ6 M, the fluorescence intensity decreased remark-
ably with a quenching efficiency up to 89%. The fluorescence
change of HPFA2 upon the addition of Cu2þ is similar to that
of Hg2þ. The obtained Ksv value for Cu2þ is 3.11 ꢀ 106 Mꢁ1
,
which is moderately higher than that for Hg2þ. The addition
of Cu2þ also led to a 97% reduction in emission intensity
when Cu2þ concentration was 10 ꢀ 10ꢁ6 M.
As the study of the pH-dependent aggregation indicated the
polymer HPFA2 may have two distinct forms at different pH
conditions (above or below pH 7.4), the ion sensing experi-
ments at pH ¼ 7.8 where the polymer is in the anionic form
was also investigated. The fluorescence quenching behavior
of HPFA2 by Hg2þ and Cu2þ was similar to that at pH ¼ 7.0
(see Fig. S1 in the Supporting Information). However, the Ksv
values of HPFA2 for Hg2þ and Cu2þ (0.20 ꢀ 106 Mꢁ1 and
FIGURE 2 pH-dependent (a) absorption and (b) emission spec-
tra of HPFA2.
deprotonated (the anionic form). The electrostatic repulsions
between anionic carboxylate groups surrounding the poly-
mer backbones reduce the unfavorable aggregation of the
polymer chains and enhance fluorescence. While at lower
pH, the carboxylate groups can be partially neutralized, lead-
ing to decreased charge density and thus a decrease in the
interchain electrostatic repulsion. Therefore, HPFA2 some-
what aggregates through hydrogen bonding between carbox-
ylic acid groups and aromatic p–p stacking of hydrophobic
backbones.
0.42 ꢀ 106
M
ꢁ1, respectively) at pH 7.8 are smaller than
that at pH 7.0, this is because the aggregation of HPFA2
Ion Sensing Properties
The ion sensing properties of HPFA2 were tested for a wide
range of metal ions including Cu2þ, Hg2þ, Agþ, Ca2þ, Al3þ
Fe3þ, Kþ, Liþ, Mg2þ, Mn2þ, Ni2þ, Pb2þ, Zn2þ, Ba2þ, and Cd2þ
,
.
The absorption and fluorescence emission spectra were
obtained in 10 mM NaH2PO4–Na2HPO4 buffer solution (pH
7.0) at a polymer concentration of 1.5 ꢀ 10ꢁ6 M in [RU].
The influence of different metal ions ([metal ion] ¼ 50 ꢀ
10ꢁ6 M) on the fluorescence intensity of HPFA2 is shown in
Figure 3. HPFA2 exhibited no significant change in the fluo-
rescence spectrum on addition of most of the tested metal
ions, while addition of Hg2þ could cause up to 95% reduc-
tion in the fluorescence intensity, which was 24-fold higher
FIGURE 3 The fluorescence response of HPFA2 upon titration
of various metal ions ([metal ion] ¼ 50 ꢀ 10ꢁ6 M) at pH 7.0 in
10 mM NaH2PO4–Na2HPO4 buffer solution.
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ARTICLE
FIGURE 4 Fluorescence response of HPFA2 to addition of (a)
Hg2þ or (b) Cu2þ at pH 7.0 in 10 mM NaH2PO4–Na2HPO4 buffer
solution. The Stern-Volmer plots are shown as an inset.
FIGURE 5 Fluorescence change of HPFA2 upon titration of
metal ion mixtures (concentration of each metal ion was 10 ꢀ
10ꢁ6 M) with or without (a) Hg2þ or (b) Cu2þ at pH 7.0 in 10
mM NaH2PO4–Na2HPO4 buffer solution.
occurred at lower pH may promote exciton diffusion and
enhanced amplification.2
It is well known that a persistent problem in fluorescence
quenching assay for specific metal ion is the interference
from other metal ions when the sensor is used in complex
media. Therefore, the selectivity of HPFA2 for Hg2þ and
Cu2þ in the presence of excess various metal ions was also
studied. As shown in Figure 5, on addition of a mixture of
metal ions including Agþ, Ca2þ, Al3þ, Fe3þ, Kþ, Liþ, Mg2þ
,
Mn2þ, Ni2þ, Pb2þ, Zn2þ, Ba2þ, and Cd2þ(concentration of
each metal ion was 10 ꢀ 10ꢁ6 M), no obvious change of
fluorescence intensity for HPFA2 was observed, When Hg2þ
(10 ꢀ 10ꢁ6 M) was added to the mixture solution, the fluo-
rescence intensity of HPFA2 could be quenched up to 76%.
Similarly, the addition of Cu2þ could cause a 93% quenching
in the fluorescence intensity of HPFA2. These results indi-
cate that HPFA2 can be used as highly selective and sensi-
tive sensor for Cu2þ and Hg2þ ions.
FIGURE 6 Fluorescence spectra of HPFA2 with Hg2þ ions and
upon titration of EDTA at pH 7.0 in 10 mM NaH2PO4–Na2HPO4
buffer solution.
To gain insight into the quenching mechanism of HPFA2 by
Hg2þ, the reversibility of HPFA2 quenching response to
Hg2þ was investigated. As shown in Figure 6, addition of
HYPERBRANCHED POLYELECTROLYTES, BAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Among these polyelectrolytes, the hyperbranched polyfluo-
rene HPFA2 with lower amount of benzene branch unit
exhibits remarkably high fluorescence quantum yield (89%)
relative to the linear counterpart, indicating that introduc-
tion of hyperbranched structures into conjugated polymers
is an efficient way to improve quantum yield of CPEs. The
fluorescence of HPFA2 in NaH2PO4–Na2HPO4 buffer solu-
tion can be efficiently quenched by Hg2þ and Cu2þ with lit-
tle interference from other various metal ions. The results
demonstrate that HPFA2 can be used as a sensitive and
selective sensor for Hg2þ and Cu2þ ions. Furthermore, good
water solubility and high quantum efficiency of this hyper-
branched polyelectrolyte makes this system suitable for the
detection of biologically relevant molecules in aqueous
solution.
This work was supported by the National Natural Science Foun-
dation of China under Grants 60537030, 90813010, and
20721061, Shanghai Pujiang Program under Grant 08PJ14011,
Shanghai Leading Academic Discipline Project, Project Number:
B113, and the Chinese Academy of Sciences.
FIGURE 7 Fluorescence intensity at different temperature rela-
tive to that at 25 ꢃC for HPFA2 with Hg2þ at pH 7.0 in 10 mM
NaH2PO4–Na2HPO4 buffer solution.
EDTA (ethylenediamine tetra-acetic acid disodium salt) to
the solution of HPFA2 containing Hg2þ ions resulted in a re-
markable recovery in PL intensity of HPFA2, which reflected
the reversible association between Hg2þ and carboxylate
groups. Therefore, the fluorescence quenching of HPFA2 by
Hg2þ should be attributed to the complex formation between
Hg2þ and carboxylate-functionalized HPFA2, followed by the
rapid migration of exciton to the quencher ‘‘binding site.’’34–36
In deed, amplified quenching with Hg2þ in conjugated
polymers has been observed in previous studies,18,19,23,37
and it was generally attributed to electron transfer pro-
cess19,38 or enhanced spin-orbit coupling.23 To further con-
firm the aforementioned speculation, the temperature-de-
pendence of the PL intensity of HPFA2/Hg2þ complex was
also investigated. The PL intensityꢃat different temperatures
was normalized to the value at 25 C. As illustrated in Figure
7, the fluorescence intensity of HPFA2 did not show obvious
change with increasing temperature, whereas the PL inten-
sity of HPFA2/Hg2þ complex increased significantly with
increasing temperature. These results indicate that HPFA2/
Hg2þ complex dissociated at elevated temperature, which is
consistent with our speculation that fluorescence quenching is
caused by the strong association between Hg2þ and anionic
carboxylate-substituted HPFA2. The quenching mechanism of
conjugated polymers by metal ions is still not well understood
at present, and it remains a challenge in the field of fluores-
cence assay of metal ions based on superquenching.
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