Novel water-soluble light-emitting materials prepared by noncovalently bonded self-assembly

Article abstract of DOI:10.1246/cl.2005.1164

Novel noncovalently connected water-soluble nanoparticles containing hydroxyl-capped polyfluorene (PFOH) and poly-(acrylic acid) (PAA) were obtained and characterized. These nanoparticles were quite stable in water and no precipitate was observed after weeks. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and laser light scattering (DLS) were used to confirm the morphology of the PFOH/PAA nanoparticles. Their optical properties were investigated and showed the similar optoelectronic properties with PFOH solid film. Copyright

Full text of DOI:10.1246/cl.2005.1164

1164
Chemistry Letters Vol.34, No.8 (2005)
Novel Water-soluble Light-emitting Materials Prepared
by Noncovalently Bonded Self-assembly
Liang-Liang Qiang, Qu-Li Fan, Zhun Ma, Zhe Zheng, Yun-Yun Wang, Guang-Wei Zhang, and Wei Huangꢀ
Institute of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, P. R. China
(Received May 6, 2005; CL-050591)
Novel noncovalently connected water-soluble nanoparticles
bonding between PF and PAA.9
containing hydroxyl-capped polyfluorene (PFOH) and poly-
(acrylic acid) (PAA) were obtained and characterized. These
nanoparticles were quite stable in water and no precipitate
was observed after weeks. Transmission electron microscopy
(TEM), scanning electron microscopy (SEM), and laser light
scattering (DLS) were used to confirm the morphology of the
PFOH/PAA nanoparticles. Their optical properties were inves-
tigated and showed the similar optoelectronic properties with
PFOH solid film.
Polyfluorenes, the high-performance blue-light-emitting
polymers with great chemical and thermal stability, have attract-
ed great interests.11 Water-soluble polyfluorenes were widely
investigated as the highly sensitive materials in biosensors.12
Those water-soluble PFs are all obtained through attaching ionic
groups on the side chains.6,8 However, as far as we known, using
noncovalently (hydrogen bond) connected water-soluble nano-
particles to achieve water-soluble light-emitting materials was
seldom reported. In this paper, aqueous nanoparticles containing
polyfluorene were successfully prepared through hydrogen
bonded self-assembly between hydroxyl-capped polyfluorene
and poly(acrylic acid) (Scheme 1). Their water-solubility,
optical properties and morphologies were investigated and the
corresponding results showed that many other types of water-
soluble light-emitting materials can be conveniently and
efficiently obtained by this method.
Phenylmethanol-capped polyfluorene (PFOH) was obtained
by Yamamoto condensation reaction, using 4-bromobenzyl alco-
hol as the end-capper. The structures of the monomer and poly-
fluorene were confirmed by 1H and 13C NMR measurements.
The molecular weight of PFOH was estimated through compar-
ing the relative integrations of total fluorene aromatic protons
(from 7.66 to 7.85 ppm) and methylene protons (Ph–CH2–O,
4.78 ppm). The number-average polymerization degree of the
PFOH was calculated to be about 15 (6400 g/mol). GPC meas-
urements were also used to characterize Mn (7010 g/mol) and
Mw (11300 g/mol) and further proved the successful synthesis
of the PFOH. Poly(acrylic acid) was synthesized by racial poly-
merization (Mn ¼ 1 ꢁ 105 g/mol), using PAA in H-form in our
study. See details in Supporting Information.
In recent years, great efforts have been devoted to the con-
jugated polymers for the application as fluorescent chemo or bio-
sensors.1 In this field, water-soluble conjugated polymers have
attracted increasing attention as good sensory materials to detect
chemical or bioactive species, such as protein, DNA, and RNA.2
To be the good candidates as biosensors, water-solubility of
those conjugated polymers must be realized since most of the
biomolecules were dissolved in water. The first water-soluble
conjugated polymer was reported in 3-substituted polythio-
phene3 and subsequently a series of conjugated polyelectrolytes
were developed, such as poly(phenylene ethylene),4 poly(pheny-
lene vinylene),5 and poly(fluorene).6 Their water-solubility was
obtained through introducing ionic functions to their side chains.
Another way to achieve water-solubility is through block co-
polymerization, combining the neutral conjugated polymer seg-
ments with the water-soluble polymer segments. Our group first-
ly reported the water-soluble conjugated-acidic and conjugated-
ionic block copolymers prepared by atom-transfer radical poly-
merization (ATRP) with the polyfluorene-based macroinitia-
tor.7,8 In recent years, noncovalently bonded self-assembly, a
facile way to produce water-soluble micelles and other nano-
structure, was used to the nanotechnology.9,10 For example,
Yoshida et al. reported the micelle formation of nonamphiphilic
diblock copolymer via noncovalently bonded cross-linking in
1,4-dioxane.10a Jiang et al. demonstrated a series of spherical mi-
celles obtained from polyimide (PI) with carboxyl ends, a rigid
polymer, and poly(4-vinylpyridine) (PVPy) in the two polymers’
common solvent.9a Unlike other methods to produce water-solu-
ble polymers mentioned above, our water-soluble materials can
be obtained directly from water-insoluble polymers assisted with
water-soluble polymers and do not need any more chemical
modification on polymer chain except the end groups that serve
as the end-capping reagent in polymerization.10 As a significant
advantage in a ‘‘block-copolymer-free’’ strategy9 for preparing
water-soluble nanoparticles, here we report a new approach for
the preparation of water-soluble light-emitting materials self-as-
sembled by hydrogen bonding between of polyfluorene (PF) and
poly(acrylic acid) (PAA). This phenomenon can be attributed to
two main factors: the rigid character of PF and the hydrogen
In a typical procedure to prepare the nanoparticles, 2 mL of
H2O was added to 0.2 mL of PAA (10 mg/mL in water) under
ultrasonic for 5 min, and 0.2 mL of PFOH (0.4 mg/mL in
THF) was added dropwise. The resulted nanoparticles were
characterized by DLS (Table 1).
The UV–vis absorption and photoluminescence emission
spectra of PFOH solution (in THF), PFOH solid film, and
PFOH/PAA nanoparticles (solution in water) were shown in
Figure 1. The solid film emission spectra were collected from
Scheme 1. A schematic illustration of the formation of PAA/
PFOH nanoparticles.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.8 (2005)
1165
Table 1. DLS characterization data of the nanoparticles and
the proportion of PFOH and PAA
gen bond. TEM and SEM observations demonstrated the mor-
phology of the nanoparticles (diameter ranged from 100–
400 nm mainly in the range 300–400 nm) obtained by the
process mentioned above and gave the direct evidence of the
nanoparticle aggregates. The DLS data about the nanoparticles
in solutions also confirmed the results.
In summary, novel water-soluble light-emitting nanoparti-
cles consisting of PFOH and PAA were successfully prepared
by noncovalently bonded self-assembly. TEM, SEM, and DLS
were used to confirm the morphology of the PFOH/PAA nano-
particles. The nanoparticles exhibit the similar optoelectronic
properties to PFOH solid film. Such a noncovalently bonded
self-assembly provides a novel and simple way to obtain wa-
ter-soluble light-emitting materials and realize biomodification
on conjugated polymers, which open opportunities for greatly
improved conjugated-polymer-based biosensors. Further studies
of the materials as biosensor are in progress in our laboratory.
Diameter
/nm
Polydispersity
Index (PI)
PAA:PF (weight
ratio, in water)
Sample
M1
M2
M3
283.1
236.4
332.5
0.30
0.26
0.40
25:1
100:1
50:1
This work was financially supported by the National Natural
Science Foundation of China under Grants 60325412,
90406021, and 50428303 as well as the Shanghai Commission
of Science and Technology under Grants 03DZ11016, and
04XD14002 and the Shanghai Commission of Education under
Grant 2003SG03.
Figure 1. UV–vis absorption and photoluminescence spectra of
PFOH solution (1 mg/mL in THF), PFOH solid film and PFOH/
PAA nanoparticle (sample M2).
solid film made by spin-coating PFOH (5 mg/mL) from THF so-
lutions on the glass substrate. The absorption maximum of
PFOH in THF solution was at 375 nm and the emission peak
was at 420 nm. The absorption maxima of PFOH solid film
and PFOH/PAA nanoparticle red-shifted from 375 nm (PFOH
in THF solution) to 380 and 402 nm, respectively. Their photo-
luminescence spectra were similar and both red-shifted from
420 nm to about 440 nm, compared with the PL spectrum of
PFOH in THF solution. The red-shift can be attributed to the for-
mation of interchain ꢀ aggregation of PFOH.7,8 The emission
peak of the nanoparticles a little more red-shifted than that of
the PFOH solid film (from 438 to 440 nm), which may be attrib-
uted to the more regular arrangement of PF chains in the nano-
particles.7,8
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Figure 2. SEM (left) and TEM (right) images of nanoparticles
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Published on the web (Advance View) July 23, 2005; DOI 10.1246/cl.2005.1164