Preparation and self-assembly of amphiphilic polymer with aggregation-induced emission characteristics

Article abstract of DOI:10.1007/s11426-012-4528-7

An amphiphilic polymer bearing tetraphenylethene (TPE) moiety was synthesized by convenient reactions. The polymer exhibits unique aggregation-induced emission (AIE) characteristics and can self-assemble to size-tunable particles in DMF/water mixtures. Th

Full text of DOI:10.1007/s11426-012-4528-7

SCIENCE CHINA
Chemistry
May 2012 Vol.55 No.5: 772–778
doi: 10.1007/s11426-012-4528-7
• ARTICLES •
· SPECIAL ISSUE · In Honor of the 80th Birthday of Professor WANG Fosong
Preparation and self-assembly of amphiphilic polymer with
aggregation-induced emission characteristics
QIN AnJun1*, ZHANG Ya2, HAN Ning3, MEI Ju1, SUN JingZhi1, FAN WeiMin3 &
TANG Ben Zhong1,4*
1MOE Key Laboratory of Macromolecular Synthesis and Functionalization; Department of Polymer Science and Engineering,
Zhejiang University, Hangzhou 310027, China
2Department of Chemistry, Zhejiang University, Hangzhou 310027, China
3College of Medicine, Zhejiang University, Hangzhou 310027, China
4Department of Chemistry, Institute of Molecular Functional Materials, State Key Laboratory of Molecular Neuroscience, The Hong Kong
University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
Received November 30, 2011; accepted December 22, 2011; published online March 12, 2012
An amphiphilic polymer bearing tetraphenylethene (TPE) moiety was synthesized by convenient reactions. The polymer ex-
hibits unique aggregation-induced emission (AIE) characteristics and can self-assemble to size-tunable particles in DMF/water
mixtures. The polymer nanoparticles can be used for cell imaging, which provides a potential stable fluorescent tool to monitor
the distribution of drugs and bioconjugates in living cells.
amphiphilic polymer, aggregation-induced emission, tetraphenylethene, self-assembly, cell imaging
molecules are virtually non-luminescent when they are dis-
1 Introduction
solved in their good solvents as isolated species but become
highly emissive when they are aggregated into nanoparticles
in their poor solvents or fabricated into thin films in the
solid state [5]. We thus coined the term of aggregation-
induced emission (AIE) for this extraordinary phenomenon,
because these molecules were induced to emit by aggregate
formation.
Since then, scientists have been actively involved in the
synthesis of new AIE systems and exploration of their po-
tential applications. A large number of AIE-active lumino-
gens and polymers have been developed for diverse appli-
cations, for example, in the areas of stimuli-responsive na-
nomaterials, and organic light-emitting diodes [6, 7]. Par-
ticularly, the AIE luminogens have shown their unique
properties in the detection of bioconjugates, such as DNA,
saccharides, and protein, based on their mechanism of re-
striction of intramolecular rotation [813] and can be re-
garded as the organic version of quantum dots.
It has been a textbook knowledge that conventional lumines-
cent materials are highly emissive in their dilute solutions
with fluorescent quantum yields reaching unity. However,
their emission could be partially or completely quenched by
the aggregation [1]. This photophysical process is well-
known as aggregation-caused quenching (ACQ), which has
greatly limited the technological application scopes of these
luminophores, because most of them are commonly used in
the solid state (e.g., as thin films) in real-world applications.
Various chemical, physical and engineering approaches
have been taken to interfere with luminophore aggregation,
but the attempts have met with only limited success [24].
Exactly opposite to the notorious ACQ effect, our group
has discovered an unusual phenomenon: propeller-shaped
*Corresponding author (email: qinaj@zju.edu.cn; tangbenz@ust.hk)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

Qin AJ, et al. Sci China Chem May (2012) Vol.55 No.5
773
However, the AIE luminogens have been rarely incorpo-
rated in amphiphilic systems, which could self-assemble to
form nanoparticles [14]. It will be extremely attractive to
prepare such nanoparticles from amphiphilic polymers that
exhibit AIE feature, which are promising to deliver drugs
and DNA, and allow us to monitor cellular distribution of
the drugs at the same time [15]. The essence of the present
work is to synthesize amphiphilic polymers bearing AIE
luminogen as their pendant, to prepare size tunable fluores-
cent nanoparticles, and to investigate the photophysical
properties of both the polymers and nanoparticles.
4.60, 4.44, 4.12, 3.45, 2.79, 2.35, 1.83, 1.59, 1.29, 1.11,
0.99, 0.86, 0.66.
2.3 MTT assay
Human breast cancer BCap37 cells were purchased from
ATCC. Drug resistant breast cancer cell line-Bats72 was
established from BCap37 cell line via a two-stage screening
strategy using paclitaxel as a selective drug. Both BCap37
and Bats72 cells were cultured in RPMI1640 (Gibco, USA)
supplemented with 10% fatal bovine serum (FBS) (Sijiqing
Biological Engineering Materials Co., Ltd.).
BCap37 and Bats72 cells were seeded onto 96-well
plates at a density of 1.2 × 104 cells/well, cultured in 100 µL
growth medium, and incubated for 24 h (5% CO2, 37 °C) to
reach 70%–80% confluency before nanoparticle addition.
The nanoparticles, which were prepared in DMF/water
mixture with 99% water fraction, were dispersed in cell
culture medium at two different concentrations (15 and 30
mg/L). When the desired cell confluency was reached, 100
µL of the pre-prepared nanoparticle solution was added into
each well following the removal of the spent growth medi-
um. Each condition was tested in 10 replicates. The plates
were then incubated for 48 h (5% CO2, 37 °C).
Next, the nanoparticle solution was removed from each
well and replaced with the mixture of 15 µL of 3-(4,5-di-
methylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT)
solution and 100 µL of fresh cell culture medium. The sam-
ples were cultivated in an incubator for a further 3 h (5%
CO2, 37 °C). After removing the culture medium and excess
MTT solution from each well, 200 µL DMSO was added
into each well to dissolve the internalized purple formazan
crystals. An aliquot of 100 µL was transferred from each
well to a new 96-well plate. The new plates were analyzed
at 570 nm using a microplate reader (BIO-RAD, Model
680, USA). The absorbance readings of the formazan crys-
tals were recorded as A570. The results were expressed as
cell viability = A570 (sample)/A570(control) × 100%.
2 Experimental
2.1 General information
The starting materials used for the synthesis of intermedi-
ates 1 and 2 as well as the PI were purchased from Alfa,
Acros and Aladdin. They were used without further purifi-
cation. All the solvents used were freshly dried. FT-IR
analysis was conducted on a BURKER VECTOR 22 FT-IR
1
spectrometer. H NMR spectra were measured on a Bruker
DMX 300 NMR spectrometer in CDCl3 using tetrame-
thylsilane (TMS,  = 0) as internal reference. Thermograv-
imetric (TGA) analyses and differential scanning calorime-
try (DSC) studies were conducted on Perkin Elmer Pyris 6
o
under nitrogen at a heating rate of 10 C/min. Photolumi-
nescence (PL) spectra were measured on a Perkin Elmer LS
55 Spectrofluorometer. Mw, Mn and Mw/Mn or PDI values of
the polymer were estimated by a Waters 150 C GPC system,
using N,N-dimethylformamide (DMF) as the eluent at a
flow rate of 1.0 mL/min and a set of monodisperse polysty-
renes as calibration standards. The dynamic light scattering
(DLS) analysis was conducted on a Brookhaven90 plus
spectrometer. SEM study was conducted on JSM-5510.
2.2 Synthesis of amphiphilic polymer PII
To a 250 mL two-necked, round-bottom flask were added
poly(N-methyldietheneamine sebacate) (PI) (3.574 g, 9.03
mmol) and cholesterol derivative (1) (4.97 g, 9.04 mmol).
Then the flask was vacuum-evacuated and refilled with dry
nitrogen three times. Freshly distilled toluene (50 mL) was
injected and the mixture was refluxed under nitrogen for
two days. Afterwards, the solution of 1-[(4-bromomethyl)-
phenyl)]-1,2,2-triphenylethene (2) (1.203 g, 2.83 mmol) in
dry toluene was injected under nitrogen. The solution con-
tinued to reflux for an additional one day. Then the mix-
ture was cooled down and added dropwise into 500 mL
Et2O under vigorous stirring. The precipitate was allowed
to stand overnight, collected by filtration, washed three
times by Et2O, and dried under vacuum at 40 °C to a con-
stant weight. Brown viscous product of PII was obtained
in a yield of 31.6%. Mw: 18800, PDI (GPC, polystyrene
calibration): 1.58. IR (KBr) v = 1736, 1463, 1243, 1116,
2.4 Cell imaging
Polylysine pre-treated cover glass was placed in each well
of a 6-well plate. BCap37 cells were seeded onto the 6-well
plate at a density of 2 × 105 cells/well, cultivated in 2 mL
of growth medium, and incubated overnight at 5% CO2,
37 °C. The next day, the spent growth medium was re-
placed with 2 mL of the pre-prepared 15 mg/L nanoparticle
solution. Afterwards, the cells were incubated with the flu-
orescent nanoparticles for 1 h, and then washed with PBS,
stained with DAPI, and fixed with 4 wt% paraformalde-
hyde in PBS. The cell-attached cover glasses were trans-
ferred from each well to glass slides, and imaged at 10 and
20-fold magnifications using confocal microscopy (Carl
Zeiss, USA). BCap37 cells cultivated without the fluores-
cent nanoparticles were tested as controls. Each condition
was performed in triplicates.
1
735, 701. H NMR (300 MHz, CDCl3, ): 7.07, 7.00, 5.35,

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Qin AJ, et al. Sci China Chem May (2012) Vol.55 No.5
resonate at  4.14, 5.39, and 7.007.07, respectively, which
3 Results and discussion
are all present in PII (labeled as a, b, and Ar respectively).
Furthermore, the methylene protons adjacent to the bromo
groups in 1 and 2 all shifted to up-field after reaction, man-
ifesting that the hydrophobic cholesterol and TPE moieties
have been successfully grafted to the PI after quaterization
reaction.
Since the peaks of Ar, b and a in the spectrum of PII are
free from overlapping by other resonances, we used them to
calculate the grafting ratio of cholesterol and TPE moieties,
which are deduced as 24.0% and 16.2%, respectively. The
appearance of resonance of carbon at  162 and 133126 in
13C NMR spectrum of PII further substantiates the conclu-
sion drawn from the 1H NMR spectra (Figure 2).
3.1 Synthesis of amphiphilic polymer PII
Among the AIE luminogens, tetraphenylethene (TPE) moi-
ety has drawn much attention because of its facile prepara-
tion, ready functionalization, good photostability, and high
PL efficiency [1623]. To demonstrate our design is ration-
al, we used TPE derivative as the AIE pendant in the am-
phiphilic polymer. The cholesterol derivative 1 and
poly(N-methyldietheneamine sebacate) (PI) were synthe-
sized according to the published procedures [15]. TPE de-
rivative 2 was synthesized according to the synthetic routes
shown in Scheme 1. Lithiation of diphenylmethane (3) fol-
lowed by reaction with 4-methylbenzophenone (4) generat-
ed 5, which underwent dehydration in the presence of
p-toluenesulfonic acid to form 1-(4-methylphenyl)-1,2,3-
triphenylethene (6). Bromination of 6 with N-bromosucc-
inimide in refluxed carbon tetrachloride readily furnished
the intermediate of 1-[(4-bromomethyl)phenyl)]-1,2,2-
triphenylethene (2). The amphiphilic polymer PII was syn-
thesized by the quaterization reaction (Scheme 2). Che-
losterol derivative 1 was first added to the toluene solution
of PI and the mixture was allowed to reflux for 48 h. Then,
TPE derivative 2 was added and the solution was refluxed
for an additional 24 h. The PII with Mw of 18800 was ob-
tained in 31.6% yield. The polymer is soluble in DMF but
partially soluble in chloroform, and is thermally stable, with
5% loss of its weight at a temperature of 222 °C.
3.3 Aggregation-induced emission
TPE is characterized by its AIE attribute: it is virtually
nonluminescent when molecularly dissolved in its good
solvents but emits intensely when aggregated in its poor
solvent or fabricated into thin solid film. Dose the
TPE-containing PII possess the similar feature? To answer
3.2 Structural characterization
Although the polymer is partially soluble in chloroform, we
1
used CDCl3 as a solvent to measure the H NMR spectra
because they still possess high resolution. Figure 1 shows
the 1H NMR spectra of PII as well as its intermediates of PI
and 1. The methylene protons adjacent to the ester groups in
1, the ethenyl protons in PI, and the aromatic protons in 2
Scheme 1 Synthetic routes to TPE derivative 2. PTSA = p-toluenesul-
fonic acid, NBS = N-bromosuccinimide.
Scheme 2 Synthesis of TPE and cholesterol containing amphiphilic polymer PII.

Qin AJ, et al. Sci China Chem May (2012) Vol.55 No.5
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Figure 2 13C NMR spectrum of PII in DMSO-d6 at room temperature.
The solvent peaks are marked with asterisks.
Figure 3 PL spectra of PII in DMF and DMF/water mixtures. Polymer
concentration: 3.252 mg/L; excitation wavelength: 340 nm.
Figure 1 1H NMR spectra of (A) PI, (B) 1, and (C) PII in CDCl3 at room
temperature. The solvent peaks are marked with asterisks.
solution, an intense PL peak centered at ~480 nm was rec-
orded (Figure 3). Furthermore, the emission was continu-
ously intensified with gradual addition water. The highest
emission was obtained in DMF/water mixtures with 99%
water fraction. These results indicate that PII is AIE-active.
The N-methyldietheneamine sebacate units in the back-
bone, the cholesterol and TPE moieties are hydrophobic, but
the quaternary ammonium salt segments are hydrophilic.
These amphiphilic characteristics make PII form aggregates
upon addition of water, in which the hydrophobic moieties
come together to form the inner core and the phenyl rotation
of TPE moieties should be remarkably restricted in the crowed
environment and its emission is turned on accordingly.
this question, we investigated the PL behaviors of PII in the
solution and aggregate states. Since PII is soluble in DMF,
we then dissolved the polymer in DMF for AIE characteri-
zation. The aggregates were prepared by adding water into
the DMF solution under vigorous stirring. The absorption
spectrum of PII in DMF exhibits a maximum peak at 340
nm that corresponds to the electronic absorption of the TPE
moiety.
When excited at 340 nm, the DMF solution is
non-emissive, manifesting that PII is a weak emitter when it
is molecularly dissolved. With addition of water into the

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Qin AJ, et al. Sci China Chem May (2012) Vol.55 No.5
3.4 Aggregate morphology
experimental errors in the DMF/water mixture with water
fraction larger than 90%, suggesting that the self-assembly
process was performed by several or even one polymer
chain.
The morphology of the aggregates was investigated by
scanning electron microscopy (SEM). The samples were
prepared by adding water into the DMF solution of PII and
the aqueous mixtures were stood for 3 h to allow the poly-
mer to be well self-assembled. Then, the aqueous mixture
was dropped on a silica grid and the solvents were allowed
to evaporate. As shown in Figure 4(a), PII aggregates into
particles in the aqueous mixtures. More interestingly, the
sizes of the aggregates are tunable and the diameter of the
aggregates ranges from ~2 m to 50 nm. Furthermore, the
particle sizes decreased and their surface turned to be
smoother with increasing the water fraction in the DMF/
water mixtures. When the polymer is dissolved in its good
solvent, the driving force for it to self-assemble is weak.
Thus, the polymer chain tends to assemble loosely and
entangle more randomly. Whereas, in the DMF/water mix-
tures with higher water fraction, the environment becomes
more hydrophilic, the polymer orientates itself in order to
remove the hydrophobic part from the hydrophilic envi-
ronment, which results in tighter aggregates. Thanks to the
AIE feature of the polymer, its aggregates are emissive.
Although the resolution is not high, bluish-green light of the
particles was observed under the fluorescent microscopy
(Figure 4(b)).
3.5 Cell imaging
With the AIE-active amphiphilic polymer in hand, we in-
vestigated its potential application for bio-imaging. We first
evaluated the cytotoxicity of its nanoparticles, which are to
be used for cell imaging and prepared in the DMF/water
mixtures with 99% water fraction, via classic MTT assay
[24]. As shown in Figure 5, the nanoparticles of PII dis-
played different cytotoxicity to Human breast cancer
BCap37 cells and multidrug-resistant Bats72 cells. The
former did not show significant decrease in cell viability
when treated with either 15 or 30 mg/L nanoparticle solu-
tion [statistical significance (p) > 0.05], whereas the addi-
To verify the size trends of the particles, we performed
the dynamic light scattering (DLS) measurement using the
aggregates prepared from DMF/water mixtures with water
fraction of 0, 40%, 90%, and 99%. The particle sizes are
obtained as 1718, 459, 51, and 59 nm, respectively, further
substantiating the observation from SEM. It is noteworthy
that the particle sizes remain almost unchanged within the
Figure 5 Viability of BCap37 and Bats72 cells after being incubated with
the nanoparticles of PII. Samples with the same number of asterisks are
significantly different (p < 0.05).
Figure 6 Cellular distribution of the nanoparticles of PII in BCap37.
(ac) control: BCap37 cells. Nuclei are stained with 4',6-diamidino-2-
phenylindole, and the cytoplasm is shown as light gray dots. (df) BCap37
cells incubated with the nanoparticles (concentration: 15 mg/L). The na-
noparticle is shown as green fluorescence in the cytosol. Scale bars: 20 µm.
Figure 4 (a) SEM image of PII particles, prepared in the DMF/water
mixture with 99% water fraction. (b) Fluorescent particles observed under
a fluorescent microscope. The sample was obtained in DMF/water mixture
with 90% water fraction.

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composite image of Figure 6(a) and 6(b). As shown in
Figure 6(df), no significant morphological change of
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In summary, the amphiphilic polymer PII with Mw of
18800 was synthesized by incorporation of hydrophobic
cholesterol and TPE moieties via facile quaterization reac-
tions. The obtained PII was thermally stable, soluble in
DMF, and can self-assemble to particles in the DMF/water
mixture with the hydrophobic moieties in the inner core,
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TPE. Furthermore, the particle sizes can be tuned with wa-
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