Responsive fluorescent core-crosslinked polymer particles based on the anthracene-containing hyperbranched poly(ether amine) (hPEA-AN)

Article abstract of DOI:10.1039/c1sm05537b

We here demonstrated a novel multistimuli responsive core-crosslinked polymer particle with fluorescence. These polymer particles with uniform size in the range of 100-300 nm were fabricated by self-assembly of the anthracene-containing amphiphilic hyperbranched poly(ether amine) (hPEA-AN) in water, followed by UV-light induced crosslinking of the core through the photodimerization of anthracene. The obtained hPEA-AN particles exhibited high fluorescence, and their size and fluorescence were responsive to external stimuli such as temperature and pH. The size, as well as the fluorescence, of the hPEA-AN particles decreased with an increase in temperature and pH. Moreover, the hPEA-AN particles can encapsulate both the hydrophobic guest molecule distytyl pyridine (DSP) and the hydrophilic guest molecule methyl orange (MO) in water, and their fluorescence can be gradually quenched after encapsulation of these guest molecules and then recovered after these guest molecules are released. Based on these characteristics of the hPEA-AN particles, we tried to develop an attractive concept in the field of drug delivery: the controlled release of guest molecules monitored by the fluorescence changes of the hPEA-AN particles.

Full text of DOI:10.1039/c1sm05537b

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PAPER
Responsive fluorescent core-crosslinked polymer particles based on the
anthracene-containing hyperbranched poly(ether amine) (hPEA–AN)†
*
Bing Yu, Xuesong Jiang and Jie Yin
Received 29th March 2011, Accepted 6th May 2011
DOI: 10.1039/c1sm05537b
We here demonstrated a novel multistimuli responsive core-crosslinked polymer particle with
fluorescence. These polymer particles with uniform size in the range of 100–300 nm were fabricated by
self-assembly of the anthracene-containing amphiphilic hyperbranched poly(ether amine) (hPEA–AN)
in water, followed by UV-light induced crosslinking of the core through the photodimerization of
anthracene. The obtained hPEA–AN particles exhibited high fluorescence, and their size and
fluorescence were responsive to external stimuli such as temperature and pH. The size, as well as the
fluorescence, of the hPEA–AN particles decreased with an increase in temperature and pH. Moreover,
the hPEA–AN particles can encapsulate both the hydrophobic guest molecule distytyl pyridine (DSP)
and the hydrophilic guest molecule methyl orange (MO) in water, and their fluorescence can be
gradually quenched after encapsulation of these guest molecules and then recovered after these guest
molecules are released. Based on these characteristics of the hPEA-AN particles, we tried to develop an
attractive concept in the field of drug delivery: the controlled release of guest molecules monitored by
the fluorescence changes of the hPEA-AN particles.
stimulus will be very attractive in controlled drug release appli-
Introduction
cations. To the best of our knowledge, however, these multi-
functional polymer particles with fluorescence and response to
stimuli are less frequently reported.40,41 In this paper, we demon-
strated novel multi-responsive fluorescent polymer particles with
crosslinked cores, the fluorescence of which is sensitive to the guest
molecules. Based on these multifunctional polymer particles, we
tried to develop an attractive concept for application of the drug
release: the controlled release of the guest molecules from polymer
particles monitored by changes in their fluorescence.
Recently much attention has been paid to polymer particles that
are responsive to environmental stimuli,1–8 which often exhibit
strong responsive behaviors dependent on external chemical and
physical stimuli, such as ionic strength, light, electric and magnetic
field, temperature and pH values.9–15 Responsive polymer parti-
cles can potentially be used as drug carriers, and their advantages
are associated with the solubilization of low-solubility drugs, the
potential for tumor targeting, and controlled drug release.16–24
However, most of the polymer particles are not stable enough
when they are transferred into organic solvent or diluted into
a concentration below their critical micelle concentration (CMC),
which limits the application of these polymer particles in
controlled drug delivery. In order to enhance the stability, the
cores of the polymer particles are often crosslinked through
different strategies,16,18,20,21,24–26 among which photo-crosslinking
is a more convenient and green way.18,25,27–29 At the same time,
polymer particles with strong fluorescence become of great
interest because of their potential biological applications such as
for biosensors and cellular imaging.30–39 Especially, fluorescent
polymer particles which also have multi-response to external
Multifunctional polymer particles were based on the amphi-
philic poly(ether amine) (PEA), which was developed by our
group recently.42–44 A series of anthracene-containing amphi-
philic hyperbranched poly(ether amine) (hPEA–AN) were
synthesized by introducing anthracene moieties into the
periphery of hPEA through a nucleophilic addition/ring-opening
reaction between the epoxy and amino groups. hPEA–AN can
self-assemble into polymer particles in aqueous solution, which
were comprised of hydrophobic anthracene moieties and PPO
chains as the core and hydrophilic PEO chains as the shell. The
core of the obtained hPEA–AN particles was further crosslinked
through photodimerization of some of the anthracene moie-
ties,45–48 and the residual anthracene moieties make the core-
crosslinked hPEA–AN particles fluorescent. The multi-respon-
sive behavior of the hPEA–AN particles to temperature, pH, and
ionic strength was investigated by DLS in detail. Moreover,
hPEA–AN particles exhibited fluorescence changes when inter-
acting with guest molecules such as hydrophobic and hydrophilic
School of Chemistry & Chemical Technology, State Key Laboratory for
Metal Matrix Composite Materials, Shanghai Jiao Tong University,
China. E-mail: ponygle@sjtu.edu.cn; Fax: +86-21-54747445; Tel: +86-
21-54743268
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1sm05537b
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dyes. These novel characteristics give hPEA–AN particles the
potential to be used in controlled drug release applications.
and ethanol (20 mL), E–AN was added in varying proportions
(shown in Table 1). The reaction mixture was refluxed for 24 h
with nitrogen protection. After cooling to room temperature, the
mixture was dialyzed against ethanol for 3 days (cellulose ester
dialysis membrane MWCO 3500), then the ethanol was removed
through rotary evaporation to get the product, which was dried
in a vacuum oven and named hPEA211–AN. hPEA321–AN and
hPEA541–AN were synthesized similarly.
Experimental
Material
9-Anthracenemethanol (hydroxyl anthracene, H–AN, Alfa
Aesar), Nile Red (NR, TCI), 3-chloro-1,2-epoxypropane (ECH,
Sinopharm Chemical Reagent), potassium hydroxide (KOH,
Sinopharm Chemical Reagent), and methyl orange (MO, Sino-
pharm Chemical Reagent) were all used without further purifi-
cation. Hyperbranched poly(ether mine) (hPEA) and distytyl
pyridine (DSP) were synthesized according to previous
reports.42,49
Preparation of hPEA–AN particles
hPEA–AN was first dissolved into THF, which is a good solvent
for both the hPEA and anthracene moieties, at a concentration
ꢁ
of 0.5 wt%. The solution was left to equilibrate at 30 C while
ultrapure water was added very slowly to the solution (5 mL of
water per hour to 1 mL of polymer solution). The polymer
solution was gently stirred during the water addition for 1 h, and
then the solution was exposed to an ultraviolet LED lamp
(Uvata) at 365 nm with an intensity about 8.4 mW cmꢀ2 to make
some of the anthracene moieties in hPEA–AN crosslink with
each other. Except for the cases specifically mentioned, the
irradiation time is 5 min. The solution with crosslinked hPEA–
AN particles was then dialyzed against water for 24 h to remove
THF using a cellulose membrane with a molecular weight cutoff
of 3500, and crosslinked hPEA–AN particles aqueous solution
with a concentration about 1 mg mLꢀ1 was obtained.
Synthesis of 9-anthracenemethoxyl glycidyl ether (E–AN)
The reaction was conducted in a two-necked flask equipped with
a nitrogen inlet and a reflux condenser. The mixture of H–AN
(0.02 mol), ECH (0.10 mol), and KOH (0.04 mol) was refluxed in
25 mL tetrahydrofuran (THF) for 24 h under a nitrogen atmo-
sphere. After cooling to room temperature, the inorganic residue
was filtered off and washed with THF several times. The filtrate
solution was rotary evaporated to remove solvent and excess
ECH. The crude product was purified by recrystallization in
1
ethanol/toluene (1 : 1) to get E–AN in a yield of about 87%. H
NMR (CDCl3, ppm): d 8.42, 8.03, 7.56, 7.48 (aromatic, 9H), 5.59
(An–CH2–, 2H), 3.92, 3.62 (–OCH2–, 2H), 3.23 (CH, 1H), 2.79,
2.64 (–CH2–, 2H). FT-IR (KBr): 2998 cmꢀ1, 2898cmꢀ1 (C–H),
1623 cmꢀ1, 1446 cmꢀ1 (C]C in anthracene), 1089 cmꢀ1 (C–O),
737 cmꢀ1 (substituents with five adjacent H in aromatic).
Characterization of hPEA–AN
1H NMR spectra was acquired with a Mercury Plus spectrometer
(Varian, Inc., USA) operating at 400 MHz using CDCl3 as the
solvent and TMS as an internal standard at room temperature.
Infrared absorption spectra (IR) measurements were carried
out with a Spectrum 100 Fourier transformation infrared
absorption spectrometer (Perkin Elmer, Inc., USA). The samples
were prepared by dropping the polymer solution onto a KBr film
and which was dried below an infrared lamp.
Synthesis of hPEA–AN
hPEA–AN was synthesized according to Scheme 1. To the
mixture of hPEA 211 (0.005 mol, in terms of its structural units)
The critical micelle concentration (CMC) of the hPEA211–AN
was determined by using NR as a fluorescent probe.50 Solutions
of hPEA211–AN at different concentrations were prepared with
ultrapure water, and about 0.1 mg NR added into each of the
samples. The solutions were sonicated for 30 min and equili-
brated in the dark overnight. The emission spectra were recorded
using
a LS-50B luminescence spectrometer (Perkin-Elmer
Company, USA). The excitation wavelength was set at 550 nm,
and the fluorescence emission spectra were recorded between 560
and 750 nm. All fluorescence spectra were recorded at room
temperature.
Characterization of hPEA–AN particles
The UV-visible spectra of the hPEA–AN particle aqueous
solution with different irradiation times were checked by a UV-
2550 spectrophotometer (Shimadzu, Japan). The hPEA–AN
particle solutions were prepared according to the method
mentioned above and diluted by 20 times before measurement.
Dynamic light scattering (DLS) measurements were per-
formed on the hPEA–AN particle aqueous solutions using
a ZS90 Zetasizer Nano ZS instrument (Malvern Instruments
Ltd., U.K.) equipped with a multi-s digital time correlation and
Scheme 1 The total synthesis process of hPEA-AN.
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a 4 mW He–Ne laser (l ¼ 633 nm) at an angle of 90ꢁ. A regu-
larized Laplace inversion (CONTIN algorithm) was applied to
analyze the obtained autocorrelation functions.
a result, hPEA can easily functionalized by anthracene with
epoxy groups to form amphiphilic anthracene-containing hPEAs
(hPEA–ANs), and the hydrophobicity of the obtained hPEA–
ANs is also tuned by the amount of hydrophobic anthracene
moieties introduced.
The fluorescence emission spectra of the hPEA–AN particles
solution were recorded under QM/TM/IM steady-state and time-
resolved fluorescence spectrofluorometer (PTI Company, USA)
equipped with a thermo cell. The excitation wavelength was set at
381 nm, and the fluorescence emission spectra were recorded
between 400 and 550 nm.
A series of anthracene containing hyperbranched poly(ether
amine)s (hPEA–ANs) were synthesized by introducing anthra-
cene moieties into the periphery of hPEA. The successful func-
tionalization of hPEAs with anthracene groups was confirmed by
1
The transmission electron microscopy (TEM) images were
obtained using a JEM-2100 (JEOL Ltd., Japan) microscope
operated at 200 kV. The sample was prepared by dropping the
hPEA–AN particle solution onto a copper grid coated with
a thin polymer film while the excess solution was removed by
filter paper, and then dried at 30 ꢁC for 24 h. No staining
treatment was performed for the measurement.
FT-IR (Fig.S1, ESI†) and H NMR (Fig. 1). Compared with
hPEA211, new peaks at 1446 cmꢀ1 and 737 cmꢀ1 assigned to
anthracene groups appeared in the FT-IR spectrum of
hPEA211–AN10, indicating that the anthracene has been
introduced to hPEA211. The structure of hPEA211–AN10 is
also supported by 1H NMR spectra, and the attribution of each
signal is also signed in Fig. 1. In comparison with hPEA211, the
existence of anthracene groups in hPEA211–AN10 is proved by
signals from the aromatic compound at ꢂ8.42, ꢂ8.03, ꢂ7.56, and
ꢂ7.48 ppm in 1H NMR spectrum of hPEA211–AN10. The
proportion of amino groups of hPEA211 modified by anthracene
is tunable by the amount of E–AN in feed during the synthesis
process. The components of the obtained anthracene-containing
hPEAs are summarized in Table 1.
The confocal microscope images were obtained using a laser
scanning confocal microscope (Leica TCS-SP5, Leica, Wetzlar,
Germany) equipped with UV lasers. Fluorescence images were
captured and processed with a digital imaging processing system
(Leica TCS software, Leica, Wetzlar, Germany). The excitation
wavelength was set at 405 nm with the X63 oil immersion
objective. The sample was prepared by dropping the hPEA–AN
particle solution onto a silicon wafer while the excess solution
ꢁ
was removed by filter paper, and then dried at 30 C for 24 h.
Formation and morphology of hPEA–AN particles
Amphiphilic hPEA–ANs are comprised of hydrophobic PPO
chains and anthracene moieties as well as hydrophilic PEO
chains, which can self-assemble into nanoparticles or micropar-
ticles in aqueous solution. As strong evidence for the self-
assembly behavior of the amphiphilic polymer into particles, the
critical micelle concentration (CMC) value for hPEA–AN was
determined by fluorescence spectroscopy using NR as a fluores-
cence probe,50 an example of which is shown in Fig. S2 (ESI†).
Drug release experiment
5 mg MO was added to 5 mL hPEA211–AN10 particle solution
with a concentration of about 10 mg mLꢀ1. After equilibrating at
30 ꢁC for 24 h, the solutions were heated to 90 ꢁC for 10 min and
centrifuged at 4000 r minꢀ1, and then the UV-vis spectra of MO
in the initial aqueous solution with hPEA211–AN10 particles as
well as in the supernatant after centrifugation were recorded.
Meanwhile, the precipitate was dispersed in 5 mL 0.1 M citrate
buffered aqueous solution with pH 3.0 and pH 8.0, and trans-
ferred into a dialysis bag (cellulose ester dialysis membrane
MWCO 3500), then dialyzed against 95 mL of the corresponding
citrate buffered aqueous solution with constant stirring at 37 ꢁC
for 120 h. At selected time intervals, 2 mL dialysate was with-
drawn and replaced with 2 mL fresh citrate buffered aqueous
solution. The amount of MO released was quantified by the UV-
vis spectra, and the fluorescence emission spectra of the solution
before dialysis as well as the residual solution in the dialysis bag
were also measured. The release experiments were carried out in
duplicate, and the averaged results were obtained.
Results and discussion
Synthesis and characterization of hPEA–ANs
Amphiphilic hyperbranched poly(ether amine)s (hPEAs) have
been already successfully synthesized through nucleophilic
addition/ring-opening reaction of commercial diglycidyl ether
and amine via a one-pot synthesis in our previous work,42 and the
hydrophilicity of the obtained hPEA is easily tuned by the mole
ratio of PPO–DE and PEO–DE in the feed. Moreover, there
were reactive amino groups in the periphery of the obtained
hPEAs, which can be further reacted with epoxy groups and this
reaction possesses the characteristics of ‘‘click-chemistry’’. As
Fig. 1 1H NMR spectra of hPEA211, E–AN, and hPEA211–AN10 in
CDCl3.
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Table 1 Composition data of hPEA-AN
Content of anthracene (mole ratio)
d
Polymera
Mole Ratio of PPO/PEO in hPEA
In feedb
In polymerc
CMC (g Lꢀ1
)
hPEA211–AN02
hPEA211–AN06
hPEA211–AN10
hPEA321–AN10
hPEA541–AN10
1 : 1
1 : 1
1 : 1
2 : 1
4 : 1
0.20
0.60
1.00
1.00
1.00
0.17
0.54
0.99
0.98
0.97
0.30
0.28
0.17
0.19
0.06
a
b
hPEA211–ANx, x/10 represent the proportion of the amino groups in hPEA211 theoretically modified by anthracene. The mole ratio in feed is
c
calculated as the mole ratio of E–AN and secondary amino groups in hPEA. The content of anthracene in hPEA–AN was determined by 1H
NMR. d CMC was measured by using NR as a fluorescent probe in ultrapure water.
ꢁ
The CMC values at room temperature of the five kinds of hPEA–
AN are summarized in Table 1, which indicates that the CMC
value of hPEA–AN is lower than that of hPEA with the same
PEO content. This phenomenon may be ascribed to the intro-
duction of the hydrophobic anthracene moieties.
mentioned above were measured at 25 C. The size distribution
of these hPEA–AN particles and the corresponding autocorre-
lation function are shown in Fig. 2a and Fig. 2b, respectively.
The Z-average diameters of hPEA541–AN10, hPEA321–AN10,
hPEA211–AN10, hPEA211–AN06, and hPEA211–AN02 parti-
cles in aqueous solutions are 266.8 ꢃ 33.3 nm, 258.3 ꢃ 62.7 nm,
128.7 ꢃ 14.7 nm, 127.4 ꢃ 40.3 nm, and 149.5 ꢃ 48.8 nm with low
polydispersity indices (PDI) of 0.066, 0.228, 0.043, 0.382, and
0.424, respectively. The diameters of the hPEA–AN particles
increase with the decrease in PEO content. This can be explained
by the fact that the decrease of PEO content can make hPEA–
AN more hydrophobic, resulting in the full aggregation of
hPEA–AN molecules. However, the proportion of anthracene
has little effect on the sizes of the hPEA–AN particles, which
might be ascribed to the fact that anthracene is more hydro-
phobic than PPO and resides deep in the cores of the hPEA–AN
particles. The morphology of hPEA211–AN02 was revealed by
TEM (Fig. 2c). The diameter of hPEA211–AN02 particles is
450.2 ꢃ 40.2 nm according to TEM images, which is bigger than
that determined by DLS analysis. This might be ascribed to the
particles being very flexible and somewhat flattened on the
copper grids in the TEM experiments.51–55 After 5 min irradiation
of 365 nm light, the obtained hPEA–AN particles are stable
enough and cannot even disassemble in good organic solvents
such as THF, indicating that the cores are tightly crosslinked
(Fig. S4†).
In order to obtain uniform aggregation of hPEA–AN, the
particles of hPEA–AN were prepared by gradually adding water
to homogeneous solutions of the polymers in THF. Because
THF was a common solvent for both parts of the hPEA–AN
molecules, the polymer could be well dissolved in THF without
obvious aggregation. Water was a good solvent for the PEO
chains but was a precipitant for the anthracene moieties and PPO
chains at 30 ꢁC. As water was added to the hPEA–AN solutions,
the solvent became progressively worse for the anthracene
moieties and PPO chains. When the water content reached
a critical value, the polymer solutions underwent microphase
separation, and the anthracene moieties and PPO chains started
to aggregate to form the cores of the particles while the PEO
chains formed the shells. As anthracene could undergo photo-
dimerization upon irradiation with light l > 300 nm, the cores of
the hPEA–AN particles could be crosslinked to make these
particles more stable, and the crosslinking degree can be esti-
mated by UV-vis spectra of the solutions of the hPEA-AN
particles. As shown in Fig. S3 (ESI†), the degree of crosslinking
increases with the irradiation time at 365 nm up to 90% at 10 min.
The whole strategy for the formation of hPEA-AN core-cross-
linked particles is illustrated in Scheme 2.
Moreover, we also find that the diameter of the hPEA–AN
particles changes with the increase of UV irradiation time.
Z-average diameters of the hPEA211–AN10 particles at different
UV irradiation times were measured by DLS and shown in
The formation of particles from hPEA–ANs was further
investigated by DLS experiments, and a series of hPEA–AN
particle aqueous solutions prepared according to the method
Scheme 2 Proposed mechanism of formation of hPEA-AN particles in aqueous solution.
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Fig. 3 (a) Z-average diameters of particles formed by hPEA211–AN10,
with different UV irradiation (365 nm) times obtained by DLS. Inset: size
distribution of particles formed by hPEA211–AN10 with different irra-
diation time determined by DLS at 25 ꢁC in aqueous solution. (b)
Fluorescence emission spectra of hPEA211–AN10 particles in aqueous
solution with different UV irradiation time. (c) Plot of the maximal
fluorescence emission intensity of hPEA211–AN10 vs. UV irradiation
time and their photographs.
Fig. 2 (a) Size distribution of particles formed by hPEA541–AN10,
hPEA321–AN10, hPEA211–AN10, hPEA211–AN06, and hPEA211–
AN02 determined by DLS at 25 ꢁC in aqueous solution and (b) the
corresponding autocorrelation functions. (c) TEM image of the particles
formed by hPEA211–AN02 in water at room temperature.
Fig. 3a, which indicates that the diameter of the particles
decreases with the increase of UV irradiation time, and this is
attributed to the photodimerization of anthracene moieties,
resulting in the cores of the hPEA–AN particles being cross-
linked and shrinking. Although most of the anthracene moieties
in the cores undergo photodimerization, there are still some of
them left, resulting in the strong fluorescence of the obtained
particles (Fig. S5†). Fig. 3b and 3c reveal the changes in
fluorescence intensity with irradiation time at 365 nm. Interest-
ingly, the fluorescence of hPEA–AN particles is not always
weakened by UV irradiation at 365 nm, which is strengthened
during the first ten minutes of irradiation and then becomes weak
with the increase of UV irradiation time. That is probably
because the concentration of anthracene moieties in the cores of
hPEA–AN particles is relatively higher, which makes them
mostly face-to-face overlapped to form excimers with the
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fluorescence emission at ꢂ465 nm,46,56 then the monomer fluo-
rescence emission of anthracene at ꢂ417 nm will appear rela-
tively weak. During the first ten minutes of UV irradiation,
dimerization mainly occurs between anthracence moieties which
are face-to-face overlapped, which leaves other anthracene
moieties isolated. As a result, the excimer peak becomes weak
while the monomer peak becomes strong accordingly. This
process can be obviously reflected in the changes of the fluores-
cence visible to the eye (inset photographs, Fig. 3c), and the
proposed mechanism is illustrated in Scheme 2. After all the face-
to-face overlapped anthracence moieties have been dimerized,
continuing UV irradiation will make the isolated anthracence
moieties dimerize with each other, resulting in the decrease of
fluorescence intensity of the hPEA–AN particles with the
increase in UV irradiation time. Taking the crosslinking density
of the core and fluorescence into consideration, we chose 5 min
irradiation of 365 nm light for the preparation of the hPEA–AN
particles.
particles dehydrated and smaller in size. However, when the pH
value is too high, and the hydrophilicity of the PEO shell may not
be sufficient to stabilize the particles, these particles will aggre-
gate to the large microparticles. Meanwhile, the fluorescence of
hPEA–AN particles can also be affected by pH value, and the
fluorescence emission intensity decreases with the increase of pH
value, which is shown in Fig. 5b and Figure 5c. This phenomenon
can also be ascribed to the fluorescence of anthracene being
quenched by tertiary amino groups, and that the fluorescence
quenching effect is weakened when amino groups are proton-
ated.46,57 As a result, the fluorescence is stronger at a lower pH.
Because ionic strength is one of important parameters of
aqueous solutions, we also checked the effect of ionic strength on
the size and fluorescence of hPEA–AN particles. As shown in
Fig. S6†, Z-average diameters and fluorescence emission inten-
sity of hPEA211–AN10 particles can not be obviously affected
by the addition of NaCl to the aqueous solution, which means
that hPEA-AN particles are not sensitive to ionic strength.
Responsive behaviors of hPEA–AN particles
Encapsulation and controlled release of dyes
Due to the hydrophilic shell of PEO short chain and amino
groups, the size of hPEA–AN particles as well as their fluores-
cence is expected to be responsive to temperature and pH. To test
the possibility of response to temperature, the size distribution of
hPEA–AN particles was checked by DLS at different tempera-
tures. As shown in Fig. 4a, the peak at 260 nm shifted to small
diameter with the increase of temperature, suggesting that the
size of hPEA321–AN10 particles decreases. The same trend was
also found for the other hPEA–AN particles (Fig. 4b). This
phenomenon should be caused by the shrinkage of the PEO shell
of the hPEA–AN particles. With the increase in temperature,
hydrogen bonds between the PEO chains and water molecules
can be destroyed, which made the PEO shell less hydrophilic and
caused them to shrink in water. It should be noted that this
process is reversible, and the mechanism is also shown in Scheme
2. Mori and Morawetz have already reported that the fluores-
cence of anthracene can be quenched by tertiary amino
groups.46,57 Therefore, with the increase of temperature, the shell
of particles shrinks, which makes the amino groups in the shells
closer to the anthracene moieties in the cores. As a result, the
excited state of anthracene can be quenched by amine more
efficiently at a higher temperature. Taking hPEA211–AN10 as
an example, the fluorescence emission spectra of hPEA211–
AN10 particles is shown in Fig. 4c, and the temperature depen-
dence of the fluorescence emission intensity of hPEA211–AN10
at 416 nm is shown in Fig. 4d, which indicates the fluorescence
emission intensity indeed decreases with the increase of
temperature.
Our previous research has showed that some amphiphilic poly
(ether amine), such as SiO1.5-g-PEA and PDMS-g-PEA, can self-
assemble into polymeric micelles, which can encapsulate both
hydrophobic and hydrophilic guest molecules in water,43,44 and it
has been reported that some of the hyperbranched polymers also
possessed an encapsulation effect.58–60 Fortunately, hPEA–AN
particles are also found to have the ability to encapsulate both
hydrophobic and hydrophilic dyes in water. Hydrophobic NR
which is insoluble in water could be dispersed immediately in
water in the presence of hPEA–ANs, which was indicated by the
CMC experiments. NR can be well encapsulated in the hydro-
phobic cores of hPEA–AN particles in water. To understand the
interaction between hPEA–AN particles and guest molecules, we
investigated the effect of encapsulation of a dye on the fluores-
cence of hPEA–AN particles. Another hydrophobic and fluo-
rescent dye DSP was encapsulated in hPEA–AN particles, which
can be obviously observed from the color of the solution shown
in Fig. 6a. We found that the fluorescence of hPEA–AN particles
is obviously weakened with an increase in the amount of DSP
encapsulated (Fig. 6b). As shown in Fig. 6c, the fluorescent
emission of hPEA–AN particles at 416 nm decreases with the
increase of DSP concentration, while the fluorescence of DSP at
530 nm was enhanced. This might be ascribed to the energy
transfer between the excited anthracene and DSP guest molecule.
We also check the possibility of encapsulating hydrophilic dye
MO in the hPEA–AN particles. The water-soluble dye MO was
dissolved in the hPEA211–AN10 particle aqueous solution,
which was then heated and centrifugated after equilibrium. As
shown in Fig. S7†, we measured the UV-vis spectra of the
hPEA211–AN10 aqueous solution before and after separation,
as well as MO in pure water and THF. Compared to the initial
MO/hPEA211–AN10 solution, the absorption of the superna-
tant decreases, which indicated that MO was sequestered in the
hPEA–AN particles as guest molecules. The MO loading
capacity of hPEA211–AN10 particles is 3.26 wt% determined
from the UV-vis spectra. Moreover, the maximum absorption
wavelength (lmax) blue-shifted from 464 nm to 453 nm after
adding hPEA211–AN10 particles, which is closer to the MO
The hPEA–AN particles also exhibit a response to pH as the
existence of amino groups in the backbone and periphery of
hPEA–AN, which can be protonated and deprotonated at
different pH. Fig. 5a shows the average diameters of the
hPEA211–AN10 particles at different pH, which indicates that
the average diameters of the hPEA211–AN10 particles decrease
from 208.0 ꢃ 76.8 nm to 134.7 ꢃ 27.8 nm with the increase of pH
value from 3.0 to 7.0. This can be explained by the fact that
deprotonation of amino groups at a higher pH reduces the
hydrophilicity of the outer shell of particles, which makes the
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Fig. 5 (a) Z-average diameters of particles formed by hPEA211–AN10,
at different pH obtained by DLS. Inset: size distribution of particles
formed by hPEA211–AN10 at different pH determined by DLS at 25 ꢁC
in aqueous solution. (b) Fluorescence emission spectra of hPEA211–
AN10 particles aqueous solution at different pH. (c) Plot of the fluo-
rescence emission intensity of hPEA211–AN10 at 416 nm vs. pH and
their photographs.
absorption in medium polar solvent THF (lmax ¼ 417 nm). This
means that there is a change to the micro-environment of the MO
and that it may be encapsulated in the hPEA211–AN10 particles.
At the same time, we also observed that the fluorescence of the
hPEA–AN particles is obviously weakened with the increase in
Fig. 4 (a) Size distribution of particles formed by hPEA321–AN10 in
ultrapure water determined by DLS at different temperatures. (b) Z-
average diameters of particles formed by hPEA541–AN10, hPEA321–
AN10, and hPEA211–AN10 at different temperatures obtained by DLS.
(c) Fluorescence emission spectra of hPEA211–AN10 particle aqueous
solution at different temperatures. (d) Plot of the fluorescence emission
intensity of hPEA211–AN10 at 416 nm vs. temperature and their
photographs.
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Fig. 7 (a) Photograph of 1 mg mLꢀ1 hPEA211–AN10 particles aqueous
solutions with different MO concentrations under natural light illumi-
nation and (b) the corresponding photograph of these solutions under
365 nm UV-light illumination in the dark. The MO concentrations are
Fig. 6 (a) Photograph of 1 mg mLꢀ1 hPEA211–AN10 particle aqueous
solutions with different DSP concentrations under natural light illumi-
nation and (b) the corresponding photograph of these solutions under
365 nm UV-light illumination in the dark. The DSP concentration is 0 mg
Lꢀ1, 2.45 mg Lꢀ1, 3.48 mg Lꢀ1, 7.15 mg Lꢀ1, and 21.3 mg Lꢀ1, left-to-right
respectively which are determined by UV-vis spectra. (c) Fluorescence
emission spectra of hPEA211–AN10 particle aqueous solution with
different DSP concentrations and UV irradiation time of 60 min.
0 mg mLꢀ1, 0.001 mg mLꢀ1, 0.01 mg mLꢀ1, 0.1 mg mLꢀ1, and 1 mg mLꢀ1
,
left-to-right respectively. (c) Fluorescence emission spectra of hPEA211–
AN10 particles aqueous solution with different MO concentrations.
Inset: plot of the fluorescence emission intensity of 1 mg mLꢀ1 hPEA211–
AN10 particles aqueous solution at 416 nm vs. the concentration of MO
encapsulated.
MO concentration, which is displayed in the photograph shown
in Fig. 7a and b. Fig. 7c shows the fluorescence emission spectra
of hPEA211–AN10 particle aqueous solution with different MO
concentrations, and the fluorescence emission intensity of the
particles at 416 nm decreases with the increase of MO concen-
tration, which is consistent with the photographs. Moreover,
separation experiments with different MO concentrations in the
aqueous solution were also carried out, and it’s observed that
more MO molecules are encapsulated with the increase of MO
concentration. As a result, the fluorescence emission intensity of
the hPEA211–AN10 particles at 416 nm is also weakened
quantitatively with the increase of the encapsulated MO content,
which is also shown in Fig. 7c.
moieties in their cores, which can be excited by UV-light at
365 nm with blue light emission at 416 nm. At the same time,
both DSP and MO molecules have a relatively strong visible
light absorption at 416 nm. When these dyes are encapsulated in
the particles, the distance between dye molecules and anthra-
cene moieties became so small that the energy of the blue light
emission from the hPEA–AN particles can be absorbed by these
dye molecules. As a result, the fluorescence emission intensity of
the particles at 416 nm decreases with the increase of dye
encapsulated. Especially, DSP is also a fluorescent dye with
light emission at 530 nm, and the increasing amount of DSP
encapsulated resulted in the enhanced fluorescence emission
at 530 nm.
The mechanism of the fluorescence changes of hPEA–AN
particles during the encapsulation of dyes is hypothesised as
follows (as illustrated in Scheme 3). hPEA–AN particles are
fluorescent as a result of the existence of isolated anthracene
Motivated by these novel characteristics of hPEA–AN
particles such as multi-response, strong fluorescence and
encapsulation of guest molecules with changes of fluorescence,
6860 | Soft Matter, 2011, 7, 6853–6862
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The UV-vis spectra of the solution of MO released from
hPEA211–AN10 particles within different time are shown in
Fig. S8†. The rate of MO release is much faster in pH 3.0 citrate
buffered aqueous solution than that at pH 8.0 at 37 ꢁC. As
shown in Fig. 8a, about 80% of MO loaded into the hPEA–AN
particles is released within 72 h at pH 3.0, while only about 15%
of MO is released over the same period at pH 8.0. These results
may be ascribed to the fact that hPEA–AN particles will swell at
a lower pH, resulting in the faster release of MO molecules.
Therefore, pH value plays an important role in the controlled
release of the model MO from hPEA–AN particles. Meanwhile,
the fluorescence of hPEA–AN particles changes during the
release process, which is shown in Fig. 8b. The fluorescence of
hPEA–AN particles is barely visible after encapsulation of MO
molecules, but is clearly observed after the MO molecules were
released over 72 h, which is reflected by the photograph and also
confirmed by the fluorescence emission spectra in Fig. 8b. This
phenomenon is also attributed to the blue light emission from
hPEA–AN particles not being completely absorbed by the MO
molecules after the MO molecules are released. As a result, the
fluorescence from the anthracene moieties in the cores of the
hPEA-AN particles is recovered, this mechanism is also illus-
trated in Scheme 3. Therefore, the controlled release of the guest
molecules can be monitored by the fluorescence changes of
hPEA–AN particles.
Conclusion
We demonstrated a novel responsive core-crosslinked polymer
particles with fluorescence, which were fabricated by self-
assembly of the anthracene-containing amphiphilic hyper-
branched poly(ether amine) (hPEA–AN) in water, followed by
UV-light induced crosslinking of core through the photo-
dimerization of anthracene. The obtained hPEA–AN particles
exhibit strong fluorescence, and their size and fluorescence are
responsive to external stimuli such as temperature and pH. The
size and fluorescence of the hPEA–AN particles decreases with
the increase of temperature and pH. Meanwhile, hPEA–AN
particles can encapsulate hydrophilic guest molecule Methyl
Orange (MO) in water, and exhibit pH value controlled release
behavior of MO. Moreover, their fluorescence can be gradually
quenched after encapsulation of MO and then recovered after
MO molecules are released. These novel characteristics give
hPEA–AN particles potential in the field of drug delivery: the
controlled release of the guest molecules can be monitored by the
fluorescence changes of hPEA–AN particles.
Fig. 8 (a) Cumulative MO release from hPEA–AN particles in pH 3.0
and pH 8.0 citrate buffered aqueous solution, at 37 ꢁC. (b) Fluorescence
emission spectra of hPEA211–AN10 particles at different time during the
release of MO in citrate buffered aqueous solution at pH 3.0 and their
photographs.
we tried to developed an attractive concept that the release of
guest molecules from hPEA–AN particles can be controlled by
external stimuli, and monitored by the fluorescent changes of
the hPEA–AN particles simultaneously. To verify this idea,
guest molecule release experiments were carried out in citrate
buffered aqueous solution with pH 3.0 and pH 8.0, at 37 ꢁC,
and the process is listed in the experimental section in detail.
Scheme 3 Proposed mechanism of the fluorescence changes of hPEA–AN particles during the encapsulation and release of dyes.
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28 A. Trajkovska, C. Kim, K. L. Marshall, T. H. Mourey and
S. H. Chen, Macromolecules, 2006, 39, 6983–6989.
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30 R. Huang, D. Y. Chen and M. Jiang, J. Mater. Chem., 2010, 20, 9988–
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Acknowledgements
We thank the National Nature Science Foundation of China
(No:
50803036),
the
Rising-Star
Foundation
(No.
11QA1403100) of Science &Technology Commission of
Shanghai Municipal Government, and the Shanghai Leading
Academic Discipline Project (No.B202) for their financial
support. X. S. Jiang is supported by the SMC Project of
Shanghai Jiao Tong University.
ꢀ
ꢀ
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