Complex self-assembly of hyperbranched polyamidoamine/linear polyacrylic acid in water and their functionalization

Article abstract of DOI:10.1021/jp810221b

This paper studied the complex self-assembly of hyperbranched polyamidoamine (h-PAMAM) and linear polyaryalic acid (1-PAA) by the facile mixing of their aqueous solutions. The complex self-assembly behavior and mechanism were investigated by the optical m

Full text of DOI:10.1021/jp810221b

J. Phys. Chem. B 2009, 113, 7729–7736
7729
ARTICLES
Complex Self-Assembly of Hyperbranched Polyamidoamine/Linear Polyacrylic Acid in
Water and Their Functionalization
Yongwen Zhang, Wei Huang,* Yongfeng Zhou, and Deyuan Yan*
School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong UniVersity, 800 Dongchuan Road, Shanghai, 200240, P.R. China
ReceiVed: NoVember 20, 2008; ReVised Manuscript ReceiVed: March 18, 2009
This paper studied the complex self-assembly of hyperbranched polyamidoamine (h-PAMAM) and linear
polyaryalic acid (l-PAA) by the facile mixing of their aqueous solutions. The complex self-assembly behavior
and mechanism were investigated by the optical microscopy, UV-vis spectrometer, TEM, and ꢀ potential
measurements. Interestingly, various self-assembled aggregates from micelles to microscaled vesicles were
obtained by adjusting the solution pH. Moreover, the hollow structure of the vesicles was successfully stabilized
by using glutaric dialdehyde (GDA) to cross-link the PAMAM layer. As expected, the resulting hollow spheres
were able to capture different noble metal ions from their aqueous solution and reduce them into nanoparticles
in situ, hence forming the hybrid hollow spheres. Such hybrid hollow spheres might have potential application
in catalytic fields.
1. Introduction
assembly of hyperbranched poly(3-ethyl-3-oxetanemethanol)-
based polymers in recent work of our group.8 Here we describe
In recent years, the self-assembly objects with nanostructures
based on polymers have attracted increasing attention due to
their wide applications in scientific and technical fields.1 Among
them, hollow spheres with various diameters (from nanometer
to micrometer scale) have aroused especially great interests
because they can be potentially used as the carriers of catalysts,
enzymers, drugs, etc.2 To date, several famous research groups
(such as Eisenberg’s,1d Meijer’s,1i Armes’,2a Bates’,2d Rotello’s,2e
and Jiang’s1g groups) have reported some novel self-assembly
strategies and fabricated a lot of charming hollow spheres. For
example, one kind of hollow sphere was studied most exten-
sively on the basis of the micellization of block or graft
copolymers in selective solvents.3 Another kind of hollow
spheres was also prepared by using templates to form core-shell
particles, subsequently removing the core.4 Besides, a peculiar
class of hollow spheres were also fabricated effectively on the
basis of some rigid-coil polymer pairs,5a positively charged
polymer pairs,5b or complementary random copolymers pos-
sessing hydrogen bonding sites.5c,d In these pioneering works,
the self-assembly process was often driven by hydrophobic
interactions, intermolecular hydrogen bonds, parallel packing
of rigid chains, and so forth. Furthermore, some approaches
based on cross-linking reaction were usually adopted to stabilize
the self-assembled hollow spheres because this is very important
to their applications.6
a novel kind of hollow sphere by the complex self-assembly of
hyperbranched polyamidoamine (h-PAMAM) and linear poly-
(acrylic acid) (l-PAA) in water. The self-assembly behavior and
mechanism in water was exploited in detail and then the
stabilization of hollow structures by the cross-linking technology
to produce functional hollow spheres was also studied. We find
that this assembly process is driven by the specific interactions
between the amino subunits in the h-PAMAM and carboxyl
groups in the l-PAA. Besides, various morphologies of the self-
assembled aggregates can be realized by adjusting the solution
pH. More importantly, the cross-linked hollow spheres can serve
as the template and reductant to capture different noble metal
ions, reduce them into nanoparticles in situ, and form hybrid
hollow spheres. As a result, this type of hollow sphere can be
potentially applied as the effective catalyst carriers.
2. Experimental Section
2.1. Materials. Polyacrylic acid (35 wt % solution in water,
Mw 100 000; acid value 698 mg of KOH/g), N,N′-methylenebi-
sacrylamide (MBA, 99%), 1-(2-aminoethyl)piperazine (AEPZ,
99%), and fluorescein isothiocyanate isomer I (FITC, 90%) were
purchased from Aldrich. Silver nitrate (AgNO3), chloroauric acid
tetrahydrate (HAuCl4 · 4H2O), palladium(II) chloride (PdCl2),
glutaric dialdehyde aqueous solution (GDA, 25 wt %), hydro-
chloric acid (HCl), and sodium hydroxide (NaOH) were
purchased from Sino-pharm Chemical Reagent Co., Ltd.
2.2. Preparation of the Hyperbranched Polyamidoamine.
The h-PAMAM with terminated amine groups was synthesized
according to our previous work.9 Briefly, the Michael addition
polymerization of MBA with AEPZ at equal feed molar ratio
was carried out in water until the vinyl groups of MBA were
Hyperbranched polymers are a kind of highly branched
macromolecule with three-dimensional architecture and numer-
ous terminal groups, which can be synthesized conveniently in
high yields. Such features have made them promising materials
for supramolecular self-assembly.7 For example, some hollow
spheres (giant vesicles) were successfully obtained by the self-
* E-mail: hw66@sjtu.edu.cn (Wei Huang), dyyan@sjtu.edu.cn (Deyue
Yan); Fax: (+86)-21-5474-1297; Tel: (+86)-21-5474-2665.
10.1021/jp810221b CCC: $40.75  2009 American Chemical Society
Published on Web 05/12/2009

7730 J. Phys. Chem. B, Vol. 113, No. 22, 2009
Zhang et al.
consumed away. The degree of branching (DB) ) 0.42; Mn )
26 000; Mw/Mn ) 2.3. The total amine value was 5200 mg of
KOH/g.10
2.3. Preparation of Complex Self-Assembled Polymer
Vesicles. All the polymer solutions were prepared in pure water,
and the concentration of each polymer solution is 0.1 wt %.
Then the aqueous solution of h-PAMAM was dropped into the
aqueous solution of l-PAA under the mild stirring. The dosage
of h-PAMAM or l-PAA solution was dependent on the complex
ratio of h-PAMAM/l-PAA in the final mixing solution (see
section 3.1.1). With addition of the h-PAMAM solution, the
mixing solution gradually showed faint blue opalescence or
white milk-like turbidity, indicated by the formation of the self-
assembled aggregates of h-PAMAM/l-PAA. The final pHs of
all mixed solutions were adjusted by adding hydrochloric acid
(HCl; 1.0, 0.5, and 0.1 mol/L) or sodium hydroxide (NaOH;
1.0, 0.5, and 0.1 mol/L) aqueous solution.
2.4. Preparation of Complex Self-Assembled Fluorescent
Polymer Vesicles. The fluorescent labeling of h-PAMAM was
prepared as follows: 2.0 g of h-PAMAM was dissolved in 20
mL of deionized water. Then 10.0 mg of FITC was added into
it, and the mixture was kept stirring at 30 °C for 60 h. The
FITC labeled h-PAMAM were obtained by precipitating in
acetone and drying in vacuum. The resulting FITC labeled
h-PAMAM was purified by reprecipitation three times.
Figure 1. Micrograph image of the mixed h-PAMAM/l-PAA solution
(pH ) 5.0) after standing for one week. R ) 1.1:1. Inset image: the
confocal micrograph of the corresponding FITC labeled-h-PAMAM/
l-PAA solution.
same TEM. (6) Atomic force microscopy (AFM) imaging was
performed with the aid of a Nanoscope III-M system operating
in tapping mode. The samples were prepared as follows: 5 µL
of an aqueous solution of self-assembled aggregates was dropped
onto freshly cleaved mica, then frozen, and dried by the same
freeze-dryer. (7) For X-ray diffraction (XRD) analysis, the glass
plates with samples were fixed on a sample holder and subjected
to XRD analysis at room temperature on a D/MAX-2200/PC
diffractometer. XRD patterns were recorded at a scanning rate
of 4°/min in the 2θ range of 25-80° with Cu KR radiation (λ
) 1.541 78 Å). (8) Laser confocal scanning microscopy (LCSM)
was performed on an AXIOPLAN 2 MOT laser confocal
scanning microscope (ZEISS, Germany). (9) The transmittance
of the solutions was measured on a GBC Cintra 10e UV-vis
spectrophotometer (fixed wavelength 500 nm; slit width 1.5 nm).
(10) Inductively coupled plasma-optical emission spectroscopy
(ICP-OES) measurements were performed on a Prodity instru-
ment (Teledyne Leeman Laboratories Inc.). The samples were
digested by chloronitrous acid for testing.
Three milliliters of the FITC labeled h-PAMAM aqueous
solution (0.1 wt %) was mixed with 2 mL of the l-PAA aqueous
solution (0.1 wt %). The pH of the mixed solution was adjusted
to 5.0, and some self-assembled aggregates formed. The
resulting aggregates were observed by laser confocal scanning
microscopy (LCSM).
2.5. Preparation of the Stable Hollow Spheres. The hollow
structure of the h-PAMAM/l-PAA vesicles can be stabilized
by cross-linking the h-PAMAM layer as follows. On the basis
of the mole number of primary amino groups in the h-PAMAM/
l-PAA hollow spheres, 110 mol % of glutaric dialdehyde was
added into the mixture and was kept stirring at room temperature
overnight. Some precipitates appeared in this process and were
collected by centrifugation. To remove the l-PAA ingredient,
the precipitates were soaked in NaOH aqueous solution (2.0
mol/L) under mild stirring overnight, followed by centrifugation
for three cycles. Then the precipitates were further washed by
pure water to remove the residual NaOH and dried. The stable
hollow spheres of cross-linked h-PAMAM were achieved.
2.6. Characterizations and Measurements. (1) 1H nuclear
magnetic resonance (NMR) and 13C NMR were tested on a
MERCURY plus-400 spectrometer (Varian, Inc.) at 20 °C in
DMSO-d6. (2) Gel permeation chromatography (GPC) was
carried out on a Perkin-Elmer Series 200 system at 70 °C (100
mL injection column, PL gel 10 µm 300 × 7.5 mm mixed-B
columns, polystyrene calibration). DMF was used as the eluent,
and the flow rate was 1.0 mL/min. (3) Fourier transform infrared
(FTIR) spectra were recorded on an EQUINOX 55 spectrometer
using a KBr window, and all the measurements were performed
at ambient temperature. (4) Thermogravimetric analysis (TGA)
was performed on a Perkin-Elmer apparatus with the heating
rate of 20 °C/min under N2 atmosphere. (5) Transmission
electron microscopy (TEM) measurements were performed on
a JEOL JEM-2100F transmission electron microscope at an
accelerating voltage of 200 kV. The samples were prepared as
follows: 5 µL of an aqueous solution of self-assembled
aggregates was dropped onto a carbon-coated copper grid, then
frozen, and dried by Freeze-Dryer ALPHA1-4 LDplus. The
energy dispersing spectrum (EDS) analysis was obtained on the
3. Results and Discussion
3.1. Morphology of the h-PAMAM/l-PAA Aggregates.
3.1.1. Effect of the Complex Ratio. Pure h-PAMAM or l-PAA
is highly soluble in water. To our surprise, an interesting self-
assembly phenomenon was observed in the process of mixing
these two aqueous solutions. We further found the self-assembly
process was affected by the complex ratio (named as R), which
was defined as the molar ratio of amino groups in h-PAMAM
to carboxyl groups in l-PAA. When the complex ratio R was in
the range from 5:1 to 1:5, the resulting mixed solutions had
faint blue opalescence. It indicates the self-assembled aggregates
of h-PAMAM/l-PAA were formed. When R was close to 1:1
(2:1 < R < 2:3), the faint blue opalescence of the mixed solutions
could be kept even if was stored for 1 week. However, some
white precipitates were separated from the mixed solutions at
the same condition when R was higher than 2:1 (R > 2:1) or
lower than 2:3 (R < 2:3). Here the final pHs of all the mixed
solutions were adjusted to 5.0.
The morphology of the self-assembled aggregates was
observed preliminarily by optical and fluorescence microscopy.
As can be seen in Figure 1, when R was equal to 1.1:1, a lot of
spherical aggregates were found with a broad size distribution,

Self-Assembly of h-PAMAM/l-PAA in Water
J. Phys. Chem. B, Vol. 113, No. 22, 2009 7731
On the basis of the above TEM results, it can be concluded
that the average diameters of the h-PAMAM/l-PAA aggregates
are quite dependent on the solution pH. The relationship curve
between them is shown in Figure 4. It is clear that the diameters
of the h-PAMAM/l-PAA aggregates experienced an increasing
first and then decreasing process with decreasing the solution
pH. Correspondingly, the h-PAMAM/l-PAA aggregates under-
went a morphology transition from solid nanoparticles to
vesicles and then back to solid nanoparticles. Therefore the
above transmittance changes of the mixed solution were
evidently attributed to such microstructural morphology changes
of the h-PAMAM/l-PAA aggregates. It is noteworthy that the
above transition process was reversible. More importantly, the
h-PAMAM/l-PAA aggregates with the different morphologies
and diameters could be obtained by adjusting the solution pH.
3.2. Explanation of the Complex Self-Assembly. 3.2.1. ꢀ
Potentials. The surface charges of the h-PAMAM/l-PAA
aggregates depending on the solution pH were investigated by
measuring the ꢀ potential of the mixed aqueous solution. As
for the h-PAMAM/l-PAA aggregates in aqueous solution, their
surface charges were changed from the positive to the negative
with increasing the solution pH. The isoelectric point appeared
at pH 4.6. This implicates that the outside surface and inner
part of the h-PAMAM/l-PAA aggregates can be inverted by
adjusting the solution pH. At the low solution pH, h-PAMAM
was the outside surface of the aggregates. On the contrary,
l-PAA became the outside surface of the aggregates at the high
solution pH.
Figure 2. Transmittance of the h-PAMAM/l-PAA aqueous solution
depended on its pH.
and most of them were in the range 1-2 µm. According to the
fluorescent images of the corresponding self-assembled ag-
gregates labeled FITC (insert image in Figure 1), their hollow
structures were evidenced by the significant decrease of
fluorescence intensity from the peripheral ring toward the center
of the spheres.5c,8d,11 However, the self-assembled aggregates
became unstable and even disappeared (see Supporting Informa-
tion S1) when R was off 1:1 gradually. So we suppose that the
strongest specific interactions could be formed when the R value
was about 1 (i.e., at the equal mole of amino groups and
carboxyl groups). Such interactions facilitate complexing of
h-PAMAM with l-PAA in water and self-assembling into
relatively stable vesicles.
3.1.2. Effect of the Solution pH. In experiments, we found
that the complex self-assembly behavior at a certain R was also
considerably affected by the final pH of the mixed solution.
Here the stable mixed solutions of h-PAMAM/l-PAA (R ) 1)
with different pHs were selected and characterized by the
spectrophotometer. The results in Figure 2 show that the
transmittance of the mixed solution is a function of its pH. When
the pH was lower than ca. 2.06 or higher than ca. 7.46, the
mixed solution was kept transparent and its transmittance was
almost close to 100%. When the pH was in the range of ca.
2.15-7.27, the mixed solution appeared totally cloudy and its
transmittance nearly decreased to zero. It is clear that two abrupt
jumps of turbidity appeared around pH 2.1 and pH 7.3.
TEM were further applied to investigate the morphologies
of h-PAMAM/l-PAA aggregates at the different pHs, and the
results are shown in Figure 3. At the basic pH (9.23), the
aggregates were irregular spherical particles with the average
diameter of 46 nm (Figure 3A) and the solution was transparent.
When the pH was decreased to 7.16, the aggregates changed
into vesicles with the average diameter of 157 nm (Figure 3B)
and the solution became opaque with some light blue opales-
cence. When the pH was further decreased to 5.63, the average
diameter of the vesicles even increased to 1400 nm, as shown
in Figure 3C, and the solution became white and turbid. When
the pH reached 2.16, the solution became opaque again with
some light blue opalescence, and the average diameter of
vesicles decreased to 369 nm. At the lower pH 2.08, the solution
again became transparent, and the vesicles became smaller with
the average diameter of 67 nm. Simultaneously, some of them
changed into spherical particles, as shown in Figure 3E. When
the pH was below 2.0, all the vesicles were changed into
spherical particles again (Figure 3F).
3.2.2. Mechanism of the Complex Self-Assembly. We
propose that the complex self-assembly process is driven by
the specific interactions between the carboxylic acid groups of
l-PAA and the various amino groups of h-PAMAM. The
morphology transition process is driven by the hydrophilic-
hydrophobic balance depending on the solution pH.
When the solution pH was below ca. 2.1, the ionization of
the carboxyl groups in l-PAA was greatly restrained. Thus,
l-PAA chains were hydrophobic and tended to collapse together
to avoid the unfavorable contact with water. However, the
various amino groups in h-PAMAM were quaternized; thus
h-PAMAM was very hydrophilic. At this condition, the complex
process was probably performed between the collapsed l-PAA
clusters and h-PAMAM molecules, which certainly resulted in
the solid nanoparticles (Figure 6A) with l-PAA molecules
forming the hydrophobic cores and the quaternized h-PAMAM
molecules forming the hydrophilic shells.
With the increase of the solution pH from 2.2 to 7.5, the
ionization degree gradually increased for l-PAA while it
gradually decreased for h-PAMAM. According to ꢀ potential
measurement, the isoelectric point was pH ) 4.6 (Figure 5).
When 2.2 < pH < 4.6, the partially ionized l-PAAs gained some
hydrophilicity and were stretched in conformation. At this
condition, the complex between l-PAA and h-PAMAM was
probably performed at the molecular level. Since the partly
ionized l-PAA was semirigid and its hydrophilicity was still
smaller than that of the positively charged h-PAMAM mol-
ecules, the l-PAAs tended to form the inner layer while
h-PAMAMs tended to form outer shell to construct the bilayer
structure.12 As a result, polymer vesicles were generated (Figure
6B). Inversely, when 4.6 < pH < 7.5, h-PAMAMs became
semirigid and partly hydrophobic, while the hydrophility of the
ionized l-PAA increased continuously. Thus, the molecular
complex self-assembly resulted in the polymer vesicles with
h-PAMAM inner layer and l-PAA outer shell (Figure 6C).

7732 J. Phys. Chem. B, Vol. 113, No. 22, 2009
Zhang et al.
Figure 3. TEM images of h-PAMAM/l-PAA aggregates at the different pH and the corresponding size distribution histogram (the solid curve is
a Gaussian fit to the data): (A) pH ) 9.23; (B) pH ) 7.16; (C) pH ) 5.63; (D) pH ) 2.16; (E) pH ) 2.08; (F) pH ) 1.78.
Figure 4. Relationship between the average diameters of the h-
Figure 5. Relationship of the ꢀ potential and the solution pH of self-
PAMAM/l-PAA aggregates and the solution pH.
assembled h-PAMAM/l-PAA aggregrates.
When pH > 7.5, l-PAAs were fully ionized and became very
hydrophilic while h-PAMAMs were completely deprotonized
and became hydrophobic. Thus, h-PAMAMs were quickly
collapsed together to form clusters. The complex self-assemlby
of h-PAMAMs clusters and fully ionized l-PAAs was performed
to construct core-shell spherical aggregates (Figure 6D) with
the collapsed hydrophobic h-PAMAM cores and the hydrophilic
l-PAAs shells.
3.3. Functionalization of the Self-Assembled Hollow
Spheres. 3.3.1. Fixation of the Self-Assembled Hollow Spheres.
It was found that the as-prepared hollow spheres by the complex
self-assembly were sensitive to environmental impacts, such as

Self-Assembly of h-PAMAM/l-PAA in Water
J. Phys. Chem. B, Vol. 113, No. 22, 2009 7733
Figure 6. Plots for the complex self-assembly mechanism of h-PAMAM and l-PAA.
Figure 8. FTIR spectra of (A) pure h-PAMAM and (B) cross-linked
h-PAMAM hollow spheres.
ingly in the cross-linked product (Figure 8). When the cross-
linked aggregates were soaked in ethanol again, their hollow
sphere structure was still kept and observed by the optical
microscopy (Figure 9A-C). However, the un-cross-linked
aggregates in water were dissociated when the ethanol was
added. Moreover, the vesicles were broken up when the water
was evaporated (Figure 9D). These results indicate that the
hollow structure of the complex self-assembly aggregate was
fixed successfully by the cross-linking reaction.
From the insert TEM images in Figure 9, numerous folds
can be found on the cross-linked aggregates in the dry state.
This is the typical characteristic of the polymeric hollow spheres.
The outermost dark shell with the thickness of 60 nm was
attributed to the cross-linked h-PAMAM layer. The obvious
difference between the inner part and the outermost part of the
aggregates could be attributed to their hollow structures. The
diameter of the aggregates was in the range from 0.5 to 2 µm.
Besides the folding hollow spheres, some irregular mottles can
also be observed. It indicated that partial self-assembled
h-PAMAM aggregates were broken due to the stirring in the
period of cross-linking reaction.
Figure 7. Chemical cross-linking of h-PAMAM/l-PAA vesicles.
solvent, pH, and temperature. For further applications, we tried
to fix their hollow structures by chemical modifications. Around
the neutral pH, some nonionized terminal primary amino groups
in h-PAMAM could be cross-linked by GDA. The schematic
illustration of this fixation process was shown in Figure 7.
Typically, GDA (110 mol % in feed) was added into the aqueous
solution of h-PAMAM/l-PAA at pH 4.6-7.3. The mixture was
kept stirring at room temperature overnight. After centrifugation,
the precipitates were collected and washed by NaOH aqueous
solution (1.0 mol/L), deionized water, and alcohol in turn to
remove the l-PAA layer. Some cross-linked h-PAMAM hollow
spheres were obtained after being dried.
The cross-linked hollow structure of the resulting h-PAMAM
spheres was confirmed by FTIR, optical microscopy, and TEM
measurements. The FTIR spectrum of pure h-PAMAM dis-
played the absorption bands of amide I, amide II, and NH
stretching at 1649, 1536, and 3268 cm-1, respectively. However,
these bands shifted to 1608, 1608, and 3430 cm-1 correspond-
3.3.2. Carriers for Noble Metals. The application of the
hollow spheres based on polymers has become a hot topic, as
mentioned above. They can be widely used as drug or catalyst
carriers.2,13 Here our cross-linked hollow spheres of h-PAMAM

7734 J. Phys. Chem. B, Vol. 113, No. 22, 2009
Zhang et al.
Figure 9. Optical microscopy images of the cross-linked aggregates soaked in ethanol, prepared from the self-assembled vesicles at the pH of (A)
5.2, (B) 6.5, and (C) 7.5. (D) Un-cross-linked aggregates at the pH of 6.5 after the water evaporated. Inset: corresponding TEM images.
Figure 10. Cross-linked hollow spheres of h-PAMAM to load silver.
could be also used as the carriers. Other than previous reports,
interestingly, our cross-linked hollow spheres could absorb the
different metal cation from water, reduced them into corre-
sponding nanoparticles in situ and produced the polymer/metal
(Ag, Au, or Pd) hybrid spheres at last.
The schematic process for the preparation of the hybrid
spheres containing silver was shown in Figure 10. When the
AgNO3 aqueous solution was added into the aqueous dispersion
of the cross-linked hollow spheres, the amino groups in the
cross-linked h-PAMAM layer were coordinated with Ag+s
effectively. Then the single electron transfer occurred and the
Ag+s were reduced in situ into Ag nanoparticles. In this process,
the cross-linked h-PAMAM layer acted as both the stabilizing
and reducing agent.10,14 Such an approach for loading metallic
nanoparticles was rather convenient and straightforward without
any extra reductant.
The TEM images in Figure 11 showed that various nano-
particles (Au, Ag, or Pd) were loaded effectively into the
h-PAMAM shell of the cross-linked hollow spheres. The Pd or
Au nanoparticles with about 10 nm diameters dispersed well in

Self-Assembly of h-PAMAM/l-PAA in Water
J. Phys. Chem. B, Vol. 113, No. 22, 2009 7735
4. Conclusion
In conclusion, a novel kind of polymeric hollow spheres was
prepared successfully by the complex self-assembly of h-
PAMAM/l-PAA in aqueous solution. The morphology transition
of the self-assembled aggregates from solid particles to vesicles
could be realized by adjusting the solution pH. When the
solution pH was lower than 2.1, the assembled aggregates were
solid nanoparticles with l-PAA as the core and the prontonated
h-PAMAM as the shell. On the contrary, when the solution pH
was larger than 7.5, the resulting solid nanoparticles consisted
of the h-PAMAM core and the solvated l-PAA shell. When the
solution pH was in the range from 2.1 to 7.3, the aggregates
became vesicles with typical hollow structures. Interestingly,
the inverting of the inner surface and outer surface occurred in
the vesicle when the solution pH equals 4.6. The driving forces
of the complex self-assembly can be ascribed to the hydro-
philic-hydrophobic balance and the specific interactions be-
tween the carboxylic acid groups in l-PAA and various amino
groups in h-PAMAM. The hollow structure of the self-
assembled vesicles can be stabilized by using GDA to cross-
link the h-PAMAM layer. The resulting cross-linked hollow
spheres can encapsulate various noble metallic cations and
reduce them into nanoparticles in situ. The loading capacity of
them for various noble metals was ca. was ca. 15.5 wt % for
Au, 12.0 wt % for Ag, and 8.2 wt % for Pd, respectively. Such
hybrid materials can be potentially applied in metal catalysis.
Figure 11. TEM images of the cross-linked hollow spheres of
h-PAMAM (A) loading Au, (B) loading Ag, (C) loading Pd, and (D)
loading nothing. Scale bars are 500 nm.
Acknowledgment. We gratefully acknowledge financial
support provided by the National Basic Research Program
(2007CB808000), National Natural Science Foundation of China
(No. 50873058, No.50633010, No. 50503012), and Shanghai
Leading Academic Discipline Project (No. B202).
Supporting Information Available: Optical microscopy
images of the self-assembled aggregates, TEM images of the
vesicles at the different solution pH, and the TGA curves of
the different cross-linked hollow spheres of h-PAMAM, and
AFM images of the hollow spheres before and after crosslinking.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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h-PAMAM (A) loading Au, (B) loading Ag, and (C) loading Pd.
the spheres. But for the Ag nanoparticles, some agglomeration
of them was found in the spheres. The morphology difference
of various metal nanoparticles may be correlative to the
formation and growth of their crystal nucleus. Besides, the cross-
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ment further confirmed the successful carrying of Au, Ag, or
Pd nanoparticles in the cross-linked hollow spheres of h-
PAMAM and the corresponding spectra were shown in Figure
12. The loading capacity of the resulting cross-linked hollow
spheres for various noble metals was ca. 15.5 wt % for Au,
12.0 wt % for Ag, and 8.2 wt % for Pd, respectively, by EDS
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% for Pd, respectively, by TGA data in Supporting Information
S3; 15.7 wt % for Au, 11.8 wt % for Ag, and 8.4 wt % for Pd,
respectively, by ICP-OES measurements).

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