Asymmetric PSt-EA/Ni-Silicate hollow microsphere with a hierarchical porous shell

Abstract

Novel asymmetric hollow microspheres with polystyrene-ethylacrylate (PSt-EA) semi-spherical cores and porous hierarchical Ni-Silicate shells have been successfully fabricated by the combination of emulsifier-free polymerization, a modified Stöber method and an in situ hydrothermal conversion reaction. During the conversion of the PSt-EA@SiO2 core/shell microspheres to the asymmetric PSt-EA/Ni-Silicate composite, the spherical PSt-EA was melted within the hollow Ni-Silicate interior to form semi-microspheres. Upon further treating the asymmetric hollow microspheres by 500 °C calcination for 5 h, hierarchical Ni-Silicate hollow spheres were obtained. The BET area of the asymmetric hollow PSt-EA/Ni-Silicate microspheres was 58.9 m2 g−1 and the pore diameter was about 10–20 nm. The large porous nature of the products enable them be used as carriers for bio-molecules, and experiments indicated that the maximum adsorption ability of the asymmetric hollow microspheres could reach 8.2 μmol g−1 when the concentration of Cytochrome C was 200 mmol L¬1.

Full text

Asymmetric PSt-EA/Ni-Silicate hollow microsphere with
a hierarchical porous shell†
Yufeng Zhou,
a
Wanquan Jiang,
*
a
Shouhu Xuan,
b
Xinglong Gong,
*
b
Fang Ye,
a
Sheng Wang
a
and Qunling Fang
*
c
Novel asymmetric hollow microspheres with polystyrene-ethylacrylate (PSt-EA) semi-spherical cores and
porous hierarchical Ni-Silicate shells have been successfully fabricated by the combination of emulsifier-
free polymerization, a modified St
¨
ober method and an in situ hydrothermal conversion reaction. During
the conversion of the PSt-EA@SiO
2
core/shell microspheres to the asymmetric PSt-EA/Ni-Silicate
composite, the spherical PSt-EA was melted within the hollow Ni-Silicate interior to form semi-
microspheres. Upon further treating the asymmetric hollow microspheres by 500

C calcination for 5 h,
hierarchical Ni-Silicate hollow spheres were obtained. The BET area of the asymmetric hollow PSt-EA/Ni-
Silicate microspheres was 58.9 m
2
g
1
and the pore diameter was about 10–20 nm. The large porous
nature of the products enable them be used as carriers for bio-molecules, and experiments indicated
that the maximum adsorption ability of the asymmetric hollow microspheres could reach 8.2 mmol g
1
when the concentration of Cytochrome C was 200 mmol L
1
.
1 Introduction
Nanostructured materials with hollow interiors have attracted
extensive attention because of their unique properties, such as
high surface area, large pore volume and well-dened pore
architecture.
1,2
Due to their outstanding structural features,
these hollow materials could be extended to versatile applica-
tions in adsorption, drug delivery, catalysis, chromatography
and optical systems.
3–12
During the past decades, various hollow
materials with different morphologies such as nanoparticles,
13
nanotubes,
14,15
spindles,
16
microspheres,
17
nanoboxes,
18
etc.
have been developed. Most of these functional hollow particles
exhibited well-dened porous nanostructures and they could be
widely used in biological elds, like medicine sustained-release
systems in blood or the body and diagnostic imaging,
19,20
which
is attributed to their multifunctional properties of good water
stability and delivery for particular proteins.
The nature of the porous shells is very important for the bio-
applications of hollow microspheres.
21
The surfactant assistant
sol–gel method was believed to be the most popular approach
for porous silica nanomaterials because of its easy preparation,
green process, and uniform pore distribution.
22–24
By controlling
the carbon chain length of the surfactant, nanoparticles with
different pore sizes from 2.4 nm to 3.4 nm were obtained. Large
pore size is favorable for immobilization, delivering, and sepa-
ration of proteins or enzymes due to their large sizes,
25–28
thus
many other methods should be developed to synthesize hollow
microspheres with large pores.
29,30
Yin et al. described a surface-
protected etching strategy by selectively etching the interior of
the silica spheres to yield porous silica hollow spheres.
31
The
pore size can reach as large as 13 nm and meet well with some
unique bio-requirements. Recently, hierarchical shells with
large pores, which were constructed from tiny nanosheets, have
drawn much research interest because of their special nano-
structures and functionalities.
32
By using SiO
2
as sacricial
templates, hollow metal Silicate spheres with hierarchical
porous shells could be effectively achieved.
33
In comparison to
the traditional silica and polymer porous hollow spheres,
these silicate based hierarchical porous nanomaterials exhibi-
ted much higher stability in various chemical environments,
high temperatures, and pressures. Moreover, Fe
3
O
4
nano/
microspheres, CdTe quantum dots, and Pd nanocrystals can be
further integrated into this system to give yolk–shell like
nanostructures and they exhibited highly promising applica-
tions in magnetic separation, imaging, catalysis, and
adsorption.
7,34,35
Asymmetric nanoarchitectures, oen dened as Janus
particles, exhibit unique properties by precisely controlling the
compositions, hydrophilicity/hydrophobicity, surface charges,
and molecular functionalities. Many asymmetric particles, such
a
Department of Chemistry, University of Science and Technology of China (USTC),
Hefei 230026, PR China. E-mail: jiangwq@ustc.edu.cn; Fax: +86-551-63600419; Tel:
+86-551-63607605
b
CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of
Modern Mechanics, USTC, Hefei 230027, PR China. E-mail: gongxl@ustc.edu.cn; Fax:
+86-551-63600419; Tel: +86-551-63600419
c
School of Medical Engineering, Hefei University of Technology, Hefei 230009, PR
China. E-mail: fql.good@hfut.edu.cn; Fax: +86-551-62904405; Tel: +86-551-62904353
† Electronic supplementary information (ESI) available . See DOI:
10.1039/c2tb00508e
Cite this: J. Mater. Chem. B, 2013, 1 ,
1414
Received 8th December 2012
Accepted 7th January 2013
DOI: 10.1039/c2tb00508e
www.rsc.org/MaterialsB
1414 | J. Mater. Chem. B, 2013, 1, 1414–1420 This journal is ª The Royal Society of Chemistry 2013
Journal of
Materials Chemistry B
PAPER

as submicron-sized uorescent spheres, superparamagnetic
Fe
3
O
4
@SiO
2
@polystyene microspheres, Fe
3
O
4
–Ag nanocrystals,
etc., have been developed for optical probes, spintronic memory
devices, and catalysis.
36,37
Very recently, Gao and Hu
38
reported a
novel method for making magnetic composite nanoparticles
with asymmetric nanostructures, which can be innovatively
applied in cancer therapy based on magnetically controlled
mechanical forces. However, most Janus particles are solid and
few works have been reported on hollow ones. In consideration
of the superior characteristics of porous materials, the devel-
opment of asymmetric hollow particles with hierarchical
porous shells is necessary due to their practical applications.
Moreover, approaches that can combine the multi-functional-
ities, uniform size, controllable components, and dened
nanostructures together would be also favorable.
In this work, novel asymmetric hollow microspheres (AHMs)
with semi-spherical PSt-EA cores and hierarchical porous Ni-
Silicate shells were acquired by using an in situ template scari-
fying–melting method. PSt-EA@SiO
2
core/shell microspheres,
whose size and the shell thickness were tunable, could be
employed as the precursor to be converted into AHMs. The
resulting composite possesses all the desirable properties, such
as hollow interior, porous shell, semi-spherical core with
functional surface, high stability, etc. The formation mecha-
nism was discussed and this is believed to be a simple and
versatile synthetic approach to construct other kind of AHMs
with unique functionalities. By using Cytochrome C as an
example, the AHMs were proven to be a potential candidate for
storing and delivering bio-molecules.
2 Experimental section
2.1 Chemicals
Styrene (St), ethyl acrylate (EA), acrylic acid (AA), potassium
persulfate (KPs) tetraethyl orthosilicate (TEOS), ammonium
chloride (NH
4
Cl), ammonium hydroxide (NH
3
$H
2
O, 25–28%),
nickel chloride hexahydrate (NiCl
2
$6H
2
O) and ethanol were of
analytical grade and purchased from Sinopharm Chemical
Reagent Co. Ltd. Cytochrome C was purchased from Sangon
Biotech (Shanghai) Co. Ltd. The monomer styrene was puri ed
by distillation. Deionized water was used for all experiments.
2.2 Synthesis of asymmetric PSt-EA/Ni-Silicate hollow
microsphere and hollow Ni-Silicate microspheres
The PSt-EA nanospheres were formed by soap-free emulsion
polymerization.
39
Typically, 34 mL styrene (St), 2.6 mL ethyl
acrylate (EA), 2.3 mL acrylic acid (AA) and 300 mL deionized
water were added to a 500 mL three-necked ask with
mechanical stirring for 30 min under nitrogen atmosphere.
Then 0.15 g potassium persulfate was introduced as initiator.
Aer mixing, the reactor was heated at 70

C in a water bath.
The PSt-EA product was obtained aer 7 h reaction and washed
with distilled water three times and then dried in a vacuum
oven at 50

C.
A modied St
¨
ober method was employed to synthesize the
core/shell PSt-EA@SiO
2
microspheres. Firstly, 35 mg PSt-EA
nanospheres were dispersed into a mixture of deionized water
(10 mL) and ethanol (5 mL) by ultrasonication for 2 h. The
solution was transferred to a 250 mL beaker with NH
3
$H
2
O
(3 mL) and deionized water (82 mL) under magnetic stirring.
Then, 2 mL solution of TEOS/ethanol (0.6 mL/10 mL) was
injected into the solution every 2 h. Aer the reaction was
performed for 10 h, the obtained products were collected by
centrifugation, washed with distilled water and ethanol
several times and dried in a vacuum oven at 50

C. Following
the same procedure, the PSt-EA@SiO
2
microspheres with
shell thickness 120 nm were prepared by injecting a solution
of TEOS/ethanol (1.2 mL/20 mL) and extending the reaction
to 20 h.
The asymmetric PSt-EA/Ni-Silicate hollow microspheres
were synthesized by a hydrothermal treatment. 40 mg PSt-
EA@SiO
2
core–shell microspheres were dispersed in 20 mL
H
2
O under sonication for 2 h. Then, NH
4
Cl (0.2673 g),
NiCl
2
$6H
2
O (0.28 g) and NH
3
$H
2
O (0.5 mL) were added to
the former solution. The solution was further transferred to a
30 mL Teon-lined autoclave, sealed and maintained at 140

C
for 12 h. Aer the autoclave cooled to room temperature
naturally, the nal product AHMs were collected by centrifu-
gation, washed with distilled water and ethanol several
times, and dried in a vacuum desiccation oven at 50

C
overnight.
In addition, Ni-Silicate hollow microspheres could be
obtained from the calcination of the AHMs at 500

C for 5 h
(Scheme 1).
2.3 Adsorption properties of the AHMs
A series of solutions with standard concentrations ranging
from 0.4 to 200 mM were prepared. In each adsorption experi-
ment, 10 mg AHMs were suspended in 20 mL of Cytochrome C
solution. The resulting mixture was le for 24 h to reach
adsorption equilibrium. The concentration of Cytochrome C
was determined spectrophotometrically at a wavelength of 409
nm using UV/vis measurement.
2.4 Measurements and characterization
X-ray powder diffraction (XRD) patterns of the products
were obtained with a Japan Rigaku DMax-gA rotating anode
X-ray diffractometer equipped with graphite mono-
chromatized Cu Ka radiation (l ¼ 0.154178 nm). Trans-
mission electron microscopy (TEM) photographs were taken
on a JEM-2011 with an accelerating voltage of 200 kV TEM.
The eld emission scanning electron microscope (FE-SEM,
20 kV) images were taken on a JEOL JSM-6700F SEM. X-ray
photoelectron spectra (XPS) were measured on an ESCALAB
250. The UV/vis spectra were recorded by a UV-365 spectro-
photometer. Infrared (IR) spectra were recorded in the wave-
number range 4000–500 cm
1
with a Nicolet Model 759
Fourier transform infrared (FT-IR) spectrometer using a KBr
wafer. The nitrogen (N
2
) adsorption–desorption isotherms at
about 77 K were studied using a Micromeritics, ASAP 2020M
system.
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Paper Journal of Materials Chemistry B

3 Results and discussion
Different from the traditional method to synthesize composites
with the core eccentrically positioned inside the coating shell,
our work developed a facile low temperature melting approach
to transform the spherical polymer core to a semi-spherical core
within a hollow interior. Interestingly, the hollow nature and
the melting of the polymer core took place at the same time,
which simplied this method. Firstly, the PSt-EA microspheres
were prepared by emulsier-free polymerization and their
surfaces were functionalized with a large amount of carbonyl
and hydroxyl groups. Therefore, by using the famous St
¨
ober's
method, a uniform SiO
2
shell could be coated onto the PSt-EA to
form well dispersed core –shell PSt-EA@SiO
2
microspheres.
Then, a hydrothermal treatment was conducted on the above
particles and hierarchical PSt-EA/Ni-Silicate AHMs with both
hollow interiors and asymmetric nanostructures were obtained.
Aer heating the AHMs powder at 500

C for 5 h, we obtained
the hollow Ni-Silicate microspheres.
The crystalline structures of products obtained at each step
were characterized by XRD. Fig. 1a shows the XRD pattern of
the PSt-EA microspheres. Similar to the previous report,
40
two
diffraction peaks located at 10

and 18

which can be attrib-
uted to the polymeric phase of the materials were observed,
indicating the formation of PSt-EA microspheres by the soap-
free emulsion polymerization. Aer coating with the SiO
2
shell, a broad peak at 24

was found as shown in Fig. 1b.
This diffraction peak corresponds to amorphous silica,
41
con-
rming that the obtained microspheres consisted of amor-
phous silica and PSt-EA. As soon as the PSt-EA@SiO
2
core–
shell microspheres were transformed into AHMs, only sharp
peaks located at 20

,34

,39

and 53

were observed (Fig. 1c)
and they were indexed to be silicate hydroxide hydrate
hexagonal phase (JCPDS no. 43-0664, Ni
3
Si
4
O
10
(OH)
2
$5H
2
O,
this substance was dened as Ni-Silicate in this work for
simplicity).
42
Denitely, the broad nature of the diffraction
peak indicates the low crystallinity of the nal silicate. To
further investigate the compositions of the obtained products,
the XPS spectra were recorded (Fig. 2). As shown in Fig. 2a,
two peaks of C1s and O1s at 285 eV and 520 eV were derived
from the PSt-EA microspheres. When the SiO
2
layer was coated
on the PSt-EA core, the intensity of the C1s signal decreased
sharply while the O1s peak increased markedly. In combina-
tion with the presence of the Si2p peak located at 104 eV, it
could be concluded that the uniform core–shell structure was
successfully achieved. Aer further hydrothermal treatment of
the PSt-EA@SiO
2
core–shell microspheres in the Ni
2+
based
solution, the SiO
2
shell was transformed to nickel silicate,
which corresponded well with the XPS spectra for the
appearance of Si2p and Ni2p peaks at 104 eV and 855 eV,
respectively.
Fig. 3a is the low magnication TEM image of the as-
prepared AHMs, which clearly shows that all the products
exhibited a spherical morphology. These microspheres were
very uniform and the average size was about 600 nm. Interest-
ingly, about half of the microsphere exhibited a gray color
within the dark black shell and the other hemi-sphere was
brighter, which indicated that these hybrid microspheres pre-
sented an asymmetric nanostructure (Fig. 3b). Fig. 3c shows a
typical TEM image of the AHMs, in which an arc interface
Scheme 1 Graphical illustration of the fabrication of hierarchical AHMs and hollow Ni-Silicate microspheres.
Fig. 1 The XRD patterns of PSt-EA microspheres (a), PSt-EA@SiO
2
core–shell
microspheres (b), and AHMs (c).
Fig. 2 The XPS spectra of PSt-EA microspheres (a), PSt-EA@SiO
2
core–shell
microspheres (b), and AHMs (c).
1416 | J. Mater. Chem. B, 2013, 1, 1414–1420 This journal is ª The Royal Society of Chemistry 2013
Journal of Materials Chemistry B Paper

between the two hemi-spheres was clearly observed. The gray
hemi-sphere was formed by melting the PSt-EA microsphere
within a spherical space thus the other part must be a hollow
interior. A TEM image with higher magnication (Fig. 3d)
clearly reveals that the hemi-spherical PSt-EA particle connected
well with the outer shell. Although no clear interface could be
found due to the similarity of the TEM image contrast, the
hemi-spherical PSt-EA core and the silicate shell were denitely
distinguished because of their different nanostructures. Shown
in Fig. 3e is the TEM image of the silicate shell located in the
hollow part, which indicates that the silicate shell was
composed of a large number of nanosheets with thicknesses of
several nanometres. These nanosheets were randomly packed
thus the hierarchical porous nanostructure was formed due to
the inter-particle attachment. Importantly, the PSt-EA core was
well conned within the hierarchical porous silicate shell
without any diffusion. These results also proved that the PSt-EA
microsphere was well cross-linked and they could be only
soened during the hydrothermal treatment. Moreover, when
the cross-section face of the semi-polymer core was parallel to
the TEM beam, the image exhibited a clear semi-hollow nature.
However, if the cross-section face of the polymer core was
perpendicular to the TEM beam, the particle would show a
core–shell like image. Fig. 3f shows a particle where the cross-
section face was moved from the parallel position, thus a
gibbous-shaped core was found.
To analyze the synthesis mechanism, both SEM and TEM
were used to track the product in each step. Fig. 4a represents
a typical SEM image of as-prepared PSt-EA microspheres,
indicating that the sample has spherical structure with
uniform diameter of about 310 nm. The PSt-EA microspheres
with ne dispersity and narrow size distribution are also
clearly observed from the TEM image (Fig. 4b), which agreed
well with the SEM analysis. The modied St
¨
ober method was
used for coating SiO
2
shells on PSt-EA microspheres. As shown
in Fig. 4c, the PSt-EA@SiO
2
composite microspheres were
about 450 nm and they could pack to form a hexagonal
structure on the copper plate, which clearly indicated the size
Fig. 3 The TEM images of the AHM microspheres with different magnifications (a–f).
Fig. 4 The SEM and TEM images of the PSt-EA microspheres (a and b), PSt-
EA@SiO
2
microspheres (c and d), AHMs (e), and hollow Ni-Silicate microspheres (f).
This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. B, 2013, 1, 1414–1420 | 1417
Paper Journal of Materials Chemistry B

distribution of the product was very uniform. Unfortunately,
some SiO
2
nanospheres with diameter of 80 nm were also
found in the SEM image, which must be formed during the
sol–gel progress due to the self-nucleation of the TEOS. From
the TEM image (Fig. 4d), we could nd the as-prepared
microspheres exhibited a clear core/shell nanostructure and
the shell thickness was about 80 nm. In this work, the shell
thickness can be controlled by simply varying the concentra-
tion of the TEOS precursor. Shown in Fig. SI1† are the PSt-
EA@SiO
2
samples with 80 nm and 120 nm shell thicknesses,
and the core/shell particles were also well dispersed without
any shell coalescence.
A hydrothermal reaction was conducted to convert the PSt-
EA@SiO
2
core–shell microspheres to PSt-EA/Ni-Silicate asym-
metric hollow microspheres. The SEM image (Fig. 4e) shows
that the average size of the AHMs was about 600 nm, in good
agreement with the TEM analysis. The surface of the micro-
spheres is composed of many sheet-like nanocrystals, thus the
product exhibited a ower-like hierarchical nanostructure. The
inset of Fig. 4e is the SEM image of a broken particle, which
clearly indicates the hollow nature of the AHMs. Here, if the
shell thickness of the PSt-EA@SiO
2
increased, both the inner
diameter of the AHMs and the nal silicate shell thickness
increased (Fig. SI2†). With further calcination of the hierar-
chical PSt-EA/Ni-Silicate microspheres at 500

C for 5 h, the PSt-
EA cores were decomposed at high temperature and only
Ni-Silicate hollow microspheres could be achieved. As shown in
Fig. 3f, the hierarchical sheet-like nanostructure of the Ni-Sili-
cate shell was well retained and the hollow interior was also
conserved.
Fig. 5a shows the FT-IR spectroscopy of the PSt-EA micro-
spheres, in which the bands at 1630 and 1710 cm
1
were
attributed to the stretching vibrations of the vinyl and carbonyl
groups, respectively. A strong characteristic absorption band at
1100 cm
1
was found in Fig. 5b and these peaks were attributed
to the Si–O–Si bond, which indicated the SiO
2
layer was coated
on the PSt-EA to form PSt-EA@SiO
2
microspheres. Aer the
further hydrothermal process, a band located at 1024 cm
1
was
present in Fig. 5c. Clearly, it was derived from the Si–O vibration
of the hierarchical Ni-Silicate shell. Fig. 6 illustrates the result of
the thermogravimetric analysis of the PSt-EA/Ni-Silicate AHMs.
The rst weight loss from 100 to 300

C resulted from the
evaporation of adsorbed water. The weight loss at higher
temperature (300–500

C) could be attributed to the decompo-
sition of PSt-EA cores, where the weight reduced to 71%. The
weight loss (300–500

C) includes several weight losses due to
the different degrees of polymerization of PSt-EA. When the
temperature was increased to 900

C, the slight weight loss
could be attributed to the residual polymer and crystallized
water.
In this work, the hydrothermal treatment played a critical
role in the preparation of the AHMs. This reaction was con-
ducted within an alkaline medium in the presence of Ni
2+
.
Under high temperature and pressure, the surface SiO
2
of the
PSt-EA@SiO
2
microspheres was rstly dissolved to form silicate
anions which would quickly react with the Ni
2+
to give nickel
silicate nucleation. These nanograins preferred to deposit on
the periphery of the PSt-EA@SiO
2
microspheres and continu-
ously grow to form the nanosheets. Thereaer, the nanosheets
randomly packed to form hierarchical porous shells and thus
the inner diameter would be equal to the diameter of the PSt-
EA@SiO
2
microsphere. As soon as the SiO
2
coating was
consumed, the PSt-EA microspheres would attach to the inner
interface of the Ni-Silicate shell. At high temperature, the PSt-EA
microspheres were soened and transformed into semi-spher-
ical particles within the spherical capsules due to gravity.
Because of the cross-linking nature of the PSt-EA and the
hydrophobic effect, these PSt-EA microspheres remained within
the hollow interior without outow. Finally, the asymmetric
hollow microspheres with porous shells were successfully
prepared by using the in situ template scarifying–melting
method.
The obtained PSt-EA/Ni-Silicate microspheres presented
semi-hollow nature and the Ni-Silicate shell was randomly
packed with a large amount of nanosheets, thus it was believed
that the product possessed excellent surface area. The nitrogen
adsorption–desorption isotherm and BJH pore plot of the
AHMs were recorded to evaluate the porous properties of the
sample (Fig. 7). Obviously, the nitrogen adsorption–desorption
isotherm shows a typical type-IV curve, indicating the
presence of interparticle and nonordered mesoporosity in the
Fig. 5 The FT-IR spectra of PSt-EA microspheres (a), PSt-EA@SiO
2
microspheres
(b), and AHMs (c).
Fig. 6 TG of the as-prepared AHM microspheres.
1418 | J. Mater. Chem. B, 2013, 1, 1414–1420 This journal is ª The Royal Society of Chemistry 2013
Journal of Materials Chemistry B Paper

sample. According to the BJH pore distribution curve (inset
in Fig. 7), the major pore size is found to be 12 nm. The
BET surface area of the asymmetric hollow microspheres is
58.9 m
2
g
1
, which must correspond to the mesopores from the
hierarchical shell and the hollow interior of the composite
particle.
Due to the large pore size, hollow interior, and the large
BET surface areas, the as-prepared PSt-EA/Ni-Silicate micro-
spheres could have potential applications in adsorption for
protein macromolecules. Here, Cytochrome C was chosen as a
typical bio-molecule for study and the adsorption plot of
Cytochrome C into the semi-hollow PSt-EA/Ni-Silicate micro-
spheres is shown in Fig. 8. A er the equilibration for 24 h, it
was found that the AHMs were effective for the adsorption.
With increasing Cytochrome C concentration, the adsorption
capability rstly increased sharply. When the Cytochrome C
concentration was greater than 100 mmol L
1
, the adsorption
tended level off and the maximum adsorption could reach
8.2 mmol g
1
when the concentration was 200 mmol L
1
. The
high surface area in the void space of the porous shell and
hollow interior make this kind of particle an ideal candidate for
bio-separation.
4 Conclusion
In summary, we demonstrated a facile preparation of asymmetric
hollow microspheres with PSt-EA semi-spherical cores and Ni-
Silicate hierarchical porous shells. AHMs with tunable size could
be obtained by varying the thicknesses of the SiO
2
coating on the
PSt-EA microspheres. By investigating the product under each
step, the in situ template scarifying–melting mechanism was
studied. Due to the hollow interior and porous shell, the asym-
metric hollow PSt-EA/Ni-Silicate microspheres exhibited high
BET surface areas. Toward the bio-adsorption application, we
found the maximum adsorption capability of the AHMs for
Cytochrome C could reach to 8.2 mmol g
1
at a Cytochrome C
concentration of 200 mmol L
1
. These asymmetric nano-
structures and their derivatives would be very interesting in the
future for assembling other new types of multi-functional plat-
forms for catalysis, sensing, and enzyme immobilization, which
have attracted much attention.
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (Grant no.21205026, 11072234, 11125210) and the
National Basic Research Program of China (973 Program, Grant
no. 2012CB937500) is gratefully acknowledged.
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