Robust platforms for creating organic-inorganic nanocomposite microspheres: Decorating polymer microspheres containing mussel-inspired adhesion layers with inorganic nanoparticles

Article abstract of DOI:10.1039/c4cc05433d

We describe a method for creating robust and stable core-shell polymer microspheres decorated with inorganic (IO) nanoparticles (NPs) by a self-organization process and heterocoagulation using a mussel-inspired polymer adhesive layer between the IO NPs an

Full text of DOI:10.1039/c4cc05433d

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Robust platforms for creating organic–inorganic
nanocomposite microspheres: decorating polymer
microspheres containing mussel-inspired adhesion
layers with inorganic nanoparticles†
Cite this: Chem. Commun., 2014,
50, 14786
Received 15th July 2014,
Accepted 10th October 2014
H. Satoh,a Y. Saitoa and H. Yabu*ab
DOI: 10.1039/c4cc05433d
www.rsc.org/chemcomm
We describe a method for creating robust and stable core–shell polymer NPs from the surface of microspheres, robust composite micro-
microspheres decorated with inorganic (IO) nanoparticles (NPs) by a self- spheres must be fabricated.
organization process and heterocoagulation using a mussel-inspired
polymer adhesive layer between the IO NPs and the microspheres.
We have developed a simple method, known as self-organized
precipitation (SORP), for preparing polymer microspheres.18 The
method involves evaporating a good solvent from a polymer solution
containing a poor solvent. Microspheres have been prepared by
SORP from a variety of materials, including conductive polymers,
engineering plastics, biodegradable polymers, polymer blends, and
block copolymers.19 Furthermore, various kinds of composite micro-
spheres containing phase-separated structures consisting of
polymer-stabilized IO nanoparticles and polymers have also been
fabricated.20,21 We have also reported that the surface charges of
microspheres can be controlled by end-functionalized polymers, and
composite microspheres containing Au nanoparticles can be formed
by heterocoagulation.22 Recently, it has been reported that the
catechol group, which is found in the adhesive proteins of mussels,
shows excellent adhesion properties for many materials, including
noble metals, metal oxides, and other ceramics.23–26 Catechol groups
have also been used as anchoring moieties to fix polymers to the
surface of NPs in order to disperse them in organic solvents.27,28 An
advantage of the catechol group over other adhesive moieties, such
as thiols and silanes, is that the adhesion does not depend on the
substrate. We have also reported that amphiphilic copolymers
containing mussel-inspired adhesive moieties show excellent
adhesion properties on IO substrates under harsh conditions
such as low pHs.29 Here, we describe a novel method for creating
robust, stable polymer microspheres decorated with IO NPs by
using a mussel-inspired polymer as an adhesive layer between the IO
NPs and the microspheres. The core–shell polymer microspheres
composed of PS cores and mussel-inspired adhesive polymer shells
were prepared by SORP. Au, Fe2O3, and Al2O3 IO NPs adhered to the
surface of the microspheres. We also investigated the stability of
composite microspheres especially for high salt conditions, which
decreased electrostatic interactions.
Polymer microspheres decorated with functional inorganic (IO) nano-
particles (NPs) have received significant attention because of their
potential applications in photonics, electronics, and biotechnology.1–4
Functional IO NPs possess unique optical, electrical, mechanical, and
chemical properties derived from their high surface-to-volume ratios
and quantum effects.5–7 Polymer microspheres may be excellent
substrates for NPs because of their high surface-to-volume ratios
and their ability to disperse readily in various solvents.8–10 The surface
decoration of functional IO NPs, such as those composed of magnetic,
noble metal, and semiconductor materials, has produced composite
particles that have demonstrated great potential for use in many
applications, from photovoltaics to cancer therapies.11,12 Electrostatic
interactions between oppositely charged IO NPs and polymer
microspheres, known as heterocoagulation, are generally used
to decorate polymer microspheres with IO NPs.13 Many types of
composite microspheres, including organic–organic, IO–organic,
and IO–IO microspheres, have been developed based on hetero-
coagulation because it is a simple, facile fabrication method.3,14,15
However, one disadvantage of heterocoagulation is its poor stability
towards environmental changes in the dispersion media, such as
the solvent polarity, temperature, and salt concentration.16 For
example, the structures of heterocoagulated composite micro-
spheres are easily damaged by high salt concentrations because
of the electrostatic shielding of surface charges, both of IO NPs and
microspheres.17 To use heterocoagulated composite microspheres
for practical applications, including under biological conditions
where salt concentrations are usually high enough to remove IO
a Institute of Multidisciplinary Research for Advanced Materials (IMRAM),
Tohoku University, 2-1-1, Katahira, Aoba-Ku, Sendai 980-8577, Japan.
E-mail: yabu@tagen.tohoku.ac.jp
Mussel-inspired adhesive copolymers were synthesized according
to literature methods.29 The detailed synthetic routes are shown in
ESI,† S-1. The compositions, molecular weights, and polydispersities
b Precursory Research for Embryonic Science and Technology (PRESTO), Japan
Science and Technology Agency (JST), Saitama, Kawaguchi 332-0014, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc05433d of the synthesized copolymers are listed in Table 1. Tetrahydrofuran
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Table 1 Synthesized copolymers of N-dodecyl acrylamide (DAA) and
N-(3,4-dihydroxyphenethyl)methacrylamide (DMA)
Table 2 Prepared composite microspheres
Shell
Core
Dh (nm)
z-Potential (mV)
DAA : DMA
Mn (kg molÀ1
)
Mw/Mn
MS-1
MS-2
MS-3
MS-4
MS-5
Polymer 1
Polymer 2
Polymer 3
Polymer 4
PS–OH
PS
PS
PS
PS
PS
336 Æ 121
756 Æ 342
215 Æ 64
À42.8
À45.9
À34.1
À38.2
À44.1
Polymer 1
Polymer 2
Polymer 3
Polymer 4
2 : 1
4 : 1
8 : 1
9.9
12.0
10.2
14.1
2.3
2.5
2.4
2.4
343 Æ 170
957 Æ 492
12 : 1
microspheres (100 mL) and incubated for 1 h at 25 1C in a water bath.
UV-Vis spectra of the mixture were obtained by using a UV-Vis-NIR
spectrometer (V-670, Jasco, Japan) after dilution with membrane-
filtered water (300 mL). To evaluate effects of the addition of salt, 2 M
KCl aq. (300 mL) was added to the mixture. UV-Vis spectra of the
dispersions containing salt were also obtained. Surface structures of
composite microspheres before and after the addition of salt were
observed by SEM. Fe2O3 NPs or Al2O3 NPs (0.5 wt%) were dispersed
in membrane-filtered water. After overnight incubation, large pre-
cipitates were removed. The dispersion (50 mL) and the MS-2
aqueous dispersion (100 mL) were mixed and stirred using a vortex
mixer overnight. The composite microspheres were collected by
centrifugation (2000 rpm, 15 min, 5 1C). The surface structures of
the composite microspheres were also observed by FE-SEM. To
examine the magnetic alignment of Fe2O3 NPs and MS-1 composite
microspheres, the aqueous dispersion of composite microspheres
(10 mL) was dropped onto a glass slide and a glass cover slip was
placed over it. The alignment of the microspheres was observed by
using an optical microscope (VHX-600, Keyence, Japan) with a
neodymium magnet placed near the sample specimen.
Typical SEM and cross-sectional TEM images of MS-2 are
shown in Fig. 2(a) and (b), respectively. Spherical particles were
obtained using a core–shell structure consisting of a PS core,
which had a higher electron density than polymer 2, and a
polymer 2 shell. The structures of the other microspheres were
also observed by TEM (ESI,† S-4). The hydrodynamic diameters
(Dh) and z-potentials of microspheres are listed in Table 2.
Submicron- to micron-sized microspheres were prepared, and
their z-potentials were negative. These results indicate that
hydrophilic moieties including catechol and hydroxyl groups
located at surfaces of microspheres. It is worth noting that MS-1
and MS-2 have larger negative values of surface z-potentials
than MS-3 and MS-4, whose content of catechol groups is less
than that of MS-1 and MS-2. These results suggest that posi-
tively charged IO NPs were suitable for forming composite
microspheres by heterocoagulation. The negatively charged
(THF) was purchased from Wako Chemical Industry, Co. Ltd, Japan.
Polystyrene (PS; 44 kg molÀ1) and hydroxyl-terminated PS (PS–OH;
7 kg molÀ1) were purchased from Polymer Source Inc., Canada.
An aqueous dispersion of citrate-reduced Au NPs (average
diameter (Dave) = 20 nm, z-potential = À40 mV) was purchased
from Sigma-Aldrich, USA. An aqueous dispersion of positively
charged Au NPs (Dave = 25 nm, z-potential = +12 mV), modified
with 2-aminoethanethiol hydrochloride, was synthesized
according to a literature procedure30 (ESI,† S-2). Fe2O3 NPs
(Dave = 87 nm, z-potential = +10 mV) and Al2O3 NPs (Dave
128 nm, z-potential = +42 mV) were purchased from Aldrich.
=
Fig. 1 shows a schematic illustration of the preparation of
core–shell microspheres (MS-1-5) and composite microspheres.
Core–shell microspheres were prepared by SORP.31 The composition
of each microsphere is shown in Table 2. THF solutions of PS and
other polymers were used to prepare 1.0 mg mLÀ1 THF solutions.
Water (1 mL) was added slowly to each 1.0 mg mLÀ1 THF solution
(0.5 mL) and the THF was evaporated for 2 days. After complete
evaporation of THF, an aqueous dispersion of particles was
obtained. It has been reported that core–shell type phase separation
was formed during precipitation of two distinct polymers whose
hydrophobicities are much different.22 Since the polymers are
precipitated as microspheres in water, the more hydrophilic polymer
forms shells of microspheres, the other forms core. Since the
particles prepared from synthesized copolymers and PS form core–
shell structures and negatively charged surfaces, PS–OH should be
located on the surface of the particles by mixing with more hydro-
phobic PS. The hydrodynamic diameters (Dh) of the particles were
measured by dynamic light scattering (DLS; Zetasizer Nano, ZS-1,
Malvern, Inc., UK) after removal of free NPs by ultracentrifugation.
The structural analysis was performed by scanning electron micro-
scopy and transmission electron microscopy (ESI,† S-3). Positively
charged Au NPs (200 mL) were added to an aqueous dispersion of
Fig. 1 Schematic illustration of the preparation of core–shell micro-
Fig. 2 Typical SEM and cross-sectional TEM image of a core–shell micro-
spheres and composite microspheres.
sphere (MS-2).
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Au NPs with citrate-stabilized surfaces did not adhere to the
surface of the negatively charged microspheres (ESI,† S-5).
Since electrostatic interaction attracted core–shell polymer
particles and inorganic nanoparticles from far field, however,
only direct chemical interaction is allowed for catechol–gold
interaction. Thus, the nanoparticles should be accumulated by
electrostatic interaction, and then, catechol moieties stabilize
further. When negatively charged gold nanoparticles mixed with
negatively charged core–shell polymer particles, electrostatic
repulsion avoided adhesion of nanoparticles.
To form composites of microspheres and Au NPs, a dispersion
of positively charged Au NPs stabilized with aminoethanethiol
hydrochloride were added to the dispersion of each microsphere.
Fig. 3 shows dispersions of MS-1 to MS-5 composite microspheres
before and after the addition of 2 M KCl. To examine the stability of
composite microspheres, 2 M KCl, which is a sufficiently high salt
concentration to coagulate any IO NPs used in this experiment, was
added to the dispersion of composite microspheres.32 MS-1, which
had a polymer 1 shell that contained the highest amount of
catechol moieties, easily aggregated after Au NPs were added to
Fig. 4 Absorption spectra of dispersions of composite microspheres
composed of MS-2 (a) and MS-5 (d) before and after the addition of 2 M
KCl solution. SEM images of composite microspheres of MS-2 (b), (c), and
MS-5 (e), (f) before (b), (e) and after (c), (f) the addition of 2 M KCl solution.
the dispersion (Fig. 3(i)), and the aggregated particles were not before and after the addition of 2 M KCl, respectively. Before
changed upon the addition of 2 M KCl (Fig. 3(ii)). Because of their the addition of 2 M KCl, there were obvious absorption peaks at
excellent adhesion properties, the large number of catechol groups around l = 530 nm arising from the plasmonic absorption band
on the surface of the microspheres resulted in each Au NP being of the Au NPs adhered to the MS-2 or MS-5 microspheres (black
strongly attached to many microspheres, and the catechol groups solid lines). After the addition of 2 M KCl solution, the
acted as cross-linkers between microspheres and Au NPs. As a plasmonic absorption peak of the MS-5 composite microspheres
result, the microspheres and Au NPs aggregated three- was substantially red-shifted, although that of the MS-2 micro-
dimensionally (ESI,† S-6). This highlights the strong adhesive spheres was unchanged (black dashed lines). These results indicate
properties of catechol moieties. In contrast, other microspheres were that the Au NPs were aggregated in the MS-5 microspheres.33
still dispersed in water after the addition of Au NPs (Fig. 3(iii), (v), Fig. 4(b) and (c) show the SEM images of the MS-2 composite
(vii) and (ix)). Furthermore, the color of the MS-2 and MS-3 compo- microspheres before and after addition of the 2 M KCl solution,
site microsphere dispersions did not change after the addition of respectively. Au NPs adhered to the surface of the MS-2 micro-
2 M KCl (Fig. 3(iv) and (vi)), whereas those of MS-4 and MS-5 spheres and were homogeneously distributed. However, the Au NPs
microspheres changed from red to purple (Fig. 3(viii) and (x)). To aggregated on the surface of MS-5 microspheres after addition of
reveal the nanoscale mechanism of these colorimetric changes, the the 2 M KCl solution (Fig. 4(f)) compared with before the addition
absorption spectra in the visible light region and surface structures of the 2 M KCl solution (Fig. 4(e)). Furthermore, Au NPs removed
of the MS-2 and MS-5 composite microspheres before and after the from the surface of MS-5 microspheres were also observed as
addition of 2 M KCl were observed. At high salt concentrations, aggregates on the substrate (inset image in Fig. 4(f)). These results
colloidal particles can be salted out because the shielding of the show that MS-2 microspheres, which had mussel-inspired
electric double layers formed on the surface of the colloidal particles adhesion polymers as the shell, have stable adhesion properties
reduces the Debye length. In the MS-5 and Au NP composite for Au NPs at high salt concentrations.
microspheres, the MS-5 microsphere and the Au NP were only
When a high concentration salt solution was added to the
bound together by electrostatic interactions. Fig. 4(a) and (d) show dispersion of the composite microspheres, the shielding of the
the absorption spectra of MS-2 and MS-5 composite microspheres electric double layers weakened the electrostatic interactions
between MS-5 microspheres and Au NPs. As a result, the Au NPs
were detached from the surface of the MS-5 microspheres, and
aggregated on their surface or the substrate. In contrast, MS-2
microspheres had a negative surface charge and the adhesion
properties of the catechol moieties. Catechol moieties strongly
adhere to Au surfaces.34 When the salt solution was added, the
electric double layers of the MS-2 composite particles were also
weakened; however, the catechol moieties were still anchored
to the Au NPs on the surface of the MS-2 microspheres. To
prove the adoptability of catechol-containing polymer particles
Fig. 3 Photograph of composite microsphere dispersions of MS-1 (i),
MS-2 (iii), MS-3 (v), MS-4 (vii), and MS-5 (ix) as-mixed Au NPs, and MS-1
for other particles, we made composite particles with Al2O3 and
Fe2O3 NPs. SEM images of the MS-2 and Al2O3 NP microspheres
(ii), MS-2 (iv), MS-3 (vi), MS-4 (viii), and MS-5 (x) after the addition of 2 M KCl
solution.
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