# Asymmetric Composite Nanoparticles with  Anisotropic Surface Functionalities

Authors: Yilong Wang; Hong Xu; Weili Qiang; Hongchen Gu; Donglu Shi
Journal: Journal of Nanomaterials
 (2009)

---

## Abstract

Asymmetric inorganic/organic composite nanoparticles with anisotropic surface
functionalities represent a new approach for creating smart materials, requiring the selective
introduction of chemical groups to dual components of composite, respectively. Here, we
report the synthesis of snowman-like asymmetric silica/polystyrene heterostructure with
anisotropic functionalities via a chemical method, creating nanostructure possibly offering
two-sided biologic accessibility through the chemical groups. Carboxyl group was introduced
to polystyrene component of the snowman-like composites by miniemulsion polymerization
of monomer on local surface of silica particles. Moreover, amino group was then grafted to
remained silica surface through facile surface modification of the composite nanoparticles.
The asymmetric shape of these composites was confirmed by TEM characterization.
Moreover, characteristics of anisotropic surface functionalities were indicated by Zeta
potential measurement and confocal laser microscopy after being labeled with fluorescent
dyes. This structure could find potential use as carriers for biological applications.


---

## Body

## 1. Introduction

Recently, there have been extensive research efforts on the surface
functionalization with controlled properties of the colloid particles [1, 2] and
the fabrication of the new programmable building blocks for assembly [3]. Several
new methods have also been reported on the design and control of the surface
functionalization, including deposition of molecules on particles [4],
surface-initiated heterophase polymerization [5],  and adsorption of colloids [6].
One of the research interests has been focused on the preparation of
asymmetrically functionalized particles stemmed from its potential applications
in biomedicine due to the fact that the anisotropic particles provide additional
functionality compared to their isotropic counterparts [7]. The previously
developed methods have been mainly utilized for synthesizing the micrometer
scale particles including microcontact printing [8], partial deposition [9, 10],
anisotropic decoration [11], gel trapping method [12], laser photochemical
deposition [13], micropatterning using a laminar flow microfluidic device [14],
layer-by-layer assembly of polymer [15], and electrohydrodynamic jetting [16, 17].
These asymmetric composites have significantly potential applications in
biomedicine and electronic displays due to their asymmetric chemical, physical, and surface
properties [18]. However, there have been a few research focused on the
development of anisotropically surface-functionalized nanostructures.  Therefore, novel methods are critically
needed to synthesize composites that are surface functionalized to exhibit high
degree of anisotropy in nanoscale.Here, we report
experimental results on the synthesis of nanometer scale, asymmetrically dual-functionalized
silica/polystyrene (PS) composites through a chemical approach. That is, a PS
nodule with surface carboxyl group and an amino-ended silica particle formed a
composite dimmer. As will be shown, surface functionalization can take place
selectively on either the inorganic or organic
surface regions of the composite nanoparticles. Indeed, the synthesized
composite nanoparticles possess pronounced structural anisotropy with tunable surface
properties.


## 2. Experimental Details

## 2.1. Materials

Tetraethoxysilane (TEOS), ammonium hydroxide (25% w/w),
absolute ethanol, triethylamine, toluene, sodium dodecyl sulfate (SDS), sodium
bicarbonate, and styrene were purchased from Shanghai Chemical Reagents Company, (Shanghai, China). Hexadecane (99%) andn-octadecyltrimethoxysilane (ODMS) were purchased
from Acros Organics (NJ, USA). (USA). 3-aminopropyl-triethoxy-silane
(APS), 4,4′-Azobis(4-cyanopentanoic
acid) (ACPA), and tetramethylrhodamine-5-isothiocyanate
(TRITC) were purchased from Sigma-Aldrich (MO, USA).
5-(and-6)-carboxyfluorescein succinimidyl ester (NHS-FITC) was purchased from Molecular Probes Inc. (OR, USA).
EDC⋅HCL was
purchased from Shanghai
Medpep Co. Ltd. (Shanghai, China). Styrene was washed with 5 wt% sodium hydroxide solution
first and then with distilled water three times, and stored at 4°C. Deionized water was used for
preparation of all aqueous solutions.


## 2.2. Synthesis and Local Surface Modification of Silica Particles

120 nm monodispersed silica particles were prepared according
to the developed Stöber method [19]. Ammonium hydroxide (0.5 M),
deionized water (2.1 M), and TEOS (0.12 M) were
introduced into 300cm3 of absolute ethanol at
ambient temperature under vigorous magnetic stirring for 6 hours. 90cm3 of TEOS was then added
dropwise to the above prepared solution. After 24-hour reaction, the silica
particles were collected by centrifugation (9500 rpm for 10 minutes) and frozen
dry for 16 hours.Preparation of
the locally modified silica particles: [20, 21] 0.4cm3 of ODMS in 20cm3 of toluene was impregnated
into 0.6 g of the
w-silica powder immersed in 0.3 g of water. After
addition of 0.8cm3 of triethylamine, the suspension was further stirred for 40 hours at room
temperature. Then the solid was collected by centrifugation (3000 rpm
for 10 minutes)
and followed by washing/centrifugation circles with ethanol for three times and
thermo treatment at 110°C for 3 hours under vacuum. The obtained locally
modified silica particles were kept in a vial until use.


## 2.3. Preparation of Asymmetric Composites with Dual Functionalities

First, the dispersion of 0.1 g locally modified
silica particles in the mixture
of 4.38 g styrene and 0.17 g hexadecane was
introduced into the mixture of 0.114 g SDS, 0.004 g
of sodium bicarbonate, and 50 g of deionized water. The
whole system was
emulsified by stirring at 150 rpm for 1 hour with purged
nitrogen. The emulsion
was miniemulsified by being ultrasonified at 350 w for
30 minutes. Then under
nitrogen protection, the carboxyl-composite particles were
obtained by
polymerization of styrene initiated by 0.4 g ACPA at70°C and finished within 3 hours.Then0.01cm3 APS was added into the
dispersion of 35.8 mg carboxyl composite particles in the mixture of 9.1cm3 water and 19.9cm3 ethanol. The reaction
system was magnetically stirred at PH 2.0 for 21 hours and then at PH 12 for 2
hours. At last, the dual-functionalized composites were obtained when the PH
was adjusted back to neutral by several circles of centrifugation and
redispersion with water.


## 2.4. Labeling the Fluorescence to the Asymmetric Composites

Labeling the TRITC to the silica part of composites: 0.5 mg
TRITC was firstly dissolved in the 11 g absolute ethyl
alcohol. 31.4 mg functionalized silica/PS polystyrene
composite
nanoparticles were dispersed in the 4.8 g absolute
ethyl alcohol. The reaction started when the alcohol dispersion
of the composites was added in
the TRITC alcohol solution rapidly and was magnetically stirred in dark place for
24 hours.Labeling the NHS-FITC to the PS
nodules of the composites: the 2.1 mgEDC⋅HCL and the 0.7 mg
NHS-FITC were
dissolved in the 3.5cm3 of absolute ethyl alcohol. The 10 mg
TRITC labeled composites were dispersed in
1.5cm3 of
absolute ethyl alcohol. The reaction runs for 5 hours with end-over-end process
in dark place after the two dispersions
were mixed.The fluorescence-labeled
composite nanoparticles were washed with each8cm3 of ethanol for eleven times to erase
the excess TRITC or NHS-FITC by centrifugation/redispersion circles (Sigma Laboratory Centrifuge 3 K 15, Harz, Germany).


## 2.5. Characterization

Transmission electron microscopy (TEM) experiments were performed
with a JEOL 2010 microscope (accelerating voltage of 200 kV).
Zeta potential
measurement was performed at Zetasizer 2000 instruments
(Malvern Co., Worcestershire, UK). Confocal micrographs
were obtained at Leica
TCS SP2 instrument, (Leica Microsystems Co. Ltd., Wetzlar, Germany).


## 3. Results and Discussion

In this case, the negatively
charged carboxyl group was loaded on PS part of the composite
through
miniemulsion polymerization of monomer initiated by an
initiator with carboxyl
group. The positive amino group was then attached on the
silica surface of the
composite via grafting with an alkylsilane.
Figure1 illustrates
the complete preparation process of the dual
functionalized asymmetric composite nanoparticles. As shown in
this figure, first,
the 120 nm silica particles are prepared. Then, in
step (1), the locally
modified silica particles are obtained via modification of
the partial surface of
silica particles by ODMS. In other words, only a
limited local region on each silica particle is modified
[20, 21]. In step
(2), the miniemulsion polymerization of the styrene monomer is performed
based
on the locally modified silica particles initiated by ACPA.
In completion of
above steps, the asymmetric silica/PS composite
nanoparticles with carboxyl
groups on polystyrene surface are prepared. In step (3),
grafting of the
coupling agent APS on the remained silica surface is
performed to introduce the
amino groups into the composite nanoparticles. Thus,
the dual-functionalized
asymmetric composite nanoparticles are obtained. As can be
seen in Figure 1, the
anisotropic surface functionalization starts from the
miniemulsion
polymerization of styrene monomer (step (2)). The asymmetric
morphology of the silica/PS
composite and the surface carboxyl groups on polystyrene
nodules is synchronously created via
the miniemulsion polymerization. As reported in our previous
work, the combination
of miniemulsion polymerization of monomer and local surface
modification of
silica particles is the key for the formation of the
asymmetric inorganic/organic
nanocomposites [22]. Due to the
selective nucleation and growth of styrene on
the partial surface of silica (steps (1) and (2)), further
functionalization of the amino group on the remained silica
surface can be realized
successfully. Thus, as depicted at step (3) in Figure
1, the amino groups are
loaded on the
silica particles of the composites through grafting of APS.Figure 1
Scheme of the pathway for preparation of
dual functionalized asymmetric composites
nanoparticles.In this experiment, the anisotropic functionality
was established based on the formation of the asymmetric
silica/PS composites
with snowman-like structure [23].
Figure 2(a) shows the TEM image
of the asymmetric composite
nanoparticles with carboxyl groups on the surface of PS nodules.
The snowman-like pair is composed of a 65 nm polystyrene
nodule and a 120 nm silica
particle. Figure 2(b) shows
the asymmetric nanocomoposites with anisotropic dual-functionalities,
carboxyl groups on PS nodules, and amino groups on surface of
silica particles,
respectively. We can see that the composite nanoparticles
possessed stable asymmetric
morphology after being dually functionalized. Moreover, the
asymmetric
composites exhibit obvious amphiphilic characteristics.
Figure 3 shows
comparison of interfacial behavior of the asymmetric composites
to that of pure
hydrophilic silica particles (w-SIO) or hydrophobic silica
particles (o-SIO) in
the water-toluene dual-phase system. It can be seen in
Figure 3(b) that asymmetric
composite nanoparticles could preferentially
exist at the dual-phase interface
[24]. In contrast, the
unmodified w-SIO particles
are dispersed in water (Figure 3(a)),
while the o-SIO particles, modified by ODMS
thoroughly, are
dispersed in toluene (Figure 3(c)).TEM images of (a) asymmetric
nanocomposites with only carboxyl group on the
polystyrene nodule, (b) dual-functionalized
asymmetric nanocomposites with carboxyl groups
on polystyrene nodules and amino groups on silica
surface.
(a)(b)Photograph of the interfacial
behavior of three kinds of particles in the
water-toluene dual-phase system: (a) w-SIO
particles, (b) asymmetric composite particles,
and (c) o-SIO particles.
(a)(b)(c)Variation in surface potential of the
dual-functionalized composites was characterized by Zeta
potential measurement.
Figure4 shows the variation
of Zeta potential as a function of PH value. As
can be seen, there is obvious difference of the isopotential
points between three kinds of aqueous dispersion of the silica
particles,
carboxyl-ended composite nanoparticles (COOH-CNP), and
dual-functionalized composite
nanoparticles (NH2/COOH-CNP).
The isopotential points are 3.2, 2.8,
and 5.2, respectively. Meanwhile, the quantity of surface
charge of
different composites at a certain PH value varies significantly
due to different
surface functionalities.
As can be seen in this figure, the maximum negative charge
value of COOH-CNP is
about −58mv at PH 10, slightly higher than
−45mv of
silica particles due to the presence of carboxyl groups. In contrast, the
positive charge value of the NH2/COOH-CNP is about 37 mv,
much
higher than that of the COOH-CNP at the PH 1.7 due
to the presence of the amino
groups.Figure 4
Zeta potential curves as a function of PH
value of different kinds of particles.To
investigate the possibilities of the as-synthesized dual-functionalized
asymmetric composite nanoparticles as the biologic dual-component
supporters,
the anisotropic functionalities of the composites were labeled
with two
different fluorescent molecules, respectively. In this case, the
dye of TRITC was first
used to label the amino group on silica surface through an
isothiocyanate
functional group that only could be coupled to the amino group
of APS specifically. The dye of NHS-FITC was then
labeled to the carboxyl group on PS nodules intermediated byEDC⋅HCL.
It is worth noting that TRITC
was superfluously used to occupy nearly all available amino
groups on the silica
surface to avoid the reactions between FITC and remained
amino groups. The
fluorescent properties of the labeled asymmetric composite
were examined by using
the confocal laser scanning microscopy (CLSM).
Figure 5 shows confocal
micrograph of the dual-functionalized
composite nanoparticles labeled with two kinds of
fluorescences. Figures
5(a) and
5(b) show the CLSM images
obtained at different individual excitation
wavelengths of 488 and 543 nm, corresponding to the
fluorescences of NHS-FITC
and TRITC, respectively. Figure 5(c),
superposition of Figures 5(a)
and 5(b), indicates
the anisotropic fluorescent property of the composite particles
with interfacial
yellow color, coexisting with the red and green visible
from both sides.Confocal laser microscopy images showing the dual-functionalized composite
particles labeled with different fluorescent molecules: (a) FITC fluorescence; (b) TRITC
fluorescence, and (c) overlay. Scale bar: 400 nm.
(a)(b)(c)The confocal results gave a direct evidence for the
anisotropic
surface functionalities of the silica/PS composites and the
reactive activity
of the anisotropic functional groups. The dual functionalized
composite nanoparticles could be used as
the selective carriers for two independent ingredients or
biological molecules
such as DNA or enzyme [25].


## 4. Conclusion

In conclusion, we have demonstrated a viable synthesis route
for making the asymmetric composite nanoparticles with dual
functionalities
through a two-step chemical approach. The method combining
the miniemulsion
polymerization of the organic component to local surface
modification of the
inorganic component may be extended to the introduction of
other chemical
groups into the asymmetric composite nanoparticles.
The as-synthesized
composite nanoparticles with controlled distribution of the
bioreactive surface
functionalities will find important applications in biosensors,
cell sorting,
and fabrication of smart display.



---

*Source: 620269-2009-01-04.xml*