Preparation of hollow silica nanoparticles using fluorinated surfactant and fluorocarbon solvents
JST: Engineering and Technology for Sustainable Development
Vol. 1, Issue 2, April 2021, 126-130
Preparation of Hollow Silica Nanoparticles Using Fluorinated Surfactant
and Fluorocarbon Solvents
Tổng hợp vật liệu nano silica rỗng sử dụng chất hoạt động bề mặt và dung môi fluorocarbon
Thanh Khoa Phung1,2, Ha N. Giang3, Khanh B. Vu1,2*
1 School of Biotechnology, International University, Ho Chi Minh City, Vietnam
2 Vietnam National University, Ho Chi Minh City, Vietnam
3 Ho Chi Minh City University of Food Industry, Ho Chi Minh City, Viet Nam
*Email: [email protected]
Abstract
In this study, we used a new class of fluorinated surfactant as a soft template for the preparation of the
hollow silica nanoparticles. The size of the hollow silica nanoparticles was enlarged by incorporating a
variety of swelling agents (perfluorodecalin, perfluorotributylamine, perfluorooctane, and perfluorooctyl
bromide) into the cores of the micelles of the fluorinated surfactant. However, once we used the
perfluorinated acids (perfluorooctadecanoic acid and perfluorodecanoic acid) as swelling agents, the
structure of silica nanoparticles is solid without the formation of hollow voids. The TEM analysis combined
with copper elemental mapping of the hollow silica loaded with copper hexadecafluorophthalocyanine
indicated that the cores of the hollow silica nanoparticles are hydrophobic. The formation mechanism of the
hollow silica nanoparticles is similar to that prepared by hydrocarbon surfactant/hydrocarbon, which was
supported by the zeta potential measurements. The prepared hollow silica nanoparticles had the type IV
isotherm with the H3 hysteresis loop.
Keywords: Hollow silica, fluorinated surfactant, fluorocarbon, micelle, swelling agent
Tóm tắt
Trong nghiên cứu này, chúng tôi sử dụng chất hoạt động bề mặt fluorocarbon làm khuôn mềm để tổng hợp
các hạt nano silica rỗng. Kích thước của các hạt nano silica rỗng được tăng lên bằng cách thêm vào các
chất gây trương nở như perfluorodecalin, perfluorotributylamine, perfluorooctane và perfluorooctyl bromide
vào lõi của mixen. Tuy nhiên, khi chúng tôi sử dụng axit perfluorooctadecanoic và axit perfluorodecanoic làm
tác nhân trương nở thì cấu trúc của hạt nano silica là rắn mà không hình thành các lỗ rỗng. Phân tích TEM
kết hợp nguyên tố đồng của silica rỗng chứa đồng hexadecafluorophthalocyanine chỉ ra rằng lõi của các hạt
nano silica rỗng có tính kỵ nước. Cơ chế hình thành của các hạt nano silica rỗng tương tự như được điều
chế bằng chất hoạt động bề mặt và dung môi hydrocarbon. Các hạt nano silica rỗng có đường hấp phụ
đẳng nhiệt loại IV với vòng trễ loại H3.
Từ khóa: Silica rỗng, chất hoạt động bề mặt fluorocarbon, mixen, tác nhân gây trương nở
1. Introduction1
Hollow nanoparticles have attracted much
attention because they exhibit many potential
applications in delivery systems, catalysts, sensors,
intermediate approach between soft and hard
templates is the nanoprecipitation of polymer to form
nanoparticle templates for growing the hollow shells
[13-15]. This nanoprecipitation is easy to be
performed and a variety of polymers can be selected.
storage
materials,
photonic
materials,
and
nanoreactors [1]. Hollow nanoparticles have been
typically synthesized by soft templates and hard
templates. The soft templates can be from the
assembly of small surfactant molecules or polymeric
chains with or without swelling agents [2-5]. The
hard templates are usually polymeric nanoparticles or
metallic oxides. The most typical hard template is
polystyrene with positive charge on its surface [6,7]
or polystyrene functionalized with siloxane groups
[8]. Other hard templates are carbon [9], silica [10],
and calcium carbonate particles [11,12]. The
The templates need to be removed from the
composite particles after the synthesis to create the
hollow structure. The removal of templates can be
performed by solvent extraction or calcination at
temperature that is suitable for the pyrolysis of
templates. The size of hollow nanoparticles can be
modified by using swelling agents in soft templates or
by varying the diameter of hard template
nanoparticles.
Usually, the polymeric hard template provides
bigger size of hollow nanoparticles than the soft
templates; however, the hard template approach gives
better particle size distribution than the soft
templates. The poor size distribution of hollow
nanoparticles using the soft templates originates from
ISSN: 2734-9381
Received: February 11, 2020; accepted: July 22, 2020
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Vol. 1, Issue 2, April 2021, 126-130
the flexible dynamics of micelles of small molecules
re-dispersed in ethanol. This purification procedure
was repeated three times to get the as-synthesized
nanoparticles. The as-synthesized sample was
calcined at 500 oC for 6h.
or polymeric chains in solution. The nature of
templates (soft or hard) in the above-mentioned
studies originates from hydrocarbon molecules
(surfactants, polymeric beads, and swelling agents).
2.2. Characterizations of materials
In this study, we aim at using a fluorinated
surfactant (FS) as a soft template and a variety of
The size of the hollow silica nanoparticles was
characterized by transmission electron microscopy
(TEM, FEI Titan G2 80-300) and dynamic light
scattering (Zetasizer Nano ZS). The elemental
mapping was performed by TEM (FEI Titan G2
80-300). The morphology was characterized by
scanning electron microscopy (SEM, FEI Quanta 200
FEG), and the textural properties were measured by
the nitrogen-sorption technique (Micromeritics ASAP
2420). Zeta potential was measured by the Zetasizer
Nano ZS.
fluorocarbon
perfluorotributylamine,
solvents
(perfluorodecalin,
perfluorooctane,
perfluorooctyl bromide, perfluorooctadecanoic acid,
and perfluorodecanoic acid) as swelling agents for the
synthesis of the hollow silica nanoparticles. We also
incorporated copper hexadecafluorophthalocyanine
into the cores of the hollow silica nanoparticles to
support the fact that the cores of the hollow silica
nanoparticles are hydrophobic, which is compatible
with copper hexadecafluorophthalocyanine.
3. Results and discussion
2. Experimental
We enlarged the size of micelles of fluorinated
surfactant by incorporating several swelling agents
into the core of the FS micelles. The swelling agents
2.1. Synthesis of materials
Synthesis of cationic fluorinated surfactant: The
details of synthesis and chemical analysis for this
surfactant can be found in the previous work (cf.
Supporting Information) [16].
we
selected
were
perfluorodecalin,
perfluorooctane,
perfluorotributylamine,
perfluorooctyl bromide, perfluorooctadecanoic acid,
and perfluorodecanoic acid. The proposed swelling
mechanism is shown in Fig. 1 and is supported by the
zeta potential measurement of the micellar solution
before and after being coated by silica species.
Synthesis of the hollow silica using
fluorocarbon surfactant (FS) and swelling agents:
fluorinated surfactant (30 mg) was dissolved in a
mixture of deionized water (15 mL) and ethanol (2
mL) to have a clear solution. The swelling agent (10
µL of perfluorodecalin, perfluorotributylamine,
perfluorooctane,
perfluorooctyl
bromide,
perfluorooctadecanoic acid, or perfluorodecanoic
acid) was added to 1 mL of the prepared surfactant
solution, and then sonication for 10 min was applied
to produce the oil-in-water emulsion. The emulsion
was cooled down to 25 °C before sodium hydroxide
0.2 M (10 µL) and tetraethyl orthosilicate (50 µL,
17% v/v in methanol) were added to this emulsion.
The hydrolysis of tetraethyl orthosilicate to silica was
performed at 25 °C for 24 h with stirring to get the
solid precipitate. Hollow silica was retrieved by the
centrifugation of the product at 6,411g for 40 min and
then re-dispersed in ethanol. This purification
procedure was repeated three times to get the as-
synthesized hollow silica nanoparticles.
Fig. 1. The proposed mechanism of the formation of
the hollow silica nanoparticles using fluorocarbon
chemistry.
The fluorinated surfactant was chosen because
the fluorinated tails assemble to form the cores of the
micelles in the aqueous solution. The fluorinated
cores of the micelles are compatible with
fluorocarbon solvent, and therefore fluorocarbon
swelling agent can diffuse into the cores of the
micelles, or the micelles re-distribute to be adsorbed
on the surfaces of fluorocarbon droplets. The zeta
potential of the fluorinated micelles in aqueous
solution is positive (+46.0 mV) at pH = 12.0 because
the fluorinated surfactant heads are positively
charged, which are compatible with the aqueous
phase. The interaction between the positively charged
micelles of fluorinated surfactant and negatively
charged silica species is direct co-condensation
following the electrostatic interaction mechanism.
Synthesis of the hollow silica loaded with
copper hexadecafluorophthalocyanine (CuF16Pc): the
mixture containing 60 mL of the prepared solution of
the surfactant (120 mg FS in a mixture of 60 mL
water and 8 mL ethanol), 0.6 mL of perfluorooctyl
bromide, and 20.9 mg of CuF16Pc was sonicated for
20 min to form a clear emulsion of oil-in-water. After
that, NaOH 0.2
M (600 µL) and tetraethyl
orthosilicate (3 mL, 20% v/v in methanol) were
added to the emulsion. The reaction mixture was
stirred at 25 °C for 24h. After that, the mixture was
centrifuged at 6,411g for 40 min and then
After
the
fluorinated
micelles
(containing
fluorocarbon) being coated with silica species, the
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zeta potential of the silica nanoparticles shows a
(standard deviation = 73 nm, min = 93 nm,
max = 364 nm). Perfluorooctyl bromide (PFOB)
swelling agent gives the mean diameter of the hollow
silica nanoparticles of 155 nm (standard
deviation = 40 nm, min = 77 nm, max = 241 nm). The
thickness of the shell of those hollow silica
nanoparticles with the presence of the swelling agent
is 16 nm.
negative value (-21.5 mV), indicating that micelles
were covered by silica species that have a negative
value of zeta potential in an alkaline solution.
The as-synthesized silica samples were
characterized by TEM, and the results are shown in
Fig. 2.
On the contrary, perfluorooctadecanoic acid
(PFOA) and perfluorodecanoic acid (PFDA) do not
play as the swelling agents, as seen in Figs. 2f and g,
because the hollow structure of the silica
nanoparticles was not observed in those cases.
Perfluorooctadecanoic acid produces the solid silica
nanoparticles with the mean diameter of 67 nm
(standard deviation = 12 nm, min = 47 nm,
max = 95 nm), and perfluorodecanoic acid produces
the solid silica nanoparticles with the mean diameter
of 23 nm (standard deviation = 3 nm, min = 18 nm,
max = 29 nm). For the ease of comparison, the outer
diameter of the hollow silica obtained with a variety
of swelling agents was summarized in Table 1.
h
i
Table 1. The outer diameter of the hollow silica
obtained with a variety of swelling agents.
PFD PFTA PFO PFOB PFOA PFDA
Fig. 2. TEM images of the as-synthesized silica
samples using (a) FS surfactant, (b)
mean 220
92
30
39
162 155
67
12
47
95
23
3
SD
107
92
73
93
40
77
FS/perfluorodecalin, (c) FS/perfluorotributylamine,
(d) FS/perfluorooctane, (e) FS/perfluorooctyl
bromide, (f) FS/perfluorooctadecanoic acid, (g)
FS/perfluorodecanoic acid, and (h) FS/perfluorooctyl
bromide with CuF16PC. (i) SEM image of the hollow
silica nanoparticles using FS/perfluorooctyl bromide
with CuF16PC.
min
max
18
29
483 142 364 241
The hollow structure of the silica nanoparticles
with swelling agents suggests that the swelling agent
has formed the nanoemulsion stabilized by the FS
surfactant in the aqueous medium. The FS surfactant
adsorbed on the surface of the nanoemulsion through
a hydrophobic - hydrophobic interaction between the
fluorinated tail of the FS surfactant and the surface of
nanoemulsion. Consequently, the FS surfactant and
swelling agent formed a positively charged surface
where the negatively charged silica species interacted
with the positively charged heads of the FS surfactant
and formed a silica layer on that surface, as
postulated in the possible mechanism in Fig. 1. On
the contrary, the acidic swelling agent consisting of
negatively charged carboxylates covered the surface
of the FS micelles through the electrostatic
interaction. Therefore, solid silica nanoparticles were
formed outside the micelles because the negatively
charged silica species and negatively charged heads
of the fluorinated acid repulse to each other.
The TEM image of silica sample without a
swelling agent is presented in Fig. 2a. This sample
exhibits hollow nanoparticles with a mean diameter
of 44 (± 5) nm and a void diameter of 5 nm. The size
of the hollow silica nanoparticles pronouncedly
changes as the swelling agent (perfluorodecalin,
perfluorotributylamine,
perfluorooctane,
or
perfluorooctyl bromide) was used, as seen through
Figs. 2b, c, d, and e.
The sizes of the hollow silica nanoparticles and
of the voids with swelling agents become much
bigger as compared with the hollow silica
nanoparticles prepared by the FS only. Particularly,
perfluorodecalin (PFD) swelling agent gives the mean
diameter of the hollow silica nanoparticles of 220 nm
(standard deviation = 107 nm, min = 92 nm,
max = 483 nm). Perfluorotributylamine (PFTA)
swelling agent gives the mean diameter of the hollow
The hollow structure of silica nanoparticles
synthesized by the FS and perfluorooctyl bromide
loaded with CuF16PC is presented in Figs. 2h and i.
We selected perfluorooctyl bromide swelling agent
for this experiment because CuF16Pc was easily
silica nanoparticles of 92 nm
(standard
deviation = 30 nm, min = 39 nm, max = 142 nm).
Perfluorooctane (PFO) swelling agent gives the mean
diameter of the hollow silica nanoparticles of 162 nm
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dissolved in this solvent in comparison with the other
often observed with aggregates of plate-like
nanoparticles giving rise to slit-shaped pores [17].
However, this type of H3 hysteresis loop has been
also observed in hollow nanoparticles with
mesoporous walls [18, 19]. The BET surface areas of
the as-synthesized and calcined samples are 288 and
915 m2/g, respectively.
swelling agents. To verify if CuF16PC has been
successfully incorporated into the hollow silica
nanoparticles, the copper elemental mapping was
performed.
The TEM images, copper and silicon elemental
mapping images of the hollow silica prepared by
FS/perfluorooctyl bromide with the presence of
CuF16PC are presented in Figs. 3a, b, c, and d. The
copper element can be clearly seen from the copper
elemental mapping image (white areas from Fig. 3b).
Fig. 4. Isotherm and pore size distribution (inset) of
the silica sample prepared using FS/perfluorooctyl
bromide with CuF16PC. The as-synthesized powder
sample was obtained by a freeze-drier. The calcined
sample was obtained with the calcination of the
as-synthesized sample at 500 oC for 6h.
In comparison with the as-synthesized sample,
an increase in the surface area of the calcined sample
indicates that the combustion of the organic
compounds (surfactant, swelling agent, and CuF16Pc)
from the as-synthesized sample creates porosity,
which leads to the enhancement of the surface area.
The pore size distributions (the inset of Fig. 4) of the
hollow silica from both as-synthesized and calcined
samples do not show any clear peaks that represent
for mean diameter of the pores. This observation
implies that the microporosity (from silica shells) and
macroporosity (from hollow voids) are likely
predominant in these materials.
Fig. 3. TEM images of the as-synthesized silica
samples using (a, c) FS/perfluorooctyl bromide with
CuF16PC, (b) its copper elemental mapping, and (d)
silicon elemental mapping.
If we superpose the TEM image with its copper
elemental mapping image, the white areas are well
overlapped with those of the hollow silica
nanoparticles. This observation indicates that
CuF16PC has been integrated into the hollow silica
nanoparticles and that nature of the cores of silica
nanoparticles is hydrophobic. Similarly, the silicon
element can be clearly seen from the silicon
elemental mapping image (white areas from Fig. 3d),
which indicates that the obtained particles are silica.
4. Conclusion
In conclusions, we have successfully used a
fluorinated surfactant to prepare the hollow structure
of the silica nanoparticles. Perfluorodecalin,
The textural properties of the hollow silica
nanoparticles synthesized by FS and perfluorooctyl
bromide loaded with CuF16PC before and after
calcination at 500 oC for 6h were measured by
nitrogen sorption analysis. The isotherms and pore
size distrutions are shown in Fig. 4. The isotherms of
the hollow silica nanoparticles from both samples
show the hysteresis loop that is characteristic for the
type IV isotherm, which is associated with capillary
condensation taking place in mesopores of the wall of
hollow silica. The hysteresis loop of this material
belongs to the type H3 loop, which does not exhibit
any limiting adsorption at high p/p0 This H3 loop is
perfluorotributylamine,
perfluorooctane,
and
perfluorooctyl bromide were successfully used as
swelling agents to enlarge the size of the hollow silica
nanoparticles. However, fluorinated acids such as
perfluorooctadecanoic acid and perfluorodecanoic
acid did not work as swelling agents because the
obtained structure of the silica nanoparticles is solid.
The repulsion of the negatively charged carboxylate
functionals in fluorinated acids with the negatively
charged silica species in aqueous solution may be
responsible for the formation of solid structure of
silica nanoparticles. The positive zeta potential of the
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JST: Engineering and Technology for Sustainable Development
Vol. 1, Issue 2, April 2021, 126-130
[9] H. Zhang, H. Xu, M. Wu, Y. Zhong, D. Wang, Z.
fluorinated surfactant and negative zeta potential of
the micelles of fluorinated surfactant coated with
silica species propose that the formation of hollow
structure of the silica nanoparticles prepared by
fluorinated surfactant/fluorocarbon (swelling agent) is
similar to that prepared by hydrocarbon
surfactant/hydrocarbon (swelling agent). The
hydrophobic nature of the cores of the silica
nanoparticles was verified by the TEM analysis
combined with the copper elemental mapping. This
hydrophobic nature of the cores indicates that they
can be loaded with other cargos than CuF16PC for
certain applications.
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Acknowledgments
This research is funded by Vietnam National
Foundation
for
Science
and
Technology
Development (NAFOSTED) under grant number
104.05-2018.47.
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