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 
126 
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: vubkhanh@gmail.com 
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, 
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 
ISSN: 2734-9381 
https://doi.org/10.51316/jst.149.etsd.2021.1.2.21 
Received: February 11, 2020; accepted: July 22, 2020 
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. 
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 
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127 
the flexible dynamics of micelles of small molecules 
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). 
In this study, we aim at using a fluorinated 
surfactant (FS) as a soft template and a variety of 
fluorocarbon solvents (perfluorodecalin, 
perfluorotributylamine, 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. 
2. Experimental 
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]. 
Synthesis of the hollow silica using 
fluorocarbon surfactant (FS) and swelling a ... 
(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. 
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 
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 
(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. 
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. 
Table 1. The outer diameter of the hollow silica 
obtained with a variety of swelling agents. 
 PFD PFTA PFO PFOB PFOA PFDA 
mean 220 92 162 155 67 23 
SD 107 30 73 40 12 3 
min 92 39 93 77 47 18 
max 483 142 364 241 95 29 
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 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 
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dissolved in this solvent in comparison with the other 
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. 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. 
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 
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. 
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. 
4. Conclusion 
In conclusions, we have successfully used a 
fluorinated surfactant to prepare the hollow structure 
of the silica nanoparticles. Perfluorodecalin, 
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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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. 
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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