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Spray-Drying Water-Based Assembly of Hierarchical and Ordered MesoporousSilica Microparticles with Enhanced Pore Accessibility for Efficient Bio-Ad-sorption
Zhangxiong Wu, Kathryn Waldron, Xiangcheng Zhang, Yunqing Li, Lei Wu,Winston Duo Wu, Xiao Dong Chen, Dongyuan Zhao, Cordelia Selomulya
PII: S0021-9797(19)30995-6DOI: https://doi.org/10.1016/j.jcis.2019.08.084Reference: YJCIS 25340
To appear in: Journal of Colloid and Interface Science
Received Date: 28 May 2019Revised Date: 22 August 2019Accepted Date: 23 August 2019
Please cite this article as: Z. Wu, K. Waldron, X. Zhang, Y. Li, L. Wu, W. Duo Wu, X. Dong Chen, D. Zhao, C.Selomulya, Spray-Drying Water-Based Assembly of Hierarchical and Ordered Mesoporous Silica Microparticleswith Enhanced Pore Accessibility for Efficient Bio-Adsorption, Journal of Colloid and Interface Science (2019),doi: https://doi.org/10.1016/j.jcis.2019.08.084
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a coverpage and metadata, and formatting for readability, but it is not yet the definitive version of record. This version willundergo additional copyediting, typesetting and review before it is published in its final form, but we are providingthis version to give early visibility of the article. Please note that, during the production process, errors may bediscovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier Inc.
1
Spray-Drying Water-Based Assembly of Hierarchical and Ordered
Mesoporous Silica Microparticles with Enhanced Pore Accessibility for
Efficient Bio-Adsorption
Zhangxiong Wu*‡,†, Kathryn Waldron†, Xiangcheng Zhang‡, Yunqing Li‡, Lei Wu‡, Winston Duo
Wu‡, Xiao Dong Chen‡, Dongyuan Zhao†,ф, and Cordelia Selomulya*†
‡ Particle Technology Engineering Laboratory, School of Chemical and Environmental
Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow
University, Suzhou City, Jiangsu 215123, P. R. China.
ф Department of Chemistry and Laboratory of Advanced Materials, Fudan University, Shanghai,
200433, P. R. China
† Department of Chemical Engineering, Monash University, Clayton Campus, Victoria 3800,
Australia.
Corresponding authors:
Zhangxiong Wu, E-mail: zhangwu@suda.edu.cn
Cordelia Selomulya, E-mail: cordelia.selomulya@monash.edu
Abstract: The fast and scalable spray-drying-assisted evaporation-induced self-assembly (EISA)
synthesis of hierarchically porous SBA-15-type silica microparticles from a water-based system is
demonstrated. The SBA-15-type silica microparticles has bowl-like shapes, uniform micro-sizes
(~ 90 µm), large mesopores (~ 9.5 nm), hierarchical meso-/macropores (20 ~ 100 nm) and open
2
surfaces. In the synthesis, soft- and hard-templating approaches are combined in a single rapid
drying process with a non-ionic tri-block copolymer (F127) and a water-insoluble polymer colloid
(Eudragit RS, 120 nm) as the co-templates. The RS polymer colloid plays three important roles.
First, the RS nanoparticles can be partially dissolved by in-situ generated ethanol to form RS
polymer chains. The RS chains swell and modulate the hydrophilic-hydrophobic balance of F127
micelles to allow the formation of an ordered mesostructure with large mesopore sizes. Without
RS, only worm-like mesostructure with much smaller pore sizes can be formed. Second, part of
the RS nanoparticles plays a role in templating the hierarchical pores distributed throughout the
microparticles. Third, part of the RS polymer form surface “skins” and “bumps”, which can be
removed by calcination to enable a more open surface structure to overcome the low accessibility
issue of spray-dried porous microparticles. The obtained materials have high surface areas (315 ~
510 m2 g-1) and large pore volumes (0.64 ~ 1.0 cm3 g-1), which are dependent on RS concentration,
HCl concentration, silica precursor hydrolysis time and drying temperature. The representative
materials are promising for the adsorption of lysozyme. The adsorption occurs at a > three-fold
faster rate, in a five-fold larger capacity (an increase from 20 to 100 mg g-1) and without pore
blockage as compared to the adsorption onto spray-dried microparticles of similar
physicochemical properties obtained without the use of RS.
Keywords: porous silica, hierarchical structure, particle processing, spray drying, bio-adsorption
1. Introduction
The evaporation-induced self-assembly (EISA) is a universal method for the assembly and
processing of mesoporous materials.1-9 For the synthesis of ordered mesoporous silicas (OMSs),
3
the EISA process involves the competition between the condensation of hydrolyzed silica
precursors and the self-assembly of micelles. During the solvent evaporation process, the template
and hydrolyzed silica precursor enriched at the vapor-liquid interface should cooperatively co-
assemble into composite micelles in order to form ordered mesophases.2, 10-14 The formation of
ordered mesophases is much dependent on a range of factors, especially the solvent type, which
has been well studied for thin-film EISA process that involves long evaporation periods to allow
sufficient mesostructure organization.2-4, 6 It is found that the formation of OMSs with water as the
solvent in EISA systems is much challenging, mainly because a high concentration of water
promotes condensation of silica oligomers rather than micelle assembly. On the other hand, the
aerosol-assisted EISA process involves very short evaporation periods in the order of seconds,
which has been applied for the synthesis of OMSs and porous metal oxides of various structures
and particle sizes.1, 9, 10, 12, 15-18 In the aerosol drying process, the effect of solvent can be more
pronounced as it impacts upon the drying rate and thus the assembly process. In both thin-film and
aerosol-assisted EISA processes, the synthesis of OMSs with non-ionic block copolymers as the
templates and water as the solvent is challenging.
Spray drying provides a well-known, rapid, and scalable preparation method for mesoporous
microparticles.1, 9, 15-23 The use of the micro-fluidic jet spray-dryer avoids the drawback of
polydisperse droplets.24 In our previous work, this method has enabled the formation of uniform
“bowl-like” mesoporous MCM-41-type silica microparticles with water as a solvent and CTAB as
a template,25 as well as uniform mesoporous SBA-15-type silica microspheres with ethanol as a
solvent and a non-ionic block copolymer (F127) as a template.26 However, spray-dried SBA-15-
type silica microparticles in a water system has not been obtained, due to the difficulty in making
the hydrolyzed silica precursor cooperatively assemble with the template in a fast evaporating
4
water system. Moreover, aerosol-dried mesoporous microparticles are often produced with a
“skin” over the surfaces of the particles which allows only small molecules to pass through.27-30
Thus regardless of the average mesopore size of the particles, large molecules are difficult to be
adsorbed. Overcoming such an issue in using aerosol-assisted EISA for producing mesoporous
materials is therefore desirable.
A hierarchical pore architecture with small pores establishing the connection between large ones
in three dimensions is desirable for various applications, because this type of structure can produce
mass transport pathways, enhance the diffusion rate of foreign molecules and promote the
interactions of various species within the pore frameworks.31 The combined soft and hard dual-
templating method is generally applicable for the synthesis of various materials with hierarchical
pore structures.32-34 Aerosol-assisted EISA has been adopted to synthesize hierarchically porous
particles,15, 35-38 but with limited focus on the assembly of hierarchical OMSs. Moreover, in
previous reports the hard templates only play a single role of generating large pores with no
obvious influence on the self-assembly behavior of the soft micelles. In a water-based EISA system
with a non-ionic block copolymer as the soft template, a unique chemical environment needs to be
designed in order to promote self-assembly to form ordered mesostructures and to produce
hierarchical structures at the same time. To achieve this purpose, based on our previous research
in synthesizing core-shell microparticles,39-40 the commercial Eudragit RS polymer, poly (ethyl
acrylate-co-methyl methacrylate-co-trimethylammonioethyl methacrylate chloride), could be an
attractive choice. The RS nanoparticles are water-insoluble, stably dispersible in water and
ethanol-soluble. They are compatible with silica sols and surfactant dispersions. Therefore, it is
hypothesized that the co-existence of RS polymer chains and RS nanoparticles can play the roles
5
of modulating the assembly behavior of block copolymers and templating the formation of
hierarchical pore structures.
In this study, a water-based spray-drying route for the synthesis of uniform silica microparticles
with ordered and hierarchical porosity and enhanced pore accessibility to large biomolecules is
presented. Uniform hierarchically porous SBA-15-type silica microparticles are fast assembled
and processed by using the microfluidic-jet spray drying method. The key to the synthesis is the
introduction of a water-insoluble polymer (Eudragit RS) colloidal nanoparticles to the water-based
precursor solution. It plays three vital roles over the EISA process, namely, partial dissolution by
the in-situ generated ethanol from hydrolysis of TEOS to form hydrophobic polymer chains to
swell the F127 micelles and decrease the hydrophilic-hydrophobic balance to promote the
assembly of ordered mesostructure with large mesopores, forming nano-sized hard template to
generating additional large-meso/macropores, and acting as a skin former on particle surface to
circumvent the accessibility issue by opening up the porous structure after calcination.39 The
material structure and physicochemical property are analyzed in detail and their variations with
several typical experimental parameters are discussed. The obtained uniform hierarchically porous
SBA-15-type microparticles are promising for bio-adsorption, as demonstrated by the significantly
improved adsorption capacity and rate toward the adsorption of lysozyme.
2. Experimental section
2.1 Materials
Tetraethyl orthosilicate (TEOS, ≥ 99.0 %), amphiphilic Pluronic block co-polymer F127, and
lysozyme from chicken egg white were purchased from Aldrich Chemicals. The 0.05 ~ 0.5 M
hydrochloric acid solution was prepared by dilution of a 1.0 M analytical reagent grade HCl
6
purchased from Univar with distilled H2O. Eudragit RS ® 30D (denoted as RS), a copolymer of
ethyl acrylate, methyl methacrylate, and a low content of methacrylic acid ester with quaternary
ammonium groups in a ratio of 1: 2: 0.1 (the molecular structure is shown in Figure S1), in a 30
wt% aqueous dispersion (supplied by Evonik Industries) has a uniform colloidal size of 120.2 ±
1.2 nm (measured from a 1.0 w/v% RS dispersion by the dynamic light scattering method on the
Malvern Zetasizer, Nano-ZS). Milli-Q de-ionized water (~ 16 MΩ) and ethanol (Merck) were used
as supplied. A sodium hydrogen carbonate buffer (pH = 10.95) was prepared from sodium
hydroxide pellets (Merck) and sodium bicarbonate pellets (Sigma Aldrich).
2.2 Precursor preparation
For the synthesis of the uniform hierarchically porous SBA-15-type silica microparticles, 13.3
g of TEOS (equivalent to 3.84 g of silica) was pre-hydrolyzed with 6.4 g of 0.2 M HCl in a glass
bottle under magnetic stirring for 2 hours. Meanwhile, 6.4 g of F127 was dissolved in 70 g of
distilled water in a water bath at 35 °C by magnetic stirring for two hours. The above two solutions
were then combined while being stirred, followed by the addition of the RS dispersion. The
solution was then diluted to 10 wt% (total solute concentration, defined as the total solid weight
percentage of SiO2, F127 pure RS in the precursor) using distilled water and stirred magnetically
for a further 30 min to obtain the final precursor. The amount of RS added varied from 0 to 70
wt% of silica added to study its influence on particle structure. For example, with the addition of
70 wt% RS indicated that the amount of pure RS added was 2.69 g. In this case, the final HCl
concertation of the precursor was about 0.01 M. In order to study the influences of another two
experimental parameters on the properties of the resultant materials, two precursors were prepared
by using initial HCl concentrations of 0.05 and 0.5 M with the RS added amount fixed at 70 wt%
7
and the TEOS pre-hydrolysis time fixed at 2.0 h, and another two precursors were prepared by
using a TEOS pre-hydrolysis time of 1.0 and 4.0 h with the RS added amount fixed at 70 wt% and
the initial HCl concertation fixed at 0.2 M, respectively.
2.3 Spray-drying assembly of microparticles
The uniform droplets were formed using the same process as stated previously.24-26 Briefly, the
precursor solution was atomized using the micro-fluidic aerosol nozzle system,24, 41 with an orifice
diameter of 75 µm. The droplets were formed via vibrating piezoceramics, monitored using a
digital SLR (Nikon D90) with a speed light (Nikon SB-400) and micro-lens (AF Micro-Nikkor 60
mm f/2.8D), and optimized via adjustment of frequency of the piezoelectric nozzle under
observation using high-speed photography. The inlet and outlet temperatures of the drying
chamber were fixed at ~160 and 115 °C, respectively. The particle collection efficiency was > 70
wt%, and the collected microparticles were then oven dried at 100 °C overnight to remove excess
water. Calcination was conducted in air environment at 550 °C for 5 hours with a 2 °C min-1 ramp
to remove the F127 and RS components. The final samples were denoted as UHPM-SBA-15-x,
wherein UHPM stands for uniform hierarchically porous microparticles, and x for the added RS
amount in weight percentage related to the silica amount, respectively. When x is 0 wt%, the
obtained sample was denoted as UPM-SBA-15 in which UPM stands for uniform porous
microparticle. Moreover, a sample for lysozyme adsorption control experiments was also prepared
under the same experimental procedure by using a precursor with the same solute compositions
but without RS and with ethanol as the solvent. The sample was denoted as UPM-SBA-15*.
Besides, two more samples spray dried at 170 and 190 °C were prepared to study the influence of
the drying temperature on the properties of the resultant materials.
8
2.4 Measurement and characterization
Samples of (intentionally) crushed and intact particles were placed on carbon tapes attached to
spindles and coated with ~ 1 nm of Pt for morphological examination of microparticles taken using
a FEI Nova Nano SEM 450 Field Emission Scanning Electron Microscope (SEM) or a FEI
Magellan 400 FEG SEM. Energy dispersive X-ray photoelectron spectrometry (EDX) was
conducted on the FEI Magellan 400 FEG SEM operated at 15 kV for a period of 15 min for the
element map distribution and 180 s for the point element quantification. To observe the pore
structure and distribution, samples were prepared by crushing a small quantity of particles in
butanol in an agate pestle and mortar to form a weak suspension of fragments. A droplet was placed
on a carbon-coated copper grid and then air-dried. The samples were examined using a FEI Tecnai
G2 T20 Twin LaB6 transmission electron microscope (TEM) or a FEI Tecnai G2 F20 S-Twin FEG
STEM, operated at 200 kV. Nitrogen sorption isotherms were obtained using a Micromeritics
ASAP 2020 analyzer. The samples were dried in an oven overnight, followed by a pre-treatment
of 30 min at 100 °C and a degas step for 240 min at 180 °C under vacuum, with the exception of
the samples after the adsorption of lysozyme which were dried in a vacuum oven at 40 °C
overnight, prior to degas conditions of 40 °C for 24 hours. Surface areas were calculated using the
Brunauer-Emmett-Teller (BET) method and pore volumes were determined by the adsorbed
amount at p/p0 = 0.99, while the pore size distribution was determined by the adsorption branch of
the isotherm with the Barret-Joyner-Halenda (BJH) method. Mercury (Hg) intrusion and extrusion
analyses were conducted over a wide range of pressures starting with vacuum up to 60,000 psi
with a Micromeritics Autopore III. Samples were oven dried overnight at 180 °C prior to analysis.
Small angle X-ray scattering (SAXS) patterns were recorded using a Nanostar U small-angle X-
9
ray scattering system using the Cu Kα radiation (40 kV, 35 mA). Small-angle XRD patterns were
measured on a Bruker D2 Phase Diffractometer (30 kV, 10 mA).
2.5 Adsorption of lysozyme
The control sample obtained without the use of RS and with ethanol as the solvent (UPM-SBA-
15*) and the representative sample UHPM-SBA-15-70wt% were selected for adsorption test
because the two samples have very similar internal ordered mesoporous structure, pore size,
surface area and pore volume. The adsorption was conducted in a buffer of pH = 10.95, close to
the isoelectric point of lysozyme to reduce protein-protein repulsion. Sub-samples were dried in a
vacuum oven at 40 °C overnight. 200 mg of particles were then added to glass bottles containing
1.2 g of lysozyme dissolved in 100 mL of 100 mM buffer. The bottles were placed in an incubator
shaker at 25 °C and 200 rpm. At specific time intervals, 5 mL suspension was removed and
centrifuged at 6000 rpm for 10 min. The concentration of lysozyme was then determined via UV-
Vis Spectroscopy where 2.5 mL aliquot was measured in a quartz cuvette at λ = 281 nm. After
each test, the aliquot was re-dispersed and added back to the bulk solution. The intraparticle
diffusion model, expressed as qt = kit0.5 wherein qt, t, and ki are respectively the time-dependent
adsorption capacity, adsorption time and rate constant, was adopted to evaluate the adsorption
rates.
3. Results and discussion
3.1 Pore structure of the representative sample
With the aid of the water-insoluble RS polymer, hierarchically porous SBA-15-type silica
microparticles have been successfully obtained via the fast spray-drying EISA from a water-based
10
precursor system. Representatively, the UHPM-SBA-15-70wt% sample obtained with the addition
of 70 wt% RS at a drying temperature of 160 ºC exhibits an ordered hexagonal mesostructure with
the presence of the 100 peak and the combination of the 110, and 200 peaks (Figure 1A, curve a),
typical of SBA-15 materials.42 The d100 spacing is calculated to be ~ 11.6 nm (Table 1). The N2
adsorption/desorption results of the sample exhibit a typical type IV plot, a narrow mesopore size
distribution centered at ~ 9.2 nm, a high surface area of 510 m2 g-1 and a large pore volume of 0.95
cm3 g-1 (Figure 1B, curve a, and Table 1). The mesopore wall thickness is calculated to be ~ 4.2
nm (Table 1). From the high-resolution SEM (HRSEM), TEM and STEM images, the sample
possesses an ordered mesoporous framework interspersed with large-meso/macropores of 20 ~
100 nm (Figure 2a-c, and 3a, d). The ordered mesopores and the large-meso/macropores are
interconnected, and the distance between adjacent large-meso/macropores is only 50 ~ 120 nm
(Figure 2c, 3d). Besides, the large-meso/macropores are also interconnected to some extent (Figure
2c-e, 3a and 3d), which could be helpful for mass diffusion. These large-meso/macropores are
originated from the RS polymer nanoparticles, but are smaller and more broadly distributed,
indicating that the RS polymer nanoparticles can be partially dissolved and shrink to some extent
during the synthesis process. The mercury porosimetry of the sample further reveals the presence
of uniform mesopores and additional broadly distributed large-meso/macropores of 20 ~ 100 nm
(Figure 4a).
3.2 Influencing factors on pore structure
The RS amount added in the precursor solution is the most important factor influencing the pore
structure of the resultant samples. This is because the water-insoluble RS polymer plays a critical
role in assembly of the ordered mesostructure and the formation of large-meso/macropores. For
11
the sample UPM-SBA-15 obtained without the use of RS, although its SAXS pattern is similar to
those of the samples obtained with the use of RS (Figure 1A), it exhibits a larger full width at half
maximum (FWHM) (Table 1), indicating that its mesostructure is not as ordered as the
counterparts obtained with the use of RS. More clearly, the HRSEM (Figure 2h), TEM and STEM
(Figure 3c, f) images of the sample UPM-SBA-15 clearly reveal the presence of a disordered
“worm-like” mesostructure, indicating a lack of complete self-organization of the surfactant-silica
micelles. This is mainly the result of the aqueous system in the droplet promoting the condensation
of the silica oligomers rather than the self-assembly of the micelles, which has been noted
previously.10, 26 Notably, from the SAXS patterns, there is an apparent increasing trend in unit cell
with the increase of RS concentration (Figure 1A and Table 1), indicating that the hydrophobic
cores of the micelles can be increasingly swollen with the increase of RS concentration. In
accordance, the mesopore size dramatically increases from 6.8 to 9.2 nm, along with significantly
increased surface areas (from 299 to 510 m2 g-1) and pore volumes (from 0.38 to 0.95 cm3 g-1),
with the added RS increasing from 0 to 70 wt% (Figure 1B, Table 1). Resultantly, upon the addition
of RS, all materials possess small pore wall thicknesses of 3.5 ~ 4.8 nm, while the UPM-SBA-15
sample has thicker walls of 5.6 nm (Table 1). Therefore, with the use of the water-insoluble RS
polymer, ordered mesoporous silica, analogue of SBA-15 structure, can be produced in an
evaporating water system, and the mesopore size can be enlarged to ~ 9.5 nm without any
hydrothermal treatment.
On the other hand, with increasing content of RS polymer in the precursor solution, the hysteresis
loops in the N2 sorption isotherms are broadened and the isotherms cannot reach the typical plateau
at high relative pressures (Figure 1B, curves a-d), indicating there is an increased presence of large-
meso/macropores. The HRSEM (Figure 2c-h), TEM (Figure 3a-c), STEM images (Figure 3d-f)
12
and mercury porosimetry (Figure 4) confirm the presence of large-meso/macropores of 20 ~ 100
nm in the UHPM-SBA-15 samples. With a higher amount of RS added, more large-
meso/macropores can be observed in the resulted samples. Meanwhile, the large-meso/macropores
in the samples obtained with the addition of 10 ~ 30 wt% RS are separated one another, while
those in the samples obtained with 50 ~ 70 wt% RS added become more interconnected, which is
beneficial for improving mass diffusion. The initial size of the RS polymer colloids is ~ 120 nm,
while there are broadly distributed pores of 20 ~ 100 nm for all the materials obtained with the use
of RS. Roughly, the mesopores in the size range of 20 ~ 50 nm are majorly originated from largely
dissolved RS polymer nanoparticles by ethanol in-situ produced from the hydrolysis of the TEOS.
The macropores of 50 ~ 100 nm are majorly originated from the less dissolved RS nanoparticles
with thermal shrinkage. For every TEOS precursor solution prepared, theoretically, a maximum
of ~ 11.8 g of ethanol can be produced if the TEOS is completely hydrolyzed. The theoretical
solubility of RS in ethanol is 1:5. In this regard, in the precursor solutions with the addition of 10
~ 50 wt% RS (0.38 ~ 1.92 g of pure RS), the RS nanoparticles can theoretically be fully dissolved
in 11.8 g of ethanol. However, complete hydrolysis of TEOS cannot be achieved in the current
synthesis, which makes much less RS dissolved. The solubility of RS in the produced ethanol from
TEOS hydrolysis may also decrease with the presence of the major solvent component water and
other solutes. On the other hand, part of the dissolved RS chains can enter into the F127 micelles,
which makes more RS dissolved. Therefore, only a small fraction of large-meso/macropores can
be observed in the sample obtained with only 10 wt% RS added. The increasing addition of RS
polymer can produce more and broadly distributed large-meso/macropores to generate hierarchical
pore structures in a controllable way.
13
With the other conditions kept the same, the influence of the HCl concertation adopted for the
TEOS precursor preparation has been studied. The same ordered hexagonal mesostructure can be
obtained with the use of a HCl concentration of 0.05, 0.2 and 0.5 M (Figure S2A, S3). The cell
parameter (~ 11.4 nm) of the sample obtained with a 0.05 M HCl is obviously smaller than those
(~ 13.5 nm) of the sample obtained at the higher HCl concentrations. N2 adsorption/desorption
results show that the surface area of the sample obtained with a 0.05 M HCl is slightly higher
compared with the other two samples, while their mesopore sizes (~ 9.7 nm) are almost the same
(Figure S2B, C). Therefore, the silica pore wall thickness of the sample obtained with a 0.05 M
HCl is obviously smaller compared with those of the other two samples, mostly because the lower
HCl concentration leads to a relatively lower degree of TEOS hydrolysis and condensation. On
the other hand, the sample obtained with a 0.05 M HCl shows a much higher pore volume (0.97
cm3 g-1) compared with the two samples obtained with 0.2 and 0.5 M HCl (Figure S2B).
Meanwhile, In the pore size distribution curves (Figure S2C), the volume of the large-
meso/macropores of 20 ~ 100 nm is obviously larger for the sample obtained with a lower HCl
concentration of 0.05 M. The HRSEM images also show that the large-meso/macropores are more
populated for the sample obtained at a lower HCl concentration of 0.05 M (Figure S3). The above
result is because the hydrolysis degree of TEOS is lower with less ethanol produced at a lower
HCl concentration, leading to less dissolved RS nanoparticles and the formation of more large-
meso/macropores in the calcined sample. Therefore, it can be confirmed that the partial dissolution
of RS by the in-situ generated ethanol plays the dual roles of modulating the mesostructure
assembly and templating the hierarchical large-meso/macropores.
With the other conditions kept the same, the influence of the TEOS hydrolysis time has also
been studied. The same ordered hexagonal mesostructure can be observed in the three samples
14
obtained with 1.0, 2.0 and 4.0 h TEOS hydrolysis time (Figure S4A). The 100 diffraction peak of
the sample obtained with a 4.0 h hydrolysis time becomes broadened, indicating that the
mesostructure ordering decreases. This is because the co-assembly between large-sized silica sols
obtained from a long hydrolysis time and the F127 micelles becomes difficult. On the other hand,
while the three samples possess almost the same mesopore sizes (~ 9.7 nm), the sample obtained
with only 1.0 h hydrolysis possesses a slightly higher surface area and a relatively larger pore
volume (Figure S4B, C). Similarly, this is because the hydrolysis degree of TEOS is lower with
less ethanol produced in a shorter hydrolysis period. As a result, less RS nanoparticles can be
dissolved in a short hydrolysis time and formation of more large-meso/macropores in the calcined
sample can be observed (Figure S5).
Finally, with the other conditions kept the same, the influence of the drying temperature (140 ~
190 ºC) has been considered. It is found that the low temperature of 140 ºC cannot make the water
droplets sufficiently dried because of the short residence time (< 2 s), leading to aggregated and
deformed microparticles (data not shown). The ordered hexagonal mesostructure can be
maintained at a drying temperature of 170 and 190 ºC (Figure S6A). The mesostructure of the
sample obtained at 190 ºC becomes less ordered, mainly because the F127 micelle assemblies
become less stable in such a high temperature. On the other hand, the sample obtained at 190 ºC
shows an obviously higher surface area and a relatively larger pore volume compared with those
of the sample obtained at 170 ºC, while their mesopore sizes are the same (Figure S6B, C),
indicating the formation of more large-meso/macropores as verified by the HRSEM images
(Figure S7). This is probably because of the higher degree of water removal and the faster
evaporation of ethanol with less RS dissolved at 190 ºC, leading to the formation of more large-
meso/macropores in the calcined sample.
15
3.3 Particle morphology and the influence of RS
All the microparticles produced via the spray-drying EISA process in this study have a particle
sizes of ~ 90 µm, and are “bowl-like” in shape, typical of skin forming materials (Figure 5 and
S8).43 Such a particle morphology is the result of the formation at the droplet surface of a
viscoelastic shell consisting of densely packed colloidal particles initially balanced by electrostatic
forces resisting the capillary forces until the electrostatic forces are overcome and the shell
buckles.44 This morphology is similar to mesoporous silica particles from our previous studies.25,
26 The addition of RS polymer nanoparticles appears to affect the external appearance of the
particle, giving the surface a “bumpy” appearance before calcination (Figure 5a, b and S8b, d and
f), while the microparticles obtained without the use of RS show smooth particle surfaces (Figure
5d-f). EDX maps and quantitative data of these “bumps” show a much higher concentration of
carbon and lower concentration of silicon than the other areas (Figure 6a-d, S9 and Table S1),
while no such element concentration variation can be found in the non-calcined sample obtained
without the use of RS (Figure 6e-g, Table S1). This result indicates that the “bumpy” regions are
organic (RS polymer/F127) enriched. Before calcination, the microparticles are also more likely
to appear stuck together (Figure S8b, d and f). This is because the polymer-rich “bumps” and skin
at the surfaces increase the stickiness and thus the adhesive strength of the microparticles.
However, after calcination, the surfaces of the microparticles are very smooth, and they are no
longer stuck together (Figure 5c, S8a, c and e), indicating that the polymer-rich “bumps” and the
skin of the microparticles can be easily removed via calcination.
Our previous work has produced microparticles from binary TEOS and RS polymer
nanoparticles in water based sols.39, 40 It is found that the RS nanoparticles can diffuse to the
16
surface, while the silica can diffuse to the center during the drying process, forming core-shell
morphology due to the difference in size of the solutes. In the current study, the point EDX data
of cross-sections and surfaces of the non-calcined microparticles show only a minor increase in
polymer (carbon) on the external surface (Table S1). The RS polymer and F127, indicated by the
carbon element, appears to be well dispersed throughout the microparticles, with exception of the
afore-mentioned “bumps” on the surface (Table S1). There is also a minor trend for increased
presence of Si in the cross-section when compared to the external surface, especially in the samples
obtained with the addition of 30 ~ 70 wt% RS (Table S1), further indicating that the surfaces of
the spray-dried particles are polymer-enriched but the cross-sections are uniform. Besides,
HRSEM images of the calcined samples show no structure differences over the cross-sections of
the microparticles and little difference can be found between the area close to the surface and the
internal structure of microparticles (Figure 2 and S10), unlike the distinctive core-shell
morphology observed previously,39 indicating that the presence of the amphiphilic F127 can bridge
the RS polymer nanoparticles and hydrolyzed TEOS to be uniformly dispersed over the assemble
process.
It should be mentioned that the HCl concentration adopted for the TESO hydrolysis, the TEOS
hydrolysis time, and the drying temperature has no obvious influences on particle morphology of
the resultant UHPM-SBA-15 silica materials (Figure S3, S5 and S7). The drying temperature at
the high temperature of 190 ºC leading to microparticles with multiple dimples on the surfaces
(Figure S7).
3.4 Formation mechanism
17
The formation mechanism of the UHPM-SBA-15 materials is analyzed in detail (Scheme 1).
Initially, the mixing of the solutions of F127, RS and pre-hydrolyzed TEOS leads to the formation
of a homogeneous suspension (Scheme 1a). As poly(propylene oxide) (PPO) is not particularly
soluble in water, F127 in water consists of PPO hydrophobic cores surrounded by the hydrophilic
poly(ethylene oxide) (PEO) chains. Then, over the stirring stage over 30 min, the solutes start to
assemble (Scheme 1b). First, the electrostatic interactions between silica oligomers and PEO
blocks drive the aggregation of F127, and micelles will be formed when the PPO-PPO attractions
dominate the PEO-PEO repulsions.45 Second, the RS nanoparticles are partially dissolved by the
in-situ formed ethanol from hydrolysis of TEOS. Experimentally, about 75 and 20 wt% of the RS
nanoparticles can be dissolved in the precursor solutions with a fixed TEOS amount of 13.3 g and
a RS amount of 10 wt% (0.38 g) and 70 wt% (2.69 g), respectively. The partial dissolution of RS
produces non-uniform RS nanoparticles and water-insoluble RS chains. It has been reported that
solubilized hydrophobic molecules can promote micellization via the expulsion of water from the
micellar cores, which decreases the hydrophilic-hydrophobic balance to make PPO-PPO
attractions dominant over PEO repulsions.45 In our synthesis, the solubilized RS chains can be
partially attracted to the hydrophobic cores of F127, decreasing the hydrophilic-hydrophobic
balance and the critical micelle concentration (CMC) and thus promoting the micellization and
self-assembly processes. Next, during the initial drying stage, the temperature increase can also
promote micellization driven by the enthalpy increase.46, 47 More importantly, with fast solvent
evaporation, micellization, micelle organization and its assembly with silica oligomers can be fast
approached at the air-liquid interface (Scheme 1c). Due to the easier evaporation of ethanol as
compared with water, part of the dissolved RS chains in ethanol forms a RS-rich surface skin along
with fast diffusion and evaporation of ethanol at the interface (Scheme 1c). The remaining RS
18
polymer nanoparticles are interspersed throughout the droplet. Finally, with the continuous solvent
(water) evaporation, the composite micelles are continuously organized with increasing
mesostructure ordering, leading to the formation of an ordered mesostructured silica network
surrounding the RS-swollen assembled F127 micelles and RS nanoparticles of various sizes inside
the dried microparticles (Scheme 1d). On the other hand, part of the large RS nanoparticles is
located on the surface because of their low diffusivity, forming the “bumps” on the surfaces of the
dried microparticles (Scheme 1d). Comparatively, without the assistance of RS, the evaporating
water system is un-favorable for ordered mesostructure assembly. This is because the in-situ
formed ethanol is not enough to optimize the hydrophilic-hydrophobic balance. Over the fast
drying process, the condensation of the silica oligomers is more promoted in the water-based sols
as compared with micellization and the self-assembly, leading to the formation of a disordered
worm-like mesostructure. Besides, without the use of RS, pore swelling and RS nanoparticle
template are absent, leading to the formation of only a small-sized mesoporous silica framework.
In our synthesis, the unique and novel aspects of the assembly process are three-fold. First, the
in-situ formed RS chains can tune the hydrophilic-hydrophobic balance and swell the micelles to
enable the success in assembling ordered mesostructure from F127 and pre-hydrolyzed TEOS in
water and forming larger mesopores without hydrothermal treatment. Second, the remaining RS
nanoparticles endow the final silica microparticles with a hierarchical structure as well as more
open surface porosity. Last but not least, the whole spray-drying EISA process finishes in ~ 2
seconds, enabling a faster approach for producing mesoporous materials compared with
conventional EISA processes conducted in ambient conditions, as well as conventional
hydrothermal synthesis of mesoporous silica materials.
19
3.5 Performance in adsorption of lysozyme
The uniform hierarchically porous SBA-15-type silica microparticles are desirable for bio-
adsorption and bio-separation.48-54 Typically, adsorption of lysozyme on the two samples UHPM-
SBA-15-70wt% and UPM-SBA-15* is compared. The former sample can adsorb significantly
more lysozyme than the latter, with almost a 5-fold increase from 20 to 100 mg g-1 (Figure 7A).
Compared with some prior research regarding the adsorption of lysozyme,48-54 the adsorption
capacity on the sample UHPM-SBA-15-70wt% is comparable. Based on the adsorption capacity
and the initial dosage, the lysozyme loading efficiency in the sample UHPM-SBA-15-70wt% is ~
1.67 %, much higher than that (~ 0.33 %) in the reference sample UPM-SBA-15*. However, the
loading efficiencies are generally low, thus the microparticles are probably not suitable for loading
costly molecules. Nevertheless, the above results clearly confirm that the hierarchical pore
structure can significantly enhance the adsorption performance. After a week of adsorption, the
SAXS pattern of the sample UHPM-SBA-15-70wt% becomes slightly less resolved but still
typical of ordered mesopores (Figure S11A). After the adsorption process, the cell parameter of
the sample keeps the same. Meanwhile, the surface area decreases significantly from 510 to 293
m2 g-1, but the pore volume and pore size increase slightly, from 0.95 to 0.97 cm3 g-1 and from 9.2
to 9.8 nm, respectively. This result indicates that the silica pore walls are generally stable but may
undergo slight dissolution and re-precipitation in the basic buffer solution (pH =10.95).55-56
Considering the low external surface area of the sample and the considerable adsorption capacity,
the lysozyme molecules are mostly adsorbed on the mesopore surfaces inside the microparticles.
On the other hand, after the adsorption process, the sample UPM-SBA-15* also well maintains the
ordered mesostrucure with only slight mesostructure ordering decreases as revealed by the
broadened 100 diffraction peak (Figure S11B, S12). Meanwhile, after the adsorption process, no
20
N2 porosity can be detected for the sample UPM-SBA-15*, indicating the surfaces of the
microparticles might be blocked probably due to lysosome adsorption and slight redistribution of
silica. Given that the molecular size of lysozyme is much smaller than the mesopore sizes of both
the two samples, the significant higher adsorption capacity on the sample UHPM-SBA-15-70wt%
compared with the sample UPM-SBA-15* can be mainly ascribed to the much improved pore
accessibility inside the microparticles. In spray dried microparticles, a dense skin of silica formed
on the surface of microparticles can be often observed, rendering the inner pores inaccessible for
large molecules. For example, Carroll et al. stated that porous structures can be inaccessible as a
result of a solid layer of materials at the surface.26 In our work, such an issue can be overcome by
introducing RS in the precursor to surface skins and bulk nano-templates which can then be
removed in the calcination step to generate an open and hierarchical porous structure.
Figure 7B shows the plots of the intraparticle diffusion model which produces multi-linearity
with each line representing a step in the adsorption process. It is evident that there are two regions
in the plots indicating two adsorption steps. The initial region correlates to the adsorption on the
external surface (film diffusion), while the second region relates to intraparticle diffusion. From a
comparison of the two samples, it is evident that there is less intraparticle diffusion for the sample
UPM-SBA-15* (Table S2). The rate of adsorption is significantly faster for the sample UHPM-
SBA-15-70wt%, with > 3-fold increases in the rate constant over both the two regions. This result
indicates that the hierarchical pore structure can significantly improve the pore accessibility, thus
promoting the mass transfer of large lysozyme molecules into the pore channels inside the
microparticles.
4. Conclusions
21
In summary, we have successfully synthesized uniform hierarchically porous SBA-15-type
silica microparticles in a water based solution by using a rapid and scalable spray-drying EISA
process. These microparticles have uniform micron sizes of ~ 90 µm, and “bowl-like” morphology.
They possess high surface areas of 315 ~ 510 m2 g-1, large pore volumes of 0.64 ~ 0.10 cm3 g-1,
ordered large mesopores of 8.3- 10.0 nm, hierarchical large-meso/macropores of 20 ~ 100 nm, and
open surfaces toward good accessibility into the inner pores. Gradual increase of RS in the
precursor solution has resulted in more swelling of the ordered mesopores, rapid increase of large-
meso/macropores, as well as enhanced surface areas and pore volumes. The ordered mesostructure
and particle morphology can be maintained over a wide range of experimental conditions. The use
of a lower HCl concentration, a shorter TEOS hydrolysis time, and a higher drying temperature all
lead to less dissolved RS nanoparticles and thus the formation of relatively higher surface areas,
large pore volumes due to the creation of more large-meso/macropores. The assembly mechanisms
of these microparticles have been analyzed in detailed. The RS nanoparticles can be partially
dissolved by in-situ generated ethanol from hydrolysis of TEOS to form solubilized hydrophobic
RS chains. These chains can swell the F127 micelles and decrease their the hydrophilic-
hydrophobic balance to promote the micelle assembly into ordered mesostructures with increased
cell parameters. The un-dissolved RS nanoparticles form large-meso/macropores interconnecting
with the ordered mesoporous framework. In addition, part of the RS chains and nanoparticles form
surface “skins” and “bumps”, the removal of which can make the particle surface more open.
Without the use of RS in the precursors, the obtained control samples display a disordered “worm-
like” mesostructure due to the condensation of silica being favored over the self-assembly of the
micelles in water. In the adsorption of lysozyme, the representative hierarchically porous SBA-15-
type silica microparticles has exhibited a > three-fold faster rate of adsorption and a five-fold
22
increase in adsorption capacity (from 20 to 100 mg g-1), and no pore blockage effect after
adsorption as compared with the control sample obtained without the use of RS. Finally, compared
with the typical previous research,1, 2, 4, 25, 26, 30, 42 this study improves the ordered mesostructure
assembly EISA process in water systems, enhances the mesopore sizes without any hydrothermal
conditions and more conveniently creates hierarchical pore structures with improved pore
accessibility of spray-dried microparticles. The method would be applicable for the synthesis of
other hierarchical and ordered mesoporous materials for adsorption, separation and catalytic
applications.
Appendix A. Supporting Information
Acknowledgements: Financial supports from the National Natural Science Foundation of China
(No. 21875153 for Z. Wu) and the Australian Research Council (DP140104062, FT140101256 for
C. Selomulya) are much appreciated. The authors would like to acknowledge Dr. Wenjie Liu for
experimental assistance. The authors acknowledge the use of facilities within the Monash Centre
for Electron Microscopy. K.W. thanks the Australian Postgraduate Award Scheme for the
provision of a scholarship.
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Figures and tables
Figure 1. SAXS patterns (A) and N2 sorption isotherms (B) and the corresponding pore size
distribution curves (Inset B) of the UHPM-SBA-15 samples obtained with the addition of 70 (a),
50 (b), 30 (c), 10 wt% (d) RS, and the UPM-SBA-15 sample obtained without the use of RS (e),
respectively.
31
32
Figure 2. HRSEM images of the UHPM-SBA-15 samples obtained with the addition of 70 (a-c),
50 (d, e), 30 (f), 10 wt% (g) RS, and the UPM-SBA-15 sample obtained without the use of RS (h),
respectively.
33
Figure 3. TEM (a-c) and STEM (d-f) images of the UHPM-SBA-15 samples obtained with the
addition of 70 (a, d) and 30 wt% (b, e), and the UPM-SBA-15 sample obtained without the use of
RS (c, f), respectively.
34
Figure 4. Mercury porosimetry isotherms and the inset pore size distribution curves of the UHPM-
SBA-15 samples obtained with the addition of 70 (a), 50 (b), 30 (c), 10 wt% (d) RS, respectively.
.
.
35
Figure 5. SEM images of the UHPM-SBA-15 sample obtained with the addition of 70 wt% RS
(a-c) and the UPM-SBA-15 sample obtained without the use of RS (d-f) before (a, b, d, e) and
after (c, f) calcination, respectively.
36
Figure 6. SEM images (a, e) and the corresponding carbon (b, f), silicon (c, g) and oxygen (d, h)
element maps of the UHPM-SBA-15 sample obtained with the addition of 70 wt% RS (a-d) and
the UPM-SBA-15 sample obtained without the use of RS (e-h) before calcination, respectively.
37
Figure 7. Time-dependent adsorption capacity curves (A) and the corresponding linear fitting
curves (B) by using the intraparticle diffusion model for the adsorption of lysozyme on the UHPM-
SBA-15 sample (a) obtained with the addition of 70 wt% RS and on the control sample UPM-
SBA-15* (b) obtained without the use of RS and with ethanol as the solvent, respectively.
38
Scheme 1. Schematic representation of the assembly process of the hierarchical porus SBA-15-
type microparticels: the initial mixing stage (a), further hydrolysis of TEOS and partial dissolution
of RS nanoparticles to form RS polymer chains (b), formation and swelling of micelles and the
structure starting to assemble in the initial drying stage (c), the ordered array of swollen micelles
and disordered packing of RS nanoparticles with some RS polymer on surface of particle to
produce “bumps” after the drying process (d).
39
Table 1. Summary of physicochemical properties of the various samples obtained at different
experimental conditions.
sampleSBET
a
(m2 g-1)
Vtb
(cm3 g-1)Dc (nm)
d100d
(nm)ae (nm)
wf (nm)
FWHMg (nm-1)
UPM-SBA-15 299 0.38 6.8 10.8 12.4 5.6 0.213
UPM-SBA-15* 537 0.76 7.9 10.8 12.4 4.5 0.128
UHPM-SBA-15-10 wt% 487 0.68 9.3 11.0 12.8 3.5 0.185
UHPM-SBA-15-30 wt% 435 0.64 8.3 11.3 13.1 4.8 0.199
UHPM-SBA-15-50 wt% 491 0.86 9.5 11.9 13.8 4.3 0.199
UHPM-SBA-15-70 wt% 510 0.95 9.2 11.6 13.4 4.2 0.185
a the BET surface area, b the total pore volume, c the pore size, d the d100 spacing, e the cell parameter of the hexagonal mesostructure calculated by the formula a = 2/ d100, f the pore wall thickness √3calculated by using the formula w = a - D, and g the full width at half maximum of the 100 peaks in the SAXS patterns, respectively. * the sample was obtained with ethanol as the solvent while the other samples were obtained with water as the solvent.
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Spray-drying-assisted EISA assembly of SBA-15-type silica microparticles with water as the solvent is achieved by using a multiple-role polymer colloid. The microparticles possess uniform micro-sizes, ordered and large mesopores, hierarchical porosities and high surface areas. They are promising for lysozyme adsorption with a fast adsorption rate and a high capacity because of the improve pore accessibility.
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