Effect of stirring speed on particle dispersion in silica synthesis

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Nano-Structures & Nano-Objects 35 (2023) 100994
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Contents lists available at ScienceDirect
Nano-Structures &Nano-Objects
journal homepage: www.elsevier.com/locate/nanoso
Effect of stirring speed on particle dispersion in silica synthesis
Shan Zhang a,∗, Chao Wang b,∗
Institute of Materials Science and Devices, Suzhou University of Science and Technology, Suzhou 215011, China
The Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University School of
edicine, Baltimore, MD 21205, United States
a r t i c l e i n f o
Article history:
Received 15 April 2023
Received in revised form 17 May 2023
Accepted 9 June 2023
Keywords:
Stirring speed
SiO2 nanoparticles
Size
PDI
a b s t r a c t
The particle size distribution of nanoparticles is an important indicator and has a crucial impact on
their applications. In this work, we investigated the effect of stirring speed on particle size and size
distribution during the synthesis of SiO2 nanoparticles and quantified the polydispersity index (PDI) for
the first time. Our results showed that there is an approximate relationship between the stirring speed
and the PDI. At a stirring speed of 400 rpm during the Stöber reaction, the PDI of SiO2 nanoparticles
reached a minimum value of 0.027, with a diameter of 90 nm, indicating excellent particle dispersion
and potential drug delivery applications.
© 2023 Elsevier B.V. All rights reserved.
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1. Introduction
Size properties, as one of the main properties of nanoparticles,
reatly determine their availability and application. For exam-
le, the size of nanoparticles used in electrocatalytic hydrogen
roduction can reach several microns [1], while nanomedicines
sed in tumor therapy are often below 100 nm [2]. Therefore,
egulating the particle size of nanoparticles is critical.
The polydispersity index (PDI) indicates the narrowness of
he particle size distribution, with values ⩽0.1 indicating a very
arrow distribution [3]. In drug delivery applications, a PDI of 0.3
r below is considered acceptable and indicates a homogenous
opulation of the nanocarrier [4]. In general, for polymers and
mall particles (micron particles, nanoparticles, etc.), the more
omogeneous the particle size, the more stable their proper-
ies. Stable properties are of great benefit for their applications.
or instance, the FDA’s ‘‘Guidance for Industry’’ concerning drug
roducts emphasizes the importance of size and size distribution
s ‘‘critical quality attributes’’ [4]. Therefore, obtaining a homoge-
eous size of nanoparticles with a small PDI is a very important
oal.
Among the nanoparticle synthesis methods, solvothermal syn-
hesis is preferred by many researchers for nanoparticle prepara-
ion due to its simple process and homogeneity of the dispersion
Abbreviations: PDI, polydispersity index; FDA, U.S. Food and Drug
Administration; TEOS, ethyl orthosilicate; TEM, transmission electronic
microscope; DLS, dynamic light scattering
∗ Corresponding authors.
E-mail addresses: zs@usts.edu.cn (S. Zhang), cwang196@jhmi.edu
C. Wang).
ttps://doi.org/10.1016/j.nanoso.2023.100994
352-507X/© 2023 Elsevier B.V. All rights reserved.
system. This makes it easier to obtain homogeneous size nanopar-
ticles compared to other synthesis methods, such as the sol–gel
method, microemulsion technique and vapor deposition [5,6].
Therefore, the solvothermal method was applied for the synthe-
sis of nanoparticles in this report and SiO2 nanoparticles were
selected as typical nanoparticles for this study due to their long
history of research, versatility [7] and their simple solvothermal
synthesis.
In the process of synthesizing nanoparticles by solvothermal
synthesis, many parameters affect the size and size distribution,
such as the ratio of ingredients, reaction time and temperature
etc. However, a very critical parameter has been neglected by
most researchers: the stirring speed of the magnetic stirring
bars [8–10]. This key parameter is often neglected in many re-
ports on the synthesis of SiO2 nanoparticles for two reasons: one
s that the stirrer chosen by the researcher is relatively rudimen-
ary and does not have a display for speed; the other is that the
esearcher subjectively ignores this parameter and considers it
nimportant in the synthesis of SiO2. However, in our work, we
ound that the stirring speed has a slight effect on the size of the
iO2 nanoparticles, while having a significant effect on the PDI
value. Based on this, our work should be of great inspiration to
researchers who need to synthesize homogeneous nanoparticles.
2. Experiments and materials
2.1. Materials and apparatus
Ethanol, ethyl orthosilicate (TEOS) and ammonia (30%) were
purchased from Sigma Alpha. Water was obtained from a deion-
ized water system. A magnetic stirring bar, a 25 ml beaker with
https://doi.org/10.1016/j.nanoso.2023.100994
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https://doi.org/10.1016/j.nanoso.2023.100994
S. Zhang and C. Wang Nano-Structures & Nano-Objects 35 (2023) 100994
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Fig. 1. TEM images (A) and corresponding size distributions (B) of SiO2 nanoparticles at different stirring speeds.
Fig. 2. Hydrated particle size for triplicate SiO2 nanoparticles synthesized at different stirring speeds.
a stirring station (Isotemp™ Hot Plate Stirrer, Ambient to 400 ◦C,
eramic) was purchased from Fisher Scientific. Transmission elec-
ronic microscope (TEM) images were obtained from a TEM sys-
em (Hitachi H-600 EM) with an accelerating voltage of 75 keV.
mageJ (version: 1.53t) software was used to analyze the size
istributions from TEM images. A Malvern particle size meter
ZEN3690; Malvern Instruments, Malvern UK) was used for DLS
article size testing.
2
2.2. Synthesis of SiO2 nanoparticles
The SiO2 nanoparticles were synthesized using a modified
Stöber method [11]. Specifically, 8 ml of ethanol was mixed with
1 ml of ammonia to make solution A, and 0.1 ml of TEOS was
mixed with 0.9 ml of ethanol to make solution B. Solution B
was added to solution A in one go and stirred for 30 min at
room temperature. The mixture was then centrifuged and washed
twice with ethanol and once with deionized water to obtain the
S. Zhang and C. Wang Nano-Structures & Nano-Objects 35 (2023) 100994
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Fig. 3. PDI values of SiO2 nanoparticles prepared at different stirring speeds.
roduct. The stirring speed was adjusted to 0, 100, 200, 300, 400,
00 and 600 rpm during the reaction. All the SiO2 nanoparticles
ere synthesized in triplicate.
. Results and discussion
All the SiO2 nanoparticles prepared by the modified Stöber
ethod are uniform spherical, as seen in the TEM images. The
article size ranges slightly from about 90 to 110 nm, despite the
ifferent stirring speeds (Fig. 1). This shows that the stirring speed
n the reaction does not significantly affect the particle size, which
s consistent with results of previous reports [12]. However, its
nfluence on nanoparticle dispersion is large. It could be observed
hat the SiO2 nanoparticles have the narrowest size distribution
at 400 rpm from the TEM image and corresponding size analysis
graph (Fig. 1). The hydrated particle size of SiO2 ranges from
05 to 141 nm (Fig. 2), and both size and trend match the silica
article size distribution in the TEM image. In Fig. 3, the PDI of
iO2 varies greatly at different stirring speeds. It is 0.088 at a
tirring speed of 0 and decreases gradually to the lowest value
f 0.027 with a stirring speed of 400 rpm, indicating its excellent
otential for drug-delivery applications. After that, it increases
ith an increase in stirring speed. The values match well with
he results of the size analysis in Figs. 1B and 2. In general,he difference in PDI can reach 3.3 times at different stirring
peeds (0 vs 400 rpm), showing obvious stirring speed-dependent
ariability. In Fig. 3, it can be deduced that the optimum stirring
peed to achieve a minimum PDI is probably between 300 and
00 rpm.
. Conclusion
Our results reveal the relationship between stirring speed and
DI of SiO2 nanoparticles prepared by Stöber’s method. There is
n optimal particle size dispersion with a PDI of 0.027 when the
tirring speed is 400 rpm, which is suitable for drug-delivery and
io-applications. This work may inspire the researchers studying
anomedicine as well as those who want to adjust the PDI of
anoparticles and polymers for other necessary uses.
3
RediT authorship contribution statement
Shan Zhang: Conceptualization, Methodology, Validation, Proje
dministration, Funding acquisition, Data curation, Investigation,
esources, Writing - review & editing. Chao Wang: Software,
ormal analysis, Visualization, Writing – original draft.
eclaration of competing interest
The authors declare that they have no known competing finan-
ial interests or personal relationships that could have appeared
o influence the work reported in this paper.
ata availability
Data will be made available on request.
cknowledgment
The work was financially supported by Open Funds of the State
ey Laboratory of Electroanalytical Chemistry (SKLEAC202206);
he Natural Science Foundation of Jiangsu Province (BK20220641);
he Natural Science Foundation of Suzhou University of Science
nd Technology (XKZ2021001); the Natural Science Research
oundation of Jiangsu Higher Education Institutions of China
21KJB150032); Ph.D. Innovation and Entrepreneurship Program
f Jiangsu Province (JSSCBS20210735).
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	Effect of stirring speed on particle dispersion in silica synthesis
	Introduction
	Experiments and Materials
	Materials and apparatus
	Synthesis of SiO2 nanoparticles
	Results and Discussion
	Conclusion
	CRediT authorship contribution statement
	Declaration of competing interest
	Data availability
	Acknowledgment
	References