464868 Microfluid-Aassisted Hollow Silica Nanoparticle Synthesis through Polyelectrolyte Template
Hollow spherical silica nanoparticles (HSNPs) have been widely proposed as a promising material in various fields including catalysis, drug delivery, cell-labeling, semiconductor and optical coatings, due to its unique properties such as biocompatibility, controllable surface areas and pore volumes, as well as chemical and thermal stability. To prepare the HSNPs, template method is a facile and straightforward approach. Hard templates such as polystyrene particles  and hydroxyapatite nanoparticles , were successfully applied to produce the uniform HSNPs in tunable void space and shell thickness. High temperature (around 500℃) was required to calcinate out these hard templates thoroughly after the formation of silica shell. Oppositely, soft templates made from emulsion micelles  and vesicles  can be easily washed away by a selective solvent at room temperature. However, to prepare the soft templates in a uniform shape usually involved at least two surfactants and tedious procedures, especially to obtain and control the uniform HSNPs under 100 nm which have higher surface area is still a challenge. The groups of Du et al.  and Wan et al.  proposed a simplified and environment-friendly approach by applying a single polyelectrolyte - poly(acrylic acid) (PAA) as the soft-template. However, the PAA globule or necklace monomers start randomly aggregation in microseconds ahead of the silica shell formation and lead to the PAA templates in broad size distribution, which is the main reason of the inconsistent void space of the HSNPs in the batch reaction. Microfluidic system is able to reach the homogeneous environment and elaborate control on the reaction conditions due to its extraordinary mass/heat transfer, expected to improve the management of the colloidal size and uniformity.
Microfluidic system with hydrodynamic focusing micromixer are introduced in this study. The result showed that successfully generated the uniform HSNPs in 50 nm and hollow wormlike nanoparticles were synthesized in a short-time scale. Microfluidic system, scaling down the reactor into micrometer scale, is able to reach the homogeneous environment and elaborate control on the reaction conditions. Its extraordinary mass/heat transfer dramatically improved the management of the colloidal size and uniformity. Hydrodynamic focusing micromixer (HFM) is one of the efficient mixers which can achieve the microsecond-mixing in a low flow rate due to the existence of both molecular and convectional diffusions. HFM possesses a continuous flow in small dead time without any clogging which usually happened in the other chaotic micromixers such as the U-shaped mixers and Tesla mixers. It can provide not only the mixing time in microsecond but also the high volume flow rate ratio (VFRR) between the focused and side streams. This high VFRR is essential in some particular nanotechnology such as our case which only allows the reactants in a small proportion of its solvent. Valencia et al.  and Jahn et al.  has been applied the HFM to obtain lipid vesicles and micelles. However, controlling the self-assembly of a single polyelectrolyte and conserving its intermediate conformations has not been reported due to their rapid aggregation.
In our microfluidic system with HFM, the fast and homogeneous mixing enable the PAA globules/necklace monomers protected instantly by evenly dispersed silica precursor. Along the further attachment of silica precursor and the rapid growth of silica shell, the aggregation of PAA monomers are efficiently avoided due to the hydroxide bond on the particle surface. A micro-channel following the HFM was applied to accomplish the growth of silica shell. By controlling the residence time of the microfluidic flow, different thickness and density of the silica shell can be obtained. In this study, a numerical model was established by using COMSOL Multiphysics to investigate the mixing profile under different flow condition. Incorporated with this computational model, we describe the experimental operation of the microfluidic chemical reactor to control the continuous synthesis of HSNPs. The effects of the mixer geometry and the operating variables on particle structure and size distribution are discussed.
- Ding, Xuefeng, Kaifeng Yu, Yanqiu Jiang, Hari-Bala, Hengbin Zhang, and Zichen Wang. "A Novel Approach to the Synthesis of Hollow Silica Nanoparticles." Materials Letters 58.27-28 (2004): 3618-621.
- Williamson, Peter A., Philip J. Blower, and Mark A. Green. "Synthesis of Porous Hollow Silica Nanostructures Using Hydroxyapatite Nanoparticle Templates." Chem. Commun. 47.5 (2011): 1568-570.
- Yildirim, Adem, and Mehmet Bayindir. "A Porosity Difference Based Selective Dissolution Strategy to Prepare Shape-tailored Hollow Mesoporous Silica Nanoparticles." J. Mater. Chem. A 3.7 (2015): 3839-846.
- Wu, Xue-Jun, and Dongsheng Xu. "Soft Template Synthesis of Yolk/Silica Shell Particles." Adv. Mater. Advanced Materials 22.13 (2010): 1516-520.
- Li, Yunqi, Bishnu Prasad Bastakoti, Masataka Imura, Jing Tang, Ali Aldalbahi, Nagy L. Torad, and Yusuke Yamauchi. "Dual Soft-Template System Based on Colloidal Chemistry for the Synthesis of Hollow Mesoporous Silica Nanoparticles." Chemistry - A European Journal Chem. Eur. J. 21.17 (2015): 6375-380.
- Du, Yi, Lunet E. Luna, Wui Siew Tan, Michael F. Rubner, and Robert E. Cohen. "Hollow Silica Nanoparticles in UV−Visible Antireflection Coatings for Poly(methyl Methacrylate) Substrates." ACS Nano 4.7 (2010): 4308-316.
- Wan, Y., and S.-H. Yu. "Polyelectrolyte Controlled Large-Scale Synthesis of Hollow Silica Spheres with Tunable Sizes and Wall Thicknesses." J. Phys. Chem. C Journal of Physical Chemistry C 112.10 (2008): 3641-647.
- Dobrynin, A., and M. Rubinstein. "Theory of Polyelectrolytes in Solutions and at Surfaces." Progress in Polymer Science 30.11 (2005): 1049-118.
- Dobrynin, Andrey V., and Michael Rubinstein. "Counterion Condensation and Phase Separation in Solutions of Hydrophobic Polyelectrolytes." Macromolecules 34.6 (2001): 1964-972.
- Valencia, Pedro M., Pamela A. Basto, Liangfang Zhang, Minsoung Rhee, Robert Langer, Omid C. Farokhzad, and Rohit Karnik. "Single-Step Assembly of Homogenous Lipid−Polymeric and Lipid−Quantum Dot Nanoparticles Enabled by Microfluidic Rapid Mixing." ACS Nano 4.3 (2010): 1671-679.
- Jahn, Andreas, Samuel M. Stavis, Jennifer S. Hong, Wyatt N. Vreeland, Don L. Devoe, and Michael Gaitan. "Microfluidic Mixing and the Formation of Nanoscale Lipid Vesicles." ACS Nano 4.4 (2010): 2077-087.
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