A PTFE helical capillary microreactor for the high throughput synthesis of monodisperse silica particles
Graphical abstract
Introduction
Since Stöber first reported on the synthesis of solid silica micro-/nanoparticles by means of hydrolysis of alkyl silicates and subsequent condensation of silanols and silicic acid in alcoholic solution in 1968, there has been enormous interest in the synthesis of silica based nanomaterials [1]. Both solid and porous silica materials have been reported and their good biocompatibility, high thermal and mechanical stability and ease of surface functionalization have all been exploited in many applications [2]. Traditionally, such silica particles have been synthesized in a batch-wise manner to obtain large quantities. Micro-structured chemical reactors provide a useful alternative to batch reactors because they offer better control over the reaction rate [3]. Here we use a helical tubular microreactor for the high throughput synthesis of monodisperse submicron silica particles. We exploit Dean’s flow to achieve both high throughput and sorting efficiency [4], [5]. Under Dean’s flow [6] the magnitude of the main stream velocity in a curved channel at the outer (larger radius) side of the channel is larger than that at the inner wall [5]. The difference in the speed of the fluids along the opposing channel walls is accompanied by a pressure gradient in the radial direction, which results in a secondary flow driving the fluid from the inner wall to the outer wall of the channel and this, in turn, leads to travelling vortices [7]. This effect can be utilized to achieve ultrafast mixing of two adjacent liquid phases in a single spiral microchannel (Fig. 1).
In general, there are two types of spiral microchannel reactors, using either planar or helical geometry. Planar spiral microreactors are typically fabricated using lithographic methods and these reactors have indeed been used for the synthesis of silica particles in the past. For example, Yuan Nie et al. demonstrated a lithographically fabricated microfluidic spiral channel device for the synthesis of submicron, hollow, silica spheres [8]. This geometry has also been used for solid-forming reactions [9], [10], ultra-fast blood plasma separation [11], and manipulation of biological particles [12], [13]. Though elegant, such devices require an extensive sequence of fabrication steps and necessitate photolithography for assembly [14]. In particular, the bonding of multiple device layers requires tedious micro-scale alignment and surface treatments, which all render commercialization difficult and expensive. Furthermore, these devices are usually based on polydimethylsiloxane (PDMS) and glass-based chips, which have intrinsically poor heat conductivity making uniform heating within the device difficult. Also, the small chip sizes amenable to photolithography make it difficult to achieve sufficiently large residence times. Hence, the complexity of the devices and the associated fabrication constraints limit its applicability severely. On the other hand, helical tubular microreactors offer improved heat and mass transport as well as enhanced mixing compared to planar spiral microreactors [15]. Mixing takes place along the length of the microreactor in planar micro-structured reactors, whereas the mixing occurs along the length of the microreactor as well as in the radial direction in the helical capillary micro-reactor [16], [17]. The efficiency of helical tubular microchannel reactors has been validated using the synthesis of Ag nanoparticles (NPs) [18], peracetic acid [19], and peroxypropionic acid [20].
Here, we propose to use this facile and inexpensive method to produce uniform SiO2 particles with controlled sizes between 100 nm and 600 nm diameter and yields up to 0.234 g/h of solid product. We achieve this with a lithography-free, microfluidic device based on a polytetrafluoroethylene (PTFE) spiral microchannel. In this device, two fluid phases undergo ultrafast mixing due to Dean flow effects. The two adjacent fluid phases consist of the silica precursor, TEOS, in a non-aqueous solvent and NH3·H2O as the catalyst in the aqueous phase. Hydrolysis of TEOS and its continuous condensation during the mixing of the two reactant flows leads to the build-up of nanoparticles in the spiral microchannel [21]. Numerical analysis is used to optimize the device dimensions in order to obtain complete mixing in less than 1.4 s.
Section snippets
Materials
Tetraethyl orthosilicate (TEOS), ammonium hydroxide solution (28%) and dry ethanol were purchased from Aladdin, Shanghai, China. 18 MΩ ultrapure Milli-Q water was used for all experiments. All chemicals were used as received without any further purification.
Fabrication of microfluidic devices
The micromixer consists of a 0.5 m long polytetrafluoroethylene (PTFE) tube with an inner diameter of 0.8 mm and an outer diameter of 1.6 mm, which is wrapped around a glass rod with 6 mm diameter. At the entrance, a Y-shaped mixer connects
Simulation of device dimensions
Computational analysis is used to determine how many turns in the helical capillary micromixer are required for complete mixing of the two liquid phases. In the laminar flow region, a parabolic velocity pattern within a capillary microreactor can be obtained through introduction of curved channels. This can be achieved by wrapping a flexible straight capillary around a cylindrical rod-shaped core. The perturbations induced by the centrifugal forces act perpendicularly to the fluid flow
Discussion
From the simulations we obtained information on the optimum micromixer dimensions. Because the Dean effect has a close relationship to both the pipe and rod radii, these parameters affect the mixing of the phases significantly.[26] The radius of the pipe affects the travel distance within the spiral linearly; in other words, the thinner the pipe, the higher the velocity, such that complete mixing is achieved more rapidly.[27], [28] However, because a long residence time is required for complete
Conclusion
In conclusion, a new PTFE based helical capillary microreactor has been developed and optimized for submicron SiO2 particle synthesis. Dean flow effects in the curved channel enable ultrafast mixing of two phases, which we have exploited to synthesize SiO2 particles based on the Stöber method. We have investigated the effect of different device geometries and process parameters on the SiO2 NP synthesis and optimized the reaction conditions for preparation of uniform silica colloids. The TEOS
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work has been funded by the Guangdong Innovative and Entrepreneurial Team Program (No. 2016ZT06C517), the Australian Government through Australian Research Council Grant CE170100026, the Science and Technology Planning Project of Guangdong Province (No. 2016B090906004), the Science and Technology Project of Guangdong Province (No. 2018A050501012), and the Special Fund Project of Science and Technology Application in Guangdong (No. 2017B020240002). E.M.A. acknowledges a Feodor Lynen
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