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Microfluidic Flow Synthesis of Functional Mesoporous Silica Nanofibers with Tunable Aspect Ratios Nanjing Hao, Yuan Nie, and John X.J. Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03527 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Sustainable Chemistry & Engineering

Microfluidic Flow Synthesis of Functional Mesoporous Silica Nanofibers with Tunable Aspect Ratios Nanjing Hao, Yuan Nie, and John X. J. Zhang* Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, New Hampshire 03755, United States. Email: [email protected] KEYWORDS: microfluidic, shape, mesoporous silica, aspect ratio, fiber

ABSTRACT: Microfluidic techniques open new frontiers for the controllable synthesis of functional micro-/nanomaterials with desired shapes for a variety of applications. In this study, miniaturized spiral-shaped microchannel with two inlets and one outlet was specially designed for the controllable flow synthesis of mesoporous silica nanofibers (MSNFs) using one inlet flow containing cetyltrimethylammonium bromide and diluted ammonia and the other inlet flow containing diluted tetraethyl orthosilicate. The aspect ratios and diameters of MSNFs can be easily tuned by changing the flow rates and/or the concentrations of reactants. In addition, fluorescent dyes, magnetic nanoparticles, therapeutic drugs, or silver nanoparticles can be simultaneously assembled into MSNFs to make them promising functional materials in bioimaging, theranostics, and catalysis fields.

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Since their discovery in early 1990s,1 mesoporous silica materials have attracted increasing and extensive attentions in various applications, such as biomedicine, water treatment, catalysis, energy, sensors, and environmental science. To maximize the application efficacy, the physicochemical properties of mesoporous silica materials, including particle size, shape, pore, and surface chemistry, have been systematically investigated.2 Given the findings from both theoretical and experimental aspects that particle shapes of micro-/nanomaterials play significant roles on their performance,3,4 in recent years, shape-controllable synthesis of mesoporous silica materials has gained considerable interests. To date, synthesis of mesoporous silica structures with various shapes, such as sphere, ellipsoid, cube, rod, film, platelet, and sheet, can be realized in different ways.5 However, there are still very limited methods to synthesize fiber-like mesoporous structures with tunable aspect ratios, which have already demonstrated their superior capacity in fields like photocatalysis.6–10 Currently, the most commonly used methods for synthesizing mesoporous silica fibers are based on electrospinning11–14 and hydrothermal dayscale treatment,15–17 which generally suffer from low product yields, high voltage power supply, time-consuming operations, products with large diameters (micrometer- or submicrometerscale), and relatively poor reproducibility. Microfluidic techniques provide an alternative promising platform for controllable micro/nanomaterials synthesis and offer a variety of unique features that conventional batch methods can hardly achieve.18–20 These features include: 1) sufficient and intensive mixing of reactants, enabling chemical reactions to be performed with high yields; 2) flexible and scalable systems, allowing low production cost and easy scale-up; 3) rapid reaction kinetics and dedicate control of reaction parameters, permitting for fast screening and optimization of material properties; 4) greatly reduced reactor dimensions and automated operations, allowing for minimizing the local


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variations and the reproducible materials synthesis. Therefore, microfluidic reactors give great and new chances to synthesize mesoporous silica structures.21–24 Herein, on the basis of spiral-shaped microfluidic reactor, we firstly developed a facile and general flow synthesis strategy to prepare mesoporous silica nanofibers (MSNFs). The effect of a series of reaction conditions, including the flow rates and the concentrations of reactants, on the morphology changes of mesoporous silica structures was then examined. We further investigated the possibilities of one-step synthesis of multifunctional MSNFs for diverse potential applications.

Figure 1. Microfluidic synthesis platform for mesoporous silica nanofibers (MSNFs). (A) Experimental set up for the microfluidic synthesis of MSNFs, with a U.S. one cent coin for scale. (B) COMSOL simulation result of mixing in microfluidic spiral channel (see simulation details


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in SI). (C) Schematic drawing for illustrating the formation of mesoporous silica fibers (Objects are not drawn to scale).

The five-run spiral-shaped microfluidic channel with two inlets and one outlet was fabricated from polydimethylsiloxane using soft lithography (Figure 1A). The width and height of the microchannel are 500 µm and 50 µm, respectively. The smallest diameter of the spiral microchannel is 5.25 mm, and then it increases from 11.0 mm to 22.2 mm with an increment of 1.4 mm for each half run. The two inlet flows, one containing cetyltrimethylammonium bromide (CTAB) and diluted ammonia (Inlet I) and the other having diluted tetraethyl orthosilicate (TEOS, Inlet II), were pumped (Pump 33 DDS, Harvard Apparatus) into the spiral microchannel at room temperature and the as-synthesized products were collected at the outlet. Due to the relatively low Reynolds number (less than 6.14, see details in SI) and transverse flow across the channel,23-25 this kind of miniaturized microreactor could give rise to efficient and intensive mixing as revealed by COMSOL analysis (Figure 1B), leading to faster reaction kinetics and higher yields than conventional batch methods. The formation of mesoporous silica products can be realized by ammonia-catalyzed hydrolysis and condensation of TEOS using CTAB as the structure-directing agent (Figure 1C). As shown in Figure 2, when the flow rates of Inlet I and Inlet II were set as 100 µL/min and 10 µL/min, respectively, well-defined silica fiber nanostructures can be obtained from the outlet of microfluidic reactor using CTAB as surfactant, ammonia as catalyst, and TEOS as silica precursor. These fibers have an average diameter of ~130 nm and an average aspect ratio of ~12, and the mesoporous channels are well-aligned and are parallel to each other (Figure 2F). Nitrogen sorption results showed that MSNFs have a type IV isotherm, suggesting typical


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mesoporous structures with uniform cylindrical pores (Figure S1A).26 The filling of frameworkconfined mesoporous occurred at relative pressures (P/P0) of 0.3-0.5 and above 0.9, indicating a high degree of texture porosity.26,27 The Brunauer-Emmett-Teller (BET) surface area and pore volume of MSNFs were measured to be 1213.6 m2 g-1 and 1.04 cm3 g-1, respectively. The pore size of MSNFs was calculated to be 2.79 nm by the Barrett-Joyner-Halenda (BJH) method (Figure S1B). In agreement with the COMSOL simulation, the product yield of MSNFs can be achieved to 96.1±3.7 %, revealing the enhanced mixing performance of microreactors compared to that of conventional batch reactors.28 In addition, the time duration for generating mesoporous nanofibers only needs less than 4 seconds, showing fast reaction kinetics via the microreactor. Because of the automatic and continuous characteristics of such microfluidic reaction system, the fiber product yield can be easily achieved to gram-scale and scale-up.


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Figure 2. As-synthesized mesoporous silica nanofibers (MSNFs). (A) Scanning electron microscopy (SEM, A-C) and transmission electron microscopy (TEM, D-F) images of assynthesized MSNFs at different magnifications. The inset in Figure A is a histogram showing the statistical distribution of aspect ratios.

To explore the tunability and formation mechanism of the mesoporous silica products from the microreactor, we next separately investigated the effect of a series of reaction conditions on the structural changes. As shown in Figure 3, both the flow rates and the concentrations of reactants play significant roles on the formation and morphology of mesoporous silica (see experimental and product details in Table S1). Increasing the flow rate of TEOS (i.e., Inlet II, 40 µL/min) reduced the aspect ratios of mesoporous silica product until it became spherical (Figure 3A), this was resulted from the shorter CTAB micelles formed under the influence of stronger shearing force at the interface of the two reaction fluids. On the contrary, decreasing the flow rate of TEOS (i.e., 5 µL/min) helped to form longer CTAB micelles and thus raised the aspect ratios of fiber product over 50 (Figure 3B). Changing the concentration of TEOS (0.35-2.46 M in Inlet II) while maintaining the same constant flow rate of Inlet II (Figures 3C-D, 10 µL/min), the size and aspect ratio of silica products were roughly positively correlated with TEOS concentration. Correspondingly, increasing the flow rate of Inlet I (i.e., 400 µL/min) generated longer CTAB micelles and thus raised the aspect ratios of silica fibers (Figure 3E), while decreasing it (i.e., 25 µL/min) reduced the aspect ratios and yielded nanospheres (Figure 3F). The aspect ratios of silica nanostructures were also positively correlated with the concentrations of CTAB (3.9-47 mM in Inlet I) while maintaining the same constant flow rate of Inlet I (Figures 3G-H, 100 µL/min). In addition, the concentration of ammonia regulated the hydrolysis and condensation


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rates of silica precursor and thus played important roles in controlling the diameters of fiber materials (Figures 3I-J, 0.18-1.10 M in Inlet I). These results suggest that the properties of mesoporous silica fibers, including aspect ratio, homogeneity, and diameter, can be well and easily tuned by the operating parameters of the microreactor (e.g., flow rate) or the inlet flow concentrations of reactants.

Figure 3. The effect of reaction conditions, including flow rate of TEOS (A-B), concentration of TEOS (C-D), flow rate of CTAB and ammonia (E-F), concentration of CTAB (G-H), and concentration of ammonia (I-J), on the morphology changes of mesoporous silica products (See experimental details in Table S1). Considering the ease of operating the microreactor and combining other functional molecules/nanoparticles, the feasibility of microfluidic flow synthesis of functional nanofibers was then examined. A series of functional molecules/nanoparticles, including fluorescent dyes (fluorescein isothiocyanate, FITC), magnetic nanoparticles (MNPs), therapeutic drugs (doxorubicin, Dox), or silver nanoparticles (AgNPs) precursors, were chosen to add into the flow of either Inlet I or Inlet II and thus endow MSNFs with more specific functionalities (Figure 4). For the synthesis of fluorescent fibers, aminosilanes (3-aminopropyltriethoxysilane) were firstly


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conjugated with FITC and then added into diluted TEOS as Inlet II. As shown in Figure 4A, well-defined fluorescent silica fibers with typical uniformly distributed green color were obtained (Figure 4Ai-ii), and the FITC dyes from MSNFs-FITC can be firmly doped into the silica matrix without leakage over 1 month in water at room temperature (Figure 4Aiii-iv). The synthesis of magnetic nanofibers was carried out using MNPs-contained Inlet I flow. As shown in Figure 4B, Fe3O4 MNPs, having an average size of ~10 nm from co-precipitation method (Figure S2),29 can be successfully integrated with silica fiber nanostructures (Figure 4Bi). It is noted that, although MNPs were randomly and nonuniformly distributed into silica fibers, MSNFs-MNPs still can stably maintain their structures after 10 days of continuous shaking (Figure 4Bii) and be separated rapidly using an external magnet (

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