Highly compressible magnetic liquid marbles assembled from hydrophobic magnetic chain-like nanoparticles

Abstract

We demonstrated a large-scale, low-cost and rapid flame synthesis of hydrophobic magnetic chain-like nanoparticles (HMCNPs) composed of core–shell Fe₂O₃@SiO₂ nanoparticles. The liquid marble made up of HMCNPs is highly compressible with rapid self-recovering ability mainly due to the intriguing elastic behavior and magnetic performance.

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Controlled manipulation of small volumes of liquid, either flow or droplet, is extremely important in miniature systems for chemical and biological applications.^1–3 Since Aussillous et al. proposed their use, liquid marbles have had the potential to effortlessly move liquids in microfluidic devices.⁴ They are non-wetting to any solid surfaces and could be manipulated by external forces, such as gravity, electrical, and magnetic, depending on their components.^5,6 They are very useful for high-throughput analyses and purifications in chemical and biological process, such as drug discovery, DNA analysis, protein crystallization, and the synthesis of molecules or particles.^7,8

Liquid marbles are formed when hydrophobic particles self-organize on the liquid–vapor interface thereby encapsulating a small volume of liquid. The hydrophobic powders play a decisive role in properties of liquid marbles.^9,10 Zhao et al.¹¹ recently reported on a magnetic liquid marbles, prepared by coating a water droplet with hydrophobic Fe₃O₄ nanoparticles, and its magnetic actuation. The magnetic Fe₃O₄ nanoparticles have dual function, as both the force mediator and the hydrophobic-coating phase. For the purpose of improved robustness of liquid marbles, Nguyen et al.¹² studied the shell morphology of liquid marbles using confocal microscopy and obtained a more detailed picture of porous and multilayer structure of liquid marbles shell. Such a structure makes the liquid marbles shell stretchable, which enables a liquid marble to withstand a certain level of mechanical impact and deformation. It is noted that most of previous studies have used hydrophobic microparticles or nanoparticles for protecting the marble. However, the robustness is still unsatisfactory, as well as the lack of functionality. In addition, the preparation process is also complicated and hard to form scalable productions.

Flame aerosol technique is so far one of the most used approaches for the fabrication of commercial quantities of nanoparticles.^13,14 Furthermore, flame methods offer more choices for the design and scalable synthesis of nanomaterials with hybrid structures and functionalization due to the use of a variety of organic–inorganic precursors.^15–18 For example, Friedlander et al.¹⁹ investigated mechanical properties of nanoparticles chain aggregates generated by flame aerosol method, indicating that the chain-like nanoparticles formed network film was deformable and showed elastic behaviors. Based on this theory, Bhosale et al.²⁰ made liquid marbles by surface-treated fumed silica nanoparticles with network structure and attributed its robustness to a nanoparticles elastic thin film that self-assembles on the liquid–vapor interface. However, liquid marbles as carriers in microfluidic devices still remain technical challenges in mechanical robustness and controlled movement.

Herein, for the first time, we developed a large-scale, low-cost and rapid flame synthesis strategy to prepare hydrophobic magnetic chain-like nanoparticles (HMCNPs), which combined flame combustion, aerosol coating and surface functionalization into a continuous gas-phase process. The HMCNPs are composed of core–shell Fe₂O₃@SiO₂ nanoparticles. These HMCNPs can act as the force mediator and the hydrophobic-coating phase for endowing liquid marbles with high mechanical robustness and magnetic controlled movement. More significantly, the liquid marble made up of HMCNPs is highly compressible with rapid self-recovering ability. The synthesis strategy of HMCNPs has been illuminated in Fig. 1. Typically, the precursor alcohol solution (FeCl₃) was fed to the vaporizer with a rate-controlled syringe pump, where air preheated to 250 °C was used as carrier gas to transport the vaporized precursor mixture to the central tube of the nozzle. This resulted in the formation of Fe₂O₃ nanoparticles in a vapour assist flame synthesis (VAFS) reactor. And then, a quench ring with 16 equispaced pores made by a stainless steel torus (see ESI†) was placed to connect to the tube. The in situ coating precursor (tetraethylorthosilicate, TEOS) vapor was supplied by introducing N₂ through TEOS bubbler, and was injected along with an accelerated air via the 16 pores. At the top of the second tube, a spraying nozzle in one side tube sprayed 1 functionalizing precursor solution of 3-methacryloxypropyltrimethoxysilane (MEMO) dispersed by air into the freshly flame-made Fe₂O₃@SiO₂ stream. The products were collected on the filter with the aid of a vacuum pump. The more detailed experimental process was described in ESI.†

Fig. 1 Schematic of preparation process of HMCNPs via flame method.

Fig. 2a shows a low-magnification TEM image of HMCNPs composed of core–shell Fe₂O₃@SiO₂ nanoparticles with an average diameter of ∼22 nm. It can be observed that the as-prepared chain-like nanoparticles possess a satisfactory dispersion. A representative chain-like nanostructure has also been supplied in Fig. 2b showing that the Fe₂O₃ has been well-covered and well-connected by a thin layer of SiO₂ in the range of 2–4 nm. No obvious amorphous SiO₂ particles can be found. The as-obtained chain-like nanostructures have been formed in flame process with the reaction temperature as high as 1800 °C, which could endow the products with higher strength and better flexibility compared to the counterparts synthesized by wet-phase methods. On the other hand, an in situ gas phase surface functionalization has also been realized in the present work, which can not only greatly alleviate the agglomeration among nanoparticles by mutual exclusion and steric hindrance effect of functional groups, but also be a convenient way to proceed the surface functionalization, such as hydrophobic. Moreover, the surface functionalization has no effect on the size of the as-obtained nanoparticles. The wettability of Fe₂O₃@SiO₂ nanoparticles has been evaluated by the method of water contact angle measurement. As shown in Fig. 2d, the HMCNPs exhibited a hydrophobic feature with a water contact angle of 142.5, which made such hydrophobic nanoparticles a good candidate to act as encapsulating agent for the formation of liquid marbles. In general, the materials with hydrophobic surfaces possess lower free energy. This characterization could be beneficial for the enhancement of liquid marbles stability.¹²

Fig. 2 (a) Low-magnification, (b) high-magnification and (c) high resolution TEM images of the as-prepared HMCNP; (d) the investigation of water contact angles and (e) the room temperature magnetization curve of the HMCNP.

The room-temperature magnetization curve of the HMCNP exhibited the superparamagnetic feature with negligible hysteresis, as shown in Fig. 2e, suggesting a minimal agglomeration of magnetite nanoparticles. The saturation magnetization value is as high as 21 emμ g⁻¹, showing excellent magnetic actuation performance for magnetic liquid marbles. Furthermore, the magnetic separability of such HMCNPs has also been tested in cyclohexane by placing a magnet near the glass bottle. The reddish-brown HMCNPs were attached on the wall near the magnet within 30 seconds (inset in Fig. 2e), and then could be easily redispersed only by slight shaking, further demonstrating that the core–shell Fe₂O₃@SiO₂ nanoparticles possessed intriguing magnetic properties.

Liquid marbles were prepared only by rolling a small volume of water into the HMCNPs powder. The nanoparticles spontaneously self-aggregate on the liquid–air interface leading to the non-wetting property. The diameters of the liquid marbles can be tuned even as large as 4 mm, as shown in Fig. 3a. It is noted that mechanical robustness of liquid marbles is an essential requirement for their practical applications, such as microfluidic devices. In our work, the as-prepared liquid marbles exhibited an excellent mechanical robustness. Fig. 3b–d provided a direct evidence based on the compressible measurements of the liquid marbles. The liquid marble can repeatedly be pressed and released by means of a glass coverslip, demonstrating highly compressible with rapid self-recovering ability. The corresponding video has also been supplied (see ESI†). Such excellent mechanical robustness is better than that of other iron oxides and fumed silica liquid marbles reported in the literature,^11,20 which mainly be attributed to the elastic behaviors and magnetic properties of HMCNPs. It is noteworthy that the shell layer of SiO₂ imparted elastic behaviors to the liquid marbles. As the marble was compressed, the network space disappeared, making it parallel chain-like structures. After withdrew the force, the squashed marble can quickly self-restore again. Such intriguing elastic behaviors could be ascribed to the mutual entanglement of chain-like nanostructures in our liquid marbles, forming an enhanced network on the liquid–air interface. And hence, the network can act like an elastic membrane to protect the liquid marble. Similar behavior was also reported, where the liquid marble was made from water and glycerol drops embedded in surface-treated fumed silica nanoparticles.²⁰ The possible schematic has been proposed in Fig. 3e and f. On the other hand, the magnetic liquid marbles also possess a remarkable ability to be opened and closed reversibly with the assistance of a magnetic field. The detailed description can be found in ESI.†

Fig. 3 (a) Liquid marbles formed by HMCNPs with different sizes, (b–d) the process of liquid marbles be compressed and recovered (see also Movie S1 of the ESI†).

In conclusion, hydrophobic magnetic chain-like nanoparticles (HMCNPs), composed of core–shell Fe₂O₃@SiO₂ nanoparticles, have been successfully realized by a large-scale, low-cost and rapid flame synthesis strategy incorporating in situ coating and surface functionalization. More significantly, the liquid marble made up of HMCNPs is highly compressible with rapid self-recovering ability mainly due to the intriguing elastic behaviors and magnetic performances. The results in the present work are important not only for the synthesis of HMCNPs, but also for the development of liquid marbles. It is also reckoned that such intriguing feature will possess very promising applications in the development of magnetically actuated channel-free microfluidic systems and smart microreactors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21236003), the Shanghai Rising-Star Program (13QA1401100), the Basic Research Program of Shanghai (11JC1403000), Program for New Century Excellent Talents in University (NCET-11-0641), the Fundamental Research Funds for the Central Universities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns and IR spectrums for HMNCPs were showed in Fig. S1 and Fig. S2. The evidence of magnetic liquid marbles has the ability to be opened and closed reversibly under the action of a magnetic field can be found in Fig. S3. See DOI: 10.1039/c3ra40998h

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