Colloidal microparticles, with polymer composites encapsulated within two separate 2D material sheets, are fabricated by autoperforation, which can carry chemical and electronic information with long-term instability in complex environments.
Conforming materials to rigid substrates with Gaussian curvature can generate strain field and guide its crack propagation. This concept has been recently demonstrated at the macroscopic scale for controlling cracking morphology and producing desired shapes1. Now reporting in Nature Materials, Pingwei Liu and colleagues2 have successfully scaled down the concept to spontaneously assemble polymer composite ink and a pair of two-dimensional (2D) material sheets into functional microparticles, via guided cracking, termed autoperforation. It is a simple yet elegant nanofabrication technique that performs well with 2D materials such as graphene, transition metal dichalcogenides and hexagonal boron nitride, allowing a precise compositional or structural control of the exterior 2D material encapsulation and interior composite fillers, to achieve various functions for practical purpose.
A schematic of autoperforation process is shown in Fig.1a. Polymer discs with an appropriate aspect ratio of radius to height, the array of which can be fabricated by inkjet printing as shown in Fig. 1b, are sandwiched by two layers of 2D materials supported on poly(methyl methacrylate) (PMMA) films. During the lift-off process, 2D materials crack along the edges of the solid templates, forming a stable colloidal suspension of encapsulated microparticles. In this platform, different types of 2D materials can be chosen as top or bottom layers to have a hybrid encapsulation, while the polymer fillers can be composited with functional nanomaterials for different applications.
The flat surface and flexible nature of 2D materials render them perfect matrix to composite a variety of nanomaterials. For example, self-assembled transition metal dichalcogenide nanoscrolls have shown high loading amount of semiconducting or biological nanomaterials3. Methods of assembling 2D materials into microstructures4 to incorporate functional nanomaterials, such as warping5, scrolling3,6,7 and layer-by-layer assembly8, are widely applied in various fields. However, most reported techniques relied on bottom-up self-assembly processes, which lack the control on individual microstructures due to the inherent stochasticity of the process, with few exception of controlled top-down fabrication9. On the other hand, the autoperforation utilizes strain field to guide the assembly of 2D materials to form fully sealed encapsulation of functional nanomaterials with high loading amount, controllability, and long-term stability. It is a perfect combination of bottom-up assembly and top-down fabrication, which is beyond the reach of conventional nanofabrication techniques that either lack fine control and versatility of configurations, or require clean-room processes.
Using proper functional nanomaterials as the interior filler, 2D materials-encapsulated microparticles could be used as electronic devices in complex gas and liquid media. As an example presented by Liu and co-workers, microparticles filled with black phosphorus nanoflake composite (0.9 wt% in polystyrene) and encapsulated with monolayer graphene can function as a non-volatile memory, where the top and bottom graphene layers act as two-terminal electrodes and information can be written and read with a potential probe. As shown in Fig. 1c, 15-bit letters of M, I and T were written on an individual microparticle via spatially selective writing/erasing with a 5-μm diameter potential probe. Ideally, the number of stored bits is only determined by the probe size (for example, 106 with a 100-nm probe). Despite the instability of black phosphorous towards oxygen and water, the digital information written in graphene-encapsulated microparticles can still be retained and read out after being dispersed and collected for several circles in water. The 2D material envelopes are remarkably stable for months in harsh liquid environments, such as the human gastrointestinal tract and organic solvents. With the superior mechanical property of graphene, these microparticle devices exhibit excellent mechanical stability. Aerosolization of the microparticle devices can yield 69% intact microparticles captured over a 30-cm distance. This is particularly impressive considering the outer layer of the microparticles is merely monolayer graphene.
The microparticle devices can easily travel through complex liquid and solid media. As a proof-of-concept application, the microparticle devices have been used to sense and extract chemical species in water and soils matrix. By distributing and collecting microparticle devices with magnetic filler (1.9 wt% of iron oxide nanoparticles in polystyrene) and proper surface functionalization (for example, with ligands that can capture metal ions), Zn2+ could be detected from soil matrix. The excellent chemical and mechanical stabilities of the microparticle devices ensure a prolonged operation time up to months.
Beyond chemical sensing, the microparticle devices could be very versatile probes in biological and medical research areas. Surface functionalization with proper receptors or biomarkers could enable in vivo microelectronic detections, capable of preserving chemical and biological information in biological environments or even living organisms. Furthermore, by introducing selective mass-transport channels (holes in the 2D materials) via functionalization or defect engineering, the encapsulated microparticle could be a new type of microextraction technique in complex liquid and solid phases to extract target chemical and biological species over a long period of time.
Although only three kinds of 2D materials (graphene, MoS2 and hexagonal boron nitride) are demonstrated, autoperforation is completely dependent on the edge-induced strain field. Therefore, there is no limitation in principle on the choice of 2D materials, or even any kind of thin materials. It has the potential to be a platform technology for versatile applications by choosing different 2D materials, changing the shape and composition of fillers, and functionalizing the surface of microparticles.
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He, Q., Zhang, H. 2D materials-wrapped microparticles. Nature Mater 17, 956–957 (2018). https://doi.org/10.1038/s41563-018-0202-6
Advanced Materials (2020)