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273202999-Oa A General Approach to Free-Standing Nanoassemblies via Acoustic Levitation Self-Assembly Qianqian Shi, 1,2 Wenli Di,3 Dashen Dong, 1,2 Lim Wei Yap, 1,2 Lin Li,3 Duyang Zang,3* and Wenlong Cheng1,2 * 1Department of Chemical Engineering, Faculty of Engineering, Monash University, Clayton 3800, Victoria, Australia. 2The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton 3168, Victoria, Australia. 3Functional Soft Matter & Materials Group, Key Laboratory of Space Applied Physics and Chemistry of Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an, Shanxi 710129, People’s Republic of China. * Address correspondence to [email protected] or [email protected] 1 Abstract Suspended droplets by acoustic levitation provide genuine substrate-free environments for understanding unconventional fluid dynamics, evaporation kinetics, chemical reactions by circumventing solid surface/boundary effects. Using fully levitated air/water interface by acoustic levitation in conjunction with drying-mediated nanoparticle self-assembly, here, we demonstrate a general approach to fabricate free-standing nanoassemblies, which can 100% avoid solid surface effects during the entire process. This strategy has no limitation of sizes/shapes of constituent metallic nanoparticle building blocks, and can also be applied to fabricate free-standing bilayered and trilayered nanoassemblies or even three-dimensional hollow nanoassemblies. We believe that our strategy may be further extended to quantum dots, magnetic particles, colloids, etc. Hence, it may lead to a myriad of homogeneous or heterogeneous free-standing nanoassemblies with programmable functionalities. Keywords: levitation, self-assembly, free-standing, nanoassembies, DNA, gold nanoparticles 2 Acoustic levitation can provide an ideal substrate-less environment by levitating materials of interest in the sound field between an emitter and a reflector.1-3 This technique can 100% avoids solid/liquid and solid/air interfaces, thus minimizing solid surface chemical/physical effects.4 Therefore, this technique has been widely used in studies of fluid dynamics,5-7 evaporation of droplets,8-10 microreactions,11, 12 or investigate the particle formation3 and assembly mechanism13 in combination with other analytic techniques. For example, the in- situ observation of the gold nanoparticles synthesis in levitated water droplets by X-ray scattering/adsorption revealed nanoscale nucleation/growth mechanism.3 Levitating droplets can shrink concentrically without contact line pinning effects associated with solid surfaces, enabling the fabrication of mesocrystals.13 Herein, we demonstrate that acoustic levitation can be a general platform to fabricate versatile free-standing nanoassemblies, which can exhibit mechanical, plasmonic, conductive, and catalytic properties that differ from their bulk or individual nanoparticles.14-21 Previous fabrication methods include Langmuir-Blodgett,15, 22 DNA based drying mediated self- assembly,20, 23 liquid-liquid interface self-assembly,24-26 air-liquid interface self-assembly,27-30 and layer-by-layer assembly.31, 32 It has to be noted that solid surfaces or containers are unavoidable in these strategies (Supporting information Figure S1), which often affect the quality, the transferability and manufacturability of free-standing nanoassemblies.33 Even boundary solid/liquid or solid/air interfaces can sometimes significantly influence the uniformity of the nanoassemblies by the known ‘‘coffee ring’’ effects during stochastic solvent evaporation.34-36 In contrast, the acoustic levitation technique can 100% avoid the solid/air and solid/liquid interface in any stage of the self-assembly process. Conventional air/water interfacial self-assembly usually leads to a monolayered nanoassemblies under appropriate conditions,28-30, 37 and it is non-trivial to obtain a bilayered structure. This is because of co-existing of air/water interface and solid/liquid interface for 3 both LB trough and sessile drops. Because of 100% air/water interface for levitating water droplet, it is possible to obtain free-standing bilayered nanoassemblies in a single step. Analysis of time-resolved evaporation process and the acoustic radiation pressure evolution show that the suction effect plays a key role in determining the bilayer structure. Further experimental results indicate that this method is generic, applicable to different kinds of building blocks and the bilayered structure can be further served as a base material to design a number of “sandwich’ trilayered or even hollow 3D nanoassemblies. We begin with Au nanocubes (NCs) as model building blocks to demonstrate the acoustic levitation self-assembly process. Following our previously published protocl,29, 30, 38, 39 Au NCs were synthesized by the seed-mediated growth40, 41 followed by grafting thiolated- polystyrene (SH-PS) to render nanoparticles hydrophobic. Then polystyrene-capped Au NCs were used in acoustic levitation self-assembly as illustrated in Figure 1a. To avoid the influence of solid substrate, the water droplet which served as the template for the assembly of Au nanoparticles was levitated by a single-axis acoustic levitator38 in one of its sound pressure nodes. Concentrated PS-Au NC suspension was subsequently drop-casted on the levitated water surface (Supporting video 1). Upon the quick evaporation of solvent, solid nanoparticle assembled around the droplets, forming a levitated liquid marble with golden reflection (Supporting information Figure S2 and Supporting video 2). With further evaporation water, the levitated golden droplet marble shrank in the vertical direction, but remained constant horizontally (Supporting video 3), and finally, a circular disk composed of a bilayer of building blocks was obtained (Figure 1b-f). Results Unlike air/water interfacial assembly on a semi-sphere water droplet, or the assembly inside a levitated droplet,13 the assembly on the acoustically levitated water droplet results in the 4 formation of a free-standing bilayered structure. Scanning electron microscopy (SEM) image of a piece of free-standing nanossemblies clearly shows two vertically stacked Au NC monolayers (Figure 1g). Since the sound wavelength of the levitator (λ ∼16.6 mm) is much larger than the size of nanoscaled building blocks, the sound wave does not affect the properties of particles as well as the interparticle actions. Therefore, such method can be further applied in different types of building blocks including Au@Ag nanobrick (NB),42-44 Au nanobipyramid (NBP),30, 45, 46 and Au trisoctahedron (TOH)47, 48 (Figure 1h-j), indicating the generality and robustness of this method. To understand the shape evolution during water evaporation, the acoustic radiation pressure PA on the droplet surface was calculated and plotted (Figure 2a). PA was not uniformly distributed, however, it was positive on the polar regions of the droplet whereas negative at the equator region. Such distribution of PA indicates there is a suction effect caused by sound at droplet equator.39 This is extremely important for the subsequent droplet shape evolution. The presence of nanoparticles could enhance the “suction effect” because of their contribution to the puddle shape of droplet. With the reduction of droplet surface area caused by evaporation, the surface density of Au nanoparticles increases which leads to a decrease in surface tension until an interfacial jamming state is reached. The reduced surface tension would weaken its counteraction with sound field, therefore, the droplet did not retract horizontally, which in turn significantly enhanced the suction effect on the droplet because of its puddle shape. In this case, the PA exerted on the sample surface not only balance gravity, but also overcome the retraction of surface tension, therefore eventually leading to the disk- like nanoassemblies (Figure 2c). It should be noted that interfacial jamming of the building blocks during evaporation also plays an essential role in the maintenance of its lateral dimension because of the mechanically robustness of the jammed particle layer. This in turn enable a stronger suction effect than spherical shape,49 which is essential to form the 5 bilayered structure. In contrast, a levitated “bare” water droplet shrinks uniformly (Fig. 2b) because its surface tension does not change during the entire evaporation process. Note that the acoustic radiation pressure is also a key factor to obtain a free-standing bilayered structure. The shape of the levitated droplet is determined by the competition between acoustic radiation force and surface tension.49 Therefore, the needed acoustic radiation pressure would depend on droplet surface tension which may be varied for different systems. However, regarding on the formation of bilayered structure, it is more important to judge from the droplet shape rather than from detailed value of acoustic radiation pressure. During particle assembly process, the droplet shape, as well as the acoustic radiation pressure over its surface, can be conveniently regulated through adjusting the emitter-reflector distance.6 Since the levitation is realized by the balance between acoustic radiation force (namely, an 50 integral of acoustic radiation pressure PA over the droplet surface) with the sample gravity, the system can levitate a wide range of materials regardless of their physical/chemical properties. This allows for adaptive fabrication of complex structures, such as adding
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