A General Approach to Free-Standing Nanoassemblies via Acoustic

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A General Approach to Free-Standing Nanoassemblies via Acoustic Levitation Self-Assembly Qianqian Shi, Wenli Di, Dashen Dong, Lim Wei Yap, Lin Li, Duyang Zang, and Wenlong Cheng ACS Nano, Just Accepted Manuscript • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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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 * 1

Department of Chemical Engineering, Faculty of Engineering, Monash University, Clayton

3800, Victoria, Australia. 2

The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton 3168, Victoria,

Australia. 3

Functional 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]

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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

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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,

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or investigate the particle formation3 and

assembly mechanism13 in combination with other analytic techniques. For example, the insitu 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,

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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,

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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


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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,

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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

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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 disklike 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


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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 integral of acoustic radiation pressure PA over the droplet surface)50 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 additional materials within bilayered structures. As the first demonstration, concentrated DNA-capped nanospheres (DNA-NSs) suspension was injected into the levitated liquid marble (Figure 3a). The reason for using DNA-NSs is because of their extreme stability in water against aggregation as demonstrated in our earlier publications.20, 23 During the water evaporation, the suspended “oval-shaped” droplet gradually evolves into “pancake-like” twodimensional (2D) water films. It has been known previously that such highly stable DNANSs will crystallize via the volume-restriction mechanism.20,

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Hence, the suspended 2D

water films “squeeze” DNA-NSs into the confined 2D plane, and they will only solidify in the last stage of drying. This led to well-defined free-standing trilayered AuNC/DNANS/AuNC nanoassemblies. Note that the static shape of the levitated droplet was determined 6


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by the acoustic radiation pressure, gravity and the liquid surface tension.51 Therefore, it can be conveniently adjusted by the sound intensity in the levitator.39 To ensure both sufficient levitation ability and droplet stability (inhibition of atomization), the sound intensity was tuned carefully as gravity changed due to the water evaporating or adding DNA-NSs. We found that the trilayered nanoassemblies are continuous to macroscopic scale while maintaining overall structural integration with golden colour (insert in Figure 3b). Highresolution SEM characterization demonstrates that all DNA-NSs are trapped and locked between AuNC nanoassemblies (Figure 3b-c). It is worth to note that DNA-NSs didn’t aggregate into 3D structures and no DNA-NS found outside of nanosheets. The sandwich structure with two layers of monolayered AuNC and embedded layers of NS can be easily distinguished from the transmission electron microscope (TEM) images at a high magnification (Figure 3d). If more DNA-NSs were added, pancake-like Au NC-DNA-NSsAu NC layer with tunable thickness could be obtained (supporting information Figure S3). This led to the blue-shift in the maximum plasmonic coupling peak Figure 3e, which may be due to the dominant coupling of DNA-NSs. We found that free-standing nanoassemblies can be uniformly deposited onto both sides of a dielectric sheet without any coffee-ring effects (supporting information Figure S4). As a proof of concept, a paper disk was inserted into suspended AuNC-covered liquid marble while keeping the entire system suspended (supporting information Figure S5). A conformal coating was achieved for both sides of paper disks after the full evaporation of water (supporting information Figure S6a). The cross-sectional optical images clearly demonstrate the sandwich structure with a layer of nanoparticles on the paper disc surface (supporting information Figure S6b). This is in sharp contrast to the first control experiment by directly dropcasting hydrophobic PS-capped nanoparticle suspension on the same type of paper disc. Clearly, nanoparticles penetrate throughout the paper matrix (supporting information Figure 7


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S6c-d). In the second control experiment, a paper disk was firstly embedded inside a droplet, followed with self-assembly of nanoparticle on the water surface (Figure S7). The result showed that only the front side of the paper disk was covered by nanoparticle assemblies. This is in contrast with the double side coating in this work. We further demonstrate the feasibility of generating three-dimensional (3D) free-standing nanoassemblies by in situ producing bubbles inside the liquid marbles. The fabrication process is illustrated in the supporting information, Figure S8a. The introduction of a small amount of cetyltrimethylammonium chloride solution (CTAC) is the key for the stabilization of bubbles inside suspended water droplets (supporting information, Figure S8b). CTAC molecules (with a hydrophilic head and a hydrophobic tail) can adsorbed at the air-water interface to form a monolayer with strong interfacial viscoelasticity, which is able to prevent bubble rupture.52-54 The framework of molecular assemblies is robust enough to survive even after full water evaporation (Supporting information, Figure S8c). This leads to a 3D hollow scaffold structure, which can sustain additional nanoassemblies without destroying overall structural integrity (Supporting information, Figure S8d-e). Moreover, we found that CTAC molecular scaffolds can also support hydrophilic nanoparticles without collapsing. As a proof of concept, DNA-NSs is introduced along with CTAC molecules forming bubbling droplet by acoustic levitation (Figure 4a-b). Then a chloroform solution of PS-Au NCs is introduced to the air/bubbled water interface (Figure 4a and c). The rapid drying of chloroform followed by slow evaporation of water didn’t destroy levitated CTAC hollow scaffold (Figure 4d). DNA-NSs are distributed throughout the CTAC molecular scaffold due to the electrostatic attraction between negatively charged DNA and positively charged CTAC. The PS-Au NCs stay outside hollow CTAT scaffold bubble with a golden reflection colour (insert in Figure 4e). SEM micrograph further proves that DNA-NSs are present inside the bubble while the Au NC layer outside (Figure 4e). 8


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Conclusions The above results clearly show that acoustic levitation self-assembly is a powerful general strategy to fabricate versatile free-standing nanoassemblies. This originates from force balances between acoustic radiation force, surface tension and gravity. Its adaptive nature enabled fabrication of versatile nanoassemblies, ranging from bilayer, trilayers, and hollow capsules. It could also provide the possibility to fabricate such nanoassemblies at the liquidliquid interfaces since liquid droplet can be levitated in liquid medium.55 We envisage that additional functional materials including quantum dots, magnetic particles, graphene, etc. may also be assembled in a programmable manner. Such heterogeneous free-standing multifunctional 2D assemblies may find wide applications in sensing, anticounterfeiting, ionic gating, nanophotonics and nanoelectronics.

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Methods The synthesis of building blocks: The synthesis of Au nanocube (NC): Au nanocubes were synthesized by a seed-mediated growth method.40,

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Firstly, a seed

solution was prepared by reducing 0.1 ml of 25 mM HAuCl4 with 0.6 ml of 0.01 M NaBH4 in 7.65 ml of 0.1 M CTAB solution, followed by stirring for 2min and keeping at 30oC for 1 hour. The obtained seed solution was diluted 10 times by adding 1ml of seed solution into 9 ml of Milli-Q water. Then, by adding 2.5 µl of diluted seeds into a growth solution containing 0.8 ml of 0.1 M CTAB, 0.1ml of 7 mM HAuCl4, 4 ml Milli-Q water and 0.6 ml of 0.1 M ascorbic acid. The solution was then kept undisturbed at 30oC overnight, followed by washing with Milli-Q water after centrifugation at 7830 rpm for 10 min. The synthesis of Au@Ag nanobrick (NB): The synthesis of Au@Ag nanobrick involved two steps.42-44 In the first step, Au seeds were prepared by adding 0.6 ml of 0.01M NaBH4 into the mixture of 5 ml of 0.5 mM HAuCl4 and 5ml of 0.2 M CTAB. Then, 12µl of seeds solution was added into a growth solution that containing 0.2 ml of 4 mM AgNO3, 5 ml of 0.2 M CTAB, 5 ml of 1 mM HAuCl4 and 0.08 ml of 0.08 M ascorbic acid. The above solution was kept undisturbed at 30oC. After two hours, the seeds grew into nanorods. The final nanorod solution was collected by centrifugation at 8000 rpm for 20 min and finally redispersed in 10 ml of 80 mM CTAC solution. Au@Ag nanobricks were synthesized by coating a layer of silver on Au nanorod. Briefly, 1 ml of 10 mM AgNO3 and 0.5 ml of 0.1 M ascorbic acid were added into 10ml of prepared Au nanorod solution. The above solution was kept stirring at 60oC for 4 hours. Finally, Au@Ag nanobircks were collected by centrifugation at 7000rpm for 10min and redispersed in 10ml of Milli-Q water for further use. 10


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The synthesis of Au nanobipyramid (NBP): Au nanobipyramids were synthesized by a seed-mediated method followed with a three-step purification.30, 45, 46 Firstly, Au seed solution was prepared by adding 1ml of 100mM NaBH4 into a 40 ml solution containing 0.25 mM HAuCl4 and 0.25 mM trisodium citrate. The solution was kept at room temperature for 2 hours with vigorous stirring. In the second step, a growth solution was prepared by mixing 2 ml of 25 mM HAuCl4, 1 ml of 10 mM AgNO3, 2 ml of 1.0 M HCl, and 0.8 ml of 0.1 M ascorbic acid. Then, 0.8 ml of seed solution was added to the above growth solution and aged at 30oC overnight. The final solution was centrifuged and redispersed in 80 mM CTAC solution. A three-step purification method was used to remove impurities in the NBP solution as reported. Briefly, 36 ml of 10 mM AgNO3 and 18ml of ascorbic acid solution were added into 70 ml of 80 mM CTAC capped NBP nanoparticle. The solution was kept at 65oC water bath for 4 hours to grow Au NBP core @Ag shell nanorod. The core-shell nanorods were washed and kept undisturbed in 50 mM CTAB. Following on, the precipitation was collected and etched with NH3.H2O and H2O2. The final NBP solution was washed and redispersed in 25 ml of 50 mM CTAB for further use. The synthesis of Au trisoctahedron (TOH): Au TOH particles were prepared according to the procedure described previously.47, 48 A seed solution was first prepared by adding 0.3 ml of 100 mM NaBH4 into 5 ml of 0.1 M CTAB and 50 µl of 25 mM HAuCl4 mixture. After 2 hours, the seed solution was further diluted into 100 fold with water. To synthesis TOH particles, 667 µl of diluted seed solution was added into a growth solution containing 1.667ml of 20mM HAuCl4 and 120ml of 22mM CTAC and 40.8ml of 38.8mM AA under constant stirring. After 5 min, the solution was centrifuged and redispersed into 40 ml of water. 11


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The synthesis of Au nanosphere (NS): Typically, 2 ml of 25 mM HAuCl4 were added into 170 ml of Milli-Q water and then heated to boiling temperature. Then, 6 ml of 34 mM sodium citrate was added into the above solution under vigorously stirring. The solution was kept in the boiling water bath for another 10 min and then cooled down to room temperature for further use. Capping Au nanospheres with DNA: Firstly, 15.78µl of thiol-terminated DNA (333µM) was added into 200ul of 50mM tris(2carboxyethyl)phosphine hydrochloride (TCEP), The solution was kept undisturbed to activate the thiolated DNA.56 After 30min, 1ml of Au nanosphere (5.26×10-9M) was added to the TCEP-DNA solution and left undisturbed for 24 hours. Then, 100µl of 1M NaCl solution was added drop-wise to the above solution slowly. The mixed solution was kept at room temperature for another 66 hours. Finally, the solution was centrifuged at 14500 rpm for 30 min twice and finally redispersed in 0.1ml of Milli-Q water for further use. The nanoparticle used for injection are concentrated from 10 ml of DNA capped nanospheres, the final concentration of the nanoparticle is 5.26×10-7M. The assembly of nanoparticles: Preparation of hydrophobic nanoparticles: The hydrophobic nanoparticles were prepared using a two-step ligand-exchange method. 30, 42, 47

As-prepared CTAC/CTAB capped nanoparticles (15ml of Au NC (3.59×10-11M), 10ml of

Au@Ag NB (3.36×10-10M), 12ml of Au NBP (2.44 x 10-11M), and 10ml of Au TOH (4.67×10-11M)) were centrifuged and redispersed in 4mg/ml of thiol-terminated polystyrene (Mn = 50, 000 for Au NC, Au@Ag NB, and TOH, and Mn = 20, 000 for Au NBP nanoparticles) solution. After ligand-exchange overnight, the samples were washed with THF and then chloroform for several times to wash off uncapped PS ligands. Finally, after 12


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discarding the supernatant and leave ~20 µl chloroform, the samples were ready for dropping on the water surface. It is important to control right concentration to achieve uniform selfassembly.57 The final concentration of nanoparticles in the chloroform solution used for levitation self-assembly is 2.69×10-8M (Au NC), 1.68×10-7M (Au@Ag NB), 1.46×10-8M (Au NBP), and 2.33×10-8M (Au TOH). Acoustic levitation: To accomplish the substrate-free condition for self-assembly, a single-axis acoustic levitator (SonoRh-1, Shengdu Ltd., China) was used to levitate the droplet which acted as the assembly template. The levitator consists of an emitter and a reflector which were arranged coaxially with respect to the gravitational direction. The frequency of the emitter was 20.7 kHz. To enhance the levitation ability and stability, the surface of the reflector was made concave, with a radius of curvature of 37.4 mm. The details can be found elsewhere.6 In order to conveniently adjust the sound intensity in the levitator, the reflector was fixed on a microlifting table (ST401ES60, Strong Precision, China) to regulate the distance between the emitter and reflector. The lifting/descending rate of the reflector can be accurately controlled by a servo motor (42BYGH47-1684B, Sihongmotor, China). In our experiments, uR was fixed at 1.0 mm/s. The assembly of bilayered nanoassembly: 10µl of ultrapure water was positioned at one of the sound pressure nodes (Figure S2(c)) using a micro-syringe. The distance between an emitter and reflector was carefully tuned to further stabilize the water droplet. Normally, the droplets were trapped at one of the pressure nodes which is located ~23 mm above the reflector. Following on, one drop of prepared concentrated chloroform-nanoparticles solution (~1µl) was spread onto the levitated water droplet. During the water evaporation process, the distance between the emitter and reflector 13


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was further adjusted to stabilize the liquid marble. After the water fully evaporated, the sample was collected by turning off the levitator power. The drying time (from the suspending of a water droplet to the fully dry of a nanoassembly) is approximately 30-60min for a 10µl water droplet under ambient conditions. The assembly of sandwich nanoassembly with DNA-nanoparticles inside: The assembly of sandwich nanoassemblies followed the same steps as the fabrication of bilayered nanoassemblies, except the injection of 2µl (or 5µl or 8µl) of DNA capped nanospheres into the droplet after dropping the concentrated hydrophobic Au NC nanoparticles. The assembly of sandwich nanoassembly with a paper disk inside: The assembly of sandwich nanoassembly followed the same steps as the fabrication of bilayered nanoassembly, except inserting a piece of paper disk (with a dimension of ~3mm) into the droplet after dropping the concentrated hydrophobic nanoparticles. The assembly of hollow nanoassembly: The assembly of hollow nanoassembly followed the same steps as the fabrication of bilayered nanoassembly, except injecting 0.6µl of CTAC into the droplet before dropping the concentrated hydrophobic nanoparticles. The CTAC hollow structure was fabricated following similar steps as the fabrication of hollow nanoassembly without dropping PS capped nanoparticles. Control experiments: The control experiments were undertaken by directly dropping PS capped (hydrophobic) nanoparticles solution on a piece of paper/on a droplet embedded with a paper disk and leaving it dry at room temperature. 14


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Characterization TEM (FEI Tecnai G2 T20 TWIN LaB6 TEM operating at 200 kV) and SEM (FEI Helios Nanolab 600 FIB-SEM operating at 5 kV) were used to characterize the morphology of the bilayered or sandwich structures. Optical properties were tested from J&M MSP210 Microscope spectrometry system which was illuminated by high-intensity fibre light source under a 20x objective. Optical images were obtained from Nikon Eclipse Ni Upright Microscope. Acknowledgements We thank financial support from Australian Research Council via Discovery Grant scheme DP170102208. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors also gratefully acknowledge the use of facilities at the Monash Center for Electron Micron Microscopy. Author Contributions W.C., D.Z. and Q.S. conceived the project and designed the experiments. Q.S. carried out experiments work on assembly and characterization of free-standing nanoassemblies and schematic drawing; W.D. carried out the simulation work; D.D. was involved in particle characterization; L.W.Y. performed part of the schematic diagram drawing; L. L. helped with experimental setup. W.C., and Q.S. analyzed the experimental data and co-wrote the paper. All authors discussed the results and commented on the manuscript. Competing financial interests The authors declare no competing financial interests.

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Supporting Information Available: Details of simulations, the temporal evolution of the assembly of sandwich and hollow nanoassemblies, and the control experiments; the videos of the fabrication process at the beginning, middle and the last stages. These materials are available free of charge via the Internet at http://pubs.acs.org.

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Figures

Figure 1 Levitation mediated self-assembly of a bilayered nanoassembly. (a) Schematic representation of the assembly of a bilayered nanoassembly in an ultrasonic standing wave between the emitter and reflector. (b-f) The temporal evolution of an evaporating droplet collected with from a camera. The droplets were trapped at one of the pressure nodes which is located ~23 mm above the reflector. SEM images of bilayered nanoassemblies that assembled from (g) Au NC; (h) Au@Ag NB; (i) Au NBP; and (j) Au TOH.

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Figure 2 Time evolution of the acoustic radiation pressure PA on the sample surface during water evaporation in (a) liquid marble and (b) “bare” water droplet. For both cases, PA was negative (suction effect) at the equator areas while positive (compression effect) at the polar regions. However, for the droplet coated with nanoparticle assemblies the suction effect was significantly enhanced because of the puddle shape of the droplet caused by interfacial jamming. (c) Schematic illustration of PA distribution on a puddle shaped droplet. 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 2D nanoassemblies. The acoustic pressure levels from left to right in figure 2(a) are 152.30 dB, 152.84 dB, 152.34 dB and 153.58 dB, respectively. R represents the projected radius of the droplet. Upper and lower refer to the top and bottom surface of the droplet, divided by the equator.

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Figure 3 Levitation mediated self-assembly of a sandwich nanoassembly with DNA-NSs inside. (a) Schematic representation of the assembly of a sandwich nanoassembly by injecting concentrated DNA-NSs suspension. (b-c) SEM images of the cross-session of a sandwich nanoassembly showing DNA-NSs located between two layers of Au NC nanoassemblies, insert shows the image of a whole piece of sandwich nanoassembly. (d) TEM images of corresponding sandwich nanoassembly. (e) Plasmonic spectra of sandwich nanoassemblies with 2µl (black curve), 5µl (red curve), and 8µl (blue curve) of DNA-NSs, inserts represent their corresponding optical microscope images under transmission mode.

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Figure 4 Levitation mediated self-assembly of a free-standing hollow nanoassembly. (a) Schematic representation of the cross-session of the sample during the assembly process. Corresponding photographs of a levitated DNA-NSs droplet with bubbles (b) before) and (c) after adding PS-Au NCs, and (d) a levitated hollowed structure after the full evaporation of water. (e) SEM image of the free-standing hollow nanoassembly, insert represents the photograph of the sample after removal from the levitator.

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A General Approach to Free-Standing Nanoassemblies via Acoustic

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