Observation of Metal Nanoparticles for Acoustic Manipulation

Use of acoustic trapping for the manipulation of objects is invaluable to many applications from cellular subdivision to biological assays. Despite remarkable progress in a wide size range, the precise acoustic manipulation of 0D nanoparticles where all the structural dimensions are much smaller than the acoustic wavelength is still present challenges. This study reports on the observation of metal nanoparticles with different nanostructures for acoustic manipulation. Results for the first time exhibit that the hollow nanostructures play more important factor than size in the nanoscale acoustic manipulation. The acoustic levitation and swarm aggregations of the metal nanoparticles can be easily realized at low energy and clinically acceptable acoustic frequency by hollowing their nanostructures. In addition, the behaviors of swarm aggregations can be flexibly regulated by the applied voltage and frequency. This study anticipates that the strategy based on the unique properties of the metal hollow nanostructures and the manipulation method will be highly desirable for many applications.

. Acoustic manipulation of NP100 with various porosities. The images versus the levitation plane were recorded by dark-field optical microscopy and shown with surface plot by using the three-dimensional (3-D) Surface Plot plugin. In all scanned images the size is 5 × 5 mm. Figure S2. Acoustic manipulation of NP150 with various porosities. The images versus the levitation plane were recorded by dark-field optical microscopy and shown with surface plot by using the three-dimensional (3-D) Surface Plot plugin. In all scanned images the size is 5 × 5 mm. Table S1. Notation and value ρ P the density of particle for pure metals Ag: 10.5 g·cm -3 ; Au: 19.32 g·cm -3 ; ρ M the density of medium for pure water: 1 g·cm -3 ; C P the speed of sound in particle for pure metals Ag: 3650 m·s -1 ; Au: 3240 m·s -1 ; C M the speed of sound in medium for pure water: 1496 m·s -1 V P the volume of particle 1.0 × 10 -21 m 3 for 100 nm nanocube P maximum acoustic pressure about 1.0 × 10 4 Pa (measured at 4.5 MHz and 10 V pp ). λ the sound wavelength about 332 μm (for 4.5 MHz in water)

Video S1
Brownian motion. In the absence of the acoustic field, 50 nm silver nanocubes suspended in the cylindrical chamber showed typical Brownian motion. The observations were conducted under the upright microscope Leica DM4000M with a HC PL Fluotar 50×/0.55 BD objective.
The movie was captured at 100 fps and was played at 100 fps.

Video S2
Solid nanostructures for acoustic manipulation. 50, 100 and 150 nm metal nanoparticles with solid nanostructures (about 1 nM) were exposed to the acoustic field (continue sine waves with a resonant frequency of 4.5 MHz and the driving voltage of 10 V PP ), respectively.
For smaller solid nanoparticles (such as 50 or 100 nm), the acoustic streaming force is dominant over the radiation force, so there is no evidence of acoustic levitation and aggregation. The observations were conducted with an N PLAN EPI 5×/0.12 BD objective.
The movie was captured at 5 fps and was played at 20 fps.

Video S3
Hollow nanostructures for acoustic manipulation. 100 nm metal nanoparticles with hollow nanostructures (NP100-4, about 1 nM) were exposed to the acoustic field (continue sine waves with a resonant frequency of 4.5 MHz and the driving voltage of 10 V PP ). It showed that the hollow nanostructures had a fast upward motion and then actually trapped in suspension, which indicated that the radiation force was dominant over the acoustic streaming force. The observations were conducted with a HCX PL FLUOTAR 10×/0.30 and an N PLAN EPI 5×/0.12 BD objective, respectively. The movie was captured at 5 fps and was played at 20 fps.

Video S4
Experiments by varying the driving voltage between 4 and 10 V pp . When the aggregation is established, the secondary radiation force arising from the acoustic particle-particle interaction produces a significant attraction, and thus the aggregation of nanoparticles is easier to maintain by the net effect of the primary and secondary acoustic radiation forces. It showed that 4 V pp and 10 V pp could be used for maintaining and growing the aggregation, respectively.
The observations were conducted with a HCX PL FLUOTAR 10×/0.30 objective. The movie was captured at 5 fps and was played at 5 fps.

Video S5
Experiments by varying the driving voltage between 10 and 0 V pp . The aggregation and dis-aggregation could be easily repeated by switching the driving voltage between 10 Vpp and 0 Vpp. The observations were conducted with a HCX PL FLUOTAR 10×/0.30 objective. The movie was captured at 5 fps and was played at 20 fps.

Video S6
Experiments by varying the applied acoustic frequency. The location of the swarm aggregation was firmly corresponded to the applied acoustic frequency, but it showed some interesting differences in the movement speed and orbit by the changes of frequency in magnitude and direction. The observations were conducted with a HCX PL FLUOTAR 20×/0.40 objective. The movie was captured at 5 fps and was played at 5 fps.

Video S7
The effect of the secondary forces in the aggregation process. The observations were conducted with a HCX PL FLUOTAR 20×/0.40 objective. The movie was captured at 5 fps and was played at 20 fps.