The Control, Assembly, Shape-dependent and Material-dependent Studies of Ultrasonically Propelled Nano- and Micromotors

Open Access

Author:

Ahmed, Suzanne

Graduate Program:

Chemistry

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

May 26, 2015

Committee Members:

• Thomas E Mallouk, Dissertation Advisor
• Ayusman Sen, Committee Member
• Benjamin James Lear, Committee Member
• Jun Huang, Committee Member

Keywords:

• Nanorod
• Nanomotor
• Ultrasonically Propelled Nanomotor
• Acoustic Nanomotor

Abstract:

Fuel free, low power, ultrasonic propulsion of nano- and microscale motors is currently among the leading candidates for use for biomedical applications such as drug delivery and holds potential for use in a variety of other applications. It does not rely on a finite supply of fuel, provides autonomous motion and promises to be biocompatible. In order to realize its potential for use in biological systems as well as other applications it must be, its biocompatibility demonstrated, the assembly behaviors of the propelled nanomotors explored, and the factors affecting motion interrogated. This document aims to shed light on these various aspects of ultrasonic motor propulsion. An overview of the field of nanomotors, including different propulsion mechanisms, types of motion and the challenges of propulsion at a low Reynolds number are provided in Chapter 1. In Chapter 2, the steering of nanorod motors in biocompatible media towards living cells was demonstrated. By incorporating a magnetic nickel segment (40±5nm) within a gold-ruthenium rod that is 300 ± 30nm in diameter and 4.3 ± 0.2 µm long, it is possible to use a small external magnetic field to define the motor path. It is possible to suppress random motion, as quantified by the decrease in the mean displacement angle and rotation diffusion coefficient of the rods, and exert relatively fine control over the steering of acoustically propelled nanomotors using a 40milliTesla external magnetic field. Rods maintain autonomy and can be selectively guided toward single cells with micron level precision. The effect of the exposure to ultrasonic power within the acoustic chamber on living cells in the presence of metallic nanorods was evaluated, and for durations as long as 20 minutes biological cells remain alive. In Chapter 3, the assembly behavior of magnetic, steerable nanorods is explored. It was found that segmented gold-ruthenium nanorods (300 ± 30 nm in diameter and 2.0± 0.2 µm in length) with thin Ni segments at one end assemble into few particle, geometrically regular dimers, trimers and higher multimers while levitated in water by ~4 MHz ultrasound at the midpoint of a cylindrical acoustic cell. The assembly of the nanorods into multimers is controlled by interactions between the ferromagnetic Ni segments. These assemblies are propelled autonomously in fluids by excitation with ultrasound and exhibit several distinct modes of motion. Multimer assembly and disassembly are dynamic in the ultrasonic field. The relative numbers of monomers, dimers, trimers, and higher multimers are dependent upon the number density of particles in the fluid and their speed, which is in turn determined by the ultrasonic power applied. The magnetic binding energy of the multimers estimated from their speed-dependent equilibria is in agreement with the calculated strength of the magnetic dipole interactions. These autonomously propelled multimers can also be steered with an external magnetic field and remain intact after removal from the acoustic chamber for SEM imaging. In Chapter 4, the shape dependent motion of micromotors in the acoustic chamber is explored. Micromotor structures are designed to interrogate the effect of various levels of shape asymmetry, aspect ratio and rotational asymmetry. Structures of precise shapes and dimensions are fabricated using a combined photolithography and electrodeposition or evaporation approach that allows for the post synthesis release of structures from the substrate for testing within the acoustic chamber. Structures with dimensions larger than 20µm were shown to not undergo random autonomous motion while structures with dimensions smaller than 10µm undergo rotational random autonomous motion. These structures show a consistent polarity of motion indicating that the shape asymmetry of the structure indeed has an effect on its motion. This is in support of the existing consensus that propulsion can be attributed to motor shape asymmetry. In Chapter 5, various factors that affect nanorod ultrasonic propulsion are explored. The effect of a bimetallic nanorod composition, nanorod size and the location of various acoustic behaviors and their response to changes in the electronic signal are studied. These observations also provide insight into the mechanism of ultrasonic propulsion. Bimetallic nanorod motors, exhibit similar behaviors as their monometallic counter parts including random autonomous motion and nodal patterns. Interestingly, bimetallic rods exhibit a consistent polarity of motion with one metal end leading axial propulsion. While propulsion has been attributed to shape asymmetry, in the presence of material asymmetry, the polarity of motion is determined by material asymmetry in cases where there is a difference in density between the two metal segments. In these cases the material segment with a lower density leads motion. Speed comparisons between rods of different densities have shown that lower density rods travel faster than higher density rods explaining the reason why the lower density segment within a bimetallic rod leads. Where only a small density difference exists between the metal segments in a bimetallic rod, the shape asymmetry at the ends of the rod determines the polarity of motion. Insight into the primary force propelling motors was gained by interrogating the length dependent speeds of rods by varying their lengths. As the rods got longer, their speed was reduced which is in support of a streaming induced drag force dominated motion. This was further supported by the correlation between the sharp reduction of acoustic streaming, as indicated by the disappearance of spinning chains, and the sharp reduction in axial propulsion speed of random autonomous motors within the levitation plane. The locations of the various nanorod behaviors within the acoustic chamber were noted.