Understanding the propulsion and assembly of autonomous nano- and micromotors powered by chemical gradients and ultrasound

Nano- and micromotors are a type of machines that turn energy into mechanical motion at the corresponding scales. Autonomous nano- and micromotors have attracted the attention of the scientific community since the initial discovery of the catalytically powered motion of Au-Pt bimetallic nanowires in hydrogen peroxide (H2O2) solution. This particular nanomotor system operates by a self-generated electric field (self-electrophoresis) through asymmetric surface catalytic reactions. However there are two challenges that greatly limit the use of such nanomotors in biological systems: low energy conversion efficiency and poor bio-compatibility of the fuel and the propulsion mechanism. The goal of my research projects is to address these challenges by modifying existing and discovering new nano- and micromotor systems.;Chapter 1 provides a general overview of the research field of nano- and micromotor, the challenges within this field, and a concise description of the research projects discussed in this dissertation.;Chapter 2 in this dissertation is dedicated to addressing the issue of the low energy conversion efficiency of the Au-Pt nanomotors, which is estimated to be on the order of 10-9. Four stages of energy loss are identified. A 10-3 energy is lost due to the non-electrochemical decomposition of H2O2 at the Pt end of the Au-Pt motor, and another 10-3 energy loss can be partially attributed to the fast diffusion of protons. The electrophoretic propulsion mechanism is found to be intrinsically inefficient, contributing to another 10-3 energy loss, while the electroosmotic flow near the charged substrate further slows down the motor. Efforts are made to improve the energy efficiency. Replacing platinum with the less catalytically active ruthenium increases the energy efficiency, and confining the proton flux with a tubular structure is also a promising way to improve the energy efficiency of catalytic motors. These two modifications combined improved the energy efficiency of bimetallic nanomotors by a factor of 12. A numerical model based on COMSOL multi-physics package provides useful information, and is discussed in more detail in Chapter 5. The energy efficiency of a few other nano- and micromotor systems is also discussed. A better understanding of the energy loss of nanomotor systems sheds light on future designs of more efficient nanomotors.;The dynamic interactions and particle assembly phenomena in the bimetallic catalytic nanomotor system are investigated in Chapter 3. Au-Pt nanomotors in H2O2 solutions were found to form doublets and triplets of staggered shapes. A combination of asymmetric pumping and electrostatic interactions between charged ionic clouds around the nanomotors is proposed to explain the binding between two bimetallic nanomotors and the staggered shapes of these doublets. The spontaneous rotation and disintegration of these doublets are attributed to an asymmetric distribution of forces. In addition, bimetallic nanomotors can attract charged microparticles to form close-packed aggregates through electrophoretic attraction. The electrophoretic migration of these charged tracer particles near nanomotors is found to agree qualitatively with simulation results. The effect of particles assembly on the motion of nanomotors is also characterized and discussed.;In Chapter 4 a new propulsion mechanism for autonomously moving metallic nanomotors is described and discussed. Ultrasonic standing waves operating in the MHz frequency range can be used to levitate, propel, rotate, align and assemble metal microrods (2-3 mum long and ∼300 nm diameter) in water. A self-acoustophoresis mechanism based on the shape asymmetry of the metal nanowires is proposed to explain the axial propulsion of the rods. Furthermore such acoustic nanomotors interact with HeLa cells and with polystyrene microspheres in different ways. The significant binding between metallic microrods and HeLa cells in an acoustic field is attributed to a combined effect of colloidal aggregation at high ionic strength and specific binding on the cell surface. When incubated with gold microrods, HeLa cells show substantial uptake of these gold rods, which remain active inside the cells when exposed to ultrasonic standing waves. The ultrasonically driven movement of the metal microrods and their interactions with live cells open up the possibility of driving and controlling metallic nanomotors in biologically relevant media.