Autonomous Motion Driven by Catalytic Nanoparticles

Abstract

Attainment of incisively directed autonomous motion of nano and microscale objects holds promise towards deterministic transportation of materials at smaller length scales. Controlled manipulation of dynamics of such objects not only opens possibilities for targeted delivery of useful biomolecules but also excavates smarter scopes for biosensing, fluidics and minimally invasive surgeries. The very first attempt towards controlled autonomous transport was to harness the self propelling ability of natural biomolecular motors, which naturally evolve and carry things within the cell with extraordinary efficiency. The approach had been to couple these proteins with inorganic structures and then to allow them to move along the cytoskeleton in a directed manner. Applications of these bio-integrated motors were, nevertheless, found to be limited not only for the need of defined environment for protein activation but also for their quick natural degradation. The challenge, therefore, lies in the fabrication of inorganic or organic micro/nano structures exhibiting two and three dimensional autonomous movements, preferably in a liquid, whose motion can be precisely steered inside the medium as desired. These structures must offer flexibility in terms of ease of synthesis and degree of scalability. Attainment of such self-propulsion would not only ensure correct understanding of small scale particle dynamics but also would find importance in targeted transport of materials at the submicronscale. Autonomous motion of micron scale objects was realized in our laboratory with polymer microstructures coated with metal nanoparticles - immersed in dilute hydrogen liquid with finite speed. Control over the motion of these objects was achieved by tuning the properties of the composite as well as that of the medium in which the particles moved. With an aim to attain controlled self-propulsion of an DallD inorganic catalytic object, we used palladium (Pd) nanoparticles incorporated cobalt ferrite (CoFe2O4) micro particles in dilute H2O2 solution. This essentially allowed observation of self propelled motion at smaller length scales, where the Brownian fluctuations were yet to dominate the dynamics. It also established the fabrication of stable magnetic microstructures capable of moving autonomously in a highly reactive medium like H2O2, for a considerably longer period of time. With Pd nanoparticles coated polymer microstructures, we finally demonstrated the first ever example of inorganic pH taxis in a liquid, where the particles were seen to migrate spontaneously from a region of low pH to a higher one. The observation not only established the fabrication of structures that mimic the bacterial behavior across a pH gradient but also marked the development of a novel, quick and efficient pH sensing method. All our observations were theoretically modeled using laws of classical physics. The calculations aided in quantifying the observations - providing estimates on the possible controls achievable over the dynamics of these objects, by manipulating different external and internal parameters. peroxide (H2O2) solution. The nanoparticles (of palladium, nickel and gold) deposited over the polymer surface were capable of decomposing H2O2 catalytically under specific conditions. This essentially generated bubbles of oxygen (O2) that remained tethered to the polymer surface after they were forme......