Micromotors powered by catalytic reactions and their applications

In this thesis the mechanism of motility and applications, of micro-sized, autonomously powered, platinum-gold (Pt-Au) motors are discussed.;In the electrochemical pathway for peroxide decomposition, H2O 2 is oxidized to O2 at the Pt anode, and reduced to H 2O at the Au cathode. Based on the bipolar fuel decomposition, a self-electrophoretic mechanism for rod motility was proposed. According to this hypothesis, the electron current from Pt anode to Au cathode in the rod is accompanied by concomitant proton flux in the surrounding fluid. As the protons migrate from one end to another an electroosmotic fluid flow occurs from Pt to Au, resulting in motor propulsion Pt end forward. Several approaches have been adopted to verify this hypothesis. The approach described in this thesis was to alter the surface charge of the motor. The experiments relating to mechanism of motor motility are discussed in chapter 2.;Synthetic Pt-Au motors could be used as agents for transport and delivery of materials in the microscale regime. In chapter 3, the application of Pt-Au motors for transport of colloidal cargo is demonstrated. A couple of methods for attachment of prototypical microsphere cargo to Pt-Au motors were explored. In the first method, electrostatic attractive forces were used to attach positively charged, amidine functionalized, microsphere cargo to negatively charged Pt-Au-Polypyrrole (PPy) rods. The PPy end of the rod bears a higher negative charge than the metallic segment and the cargo preferably attached to that end. For the second mode of cargo attachment, the more specific biotin-streptavidin based interaction was used. A monolayer of a biotin terminated dithiol was formed on the gold end. Streptavidin-coated microsphere cargo was then bound to the biotin functionalized end of the rod. Motor-cargo doublets were found to exhibit motility. Subsequently, a quantitative study of the effect of cargo diameter on motor speed was performed.;For maneuvering matter in the mesoscale, in addition to cargo transport the ability to deliver the payloads at desired locations is essential. In chapter 4 strategies for UV-light induced drop-off of cargo attached to motors are described. Applications of Pt-Au motors for delivery of materials or assembly of structures in the meso scale require transport and delivery of cargo. Two strategies for cargo drop-off were explored. In both methods, the link holding the motor and cargo snapped upon exposure to UV light. In the first approach the motor design incorporated Ag segments. Pt-Au-Ag-Au-PPy motors were attached to amidine functionalized cargo. This mode of cargo drop-off was based on the rapid dissolution of the Ag segment in the presence of UV light and chloride ions. In the second mode of cargo drop-off, a bifunctional linker with a photocleavable moiety was synthesized. The photocleavable moiety of the molecule was flanked by an amino group on one end and biotin on the other end. Pt-Au-PPy-PPyCOOH motors were synthesized.;In chapter 5, the feasibility of powering microscale objects using enzymatic reactions is explored. In earlier chapters, the catalytic micromotor system was based on the bimetallic Pt-Au catalytic system. Motor performance can be improved by using better catalysts. Naturally occurring biocatalysts---enzymes, are among the most efficient catalysts known. In addition to the efficacy in their action, enzymes catalyze a wide range of reactions. The plethora of enzymatic reactions can be explored for design of future chemical locomotors. The enzyme catalase, which catalyzes the decomposition of H2O 2 to H2O and O2 was asymmetrically functionalized on 0.5 microm polystyrene microspheres. The diffusion coefficients of the particles were tracked in the presence and absence of H2O 2 fuel solutions. In this system, no directed motility or enhancement in diffusion coefficient of the catalase functionalized particles was observed in the presence of substrate. The most likely cause for this may be the poor activity of the enzyme, post-immobilization. Further investigation of this system is required to determine whether asymmetric gradients of neutral species can power microscale objects. (Abstract shortened by UMI.)...