Micromotors Powered by Catalytic Reactions and Their Applications

Open Access
Author:
Sundararajan, Shakuntala
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 20, 2010
Committee Members:
  • Ayusman Sen, Dissertation Advisor
  • Ayusman Sen, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Mary Elizabeth Williams, Committee Member
  • Jeffrey M Catchmark, Committee Member
Keywords:
  • autonomous micro/nanomotors
  • applications for micro/nanomotors
  • catalytic micro/nanomotors
Abstract:
ABSTRACT In this thesis the mechanism of motility and applications, of micro-sized, autonomously powered, platinum – gold (Pt-Au) motors are discussed. The Sen group had previously reported powered motion of striped Pt-Au motors (2 µm in length and 360 nm in diameter) in aqueous hydrogen peroxide (H2O2) ‘fuel’ solutions. The motors move at speeds of 5 – 10 µm/s. Transduction of the chemical energy derived from the catalytic conversion of H2O2 to products (H2O and O2), on the bimetallic motor surface results in the observed motility. The motility of these synthetic micromotors is comparable to that of micro-sized flagellar bacteria that convert chemical energy from ATP hydrolysis to power themselves at speeds of few body lengths per second. Initially, the catalytic decomposition of the peroxide substrate was thought to occur at Pt end alone and the motility was ascribed to interfacial effects. According to this hypothesis, the hydrophobicity of the Au end powered the motor Pt end forward due to the lower interfacial tension at the O2 rich environment. However, further investigation revealed the possibility of bipolar H2O2 decomposition, with both Pt and Au participating in fuel consumption. When bulk Au and Pt wires were placed in aqueous H2O2 solutions a current was observed indicating an electrochemical pathway for fuel decomposition. In the electrochemical pathway for peroxide decomposition, H2O2 is oxidized to O2 at the Pt anode, and reduced to H2O 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. By analogy to electrophoresis where oppositely charged colloidal particles move in opposing directions in presence of an external field, self-electrophoretic micro-motors of opposite charges should also move in opposite directions. Negatively charged Pt-Au rods move Pt end forward. On altering the surface charge of the motors and rendering them positively charged, the rods were found to move Au end forward. 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. As the cargo diameter was increased, the motor-cargo doublet speed decreased due to increasing drag contribution from larger cargo. The drag force on the doublets was computed and the speeds predicted by theory were found to be in close agreement with empirically observed speeds. 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. Carboxyl groups incorporated in the polymeric segment of the rods facilitated covalent attachment to the amine terminus of the bifunctional linker molecule via amide bond formation. The biotin end of the linker molecule was used to attach streptavidin-coated cargo to the motors. Upon exposure to UV light, the photocleavage of the linker molecule occurred, releasing the cargo from the motor. 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 H2O2 to H2O and O2 was asymmetrically functionalized on 0.5 µm polystyrene microspheres. The diffusion coefficients of the particles were tracked in the presence and absence of H2O2 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.