Designs for Directing Motion at Nano and Microscale

Open Access
Author:
das, sambeeta
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 07, 2016
Committee Members:
  • Ayusman Sen, Dissertation Advisor
  • Ayusman Sen, Committee Chair
  • Thomas E Mallouk, Committee Member
  • John V Badding, Committee Member
  • Darrell Velegol, Outside Member
Keywords:
  • Micromotor
  • nanomotor
  • Active Matter
  • colloid
  • Enzymes
Abstract:
Motion is one of the defining characteristics of life. Inspired by the biological motors at the micron and nanoscale, scientists have developed artificial machines that self propel in solutions and are on the same length scale as biological motors. However, motion at such low length scales is fraught with many challenges. The focus of this thesis is on devising some field free and autonomous strategies for addressing those challenges. This dissertation starts with a literature review of the field with a discussion about the various self-propelled prevalent in the scientific community. Then the different challenges of motion at low Reynolds number and their applicability to the motors are discussed. One of the more popular categories of catalytic micromotors is Janus motors which move in a solution of hydrogen peroxide due to the catalytic activity of Platinum. However it is very hard to introduce directionality in these motors due to the Brownian randomizations prevalent in low Reynolds number regime. The first chapter of this thesis introduces a method of overcoming this challenge by exploiting the interaction of Janus micromotors with physical features. It was observed that Janus micromotors could move linearly in geometric "tracks" as long as they have fuel available for their motion, thus overcoming Brownian diffusion rotations. The second system discussed here takes its inspiration from biological motors and is based on cilia and flagella seen in microorganisms. A design for synthesizing artificial cilia is outlined. This system consists of polymeric rods with a catalytic disk on top which undergo oscillations due to reversible chemical reactions. These synthetic ciliary systems represent an efficient way of mixing fluid at the microscale and overcome the challenges associated with laminar flow at such regimes. The discussion then moves to the nanoscale with the introduction of enzyme nanomotors in Chapter 4. Enzyme molecules not only show enhanced diffusion in the presence of their substrate such as urea for Urease or hydrogen peroxide for catalase; but they also show chemotactic behavior towards their substrate gradient. This is directly parallel to the chemotactic behavior shown by bacteria. This property is exploited to design a system of separating two enzymes with the same mass and isoelectric point. The separation is observed within a two-inlet, five-outlet microfluidic network, designed to allow mixtures of active (ones that catalyze substrate turnover) and inactive (ones that do not catalyze substrate turnover) enzymes, labeled with different fluorophores, to flow through one of the inlets. Substrate solution prepared in phosphate buffer was introduced through the other inlet of the device at the same flow rate. In the presence of a substrate concentration gradient, active enzyme molecules migrated preferentially toward the substrate channel leading to separation of the enzymes. As discussed above enzyme molecules undergo enhanced diffusion in the presence of their substrate. Additionally immobilizing these enzymes on a surface leads to a pump which can pump fluid in all directions in the presence of the respective substrate of the enzyme. This concept has been utilized in Chapter 4 to make a directional microscale pump which can move fluid from one specific point to another. An asymmetrical pattern of enzyme pumps was developed and a substrate gradient was introduced. Due to the asymmetry of the pattern, the fluid being pumped had a particular direction to it which was determined by the direction of gradient. This kind of architecture overcomes the problem of directionality seen in microscale pumps. Finally, the principle above was adapted for use in nano-biosensors. The above enzyme based pump can be used to drive fluid from point A to point B, thus it can also drive analytes in the system from point A to point B. Therefore the analytes could be concentrated at a particular point increasing the signal at that point.