Electrostatic Confinement, Patterning, and Manipulation of Charged Nanoparticles by Combining Nanostructured Surfaces and Ionic Charge Regulation.

Electrostatic forces are amongst the most versatile when applied to mediate the interactions between nanostructured interfaces. Depending on the experimental conditions, these forces can be either attractive or repulsive, and their directionality can be controlled dynamically. In this dissertation, we employ these forces to confine and manipulate charged nanoparticles using nanostructured interfaces. The various methodologies discussed herein inform and complement each other while opening pathways for diversified applications. Electrostatic confinement of nanoscale species in solution has far-reaching effects in fields as diverse as biophysics, gene therapy, single-particle motion monitoring studies and bottom-up fabrication of nanostructures. We present a methodology to uniaxially confine charged nanoparticles on one-dimensional electrodes without the usage of geometrical barriers. An actively-tunable, engineered model system for electrostatic binding interactions is demonstrated and interaction characteristics are discussed in relation to mimicking the natural biological interaction between charged species. We further investigate the electrostatic interactions between nanoparticles and patterned sinusoidal-void structures. A size-selective nanoparticle confinement and patterning technique is demonstrated. In addition, ionic charge regulation in the electrical double layer, its ramifications and its applications are discussed. In many particle-fractionation applications, complementary geometries are critical for understanding confinement characteristics and so a novel methodology is introduced to detect and visualize relative size variations in pre-characterized nanoparticle ensembles. This capped particle optical-sizing methodology is easily accessible, has high-throughput, and is relatively facile when compared to existing size-characterization techniques. Finally, a nanoparticle-manipulation-based transparent display concept is demonstrated that has been supplemented by our enhanced understanding of the above mentioned confinement methodologies of electrostatically confining charged nanoparticles in solution.PhDMacromolecular Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120879/1/ashwinp_1.pd

Measurement of the Membrane Electric Field and the Swimming Behavior of Chlamydomonas reinhardtii: Experiments and Analytical Modeling

The green alga Chlamydomonas reinhardtii has evolutionarily developed a range of receptors to detect light, chemical, or mechanical stimuli for its survival. It employs its two cilia, identified as cis and trans with respect to its photoreceptor-containing eye, to achieve appropriate behavioral responses. The sensory signals are relayed to the dynein motors in the cilia through complex networks of signal transduction pathways that have yet to be fully characterized. The first part of this work is an experimental study of one such signal transduction pathways, the membrane electric field. In the study, an experimental method is developed to monitor the membrane electric field transients in response to an external stimulus. The method is non-invasive and allows monitoring the membrane electric field of cell population over extended periods of time by using a voltage-sensitive fluorescence probe, di-8-ANNEPS. The method is also insensitive to cell orientations and is suitable for studying the effect of any stimuli that may influence the behavior of cells by changing the membrane electric field. In this work two such types of stimuli, green light and sound, are used. In response to impulses of green light, the membrane electric field was found to change in the same way for both positively and negatively phototactic strains, and all the processing due to green light detection at the eye appeared to take place in the cilia. In response to sound stimuli, amplitude-modulated as 1-second-on-1-second-off or sine waves at 8.0 Hz, no change in the membrane electric field was observed. The second part of this work is devoted to tracking experiments of swimming Chlamydomonas reinhardtii cells. The measured cell trajectories are quantified using a suitable implementation of the cell-motility model developed in the third part of this work. Through quantifying the cell trajectories using the motility model, the activity of IC138 component of cilium\u27s inner I1/f-dynein arm is characterized. This unit has a regulatory role in motility. When it is phosphorylated (due to increased level of cAMP), the probability of it acting like a transient brake or an extra drag on the trans-cilium increases. This in turn causes a low-amplitude extra beat relative to the cis-cilium that maintains a steady beat. The extra low amplitude beat causes the cell to change direction more frequently, which makes the motion less ballistic and more diffusive. This ballistic-diffusive ratio affects the behavior associated with mating and searching for food and light in opposing manners. More frequent activation of the brake, for example, worsens search for food and light but increases chances of mating. In order to quantify this regulatory mechanism, which is a part of the braking signal transduction network, individual tracks of six Chlamydomonas reinhardtii strains were recorded and the data was fit to the above mentioned motility model at the population level. Among the obtained set of statistical parameters from fitting, the persistence time was found to be the most suitable one for characterizing the activity of IC138. In addition to this, a special realization of the cell-motility model, suitable for studying the effect of an external periodic force on motility, is also developed in the third part of this work. This realization provides a quantitative mean to discern between the pure mechanical effect of an external periodic force, such as sound, and its sensory detection on the cell behavior

Nanoparticle Charge and Shape Measurements using Tuneable Resistive Pulse Sensing

Accurate characterisation of micro- and nanoparticles is of key importance in a variety of scientific fields from colloidal chemistry to medicine. Tuneable resistive pulse sensing (TRPS) has been shown to be effective in determining the size and concentration of nanoparticles in solution. Detection is achieved using the Coulter principle, in which each particle passing through a pore in an insulating membrane generates a resistive pulse in the ionic current passing through the pore. The distinctive feature of TRPS relative to other RPS systems is that the membrane material is thermoplastic polyurethane, which can be actuated on macroscopic scales in order to tune the pore geometry. In this thesis we attempt to extend existing TRPS techniques to enable the characterisation of nanoparticle charge and shape. For the prediction of resistive pulses produced in a conical pore we characterise the electrolyte solutions, pore geometry and pore zeta-potential and use known volume calibration particles. The first major investigation used TRPS to quantitatively measure the zeta-potential of carboxylate polystyrene particles in solution. We find that zeta-potential measurements made using pulse full width half maximum data are more reproducible than those from pulse rate data. We show that particle zeta-potentials produced using TRPS are consistent with literature and those measured using dynamic light scattering techniques. The next major task was investigating the relationship between pulse shape and particle shape. TRPS was used to compare PEGylated gold nanorods with spherical carboxylate polystyrene particles. We determine common levels of variation across the metrics of pulse magnitude, duration and pulse asymmetry. The rise and fall gradients of resistive pulses may enable differentiation of spherical and non-spherical particles. Finally, using the metrics and techniques developed during charge and shape investigations, TRPS was applied to Rattus rattus red blood cells, Shewanella marintestina bacteria and bacterially-produced polyhydroxyalkanoate particles. We find that TRPS is capable of producing accurate size distributions of all these particle sets, even though they represent nonspherical or highly disperse particle sets. TRPS produces particle volume measurements that are consistent with either literature values or electron microscopy measurements of the dominant species of these particle sets. We also find some evidence that TRPS is able to differentiate between spherical and non-spherical particles using pulse rise and fall gradients in Shewanella and Rattus rattus red blood cells. We expect TRPS to continue to find application in quantitative measurements across a variety of particles and applications in the future