This thesis explores the manipulation of DNA and colloids in suspension and at surfaces. Specifically: (1) we investigate the evaporative self-assembly of DNA and colloidal particles at surfaces; (2) we explore the like-charge attraction between DNA and colloids in suspension; (3) we examine Janus particles with varied surface chemistry and inner structures as model systems to understand dielectrophoresis (DEP) of anisotropic particles.
In chapter 2, we investigate multiple-ring formation from an evaporating DNA droplet, and demonstrate that an internal stagnation flow is responsible for the formation of precursor ring and repeated stick-slip motion leads to multiple-ring formation. We further examine DNA-colloid binary suspensions as model systems to understand evaporation-induced interfacial hydrodynamics and self-assembled morphology of multi-component systems. The results demonstrate the feasibility of fabricating periodic self-assembled hybrid structures via one-step evaporation of droplets consisting of multiple components.
In chapter 3, we investigate like-charge interaction of ÌøåÂ-DNA and colloids in aqueous suspension and the resultant DNA-colloid complex structures. Direct microscopic observations demonstrate enhanced adsorption of DNA on like-charge colloids with the addition of monovalent or divalent salts. Zeta potential measurements indicate that functionalized colloids remain negatively charged with increasing ionic strength, suggesting that screened electrostatic repulsion can account for enhanced DNA adsorption in high ionic strength media, despite the possible hydrophobic origin for DNA adsorption in low ionic strength media. Additionally, DNA-mediated colloid clustering is observed and the optimal DNA and salt concentration ranges are explored to predict the formation of considerable fractions of DNA-induced colloidal doublets, which could lead to a simple and rapid method to synthesize colloidal clusters of desirable shape and bio-functionality.
In chapter 4, we examine Janus particles as model systems to understand dielectrophoresis of anisotropic particles. Janus particles with gold-coated hemisphere or with assembled COOH-end thiol monolayer on gold-capped hemisphere always experience positive DEP. Janus particles with assembled CH3-end thiol monolayer on gold-capped hemisphere exhibit negative DEP to positive DEP (n-to-p) crossover frequencies at high medium conductivities because interfacial polarization dominates at low frequencies and conductive gold layer dominates at high frequencies.