Deposition, Oxidation, and Adhesion Mechanisms of Conformal Polydopamine Films
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The oxidation of dopamine in aqueous solutions deposits thin conformal films on a wide variety of material surfaces. These films consist of a material known as polydopamine (PDA), and they exhibit chemical and structural similarities to melanin pigments and adhesive proteins secreted by mussels. The facile synthesis and versatile adhesion of PDA enable the functional modification of numerous material surfaces for applications in biomedical devices, energy storage, and water purification. This thesis details fundamental investigations into the deposition, oxidation, and adhesive mechanisms of PDA films. Depositing PDA films on substrates with different controlled chemistries revealed the importance of solution pH and initial deposition rates on the morphology of the films. The deposition of PDA molecules with increasing pH depends on two competing factors: increased generation rate of PDA molecules versus increased solubility due to catechol ionization. The areal density and coverage of three-dimensional PDA islands is influenced by the surface charge and hydrophobicity of the substrate in aqueous solutions. Spectroscopic and electrochemical characterization of PDA films revealed that redox-inactive metal cations can accelerate the oxidation of PDA. The generation of radicals of 5,6-dihydroxyindole were monitored in situ via ultraviolet-visible spectroscopy as a function of cation concentration and pH. The extent of oxidation was quantified by cyclic voltammetry. The resulting oxidation modifies the metal sorption properties of PDA by generating more carboxylic acid groups and enhancing the iron chelation of the films. The adhesive stability of PDA films was characterized by delamination kinetics of films on SiO2 and indium tin oxide (ITO). PDA film adhesion is a substrate, salt, and oxidation-dependent phenomenon. Long-term adhesive stability of PDA films can be promoted by use of higher dopamine concentrations during synthesis, incorporation of multivalent cations, and avoiding alkaline conditions and strongly oxidizing electrical bias. Elastic moduli of PDA films were quantified by compressive thin film wrinkling, and the measured value of 2.0 ± 0.9 GPA agrees with simulations of PDA based on an oligomeric aggregate model. This thesis helps develop a framework for understanding the synthesis, composition, microstructure, and stability of PDA films.