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Adhesion and formation of coatings on a surface under water, such as physiological fluids and seawater, is limited by high salt concentrations, pH, and hydration. Inspired by mussels, polydopamaine (PDA) films overcome these limitations. Mussels are marine organisms that adhere to any kind of surface using byssus. The byssus is made of mussel foot proteins (Mfp) that are rich in catechol and amine contents, the key combination for the adhesion. PDA films, formed by oxidative polymerization of dopamine (DA), are a great tool for surface modification. Having both catechol and amine functional groups in a monomer, DA is a great building block for the PDA films that can coat any kind of surfaces. PDA films also have a wide variety of applications as they can function as versatile “primers” for further surface modification and attachment. The mechanism of PDA polymerization and film formation is not yet fully understood. However, it is widely accepted that it is initiated by the oxidation of DA monomer to yellow dopaminequinone (DQ) which is followed by cyclization and oxidation into orange dopaminechrome (DC) and 5,6-dihydroxyindole (DHI). Typically, PDA films are prepared by the dip-coating method where a substrate of interest is submerged in a basic solution containing DA for an extended period of time, utilizing oxygen in air as the oxidant. Another method is by spraying DA solutions on the substrates of interest. In this research, the adsorptive spin coating was evaluated, and the optimal condition was explored to prepare PDA films with sufficient thickness on native (SiO2) and polymer-coated silicon wafers. Polydimethylsiloxane (PDMS) and amine-containing polydimethylsiloxane (PADMS) were chosen to provide a wide range of interactions between a substrate and PDA. The spin-coating method allows coating only on a surface of interest without submerging the entire object, requiring a small amount of materials, and is a great model to study surface adhesion of PDA films. In order to find the optimal condition of spin coating PDA films, reaction parameters including the type and amount of oxidant, pH values of DA solutions, DA solution aging time, DA concentration, adsorption time, and surface chemistry were varied. To better understand the effect of oxidant and pH on (P)DA chemistry in solutions, DA solutions containing atmospheric oxygen (O2) at pH 8.5 and sodium periodate (SP) at pH 3.4, 4.9, and 5.9 were analyzed. Color changes of the solutions, precipitate formations, UV-Vis spectrophotometry, dynamic light scattering (DLS), and spin capturing indicated accelerated reaction kinetics when SP was used as the oxidant at a higher pH value. This follows the Le Chatelier’s Principle. As protons are produced during the DA oxidation, the product formation is favored at higher pH values. To correlate (P)DA chemistry in solutions with the surface adhered PDA, thicknesses of PDA films obtained from DA solutions with different molar ratios of SP to DA, at different pH values, in different salt (NaCl and NaNO3) solutions, and with different acetate buffer concentrations were compared. The PDA films obtained using the dip-coating method showed insignificant dependence on the ion type but indicated the importance of keeping the molar ratio between SP and DA at 2:1 and the pH of the solution higher (5.9) to achieve thicker PDA films. Spin coating DA solutions at pH 5.9 with a higher buffer concentration also produced thicker PDA films. While adsorption time was not an independent parameter due to solvent evaporation, aging time was the essential parameter of PDA film formation. The highest DA concentration studied of 4 mg/mL gave the thickest PDA films at all aging times using the spin-coating method. The PDA film thickness went through a maximum value as a function of aging time under all other experimental conditions examined. While PDA thickness is independent of substrate type, the films have speck to continuous morphologies on SiO2, and PADMS while “island” morphologies on PDMS. The stability of the PDA films obtained from coating different DA solutions was tested in 0.1 M HCl solution and dimethyl sulfoxide (DMSO). The decrease in thickness showed that the PDA films prepared using SP and the spin-coating method were more stable than those obtained using O2 and the dip-coating method, respectively. The results showed that the adhesiveness and size of the PDA particles go through a maximum value as functions of aging time in solution. The dip-coating method allows equal adhesion time and aging time, as the substrate of interest is introduced to the solution as soon as DA starts to oxidize and aggregate in solution. This leads to less adhesive and unstable particles forming the base films on the substrates. The larger and more adhesive particles that forms at later aging times would adhere on top, affecting only the thickness not stability. The spin-coating method allows us to capture a moment of the reactions in solution to introduce PDA particles to a substrate. It not only provides a great model to control and study reaction kinetics and surface chemistry of PDA, but also gives more stable base layer of PDA films by capturing the moment when PDA particles have the most optimal adhesiveness and size.



polydopamine, spin coating, thin film, surface modification, surface adhesion, materials science, surface chemistry