INVESTIGATING POLYDOPAMINE SOLUTION CHEMISTRY AND EVALUATING SPIN COATING AS A FABRICATION METHOD TO STUDY POLYDOPAMINE ADHESION
Possessing a very similar structure to L-DOPA (a compound found in the mussel’s protein that helps this marine creature adhere to virtually all kinds of surfaces), dopamine was first brought to attention by Lee et al. as a fundamental building block for the surface modification of bulk materials. Under an alkaline environment, dopamine can undergo oxidation and self- polymerization to produce polydopamine (PDA): supramolecular aggregates that can adhere to and form a thin film on various substrates. Since dopamine was introduced, scientists have conducted much research to study its adhesion mechanism and potential applications. However, the most popular procedure for the fabrication of PDA thin films has two major limitations. First, dopamine solution is prepared in an alkaline environment to promote the spontaneous reaction between dopamine and dissolved oxygen from air. Since oxygen is a weak oxidant, the oxidation process usually results in a low yield of the dopamine oxidative products, which leads to the film’s low thickness and poor stability. Second, each substrate is coated by being immersed in a dopamine solution for a certain amount of time. This practice, although simple, has many drawbacks. First, the volume of the solution must be high enough to accommodate the entire object, yet the film formed is only nanoscale in thickness, causing a significant waste of the materials. Moreover, during the immersion, both PDA formation in solution and surface adhesion take place simultaneously, making it difficult to precisely control the composition of the final coating. In this project, we aimed to overcome these two major limitations by using a stronger oxidant and by utilizing a different thin film coating method. The oxidant chosen was sodium periodate (SP) and the coating method employed was spin coating. To understand the impact of using a strong oxidant on the PDA formation, the solution chemistry was characterized and compared with the trials using oxygen as oxidant. Based on the solution color, the difference in these solutions’ compositions was observed. Fresh dopamine solution is colorless; however, it changes color as the oxidation takes place. In the oxygen trial, the solution started off as light pink, slowly changed to green-brown and then became more opaque as the reactions proceeded. In the SP trial, the solution color went from yellow (dopamine quinone) to red-orange (dopamine chrome) and then became completely black after just a few minutes. From UV-Vis spectra, it was confirmed that the oxidation of dopamine by oxygen happened very slowly and very little of dopamine oxidative products were achieved after the course of 24 h. On the other hand, the use of SP impressively accelerated the oxidation reactions and the yield of dopamine oxidative products, with a significant amount of the dopamine being oxidized as soon as the solution was made. To develop the procedure for spin coating PDA films, three different parameters—oxidant, solution aging time, and solution pH—were varied to achieve the optimal solution chemistry before it came in contact with the substrate. Three different substrates—silicon (Si), polydimethylsiloxane (PDMS), and amine-functionalized PDMS (AMS)—were utilized to investigate the versatility of this procedure. Using a 4 mg/mL dopamine solution with dopamine/SP molar ratio of 1:2 aged for less than 5 mins, we were able to obtain 10 nm thick PDA films on all three substrates. Moreover, these films were achieved within ~ 10 mins of experiment using at most 500 μL of stock solution, while the conventional dip coating method using oxygen from air could take up to 24 h and 8 mL of stock solution to grow films of comparable thickness. Lastly, the stability of these films was tested in 0.1 M HCl solution and in dimethyl sulfoxide (DMSO). Due to the higher extent of oxidation using SP, the PDA films showed much better resilience toward the extreme environments than those obtained via the conventional method. From these results, we concluded that using SP as an alternative for oxygen promotes the oxidation extent of dopamine and enhances not only the rate of PDA film growth but also the stability of the films. Moreover, spin coating was confirmed to be a promising coating method for fabricating PDA thin films. Not only does spin coating save time and materials, it also provides the ability to take “snapshots” of the solution being deposited, offering the ability to precisely control PDA reaction kinetics and surface chemistry of the PDA- coated substrates.