Influence of Bases on Surface Functionalities of Polydopamine Nanoparticles: Impact on Radical Trapping Properties

Melanin is a biological nanomaterial with a variety of functions, for example, its ability to quench free radicals. Even though the surface chemistry of melanin is important for these properties, this area has remained relatively unexplored. Here, we compare differences in surface properties of polydopamine (PDA, synthetic mimic of natural melanin) nanoparticles synthesized using three different bases commonly reported in the literature. We use a fluorescence assay and X-ray photoelectron spectroscopy (XPS) to characterize the surface functionalities of nanoparticles synthesized using these three bases. Fluorescence measurements reveal that the PDA synthesized using tris and bicine bases had higher concentrations of amine and carbonyl groups compared to PDA synthesized using ammonium hydroxide. XPS measurements confirmed the presence of carbonyl and amine groups. However, this technique was not able to distinguish the differences in surface chemistry that we observed using fluorescence spectroscopy. Using a 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical trap assay, we demonstrated that the PDA nanoparticles synthesized using tris and bicine were more effective in quenching free radicals compared to PDA synthesized using ammonium hydroxide, correlating with a higher fraction of carbonyl groups on the surface of the PDA nanoparticles.