Melanin is a ubiquitous pigment with a variety of biological functions. Since natural melanin is difficult to extract, various synthetic routes have been developed and these materials exhibit many similar properties such as broad band UV absorption and ability to quench free radicals. However, not much is known about the surface properties of these synthetic melanin. Here, we compare differences in surface properties of melanin synthesized using three different bases and buffers commonly reported in the literature. We use fluorescence assay and x-ray photoelectron spectroscopy (XPS) to quantify surface functionalities of these three polydopamine chemistries. From fluorescence measurements, it was determined that PDA synthesized with tris and bicine buffers had a greater quantity of amine and carbonyl groups compared to PDA synthesized using ammonium hydroxide base. XPS confirmed the presence of amine and carbonyl groups. However; we were not able to confirm the differences in the quantity of these functional groups as a function of PDA chemistry. We characterize the physical properties of the different polydopamine particles using Brunauer-Emmett-Teller and scanning electron microscopy. Using 2,2-Diphenyl-1-picrylhydrazyl (DPPH) antioxidant assay, we demonstrated that the PDA particles synthesized using tris and bicine buffers were more effective free radical quenchers compared to PDA synthesized using ammonium hydroxide due to the greater quantity of carbonyl groups on the surface of the PDA particles.

Determination of surface functional groups on polydopamine particles and its influence on antioxidant properties

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