Tuning the Absorption Spectrum of Polydopamine via Post-Synthetic Oxidation with Bobbit’s Salt

Polydopamine (PDA) is a promising biomimetic material, but its structural complexity hinders rational control over its light absorption properties. The purpose of this study was to develop a simple post-synthetic method to tune the absorption spectrum of PDA using Bobbit’s salt (4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxoammonium salt) as a mild oxidant. Conventional PDA nanoparticles were treated with Bobbit’s salt either in pure water or in a 1:1 methanol–water mixture to obtain two modified samples. Structural analysis conducted using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and mass spectrometry demonstrated that Bobbit’s salt selectively oxidized catechol units to ortho-benzoquinone moieties, with the C–O/C=O ratio decreasing from 71:29 in the untreated PDA to 51:49 in the water-treated sample, while nitrogen functionalities remained unchanged. Consequently, the sample prepared in pure water showed generally lower absorbance across the visible–near-infrared range, whereas the sample prepared in the methanol–water mixture exhibited enhanced ultraviolet absorption but reduced near-infrared absorption. When coated onto polyvinylidene fluoride membranes, the water-treated PDA produced a brighter and more reddish-yellow appearance. On transparent poly(methyl methacrylate) substrates, the same coating also enhanced ultraviolet blocking and reduced visible transmittance. These findings conclude that Bobbit’s salt is an effective and selective reagent for tailoring the optical properties of PDA, with potential applications in protective coatings and light-modulating materials.