Mapping polydopamine nanoparticle maturation over 1-240 hours: Linking morphology, surface accessibility, and redox response

Polydopamine nanoparticles (PDA NPs) are widely used in interface-mediated technologies, yet how polymerization time governs the surface accessibility of redox-active phenolic motifs over extended synthesis windows remains insufficiently quantified. Here, bare PDA NPs were prepared by oxidative dopamine polymerization and collected from 1 to 240 h to build a time-resolved maturation map linking morphology, chemical fingerprints, and functional redox response. Scanning electron microscopy shows that particles become uniform and quasi-spherical within 24 h and remain morphologically stable up to 240 h, with only minor densification at prolonged times. UV–Vis, FTIR and Raman analyses consistently indicate progressive oxidation/crosslinking, the development of eumelanin-like broadband absorption, and spectral similarity of the 240 h sample toward Sepia melanin. Surface-normalized phenolic density (TPC/SBET) exhibits a pronounced maximum at intermediate polymerization times (48–96 h), followed by a sharp decline at 240 h, paralleled by DPPH inhibition. Overall, time governs the balance between chemical maturation and interfacial redox accessibility in PDA NPs.