To date, synthetic melanin has received considerable attention in the development of functional biomaterials inspired by natural systems. While the integration of synthetic catechol-based materials with metals has been widely explored for diverse applications, few studies have focused on the fundamental chemical properties of these materials such as polymerization kinetics, influence the identity of the metal ion on polymerization rates, upper limits of metal ion incorporation into synthetic melanin, magnetic properties, and their relationships to MRI. Herein, we utilized self-oxidation polymerization to prepare a series of metal- loaded synthetic melanin nanoparticles (SMNPs) via a pre-doping strategy that involves mixing dopamine with different metal ion salts prior to dopamine polymerization. The resulting SMNPs show a notably higher level of metal content compared with post-synthetic metal doping strategies. We choose Fe(III)-loaded SMNPs as an example, to investigate the parameters that influence the final particles’ morphology as well as the Fe(III) loading. Importantly, we found that the high and controllable content of iron enables the systematic investigation of magnetic properties via magnetrometry which reveals details of the local anisotropy and coupling interactions that are vital to understanding the magnetic nature these materials and how they can be optimized for MRI contrast. In addition, we developed a metal displacement method to prepare a series of Gd(III)-loaded SMNPs with tunable metal loadings and ultra-high relaxivity (75 mM−1 s −1 and 10.3 mM−1 s −1 at 1.4 T and 7 T, respectively). We performed a systematic investigation to demonstrate the high relaxivities are attributed to a high hydration state of the Gd(III) at the particle surface, fast rate of water exchange and negligible antiferromagnetic coupling between Gd(III) centers throughout the particles.

Synthesis and characterization of metal-doped synthetic melanin nanoparticles

253rd National Meeting (2017)

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