Synthesis and characterization of synthetic melanin nanoparticles (SMNPs). (A) Molecular structure of dopamine and route to nanoparticles, (B) synthesis of Gd(III)-doped SMNPs, (C) TEM micrograph of SMNPs shows spherical morphology and uniform size, (D) TEM micrograph of Gd(III)-doped SMNPs shows spherical morphology and uniform size, (E) DLS indicates a low dispersity solution and a particle size of 150 nm for SMNPs, (F) DLS data indicate a particle size of 160 nm for Gd(III)-doped SMNPs.