EPR and Structural Characterization of Water-Soluble Mn²⁺-Doped Si Nanoparticles

Water-soluble poly(allylamine) Mn²⁺-doped Si (Si_Mn) nanoparticles (NPs) were prepared and show promise for biologically related applications. The nanoparticles show both strong photoluminescence and good magnetic resonance contrast imaging. The morphology and average diameter were obtained through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM); spherical crystalline Si NPs with an average diameter of 4.2 ± 0.7 nm were observed. The doping maximum obtained through this process was an average concentration of 0.4 ± 0.3% Mn per mole of Si. The water-soluble Si_Mn NPs showed a strong photoluminescence with a quantum yield up to 13%. The Si_Mn NPs had significant T₁ contrast with an r₁ relaxivity of 11.1 ± 1.5 mM^-1 s^-1 and r₂ relaxivity of 32.7 ± 4.7 mM^-1 s^-1 where the concentration is in mM of Mn²⁺. Dextran-coated poly(allylamine) Si_Mn NPs produced NPs with T₁ and T₂ contrast with a r₁ relaxivity of 27.1 ± 2.8 mM^-1 s^-1 and r₂ relaxivity of 1078.5 ± 1.9 mM^-1 s^-1. X-band electron paramagnetic resonance spectra are fit with a two-site model demonstrating that there are two types of Mn²⁺ in these NP's. The fits yield hyperfine splittings (A) of 265 and 238 MHz with significant zero field splitting (D and E terms). This is consistent with Mn in sites of symmetry lower than tetrahedral due to the small size of the NP's.