Superparamagnetic Resonance of de novo Biomagnetic Nanoparticles

Magnetite nanoparticles of 12.7 nm in diameter were studied by superparamagnetic resonance (SPR) at X-band frequency (9.4 GHz). Two methods were used that yielded particles with very different SPR spectra. The particles were studied with ligands, such as salts and proteins, immobilized onto the particle surface. The characteristic lines and line-shapes of Fe
²⁺
and Fe
³⁺
highlight the sensitivity of SPR to changes in the coordination sphere of the particles. For example, the presence of the g∼4 resonances reveals distortions to the packing of the F
³⁺
ions within the nanoparticles. The SPR spectra of magnetite appear to be acutely sensitive to alteration of the chemical environment. Lineshape parameters relating to the anisotropy and crystal structure (ΔB
eff
, g
eff
, A) detail ligand-dependent changes within the SPR spectra.