(566h) FORC Diagrams as a Diagnostic Tool for Evaluating Nanoparticle Coating

The use of magnetic nanoparticles has become important in a wide range of biomedical and polymer nanoparticle applications. Particles are frequently coated to improve thermal stability and functionality but also clustering and therefore retention of superparamagnetic (SP) behavior. The effectiveness of coating is often evaluated using electron microscopy and magnetic hysteresis loops. First-order reversal curves (FORC) allow for a more rigorous evaluation of effective particle size and interaction. Predominantly γ-Fe₂O₃ particles with low amounts of α-Fe₂O₃ were produced by scalable flame technology and the resulting aerosol was in situ coated with SiO₂ by oxidation of hexamethyldisiloxane (HMDSO) vapor.^[1,2] The individual Fe₂O₃ grains were encapsulated by thin SiO₂ films and the coating thickness was controlled by the HMDSO concentration in the reactor. The mean particle size obtained from TEM images is 22.4 nm for uncoated and slightly higher for coated particles, although particle size can vary from about 10 to 80 nm. Particles were also produced by co-oxidizing Fe and Si precursors in the flame reactor. Co-oxidation reduces the Fe₂O₃ grain size to about 8 nm and results in particles segregated in crystalline Fe₂O₃ and amorphous SiO₂ domains. FORC analysis shows that co-oxidized particles with 7? 46 wt% SiO₂ display predominantly SP behavior due to the small Fe₂O₃ grain size. The uncoated Fe₂O₃ and 7 ? 23 wt% SiO₂-coated particles show a broader coercivity spectrum and therefore a larger spectrum of effective particle size; there is also a broader range of interaction fields. The FORC diagrams for samples with higher wt% SiO₂ coating and thus higher coating thickness suggest SP behavior with a narrow grain size spectrum and low degree of particle interaction. FORC diagrams are a powerful tool for assessing the effectiveness of coating treatment in the production of non-interacting SP nanoparticles.

[1] Teleki, A., M. Suter, P. R. Kidambi, O. Ergeneman, F. Krumeich, B. J. Nelson, and S. E. Pratsinis, Hermetically-coated superparamagnetic Fe₂O₃ particles with SiO₂ nanofilms, Chem. Mat. in press, (2009).

[2] Teleki, A., M. C. Heine, F. Krumeich, M. K. Akhtar, and S. E. Pratsinis, In situ coating of flame-made TiO₂ particles with nanothin SiO₂ films, Langmuir 24, 12553 (2008).