Synthesis and characterization of functionalized silica-coated Fe3O4 superparamagnetic nanocrystals for biological applications

Superparamagnetic Fe3O4 nanocrystals were prepared by a chemical coprecipitation method with a thin thickness-adjustable silica layer coated on the surface by hydrolysis of tetraethyl orthosilicate. The silica-coated Fe3O4 nanocrystals were well dispersed and consisted of a 6-7 nm diameter magnetic core and a silica shell about 2 nm thick, according to transmission electron microscopy observations. Fourier transform infrared spectra revealed that amino (-NH2) groups were successfully covalently bonded to the silica-coated Fe3O4 and then carboxyl (-COOH) groups were functionalized to the surface through the reaction of -NH2 and glutaric anhydride. The synthesized nanocrystals have a cubic spinel structure as characterized by x-ray diffraction, electron diffraction and high-resolution transmission electron microscopy. Their magnetic properties were carefully investigated by a SQUID magnetometer. The results showed that the nanocrystals were superparamagnetic and the blocking temperature TB shifted from 131 K down to 92 K after they were coated with a thin nonmagnetic layer, since this layer can effectively suppress the magnetic dipolar interaction between particles; the chemically inert silica layer can limit the outside environment effect on the Fe3O4 cores quite well due to the excellent magnetic reproducibility of the coated nanocrystals after ageing for 7 months at room temperature. In addition, the dependence of their high-field specific magnetization on temperature has a T-2 relationship. These functionalized silica-coated Fe3O4 superparamagnetic nanocrystals have great potential in biomagnetic applications.