Ferrite plating on porous silica microspheres for ultrasonic contrast agents

Porous silica microspheres, 1-3 /spl mu/m in diameter, were coated with Fe/sub 3/O/sub 4/ by "reactor method" ferrite plating. The polycrystalline Fe/sub 3/O/sub 4/ coating was synthesized from an aqueous solution (pH=7.3) of FeCl/sub 2/ at 60-70/spl deg/C, using oxidizing solution of NaNO/sub 2/ mixed with CH/sub 3/COONH/sub 4/ (pH buffer). The magnetite capsulated silica microspheres can be potentially used for contrast agents for clinical ultrasonography, especially for blood flowmetry; the air trapped in the silica pores enhances the ultrasound echogenicity. The contrast agents are controllable by magnetic fields, and their virtual density can be adjusted to that of ambient fluid by changing the thickness of the Fe/sub 3/O/sub 4/ coating.>