Ferrite-Capsulated Porous Silica Microspheres for Ultrasonic Contrast Agents

Abstract
Porous silica microspheres, ~2.2 µm in average diameter, were encapsulated with magnetite (Fe₃O₄) by ferrite plating in an aqueous solution of FeCl₂ at 65°C. Using the resultant ferrite-capsulated particles as ultrasonographic contrast agents (in which air trapped in the pores scatters ultrasonic waves), we tried to visualize still and flowing waters in holes and a channel perforated in agar blocks by using a 7.5MHz B-mode echograph. Clear image enhancement was successfully obtained only when the ferrite-capsulated particles were evaporated in air before they were dispersed in the waters. This is because, during ferrite plating in the aqueous solution, the pores were permeated with water, which was expelled by the evaporation and, then, substituted by air when the particles were exposed to air again.