Nanofabricated devices designed to carry drug and contrast agents to breast cancer cells are surface modified with targeting moieties that recognize unique or abundantly expressed molecules on the surface of tumor cells. However, major obstacles such as enzymatic degradation, uptake by professional phagocytes and the reticular endothelial system, and the vascular endothelium could hinder the ability of nanoparticles to reach the tumor site. To overcome these biological barriers, we have developed a multistage delivery system comprised of biocompatible porous silicon particles that encapsulate iron oxide nanoparticles for protection and transport to tumor-associated vasculature for magnetic resonance imaging of the tumor cell.