Nanoparticles promise to illuminate tiny tumors and precisely deliver drugs, but many worry about the safety of nano-scale materials. Now scientists have created minuscule flakes of silicon that glow brightly, slowly release cancer drugs, then break down into harmless by-products. They are the first luminescent nanoparticles to be purposely designed to minimize toxic side effects and this is the first study to image tumors using biodegradable silicon nanoparticles, the scientists say.