Silicon nanocrystals, also called quantum dots, have unique optical properties when in the quantum-confinement regime.::These optical properties make silicon nanocrystals promising materials for a wide variety of applications ranging from optoelectronic devices to fluorophores in biological imaging. A liquid-phase synthetic approach is reported using surfactant molecules to control particle growth, producing highly monodisperse silicon particles. The surface of the nanocrystals, are capped by functional organic molecules that passivate and protect the silicon particles from oxidation, enabling the particles to be used in hydrophobic and hydrophilic applications. The use of hydrophilic silicon quantum dots as optical probes is illustrated by the imaging of Vero cells.