The development of new imaging probes to noninvasively detect, diagnose and guide therapy for a large variety of diseases is needed. Quantum dots are highly fluorescent and photostable nanoparticles that are finding increasing applications as imaging probes. We have conjugated quantum dots to annexin A5, in order to develop a sensor for detecting apoptotic cells. The sensor specifically recognizes phosphatidylserine on the extracellular leaflet of the plasma membrane through the annexin A5. This quantum dot-based sensor allows increased sensitivity of detection and continuous monitoring of apoptotic cells over time periods that are longer than previously possible.