As ultrafine particles, quantum dots can be delivered to any location in the body and are thus suited to biomedical applications such as medical imaging and biosensors. It is also possible to disperse quantum dots in blood by coating them with materials such as biocompatible polymers. Quantum dots can also be bound to antibodies or other specific molecules for use on target cells. Current fluorescence-based biosensors use organic dyes with a broad spectral width, but they face restrictions such as few effective colors and short label life. By contrast, quantum dots are superior to conventional organic dyes because of their ability to emit light in a wider spectral range, with high luminance and long fluorescence lifetime. The creation of quantum dots that are capable of infrared absorption and emittance and that feature physiologically safe composition could enable thermal treatment of malignant tumors through means such as drug delivery systems.