| By varying the chemical composition in a semiconductor on a scale of nanometers, it is possible to confine some of the electron states to specific regions and create a variety of new quantum effects. The best known examples of this are epitaxially-grown quantum-well superlattices, in which the charge carriers are confined in one dimension to layers often only a few nanometers thick. Many useful devices are based on these materials, including semiconductor lasers, and optical receivers and modulators. Successes such as these as well as interest in fundamental phenomena have stimulated the development of materials in which the electrons are confined in all three dimensions. Such materials are known as quantum dots .
Quantum dot nanoparticles are molecular-sized semiconductor material that light up like LEDs and enable the detection and barcoding of biological materials from DNA to proteins. This powerful tool enables dramatic advances in genetic analysis, flow cytometry, high-throughput screening, fluorescence microscopy, drug discovery, and diagnostics.
The remarkable size-dependent optical properties of semiconductor nanocrystals, or quantum dots were discovered in the early 1980s by several research groups. Liquid phase methods for synthesizing supsensions of nanocrystals in solvents such as water and acetonitrile were developed. In the subsequent decade and a half, the synthesis of these particles has been refined to the point where a number of metal-semiconductor salts can be grown in suspensions. By tailoring the size and composition of these nanocrystals, it is possible to engineer fluorescent probes with new and specific properties, opening up all types of industrial and biological applications. | 
