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Due to their interesting properties, research on colloidal nanocrystals has moved in the last few years from fundamental research to first applications in materials science and life sciences. In this review some recent biological applications of colloidal nanocrystals are discussed, without going into biological or chemical details. First, the properties of colloidal nanocrystals and how they can be synthesized are described. Second, the conjugation of nanocrystals with biological molecules is discussed. And third, three different biological applications are introduced: (i) the arrangement of nanocrystal–oligonucleotide conjugates using molecular scaffolds such as single-stranded DNA, (ii) the use of nanocrystal–protein conjugates as fluorescent probes for cellular imaging, and (iii) a motility assay based on the uptake of nanocrystals by living cells.

Colloidal nanocrystals are building blocks of the “nanoworld”. Their electronic properties enable the building of single-electron transistors, their optical properties can be used to generate fluorescence labels with many different colors. Based on the principles of molecular recognition and self assembly biological molecules can be used to arrange nanoscale building blocks. Two applications will be discussed. Colloidal gold nanocrystals were conjugated with a controlled number of DNA molecules per nanocrystal. By using complementary sequences of DNA molecules that were attached to different nanocrystals, small groupings of gold nanocrystals could be formed. Biomolecule conjugated colloidal semiconductor nano-crystals also have been used to fluorescence label structural compartments of cells. These nanocrystals were found to be actively incorporated by living cells. It will be described how cells “eat” nanocrystals and an assay for cell mobility based on this fact will be introduced.