Water-soluble fluorescent nanocrystals

A water-soluble semiconductor nanocrystal comprising:

• a semiconductor material having a selected band gap energy and being a nanocrystal;
• and
• an outer layer comprising a compound having at least one linking group for attachment of the compound to a surface of the nanocrystal and at least one hydrophilic group spaced apart from the linking group by a hydrophobic region sufficient to prevent electron charge transfer across the hydrophobic region, wherein the compound is crosslinked or polymerized.

A water-soluble semiconductor nanocrystal comprising:

• a semiconductor material having a selected band gap energy and being a nanocrystal;
• and
• a bilayer, the bilayer comprising an inner layer having affinity for a surface of the nanocrystal, and an outer layer having a hydrophilic group spaced apart from the inner layer by a hydrophobic region adjacent to the inner layer.

A composition, comprising:

• a water-soluble nanocrystal including a semiconductor material having a selected band gap energy and being a nanocrystal and an outer layer comprising at least one water-solubilizing moiety selected from the group consisting of a multidentate compound, a bilayer, a crosslinked compound or polymerized compound, the water-solubilizing moiety having at least one linking group for attachment of the compound to the overcoating layer and at least one hydrophilic group spaced apart from the linking group by a hydrophobic region sufficient to prevent electron charge transfer across the hydrophobic region, the water soluble nanocrystal dispersed or dissolved in an aqueous medium.