There has been a recent interest in the use of biocompatible nanocrystals as in situ fluorescent labels. These microcrystalline particles have interesting fluorescent properties that vary with their composition (CdSe, InP, InAs) and size [70,71]. Their emission spectra are narrow (25-35 nm at half-height) and symmetrical with peak maxima shifting from blue to infrared (460-1800 nm) as the sizes of the particles increase from 2 to 6 nm in diameter. Their emission is long lived (>100 ns), which allows for time gating to reduce autofluorescent backgrounds. While these nanocrystals can be efficiently excited at any wavelength below their emission peak, all nanocrystals share common absorption bands in the 350-450 nm range. Therefore, an ensemble of different nanocrystals can be simultaneously excited with light of only one wavelength, to yield emission peaks that can be tuned in 1-2 nm increments across the entire 460-1800 nm range. This allows considerable choice in simultaneous multicolor labeling  to maximize non-overlap. There has been considerable progress on functionalizing the surfaces of nanocrystals for water solubility, biological compatibility, and enablement of bioconjugation chemistries. These derivatized nanocrystals have already been used for receptor internalization and microstructure studies in cells . The ability to tune the emission of nanocrystals to red wavelengths and their long emission lifetimes will allow them to be applied to difficult FRET configurations where inner filter, optical quenching, and autofluorescence are problematic in HTS.
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