A novel encoding system, compositions for use therein and methods for
determining the source, location and/or identity of a particular item or
component of interest is provided. In particular, the present invention
utilizes a collection of one or more sizes of populations of semiconductor
nanocrystals having characteristic spectral emissions, to "track" the
source or location of an item of interest or to identify a particular item
of interest. The semiconductor nanocrystals used in the inventive
compositions can be selected to emit a desired wavelength to produce a
characteristic spectral emission in narrow spectral widths, and with a
symmetric, nearly Gaussian line shape, by changing the composition and
size of the semiconductor nanocrystal. Additionally, the intensity of the
emission at a particular characteristic wavelength can also be varied,
thus enabling the use of binary or higher order encoding schemes.
