Spectral bar coding of polystyrene microbeads using multicolored quantum dots

This paper focuses on encoding polystyrene microbeads, 10-100 microm in diameter, with a luminescent spectral bar code composed of mixtures of quantum dots (QDs) emitting at different wavelengths (colors). The QDs are encapsulated in the bead interior during the bead synthesis using a suspension polymerization, and the bar code is constructed by varying both the number of colors included in the bead and, for each color, the number of QDs of that color. Confocal laser scanning microscopy images of the beads demonstrate that the multicolored QDs are pushed together into inclusions within the bead interior. The encoded bead emission spectrum indicates that the peak position of the included colors does not shift relative to the corresponding peaks of the spectra recorded for the nonaggregated QDs at identical loading concentrations. Due to the spatial proximity of the QDs in the inclusions, electronic energy transfer from the lower wavelength emitting QDs to the higher emitting QDs changes the relative intensities of the colors compared to the values in the nonaggregated spectra. We show that this energy transfer does not obscure the spectral uniqueness of the different codes. Ratiometric encoding, in which the bar code is read as relative color intensity, is shown to remove the dependence of the code on the bead size.