The focus of this presentation is the development of optically barcoded polymer beads for use in high-throughput, multiplexed screening applications such as protein microarrays or flow cytometry. Luminescent semiconductor nanoparticles (or quantum dots (QDs)) with different emission wavelengths (colors), and incorporated in different compositions in polystyrene (PS) beads are used to define an optical barcode. The incorporation is undertaken by copolymerizing the PS beads with hydrophobically capped, core-shell, CdSe/ZnS QDs, using a spraying suspension polymerization procedure. Confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM) images of the beads indicate that the QDs are segregated into inclusions distributed throughout the bead. The segregation of the QDs inside the polymer beads is due to enthalpy and entropy-driven rejection of the QDs from polymerizing loci as the polymerization proceeds. CLSM and fluorometer measurements of the emission spectra of PS beads embedded with three color QDs in varying concentrations are reported which verify that distinguishable optical ratio-metric barcodes derived from the spectral scans of the barcoded beads can be obtained by this technique.

Comparison of the emission profiles of the barcoded beads with that of the neat QDs in styrene indicates Fluorescence Energy Transfer from the lower wavelength QDs to the higher wavelength QDs, providing evidence that the QDs are situated within nanometers of each other in the inclusions. The fluorescence energy transfer limits our ability to obtain significant number of ratio-metric barcodes for use in multiplexing applications. We observed that the energy transfer could be reduced by separation of the segregating QDs from one another during the polymerization process. This separation is achieved by addition of low molecular weight polyethylene (PE) particles along with the QDs in styrene followed by the polymerization reaction. Both the QDs and PE particles are rejected from the polymerizing matrix during polymerization. TEM images of the bead interiors indicate that the QDs are present at the interface between the PS matrix & segregated PE domains, and are well separated from one-another. CLSM emission profiles of these barcoded beads in comparison with that of the beads without PE particles, show reduction in the energy transfer.