(358e) Encapsulation of Multicolored Quantum Dots In Polystyrene Beads Using Microfluidic Devices

This study develops a microfluidic platform to fabricate polymer microbeads with encapsulated semiconductor nanocrystals (quantum dots or Qds). Upon excitation, Qds luminesce, and their emission wavelength (color) is dependent on their size. Qds with different colors (sizes) are incorporated in the microbeads to label the beads with a spectral barcode. In this encoding strategy, the luminescent spectrum of the encapsulated Qds consists of a set of peaks which is dependent on which colors have been incorporated in the bead. The intensity of each peak is dependent on the number of Qds encapsulated with that color. Hence a unique spectral code can be developed by varying the composition of the Qds incorporated in the bead. Spectrally encoded beads find applications in bead based platforms for the high-throughput, multiplexed screening of the binding interactions of biomolecules, In this context, the surface of the beads are bound with probe molecules which are potential ligands to a particular target molecule of interest, and the spectral label serves to identify the particular probe molecule on the bead surface.

The quantitative incorporation of the nanocrystals in the microbead is necessary for constructing a reproducible label, and is most effectively undertaken by encapsulating the nanocrystals in the polymer beads during their fabrication step using a suspension polymerization. In this method, liquid monomer, with dispersed Qds of a prescribed composition, is formed into droplets in a continuous phase immiscible with the monomer phase. The polymerization is then initiated on this suspension, and the monomer droplets are converted into particles. To disperse the Qds in the monomer, the monomer phase must be able to solvate the Qds. This is accomplished by using Qds which have a surface chemistry of similar polarity to the monomer phase, and of necessity, of opposite polarity to the immiscible continuous phase of the suspension. The Qds present in the polymerizing droplets are effectively “locked into” the beads since they are insoluble in the continuous phase. As such, the composition charged into the droplets is preserved in the polymerized particle. In addition each particle has the same composition since they are formed from the same monomer phase.

In a suspension polymerization, the formation of the emulsion of the monomer phase is usually undertaken by either the controlled mixing of the monomer into the continuous phase or the spraying of the monomer into a stirred bath of the continuous phase. Both procedures have the common problem that the droplets formed are not monodisperse in size, and hence the polymerized particles have a broad size distribution which can adversely affect the reading of the intensity part of the spectral code. Microfluidic devices offer the ability to synthesize highly monodisperse suspensions in the 10-100um range by reproducibly forming the droplets one at a time, at the tip of orifice situated in a microfluidic channel. The droplets then move single file down the channel where they can be polymerized, and collected. In this presentation, we demonstrate this approach by forming polystyrene beads from nonpolar styrene monomer droplets dispersed with Qds with a hydrophobic coat and moving through a continuous water phase. UV light is used to initiate a free radical polymerization, and in situ confocal laser scanning microscopy is used to record the spectral code and analyze the distribution of quantum dots in the bead.

Aside from their monodispersity, this format for the production of the particles has the added advantages that (i) the spectral codes can be read directly on the device as they pass through the channel by embedding optical fibers in the channel to excite and detect the luminescent spectrum, (ii) batches with different codes can easily be fabricated by adjusting the QD composition in the inlet to the orifice and (iii) probe molecules can be conjugated to the bead surfaces after the polymerization and while the beads are still in the microchannel facilitating their use in bead based biomolecular assays.