(236a) Multi-Color Nanoparticles for Single Particle Tracking

With their increased photostability, narrow band-widths, broad excitation spectra, and bright photoluminescence, quantum dots have revolutionized biological imaging and single particle tracking. However, quantum dots exhibit an intermittent loss of fluorescence, known as ?blinking?, which has limited their use in the latter application. Blinking is believed to result from defects in the underlying crystal structure that enhance non-radiative decay and surface trapping of electrons. Several groups have attempted to eliminate blinking through the application of protective surface coatings or manipulation of the quantum wavefunction. However, this phenomenon, while detrimental to tracking, also has a positive function, serving as a tool for discriminating small clusters of particles (1-4) from large aggregates (>4). Here we present composite quantum dot nanoparticles that exhibit changes in emission color that permit small clusters to be identified from aggregates, but that also exhibit significantly reduced blinking.

Composite quantum dot nanoparticles consist of quantum dots with differing emission wavelengths (here green and red) encapsulated in a polymer micelle. When both quantum dots are in a fluorescent state, the composite appears yellow (green and red on). However, when one of the constituent particles ?blinks off? the fluorescence from the remaining particles is emitted only (e.g., red off, green on = green fluorescence). Because blinking is a stochastic process, it is highly unlikely that both particles will be in the dark state at the same time. We have demonstrated continuous fluorescence and particle tracking for over 2 minutes with concomitant changes in nanoparticle color from yellow to red to green. Additionally, because nanoparticles exhibit near continuous fluorescence, it is now possible to distinguish loss of fluorescence as a result of particle motion out of the focal plane from loss of fluorescence as a result of blinking, eliminating a long-standing problem in single particle tracking. Composite quantum dots offer a facile method to discern aggregates from individual particles and small particle clusters, while providing near continuous fluorescence for particle tracking. They could significantly impact biological imaging, fluid dynamics studies, and molecular separations.