Alternating-color quantum dot nanocomposites for particle tracking.

Because of their extraordinary brightness and photostability, quantum dots (QDs) have tremendous potential for long-term, particle tracking in heterogeneous systems (e.g., living cells, microfluidic flow). However, one of their major limitations is blinking, an intermittent loss of fluorescence, characteristic of individual and small clusters of QDs, that interrupts particle tracking. Recently, several research groups have reported "nonblinking QDs". However, blinking is the primary method used to confirm nanoparticle aggregation status in situ, and single or small clusters of nanoparticles with continuous fluorescence emission are difficult to discern from large aggregates. Here, we describe a new class of quantum dot-based composite nanoparticles that solve these two seemingly irreconcilable problems by exhibiting near-continuous, alternating-color fluorescence, which permits aggregation status discrimination by observable color changes even during motion across the focal plane. These materials will greatly enhance particle tracking in cell biology, biophysics, and fluid mechanics.