Single particle tracking has provided a wealth of information about biophysical processes such as motor protein transport and diffusion in cell membranes. However, motion out of the plane of the microscope or blinking of the fluorescent probe used as a label generally limits observation times to several seconds. Here, we overcome these limitations by using novel non-blinking quantum dots as probes and employing a custom 3D tracking microscope to actively follow motion in three dimensions (3D) in live cells. Signal-to-noise is improved in the cellular milieu through the use of pulsed excitation and time-gated detection.
Understanding and controlling the emission efficiency and recombination dynamics of bi-excitonic states (i.e. QBX and τBX) in semiconductor nanocrystal quantum dots (NQDs) holds the key to many novel technological applications including optical amplification, entangled photon-pair generation and carrier multiplication. Here we present novel single particle spectroscopy approaches capable of measuring these parameter directly. Our approaches are based on second order photon correlation spectroscopy (g(2) (τ)) and can also be applied to small clusters of NQDs to determine the number of NQDs in a cluster together with average value of QBX and τBX. Specifically, first we demonstrate that the ratio of the areas of center and side peaks of the g(2) (τ) function of the spectrally integrated PL of a single NQD provide a precise measure of the ratio of the quantum yield of single and bi-exciton states. Next, we present a time gated photon correlation spectroscopy approach that allows separation of the effects of multi-exciton emission and NQD clustering in g(2) measurements. Finally, we present how the emission of bi-excitons can be separated in g(2) (τ) measurements and extract decay dynamics of bi-excitons without any ambiguity.