Probing site-exclusive binding of aqueous QDs and their organelle-dependent dynamics in live cells by single molecule spectroscopy

Understanding the biophysical and chemical interactions of nanoprobes and their fate upon entering live cells is critical for developing fundamental insights related to intracellular diagnostics, drug delivery and targeting. In this article we report herein a single molecule analysis procedure to quantitate site-specific exclusive membrane binding of N-acetyl-l-cysteine (NAC)-capped cadmium telluride (CdTe) quantum dots (QDs) in A-427 lung carcinoma cells (k_eq = 0.075 ± 0.011 nM^-1), its relative intracellular distribution and dynamics using fluorescence correlation spectroscopy (FCS) combined with scanning confocal fluorescence lifetime imaging (FLIM). In particular, we demonstrate that the binding efficacy of QDs to the cell membrane is directly related to their size and the targeting of QDs to specific membrane sites is exclusive. We also show that QDs are efficiently internalized by endocytosis and enclosed within the endosome and organelle-dependent diffusion dynamics can be monitored in live cells.