Spatiotemporal Multicolor Labeling of Individual Cells Using Peptide-Functionalized Quantum Dots and Mixed Delivery Techniques

Multicolor fluorescent labeling of both intra- and extracellular structures is a powerful technique for simultaneous monitoring of multiple complex biochemical processes. This approach remains extremely challenging, however, as it often necessitates the combinatorial use of numerous targeting probes (e.g., antibodies), multistep bioconjugation chemistries, different delivery strategies (e.g., electroporation or transfection reagents), cellular fixation coupled with membrane permeabilization, and complex spectral deconvolution. Here, we present a nanoparticle-based fluorescence labeling strategy for the multicolor labeling of distinct subcellular compartments within live cells without the need for antibody conjugation or cellular fixation/permeabilization. This multipronged approach incorporates an array of delivery strategies, which localize semiconductor quantum dots (Qps) to various subcellular structures. QD uptake is implemented in a spaciotemporal manner by staggering the delivery of QD-peptide composites and exploiting various innate (peptide-mediated endocytosis, peptide-membrane interaction, polymer-based transfection) along with physical (microinjection) cellular delivery modalities to live cells growing in culture over a 4 day period. Imaging of the different intracellular labels is simplified by the unique photophysical characteristics of the QDs in combination with Forster resonance energy transfer sensitization, which allow for multiple spectral windows to be accessed with one excitation wavelength. Using this overall approach, QDs were targeted to both early and late endosomes, the cellular cytosol, and the plasma membrane in live cells, ultimately allowing for simultaneous five-color fluorescent imaging.