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To realize their full potential as intracellular imaging and sensing reagents, robust and efficient methods for the targeted
cellular delivery of luminescent semiconductor quantum dots (QDs) must be developed. We have previously shown
that QDs decorated with histidine-terminated polyarginine cell-penetrating peptides (CPP) are rapidly and specifically
internalized via endocytosis by several mammalian cell lines with no cytotoxicity. Here we demonstrate the long-term
intracellular stability and fate of these QD-peptide conjugates. We found that the QD-peptide conjugates remain
sequestered within endolysosomal vesicles for up to three days after delivery. However, the CPP appeared to remain
stably associated with the QD within these acidic vesicles over this time period. Hence, we explored a number of
techniques to either actively deliver QDs directly to the cytosol or to facilitate the endosomal release of endocytosed
QDs to the cytosol. Active methods (e.g., electroporation) delivered only modest amounts of QDs to the cytosol that
appeared to form aggregates. Delivery of QDs using polymer-based transfection reagents resulted primarily in the
endosomal sequestration of the QDs, although one commercial polymer tested delivered QDs to the cytosol but only
after several days in culture and with a considerable degree of polymer-induced toxicity. Finally, a modular,
amphiphilic peptide containing functionalities designed for cell penetration and vesicular membrane interaction
demonstrated the ability to deliver QDs in a well-dispersed manner to the cytosol. This peptide mediated rapid QD
uptake followed by a slower efficient endosomal release of the QDs to the cytosol that peaked at 48 hours post-delivery.
Importantly, this QD-peptide conjugate elicited minimal cytotoxicity in two cell lines tested. A more detailed
understanding of the mechanism of the peptide's uptake and endosomal escape attributes will lead to the design of
further QD conjugates for targeted imaging and sensing applications.
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