Fig. 2.

Cellular internalization of QD conjugates by chemical/biological and physical approaches. (a) Unconjugated QDs typically are uptaken via non-specific endocytosis, resulting in aggregation in the cytosol, shown as punctate fluorescence staining.⁶³ (b) Delivery assisted by transfection agent (liposome, micelle or polymer) or ligand-modification on the QDs is usually more specific and efficient than non-specific endocytosis alone.⁶⁴ (c) The electrical field in electroporation temporarily permeabilizes the cell membrane, allowing the QD conjugates to be delivered directly into the cells to achieve high gene expression. However, aggregations are still observed in the endolysosomes and cytosol for both transfection agent and electroporation assisted deliveries.⁶⁴ (d) Microinjection or needle penetration, allows QDs to be delivered directly into the cytoplasm or even nucleus, and bypass the endosome/lysosome, thus avoiding enzymatic degradation.⁶⁴ QD conjugates delivered through different approaches result in contrasting patterns in cell labeling. Figures reprinted with permission from Wiley-VCH Verlag GmbH & Co.