The penetration of QD through skin has been addressed by our research group employing an ex vivo porcine skin model. QD565 and QD655 have a cadmium/selenide (CdSe) core surrounded by a zinc sulfide (ZnS) shell. By TEM, QD565 are spherical with a diameter of 4.6 nm, while QD655 are ellipsoid with a size of . The hydrodynamic diameters for QD565 are 35 nm for the QD coated with polyethylene glycol (PEG), uncharged, 14 nm for the QD coated with carboxylic acid (COOH), negatively charged, and 15 nm for the QD coated with PEG-amine (), positively charged. The hydrodynamic diameters of the coated QD655 were 45 nm (PEG), 18 nm (COOH), and 20 nm (). The penetration of QD 565 and 655 with the three different coatings were investigated in porcine flow-through diffusion cells, followed by skin sectioning and CLSM imaging.12 QD were excited by ultraviolet light or by 488 nm argon laser in CLSM; QD565 can be filtered and imaged at the emission wavelength of 565 nm, while QD655 image at the emission wavelength of 655 nm. It was noted that PEG and COOH coated QD 565 were localized within the epidermis at 8 h, while coated QD 565 were localized within the dermis. QD 655 with PEG and coatings were found within the epidermal layers after 8 h. In comparison, the penetration of QD655COOH coating was relatively slow and present within the epidermis at 24 h.12 Therefore, the surface coatings can greatly influence the depth of skin penetration of the NP.