A recent in vivo study published in NanoLetters focuses on the impact of ultraviolet radiation on the ability of Quantum Dots (QD) to penetrate skin.

L. Mortensen, et al., "In Vivo Skin Penetration of Quantum Dot Nanoparticles in the Murine Model: The Effect of UVR," NanoLetters, Vol. 8, No. 9, pp. 2779-2787 (August 2008).

The article begins by noting that "[n]anoparticles (NP) are commonly used in sunscreens and other cosmetics, and since consumer use of sunscreen is often applied to sun damaged skin, the effect of UVR on NP skin penetration is a concern due to potential toxicity;" and "[t]he question of whether or not NP can penetrate the healthy stratum corneum skin barrier in vivo remains largely unanswered."

The authors note that there are conflicting results from recent studies in this area, which they attribute to different researchers using different nanoscale materials with different sizes/diameters. The authors point out that the inconsistent results of prior studies "highlight the need for standardization of experimental techniques in ex vivo skin models are to be useful."

Despite the authors’ stated concern with possible dermal penetration of NP contained in cosmetics and sunscreens, they chose to test a nanoscale material which is not used in any cosmetic or sunscreen – Quantum Dots.

The authors explain:

We selected to investigate QD, as they posses ideal characteristics for in vivo experimentation including broad excitability, narrow emission bandwidth, high fluorescence quantum yield, photostability, and ease of surface functionalization. Moreover, QD are of a similar size to TiO2 NP used in sunscreen applications, they intrinsically generate ROS species, and the carboxyl terminated QD have a similar negative oxide surface chemistry to the TiO2 and ZnO raw materials often used in sunscreen applications.

There are no lack of manufacturers selling the very type of nanoscale zinc oxide and titanium oxide used in consumer cosmetics and sunscreens. Quantum dots are very remote cousins to these particles.  Why not test the substances themselves rather than a surrogate?

Regarding the animals used in the experiment, the researchers selected 6-7 week old SKH-1 hairless, albino mice.  The QD mouse application vehicle was a solution of 75% glycerol/25% carboxyl QD Stock Solution (pH=9.0 borate buffer, 8μm QD).  One half of the mice received an acute single UVR dose, which was administered by using UVA Sun 340 lamps (320-400 nm (UVA)) (290-320 nm (UVB)). 

The scientists found increased QD penetration for 8 hr and 24 hr treatment conditions after UVR exposure.  However, "[m]ost strikingly, under no circumstances is there evidence for massive QD penetration, even for UVR exposed mice 24 hr after QD application." "[N]one of the penetration observed was at a very high level."

The scientists conclude that "[t]hese studies demonstrate the importance of skin condition to effect the penetration of QD nanoparticles . . . in the . . . mouse model.  We have shown that QD work their way between corneocytes of the stratum corneum and penetrate deep in the epidermis and dermis of an in vivo model with UVR penetration exacerbation." "The minimal QD penetration observed in our study on barrier intact (non-UVR exposed) skin suggests the preponderance of current literature suggesting TiO2 and ZnO NP used in commercial sunscreens exhibit limited penetration in layers below the lower SC."