Exposure to Zinc Oxide and Titanium Dioxide Nanoparticles in Sunscreen

1. Exposure to Zinc Oxideand Titanium Dioxide Nanoparticles in Sunscreen Heidi Nelson December 9, 2011 CHEM 4101

2. Nanoparticles in Sunscreen As the field of nanotechnology develops, nanoparticles appear more frequently in consumer products. In sunscreen, zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles are used to scatter ultraviolet light. The advantage of these over larger particles is that sunscreen containing nanoparticles is transparent when applied. However, the potential health risks of exposure to nanoparticles are largely unknown.1 ZnO nanoparticles and sunscreen. Consumer Reports, www.consumerreports.org Human subjects after application of sunscreen containing nanoparticles (left) and sunscreen containing larger particles (right). 1

3. Problem When sunscreen containing nanoparticles is used, do the particles penetrate into human skin? How far/how much? Hypothesis Nanoparticles will only penetrate into the upper layers of skin, and not in significant amounts. Cross section of pig skin labeled with TiO2 particle distribution.2

4. Experiment • Apply sunscreen containing ZnO and TiO2 nanoparticles to human subjects • Monitor penetration of ZnO and TiO2 over time • Controls: sunscreen with larger particles of ZnO or TiO, sunscreen with no ZnO or TiO2 • Use an imaging technique to determine the depth of nanoparticle penetration • Use a quantitation technique to determine how much ZnO/TiO2 is absorbed into skin

5. Imaging Techniques References 3-4

6. Quantitative Techniques Inductively coupled plasma-Mass spectrometry (ICP-MS) • Measures atomic Zn and Ti • Multiple peaks due to different isotopes • Low background • LOD: 0.1 to 10 ppb Inductively coupled plasma-Atomic emission spectroscopy (ICP-AES) • Measures atomic Zn and Ti • Multiple peaks due to different energy transitions • Potential high background • LOD < 10 ppb UV-vis absorbance • Detects ZnO and TiO2 particles • Optical properties depend on size/shape of particles • Need to remove intact particles from skin matrix • LOD is generally higher (can’t directly compare to atomic techniques) References 5-6

7. Sample preparation Fluorescence confocal microscopy3 • Virtually no preparation needed for non-invasive in vivo imaging • Put drop of water, cover slip, immersion oil on skin ICP-MS2,4 • Separate dermis and epidermis with dry heat (63 °C for two minutes) • Combine samples with 4:1 HNO3:HF • Microwave dissolution for 35 minutes: 300 W, 200 °C, 220 psi • Dilute with 2% HNO3 • Add Sc or Y internal standard No chromatography or additional purification is necessary.

8. Fluorescence Confocal Microscopy The excitation of ZnO and TiO2 fluorescence by two IR photons (instead of one UV photon) reduces scattering background and increases imaging depth. Nanoparticle fluorescence and skin autofluorescence background are collected separately, using two detectors with different filters. The focal spot is scanned across the sample and the two signals are overlaid to create a contrast image.3 Nikon Microscopy U, www.microscopyu.com

9. ICP-MS Plasma containing Ti and Zn ions The sample is atomized and ionized in an argon plasma. Ions are sorted by a quadrupole mass analyzer. Their concentrations are determined by comparing signal intensity to an internal standard.2,4 Ions sorted by mass and charge detector Ar Sample in aq. HNO3 Agilent 7500a LOD ~10 ppt Linear range 9 orders of magnitude Mass range 2-260 amu http://www.chem.agilent.com/Library/Support/Documents/F05009.pdf

10. Expected Results Above: Fluorescence images of ZnO nanoparticles, shown in red, in human skin at different depths (scale bar 20 μm).3 Right: Levels of Ti from different types of sunscreens measured by ICP-MS in the epidermis (top) and dermis of pig skin.2

11. Conclusions Fluorescence confocal microscopy is a suitable technique for imaging ZnO and TiO2 nanoparticles in skin and determining the depth of their penetration. Skin can be imaged in vivo at different depths, and two-photon excitation reduces background. ICP-MS is a suitable technique for quantifying the levels of Zn and Ti present in skin. After microwave dissolution of skin samples, these elements can be detected at low levels with few interferences. Previous papers with similar experiments showed results consistent with my hypothesis, that nanoparticles penetrate skin to relatively shallow depths and in relatively small amounts.

12. References • Wolf, L. K. Scrutinizing Sunscreens. Chem. Eng. News2011, 89, 44-46. • Sadrieh, N. et al. Lack of Significant Dermal Penetration of Titanium Dioxide from Sunscreen Formulations Containing Nano- and Submicron-Size TiO2 Particles. Toxicol. Sci. 2010, 115, 156-166. • Zvyagin, A. V. et al. Imaging of zinc oxide nanoparticle penetration in human skin in vitro and in vivo. J. Biomed. Opt. 2008, 13, 064031. • Monteiro-Riviere, N. A. et al. Safety Evaluation of Sunscreen Formulations Containing Titanium Dioxide and Zinc Oxide Nanoparticles in UVB Sunburned Skin: An In Vitro and In Vivo Study. Toxicol. Sci. 2011, 123, 264-280. • Contado, C.; Pagnoni, A. TiO2 in Commercial Sunscreen Lotion: Flow Field-Flow Fractionation and ICP-AES Together for Size Analysis. Anal. Chem.2008,80, 7594-7608. • Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed., 2006.