Widespread applications of ZnO nanoparticles (NP) in sun-blocking cosmetic products have raised safety concerns related to their potential transdermal penetration and resultant cytotoxicity. Nonlinear optical microscopy provides means for high-contrast imaging of ZnO NPs lending in vitro and in vivo assessment of the nanoparticle uptake in skin, provided their nonlinear optical properties are characterized. We report on this characterization using ZnO NP commercial product, Zinclear, mean-sized 21 nm. Two-photon action cross-section of this bandgap material (E_bg = 3.37 eV, λ_bg = 370 nm) measured by two techniques yielded consistent results of ${\eta }_{ZnO}{\sigma }_{ZnO}^{\left(2ph\right)}$ = 6.2 ± 0.8 μGM at 795 nm, and 32 ± 6 μGM at 770 nm per unit ZnO crystal cell, with the quantum efficiency of ${\eta }_{ZnO}$ = (0.9 ± 0.2) %. In order to demonstrate the quantitative imaging, nonlinear optical microscopy images of the excised human skin topically treated with Zinclear were acquired and processed using ${\sigma }_{ZnO}^{\left(2ph\right)}$ and ${\eta }_{ZnO}$values yielding nanoparticle concentration map in skin. Accumulations of Zinclear ZnO nanoparticles were detected only on the skin surface and in skin folds reaching concentrations of 800 NPs per μm³.

© 2011 OSA

1. Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, “Room-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin films,” Appl. Phys. Lett.72(25), 3270–3272 (1998). [CrossRef]
2. S. E. Cross, B. Innes, M. S. Roberts, T. Tsuzuki, T. A. Robertson, and P. McCormick, “Human skin penetration of sunscreen nanoparticles: in-vitro assessment of a novel micronized zinc oxide formulation,” Skin Pharmacol. Physiol.20(3), 148–154 (2007). [CrossRef] [PubMed]
3. F. Pflücker, V. Wendel, H. Hohenberg, E. Gärtner, T. Will, S. Pfeiffer, R. Wepf, and H. Gers-Barlag, “The human stratum corneum layer: an effective barrier against dermal uptake of different forms of topically applied micronised titanium dioxide,” Skin Pharmacol. Appl. Skin Physiol.14(Suppl 1), 92–97 (2001). [CrossRef] [PubMed]
4. Zs. Kertesz, Z. Szikszai, and A. Z. Kiss, “Quality of skin as a barrier to ultra-fine particles. Contribution of the IBA Group to the Nanoderm EU-5 Project in 2003-2004,” in ATOMKI Annual Report 2004 (Institute of Nuclear Research of the Hungarian Academy of Sciences, 2005), p. 70.
5. T. Xia, M. Kovochich, M. Liong, L. Mädler, B. Gilbert, H. B. Shi, J. I. Yeh, J. I. Zink, and A. E. Nel, “Comparison of the mechanism of toxicity of zinc oxide and cerium oxide nanoparticles based on dissolution and oxidative stress properties,” ACS Nano2(10), 2121–2134 (2008). [CrossRef] [PubMed]
6. Y. B. Zhang, W. Chen, S. P. Wang, Y. F. Liu, and C. Pope, “Phototoxicity of zinc oxide nanoparticle conjugates in human ovarian cancer NIH: OVCAR-3 cells,” J. Biomed. Nanotechnol.4(4), 432–438 (2008). [CrossRef]
7. C. G. J. Hayden, S. E. Cross, C. Anderson, N. A. Saunders, and M. S. Roberts, “Sunscreen penetration of human skin and related keratinocyte toxicity after topical application,” Skin Pharmacol. Physiol.18(4), 170–174 (2005). [CrossRef] [PubMed]
8. B. C. Heng, X. X. Zhao, S. J. Xiong, K. W. Ng, F. Y.-C. Boey, and J. S.-C. Loo, “Toxicity of zinc oxide (ZnO) nanoparticles on human bronchial epithelial cells (BEAS-2B) is accentuated by oxidative stress,” Food Chem. Toxicol.48(6), 1762–1766 (2010). [CrossRef] [PubMed]
9. C. Hanley, A. Thurber, C. Hanna, A. Punnoose, J. H. Zhang, and D. G. Wingett, “The Influences of Cell Type and ZnO Nanoparticle Size on Immune Cell Cytotoxicity and Cytokine Induction,” Nanoscale Res. Lett.4(12), 1409–1420 (2009). [CrossRef] [PubMed]
10. A. V. Zvyagin, X. Zhao, A. Gierden, W. H. Sanchez, J. A. Ross, and M. S. Roberts, “Imaging of zinc oxide nanoparticle penetration in human skin in vitro and in vivo,” J. Biomed. Opt.13(6), 064031 (2008). [CrossRef] [PubMed]
11. M. J. Osmond and M. J. McCall, “Zinc oxide nanoparticles in modern sunscreens: an analysis of potential exposure and hazard,” Nanotoxicology4(1), 15–41 (2010). [CrossRef] [PubMed]
12. M. S. Roberts, M. J. Roberts, T. A. Robertson, W. Sanchez, C. Thörling, Y. Zou, X. Zhao, W. Becker, and A. V. Zvyagin, “In vitro and in vivo imaging of xenobiotic transport in human skin and in the rat liver,” J Biophotonics1(6), 478–493 (2008). [CrossRef] [PubMed]
13. A. P. Popov, A. V. Zvyagin, J. Lademann, M. S. Roberts, W. Sanchez, A. V. Priezzhev, and R. Myllylä, “Designing inorganic light-protective skin nanotechnology products,” J Biomed Nanotechnol6(5), 432–451 (2010). [CrossRef] [PubMed]
14. L. L. Lin, J. E. Grice, M. K. Butler, A. V. Zvyagin, W. Becker, T. A. Robertson, H. P. Soyer, M. S. Roberts, and T. W. Prow, “Time-correlated single photon counting for simultaneous monitoring of zinc oxide nanoparticles and NAD(P)H in intact and barrier-disrupted volunteer skin,” Pharm. Res.28(11), 2920–2930 (2011). [CrossRef] [PubMed]
15. B. Gulson, M. McCall, M. Korsch, L. Gomez, P. Casey, Y. Oytam, A. Taylor, M. McCulloch, J. Trotter, L. Kinsley, and G. Greenoak, “Small amounts of zinc from zinc oxide particles in sunscreens applied outdoors are absorbed through human skin,” Toxicol. Sci.118(1), 140–149 (2010). [CrossRef] [PubMed]
16. B. Nanda and R. S. R. Murthy, “Preparation and characterization of chitosan lactate nanoparticles for the nasal delivery of enalaprilat,” J. Biomed. Nanotech.3(1), 45–51 (2007). [CrossRef]
17. A. P. Popov, J. Lademann, A. V. Priezzhev, and R. Myllylä, “Effect of size of TiO2 nanoparticles embedded into stratum corneum on ultraviolet-A and ultraviolet-B sun-blocking properties of the skin,” J. Biomed. Opt.10(6), 064037 (2005). [CrossRef] [PubMed]
18. J. Lademann, H. J. Weigmann, C. Rickmeyer, H. Barthelmes, H. Schaefer, G. Mueller, and W. Sterry, “Penetration of titanium dioxide microparticles in a sunscreen formulation into the horny layer and the follicular orifice,” Skin Pharmacol. Appl. Skin Physiol.12(5), 247–256 (1999). [CrossRef] [PubMed]
19. B. R. Masters and P. T. C. So, “Confocal microscopy and multi-photon excitation microscopy of human skin in vivo,” Opt. Express8(1), 2–10 (2001). [CrossRef] [PubMed]
20. R. M. Williams, A. Flesken-Nikitin, A. Y. Nikitin, and W. R. Zipfel, “Multiphoton microscopy of intrinsic tissue emissions for cancer research,” FASEB J.21, A601 (2007).
21. W. R. Zipfel, R. M. Williams, and W. W. Webb, “Nonlinear magic: multiphoton microscopy in the biosciences,” Nat. Biotechnol.21(11), 1369–1377 (2003). [CrossRef] [PubMed]
22. B. R. Masters and P. T. C. So, “Multi-photon excitation microscopy and confocal microscopy imaging of in vivo human skin: A comparison,” Microsc. Microanal.5(4), 282–289 (1999). [PubMed]
23. J. A. Palero, H. S. de Bruijn, A. van der Ploeg van den Heuvel, H. J. Sterenborg, and H. C. Gerritsen, “Spectrally resolved multiphoton imaging of in vivo and excised mouse skin tissues,” Biophys. J.93(3), 992–1007 (2007). [CrossRef] [PubMed]
24. D. C. Dai, S. J. Xu, S. L. Shi, M. H. Xie, and C. M. Che, “Efficient multiphoton-absorption-induced luminescence in single-crystalline ZnO at room temperature,” Opt. Lett.30(24), 3377–3379 (2005). [CrossRef] [PubMed]
25. Z.-W. Dong, C.-F. Zhang, G.-J. You, X.-Q. Qiu, K.-J. Liu, Y.-L. Yan, and S.-X. Qian, “Multi-photon excitation UV emission by femtosecond pulses and nonlinearity in ZnO single crystal,” J. Phys. Condens. Matter19(21), 216202 (2007). [CrossRef]
26. X. J. Zhang, W. Ji, and S. H. Tang, “Determination of optical nonlinearities and carrier lifetime in ZnO,” J. Opt. Soc. Am. B14(8), 1951–1955 (1997). [CrossRef]
27. S. K. Das, M. Biswas, D. Byrne, M. Bock, E. McGlynn, M. Breusing, and R. Grunwald, “Multiphoton-absorption induced ultraviolet luminescence of ZnO nanorods using low-energy femtosecond pulses,” J. Appl. Phys.108(4), 043107–043112 (2010). [CrossRef]
28. C. Y. Liu, B. P. Zhang, N. T. Binh, and Y. Segawa, “Second harmonic generation in ZnO thin films fabricated by metalorganic chemical vapor deposition,” Opt. Commun.237(1-3), 65–70 (2004). [CrossRef]
29. L. Guo, S. H. Yang, C. L. Yang, P. Yu, J. N. Wang, W. K. Ge, and G. K. L. Wong, “Highly monodisperse polymer-capped ZnO nanoparticles: Preparation and optical properties,” Appl. Phys. Lett.76(20), 2901–2903 (2000). [CrossRef]
30. L. Irimpan, V. P. N. Nampoori, P. Radhakrishnan, B. Krishnan, and A. Deepthy, “Size-dependent enhancement of nonlinear optical properties in nanocolloids of ZnO,” J. Appl. Phys.103(3), 033105 (2008). [CrossRef]
31. L. Irimpan, A. Deepthy, B. Krishnan, L. M. Kukreja, V. P. N. Nampoori, and P. Radhakrishnan, “Effect of self assembly on the nonlinear optical characteristics of ZnO thin films,” Opt. Commun.281(10), 2938–2943 (2008). [CrossRef]
32. M. H. Majles Ara, Z. Dehghani, and S. Saievar Iranizad, “Synthesis, characterization and single-beam Z-scan measurement of the third-order optical nonlinearities of ZnO nano-particles,” Int. J. Mod. Phys. B22(18 & 19), 3165–3171 (2008). [CrossRef]
33. M. G. Vivas, T. Shih, T. Voss, E. Mazur, and C. R. Mendonca, “Nonlinear spectra of ZnO: reverse saturable, two- and three-photon absorption,” Opt. Express18(9), 9628–9633 (2010). [CrossRef] [PubMed]
34. J. H. Lin, Y. J. Chen, H. Y. Lin, and W. F. Hsieh, “Two-photon resonance assisted huge nonlinear refraction and absorption in ZnO thin films,” J. Appl. Phys.97(3), 033526 (2005). [CrossRef]
35. T. Tsuzuki and P. G. McCormick, “ZnO nanoparticles synthesised by mechanochemical processing,” Scr. Mater.44(8-9), 1731–1734 (2001). [CrossRef]
36. M. Kauert, P. C. Stoller, M. Frenz, and J. Rička, “Absolute measurement of molecular two-photon absorption cross-sections using a fluorescence saturation technique,” Opt. Express14(18), 8434–8447 (2006). [CrossRef] [PubMed]
37. M. Schubnell, I. Kamber, and P. Beaud, “Photochemistry at high temperatures - Potential of ZnO as a high temperature photocatalyst,” Appl. Phys., A Mater. Sci. Process.64(1), 109–113 (1997). [CrossRef]
38. T. M. Stachelek, T. A. Pazoha, W. M. McClain, and R. P. Drucker, ““Detection and assignment of the “phantom” photochemical singlet of trans-stilbene by two-photon excitation,” J. Chem. Phys.66(10), 4540–4543 (1977). [CrossRef]
39. M. A. Albota, C. Xu, and W. W. Webb, “Two-photon fluorescence excitation cross sections of biomolecular probes from 690 to 960 nm,” Appl. Opt.37(31), 7352–7356 (1998). [CrossRef] [PubMed]
40. W. R. Zipfel, R. M. Williams, R. Christie, A. Y. Nikitin, B. T. Hyman, and W. W. Webb, “Live tissue intrinsic emission microscopy using multiphoton-excited native fluorescence and second harmonic generation,” Proc. Natl. Acad. Sci. U.S.A.100(12), 7075–7080 (2003). [CrossRef] [PubMed]

ACS Nano

• T. Xia, M. Kovochich, M. Liong, L. Mädler, B. Gilbert, H. B. Shi, J. I. Yeh, J. I. Zink, and A. E. Nel, “Comparison of the mechanism of toxicity of zinc oxide and cerium oxide nanoparticles based on dissolution and oxidative stress properties,” ACS Nano2(10), 2121–2134 (2008). [CrossRef] [PubMed]

Appl. Opt.

• M. A. Albota, C. Xu, and W. W. Webb, “Two-photon fluorescence excitation cross sections of biomolecular probes from 690 to 960 nm,” Appl. Opt.37(31), 7352–7356 (1998). [CrossRef] [PubMed]

Appl. Phys. Lett.

• Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, and Y. Segawa, “Room-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin films,” Appl. Phys. Lett.72(25), 3270–3272 (1998). [CrossRef]
• L. Guo, S. H. Yang, C. L. Yang, P. Yu, J. N. Wang, W. K. Ge, and G. K. L. Wong, “Highly monodisperse polymer-capped ZnO nanoparticles: Preparation and optical properties,” Appl. Phys. Lett.76(20), 2901–2903 (2000). [CrossRef]

Appl. Phys., A Mater. Sci. Process.

• M. Schubnell, I. Kamber, and P. Beaud, “Photochemistry at high temperatures - Potential of ZnO as a high temperature photocatalyst,” Appl. Phys., A Mater. Sci. Process.64(1), 109–113 (1997). [CrossRef]

Biophys. J.

• J. A. Palero, H. S. de Bruijn, A. van der Ploeg van den Heuvel, H. J. Sterenborg, and H. C. Gerritsen, “Spectrally resolved multiphoton imaging of in vivo and excised mouse skin tissues,” Biophys. J.93(3), 992–1007 (2007). [CrossRef] [PubMed]

FASEB J.

• R. M. Williams, A. Flesken-Nikitin, A. Y. Nikitin, and W. R. Zipfel, “Multiphoton microscopy of intrinsic tissue emissions for cancer research,” FASEB J.21, A601 (2007).

Food Chem. Toxicol.

• B. C. Heng, X. X. Zhao, S. J. Xiong, K. W. Ng, F. Y.-C. Boey, and J. S.-C. Loo, “Toxicity of zinc oxide (ZnO) nanoparticles on human bronchial epithelial cells (BEAS-2B) is accentuated by oxidative stress,” Food Chem. Toxicol.48(6), 1762–1766 (2010). [CrossRef] [PubMed]

Int. J. Mod. Phys. B

• M. H. Majles Ara, Z. Dehghani, and S. Saievar Iranizad, “Synthesis, characterization and single-beam Z-scan measurement of the third-order optical nonlinearities of ZnO nano-particles,” Int. J. Mod. Phys. B22(18 & 19), 3165–3171 (2008). [CrossRef]

J Biomed Nanotechnol

• A. P. Popov, A. V. Zvyagin, J. Lademann, M. S. Roberts, W. Sanchez, A. V. Priezzhev, and R. Myllylä, “Designing inorganic light-protective skin nanotechnology products,” J Biomed Nanotechnol6(5), 432–451 (2010). [CrossRef] [PubMed]

J Biophotonics

• M. S. Roberts, M. J. Roberts, T. A. Robertson, W. Sanchez, C. Thörling, Y. Zou, X. Zhao, W. Becker, and A. V. Zvyagin, “In vitro and in vivo imaging of xenobiotic transport in human skin and in the rat liver,” J Biophotonics1(6), 478–493 (2008). [CrossRef] [PubMed]

J. Appl. Phys.

• L. Irimpan, V. P. N. Nampoori, P. Radhakrishnan, B. Krishnan, and A. Deepthy, “Size-dependent enhancement of nonlinear optical properties in nanocolloids of ZnO,” J. Appl. Phys.103(3), 033105 (2008). [CrossRef]
• J. H. Lin, Y. J. Chen, H. Y. Lin, and W. F. Hsieh, “Two-photon resonance assisted huge nonlinear refraction and absorption in ZnO thin films,” J. Appl. Phys.97(3), 033526 (2005). [CrossRef]
• S. K. Das, M. Biswas, D. Byrne, M. Bock, E. McGlynn, M. Breusing, and R. Grunwald, “Multiphoton-absorption induced ultraviolet luminescence of ZnO nanorods using low-energy femtosecond pulses,” J. Appl. Phys.108(4), 043107–043112 (2010). [CrossRef]

J. Biomed. Nanotech.

• B. Nanda and R. S. R. Murthy, “Preparation and characterization of chitosan lactate nanoparticles for the nasal delivery of enalaprilat,” J. Biomed. Nanotech.3(1), 45–51 (2007). [CrossRef]

J. Biomed. Nanotechnol.

• Y. B. Zhang, W. Chen, S. P. Wang, Y. F. Liu, and C. Pope, “Phototoxicity of zinc oxide nanoparticle conjugates in human ovarian cancer NIH: OVCAR-3 cells,” J. Biomed. Nanotechnol.4(4), 432–438 (2008). [CrossRef]

J. Biomed. Opt.

• A. P. Popov, J. Lademann, A. V. Priezzhev, and R. Myllylä, “Effect of size of TiO2 nanoparticles embedded into stratum corneum on ultraviolet-A and ultraviolet-B sun-blocking properties of the skin,” J. Biomed. Opt.10(6), 064037 (2005). [CrossRef] [PubMed]
• A. V. Zvyagin, X. Zhao, A. Gierden, W. H. Sanchez, J. A. Ross, and M. S. Roberts, “Imaging of zinc oxide nanoparticle penetration in human skin in vitro and in vivo,” J. Biomed. Opt.13(6), 064031 (2008). [CrossRef] [PubMed]

J. Chem. Phys.

• T. M. Stachelek, T. A. Pazoha, W. M. McClain, and R. P. Drucker, ““Detection and assignment of the “phantom” photochemical singlet of trans-stilbene by two-photon excitation,” J. Chem. Phys.66(10), 4540–4543 (1977). [CrossRef]

J. Opt. Soc. Am. B

• X. J. Zhang, W. Ji, and S. H. Tang, “Determination of optical nonlinearities and carrier lifetime in ZnO,” J. Opt. Soc. Am. B14(8), 1951–1955 (1997). [CrossRef]

J. Phys. Condens. Matter

• Z.-W. Dong, C.-F. Zhang, G.-J. You, X.-Q. Qiu, K.-J. Liu, Y.-L. Yan, and S.-X. Qian, “Multi-photon excitation UV emission by femtosecond pulses and nonlinearity in ZnO single crystal,” J. Phys. Condens. Matter19(21), 216202 (2007). [CrossRef]

Microsc. Microanal.

• B. R. Masters and P. T. C. So, “Multi-photon excitation microscopy and confocal microscopy imaging of in vivo human skin: A comparison,” Microsc. Microanal.5(4), 282–289 (1999). [PubMed]

Nanoscale Res. Lett.

• C. Hanley, A. Thurber, C. Hanna, A. Punnoose, J. H. Zhang, and D. G. Wingett, “The Influences of Cell Type and ZnO Nanoparticle Size on Immune Cell Cytotoxicity and Cytokine Induction,” Nanoscale Res. Lett.4(12), 1409–1420 (2009). [CrossRef] [PubMed]

Nanotoxicology

• M. J. Osmond and M. J. McCall, “Zinc oxide nanoparticles in modern sunscreens: an analysis of potential exposure and hazard,” Nanotoxicology4(1), 15–41 (2010). [CrossRef] [PubMed]

Nat. Biotechnol.

• W. R. Zipfel, R. M. Williams, and W. W. Webb, “Nonlinear magic: multiphoton microscopy in the biosciences,” Nat. Biotechnol.21(11), 1369–1377 (2003). [CrossRef] [PubMed]

Opt. Commun.

• C. Y. Liu, B. P. Zhang, N. T. Binh, and Y. Segawa, “Second harmonic generation in ZnO thin films fabricated by metalorganic chemical vapor deposition,” Opt. Commun.237(1-3), 65–70 (2004). [CrossRef]
• L. Irimpan, A. Deepthy, B. Krishnan, L. M. Kukreja, V. P. N. Nampoori, and P. Radhakrishnan, “Effect of self assembly on the nonlinear optical characteristics of ZnO thin films,” Opt. Commun.281(10), 2938–2943 (2008). [CrossRef]

Opt. Express

• M. G. Vivas, T. Shih, T. Voss, E. Mazur, and C. R. Mendonca, “Nonlinear spectra of ZnO: reverse saturable, two- and three-photon absorption,” Opt. Express18(9), 9628–9633 (2010). [CrossRef] [PubMed]
• M. Kauert, P. C. Stoller, M. Frenz, and J. Rička, “Absolute measurement of molecular two-photon absorption cross-sections using a fluorescence saturation technique,” Opt. Express14(18), 8434–8447 (2006). [CrossRef] [PubMed]
• B. R. Masters and P. T. C. So, “Confocal microscopy and multi-photon excitation microscopy of human skin in vivo,” Opt. Express8(1), 2–10 (2001). [CrossRef] [PubMed]

Opt. Lett.

• D. C. Dai, S. J. Xu, S. L. Shi, M. H. Xie, and C. M. Che, “Efficient multiphoton-absorption-induced luminescence in single-crystalline ZnO at room temperature,” Opt. Lett.30(24), 3377–3379 (2005). [CrossRef] [PubMed]

Pharm. Res.

• L. L. Lin, J. E. Grice, M. K. Butler, A. V. Zvyagin, W. Becker, T. A. Robertson, H. P. Soyer, M. S. Roberts, and T. W. Prow, “Time-correlated single photon counting for simultaneous monitoring of zinc oxide nanoparticles and NAD(P)H in intact and barrier-disrupted volunteer skin,” Pharm. Res.28(11), 2920–2930 (2011). [CrossRef] [PubMed]

Proc. Natl. Acad. Sci. U.S.A.

• W. R. Zipfel, R. M. Williams, R. Christie, A. Y. Nikitin, B. T. Hyman, and W. W. Webb, “Live tissue intrinsic emission microscopy using multiphoton-excited native fluorescence and second harmonic generation,” Proc. Natl. Acad. Sci. U.S.A.100(12), 7075–7080 (2003). [CrossRef] [PubMed]

Scr. Mater.

• T. Tsuzuki and P. G. McCormick, “ZnO nanoparticles synthesised by mechanochemical processing,” Scr. Mater.44(8-9), 1731–1734 (2001). [CrossRef]

Skin Pharmacol. Appl. Skin Physiol.

• J. Lademann, H. J. Weigmann, C. Rickmeyer, H. Barthelmes, H. Schaefer, G. Mueller, and W. Sterry, “Penetration of titanium dioxide microparticles in a sunscreen formulation into the horny layer and the follicular orifice,” Skin Pharmacol. Appl. Skin Physiol.12(5), 247–256 (1999). [CrossRef] [PubMed]
• F. Pflücker, V. Wendel, H. Hohenberg, E. Gärtner, T. Will, S. Pfeiffer, R. Wepf, and H. Gers-Barlag, “The human stratum corneum layer: an effective barrier against dermal uptake of different forms of topically applied micronised titanium dioxide,” Skin Pharmacol. Appl. Skin Physiol.14(Suppl 1), 92–97 (2001). [CrossRef] [PubMed]

Skin Pharmacol. Physiol.

• S. E. Cross, B. Innes, M. S. Roberts, T. Tsuzuki, T. A. Robertson, and P. McCormick, “Human skin penetration of sunscreen nanoparticles: in-vitro assessment of a novel micronized zinc oxide formulation,” Skin Pharmacol. Physiol.20(3), 148–154 (2007). [CrossRef] [PubMed]
• C. G. J. Hayden, S. E. Cross, C. Anderson, N. A. Saunders, and M. S. Roberts, “Sunscreen penetration of human skin and related keratinocyte toxicity after topical application,” Skin Pharmacol. Physiol.18(4), 170–174 (2005). [CrossRef] [PubMed]

Toxicol. Sci.

• B. Gulson, M. McCall, M. Korsch, L. Gomez, P. Casey, Y. Oytam, A. Taylor, M. McCulloch, J. Trotter, L. Kinsley, and G. Greenoak, “Small amounts of zinc from zinc oxide particles in sunscreens applied outdoors are absorbed through human skin,” Toxicol. Sci.118(1), 140–149 (2010). [CrossRef] [PubMed]

Other

• Zs. Kertesz, Z. Szikszai, and A. Z. Kiss, “Quality of skin as a barrier to ultra-fine particles. Contribution of the IBA Group to the Nanoderm EU-5 Project in 2003-2004,” in ATOMKI Annual Report 2004 (Institute of Nuclear Research of the Hungarian Academy of Sciences, 2005), p. 70.

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