Heidi Nelson - Analytical Problem: Exposure to Zinc Oxide and Titanium Dioxide Nanoparticles in Sunscreen
Nanotechnology is a rapidly growing field with great potential for many applications. One of the most common commercial uses of nanoparticles is in sunscreen, which often includes zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles instead of using them in bulk form. As nanoparticles, these materials are transparent to visible light instead of leaving a white paste on the skin when applied, and they may also be more efficient at scattering UV light.
However, the impact of nanoparticles on human health and the environment is not fully understood. In general, the high surface-area-to-volume ratio of nanoparticles gives them different properties from bulk materials, and their size can enable them to permeate cells. Some forms of TiO2 are photocatalytic, generating free radicals when exposed to UV light and potentially damaging nearby molecules or cells.
To determine whether nanoparticles are safe for commercial applications like sunscreen, it is important to understand what happens when humans are exposed to them. Previous studies have found a small concentration of Zn ions, but no nanoparticles, in blood after exposure to sunscreen containing ZnO. Also, TiO2 nanoparticles have been shown to aggregate in the top layers of skin (stratum corneum) with little penetration deeper into the epidermis.
My hypothesis is that nanoparticles will not be detected in blood, and that both ZnO and TiO2 nanoparticles will only be present in the top layers of skin after sunscreen is applied. I would expect the amount and depth of penetration to depend on the frequency and duration of sunscreen application, nanoparticle concentration, and nanoparticle size. The analytes in this problem are ZnO and TiO2 nanoparticles of varying sizes, and the matrices under consideration are human skin and blood.
1. Biello, D. Do Nanoparticles and Sunscreen Mix? Scientific American, August 20, 2007.
2. Wolf, L. K. Scrutinizing Sunscreens. Chem. Eng. News 2011, 89, 44-46.
Blog 2: UV-vis Absorption Spectrometry
Metal oxide nanoparticles absorb light in the UV range, but they also scatter light. This property makes their UV-visible absorption spectra very complicated. The maximum wavelengths of absorption depend not only on the material, but also on the particle size and shape (3). Metal oxide nanoparticles are also difficult to quantify using UV-vis absorbance spectrometry, since their molar absorptivity also varies significantly with particle size and shape (4). Consequently, there are no convenient reference values for the maximum absorption wavelengths and molar absorptivities, although some size-dependent data and sample values were found.
3a) Bulk ZnO absorbs at 365 nm, and smaller nanoparticles in ethanol absorb at shorter wavelengths (about 340 nm for 5-nm ZnO, and 310 nm for 2.5-nm ZnO) (3). TiO2 nanoparticles of the same size in water absorb in a similar range, 340 to 370 nm (5).
3b) The value of ε for ZnO nanoparticles ranges from 35 m3/kmol mm (350 M-1cm-1) for 2-nm particles to 55 m3/kmol mm (550 M-1cm-1) for 10-nm particles (3). I could not find a value for comparable TiO2 nanoparticles.
3. Segets, D.; Gradl, J.; Taylor, R. K.; Vassilev, V.; Peukert, W. Analysis of Optical Absorbance Spectra for the Determination of ZnO Nanoparticle Size Distribution, Solubility, and Surface Energy. ACS Nano 2009, 3, 1703-1710.
4. 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.
5. Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Preparation and Characterization of Quantum-Size Titanium Dioxide. J. Phys. Chem. 1988, 92, 5196-5201.
Blog 3: Similar Analytical Problems
Nanoparticles Accumulate in the Food Chain (Nate Vetter): The hypothesis is that silver nanoparticles in wastewater end up being transferred through the food chain. The analyte is silver nanoparticles and the matrix is wastewater, insects, and animals (not sure of specifics). This problem and my analytical problem both involve detecting nanoparticles in a biological matrix, so we will likely use many of the same techniques. Both types of nanoparticles display size-dependent UV-vis properties. However, the nanoparticle material is different and the particle size may also be different. The behavior of noble metal and metal oxide nanoparticles is likely to be similar but not identical, so some different analytical techniques may be used.
Titanium Dioxide in Masterbatch (Revy Saerang): The hypothesis is that TiO2 particles in Masterbatch affect the uniformity of mixing and pigment distribution. The analyte is TiO2 nanoparticles and the matrix is a mixture of polymer resin, additives, and other pigments. Both of our analytical problems involve TiO2 nanoparticles used commercially, so it is very likely that the same techniques would be used for both problems. However, the context and matrices are different. While Revy is considering how TiO2 nanoparticles affect the properties of a mixture used in industry, I'm looking at whether TiO2 nanoparticles are absorbed through human skin. Detecting TiO2 nanoparticles in Masterbatch may require different techniques from detecting them in blood and skin, especially since skin is a solid.
Blog 6: Chemical structure and Standards
ZnO has the wurtzite crystal structure in its most common form. TiO2 commonly exists in both rutile and anatase structures, although rutile is more stable and consequently more common. Most of the references I have previously cited used wurtzite ZnO or rutile TiO2 nanoparticles, although some extracted nanoparticles from commercial sunscreens and didn't specify the crystal structure.
Images from Wikipedia: http://en.wikipedia.org/wiki/File:Rutile-unit-cell-3D-balls.png and http://en.wikipedia.org/wiki/File:Wurtzite_polyhedra.png
ZnO and TiO2 nanoparticles are commercially available in the 10-30 nm size range used in sunscreen:
Nanostructured & Amorphous Materials, Inc.
spherical ZnO nanoparticles (99.5%, average diameter 20 nm) - stock # 5810HT, $70/100 g
needle-like TiO2 nanoparticles (rutile, >98%, 10x40 nm) - stock # 5480MR, $80/100 g
SkySpring Nanomaterials, Inc.
spherical ZnO nanoparticles (99.8%, diameter 10-30 nm), product # 8410DL, $64/100 g
spherical TiO2 nanoparticles (rutile, 99.5%, diameter 10-30 nm), product # 7920DL, $64/100 g