Nowadays, the science of protecting women's skin has gone from pure plant extracts to high techniques. Skin care products regardless the price range are adopting not only the language of nano-technology, but also the sophisticated techniques themselves. Sun-block products, for instance, start to use titanium dioxide, zinc oxide and silica particles as a substitution for organic chemical compounds which may cause irritations and allergies. Nano-particles (sub-micron size) mixed with lotion can provide a broad-spectrum protection against UV radiation (e.g. UVA and UVB) and are more acceptable by customers who has sensitive skin.

A successful sun-proof product should be able to block a broadband UV radiation, yet transparent to visible so that it looks fresh-toned when applied on the skin. The motivation behind this project is to use scanning electron microscope (SEM) to analysis the nano-particles in the lotion and predict the performance of the product. Two products from Neutrogena (Ultra Sheer SPF45 and Sensitive Skin SPF30) are studied and compared.

The size of the specimen can reach down to 100 nm and under, thus the sample holder has to be optical flat. Polished silicon wafer is chosen because it has a RMS roughness of ~ 2 nm and a good conductivity. Lotion is cream-based which can not survive in the vacuum system; moreover it will contaminate the chamber. In order to separate the grease and the samples, a weak base solution is used to rinse the grease away. The wafer is placed at an angle to the table so that the particles will not stack up but rather form a monolayer. Acetone is used to clean the sample and ethanol is applied to accelerate the evaporation procedure. The sample is then baked to enforce the attachment between particles and the surface. A compressed air gun helps to remove particles that are not securely adhered.

• Secondary Electron (SE) Imaging

The amount of particles bind on surface is small, which brings significant difficulties on finding the specimen. A mark is made to minimize the searching area. To avoid the charging effect a low accelerating voltage should be applied, however, a better image can be obtained by using 20 keV. It might because the specimen is relatively thin so that the electrons can penetrate into substrate. At a magnification of 2,000, some micro-spheres are founded with diameters of 1 um~4 um which can effectively scatter UV light. However, they are capable to reflect visible light as well when applied on the skin, which is not preferred by most of the female customers. Once increase the magnification to 100,000, particles with a variety of sizes from 100 nm down to ~20 nm can be observed. The spheres with a diameter of ~100 nm can still absorb UV radiation, and in addtion it looks transparent to visible light. Whereas particles with size going under 20 nm might be considered as not necessary since the absorbing efficiency of UVA and UVB could be low. To determine the composition of the particles, an X-ray analysis is operated.

xx

• X-Ray Spectroscopy

An accelerating voltage of 15 keV is used for x-ray spectroscopy. The spectrum provides information of only O and Si elements, which is consistent with the ingredients notified on the product. The high peak Si indicates that a majority of the characteristic x-rays are from the substrate and the low peak of oxygen is from the silica particles. The silicon wafer has a native oxide of 2.6 nm thick which is neglectable compared to the silica nano-particles. In addition, an EDAX mapping is done for oxygen. We can see from the graph that the oxygen is limited distributed while the native oxide of silicon should be uniformly distributed.

 

xxxx

An x-ray spectroscopy is carried for comparison with the particles obtained from another product from the same brand (Sensitive Skin SPF 30). The spectrum shows a high peak at titanium which indicates the composition of the particles are titanium oxide instead of silica.

The optical microscope can resolve sub-micron features which is the limit of visible light microscope. This technique is implemented to make sure that particles can stick to the surface so that SEM can be done afterwards.

Nano-particles obtained from sun-screen lotion are observed and characterized in this project. They have the capability of protecting the skin from harmful UV radiation and nearly invisible when applied on skin (also proved by personal experience :)). The cost of the particles is low which makes the price of these products competitive.