Cryo-SEM analysis of UV light stimulated sun screen lotion

Pum, Dietmar; Mimietz-Oeckler, Saskia

March 06, 2019

Cryo-SEM analysis of UV light stimulated sun screen lotion

High pressure freezing with light stimulation

The EM ICE is the only available high-pressure freezer combining light stimulation with fast cryo fixation, a module mostly used for optogenetic approaches in neurobiology studies. However, this approach can be also very interesting for pharmaceutical or cosmetic industries to study the influence of UV light exposure on their target structures. This article shows an example on how UV light exposure changes the ultrastructure of sun screen lotion.

High Presure Freezing

Sun screen lotion* was carefully filled in the 100 μm incision of a 3 mm copper/ gold plated carrier and covered with a 3 mm sapphire disk. The sun screen lotion sample was then high pressure frozen with the Leica EM ICE with and subsequently without light stimulation. The light stimulated samples were exposed to a UV light for 500 milliseconds (continuous exposure) before the actual freezing.

Experiment details

Light stimulation Intensity (mW/mm²) (at the sample surface)	6.0
LED colour:	UV
LED wave length (nm)	385
Period (ms) (total duration of the experiment)	500
Pulse (ms) (exposure to light stimulation)	500

Transfer, Coating and SEM

The frozen sample sandwiched between the 3 mm carrier and the sapphire disk was mounted on a universal sample holder in a cooled Leica EM VCM loading station and transferred into a Leica EM ACE600 using a Leica EM VCT500 shuttle. Samples were fractured by pushing off the sapphire disk (top side) with the fracturing knife at -110°C (after 10 minutes waiting period).

Next, the sample was partially freeze-dried at -100°C (15 seconds hold time) and sputter coated with a 2nm layer of platinum. Samples were then transferred onto the Leica EM VCT500 cryo-stage in a FEI Inspect S50 scanning electron microscope (SEM) using the Leica EM VCT500 shuttle keeping the specimen under high-vacuum conditions and actively cooled. Secondary electron Images were acquired at -120°C (High tension: 30kV).

Fig.1: Sun Screen Lotion High Pressure Frozen without UV light exposure

Result

Sun screen structure in general allows absorption of high-energy ultraviolet rays and releases the energy as lower-energy rays. The UV-attenuating efficacy will depend on the particle size of the inorganic particles and the nature of the organic components as well as the specific proportion of each component. This composition specificity will define the ratio of absorption and scattering of UV light by the sun screen system. The images depict the structural changes in a sun screen sample exposed to UV light for 500 milliseconds in comparison to the same sample without UV exposure.

Fig.2: Sun Screen Lotion High Pressure Frozen 5 seconds after UV light exposure

Fine print

Samples prepared May, 2015 by Cveta Tomova Ph.D. and Saskia Mimietz-Oeckler Ph.D. Leica Microsystems, Vienna, Austria. Pictures taken May, 2015 by Dietmar Pum Prof., University of Natural Resources and Life Sciences, Vienna, Austria.

*The trade mark and SPF (sun protection factor) of the product are not mentioned intentionally.