Nano Sunscreen

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Slide 30

Inorganic Ingredients: Cluster Size

Inorganic ingredients come in different cluster sizes (sometimes called “particles”)

Different number of ions can cluster together

Must be a multiple of the formula unit

ZnO always has equal numbers of Zn and O atoms

TiO2 always has twice as many O as Ti atoms

~100 nm TiO2 particle ~200 nm TiO2 particle

Source: Images adapted from http://www.cse.clrc.ac.uk/msi/projects/ropa.shtml

Slide 31

Inorganic Ingredients: UV Absorption

Inorganics have a different absorption mechanism than organics

Absorb consistently through whole UV range up to ~380nm

How is the absorption pattern different than for organics?

Source: Graph adapted from http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html

Slide 32

If inorganic sunscreen ingredients block UVA light, why doesn’t everybody use them?

Source: http://www.loc.gov/rr/scitech/mysteries/images/sunscreen2.jpg

Slide 33

Appearance Matters

Traditional inorganic sunscreens have appear white on our skin

Many people don’t like how this looks, so they don’t use sunscreen with inorganic ingredients

Of the people who do use them, most apply too little to get full protection

Source: http://www.4girls.gov/body/sunscreen.jpg

Slide 34

Why Do They Appear White?

Traditional ZnO and TiO2 clusters are large

(> 200nm)

Large clusters scatter visible light

(400-700 nm)

Maximum scattering occurs for wavelengths twice as large as the clusters

The scattered light is reflected to our eyes, appearing white

Source: Original image

Slide 35

Why don’t organic sunscreen ingredients scatter visible light?

Source: Adapted from http://www.loc.gov/rr/scitech/mysteries/images/sunscreen2.jpg

Slide 36

Organic Sunscreen Molecules are Too Small to Scatter Light

~200 nm TiO2 particle Methoxycinnamate

(Inorganic) (Organic)

(Note that these images are not drawn to scale)

Source: Images adapted from http://www.cse.clrc.ac.uk/msi/projects/ropa.shtml and http://www.3dchem.com/molecules.asp?ID=135#

Slide 37

Waves and obstacles

Waves go around small obstacles

Waves scatter all around from obstacles of sizes comparable to a wavelength