1. Organic Ingredients: The Basics
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Organic Ingredients: The Basics
Organic = Carbon Atoms
Hydrogen, oxygen & nitrogen atoms are also often involved
Structure
Covalent bonds
Exist as individual molecules
Size
Molecular formula determines size
Typical a few to several dozen Å (<10 nm)
Octyl methoxycinnamate (C18H26O3)
an organic sunscreen ingredient
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2. Organic Ingredients: UV Absorption
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Organic Ingredients: UV Absorption
Electrons capture the energy from UV rays
They jump to higher energy levels
The energy is released as infrared rays which are harmless (each ray is low in energy)
hf=2.48 eV
3hf=2.48 eV
Source: Adapted from
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3. Organic Ingredients: Absorption Range
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Organic Ingredients: Absorption Range
Organic molecules only absorb UV rays whose energy matches difference between electron energy levels
Different kinds of molecules have different peaks and ranges of absorption
Using more than one kind of ingredient (molecule) gives broader protection
One Ingredient Two Ingredients Three Ingredients
Source: Graphs adapted from
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4. Organic Ingredients: Absorption Range cont.
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Organic Ingredients: Absorption Range cont.
Most organic ingredients that are currently used were selected because they are good UVB absorbers
The FDA has approved 15 organic ingredients
Sunscreen makers are trying to develop organic ingredients that are good UVA blockers
Avobenzone (also known as Parasol 1789) is a new FDA approved UVA blocker
Source:
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5. UMass STEM Education Institute
How are inorganic sunscreen ingredients different from organic ones?
How might this affect the way they absorb UV light?
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6. Inorganic Ingredients: The Basics
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Inorganic Ingredients: The Basics
Atoms Involved
Zinc or Titanium
Oxygen
Structure
Ionic attraction
Cluster of ions
Formula unit doesn’t dictate size
Size
Varies with # of ions in cluster
~10 nm – 300 nm
Detail of the ions in one cluster
Group of TiO2 particles
Source: and image adapted from
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7. Inorganic Ingredients: Cluster Size
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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
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8. Inorganic Ingredients: UV Absorption
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Inorganic Ingredients: UV Absorption
Inorganics have a different absorption mechanism than organics
Absorb consistently through whole UV range up to ~380nm
Source: Graph adapted from
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9. UMass STEM Education Institute
Why not use inorganics?
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:
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10. Why Do They Appear White?
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Why Do They Appear White?
Traditional ZnO and TiO2 clusters are large
(> 200nm)
Large clusters scatter visible light
( 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
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11. Organic Sunscreen Molecules are Too Small to Scatter Light
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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 and
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Waves and obstacles
Waves go around small obstacles
Waves scatter all around from obstacles of sizes comparable to a wavelength
Water wave (ripple tank) simulation:
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13. UMass STEM Education Institute
What could we do to inorganic clusters to prevent them from scattering visible light?
Source: Adapted from
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14. Nanosized Inorganic Clusters
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Nanosized Inorganic Clusters
Maximum scattering occurs for wavelengths twice as large as the clusters
Make the clusters smaller (100 nm or less) and they won’t scatter visible light
Source: Graph adapted from
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15. Nano-Sunscreen Appears Clear
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Nano-Sunscreen Appears Clear
Nanosized ZnO particles
Large ZnO particles
Source:
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16. UMass STEM Education Institute
In Summary…
Nanoparticle sunscreen ingredients are small inorganic clusters that:
Provide good UV protection by absorbing both UVB and UVA light
Appear clear on our skin because they are too small to scatter visible light
Source:
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17. Essential Questions: Time for Answers
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Essential Questions: Time for Answers
What are the most important factors to consider in choosing a sunscreen?
How do you know if a sunscreen has “nano” ingredients?
How do “nano” sunscreen ingredients differ from other ingredients currently used in sunscreens?
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18. Testing sunscreen activity
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Testing sunscreen activity
Use UV sensitive beads
Compare opacity/transparency of samples for visible light and UV light
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