Ozone, UV, and Nanoparticles
Mort SternheimSTEM Education Institute
STEM ED/CHM Nanotechnology 2012
Today’s agenda
1. Ozone and ultraviolet light2. Nanoparticles and sunscreen3. Hands on activity
Sunscreen PowerPoint and activities based on
NanoSense web site: http://nanosense.org/activities/
clearsunscreen
1. Ozone and ultraviolet light2. Nanoparticles and sunscreen3. Hands on activity (brief)
Sunscreen PowerPoint and activities based on
NanoSense web site: http://nanosense.org/activities/
clearsunscreen
The big ideas
• Ultraviolet light causes skin damage and cancer
• Ozone in the stratosphere blocks UV• Sunscreen blocks UV, partly• Nanoparticles in sunscreen improve
blocking
1. Ozone and Ultraviolet LightWhat is ozone?
• Ordinary oxygen gas: O2 (2 oxygen atoms)
• Ozone: O3 (3 oxygen atoms)
• Polar molecule, like water
• Ozone is much more reactive, unstable
• Pale blue, poisonous gas Bad!
• Absorbs ultraviolet radiation! Good!
The Sun’s radiation spectrum
• ~ 43% is in the visible range
• ~ 49% is in the near infrared range
• ~ 7% is in the ultraviolet range
• < 1% is x-rays, gamma rays, radio waves
.
Most of the sun’s radiation is Ultraviolet (UV), Visible & Infrared (IR) :
Source: Adapted from http://www.ucar.edu/learn/imgcat.htm
Some types of electromagnetic radiation
• The sun emits several kinds of electromagnetic radiation: Visible (Vis), Infrared (IR) and Ultra Violet (UV). Note the split into UVA, UVB, UVC
• Each kind is distinguished by a characteristic wavelength, frequency and energy
• Higher energy radiation can damage our skin
Source: http://www.arpansa.gov.au/is_sunys.htm
High Energy Low Energy
What is Radiation?
• Light radiation is often thought of as a wave with a wavelength (), speed (c), and frequency (f) related by
Source: http://www.pueblo.gsa.gov/cic_text/health/sun_uv/sun-uv-you.htm
• Since c (the speed of light) is constant, the wavelength and frequency are inversely related
• This means that light with a short wavelength will have a high frequency and visa versa.
Radiation energy comes in packets or photons
• The size of an energy packet or photon (E) is determined by the frequency of the radiation (f)
E fE
f
• Radiation with a higher frequency has more energy in each packet
• The amount of energy in a packet determines how it interacts with our skin
• Very high energy radiation (UVC) is currently blocked by the ozone layer
• High energy radiation (UVB) does the most immediate damage (sunburns)
• But lower energy radiation (UVA) can penetrate deeper into the skin, leading to long term damage
Source: N.A. Shaath. The Chemistry of Sunscreens. In: Lowe NJ, Shaath NA, Pathak MA, editors. Sunscreens, development, evaluation, and regulatory aspects. New York: Marcel Dekker; 1997. p. 263-283.
Skin Damage
Ozone layer
• Ozone in stratosphere, 10 to 50 km above surface• Ozone Can be depleted by free radical catalysts –
NO, OH, Cl, Br – from natural / human sources (CFC’s)
• Stratospheric ozone levels decreasing ~4% per year since ’70’s
• More skin cancer?• Larger seasonal decrease in lower altitudes
(troposphere) in polar regions: the ozone hole• CFC’s phased out globally by 1996 (Montreal
Protocol, 1987) – will take decades to leave atmosphere
• Ozone levels have stabilized• Recovery will take decades
Good ozone
• In the stratosphere, absorbs 97+ % of solar UV, protecting life from harm
• Produced by solar UV light from O2 :
– O2 + UV (radiation < 240 nm) → 2 O
– O + O2 → O3
• Ozone – oxygen cycle:
– O3 + UV (< 320 nm) → O2 + O
• This cycle heats the atmosphere slightly, so ozone is a minor greenhouse gas
2. Nanoparticles and sunscreen
• Nanoparticles: 1 to 100 nm in diameter, or about 10 to 1000 atomic diameters
• Number of products using nanomaterials is growing very rapidly
• Clothing, food and beverages, sporting goods, coatings, cosmetics, personal care, electronics
• Sunscreens: many use nanomaterials– Some labeled as containing nanoparticles– Some not labeled
http://www.nanotechproject.org/inventories/consumer/analysis_draft/
Why Use Sunscreen?
Too much unprotected sun exposure leads to:
• Premature skin aging (e.g. wrinkles)
• Sunburns
• Skin cancer
Sources: http://www.oasishospital.org/previousnews.html; http://wohba.com/archive/2005_03_01_archive.html
Skin Cancer Rates are Rising Fast
Skin cancer:• Is ~50% of all cancer
cases• Has > 1 million cases
diagnosed each year• Causes 1 person to
die every hour
Probability of getting skin cancer:1930 : 1 in 5,0002004 : 1 in 652050 : 1 in 10…
http://www.skincarephysicians.com/skincancernet/whatis.html; http://www.msu.edu/~aslocum/sun/skincancer.htm
Causes of the increase:• Decreased ozone protection• Increased time in the sun• Increased use of tanning beds
Sources: http://www.msnbc.msn.com/id/8379291/site/newsweek/ ;
Sources: http://www.bbc.co.uk/wiltshire/content/articles/2005/05/05/peoples_war_feature.shtml http://www.arpansa.gov.au/is_sunys.htm
A Brief History of Sunscreens: The Beginning
• First developed for soldiers in WWII (1940s) to block “sunburn causing rays”
Shorter wavelengths (more energy) called UVC
Longer wavelengths (less energy) called UVA
These were called UVB rays
WWII soldier in the sun
Sources: http://www.shop.beautysurg.com/ProductImages/skincare/14521.jpg and http://www.shop.beautysurg.com/ProductImages/skincare/14520.jpg
A Brief History of Sunscreens: The SPF Rating
• SPF (Sunscreen Protection Factor) Number– Measures the strength of UVB
protection only– Higher SPF # = more
protection from UVB – Doesn’t tell you anything
about protection from UVA– UVA causes cancer, skin
aging– No official UVA ratings
until now
• Sunscreens first developed to prevent sunburn– Ingredients were good UVB blockers
New FDA UVA Ratings (2012)
• The phrase “broad spectrum” is meant to indicate protection against UVA
• Products labeled “broad spectrum” will have to provide equal protection against UVB and UVA
• Bathing suits: 3 tbsp every 2 hours
How much UV is there today
• EPA UV index http://www.epa.gov/sunwise/uviscale.html
• 1 = low risk, 11+ = extreme risk• Sometimes included in weather reports• Reflective surfaces (concrete, sand, snow,
water) may make actual level higher than reported
• Free Smartphone App: myUV Alert
Clothing
• Ordinary clothing provides a good sun shield when dry (the tighter the weave, the better) but little or no protection when wet
• Special sun-protective clothing is costly but works well wet or dry; it is a wise investment for children who tend to stay in or around water for hours. http://img.consumersearch.com/files/cs/imagecache/blog-
entry/images/blog/sunscreenclothing.jpg
Know Your Sunscreen:Look at the Ingredients
• Lotion has “inactive ingredients”– Don’t block UV light
• UV blocking agents are “active ingredients”– Usually have more
than one kind present
Source: Original Image
• UV blocking agents suspended in a lotion– “Colloidal suspension”
• Two kinds of active ingredients– Organic ingredients and inorganic ingredients
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– Typically < 10 nm
Sources: http://www.3dchem.com/molecules.asp?ID=135# and original image
Octyl methoxycinnamate (C18H26O3)
an organic sunscreen ingredient
Organic Ingredients: UV Absorption
1. Electrons capture the energy from UV rays
2. They jump to higher energy levels
3. The energy is released as infrared rays which are harmless (each ray is low in energy)
Source: Adapted from http://www.3dchem.com/molecules.asp?ID=135#and http://members.aol.com/WSRNet/tut/absorbu.htm
hf=2.48 eV 3hf=2.48 eV
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 & ranges of absorption; usually in UVB region
– Using more than one kind of ingredient (molecule) gives broader protection
One Ingredient Two Ingredients Three Ingredients
Source: Graphs adapted from http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html
Inorganic Ingredients: The Basics• Atoms: Zinc or Titanium,
Oxygen• Structure
– Ionic molecules: ZnO, TiO2
– Cluster of ions– Formula unit doesn’t dictate
size• Cluster (particle) size
– Varies with # of ions in cluster– ~10 nm – 300 nm
• Absorb thru whole UV spectrum up to 380 nm
Source: http://www.microspheres-nanospheres.com/Images/Titania/TIO2%20P7.jpg and image adapted from http://www.cse.clrc.ac.uk/msi/projects/ropa.shtml
Group of TiO2 particles
Detail of the ions in one cluster
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: http://www.4girls.gov/body/sunscreen.jpg
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
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:
http://www.falstad.com/ripple/
• Organic sunscreen molecules are too small to scatter light ( < 10 nm)
• How does absorption of light by inorganic compounds differ from absorption by organic molecules?
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Inorganic Compounds: Energy Levels
• Inorganic ingredients exist as particle clusters– Very large number of atoms involved– Electrons’ energy depends on their position in
relation to all of them• Huge number of different energy levels
possible
~200 nm TiO2 particle
Source: Images adapted from http://www.cse.clrc.ac.uk/msi/projects/ropa.shtml
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Inorganic Compounds: Absorption I
• In each band, there are many different energies that an electron can have– The energy spacing
between the two bands is called the "energy gap” or "band gap“
Source: Original Image
• Because the energy levels are so closely spaced, we talk about them together as energy “bands”– Normal energy band for electrons (ground
states) is called the “valence band” – Higher energy band (electrons are more
mobile) is called the “conduction band”
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• Electrons can “jump” from anywhere in the valence band to anywhere in the conduction band– Inorganic Compounds are able to absorb all
light with energy equal to or greater than the band gap energy
Source: Original Images
Inorganic Compounds: Absorption II
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• This is the same as saying that all light absorbed must have a wavelength equal to or less than the wavelength corresponding to the band gap energy
• Absorption curves have sharp cutoffs at this – Cutoff is characteristic of the kind of
compound– Doesn’t depend on size of the cluster
Inorganic Compounds: Absorption Curve
Source: Graph adapted from http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html
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• Inorganic Compounds with cut off wavelengths around 400 nm (ZnO and TiO2) are able to absorb almost the whole UV spectrum
Inorganic Compounds: UV Protection
Source: Graph adapted from http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html
– Can be the only active ingredient in a sunscreen
– Can also be combined with other ingredients for reasons such as appearance or cost
– True for both nano and traditional forms (not dependant on size)
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Absorption Summary
Atoms Organic Molecules
Inorganic Compounds
Energy Levels
Absorption
Spectrum
Nanosized Inorganic Clusters
Source: Graph adapted from http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html
• 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
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: http://www.smalltimes.com/images/st_advancednanotech_inside_.jpg
3. Testing sunscreen activity
• Use UV sensitive beads
• Compare opacity/ transparency of samples for visible light and UV light
• Beads absorb UV from 300 nm to 360 nm (UV A is 320 – 400 nm, UV B is 280 – 320 nm)
• Make UV detector necklaces
Teacher note highlights
• Safety. Do not look directly at the UV lamp
• Liquids. Apply directly to bead or to clear plastic strips
• Purchasing. See teacher notes handout
• Making the testers. Melt beads in oven, glue to sticks
http://graphics8.nytimes.com/images/2011/06/15/science/SUNSCREEN/
SUNSCREEN-articleLarge.jpg
Available documents
Handouts
• This PowerPoint• Teacher guide• Student write-up• Consumer Reports
sunscreen ratings
More on the website• UV violet color guide• Controlled
Experiment On The Transmission Of Ultra-Violet Radiation (Jennifer)
• Web links– UVA standards articles– Sunscreen clothing –
does it work?