Ozone, UV, and Nanoparticles Mort Sternheim STEM Education Institute [email protected] STEM ED/CHM Nanotechnology.

Ozone, UV, and Nanoparticles

Mort SternheimSTEM Education Institute

[email protected]

STEM ED/CHM Nanotechnology

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

Sunscreen PowerPoint and activities based onNanoSense web site:

http://nanosense.sri.com/activities/clearsunscreen/index.html

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 (), speed (c), and frequency (f); c = f x

• Higher energy radiation can damage our skin

Source: http://www.arpansa.gov.au/is_sunys.htm

High Energy Low Energy

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/

October 2013: 1628 total products

http://www.nanotechproject.org/cpi/about/analysis/

http://www.nanotechproject.org/cpi/about/analysis

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

Truck driver who did not use sunscreen

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

• Adopted after decades of discussion

• Products labeled “broad spectrum” have to provide equal protection against UVB and UVA

• Bathing suits: 3 tbsp every 2 hours (1 tsp per limb etc.)

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: EPA UV Index

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

3. 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?

30

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 (band)

~200 nm TiO2 particle

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

31

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”

32

• 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

33

• 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

34

• 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)

35

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

Question: Can you tell if a material will block UV by looking at it?• 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• Educational

Innovations handout on UV beads

• 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– NanoSense