Ozone, UV, and Nanoparticles Mort Sternheim STEM Education Institute [email protected] 6/29/2008.

Ozone, UV, and Nanoparticles

Mort SternheimSTEM Education Institute

[email protected]

6/29/2008

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

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. What 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

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

11

UVc - 100% AbsorptionUVb - 90% Absorption

UVa - 50% Absorption & Scattering

Ozone is the Earth’s natural sunscreen

Ozone (parts per million)

0

20

40

60

80

100

Alt

itud

e (k

m)

Troposphere

Mesosphere

Thermosphere

Altitude (m

iles)

10

0

20

30

40

50

60

0 2 4 6 8

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 sources

• Also from chlorofluorocarbons (CFCs) (freons) and bromofluorocarbons (halons)– UV light produces free Cl, Br radicals– Cl, Br catalyze chain reactions destroying

~100,000 ozone molecules• Used in aerosols, refrigerators, air

conditioners, fire extinguishers

16%

32%

23%

12%

7%5%

1%4%

0

3400

3000

2000

1000

(CH3CCl3)

(e.g., HCFC-22 = CHClF2)

(CCl2FCClF2)

Naturalsources

Other gasesMethyl chloroform

HCFCs

CFC-113

Carbon tetrachloride (CCl4)

CFC-11 (CCl3F)

CFC-12 (CCl2F2)

Methyl chloride (CH3Cl)

0

20

15

10

5

15%

27-42%

5-20%20%

14%

4%

Methyl bromide (CH3Br)

Halon-1211 (CBrCIF 2)

Halon-1301 (CBrF3)

Other halons

Very-short lived gases (e.g., bromoform = CHBr3)

Chemicals that Destroy Stratospheric Ozone

• Cl is much more abundant than Br• Br is about 50 times more effective at O3 destruction

From Ozone FAQ - see http://www.unep.org/ozone/faq.shtmlFrom Ozone FAQ - see http://www.unep.org/ozone/faq.shtml

Ozone depletion

• 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

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– Doubling every year?

• Clothing, food and beverages, sporting goods, coatings, cosmetics, personal care

• Sunscreens: many use nanomaterials– Some labeled as containing nanoparticles– Some not labeled

http://www.masspolicy.org/pdf/

workshop/rejeski.pdf

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/ ;

Radiation

Type

Characteristic Wavelength

(

Energy per Photon

% of Total

Radiation Reaching

Earth

Effects on Human

Skin

Visible to

Human Eye?

UVC ~200-290 nm(Short-wave UV)

High Energy

~0%

(<1% of all UV)

DNA Damage

No

UVB ~290-320 nm(Mid-range UV) Medium Energy

~.35%

(5% of all UV)

SunburnDNA

DamageSkin Cancer

No

UVA ~320-400 nm (Long-wave UV)

Low Energy

~6.5%

(95 % of all UV)

TanningSkin Aging

DNA Damage

Skin Cancer

No

Vis ~400-700 nmLower Energy

~43 % Yes

IR ~700-120,000 nm Lowest Energy

~49% No

Sun Radiation Summary

Increasing Energy

Increasing Wavelength

Which Sunscreen Should You Use???

New and Improved

Now with Nano-Z

SPF 50Goes on Clear

Safe for Children

Broadband Protection

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

• Sunscreens first developed to prevent sunburn– Ingredients were good UVB blockers

A Brief History of Sunscreens: The UVA Problem

• UVA rays have no immediate visible effects but cause serious long term damage – Cancer– Skin aging

• Sunscreen makers working to find UVA blockers– No official rating of UVA

protection yetSource: http://www.cs.wright.edu/~agoshtas/fig8.jpg

Twenty different skin cancer lesions

How do you know if your sunscreen is a good UVA

blocker?

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– Typical a few to several dozen Å

(<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 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 http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html

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: http://jchemed.chem.wisc.edu/JCEWWW/Features/MonthlyMolecules/2004/Oct/JCE2004p1491fig4.gif

How are inorganic sunscreen ingredients different from organic

ones?

How might this affect the way they absorb UV light?

Inorganic Ingredients: The Basics

• Atoms involved– 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

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

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

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 http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html

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

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#

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/

What could we do to inorganic clusters to prevent them from

scattering visible light?

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

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

Essential Questions: Time for Answers

1. What are the most important factors to consider in choosing a sunscreen?

2. How do you know if a sunscreen has “nano” ingredients?

3. How do “nano” sunscreen ingredients differ from other ingredients currently used in sunscreens?

3. Testing sunscreen activity

• Use UV sensitive beads

• Compare opacity/ transparency of samples for visible light and UV light