Transcript Slide 1

STEM ED/CHM Nanotechnology 2012
Ozone, UV, and Nanoparticles
Mort Sternheim
STEM Education Institute
[email protected]
Today’s agenda
1. Ozone and ultraviolet light
2. Nanoparticles and sunscreen
3. 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 Light
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
Most of the sun’s radiation is Ultraviolet (UV),
Visible & Infrared (IR) :
• ~ 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
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
High Energy
Low Energy
• 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
What is Radiation?
• Light radiation is often thought
of as a wave with a wavelength
(l), speed (c), and frequency (f)
related by
• 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.
Source: http://www.pueblo.gsa.gov/cic_text/health/sun_uv/sun-uv-you.htm
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
•
•
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
Ef
f
Skin Damage
• 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.
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
Probability of
getting skin cancer:
1930 : 1 in 5,000
2004 : 1 in 65
2050 : 1 in 10…
Skin cancer:
• Is ~50% of all cancer
cases
• Has > 1 million cases
diagnosed each year
• Causes 1 person to die
every hour
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/ ;
http://www.skincarephysicians.com/skincancernet/whatis.html; http://www.msu.edu/~aslocum/sun/skincancer.htm
A Brief History of Sunscreens:
The Beginning
• First developed for soldiers
in WWII (1940s) to block
“sunburn causing rays”
These were called UVB rays
WWII soldier in the sun
Shorter wavelengths
(more energy) called UVC
Longer wavelengths
(less energy) called UVA
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 SPF Rating
• Sunscreens first developed to prevent sunburn
– Ingredients were good UVB blockers
• 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
Sources: http://www.shop.beautysurg.com/ProductImages/skincare/14521.jpg and
http://www.shop.beautysurg.com/ProductImages/skincare/14520.jpg
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/blogentry/images/blog/sunscreenclothing.jpg
Know Your Sunscreen:
Look at the Ingredients
• UV blocking agents suspended in a lotion
– “Colloidal suspension”
• Lotion has “inactive
ingredients”
– Don’t block UV light
• UV blocking agents are
“active ingredients”
– Usually have more than
one kind present
• Two kinds of active ingredients
– Organic ingredients and inorganic ingredients
Source: Original Image
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
hf=2.48 eV
3. The energy is
released as infrared
rays which are
harmless (each ray
is low in energy)
3hf=2.48 eV
Source: Adapted from http://www.3dchem.com/molecules.asp?ID=135#and http://members.aol.com/WSRNet/tut/absorbu.htm
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
Detail of the ions in
one cluster
Group of TiO2 particles
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
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?
29
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
30
Inorganic Compounds: Absorption I
• 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”
• 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
31
Inorganic Compounds: Absorption II
• 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
32
Inorganic Compounds: Absorption Curve
• 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 l
– Cutoff l is characteristic of the kind of compound
– Doesn’t depend on size of the cluster
Source: Graph adapted from http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html
33
Inorganic Compounds: UV Protection
• Inorganic Compounds with cut off wavelengths around
400 nm (ZnO and TiO2) are able to absorb almost the
whole UV spectrum
– 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)
Source: Graph adapted from http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html
34
Absorption Summary
Atoms
Energy
Levels
Absorption
Spectrum
Organic
Molecules
Inorganic
Compounds
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 http://www.aims.gov.au/pages/research/projects/sunscreens/pages/sunscreens02.html
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/20
11/06/15/science/SUNSCREEN/SUNSC
REEN-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?