Scienceshow/Nanotechnology

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Nanotechnology Demonstration Experiments and Hands-on Activities

Completed Guides[edit]

  • Under preparation in English, but there are numerous in the danish version

All guides are listed on the overview page

Show sequences[edit]

Show: Intro to nanotech[edit]

This show sequence contains material for easily more than 2 hours of stand-up nanotechnology show with plenty of hands-on activities to keep the audience occupied for much longer. There is a kind of red thread going through the show, but emphasis is on showing the wide variety of nanoscale phenomena people will see in their every day life or that can be demonstrated without complicated setups.

Why things are white[edit]

This is not really nano, it only shows is that microstructured surfaces can give the white color and is different from the rayleigh scattering in nanoparticles described later.

Light scattering in transparent materials[edit]

  • Large transparent crystals and small white crystals (salt, sand, glass...) pieces are white, bulk is transparent.
  • Refraction in a prism gives a rainbow, many refractions in disordered materials give white light.
  • refraction in a droplet of water or a round-bottom bottle of water can make a rainbow [1]
  • Guide: Making Clouds with Liquid Nitrogen. Clouds, make them with liquid nitrogen, could give us a rainbow, but are white because of multiple refractions.

Index matching[edit]

How to make white things disappear

  • Guide: Transparent Wool. Wool and silk, make wool transparent by submerging it in benzyl alcohol that matches the refractive index of the wool.
  • Guide: Invisible Glass. Invisible glass in an index matching transparent oil. Cooking oil and pyrex glass work well. You can make fun things like taking a pieces of a broken glass and put them in a jar with oil and then pull out a whole glass you have put in there previously to magically repair the broken glass.
Index of refraction of transparent materials and liquids
Material n index matching
Solid
Pyrex 1.47 Cooking oil
Quartz 1.544-1.553
PMMA (Acrylic glass) 1.4893-1.492
Nylon 1.53-1.566
polystyrene 1.55- 1.60
Polycarbonate 1.584-1.586
Wool 1.553 Benzyl alcohol
Cotton/linen 1.515
Paper/cellulose
Cellulose acetate 1.4750
Kitchen Salt (NaCl) 1.516-1.544
Sugar/Sucrose 1.5376
Diamond 2.417
Gallium phosphide 3.50
Liquids
water 1.33
ethanol 1.36
Paraffin 1.332-1.412
Acetonitrile 1.3441
Polyunsaturated oils 1.371-1.432
Cyclohexane 1.4260
Ethylene Glycol 1.4318
Glycerol/glycerin 1.4729
Sunflower oil 1.474-1.475
Sugar solution (80%) 1.49
Benzene 1.5010
Benzyl alcohol 1.538-1.540
2-Iodoethanol 1.5720
Carbon disulfide 1.63
Methylene iodide 1.74

Mixing liquids like glycerol and water can make solutions with intermediate refractive indexes.

Graded index matching[edit]

By gradually changing the index of refraction, reflections can be avoided. This can be done by nanostructuring surfaces. For instance silicon surface etched into small spikes becomes black (silicon black) unlike the silvery shine of the smooth surface.

The blue sky[edit]

  • Small nanoparticles, smaller or about the wavelength of light, scatter light depending on the wavelength to the fourth power - so blue light is scattered very much while red isn't.
  • Guide: Colloidal Sunset. We make sulfur nanoparticles in a vial on a overhead projector, and you can see the red light going through while the blue gets scattered. SO the sunset you see and the blue sky above you are caused by nanoparticles in the air (salt, clay, smoke...). A too dense solution of nanoparticles give white light (ok, slightly yellow because sulfur is yellow).
    • See the difference in polarization in the blue light
    • Polarizer film NT45-668 8.5x15" 35 USD
  • An easy way to make nanoparticles is also to add a couple of sulfur crystals to ethanol and leave it to make a saturated solution (probably a few mg pr ml is enough to saturate it). The add drops of the solution to water and sulfur nanoparticles form instantly. Propylene glycol should make it possible to make more concentrated solutions (Stable Colloidal Sulfur Solutions, M. W. Brenner; Joseph L. Owades, Science, New Series, Vol. 119, No. 3104. (Jun. 25, 1954), p. 911.). Zeta potential of sulfur nanoparticles from isopropanol solution is -50mV.
  • Its a similar effect you see in some alcoholic drinks like Ouzo, where the anis oil forms oil droplets in the alcohol when too much water is added.
  • Guide: Milk and Nanoparticles. Diluted low-fat homogenized milk show the same effect, so here we have nanoparticles of fat that give the white color. Laser scattering in milk

Silver nanoparticles[edit]

  • Guide: Silver Mirror Coating.
    • Synthesize silver mirror to show how nanoscale metal films are still metallic [2].
  • Guide: Silver Nanoparticle Synthesis. Synthesize silver nanoparticles and show how light going through a dilute solution is red while the reflected (backscattered) light is green-blue. These particles have a plasmon resonance in the yellow frequency range, so they scatter blue to yellow light and let the red pass through.
  • Metal nanoparticles are used for many things - the red color in some pregnancy tests is from gold nanoparticles.

Gold nanoparticles[edit]

Dyes and fluorescent molecules[edit]

  • These are not really nanostructures, but chemical compunds that have unique optical properties (but quite a lot of chemistry is being called 'nano' these days)
  • Guide: Nile Blue Fluorescence. Nile blue in alcohol - show the red fluorescence when you shine a green laser pointer on it. The molecule absorbs green to red light and can also emit red light when it has absorbed green.
  • Guide: Fluorescent Dyes. Rhodamin dyes and other fluorescent dyes ( Quinine in tonic water)

Spectrometers and diffraction[edit]

  • We need a method to distinguish all these strange optical phenomena, and luckily nanostructured surfaces can be used to do so.
  • Guide: Compact Disc Spectrometer. Compact discs - the diffraction in the 'morse-code' bit pattern on the CD surface gives interference of the scattered light that can be seen as bright rainbows. Using the CD you can get an idea about what colors are coming from different light sources. Try looking at a old-fashioned lamp and compare it to a fluorescent tube - the tube has distinct bands from the fluorescent molecules on the inside coating of the tube.
  • Guide: Diffraction Grating Spectrometer. Using diffraction gratings it is easier to see the effect, and you can investigate for instance discharge tubes and light enmitting diodes.

Subwavelength optical elements[edit]

  • Monolayers of polystyrene spheres can give nice diffraction patterns.
  • Liquid crystals and polarization effects

Tuning the material properties[edit]

  • Guide: Light Emitting Diodes. The light emitting diodes is an example of a material where the properties have been change by changing the composition. By adding different substances to a crystal that emit light, different wavelengths can be obtained.
  • Adding fluorescnt dyes to a blue emitting diode can give white light.
  • Activity guide [4]

Different materials absorption, transmission and reflection properties[edit]

  • Guide: Absorption, Transmission and Reflection of Light and Heat. This is not really about nanostructures, but show how different wavelenghts behave in different materials. Its a good starting point to talk about optical properties of materials and their bandgaps etc.
  • Using a lamp and an infrared heat source you can see and feel how:
    • Glass transmits visible light but not heat
    • Thin silicon and germanium transmit heat but not visible light (sapphire can also do the trick)
    • A thick silicon wafer absorbs heat and gets quickly hot
    • A gold coated silicon wafer reflects heat and stays fairly cold. The electromagnetic radiation only reahes a few nanometers into the gold (skin depth) and is reflected very well from sub 100nm gold films.
    • A thin water film (add some soap) on a metal mesh stops the heat (thats why chicken lamps warms your skin so comfortably, but you dont really get much warmer from them :-) and why toasting bread toasts the surface and not the inner parts. In the secion on soap films we will see that such films can be sub 100nm thick (I haven't checked up on how thick it actually needs to be to stop heat radiation so much you can feel it, but its probably about a micrometer).
    • try out aerogels...

Photonic Crystals[edit]

Ionization[edit]

  • Light emission from ionization - The light emission from the fluorescent lamps, plasma balls and discharge tubes can be used to analyze the gas composition
  • Guide: Plasma Ball.

Field Ionization[edit]

The ionization process can also be used in itself to analyze what gasses are present and it can be made better with nanoscale tips, field ionization is also used to make ions in air purifiers (that actually also produce a lot of ozone that isn't healthy)

  • Rub balloons and make them stick on the wall.
  • Guide: Van de Graaff Generator. On a Van de Graaff generator, explain the charging just like rubbing a balloon continuously on your shirt - and demonstrate discharging of cotton pieces the bounce back and forth and stick temporarily by electrostatic forces.
  • Discuss how the high electrical fields break down molecules
  • Guide: Van de Graaff Generator. The show the field emission from a rotator with angled tips and explain how the tip makes huge electrical fields and generate ions and ozone.
  • Guide: Ion Spray. Demonstrate the de-ionization spray gun - smell the ozone and feel the ionized air coming from it. Try using it to detach polystyrene spheres sticking to plastic and other things. They also get charge just like a balloon being rubbed, and stick like the cotton. The ion-spray makes ions go to where there are high charge densities and neutralize it and then the rest of the ions will adhere more or less evenly everywhere and reduce the electrostatic 'dust-attracting' forces. Some hair dryers have 'ion-sprays' built in to reduce the electrostatic feeling in dry hair.
  • Vizualize the ion motion in air with clouds repelled and attracted by VdG [7]

Molecular Surface Forces[edit]

  • Like the charge differences can make things adhere, differently charge parts of molecules will also attract each other, for instance the H a water molecule H2O like to be near the O of other water molecules. If there is no static charge in the molecule, there will still be a weak attraction because the overall charge neutrality is only there when we look at a time average. At any instant the electrons will be in specific positions and there will be s small polarization, but this will change quickly with time. However two molecules will affect each other because of the quickly varying polarizations and this gives rise to an attractive force, called the van der Waals force.
    • Paraffin and stearine and wax are examples of materials that are solid because of the VdW forces.
    • Water is an example of a statically charged molecule.
  • The charged molecules attract each other so well that they do not like to mix with the weakly attracting VdW substances -oil and water do not mix well.
  • Guide: Capillary Pressure. Capillary pressure sucking a colored water into small glass tubes but not as much into plastic/teflon tubes. What about oil and soap water?
  • Examples
    • Teflon coatings in the kitchen
    • Superhydrophobic coatings
      • The lotus effect (see below)
    • Super hydrophilic and self cleaning surfaces (see below)

Superhydrophilic surfaces[edit]

  • Titanium oxide coatings on windows makes them superhydrophilic and self cleaning [8], [9], [10]

Sticktion and Adhesion[edit]

Oil thin films[edit]

Estimate the size of moleculs from oil droplets spreading on water[12].

  • Guide: Oil on Water. oil on water makes a nanometer thin film with interference effects

Soap thin films[edit]

In soap bubbles we can see such effects ([SoapBubbler.com website with soap bubbles]).

  • Guide: Soap Films. In a soap film suspended in a metal wire loop that is held vertically you can see rainbow olors in light reflected in it. The top part eventually turns transparent and that is when the film is thinner than 1/4 of the wavelength -and just before the thin film pops.

[14]

Lipid bilayers[edit]

  • The essential part of a living cell [15]

'Superhydrophobic' coatings[edit]

Coated particles[edit]

  • Water coating
    • Guide: Non-Newtonian fluid. Maize flour or potato flour experiment. Maize starch is usually best. About equal volumes of flour and water gives a good fluid. Typical grain size is 10 microns giving a 5 micron water film coating the particles if 1 g of water is mixed with 1.5 g of maizens (best result). So its not that nano, but more micro.

If you boil it, the starch glycose polymer chains will dissolve into the water.

  • Surfactants
    • The the silver particles mentioned earlier are coated with a charged film of molecules.

Brownian Motion[edit]

The motion of matter due to its thermal energy. This is why ferrofluids do not just clump together but stay fluid.

  • Microscope demonstration using colloidal graphite or just a little milk (add some food coloring to see it better, maybe a hydrophobic colorant like paprika)[17]
  • (I have not succeeded in making this experiment:)An overhead projector can also be used - here with sulphur particles [18]
  • Demonstration of gas diffusion through soap film [19]

Iron nanoparticles[edit]

Combustion[edit]

  • iron oxalate based iron nanoparticles for burning fountain in air. Decomposes on heating into ultra fine iron powder and carbon dioxide. The particles ignite spontaneously when poured from the tube into the surrounding air. [20]
    • The process of forming the nanoparticles depends on temperature and many different compunds can be formed at different temperatures [21]
  • Another option is Raney Nickel [22] which can be painted on paper where it burns.

Ferrofluids[edit]

  • Guide: Ferrofluid. A ferrofluid - iron particles coated and dispersed in a fluid
  • ferrofluid recipe using oleic acid surfactant dissolved in kerosene.
  • and a simple one
  • These particles act just like magnets we see in everyday life - or maybe rather like a box of nails that you can stick your magnet into and pull out long strings of nails attracting each other.
  • Ferrofluids are for instance used in loudspeakers to ensure good heat dissipation between the magnet and the moving coil and some damping.
  • The above magnetic experiment shows nanoscale ferromagnetism that was very much like macroscale magnetism (that's just these particles - a lot of research is going into control nanoscale magnetism for data storage on hard disks etc).
  • Ferrofluids can be used to make the magnetic patches on a hard disk visible under an optical microscope.

Electrically controllable particles[edit]

Like the magnetic fields that control ferrofluids - electrical fields can be used to control small particles dispersed in liquids.

Quantized conductivity[edit]

  • When it comes to electrical conductivity, strange things can happen on the nanoscale: Quantized conductivity.
  • Guide: Quantized Conductivity. Experiment in preparation
  • 12K ohm - thats the resistance for one channel for electrons.

Aerogels, another kind of glass[edit]

  • Use it to observe Rayleigh scattering. Talk about the insulation and energy balance when using it in windows. See how capillary forces crush it when its wetted. Examine how much weight it can carry and how 'projectiles' can be caught in it (as in the stardust satellite).
    • I guess another way to make a 'gel' is by making a Calcium acetate saturated solution (412g in 1L water, stir for two days to dissolve well), take 10ml of this solution and mix with alcohol (100ml) that gives an amorphous gel in 10 secs looking very much like the aerogel - the aerogel is then made be critical point drying of such a liquid gel to avoid it collapsing because of capillary effects. Capillary effects are also what causes the whitening of the gel if you touch it with your fingers, where water from your skin is absorbed into the fine capillaries of the aerogel which the collapse.

Mechanical properties of clean surfaces[edit]

  • Optical fibers don't break
  • An HF etched glass rod also can sustain much more strain than an untreated [26] because the surface is free of microscopic crack initiating fractures.

The different types of Carbon[edit]

Carbon is a good example of a material that can have numerous different nanostructures:

  • Graphite - see it in you pen. Both dark and metallic at the same time. Chemically inert (well, it can be burnt) and used for electrodes in batteries.
  • Graphene sheets - how do they make them? Can it be demonstrated?
  • Pyrolithic graphite - See the diamagnetic properties below. can also be used to cut ice due to the high thermal conductivity. And its quite strong mechanically.
  • Highly oriented pyrolithic graphite - HOPG -make an STM image of the ordered stucture
  • Carbon black -used in ink and as coloring for black rubber. can be used to demonstrate brownian motion.
  • Diesel Sooth - probably the most abundant nanoparticle source in city areas. Many environmental and safety aspects to talk about (why worry about a little nanoparticles in the lab when its all in the air from the traffic outside?).
  • Carbon 60 - buckyballs - the football molecule. Any demonstrations?
  • Carbon nanotubes - rolled up graphene sheets with numerous interesting properties. Can we make a demonstration of acutation of CNT-paper electrodes in water [27]? Talk about science fiction space elevators and nanoelectronics.
  • Diamond -the hardest material. does not conduct electricity but does conduct heat if i remember right... any demonstrations we can do with diamond powder for instance?
  • Potassium graphite [28]

Diamagnetism[edit]

  • Guide: Magnetic Induction in a Tube. Induction recap by having a magnet fall in a aluminium tube
  • Guide: Magnetic Induction in Copper Plates. Moving a magnet over a copper plate cooled by liquid nitrogen
  • Guide: Pyrolithic Graphite. Talk about the graphene sheet and how the electrons can move in it. Let people play with pyrolitic graphite hovering over magnets.
  • Guide: Superconductors. To show a stronger effect, demonstrate magnets hoovering over superconductors. And while you're at it you can also show how liquid oxygen is paramagnetic (and burns well).
  • Compare to ferrofluids

Amorphous materials[edit]

  • Guide: Freezing with Liquid Nitrogen. Quick cooling gives small crystals
  • LN2 is used in the industry to make small ice crystals that ensure a better taste of food. Its used to store biological samples without cracking due to freezing and thawing
  • Guide: Freezing with Liquid Nitrogen. Freeze flowers in liquid nitrogen
  • Guide: Supersaturated Sodium Acetate. Demonstrate sodium acetate super saturated solution crystallisation, giving a white crystal (About 650g CH3COONa cab be dissolved in 250 ml water at 100°C, cool very slowly in an undisturbed place).
  • Guide: Amorphous Metals. Demonstrate amorphous metals with unique mechanical properties, see also [29]
  • Guide: Liquid Nitrogen Marshmallows. Serve cryogenic marshmallows
  • Guide: Liquid Nitrogen Ice Cream. Make ice cream with liquid nitrogen - we are back to the thing we started with, white stuff with small particles!

Memory metals[edit]

Catalysis[edit]

Chemical reaction can often occur at much lower temperatures and more efficiently if catalytic materials are present. For example, various enzymes are used to enhance biochemical processes in detergents and washing powders so you can wash clothes at lower temperatures and with less power and detergent consumption. Catalysts are also heavily used in industrial chemical processes. Often the catalytic material is very expensive and only the small reactive sites on the material surface are actually catalytically active. For these reasons it is much more efficient to use nanostructured catalytic substances that maximize the catalyst area with many reactive sites and a minimal consumption of the material itself.

  • Car exhaust catalyst
  • Demonstration of Methanol catalysis with platinum wire (see also Journal of Chemical Education, vol. 71, no.4, pp 325 -327.)
    • [31]
      • Heat 10ml Methanol in an Erlenmeyer flask (you can color it with methyl blue). Can this be done with ethanol which is less hazardous than methanol?
      • Heat a platinum wire until glowing and hang it into the the Erlenmeyer flask
      • The wire will glow weakly and then more brightly as it is heated by the exothermic catalytic oxidation of the methanol to formaldehydeCH3OH + 1/2 O2 =CH2O + H2O
      • When the wire is hot enough, it will ignites the methanol/air mixture that starts burning with a green flame CH3OH + 1.5 O2 =CO2 + 2 H2O, until there is no more oxygen in the flask.
      • The reaction will oscillate, restarting when fresh oxygen gets into the flask[32]
      • See also [33]
      • The flames can be made brighter green if boric acid and concentrated sulfuric acid (catalyst) are added to the methanol (Forming volatile boric acid trimethyl ester, which burns with a green flame: B2O3 + 6 CH3OH = 2 B(OCH3)3 + 3 H2O
  • [34] Catalytic oxidation of ammonia (seems to be a more benign reaction than methanol combustion, though it produces NOx, which require good ventilation)
    • 25 mL of concentrated aqueous ammonia solution in an Erlenmeyer flask. Lower the glowing Pt wire down to a couple of cm above the liquid and it will glow bright. You can also try with copper that will melt a drip into the liquid forming cupperoxide and a complex with the ammonia [Cu(NH 3) 4] 2+. [B. Z. Shakhashiri, Chemical Demonstrations, A Handbook for Teachers of Chemistry, Wisconsin, 1989, Vol.2, p.214-215][Gilbert, Alyea, Dutton, Dreisbach, Tested Demonstrations in Chemistry, Published by arrangement with the Journal of Chemical Education, 1994, Vol. I, p.I-35]. See also [35].
  • Example of a reaction where the catalyst is temporarily consumed in the reaction by eventually is recovered and ready for a new reaction cycle -When concentrated H2O2 is added to a yellow-orange aqueous solution of Fe3+ [36].
  • [37] Simple Cu catalyzed Zn+Acid reaction and a discussion about the underlying chemistry... could a nanoscaled catalyst help?

Polymers[edit]

  • Guide: Superabsorbing Polymers. Superhydrofile superabsorbents. Luquasorb 3746sx 0.1g absorbs 14ml ordinary before becoming fluid.
  • [38] poly vinyl alcohol slime, see also [39]
  • Alginic acid mixed with food coloring. Add the colored Alginic acid to a solution of Calcium Chloride to form worm-like structures.
  • Polyurethane foam is made by mixing two polymers that grow when mixed [40] and [41].
  • Polydimethylsiloxane (PDMS) Elastomer
  • Sulfur can also be made into a kind of polymer [42]

DNA[edit]

  • Extract DNA from vegetables [43]
  • Discuss DNA X-ray diffraction [44] REMOVED
  • We can show chromosome-1 in a high mag microscope. Can different chromosomes be coloured differently?

Enzymes[edit]

  • catalysis of oxygen from hydrogen perioxide with peroxidase
  • Enzymes are used in washing powders to
    • make clothes clean at low temperatures (clothes last longer and it saves energy)
    • Dishwashing liquids that also remove starch
    • Leather conditioning with less harsh chemicals
    • Bio fuel production
    • Most chemical processes in your body needs various enzymes to run smoothly, prepare reagents and remove the waste efficiently.

Food[edit]

A welath of dispersions and nanosize particles can be found in food. See e.g. the milk experiment with Rayleigh scattering.

  • Review Physics of Foodstuffs Athene M Donald 1994 Rep. Prog. Phys. 57 1081-1135 doi:10.1088/0034-4885/57/11/001

Complete dialogue show examples[edit]

The colors in the sky[edit]

This show aims at demonstrating a wide variety of nanoscale phenomena that appears in everyday life or relate to everyday life. It is the intention the audience should get several hands-on experiments during the show and participate to some extent. A different sequence is naturally possible and experiments can easily be added or left out depending on the audience and available materials. Depending on the school level of the audience you will probably want to modify this sequence. 'to show' in this description means to pass the sample around whenever possible.

White Nanostructures[edit]

Walking around you will actually see a wealth of effects in you daily life that comes from nanoscale phenomena and structures. For instance, you might wonder why things are white. What do you know thats is normally white? Do you know why?

Clouds - If you think of the clouds, they can give us both rainbows or be havy gray rain clouds or white fluffy clouds.

Rainbows come from refraction of light in the droplet, just like the good old Pink Floyd cover where a white light beam is split into a rainbow. If the droplets are too dense, the rainbow colours will be scattered in many droplets and eventually we will see a diffuse mixture of all colors which gives us white light.

We can make nice clouds by using liquid nitrogen.

  • Guide: Liquid Nitrogen. In this cup (two plastic single use cups put inside each other to work as a cheap and surprisingly effective dewar) I have liquid nitrogen, probably the substance you know better than anything else in your life because 80% of the air your breathe is nitrogen. If we cool it to -196 C it turns into a liquid, just like water vapor can condense on cold things.
  • Guide: Liquid Nitrogen . Pouring it on the floor gives a lot of 'smoke' and leaves no liquid because it boils violently when it reaches something that is 200C over its boiling point. It makes you wonder if nitrogen is grey... but that is a bit strange if 80% of the air is made from it and quite transparent.
  • Guide: Making Clouds with Liquid Nitrogen. If I put a plexiglas tube into the liquid nitrogen, you can see it boil and the 'steam' rising up in the tube. The nitrogen in the tube is quite clear; it is only when it reached the surrounding air it turns into an opaque cloud. And that's what it is - condensation of the water vapor in the air gives real clouds right here for us to play with.
  • Guide: Making Clouds with Liquid Nitrogen. Making better clouds is easy if we take some boiling water where we have a lot of water vapor. Pouring 1 dL liquid nitrogen into 0.5L boiling water in a electric kettle (that can be opened *completely* to avoid bursts of boiling water and nitrogen) we get a thick and heavy cloud of cold condensed water vapor. Its fun to play with, nice and cold - only be careful not to play too much and spill boiling water over you.

So things can be white even if they are clear and transparent as 'bulk' materials, simply because the are cut into small pieces that refract light in all directions. We know that from salt as well, where the individual salt grain can be a nice clear crystal, while a pile of such grains is white.

The refraction of light in the particles depends on the change in refractive index between the surroundings and the material.

Wool is another example of a white material, and here the white color comes from the roughness of the practically transparent wool fibres (when they are from white sheep).

  • Guide: Transparent Wool. If we submerge wool intro benzyl alcohol (has a refractive index of 1.540.) which has the same refractive index as the wool fibres, the light will not be refracted at the fiber interface. And as we can see it becomes almost completely transparent. If you put white silk into it, it will not be transparent, because silk has a different index of refraction. You can also see this effect sometimes if you get greasy food in some paper bags, that become almost transparent.

So it is probably not a good idea to make swimsuits from textiles that match the refractive index of water :-)

The effect does not always have to be nano - salt is white even when the particles are large enough to be visible.

(*Guide: Invisible Glass. we can also show the indexmatching effect by submerging a glass filled with clear oil or a glass figure in an clear oil bath also kept in transparent glas, and ask people to guess whats in it - its invisible until we pull it up)

The Blue Sky[edit]

Now we will turn into an effect that does require nanoscale particles and you hopefully see almost every day.

Nanoparticles are present in the sky above us - small water droplets, grains of salt from the sea...

When the particles become smaller than the wavelength of light the begin to scatter light different than the big water droplets making rainbows. It is called Rayleigh scattering and is very dependent on the wavelength of the light.

To illustrate this we will make some sulfur nanoparticles. Sulfur is yellow (show a vial of yellow sulfur crystals), but when made into nanoparticles they can take on many colors.

  • Guide: Colloidal Sunset. In a tall glass on an overhead projector, mix diluted sulfuric acid with sodium thiosulfate 1:1 vol, and wait until a pale shine appears, then dilute 10 times or more with water. (Its also called the tyndall-rayleigh experiment/scattering) [45]

another recipe uses 6.75L water, 135g sodium thiosulfate and 5 ml Conc. Acid but I dont like to have conc. acid in class demonstrations)

As the particles grow because sulfur is precipitated from the solution and aglomerate to form nanoparticles, you see a change in color. The light coming from the sides of the vial is blue while the light going through it is red - you can see what happens when you look into the sky on a bright day and the blue part of the sunlight passing over you is scattered down towards you while the red continues out towards the people looking at the sunset in the horizon. If there is too much sulfur available, at some point the particles become so dense no light passes through the vial, then you should use less liquid to start the reaction. After some time the particles grow so big that they scatter white light like the white cloud (possibly a slightly yellowish one due to the color of sulfur). The blue sky is definitely a nano-scale phenomena.

  • Guide: Milk and Nanoparticles. You actually meet a similar effect every day if you drink milk products. If you take a low fat homogenized milk and pour a little into a large vial with water, you will see the same effect. Milk is white because of the fat in the milk making nanoparticles scattering light. If the fat content is too high or it isnt homogenized it can be difficult to show a clear effect.

Not only do we inhale nanoparticles and enjoy their colors in the sunset and blue sky, we drink them very often.

Silver nanoparticles[edit]

There are of course many types of nanoparticles - harmless, healthy, dangerous, and useful ones, just like there are different materials around us.

A quick way to show nanoparticles is to make silver nanoparticles by mixing silver nitrate solution with sodium citrate and boil it reducing the silver to free metallic silver. The citrate stabilizes the silver which assembles in charged thin-film coated nanoparticles.

First we will show what happens when you reduce silver without a stabilizing agent.

Before the show: Add one drop 0.4 M NaOH (aq) per ml of 5%wt silver nitrate solution and shake gently. Then add 1M NaOH dropwise while shaking until the precipitate just dissolves. This is Tollen's reagent, a diaminesilver(I) complex. Use it within a couple of hours. Do not store Tollens reagent [46]because it can form silver nitride which is highly explosive if left for longer periods. After use, neutralise it with dilute nitric acid and flush out in drain if permitted. Tollens reagent recipe tollens reagent

We have a silver solution called tollens reagent and we add a droplet of dextrose that will reduce the silver but not protect the silver from assembling in large structures.

(Put the test tube in a warm water bath and add dextrose solution by letting a droplet slide down the side of the glass, and do not touch it until the reaction has made a nice silver mirror. (aldehydes give a silver mirror, ketones a yellowish mirror and alkynes with a triple bond in the 1-position give a yellow precipitate of silver carbide.))

And so we get a beautiful silver mirror.

  • Guide: Silver Nanoparticle Synthesis. In the case of citrate, the citrate ions form a kind of soap film around the silver as it reacts and the charged part of the citrate molecules will make silver particles repel each other so they do not accumulate and precipitate as metallic film.

When we dilute the silver particle solution so we can look through it, se can by holding it in front and behind a lamp see that light comming through it or 'reflected' from it has different colors. Red/orange light pass through while green-blue light is reflected.

The silver nano particles scatter light the has short wavelenghts and leaves most of the yellow and red light pass through. If you make them with gold you get red nanoparticles instead (but it is more expensive chemicals).

The silver and gold nanoparticles are used for many things. Gold nanoparticles is widely used for makring chemical reactions -for instance the red color in pregnancy tests often comes from gold nanoparticles. Silver nanoparticles can be used to kill bacteria and you can for instance buy socks with silver nanoparticles, and at some point people even believed they should be healthy to drink... I wouldn't try to do that.

Resources[edit]