# Nanoparticles and Nanotechnology

## Key Concepts

• Nanotechnology offers the potential to construct new materials by manipulating individual atoms to form complex structures.1
• Nanotechnology is the application of nanoscience.
• Nanoscience is the study of nanoparticles.
• Nanochemistry uses the tools and methodologies of chemistry to synthesize nanoparticles.
• Nanoparticles are particles with a diameter between 1 and 100 nanometres (1-100 nm)
• 1 nanometre = 10-9 metres 2

1 nm = 10-9 m

• Nanoscale or nanolevel is the term used to refer to objects that are 1 nm to 100 nm.
• "Bulk" material is the term used to describe large, "everyday" amounts of material.
• Materials at the nanolevel, or nanoscale, have significantly different properties to the same materal at a larger level (bulk material) due to:

(i) surface effects

(ii) quantum effects

• Manufacturing at the nanolevel is known as nanomanufacturing. There are two approaches to nanomanufacturing

(i) top-down

(ii) bottom-up

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## Nanolevel (Nanoscale)

The SI3 unit of measurement of length is the metre which has been given the symbol m.
Compared to the size of atoms and molecules, a metre is an enormous unit of length. So Chemists routinely use smaller units of measurement, such as the nanometre.

The prefix "nano" indicates a billionth (a thousand millionth), that is, 1 divided by a 1,000,000,000, which is represented as:

 nano = 1           1,000,000,000

So 1 nanometre is 1 billionth of a metre, or,

1 nanometre =
 1           1,000,000,000
metre = 0.000000001 m

For very small numbers like 0.000000001 scientists prefer to use scientific notation (exponential notation) to represent the number:
1 nanometre = 0.000000001 m = 1 × 10-9 m

When using nanometres as a unit of measurement, nanometre is given the symbol nm:
1 nanometre = 1 nm

The table below gives the names and abbreviations used for common measurements of length used in chemistry:

scientific notation 10-10 m 10-9 m 10-8 m 10-7 m 10-6 m 10-5 m 10-4 m 10-3 m 10-2 m 10-1 m 100 m
unit name
(abbreviation)
Angstroms
(Å)
nanometres
(nm)

micrometres4
(μm)

millimetres
(mm)
centimetres
(cm)
decimetres
(dm)
metres
(m)
trend smallest → → → → → → → → → → → largest
example
(approximate size)
atomic diameter
(0.3 - 3 Å)
carbon nanotube
(width = 1 nm)
transistor on computer chip
(32 nm)
bacteria
(0.2 μm)
smoke particle
(1 μm)
mold spore
(20 μm)
human hair
(0.1 mm)
sand grain
(1 mm)
pen diameter
(1 cm)
DL envolope
(height = 1.1 dm)
( 2 m)
Can you see it? not visible to the naked eye visible to the naked eye

A nanoparticle has a diameter between 1 nm and 100 nm.
1 nm = 10-9m
100 nm = 100 × 10-9 m = 10-7 m

So, a nanoparticle has a diameter between 10-9 m and 10-7 m and is not visible to the naked eye.
 An atom with a diameter ~ 2 Å, or 2 × 10-10 m, is smaller than a nanoparticle with a minimum diameter of 1 nm or 1 × 10-9 m. atomic diameter = 2 Åatomic diameter = 2 × 10-10 m

How many atoms could you place side by side to make up a linear nanoparticle with length of 1 nm ?

 length of line = no. atoms × atomic diameter Rearrange equation by dividing both sides by atomic diameter length of lineatomic diameter = no. atoms × atomic diameteratomic diameter Which gives us: no. atoms = length of lineatomic diameter length of line = 1 nm = 1 × 10-9 m atomic diameter = 2 Å = 2 × 10-10 m Substitute these values into the equation and solve: no. atoms = 1 × 10-9 m2 × 10-10 m no. atoms = 5

 1 nm = 1 × 10-9 m = 5 atoms |2 × 10-10m| 2 × 10-10m| 2 × 10-10m| 2 × 10-10m| 2 × 10-10m|

How many atoms could be placed in a line to make a line 100 nm long?

 no. atoms = length of lineatomic diameter length of line = 100 nm = 100 × 10-9 m = 1 × 10-7 m atomic diameter = 2 Å = 2 × 10-10 m Substitute these values into the equation and solve: no. atoms = 1 × 10-7 m2 × 10-10 m no. atoms = 500

So scientists studying nanoparticles are studying very small particles that are not visible to the naked eye and are made up of a relatively small number of atoms.

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## Surface Effects

At the nanolevel, a nanoparticle has a greater proportion of its atoms at the surface compared to a larger particle made up of the same atoms.

Dimensions of particle length cm × height cm × breadth cm Total no. "atoms" used No. "atoms" at center No. "atoms" at surface % "atoms" at surface Imagine you have been given cubes that are 1 cm × 1 cm × 1cm (that is, each cube has a volume of 1 cm3) to represent atoms. You now use these cubes to build bigger particles (bigger cubes) and: count the number of "atoms" (1 cm3 cubes) used in total count the number of these "atoms" at the surface of the cube count the number of "atoms" in the centre of the cube calculate the percentage of the total number of "atoms" that occur at the surface 2 cm × 2 cm × 2 cm 8 0 8 8/8 × 100 = 100 % 3 cm × 3 cm × 3 cm 27 1 26 26/27 × 100= 96 % 4 cm × 4 cm × 4 cm 64 8 56 56/64 × 100 = 88% 5 cm × 5 cm × 5 cm(≈ 1 nm3 using the same scale as above, 5 atoms to 1 nm) 125 27 98 98/125 × 100 = 78% 500 cm × 500 cm × 500 cm(≈ 100 nm3 using the same scale as above, 5 atoms to 1 nm) 125,000,000 123,505,992 1,494,008 1494008/125000000 × 100 = 1 % As the size of the particle increases from 8 to 125,000,000 atoms, the percentage of atoms at the surface decreases from 100% to 1%. Or, as the particle size decreases from 125,000,000 to 8 atoms, the percentage of atoms at the surface increases from 1% to 100%. So we can generalise and say that a very small particle such as a nanoparticle will have a greater percentage of its atoms at the surface compared to a much larger particle.

Now atoms are usually modelled as spheres not cubes, and when atoms cluster together to form a nanoparticle it is also thought of as being spherical, so scientists usually talk about the ratio of the surface area of the spherical particle to its volume when they want to describe the proportion of atoms at the surface of the sphere:

 Recall these maths formulae: surface area of a sphere = 4πr2 volume of a sphere = 4/3πr3 Ratio of surface area to volume: surface area : volume Substitute values for sphere: 4πr2 : 4/3πr3 Divide both sides of the ratio by 4πr2: 4πr2 : 4/3π r 3 4πr2 : 4/3π r 3 1 : 1/3r Multiply both sides of the ratio by 3: 3 : r Or, put another way: surface area volume = 3   r As the radius, r, of the sphere increases, the surface area to volume decreases, that is, there will be a lower percentage of the total atoms at the surface.

A 3 nm particle has about 50% of all its atoms at the surface, but a particle tens time larger, a 30 nm particle, only has about 5% of all its atoms at the surface.
Since the rate of a chemical reaction is dependent on the size of solid particles used, that is it is dependent on the number of atoms at the surface of the particle, we can easily assume that using solid nanoparticles will greatly increase the rate of a chemical reaction when compared to the bulk material, that is, compared to the solid material reacting but using larger particles.

Rate of reaction is increased by using nanoparticles rather than the bulk material.

Silver is known to kill bacteria, but the cost of using bulk silver for this purpose is high. However, the antibacterial properties of silver are enhanced as the particle size decreases, so nanoparticles of silver can be cost effectively incorporated into medical dressings.

The chemical activity of a substance can be enhanced so much at the nanoscale that a material we usually think of as being unreactive becomes reactive!
We see this in the apparent chemical stability of bulk coal. A lump of coal that you can hold in your hand is quite stable in air, it does not react spontaneously, however, if you suck up very fine carbon powder, such as that used in photocopiers, with a vacuum cleaner then the carbon reacts spontaneously in air and can cause an explosion.

Similarly, when you buy bulk flour at the supermarket you do not expect it to react spontaneously with air because the particles of flour are quite large, but, very fine flour "smoke" particles at the mill can ignite spontaneously in air.

If you add silver as a bulk material such as a strip of silver foil to hydrochloric acid, you won't see bubbles of hydrogen gas given off, there is no apparent reaction. If, however, you grind down the bulk silver foil to produce silver nanoparticles you will find that the silver nanoparticles rapidly react with hydrochloric acid.

Similarly, using nanoparticle-sized catalysts would improve the rate at which the catalyst converts reactants to products because it provides a greater surface area on which the reaction can take place. This means you could use less mass of expensive catalysts, reducing the costs of manufacturing a product. Cobalt nanoparticles are used in the production of adipic acid (hexanedioc acid) which is then used to produce the polymer nylon-6,6 on an industrial scale. It is hoped that the use of nanoparticle catalysts will improve the efficiency and cost effectiveness of future fuel cells.

Since nanoparticles have a greater percentage of their atoms at the surface compared to the bulk material, it follows that nanoparticles have a greater proportion of their mass at the surface compared to the bulk material.
This means nanoparticles are extremely easy to accelerate, scientists say they have negligible inertia, and so nanoparticles are in constant motion. Since particles must collide in order to react, nanoparticles which are in constant motion are more likely to react than bulk materials. It can also mean that nanoparticles will collide with each other more often and stick together due to enhanced intermolecular forces between the particles, resulting in the building up of larger particles.

Having a larger proportion of their atoms at the surface results in nanoparticles interacting more strongly with solvent molecules compared to interactions between bulk material and solvent. So, while a more dense bulk material will sink if added to a less dense solvent, nanoparticles of the same material interact strongly with the molecules making up a solvent so that even very dense nanoparticles can be suspended in a less dense solvent.5

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## Quantum Effects

Nanoparticles are so small that their electrons are confined, resulting in what scientists refer to as quantum effects. At the nanolevel, properties such as melting point, fluorescence, electrical conductivity, and magnetic permeability, as well as chemical reactivity, change as a function of the size of the particle. Bulk gold, for example, melts at 1064oC, but 2.5 nm gold particles melt at the much lower temperature of approximately 300oC. Nanoparticles interact with light in a different way compared to the bulk material so that nanoparticles tend to absorb more solar radiation than the bulk material.

One of the physical properties of bulk gold is its yellow colour.
But, nanoparticles of gold look red!
This is because the motion of gold's electrons is confined when the particles of gold become so small.
This restricted electron movement means that gold nanoparticles interact differently with light compared to larger-scale particles of gold (bulk gold).
Much of the beautiful colour seen in the stained glass of medieval European cathedrals arises from the use of nanoparticles dispersed in the glass.
Gold nanoparticles are being used in medical diagnosis because they selectively accumulate in tumours. Using the special optical properties of these nanoparticles we can get a precise image of the tumour cells, and target these cells for destruction using a laser, while avoiding healthy cells.

Zinc oxide in the bulk material is white, and is the main component in "zinc cream" that has been keeping Australian's noses sun-burn free for decades by absorbing ultraviolet radiation.
Nanoparticles of zinc oxide, on the other hand, are colourless but still retain their ability to absorb UV radiation so they can be included in a wide range of cosmetic products, such as moisturisers. These products can be colourless or various pigments can be added to provide skin-tone shades, and, because the zinc oxide nanoparticles absorb UV light they provide protection from UV radiation.

Similarly, titanium dioxide in the bulk material is a white solid and is commonly found in house paint because of its ability to reflect visible light.
Nanoparticles of titanium dioxide do not, however, reflect visible light or even ultraviolet light, instead they absorb UV light, so these colourless nanoparticles can be used in sunscreen (or sunblock) products which absorb ultraviolet radiation.

Nanoparticles made up of cadmium and selenium are used to produce quantum dots. Quantum dots6 are semiconducting fluorophores, that is, they absorb light at one wavelength and emit light of a different wavelength. The wavelength of the light emitted depends on the size of the quantum dot, a larger quantum dot emits light of a longer wavelength:

 trend in quantum dot size colour emitted wavelength emitted / nm trend in wavelength smaller → → → → → larger 480 nm 500 nm 510 nm 520 nm 565 nm 590 nm 655 nm smaller → → → → → longer
Quantum dots are currectly used by scientists to tag and follow molecules, an important use is in biological systems.
Because quantum dots produce monochromatic light, they have been used to improve the quality of the colour emitted by light-emitting diodes (LEDs) resulting in better television and computer screen displays.
It is thought that in the future quantum dots will be able to increase the efficiency and reduce the cost of future photovoltaic cells.

Bulk material of carbon such as diamond is an electrical insulator, while graphite is conducts electricity in one direction. Carbon nanotubes, long tubes made of carbon but only a nanometre in diameter, are semiconductors, sometimes they conduct electricity and sometimes they don't, which means they could be used to act like switches on computer chips.
Carbon nanotubes are also incredibly strong, stronger than steel. Carbon nanotubes are used in sporting equipment like tennis rackets and bicycles where you need low mass but great stength. These properties also make carbon nanotubes an ideal material for the aerospace industries.

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## Nanomanufacturing

There are two main approaches to nanomanufacturing. These are known as:

• top-down approach
Top-down manufacturing reduces large pieces of materials until they reach nanoscale particles.
Large amounts of material are required to begin the process and can lead to waste if excess material is discarded.

• bottom-up approach
In the bottom-up approach to nanomanufacturing, products are created by building them up from atomic and molecular scale components which can be time-consuming.

The bottom-up approach to manufacturing was first demonstrated in 1989 by a group of IBM scientists who used a scanning probe microscope to move 35 individual xenon atoms around on a copper surface to spell out IBM, demonstrating that it is possible for us to build up nanostructures one atom at a time. It took a whole day to this, so it was a very time-consuming demonstration!
Our current level of manufacturing technology can produce a transistor, a switch that is either on or off, that is about 32 nm, and a computer chip contains about 1,000,000,000 transistors. Scientists have shown that it is possible to make a transistor out of a single atom or molecule by inserting a phosphorus atom into a group of silicon atoms. When a voltage is applied to the phosphorus atom it switches back and forth.
Silicon nanowires can be grown using chemical vapour deposition and gold as a catalyst. As the silicon nanowire gets longer, the gold that was on the surface gets pushed up resulting in this "forest" of what look like trees but are actually gold tipped silicon nanowires (since the nanoparticles are transparent to visible light, this image on the right has been coloured after it was taken to highlight the "tree" effect).

There is much interest in the idea of "self-assembly", a process in which molecular-scale components can be brought together and will spontaneously "self-assemble" from the bottom up into ordered structures.
There a number of "self-assembly" processes that occur naturally.
Snowflakes also form by self-assembly in nature, a minute dust particle acting as a nucleus for water molecules to crystallize around.
Soap bubbles also "self-assemble" as the soap and water molecules organise themselves to form a layer.
In nature there are viruses that "self-assemble" nanoscale particles to produce functional viral particles.
Bacteria "self-assemble" a surface layer, called the S-layer, made out of identical proteins and only 5-25 nm thick. Nanoscientists can use the S-layers to make nanowires which could be used in rechargeable batteries. Compared to bulk material currently used in the manufacture of batteries, nanowires will result in longer lasting batteries that hold more of a charge.
Self-assembled nanoparticles can also be used to stabilise emulsions. These nanoparticles, called Janus particles7, incorporate both hydrophilic (water loving) and hydrophobic (water hating) particles so that when they self-assemble at the interface between water and oil they act as a solid surfactant8. These nanoparticles have been used to produce stain-resistant fabrics. Nanotex is a company that has developed a nanotechnological process to treat fabrics so that they resist stains, and, the treatment does not wash away when you wash your clothes.

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## Safety of Nanoparticles

Are nanoparticles harmful to us or to the environment?
We don't really know...
We do know that nanoparticles are small enough to pass through many of the biological membranes designed to exclude larger particles, but ....
We have been exposed to nanoparticles our whole life, and our parents, and their parents, etc, were also exposed to nanoparticles. Nanoparticles are not new, only our scientific study of them is new. But it is entirely likely that during your lifetime you will be exposed to a greater concentration and variety of nanoparticles than your parents were.

We know that the level of "harm" caused to us by bulk substances depends on a number of factors such as what the substance is made up of, what size and shape the particles are, and how the particles enter the body, but nanoparticles exhibit different properties to the bulk material.

Particles can enter the human body when we eat, drink or breathe, and, because nanoparticles are so small they may also be absorbed through the skin.
Breathing in small particles can irritate the lungs and potentially cause life-threatening diseases such as miners who contract silicosis caused by the inhalation of dust particles containing silica, or smokers contracting emphysema.
But what happens to nanoparticles in the lungs? Are they so small that they can pass straight through the lining of the lungs, or do they stick to the lining and aggregate where they have the potential to do us harm?

If we take in particles with our food and drink, what happens? We know that nanoparticles have a much greater reaction rate than bulk materials, will they speed up biochemical reactions in an unpredictable way? Or can they provide alternative biochemical pathways that result in toxic products?

Nanoparticles will increasingly be ingested via the consumption of agricultural products because nanoparticles hold particular appeal for agriculture, the smaller the particles of the herbicides, pesticides, and fungicides used, the less the mass of these chemicals required would be, lowering the overall cost of agricultural production.
Nanoparticles can also be added, intentionally or unintentionally, to agricultural products via fertiliser. Treated solid waste from sewage treatment plants can be sold as fertiliser. Nanoparticles such as zinc oxide which is commonly used in the cosmetics industry can end up as a contaminant in this product. These particles can adversely affect plant growth. But what impact will they have when the affected plant is eaten and moves on through the food chain?

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## Future of Nanotechnology

We are already seeing increased government and industrial spending on nanotechnology research and development.
Nanotechnology as a science is still in its infancy, there are enormous opportunities for future scientific investigation not only of the properties of nanoparticles, but also the study of how to produce the nanoparticles, how to apply this knowledge to the production of engineered nanoparticles, and to the investigation of the biological and environmental effects of nanoparticles.

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1 The term was coined in 1974 by Norio Taniguchi who referred to this as the ability to engineer materials preceisely at the nanometre scale.

2 You will also see the alternative spelling for metres as meters. In general, it is better to use "metre" when referring to the measurement, and "meter" for an instrument that measures (such as spectrometer or voltmeter) or for a word indicating a specified number of measures (such as diameter).

3 SI is the abbreviation for Système Internationale d'Unités. It is a unified version of the metric system agreed upon by the General Conference of Weights and Measures in 1960 and is used world-wide. 4 In 1967 the 13th CGPM abolished the name "micron" in favour of the SI term "micrometre" (you will also see the alternative spelling micrometer).
You will still see, and hear, people refer to microns.

5 This type of mixture is referred to as a colloidal dispersion, and is a two phase system. The nanoparticles are referred to as the "dispersed phase" and the liquid which we referred to as the solvent should technically be referred to as the dispersion medium.

6 The term "quantum dot" was coined n 1988 by Mark Reed and is often abbreviated to QD.

7 Janus, the Roman god of gates and beginnings, is shown with two faces, one on the front of his head and the other on the back of his head.

8 The term "surfactant" is a contraction of the term "surface active agent" and is commonly applied to the molecules responsible for the cleaning action of soaps and detergents.