Nanotechnology and Jewelry

By Roland Piquepaille

Today, we'll look at nanotechnology under an unusual angle: the impact on the jewelry industry. In this long article, "The Weird World of Precious Metal Nanotechnology," published by AJM Magazine (The Authority on Jewelry Manufacturing), Michael Cortie, professor at the University of Sydney, Australia, explains why gold is often used by nanotechnologists. Not only gold exhibits very interesting properties at the nanoscale level, but it's also a bargain when compared to current prices of carbon nanotubes. And gold -- and silver -- nanoparticles can offer a range of unusual colors, suitable for fine jewelry or luxurious coatings. Finally, Cortie envisions smart jewelry made possible through the use of nanotechnology, such as a pendant that could include cell phone capabilities.

Please read the whole article for many more details about the birth of nanotechnology and let's jump to the section explaining why gold is so often used by nanotechnologists. Here are two important paragraphs.

Thousands of technologists have independently arrived at this conclusion. As a result, gold particles, wires, and surfaces are at the heart of much of nanotechnology. At this scale, the inherent softness of pure gold is not an issue, nor is its high intrinsic value. In addition to resistance to corrosion, gold's electrical conductivity and special affinity for sulfur-containing organic molecules are also particularly attractive features. These properties allow chemists to design molecules that can stick onto the gold in a controlled fashion, and then be probed by electrical currents. This permits the bottom-up assembly of quite interesting and promising structures, such as ultra-sensitive biosensors.

It is important to note that the relatively high value of gold is not expected to impede its penetration into the high tech markets. The value of the tiny amounts of gold used in existing or anticipated nanotech products is completely swamped by the overall added value of the product. Manufacturers will use gold when it provides the best technological performance, and they will not be overly concerned by its price. A $20 medical test kit or sensor might contain gold worth only 50 cents, yet it may be this critical ingredient that makes the whole device possible. In any case, gold is far cheaper than the highly touted carbon nanotube, the other material frequently associated with nanotechnology. Single-wall carbon nanotubes cost $400 per gram when in reasonable purity. The cost increases to $1,500 per gram or $46,000 per troy ounce for highly processed carbon nanotubes. Gold is a bargain compared to this.

Now, it's time to look at the unusual colors exhibited by gold nanoparticles.

	"Dispersions of discrete gold nanoparticles in transparent media have an interesting and flexible color gamut that has only recently been exploited for paints and coatings. These colors depend on how the particles are viewed and on their shape. The gold particles in the test tubes above are being viewed in transmitted light." (Credit for picture and legend: Michael Cortie)
	"The same gold nanoparticles shown [in the figure above] are pictured here in reflected light. Contents of test tubes one and three (from left to right) are now a golden-orange. Tube two has become inky-purple, and tube 4 a light purple-pink." (Credit for picture and legend: Michael Cortie)

After the images, here is an explanation.

One of the features of gold and silver nanoparticles is that they possess a range of quite unusual colors. Bulk gold has a familiar yellow color, which is caused by a reduction in the reflectivity of light at the blue end of the spectrum. However, if we subdivide the gold into smaller and smaller particles, there comes a point at which the particle size becomes smaller than the wavelength of incident light. New modes of interaction between the radiation and the gold become prominent, in particular interactions involving electronic oscillations called surface plasmons. When the particles of gold are small enough, they are ruby red in color. This coloration is due to the gold particles' strong absorption of green light, corresponding to the frequency at which a resonance occurs with the gold.

Will these unusual colors be used for real jewels one day?

The jury is still out on this question. Certainly, to be of value in fine jewelry, the karatage of the colored gold should be high. This probably excludes many of the commonly prepared colored glasses as possible materials from which to produce a piece of jewelry. But it is worth noting that, in theory, interesting colors are possible up to about 23 karats. This is because of the high density of gold relative to the various candidate transparent matrix materials. The trick will be to find a matrix to hold the precious metal nanoparticles. However, the availability of gold gilding pastes and paints of very high metal content shows that there is no theoretical limitation that prevents this possibility.

Finally, Cortie looks at a future where we could carry 'smart' jewels.

Will there be a general trend toward integrating some technological devices into items of jewelry? It is certainly becoming possible. Candidate functionalities include bracelets that could record their owner's blood pressure and heartbeat, or a pendant that could include cell phone capabilities. There are problems of hallmarking, of course, and no doubt many would see such items as tawdry. However, a small market already exists for color-change and other novelty jewelry, so it is possible, for example, that an integration of electronic "smarts" with a gold nanoparticle color change functionality might appeal to some markets.

For more information, an extended -- and more technical -- version of Cortie's work has been published in June 2004 by Gold Bulletin under the title "The Weird World of Nanoscale Gold" (PDF format, 8 pages with diagrams, 120 KB).

Source: Michael Cortie, for AJM Magazine (The Authority on Jewelry Manufacturing), March 2005

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• Fashion
  
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• Technology