For once, instead of encouraging you to think big, we’re going to ask you to think small—really small. The field of nanotechnology—the science of controlling materials at the atomic or molecular level—is currently attracting keen interest. New start-ups and scientific discoveries seem to be announced daily, and books and movies on the subject keep popping up. All the excitement about this “weird science” is motivated by the promise of faster computing, better medicines, and “smarter” products. Some aspect of nanotechnology has been used to create products in markets as diverse as the building, electronics, cosmetics, food, and pharmaceuticals industries. Which begs the question, What is the potential impact of nanotechnology on the jewelry industry?

The short answer is that nanotechnology has already penetrated the jewelry industry in many forms, discreetly playing a useful role in a number of applications—especially with respect to precious metals. But it’s really only scratched the surface. There’s much to come!

FIGURE 1: A molecular simulation of a gold slab with an attached organic molecule shows individual atoms and their associated electrons.

The birth of nanotechnology
Nanotechnology was officially “born” in 1959, when renowned physicist Richard Feynman gave a lecture in which he predicted that, sooner or later, engineers and scientists would find ways to build really tiny structures, machines, and devices out of individual atoms and molecules. Not much happened until 1981, the year Nobel prize winners Gerd Binnig and Heinrich Rohrer demonstrated their scanning probe microscope. This remarkable tool allowed technologists to poke and push individual atoms and molecules. The device was used in an early attempt at nanoscale advertising, in which the initials of a well-known computer company were spelled out on a flat surface using a dozen or so atoms of the inert gas xenon.

Up until that time, the common tools of chemistry, such as the analytical balance, spectroscopy, and X-ray diffraction, were not particularly useful for studying structures at the nanoscale, so the availability of the new equipment really got people thinking. Interest in nanotechnology has grown enormously since then, as revealed by a casual search on the Internet. There are many aspects of nanotechnology that are attracting interest, particularly in the fields of biotechnology and chemistry, but here I will focus on the very special role that the precious metals gold, silver, and platinum have in the field. Put the terms “gold” and “nano” into the Google search engine and you’ll be swamped by nearly half a million hits. The reason for this phenomenon is explained below.

Four gold atoms side-by-side
First of all, let’s clarify the term “nanotechnology.” A nanometer is one billionth of a meter. About four gold atoms lined up side-by-side make up a nanometer. We can’t see an atom because it is too small for the properties of surface and color to have any physical meaning, but we can create images showing some aspect of its presence, such as its electron density (figure 1). A troy ounce of gold contains about 95,090,000,000,000,000,000,000 atoms of gold, a number so big that it’s beyond human conception. I will try to put it into perspective. A typical period at the end of a sentence is about 300 micrometers in diameter. This is already pretty small. A little machine the size of a period would be amazing. But this is not nanotechnology. Although a bacterium cell is 100 times smaller than a period, we need to go 1,000 times smaller than even the cell—so small that the nano-thing is to a single bacterium cell as the period is to us. That is the nanoscale. We can broadly define nanotechnology as the business of making, studying, and exploiting useful nanoscale structures.

Why the excitement?
Matter behaves rather differently at the nanoscale. For example, interatomic interactions and factors that are quite unimportant at macro-scales become dominant at the nanoscale. In addition, many material parameters change markedly from those of a bulk material. Part of the excitement about nanotechnology is motivated by the observation that control of matter at the nanoscale is the key to faster computing, better medicines, materials with in-built functionalization (so-called “smart materials”), and new and exciting properties.

Unfortunately, there is a lot of hype around nanodevices and nanotechnology, much of it initially stimulated by certain futuristic predictions of self-replicating nanorobots and now perpetuated by a progression of new startups, some offering hypeware of the most improbable nature. I want to emphasize that at this stage scientists do not know how to design, make, power, and actuate self-replicating nanobots. (Person-ally, I think we have many more important issues in our lives to worry about than this distant possibility.) However, there are many genuine areas in which the application of nanotechnology has led, or will in time lead, to socially significant breakthroughs in the areas of medicine, solar energy, pollution abatement, computing, materials of construction, textiles, domestic hygiene, and biotechnology. For example, the idea of using tiny gold nanoparticles to selectively target and destroy cancer cells is currently receiving a lot of attention, and some therapies are already in the first phase of testing. The U.S., Japanese, and EU governments have earmarked billions of dollars for nanotechnology research—and a commitment that big is always matched by the expectation of meaningful returns.

Building blocks of gold
As mentioned previously, nanotechnology involves assembling individual atoms or molecules in-to useful nanoscale structures. It turns out that this is very hard to do with metals in general because they oxidize rapidly under atmospheric conditions. For example, a nanoparticle of iron becomes a particle of iron oxide rust in only a few seconds. Although some metals can be protected for a while by the formation of a thin oxide film, it’s better to use metals that are inert in the atmosphere—and what finer example is there than gold?

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.

However, gold also offers many of the top-down fabrication advantages of silicon, and it can be processed using electroless or electrolytic deposition, lithography, and etching—techniques that are all familiar to the world’s huge integrated circuit industry. Regrettably, no simple way of making gold nano-wires has been found—yet.

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.

Some nanoscale weirdness
In the bulk form, gold is a soft, yellow metal with a face-centered cubic crystal structure, a melting point of 1,064°C/1,947°F, and excellent electrical conductivity. However, not one of these “facts” necessarily applies at the nanoscale. Similar odd behavior is observed in most other elements and compounds. The new properties and phenomena occurring at the nanoscale are some of the things that the practitioners in the young field of nanotechnology seek to exploit. Here we will have space to explore only a few of the areas of nanotechnology that might impact the jewelry industry, and I will say little about the very significant applications of precious metal nanoparticles in chemical catalysis, biosensors, and the electronics industry.

FIGURE 2: Glass coated with specially aggregated
gold nanoparticles has a pleasant bluegrey
hue and can block heat from the sun.

Strange colors. 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.

This effect has been used to color glass, even as far back as Roman times. However, it is a transmission color; if the same nanoparticles are viewed in reflected light, they will appear orange or green-brown. This gives an interesting color-shift to translucent artifacts colored with metallic nanoparticles.

These colors may also be readily varied by adjusting the shape or state of agglomeration of the metallic nanoparticles. A particularly useful color change is produced when discrete gold nanoparticles agglomerate into groups that resemble bunches of grapes. The color of a suspension or coating of such particles changes from burgundy to inky blue. This phenomenon has been invoked as a colorimetric indicator in home pregnancy tests and in testing for specific genetic sequences. It is currently being researched as the basis for solar screen coatings for windows (figure 2). Some idea of the range of colors possible with gold is shown in figures 3 and 4.

Actually, many other metals besides gold and silver display this type of spectrally selective resonance, too, but in general the resonant frequency lies out of the visible range, in the ultraviolet. Furthermore, gold is one of the very few metals noble enough to survive as a nanoparticle under atmospheric conditions. This serendipitous combination of properties has encouraged its use in a diverse range of niche applications.

Can these strange colors be exploited in a jewelry application? 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. Watch this area for future developments.

FIGURE 3: 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.

Anomalous bulk properties. Bulk gold melts at 1,064°C/1,947°F. On the other hand, the melting point of gold nanoparticles can be as low as 300°C/572°F. This phenomenon is the result of the huge increase in surface area of gold nanoparticles. However, it seems that the exploitation of this property for jewelry manufacturing, such as making gold solder, is not easy. As soon as two molten nanoparticles of gold touch, they weld together and merge. This decreases their surface area and causes them to freeze again.

On the positive side, the strength and toughness of metals can be enormously enhanced if they are made out of nanoscale crystallites rather than the usual micron-size grains. This effect is already widely exploited to make superior ceramics and tungstencarbide-cobalt composites. There is a similar effect in metal systems. Nanocrystalline copper, for example, is as hard as fully cold-worked copper, yet it retains a significant degree of ductility. Full exploitation of this phenomenon has yet to take place in the jewelry industry.

Coatings de luxe. Nanotechnology need not be applied in a bulk material only. It can also be applied within a coating of some kind. Of course, as a general rule, the precious jewelry industry is not keen on coatings, preferring instead that jewelry products be based on a combination of high value bulk materials and excellent design. Certainly, coatings in high wear regions of a piece are especially problematic if their slow removal is associated with a color change. Nevertheless, in low wear products, such as pendants and earrings, nanoscale coatings of colored compounds of precious metals are definitely feasible. As mentioned in a recent MJSA Journal article by Dr. Christopher Corti  (“Breaking Tradition,” July 2004), colored gold coatings are already on the market in a limited way.

Purple gold, AuAl2, may be the best known coating. It can be deposited onto an 18k substrate to yield patterns or swirls of purple that are hallmarkable as 18 karat in their own right. Bulk AuAl2 is relatively soft and brittle, and it should be treated in use as one might treat a pearl. On the other hand, a nanoscale coating of AuAl2 is both durable and attractive. It can be readily prepared by physical vapor deposition (a clean, dry vacuum method in which a work piece is subjected to plasma bombardment to ensure a dense, hard coating) onto a suitably cleaned substrate, and it must be only a few tens of nanometers thick to yield the desired color. The best method seems to be to codeposit the gold and aluminum simultaneously, as this application can be tailored to yield the purple color without any further requirement for heat treatment.

There are other more exotic possibilities in the pipeline. For example, some way to impart scratch-resistance to lustrous polished surfaces of gold or platinum alloys would be of great interest. In general, the various techniques developed to date have focused on increasing the hardness of the bulk alloy, with some attendant complications regarding subsequent manufacture. An alternative line of thought is to apply a nanoscale coating of a hard, transparent compound to the jewelry piece.

Chemical vapor deposition diamond films are one candidate for this application, but there are many practical problems to be solved before this would become generally feasible. However, some lesser-known nitride compounds of the transition metal elements seem to offer interesting prospects for high hardness in a coating that is too thin to materially change the karatage. Unfortunately, the best known compound, titanium ni-tride, has a strong golden color of its own that would mask the color of the underlying precious metal alloy. Time will tell if a hard, transparent alternative can be found.

FIGURE 4: The same gold nanoparticles shown in figure 3 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.

Smart jewelry?
The whole point of nanotechnology is that it is becoming possible to make useful, functional devices that are smaller than anything ever made before. 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.

The future
In this article, I have only skimmed through the subject of nanotechnology and its potential application to the world of jewelry. The emphasis here has been on showing how and why materials and phenomena are different at the nanoscale, and how this affects the properties of the precious metals. Gold, in particular, has a special place in nanotechnology due to its nobility. In addition, it has a variety of unexpected nanoscale properties that are not observed at the macro-scale. The strange and variable colors of gold nanoparticles are particularly interesting and may offer creative possibilities for an adventurous designer.