Gold is an oft coveted material. Countries have been conquered in pursuit of gold, and men have been browbeaten to buy jewelry constructed from it. To the technically minded, however, gold is more than currency, more than a neckalace to show off to your friends. Electronics enthusiasts already know that gold is very practical, found in everything from electronic contacts to thin films on DVDs. As a grad student, I learned to look at gold in a completely different light - as an optical bio-sensor for small distances.

One of the many reasons why biology is such a complex field is that biological interactions occur at such small scales. The distances that are relevant to a biological machine are typically on the order of nanometers, not the kind of thing one can pull out a ruler to measure. Throw into that an aqueous environment and rapid timescales, and most conventional tools are rendered useless in its study. Recently, research scientists at the University of California at Berkeley exploited one of the quirks of small (nanometer) gold particles, the plasmon resonance, to develop what they term a 'plasmon ruler' - a novel tool which they propose to use to study these small distances [ref]. I had the good fortune to be involved in its initial calibration and learn more about the physics that makes it work.

Metals are very conductive materials, their electrons move freely in response to an applied electric field. If the applied electric field oscillates, as it does in light, these free electrons will oscillate as well. Most physical systems have certain frequencies, or resonances, at which they are very efficient at transfering energy. Anyone building a circuit knows to avoid operating near these resonances, or they will likely have to keep a large supply of fuses at hand. When free electrons in a metal are driven at their resonance frequency, or plasmon frequency, they interact with light through what is termed a 'plasmon response'. This effect manifests itself in different phenomenon, depending on the size, shape, and composition of the particle interacting with the light. For very small particles (40-100 nm.), light at the plasmon frequency is reflected. Gold nanoparticles, and most noble metal nanoparticles, are particularly intriguing as their plasmon frequencies are in the visible regime of light - this means that this reflected light can be captured and quantitated with a relatively simple (on the scale of research experiments) light source and a camera sensitive to visible light.

Plasmon frequencies are all well and good, but how can they possibly be used to measure distance? Most oscillators can be coupled to each other - the sum of which can simply be thought of as another oscillator with a different resonance. While a single 40 nm. gold particle will reflect green light, for instance, two gold particles which are placed 5 nm. apart will reflect light in the redder part of the spectrum. The closer the particles are to each other, the stronger the coupling, and the greater the shift in the plasmon frequency. Although this is an oversimplification of the underlying physics, it is not a stretch to say that two gold nanoparticles can be used as a plasmon ruler - just shine light on them and observe the frequency of light reflected back to determine the distance they are apart.

Although this effect is relatively new in the field of biosciences, metallurgists have known about it as far back as the fourth century, as demonstrated in the ancient Roman goblet (a Lycurgus cup) pictured above. The glass contains gold particles with a plasmon frequency in the green. If it is viewed with a light source placed on the outside of the glass (left), the gold particles reflect green light and the glass appears green. If the light source is placed inside the cup (right), the green light is reflected inwards and the glass takes on the character of the transmitted red light.