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The Lycurgus Cup

Nanoscience/nanotech is a hot research topic these days, in fact red hot, and it should really be called Red Sunset Hot. Why? Well, let’s see …

We have all seen orange red-sunsets, red-sunrises, the midday yellow sun, and the blue sky. We have all been taught at Pui-Ching why these things have specific colors.

Here is a photo of sunset at Newport Beach, CA one December day, and the orange-red color is obvious. Do you still remember why it looks the way it looks?

We learned at Pui-Ching that sunlight is scattered and only gives off red/orange color when the sun is near the horizon, Remember? Well, what we did not learn was that nano-size particles in the atmosphere scatter away most of the other colors with high frequencies, and only red and orange colors with low frequencies reach our eyes, or in the lens of the camera. The scientific term for this selective scattering is called “Rayleigh Scattering”, the intensity of which depends on the particle size, frequency of the photons, and the angle of scattering and the refractive index of the particle.

All these mumbo-jumbos are to say that nano-size related phenomena and science have been around since time began, not to mention all the single-cell and/or simple bacteria are “nano” in size. In this case it means nanometer in size, or 10-9 meters. You may say, hey, no fair! This is a natural phenomenon. True, air (O2 and N2) molecules are natural light scatterers, but industrial pollutants, smog particles, smoke and the like are not.

Let us fast forward from the beginning of time to the 4th century. It was Roman time, and we have here the Lycurgus cup (after King Lycurgus), a glass cup housed in the British Museum. The cup appears green when it is illuminated from the outside. When it is illuminated from the inside, it glows red.

It turns out that the red color is due to very small of gold/silver alloy powder (~70 nanometer, nm, in size. Nano = 10-9 .) mixed into the glass. Bacteria are about 100 times bigger than these particles. When light passes through the glass, it is again scattered, and this is time it is due to the “Mie scattering”, which becomes important when the particle size is smaller than the wavelength of interest, but not as small as gas molecules or atoms. Certain colors (different wavelengths) of the light are absorbed as light strikes the particles, as a result of something call “ surface plasmon resonance”, SPR.

Surface plasmon is the collective oscillation wave of the free electron gas in the particle, and the oscillation frequency is size and refractive index dependent. It so happens that the Au particles of ~70 nm in size absorb most of the other colors (absorption peak around 520 nm), and only red light passes through the cup, therefore, the cup appears red. In Roman time, they knew how to make these fine Au particles, which are tough to make to be uniform in size. One cannot see them with naked eyes. Grinding Au coins would not yield nano-size Au powders readily.

So it appears that nanotech has been around for a long long time.

Here is another example of color glass in England, “Labors of the Months” Norwich, England, ca. 1480. The ruby color is probably due to embedded gold nanoparticles.

What good are these metal nanoparticles other than giving colors to glass and whatnots? One of the interesting applications is to use these particles as sensors to detect DNA and other bio-related materials. The way it works is when DNA strands are attached onto these particles, the refractive index of the particle changes, and so thus the surface plasmon resonance frequency, and the color of the colloid.

Here we have a gold color colloid with fine Au powder before DNA strands are linked to the Au particle. After attaching DNA strands, the resonance frequency changes, and the color turns blue. In this way, nano-particles serve as sensors to detect, say, very very small biological matters by just looking at the color of the colloid (see the following figure).

Schematic of DNA linked to Au nanoparticles, causing the frequency of SPR to shift, and the coloration.

So is nanotechnology going to lead to a new industrial revolution? I don’t know, only time can tell.

I can say that when Professor Rabi at Columbia University predicted and observed nuclear magnetic resonance, NMR, in 1937, it was perhaps purely for the interest of physics. He did not know the impact that was to come in clinical medicine as one of the most powerful medical diagnostic tools.