Here at La Livella Magazine, though, we are vulnerable to the charm of the past. Therefore, for this issue, I thought of dealing with an application of nanotechnology that is more than a thousand years old.
It is the pictures of minuscule robots cutting and sewing DNA, of futuristic-looking electric circuits, of microscopic spaceships capable of attacking single infected cells, and more, that appear when researching the word “nanotechnology” on Google. And, indeed, these examples are emblematic of what, today, scientists are trying to create. The miniaturization of circuits is what allowed the evolution of computers that, in the 50s occupied an entire room, all the way to the smartphone that we carry in our pockets. The use of nanoparticles in medicine allows (and will allow) less invasive diagnostic procedures, drug administration, and removal of tumors. Here at La Livella Magazine, though, we are vulnerable to the charm of the past. Therefore, for this issue, I thought of dealing with an application of nanotechnology that is more than a thousand years old.
Indeed, the first use of nanoparticles goes back to the IV century AD, during Roman times. It is a very precious glass goblet, the “Lycurgus Cup”, decorated with high-relief shapes of Dionysius, his sworn enemy Lycurgus, and the god Pan. However, beyond the mythological tale, the peculiarity of the cup is in the fact that it shines in different colors depending on how it is illuminated. In fact, reflected light makes the cup appear with a sharp green color. In contrast, transmitted light makes it shine with a burning red. This phenomenon is due to the presence of gold and silver nanoparticles scattered inside the glass of the goblet. This is because gold nanoparticles absorb blue light, letting red light pass through, whereas silver ones reflect green light.
However, this explanation is clearly incomplete. To say which particles are responsible for the various colors says nothing about how the phenomenon actually works, nor it explains the dimensions of the nanoparticles: why is it necessary for them to be nano to produce such colors?
First, we should establish what ‘nano’ means. This designation stands for ‘billionth’. Therefore, a nanometer means a billionth of a meter, that is a thousandth of a thousandth of a thousandth of a meter, or, equivalently, a millionth of a millimeter. To fully understand how small a nanometer is, consider the thickness of a human hair, which is somewhere near one hundred thousand nanometers. A nanoparticle is a particle of the size of a few tens or hundreds of nanometers, meaning about one thousand times smaller than a human hair.
Having clarified what scales are in play, we can question why matter, when assuming such small dimensions, manifests so peculiar optical properties. In order to answer this, we need to consider that light is a wave (electromagnetic) and that, as one, it has a wavelength, given by the distance of two consecutive peaks. It so happens that light, meaning the one responsible for the colors that we humans can see with the naked eye, has wavelengths that go from five hundred nanometers (blue light) to eight hundred nanometers (red light). When a light beam shines on a particle whose dimension is similar or inferior to its wavelength, we begin noticing ‘special’ optical effects.
Indeed, the electrons contained in the nanoparticle begin to wobble coherently and collectively, following the wave effect of the light passing through them. It is as if the particle’s electrons all flutter together, letting themselves be carried away by light. If the particle was bigger, the electrons would not swing together, and this collective effect would be lost. The amplitude of the electron’s wave motion is determined by the deployed wavelength. Not only, but the wavelength that maximizes this phenomenon can also vary by modifying the geometry of the nanoparticle, its dimensions, and the nature of the atoms composing it. In this way, we can obtain nanoparticles that absorb red light, as well as green, yellow, or blue, creating the shimmering effect we can appreciate, for example, in Lycurgus Cup. The same phenomenon is visible on the stained glass of some Medieval churches. So, next time you happen to visit a church with windows that appear exceptionally bright and iridescent, you can explain to the person who accompanied you that it is due to the effect of the nanoparticles scattered in the glass – even if it were not the case for that specific window, who could ever lecture you otherwise?