For the past several years, biologists have watched proteins and other molecules travel in and around cells and tissues due to the invention of tiny little fragments of matter known as quantum dots. A blast of light makes these fragments glow different colors depending on their size, allowing the masterpiece of life to become visible and coded with color. The current quantum dots available, however, contain poisonous Cadmium and hence are not suitable from the point of view of biology. But now, researchers at Clemson University (SC) have come up with a revolution kind of in chemistry and have made quantum dots from carbon which is a new step in quantum dot-tery (J. Am. Chem. Soc. 2006, 128, 7756–7757).

Seen on the large scale, we can easily find some materials that are fluorescent which means that they give off light when a different colored light is put on them. For example, the light from a fluorescent light bulb is originally ultraviolet which is not apt for humans. But, after it hits the phosphors that coat the inside of the glass of the bulb, the UV is absorbed and the light emitted thereafter is what we can see and use.

When the above absorption is taking place, some electrons, which are in a ground state, move into a higher energy excited state. This leads to an empty spot called a hole where the electrons had been. The electron and the hole can both move from place to place, but as soon as they face each other again, the electron comes back to its original ground state, fills the hole and the excess energy is emitted in the form of light.

The color of the light so emitted is respective to the energy difference between the electron's excited state and its ground state which in normal materials are properties of the materials themselves. So, if a material fluoresce red it will always glow red and not any other color. To change the color of the light emitted, use of a different fluorescent material is required.

However, things are not the same at the nanoscale. Quantum dots are so small that the size of the dot determines the energy levels that electrons can obtain. Thus, the size and not the material control what colors are produced.

A mention of quantum dots brings to mind those made from cadmium selenide (CdSe) which have been researched extensively to obtain quantum fluoresce.

The quantum dots made from carbon have a different mechanism than their CdSe counterparts, as explained by Sun Ya Ping, one of the researchers at Clemson University. This signifies that in the case of CdSe, the semi-conducting nature of the material decides how light is made, but for carbon something else does.

To make carbon quantum dots, laser ablation of graphite power and cement was used to make carbon particles which made the particles form aggregations (and hence no fluoresce). The particles were than coated with simple organic molecules which passivated the surface which led to bright luminescence.

The simple organic molecules used, such as diamine-terminated oligomeric poly(ethylene glycol) and poly(propionylethylene-imine-co-ethyleneimine), did not have any luminescent activity alone but when put along with the carbon dots, the carbon dots became luminescent.

Probably, the best thing of the carbon quantum dots is that they do no blink. This means that unlike their other counterparts, they do not turn on and off at intervals which is a major problem. This discovery could be used in medicine where cadmium-based quantum dots are unsuitable. Other uses include LEDs, lasers, and lighting.