1. What are carbon dots?
  2. How were they discovered?
  3. How do they work and why do they produce light?
  4. Why are carbon dots an interesting research object?
  5. Do they have any medical applications?
  6. How else can they be used?
  7. How are they synthesized?
  8. What studies are conducted in the field

What are carbon dots?

They are carbon nanomaterials, just like graphene or carbon nanotubes, made up of carbon atoms under 100 nanometers. However, while graphene and carbon nanotubes are allotropes of carbon (in them, atoms form two different crystal structures), carbon dots can contain all kinds of carbon bonds apart from those characteristic of diamonds.

Their one defining feature is their ability to absorb and then radiate light, luminescing in the range from blue to red and even in the infrared spectrum.

How were they discovered?

Curiously, it happened by accident in 2004 during research of graphene and carbon nanotubes, which were the next big thing at the time. After one such experiment, the researchers observed luminescence when they were cleaning nanotubes – and that’s how luminescing carbon nanostructures stole the spotlight.

How do they work and why do they produce light?

It’s actually quite uncommon for carbon to radiate light, which is why the exact mechanism behind this effect and the structure of carbon nanoparticles is yet to be discovered. According to some researchers, luminescence has to do with a quantum size effect, while others argue that it is due to molecular compounds similar to organic colorants.

Why are carbon dots an interesting research object?

One of their unquestionable benefits is that they can be made affordably out of almost any material, including juice, citric acid, plant leaves, and other organic substances.

Moreover, the properties of nanoparticles (their size, solubility, the brightness and color of their radiation, etc.) are easy to control not only through manipulating precursors during their synthesis, but also by changing their excitation parameters, such as wavelength.

Thanks to their organic origin, carbon dots are completely eco-friendly, unlike, for instance, perovskite nanocrystals or quantum dots, which contain ions of heavy metals.

Most importantly, however, carbon dots are water-soluble, biocompatible, and have low toxicity.

Do they have any medical applications?

Yes, thanks to their biocompatibility, researchers are considering using them as biosensors, luminescent markers, or targeted drug delivery probes.

For instance, researchers from ITMO have recently synthesized chiral carbon dots (in other words, incompatible with their mirror copies), which is an important milestone because chirality is an intrinsic property of various biological structures, including DNA, RNA, and protein amino acids. This means that chiral carbon dots may be less likely to be rejected by the human body, while at the same time acquiring the capacity to selectively interact with biological objects. Thanks to these properties, they can be used in diagnostics, including that of oncological diseases, as well as to identify DNA defects and deliver treatment to affected cells.

Another research group from ITMO succeeded in creating nanodots that can absorb and radiate light within the infrared range, which is highly useful given that human tissues are only transparent to waves in this range (800-1,000 nanometers). Such dots can help produce high-quality, detailed images of biological tissues, for example, tumors, inside the body.

How else can they be used?

Apart from medicine, carbon dots can be used in biosensorics (e.g., to identify compounds of biological liquids or observe vital processes in living cells), environmental monitoring (detecting impurities in air or water), chemical probing (e.g., to evaluate oil quality or metal corrosion), and food quality control. Their applications also include fluorescent indicators and ink.

Another important area of application is using the optical radiation of these particles in optoelectronics, where they can be applied in LEDs on par with semiconductors thanks to their photostability and brightness.

How are they synthesized?

Both the chemical compounds and the synthesis methods of carbon dots are subject to experiments. The easiest way to synthesize them is by using bottom-up methods based on heating organic molecules. For instance, this technique has produced unique dots that are capable of identifying the pH and solvent polarity of mixed biological liquids. Alternatively, a research group from ITMO and Ioffe Institute has demonstrated that carbon dots can be synthesized from organic dyes placed in the pores of silica microspheres. The suggested method doesn’t require additional processing or cleansing, while different dyes can be used to control particle radiation.

What studies are conducted in the field

These days, researchers are mainly aiming to expand the radiation range to include the near-infrared spectrum, as well as increase the luminescence quantum yield. For example, scientists from ITMO have recently demonstrated that carbon dots’ optical properties change when dimers of organic dyes form inside them. This makes their radiation shift into the red area of the spectrum as its intensity drops.

In another study, the researchers showed that optical centers of bright red radiation can form on the surface of nanoparticles. Carbon dots acquired by heating citric acid in formamide were observed to possess both green and red luminescence, with optical centers forming both inside nanoparticles and on their surface.