Applications of photoluminescent semiconductors in biomedicine.

Hello community, this time I have the opportunity to write about a new kind of semiconductor nanomaterials named carbon dots, which has a lot of applications especially in biomedicine. In this post, I will explain you what carbon dots are and why their different properties make them very useful.

What are carbon dots?

Carbon dots (CDs) are colloidal semiconductor crystals form by hundreds or thousands of atoms arranged into a crystalline structure. The essential characteristic of CDs is that the electrons that compound them are forced to be confined in the three dimensions, which generates diverse quantum phenomena.

Carbon dots have drawn increasing attention in recent years because of their exceptional advantages such as high optical absorptivity, chemical stability, biocompatibility and low toxicity, what is even better the different properties such as fluorescence and photostability can be modified just changing the material used as precursor or the conditions of the synthesis.

The superior properties of carbon dots distinguish them from traditional fluorescent material and their analogous quantum dots, and make them promising candidate for numerous exciting applications such as bio-imaging, medical diagnosis, catalysis, and photovoltaic devices.

How can we change carbon dots properties?

The properties of carbon dots are related to superficial functional groups present on the nanostructure like C=O and C=N and to the size of the nanomaterial. So as bigger size the fluorescence shifts to red in the visible spectrum and vice versa, this can be appreciated in the figure 2. As I mentioned above just changing one of this characteristic you can obtain carbon dots with different properties. Personally, I have obtained carbon dot from ionic liquids through microwaving and the different size of nanostructure is easily obtained just changing time of exposure to microwave irradiation.

Why are carbon dots applicable in biomedicine?

In order to be use for biomedical applications, carbon dots need to have superior biocompatibility, photostability and low cytotoxicity. This properties have been studied several times using carbon dots obtained by different methods of synthesis such as arc discharge, laser ablation, electrochemical oxidation, supported synthesis, or microwave treatment and so on, and using different substance as precursors such as Ionic liquids, carbohydrates, various bulk carbon materials and others substances.

The fact is that carbon dots obtained in all cases have reported excellent stability, which is observed trough the study of size distribution using techniques like dynamic light scatting (DLS) and transmission electronic microscopy (TEM), where even after a week the size distribution of nanostructures was almost unchanged which means that carbon dots almost do not degrade. Moreover, photoluminescence intensity of carbon dots remain stable at high ionic strength and under change of pH (pH 4-11). carbon dots exhibit better photostability compared to CdSe/ZnS quantum dots, it is demonstrated observing the photoluminescence intensity after one hour of UV irradiation where quantum dots quickly quenched due the photobleaching while CDs preserve around 86% of the initial intensity of photoluminescence, which means CDs are extremely stable. Remarkable photostability is attributed to the composition of the surface`s structure and the general composition of carbon dots. Finally, the cytotoxicity of carbon dots have been investigated by the standard MTT assay. Obtaining that more than 92% cells survive for most of the cases after incubation with carbon dots during 24 hours, even changing concentration of carbon dots from 1 to 8 percent has similar results indicating the low cytotoxicity of CDs. These properties demonstrate that CDs have great potential for biomedical applications including bioimaging and biosensing.

Carbon dots in Bioimaging.

In vitro and in vivo imaging provide abundant information on the imaging ability, distribution and cytotoxicity of probes in cell. Taking it in count has been demostrated that carbon dots can be used successfully as fluorescent probes for in vitro and in vivo bioimaging of different kind of cells such as HeLa, human neural stem cells, HepG-2 and others obtaining excelent results. Carbon Dots also have been used as a fluorescence contrast agent for imaging microorganisms such as E. Coli, nematodes and even animals like mice. An advantage of carbon dots is that mainly enter into cells through endocytosis, and concentrate in the cytoplasm which has allowed scientists to study the structure, behavior and others properties of different microorganisms and cells that in other way would be harder or even impossible due to the translucent nature of them. In my own experience, I have used carbon dots as biomarker for bioimaging nematodes (figure 4) and using a microscope of epi-fluorescence it is easier to see all internal parts of the nematodes body what in most of the cases is very useful to determinate the species to which they belongs.

Carbon dots in Biosensing.

The fascinating photoluminescence, tunable organic surface functionalities and high stability of carbon dots allowed developing a wide variety of biological-chemical sensor. As biosensor CDs can be applied for monitoring concentration of cellular Cu²⁺, Hg ²⁺, glucose, biological thiols and others. This kind of sensor also can be applied for detecting DNA, vitamins, carbohydrates and proteins. However, to the best of knowledge, the use of CDs for monitoring and detection of different kind of substance remains at an early stage of investigation.

Carbon dots have proven to be novel nanomaterilals that have a large number of applications not only in the area of biomedicine but also in those of optoelectronics and photocatalysis. Therefore, its unique properties will allow us to develop new technologies and improve existing ones.

I hope in other opportunity to expand the information about the most recent applications of this type of nanomaterial and I hope that this post will be useful for you, greetings.

References.

1. Carbon quantum dots
2. Fluorescent Carbon Dots for Bioimaging and Biosensing Applications
3. Carbon Dots for Bioimaging and Biosensing Applications