The fluorescent nanoparticles of carbon, known as carbon nanodots may be biocompatible and cheap alternative to the quantum dots in the detection of chemical substances present in living cells. Received to date carbon dots typically emit light in the range from blue to green.In the new study, the researchers reported about receiving carbon dots that emit over the entire range of visible light.Minjie Li and Sean Xiao-An Zhang from Jilin University used multicolor colored carbon points for pH measurements in solution and inside dots. The researchers note that received carbon dots can become effective as well as chemical sensors for application in living cells.

Oxygen and nitrogen containing functional groups on the surface of carbon nanoparticles allow them to emit the light  from blue to intensely red spectrum during the absorption of certain wavelengths of light (wave numbers are indicated in red).Researchers believe that carbon dots may be more suitable for biological studies than quantum dots, as they are more soluble in water than the quantum dots and do not contain toxic heavy metals such as cadmium and plumbum. In order to obtain different colors the researchers were changing sizes, crystalline structure and surface properties of the particles. However, Li noted that such features change even such option as radiation intensity and frequency, but also the rate of these particles penetration in living cells, which may affect the reliability and reproducibility of the results obtained with heterogeneous by size particles.

To avoid complexity in the use of particles with different sizes, the researchers from the University of Jilin wanted to obtain bright intense radiation of different colors from single carbon dot. To solve this task, the researchers heated mixture of diethylamine and chloroform, separating formed during this process nanoparticles. Heating the mixture for one hour allowed to get blue carbon dots with diameter from 1 to 3 nanometers, and during the heating for 60 hours – they obtained colored quantum dots with a diameter from 2 to 4 nanometers. Recent nanoscale objects emit light throughout the range of visible area - they emit a blue light with a wavelength of 470 nm when excited by light with a wavelength of 390 nm and a red wavelength of 670 nm upon the absorption of light with a wavelength of 650 nm.

The researches analyzed and compared chemical compound of carbon dots showing the blue and multi-color fluorescence using infrared spectroscopy. It has been found that the surface of dots, which give fluorescence in different colors , contains functional groups with the double bonds C = O and C = N. Removal of these groups by treatment with sodium borohydride led to decrease of emission intensity in some areas. This made it possible to assume that exactly functional groups are responsible for emission of carbon dots in different areas of the spectrum.On the basis of carbon dots emitting across the whole area of visible spectrum, researchers obtained the sensors on pH, which were used for measuring pH in model solutions and human tumor cells.  

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