The Investigation of the Dependence of Optical, Luminescent, and Emission Properties of Carbon Nanoparticles on pH of the Medium

Abstract: The effect of pH of the medium on the optical, luminescent, and emission parameters of various types of carbon nanoparticles (CNPs) in aqueous solutions is studied. It is shown that the parameters of the optical absorption and photoluminescence spectra, as well as the value and stability of the quantum yield of emission, significantly depend on the value of pH of the medium and the properties of the functional groups and carbon cores of the carbon nanoparticles. The main contribution to the change in the specified parameters of carbon nanoparticles is made by their functional groups. The strongest and most vivid change in the optical and luminescent parameters of all the types of carbon nanoparticles is observed in the ranges of pH of an aqueous solution of 10–13 and 0.1–3. The change in the parameters of carbon nanoparticles is closely related to the processes of protonation and deprotonation of the functional groups of the particles of the CNPs–COOH, CNPs–OH, and CNPs–NH2 types as well as to photostimulated processes. A growth and a decline in the absorption and emission, appearance and disappearance of absorption bands, change in the symmetry and width of the contours, and also the bathochromic and hypsochromic shifts of the absorption and photoluminescence bands are often observed in the absorption and photoluminescence spectra with the change in pH of the medium of carbon nanoparticles. It is shown that the mechanism of the effect of nitrogen and oxygen heteroatoms of the functional groups on the photoluminescence and quantum yield of emission is associated with the complex interactions of the electrons of the unshared pairs of the N and O atoms with the π-system of the aromatic rings of the carbon core of the nanoparticles. © 2020, Pleiades Publishing, Ltd.