In this study, the optical and structural properties of carbon dots (CDs) synthesized using a hydrothermal method were investigated. CDs were prepared from various precursors such as citric acid (CA), glucose, and birch bark soot. The SEM and AFM results show that the CDs are disc-shaped nanoparticles with dimensions of ~7 nm × 2 nm for CDs from CA, ~11 nm × 4 nm for CDs from glucose, and ~16 nm × 6 nm for CDs from soot. The TEM images of CDs from CA showed stripes with a distance of 0.34 nm between them. We assumed that the CDs synthesized from CA and glucose consisted of graphene nanoplates located perpendicular to the disc plane. The synthesized CDs contain oxygen (hydroxyl, carboxyl, carbonyl) and nitrogen (amino, nitro) functional groups. CDs have strong absorption in the ultraviolet region in the range of 200–300 nm. All CDs synthesized from different precursors displayed bright luminescence in the blue-green region of the spectrum (420–565 nm). We found that the luminescence of CDs depended on the synthesis time and type of precursors. The results show that the radiative transitions of electrons occur from two levels with energies ~3.0 eV and ~2.6 eV, which are due to the presence of functional groups.

Carbon dots (CDs) demonstrate unique luminescence properties due to conjugated π domains of the sp²/sp³ hybridized carbon core, quantum-size effects, and passivation of CD surfaces by functional groups. In this work we present hydrothermal synthesis of nitrogen-doped and boron-doped CDs, their morphological, structural, optical characteristics, and luminescent properties. The CDs are first synthesized from trans-aconitic acid. X-ray photoelectron and infrared spectroscopy data indicate that prepared CDs contain nitrogen and boron atoms. The photoluminescence quantum yield of N-doped and B-doped CDs is 26% and 9%, respectively.

Herein, we report the characterization of two types of luminescent carbon dots (CDs) synthesized by the hydrothermal treatment of citric acid and trans-aconitic acid by using ammonia solution as a nitrogen dopant. The lateral size range of nanoparticles for CDs lies in the range of 3-15 nm. The intense blue photoluminescence (PL) was emitted by the CDs at around 409-435 nm under the excitation of 320 nm. The PL quantum yield of the synthesized CDs ranged from 26.4 to 51%. Our results of the structural and optical properties of CDs imply that molecular fluorophores are an important part of the structure; in particular, the main contribution to the PL is carried by the fluorophores based on citrazinic acid derivatives, which formed during the synthesis of CDs.

The pH dependences of the photoluminescence of carbon dots synthesized by the hydrothermal method with polyfunctional and monofunctional (carboxylated and hydroxylated) surfaces have been studied. As a result of the analysis of the obtained photoluminescence and absorption spectra of aqueous suspensions of all studied types of carbon dots at different pH values, a significant effect of the acidity of the environment of nanoparticles on their optical properties was found. It has been found that the greatest changes in the spectral characteristics of absorption and photoluminescence of carbon dots with COOH, OH, and NH₂ surface groups appear in the pH ranges of 2-5 and 8-12. The obtained results are explained are explained by the processes of protonation/deprotonation of the surface groups of carbon dots. Keywords: carbon dots, surface functionalization, absorption spectroscopy, photoluminescence, hydrogen index, deprotonation.