Dependence of photoluminescence of carbon dots with different surface functionalization on hydrogen factor of water

Abstract

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.

Quenching of Photoluminescence of Carbon Dots by Metal Cations in Water: Estimation of Contributions of Different Mechanisms

“…It was previously shown that the main reason for the change of photoluminescence of CDs obtained by hydrothermal synthesis with the change of pH of the solution, , similar to other carbon nanoparticles–nanodiamonds, is (de)­protonation of surface groups. Deprotonation of surface groups occurs in the pH range from p K a – 1 to p K a + 1, where p K a is the decimal logarithm of the acid (or alkaline) K a index of certain surface groups.…”

“…This means that small aggregation of nanoparticles occurs in water due to the interaction of richly developed surface groups of various nanoparticles with each other. It should be noted that three types of the main chemically active functional groups on the surface of CDs (carboxyl, hydroxyl and amide) at the specified measured pH value are in different states: carboxyl groups are deprotonated (−COO – ), hydroxyl groups are neutral (−OH), amide groups on sp 2 - and sp 3 -hybridized bases are excessively protonated (−NH 3 + ) and neutral (−NH 2 ), respectively ,− (for more details, see section ). It is most likely that the negative zeta potential of CDs is caused by the quantitative advantage of deprotonated carboxyl groups over positively charged groups.…”

“…In addition, the surface photoluminescence caused by local surface states is typical for all CDs . These electronic states are significantly affected by the environment: solvent, pH, , temperature, interactions with ions and molecules in the environment, , etc. One of the applications of such CDs’ properties is the sensorics of the environment .…”