Aminolysis-assisted hydrothermal conversion of waste PET plastic to N-doped carbon dots with markedly enhanced fluorescence

Abstract

While the possibility of waste bottles’ PET conversion to fluorescent materials has been recently demonstrated, optimization of the conversion protocol based on rational material design may greatly enhance key characteristics of the upcycled material. In this study, conversion of PET to fluorescent nanomaterials by means of hydrothermal (HT) treatment of PET and products of PET pyrolysis, glycolysis, and aminolysis in the presence and in the absence of an oxidizing agent was systematically investigated to compare fluorescent properties of HT products. Among studied conversion routes, the direct HT conversion of PET or a combination of the HT treatment with the PET pre-treatment by either pyrolysis or glycolysis resulted in non-fluorescent of weakly fluorescent products, whereas aminolysis of PET bottle plastics followed by the HT treatment of the aminolysis products increased dramatically, up to tens to hundreds fold, the fluorescence of the products. Admixing of several percent of an oxidant, H2O2, to HT mixture further enhanced the product fluorescence yielding fluorescent nanoparticles (nitrogen-doped carbon dots, NCDs) with 25.3% conversion yield and the quantum yield of 9.1%. Based on 1H and 13C NMR, FTIR, XPS, and Raman spectroscopy data, the mechanism of NCDs formation was proposed. The N-doped carbon dots obtained by waste PET upcycling were used for detection and quantification of Cu2+ ion at ppm and Fe3+ ion at ppb concentrations.