Photoluminescence mechanism and agricultural application of carbon dots

Abstract

Due to the prominent characteristics of carbon dot (CDs), such as inexpensive precursors, excellent hydrophilicity, low toxicity, and intrinsic fluorescence, these nanomaterials are regarded as potential candidates to replace traditional quantum dots in some applications. However, there are still a few things that remain unclear about CDs. The photoluminescence mechanism is under debate, and a few hypotheses have been raised, such as quantum confinement effect, surface defects, crosslink-enhanced emission and molecular fluorophores. Additionally, there is no systematic way to tune the fluorescence of CDs, because CD syntheses involve many parameters, and it is not clear how each parameter contributes to CD properties. In this thesis, the photoluminescence mechanisms of polymeric CDs were explored. Additionally, machine learning was used to study the synthesis parameters and make prediction about the emission of CDs. Lastly, the application of CDs in agriculture was explored, and its toxicity to plants was evaluated. In the first chapter, an overview comparing different types of CDs and their toxicity is presented. In the second chapter, multicolor CDs were synthesized and applied in bioimaging of trout gill cells. A few molecular fluorophore candidates were proposed for computational simulations. It was found that their fluorescence shifts when the fluorophores are integrated into the polymer structure of CDs. Furthermore, in the third chapter, a series of analytical tools were utilized to identify the structure of a new blue fluorophore important in a common CD formulation. This fluorophore was found to be nonconjugated and has a different fluorescence mechanism compared to traditional fluorophores. The intermolecular interactions play a significant role in tuning its fluorescence. In the fourth chapter, machine learning was used to study a database of CD synthesis parameters and make predictions about CD emission color and wavelength. It is interesting that when the emission color was incorporated as a feature, the prediction accuracy of the wavelength increased. Lastly, in the fifth chapter, the complexation ability of CDs to copper ion was studied, and it was found that CDs are nontoxic and they can enhance tomato growth at some concentrations.