Are "Carbon Dots" Always Carbon Dots? Evidence for their Supramolecular Nature from Structural and Dynamic Studies in Solution and in the Pure Solid

A model for the morphology (size, shape, and crystallinity) of carbon dots (CDs) in the solid state consistent with the observed photoluminescence in solution is proposed herein. Overwhelming evidence has been collected that links the data coming from solid-state analysis (high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS)) to that of solution (pulsed-field gradient (PFG)-NMR spectroscopy, time-resolved fluorescence anisotropy (TRFA), and steady-state/time-resolved fluorescence), allowing the establishment of an overall structural model for CDs. According to this model, the so-called carbon dots, observed under HRTEM imaging, are in fact supramolecular organized structures dynamically assembled from small to medium-sized molecular species when the solvent is removed to give the solid form. In this way, the imaged nanoparticles (TEM/AFM) are not covalently bound entities formed during the synthetic process, but instead supramolecular entities formed by noncovalent interactions. These particles, if at all present in solution, have the form of loose associations of relatively small molecules. This study was conducted on CDs obtained from the hydrothermal carbonization (HTC) of a biomass waste (olive wet pomace). A model unraveling the supramolecular nature of the so-called carbon dots (CDs) is proposed, in which small to medium-sized molecular species present in dilute solution dynamically assemble into supramolecular organized structures when highly concentrated or in the solid state. (HR-TEM: high-resolution transmission electron microscopy, PFG-NMR: pulsed field gradient-NMR, TRFA: time-resolved fluorescence anisotropy).image