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 solid state consistent with the observed photoluminescence in solution is herein proposed. Overwhelming evidence has been collected which 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)-nuclear magnetic resonance (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 size molecular species when removing the solvent to obtain 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. The present study was conducted on CDs obtained from the hydrothermal carbonization (HTC) of a biomass waste (olive wet pomace).