The Elusive Nature of Carbon Nanodot Fluorescence: An Unconventional Perspective

The burgeoning interest raised by carbon dots (CDs) is an epitome of the urgency to develop green and biocompatible alternatives to inorganic and hybrid quantum dots. The fast-paced development of synthetic approaches for CDs over the past few years has left many open questions on their characterization. Herein, we further confirm how the standardization of CDs cannot disregard the presence of fluorescent molecular byproducts. On the contrary, we show how the emissive properties of our solvothermally synthesized CDs largely stem from free molecular adducts produced during the synthesis of carbon cores. The presence of these small molecules, not detectable by electron microscopy, could be deceptive for a reliable characterization of the CDs and could lead to an overestimate of their optical properties. Hence, we propose to introduce more bias-free structure-property correlations based on spectroscopy techniques capable of giving direct insights into their structural properties. Through a combination of standard and unconventional characterization techniques, such as fluorescence correlation spectroscopy (FCS) and time-resolved electron paramagnetic resonance (TREPR), we demonstrate how molecular byproducts dominate the emission properties: these are freely moving in the solution rather than decorating a carbonaceous scaffold. Considering carefully the possibility that newly synthesized CDs are heterogeneous solutions will boost the research and optimization of CDs, thereby paving the way to the large-scale production of cheap and biocompatible light-emissive nanostructures.