Artifacts and Errors Associated with the Ubiquitous Presence of Fluorescent Impurities in Carbon Nanodots

Fluorescent carbon dots have attracted tremendous attention owing to their superlative optical properties which suggest opportunities for replacing conventional fluorescent materials in various application fields. Not surprisingly, the rapid pace of publication has been accompanied by a host of critical issues, errors, controversies, and misconceptions associated with these emergent materials, which present significant barriers to elucidating their true nature, substantially hindering the extensive exploitation of these nanomaterials. Of particular interest are expedient, bottom-up pathways to carbon dots starting from molecular precursors (e.g., citric acid, amino acids, and alkylamines), although such routes are associated with generation of a ubiquity of small molecular weight or oligomeric fluorescent byproducts. A primary obstacle to progress is the inadequacy of purification in reported studies, an omission which gives rise to misconceptions about the nature and characteristics of the carbon dots. In this work, we conducted a series of carbon dot syntheses using facile hydrothermal and microwave routes employing citric acid (paired with urea or ethylenediamine as a nitrogen source), followed by dialysis or ultrafiltration purification steps. Careful comparison and analysis of the optical properties of the resulting purification products (i.e., dialysate/filtrate versus retentate fractions) affirms the formation of molecular fluorophores (potentially oligomeric or polymeric in nature) during the bottom-up chemical synthesis which contribute a majority of the emission from carbon dot samples. We provide clear evidence showing that the fluorescent impurities produced as byproducts of carbon dot synthesis must be rigorously removed to obtain reliable results. On the basis of our findings, the inadequate purification in many reports calls into question published work, suggesting that many previous studies will need to be carefully revisited using more rigorous purification protocols. Of course, deficiencies in purification in prior studies only add to the ongoing debate on carbon dot structure and the origin of their emission. Moving forward, rigorous and consistent purification steps will need to be uniformly implemented, a tactical change that will help pave the way toward the development of carbon dots as next-generation agents for cellular imaging, solid-state and full-color lighting, photovoltaics, catalysis, and (bio)sensing.