1 research outputs found
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