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.

Hybrid supercapacitor electrodes with remarkable specific capacitance have been fabricated by coaxially coating manganese oxide thin films on a vertically aligned carbon nanofiber array. Ultrathin manganese oxide layers are uniformly coated around each carbon nanofiber via cathodic electrochemical deposition, likely based on water electrolysis initiated electrochemical oxidation. This results in a unique core-shell nanostructure which uses the three-dimensional brush-like vertical carbon nanofiber array as the highly conductive and robust core to support a large effective surface area and provide reliable electrical connection to a thin redox active manganese oxide shell. The pseudo-capacitance of 313 F/g in addition to the electrical double layer capacitance of 36 F/g is achieved by cyclic voltammetry at a scan rate of 50 mV/s and maintains at this level as the scan rate is increased up to 2000 mV/s. A maximum specific capacitance of 365 F/g has been achieved with chronopotentiometry in 0.10 M Na2SO4 aqueous solution with ∼7.5 nm thick manganese oxide. This hybrid core-shell nanostructure demonstrates high performance in maximum specific energy (32.5 Wh/kg), specific power (6.216 kW/kg), and cycle stability (∼11% drop after 500 cycles), which are derived from cyclic voltammetry and galvanostatic charge−discharge measurements. This new architecture can be potentially developed as multifunctional electrical energy storage devices.

Nanoscale carbon dots are promising candidates as inexpensive and sustainable alternatives to conventional materials for fabricating competitive solar photovoltaic devices.

We have demonstrated an easy, economic, one-step synthetic route to water-soluble fluorescent carbon dots derived from the thermal upcycling of urine.