Carbon dots are an emerging class of nanoparticle with puzzling fluorescence properties. Instead of adding to the wealth of data in the field, I have tried to unravel conflicting reports by probing for issues with terminology, synthesis and characterisation.
As the terms currently used in the field are vague and multifarious, I proposed a new, robust definition set, including the “mixed carbon dots” sub-category which are quasi-spherical with mixed sp2/sp3 bonding. I chose to investigate two of the most cited papers in this sub-field and found both were replicable by control experiments and instead characterise contaminants retained during purification or passivation steps.
The first paper used para¬-phenylenediamine and urea and after hydrothermal synthesis contained high concentrations of small fluorescent molecules. I showed column chromatography fractionation could modulate the exhibited fluorescence of control samples through exposure to silica and solvents. I hypothesised and substantiated the existence of fluorescent colloidal contaminants for the first time using the same synthesis, and showed that this effect prevented dialysis from fully purifying the solution by comparing the retentate and dialysate. I then demonstrated a novel purification method using solvent extraction and dialysis, unmasking a new fluorescence profile.
Finally, I examined a paper which improved their mixed carbon dots by passivating them with amine-terminated PEG1500. By conducting control experiments I showed that the amine-terminated PEG1500 was fluorescent and that the quantum yield improved after thermal treatment, fully replicating the results in the paper absent of any carbon dots. This raises serious doubts as to the point of surface passivation.
These studies indicate that much of the preceding literature in this field is compromised and that the field has been accidentally studying mixtures including fluorescent dyes. I hope that by unravelling these obfuscating layers, the fluorescence puzzle is easier for future researchers to investigate.