Disentangling the Fluorescence Origin in Flame-Formed Carbon Dots

Abstract

Flame synthesis provides a sustainable and continuous route for producing carbon dots (CDs) with tunable photoluminescence; however, the fluorescence origin in flame-formed CDs remains poorly understood, limiting their rational design. In the present paper, we elucidate the fluorescence mechanism of blue-, green-, and yellow-emitting CDs (B-, G-, and Y-CDs) extracted from particulate matter collected in a rich premixed ethylene flame by tuning residence time and extraction solvent without postsynthetic treatments. Advanced spectroscopic and structural characterization, including steady-state and time-resolved fluorescence, fluorescence anisotropy, transmission electron microscopy (TEM), size-exclusion chromatography (SEC), UV–vis absorption, Raman spectroscopy, and laser desorption/ionization mass spectrometry, reveals the interplay between the CD structure and emission behavior. The CDs exhibit a dual fluorescence mechanism arising from emission associated with molecular-like emissive states and emission from surface-state/π-conjugated domains, with the emission color governed by the relative abundance of the corresponding fluorophores. B-CDs are characterized by a predominant contribution from molecular-like emissive states. Such states may arise from aromatic molecular subunits embedded in, or connected to, the carbonaceous nanostructure. Differently, G- and Y-CDs exhibit a larger contribution from emission associated with extended aromatic domains and, particularly in Y-CDs, with surface state. The redshift emission observed for Y-CDs is attributed to increased surface oxidation, as confirmed by laser-desorption mass spectrometry. TEM, SEC, and fluorescence anisotropy further indicate that the apparent size of all of the CDs increases from solution to solid state and mainly originates from dynamic aggregation of fluorophores into supramolecular assemblies. These findings disentangle the fluorescence origin in flame-formed CDs and establish design principles for tuning emission through flame conditions and surface chemistry, enabling scalable production of tailored CDs for optical and sensing applications.