Surface modifications of carbon nanodots reveal the chemical source of their bright fluorescence
Fluorescent carbon nanodots (CNDs) have drawn increasing attention in recent years due to their bright fluorescence. CND have been investigated as cost-effective and eco-friendly promising nanomaterials for electrooptic and bioimaging applications. CNDs are spherical and core made of sp2 carbon implanted on sp3 carbon backbone, hydrophilic functional groups imparts high solubility in water. The tunable fluorescence and excitation- dependent emission properties make CNDs optical performances more versatile. However, the chemical source stimulating their strong fluorescence has not been yet completely identified. Importantly, the chemical origin of the two absorption peaks (340 nm and 405 nm) observed in the visible range, which remained unclear. In this study, we applied selected chemical modifications to CNDs in order to elucidate the correlation between chemical structure and optical behavior of CNDs. We added acetic acid as a carbon and oxygen source. Interestingly, varying the amounts of acetic acid in the synthesis process produces different effects on the photo-spectra differently. Specifically, at small concentrations, the fluorescence is enhanced and dramatically red shifted from 340 to 405 nm. Comprehensive characterization of the chemical modification (FTIR AND XPS) allows identification of the role of acetic acid in the reaction mechanism leading to the modified photoactivity. Understanding the molecular origin of CNDs’ fluorescence may promote the design and control of effective CNDs fluorescence in optical applications.