Photobleaching and Recovery Kinetics of a Palette of Carbon Nanodots Probed by In Situ Optical Spectroscopy

Carbon dots (CDs) are a family of fluorescent nanoparticles displaying a wide range of interesting properties, which make them attractive for potential applications in different fields like bioimaging, photocatalysis, and many others. However, despite many years of dedicated studies, wide variations exist in the literature concerning the reported photostability of CDs, and even the photoluminescence mechanism is still unclear. Furthermore, an increasing number of recent studies have highlighted the photobleaching (PB) of CDs under intense UV or visible light beams. PB phenomena need to be fully addressed to optimize practical uses of CDs and can also provide information on the fundamental mechanism underlying their fluorescence. Moreover, the lack of systematic studies comparing several types of CDs displaying different fluorescence properties represents another gap in the literature. In this study, we explored the optical properties of a full palette of CDs displaying a range from blue to red emissions, synthesized using different routes and varying precursors. We investigated the photostability of different CDs by observing in situ their time-resolved fluorescence degradation or optical absorption changes under equivalent experimental conditions and laser irradiation. The results about different PB kinetics clearly indicate that even CDs showing comparable emission properties may exhibit radically different resistances to PB, suggesting systematic connections between the resistance to PB, the characteristic spectral range of emission, and CD quantum yields. To exploit the PB dynamics as a powerful tool to investigate CD photophysics, we also carried out dedicated experiments in a partial illumination geometry, allowing us to analyze the recovery of the fluorescence due to diffusion. Based on the experimental results, we conclude that the nature of the CD fluorescence cannot be solely ascribable to small optically active molecules free diffusing in solution, contributing to shed light on one of the most debated issues in the photophysics of CDs.