Metamorphosis of Ruthenium-Doped Carbon Dots: In Search of the Origin of Photoluminescence and Beyond

Carbon dots (CDs) are known to have a wide range of applications, yet our understanding of their structures and chemistry remains uncertain because of their highly complex nanostructured framework. Here we attempt to elucidate the molecular structure and intrinsic mechanisms governing photoluminescence (PL) of CDs by trapping seven visibly distinct colored intermediates that evolved during pyrolytic metamorphosis of citric acid with dopant Ru(III). The “excitation-dependent” PL of doped CDs, Ru:CDs, can be tuned by ethylenediamine (EDA), yielding “excitation-independent” highly fluorescent nanodots, Ru:CNDEDAs. To mimic the optical and chemical properties of CDs, we devise a unique model cocktail comprising multiple fluorogenic molecules that truly supports the existence of chemically switchable conjugated moieties in CDs. We propose a plausible molecular level framework of CDs on the basis of spectroscopic findings and existing literature regarding thermal decomposition of CA. The PL of chemically engineered Ru:CNDEDAs is quenched efficiently by photoinduced electron transfer (PET) phenomenon. By exploiting the PET process, we also develop an important sensing platform for quantifying toxic and carcinogenic quinone derivatives in live HeLa cells that can be used for drug screening. Moreover, the distribution pattern of these photoluminiscent nanodots in HeLa cells is studied to demonstrate their utilities as endosomal markers.