In recent years, nanomaterials (defined as materials < 100 nm in a single dimension) have garnered a significant interest for the development of novel applications in the physical and life sciences. One such class of nanomaterials are carbon dots, a relatively new member in the carbon allotrope family, which possesses versatile optical properties rendering it attractive for sensing, catalysis, bioimaging and drug delivery applications, to name a few. Their ultra-compact size, low cytotoxicity, resistance to photo-bleaching/blinking and tunable photoluminescence, combined with simple, environmentally-friendly and low-cost synthesis, makes them ideal candidates for study. We synthesize carbon dots via bottom-up synthesis methods, with simple organic precursors (e.g. citric acid) as the carbon source. We passivate the surface of our carbon dots to achieve high fluorescence quantum yields. Moreover, our work focuses on trying to elucidate the fluorescence mechanisms in carbon dots, which remain a subject of debate. Finally, we exploit their optical properties in order to design multifunctional materials as chiral, pH or temperature sensors, as well as heterogeneous catalysts for green energy applications.