Study of the reactivity and properties of fluorescent carbon dots

Carbon dots (CDs) are carbon-based nanoparticles that display several advantageous optical and electronic properties. Amongst these are included a high photoluminescence, broadband optical absorption, low toxicity, simple and low cost production, photostability, photo-chemical stability, chemical inertness and good water solubility. CDs can be used for several applications such as LEDs, bioimaging, sensing and biosensing, photocatalysis, photodynamic therapy, among others. One of the gaps in the literature regarding CDs is information about the environmental impact caused during their life cycle. Given that the majority of the impacts originated during the life cycle of a nanomaterial result from their synthesis, a life cycle assessment (LCA) was made for the synthesis of CDs. This was done considering the most commonly used synthetic strategies while employing citric acid (an extremely common precursor) with the ocasional addition of urea as precursors. It was observed that in general, when the functionality of the CD is considered (under the form of the quantum yield of fluorescence, an important parameter for CDs) and used as a base for the functional unit of the study, synthesis of CDs based in hydrothermal treatment cause the most pronounced environmental impacts. Furthermore, the addition of urea, which causes a great increase in the CD quantum yield of fluorescence, largely diminishes the relative environmental impact associated to the synthesis of the particle. Recently, some authors reported that some of the photoluminescence previously attributed exclusively to CDs results from molecular fluorescent by-products (impurities) produced during the CD synthesis. A CD, obtained through the microwave-treatment of an aqueous solution of citric acid and urea, and the resulting impurities, were characterized using a range of techniques including HR-TEM, AFM, XPS, FT-IR, UV-Vis absorption, fluorescence and ESI-MS. The role of these fluorescent by-products in the fluorescence of CDs was studied in the presence of nitromethane (an electronwithdrawing molecule) and diphenylamine (an electron-donor). It was observed that, when present together in the same solution, the CD and the impurities do not behave as two separate species. The result from the co-existence of these components is more than just a simple additive phenomenon of their properties. Instead, they display a synergistic behaviour in which the presence of the impurities affect the optical properties of the nanoparticle and vice-versa. Lastly, the use of CDs for catalytic applications was studied. The epoxide ringopening reaction is an important preliminary step that theoretically can be followed by several reactions, such as conjugation with CO2 or aminolysis reactions with aminated FCUP Study of the reactivity and properties of fluorescent carbon dots IX compounds. The capacity of a 4-aminopyridine-based CD favoring the ring-opening in a model epoxide (propylene oxide) was assessed and evaluated by RP-HPLC-DAD and fluorescence studies. Additionally, the possibility of this ring-opening being followed by an aminolysis reaction when in the presence of aniline (aminated compound with an NH2 group) was also studied. The effect of the CD presence in the reaction outcome was evaluated by GC-MS studies.