Functional Groups Accessibility and the Origin of Photoluminescence in N/O-containing Bottom-up Carbon Nanodots

Chemical functionalization of carbon nanodots (CNDs) offers a valuable opportunity to tailor multifunctionality in these nanocarbons, by engineering the composition of their molecular surface. Therefore, it is important to elucidate the type and amount of CNDs functionalization to be able to design their properties accurately. CNDs are often functionalized through amide coupling without validating the degree of functionalization. As a measure of functionalization, the amounts of primary amines via Kaiser test (KT) or imine reactions of the bare CNDs is often considered. However, this may lead to overestimating the degree of functionalization obtained by the pure amide coupling due to different reaction mechanisms and involved intermediates. Herein, four different CNDs prepared by microwave-assisted synthesis from arginine or citric acid with varying amounts of ethylenediamine are presented. We resorted to a combination of physicochemical methods to provide elemental, structural, and optical information. By that, we developed a method to quantify the degree of functionalization by amide coupling and show that the functionalization is lower than one would expect. Comparing experimental optical features of the CNDs with different computed model systems further allows us to provide a more advanced vision of structure-property relationships in these still elusive nanocarbons.