Despite many efforts focused on the emission origin of carbon dots (CDs), it is still a topic of debate. This is mainly due to the complex structure of these nanomaterials. Here, we developed an innovative method to evaluate the number and spectral characterizations of various emission centers in CDs. We monitored the photostability of a series of column-separated CDs under UV irradiation to obtain three-dimensional data sets and resolve them using multivariate decomposition methods. The obtained results clearly revealed the presence of three different types of emission centers in CDs, including molecular states, aromatic domain states, and carbon-core states so that their single or coexisting appearance was found to be deeply dependent on the reaction temperature. Furthermore, density functional theory and time-dependent density functional theory were used to investigate the electronic and optical properties of some different aza-polycyclic and corannulene molecules as possible polycyclic aromatic hydrocarbons responsible for the above-mentioned aromatic domain states.

Four fluorescent DNA-stabilized fluorescent silver nanoclusters (DNA–AgNCs) were designed and synthesized with differences in lengths of cytosine-rich DNA strand (as the stabilizing agent) and target-specific strand DNA aptamers for adenosine triphosphate (ATP) and cytochrome c (Cyt c). After their nanohybrid formation with graphene oxide (GO), it was unexpectedly found that, depending on the composition of the base and length of the strand DNA aptamer, the fluorescence intensity of three of the nanohybrids significantly enhanced. Our experimental observations and quantum mechanical calculations provided an insight into the mechanisms underlying the behavior of DNA–AgNCs/GO nanohybrids. The enhanced fluorescence was found to be attributed to the aggregation-induced emission enhancement (AIE) characteristic of the DNA–AgNCs adsorbed on the GO surface, as confirmed evidently by both fluorescence and transmission electron microscopies. The AIE is a result of hardness and oxidation properties of GO, which lead to enhanced argenophilic interaction and thus to increased Ag­(I)–DNA complex shell aggregation. Consequently, two of the DNA–AgNCs/GO nanohybrids were successfully extended to construct highly selective, sensitive, label-free, and simple aptasensors for biosensing of ATP (LOD = 0.42 nM) and Cyt c (LOD = 2.3 nM) in lysed Escherichia coli DH5 α cells and mouse embryonic stem cells, respectively. These fundamental findings are expected to significantly influence the designing and engineering of new AgNCs/GO-based AIE biosensors.

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