Blue-Emissive Nitrogen-Doped Carbon Dots for Picric Acid Detection: Molecular Fluorescence Quenching Mechanism

We report a molecular fluorophore-based concept for the first time, to explain the quenching mechanism of the blue-emissive N-doped carbon dots (NCDs) exhibiting molecular fluorescence (quantum yield = 37%), synthesized through microwave-assisted pyrolysis of malic acid and urea (in 1:3 mole ratio), which has been devised as a fluorescent probe for the sensitive and selective detection of picric acid (PA). A linear Stern–Volmer plot for the fluorescence quenching of NCDs in the PA detection has been observed in the concentration range of 0–1.6 μM, and the limit of detection (LOD) is found to be 33 nM (7.56 ppb). Furthermore, molecular fluorescence in NCDs has been realized to originate from a putative pyridinic (2-pyridone moiety)-type molecular fluorophore, which has been manifested by the inferences drawn from HRMS, 1H NMR, 13C NMR, and XPS studies. The chemical structure of the molecular fluorophore was further validated by correlating the simulated absorption and emission spectra of the molecular fluorophore generated by TD-DFT, with those obtained from NCDs’ experimental data. The underlying photophysical property involved in the simultaneous occurrence of FRET and ET quenching mechanism has been illustrated based on the molecular fluorophore, along with the conventional approach. Here, the role of the molecular fluorophore in the NCDs has been demonstrated as a donor in the ET process involving the NCDs–nitroaromatics pair, where the acid–base interaction between the acceptor (nitroaromatics) and donor has been considered as the essence of the ET process.