Three kinds of nitrogen-doped carbon quantum dots (N-CQDs) were successfully fabricated through a one-pot hydrothermal reaction at 180 °C for 12 h. L-lactic acid served as the carbon precursor, while three phenylenediamine isomers (o-phenylenediamine, m-phenylenediamine, p-phenylenediamine) were employed as nitrogen dopants, yielding samples denoted as OPD-LA, MPD-LA, and PPD-LA. All as-prepared N-CQDs presented uniformly dispersed spherical nanostructures, with average particle sizes of 8.2 nm (OPD-LA), 9.3 nm (MPD-LA), and 10.5 nm (PPD-LA). Abundant surface functional groups, including hydroxyl, carboxyl, amino, and amide moieties, endowed these N-CQDs with outstanding water solubility and tailorable fluorescence emission. The maximum emission wavelengths were centered at 550 nm, 505 nm, and 450 nm for OPD-LA, MPD-LA, and PPD-LA, respectively, exhibiting excitation-independent emission positions yet excitation-dependent intensity. MPD-LA delivered the highest fluorescence quantum yield of 9.59%, and the incorporation of lactic acid significantly elevated the quantum yield of all samples. The N-CQDs maintain high fluorescence intensity and favorable stability within the pH range of 4–11, possessing outstanding salt resistance and stable storage performance for six months. Their fluorescence was effectively quenched upon exposure to Fe3+, with a linear detection range of 10–100 μM and a low limit of detection (LOD) of 1.49 μM. These lactic acid-derived N-CQDs hold great promise as functional fluorescent probes for Fe3+ sensing applications.
Wang et al. (Wed,) studied this question.