L-Lactic Acid-Based N-Doped Carbon Quantum Dots with Phenylenediamine Isomers as a Nitrogen Source for the Highly...

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.