Propanoic Acid-Derived Nitrogen-Doped Carbon Nanoparticles with Enhanced Fluorescence for Cellular Imaging

Carbon nanoparticles (CNPs) have attracted considerable attention for biomedical applications owing to their excellent biocompatibility, tunable optical properties, and low toxicity. In this study, we report a simple, rapid, and cost-effective one-pot bottom-up synthesis of fluorescent carbon nanoparticles using propanoic acid as a novel liquid carbon precursor. Unlike conventional approaches that often require high temperatures, lengthy reaction times, or complex procedures, the proposed method enables efficient nanoparticle synthesis under mild thermal conditions. The use of propanoic acid enabled carbonization under relatively mild conditions, yielding nanoparticles with excellent aqueous dispersibility. Following the synthesis of CNPs using propanoic acid as the carbon precursor, the nanoparticles were treated with ethylenediamine (EDA) as a nitrogen source and surface-passivating agent, yielding nitrogen-doped carbon nanoparticles (NCNPs). The resulting NCNPs exhibited approximately fourfold higher fluorescence intensity than the undoped CNPs. The NCNPs exhibited strong visible photoluminescence with a dominant emission band in the green spectral region (490–540 nm), which is advantageous for cellular imaging due to reduced background interference and improved imaging selectivity. Furthermore, the NCNPs demonstrated high aqueous solubility, cellular permeability, and excellent photostability under the in vitro imaging conditions investigated. The bioimaging capability of the NCNPs was evaluated using human lung fibroblasts and lung cancer cells. Confocal fluorescence microscopy revealed efficient cellular uptake and successful nuclear labeling without observable photobleaching and morphological alterations. These findings demonstrate the feasibility of using propanoic acid-derived NCNPs as fluorescent probes for cellular imaging and suggest their potential for future applications in bioimaging, biosensing, and related biomedical research. Further studies are warranted to evaluate their long-term biological safety and performance.