Green synthesis of eco-friendly fluorescent carbon dots from Polyalthia longifolia leaves for enhanced photoactivated antimicrobial activity and biocompatibility

The threat of antibiotic-resistant pathogens has created an urgent demand for novel, and sustainable antimicrobial agents. Carbon dots (C-Dots), due to their unique physicochemical properties and ease of surface functionalization, have emerged as promising candidates in nanomedicine. This study focuses on the green synthesis of C-Dots using Polyalthia longifolia leaves, a medicinal plant known for its therapeutic properties, through a simple and cost-effective pyrolysis method. The synthesized C-Dots exhibited strong blue-green fluorescence under UV light, high water dispersibility, and a quantum yield of 8.88%. Physicochemical characterization using spectroscopic and microscopic techniques revealed that the C-Dots exhibited a predominantly spherical morphology, were nanoscale in size, negatively charged, and rich in functional groups such as hydroxyl (–OH), carboxyl (–CO), sulfonate (–SO), amine (–NH), and methyl (–CH), all of which contribute to their stability and bioactivity. The antimicrobial potential of the C-Dots was assessed against common bacterial and fungal pathogens using the Kirby-Bauer disk diffusion method, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and time-kill assays. Results indicated that the C-Dots possessed significant antibacterial activity, particularly against Staphylococcus aureus , with enhanced effects observed under photoactivation. Scanning electron microscopy (SEM) analysis confirmed the morphological damage to bacterial cells, supporting the observed antimicrobial effects. Additionally, the cytotoxicity of the C-Dots was evaluated using the MTT assay on HeLa cells, yielding an IC₅₀ value of 8.23 mg/mL, indicating good biocompatibility. Overall, this research demonstrates that green-synthesized C-Dots from P. longifolia leaves hold substantial promise as multifunctional nanomaterials for antimicrobial applications and potentially broader biomedical use, aligning with sustainable and eco-friendly approaches in nanotechnology.