DFT-guided photostable chitosan-derived carbon quantum dots as colloidal antibacterial and bioimaging agents.

Environmentally friendly and biocompatible nanomaterials are essential for biomedical applications demanding long-term safety, stability, and functional performance. In this study, we present a simple hydrothermal synthesis of chitosan-derived fluorescent carbon quantum dots (CQDs), utilizing chitosan as a biodegradable carbon precursor. The synthesized CQDs displayed uniform spherical morphology (~ 3 ± 1 nm), a positive surface charge (+ 12.6 mV), and strong cyan-blue photoluminescence (λem ≈ 473 nm, PLQY ≈ 17.5%) with excitation-dependent emission. Their structure and optical properties were characterized using TEM, XRD, FTIR, UV–Vis, PL, DLS, and cyclic voltammetry. Density functional theory (DFT) simulations supported experimental data, showing stable nanocluster formation through chitosan–acetic acid interactions and predicting a band gap of ≈ 3.41 eV, consistent with the experimental 3.45 eV. Nitrogen atoms were identified as major contributors to optical transitions. Biological studies revealed dose-dependent cytocompatibility in Vero cells, strong antioxidant activity (DPPH assay), and antibacterial efficacy against E. coli and S. aureus via reactive oxygen species generation and electrostatic interactions. Fluorescence microscopy confirmed efficient cellular uptake and cytoplasmic distribution. Overall, this integrated experimental–theoretical approach highlights the CQDs’ potential for sustainable applications in bioimaging, drug delivery, and theranostics.