Abstract Chitosan–cinnamaldehyde biopolymer nanoparticles were synthesized via a green and facile Schiff-base approach and subsequently evaluated for their structural and antibacterial properties. Structural characterization using FTIR confirmed imine (C = N) formation, while XRD analysis revealed a noticeable reduction in crystallinity from approximately ~ 40–45% for pristine chitosan to ~ 28–32% for the modified nanoparticles, indicating disruption of the native polymer packing. The average cr