NANOCUBI DI BLU DI PRUSSIA RICOPERTI DA POLIDOPAMINA DOPATA CON CATIONI RAMEICI: PROPRIETÀ ANTIMICROBICHE SINERGICHE

ABSTRACT The alarming rise in antimicrobial resistance (AMR) and the limitations of traditional sterilization methods have underscored the need for drug-independent antimicrobial strategies. Among these, nanomaterials with photothermal activity have emerged as promising candidates due to their ability to locally convert light into heat and exert antibacterial effects via thermal ablation and ion release. In this work, we synthesized and characterized Prussian Blue nanoparticles (PBNPs), a coordination polymer based on Fe²⁺/Fe³⁺ cations coordinated by CN- anions, known for its intense absorption in the near-infrared (NIR) region and excellent photothermal conversion. The PBNPs were obtained via a citrate-assisted aqueous method and were found to form nanocubes with a characteristic absorption maximum around 712 nm. Their high photothermal stability and non-toxic profile make them suitable for biomedical applications. The synthesized nanoparticles were first characterized by TEM, DLS, and UV-Vis spectroscopy, confirming their cubic morphology, their dimensions and the presence of the characteristic charge-transfer absorption band. The PBNPs were then coated with a shell of polydopamine (PDA), a versatile bioinspired polymer obtained from polymerization of dopamine, thus obtaining PB@PDA samples. The coating was optimized by varying dopamine concentration and polymerization time, two parameters that consent to tune the shell thickness. Samples were characterized by TEM and DLS and the formation of the PDA layer was confirmed by the increase in hydrodynamic diameter, shift in Z-potential, and TEM images. Thus, we demonstrated that the thickness of the PDA layer can be controlled in a reproducible way. Figure A TEM images of PBNPs and of PB@PDA samples having different thickness of PDA layer The PDA layer served as a substrate for subsequent coordination of Cu²⁺ ions, which were loaded via chelation on the functional groups of PDA (PB@PDA-Cu samples). The successful chelation of Cu²⁺ ions was confirmed by EDX elemental analysis and ICP analysis. Moreover, with these techniques, we demonstrated that different and controllable amount of copper ions can be loaded depending on the PDA layer thickness, in quantities which are safe for use in human body. Figure B The overall scheme of PB@PDA-Cu samples preparation The resulting PB@PDA-Cu nanoparticles exhibited excellent photothermal properties under NIR laser irradiation (808 nm), rapidly converting light into heat. Upon irradiation, the nanoparticle suspensions showed a marked temperature increase sufficient to overcome the thermal threshold required for bacterial inactivation within 30 minutes. The antibacterial efficacy of the nanocomposites was evaluated against E. coli (Gram –) and S. aureus (Gram +). Both PB@PDA and the PB@PDA-Cu sample bringing the thicker PDA layer and subsequently the higher copper ions amount, were tested and both exhibited negligible toxicity under non-irradiated conditions. However, under NIR exposure, a reduction in viable colony-forming units (CFU) was recorded for PB@PDA-Cu samples, confirming the synergistic antibacterial effect of photothermal heating and Cu²⁺ ion release. These results demonstrate the ability of PB@PDA_Cu nanoparticles to act as dual-action antibacterial agents, exploiting both heat generation and metal ion toxicity in a controlled, biocompatible system.