Evidence of aggregation-assisted antibacterial photodynamic activity against S. aureus and E. coli using amphiphilic Ru(II) polypyridyl complexes

Abstract

Antimicrobial resistance represents a critical public health challenge, driving the search for therapeutic strategies that bypass conventional resistance mechanisms. Antimicrobial photodynamic therapy (aPDT) offers a promising alternative that is based on light-triggered non-specific oxidative damage. Herein, we report four new bis-heteroleptic Ru(II) complexes, with the general formula [Ru(Ν-Ν)₂(Ν-ΝΧ)]Cl₂, where N-N are the ancillary ligands 2,2’-bipyridine (bpy) or 4,7-Diphenyl-1,10-phenanthroline (DIP), and Ν-ΝΧ is a polyether-functionalized phenanthroline ligand. This design preserves the favourable photophysical characteristics of the [Ru(bpy)₃]²⁺ core while enabling lipophilicity modulation. Dynamic light scattering and emission lifetime studies support that the complexes bearing the DIP ligand (Ru-DIP-O3 and Ru-DIP-O4) self-assemble into nanoaggregates in aqueous media, due to their amphiphilic nature, whereas their bpy analogues remain in their monomeric form. We propose a previously undescribed aggregate architecture in which the Ru(II) core is shielded within the hydrophobic interior, while the polyether chains remain solvent-exposed. Biological evaluation of the complexes against S. aureus strains reveals that Ru-DIP-O3 and Ru-DIP-O4 significantly inhibited bacterial growth, while the bpy derivatives exhibit negligible activity. Notably, Ru-DIP-O4 demonstrates at least a 16-fold enhancement in bacteriostatic rate upon irradiation relative to dark conditions. Scanning electron microscopy studies provide evidence of membrane disruption in irradiated bacteria treated with Ru-DIP-O4. We attribute the enhanced photodynamic activity of the DIP-based complexes on aggregation-driven interactions with the bacterial membrane. Collectively, these findings underscore the therapeutic potential of rationally designed Ru(II) complexes for photodynamic applications and highlight the role of amphiphilicity and nanoscale self-assembly as a key design parameter in next generation aPDT agents.