Impact of structural changes in heteroleptic bismuth phosphinates on their antibacterial activity in Bi-nanocellulose composites

To study and evaluate the effect of ligand choice and distribution in bismuth phosphinates on toxicity and antibacterial activity, a series of novel diphenyl mono-phosphinato bismuth complexes, [BiPh₂(O(O-)P(H)Ph)]1, [BiPh₂(O(O-)PPh₂)]2, [BiPh₂(O(O-)PMe₂)]3and [BiPh₂(O(O-)P(p-MeOPh)₂)]4, were synthesised, characterised and structurally authenticated by X-ray crystallography. Evaluation of their antibacterial activity towardsStaphylococcus aureus(S. aureus), methicillin-resistantS. aureus(MRSA), vancomycin-resistantEnterococci(VRE),Escherichia coli(E. coli) andPseudomonas aeruginosa(P. aeruginosa) showed all four mono-phosphinato bismuth complexes to be highly active. However, unlike their less soluble bis-phosphinato analogues, they displayed an increased level of toxicity towards mammalian cells (COS-7, human and murine fibroblasts), where it was shown the complexes disrupt cellular membranes leading to cytotoxicity. The mono-phosphinato bismuth complexes were used to produce antibacterial nanocellulose composites. Leaching studies showed that complex1had the highest levels of leaching, at 15% of the total available bismuth when the composite was soaked in water. The aqueous leachates of1were bacteriostatic towards MRSA and VRE at concentrations between 4.0 and 4.6 μM, while being bactericidal towardsE. coliabove 2.8 μM. At similar concentrations the complex showed toxicity towards human fibroblast cells, with cell viability reduced to 2% (1, 2.4 μM). The possibility to control leaching of the bismuth complexes from cellulose composites through structural changes is evidence for their potential application in antibacterial surfaces and materials.