Antibiotic-resistant bacteria represent a growing threat to global health, and there is a urgent need to address this issue. Carbon monoxide (CO) has been identified as an inhibitor of bacteria proliferation and a potent antibacterial agent. Its controlled delivery by photoactivatable CO-releasing molecules (photoCORMs) could be an attractive alternative to conventional antibiotics [1], especially if these molecules are incorporated into nanomaterials with the aim to increase their biocompatibility.
In the present work, a tricarbonylrhenium(I) complex (Re-Phe(TPP)) and a hydrophobic analogue substituted by an adamantyl moiety (Re-Ada(TPP)) were developed. These complexes were integrated to a biocompatible cellulose nanocrystal (CNC) matrix [2]. When irradiated in the near UV, the free photoCORMs generate rapidly one molecule of CO and small amounts of singlet oxygen (1O2). Their decarbonylated photoproducts generate only 1O2, on a prolonged period of time. In the CNC material, the diffusive species (CO and 1O2) were the most active ones. None of these systems showed bactericidal activity against Pseudomonas aeruginosa. In contrast, Re-Ada(TPP) showed clear photochemical activity against Staphylococcus aureus, while Re-Phe(TPP) was a very good antibacterial agent in the dark. The decarbonylated photoproduct D-Re-Phe(TPP) showed moderate activity, suggesting that part of the efficiency is linked to the production of CO. The photoCORMs adsorbed on CNCs were rather ineffective. This suggests that the direct biological action of photoCORM and/or the generation of CO and 1O2 near their biological target are necessary for good antibacterial activity [3]. This work allows to identify the limits of Re(I) photoCORMs and corresponding nanomaterials, and it provides good indications on how to improve their design.