Deciphering the Mechanisms of Bacterial Inactivation on HiPIMS Sputtered CuxO-FeOx-PET Surfaces: From Light Absorption to Catalytic Bacterial Death

The production of nontoxic, affordable, and efficient antibacterial surfaces is key to the well-being of our societies. In this aim, antibacterial thin films have been prepared using earth-abundant metals deposited using high-power impulse magnetron sputtering (HiPIMS). The sputtered FeOx, CuxO, and mixed CuxO-FeOx films exhibited fast bacterial inactivation properties under exposure to indoor light (340-720 nm) showing total bacterial inactivation within 180, 120, and 60 min, respectively. The photocatalytic mechanisms of these films were investigated, from the absorption of photoOns up to the bacteria's fate, by means of ultrafast transient spectroscopy, flow cytometry, and malondialdehyde (MDA) quantification justifying the cell wall disruption. The primary driving force leading to bacterial inactivation was found to be the oxidative stress at the interface between the sputtered thin films and the microorganism. This was justified by using engineered porinless bacteria disabling the possible ion diffusion leading to internal bacterial inactivation. Such stress is a direct consequence of the photogenerated electron-hole pairs at the interface of the sputtered layers. By diffuse reflectance spectroscopy, we found that both FeOx and CuxO present a band gap of similar to 2.9 eV (>425 nm), while the mixed CuxO-FeOx, thin film has a band gap below 2.3 eV (>540 nm). The structure and atomic composition of the films were characterized by energy-dispersive X-ray, X-ray photoelectron, and optical spectroscopy. While the composition and metal oxidation states are distinct in all three films, the difference in photocatalytic efficiency can, at first sight, be explained as the direct consequence of their absorbance and the unique interaction between Fe and Cu oxides in the composite film.