<i>Citrus maxima</i> extract-mediated synthesis of pristine and Cu-doped TiO₂: Impact of the structural and optical properties on the antimicrobial activity

Anatase is the most active among all TiO2 polymorphs; however, its thermal instability and its absorption confined in the UV range constitute its main limitations. Alternatively, plant-based synthesis arose as a promising approach to address the thermal instability issue, while transition-metal doping is recognised to extend its response to the visible range. In this study, Citrus maxima extract was used as a reducing/capping agent for the preparation of both pristine and copper-doped TiO2 nanostructures. Several techniques, including X-ray diffraction (XRD), Fourier-transform infrared (FTIR), UV-visible spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), were used to explore the physico-chemical features of the prepared TiO2 samples. XRD results showed that TiO2 samples, sintered at 600°C for 2 hrs, exhibit predominantly an anatase phase with a crystallite size of 15.3 nm and 23.5 nm for pristine and Cu-doped TiO2, respectively. XPS analysis revealed that both samples are inherently doped with carbon originating from the organic phase of the extract. Combined analysis using XRD, XPS, and UV-vis shows that the copper element was successfully inserted in the TiO2 lattice, causing an optical absorption red shift, as well as a structural transformation consisting of a mixture of anatase-rutile phases. The inhibitory potentials of the prepared samples were evaluated in terms of minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and disc diffusion measurements over several microbial pathogens, namely Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Candida albicans. Remarkably, both pristine and Cu-doped have demonstrated variable inhibition power against all the tested microbes, with a zone of inhibition ranging from 10 to 20 mm, depending on the microbe type.