Microstructure of CeO2 Nanoparticles Loaded with Different Amounts of Ag and Their Antimicrobial Activity

Nanomaterials are widely used in environmental remediation and biomedical applications. Mainly in fields such as the textile industry, water treatment, medicine, and food packaging for controlling bacterial proliferation, which is a severe concern of everyday life [1]. Metal oxide nanomaterials such as CeO2 nanoparticles (NPs) are well-known oxidant and antioxidant agents due to their interesting physicochemical properties, which confer antimicrobial activity through the reactive oxygen species (ROS) release [2]. Furthermore, it has been shown that the addition of noble metals such as silver on CeO2 NPs enhances the antibacterial activity due to the silver release (Ag+) ions in the aqueous solution [3]. In this study, Ag/CeO2 NPs were synthesized by a co-precipitation method, varying the amount of silver precursor. Firstly, a 0.02 M aqueous solution of cerium nitrate hexahydrate (Ce(NO3)3 6H2O) was prepared with 5 wt% of polyvinylpyrrolidone (PVP) regarding cerium nitrate. The dissolution remained in stirring at 700 rpm for 10 min. at room temperature. Then, ammonium hydroxide (28%) was added dropwise until the dissolution reached a pH = 9. After 30 minutes of stirring, the silver salt precursor (AgNO3) was added in quantities of 0, 1, 3, and 5 wt%. After 5 h of reaction, the obtained precipitate was washed several times with deionized water. The powders were recovered by centrifugation, dried at 110 °C for 12 h, and annealed at 400 °C for 2 h. The Ag/CeO2 NPs were characterized by different techniques, and their antibacterial activity was evaluated against P. aeruginosa (ATCC 27853) and S. aureus (ATCC 25923) according to the ASTM E2149-20 standard method. In this procedure, 48 μg/mL of NPs was the concentration used to evaluate the antibacterial activity of samples. To confirm the obtained phases in the NPs, X-Ray (XRD) diffraction patterns were acquired for all samples. The patterns were indexed with the ICDD card 00-034-0394, that confirms the cubic structure of CeO2. Intensities belonging to the silver were not observed. However, the shifts towards smaller angles were observed in the 2θ position of the (111) intensity, which indicates the presence of the impurity in the CeO2 crystal. Furthermore, the crystal size was calculated by the Debye Scherrer equation, and crystal size's decrease with the silver amount's increase was determined. Figure 1 shows the images of the NPs acquired by transmission electron microscopy (TEM). The shape of the CeO2 NPs is spherical, and their size decreases with the increase of silver amount. The size of the particles is between 7 nm and 9 nm. All images show particles (marked with a red circle) with sizes between 30 and 40 nm. The particles’ elemental composition and structure were investigated by elemental mapping EDS and Select Area Electron Diffraction (SAED), respectively. These results identified the larger particles of silver NPs (Figure 1-d). On the other hand, the antibacterial activity was tested by a quantitative method. Figure 2 shows the antibacterial activity of NPs as a percentage of the reduction rate for both strains. According to the plot, the pristine CeO2 NPs showed no bacterial cell reduction. The Ag/CeO2 NPs showed a higher antibacterial activity against P. aeruginosa, and this activity against S. aureus increases with the silver amount increase. Bright-field STEM images of Ag/CeO2 NPs. (a) 1%, (b) 3%, (c) 5% of silver precursor. (d) SAED pattern of 1%Ag/CeO2 sample Antibacterial activity of Ag/CeO2 NPs evaluated against P. aeruginosa and S. aureus, expressed as a percentage of reduction rate.