Morphology-dependent bactericidal activities of Ag/CeO2 catalysts against Escherichia coli
Graphical abstract
The Ag/CeO2 nanomaterials could induce the production of extracellular reactive oxygen species (ROS). Intracellular ROS induced by extracellular ROS and Ag+ eluted from Ag/CeO2 was a candidate mediator for cell death. The combination effects of ROS and Ag+ were responsible for the disruption of cell membrane and inactivation of bacteria.
Introduction
The unique properties of metal nanoparticles have a great potential in research and diverse applications [1], [2], [3]. These properties include chemical, mechanical, electrical and optical characteristics as well as catalytic and biological activities. The antimicrobial properties of nanoscale metal and metal oxide particles such as Ag, TiO2, ZnO, and MgO have been the focus of research and application in antimicrobial coatings. Such metal nanoparticles interact with microbial cells through multiple biochemical pathways, for instance, via the production of reactive oxygen species (ROS) such as OH, H2O2, and O2−, which can damage cell structures and ultimately cause cell death [4], [5], [6], [7], [8]. Generally, photocatalysts such as TiO2 can effectively produce ROS when applied in water [9], [10]. However, photocatalysis technology requires the use of photon energy and complex devices. Therefore, the development of non-photocatalysis procedures containing abundant ROS formation is necessary for disinfection. According to the literature, many inorganic bactericidal materials such as MgO, CaO, and silver loaded materials can inactivate microorganisms through catalytic oxidation processes involving ROS [4], [6], [11], [12], [13], [14], [15], [16]. For pure oxides such as MgO and CaO, the bactericidal activity is low [12]. Although the bactericidal activity of silver-loaded materials is high, Ag+ elution is an issue [16]. To improve bactericidal activity and suppress the elution of Ag+, it is necessary to develop a new inorganic material to effectively produce ROS and reduce Ag+ elution.
Due to the high oxygen transport and storage capacities of ceria (CeO2), notable surface oxygen species form on CeO2 nanomaterials [17] widely employed in heterogeneous catalysis [18], [19], [20]. Ceria is an interesting oxide because oxygen vacancy defects can be rapidly formed, thus O vacancies and ROS in CeO2 are naturally anticipated in catalytic processes. Since ROS play an important role in the catalytic bactericidal process [14], [15], [16], CeO2 is a potential bactericidal material through ROS formation either as a catalyst or catalyst support, and thus it is important to investigate its bactericidal activity. The toxicity effects of CeO2 nanoparticles on bacteria have been studied recently [21], [22]. However, little research has been conducted on the role of ROS and related bactericidal activity of CeO2. Few reports are available on the effect of CeO2 shape on bactericidal activity although the catalytic activity of CeO2 is usually related to its shape and size [23], [24]. Furthermore, considering the high catalytic oxidation ability of Ag/CeO2, strong interaction of Ag with CeO2, and unusual sinter resistance [25], [26], Ag/CeO2 catalysts were prepared based on as-prepared CeO2 to effectively decrease Escherichia coli survival through catalytic oxidation at room temperature and to decrease Ag+ elution depending on strong interaction. Hereby, bactericidal activities of CeO2 nanorods, nanocubes, and nanoparticles prepared by facile hydrothermal synthesis and precipitation methods, and CeO2 supported silver (Ag/CeO2) catalysts were tested in this study. The reasons for different bactericidal activities of CeO2 correlated with different shapes and for largely improved bactericidal activity of Ag/CeO2 were explored. In addition, the formation of ROS was confirmed and the catalytic bactericidal mechanism was proposed.
Section snippets
Preparation of catalysts
The CeO2 nanorods and nanocubes were synthesized by a solution-based hydrothermal method, whereby Ce(NO3)3·6H2O (3.0 g, AR grade, Tianjin Fuchen Chemical Reagent Factory, China) was dissolved in deionized water, and then mixed with proper amounts of 10 and 1 mol/L NaOH solution in a 100-mL Teflon bottle, respectively. The Teflon bottle was then placed in a stainless steel autoclave heated at 100 °C for 12 h. The CeO2 nanoparticles were prepared by traditional precipitation, whereby Ce(NO3)3·6H2O
Bactericidal activity of CeO2 and Ag/CeO2
For comparison purposes, CeO2, 1 wt.% Ag/CeO2, and 2 wt.% Ag/CeO2 were prepared. The actual contents of Ag in Ag/CeO2 products were close to the prospective contents in the preparation process (Table 1). The bactericidal activities of CeO2, 1 wt.% Ag/CeO2, and 2 wt.% Ag/CeO2 are shown in Fig. 1. Among the three CeO2 shapes, bactericidal activity was in the order of CeO2 nanocubes ≈ nanorods > nanoparticles. Bactericidal activity was significantly improved after a small amount of silver was loaded. In
Conclusion
In conclusion, difference in the exposed crystal planes as well as oxidation ability among CeO2 nanocubes, nanorods and nanoparticles resulted in much higher bactericidal activities of CeO2 nanocubes and nanorods than that of nanoparticles. When CeO2 was loaded with a small amount of Ag, the bactericidal activities largely increased with the increase of oxidation ability. Extracellular ROS with strong oxidative capabilities, such as OH and O2−, were successfully detected by ESR at room
Abbreviations
- ROS
reactive oxygen species
- ICP-OES
inductively coupled plasma optical emission spectrometer
- XRD
X-ray diffraction
- HRTEM
high-resolution transmission electron microscope
- EDS
energy dispersive spectroscopy
- XPS
X-ray photoelectron spectroscopy
- ESR
electron spin resonance
- H2-TPR
temperature-programmed reduction of H2
- LB
lactose broth
- CFU/mL
colony forming units per milliliter
- DCFH-DA
2′,7′-dichlorofluorescin- diacetate
- PBS
phosphate-buffered saline
- TEM
transmission electron microscopy
- PI
propidium iodide
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 51208497) and the National High Technology Research and Development Program of China (Nos. 2010AA064905, 2012AA062702).
References (40)
- et al.
Curr. Opin. Biotechnol.
(2007) - et al.
Trends Biotechnol.
(2008) - et al.
J. Inorg. Biochem.
(2002) - et al.
Adv. Environ. Res.
(2003) - et al.
J. Photochem. Photobiol. A
(1997) - et al.
Water Res.
(2004) J. Microbiol. Methods
(2003)- et al.
Catal. Commun.
(2004) - et al.
J. Inorg. Biochem.
(2007) - et al.
J. Inorg. Biochem.
(2008)
Catal. Today
J. Colloid Interface Sci.
Appl. Catal. B
J. Catal.
Biomaterials
J. Catal.
Surf. Sci.
Surf. Sci.
Chem. Eng. J.
Crit. Rev. Microbiol.
Cited by (75)
Insights into the antibacterial and antiviral mechanisms of metal oxide nanoparticles used in food packaging
2023, Food Packaging and Shelf LifeNanoparticles-based therapeutics for the management of bacterial infections: A special emphasis on FDA approved products and clinical trials
2023, European Journal of Pharmaceutical SciencesNanoceria and hybrid silver–ceria nanoparticles fabricated by liquid-mediated laser ablation as antimicrobial agents
2023, Nano-Structures and Nano-Objects