Cerium oxide nanoparticles (CeO2 NPs) are a sought-after material in numerous fields due to their potential applications such as in catalysis, cancer therapy, photocatalytic degradation of pollutants, sensors, polishing agents. Green synthesis usually involves the production of CeO2 assisted by organic extracts obtained from plants, leaves, flowers, bacteria, algae, food, fruits, etc. The phytochemicals present in the organic extracts adhere to the NPs and act as reducing and/or oxidizing agents and capping agents to stabilize the NPs, modify the particle size, morphology and band gap energy of the as-synthesized materials, which would be advantageous for numerous applications. This review focuses on the green extract-mediated synthesis of CeO2 NPs and discusses the effects on CeO2 NPs of various synthesis methods that have been reported. Several photocatalytic, antimicrobial, antioxidant and cytotoxicity applications have been evaluated, compared and discussed. Future prospects are also suggested.

Green-synthesized CeO2 nanoparticles for photocatalytic, antimicrobial, antioxidant and cytotoxicity activities

Adsorption promoted visible-light-induced photocatalytic degradation of antibiotic tetracycline by tin oxide/cerium oxide nanocomposite

A mixed-valency bimetallic Ce/Zr MOF with Ce3+/Ce4+ ions incorporated and an oxygen vacancy-rich single-component photocatalyst have been designed through the one-step solvothermal route to harness photons from the visible-light spectrum for green energy (H2) generation and ciprofloxacin (CIP) degradation. The one-pot-engineered bimetallic Ce/Zr MOF shows visible-light-active characteristics accompanied by a narrower band gap, along with enhanced exciton separation and superior ligand-to-metal charge transfer (LMCT), due to the presence of an interconvertible Ce3+/Ce4+ ions pair in comparison to its pristine MOF counterpart. The Ce ion insertion led to increase in electron density around the Zr4+ ion, along with generation of some oxygen vacancies (OV), which cumulatively led to the rise in the photo-reaction output. The synthesized UNH (Ce/Zr 1:1) MOF displayed a boosted photocatalytic H2 production rate of 468.30 μmol h-1 (ACE = 3.51%), which is around fourfolds higher than that of pristine MOFs. Moreover, for CIP photodegradation, the UNH (Ce/Zr 1:1) shows an enhanced efficiency of 90.8% and follows pseudo-first-order kinetics with a rate constant of 0.0363. Typically, the active species involved in the photo-redox reaction of the CIP photodegradation follows the order hydroxyl radical (OH•) < superoxide radical (O2•-), as confirmed by the TA and NBT tests. Consequently, the bimetallic Ce/Zr MOF can be readily employed as a robust photocatalyst with enhanced tendencies towards CIP degradation and H2 evolution.

Mixed-Valence Bimetallic Ce/Zr MOF-Based Nanoarchitecture: A Visible-Light-Active Photocatalyst for Ciprofloxacin Degradation and Hydrogen Evolution.

Rapid transmission of infectious microorganisms such as viruses and bacteria through person-to-person contact has contributed significantly to global health issues. The high survivability of these microorganisms on the material surface enumerates their transmissibility to the susceptible patient. The antimicrobial coating has emerged as one of the most interesting technologies to prevent growth and subsequently kill disease-causing microorganisms. It offers an effective solution a non-invasive, low-cost, easy-in-use, side-effect-free, and environmentally friendly method to prevent nosocomial infection. Among antimicrobial coating, zinc oxide (ZnO) stands as one of the excellent materials owing to zero toxicity, high biocompatibility to human organs, good stability, high abundancy, affordability, and high photocatalytic performance to kill various infectious pathogens. Therefore, this review provides the latest research progress on advanced applications of ZnO nanostructure-based antibacterial coatings for medical devices, biomedical applications, and health care facilities. Finally, future challenges and clinical practices of ZnO-based antibacterial coating are addressed. 

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ZnO-based antimicrobial coatings for biomedical applications

S-scheme photocatalysis induced by ZnIn2S4 nanoribbons-anchored hierarchical CeO2 hollow spheres for boosted hydrogen evolution.

Cerium oxide (CeO2) and 1%, 5% and 10% zirconium/tin-dual doped CeO2 nanoparticles (Zr/Sn-dual doped CeO2 NPs) were synthesized using an aqueous leaf extract of Pometia pinnata. By using UV-visible diffuse reflectance spectroscopy, the band gap energies of these materials were found to be in the range of ∼2.49 to 2.66 eV. The average crystallite sizes of the fluorite phase obtained from X-ray diffraction were between 7 and 16 nm. X-ray photoelectron spectroscopy (XPS) analysis further confirmed the synthesis of CeO2 and Sn-doped CeO2 NPs. Almost spherical shapes of the nanomaterials with an average particle size of 12–17 nm were determined using scanning electron microscopy and transmission electron microscopy studies. Photoantioxidant activities of the synthesized materials showed enhanced photoantioxidant response under visible light irradiation in comparison with those under dark conditions in both dose- and time-dependent manner. The CeO2 NPs exhibited a significant concentration-dependent antibiofilm activity against the Gram-positive bacteria Staphylococcus aureus (S. aureus) and Listeria monocytogenes (L. monocytogenes). Only the 10% Zr/Sn-dual doped-CeO2 NPs were found to inhibit S. aureus biofilm formation at higher concentrations. All Zr/Sn-dual doped CeO2 NPs exhibited a concentration-dependent biofilm inhibition of L. monocytogenes and also bactericidal activity towards S. aureus. These nanomaterials exhibited enhanced photoantioxidant activities and antibacterial properties, which make them suitable for various biological applications.

Green synthesis of CeO2 and Zr/Sn-dual doped CeO2 nanoparticles with photoantioxidant and antibiofilm activities.

Cerium oxide (CeO2) and 1%, 5% and 10% tin-doped CeO2 nanoparticles (Sn-doped CeO2 NPs) were synthesized using aqueous leaf extracts of Pometia pinnata. UV-Vis diffuse reflectance spectroscopy showed band gap energies of 2.47–2.66 eV. X-ray diffraction results confirmed the formation of a fluorite phase of CeO2 and Sn-doped CeO2 NPs with average crystallite sizes between 6 and 19 nm. X-ray photoelectron spectroscopy (XPS) analysis further established the synthesis of CeO2 and Sn-doped CeO2 NPs. The density of states was proposed accordingly using the valence band-XPS data. Scanning electron microscopic and transmission electron microscopic studies showed almost spherical shaped particles with average particle sizes around 15–20 nm. All synthesized nanomaterials were tested for photoantioxidant activity under visible light and in the dark and showed improved photoantioxidant activity under visible light irradiation for dose and time-dependent studies. The synthesized nanomaterials were also tested for antibiofilm activities and exhibited significant concentration-dependent antibiofilm activity against Gram-positive pathogenic bacteria such as Staphylococcus aureus and Listeria monocytogenes. The 10% Sn-doped CeO2 NPs and S-CeO2 NPs also exhibited bactericidal activity against Staphylococcus aureus. These studies showed that S-CeO2 and Sn-doped CeO2 NPs have both good antioxidant and antibacterial properties.

Photoantioxidant and antibiofilm studies of green synthesized Sn-doped CeO2 nanoparticles using aqueous leaf extracts of Pometia pinnata

Structural, Morphological and Optical Studies of CeO2 Nanoparticles Synthesized Using Aqueous Leaf Extract of Pometia pinnata

Synthesized cerium oxide nanoparticles (S-CeO2 NPs) and 1%, 5% and 10% zirconium doped CeO2 (Zr-doped CeO2) NPs were fabricated using aqueous leaf extract of Pometia pinnata. The synthesized NPs were characterized using standard techniques which confirmed successful synthesis of NPs with particle size ranging from 12 to 23 nm and band gap energy of 2.54–2.66 eV. Photoantioxidant activities showed enhanced activities under visible light irradiation in comparison to the dark condition in the dose-dependent study. Biofilm inhibition studies showed ~ 73% biofilm inhibition of Staphylococcus aureus at 512 µg/mL for S-CeO2, whereas 10% Zr-doped CeO2 NPs showed biofilm inhibition of 52.7%. The bactericidal tests showed killing properties at 1024 µg/mL of S-CeO2 NPs and at 512 µg/mL of 1% Zr-doped CeO2. Reduced bactericidal activities were observed for 5% and 10% Zr-doped CeO2. These studies showed that the fabricated NPs have both good photoantioxidant and antibacterial properties.

Siti Najihah Naidi

Papers

Green-synthesized CeO2 nanoparticles for photocatalytic, antimicrobial, antioxidant and cytotoxicity activities

Green synthesis of CeO2 and Zr/Sn-dual doped CeO2 nanoparticles with photoantioxidant and antibiofilm activities.

Photoantioxidant and antibiofilm studies of green synthesized Sn-doped CeO2 nanoparticles using aqueous leaf extracts of Pometia pinnata

Structural, Morphological and Optical Studies of CeO2 Nanoparticles Synthesized Using Aqueous Leaf Extract of Pometia pinnata

Effect of Zr doping on photoantioxidant and antibiofilm properties of CeO2 NPs fabricated using aqueous leaf extract of Pometia pinnata