Cerium-oxide nanoparticles as nanozymes
Introduction In recent years, nanotechnology has shown numerous revolutionary developments in the area of modern research. Nanotechnology deals with the materials that have at least one dimension in nanoscale (less than 100 nm). These materials are known as nanoparticles (NPs)1. NPs possess unique physicochemical properties which are associated with size range and surface charge of metal atoms. These properties make them an attractive material for therapeutic and diagnostic applications. NPs, which have a different physical and chemical property than their bulkier forms are continuously proving themselves as a future essential2. Since the ancient time to till date these NPs are continuously growing and researchers have found out different types of materials from which they can synthesize NPs. NPs can be synthesized by various routes such as top down and bottom up which have subtypes like fabrication, chemical synthesis, laser ablation, chemical and physical vapour deposition and so on. NPs can be differentiated into metal, metal oxide, magnetic, and other organic NPs based on the type of core materials used for the NPs synthesis3. NPs have a wide range of applications like, biomedical, energy generation, catalyst, and food and agriculture industry which depends on its synthesis routes and care materials. Various NPs such as gold (Au), silver (Ag), copper (Cu), cerium oxide (CeO2/Ce2O3), iron oxide (Fe2O3), organic NPs such as liposomes have shown a wide range of application in drug delivery, protecting agent, biosensing, nanozymes, diagnosis, prosthesis and implants4. In the biological system, a number of free radicals are generated as a result of oxygen metabolism and these radicals are highly unstable molecules which have a tendency to react with proteins, DNA/RNA, or other biomolecules of the body. Free radicals are the molecules that are produced as a by-product of normal metabolic reactions. They are highly unstable and have the capability to harm the biomolecules. They are necessary part of many cellular reactions but when their level exceeds, they can be hazardous5. Naturally, antioxidants are produced in the biological system to control the level of free radicals but when there is imbalance between the level of free radicals produced and antioxidants generated, then a condition arises, which is known as oxidative stress6. Reactive oxygen species (ROS) and Reactive oxygen species (RNS) are two major types of free radicals generated in the living system. Since last decade, a number of metal oxide NPs (MONPs) such as iron oxide, cerium oxide, zirconium oxide, aluminium oxide, copper oxide etc. have the potential to scavenge these ROS species and behave much like natural antioxidants7. Advantages of nanozymes The NPs which can behave like natural enzymes are known as nanozymes. Compared with natural enzymes, nanozymes are advantageous in several aspects: Low Cost: Nanozymes are much cheaper than natural enzymes. Robustness to harsh environments: Nanozymes can survive in harsh environmental conditions such as high temperature, pH etc. Easy for mass production: Nanozymes can easily be scaled up at high level as compared to natural enzymes. High stability: Nanozymes are comparatively more stable than natural enzymes even at room temperature. Long-term storage: Nanozymes can be stored for a long time. Size/composition dependent activity: Nanozymes show size and composition dependent activity and we can alter the properties of nanozymes as per our need. Cerium oxide nanoparticles as nanozymes Cerium oxide nanoparticles (CeNPs) can behave like various enzymes and exhibit excellent antioxidant properties8. Cerium is a rare earth element and when it combines with oxygen, it attains fluorite (FCC) structure with defects on its surface9. But unlike other rare elements, cerium oxide has two oxidation states (3+ and 4+) and these two states coexist on the surface of NPs. The ratio of these two states on the surface determines the antioxidant properties of the CeNPs. The oxygen vacancies present on the surface allows the interconversion of these two oxidation states of CeNPs10. CeNPs have been reported for exhibiting various enzyme-like activities such as superoxide dismutase (SOD), phosphatase, catalase, phosphotriesterase, peroxidase, oxidase etc. Based on the ROS elimination or generation, CeNPs are divided in two groups: antioxidants and pro-oxidants. CeNPs as antioxidants CeNPs exhibit antioxidant behaviour and can eliminate two major free radicals (superoxide radicals and hydrogen peroxide) from the body. Superoxide dismutase (SOD) are the enzymes which leads to dismutation of harmful superoxide free radicals (O2-) into hydrogen peroxide (H2O2) and molecular oxygen (O2)11. The reaction catalysed by SOD enzymes is shown below in equation 1: 2O_2^(-)+2H^+→ 〖2H〗_2 O_2+ O_2 Equation (1) The SOD-like activity of CeNPs was firstly reported by Korswik et al. in 200712. CeNPs with higher 3+/4+ ratio on the surface exhibit excellent SOD-like activity. Catalases are the enzymes which result in degradation of H2O2 into oxygen and water13. The reaction catalysed by catalases is shown below in equation 2: 〖2H〗_2 O_2→2H_2 O+ O_2 Equation (2) The catalase-like activity of CeNPs was first reported by Pirmohamed et al. in 201014. CeNPs with higher 4+/3+ ratio on the surface, exhibit good catalase-like activity. CeNPs as pro-oxidant CeNPs can also generate free radicals and exhibit pro-oxidant behaviour. Oxidases are the oxidoreductases enzymes which catalyse oxidation reaction where molecular oxygen acts as electron acceptor15. The reaction catalysed by oxidases is shown in equation 3: Substrate (red)+ O_2+2e^-+2H^+ →Substrate (oxi)+ H_2 O Equation (3) Asati et al. firstly reported the oxidase-like activity of CeNPs. They showed that oxidase-like activity of CeNPs is pH dependent and maximum activity was observed at acidic pH (pH=4)16. Peroxidases are the enzymes which catalyse the degradation of H2O2 and generate hydroxyl radicals15. The reaction catalysed by peroxidases is shown below in equation 4: Substrate (red)+〖2H〗_2 O_2 →Substrate (oxi)+ 2H_2 O Equation (4) Asati et al. reported firstly the peroxidase-like activity of CeNPs in 201117. Based on the peroxidase-like activity of CeNPs they also developed a colorimetric method for glucose sensing. Apart from these major enzyme-like activities CeNPs also exhibit phosphatase and phosphotriesterase-like activities18. Based on these enzyme-like activities CeNPs emerged as a fascinating and remunerative material with a number of bio sensing and therapeutic applications19. References 1. Murthy, S. K., Nanoparticles in modern medicine: state of the art and future challenges. International journal of nanomedicine 2007, 2 (2), 129. 2. Khan, I.; Saeed, K.; Khan, I., Nanoparticles: Properties, applications and toxicities. Arabian journal of chemistry 2019, 12 (7), 908-931. 3. 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