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Nanoscience & Nanotechnology
Inorganic nanoparticles, such as CeO3, TiO2 and Fe3O4 could be served as a platform for their excellent performance in antioxidant effect. They may offer the feasibility to be further developed for their smaller and controllable sizes, flexibility to be modified, relative low toxicity as well as ease of preparation.
- inorganic nanoparticles
- antioxidant
- applications
- mechanism
- toxicity
1. Introduction
Nanotechnology functions as a platform to bridge the materials from the molecular and atomic levels to bulk, which has been widely developed in many fields such as energy, environment protection, and healthcare [1]. The preparation of the nanoparticles could be mainly divided into two different strategies: bottom-up and top-down [2]. Also, the system engineering was developed to scale up the nanoparticle manufacturing from bench to industry for their further applications [3]. Depending on the forming materials, these nanoparticles could be mainly classified into organic polymer formed nanoparticles and the inorganic nanoparticles such as metal oxidants nanoparticles. Among them, most of the polymers (including a natural polymer and synthetic polymers) formed nanoparticles could be applied as carriers to encapsulate bioactive agents for the purpose of improving their stability and efficacy. The inorganic nanoparticle was recognized as nano-scaled particles to be applied as carriers or exhibit bioactivities, which have drawn much attention to be applied in fields such as medicine, cosmetics, agriculture, functional foods development and packaging [4,5,6,7,8,9]. More research was performed regarding the inorganic particles due to their good performance, such as physical, chemical, mechanical stability, compatibility with other compounds (e.g., synthetic polymers) and whether they were easy to prepare or modify [10,11,12].
Oxidant is recognized as one class of the factor for series of diseases and ageing [13,14,15,16,17]. Reactive oxygen species (ROS) are commonly produced by the natural oxidative process, which is formed by reduction–oxidation reactions or by electronic excitation [18]. Evidence suggested that aging may relate to reactive oxygen species (ROS) damage. The popular theory regarding aging in recent years was the free radical theory of aging, focusing on mitochondria as a source as well as a target of ROS [14,19,20]. The recent research was mostly focused on how to reduce these oxidant effects. The nano-scaled inorganic particles could be well developed for their further application of antioxidant effects in many fields, such as medicine, cosmetic and functional foods additives. The smaller sizes of inorganic nanoparticles could offer more feasibility to function as antioxidants due to the high surface to volume ratio. Although metallic and metallic oxide nanoparticles were considered to potentially induce oxidative stress and lead to undesirable health problems, some of the nanoparticles exhibit excellent behavior in antioxidants, such as TiO2, cerium oxide nanoparticles and Fe3O4 in recent research works [21]. TiO2 were widely used in many products due to their low toxicity, and has to be considered as a nano-material on cellular antioxidant defense [22]. Also, cerium oxide nanoparticles may function as a free radical scavenger regeneratively to defect oxygen with their lattice structure [23]. Fe3O4 could be further modified to carbon paste electrode to form nanoparticles for electrosensitive determination of antioxidant components, such as sinapic acid, syringic acid and rutin [24].
2. Mechanism of Antioxidant Effects
2.1. Antioxidant Effects of the Modified Nanoparticles
The modification of the inorganic nanoparticles could immobilize some functional groups, which could provide antioxidant activity. The nanoparticle could be modified by chemical reaction or by some other method, such as the self-assembly method [56,57,58]. One example is that the 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionic acid (DPPH), which is one kind of phenol antioxidant organic compound, could be modified on the surface of the ZnO to scavenge the radicals. The linkage could be formed between DBHP and ZnO nanoparticles [59]. Also, the DBHP-ZnO nanoparticles could improve the efficiency of DBHP in scavenging the radicals produced by the oil oxidation process [60]. Convent coupling of the antioxidant functional moieties or entrapping the functional bioactivates on the surface of the inorganic nanoparticles could serve as one of the fantastic strategies to combine the surface activities of the nanoscaled particles together with the antioxidant effects of incorporated functional moieties [61,62,63]. The nano-formulated quercetin was loaded in calcium phosphate nanoparticles, which could exhibit pH indicator, fluorophore and antioxidant effects. The pretreatment of these formulations could protect the cells from H2O2-mediated toxicity [61]. Also, the antioxidant functional substance, which could help to form the inorganic nanoparticles, such as some natural fruit extract, could also be absorbed on the surface of the nanoparticulate to scavenge the free radical for its antioxidant effects. Gold nanoparticles were fabricated by ripened Capuli (Prunus serotina Ehrh. var. Capuli) fruit-derived extracts in an ecofriendly way. The formed nanoparticles were spherical and triangular in shape with the size ranging from 30 to 400 nm. These nanoparticles could exhibit a 46.12% inhibition percentage of DPPH for 30 min, which was mostly attributed to the adsorption/binding of the extract phytocompounds on the surface of the gold nanoparticles [64]. Some other metal oxide nanoparticles, such as Fe2O3, TiO2 and CuO, also have the antioxidant effect due to the incorporation of natural extracts from plants or fruit on their surface, which could provide the bioactive function [65].
2.2. Antioxidant Enzyme-Mimetic Activity
Oxidative stress is the main factor which could induce related diseases associated with the imbalanced ROS and antioxidant defenses such as the free radical cleavage substances. Recently, the research works involved in the inorganic nanoparticles in antioxidant effects have explained the mechanism of their activities. The effects of the antioxidant could be explained as an antioxidant enzyme-mimetic mechanism. For example, the superoxide dismutase(SOD) mimetics could function as catalytic agents to remove superoxide and peroxynitrite [66]. Korsvik et al. first revealed that Ce Nanoparticles (CNPS) could exhibit the effects of the SOD enzyme mimetic activities [67]. They speculate that the mechanism of the CNPS to the O2− was catalyzed as Equation (1) as below:
O2− + Ce4+ → O2 + Ce3+
O2− + Ce3+ + 2H+ → H2O2 + Ce4+
O2− + Ce3+ + 2H+ → H2O2 + Ce4+
It is obvious that the Ce3+/Ce4+ could regenerate during their functions. Also, nanoparticles could offer more possibility to increase the reactivity due to the large surface to volume ratio [68]. The ceria nanoparticles with higher Ce3+/Ce4+ could achieve SOD mimetic activity which was assayed using ferricytochrome C [67]. The Ce oxidant nanoparticles could have catalase mimetic activity on the H2O2, which was much more damaging due to the inducing of OH. Most of their effects depended on the ratio of the Ce3+ to Ce4+, as well as other parameters, such as the preparation procedure [69]. Additionally, the CNPs could produce a series of enzyme-like activities, such as phosphatase mimetic [70], oxidase mimetic [71], peroxidase mimetic [72] and ATPase mimetic effects [73], and allow them to be applied in broad fields, such as medicine, functional additives and environmental sciences [74,75].
Also, glutathione peroxidase (GPx)-like enzymes are known to affect the H2O2 level intra- and intercellularly, which involves glutathione(GSH) as a co-factor. The V2O5 nanowires could exhibit the GPx enzymes activity in presence of GSH, thus prohibiting the processing of the cell oxidant damage. The variations in the GPx enzyme-like activity may be attributed to the difference in the rate of the V-peroxide species formation on the surface of the nanostructure [76].
2.3. Antioxidant ROS/RNS Scavenging Activity
Reactive oxygen species (ROS) are usually generated by breaking covalent bonds of molecules during natural oxidative processes [77]. Formation of the different ROS, which included molecules derived from molecular oxygen by reduction-oxidation reaction, as well as electronic excitation, could induce the molecular damage [17]. ROS are generated by various sources both endogenous and exogenous. For example, H2O2 was one of the major ROS-induced substances and could maintain its effects even at low nano-molar under stimulated stress, e.g., growth factors, chemokines or other stressors. Some of the inorganic nanoparticles could exhibit their effects by enzyme mimetic behavior, such as SOD-mimetic activity, to function as reducing agents. They could also scavenge the ROS effectively. It has been suggested that the CNPs could remove the OH produced by H2O2, most of which were also based on the ratio of the Ce3+ [78]. The switch from Ce3+ to Ce4+ could well explain the scavenging activity of the antioxidant inorganic nanoparticle in ROS. Also, the CNPs were considered as activities in reducing the nitrosative stress, such as NO and O2NO− [79,80,81]. In addition to Ce oxidant nanoparticles, other inorganic nanoparticles could also have the efficiency in scavenging of the ROS/RNS. Some of the effects were due to the special electronic configuration such as La element-based oxidant nanoparticle. The surface modification could be recognized as one of the strategies to couple with functional moieties or coating on the nanoparticles for the purpose of exhibiting antioxidant effects. Also, selenium could function as the bioactive element to inhibit the damage of ROS scavenging. The selenium nanoparticles could scavenge the free radicals, and most of the selenium could participate in the activities of important antioxidant enzymes [82].
This entry is adapted from the peer-reviewed paper 10.3390/antiox11040791
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