Reactive oxygen species (ROS) play an important role in proper cellular functions and adaptation. However, an excess of free ROS in biological systems can lead to oxidative stress-related diseases such as inflammatory disorders, neurological diseases, aging-related diseases, and cancers . The human body naturally defends itself against oxidative stress by using antioxidant biomolecules. With the excellent ROS scavenging effect, antioxidants significantly contribute to the balance of ROS and protect the human body from free radicals, which are produced either by normal metabolism or an effect of exposure to external factors . However, the natural antioxidants in our body do not always work efficiently because ROS are so pervasive. Although antioxidant supplements from natural sources such as plants and animals are considered an effective strategy to combat oxidative stress, antioxidants from natural sources still have some notable limitations. These are low stability, low bioavailability, difficult long-term storage, and high cost of large-scale production .

Recently, nanomaterials have emerged as a promising strategy to overcome the limitations of natural antioxidants because of their high stability, easy storage, time effectiveness, and low cost. Also, progress in nanotechnology enables us to easily control size, morphology, surface coating, and chemical configuration, which are highly related to the antioxidant activities. The integration of biomedicine and nanotechnology for developing new types of antioxidants has witnessed a technological breakthrough that resulted in extraordinary progress in the pharmaceutical and biotechnology industries . Moreover, many categories of nanomaterials have shown greater antioxidant activity and more resistance to severe environments than the antioxidants originating from plants and animals. More interestingly, through nanoencapsulation and nanodelivery, antioxidant nanomaterials improve the pharmacokinetics of natural antioxidants by preventing their degradation under stress conditions .

Antioxidant nanomaterials can be synthesized from carbon-based compounds, polymeric compounds, and metal-based compounds. Metal-based nanoantioxidants exhibit strong reactivity because there are atoms with unpaired electrons on the surface. Therefore, metal-based nanoantioxidants have a significant advantage over natural antioxidants as well as other types of nanoantioxidants with lower reactivity and stability. As a result, metal-based nanoantioxidants have gathered significant attention from scientists. Many efforts have been made to develop new metal-based nanoantioxidants, understand the mechanisms of action, and expand their applications, especially in medicine and healthcare. For example, the question of why nanoparticles with a majority of Ce3+ on the surface have stronger antioxidant activity than those with Ce4+ has recently been answered by Dutta and co-workers . Ce3+ nanoparticles have a single 4f1 electron, which can be easily given up in a reaction with ROS. In contrast, Ce4+ nanoparticles have an octet-filled xenon configuration leading to less chemical activity. Understanding the mechanisms of metal-based nanoantioxidants is vitally important because it helps to rationally design and safely apply these nanomaterials for human healthcare, which strictly require assessment regarding quality control, safety, and efficacy. Many in vitro and in vivo assessments have been reported to prove the potential of metal-based nanomaterials for scavenging free radicals. FeO nanoparticles have 81% DPPH inhibition and exhibit in vivo hepatoprotective properties in laboratory mice . The combination of copper and cerium in nanoparticle structures enhances antioxidant activity through a synergistic effect. CuCe nanoparticles have been demonstrated for therapeutic effects in ischemic vascular diseases .

Current review articles have been published mainly focusing on the general mechanisms of the nanoantioxidants and their applications in various fields . In addition to these works, this review tries to take a deeper look at nanoantioxidants and only focuses on the mechanisms of metal-based nanoantioxidants and their application in medicine and healthcare in the last ten years. First, we summarize some novel metal-based nanoantioxidants and classify them into two main categories, namely chain-breaking and preventive antioxidant nanomaterials. Some representative examples of metal-based nanoantioxidants of each type are described focusing on the mechanism of action, antioxidant activities, and individual properties. Second, the application of metal-based nanoantioxidants in medicine and healthcare is carefully discussed in order to demonstrate the potential of nanotechnology, especially for reducing oxidative stress.

Figure 1: Classification of antioxidant nanomaterials.

CAT mimics: Nanomaterials exhibiting CAT activity mainly employ the properties of transition metals and metal oxides (e.g., cobalt, iron, cerium, and gold), which can generate a cycle of reduction and oxidation stages . Among these metal oxides, cerium oxide-based nanomaterials have been deeply studied with regard to the mechanisms of CAT activity. Cerium-based nanomaterials exhibit CAT activity through the ability to decompose H2O2 into O2 and H2O. Cerium oxide consists of mixed valence stages of Ce3+ (reduced) and Ce4+ (fully oxidized), which allows for the generation of redox cycles for exhibiting CAT activity in the presence of H2O2. The redox cycles start when H2O2 binds to two Ce4+, reducing two Ce4+ into two Ce3+ while H2O2 is decomposed into O2 and two H+. Subsequently, two Ce3+ decompose the second H2O2 and are oxidized into two Ce4+. The redox cycle of Ce4+/Ce3+ for H2O2 decomposition can be described as follows:

Chain-breaking antioxidants slow down or stop oxidation reactions after they begin. Chain-breaking antioxidants can directly trap, scavenge, or convert ROS into more stable and nonradical products and, thus, compete with the propagation reactions. In other words, ROS tend to react with chain-breaking antioxidants more rapidly than with the substrate . Phenolic compounds are among the most effective chain-breaking antioxidants that appear in nature and can also be synthesized. For example, α-tocopherol (vitamin E) is a chain-breaking antioxidant appearing in low-density lipoprotein and human blood plasma contributing to the decrease of oxidative stress. Its mechanism of action relies on the ability to transfer the phenolic H atom to a peroxyl radical (ROO•) much more rapidly than the propagation reactions . Despite the promising application of chain-breaking antioxidants for scavenging ROS, most chain-breaking antioxidants have unsaturated bonds and strong antioxidant activity, which is greatly affected by pH variations, heat, light, and the amount of metal and oxygen. Moreover, chain-breaking antioxidants showed limited stability during long-term storage .

Relying on the mechanism of chain-breaking antioxidants, nanomaterials can be synthesized to directly react with ROS after the oxidative reaction begins. Chain-breaking antioxidant nanomaterials are materials or chemical substances having a size between 0–100 nm that display chain-breaking antioxidant activities . The incorporation of nanomaterials and natural chain-breaking antioxidants is one of the most efficient strategies to combine the advantages of nanomaterials and natural antioxidants. For example, the natural phenolic compound gallic acid was covalently grafted on the surface of SiO2 nanoparticles. SiO2 nanoparticles provided thermal stability and chemical inertness while gallic acid provided chain-breaking antioxidant properties. By grafting antioxidant compounds on SiO2 nanoparticles, the deterioration can be decreased . Similarly, gallic acid was covalently grafted on magnetite with an average size of 5 and 8 nm. The functionalization of ultrasmall magnetite with gallic acid increased free radical scavenging two- to fourfold compared to free magnetite . Centurion et al. investigated the assembly of natural polyphenolic compounds triggered by liquid metals, leading to the formation of phenolic nanocoatings on substrates. In detail, nanocoatings of the phenolic compound pyrogallol deposited on polystyrene, glass beads, and paper yielded free radical scavenging activities of ca. 67%, ca. 84%, and ca. 92%, respectively .

Superoxide dismutase (SOD) mimics: SOD is a natural enzyme produced by organisms to protect themselves from oxidative stress and maintain normal physiological activity by neutralizing superoxide radicals. SOD degrades superoxide radicals into less harmful elements . SOD is classified as a chain-breaking antioxidant because of its ability to directly react with ROS. However, SOD can also be considered as a preventive antioxidant because superoxide radicals can act as precursors to initiate peroxidation in some cases . The main biochemical reactions of SOD relating to the decomposition of superoxide radicals into H2O2 and O2 occur at the active site. SOD is a promising antioxidant when it is used together with CAT and GPx for the decomposition of H2O2 into O2 and H2O.

SOD-mimicking nanomaterials have been synthesized to scavenge superoxide radicals. Encouraged by the discovery of the radical reaction of C60 fullerene in 1991 , C60 derivates were investigated for scavenging various free radicals including alkoxyl radicals, alkylperoxyl radicals, hydroxyl radicals, benzyl radicals, and superoxide radicals . Since then, a large number of SOD-mimicking nanomaterials have been reported. Most of them employ transition metals such as iron , copper , cobalt , gold , manganese , platinum , or cerium as main elements. Most transition metals have various oxidation states allowing them to generate cycles of redox reactions that are involved in superoxide radical scavenging. For example, cerium(IV) reacts with a superoxide radical to generate cerium(III) and oxygen. Then, cerium(III) reacts with the second superoxide radical to cerium(IV). The degradation of superoxide radicals by cerium nanomaterials can be described as follows:

Cerium nanomaterials with higher Ce4+ content are known to have higher CAT activity, while cerium nanomaterials with greater Ce3+ content enhance SOD activity. It was reported that Ce3+ has more oxygen vacancies providing more active sites for the binding of superoxide radicals on the surface . By controlling the Ce3+/Ce4+ ratio, the antioxidant activities of cerium nanomaterials can be flexibly designed to serve specific purposes.

Table 1: Summarization of metal-based nanoantioxidants and their remarkable features.

Inflammation is a complex biological process occurring in response to harmful stimuli causing injury to healthy tissue. Oxidative stress resulting from a high level of ROS is one of the key contributors to inflammation . Therefore, balancing the ROS level via antioxidant supplementation is a promising strategy for treating inflammatory diseases. Recently, Kim et al. introduced ultrasmall antioxidant cerium oxide nanoparticles (CeONPs) with strong SOD and CAT activities, which were used to decrease ROS levels and suppress the production of inflammatory cytokines (TNFα and IL-1β) in macrophages. CeONPs also decrease monocyte recruitment at inflammatory sites by balancing ROS levels . Another modern strategy for developing anti-inflammatory nanodrugs is coupling transition metals with natural antioxidants. For example, Yuan et al. synthesized Fe-curcumin nanoparticles (Fe-Cur NPs) possessing intracellular ROS scavenging and anti-inflammatory capability for curing acute lung injury . Fe-Cur NPs acted as an anti-inflammatory agent by downregulating the level of proinflammatory cytokines (TNF-α, IL-1β, and IL-6), suppressing NF-κB signaling pathways, inhibiting NLRP3 inflammasomes, and reducing intracellular Ca2+ in cells and tissues. One of the most important criteria of anti-inflammatory drugs is the direct delivery to the inflamed tissue .

To increase the targeting ability, anti-inflammatory agents can be wrapped with a cell membrane camouflage technique . For example, Ma et al. constructed carbon dot-SOD nanozymes and CD98 CRISPR/Cas9 plasmids encapsulated in a metal-organic framework (MOF), and then camouflaged it with macrophage membrane . In this system, the macrophage membrane guided the encapsulated nanozymes and CD98 CRISPR/Cas9 plasmids to accumulate at the inflammatory sites. The acidic environment at the inflammatory sites induced MOF disaggregation, resulting in the release of carbon dot-SOD nanozymes and CD98 CRISPR/Cas9 plasmids. The released nanozymes and plasmids contributed to ROS scavenging and reduced the CD98 gene expression, respectively. Another study employed CD44–hyaluronic acid interaction to endow a diselenide-bridged hyaluronic acid nanogel (SeNG) with the ability to specifically accumulate at CD44-overexpressed inflammatory cells . Both in vitro and in vivo experiments demonstrated that the designed SeNG could not only eliminate ROS, but also up-regulated the Nrf2/HO-1 pathway to maintain redox homeostasis. Depending on the location of the accumulation, nanoantioxidants need to employ different encapsulating or coating strategies. Several studies used neutrophil membranes to increase the specificity of antioxidant delivery to ischemia-reperfusion (I/R) injury kidneys . Neutrophils play a vital role in resolving inflammation, tissue repair, and initiating I/R-induced acute kidney injury progression. Moreover, I/R injury is characterized by a massive influx of neutrophils, which highly contribute to the pathophysiology of post-ischemic renal failure. Therefore, by coating with a neutrophil membrane, nanoantioxidants can be specifically delivered to kidneys with I/R injury.

A majority of reported anti-inflammatory nanomaterials mainly focus on the degradation of ROS. However, nanomaterials possessing the capability to degrade reactive nitrogen species (RNS) are rare. In response, Miao et al. synthesized polyethylene glycol-coated ultrasmall rhodium nanodots (Rh-PEG NDs) for scavenging both ROS and RNS in order to decrease inflammation . Rh-PEG NDs exhibited O2•–, HO•, NO•, ONOO–, and H2O2 scavenging capability and were used to decrease the level of proinflammatory cytokines, including TNF-α and IL-6, at the inflammatory sites. As another example, an alendronate-coated nanoceria (CeAL) nanozyme was reported by Zhou et al. for both ROS and RNS scavenging . Other RNS scavenging nanomaterials such as Co-doped Fe3O4 nanozymes , WS2, MoSe2, WSe2 nanosheets , carbogenic nanozymes , and Pt/CeO2 nanozymes also showed excellent RNS scavenging capability.

Table 2: In vitro and in vivo experiments using antioxidant nanomaterials for the treatment of oxidative stress-related diseases.

Aging is a natural process defined as a time-related deterioration of the physiological functionalities that are necessary for fertility and survival . The free radical theory of aging proposes that aging is caused by the accumulation of ROS during the metabolism. Although the natural antioxidant defense systems in the human body including CAT, SOD, and GPx can neutralize ROS, the system cannot completely stop the aging process . In this context, antioxidant supplementation is regarded as an efficient strategy to defend humans against aging. Natural compounds from plant-derived extracts such as polyphenols, tocopherols, carotenoids, and ascorbic acid are the most important and common ingredients of antioxidant supplements. However, these compounds have low bioavailability related to low stability, solubility, and absorbance in the gastrointestinal tract . Recently, with extraordinary progress in nanotechnology development, nanodelivery systems have been considered as an effective solution because of their ability to overcome the physical and chemical barriers in the gastrointestinal tract. Such barriers are the intestinal mucosal barrier, acidic conditions in the stomach, and selectively permeable membranes of enterocytes. Moreover, natural compounds delivered by nanocarriers have various therapeutic advantages such as no or minimized side effects, long storage life, enhanced residence time, extended circulation time, increased half-life, and decreased dose .

Nanodelivery systems for natural antioxidants can be divided into two main classes, namely natural and synthetic nanocarriers. Because of the advantages of synthetic nanocarriers regarding easier customization of size, surface properties, charge, and morphology, we will focus on synthetic nanocarriers. Various types of synthetic nanocarriers have been developed to deliver natural antioxidants: (1) nanoliposomes, which are a nanoscale bilayer lipid vesicle ; (2) nanocapsules, which consist of an inner aqueous core surrounded by a nontoxic polymeric membrane ; (3) solid lipid nanoparticles, which consist of a solid lipid core stabilized by a surfactant ; and (4) nanocrystals, which are a cluster of hundreds to thousands atoms aggregated together . The potential of natural antioxidant nanodelivery systems for treating age-related metabolic disorders has been proved in both in vivo and in vitro studies. For example, a nanoparticle-based formulation of curcumin exhibited better anti-aging properties than natural curcumin. By encapsulating curcumin in nanocarriers or by conjugating it to metal oxide nanoparticles, the solubility and bioavailability of curcumin have been substantially improved, leading to a rise in its pharmacological efficiency . Pharmacokinetic analysis of curcumin-loaded polymeric nanoparticles after oral delivery in mice demonstrated a 20-fold decrease in dose requirement compared to natural curcumin . Both experimental and molecular dynamics simulation studies suggested an optimal ferulic acid (an antioxidant in plants) concentration of 0.5 wt % for the successful formation of ferulic acid-loaded lipid-based nanoparticles . The ferulic acid-loaded nanoparticles with improved bioavailability can be useful for skin care products and human skin cancer treatment .

The skin is the outermost layer, the largest organ, and the first barrier protecting our body against toxic elements, infections, and dehydration, which makes it vulnerable. A major skin injury can cause severe problems to human health such as increased risk of infections, dehydration, and immune system disorders . Wound repair is a crucial process for the recovery of injured skin by which the integrity of the wounded area is restored and regenerated. In wound repair, ROS acts as a double-edged sword because ROS have both positive and negative effects on wound repair. At a low ROS level, the wound repair process benefits from ROS because ROS can prevent microbial infections. However, a high level of ROS increases the deleterious effects on wound repair by increasing the inflammation . Therefore, nanomaterials with antioxidant effects are proposed to balance the ROS level in the wound repair process. As a typical example, nanofibers composed of polygalacturonic acid, hyaluronic acid, and embedded silver nanoparticles were applied to recover wounded areas of albino rats in vivo . In this nanocomposite, silver nanoparticles acted as an antioxidant, polygalacturonic acid acted as a reducing agent for generating silver nanoparticles from silver ions, whereas hyaluronic acid enhanced hydrophilicity and strain activities.