1. Introduction
Cerium, a rare earth metal, is the first element of the lanthanide series in the periodic table. The 4f orbitals of rare earth metals are adequately shielded by 5p and 4d electrons, leading to interesting catalytic properties [
1]. Unlike most rare earth metals, cerium can exist in both 3+ and 4+ states [
2]. Thus, cerium oxide exists as both CeO
2 and Ce
2O
3 in the bulk state. At the nanoscale, however, cerium oxide has a mix of cerium in the 3+ and 4+ states on the nanoparticle surface. With a decrease in nanoparticle diameter, the number of 3+ sites on the surface increase and oxygen atoms are lost (oxygen vacancies) [
3,
4]. This is depicted by an overall structure of CeO
2−x.
Cerium oxide nanoparticles (CeNPs, nanoceria) are widely used in chemical mechanical polishing/planarization [
5], corrosion protection [
6], solar cells [
7], fuel oxidation catalysis [
8], and automotive exhaust treatment [
9]. Pertinent to this review, CeNPs also display many bio-relevant activities-mimicking superoxide dismutase (SOD) [
10], catalase [
11], peroxidase [
12], oxidase [
13], and phosphatase [
14], and scavenging hydroxyl radicals [
15], nitric oxide radicals [
16], and peroxynitrite [
17].
Reactive Oxygen Species (ROS) are released as by-products in aerobic metabolism and they are routinely linked to oxidative stress (increased levels of intracellular ROS contributing to many diseases). However, ROS primarily act as signaling molecules in physiological processes. For a current and detailed understanding of ROS, readers are referred to an extensive review by Schieber et al. [
18] that describes the two faces of ROS—redox signaling and oxidative stress. In this context, antioxidants can be defined as substances that scavenge ROS or inhibit their production. Interest in studying antioxidants grew after a study that described the potential benefits of vitamin E on cardiac health [
19]. The activity of metal and metal-based nanoparticle systems and their interactions with ROS depend on their microenvironment. It is well established that metal and metal oxide nanoparticles exhibit antioxidant properties [
20]. Since naturally occurring small antioxidant molecules have limited absorption into the body [
19], nanoparticles have been investigated as carriers for antioxidant molecules [
20]. On the other hand, metal and metal-based nanoparticle systems can also be used for prooxidant treatment strategies [
21,
22,
23,
24].
The bio-relevant activities of CeNPs earmark them for use in potential pharmacological agents [
25], drug delivery [
26,
27,
28], and bioscaffolding [
29,
30]. The basis for these activities of CeNPs is the thermodynamic efficiency of redox-cycling between 3+ and 4+ states on their surface [
10] and their unique ability to absorb and release oxygen [
31]. While it was initially thought that both oxygen vacancies and the redox-cycling between cerium in 3+ and 4+ states are involved in the antioxidant activities of CeNPs [
6,
10], it is now accepted that redox-cycling is solely responsible for all antioxidant properties [
32]. This suggests that the surface ratio of Ce
3+/Ce
4+ plays a key role in all of the bio-relevant activities of CeNPs. It is worth noting that CeNPs can also show prooxidant properties at lower pH values [
13] and high doses [
33], and they are known to exhibit potential toxicity based on their synthesis method, concentration, and exposure time, as detailed in a review by Yokel et al. [
34]. As explained by Xu et al. [
35] in their review on CeNPs and their applications, cerium is not found in the human body and there are no known clearance mechanisms for it. This implies that exposure to cerium would lead to systemic toxicity. These reasons necessitate careful optimization of synthesis parameters to generate non-toxic CeNPs that have either prooxidant or antioxidant properties that are based on the treatment strategy being used.
The interactions of a nanoparticle system with its microenvironment need to be considered while designing effective nanocarriers. It is important to note that polymeric nanocarriers and smart polymer systems can be used to encapsulate enzymes for drug delivery applications [
36,
37,
38,
39,
40,
41,
42] and offer good biocompatibility. Such systems can also offer dual responsive programmable drug release [
43]. However, CeNP-based treatment strategies have a unique advantage in that they have a self-regenerative antioxidant capability [
44].
In the past decade, there have plenty of studies that demonstrate antibody-directed targeted delivery of antioxidant enzymes, like superoxide dismutase and catalase [
45,
46,
47,
48]. To achieve similar goals of targeted delivery, there are studies that have demonstrated the use of functionalizing CeNPs with surface groups and stabilizers so that they can be applied for targeted delivery into the body (reviewed by Nelson et al. [
44]). Such functionalization though, needs to be fine-tuned to suit the needs of a targeting strategy and ensure that the CeNPs that are involved can self-regenerate their surface. Clearance from the body also needs to be considered while designing effective CeNP-based treatment strategies. Readers are directed to a review by Walkey et al. [
49] for detailed information on targeting strategies and the possible routes of clearance.
This review begins with a brief overview of the methods that are routinely used to synthesize CeNPs. Green synthesis methods are highlighted because of their use of biocompatible stabilizers that may provide non-toxic preparation routes. We then briefly describe known enzyme-mimetic and antioxidant activities of CeNPs. Lastly, recent evidence from both in vitro and in vivo studies is presented to provide the reader with an up-to-date account of the potential biomedical applications of CeNPs.
2. Synthesis of Cerium Oxide Nanoparticles
The physicochemical properties of any nanoparticle depend on the method of synthesis. For a bio-relevant nanoparticle, synthesis parameters need to be carefully optimized to select for beneficial physicochemical properties in vivo. Different methods of synthesis result in CeNPs of varying size, morphology, and agglomeration. In general, using a polymer or surfactant during synthesis or a coating post-synthesis results in lowered agglomeration of CeNPs in bio-relevant solutions.
An important consideration while synthesizing nanoparticles for use in vivo is the formation of a protein corona that affects both the uptake and clearance of the nanoparticle. Readers are encouraged to read a review by Lynch et al., on the nanoparticle-protein corona [
50]. Common methods used to synthesize CeNPs, including recent green synthesis methods, are listed in
Table 1 with a relevant example for each method.
2.1. Traditional Synthesis Methods
Numerous methods for the synthesis of CeNPs have been reported. These include solution precipitation [
51], hydrothermal [
52], solvothermal [
53], ball milling [
54], thermal decomposition [
55], spray pyrolysis [
56], thermal hydrolysis [
57], and sol-gel methods [
58,
59,
60]. While these methods can help to determine the shape and size, Dowding et al. [
14] were the first to report fine-tuned control over the surface ratio of Ce
3+/Ce
4+. As expected, many of the traditional methods suffer from low biocompatibility. In general, biocompatible coatings of nanoparticles provide greater stability, longer retention times, and lower toxicity by decreasing non-specific interactions. CeNPs have been functionalized using a variety of coatings–polyacrylic acid [
61], polyethylene glycol (PEG) [
62], dextran [
63], polyethyleneimine [
64], cyclodextrin [
27], glucose [
26], and folic acid [
28]. Additionally, CeNPs can also be doped with chelating MRI contrast agents, such as gadolinium, to improve their safety while also displaying antioxidant properties [
65]. For a comprehensive review of traditional synthesis methodologies and the associated physicochemical properties, readers are directed to a review by Das et al. [
66].
2.2. Green Synthesis Methods
Recently, bio-directed CeNP synthesis methods that use natural matrices as stabilizing agents have gained importance because they help alleviate concerns of bio-compatibility. Such green chemistry methods provide safer routes for preparing CeNPs [
59,
60] and they are potentially useful for pharmaceutical applications [
67]. In general, these methods provide low-cost and simpler alternatives to traditional synthesis methods. However, conclusions about the biocompatibility of a given green synthesis method should only be made after assessing the protein corona formation for synthesized CeNPs in biological fluid environments. The effect of the surface ratio of Ce
3+/Ce
4+ on the biological properties of CeNPs synthesized via green chemistry methods also needs to be investigated [
67].
The main strategies involved in the green synthesis of CeNPs are plant-mediated synthesis, fungus-mediated synthesis, polymer-mediated synthesis, and nutrient-mediated synthesis. Plant-mediated methods (phytosynthesis), where plant extracts act as stabilizing and capping agents, result in relatively large CeNPs [
68] that are currently not appropriate for biomedical applications [
69]. Fungus-mediated methods (mycosynthesis) resolve this by producing smaller CeNPs [
70] that are more stable, have higher water dispersibility, and high fluorescent properties [
71]. Natural polymers can also aid in the green synthesis of CeNPs and act as stabilizers [
72]. An example is the use of PEG to create dispersible nanopowders in aqueous solutions [
73]. Nutrient-mediated synthesis, such as in the use of egg white as a substrate to synthesize CeNPs, are extremely cost-effective [
72]. Egg white proteins act as stabilizers that result in controlled isotropic growth of small CeNPs. For further information on the green synthesis methods that are available for CeNPs, readers are directed to a review by Charbgoo et al. [
67].
As seen from the table, the sheer breadth of techniques available and the resulting physicochemical properties of synthesized CeNPs warrant the development of a reference CeNP material that has well-characterized properties and can be used to maintain consistency across studies.
5. Conclusions and Future Perspectives
Oxidative stress is implicated in the development and progression of many diseases. The broad range of CeNPs’ antioxidant activity and their ability to self-regenerate their surface makes them strong candidates for use as in vivo ROS scavengers. However, to be considered as potential therapeutic agents, it is necessary to optimize their synthesis methods, surface chemistry, and concentration to select the beneficial physicochemical properties. For consistency, such endeavors must begin by establishing CeNP reference materials for pertinent disease models. Eventually, all toxicological concerns also need to be addressed.
It is important to note that the physicochemical properties of CeNPs reported in vitro differ from those under physiological conditions. In particular, the natural protein corona associated with CeNPs in vivo is primed to be the focus of studies that describe existing/identify new biomedical applications. Any pharmacokinetic improvement will involve CeNPs with biocompatible coatings and effective targeting strategies.
Green synthesis methods that use biocompatible stabilizers are increasingly gaining relevance in the production of CeNPs and their biomedical applications. Undoubtedly, interdisciplinary studies that are based in material science and tailored towards identifying new biomedical applications of CeNPs will continue to clarify their physicochemical properties and catalytic activities.