Nanotechnology has transformed material science and engineering, enabling the design and application of materials at the atomic and molecular levels. Among these materials, cerium oxide nanoparticles (CeO₂ NPs) have emerged as one of the most promising and versatile nanomaterials due to their unique physicochemical properties and wide applicability across multiple scientific domains [1]. CeO₂ NPs, derived from cerium, a rare earth element, exhibit exceptional redox properties, high oxygen storage capacity, and the ability to alternate between Ce³⁺ and Ce⁴⁺ oxidation states. These attributes render them ideal candidates for catalytic, environmental, biomedical, and energy applications [2]. The widespread interest in CeO₂ NPs can be attributed to their remarkable reactivity and adaptability in different functional environments [3]. Their redox potential, combined with their ability to scavenge free radicals, positions them as effective agents in mitigating oxidative stress, an underlying factor in many degenerative diseases. In addition to their biomedical utility, CeO₂ NPs have demonstrated efficacy in catalysis, particularly in automotive catalytic converters, where they facilitate the oxidation of harmful gases, contributing to pollution control [4]. Furthermore, their role in energy applications, such as super capacitors and solid oxide fuel cells, underlines their importance in addressing critical energy and environmental challenges. Synthesis techniques for CeO₂ NPs have evolved significantly to meet the growing demand for precise control over their size, morphology, and functional properties [5]. Traditional physicochemical methods, including sol-gel, hydrothermal, and precipitation techniques, offer high precision but are often energy-intensive and reliant on hazardous chemicals, which raises concerns about their environmental footprint. Consequently, researchers have turned to green synthesis approaches, which employ plant extracts, microorganisms, and other bio-based agents [6]. These eco-friendly methods not only reduce environmental impact but also impart biocompatibility and enhanced functional properties to the nanoparticles, making them suitable for biomedical applications. The applications of CeO₂ NPs span diverse fields, demonstrating their multifunctionality and transformative potential [7]. In environmental remediation, they are widely used in photo catalytic processes for degrading organic pollutants and improving water quality. In energy technology, CeO₂ NPs contribute to the development of advanced electrodes for super capacitors, hybrid batteries, and fuel cells [8]. In biomedicine, their antioxidant and anti-inflammatory properties have led to their use in therapies targeting oxidative stress, neurodegenerative diseases, and cancer [9]. The development of composite materials incorporating CeO₂ NPs has further expanded their applications, providing solutions for advanced coatings, sensors, and medical implants [10]. Despite their immense potential, several challenges hinder the widespread adoption and optimal utilization of CeO₂ NPs. Scalability remains a significant hurdle, as conventional synthesis methods often struggle to meet industrial-scale production requirements without compromising quality [11]. Additionally, achieving consistent surface functionalization and addressing potential cytotoxicity concerns are critical for advancing their biomedical applications. Furthermore, the environmental and economic sustainability of CeO₂ NP production requires further exploration, particularly in light of the increasing demand for rare earth elements [12]. This review aims to provide a comprehensive analysis of the synthesis strategies, properties, and applications of CeO₂ NPs while addressing the challenges and opportunities in this rapidly evolving field. By integrating insights from recent research, this work highlights the transformative potential of CeO₂ NPs in catalysis, energy, biomedicine, and environmental applications [13]. Moreover, it identifies key gaps in current knowledge and proposes future research directions to unlock the full capabilities of CeO₂ NPs. Through this effort, the review seeks to underscore the critical role of CeO₂ NPs in advancing nanotechnology and driving innovation in sustainable and interdisciplinary scientific domains.

Unique Properties of Cerium Oxide Nanoparticles

Cerium oxide nanoparticles (CeO₂ NPs) possess unique properties that underpin their wide-ranging applications. Their rapid redox cycling between Ce³⁺ and Ce⁴⁺ oxidation states enables them to participate efficiently in catalytic reactions by readily donating or accepting electrons, significantly enhancing catalytic performance in processes such as pollutant degradation and automotive catalytic conversions [14]. Additionally, CeO₂ NPs demonstrate the ability to reversibly store and release oxygen within their lattice structure, a characteristic crucial for applications in three-way catalysis and solid oxide fuel cells [15]. This oxygen buffering capacity supports the simultaneous oxidation of hydrocarbons and carbon monoxide while reducing nitrogen oxides, playing a critical role in energy and environmental technologies [16]. At the nanoscale, cerium oxide exhibits a high surface area with abundant surface-active sites, which enhances molecular interactions and improves the efficiency of catalytic and adsorption processes. This increased surface area is essential for applications in environmental remediation, energy conversion, and storage technologies [17]. Furthermore, emerging research highlights the biocompatibility of CeO₂ NPs, emphasizing their potential in biomedical applications. Their inherent antioxidant properties allow them to scavenge reactive oxygen species (ROS), mitigating oxidative stress and offering therapeutic potential for conditions such as neurodegenerative diseases and cancer [18]. This biocompatibility also extends their utility in diagnostics and drug delivery systems. These remarkable properties make CeO₂ NPs a transformative material, bridging critical advancements across catalysis, energy, biomedicine, and environmental science. Their multifunctionality positions them as an indispensable component in the development of sustainable and innovative Nano technological solutions [19].

Synthesis methods significantly influence the physico-chemical properties—such as crystallite size, shape, surface defects, and oxygen vacancy concentration—of CeO₂ NPs. Below are some of the most widely adopted techniques.

Precipitation Method

The precipitation method, also known as co-precipitation, is a widely employed technique for synthesizing cerium oxide nanoparticles (CeO₂ NPs) due to its simplicity, cost-effectiveness, and scalability [20]. In this process, a cerium precursor, such as cerium nitrate or cerium chloride, is dissolved in an aqueous solution. Subsequently, a base, commonly ammonia or sodium hydroxide, is introduced to the solution, triggering the precipitation of cerium hydroxide [21]. This intermediate is then thermally treated to yield cerium oxide nanoparticles. This method is particularly attractive because of its straightforward procedure and low operational costs, making it suitable for large-scale production [22]. However, there are inherent challenges associated with the technique. Achieving precise control over particle size distribution can be difficult, and the resulting nanoparticles often exhibit a tendency to agglomerate, which may limit their applicability in certain advanced fields. Despite these limitations, careful tuning of synthesis parameters can mitigate these issues and significantly enhance the quality of the resulting nanoparticles [23]. Key parameters that influence the precipitation process include temperature, pH, and the choice of cerium precursor [24]. Temperature plays a critical role in determining the kinetics of the precipitation reaction and subsequent crystal growth. Lower temperatures may yield smaller particles, whereas higher temperatures can promote crystallinity [25]. The pH of the reaction medium directly impacts the rate of nucleation and growth, with optimal pH levels required to produce uniform and monodisperse nanoparticles [26]. Additionally, the type of precursor used can influence the solubility, purity, and morphology of the final CeO₂ NPs, necessitating careful selection based on the intended application [27]. By fine-tuning these parameters, researchers can improve particle uniformity, minimize agglomeration, and tailor the properties of CeO₂ NPs for specific applications. Despite its limitations, the precipitation method remains a cornerstone in nanoparticle synthesis, offering a balance between cost-efficiency and the ability to produce high-quality materials at scale [28].

Hydrothermal and Solvothermal Techniques

Hydrothermal and solvothermal synthesis methods have become indispensable for the production of high-quality cerium oxide nanoparticles (CeO₂ NPs) due to their ability to offer precise control over nanoparticle properties [29, 30]. These methods utilize sealed, high-pressure environments, typically within an autoclave, where reactions occur under elevated temperatures and pressures. The hydrothermal method employs water as the reaction medium, whereas solvothermal synthesis extends to the use of organic solvents, enabling a broader range of reaction conditions and tunability [31]. The controlled reaction environment in hydrothermal and solvothermal methods significantly enhances the crystallinity of CeO₂ NPs [32]. Under high-temperature and high-pressure conditions, uniform nucleation and growth processes occur, suppressing the formation of structural defects and improving the nanoparticles' overall stability and performance. These conditions also allow the fine-tuning of nanoparticle size and shape, which is critical for tailoring their functionality for specific applications [33]. By adjusting synthesis parameters such as temperature, pressure, reaction time, and solvent type, researchers can produce nanoparticles with various morphologies, including nanocubes, nanorods, nanospheres, and hierarchical structures [34]. Each morphology provides unique surface properties and interaction dynamics, making these techniques highly versatile for customizing nanoparticles to optimize their performance in diverse fields such as catalysis, energy storage, and biomedical applications [35]. Advantages of hydrothermal and solvothermal methods are particularly evident in their ability to produce nanoparticles with uniform size distribution and highly crystalline structures [36]. These properties are essential for applications that demand high surface activity and stability, such as photo catalysis, fuel cell technology, and antioxidative therapies. The solvothermal method, in particular, allows for the use of non-aqueous solvents, which can influence the solubility and reactivity of precursors, providing additional control over the properties of the synthesized nanoparticles [37].However, these methods are not without limitations. The requirement for specialized equipment, such as high-pressure autoclaves, increases the initial cost and complexity of these techniques, potentially limiting their accessibility [38]. Moreover, the reaction times are typically long, often extending to several hours or even days, and the high energy demands for maintaining elevated temperatures and pressures can increase operational costs. These factors make hydrothermal and solvothermal methods less favorable for large-scale, cost-sensitive production scenarios [39]. Despite these challenges, the unique advantages of hydrothermal and solvothermal techniques make them a cornerstone in the synthesis of cerium oxide nanoparticles [40]. Their ability to deliver superior crystallinity, tailored morphologies, and controlled particle sizes makes them ideal for advanced applications. Ongoing research into optimizing these methods aims to address their current limitations, focusing on reducing energy consumption, developing alternative low-cost autoclave designs, and exploring hybrid methods that combine hydrothermal or solvothermal processes with other techniques to enhance scalability [41].

Hydrothermal and solvothermal techniques represent a highly effective approach for the synthesis of CeO₂ NPs, offering unparalleled control over nanoparticle properties. By enabling the precise manipulation of particle size, shape, and crystallinity, these methods continue to drive innovation in the development of nanomaterials for cutting-edge applications in catalysis, energy, biomedicine, and environmental science [42].

Sol-Gel Method

The sol-gel technique is a widely utilized method for synthesizing cerium oxide nanoparticles (CeO₂ NPs), offering excellent control over chemical composition and homogeneity. This technique involves a two-step transformation of a liquid precursor solution ("sol") into a solid three-dimensional network ("gel") through hydrolysis and condensation reactions [43]. The process is particularly attractive for producing highly pure and uniform nanoparticles, making it a preferred approach in advanced material synthesis [44]. The process begins with the hydrolysis of metal alkoxide or metal salt precursors, during which the metal reacts with water to form hydroxyl (M–OH) bonds [45]. This step is followed by condensation reactions, where M–OH groups undergo cross-linking to form a continuous gel-like network. The resulting gel is then aged, dried, and subjected to thermal treatment to produce cerium oxide nanoparticles [46]. This approach provides a high degree of control over particle composition and structure, enabling the synthesis of CeO₂ NPs with desirable properties tailored for specific applications. One of the key advantages of the sol-gel technique is its ability to achieve exceptional chemical homogeneity, ensuring uniform distribution of the precursor materials at the molecular level [47]. Additionally, the process typically occurs at relatively low processing temperatures compared to other synthesis methods, reducing energy consumption and allowing for the integration of temperature-sensitive additives [48]. These features make the sol-gel technique particularly suitable for applications where precise control over material properties is critical, such as in catalysis, coatings, and optical materials. Its benefits, the sol-gel technique has certain drawbacks [49]. The process is time-intensive, requiring extended aging and drying periods to achieve gel stability and prevent structural collapse. Moreover, the drying stage poses a significant risk of cracking due to shrinkage, particularly when dealing with large-scale gels or thick films [50]. These challenges necessitate careful optimization of synthesis parameters, such as the precursor concentration, water-to-precursor ratio, and drying conditions, to minimize defects and improve yield. The sol-gel technique is a versatile and effective method for synthesizing CeO₂ NPs, offering superior control over chemical composition and structural properties [51]. While it demands longer processing times and careful handling during drying, the benefits of chemical homogeneity, low processing temperatures, and precision make it a valuable tool for producing high-quality nanoparticles for advanced applications in catalysis, energy storage, and functional coatings [52].

Thermal Decomposition

Thermal decomposition is a straightforward and efficient method for synthesizing cerium oxide nanoparticles (CeO₂ NPs), widely used for its simplicity and scalability [53]. The process involves the thermal treatment of cerium precursors, such as cerium acetate or cerium oxalate, under controlled conditions. When heated, these precursors decompose to release gaseous byproducts, leaving behind cerium oxide in its crystalline form [54]. This method is particularly well-suited for producing nanoparticles on a large scale, making it attractive for industrial applications. The primary advantage of thermal decomposition is its simplicity. The process requires relatively minimal equipment and straightforward operational steps, which lowers production costs and enables scalability. Additionally, the direct conversion of precursors into cerium oxide makes this method efficient for producing high-purity nanoparticles with minimal contamination. The ability to scale up production further enhances its applicability in areas such as catalysis, coatings, and energy storage [55]. This method has several challenges that need to be addressed for optimal outcomes. Precise temperature control is critical to ensure the complete decomposition of precursors and to prevent the formation of undesired phases, such as cerium hydroxide or intermediate oxides. Failure to maintain accurate temperature settings can lead to impurities, reducing the performance and functionality of the nanoparticles. Moreover, high temperatures used during decomposition often promote agglomeration of nanoparticles, resulting in larger particles with reduced surface area, which can diminish their catalytic efficiency and other desirable properties [56]. To mitigate these drawbacks, several strategies can be employed. Temperature ramping techniques, where the temperature is gradually increased, can help achieve uniform decomposition and minimize agglomeration. Additionally, incorporating stabilizing agents or surfactants during the synthesis process can further reduce particle aggregation, ensuring a more uniform product. Thermal decomposition offers a straightforward and scalable approach for synthesizing cerium oxide nanoparticles, making it a practical choice for industrial-scale production. While challenges such as phase control and agglomeration at high temperatures exist, careful optimization of synthesis parameters can enhance the quality and performance of the nanoparticles. This method remains a valuable tool in the development of CeO₂ NPs for a variety of applications in catalysis, energy storage, and environmental remediation [57].

Green Synthesis

Green or bio-inspired synthesis represents a sustainable and environmentally conscious method for the production of cerium oxide nanoparticles (CeO₂ NPs). This approach utilizes biological resources, such as plant extracts, microorganisms, and other biogenic materials, to reduce cerium salts into nanoparticles [58]. The natural components in these biological agents serve dual roles as reducing and stabilizing agents, facilitating the formation of nanoparticles under mild reaction conditions. This method aligns with the principles of green chemistry, emphasizing minimal environmental impact and reduced use of hazardous reagents [59]. The process typically involves the use of phytochemicals in plant extracts, enzymes in microbial systems, or other biogenic compounds to mediate the synthesis. Phytochemicals such as polyphenols, alkaloids, and flavonoids reduce cerium ions while simultaneously acting as capping agents to stabilize the nanoparticles. The mild reaction conditions, including ambient temperatures and neutral pH, contribute to the eco-friendly nature of this synthesis method. These characteristics make green synthesis particularly suited for applications in fields requiring biocompatible and non-toxic materials, such as biomedicine and environmental remediation [60]. The advantages of green synthesis are significant. It offers an environmentally sustainable alternative to conventional methods by eliminating the need for harsh chemicals and reducing energy consumption. The inherent biocompatibility of nanoparticles synthesized using biological agents enhances their suitability for therapeutic applications, including drug delivery, antioxidant therapies, and biomedical imaging [61]. The surface functionalization imparted by natural capping agents improves the colloidal stability and reactivity of the nanoparticles, broadening their applicability across various fields. These properties make green synthesis an attractive method for producing nanoparticles that meet both ecological and functional criteria [62]. Challenges associated with green synthesis include variability in biological resources, which can lead to inconsistencies in nanoparticle properties such as size, shape, and yield [63]. The composition of plant extracts and microbial systems is influenced by factors like geographical origin, seasonal changes, and environmental conditions, introducing variability into the synthesis process. This lack of reproducibility poses difficulties for standardization, especially in applications that demand precise control over nanoparticle characteristics [64]. Additionally, scaling up the synthesis for industrial applications requires significant quantities of biological materials, which may be resource-intensive and introduce logistical challenges [65]. Recent research focuses on standardizing the synthesis process by utilizing engineered biological systems or synthetic analogs of natural compounds to enhance reproducibility. The development of robust and reproducible protocols, including the use of well-characterized and consistent biological resources, is essential for addressing these challenges [66]. Combining green synthesis with complementary techniques, such as hydrothermal or solvothermal methods, is being explored to overcome scalability limitations and achieve improved control over nanoparticle properties. Green or bio-inspired synthesis offers a promising alternative to traditional methods for producing cerium oxide nanoparticles. Its emphasis on sustainability, reduced toxicity, and compatibility with biomedical applications highlights its potential for addressing current demands in nanotechnology and material science. Advances in standardization and hybridization of methods are likely to enhance the feasibility and effectiveness of this approach for large-scale and interdisciplinary applications [67].

Cerium oxide nanoparticles (CeO₂ NPs) exhibit remarkable catalytic, antioxidant, and regenerative properties, making them highly versatile in various applications. They are widely used in biomedical fields for drug delivery, neuroprotection, and antioxidant therapies. Their catalytic activity finds relevance in environmental protection, such as wastewater treatment and fuel cells [68]. Additionally, CeO₂ NPs are utilized in industrial applications, including UV shielding, sensors, and as additives in cosmetics and coatings. Some of its applications are as follows:

Catalysis

Cerium oxide nanoparticles (CeO₂ NPs) have emerged as critical materials in catalysis due to their exceptional oxygen storage/release capacity, reversible redox behavior (Ce³⁺/Ce⁴⁺), and high thermal stability [53]. In automotive applications, CeO₂ NPs are integral to three-way catalytic converters, where they dynamically buffer the oxygen concentration by alternately storing and releasing oxygen. This property facilitates the efficient oxidation of carbon monoxide (CO) and hydrocarbons (HCs) into carbon dioxide (CO₂) and the reduction of nitrogen oxides (NO_x) into nitrogen (N₂) [69]. The unique redox capability of CeO₂ thus enhances the catalytic efficiency and contributes to stringent emission control standards. In industrial catalysis, CeO₂ NPs serve as active catalysts or catalyst supports in reactions such as the water-gas shift reaction (WGS) for hydrogen production and oxidative desulfurization for sulfur removal from fuels. The catalytic activity of CeO₂ is significantly augmented by doping with metals like zirconium (Zr), copper (Cu), or other rare earth elements [70]. These dopants enhance the structural properties, improve oxygen mobility, and increase surface area, which collectively optimize reaction kinetics and stability under harsh operating conditions. The ability of CeO₂ to undergo reversible oxygen exchange, combined with its structural versatility, positions it as a cornerstone material in both automotive and industrial catalytic processes, advancing sustainable energy and environmental technologies [71].

Energy Storage and Conversion

Cerium oxide nanoparticles (CeO₂ NPs) are increasingly recognized for their critical role in energy storage and conversion technologies due to their unique redox properties, high ionic conductivity, and excellent thermal stability. These attributes make CeO₂ a material of choice in advanced systems like solid oxide fuel cells (SOFCs) and solar-driven thermochemical cycles, which are pivotal in the shift toward sustainable energy solutions. The context of solid oxide fuel cells (SOFCs) is that doped CeO₂ serves as a high-performance electrolyte or interlayer material. SOFCs rely on the transport of oxygen ions across the electrolyte to facilitate electrochemical reactions that convert chemical energy into electrical energy [72]. Conventional SOFCs, which use materials like yttria-stabilized zirconia (YSZ) as electrolytes, require operating temperatures exceeding 800°C. However, these high temperatures often result in thermal degradation and reduced system longevity. CeO₂, particularly when doped with trivalent ions such as gadolinium (Gd³⁺) or samarium (Sm³⁺), exhibits significantly higher ionic conductivity at intermediate temperatures (500–700°C). This improvement arises from the increased concentration of oxygen vacancies introduced by doping, which enhances oxygen ion transport [73]. By enabling SOFCs to operate efficiently at lower temperatures, doped CeO₂ reduces thermal stress, improves durability, and lowers manufacturing and operational costs. Furthermore, its role as an interlayer minimizes undesired reactions between other cell components, further enhancing the performance and longevity of the SOFC system. Solar-driven thermochemical cycles, CeO₂ stands out for its ability to harness concentrated solar energy for the splitting of water (H₂O) and carbon dioxide (CeO₂). These cycles operate by leveraging the redox flexibility of cerium oxide, which alternates between its reduced (Ce³⁺) and oxidized (Ce⁴⁺) states. Under high-temperature solar irradiation, CeO₂ undergoes a thermochemical reduction, releasing oxygen and creating oxygen vacancies within its lattice structure [74]. In the subsequent oxidation step, the reduced CeO₂ reacts with H₂O or CO₂ to produce hydrogen (H₂) or carbon monoxide (CO), respectively. The hydrogen produced can serve as a clean fuel, while the carbon monoxide can be combined with hydrogen to synthesize syngas, a versatile precursor for a variety of fuels and chemicals. This process not only provides a pathway for storing solar energy in chemical form but also offers a means for recycling CO₂, contributing to the development of carbon-neutral energy technologies. The high thermal stability of CeO₂, combined with its rapid redox kinetics and scalability, makes it particularly well-suited for integration into large-scale solar reactors. Furthermore, advancements in material engineering, such as nanoscale structuring and doping with elements like zirconium (Zr) or hafnium (Hf), have further optimized its performance by enhancing its redox capacity and reaction rates. These innovations ensure that CeO₂-based thermochemical systems can operate efficiently under the extreme conditions required for solar fuel production. The versatility and efficiency of CeO₂ NPs in energy storage and conversion underscore their importance in advancing clean energy technologies. Their integration into SOFCs and solar thermochemical cycles represents a significant step toward achieving sustainable energy systems, highlighting the material’s transformative potential in addressing global energy challenges [75].

Environmental Remediation

Cerium oxide nanoparticles (CeO₂ NPs) have demonstrated significant potential in environmental remediation due to their surface chemistry, redox properties, and photo catalytic capabilities [76]. Their high reactivity and ability to neutralize pollutants make them an effective material for mitigating environmental contamination in water, soil, and air. One of the key applications of CeO₂ NPs is the adsorption of pollutants. The surface of CeO₂ exhibits a high density of oxygen vacancies, which act as active sites for binding various contaminants. These include heavy metal ions such as lead (Pb²⁺), mercury (Hg²⁺), and arsenic (As³⁺), as well as organic dyes like methylene blue and rhodamine B, which are prevalent in industrial wastewater [77]. The nanoscale dimensions of CeO₂ provide a high surface area, enhancing its adsorption efficiency and enabling effective removal of pollutants from contaminated media.CeO₂ NPs also exhibit strong photo catalytic activity, making them suitable for the degradation of organic pollutants under UV or visible light. Upon light exposure, CeO₂ generates electron-hole pairs, which interact with water and oxygen molecules to produce reactive oxygen species (ROS), such as hydroxyl radicals (•OH) and superoxide anions (O₂^-•). These ROS oxidize and break down complex organic molecules into simpler, less harmful compounds, ultimately mineralizing them into CO₂and water [78]. CeO₂ is effective in degrading persistent pollutants like pesticides, pharmaceuticals, and aromatic hydrocarbons. The photocatalytic efficiency of CeO₂ can be enhanced by doping with metals such as zinc or titanium or coupling with other semiconductors like TiO₂ or ZnO, which extend its light absorption range into the visible spectrum and improve its performance under natural sunlight. The redox flexibility of CeO₂, enabled by the reversible transition between Ce³⁺ and Ce⁴⁺ states, allows it to catalytically reduce toxic substances such as nitrogen oxides (NO_x) or sulfur oxides (SO_x) in gaseous emissions. This makes CeO₂ an effective material for both water treatment and air purification, as well as for controlling industrial emissions [79]. The multifunctional properties of CeO₂ NPs in pollutant adsorption and photocatalysis highlight their significance in environmental remediation. Research efforts aimed at improving their efficiency through doping, nanoscale engineering, and composite development are likely to expand their applications in sustainable environmental management [80].

Gas Sensing

Cerium oxide nanoparticles (CeO₂ NPs) are highly effective materials for gas sensing applications due to their exceptional redox properties and surface reactivity. The redox cycling of cerium ions between Ce³⁺ and Ce⁴⁺ states enables dynamic detection of gaseous analytes, such as carbon monoxide (CO), hydrogen (H₂), and volatile organic compounds (VOCs) [81]. This redox activity facilitates gas adsorption and reaction on the CeO₂ surface, leading to measurable changes in electrical conductivity or other properties, which form the foundation of gas-sensing mechanisms [82]. The performance of CeO₂-based gas sensors is strongly influenced by particle size, defect density, and surface characteristics. Smaller particle sizes increase the specific surface area, providing more active sites for gas interactions. High oxygen vacancy density enhances adsorption and promotes redox reactions with target gases, improving sensitivity and reducing detection limits. Optimizing these structural features is crucial for achieving efficient and reliable gas sensing [83].

Material modifications often improve sensor performance. Doping CeO₂ with transition metals such as copper or iron, or rare earth elements like gadolinium or samarium, enhances redox activity and electronic properties. These modifications increase oxygen vacancies and alter the electronic structure, leading to stronger and more specific interactions with certain gases. Coupling CeO₂ with other semiconductors, such as ZnO, SnO₂, or TiO₂, creates heterojunctions that facilitate charge transfer, improve response times, and enhance selectivity.CeO₂-based sensors are known for their stability and ability to operate under varying environmental conditions, including fluctuations in temperature and humidity [84]. This versatility makes them suitable for a wide range of applications, such as industrial safety, environmental monitoring, and healthcare diagnostics, where detecting harmful gases or air composition changes is critical. CeO₂’s tunable properties, high stability, and capability to detect multiple gases make it a key material for gas-sensing technologies [85]. Advances in nanotechnology, such as the synthesis of ultra-small CeO₂ particles or hybrid nanostructures, continue to enhance sensor performance. These developments strengthen the role of CeO₂ NPs in next-generation gas sensors, contributing to improved safety, environmental sustainability, and public health [86].

Biomedical Applications

Cerium oxide nanoparticles (CeO₂ NPs) have garnered significant interest in biomedical research due to their unique redox properties, biocompatibility, and ability to interface with complex biological systems [87]. These properties have enabled their exploration for diverse applications, including antioxidant therapy, drug delivery, and anti-inflammatory treatments, demonstrating their potential to address a range of challenging medical conditions. The antioxidant properties of CeO₂ NPs are derived from their reversible redox cycling between Ce³⁺ and Ce⁴⁺ states, enabling them to act as highly effective scavengers of reactive oxygen species (ROS) such as superoxide anions, hydroxyl radicals, and hydrogen peroxide [88]. ROS are implicated in the pathogenesis of various oxidative stress-related diseases, including neurodegenerative disorders like Alzheimer’s and Parkinson’s, cardiovascular diseases, and certain cancers. CeO₂ NPs exhibit a self-regenerating capacity to continuously neutralize ROS, maintaining oxidative balance over extended periods [89]. This capability positions them as promising therapeutic agents for managing oxidative stress and its associated diseases. Their nanoscale size allows for cellular uptake and effective interaction with intracellular ROS, offering an innovative approach to mitigating oxidative damage in biological systems. CeO₂ NPs also show significant potential in drug delivery applications, leveraging their intrinsic biocompatibility and ability to be functionalized for specific biomedical purposes [90]. Surface modifications with biopolymers, peptides, antibodies, or ligands enhance their stability, targeting ability, and therapeutic payload capacity. This functionalization enables the nanoparticles to recognize and bind to specific cellular receptors or biomarkers, facilitating targeted drug delivery with high precision. For example, functionalized CeO₂ NPs have been explored for delivering chemotherapeutic agents directly to tumor sites, minimizing systemic toxicity and improving treatment efficacy [91]. Their ability to respond to environmental stimuli, such as pH changes in the tumor microenvironment, further enhances their role in controlled drug release. Additionally, CeO₂ NPs can be engineered for dual functionality in theranostics, combining drug delivery with imaging capabilities for real-time monitoring of therapeutic outcomes [92].

Emerging research indicates that CeO₂ NPs also possess anti-inflammatory properties, making them a potential tool for treating inflammatory diseases. By modulating cellular signaling pathways, CeO₂ NPs can suppress the release of pro-inflammatory cytokines and reduce the activation of immune cells involved in chronic inflammation [93]. This modulation is attributed to their ROS-scavenging activity, which interrupts oxidative signalling cascades that exacerbate inflammation. These anti-inflammatory effects make CeO₂ NPs promising candidates for managing conditions such as rheumatoid arthritis, inflammatory bowel disease, and chronic obstructive pulmonary disease (COPD). The ability of CeO₂ NPs to reduce inflammation without compromising immune system function enhances their appeal as a novel therapeutic strategy. The versatility of CeO₂ NPs stems from their unique physicochemical properties, tunability through surface modifications, and biocompatibility [94]. Their multifunctional nature allows for integration into a wide array of biomedical applications, from therapeutic interventions to diagnostic tools [95]. However, further research is required to optimize their synthesis, ensure safety and efficacy, and understand their long-term biological interactions. As advancements continue, CeO₂ NPs hold the potential to revolutionize the treatment of oxidative stress-related disorders, inflammatory diseases, and targeted drug delivery systems, paving the way for innovative approaches in modern medicine [96].

Cerium oxide nanoparticles (CeO₂ NPs) have emerged as a versatile material with diverse applications spanning catalysis, energy storage, environmental remediation, gas sensing, and biomedicine. Their exceptional redox properties, high oxygen storage capacity, thermal stability, and tunable surface chemistry make them indispensable for various technological and industrial advancements. The synthesis methods, including precipitation, hydrothermal, sol-gel, thermal decomposition, and green synthesis, offer tailored approaches to optimize the size, shape, and functionality of CeO₂ NPs, enabling their adaptation to specific applications. In catalysis, CeO₂ NPs have demonstrated superior performance in automotive exhaust systems and industrial chemical reactions, while their role in energy storage and conversion highlights their potential to advance sustainable energy solutions, including solid oxide fuel cells and solar thermochemical cycles. In environmental remediation, CeO₂ NPs are effective in pollutant adsorption and photo catalysis, making them invaluable in water and air purification. Their sensitivity to gaseous analytes underpins their utility in gas sensing applications, and their biocompatibility, coupled with antioxidant and anti-inflammatory properties, establishes them as promising candidates for biomedical innovations. Despite these advancements, several challenges remain. The long-term environmental and biological safety of CeO₂ NPs requires comprehensive evaluation to ensure their sustainable use, particularly in biomedical and environmental applications. Scalability and cost-effectiveness of synthesis methods also need improvement to facilitate industrial adoption. Moreover, a deeper understanding of their interaction with biological systems and their fate under environmental conditions is crucial for realizing their full potential. Future research should focus on developing more efficient and eco-friendly synthesis methods, such as green synthesis, to minimize environmental impact. Advances in surface engineering and doping strategies can further enhance the performance of CeO₂ NPs for specific applications, such as extending their photo catalytic activity into the visible light range or improving their ionic conductivity for energy applications. In the biomedical field, efforts should be directed toward achieving precise control over particle size, surface charge, and functionalization to maximize therapeutic efficacy while minimizing potential cytotoxicity. In conclusion, the multifunctional nature of CeO₂ NPs, combined with ongoing innovations in their synthesis and application, positions them as a cornerstone material in addressing global challenges related to energy, the environment, and health. By addressing current limitations and exploring new frontiers, CeO₂ NPs hold immense promise for driving transformative advances across a wide spectrum of scientific and industrial domains.

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