Synthesis and Biomedical Applications of Zirconium Nanoparticles: Advanced Leaps and Bounds in the Recent Past

Many synthetic routes manufacture zirconium nanoparticles in metal oxide, nitride, and other combination forms. Coupled with other variables such as concentration, pH, and form of precursor used, the various synthetic methods support synthesizing the zirconium metal oxide nanoparticles with changed features. Various synthetic methods were studied, such as sol-gel, hydrothermal, laser ablation, and precipitation. All have different synthetic routes, different precursors and solvents were sued, and the product was characterized by SEM, TEM, photo luminance spectroscopy, UV-absorption spectroscopy, and powder X-ray diffraction. X-ray diffraction determined the crystal structure by identifying the crystal shape, arrangement of atoms, and spacing between them. SEM and TEM studied the particle size and morphology of nanoparticles. UV-visible absorption spectroscopy and PL spectroscopy were used for the determination of optical properties of nanoparticles. Zirconium oxide nanoparticles have many applications in the medical field. The review study primarily focuses on the efficient combination of zirconium dioxide with other additive materials and functionalization techniques used to improve the material's properties, assisting the use of the material in hip arthroplasty and bone tissue applications. The development of sophisticated near-infrared (NIR) absorbing small molecules for useful phototheranostic applications was discussed in this paper.


Introduction
Nanoparticles have a diameter of 1-100 nm in range. Although nanoparticles have great chemical, mechanical, thermal, magnetic, and electrical capabilities, scientists still encounter difficulties identifying, creating, and analyzing the potential of these nanostructured objects [1]. Different metal oxides of nanoparticles synthesized through joining two metals in the form of oxide give the manufactured product enhanced fea-tures. The metal oxide nanoparticles differ in various physical properties and chemically and morphological characteristics and are applied in different fields because they have different uses [2]. Nanomaterials deal with the branch of science which studies the particles at a nanoscale level ranging from 10 to 100 nm. The nanoparticles' unique properties depend upon the particles' small size [3]. Zirconium belongs to the class of strong transition metals, their properties resembling titanium due to strong action against corrosion. Mostly, zirconium metal is obtained from minerals such as zirconium which can be refined with chlorine organic and inorganic compounds and can also be synthesized zirconium metal [4]. Zirconium nanoparticles have been used for different syntheses due to their various assets, such as extraordinary fracture toughness, high tensile strength, and hardness. Naturally, zirconium can exist in five different isotopes form; out of five isotopes, 90 Zr exists abundantly in nature almost (51.45%) [5]. Various synthetic paths have existed to make nanoparticles of metal oxide and mixed metal oxide. Different technical methods have assembled all of these synthetic paths. Zirconium metal complexes have catalytic use [6]. The growing of TWCNs in the size of 2D and 3D graphene layer zirconium nanoparticles (NP) is used as a catalyst. For the formation of good-quality carbon nanotubes (CNTs), using zirconium as a catalyst is the best for such high-purity carbon nanoparticles [7]. Kroll method was good for synthesizing zirconium nanoparticles; when zirconium tetrachloride is treated with magnesium metal, an active metal, then the temperature is maintained at 800-900°C. In this article, we covered the various synthetic methods for creating zirconium nanoparticles [8] in cubic, monoclinic, and mixed phases, including the sol-gel method, the hydrothermal method, the laser ablation method, the wire explosion process, hybrid transparent coating, the chemical polymerization, and the coprecipitation method [9].

Synthesis of Zirconium Metal Oxide Nanoparticles in Mixed Form
The various synthetic routes applied to manufacture nanoparticles of metal oxide are also in the form of mixed metal oxide. Under a different technological method, every synthetic method has been clustered (Table 1). In unity with the growth of crystals, such as thin film growth [10], thin film growth is classified into two types: one is vapor phase growth (VPG), and second is liquid phase growth (LPG); other growth methods are the formation of solid phase and the hybrid growth of the crystal, which form the nanoparticles with the help of colloidal treating, nanoparticles utilizing rapid growth like template-based growth and thin films growth [11] that have a thickness range from the friction of nanometer to several microns by molecular beam epitaxy [12] and the nanostructure in the form of bulk materials, and also in conflict with the models, a changed practical methodology could be considered [13]. The top-down synthesis approach involves reducing the bulk materials into a nanoscale dimension utilizing a physical approach, but the bottom-up approach is in which the smaller unit is combined to form a bulk unit ( Figure 1) [14].
2.1. Cubic form Zirconium Oxide (ZrO 2 ) nanoparticles. The cubic shape of zirconium oxide nanoparticles was manufactured with the reaction of xylene, zirconium oxychloride, and N-cetyl-N, N, N-trimethylammonium bromide taken in different amounts and added to the distilled water of 100 ml and stirred the mixture almost for 2 h [22]. To maintain the pH at 9, sodium hydroxide (0.5 M) solution was added. After that, the white precipitate was obtained, and the mixture was refluxed for 3 h at 85. The reaction mixture was cool and then stirred strongly for 24 h. The obtained mixture was like a suspension, and the white precipitate was obtained [23], and then, they can be filtered, diluted, and washed with water (H 2 O) and ethanol (C 2 H 5 OH) to separate the xylene and CTAB. The obtained precursor was dried for 17 to 18 h at 100°C. After that, they can be calcinated for 4 h at 500°C [24].
2.2. Monoclinic and Cubic Phase Nanoparticles. The mixed monoclinic and cubic phase zirconium oxide nanoparticles were synthesized zirconium oxychloride (ZrOCl 2 .8H 2 O), 1hexanol, and oxalic acid (COOH) 2 . Zirconium oxychloride and oxalic acid were taken equal to 1 mol/l, but 1-hexanol (C 6 H1 4 O) was taken at 2 mol/l [25]. These chemicals were mixed utilizing the pestle and mortar, and the sample was equipped in the form of rheological from the mixture [26]. Then, the mixture was reacted at 98°C for 13 h. It can be washed with ethanol (C 2 H 5 OH) and water and then dried in an oven for 4 h and adjusted to the temperature of 500°C. X-ray powder diffractometers were used to determine the crystallographic features of the sample by using Ge and Cu monochromatized Kα radiation by using Bragg's relation [26], where λ = 1:54287Å and 2θ range of 20-80°.
The morphologic properties of the crystal are determined by using SEM and TEM. For the determination of optical absorption spectra, spectrometer was used at room temperature. Optical spectra were recorded in 200-800 nm wavelength regions [27]. At room temperature, identification of photoluminescence was achieved by employing a Dilor XY device, which has the capability of being fitted with an additional components monochromator. Electrochemical properties of ZrO 2 nanoparticles were analyzed by silver-silver and counter electrode with the reference electrode method [4].

Ultrafast Laser
Ablation. The metallic form of Zr nanoparticles manufactured by ultrafast laser ablation of a zirconium rod in isopropyl alcohol has been defined in the literature. This process forms a colloidal form of solution of zirconium nanoparticles. In this process, using the femtosecond laser pulses for ablation, the proportion dispersal of nanoparticles can be significantly concentrated [28]. For the development of 50 nm metallic zirconium nanoparticles, the setup of plasma-induced cathodic discharge electrolysis under argon (Ar) gas in melted salt was also used. Implants are also made of titanium and zirconium [29]. Zirconium is extremely flammable in powder form and thus has armed uses, such as in manufacturing explosive resources for military capability. The zirconium oxide (ZrO) was then characterized by powder X-ray diffraction [30].

Wire Explosion
Process. The zirconium nitride (ZrN) nanoparticles were manufactured by the wire explosion method; in this process, the amount of energy (E) accumulated on the wire and the ambient pressure show a significant character in the size of the shaped particles [31]. The qualitative analysis of the synthesized particles can be analyzed by this method. It is possible to manufacture nitride nanoparticles by giving sufficient energy to the electrode for evaporation and by keeping an effective standard for nitridation [32]. To obtain zirconium, a systematic experimental study was performed. By the explosion of the wire, as well as the blowing up of zirconium nanoparticles while they were being conducted in a nitrogen (N 2 ) atmosphere, nitride nanoparticles of a much smaller scale were created.The X-ray diffraction (XRD) and designated region electron deflection analysis characterized the synthesized powders [33]. A TEM was used to examine the size and shape of the constituent part formed by the wire explosion method. Particle bulk dispersal experiments were conducted by following the distribution of log-normal possibility. The connection between the size of nanoparticles, the wire explosion method produced, and the nitrogen ambient pressure was investigated [34].

Hybrid Transparent
Coating. The hybrid transparent zirconium nanoparticle filler-based coatings and epoxy resin have improved the scratch conflict of goods polymers without losing their visual properties. The nanocrystalline ZrO 2 suspensions in the benzyl alcohol have been manufactured using ZrCl 4 as a precursor through a flexible nonhydrolytic Table 1: Different methods for the preparation of zirconium oxide complexes.

Synthesis Benefits Drawbacks Morphology
Coprecipitation method Preparation is straightforward; minimal equipment is required, and it is commercialized. A representative method for producing heterogeneous catalyst powders with good homogeneity and a relatively low calcination temperature is the usual coprecipitation techniques for producing heterogeneous catalyst powders 50 with good homogeneity and a relatively low calcination temperature coprecipitation technique [15].
One of this method's biggest flaws is the inability to regulate the precipitating particle sizes and subsequent aggregation. Therefore, hybrid approaches that control the size of the particle by deagglomerating the generated nanomaterial should be considered [16]. Under ultrasound irradiation, the precipitated gel undergoes extremely high shear pressures and cavitation heating, creating nanoparticles and high-phase purity in complicated metal oxides. Therefore, agglomeration, poor yield, and finished product purity affect many factors, such as pH values, washing solvents, and dry method.
According to morphological research, the grains are compact and randomly organized, and as the temperature of sintering rises, so does grain size. X-ray photoelectron spectroscopy analysis has been used to investigate the compositional analyses of metal nanoparticles [17]. To explore the surface morphology and particle size fluctuations in metal oxide nanoparticles, the pH value of the solution was changed.

Sol-gel synthesis
The sol-gel method is superior to other methods for creating metal oxide nanoparticles because it is straightforward and inexpensive and uses low temperatures and pressure [18]. Easy to use; high purity; uniform distribution; commercialized, perfect, high purity and time-saving. Newly, alumina membranes with pore sizes of around 3 to 4 nm, thicknesses of 1 to 10 μm, and porosities of about 50% have been created using the sol-gel process.
The relatively substantial shrinkage associated with the gelation process and the drying of gels, the presence of high pore concentrations, and the removal of unwanted residuals such as hydroxyls and organics are some of the key drawbacks of the sol-gel method [19]. It takes a long time to sinter; it has weak sintering characteristic.
The experimental circumstances and processing factors used during the solgel synthesis process can have an impact on the materials characteristics.
In order to tailor the numerous properties displayed by the produced nanoparticles to the required applications, processing parameters are the operating conditions that must be taken into account throughout the synthesis process of nanoparticles [20]. Effects of variables on the morphology and optical qualities include the pH of the sol, additives (such as capping agents and surfactants), annealing temperature, and calcination.

Hydrothermal method
Metal nanoparticle application in biomedicine and related fields is constantly growing globally. Applications for hydrothermally produced nanoparticles include those in optics, medicine, electronics (including sensors, information, and communication technologies), catalysis, devices (including fuels for energy conversion and storage), and electronics [21].
These techniques produce nanoparticles with less precise size distributions, compositional control, and optoelectronic characteristics. According to Xu et al., hydrothermally created CIS NCs coated in glutathione and exhibiting multiple emission bands in the photoluminescence (PL) spectrum.
At every stage of the process, from the unit cell to the crystallite size to the size and shape of the nanoparticles, the surfactants and dopants have a significant impact [21].
3 BioMed Research International sol-gel process [35]. The ZrO 2 nanoparticles displayed a tetragonal phase-attributable crystalline structure with a normal particle size of 2 nm. A commercial epoxy resin was mixed with ZrO 2 nanoparticles adjourned in tertbutanol. The biological solvent was then vaporized, and the polycarbonate and hybrid composites were deposited [36]. There was a homogeneous adhesion and a marginal effect on the clearness of polymers in the coatings. The improved scratch resistance was achieved by increasing the content of ZrO 2 nanoparticles.
2.6. Synthesis of Crystalline Zirconium Nanoparticles. Due to their atom-like size-dependent property, semiconductor particles in the nanometer mass regime have been involved in considerable desirability. Recently, several improved routes have been reported for the amalgamation of such type's nanomaterial [37] with tunable characteristics. Ceramics among the zirconium are an unusual form of transition-metal semiconductor with the functional properties of pathetic acidic and basic goods. Fast combination and categorization of cubic form ZrO 2 nanoparticles with hydrothermal influence over the morphological and crystallinity properties [38]. The temperature is constantly used to produce cubic form ZrO2 nanoparticles at the lowest rate. In the synthesis process, zirconium isopropoxide, 1 g was taken in the flask made up of Teflon, and then, analytical graded ethanol 6 ml was added. After that, the flask was placed in a desiccator chamber [39]. The ZrO 2 precipitation was obtained lower than the saturated atmospheric condition by introducing a Petri plate with the water at the end of the desiccator. After 12 h, the reaction was immobile, and 25 ml of sodium hydroxide (NaOH) was added. Then, the whole reaction was sealed in the hydrothermal stainless steel for 18 h at 180°C. When the autoclave was done, the solution mixture was cooled at room heat; the mixture solution was continuously washed with 0.1 M solution of nitric acid (HNO 3 ) and 1 N hydrochloric acid (HCl) solution [40]. A fibrous and white soft like powder was achieved after drying for almost 3 h under the vacuum. The HREM examined the obtained crystals, which can be synthesized by the hydrothermal method. The image of synthesized zirconium oxide (ZrO 2 ) nanoparticles was taken with the help of TEM by this constituent part mass of nanoparticles from 5 to 6 nm [41].

Synthesis of Nanocrystalline by Sol-Gel Method with
Organic Precursor. In manufacturing zirconium nanoparticles, the current research developed the sol-gel process with organic extracts such as glucose and fructose (an example of monosaccharides) as two organic additives [42]. The existence of these organic precursors created some positive effects. The transition phase of the crystal altered from tetragonal to monoclinic, determining the shape and crystallinity bulk of the nanoparticles. Zirconium nanoparticles were synthesized using various techniques, including precipitation, combustion analysis, inert gas condensation, sol-gel, and thermal decomposition. However, the sol-gel method is the most effective for synthesizing nanosize particles, including nanoceramics, nanozirconium, and nanotitanium [43]. The crystalline size, phase, and other characteristics of zirconium nanoparticles depend on the precursor, thermal property, and pH during hydrolysis. By the sol-gel method, zirconium's physical properties, homogeneity, and purity are controllable at low temperatures [44]. Hydrolysis of sucrose in the presence of glucose and fructose yielded cubic phase zirconium nanoparticles; sucrose hydrolyzed into gluconic acid, a chelating form, which then turned into a complex form when combined with metals. The layer form of pectin surrounds this obtained complex, so by this, the process of agglomeration was stopped. Glucose and fructose are comparatively inexpensive [45] and are proposed as organic additives. The additives proposed are also nontoxic, easy to store at room temperature and properly delivered, and polar, so water-soluble. The proposed sol-gel solution is naturally approachable and requires 15-20 h, two major advantages. Organic additives cause particles to take on a spherical shape, reach a reasonably constant size, and reduce the crystalline size in the range of 10-30 nm. Then, the agglomeration of the particles would be stopped because there are efficient agents for capping [46]. Because of this, the size of the particles is reduced, and as a result, the phase transition that occurs at high temperatures where the structural property changes from tetragonal form to monoclinic form is avoided.

BioMed Research International
Zirconium salt used is zirconium propoxide which is cheaper than others such as zirconium nitrate and zirconium chloride. Initially, n-propanol and n-isopropoxide were mixed in such a way zirconium in the form nisopropoxide was diluted by using the zirconium precursor, which was 30 wt% [47]. Then, adding ammonia (NH 3 ) and water, the mixture was diluted with distilled water in the 9-10 pH range. For the hydrolysis, the volume ratio was 1 : 4 volume zirconium in n-propoxide form and water (H 2 O) was used. The mass ration of 1 : 1 of glucose to fructose was added in the mixture during vigorous stirring after the homogenization of the solution mixture at continuously stirred at low temperature for several hours [48]. The obtained gel was then polymerized and dried in the oven for 12 h at 110°C temperature. The temperature ranged from 300 to 700°C. All the sample mixture was calcinated. The characterization techniques used are XRD, SEM, transmission electron microscope [49], photoluminescence, and infrared for characterizing the synthesized sample.

Synthesis of Zirconium Nanoparticles by Coprecipitation
Method. The process of coprecipitation involves using a precipitating medium to precipitate the oxo-hydroxide phase from a salt precursor such as chloride and nitride of metal salt solution in a solvent like water and sodium hydroxide. There is a period of nucleation that is accompanied by a stage of growth of nanoparticles which can be attained at the point of critical species concentration of the solution. The ZrO 2 nanoparticles have been created with coprecipitation [50]. The ZrO 2 -CeO was synthesized by two different methods: first one is by sol-gel method, and second one is by coprecipitation method. In the coprecipitation and solgel synthesis route of the obtained materials, it was reported that the texture and structural properties depend on the precursor and synthetic routes. The research, classification, and antiseptic features of MgO-ZrO 2 oxide nanoparticles have been examined [51]. The antiseptic analysis was recorded after the experiment to demonstrate that the blended nanoparticles have been used to produce infectious diseases by E. coli. The usefulness of suspension desiccating in preventing powder agglomeration and the development of nanostructure in moderately stabilized zirconium form solid solutions established by coprecipitation were studied [52].
2.9. Hydrothermal Method. The hydrothermal process is a solvothermal synthetic route used to formulate multiple nanomaterials. This process involves separating the preliminary material in an acceptable solvent and then subjecting it to temperatures (T) and pressures (p) that are reasonably high. This creates the conditions that lead to the formation of nanoparticles. Machmudah et al. investigated that hydrothermal synthesizing cesium and zirconium combine oxide and stated that the greater part of the units produced is subject to the temperature provided. Zheng et al. studied that zirconium in the form of mixed oxide organized by constant hydrothermal synthesis method has been investigated utilizing a catalyst in supercritical water [53]. Due to its thinly agglomerated morphology and its potential use as a catalytic agent, it has been stated that the supercritical creation can clue to zirconium mixed oxides with greater thermal strength and enhanced oxygen storing capacity. Various characterization methods have defined the dimension, crystal configuration, and morphology of the manufactured zirconium metal oxide nanoparticles [54], such as SEM, TEM, and X-ray diffraction (XRD). Researchers used different characterization tools for the characterization of prepared sample materials.
2.10. Medical Application of Zirconium Nanoparticles. Due to an increase in accidents and natural disasters that cause flaws in bone structure, the world's population is currently experiencing serious issues. Replacement or repair of bone defects is essential for their proper functioning. This need has spurred the extensive study of orthopedic implants, hip arthroplasty, partial and complete denature removal, radio pacifying agents, bone tissue engineering, and bone cement for biomedical applications (Figure 2) [55].
2.11. Orthopedic Implants. Orthopedic implants age more quickly due to crumpling and microcracking, affecting how well the implants wear over time. In the periprosthetic tissues, the particles produced by zirconia wear at contact surfaces cause macrophages to react, which causes the production of proinflammatory cytokines that stimulate osteoclastic bone resorption and osteolysis loosening of orthopedic implants. The reduction in hardness from 18 GPa to 11 GPa ended the material's microcracking. Zirconia's ageing can be stopped by paying attention to fundamental material characteristics, including density, grain size, and uniformity of the phase distribution, which are crucial for maintaining the cohesiveness of the material's particles and observing its mechanical properties [56].
2.12. Hip Arthroplasty. Total hip prosthesis wear is a serious clinical issue affecting many patients. Wear experiments are carried out on new biomaterials to lengthen material life in orthopedic implants to learn more about the tribological phenomena involving hip prostheses [21]. Wear debris and corrosion of biomaterial implants have been identified as a significant factor in the failure of medical implants in a recent study on total hip arthroplasty, hip prosthesis, and joint replacement. According to a study, hip replacements are predicted to rise to 5 lac by 2030 [57]. The electrochemical behavior of anodic zirconium dioxide in stimulated bodily fluid was demonstrated by a research reported by Wang and Luo. The experimental findings concluded that the increased performance of zirconia nanotubes in stabilizing zirconium-containing metals led to their application in corrosion resistance for hip arthroplasty [58].
2.13. Radio Mollifying Agents. The primary treatment for treating pulpal and periapical disorders is root canal therapy. It should be noted that root canal obturation affects the outcome of treatment. The main method used during the procedure to assess the root canal obturation is X-ray film. So good radiopacity is required for the intended root filling substance. Some metal dioxides, such as bismuth oxide, niobium oxide, and ZrO 2 , can be suggested as excellent options for the radiopacifiers in cement-base filling material [59]. 5 BioMed Research International 2.14. Bone Tissue Engineering. To restore, maintain, or enhance organ function, tissue engineering typically incorporates seed cells, engineering techniques, biomaterial scaffolds, and physical and chemical variables. And the best option for repairing bone abnormalities is known as bone tissue engineering. To satisfy the stressed zone criterion, biomaterial scaffolds must have sufficient mechanical strength [60]. Additionally, in order to enable the development of osteoblasts, vasculature, and new bone, these scaffolds must possess specific osteoinductivity and cytocompatibility qualities as well as the connectivity structure. Through the replication process, researchers have created porous nano-ZrO 2 scaffolds (Figure 3) [42].
Various materials are available for the functionalization of zirconium dioxide utilised in bone and tissue applications. The investigation's main goal is to use various functionalization techniques to promote the regeneration of broken bone or tissue, which will substantially impact the usage of ceramic materials in biomedical applications. The modified regenerated bone and tissue's biological and mechanical characteristics have been thoroughly discussed. The entire overview of the resorption process has been provided, taking into account the reactions to the resorbed products in vivo; on the contrary, the bone resorption of implants has been thoroughly examined [61].

Bone Resorption.
Wear debris and periprosthetic osteolysis of medical implants have a significant role in loosening prosthetic implants. Zirconium dioxide is typically used to manage debris and inflammatory reactions to wear products. Comparing osteoblast, fibroblast, and macrophage cell proliferation to CoCrMo-alloy, zirconia particles perform better [62]. In osteoarthritis and rheumatoid synovial cells, Liagre et al. investigated the effects of zirconia or aluminum particles on the production of proinflammatory interleukin and the metabolism of arachidonic acid. In this study, Ramaswamy et al. investigated the attachment of human osteoclasts grown in vitro to resorbable bioceramics with various surface properties and chemical compositions to affect osteoclast resorption and the creation of resorption lacunae. Specifically, the researchers were interested in how these factors could affect the resorption of bone. Using a mouse coculture with osteoblast-like cells on bone slices, Sabokbar et al. showed how the addition of zirconium dioxide nanoparticles increased tartrate-resistant acid phosphate (TRAP) expression and bone resorption in the system [63].

Conclusion
After studying the different literature for synthesizing zirconium nanoparticles, various synthetic routes were identified,   BioMed Research International such as hydrothermal, sol-gel, laser ablation, coprecipitation, hybrid transparent coating, and wire explosion process. The materials' various properties such as shapes, size, and other desired features are controlled and regulated by precursor, solvent, and cosolvents used in synthetic routes and temperature. Different characterization tools were used, such as Xray diffraction, scanning electron microscopy, transmission electron microscopy, and FTIR. The bulk of the synthesized nanoparticles was determined by TEM. The combination of metal oxide has synthesized zirconium and metal nitride. These have various applications in different fields, such as antimicrobial and industrial applications.

Data Availability
The data will be available to the readers upon request from the first author.

Conflicts of Interest
The authors declare there is no conflict of interest.