key: cord-0784042-072mt2y7
authors: Tran, Thuan Van; Nguyen, Duyen Thi Cam; Kumar, Ponnusamy Senthil; Din, Azam Taufik Mohd; Jalil, Aishah Abdul; Vo, Dai-Viet N.
title: Green synthesis of ZrO(2) nanoparticles and nanocomposites for biomedical and environmental applications: a review
date: 2022-01-08
journal: Environ Chem Lett
DOI: 10.1007/s10311-021-01367-9
sha: b94dc2ebf6d427d67014b58209cf82a7c41ebe49
doc_id: 784042
cord_uid: 072mt2y7

Pollution and diseases such as the coronavirus pandemic (COVID-19) are major issues that may be solved partly by nanotechnology. Here we review the synthesis of ZrO(2) nanoparticles and their nanocomposites using compounds from bacteria, fungi, microalgae, and plants. For instance, bacteria, microalgae, and fungi secret bioactive metabolites such as fucoidans, digestive enzymes, and proteins, while plant tissues are rich in reducing sugars, polyphenols, flavonoids, saponins, and amino acids. These compounds allow reducing, capping, chelating, and stabilizing during the transformation of Zr(4+) into ZrO(2) nanoparticles. Green ZrO(2) nanoparticles display unique properties such as a nanoscale size of 5–50 nm, diverse morphologies, e.g. nanospheres, nanorods and nanochains, and wide bandgap energy of 3.7–5.5 eV. Their high stability and biocompatibility are suitable biomedical and environmental applications, such as pathogen and cancer inactivation, and pollutant removal. Emerging applications of green ZrO(2)-based nanocomposites include water treatment, catalytic reduction, nanoelectronic devices, and anti-biofilms.

Environmental pollution and climate change are presenting as two of the most globally critical issues in the context of the coronavirus pandemic and circular economy (Muhammad et al. 2020; Ufnalska and Lichtfouse 2021) . The advent of nanoscience and nanotechnology can solve these problems; thereby, holding the key to economic recovery, and environmental mitigation (Weiss et al. 2020; Tang et al. 2021) . Nanomaterials play a central role in nanotechnology, raising the demands for sustainable development (Srivastava et al. 2021) . The chemical synthesis of nanomaterials chiefly does not satisfy the strict requirements for this trend, and also mismatches with the principles of green chemistry. Currently, the green approaches for the synthesis of nanomaterials are of remarkable significance due to their eco-friendliness as well as cost-effectiveness (Tran et al. 2020a ). This also pays the way for multiple applications of nanomaterials in the environmental mitigation, biomedical, and catalysis fields.

Zirconium (Zr) is classified as a transition metal element (d-block) in the titanium family (group IV) with the Thuan Van Tran and Duyen Thi Cam Nguyen have contributed equally to this work. atomic number 40. Zr does not basically capture neutrons, offering a potential metallic cladding for the fuel rods in the nuclear reactors (Cazado et al. 2021) . Zirconium dioxide (ZrO 2 ) or zirconia is one of the highly stable oxides, created by thermalizing zirconium compounds (Hassan and Jalil 2022) . Depending on the various synthesis routes, ZrO 2 can present in the crystalline phases involving monoclinic, tetragonal, and cubic (Zhang et al. 2018) . Bulk ZrO 2 has a wide bandgap energy, typically ranging from 5.0 to 7.0 eV. Because of the ultrahigh stability and very low toxicity, ZrO 2 has exhibited an intensive range of practical technologies for heat-resistant ceramic superalloys (Wang et al. 2021) , dental restorations , fuel cells (Rambabu et al. 2020) , and heterogeneous catalysis (Jiang et al. 2020) . Such promising utilizations make ZrO 2 an ideal nanomaterial, promoting the green strategy for synthesizing ZrO 2 nanoparticles.

In general, there are two major approaches to synthesize the ZrO 2 nanoparticles, involving top-down and bottom-up (Jadoun et al. 2021) . The former implies the conversion of bulk material into thinner crystallites by the physical route. This means that it needs the participation of enormous mechanical energy sources such as milling, and ionic sputtering (Shrimal et al. 2020) . As a result, top-down strategy brings many inevitable drawbacks of causing secondary impressions or intermediates, altering the physicochemical property and surface chemistry of as-synthesized nanoparticles (Indiarto et al. 2021 ). More importantly, nano-sized particles are mostly unattainable by top-down approach. Meanwhile, the latter implies the formation of particles by creating building blocks from ultra-small particles such as atoms or molecules, and then assembling them together. By this way, the nanostructured particles can be intentionally attainable under the control of fabrication conditions (Rana et al. 2020) . The bottom-up strategy involves the physical (e.g., vapor decomposition, plasma irradiation, and ultrasonication), chemical (e.g., sol-gel, co-precipitation, chemical reduction, and hydrothermal), and biological (e.g., plant, fungi, algae, and bacteria) methods. However, the physical bottom-up method requires a high investment cost for operating instruments and consumes a large amount of heating or electric energies (Bolade et al. 2020 ). The chemical bottom-up method necessarily adopts toxic chemicals for its protocols, resulting in adverse impacts on the environment. Therefore, it may be greatly difficult for physical and chemical methods to attain a critical green strategy.

The biological bottom-up method is the most adequate for the green synthesis of ZrO 2 nanoparticles because it offers an effective, tunable, and eco-friendly approach (Shafey 2020) . The biogenic synthesis uses low-cost and locally available sources such as plants or other biocompatible sources such as fungi, algae, and bacteria for ZrO 2 fabrication (Rana et al. 2020; Nguyen et al. 2021d ). The biomolecules extracted from the biological sources play a vital role as extremely efficient bioreducing, biocapping, and biostabilizing agents, bringing excellent ZrO 2 production yields (Bandeira et al. 2020) . Such biological substrates can safely replace almost highly expensive and toxic chemicals or energy-consuming physical instruments (Agarwal et al. 2017) . Biological method for synthesis of ZrO 2 nanoparticles also match well with principle of sustainable and green chemistry (Yadi et al. 2018) . Because of diminishing the potential risks from chemical and physical methods, generating no hazardous intermediates, and secondary pollutions, biogenic synthesis of ZrO 2 nanoparticles can be therefore considered as the green synthesis (Jadoun et al. 2021) .

To our knowledge, there is only a limited number of relevant works (about twenty research articles) reported the green synthesis of ZrO 2 nanoparticles using various biological sources such as bacteria, fungi, and plant extract. Moreover, there is currently no comprehensive review on the systematic evaluations from the green synthesis to the applications of ZrO 2 nanoparticles and their nanocomposites. Therefore, the main objectives of this study are to discuss the current findings on green ZrO 2 preparation techniques and highlight their performance in the biomedical field and environmental remediation. Specifically, this review has the following organization: (i) green synthesis methods for ZrO 2 nanoparticles; (ii) structural characterizations of green ZrO 2 nanoparticles; (iii) biomedical, adsorption, catalytic, and other applications of green ZrO 2 nanoparticles; and (iv) emerging applications of green ZrO 2 -based nanocomposites (Fig. 1) . We briefly elucidate some challenging and prospective aspects of green ZrO 2 nanocomposites for expanding their potential applications.

Bacteria are among the fastest-growing microorganisms under mild cultivation conditions in temperature, pressure, and pH (Ram et al. 2020) . They are therefore the favorable biofactories for the synthesis of ZrO 2 nanoparticles. During the metabolism activities, bacteria secrete some enzymes and proteins extracellularly or intracellularly, which possibly aid the bioreduction, biocapping, and biostabilization of zirconium ions (Fariq et al. 2017) . Underlying mechanism of forming ZrO 2 nanoparticles by bacterial communities endures further complex steps including (i) biosorption and (ii) bioreduction (Saravanan et al. 2021) . The former step initiates with trapping zirconium ions on the bacterial cell surface through the physical and chemical interactions (e.g., electrostatic, hydrogen bonding, ionic exchanging, and chelating) (Gahlawat and Choudhury 2019) . Specifically, the secretion of macromolecules such as proteins, polysaccharides containing many negatively charged functional groups enriches the surface chemistry of bacterial cell wall (Patil and Kim 2018) . This creates the attraction of Zr 4+ ions towards bacterial cell surface. The latter step takes main responsibility for reducing Zr 4+ ions into ZrO 2 nanoparticles. Bioactive compounds such as acid amines, proteins, etc. in the extracellular or intracellular environment can participate in the formation and stabilization of ZrO 2 nanoparticles (Narayanan and Sakthivel 2010) . Suriyaraj et al. (2019) reported the one-step biofabrication of ZrO 2 nanoparticles using extremophilic Acinetobacter sp. bacterial community. This green source was firstly cultivated in glucose solution, and added by 100 mM ZrOCl 2 ·8H 2 O for incubation for 12 h. Interestingly, highly crystalline ZrO 2 nanomaterial was facilely precipitated under room-temperature condition (37 °C). Moreover, this Fig. 1 Green synthesis of ZrO 2 nanoparticles and their applications in biomedical, adsorption, catalysis, and nanocomposite fabrication. The plant tissues including flowers, fruits, seeds, leaves, roots, etc. possess many phytochemicals such as polyphenols, saponins, quercetin, and gallic acid. Microorganisms including bacteria, fungi, algae, etc. can secrete biomolecules, metabolites, enzymes, and proteins. These phytochemicals and biomolecules participate in the green synthesis of ZrO 2 study found biocompatibility and ineligible cytotoxicity with mouse fibroblast cells. The fluorescein staining was carried out to confirm the major role of extracellular proteins released from Acinetobacter sp. for biosynthesis of ZrO 2 . Very recently, Ahmed et al. (2021) successfully synthesized zirconia nanoparticles using Enterobacter sp. strain. Notably, the bacterial strain was facilely isolated from the paddy soils, and then, incubated at room temperature. After the bacterial purification, 5 mM ZrOCl 2 ·8H 2 O was addedto the supernatant under agitating incubation for 24 h. Visible precipitation was finally formed at the bottom of the flask. It was proposed that nitrate reductase enzyme was in charge of reducing Zr 4+ ions into zirconia and protein secreted by Enterobacter sp. acts as a capping agent (Fig. 2) . Asbiosynthesized ZrO 2 nanoparticles revealed the promising antifungal activity with 75.5% inhibition against Pestalotiopsis Versicolor disease pathogen. These publications have demonstrated the great potentials and biocompatibility of the bacterially synthesized ZrO 2 nanoparticles.

Fungi include the microbial organisms that absorb nutrients by decomposing organic matters, without the photosynthesis pathways (Chu et al. 2021) . They play the principal role in nutrient cycling and exchanging as well as mitigation of metals-contaminated soils (Riaz et al. 2021) . During these processes, fungal communities secrete digestive enzymes into their foods to acquire the nutrients essential for growth (Ferreira et al. 2020) . Taking advantage of such features, fungi can be ideal biotemplates for the synthesis of ZrO 2 nanoparticles. Basically, the mechanism for green synthesis of ZrO 2 nanoparticles using fungal species is closely similar to that using bacterial species. However, the utilization of fungi for ZrO 2 production offers many advantageous points (Narayanan and Sakthivel 2010) . Firstly, they can endure the harsh synthesis conditions such as flow pressure or agitation in the bioreactor, easily handling the fabrication of ZrO 2 (Abinaya et al. 2021) . Secondly, fungi have accelerated growth in controllable ways, i.e., through adjusting their nutrient ingredients. Ultimately, almost all fungi are better resistant to genetic or environmental mutations, facilitating the culturing process (Bansal et al. 2011 ). These features of Fig. 2 Extracellularly biosynthesized mechanism of zirconia nanoparticles using Enterobacter sp. strain. Bacteria extracellularly secrete some specific enzymes such as reductase, which take responsibility for enzymatic bioreduction of Zr 4+ ions. The underlying mechanism is attributable to the redox of nicotinamide adenine dinucleotide (NAD + /NADH), which supplies electrons to the reduction of Zr 4+ ions during nucleation. Capping proteins can also be released into the extracellular environment and aid the biocapping and biostabilization of zirconia nanoparticles. Reprinted with the permission of Elsevier from reference (Ahmed et al. 2021) . Abbreviations: ZrONPs, zirconia nanoparticles; NAD + , an oxidized form of nicotinamide adenine dinucleotide; NADH, a reduced form of nicotinamide adenine dinucleotide fungal species are more favorable for biosynthesis of ZrO 2 than other biological synthesis methods such as bacteria and plants. Ghomi et al. (2019) reported that the formation of ZrO 2 by extracellular secretion of Penicillium fungal species including P. aculeatum, P. notatum, and P. purpurogenome. Under the optimum conditions at pH 9, and 1.5 mM Zr 4+ , the solution color changed from pale yellow into deep yellow after seven-day incubation, affirming the generation of colloidal ZrO 2 . In terms of the ZrO 2 biofabrication mechanism, the enzymes presenting in the fungal supernatants may aid to transform Zr 4+ ions into ZrO 2 nanoparticles. Bansal et al. (2004) cultured another fungal species named Fusarium oxysporum for the successful biosynthesis of ZrO 2 nanoparticles from the ZrF 6 2− source at room temperature. To gain insight into the major role of proteins secreted from this fungus for the extracellular hydrolysis of ZrF 6 2− , the resulting filtrates were lyophilized to test for hydrolytic activity. The findings confirmed the cationic nature of proteins to bind easily to ZrF 6 2− anions, which could not be found in other fungal species (e.g., C. lunata, C. gloeosporioides, Phomopsis sp., A. niger). More importantly, ZrF 6 2− anions showed a non-toxicity to Fusarium oxysporum, opening the enormous prospects for large-scale biosynthesis of ZrO 2 nanoparticles. To sum up, the highlighted eco-friendly, biocompatible, and energy-conserving advantages of the fungal biosynthesis of ZrO 2 nanoparticles are superior to chemical methods, and hence, should not be overlooked.

Great difference from fungal species, algae are a group of dominantly aquatic, eukaryotic, and autotrophic organisms that acquires their nutrients from the photosynthetic processes. Algae use chlorophyll for their primary photosynthesis; therefore, they are sometimes considered as terrestrial plants. Some algal species such as brown algae, or seaweeds excrete fucoidans from their cell walls (Filote et al. 2021) . Fucoidans refer to multifunctional polysaccharides containing a substantial amount of fucose and sulfated esters, exhibiting many bioactivities involving antiviral, antioxidant, and anticoagulant (Ponce and Stortz 2020) . These biomolecules can therefore act as ideal bioreducers for the green synthesis of ZrO 2 . Indeed, Kumaresan et al. (2018) successfully bioprepared tetragonally nanostructured ZrO 2 from the aqueous extract of seaweed Sargassum wightii. The biosynthesis procedure was tunable with the grind of ZrO(NO 3 ) 2 ·H 2 O with Sargassum wightii extract within 20 min. ZrO 2 nanoparticles could be formed after the calcination at 400 °C. This nanomaterial showed the spherical morphology with the particle size of 5 nm and was utilized for investigating several potential antibacterial activities against gram-positive and gram negative-bacteria. The researchers demonstrated that large surface area and nanosize (4.8 nm) of ZrO 2 obtained from this green strategy contributed considerably to enhancing growth inhibitory effects against antibacterial pathogens. Although many works reported the use of algae for biosynthesizing various nanoparticles, there is still an exception for ZrO 2. It is also necessary to elucidate the underlying mechanisms for the formation of ZrO 2 nanoparticles using algal species.

Plants are the most prevalent, locally available, highly costeffective resource for biosynthesis of ZrO 2 nanoparticles (Yadi et al. 2018) . Compared with other green materials such as bacteria, fungi, and algae, the use of plants as biotemplates attains many notable benefits (Saravanan et al. 2021) . Firstly, there are less risks to health than microbial ZrO 2 synthesis strategies. Almost plants are substantially more benign than bacteria or fungi. Some microbial species can even secrete toxins or metabolites during cultivation (Yang and Chiu 2017; Xu et al. 2020) . Secondly, the botanical synthesis of ZrO 2 nanoparticles is considerably accelerated in comparison with microbial routes. The bioreduction of Zr 4+ by biomolecules from plant extract occurs within a few minutes to several hours, while microbial communities mostly prolong these bioprocesses by many days under ambient incubation (Vijayaraghavan and Ashokkumar 2017) . This results in the higher kinetic of overall ZrO 2 production process. Thirdly, the synthesis of ZrO 2 nanomaterial by plant extracts is controllable and easy to manipulate. Diverse parts of plants such as leaves, barks, roots, flowers, etc. can be effortlessly collected for biosynthesis of ZrO 2 nanoparticles. The green solvents for extracting phytochemical compounds from plant tissues are mainly water and ethanol. Meanwhile, the microbial culture needs to be carried out under strict conditions to avoid the generation of colonies, unfavorable to ZrO 2 fabrication.

Phytochemicals involving reducing sugars, polyphenols, flavonoids, saponins, and amino acids are inherent in the plant tissues such as flowers, leaves, and roots (Fig. 3a) . These compounds are known as strong antioxidants or bioreductants, which can play pivotal roles as biocapping, biochelating, bioreducing, and biostabilizing agents for the synthesis of ZrO 2 nanoparticles (Fig. 3b) . The phytochemicals participate in the formation and stabilization of octahedral complex of Zr 2+ -phytochemicals. The presence of phytochemicals also aids to hamper the aggregation of ZrO 2 nanoparticles during thermal calcination. This may boost the surface area of ZrO 2 nanoparticles, which are conducive to their catalytic and adsorption properties. Consequently, the phytochemicals in plant extract can exhibit a range of important functionalities including replacing hazardous, Fig. 3 Phytochemicals in plant extracts (a), and proposed mechanism for the biosynthesis of ZrO 2 nanoparticles using these phytochemicals (b). Phytochemicals act as bioreductants to convert Zr 4+ into octahedral complex [Zr(H 2 O) 6 ] 2+ . This enhances the stability and hinders the clustering of zirconium complexation during chelation with phytochemicals Zr 2+ -phytochemicals. The calcination of Zr 2+ -phytochemicals complex produces ZrO 2 nanoparticles in tandem with the release of CO 2 , H 2 O, N 2 , and many decomposed products Gowri et al. (2015) expensive chemical antioxidants or reductants, and shortening the number of complicated synthesis steps. The general botanical biosynthesis of ZrO 2 nanoparticles initiates with the collection and pretreatment of plant tissues to eliminate the irrelevant parts (Table 1 ). The procedure is followed by extracting phytochemicals using several common kinds of solvents such as water, ethanol, or hexane from cell walls (Saraswathi and Santhakumar 2017; Goyal et al. 2021; Alagarsamy et al. 2022) . Water is a prevalently used, and highly polar extractor which recovers the aqueously soluble phytochemical compounds such as gallic acid, polyphenols, glucose, etc. while ethanol exhibits a poorer polarity than water, but higher than hexane. There are two major steps including complexation and calcination to reach the final ZrO 2 products. The former is carried out under the incorporation of plant extract into zirconium source, mainly ZrOCl 2 ·8H 2 O. The proper temperature and time for forming Zr 2+ -phytochemicals are 55-75 °C, and 1-4 h, respectively. This step can also be assisted by microwave irradiation (900 W) to accelerate the complexation rate up to 15 min (Shinde et al. 2018) , or by alkaline addition (Joshi et al. 2021 ). The Zr 2+ -phytochemicals complex is totally separated by centrifugation or paper filtration (Gowri et al. 2014) . The latter experiences the thermal calcination at high temperature (500-800 °C) for 2-4 h to convert this complex into ZrO 2 nanoparticles.

Bulk ZrO 2 nanoparticles exist in three common crystalline phases involving monoclinic, tetragonal, and cubic. Biologically synthesized ZrO 2 nanoparticles can also attain such crystalline phases based on X-Ray diffraction analysis (Table 2) . Specifically, Davar et al. (2018) reported a cubic phase of ZrO 2 nanoparticles bioproduced from Salvia Rosmarinus plant extract with the reflecting planes at (111), (200), (220), (311), (222), and (400). Another study indicated the monoclinic phase of ZrO 2 synthesized from Helianthus annuus plant extract (Goyal et al. 2021 ). However, tetragonal phase is the most prevalent single-crystal phase of green ZrO 2 nanoparticles. Green ZrO 2 nanoparticles can also have multiphase crystallinity, which includes at least two phases (Sathishkumar et al. 2013; Debnath et al. 2020; Isacfranklin et al. 2020) . The presence of crystal phase in green ZrO 2 possibly affects their electronic properties, optical band gap, and surface energy, and stability. It is therefore essential for further studies to clarify the role of synthesis conditions, e.g., the ratio between zirconium and plant extract in the formation of ZrO 2 phase.

The bandgap energy of a semiconductor reflects the energy disparity between the top of valence band and the bottom of conduction band, or the minimum energy to excite an electron from the valence band to the conduction band (Makuła et al. 2018) . It can be determined from diffuse reflectance spectra. While the bulk ZrO 2 nanoparticles have a wide bandgap energy (~ 5.0 eV), green ZrO 2 nanoparticles exhibit values ranging from 3.7 to 5.5 eV (Table 2) . Indeed, Goyal et al. (2021) fabricated ZrO 2 nanoparticles using methanolic extract of Helianthus annuus seeds with very narrow bandgap (3.7 eV). Al-Zaqri et al. (2021) found the same small value of ZrO 2 nanoparticles biosynthesized from Wrightia tinctoria leaf extract. They also gave an assumption about more contribution from external surface defects ZrO 2 nanoparticles. Meanwhile, obtaining higher bandgap energies (5.4-5.5 eV) of other green ZrO 2 nanoparticles may be ascribed by quantum confinement effects (Gowri et al. 2014; Alagarsamy et al. 2022) . The bandgap augments for the smaller size nanoparticles due to the deformity phenomena. Overall, biologically synthesized ZrO 2 nanoparticles acquire sufficient gap energies, capable of exhibiting their excellent catalytic and biomedical activities through the formation of reactive oxygen species.

Many physicochemical techniques such as X-Ray diffraction, dynamic light scattering, transmission electron microscopy, etc. can be used to measure the average particle size of ZrO 2 nanoparticles. From the database of previous studies (Table 2), the particle size of ZrO 2 nanoparticles is distributed from 5 to 150 nm, commonly less than 50 nm. Compared with chemical approaches, biological methods generally create ZrO 2 nanoparticles with considerably smaller sizes (Suriyaraj et al. 2019; Ahmed et al. 2021) . It can be understandable that biomolecules from the secretion of microbial sources, or phytochemicals from plant extracts lessen the assembly of ZrO 2 nanoparticles (Kumaresan et al. 2018; Silva et al. 2019; Debnath et al. 2020 ). Small-size ZrO 2 nanoparticles offer many biomedical benefits since they facilitate the penetration into cell walls, performing better antibacterial, antifungal, and anticancer activities (Khatoon et al. 2015) .

The shape of ZrO 2 nanocrystals biosynthesized from biological routes can be explored by scanning/transmission electron microscopy analysis. Accordingly, green ZrO 2 nanoparticles can present a wide range of morphological properties such as nanospheres (mainly), nanochains, nanorods, semi-nanospheres, nano-sized ovals, and nanoflakes (Fig. 4) . Generally, the ZrO 2 morphology is significantly dependent on the synthesis conditions including the ratio between zirconium source and green extract. Spherical ZrO 2 can be synthesized by using microbial sources with the long incubation (1-3 days) at room temperature or by several botanical sources such as Euclea natalensis roots, Aloe vera leaves, Ficus benghalensis leaves, Moringa oleifera leaves, Helianthus annuus seeds, Tinospora cordifolia leaves, and Laurus nobilis leaves (Table 2) . Meanwhile, other ZrO 2 morphologies are relatively rare, mostly synthesized by plant sources. For example, Sathishkumar et al. (2013) obtained ZrO 2 nano-chains by the hydrolysis of ZrF 6 2− in the presence of Curcuma longa tuber extract. Gowri et al. (2015) also successfully biosynthesized ZrO 2 nanoflakes by the hydrolysis of ZrOCl 2 ·8H 2 O using aqueously soluble carbohydrates extracted from Nyctanthes arbor-tristis flower. Semi-spherical and oval ZrO 2 can be formed from many plant extracts such as Salvia Rosmarinus and Lagerstroemia speciose, respectively (Saraswathi and Santhakumar 2017; Davar et al. 2018 ). More importantly, diverse morphologies of ZrO 2 morphologies can bring many benefits for practical applications. As an example, (Isacfranklin et al. 2020) indicated that ZrO 2 nanorods possess sufficient cellular uptake and toxicity, offering the prospect for biomedical application.

The surface functionalization by chemical groups aids to extend the applications of nanomaterials (Tran et al. 2021 ).

Surface chemistry of ZrO 2 nanoparticles can be identified by several physicochemical techniques such as Fouriertransform infrared spectroscopy and X-ray photoelectron spectroscopy or quantified by Boehm titrations (Nguyen et al. 2021b) . The surface functional groups on ZrO 2 nanoparticles may be originated from the phytochemical compounds. Considering the calcination of the complex of Zr 2+ -phytochemicals, almost all organic components are decomposed into volatiles or simple products such as CO 2 , N 2 , and H 2 O. A tiny proportion of phytochemicals may still remain due to the chelating with zirconium ions. They can be thermally modified to generate simpler biomolecules with more diverse functional groups such as carbonyl, amine, carboxylate, and alkyl (Table 2) . However, hydroxyl groups are detected partly owing to H 2 O-adsorbed surface of ZrO 2 (Kumaresan et al. 2018; Silva et al. 2019; Annu et al. 2020 ). The functionalization of ZrO 2 surface possibly can lead to the improvement of adsorption efficiency towards many pollutants through many key interactions such as electrostatic attraction, hydrogen bond, Yoshida hydrogen bond, and π-π interaction (Suresh et al. 2015; Nguyen et al. 2021c ). 

Theoretically, surface area of nanomaterials links closely to the number of active sites on the edges, resulting in a significant influence on their catalytic activity (Jiang et al. 2011) . The higher surface area of ZrO 2 nanoparticles can also facilitate the better favorable adsorption process. The bio compounds such as proteins, enzymes, polysaccharides, polyphenols, etc. present in the biological source extract chelating with zirconium ions can prevent the aggregation of ZrO 2 nanoparticles during the thermolysis. This enhances the surface area and the number of active sites of ZrO 2 nanoparticles. For example, Shinde et al. (2018) reported the surface area (88 m 2 /g) of ZrO 2 nanoparticles biosynthesized from Ficus benghalensis leaf extract using BET (Brunauer, Emmett and Teller) analysis. It was also witnessed an enhancement of the photocatalytic activities of ZrO 2 to methylene blue (91%) and methyl orange (69%) for 240 min. These results of green ZrO 2 were remarkably higher than those obtained by the chemical methods such as anodization in H 2 O 2 /NH 4 F/ethylene glycol electrolyte (Rozana et al. 2017) , decomposition of Zr(OH) 4 -urea complex (Sudrajat et al. 2016) , polymer-assisted sol-gel synthesis (Dhandapani et al. 2016 ).

Compared with chemically produced nanomaterials, biologically produced ZrO 2 nanoparticles not only exhibit good dispersion but also show better stability. For example, Sathishkumar et al. (2013) found the surprising stability of zirconia nanoparticles up to five months after green fabrication. This phenomenon may be elucidated by the presence of phytochemicals acting as efficient biostabilizers and biocapping agents. The stability of ZrO 2 nanoparticles can be assessed by the zeta potential measurements. Basically, the zeta potential values are different with zero, normally minimum ± 30 mV, indicating the high stability of ZrO 2 nanoparticles (Jameel et al. 2020). Indeed, Suriyaraj et al. (2019) demonstrated the well-dispersed, anti-clustering, and nanostructured ZrO 2 suspension by the measurement of zeta potential (36.5 ± 5.5 mV). Chau et al. (2021) also measured the high zeta potential (− 32.8 mV) of ZrO 2 nanoparticles biosynthesized from Laurus nobilis leaf extract by dynamic light scattering analysis, proposing the good anti-sedimentation of the nanomaterials for antimicrobial activities.

Bacterial and fungal species attack the immune system of humans and animals and become the major factors causing infections and even deaths (Humbal et al. 2018) . Many antibiotics and antifungal pharmaceuticals have been developed over the centuries, but mankind is increasingly encountering the huge problems of antibiotic resistance (Kovalakova et al. 2020 ). Among the efforts to search for alternatives to antibiotics, bionanotechnology exhibits its high reliability and performance contributing proactively to the abatement of infections (Ong et al. 2018) . Many metallic nanomaterials (e.g., silver nanoparticles) possess their inherent antibacterial and antifungal activities; hence, they are widely utilized in biomedical fields (Abbasi et al. 2014 ). However, the lack of biocompatibility, biostability, and effectivity may restrain the biomedical application of these nanomaterials. Green ZrO 2 nanoparticles biosynthesized from environmentally friendly botanical and microbial sources show unique properties, adequate for fabricating biomedical devices. Based on the published literature, this section will discuss some potential activities of green ZrO 2 nanoparticles.

The plasma membrane of bacteria is mostly constituted of proteins (e.g., peptidoglycan macromolecules) that charge negatively (Li et al. 2019) . ZrO 2 nanoparticles charge positively on their surface, leading to electrostatic interactions with bacterial membranes (Xing et al. 2018 ). This facilitates the biosorption and bioaccumulation of ZrO 2 nanoparticles on the cell walls. With the nanoscale size, high surface area, and good biocompatibility of green ZrO 2 nanoparticles, they easily pass through cell membrane, inhibit the key metabolic functions, and finally deactivate bacterial cells (Kumaresan et al. 2018) . It is suggested that the generation of reactive oxygen species (e.g., ·O 2 − , and ·OH) is the underlying mechanism (Akintelu and Folorunso 2020) . Accordingly, these species damage the genetic material such as deoxyribonucleic and ribonucleic acids, disordering the transcription and translation processes in bacteria (Slavin et al. 2017 ). Bacterial death occurs as a result of cell division failure.

There are many publications that reported the excellent antibacterial activity of green ZrO 2 nanoparticles against both gram-negative and positive bacteria (Table 3) . For example, Gowri et al. (2015) investigated the antibacterial activity against E. coli and S. aureus using ZrO 2 biosynthesized from Nyctanthes arbor-tristis plant extract. The 

Basically, the ZrO 2 nanoparticles may have the inhibition mechanism against fungi as same as that against bacteria (Ahmed et al. 2021) . The main mechanism of the disruption of fungal cell division may rely on the generation of reactive oxygen species (e.g., ·O 2 − , and ·OH) and free radicals (Akintelu and Folorunso 2020). With the high biocompatibility, green ZrO 2 nanoparticles can pervade through the fungal cell membranes by endocytosis. In the intracellular environment, ZrO 2 nanoparticles account for detrimental functions such as ribosomal, chromosomal, and mitochondrial damages through the formation of reactive oxygen species (Fig. 5) . Moreover, ZrO 2 nanoparticles possibly break the integrity of fungal cell by changing the physicochemical conditions to cause cell stress and leaking the intracellular components (Reddy et al. 2015) .

Although the mechanisms of damaging the fungal cells by ZrO 2 nanoparticles have not been fully understood, many attempts were still made to demonstrate their antifungal efficiency. Ahmed et al. (2021) obtained the promising practical outcomes of ZrO 2 nanoparticles biosynthesized from Enterobacter sp. against bayberry fungal pathogen. Moreover, the antifungal inhibition zones were very wide 25.18 ± 1.52 mm at the dilute concentration of ZrO 2 (only 20 μg/mL). They also observed the clear deconstruction of P. Versicolor fungal cell by extracellular leakage of deoxyribonucleic acid (DNA) and proteins under microscopy imaging techniques. Several works clarified the strength of green ZrO 2 for inhibiting both fungal and bacterial species (Chau et al. 2021; Joshi et al. 2021) . Regardless of some limitations of the scope of fungal and bacterial species, those may The cells viability with 18% inhibition was observed at 500 μg/mL of zirconia nanoparticles Saraswathi and Santhakumar (2017) Nearly 30-40% of the cells showed blebbing provide more insightful evidence into the antifungal activities of green zirconia nanoparticles. It is also recommended to better elucidate the underlying mechanisms of inhibiting the fungal cells by green ZrO 2 nanoparticles for future researches (Tables 4 and 5) .

On average, one-third people are diagnosed with cancer in their lifetime, causing one of the world's leading deaths (Sung et al. 2021) . The evolution of cancer disease is the growth of abnormal cells to invade other tissue of the body. Many therapies such as surgery, chemotherapy, stem cell, and radiotherapy have been developed to treat this disease (Mooney et al. 2018 ). Bionanotechnology has recently played a vital role in designing novel anticancer agents and nanocarriers for drug delivery (Dai et al. 2017) . Accordingly, nanoparticles can preferentially approach and accumulate in tumors thanks to the permeable effects, but this accumulation process is mostly influenced by physicochemical factors in the body (Beik et al. 2019) . Green ZrO 2 nanoparticles have several emerging advantages such as nanoscale size, high surface area, stability, and good biocompatibility to overcome these barriers. On the tumor surface, green ZrO 2 nanoparticles bind to the macromolecules such as proteins and then penetrate into the tumor cells (Ranji-Burachaloo et al. 2018) . The chief mechanisms of cell damage by green ZrO 2 nanoparticles are not elucidated insightfully in the published literatures. According to Table 3 , there are a limited number of works that evaluated the cytotoxicity activity of green ZrO 2 nanoparticles biosynthesized from Lagerstroemia speciose leaf and Nephelium lappaceum fruit extracts against the breast cancer cell lines (Saraswathi and Santhakumar 2017; Isacfranklin et al. 2020) . Therefore, many further investigations of anticancer activity of green ZrO 2 nanoparticles are required. Abbreviations: TEM, transmission electron microscope; FTIR, Fourier-transform infrared spectroscopy; XRD, X-ray diffraction; EDS, energy-dispersive X-ray spectroscopy; SEM, scanning electron microscopy; UV-Vis, ultraviolet-visible spectroscopy; ZrONPs, zirconia nanoparticles; RNT10, an isolated strain of Enterobacter

Environmental pollution is currently presenting as one of the most globally critical issues, demanding many urgent actions (Crini and Lichtfouse 2019) . In particular, the presence of emerging pollutants such as antibiotics can reduce the quality of water sources, and generate many potential threats for human and aquatic animals (Tahrani et al. 2015; Oberoi et al. 2019; Kovalakova et al. 2020) . Several works reported the evolution of antibiotic resistance genes in rivers, hospital effluents, wastewater treatment plants, and sediments (Kairigo et al. 2020; Böger et al. 2021) . Therefore, the treatment of antibiotic pharmaceuticals from water media by effective methods is necessarily required. Considering the non-degradable and chemically stable structure of antibiotics, adsorption may be the best remediation method due to its enormous advantages including high efficiency, fast kinetic, and tunable procedure (Tran et al. 2019b (Tran et al. , 2020d Dang et al. 2021; Nguyen et al. 2021e ). However, the selection and development of novel advanced materials for water treatment are still challenging. Green ZrO 2 nanoparticles can meet the requirements for a good adsorbent since they obtain many outstanding properties. Indeed, biologically synthesized ZrO 2 nanoparticles have proved their superior functionality of the surface, stability, and eco-friendly synthesis, and hence, exhibiting their capacity for antibiotic adsorption (Table 4) . Silva et al. (2019) optimized the tetracycline removal efficiency of ZrO 2 nanoparticles biosynthesized from Euclea natalensis roots by response surface methodology. In this experiment, the conditions of ZrO 2 nanofabrication were set up 2 3 factorial design for extract content, precursor content, and pyrolysis temperature. The calcination temperature at 550 °C was found the most suitable. Under optimized removal conditions at initial concentration of 20 mg/L and adsorbent dose of 0.5 g/L, green ZrO 2 nanoparticles obtained the tetragonal phase with extremely small particle size (5.90-8.54 nm) and exhibited the best adsorptive capacity of 30.45 mg/g for tetracycline. However, this capacity value was relatively lower than that obtained by most adsorbents. To improve such situation, Debnath et al. (2020) investigated the synthesis of green ZrO 2 nanoparticles by bacterial community, specifically, Pseudomonas aeruginosa species for the treatment of tetracycline in aqueous solutions. Green ZrO 2 owned both monoclinic and tetragonal crystal phases with small size (6.41 nm). They optimized the operating tetracycline adsorption parameters such as pH 7 and contact time of 15 min to reach the very promising capacity of 526.32 mg/g. Importantly, green ZrO 2 could be easily regenerated with dilute NaOH solution and distilled water, bringing good reusability up to 5 cycles. The authors also proposed the major role of electrostatic interaction for their adsorption mechanisms. With the high performance of stability, reusability, and removal efficiency, green ZrO 2 nanoparticles can be ideal nanomaterials for water purification.

In addition to the contamination of antibiotics in water, many textile dyes are becoming persistent pollutants. It was estimated that a large amount of textile dyes is yearly discharged into water sources, causing many adverse impacts on human health (Zhou et al. 2019) . They are also known to dismiss the solar light penetration, resulting in the reduction of photosynthesis efficiency as well as metabolism processes in several aquatic species (Tran et al. 2019a (Tran et al. , 2020c . As same as antibiotics, most of these organic dyes are chemically stable, and difficult to degrade by biological methods such as phytoremediation (Nguyen et al. 2021f ). However, another feasible solution is to apply the nanomaterials for the remediation of textile dyes through the adsorption and photocatalytic routes (Tran et al. 2020e; Nguyen et al. 2021c, a) . Table 4 summarizes the main findings of recent publications on the treatment of dyes using green ZrO 2 nanoparticles.

By adsorption pathway, Alagarsamy et al. (2022) demonstrated the high efficiency of green ZrO 2 nanoparticles biosynthesized from the pericarp extract of Sapindus mukorossi for the removal of methylene blue. They optimized the adsorption conditions such as pH 10, ZrO 2 dose of 0.3 g/L, and 20 mg/L of tetracycline to remove 94% methylene blue for 300 min. Although the adsorption capacity was low (23.25 mg/g), this adsorbent could be reused for three consecutive cycles. In terms of catalytic degradation by green ZrO 2 nanoparticles, textile dyes were investigated more widely (Table 4 ). Take the study by Shinde et al. (2018) for example, both methylene blue and methylene orange dyes could be sufficiently degraded (61-91%) by green ZrO 2 nanoparticles biosynthesized from the Ficus benghalensis leaves under ultraviolet light irradiation. Moreover, the authors reported the catalytic optimal conditions at pH 7 and catalyst loading of 1.5 g/L. The plausible catalytic mechanism was suggested by the incorporation of reactive oxygen species such as ·O 2 − , and ·OH into the degradation process of dyes. However, carrying out the dyes degradation under ultraviolet light irradiation as mentioned by Shinde et al. (2018) may be unfavorable for practical applications. Al-Zaqri et al. (2021) improved this harsh input by using green ZrO 2 nanoparticles biosynthesized from Wrightia tinctoria leaves. The outcomes were very promising because this dye was mostly removed with the high first-order rate constant (k 1 = 0.9837 min −1 ) in the presence of sunlight irradiation. The plant extract may be contributed significantly to the formation of green ZrO 2 nanoparticles with smaller particle size (17 nm) as well as narrower bandgap energy (3.78 eV), which exhibited better catalytic performance.

The presence of other emerging pollutants such as fluoride can cause several serve diseases including skeletal fluorosis, teeth mottling, bones deformation, and many neurological issues (Affonso et al. 2020) . The process of chemical fertilizing using hydrofluoric acid generates the amount of fluoride ions in water. It is therefore necessary to treat this pollutant before being discharged from the aqueous media. Prasad et al. (2014) assessed the adsorptive efficiency of adopting green ZrO 2 nanoparticles from Aloe vera extract. Nearly 99% of Fions were adsorbed ZrO 2 -based adsorbent, and fluoride chemisorption capacity was obtained at 96.58 mg/g. The authors also found the exothermic, spontaneous, and recyclability nature of the fluoride adsorption process, suggesting the potential of green ZrO 2 in fluoride decontamination. Although the high performance of ZrO 2 nanoparticles produced from green extract sources for the treatment of antibiotics, textile dyes, and fluoride has been presented in the previous works, their great potentials for removing other emerging pollutants such as heavy metal ions, chlorinated compounds, persistent organic pollutants, pharmaceuticals, and personal care products have not yet exploited. It is recommended for further studies to explore the strength of green ZrO 2 nanoparticles for the treatment of other emerging pollutants in the environmental remediation field.

Silicon and many commercialized semiconductors-based solar cells have exhibited their highly efficient performance, but their production process is generally expensive (Roy et al. 2020) . Moreover, these processes use chemically synthesized protocols, hindering sustainable development. To reach the purposes of green production, the biological synthesis of low-cost dye-sensitized solar cells should be of great interest (Alami et al. 2019) . Green ZrO 2 nanoparticles can become one of the prospective electronic nanomaterials for solar energy harvesting. Several studies reported the biosynthesis of green ZrO 2 nanoparticles for solar cell applications. For example, Majedi et al. (2016) synthesized the cubic-phase zirconia nanoparticle with small particle size (21 nm) using lemon juice. They reported the electrical conductivity and activation energy of ZrO 2 nanoparticles of 0.0034 S/cm, and 31.015 kJ/mol, respectively. These results exhibited a relatively electronic performance of green ZrO 2 as potential electrolytes. Vennila et al. (2018) also have investigated the synthesis of ZrO 2 using Gloriosa Superba tuber powder and found the potential to create high-efficiency electrodes. However, the applications of green ZrO 2 nanoparticles as solar cells are still limited. The reaction mechanism, as well as the ways to improve the energy harvesting efficiency, are sufficiently unexplored. It is recommended that further studies should be investigated.

The incorporation of dopants can considerably change the properties of ZrO 2 nanoparticles in terms of particle size, uniformity, electronic nature, and morphologies. Green synthesis of ZrO 2 -based nanocomposites allows improving some disadvantages of bandgap energy, surface area, surface chemistry, and stability of inherent ZrO 2 nanoparticles. To reach a green and sustainable approach, the synthesis of ZrO 2 -based nanocomposites uses microbial or botanical sources, which play a key role in the bioreduction, biocapping, biochelating, and biostabilizing process for the transformation into composites. For example, Vivekanandhan et al. (2015) functionalized the surface of ZrO 2 nanocrystallites with silver nanoparticles using Anacardium occidentale leaf extract (Fig. 6) . The bio compounds from this extract might participate in the bioreduction of Ag + into Ag and nucleation of Ag nanospheres (5-20 nm) on ZrO 2 surface. The authors also reported the inhibition of the deformation of Ag/ZrO 2 nanocomposites in the larger presence of leaf extract. This phenomenon may be attributable to swift bioreduction of Ag + ions before their dispersion on the surface of ZrO 2 . Therefore, a sufficient amount of plant extract is important to obtain the structure of ZrO 2 -based nanocomposites. Table 5 summaries several potential applications of green ZrO 2 -based nanocomposites.

Because ZrO 2 nanoparticles exhibit a wide bandgap (~ 5.0 eV), the absorption of visible light is difficult, hampering their photocatalytic and nanoelectronic applications. The modification of ZrO 2 nanoparticles aims to suffer this major drawback. For example, Yadav et al. (2021) inserted an amount of transition metal nickel (Ni) into ZrO 2 nanoparticles using rubber latex extract to obtain monoclinic, tetragonal, and cubic Ni-doped ZrO 2 polycrystalline. The band gaps were very narrowed between 2.4 and 2.75 eV, making Ni-doped ZrO 2 as potential nanoelectronic devices. Rasheed et al. (2020) reported the utilization of Daphne alpine leaf extract to synthesize green V 2 O 5 /ZrO 2 nanocomposite. They not only observed the increase in surface area (214 m 2 /g) but also found the improvement of bandgap energy (3.93 eV) and thermal stability (1000 °C). It was suggested that V 2 O 5 efficiently enhanced the electron separation, resulting in good photocatalytic activities against orange (76.9%) and picloram (86%) of green V 2 O 5 /ZrO 2 .

Doping other transition metals (Sm, Cu, Ag) significantly changes the electronic nature and crystalline phases of the nanocomposites, leading to the enhancement of their stability, recyclability, and photocatalytic activities. Indeed, Gurushantha et al. (2016) fabricated the green ZrO 2 with samarium dopant (Sm 3+ /ZrO 2+ = 3-11 mol.%) using Leucas Aspera extract. Sm/ZrO 2 composites with the doping of 11% dopant obtained highly symmetric nanocubics, and the highest photocatalytic degradation of acid green with six consecutive recycles in the presence of solar energy. This suggested that the Sm/ZrO 2 composites are highly stable and reusable. Hamad et al. (2019) demonstrated the presence of 7-hydroxy-4´-methoxy-isoflavon from Commelina diffusa leaves reduced the agglomeration of Cu/ZrO 2 nanoparticles (18-25 nm) with good stable ability during the post-synthesis. Moreover, the green Cu/ZrO 2 nanoparticles catalyzed the reduction and degradation of 2,4-dinitrophenylhydrazine, and organic dyes including congo red, nigrosin, methyl orange with ultrafast reaction times (1-150 s). In another study, Maham et al. (2020) obtained green Ag/ZrO 2 nanocomposites (50 nm) using Ageratum conyzoides plant extract. The catalyst could reduce and degrade a range of substrates such as 2,4-dinitrophenylhydrazine, 4-nitrophenol, nigrosin, and congo red, indicating the greatly catalytic performance of green Ag/ZrO 2 nanocomposite.

Although green ZrO 2 nanoparticles exhibit their very high catalytic performance, the separation and recovery of these non-magnetically small-size nanomaterials from the aqueous post-reaction is disadvantageous. The intercalation of magnetic components (e.g., Fe 3 O 4 ) into ZrO 2 nanomaterials should be therefore required. Inspired by this idea, Rostami-Vartooni et al. (2019) have successfully the Centaurea cyanus-biofabricated Ag/Fe 3 O 4 /ZrO 2 nanocomposites with average particle size between 30 and 90 nm. The nanocomposite could be easily separated from the reaction solutions because its saturation magnetization value was obtained at 10 emu/g. The authors also reported some good results of catalytic reduction of 4-nitrophenol and degradation of methyl orange green Ag/Fe 3 O 4 /ZrO 2 with a swift reaction time between 7.5 and 8 min. Therefore, this advantage may urge the recyclability experiments of magnetic ZrO 2 nanocomposites.

ZrO 2 -based nanocomposites biosynthesized from the green extract sources can offer a high diversity of morphology and open new applications. For example, Pandiyan et al. (2018) employed the green synthesis of CeO 2 doped ZrO 2 nano sticks with small particle size (10-15 nm) and bandgap (3.37 eV) from Justicia adhatoda leaf extract. This nanocomposite exhibited high antibacterial activities against S. aureus, and E. coli and high antioxidant activity. They also reported the application of CeO 2 /ZrO 2 nano sticks as potential biofilms against many pathogeneses caused by bacteria such as S. marcescens. Renuka et al. (2016) successfully created the hollow structure of Mg-doped ZrO 2 microspheres using Aloe Vera leaf extract. They assumed that bio compounds such as polysaccharides from Aloe Vera leaf might chelate with Zr 4+ and Mg 2+ ions during the formation of Mg/ZrO 2 microspheres. The authors assumed that Fig. 6 The bioreduction of Ag + into Ag and nucleation of on ZrO 2 surface for the biosynthesis of green Ag/ZrO 2 nanocomposites using Anacardium occidentale leaf extract (a); the photograph of as-synthesized green ZrO 2 and Ag/ZrO 2 nanocomposites (b); scanning electron microscopy (c, d) and transmission electron microscopy (e, f) microphotographies of green Ag/ZrO 2 nanocomposites. The pres-ence of leaf extract efficiently leads to the good dispersion and high biostabilization of Ag nanospheres (5-20 nm) on the surface of ZrO 2 . However, larger addition of leaf extract inhibits the formation of Ag/ ZrO 2 nanocomposites. Reproduced and adapted with the permission of Elsevier from reference (Vivekanandhan et al. 2015) . Abbreviations: AgNP, silver nanoparticles bio compounds such as polysaccharides from Aloe Vera leaf might chelate with Zr 4+ and Mg 2+ ions during the reaction of complexation. The burning calcination of these complexes allowed to acquire the hollow structure of Mg/ZrO 2 microspheres. They found the Mg/ZrO 2 composites with the magnesium doping rate of 2 mol% gave the highest photocatalytic activity against Rhodamine B (93%) in the presence of ultraviolet irradiation. Controlled oxygen vacancies on active sites may be attributable to the enhanced catalytic performance of green Mg/ZrO 2 .

Green synthesis of ZrO 2 nanoparticles from microbial and botanical sources has gained great attention because they offer many advantages such as eco-friendly and low-cost manufacturing compared with chemical synthesis methods. Green ZrO 2 nanoparticles possess many distinctive structure properties including small size, good dispersion, diverse surface chemistry, large surface area, and sufficient bandgap energy, making them extend excellent applications regarding biomedical and environmental fields. In many cases, the green ZrO 2 nanoparticles experimentally exhibit better antibacterial, antifungal, catalytic and adsorptive performance than chemically synthesized ZrO 2 nanomaterials. These superior prospects of green ZrO 2 nanoparticles may pay the way for developing advanced ZrO 2 -based nanosensors for smart agriculture, high-efficiency solar cells for energy harvesting, and electronic nanodevices for many high-technological manufacturing industries.

Although green ZrO 2 nanoparticles hold many key advantages, there are still several limitations that need further investigation. Firstly, regarding the synthesis procedure for ZrO 2 nanoparticles microbial sources, the control of biomass growth rate, metabolite generation, and biochemical properties of cultivation environment should be investigated vigorously. For the botanical sources, the determination of bioactive compound ingredients in the plant extract should be conducted to better elucidate the formation mechanism of ZrO 2 nanoparticles. Moreover, the formation of green ZrO 2 nanoparticles should be optimized to obtain their expectable properties of particle size, charge nature, surface energy, and surface chemistry. Secondly, regarding the treatment of pollutants such as antibiotics, and textile dyes, the treatment of reused ZrO 2 nanoparticles has not been addressed. This action may generate secondary pollution; hence, it is necessary to solve the post-treatment problems. Moreover, most studies on the application of green ZrO 2 nanoparticles only reported their findings under laboratory modes, resulting in the difference between simulated and practical results. This drawback can be therefore addressed if the tests are conducted under practical conditions. Thirdly, to the best of our knowledge, the potential of green ZrO 2 nanoparticles is substantially unexploited in the many environmental fields, e.g., the treatment of emerging persistent organic pollutants, pharmaceuticals and personal care products, nonsteroidal anti-inflammatory drugs, and toxic gas detection. With the excellent properties of green ZrO 2 nanoparticles, it is expected that they will contribute significantly to the development of new environmental remediation approaches. Ultimately, the combination of dopants with green ZrO 2 nanoparticles can accelerate their inherent properties of the nanocomposites, expanding many promising applications. However, it is generally difficult to control the interactions of green extract with the dopants during the ZrO 2 based nanocomposites. The optimization of the biomass proportion should be evaluated to avoid these interactions. Even though the formation mechanism and application of green ZrO 2 based nanocomposites have been better understood in some published works, the number of researches is still limited. The knowledge of the overall interactions between ZrO 2 nanomaterials and the environment is just burgeoning. The profound evaluations of the ecotoxicity of green ZrO 2 nanoparticles should be continuously conducted to avoid possible future challenges.

The present review addressed some aspects of synthesis, properties, and potentials of green ZrO 2 nanoparticles and their nanocomposites for biomedical, environmental remediation, and other applications. The bioactive compounds derived from microbial and botanical extracts are attributable to the bioreduction, biocapping, biochelating, and biostabilization during the transformation of zirconium sources into green ZrO 2 nanoparticles. Through the structural characterization techniques, green ZrO 2 nanoparticles are found with many morphologies such as nanospheres, nanorods, nano chains. They possess diverse surface chemistry, high stability, and wide bandgap energy possibly due to the effect of green extracts. Some applications regarding the antibacterial, antifungal, anticancer, photocatalytic, and adsorptive activities of green ZrO 2 nanoparticles have been overviewed and discussed systematically. The advanced properties and applications of green ZrO 2 -based nanocomposites were also mentioned. To orientate the future researchers, some discussions of knowledge gaps and prospects of green ZrO 2 nanoparticles have been clarified. The green ZrO 2 nanoparticles and their nanocomposites are highly expected to obtain many good results in various applications. Funding There was no external funding for this study.

The authors declare that all data and materials support their published claims and comply with field standards.

The authors declare that software application or custom code supports their published claims and comply with field standards.

The authors declare that there are no conflict of interest.

The manuscript has not been published anywhere nor submitted to another journal. The manuscript is not currently being considered for publication in any another journal. All authors have been personally and actively involved in substantive work leading to the manuscript, and will hold themselves jointly and individually responsible for its content.

Human and animal rights Research does not involve any Human Participants and/or Animals.

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