Introduction

Environmental pollution and climate change are presenting as two of the most globally critical issues in the context of the coronavirus pandemic (COVID-19) 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 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 (ZrO2) or zirconia is one of the highly stable oxides, created by thermalizing zirconium compounds (Hassan and Jalil 2022). Depending on the various synthesis routes, ZrO2 can present in the crystalline phases involving monoclinic, tetragonal, and cubic (Zhang et al. 2018). Bulk ZrO2 has a wide bandgap energy, typically ranging from 5.0 to 7.0 eV. Because of the ultrahigh stability and very low toxicity, ZrO2 has exhibited an intensive range of practical technologies for heat-resistant ceramic superalloys (Wang et al. 2021), dental restorations (Chen et al. 2021), fuel cells (Rambabu et al. 2020), and heterogeneous catalysis (Jiang et al. 2020). Such promising utilizations make ZrO2 an ideal nanomaterial, promoting the green strategy for synthesizing ZrO2 nanoparticles.

In general, there are two major approaches to synthesize the ZrO2 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 ZrO2 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 ZrO2 fabrication (Rana et al. 2020; Nguyen et al. 2021d). The biomolecules extracted from the biological sources play a vital role as extremely efficient bioreducing, biocap**, and biostabilizing agents, bringing excellent ZrO2 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 ZrO2 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 ZrO2 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 ZrO2 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 ZrO2 nanoparticles and their nanocomposites. Therefore, the main objectives of this study are to discuss  the current findings on green ZrO2 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 ZrO2 nanoparticles; (ii) structural characterizations of green ZrO2 nanoparticles; (iii) biomedical, adsorption, catalytic, and other applications of green ZrO2 nanoparticles; and (iv) emerging applications of green ZrO2-based nanocomposites (Fig. 1). We briefly elucidate some challenging and prospective aspects of green ZrO2 nanocomposites for expanding their potential applications.

Fig. 1
figure 1

Green synthesis of ZrO2 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 ZrO2

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Green synthesis of ZrO2 nanoparticles

Bacteria

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 ZrO2 nanoparticles. During the metabolism activities, bacteria secrete some enzymes and proteins extracellularly or intracellularly, which possibly aid the bioreduction, biocap**, and biostabilization of zirconium ions (Fariq et al. 2017). Underlying mechanism of forming ZrO2 nanoparticles by bacterial communities endures further complex steps including (i) biosorption and (ii) bioreduction (Saravanan et al. 2021). The former step initiates with trap** 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 Zr4+ ions towards bacterial cell surface. The latter step takes main responsibility for reducing Zr4+ ions into ZrO2 nanoparticles. Bioactive compounds such as acid amines, proteins, etc. in the extracellular or intracellular environment can participate in the formation and stabilization of ZrO2 nanoparticles (Narayanan and Sakthivel 2010).

Suriyaraj et al. (2019) reported the one-step biofabrication of ZrO2 nanoparticles using extremophilic Acinetobacter sp. bacterial community. This green source was firstly cultivated in glucose solution, and added by 100 mM ZrOCl2·8H2O for incubation for 12 h. Interestingly, highly crystalline ZrO2 nanomaterial was facilely precipitated under room-temperature condition (37 °C). Moreover, this 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 ZrO2. 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 ZrOCl2·8H2O 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 Zr4+ ions into zirconia and protein secreted by Enterobacter sp. acts as a cap** agent (Fig. 2). As-biosynthesized ZrO2 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 ZrO2 nanoparticles.

Fig. 2
figure 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 Zr4+ ions. The underlying mechanism is attributable to the redox of nicotinamide adenine dinucleotide (NAD+/NADH), which supplies electrons to the reduction of Zr4+ ions during nucleation. Cap** proteins can also be released into the extracellular environment and aid the biocap** 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

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Fungi

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 ZrO2 nanoparticles. Basically, the mechanism for green synthesis of ZrO2 nanoparticles using fungal species is closely similar to that using bacterial species. However, the utilization of fungi for ZrO2 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 ZrO2 (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 fungal species are more favorable for biosynthesis of ZrO2 than other biological synthesis methods such as bacteria and plants.

Ghomi et al. (2019) reported that the formation of ZrO2 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 Zr4+, the solution color changed from pale yellow into deep yellow after seven-day incubation, affirming the generation of colloidal ZrO2. In terms of the ZrO2 biofabrication mechanism, the enzymes presenting in the fungal supernatants may aid to transform Zr4+ ions into ZrO2 nanoparticles. Bansal et al. (2004) cultured another fungal species named Fusarium oxysporum for the successful biosynthesis of ZrO2 nanoparticles from the ZrF62− source at room temperature. To gain insight into the major role of proteins secreted from this fungus for the extracellular hydrolysis of ZrF62−, the resulting filtrates were lyophilized to test for hydrolytic activity. The findings confirmed the cationic nature of proteins to bind easily to ZrF62− anions, which could not be found in other fungal species (e.g., C. lunata, C. gloeosporioides, Phomopsis sp., A. niger). More importantly, ZrF62− anions showed a non-toxicity to Fusarium oxysporum, opening the enormous prospects for large-scale biosynthesis of ZrO2 nanoparticles. To sum up, the highlighted eco-friendly, biocompatible, and energy-conserving advantages of the fungal biosynthesis of ZrO2 nanoparticles are superior to chemical methods, and hence, should not be overlooked.

Algae

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 ZrO2. Indeed, Kumaresan et al. (2018) successfully bioprepared tetragonally nanostructured ZrO2 from the aqueous extract of seaweed Sargassum wightii. The biosynthesis procedure was tunable with the grind of ZrO(NO3)2·H2O with Sargassum wightii extract within 20 min. ZrO2 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 ZrO2 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 ZrO2. It is also necessary to elucidate the underlying mechanisms for the formation of ZrO2 nanoparticles using algal species.

Plants

Plants are the most prevalent, locally available, highly cost-effective resource for biosynthesis of ZrO2 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 ZrO2 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 ZrO2 nanoparticles is considerably accelerated in comparison with microbial routes. The bioreduction of Zr4+ 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 ZrO2 production process. Thirdly, the synthesis of ZrO2 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 ZrO2 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 ZrO2 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 biocap**, biochelating, bioreducing, and biostabilizing agents for the synthesis of ZrO2 nanoparticles (Fig. 3b). The phytochemicals participate in the formation and stabilization of octahedral complex of Zr2+–phytochemicals. The presence of phytochemicals also aids to hamper the aggregation of ZrO2 nanoparticles during thermal calcination. This may boost the surface area of ZrO2 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, expensive chemical antioxidants or reductants, and shortening the number of complicated synthesis steps.

Fig. 3
figure 3

Phytochemicals in plant extracts (a), and proposed mechanism for the biosynthesis of ZrO2 nanoparticles using these phytochemicals (b). Phytochemicals act as bioreductants to convert Zr4+ into octahedral complex [Zr(H2O)6]2+. This enhances the stability and hinders the clustering of zirconium complexation during chelation with phytochemicals Zr2+–phytochemicals. The calcination of Zr2+–phytochemicals complex produces ZrO2 nanoparticles in tandem with the release of CO2, H2O, N2, and many decomposed products

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The general botanical biosynthesis of ZrO2 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 ZrO2 products. The former is carried out under the incorporation of plant extract into zirconium source, mainly ZrOCl2·8H2O. The proper temperature and time for forming Zr2+–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 Zr2+–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 ZrO2 nanoparticles.

Table 1 Synthesis of green ZrO2 nanoparticles using plants
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Properties of green ZrO2 nanoparticles

Crystallinity

Bulk ZrO2 nanoparticles exist in three common crystalline phases involving monoclinic, tetragonal, and cubic. Biologically synthesized ZrO2 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 ZrO2 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 ZrO2 synthesized from Helianthus annuus plant extract (Goyal et al. 2021). However, tetragonal phase is the most prevalent single-crystal phase of green ZrO2 nanoparticles. Green ZrO2 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 ZrO2 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 ZrO2 phase.

Table 2 Properties of green ZrO2 nanoparticles
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Optical bandgap

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 ZrO2 nanoparticles have a wide bandgap energy (~ 5.0 eV), green ZrO2 nanoparticles exhibit values ranging from 3.7 to 5.5 eV (Table 2). Indeed, Goyal et al. (2021) fabricated ZrO2 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 ZrO2 nanoparticles biosynthesized from Wrightia tinctoria leaf extract. They also gave an assumption about more contribution from external surface defects ZrO2 nanoparticles. Meanwhile, obtaining higher bandgap energies (5.4–5.5 eV) of other green ZrO2 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 ZrO2 nanoparticles acquire sufficient gap energies, capable of exhibiting their excellent catalytic and biomedical activities through the formation of reactive oxygen species.

Particle size

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 ZrO2 nanoparticles. From the database of previous studies (Table 2), the particle size of ZrO2 nanoparticles is distributed from 5 to 150 nm, commonly less than 50 nm. Compared with chemical approaches, biological methods generally create ZrO2 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 ZrO2 nanoparticles (Kumaresan et al. 2018; Silva et al. 2019; Debnath et al. 2020). Small-size ZrO2 nanoparticles offer many biomedical benefits since they facilitate the penetration into cell walls, performing better antibacterial, antifungal, and anticancer activities (Khatoon et al. 2015).

Morphology

The shape of ZrO2 nanocrystals biosynthesized from biological routes can be explored by scanning/transmission electron microscopy analysis. Accordingly, green ZrO2 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 ZrO2 morphology is significantly dependent on the synthesis conditions including the ratio between zirconium source and green extract. Spherical ZrO2 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 ZrO2 morphologies are relatively rare, mostly synthesized by plant sources. For example, Sathishkumar et al. (2013) obtained ZrO2 nano-chains by the hydrolysis of ZrF62− in the presence of Curcuma longa tuber extract. Gowri et al. (2015) also successfully biosynthesized ZrO2 nanoflakes by the hydrolysis of ZrOCl2·8H2O using aqueously soluble carbohydrates extracted from Nyctanthes arbor-tristis flower. Semi-spherical and oval ZrO2 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 ZrO2 morphologies can bring many benefits for practical applications. As an example, (Isacfranklin et al. 2020) indicated that ZrO2 nanorods possess sufficient cellular uptake and toxicity, offering the prospect for biomedical application.

Fig. 4
figure 4

source 3:1 (b), and 6:1 (c). The role of plant extract is to improve the dispersion of ZrO2 nanoparticles during biosynthesis. Reproduced and adapted with the permission of Elsevier from reference (Davar et al. 2018). Transmission electron microscopy photographs of ZrO2 nanospheres (d), nano chains (e), and nanorods (f) biosynthesized from Acinetobacter sp. bacterial community (Suriyaraj et al. 2019), Curcuma longa tuber extract (Sathishkumar et al. 2013), and Nephelium lappaceum fruit extract (Isacfranklin et al. 2020), respectively. Reproduced and adapted with the permission of Elsevier from references (Sathishkumar et al. 2013; Suriyaraj et al. 2019; Isacfranklin et al. 2020)

Field emission scanning electron microscope photographies of ZrO2 nanoparticles produced at 600 °C without Rosmarinus officinalis leaf extract (a), and from Rosmarinus officinalis leaf extract with the ratio between the extract and zirconium

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Surface chemistry

The surface functionalization by chemical groups aids to extend the applications of nanomaterials (Tran et al. 2021). Surface chemistry of ZrO2 nanoparticles can be identified by several physicochemical techniques such as Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy or quantified by Boehm titrations (Nguyen et al. 2021b). The surface functional groups on ZrO2 nanoparticles may be originated from the phytochemical compounds. Considering the calcination of the complex of Zr2+–phytochemicals, almost all organic components are decomposed into volatiles or simple products such as CO2, N2, and H2O. 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 H2O-adsorbed surface of ZrO2 (Kumaresan et al. 2018; Silva et al. 2019; Annu et al. 2020). The functionalization of ZrO2 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).

Surface area

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 ZrO2 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 ZrO2 nanoparticles during the thermolysis. This enhances the surface area and the number of active sites of ZrO2 nanoparticles. For example, Shinde et al. (2018) reported the surface area (88 m2/g) of ZrO2 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 ZrO2 to methylene blue (91%) and methyl orange (69%) for 240 min. These results of green ZrO2 were remarkably higher than those obtained by the chemical methods such as anodization in H2O2/NH4F/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).

Stability

Compared with chemically produced nanomaterials, biologically produced ZrO2 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 biocap** agents. The stability of ZrO2 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 ZrO2 nanoparticles (Jameel et al. 2020). Indeed, Suriyaraj et al. (2019) demonstrated the well-dispersed, anti-clustering, and nanostructured ZrO2 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 ZrO2 nanoparticles biosynthesized from Laurus nobilis leaf extract by dynamic light scattering analysis, proposing the good anti-sedimentation of the nanomaterials for antimicrobial activities.

Applications of green ZrO2 nanoparticles

Biomedical applications

Biomedical compatibility of green ZrO2 nanoparticles

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 ZrO2 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 ZrO2 nanoparticles.

Antibacterial activity

The plasma membrane of bacteria is mostly constituted of proteins (e.g., peptidoglycan macromolecules) that charge negatively (Li et al. 2019). ZrO2 nanoparticles charge positively on their surface, leading to electrostatic interactions with bacterial membranes (**, biochelating, and biostabilizing process for the transformation into composites. For example, Vivekanandhan et al. (2015) functionalized the surface of ZrO2 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 ZrO2 surface. The authors also reported the inhibition of the deformation of Ag/ZrO2 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 ZrO2. Therefore, a sufficient amount of plant extract is important to obtain the structure of ZrO2-based nanocomposites. Table 5 summaries several potential applications of green ZrO2-based nanocomposites.

Fig. 6
figure 6

The bioreduction of Ag+ into Ag and nucleation of on ZrO2 surface for the biosynthesis of green Ag/ZrO2 nanocomposites using Anacardium occidentale leaf extract (a); the photograph of as-synthesized green ZrO2 and Ag/ZrO2 nanocomposites (b); scanning electron microscopy (c, d) and transmission electron microscopy (e, f) microphotographies of green Ag/ZrO2 nanocomposites. The presence of leaf extract efficiently leads to the good dispersion and high biostabilization of Ag nanospheres (5–20 nm) on the surface of ZrO2. However, larger addition of leaf extract inhibits the formation of Ag/ZrO2 nanocomposites. Reproduced and adapted with the permission of Elsevier from reference (Vivekanandhan et al. 2015). Abbreviations: AgNP, silver nanoparticles

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Because ZrO2 nanoparticles exhibit a wide bandgap (~ 5.0 eV), the absorption of visible light is difficult, hampering their photocatalytic and nanoelectronic applications. The modification of ZrO2 nanoparticles aims to suffer this major drawback. For example, Yadav et al. (2021) inserted an amount of transition metal nickel (Ni) into ZrO2 nanoparticles using rubber latex extract to obtain monoclinic, tetragonal, and cubic Ni-doped ZrO2 polycrystalline. The band gaps were very narrowed between 2.4 and 2.75 eV, making Ni-doped ZrO2 as potential nanoelectronic devices. Rasheed et al. (2020) reported the utilization of Daphne alpine leaf extract to synthesize green V2O5/ZrO2 nanocomposite. They not only observed the increase in surface area (214 m2/g) but also found the improvement of bandgap energy (3.93 eV) and thermal stability (1000 °C). It was suggested that V2O5 efficiently enhanced the electron separation, resulting in good photocatalytic activities against orange (76.9%) and picloram (86%) of green V2O5/ZrO2.

Do** 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 ZrO2 with samarium dopant (Sm3+/ZrO2+ = 3–11 mol.%) using Leucas Aspera extract. Sm/ZrO2 composites with the do** 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/ZrO2 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/ZrO2 nanoparticles (18–25 nm) with good stable ability during the post-synthesis. Moreover, the green Cu/ZrO2 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/ZrO2 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/ZrO2 nanocomposite.

Although green ZrO2 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., Fe3O4) into ZrO2 nanomaterials should be therefore required. Inspired by this idea, Rostami-Vartooni et al. (2019) have successfully the Centaurea cyanus-biofabricated Ag/Fe3O4/ZrO2 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/Fe3O4/ZrO2 with a swift reaction time between 7.5 and 8 min. Therefore, this advantage may urge the recyclability experiments of magnetic ZrO2 nanocomposites.

ZrO2-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 CeO2 doped ZrO2 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 CeO2/ZrO2 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 ZrO2 microspheres using Aloe Vera leaf extract. They assumed that bio compounds such as polysaccharides from Aloe Vera leaf might chelate with Zr4+ and Mg2+ ions during the formation of Mg/ZrO2 microspheres. The authors assumed that bio compounds such as polysaccharides from Aloe Vera leaf might chelate with Zr4+ and Mg2+ ions during the reaction of complexation. The burning calcination of these complexes allowed to acquire the hollow structure of Mg/ZrO2 microspheres. They found the Mg/ZrO2 composites with the magnesium do** 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/ZrO2.

Perspective

Green synthesis of ZrO2 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 ZrO2 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 ZrO2 nanoparticles experimentally exhibit better antibacterial, antifungal, catalytic and adsorptive performance than chemically synthesized ZrO2 nanomaterials. These superior prospects of green ZrO2 nanoparticles may pay the way for develo** advanced ZrO2-based nanosensors for smart agriculture, high-efficiency solar cells for energy harvesting, and electronic nanodevices for many high-technological manufacturing industries.

Although green ZrO2 nanoparticles hold many key advantages, there are still several limitations that need further investigation. Firstly, regarding the synthesis procedure for ZrO2 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 ZrO2 nanoparticles. Moreover, the formation of green ZrO2 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 ZrO2 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 ZrO2 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 ZrO2 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 ZrO2 nanoparticles, it is expected that they will contribute significantly to the development of new environmental remediation approaches. Ultimately, the combination of dopants with green ZrO2 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 ZrO2 based nanocomposites. The optimization of the biomass proportion should be evaluated to avoid these interactions. Even though the formation mechanism and application of green ZrO2 based nanocomposites have been better understood in some published works, the number of researches is still limited. The knowledge of the overall interactions between ZrO2 nanomaterials and the environment is just burgeoning. The profound evaluations of the ecotoxicity of green ZrO2 nanoparticles should be continuously conducted to avoid possible future challenges.

Conclusion

The present review addressed some aspects of synthesis, properties, and potentials of green ZrO2 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, biocap**, biochelating, and biostabilization during the transformation of zirconium sources into green ZrO2 nanoparticles. Through the structural characterization techniques, green ZrO2 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 ZrO2 nanoparticles have been overviewed and discussed systematically. The advanced properties and applications of green ZrO2-based nanocomposites were also mentioned. To orientate the future researchers, some discussions of knowledge gaps and prospects of green ZrO2 nanoparticles have been clarified. The green ZrO2 nanoparticles and their nanocomposites are highly expected to obtain many good results in various applications.