Nowadays, metal oxide nanomaterials are found to have peculiar uses in the area of catalysis, ceramics, semiconductors, space industry, medical science, agriculture, capacitors, batteries, absorbents, defense, chemical and biological sensors, optoelectronics, textile and food industry [1-34]. Among all known metal oxide nanomaterials, Al2O3 NPs have drawn remarkable attention in the cutting edge of particular innovation, in the formulation and designing of recent antimicrobial agents for sustainable biomedical applications; because Al2O3 NPs are chemically bio-inert and hydrolytically more stable . The biocompatibility of Al2O3 ceramic has already been mentioned by many researchers . Al2O3 NPs with high purity were the first bio-ceramics widely utilized in clinical application, and it was recommended that the lifespan of Al2O3 is longer than the concerned patients . Accordingly, Al2O3 NPs have been utilized in several branches (Fig. 1.) consisting of structural ceramics , catalysis , textiles , wastewater treatment  and protein separation/purification . Moreover, Al2O3 NPs also find extensive biomedical applications in biosensors , bio-filtration  and drug delivery .
Al2O3 NPs can be easily synthesized using several methods such as combustion , hydrothermal , laser ablation , mechanochemical , sol-gel , template method , microwave-assisted , pechini method , precipitation method , solvothermal , pyrolysis  and ball milling . However, these synthetic routes are quite expensive, potentially hazardous and require long reaction time, perilous chemical precursors and special instruments for experimental work. Therefore, these routes create a bad impact on the ecosystem. This enhances the urgent need to replace or modify chemical preparation methodology and develop a sustainable, clean, non-toxic, cost effective and environmentally gracious process through green synthesis and other biological approaches. It is one of the promising pathways for fabrication of NPs as it is free from perilous chemicals as well as providing natural or herbal capping agents such as plant extracts, algae, fungi, bacteria, sugars, biodegradable polymers for the stabilization of Al2O3 NPs.
The present review article highlights the current scenario and knowledge concerning the capability of various plant materials for eco-benevolent synthesis of Al2O3 NPs and presents a database that future researchers may be based on the biosynthesis of Al2O3 NPs using various plant material sources.
GREEN SYNTHESIS OF Al2O3 NPs
Nowadays, several methods have been successfully used to fabricate the Al2O3 NPs, however, they have some demerits such as the higher cost of the method and not being eco-benevolent since they make lots of pollution in the ecosystem because of using perilous solvents and toxic reducing agent. To mitigate these drawbacks, green chemistry approaches have been employed for the fabrication of Al2O3 NPs which are sustainable, less energy-intensive, eco-accommodating and increase the efficiency of the methods. Although chemical stabilizers are utilized more than plants part extract, that materials are not safe for the ecosystem and aspects of human health. The stabilization of Al2O3 NPs is dependent on biomolecules such as amino acids, enzymes, proteins, steroids, phenols, tannins, sugar and flavonoids, which are already present in the plant extracts having medicinal importance and are eco-benign [2-3]. The main principle in the green chemistry approaches (Fig. 2.) is that the phytoconstituents are present in the plant parts serve the dual role of a natural reducing agent and a NP stabilizer. Some plants are already reported to facilitate Al2O3 NPs biosynthesis and all of them are described in this review (Table 1). The various parts of plant such as leaves, seed fruit and flower are used to fabricate Al2O3 NPs in different morphologies and sizes by biological approaches. The aqua soluble heterocyclic constituents are mainly accountable for formation and stabilization of nanoparticles. Thereafter, the biosynthesized NPs need to be characterized by using numerous techniques.
PROTOCOL FOR BIOSYNTHESIS OF Al2O3 NPs
Bio-fabrication of Al2O3 NPs is an effortless, rapid, one pot synthesis and eco-friendly route without participation of any harmful and perilous chemical. Al2O3 NPs are synthesized using distinctive parts of plants such as leaves, fruit, seed and flower (Table 1). A completely easy and clean protocol is implemented for their biosynthesis (Fig. 3). The plant parts such as leaves, flowers, seeds, fruits, etc. are collected from distinctive sources and thoroughly washed with ordinary water as well as double distilled water to remove other undesirable materials. The plant materials are either grinded or dried to form the fine powder or used directly to obtain extract. The plant parts are hewed into small pieces or ground to fine powder and boiled in different special solvents (ethanol, water, and many others) and boiled at a suitable temperature to acquire extract. Different concentrations of aluminum salts as a metallic precursor and as-prepared plant extract can be used for the biosynthesis of Al2O3 NPs. There may be no need to add external chemical reducing agents or stabilizers, simply plant extract is mixed with aluminum salt solution and the phytochemical present in plant extract acts as a bio-reducing agent as well as stabilizing agent for the biosynthesis of Al2O3 NPs. The precise protocol of biosynthesis of Al2O3 NPs by Cymbopogon citratus leaf extract is mentioned by authors reported in literature . The synthesized Al2O3 NPs solution is further centrifuged to separate out the NPs at excessive rpm, and wash thoroughly with suitable solvents. A fine powder of Al2O3 NPs is obtained and this is carefully collected for further characterization purposes.
CHARACTERIZATION TECHNIQUES FOR Al2O3 NPs
To study the effect of synthesized Al2O3 NPs on ecosystem and human health, and affirmation in their formation, diverse routes of their formation and monitoring their typical effect are needed. Different instrumental techniques are used to characterize synthesis of Al2O3 NPs.
There are various methods to measure crystalline particles size of Al2O3 NPs. X-ray Diffraction (XRD) is also used to determine the particle size and exact phase identification of Al2O3 NPs. The size of suspended NPs in liquid phase is described by dynamic light scattering (DLS).
X-ray diffraction (XRD) is used to determine each and every crystal structure of Al2O3 NPs.
Accurate morphology of Al2O3 NPs may be examined by using electron microscopies such as transmission electron microscope (TEM), atomic force microscopy (AFM) and scanning electron microscope (SEM).
Specific surface Area
The nitrogen absorption technique based on Brunauer–Emmett–Teller (BET) isotherm is most commonly used for solid state, and nuclear magnetic resonance (NMR) technique is among the techniques could be used for liquid state.
Mass spectrometry (MS), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS) and atomic emission spectroscopy (AES) could be used to examine purity and elemental composition of Al2O3 NPs.
APPLICATIONS OF BIOGENICALLY SYNTHE-SIZED Al2O3 NPs
Al2O3 NPs have many captivated applications in several branches of science and technology. However, the ceramics, textiles, biosensor and antimicrobial activities of the biosynthesized Al2O3 NPs are very prominent nowadays. Accordingly, their peculiar applications are described here as a guidance to new researchers for future prospects.
Jalal et al. reported the Cymbopogon citratus leaf extract mediated Al2O3 NPs with the size of 34.5 nm and investigated the antifungal activity of Al2O3 NPs against various Candida spp. isolated from oropharyngeal mucosa of HIV+ patients .
Ansari et al. reported the biosynthesis of Al2O3 NPs using leaf extract lemongrass and analyzed the antibacterial activity of the prepared NPs. These Al2O3 NPs exhibited an excellent antibacterial activity against MDR strains of P. aeruginosa, indicating their compatibility for pharmaceutical and other biomedical applications .
Besides, Manikandan et al. reported plant mediated synthesis of Al2O3 NPs using Prunus xyedonesis and examined the antibacterial activity of Al2O3 NPs against pathogenic bacteria. These biosynthesized Al2O3 NPs displayed effective antibacterial activity against gram-positive S. aureus and gram-negative E. coli bacteria. The synthesized Al2O3 NPs also showed nitrate removal ability. From the results, green synthesized Al2O3 NPs is found to have promising applications in pollutant ion removal from aquatic systems .
This review has summarized the current scenario of the research work in the area of green synthesis of Al2O3 NPs by using distinctive plant parts. This literature surveys displayed the multifarious experimental works on biosynthesis of NPs of silver, zinc, gold and copper NPs in comparison to Al2O3 NPs. Therefore, special attention of scientific community is required to develop this efficient, swift, sustainable, noxious, affordable and environmentally gracious method for biosynthesis of Al2O3 NPs through this green chemistry bottom to top approach. Furthermore, multifarious plant species could be exploited in future era towards completely facile and rapid biosynthesis of metal oxide NPs. Further research needs to develop outstanding applications, use of distinctive plant parts for fabrication and highlight the exact mechanism behind the synthesis of Al2O3 NPs.
CONFLICT OF INTEREST
No potential conflict of interest was reported by the author.