Recent progress of phytogenic synthesis of ZnO, SnO2, and CeO2 nanomaterials

Bioprocess and Biosystems Engineering https://doi.org/10.1007/s00449-022-02713-z CRITICAL REVIEW Recent progress of phytogenic synthesis of ZnO, SnO2, and CeO2 nanomaterials Mohammad Mansoob Khan1 · Shaidatul Najihah Matussin1 · Ashmalina Rahman1 Received: 14 December 2021 / Accepted: 14 February 2022 © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2022 Abstract A critical investigation on the fabrication of metal oxide nanoparticles (NPs) such as ZnO, SnO2, and CeO2 NPs synthesized from green and phytogenic method using plants and various plant parts have been compiled. In this review, different plant extraction methods, synthesis methods, characterization techniques, effects of plant extract on the physical, chemical, and optical properties of green synthesized ZnO, SnO2, and CeO2 NPs also have been compiled and discussed. Effect of several parameters on the size, morphology, and optical band gap energy of metal oxide have been explored. Moreover, the role of solvents has been found important and discussed. Extract composition i.e. phytochemicals also found to affect the morphology and size of the synthesized ZnO, SnO2, and CeO2 NPs. It was found that, there is no universal extraction method that is ideal and extraction techniques is unique to the plant type, plant parts, and solvent used. Keywords Green synthesis · Phytogenic synthesis · Metal oxides · Zin oxide · ZnO · Tin oxide · SnO2 · Cerium oxide · CeO2 · Nanomaterials · Plant extract · Leaf extract Introduction Green chemistry offers the utilization of green synthesis method for nanoparticles (NPs) fabrication, which is relatively simpler, cost-effective, energy efficient and ecofriendly process when compared to the conventional chemical methods [1]. Green synthesis of nanoparticles involves the choice of solvent medium, option of using environmentally reducing agent and the use of nontoxic stabilizing agent [2]. Green synthesis is a method that does not require high energy consumption and makes the use of green starting materials as reducing and capping or stabilizing agents. Different green synthesis method of metal oxides have been reported in literature, such as those that were synthesized from plant parts (such as stems, flowers, leaves, seeds etc.) and microorganisms (fungi, yeast and bacteria). The synthesis of metal oxides such as ZnO, SnO2, CeO2 etc. is driven by the presence of active biochemicals in plants and microorganisms that can act as stabilizing, capping and strong * Mohammad Mansoob Khan mmansoobkhan@yahoo.com; mansoob.khan@ubd.edu.bn 1 Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong BE 1410, Brunei Darussalam reducing/oxidizing agents to reduce/oxidize/stabilize the metal ions during synthesis (Fig. 1) [1, 3–5]. Unlike green synthesis, chemical approaches may require the involvement of hazardous chemicals and they may have negative effects on human beings as well as the environment. Moreover, physical methods tend to involve high cost and high energy consumption equipment and also require conditions such as high pressure and temperature. Hence, green synthesis of metal oxide NPs has increasingly become the subject of interest among the scientific community. In addition to the selection of suitable plant extract in green synthesis, other factors may also influence the properties of these nanomaterials. To obtain nanomaterials of desired shape, sizes and functionalities, various reaction parameters such temperature, solvent, reaction conditions (acidic or basic), concentration or precursors, etc. should also be taken into account and optimized. Figure 2 shows some of the nanomaterial properties that may be affected based on the synthesis parameters used. Metal oxide nanostructures such as ZnO, SnO2 and CeO2 in particular display excellent properties when compared to their bulk counterparts and have been utilized widely in various field such as biomedical technology, water treatment, and energy and personal care products [6, 7]. ZnO is amongst the most widely synthesized metal oxides, and it is 13 Vol.:(0123456789) Bioprocess and Biosystems Engineering Fig. 1 Green synthesis of ZnO, SnO2 and CeO2 using different parts of plant Fig. 2 Factors affecting the properties of green synthesized materials known to be a multifunctional material having applications in a wide variety of fields [8]. ZnO is an II–VI semiconductor and is reported to occur in three different structures: wurtzite, zinc-blende and rock salt. However, ZnO occurs almost exclusively in the wurtzite type structure. Generally, at room temperature, chemically synthesized ZnO has a direct energy band gap of approximately 3.37 eV (could only absorb UV light) and a relatively large exciton binding energy of about 60 meV [4]. 13 One the other hand, SnO2 is an n-type metal oxide semiconductor with a band gap energy of ∼ 3.6 eV. It has a rutiletype (P42/mnm) tetragonal structure. SnO2 has various polymorphs, such as CaCl2-type (Pnnm), α-PbO2-type (Pbcn), pyrite-type (Pb3), ZrO2-type orthorhombic phase I (Pbca), and fluorite-type (Fm3m) [3]. In addition, CeO2 is also a semiconductor with a band gap energy of ~ 3.14 eV and exhibits high excitation energy. CeO2 is a rare earth metal oxide and does not show any crystallographic changes with Bioprocess and Biosystems Engineering increase in temperature up to its melting point. Cerium exists in dual oxidation states, Ce3+ and Ce4+. Ce4+ is considered more stable than Ce3+ due to the noble gas like electronic configuration of Ce4+ [Xe]4f0 as compared to that of Ce3+ [Xe]4f1 [5]. This review focuses on the green and phytogenic synthesis of ZnO, SnO2, and CeO2 NPs using different types of plants and plant parts. This review discusses the past and present literature associated with the synthesis of ZnO, SnO2, and CeO2 NPs and its effect on their size, morphologies and optical band gap energy, especially those that have been synthesized through green and phytogenic synthesis methods. This review also emphasizes several parameters such as precursor’s concentration, type of capping and reducing agents, temperature, and solvents that have the ability to tune the physical, chemical and optical properties of the ZnO, SnO2 and CeO2 NPs. At the end, future prospects for plant extract assisted synthesis of ZnO, SnO2 and CeO2 NPs are provided. Green synthesis of ZnO NPs Different parts of plant such as leaf, stem, root, fruit, and seed have been used for ZnO NPs synthesis because of the phytochemicals that they produce. Most commonly applied method to prepare the plant extract is that the desired part of the plant is first collected and cleaned thoroughly with tap water to remove dust particles and sterilized using double distilled water. It is then air-dried to get rid of residual moisture. The dried samples are then weighed and ground using a blender/grinder into fine pieces. There are different extraction methods have been utilized to separate the insoluble plant residues from the soluble plant metabolites. These methods are intended to release the soluble phytochemicals by softening and breaking the plant’s cell wall. Other methods for extraction are maceration and decoction. Decoction uses the same principle as maceration but for decoction, the desired plant materials (coarse or powdered) is soaked boiled in a specified volume of solvent (e.g. 1:10 coarsely powdered plant material to water ratio) for a defined time [9]. Based on this decoction method, the cell wall of plants is swelled as a result of applied heat and moisture and then plant constituents are hydro-dispersed from the swollen membranes [10]. Moreover, the selection of solvent is also important. Plant extract can be prepared by the addition of the desired solvent to the weighed samples and the solution is then either filtered using filter paper or centrifuged to remove the residual solids to obtain a clear solution. Figure 3 shows the widely used method for synthesizing ZnO. A measured volume of extract is heated at a desired temperature (normally 60–80 °C as reported in previous Fig. 3 Common method used for ZnO NPs fabrication using plant extract 13 Bioprocess and Biosystems Engineering studies). Upon reaching the desired temperature, a measured amount of the zinc precursor is added into the heated extract. Some of the reported precursors are zinc nitrate hexahydrate, zinc acetate dehydrate, zinc gluconate hydrate, zinc chloride, and zinc sulphate. Different factors such as temperature, pH, extract volume, amount of precursor added, and, time of synthesis, can be varied to obtain the optimized conditions. The resulting product is then calcined at 400–600 °C for a certain number of hours. The incubation period causes the color of the mixture to change, indicating the formation of ZnO NPs. Lastly, the synthesized ZnO are characterized using different techniques and tested for various applications. Due to the increasing popularity of green methods, different works had been done to synthesize ZnO using different types of plants and plant parts. Table 1 tabulated the summary of previous work on the use of plants in the syntheses of ZnO NPs. Different types of leaf extract used for the synthesis of ZnO NPs Among the different parts of a plant, leaf is the most commonly used for the synthesis of ZnO NPs. This may be because of its availability and abundance in nature as well as it is easily extractable. Leaves have broader surface area, contain chlorophyll that carry out photosynthesis and it was reported to contain higher concentration of phytochemicals [76, 77]. Steffy et al. demonstrated an environmentally safe approach on biosynthesis of ZnO NPs from a well-known medicinal plant Aristolochia indica. The estimated optical direct band gap energy from Tauc’s plot was found to be 3.37 eV. The green synthesized ZnO NPs of size 22.5 nm exhibited strong antibacterial properties against clinical multi-drug resistant strains isolated from diabetic foot ulcer. Thus, the study provides a new insight of using ZnO NPs as an alternate therapeutic agent [19]. In another study, Balaji et al. have also successfully synthesized spherical shaped ZnO NPs with average particle size of 11.6 nm using leaf extract of Eucalyptus globulus under ambient conditions. UV–visible spectrum of the synthesized material revealed the characteristic peak at 361 nm indicating the formation of ZnO NPs. XRD confirmed that the synthesized ZnO NPs are crystalline with stable hexagonal phase. EDX revealed the formation of highly pure ZnO NPs with the peaks of only Zn and O atoms. These biosynthesized ZnO NPs exhibited photocatalytic and antioxidant properties [36]. The formation of spherical ZnO NPs have also been found in a study carried out by Zare et al., where the authors have prepared ZnO NPs using a combination of biosynthesis and hydrothermal (bio-hydrothermal) methods, utilizing garden thyme known as Thymus vulgaris as reducing and stabilizing agents. They have also confirmed the presence of flavonoids, 13 phenols and saponins in the leaf extract of Thymus vulgaris. Their study revealed that the synthesized ZnO have a size range of 50–60 nm with an irregular morphology. From UV–Vis spectroscopic studies, the estimated band gap values are found to be 3.2, 3.4 and 3.5 eV for 0.0, 0.5 and 1.0 mL of extract added, respectively. They also reported that the higher concentration of Thyme leaf extract reduces agglomeration of particles which resulted in an increase in band gap energy [73]. In some of the literature review, they are also reports of green synthesized ZnO NPs with quasi-spherical shape. It has been reported that quasi-spherical NPs have shown a range of advantageous properties especially for catalytic applications [78]. In previous study, Solanum nigrum leaf extract has been successfully used for the synthesis of ZnO NPs as reported by Ramesh et al. TEM images revealed that most of the ZnO NPs possessed quasi-spherical shaped and their diameter is about 29.79 nm. The authors investigated the antibacterial activities of the synthesized ZnO and the highest antimicrobial activity was found to be against Salmonella paratyphi (with zone of inhibition 17 mm) in comparison to standard tablet [65]. Recently, Muthuvel et al. have used similar extract (Solanum nigrum) and obtained spherical shaped particles with smaller average particle size of 22 nm [68]. The difference may be due to the use of different precursor, different temperature utilize in leaf extraction method and amount of leaves added. In another study, natural extract of Agathosma betulina used in the synthesis of ZnO also results in the formation of quasi-spherical agglomerated nano-scaled particles with an average size of about 15.8 nm. Thema et al. demonstrated for the first time the use of A. betulina’s extract as an effective oxidizing/ reducing chemical agent [15]. In another study, Anbuvannan et al. reported the use of different volumes of Anisochilus carnosus leaf extract i.e. 30, 40 and 50 mL for synthesizing ZnO NPs. Based on the UV–Vis spectra, the synthesized ZnO exhibited indirect band gap of 3.47, 3.51 and 3.56 eV, respectively for 30, 40 and 50 mL of extract addition. ZnO NPs synthesized using 50 mL of extract was used for degradation of methylene blue (MB) under UV irradiation. Biosynthesized ZnO showed great photocatalytic activity due to its wide band gap because it can absorb UV light [18]. Flower extracts used for the synthesis of ZnO NPs The primary purpose of flower is for reproduction [79]. They are of great aesthetic value and they are mainly utilized for their beauty as their petals radiate different colors to the surroundings [46]. Different type of flowers have different effect towards the morphology and size of ZnO. This is because as different parts of the plants have different phytochemicals and the amount varies in each part [80]. 13 Plants used Plant parts used Extraction method and solvents used Size and shape References Abelmoschus esculentus Mucilage Distilled water (soaked overnight and filtered) [11] Acacia caesia Flower Distilled water (refluxed at 60 °C and centrifuged) Acalypha fruticosa L. Leaf Water (heated at 60 °C and filtered) Achyranthes aspera Agathosma betulina Aloe barbadensis Mill. Leaf Leaf Leaf Double distilled water (heated at 110 °C and filtered) Deionized water (heated at 100 °C and filtered) Deionized water (boiled and filtered) Aloe socotrina Anisochilus carnosus Aristolochia indica Artocarpus gomezianus Artocarpus gomezianus Artocarpus heterophyllus Averrhoa bilimbi L. Bauhinia tomentosa Leaf Leaf Leaf Fruit Fruit Ripened leaf extract Fruit Leaf Water (heated at 70 °C and filtered) Distilled water (heated at 100 °C and filtered) Double distilled water (heated at 60 °C and filtered) Double distilled water (boiled and filtered) Water (refluxed at 100 °C and filtered) Deionized water (boiled and filtered) Distilled water (boiled and filtered) Distilled water (heated at 60 °C and filtered) Beta vulgaris The outer layer and the roots Natural extract and no solvent added Canthium dicoccum L. Leaf Capparis zeylanica Leaf Double distilled water (agitated at room temperature and filtered) Distilled water (heated at 60 °C and filtered) Capsicum annuum Fruit Distilled water (heated at 80 °C and filtered) Cardiospermum halicacabum Carica papaya Leaf Leaf Conyza canadensis Leaf De-ionized water (heated at 85 °C and filtered) Double distilled water (refluxed at 100 °C and filtered) Distilled water (heated at 80 °C and filtered) Uniform spheres (29 nm) and elongated and rod-like structures (70 nm) Distinct attachment of spherical Ag NPs (7 nm) and Cu NPs (12 nm) anchored over the ZnO surface Spherical and hexagonal shaped with average particle size of 50 nm ZnO consists of aggregated flake-like NPs 30–40 nm Quasispherical with particle size 15.8 nm Chemically synthesized → spherical with aggregations 42–56 nm Green synthesized → spherical without aggregation 30–40 nm Spherical in shape with size range of 15–50 nm Quasi-spherical and 30–40 nm Agglomerated quasi-spherical shaped NPs 22.5 nm Spherical shape with size 10–30 nm Almost spherical with aggregations Spherical particles 10–15 nm Spherical morphology with size around 37.5 nm Separated hexagonal-shaped ZnO in the range of 22–94 nm Agglomerated to form sponge-like structures having defined pores Rod-shaped and agglomerated with size range of 20–40 nm Dispersed without agglomeration with particle size in the range of 28–30 nm Agglomerated cluster to form spongy cave-like structures with size 500 nm to 1 µm Hexagonal structure with some surface agglomeration 50 nm Cordia myxa Leaf Distilled water (maceration at 30–33 °C and filtered) ZnO NPs synthesized at 80 °C → smaller in size and have high and uniform dispersion ZnO NPs synthesized at 30 °C → aggregation and cluster formation, are larger in size, show irregular morphology and uneven distribution Pure extract → Hexagonal wurtzite 10% extract concentration → spherical with layer-by layer discs like entities 25% extract concentration → micro/nano sized triangular entities [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] Bioprocess and Biosystems Engineering Table 1 Green and phytogenic synthesis of ZnO NPs using different types of plants 13 Table 1 (continued) Plants used Plant parts used Extraction method and solvents used Size and shape References Costus woodsonii Bulb (flower) Leaf Spherical and hexagonal shape with size range of 20–25 nm Rectangular and spherical shapes [33] Costus woodsonii Couroupita guianensis Deverra tortuosa Eucalyptus globulus Eucalyptus spp. Leaf Flower Leaf Leaf Double distilled water (with and without heating at 60 °C) Double distilled water (with and without heating at 60 °C) Double distilled water (heated at 110 °C and filtered) Distilled water (cold maceration) Deionized water (heated at 80 °C and filtered) Distilled water (heated at 90 °C and filtered) [14] [35] [36] [37] Euphorbia hirta Leaf Euphorbia jatropha Crude latex Garcinia gummi-gutta Seed Garcinia mangostana Garcinia xanthochymus Hibiscus rosa-sinensis Fruit pericarp Fruit Leaf Water (refluxed at 90–100 °C then centrifuged and concentrated) Deionized water (heated at 70–80 °C and filtered) Water (refluxed at 100 °C and centrifuged) Deionized water (heated at 50 °C) Hibiscus sabdariffa Flower Water (heated at 60 °C and filtered) Hylotelephium telephium Jacaranda mimosifolia Justicia adhatoda Flower Flower Leaf De-ionized water (ground and filtered) Deionized water (heated at 90 °C and filtered) Ethanol (using soxhlet extractor at 60 °C then filtered and concentrated below 40 °C by solvent evaporation) Melastoma malabathricum L. Leaf Distilled water (stirred at room temperature) Melia azedarach Leaf Distilled water (heated at 80 °C and filtered) Melissa officinalis L. Leaf Moringa oleifera Moringa oleifera Nyctanthes arbor-tristis Seed Natural extract Flower Double distilled water (heated at 100 °C and centrifuged) Distilled water (heated at 40 °C and centrifuged) Natural extract and no solvent added Double distilled water (boiled and filtered) ZnO contains spherical shaped NPs 5–10 nm Hexagonal with particles size 9.26–31.18 nm Spherical with size of 11.6 nm Irregular in shape with particle size in the range of 20–40 nm Spherical in shape with aggregations and particle size in the range of 20–25 nm Hexagonal with slight variation in thickness 50–200 nm Agglomerated and irregular shaped morphology with spongy cave like 10–20 nm Spherical in shape with an average size of 21 nm Spongy cave-like structures Pure ZnO → spherical in shape with size 35–225 nm Fe-doped ZnO → 15–170 nm 1% of extract → spherical shapes with few agglomerations and particle size in the range 20–40 nm 4 and 8% of extract → agglomerations with size range of 5–12 nm Irregular morphology with particle size 20–44 nm Spherical with size range from 2 to 4 nm ZnO → flower-like structure Both Ag-doped and Au-doped ZnO → nanorod structure with uniform morphologies Ag/Au-doped ZnO NPs → nanostick shaped with particle size 20–25 nm Agglomerated spherical shaped particles (no size mentioned) Hexagonal and spherical in shape with size range of 33–96 nm Spherical, narrow sized (nearly 10–20 nm) CuO NPs are attached to ZnO NPs Spherical with agglomerations Spherical in shape with the average size of 20–50 nm Formation of agglomerates with size 12–32 nm Ethanol (extracted using soxhlet extraction method and evaporated using rotary evaporator) Crude [34] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [49] [50] [51] [52] [53] Bioprocess and Biosystems Engineering [48] Plants used Plant parts used Extraction method and solvents used Ocimum tenuiflorum Leaf Double distilled water (heated at 60 °C and filtered) Ocimum tenuiflorum (Tulsi) Seed Palm pollen grain Prosopis juliflora Pollen suspension Leaf Psidium guajava Leaf Quince Seed mucilage Rambutan (Nephelium lappaceum L.) Peel Raphanus sativus Root Raphanus sativus (white radish) Root Ricinus communis Ruta graveolens (L.) Solanum nigrum Selaginella convolute Seed Stem Leaf Leaf Sida rhombifolia Leaf Solanum nigrum Leaf Size and shape 13 Undoped ZnO → hexagonal like grains Zn0.98Mn0.02O → elongated, hexagonal and rodshaped structure Zn0.96Mn0.02Mg0.02O → agglomerated with each other Double distilled water (boiled and filtered) Pure Zn NPs → flakes shaped with size 30–40 nm Different concentration of Ag in Ag/ZnO nanocomposites (NCs) → both hexagonal and spherical shapes with few aggregation with different size 0.5%: 25–35 nm 1.0%: 15–22 nm 2.0%: 50–60 nm Distilled water (stirred at room temperature) Spherical and polydispersed ZnO NP 15.57 ± 2.5 nm Distilled water (heated at 80 °C and filtered) Spherical and hexagonal shaped particles Leaf extract assisted → 65 nm Citric acid assisted → 75 nm Double distilled water (heated at 80 °C and filtered) ZnO → Trigonal Mg-doped ZnO → spherical The average sizes of both NPs are 78–100 nm Distilled water (heated at 60 °C and centrifuged) Fairly uniform distribution and the average size is about 25 nm Double distilled water and ethanol 2:1 ratio (heated at Spherical with size range from 25 to 40 nm 80 °C and filtered) Water (boiled) Spherical in shape and arranged like a chain with size range from 25 to 40 nm Natural extract and no solvent added (blended and Pure ZnO (chemically synthesized) and RC-ZnO NPs centrifuged) (biosynthesized using root extract) → spherical in shape and 60–100 nm R-ZnO NPs (biofunctionalization using root extract) → aggregated and totally covered with plant extract Water (squashed and filtered) Non-uniform and size in the range 10–30 nm Deionized water (heated at 60–70 °C and centrifuged) Spherical with size range of 20–30 nm Distilled water (boiled and filtered) Quasi-spherical in shape with particle size 29.79 nm Water (heated at 60 °C and filtered) Spherical and uniform in shape with particle size in range of 40–60 nm Water (heated at 80 °C and filtered) ZnO → rod-like and spherical Ag-doped ZnO → agglomerated Deionized water (heated at 80 °C and filtered) Spherical shaped particles and average particles size of Chemically synthesized ZnO → 30 nm Green synthesized ZnO → 22 nm References [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] Bioprocess and Biosystems Engineering Table 1 (continued) [75] [74] [73] [70] [71] [72] [69] Distilled water (stirred at room temperature) Leaf Leaf Fruit Seed bark Leaf Flower Leaves Stachytarpheta jamaicensis Tabernaemontana divaricata Terminalia chebula Theobroma cacao Thymus vulgaris Trifolium pratense Ziziphus mauritiana (L.) Agglomerated spherical shaped particles with particle size about 40 nm Deionized water (heated at 80 °C and filtered) Spherical shape ZnO NPs 20–50 nm Deionized water (refluxed at 80 °C and filtered) Spherical (size is not mentioned) Methanol (macerated, filtered then concentrated using Spherical shape and 20–50 nm vacuum rotary evaporator and fractionated with water and n-hexane) *Aqueous fraction was used for the synthesis Double distilled water (heated at 60 °C and centri50–60 nm size with an irregular shape fuged) Double distilled water (heated at 80 °C) Agglomerations with a particle size ranging from 100 to 190 nm Distilled water (stirred at room temperature) Aggregated spherical shape with particle size between range of 0.1–6 µm Extraction method and solvents used Plant parts used Plants used Table 1 (continued) Size and shape References Bioprocess and Biosystems Engineering 13 Recently, Hylotelephium telephium flower extract has been reported to assist the synthesis of ZnO. The synthesized ZnO exhibited particle size 20–44 nm with irregular shaped particles. The antibacterial activities of synthesized ZnO were tested against clinical pathogens namely S. aureus and K. pneumoniae using disk diffusion method. The synthesized ZnO showed zone inhibition of 16 ± 0.24 and 14 ± 0.38 mm against S. aureus and K. pneumoniae, respectively [45]. The use of Costus woodsonii bulb (flower) extract was demonstrated by Khan et al. to synthesize ZnO NPs. ZnO NPs were prepared by using two different bulb extracts i.e. unboiled and boiled bulb extracts. The authors also investigated effect of adding bulb (unboiled and boiled) extracts to zinc nitrate (precursor) at different ratios (1:1; 2:1 and 3:1) and they found out the ratio that gave the optimal yield was 1:1 extract to precursor ratio. Based on the TEM images, the synthesized ZnO possessed wurtzite structure, which is consistent with the XRD results. According to the authors, the calculated band gap energies of biosynthesized ZnO NPs are 2.66–2.73 eV and 2.66–2.79 eV for unboiled and boiled bulb extracts used, respectively. These band gap values were much lower than the commercial ZnO (3.15 eV) which may be due to the surface coating of the as-synthesized ZnO NPs [33]. In another study, Sharma et al. have synthesized ZnO NPs using Jacaranda mimosifolia flowers extract to investigate its optical properties. They were also comparing both structural and optical properties of ZnO synthesized using with and without the extract. HRTEM images presented the synthesized ZnO NPs with the size range of 2–4 nm, whereas ZnO NPs prepared without the use of extract are larger with an average size of 8–11 nm. Both synthesized materials have spherical morphology. Based on UV–Visible spectra, the calculated band gap energies were found to be 4.07 and 3.74 eV for ZnO NPs synthesized with and without Jacaranda mimosifolia extract, respectively. The authors reported that, this may be due to quantum confinement which is, when the size of particles decreases, there is an increase in the energy gap of electronic transitions [46]. ZnO NPs (particle size 100–190 nm) synthesized using Trifolium pretense flower extract was reported by Dobrucka et al. The antibacterial properties of the synthesized materials were evaluated against standard (S. aureus ATCC 4163, E. coli ATCC 25922 and P. aeruginosa ATCC 6749) and clinical (S. aureus and P. aeruginosa) strains of Gram negative and Gram positive bacteria using the agar well diffusion method. Based on their results, the synthesized materials (516 µg/mL) showed better antibacterial activity against clinical strain P. aeruginosa than same concentration of gentamicin (control) with inhibition zones of 26 and 17 mm, respectively [74]. Biosynthesis of ZnO NPs from the aqueous flower extract of Nyctanthes arbor-tristis has also been achieved. XRD confirmed the crystalline nature of the nanoparticles. TEM Bioprocess and Biosystems Engineering images revealed the synthesized materials are present in the form of aggregates with particle size 12–32 nm. The antifungal potential of the biosynthesized ZnO were tested against five phytopathogens (A. alternate, A. niger, B. cinerea, F. oxysporum and P. expansum) and were found to be most sensitive against A. niger with lowest MIC value (16 µg/mL), while highest MIC value (128 µg/mL) was observed for both B. cinerea and P. expansum [53]. hexagonal wurtzite structure for their synthesized ZnO NPs. The UV–Vis absorption spectrum exhibited a sharp absorption peak which confirmed the presence of ZnO in the synthesized sample. FT-IR revealed the presence of ZnO and other phytochemical constituents originated from the Moringa oleifera seed extract such as amines, phenolics, carboxylic acid and alcohol on ZnO surface [51]. Fruit extract used for the synthesis of ZnO NPs Seed extracts used for the synthesis of ZnO NPs The three stored constituents in seeds are carbohydrates, proteins and lipids. These compounds act as the source of energy as well as growth compounds for germination [81]. For instance, Ansari et al. stated that the Phoenix dactylifera (date palm) seeds contain a large number of nutritionally important functional compounds, such as protein, fatty acids, sugars, fibers, ash, minerals, and vitamins as well as high amounts of phenolic and flavonoids [82]. Different morphologies of ZnO were observed in a study carried out by Panchal et al. They have used Ocimum tenuiflorum (Tulsi) plant seed extract for the biosynthesis of pure ZnO NPs and Ag–ZnO nanocomposites. Based on TEM images, pure ZnO NPs showed flakes shape with an average size in the range of 30–40 nm. While, 0.5%, 1.0% and 2.0% Ag–ZnO nanocomposites exhibited both hexagonal and spherical shapes with different size. The particle size of 0.5% Ag–ZnO is in the range of 25–35 nm, 1.0% Ag–ZnO has slightly smaller size which is within 15–22 nm and for 2.0% Ag–ZnO, the size is 50–60 nm with few aggregation. The photocatalytic performance of synthesized materials were evaluated. The pure ZnO showed about 50% degradation of MB. While, composites of 0.5%, 1% and 2% Ag–ZnO degraded approximately 74.41%, 94.27% and 49.2%, respectively. Under solar light, the 1% Ag/ZnO nanocomposite exhibited maximum degradation efficiency of 94.27% to degrade 10 ppm MB in 120 min [55]. Smaller particle size of ZnO NPs were obtained in a study reported by Shobha et al. The authors have utilized seeds extract of Ricinus communis plant in biosynthesis of ZnO NPs. Zinc nitrate hexahydrate was used as a precursor, and the size range of ZnO NPs obtained was between 10 and 30 nm. The authors employed XRD, FESEM/EDX and FTIR for the characterization of synthesized ZnO. Both powder XRD and FE-SEM results showed the formation of highly crystalline ZnO NPs with distinct morphologies. The bioactive molecules from the Ricinus communis seed extract were reported to appear present in the FTIR spectrum. The authors stated that the as-prepared ZnO NPs displayed good anticancer, antibacterial and antioxidant activities and the results are found to be size and morphology dependent [63]. Synthesis of ZnO NPs using Moringa oleifera seeds extract with zinc nitrate as precursor was described by Rajeswari et al. XRD confirmed Fruits play a significant role in human nutrition by providing nutrients such as vitamins, minerals and dietary fiber [83]. Extracts from fruit have also been employed for the synthesis of ZnO. Ramanarayanan et al. reported that the use of fruit extract of Averrhoa bilimbi L. to synthesize ZnO NPs. Zinc acetate dihydrate was used as the precursor. The fruit extract was used as the reducing and capping agent. The calculated band gap energy from UV–Vis spectrum was 3.3 eV. XRD confirmed that the synthesized material was ZnO of hexagonal wurtzite phase. The authors performed Electrochemical Impedance and Linear Sweep Voltammetry studies on the ZnO NPs to investigate the photoelectrochemical properties of the nanomaterials. The results showed that the ZnO NPs have good photoelectrode characteristics for solar cell applications [23]. According to Aminuzzaman et al. preparation of ZnO NPs was efficient and economically viable by using Garcinia mangostana fruit pericarp extract. TEM confirmed that synthesized ZnO NPs were mostly spherical in shape with an average size of 21 nm. Based on UV–Vis spectrum, the calculated band gap energy of biosynthesized ZnO NPs was 3.32 eV which is lower compared to bulk ZnO (3.37 eV). The photocatalytic activity of the synthesized ZnO was investigated and the nanoparticles have shown an excellent photocatalytic performance by degrading 99% of Malachite Green (MG) dye on exposure to natural sunlight for 180 min. [41]. Terminalia chebula fruit extract was used to successfully to synthesize ZnO NPs as reported by Subhash et al. [71]. The XRD results showed that the synthesized nanoparticles have stable hexagonal wurtzite structure, and were roughly spherical in shape. The optical band gap of the synthesized ZnO NPs is estimated using the Tauc’s equation and the obtained value is 3.22 eV. The photocatalytic performance of the synthesized ZnO was assessed by degradation of Rhodamine B (RhB) dye. After 5 h irradiation of UV light, 1 g/L of synthesized ZnO degraded about 70, 60, 15 and 5% for initial dye concentration of 5, 10, 20 and 30 ppm, respectively. The rate constants for 5, 10, 20 and 30 ppm of RhB dye concentrations were measured to be 0.228, 0.183, 0.033 and 0.009 h−1, respectively. They reported that the decrease in the value of rate constant with the increase in RhB concentration may due to the reduction in the availability of 13 Bioprocess and Biosystems Engineering active reaction sites of the synthesized materials for the photocatalytic reaction [71]. Nethravathi et al. described Garcinia xanthochymus fruit extract for the preparation of ZnO NPs. The extract was found to contain significantly high amounts of polyphenols and flavonoids. XRD confirmed pure wurtzite structure of the synthesized ZnO NPs. The calculated band gap energy was 3.33 eV. SEM studies showed the formation of spongy cave like structures. The PL spectra revealed four emission edges at 397, 436, 556 and 651 nm upon excitation at 325 nm because of oxygen deficiencies and zinc interstitials. The synthesized ZnO NPs exhibited antioxidant activity by inhibiting the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radicals with IC50 value of 8000 µg/mL [42]. Anitha et al. have prepared different volume of citrate containing 10% Artocarpus gomezianus fruit extract (5–15 mL) for the synthesis of ZnO NPs. The synthesized ZnO NPs were in sizes ranging from 10 to 30 nm and spherical in shape. The optical band gap of the ZnO NPs was acquired to be 3.39 eV. The biosynthesized ZnO NPs demonstrated significant antibacterial activity against Staphylococcus aureus and antifungal activity against Aspergillus niger [20]. Other parts of the plant used for the synthesis of ZnO NPs Extracts of different plants part i.e. mucilage, latex, root, stem, etc. have also been used for the synthesis of ZnO NPs as they act as stabilizing, oxidizing and reducing agents. Prasad et al. synthesized ZnO NPs using Abelmoschus esculentus mucilage without employing additional solvents, catalysts, or templates. Mucilage is a thick and slimy hydrocolloid, which is a common constituent present in plants. The green synthesized ZnO NPs were able to completely degrade the target cationic dyes under irradiation of UV light. As reported, 125 mg of the catalyst was required to degrade methylene blue solution (32 mg/L) within 60 min and 100 mg was needed for the complete removal of rhodamine B (9.5 mg/L) within 50 min [11]. Based on the UV–Vis DRS spectrum, the calculated band gap energy of the synthesized ZnO was 3.10 eV while from Tauc’s plot, the estimated band gap energy was 3.12 eV. Mucilage from Quince seed also has been used as a stabilizing agent in the synthesis of ZnO [59]. FESEM image displayed the prepared ZnO with a fairly uniform distribution with some agglomerates and the average particle size was about 25 nm. Moreover, the calculated band gap energy of ZnO nanosheets was 3.28 eV. Photocatalytic degradation of MB dye using the synthesized ZnO was carried out in dark and in the presence of UV light. Under UV light irradiation, 50 mg ZnO could remove over 80% of MB (2.87 × 10–5 M) within 2 h. In a study reported by Azizi et al. ZnO NPs were successfully prepared by a simple and cost-effective method using palm pollen grain. 13 The palm pollen played important roles as a reaction media for the formation of ZnO and also as a stabilizer. FE-SEM analysis showed that the ZnO prepared were polydispersed with average particle size less than 20 nm. From the UV–Vis reflection spectrum, the calculated band gap energy of the synthesized ZnO NPs is 3.29 ± 0.05 eV [56]. ZnO NPs were biosynthesized by using Euphorbia jatropa latex as reducing agent. The size of biosynthesized ZnO is between 50 and 200 nm which is confirmed by TEM analysis. Rietveld refinement showed the structure has hexagonal crystal system with point group 6 mm. Based on their UV–visible spectra, the band gap energy was 3.4, 3.45 and 4.00 eV for Jatropa extract of 2, 4 and 6 mL, respectively. PL showed prominent peaks at 392, 520 and 651 nm, respectively [39]. Biological synthesis of ZnO NPs (spherical and agglomerated particles with size in the range of 20–30 nm) using Ruta graveolens (L.) stem extract was reported by Lingaraju et al. [64] The estimated band gap energies were found to be in the range 3.26–3.33 eV. The synthesized ZnO NPs exhibited significant bactericidal activity against gram negative bacterial strains such as Klebsiella aerogenes (K. aerogenes), Pseudomonas aeruginosa (P. aeruginosa), Escherichia coli (E. coli) and gram positive Staphylococcus aureus (S. aureus). Biosynthesis of ZnO NPs using rambutan (Nephelium lappaceum L.) peel extract was reported by Karnan et al. The formation of hexagonal structure was confirmed by XRD and an estimated bandgap energy of 3.32 eV was confirmed by UV–Vis DRS. FESEM and HR-TEM revealed the spherical shape of synthesized ZnO NPs with size in the range of 25–40 nm. They also reported that biosynthesized ZnO NPs possessed surface area of 69.01 m2/g showing great photocatalytic activity against MO dye. Their results showed that the biosynthesized ZnO can effectively degrade 83.99% within 120 min under irradiation of UV light [60]. Kumar et al. have compared the antibacterial activities of ZnO synthesized via chemical and green synthesis methods. They have successfully synthesized pure ZnO (chemically synthesized), R-ZnO (biosynthesized using Raphanus sativus root extract) and RC-ZnO NPs (prepared by simple bio-functionalization of pure ZnO using Raphanus sativus root extract) [62]. The antimicrobial activity of the three samples were carried out against Escherichia fergusonii (MDR) and Escherichia coli strains. Based on the results, they found out that RC-ZnO showed the superior antimicrobial properties amongst other prepared samples. Aqueous extract of Deverra tortuosa (aerial parts) was also used to synthesize ZnO NPs with size in range of 9.26–31.18 nm (based on TEM images). The potential anticancer activity of the synthesized ZnO was studied against two cancer cell lines (human colon adenocarcinoma (Caco-2) and human lung adenocarcinoma (A549). Both the plant extract and biosynthesized ZnO exhibited a selective cytotoxic effect Bioprocess and Biosystems Engineering towards the Caco-2 and A549 cancer cell lines. IC50 of A549 cells were 193.12 and 83.47 while, IC50 values of Caco-2 cells were 136.12 and 50.81 μg/mL by the extract and ZnO NPs, respectively. These results showed that biosynthesized ZnO had the most potent cytotoxic activity and was more sensitive towards Caco-2 than A549 [35]. The effect of phytogenic synthesized ZnO on its morphology and optical band gap energy In an experiment conducted by Saif et al., they were varying the concentration of Cordia myxa leaf extract to synthesize ZnO NPs (pure (0.1 mg/mL), 25% extract (prepared by mixing 25 mL pure extract and 75 mL water), and 10% extract (prepared by 10 mL pure extract + 90 mL water) [84]. The SEM images revealed hexagonal structures when pure extract was used. For 25% extract concentration, hexagonal shaped particles were obtained, which comprised of micro and nanosize triangular entities. While at 10% extract concentration, spherical shaped particles comprising of layerby-layer discs like entities were observed. Based on the extrapolation of the UV–Vis results, the band gap energy of ZnO for each concentration were calculated to be 3.36 eV, 3.5 eV, and 3.62 eV for pure, 25% and 10% leaves extract, respectively. Thus, it can be concluded that when higher concentration of Cordia myxa leaf extract was used in synthesizing ZnO, a smaller band gap energy will be obtained. In another study reported by Ali et al., the synthesis of ZnO NPs using Conyza canadensis leaf extract was conducted at two different temperatures i.e., 30 and 80 °C. The SEM images clarified that the ZnO NPs synthesized at 30 °C have aggregations, have no specified shape and present in the form of clusters. It is due to the fact that a very low temperature (30 °C) provided low energy which is insufficient to break the interactions between capping agents and nanoparticles which results in aggregations. While the biogenic ZnO NPs synthesized at 80 °C were spherical in shape with no aggregations and are smaller in size. Based on the UV–Vis spectra, the sample obtained at 30 °C has a much lower band gap energy (2.66 eV), while the sample obtained at 80 °C has high band gap energy of 3.08 eV [31]. Hence, it can be concluded that temperature used for the synthesis of ZnO NPs has an effect on both the morphology and optical properties of the synthesized material. Chandraprabha et al. described the use of Artocarpus heterophyllus ripened leaf to prepare the extract for the synthesis of ZnO NPs. The particles were calcined at different temperatures i.e. 400, 600 and 800 °C for 1 h. Powder XRD results showed the ZnO NPs calcined at different temperature are in hexagonal wurtzite phase. The TEM showed that ZnO NPs calcined at 400 °C to be in the range of 10–15 nm, at 600 °C the particles are in the range of 15–25 nm and biogenic ZnO NPs calcined at 800 °C have size of 25–30 nm, which are in good agreement with the SEM observation. The band gap energies were calculated from UV diffuse-reflectance spectra and found to be 3.42, 3.38 and 3.35 eV for 400, 600 and 800 °C respectively. FTIR spectra of leaf extract revealed the presence of phytochemicals such as amines, amides, quinines [22]. Comparison study between chemically synthesized ZnO (prepared by direct precipitation method using zinc nitrate and KOH as precursor) and biosynthesized ZnO using Cardiospermum halicacabum was carried out by Nithya et al. The SEM images demonstrated that the particles were hexagonal wurtzite structure with some surface agglomeration. The effect of chemical synthesis and biosynthesized ZnO on the optical band gap values was investigated. The calculated band gap energies were found to be 3.02, 3.05, 3.09 and 3.13 eV for the samples of chemically synthesized ZnO NPs, and biosynthesized using C. halicacabum leaf extract (0.2, 0.4 and 0.6 mL) ZnO NPs samples respectively. The increase in band gap energy from 3.02 to 3.13 eV may be due to the change of crystal structure of the synthesized samples [29]. The difference in band gap energy of chemically synthesized and green synthesized ZnO NPs was also discussed by Singh et al. The authors have successfully synthesized ZnO NPs using Aloe barbadensis Mill leaf extract. The average size of green synthesized ZnO NPs was 35 nm which was smaller than that prepared by conventional chemical methods (48 nm). Both ZnO NPs possessed spherical shape with high crystallinity. Based on the optical absorption spectra of both (green and chemical) ZnO NPs, the estimated band gap energy value for green synthesized ZnO NPs is 3.44 eV while chemically synthesized ZnO NPs exhibited band gap of 3.35 eV. They also reported that this may be due to the quantum confinement range, in which an increase in band gap energy shifts the absorption band towards a lower wavelength with decreasing particle size. Therefore, the higher band gap energy of the green synthesized ZnO NPs indicates that they are smaller than the chemically synthesized ZnO NPs. Their comparatively smaller size might result from the strong capping and reducing ability of the aloe vera leaf extract [16]. Comments and discussion on biosynthesis of ZnO nanoparticles using plant extracts Based on the reviewed findings, when different parameters were taken into considerations (i.e. the used of different types/volume of plant extracts used, different temperatures in synthesizing the materials, different calcination temperatures used), it results in the production of ZnO NPs with different chemical, physical, structural, morphological, electronic and optical properties have been compiled, discussed and compared. 13 Bioprocess and Biosystems Engineering Most of the researchers have highlighted the effect of plant extract on the particles size and morphology of green synthesized ZnO in their studies. For example, an ecofriendly synthesis of ZnO NPs utilizing Bauhinia tomentosa leaf extract as bio-reducing agent was reported by Sharmila et al. XRD pattern confirmed the heaxagonal wurtzite structure of the synthesized ZnO. TEM images revealed hexagonal shaped particles in the range of 22–94 nm [24]. Different morphology of ZnO was observed when it was synthesized using Carica papaya leaf extract as reported by Rathnasamy et al. From their FESEM images, the synthesized ZnO exhibited spherical shape with particles size of approximately 50 nm. It is used as photocatalyst for methylene blue dye degradation and as photo anode for dye sensitized solar cells [30]. Similarly, the green synthesized ZnO NPs using leaf extract of Tabernaemontana divaricate was reported for the first time by Raja et al. From their XRD analysis confirmed that the prepared ZnO exhibited a pure hexagonal wurtzite crystalline structure. The TEM images revealed that ZnO NPs showed spherical shape with size ranging from 20 to 50 nm. They also suggested that the ZnO NPs obtained were stabilized by the interactions of various phytochemicals such as steroids, terpenoids, flavonoids, phenyl propanoids, phenolic acids and enzymes in the leaf extract based on the FTIR analysis [70]. Theoretically, the band gap energy of ZnO is approximately 3.37 eV [85]. Only few researchers have reported the effect of plant extract on the optical properties of ZnO. For instance, Khan et al. have succesfully prepared narrow band gap of ZnO using Costus woodsonii leaf extract. Khan et al. [34] have synthesized the ZnO NPs under two different synthesis conditions i.e. BLE (boiled leaf extract) and ULE (unboiled leaf extract). TEM images revealed that ZnO synthesized under both conditions had rectangular and spherical shapes. The optical band gap energies of ZnO–C and assynthesized ZnO NPs (ZnO-UL1 and ZnO-BL1) were calculated to be 3.18 eV, ~ 2.68 eV, and ~ 2.77 eV, respectively. Fig. 4 Common method for producing SnO2 NPs using plant extract [89] 13 Thus, it can be concluded that the use of Costus woodsonii leaf extract reduces the optical band gaps. In another study, Rahman and co-workers have also successfully synthesized ZnO with narrow band gap energy of 3.30 eV using aqueous leaf extract of Ziziphus mauritiana (L.) [75]. They reported that the green synthesized ZnO exhibited good antioxidant and antibacterial activities. The same research group have also modified the green synthesized ZnO by doping with Mg [86], Cu [87], and Mg/Cu dual-doping [88] to further reduce its band gap energy. They observed that the optical band gap energy of ZnO has narrowed from 3.11 eV to 3.08 and 3.03 eV when doped with 1% Mg and 5% Mg, respectively. UV–visible diffuse reflectance spectra of Cu-doped ZnO also showed the reduction of band gap energies from 3.11 to 2.54 eV as the amount of Cu doping increases. Moreover, the band gap energy of Mg/Cu-doped ZnO was greatly reduced from 3.11 to 2.76 eV when Mg/Cu dopant was introduced. They reported the effect of light on the antioxidant and antibacterial activities of the synthesized materials due to their narrow band gap energy. The prepared materials were able to scavenge DPPH⋅ in the dark and showed slight enhancement under irradiation of visible light. These materials also showed good antibacterial activities and were found to be more effective against S. aureus than E. coli. Therefore, further study is needed to investigate the effect of plant extract (comparison using different extract concentration or by mass) to deduce its effect on the optical band gap of ZnO. Green synthesis of SnO2 nanoparticles Biosynthesis of SnO2 nanoparticles using plant extracts The common process for synthesizing SnO2 NPs is fairly straightforward as shown in Fig. 4 [89]. The primary step includes assembling the plants, separating the parts required Bioprocess and Biosystems Engineering for experiments, followed by cleaning and drying prior to use. The plants can then be ground, dispersed and heated in some cases in distilled water, and later, the solution mixture can be filtered off to remove fibre or suspension in order to obtain a clear plant extract. Different amounts of precursor will be added into plant extract, or vice versa to produce SnO2 and finally calcined at high temperature (600–800 °C) to form the SnO2 NPs. Many plants species have been used and reported to produce SnO2 NPs successfully (Table 2). The plant parts used are mostly peels, leaves, buds, flowers, barks, fruits and seeds. The diameter/sizes of the synthesized SnO2 NPs varied over a large range from 2 to 47 nm, and most of them formed spherical shape. The common applications reported for these NPs include antibacterial activities, antifungal activities, antioxidant activities, photocatalysis, and sensors. Figure 5 shows some reported plant species used for the preparation of SnO2 NPs. For instance, the biosynthesis of small and spherical shaped SnO2 NPs with average size of 17 nm were obtained using Aloe barbadensis Miller Table 2 Plants and plant parts used for the fabrication of SnO2 NPs Plants Plant part used Particle size (nm)/shape Aloe barbadensis Miller Annona squamosa Annona squamosa Linn. Aspalathus linearis Peels Peels Leaves Leaves Baringtoria acutagularis Brassica oleracae L. var. botrytis Cyphomandra betacea Calotropis gigantean Camellia sinensis Catunaregam spinosa Cicer arietinum L. Citrus aurantifolia Citrus sinensis Cleistanthus collinus Clerodendrum ineume Cyclea pelitata Daphne alpine Ficus benghalensis Ficus carica Litsea cubeba Leaves Leaves Fruits Leaves Leaves Root bark Seeds Peels Peels Leaves Leaves Leaves Leaves Leaves Leaves Fruits Parkia speciosa hassk Persia americana Phaseolus lunatus L. Piper betle Piper nigrum Plectranthus ambonicus Psidium guajava Punica granatum Stevia rebaudiana Stevia rebaudiana Saccharum officinarum Saccharum officinarum Pods Seeds Leaves Leaves Seeds Leaves Leaves Seeds Leaves Leaves Juice Juice Saraca indica Tamarindus indica Tradescantia spathacea Trogonelle foenumgraecum Flower buds Leaves Leaves Seeds 17/spherical 27.5/spherical –/irregular 2.1–19.3/spherical Use/application Antibacterial and antifungal activities Cytotoxicity effect against hepG2 – Congo red/methylene blue and eosin degradation –/irregular – 3.55–6.34/– Methylene blue degradation 15–20/spherical Methylene blue degradation 35/irregular Methyl orange degradation 20/spherical – 47/spherical Diazo dye degradation 6/spherical NO2 gas sensing 32/– – 4.5–5.5/quasi-spherical Methylene blue dye degradation 49.26/cubic Antibacterial and antioxidant activities 30/spherical Antitumor and anticancer 37/irregular – 19.5–27.2/elongated Adsorption of Cd2+ –/irregular – 128/spherical Hg2+ electrochemical sensor 30/irregular Congo red degradation and antioxidant activities ~ 1.9/spherical Acid yellow 23 degradation 4/fine flakes Phenol red degradation 18/porous Alizarin red S degradation 8.4/spherical RY 186 dye degradation 10–15/spherical Cytotoxicity studies 63/cluster Rhodamine B degradation 8–10/spherical RY 186 dye degradation 2–16.2/spherical and rectangular Antioxidant and antibacterial activities 20–30/spherical Antibacterial activities – Antibacterial activities 3/irregular – 9/spherical Methylene blue/RB/4-nitrophenol degradation and antimicrobial 2.1–4.1/spherical Antibacterial and anti-oxidant activities 29/irregular – 46–89 nm/spherical Photoantioxidant activities 2.2–3.2/spherical Antibacterial activities References [90] [91] [92] [93] [92] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [92] [104] [92] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [92] [120] [121] 13 Bioprocess and Biosystems Engineering Fig. 5 Common plants used in the fabrication of the SnO2 NPs aqueous extract as reported by Ayeshamariam et al. [122]. The authors used tin nitrate as the precursor and dissolving it into a mixture of distilled water and nitric acid with the presence of known amount of urea. Urea was used as a fuel to release maximum of energy during combustion. XRD analysis confirmed the formation of tetragonal rutile structure of SnO2 and sharper diffraction peaks were obtained compared to SnO2 NPs produced without Aloe vera extract. Furthermore, the authors stated that the peaks shifted between the two spectra of SnO2 NPs synthesized from Aloe vera extract and that without the extract. The particle size obtained from TEM was greater than 100 nm as opposed to the as-prepared SnO2 powder with average particle size of 17 nm. The difference was concluded to be due to the disturbances in the diffraction planes [90]. In another study, Diallo et al. carried out an experiment with Aspalathus linearis extract and produced spherical shape of SnO2 NPs with an average particle size ranging from 2 to19 nm. SnCl4 precursor was used and the crystallinity of SnO2 NPs produced was reported to increase with increasing calcination temperatures at 400 °C, 700 °C and 900 °C. The broad XRD peaks mirrors their nanosized scale. The TEM image showed that there is no direct contact between the NPs, suggesting some of the Aspalathus linearis compounds act as capping agent and prevents the agglomeration of the NPs. The XPS study was done on annealed powder at 400 °C. It precisely reports the characteristics of 13 Sn 3d and the O 1s peak and its corresponding binding energies. The Sn 3d3/2 and Sn 3d5/2 are centered at about ~ 487.1 and ~ 495.5 eV while O 1s is centered at about 31.5 eV. The distance between the 3d3/2 and 3d5/2 is in very good agreement with the energy splitting reported for nano-scaled SnO2. However, it should be pointed out that the binding energies of O 1s for SnO and SnO2 are very close. Nevertheless, the XRD studied can be used to confirm that the dominant Sn valence is Sn4+ as SnO and SnO2 have different lattice constants; for SnO, (a) ~ 3.850 Å, (c) ~ 4.901 Å while for SnO2; (a) ~ 4.738 Å, (c) ~ 3.188 Å respectively. The latter set of values are in conformity with the experimental values, it can be concluded that the electronic status of Sn is exclusively valence 4+ [93]. Recently Osuntokun et al. reported biosynthesis of SnO2 using Brassica oleracea L. var. botrytis extract. The SnO2 was produced by mixing boiled extract with SnCl2 which was stirred and heated at 60 °C for 6 h. The precipitate was centrifuged and washed followed by drying at 75 °C for 8 h. The dried product was annealed at two different temperatures i.e. 300 and 450 °C. The authors successfully produced small size spherical particles ranging from 3 to 6 nm. Pronounced and narrower XRD peaks were observed for SnO2 NPs annealed at 450 °C than at 300 °C indicating that the crystallinity was associated with increased temperature. However, the authors stated that the size increased with the increase in annealing temperature as observed from TEM Bioprocess and Biosystems Engineering analysis. The band gap was estimated from extrapolating the absorption spectra. The band gap decreased with increased in annealing temperature which are 4.3 eV decreased to 3.90 eV [94]. Elango et al. reported the use of Cyphomandra betacea methanolic plant extract to prepare SnO2 NPs [95]. The particle size of the NPs was found to vary from 20 to 50 nm with spherical morphology as confirmed by TEM analysis. The SEM image illustrated the SnO2 NPs as cluster of particles with sharp edges. In another study, Bhosale et al. reported the synthesis of SnO2 NPs using aqueous leaf extract of Calotropis gigantean. SnCl2 was mixed in the leaf extract and heated at 60 °C for 6 h. The precipitate was obtained by centrifugation and annealed at 300 °C. SnO2 NPs with an average crystallite size of 35 nm was obtained using Debye–Scherer equation. TEM image showed irregularly agglomerated particles. The authors explained that the agglomeration was mainly due to the formation of small size NPs. Nevertheless, the XRD analysis confirmed the formation of rutile structure [96]. Selvakumari et al. reported the presence of –OH groups from flavanol derivatives in Camellia sinensis act as stabilizing and capping agents in the synthesis of SnO2 NPs. This work produced spherical shaped NPs confirmed by TEM image with particle size ranging between 5 and 30 nm. Broad XRD peaks were also observed indicating formation of small particle size. The SnO2 NPs synthesized in this case was found to possess tetragonal rutile structure [97]. In another study, the root barks of Catunaregam spinosa were used for the green synthesis of SnO2 as reported by Haritha et al. [98]. The authors studied the components of Catunaregam spinosa using GC–MS and found that the presence of organic molecule of 7-hydroxy-6-methoxy2H-1benzopyran-2-one played the role as capping agent for the conversion of stannous salt into SnO2 NPs. It was also reported that SnCl2 precursor was mixed in C. spinosa aqueous extract, water bathed for 2 h at 60 °C. The solution was centrifuged at 4000 rpm for 10 min and the precipitate was annealed at 450 °C. Authors have synthesized spherical structure of SnO2 NPs with an average particle size of 47 nm as observed from SEM. The nature of the synthesized SnO2 was confirmed to be tetragonal rutile structure and crystalline in nature according to XRD patterns. The crystallite size was estimated to be 46 nm. Cicer arietinum L. extract were also used to fabricate SnO2 thin films as reported by Gattu et al. [99] The XRD confirmed the formation of tetragonal rutile structure and the average crystallite size of SnO2 NPs was found to be 11 nm. SEM showed spherical shaped NPs with narrow size distribution which was further confirmed using TEM. The FTIR spectra of the extract was claimed to resemble the IR spectra of pectin present in the extract. An obvious absorption peak at about 514 cm−1 was found which is a typical IR absorption peak originates from stretching mode of the Sn–OH [99]. On the other hand, SnO2 thin films were synthesized by aqueous peel extract of Citrus aurantifolia. According to Senthikumar et al. spin-coating system was utilized in this procedure to make SnO 2 thin films and annealed at different temperatures i.e. 100 and 200 °C for 2 h. The crystallite size of the materials calculated from XRD was 32 nm for SnO 2 NPs annealed at 200 °C and decreased to 26 nm as the annealed temperature decreased. Nevertheless, the XRD confirmed the formation of tetragonal structure of SnO 2 NPs [100]. SnO 2 NPs have also been prepared using Citrus sinensis peel extract [101]. A high purity of ultra-small quasi spherical SnO2 NPs with an average particle size between 4.5 and 5.5 nm were obtained. The band gap was shown to be 3.49 eV with efficient MB photocatalytic degradation activity reaching 94.4% contaminant removal within 120 min. Kamaraj et al. reported the Cleistanthus collinus methanolic plant extract mediated fabrication of SnO2 NPs. The mixture of dried leaf powder in ethanol was heated at 60 °C for 2 h. It was filtered and the extract was mixed with tin precursor solution. It was treated with heat at 80 °C for 1 h. The change in color from greenish color to pale yellow indicates the formation of tin oxide NPs. The formation of tetragonal rutile structure of SnO2 NPs was derived from XRD. According to Debye–Scherrer’s equation the average crystallite size of the synthesized SnO2 NPs was about 49.26 nm. The SEM image presented hexagonal shaped SnO2 NPs with average particles size in the range of 20–40 nm. The EDX study also confirmed the formation of SnO2 [102]. A recent study on the synthesis of SnO2 NPs using Clerodendrum inerme leaf extract was conducted by Khan et al. The leaf extract was obtained using Soxhlet extraction and the extract was made concentrated by rotary flash evaporator under vacuum. The SnO2 NPs synthesis using the extract was prepared through green solution combustion method. The XRD showed sharp intense peaks which proved the NPs possess crystalline nature. The average crystallite size was found to be 30 nm through Debye–Scherrer equation. According to SEM, the synthesized SnO2 NPs were spherical in shape with particle size of below 50 nm which is consistent with XRD, TEM and DLS results [103]. Haq et al. has reported a plant mediated SnO2 NPs synthesized using Daphne alpine leaf extract. The SEM image clearly showed uniform distributed SnO2 NPs with slightly elongated in shape. The particle size was calculated from SEM varies from 19.5 to 27.2 nm. On the other hand, the XRD patterns showed SnO2 NPs with tetragonal and orthorhombic geometries [104]. Ficus carica acts as stabilizing agent for production of SnO2 NPs as reported by Hu et al. An average size of the NPs was found to be 128 nm according to the SEM. This could be due to agglomerated spherical shape seen in the SEM. Meanwhile, the XRD 13 Bioprocess and Biosystems Engineering exhibited a series of well-defined diffraction peaks which showed tetragonal geometry [105]. Hong et al. described the use of Litsea cubeba fruit extract to synthesize SnO2 NPs. An average particle size of 30 nm was found, however the shape was observed to be irregular according to the SEM image [106]. Begum et al. produced SnO2 quantum dots approximately 1.9 nm by using Parkia speciosa Hassk pods extract. The TEM image revealed spherical morphology of the particles and consistent particle size [107]. Persia americana seed extract assisted synthesis of SnO2 NPs exhibits fine flakes with less agglomerates morphology with size of approximately 4 nm as reported by Roopan et al. Methanol extract of Persia americana was mixed with 1 mM of stannous chloride solution and heated at constant temperature of 60 °C for 12 h [123]. On the other hand, NPs with approximately 8 nm size was observed by Singh et al. using Piper betle leaves extract. The extract and SnCl2 solutions were mixed and stirred at 60 °C for 4 h. The SEM image showed spherical morphology and uniform size distribution [110]. In another study, Fu et al. selected Plectranthus amboinicus leaves for the biosynthesis of SnO2 NPs. When P. amboinicus leaf extract was reacted with the aqueous SnCl2 and heated to 200 °C for 3 h, they produced sediments which later were centrifuged, washed and dried. The synthesized SnO2 NPs had a cluster morphology due to high agglomeration with an average size of 64 nm [112]. In another report by Kumar et al. Psidium guajava was effective in producing smaller size of SnO2 NPs as the authors reported production of spherical-shaped of 8–10 nm NPs [113]. Kumari et al. reported the use of Punica granatum seeds extract and SnCl4 solution as the precursor for the production of SnO2 NPs. The synthesized NPs were reported to form less agglomerative-spherical and rectangular shaped particles with increase in annealing temperature. The XRD spectrum was indexed to the tetragonal rutile crystalline phase. The diffraction peaks get narrower when annealed at 300, 600 and 900 °C. The size of the NPs were in the range of 4–42.2 nm as illustrated in the SEM image [114]. Ethanolic extract of Stevia rebaudiana was mixed with SnO2 solution and heated at 80 °C for 2 h forming pale yellow solution indicated the formation of SnO2 NPs as reported by Merlin et al. SEM image proved the formation of well-dispersed spherical NPs which ranges from 20 to 30 nm [124]. In another report by Nathan et al. the use of S. rebaudiana leaves extract was also reported. The synthesis of SnO2 NPs was observed as pale yellow solution was seen after 2 h stirring and heating at 80 °C. The formation of SnO2 was confirmed by UV–Vis–NIR analysis which showed maximum absorption around 334 nm [116]. Begum et al. reported the biosynthesis of SnO 2 NPs using Saccharum officinarum. The Saccharum officinarum 13 juice was collected and used in the synthesis. It was reported that spherical shaped NPs with average size of 9 nm size were successfully produced. In another paper by Begum et al. where they also reported using the same plant species, S. officinarum juice. It was observed that shape of the particles was non-uniformly distributed. However, according to the histogram of particle size distribution evaluated from TEM image, the average size of the NPs was found to be 3 nm [117, 118]. Vidhu et al. synthesized SnO2 NPs using Saraca indica flower extract which resulted in smaller size NPs ranging from 2.1 to 4.1 nm. The formation of SnO2 NPs was confirmed using XRD as the pattern was indexed to tetragonal structure crystalline phase [119]. Sudhaparimala et al. reported the synthesis of SnO 2 NPs using five potent plants namely Tamrindus indica, Ficus bengalhensis, Baringtoria acutagularis, Annona squamosa Linn. and Cyclea peltata [125]. The purpose of their study is for drug formulation in the treatment of infection diseases. The authors stated that they used trituration method (ground with the plant extract intensively in pestle and mortar) to produce SnO2 NPs. The tin precursor was triturated with the leaf extract until a homogenous paste was formed and the sample was calcined at 400 °C in the first step. Subsequently, the product was subjected to trituration with the second plant extract followed by calcinations [92]. Spherical SnO2 NPs have been prepared by Matussin et al. using aqueous leaf extract of Tradescantia spathacea [120]. The SnO2 NPs was prepared via two different methods, namely; centrifuge and paste method. The concentration of precursor solution was also varied in the synthesis as well as the amount of aqueous leaf extract. These variations were found to influence the structural and morphological properties of SnO2 NPs. According to the report, the presence of flavonoid and phenolic in Tradescantia spathacea were reported to have antioxidant activities, therefore, antioxidant activity was also carried out using conventional DPPH assay method. However, due to the narrow band gap of the synthesized SnO2 (2.51–3.3 eV), photoantioxidant activity of the synthesized SnO2 NPs was also studied. Antioxidant activities of the synthesized SnO2 NPs were found enhanced under irradiation of visible light. Matussin et al. also reported Ni-doped SnO2, Co-doped SnO2 as well as Co, Ni co-doped SnO2 NPs using the same plant species via paste method [126–128]. Synthesis of SnO2 NPs of size in the range 2.2–3.2 nm using Trigonella foenum-graecum seeds extract was reported by Vidhu et al. The extract was obtained by boiling the seeds at 100 °C for 2 min. It was then mixed with SnCl4 solution to form light brown color precipitate which indicates the formation of SnO2 NPs. The morphology of the synthesized NPs were observed to be spherical as shown in the TEM images [121]. Bioprocess and Biosystems Engineering Comments and discussion on biosynthesis of SnO2 nanoparticles using plant extracts The choice of solvent for plant (leaf, flower, seed etc.) extraction of any phytochemicals depends on its solubility in that solvent. Solvents used to extract plants were mostly water, methanol, ethanol and/or its mixture. Generally, polar solvents are more effective in the extraction of bioactive components [9]. However, water presented to have similar effectiveness as ethanol except there is no trace of alkaloids was presence in the water extract as discussed by Gayathri et al. [129]. Nevertheless, as water is a non-toxic renewable resource, it will help to reduce the usage of toxic organic and reduce the amount of toxic waste which can be detrimental to our health. Both water and alcohol as solvents showed small particles sizes ranging from 1.9 to 49.26 nm. Nonetheless, water extraction has successfully synthesized more spherical shaped SnO2 NPs compared to methanolic or ethanolic extraction methods. Investigators mostly emphasized the effect of plant extract on the particles size of SnO2 obtained in their studies. They successfully produced SnO2 with average particle sizes in the range of 3–70 nm according to the SEM images and supported by other study such as XRD and TEM. The extract thus have effects on their surface which acts as capping and stabilizing agents. For instance, the TEM images of SnO2 synthesized using Aspalathus linearis suggesting that some of the Aspalathus linearis compounds act likely as capping agents and prevents the agglomeration of the NPs as the extract contains bioactive compounds such as nothofagin and aspalathin which are believed to be potential chemical chelating agents. The extracts also suppress the growth of the nucleated particles as illustrated by Saccharum officinarum extract. Most studies showed the existence of C–N and N–H stretching vibrations of amines and aldehydes which came from the plant extracts. These were confirmed by FTIR analysis of SnO2 NPs. As the constituents of the extracts may coated on the surface of the SnO2 NPs, the shape of the NPs were greatly influenced. Leaf extracts mediated synthesis showed regular spherical shape of SnO2 NPs whereas, stem, flower and seed extracts mediated showed highly agglomerated irregular shape of SnO2. This could be due to different phytochemical presence in different parts of the plants. As the authors used different plant extracts concentrations, the amounts of phytochemical presence were varied hence, affecting the morphology of the synthesized SnO2 NPs. Few researchers have reported the band gap energies of the plant extract assisted synthesis of SnO2 and showed obvious effects on the band gap energy using plant extract mediated synthesis. Normally, the use of plant extract in the synthesis of SnO2 was to decrease the band gap energy as reported in some studies. However, the band gap energy were mostly higher than the theoretical band gap energy value of 3.6 eV which are in the range of 3.1–4.5 eV. Therefore, the plant extract alone cannot be presumed to influence the narrowing of band gap energy. Green synthesis of CeO2 nanoparticles Biosynthesis of CeO2 nanoparticles using plant extracts A typical biosynthesis method of CeO2 NPs using plant extract involves heating or boiling the leaves in a solvent, most commonly water. Different ratios of Ce precursors added to the extracts were, heated with stirring until precipitation occurred. Gel like precipitation obtained were then calcined using varying temperatures but most commonly at 400–500 °C. After calcination, solid CeO2 NPs obtained were ground into fine powder. There are other methods used such as drying and grinding to achieve powdered plant, [130] using different types of solvents for extraction, [130–132] and altering the experimental conditions during the reaction of the precursor and extracts [5, 133, 134]. Figure 6 shows the common method for synthesizing CeO2 NPs using plant extract. Chen et al. reported the use of powdered extract obtained from ground dry leaves for the extraction process [135]. Studies on the effects of extraction from dry samples with water reported higher yield of phytochemicals than wet samples due to breakage and destruction of cell walls and the formation of large cavities and intercellular spaces allowing solvents to penetrate the cells and extract the phytochemicals more easily [136–138]. However, one of the advantages of this technique is the time spent drying the samples which can take several days without using an oven. While there are some reported studies that used other solvents used in extractions such as ethanol and methanol, water as a solvent is safer from a medical and ecological standpoint. It is also a renewable resource that is abundant in nature thus cost effective. Generally, biogenic synthesized metal oxides uses green extracts using leaves. However, an article by Patil et al. reported on the utilization of Indian red pomelo (Citrus maxima) fruit peels. These peels are agro-food waste, usually thrown away once the fruit was taken. Pectin was extracted from these peels for the biosynthesis of CeO2 NPs [139]. Furthermore, some articles report on biosynthesized CeO2 NPs using different methods such as hydrothermal method using an autoclave or adding an additional step of using centrifuge to form a CeO2 pellet using the precipitate before calcining [140, 141]. However, this method is not considered as a cost effective method. There have been reports whereby the material used was not extracted, instead 13 Bioprocess and Biosystems Engineering Fig. 6 Common method used for CeO2 NPs fabrication they were used directly with Ce precursor. For instance, Zamani et al. reported the addition of walnut shells into a Ce precursor solution and the resulting solid obtained after rotary evaporation of the mixture, was calcined to give CeO2 NPs [142]. Similarly, another study by Maensiri et al. reported the use of diluted egg whites to react with Ce precursor solution [143]. Biosynthesis of CeO2 NPs using food‑based products Other than plant extracts, food-based products have also been used as extracts to synthesize CeO2 (Table 3). Ferreira et al. used Cassava starch to synthesize CeO2 NPs and it was reported that the average crystallite size of the CeO2 NPs increases with increase in the calcination temperature: 200–500 °C. Similar observation was reported for Sangsedifi’s sugar mediated CeO2 NPs. At higher temperatures, the crystallization rate increases due to the large movement of atoms, which accelerates the arrangement of the crystalline structure and consequent aggregation of the crystallites to minimize the interfacial surface energy. Valence band spectra was carried out by XPS. Chemical composition in samples showed no elements other than C, Ce and O were present in the samples, confirming the high chemical purity of CeO2 NPs [144–146]. Results of FT–IR spectra confirmed the existence of electrostatic interactions between fructose and glucose which act as capping agent to the CeO2 nanostructures. The stretching bands for C–O–H shifted from 1154 cm−1 to 1114.05 cm−1 for fructose and a broad band was observed at 883 cm−1, indicating the presence of Ce–O–C mode for glucose [145, 146]. Arasu et al. have used an interesting combination of seashell, Aloe vera, grapes and pomegranates for the synthesis of CeO2 NPs. In brief, cerium precursor was mixed with Aloe vera leaf extract and seashell powder. Grape and pomegranate solution were then added to the mixture and the mixture was heated in an oven and later was calcined. Optical studies were performed using UV–Vis DRS. Band gap energy of the particles was estimated by applying Kubelka–Munk function and its value was calculated to be 3.78 eV which complement the UV–Vis spectrograph’s Table 3 Food-based extracts for the green synthesis of CeO2 NPs, solvent used and its characteristics Extract source Solvent used Characteristics Morphology References Cassava starch Water Spherical [144] Citrus maxima peels Water, ethanol Spherical [139] Gelatin, glucose, lactose and fructose – Spherical [145, 146] Watermelon juice Water Spherical [148] Seashell, Aloe vera, grapes and pomegranate Water Spherical [147] Walnut Water Larger particle size at higher calcination temperature (8.1 nm–12.7 nm) Particle size ≤ 40 nm Band gap energy 3.59 eV Smaller particle size at higher temperature and longer time during hydrothermal Crystallite size 36 nm Band gap energy 5.57 eV Crystallite size 42 nm Particle size 20–60 nm Crystallite size 12–18 nm Spherical [142] 13 Bioprocess and Biosystems Engineering band gap value of 3.85 eV. Brunauer–Emmett–Teller (BET) measurements were performed to examine the specific surface area of the CeO2/A. vera/ Seashell sample. The specific surface area and average pore size of CeO2/A. vera/ Seashell were found to be 10 m2/g and 1.2 nm, respectively. Strong IR peaks were observed at 1573, 1408, 1053 and below 669 cm−1. The intense IR bands at 1573 cm−1 related to the δ (OH) mode of (H-bonded) water molecules. Peaks observed at 1573, 1408, 1053, 669 and 558 cm−1 are due the C–O, C–C, C=C and C–H stretching bands of the organic compounds present in Aloe vera extract [147]. Effect of extract to precursor ratio, reaction time, reaction temperature and calcination temperature to the particle size and shape of CeO2 NPs Synthesized CeO2 NPs typically possess cubic fluorite structures with XRD patterns correspond to (111), (200), (220), (311), (222), (400), (331) and (420) planes. There has been an observation on the effects on particle size when the extract to precursor ratios were varied. According to a comparative study by Elahi et al., the effects of increasing amount of extracts (Salvia macrosiphon Boiss seeds extract) showed an inverse relationship with particle size [133]. Elahi’s varied the extract to precursor ratio by 0.5:1, 1:1 and 1.5:1. FE-SEM results reported showed synthesized CeO2 particle sizes between 20 and 47 nm whereby the sizes decreases with increase amount of extract. The particle size of CeO2 NPs produced from Salvia macrosiphon boiss leaf extract [13] is comparable with CeO2 NPs biosynthesized with Gloriosa superba leaf extract (5 nm), [45] Hibiscus sabdariffa leaf extract (3.9 nm), [47] Jatropha curcas leaf extract (2–5 nm), [50] Picrasma quassioides stem bark extract (10–80 nm), [69] Moringa oleifera plant extract (45 nm) [72] and Cymbopogon citratus plant extract (10–30 nm) [73]. There are other factors that can also tune the size of nanostructures. Sangsefidi et al. conducted investigation on the effects of the synthesis by changing several parameters such as reaction time, reaction temperature, calcination temperature and different capping agents [145]. In their reported protocol, CeO2 was synthesized with varying capping agents of sugars (lactose, fructose, glucose) and the synthesis were microwave assisted. Reaction time (5, 10 and 15 min) with microwave was investigated and through SEM, particle sizes recorded ranges from 10 to 40 nm. It was observed that longer reaction time resulted in the formation of densely agglomerated NPs with a larger particle size. It was explained that during the production of CeO2 NPs, the supersaturation of the system rapidly increased and the product gradually began nucleation and growth. By prolonging the reaction time, the thermodynamic effect leads to nonuniformity at the surface of samples [145]. In a different study, Sangsefidi et al. synthesized CeO2 with lactose, fructose, glucose by using hydrothermal method [146]. The effect of different reaction temperatures (100, 140 and 180 °C) was investigated. Glucose and fructose mediated CeO2 resulted in smaller and dispersed NPs. Unlike previous study by them using a microwave, CeO2 NPs synthesized using the hydrothermal method yielded smaller particles. This could be due to higher temperature that could prepare enough energy to separate particles and create very tiny NPs. Different calcination temperatures do affect the size of NPs formed. This was addressed by several studies. Maensiri et al. reported CeO2 NPs prepared by adding Ce precursor into lemongrass extract solution with vigorous stirring at 80 °C until viscous organic gel was formed and then calcined with three different calcination temperatures (400, 500 and 600 °C) [134]. The trend observed was with increase in calcination temperature, the size of NPs increases from 10 to 30 nm. Similar results were obtained by Ferreira et al. on the study of the synthesis of Cassava-starch assisted CeO2 NPs [144]. Ferreira’s synthesized CeO2 was subjected to calcination temperatures from 200 to 500 °C and the nanoparticle sizes produced increase from 8.1 to 12.7 nm with respect to increasing calcination temperature. Moreover, Elahi et al. explored the effects of calcination temperature using CeO2 synthesized with Cydonia oblonga miller seeds extract. Similarly, calcination temperatures (400, 500 and 600 °C) were used during CeO2 synthesis. FESEM showed that smallest particle size was obtained when calcined using 400 °C. Higher temperature resulted to the particle agglomeration and overall, bigger particle size was achieved [153]. Observation is further supported by Sangsefidi et al. [146]. The effect of capping agent on the morphology of CeO2 NPs in the presence of lactose, fructose, glucose was researched by Sangsefidi et al. [34]. It was observed that lactose molecule being larger than fructose and glucose in terms of molecular mass, decreases aggregation. As a capping agent, fructose has the smaller particle size hence gives the best morphology compared to the other two sugars [145]. While the most commonly observed shape of CeO 2 NPs is spherical, there have been different morphologies reported. Picrasma quassioides extract assisted CeO2 NPs synthesis showed a mixture of spherical, trigonal, tetragonal and octahedral shapes and the aggregation of particles were decreased with the increase of calcination temperature [151]. Nevertheless, spherical CeO2 NPs were obtained using Pometia pinnata aqueous leaf extract as reported by Naidi et al. [154]. The synthesis methodology can be briefly explained as follows: the leaves of Pometia pinnata were crushed and blended before the leaves were mixed in water and stirred at room temperature. The extract was filtered off and the filtrate was mixed with cerium precursor solution. 13 Bioprocess and Biosystems Engineering The reaction temperature was set at 80 °C until a paste was formed. The paste was calcined at 500 °C for 2 h. XRD results confirmed the formation of fluorite phase CeO2 with average crystallite sizes between 6 and 19 nm. The average particle size from TEM images were found to be 15–20 nm. This might be due to low temperature was used in the reaction synthesis. The synthesized CeO2 NPs were tested for photoantioxidant activity under visible light as well as antibiofilm activities. The synthesized CeO2 NPs showed improved photoantioxidant activity. Moreover, the synthesized nanomaterials also exhibited bactericidal activity against Staphylococcus aureus. The fabrication of Sndoped CeO2 and Zr/Sn-dual doped CeO2 NPs were reported using the same plant extract [154, 155]. The biosynthesized CeO2 NPs using various plants and plants parts were shown in Table 4. Comments and discussion on biosynthesis of CeO2 nanoparticles using plant extracts In short, there are several factors that are influencing the structural and morphological properties of CeO2. Most of them have already been discussed earlier (Sects. 2.7 and 3.2). Nevertheless, the most common factors that affecting the properties of CeO2 as reported are the calcination temperature, the concentration of leaf extract, the involvement of capping agents found in the extract as well as the reaction temperature. Calcination temperature were found to have Table 4 Summary of work on plant extract assisted synthesis of CeO2 NPs Plants used Parts used Solvent used Particle size for extracts Shape References Acalypha indica Aloe vera Aloe vera Azadirachta indica Leaves Leaves Leaves Leaves Water Water Water Water Spherical Spherical Spherical Spherical 156 157 158 159 Camellia sinens Ceratonia siliqua Cydonia oblonga Leaves Leaves Seeds Water Water Water Almost hexagonal Spherical 135 160 153 Euphorbia hirta Gloriosa superba Leaves Leaves Ethanol Water Monoclinic Spherical 130 149 Hibiscus sabdariffa Jatropha curcas Leaves Leaves Water Water Spherical Spherical 150 140 Lemon grass Moringa oleifera Olea europaea Origanum majorana Picrasma quassioides Leaves Peel Leaves Leaves Bark Water Methanol Water Water Water 134 132 161, 162 163 151 Pisonia alba Leaves Water Spherical Spherical Spherical Spherical Spherical, trigonal, tetragonal, octahe- drons Spherical Prosopis juliflora Leaves Water Spherical 165 Pometia pinnata Leaves Water Spherical 154 Rape pollen Pollen Rubia cordifolia L. Leaves Salvia macrosiphon boiss Leaves Sida acuta Sida acuta Stevia rebaudiana 13 Leaves Leaves Leaves Ethanol Water Water Water Water Water Particle size 8–54 nm Particle size 2–3 nm Particle size < 5 nm Particle size 10–15 nm Band gap energy 2.57 eV – Particle size 22 nm With increased calcination temperature, band gap increased (2.4–3.1 eV) – Particle size 5 nm Band gap energy 3.78 eV Particle size 3.9 nm Particle size 2–5 nm Band gap energy 3.44–3.64 eV Particle size 8.94 nm Particle size 45 nm Particle size 24 nm Particle size ~ 20 nm Grain size 10–80 nm Particle size 10 nm Band gap energy 2.97 eV Particle size 15 nm Band gap energy 3.62 eV Particle size 15–20 nm Band gap energy 2.66 eV Particle size 10–15 µm Particle size 22–26 nm With increased extract volume, particle size decreased, band gap increased (2.5–3.5 eV) Particle size 3.61–24.40 nm Particle size 8.2 nm Particle size 8–10 nm 164 Hollow microsphere 166 Small spherical and hexagonal 167 Spherical 133 Spherical Spherical Spherical 168 169 170 Bioprocess and Biosystems Engineering effects on the overall particle size distribution of synthesized CeO2. For example, calcination temperatures (400, 500 and 600 °C) were used during CeO2 synthesis and it was shown that smallest particle size was obtained when calcined using 400 °C. Higher synthesis temperature often results to the particle agglomeration and overall, bigger particle size. Nonetheless, high concentration of leaf extract is found to produce smaller particle size. This supports that the contents of the extract act as capping agents in which they suppress the growth of the particles. Longer synthesis reaction time also often results in the formation of densely agglomerated NPs with a larger particle size. It might be due to the supersaturation of the system that is rapidly increased and the CeO2 gradually begins to nucleate and grow. By prolonging the reaction time, the thermodynamic effect leads to nonuniformity at the surface of samples. Future outlook Plant extract has been proven to possess high efficiency as stabilizing and reducing agents for the synthesis of controlled ZnO, SnO2 and CeO2 (i.e., controlled shapes, sizes, structures, and other specific features). However, there are a few notable research gaps that should be taken into account: 1. To identify active phytochemicals present on the surface of the green-synthesized ZnO, SnO2 and CeO2 for better understanding of their formation mechanism. 2. To study the effects of green synthesized ZnO, SnO2 and CeO2 in comparison to the chemically synthesized ZnO, SnO2 and CeO2 in various applications. 3. To study the difference between ZnO, SnO2 and CeO2 synthesized using boiled and unboiled plant extracts. 4. To study and compare ZnO, SnO2 and CeO2 synthesized using extracts from different parts of the same plant type. 5. More in-depth studies on the effects of temperature, reaction time, pH, and other synthesis conditions of ZnO, SnO2 and CeO2 would make a significant contribution to the research. Conclusion The growing interest on the fabrication of green and phytogenic fabrication of NPs is due to their ability to provide a simple, inexpensive and eco-friendly method. The green and phytogenic fabrication of ZnO, SnO2, and CeO2 NPs using plant extracts is a clean, low-cost, environmentally friendly, non-toxic, reliable, and safe method. Furthermore, their potentials in controlling morphology and particle size of the NPs to ultra-nanoscale level compared to the conventional methods is driven by the presence of natural reducing, oxidizing, capping and stabilizing agents in plant extracts. Apart from that, size, morphology and optical band gap energy of the selective metal oxides in particular ZnO, SnO2, and CeO2 NPs can be easily controlled by the manipulation of several parameters such as concentration of precursors, reducing agents, temperature, solvents and type of capping agents. Additionally, the role of solvents has been found important and discussed. Since this also affects the effectiveness in the extraction of composition of the phytochemicals which ultimately affects the synthesized ZnO, SnO2 and CeO2 NPs. Hence, it can be concluded that there is no universal extraction method that is ideal and extraction procedures is distinctive to the type of plants, plant parts and solvent used. Acknowledgements The authors would like to acknowledge the FIC block grant UBD/RSCH/1.4/FICBF(b)/2021/035 received from Universiti Brunei Darussalam, Brunei Darussalam. Declarations Conflict of interest The authors declare that there is no conflict of interest. References 1. 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