Introduction

Silica nanoparticles (SiNPs) and porous SiNPs have shown wide range of applications in various technological fields ranging from nanocomposites and ceramics to diagnosis tools and drug delivery [1]. Stöber et al. synthesized silica nanoparticles in 1962 by using tetraethyl orthosilicate (TEOS) as the silica precursor, ethyl alcohol and water as solvents and ammonia as an alkaline catalyst [2]. The scheme of synthesis of SiNPs by Stöber’s method using TEOS as silica source is shown, in Fig. 1. Then onwards, there are several reports in the literature about the synthesis of SiNPs. The synthesis was done by modifying the reaction conditions, varying the base or catalyst and by using different precursors [3, 4]. The nanoparticles can be synthesized by “top-down” and “bottom-up” approaches [5]. The bottom-up methods use various hazardous chemicals and expensive processes that can cause potential environmental and biological hazards [6]. The SiNPs can also be synthesized by different physical and chemical methods, such as sol–gel synthesis [7], chemical vapour condensation [8], flame synthesis [9], laser ablation [10], reverse microemulsion synthesis, etc. [11].

Fig. 1
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Basic scheme of synthesis of silica nanoparticles

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However, these methods have some disadvantages. For instance, sol–gel and hydrothermal synthesis methods need costly raw materials and they also need very high-temperature furnace or heating devices [12]. The chemical vapour condensation method needs high temperatures, it has slower growth rates and it uses toxic reagents like Ni(CO)4, B2H6, SiCl4, etc. Some of these reagents are explosive and are also corrosive in nature [13]. The main disadvantage of flame synthesis is control over the particle size, morphology and phase composition [14]. The laser ablation method has a limitation in wavelength of laser impinging metallic target, duration of laser pulses, laser fluency, duration of ablation and an effective liquid medium with or without the presence of surfactants [15]. Reverse microemulsion methods are costly and the removal of surfactants from final products is a difficult task [5].

The green methods for synthesis of SiNPs offer several advantages and they can also overcome some of the drawbacks of other synthesis techniques mentioned above [16]. Importantly, the SiNPs synthesized by green methods are of similar quality as prepared by the chemical methods. The SiNPs produced by green methods can be used in medicine, pharmaceuticals, electrical, paints, cosmetics and in many other technical applications. In green synthesis methods, eco-friendly reagents replace the harmful chemical regents. Hence, the particles are less toxic and reasonably pure than those prepared by the chemical techniques. In the case of green synthesis of SiNPs, natural precursors of silica and sustainable bases are used. Many minerals, rocks, clays; agricultural wastes like rice husk, hull, straw; industrial waste like sugarcane bagasse and some plants are good sources of silica in various forms for the synthesis of nanoparticles. The leaf extracts, fruit pulps as well as aqueous extracts of ash of some plant parts can be used as alkaline catalysts for the synthesis of SiNPs. The green synthesis of SiNPs by using natural resources is an efficient, economical, time saving and sustainable process [7]. Recently, there are many reports about chemical and green synthesis of various nanoparticles [17] such as silicon oxide [18], calcium oxides [19], copper oxides [20, 21], gold [22], iron oxides [23,24,25], silver [15], zinc oxides [26,27,28,29,30] and titanium oxides [31, 32] for various applications. Instead, a very little investigation has been carried out on the eco-friendly green synthesis of SiNPs. The different types of natural resources that can be used for the green synthesis of silica nanoparticles are shown in Fig. 2.

Fig. 2
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Resources for the green synthesis of silica nanoparticles

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Due to the wide range of applications of SiNPs in various technological fields, their demand in coming days is going to increase. The applications in fields such as biomedicine, food, catalysis, smart pesticides and in the agriculture (as mineral carrier for plants) demand large quantities of SiNPs. In addition, SiNPs also show applications as an additive for the manufacture of rubber and plastics, as strengthening filler for concrete and other construction composites. These fields also demand huge quantities of SiNPs. The large-scale production of SiNPs by chemical routes may cause considerable harmful effects to the environment. In this review, collective information about different green methods for the synthesis of SiNPs is discussed with examples from the literature. Almost all green methods that can be used to synthesize SiNPs are covered in the discussion. Some future prospects and challenges are also pointed out and discussed.

Green methods for the synthesis of SiNPs

Agricultural waste

Agricultural waste products are utilized for the preparation of green fertilizers, as fuel and in some cases for the extraction of useful chemicals. The agricultural waste is used as precursor for synthesis of SiNPs. The main advantage of using agricultural waste is its availability in abundance at the end of each harvesting season. Hence, the nanoparticle synthesis methods that utilize agricultural wastes are always economical compared to other methods. Rice hull is a waste product of agriculture, which contains bulk of silica. There are many reports in the literature about use of this material to produce SiNPs with good quality. For instance, Jansomboon et al. synthesized silica micro- and nanoparticles by using rice hull for first time [33]. Lu et al. used rice straw as a source of silica and synthesized highly pure amorphous silica nano-discs [34]. Figure 3a, b shows SEM and TEM images of SiNPs obtained from rice straw ash. Figure 3c shows distribution of diameter of SiNPs obtained by using rice straw ash, which was determined from more than 200 particles in their TEM images. Sugarcane bagasse is a major waste product produced in the sugar industry. Mohd et al. reported the production of SiNPs from sugarcane bagasse [35]. This green approach gives a way for agricultural waste utilization by economical route. G Falk et al. used sugarcane bagasse ash to synthesize SiNPs and nano-silicon by magnesiothermic reactions [36]. Sankar et al. used sticky, red and brown rice husk ash and synthesized biogenerated SiNPs [37]. Vaibhav et al. synthesized SiNPs from agricultural waste like rice husk, bamboo leaves, sugarcane bagasse and groundnut shell and recovered 52–78% silica from respective wastes [38]. Gonzalez et al. synthesized crystalline SiNPs via Annelid bioprocessing of agricultural waste. These agro-wastes were subjected to Annelid treatment by using Eisenia foetida species to produce humus and then SiNPs were obtained by calcination and acid treatment [39]. Gurbani et al. reported a novel method to synthesize silica nanoparticles by using sedge. The sedge Carex riparia is a weed which produces large amount of agro-waste. The synthesis of SiNPs from it provided an excellent way to obtain useful nanomaterials from waste [40]. Sorghum bicolor also produces agricultural residue, but these agricultural residues were not disposed properly hence they create pollution. Sorghum residues consist high amount of silica, so it can be used as precursor for the synthesis of SiNPs. Athinarayan et al. synthesized silica nanoparticles using leaves of Sorghum bicolor for various applications in food industry [41]. Nanosilica was also synthesized by using Sorghum vulgare seed head by Balamurugan et al. [42]. Sivakumar et al. synthesized pure silica from waste material like cow-dung ash by using sol–gel process. They obtained SiNPs of 100% purity having spherical shape [43]. Teff straw is an agricultural residue available in high amount in Ethiopia. Wassie et al. reported the synthesis of nanosilica by using Teff straw. They extracted nano-silica by using heat and acid treatment [44]. Therefore, by looking at all above examples from the literature, it can be concluded that agricultural waste emerges as a good source for the synthesis of SiNPs.

Fig. 3
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Reproduced with permission from Lu et al. © 2012 Elsevier

SEM (a) and TEM (b) micrographs of silica from rice straw ash. The inset in (a) is a higher-magnification SEM image and the one in (b) is the selected area electron diffraction (SAED) pattern of silica. The diameter distribution (c) of silica particles was determined from more than 200 individual particles in TEM images.

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Industrial waste

Some of the industrial waste products were turned to be useful materials for synthesis of SiNPs with minimum efforts. A classic example is the conversion of fly ash into nanosilica. The coal fly ash is a waste or by-product of coal-fired power plants. Yadav et al. reported the synthesis of amorphous silica nanoparticles from fly ash which is rich in silica. This is an efficient and economical method for synthesis of SiNPs [45]. Yan et al. derived a green method in which silica nanoparticles were obtained from coal fly ash [46]. This green protocol gave a new way to reduce the industrial waste which can harm the nature. The nanosilica derived from the fly ash was tested for its CO2 capturing ability and it showed excellent results. The SEM and TEM images of the obtained particles are shown in Fig. 4. Figure 4a, b is obtained at magnification × 10,000 and × 50,000, respectively. This report also proves that the nanosilica obtained from waste products by green approach shows excellent properties compared to that of SiNPs synthesized from chemical methods.

Fig. 4
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Reproduced with permission from Yan et al. © 2017 American Chemical Society

a, b SEM images, c TEM image and d HR-TEM image of “nano-SiO2-2%”. Magnification factor: × 10,000 for (a) and × 50,000 for (b).

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Clay/minerals

The clay and minerals have been also used to prepare nanosilica. Malatya Pyrophyllite is a clay mineral that contains a significant amount of silica. Sarikoya et al. used Malatya Pyrophyllite and synthesized nano-amorphous silica nanoparticles [47]. Zulfiqar et al. synthesized silica nanoparticles by using bentonite clay [48]. Silica-rich clay was prepared from bentonite by a sequence of acid and thermal processes. It was later treated with sodium hydroxide solution to prepare sodium silicate. Silica particles were obtained by hydrolysing sodium silicate solution with nitric acid in the presence of ethanol. In this report, effect of acid concentration on particle size was also studied which shows that particle size can be tuned by varying the concentration of the acid used. Mourhly et al. synthesize mesoporous silica nanoparticles from Pumice rock, which is a silica-rich material [49]. Many researchers obtained nanosilica from different types of minerals. Stopic et al. derived silica nanoparticles from olivine mineral through carbonation under high pressure in autoclave [50]. Figure 5 shows the TEM analysis of the obtained SiNPs. The results confirmed that the particles have a spherical shape. The diameters of the particles were approximately 400–500 nm and the particles were amorphous in nature.

Fig. 5
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Reproduced with permission from Stopic et al. © 2019 MDPI

The TEM and STEM images of nanosilica obtained by olivine mineral carbonation under high pressure in an autoclave.

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Plants

Recently, there are a huge number of reports in the literature about the green synthesis of nanoparticles by using different plants. As pointed out earlier, nanoparticles of different materials and from various categories can be synthesized by using various parts of the plants. The synthetic methods have used the extracts of various parts of plants from different origins as reducing or hydrolysing agents for the synthesis of nanomaterials. This eco-friendly approach is helpful to avoid use of chemicals needed for the synthesis of various nanomaterials. Some classic examples of green synthesis of SiNPs using plants are as follows. Babu et al. used Cynodon dactylon plant as source of silica and produced SiNPs for antimicrobial study [51]. Sethy et al. used bamboo leaf ash to synthesize silica nanoparticles and incorporate in polydimethylsiloxane (PDMS) membrane to enhance its separation properties [52]. Most of the studies have used extract of plant parts as a base in the synthesis of SiNPs in the alkaline hydrolysis step (refer to Fig. 1). Durairaj et al. produced silica nanoparticles by well-known sol–gel method using Bambusa vulgaris leaves ash and used it for environmental applications [53]. Jabeen et al. reported the synthesis of silica nanoparticles using Azadirachta indica (Neem) leaves extract which acts as an effective chelating agent [54]. Azawi et al. prepared silica nanoparticles by using hot aqueous extract of Thuja orientalis leaves and study its effect on biofilm formation [55]. Irzaman et al. used bamboo leaves to synthesize SiO2 with 99.9%* purity [56]. Yusaldi et al. extracted silica from banana stem by using acid leaching method [57]. San et al. synthesized silica nanoparticles from sugarbeet bagasse by novel one-step method using laser ablation and reported environmental benign procedure to produce SiNPs [10]. Their results showed that the SiNPs obtained by laser ablation are significantly smaller (38–190 nm) or comparable to those prepared using chemical routes. The analysis results of the particles are shown in Fig. 6. Agunsoye et al. in their work investigated the utilization of Cassava periderm as new source of silica. They recovered silica nanoparticles from Cassava periderm by reduction method [58]. Fahmi et al. explored coconut husk waste as raw material for silica synthesis. They extract silica from coconut husk by chemical extraction method [59]. Subramanian et al. synthesized silica nanoparticles from coconut shell [60]. All these examples prove the worth of various plants in the green synthesis of SiNPs.

Fig. 6
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Reproduced with permission from San et al. © 2014 Elsevier

a TEM image of SiNPs, inset: size distribution, b XPS analysis recorded from SiNPs, c amorphous structure of SiNPs and d nanocrystal (NC) structure of SiNPs.

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Bacteria and yeast

The handling and genetic manipulation of various bacteria is possible due to recently invented techniques hence they can be used in the synthesis of useful nanomaterials. In fact, they are considered as the most potent eco-friendly nanofactories [61]. There are some reports about bacteria-assisted synthesis of SiNPs. Vetchinkina et al. used bacterial culture liquids to synthesize silicon nanospheres having a size ranging from 5 to 250 nm [62]. Silicon/silica nanocomposites were produced by Singh et al. by using bacterium Actinobacter species. In this report, silica precursor K2SiF6 and bacteria were exposed together. The reductase and oxidase enzymes secreted by the bacteria led to the synthesis of silica nanoparticles [63]. Show et al. derived a green protocol to produce SiNPs by using thermophilic bacteria BKH1 and inorganic magnesium trisilicate precursor and organic tetraethyl orthosilicate precursor [64]. Figure 7 shows the SEM image of the particles obtained. The EDS analysis of the particles showed the presence of only two major peaks of Si and O which confirmed the high purity of the particles obtained. Yeasts are simple eukaryotic organisms which can be used for production of nanoparticles. Liu et al. synthesize silica nanoparticles by a photochemical method using diphenyl iodonium hexafluorophosphate as a super acid generator and TEOS as silicon source [65]. The generated acid was responsible for the initial hydrolysis of the silica source (TEOS) which subsequently led to the particle formation. Zamani et al. used Saccharomyces cerevisiae yeast for the biochemical synthesis of silica nanoparticles for its application in enhanced oil recovery [66].

Fig. 7
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FE-SEM image of SiO2-NPs formed by the interaction of BKH1 cell with magnesium trisilicate hydrates solution

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Fungi, worms and actinomycetes

Recently, fungi have gained more attention as nanofactories for the biosynthesis of various nanoparticles. The use of fungi for synthesis of nanoparticles is potentially exciting since they secrete large amounts of enzymes and are simpler to deal with in the laboratory. Studies indicate that the genera Fusarium, Aspergillus and Penicillium have promising potential for the extracellular bio-production of different metal nanoparticles and Verticillium Sp. could be harvested for intracellular synthesis of nanoparticles. There are considerable numbers of reports in the literature about synthesis of SiNPs using fungi. For instance, Aleksandra Pieta et al. obtained the silica nanoparticles of defined size and morphology from bioconversion of corn cob husk using fungus of the Fusarium culmorum [67]. Bansal et al. derived a novel method to produce silica nanoparticles by bioleaching of sand using fungus Fusarium oxysporum. This Bioleaching method is quite rapid and takes place within one day of reaction of the Biomass with sand [68]. In another report, Bansal et al. biosynthesized silica nanoparticles from fungus Fusarium oxysporum using aqueous and anionic complex of SiF6−2. The detailed study showed that the exposure of fungus to silicon complex induce secretions of proteins which are capable of hydrolysing SiF6−2 to produce SiNPs [69]. Bansal et al. derived a new way to synthesize nanosilica from rice husk. In this report, fungus Fusarium oxysporum is used to bioleach silica from rice husk. This fungal species is capable of rapid biotransformation of the naturally occurring biosilica from rice husk into crystalline silica nanoparticles [70]. Figure 8 shows TEM images of SiNPs produced by using fungus Fusarium oxysporum upon exposure to rice husk. Many research reports suggest agro-waste rice husk as a better natural source of silica. In one of the reports from Rohtagi et al., they have synthesized SiNPs from rice husk by microbial fermentation using white rot fungus Python [71]. Zielonka et al. produced silica nanoparticles from agro-waste rice husk by fungal mediated biotransformation using Aspergillus parasiticus species, which gives SiNPs having size 3–400 nm [72]. In an interesting report, Estevez et al. produced silica nanoparticles through biodigestion of rice husk using Californian red worm. The worms are fed with rice husk and water which produce humus. After different treatments, silica nanoparticles are obtained within the range 55–250 nm [73].

Fig. 8
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Reproduced with permission from Bansal et al. © 206 American Chemical Society

TEM micrographs at different magnifications of silica nanoparticles synthesized by the exposure of rice husk to the fungus Fusarium oxysporum before (a and b; text for details) and after calcination at 400 °C for 2 h (c and d).

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Actinomycetes are gram-positive, anaerobic, mycelial bacteria. They can be also harvested for the synthesis of nanoparticles. Biosilification is economically beneficial, energy-saving and eco-friendly method for commercial synthesis of oxide nanomaterials. Kaur et al. synthesized SiO2 nanocomposites from rice husk and wheat bran by using various eukaryotic micro-organisms such as Actinomycetes, Fusarium oxysporum, Aspergillus niger, Trichoderma species and Penicillium species. All of these micro-organisms biotransform amorphous silica from natural source to crystalline silica within 24 h in the size range of 232 to 250 nm [74]. The brief comparative analysis of all the methods discussed above with their advantages and disadvantages is given in Table 1. Table 2 shows the role of various green sources or reagents in synthesis of SiNPs. The SiNPs obtained by use of micro-organisms are pure. Their separation and purification can be done by performing unit operations on the crude products. The separation of biomass and other impurities can be done by filtration, centrifugation and drying. The purity of the SiNPs obtained by various green methods mentioned above is considerably high compared to the SiNPs obtained by chemical methods, as there is no any possibility of formation of toxic by-products during their synthesis.

Table 1 Comparative analysis of green synthesis methods for SiNPs
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Table 2 Role of different sources in green synthesis of SiNPs
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Expected future advances

Recently, there is an increased interest in the synthesis of various nanoparticles including SiNPs using advanced synthetic techniques such as continuous flow synthesis in microfluidic devices (laboratory on chip concept) [75,76,77,78], by laser ablation [79] and microwave-assisted synthesis methods [80]. In these methods, the particle size and morphology can be controlled via parameters such as temperature, residence time, process chemistry, flow rates, irradiation times and precursor concentrations. These methods yield monodisperse nanoparticles and offer great control over the size of the particles. Additionally, the large-scale production of nanoparticles is also possible by using these methods. In particular, the continuous flow microfluidic chemistry has demonstrated relatively superior process performance over conventional nanoparticle synthesis technologies. At present, these modern nanoparticle synthesis methods are using the same chemicals used in the synthesis of the nanoparticles by other conventional techniques. As a future advancement in this field, it is possible to replace the harmful chemicals used in these modern techniques with green alternatives. Natural precursors, precursors derived from wastes and green catalysts can be used. The bases used in the hydrolysis of silica precursors to produce SiNPs can be easily replaced with plant extracts mentioned before. Some microfluidic devices can also be invented for biological synthesis of nanoparticles where micro-organisms (nanofactories) can flow through the channels easily, survive the conditions and produce nanoparticles. Such sustainable modern synthetic technique with the capability of large-scale production that utilizes green precursors, solvents and catalysts is the demand of future.

Conclusions and future outlook

The green methods for synthesis of nanoparticles eliminate use of unsafe chemicals and reduce the risks to the environment. All the examples of green synthesis of SiNPs discussed above show that variously shaped nanoparticles of silica can be prepared by simple biological and eco-friendly methods by using green resources. Many natural resources such as plants and minerals, micro-organisms and waste products from agriculture and industries can be used to prepare SiNPs. The green synthetic methods do not produce any hazardous by-products, provide better control over shape, size and morphology of the particles prepared. Further, in the case of micro-organism-assisted synthesis, the purity of the nanoparticles obtained is very high and there is no need for purification of the obtained nanoparticles by complex operations. The purification step is rather time-consuming and requires costly solvents in the chemical methods of synthesis of nanoparticles. This eliminates the use of large amounts of hazardous organic solvents used for the purification of the nanomaterials. In spite of the good progress in the field of green synthesis of SiNPs, several issues are still needed to be sorted out in near future. There is a need to develop even more “green” synthesis protocols for the synthesis of SiNPs than existing methods which can utilize various other available agricultural and industrial wastes. The demand of SiNPs is going increase in future; therefore, large-scale production of SiNPs will be needed. Presently, the large-scale production of SiNPs is done mostly by chemical routes due to high yields and industrial feasibility of the technology. There is still a lot of scope to develop and further modify presently invented green synthesis methods for the large-scale production of SiNPs. Thus, despite the current research, new innovations and further research is needed to upgrade the current green methods and processes to prepare SiNPs in future.