The species belonging to the Polypodiopsida class have been part of human culture since the beginning of civilization, often being utilized due to the presence of antimicrobial substances (such as alkaloids, terpenes as tannins, saponins, anthraquinones, cardiac glycosides, etc.). Application of the ferns in nanotechnology is presented with practical examples.
1. Introduction
The term “nano” is encountered in all aspects of our daily life, often being regarded as the ultimate “bottleneck” breaker in various technological areas
[1][2][3]. Day by day, nanomaterials represent a more and more a common aspect of our lives, in as much as we are becoming accustomed to nanomaterials-based personal care products, pharmaceutical products or even agricultural products
[4]. The “nano” approach can help to improve the solubility of poorly water-soluble active substances and increase their bioavailability
[5], increase storage stability of active substances, and develop more efficient drug carriers, thus leading to next generation nanomedicine tools
[6].
2. Polypodiopsida and the Nanomaterials
Although ferns are not as commonly encountered as other classes of plants in the nanotechnology area
[7], the literature study revealed several important studies in this field, suggesting a potential field of application for this underutilized class, particularly in the nanomaterials phytosynthesis process. The phytosynthesis mechanism involves the reduction and stabilization of the metallic nanoparticles by the phytoconstituents of the vegetal material (
Figure 1). As in the case of any phytosynthesis process, in the case of fern extracts, applications can be distinguished several factors influencing the morphology of the final nanoparticles (NPs) and thus their potential applications
[7]:
Figure 1. Phytosynthesis process and some of the involved phytoconstituents of ferns.
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factors related to the vegetal extract used: the intrinsic properties of the plants, related to their phytocomponents, the part of the plant used, extraction procedure, used solvents, the vegetal material to solvent ratio, plant pre-treatment, etc.;
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factors related to the phytosynthesis process: concentration of the metallic salt precursors, reaction conditions (temperature, pH, reaction time), extract to metallic salt ratio, etc.
As all these factors can affect the NPs’ properties, studies should be considered for comparative evaluation of their influence. In the case of other plant classes, it is not surprising to identify studies on similar vegetal materials, with different results, as the authors used slightly different conditions; even the geographical region from which the vegetal material is collected can influence the NPs’ characteristics, as the plants’ composition can be influenced by the environmental factors
[8].
The general process for the phytosynthesis of NPs using fern extracts, in which the main role for the reduction and capping of the metals is assigned to different phytoconstituents of the plants in the Polypodiopsida class, is depicted in Figure 1.
2.1. Nanoparticle Phytosynthesis Using Ferns
As the
terrestrial ferns are the most encountered, their use for the nanoparticle phytosynthesis is also more frequent. Several authors present the phytosynthesis of different types of nanoparticles, most often silver or gold. Other types of NPs (copper oxide, iron) or composites are also encountered, although to a lesser extent. Among those nanoparticles, AgNPs are the most common subject of research, due to the well-known antimicrobial potential of silver, well-known from ancient times
[9]. The differences between silver in the nanoparticle and in its ionic form, in terms of interaction with living cells, were recently discussed by other authors
[9]. Elaborating the aspects detailed in the cited work, the use of silver in its nanoparticle form can be considered advantageous over the application of silver ions (even though some studies report inferior antibacterial properties for NPs, compared with silver ions
[10]), due to several aspects:
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silver ions can bind to form different insoluble precipitates, which can negatively affect their properties
[9];
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particularly for the case of phytosynthesized nanoparticles, the use of different phytocomponents as capping agents can not only contribute to an increase in their antimicrobial or antioxidant potential (for example)
[7], but can decrease their toxic potential against non-target organisms
[7], which is actually lower for NPs, compared with silver ions
[11];
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the large surface area to volume ratio of nanoparticles (an element common for all types of NPs) provides better contact with microorganisms, thus increasing their antimicrobial potential, as well as contributing to their successful application in other areas
[12].
2.2. Potential Applications of Phytosynthesized Nanoparticles
The phytosynthesis of the NPs leads to the attainment of nanoparticles with characteristics depending on the extract used. Being closely correlated with the natural extract, the phytosynthesized NPs finds applications in areas in which the extracts have a historical use, such as antioxidant or antimicrobial fields, in which various phytoconstituents (such as phenolic acids, flavonoids, terpenes, carotenoids and proanthocyanidins) have proven applicability.
The main area in which the fern-phytosynthesized NPs are expected to find application is represented by antimicrobial applications (and in this area are found most of the published studies on fern-mediated NPs). The antimicrobial mechanism of the phytosynthesized NPs is well established, being previously presented by the group
[7], mainly involving the disruption of cellular membrane and on the generation of ROS (reactive oxygen species). Silver nanoparticles (a common subject of antimicrobial studies) represent the main subject regarding nanoparticles phytosynthesis using ferns. Multiple studies evaluated the antimicrobial potential of AgNPs obtained using fern extracts. Results (
Table 1) presented either as inhibition zones or as MIC/MCBE (minimum inhibitory concentration/minimal concentration values for biofilm eradication) values are usually close to the positive control used for the experiments (a commercial antimicrobial). Significant results were obtained by Miljković et al.
[13] using AgNPs obtained by
Equisetum arvense L. especially against the Gram-negative bacteria, with an MIC of 0.72 mg/L.
Table 1. Potential applications of nanoparticles phytosynthesized using ferns 1.
1 where: 3T3-L1—adipocyte cell lines; A549—adenocarcinomic human alveolar basal epithelial cells line; AAE—ascorbic acid equivalents; ABTS—azinobis 3-ethylbenzothiazoline-6-sulfonate; b.w.—body weight; CAT—catalase; DPPH—2,2-diphenyl-1-picrylhydrazyl; EDTA—ethylenediaminetetraacetic acid; ER—effective repellence; GOT—glutamate oxalate transaminase; GPT—glutamate pyruvate transaminase; GSH-Px—glutathione peroxidase; HEK293—human embryonic kidney 293 cells; HeLa—human cervical cancer cell line; HepG2—human liver cancer cell line; HPSA—hydrogen peroxide scavenging activity; IC50—concentration required to result in a 50% antioxidant activity; LC50—LC50 lethal concentration that kills 50% of the exposed organisms; LDH—lactate dehydrogenase; HSA—OH– scavenging activity; L929—normal subcutaneous areolar adipose tissue cellular lines; MB—methylene blue; MBC—minimum bactericidal concentration; MC3T3-E1—mouse pre-osteoblast cells; MCBE—minimal concentration values for biofilm eradication; MCF-7—breast cancer cell line; MDA—malondialdehyde; MI—mitotic index; MIC—minimum inhibitory concentration; MP—mitotic phases; MRSA—Methicillin-resistant Staphylococcus aureus; M.V. 6B—Methyl Violet 6B; NOx—nitric oxide; PA-1—human ovarian teratocarcinoma cell line; RAW264.7—macrophage, Abelson murine leukaemia virus transformed cells line; RB—Rose Bengal; RhB—rhodamine B; ROS—reactive oxygen species; RWC—relative water content; SOD—superoxide dismutase; SORS—Superoxide Radical Scavenging; SVI—Seedling Vigour Index.
2.3. Development of Biogenic Nanoparticles
Another nanotechnological-related potential application of the ferns is represented by the attainment of biogenic nanoparticles (especially silica). This application is related to the capacity of ferns to take up different metals or metalloids, followed by the processing of the vegetal material, in order to obtain the amorphous, semi- or highly crystalline nanoparticles. As such, the vegetal material can be considered a source of metalloids (in the case of silica). For example, Mattos et al.
[44] obtained amorphous spherical SiO
2 nanoparticles (7 nm) using horsetail fern (
E. arvense) stems, by acid leaching (2% sulfuric acid, acid:solid ratio 10:1, temperature 100 °C), filtration and washing to neutral pH, drying (103 °C), and finally calcination for 1h at 650 °C in an air atmosphere. The authors used the biogenic silica as a carrier of a neem bark extract cross-linked with polycarboxylic acids biocide
[45].
The same group used a three-step procedure to obtain agglomerated, irregular sphere-like silica nanoparticles from the leaves and stems of the same fern: hydroalcoholic extraction (1:1 (v/v) H
2O: EtOH, solid:liquid ratio of 1:10, 24 h); hydrolysis of the pre-extracted biomass (diluted sulfuric acid or water, solid:liquid ratio 1:10, heated at different temperatures and times); calcination of the washed solid after the hydrolysis for 1 h.
[46].
Hosseini Mohtasham and Gholizadeh
[47] also used the horsetail (entire plant) to obtain semi-crystallin silica nanoparticles using a procedure involving acid leaching (4 M hydrochloric acid, acid:solid ratio 50:1, refluxed 2 h); the treated vegetal material was subsequently centrifuged and washed to neutral pH, dried (50 °C) and calcinated (air atmosphere, heating rate of 1 degree/min, 2 h at 500 °C). A final composite, consisting of H
3PW
12O
40 loaded on the ethylenediamine immobilized on an epibromohydrin-functionalized Fe3O4@SiO
2 support, was tested by the authors; magnetite deposited on the silica support was applied by the authors for the one-pot synthesis of dihydropyrano [2,3-c] pyrazole derivatives with a 99% yield
[47].
Adinarayana et al.
[48] used the water horsetail extract microwaved–pyrolyzed at 200 °C (30 min., using household microwave oven) for the attainment of highly crystalline silica nanoparticles (average size 2.5 nm) applied for the fluorescence detection of Fe
3+ ions in water. Another species of the
Equisetum genus (
Equisetum myriochaetum Schltdl. and Cham., 1830—Mexican giant horsetail) was also used to obtain silica nanoparticles. Dried stems and branches were acid-digested (concentrated HNO
3/H
2SO
4 = 4:1, solid:acid ratio = 1:40, 48 h.), washed to a pH of 5, lyophilized and calcinated (air atmosphere, 650 °C, 5 h, heating rate 10 °C/min) by Sola-Rabada et al.
[49] to obtain amorphous, 5-nm biogenic silica, used for enzyme immobilization. Following the same recipe, Bogireddy et al.
[50] obtained biogenic amorphous silica from the stems of the giant horsetail, functionalized it with APTES and grown PtNPs on the obtained substrate (using a chemical route). The composite was applied for the catalytic reduction of 4-nitrophenol to 4-aminophenol, obtaining a complete reduction in 90 s. at room temperature
[50].