Indigenous north eastern India fern mediated fabrication of spherical silver and anisotropic gold nano structured materials and their efficacy for the abatement of perilous organic compounds from waste water-A green approach

Abstract

Herein, we presented a hitherto unexplored native fern of north eastern India; Diplazium esculentum mediated biogenic fabrication of spherical silver and anisotropic gold nano structured materials. The effects of various reaction parameters such as concentration and temperature were investigated in detail and the results revealed the formation of silver nano structures with spherical morphology and gold nano structured materials of spherical, triangular and decahedral shapes. Using a suite of analyzing techniques, the intrinsic crystallinity, size, morphology, elemental composition and the functional moieties associated with the reduction and surface stabilization were also disclosed. Significantly, the catalytic properties of these synthesized nano structured materials for the remediation of two carcinogenic and lethal textile dyes, Methyl Violet 6B and Rose Bengal and one hazardous phenolic compound, 4-nitro phenol from aqueous solution were evaluated and plausible mechanisms that drive these critical processes were also proposed. Approximately, 98.4 and 98.2% of Methyl Violet 6B and Rose Bengal dye were degraded within 240 minutes using the synthesized silver nano structured materials and 98 and 98.9% of Methyl Violet 6B and Rose Bengal dye were degraded within 180 minutes employing gold nano structures as the catalyst, respectively. While about 96.8 and 97.3% of 4-nitro phenol was reduced within 70 and 80 minutes respectively using silver and gold nano structured materials. Additionally, the exhausted nano structured materials were regenerated and their photo catalytic capability was evaluated for three continuous rounds of cycles. The exhausted nano structured materials and the intermediates of the degradation process were respectively analysed using TEM and LC-MS techniques. Hence, the present study has unfastened a pioneering way for synthesizing nano structured materials of different sizes and morphologies and their applicability for the remediation of hazardous compounds make these nano structured materials ideal candidates for waste water treatment.

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1. Introduction

Anisotropic noble nano structured materials have evolved as one of the most explored fields of nanomaterial research owing to their fascinating optoelectronic and physiochemical properties which make them different from their bulk counterparts.¹ Amongst these noble nano structured materials, silver (Ag) and gold (Au) nano structured (NS) materials are the most extensively studied materials due to their widespread applications in electronics, catalysis, sensors, plasmonics, pharmaceuticals, biomedical devices and waste water treatment.² However, all the conventional synthetic protocols are capital and energy intensive; and involve the utilization of hazardous, toxic and non environment friendly chemicals.³ Consequently, a need for developments of clean, non-hazardous, biocompatible, reliable and environmental begin procedures incorporating green principles is an urgent necessitate for the scientific fraternities.

One of the methods which stands on a strong footing and have been considered as greener, economic, cleaner and environment friendly procedure is the biosynthesis of NS materials. The biosynthetic approach utilizes the biological resources available in nature such as plants, plant products, fungi, algae, viruses, bacteria and yeast and minimizes or avoids the use of harmful substances. However, the exploitation of microorganisms for synthesis protocol is also undesirable as it requires not only tedious maintenance of microbial cell culture but also have difficulty in implementation on a large scale.⁴ Alternatively, plant extract mediated fabrication is quiet simple, scalable and practical. Consequently, plant extract mediated fabrication is extensively explored. Hence, in this perspective we utilized the extracts of an indigenous vegetable fern of north eastern India, Diplazium esculentum for the fabrication of Ag and anisotropic Au NS by controlling various reaction parameters.

Diplazium esculentum is an edible fern and belongs to the family Athyriaceae. The main phytochemicals in Diplazium esculentum are high quality of steroids, triterpenoids, glycosides, saponins, alkaloids, flavonoids, phenolic compounds, tannins, lignins, lipids and amino acids.⁵ It is believed that the compositional abundance of polyphenolic compounds especially flavonoids and proteins are involved in reduction and stabilization of the NS materials.⁶

Furthermore, with the increase of urbanization, modernisation and industrialization, air and water pollution has been enhanced and a major portion is constituted by the effluents from the dyeing industries and hazardous aromatic nitro compounds. Moreover, dyes exhibit numerous applications in our daily life and are comprehensively utilized in various industries and are considered as the primary contaminant in industrial waste water.⁷ They are carcinogenic and lethal in nature and are a substantial source of non-aesthetic pollution and eutrophication that produces detrimental by-product by further hydrolysis, oxidation or other chemical reactions in the waste water.⁸ Additionally, the existence of dyes in water can reduce the light penetration resulting in less photosynthetic activity, thus making oxygen unavailable for biodegradation of microorganisms in the water.⁹ Hence, it possesses a menace to both marine ecosystems and human begins.¹⁰ Therefore, their complete dislodgment is a mandatory and a cumbersome assignment owing to their complicated structure and high stability.

In this article, two dyes Methyl Violet 6B (MV6B) and Rose Bengal (RB) were selected. MV6B is a water soluble dye and is used in various industries such as paper, textile, printing ink and paints. It is a carcinogen, mutagen and miotic poison. Whereas, RB is a xanthene class of dye and is extensively used as a dyeing material, insecticides, and in printing industries. The dissipation of RB causes several harmful diseases to the liver and stomach of human beings.

Hence, both MV6B and RB are health hazardous and are threat to both human and aquatic life and their abatement is essential and photocatalytic degradation in presence of a suitable nanocatalyst is found to be the most effective method for their remediation.

Besides dyes, nitro-phenol and its derivatives are also peril to environment. para-Nitro phenol (4-NP) is known to be water soluble, stable, toxic, anthropogenic and inhibitory in nature.¹¹ Its exposure causes nausea, headache, cyanosis, drowsiness and damage to central nervous system, liver, kidney, and both human and animal blood. Therefore, their complete deportation is mandatory on account of environmental concern. Scientific fraternities have developed several techniques or methodologies for their abatement but all these techniques are either energy consuming or require harmful organic solvents.¹²

Hence, it is obligatory to develop a technique or reaction which can be easily studied or developed and is trustworthy, convincing and requires easy experimental set up. So, far only one reaction which stands on a strong footing and has been qualified is the catalytic reduction of 4-NP to 4-AP (para-aminophenol) in aqueous media in presence of sodium borohydride (NaBH₄) using Ag or Au NS as catalyst.

Consequently, increased environmental pollution has evoked our research attention to utilize materials or to design processes that are friendly to both environment and human health.

Thus, in this perspective, the present work addresses a green, facile, environment friendly and cost-effective method for the fabrications of Ag and anisotropic Au NS materials employing a indigenous fern of north eastern India and their efficacy as a catalyst for the abatement of hazardous compounds (MV6B, RB and 4-NP).

The fabricated NS materials were characterized using various techniques, such as UV-Visible spectroscopy (UV-Vis spectroscopy), Fourier Transformer Infrared Spectroscopy (FTIR spectroscopy), Transmission Electron Spectroscopy (TEM), Selected Area Electron Diffraction (SAED) pattern and Energy Dispersive X-ray Spectroscopy (EDAX).

2. Experimental

2.1 Materials

Silver nitrate (AgNO₃), hydrogen tetrachloroaurate monohydrate (HAuCl₄·3H₂O), Methyl Violet 6B (MV6B), Rose Bengal (RB), 4-nitro phenol (4-NP) and sodium borohydride (NaBH₄) of AR grade was procured from Sigma Aldrich and used as received. Double distilled water was used in all the experiments. Diplazium esculentum (DE) fern was collected from the local market and was washed thoroughly with double distilled water and then dried in oven at 60 °C until constant weight. The dry biomass was then grounded in a stainless steel grinder and sieved.

2.2 Preparation of aqueous extract of DE fern

Fine powdered DE fern (10 g) was placed in a 500 ml Erlenmeyer flask containing 450 ml distilled water, and then heated at 70 °C for 20 minutes (min). This was followed by centrifugation at 4000 rpm for 20 min and the supernatant was then filtered. The extract was stored in a refrigerator at 4 °C for further use. The prepared extract was used within one week of preparation.

2.3 Synthesis of Ag and Au NS materials

An aliquot (20 ml) of DE fern extract of various concentrations (10%, 20%, 30% and 40%) were added separately to 0.1 M (20 ml) AgNO₃ and 0.1 M (20 ml) HAuCl₄·3H₂O and heated at 70 °C for 20 min followed by slow cooling at room temperature for the fabrications of Ag and Au NS materials, respectively.

Another set of Ag and Au NS materials were respectively synthesized at different reaction temperatures (40 °C, 60 °C, 80 °C and 100 °C) by heating a mixture of 10% (20 ml) DE fern extract with 0.1 M (20 ml) AgNO₃ or 0.1 M (20 ml) HAuCl₄·3H₂O solution for 20 min. These solutions were then allowed to stabilize for 1 day. After 1 day, the solution with brown sediment for Ag NS materials and ruby red sediment for Au NS materials were formed at the bottom of the container. These were then centrifuged, filtered and the residues were washed several times with double distilled water to remove unbound polymers to yield NS materials.

2.4 Evaluation of photocatalytic activity

Photodegradation of MV6B and RB dyes was assessed by dispersing separately 10 mg of Ag and Au NS materials in 200 ml of 10⁻⁴ M aqueous solutions of these dyes. The suspended solutions were then allowed to stand for 1 h in the dark before solar irradiation for attaining the adsorption–desorption equilibrium of dye on the surface of the NS materials. These dyes were then exposed to sunlight. The experiments were then conducted on a sunny day at Silchar city between 10 a.m. to 3 p.m. (atmospheric temperature 32–36 °C). 4 ml of suspensions were withdrawn and immediately centrifuged at regular intervals of time. At regular intervals of time, the progress of the reaction was examined using UV-Vis spectroscopy.

2.5 Catalytic activity of the synthesized NS materials

To evaluate the efficiency of the fabricated NS materials as catalyst, the conversion of 4-NP to 4-AP in aqueous medium in presence of NaBH₄ was carried out at room temperature. In a standard quartz cuvette having 1 cm path length, 2.6 ml of water and 60 μl of 6.07 × 10⁻³ M 4-NP were taken separately and the absorbance was recorded using the UV-visible spectrometer. To this 4-NP solution, 350 μl of aqueous NaBH₄ (0.1 M) was added and the absorbance was noted. Thereafter, 150 μl of 0.01 g of Ag and Au NS materials were separately added to that mixture and the absorbance was until the absorbance due to the 4-NP was no longer detected.

2.6 Characterization of the synthesized NS materials

An UV-Vis spectrophotometer (Cary-100 BIO) was employed for absorption measurement. The TEM, HRTEM micrographs, and SAED pattern were recorded using JEOL-JEM 2100 transmission electron microscope operated at an accelerating voltage of 200 kV. The TEM samples of the NS materials were prepared by placing the solution drops over the carbon coated copper grids and allowing the solvent to evaporate at room temperature. An energy dispersive X-ray spectroscopy (EDX) analyzer was attached to the TEM operating mode and was used to analyze the components in the NS materials. The FTIR spectra were measured using Bruker Hyperion 3000 FTIR spectrometer, using thin, transparent KBr pellets prepared by pressing a mechanically homogenized mixture of dried sample with dehydrated KBr. The XRD pattern was recorded using a Phillips X'Pert Pro Diffractometer with Cu Kα radiation of wavelength 1.5418 Å.

3. Results and discussion

3.1 UV-Vis spectral analysis

Ag and Au NS materials are known to exhibit a unique phenomenon known as surface plasmon resonance (SPR) which arises due to the collective oscillations of electrons in the conduction band with that of electromagnetic radiation owing to which it gives absorption in the UV-Vis region. The alteration in position of these bands gives information about the particle size, dielectric constant morphology, and adsorbed species on the surface.¹³

(Fig. 1(a) and (b)) respectively displayed the absorption spectra of the fabricated Ag NS materials employing different concentrations of DE fern extracts (10%, 20%, 30% and 40%) and at different heating temperatures (40 °C, 60 °C, 80 °C and 100 °C).

Fig. 1 (a) represented the UV-Vis spectra of the Ag NS materials fabricated at different concentrations of DE fern extracts (10%, 20%, 30% and 40%). (b) displayed the UV-Vis spectra of the Ag NS materials synthesized employing DE fern extracts at different temperatures (40 °C, 60 °C, 80 °C and 100 °C). (c) represented the UV-Vis absorption spectra of the Au NS materials formed at different concentrations (10%, 20%, 30% and 40%) of the DE fern extract. (d) depicted the absorption spectra of the Au NS materials at different temperatures (40 °C, 60 °C, 80 °C and 100 °C).

The spectra (Fig. 1(a)) revealed that at lower concentration (10%), the absorption onset occurred at about ∼440 nm, whereas at higher concentration (40%) the absorption owing to the SPR of Ag NS materials were observed at ∼445 nm with increased intensity and broadness indicating a bathochromic shift with enhancement of concentration.

Generally, a bathochromic shift is associated with enhancement of particle size or withdrawal of electron density from the surface.¹⁴

The lower SPR absorption at lower concentration is an indication of formation of spherical NS materials of smaller particle size while at higher concentration, the enhancement of broadness of SPR band is a signal of formation of either anisotropic NS materials or aggregation of spherical NS materials.¹⁵ Usually, the aggregation or formation of anisotropic NS materials occurred owing to the fact that beyond a certain limit the biomolecules present in the extract ceases to act as a stabilizing agent.¹⁵ Consequently, this result indicated that with the increase in concentration of the DE fern extract the particle size can be tuned.

It is evident from the spectra (Fig. 1(b)), that at lower temperature (40 °C), the SPR owing to the formation of Ag NS materials appeared at about ∼426 nm which shifted to approximately ∼430 nm with increased intensity and broadness at higher temperature (100 °C).

This enhancement of absorption band at higher temperature is a clear indication of higher productivity of Ag NS materials at higher temperature.¹⁶ From previous studies, it is relevant that this phenomenon may appear owing to the efficient crystal growth of (111) faces by deposition of Ag atoms on cubic (100) faces than the nucleation of new Ag crystals at elevated temperatures.¹⁷ Additionally, it is also established that the enhancement in SPR band with enhancement of temperature is a sign of positive correlation between the yield of the NS materials and the temperature.¹⁸ Therefore, the study indicated that temperature is a crucial factor for the formation of the Ag NS materials.

The optical spectra of the Au NS materials at different concentrations (10%, 20%, 30% and 40%) and at different temperatures (40 °C, 60 °C, 80 °C and 100 °C) are respectively represented in (Fig. 1(c) and (d)).

Two significant observations were noticed from the spectrum (Fig. 1(c)).

The initial is that as the concentration of DE fern extract increased the SPR band centred at ∼536 nm owing to spherical Au NPs increased monotonically indicating that the number of spherical NPs got enhanced. While the second observation was that at higher concentrations (30% and 40%), in addition to band at ∼536 nm, a new band in the NIR region was found that also amplified with the increase in concentration.

Normally, the peak at ∼536 nm corresponded to transverse (out of plane) SPR component, while the peak at NIR region corresponded to longitudinal (in plane) SPR component of triangular Au NPs.¹⁹ The out of plane transverse absorbance more or less coincided with the SPR of spherical Au NPs, while the in-plane longitudinal absorption is a function of edge length of triangles.²⁰

Consequently, the increase in intensity of the peak at ∼536 nm with increase in concentration is due to the changes in the dielectric properties of the layer surrounding the Au NS materials, while the peak in the NIR region is due to the sintering of some of the spherical NS materials leading to the formation of single crystalline anisotropic NS materials.²¹

The absorption spectrum (Fig. 1(d)) at different temperatures also revealed two peaks one corresponding to transverse SPR component (∼536 nm) which increased continuously with temperature and another owing to longitudinal SPR component that decreased to shorter wavelength with increase in temperature (at NIR region for lower temperature and other at ∼612 nm for higher temperature (80 °C and 100 °C)) depicting the formation of anisotropic nano structured materials. This shift to shorter wavelength of the longitudinal SPR with increase of temperature is due to the decrease of the edge length of the anisotropic NS materials with the increase of temperature and can be borne by the TEM micrographs²² (Fig. 5(a) and (d)).

Hence, these results showed that both concentration and temperature are essential factors for the formation and size distribution of NS materials.

3.2 TEM and SAED studies

The morphology, size and crystallinity of the fabricated NS materials were investigated by TEM and SAED measurements. (Fig. 2(a) and (d)) corresponded to the TEM images of the synthesized Ag NS materials employing 10% and 40% extract solutions of DE fern. The SAED pattern of (Fig. 2(a) and (d)) are shown in (Fig. 2(c) and (f)), respectively. The TEM images revealed that the particles are spherical in morphology. It could also be seen that the NPs are well separated from each other indicating good capping and absence of aggregation. The Ag NS materials formed using 10% and 40% extract solutions of DE fern had an average size of 10–17 nm and 15–25 nm, respectively. This indicated that the particle size increased with increase in concentration of the DE fern extract.

Fig. 2 (a) and (d) represented the TEM images of the Ag NPs formed using different concentrations of DE fern extract (10% and 40%), ((b) and (e)) represented the HRTEM images and ((c) and (f)) depicted the SAED pattern of the Ag NPs formed using 10% and 40% of the extract respectively.

High resolution TEM images (HRTEM) indicated clear lattice fringes with fringe spacing of 0.24 nm (Fig. 2(b)) and 0.25 nm (Fig. 2(e)) that matches well with the (111) plane of fcc Ag (JCPDS 04-0783).

SAED analysis (Fig. 2(c) and (f)) of one of the NPs displayed concentric diffraction rings that indicated the polycrystalline nature of the Ag NS materials. The d-spacings established from SAED pattern (Fig. 2(c) and (f)) were 2.34, 2.05, 1.41 and 1.19. These could be indexed as (111), (200), (220) and (311) reflections (JCPDS 04-0783) that corresponded to fcc Ag. Thus, it is perceptible that the particles dimension can be tuned with change in concentration of the extract solution.

TEM images of the Ag NS materials formed at different temperature (40 °C and 100 °C) are depicted in (Fig. 3(a) and (d)), respectively. (Fig. 3(b) and (e)) represented the HRTEM images and (Fig. 3(c) and (f)) depicted the SAED pattern of the Ag NS materials formed at different temperatures (40 °C and 100 °C) respectively.

Fig. 3 (a) and (d) represented the TEM images of the Ag NPs formed at different temperatures (40 °C and 100 °C), ((b) and (e)) represented the HRTEM images and ((c) and (f)) depicted the SAED pattern of the Ag NPs formed at 40 °C and 100 °C respectively.

TEM micrographs (Fig. 3(a) and (d)) illustrated the formation of Ag NS materials mainly of spherical morphology with dimension in the range of 8–12 nm and 9–15 nm respectively.

The HRTEM images displayed clear lattice fringes with fringe spacings of 0.25 nm (Fig. 3(b)) and 0.23 nm (Fig. 3(e)) that are in agreement with the (111) lattice plane of fcc Ag (JCPDS 04-0783).

SAED pattern (Fig. 3(c) and (f)) showed concentric diffraction rings with d-spacings of 2.34, 2.05, 1.41 and 1.19 and could be marked as (111), (200), (220) and (311) reflections (JCPDS 04-0783) that corresponds to fcc Ag.

The TEM images of Au NS materials at 10% and 40% concentrations of the DE fern extract are respectively represented in (Fig. 4(a) and (d)). (Fig. 4(b) and (e)) represented the HRTEM images of (Fig. 4(a) and (d)) respectively while the SAED pattern of Fig. 4(a) is represented by Fig. 4(c).

Fig. 4 (a) and (d) represented the TEM images of the Au NPs formed using different concentrations of DE fern extract (10% and 40%), ((b) and (e)) represented the HRTEM images and (c) depicted the SAED pattern of the Au NPs formed using 10% of the extract.

The TEM study clearly revealed the formation of spherical NS materials at lower concentration (Fig. 4(a)) with average particle diameter in the range of 35–40 nm. While at higher concentration (Fig. 4(d)), triangular NS materials were found to coexist along with smaller percentage of spherical NS materials indicating that the absorption in the NIR region is due to considerable anisotropy in the NS materials, a result that was constant with TEM analysis of particles that depicted triangular nano structured materials of high density and not due to agglomeration of spherical NPs.¹⁹ The average edge lengths of the triangular NS materials were found to be in the range of 45–65 nm.

The HRTEM (Fig. 4(b) and (e) respectively) result corresponded to d-spacings which were consistent with (200) and (111) lattice planes of fcc Au.

The SAED pattern (Fig. 4(c)) represented polycrystalline nature of the particles and the spots could be indexed corresponding to the reflections from the (111), (200), (220), and (311) lattice planes of fcc Au.

The TEM spectra (Fig. 5(a) and (d)) respectively disclosed that the Au nano structured materials at lower temperature (40 °C) were predominantly triangular in morphology with average edge length in the range of 50–75 nm whereas at higher temperature (100 °C), particles were predominantly decahedral in shape with average edge length in the range of 45–55 nm.

Fig. 5 (a) and (d) respectively depicted the TEM images of the synthesised Au NPs at different temperatures (40 °C and 100 °C), and ((b) and (e)) are the magnified images of ((a) and (d)) respectively and the SAED pattern of ((a) and (d)) are respectively shown in ((c) and (f)).

The HRTEM images (Fig. 5(b) and (e)) respectively showed that triangular Au nano structured materials with lattice spacing of 0.24 nm corresponding to (111) plane of fcc Au and a decahedral NS materials have been formed.

An ideal decahedral structure involves equilateral triangles on its surface (Fig. 5(e)) with pentagonal projections containing slightly rounded faces. A decahedron consists of fivefold twinned structure, comprising of five tetrahedral crystallites with continuous lattice fringes and is one of the most stable structures in nanoscopic size ranges.

The SAED pattern (Fig. 5(c)) of the Au nanotriangles (Fig. 5(a)) supported the single crystalline nature of the particle and the spots were found to be hexagonal in nature indicating that the nanotriangles are highly {111} oriented. Three sets of spots could be noticed and corresponded to the 1/3{422}, {220} and {311} Bragg reflection.

The SAED pattern of the decahedron was represented in Fig. 5(f). The spectra revealed a set of spots with five-fold symmetry as well as spots from various fcc reflections and could be labeled corresponding to reflections from planes 11̄1 and 1̄11 of decahedral structure.

Hence, it was found that the TEM results were in accordance with the absorption spectral results and both concentration and temperature are crucial factors for the fabrication of both Ag and Au NS materials.

3.3 FTIR studies

The FTIR spectra of DE fern extract and DE stabilized NS materials were represented in Fig. 6. The FTIR analysis was carried out to identify the possible biomolecules responsible for the reduction and capping of the bioreduced NS materials fabricated by the fern extract. The assignments of FTIR bands of the DE fern extract and the synthesized NS materials are summarized in Table 1.

Fig. 6 Represented the FTIR spectra of the DE fern extract (represented in black color), and the spectra of the fabricated Au (red color) and Ag (blue color) NS materials.

Table 1 FTIR spectra of DE fern extract and fabricated NS materials

FTIR bands (cm^−1)
Samples	νO–H with νN–H	νC=C asymmetric stretching	νC–O stretching of amide and νNH2	νN–H wagging/out of plane νO–H bending
DE fern extract	3460	2039	1631	645
Ag NS materials	3433	2066	1644	685
Au NS materials	3473			700

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The DE fern extract revealed strong or medium absorption bands centered at about 3460, 2039, 1631 and 645 cm⁻¹ indicative of hydrogen-bonded O–H stretching vibration overlapped with N–H stretching, C=C asymmetric stretching, C=O stretching of amide and NH₂, N–H wagging or out of plane O–H bending vibration respectively.²⁰

However, for the DE fern extract stabilized Ag and Au NS materials the peak for hydrogen-bonded O–H stretching vibration overlapped with N–H stretching respectively shifted to 3433 and 3473 cm⁻¹ and became relatively broad and strong. In addition, for both the Ag and Au NS materials, the band due to asymmetric C=C stretching and C=O stretching of amide shifted to 2066 cm⁻¹ and 1644 cm⁻¹.

Also the peaks assigned due to NH₂, N–H wagging or out of plane O–H bending vibration was shifted to 685 cm⁻¹ for Ag NS materials and 700 cm⁻¹ for Au NS materials. The above mentioned data indicated that O–H, C=C, amide C=O and N–H could be present in the DE fern extract. These functional groups could be attributed to the presence of polyphenolic compounds such as flavonoids and also to proteins.²⁰ The peak alteration in the IR spectra of the fabricated NS materials were related to the NH₂ and OH moieties depicting that these functional moieties are involved in their fabrication as well as in capping.

3.4 XRD studies

The XRD analysis was carried out to determine the crystal structure of the biosynthesized NS materials. The Bragg diffraction peaks of Ag NS materials were noticeable at positions 38.2, 46.2, 64.5 and 76° that corresponded to d-spacings of 2.33, 2.03, 1.41 and 1.24 Å, respectively, and could be attributed to (111), (200), (220) and (311) planes of fcc Ag, respectively, (JCPDS 04-0783) (Fig. 7(a)).

Fig. 7 (a) represented the XRD pattern of the fabricated Ag NS materials. (b) represented the XRD pattern of the fabricated Au NS materials.

The results were found to be in accordance with the SAED pattern and no additional unassigned peaks owing to crystals of bio-organic phases were observed.²³

Moreover, the reflections from (111) plane was comparatively broader and more intense compared to reflections from (200), (220) and (311) planes. This results indicated that the Ag NS materials were (111) plane oriented as confirmed by the HRTEM results.²⁴

For Au NS materials, the diffraction peaks at 2θ values of 39.5°, 45.4°, 63° and 74° were noticeable that attributed to reflections from (111), (200), (220) and (311) lattice planes of fcc Au and corresponded to d-spacings 2.35, 2.04, 1.44 and 1.23 Å, respectively (JCPDS 04-0784) (Fig. 7(b)).

The results were found to be in agreement with the SAED pattern and no additional unassigned peaks owing to crystals of bio-organic phases were noticed.²³

3.5 EDAX studies

EDAX is a semi quantitative technique used to identify the elements present in the synthesized NS materials. Fig. 8(a) represented the EDAX spectra of the Ag NS materials fabricated employing DE fern extract. The spectra clearly depicted peaks at around 3 keV attributed to the SPR of Ag NPs.²⁴ However, some weak signals due to Cu, O and Mg were also noticed. The signal of Cu was due to the copper grids used in the analysis while the rest of the elements were due to the biomolecules that are capping the Ag NS materials.

Fig. 8 (a) represented the EDAX pattern of the synthesized Ag NS materials. (b) represented the EDAX pattern of the synthesized Au NS materials.

The EDAX spectra (Fig. 8(b)) clearly revealed peaks at 1.5, 2, 8.5 and 9.7 keV owing to the SPR of Au NS materials.²⁵ However, some weak signals due to Cu, O, N, Mg and Cl were also noticed. The signal of Cu was due to the copper grids used in the analysis while the rest of the elements were due to the biomolecules that capped the Au NS materials.

3.6 Plausible mechanism for the formation of the NS materials

In recent times, several speculations have been intended for the mechanism of the NS materials.²⁶ Many researchers have reported the involvement of proteins, polyols, polyphenols, flavonoids and terpenoids in the reduction and stabilization of these NS materials.^27,28 However, to propose exact mechanism for the phytosynthesis of these NS materials is typically a cumbersome task.

It is already mentioned that DE fern contains phytochemicals such as steroids, triterpenoids, glycosides, saponins, alkaloids, flavonoids, phenolic compounds, tannins, lignins, lipids and amino acids and moreover, from the FTIR studies (Fig. 6), it is previously ascertained that flavonoids and proteins are the main components involved in the fabrication of these NS materials.⁵

So, in the present study the mechanistic pathway involves the participation of the flavonoids and proteins. There are several reports available where the involvement of flavonoids in the synthesis is described.^28–30 It is believed that flavonoids acts as a reducing agent and are liable for the reduction of the metal salts and the carboxylate group present in the protein can behave like surfactant and adhere to the surface of the NS materials thereby stabilizing it through electrostatic stabilization.³¹

Hence, based on all these previously reported studies and FTIR results, it is perceived that in the present study, the polyphenolic compounds especially flavonoids might be involved in the reduction of the metal ions into metal NS materials owing to their unique ability to chelate metal ions and donate electrons and hydrogen atoms.³² While the proteins were responsible for the stabilization of NS materials due to the presence of carboxylate and amine moieties.³² Therefore, the fabrication and stabilization of NS materials can be represented by the following steps (Scheme 1).

(i) Complexation of flavonoids with Ag/Au metal salts,

(ii) Simultaneous reduction of Ag/Au metal and

(iii) Capping with oxidized polyphenols/proteins.

Scheme 1 Mechanism for the formation of NS materials.

Hence, it was observed that the polyphenols and proteins present in the DE fern extract resulted in the formation of NS materials without the need of any reducing or stabilizing agent and this method can be suitably scaled up for large-scale synthesis of NS materials.

3.7 Evaluation of the photocatalytic activity of the synthesized NS materials

Two different dyes, namely Methyl Violet 6B and Rose Bengal were chosen for evaluating the photocatalytic activities of the synthesized Ag NS and Au NPS materials in aqueous medium under solar irradiation. The dye degradation doesn't start immediately. The dye degradation processes were monitored by witnessing the alterations in the UV spectrum of the reaction mixture, which was made free from the catalyst by centrifugation.

It was scrutinized that as the exposure time increased, the absorption peak corresponding to different dye depreciated gradually and reached their minimum. In (Fig. 9(a) and (b)), the absorption peaks at 580 and 540 nm, corresponding to Methyl Violet 6B and Rose Bengal confirmed rapid degradation and disappeared after 240 and 240 min, respectively. To substantiate the photocatalytic activity of the synthesized NS materials, a control experiment was also carried out. It was noticed that when the dye solutions were kept under sunlight in the absence of NS materials, the dye showed no degradation. Similarly, dye showed almost negligible degradation when placed in dark without sunlight in presence of NS materials.

Fig. 9 (a) represented the absorption spectra for photocatalytic degradation of MV6B using Ag NS materials. (b) represented the absorption spectra for photocatalytic degradation of RB using Ag NS materials. (c) represented the degradation capacity of the manufactured Ag NS materials for MV6B and RB. (d) represented the plot of ln(C₀/C_t) vs. irradiation time t for the degradation of dyes employing Ag NS materials as catalyst.

Fig. 9(c) depicted the degradation capability of the synthesized Ag NS materials for Methyl Violet 6B and Rose Bengal, which reached to 98.4 and 98.2%, respectively. In the present experiment, although the morphologies of NS materials used were the same, but the rate of degradation was found to be different for different dyes and it exclusively depended upon the chemical structure of the target dye.

The rates of degradation of these dyes in presence of NS materials were according to pseudo-first order reaction and their kinetics may be expressed as follows.³³

ln(C₀/C_t) = kt (i)

where, C_t and C₀ are the concentration of the dyes at time t and 0, respectively, k = pseudo-first order rate constant and t = time in min.

Fig. 9(d) represented the plot of ln(C₀/C_t) vs. irradiation time t for the degradation of dyes. The plot represents a linear relationship and hence, slope of the line represents the rate constant (k) for the degradation of dyes. The value of k was found to be, 1.54 × 10⁻² and 1.77 × 10⁻² min⁻¹ for Methyl Violet 6B and Rose Bengal respectively.

Similarly, degradation of these dyes using Au NS materials were also carried out (Fig. 10(a) and (b)) and it was observed that the dyes degraded completely within 180 min for both Methyl Violet 6B and Rose Bengal. The degradation capability was found to be 98 and 98.9% for Methyl Violet 6B and Rose Bengal, respectively (Fig. 10(c)) and the rate was found to be 2.37 × 10⁻², 2.4 × 10⁻² min⁻¹, respectively (Fig. 10(d)). Thus, from these results it can be concluded that the dye degradation rate for Au NS materials were higher as compared to that of Ag NS materials.

Fig. 10 (a) represented the absorption spectra for photocatalytic degradation of MV6B using Au NS materials. (b) represented the absorption spectra for photocatalytic degradation of RB using Au NS materials. (c) represented the degradation capacity of the manufactured Au NS materials for MV6B and RB. (d) represented the plot of ln(C₀/C_t) vs. irradiation time t for the degradation of dyes employing Au NS materials as catalyst.

A comparison of dye degradation between the present study and other investigations reported in the literature is shown in Table 2. The comparison also showed the complete degradation time of dyes using various nanoparticles. It is evident that from the Table 2 that the catalysis reaction rate is either equivalent or improved in our present procedure compared to most of the reported literature data. In our present case, we believe that the electron transfer rate becomes much faster which in turn increased the catalysis reaction rates.³⁶

Table 2 Comparative analysis of the experimental output of the present study and studies conducted by other researches for the photocatalytic degradation of dyes using NS materials

NS materials	Dye	Volume (ml)	% degradation	Time (min)	Catalyst conc. (g)	Ref.
Ag	MV6B	200	98.4	240	0.10	Present study
Ag	RB	200	98.2	240	0.10
Au	MV6B	200	98	180	0.10
Au	RB	200	98.9	180	0.10
Graphen-e-polyanili-ne	RB	100	56	180	0.10	34
Ag–ZnO	MV6B	100	93	210	0.50	35
ZnO	MV6B	100	68	210	0.50	35

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3.8 Probable mechanism for the photocatalytic activity of the NS materials

A photocatalytic mechanism involves two components: photo and catalysis. The former portion is correlated with interaction of light material which consists of photon absorption, charge creation, dynamics, and surface trapping. Where as the later part is associated with surface reactivity and surface radical formation that is association with H₂O, O₂, and organic pollutants.³⁷ Consequently, the photocatalytic mechanism can be figured up as follows:

Initially, the solar irradiation is absorbed by the NS materials which are photo excited and experiences plasmonic decay by three mechanisms:³⁸

1. The absorbed molecules absorb photon and gains energy from the plasmonic structure of NS materials and the process is known as elastic radiative re-emission of photons.

2. Next, a non radiative Landau damping is experienced by the photon energy which converts it to a single e⁻/h⁺ pair excitations. Then via columbic inelastic scattering, the excited primary electrons generate many other electrons.

3. Lastly, due to the interaction between the adsorbate and the excited surface plasmons, the induction of a direct electron injection into the adsorbate takes place.

Secondly, owing to plasmonic decay, the electrons and holes generated can react with O₂ and H₂O molecules to furnish active species; anionic super oxide radical (O₂⁻˙) and hydroxyl radical (OH˙), respectively.

In the next step, protonation of the superoxide ion (O₂⁻˙) occurs which leads to the formation of hydro peroxyl radical (HO₂˙). These hydro peroxyl radical then converts to H₂O₂ which ultimately dissociates into highly reactive hydroxyl radicals (OH˙).

Lastly, both reduction as well as oxidation takes place on the surface of the photocatalyst.

Therefore, the complete degradation process can be represented by the Scheme 2, and the related reactions are shown in eqn ((1)–(9)).

NS + hν → h⁺ (Au) + e⁻ (NS) (1)

H₂O (ads) + h⁺ → OH˙ + H⁺ (ads) (2)

O₂ + e⁻ → O₂⁻˙ (ads) (3)

O₂⁻˙ (ads) + H⁺ ⇄ HOO˙ (ads) (4)

2HOO˙ (ads) → H₂O₂ (ads) + O₂ (5)

H₂O₂ (ads) → 2OH˙ (ads) (6)

Dye + OH˙ → CO₂ + H₂O (dye intermediates) (7)

Dye + h⁺ → oxidation products (8)

Dye + e⁻ → reduction products (9)

Scheme 2 Schematic representation of the photodegradation process employing NS materials under solar irradiation.

3.9 Evaluation of photostability of the NS materials

To illustrate the photo stability of the catalyst, recycling reactions were carried out for the degradation of both MV6B and RB. In each test, NS materials (catalyst) were separated from the solution, washed with ethanol and dried in vacuum.³⁹ It was found that the catalyst exhibits excellent stability even after 3 cycles and the results are depicted in ESI (S1).†

3.10 Identification of the intermediate products of dye degradation

The intermediates generated during the degradation process were analyzed using liquid chromatography-mass spectroscopy (LC-MS) technique and were identified by comparison with commercial standards and by interpretation of their fragment ions in the mass spectra.

All the identified intermediate products of the dye degradation are presented in the ESI (S2).†

3.11 Catalytic reduction of 4-NP to 4-AP employing NS materials as catalyst in aqueous medium

To explore the catalytic efficiency of the fabricated NS materials, the reduction of 4-NP to 4-AP was examined in the presence of NaBH₄. 4-NP in aqueous medium exhibited an absorption band centered at 317 nm (Fig. S3(a)†). However, on subsequent addition of freshly prepared solution of NaBH₄ to the 4-NP solution led to a bathochromic shift to 403 nm (Fig. S3(a)†) and the light yellow colour solution changed to intense yellow owing to the formation of 4-nitrophenolate ions in alkaline condition.⁴⁰ In absence of any catalyst, the peak owing to 4-nitrophenolate doesn't undergoes any reduction and the peak at 403 nm remains unaltered even up to 5 days. The yellow colour of 4-NP slowly faded after the addition of 150 μl of 0.01 g of NS materials and finally completely vanished on complete reduction of 4-NP. This decolourization reaction was monitored spectrophotometrically with time. It has been noticed that the peak owing to 4-NP successively decreases with time and there is a simultaneous appearance of a new peak due to 4-AP at 298 nm with progressive increase in intensity (Fig. 11(a) and (b)). The complete reduction of 4-NP was completed within 80 min in case of Ag NS materials and within 70 min in case of Au NS materials.

Fig. 11 (a) represented the absorption spectra for the reduction of 4-NP by NaBH₄ in aqueous medium in presence of synthesized Ag NS materials as catalyst. (b) represented the absorption spectra for the reduction of 4-NP by NaBH₄ in aqueous medium in presence of synthesized Au NS materials as catalyst. (c) depicted the plot of ln(C₀/C_t) versus time required for the reduction of 4-NP using NS materials as catalyst in presence of NaBH₄ in aqueous medium. (d) represented the degradation capacity of the manufactured NS materials for removal of 4-NP.

Considering much higher concentration of NaBH₄ compared to 4-NP, the reaction was found to follow pseudo-first order kinetics and the rate constant was estimated by a linear plot of ln(C_t/C₀) vs. reduction time in minutes and the rate constant was found to be 4.8 × 10⁻² and 5.71 × 10⁻² min⁻¹ respectively for Ag and Au NS materials (Fig. 11(c)). About 96.8 and 97.3% of 4-NP was reduced to 4-AP employing NaBH₄ in presence of Ag and Au NS materials respectively as catalyst (Fig. 11(d)). Hence, both Ag and Au NS materials were found as an efficient catalyst in the reduction of 4-NP in presence of NaBH₄ and analogous to dye degradation rate, the rate of reduction of Au NS materials were found to be higher as compared to Ag NS materials.

3.12 Mechanism of reduction of 4-NP to 4-AP

The mechanism of reduction of 4-NP to 4-AP by NaBH₄ in presence of NS materials is illustrated and represented in ESI (S4).†

4. Conclusions

Herein, we exploited a native fern of north eastern India, Diplazium esculentum for the rapid fabrication of spherical Ag and anisotropic Au NS materials by controlling various reaction parameters. It was perceived that the polyphenolic compounds especially flavonoids have been involved in the reduction of the metal ions into metal NS materials while the proteins were found to be active in the synthesis and stabilization of NS materials owing to the presence of carboxylate and amine moieties. The present study was thus free from the use of any surfactant, solvent, capping agent, template and revealed the dual functional ability of the fern extract solutions. Hence, the aforesaid fabrication methodology is green, simple, economic and environment friendly method which is an improvisation in exploring the plant material for the development of NS materials for sustainable environmental applications.

The NS materials so developed were explored for the remediation of hazardous compounds (MV6B, RB and 4-NP).

The degradation of MV6B, RB and 4-NP followed pseudo first order kinetics with a degradation efficiency of 98.4, 98.2 and 96.8% employing Ag NS materials and 98, 98.9 and 97.3% respectively using Au NS materials as catalyst. The degradation products were analyzed using LC-MS technique and it was observed that the dye initially undergoes cleavage of one or more of the methylene groups substituent on the amine group in case of MV6B while removal of oxy group occurred in case of RB resulting in the formation of some intermediate products. The exhausted NS materials were effectively regenerated and were analyzed using TEM micrographs. The renewed NS materials also demonstrated dye removal efficiency of 98, 97.5 and 97% for MV6B and 97.5, 97 and 96% for RB employing Ag NS materials while 97.5, 96 and 95.7% for MV6B and 98.5, 97.3 and 96.2% for RB using Au NS materials respectively for 1^st, 2^nd and 3^rd rounds of regeneration cycles.

These high competences of the NS materials have presented a promising and effective treatment methodology for the removal of dyes and phenols from the industrial effluents.

Therefore, the present study has unfastened a pioneering way for synthesizing NS materials of varied size and morphology and their applicability for the remediation of hazardous compounds. Hence, the fabrication of these NS materials by this technique and their utilization in the abatement of industrial effluents are quite justified.

Acknowledgements

Tanur Sinha is grateful to NIT Silchar for providing the financial assistance and SAIF-NEHU Shillong, SAIF-IIT Bombay and CSMCRI-Gujarat for providing the TEM, FTIR, LC-MS EDAX and XRD facilities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26124d

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