Green and Environmentally Sustainable Fabrication of Ag-SnO2

Research Article pubs.acs.org/journal/ascecg

Green and Environmentally Sustainable Fabrication of Ag-SnO2 Nanocomposite and Its Multifunctional Efficacy As Photocatalyst and Antibacterial and Antioxidant Agent Tanur Sinha,*,† Md. Ahmaruzzaman,‡ Partha Pradip Adhikari,§ and Rekha Bora∥ †

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra−400076, India Department of Chemistry, National Institute Of Technology Silchar, Silchar−788010, Assam, India § Genoine Research Laboratory Pvt. Ltd., Subhash Nagar, Karimganj, Assam−788710, India ∥ Laboratory of Ethnobotany & Medicinal Plants Conservation, Department of Ecology & Environmental Science, Assam University, Silchar−788011, India ‡

S Supporting Information *

ABSTRACT: Herein, we describe a phytosynthetic, additive-free, economically viable, environmentally sustainable and rapid methodology for the formation of sphere-shaped Ag-SnO2 nanocomposites of 9 nm average particle size employing the stem extracts of Saccharum officinarum. Employing various spectroscopic techniques, the morphology, size, crystallinity, elemental conformation, and functional groups liable for surface stabilization as well as capping were depicted. Considerably, the Ag-SnO2 nanocomposite in aqueous phase revealed excellent removal efficiency for the abatement of four industrially emerging pollutants (Methylene Blue, Rose Bengal, Methyl Violet 6B, and 4-nitrophenol) and probable mechanisms were also suggested. Nearly, 99.1, 99.6, 99.5, and 98.4% of Methylene Blue, Rose Bengal, Methyl Violet 6B, and 4-nitrophenol were eradicated respectively, within 60, 75, 75, and 58.3 min using the synthesized nanocomposite. Moreover, the spent nanocomposites were renewed and their photocatalytic proficiencies were assessed for three consecutive cycles. The spent nanocomposite and the degraded products were respectively analyzed using X-ray diffraction and liquid chromatography-mass spectrometry spectroscopic methods. Additionally, the nanocomposite displayed comparative antimicrobial action against Pseudomonas aeruginosa, Escherichia coli, and Bacillus subtilis and indicated fair activity on 2,2-diphenyl-1-picrylhydrazyl scavenging with IC50 values 0.73 mM depicting its efficient antimicrobial and antioxidant activity. Thus, the present article has disclosed a revolutionary way for fabricating Ag-SnO2 nanocomposites and depicted their multifunctional efficacy as photocatalysts and reducing and prospective antibacterial and antioxidant agents. KEYWORDS: Phytosynthesis, Photocatalyst, Reduction, Antibacterial and antioxidant

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INTRODUCTION Currently, nanocomposite fabrications have gained the focus of the researchers worldwide, the most notable being the nanocomposite formed by combining a semiconductor metal oxide with a noble metal nanoparticle.1−8 Consequently, the silver-stannous oxide (Ag-SnO2) nanocomposites have attracted considerable attention because of their wide range of potential applications arising owing to their distinctive optical and electrical properties.9−11 Not only this, these nanocomposites have good stability, nontoxicity, low cost, chemical inertness, and high activity but also have adequate arc resistance, enough protection regarding contact welding, lower material migration, excellent over temperature perform© 2017 American Chemical Society

ance, jointing properties, and practical processing and serve as excellent photocatalysts for the eradication of detrimental organic substances.12 With the development of nanotechnology, there have been numerous reports on their fabrications employing synthetic methodologies,13 and these methodologies are not only energy and capital intensive but also utilize hazardous chemicals and solvents whose discharge into the environment affect the biosphere and cause human health hazards. Henceforth, there is Received: December 27, 2016 Revised: March 29, 2017 Published: April 13, 2017 4645

DOI: 10.1021/acssuschemeng.6b03114 ACS Sustainable Chem. Eng. 2017, 5, 4645−4655

Research Article

ACS Sustainable Chemistry & Engineering

as well as very stable in water. Henceforth, its complete remediation is compulsory owing to ecological distress. Scientist have established several methodologies for the its removal, but all these procedures either employ toxic solvents or are capital and energy intensive. So, far only one reaction which is found to be simple and convincing is the catalytic reduction of 4-NP to 4-AP (4-aminophenol) in aqueous medium employing fabricated nanostructured materials in the presence of sodium borohydride (NaBH4) as a reducing agent. Additionally, at present awful escalation of multidrug resistant strains has increased considerably which is found to be dreadful to human health. These outrageous strains not only have the capacity to quench the strength of antibiotics but also sometimes negative effects of present day antibiotics are observed in patients. Therefore, there is an urgent necessity to search for effective antimicrobial substances those can overcome the aforementioned limitations. In this perspective, antimicrobial screening is found to be a new effective antimicrobial agent which have less negative effect on human body. Further, currently, many hazardous human diseases like cancer, liver and kidney damage, atherosclerosis, inflammatory joint disease, asthma, and gastritis have been increased and antioxidants can act as a safeguard to the human body by protecting it from active oxygen species and free radicals that can inhibit or slow down the expansion of these dreadful diseases.25 Consequently, to overcome this risk, there is an urgent need to avail ourselves of such antioxidants in the present day pharmacological field to strengthen human health. Therefore, in this study we present a facile, environmentally friendly, green, and economical process for the fabrication of Ag-SnO2 nanocomposites employing juice extract of Saccharum and evaluated their applicability as catalytic agents for the eradication of perilous compounds (MB, RB, MV6B, and 4NP). The antioxidant ability and antibacterial capabilities of the fabricated nanocomposites were also portrayed. Further, the manufactured nanocomposites were analyzed via numerous techniques, such as UV−visible spectroscopy (UV− vis spectroscopy), Fourier transformer infrared spectroscopy (FTIR spectroscopy), transmission electron microscopy (TEM), selected area electron diffraction (SAED) studies, and energy dispersive X-ray spectroscopy (EDAX).

a growing need to develop greener protocols for metal−metal oxide fabrications that avoid the utilization of stoichiometric reagents and solvents, circumvent waste byproducts, and exploit the available renewable resources available in nature.14−16 In this context, the phytosynthesis of nanocomposites, which uses plants for fabrication, has become an advantageous and profitable approach as compared to methods that are synthetic or employ electrochemically active microorganisms.17 Further, this method is more rapid and economically favorable and effortlessly could be employed for the large scale fabrication of the nanocomposite.18 Subsequently, for nanocomposites, the phytosynthetic protocols are found to be more advantageous over routine synthetic protocols. Hence, herein, Ag-SnO2 nanocomposites were fabricated employing a phytosynthesis procedure that exploits the stem extract of Saccharum officinarum. Saccharum officinarum, a crop, is a native to the family poaceae. They have firm interconnected stems which are enriched in sugar and are essentially gathered at internodes. The juice comprises of reducing, nonreducing sugars, organic acids like oxalic acid, malic acid, succinic acid, citric acid, and Dgluconic acid, and inorganic ions such as sodium, calcium, and chloride which may be envisaged for the formation of the AgSnO2 nanocomposite.19,20 Moreover, at present, fast urbanization, rapid industrialization, and modified agricultural activities have enhanced the interaction of mankind with the environment leading to contamination of air, water, and soil thereby affecting the human health and causing ecological and environmental disarray.21 A major portion of such water and air pollutants is constituted by the effluents from the textile industries and by aromatic hazardous nitro compounds which are considered as the primary contaminant in industrial wastewater. Moreover, these compounds are carcinogenic and hazardous and are a substantial basis for nonaesthetic contamination. Further, the existence of these perilous compounds in water can deteriorate the light infiltration ensuing in lesser photosynthetic capacity, thereby making oxygen inaccessible for biological degradation of microbes in water which yields harmful waste materials via further oxidation, hydrolysis, or other chemical reactions in the wastewater.22 Hence, they are menace to both humanoid and aquatic biomes. Consequently, their entire disarticulation is compulsory. Hence, herein we selected three different categories of dyes, namely; Methylene Blue (MB), a basic aniline dye which is lethal and once inhaled causes nausea, vomiting, cardiovascular, genitor urinal and hematological problem; Rose Bengal (RB), an Xanthene dye, whose dissipation causes numerous harmful diseases to the stomach and liver; and Methyl Violet 6B (MV6B), a triphenyl methane dye, which is carcinogenic and mutagenic and a mitotic poison.23 Subsequently, all these three dyes are a menace to all existing creatures in the environment and hence their remediation is obligatory and photodegradation in the presence of an apt nanocatalyst is considered as the most operative methodology for their abatement. Besides these toxic textile dyes, nitrophenol and its derivatives especially 4-nitrophenol (4-NP) are also a threat to the environment. 4-NP is recognized to be anthropogenic, noxious, and inhibitory in nature.24 Their revelation leads to headaches, vomiting, lethargy, cyanosis, and impairment to liver, central nervous system, kidney, and damage to blood of both human beings and animals. Additionally, 4-NP is soluble

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EXPERIMENTAL METHODS

Materials. Stannous chloride dihydrate (SnCl2·2H2O) and silver nitrate (AgNO3), Rose Bengal (RB), Methyl Violet 6B (MV6B), Methylene Blue (MB), and sodium borohydride (NaBH4) of AR grade were acquired from Sigma-Aldrich and was used as obtained without further purification. In all the experiments, double distilled water was used. Moreover, for antibacterial study, bacterial strains of Gram-negative bacteria Pseudomonas aeruginosa (MTCC 1688) and Escherichia coli (MTCC 1195) and Gram-positive bacteria Bacillus subtilis (MTCC 1427) and Streptococcus pneumoniae (MTCC 2672) were acquired from IMTECH (Institute of Microbial Technology), Chandigarh, Punjab, and Haryana, India. The stains were subcultured, based on requirements, by MHB (Muller Hinton Broth) and maintained at 4 °C. Saccharum officinarum extract was made as per the process mentioned here. Preparation of Saccharum officinarum Extract. Sugar cane juice was obtained by crushing local Saccharum officinarum (sugar cane) cane in a double roller juice crushing machine. Collected juice was filtered using Whatman filter paper no. 41. The filtered raw juice so obtained was used for the synthesis. 4646


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was added to each well and incubated for 37 °C for 1 day. After incubation, the progress of inhibition was observed on the well dishes, where the minimum concentration that did not have any turbidity was taken as MIC. Minimum Bactericidal Concentrations (MBC). For MBC, 10 μL of broth medium were taken from each well of the MIC micro plate. These were spread on sterile MHA plates to incubation at 37 °C for 1 day. The lower most concentration was taken for bacteriological progress on the agar plates and selected for MBC. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) Radical Scavenging Assay. The free radical scavenging action of Ag-SnO2 nanocomposite was estimated by means of the Brand−Williams method. The reaction was done in a dark place at room temperature maintaining an incubation period of 30 min. Absorbance was observed at 517 nm. The experiments were carried out in triplicates using the following formula.

Synthesis of Ag-SnO2 Nanocomposite. At the outset, 200 mL of 0.01 M SnCl2·2H2O and 0.0001 M AgNO3 were treated with 20 mL of Sugar cane extract in a 500 mL round bottomed flask maintained at pH = 10 and refluxed with constant heating and stirring at 100 °C for 4 h. A dark brown precipitate was observed which was stabilized at room temperature for 24 h. Thereafter, the dark brown precipitate was centrifuged and washed thrice with distilled water. The precipitate was desiccated at 60 °C and calcined at 200 and 400 °C for 2 h. Characterization of the Nanocomposite. A Cary 100 Bio spectrophotometer (k max in nanometers) laced with 1 cm quartz cell was utilized to record the optical absorption spectra of the fabricated nanocomposite. The micrographs of TEM and HR-TEM, as well as the SAED pattern, were noted employing JEOL-JEM 2100 transmission electron microscope and a FEI Tecnai G2 F 20 run at an accelerating voltage of 200 kV. The elements in the nanocomposite were analyzed using an energy dispersive X-ray (EDX) spectral analyzer. The Bruker Hyperion 3000 FTIR spectrometer was used to record the FTIR spectra. The XRD patterns were studied and recorded with the help of Phillips X’Pert Pro diffractometer (Cu Ka radiation of wavelength 1.5418 Å). Evaluation of the Photocatalytic Activity. The aqueous solutions of MB, RB, and MV6B were taken to assess the photocatalytic competency of the synthesized Ag-SnO2 nanocomposite. Herein, 10 mg of the fabricated composite were dispersed separately in 200 mL of 10−4 M solutions of the three different dyes. The suspended solutions were then kept in the dark for 1 h, to achieve adsorption−desorption equilibrium of the dye on the surface of the composite. Thereafter, these dyes were exposed to solar irradiation (average intensity ∼40−50 mW cm−2) on a bright sunny day at Silchar city (24° 49′ N, 92° 48′ E) between 10 a.m. and 3 p.m. (atmospheric temperature 32−36 °C). At set time intervals, 4 mL of aliquots were drawn out and centrifuged at once. The advancement of the reaction was observed with the help of UV−vis spectroscopy at regular durations. Catalytic Activity of the Nanocomposite. To assess the efficiency of the present nanocomposite as catalyst, the transformation of 4-NP to 4-AP in aqueous medium was executed in the presence of NaBH4 at room temperature. The absorbance of 4-NP (60 μL, 6.07 × 10−3 M) in 2.6 mL of water (taken in a standard quartz cuvette having 1 cm path length) was monitored with the assistance of UV−visible spectrometer. An aqueous solution of NaBH4 (350 μL, 0.1 M) was added to the aforementioned 4-NP solution, and the absorption spectra was recorded. Thereafter, the said synthesized nanocomposite (150 μL, 0.01 g) was added to that mixture and the absorbance was recorded until the peak on account of 4-NP faded out completely. Antimicrobial Assay. Antibacterial assay was performed using agar well diffusion technique, with minor modifications. Freshly prepared molten MHA (Muller Hinton Agar) media was put in 9 cm Petri dishes of uniform depths (5 mm) and set to cool at room temperature. After solidification, wells were made on MHA media by 6 mm sterilized cork borer. Here, 1−2 drops of MHA were poured in the bottom with the help of sterile micropipette. A 3 × 108 CFU/mL portion of inoculums were dispersed widely by sterile swab on the solid agar plates. A 50 μL portion of samples of three different concentrations were then filled in three wells. Four antibiotics were taken as positive control i.e., Ciprofloxacin, Ceflexin, Azithromycin, and Metronidazole; whereas, DMSO was taken as negative control. The dishes were then incubated for 24 h, at 37 °C. On completion of incubation, areas of inhibition formed around the wells were measured by a transparent ruler in millimeters. For comparison of the antimicrobial activity of the sample with that of the standards (antimicrobial index) the following formula was used. antimicrobial index =

% inhibition =

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absorbance of control − absorbance of sample × 100 absorbance of control

RESULTS AND DISCUSSIONS UV−vis Spectroscopy Analysis. To analyze the optical properties of the fabricated Ag-SnO2 nanocomposite, the UV− vis absorption spectra were listed. Figure 1 depicts the UV−vis absorption spectra of the Ag-SnO2 nanocomposites synthesized at varied temperatures (200 and 400 °C), respectively. One peak at ∼285 nm and a wide absorption band near the SPR of Ag NPs at about 350−450 nm with increased intensity with the enhancement of temperature were depicted by the spectra.26 In fact, the peak at ∼285 nm was due to SnO2 while the band at

inhibition zone of sample × 100 inhibition zone of standard

Minimum Inhibitory Concentration (MIC). The MIC values were evaluated through the broth dilution method (NCCLS, 2000) with few modifications. Serial dilutions were performed in 96 well micro plates, 50 μL of each concentration was pipetted to individual wells and overnight bacterial culture (1.5 × 108 CFU/mL) on broth

Figure 1. (a) UV−vis absorption spectra of the Ag SnO 2 nanocomposites fabricated at different temperatures (200 and 400 °C). (b) Tauc plot for the band gap of Ag-SnO2 nanocomposites. 4647


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Figure 2. (a−c) TEM and HRTEM and SAED pattern of the synthesized Ag-SnO2 nanocomposite at 200 °C. (d) FTIR spectra of the bare sugar cane extract (represented in black color) and the synthesized Ag-SnO2 nanocomposite (represented in red color). (e) XRD pattern of the synthesized Ag-SnO2 nanocomposite. (f) EDAX spectra of the synthesized Ag-SnO2 nanocomposite.

350−450 nm was attributed to silver particles.27 Thus, the absorption spectra clearly revealed the formation of Ag-SnO2 nanocomposites. Moreover, the band gap energy of the fabricated nanocomposite was assessed using the Tauc equation which is given as α hν = C(hν − Eg )2

sented by Figure 2a and b, respectively. The TEM pattern clearly depicted the formation of sphere shaped Ag-SnO2 nanocomposite having an average particle diameter of 8−10 nm, while the HRTEM pattern disclosed the typical profile view of the Ag NPs decorated over the SnO2 matrix. Additionally, the (111) plane of the face centered cubic structure of Ag with a gap between contiguous planes of 0.24 nm and the (110) plane of tetragonal structure of SnO2 with spacings between adjacent lattice of 0.32 nm was revealed by the HRTEM micrographs. Therefore, the HRTEM images clearly illustrated that the Ag NPs are decorated over the SnO2 surface through an interface having continuous lattice fringes between the Ag NPs and SnO2 NPs. The SAED pattern of Ag-SnO2 nanocomposite is depicted by Figure 2c. The pattern clearly depicted concentric diffraction

(1)

Where, α = absorption coefficient, C = constant, hν = photon energy, and Eg = band gap energy. The plot of (αhν)1/2 vs hν is depicted in Figure 1b. The band gap of Ag-SnO2 nanocomposite was found to be 2.8 eV, which was in accordance with the previous studies.27 TEM and SAED Studies. The TEM and HRTEM micrographs of the synthesized nanocomposite were repre4648


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ACS Sustainable Chemistry & Engineering Table 1. FTIR Spectra of Saccharum officinarum Stem Extracts and Ag-SnO2 Nanocomposite FTIR Bands (cm−1) samples Saccharum officinarum stem extracts Ag-SnO2 nanocomposite

νO−H with νN−H

νCO of amide

νC−O−H bending of carboxylic acid/in plane νO−H bending

νN−H wagging/out of plane νO−H bending

3385

1625

1012

562

3398

1652

1106

602 (Sn−O−Sn stretching)

tetragonal crystalline structure of SnO2 (JCPDS 41-1445) while the other peaks at 2θ values = 38.2, 66.4, and 78.7°, marked with # were assigned to the (111), (220), and (311) lattice planes of face centered cubic structure of Ag (JCPDS 040783).31,32 This indicated the immobilization of Ag NPs on the surface of SnO2 NPs. Moreover, some other additional peaks were also obtained, which was due to the crystallization of the bioorganic phases owing to that of the Saccharum officinarum stem extract.33 Furthermore, the average crystallite size of the nanocomposite was measured employing the Debye−Scherrer equation:34

circles that exhibited the polycrystalline behavior of the nanocomposite. Two phases were noticed; one analogous to fcc structure of Ag-NPs which could be labeled as (111) and (200) lattice planes (JCPDS 04-0783) while the other corresponds to reflections from (110), (101), and (211) lattice planes of tetragonal crystal structure of SnO2 (JCPDS 411445). Thus, the SAED results were in accordance with the TEM pattern and indicated the formation of Ag-SnO 2 nanocomposite with fcc and tetragonal structure of Ag and SnO2 NPs, respectively. FTIR Studies. The FTIR studies were executed to identify the functional groups accountable for the fabrication and stabilization of the Ag-SnO2 nanocomposite (Figure 2d). The FTIR peaks accounted for the stem extract and the manufactured Ag-SnO2 nanocomposite are portrayed in Table 1. The important bands that were recognized in the IR spectra of the Saccharum officinarum stem extracts were at 3385, 1625, 1012, and 562 cm−1 and were attributed to hydrogen bonded O−H stretching vibration of phenol superimposed on N−H stretching, CO stretching due to biomolecules containing −COO groups or maybe owing to amide, in plane O−H bending vibration or C−O−H bending of carboxylic acid, and out of plane O−H bending vibration or N−H wagging, respectively.28 However, for the fabricated Ag-SnO2 nanocomposite, the band ascribed to the hydrogen bonded O−H stretching vibration of phenol superimposed on N−H stretching altered to 3398 cm−1 and became comparatively strong and extensive. Moreover, the peak owing to CO stretching of −COO groups or amide and in plane O−H bending vibrations or C− O−H bending of carboxylic acid changed to 1652 and 1106 cm−1, respectively, for the fabricated nanocomposite. Moreover, the band at 562 cm−1 transformed to 602 cm−1 demonstrating that N−H wagging or out of plane O−H bending vibration has been merged with the Sn−O−Sn stretching of surface bridging oxide developed through the condensation of neighboring hydroxyl moieties depicting the existence of SnO2 moiety.29 Henceforth, the aforementioned data showed the existence of O−H, COOH, NH2 functional moieties in the Saccharum officinarum stem extracts. These functional moieties were due to glucose, sucrose and various organic acids, like oxalic acid, malic acid, succinic acid, citric acid, and D-gluconic acid present in the Saccharum officinarum stem extract,30 and the peak transformations were related to O−H, COOH, and NH2 groups demonstrating that these functional moieties were responsible for the synthesis and stabilization of the Ag-SnO2 nanocomposite. XRD Studies. The crystal structure and the composition of the Ag-SnO2 nanocomposite were characterized employing the XRD pattern (Figure 2e). Two sets of XRD patterns were noticed. The peaks at 2θ values = 26.8, 34, and 51.9°, marked with * were ascribed to the (110), (101), and (211) planes of

D = Kλ /β cos θ

(2)

where, D = average crystallite diameter, β = full width halfmaximum (fwhm) of the highest intensity peak ((110) peak), K = shape factor of particle (= 0.89), θ = diffraction angle, and λ = wavelength of X-rays. The average crystallite diameter was found to be 9 nm, in accordance with the TEM results. EDAX Studies. To identify the elements present in the nanocomposite, the EDAX analysis was carried out (Figure 2f). The spectra clearly depicted strong signals at 3.4 and 0.5 keV attributed to tin (Sn) and O, respectively.35 Moreover, a weak peak at 2.8 keV was also noticed and was owing to the surface plasmon resonance of Ag NPs.36 The atomic and weight proportions of elements achieved from the EDAX studies were displayed in Table 2, and the outcomes were found to be in decent accordance with the modified content. Table 2. EDAX Elemental Analysis of Ag-SnO2 Nanocomposite sample name

weight %

atomic %

keV

Ag-SnO2

O = 62.07 Sn = 37.09 Ag = 0.85

O = 60.00 Sn = 39.05 Ag = 0.95

O = 0.5 Sn = 3.4 Ag = 2.8

X- Ray Elemental Mapping. The X-ray elemental plotting of Sn, O, and Ag was carried out by EDS area scanning to know further about the elemental distribution of the fabricated AgSnO2 nanocomposite and was represented by Figure 3. The spectra clearly revealed that the Ag NPs was homogeneously distributed over the SnO2 surface. Also, the X-ray elemental mapping of Sn, O, and Ag were well-defined with sharp contrast. Accordingly, these results provided evidence that the SnO2 surface was successfully decorated by Ag and confirmed the formation of Ag-SnO2 nanocomposite. Role of Saccharum officinarum Extract. The biological molecules available in the Saccharum officinarum juice not only acts as a capping agent but also as complexing agent. Actually, the biological molecules can behave as ligand and can associates along the precursor metal ion forming complex.37 These biomolecules when acted as capping agents then they 4649


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Figure 3. X-ray elemental mapping of (a) Sn−L, (b) O−K, and (c) Ag−L.

where Ct = concentration of the dyes at time t and C0 = concentration of the dyes at time 0, k = rate constant, and t = time (min). Figure 4f represented the graph of ln(C0/Ct) vs radiation time t for the decay of dyes. The sketch displayed a linear design, and thus, the slope of the line is represented the rate constant (k) for the decay of dyes. The value of k was found to be 9.2 × 10−2, 7.6 × 10−2, and 7.9 × 10−2 min−1 for MB, RB, and MV6B, respectively. Here, the rates for photodegradation of dyes employing AgSnO2 nanocomposite were highest compared to any other metallic or bimetallic nanostructured materials.39 It was assumed that the Ag acts as an electron basin and increased the isolation of the photogenerated electron−hole pairs considerably and inhibited their reunion resulting in higher catalytic activity of the Ag-SnO2 nanocomposite compared to other nanostructured materials.39 Probable Mechanism for the Photocatalytic Activity of the Fabricated Ag-SnO2 Nanocomposite. The mechanism liable for the abatement of dye contaminants employing Ag-SnO 2 nanocomposite underneath solar radiation is suggested and demonstrated in Scheme 1. Usually, when two substances with dissimilar work functions are united, a Schottky barrier is formed. As a result of which, electrons are transferred to create a fresh Fermi energy level until the two levels attain equilibrium.40 Next, in the space charge area nearby the interface a built-in electric field is created owing to the equilibrium orientation of the Fermi level of the metal oxide and metal nanocomposite. Subsequently, the distance between the photogenerated electrons and holes increases which ultimately improves the photocatalytic capability. When exposed to solar radiation, the Ag-SnO2 nanocomposite is photoexcited owing to the SPR of Ag NPs and undergoes plasmonic decay.41 Then, due to decay of the plasmon, the holes and electrons are produced. Next, through the interface between the Ag and SnO2, the photoexcited electrons of the Ag get rapidly transferred to that of SnO2 surface and then these injected electrons are trapped by the with O2 and H2O molecules to generate active particles; anionic super oxide radical (O2−·) and hydroxyl radical (OH·), respectively. Lastly, the superoxide ion (O2−·) undergoes protonation forming hydro peroxyl radical (HO2·) which then converts to H2O2 and eventually splits into extremely active hydroxyl radicals (OH·). Finally, on the surface of the photocatalyst, both oxidation and reduction occurs. On the other hand, few photoinduced holes on the surface of the Ag can react with H2O molecule to furnish hydroxyl radical which induces the mineralization of the dyes.

suppressed the growth of the nucleated particles and led to the formation of the nanocomposite.38 The nucleated ligands were set apart from one another by the steric repulsive forces which prevented the aggregation. Hence, these biomolecules not only stabilized the fabricated nanocomposite but also prevented these from agglomeration and the results were also supported by the FTIR studies. Therefore, the biomolecules available in the juice played the twofold role of capping and stabilizing agent in the fabrication of Ag-SnO2 nanocomposite. Evaluation of the Photocatalytic Activity of the Fabricated Ag-SnO2 Nanocomposite. The photocatalytic activity of the synthesized Ag-SnO2 nanocomposite was assessed by choosing three distinct categories of dyesMB, RB, and MV6Bin water medium underneath solar radiation. The decay of dyes did not begin instantaneously, and the process was noted by observing variations in the UV−visible spectra of the reaction mixture, which was made free from the catalyst via centrifugation. At the outset, a control test was performed to validate the photocatalytic capability of the synthesized Ag-SnO2. It was witnessed that in dark environments in the presence of the nanocomposite, all the dye solutions exhibited minor degradation (Supporting Information Figure S1a, c, and f). Moreover, when the degradation procedure was scrutinized in the presence of solar radiation deprived of any catalyst, no degradation, i.e., no photochemical reaction, was detected (Figure S1b, d, and e). Henceforth, the outcome showed that sunlight and photocatalyst are equally necessary for the proficient remediation of these toxic dyes. Thus, the degradation process was carried out in the presence of the nanocomposite underneath sunlight and the absorbance was recorded. It was observed that the optical absorption bands corresponding to MB, RB, and MV6B at 664, 540, and 580 nm showed rapid degradation and decolourized completely after 60, 75, and 75 min, respectively (Figure 4a−c). Figure 4d displayed the percentage proficiency of photodegradation of dye with time. It was perceived that around 99.1%, 99.6%, and 99.5% of MB, RB, and MV6B was removed, respectively, within 60, 75, and 75 min in the presence of solar radiation employing the synthesized Ag-SnO2 nanocomposite. Therefore, it was apparent that the rate of decay of dyes exclusively relied on the chemical framework of the target dye. The rates of decay of these dyes in the presence of nanocomposite followed a pseudo-first order reaction, and their kinetics may be represented as follows:38 ln(C0/Ct ) = kt

(3) 4650


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Figure 4. Absorption spectra for photocatalytic degradation of (a) MB, (b) RB, and (c) MV6B using Ag-SnO2 nanocomposite. (d) Degradation capacity of the manufactured Ag-SnO2 for MB, RB, and MV6B. (e) Plot of ln(C0/Ct) vs irradiation time t for the degradation of dyes. (f) Absorbance spectra of 4-NP and 4-NP + NaBH4 in aqueous medium. (g) Absorption spectra for the reduction of 4-NP by NaBH4 in aqueous medium in the presence of synthesized Ag-SnO2 nanocomposite as catalyst. (h) Plot of ln(C0/Ct) versus time required for the reduction of 4-NP using Ag-SnO2 nanocomposite as catalyst in the presence of NaBH4 in aqueous medium. (i) Degradation capacity of the manufactured Ag-SnO2 nanocomposite for removal of 4-NP.

Hence, the whole degradation procedure can be represented by the Scheme 1, and the associated reactions are depicted in eqs 4−12.

O2 + e− → O2− ·(ads)

(6)

O2− ·(ads) + H+ ⇄ HOO·(ads)

(7)

NS + hν → h+(NS) + e−(NS)

(4)

2HOO· (ads) → H 2O2 (ads) + O2

(8)

H 2O(ads) + h+ → OH ·+H+(ads)

(5)

H 2O2 (ads) → 2OH·(ads)

(9)

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ACS Sustainable Chemistry & Engineering Scheme 1. Photodegradation Process Employing Ag-SnO2 Nanocomposite under Solar Irradiation

Figure 5. (a) Antibacterial activity of Ag-SnO2 against (A) Bacillus subtilis, (B) Escherichia coli, (C) Pseudomonas aeuginosa, (D) Streptococcus pneumonia where concentrations are (a) 0.01; (b) 0.09; (c) 0.116 M. (b) Effect of antibiotics on four human pathogens Bacillus subtilis (Bs), Escherichia coli (Ec), Pseudomonas aeuginosa (Pa), and Streptococcus pneumonia (St pn). CP = Ciprofloxacin, CX = Ceflexin, AZ = Azithromycin, and MT = Metronidazole. (c) Effect of Ag-SnO2 nanocomposite on four human pathogens Bs, Ec, Pa, and St pn. (d) DPPH radical scavenging activity of Ag-SnO2 nanocomposite. (e) DPPH radical scavenging activity of Quercetin. The data presents the percentage of inhibition of the standard antioxidant.

dye + OH· → CO2 + H 2O

(dye intermediates)

dye + h+ → oxidation products

(10) 4652

(11)


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ACS Sustainable Chemistry & Engineering Table 3a. Antimicrobial Index of Ag-SnO2 Nanocompositea Ciprofloxacin (0.1 M)

a

Ceflexin (1 M)

Azithromycin (1 M)

Metronidazole (0.1 M)

strains

0.01 M

0.09 M

0.116 M

0.01 M

0.09 M

0.116 M

0.01 M

0.09 M

0.116 M

0.01 M

0.09 M

0.116 M

Bacillus subtilis Escherichia coli Pseudomonas aeuginosa Streptococcus pneumoniae

ND ND ND ND

ND 41.45 ND 41.45

98.80 107.31 100 96.34

ND ND ND ND

ND 73.90 ND 82.94

ND 191.32 208.55 192.75

ND ND ND ND

ND 41.45 ND 44.72

ND 107.31 124.04 103.94

ND ND ND ND

ND ND ND 106.28

ND ND 150.78 246.99

All data are expressed in percentage. ND = not detected.

dye + e− → reduction products

Evaluation of the Stability and Recyclability Test of 4NP. The stability and recyclability test of 4-NP is illustrated in the Supporting Information (S6). Antibacterial Activity of the Fabricated Ag-SnO2 Nanocomposite. Cell wall of Gram negative bacterium is multilayered while Gram positive bacterium is single layered.44 This may be the reason that in most of the cases Gram positive bacterium are more vulnerable to substances having antibiotic property than the Gram negative bacterium.45,46 In fact, in this study Ag-SnO2 nanocomposite showed more or less similar activity against the four pathogenic bacteria. The outcomes of the current work unveil that the fabricated nanocomposite have an excellent antibacterial property which can able to serve as a bactericidal agent. Effects of Ag-SnO2 nanocomposite on the four human pathogenic bacteria are shown in Figure 5, and the antibacterial index is shown in Table 3a. Activities of four standard antibiotics are presented in Figure 5b. MBC and MIC values were examined and are tabulated in Table 3b. The antimicrobial index of the samples against four standard antibiotics was also calculated those were presented in the Figure 5c.

(12)

Evaluation of the Photostability of the Ag-SnO2 Nanocomposite. Recycling reactions were executed for degradation of MB, MV6B, and RB to elucidate the photostability of the catalyst. In each investigation, from the solution, Ag-SnO2 nanocomposite (catalyst) was isolated, washed using ethanol and desiccated in vacuum.42 Ag-SnO2 nanocomposite exhibited exceptional stability even after three consecutive turns and the outcomes are described in the Supporting Information (S2). Identification of the Intermediate Products of Dye Degradation. Employing the LC-MS (liquid chromatography−mass spectroscopy) method, degraded products were investigated and identified by relating with commercial standards and from the fragments of the mass spectra. All the recognized degraded products are described in the Supporting Information (S3). Catalytic Reduction of 4-NP to 4-AP Employing AgSnO2 as a Catalyst in Aqueous Medium. The catalytic reductive proficiency of the synthesized nanocomposite was scrutinized by studying the transformation of 4-NP to 4-AP in attendance of NaBH4. An absorption peak at 317 nm was revealed by 4-NP in water medium. (Figure 4f). Further, on successive adding of newly synthesized solution of NaBH4 to the 4-NP solution led to a red shift near 403 nm (Figure 4f) and due to the generation of 4-nitrophenolate ions in basic condition, the pale yellow color solution transformed to deep yellow colored solution.43 Without any catalyst, the band owing to 4-nitrophenolate does not undergo any transformation and the band at 403 nm remain unchanged even for 5 days (S4). When the nanocomposite was added (0.01 g, 150 μL), the yellow colored solution of 4-NP gradually diminished and entirely disappeared on whole transformation of 4-NP to 4-AP. This reaction was supervised using UV−visible absorption spectroscopy with time. The band due to 4-NP successively diminishes with time and an instantaneous arrival of a new band at 298 nm owing to 4-AP with progressive increase in intensity was noticed. The Ag-SnO2 nanocomposite was found to completely reduce the 4-NP to 4-AP within 3500 s−1 (Figure 4g). Not only this, the reduction employing Ag-SnO2 nanocomposite also followed pseudo-first order kinetics with rate constant as 1.9× 10−3 s−1 (Figure 4h). It was noted that 98.4% 4-NP was transformed to 4-AP using NaBH4 in attendance of Ag-SnO2 nanocomposite (catalyst) (Figure 4i). Therefore, Ag-SnO2 was established as an effective catalyst in the transformation of 4-NP to 4-AP. Mechanism of Reduction of 4-NP to 4-AP. The reductive mechanism for transformation of 4-NP to 4-AP by NaBH4 in attendance of Ag-SnO2 nanocomposite is demonstrated as well as presented in Supporting Information (S5).

Table 3b. Minimum Inhibitory Concentration (MIC) Minimum and Bactericidal Concentrations (MBC) of AgSnO2 Nanocompositea sample Ag-SnO2 nanocomposite

a

bacterial strains Bacillus subtilis Escherichia coli Pseudomonas aeuginosa Streptococcus pneumoniae

MIC (M)

MBC (M)

0.075 ± 0.005 0.04 ± 0.01 0.07 ± 0.01

0.1075 ± 0.0005 0.072 ± 0.002 0.1095 ± 0.0005

0.025 ± 0.015

0.055 ± 0.005

The values are mean of two independent replication ± SE.

Antioxidant Study (DPPH Radical Scavenging Study). The study indicates the average activity of the fabricated nanocomposite. The result revealed that the IC50 value is 0.73 mM as illustrated in Figure 5d. In this test, Quercetin, a standard antioxidant was also taken for comparing with the result of the sample (Figure 5e). The IC50 value of Quercetin was found to be 0.025 mM. The radical scavenging activity of the nanocomposite is presented in the Figure 5d.

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CONCLUSIONS This work reports a green, ecofriendly, economically viable, and additive free (capping agent, solvent, templates) methodology for the rapid fabrication of Ag-SnO2 nanocomposite employing the stem extracts of Saccharum officinarum. The juice comprises of reducing, nonreducing sugars, organic acids like oxalic acid, malic acid, succinic acid, citric acid, and D-gluconic acid, and inorganic ions such as sodium, calcium, and chloride which may be envisaged for the formation of the Ag-SnO2 nanocomposite. 4653


ACS Sustainable Chemistry & Engineering Thus, the study depicted the multifunctional abilities of the juice extract. Hence, the phytosynthetic protocols are an improvisation in employing the plant material for the synthesis of nanocomposite for sustainable ecofriendly applications. The Ag-SnO2 nanocomposite thus synthesized was exploited for the abatement of four industrially emerging pollutants (MB, RB, MV6B, and 4-NP) from aqueous phase and was also used as an antibacterial and antioxidant agent. The rates for the eradication of the contaminants proceeded via pseudo-first order kinetics with removal efficiency of 99.1, 99.6, 99.5 and 98.4% respectively for MB, RB, MV6B, and 4-NP. Further using LC-MS techniques, the degraded products were analyzed, and it was assumed that for MB, RB, and MV6B, respectively, the dye first experiences N-demethylation, elimination of the oxy group and demethylenation of the substituent on the amine moiety resulting in further mineralization compounds. The spent nanocomposite were renewed and evaluated employing XRD technique. The renewed nanocomposite revealed dye abatement proficiency of 98.7%, 97.8%, 96.5% for MB, 98.8%, 97.9%, 96.7% for RB, and 98.5%, 97.4%, 96.2% for MV6B, respectively, for the first, second, and third consecutive cycles. Additionally, the nanocomposite displayed comparative action for antimicrobial action against Pseudomonas aeruginosa, Escherichia coli, and Bacillus subtilis and indicated fair activity on DPPH scavenging with IC50 values 0.73 mM. These great proficiencies of the nanocomposite have demonstrated a favorable and efficient treatment procedure for the remediation of phenols and dyes from the industrial emissions. Hence, the current work has opened a revolutionary way for fabricating Ag-SnO2 nanocomposite and their proficiencies for the abatement of perilous compounds and their capability as antibacterial and antioxidant agents. Consequently, the synthesis of Ag-SnO2 nanocomposite by this procedure and its applicability in the removal of industrial discharges and as antibacterial and antioxidant agents are fairly justified.

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ACKNOWLEDGMENTS

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REFERENCES

T.S. is thankful to NIT Silchar for financial assistance and SAIFNEHU Shillong, SAIF-IIT Bombay, and CSMCRI-Gujarat for providing the TEM, FTIR, LC-MS EDAX, X-ray elemental mapping, and XRD facilities. Meanwhile P.P.A. and R.B. are thankful to Institutional Biotech Hub, D.K. College, Mirza, for providing lab facility for antibacterial and antioxidant activities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03114. Evaluation of the photocatalytic activity of the fabricated Ag-SnO2 nanocomposite, photostability of the Ag-SnO2 nanocomposite as catalyst, identification of the intermediate products of dye degradation, absorption spectra for the reduction of 4-NP by NaBH4 in aqueous medium in the absence of any catalyst, mechanism of reduction of 4-NP to 4-AP, evaluation of the stability and recyclability test of 4-NP, table with the recyclability test for Ag-SnO2 nanocomposite. (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tanur Sinha: 0000-0003-4590-9587 Notes

The authors declare no competing financial interest. 4654


Research Article


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Green and Environmentally Sustainable Fabrication of Ag-SnO2

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