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Dec 27, 2016 - Department of Fuel & Mineral Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, 826004 Jharkhand,. India. â...
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Cross-Linked Biopolymer Stabilized Exfoliated Titanate NanosheetSupported AgNPs: A Green Sustainable Ternary Nanocomposite Hydrogel for Catalytic and Antimicrobial Activity Amit Kumar Sarkar,† Arka Saha,⊥ Lipi Midya,† Chiranjib Banerjee,‡ Narayan Mandre,§ Asit Baran Panda,*,⊥ and Sagar Pal*,† †

Polymer Chemistry Laboratory, Department of Applied Chemistry, ‡Department of Environmental Science and Engineering, and Department of Fuel & Mineral Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, 826004 Jharkhand, India ⊥ Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSIR), G. B. Marg, Bhavnagar,364002 Gujarat, India §

S Supporting Information *

ABSTRACT: For the first time, we report in situ exfoliated titanate nanosheet-supported silver nanoparticles (AgNPs) toward environmental sustainability through rapid catalytic reduction of p-nitrophenol (4-NP), organic dyes for decoloration, as well as by inhibiting the growth of microbes. The ternary nanocomposite hydrogel has been synthesized through stabilization of an anionically charged titanate sheet by threedimensional amino-functionalized chemically cross-linked amylopectin for proper growth and stabilization of AgNPs. Here, titanate nanosheets act as an excellent solid support for proper fabrication of AgNPs that could restrict its agglomeration and rapid leaching of AgNPs from the ternary nanocomposite hydrogel. The structural confirmation as well as the stability of titanate nanosheets along with AgNPs have been studied using various characterization techniques. The preparaed nanocomposite hydrogel demonstrates excellent catalytic efficacy and recycling ability toward rapid reduction of toxic 4-NP and decoloration of organic dyes. Notably, the complete reduction of 4-NP could be accomplished within 16 s using 5 mg of as synthesized cl-AP/exf.LT-AgNPs in the presence of excess NaBH4. The excellent catalytic efficiency of the ternary nanocomposite hydrogel arises from the synergistic effects of cross-linked amylopectin stabilized titanate nanosheets and in situ fabrication of AgNPs. Moreover, the strong bactericidal activity (3.2 mg/mL for 107 cells/mL of Escherichia coli and Bacillus subtilis) of the ternary nanocomposite hydrogel would overcome the limitations for removal of water-soluble organic pollutants and microbial contaminants owing to future perspective on environmental sustainability. KEYWORDS: AgNPs, Antimicrobial activity, Catalytic reduction, Ternary nanocomposite hydrogel, Exfoliated titanate nanosheets, 4-NP

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restricted owing to their high surface charges9 and easy aggregation of assembled NPs, which creates low negative Fermi potential with consecutive loss of inherent properties like catalytic activity.10 Among various metal nanoparticles, in particular nanosizedsilver has received ample attention in recent years owing to its low cost, high surface to volume ratio, and excellent catalytic properties and antibacterial activity.11,12 However, simple filtration or centrifugation techniques could not remove AgNPs from reaction media, and their aggregation indeed restricts their wide applicability in environmental remedia-

INTRODUCTION Ever increasing water contamination because of soluble toxic organic contaminants and microbes are creating a scarcity of safe water and waste management.1,2 Hence, it is imperative to develop an environmentally friendly methodology using green, sustainable, and recyclable materials to treat industrial effluents and control the growth of microbial contaminants. In this regard, nanomaterials and metal−organic frameworks have garnered special attention owing to their potential applications in environmental sustainability.3−6 Therefore, the scientific community has given considerable attention to the continuous development of noble metal nanoparticles (NPs)-based sustainable materials because of their unique physical and chemical properties that are significantly different from their bulk.7,8 However, the direct use of metal NPs are certainly © 2016 American Chemical Society

Received: October 28, 2016 Revised: December 16, 2016 Published: December 27, 2016 1881

DOI: 10.1021/acssuschemeng.6b02594 ACS Sustainable Chem. Eng. 2017, 5, 1881−1891

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ACS Sustainable Chemistry & Engineering tion.11 Furthermore, after application, the rapid release of AgNPs into the environment can cause adverse health issues.13 For addressing these concerns, polymeric materials/inorganic solid stabilizer-supported AgNPs provide an alternative choice for stabilization and easy separation of AgNPs. Consequently, the inorganic solid stabilizer not only supports and stabilizes the metal NPs but also enhances the thermal and chemical stability of supported nanocomposites.14,15 For example, solid supports related to graphene and noble metal NP hybrid composites are fabricated to overcome these challenges.16−18 However, extensive research has been performed on graphene-supported metal NPs through proper covalent and noncovalent modifications.19,20 Similarly, layered titanate (LT), an inorganic solid stabilizer, has drawn ample attention owing to its chemical stability and environmentally friendly behavior.21−23 In the recent past, layered titanate-based metal NPs have been fabricated for visible light active photocatalysis behavior,24,25 but properties of LT are highly dependent on the extent of exfoliation of individual nanosheets. The complete exfoliation of the layer through proper ionic interactions is certainly restricted as a result of the presence of higher surface charge of LT. The properties of these LT-based nanocomposites are extremely dependent on the extent of exfoliation of stacked layers to individual layers and interaction of the negatively charged inorganic layer with polymer moiety.26 Thus, before hybridization with polymer, exfoliation of layers is essential, which is dependent on the interlayer charge density. However, the exfoliation of LT is difficult because of its high surface charge density. Fuse et al.27 reported a UV light-durable epoxy resin-layered titanate composite. Asai et al.28 reported a poly(vinylidine fluoride)-layered titanate composite. Guo et al.29 reported a photocatalytically active composite with polyaniline. Sukpirom and Lerner30 reported PEO- and PVP-based nanocomposites. In most of the reported strategies, the exfoliation of the layer was achieved through intercalation of a cationic surfactant-mediated ion exchange mechanism with polymer incorporation. Thus, in the reported composites, the full exfoliation of layers was difficult due to weaker interactions between polymer and titanate sheet. For the first time, we were able to develop stabilized titanate nanosheets through in situ polymerization and exfoliation.26 Amylopectin, a highly branched part of starch, is a renewable, biodegradable polysaccharide31 that acts as a promising biomaterial. The functionalization of amylopectin through cross-linking and stabilization of in situ-formed nanosheets followed by proper fabrication of antibacterial AgNPs is expected to be a sustainable and recyclable green ternary nanocomposite hydrogel. Among the nitoaromatic compounds, p-nitro phenol (4-NP) is considered a prime pollutant owing to its nonbiodegradability and stability in the environment.32 However, in its reduced form, p-amino phenol (4-AP) is extensively used in dye, pigment, pesticide, corrosion industries, and most importantly as an intermediate for synthesis of paracetamol, an analgesic drug.33 Similarly, organic dyes are an important part of modern civilization; however, 100% color removal of organic dyes from aqueous solution with elimination of dye disposal problem is a major challenge, requires more time, or the involvement of a variety of consecutive steps like adsorption along with degradation.34,35 Thus, catalytic reduction is regarded as a useful technique for quick remediation of the hazardous effects of 4-NP and decoloration of organic dyes. Furthermore, the transition metal-catalyzed catalytic reduction of 4-NP is known as a classic reaction to scrutinize the catalytic

activity of noble metal nanoparticles (NPs) in the presence of sodium borohydride.33,36 Polymeric hydrogel-directed, in situfabricated metal NPs were used as catalyst in the presence of NaBH4 (acting as hydrogen source).37 Despite advantages such as the easy handling of hydrogel-based nanocomposite materials, the increment of time toward catalytic reduction has somehow restricted its potential applications,38 and thus, it is imperative to develop an efficient nanocomposite. Beyond the hydrogel based nanocomposites, the porous Au−Pd NPs demonstrate rapid catalytic reduction of 4-NP.39 Very recently, a novel binary nanocomposite system was reported for the adsorption of dyes and reduction of toxic 4-NP to nontoxic 4AP.40 For the dye reduction study, the synthesized AgNPentrapped hydrogel took 30 min for decoloration of organic dyes through reduction.41 The increased reduction time as well as maximum recyclability up to certain cycles restricts the potential application as an efficient catalyst. Thus, proper modification is required to introduce the best properties in hydrogel-based nanocomposite materials. In the present research, stabilization of AgNPs was initiated after titanate nanosheet formation that was stabilized by a porous cross-linked polymeric network. The incorporation of a hydrogel matrix integrates the titanate nanosheet-supported AgNPs in favor of easy handling of the nanocatalyst for recycling without decreasing the efficiency of catalytic reduction. Here, for the first time, we have established a novel synthesis route for the preparation of a ternary nanocomposite hydrogel (cl-AP/exf.LT-AgNPs), which is comprised of a fully exfoliated titanate nanosheet stabilized by amino-functionalized chemically cross-linked amylopectin (i.e., amylopectin cross-linked with [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) in the presence of diethylene glycol dimethacrylate cross-linker, cl-AP) supporting in situ generation of AgNPs. Finally, catalytic efficiency of the nanocomposite was examined toward reduction of 4-NP, methylene blue (MB), and methyl orange (MO) in the presence of excess NaBH4. The observed excellent catalytic activity of the nanocomposite is due to the enhanced area of interaction of titanate nanosheet-supported AgNPs originating from its suitable support for rapid electron transfer into substrate 4-NP/MB/MO. The AgNPs grow steadily under the solid support of titanate nanosheets that were fully exfoliated in the presence of cross-linked amylopectin and prevent aggregation along with leaching of the AgNPs. Hence, the fabricated ternary nanocomposite hydrogel demonstrated outstanding catalytic efficiency toward reduction of 4-NP, MB, and MO, which is probably beyond the state-of-the-art available in the literature.38−52 The sustainability of the nanocomposite hydrogel was further established by removing color from textile industry wastewater. The antibacterial activity of the ternary nanocomposite hydrogel has also been studied against E. coli (Gram-negative) and B. subtilis (Gram-positive) cells by disc diffusion assay and found to be effective toward reducing the total microbial load in pond water.

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EXPERIMENTAL SECTION

Materials. Amylopectin (source: maize) was purchased from Fluka, Switzerland. Titanium isopropoxide, silver nitrate, and [2(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) were procured from Sigma-Aldrich, USA. Potassium persulfate (KPS) was received from Qualigens, Mumbai, India. Diethylene glycol dimethacrylate (DEGDMA) was obtained from TCI, Tokyo, Japan. Acetone, ammonia, ammonium carbonate, sodium hydroxide (E. 1882

DOI: 10.1021/acssuschemeng.6b02594 ACS Sustainable Chem. Eng. 2017, 5, 1881−1891

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Scheme 1. Schematic Illustration for the Synthesis of Fully Exfoliated Titanate-Layered Supported AgNP Ternary Nanocomposite Hydrogel (cl-AP/exf.LT-AgNPs)

Merck, Mumbai, India), p-nitro phenol (4-NP), methylene blue (MB), methyl orange (MO) (Loba Chemie Pvt. Ltd., Mumbai, India), and sodium borohydride (Spectrochem India Pvt. Ltd., India) were of analytical grade and used as received. Millipore double distilled water was used for all experimental work. Synthesis. Preparation of Cross-Linked Amylopectin (cl-AP). A grafting-from polymerization approach initiates the formation of crosslinked hydrogel. The polymerization was carried out in the presence of nitrogen atmosphere using a three-necked round-bottom (RB) flask. The RB flask was placed on an oil bath with a thermometer and temperature controller to dissolve the reactants. The whole system was attached with an electrically operated magnetic stirrer (model: Spinot Digital, Tarsons). Then, 0.00617 mol of amylopectin was dissolved in 60 mL of distilled water with slow stirring. The reaction temperature was maintained at 70 °C throughout the synthesis process with

constant stirring of 400 rpm. Initially, an inert atmosphere was created in the RB flask by purging nitrogen gas for 5 min. Afterward, 2.95 × 10−5 moles KPS followed by 0.026 mol METAC were added to the reaction system. After 10 min, 6.69 × 10−4 moles DEGDMA crosslinker was added to the RB flask. The copolymerization/cross-linking reaction was continued for another 3 h at the same temperature and rotation speed. Then, the cross-linking/copolymerization reaction was terminated with the addition of a saturated solution of hydroquinone. Subsequently, the reaction mixture was cooled to room temperature, followed by soaking in acetone (400 mL) to remove unreacted monomer and homopolymer (if formed). Finally, the product was dried to a constant weight in a vacuum oven at 50 °C. Synthesis of cl-AP/exf.LT-AgNPs Ternary Nanocomposite Hydrogels. First, 0.01 mol titanium isopropoxide was dissolved in 10 mL of ethanol (100 mL RB flask) at room temperature. Then, 40 mL of 1883

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AgNPs under continuous stirring at room temperature. The textile wastewater was collected from Ranchi, Jharkhand, India. The pond water was collected from GPS coordinates (23.8144° N, 86.4412° E), and the effect of the ternary nanocomposite hydrogel was analyzed toward the cidal effect on microbial load in the pond water. A serial dilution test was performed; 0.1 mL was plated from each 10−5 dilution series, and the requisite amount of nanocomposite was added to the sample according to the MIC result. The nanocomposite was analyzed with different concentrations, including 400, 1600, and 3200 μg/mL. After addition of the nanocomposite, the samples were incubated for 1 h at 37 °C in dark. After incubation, 100 μL of sample was plated in a Luria−Bertani (LB) agar plate. The CFU was further determined after incubating the LB plate at 37 °C for 24 h. No nanocomposite hydrogel was added in the control experiment.

ammoniacal water was added dropwise into the reaction system. Just after addition of ammoniacal water, a white precipitate appeared. Then, stirring was continued for another 15 min. The precipitate was collected after centrifugation followed by washing 4 times with double distilled water. In due course, 25 mL of 1.6 M ammonium carbonate was added to the precipitate with constant stirring of 400 rpm, and then 3 mL of 30% H2O2 was introduced to the reaction mixture.26 The reaction was continued for 30 min at 70 °C with constant stirring. On the other hand, graft copolymerization between AP and METAC followed by cross-linking with DEGDMA was carried out in a 250 mL RB flask under a nitrogen atmosphere. Once the polymerization mixture became viscous in nature, then a freshly prepared light yellow colored reaction mixture was added to the reaction system under continuous nitrogen purging. Subsequently, 10 mL of an aqueous solution of silver nitrate (0.040 mol) was added dropwise to the RB flask, and the reaction was continued for up to 3 h. Then, an aqueous solution of NaBH4 (0.080 mol in 20 mL of water) was added very slowly (added dropwise for up to 4 h) to the reaction mixture until a color change from light yellow to brown is accomplished. The final step of the reaction was carried out for 12 h at 50 °C with continuous stirring at 400 rpm. Afterward, the reaction mixture was cooled to room temperature and precipitated in 400 mL of acetone. Then, the precipitate was separated and dried in a vacuum oven at 40 °C for 24 h. For comparison, a AgNP-based nanocomposite hydrogel (cl-AP/ AgNPs) (without using titanium peroxo carbonate precursor), layered titanate-supported AgNPs (LT/AgNPs) (without using cl-AP), and clAP/exf.LT ex situ AgNP ternary nanocomposite hydrogels were also synthesized. The detailed synthetic procedure are provided in the Supporting Information. Characterization. The ternary nanocomposite hydrogel was characterized using UV−vis spectroscopy, XRD, 13C NMR, FTIR, FESEM, EDAX, elemental mapping, HRTEM, TGA, and zeta potential analyses. The instrumental details, procedures, and sample preparation technique for HRTEM analysis are elucidated in the Supporting Information. Catalytic Study and Recyclability of the Nanocomposites. For the catalytic activity to be investigated, a small piece (5 mg) of clAP/exf.LT-AgNPs was used as catalyst for the catalytic reduction of 4NP in aqueous solution (with pH 9.60) using NaBH4 as reducing agent. The detailed experimental procedure and recyclability study are discussed in the Supporting Information. To investigate the driving factor for the excellent catalytic activity of the cl-AP/exf.LT-AgNPs, we carried out three controlled reactions, i.e., cross-linked cationically functionalized AP with AgNPs (without adding titanium peroxo carbonate precursor, cl-AP/AgNPs), layered titanate supported AgNPs (in the absence of cl-AP, LT/AgNPs), and in situ-exfoliated layered titanate by cl-AP followed by ex situ addition of AgNO3 precursor (clAP/exf.LT-ex situ AgNPs). All of the reactions were carried out by maintaining the proper reaction conditions (detailed experimental procedures are given in the Supporting Information). Furthermore, the catalytic reduction of MB/MO dyes (in MB solution with pH 6.54 and MO solution pH 6.06) was carried out to support the catalytic activity of cl-AP/exf.LT-AgNPs as catalyst. The detailed procedure of the catalytic reduction is explained in the Supporting Information. Antimicrobial Assay. Mueller Hinton (MH) agar culture media (Himedia) was poured into sterile Petri plates, and freshly grown E. coli and B. subtilis (OD600 = 0.62 equiv to 107 cells/ml) cultures were spread on the agar plate surface using sterile cotton swabs. The plates were incubated at 37 °C for 24 h, and the minimum inhibitory concentration (MIC) was examined by calculating the zone of inhibition (in mm). The antibacterial activity was analyzed by measuring the difference between sample and zone of inhibition. The standard drug Gentamicin (100 μg) was taken as positive control, and 0 μg of ternary nanocomposite hydrogel was taken as negative control. Wastewater Treatment. The decoloration of textile wastewater was carried out in the presence of excess NaBH4 with cl-AP/exf.LT-

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RESULTS AND DISCUSSION

Synthesis of cl-AP/exf.LT-AgNP Ternary Nanocomposite Hydrogels. The synthetic strategy is based on exfoliation of the titanate layer to a single titanate sheet through continuous generation of cross-linked cationically functionalized amylopectin followed by growth and stabilization of AgNPs, which is novel. The synthetic route for the formation of cl-AP/exf.LT-AgNPs is based on graft copolymerization of METAC on amylopectin in the presence of DEGDMA crosslinker at 70 °C, which stabilized the single titanate sheet followed by generation of AgNPs to form a cross-linked ternary nanocomposite hydrogel (cl-AP/exf.LT-AgNPs). The polymerization of TiO6 octahedra provides anionically charged sheetlike structure, which basically supports the fabrication of AgNPs after addition of NaBH4 that were further stabilized in the presence of cationically functionalized cl-AP to restrict the self-stacking between individual titanate layers (Scheme 1). The intense color change from light yellow to brown along with a UV−vis peak at 400 nm confirms the formation of AgNPs (Figure S1). With an increase in the reaction time of copolymerization, cross-linking along with decomposition/hydrolysis of titanium peroxo carbonate with homogeneous immobilization of AgNPs took place simultaneously. Characterization. FTIR Spectra. cl-AP (Figure S2a) contains a few additional peaks with all the characteristics peaks of AP.53 A new absorption peak at 1738 cm−1 was due to the presence of carbonyl carbon present in METAC. Furthermore, the intense peak at 1643 cm−1 confirms the stretching frequency of the carbonyl carbon of the cross-linker. Additionally, another peak at 1472 cm−1 demonstrates the C− H bending of the methyl group attached with the ammonium moiety. These observations suggest the formation of crosslinked cationically functionalized AP. However, the FTIR spectrum of cl-AP/exf.LT-AgNPs nanocomposite reveals the presence of all the characteristics peak of cl-AP. However, it was observed that all the peak positions shifted toward lower wavenumbers (Figure S2b), indicating the probable electrostatic interactions between the titanate sheet-supported AgNPs and cationically functionalized cross-linked AP.54 13 C NMR Analysis. Solid-state 13C NMR spectrum of cl-AP shows a peak at δ = 177.5 corresponding to the carbonyl carbon of poly(METAC) (Figure S3a) along with the other peaks of bare AP.31 Additionally, the extra peak at δ = 19.0 suggests the presence of a methyl carbon of the cross-linker. The peaks at 55.3 and 46.1 ppm are responsible for the formation of sp3-hybridized carbon atoms, which formed during the polymerization of METAC (Figure S3a). On the contrary, it is obvious that the corresponding spectrum of cl-AP/exf.LT1884

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Figure 1. (a) XRD pattern of in situ-exfoliated cl-AP/exf.LT-Ag NPs; TEM and HR-TEM images of a (b−d) cl-AP/exf.LT-AgNPs ternary nanocomposite hydrogel.

Figure 2. FE-SEM images of (a) bare AP, (b) pure cl-AP, (c) cl-AP/exf.LT-AgNPs, and (d) closer view of cl-AP/exf.LT-AgNPs.

(200) plane of inter layer distance21 (also present in the bare layered titanate when synthesized in the absence of polymer, i.e., cl-AP; Figure S4) indicates exfoliation of the layered structure and suggests the absence of stacked layers. The broad peak at ∼20° was observed for cross-linked copolymer, i.e., clAP (Figure S5). Here, it is essential to mention that other low intensity peaks corresponding to titanate sheets do not appear in the diffractograms of cl-AP/exf.LT-AgNPs, most likely due to the presence of a large amount of amorphous cross-linked polymer. Panels b and c in Figure 1 represent the HR-TEM images of the synthesized cl-AP/exf.LT-AgNPs nanocomposite hydrogel. The TEM image (Figure 1b) shows randomly distributed wirelike structures that can be ascribed to the individual LT

AgNPs contains all the peaks of cl-AP (Figure S3b); however, the characteristics peak values were shifted. The observed phenomenon reveals that electrostatic interactions predominate between the positively charged cross-linked hydrogel with negatively charged titanate sheet-supported AgNPs, which confirms the formation of the ternary nanocomposite hydrogel as proposed in Scheme 1. XRD and HR-TEM Analyses. Figure 1a represents the XRD pattern of cl-AP/exf.LT-AgNPs. In the diffraction pattern, observed distinct diffraction peaks at 2θ = 38.1°, 44.0°, 64.10°, and 77.34° can be ascribed to the (111), (200), (220), and (311) planes of metallic Ag (JCPDS no. 65−2871). The absence of any peak at ∼10° that is generally observed in layered titanates with the highest intensity responsible for the 1885

DOI: 10.1021/acssuschemeng.6b02594 ACS Sustainable Chem. Eng. 2017, 5, 1881−1891

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Figure 3. (a) UV−vis spectra of p-nitro phenol reduction (i) before treatment and (ii) 16 s after treatment of cl-AP/exf.LT-AgNPs. (b) Stability of the cl-AP/exf.LT-AgNPs over 16 cycles for p-nitro phenol reduction, (c) in situ MB reduction, and (d) in situ MO reduction.

probably because the titanate nanosheets and AgNPs are embedded into the cross-linked amylopectin to form a stable composite that results in the collapse of the three-dimensional network. The elemental mapping of Ti and Ag are uniform in the observed area, which indicates the good dispersion of AgNPs and exfoliated LT on the hydrogel matrix (Figure S8a−e). TGA and UV−Vis NIR Analyses. The comparative TG analysis of cl-AP and cl-AP/exf.LT-AgNPs is shown in Figure S9. In cl-AP, four-step weight loss zones were observed. The initial weight loss within 150 °C was due to the removal of moisture from the sample. The weight loss in the region of 150−260 °C was due to the degradation of the cross-linker moiety. The third degradation zone was attributed to the decomposition of the polysaccharide backbone. The final weight loss region, observed in the range of 420−450 °C, was due to the degradation of grafted poly(METAC) chains. Although the TGA result of cl-AP/exf.LT-AgNPs suggests higher thermal stability than that of the corresponding hydrogel. It is apparent that cl-AP/exf.LT-AgNPs exhibit all four stages of degradation with an increase in stability range. Moreover, UV−vis reflectance spectra of cl-AP exf.LT-AgNPs demonstrated a band centered at 415 nm, revealing that AgNPs are present in the dried matrix of the ternary nanocomposite hydrogel (Figure S10). Stability Study of the Ternary Nanocomposite Hydrogel. Zeta potential was measured to explain the stability of the nanocomposite hydrogel. Electrostatic repulsions among the similarly charged particles signify the stability of the colloidal suspensions. Therefore, the stability of the suspension might be understood by higher positive or negative magnitudes of the zeta potential value.35 The suspension of ternary nanocomposite hydrogel was used for zeta potential analysis for a period of 0−7 days with continuous stirring (Figure S11). The initial zeta potential value was found to be +66.07 mV. The high positive zeta potential value indicates the stability of the suspension. Furthermore, it was observed that there was no sharp decline of zeta potential value up to 7 days (+52.37 mV),

sheets, confirming the formation of exfoliated LT through adopted in situ synthesis and supporting the XRD result. The magnified image depicts the presence of high contrast dark spherical particles (Figure 1c) attributed to metallic AgNPs, which is a common phenomenon of metallic particles. In the HR-TEM image, the presence of distinct lattice fringes in the dark spherical particles had an interplanar distance of 0.23 nm attributed to the (111) plane of metallic Ag. This confirms the formation of crystalline metallic AgNPs with spherical morphology (Figure 1d and Figure S6b). In the same HRTEM image, lattice fringes in low contrast area with interplanar distance of 0.35 nm correspond to (101) planes of LT horizontally lying in the surface of the composite, confirming the presence of LT in the composite hydrogel (Figure 1d and Figure S6a).55 In the HR-TEM image, the presence of metallic AgNPs in the surface of horizontally lying LT surface supports the stabilization of AgNPs by LT. It is obvious from the HRTEM analysis that cationically functionalized cross-linked amylopectin is primarily responsible for stabilization of anionically charged titanate sheets, whereas the stabilized titanate nanosheets could hold and catalyze the proper growth of AgNPs. Moreover, the EDAX analysis (in HR-TEM) reveals that both Ag and Ti are present in the composite hydrogel (Figure S7). FE-SEM Analysis and Elemental Mapping. The surface morphologies of bare AP, cl-AP, and cl-AP/exf.LT-AgNPs are shown in Figure 2. AP shows a granular-shaped morphology (Figure 2a), and the cl-AP hydrogel demonstrates a porous morphology with the presence of spherical pores (Figure 2b). The surface morphology of the cl-AP hydrogel was drastically changed after introduction of exfoliated titanate sheetsupported AgNPs. After closer inspection of the cl-AP/ exf.LT-Ag composite, it is obvious that the spherical particles are embedded on the surface (Figure 2c,d), i.e., no surface deposition took place and the formed spherical particles are stabilized by exfoliated titanate sheets, which are highly dispersed in the hydrogel matrix. Here, it is essential to mention that the three-dimensional network structure is not obvious in the FESEM image of the nanocomposite. This is 1886

DOI: 10.1021/acssuschemeng.6b02594 ACS Sustainable Chem. Eng. 2017, 5, 1881−1891

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ACS Sustainable Chemistry & Engineering signifying the stability of the nanocomposite hydrogel in suspension form. Catalytic Reduction of p-Nitro Phenol and MB/MO Dyes. The reduction of 4-NP to 4-AP is thermodynamically favorable (E° = −0.76 V) at ambient conditions when NaBH4 acts as reducing agent in aqueous media (E° = −1.33 V).56 However, the kinetic barrier of the reduction process is a main hurdle to prevent the forward reaction under the experimental time scale. The barrier can be overcome in the presence of metal NPs that have a predominant role as catalyst for complete reduction.57 To examine the catalytic efficiency of clAP/exf.LT-AgNP ternary nanocomposite hydrogel, we carried out the reduction of 4-NP with nanocomposite material in the presence of NaBH4 as hydrogen source. With addition of NaBH4 on 4-NP, the formed p-nitrophenolate ion with a high spectrophotometric sensitivity and can be easily monitored by UV−vis spectroscopy. As is obvious from Figure 3, with progress of the reaction, the peak for 4-NP (bright yellow) at 400 nm declined sharply with the generation of a new peak at 295 nm (colorless), suggesting the formation of 4-AP, a reduced product.56 It was observed that only 16 s was required to complete the reduction process with freshly prepared NaBH4 solution (3 mL, 10 × 10−3 M). The rapid color change indicates the complete reduction of 4-NP to 4-AP (Figure 3a, inset). Furthermore, the stability of the catalyst was examined by repeated experiments with 4-NP. After each cycle, the catalyst was easily reused for more than 15 times (Figure 3b) by taking out the macroscopic bead from the reaction system using a spatula, followed by drying. The reaction proceeded with a six electron transfer process in the presence of NaBH4, a hydrogen source. During the reaction, electron transfer took place in basic media where both borohydride ion and nitrophenolate ion adsorbed on the surface of the catalyst. Several scientific groups described evidence for the reduction mechanism. According to Herves et al., the reduction occurs via adsorption of both the reactants on the metal (Au/Pd NPs) surfaces, which were immobilized on polyelectrolyte brushes.58 Similarly, Li et al. described the synergistic effect of graphene and involvement of noble NPs for higher catalytic activity of 4-NP.59 The involvement of surface area, surface functional groups, or leached gold species also influenced the higher catalytic conversion of 4-NP.60,61 Zhang et al. demonstrated transfer of hydride to the AgNPs in lieu of NaBH4 for TiO2-supported AgNPs.62 In the present study, the outstanding catalytic activity of clAP/exf.LT-AgNPs composite is mainly endorsed by the stabilization of exfoliated LT in the presence of cl-AP, which could restrict the formation of the layer by self-stacking of layered titanate, which effectively inhibited the agglomeration of AgNPs and acted as a superior supportive material for excellent catalytic activity. Moreover, the synergistic effect of clAP/exf.LT-AgNPs arises due to electrostatic interactions between 4-NP with poly(METAC), which brought 4-NP closer to titania nanosheet-supported AgNPs and increased the rapid electron transfer to 4-NP in basic media for efficient catalytic reduction. For establishing the fact of proper endorsement and identification of the role of LT-supported AgNPs, we performed some controlled catalytic reduction reactions under the same experimental conditions. The conversion time was increased to 1020 s when cl-AP/AgNPs were used under the same reaction conditions (Figure 4) with rapid release of AgNPs. Similarly, for LT-supported AgNPs and cl-AP/exf.LT-

Figure 4. Comparative reduction times of 4-NP using different catalysts.

ex situ AgNPs, the composites took 180 and 320 s to complete the reduction process (Figure 4). The reduced reduction time of cl-AP/exf.LT-AgNPs clearly states that AgNPs received proper solid support as titanate nanosheets for suitable immobilization and show superior catalytic activity. Thus, the combined effect of both components (LT and AgNPs) plays a crucial role for stabilization of the composite hydrogel as well as efficient catalytic reduction. Similarly, the property of LT is highly dependent on the extent of exfoliation of the layers. Thus, it is apparent that the exfoliation of the layer in LT using cationically functionalized AP along with proper support of AgNPs is the main driving factor for efficient and rapid catalysis. Finally, appropriate growth and immobilization of AgNPs occurred in the presence of the titanate sheet, which was already stabilized by cationically functionalized cl-AP, which boosts the overall efficiency of cl-AP/exf.LT-AgNPs as catalyst for rapid hydrogenation reduction of 4-NP to 4-AP. Furthermore, to obtain the optimum catalyst dose, the concentration of nanocomposite hydrogel was varied from 1 to 15 mg. It was observed that, with a dosage of 5 mg, the catalyst showed the best reduction efficacy (16 s) (Figure S12). Additionally, rapid catalytic reduction was also observed toward the decoloration of MO (90 s) and MB (80 s) dyes at room temperature in aqueous media (Figure 3c,d). The formation of metal hydride on the catalyst surface after adsorption of NaBH4 catalyzed the reduction of MO/MB on the metal surface. Then, desorption leads to the creation of a vacant space for further MO/MB adsorption, continuing the reduction reaction.63 The ternary nanocomposite hydrogel catalyst gives excellent catalytic efficiency up to the fifth cycle (Figure S13). For the catalytic efficiency of cl-AP/exf.LT-AgNPs for reduction of 4-NP, MB, and MO in the presence of NaBH4 to be elucidated, the effective catalysis time and amount of nanocatalyst were compared with those of various reported catalysts.14,38−52 However, it is very difficult to directly correlate the catalytic efficiency of cl-AP/exf.LT-AgNPs with reported literature statistics because of their dissimilar experimental conditions. Table 1 discloses the fact that the cl-AP/exf.LTAgNPs can be considered as one of the most promising catalysts reported to date (it took only 16 s for reduction of 4NP using 5 mg of cl-AP/exf.LT-AgNPs catalyst). Assessment of Antimicrobial Activity. Antibacterial materials have received great interest in packaging industries and medical science. Therefore, cl-AP/exf.LT-AgNPs was examined for in vitro antibacterial activity against E. coli and B. subtilis as model microbes. It has been well established that metallic Ag exhibits excellent antimicrobial activity.11,12 Here, a zone of inhibition test was performed to analyze the extent of antimicrobial activity of the ternary nanocomposite hydrogel. 1887

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Research Article

ACS Sustainable Chemistry & Engineering Table 1. Comparative Review of Different Catalysts for 4-NP and Organic Dye Reduction amount of catalyst for p-NP reduction

catalyst PAM/graphene/Ag ternary hydrogel14 Ag NP-embedded semi-IPN hydrogel38 porous Au-PdNPs39 Cu2O-Ag40 Ag-entrapped hydrogel41 acrylic acid-amidodiol/Ag hydrogel (SPAG)42 Gold NP-deposited cellulose nanocrystal (Au NPs@CNs)43 Ag-alginate biohydrogel44 Ag NPs-poly(vinyl alcohol) [PVA]45 alginate-based biohydrogels (Ag@ AMH)46 CNC (cellulose nanocrystals)PAMAM(polyamidoamine)-Au-147 Ag-Fe3O4@chitin microspheres48 carbon nanofiber/silver nanoparticle nanocomposite (CNFs/AgNPs)49 Fe3O4@TiO2@Au magnetic microsphere50 Ag NP-immobilized Fe3O4@C (MFC) nanocomposites51 Pd-rGO-CNT52 cl-AP/exf.LT-AgNPs(this study)

p-NP reduction (time)

0.13 g 10 mg 1 mL of catalyst solution (0.5 mg/mL) 1 mg

90 s 18 min 12 s 7 min

20 mg

10 min

(1 mL; total Au content: 0.2 μmol)

14 min

polymer thin film (thickness: ∼1.3 mm) four beads of Ag@AMH

10 min 10 min 13 min

50 μL of 0.009% w/w CNC-PAMAM-Au (Au content: 0.02 μmol) 1.5 mL of an aqueous dispersion of MRChS (AgFe3O4@chitin microspheres) at 50 mg L−1 1.0 mg

organic dye reduction time

0.13 g

80 s (MB)

10 mg

30 min for MB

270 s 10 min 8 min

1.0 mL 5 wt %

6 min

20 mg

4 min

5 mg 5 mg

amount of catalyst for organic dye reduction

20 s 16 s

10 min

10 mg

90 s (MO), 80 s (MB)

Figure 5. Inhibition zone toward (a) positive control (100 μg of Gentamicin), (b) negative control, (c) different nanocomposite concentrations of 400, 800, 1600, and 3200 μg/mL for E. coli, (d) different nanocomposite concentrations of 400, 800, 1600, and 3200 μg/mL for B. subtilis, and (d) MIC of the tested compound against E. coli and B. subtilis.

Gentamicin (100 μg of Himedia) was taken as positive control (Figure 5a) in which a clear zone of inhibition was found against E. coli and B. subtilis cells. However, the zone of inhibition was absent in the negative control (Figure 5b). In this study, the antimicrobial activity was started from 400 μg/ mL, but the ternary nanocomposite hydrogel was not able to kill all the initially inoculated cells (Figure 5c,d). Alternatively, no zone was found below a nanocomposite concentration of 400 μg/mL (Figure 5e); however, the ternary nanocomposite hydrogel concentration of 3200 μg/mL was found to be effective for both E. coli and B. subtilis cells (Figure 5e). MIC

reveals that the compound has significant biological activity (Figure 5e) at 400 μg/mL concentration. The effective concentration was found to be 3200 μg/mL, which relies on previously reported studies.11 It can be demonstrated from the results that AgNPs present in the ternary nanocomposite hydrogel are able to kill E. coli and B. subtilis through diffusion of silver into the media. The excellent antimicrobial effect of the material at 3200 μg/mL is because of the presence of a comparatively higher amount of silver from other nanocomposite concentrations and the extent of silver release from the ternary nanocomposite hydrogel. The exact 1888

DOI: 10.1021/acssuschemeng.6b02594 ACS Sustainable Chem. Eng. 2017, 5, 1881−1891

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ACS Sustainable Chemistry & Engineering mechanism of silver on the microbes is still unclear.64 However, the probable mechanism is based on the interaction of AgNPs with the cell membrane of the micobes.65 Actually, AgNPs have a tendency to react with thiol-containing proteins to inhibit growth by interrupting the respiration mechanism, leading to cell death.66 Wastewater Treatment. The treatment of industrial effluent is a requisite criteria for maintaining environmental sustainability. The fabricated ternary nanocomposite hydrogel exhibits rapid removal of toxic organic contaminants from synthetic wastewater.35 Subsequently, the applicability of the material was investigated using the colored textile wastewater. After 10 min of continuous stirring of textile effluent in the presence of NaBH4 and cl-AP/exf.LT-AgNPs, the effluent became colorless (Figure S14). Thus, it is obvious that cl-AP/ exf.LT-AgNPs nanocomposite hydrogel demonstrates environmental remediation through decoloration of textile effluent. The synthesized ternary nanocomposite hydrogel also exhibits excellent bactericidal activity toward the microorganism present in pond water. With increasing nanocomposite concentration, the colony was found to decrease in number. In the control LB plate, the concentration of bacterial cells was too high to count (TNTC); when the concentration increased to 3200 μg/mL, the colony-forming unit (CFU) was reduced to 90%, indicating a bactericidal affect (Figure S15). Thus, it is apparent that the ternary nanocomposite hydrogel has a potential impact toward the reduction of microbial load from high microbial contaminated water.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +91-278-2567760, ext. 704; E-mail: [email protected]. *Tel: +91-326-2235769; E-mail: [email protected]. ORCID

Sagar Pal: 0000-0002-3425-7010 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.S. earnestly acknowledges financial assistance from the Indian Institute of Technology (Indian School of Mines), Dhanbad for a Senior Research fellowship. Authors also acknowledge the various instrumental facilities of CRF of Indian Institute of Technology (Indian School of Mines), Dhanbad. The authors also acknowledge help given by the Analytical Discipline and Centralized Instrument Facility of CSIR-CSMCRI for characterization of materials.

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CONCLUSIONS In summary, we have developed a novel cationically modified cross-linked amylopectin, which served as an efficient stabilizer for the formation of an exfoliated single sheet structure that further acts as support material for homogeneous dispersion and controlled growth of AgNPs. The in situ-formed titanate nanosheets can efficiently prevent the aggregation and rapid leaching of AgNPs along with usable active area, which assists the overall stability of the ternary nanocomposite hydrogel. The prepared cl-AP/exf.LT-AgNPs material showed excellent catalytic and highly recyclable ability toward reduction of 4NP, MB, and MO via the synergistic effect of cross-linked natural polymer-stabilized nanosheet-supported AgNPs. Moreover, the antibacterial disc assays toward E. coli and B. subtilis demonstrated the excellent antibacterial activity of ternary nanocomposite hydrogel, which is effective toward a variety of practical applications. Considering the various aspects of environmental ecology, the ternary nanocomposite hydrogel was acclaimed as an excellent sustainable green material. Therefore, the synthetic approach may open a new possibility for integrating titanate nanosheets supporting other metal nanoparticles for advanced studies.

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analysis, EDAX analysis, elemental mapping, TGA analysis, UV−vis NIR analysis, zeta potential analysis, data on comparative reduction time of 4-NP with variation of cl-AP/exf.LT-AgNPs dosage and catalytic reduction of MB dye up to seventh cycle and MO dye reduction up to seventh cycle, treatment of textile wastewater, graph of number of colonies with different concentrations of ternary nanocomposite hydrogel toward pond water, and variations of colony formation assay with nanocomposite concentrations (PDF)

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REFERENCES

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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.6b02594. Synthesis procedure of nanocomposite formation (synthesis of cl-AP/AgNPs, cl-AP/exf.LT-ex situ AgNPs, LT/ AgNPs), details of characterization techniques, catalytic study of p-NP, M,B and MO, results of UV−vis spectrum, FTIR spectra, XRD analysis, HR-TEM 1889

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