Cross-Linked Biopolymer Stabilized Exfoliated Titanate Nanosheet

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Crosslinked Biopolymer Stabilized Exfoliated Titanate Nano Sheet Supported AgNPs: A Green Sustainable Ternary Nanocomposite Hydrogel for Catalytic and Antimicrobial Activity Amit Kumar Sarkar, Arka Saha, Lipi Midya, Chiranjib Banerjee, Narayan R. Mandre, Asit Baran Panda, and Sagar Pal ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02594 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016

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ACS Sustainable Chemistry & Engineering

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Crosslinked Biopolymer Stabilized Exfoliated Titanate Nano Sheet Supported 6 8

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AgNPs: A Green Sustainable Ternary Nanocomposite Hydrogel for Catalytic 9 1

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and Antimicrobial Activity 13

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Amit Kumar Sarkar†, Arka Saha┴, Lipi Midya†, Chiranjib Banerjee§, Narayan Mandre‡, Asit 14 15

Baran Panda┴*, Sagar Pal †* 18

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†

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Polymer Chemistry Laboratory, Department of Applied Chemistry, Indian Institute of

Technology (Indian School of Mines), Dhanbad – 826004, Jharkhand, India. 21 23

2 §

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Department of Environmental Science and Engineering, Indian Institute of Technology (Indian

School of Mines), Dhanbad – 826004, Jharkhand, India. 27

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‡

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Department of Fuel & Mineral Engineering, Indian Institute of Technology (Indian School of

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Mines), Dhanbad – 826004, Jharkhand, India. 32

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┴

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Discipline of Inorganic Materials and Catalysis, Central salt and Marine Chemicals Research

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Institute (CSIR), G. B. Marg, Bhavnagar-364002, Gujarat, India. 36 37 38 40

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AUTHOR INFORMATION 42

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†*

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Corresponding author:

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Tel: +91-326-2235769; E-mail: [email protected] 46 47 48 49 50 52

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RECEIVED DATE (to be automatically inserted after your manuscript is accepted if 53 54

required according to the journal that you are submitting your paper to) 5 56 57

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1 ACS Paragon Plus Environment


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ABSTRACT 5

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For the first time, we report an in-situ exfoliated titanate nano sheets supported silver nanoparticles 7

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(AgNPs) towards environmental sustainability through rapid catalytic reduction of p-nitrophenol 8 9

(4-NP), organic dyes for decoloration as well as by inhibiting the growth of microbes. The ternary 10 12

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nanocomposite hydrogel has been synthesized through stabilisation of anionically charged titanate 14

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sheet by three dimensional amino functionalized chemically crosslinked amylopectin for proper 15 17

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growth and stabilization of AgNPs. Here, titanate nanosheets act as an excellent solid support for 19

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proper fabrication of AgNPs that could restricts its agglomeration and rapid leaching of AgNPs 20 21

from the ternary nanocomposite hydrogel. The structural confirmation as well as the stability of 2 24

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titanate nanosheets along with AgNPs have been studied using various characterization techniques. 26

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The preparaed nanocomposite hydrogel demonstrates excellent catalytic efficacy and recyclable 27 28

ability towards reduction of toxic 4-NP and decoloration of organic dyes rapidly. Notably, the 29 31

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complete reduction of 4-NP could be accomplished within 16 s using 5 mg of as synthesized cl3

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AP/exf.LT-AgNPs in presence of excess NaBH4. The excellent catalytic efficiency of the ternary 34 36

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nanocomposite hydrogel arises from the synergistic effects of crosslinked amylopectin stabilized 38

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titanate nanosheets and in-situ fabrication of AgNPs. Moreover the strong bactericidal activity (3.2 40

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mg/mL for 107 cells/mL of Escherichia coli and Bacillus subtilis cell) of the ternary 41 43

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nanocomposite hydrogel would overcome the limitations for removal of water soluble organic 45

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pollutants and microbial contaminants owing to future perspective on environmental 46 47

sustainability. 48 49 50 52

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Keywords: AgNPs; Antimicrobial activity; Catalytic reduction; Ternary nanocomposite hydrogel; 53 54

Exfoliated titanate nanosheets; 4-NP. 5 56 57

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INTRODUCTION 5

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The ever increasing water contamination because of soluble toxic organic contaminants and 7

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microbes are creating scarcity of safe water and waste management.1-2 Hence it is imperative to 8 10

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develop an environment friendly methodology using green, sustainable and recyclable materials 12

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to treat the industrial effluents and control the growth of microbial contaminants. In this regard, 13 14

nanomaterials and metal-organic frameworks have garnered special attention owing to their 15 17

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potential application in environmental sustainability.3-6 Therefore the scientific community 19

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have emerged considerable attention for continuous development of noble metal 20 21

nanoparticles (NPs) based sustainable materials because of their unique physical or chemical 2 24

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properties that are significantly different from their bulk.7-8 However, the direct use of metal NPs 26

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are certainly restricted owing to their high surface charges,9 assembly of NPs easily aggregated 27 29

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which created low negative Fermi potential with consecutively losing their inherent properties like 31

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catalytic activity.10 32 3

Among various metal nanoparticles, particularly nano sized-silver has received ample attention in 34 36

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recent years owing to its low cost, high surface to volume ratio along with excellent catalytic 38

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properties and antibacterial activity.11-12 However, simple filtration or centrifugation techniques 39 40

could not remove AgNPs from reaction media and their aggregation indeed restricts its wide 41 43

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applicability in environmental remediation.11 Also after application, the rapid release of AgNPs 45

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into the environment can cause adverse health issues.13 To address the above mentioned concerns, 46 48

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polymeric materials/inorganic solid stabilizer supported AgNPs provide a prior alternative choice 50

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for stabilization and easy separation of AgNPs. 52

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In consequence, the inorganic solid stabilizer not only support and stabilize the metal NPs but also 53 54

enhanced the thermal and chemical stability of supported nanocomposites.14-15 For example, solid 57

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support related with graphene and noble metal NPs hybrid composites are fabricated to overcome 60

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the challenges.16-18 However, extensive research has been made on graphene supported metal NPs 5

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by proper covalent and non-covalent modifications.19-20 Similarly, layered titanate (LT), an 6 8

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inorganic solid stabilizer has drawn ample attention owing to its chemical stability and 10

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environment friendly behaviour.21-23 In recent past, layered titanate based metal NPs are fabricated 12

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for visible light active photocatalysis behaviour, 24-25 but properties of LT are highly dependent on 13 15

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the extent of exfoliation of individual nano sheets. The complete exfoliation of layer through 17

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proper ionic interaction is certainly restricted as a result of the presence of higher surface charge 18 19

of LT. The properties of these LT based nanocomposites are dependent extremely on the extent of 20 2

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exfoliation of stacked layers to individual layers and interaction of negatively charged inorganic 24

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layer with polymer moiety.26 Thus, before hybridization with polymer, exfoliation of layers are 25 27

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essential, which is well dependent on inter layer charge density. However, the exfoliation of LT is 29

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difficult because of its high surface charge density. Fuse et al. 27 reported UV light durable epoxy 31

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resin-layered titanate composite, Asai et al. 28 reported poly (vinylidine fluoride)-layered titanate 32 3

composite, Guo et al. 29 reported photo-catalytically active composite with polyaniline, Sukpirom 36

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and Lerner 37

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reported PEO, PVP based nanocomposites. In most of the reported strategies, the

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exfoliation of layer was achieved through intercalation of cationic surfactant mediated ion 39 41

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exchange mechanism with polymer incorporation. Thus in the reported composites, the full 43

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exfoliation of layers was difficult due to weaker interactions between polymer and titanate sheet. 4 45

For the first time, we were able to develop stabilized titanate nanosheets through in-situ 46 48

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polymerization and exfoliation.26 50

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Amylopectin, a highly branched part of starch is a renewable, biodegradable polysaccharide, 51

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which acts as a promising biomaterial. The functionalization of amylopectin through crosslinking 5

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and stabilization of in-situ formed nanosheets followed by proper fabrication of antibacterial 56 57

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AgNPs is expected to be a sustainable and recyclable green ternary nanocomposite hydrogel. 5

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Among the nitoaromatic compounds, p-nitro phenol (4-NP) is considered as prime 6 7

pollutant owing to its non-biodegradability and stability in the environment.32 However, it’s 10

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reduced form, p-amino phenol (4-AP) is extensively used in dye, pigment, pesticide, corrosion 1 12

industries and most importantly as an intermediate for synthesis of paracetamol, an analgesic 13 15

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drug.33 Similarly, organic dyes are important part of modern civilization, however 100% colour 17

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removal of organic dyes from aqueous solution with elimination of dye disposal problem is a major 18 19

challenge, requires lengthy time or varieties of consecutive steps like adsorption along with 20 2

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degradation are involved.34-35 Thus, catalytic reduction is regarded as useful technique for quick 24

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remediation of hazardous effects of 4-NP and decoloration of organic dyes. Also, the transition 25 27

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metal-catalyzed catalytic reduction of 4-NP is known as a classic reaction to scrutinize the 29

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catalytic activity of noble metal nanoparticles (NPs) in presence of sodium borohydride. 33,36 31

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Polymeric hydrogel directed in-situ fabricated metal NPs were used as catalyst in presence of 32 3

NaBH4 (acts as hydrogen source).37 In spite of advantages like easy handling of hydrogel based 36

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nanocomposite materials, the increment of time towards catalytic reduction has somehow 38

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restricted its potential application38 and thus it is imperative to develop an efficient nanocomposite. 39 41

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Beyond the hydrogel based nanocomposites, the porous Au-Pd NPs demonstrate rapid catalytic 43

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reduction of 4-NP.39 Very recently, a novel binary nanocomposite system has been reported for 4 45

the adsorption of dyes and reduction of toxic 4-NP to non-toxic 4-AP.40 For dye reduction study, 46 48

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the synthesized AgNPs entrapped hydrogel took 30 min for decoloration of organic dyes through 50

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reduction.41 The increased reduction time as well as maximum recyclability up to certain cycles 51 53

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restricts the potential application as efficient catalysts. Thus, a proper modification is required to 5

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introduce the best properties in hydrogel based nanocomposite materials. 56 57

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In the present research, stabilization of AgNPs was initiated after titanate nanosheets formation 5

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that was stabilized by porous crosslinked polymeric network. The incorporation of hydrogel matrix 6 8

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integrates the titanate nanosheet supported AgNPs in favor of easy handling of the nanocatalyst 10

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for recycling without decreasing the efficiency of catalytic reduction. Here for the first time, we 1 12

have established a novel synthesis route for the preparation of ternary nanocomposite hydrogel (cl13 15

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AP/exf.LT-AgNPs), which comprised of fully exfoliated titanate nanosheet stabilized by amino 17

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functionalized chemically crosslinked amylopectin (i.e. amylopectin crosslinked with [218 19

(methacryloyloxy)ethyl] trimethyl ammonium chloride (METAC) in presence of diethylene glycol 20 2

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dimethylacrylate crosslinker, cl-AP) supported in-situ generation of AgNPs. Finally, catalytic 24

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efficiency of the nanocomposite was examined towards reduction of 4-NP, methylene blue (MB), 25 27

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and methyl orange (MO), in presence of excess NaBH4. The observed excellent catalytic activity 29

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of the nanocomposite is due to the enhanced area of interaction of titanate nanosheets supported 31

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AgNPs originating to its suitable support for rapid electron transfer into substrate 4-NP/MB/MO. 32 34

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The AgNPs grow steadily under the solid support of titanate nanosheets that was fully exfoliated 36

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in presence of crosslinked amylopectin and prevent the aggregation along with the leaching of 37 38

AgNPs. Hence, the fabricated ternary nanocomposite hydrogel demonstrated outstanding catalytic 39 41

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efficiency towards reduction of 4-NP, MB and MO, which is probably beyond the state-of-the-art 43

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available in the literature.38-52 The sustainability of the nanocomposite hydrogel was further been 4 45

established by removing colour from textile industry wastewater. The antibacterial activity of the 46 48

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ternary nanocomposite hydrogel has also been studied against E. coli (gram negative) and B. 50

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subtilis (gram positive) cells by disc diffusion assay and found to be effective towards reducing of 51 53

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the total microbial load in pond water. 54 5 56 57

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EXPERIMENTAL SECTION 4 6

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Materials. 8

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Amylopectin (source: maize) was purchased from Fluka, Switzerland. Titanium isopropoxide, 9 10

silver nitrate and [2-(methacryloyloxy) ethyl] trimethyl ammonium chloride (METAC) were 1 13

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procured from Sigma Aldrich, USA. Potassium persulfate (KPS) was received from Qualigens, 15

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Mumbai, India. Diethylene glycol dimethacrylate (DEGDMA) was obtained from TCI, Tokyo, 16 17

Japan. Acetone, ammonia, ammonium carbonate, sodium hydroxide (E. Merck, Mumbai, India), 18 20

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p-nitro phenol (4-NP), methylene blue (MB), methyl orange (MO) (Loba Chemie Pvt. Ltd., 2

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Mumbai, India) and sodium borohydride (Spectrochem India Pvt. Ltd., India) were of analytical 23 25

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grade and used as received. Millipore double distilled water was used for all experimental work. 27

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Synthesis. 28 29

Preparation of crosslinked amylopectin (cl-AP). 30 32

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A grafting from polymerization approach initiates the formation of crosslinked hydrogel. The 34

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polymerization was carried out in presence of nitrogen atmosphere using three necked round35 36

bottom (RB) flask. The RB flask was placed on an oil bath with a thermometer and temperature 37 39

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controller to dissolve the reactants. Then the whole system was attached with an electrically 41

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operated magnetic stirrer (Model: Spinot Digital, Tarsons). 0.00617 moles of amylopectin was 42 43

taken in 60 mL of distilled water and stirred slowly to dissolve it. Reaction temperature was 4 46

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maintained at 70 °C throughout the synthesis process with constant stirring of 400 rpm. Initially, 48

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inert atmosphere was created in RB flask by purging nitrogen gas for 5 min. Afterwards, 2.95×1049 50 5

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moles of KPS followed by 0.026 moles of METAC were added into the reaction system. After

10 min, 6.69×10-4 moles of DEGDMA crosslinker was added into the RB flask. The 54 5

copolymerization/crosslinking reaction was continued for another 3 h at same temperature and 56 57

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rotation speed. Then, the crosslinking/copolymerization reaction was terminated with addition of 5

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saturated solution of hydroquinone. Subsequently, reaction mixture was cooled to room 6 8

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temperature, followed by soaked in acetone (400 mL) to remove unreacted monomer and 10

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homopolymer (if formed). Finally, the product was dried to constant weight in a vacuum oven at 1 12

50 °C. 13 15

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Synthesis of cl-AP/exf.LT-AgNPs ternary nanocomposite hydrogel. 17

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0.01 mole of titanium isopropoxide was dissolved in 10 mL ethanol (100 mL RB flask) at room 18 19

temperature. 40 mL ammonical water was added drop wise into the reaction system. Just after 20 2

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addition of ammonical water, a white precipitate was appeared. Then, the stirring was continued 24

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for another 15 min. The precipitate was collected after centrifugation followed by washed 4 times 25 27

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with double distilled water. In due course, 25 mL of 1.6 M ammonium carbonate was added to the 29

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precipitate with constant stirring of 400 rpm and then 3 mL of 30% H2O2 was introduced in to the 31

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reaction mixture.26 The reaction was continued for 30 min at 70 °C with constant stirring. 32 34

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On the other hand, graft copolymerization between AP and METAC followed by crosslinking with 36

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DEGDMA was carried out in 250 mL RB flask under nitrogen atmosphere. Once the 37 38

polymerization mixture became viscous in nature, then freshly prepared light yellow coloured 39 41

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reaction mixture was added into the reaction system under continuous nitrogen purging. 43

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Subsequently, drop wise 10 mL aqueous solution of silver nitrate (0.040 mol) was added into the 4 45

RB flask and the reaction was continued upto 3 h. Then aqueous solution of NaBH4 (0.080 mol in 46 48

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20 mL water) was added very slowly (dropwise added upto 4 h) into the reaction mixture until the 50

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colour change from light yellow to brownish can accomplished. The final step of the reaction was 51 53

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carried out for 12 h at 50 °C with continuous stirring at 400 rpm. Afterwards, the reaction mixture 54 5 56 57

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was cooled to room temperature, precipitated in 400 mL acetone. Then the precipitate was 5

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separated and dried in vacuum oven at 40 °C for 24 h. 6 8

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For comparison, AgNPs based nanocomposite hydrogel (cl-AP/AgNPs) (without using titanium 10

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peroxo carbonate precursor); layered titanate supported AgNPs (LT/AgNPs) (without using cl-AP); 1 12

and cl-AP/exf.LT-ex-situ AgNPs ternary nanocomposite hydrogels were also synthesized. The 13 14

detailed synthetic procedure has been given in “Supporting Information”. 17

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Characterization. 18 20

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The ternary nanocomposite hydrogel was characterized using UV-vis spectroscopy, XRD, 2

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C

NMR, FTIR, FESEM, EDAX, elemental mapping, HRTEM, TGA, and zeta potential analyses. 23 25

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The instrumental details, procedures and the sample preparation technique for HRTEM analysis 27

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have been elucidated in “Supporting Information”. 28 29

Catalytic study and recyclability of the nanocomposites. 30 32

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To investigate the catalytic activity, a small piece (5 mg) of cl-AP/exf.LT-AgNPs was used 3 34

as catalyst for the catalytic reduction of 4-NP in aqueous solution (with pH: 9.60) using 35 37

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NaBH4 as reducing agent. The detailed experimental procedure and recyclability study have 39

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been discussed in “Supporting Information”. To investigate the driving factor for the 40 42

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excellent catalytic activity of the cl-AP/exf.LT-AgNPs, we carried out three controlled 4

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reactions i.e. crosslinked cationically functionalized AP with AgNPs (without adding 46

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titanium peroxo carbonate precursor, cl-AP/AgNPs), layered titanate supported AgNPs (in 47 49

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absence of cl-AP, LT/AgNPs) and in-situ exfoliated layered titanate by cl-AP, followed by 51

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ex-situ addition of AgNO3 precursor (cl-AP/exf.LT-ex-situ AgNPs). All the reactions were 52 53

carried out by maintaining the proper reaction conditions (detailed experimental procedure 54 5

has been given in “Supporting Information”). 57

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Also the catalytic reduction of MB/MO dyes (in solution with pH of MB solution: 6.54 and 5

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that of MO: 6.06) was carried out to support the catalytic activity of cl-AP/exf.LT-AgNPs 6 7

as catalyst. The detailed procedure of catalytic reduction has been explained in “Supporting 10

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Information”. 1 12

Antimicrobial assay. 13 14

Mueller Hinton (MH) agar culture media (Himedia) was poured into sterile petri plates and freshly 15 17

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grown E. coli and B. subtilis (OD600=0.62 equivalent to 107 Cells/ml) cultures were spreaded on 19

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the agar plate surface using sterile cotton swab. Plate was incubated at 37 °C for 24 h and the 20 21

minimum inhibitory concentration (MIC) was examined by calculating the zone of inhibition (in 2 24

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mm). The antibacterial activity was analyzed by measuring the difference between sample and 26

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zone of inhibition. Standard drug Gentamicin (100 µg) was taken as positive control and 0 µg of 27 29

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ternary nanocomposite hydrogel was taken as negative control. 31

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Wastewater treatment. 3

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The decoloration of textile wastewater was carried out in presence of excess NaBH4 with cl34 35

AP/exf.LT-AgNPs under continuous stirring at room temperature. The textile wastewater was 36 38

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collected from Ranchi, Jharkhand, India. 40

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The pond water was collected from GPS coordinate (23.8144° N, 86.4412° E) and the effect of 41 42

ternary nanocomposite hydrogel was analyzed towards the cidal effect on microbial load in the 43 45

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pond water. A serial dilution test was performed and 0.1 mL was plated from 10-5 dilution series 47

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each and the requisite amount of nanocomposite was added to the sample according to the MIC 48 50

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result. The nanocomposite was analyzed with different concentrations viz. 400, 1600, 3200 µg/mL. 52

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After addition of nanocomposite, it was incubated for 1 h at 37 °C in dark. After incubation, 100 53 54

µL of sample was plated in Luria Bertani (LB) agar plate. The CFU was further determined after 5 56 57

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incubating the LB plate at 37 °C for 24 h. No nanocomposite hydrogel was added in the controlled 5

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experiment. 6 8

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RESULTS AND DISCUSSIONS. 9 10

Synthesis of cl-AP/exf.LT-AgNPs ternary nanocomposite hydrogel. 1 13

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The synthetic strategy is based on exfoliation of titanate layer to single titanate sheet 15

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through continuous generation of crosslinked cationically functionalized amylopectin 16 17

followed by growth and stabilization of AgNPs, that is novel. The synthetic route for the 18 20

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formation of cl-AP/exf.LT-AgNPs is based on graft copolymerization of METAC on 2

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amylopectin in presence of DEGDMA crosslinker at 70 °C, which stabilized the single 23 25

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titanate sheet followed by generation of AgNPs to form a crosslinked ternary 27

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nanocomposite hydrogel (cl-AP/exf.LT-AgNPs). 29

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The polymerization of TiO6 octahedra provides anionically charged sheet like structure, 30 32

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which basically supports the fabrication of AgNPs after addition of NaBH4 that were further 34

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stabilized in presence of cationically functionalized cl-AP to restrict the self-stacking 35 36

between individual titanate layers (Scheme 1). The intense colour change from light yellow 37 39

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to brown along with a UV-Vis peak at 400 nm confirms the formation of AgNPs (Figure 41

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S1, Supporting Information). With increase in reaction time of copolymerization, 42 43

crosslinking along with decomposition/hydrolysis of titanium peroxo carbonate with 4 46

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homogeneous immobilization of AgNPs took place simultaneously. 47 48 49 50 51 52 53 54 5 56 57

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1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 52

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Scheme 1. Schematic illustration for the synthesis of fully exfoliated titanate layered supported 54

AgNPs ternary nanocomposite hydrogel i.e. cl-AP/exf. LT-AgNPs. 5

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CHARACTERIZATION. 5

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FTIR spectra. 6

cl-AP (Figure S2a, Supporting Information) contains few additional peaks with all the 7 9

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characteristics peaks of AP.53 A new absorption peak at 1738 cm-1 was due to the presence of ester 1

10

carbonyl carbon present in METAC. Also, the intense peak at 1643 cm-1 confirms the stretching 12 13

frequency of carbonyl carbon of crosslinker. Additionally, another peak at 1472 cm-1 demonstrates 14 16

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the C-H bending of methyl group attached with ammonium moiety. These observations suggest 18

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the formation of crosslinked cationically functionalized AP. However, the FTIR spectrum of cl19 21

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AP/exf.LT-AgNPs nanocomposite reveals the presence of all the characteristics peak of cl-AP. 23

2

But, it was observed that all the peak positions shifted towards lower wavenumber (Figure S2b, 24 25

Supporting Information), indicating the probable electrostatic interactions between the titanate 26 28

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sheets supported AgNPs and cationically functionalized crosslinked AP. 54 30

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13C

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NMR analysis.

Solid state 13C NMR spectrum of cl-AP shows the peak at δ= 177.5, corresponds to the ester carbon 3 35

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of poly (METAC) (Figure S3a, Supporting Information) along with the other peaks of bare AP.31 37

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Additionally, the extra peak at δ= 19.0 suggests the presence of methyl carbon of crosslinker. The 38 39

peaks at 55.3 and 46.1 ppm are responsible for the formation of sp3 hybridized carbon atoms, 40 42

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which formed during the polymerization of METAC (Figure S3a, Supporting Information). On the 4

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contrary, it is obvious that the corresponding spectrum of cl-AP/exf.LT-AgNPs contains all the 45 47

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peaks of cl-AP (Figure S3b, Supporting Information), however the characteristics peak values 49

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were shifted. The observed phenomenon reveals that electrostatic interactions predominate 50 51

between the positively charged crosslinked hydrogel with negatively charged titanate sheet 52 54

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supported AgNPs that confirms the formation of ternary nanocomposite hydrogel as proposed in 56

5

Scheme 1. 57

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XRD and HR-TEM analyses. 5

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Figure 1a represents the XRD pattern of cl-AP/exf.LT-AgNPs. In the diffraction pattern, observed 6 7

distinct diffraction peaks at 2θ=38.1°, 44.0°, 64.10°, 77.34° can be ascribed to the (111), (200), 10

9

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(220) and (311) planes of metallic Ag (JCPDS no. 65-2871). While the absence of any peak at ~ 1 12

10° that are generally observed in layered titanates with highest intensity, responsible for (200) 13 15

14

plane of inter layer distance 21 (also present in the bare layered titanate when synthesized in absence 17

16

of polymer i.e. cl-AP, Figure S4, Supporting Information), indicates the exfoliation of layered 18 19

structure and suggest the absence of stacked layers. The broad peak at ~ 20° was observed for 20 2

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crosslinked copolymer i.e. cl-AP (Figure S5, Supporting Information). Here, it is essential to 24

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mention that other low intense peaks correspond to titanate sheets are not appeared in the 25 27

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diffractograms of cl-AP/exf.LT-AgNPs, most probably due to the presence of large amount of 29

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amorphous grafted polymer. 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57

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Figure 1. (a) XRD pattern of in-situ exfoliated cl-AP/exf.LT-Ag NPs; TEM and HR-TEM 37

images of (b)-(c)-(d) cl-AP/exf. LT-Ag NPs ternary nanocomposite hydrogel 38 39 40 42

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Figure 1b-c represents the TEM and HR-TEM images of the synthesized cl-AP/exf.LT4

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AgNPs nanocomposite hydrogel. The TEM image (Figure 1b) shows randomly distributed 45 46

wire like structures that can be ascribed to the individual LT sheets, confirming the 47 49

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formation of exfoliated LT through adopted in-situ synthesis and supports the XRD result. 51

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Magnified image depicts the presence of high contrast dark spherical particles (Figure 1c), 52 53

attributed to metallic AgNPs, common phenomenon of metallic particles. In the HR-TEM 54 56

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image, presence of distinct lattice fringes in the dark spherical particles with interplaner 57

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distance of 0.23 nm attributed to the (111) plane of metallic Ag. This confirms the formation 5

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of crystalline metallic AgNPs with spherical morphology (Figure 1d and Figure S6b, 6 8

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Supporting Information). In the same HR-TEM image, lattice fringes in low contrast area 10

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with interplaner distance of 0.35 nm corresponds to (101) planes of LT horizontally lying 1 12

in the surface of composite, confirmed the presence of LT in the composite hydrogel 13 15

14

(Figure 1d and Figure S6a, Supporting Information).55 In the HR-TEM image, the presence 17

16

of metallic AgNPs in the surface of horizontally lying LT surface supports the stabilization 18 19

of AgNPs by LT. It is obvious from the HR-TEM analysis that cationically functionalized 20 2

21

cross-linked amylopectin is primarily responsible for stabilization of anionically charged 24

23

titanate sheets, while the stabilized titanate nano sheets could hold and catalyzed the proper 25 27

26

growth of AgNPs. Besides, the EDAX analysis (in HR-TEM) reveals that both Ag and Ti 29

28

are present in the composite hydrogel (Figure S7, Supporting Information). 31

30

FE-SEM analysis and elemental mapping. 32 34

3

The surface morphology of bare AP, cl-AP and cl-AP/exf.LT-AgNPs are shown in Figure 2. AP 36

35

shows granular shaped morphology (Figure 2a), while cl-AP hydrogel demonstrates porous 37 38

morphology with the presence of spherical pores (Figure. 2b). The surface morphology of cl-AP 39 41

40

hydrogel was drastically changed after introduction of exfoliated titanate sheets supported AgNPs. 43

42

After closer inspection of cl-AP/exf.LT-Ag composite, it is obvious that the spherical particles are 4 45

embedded on the surface (Figure. 2c-d), i.e. no surface deposition took place and the formed 46 48

47

spherical particles are stabilized by exfoliated titanate sheets, which are highly dispersed in the 50

49

hydrogel matrix. Here it is essential to mention that the three-dimensional network structure is not 51 53

52

obvious in the FESEM image of nanocomposite. This is probably because the titanate nanosheets 54 5 56 57

60

59

58


Page 17 of 38


1 3

2

and AgNPs are embedded into the crosslinked amylopectin to form a stable composite that results 5

4

the collapse of three-dimensional network. 6 8

7

The elemental mapping of Ti and Ag are uniform in the observed area, which indicates the well 10

9

dispersion of AgNPs and exfoliated LT on the hydrogel matrix (Figure S8a-e, Supporting 1 12

Information). 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 48

47

Figure. 2. FE-SEM images of (a) bare AP (b) pure cl-AP (c) cl-AP/exf.LT-AgNPs (d) closer 50

view of cl-AP exf.LT-AgNPs. 51

49

52 53 54 5 56 57

60

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Page 18 of 38

1 3

2

TGA and UV-Vis NIR analyses. 5

4

The comparative TG analysis of cl-AP and cl-AP/exf.LT-AgNPs are shown in Figure. S9, 6 8

7

Supporting Information. In cl-AP, four steps weight loss zones were observed. The initial weight 10

9

loss within 150 °C was due to the removal of moisture from the sample. The weight loss in the 1 12

region of 150-260 °C was owing to the degradation of crosslinker moiety. The third degradation 13 15

14

zone attributed to the decomposition of polysaccharide backbone. The final weight loss region was 17

16

observed in the range of 420-450 °C that is due to the degradation of grafted poly (METAC) chains. 18 19

While the TGA result of cl-AP/exf.LT-AgNPs suggests the higher thermal stability than the 20 2

21

corresponding hydrogel. It is apparent that cl-AP/exf.LT-AgNPs exhibits all the four stages of 24

23

degradation with increase in stability range. 25 27

26

Besides, UV-Vis reflectance spectra of cl-AP exf.LT-AgNPs demonstrated a band centered 29

28

at 415 nm, reveals that AgNPs are present in the dried matrix of ternary nanocomposite 31

30

hydrogel (Figure S10, Supporting Information). 32 34

3

Stability study of ternary nanocomposite hydrogel. 36

35

Zeta potential was measured to explain the stability of the nanocomposite hydrogel. The 38

37

electrostatic repulsions among the similarly charged particle signify the stability of the colloidal 39 41

40

suspensions. Therefore, the stability of the suspension might be understand by higher positive or 43

42

negative magnitude of zeta potential value.35 The suspension of ternary nanocomposite hydrogel 4 45

was used for zeta potential analysis for a period of 0-7 days with continuous stirring (Figure S11, 46 48

47

Supporting Information). The initial zeta potential value was found to be +66.07 mV. The high 50

49

positive zeta potential value indicates the stability of the suspension. Also, it was observed that 51 52

there was no sharp decline of zeta potential value up to 7 days (+ 52.37 mV), signifying the stability 53 5

54

of the nanocomposite hydrogel in suspension form. 56 57

60

59

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Page 19 of 38


1 3

2

Catalytic reduction of p-nitro phenol and MB/MO dyes. 5

4

The reduction of 4-NP to 4-AP is thermodynamically favorable (E° = -0.76 V) at ambient 6 7

condition when NaBH4 acts as reducing agent in aqueous media (E° = -1.33 V).56 But, the 10

9

8

kinetic barrier of the reduction process is a main hurdle to prevent the forward reaction 1 12

under the experimental time scale. The barrier can be overcome in presence of metal NPs 13 15

14

that has a predominant role as catalyst for complete reduction. 57 To examine the catalytic 17

16

efficiency of cl-AP/exf.LT-AgNPs ternary nanocomposite hydrogel, we carried out the 18 19

reduction of 4-NP with nanocomposite material in presence of NaBH4 as hydrogen source. 20 2

21

With addition of NaBH4 on 4-NP, the formed p-nitrophenolate ion experienced with a high 24

23

spectrophotometric sensitivity and can be monitored easily by UV-Vis spectroscopy. As 25 27

26

obvious from Figure 3, with progress of reaction, the peak for 4-NP (bright yellow) at 400 29

28

nm declined sharply with the generation of new peak at 295 nm (colourless), suggesting 31

30

the formation of 4-AP, a reduced product.56 It was observed that only 16 s was required to 32 3

complete the reduction process with freshly prepared NaBH4 solution (3 mL, 10×10-3M). 36

35

34

The rapid colour change indicates the complete reduction of 4-NP to 4-AP (Figure 3a, 37 38

inset). Also, the stability of the catalyst was examined by repeated experiments with 4-NP. 39 41

40

After each cycle, the catalyst was easily reused for more than 15 times (Figure 3b) by taking 43

42

out the macroscopic bead from the reaction system using spatula, followed by drying. The 4 45

reaction undergoes with six electron transfer process in presence of NaBH4, a hydrogen 46 48

47

source. During the reaction, electron transfer took place in basic media where both 50

49

borohydride ion and nitrophenolate ion adsorbed on the surface of catalyst. Several 51 53

52

scientific groups described the evidence for reduction mechanism. According to Herves et 5

54

al., the reduction occurs via adsorption of both the reactants on the metal (Au/Pd NPs) 56 57

60

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Page 20 of 38

1 3

2

surfaces, which was immobilized on polyelectrolyte brushes.58 Similarly Li et al. described 5

4

the synergistic effect of graphene and involvement of noble NPs for higher catalytic activity 6 7

of 4-NP.59 The involvement of surface area or surface functional groups or leached gold 10

9

8

species also influenced for higher catalytic conversion of 4-NP.60-61 Zhang et al. represents 12

1

transfer of hydride to the AgNPs in lieu of NaBH4 for TiO2 supported AgNPs.62 13 15

14

In the present study, the outstanding catalytic activity of cl-AP/exf.LT-AgNPs composite is mainly 17

16

endorsed by the stabilization of exfoliated LT in presence of cl-AP, which could restricts the 18 19

formation of layer by self-stacking of layered titanate, that was effectively inhibit the 20 2

21

agglomeration of AgNPs and acts as superior supportive material for excellent catalytic activity. 24

23

Moreover, the synergistic effect of cl-AP/exf.LT-AgNPs arises due to electrostatic interactions 25 27

26

between 4-NP with poly (METAC), which brings 4-NP closer towards titania nanosheets 29

28

supported AgNPs that increased the rapid electron transfer to 4-NP in basic media for efficient 31

30

catalytic reduction. 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57

60

59

58


Page 21 of 38


1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 29 30 32

31

Figure. 3. (a) UV-Vis spectra of p-nitro phenol reduction (i) before treatment (ii) 16 s 3 34

after treatment of cl-AP/exf.LT-AgNPs, (b) Stability of the cl-AP/exf.LT-AgNPs during 35 37

36

16 cycles for p-nitro phenol reduction, (c) in-situ MB reduction, and (d) in-situ MO 39

38

reduction. 40 41 42 4

43

For establishing the fact of proper endorsement and identification of the role of LT supported 46

45

AgNPs, we performed some controlled catalytic reduction reactions under same experimental 47 49

48

condition. The conversion time was increased to 1020 s when cl-AP/AgNPs was used under the 51

50

same reaction condition (Figure 4) with rapid release of AgNPs. Similarly, for LT supported 52 53

AgNPs and cl-AP/exf.LT-ex situ AgNPs composite took 180 s and 320 s to complete the reduction 54 56

5

process (Figure 4). The reduced reduction time of cl-AP/exf. LT-AgNPs clearly states that AgNPs 57

60

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58



Page 22 of 38

1 3

2

got proper solid support as titanate nanosheets for suitable immobilization and shows superior 5

4

catalytic activity. So, the combined effect of both components (LT and AgNPs) plays crucial role 6 8

7

for stabilization of composite hydrogel as well as efficient catalytic reduction. Similarly, the 10

9

property of LT is highly dependent on the extent of exfoliation of layers. Thus, it is apparent that 1 12

the exfoliation of layer in LT using cationically functionalized AP along with proper support of 13 15

14

AgNPs is the main driving factor for efficient and rapid catalysis. Finally, appropriate growth and 17

16

immobilization of AgNPs was occurred in presence of titanate sheet, which was already stabilized 18 19

by cationically functionalized cl-AP, that boost up the overall efficiency of cl-AP/exf.LT-AgNPs 20 2

21

as catalyst for rapid hydrogenation reduction of 4-NP to 4-AP. Further, to obtain the optimum 24

23

catalyst dose, the concentration of nanocomposite hydrogel was varied from 1-15 mg. It was 25 27

26

observed that with dosage of 5 mg, the catalyst showed best reduction efficacy (16 s) (Figure S12, 29

28

Supporting Information). 30 31 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 54

53

Figure 4. Comparative reduction time of 4-NP using different catalysts. 5 56 57

60

59

58


Page 23 of 38


1 3

2

Additionally, rapid catalytic reduction was also observed towards the decoloration of MO (90 s) 5

4

and MB (80 s) dyes at room temperature in aqueous media (Figure 3c-d). Formation of metal 6 8

7

hydride on catalyst surface after adsorption of NaBH4 catalyzed the reduction of MO/MB on metal 10

9

surface. Then desorption leads to create a vacant space for further MO/MB adsorption, continuing 12

1

the reduction reaction.63 The ternary nanocomposite hydrogel catalyst gives excellent catalytic 13 15

14

efficiency upto 5th cycle (Fig. S13, Supporting Information). 17

16

To elucidate the catalytic efficiency of cl-AP/exf.LT-AgNPs for reduction of 4-NP, MB and MO 18 19

in presence of NaBH4, the effective catalysis time and amount of nano-catalyst were compared 20 2

21

with various reported catalysts.14, 38-52 However, it is very difficult to directly correlate the catalytic 24

23

efficiency of cl-AP/exf.LT-AgNPs with reported literature statistics because of their unlike 25 27

26

experimental conditions. Table 1 discloses the fact that the cl-AP/exf.LT-AgNPs can be considered 29

28

as one of the most promising catalysts reported so far (It took only 16 s for reduction of 4-NP using 31

30

5 mg of cl-AP/exf.LT-AgNPs catalyst). 32 3 34 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57

60

59

58



Page 24 of 38

1 3

2

Table 1. Comparative review on different catalyst for 4-NP and organic dye reduction 6

5

4

Catalyst

Amount of catalyst for pNP reduction

p-NP reduction (time)

PAM/graphene/Ag ternary hydrogel14 Ag NPs embedded semi-IPN hydrogel38 Porous Au- PdNPs39

0.13g 10 mg 1 ml of catalysts solution (0.5 mg/ml) 1 mg -

90 s 18 min. 12 s

7 min -

10 mg

20 mg

10 min.

-

30 min. for MB -

(1 mL, total Au content: 0.2 μmol)

14 min.

-

-

10 min. 10 min.

-

-

Four beads of Ag@AMH 50 μL 0.009% w/w CNCPAMAM-Au (Au content: 0.02 μmol) 1.5 mL of an aqueous dispersion of MRChS (Ag– Fe3O4@chitin microspheres) at 50 mg L−1 1.0 mg

13 min.

-

-

270 s

-

-

10 min.

-

-

8 min.

-

-

1.0 mL 5 wt%

6 min.

-

-

20 mg

4 min

-

10 min.

5 mg 5 mg

20 s 16 s

10 mg

90 s (MO) 80 s (MB)

7 8 9 10

15

14

13

12

1

16 18

17

21

20

19

Cu2O-Ag40 Ag entrapped hydrogel41 Acrylic acid - Amidodiol /Ag hydrogel (SPAG) 42 Gold NPs deposited cellulose nanocrystal(Au NPs@CNs)43 25

24

23

2

26

Ag–Alginate biohydrogel44 Ag NPs – poly(vinyl alcohol) [PVA]45 28

27 29 30 31 3

32

Alginate based biohydrogels (Ag@AMH)46 CNC (cellulose Nanocrystals) – PAMAM(polyamidoamine)-Au-147 38

37

36

35

34

39 41

40

Ag–Fe3O4@chitin microspheres48 42 43 4 45 46 47 49

48

Carbon nanofibers/silver nanoparticles nanocomposite (CNFs/AgNPs)49 Fe3O4@TiO2@Au magnetic microsphere50 Ag NPs immobilized Fe3O4@C (MFC) nanocomposites51 Pd–rGO–CNT52 cl-AP/exf. LT-AgNPs This study 57

56

5

54

53

52

51

50

Polymer thin film film (thickness ca. 1.3 mm)

60

59

58


Amount Organic dye of reduction catalyst time for organic dye reduction 0.13g 80 s (MB) -

Page 25 of 38


1 3

2

Assessment of antimicrobial activity. 5

4

Antibacterial materials have received great interest in packaging industries and medical science. 6 8

7

Therefore, cl-AP/exf. LT-AgNPs was examined for in vitro antibacterial activity against E. coli 10

9

and B. subtilis as model microbes. It has been well established that metallic Ag exhibits excellent 12

1

antimicrobial activity.11-12 Here, zone of inhibition test was performed to analyze the extent of 13 15

14

antimicrobial activity of the ternary nanocomposite hydrogel. Gentamicin (100 µg Himedia) was 17

16

taken as positive control (Figure 5a) in which a clear zone of inhibition was found against E. coli 18 19

and B. subtilis cells. However, the zone of inhibition was absent in negative control (Figure 5b). 20 2

21

In this study, the antimicrobial activity was started from 400 µg/mL but the ternary nanocomposite 24

23

hydrogel is not able to kill all the initially inoculated cells (Figure 5c-d). Whereas, no zone was 25 27

26

found below nanocomposite concentration of 400 µg/mL (Figure 5e). But the ternary 29

28

nanocomposite hydrogel concentration of 3200 µg/mL was found to be effective for both E. coli 31

30

and B. subtilis cells (Figure 5e). MIC reveals that compound has significant biological activity 32 34

3

(Figure 5e) at 400 µg/mL concentration. The effective concentration was found to be 3200 µg/mL 36

35

that relies with previously reported studies.11 It can be demonstrated from the results that AgNPs 37 38

present in the ternary nanocomposite hydrogel are able to kill E. coli and B. subtilis through 39 41

40

diffusion of silver in to the media. The excellent antimicrobial effect of the material at 3200 µg/mL 43

42

is because of the presence of comparatively higher amount of silver from other nanocomposite 4 45

concentrations and extent of silver release from the ternary nanocomposite hydrogel. The exact 46 48

47

mechanism of silver on the microbes is still unclear.64 But, the probable mechanism is based on 50

49

the interaction of AgNPs with cell membrane ofmicobes.65 Actually AgNPs has tendency to react 51 53

52

with thiol containing proteins through inhibiting the growth by interrupting the respiration 5

54

mechanism leading to the cell death.66 56 57

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Page 26 of 38

1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 2 23 24 25 26 27 28 30

29

Figure 5. Inhibition zone towards (a) positive control (100 µg Gentamicin), (b) negative control, 32

31

(c) different nanocomposite concentration of 400 µg/mL, 800 µg/mL, 1600 µg/mL and 3200 3 34

µg/mL for E. coli, (d) different nanocomposite concentration of 400 µg/mL, 800 µg/mL, 1600 35 37

36

µg/mL and 3200 µg/mL for B. subtilis, and (d) MIC of the tested compound against E coli and 38 39

B. subtilis. 40 41 42 4

43

Wastewater treatment 46

45

The treatment of industrial effluent is a very requisite criteria for maintaining environmental 47 49

48

sustainability. The fabricated ternary nanocomposite hydrogel exhibits rapid removal of toxic 51

50

organic contaminants from synthetic wastewaters.35 Subsequently, the applicability of the material 53

52

was investigated using the coloured textile wastewater. After 10 min of continuous stirring of 54 56

5

textile effluent in presence of NaBH4 and cl-AP/exf.LT-AgNPs, the effluent became colourless 57

60

59

58


Page 27 of 38


1 3

2

(Fig. S14, Supporting Information). Thus, it is obvious that cl-AP/exf.LT-AgNPs nanocomposite 5

4

hydrogel demonstrates environmental remediation through decoloration of textile effluent. 6 8

7

The synthesized ternary nanocomposite hydrogel also exhibits excellent bactericidal activity 10

9

towards the microorganism present in pond water. With increase in nanocomposite concentration, 1 12

the colony was found to decrease in number. In the control LB plate, the concentration of bacterial 13 15

14

cells were to be too numerous to count (TNTC), while as the concentration increased to 3200 17

16

µg/mL, the colony forming unit (CFU) was reduced to 90%, indicating the bactericidal affect (Fig. 18 19

S15, Supporting Information). Thus, it is apparent that the ternary nanocomposite hydrogel has 20 2

21

potential impact towards the reduction of microbial load from high microbial contaminated water. 24

23

CONCLUSION. 25 27

26

In summary, we have developed a novel cationically modified crosslinked amylopectin, which 29

28

served as efficient stabilizer for formation of exfoliated single sheet structure that further acts as 30 32

31

support material for homogeneous dispersion and controlled growth of AgNPs. The in-situ formed 34

3

titanate nanosheets can efficiently prevent the aggregation and rapid leaching of AgNPs along with 35 36

usable active area, which assists the overall stability of the ternary nanocomposite hydrogel. The 37 39

38

prepared cl-AP/exf.LT-AgNPs material showed excellent catalytic and highly recyclable ability 41

40

towards reduction of 4-NP, MB and MO via synergistic effect of crosslinked natural polymer 42 43

stabilized nano sheets supported AgNPs. Besides, the antibacterial disc assays towards E. coli and 4 46

45

B. subtilis demonstrated the excellent antibacterial activity of ternary nanocomposite hydrogel 48

47

which is effective towards varieties of practical applications. Considering the various aspects of 49 51

50

environmental ecology, the ternary nanocomposite hydrogel acclaimed as an excellent sustainable 53

52

green material. Therefore, the synthetic approach may open a new possibility for integrating 54 5

titanate nanosheets supported other metal nanoparticles for advanced studies. 56 57

60

59

58



Page 28 of 38

1 3

2

SUPPORTING INFORMATION 4 6

5

Synthesis procedure of nanocomposites formation (Synthesis of cl-AP/AgNPs, cl-AP/exf. LT/ex8

7

situ AgNPs, LT/AgNPs), details of characterization techniques, catalytic study of p-NP, MB and 9 10

MO, the results of UV-Vis spectrum, FTIR spectra, XRD analysis, HR-TEM analysis, EDAX 1 13

12

analysis, Elemental mapping, TGA analysis, UV-Vis NIR analysis, zeta potential analysis, data on 15

14

comparative reduction time of 4-NP with variation of cl-AP/exf.LT-AgNPs dosage and catalytic 16 17

reduction of MB dye upto 7th cycles and MO dye reduction up to 7th cycles; Treatment of textile 18 20

19

wastewater; Graph between number of colonies with different concentration of ternary 2

21

nanocomposite hydrogel towards pond water; Variation of colony formation assay with 23 25

24

nanocomposite concentration. This material is available free of charge via the Internet at 27

26

http://pubs.acs.org. 28 29 30 32

31

Acknowledgement 34

3

The first author earnestly acknowledge the financial assistance from Indian Institute of Technology 35 36

(Indian School of Mines), Dhanbad for Senior Research fellowship. Authors also acknowledge the 37 39

38

various instrumental facilities of CRF of Indian Institute of Technology (Indian School of Mines), 41

40

Dhanbad. The authors also acknowledge the help given by the Analytical Discipline and 42 43

Centralized Instrument Facility of CSIR-CSMCRI for characterization of materials. 4 45 46 48

47 ┴*Corresponding

49

Author: Tel: +91-278-2567760, Ext-704; E-mail: [email protected]

50 51 52 53 54 5 56 57

60

59

58


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1 3

2

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Crosslinked Biopolymer Stabilized Exfoliated Titanate Nano Sheet Supported AgNPs: A Green Sustainable Ternary Nanocomposite Hydrogel for Catalytic and Antimicrobial Activity 12

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Amit Kumar Sarkar, Arka Saha, Lipi Midya, Chiranjib Banerjee, Narayan Mandre, Asit Baran Panda, Sagar Pal 17

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Crosslinked biopolymer based ternary nanocomposite hydrogel, a novel sustainable material has been developed towards the control of environmental remediation via catalytic reduction of toxic contaminants and antimicrobial activity. 24

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