Aminated polyethersulfone-silver nanoparticles (AgNPs-APES) composite membranes with controlled silver ion release for antibacterial and water treatment applications

Materials Science and Engineering C 62 (2016) 732–745

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Aminated polyethersulfone-silver nanoparticles (AgNPs-APES) composite membranes with controlled silver ion release for antibacterial and water treatment applications M. Salman Haider a,d, Godlisten N. Shao b,c, S.M. Imran b, Sung Soo Park b, Nadir Abbas b, M. Suleman Tahir d, Manwar Hussain b, Wookeun Bae a, Hee Taik Kim b,⁎ a

Department of Civil and Environmental Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea c Department of Chemistry, University of Dar es Salaam, Mkwawa College, Iringa, United Republic of Tanzania d Department of Chemical Engineering, University of Gujrat, HH Campus, Punjab, Pakistan b

a r t i c l e

i n f o

Article history: Received 14 October 2015 Received in revised form 30 December 2015 Accepted 8 February 2016 Available online 10 February 2016 Keywords: Silver nanoparticles Aminated polyethersulfone Antibacterial activities Leaching Composite membranes

a b s t r a c t The present study reports the antibacterial disinfection properties of a series of silver nanoparticle (AgNP) immobilized membranes. Initially, polyethersulfone (PES) was functionalized through the introduction of amino groups to form aminated polyethersulfone (NH2-PES, APES). AgNPs were then coordinately immobilized on the surface of the APES composite membrane to form AgNPs-APES. The properties of the obtained membrane were examined by FT-IR, XPS, XRD, TGA, ICP-OES and SEM-EDAX analyses. These structural characterizations revealed that AgNPs ranging from 5 to 40 nm were immobilized on the surface of the polymer membrane. Antibacterial tests of the samples showed that the AgNPs-APES exhibited higher activity than the AgNPs-PES un-functionalized membrane. Generally, the AgNPs-APES 1 cm × 3 cm strip revealed a four times longer life than the un-functionalized AgNPs polymer membranes. The evaluation of the Ag+ leaching properties of the obtained samples indicated that approximately 30% of the AgNPs could be retained, even after 12 days of operation. Further analysis indicated that silver ion release can be sustained for approximately 25 days. The present study provides a systematic and novel approach to synthesize water treatment membranes with controlled and improved silver (Ag+) release to enhance the lifetime of the membranes. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Improving the lifetime of water treatment membranes can be achieved through precise control of active disinfectant release. There has been worldwide attention in using desalination technologies to treat seawater, brackish water and wastewaters [1,2]. Growing global demand for reliable clean water, ease of operation, and increasing environmental concerns makes polymer-based membrane filtration more prudent [3]. Polymer membrane water treatment technology has been the most widely used technology, especially for organizations seeking to reduce their water footprint by reusing wastewater [4]. Recent advances in technology have significantly reduced the cost of membrane-based systems. Installation costs are lower because membrane systems do not require large buildings or as much land as conventional systems. Operating costs can also be reduced since today's membranes produce more water and remove more impurities while

⁎ Corresponding author. E-mail addresses: [email protected] (M. Hussain), [email protected], [email protected] (H.T. Kim).

http://dx.doi.org/10.1016/j.msec.2016.02.025 0928-4931/© 2016 Elsevier B.V. All rights reserved.

using less energy [5]. However, membrane biofouling remains an inevitable complication in the membrane process, causing a decline in the membrane performance and consequently higher operation and maintenance costs for cleaning and replacement [6]. Biofouling is referred to as the unwanted deposition and growth of biofilms. A biofilm is an accumulation of microbial cells belonging to attached-growth systems that are irreversibly associated (not removed by gentle rinsing) with a surface and enclosed in a matrix of extracellular polymeric substances (EPS) [7]. Microorganisms are present in nearly all water systems and are capable of colonizing almost any surface [8]. Water contains various dissolved salts and nutrients that are converted to immobilize structures, from solution to semisolid state by these microorganisms resulting in blocking/fouling of the membrane [9]. This biofouling affects the efficiency of the membrane by decreasing the permeate flux and salt rejection [10]. Hence, developing anti-fouling membranes has become the main focus of many researchers in academics and industry. In recent years, polymer nanocomposites have attracted considerable attention as a result of their extraordinary performance, improved properties compared to constituent's parts, design flexibility, lower life-cycle costs, and uniquely large applicability of nanocomposites to various industrial fields [11].

M.S. Haider et al. / Materials Science and Engineering C 62 (2016) 732–745

Polymer nanocomposites are advanced functional materials composed of nanoparticles coated or dispersed inside a polymer matrix [12] As a result, the produced material combines suitable properties of both components [13]. Among the several nanoparticles that have been used as a polymer-functionalizing agent, silver nanoparticles signify the most sought-after (nano) material. This is primarily due to their unique optical [14], electric [15], catalytic [16], and particularly, antimicrobial properties [17,18], which are well established and extensively investigated in polymer nanocomposites membranes. AgNPs with well-known antimicrobial activity, broad antimicrobial spectrum, low human toxicity, and ease of use make it a promising choice for water disinfection and microbial control. However, numerous practical applications of silver nanoparticles require their immobilization on various substrates and matrices [19], which make polymer materials the best possible combination because of their specific morphology, chemical and structural nature with long polymeric chains allowing for the incorporation and fine dispersion of nanoparticles. Recently, AgNPs have been studied with their effect in different polymeric materials, such as cellulose acetate [20], polyimide [21], polyamide [22], and poly (2-ethyl-2-oxazoline) [23] membranes. Less research has been conducted to determine the properties of polyethersulfone (PES) membranes, which are prominent for their widespread application in water filtration (microfiltration, ultrafiltration, or nanofiltration membranes). Recently, incorporation of AgNPs in a sulfonated PES membrane [24], silver-filled PES membranes [25, 26] and biogenic AgNPs incorporated in PES [18], for antifouling application shows the revival of PES membrane significance. However, the long-term antimicrobial effects of AgNPs during continuous filtration have not been clearly addressed and the AgNP mechanism of the antimicrobial activity is also not fully understood. The three most common suggested mechanisms are: (i) gradual release of Ag+ ions that disrupt the ATP production and DNA replication, (ii) direct destruction of cell membranes by AgNPs, and (iii) AgNPs and silver ion generation of a reactive oxygen species [27,28]. It is well known that the antimicrobial activity of AgNPs largely depends on the release of silver ions. Kumar and Münstedt [27] made substantial efforts toward the investigation of ion release from silver/ polyamide composites. Kumar et al. suggested the importance of having ions flow from the silver component toward the bacteria or vice versa, and they further reported two different physical mechanisms that affect the mass transfer: (i) diffusion and (ii) embedding (intermolecular forces between the nanoparticles and polymer). High silver release from the composite membrane is good for antimicrobial activity, but the rapid depletion of silver in the membrane reduces the life of the membranes [18,20,26,29,30]. Thus, the development of AgNP bonded membranes (with strong intermolecular attraction between the nanoparticle and host polymer) can allow for the appropriate release of silver ions. Zhao et al. [31] suggests that coordinate bonding occurs between the silver and amine groups grafted on the polymer surface and subsequent reduction of silver by a strong reducing agent can facilitate the formation of composites with the controlled release of silver nanoparticles. Based on this suggestion, few researchers have immobilized silver on different substrates by coordinately bonding them to the amine group (− NH2), which have shown promising results in both gradual silver release and disinfection [32,33]. Quang et al. [34] reported that amino groups can be functionalized on silica surface micro beads by a dry method to support AgNPs. However, a study on immobilization of AgNPs on an amine group (\\NH2) grafted polymer for water treatment membranes has been overlooked. In the present study, polyethersulfone (PES) containing amine groups on the backbone was synthesized and then AgNPs were immobilized on the surface of the obtained polymer membrane. The attachment of the AgNPs on the amino groups of the polymer can act as a protracted silver releasing membrane for antimicrobial purposes in water disinfection processes. PES is one of the most suitable

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polymeric materials for grafting amine due to its outstanding thermal tolerance, chemical stability, oxidation resistance, and mechanical characteristics [24,35]. The effect of silver leaching and anti-biofouling in the AgNPs-APES samples was systematically investigated. The disinfection potential of this antibacterial material was evaluated against Escherichia coli bacterial strains acquired from the American type culture collection under the code ATCC 25922, using zone of inhibition [18] and concentration-contact testing methods [36]. Furthermore, silver leaching tests were also performed to examine the leaching of silver nanoparticles from the polymer membrane. To the best of our knowledge, no studies have reported on the formation of promising amino group functionalized polymer membranes for controlling silver release. 2. Experiment 2.1. Materials The polymer (PES) was obtained from BASF (Ultrason-S6010, Korea). Tin (II) chloride (SnCl2·2H2O), sodium iodide (NaI), silver nitrate (AgNO3), sodium hydroxide (NaOH), silver nanoparticles (≤80 nm) and glacial acetic acid were purchased from Sigma Aldrich® Chemical Co. and used without further purification. Chloroform (CHCl3) was procured from Merck Schuchardt OHG Germany. Nitric acid (HNO3) and dimethyl sulfoxide (DMSO) were acquired from Junsei Chemical Co., Ltd., Korea. Sulfuric acid (H2SO4), hydrochloric acid (HCl), and common reagents such as methanol were obtained from Dae-Jung Chemicals & Metals Co., Ltd., Korea and used without further purification. Nexpure® RO 2000 Korea distilled water was used. 2.2. AgNP embedded polyethersulfone (AgNPs-PES) (M2) In order to produce a homogeneous dispersion of AgNPs in the PES matrix (M2), 0.01 g of AgNPs (9.27 × 10−5 mol) were added to DMSO (5 mL) and dispersed by an ultrasonic processor (SONICS, model VCX750, USA) for 1 h. Simultaneously, 5 g of PES (0.022 mol) were mixed in DMSO (10 mL) and stirred at 1000 rpm (DAIHAN Scientific Co., Ltd., Model SMHS-6, Korea) for 1 h. The AgNPs/DMSO solution was slowly poured into the PES/DMSO solution and the mixture was stirred for 30 min. The flat sheet membranes were prepared by phase inversion via an immersion precipitation technique [29,37]. The homogeneous polymer solution was heated at 190 °C to evaporate excess DMSO. Subsequently, the solution was cast on the glass surface using a casting knife (Elcometer 3580, UK) with a 210 μm thickness. This was immediately moved to a non-solvent bath containing water at room temperature without any evaporation. The compositions of the casting solution are shown in Table 1. This membrane was prepared with ≤80 nm AgNPs in the casting solution and was named M2. M1 is pure PES membrane (Table 1). 2.3. Preparation of AgNPs-APES membrane (M3 and M4) 2.3.1. Nitration of polyethersulfone (NO2-PES) 5 g of PES (0.022 mol) beads were added to a nitrating mixture of HNO3 (10 mL) and H2SO4 (25 mL) in an oil bath at 60 °C and stirred at a constant speed of 900 rpm for 4 h. Upon completion, the reaction mixture was poured over cold water in a refrigerated bath, and washed several times with distilled water to remove excess acid. The obtained

Table 1 Composition of as-synthesized membranes. No.

Composition

PES (wt.%)

DMSO (wt.%)

AgNO3 (wt.%)

AgNPs (wt.%)

M1 M2 M3 M4

PES PES-AgNPs AgNPs-APES AgNPs-APES

18.5 18.5 18.5 18.5

81.5 81.42 81.4 81.3

0 0 0.1 0.2

0 0.08 0 0

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flakes were separated by vacuum filtration and sequentially washed with distilled water, NaOH (2 M), and then again with distilled water. The flakes were dried in a vacuum oven for 24 h. Finally, 5.5 g of nitrated polyether sulfone (NO2-PES) were obtained.

for the remainder of the phase-inversion process. AgNPs-APES membranes were thoroughly washed with de-ionized water and dried in a vacuum oven for 12 h. 2.4. Membrane characterization

2.3.2. Amination of polyethersulfone (APES) NO2-PES was further reduced by the following procedure. A mixture of 5 g of NO2-PES and chloroform (25 mL) was poured into a threenecked round bottom flask equipped with a reflux condenser. Then, 25 g of SnCl2·2H2O (0.11 mol) and 1.67 g of NaI (0.011 mol) in a (2:1) mixture of HCl and glacial acetic acid (100 mL) were slowly added to this solution over 5 min at 60 °C with stirring. The polymer started to precipitate 20 min after the addition. To avoid precipitation, methanol was added until the solution became clear. This mixture was further refluxed for 6 h and cooled to room temperature. The polymer was poured in a NaOH (2 M) solution, and the precipitated polymer was vacuum filtered and washed with distilled water until the base was completely removed. APES or (NH2-PES) was dried for two days at 45 °C under vacuum. 2.3.3. In situ attachment/immobilization of AgNPs on APES Two membrane samples (M3 and M4) were prepared by dissolving 3 g of aminated polyethersulfone (NH2-PES) for each sample in DMSO (10 mL). Then, 0.5 mL of the AgNO3 (0.2 M) solution for M3 and 0.5 mL of AgNO3 (0.4 M) solution for M4 were added to the solution and the mixture was stirred for 2 h at room temperature. The polymer membrane was prepared using the wet phase-inversion process. The composite polymer solution was heated at 190 °C until the excess DMSO was evaporated and cast on a support material (glass, polymer, metal, non-woven) with a casting knife (Elcometer 3580, UK) using a gap setting of 300 μm at an appropriate casting shear. The thickness of the non-woven support layer was 210 μm and the initial thickness of the composite membrane was approximately 250 μm. The glass plate and membrane film were quickly transferred to a water bath at room temperature for 1 min and then transferred to NaBH4 (0.2 M) solution

Fourier transform infrared spectroscopy was used to assess the bonding pattern of the synthesized materials using an FT-IR spectrometer (Avatar 360 E.S.P., Nicolet). The IR spectrometer was equipped with a DTGS KB detector and the transmittance measurement was performed by making KBr pellets containing 2 wt.% of the sample to be analyzed. An average of 64 scans with a wavenumber resolution of 4 cm−1 and optical velocity of 0.6334 cm−1 was collected from 400 to 4000 cm−1. The 1H NMR spectra were recorded on a Bruker (400 MHz) instrument using DMSO-D6 (2.5 ppm) as solvent. The initial changes in the elemental composition of all the as-synthesized membranes were confirmed by an X-ray photoelectron spectrometer (XPS; UVS-20-A, SPECS, Germany) using an Al Kα X-ray source. AgNPs formations were further confirmed by X-ray diffraction (XRD) measurements using a Rigaku rotating anode X-ray diffractometer (D/MAX-2500/PC, Rigaku, Japan) with a scanning speed of 5°/min from 10° to 60° equipped with a CuKα radiation source (λ = 0.15418 nm) at an accelerating voltage of 50 kV and a current of 100 mA. Field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan) with an accelerating voltage of 15.0 kV was used to study the morphology of the composite. The FE-SEM was coupled with energy dispersive spectroscopy (EDS) to assess the purity and elemental composition of the synthesized samples. The samples for FE-SEM characterization were scattered lightly with a spatula on carbon tape that was affixed to an aluminum stub. The stub was placed into a sputter coater for 10 min to allow for a thin layer of Pt. Thermogravimetric analysis (TGA, TA Instruments, Q500, USA) was performed to check the compatibility of the polymer. The polymers were dried in a vacuum oven at 80 °C under − 0.8 bar to remove moisture, and TGA of the nanocomposites was performed at a heating rate of 10 °C/min to 25–800 °C under a nitrogen environment.

Scheme 1. Schematic illustration of aminated-polyethersulfone (APES) decorated with silver nanoparticles.

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Fig. 1. FT-IR of (a) PES, (b) NO2-PES, (c) APES (NH2-PES) and (d) AgNPs-APES.

2.5. Silver leaching The static release of silver ions from the aminated polymer membrane was studied using an immersion technique [18]. Rectangularshaped 1 cm × 3 cm stripes were immersed in 10 mL of deionized water and after 24 h, the membrane was transferred to a fresh DI water vial of the same volume. This step was performed for 12 days at room temperature. It was estimated that the 1 cm × 3 cm strip area out of the 2 cm × 6 cm strip should contain 1/4th the total AgNPs available in the entire polymer strip. Silver ion leaching was quantified with an inductively coupled plasma optical emission spectrometer (ICP OES) with axial and radial viewing plasma configuration Model Optima 8000 (Perkin Elmer, USA) operating at a 40 MHz. The nebulization system, with a chemical resistant concentric glass nebulizer coupled to a glass cyclonic spray chamber was utilized. The polychromatic device, with spectral range of 160 to 900 nm and the system was provided with an S 10 auto sampler (Perkin-Elmer). The operating conditions used are presented in Table S5.

2.6. Antibacterial activity (zone of inhibition test) The antibacterial activity of the synthesized polymers was studied against the gram-negative bacterial strain E. coli [18]. The strain, acquired from the American type culture collection under the code ATCC 25922, was maintained in nutrient broth (NB) at 37 °C with continuous shaking at 200 rpm for 24 h. The bactericidal effect of the composite polymers was studied in nutrient agar medium using a zone of inhibition method. Circular discs 6 mm in diameter and 0.65 mm thick were prepared out of the polymer membrane samples (M1, M2, M3 and M4). The disc samples were washed in sterile water and dried. 20 μL of the E. coli broth culture were spread on the solidified nutrient agar. Three sample discs from the same polymer were gently placed on the

agar culture plates and incubated at 37 °C. A similar procedure was performed for all the prepared membranes with different particle concentrations. A control sample was made by inoculating plain pure PES polymer discs (M1) on the E. coli agar culture plates. 2.7. Concentration-contact testing In addition to the zone of inhibition test, concentration-contact testing or the quantitative suspension test method was also performed to further verify the antibacterial effect of the as-synthesized membrane [36]. In this particular experiment, 5 mL of E. coli (OD600 0.091/mL under the code ATCC 25922) suspended NB cultures were made in sterile culture tubes. Then, 1 g each of M1, M2, M3 and M4 was suspended in the bacterial cultures and incubated for 20 min. 100 μL of the culture suspension and four samples were collected at the 0th, 5th, 10th and 20th minute of cultivation to investigate the influence of contact time on bacteria. Plating was performed with 100, 102 and 104 serial dilutions of the supernatant fractions using NB. 10 μL of each dilution were plated, in triplicate, on the nutrient agar and incubated at 37 °C for 24 h. The number of colonies formed were counted and converted to CFU/mL. Pure PES (M1) was also tested under the same procedures. 3. Results and discussion 3.1. Membrane characterization In the present study, silver nanoparticles were coordinately attached to aminated PES (AgNPs-APES) membrane by a three-step process (Scheme 1). The process includes the initial nitration of pure PES (NO2-PES), reduction of the nitro group to amine (NH2-PES) and then coordination between NH2 and AgNPs by the reduction of AgNO3 with NaBH4 (AgNPs-APES). The molecular structure and functional groups on the polymer were confirmed by specific absorption peaks of FT-IR

Scheme 2. Preparation of aminated-polyethersulfone (APES).

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Fig. 2. 1H NMR of (a) PES, (b) NO2-PES and (c) APES (NH2-PES).

spectroscopy. Fig. 1 presents the FT-IR spectra for the confirmation of nitration, amination and silver bonding stages, i.e., (a) PES to (b) NO2-PES to (c) APES (NH2-PES) and then to (d) AgNPs-APES. Spectrum (a) confirms the characteristic peaks of the pure PES molecular structure. Pure PES structure contains a benzene ring, a sulfone, and an ether bond. The C\\H stretching peak of the benzene ring was positioned at 3058.39– 3080.19 cm−1 [38,39]. Three peaks of aromatic skeletal vibration were observed between 1600 and 1400 cm−1. The C\\O\\C stretching peaks were situated at 1324 and 1239 cm−1. The S_O stretching peaks were observed at 1151 and 1105 cm−1 [40, 41]. The Fig. 1(b) spectrum shows the same characteristic peaks of PES, and the N\\O stretching peak can be observed at 1536.92 cm−1, suggesting the attachment of NO2 on the PES molecular structure. The attachment of NH2 on PES can be observed in Fig. 1(c) based on the formation of the two bands of N\\H appearing at 3390.53 and 3416.13 cm−1 and the deformation band of \\NH2 at 1618.92 cm−1 [35,42]. Fig. 1(d) shows the same PES signature peaks along with the N\\H peaks at 3350.23 and 3378.86 cm−1 while the deformation peak is at 1604.88. In spectrum (d), the position and relative intensity of these bands changed during amine metal particle complex formation. For example, the shifts of the band at 3390.53 and 3416.13 cm−1 in APES to a lower wavenumber 3350.23 and 3378.86 cm−1 in AgNPsAPES were observed [43,44]. These shifts indicate the preferable arrangement of the amine coordination bond between the metal particle surfaces. The larger band shift for the complex crystal indicates a stronger interaction between the metal ion and amine group [35,42]. Scheme 2 describes the two-step attachment process of functional NO2 and NH2 on the PES surface. Next the functionalization of PES by nitro group (later was reduced to NH2 group) was characterized by NMR spectroscopy and the 1H NMR spectra of PES, PES-NO2 and APES in DMSO-D6 are presented in Fig. 2 and Fig. S1. Fig. 2(a), shows two distinct peaks of aromatic ring, up-field doublet peak represent the hydrogen (Hb) at ortho-position to the alkoxy group while down-field doublet peak is from hydrogen (Ha) at ortho-position to sulfonyl group in PES [45]. After nitration of un-functionalized PES, 1H NMR revealed the presence of nitro group on aromatic ring by showing a very down field singlet peak at δ-8.68 in Fig. 2(b). This down-field shift is attributed to the presence of aromatic-proton (Hc) at ortho-position of two highly electron

withdrawing sulfonyl and nitro groups. Presence of some additional peaks in the spectrum credited to the presence of nitro group on aromatic ring which resulted in shifting of δ-values. After reduction of PES-NO2 to APES, 1H NMR depicted the presence of Aryl-NH2 group in Fig. 2(c) by showing broad singlet peak of amino group at δ-5.71 and the singlet peak at δ-8.68 (Hc) in PES-NO2 [Fig. 2(b)] was shifted to δ-8.33 (Hd) due to the electron withdrawing nature of amino group [46]. The compositions of all the prepared membranes (M1, M2, M3 and M4) are shown in Table 1. Two AgNPs-APES membranes (M3 and M4) with different concentrations of AgNO3 were prepared to assess the effect of concentration on size, distribution, disinfection, and leaching. XPS analyses of unmodified and modified PES membranes were performed in order to confirm amine attachment in APES (NH2-PES) and silver coordination on AgNPs-APES (M3 and M4) membrane surfaces. The surface compositions, atomic percentages and assignment of binding energies of C1s, O1s, N1s, S2p and Ag3d in pure PES, APES and AgNPs-APES membranes are given in Table 2. The main spectrum and core level spectrums of C1s, O1s, N1s, S2p and Ag3d XPS spectra are shown in Figs. 3 and 4, respectively. Fig. 3(a) depicts the wide range spectra of modified and unmodified PES. It is well-known that the bands centered at 168, 284, and 531 eV are associated with the core level spectrum of S2p, C1s and O1s, respectively, which confirms the signature structure of unmodified PES in

Table 2 Assignment of binding energies of main XPS regions and atomic percentages. Peaks

PES (eV)

at.%

NH2-PES (eV)

at.%

AgNPs-APES (eV)

at.%

Assignment

C1s

70.43

–

–

Ag3d

–

–

283.69 284.95 288.22 531.09 533.40 167.01 398.53 – 367.91 373.96

66.27

N1s

283.36 284.64 287.06 531.73 533.51 167.10 398.48 400.18

66.05

S2p

285.87 286.31 287.98 531.23 533.10 169.17

O1s

20.44 9.13

–

20.12 9.21 4.62 –

7.69

C\ \C and C\ \H C\ \S Aromatic ring O_S_O O\ \C Sulfone

1.67

N\ \H

4.99

Silver

19.37

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Fig. 3. XPS main survey spectrum of (a) PES, (b) APES (NH2-PES) (c) AgNPs-APES.

Fig. 3(a) [47]. The atomic percentages of the unmodified PES membrane are 70.43% for carbon, 20.44% for oxygen and 9.13% for sulfur [48]. With the appearance of the N1s band at 398 eV in Fig. 3(b), the attachment of the amine group on the PES structure was confirmed [48–50]. Fig. 3(c) confirms the coordination of AgNPs with the amine group, the Ag3d band appears at 367.91 and 373.96 eV [51,52]. Additionally, the atomic percentage of N\\H decreases from 4.62% in APES to 1.67% in AgNPs-APES (Table 2) [48]. Fig. 4 shows the core level spectrum of the samples. The S2p corelevel spectrum of the pure PES membrane can be deconvoluted into only one peak component with a binding energy at 169.17 eV (Fig. 4a) [53], which is associated with the sulfone group of the PES membrane

[48]. The atom percentage of sulfone is 9.13% (Table 2). Fig. 4(b) shows that the C1s core-level spectrum of PES can be deconvoluted into three peak components. Carbon atoms at the PES surface exhibit binding energies of 285.87 eV for the C\\H and C\\C species, 286.31 eV for the C\\O species and 287.98 eV for the C\\C species on the aromatic benzene rings [54]. Fig. 4(c) clearly shows that the N1s band is not present in the PES sample [49]. The O1s core-level spectrum of PES can be deconvoluted into two peak components at 531.23 eV for the O_S species and 533.1 eV for the O\\C species (Fig. 4d) [32]. The new peak at 398.48 eV of the N1s region associated with N\\H was observed in the APES membrane, verifying that the amine has been grafted to the PES membrane surface (Fig. 4g) [32,48].

Fig. 4. Deconvoluted spectra of all peaks in Fig. 2 (a) S2p, (b) C1s, (c) N1s and (d) O1s region spectra of virgin PES. (e) S2p, (f) C1s, (g) N1s and (h) O1s region spectra of APES (NH2-PES). (i) S2p, (j) C1s, (k) Ag3d and (l) O1s region spectra of AgNPs-APES (M3).

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Fig. 5. XRD analysis of (a) M1, (b) M2, (c) M3 and (d) M4.

Sulfur, carbon, and oxygen were still observed in the spectrum after amine grafting to the PES membrane surface that is observed in Fig. 4(e, f, h), respectively. As shown in Fig. 4(k), two new peaks at 367.91 and 373.96 eV of the Ag3d region along with a low intensity peak of the N1s (Fig. 4c) region were observed, confirming the presence of silver and a minuscule amount of N\\H [32,52]. This specifies the decrease in N\\H atomic % after AgNP coordination with the amine group in the AgNPs-APES membrane, and sulfur, carbon, and oxygen were still observed in the spectrum (Fig. 4i, j and l). XPS results are in agreement with the FT-IR results, confirming the grafting of NH2 and AgNPs. The chemical composition of the resultant nanocomposites (M1, M2, M3 and M4) was also confirmed by XRD and the results are shown in Fig. 5. The amorphous structure of PES was evident because of the rigid benzene ring and the flexible ether bond structure showing a broad peak at 2θ = 18.1° in Fig. 5(a) [40]. The prominent peaks of the corresponding silver nanoparticles in Fig. 5(b, c, d) were observed at 38.1°, 44.3°, 64.5° and 77.5°, which can be indexed as (1 1 1), (2 0 0), (2 2 0), and (3 1 1) diffractions for Ag (JCPDS No. 04–0783 “Joint Committee on Powder Diffraction Standards”), respectively [55]. The XRD results revealed that the composites were composed of PES and AgNPs. In Fig. 5(b) (M2), the corresponding silver peaks are broader than those of M3 and M4, which means the particle size of the AgNPs is larger in M2 [56]. These results are in agreement with a previous study reporting the change in the size of AgNPs with varying concentrations of AgNO3 [56–66]. The Scherrer–Debye equation was used to estimate the particle size of the AgNPs in both M3 and M4 using XRD data and the results are summarized in Table 3 [56,61]. Figs. 6 and 7 show the FE-SEM images of the pure PES (M1), PESAgNPs (M2) and AgNPs-APES (M3 and M4) membrane, respectively. Fig. 6(a) reveals the cross-sectional area of M1, M2, M3 and M4, it can be seen that on micro level typical PES membrane cross-sectional morphology was observe in all the samples [30]. In case of M3 and M4 the morphology of membrane was not affected because AgNPs were Table 3 Particle size of all runs calculated by Scherrer equation. Parameters

FWHM (degrees)

Lattice constant (Å)

Particle size (nm)

M2 M3 M4

0.121 0.351 0.295

4.05 4.05 4.05

69.39 23.66 28.89

attached to the polymer surface after the formation of characteristic morphology (porous structure) or in other words because of in situ AgNPs generation. The PES membrane exhibits a characteristic asymmetric porous structure containing fine and dense upper skin layers primarily functioning as the primary filter, i.e., permeation and retention of solutes. This layer is formed by immersion of the polymer solution film in water, which causes fast solvent–non-solvent exchange across the interface, leading to the sudden precipitation of the polymer at the interface. Because of this a porous finger-like sub layer is structured and at the end a macro void structure for mechanical support is formed [18,29]. Fig. 6(b) shows the clear porous structure of the M1, M2, M3 and M4 membranes. The AgNP attachment did not alter the visible porous structure of the PES membrane, as no difference was observed at this micro level, but a clear change was visible at the nano level. Fig. 7(a) (M3 and M4) explains the nanosize surface morphology of the as-synthesized polymer membrane, which clearly demonstrates that the AgNPs are attached to the surface of the polymer in coordination with the amino group. When the AgNO3 solution is mixed with NH2-PES for 30 min (before starting the casting process), it establishes coordinate bonding between the amine and Ag+ [32] that is then reduced to AgNPs by NABH4. The AgNPs were evenly distributed on the polymer surface/inside the channels through which the water flows. In the present study, no agglomeration of AgNPs was observed, as compared to the previously reported studies in which chemically-synthesized AgNPs formed a cluster on the polymer surface either by direct coating or by embedding in the polymer matrix [18,25,62]. As shown in Fig. 7(b), PES-AgNPs (M2) in which 0.2% by weight AgNPs (≤80 nm) were embedded in the PES membrane, no visible AgNPs were observed on the surface of the polymer except few large particles, however XRD and EDS confirmed the same amount of silver in M2 as that of M3 and M4 membrane. It is conceivable that AgNPs in M2 [Fig. 7(a)] are encapsulated by the polymer matrix hence forming blister like spherical structures on the surface of polymer. On the other hand, AgNPs in M2 were also not attached to the polymer, which will greatly affect its desirable properties that in this case are antibacterial activity and controlled release [30]. Antibacterial properties and controlled leaching abilities will be discussed in further sections. Fig. 8 depicts the elemental analysis performed by EDX of the pure PES (M1), PES-AgNPs (M2), AgNPs-APES (0.1 wt.%) (M3) and AgNPsAPES (0.2 wt.%) (M4) membranes. EDX results support the morphology

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Fig. 6. FESEM images of as synthesized membrane morphologies. (a) Cross sectional area; (b) porous structure.

images of FESEM and the experimental concentrations that are given in Table 1. As shown in the EDS images, M3 has a lower concentration of silver than M4, which also demonstrates that the AgNPs size is smaller in M3 than in M4 [52]. The particle size of AgNPs in M3 is less than in M4 because of the different concentrations of precursors used in the membrane synthesis. According to previous studies [63,64], the AgNPs ≤ 20 nm are more toxic to bacteria such as E. coli. Elechiguerra et al.

[65] explained that AgNPs ranging from 1 to 10 nm inhibit certain viruses from binding to the host cell by preferentially binding to the virus. Fig. 9 shows the size distribution of AgNPs in M3 and M4. M3 has more particles ≤20 nm (N40%) that makes it more effective for antibacterial properties. Alternatively, M4 has more particles, but more than 20% of the particles are ≤20 nm, resulting in a higher density of particles due to the increased silver loading [52]. The average particle size

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Fig. 7. SEM images of as synthesized membrane (a and b) surface morphology.

manually calculated by the FESEM images were 25.9 nm and 30.2 nm for M3 and M4, respectively, which is nearly similar to the average particle size calculated by the XRD data using the Scherrer equation given in Table 3. Therefore, the XRD and FESEM results were congruent, indicating that this technique is a suitable approach to control the distribution and particle size of AgNPs bonded to the PES surface. PES composite membranes show good thermal stability at high temperatures in a nitrogen atmosphere. Fig. 10 depicts the thermo oxidation stabilities of pure PES (M1), PES-AgNPs (M2), AgNPs-APES

(0.1 wt.%) (M3) and AgNPs-APES (0.2 wt.%) (M4). The single step decomposition of all three composite membranes indicates good compatibility with the pure PES membrane [66]. The main weight loss at approximately 450–550 °C was assigned to the degradation of the polymer main chain and the total weight loss below 800 °C was due to the polymer decomposition [67]. In the case of M1 and M2, previous studies showed the same results [18,26,35]. The thermogravimetric results indicate that no notable change occurred in the physical properties of the as-synthesized membranes.

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Fig. 8. EDX analysis of virgin PES (M1), PES-AgNPs (M2), AgNPs-APES (0.1 wt.%) (M3) and AgNPs-APES (0.2 wt.%) (M4) membranes.

3.2. Silver release analysis The static release of Ag+ was studied using an immersion technique [18] and samples were analyzed by ICP-OES; the results are summarized in Table 4 and Fig. S2. Based on the XRD and FESEM results, AgNPs were well distributed on the surface of the polymer membrane. As shown in Fig. S2, the M2 silver release is fast and sudden, where nearly 70–80% silver is released in 4–5 days, these results are in agreement to the previous studies [20,30] reporting the fast depletion of silver that eventually reduces the antibacterial effect of the polymer membrane. Alternatively, M3 and M4 exhibits slow and consistent silver release of 44.6% and 53.74%, respectively, after 6 days. Even after 12 days, more than 35% (M3) and 29% (M4) of the silver remained in the membrane. Based on extrapolation of the static silver leaching graph, leaching

from the 2 cm × 6 cm strip will continue for approximately 25 days. According to the WHO standards, 0.1 mg/L is greater than the silver concentrations in drinking water [68], but in the as-synthesized low concentration AgNP polymer composite membrane, Ag+ leaching is in a non-hazardous zone. Fig. 11 explains the rate of silver release from the composite membrane, where a clear difference between the embedded silver and coordinately attached silver on the surface of the polymer is observed. Theoretically, M3 averages silver release (Ag+) at 5.1 μg L−1 h−1 and M4 releases silver at 7.5 μg L−1 h−1 over 12 days. While M2 shows a higher average silver release of approximately 35 μg L−1 h−1 over a 5 day period. As the antibacterial property of the membrane depends on the release of Ag+ from the solution, consistent silver release can prolong the membrane ability [30,69]. Additionally, the M3 and M4 Ag+ release rate is nearly constant after the initial 2 days. Based on this steady silver release rate, biofouling and antibacterial properties of the polymer membrane will be prolonged. Hence, this approach to attach AgNPs in coordination with the amine group at the PES surface can improve and prolong the life of the water treatment membrane. This result concludes that the as-synthesized nanocomposite membrane (M3 and M4) has a 40% slower silver release rate that makes its life-time four times longer than the embedded AgNPs membrane (M2). Release studies were further evaluated to determine the chemical kinetics of the reaction involved. It was found that Ag+ ion release exhibits the first order chemical kinetics model (see S4). Coefficient of determination (R2) values from the silver release data were 0.96 (M3) and 0.93 (M4), which confirms the first order chemical kinetics of Ag+ ion release (Fig. S4). Furthermore, longer polymer composite membrane performance is being studied using ultrafiltration and reverse osmosis filtration units. 3.3. Antimicrobial activity

Fig. 9. Size distribution in percentage of AgNPs attached on aminated polyethersulfone (AgNPs-APES) in M3 and M4, estimated from FESEM.

E. coli, a gram-negative bacteria, is a common water quality indicator that was selected to test the antibacterial activity of the as-synthesized

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Fig. 10. Thermogravimetric curves of virgin PES (M1), PES-AgNPs (M2), AgNPs-APES (0.1 wt.%) (M3) and AgNPs-APES (0.2 wt.%) (M4) composite membranes.

composite membranes. Polymer water treatment disinfection primarily depends on the contact of bacteria with disinfecting material (in this case, silver), which is derived by diffusion of bacteria or a disinfecting material [11,17,69]. The antibacterial activities of aminated PES-AgNPs were evaluated by determining the presence of the inhibition zone [59]. Fig. S3 shows the zone inhibition tests performed using E. coli in the presence of pure PES (M1), PES-AgNPs (M2), AgNPs-APES (0.1 wt.%) (M3) and AgNPs-APES (0.2 wt.%) (M4). After incubation at 37 °C for 24 h, M1 does not show any antibacterial activity; instead, bacterial growth was observed on the polymer surface. M2 samples displayed reduced antibacterial effect that indicates the encapsulation effect of AgNPs in the polymer matrix. The M2 membrane contains a considerable amount of AgNPs compared to M3 and M4, but this encapsulation reduces the bacterial/AgNPs contact potentials, reducing its disinfecting efficiency. M3 and M4 exhibit the highest activities, with the aminated-PES-AgNPs (AgNPs-APES) composite membrane displaying noticeable inhibition rings even at a low concentration. The reason for this enhanced activity is the surface attachment of silver. The FESEM images confirm the attachment of AgNPs on the PES surface. The AgNPs attached on the surface of aminated PES increased the chance of bacterial/silver contact. The controlled and prolonged release of silver from PES is another reason for this superior activity. As observed from the leaching test (Fig. S2), normal mixing of AgNPs with PES (M2) shows a sudden release of silver that results in the depletion of Ag+ ions. Alternatively, M3 and M4 show prolonged and consistent release of silver ions, enhancing the disinfection ability and performance of the PES membrane. The average diameter of the silver nanoparticles in the M3 and M4 was approximately 25 and 30 nm, respectively. M2 contains more ≤20 nm particles (N40%), which is the optimum particle size for materials suitable for biological applications as previously reported. The results of the zone of inhibition analysis are summarized in Fig. 12. The clearance area of M3 and M4 was enhanced as compared to M2, suggesting that AgNPs-APES exhibited better antibacterial activity due to their superior properties. The antibacterial properties of the Table 4 Percentage of Ag+ released from the tested samples. Time (days)

M2 (%)

M3 (%)

M4 (%)

1 6 12

29.14 82.00 100

6.6 44.6 63.25

6.25 53.74 71.71

M2 sample were less than that of the M3 and M4 samples. M1 shows growth of E. coli bacteria on the polymer surface, which is considered to have a negative antibacterial effect. M3 shows the highest zone of inhibition, even with the lower concentration of the AgNPs, signifying that various metallic nanoparticles with antibacterial activities can be attached to the surface of PES using this technique. In order to verify the antibacterial properties of the composite polymer membrane in this particular study, the rate of bacterial inactivation was quantified by concentration/contact testing of E. coli colonies [36] and the results are shown in Fig. 13. The rate of inactivation of E. coli was investigated to determine the disinfection rate of M1, M2, M3 and M4. In each test, 5 mL of E. coli bacteria were initially tested against the samples. M1 showed an increase in the count of bacterial colonies that can also be observed in the zone of inhibition test, while M2, M3 and M4 showed a decrease in the count of bacteria. The same encapsulation effect can be observed in M2, as a result of which, less antibacterial effects were observed in M2. M3 and M4 show the highest disinfection potential by reducing the colony count to zero in 20 min. These results are in agreement with the results of the zone of inhibition (Fig. S3). The antimicrobial mechanism of the silver nanoparticles is prudently related to their interaction with sulfur and phosphorus, most notably the thiol groups (S\\H) present in cysteine and other compounds [70]. Ionic silver (which is released from AgNPs) interacts with the thiol group and form S\\Ag or disulfide bonds that destroys bacterial proteins, interrupts the electron transport chain, and dimerize DNA [71–73]. Similarly, the antiviral properties of silver ions involve interactions with viral DNA and thiol groups in proteins [74]. 4. Conclusions In the present study silver nanoparticles were immobilized on an aminated polyethersulfone surface. The experimental results demonstrated that the formed materials possess promising antibacterial ability and prolonged silver release which can improve the life time of water treatment membranes. Even with a low concentration of silver (M3), effective antibacterial properties were achieved. FE-SEM results confirmed even distribution of AgNPs with particle size ranging from 5 to 40 nm were mobilized on the APES (NH2-PES) surface. TGA profiles confirmed the compatibility of the AgNPs-APES composite membrane with a pure PES membrane. The leaching test of a 2 cm × 6 cm strip of M3 and M4 membranes confirmed the slow release of silver after 12 days at 63.25% and 71.71% released, respectively, which is

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Fig. 11. The release rate of silver (Ag+) during 12 days.

almost 40% less than the M2 membrane. This substantiate that the assynthesized membrane has a four times longer life than an embedded AgNPs polymer membrane. Silver nanoparticles immobilized on the water treatment polymer membrane could minimize toxicity issues toward mammalian cells by avoiding excess release of AgNPs into the filtrate and increasing the life of the water treatment membrane. This technique to immobilize AgNPs on the polymer membrane can be further applied to other surfaces to improve the usage time of the equipment for antimicrobial purposes. Acknowledgment This work was supported by the Human Resources Development program (Grant No. 20124030200130) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP); the grant was funded by the Korean government through the Ministry of Trade, Industry and Energy. Also we are indebted to the support of the Hanyang University Research Fund (HY-2015-P). Authors gratefully recognize the Higher Education Commission, government of Pakistan for PhD funding. Fig. 12. Zone of inhibition in millimeters for M2, M3 and M4, using E. coli bacteria.

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.msec.2016.02.025. References

Fig. 13. The concentration/contact test of E. coli colonies on virgin PES (M1), PES-AgNPs (M2), AgNPs-APES (0.1 wt.%) (M3) and AgNPs-APES (0.2 wt.%) (M4) composite membranes.

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