Development of Silver-Nanoparticle-Decorated Emulsion-Templated

Jun 23, 2017 - Water, the driver of nature, has always been polluted by the blind hurling of highly toxic contaminants, but human-friendly science has...
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Development of Silver-Nanoparticle-Decorated Emulsion-Templated Hierarchically Porous Poly(1-vinylimidazole) Beads for Water Treatment Muhammad Ahmad Mudassir,†,‡,§ Syed Zajif Hussain,† Asma Rehman,∥ Wasif Zaheer,† Syeda Tasmia Asma,†,¶ Asim Jilani,⊥ Mohammad Aslam,# Haifei Zhang,§ Tariq Mahmood Ansari,*,‡ and Irshad Hussain*,† †

Department of Chemistry, SBA School of Science & Engineering (SBASSE), Lahore University of Management Sciences (LUMS), Lahore - 54792, Pakistan ‡ Institute of Chemical Sciences, Bahauddin Zakariya University, Multan - 60800, Pakistan § Department of Chemistry, University of Liverpool, Oxford Street, Liverpool - L69 3BX, U.K. ∥ National Institute for Biotechnology & Genetic Engineering (NIBGE), Faisalabad, Pakistan ⊥ Center of Nanotechnology and #Centre of Excellence in Environmental Studies, King Abdulaziz University (KAU), Jeddah - 21589, Saudi Arabia ¶ Department of Biology, Lahore Garrison University, Lahore - 54810, Pakistan S Supporting Information *

ABSTRACT: Water, the driver of nature, has always been polluted by the blind hurling of highly toxic contaminants, but human-friendly science has continuously been presenting better avenues to help solve these challenging issues. In this connection, the present study introduces novel nanocomposites composed of emulsion-templated hierarchically porous poly(1-vinylimidazole) beads loaded with the silver nanoparticles generated via an in situ approach. These nanocomposites have been thoroughly characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer−Emmett−Teller, and field emission scanning electron microscopy. The appropriate surface chemistry, good thermal stability, swelling behavior, porosity, and nanodimensions contributed to achieve very good performance in water treatment. Owing to their easier handling and separation, these novel nanocomposites are highly efficient to remove arsenic and eriochrome black T with decent adsorption capacities in addition to the inactivation and killing of Escherichia coli (Gramnegative) and Staphylococcus aureus (Gram-positive) bacteria. KEYWORDS: porous poly(1-vinylimidazole) beads, silver nanoparticles, arsenite, eriochrome black T, bacterial inactivation, water treatment

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and carcinogenic aromatic amines, respectively,13 whereas E. coli and S. aureus bacteria are indicators of the fecal contamination in drinking water and the major source of fatal diseases in human.14,15 To alleviate the lethal effects of these deadly water toxicants, several methods such as ion exchange, filtration, coagulation and flocculation, electrocoagulation, chlorination, direct irradiation with ultraviolet C rays, and photochemical disinfection have been tried.7,16−19 However, adsorption is found to be the most attractive and smart approach among the existing

INTRODUCTION

Water, the elixir of life and a great blessing,1 is continuously being contaminated by human activities,2 sewage,3 underground water leakages, agricultural runoff, radioactive isotopes,4 industrial effluents,5 and oil spilling6 in the forms of inorganic, organic, and biological contaminants. These contaminants include toxic metals including arsenic,7 dyes including eriochrome black T (EBT),8 and bacteria including Escherichia coli and Staphylococcus aureus,9,10 which in turn are alarmingly harming the nature and threatening the human life. Arsenic, EBT, E. coli, and S. aureus have been declared to be the foremost water contaminants because of their hazardous11 and pathogenic nature.12 Under specific conditions, arsenic and EBT are converted into carcinogenic ions (arsenite or arsenate) © 2017 American Chemical Society

Received: April 17, 2017 Accepted: June 23, 2017 Published: June 23, 2017 24190

DOI: 10.1021/acsami.7b05311 ACS Appl. Mater. Interfaces 2017, 9, 24190−24197

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of Ag NPs generated in the PVI beads via an in situ approach. sunflower oil, and LB (Luria−Bertani) nutrient broth were purchased from Sigma-Aldrich (Germany). The E. coli (ACCN, KJ880039) and S. aureus (ACCN, KY635411) strains were used for antibacterial experiments. Deionized water purified by Milli-Q Plus system (Millipore, Bedford, MA, USA) was used for the whole research study. Methods. Synthesis of PVI Beads. The emulsion-templated porous PVI beads were prepared by using a reported method36,37 with slight modifications. According to this method, a clear monomer solution was obtained by sonicating 1.27 mL of VI, 0.24 g of BAM, and 0.06 g of PVA in 2 mL of deionized water and was mixed with 0.65 mL of Triton X-405 under vigorous stirring at room temperature. To obtain HIPE, 3.5 mL of vegetable/sunflower oil containing 12 μL of TMEDA was gradually added to the mixed monomer solution and stirred approximately at 160 rpm with an overhead paddle stirrer. After the formation of a stable HIPE, 0.6 mL of aqueous solution containing 0.018 g of APS was also added to initiate polymerization. The stirred HIPE was then injected dropwise into the sedimentation column containing hot vegetable oil (∼85 °C) set by an electric hot plate, and this temperature was maintained for about 3 h to achieve complete polymerization. The PVI beads were filtered and immersed in nhexane for at least 24 h at room temperature to extract internal oil phase. Likewise, the beads were washed with acetone followed by nhexane thrice for about 1 h and then soaked in sufficient isopropanol overnight. After washing, the beads were oven-dried at 85 °C overnight and used for subsequent studies. Preparation of PVI−Ag NCs. An in situ approach was used to generate Ag NPs inside of the porous PVI beads, in which about 10 mg of beads was soaked in a centrifuge tube containing 10 mL of AgNO3 (1.36 mg, 8 μmol) solution. The opening of the tube was immediately closed/sealed after argon purging, and the tube was allowed to shake gently for about 24 h. The soaked beads were then taken out and directly placed into the boiling aqueous NaBH4 solution (24.2 mg, 640 μmol, 20 mL) for about 15 min (Figure 1). The Ag NPloaded PVI beads were filtered, washed with deionized H2O, and dried in a vacuum oven at about 70 °C. Characterization. The Fourier transform infrared (FTIR) spectra were recorded using an attenuated total reflectance−infrared (ATR− FTIR, Alpha, Bruker, Germany) spectrometer. Thermogravimetric analysis (TGA) curves were taken with a TA Instruments device (model Q600-SDT). Nitrogen adsorption−desorption isotherms were determined on a surface area analyzer (3Flex, Micromeritics ASAP 2420) to measure the Brunauer−Emmett−Teller (BET) surface area. Scanning electron microscopy (SEM)/transmission electron microscopy (TEM) images and energy-dispersive X-ray (EDX) pattern were obtained by using a field emission scanning electron microscope (NovaNano) equipped with a transmission detector and an EDX analyzer. The particle size and pore size distributions were calculated from SEM images by using ImageJ software. The silver leaching and arsenic adsorption analyses were performed by using an inductively coupled plasma-optical emission spectrometer, whereas the absorption study of PVI and PVI−Ag, adsorption analysis of EBT, and the optical density (OD) measurement were carried out by an ultraviolet−visible (UV−vis) light spectrophotometer (Shimadzu UV-1800). Swelling Behavior. The swelling capacity of PVI and PVI−Ag was determined based on gravimetric measurement by soaking these materials in deionized water for different time intervals (15, 30, 60, 120, and 240 min) at room temperature and also at different

methods because of its low cost, high efficiency, ease of processing, and versatility for different water streams.6,17 A wide range of materials including zeolites,20 clays,21 activated carbon,22 and eggshell particles23 have been used as adsorbents, but all of these materials have their own limitations including low surface area.24 However, polymer materials have been proved to be the efficient adsorbents among other materials based on their simple preparation methods, rich chemistry, easy handling, higher adsorption capacity, faster removal rate, multipurpose applications, and so forth. Among various polymers, poly(1-vinylimidazole) (PVI) has been extensively studied as a ligand to selectively bind different metal ions in solution. In some cases, PVI has simply been used because of its complex forming ability,25 whereas in other cases, this polymer has been investigated for the surface grafting of nanoparticles.26 It has also been employed as an ion-imprinted organic−inorganic hybrid for the removal of heavy metal ions,27 and 1-vinylimidazole (VI) carrying metal-chelated beads for their use in yeast invertase adsorption.28 Nonetheless, some highly ordered porous materials prepared via emulsion templating29−31 have recently received much scientific attention because of their potential applications in tissue engineering and cell culture,32 enzyme immobilization,33 hydrogen storage,34 water treatment, and so forth.35 Owing to their immense potential in wastewater remediation, the emulsion-templated piperazine-functionalized 4-nitrophenyl acrylate materials have been used to sequester atrazine from polluted water. Similarly, styrene/divinylbenzene polyHIPEs (high internal phase emulsions) have shown tremendous performance to reduce (up to 99%) poliovirus type I from a contaminated water source.35 Keeping in view the excellent adsorptive behavior of PVI polymer, promising potential of the emulsion-templated porous materials, and antimicrobial nature of the silver nanoparticles (Ag NPs), we set out to develop Ag NP-loaded emulsiontemplated and hierarchically porous PVI beads. These nanocomposites are highly valuable and quite efficient to treat model contaminants from water such as arsenic, EBT, E. coli, and S. aureus. In addition to their multipurpose applications to remove inorganic, organic, and biological water contaminants, these nanocomposites can easily be handled, processed, and separated during and after the adsorption studies and are quite stable to be stored for a longer period of time.

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EXPERIMENTAL SECTION

Materials. The analytical-grade chemicals including silver nitrate (AgNO3, 99.0%), sodium borohydride (NaBH4, 99.99%), sodium (meta)arsenite (NaAsO2, 99%), EBT (ACS reagent), VI (99%), N,N′methylenebisacrylamide (BAM, 99%), ammonium persulfate (APS, 98%), N,N,N′,N′-tetramethylethylenediamine (TMEDA, 99%), poly(vinyl alcohol) (PVA, Mw 22 000), Triton X-405 (70% in H2O), acetone (99.9%), isopropanol (99%), n-hexane (95%), vegetable/ 24191

DOI: 10.1021/acsami.7b05311 ACS Appl. Mater. Interfaces 2017, 9, 24190−24197

Research Article

ACS Applied Materials & Interfaces temperatures (25, 35, 45, 55, and 65 °C) for 1 h, followed by carefully removing the excess water from their surfaces by tissue papers. Adsorption Study. The variation in adsorption of arsenite and EBT was studied under different experimental conditions including the pH (2−7), adsorbent dosage (1−5 mg), pollutant concentration (25, 50, 100, 500, and 1000 ppm), adsorption time (10−60 min), and temperature (25−65 °C). Using the optimized values of pH (2 and 3), adsorbent dose (5 mg), and pollutant concentration (25 ppm) under room temperature conditions for 1 h, the removal efficiencies of PVI and PVI−silver nanocomposites (Ag NCs) were calculated. The adsorption capacities of PVI and PVI−Ag NCs were calculated by using variable pollutant concentrations (25, 50, 100, 500, and 1000 ppm) at pH (2 and 3), adsorbent dose (10 mg), and room temperature conditions for 1 h. The adsorption mechanism was studied by applying the Langmuir and Freundlich isotherms, whereas the kinetics parameters were further estimated by employing the pseudo-first-order and pseudo-second-order rate equations. Bioactivity Analysis. The growth inhibition and bactericidal properties of PVI and PVI−Ag NCs were investigated against the model S. aureus and E. coli bacterial strains by using the OD and colony forming unit (CFU) counting methods, respectively. Fresh Bacterial Strain Culturing. The LB broth medium (containing tryptone, yeast extract, and NaCl) was sterilized at 121 °C and 15 psi for 15 min. The S. aureus and E. coli bacteria were inoculated into a freshly prepared sterile LB broth medium and incubated at 37 °C and 150 rpm for 24 h to get the fresh bacterial cultures. The exponentially growing bacterial cells were collected by centrifugation at 5000 rpm for 5 min and washed with 0.9% normal saline (NS) to remove macromolecules and bacterial dead cell mass. The cell suspensions of both the bacterial strains were prepared side by side in 0.9% sterile NS until an OD of 0.1 at 600 nm was adjusted to achieve 107 CFU/mL concentrations. For further confirmation of 107 CFU/mL concentrations, 10-fold dilutions were prepared and 100 μL from each dilution was plated onto sterile agar plates and subsequently grown at 37 °C in an incubator for 24 h. OD Measurement. To analyze the growth kinetics and inactivation of E. coli and S. aureus bacteria, different doses (0.71, 1.43, and 2.14 mg/mL) of PVI and PVI−Ag NCs were suspended in separate sterilized vials/flasks (each containing 7 mL of cell suspension of an approximate 107 CFU/mL density), which were shaken at 150 rpm in a shaking incubator set at 37 °C. An aliquot (2 mL) of each suspension mixture was taken after a regular time interval of 30 min for 24 h to record its OD at 600 nm and then resuspended back to the respective vial/flask. Furthermore, the OD values for the positive controls (containing PVI or PVI−Ag NCs and LB nutrient media) and the negative controls (containing inoculum and LB nutrient media) were also measured at 600 nm for comparisons.38 The minimum inhibitory concentration (MIC, mg/mL) sufficient to prevent bacterial growth was determined by the growth inhibition profiles of both bacteria. CFU Counting. The quantitative analysis of the antibacterial activity of PVI and PVI−Ag was made via CFU counts. The dose-dependent counting was performed after 24 h by using different dosages (0.71, 1.43, and 2.14 mg/mL) of PVI and PVI−Ag, whereas the timedependent assay was carried out after every 30 min for 24 h by using 0.71 mg/mL each of PVI and PVI−Ag. The aliquots (100 μL) of all suspension mixtures (each of 7 mL) were separately obtained, serially diluted (10-fold) in NS, plated (100 μL of each dilution) onto the freshly prepared sterile LB agar plates, and incubated for 24 h at 37 °C. Afterward, the total number of CFUs grown/recovered in the sample LB agar plates was counted and compared with those in the control LB agar plates to determine the efficiencies of PVI and PVI−Ag to kill S. aureus and E. coli bacteria. The minimum concentration, sufficient to kill the bacteria, was nominated to be minimum bactericidal concentration (MBC, mg/mL).

Figure 2. FTIR spectra of [A(a)] PVI and [A(b)] PVI−Ag NCs. TGA curves of [B(a)] PVI and [B(b)] PVI−Ag NCs.

linker in the main chain were observed at 3366 and 3250 cm−1, whereas the presence of aromatic hydrogens bound to the imidazole ring was confirmed by stretching vibrations of C C−H and NC−H at 3150, 3116, and 3100 cm−1.39−41 The CO stretching vibration appeared at 1744 cm−1,42,43 whereas the signals at 1640, 1544, 1497, and 1451 cm−1 can be designated to CC stretching vibrations.43−45 The appearance of peaks at 1285 cm−1 (imidazole ring) and 1233 and 1146 cm−1 (polymer chain) can be attributed to C− N stretching vibrations,46 whereas the peaks at 1081, 964, 916, 840, 825, 737, and 664 cm−1 can further be assigned to the C− H bending vibrations of imidazole rings.45,47−49 In PVI−Ag NCs [Figure 2A(b)], the suppression of all PVI peaks was observed, accompanied by the inaccessibility of some signals at 3366, 1146, and 964 cm−1, which may be associated with the interaction of the Ag NPs with the PVI matrix. The loading of silver into the porous PVI scaffolds was further manifested from the TGA. At about 542 °C, the TGA curves showed complete weight loss (≈100%) of PVI [Figure 2B(a)], whereas PVI−Ag NCs [Figure 2B(b)] lost approximately 89.2% of the total weight that can be assumed by the decomposition of PVI, and the remaining weight can be ascribed to the inorganic Ag, which was incorporated into the PVI matrix. Considering these results, the estimated amount of the loaded silver came out to be 108 mg/g of PVI−Ag NCs (10.8 wt %). The swelling capacities of PVI and PVI−Ag were determined by the gravimetric method at room temperature for different time intervals (15−240 min) and at different temperatures (25−65 °C) for 1 h. A rapid increase in the swelling capacity was observed from 15 to 120 min followed by a slower increase until the equilibrium stage. Similarly, the degree of swelling was enhanced with an increase in temperature from 25 to 65 °C. In general, PVI showed a relatively greater degree of swelling than the PVI−Ag NCs for both the time and the temperature as variable factors (Figure 3). The porous structure of PVI−Ag NCs, entrapment of S. aureus and E. coli bacteria into the pores of PVI−Ag NCs, and morphology and diameter of Ag NPs were also investigated by

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RESULTS AND DISCUSSION Characterization. The chemical nature of PVI and PVI−Ag was studied by ATR−FTIR spectroscopy. For PVI [Figure 2A(a)], the N−H stretching vibrations of bisacrylamide cross-

Figure 3. Swelling behavior of PVI (a) and PVI−Ag (b) at different time intervals (A) at 25 °C and (B) at different temperatures for 1 h. 24192

DOI: 10.1021/acsami.7b05311 ACS Appl. Mater. Interfaces 2017, 9, 24190−24197

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ACS Applied Materials & Interfaces

Figure 4. SEM images (cross-sectional view) of PVI−Ag NCs. (A) Porous structure of PVI−Ag NCs with histogram in the inset. (B) Entrapment of S. aureus and (C) E. coli bacteria into the pores of PVI−Ag NCs. (D) Distribution of Ag NPs with histogram in the inset.

SEM. The cross-sectional view of SEM images revealed the porous nature of PVI−Ag NCs (Figure 4A) and the distribution of Ag NPs (Figure 4D) with a mean diameter of 18.72 ± 6.53 nm (Figure 4D, inset), confirming the greater potential of NCs to inactivate the S. aureus (Figure 4B) and E. coli (Figure 4C) bacteria. The presence of Ag NPs on the fractured parts of PVI matrix was also ascertained by TEM images (Figure S1) and further confirmed by the EDX pattern (Figure S2). Furthermore, the pore sizes of PVI (Figure S3) and PVI−Ag NCs (Figure 4A, inset) were also measured from SEM images, wherein both PVI and PVI−Ag NCs displayed the hierarchical macroporosity (>50 nm).50 The surface areas of PVI and PVI−Ag NCs were determined by N2 sorption isotherms at 77 K. In the case of PVI, the specific surface area of PVI was estimated to be 10.22 m2/g, whereas PVI−Ag exhibited comparatively enhanced BET surface area of 21.77 m2/g. The overall lower surface areas of PVI and PVI−Ag may be associated with their large number of enlarged pores (macropores) resulting from the size and volume fraction of the oil (porogen) globules entrapped in the voids of the cross-linked PVI during the polymerization process.51−53 However, a relatively greater surface area of PVI−Ag may be correlated with the high surface-to-volume ratio of nonaggregated Ag NPs immobilized throughout the surface and interior of PVI beads.54 Adsorption Study. Factors Affecting Adsorption. The adsorption behavior of As(III) and EBT was studied under the key factors including pH, adsorbent dose, pollutant concentration, time, and temperature. On the basis of the pKa value of an imidazole group,55 a pH range of 2−7 was selected, across which PVI and PVI−Ag showed higher adsorption potential for As(III) at pH ≈ 2, whereas PVI and PVI−Ag exhibited the best results for EBT at pH 2 and 3, respectively (Figure 5A). The relatively higher adsorption can be associated with the maximum protonation of imidazole groups at these particular pH values, which may boost the electrostatic interaction as well as complexation between the adsorbents and adsorbates. The increment in the dosage from 1 to 5 mg of adsorbent increased the percentage removal of As(III) and EBT while decreasing their corresponding adsorption capacities by both

Figure 5. (A) Effect of pH (5 mg, 1 h, and 25 °C), (B) adsorbent dose (pH = 2 and 3, 1 h, and 25 °C), (C) contact time (pH = 2 and 3, 5 mg, 1 h, and 25 °C), and (D) temperature (pH = 2 and 3, 5 mg, and 1 h) on the removal (%) of EBT and As(III) by PVI (a,c) and PVI−Ag (b,d), respectively.

PVI and PVI−Ag. However, after using equal doses of PVI and PVI−Ag, their removal efficiencies were lower for EBT as compared to As(III) (Figure 5B). The time course study showed that the number of pollutant ions/molecules (As and EBT) in contact with both PVI and PVI−Ag NCs was enhanced because of the increase in the penetration of pollutant ions/molecules into the pores of adsorbents with the increase in the degree of swelling of PVI with the passage of time (10−60 min) (Figure 5C). The increasing temperature also improved the removal of pollutants by the adsorbents, which can be related to the increase in their swelling capacities at higher temperatures; however, the trend was a bit different for As(III) and EBT. In the case of As(III), the removal efficiency was the highest even at room temperature, and therefore, the increase in temperature did not cause much improvement in its removal. For PVI, a sharp but regular improvement in the removal of EBT was 24193

DOI: 10.1021/acsami.7b05311 ACS Appl. Mater. Interfaces 2017, 9, 24190−24197

Research Article

ACS Applied Materials & Interfaces observed in the temperature range of 25−65 °C, whereas for PVI−Ag, EBT removal was rapid from 25 to 45 °C, which then became slower after 45 °C (Figure 5D). Using the optimum values of pH, adsorbent dose, and time under room temperature conditions, the influence of the initial concentrations of As(III) and EBT (25, 50, 100, 500, and 1000 ppm) on the adsorption capacity of PVI and PVI−Ag NCs was also investigated. Consequently, the adsorption capacities were not up to the mark for the pollutant (As and EBT) concentrations from 25 to 100 ppm. However, the use of 500 ppm As(III) and EBT markedly increased the adsorption capacity of PVI and PVI−Ag NCs, particularly in the case of As(III). Likewise, the adsorption capacities of PVI and PVI−Ag reached to their maximum levels (qmax) of 236.91 and 333.36 mg/g for 1000 ppm As(III), respectively, and 63.04 and 81.14 mg/g for EBT (1000 ppm), respectively (see Table S2). While comparing the adsorption capacities of PVI and PVI− Ag for As(III) and EBT in general, PVI−Ag NCs showed better performance to remove As(III) and EBT as compared to PVI, which can be associated with the availability of more active sites and a higher surface positive charge, confirmed by the zeta potential values (see Table S1). On the other hand, the better removal of As(III) than EBT by both PVI and PVI−Ag may be due to the chelating behavior of imidazole ligand toward As(III).56 Furthermore, the relatively better surface area and the possible complexation reaction between Ag and As(III) in combination to the chelation by the imidazole groups can additionally augment the adsorption capacity of PVI−Ag NCs for As(III) as compared to PVI.57 Estimating the Adsorption Mechanism. The mechanism of adsorption between adsorbents (PVI and PVI−Ag) and adsorbates [As(III) and EBT] was probed by applying the Langmuir and Freundlich isotherms. The results (Table S2) supported that the Langmuir isotherm model can be best fit based on comparatively higher values of the linear regression coefficient, which depicts the monolayer adsorption of As(III) and EBT on both PVI and PVI−Ag. Between PVI and PVI−Ag, PVI−Ag exhibited the maximum adsorption capacity (qmax = 333.36 ± 13.89 mg/g) for As(III), wherein the value of the separation factor or equilibrium parameter (RL = 0.92) was also favorable to adsorption. Approximating the Kinetics Parameters. The kinetics parameters of adsorption were further approximated by employing the pseudo-first-order and pseudo-second-order rate equations. The higher values of the linear regression coefficient and the reasonable agreement between the calculated and experimental qe values favored the pseudo-firstorder for EBT adsorption and the pseudo-second-order for As(III) adsorption by both PVI and PVI−Ag as the best models to explain the kinetics mechanism (Table S3). Evaluating the Growth Inhibition and Bactericidal Properties. The growth inhibition properties of PVI and PVI−Ag NCs (0.71, 1.43, and 2.14 mg/mL) were tested against two bacterial strains S. aureus and E. coli over different time intervals (0.5−24 h) and then compared with those of the positive and negative controls. It was observed that PVI beads alone were also able to slightly inactivate S. aureus and E. coli. This extremely low inhibition effect of PVI was, however, gradually increased after increasing its concentrations (0.71− 2.14 mg/mL) and the incubation time (0.5−24 h). In the case of PVI−Ag, 0.71 mg/mL was found to be the MIC, at which the growth of bacteria was, however, increased with the passage of time, whereas 1.43 mg/mL dose of PVI−Ag fairly inhibited

the growth of both the bacteria within even 30 min. The growth of S. aureus increased a bit with the passage of time, but the inhibition profile of E. coli remained almost constant up to 24 h. Nonetheless, a 2.14 mg/mL dosage of PVI−Ag caused a significant inhibition of S. aureus (Figure 6A) and complete inactivation of E. coli (Figure 6B) within 24 h.

Figure 6. Growth inhibition profile of (A) S. aureus and (B) E. coli bacteria after using 0.71 (c), 1.42 (e), and 2.14 mg/mL (g) of PVI and 0.71 (d), 1.42 (f), and 2.14 mg/mL (h) of PVI−Ag and their comparison with the negative (a) and positive (b) controls over different time intervals along with their photographs (insets). The experiments were performed in triplicate, and the error bar shows the standard error of mean.

Furthermore, the CFU counting results showed that 0.71 mg/mL dose of PVI was unable to kill S. aureus but caused the death of only 0.12% E. coli. By using a higher dose, that is, 2.14 mg/mL, the bactericidal efficiency of PVI against S. aureus and E. coli was further improved up to 0.30 and 0.37% within 24 h, respectively. However, PVI−Ag exhibited an excellent bactericidal potential against both the tested bacterial strains. The use of 0.71 and 1.43 mg/mL doses of PVI−Ag killed ∼65 and ∼90% S. aureus bacteria, respectively, whereas the higher dosage, that is, 2.14 mg/mL PVI−Ag caused ∼96% killing of S. aureus after 24 h. However, the MBC (0.71 mg/mL) of PVI− Ag sufficiently killed ∼90% of E. coli bacteria within 24 h. The efficiency of PVI−Ag to kill E. coli, however, reached up to ∼98 and ∼100% after using its 1.43 and 2.14 mg/mL doses, respectively, within 24 h (Figures S4 and S5). The growth inhibition/antibacterial assays showed, however, very weak ability of PVI to inhibit/kill the tested S. aureus and E. coli strains. The antibacterial nature of PVI could somehow be explained by its positively charged surface to electrostatically interact and adsorb overall negatively charged bacteria. Subsequently, development of an electrostatic interaction between PVI and the lipid headgroups may lead to the formation of interfacial complexes, induce flip−flop of lipid molecules toward the outer leaflet in the bilayer membrane, cause disruption and phase separation of the lipid bilayers, and leak cytoplasmic ingredients and thus result in bacterial cell lysis.58−61 Better antibacterial activity of PVI−Ag may be associated with the release of Ag in media (Figure S6) because both the bioactivity and Ag leaching of PVI−Ag followed almost a similar enhancement trend by increasing the dosage/concentration of PVI−Ag and the incubation period. Although PVI− Ag showed good inhibition/bactericidal effect against both S. aureus and E. coli, it was more effective against E. coli (Gramnegative) than S. aureus (Gram-positive), which may be attributed to the thicker peptidoglycan layer of S. aureus that limits the penetration of Ag into its cytoplasm,62,63 which is due to the difference in membrane structure, chemistry of certain bacterial species,64 and possible electrostatic interaction 24194

DOI: 10.1021/acsami.7b05311 ACS Appl. Mater. Interfaces 2017, 9, 24190−24197

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ACS Applied Materials & Interfaces Notes

between the negatively charged cell membrane of E. coli and positively charged Ag nanoparticles.65,66 The exact mechanism of bacterial inactivation/killing by Ag NPs is, however, not much clear and needs to be authenticated. However, it is generally accepted that the smaller sized Ag NPs having higher surface area may easily attach and penetrate the bacterial cell membrane and disturb its power functions (permeability and respiration). Subsequently, Ag (possibly in the forms of nanoparticles, ions, and radicals) may interact with the phosphorous- and sulfur-containing compounds (DNA and proteins) inside of the bacteria to degrade their enzyme activity, deactivate cellular protein, disrupt adenosine 5′-triphosphate production, break DNA replication, and cause cell leakage and lysis, which ultimately lead to the bacterial death.67−70

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by the Higher Education Commission (HEC), Pakistan, and the US National Academy of Sciences (joint Pak−US collaborative project). I.H. thanks SBA School of Science & Engineering (SSE), LUMS, for startup funds to initiate nanomaterials research at LUMS.

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CONCLUSIONS To summarize, these findings present the potential applications of emulsion-templated porous materials to simultaneously treat the inorganic, organic, and biological water contaminants such as As(III), EBT, E. coli, and S. aureus by controlling the chemistry of polymer matrix and developing their composites with Ag NPs. The PVI−Ag NCs seem reasonably set to remove EBT and to inactivate/kill S. aureus bacteria but are comparatively more efficient to remove As(III) and inactivate/kill E. coli bacteria, probably because of the difference in the chemistry of polymer matrix and their complexation/ chelation properties and electrostatic interaction with the pollutants to be removed. These NCs have good potential to real-life applications because of their macrosize (1.80 mm ± 0.15 mm), hierarchical porosity, easy handling and separation, and long-term stability and demonstrate a fairly new approach to develop emulsion-templated porous materials to remove a variety of pollutants by controlling the chemistry of the polymer matrix and the size and nature of inorganic nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05311. TEM images and EDX spectrum of PVI−Ag NCs, SEM and TEM images of PVI, general characterization data of PVI and PVI−Ag NCs, isotherm parameters for the removal of As(III) and EBT by PVI and PVI−Ag NCs, kinetics parameters for the removal of As(III) and EBT by PVI and PVI−Ag NCs, dose-dependent bactericidal efficiencies of PVI and PVI−Ag NCs against S. aureus and E. coli, time-dependent bactericidal efficiencies of PVI and PVI−Ag NCs against S. aureus and E. coli, leaching analysis of total silver from PVI−Ag NCs, and UV−vis absorption spectra of PVI and PVI−Ag NCs (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.M.A.). *E-mail: [email protected] (I.H.). ORCID

Syed Zajif Hussain: 0000-0002-3834-6061 Irshad Hussain: 0000-0001-5498-1236 24195

DOI: 10.1021/acsami.7b05311 ACS Appl. Mater. Interfaces 2017, 9, 24190−24197

Research Article

ACS Applied Materials & Interfaces

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