Article pubs.acs.org/JPCC
Insight into the Mechanism of Decontamination and Disinfection at the Functionalized Carbon Nanotube−Polymer Interfaces Debmalya Roy,* Neeru Tiwari, S. Kanojia, K. Mukhopadhyay, and A. K. Saxena Nanoscience and Technology Division, DMSRDE, GT Road, Kanpur, India-208013 S Supporting Information *
ABSTRACT: The role of different functional groups and the nature of the functional group on multiwalled carbon nanotube (MWCNT) surface were thoroughly studied for silver nanoparticles (AgNPs) loading and on the mechanism of decontamination and disinfection. The surfactant free method for grafting of AgNPs on MWCNT surface followed by vacuum annealing was adapted to enhance the interfacial interactions of nanomaterials with bacteria. The best performing functionalized MWCNT was selected for the fabrication of functional composite membrane for further insight into the interfacial interaction of polymer− nanomaterials. It has been shown that at an optimized weight percentage loading of functionalized MWCNTs, nanotube scaffolds were generated inside the pores of polysulfone membrane to sieve out toxic metal ions and bacteria by physical and chemical elimination without compromising the flux rate of filtration. The structure property relationship of the nanocomposite membrane has been thoroughly evaluated by the morphological, surface area, and contact angle measurement studies. The modified surface of MWCNTs by Ag nanoparticles and polar functional groups placed on the pores of the membrane was thus further exposed for interfacial interaction with the decontaminated and disinfected water, which in turn enhances the efficiency of filtration.
1. INTRODUCTION The advent of nanoscale science and technology provides an unprecedented opportunity to tailor make the polymer and nanomaterials interfaces for multifarious activities. Nanomaterials like, carbon nanotubes (CNTs), metal/metal−oxide nanoparticles and nanowires, zeolites, dendrimers, and polymer nanostructures show the excellent adsorbent and catalytic activities due to their large specific surface area and high reactivity.1,2 The unusually high aspect ratio, atomically smooth hydrophobic graphitic surface and nanoscale inner diameter of CNT makes it unique for efficient transport of water molecules through the intra and interstitial molecular pores. The hydrophobic effect, π to π interactions, hydrogen bonding, and electrostatic interactions of the CNT−polymer nanocomposites facilitate the adsorption of analytes on the membrane with good selectivity and reproducibility.1−3 CNTs have been shown for higher efficiency than activated carbon in adsorption of various organic chemicals and toxic metal ions.4 The functionalization of CNT opens up an excellent platform to gain the potential benefits of debundled CNTs with improved separation and sensing characteristics. © XXXX American Chemical Society
The choice of attaching organic moieties on the surface of CNTs leads to the major adsorption sites for metal ions mainly through electrostatic attraction and chemical bonding. It has been demonstrated that conventional disinfectants like chlorine or ozone could produce toxic carcinogenic byproducts whereas UV irradiation generates less side effect however requires a very high dose. The potential disinfection and microbial control of the nanomaterials did not exhibit any strong oxidation and hence have lower the tendency to form toxic byproducts.5 CNTs have been known for killing bacteria by causing physical perturbation of the cell membrane or by disruption of a specific microbial process via oxidizing a vital cellular component.6 On the other hand, silver nanoparticle (AgNPs) has widely been used for the controlled release of silver ions, which could bind to the thiol groups of microbial proteins, resulting in disinfectants.7 Received: April 29, 2015 Revised: June 19, 2015
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DOI: 10.1021/acs.jpcc.5b04114 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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elevated temperature to get the further purified CNTs. The average lengths of the MWCNTs are 4−5 μm with the inner and outer diameters are 25 and 35 nm, respectively. The chemical derivatizations of purified MWCNT with different functional groups were carried out by a modified procedure which is described elsewhere.15 In brief, the acid derivative of MWCNTs was prepared by refluxing the MWCNTs with the mixture of nitric and sulfuric acid, followed by the conversion of acid derivative to acid chloride derivative by reacting with the thionyl chloride in the presence of catalytic amount of dimethylformamide (DMF). The both acid and acid chloride derivatives were then subjected to covalent functionalization with oleyl amine (OA) [unsaturated long chain aliphatic amine], octadecyl amine (ODA) [saturated long chain aliphatic amine], and octadecyl thiol (ODT) in the DMF medium. The process of different functionalization methods of MWCNT is schematically represented by Figure S1 in Supporting Information. 2.1.2. Method for Decoration of Silver Nanoparticles on the Nanotube Surface. Ag nanoparticles were decorated on pristine and functionalized nanotube surfaces using the standard procedure17 and by the modified procedure. In the modified procedure, the Ag nanoparticles decoration on nanotube surface was carried out by reducing the silver nitrate solution using polyol without any use of surfactant.18 CNTs were first sonicated in the mixture of ethylene glycol (EG) and deionized (DI) water in 7:3 ratio for 15 min followed by the addition of 0.1 molar silver nitrate solution which was then again sonicated for another 15 min. The final solution was heated at 120 °C for 2 h under inert atmosphere with vigorous stirring followed by keeping the solution undisturbed for 2 days at inert atmosphere to deposit the produced Ag nanoparticles on CNT surface. The reaction mixture was filtered through coarse filter paper, and the residue was washed with ethanol and DI water several times and then was dried in vacuum at 120 °C as illustrated in Figure S2 of the Supporting Information. 2.1.3. Study of Heavy Metal Removal and Antibacterial Testing on Functionalized MWCNTs. CNTs and functionalized CNTs were packed in a Whatman high-purity glass microfiber extraction thimble of 30 × 100 mm and placed on a funnel. Water-soluble metal salts of K2Cr2O7 and Pb(NO3)2 were dissolved in DI water to make concentration of 10 ppm. The 100 mL solution of metal salt was allowed to pass through the thimble packed with 10 mg of the nanotube sample in 05 h followed by a washing with 100 mL of DI water. This process is repeated for four times and a small part of CNTs were taken out, dried in vacuum at 60 °C followed by the evaluation of metal ions capture by the functionalized CNTs. First, 50 μL of the Escherichia coli (E. coli) cell suspension (∼108 colony forming units mL−1) were diluted in 500 μL of functionalized and pristine CNT solution in DI water with various concentrations from 0.5 to 2.0 mg/mL and allowed to incubate at 37 °C and 200 rpm for 1h. After incubation, 1.45 mL of bacterial culture medium (Agar broth) was added to each solution for the final volume of 2 mL. Then, 100 μL aliquots were taken from the solutions at every 30 min for the next 5 h and tested for optical density. The growth curves were created by subtracting the absorbance of the control solution containing the same concentration of nanotube as the experimental sample. 2.2. Polymer Nanocomposite Membrane. 2.2.1. Fabrication. Polysulfone beads were dissolved in DMF by keeping the mixture at 50 °C for overnight in an inert atmosphere
Although the CNT-based membrane technologies showed all the promises of effective decontamination and disinfection, however, there are still numerous technological and scientific challenges which still need to be addressed. The major bottleneck of the CNT-based membrane technology is the inherent trade-off between membrane selectivity and permeability due to the difficulties in controlling the inter tube distance and aspect ratios of CNTs. The CNT membranes thus suffer the complexity of the process design and operation which reduces the lifetime of membranes and increases the effective cost of the membrane modules.8 The flexible, scalable, modular, and relatively easy to operate polymer membranes have been thus become the dominant technology for decontamination and disinfection.9 The large numbers of studies have been carried out to understand the structure−property relationships of polymers in the membranes microstructures. Polysulfone is widely used as filtration membrane as it has good thermal as well as chemical stabilities and the pore size of the membrane morphology could be tailor-made to control the permeability or physical separation of solute from dissolved water.10,11 The hydrophobic surface of polymeric membrane causes severe fouling especially organic or biofouling and that in turn reduces the flux rate of the filtration process with the increase of permeation time. The incorporation of nanomaterials into membranes offers a great opportunity to tailor make the multifarious functionalities of the membrane for achieving multiple treatment goals in one reactor while minimizing fouling. The morphology of the polymer membrane could be engineered by adding AgNPs or CNTs into the polymer matrix and the functional moieties on the nanotube surface exhibit excellent hydrophilicity which increase their biocompatibility with reduced fouling and increased decontamination.12,13 A large number of nanomaterials including CNTs and AgNPs have been used as filler to modify the physical as well as chemical characteristics of the polymeric membranes and the multifunctional composite membranes showed improved decontamination and disinfection of water.3,7,12,13 However, the uniform dispersion of nanomaterials in the polymer matrix and the sustainability of the unique features of the nanomaterials in the matrix is a challenge.14,15 In this study we have carried out the series of experiments on MWCNTs by changing the functional groups on the surface to select the best candidate for the decontamination and disinfection. The better performing AgNPs decorated functionalized MWCNT has then been used for the fabrication of functional polysulfone composite membrane for structure property correlation. It has been shown that at an optimum weight percentage loading, a mesh of CNT networks could be formed over the pores of composite membrane to enhance the interfacial interactions and to maintain the sustainability of nanomaterials in the polymeric matrix.
2. EXPERIMENTAL SECTION 2.1. Selection of Better Decontaminating and Disinfecting AgNPs Decorated Functionalized MWCNTs. 2.1.1. Synthesis of Functionalized MWCNT. The all chemicals used for this study were purchased from Sigma-Aldrich Chemical and were used as received without any purification. The starting pristine MWCNTs used were synthesized in our laboratory by catalytic chemical vapor deposition (CCVD) using an iron and magnesium bimetallic catalyst.16 The as grown MWCNTs were purified by following the standard procedure, and the resultant MWCNTs were annealed at B
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Figure 1. TEM images of the OA functionalized MWCNT are represented in the case of silver ions which were reduced by DMF (A) and EG/DI water (B) where the scale bar in the spectra are 5 and 10 nm, respectively.
with flat square sheet test membrane of 47 mm where the volume of the apparatus was 100 cm3 and the operation pressure was 7 psi by applying compressed air. The metal ion removal experiments were carried out using a 300 mL test solution of 10 ppm initial concentration and it was allowed to permeate all the inlet solution to the membrane, which took 2 h to complete the filtration. The metal ions in the filtered solution were analyzed in every 15 min by both inductively coupled plasma-optical emission spectroscopy (Shimazu ICPE9000) and by UV−vis absorption spectroscopy (Jasco V-630 double beam UV−vis spectrophotometer). A 500 μL sample of E. coli cell suspension (∼108 colony forming units ml−1) was diluted with DI water to make the final volume of 300 mL and allowed to permeate the square shape test membrane of 47 mm for 2 h and the optical density at 600 nm of the filtered solution were measured in every 15 min. The E. coli bacteria culture was carried out on the covered Petri culture dish in the presence of test membrane of 45 mm diameter and bacterial culture medium. Then, 100 μL of E. coli bacteria stock solution was diluted with DI water to make a final volume of 15 mL where 5 mL of Agar broth was mixed and allowed to incubate at 37 °C and 200 rpm for 30 min and then added onto the pristine and composite membrane and the bacterial growth on the test membrane were monitored after 1 day. 2.3. Characterizations. The scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images were recorded on a SUPRA 40 VP, Gemini, Carl Zeiss scanning electron microscope and Philips CM200 TEM, operating at a 200 kV voltage. The EDX spectra were recorded on a SUPRA 40 VP, Gemini, Carl Zeiss scanning electron microscope. The silver loading was also verified by the thermal analysis study using a TA Instrument Hi-Res TGA 2950 thermogravimetric analyzer (TGA) attached to a Thermal Analyst 2100 (Du Pont Instruments) thermal analyzer at a 10 °C/min heating rate in air.
where the moisture content was less than 1 ppm (glovebox). The right viscosity of the slurry was obtained at an optimum solid content of 40% of polysulfone in DMF where nanotube could be uniformly dispersed in the matrix. The polysulfone solution was mixed thoroughly by a hand blender where the AgNPs decorated thiol-functionalized MWCNTs which was predispersed in DMF were slowly introduced into the polysulfone matrix with continuous stirring. The well dispersed AgNPs decorated functionalized CNTs in polysulfone slurry was then subjected to membrane fabrication by the well established phase inversion membrane casting technique like the doctor’s blade method.18 2.2.2. BET and Contact Angle Measurements of Membranes. Surface areas of pristine and composite membranes were determined using an IGA-002 gravimetric system (Hiden Isochema, U.K.) by physisorption and desorption of nitrogen. The test membrane, all tubings and chambers were degassed at 120 °C for 4 h by applying vacuum at 10−3 mbar pressure. The pore size distribution and amount of porosity of the pristine and the composite membrane were further characterized by the molecular weight cutoff (MWCO) curves using water and successive permeation of dilute solutions containing a single solute of various molar masses which was discussed detailed in Figure S3 of the Supporting Information. The contact angles of the membranes were measured at room temperature using an OCA20 contact angle measurement system (Dataphysics, Germany). The static contact angle was determined by sessile drop method using a water drop of 5 μL by a needle tip. The images of the droplet were recorded in every 10 s for 2 min. The final data was an average of five measurements for pristine and eight for composite membrane to cancel out the nonuniformity in the composite membrane (if any). 2.2.3. Heavy Metal Removal and Antibacterial Testing of Membrane. Water-soluble metal nitrate salts of Cu(I), Zn(II), Pb(II), and Cr(VI) were dissolved in DI water to make concentration of 10 ppm and the pH of the final metal ion solutions were adjusted to 8.0 by ammonium hydroxide. Batch membrane process step was carried out in an apparatus built C
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Figure 4. Optical density values at the 600 nm have been represented for the blank (green squares) pristine (black triangles), acid (olive pentagons), acid chloride (magenta diamonds), OA (blue circles), ODT (red triangles) and silver nanoparticles decorated ODT (violet stars) functionalized MWCNTs where in the inset the optical density values at the 600 nm for the bacterial culture with 0 (green squares), 0.5 (black triangles), 1 (blue triangles), 1.5 (red triangles), and 2 (magenta triangles) mg/mL concentration of thiol ester functionalized MWCNT have been illustrated.
Figure 2. TGA of the Ag nanoparticles decorated acid (magenta dash dot dot line), acid chloride (red dash dot line), ODA (blue dotted line), OA (light green thin solid line), and ODT (dark green thick solid line) functionalized MWCNTs by the EG/DI water method. The TGA of Ag nanoparticles decorated OA functionalized MWCNT by the DMF/SDS method (black color dashed line) is also compared and it is found that the Ag loading is less in the case of the DMF/SDS method compared to the EG/DI water method. In the inset, the char yields of silver were expanded for better clarity.
nanotube surface enable us to try the reduction of Ag ions in water medium followed by deposition of AgNPs on MWCNTs.23 The optimized amount of DI water and EG mixture provides the right viscosity to disperse the functionalized nanotubes in solvent and the slow reduction of the silver ions by the EG gives a better control over the distribution of size and shape of AgNPs which is described in Figure S4 of the Supporting Information. 3.2. Effect of Vacuum Annealing on the Morphology of Nanotube. The AgNPs are not generally chemically bound to the nanotube surface and hence the chance of releasing the AgNPs or Ag ions in water would be high. We have thus annealed the AgNPs decorated MWCNTs in vacuum at temperature of 120 °C which lead to partial embedment of Ag nanoparticles in the MWCNT surface (Figure 1A). The
3. RESULTS AND DISCUSSION 3.1. AgNPs Decoration on MWCNT without Any Use of Surfactant. The synthesis of silver nanoparticles with tailormade size and controllable shape has been reported in the past by various research groups.19,20 It has been demonstrated that the most successful method for the metal nanoparticles synthesis with controlled shape and size is the reduction of metals by polyols.21,22 The earlier method for producing AgNPs and decorating them on CNT was achieved by reducing the silver ions by DMF in the presence of a surfactant or by supercritical carbon dioxide.19,22 The fast reduction of Ag ions in DMF leads to a wide distribution of size and shape of AgNPs (Supporting Information). The polar functional groups on the
Figure 3. Part A represents the char yield of Cr in the TGA of pristine (black triangles), acid (brown pentagons), acid chloride (magenta diamonds), ODA (green hexagons), ODT (blue circles) and OA (red stars) functionalized MWCNTs whereas in the inset the char yield portions were expanded for better clarity. Part B demonstrates the weight percentage removal of Cr and Pb ions by the pristine (black squares and magenta circles respectively) and OA-functionalized MWCNT (blue triangles and red stars respectively) in the different steps of filtration process. D
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Figure 5. SEM images of the composite polysulfone membrane with 5 wt %ages of AgNPs decorated (A) pristine and (B) functionalized MWCNTs. The weight percentages of polysulfone in the DMF solution are 60% (A and B), 50% (C), and 40% (D). The scale bars of all SEM images are 1 μm.
unsaturated side chain was used for nanotube functionalization (Figure 2). The OA [H2NCH2(CH2)7CHCH(CH2)7CH3] has a structure simiar to ODA [H2NCH2(CH2)7CH2− CH2(CH2)7CH3]; however, OA has a double bond in the middle of the chain and this isolated double could assist the solvation of the electrons from EG for reduction of Ag ions.24,25 The main feature of the introduction of polar functional groups on the surface of nanotubes is the potential electrostatic or ionic interaction with the toxic metal ions.26−28 The pristine and different functionalized MWCNTs were treated with 10 ppm solution of potassium dichromate and it was found that the highest amount of chromium ions, 27.6% was adsorbed by OA functionalized MWCNT whereas pristine, acid, acid chloride, ODA and ODT functionalized MWCNT adsorbed 20.0, 22.8, 23.2, 26.3 and 26.6% chromium ions respectively (Figure 3A). To confirm that the potassium ions are not interfering in the calculation of chromium ion adsorption data, EDX measurement were carried out on metal adsorbed MWCNT sample and the results were found comparable with TGA data. We have found that the trends in adsorption of metal ions by the functional moieties on nanotube are in order of their nucleophilicity,29 and the only exception is acid chloride. Our study also revealed the importance of the nature of the side chain of functional groups as the long side chain in OA, ODA, and ODT helps to physically trap the metal ions along with the electrostatic interactions by polar functional groups. The highest metal ion elimination was found when the long unsaturated aliphatic side (OA) was used indicating that the isolated double bonds in the side chain also participated in the electrostatic interactions (Figure 3A). To further insight into the interaction of the metal ions with the functional group on nanotube surface, we have selected two metal ions, viz Pb2+ and
functionalization of nanotube leads to amorphization of outermost walls and the thickness of the amorphous layer depends upon the degree of derivatization as shown in Figure S5 of the Supporting Information.15 The silver nanoparticles actually diffused into this amorphous layer upon heating which helps to control the excess release of AgNPs in water and makes the AgNPs decorated functionalized MWCNTs reusable for a longer time. It has also been observed that the surfactant used for dispersing CNT and for capping AgNPs leads to a thin coating of the surfactant on the surface of AgNPs and nanotubes even after washing with water and alcohol. The TEM image of the AgNPs deposited on nanotube surface by SDS/DMF method clearly shows that AgNPs are embedded in the thin coating of surfactant which render the direct contact of water with the surface of silver or CNT (Figure 1B). Our modified process of Ag deposition on nanotube surface by EG/DI water does not use any surfactant and hence Ag and nanotube surface are both exposed to the incoming water for antibacterial and heavy metal removal purpose (Figure 1A). 3.3. Role of Functional Group on Nanotube Surface for Ag Loading, Metal Ion Removing, and Antibacterial Property. The amount of Ag loading on the nanotube depends on the nature of functional groups on the nanotube surface which has been systematically studied and represented in Figure 2. Among the functional groups, one from each category like acid, acid chloride, nitrogen containing functional group (amide) and sulfur containing functional group (thiol) are selected to study the effect of derivatization on MWCNT. Because of the strong affinity of thiol group toward noble metals, the maximum amount of Ag was deposited when thiol groups were present on the nanotube surface (Figure 2). The higher amount of Ag decoration was observed when E
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Figure 6. SEM images of the composite polysulfone membrane with 0 (A), 1 (B), 2 (C), 4 (D), 5 (E), and 7 (F) weight percentages of silver nanoparticles decorated functionalized MWCNTs. The weight percentage of polysulfone in the DMF solution for each membrane is 40%. The scale bar for each SEM images is 200 nm.
Cr6+, with contrasting ionic radii (133 and 58 pm, respectively) and charges (+2 and +6, respectively). It has been illustrated in Figure 3B that the amount of metal ion elimination is higher in Cr6+ compared to Pb2+ by treating the same concentration of lead and chromium salts on the OA functionalized MWCNTs. The critical factor for higher amount of elimination of metal ions by functionalized MWCNTs is thus charge on the metal ion and not the size of the ion. It has also been noticed that the elimination of metal ions in repeated exposure was much effective with the larger metal ion size as the larger metal ion radius provides a higher dimension of ionic clusters on the porous medium which consequently trap more metal ions (Figure 3B). The outer layer of CNT has been demonstrated for toxicological perspective where SWCNTs were found to be more effective compared to MWCNTs.6,30−32 AgNPs were thus grafted on the MWCNT surface to increase the cytotoxicity. The functional group on MWCNT furnishes additional
opportunity to make the surface of nanotube further reactive toward bacteria.32 The pristine and functionalized MWCNTs were subjected to antimicrobial testing against the E. coli Gramnegative bacteria and the growth of the culture was monitored by OD600 values. Among the functional groups, the thiol ester group was found to be most effective as the most of the sulfur containing compounds are known to exhibit widespread antimicrobial activities.33,34 It has been well established that the Gram-negative bacteria are much more susceptible to Ag+ compared to Gram-positive bacteria due to the fact that the cell wall of Gram-positive bacteria is thicker than that of Gramnegative bacteria.35 The highest antibactericidal activity of the AgNPs decorated thiol ester derivative of MWCNT indicates that both the thiol and AgNPs on the nanotube surface participated in the antibacterial activity (Figure 4). The cell growth curves were found to be decreased by increasing the concentration of nanotubes and the distinct growth to plateau like transition was observed in 1.5 mg/mL concentration of F
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Figure 7. N2 adsorption and desorption isotherm for pristine (closed and open blue squares respectively) and composite (closed and open red circles respectively) membranes are represented in part A. Part B illustrates the time dependent contact angles at every 10 s of the pristine (blue squares) and composite (red circles) membranes, which were measured for 2 min. The differences from the mean contact angle were represented by an error bar in graph B.
Figure 8. (A) Metal ion concentration of initial (C0) and filtered water (C) for pristine (open symbol) and composite (closed symbol) membranes at every 15 min up to 2 h, where Cu(I), Zn(II), Pb(II) and Cr(VI) were represented by blue circles, red stars, magenta triangles and green squares. Part B illustrates the ratio of absorbance at 600 nm of the disinfected water of initial (A0) and filtered water (A) for pristine (blue squares) and composite (red circles) membranes at every 15 min up to 2 h, respectively.
pores of polysulfone matrix (Figure 6). We observed that at an optimum weight percentage addition of AgNPs decorated functionalized MWCNTs into the polymer matrix, the CNT scaffolds could be constructed on the pores of polymer matrix (Figure 6E). One of the serious technological challenges of the membrane-based filtration is the optimization of the pore size of the polymeric membrane as the lowering of pore size will lead to efficient decontamination however at much lower flux rate.36,37 A mesh of CNT network on the pores of polymeric membrane will thus help to physically eliminate the toxic metal without compromising the flux rate (Figure 6E). The flux rates at 7 psi pressure of the 1.7 mm test cell membrane based on pristine and AgNPs-grafted thiol-functionalized MWCNTsbased polysulfone were found to be 74.79 and 76.13 L/m2 h. The N2 adsorption of the pristine polysulfone membrane gives rise to a lower BET surface area of 55 m2 g−1 compared to the 130 m2 g−1 of the composite membrane. A steep uptake of N2 at high pressure was seen in the composite membrane due to the higher interspatial volume between the nanotubes which is accessed by the N2 (Figure 7A). The molecular weight cut off
thiol ester functionalized MWCNT at the much lower absorbance value (inset of Figure 4). The similar observation was also made for E. coli bacterial growth in control and different functionalized MWCNTs at 50 μL concentration after 1 day on the covered Petri culture dish as illustrated in Figure S6 of the Supporting Information. 3.4. Fabrication of Nanocomposite Polymeric Membranes. 3.4.1. Morphology of Membrane. The composite membranes of nanomaterials mixed in polymer matrix are known for multifunctional activities, however, the uniform dispersion of CNTs in the polymeric membrane is a challenge. Figure 5 demonstrates that the functionalization of CNTs by polar group with a long side chain helps to disperse the nanotubes well into the polymeric matrix. Another key issue for the dispersing nanotube into the polymeric membrane is the viscosity of the resultant polymeric solution for casting of film (Figure 5B-5D). At higher viscosity of polysulfone, most of the nanomaterials are embedded in the matrix which are no longer available for decontamination and disinfection. We found that at 40 wt % polysulfone in DMF, most of the CNTs are introduced into the G
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Figure 9. SEM images of the top surface morphology of the pristine (A and C) and Ag nanoparticles decorated thiol functionalized MWCNTs (B and D) based polysulfone membranes which were subjected to constant filtration for 10 days with the Egyptian blue dye of 10 ppm initial concentration (A and B) and with E. coli bacteria stock solution (C and D). The scale bars for SEM images A and B are 2 μm and for C and D are 10 μm, respectively.
an important role in electrostatic interaction for adsorbing cationic species. The metal ions are generally remain cationic below pH 8 and thus the pH of the overall test solution is kept 8 to avoid hydroxide formation.27,39,40The trend of the metal ion removal by the composite membrane is similar to that of results obtained with functionalized MWCNTs without polymer matrix (Figure 8A). The higher ionic charges and the large size metal ions efficiently eliminated due to the electrostatic interactions and for the formation of larger ionic clusters in the pores of the membrane at the successive steps of filtration. The CNT scaffolds which were generated inside the pores of filtration membrane would help to sieve out the toxic metal ions and metals by both the physical and chemical interactions. Unlike the metal ion removal, the bacteria elimination by the membrane was found to be more efficient as the time progresses (Figure 8B). This phenomenon is due to the higher contact time allowed for the infected water to interact with the composite membrane and it can be concluded that the AgNPs and the surface of functionalized CNT in the pores of membrane actually helps to remove bacteria (Figure 8B). This observation was further confirmed by the E. coli bacteria culture on the pristine and composite membranes where the lower bacteria colony was found on the composite membrane after 1 day as illustrated in Figure S7 of the Supporting Information. The top surface morphology of the AgNPs decorated thiolfunctionalized CNT-based polysulfone membrane after 10 days of filtration by E. coli bacteria stock solution reveals higher bacteria counts compared to that of pristine membrane due to the strong interaction of the bacterial cell cellulose membrane with the thiol, Ag and nanotube inside the pores of membrane
(MWCO) experiments also confirms the fact that nanotube introduction in the polysulfone membrane increases the porosity of the composite membrane and the relatively higher amount of larger pores in composite membrane practically determines the membrane performance which critically influences the flux rate of filtration process as discussed in Figure S3 of the Supporting Information. The grafting of polar groups on the surface of nanotube increases the hydrophilicity of MWCNTs which in turn increase the hydrophilicity of the composite membranes.14 The contact angle measurements of the pristine and the composite membranes were carried out to assess the changes in the wetting characteristics of the membrane surface and it was found that the static contact angle of composite membrane was reduced to 33.0 from 68.9 in case of pristine membrane. The time dependent contact angle of the pristine and composite membrane was measured for 2 min, and data were collected every 10 s (Figure 7B). The gradual decay of contact angle within the measurement time is more evident in composite membrane where the deviations were shown by error bars in the graph (Figure 7B). This reduction in contact angles has been expressed by diffusion controlled reaction38 and the higher hydrophilic surface of the composite membrane due to the incorporation of functionalized MWCNTs leads to the decrease of contact angle from 33 to 31.11 within the measurement time (Figure 7B). For the toxic metal ion removal study, different ionic charges [Cu(I), Zn(II), Pb(II), and Cr(VI)] with different atomic radii [145, 142, 154, and 166 pm, respectively] metals were used at pH 8. The pH value of the solution critically determine the density of negative charges on the nanotube surface which plays H
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and hence the amount of sieving of bacteria is expected to be high on composite membrane (Figure 9, parts C and D).18,31 The insertion of hydrophilic functionalized nanotubes in the hydrophobic polymeric matrix resulted higher hydrophilicity of the membrane which resulted decreased bio and organic fouling (Figure 9, parts A and B).14,31 The degree of fouling in the composite membrane was found to be much lower after 10 days of filtration where the initial concentration of Egyptian blue dye was 10 ppm. The modification of the MWCNT by polar functional groups and the grafting of AgNPs on nanotube surface thus provide us an opportunity to tailor make the properties of the functional membrane for better interaction with contaminated and infected water.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge help from Prof. Ashutosh Sharma and Mr. Alok Srivastava, DST Unit of Nanoscience, IIT, Kanpur, India, for the SEM experiments. We thank Prof. A. R. Bhattacharya, IIT, Mumbai, India, for the TEM experiments. The authors acknowledge the help and support of the scientists, research scholars, and the staff members of the Nano Science and technology division for the experimentations and characterizations. The help and support from Dr. Vineet Sharma, Apeejay STYA University, Gurgaon, India, is gratefully acknowledged. The authors also thank Dr. Chetan Malhotra, Tata Research Development & Design Centre, Pune, India, and Mr. Subhash Devi, Membrane Filters (lndia) Pvt. Ltd., Pune, India, for collaboration, helpful discussions, and suggestions. The authors are grateful to the Director, DMSRDE, Kanpur, India, for help, support, and guidance and for permitting us to publish our experimental findings.
4. CONCLUSIONS The method for the grafting of AgNPs on the MWCNTs surface has been modified for the controlled and higher amount of Ag decoration on the surface of MWCNTs. The surfactant was not used for the synthesis of AgNPs-decorated MWCNT to enhance the interfacial interactions of the bare surface of Ag nanoparticles, polar functional groups and MWCNT surface with the toxic heavy metal ions and bacteria. For the decontamination of the toxic heavy metal ions by the electrostatic interaction with the polar group on nanotube surface, it has been noticed that the oxidation state of the metal ion is the determining factor over the size of the metal ion. However, the higher ionic clusters help to eliminate the larger size metal ions in the successive steps of filtration. The characteristic affinity of the thiol functional group toward the noble metals enables us to load the higher amount of silver nanoparticles on the nanotube surface. At an optimum weight percentage addition of the AgNPs decorated thiol functionalized MWCNT in polysulfone matrix, the CNT scaffolds were seen to be generated inside the pores of filtration membrane which sieve out the toxic metal ions and metals by both the physical and chemical elimination. The modification of the CNT surface by polar functional group also helps to increase the bio and organic fouling characteristics due to the enhanced surface area and hydrophilicity of the composite membrane. The grafting of AgNPs on nanotube derivative thus provides us an opportunity to tailor the molecular level interactions in the functional polymeric membrane for improved decontamination and disinfection.
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ASSOCIATED CONTENT
S Supporting Information *
Schematic process of different functionalization methods (Figure S1), schematic process of grafting of Ag nanoparticles on MWCNTs (Figure S2), molecular weight cutoff (MWCO) curves of pristine and composite membrane (Figure S3), SEM and TEM images of silver nanoparticles decoration of CNT by different methods (Figure S4), surface morphology of pristine MWCNT and amidized MWCNT (Figure S5), and bacterial growth culture in the presence of pristine and functionalized MWCNT solutions (Figure S6) as well as on pristine and composite membranes (Figure S7) respectively. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04114.
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DOI: 10.1021/acs.jpcc.5b04114 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
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DOI: 10.1021/acs.jpcc.5b04114 J. Phys. Chem. C XXXX, XXX, XXX−XXX
