Dual applications of silver nanoparticles incorporated functionalized MWCNTs grafted surface modified PAN nanofibrous membrane for water purification

Abstract

An appropriate choice of material and tenacious substrate is imperative to accomplish the development of membrane based point-of-use materials. Herein, we have developed silver nanoparticles (Ag NPs) incorporated carboxylated multiwalled carbon nanotubes (MWCNTs) grafted aminated polyacrylonitrile (APAN) based nanofibrous membrane pertinent for the removal of toxic heavy metals and bacteria present in water. The intermediate product formed in the interim of the membrane preparation has the potential for the filtration and adsorption of heavy metals whereas, the final nanofibrous membrane is found to have exceptional antibacterial properties as well as filtration capability. Analytical techniques such as field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) corroborated the functional modifications and integration of Ag/PEGylated MWCNT–COOH nanocomposites with the APAN nanofiber mats. Thermal and mechanical studies revealed improved stability and tensile strength of the nanofibrous membrane. The kinetic and isotherm model investigations indicated the adsorption properties of the APAN nanofibers. Bacterial viability assay, fluorescence microscopy and spectroscopy analysis, disc diffusion studies and electron microscopic imaging of the membrane after the filtration was investigated. Comprehensively, our results put forth the candidature of APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane to be used as an impending point-of-use water purification material.

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Introduction

One of the most ubiquitous problems today that mankind is facing throughout the world is the poor access to clean drinking water and freshwater.¹ With an increase in the demand for high quality drinking water, several modern technologies are being established for water purification to cater for the potable and non-potable use. Looking at the challenges associated with the customary purification materials, solutions based on state-of-the-art science and technology plays a pivotal role for the availability of safe drinking water. In recent decades, interest has arisen in the use of nanotechnology for water purification.²

A polymer based nanofibrous membrane plays an important role in the commercial water purification systems.^3,4 These systems are exquisitely based on the filtration process which are effective but, most of the membranes lack adsorption and antibacterial properties. Surface functional nanofibrous membrane become expedient for small scale point of use applications in lieu of centralized water purification network where rural population of the developing nations depends on it. Such nanofibrous membranes should be profoundly simple to use, portable, non-toxic and should require zero energy consumption.

Our approach comprises formerly of the surface modification of as-spun PAN nanofibers with amine groups to adsorb the heavy metals. The adsorption behaviour of APAN nanofibers has already been investigated on the wide range of heavy metals such as Ag, Cu, Fe and Pb.^5–7 Several functional nanomaterials having the antibacterial properties have been exercised for the fabrication of hybrid nanofibrous membrane. For centuries, silver have been used as an effective antibacterial agent.⁸ Silver nanoparticles (Ag NPs) are well known for their antibacterial properties for use in water filtration applications.^9–12 Point of use filtration applications include nano-silver impregnated onto the blotting paper,¹³ in polyurethane foams,¹⁴ ceramic filters¹⁵ and silver nanoparticles decorated PAN nanofibrous membrane for the detection of biological and chemical species.¹⁶ The enhanced antibacterial property of the carboxylated multiwalled carbon nanotubes (MWCNT–COOH) incorporated with the silver nanoparticles has been reported.¹⁷ In this work, a hybrid nanofibrous membrane was fabricated progressively and it was characterized by several analytical techniques and its antibacterial and filtration properties were explored scientifically. Hence forth, the present research work proposed the nanofiber based functionalized materials having dual applications by means of adsorption, antibacterial and filtration methods.

Materials and methods

Polyacrylonitrile (PAN) (M_w: 150 000) and polyethylene glycol (PEG) (M_w: 6000) were purchased from Sigma-Aldrich, silver nitrate was purchased from Merck Pvt. Ltd., India and carboxylated multiwalled carbon nanotubes (MWCNTs) (OD: 30–50 nm) was procured from Sisco Research Laboratories (SRL) Pvt Ltd., India and were stored at appropriate temperature until used. Dimethylformamide (DMF) was acquired from RANKEM laboratories, India. Ethylenediamine, N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide (EDC) and 1,6-hexamethylene diisocyanate were obtained from Sigma-Aldrich. Gram positive bacterial strain Staphylococcus aureus (S. aureus) (MTCC 737) was obtained from IMTECH, India and recombinant green fluorescence protein expressing Escherichia coli (GFP E. coli) used as Gram negative strain. Bacterial growth medium, such as Luria–Bertani (LB) medium and nutrient broth (NB) medium were purchased from Merck (Germany) and Himedia (India), respectively. Ampicillin antibiotic was procured from Sisco research laboratories (India). All the chemicals were of analytical grade and used as received. All the preparations were made in deionized water.

Electrospun PAN nanofibers

Standard electrospinning apparatus procured from ESPIN Nano (Physics Equipment and Company, India) was used for the fabrication of nanofibers. 8% PAN nanofibers was synthesized by dissolving polyacrylonitrile (PAN) in dimethylformamide (DMF) followed by magnetic stirring (i.e. 450 rpm) at 80 °C for 10 h. After curing for 2 h to bring the polymer solution to room temperature, the PAN solution was loaded into the 2 mL syringe and electrospun onto the aluminium foil. The spinneret tip was connected to a variable high voltage power supply to dissipate charge into the polymer solution. The instrument was operated at 11 kV with a flow rate of 0.35–0.4 mL h⁻¹, respectively. The nanofibers generated in the process were collected over grounded stationary metal collector positioned at a distance of 15 cm from the spinneret. The nanofiber deposition was carried out for a stipulated time of approximately 4 h for uniform thickness under a controlled cabinet temperature of 30 °C and a constant relative humidity of 50–55%.

Synthesis of aminated PAN (APAN) nanofibers

The as synthesized PAN nanofibers was immersed in the 20% of ethylenediamine (EDA) solution under magnetic stirring at 150 rpm for 24 h in the room temperature. Then the APAN nanofibrous mat was washed with deionized water for several times and the pH was adjusted to 7.0 using acid. The percentage conversion of the nitrile group of PAN into the amine group (i.e. in%) on the surface of the nanofibers was calculated by using the equation shown below.where, C_n is the nitrile conversion in percentage, W₀, W₁ are the initial and final weights of the PAN nanofiber mat after the amination, M₀ and M₁ are the molecular weights of the acrylonitrile monomer (i.e., 53 g mol⁻¹) and ethylenediamine (EDA) (i.e., 60 g mol⁻¹). The sample weights are measured using a Sartorius BS 224S digital balance with a measurement resolution of 0.1 mg.

Adsorption behaviour and filtration studies

Batch adsorption studies of As(V) ions from aqueous solutions were carried out with APAN nanofibrous mat each weighing 0.1 g in 100 mL of As(V) solutions in flasks having different concentrations viz., 100, 200, 300, 400 and 500 μg L⁻¹, respectively. The As(V) solutions in the flasks were kept in a thermostatic incubator at 100 rpm and the adsorption of As(V) with the effect of contact time was investigated at a constant pH value of 7. At an intermittent time points, a 2 mL solution of each samples were withdrawn and the As(V) ion concentration has been quantified by inductive coupled mass spectroscopy (ICP MS). The adsorption capacity (q, in μg L⁻¹) of the nanofibrous adsorbent has been determined using the following equation.¹⁸where, C_N0 and C_Ne represents the initial and the equilibrium concentration of the As(V) ion (μg L⁻¹), vol.; total volume of the testing solutions (L), and M; mass of the adsorbent (0.1 g).

The isothermal studies of the As(V) ion were performed by placing different nanofiber mats (0.1 g each) in flasks having concentrations ranging from 100 to 500 μg L⁻¹. A constant pH value of 7 has been maintained for different solutions in the flasks and the same were kept in a thermostatic shaker at 35 °C and 100 rpm. After 6 h of the contact time, 2 mL of each samples were withdrawn and As(V) concentration has been determined. In an another experiment, filtration efficiency of the APAN nanofibrous mats was evaluated by using three different As(V) ion concentrations such as 100, 200 & 300 μg L⁻¹, respectively. The sample solutions from both the studies were analysed to quantify the metal ion concentration by ICP-MS.

PEGylation of MWCNT–COOH

PEGylation was done by prior sonication of carboxylated multiwalled carbon nanotubes (MWCNT–COOH) in acetone for 1 h and magnetically stirred at 50 °C for 15 min under N₂ atmosphere purged at the rate of 60 mL min⁻¹. The above MWCNT–COOH suspension was supplemented with 200 μL of 1.25 mM hexamethylene diisocyanate solution and the mixture was heated at 50 °C for 3 h under magnetic stirrer. Then, 1.5 mM of polyethylene glycol (PEG) was added into the mixture under magnetic stirring for another 2 h at the same temperature under N₂ atmosphere. Eventually, the resulting PEGylated MWCNT–COOH suspension has been repeatedly washed with deionized water and vacuum dried to collect the particles.

Synthesis of Ag/PEGylated MWCNT–COOH nanocomposites

Silver nanoparticles were incorporated onto the surface of the PEGylated MWCNT–COOH by the reduction reaction of Ag⁺ ions by the PEGylated MWCNT–COOH.¹⁹ Briefly, 50 mg of PEGylated MWCNT–COOH has been sonicated in 10 mL of deionized water and 20 mL of 0.01 M silver nitrate (AgNO₃) aqueous solution was added to the PEGylated MWCNT–COOH mixture. It was then heated for 3 h at 60 °C under magnetic stirring at 300 rpm. The aqueous solution was termed as silver nanoparticles incorporated PEGylated MWCNT–COOH nanocomposites and it was washed for several times using deionized water to remove the unreacted substances.

Fabrication of APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane

25 mL of aqueous Ag/PEGylated MWCNT–COOH nanocomposites was increased up to 100 mL with deionized water and it was sonicated for 30 min and immersed into APAN nanofibers. Further, 0.2 g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) was added and then the mixture was magnetically stirred at room temperature for 12 h. The Ag/PEGylated MWCNT–COOH nanocomposites mixture grafted over the APAN nanofibers has been denoted as APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane, it was then washed with deionized water and dried to evaluate the filtration performance and its bactericidal efficiency and reusability. The sustained release of silver ions from the nanofibrous membrane was subsequently determined by ICP-MS and the total concentration of silver ions was also determined.

Characterization of PAN, APAN nanofibers and APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane

The morphology of the PAN alone, APAN and the APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane was analyzed by Ultra plus-Carl Zeiss (Germany) field emission-scanning electron microscope (FE-SEM) operated at 15 kV and FE-SEM (FEI Quanta 200F, USA) equipped with energy dispersive X-ray detector (EDX) operated at an accelerating voltage of 15–20 keV. The nanofibers were gold coated for 80 s in Denton gold sputtering unit before being mounted on FE-SEM stage. The nanofibers images were captured and were further processed by analysis software ImageJ to determine the mean diameter and size distribution of the nanofibers. The morphology of the bare PEGylated MWCNT–COOH and the APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane were studied by TEM (FEI TECHNAI G2, USA) at an operating voltage 200 kV. 2 mg of the membrane was sonicated with 10 μL of deionized water and drop casted onto the carbon coated copper grid for TEM analysis. The Fourier transform infrared (FTIR) spectra of the different stages of the membrane were acquired by Thermo Nicolet spectrometer (USA) using KBr pellets in the range 4000–400 cm⁻¹. FTIR spectra of control samples such as PAN nanofiber, MWCNT–COOH were also acquired for the interpretation of peaks. TG-DTA analysis of the nanofiber membranes were carried out to analyze the thermal degradation profile and its stability. Stipulated quantity of 15 mg of the nanofibrous membrane was heated from 0 °C to 800 °C at a constant rate of 10 °C min⁻¹ in inert atmosphere using the EXSTAR TG/DTA 6300 (Hitachi, Japan). The purity and crystalline phase structure of the pristine PAN nanofiber, MWCNT–COOH and the APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane were determined using advance powder X-ray diffractometer (Bruker AXSD8, USA) (Cu-K_α radiation, λ = 1.5406 Å) with θ value in between 5–80° at a scan speed of 0.2° min⁻¹.

Tensile analysis

Tensile strength of the nanofibrous membrane was measured using a high-load cell testing machine (Bose ElectroForce® 3200 series III) acquired from Bose Corporation, USA. Samples used were according to the American Standard for Testing Materials (ASTM) standard D638 with a rate of grip separation of 0.1 mm s⁻¹. Sample dimensions were 25 mm and 10 mm with a gauge length of 7 mm measured using Vernier calliper and the thickness varies from 0.120 to 0.180 mm. Tensile test was done initially with the pristine PAN nanofiber and latter using the nanofibrous membrane. In brief, the samples were clamped on a 450 N load cell and a maximum load of 200 N was applied at a speed of 0.1 N s⁻¹. Signals related to force and displacement were recorded throughout the experiments. The ultimate tensile strength (UTS), Young's modulus and strain failure point (%) were evaluated using stress–strain plot.

Antibacterial study

The bactericidal property of the APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane was tested against a non-pathogenic strain of GFP E. coli, a Gram negative bacteria and against S. aureus (MTCC 737), a Gram positive bacteria and the bacterial cultures without the treatment of nanofiber membrane was used as control. The inoculated flasks were kept for incubation at 220 rpm at 37 °C for overnight. The samples were withdrawn intermittently from the flasks to analyze the time dependent antibacterial activity of the nanofiber membrane using the UV-visible spectrophotometry and were compared with the control experiment. The treated and untreated bacterial cells were obtained by centrifuging 1 mL of bacterial culture each for 10 min at 3000 rpm and were resuspended in water for the fluorescence microscopic analysis. Additionally, the bacteria was plated on the respective LB and NB agar plates and were incubated overnight at 37 °C for 24 h and the number of bacterial colonies were counted.

Filtration study of nanofibrous membrane

S. aureus and GFP E. coli were the model bacterial systems chosen to investigate the filtration efficiency of the nanofibrous membrane. Both the target bacteria were cultured to a midlog phase and the cells were harvested by centrifugation. A 10 mM solution of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) and 1% dextrose with pH adjusted to 7 was used as the medium for the harvested bacterial cells during the filtration process. The dextrose present in the medium act as a nutrient source to reduce the bacterial cell death, since large number of viable bacterial cells are prerequisite for the filtration studies. The bacterial cells in the medium were poured over the special filtration unit. It consisted of an upper chamber, in which the medium containing bacterial cells can be fed, an intermediate porous plastic filter acting as a template in which the nanofibrous membrane was clinched and the outlet has the provision to connect with a glass bottle to collect the filtrate. Then the presence of bacterial colonies in the filtrate was further analysed.

Statistical analysis

The values are indicated in mean ± S.E.M. for each experiments. The data were analyzed using Student's t test or by one-way ANOVA, using GraphPad Prism 6.0, and statistically significant values are denoted by *p < 0.05, **p < 0.005, and ***p < 0.001.

Results and discussion

PAN nanofibers

Polyacrylonitrile (PAN) nanofibers were obtained by electrospinning of 8% PAN in dimethylformamide (DMF) solution at 10 kV with a tip-to-collector distance of 12 cm. The PAN nanofibers mats with rugged surfaces are evident in the FESEM image (Fig. 1(a) and (b)). The average size distribution of the PAN nanofibers was found to be 382 ± 36 nm in diameter (inset of Fig. 1(b)).

Fig. 1 FE-SEM images of PAN nanofibers (a and b) pristine PAN nanofiber (c and d) aminated PAN nanofiber (APAN).

Surface modified PAN nanofibers

FESEM investigations over the morphological transformation of the as-spun PAN nanofibers after the amination reaction by ethylenediamine (EDA) found that the APAN nanofibers was smooth with sticky wet surface (Fig. 1(c) and (d)). The graft yield of the amine group after the reaction was estimated to be 31.5% by the eqn (1) and the chemical transition of the as-spun PAN nanofibers into APAN nanofibers is represented in (Fig. 2(a)). The functional changes from the nitrile to amine group was confirmed by FTIR. The post amination reaction found that the intensity of the absorption peaks at wavenumbers 2245 and 1732 cm⁻¹ decreased owing to stretching and bending vibration of the nitrile functional group (–CN) of the pristine PAN nanofibers. On the other hand, the APAN nanofibers showed significant changes with decrease in the intensity of the peaks at 2245 and 1253 cm⁻¹ which is the testimony for the conversion of nitrile into the amine and amino group onto the surface of the polymer (Fig. 2(c)). Besides, there were broad bands at 3361 and 3428 cm⁻¹, respectively which can be attributed to the N–H stretching vibration. Subsequently, successive peaks such as 1389, 1474, 1580 and 1654 cm⁻¹ were observed demonstrating the presence of primary amine groups.

Fig. 2 Schematic of (a) amination and (b) PEGylation reactions. FTIR of (c) PAN and aminated PAN (APAN) nanofiber (d) MWCNT–COOH and PEGylated MWCNT–COOH.

PEGylation of MWCNT–COOH

The PEGylation of carboxylated multiwalled carbon nanotubes (MWCNT–COOH) using polyethylene glycol (PEG) was assessed by FTIR spectroscopy. MWCNT is considered to be a suitable carbon based materials for various applications such as electrocatalysis, sensors and energy devices etc.^20–22 In this process, the MWCNT–COOH of diameter 30–50 nm was PEGylated in order to increase its hydrophilicity. The covalent grafting of PEG with the pristine carboxylated MWCNTs was carried out by using a linker 1,6-hexamethylene diisocyanate. The PEGylation reaction was explicitly illustrated in (Fig. 2(b)). To clearly interpret the functional changes, FTIR spectra of bare MWCNT–COOH was also acquired apart from the PEGylated MWCNT–COOH (Fig. 2(d)). Both exhibits sp³-C–H stretching absorptions from the alkyl chain appeared at 2916 and 2971 cm⁻¹. Similarly, the absorption attributed to the carboxylic acid group was represented by a broad peak at 3440 cm⁻¹ from the –OH group and the peak at 1630 cm⁻¹ from the carbonyl group (C=O). At the same time, in the spectra of the PEG grafted MWCNTs–COOH, the stretching vibrations shifted to the lower frequency bands between 2860–2925 cm⁻¹ and subsequently the stretching mode of carbonyl (C=O) and C–O–C group was observed at 1630 and 1106 cm⁻¹, respectively. The grafted PEG plays crucial role in this process because, it acts both as the reduction site for the formation of Ag NPs as well as enhancing the hydrophilic properties of the MWCNT–COOH.

Fabrication of APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane

The FESEM examination revealed the morphological transition of the fabricated APAN grafted with Ag NPs incorporated PEGylated MWCNT–COOH nanofibrous membrane (Fig. 3(a) and (b)). Individual shattered nanofibers with MWCNT–COOH over the surface was apparently witnessed and the elemental analysis substantiated the presence of Ag NPs incorporated onto the PEGylated MWCNT–COOH (Fig. 3(c) and (d)) with silver 3.8% by weight (inset of Fig. 3(c)). The grafting of silver nanoparticles (Ag NPs) incorporated PEGylated MWCNT–COOH over the APAN nanofibrous mat was carried out by EDC chemistry and the exhaustive grafting protocol of the membrane is illustrated in (Fig. 3(e)). FTIR analysis of the nanofibrous membrane revealed that the wavenumbers at 2921 and 2860 cm⁻¹ is attributed to the C–H stretching vibrations. Similarly, carbonyl group stretching of the carboxylic acid group of MWCNTs was observed at 1620 cm⁻¹ and successive amide group vibrations explicitly ascertained by sharp peaks between 1350–1580 cm⁻¹ of the amination of PAN nanofiber. In addition, C–N stretching vibrations at 1262 and 1032 cm⁻¹ are found which was contributed by the linking agent EDC (IR spectra not shown).

Fig. 3 APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane (a and b) FESEM (c and d) EDX elemental analysis and (e) grafting reaction.

TEM analysis

TEM micrographs showing the PEGylated MWCNT–COOH (Fig. 4(a)) and the Ag NPs dispersion over the nanofibrous membrane (Fig. 4(b)). The elemental analysis affirmed the presence of Ag NPs presence on the nanofibrous membrane (Fig. 4(c)). Some of the Ag NPs incorporated MWCNT–COOH nanocomposites gets detached from the nanofiber (Fig. 4(b)) which was due to the ultrasonic vibrations generated during the sample preparation. In addition, TEM analysis also substantiated that, grafting Ag/MWCNT–COOH onto the APAN nanofibers form a three dimensional structure (Fig. S1†).

Fig. 4 TEM micrographs (a) PEGylated MWCNT–COOH, (b and c) nanofibrous membrane and its elemental analysis and (d) HRTEM micrographs of nanofibrous membrane (insets in (d) represent the inverse FFT images of the selected region corresponding to Ag NPs and its dispersion onto MWCNT–COOH).

XRD analysis

The X-ray diffraction pattern of the pristine PAN nanofibers, MWCNT–COOH and the nanofibrous membrane is shown in (Fig. 5(C)(a) and (b)). The broad peak at 26° can be attributed to the (002) plane of the COOH group of the MWCNT²³ and the nanofibrous membrane possessed sharp low intense peak at 38.1° is the characteristics of the (111) diffraction plane of the silver (PDF no: 001-1167). Ag NPs dispersion on carboxylated MWCNT as well as over the surface of the nanofibers were observed (Fig. 4(b)) in which the particles were formed due to the reduction reaction of both PEG and COOH groups.²⁴ A higher resolution HRTEM image of an individual silver nanoparticle with lattice distance of 0.267, which was in concordance with that of the (111) plane distance (Fig. 4(d)).

Fig. 5 APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane showing (A) DTA, (B) TGA (C) XRD spectra (D) tensile analysis.

DTA/TG analysis

The thermal perceptivity of the pristine PAN, APAN and the nanofibrous membrane was determined by DTA-TG analysis (Fig. 5(A) and (B)). The DTA thermogram of the pristine PAN nanofibers exhibited a proportionately large and extensive sharp exothermic peak at about 320 °C. In the modified nanofibers, although the exothermic peaks shifted to the lower temperature, yet the peak positions remains same with the presence of amine group and MWCNT on the functionalized nanofibers (Fig. 5(A)(b) and (c)). Also, the intensity of the peak area decreased due to the functional groups. It was generally agreed that the sharp exothermic peak of the pristine PAN was due to the complex chemical reactions conventionally associated with the oxidative stabilization of aforementioned nanofibers.²⁵ Similarly, the downshift of the exothermic peaks of the modified nanofibers given the clue that the amine group and the MWCNT onto the nanofibers accelerate the oxidative stabilization. Decrease in temperature was due to the interaction between the PAN and the functionalized materials which may decrease the free radical formation on the nitrile group and their recombination which leads to decrease in the reaction temperature. Thermo gravimetric (TG) analysis of the pristine PAN (Fig. 5(B)(a)) revealed that the decomposition occurs predominantly at 320 °C. Compared to the pristine PAN, both APAN and the nanofibrous membrane (Fig. 5(B)(b) and (c)) decomposed partially at low temperatures whereas, 32% and 49% of residuals are left behind at 800 °C. Since the pristine PAN nanofibers which are not carbonized showed less than 1% of residuals. The increased residuals in the former functionalized nanofibers were due to the presence of amine group, Ag NPs and MWCNT over the membrane. The detailed results of the thermal behaviour of the nanofibers is briefed in (Table S1†).

Mechanical analysis

Mechanical strength is one of the important factor for the nanofibrous membrane which is to be used for the filtration goals. Stress–strain curves of the PAN and APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane are shown in (Fig. 5(D)). It was noted that the grafting of PEGylated MWCNT–COOH onto the APAN nanofibers enhanced the ultimate tensile strength (UTS) and there is a constant elongation at break by 57%, further the elastic modulus of the nanofibrous membrane is doubled in comparison to the PAN alone nanofibers. The silver nanoparticles incorporated PEGylated MWCNT–COOH is oriented along the APAN nanofibers and hence this grafting enhances the mechanical strength of the membrane. From the tensile testing results (Table S2†), it can be clearly interpreted that the nanofibrous membrane has superior mechanical property appropriate for the filtration applications.

Adsorption study

Effect of contact time. The adsorption of As(V) ions onto the aminated PAN nanofibers with five different concentrations from 100–500 μg L⁻¹ as the function of time (until 6 h) and C_n is shown in (Fig. 6(a)). The adsorption capacity of As(V) ions increases when the contact time t was increased. The adsorption capacities of different concentrations were observed as 91.4, 187.6, 268.4, 359.9 and 460.68 μg g⁻¹, respectively. The % adsorption of each concentration of As(V) ions onto the aminated nanofibers is determined (Fig. 6(b)). Further the adsorption kinetics and isotherm models for As(V) ions have been investigated and the results were correlated with the experimental results.

Fig. 6 Adsorption kinetics of As(V) onto the APAN nanofibers mats (a) effect of contact time and (b) % adsorption of As(V) (c) first order kinetics and (d) second order kinetics.

Adsorption kinetics. Kinetic studies was performed to investigate the adsorption kinetics of the nanofibers using five different concentrations of As(V) ions from 100 to 500 μg L⁻¹, respectively. The pseudo-first-order kinetic model suggests that the approximation of only the high concentration reactant for the reaction. The model in the equation form was shown below (3).where, q_t; adsorption capacity of the adsorbent at time t (μg g⁻¹) and k_t; pseudo first-order rate constant (min⁻¹). By plotting log(q_e − q_t) vs. t (Fig. 6(c)), the equilibrium adsorption capacity (i.e., q_e, cal) and the rate constant k_t can be determined from the slope and intercept along with the correlation coefficient (r²).

Similarly, the adsorption rate could also be determined using pseudo-second-order kinetic model. This model suggests that the second order rate constant k₂ depends on the parameters such as concentration, temperature and pH, respectively. The mathematical form of the pseudo second order model was shown in the eqn (4).

where, k₂; second-order rate constant (g μg⁻¹ min⁻¹). Through the linear plot between t/q_t and t (Fig. 6(d)), the calculated equilibrium adsorption capacity (i.e., q_e, cal) and rate constant k₂ can be obtained along with the correlation coefficient (r²).

On the basis of the values of the correlation coefficient (r²) (Table 1), the experimental data fitted better with the pseudo second-order kinetic model and followed the chemisorption process due to the presence of functional group. Additionally, the q_e, cal values were observed to be very close to the experimental values of the second order model.

Table 1 Kinetic parameters comparing the adsorption of As(V) onto the APAN nanofiber mats by experimental, first order and second order kinetics model

Adsorbent	Concentration (μg g^−1)	qe, cal (μg g^−1)	Pseudo first order			Pseudo second order
			qe, cal (μg g^−1)	k1 (min^−1)	r^2	qe, cal (μg g^−1)	k2 (min^−1)	r^2
Aminated PAN nanofiber mats	100	91.4	70.3	0.018424	0.9694	98.03	4.79 × 10^−4	0.9987
	200	187.6	159	0.015890	0.8778	200	1.36 × 10^−4	0.9902
	300	268.4	218.2	0.018424	0.9917	294.1	1.34 × 10^−4	0.9997
	400	359.9	295.1	0.017502	0.9925	400	8.58 × 10^−5	0.9994
	500	460.6	366.4	0.016351	0.9867	500	7.28 × 10^−5	0.9995

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Adsorption isotherm. Several mathematical models are available in the literature to investigate the maximum adsorption capacity of As(V) ions. The most stereotyped one is the Langmuir model, which is expedient for the adsorption studies and the equation is given below,

The maximum adsorption capacity (q_m) and the rate constant (K_L) were determined from the linear plot between C_e/q_e and C_e. The adsorption isotherm plots of the experimental and the model were obtained by using the linear method²⁶ (Fig. S2†) and the isotherm parameters for As(V) adsorption are summarized in Table S3.† From these results, it was ascertained that the adsorption data fitted well with the Langmuir isotherm with the correlation coefficient close to 1 (r² > 0.99). Thus, monolayer adsorption takes place on the adsorbent. From the model results (Table S3†), the maximum adsorption capacities of the nanofibrous mat using different concentrations of As(V) ions were found to be 58.47, 114.9, 227.2, 294.2 and 370.3 μg g⁻¹, respectively. The obtained adsorption capacities of As(V) ions is compared with other adsorbents reported earlier and found to be significant.^27–29 When the model results were correlated with the experimental results, the adsorption capacities are higher in the latter with equal weight (0.1 g) of the adsorbent used over different concentrations (Table 1). Consequently it was proposed that the precursor of the nanofibrous membrane, the APAN nanofibers can be used as a credible adsorbent for the removal of As(V) from the aqueous solution.

Filtration study

The filtration capability of the nanofibrous membrane was investigated using three different concentrations of As(V) ions such as 100, 200 and 300 μg L⁻¹, respectively. The filtration setup in which the APAN nanofibrous membrane is fixed on the underlying portion of the chamber is illustrated in Fig. S3.† The initial and final concentration of the As(V) before and after filtration was measured by ICP-MS and found to be 90% filtration efficiency in all three concentrations (Fig. 7).

Fig. 7 % filtration of As(V) by the APAN nanofibers. BF – before filtration, AF – after filtration. The experiments were conducted in triplicate and values are expressed as mean ± S.E.M. Statistical significance were carried out by one-way ANOVA with Tukey's multiple comparisons test between groups using GraphPad Prism 6.0 software. Statistical significant values were denoted by *(p < 0.05), **(p < 0.005), and ***(p < 0.001). Statistically insignificant values were represented by “ns”.

Antibacterial studies

The antibacterial property of the APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane was investigated substantially. The bacterium GFP E. coli and S. aureus were selected as model bacterial systems and the results were interpreted by the optical density (OD₆₀₀, absorbance) measurements. Both the bacterium were inoculated into the growth medium treated with the nanofibrous membrane and the decline in the absorbance of the culture with respect to the incubation time was measured using UV-visible spectrophotometry at 600 nm. The time dependent antibacterial effect of nanofibrous membrane against both the bacteria was assessed. The analogy between the nanofibrous treated and untreated bacterium was performed and observed that there was no increase in the absorbance of the treated sample, whereas, in the latter there is an increase in absorbance with time confirming the exponential growth of bacteria in the untreated sample (Fig. S4(a) and (b)†). The results imply that the 3D nanofibrous APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane possess bactericidal property against both the bacterial strains. The compiled results of the antibacterial study are defined in the following discussions. The antibacterial implications in the GFP E. coli was analysed by means of its fluorescent intensity using fluorescence spectrophotometer. The time dependent fluorescent spectral study and the digital image of control and treated GFP E. coli is shown in (Fig. 8(a) and (b)). For the quantitative analysis, the disc diffusion method was adopted in which the LB and NB agar were spread onto the bacterial plates and the nanofibrous membrane of 2.2 cm diameter was placed in the centre of the plate. After 12 h of incubation, there was a remarkable difference in the inhibition zones of plates between the control and the nanofibrous membrane (Fig. 8(c)–(f)) which is significantly greater than the silver nanoparticles alone composite nanofibers reported earlier.³⁰ Conjointly, the bacterial extermination by the nanofibrous membrane was resolved by fluorescence microscopy showing the images of the control and treated samples of GFP E. coli and S. aureus (Fig. 8(g)–(j)). The FESEM analysis of the control and the nanofibrous membrane treated bacterial cells are represented in (Fig. 8(k)–(n)). It was identified that the cell membrane of the treated bacteria get disintegrated and cellular components were exposed (Fig. 8(l) and (n)). From this analysis, it was comprehensively postulated that the Ag NPs which were incorporated on MWCNT–COOH grafted over the APAN nanofibers were the only proprietors for the bactericidal activity.

Fig. 8 Complete antibacterial study of nanofibrous membrane against bacteria (a) time dependent fluorescence spectra of GFP E. coli control and treated (b) digital image of GFP E. coli (c–f) zone of inhibition analysis (ZOI) (g–j) microscopic analysis and (k–n) FESEM analysis of control and treated (S. aureus & GFP E. coli).

Bacterial filtration study

From the past decade, earlier investigations on the filtration efficiency of any membrane were carried out by externally applied pressure to the filtration cell. With the incentive of ease in operation, the filtration was executed through the gravity flow of the bacterial suspension using a simple Tarson made filtration set up as mentioned earlier. The series of events during the filtration study are illustrated in Fig. S5† and the video recorded during the filtration process is also attached in the ESI.† It was ascertained that the upper chamber was having the turbid bacterial suspension whereas the permeate was transparent as water. The studies were carried out in three successive cycles to evaluate the membrane's potential reusability. At the end of each cycle, the bacterial lawn over the surface which was rejected by the membrane during filtration was mildly washed using deionized water which can be conducive for the filtration of succeeding cycles. Conjointly, the presence of any bacterial colonies in the permeate was investigated. The quantitative measurements in terms of absorbance (OD_{600 nm}) of the growth of S. aureus in both the medium were depicted in (Fig. S6†). Qualitatively, the fluorescent microscopic images depicted the number of bacterial colonies before and after filtration (Fig. 9). The imaging of the bacterial cells have been effectuated by using dextrose as well as LB medium in order to correlate the filtration competency in both these medium (Fig. S7(a)–(h)†). From the preceded microscopic images, it was evident that no bacterial cells were found in any of the three cycles tested. The permeate water samples were spread onto the agar plates in order to corroborate the presence of bacterial colonies and witnessed that there were no traces of bacterial colonies in the plate (Fig. S8†). In a nutshell, the experimental setup followed by the sequence of events was illustrated in (Fig. 10(a)–(g)). Subsequently after the filtration, the membrane was freeze dried for 12 h and its morphology was investigated using FESEM in all the perspectives (Fig. 11(a)–(d)). These examinations revealed that the hybrid nanofibrous membrane is flamboyant in bacterial filtration and no bacterial cells were found in the permeate as evident from the microscopic and spectroscopic analysis. Hence, this hybrid nanofibrous membrane could be used as a prominent portable membrane filter.

Fig. 9 Fluorescence microscopic images of filtration study against S. aureus (a) before filtration, (b–d) after filtration with three repeated cycles 1, 2 & 3 and against GFP E. coli (e) before filtration, (f–h) after filtration with three repeated cycles 1, 2 & 3 (scale bar: 100 μm).

Fig. 10 (a) Experimental setup of filtration unit (b and c) digital image and (d and e) microscopic images of before and after filtration (f) fluorescence spectra and (g) digital micrograph of control and treated GFP E. coli.

Fig. 11 FESEM analysis of nanofibrous membrane after filtration represented as pseudo coloured images (a) S. aureus and (c) GFP E. coli (with insets showing bacteria on the membrane ((b) and (d)).

Release study of silver ions

While contemplating the negative impacts of the silver toxicity of human health, the silver ions released from the nanofibrous membrane to the environment was analyzed by ICP-MS. Initially, 100 mg of the membrane was immersed in 100 mL of deionized water and the total silver released for 96 h consecutively was found to be 50.2 μg L⁻¹, whereas the total silver composition of the nanofibrous membrane (5 mg) was determined to be around 600 ± 50 μg L⁻¹. The cumulative release of silver ion concentration from the membrane at time t meets the US-EPA guideline for drinking water of less than 100 μg L⁻¹ shown in (Fig. S9†).

Conclusion

In summary, we have fabricated a novel hybrid bi-functional 3D APAN–Ag/PEGylated MWCNT–COOH nanofibrous membrane. Prior to its final form, an intermediate surface modified APAN nanofiber mats was used for the filtration and adsorption applications. Heavy metal such as arsenic was permeated apparently and adsorption studies revealed that the APAN nanofibrous mat is befitted for both the filtration and adsorption of all the toxic heavy metals present in the water. The kinetics and isotherm models backed the adsorption behaviour of the APAN nanofibers mats. In order to remove the bacterial content in the water, the inclusion of the Ag NPs incorporated PEGylated MWCNT–COOH as a bactericidal agent, in the nanofibrous membrane was carried out. Hence, the 3D nanofibrous membrane in its final form is an emphatic filter as well as an adversary to both Gram positive and Gram negative bacteria which were corroborated from the filtration and antibacterial studies of both S. aureus and GFP E. coli. The hybrid nanofibrous membrane is exceptional in both the filtration and antibacterial applications as evident from several quantitative and qualitative studies discussed earlier. Comprehensively, from this work, a versatile nanofibrous membrane was developed which will benefit a large number of rural population from the developing countries where still clean drinking water is a far reaching basic need.

Acknowledgements

We give sincere thanks to the Department of Science and Technology (Water Technology Initiative-Project No. DST/TM/WTI/2K13/94 (G)), Government of India, for the financial support. R. K. S. is thankful to the Ministry of Human Resource Development, Government of India, for the fellowship. Department of Chemistry and Institute Instrumentation Centre, Indian Institute of Technology Roorkee are sincerely acknowledged for providing various analytical facilities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22735j

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