Electrospun Polyacrylonitrile (PAN) Templated 2D Nanofibrous Mats: A Platform toward Practical Applications for Dye Removal and Bacterial Disinfection.

The fabrication of polymeric nanofibers and its potential versatility instigated to foster smart hybrid nanomaterials for the removal of environmental pollutants. In this pursuit, in this research work, polyacrylonitrile (PAN)-based two-dimensional (2D) nanofibrous mats with polyethyleneimine (PEI)/Fe and quaternary ammonium (QA)/Fe as hybrid fillers were prepared by the electrospinning process for the effective dye removal and bacterial disinfection. The characteristics of the fabricated nanomaterials were extensively explored by several analytical techniques such as field emission-scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and Brunauer-Emmett-Teller analysis. Magnetic and thermal properties were investigated by superconducting quantum interference device and thermogravimetric measurements. The kinetic and isothermal models affirmed the adsorption behavior of the PAN-PEI/Fe nanofibers, and further regenerative studies substantiated the sustainability of the mats for the removal of industrial dye effluents. Subsequently, the magnetic-QA-loaded PAN nanofiber mats exhibited bactericidal killing efficacy of 99 and 89.5% in both Staphylococcus aureus and green fluorescence protein expressing Escherichia coli bacterial models evaluated from the conventional quantitative bacterial colony-counting assay. Disk diffusion method and microscopic investigations corroborated the disinfection efficacy with zone of inhibitions of ∼23 and 33 mm, respectively. Interestingly, in vitro cell culture studies conducted in BHK-21 and NIH 3T3 cell lines demonstrated the cytocompatibility, and the in vivo toxicity investigations using the zebrafish models necessitated the real-time application of these nanofibrous mats. Therefore, the comprehensive study of the fabricated PAN-templated functionalized 2D nanofibrous mats affirmed to be competent for the remediation of industrial dye effluents and bacteria in water bodies.

Introduction

Clean water is the major topic of the current research because it is an important source of life for humans and environment. Microbial contamination is the major cause of waterborne diseases such as diarrhea, which causes up to 1.8 million deaths annually.[1] Similarly, due to industrialization, large-scale discharge of effluents containing toxic dyes and heavy metal ions from manufacturing industries such as cosmetic, leather, paper, textile, pharmaceuticals, and so on to nearby water bodies is also highly detrimental to human health and environment.[2] Hence, researchers are in inevitable need to put forth a sturdy solution to get rid of these noxious contaminants from the environment.

The customary treatment processes for obtaining safe water from the industrial effluents and wastewater treatment plants involve mechanical aeration and disinfection of microbes by chlorination, among others. Furthermore, other conventional treatment technologies, such as ozonization, filtration, electrochemical processes, and other biological methods using microorganisms are facing extensive drawbacks due to their ineffectiveness, thus leading to the formation of intermediate byproducts whose treatment becomes complex and requires posttreatment separation techniques and machineries.[3−5] Again, the light source mediated treatment process such as ultraviolet irradiation is completely dependent on higher energy consumption, making the treatment expensive.[6] To address these problems, the development of robust and smart functionalized nanomaterials in a uniform two-dimensional (2D) platform with practical feasibility which could act as an effective adsorbent for removal of toxic dyes, having innate bactericidal properties for disinfection and biocompatibility towards human and other living organisms present in the water bodies to maintain the sustained ecosystem is necessitated.

Ample number of studies have been reported on metals and metal oxide as cost-effective nanoadsorbents for the adsorption of dyes,[7−11] polymer-based hybrid adsorbents, and polymeric nanocomposites for the removal of heavy metals and dyes.[12,13] In polymer-templated metal oxide nanostructures, unforeseen conditions that lead to the release of metal oxide nanoparticles (NPs) to the environment may arise due to the hydrophilic polymer templates. Taking into account the aforementioned drawback and the eventual real-time applicability, several electrospun metal oxide doped nanofibrous adsorbents for decolorization of dyes were developed;[14−19] recently, a dual functional hybrid polyacrylonitrile (PAN) nanofiber (Nf)-templated nanofibrous membrane having high tensile properties was reported.[20] Following the above work, the current research explores the dye removal and bacterial disinfection applications using the aforementioned PAN-templated nanofibers functionalized with composite nanomaterials.

In recent times, polyethyleneimine (PEI) has fascinated the researchers due to the high density of amine group, which has a greater affinity toward the adsorption of heavy metals and other cationic dye pollutants, present at the end of the chain. Several studies have been reported on the polymer-coated iron oxide nanoparticles (NPs) showing promising results in removing the environmental contaminants.[21−23] Moreover, hydrophilic PEI stabilized by the silica nanocomposites for the adsorption of both cationic and anionic dyes has been reported.[24] Therefore, the preliminary work elucidated the hydrothermal synthesis of PEI–Fe functionalized PAN-templated nanofibers and explicated the adsorption properties toward cationic congo red (CR) dye. Furthermore, the sustainability studies corroborated the robust nature of the proposed nanofibrous adsorbent suitable for the real-time applications. Quaternary ammonium (QA) complexes are surface active agents that were first synthesized and identified to have antibacterial property several years ago.[25] These are cationic detergents that form micelles by reducing the surface tension and are hydrophilic in nature, which allows their easy dispersion in liquid. QAs occur in different structures, with the central region consisting of a cationic nitrogen attached to four groups of different structures and an anionic bromine linked to the nitrogen to form the QA salt.[26] Currently, these compounds are widely used as commercial products in healthcare and food industries for sanitizing, disinfecting, and cleaning agents.[27] The QAs are classified based on the nature of the alkyl group with number of nitrogen atoms, the number of carbon chains attached, and the occurrence of aromatic groups. These moieties determine the antibacterial activity of the QAs against different microorganisms. The length of the alkyl group of these compounds also affects their antimicrobial activity,[28] as the methyl groups having carbon chains of length 12–16 usually show the highest antibacterial activity compared with compounds with shorter chains.[29] Furthermore, the mechanism of action of the quaternary ammonium compounds toward bacteria was reported in earlier studies,[30,31] and the dose-dependent toxic effect of QAs toward several microorganisms such as bacteria, viruses, spores, and so on was also reported. The broad spectrum of the antibacterial action of different formulations of QAs towards different bacterial systems[25] and also antibiotic resistant strains is already studied.[32] The latter section of this research work focuses on the utilization of these robust antibacterials as disinfecting agents into the PAN nanofibers. In brief, QAs were formulated in solution form by loading stipulated concentrations of QA into the PAN polymer solution, which will act as the template for the nanofibers and these compounds are further coated over the iron oxide nanoparticles before loading in the solution to make the separation process easier after disinfection. Because the iron oxide nanoparticles had hardly any antibacterial property, the QAs, which were loaded into the nanofibers, are the only proprietary for the disinfection activity. This explains that the main significance of the iron oxide nanoparticles used in the fabrication was to accomplish the magnetic levitation of the nanofibrous mats using strong magnets after disinfection procedures. Further, the cell viability of both PAN–PEI/Fe and magnetic-QA-loaded PAN nanofibrous mats was investigated to understand their biocompatibility toward animal cells to rationalize their commercial applicability. Also, in vivo animal toxicity studies of the nanofibrous mats were also performed using zebrafish as the model organisms to ascertain their fate of toxicity.

Herein, the nanofiber-based nanomaterials were fabricated in 2D form by using the same platform of PAN nanofibers for the industrial dye removal and bacterial disinfection simultaneously, which makes the work unique from all of the other earlier studies. The desired advantages of the proposed functionalized 2D nanofibrous materials met all of the criteria such as reusability, biocompatibility, low in vivo toxicity, effective bactericidal property, and high stability in water that are greatly required for any commercial water purification systems.

Results and Discussion

Preparation and Characterization

Procedural steps in the fabrication of nanofibers are illustrated in the Supporting Information (Figure S1A,B). Iron acetylacetonate is the precursor salt used in the formation of iron oxide immobilized PAN–PEI nanofibers (Figure S1A), whereas the PAN nanofibers in the latter are magnetized by loading presynthesized magnetic-QA complex into the nanofibers to accomplish easier separation after disinfection.

Morphological Investigations

The electrospun two-dimensional nanofibrous structures actually open up the real perspectives for commercialization. Nanofibrous network with intrinsic properties of the filler materials plays a significant role in the application-driven strategies. Moreover, there was no change in the structure or the adsorption and disinfection properties of both as-prepared and heat-treated nanofibers. Due to the aforementioned properties and the robust nature of nanostructures, 2D nanofibers mats were employed in the present work. The field emission-scanning electron microscopy (FE-SEM) morphology of the hydrothermally carbonized PAN–PEI/Fe nanofibers is shown in Figure a–c, with an average diameter of 771 ± 101 nm (in Figure b). The elemental analysis constituting carbon (∼65%), oxygen (∼31%), and iron (∼3%) is represented in the Supporting Information (Figure S2a,b). Meanwhile, the FE-SEM image of the interim material of QA-coated Fe3O4 NPs is shown in Figure S3a, and the energy-dispersive X-ray (EDX) analysis confirmed the presence of elemental carbon (∼5%), oxygen (∼37%), nitrogen (∼0.87%), and iron (∼56%) (Figure S3b,c). The final QA-loaded PAN and magnetic-QA-loaded PAN nanofibers (Nfs) are illustrated in Figure a,b, and the difference in their size distribution is clearly depicted in Figure c,d. The elemental mapping of magnetic-QA-loaded PAN nanofibers represented in Figure a,b substantiated the occurrence of iron implied by the marked nodes over the surface of nanofibers (represented in dotted circles). Figure c demonstrates the frequency map of magnetic-QA-loaded PAN nanofibers operated by the EDX line scan represented in different colors. Transmission electron microscopy (TEM) analysis determines the interface between the coated QA complex and Fe3O4 NPs shown in Figure d, and the selected area electron diffraction pattern (inset of Figure e) pinpointed the crystalline nature of iron oxide nanoparticles having cubic structure. Further, TEM differentiated the QA-coated outer shell of uniform thickness of 16 ± 0.5 nm surrounding the core Fe3O4 nanoparticle represented in Figure f, and the elemental analysis affirmed the presence of carbon, oxygen, and iron implied by the predominant peaks (inset of Figure f).

Figure 1

(a–c) FE-SEM analysis of PAN–PEI/Fe nanofibers.

Figure 2

(a, b) FE-SEM and (c, d) size distribution of QA- and magnetic-QA-loaded PAN nanofibers.

Figure 3

(a) Elemental mapping and (b) FE-SEM analysis of the composite nanofibers. (c) EDX line frequency maps of C, O, and Fe elements of magnetic-QA-loaded PAN nanofiber. (d–f) TEM micrographs of magnetic-QA complex.

X-ray Diffraction (XRD)

The structural property and crystalline nature of the nanomaterials were characterized by powder X-ray diffraction (XRD) measurements and analyzed using PANalytical X’Pert High Score Plus. Figure a represents the XRD pattern of PAN/PEI–Fe nanofibers, with the characteristic diffraction peaks indexed to 2θ at 15, 18.39, 30.27, 35.59, 57.16, and 62.72° corresponding to (110), (111), (220), (311), (511), and (440) planes, respectively, in accordance with the JCPDS file (PDF-004-0755) with face-centered cubic lattice ascribed to the maghemite phase (γ-Fe2O3). Similarly, the characteristic peaks of bare Fe3O4 NPs, magnetic-QA complex, and magnetic-QA-loaded PAN nanofibers found at 2θ are 18.27, 30.06, 35.45, 37.12, 53.54, 57.16, 62.72, 70.78, and 73.99°, respectively, with JCPDS file (PDF-001-1111) shown in Figure b. The crystalline size of the nanoparticles determined by Debye–Scherrer equation (D(1/4)Kl/b cos q, with K(1/4)0.9) is 180 ± 50 nm, which is correlated with the FE-SEM and TEM investigations. However, the intensity of the diffraction peaks of magnetic-QA-loaded PAN nanofibers (Figure b) decreases with varying concentrations of Fe3O4 NPs and the polymeric nanofibrous network reported in earlier works.[33,34]

Figure 4

(a, b) XRD and (c, d) Fourier transform infrared (FTIR) analyses of PAN–PEI/Fe and magnetic-QA-loaded PAN nanofibrous mats.

FTIR

The FTIR profiles of PAN−PEI/Fe nanofibers in Figure c show the characteristic peaks at 2357 and 1573 cm–1 associated with the stretching vibrations of the nitrile groups (CN−), which got reduced after hydrothermal treatment, stretching and bending vibrations of the methylene (−CH2−) groups, respectively. Additional peaks at 3448 and 1643 cm–1 are attributed to the stretching vibrations of (N–H) and carbonyl groups, and the peak at 1378 cm–1 indicates the C=O symmetric stretching bond frequency of the carboxylate salt (COO−). Furthermore, the low-intensity absorption peak at 657 cm–1 that appeared after the hydrothermal treatment of the PAN–PEI/Fe nanofibers was attributed to the (Fe–O) stretching vibration of the iron precursor; upon heat treatment, the peak got diminished due to the higher iron loading onto PAN–PEI/Fe nanofibers. Subsequently, the FTIR spectra of bare Fe3O4 NPs, magnetic-QA complex and magnetic-QA-loaded PAN nanofibers are shown in Figure d. Wavenumbers from 2930 to 2855 cm–1 indicate the asymmetric and symmetric stretching vibrations of methylene (CH2−) groups present in the PEI. The broad and intense band at 1636 cm–1 assigned to the carbonyl (C=O) group is attributed to the cyclic nature of the quaternary ammonia compounds, and the wide range of absorption band peaks between 1140 and 1040 cm–1 is due to the stretching vibrations of the nitrile (−CN) group.[35] The characteristic peaks of the remaining spectra were diminished due to the polymer matrix of the electrospun nanofibers.

Thermogravimetric Analysis

The thermogravimetric analysis of PAN–PEI/Fe nanofibers was performed to study the thermal degradation behavior, and the representative spectra are shown in Figure S4a. The nanofibers follow the uniform degradation profile before and after hydrothermal treatment in which the former undergoes a mild two-step degradation between 250 and 350 °C due to the presence of cross-linked polyethyleneimine (PEI) and the latter undergoes a single-step degradation. The quantity of iron oxide nanoparticles grown in situ onto the nanofibers is very small, where only 7 and 2% of the residuals are left behind after heating up to 800 °C. Similarly, thermal behavior of the magnetic-QA-loaded PAN nanofibers was also analyzed as shown in the Supporting Information (Figure S4b). The initial weight loss at 200 °C is due to the physiochemical absorption of moisture and the QA compounds, followed by 70% residue left in the case of magnetic-QA complex at the end of 800 °C due to the iron oxide nanoparticles, and finally the magnetic-QA-loaded PAN nanofibers undergo complete degradation with increase in the weight loss, but the thermal stability of the composite nanofibers is greatly enhanced at higher temperatures due to the transition iron oxide nanoparticles loaded onto the polymeric nanofibrous network.[36,37]

Surface Area Measurement

To investigate the surface area and porosity of the magnetic-QA complex, N2 adsorption–desorption isotherm was performed (Figure a). From the figure, the isotherm can be classified as type III hysteresis loop, which is the characteristic of the nonporous structures of the iron oxide nanoparticles between the range 0.1 and 0.9 of relative pressure that possess the multilayer adsorption property having the surface area 29.09 m2/g using multipoint Brunauer–Emmett–Teller (BET) measurements which is higher than the as-prepared cubic structured nanoparticles reported.[38] The pore radius and pore volume of the magnetic-QA complex are tabulated in the Supporting Information (Table S2). Further, the separation property of the magnetic-QA-loaded PAN nanofibers are discussed in the following experiments.

Figure 5

(a) N2 adsorption–desorption isotherms and (b) superconducting quantum interference device (SQUID) analysis.

Magnetic Properties

The magnetization (M) characteristics of the field- and temperature-dependent properties of the bare Fe3O4 NPs, magnetic-QA complex, and magnetic-QA-loaded PAN nanofibers were determined by superconducting quantum interference device (SQUID) applying the magnetic field (H) ranging from −50 000 to +50 000 Oe at room temperature (26 °C). Figure b illustrates the hysteresis loop of the aforementioned nanomaterials having the saturation magnetizations (Ms) of bare Fe3O4 NPs (83.37 emu/g) and magnetic-QA complex (78.79 emu/g). However, the magnetization of magnetic-QA-loaded PAN nanofiber was decreased to a larger extent (5.65 emu/g) due to the deep-seated encapsulation of the QA complex over the Fe3O4 NPs within the acrylic polymer matrix. The presence of steep slopes of magnetization (Figure S4c,d) when the applied external magnetic field is close to zero is called remnant magnetization (Mr), which exquisitely implies the supraparamagnetic behavior, with strong magnetic signals having Mr values of 10.5, 9.5, and 0.528 emu/g that can be desirable for the practical applications. Hence, the magnetic-QA-loaded PAN nanofibers can be stimulated by the external magnetic field with such low magnetization values that will be required for the constructive removal of pollutants such as heavy metal, bacteria, azo dyes, and so on.

Adsorption Experiments

Adsorption experiments were performed to investigate the effect of contact time and adsorption efficiency of the nanofibers using congo red (CR) as a model dye (Figure a,b). The residual dye concentration was determined from the calibration curve between the different concentrations and their corresponding absorbance as reported.[39] Digital photographs of dye solutions before and after adsorption is shown in the inset of Figure b. The studies on the effect of adsorption time, desorption, and reusability were carried out using the PAN–PEI/Fe nanofibers. Simulated dye solutions of different concentrations ranging from 20 to 60 mg/L, pH ∼6.5, and 3 h adsorption time were used in the regenerative studies. The adsorption isotherm was carried out using PAN–PEI/Fe nanofibrous adsorbent to determine the maximum adsorption capacity, and the results were compared with three different isotherm models namely the Langmuir isotherm, Freundlich isotherm, and Dubinin–Radushkevich isotherm models, respectively. Isothermal plots of Langmuir and Freundlich models are shown in Figure c,d, and the outcome of other model parameter results are tabulated in the Supporting Information (Table S3). From the model results, it was ascertained that the correlation coefficient (R2 = 0.9877) of Langmuir plot yielded better fit compared with the Freundlich and Dubinin–Radushkevich models having the correlation coefficient values of 0.971 and 0.967, respectively. Additionally, the characteristic feature of Langmuir isotherm can be expressed in terms of a dimensionless constant (RL). The value of RL lies between 0 and 1, which is the favorable condition for the monolayer adsorption process. Meanwhile, the model parameter results of Langmuir isotherm having the maximum adsorption capacity (qm) value of 77.5 mg/g, KL = 0.190, and RL = 0.34–0.37 were correlated with those in the literature. These investigations imply that the adsorption property of the PAN–PEI/Fe nanofiber mats toward the CR dye is more effective compared to that of the other nanofibrous adsorbents reported earlier.[40]

Figure 6

UV–vis analysis of (a) time-dependent adsorption study. (b) Adsorption capacity and percent removal with time (adsorption of the dye marked by change of color from dark red to clear solution, shown in the inset of Figure b). Adsorption studies: (c) Langmuir plot, (d) Freundlich plot, (e) first-order kinetics, and (f) second-order kinetics.

Figure 7

Sustainability studies. (a) Adsorption–desorption and (b) reusability studies.

Adsorption Kinetics

Adsorption mechanism of the adsorbent was studied by the kinetic models of pseudo first- and -second-order plots (Figure e,f), and the results are illustrated in Table S4. The outcome of these studies suggested that the PAN–PEI/Fe nanofibrous adsorbent follows rapid adsorption kinetics and the adsorption property may be attributed to the electrostatic interaction between the positively charged amine group of PEI and iron oxide nanoparticles on PAN nanofibers with the cationic dye used. In comparison to the other adsorbents, the PAN–PEI/Fe nanofibers possess remarkable kinetic behavior that is appropriate for industrial applications.

Regenerative Studies

Desorption and Reusability Assay

The adsorptive performance of the adsorbent was tested in a heuristic approach by the successive adsorption–desorption cycles for 10 times continuously. In brief, 25 mg of each adsorbent was taken in tubes containing different concentrations of the dye (i.e., 20, 40, and 60 mg/L); after adsorption, the adsorbents were separately treated with alkali for 2 h of desorption and the regenerated nanofibrous adsorbents were then ready for successive adsorption studies. Similarly, the reusability tests were done without performing any postadsorption treatment procedures, and 10 repeated cycles were carried out for both the assays. Bar graphs shown in Figure a,b pin point the obvious change in the untreated and treated cycles, supporting the desorption and reusable capability leveraged by the nanofibrous adsorbent.

Disinfection Control Experiments

Colony-Counting Method

Preceding the disinfection studies, the concentration of QA complex was optimized (Table S1) by using the QA-alone and magnetic-QA-loaded PAN nanofibers (samples 1 and 2 with QA concentration of 5 mg/L), which were predominantly utilized for all of the antibacterial assays. The rationale behind this selection is that loading higher concentration of QA leads to the formation of self-assembled three-dimensional nanofibrous sponges (see Figure S1B), which affects the 2D nanofibrous membrane morphology and stability in water. The antibacterial efficacy of the magnetic-QA-loaded PAN nanofibers toward Staphylococcus aureus and green fluorescence protein (GFP) expressing Escherichia coli was evaluated by the traditional colony-counting method by the enumeration of bacterial colonies by two different methods, namely, estimated (using heuristic approximation) and experimental method (experimental procedures are elucidated in Figure S5). Bacterial cell population was determined by aliquoting the parent culture using serial dilution with a minimum of five dilutions having 108–104 colony-forming units per milliliter (CFUs/mL), and each dilution was treated with nanofibers of dimensions 1 cm × 1 cm (sample 2). Figure a,b shows the decreased trend of the bacterial population after treatment for 8 h. Interestingly, in the experimental method, significant change in the number of bacterial colonies between the untreated and treated cultures (dilutions of 108 and 107 CFU/mL) followed by an intense decrease in the colonies of the treated cultures due to dilutions (106–104 CFU/mL). These results clearly show that the magnetic-QA-loaded PAN nanofibers exhibited more than 90% killing efficacy with increasing efficacy toward GFP E. coli because of their antibiotic resistance (Figure S6). The digital photographs of bacterial plates having colonies of untreated and treated cultures with different dilutions of both bacterial models are illustrated in the Supporting Information (Figure S7A,B).

Figure 8

Bacterial enumeration by colony-counting method in (a) S. aureus and (b) GFP E. coli. Antibacterial investigation nanofiber mats. (c) Disk diffusion assay. (d) Histogram showing zone of inhibition against S. aureus and GFP E. coli. Sample details: 1, PAN nanofibers alone; 2, magnetic-QA-loaded PAN nanofibers; 3, QA-PAN nanofibers (mean diameter of the nanofiber disk is 13 mm) (estd, estimated; exp, experimental).

Optical Density (OD) Measurements

The bacterial inhibition property of the various formulations of nanofibers was assessed by UV–vis spectrophotometer measurements of optical density (OD) at 600 nm. Histogram in Figure S8 elucidate the decreased tendency in the treated nanofibers samples compared with the untreated bacterial sample, which reiterated that the killing efficacy of the nanofibers is not affected in both S. aureus and GFP E. coli irrespective of the concentration of QA-loaded onto the PAN nanofibers.

Disk Diffusion Assay

The nanofiber samples used for this assay are alone PAN nanofibers, QA-loaded PAN, and magnetic-QA-loaded PAN nanofibers against S. aureus and GFP E. coli shown in Figure c. The results show that the alone PAN does not show any bacterial inhibition; however, effective inhibition zones of average diameter of ∼17.5 and ∼24.5 mm (S. aureus) and 22.4 and 32.5 mm (GFP E. coli) were observed in the nanofiber-treated samples represented in Figure d. These investigations implied that the iron oxide nanoparticles were employed only for the purpose of magnetic separation, which gives the clear evidence that the QA complexes are crucial bacterial scavengers. Contemplating the release of QA complex to the environment, these bacterial scavengers are embedded in the polymeric nanofiber matrix in which the release is regulated unconditionally using the optimized concentration of QA-loaded nanofibers (as discussed earlier) as per the desired membrane morphology and physical nanofiber mat formation. The release of the QA complex into the water (Figure S9) is identified by the formation of peak at 362 nm in UV–vis spectrophotometer, whereas no peak is observed in the QA-loaded PAN nanofibers even after treated for 24 and 48 h, respectively. These properties necessitated the researches for the selection of synthetic polymers such as PAN to be used as templates for antibacterial membrane fabrication, and its sustainable bactericidal property is also assessed by performing repeated cyclic assays; no colonies were observed in all of the cycles (data not shown). Further, the nanofibers were separated by external magnet after disinfection and regenerated by repeated washing and used for further study. These investigations implied the reliability of the proposed nanofibrous materials possessing antibacterial properties.

Disinfection Mechanism

The mode of bactericidal action of the quaternary ammonium (QA) complex was identified from earlier reports such as electrostatic interaction between the cationic QA salts and the anionic bacterial cell membrane, followed by permeation, leakage of intracellular components, and lysis of the cell.[41] Eventually, the fluorescence microscopic investigations substantiated the bactericidal property of the alone QA-loaded and magnetic-QA-loaded PAN nanofibers (Figure a–f). Furthermore, the nanofiber-treated bacterial cultures grown on the agar plates found no bacterial colonies, supporting the fluorescence microscopic studies.

Figure 9

Microscopic investigations of control and nanofiber-treated S. aureus and GFP E. coli. Sample details: (a, d) control, (b, e) QA–PAN nanofibers alone, and (c, f) magnetic-QA-loaded PAN nanofibers (dimension of the nanofibers: 1 cm × 1 cm). Scale bar: 20 μm.

Biocompatibility Assay

Appropriate percentage of cell viability (85%) was achieved with PAN–PEI/Fe and different extents of QA-loaded PAN nanofibers (5–20 mg/L) at the end of 48 h (Figure ). The sample details of different nanofiber formulations are elucidated in the Supporting Information (as mentioned earlier). Nonetheless, decreased viability was observed in sample 4 due to the maximum concentration of QA loaded onto the nanofibers (20 mg/L), which exhibited mild toxicity toward both the cell lines baby hamster kidney (BHK-21) and mouse embryonic fibroblast (NIH 3T3). Representative fluorescence microscopic images in Figure demonstrate the biocompatibility of the different formulations of nanofiber after 48 h against the animal cell lines by using Hoechst 33342 (blue) and rhodamine B (Rho B, red) dyes, both of which stain the respective intracellular components of the cell such as nuclei and cytosol. No significant decrease in cell number and fluorescence intensity and changes in morphology were hardly observed. These investigations profoundly advocated the compatibility of the nanofibers toward the cell lines, leading to the practical disinfection applications.

Figure 10

Cytocompatibility evaluation of different formulations of nanofiber mats on baby hamster kidney (BHK-21) and mouse embryonic fibroblast (NIH 3T3) animal cell lines by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for 24 h.

Figure 11

Fluorescent microscopic images of nanofiber-treated BHK-21 and NIH 3T3 cell lines stained with Hoechst 33342 (blue) and costained with rhodamine B (red). Scale bar: 100 μm.

In Vivo Toxicity Studies

Practical applicability of the nanofibrous material was studied by assessing the fate of these nanomaterials in the environment, and it is highly necessary to address the implications and the potential outcome of the studies.[42] Zebrafish models acquired several advantages such as transpicuous and rapid embryogenic development and being genetic analogous to humans, which make them better animal models and suitable candidate for investigating the adverse effects posed by the proposed nanofibrous material. The zebrafish models were treated with nanofibers samples at different stages for every 24 h up to 144 h postfertilization (hpf) and observed more than 85% of survival rate in the samples 1, 2, 3, and 5 except in the sample 4, where the maximum concentration of QA was loaded into nanofibers and the extent of the survival rate was compared with the untreated zebrafish embryos, which act as the control sample (Figure ). The microscopic images of the zebrafish embryos were recorded every 24 h to substantiate the phenotypic transformations after nanofiber treatment (Figure ), and the digital photographs of the zebrafish are shown in the Supporting Information (Figure S10b,c). Interestingly, no malformations were found in the zebrafish subjected to the nanofiber treatment, exempting sample 4 in which the dead embryos are found with deformations such as bent spine and pericardial edema after 48 h owing to the maximum loading of QA and iron oxide nanoparticles (images of dead embryos are shown in Figure S10d–f). However, the morphology of the other treated samples are quite identical to those of the control (row 1 in Figure ), where the embryos are grown irrespective of the addition of double number of nanofiber disks after 72 h in water. Comprehensively, the toxicity and deformations of the zebrafish models were hardly visible after treatment with the alone QA-loaded and magnetic-QA-loaded PAN nanofibers, which profoundly suggested that the proposed nanomaterials are highly biocompatible and excellent disinfectant for the removal of bacteria and other microorganisms compared with other metal-based nanoparticles (AgNPs).[43]

Figure 12

Survival percent of zebrafish after treatment with different formulations of nanofibers. Sample details: control, S1, QA-PAN nanofibers (5 mg/mL); S2–S4, magnetic-QA-loaded PAN nanofibers ((5 + 5), (5 + 10), (5 + 20) mg/mL); S5, PAN–PEI/Fe nanofibers.

Figure 13

Representative microscopic images of different formulations of nanofibers treated with zebrafish models. Sample details: R1, control; R2, QA-loaded PAN nanofibers (5 mg/mL); R3 and R4, magnetic-QA-loaded PAN nanofibers ((5 + 5), (5 + 10) mg/mL); R5, PAN–PEI/Fe nanofibers.

Conclusions

In this work, PAN-templated 2D nanofibrous mats were developed by electrospinning method for the simultaneous removal of toxic dyes and bacterial disinfection. Rather than the theoretical considerations traditionally followed in earlier works, this work leaps forward by putting forth a pragmatic approach by designing a robust platform in the form of two-dimensional nanofibrous mats that can be highly preferred for commercial applications. Salient features of the PAN–PEI/Fe nanofibers include an effective adsorption efficiency and regenerative and reusability properties. However, the magnetic-QA-loaded PAN nanofibers quantitatively exhibited higher disinfection action toward both S. aureus and GFP E. coli, with more than 90% killing efficacy in both the bacterial systems (evaluated from the colony-counting method), supraparamagnetic property for separation, high biocompatibility through in vitro animal cell culture studies, and the desired less in vivo toxicity toward zebrafish models. These results corroborated the feasibility of the proposed PAN 2D nanofibrous mats for the practical consideration for potential remediation of industrial dye wastes and bacterial disinfection.

Experimental Section

Materials and Methods

Polyacrylonitrile (PAN) (Mw: 150 000), polyethyleneimine (PEI) (Mw: 25 000), iron(III) acetylacetonate, iron(III) oxide, 25% glutaraldehyde, and ethylenediamine (EDA) were purchased from Sigma-Aldrich. N,N-Dimethyldodecylamine (97%), 1,5-dibromopentane (97%), and Hoechst 33342 were also procured from Sigma-Aldrich. Rhodamine B (Rho B) was acquired from Life Technologies and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was obtained from Amresco Life Science, and used as per manufacture’s procedures. Dimethylformamide (DMF), congo red (Mw = 696.68), ethyl alcohol, and sodium hydroxide (NaOH) pellets were acquired from Sisco Research Laboratories (SRL), India. Gram-positive bacterial strain S. aureus (microbial type culture collection (MTCC) 737) was obtained from IMTECH, India, and recombinant green fluorescence protein expressing E. coli (GFP E. coli) was used as the gram-negative strain. Bacterial growth medium such as Luria–Bertani and nutrient broth were purchased from Merck (Germany) and Himedia (India), respectively. Ampicillin antibiotic was procured from Sisco Research Laboratories (SRL), India. Analytical grade chemicals and double-distilled water, DDW (18.3 mΩ).

Electrospun PAN–PEI/Fe Nanofibers

Standard electrospinning apparatus procured from ESPIN Nano (Physics Equipment and Company, India) was used for the fabrication of nanofibers. Iron(III) acetylacetonate (1%) and PEI (0.8%) were dissolved in 7% PAN–DMF solution under constant magnetic stirring at 60 °C under 400 rpm for 12 h. After curing, the polymer blend was fed into 2 mL syringe and fixed into the electrospinning apparatus with an independent programmable peristaltic microsyringe pump. The apparatus was operated at 14 kV with the flow rate maintained between 0.40 and 0.45 mL/h, and the deposition of nanofibers was prolonged for a stipulated time under room temperature with a constant relative humidity of 55% and collected over grounded stationary metal collector positioned at a distance of 14 cm from the spinneret.

Fabrication of Iron Oxide Immobilized Nanoparticles on the PAN–PEI Nanofibers

The PAN–PEI/Fe composite nanofibers were pretreated with glutaraldehyde (50% v/v) vapors for 2 h and dried in hot air oven at 45 °C to remove the remnant unreacted glutaraldehyde from the nanofibers. The cross-linked nanofibers were identified by the colorimetric change in the mat from orange to brown, and then it was UV sterilized before being carried over for further applications. The nanofiber mat was initially immersed in 70 mL of deionized water and the pH was adjusted to 10–11 using ethylenediamine (EDA) and transferred to the Teflon-lined autoclave for hydrothermal treatment at 150 °C for 12 h followed by repeated washing with deionized water to remove the unreacted amine group and dried for further use.

Development of Magnetic-QA Complex

Synthesis of QA Complex

Quaternary ammonium complex was synthesized as reported elsewhere,[44] and the scheme for synthesis is elucidated in Figure S11. In brief, 37.8 mL of N,N-dimethyldodecylamine was added dropwise to 21.8 mL of 1,5-dibromopentane (0.14 mol each) at 60 °C under N2 atmosphere with constant magnetic stirring under reflux for 12 h and the unreacted amine reagents were removed. The obtained yellowish sticky compound was dissolved in 50 mL of ethanol and added to 0.1 g of PEI in ethanol at 70 °C under N2 atmosphere and refluxed for 48 h continuously. The unreacted compounds were removed by rotary evaporation to obtain QA compounds and stored in −20 °C before being used.

Preparation of Magnetic-QA Complex

Magnetic-QA complex were prepared by a simple coating process. In brief, 0.5 g of QA compounds was dissolved in 20 mL ethanol at 55 °C followed by the addition of 1 g of Fe3O4 NPs under N2 atmosphere, and the reaction was kept for 12 h to form magnetic-QA complex. Then, it was magnetically separated and washed with deionized water and dried.

Fabrication of Electrospun Magnetic-QA-Loaded PAN Nanofibers

The magnetic-QA-loaded PAN polymer blend was prepared by solvent homogenization process. In brief, 20 mg of presynthesized magnetic-QA complex was dissolved in DMF and sonicated for 10 min with a 3 s “on” and 2 s “off” condition and added to the preprepared 7% PAN solution. The magnetic-QA-loaded polymer solution was electrospun to form the continuous nanofibers onto the collector using the aforementioned parameters.

Characterization of PAN/PEI–Fe Composite Nanofibers and Magnetic-QA-Loaded PAN Nanofibers

The morphologies of PAN–PEI/Fe nanofibers, Fe3O4 NPs, magnetic-QA complex, and magnetic-QA-loaded PAN nanofibers were analyzed by Ultra Plus-Carl Zeiss (Germany) field emission-scanning electron microscope (FE-SEM) operated at 15 kV and FE-SEM (FEI Quanta 200 F) equipped with energy-dispersive X-ray (EDX) detector 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 nanofiber images were processed by analysis software ImageJ to determine the mean diameter and size distribution of the nanofibers. The morphology of the magnetic-QA complex was studied by TEM (FEI Tecnai G2) at an operating voltage 200 kV. The functional changes in the nanomaterials were investigated by Fourier transform infrared (FTIR) spectroscopy, where the measurements were acquired by Thermo Nicolet spectrometer using KBr pellets in the range between 4000 and 400 cm–1. Thermogravimetric analysis of the nanofibers was carried out to study the thermal degradation profile and their thermal stability by heating from 0 to 800 °C at a constant rate of 10 °C/min under inert atmosphere using the EXSTAR TG/DTA 6300 (Hitachi, Japan). The purity and crystalline phase structure of the nanofiber mats were studied using advance powder X-ray diffractometer (Bruker AXSD8) (Cu Kα radiation, λ = 1.5406 Å), with θ value between 5 and 80° at a scan speed of 0.2°/min. The magnetic behavior of the nanomaterials was analyzed using a supraparamagnetic quantum interference diffractometer (SQUID) with varying magnetic field from −50 000 to +50 000 Oe. Brunauer–Emmett–Teller (BET) measurements studied the surface area and pore volume of magnetic-QA complex through N2 adsorption–desorption isotherms recorded using a Quantachrome NOVA 2200e high-speed automated surface area analyzer.

Batch Adsorption Studies

Stock solution (1000 mg/L) of congo red dye was prepared and the desired concentrations were prepared by serial dilution for the experiments. Batch adsorption studies were conducted with contact time (0–180 min) at an initial dye concentration (100 mg/L), and the nanofibrous adsorbent of 20 mg was used in 20 mL of dye solution. Kinetics and isotherm studies were carried out to investigate the adsorption mechanism of the PAN–PEI/Fe nanofibers weighing 20 mg in an aqueous dye solution. Earlier, the experimental conditions such as dye concentration, equilibrium adsorption time, and temperature were predetermined and the residual concentration of the dye was analyzed by UV–vis spectrophotometer by measuring the absorbance at 500 nm. The quantity of the CR dye adsorbed per unit mass of the nanofiber mats was determined by the following equation Qe = (Ci – Cf)V/M, where Qe (mg/g) is the amount of dye adsorbed per gram of adsorbent at equilibrium, V is the volume of testing solution (L), and M is the weight of the adsorbent (g). Simulated dye solutions were prepared by diluting the stock using deionized water having aliquots of varying concentrations (20, 40, and 60 mg/L). The desorption study was carried out by immersing the dye-adsorbed nanofiber mats into 10 mL of 0.1 M NaOH solution for 3 h, and adsorption test was repeated for further cycles after thorough washing. Similarly, the reusability study was conducted without any postchemical treatment after subsequent adsorption and monitored for 10 repeated cycles.

Disinfection Studies

Bacterial Sample Preparation

Stock cultures of a recombinant green fluorescence protein expressing E. coli (GFP E. coli) and S. aureus (MTCC 737) acquired from IMTECH, India, are the bacterial models used for the study. The secondary cultures were reinoculated into fresh medium and grown until OD at 600 nm reaches 0.3 with cell population of approximately 2.4 × 108 CFU/mL; it was further serial diluted having different cell numbers ranging from 108 to 103 CFU/mL. The information related to the different formulations of nanofibers of having different concentrations of QA loaded is extensively illustrated in the Supporting Information (Table S1), and sample 3 was used for all of the disinfection experiments; as a precautionary measure, all of the glasswares used in the experiments were sterilized at 121 °C for 20 min before usage.

Microscopic Analysis

The control and treated bacterial cultures (number of cells ∼108–107 CFU/mL) are resuspended in deionized water followed by imaging through a fluorescence microscope to differentiate the bacterial population in the control and the treated samples operated at 100× magnification.

Bacterial Killing Test by Colony-Counting Method

To quantify the disinfection efficacy of the nanofibers, the bacterial cultures of different cell density (108–103 CFU/mL as mentioned earlier) in the vials were used as control, whereas 30 μL of culture from each vial was inoculated into the fresh media containing nanofibers (1 cm × 1 cm dimension) followed by incubation for 8 h at 37 °C and plated on agar plates for the growth of bacterial colonies. The conventional colony-counting method was adopted to determine the bacterial removal efficiency of the magnetic-QA-loaded PAN nanofibers by enumerating the number of colonies grown in the plates. The disinfection efficiency (%) of the nanofiber was evaluated by the equation as follows:where CFUi and CFUf are the initial and final number of bacterial colonies grown in the plates, respectively.

Disinfection Experiments by Disk Diffusion Method

Cultures of S. aureus and GFP E. coli grown overnight were streaked over the agar plates and, subsequently, three different types of nanofiber mats of diameter 13 mm each were placed and kept for incubation at 37 °C for 24 h. The antibacterial action was visualized by the inhibition of bacterial lawn in a circular path and the efficacy was evaluated by measuring the diameter of the disk. The experiments were performed in triplicates to overcome the artifacts present in the circular nanofiber disks, and the mean diameter was used for the evaluation.

MTT Assay

Cell lines BHK-21 and NIH 3T3 were procured from the cell repository of National Centre for Cell Science, India. Cells were maintained in Dulbecco’s modified Eagle’s medium (high-glucose) medium supplemented with 10% fetal bovine serum, 50 U/mL penicillin, and 50 mg/mL streptomycin in a humidified atmosphere in 5% CO2 at 37 °C. MTT assay was performed on animal cell lines using the procedures mentioned elsewhere[45] to substantiate the biocompatibility of the different types of nanofibers such as QA-loaded PAN nanofibers, combined use of QA and magnetic-QA-loaded nanofibers with varying concentrations of QA loaded onto PAN nanofibers (discussed earlier in the Supporting Information), and the PAN–PEI/Fe nanofibers. The morphology of the treated cells was captured by EVOS cell imaging system (Life Technologies) with red and blue filters, respectively.

Animal Toxicity Experiments

Danio rerio (zebrafish) embryos (ASWT strain) obtained from the Institute of Genomics and Integrated Biology, Delhi, was used for the study. The fresh embryos were collected onto the microinjection embryo tray just before the experiment, and the nanofiber samples of different formulations consisting of different concentrations of the QA loaded on to the PAN nanofibers (samples 1–4) and the PAN–PEI/Fe nanofiber were used. A 24-well plate was filled with 4 mL of water in each well as the medium for the growth of the zebrafish embryos. A total of 10 embryos in each well were used with duplicates, and the percentage survival of embryos was determined by counting the number of live fish after every 24 h. The microinjected embryos were transferred onto the Petri dish filled with system water and incubated at 28 °C in the dark. For the in vivo toxicity tests, live embryos were counted each day until 144 hpf. After 144 hpf, the developed embryos were live imaged by using Leica M205 FA attached with Leica DFC-7000 T camera without using tricaine methane sulfonate to visualize any phenotypic changes. All of the images were captured by Planapo 2.0× objective lens.