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Article Cite This: ACS Omega 2019, 4, 5741−5749
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Performance of Polyaniline Nanofibers (PANI NFs) as PANI NFs-Silver (Ag) Nanocomposites (NCs) for Energy Storage and Antibacterial Applications Rahul S. Diggikar,*,† Shamkumar P. Deshmukh,‡ Tukaram S. Thopate,† and Santosh R. Kshirsagar§
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†
Department of Chemistry and Research Centre, New Arts, Commerce and Science College, Parner, Ahmednagar, Maharashtra 414302, India ‡ Department of Chemistry, D.B.F. Dayanand College of Arts and Science, Solapur, Maharashtra 413002, India § Department of Chemistry, R.B.N.B. College, Shrirampur, Ahmednagar, Maharashtra 413709, India S Supporting Information *
ABSTRACT: The conductive property of polyaniline nanofibers (PANI NFs) has attracted great attention due to their higher capacitance, high flexibility, low cost, and ease of synthesis. Herewith, it is demonstrated that at room temperature, the template-free synthesized PANI NFs show the properties of PANI NFs−silver nanocomposites (Ag NCs) without a silver precursor with a new physical insight into energy storage devices and antibacterial applications. The diffraction peak at 2θ = 19.1° using X-ray diffraction analysis indicates the silver nature. UV−vis absorption peaks at 408 and 369 nm indicate the silver absorption (HCl-doped). The spectroscopic studies by FTIR, Raman, and NMR indicate that the PANI NFs behave in a similar manner to PANI NFs-Ag NCs with 48.47 S cm−1 conductivity and 570 F g−1 capacitance. Morphological studies by FESEM (size: 50 nm) and HRTEM reveal that the growth of PANI NFs is one-dimensional and more preferable. It also shows higher bacterial inhibition at lower values of minimum inhibitory concentration and minimum bacterial concentration.
1. INTRODUCTION Polyaniline (PANI) as a conducting polymer has been immensely researched due to its conducting nature and tunable chemical properties for electrochemical devices as energy storage devices (ESDs), antistatic coating films, sensors actuators, and antibacterial (AB) activity applications.1,2 A variety of approaches have been engaged to synthesize PANI NFs including template-assisted polymerization, interfacial polymerization, seeding polymerization, etc.3 The percentage of doping of acids during synthesis of PANI nanofibers (NFs) is the key factor responsible for the conducting polymers.2−8 The formation of PANI NFs is mainly dependent on the homogeneous nucleation process, and by taking advantage of this nucleation mechanism, significant research efforts have been directed in the past few years toward the improvement in various syntheses of PANI NFs.4−6 In recent reports, analysis of the controlled growth and morphology of PANI has been carried out by using surfactants to minimize the above issues. However, small enlargements in the NFs may act as nucleation points for further growth of nanorods (NRs) during polymerization.7,8 Nowadays, many researchers have focused their work on preparation of different metal−polymer nanocomposites (NCs) to improve the ESDs and AB applications.9−12 The synthesis of mixed morphologies (NFs, nanotubes, nongranular particles) of PANI using a template © 2019 American Chemical Society
with rapid polymerization at high temperature was reported by Du et al.,8 Tran et al.,5 Qiu et al.,13 Ma et al.,14 and Pan et al.15 However, Sapurina et al.16 suggested that nanostructures of PANI can act as NCs of PANI−metal by changing the synthesis method.17 Poole et al.18 reported that the PANI NFs can play the role of silver-PANI NFs NCs. Factors like temperature, post-polymerization, monomer and acid concentration, mechanical disturbances, seeding strategy, and percentage of dopant can affect the morphology of PANI.8 Our other group also presented PANI-SVO (silver vanadium oxide, silver vanadium sulfide) and lithium vanadium oxide− silver (LVO/Ag) NCs for ESDs and AB applications.19−22 We tuned the reported time-dependent methods of synthesis for HCl-doped PANI nanostructures for the first time; results predict that, as variation in polymerization period (24 h) is implemented at room temperature, the HCl-doped PANI NFs show similar activity to PANI NFs-Ag NCs without using a silver precursor, which is excellent for ESDs and AB activity applications.12,25 For comparison, a sample of PANI NFs doped with nitric acid (HNO3), which is one of the highly oxidizing agents, has also been synthesized. Ag nanoparticles Received: October 16, 2018 Accepted: January 14, 2019 Published: March 22, 2019 5741
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are routinely used as an AB agent, but in the present context, PANI NFs act the role of PANI NFs-Ag NCs, which is more economical and efficient than the methods used earlier for synthesis of PANI NFs-Ag NCs. This study opens up a new avenue for the scientific community.
2. RESULTS AND DISCUSSION 2.1. X-ray Diffraction (XRD) Analysis. Figure 1 reveals the XRD patterns of HCl- and HNO3-doped PANI NFs and
Figure 2. UV−vis spectra of (a) ST-PANI NSs, (b) HNO3-doped PANI NFs, and (c) HCl-doped PANI NFs.
Figure 1. XRD patterns of the HCl- and HNO3-doped PANI NFs and HCl-doped ST-PANI NSs.
HCl-doped PANI NFs, respectively. The characteristic bands of ST-PANI NSs and PANI NFs appear at 357, 428, and 812 nm with a free carrying tail extending to the near-infrared region. These bands are attributed to the π−π* electronic transition of benzene rings, polaron−π* transition, and π− polaron transition, respectively (Figure 2a). However, the characteristic peaks at 357 and 428 nm are absent in Figure 2b,c, while the presence of 408 and 369 nm bands indicates similar absorption of silver in HCl-doped PANI NFs. The absorption band of the Ag usually appears at around 400 nm, which is caused by surface plasmon resonance. It is difficult to differentiate these bands due to the merging of two bands.32,33 2.3. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra of PANI NFs and PANI NSs are shown in Figure 3. The characteristic peaks at 1477 and 1558 cm−1 belong to the CC stretching vibration mode of benzenoid and quinonoid rings, respectively. The absorption bands at 1282, 1100, and 1230 cm−1 correspond to the C−N stretching, C−H bending, and the protonated C−N group, respectively. The characteristic bands at about 1601 and 1494 cm−1 are the sign of the PANI NF backbone due to the stretching modes of the protonated metal PANI NFs NCs.31 The high intensity peaks in Figure S1a indicate that the activity of HCl-doped PANI NFs is similar to the NC of PANI NFs-Ag.30,31 2.4. Field Emission Scanning Electron Microscopy (FESEM). Figure 4a,b shows the morphology of HCl-doped and HNO3-doped PANI NFs, respectively, at different magnifications. The ST-PANI morphology is presented in Figure 4e,f. The use of different monobasic acids is seen to significantly affect the morphology of PANI NFs. The growth of PANI NFs is one-dimensional (1D) (Figure 4b) instead of two-dimensional (2D).34 In Figure 4a, the bulk quantity of PANI in the form of entangled chains appears as worm-like nanofibers (NFs). The diameter of the nanofibers is ∼50 nm with lengths up to several micrometers, and most of the NFs show the aspect ratio 1:4 (Figure S2). In Figure 4b, the PANI NFs are in the form of a cluster (aspect ratio, 1:2). In Figure 4c, the PANI
HCl-doped ST-PANI NSs. XRD patterns of HCl- and HNO3doped PANI NFs show the characteristic peaks centered at 2θ = 9.76° (001), 14.71° (011), 21.53° (100), and 25.96° (110). All peaks are in good agreement with results by Pouget et al.3 The sharp peak in HCl-doped PANI NFs at 6.34° specified by an asterisk (*) corresponds to the organization of lamellae of PANI NFs23 as well as the formation of silver-PANI NFs NCs. The XRD peak of silver is obtained at 19.1° denoted by a number sign (#) corresponds to JCPDS-011164. The two peaks that appeared in 6.34° and 19.1° are the indication of the silver behavior of the PANI NFs.24,25 The simultaneous appearance of the two peaks at 2θ = 19.1° and 21.53° are not yet reported, providing the evidence of formation of a new phase of PANI NFs synthesized by HCl doping polymerization. Peaks at 2θ = 21.53° and 25.53° are the characteristic peaks of silver-PANI NFs NCs observed in HCl-doped PANI NFs.26,27 Park et al.28 presented that each XRD pattern gives the individual components of PANI NFs. The peak at (110) decreases in HCl-doped PANI NFs, HNO3-doped PANI NFs, and ST-PANI NSs. The results indicate that the structural changes are obtained on successive exposure to HCl as proven by NMR studies.15,29 It is important that the broad peaks at 2θ = 10° to 35° are found in the amorphous nature of PANI, but the sharp peak at 19.1° is only obtained from PANI NFs-Ag NCs.28,30 The same results are presented by Wei et al.31 in the nanocomposites of PANI-WO3 (tungsten oxide). In ST-PANI NSs, the conductive phase is formed, but it is difficult to recognize other phases (Figure 1) since the spectrum is very broad and the peak resolution is poor. The similarity of XRD patterns of PANI prepared by using different dopants indicates the presence of emeraldine salt form of the product. All these results confirm that the products with good crystallinity can be synthesized at room temperature with same morphologies instead of the system at high temperatures and by rapid polymerization.5,8 2.2. UV−Vis Spectroscopy. Figure 2a−c demonstrates the UV−visible spectra of ST-PANI NSs and HNO3-doped and 5742
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NSs, suggesting that HCl-doped PANI NFs indicate higher conjugation.8 The decrease in intensities and slight shift in C− N and N−H bands of HCl-doped PANI NFs (Figure 3a) are due to the activities similar to silver-PANI NFs NCs.32 2.5. Raman Spectroscopy Studies. Figure S1a,b (Supporting information) reveals Raman studies of HCldoped and HNO3-doped PANI NFs, respectively. The intensity of several sharp peaks in HCl-doped PANI NFs is higher than that of HNO3-doped PANI NFs. It indicates that the surface enhanced with laser excitement is due to the behavior of the form of a network of NFs with an aspect ratio of 1:8, and the same is also observed in Figure 4d. The morphology of ST-PANI NSs is observed in the form of agglomeration/nanospheres (Figure 4e,f) because supersaturation is not achieved and heterogeneous nucleation dominates, which can form PANI NFs by a long period of polymerization.5 The isolated HCl-doped PANI NFs show higher aspect ratios (Figure 4b) than HNO3-doped PANI NFs.34 This is quite similar to the report of Bober’s group (Figure S3).35 In order to obtain isolated PANI NFs, the use of HCl as a dopant is preferable. The PANI NFs are showing a random, interconnected, and flat web structure having ∼30−70 nm in diameter and 0.4−3 μm in length. This highly interconnected microstructure is the identity of the formation of PANI NFs-Ag NCs.31,33,36 2.6. Transmission Electron Microscopy (TEM). Figure 5a,b represents the TEM images of HCl-doped and HNO3-
Figure 3. FTIR spectra of (a) HCl-doped PANI NFs, (b) HNO3doped PANI NFs, and (c) ST-PANI NSs.
Figure 4. FESEM images of the (a,b) HCl-doped PANI NFs, (c,d) HNO3-doped PANI NFs, and (e,f) ST-PANI NSs.
Figure 5. TEM images of (a,b) HCl-doped PANI NFs and (c,d) HNO3-doped PANI NFs.
NFs are in the quinonoid ring and the benzenoid ring. The band at 1277 cm−1 can be assigned to the C−N stretching mode in a secondary aromatic amine. Therefore, the results of FTIR spectrum demonstrated that the PANI NFs and ST-PANI NSs exist in the conducting emeraldine form. However, these spectra exhibited a difference in their intensities. The integrated band intensities are minimum in HCl-doped PANI NFs (Figure 3a).26 However, slightly higher intensity bands are observed in HNO3-doped PANI NFs and ST-PANI NSs (Figure 3b,c). The FTIR spectra of the HCl-doped PANI NFs NCs and other two nanostructures of PANI (NFs and NSs) are quite similar (irrespective of band intensities) and indicate the formation of PANI backbone. However, in the HCl-doped PANI NFs, the integrated peak intensity and appearing peaks of benzenoid ring against quinonoid ring are significantly different with those in the HNO3-doped PANI NFs and PANI
doped PANI NFs, respectively. NFs synthesized at 60 °C are found to be thinner and broader in size according to the FESEM investigations (Figure 4a,b and Figure S4) of Du et al.8 and Patil et al.34 Figure 5c,d reveals the TEM images of HNO3-doped PANI NFs showing a tubular network. The thickness of HNO3-doped PANI NFs is observed to be higher than HCl-doped PANI NFs. In addition, the TEM image indicates that the PANI NFs have a coarse surface. The coarse surface has a positive effect on the specific surface area of PANI NFs. All these factors should be considered in designing PANI NF DE with higher capacitance.6,34 Morphological studies show that the experimental conditions affect the morphology of PANI and longer periods of polymerization formation of smoother and thinner PANI NFs (Figure 5b) and longer PANI NFs are possible.8 White circles indicate one of the cross-link points between each PANI NF within a network structure common in silver-polymer NC.36,37 5743
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Figure 6. Thermogravimetric analysis (TGA) of (a) HCl-doped and (b) HNO3-doped PANI NFs.
Figure 7. Cyclic voltammetry (CV) of (a) HCl-doped and (b) HNO3-doped PANI NFs and (c) ST-PANI NSs.
loss is observed at 550 °C, while it is extended up to 600 °C in HCl-doped PANI NFs, indicating high thermal stability. This is due to the PANI NFs-Ag NCs behavior of the HCl-doped PANI NFs.32 2.8. Electrochemical Studies. Figure 7 reveals the electrochemical studies of HCl-, HNO3-doped PANI NFs, and ST-PANI NSs. The cyclic voltammetry results obtained from ST-PANI NSs and HCl- and HNO3-doped PANI NFs revealed several common features such as (1) the sharp exponential rise of anodic current beyond +1.06 V, (2) weak shoulder at +1.0 V, (3) the reduction peak observed at about 0.15 V that results from a surface-confined electrochemical reaction, and (4) the peak current that is directly proportional to the scan rate.38 The specific capacitance values of HCl- and HNO3-doped PANI NFs and ST-PANI NSs are observed to be 570, 320, and 270 F g−1, respectively. The capacitance of HCl-doped PANI NFs, which behave like PANI NFs-Ag NCs, is 570 F g−1 higher than that of the pure PANI, as capacitance depends on
In Figure 5a,b, the thinner PANI NFs along with the bridging (in white circles) indicate similar behavior of PANI NFs-Ag NCs.30 2.7. Thermal Studies. Thermal behavior was studied by (TGA, SETRAM) in the range of 50−650 °C with a heating rate of 10 °C min−1 under an inert atmosphere. In Figure 6a,b, the major similarity is the presence of three-step thermal decomposition patterns. The first step of thermal decomposition is observed at 50−100 °C due to the loss of absorbed water molecules present in the PANI NFs. In Figure 6a,b, the second step of thermal decomposition starts at ∼140 °C and continued up to 240 °C followed by the third step of the thermal decomposition pattern. The weight loss is mainly due to the loss of a dopant ion (thermal dedoping of the polymer) from the polymer chains (second stage). There is not much variation in the first and second steps of the thermal decomposition in HCl- and HNO3-doped PANI NFs. However, the third step of thermal decomposition is found to be varied; in HNO3-doped PANI NFs (Figure 6b), the 99% 5744
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suggesting that interfacial charge transfer is low and the conductivity is high, which can be attributed to the behavior of PANI NFs-Ag NCs. Except for the low electrical resistance, HNO3-doped PANI NFs and ST-PANI NSs also exhibit short and equal diffusion path lengths of ions in the electrolyte, which can be seen from the negligible Warburg region on the Nyquist plots. This may be illuminated by the unique morphology of the PANI NFs.43 Figure 9 shows the charge/discharge curves of HCl-doped and HNO3-doped PANI NFs and HCl-doped ST-PANI NSs.
morphology, metallic behavior, as well as percentage of dopant. The polymerization time for HCl-doped PANI NFs is 24 h.39−41 This capacitance is higher than those reported for silver vanadium oxide (SVO)-PANI NCs (365.5 F g−1) and silver vanadium sulfide (SVS)-PANI NCs (440 F g−1). It is also greater than that reported for lithium vanadium oxide/silver NCs (124 F g−1).21 Nonfibrillar PANI synthesized by a conventional method shows a capacitance of 33 F g−1.40,42 In Figure 7a,b, it is observed that the current increases with scan rate due to the increase in nearby surface and the number of rechargeable redox sites created by PANI NFs but not by ST-PANI NSs (Figure 7c). It is also evident that the assynthesized PANI NFs show similar activity to PANI NFs-Ag NCs, which is supported by XRD, FTIR, and UV−vis spectroscopy. The electrochemical stability of the HCl-doped behavior of PANI NFs-Ag NCs and HNO3-doped PANI NFs and STPANI NSs was examined in 1 M HClO4 aqueous electrolyte for consecutive cycles at a current density of 1 A g−1. The better stability of HCl-doped PANI NFs came from the synergistic effect of PANI NFs-Ag NCs, which can avoid damaging of the electrode.
Figure 9. Charge/discharge cycling curves of electrodes (a) HCldoped PANI NFs behaving like PANI NFs-Ag NCs, (b) HNO3doped PANI NFs, and (c) HCl-doped PANI NSs at a current density 1 A g−1.
All curves exhibit an equilateral triangle shape, showing that the potential of charge/discharge is a linear response to time, indicating a good reversibility during the charge/discharge processes. On the basis of the charge/discharge curves, the specific capacitance values for electrodes HCl-doped and HNO3-doped PANI NFs and ST-PANI NSs are summarized in Figure S6. It can be seen that at a current density of 1 A g−1, the HCl-doped PANI NFs increase the specific capacitance and charge/discharge curves. Similar results are also found in our graphene-based PANI NCs.44 2.9. Growth, Mechanism, and Work for Energy Storage Devices of PANI NFs. We reported the formation of fibrillar morphology by long period of polymerization of aniline in the dopant of HCl and HNO3. Two important factors are common to long period of polymerization: (i) an induction period followed by a rather rapid formation of a precipitate, and (ii) the influence of inert surfaces (walls of the reaction flask) on the progress of the reaction. We believe that polymerization first occurs on the surface whose morphology is mirrored by the growing polymer chain. Indeed, a blue-green film of pernigraniline salt is formed on the walls of the reaction flask well before a precipitate is observed in bulk. Due to in situ deposition of pernigraniline, a (fibrillar) salt seed fresh polymer growth triggering in a continuous seeding process resulting in a black precipitate, the nanoscale morphology of the original is obtained in various lengths.45−47 It is also supported by Tran et al.5 and our experimental observations during polymerization of aniline. We have also noticed the significantly decreasing polymerization rate. Secondary growth leads to agglomeration not seen in PANI NFs except in ST-PANI NSs. These results reveal important insights into a semirigid rod nucleation phenomenon that has
Figure 8. Schematic representation of how the required diffusion length is reduced in DE.
The electrochemical impedance (EI) data of HCl-doped and HNO3-doped PANI NFs and ST-PANI NSs were analyzed using Nyquist plots given in Figure S6. These plots show the frequency response of the electrode/electrolyte system and the imaginary component (Z″) of the impedance against the real component (Z′). The more vertical behavior of the curve corresponds to a cell that is more similar to an ideal capacitor. The Nyquist plots of HCl- and HNO3-doped PANI NFs and ST-PANI NSs exhibit squares over the high-frequency range, followed by a straight sloped line in the low-frequency region of HCl-doped PANI NFs behaving like PANI NFs-Ag NCs also obtained in graphene-based conducting polymer nanocomposites.43,44 Large data observed for these electrodes are indicative of high interfacial charge-transfer resistance, which can be attributed to the poor electrical conductivity of these materials. The sloped portions of the Nyquist plots are the Warburg resistance resulting from the frequency dependence of ion diffusion/transport in the electrolyte. The larger Warburg region of these electrodes indicates greater variations in iondiffusion path lengths and increased obstruction of ion movement. Different from HNO3-doped PANI NFs and STPANI NSs, the squares at high frequency are not detected for HCl-doped PANI NFs behaving like silver-PANI NFs NCs, 5745
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Here, we found that the conductivity of HCl-doped PANI NFs is approximately 38 times higher than that generally found in nonfibrillar PANI. The nanofibrillar composite materials exhibit enhanced conductivities with values up to 95 S cm−1.39 In ST-PANI NSs, the conductivity is low, indicating low doping of HCl (Table S1). Recently, Shi et al.51 reported that the polymerization time and dopant concentration play the major role on the electronic conductivity of PANI/PANI NFs/PANI NCs. In the present work, a network of PANI NFs behaved like silver-PANI NFs NCs. The formation of new phases and the doping level of acids due to post-polymerization show the positive effect on conductivity.52 2.13. Justification of Behavior of PANI NFs as PANI NFs-Ag NCs. According to Jia et al.24 polyaniline shows several peaks around 2θ = 20° that are probably induced by the dopant. The silver peaks obtained around 2θ = 20° are disappearing in silver−polyaniline nanofibers NC synthesized by using surfactant-like poly(styrene sulfonate) (PSS). In XRD (Figure 1, HCl dopant), the peaks appear around 2θ = 19.1° indicate the behavior of PANI NFs-Ag NCs synthesized by a template-free method similar to the work of Huang et al.1 Recently, Ghani et al.33 reported that the preparation method of PANI can affect the morphology and properties of it. 2.14. Antibacterial Studies. PANI NFs behaving like PANI-Ag NCs is used for antibacterial studies. PANI NFs are used to determine the MIC and MBC of Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. PANI NFs and Ag nanoparticles are used as antibacterial agents. Bacterial inhibition zones of E. coli and S. aureus are shown in Figure 10. The PANI NFs NCs display zones of inhibition at
hitherto been little explored. When PANI nucleates homogeneously, surface energy necessitates the formation of ordering nuclei, which leads to the directional polymerization of aniline. This ultimately leads to the formation of 1D nanofibrillar morphology of the final product. This interesting point noted that the HCl-doped polyaniline nanofibers lead to the formation of silver−polyaniline nanofibers NCs. Li et al.48 reported that PANI NFs perform like a concentric cable where only a small part of PANI NFs is involved in the cyclic voltammetry. The nanometer-sized PANI NFs in the form of fibers formed a well-organized pipe-like structure and yield a high electrochemical capacitance performance. This is the reason for such structure electrode material showing the best capacitance (Figure 8). 2.10. Electrodes with Numerous Pipe-like Channels of PANI NFs. In Figure 8, the faint gray shield stands for the part where the insertion/expulsion of HCl/HNO3 can take place, and the dark green core stands for the conductive part of NFs where no change occurs during the cycles. Therefore, the dark green core has no contribution to the enhancement in the capacitance, and only the faint gray shield is available for deciding the capacitance. The ratio of the dark green core to the whole NF represents the proportion of PANI generating the capacitance.48 The PANI NFs must be conducted for electron transfer during the redox reaction. As mentioned in FESEM and TEM analyses, the PANI NFs have a 1D structure (Figure 5a−d and Figures S2−S4). Hence, we believe that data presented in this paper as well as those discussed regarding the capacitance of PANI NFs are the most reliable. Template-free synthesized PANI NFs could play a pioneering role and replace nonfibrillar PANI in the development of next-generation ESDs as it behaves like a metal-PANI NC with higher capacitance. This synthetic procedure is quite economical and facile. 2.11. Correlation of FTIR and NMR Spectroscopy with CV. The electrochemistry of nitrogen compounds is of particular interest because of the availability of two reaction sites for removal and addition of an electron in PANI NFs, but very limited work has been done with these systems. We take this trend as evidence that the site of electron abstraction in the PANI NFs is the lone pair on nitrogen delocalized into the aromatic ring.49 Due to the presence of phenazine rings in PANI NFs and in ST-PANI NSs, peaks I and II show the formation of the cross-linked PANI NFs. The FTIR peak of the products aniline cation (C6H5NH+) and nitrenium ion (C6H4N+) of the PANI NFs corresponds to ∼1477 cm−1, whereas 13C NMR signals at 7−7.5 ppm and 1H signals with chemical shifts at 7.0−8.5 ppm (14H) are associated with protons that are directly bonded to aromatic rings due to the chemical shift (Figure S5). Studies of the evolution of the voltammetry curves of HCldoped and HNO3-doped PANI NFs and ST-PANI as a function of the upper potential limit are supported by by FTIR and NMR studies. In the corresponding PANI NFs, the middle peak (I) frequently appears in the cyclic voltammetry curves that correlate to the formation of cross-linked PANI NFs (Figure 5a−d) through the substitution of a nitrenium cation into each other and that of ST-PANI due to agglomeration. However, peak II is not observed due to the lower concentration of nitrenium ion (C6H4N+).49,50 2.12. Electrical Conductivity. The conductivities of HCldoped and HNO3-doped PANI NFs and ST-PANI NSs were observed to be 48.47, 32.08, and 11.9 S cm−1, respectively.
Figure 10. Zones of inhibition of bacteria at 10 and 20 μL/mL of (a) Ag NPs of S. aureus, (b) PANI NFs of S. aureus, (c) Ag NPs of E. coli, and (d) PANI NFs of E. coli.
20 μL/mL of 15 and 19 mm for S. aureus and E. coli, respectively, which were higher than other concentrations and silver nanoparticles, as shown in Table 1. The formation of the zones of inhibition indicates AB activity. In addition, the zone of inhibition of E. coli is elevated than S. aureus in PANI NFs as well as for Ag nanoparticles. This is due to positively charged ions binding with negatively charged thiols, carboxylates, amides, and indoles, destructing and disabling the DNA and cellular enzyme.53 This indicates the PANI NFs play the role of PANI NFs-Ag NCs without Ag nanoparticles. The nanostruc5746
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free-flowing powder of HCl-doped PANI NFs was obtained and used for further characterization. 3.3. Synthesis of HNO3-Doped PANI NFs. For synthesis of HNO3-doped PANI NFs, the same experimental protocol for HCl-doped PANI NFs was used. Instead of HCl, an appropriate amount of HNO3 was used. 3.4. Synthesis of HCl-Doped PANI Nanospheres (Short-Time (ST)-PANI NSs). HCl-doped PANI nanospheres were synthesized by employing a similar procedure adopted for HCl-doped PANI NFs and by decreasing the time of addition to 30 min and the polymerization time to 1 h. 3.5. AB Activity: Strain Selection and Inoculum Preparation. The Gram-positive bacteria used are S. aureus (NCIM-2121), while the Gram-negative bacteria used are E. coli (NCIM-2110). Each of the strain was grown in nutrient broth (NB) medium (HiMedia, India) for about 24 h. The culture was then suspended in physiological saline using a turbidity of McFarland standard no. 2. From that, an aliquot of 50 μL was added to 5 mL of sterile nutrient broth, resulting in an inoculum of approximately 1 × 106 CFU/mL. 3.6. Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) Determination. The catalyst PANI NFs were treated for 24 h with B cultures, which was diluted 1 × 106 CFU/mL, and cell strain was determined with an absorbance at 600 nm. In the control, a set of B cultures is grown in the absence of PANI NFs, and for sterility testing, a sterile uninoculated nutrient broth of the same set was used. Minimum B concentration of the PANI NFs was determined by spreading the catalyst with a culture broth solution of 1 × 106 CFU/mL in the range of 100 to 1000 μg/mL. The ratio of the bacterial culture with catalyst solution was optimized to 1:1 and incubated at 35−37 °C for 18−24 h. The MIC is measured as the lowest concentration of an antimicrobial agent that resulted into no any visible growth. MBC is also determined after determination of MIC. Thus, for MBC, 50 μL of the bacterial culture was collected sterilely from the incubated set of the MIC determination and was uniformly distributed on the nutrient agar plate. These plates were incubated for 24 h at 37 °C with the reduction of CFU of the inoculum by 99.9% or more determined. The above protocols were repeated in triplicates. The electrochemical study was also performed using a button-type cell configuration with the help of a computercontrolled Autolab PGSTAT30 (Eco Chemie) potentiostat/ galvanostat system. The working electrode was prepared by mixing the HCl-doped and HNO3-doped PANI NF and PANI NS powder (separately), carbon black, and Nafion (70:25:5 by weight) followed by compacting and drying under a primary vacuum for 2 h at 600 °C. The electrolyte used was 1 M HClO4 in propylene carbonate (PC), and a lithium foil was used as the anode. The PANI NFs were also characterized using nuclear magnetic resonance (NMR) spectroscopy (1H and 13C) for percentage determination of doping of acids. Conductivity was calculated by using the resistance measured using a Keithley electrometer. 3.7. Characterization. X-ray diffraction (XRD) patterns of HCl-doped and HNO3-doped PANI NFs were recorded on Rigaku (Japan) D/MAX-2400. The morphology was examined by field emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL, JEM 2100). For detection of functional groups and measurement of order and disorder in PANI, Fourier transform infrared (FTIR, PerkinElmer) techniques have
Table 1. Zone of Inhibition Measured by Disc Diffusion Method zone of inhibition (mm) E. coli
S. aureus
catalyst/bacteria
10 μL/mL
20 μL/mL
10 μL/mL
20 μL/mL
Ag NPs PANI NFs
7 14
10 19
8 13
9 15
tures of PANI NFs can play a major role in the antibacterial performance because of their high atom density.51 Minimal inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) are determined by the standard methods. MIC is the lowest concentration where there is no noticeable growth of bacteria, and the complete absence of colony noticed at the lowest concentration is known as MBC. Here, the MBCs at 600 and 700 μg/mL of E. coli and S. aureus, respectively, were considered for their study. The antibacterial agents having ability to kill bacteria at lower concentration. Herein, the lower values of MIC (200−300 μg/ mL) of E. coli and S. aureus bacteria exhibiting efficient antibacterial agent. Tolerance factor is used to determine bactericidal or bacteriostatic nature of an antibacterial agent by using MIC and MBC values. E. coli has a tolerance factor of 3, and S. aureus has a tolerance factor of 2.3. As per literature reports, the agents that kill microorganisms are called bactericidal, while the agent that inhibits the bacterial development is called bacteriostatic. Therefore, the tolerance factors conclude that the antibacterial agent has a bacteriostatic or bactericidal nature by taking into account its MBC/MIC ratio. When the MBC/MIC ratio is larger or identical to 16, it is bacteriostatic, and when the MBC/MIC ratio is less than 4, it is believed to be bactericidal.54 The antibacterial agent is also bactericidal if colony-forming units (CFU)/mL is reduced by 99.9% after the incubation period. In the present context, PANI NFs encompass a proficient and complete reduction of 1 × 106 CFU/mL. Therefore, the PANI NFs demonstrate the bactericidal consequence against these bacteria because of the lower value of the MBC/MIC ratio. This outcome indicates that the bactericidal effect on both bacteria and is concordance with earlier reports.55−57
3. EXPERIMENTAL SECTION 3.1. Materials. All the precursors and chemicals used were of analytical reagent (A.R.) grade, purchased from commercial sources (Fluka, Aldrich), and used without further purification except for aniline. Aniline was purified by double distillation method. The two bacterial species were purchased from the National Collection of Industrial Microorganisms (NCIM), Pune Maharashtra, India. 3.2. Synthesis of HCl-Doped PANI NFs. In a roundbottom flask, 100 ml of 1 M HCl solution and 1 mL of aniline were slowly mixed under constant magnetic stirring at a speed of 1200 rpm on a Remi magnetic stirrer for 12 h. During constant stirring, 0.25 M APS was slowly added to the above mixture and continued for 48 h at room temperature. Finally, the formation of dark green emeraldine salt and a colorless filtrate confirmed the formation of PANI. The precipitate was washed with deionized distilled water several times and then with acetone and dried in oven at 45 °C for 24 h afterward. A 5747
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been used with sufficient loading of samples. UV−vis spectroscopy measurement was recorded by using PerkinElmer Lambda 950 in the range of 200−1000 nm in aqueous medium. Thermal behavior was studied by (TGA, SETRAM) in the range of 50−650 °C under an inert atmosphere.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02834.
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REFERENCES
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4. CONCLUSIONS In the present investigation, we have demonstrated an economical, room-temperature, and template-free method for fabrication of PANI NFs. The HCl-doped PANI NFs show similar properties of PANI NFs NCs with an enhanced capacitance (570 F g−1) compared with PANI (33 F g−1) and PANI NFs (122 F g−1). The presented PANI NFs can promote remarkable enhancement in the performance of the DE. Morphological studies indicate that the size of PANI NFs is around ∼50 nm in diameter and few micrometers in length with good aspect ratio. The XRD pattern of HCl-doped PANI NFs shows that additional peaks at 2θ = 6.34° and 19.1° indicate the formation of silver, indicating that the corresponding NFs behave like silver−PANI NFs NCs. The detailed mechanism of formation of PANI NFs has also been interpreted. The high capacitance of PANI NF electrodes that are behaving like PANI NFs-Ag NCs is making them attractive for DE applications. On the basis of this hypothesis, it is easy to alter elongated 1D NFs to 3D hollow nanospheres without any template. The PANI NFs exhibited the efficient antibacterial agent as compared to only Ag nanoparticles, which revealed that the PANI NFs are acting as a competent antibacterial agent than PANI NFs-Ag NCs. Significantly, water-soluble PANI NFs are useful for applications in biosensors, electronics, and electrical and optical devices. Moreover, as PANI can act as a synthetic metal, the PANI NFs can also act as PANI NFs-Ag NCs. This opens up a new spectrum of research in NFs of other conducting polymers like polypyrrole and polythiophene and their metallic behavior.
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Article
Raman spectroscopy, FESEM images of HCl-doped PANI NFs, FESEM image of HNO3-doped PANI-NFs, TEM image of HCl-doped PANI-NFs, NMR study, EI study, and conductance and specific capacitance (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Rahul S. Diggikar: 0000-0001-8573-3999 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financed by the University Grants Commission, New Delhi, India under Major Research Project Scheme (file no. 43/ 408/2014 (SR)). The authors are deeply acknowledged to Dr. Bharat B. Kale, Director, C-MET, Pune, Maharashtra, India for constant support and motivation. 5748
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