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A novel, rapid synthetic protocol for controllable sizes, conductivities and monomer units of soluble polypyrrole
European Polymer Journal 71 (2015) 596–611
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
A novel, rapid synthetic protocol for controllable sizes, conductivities and monomer units of soluble polypyrrole Subrata Mondal, M.V. Sangaranarayanan ⇑ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e
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Article history: Received 12 March 2015 Received in revised form 31 July 2015 Accepted 20 August 2015 Available online 28 August 2015 Keywords: Polymerization Polypyrrole Sodium tetradecyl sulfate Nanospheres
a b s t r a c t We report a facile synthesis of polypyrrole spheres with controllable sizes, monomer units and conductivities using sodium nitrite and nitric acid in presence of the anionic surfactant sodium tetradecyl sulfate. The rapid synthesis of polypyrrole spheres was accomplished without employing any explicit oxidizing agents. The characterization of polypyrrole (PPy) is carried out using FTIR, XRD, SEM, TEM and TGA while the molecular weight is estimated from the MALDI TOF analysis. The precise tuning of various physicochemical properties is accomplished by altering the concentrations of sodium nitrite and is attributed to the in situ generation of NO2. The size of the PPy spheres varies from 45 nm to 350 nm depending upon the concentration of NaNO2. The dynamic light scattering studies provide mechanistic insights into the role of sodium nitrite in altering the pyrroleincorporated micellar size of the surfactant, leading to different diameters of the PPy spheres. The surfactant-induced solubility of PPy is rationalized with the help of the X-ray diffraction patterns. The solution of PPy in dimethylsulfoxide has a remarkable ability of selectively detecting Cu2+ and Hg2+ among a host of other bimetallic ions, as inferred from colorimetry and UV–Visible absorption spectroscopy. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Conducting polymers having metal like conductivity and polymeric properties render themselves as potential candidates for a wide range of applications in sensors [1,2], electrochromic devices [3], batteries [4], supercapacitors [5], optical devices [6], energy storage [7], etc. Conducting polymers are insulators in their neutral states and can be converted into conductive states by doping [8] with conductivities spanning a range of 103 to 104 S cm1. This doping can be achieved by chemical or electrochemical oxidations, wherein the positive charges are acquired by the polymer backbone and to maintain electroneutrality, the oppositely charged counter anions enter the polymer matrix. This leads to impressive alterations in optical as well as physico-chemical properties. Several conducting polymers viz polypyrrole (PPy), polyaniline, polythiophene, polyindole, polyethylenedioxy thiophene have been widely employed during the past few decades [9–14]. Among these conducting polymers, PPy has attracted great interest due to its ease of preparation, wide conductivity range (102 to 104 S cm1), satisfactory environmental stability and biocompatibility. PPy has been employed in sensors [15,16], batteries [17], supercapacitors [18], electrochromic devices [19], etc. In order to enhance the performance in various applications, PPy nanostructures in conjunction with simpler synthetic strategies are required. PPy nanomaterials provide large surface areas as well as
⇑ Corresponding author. E-mail address: [email protected] (M.V. Sangaranarayanan). http://dx.doi.org/10.1016/j.eurpolymj.2015.08.027 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.
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unique properties which facilitate the interaction of PPy with the species of interest, thereby rendering them as suitable materials in diverse contexts. The chemical polymerization of pyrrole can be accomplished using dispersive polymerization [20], interfacial polymerization [21,22], seeding techniques [23,24], emulsion polymerization [25,26], etc. Surfactant-assisted emulsion techniques offer facile routes for synthesizing PPy whereby interesting optical, electrical and morphological properties can be achieved in conjunction with easy removal of templates. Further, the surfactants enable tuning of the morphologies and dimensions of the polymers via micelle formation through interactions between the reactants and solvent molecules [27]. Depending upon the nature of the surfactants, the morphologies of PPy vary from nanospheres, nanotubes to nanofibers [27–29]. The nanofibers and nanospheres of PPy have been synthesized using the cationic surfactant Cetyltrimethylammonium bromide (CTAB) which influences the solubility and conductivity [28,30]. Indeed, the anionic surfactants are also interesting since they act both as surfactants and counterions. Thus, diverse morphologies of PPy can be synthesized using sodium dodecyl sulfate, dodecylbenzenesulphonic acid, azobenzenesulphonic acid, etc. [31,32]. The chemical polymerization process also offers the simplest method of tuning the conductivities by doping with strong acids such as HCl, H2SO4, HNO3 or HClO4 [33,34]. However, the formation of PPy requires in general, a time duration ranging from 6 to 12 h due to the initially sluggish oxidation of pyrrole monomers. Thus simple, robust and rapid synthetic protocols for PPy with tunable physiochemical properties are especially challenging. Here we report a novel methodology for the synthesis of polypyrrole (PPy) nanomaterials with controllable sizes, monomer units and conductivities. The synthesis is carried out with sodium nitrite in acidic medium (HNO3) in contrast to the customarily used oxidizing agents for polymerization. Since surfactants often yield interesting morphological and spectral features, PPy is synthesized using a new anionic surfactant sodium tetradecyl sulfate (STS). We demonstrate that the sizes, monomer units and conductivities of PPy can be altered by solely varying the concentration of NaNO2. Further, PPy prepared in the presence of STS is soluble in various solvents viz dimethyl sulphoxide (DMSO), dimethyl formamide (DMF), dimethyl acetamide (DMA), etc. The origin of the solubility is rationalized using the XRD data. We also establish that the PPy solution is capable of selectively detecting Cu2+ and Hg2+ ions among various bimetallic ions. For quantitative estimation of these ions, the UV–Vis spectroscopy has been employed.
2. Experimental section 2.1. Chemicals Pyrrole (SRL chemicals India) was distilled at 140 °C under nitrogen atmosphere at reduced pressure and stored in the refrigerator. Sodium nitrite (NaNO2), nitric acid (HNO3), potassium chloride (KCl), dimethyl sulphoxide (DMSO) cupric sulfate (CuSO4) and mercuric sulfate (HgSO4) (SRL chemicals India) were employed without further purification. Sodium tetradecyl sulfate [CH3 (CH2)13OSO3Na] procured from Sigma Aldrich was used as received. All experiments were performed using triply distilled water.
2.2. Synthesis of polypyrrole In a typical synthesis, 0.031 g of STS was added to 20 mL of triple distilled water in a 100 mL beaker and stirred using a mechanical stirrer for five minutes. Subsequently, 0.77 g of distilled pyrrole and 60 lL of 12 M HNO3 were added and then sonicated for 10 min in order to obtain a clear solution, followed by the drop wise addition of 20 mL of 0.25 M solution of NaNO2 with continuous stirring. The instantaneous appearance of a black precipitate due to the addition of NaNO2 indicates the formation of polypyrrole. The entire reaction was carried out at a temperature of 30 ± 1 °C. After five minutes, the precipitate was filtered using a Whatman 42 filter paper and washed repeatedly with distilled water so as to remove all the soluble oligomers and other side products. The black precipitate was then dried in an oven at 50 °C for four hours. The same procedure was also employed for synthesizing PPy in the absence of the surfactant STS. The PPy synthesized by adopting the above procedure using the aforementioned composition in presence and absence of surfactant are designated as WSPPy and WOSPPy respectively. The synthesis of PPy was carried out in the presence and absence of the anionic surfactant STS using identical concentrations of pyrrole and other chemicals viz. NaNO2, HNO3 in order to study the differences in their properties. For comprehending the effect of NaNO2, different compositions of pyrrole and NaNO2 were employed by altering the NaNO2 concentrations. Since our objective is to analyze the surfactant-induced polymerization, different PPy samples were synthesized by altering the pyrrole: NaNO2 concentration in the range of 1: 2/3, 1: 4/3, 1: 5/3, 1: 8/3 using the concentration of STS and HNO3 as 0.00125 M and 0.018 M respectively. These different compositions were made by keeping a fixed concentration of pyrrole (as 0.075 M) while NaNO2 concentrations were varied from 0.05 M to 0.20 M. However, the concentrations of STS and HNO3 were kept constant in general. The PPy prepared in presence of STS using the above-mentioned different composition ratios of pyrrole and NaNO2 are denoted as WSPPy 1, WSPPy 2, WSPPy 3 and WSPPy 4.
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2.3. Instrumentation UV–Visible absorption spectra were recorded on a Ocean Optics GmbH DT-MINI-2-GS UV–Vis spectrometer. The SEM images and energy dispersive X-ray analysis (EDAX) were acquired using FEI Quanta FEG 200. The FTIR spectra were recorded using Jasco FTIR-4100 spectrometer (resolution = 0.9 cm1) with KBr pellets, while the X-ray diffraction (XRD) analysis was carried out with Bruker D8 system equipped with Cu Ka radiation (1.5405 Å). Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer TGA7 with 20 °C rate of N2, controller from 0 °C to 900 °C and the derivative weight loss was estimated using the commercial software provided with the instrument. Matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) using the Bruker ultraflextreme was employed to estimate the molecular weight as well as the number of monomer units present in PPy. The spectrum is recorded using the PPy solution (in DMSO) in the presence of a suitable matrix. The matrix used in the MALDI analysis is a-cyanohydroxy cinnamic acid (CHCA). In order to elucidate the mechanism of synthesizing the different sized PPy and the influence of NaNO2 on the micelles, the dynamic light scattering (DLS) measurements were performed during the polymerization process using Malvern Zetasizer Nano Zs 90 instrument. The DLS study also enables the estimation of the hydrodynamic radius (Dn) of various PPy samples. The conductivities of the different PPy samples were measured from the Nyquist plot with the help of a multi-channel potentiostat after making suitable pellets. All the characterization studies were carried out at a temperature of 30 ± 1 °C. 3. Results and discussion 3.1. Reaction mechanism In the presence of HNO3, sodium nitrite initially forms nitrous acid (HNO2) which upon further reaction with the former forms N2O4 which dissociates to form NO2. The addition of NaNO2 to the solution of pyrrole in presence of acids yields initially a reddish brown color and then a black precipitate. The initial reddish brown color is due to the formation of soluble oligomers while the appearance of the black precipitate indicates the formation of PPy. The in situ formation of HNO2 and NO2 has been reported earlier [35]. NO2 acts as an oxidizing agent for the polymerization and possesses excellent catalytic activity in this context. Consequently it enhances the rate of formation of the pyrrole radical cations. The coupling of the two pyrrole radical cations and loss of two hydrogen ions generates bipyrrole. In the propagation step, further re-oxidation, coupling and deprotonation processes continue thereby yielding polypyrrole. On account of the in situ formation of NO2, the synthesis of PPy is rapid in contrast to the other methods employing ferric chloride (FeCl3), ammonium persulfate (APS), etc. It is of interest to point out that the mechanism involved in the coupling of pyrrole radical cations has been investigated earlier using computational approaches too [36]. A plausible mechanism for the formation of PPy is provided in Scheme 1. It may be emphasized that a facile rapid chemical synthesis of PPy is a challenging task. While the oxidative polymerization can be accelerated with FeCl3 by an optimal choice of the reaction conditions, the synthetic methodology proposed here demonstrates the use of a small catalytic amount of NaNO2 at room temperature. The acidic medium not only accomplishes effective doping, but also facilitates the in situ generation of NO2. The combination of NaNO2 and HNO3 leads to the formation of NO2 which enhances the rate of polymerization and subsequently enables the incorporation of the counter anions into the polymer matrix. The choice of HNO3 is deliberate since anions play an important role in dictating the size of the PPy, lower 2 sized NO 3 being more preferable than ClO4 , SO4 , camphor sulfonate anions etc due to enhanced diffusion. 3.2. Characterization of PPy 3.2.1. FTIR studies The FTIR spectra of PPy depicted in Fig. 1(A) indicate various typical absorption bands pertaining to WSPPy and WOSPPy. The broad absorption band at 3388 cm1 due to the NAH stretching vibration is noticed in both cases. In the case of WSPPy, two additional absorption bands at 2914 cm1 and 2846 cm1 arise on account of CAH stretching frequency indicating the presence of STS in the PPy sample while for WOSPPy, these two peaks are absent. The sharp absorption band at 1564 cm1 is attributed to the C@C stretching vibration of the pyrrole ring. The CAN stretching vibration band is observed at 1417 cm1. The bands at 950 cm1 and 750 cm1 are due to CAH out of plane ring deformation vibrations. These absorption bands are consistent with other FTIR studies of polypyrrole [37]. The FTIR spectra of different WSPPy samples (i.e. WSPPy 1 to WSPPy 4) are provided in Fig. 1(B) wherein all the samples exhibit absorption bands in their corresponding regions as mentioned above. Since the band intensities increase from WSPPy 1 to WSPPy 4, the extent of the polymerization is higher. This change in band intensities may either reflect the variation in the number of monomer units or the incorporation of counter anions into the polymer moiety [38]. For all samples, the absorption bands from the ACH stretching vibration appearing in the range around 2900 cm1 are attributed to the presence of the surfactant STS. The absorption bands at 1575 cm1 and 1435 cm1 are due to the C@C and CAN stretching vibration respectively which are more intense for WSPPy 4 indicating the larger number of monomer units than others (WSPPy 1 to WSPPy 3). A plausible interpretation for this observation is provided in the Supporting Information (Fig. S1, SI). This behavior is consistent with the conductivity data as well as MALDI studies (vide infra).
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Scheme 1. Mechanism of polymerization of pyrrole in presence of NaNO2.
Fig. 1. FTIR spectra of PPy for (A) WSPPy and WOSPPy and (B) WSPPy 1 to WSPPy 4.
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3.2.2. X-ray diffraction analysis The XRD patterns of WSPPy and WOSPPy are depicted in Fig. 2(A) wherefrom it is noticed that the surfactant does not influence the crystallinity. Both samples are essentially amorphous with the appearance of broad, intense peaks at 2h = 21.78° and 23.52° respectively, for WSPPy and WOSPPy. The higher 2h value for WOSPPy indicates the lower d spacing while the particle sizes calculated from the Debye-Scherer equation [39] are larger than for WSPPy. In both cases, the less intense peaks observed at 9.24° and 9.98° are attributed to the low molecular weight oligomers. From the Bragg’s equation [40], the d-spacing between the crystal planes is estimated from the equation
d ¼ nk=2 sin h where n is the integer determined by the order given (here n = 1), k denotes the wavelength of the Cu Ka radiation source (k = 1.5409 Å), h being the angle between the scattering planes and the incident ray. The interplanar spacing values are (i) 4.07 Å and 9.56 Å for WSPPy with respective 2h values being 21.78° and 9.24° and (ii) 3.77 Å and 8.89 Å for WOSPPy with the 2h values being 23.52° and 9.98°. The XRD patterns of PPy synthesized in the presence of STS with different composition ratios (of pyrrole and NaNO2) are shown in Fig. 2(B). From these XRD patterns, we note that the location of the peak is influenced by the concentration of NaNO2. The 2h values shifted towards higher angles in conjunction with the decrease in peak intensity when the concentration of NaNO2 increases. The increase in 2h values reflects the decrease in d-spacing implying that the extent of doping depends on the concentration of NaNO2. The d-spacing values for WSPPy N calculated from the Bragg’s equation are shown in Table 1. From these XRD patterns, we can also estimate the size of PPy spheres; since the 2h values shifted towards higher angles, the mean sizes calculated from the Debye-Scherer equation increases with the concentration of NaNO2 and are consistent with those deduced from the SEM images (vide infra). 3.2.3. Electron microscopic studies Fig. 3 depicts the SEM images of PPy prepared in the presence of STS with different compositions of pyrrole and NaNO2 wherein spheres of various sizes are noticed along with uniform dispersion. The diameters of these spheres vary from 45 to 350 nm. In the absence of STS, a random arrangement of rods and spheres is noticed (Fig. S2, SI). This indicates that the surfactant (STS) induces the formation of spheres in an orderly manner. The formation of such ordered PPy spheres can be comprehended as follows. In aqueous medium, the anionic surfactant STS forms spherical micelle via hydrophilichydrophobic interactions since its concentration is higher than the critical micellar concentration (CMC). The pyrrole monomers are incorporated into the micelle cores. The addition of NaNO2 in presence of HNO3 forms NO2 which initiates the radical polymerization. The in situ formation of NO2 oxidizes the monomer in the micelles and it readily polymerizes via the coupling and de-protonation process as shown in Scheme 1. Hence pyrrole monomers incorporated within the STS micelles have turned into the oligomers in the micelles and the color of the solution changes from colorless to reddish brown. Further, the monomers present in the droplet diffuse to the growing particle and finally these oligomers get converted into PPy and the color of the solution becomes black as anticipated. Due to agglomeration, a few- PPy nanospheres form chain-like morphologies as observed from the SEM images. From the TEM images depicted in Fig. 4, the spherical morphology of PPy is further confirmed. Further evidence for this mechanism is provided by the DLS studies. It is noticed from Fig. 3 that the sizes of the PPy spheres increase with the concentration of NaNO2. Although the uniform distribution of PPy spheres is inferred from the SEM images, inhomogeneous sizes are observed. This anisotropic size distribution of PPy may arise from the rapid polymerization of pyrrole in presence of NaNO2 and HNO3. In this context, we speculate that the oxidative growth of PPy spheres from monomers to oligomers into the micelle core of STS is quite rapid and this causes the non-uniform size distribution. Although the diameters of these spheres are not uniform, we can estimate their mean values. This variation can be interpreted in the following manner. At higher concentrations of NaNO2, the diameter of the spherical micelles of STS gets enhanced on account of the increase in ionic strength of the solution (larger concentration of Na+ ions). Hence the droplet sizes of the monomers increase whereby larger numbers of these get incorporated into the micelles core leading to a more facile polymerization vis a vis formation of larger sized PPy spheres. With increase in the concentration of NaNO2, the in situ formation of NO2 is also enhanced causing efficient diffusion of the pyrrole monomers into the growing particles viz. oligomers. Thus, the larger concentrations of NaNO2 facilitate the in situ formation of NO2 to a higher extent resulting in the efficient degree of polymerization. The variation in the diameter of the micelles core with concentration of NaNO2 is depicted in Scheme 2. Indeed, the quantification of the surfactant present in PPy is essential in order to obtain additional insights and while the exact amount is difficult to measure, an approximate value is deducible from the EDAX spectrum. However, this value is not accurate since the elements oxygen and carbon in STS also exist in NaNO2 and PPy respectively. Consequently, the presence of S in the EDAX spectrum is an indication of the quantity of the surfactant. Since ca 2–3% ‘S’ is present as deduced from EDAX data (Fig. 5), this value is an approximate estimate of surfactant. An interesting observation deserves mention in this context viz the quantity of the surfactant varies for different PPy samples. The weight% of STS decreases from WSPPy 1 to WSPPy 4 and is attributed to a higher extent of doping of NO 3 ions than the bulkier counter anions of STS. In order to investigate whether the surfactant can be entirely removed from the polymer moiety, additional experiments were carried out, as described in Appendix A.
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Fig. 2. The X-ray diffraction patterns of PPy: (A) WSPPy and WOSPPy and (B) WSPPy 1 to WSPPy 4. The inset of Fig. 2(B) depicts the shifts in 2h values.
3.2.4. Dynamic light scattering (DLS) studies The diameters of PPy spheres may also be correlated with those obtained from the DLS studies. For this purpose, we have carried out DLS studies (Fig. 6) of WSPPy 1 to WSPPy 4 in acetonitrile (wherein PPy is sparingly soluble). The diameters follow the trend as: WSPPy 1 (85 nm) < WSPPy 2 (175 nm) < WSPPy 3 (254 nm) < WSPPy 4 (347 nm). While the interpretation from DLS analysis is qualitatively consistent with the inference from the SEM images, the estimates from the DLS are higher due to solvation effects. The PPy molecules are solvated in the solution phase and hence the diameters are larger; furthermore, since the influence of solvation is higher for lower sized PPy spheres, the dimensions estimated from DLS and SEM are significantly at variance. On the other hand, for larger sized PPy spheres, the values estimated from DLS data and SEM images are nearly identical. Further, the DLS studies provide more insights into the formation mechanism of different sized PPy spheres. The average diameter of the pyrrole-incorporated STS micelles is ca. 7.2 nm. The growth of the polymer was studied during the polymerization at different concentrations of NaNO2. Since the synthetic protocol is rapid with only a catalytic amount of NaNO2 being required for polymerization, a very small volume of NaNO2 (5 lL) was added to the reaction mixture so as to avoid the complete polymerization (formation of the black precipitate). As the polymerization progresses, the diameters of the micelles-incorporated PPy spheres increase which are monitored for each concentration of NaNO2. In the presence of HNO3, the average diameter of micelles increases with concentration of NaNO2 as shown in Fig. 7. This behavior can be comprehended by invoking a plausible assumption. The addition of NaNO2 in presence of HNO3 forms NO2 as well as free Na+ ions wherein NO2 induces the polymerization process while Na+ ions interact with the hydrophilic part of the micelles. In the presence of HNO3, the higher concentration of NaNO2 leads to the formation of NO2 and the release of the Na+ ions occurs to a larger extent as visualised in Scheme 1. This increase in the concentration of Na+ leads to a higher concentration
Table 1 The physicochemical properties of different PPy samples. Sample
WSPPy 1 WSPPy 2 WSPPy 3 WSPPy 4 WOSPPy
Pyrrole: NaNO2
Size of PPy nanospheres
1: 1: 1: 1: 1:
45–50 nm 90–120 nm 150–200 nm 300–350 nm Spheres + rods
0.67 1.33 1.66 2.67 2.67
XRD analysis 2h (°)
d-spacing (Å)
21.58 21.84 22.17 22.35 23.52
4.11 4.06 4.01 3.97 3.78
Molecular weight and monomer units
Conductivity (in 103 S/cm)
1066.5; 1143.4; 1177.6; 1350.7; –
0.10 0.24 0.31 0.85 –
16 17 17 20
units units units units
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Fig. 3. SEM images of WSPPy 1 (A and B); WSPPy 2 (C and D); WSPPy 3 (E and F) and WSPPy 4 (G and H) at two different magnifications.
of the counter ions (Na+) of the surfactant STS and decreases the fractional charge of the surfactant. This decrease may be attributed to the charge neutralization of the micelles caused by the enhanced condensation of counter ions (i.e. Na+ ions) at the surface which may in turn enhance the aggregation numbers of the micelles and increase the micellar size. For validating the above mechanism, we have carried out additional DLS experiments wherein different concentrations of NaNO2 added to the solution of STS (i.e. without pyrrole and HNO3). The spherical micelles become enlarged upon adding NaNO2 and the extent of increase is larger at higher concentrations due to higher aggregation numbers. Therefore, by merely
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Fig. 4. TEM images of PPy prepared in presence of the surfactant (WSPPy) at two different magnifications.
Scheme 2. Influence of the concentration of NaNO2 on the diameter of the micelles.
Fig. 5. EDAX patterns of WSPPy 1, WSPPy 2 and WSPPy 4 (the EDAX spectrum for WSPPy 3 is almost similar to that of WSPPy 2).
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Fig. 6. The DLS data for WSPPy 1 to WSPPy 4 in acetonitrile.
altering the concentration of NaNO2, the change in micellar size occurs which eventually forms different sized PPy spheres. The experimental details for the corresponding DLS studies are provided in the Supporting Information (Fig. S3, SI).
3.2.5. Matrix assisted laser desorption ionization (MALDI) studies A positive mode MALDI TOF spectrum is recorded in the m/z values ranging from 500 to 2000. Fig. 8 depicts the MALDI TOF spectrum of WSPPy 2 and WSPPy 4 for different composition ratios of pyrrole: NaNO2 wherein a variety of peaks is noticed from the fragmentation of PPy. The maximum peak intensities observed in the range of m/z = 850–870 are attributed to the higher ionization of PPy in this range. The PPy solution prepared in DMSO yields peaks with m/z values ranging from 1100 to 1400 denoting the existence of 15–20 monomer units in PPy. From the MALDI TOF spectra, it is observed that the concentration of NaNO2 dictates the molecular weight of PPy. The higher concentrations of NaNO2 enable a higher degree of polymerization yielding the formation of PPy with enhanced molecular weight. Further, the experimental strategy adopted here is capable of forming different monomer units viz hexadecamers (16 units), heptadecamers (17 units) and eicosamers
Fig. 7. DLS measurements demonstrating the effect of NaNO2 on the pyrrole encapsulated STS micelles in presence of HNO3. The measurements were carried out with 5 lL addition of NaNO2 from 0.05 M (black), 0.1 M (red), 0.125 M (blue) and 0.2 M (green) respectively to the reaction mixture. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. The MALDI TOF mass spectra of WSPPy 2 and WSPPy 4.
(20 units), thus highlighting the novelty of this procedure. For heptadecamers and eicosamers, the MALDI TOF spectra are shown in Fig. 8 while the spectra for the other samples are provided in the Supporting Information (Fig. S4, SI). Although the molecular weight is 103 at present, it can be increased further by suitable factorial design of experiments. The non-occurrence of larger molecular weights may be ascribed to the nitrosation of pyrrole. The oxidation of pyrrole monomers initially yields radical cations which undergo coupling, resulting in dimers. This is followed by re-oxidation, re-coupling and removal of H+ ions, forming PPy. In the propagation step, there might be a chance of nitrosation of oligomers or polymers which inhibits further polymerization reaction. This interpretation is consistent with the FTIR spectral data wherein the absorption band at 2200 cm1 is attributed to N-nitroso group, in conformity with an earlier study [41]. Interestingly, the band corresponding to N-nitroso group in the FTIR spectrum disappears while passing from WSPPy 1 to WSPPy 4, implying that the molecular weight is less influenced by N-nitrosation, with a higher molecular weight for WSPPy 4. In this context, we may mention the synthesis of polyaniline using H2O2 in presence of NaCl/HCl system yielding sixteen monomer units [42].
3.2.6. UV–Visible spectra of PPy While the foregoing characterizations imply that the properties of PPy synthesized with and without STS are not significantly different, the solubility analysis provides an entirely unexpected behavior. PPy synthesized in the absence of STS (WOSPPy) is insoluble in various solvents, while the PPy prepared in the presence of STS (WSPPy as well as WSPPy 1 to WSPPy 4) dissolves in DMSO (dimethyl sulphoxide), DMF (dimethyl formamide), DMA (dimethyl acetamide), m-cresol, etc. The UV–Vis spectra of PPy in DMSO solution is shown in Fig. 9 wherein an intense peak with kmax = 331 nm is observed along with a broad absorption band at 540 nm. The band at kmax = 331 nm is due to strong p ? p⁄ transitions [43] while the broad band at 540 nm is from the highest energy polaron transitions [44]. From Fig. 9, it is seen that the absorption peak for the p ? p⁄ transition as well as that for the polaron–bipolaron transition increases with the concentration of NaNO2. A higher concentration of NaNO2 enhances the chain length and the doping level, thereby altering the peak intensity at the high and low energy transitions. The extent of doping can be further increased with higher concentrations of HNO3 since the latter causes more entrapment of anions into the polymer matrix. Hence the intensity of the low energy absorption band corresponding to the polaron–bipolaron transition increases as noticed from Fig. 7 (WSPPy 2A). The insolubility of WOSPPy can also be interpreted in the following manner: From the XRD pattern of PPy, it is inferred that the 2h value is larger for WOSPPy. The higher 2h value indicates lower inter-planar distances for WOSPPy and in the case of WSPPy, the lower 2h values being associated with higher inter-planar distances,
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Fig. 9. The UV–Vis spectra of PPy in DMSO. The spectrum for WSPPy 2(A) refers to WSPPy 2 prepared using 0.054 M concentration of HNO3.
the solvent molecules can easily enter into the lattice, thus facilitating the solubility. In the case of WOSPPy, the lower interplanar distances hinder this movement of the solvent molecules into the lattice thus making PPy insoluble. 3.2.7. Thermogravimetric analysis (TGA) The TGA indicates that WSPPy has a slightly higher thermal stability than the WOSPPy and among various PPy synthesized using STS with different concentration of NaNO2, the maximum weight loss is observed for WSPPy 1. Additional TGA data have been provided in the Supporting Information (Fig. S5 of SI). 3.2.8. Conductivity measurements The conductivity of the PPy nanomaterials was estimated using the four probe measurement with compressed pellets. All the PPy samples exhibit conductivities in the range of 103 S cm1 (Fig. S6, SI). Depending upon the concentration of NaNO2, the conductivity varies emphasizing the influence of different chain lengths of PPy as well as varying doping levels of NO3 ions. In this study, the highest conductivity of the PPy occurs when the concentration of HNO3 is increased and this behavior is due to enhanced doping of counterions. The conductivity of PPy increases by an order of magnitude (1.27 103 S/cm) if the concentration of HNO3 is three times larger than that for the WSPPy 2 sample. For the other PPy materials too, the conductivities can be increased at higher concentrations of HNO3. The conductivities of PPy spheres synthesized here are lower than other studies [16,25]. The decrease in conductivity herein is ascribed to the lower extent of doping of nitrate ions and is interpreted on the basis of (i) the dopant concentration and (ii) the reaction time. The presence of HNO3 and NaNO2 leads to the in situ formation of NO2 which lowers the concentration of NO3 ions and hence their doping into the polymer matrix. This may result in the lower conductivity of PPy. However the conductivity can be moderately enhanced by altering the concentrations of NaNO2 and HNO3 as shown in Table 1. The lower conductivity of PPy may also be due to the lower reaction time. Since the synthetic protocol developed here is rapid, the diffusion of counter anions into the matrix is less facile. While the optimization of the reaction conditions may further enhance the conductivity, a trade-off with the time of polymerization and morphological structures becomes inevitable and further studies are required. Table 1 provides the different physiochemical properties of various PPy samples. 3.3. Selective detection of metal ions using PPy solution In order to study the spectral characteristics of PPy and to analyze its efficacy for quantitative estimation of metal ions colorimetrically, 2 mg of WSPPy 4 is dissolved in 50 mL DMSO and is diluted with an equal amount of water. The UV–Vis absorption spectrum for the corresponding PPy solution in 1:1 DMSO and water is provided in Supporting Information (Fig. S8, SI). The addition of different metal ions to the PPy solution leads to interesting color changes thus indicating the feasibility of a colorimetric detection. In the presence of a trace amount (2 lM) of Cu2+ and Hg2+ ions, the color of the PPy solution changes from light pink to yellow1 and green respectively as shown in Fig. 10. The other bi-valent metal ions viz. Sn2+, Ni2+, Co2+, Cd2+ and Pb2+ do not show any color changes after addition to the PPy solution. Hence this PPy solution can be employed for the selective detection of Cu2+ and Hg2+ ions at micromolar concentrations. Furthermore, UV–Vis spectroscopy enables a quantitative estimation. Although all the WSPPy samples viz. WSPPy (1–4) enable the detection, the estimation of Cu2+ and Hg2+ is carried out here using the higher molecular weight sample i.e. (WSPPy 4) for improving the extent of detection. 1
For interpretation of color in Fig. 10, the reader is referred to the web version of this article.
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Fig. 10. Influence of different metal ions upon addition to the PPy solution.
Fig. 11. Effect of addition of Cu2+ and Hg2+ ions to the PPy solution: (A) and (C) UV–Vis spectrum for Cu2+ and Hg2+ respectively. (B) and (D) changes in absorption intensity at 272 nm (Red) and 380 nm (Black) and 252 nm (Red) and 331 nm (Black) for Cu2+ and Hg2+ respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The UV–Vis absorption spectrum of PPy upon the successive addition of CuSO4 is shown in Fig. 11(A). Upon addition of Cu2+, the absorption band at 380 nm decreases while a high energy absorption band at 272 nm arises. Simultaneously, the color of the solution changes from light pink to yellowish green. A well-defined isosbestic point is observed at 300 nm indicating the complex formation with PPy. The change in the absorption spectrum at 270 nm and 372 nm with the addition of
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Cu2+ ions is shown in the calibration graph of Fig. 11(B). The band located at 380 nm indicates the decrease in the absorption intensity while the band centered at 272 nm shows an increase with successive additions of Cu2+ ions as depicted in Fig. 11 (B). The absorption band at 272 nm reaches a plateau after 600 lM concentration of Cu2+ has been added. In an analogous manner, the detection of Hg2+ ions using PPy solution is investigated (Fig. 11(C) and (D)) since the addition of Hg2+ ions also leads to a color change of the PPy solution. The UV–Vis spectrum of PPy in DMSO-H2O (1:1) upon the gradual addition of Hg2+ ions is depicted in Fig. 11(C). The addition of Hg2+ to the PPy solution decreases the absorption band intensity at 331 nm while another high energy band at 252 nm is also noticed. The appearance of a well-defined isosbestic point at 270 nm suggests the formation of a new complex. Furthermore, the change in the absorption band at 331 nm and 252 nm with the successive additions of Hg2+ ions is employed in constructing the calibration plot of Fig. 11 (D). The absorption band centered at 252 nm reaches a plateau after 150 lM concentration of Hg2+ has been added. A blue shift in the absorption band is observed upon the addition of these ions. This behavior has its origin in the binding of metal ions with the soft N-center of PPy which decreases the charge transfer transition thereby causing the blue shift. The selective detection of Hg2+ and Cu2+ in the presence of other bivalent ions using (i) colorimetric and (ii) UV–Vis spectral analysis requires further investigations. From the colorimetric method, it is inferred that other ions such as Zn2+, Ni2+, Co2+, Sn2+, Pb2+ do not interfere during the detection of Cu2+ and Hg2+. The color change of the PPy solution upon the addition of Hg2+ ions (from light pink to green) remains unaffected in presence of Zn2+, Ni2+, Co2+, Sn2+ and Pb2+. Analogously, the addition of Cu2+ ions to the PPy solution leads to a color change (from light pink to yellowish green) which is not altered due to the presence of Zn2+, Ni2+, Co2+, Sn2+ and Pb2+. Hence the present method is capable of selectively detecting Cu2+ and Hg2+ colorimetrically in presence of a few other bivalent metal ions. In the case of UV–Visible spectral method of detection of Cu2+ and Hg2+, the absorption bands appear in the same region (as shown in Fig. 12(A)) even in the presence of Zn2+, Ni2+, Co2+, Sn2+ and Pb2+. However, the intensities get slightly altered and thus, the colorimetric method enables selective detection. The response of different metal ions towards the PPy solution is shown in Fig. 12(B). The detection of Hg2+ and Cu2+ ions is a frontier area of research in analytical chemistry and diverse colorimetric [45], fluorimetric [46] and electrochemical [47] techniques exist. What distinguishes the present study is the remarkable ability of the PPy solution to accomplish a qualitative detection and quantitative estimation. The limit of detection (LOD) for Cu2+ and Hg2+ ions was estimated respectively as 25 lM and 5 lM from UV–Visible spectral data. Although the limit of detection (LOD) is not satisfactory at present, the effectiveness of a conducting polymer solution for selective detection of ions is demonstrated here for the first time. The possible mode of binding of these metal ions with the PPy is represented in Scheme 3. While the PPy solution has the ability to selectively detect Cu2+ and Hg2+ ions colorimetrically, it is not capable of detecting other bivalent metal ions (Cd2+, Co2+, Ni2+, Sn2+ and Pb2+). Obviously, the color change of the PPy solution upon the addition of metal ions occurs due to their complex formation by binding with the soft center of PPy (here N-atom) via the soft-soft interaction. Although the colorimetric detection of these metal ions (especially Cu2+ and Hg2+) using substituted pyrrole or pyrrole based Schiff bases has been reported earlier [48,49], the ability of the PPy solution towards the selective colorimetric detection is hitherto-unknown. The complexation usually occurs by the ligand to metal charge transfer. Therefore the partial reduction of metal ions leads to their complex formation with PPy. Hence one anticipates that the inherent ability of M2+ (M = Cu, Hg) involved in electron transfer should be invoked in the selective detection through their respective standard reduction potentials. In the electrochemical series, the standard reduction potentials (E0) of Cu2+ and Hg2+ are more positive than those of others (Cd2+, Co2+, Ni2+, Sn2+ and Pb2+). Thus, the electron transfer from PPy to these two ions is more feasible. For other metal ions, since their standard reduction potentials are negative, their complexation is probably hindered due to unfavourable electron transfer from PPy to these ions. A selective detection of Cu2+ and Hg2+ ions using pyrrole-based Schiff bases has recently been reported and is ascribed to the large binding constants for the complexation process [48]. Table 2 provides a comparison of typical synthetic methodology of PPy.
4. Perspectives and conclusions The chemical synthesis of conducting polymers in general involves oxidizing agents such as FeCl3, and APS in acidic medium [25,50]. In these cases, the synthesis requires a long time duration ranging from 6 to 20 h in general. In contrast, the methodology proposed here for the synthesis of PPy involves the use of NaNO2 in the presence of HNO3 and the polymerization is rapid, requiring less than one minute. Due to the reaction of NaNO2 and HNO3, the in situ formation of NO2 occurs; the latter functions as an oxidizing agent and as a catalyst in this context. Since the chemical polymerization of pyrrole is a sluggish reaction, diverse attempts have been made to accelerate the rate of polymerization, employing various initiators and oxidizing agents [51]. However, we have accomplished the rapid synthesis of PPy without employing any explicit oxidizing agents or initiators. Further, this synthetic protocol also utilizes a catalytic amount of NaNO2 in contrast to the other methodologies. The present synthetic strategy may also be applicable for polymerization of various types of monomers. A hitherto-unknown anionic surfactant (STS) plays a central role in inducing the solubility of PPy in different non-aqueous solvents apart from forming uniformly distributed PPy nanospheres with controllable sizes and monomer units. It is of interest to point out here a rapid synthesis of PPy by Liao et al. wherein the time of polymerization was 30 s [51], in contrast to 60 s herein. On account of the absence of any initiators in this study, our synthesis has consumed a longer time than the previous study [51].
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Fig. 12. (A) UV–Vis spectra of the PPy solution (DMSO/H2O) in the presence of different metal ions along with Hg2+ and Cu2+ ions. (B) Response of various metal ions upon addition to PPy solution as deduced from UV–Vis absorption data.
Scheme 3. The possible modes of binding of Cu2+ and Hg2+ with PPy.
Table 2 Typical synthetic procedures for PPy and their physicochemical properties. Sr. no.
Composition of the polymerization medium
Reaction time (min)
Shapes/sizes
Conductivity (S/cm)
Reference
1 2 3 4 5
Pyrrole + APS(or FeCl3) + azobenzenesulphonic acid Pyrrole + PVC + FeCl3 Pyrrole + FeCl3 + HCl + 2,4-diaminodiphenylamine Pyrrole + K2S2O8 Pyrrole + NaNO2 + HNO3 + STS
480 120 1 20 1
Spheres Spheres Spheres Spheres Spheres
103 to 101 101 to 10 104 – 103
[25] [16] [51] [52] This work
(150–400 nm) (20–100 nm) (80–300 nm) (80 nm) (45–350 nm)
The chemical synthesis of conducting polymers using the emulsion polymerization [25] involves alterations in the surfactant concentrations for obtaining different physicochemical properties. In the present study has demonstrated a facile method of controlling sizes and monomer units by merely altering the concentration of NaNO2. An interesting application of the solution of PPy in DMSO for the selective detection of Cu2+ and Hg2+ has been reported using UV–Vis absorption spectroscopy. The selective detection of these ions is accomplished by visual color changes of the PPy solution. To the best of our
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knowledge, this is not only the first time that PPy solution is used as an analytical reagent but also the simplest method of selective detection. In summary, we have reported a novel, rapid chemical synthesis of polypyrrole using sodium nitrite and nitric acid in presence of the surfactant STS. Depending upon the concentration of NaNO2, the diameters of the PPy spheres range from 50 nm to 350 nm and the number of monomer units varies from 16 to 20. The dynamic light scattering measurements demonstrate the influence of the ionic strength on the micellar size of the surfactant vis a vis the number of monomers. The XRD patterns reveal an interesting correlation between the inter-planar distance and solubility. The solution of PPy in DMSO serves as an effective analytical reagent for the quantitative estimation of Cu2+ and Hg2+ ions. The mechanism of selective detection is attributed to the facile interaction of these ions with the soft N-center of PPy. Acknowledgements We thank the reviewers for their valuable comments. We also thank Dr. Edamana Prasad, Department of Chemistry, IIT Madras for Dynamic Light Scattering measurements and Dr. S. Ravichandran, CSIR-CECRI, Karaikudi for helpful suggestions. This work was supported by the Department of Science and Technology, Government of India. Appendix A In this appendix, we indicate the possibility of removing the surfactant from the polymer matrix. The removal of the cations (here Na+ ions) is possible by repeatedly washing with water and subsequently confirming their absence from the EDAX patterns. However, the counter anions cannot be removed since they are entrapped in the polymer matrix on account of electroneutrality. In our synthetic procedure, upon treatment of WSPPy 1 with 0.05 M NaOH solution, followed by filtration and drying, the amount of the surfactant within the polymer moiety was minimized. Consequently, the weight% of S deduced from EDAX, decreased from ca 3.41 (before treatment with NaOH) to ca 0.95 (after treatment). Hence about 75% of the surfactant gets removed by this strategy. For complete removal of the surfactant, prolonged treatment with alkaline solutions is required. The EDAX spectra of the polymer before and after NaOH treatment is provided in Fig. S8 of the Supporting Information. Appendix B. Supplementary material Experimental scheme, additional FTIR and SEM images, DLS studies, additional MALDI spectrum, UV–Vis spectrum and conductivity data are provided as supplementary data. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2015.08.027. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
J.M. Pernaut, J.R. Reynolds, J. Phys. Chem. B 104 (2000) 4080–4090. K. Ramanathan, M.A. Bangar, M. Yun, W. Chen, N.V. Myung, A. Mulchandani, J. Am. Chem. Soc. 127 (2005) 496–497. R.J. Mortimer, Chem. Soc. Rev. 26 (1997) 147–156. R. Holze, Y.P. Wu, Electrochim. Acta 122 (2014) 93–107. K.S. Ryu, S.K. Jeong, J. Joo, K.M. Kim, J. Phys. Chem. B 111 (2007) 731–739. M. Ferenets, A. Harlin, Thin Solid Films 515 (2007) 5324–5328. G. Xu, B. Ding, P. Nie, L. Shen, J. Wang, X. Zhang, Chem. Eur. J. 19 (2013) 12306–12312. C. Li, H. Bai, G. Shi, Chem. Soc. Rev. 38 (2009) 2397–2409. H. Jia, J. Wang, X. Zhang, Y. Wang, ACS Macro Lett. 3 (2014) 86–90. M. Zhao, X. Wu, C. Cai, J. Phys. Chem. C 113 (2009) 4987–4996. V. Dua, S.P. Surwade, S. Ammu, X. Zhang, S. Jain, S.K. Manohar, Macromolecules 42 (2009) 5414–5415. R. Hassanien, M.A. Hinai, S.A.F. Al-Said, R. Little, L. Siller, N.G. Wright, A. Houlton, B.R. Horrocks, ACS Nano 4 (2010) 2149–2159. R.K. Pandey, V. Lakshminarayanan, J. Phys. Chem. C 114 (2010) 8507–8514. H.R. Tantawy, A.T. Weakley, D.E. Aston, J. Phys. Chem. C 118 (2014) 1294–1305. S. Mondal, M.V. Sangaranarayanan, Sens. Actuators B 177 (2013) 478–486. O.S. Kwon, J.Y. Hong, S.J. Park, Y. Jang, J. Jang, J. Phys. Chem. C 114 (2010) 18874–18879. H.K. Song, G.T.R. Palmore, Adv. Mater. 18 (2006) 1764–1768. S.S. Shinde, G.S. Gund, V.S. Kumbhar, B.H. Patil, C.D. Lokhande, Eur. Polym. J. 49 (2013) 3734–3739. S. Takagi, S. Makuta, A. Veamatahau, Y. Otsuka, Y. Tachibana, J. Mater. Chem. 22 (2012) 22181–22189. M. Acik, C. Baristiran, G. Sonmez, J. Mater. Sci. 41 (2006) 4678–4683. P. Dallas, D. Niarchos, D. Vrbanic, N. Boukos, S. Pejovnik, C. Trapalis, D. Petridis, Polymer 48 (2007) 2007–2013. S.D. Vito, C.R. Martin, Chem. Mater. 10 (1998) 1738–1741. X. Zhang, S.K. Manohar, J. Am. Chem. Soc. 126 (2004) 12714–12715. X. Zhang, W.J. Goux, S.K. Manohar, J. Am. Chem. Soc. 126 (2004) 4502–4503. M.J. Antony, M. Jayakannan, J. Phys. Chem. B 111 (2007) 12772–12780. K. Leonavicius, A. Ramanaviciene, A. Ramanavicius, Langmuir 27 (2011) 10970–10976. X. Zhang, J. Zhang, W. Song, Z. Liu, J. Phys. Chem. B 110 (2006) 1158–1165. A.D.W. Carswell, E.A. O’Rear, B.P. Grady, J. Am. Chem. Soc. 125 (2003) 14793–14800. J. Jang, H. Yoon, Langmuir 21 (2005) 11484–11489. W. Zhong, S. Liu, X. Chen, Y. Wang, W. Yang, Macromolecules 39 (2006) 3224–3230. J. Hazarika, A. Kumar, Synth. Met. 175 (2013) 155–162. J. Kopecka, D. Kopecky, M. Vrnata, P. Fitl, J. Stejskal, M. Trchov, P. Bober, Z. Moravkov, J. Prokesc, I. Sapurinad, RSC Adv. 4 (2014) 1551–1558.
S. Mondal, M.V. Sangaranarayanan / European Polymer Journal 71 (2015) 596–611 [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]
A. Wu, H. Kolla, S.K. Manohar, Macromolecules 38 (2005) 7873–7875. S.H. Lee, S. Lee, H.W. Ryu, H. Park, Y.S. Kim, J.H. Kim, J. Polym. Sci., Part A: Polym. Chem. 52 (2014) 2329–2336. H. Osato, M. Kabaki, S. Shimizu, Org. Process Res. Dev. 15 (2011) 581–584. J.C. Lacroix, R.J. Valente, F. Maurel, P.C. Lacaze, Chem. Eur. J. 4 (1998) 1667–1677. K. Majid, R. Tabassum, A.F. Shah, S. Ahmad, M.L. Singla, J. Mater Sci.-Mater Electron. 20 (2009) 958–966. S.B. Kulkarni, U.M. Patil, R.R. Salunkhe, S.S. Joshi, C.D. Lokhande, J. Alloys Compd. 509 (2011) 3486. B.D. Cullity, Elements of X-ray diffraction, Addison-Wesley Publishing Company Inc., London, 1978. R. Rudel, F. Zite-Ferenczy, J. Physiol. 290 (1979) 317–330. X.G. Li, M.R. Huang, M.F. Zhu, Y.M. Chen, Polymer 45 (2004) 385–398. S.P. Surwade, S.R. Agnihotra, V. Dua, N. Manohar, S. Jain, S. Ammu, S.K. Manohar, J. Am. Chem. Soc. 131 (2009) 12528–12529. J. Arjomandi, A.H.A. Shah, S. Bilal, H.V. Hoang, R. Holze, Spectrochim. Acta Part A 78 (2011) 1–6. G. Zotti, G. Schiavon, Synth. Met. 30 (1989) 151–158. F. Sancenon, R.M. Manez, J. Soto, Chem. Commun. 21 (2001) 2262–2263. X.L. Zhang, Y. Xiao, X.H. Qian, Angew. Chem., Int. Ed. 47 (2008) 8025–8029. P.K. Aneesh, S.R. Nambiar, T.P. Rao, A. Ajayaghosh, Phys. Chem. Chem. Phys. 16 (2014) 8529–8535. D. Udhayakumari, S. Velmathi, Spectrochim. Acta Part A 122 (2014) 428–435. M.P. Algi, Z. Oztas, F. Algi, Chem. Commun. 48 (2012) 10219–10221. Q.F. Wu, K.X. He, H.Y. Mi, X.G. Zhang, Mater. Chem. Phys. 101 (2007) 367–371. Y. Liao, X.G. Li, R.B. Kaner, ACS Nano 4 (2010) 5193–5202. S.P. Palaniappan, P. Manisankar, Mater. Chem. Phys. 122 (2010) 15–17.
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A novel, rapid synthetic protocol for controllable sizes, conductivities and monomer units of soluble polypyrrole Subrata Mondal, M.V. Sangaranarayanan ⇑ Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e
i n f o
Article history: Received 12 March 2015 Received in revised form 31 July 2015 Accepted 20 August 2015 Available online 28 August 2015 Keywords: Polymerization Polypyrrole Sodium tetradecyl sulfate Nanospheres
a b s t r a c t We report a facile synthesis of polypyrrole spheres with controllable sizes, monomer units and conductivities using sodium nitrite and nitric acid in presence of the anionic surfactant sodium tetradecyl sulfate. The rapid synthesis of polypyrrole spheres was accomplished without employing any explicit oxidizing agents. The characterization of polypyrrole (PPy) is carried out using FTIR, XRD, SEM, TEM and TGA while the molecular weight is estimated from the MALDI TOF analysis. The precise tuning of various physicochemical properties is accomplished by altering the concentrations of sodium nitrite and is attributed to the in situ generation of NO2. The size of the PPy spheres varies from 45 nm to 350 nm depending upon the concentration of NaNO2. The dynamic light scattering studies provide mechanistic insights into the role of sodium nitrite in altering the pyrroleincorporated micellar size of the surfactant, leading to different diameters of the PPy spheres. The surfactant-induced solubility of PPy is rationalized with the help of the X-ray diffraction patterns. The solution of PPy in dimethylsulfoxide has a remarkable ability of selectively detecting Cu2+ and Hg2+ among a host of other bimetallic ions, as inferred from colorimetry and UV–Visible absorption spectroscopy. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Conducting polymers having metal like conductivity and polymeric properties render themselves as potential candidates for a wide range of applications in sensors [1,2], electrochromic devices [3], batteries [4], supercapacitors [5], optical devices [6], energy storage [7], etc. Conducting polymers are insulators in their neutral states and can be converted into conductive states by doping [8] with conductivities spanning a range of 103 to 104 S cm1. This doping can be achieved by chemical or electrochemical oxidations, wherein the positive charges are acquired by the polymer backbone and to maintain electroneutrality, the oppositely charged counter anions enter the polymer matrix. This leads to impressive alterations in optical as well as physico-chemical properties. Several conducting polymers viz polypyrrole (PPy), polyaniline, polythiophene, polyindole, polyethylenedioxy thiophene have been widely employed during the past few decades [9–14]. Among these conducting polymers, PPy has attracted great interest due to its ease of preparation, wide conductivity range (102 to 104 S cm1), satisfactory environmental stability and biocompatibility. PPy has been employed in sensors [15,16], batteries [17], supercapacitors [18], electrochromic devices [19], etc. In order to enhance the performance in various applications, PPy nanostructures in conjunction with simpler synthetic strategies are required. PPy nanomaterials provide large surface areas as well as
⇑ Corresponding author. E-mail address: [email protected] (M.V. Sangaranarayanan). http://dx.doi.org/10.1016/j.eurpolymj.2015.08.027 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.
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unique properties which facilitate the interaction of PPy with the species of interest, thereby rendering them as suitable materials in diverse contexts. The chemical polymerization of pyrrole can be accomplished using dispersive polymerization [20], interfacial polymerization [21,22], seeding techniques [23,24], emulsion polymerization [25,26], etc. Surfactant-assisted emulsion techniques offer facile routes for synthesizing PPy whereby interesting optical, electrical and morphological properties can be achieved in conjunction with easy removal of templates. Further, the surfactants enable tuning of the morphologies and dimensions of the polymers via micelle formation through interactions between the reactants and solvent molecules [27]. Depending upon the nature of the surfactants, the morphologies of PPy vary from nanospheres, nanotubes to nanofibers [27–29]. The nanofibers and nanospheres of PPy have been synthesized using the cationic surfactant Cetyltrimethylammonium bromide (CTAB) which influences the solubility and conductivity [28,30]. Indeed, the anionic surfactants are also interesting since they act both as surfactants and counterions. Thus, diverse morphologies of PPy can be synthesized using sodium dodecyl sulfate, dodecylbenzenesulphonic acid, azobenzenesulphonic acid, etc. [31,32]. The chemical polymerization process also offers the simplest method of tuning the conductivities by doping with strong acids such as HCl, H2SO4, HNO3 or HClO4 [33,34]. However, the formation of PPy requires in general, a time duration ranging from 6 to 12 h due to the initially sluggish oxidation of pyrrole monomers. Thus simple, robust and rapid synthetic protocols for PPy with tunable physiochemical properties are especially challenging. Here we report a novel methodology for the synthesis of polypyrrole (PPy) nanomaterials with controllable sizes, monomer units and conductivities. The synthesis is carried out with sodium nitrite in acidic medium (HNO3) in contrast to the customarily used oxidizing agents for polymerization. Since surfactants often yield interesting morphological and spectral features, PPy is synthesized using a new anionic surfactant sodium tetradecyl sulfate (STS). We demonstrate that the sizes, monomer units and conductivities of PPy can be altered by solely varying the concentration of NaNO2. Further, PPy prepared in the presence of STS is soluble in various solvents viz dimethyl sulphoxide (DMSO), dimethyl formamide (DMF), dimethyl acetamide (DMA), etc. The origin of the solubility is rationalized using the XRD data. We also establish that the PPy solution is capable of selectively detecting Cu2+ and Hg2+ ions among various bimetallic ions. For quantitative estimation of these ions, the UV–Vis spectroscopy has been employed.
2. Experimental section 2.1. Chemicals Pyrrole (SRL chemicals India) was distilled at 140 °C under nitrogen atmosphere at reduced pressure and stored in the refrigerator. Sodium nitrite (NaNO2), nitric acid (HNO3), potassium chloride (KCl), dimethyl sulphoxide (DMSO) cupric sulfate (CuSO4) and mercuric sulfate (HgSO4) (SRL chemicals India) were employed without further purification. Sodium tetradecyl sulfate [CH3 (CH2)13OSO3Na] procured from Sigma Aldrich was used as received. All experiments were performed using triply distilled water.
2.2. Synthesis of polypyrrole In a typical synthesis, 0.031 g of STS was added to 20 mL of triple distilled water in a 100 mL beaker and stirred using a mechanical stirrer for five minutes. Subsequently, 0.77 g of distilled pyrrole and 60 lL of 12 M HNO3 were added and then sonicated for 10 min in order to obtain a clear solution, followed by the drop wise addition of 20 mL of 0.25 M solution of NaNO2 with continuous stirring. The instantaneous appearance of a black precipitate due to the addition of NaNO2 indicates the formation of polypyrrole. The entire reaction was carried out at a temperature of 30 ± 1 °C. After five minutes, the precipitate was filtered using a Whatman 42 filter paper and washed repeatedly with distilled water so as to remove all the soluble oligomers and other side products. The black precipitate was then dried in an oven at 50 °C for four hours. The same procedure was also employed for synthesizing PPy in the absence of the surfactant STS. The PPy synthesized by adopting the above procedure using the aforementioned composition in presence and absence of surfactant are designated as WSPPy and WOSPPy respectively. The synthesis of PPy was carried out in the presence and absence of the anionic surfactant STS using identical concentrations of pyrrole and other chemicals viz. NaNO2, HNO3 in order to study the differences in their properties. For comprehending the effect of NaNO2, different compositions of pyrrole and NaNO2 were employed by altering the NaNO2 concentrations. Since our objective is to analyze the surfactant-induced polymerization, different PPy samples were synthesized by altering the pyrrole: NaNO2 concentration in the range of 1: 2/3, 1: 4/3, 1: 5/3, 1: 8/3 using the concentration of STS and HNO3 as 0.00125 M and 0.018 M respectively. These different compositions were made by keeping a fixed concentration of pyrrole (as 0.075 M) while NaNO2 concentrations were varied from 0.05 M to 0.20 M. However, the concentrations of STS and HNO3 were kept constant in general. The PPy prepared in presence of STS using the above-mentioned different composition ratios of pyrrole and NaNO2 are denoted as WSPPy 1, WSPPy 2, WSPPy 3 and WSPPy 4.
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2.3. Instrumentation UV–Visible absorption spectra were recorded on a Ocean Optics GmbH DT-MINI-2-GS UV–Vis spectrometer. The SEM images and energy dispersive X-ray analysis (EDAX) were acquired using FEI Quanta FEG 200. The FTIR spectra were recorded using Jasco FTIR-4100 spectrometer (resolution = 0.9 cm1) with KBr pellets, while the X-ray diffraction (XRD) analysis was carried out with Bruker D8 system equipped with Cu Ka radiation (1.5405 Å). Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer TGA7 with 20 °C rate of N2, controller from 0 °C to 900 °C and the derivative weight loss was estimated using the commercial software provided with the instrument. Matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) using the Bruker ultraflextreme was employed to estimate the molecular weight as well as the number of monomer units present in PPy. The spectrum is recorded using the PPy solution (in DMSO) in the presence of a suitable matrix. The matrix used in the MALDI analysis is a-cyanohydroxy cinnamic acid (CHCA). In order to elucidate the mechanism of synthesizing the different sized PPy and the influence of NaNO2 on the micelles, the dynamic light scattering (DLS) measurements were performed during the polymerization process using Malvern Zetasizer Nano Zs 90 instrument. The DLS study also enables the estimation of the hydrodynamic radius (Dn) of various PPy samples. The conductivities of the different PPy samples were measured from the Nyquist plot with the help of a multi-channel potentiostat after making suitable pellets. All the characterization studies were carried out at a temperature of 30 ± 1 °C. 3. Results and discussion 3.1. Reaction mechanism In the presence of HNO3, sodium nitrite initially forms nitrous acid (HNO2) which upon further reaction with the former forms N2O4 which dissociates to form NO2. The addition of NaNO2 to the solution of pyrrole in presence of acids yields initially a reddish brown color and then a black precipitate. The initial reddish brown color is due to the formation of soluble oligomers while the appearance of the black precipitate indicates the formation of PPy. The in situ formation of HNO2 and NO2 has been reported earlier [35]. NO2 acts as an oxidizing agent for the polymerization and possesses excellent catalytic activity in this context. Consequently it enhances the rate of formation of the pyrrole radical cations. The coupling of the two pyrrole radical cations and loss of two hydrogen ions generates bipyrrole. In the propagation step, further re-oxidation, coupling and deprotonation processes continue thereby yielding polypyrrole. On account of the in situ formation of NO2, the synthesis of PPy is rapid in contrast to the other methods employing ferric chloride (FeCl3), ammonium persulfate (APS), etc. It is of interest to point out that the mechanism involved in the coupling of pyrrole radical cations has been investigated earlier using computational approaches too [36]. A plausible mechanism for the formation of PPy is provided in Scheme 1. It may be emphasized that a facile rapid chemical synthesis of PPy is a challenging task. While the oxidative polymerization can be accelerated with FeCl3 by an optimal choice of the reaction conditions, the synthetic methodology proposed here demonstrates the use of a small catalytic amount of NaNO2 at room temperature. The acidic medium not only accomplishes effective doping, but also facilitates the in situ generation of NO2. The combination of NaNO2 and HNO3 leads to the formation of NO2 which enhances the rate of polymerization and subsequently enables the incorporation of the counter anions into the polymer matrix. The choice of HNO3 is deliberate since anions play an important role in dictating the size of the PPy, lower 2 sized NO 3 being more preferable than ClO4 , SO4 , camphor sulfonate anions etc due to enhanced diffusion. 3.2. Characterization of PPy 3.2.1. FTIR studies The FTIR spectra of PPy depicted in Fig. 1(A) indicate various typical absorption bands pertaining to WSPPy and WOSPPy. The broad absorption band at 3388 cm1 due to the NAH stretching vibration is noticed in both cases. In the case of WSPPy, two additional absorption bands at 2914 cm1 and 2846 cm1 arise on account of CAH stretching frequency indicating the presence of STS in the PPy sample while for WOSPPy, these two peaks are absent. The sharp absorption band at 1564 cm1 is attributed to the C@C stretching vibration of the pyrrole ring. The CAN stretching vibration band is observed at 1417 cm1. The bands at 950 cm1 and 750 cm1 are due to CAH out of plane ring deformation vibrations. These absorption bands are consistent with other FTIR studies of polypyrrole [37]. The FTIR spectra of different WSPPy samples (i.e. WSPPy 1 to WSPPy 4) are provided in Fig. 1(B) wherein all the samples exhibit absorption bands in their corresponding regions as mentioned above. Since the band intensities increase from WSPPy 1 to WSPPy 4, the extent of the polymerization is higher. This change in band intensities may either reflect the variation in the number of monomer units or the incorporation of counter anions into the polymer moiety [38]. For all samples, the absorption bands from the ACH stretching vibration appearing in the range around 2900 cm1 are attributed to the presence of the surfactant STS. The absorption bands at 1575 cm1 and 1435 cm1 are due to the C@C and CAN stretching vibration respectively which are more intense for WSPPy 4 indicating the larger number of monomer units than others (WSPPy 1 to WSPPy 3). A plausible interpretation for this observation is provided in the Supporting Information (Fig. S1, SI). This behavior is consistent with the conductivity data as well as MALDI studies (vide infra).
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Scheme 1. Mechanism of polymerization of pyrrole in presence of NaNO2.
Fig. 1. FTIR spectra of PPy for (A) WSPPy and WOSPPy and (B) WSPPy 1 to WSPPy 4.
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3.2.2. X-ray diffraction analysis The XRD patterns of WSPPy and WOSPPy are depicted in Fig. 2(A) wherefrom it is noticed that the surfactant does not influence the crystallinity. Both samples are essentially amorphous with the appearance of broad, intense peaks at 2h = 21.78° and 23.52° respectively, for WSPPy and WOSPPy. The higher 2h value for WOSPPy indicates the lower d spacing while the particle sizes calculated from the Debye-Scherer equation [39] are larger than for WSPPy. In both cases, the less intense peaks observed at 9.24° and 9.98° are attributed to the low molecular weight oligomers. From the Bragg’s equation [40], the d-spacing between the crystal planes is estimated from the equation
d ¼ nk=2 sin h where n is the integer determined by the order given (here n = 1), k denotes the wavelength of the Cu Ka radiation source (k = 1.5409 Å), h being the angle between the scattering planes and the incident ray. The interplanar spacing values are (i) 4.07 Å and 9.56 Å for WSPPy with respective 2h values being 21.78° and 9.24° and (ii) 3.77 Å and 8.89 Å for WOSPPy with the 2h values being 23.52° and 9.98°. The XRD patterns of PPy synthesized in the presence of STS with different composition ratios (of pyrrole and NaNO2) are shown in Fig. 2(B). From these XRD patterns, we note that the location of the peak is influenced by the concentration of NaNO2. The 2h values shifted towards higher angles in conjunction with the decrease in peak intensity when the concentration of NaNO2 increases. The increase in 2h values reflects the decrease in d-spacing implying that the extent of doping depends on the concentration of NaNO2. The d-spacing values for WSPPy N calculated from the Bragg’s equation are shown in Table 1. From these XRD patterns, we can also estimate the size of PPy spheres; since the 2h values shifted towards higher angles, the mean sizes calculated from the Debye-Scherer equation increases with the concentration of NaNO2 and are consistent with those deduced from the SEM images (vide infra). 3.2.3. Electron microscopic studies Fig. 3 depicts the SEM images of PPy prepared in the presence of STS with different compositions of pyrrole and NaNO2 wherein spheres of various sizes are noticed along with uniform dispersion. The diameters of these spheres vary from 45 to 350 nm. In the absence of STS, a random arrangement of rods and spheres is noticed (Fig. S2, SI). This indicates that the surfactant (STS) induces the formation of spheres in an orderly manner. The formation of such ordered PPy spheres can be comprehended as follows. In aqueous medium, the anionic surfactant STS forms spherical micelle via hydrophilichydrophobic interactions since its concentration is higher than the critical micellar concentration (CMC). The pyrrole monomers are incorporated into the micelle cores. The addition of NaNO2 in presence of HNO3 forms NO2 which initiates the radical polymerization. The in situ formation of NO2 oxidizes the monomer in the micelles and it readily polymerizes via the coupling and de-protonation process as shown in Scheme 1. Hence pyrrole monomers incorporated within the STS micelles have turned into the oligomers in the micelles and the color of the solution changes from colorless to reddish brown. Further, the monomers present in the droplet diffuse to the growing particle and finally these oligomers get converted into PPy and the color of the solution becomes black as anticipated. Due to agglomeration, a few- PPy nanospheres form chain-like morphologies as observed from the SEM images. From the TEM images depicted in Fig. 4, the spherical morphology of PPy is further confirmed. Further evidence for this mechanism is provided by the DLS studies. It is noticed from Fig. 3 that the sizes of the PPy spheres increase with the concentration of NaNO2. Although the uniform distribution of PPy spheres is inferred from the SEM images, inhomogeneous sizes are observed. This anisotropic size distribution of PPy may arise from the rapid polymerization of pyrrole in presence of NaNO2 and HNO3. In this context, we speculate that the oxidative growth of PPy spheres from monomers to oligomers into the micelle core of STS is quite rapid and this causes the non-uniform size distribution. Although the diameters of these spheres are not uniform, we can estimate their mean values. This variation can be interpreted in the following manner. At higher concentrations of NaNO2, the diameter of the spherical micelles of STS gets enhanced on account of the increase in ionic strength of the solution (larger concentration of Na+ ions). Hence the droplet sizes of the monomers increase whereby larger numbers of these get incorporated into the micelles core leading to a more facile polymerization vis a vis formation of larger sized PPy spheres. With increase in the concentration of NaNO2, the in situ formation of NO2 is also enhanced causing efficient diffusion of the pyrrole monomers into the growing particles viz. oligomers. Thus, the larger concentrations of NaNO2 facilitate the in situ formation of NO2 to a higher extent resulting in the efficient degree of polymerization. The variation in the diameter of the micelles core with concentration of NaNO2 is depicted in Scheme 2. Indeed, the quantification of the surfactant present in PPy is essential in order to obtain additional insights and while the exact amount is difficult to measure, an approximate value is deducible from the EDAX spectrum. However, this value is not accurate since the elements oxygen and carbon in STS also exist in NaNO2 and PPy respectively. Consequently, the presence of S in the EDAX spectrum is an indication of the quantity of the surfactant. Since ca 2–3% ‘S’ is present as deduced from EDAX data (Fig. 5), this value is an approximate estimate of surfactant. An interesting observation deserves mention in this context viz the quantity of the surfactant varies for different PPy samples. The weight% of STS decreases from WSPPy 1 to WSPPy 4 and is attributed to a higher extent of doping of NO 3 ions than the bulkier counter anions of STS. In order to investigate whether the surfactant can be entirely removed from the polymer moiety, additional experiments were carried out, as described in Appendix A.
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Fig. 2. The X-ray diffraction patterns of PPy: (A) WSPPy and WOSPPy and (B) WSPPy 1 to WSPPy 4. The inset of Fig. 2(B) depicts the shifts in 2h values.
3.2.4. Dynamic light scattering (DLS) studies The diameters of PPy spheres may also be correlated with those obtained from the DLS studies. For this purpose, we have carried out DLS studies (Fig. 6) of WSPPy 1 to WSPPy 4 in acetonitrile (wherein PPy is sparingly soluble). The diameters follow the trend as: WSPPy 1 (85 nm) < WSPPy 2 (175 nm) < WSPPy 3 (254 nm) < WSPPy 4 (347 nm). While the interpretation from DLS analysis is qualitatively consistent with the inference from the SEM images, the estimates from the DLS are higher due to solvation effects. The PPy molecules are solvated in the solution phase and hence the diameters are larger; furthermore, since the influence of solvation is higher for lower sized PPy spheres, the dimensions estimated from DLS and SEM are significantly at variance. On the other hand, for larger sized PPy spheres, the values estimated from DLS data and SEM images are nearly identical. Further, the DLS studies provide more insights into the formation mechanism of different sized PPy spheres. The average diameter of the pyrrole-incorporated STS micelles is ca. 7.2 nm. The growth of the polymer was studied during the polymerization at different concentrations of NaNO2. Since the synthetic protocol is rapid with only a catalytic amount of NaNO2 being required for polymerization, a very small volume of NaNO2 (5 lL) was added to the reaction mixture so as to avoid the complete polymerization (formation of the black precipitate). As the polymerization progresses, the diameters of the micelles-incorporated PPy spheres increase which are monitored for each concentration of NaNO2. In the presence of HNO3, the average diameter of micelles increases with concentration of NaNO2 as shown in Fig. 7. This behavior can be comprehended by invoking a plausible assumption. The addition of NaNO2 in presence of HNO3 forms NO2 as well as free Na+ ions wherein NO2 induces the polymerization process while Na+ ions interact with the hydrophilic part of the micelles. In the presence of HNO3, the higher concentration of NaNO2 leads to the formation of NO2 and the release of the Na+ ions occurs to a larger extent as visualised in Scheme 1. This increase in the concentration of Na+ leads to a higher concentration
Table 1 The physicochemical properties of different PPy samples. Sample
WSPPy 1 WSPPy 2 WSPPy 3 WSPPy 4 WOSPPy
Pyrrole: NaNO2
Size of PPy nanospheres
1: 1: 1: 1: 1:
45–50 nm 90–120 nm 150–200 nm 300–350 nm Spheres + rods
0.67 1.33 1.66 2.67 2.67
XRD analysis 2h (°)
d-spacing (Å)
21.58 21.84 22.17 22.35 23.52
4.11 4.06 4.01 3.97 3.78
Molecular weight and monomer units
Conductivity (in 103 S/cm)
1066.5; 1143.4; 1177.6; 1350.7; –
0.10 0.24 0.31 0.85 –
16 17 17 20
units units units units
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Fig. 3. SEM images of WSPPy 1 (A and B); WSPPy 2 (C and D); WSPPy 3 (E and F) and WSPPy 4 (G and H) at two different magnifications.
of the counter ions (Na+) of the surfactant STS and decreases the fractional charge of the surfactant. This decrease may be attributed to the charge neutralization of the micelles caused by the enhanced condensation of counter ions (i.e. Na+ ions) at the surface which may in turn enhance the aggregation numbers of the micelles and increase the micellar size. For validating the above mechanism, we have carried out additional DLS experiments wherein different concentrations of NaNO2 added to the solution of STS (i.e. without pyrrole and HNO3). The spherical micelles become enlarged upon adding NaNO2 and the extent of increase is larger at higher concentrations due to higher aggregation numbers. Therefore, by merely
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Fig. 4. TEM images of PPy prepared in presence of the surfactant (WSPPy) at two different magnifications.
Scheme 2. Influence of the concentration of NaNO2 on the diameter of the micelles.
Fig. 5. EDAX patterns of WSPPy 1, WSPPy 2 and WSPPy 4 (the EDAX spectrum for WSPPy 3 is almost similar to that of WSPPy 2).
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Fig. 6. The DLS data for WSPPy 1 to WSPPy 4 in acetonitrile.
altering the concentration of NaNO2, the change in micellar size occurs which eventually forms different sized PPy spheres. The experimental details for the corresponding DLS studies are provided in the Supporting Information (Fig. S3, SI).
3.2.5. Matrix assisted laser desorption ionization (MALDI) studies A positive mode MALDI TOF spectrum is recorded in the m/z values ranging from 500 to 2000. Fig. 8 depicts the MALDI TOF spectrum of WSPPy 2 and WSPPy 4 for different composition ratios of pyrrole: NaNO2 wherein a variety of peaks is noticed from the fragmentation of PPy. The maximum peak intensities observed in the range of m/z = 850–870 are attributed to the higher ionization of PPy in this range. The PPy solution prepared in DMSO yields peaks with m/z values ranging from 1100 to 1400 denoting the existence of 15–20 monomer units in PPy. From the MALDI TOF spectra, it is observed that the concentration of NaNO2 dictates the molecular weight of PPy. The higher concentrations of NaNO2 enable a higher degree of polymerization yielding the formation of PPy with enhanced molecular weight. Further, the experimental strategy adopted here is capable of forming different monomer units viz hexadecamers (16 units), heptadecamers (17 units) and eicosamers
Fig. 7. DLS measurements demonstrating the effect of NaNO2 on the pyrrole encapsulated STS micelles in presence of HNO3. The measurements were carried out with 5 lL addition of NaNO2 from 0.05 M (black), 0.1 M (red), 0.125 M (blue) and 0.2 M (green) respectively to the reaction mixture. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. The MALDI TOF mass spectra of WSPPy 2 and WSPPy 4.
(20 units), thus highlighting the novelty of this procedure. For heptadecamers and eicosamers, the MALDI TOF spectra are shown in Fig. 8 while the spectra for the other samples are provided in the Supporting Information (Fig. S4, SI). Although the molecular weight is 103 at present, it can be increased further by suitable factorial design of experiments. The non-occurrence of larger molecular weights may be ascribed to the nitrosation of pyrrole. The oxidation of pyrrole monomers initially yields radical cations which undergo coupling, resulting in dimers. This is followed by re-oxidation, re-coupling and removal of H+ ions, forming PPy. In the propagation step, there might be a chance of nitrosation of oligomers or polymers which inhibits further polymerization reaction. This interpretation is consistent with the FTIR spectral data wherein the absorption band at 2200 cm1 is attributed to N-nitroso group, in conformity with an earlier study [41]. Interestingly, the band corresponding to N-nitroso group in the FTIR spectrum disappears while passing from WSPPy 1 to WSPPy 4, implying that the molecular weight is less influenced by N-nitrosation, with a higher molecular weight for WSPPy 4. In this context, we may mention the synthesis of polyaniline using H2O2 in presence of NaCl/HCl system yielding sixteen monomer units [42].
3.2.6. UV–Visible spectra of PPy While the foregoing characterizations imply that the properties of PPy synthesized with and without STS are not significantly different, the solubility analysis provides an entirely unexpected behavior. PPy synthesized in the absence of STS (WOSPPy) is insoluble in various solvents, while the PPy prepared in the presence of STS (WSPPy as well as WSPPy 1 to WSPPy 4) dissolves in DMSO (dimethyl sulphoxide), DMF (dimethyl formamide), DMA (dimethyl acetamide), m-cresol, etc. The UV–Vis spectra of PPy in DMSO solution is shown in Fig. 9 wherein an intense peak with kmax = 331 nm is observed along with a broad absorption band at 540 nm. The band at kmax = 331 nm is due to strong p ? p⁄ transitions [43] while the broad band at 540 nm is from the highest energy polaron transitions [44]. From Fig. 9, it is seen that the absorption peak for the p ? p⁄ transition as well as that for the polaron–bipolaron transition increases with the concentration of NaNO2. A higher concentration of NaNO2 enhances the chain length and the doping level, thereby altering the peak intensity at the high and low energy transitions. The extent of doping can be further increased with higher concentrations of HNO3 since the latter causes more entrapment of anions into the polymer matrix. Hence the intensity of the low energy absorption band corresponding to the polaron–bipolaron transition increases as noticed from Fig. 7 (WSPPy 2A). The insolubility of WOSPPy can also be interpreted in the following manner: From the XRD pattern of PPy, it is inferred that the 2h value is larger for WOSPPy. The higher 2h value indicates lower inter-planar distances for WOSPPy and in the case of WSPPy, the lower 2h values being associated with higher inter-planar distances,
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Fig. 9. The UV–Vis spectra of PPy in DMSO. The spectrum for WSPPy 2(A) refers to WSPPy 2 prepared using 0.054 M concentration of HNO3.
the solvent molecules can easily enter into the lattice, thus facilitating the solubility. In the case of WOSPPy, the lower interplanar distances hinder this movement of the solvent molecules into the lattice thus making PPy insoluble. 3.2.7. Thermogravimetric analysis (TGA) The TGA indicates that WSPPy has a slightly higher thermal stability than the WOSPPy and among various PPy synthesized using STS with different concentration of NaNO2, the maximum weight loss is observed for WSPPy 1. Additional TGA data have been provided in the Supporting Information (Fig. S5 of SI). 3.2.8. Conductivity measurements The conductivity of the PPy nanomaterials was estimated using the four probe measurement with compressed pellets. All the PPy samples exhibit conductivities in the range of 103 S cm1 (Fig. S6, SI). Depending upon the concentration of NaNO2, the conductivity varies emphasizing the influence of different chain lengths of PPy as well as varying doping levels of NO3 ions. In this study, the highest conductivity of the PPy occurs when the concentration of HNO3 is increased and this behavior is due to enhanced doping of counterions. The conductivity of PPy increases by an order of magnitude (1.27 103 S/cm) if the concentration of HNO3 is three times larger than that for the WSPPy 2 sample. For the other PPy materials too, the conductivities can be increased at higher concentrations of HNO3. The conductivities of PPy spheres synthesized here are lower than other studies [16,25]. The decrease in conductivity herein is ascribed to the lower extent of doping of nitrate ions and is interpreted on the basis of (i) the dopant concentration and (ii) the reaction time. The presence of HNO3 and NaNO2 leads to the in situ formation of NO2 which lowers the concentration of NO3 ions and hence their doping into the polymer matrix. This may result in the lower conductivity of PPy. However the conductivity can be moderately enhanced by altering the concentrations of NaNO2 and HNO3 as shown in Table 1. The lower conductivity of PPy may also be due to the lower reaction time. Since the synthetic protocol developed here is rapid, the diffusion of counter anions into the matrix is less facile. While the optimization of the reaction conditions may further enhance the conductivity, a trade-off with the time of polymerization and morphological structures becomes inevitable and further studies are required. Table 1 provides the different physiochemical properties of various PPy samples. 3.3. Selective detection of metal ions using PPy solution In order to study the spectral characteristics of PPy and to analyze its efficacy for quantitative estimation of metal ions colorimetrically, 2 mg of WSPPy 4 is dissolved in 50 mL DMSO and is diluted with an equal amount of water. The UV–Vis absorption spectrum for the corresponding PPy solution in 1:1 DMSO and water is provided in Supporting Information (Fig. S8, SI). The addition of different metal ions to the PPy solution leads to interesting color changes thus indicating the feasibility of a colorimetric detection. In the presence of a trace amount (2 lM) of Cu2+ and Hg2+ ions, the color of the PPy solution changes from light pink to yellow1 and green respectively as shown in Fig. 10. The other bi-valent metal ions viz. Sn2+, Ni2+, Co2+, Cd2+ and Pb2+ do not show any color changes after addition to the PPy solution. Hence this PPy solution can be employed for the selective detection of Cu2+ and Hg2+ ions at micromolar concentrations. Furthermore, UV–Vis spectroscopy enables a quantitative estimation. Although all the WSPPy samples viz. WSPPy (1–4) enable the detection, the estimation of Cu2+ and Hg2+ is carried out here using the higher molecular weight sample i.e. (WSPPy 4) for improving the extent of detection. 1
For interpretation of color in Fig. 10, the reader is referred to the web version of this article.
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Fig. 10. Influence of different metal ions upon addition to the PPy solution.
Fig. 11. Effect of addition of Cu2+ and Hg2+ ions to the PPy solution: (A) and (C) UV–Vis spectrum for Cu2+ and Hg2+ respectively. (B) and (D) changes in absorption intensity at 272 nm (Red) and 380 nm (Black) and 252 nm (Red) and 331 nm (Black) for Cu2+ and Hg2+ respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The UV–Vis absorption spectrum of PPy upon the successive addition of CuSO4 is shown in Fig. 11(A). Upon addition of Cu2+, the absorption band at 380 nm decreases while a high energy absorption band at 272 nm arises. Simultaneously, the color of the solution changes from light pink to yellowish green. A well-defined isosbestic point is observed at 300 nm indicating the complex formation with PPy. The change in the absorption spectrum at 270 nm and 372 nm with the addition of
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Cu2+ ions is shown in the calibration graph of Fig. 11(B). The band located at 380 nm indicates the decrease in the absorption intensity while the band centered at 272 nm shows an increase with successive additions of Cu2+ ions as depicted in Fig. 11 (B). The absorption band at 272 nm reaches a plateau after 600 lM concentration of Cu2+ has been added. In an analogous manner, the detection of Hg2+ ions using PPy solution is investigated (Fig. 11(C) and (D)) since the addition of Hg2+ ions also leads to a color change of the PPy solution. The UV–Vis spectrum of PPy in DMSO-H2O (1:1) upon the gradual addition of Hg2+ ions is depicted in Fig. 11(C). The addition of Hg2+ to the PPy solution decreases the absorption band intensity at 331 nm while another high energy band at 252 nm is also noticed. The appearance of a well-defined isosbestic point at 270 nm suggests the formation of a new complex. Furthermore, the change in the absorption band at 331 nm and 252 nm with the successive additions of Hg2+ ions is employed in constructing the calibration plot of Fig. 11 (D). The absorption band centered at 252 nm reaches a plateau after 150 lM concentration of Hg2+ has been added. A blue shift in the absorption band is observed upon the addition of these ions. This behavior has its origin in the binding of metal ions with the soft N-center of PPy which decreases the charge transfer transition thereby causing the blue shift. The selective detection of Hg2+ and Cu2+ in the presence of other bivalent ions using (i) colorimetric and (ii) UV–Vis spectral analysis requires further investigations. From the colorimetric method, it is inferred that other ions such as Zn2+, Ni2+, Co2+, Sn2+, Pb2+ do not interfere during the detection of Cu2+ and Hg2+. The color change of the PPy solution upon the addition of Hg2+ ions (from light pink to green) remains unaffected in presence of Zn2+, Ni2+, Co2+, Sn2+ and Pb2+. Analogously, the addition of Cu2+ ions to the PPy solution leads to a color change (from light pink to yellowish green) which is not altered due to the presence of Zn2+, Ni2+, Co2+, Sn2+ and Pb2+. Hence the present method is capable of selectively detecting Cu2+ and Hg2+ colorimetrically in presence of a few other bivalent metal ions. In the case of UV–Visible spectral method of detection of Cu2+ and Hg2+, the absorption bands appear in the same region (as shown in Fig. 12(A)) even in the presence of Zn2+, Ni2+, Co2+, Sn2+ and Pb2+. However, the intensities get slightly altered and thus, the colorimetric method enables selective detection. The response of different metal ions towards the PPy solution is shown in Fig. 12(B). The detection of Hg2+ and Cu2+ ions is a frontier area of research in analytical chemistry and diverse colorimetric [45], fluorimetric [46] and electrochemical [47] techniques exist. What distinguishes the present study is the remarkable ability of the PPy solution to accomplish a qualitative detection and quantitative estimation. The limit of detection (LOD) for Cu2+ and Hg2+ ions was estimated respectively as 25 lM and 5 lM from UV–Visible spectral data. Although the limit of detection (LOD) is not satisfactory at present, the effectiveness of a conducting polymer solution for selective detection of ions is demonstrated here for the first time. The possible mode of binding of these metal ions with the PPy is represented in Scheme 3. While the PPy solution has the ability to selectively detect Cu2+ and Hg2+ ions colorimetrically, it is not capable of detecting other bivalent metal ions (Cd2+, Co2+, Ni2+, Sn2+ and Pb2+). Obviously, the color change of the PPy solution upon the addition of metal ions occurs due to their complex formation by binding with the soft center of PPy (here N-atom) via the soft-soft interaction. Although the colorimetric detection of these metal ions (especially Cu2+ and Hg2+) using substituted pyrrole or pyrrole based Schiff bases has been reported earlier [48,49], the ability of the PPy solution towards the selective colorimetric detection is hitherto-unknown. The complexation usually occurs by the ligand to metal charge transfer. Therefore the partial reduction of metal ions leads to their complex formation with PPy. Hence one anticipates that the inherent ability of M2+ (M = Cu, Hg) involved in electron transfer should be invoked in the selective detection through their respective standard reduction potentials. In the electrochemical series, the standard reduction potentials (E0) of Cu2+ and Hg2+ are more positive than those of others (Cd2+, Co2+, Ni2+, Sn2+ and Pb2+). Thus, the electron transfer from PPy to these two ions is more feasible. For other metal ions, since their standard reduction potentials are negative, their complexation is probably hindered due to unfavourable electron transfer from PPy to these ions. A selective detection of Cu2+ and Hg2+ ions using pyrrole-based Schiff bases has recently been reported and is ascribed to the large binding constants for the complexation process [48]. Table 2 provides a comparison of typical synthetic methodology of PPy.
4. Perspectives and conclusions The chemical synthesis of conducting polymers in general involves oxidizing agents such as FeCl3, and APS in acidic medium [25,50]. In these cases, the synthesis requires a long time duration ranging from 6 to 20 h in general. In contrast, the methodology proposed here for the synthesis of PPy involves the use of NaNO2 in the presence of HNO3 and the polymerization is rapid, requiring less than one minute. Due to the reaction of NaNO2 and HNO3, the in situ formation of NO2 occurs; the latter functions as an oxidizing agent and as a catalyst in this context. Since the chemical polymerization of pyrrole is a sluggish reaction, diverse attempts have been made to accelerate the rate of polymerization, employing various initiators and oxidizing agents [51]. However, we have accomplished the rapid synthesis of PPy without employing any explicit oxidizing agents or initiators. Further, this synthetic protocol also utilizes a catalytic amount of NaNO2 in contrast to the other methodologies. The present synthetic strategy may also be applicable for polymerization of various types of monomers. A hitherto-unknown anionic surfactant (STS) plays a central role in inducing the solubility of PPy in different non-aqueous solvents apart from forming uniformly distributed PPy nanospheres with controllable sizes and monomer units. It is of interest to point out here a rapid synthesis of PPy by Liao et al. wherein the time of polymerization was 30 s [51], in contrast to 60 s herein. On account of the absence of any initiators in this study, our synthesis has consumed a longer time than the previous study [51].
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Fig. 12. (A) UV–Vis spectra of the PPy solution (DMSO/H2O) in the presence of different metal ions along with Hg2+ and Cu2+ ions. (B) Response of various metal ions upon addition to PPy solution as deduced from UV–Vis absorption data.
Scheme 3. The possible modes of binding of Cu2+ and Hg2+ with PPy.
Table 2 Typical synthetic procedures for PPy and their physicochemical properties. Sr. no.
Composition of the polymerization medium
Reaction time (min)
Shapes/sizes
Conductivity (S/cm)
Reference
1 2 3 4 5
Pyrrole + APS(or FeCl3) + azobenzenesulphonic acid Pyrrole + PVC + FeCl3 Pyrrole + FeCl3 + HCl + 2,4-diaminodiphenylamine Pyrrole + K2S2O8 Pyrrole + NaNO2 + HNO3 + STS
480 120 1 20 1
Spheres Spheres Spheres Spheres Spheres
103 to 101 101 to 10 104 – 103
[25] [16] [51] [52] This work
(150–400 nm) (20–100 nm) (80–300 nm) (80 nm) (45–350 nm)
The chemical synthesis of conducting polymers using the emulsion polymerization [25] involves alterations in the surfactant concentrations for obtaining different physicochemical properties. In the present study has demonstrated a facile method of controlling sizes and monomer units by merely altering the concentration of NaNO2. An interesting application of the solution of PPy in DMSO for the selective detection of Cu2+ and Hg2+ has been reported using UV–Vis absorption spectroscopy. The selective detection of these ions is accomplished by visual color changes of the PPy solution. To the best of our
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knowledge, this is not only the first time that PPy solution is used as an analytical reagent but also the simplest method of selective detection. In summary, we have reported a novel, rapid chemical synthesis of polypyrrole using sodium nitrite and nitric acid in presence of the surfactant STS. Depending upon the concentration of NaNO2, the diameters of the PPy spheres range from 50 nm to 350 nm and the number of monomer units varies from 16 to 20. The dynamic light scattering measurements demonstrate the influence of the ionic strength on the micellar size of the surfactant vis a vis the number of monomers. The XRD patterns reveal an interesting correlation between the inter-planar distance and solubility. The solution of PPy in DMSO serves as an effective analytical reagent for the quantitative estimation of Cu2+ and Hg2+ ions. The mechanism of selective detection is attributed to the facile interaction of these ions with the soft N-center of PPy. Acknowledgements We thank the reviewers for their valuable comments. We also thank Dr. Edamana Prasad, Department of Chemistry, IIT Madras for Dynamic Light Scattering measurements and Dr. S. Ravichandran, CSIR-CECRI, Karaikudi for helpful suggestions. This work was supported by the Department of Science and Technology, Government of India. Appendix A In this appendix, we indicate the possibility of removing the surfactant from the polymer matrix. The removal of the cations (here Na+ ions) is possible by repeatedly washing with water and subsequently confirming their absence from the EDAX patterns. However, the counter anions cannot be removed since they are entrapped in the polymer matrix on account of electroneutrality. In our synthetic procedure, upon treatment of WSPPy 1 with 0.05 M NaOH solution, followed by filtration and drying, the amount of the surfactant within the polymer moiety was minimized. Consequently, the weight% of S deduced from EDAX, decreased from ca 3.41 (before treatment with NaOH) to ca 0.95 (after treatment). Hence about 75% of the surfactant gets removed by this strategy. For complete removal of the surfactant, prolonged treatment with alkaline solutions is required. The EDAX spectra of the polymer before and after NaOH treatment is provided in Fig. S8 of the Supporting Information. Appendix B. Supplementary material Experimental scheme, additional FTIR and SEM images, DLS studies, additional MALDI spectrum, UV–Vis spectrum and conductivity data are provided as supplementary data. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2015.08.027. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
J.M. Pernaut, J.R. Reynolds, J. Phys. Chem. B 104 (2000) 4080–4090. K. Ramanathan, M.A. Bangar, M. Yun, W. Chen, N.V. Myung, A. Mulchandani, J. Am. Chem. Soc. 127 (2005) 496–497. R.J. Mortimer, Chem. Soc. Rev. 26 (1997) 147–156. R. Holze, Y.P. Wu, Electrochim. Acta 122 (2014) 93–107. K.S. Ryu, S.K. Jeong, J. Joo, K.M. Kim, J. Phys. Chem. B 111 (2007) 731–739. M. Ferenets, A. Harlin, Thin Solid Films 515 (2007) 5324–5328. G. Xu, B. Ding, P. Nie, L. Shen, J. Wang, X. Zhang, Chem. Eur. J. 19 (2013) 12306–12312. C. Li, H. Bai, G. Shi, Chem. Soc. Rev. 38 (2009) 2397–2409. H. Jia, J. Wang, X. Zhang, Y. Wang, ACS Macro Lett. 3 (2014) 86–90. M. Zhao, X. Wu, C. Cai, J. Phys. Chem. C 113 (2009) 4987–4996. V. Dua, S.P. Surwade, S. Ammu, X. Zhang, S. Jain, S.K. Manohar, Macromolecules 42 (2009) 5414–5415. R. Hassanien, M.A. Hinai, S.A.F. Al-Said, R. Little, L. Siller, N.G. Wright, A. Houlton, B.R. Horrocks, ACS Nano 4 (2010) 2149–2159. R.K. Pandey, V. Lakshminarayanan, J. Phys. Chem. C 114 (2010) 8507–8514. H.R. Tantawy, A.T. Weakley, D.E. Aston, J. Phys. Chem. C 118 (2014) 1294–1305. S. Mondal, M.V. Sangaranarayanan, Sens. Actuators B 177 (2013) 478–486. O.S. Kwon, J.Y. Hong, S.J. Park, Y. Jang, J. Jang, J. Phys. Chem. C 114 (2010) 18874–18879. H.K. Song, G.T.R. Palmore, Adv. Mater. 18 (2006) 1764–1768. S.S. Shinde, G.S. Gund, V.S. Kumbhar, B.H. Patil, C.D. Lokhande, Eur. Polym. J. 49 (2013) 3734–3739. S. Takagi, S. Makuta, A. Veamatahau, Y. Otsuka, Y. Tachibana, J. Mater. Chem. 22 (2012) 22181–22189. M. Acik, C. Baristiran, G. Sonmez, J. Mater. Sci. 41 (2006) 4678–4683. P. Dallas, D. Niarchos, D. Vrbanic, N. Boukos, S. Pejovnik, C. Trapalis, D. Petridis, Polymer 48 (2007) 2007–2013. S.D. Vito, C.R. Martin, Chem. Mater. 10 (1998) 1738–1741. X. Zhang, S.K. Manohar, J. Am. Chem. Soc. 126 (2004) 12714–12715. X. Zhang, W.J. Goux, S.K. Manohar, J. Am. Chem. Soc. 126 (2004) 4502–4503. M.J. Antony, M. Jayakannan, J. Phys. Chem. B 111 (2007) 12772–12780. K. Leonavicius, A. Ramanaviciene, A. Ramanavicius, Langmuir 27 (2011) 10970–10976. X. Zhang, J. Zhang, W. Song, Z. Liu, J. Phys. Chem. B 110 (2006) 1158–1165. A.D.W. Carswell, E.A. O’Rear, B.P. Grady, J. Am. Chem. Soc. 125 (2003) 14793–14800. J. Jang, H. Yoon, Langmuir 21 (2005) 11484–11489. W. Zhong, S. Liu, X. Chen, Y. Wang, W. Yang, Macromolecules 39 (2006) 3224–3230. J. Hazarika, A. Kumar, Synth. Met. 175 (2013) 155–162. J. Kopecka, D. Kopecky, M. Vrnata, P. Fitl, J. Stejskal, M. Trchov, P. Bober, Z. Moravkov, J. Prokesc, I. Sapurinad, RSC Adv. 4 (2014) 1551–1558.
S. Mondal, M.V. Sangaranarayanan / European Polymer Journal 71 (2015) 596–611 [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]
A. Wu, H. Kolla, S.K. Manohar, Macromolecules 38 (2005) 7873–7875. S.H. Lee, S. Lee, H.W. Ryu, H. Park, Y.S. Kim, J.H. Kim, J. Polym. Sci., Part A: Polym. Chem. 52 (2014) 2329–2336. H. Osato, M. Kabaki, S. Shimizu, Org. Process Res. Dev. 15 (2011) 581–584. J.C. Lacroix, R.J. Valente, F. Maurel, P.C. Lacaze, Chem. Eur. J. 4 (1998) 1667–1677. K. Majid, R. Tabassum, A.F. Shah, S. Ahmad, M.L. Singla, J. Mater Sci.-Mater Electron. 20 (2009) 958–966. S.B. Kulkarni, U.M. Patil, R.R. Salunkhe, S.S. Joshi, C.D. Lokhande, J. Alloys Compd. 509 (2011) 3486. B.D. Cullity, Elements of X-ray diffraction, Addison-Wesley Publishing Company Inc., London, 1978. R. Rudel, F. Zite-Ferenczy, J. Physiol. 290 (1979) 317–330. X.G. Li, M.R. Huang, M.F. Zhu, Y.M. Chen, Polymer 45 (2004) 385–398. S.P. Surwade, S.R. Agnihotra, V. Dua, N. Manohar, S. Jain, S. Ammu, S.K. Manohar, J. Am. Chem. Soc. 131 (2009) 12528–12529. J. Arjomandi, A.H.A. Shah, S. Bilal, H.V. Hoang, R. Holze, Spectrochim. Acta Part A 78 (2011) 1–6. G. Zotti, G. Schiavon, Synth. Met. 30 (1989) 151–158. F. Sancenon, R.M. Manez, J. Soto, Chem. Commun. 21 (2001) 2262–2263. X.L. Zhang, Y. Xiao, X.H. Qian, Angew. Chem., Int. Ed. 47 (2008) 8025–8029. P.K. Aneesh, S.R. Nambiar, T.P. Rao, A. Ajayaghosh, Phys. Chem. Chem. Phys. 16 (2014) 8529–8535. D. Udhayakumari, S. Velmathi, Spectrochim. Acta Part A 122 (2014) 428–435. M.P. Algi, Z. Oztas, F. Algi, Chem. Commun. 48 (2012) 10219–10221. Q.F. Wu, K.X. He, H.Y. Mi, X.G. Zhang, Mater. Chem. Phys. 101 (2007) 367–371. Y. Liao, X.G. Li, R.B. Kaner, ACS Nano 4 (2010) 5193–5202. S.P. Palaniappan, P. Manisankar, Mater. Chem. Phys. 122 (2010) 15–17.
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