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Differential multiwavelength Raman study of electrochemical redox transitions of polyaniline in solutions of different acidities
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 229 (2020) 117991
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Differential multiwavelength Raman study of electrochemical redox transitions of polyaniline in solutions of different acidities Regina Mažeikienė, Gediminas Niaura, Albertas Malinauskas ⁎ Department of Organic Chemistry, Center for Physical Sciences and Technology, Sauletekio av. 3, LT-10257 Vilnius, Lithuania
a r t i c l e
i n f o
Article history: Received 4 September 2019 Received in revised form 9 December 2019 Accepted 21 December 2019 Available online 28 December 2019 Keywords: Polyaniline Raman spectroscopy Differential spectra Spectroelectrochemistry Multiwavelength Redox processes
a b s t r a c t A detailed Raman spectroelectrochemical study on polyaniline has been performed with the use of different laser lines ranging from UV (325 nm) through blue (442 nm) and green (532 nm) up to the red (633 nm) for spectra excitation, and within a broad range of solution pH from 1 to 9. From the data obtained, differential Raman spectra showing spectral changes occurring by a partial electrooxidation of polyaniline from its reduced to semioxidized, and by full oxidation to the fully oxidized states, were derived and analyzed. Different spectral features, appearing in electrochemical processes, were related to structural changes occurring during a partial or full electrooxidation of this polymer, to changes in its protonation state, and to resonance enhancement at different excitation wavelengths for different redox forms. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Polyaniline and its derivatives have been widely studied during a few recent decades due to their unusual properties like electric conductance and redox transformations taking place in electrochemical systems [1]. One of the most useful tools for the study of redox processes related to polyaniline taking place in aqueous solutions is Raman spectroscopy and its fusion with electrochemistry – Raman spectroelectrochemistry. This technique allows one i) to perform the studies in aqueous solutions, ii) to get vibrational spectra directly from electrodes modified with these materials, and iii) to get spectra at any reaction conditions chosen, including in situ spectra in the course of electrochemical reactions. Basically, there are three sets of experimental variables used in elucidation of changes in molecular structure and properties taking place during electrochemical redox transformations of polyanilines. First, changing the electrochemical potential results in a variety of redox forms for this polymer. Three basic redox forms of polyaniline, the reduced form leucoemeraldine, the half-oxidized emeraldine, and a fully oxidized pernigraniline form are usually accepted, although intermediate redox forms are also possible [1]. The second variable is the solution acidity, responsible for protonation level of these polymers. Either of the redox forms mentioned above can exist in a fully or partially protonated (proton-doped), or deprotonated form. The combination of these two sets leads to a plenty of different forms. Among them, the protonated ⁎ Corresponding author. E-mail address: [email protected] (A. Malinauskas).
https://doi.org/10.1016/j.saa.2019.117991 1386-1425/© 2019 Elsevier B.V. All rights reserved.
(proton doped) half-oxidized emeraldine form is of the greatest interest due to its electric conductivity. The third set of variables within the scope of present study relates to laser line wavelength used in excitation of Raman spectra. Different redox forms of polyaniline differ in their colour, thus, selected excitation wavelengths could fall into a resonance with optical absorbance of definite forms, resulting in a great enhancement of spectral intensity for those particular forms. As a result, Raman spectra greatly differing in their features could be obtained for the same particular specimen of polyaniline, depending on excitation wavelength used. A number of earlier works has been done on Raman spectroscopy and spectroelectrochemistry of polyaniline and its derivatives with different spectra excitation wavelengths [2–7]. The reduced leucoemeraldine form shows an optical absorbance maximum in UV range around 320 nm [8]. Therefore, the blue laser line excitation at 457 nm reveals only this reduced form, whereas the blue colored oxidized form appears invisible [2]. Also, no redox or protonation processes related to electrooxidation of polyaniline could be observed with this excitation [2]. The gradual electrooxidation of polyaniline layer leads to an increase of optical absorbance in the blue range of visible spectrum beyond 600 nm [8]. Thus, same authors [2] found a strong resonance for this blue colored oxidized form with the use of near infrared excitation at 1064 nm. Again, a strong resonance with oxidized blue form was found for red (676 nm) and infrared (1064 nm) excitations, making it impossible to follow up the oxidation and protonation processes by changing of electrode potential and solution acidity [3,4]. As opposite to blue and red or near infrared excitations, the use of green laser line at 514 nm and 488 nm allows to obtain well defined Raman spectra
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both from reduced and oxidized forms of polyaniline, thus making this green excitation the most suitable for these studies [3]. Until now, Raman spectroscopy remains a useful tool for elucidation of various redox and protonation processes taking place in polyaniline [9–11]. Also, Raman spectroscopy often helps in many practically oriented applications of polyaniline and its composites. A gold nanorod and polyaniline composite prepared by microfluidic synthesis has been reported as Raman scattering nanosensor for mercury detection [12]. Similarly, gold and polyaniline nanocomposite has been used for Raman imaging of cancerous tissue [13]. Polyaniline has been used as a probe for biological pH sensing based on Raman spectroscopy [14]. Possibilities for laser beam writing on polyaniline film were studied by Raman technique [15]. A number of composite materials for different potential applications based on polyaniline have been elaborated and studied by Raman spectroscopy, including a composite with interlaced silver nanosheets for molecule sensing [16], a composite with clay minerals [17], with graphene nanoplatelets [18], or a core-shell type goldpolyaniline nanocomposite for surface enhanced Raman scattering study [19]. In our previous work [20], we performed a detailed study on different redox and differently protonated forms of polyaniline with the use of a green laser line excitation at 532 nm. Later, we extended our studies using UV (at 325 nm), blue (at 442 nm), green (at 532 nm), red (at 633 nm), and near infrared (at 785 nm) laser line excitations [21–23]. These detailed studies revealed the existence of different redox and protonated forms of polyaniline and their potential- and acidity-driven interconversions. Aiming to show spectral and structural changes occuring with a stepwise electrooxidation of polyaniline layer, we performed a separate study on differential multiwavelength Raman spectroelectrochemistry [24]. Since the difference multiwavelength Raman spectroscopy happened to be very useful in extended studies of redox transformations of polyaniline, we decided to extend these studies. The present work has been aimed to study of redox transformations of polyaniline between its reduced, semioxidized and fully oxidized forms within a broad range of solution acidity. For this purpose, we combined a broad set of Raman excitation wavelengths ranging from UV to red lines with a broad range of solution pH from 1 to 9. At the same time, we restricted the number of redox steps to the two most essential transformations, reduced to oxidized, and reduced to semioxidized forms.
conditions by applying a relatively low controlled potential of 0.8 V for a relatively long period of 20 min. After that, the electrode was carefully rinsed with water and placed into spectroelectrochemical cell. As a working electrolyte, Britton-Robinson buffer solutions of pH 2.0 or 8.0, containing 0.1 M of KCl, were used. Raman spectra were recorded with a confocal microspectrometer inVia (Renishaw, UK) equipped with thermoelectrically cooled at ˗70 °C CCD camera and microscope. For excitation of Raman spectra, the following lasers were used: a continuous-wave 325 nm (He-Cd laser, 1 mW, 2400 lines/mm grating), 442 nm (He-Cd laser, 4 mW, 2400 lines/mm grating), 532 nm (diode-pumped solid state laser, 3 mW, 1800 lines/mm grating), and 632.8 nm (He-Ne laser, 1 mW, 1200 lines/mm grating). The 5×/0.12 NA objective lens was used for excitation and collection of Raman spectra excited with all visible laser lines, and 15×/0.32 NA objective lens was used for UV laser line excited spectra. 3. Results and discussion It is well known that the colour of a thin polyaniline layer depends greatly on the actual protonation level and oxidation state of this polymer. Generally, the protonated form of polyaniline appears pale yellow in its reduced leucoemeraldine state, and gradually changes to green, bluish green, greenish blue, and finally deep blue by a consecutive electrochemical oxidation to emeraldine and pernigraniline state. By taking Raman spectra from polyaniline layer, the exciting laser line could fall into a resonance with an optical absorbance of a particular redox form of this polymer, and thus characteristic features of this particular form could be resonant enhanced at the expense of other forms existing parallel within the same layer at the conditions used. Therefore, a detailed analysis of the data obtained with different laser line excitations to the same polymer film is required. The general optical absorbance spectra for protonated form of polyaniline in its different redox states have been presented and analyzed in [8]. Fig. 1 in [8] presents UV–Vis absorbance spectra for polyaniline layer at different electrode potentials ranging from 0.0 to 0.8 V vs. NHE, thus showing changes in absorbance spectra proceeding during oxidation and reduction processes. From
2. Experimental All spectroelectrochemical experiments have been done in a cylinder-shaped electrochemical cell. A gold electrode of 5 mm in diameter, press-fitted into a Teflon rod holder and placed at about 1–2 mm distance from the cell optical window, was used as a working electrode. Throughout the experiments, the spectroelectrochemical cell has been continuously periodically moved with respect to laser beam at a rate of about 20 mm/s using a custom built equipment. This arrangement was described in details in [25,26], and used in most of our works dealing with labile organic layers covered at an electrode surface in order to avoid or at least diminish laser-induced decomposition of organic layer and to avoid possible photo- and thermoeffects. Platinum wire and saturated Ag/AgCl electrode were used as a counter and reference electrodes, respectively. All electrode potential values reported below refer to this reference electrode. For each experimental set, the working gold electrode was cleaned with a Piranha solution (a mixture of 30% hydrogen peroxide solution and concentrated sulfuric acid, 3:1 by volume), then ultrasonicated in water-ethanol solution for 30 s, and carefully rinsed with water. The cell was driven by BASi-Epsilon model (Bioanalytical systems Inc., USA) electrochemical workstation. Polyaniline layer has been deposited on the working electrode from a solution containing 0.05 M of aniline and 0.5 M sulfuric acid. In order avoid a partial electrochemical degradation of electrosynthesized polyaniline layer, electropolymerisation has been performed at mild
Fig. 1. Raman spectra of polyaniline layer deposited at a gold electrode, obtained with UV laser excitation at 325 nm in pH 5 solution at a controlled electrode potential of 0.3 V (A) and −0.3 V (B), and the difference spectrum obtained by subtraction of spectrum (B) from spectrum (A), depicted as (C).
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these data, we recalculated the optical absorbances for protonated polyaniline form at selected wavelengths, and presented these results in a form of absorbance ratio for oxidized to reduced (Ox/Red), and semioxidized (half-oxidized) to reduced (Semiox/Red) forms in Table 1. The reduced protonated leucoemeraldine form of polyaniline shows an absorbance maximum around 315–320 nm [8]. The absorbance decreases gradually by extending of the oxidation degree for this layer, resulting in an absorbance ratio lower than 1.0. Particularly, for Ox/ Red and Semiox/Red transitions, the absorbance ratio of 0.41 and 0.54 has been obtained (Table 1). Therefore, the UV laser generating 325 nm line has been chosen to get into a resonance with this leucoemeraldine form. Earlier, the formation of an intermediate redox form of polyaniline characterized by absorbance maximum around 420 nm was observed by shifting the electrode potential to higher positive values [8]. Because the optical absorbance of this intermediate semioxidized form reaches its maximum for this particular wavelength and drops by further increase of oxidation degree, the absorbance ratio for Ox/Red appears somewhat lower than for Semiox/Red (Table 1). Thus, in order to get into a resonance with this intermediate form, we used for excitation of Raman spectra a blue laser line at 442 nm, located in the vicinity of its optical absorbance. The lowest optical absorbance within an entire UV–Vis spectrum of leucoemeraldine form of polyaniline is observed within the green spectral range [8]. Here, the absorbance increases greatly by extending the oxidation degree, and, as a result, relatively high ratio for Semiox/Red, and especially for Ox/Red is obtained within the reach of a green laser line at 532 nm used (Table 1). At last, a broad and intense absorbance band in the red range of visible spectrum is observed for oxidized forms of polyaniline [8], and a high ratio for Semiox/Red and especially Ox/Red is observed, enabling potentially to get into a strong resonance with oxidized form of this polymer with the use of 633 nm red laser line excitation (Table 1). In general, the use of four laser lines for Raman spectra excitation ranging from UV at 325 nm through the blue at 442 nm and green at 532 nm up to the red at 633 nm covers nearly all redox forms of polyaniline. 3.1. UV laser line excitation at 325 Nm Fig. 1 depicts an example of getting the difference Raman spectra used in the present study. Here, the original Raman spectra A and B were recorded at an upper (0.3 V), and lower (−0.3 V) electrode potential limits in pH 5.0 solution, thus representing a fully oxidized and fully reduced forms of polyaniline, respectively. It is well seen that the relative intensity for two most prominent Raman bands located around 1620 cm−1 and 1186 cm−1 appears higher for the reduced form, whereas the intensity for the band located around 1513 cm−1 appears higher for oxidized form of polyaniline. As a result, subtraction of spectrum B from spectrum A results in a difference spectrum C that contains negative difference bands at 1620 cm−1 and 1186 cm−1, and a positive difference band at 1513 cm−1. Fig. 2 shows difference Raman spectra obtained at UV laser excitation at 325 nm for solution pH ranging from 1 to 9. The difference in electrochemical potentials for oxidized and reduced forms of polyaniline in Fig. 2 was chosen at 0.6 V, ranging from 0.8 V (for Ox form) to 0.2 V (for Red form) at the lowest pH values up to 0.3 to
Table 1 Optical absorbance ratio at selected wavelengths for oxidized to reduced (Ox/Red) and semioxidized to reduced (Semiox/Red) forms of polyaniline deposited at ITO electrode, as obtained in a solution of 0.5 M of sulfuric acid. The data for calculations were taken from original spectra published in [8]. Absorbance ratio for
Ox/Red Semiox/Red
Wavelength 315 nm (λ max for Red)
420 nm (λ max for Semiox)
532 nm
633 nm
0.41 0.54
2.52 3.37
10.5 3.00
25.3 5.70
3
Fig. 2. Difference Raman spectra of polyaniline layer deposited at a gold electrode, calculated by subtracting spectra for the reduced form from those for the oxidized form (Ox-Red), as obtained with UV laser excitation at 325 nm in solutions of different pH ranging from 1 to 9 (as indicated).
−0.3 V for the highest pH values. The highest potential of 0.8 V chosen does not exactly correspond to a fully exhaustive electrooxidation of polyaniline; however, this somewhat lower potential ensures the electrochemical stability of polyaniline layer, since its degradation is known to proceed relatively fast at higher potentials. In common, the spectra appear poor in regard to the number of Raman bands. From these, two bands are of a great interest. The band located around 1620 cm−1 corresponds to C\\C stretching vibrations in benzene type rings. In an entire pH range studied, this difference band is negative, meaning that its intensity diminishes by going from the reduced to oxidized form. Indeed, electrooxidation of polyaniline leads to a decrease of the number of benzene type rings at the expense of increasing number of quinonediimine rings. It is seen from Fig. 2 that this negative difference band appears low in intensity at low solution pH, but increases by shifting pH to higher values reaching its maximum around pH 5, and then decreasing gradually throughout the rest of solution pH range. Also, a small shift of its frequency from 1620 to 1625 cm−1 could be noted by increasing pH. Close similarly, a negative difference Raman band at 1186 cm−1, which follows same pH-dependent tendencies in its magnitude as the band at 1620 cm−1, is well observed (Fig. 2). This difference band relates to C\\H bending vibrations in leucoemeraldine, and it appears negative obviously because of a decrease in the content of leucoemeraldine form by progressive electrooxidation. The only remarkable positive difference Raman band appears at 1513–1505 cm−1, and corresponds probably to phenazine-type minor side structures present within the polymer structure [27,28]. The most of difference Raman bands observed and their assignments based on previous studies [3–7,27–34] are summarized in Table 2. Noteworthy, no well recognized difference Raman bands characteristic for oxidized form of polyaniline are observed in an entire range of solution pH, even the band around 1585 cm−1, representing C_C stretching vibrations in quinonediimine type rings of oxidized polyaniline structure. This means that UV laser line used in obtaining of the spectra shown in Fig. 2 is in a strong resonance with the reduced leucoemeraldine form, not allowing the development of any detectable feature for oxidized form.
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Table 2 Tentative assignments of difference Raman bands as obtained for different spectra excitation wavelengths. Difference Raman bands (in cm−1) for different excitation wavelengths and solution pH (in parentheses) 325 nm
442 nm
Ox-Red
Ox-Red
1620↓ (5) 1625↓ (9)
1624↓ (1–4)
532 nm
633 nm
Semiox-Red
Ox-Red
Ox-Red
1625↓ (2) 1622↑ (3–9)
1628↓ (2) 1610↓ (9) 1582↑ (2) 1584↑ (9) 1552↑ (2) 1550↑ (9)
1582↑ (5–9)
1513↑ (5) 1505↑ (9)
1186↓ (2–9)
1488↑ (4)
1187↓ (2) 1182↓ (5) 1162↑ (5–9)
1192↑ (2–9) 1185↓ (2)
885↓ (2) 830↓ (2) 780↑ (2–9)
880↑ (5) 825↑ (5)
1453↑ (2) 1485↑ (5) 1470↑ (9) 1380↑ (2–9) 1324↑ (2–9) 1224↑ (2) 1218↑ (5–9) 1190↓ (2) 1184↓ (5) 1162↑ (2–9) 849↑ (2–9) 778↑ (2, 9) 792–778↑ (5) 750↑ (2–9) 525↑ (2–9)
Tentative assignments
Semiox-Red
1585↑ (2–5)
1626↑ (2)
C\ \C stretching in B (8a)
1586↑ (2)
C_C stretching in Q (8a)
1476↑ (2)
Phenazine type fragments C_N stretching in emeraldine base (imine sites)
1343–1320↑ (2) 1218↑ (5)
C\ \N+ stretching in polaronic form (polarons) C\ \N+ stretching in polaronic form (polarons) C\ \N stretching in emeraldine (amine)
1554↑ (2–5)
1471↑ (2) 1476↑ (5) 1462↑ (9)
1220↑ (2–9)
C\ \H bending in leucoemeraldine (9a) 1162↑ (2) 1165↑ (5–9) 852↑ (2–5)
1168↑ (2) 1162↑ (5)
800–780↑ (2–5)
805↑ (2) 786↑ (5)
753↑ (2–5)
C\ \H bending in emeraldine (9a) B deformation (1) Q deformation Imine deformation (C\ \N\ \C bending) Amine in-plane deformation
Assignments are based mainly on the known data [3–7,27–34]. Spectra denoted by Ox-Red were obtained by subtracting of spectra for the reduced form from those for oxidized form, and spectra denoted by Semiox-Red – by subtracting of spectra for the reduced form from those for a half-oxidized (semioxidized) form. For solution pH of 2, 5, and 9, the electrode potentials for oxidized, semioxidized and reduced forms were chosen same as indicated in the corresponding Figures. The arrows (↓) and (↑) indicate the tendencies for a decrease or an increase of intensity for spectral bands, respectively, the symbols (1), (8a), and (9a) denote Wilson‘s notations of vibrational modes for benzene derivatives, and B and Q denote benzene and quinonediimine type rings, respectively.
3.2. Blue laser line excitation at 442 Nm Somewhat similar difference Raman spectra were obtained with the blue laser line excitation, presented in Fig. 3. Again, intense negative difference bands around 1624 cm−1 appear dominating over the whole spectrum, indicating a decreasing number of benzene type rings upon
Fig. 3. Same as in Fig. 2, as obtained with blue laser excitation at 442 nm.
electrooxidation. As distinct from UV laser line excitation, the most intense negative difference bands for 442 nm excitation appear at the lowest solution pH values. At pH over 3, the intensity drops greatly, and at pH exceeding 8, this difference band cannot be detected (Fig. 3). Another well expressed feature is an intense negative difference band at 1187–1182 cm−1, representing C\\H band bending vibrations in leucoemeraldine form (Table 2). This difference band is best expressed in acidic solutions, and indicates a decrease in the content of leucoemeraldine form upon electrooxidation. The next feature, distinct from that observed at 325 nm excitation, is the development of two negative mid-intense difference bands at 885 cm−1 and 830 cm−1, well seen in acidic solutions up to pH of 4 (Fig. 3). These difference bands represent benzene ring deformations. As distinct from UV laser excitation, some spectral features characteristic for oxidized form are also observed. First, the positive (i.e. growing upon electrooxidation) difference band at 1582 cm−1 is seen at solution pH over 5 (Fig. 3). This positive band presents C_C stretching vibrations of quinonediimine type rings as the most characteristic feature for oxidized form of polyaniline. Over pH of 4, a new positive band at 1488 cm−1 appears. This difference band shows an increasing number of C_N stretchings in an oxidized form, the emeraldine base form (Table 2). Again, a new positive difference band around 1162 cm−1 appears within pH range of 5 to 9, representing C\\H bending vibrations in emeraldine. This difference band grows up with increasing pH along with the decrease of a negative difference band around 1187–1182 cm−1, characteristic for similar C\\H bending mode for leucoemeraldine. A positive difference band around 780 cm−1 should be noted, representing quinone ring vibrations. In summary, the blue laser line excitation, as similar to UV excitation, results mainly in developing of features characteristic for leucomeraldine form and thus diminishing in their intensities upon electrochemical oxidation. As distinct from UV excitation, however, blue laser line excitation reveals also the development of features characteristic for emeraldine form upon electrooxidation of polyaniline layer.
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A different behaviour was observed for difference spectra between semioxidized and reduced forms, as depicted in Fig. 4. While Ox-Red spectra presented in Fig. 3 were calculated by the difference between fully oxidized and reduced forms, as obtained at a high electrode potential difference reaching 0.8 V, the Semiox-Red difference spectra of Fig. 4 relate to a half of this electrode potential difference of 0.4 V. For calculations of Semiox-Red differences, spectra for reduced forms were taken same as for Ox-Red spectra, whereas spectra for half-oxidized forms belong to a middle electrochemical potential. At a low solution pH of 1 and 2, as it could be expected, negative difference band at 1625 cm−1 is observed indicating a decrease in a number of benzene rings in the course of electrooxidation. However, an increase in solution pH up to 3 results unexpectedly in a positive difference band at 1622 cm−1. This positive difference band reaches its maximum around pH of 4, then gradually diminishes, and retains through the whole pH range studied up to 9 (Fig. 4). Changes in relative intensities of difference Raman bands with varying of solution pH for UV and blue laser line excitations are presented in Fig. 5. This unexpected behaviour could be understood taking into account that the light absorbance ratio for Semiox-Red forms is higher than for Ox-Red forms at the absorbance maximum (420 nm) for the half-oxidized form (Table 1). This means that a resonance enhancement of Raman scattering is possible for the half-oxidized form at given experimental conditions with the use of 442 nm laser line excitation that appears in the vicinity of maximum absorbance of this semioxidized form. This resonance enhancement leads to increasing intensity of 1622 cm−1 band and thus to the development of positive difference band at these particular conditions, despite of the decreasing number of reduced fragments (benzene type rings) upon electrooxidation. We discussed this phenomenon in our previous paper [21]. Another feature that is observed with 442 nm excitation in an acidic solution is the presence of a negative difference band around 1185 cm−1 (Fig. 4). Similarly as for Ox-Red difference spectra, this negative difference band, presenting C\\H bending vibrations in leucoemeraldine (Table 2), indicate a decreasing number of reduced sites upon electrooxidation. However, as distinct from Ox-Red difference spectra, a new positive Raman band at slightly higher frequencies around 1192 cm−1 starts to develop in Semiox-Red difference spectra even at low pH values, and grows in intensity with increasing pH. The pH behaviour of this positive difference band resembles that of the
5
Fig. 5. Dependence of relative intensity for difference Raman bands on solution pH as obtained for 325 nm and 442 nm laser excitations (as indicated) for Ox-Red and Semiox-Red difference Raman spectra.
1622 cm−1 band, therefore, it could be assumed that this band presents C\\H bendings in semioxidized form. Also, as distinct from Ox-Red spectra, almost no positive difference band around 1162 cm−1, corresponding to C\\H bendings in emeraldine, is observed, at least up to the neutral pH values, indicating a small content of emeraldine formed during this partial electrooxidation. A closely similar behaviour should be noted regarding the difference band around 780 cm−1 and representing quinone ring deformations, characteristic for oxidized form. Well seen as a positive difference band for Ox-Red spectra in an entire pH range of 2 to 9, this band appears of low intensity or even undetectable for Semiox-Red spectra, thus indicating a small content of emeraldine form in the latter case. In accordance with assumption of resonance enhancement with 442 nm excitation, two positive difference bands around 880 and 825 cm−1, corresponding to benzene ring deformations and thus characteristic for the reduced form, are observed in Semiox-Red spectra within the pH range of 3 to 5. For Ox-Red spectra, as expected, both these difference bands are negative (cf. Fig. 3 and Fig. 4). 3.3. Green laser line excitation at 532 Nm
Fig. 4. Same as in Fig. 3, calculated by subtracting spectra for the reduced form from those for the semioxidized form (Semiox-Red).
The most rich in their features appear difference Raman spectra obtained at a green laser excitation (532 nm), as presented in Fig. 6. Because of moderately high difference in light absorbance ratio for Ox and Red forms (Table 1), the most features result in positive difference bands and thus relate to changes occuring along with electrooxidation. Among them, C_C double bond stretchings in quinonediimine rings around 1584–1582 cm−1, C\\N single bond stretchings in emeraldine around 1224–1218 cm−1, and C\\H bendings in emeraldine at 1162 cm−1 are most characteristic and showing electrooxidation of polyaniline to proceed. One of the most prominent features of oxidized form are stretching vibrations of a double C_N bond in emeraldine base, well seen as a broad intense difference band at pH exceeding 1, where this basic form exists. The position of this positive difference band, however, changes to a remarkable extent depending on solution pH. In acidic solutions, this difference band appears at 1453 cm−1, but shifts toward higher frequency up to 1485 cm−1 by changing pH up 4 or 5, and again shift to lower frequency of 1470 cm−1 by going to pHneutral and basic solution (Table 2). Very probably, this frequency
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Fig. 6. Same as in Fig. 2, as obtained with green laser excitation at 532 nm.
recurrency pattern is caused by the presence of a few different protonation states for emeraldine base. While other difference Raman bands are summarized in Table 2, the presence of the two negative difference bands should be noted. One of them relates to C\\C stretchings in benzene type rings, and shows the decreasing number of benzene rings at electrooxidation. This negative difference band appears around 1628 cm−1 for acidic solutions, and shift up to 1610 cm−1 upon increase of solution pH up to 9 (Fig. 6). An another negative difference band relates to C\\H bendings in leucoemeraldine, and appears in acidic and neutral solutions around 1190–1184 cm−1 (Table 2).
Fig. 7. Same as in Fig. 2, as obtained with red laser excitation at 633 nm.
intense positive difference band around 1626 cm−1, corresponding to benzene rings, is well seen in acidic solutions at pH not exceeding 4 (Fig. 8). When going by 0.3 V toward the positive potentials, the number of benzene rings in the polymer structure decreases, although not as much as for a bigger potential step of 0.6 V. At the same time, however, the optical absorbance increases by 5.7 times at this particular wavelength of 633 nm (Table 1). Thus, a combination of a decreasing number of benzene rings and a resonance enhancement results in an increase of a relative intensity for this difference Raman band, i.e. in a positive difference band. Next, the presence of a positive difference band at
3.4. Red laser line excitation at 633 Nm At a red laser line excitation of 633 nm, the light absorbance ratio for Ox/Red forms appears very high (Table 1), therefore, a resonance enhancement of Raman spectra for oxidized form of polyaniline occur. Because of a high light absorbance in the red spectral region for Ox forms, all factors influencing the absorbance properties of polyaniline should influence the resulting resonance Raman spectra. Among others, the protonation level and its changes by changing the solution pH seem to be of a great importance. As for fully oxidized form pernigraniline, the reversible deprotonation was reported to proceed at low pH values of 0.5–1.0, whereas the resulting deprotonated form possesses somewhat different light absorbance profile, thus potentially influencing the resonance conditions [35]. The most positive difference bands related to features appearing upon electrooxidation are related to those observed at a green laser line excitation, and are presented in Fig. 7, and summarized in Table 2. Because of the said high difference in optical absorbance of Ox and Red forms and accompanying resonance enhancement, the intensity of both negative difference bands appear low. However, even with 633 nm enhancement, a negative difference band in the vicinity of 1620 cm−1 is clearly seen within a broad range of solution pH (Fig. 7). Fig. 8 presents difference Raman spectra for half-oxidized minus reduced forms of polyaniline (Semiox-Red). While the electrochemical potential difference for Ox-Red spectra presented in Fig. 7 was chosen as high as 0.6 V in order to ensure a full electrooxidation, a much lower difference of 0.3 V was set in obtaining the spectra presented in Fig. 8. Some important spectral differences between Ox-Red (as in Fig. 7), and Semiox-Red (as in Fig. 8) should be noted. First, relatively
Fig. 8. Same as in Fig. 4, calculated by subtracting spectra for the reduced form from those for the semioxidized form (Semiox-Red), as obtained with red laser excitation at 633 nm.
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1586 cm−1 seems to be an expected consequence of increasing number of quinonediimine-like fragments formed during this partial electrooxidation. The most striking appears an interplay between the two vibrational modes – C_N stretches in emeraldine, representing imine sites characteristic for oxidized forms, and C\\N+ stretches representing polaronic sites in a half-oxidized structure. As for a big potential step of 0.6 V (Ox-Red), the C_N stretches manifests as a very strong and dominating by its intensity positive difference band around 1476–1471 cm−1 just at pH of 2 and over that (Fig. 7). For the small potential step of 0.3 V (Semiox-Red), again, a positive difference band at 1476 cm−1 is observed, however, its intensity remains relatively low at solution pH 1 up to pH 3 (Fig. 8). At pH of 4, this difference band grows highly and again turns dominating over the spectrum (Fig. 8). At the same time, a couple of positive difference bands appear within the range of 1343–1320 cm −1 , representing polaron sites in a semioxidized structure [27]. Noteworthy, for a big potential step of 0.6 V, only low-intense positive difference bands within same wavenumber range are observed, thus indicating a smaller number of polaronic sites in a fully oxidized form as compared to a semioxidized one (cf. relative intensities for difference bands within the spectral range of 1400–1300 cm −1 in Fig. 7 and Fig. 8). Most pronounced for the lowest pH of 1 as two well separated difference bands, these bands tend to merge and decrease in intensity by changing of solution pH from 1 to 3 (Fig. 8). At the same time, a decrease of difference bands for polaronic form with increasing pH up to 3 clearly appears to be accompanied by synchronic increase of imine band at 1476 cm−1, meaning that a decrease in a number of polaronic sites proceeds synchronic with an increase in a number of imine sites with increasing solution pH. Finally, at pH of 4, the polaronic form almost disappears from difference spectra, whereas the positive band for imine form grows greatly and again appears dominating over an entire spectrum (Fig. 8). These observations correlate well with known changes occuring in polaronic form of polyaniline upon protonic dedoping of polyaniline, viz. upon increasing pH over ca. 4, especially with the loss of electric conductivity at pH above 3 or 4, and other properties as well [1].
4. Conclusions Specific Raman features were disclosed by the differential Raman spectroscopy for redox transformations of polyaniline from its oxidized to semioxidized, and to reduced forms within a wide solution pH range. The observed Raman features were shown to depend drastically on the spectra excitation wavelength. For UV laser line excitation at 325 nm, the features of the reduced form are observed, resulting in negative difference Raman bands (Ox-Red) within an entire pH range studied from pH 1 to pH 9. At a blue excitation at 442 nm, both positive and negative difference bands for Ox-Red spectra are observed that are dependent on solution pH, whereas an inversion from negative to positive difference bands for Semiox-Red difference spectra was observed by increasing solution pH, indicating a possible resonance with an intermediate half-oxidized form of polyaniline. Characterisctic features were identified for the green (532 nm) and red (633 nm) excitations taking place upon electrooxidation and reversible protonation of polyaniline layer. At a red excitation, characteristic features for polarons were observed for Semiox-Red difference spectra that are absent in the corresponding Ox-Red difference spectra. The positive difference bands for polarons appear at a low solution pH of 1. An increase of solution pH results in a diminution of these difference bands of polarons that disappear at pH above 4.
7
Author contribution 1. Regina Mazeikiene: Methodology, Investigation 2. Gediminas Niaura: Validation, Data Curation, Writing – Reviewing and Editing 3. Albertas Malinauskas: Conceptualization, Supervision, Writing – Original Draft preparation Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] G. Ćirić-Marjanović, Recent advances in polyaniline research: polymerization mechanisms, structural aspects, properties and applications, Synth. Metals 177 (2013) 1–47. [2] M. Lapkowski, K. Berrada, S. Quillard, G. Louarn, S. Lefrant, A. Pron, Electrochemical oxidation of polyaniline in nonaqueous electrolytes: in situ Raman spectroscopic studies, Macromolecules 28 (1995) 1233–1238. [3] S. Quillard, K. Berrada, G. Louarn, S. Lefrant, M. Lapkowski, A. Pron, In situ Raman spectroscopic studies of the electrochemical behavior of polyaniline, New J. Chem. 19 (1995) 365–374. [4] M. Lapkowski, K. Berrada, S. Quillard, G. Louarn, S. Lefrant, Study of the redox properties of polyaniline in non-aqueous media by Raman diffusion, J. Chim. Phys. Phys.Chim. Biol. 92 (1995) 915–918. [5] M. Boyer, S. Quillard, G. Louarn, S. Lefrant, E. Rebourt, A.P. Monkman, Oxidized model compounds of polyaniline studied by resonance Raman spectroscopy, Synth. Metals 84 (1997) 787–788. [6] M.C. Bernard, A. Hugot-Le Goff, Raman spectroscopy for the study of polyaniline, Synth. Metals 85 (1997) 1145–1146. [7] M.C. Bernard, S.C. De Torresi, A. Hugot-Le Goff, In situ Raman study of sulfonatedoped polyaniline, Electrochim. Acta 44 (1999) 1989–1997. [8] A. Malinauskas, R. Holze, Cyclic UV-Vis spectrovoltammetry of polyaniline, Synth. Metals 97 (1998) 31–36. [9] Z. Moravkova, E. Dmitrieva, Structural changes in polyaniline near the middle oxidation peak studied by in situ Raman spectroelectrochemistry, J. Raman Spectrosc. 48 (2017) 1229–1234. [10] G.M. Do Nascimento, Raman dispersion in polyaniline nanofibers, Vibrat. Spectrosc. 90 (2017) 89–95. [11] M. Blaha, J. Zednik, J. Vohlidal, Self-doping of polyaniline prepared with the FeCl3/ H2O2 system and the origin of the Raman band of emeraldine salt at around 1375 cm−1, Polym, Internat 64 (2015) 1801–1807. [12] Y. Wang, M.X. Shang, Y.N. Wang, Z.R. Xu, Droplet-based microfluidic synthesis of (au nanorod@Ag)-polyaniline Janus nanoparticles and their application as a surfaceenhanced Raman scattering nanosensor for mercury detection, Anal. Methods 11 (2019) 3966–3973. [13] Z.C. Li, L. Xia, G.K. Li, Y.L. Hu, Raman spectroscopic imaging of pH values in cancerous tissue by using polyaniline@gold nanoparticles, Microchim. Acta 186 (2019) https:// doi.org/10.1007/s00604-019-3265-4. [14] S.Y. Li, Z.M. Liu, C.K. Su, H.L. Chen, X.X. Fei, Z.Y. Guo, Biological pH sensing based on the environmentally friendly Raman technique through a polyaniline probe, Anal. Bioanal. Chem. 409 (2017) 1387–1394. [15] Z. Moravkova, P. Bober, Writing in a polyaniline film with laser beam and stability of the record: a Raman spectroscopic study, Internat. J. Polym. Sci. (2018) Art. No. 1797216. [16] F.G. Xu, S. Xie, H. Xu, X. Chen, H. Yu, L. Wang, Interlaced silver nanosheets grown on polyaniline coated carbon foam as efficient three dimensional surface enhanced Raman scattering substrate for molecule sensing, Appl. Surf. Sci. 410 (2017) 566–573. [17] G.M. Do Nascimento, N.A. Pradie, Deprotonation, Raman dispersion and thermal behavior of polyaniline-montmorillonite nanocomposites, Synth. Metals 217 (2016) 109–116. [18] N. Badi, S. Khasim, A.S. Roy, Micro-Raman spectroscopy and effective conductivity studies of graphene nanoplatelets/polyaniline composites, J. Mater. Sci. – Mater. Electron. 27 (2016) 6249–6257. [19] S. Dutt, P.F. Siril, V. Sharma, S. Periasamy, Gold(core)-polyaniline(shell) composite nanowires as a substrate for surface enhanced Raman scattering and catalyst for dye reduction, New J. Chem. 39 (2015) 902–908. [20] R. Mažeikienė, V. Tomkutė, Z. Kuodis, G. Niaura, A. Malinauskas, Raman spectroelectrochemical study of polyaniline and sulfonated polyaniline in solutions of different pH, Vibrat. Spectrosc. 44 (2007) 201–208. [21] R. Mažeikienė, G. Niaura, A. Malinauskas, Raman spectroelectrochemical study of polyaniline at UV, blue, and green laser line excitation in solutions of different pH, Synth. Metals 243 (2018) 97–106. [22] R. Mažeikienė, G. Niaura, A. Malinauskas, Red and NIR laser line excited Raman spectroscopy of polyaniline in electrochemical system, Synth. Metals 248 (2019) 35–44. [23] R. Mažeikienė, G. Niaura, A. Malinauskas, A comparative multiwavelength Raman spectroelectrochemical study of polyaniline: a review, J. Solid State Electrochem. 23 (2019) 1631–1640.
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[24] R. Mažeikienė, G. Niaura, A. Malinauskas, Study of redox and protonation processes of polyanilineby the differential multiwavelength Raman spectroelectrochemistry, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 221 (117147) (2019). [25] G. Niaura, A.K. Gaigalas, V.L. Vilker, Moving spectroelectrochemical cell for surface Raman spectroscopy, J. Raman Spectrosc. 28 (1997) 1009–1011. [26] A. Bulovas, N. Dirvianskyte, Z. Talaikyte, G. Niaura, S. Valentukonyte, E. Butkus, V. Razumas, Electrochemical and structural properties of self-assembled monolayers of 2-methyl-3-(ω-mercaptoalkyl)-1,4-naphthoquinones on gold, J. Electroanal. Chem. 591 (2006) 175–188. [27] M.M. Nobrega, C.H.B. Silva, V.R.L. Constantino, M.L.A. Temperini, Spectroscopic study on the structural differences of thermally induced cross-linking segments in emeraldine salt and base forms of polyaniline, J. Phys. Chem. B 116 (2012) 14191–14200. [28] I. Šedĕnková, M. Trchová, J. Stejskal, Thermal degradation of polyaniline films prepared in solutions of strong and weak acids in water – FTIR and Raman spectroscopic studies, Polym. Degrad. Stabil. 93 (2008) 2147–2157. [29] G. Niaura, R. Mažeikienė, A. Malinauskas, Structural changes in conducting form of polyaniline upon ring sulfonation as deduced by near infrared resonance Raman spectroscopy, Synth. Metals 145 (2004) 105–112.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Differential multiwavelength Raman study of electrochemical redox transitions of polyaniline in solutions of different acidities Regina Mažeikienė, Gediminas Niaura, Albertas Malinauskas ⁎ Department of Organic Chemistry, Center for Physical Sciences and Technology, Sauletekio av. 3, LT-10257 Vilnius, Lithuania
a r t i c l e
i n f o
Article history: Received 4 September 2019 Received in revised form 9 December 2019 Accepted 21 December 2019 Available online 28 December 2019 Keywords: Polyaniline Raman spectroscopy Differential spectra Spectroelectrochemistry Multiwavelength Redox processes
a b s t r a c t A detailed Raman spectroelectrochemical study on polyaniline has been performed with the use of different laser lines ranging from UV (325 nm) through blue (442 nm) and green (532 nm) up to the red (633 nm) for spectra excitation, and within a broad range of solution pH from 1 to 9. From the data obtained, differential Raman spectra showing spectral changes occurring by a partial electrooxidation of polyaniline from its reduced to semioxidized, and by full oxidation to the fully oxidized states, were derived and analyzed. Different spectral features, appearing in electrochemical processes, were related to structural changes occurring during a partial or full electrooxidation of this polymer, to changes in its protonation state, and to resonance enhancement at different excitation wavelengths for different redox forms. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Polyaniline and its derivatives have been widely studied during a few recent decades due to their unusual properties like electric conductance and redox transformations taking place in electrochemical systems [1]. One of the most useful tools for the study of redox processes related to polyaniline taking place in aqueous solutions is Raman spectroscopy and its fusion with electrochemistry – Raman spectroelectrochemistry. This technique allows one i) to perform the studies in aqueous solutions, ii) to get vibrational spectra directly from electrodes modified with these materials, and iii) to get spectra at any reaction conditions chosen, including in situ spectra in the course of electrochemical reactions. Basically, there are three sets of experimental variables used in elucidation of changes in molecular structure and properties taking place during electrochemical redox transformations of polyanilines. First, changing the electrochemical potential results in a variety of redox forms for this polymer. Three basic redox forms of polyaniline, the reduced form leucoemeraldine, the half-oxidized emeraldine, and a fully oxidized pernigraniline form are usually accepted, although intermediate redox forms are also possible [1]. The second variable is the solution acidity, responsible for protonation level of these polymers. Either of the redox forms mentioned above can exist in a fully or partially protonated (proton-doped), or deprotonated form. The combination of these two sets leads to a plenty of different forms. Among them, the protonated ⁎ Corresponding author. E-mail address: [email protected] (A. Malinauskas).
https://doi.org/10.1016/j.saa.2019.117991 1386-1425/© 2019 Elsevier B.V. All rights reserved.
(proton doped) half-oxidized emeraldine form is of the greatest interest due to its electric conductivity. The third set of variables within the scope of present study relates to laser line wavelength used in excitation of Raman spectra. Different redox forms of polyaniline differ in their colour, thus, selected excitation wavelengths could fall into a resonance with optical absorbance of definite forms, resulting in a great enhancement of spectral intensity for those particular forms. As a result, Raman spectra greatly differing in their features could be obtained for the same particular specimen of polyaniline, depending on excitation wavelength used. A number of earlier works has been done on Raman spectroscopy and spectroelectrochemistry of polyaniline and its derivatives with different spectra excitation wavelengths [2–7]. The reduced leucoemeraldine form shows an optical absorbance maximum in UV range around 320 nm [8]. Therefore, the blue laser line excitation at 457 nm reveals only this reduced form, whereas the blue colored oxidized form appears invisible [2]. Also, no redox or protonation processes related to electrooxidation of polyaniline could be observed with this excitation [2]. The gradual electrooxidation of polyaniline layer leads to an increase of optical absorbance in the blue range of visible spectrum beyond 600 nm [8]. Thus, same authors [2] found a strong resonance for this blue colored oxidized form with the use of near infrared excitation at 1064 nm. Again, a strong resonance with oxidized blue form was found for red (676 nm) and infrared (1064 nm) excitations, making it impossible to follow up the oxidation and protonation processes by changing of electrode potential and solution acidity [3,4]. As opposite to blue and red or near infrared excitations, the use of green laser line at 514 nm and 488 nm allows to obtain well defined Raman spectra
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both from reduced and oxidized forms of polyaniline, thus making this green excitation the most suitable for these studies [3]. Until now, Raman spectroscopy remains a useful tool for elucidation of various redox and protonation processes taking place in polyaniline [9–11]. Also, Raman spectroscopy often helps in many practically oriented applications of polyaniline and its composites. A gold nanorod and polyaniline composite prepared by microfluidic synthesis has been reported as Raman scattering nanosensor for mercury detection [12]. Similarly, gold and polyaniline nanocomposite has been used for Raman imaging of cancerous tissue [13]. Polyaniline has been used as a probe for biological pH sensing based on Raman spectroscopy [14]. Possibilities for laser beam writing on polyaniline film were studied by Raman technique [15]. A number of composite materials for different potential applications based on polyaniline have been elaborated and studied by Raman spectroscopy, including a composite with interlaced silver nanosheets for molecule sensing [16], a composite with clay minerals [17], with graphene nanoplatelets [18], or a core-shell type goldpolyaniline nanocomposite for surface enhanced Raman scattering study [19]. In our previous work [20], we performed a detailed study on different redox and differently protonated forms of polyaniline with the use of a green laser line excitation at 532 nm. Later, we extended our studies using UV (at 325 nm), blue (at 442 nm), green (at 532 nm), red (at 633 nm), and near infrared (at 785 nm) laser line excitations [21–23]. These detailed studies revealed the existence of different redox and protonated forms of polyaniline and their potential- and acidity-driven interconversions. Aiming to show spectral and structural changes occuring with a stepwise electrooxidation of polyaniline layer, we performed a separate study on differential multiwavelength Raman spectroelectrochemistry [24]. Since the difference multiwavelength Raman spectroscopy happened to be very useful in extended studies of redox transformations of polyaniline, we decided to extend these studies. The present work has been aimed to study of redox transformations of polyaniline between its reduced, semioxidized and fully oxidized forms within a broad range of solution acidity. For this purpose, we combined a broad set of Raman excitation wavelengths ranging from UV to red lines with a broad range of solution pH from 1 to 9. At the same time, we restricted the number of redox steps to the two most essential transformations, reduced to oxidized, and reduced to semioxidized forms.
conditions by applying a relatively low controlled potential of 0.8 V for a relatively long period of 20 min. After that, the electrode was carefully rinsed with water and placed into spectroelectrochemical cell. As a working electrolyte, Britton-Robinson buffer solutions of pH 2.0 or 8.0, containing 0.1 M of KCl, were used. Raman spectra were recorded with a confocal microspectrometer inVia (Renishaw, UK) equipped with thermoelectrically cooled at ˗70 °C CCD camera and microscope. For excitation of Raman spectra, the following lasers were used: a continuous-wave 325 nm (He-Cd laser, 1 mW, 2400 lines/mm grating), 442 nm (He-Cd laser, 4 mW, 2400 lines/mm grating), 532 nm (diode-pumped solid state laser, 3 mW, 1800 lines/mm grating), and 632.8 nm (He-Ne laser, 1 mW, 1200 lines/mm grating). The 5×/0.12 NA objective lens was used for excitation and collection of Raman spectra excited with all visible laser lines, and 15×/0.32 NA objective lens was used for UV laser line excited spectra. 3. Results and discussion It is well known that the colour of a thin polyaniline layer depends greatly on the actual protonation level and oxidation state of this polymer. Generally, the protonated form of polyaniline appears pale yellow in its reduced leucoemeraldine state, and gradually changes to green, bluish green, greenish blue, and finally deep blue by a consecutive electrochemical oxidation to emeraldine and pernigraniline state. By taking Raman spectra from polyaniline layer, the exciting laser line could fall into a resonance with an optical absorbance of a particular redox form of this polymer, and thus characteristic features of this particular form could be resonant enhanced at the expense of other forms existing parallel within the same layer at the conditions used. Therefore, a detailed analysis of the data obtained with different laser line excitations to the same polymer film is required. The general optical absorbance spectra for protonated form of polyaniline in its different redox states have been presented and analyzed in [8]. Fig. 1 in [8] presents UV–Vis absorbance spectra for polyaniline layer at different electrode potentials ranging from 0.0 to 0.8 V vs. NHE, thus showing changes in absorbance spectra proceeding during oxidation and reduction processes. From
2. Experimental All spectroelectrochemical experiments have been done in a cylinder-shaped electrochemical cell. A gold electrode of 5 mm in diameter, press-fitted into a Teflon rod holder and placed at about 1–2 mm distance from the cell optical window, was used as a working electrode. Throughout the experiments, the spectroelectrochemical cell has been continuously periodically moved with respect to laser beam at a rate of about 20 mm/s using a custom built equipment. This arrangement was described in details in [25,26], and used in most of our works dealing with labile organic layers covered at an electrode surface in order to avoid or at least diminish laser-induced decomposition of organic layer and to avoid possible photo- and thermoeffects. Platinum wire and saturated Ag/AgCl electrode were used as a counter and reference electrodes, respectively. All electrode potential values reported below refer to this reference electrode. For each experimental set, the working gold electrode was cleaned with a Piranha solution (a mixture of 30% hydrogen peroxide solution and concentrated sulfuric acid, 3:1 by volume), then ultrasonicated in water-ethanol solution for 30 s, and carefully rinsed with water. The cell was driven by BASi-Epsilon model (Bioanalytical systems Inc., USA) electrochemical workstation. Polyaniline layer has been deposited on the working electrode from a solution containing 0.05 M of aniline and 0.5 M sulfuric acid. In order avoid a partial electrochemical degradation of electrosynthesized polyaniline layer, electropolymerisation has been performed at mild
Fig. 1. Raman spectra of polyaniline layer deposited at a gold electrode, obtained with UV laser excitation at 325 nm in pH 5 solution at a controlled electrode potential of 0.3 V (A) and −0.3 V (B), and the difference spectrum obtained by subtraction of spectrum (B) from spectrum (A), depicted as (C).
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these data, we recalculated the optical absorbances for protonated polyaniline form at selected wavelengths, and presented these results in a form of absorbance ratio for oxidized to reduced (Ox/Red), and semioxidized (half-oxidized) to reduced (Semiox/Red) forms in Table 1. The reduced protonated leucoemeraldine form of polyaniline shows an absorbance maximum around 315–320 nm [8]. The absorbance decreases gradually by extending of the oxidation degree for this layer, resulting in an absorbance ratio lower than 1.0. Particularly, for Ox/ Red and Semiox/Red transitions, the absorbance ratio of 0.41 and 0.54 has been obtained (Table 1). Therefore, the UV laser generating 325 nm line has been chosen to get into a resonance with this leucoemeraldine form. Earlier, the formation of an intermediate redox form of polyaniline characterized by absorbance maximum around 420 nm was observed by shifting the electrode potential to higher positive values [8]. Because the optical absorbance of this intermediate semioxidized form reaches its maximum for this particular wavelength and drops by further increase of oxidation degree, the absorbance ratio for Ox/Red appears somewhat lower than for Semiox/Red (Table 1). Thus, in order to get into a resonance with this intermediate form, we used for excitation of Raman spectra a blue laser line at 442 nm, located in the vicinity of its optical absorbance. The lowest optical absorbance within an entire UV–Vis spectrum of leucoemeraldine form of polyaniline is observed within the green spectral range [8]. Here, the absorbance increases greatly by extending the oxidation degree, and, as a result, relatively high ratio for Semiox/Red, and especially for Ox/Red is obtained within the reach of a green laser line at 532 nm used (Table 1). At last, a broad and intense absorbance band in the red range of visible spectrum is observed for oxidized forms of polyaniline [8], and a high ratio for Semiox/Red and especially Ox/Red is observed, enabling potentially to get into a strong resonance with oxidized form of this polymer with the use of 633 nm red laser line excitation (Table 1). In general, the use of four laser lines for Raman spectra excitation ranging from UV at 325 nm through the blue at 442 nm and green at 532 nm up to the red at 633 nm covers nearly all redox forms of polyaniline. 3.1. UV laser line excitation at 325 Nm Fig. 1 depicts an example of getting the difference Raman spectra used in the present study. Here, the original Raman spectra A and B were recorded at an upper (0.3 V), and lower (−0.3 V) electrode potential limits in pH 5.0 solution, thus representing a fully oxidized and fully reduced forms of polyaniline, respectively. It is well seen that the relative intensity for two most prominent Raman bands located around 1620 cm−1 and 1186 cm−1 appears higher for the reduced form, whereas the intensity for the band located around 1513 cm−1 appears higher for oxidized form of polyaniline. As a result, subtraction of spectrum B from spectrum A results in a difference spectrum C that contains negative difference bands at 1620 cm−1 and 1186 cm−1, and a positive difference band at 1513 cm−1. Fig. 2 shows difference Raman spectra obtained at UV laser excitation at 325 nm for solution pH ranging from 1 to 9. The difference in electrochemical potentials for oxidized and reduced forms of polyaniline in Fig. 2 was chosen at 0.6 V, ranging from 0.8 V (for Ox form) to 0.2 V (for Red form) at the lowest pH values up to 0.3 to
Table 1 Optical absorbance ratio at selected wavelengths for oxidized to reduced (Ox/Red) and semioxidized to reduced (Semiox/Red) forms of polyaniline deposited at ITO electrode, as obtained in a solution of 0.5 M of sulfuric acid. The data for calculations were taken from original spectra published in [8]. Absorbance ratio for
Ox/Red Semiox/Red
Wavelength 315 nm (λ max for Red)
420 nm (λ max for Semiox)
532 nm
633 nm
0.41 0.54
2.52 3.37
10.5 3.00
25.3 5.70
3
Fig. 2. Difference Raman spectra of polyaniline layer deposited at a gold electrode, calculated by subtracting spectra for the reduced form from those for the oxidized form (Ox-Red), as obtained with UV laser excitation at 325 nm in solutions of different pH ranging from 1 to 9 (as indicated).
−0.3 V for the highest pH values. The highest potential of 0.8 V chosen does not exactly correspond to a fully exhaustive electrooxidation of polyaniline; however, this somewhat lower potential ensures the electrochemical stability of polyaniline layer, since its degradation is known to proceed relatively fast at higher potentials. In common, the spectra appear poor in regard to the number of Raman bands. From these, two bands are of a great interest. The band located around 1620 cm−1 corresponds to C\\C stretching vibrations in benzene type rings. In an entire pH range studied, this difference band is negative, meaning that its intensity diminishes by going from the reduced to oxidized form. Indeed, electrooxidation of polyaniline leads to a decrease of the number of benzene type rings at the expense of increasing number of quinonediimine rings. It is seen from Fig. 2 that this negative difference band appears low in intensity at low solution pH, but increases by shifting pH to higher values reaching its maximum around pH 5, and then decreasing gradually throughout the rest of solution pH range. Also, a small shift of its frequency from 1620 to 1625 cm−1 could be noted by increasing pH. Close similarly, a negative difference Raman band at 1186 cm−1, which follows same pH-dependent tendencies in its magnitude as the band at 1620 cm−1, is well observed (Fig. 2). This difference band relates to C\\H bending vibrations in leucoemeraldine, and it appears negative obviously because of a decrease in the content of leucoemeraldine form by progressive electrooxidation. The only remarkable positive difference Raman band appears at 1513–1505 cm−1, and corresponds probably to phenazine-type minor side structures present within the polymer structure [27,28]. The most of difference Raman bands observed and their assignments based on previous studies [3–7,27–34] are summarized in Table 2. Noteworthy, no well recognized difference Raman bands characteristic for oxidized form of polyaniline are observed in an entire range of solution pH, even the band around 1585 cm−1, representing C_C stretching vibrations in quinonediimine type rings of oxidized polyaniline structure. This means that UV laser line used in obtaining of the spectra shown in Fig. 2 is in a strong resonance with the reduced leucoemeraldine form, not allowing the development of any detectable feature for oxidized form.
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Table 2 Tentative assignments of difference Raman bands as obtained for different spectra excitation wavelengths. Difference Raman bands (in cm−1) for different excitation wavelengths and solution pH (in parentheses) 325 nm
442 nm
Ox-Red
Ox-Red
1620↓ (5) 1625↓ (9)
1624↓ (1–4)
532 nm
633 nm
Semiox-Red
Ox-Red
Ox-Red
1625↓ (2) 1622↑ (3–9)
1628↓ (2) 1610↓ (9) 1582↑ (2) 1584↑ (9) 1552↑ (2) 1550↑ (9)
1582↑ (5–9)
1513↑ (5) 1505↑ (9)
1186↓ (2–9)
1488↑ (4)
1187↓ (2) 1182↓ (5) 1162↑ (5–9)
1192↑ (2–9) 1185↓ (2)
885↓ (2) 830↓ (2) 780↑ (2–9)
880↑ (5) 825↑ (5)
1453↑ (2) 1485↑ (5) 1470↑ (9) 1380↑ (2–9) 1324↑ (2–9) 1224↑ (2) 1218↑ (5–9) 1190↓ (2) 1184↓ (5) 1162↑ (2–9) 849↑ (2–9) 778↑ (2, 9) 792–778↑ (5) 750↑ (2–9) 525↑ (2–9)
Tentative assignments
Semiox-Red
1585↑ (2–5)
1626↑ (2)
C\ \C stretching in B (8a)
1586↑ (2)
C_C stretching in Q (8a)
1476↑ (2)
Phenazine type fragments C_N stretching in emeraldine base (imine sites)
1343–1320↑ (2) 1218↑ (5)
C\ \N+ stretching in polaronic form (polarons) C\ \N+ stretching in polaronic form (polarons) C\ \N stretching in emeraldine (amine)
1554↑ (2–5)
1471↑ (2) 1476↑ (5) 1462↑ (9)
1220↑ (2–9)
C\ \H bending in leucoemeraldine (9a) 1162↑ (2) 1165↑ (5–9) 852↑ (2–5)
1168↑ (2) 1162↑ (5)
800–780↑ (2–5)
805↑ (2) 786↑ (5)
753↑ (2–5)
C\ \H bending in emeraldine (9a) B deformation (1) Q deformation Imine deformation (C\ \N\ \C bending) Amine in-plane deformation
Assignments are based mainly on the known data [3–7,27–34]. Spectra denoted by Ox-Red were obtained by subtracting of spectra for the reduced form from those for oxidized form, and spectra denoted by Semiox-Red – by subtracting of spectra for the reduced form from those for a half-oxidized (semioxidized) form. For solution pH of 2, 5, and 9, the electrode potentials for oxidized, semioxidized and reduced forms were chosen same as indicated in the corresponding Figures. The arrows (↓) and (↑) indicate the tendencies for a decrease or an increase of intensity for spectral bands, respectively, the symbols (1), (8a), and (9a) denote Wilson‘s notations of vibrational modes for benzene derivatives, and B and Q denote benzene and quinonediimine type rings, respectively.
3.2. Blue laser line excitation at 442 Nm Somewhat similar difference Raman spectra were obtained with the blue laser line excitation, presented in Fig. 3. Again, intense negative difference bands around 1624 cm−1 appear dominating over the whole spectrum, indicating a decreasing number of benzene type rings upon
Fig. 3. Same as in Fig. 2, as obtained with blue laser excitation at 442 nm.
electrooxidation. As distinct from UV laser line excitation, the most intense negative difference bands for 442 nm excitation appear at the lowest solution pH values. At pH over 3, the intensity drops greatly, and at pH exceeding 8, this difference band cannot be detected (Fig. 3). Another well expressed feature is an intense negative difference band at 1187–1182 cm−1, representing C\\H band bending vibrations in leucoemeraldine form (Table 2). This difference band is best expressed in acidic solutions, and indicates a decrease in the content of leucoemeraldine form upon electrooxidation. The next feature, distinct from that observed at 325 nm excitation, is the development of two negative mid-intense difference bands at 885 cm−1 and 830 cm−1, well seen in acidic solutions up to pH of 4 (Fig. 3). These difference bands represent benzene ring deformations. As distinct from UV laser excitation, some spectral features characteristic for oxidized form are also observed. First, the positive (i.e. growing upon electrooxidation) difference band at 1582 cm−1 is seen at solution pH over 5 (Fig. 3). This positive band presents C_C stretching vibrations of quinonediimine type rings as the most characteristic feature for oxidized form of polyaniline. Over pH of 4, a new positive band at 1488 cm−1 appears. This difference band shows an increasing number of C_N stretchings in an oxidized form, the emeraldine base form (Table 2). Again, a new positive difference band around 1162 cm−1 appears within pH range of 5 to 9, representing C\\H bending vibrations in emeraldine. This difference band grows up with increasing pH along with the decrease of a negative difference band around 1187–1182 cm−1, characteristic for similar C\\H bending mode for leucoemeraldine. A positive difference band around 780 cm−1 should be noted, representing quinone ring vibrations. In summary, the blue laser line excitation, as similar to UV excitation, results mainly in developing of features characteristic for leucomeraldine form and thus diminishing in their intensities upon electrochemical oxidation. As distinct from UV excitation, however, blue laser line excitation reveals also the development of features characteristic for emeraldine form upon electrooxidation of polyaniline layer.
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A different behaviour was observed for difference spectra between semioxidized and reduced forms, as depicted in Fig. 4. While Ox-Red spectra presented in Fig. 3 were calculated by the difference between fully oxidized and reduced forms, as obtained at a high electrode potential difference reaching 0.8 V, the Semiox-Red difference spectra of Fig. 4 relate to a half of this electrode potential difference of 0.4 V. For calculations of Semiox-Red differences, spectra for reduced forms were taken same as for Ox-Red spectra, whereas spectra for half-oxidized forms belong to a middle electrochemical potential. At a low solution pH of 1 and 2, as it could be expected, negative difference band at 1625 cm−1 is observed indicating a decrease in a number of benzene rings in the course of electrooxidation. However, an increase in solution pH up to 3 results unexpectedly in a positive difference band at 1622 cm−1. This positive difference band reaches its maximum around pH of 4, then gradually diminishes, and retains through the whole pH range studied up to 9 (Fig. 4). Changes in relative intensities of difference Raman bands with varying of solution pH for UV and blue laser line excitations are presented in Fig. 5. This unexpected behaviour could be understood taking into account that the light absorbance ratio for Semiox-Red forms is higher than for Ox-Red forms at the absorbance maximum (420 nm) for the half-oxidized form (Table 1). This means that a resonance enhancement of Raman scattering is possible for the half-oxidized form at given experimental conditions with the use of 442 nm laser line excitation that appears in the vicinity of maximum absorbance of this semioxidized form. This resonance enhancement leads to increasing intensity of 1622 cm−1 band and thus to the development of positive difference band at these particular conditions, despite of the decreasing number of reduced fragments (benzene type rings) upon electrooxidation. We discussed this phenomenon in our previous paper [21]. Another feature that is observed with 442 nm excitation in an acidic solution is the presence of a negative difference band around 1185 cm−1 (Fig. 4). Similarly as for Ox-Red difference spectra, this negative difference band, presenting C\\H bending vibrations in leucoemeraldine (Table 2), indicate a decreasing number of reduced sites upon electrooxidation. However, as distinct from Ox-Red difference spectra, a new positive Raman band at slightly higher frequencies around 1192 cm−1 starts to develop in Semiox-Red difference spectra even at low pH values, and grows in intensity with increasing pH. The pH behaviour of this positive difference band resembles that of the
5
Fig. 5. Dependence of relative intensity for difference Raman bands on solution pH as obtained for 325 nm and 442 nm laser excitations (as indicated) for Ox-Red and Semiox-Red difference Raman spectra.
1622 cm−1 band, therefore, it could be assumed that this band presents C\\H bendings in semioxidized form. Also, as distinct from Ox-Red spectra, almost no positive difference band around 1162 cm−1, corresponding to C\\H bendings in emeraldine, is observed, at least up to the neutral pH values, indicating a small content of emeraldine formed during this partial electrooxidation. A closely similar behaviour should be noted regarding the difference band around 780 cm−1 and representing quinone ring deformations, characteristic for oxidized form. Well seen as a positive difference band for Ox-Red spectra in an entire pH range of 2 to 9, this band appears of low intensity or even undetectable for Semiox-Red spectra, thus indicating a small content of emeraldine form in the latter case. In accordance with assumption of resonance enhancement with 442 nm excitation, two positive difference bands around 880 and 825 cm−1, corresponding to benzene ring deformations and thus characteristic for the reduced form, are observed in Semiox-Red spectra within the pH range of 3 to 5. For Ox-Red spectra, as expected, both these difference bands are negative (cf. Fig. 3 and Fig. 4). 3.3. Green laser line excitation at 532 Nm
Fig. 4. Same as in Fig. 3, calculated by subtracting spectra for the reduced form from those for the semioxidized form (Semiox-Red).
The most rich in their features appear difference Raman spectra obtained at a green laser excitation (532 nm), as presented in Fig. 6. Because of moderately high difference in light absorbance ratio for Ox and Red forms (Table 1), the most features result in positive difference bands and thus relate to changes occuring along with electrooxidation. Among them, C_C double bond stretchings in quinonediimine rings around 1584–1582 cm−1, C\\N single bond stretchings in emeraldine around 1224–1218 cm−1, and C\\H bendings in emeraldine at 1162 cm−1 are most characteristic and showing electrooxidation of polyaniline to proceed. One of the most prominent features of oxidized form are stretching vibrations of a double C_N bond in emeraldine base, well seen as a broad intense difference band at pH exceeding 1, where this basic form exists. The position of this positive difference band, however, changes to a remarkable extent depending on solution pH. In acidic solutions, this difference band appears at 1453 cm−1, but shifts toward higher frequency up to 1485 cm−1 by changing pH up 4 or 5, and again shift to lower frequency of 1470 cm−1 by going to pHneutral and basic solution (Table 2). Very probably, this frequency
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Fig. 6. Same as in Fig. 2, as obtained with green laser excitation at 532 nm.
recurrency pattern is caused by the presence of a few different protonation states for emeraldine base. While other difference Raman bands are summarized in Table 2, the presence of the two negative difference bands should be noted. One of them relates to C\\C stretchings in benzene type rings, and shows the decreasing number of benzene rings at electrooxidation. This negative difference band appears around 1628 cm−1 for acidic solutions, and shift up to 1610 cm−1 upon increase of solution pH up to 9 (Fig. 6). An another negative difference band relates to C\\H bendings in leucoemeraldine, and appears in acidic and neutral solutions around 1190–1184 cm−1 (Table 2).
Fig. 7. Same as in Fig. 2, as obtained with red laser excitation at 633 nm.
intense positive difference band around 1626 cm−1, corresponding to benzene rings, is well seen in acidic solutions at pH not exceeding 4 (Fig. 8). When going by 0.3 V toward the positive potentials, the number of benzene rings in the polymer structure decreases, although not as much as for a bigger potential step of 0.6 V. At the same time, however, the optical absorbance increases by 5.7 times at this particular wavelength of 633 nm (Table 1). Thus, a combination of a decreasing number of benzene rings and a resonance enhancement results in an increase of a relative intensity for this difference Raman band, i.e. in a positive difference band. Next, the presence of a positive difference band at
3.4. Red laser line excitation at 633 Nm At a red laser line excitation of 633 nm, the light absorbance ratio for Ox/Red forms appears very high (Table 1), therefore, a resonance enhancement of Raman spectra for oxidized form of polyaniline occur. Because of a high light absorbance in the red spectral region for Ox forms, all factors influencing the absorbance properties of polyaniline should influence the resulting resonance Raman spectra. Among others, the protonation level and its changes by changing the solution pH seem to be of a great importance. As for fully oxidized form pernigraniline, the reversible deprotonation was reported to proceed at low pH values of 0.5–1.0, whereas the resulting deprotonated form possesses somewhat different light absorbance profile, thus potentially influencing the resonance conditions [35]. The most positive difference bands related to features appearing upon electrooxidation are related to those observed at a green laser line excitation, and are presented in Fig. 7, and summarized in Table 2. Because of the said high difference in optical absorbance of Ox and Red forms and accompanying resonance enhancement, the intensity of both negative difference bands appear low. However, even with 633 nm enhancement, a negative difference band in the vicinity of 1620 cm−1 is clearly seen within a broad range of solution pH (Fig. 7). Fig. 8 presents difference Raman spectra for half-oxidized minus reduced forms of polyaniline (Semiox-Red). While the electrochemical potential difference for Ox-Red spectra presented in Fig. 7 was chosen as high as 0.6 V in order to ensure a full electrooxidation, a much lower difference of 0.3 V was set in obtaining the spectra presented in Fig. 8. Some important spectral differences between Ox-Red (as in Fig. 7), and Semiox-Red (as in Fig. 8) should be noted. First, relatively
Fig. 8. Same as in Fig. 4, calculated by subtracting spectra for the reduced form from those for the semioxidized form (Semiox-Red), as obtained with red laser excitation at 633 nm.
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1586 cm−1 seems to be an expected consequence of increasing number of quinonediimine-like fragments formed during this partial electrooxidation. The most striking appears an interplay between the two vibrational modes – C_N stretches in emeraldine, representing imine sites characteristic for oxidized forms, and C\\N+ stretches representing polaronic sites in a half-oxidized structure. As for a big potential step of 0.6 V (Ox-Red), the C_N stretches manifests as a very strong and dominating by its intensity positive difference band around 1476–1471 cm−1 just at pH of 2 and over that (Fig. 7). For the small potential step of 0.3 V (Semiox-Red), again, a positive difference band at 1476 cm−1 is observed, however, its intensity remains relatively low at solution pH 1 up to pH 3 (Fig. 8). At pH of 4, this difference band grows highly and again turns dominating over the spectrum (Fig. 8). At the same time, a couple of positive difference bands appear within the range of 1343–1320 cm −1 , representing polaron sites in a semioxidized structure [27]. Noteworthy, for a big potential step of 0.6 V, only low-intense positive difference bands within same wavenumber range are observed, thus indicating a smaller number of polaronic sites in a fully oxidized form as compared to a semioxidized one (cf. relative intensities for difference bands within the spectral range of 1400–1300 cm −1 in Fig. 7 and Fig. 8). Most pronounced for the lowest pH of 1 as two well separated difference bands, these bands tend to merge and decrease in intensity by changing of solution pH from 1 to 3 (Fig. 8). At the same time, a decrease of difference bands for polaronic form with increasing pH up to 3 clearly appears to be accompanied by synchronic increase of imine band at 1476 cm−1, meaning that a decrease in a number of polaronic sites proceeds synchronic with an increase in a number of imine sites with increasing solution pH. Finally, at pH of 4, the polaronic form almost disappears from difference spectra, whereas the positive band for imine form grows greatly and again appears dominating over an entire spectrum (Fig. 8). These observations correlate well with known changes occuring in polaronic form of polyaniline upon protonic dedoping of polyaniline, viz. upon increasing pH over ca. 4, especially with the loss of electric conductivity at pH above 3 or 4, and other properties as well [1].
4. Conclusions Specific Raman features were disclosed by the differential Raman spectroscopy for redox transformations of polyaniline from its oxidized to semioxidized, and to reduced forms within a wide solution pH range. The observed Raman features were shown to depend drastically on the spectra excitation wavelength. For UV laser line excitation at 325 nm, the features of the reduced form are observed, resulting in negative difference Raman bands (Ox-Red) within an entire pH range studied from pH 1 to pH 9. At a blue excitation at 442 nm, both positive and negative difference bands for Ox-Red spectra are observed that are dependent on solution pH, whereas an inversion from negative to positive difference bands for Semiox-Red difference spectra was observed by increasing solution pH, indicating a possible resonance with an intermediate half-oxidized form of polyaniline. Characterisctic features were identified for the green (532 nm) and red (633 nm) excitations taking place upon electrooxidation and reversible protonation of polyaniline layer. At a red excitation, characteristic features for polarons were observed for Semiox-Red difference spectra that are absent in the corresponding Ox-Red difference spectra. The positive difference bands for polarons appear at a low solution pH of 1. An increase of solution pH results in a diminution of these difference bands of polarons that disappear at pH above 4.
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Author contribution 1. Regina Mazeikiene: Methodology, Investigation 2. Gediminas Niaura: Validation, Data Curation, Writing – Reviewing and Editing 3. Albertas Malinauskas: Conceptualization, Supervision, Writing – Original Draft preparation Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] G. Ćirić-Marjanović, Recent advances in polyaniline research: polymerization mechanisms, structural aspects, properties and applications, Synth. Metals 177 (2013) 1–47. [2] M. Lapkowski, K. Berrada, S. Quillard, G. Louarn, S. Lefrant, A. Pron, Electrochemical oxidation of polyaniline in nonaqueous electrolytes: in situ Raman spectroscopic studies, Macromolecules 28 (1995) 1233–1238. [3] S. Quillard, K. Berrada, G. Louarn, S. Lefrant, M. Lapkowski, A. Pron, In situ Raman spectroscopic studies of the electrochemical behavior of polyaniline, New J. Chem. 19 (1995) 365–374. [4] M. Lapkowski, K. Berrada, S. Quillard, G. Louarn, S. Lefrant, Study of the redox properties of polyaniline in non-aqueous media by Raman diffusion, J. Chim. Phys. Phys.Chim. Biol. 92 (1995) 915–918. [5] M. Boyer, S. Quillard, G. Louarn, S. Lefrant, E. Rebourt, A.P. Monkman, Oxidized model compounds of polyaniline studied by resonance Raman spectroscopy, Synth. Metals 84 (1997) 787–788. [6] M.C. Bernard, A. Hugot-Le Goff, Raman spectroscopy for the study of polyaniline, Synth. Metals 85 (1997) 1145–1146. [7] M.C. Bernard, S.C. De Torresi, A. Hugot-Le Goff, In situ Raman study of sulfonatedoped polyaniline, Electrochim. Acta 44 (1999) 1989–1997. [8] A. Malinauskas, R. Holze, Cyclic UV-Vis spectrovoltammetry of polyaniline, Synth. Metals 97 (1998) 31–36. [9] Z. Moravkova, E. Dmitrieva, Structural changes in polyaniline near the middle oxidation peak studied by in situ Raman spectroelectrochemistry, J. Raman Spectrosc. 48 (2017) 1229–1234. [10] G.M. Do Nascimento, Raman dispersion in polyaniline nanofibers, Vibrat. Spectrosc. 90 (2017) 89–95. [11] M. Blaha, J. Zednik, J. Vohlidal, Self-doping of polyaniline prepared with the FeCl3/ H2O2 system and the origin of the Raman band of emeraldine salt at around 1375 cm−1, Polym, Internat 64 (2015) 1801–1807. [12] Y. Wang, M.X. Shang, Y.N. Wang, Z.R. Xu, Droplet-based microfluidic synthesis of (au nanorod@Ag)-polyaniline Janus nanoparticles and their application as a surfaceenhanced Raman scattering nanosensor for mercury detection, Anal. Methods 11 (2019) 3966–3973. [13] Z.C. Li, L. Xia, G.K. Li, Y.L. Hu, Raman spectroscopic imaging of pH values in cancerous tissue by using polyaniline@gold nanoparticles, Microchim. Acta 186 (2019) https:// doi.org/10.1007/s00604-019-3265-4. [14] S.Y. Li, Z.M. Liu, C.K. Su, H.L. Chen, X.X. Fei, Z.Y. Guo, Biological pH sensing based on the environmentally friendly Raman technique through a polyaniline probe, Anal. Bioanal. Chem. 409 (2017) 1387–1394. [15] Z. Moravkova, P. Bober, Writing in a polyaniline film with laser beam and stability of the record: a Raman spectroscopic study, Internat. J. Polym. Sci. (2018) Art. No. 1797216. [16] F.G. Xu, S. Xie, H. Xu, X. Chen, H. Yu, L. Wang, Interlaced silver nanosheets grown on polyaniline coated carbon foam as efficient three dimensional surface enhanced Raman scattering substrate for molecule sensing, Appl. Surf. Sci. 410 (2017) 566–573. [17] G.M. Do Nascimento, N.A. Pradie, Deprotonation, Raman dispersion and thermal behavior of polyaniline-montmorillonite nanocomposites, Synth. Metals 217 (2016) 109–116. [18] N. Badi, S. Khasim, A.S. Roy, Micro-Raman spectroscopy and effective conductivity studies of graphene nanoplatelets/polyaniline composites, J. Mater. Sci. – Mater. Electron. 27 (2016) 6249–6257. [19] S. Dutt, P.F. Siril, V. Sharma, S. Periasamy, Gold(core)-polyaniline(shell) composite nanowires as a substrate for surface enhanced Raman scattering and catalyst for dye reduction, New J. Chem. 39 (2015) 902–908. [20] R. Mažeikienė, V. Tomkutė, Z. Kuodis, G. Niaura, A. Malinauskas, Raman spectroelectrochemical study of polyaniline and sulfonated polyaniline in solutions of different pH, Vibrat. Spectrosc. 44 (2007) 201–208. [21] R. Mažeikienė, G. Niaura, A. Malinauskas, Raman spectroelectrochemical study of polyaniline at UV, blue, and green laser line excitation in solutions of different pH, Synth. Metals 243 (2018) 97–106. [22] R. Mažeikienė, G. Niaura, A. Malinauskas, Red and NIR laser line excited Raman spectroscopy of polyaniline in electrochemical system, Synth. Metals 248 (2019) 35–44. [23] R. Mažeikienė, G. Niaura, A. Malinauskas, A comparative multiwavelength Raman spectroelectrochemical study of polyaniline: a review, J. Solid State Electrochem. 23 (2019) 1631–1640.
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