Abstract
Conducting polymers, such as polyaniline and polypyrrole, have frequently been discussed in the literature due to ease of preparation and high application potential. These polymers have been observed to interact with organic dyes because of the similarity in the conjugated molecular structure of both moieties. The interaction manifests itself in three fundamental directions that have been so far treated separately. The first is represented by the conductivity enhancement and morphology control when using organic dyes as templates in polypyrrole preparation. The adsorption of dyes on conducting polymers is the second field oriented at the water pollution treatment. Finally, the photocatalytic decomposition of organic dyes aims at the similar environmental target. The last two applications do not require the presence of conductivity which, on the other hand, is a key parameter of conducting polymers. The future design of advanced adsorbents, however, has to exploit both the conductivity and electroactivity in the control of pollutant adsorption or degradation. For this reason, all these interactions and their practical impact are considered in the present review.
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Preamble
The oxidation of aniline as early as in 1834 afforded a green product called polyaniline in present terminology (Rasmussen 2018). This is an example of probably the first man-made polymer. Its conductivity, however, was revealed only much later (MacDiarmid 2001). The first commercial synthetic dye, mauveine, was discovered serendipitously by William Henry Perkin in 1856 also by the oxidation of aniline (Sousa et al. 2008). Other aniline dyes soon followed: fuchsin, safranin and induline. Many synthetic organic dyes have since been prepared. Conducting polymers, such as polyaniline, and dyes thus have a common origin and related history. It is proposed that they may have also a promising joint future.
Introduction
The present review is based on the assumption that conducting polymers and organic dyes interact due to the similar features of their molecular structure. Such interactions manifest themselves in three directions: (1) by the effects of dyes on the preparation of conducting polymers, (2) in the adsorption of dyes on conducting polymers and (3) by the photocatalytic degradation of dyes using conducting polymers. It thus connects three seemingly unrelated research directions and points out to their common roots. Among conducting polymers, polyaniline and polypyrrole have been selected as the main objects of concern due to the simplicity and similarity of their preparation and large number of studies that have already appeared in the literature. They both include aromatic structure and nitrogen atoms in the constitutional units, which are likely to play role in their interactions with dyes.
It is not the purpose of this review to enumerate quantitatively the results presented in the literature but rather to organize them according to the type of experiments done and the dyes involved in them. The review is thus meant to provide the guidance to the researchers interested in concrete systems by listing the studies that have already been done on them and to point out the directions that are still a challenge to future research.
Conducting polymers
The design of conducting organic systems, represented here by polyaniline (Stejskal et al. 2010, 2015), polypyrrole (Sapurina et al. 2017; Stejskal and Trchová 2018) and their composites, is highly desirable for new applications (Bhadra et al. 2009; Inzelt 2017) in energy-conversion and energy-storage devices (Meng et al. 2017), analytical sciences (Jain et al. 2017), corrosion protection of metals (Yang et al. 2015; Ates 2016), in biomedicine (Humpolíček et al. 2018; Nair et al. 2019; Runsewe et al. 2019) and in environmental issues (Nasar and Mashkoor 2019). The last uses of conducting polymers, such as water pollution treatment, have recently come to the forefront (Zare et al. 2018a) despite the fact that the conductivity, the key parameter of conducting polymers, has no impact on this type of applications.
Conducting polymers represent unique class of semiconducting, electroactive and responsive materials with a benefit of easy and economic preparation. Due to the limited processability, conducting polymers have often been used in the composites with organic and inorganic materials that improved the processing and utility properties (Khatoon and Ahmad 2017). Especially one-dimensional conducting-polymer nanofibers and nanotubes has received attention in the field of nanotechnologies (Long et al. 2011; Baker et al. 2017; Huang et al. 2017) as demonstrated by the number of papers published on polypyrrole nanotubes (Stejskal and Trchová 2018). Polyaniline and polypyrrole (Fig. 1), are easily prepared by the chemical oxidation of respective monomers in acidic aqueous medium. Ammonium peroxydisulfate is favoured oxidant of aniline while iron(III) chloride is the typical oxidant of pyrrole. Conducting polymers are received as polymer salts. It should be kept in mind that the properties of these polymers substantially vary for different counter-ions A− afforded by protonating acids (Stejskal et al. 2004a, 2008a, 2016), sometimes called as “dopants”.
Polyaniline (emeraldine form) and polypyrrole are prepared as salts with various acids, HA (depicted in red)
Conducting polymers and organic dyes have one feature in common. They are coloured due to the presence of conjugated double bonds in the molecular structure and consequent selective absorption of light in visible region. While the former group has a polymer character including polarons as charge carriers responsible for the conductivity, the dyes are low-molecular-weight compounds rated as electric insulators.
Organic dyes
Organic dyes are by definition used in dyeing of various substrates. Similarly to conducting polymers, their molecular structure is based on the system of conjugated double-bonds afforded by chromogenic groups (Fig. 2). Many other organic compounds are also coloured while they are not regarded as dyes in the technological sense. They are represented by various drugs (cytostatics, antibiotics, antibacterial agents, etc.), herbicides and pesticides, acidobasic or redox indicators, and they appear in natural products, such as in fruits or vegetables, etc. From the point of molecular structure, they also have a conjugated system of bonds responsible for the light absorption even though they are not regarded as dyes. For that reason they are also marginally considered in this review.
Typical chromogenic segments that appear in organic dyes: phenazine (phz), phenothiazine (ptz), dibenzopyran (dbp), anthraquinone (anq), azo, and triphenylmethane (tpm)
Dyes are intensely coloured organic substances which impart colour to a substrate by selective absorption of light. They are usually well soluble in aqueous media. The use of synonyma and commercial trademarks for the dye names somewhat complicates the survey of the literature. For example, Congo red is also known as Congo Red 4B, Cosmos Red, Cotton Red B, Cotton Red C, Direct Red R, Direct Red Y, etc. For that reason the Colour Index (https://www.colour-index.com) generic name, here Direct Red 28, has been introduced and used in this review when available. Generic name identifies a commercial product by its usage class, its colour shade and a number reflecting the chronological order of the registration with the Colour Index. The examples of dye classes are: Acid, Basic, Direct, Disperse, Food, Mordant and Reactive, but others have also been in use. The colour shades are Black, Blue, Green, Orange, Red, Violet and Yellow.
The dyes can be divided in two basic groups. Anionic dyes carry one or more negatively charged groups (Fig. 3), typically sulfonate or carboxyl ones that promote the dye solubility in aqueous media. They are usually delivered as sodium salts. Methyl orange (Fig. 3) is probably the most important member of this family when it comes to the morphology control of polypyrrole. There is a large variety especially of azo dyes that have been investigated for their interaction with conducting polymers in adsorption experiments. Cationic dyes include a positive charge, usually localized on nitrogen atom, which is balanced by a counter-ion. Among cationic dyes, methylene blue, a photosensitizer used to produce singlet oxygen when exposed to both light and oxygen, and rhodamine B (Fig. 3) have been the dyes of choice in photocatalytic degradation. Safranin also is an important example of a cationic dye.
Examples of cationic dyes (safranin, methylene blue, rhodamine B, crystal violet) and anionic dyes (Acid Blue 25, methyl orange)
Dyes have frequently been observed to interact with conducting polymers. Two classes of experiments have to be distinguished. In the first, the dyes are present during the preparation of conducting polymers and, as a rule, they affect both their morphology or conductivity or both. The second group reports the preparation of conducting polymers followed by the interaction with organic dyes. This is illustrated by numerous studies on adsorption of dyes on conducting polymers or by photocatalytic decomposition of dyes. The present contribution reviews the relevant papers in order to elucidate nature of the interaction and ways of its exploitation.
Principles of interaction
There are four fundamental ways how conducting polymers and dyes interact:
- 1.
The π–π interaction between the aromatic rings is based on sharing the π-electrons by individual molecules and constitutional polymer units, which is favourable from the energy point of view. As the conducting polymers and dyes share the common features of molecular structure, the π–π interaction between their constitutional units and dyes is possible and probably dominating (Fig. 4). The charge-transfer complexes between chemisorbed dyes and the chains of conducting polymers are expected to exist in analogy with phenosafranin dye and carbon nanotubes (Curran et al. 2004). The formation of one-dimensional liquid-crystalline J-aggregates produced by organic dyes themselves is based on the similar principle (Harrison et al. 1996; Collings et al. 2010; Würthner et al. 2011; Gospodinova and Tomšík 2015).
Fig. 4 
Illustration of π–π, ionic, and hydrogen bonding interactions between conducting polymer, polypyrrole, and organic dye, methyl orange
- 2.
The electrostatic ionic interactions have also to be considered (Fig. 4). Conducting polymers discussed here are polycations (Fig. 1) and their ionic coupling with sulfo groups in anionic dyes is anticipated (Stejskal et al. 2008a; Wang et al. 2019a; Yang et al. 2019), i.e. they may constitute, in principle, the mutual salts. This concerns especially the cases when polyaniline or polypyrrole bases would interact with anionic dyes in acid form and not with sodium salts. The experimental support of this type of interaction is still scarce (Mahanta et al. 2008; Janaki et al. 2012a), but was proposed (Prasad and Joseph 2017), e.g., to manifest itself by cross-linking with dyes carrying multiple sulfo groups (Yang et al. 2019).
- 3.
The hydrogen bonding is represented by interactions of hydrogen atoms with nitrogen atoms in conducting polymers and nitrogen- or oxygen-containing groups in dyes (Fig. 4) (Li et al. 2017a; Sarkar et al. 2018). The intermolecular hydrogen bonding between amino –NH– and imine –N = nitrogens, which was well established by FTIR spectroscopy (Trchová and Stejskal 2011), is probably the reason for the insolubility of conducting polymers in aqueous media. Such interaction is likely to constitute the basis of dye adsorption on conducting polymers. We can speculate that the nitrogen atoms in dyes would be involved in similar manner.
- 4.
The hydrophobic interactions may also take place (Ren et al. 2018). It is important to realize that the dyes resemble surfactants (Shi et al. 2017; Bai et al. 2018). They similarly have a hydrophilic ionic group that promotes the solubility in water and large hydrophobic part (Fig. 3). The latter part has a tendency to self-assemble to micellar objects or to interact with hydrophobic parts of conducting-polymer chains. In the contrast to classical surfactants, where the hydrophobic part is represented by flexible chains and promotes the formation of micelles, the stiff part in dyes favours the formation of one-dimensional self-assembled aggregates. The list of interactions may be extended to dipole–dipole interactions, dispersion forces and other weak interactions of van der Waals type.
The possibility of covalent bonding was exceptionally proposed in the literature (Jangid et al. 2014) but the chemical reaction between conducting polymers and dyes is hardly expected to occur spontaneously. The synthetic procedure aimed at the covalent attachment of pendant Rhodamine 6G and Azure B to polyaniline chain, however, has been reported (Jangid et al. 2015). From the formal point of view the copolymerization of aniline with some dyes bearing amino groups might fall into this category, too. Such situation might appear when polyaniline is prepared in the presence of organic dyes.
As mentioned above, the interaction between conducting polymers and dyes manifests itself in four ways:
- 1.
Control of polymer morphology. The morphology of polyaniline prepared by the oxidative polymerization of aniline depends on the reaction conditions; globules, nanotubes and nanofibres being the most common forms (Sapurina and Stejskal 2008; Stejskal et al. 2008b, 2010). Polypyrrole is typically produced also in globular form (Fig. 5a). Only after introduction of some dyes, such as methyl orange, its morphology converts to nanotubes (Fig. 5b). The search for the dyes that guide one-dimensional growth is of importance (Sapurina et al. 2017). One-dimensional objects are better suited for the preparation of conducting composites because the percolation threshold can be reached at considerably lower volume fraction compared to three-dimensional globular objects (Li et al. 2009). Nanotubes have also higher specific surface areas than globules (Zeng et al. 2013; Stejskal and Trchová 2018). One-dimensional polymer structures are also more suitable to act as adsorbents (Stejskal and Trchová 2018). The polymer nanotubes and nanofibers are preferred morphologies in biomedical applications (Nair et al. 2019).
Fig. 5 
a Globular polypyrrole and b polypyrrole nanotubes produced in the presence of methyl orange (Stejskal et al. 2016)
- 2.
Conductivity enhancement. When the dyes participate in the preparation of conducting polymers, they significantly affect the resulting conductivity both in negative or positive manner (Sapurina et al. 2017). For instance, methyl orange increased the conductivity of polypyrrole at low concentrations but the conductivity dropped when the dye concentration became high (Fig. 6). The conductivity increase was often substantial, from 1 S cm−1 to 100 S cm−1 for polypyrrole nanotubes (Li et al. 2017b). The link between morphology and conductivity, however, is definitely not straightforward. It is tempting to assume that dyes assist in intermolecular charge transport between polymer chains due to their conjugated molecular structure, but convincing support of such hypothesis has not yet been provided in the literature. In addition, it has recently been observed that dyes alone display low, but non-negligible conductivity, up to 10−7 S cm−1. The interaction of two different semiconductors thus may provide materials with new physical properties.
Fig. 6 
Conductivity of polypyrrole in dependence on methyl orange concentration in the reaction mixture (adapted from Sapurina et al. 2017)
- 3.
Dye adsorption. The interaction between dyes and conducting polymers manifests itself in adsorption phenomena (Fig. 7). There are numerous papers reporting the role of conducting polymers as dyes adsorbents applicable in water-pollution treatment (Ayad et al. 2018a) as reviewed below. The dye removal is conveniently followed by the decrease in the optical adsorption of solutions (Fig. 8). The process has usually been described in terms of adsorption isotherms in detail, but the understanding of the adsorption nature is still open to discussion. In the contrast to classical adsorbents, controlled adsorption and release on conducting polymers could be achieved by pH control and possibly also by exploiting electrochemical activity of conducting polymers.
Fig. 7 
Conducting polyaniline/poly(vinyl alcohol) aerogel adsorbs methylene blue from aqueous solution (Islam Minisy, unpublished results)
Fig. 8 
The experiment shown in Fig. 7 can be followed by decrease in optical absorption (Islam Minisy, unpublished results)
- 4.
Photocatalytic dye degradation. The deposition of conducting polymers on various inorganic photocatalysts, such as metal oxides, has led to the improved photocatalytic degradation dyes. The literature on above four directions is reviewed below in more detail.
Dyes in the preparation of conducting polymers
The organic dye used in the preparation of conducting polymers can fulfil several roles, the generation of the template for deposition of polyaniline or polypyrrole being the most currently met. In addition, the dyes that include primary amino group in their molecular structure may polymerize alone or copolymerize with aniline or pyrrole.
Polyaniline
Influence of the organic dyes on the morphology of polyaniline produced in their presence has not been systematically studied (Table 1). The “standard” polymerization of aniline hydrochloride (Stejskal and Gilbert 2002) yields globular morphology of polyaniline (Stejskal et al. 2008b). Such polymerization proceeds under strongly acidic conditions at pH < 2.5 (Sapurina and Stejskal 2008). On the contrary, it is known that the oxidation of liquid aniline in water, i.e. under mild acidity conditions, yields polyaniline nanotubes (Trchová et al. 2006; Stejskal et al. 2008b; Trchová and Stejskal 2011). The same applies to the polymerization of aniline in the solutions of weak organic acids (acetic, citric, succinic, etc.) (Stejskal et al. 2008b; Trchová and Stejskal 2011; Jeong et al. 2014; Mondal et al. 2019a). This was explained by the in-situ formation of aniline oligomers, resembling safranin dye (Fig. 3), that serve as a template for the nanotubular growth.
The similar polymerizations carried out under low acidity in the presence of Acid Green 25 (Amer et al. 2018) or methyl orange (Amer et al. 2019a) have also yielded the nanotubes (Table 1), but these would be most probably obtained even in the dye absence (Trchová et al. 2006; Rakić et al. 2011). When the oxidation of aniline was carried out in the presence of methyl orange in the solution of hydrochloric acid, nanotubes were observed only when the acid concentration was reduced below 0.034 M (Ren et al. 2009), i.e. again at relatively low acidity. The assignment of the template role to these dyes under low acidity conditions is, therefore, ambiguous. The syntheses of polyaniline were limited to anioninic dyes, no cationic dyes have been tested.
When the polymerization of aniline took place under acidic conditions, coral-like morphology of polyaniline was produced in the presence of Sunset Yellow FCF (Shi et al. 2017) or amaranth dye (Acid Red 27) (Shi et al. 2018). The formation of one-dimensional morphology, however, could be a result of high dilution of reactants used by the authors, which is a method how to prepare polyaniline nanofibers (Chiou and Epstein 2005; Rakić et al. 2011). Preliminary experiments in the author’s group indeed suggest that the influence of the dyes on the morphology and conductivity of polyaniline is marginal, in the dramatic contrast to polypyrrole.
Polypyrrole
The present review was also motivated by the observation that the presence of dyes during the preparation of polypyrrole affects both its morphology and conductivity (Stejskal 2018; Stejskal and Trchová 2018). In the classical synthesis, pyrrole is oxidized with iron(III) chloride, and polypyrrole is obtained as globules (Fig. 5a). The growth of polypyrrole nanotubes (Figs. 5b, 9) supported by methyl orange in the reaction mixture is probably the best known way how to prepare one-dimensional polymer morphology (Ćirić-Marjanović et al. 2014, Kopecká et al. 2014; Stejskal et al. 2016; Kopecký et al. 2017; Li et al. 2017b; Varga et al. 2017; Sapurina et al. 2017).
Transmission electron micrograph of polypyrrole nanotubes (Sapurina et al. 2017)
Methyl orange has routinely been used in the preparation of nanotubes since 2004 (Hu et al. 2004; Yang et al. 2005) in many follow-up papers. Examples of such synthesis, called the method of “self-degraded template”, can also be found in recent literature, when polypyrrole nanotubes have been prepared as a powder or deposited on a suitable support (Škodová et al. 2013; Park et al. 2014; Alekseeva et al. 2015; Bober et al. 2015; Rudajevová et al. 2015; Stejskal et al. 2015; Varga et al. 2015; Wei et al. 2015; Xu et al. 2015; Kang et al. 2019; Kopecká et al. 2016; Sapurina et al. 2016; Babayan et al. 2017; Chen et al. 2017a; Galář et al. 2017; Ivanova et al. 2017; Xin et al. 2017; Zhang et al. 2017; Bober et al. 2018a; Lee et al. 2018; Li et al. 2018a; Lin et al. 2018; Pei et al. 2018; Stejskal 2018; Trchová and Stejskal 2018; Tuo et al. 2018; Xiao et al. 2018; Xin et al. 2018; Wang et al. 2018a; Wei et al. 2018; Wu et al. 2018; de Lazzari et al. 2019; Dong et al. 2019a; Dubal et al. 2019; Hryniewicz et al. 2019; Le et al. 2019; Mao et al. 2019; Pei et al. 2019; Prokeš et al. 2019; Yang and Chen 2019; Yashas et al. 2019; Zhang et al. 2019a, 2019b). For older papers reporting the use of methyl orange in polypyrrole synthesis the readers are referred to relevant review article (Stejskal and Trchová 2018). Methyl orange became incorporated in polypyrrole (Alekseeva et al. 2015; Li et al. 2017b) and this was the case also with other dyes, such as methylene blue (Phan et al. 2017). The conductivity of polypyrrole prepared in the presence of methyl orange increased several times at the same time (Fig. 6).
Various models of the nanotubular growth in polypyrrole have recently been proposed (Yang et al. 2005; Kopecká et al. 2014; Joulazadeh and Navarchian 2015). One of them (Stejskal 2018) (Fig. 10) assumes that the aggregation of pyrrole oligomers is followed by the growth of polypyrrole chains (black curves) to produce globules. When the template molecules are present (blue squares), they randomly aggregate at the same time and serve as loci for adsorption of pyrrole oligomers; composite globules are obtained. Nanotubes are formed if the template molecules order to one-dimensional aggregates and become coated by polypyrrole. It should be stressed that nanotubes were prepared also by electrochemical oxidation (Hryniewicz et al. 2019) and the chemical nature of oxidant in chemical oxidation thus cannot be decisive. Nanofibers, which can be regarded as a special case of nanotubes when the internal cavity is not discernible, have also been synthesized (Feng et al. 2014; Bober et al. 2018a). Among experimental studies, especially Raman spectroscopy proved to be powerful tool in the assessment of nanotubular structure (Trchová and Stejskal 2018). By using various excitation laser wavelengths, the beam penetration depth can be varied and detailed information about the nanotube profile may be obtained.
The formation of polypyrrole a globules, b composite globules, and c nanotubes by aggregation of pyrrole oligomers (orange circles) and template dye molecules (blue squares) followed by the growth of polypyrrole chains (black curves). Reprinted from (Stejskal 2018)
Other organic dyes had generally an influence on the morphology and conductivity of polypyrrole but their effect on the formation of polypyrrole is hardly predictable (Table 2). Some dyes had also promoted the formation of one-dimensional morphology despite of different molecular structure compared with methyl orange. This concerned especially anionic dyes, such as Acid Red 1 (Yan and Han 2007; Valtera et al. 2017), Acid Blue 25 (Wang et al. 2011; Bober et al. 2018a; Stejskal 2018), Acid Red 249 (Feng et al. 2009), Direct Blue 2 (Yang et al. 2019), Direct Blue 14 (Yang et al. 2019) and indigo carmine (Li et al. 2016; Hu et al. 2018), which all supported the growth of nanofibers or nanorods. Among cationic dyes, the formation of one-dimensional nanostructures was observed with rhodamine B (Xue et al. 2010), methylene blue/heparin (Wei et al. 2010), methylene green (Bonfin et al. 2019) phenosafranin (Minisy et al. 2019a), safranin (Stejskal 2018; Minisy et al. 2019a) (Fig. 11a) and acriflavine (Fig. 11b). Last three dyes are closely related chemicals of phenazine type, methylene blue and methylene green of phenothiazine type. The morphology of polypyrrole was also affected by the presence of acid fuchsin (Dong et al. 2019b). We conclude that both the morphology and conductivity are strongly influenced by the presence of dyes during polymer preparation, in contrast to polyaniline. It also seems that the electrostatic interactions between polypyrrole and dyes are not operational.
Polypyrrole nanorods/nanofibers prepared in the presence of a safranin and b acriflavine (unpublished results)
The presence of other dyes produced polypyrrole globules of various sizes. These were Acid Blue 324 (Wang et al. 2013a), Acid Blue 129 (Bober et al. 2018a), Acid Green 25 (Sapurina et al. 2017), alizarin red S (Zang et al. 2018), cresol red (Sapurina et al. 2017), Direct Violet 1 (Yang et al. 2019), ethyl orange (Li et al. 2017b), indigo carmine (Sapurina et al. 2017), Orange G (Valtera et al. 2017), methylene blue (Phan et al. 2017), Reactive Black 5 (Sapurina et al. 2017), tartrazine (Wang et al. 2019b) and thymol blue (Sapurina et al. 2017). Sunset Yellow FCF yielded exceptional hierarchical polypyrrole microbarrels (Valtera et al. 2017) (Fig. 12), and methylene blue promoted the formation of hollow spheres (Sapurina et al. 2017).
Polypyrrole prepared in the presence of Sunset Yellow FCF dye (two magnifications). Reprinted from (Valtera et al. 2017)
A group of papers reports the electrochemical oxidation of pyrrole to polypyrrole in the solutions of various organic dyes. Polypyrrole prepared along with methyl orange was used as a pH sensor base on Raman spectroscopy (Czaja et al. 2019). The presence of indigo carmine displayed improved photocurrent density under polychromatic illumination (Girotto et al. 2002) or the potential to be applied as counter electrode in dye-sensitized solar cells (Loguercio et al. 2016). The same system including indigo carmine was also used for the reduction of tetrachloroauric acid to gold nanoparticles (Luguercio et al. 2015). The dye was proposed to increase the ordering degree and to provide better electronic charge transfer in the bulk. Polypyrrole was also prepared in the presence of malachite green, which was expected to produce molecularly imprinted composite film on the electrode (Xu et al. 2019a). The electrochemical synthesis of polypyrrole in the presence of bromophenol blue led to globules or rods applicable in the sensing of explosives (Eslami and Alizadeh 2019). When polypyrrole was prepared electrochemically in the presence of Eriochrome cyanine R, the dye was incorporated in the polymer film (Tavoli and Alizadeh 2014). The electrochromic properties were enhanced compared with the film prepared in the dye absence. The presence of alizarin red reduced the electropolymerization potential in the preparation of polypyrrole and affected the morphology of films (Chen and Zhitomirsky 2013; Rounaghi et al. 2015). The presence of a dye in the electropolymerization of pyrrole thus has influence on the properties of polypyrrole films. It has to be stressed that the film morphology depends also on the concentrations of reactants, their proportions, acidity conditions and other factors.
Dyes as monomers
It was proposed that the oxidation of some dyes carrying primary amino groups could be a novel approach to the synthesis of conducting polymers (Jiang et al. 2018). This is a reasonable expectation because the conjugated molecular structure of dyes extended to polymer chain could indeed to have such result. While the polymerization might have indeed taken place, the convincing illustration of reasonable conductivity has not been reported.
The chance to obtain a conducting polymer might be expected after the oxidation of dyes containing primary amino group(s) on benzenoid ring (Gouveia-Caridade et al. 2013), in analogy of aniline oxidation to polyaniline. The chemical oxidation of safranin (Basic Red 2), however, led to non-conducting oligomeric rather than conducting polymeric products (Ćirić-Marjanović et al. 2007). The chemical oxidation of 1,5-diaminoanthraquinone produced the electroactive polymer in nanotubular form (Liu and Liu 2019), but the conductivity was not determined. The chemical oxidation of azulene with iron(III)chloride yielded electroactive polyazulene (Gradzka et al. 2018), but its conductivity has not again been reported. In contrast to current dyes, azulene is a bicyclic hydrocarbon that does not contain any nitrogen atoms. Similarly, the joint oxidation of aniline and thymol blue, where the dye was expected to be a comonomer, was claimed to yield a corresponding copolymer (Ponprapakaran et al. 2017). The dye, however, does not again contain any amino group and the formation of a copolymer is thus unlikely.
The electrochemical oxidation of safranin produced a non-conducting polymer (Pauliukaite et al. 2009; Lavanya et al. 2015; Yang 2016), similarly like neutral red (Gouveia-Caridade et al. 2013; Broncová et al. 2004, 2008, 2016; Pauliukaite and Brett 2008). The phenoxazine dyes, such as Brilliant Cresyl Blue or Nile blue A, have also been reported to produce polymer upon the electrooxidation (Gouveia-Caridade et al. 2013) similarly like phenothiazine dye, Azure B (Stoikov et al. 2019). The electrochemical oxidation of alizarin, which does not contain any amino group, yielded the corresponding polymer (Jiang et al. 2018). Glassy carbon electrode was modified by poly(methyl orange) (Chiwunze et al. 2019) or poly(phenol red) (Karabiberoglu and Dursun 2017) by the electropolymerization of corresponding dyes. These dyes also do not contain primary amino groups, which would account for the formation of a polymer. The insolubility of the product has often been regarded as a proof of polymerization in electrochemistry but this need not be convincing for a polymer chemist.
Another strategy consists in the attachment of a polymerizable group to a dye. For example, methyl red was covalently bonded to nitrogen atom in pyrrole, which was subsequently electropolymerized (Almeida et al. 2017). The Disperse Red 1 was similarly functionalized with an acrylic group followed by its polymerization (Spiridon et al. 2018). In such cases, the dye is a pendant moiety and does not produce the main chain.
Conducting polymers as adsorbents of dyes
Basic principles
The uses of conducting polymers in highly topical environmental uses, such as removal of organic dyes and heavy-metal ions from wastewater, have recently been reviewed (Zare et al. 2018a). It should be observed that this important application does not exploit the most important property, the conductivity, even though the contribution of the ionic conductivity might affect the kinetics of adsorption. In typical studies, the effects of dye concentration, the adsorbent dosage, adsorption kinetics, temperature and pH have been investigated for conducting polymers or their composites and dyes. Due to the plethora of existing dyes, many reports have been published and still will appear in the literature. Adsorption of the dyes is easily followed spectrophotometrically by the decrease in the colouration of dye solution after addition of adsorbent (Figs. 7, 8).
The adsorption of dyes on conducting polymers is routinely assessed by the series of experiments (Ayad et al. 2012; Salem et al. 2016; Tayebi et al. 2016; Tanzifi et al. 2017; Zhou et al. 2017; Kaushal et al. 2018; Kumar et al. 2018; Lyu et al. 2018; Maruthapandi et al. 2018; Shahriman et al. 2018; Amer et al. 2019a; Bagheri and Mardani 2019). The relative dye removal efficiency in % or absolute adsorption capacity (adsorptivity) in mg g−1 is the fundamental characteristics reported in the literature. Adsorption isotherms, i.e. the dependences of the adsorbed amount on the adsorbate concentration at fixed temperature, have been used in the literature to describe the adsorption mechanism. Langmuir, Dubinin-Raduskievich, Freundlich, Halsey and Temkin isotherms are the best known types and the reader is referred to a review article (Huang et al. 2014) for their survey. Time dependence of adsorption, i.e. the adsorption kinetics, has usually been rated as the pseudo-first order or pseudo-second order type, or discussed in terms of Elovich and intraparticle-diffusion models (Huang et al. 2014). Temperature dependence of adsorption parameters and of equilibrium concentrations of adsorbates allows for the determination of adsorption thermodynamics expressed in quantities, such as free energy or enthalpy. Adsorption experiments are usually performed in water close to neutral conditions but the dependence of adsorption parameters on pH is of importance for the recovery of adsorbents.
The quantitative parameters of adsorption isotherms, kinetics or thermodynamics have often been extensively reported in the literature. Especially for the composites, however, they are difficult to compare for several reasons: (1) Components of the composite may adsorb dyes independently and the contribution conducting polymers alone has not been specified, (2) the composite composition is often unknown, (3) it was not specified if the adsorption capacity concerns the whole composite or any part only, (4) the experimental conditions, especially adsorbent content and dye concentrations ranges, differ and (5) other parameters, especially pH or temperature, also vary in individual studies. Especially the studies of conducting polymers alone are scarce. The conducting polymer deposition was usually reported to improve the adsorption of the current adsorbent.
Adsorbent forms and properties
The use of composites dominates the adsorption studies for practical reasons. Conducting polymers have been as a rule deposited at the surface of substrates by in-situ polymerization (Stejskal et al. 1999; Fedorova and Stejskal 2002; Omastová and Mičušík 2012), sometimes referred to as surface polymerization, i.e. when the substrates were immersed in the aqueous reaction mixture used for the preparation of conducting polymers. It has to be stressed that this approach results indeed in the coating of substrates rather than in a mere mixture of a substrate and conducting polymer. It has been established that any interface immersed in the reaction mixture used for the oxidative polymerization of aniline or pyrrole becomes coated with a thin film of conducting polymer (Stejskal et al. 2010). The oligomers produced at early stage of monomer oxidation are adsorbed at available interfaces at first and stimulate the following growth of conducting-polymer chains. The typical thickness of the coating is 100–200 nm (Stejskal and Sapurina 2005). The film thickness is reduced when the water-soluble polymers (Riede et al. 2002) or surfactants (Castillo-Reyes et al. 2015) are present in the reaction mixture. The experiments suggest the brush-like ordering of polymer chains and enhanced conductivity in the coatings due to improved inter-chain charge-transfer compared with common globular morphology (Fig. 13). For that reason, the composites composed of surface-coated substrates, may have higher conductivity than globular conducting polymer or the substrate alone (Acharya et al. 2018) and may perform better in other respects, e.g., in adsorption. The dye adsorption is likely to be little dependent on the substrate nature and will be dominated by the conducting polymer coating including its specific surface area. The role of the material used for the polymer deposition, however, would be of importance in photocatalysis, because the polymer coating is penetrated by UV–visible light and the catalytic performance of the substrate alone may be exploited.
Random chains in globular conducting polymer, and ordered chains in coatings deposited on the substrates or in nanotubes
Various organic and inorganic substrates coated with conducting polymers proved to be better adsorbents or photocatalysts than the individual components (Mukhta et al. 2007; Zhang et al. 2014; Elsayed and Gobara 2016; An et al. 2018; Mohamed et al. 2018; Nerkar et al. 2018; Chatterjee et al. 2019; Chen et al. 2019a, 2019b). The often-mentioned synergistic effect can be explained by the improved brush-like ordering of the polymer chains deposited at the substrate surface (Sapurina et al. 2001, 2002; Stejskal et al. 2019), although some authors suggest the interfacial heterojunctions to be responsible for this effect (Megha et al. 2019). In addition to enhanced dye adsorption or photodegradation, such ordering manifests itself similarly in the synergistic conductivity increase (Acharya et al. 2018; Megha et al. 2019). Please note that the formation of nanotubes can be regarded as the surface coating of the template afforded, e.g., by dyes (Figs. 10, 13) and, for that reason, the nanotubes may also have higher conductivity (Li et al. 2017b) or dye adsorptivity (Ayad and Abu El-Nasr 2010) compared with classical globular polymers.
Among non-conventional approaches, plasma deposition technique has been used for surface modification of titanium dioxide (Subramaniam et al. 2019). In this case, the coating is suspected to consist rather of aniline oligomers than polyaniline. The solution of polyaniline base in N-methylpyrrolidone was used for the surface modification of graphene/wolframate composite (Biswas et al. 2019). The opposite strategy when the surface of a conducting polymer instead of the substrate was altered has been mentioned only rarely. For example, polyaniline was used as a reductant of silver ions to silver nanoparticles (Stejskal 2013) and the resulting composite was used as an adsorbent of brilliant green (Salem et al. 2016).
When it comes to the individual application forms, powders have been most frequently investigated. Although they are best suited for the academic studies, other material forms have been sought for the practice. In the composites, one of the components is often used to provide the mechanical properties and integrity. For example, conducting polymers were deposited on fibrillar materials of organic nature, such as, cotton textile (Fan et al. 2017; Ayad et al. 2018a, b; Gamal and Attia 2019), electrospun mats (Qi et al. 2016; Gorza et al. 2018), chitosan (Sananmuang et al. 2017) and kapok fibres (Zheng et al. 2012; Mu et al. 2015), paper (Liu et al. 2015), polyamide (Li et al. 2019a) and polyamide nanofibres (Li et al. 2015; Zarrini et al. 2017), polyimide (Ding et al. 2019) or polyester fabric (Ghaemi and Safari 2018; Gamal and Attia 2019). The inorganic nanofibers have been used rarely and they are represented by carbon nanotubes (Zeng et al. 2013) or silicon carbide nanofibres (Koysuren 2019). Titanium dioxide membranes (Li et al. 2018b) and stainless-steel mash (Yihan et al. 2018) similarly served as substrates for in-situ polymer deposition.
Composite hydrogels (Yan et al. 2015; Stejskal 2017; Stejskal and Bober 2018; Yao et al. 2018; Song et al. 2019; Xing et al. 2019) or macroporous cryogels based on conducting polymers (Stejskal et al. 2017) represent three-dimensional forms worth of investigation. When the conducting polymers have been prepared in the solutions of water-soluble polymers, colloidal dispersions with conducting polymer core and supporting polymer shell are produced as a rule (Stejskal 2001; Stejskal and Sapurina 2005; Stejskal and Bober 2018; Song et al. 2019). The composite material obtained after their precipitation and drying has only exceptionally been used in dye-adsorption studies (Abbasian et al. 2017). Such materials have been regarded not quite precisely as graft copolymers with conducting polymer backbone.
The simple mixtures of substrates with separately prepared conducting polymers have been used exceptionally. Polyaniline combined with zirconium dioxide adsorbed methylene blue (Agarwal et al. 2016), polyaniline mixture with strontium stannate photocatalysed the decomposition of the same dye (Faisal et al. 2019). Polyaniline mixture with poly(vinyl chloride) was similarly applied to the decolorization of methyl orange (Bahrudin et al. 2018a). The combination of titanium dioxide with polyaniline colloid was tested in the photocatalytic decomposition of rhodamine B (Zhou et al. 2019) or methyl orange (Bahrudin et al. 2018b, 2019). The blend of polyaniline with custom-made orange dye was exploited in impedance humidity sensor (Chani et al. 2013). Polyaniline solution in tetrahydrofuran was used to modify zinc oxide particles (Poorarjmand et al. 2019). The reasoning of such approach instead of an in-situ coating method has to be still justified.
Specific surface area is a key parameter in the adsorption experiments. It is relatively low for polyaniline, 3.6 m2g−1 (Xu et al. 2019c), 8 m2g−1 (Tanzifi et al. 2018a), 9.1 m2g−1 (Vidya and Balamurugan 2019a), 18 m2g−1 (Hasan et al. 2019) or 20 m2g−1 (Ayad and Zaghlol 2012). It increased to 17–31 m2g−1 when polyaniline was prepared in the presence of surfactants (Ahmed et al. 2016) that reduced the size of globules. The annealing of polyaniline base at 180 °C associated with cross-linking led to the increase in specific surface area to 349 m2g−1 (Ayad and Zaghlol 2012). Analogous values have been reported for polypyrrole, 6.9 m2g−1 (Chen et al. 2019b), 10 m2g−1 (Kopecký et al. 2017) or 26 m2g−1 (Acharya et al. 2018). The specific surface area may be increased by the control of morphology (Meng et al. 2014). Polypyrrole nanotubes have about one order of magnitude higher surface area than polypyrrole globules (Li et al. 2017b; Kopecký et al. 2017; Stejskal and Trchová 2018) and the behaviour of both morphologies with respect to dye adsorption may differ. Polyaniline nanoparticles have been more efficient adsorbents of methylene blue than the conventional globular polyaniline (Ayad et al. 2013). When two-dimensional substrates with a high specific surface area, such as graphene of molybdenum disulfide, are coated with conducting polymers, such composites may become promising adsorbents. On the other hand, when the substrates are porous, the micropores may be filled with conducting polymers during their deposition, and the specific surface area would be reduced (Jia et al. 2012).
The role of pH
The pH in adsorption experiments is important and adsorption parameters were found to depend on pH (Baseri et al. 2013; Ovando-Medina et al. 2015a; Gorza et al. 2018; Ma et al. 2018; Ayad et al. 2018b; Chen et al. 2019b). In practice the adsorption from aqueous solutions takes place at neutral conditions but the change in pH may be of importance for the adsorbent regeneration or recovery (Ayad et al. 2018a; Zare et al. 2018b). Depending on pH, polyaniline exists under acidic conditions as a conducting salt and above ca pH 4–6 it converts to a corresponding non-conducting base (Fig. 14). The base is less hydrophilic than the salt (Stejskal et al. 2008a) and it is thus better suited to interact with hydrophobic part of a dye by hydrophobic interactions. This explains the enhanced dye adsorption observed often at high pH. The similar salt–base transition is found also in polypyrrole (Stejskal et al. 2016) but generally polypyrrole is less sensitive to pH than polyaniline.
Green conducting polyaniline salt converts to blue non-conducting polyaniline base in alkaline media
Many dyes also exist in two forms depending on pH, typically as acids and salts, acidobasic indicators being the best examples. For instance, methyl orange is present as a red hydrophobic acid of limited solubility in water below pH 3.5 and above this pH as well-soluble yellow salt (Ren et al. 2009; Li et al. 2017b) (Fig. 15). The adsorption of the dye thus would be affected by the acidity of the medium. For example, the adsorption efficiency of methyl orange on polyaniline at pH 6 was 90% and decreased to 64.5% at pH 2 (Tanzifi et al. 2017). Up to three combinations of polymer and dye forms may interact depending on pH (Fig. 16).
Methyl orange salt, sodium 4-((4-dimethylamino)phenyldiazenyl)benzene-1-sulfonate, converts to a corresponding acid below pH 3.5 (Sapurina et al. 2017)
a Red methyl orange acid interacts with green polyaniline salt at pH < 3.5, b yellow methyl orange salt interacts still with polyaniline salt within 3.5 > pH < 5 range and c MO salt interacts with blue polyaniline base at pH > 5. The transitions are gradual in the reality
For example, adsorption of methylene blue on polypyrrole increased three times when the pH 3.5 was changed to 10.5 (Ayad et al. 2018a; Wang et al. 2018b). The better adsorption on polyaniline composite under alkaline conditions was reported for methylene blue, neutral red and crystal violet (all cationic dyes) while opposite trend was found for Eriochrome Black T (anionic dye) (Kaushal et al. 2018). Both the conducting polymers and cationic dyes are not protonated under alkaline condition, their hydrophobicity is enhanced and hydrophobic interactions thus seem to be driving or supporting forces in the adsorption. Improved adsorption of anionic dyes on polymer bases was indeed better compared to protonated forms (Aliabad and Mahmoodi 2018). Optimum adsorption on polyaniline nanotubes was at pH 9 for cationic dye and pH 5 for anionic one (Amer et al. 2018), i.e. when the dissociation of dyes was limited.
Recycling of the adsorbents is of importance. Treatment based on the variation of acidic/basic pH is the most commonly used to remove sorbates (Fig. 17) and to recover the adsorbent (Li et al. 2012; Zheng et al. 2012; Kumar et al. 2016; Ayad et al. 2018a; Lyu et al. 2018). One has to keep in mind that the deprotonation/reprotonation cycles introduce the changes in hydrogen bonding in the conducting polymer that manifest themselves in incomplete reversibility of conductivity (Prokeš et al. 2019) with expected impact also on other polymer properties. The extraction with organic solvent served for the recycling, too (Shanehsaz et al. 2015; Aliabad and Mahmoodi 2018; Shahriman et al. 2018).
Illustration of the reusability of cotton textile coated with polypyrrole in adsorptive removal of cationic dye, methylene blue (MB), and anionic dye, Acid Green 25 (AG), using changes in acidity of the medium. For details cf. (Ayad et al. 2018a)
Experimental methods in dye adsorption
The absorption of light in visible region is the inherent property of dyes. The monitoring of the decrease in the optical absorbance of the dye solution is the most easy and convenient method how to follow the course and extent of dye adsorption (Fig. 18) (Ayad et al. 2012, 2018a; Kumar et al. 2016; Kaushal et al. 2018; Maruthapandi et al. 2018; Amer et al. 2019a; Chatterjee et al. 2019; Yuan et al. 2019). The experimental simplicity is the reason for the large number of studies in this area.
Time dependence of absorption spectra of methylene blue (3.9 mg L‒1) with 50 mg cotton textile coated with polypyrrole. Reprinted from Ayad et al. 2018a
The quartz microbalance seems to be a suitable method for the testing of dye adsorption on conducting polymer films and its kinetics. This has been demonstrated for methylene blue adsorption on polyaniline film deposited on quartz crystal (Ayad and Abu El-Nasr 2010). Despite its simplicity, the method has not been routinely used. Liquid chromatography combined with mass spectrometry has frequently been applied as an additional method, especially in the photodegradation of dyes when the fate of the decomposition products had to be identified.
Adsorption of dyes on conducting polymers
This section lists various combinations of conducting polymers, substrates and dyes reported in the literature trying to organize them as a guide for future adsorption studies. While the entries in tables are sorted by the organic dyes, the type of the composite adsorbents has been used in the text. No quantitative comparison has been attempted except for quoting occasionally maximum adsorption capacities. These can be rated as relatively poor below 10 mg g−1, fair for 10–100 mg g−1, good above 100 mg g−1 and exceptional above 1000 mg g−1. Several tendencies can be traced in the design of new adsorbents: (1) The surface modification of various natural, often waste materials with conducting polymers, (2) the analogous deposition of conducting polymers on inorganic substrates with high specific surface area, (3) the use of substrates active also as photocatalysts and (4) incorporation of component that allows the easy separation, e.g., ferromagnetic one (iron, iron oxides, magnetite, ferrites) by magnetic field.
The papers devoted to the adsorption of dyes on neat conducting polymers are rare (Majumdar et al. 2019). Some studies reporting the dye adsorption on conducting polymer composites include also the behaviour of its components. They demonstrate that conducting polymers alone perform as adsorbents of organic dyes. The vast majority of papers concerns the surface modification of various substrates with conducting polymers and reports the improvement of adsorption capacities; the conducting polymers alone, however, have not usually been considered.
Polyaniline
For the adsorption of various organic dyes on polyaniline and its composites, the reader is referred to a recent review (Nasar and Mashkoor 2019). Anionic dyes have been represented especially by methyl orange (Table 3). The maximum capacity of methyl orange adsorption was 76 mg g−1 (Tanzifi et al. 2017). Polyaniline nanofibres prepared by the interfacial polymerization method adsorbed up to 25 mg g−1 of this dye (Duhan and Kaur 2019). Polyaniline prepared in the presence of sodium dodecylbenzenesulfonate had 76 mg g−1 capacity for methyl orange adsorption (Karri et al. 2018). High adsorption of methyl orange, 385 mg g−1, was found on polyaniline microspheres (Guo et al. 2011) but, considering the preparation protocol, they were composed most likely rather by aniline oligomers (Stejskal et al. 2008a).
Other anionic dyes have also been well adsorbed. For example, Acid Violet 90 complex with chromium(III) was separated by polyaniline from aqueous solutions, the adsorption capacity being 153 mg g−1 (Akti and Okur 2018). Polyaniline was a good adsorbent of Congo red from wastewaters (Chafai et al. 2017a, b). Polyaniline treated with copper(II) chloride adsorbed reactive azo dye Cibacron Navy P-2R-01 (Bingöl et al. 2012). Finally, adsorption capacities reached 300 mg g−1 for indigo carmine (Mondal et al. 2019a), 310 mg g−1 for Acid Orange G (Lyu et al. 2019) and even 435 mg g−1 for Reactive Black 5 (Bhaumik et al. 2016). Adsorption up to 5950 mg g−1 has also been observed with Acid Red 94 (Xu et al. 2019c) but needs independent confirmation.
Relatively little attention has been paid to the adsorption of cationic dyes. Methylene blue has been the most popular representative of this group (Table 3) but the experimental results differ due to varying conditions. The adsorption was spontaneous and the capacity was 19 mg g−1 (Maruthapandi et al. 2018) or 192 mg g−1 (Agarwal et al. 2016) but reached even above 413 mg g−1 (Rafiqi and Majid 2017). Both the Langmuir and Freundlich isotherms fitted the data well (Ayad and Abu El-Nasr 2010; Ayad et al. 2012; Ayad and Zaghlol 2012; Chafai et al. 2017a; Rafiqi and Majid 2017; Amer et al. 2018, 2019a; Kaushal et al. 2018; Maruthapandi et al. 2018). Polyaniline adsorbed up to 48 mg g−1 of Basic Blue 3 (Muhammad et al. 2019a), 265 mg g−1 of Acid Blue 40 (Muhammad et al. 2019b) or 335 mg g−1 of Eosin Y (Majumdar et al. 2019).
Several papers have compared the performance of both dye types. They generally agree that the type does not play a significant role, i.e. the adsorption is not controlled by the ionic interactions. For example, adsorption capacity 467 mg g−1 was found for cationic methylene blue and 440 mg g−1 for anionic rose bengal dye (Shen et al. 2018). A chemically modified polyaniline adsorbed a cationic dye, crystal violet and an anionic dye, methyl orange, also with comparable capacity, 245 and 220 mg g−1, respectively (Sharma et al. 2016). Polyaniline nanotubes also adsorbed both anionic and cationic dyes, Acid Green 25 (Amer et al. 2018) and methylene blue (Amer et al. 2019a), to similar extent. It can be concluded that the ionic interactions do not play a decisive role in the adsorption on conducting polymers.
Polyaniline base was more efficient in adsorption of anionic dyes than the corresponding protonated form, polyaniline salt (Tanzifi et al. 2017; Aliabad and Mahmoodi 2018). This is associated with higher hydrophobicity of polyaniline base compared with the corresponding salt (Stejskal et al. 2008a). Also various polyaniline salts substantially differ in hydrophobicity depending on the counter-ions (Stejskal et al. 2008a) and, consequently, also in adsorption capacity (Salem 2010).
Polyaniline composites
Polyaniline composites have been often used instead of neat polyaniline in the search of various application forms (Table 4). Table 4 is ordered by the adsorbed dyes, the following text by the type of adsorbents. The composites contained up to three components as a rule: (1) a conducting polymer, (2) a polymer providing the processability or mechanical support and (3) an inorganic part, which had independent adsorption ability or another value-added property, such as magnetic one. Comparison of the reported data is possible only on the relative basis because the adsorption on the individual composite components had often been missing, as well as the composition of composites.
The composites are classified by the type of a non-conducting component as follows:
Ag Polyaniline decorated with silver nanoparticles using the reductive properties of this conducting polymer on silver ions was applied for the adsorption of brilliant green (Salem et al. 2016; Gouthaman et al. 2019). The presence of silver hardly affected the adsorption behaviour but should have other, e.g., antibacterial value-added properties.
Al Polyaniline/aluminium potassium sulfate adsorbed the anionic dyes with a preference compared with cationic ones (Patra and Majhi 2015). Polyaniline/alumina composite was tested for adsorption of various anionic dyes (Javadian et al. 2014).
Bi Bismuth vanadate particles coated with polyaniline were reported to be an efficient adsorbent of methyl orange (Vidya and Balamurugan 2019b; Vidya et al. 2019). The composite performed better than polyaniline alone.
C Carbon in various forms was often present in the composites. The deposition of polyaniline on activated carbon provided the adsorbent of methyl orange with the maximum capacity 285 mg g−1. The coating of carbon nanotubes with polyaniline improved the adsorption of malachite green compared with neat polyaniline (Zeng et al. 2013). Here, the nanotubes afforded the high specific surface area. Polyaniline/graphene oxide efficiently adsorbed cationic methylene blue at 962 mg g−1 capacity as well as anionic methyl orange, 885 mg g−1, the high specific surface area being again of benefit (Wang et al. 2018b). The fact that both cationic and anionic dyes were adsorbed in comparable amounts again suggests the limited role of the electrostatic interactions over, e.g., π–π ones. Polyaniline/graphene oxide composite adsorbed cationic dyes, malachite green and rhodamine G, or anionic dye, Congo red, practically completely within tens of minutes (Mitra et al. 2019). Polyaniline-coated graphene oxide/multiwall carbon nanotubes composite adsorbed Congo red and simultaneously reduced Cr(VI) ions (Ansari et al. 2017). Finally, polyaniline/reduced graphene oxide afforded maximum adsorption capacity of 667 mg g−1 for a cationic dye, malachite green (Ghahramani et al. 2019).
Fe The incorporation of magnetic component, such as iron, iron oxides, magnetite or ferrites, may be conveniently used in the separation of adsorbent by magnetic field (Fig. 19). Polyaniline/iron composite was tested in the removal of Congo red (Bhaumik et al. 2014, 2015) and rhodamine B (Guo et al. 2016) from aqueous medium. Polyaniline/iron/attapulgite composite adsorbed preferentially azo dyes than non-azo type (Xu et al. 2019b). Polyaniline/iron oxide adsorbed both anionic and cationic dyes, methyl orange and rhodamine B (Xie et al. 2017). Magnetite coated with polyaniline was used for the removal of Acid Blue 40 (Muhammad et al. 2019b), Basic Blue 3 (Muhammad et al. 2019a), brilliant green (Mu et al. 2016, 2017), malachite green (Mahto et al. 2014), methylene blue (Zhao et al. 2013a; Mu and Wang 2015; Mu et al. 2016, 2017), methyl orange (Zhao et al. 2013a; Mahto et al. 2015), Reactive Red 198 (Tayebi et al. 2016), Reactive Black 5 (Hamzehloo et al. 2019) and rhodamine B (Shahriman et al. 2018). Adsorption capacities up to 400 mg g−1 were reported (Zhao et al. 2013a). A similar composite with copper ferrite adsorbed up to 346 mg g−1 of methyl orange (Kharazi et al. 2019). The adsorption of methylene blue on polyaniline/nickel ferrite, however, was reported to be only 6.6 mg g−1 (Patil and Shrivastava 2016; Singh et al. 2019). The composite with sodium nitroprusside also falls into the category of iron compounds (Rafiqi and Majid 2017) and was used for the removal of methylene blue.
Illustration of a conducting polymer cryogel containing ferrite particles (left). Magnetic aerogel obtained after freeze-drying is attracted to a magnet (right) (Islam Minisy, unpublished results)
Mo Polyaniline/molybdenum disulfide removed Congo red from the aqueous medium (Kumar et al. 2018) with adsorption capacity reaching 71 mg g−1. The composite with molybdenum trioxide adsorbed up to 36 mg g−1 of cationic rhodamine B and 76 mg g−1 of anionic Congo red (Dhanavel et al. 2016).
Ni Polyaniline/petaline/NiO was proved to be an efficient absorbent of Reactive Brilliant Red X-3B (Zhong et al. 2018).
Si This group is represented by traditional adsorbents such as silica, silicates and aluminosilicates. Polyaniline/silica rapidly removed Amido Black 10B (Tanzifi et al. 2018a) or Reactive Orange 16 (Aghajani and Tayebi 2017) from aqueous solutions. Polyaniline base/silica adsorbed 5 mg g−1 of methylene blue (Ayad et al. 2012). The highest capacity of polyaniline/mesoporous silicate for Acid Blue 62 adsorption was 55 mg g−1 (Torabinejad et al. 2017; Binaeian et al. 2018). Polyaniline/clinoptilolite composite had a higher adsorption capacity of Acid Violet 90, 72 mg g−1 (Akti and Okur 2018) and a composite with bentonite adsorbed 202–258 mg g−1 of Reactive Red 2 (Tie et al. 2017). Polyaniline/starch/montmorillonite was tested for adsorption of Reactive Blue 194 with 92 mg g−1 capacity (Olad et al. 2014).
Ti Titanium oxide is widely used as a photocatalyst but its composites were tested also for adsorption. Titanium dioxide coated with polyaniline adsorbed tartrazine dye (Elsayed and Gobara 2016) or Acid Red G (Wang et al. 2015a). The adsorption capacity of latter dye reached 455 mg g−1. The composite performed better than the individual components alone. Polyaniline/carboxymethylcellulose/titanium(IV) oxide adsorbed Congo red (Tanzifi et al. 2018b).
Zn The polyaniline-coated zinc oxide removed methyl orange from the aqueous medium more efficiently than any of the components alone (Nerkar et al. 2018), displaying again the marked synergistic effect. Polyaniline/poly(ethylene oxide) composite was mixed with zinc oxide/silver nanoparticles and subsequently used in the removal of brilliant green with maximum 95 mg g−1 capacity (Gouthaman et al. 2018, 2019).The composite with zinc oxide and seaweed was used for the adsorption of methylene blue (Pandimurugan and Thambidurai 2016). Finally, the ternary composite polyaniline/chitosan/zinc oxide adsorbed up to 476 mg g−1 of Reactive Orange 16 (Kannusamy and Sivalingam 2013). The adsorption capacity of tartrazine on polyaniline/Zn-Fe double hydroxides reached 488 mg g−1 (Sahnoun et al. 2018).
Zr Polyaniline/zirconium(IV) silicophosphate (Gupta et al. 2014) adsorbed methylene blue; a similar phosphoborate (Kaushal et al. 2018) adsorbed both anionic and cationic dyes. The typical capacities were of the order of tens mg g−1 (Gupta et al. 2014; Herrera et al. 2018). The composite performed better than neat polyaniline; the non-conducting component alone might have been an efficient adsorbent. On the contrary, adsorption of polyaniline/zirconium dioxide was less efficient compared to polyaniline alone (Agarwal et al. 2016).
Natural polymers Cellulose in various forms has often been used as a support for conducting polymers. Filter paper coated with polyaniline adsorbed well Coomassie Brilliant Blue (Liu et al. 2015). Polyaniline/cellulose (cotton linters) adsorbed 117 mg g−1of Acid Red 4 or 56 mg g−1 Direct Red 23 (Abbasian et al. 2017a). Cotton textile coated with polyaniline was used for the removal of Acid Red 18 (Gamal and Attia 2019). Other authors used the composite with cellulose (Janaki et al. 2013), bacterial polysaccharide (Janaki et al. 2012a, b), starch (Janaki et al. 2012c; Olad et al. 2014) or chitosan (Janaki et al. 2012d; Abbasian et al. 2017b) for the adsorption of reactive dyes and typical adsorption capacities were 100 mg g−1. Polyaniline/lignocellulose composite had maximum adsorption capacity 312 mg g−1 of Reactive Black 5 (Ballav et al. 2015) or as high as 1673 mg g−1 for Congo red (Debnath et al. 2015a). The composite was also tested for the adsorption of eosin yellow (Debnath et al. 2015b). Exceptionally high capacities up to 10,560 mg g−1 have been observed for adsorption of Acid Red 94 on polyaniline/calcium lignosulfonate composite (Xu et al. 2019c). Such result, however, would need independent verification. The polyaniline/chitosan composite adsorbed both anionic and cationic dyes (Minisy et al. 2019b). Polyaniline containing carboxymethylcellulose/gelatin hydrogel selectively adsorbed acid fuchsin compared with other dyes (Xing et al. 2019).
There was the obvious trend to convert various organic waste products into useful adsorbents for water-pollution treatment (Suba and Rathika 2016), e.g., by in-situ deposition of conducting polymers. Polyaniline-coated kapok fibres adsorbed 41–192 mg g−1 of various dyes (Zheng et al. 2012; Mu et al. 2015) and 76 mg g−1 of methyl orange (Herrera et al. 2018). Sawdust coated with polyaniline adsorbed Acid Green 25 (Ansari et al. 2011), up to 144 mg g−1 Reactive Orange 4 (Baseri et al. 2013), methyl orange (Ansari and Mosayebzadeh 2011) or 213 mg g−1 of Acid Red G (Lyu et al. 2018). Peanut-hull waste coated with polyaniline was used as adsorbent of crystal violet (Tahir et al. 2017) and plant leaves for adsorption of diamond green (Kanwal et al. 2018). The biomass of Scenedesmu strain combined with polyaniline proved to be a good adsorbent of Reactive Red 120 with the capacity reaching 753 mg g−1 (Maqbool et al. 2019). Polyaniline/seaweed was tested for adsorption of methylene blue (Pandimurugan and Thambidurai 2016). Another natural material, rice husk, modified with polyaniline adsorbed Acid Red 18 (Shabandokht et al. 2016), rice bran was used for adsorption of Acid Orange 7. Polyaniline-coated activated carbon produced from natural organic material removed Direct Red 23 (Gopal et al. 2014) or rhodamine B (Gopal et al. 2016) from the aqueous solution. Polyaniline/xanthan gum composite similarly adsorbed maximum 22 mg g−1 of methylene blue (Tanzifi et al. 2019).
Synthetic polymers The composites of conducting polymers with another synthetic polymer have been studied only exceptionally, electrospun polyaniline/polystyrene mats being an example applied for efficient Congo red removal (Gorza et al. 2018) and polyaniline/polyester fabrics for Acid Red 18 separation (Gamal and Attia 2019). Poly(ethylene oxide) was a component in a complex composite used for the adsorption of brilliant green (Gouthaman et al. 2019). Polyaniline deposited on polyamide nanofibres had capacity up to 370 mg g−1 for methyl orange adsorption (Zarrini et al. 2017). Polyaniline particles stabilized with poly(N-vinylpyrrolidone) removed methyl orange from the aqueous solution completely (Prasad and Joseph 2017); methyl orange was removed selectively from the mixture with methylene blue. An original approach is represented by the filtration of a dye solution through the polyurethane sponge coated with polyaniline (Fig. 20). Polyaniline hydrogel prepared with the assistance of phytic acid removed methylene blue from the aqueous media at the maximum adsorption capacity 71 mg g−1 (Yan et al. 2015).
Dye removal by spontaneous filtration of Reactive Black 5 solution (50 ppm) through polyaniline-coated polyurethane sponge (Wei Lyu, unpublished results)
Polyaniline-related materials
In addition to polyaniline alone and its composite, related materials are considered: (1) aniline oligomers, (2) polymers of substituted anilines, (3) aniline copolymers, (4) chemically modified polyaniline and (5) carbonized polyanilines. They are all poorly conducting compared with polyaniline but that does not disqualify them for the good position among adsorbents.
The oxidation of aniline under specific conditions yields aniline oligomers with a spectacular flower-like (Zhao et al. 2013b) (Fig. 21) or urchin-like morphology (Zhao et al. 2013c). Such oligomers are also able to act as dye adsorbents and 181 mg g−1 capacity was reported was for crystal violet (Zhou et al. 2015). The oligomeric oxidation product of p-aminodiphenylamine adsorbed anionic dye of anthraquinone type, alizarin red S, as a simple model compound mimicking polyaniline (Liu et al. 2018a). Classical aniline oligomers prepared by the oxidation of aniline under alkaline conditions (Stejskal et al. 2008b) have not been so far tested as adsorbents.
Flower-like morphology of aniline oligomers (Yanchai Zhao, unpublished results)
Substituted polyanilines have been used in adsorption experiments only rarely. Poly(N-methylaniline)/cellulose and poly(N-ethylaniline)/cellulose composites adsorbed anionic azo dyes, Acid Red 4 and Direct Red 23, but less than polyaniline analogue (Abbasian et al. 2017a). Poly(chitosan-graft-N-metylaniline) adsorbed these dyes at 98 and 112 mg g−1 maximum capacity, respectively (Abbasian et al. 2017b). Poly(o-methylaniline), poly(m-methylaniline) and poly(N-methylaniline) prepared similarly in the presence of chitosan were used for the adsorption of Reactive Red 198 (Sayyah et al. 2015).
The oxidation products of phenylenediamine isomers, polyphenylenediamines (Stejskal 2015), represent a promising class of eco-friendly adsorbents due to the similarity of the molecular structure. In contrast to polyaniline, they are non-conducting or their conductivity is low. For simple applications in adsorption, however, this does not seem to be a drawback. Poly(o-phenylenediamine)/ferrite composites served for the adsorption of Congo red, as well as for removal of lead(II) and chromium(III) ions (Archana et al. 2016). Poly(m-phenylenediamine) alone was reported to have outstanding adsorption capacity up to 470 mg g−1 Orange G (Zhang et al. 2012; Meng et al. 2014). Another adsorbent, poly(m-phenylenediamine)/graphene oxide/nickel ferrite was tested on Congo red, methyl orange and methyl blue (Wang et al. 2017). Poly(m-phenylenediamine)/dextrin/graphene oxide composite was found to adsorb methylene blue at 76 mg g−1 capacity (Zare et al. 2018b), i.e. much less than polymer alone. Poly(p-phenylenediamine) nanocomposite with magnetite adsorbed bromocresol blue, methyl orange (Yang et al. 2014) and Reactive Blue 19 (Liu et al. 2016).
Aniline copolymers are represented by poly(aniline-co-anthranilic acid)/magnetite composite which adsorbed both anionic and cationic dyes, methyl red and methylene blue, respectively (Zoromba et al. 2017). The comonomer, anthranilic acid (o-aminobenzoic acid), introduced reactive carboxylic groups and ferromagnetic component allows for the easier separation of adsorbent. Simultaneous oxidation of aniline and pyrrole was reported to produce interconnected polyaniline/polypyrrole nanofibres (Bhaumik et al. 2013) but, in fact, they were rather statistical copolymers of aniline and pyrrole (Stejskal et al. 2004b). They nevertheless adsorbed up to 270 mg g−1 of Congo red (Bhaumik et al. 2013).
Polyaniline chemically modified with diiodomethane, which caused cross-linking of polyaniline chains, was reported to have a high specific surface area 1083 m2g−1 (Sharma et al. 2016). It adsorbed a cationic dye, crystal violet and an anionic dye, methyl orange, to comparable extent, 245 and 220 mg g−1, respectively.
Polyaniline converts to a nitrogen-containing carbon when exposed to ca. 500–600 °C in inert atmosphere (Trchová et al. 2009; Ćirić-Marjanović et al. 2013). Such materials could be used as dye adsorbents but the studies in this direction are so far missing.
Polypyrrole
The adsorption of dyes on polypyrrole alone (Table 5) has been studied only exceptionally within the testing of polypyrrole composites. For example, the adsorption of methylene blue on polypyrrole was 19.3 mg g−1 (Maruthapandi et al. 2018) or 11.2 mg g−1 (Zhou et al. 2017). Other dyes, such as malachite green, amaranth, Acid Orange 10, Congo red or indigo carmine were adsorbed marginally at 10–20 mg g−1 capacity (Zhou et al. 2017). Polypyrrole proved to be a potent adsorbent for Acid Red G, the maximum capacity being 122 mg g−1 (Feng et al. 2014) and for Congo red (Chafai et al. 2017a). The highest adsorption capacity, 237 mg g−1, was reported for methyl orange (Ye et al. 2019b). It should be recalled that this dye plays the most important role in the synthesis of polypyrrole nanotubes (vide supra) when methyl orange becomes incorporated into polymer structures.
Polypyrrole composites
On the other hand, the adsorption of dyes on polypyrrole composites has been reported in a number of papers (Table 6). Enhanced adsorption has been typically observed after in situ deposition of polypyrrole on various substrates, and synergistic adsorption effect has been noted (Zeng et al. 2013; Nerkar et al. 2018; Chen et al. 2019b). It seems that more ordered polypyrrole brushes constituting the coating are better adsorbents compared with disordered globular form (Fig. 13).
Depending on the type of the non-conducting substrates the composites may be organized as follows:
Al Alumina coated with polypyrrole adsorbed up to 135 mg g−1 of methylene blue (Chen et al. 2016).
C The composite of polypyrrole-coated carbon nanotubes performed better than polypyrrole alone in the adsorption of Sunset Yellow FCF and Congo red (Aliabad and Mahmoodi 2018). Adsorption of Acid Orange 7 on polypyrrole/activated carbon was also investigated (Supriya and Palanisamy 2016). Electropolymerized polypyrrole/graphene oxide allowed for the adsorption of Acid Red 1 and its controlled liberation (Haque and Wong 2017).
Fe The composites containing ferrite or other ferromagnetic particles have been proposed as adsorbents separable by the magnetic field. Adsorptivity of anionic Congo red, 120 mg g−1, was found for polypyrrole/ferrite composite (Karamipour et al. 2016; Aigbe et al. 2018), while polypyrrole/magnetite adsorbed only 23 mg g−1 of cationic rhodamine B (Yao et al. 2018), but 116 mg g−1 of alizarine (Gholivand et al. 2015). The adsorption capacity of methylene blue, methyl orange or acid fuchsin on polypyrrole/carbon nanotubes/ferrite was 110–140 mg g−1 (Li et al. 2017c). Magnetite has also constituted a part of similar polypyrrole composites (Bai et al. 2015; Shanehsaz et al. 2015; Chen et al. 2016; Feng et al. 2017; Diaz-Flores et al. 2019).
Si Silicates have been used as substrates for polypyrrole coating. The high adsorption capacities exceeding 1000 mg g−1 were reported only exceptionally, e.g., 1429 mg g−1 for Acid Blue 62 on polypyrrole-coated mesoporous silica SBA-15 (Akhbartabar et al. 2017). The adsorption capacity of the same dye on polypyrrole/mesoporous MCM-41 was only 55 mg g−1 (Binaeian et al. 2018). Another study announced the adsorption capacity 215 mg g−1 for methyl orange on polypyrrole/halloysite (Zhang et al. 2018). Polypyrrole-modified attapulgite clay served for the adsorption of Naphthol Green B (Chen et al. 2019a).
Ti Titanium dioxide coated with polypyrrole was used as an adsorbent of Acid Red G (Li et al. 2012, 2019b) or methylene blue (Li et al. 2013, 2019b), and the adsorption capacities up to 425 and 440 mg g−1 were determined, respectively.
Zn Polypyrrole-coated zinc oxide adsorbed brilliant green up to 140 mg g−1 (Zhang et al. 2019c) or Congo red (Karamipour et al. 2016).
Natural polymers Polypyrrole deposited in situ on natural polymers ranks among the most studied composites, similarly like polyaniline, cellulose being the polymer of the choice. The conducting polymers deposited on cotton textile represent one of suitable forms of adsorbents (Fan et al. 2017; Ayad et al. 2018a). The sorption capacity for methylene blue was lower for polypyrrole/cotton composite, 6.63 mg g−1, due to reduced amount of polypyrrole (Ayad et al. 2018a) but was more efficient in other experiments (Fan et al. 2017). Cotton alone was an adsorbent but its adsorptivity increased after the coating with polypyrrole (Fig. 22). Cellulosic waste was similarly coated with polypyrrole and the composite adsorbed 68 mg g−1 of a dye (Ovando-Medina et al. 2014). A similar material, sawdust coated with polypyrrole, adsorbed Reactive Violet 5 at 10–13 mg g−1 capacity (Supriya and Palanisamy 2017). The polypyrrole-coated peanut hull biomass was tested for adsorption of crystal violet (Tahir et al. 2017). Other authors have successfully used chitosan and lignosulfonate as supports for polypyrrole coating (Zhou et al. 2017). The composites were generally better adsorbents than polypyrrole alone. The combination of activated carbon with fruit of gardening plant was tested for adsorption of Direct Green 6 (Geetha and Palanisamy 2016). The polypyrrole/chitosan composites served as model adsorbents of Acid Red 18 (Chen et al. 2019b).
The colouration of methylene blue solution decreases after the introduction of cotton textile and disappears when the textile was coated with polypyrrole (Ayad et al. 2018a)
Synthetic polymers Polypyrrole/poly(vinylidene fluoride) adsorption capacity reached 370 mg g−1 for cationic methylene blue and 385 mg g−1 for anionic Congo red (Ma et al. 2018), again illustrating the independence on the dye type. The filtration of dye solution (50 mg L−1) through polyester membrane coated with polypyrrole completely removed both the anionic and cationic dyes (Ghaemi and Safari 2018). Electrospun polyamide mats coated with polypyrrole served for the adsorption of Basic Orange 2 (Li et al. 2015).
Polypyrrole-related materials
The materials based of substituted polypyrroles have not been so far investigated and this section is limited to carbonized polypyrrole. Polypyrrole nanotubes prepared by standard method using methyl orange template were converted to nitrogen-containing carbon nanotubes by pyrolysis in inert atmosphere (Ćirić-Marjanović et al. 2014; Kopecká et al. 2016; Sapurina et al. 2016; Xin et al. 2017; Kang et al. 2019; Minisy et al. 2019c). Such carbonaceous material has been tested for adsorption of methylene blue, rhodamine B, Congo red and Orange G with adsorption capacities 609, 572, 151 and 122 mg g−1, respectively (Xin et al. 2017).
Photocatalytic decomposition of dyes
In addition to adsorbents, the photocatalytic degradation of dyes in the presence of conducting polymers has often been reported in environmental issues, such as water-pollution treatment. Recent developments in this direction have been reviewed (Lee and Chang 2019) and the synergistic effect between semiconducting inorganic substrates and conducting polymers has been noted. In photocatalytic experiment, the dye removal by adsorption in dark is improved by the subsequent photocatalytic decomposition of adsorbed dye under illumination (Wei et al. 2015; Lakshmi and Rajagopalan 2016; Abukhadra et al. 2018c). The process is based on the catalytic generation of reactive hydroxyl radicals from oxygen, reactive oxygen species, from water molecules during the illumination by visible light or UV-irradiation on heterogeneous substrates, typically on metal oxides (Sushma and Kumar 2017; Abukhadra et al. 2018a; Samai and Bhattacharya 2018; Sobhani-Nasab et al. 2019). The reaction of the active species with organic dyes thus results in the dye modification, degradation or decomposition. Conducting polymers are good absorbents of light in UV–visible region and this fact is probably of importance in the photocatalytic effect supported by these polymers.
From the environmental point of view, the reduction or disappearance of coloration, sometimes called decolorization (Phan et al. 2017), is correctly interpreted as the dye decomposition but that does not mean that the potentially harmful reaction products have been removed. The degradation products have been sometimes identified by liquid chromatography/mass spectrometry (Wei et al. 2015). The degradation of a dye molecule need not be complete, because the destruction even of a single bond only in dye molecule may result in the loss of conjugation and conversion to a colourless compound. It should also be noted that the reverse process illustrated by the photocatalytic oxidative coupling of molecules may take place in the presence of conducting polymers (Kong et al. 2019a). The chemistry of the photocatalytic processes thus offers stimulating field of future studies.
The coating of various metal oxides that are active in photocatalysis of dye decomposition (Janaki et al. 2012d) is expected to improve the adsorption of dyes that is a prerequisite for the subsequent catalytic performance. In addition, the electroactivity of conducting polymers enables them to participate in electron transfers between the dyes and a catalyst that is necessary for the degradation reaction. Such role of conducting polymers to act in electron and proton transfers is also well known in corrosion protection of metals (Yang et al. 2015; Kohl et al. 2017; Sazou and Deshpande 2017; Umoren and Solomon 2019).
Polyaniline
Polyaniline alone was active in photocatalysis of dyes as a photosensitizer (Table 7). The synergistic photocatalytic effect was clearly demonstrated also for the composites, polyaniline-coated bismuth selenide (Chatterjee et al. 2019), carbon nitride (Zhang et al. 2014), manganese ferrite (Zeng et al. 2016), nickel oxide (Vidya and Balamurugan 2019a) or titanium dioxide (An et al. 2018; Sun et al. 2019a) when the photocatalytic activity of composites was higher compared with the performance of any component alone.
Polyaniline composites
The photocatalytic degradation of selected dyes with polyaniline-coated inorganic materials has recently been reviewed (Chatterjee et al. 2019). A typical photocatalyst is represented by a photoactive inorganic compound modified at the surface with a conducting polymer, which takes over the role of a photosensitizer (Table 8). Two or even three inorganic components appear simultaneously in some studies (Amer et al. 2019b; Ma et al. 2019; Zhao et al. 2019a).
The recent survey of the literature provided the following examples, which are listed below according to the nature of inorganic component:
Ag Sulfonated polystyrene microspheres were coated with polyaniline and combined with silver iodide to produce a photocatalyst for the degradation of rhodamine B (Liu et al. 2019). A ternary composite polyaniline/silver phosphate/nickel ferrite served for the photocatalytic removal of rhodamine B and methyl orange (Chen et al. 2019c).
Bi Bismuth vanadate (Shang et al. 2009), bismuth molybdate (Feng et al. 2019), bismuth wolframate (Wang et al. 2014; Zhao et al. 2019b), bismuth oxide (Wang et al. 2019c), bismuth oxychloride (Wang et al. 2013b; Tang et al. 2019b; Tanwar and Mandal 2019; Wang et al. 2019d), bismuth oxybromide (Hao et al. 2017; Liu and Cai 2019) and bismuth oxyiodide (Yan et al. 2018; Habibi-Yangjeh and Shekofteh-Gohari 2019; Wang et al. 2019c) coated with polyaniline have been applied for the photodegradation of methylene blue, methyl orange or rhodamine B under visible light. The composites often contained additional components. The composite with bismuth selenide was photocatalytically active in the decomposition of methyl orange, rhodamine B, or malachite green (Chatterjee et al. 2019). Another composite with bismuth phosphate was similarly used for the decomposition of methylene blue (Yu et al. 2018).
C Among carbonaceous materials, graphene coated with polyaniline performed well in the photocatalytic degradation of rose bengal (Ameen et al. 2012) or in the combination with complex wolframate in decomposition of methyl orange (Biswas et al. 2019). The composite comprising polyaniline and graphene oxide along with two additional inorganic components catalysed the decomposition of methylene blue (Zhao et al. 2019a) or malachite green (Zhang and Ma 2019). Carbon nitride coated with polyaniline displayed marked photocatalytic effect in the decomposition of both methyl orange and methylene blue, the efficiency of the former dye being higher (Zhang et al. 2014). Silicon carbide (Koysuren 2019) and silicon nitride (Wu et al. 2019) coated with polyaniline were similarly been used for the degradation of methylene blue.
Cd Cadmium sulfide dispersed on polyaniline stabilized with hydrolyzed pectin was used for the photocatalytic degradation of rhodamine B (Alipour and Lakouarj 2019).
Ce Cerium(IV) dioxide coated with polyaniline was employed in photocatalytic UV-light degradation of rhodamine B (Samai and Bhattacharya 2018) and of methylene blue under visible light irradiation (Vidya and Balamurugan 2019b).
Fe The composite of polyaniline with NiZn ferrite was used as a photocatalyst for the degradation of methyl orange (Chen et al. 2019c; Tanwar and Mandal 2019), Orange II (Pant et al. 2018) and rhodamine B (Chen et al. 2019c). An analogous composite with MgZn ferrite was applied for decolorization of methylene blue and methyl orange solutions (Arshadnia et al. 2017). Polyaniline/CoMn ferrite was another photocatalyst for degradation of methyl orange (Jung et al. 2019). The incorporation of ferromagnetic components, such as iron (Das et al. 2017; Tanwar et al. 2017), ferrite (Zeng et al. 2016; Arshadnia et al. 2017; Kim et al. 2017; Abukhadra et al. 2018a; Aigbe et al. 2018; Bashir et al. 2019; Chen et al. 2019c; Mesdaghi et al. 2019) or magnetite (Habibi-Yangjeh and Shekofteh-Gohari 2019), allowed for the separation of the catalyst by applying magnetic field. Iron ions improved the adsorption capacity of polyaniline/titanium dioxide with respect to photodegradation of methylene blue (Koysuren and Koysuren 2019).
Mg Polyaniline/magnesium indium sulfide was active in the photodegradation of methyl orange and photoreduction of chromium(VI) ions (Jing et al. 2019).
Mn Polyaniline-coated manganese dioxide was used as catalyst for the degradation of Acid Blue 25 by hydrogen peroxide (Gemeay et al. 2012), which increased in the presence of UV-light.
Mo Molybdenum disulfide coated with polyaniline by in-situ polymerization of aniline was tested in the degradation of methylene blue (Saha et al. 2019).
Ni The decomposition of Reactive Orange 16 using polyaniline composite with nickel nanoparticles has recently been illustrated (Ahmad et al. 2019) but the nickel oxides have been used more often. For example, polyaniline/nickel(II) oxide catalysed the photodecomposition of methylene blue (Vidya and Balamurugan 2019a). It was demonstrated that ternary polyaniline/heulandite/nickel oxide composite catalysed the oxidative degradation of not only safranin but also of other dyes, such as Congo red, crystal violet, methyl orange, methylene violet or malachite green, alone or in mixtures (Abukhadra et al. 2018a). Safranin was also photocatalytically removed using polyaniline/bentonite/nickel(III) oxide composite (Abukhadra et al. 2018b).
Ru A polyaniline-coated mixed ruthenium/titanium dioxide catalysed the degradation of methyl orange both in dark and in simulated sunlight (Mousli et al. 2019). This means that the composite was active not only as a photocatalyst but also as a degradation catalyst. The explanation based on a dye adsorption could also be considered in this case.
Si While silicon compounds have been found to be good adsorbents, they have not practically been used in the photocatalytic experiments. A mixture of polyaniline with heulandite was applied in the photocatalyzed decomposition of Light Green SF (Abukhadra et al. 2018c).
Sn The composite polyaniline/tin dioxide/biotemplate has been used to catalyse the photodegradation of Reactive Yellow 15 (Karpuraranjith and Thambidurai 2016) similarly like analogous composite with diatomite (Akti 2018). Polyaniline/strontium stannate proved to be a good photocatalyst of methylene blue decomposition (Faisal et al. 2019).
Ti Titanium dioxide is the key substrate in the photocatalytic experiments due to its ability to absorb light in UV-region. After coating with polyaniline, which extended light absorption to visible region, the composite has been used for the degradation of Acid Blue 90 (Ansari et al. 2015), Acid Red 73 (Vaez et al. 2018), allura red (Salem et al. 2009), Congo red (Guo et al. 2014; Mousli et al. 2019), eosin yellow (Debnath et al. 2015c), methylene blue (Min et al. 2007, 2008; Wang et al. 2010; Radoičić et al. 2013; Jeong et al. 2014; Reddy et al. 2016; Radoičić et al. 2017; Song et al. 2019; Zhao et al. 2019a), methyl orange (Guo et al. 2014; Mohammadi et al. 2019; Song et al. 2019; Sun et al. 2019a), naphtol blue black (Debnath et al. 2015c), Orange II (An et al. 2018), quinoline yellow (Salem et al. 2009), Reactive Black 5 (Subramaniam et al. 2019), Reactive Red 4 (Razak et al. 2014), Reactive Red 45 (Gilja et al. 2017), or rhodamine B (Radoičić et al. 2013; Ansari et al. 2015; Reddy et al. 2016; Deng et al. 2017; Radoičić et al. 2017). The combination with graphene or carbon dots (Feizpoor et al. 2018; Mohammadi et al. 2019) still improved the photocatalytic properties. The mixture of titanium dioxide with a polyaniline colloid was active in photodegradation of rhodamine B (Zhou et al. 2019) or methyl orange (Bahrudin et al. 2018b, 2019).
Ta Tantalum nitride modified by chemisorption of polyaniline from tetrahedran solution was more efficient in the photodegradation of rhodamine B than nitride alone (Niu and Xu 2019).
Tm Polyaniline/thulium titanate was used in the sonophotocatalytic decomposition of rhodamine B, eosin Y and phenol red (Sobhani-Nasab et al. 2019).
W Polyaniline prepared in the presence of tungstophosphoric acid catalysed the photodecomposition of four selected dyes better than any of the composite components (Salavati and Kohestani 2013). Complex polyaniline/wolframate/graphene composite catalysed the photodecomposition of methyl orange (Biswas et al. 2019).
Zn Polyaniline/zinc oxide was investigated in the decomposition of Congo red (Poorarjmand et al. 2019), methyl orange (Saravanan et al. 2016) and methylene blue (Saravanan et al. 2016; Sahu et al. 2019). The composite with zinc selenide was also tested in the removal of last dye (Shirmardi et al. 2018), with zinc oxide/chitosan for removal of Reactive Orange 16 (Pandiselvi and Thambidurai 2013; Kannusamy and Sivalingam 2013), and the composite of zinc oxide/activated charcoal for degradation of rhodamine B (Selvin et al. 2018). Polyaniline/zinc sulfide was tested for the photocatalytic decomposition of rhodamine 6G (Allahveran and Mehrizad 2017).
Zr The photocatalytic decomposition of rhodamine B on zirconium dioxide was improved after coating the photocatalyst with polyaniline (Carević et al. 2018; Shah et al. 2018).
Natural polymers Cellulose (Ahmad et al. 2019), chitosan (Pandiselvi and Thambidurai 2013) or pectin (Alipour and Lakouarj 2019) participated in some composites tested in this direction.
Synthetic polymers Modified titanium dioxide on polyaniline-coated polyimide fabric degraded methylene blue, Reactive Blue 19 and Reactive Red 241 (Ding et al. 2019).
Polyaniline-related materials
In addition to the adsorption of dyes, polymers of phenylenediamines (Stejskal 2015) have also been used for the catalytic decomposition of dyes. Poly(o-phenylenediamine) deposited on bismuth or lanthan vanadates removed photocatalytically methylene blue from the aqueous media (Sivakumar et al. 2019). Poly(o-phenylenediamine)/zinc wolframate/fly ash displayed a photocatalytic activity in the degradation of coloured antibiotics, tetracycline (Ye et al. 2019a). Another composite with cobalt ferrite photocatalyzed the degradation of malachite green (Riaz et al. 2016). Finally, titanium dioxide with polypyrrole shell was active in the decomposition of methylene blue (Wang et al. 2012, 2013c) and Acid Orange 7 (Archana et al. 2016). It should be noted that the products of o-phenylenediamine oxidation are rather oligomers than polymers (Stejskal 2015). Poly(m-phenylenediamine) deposited on zinc oxide degraded under illumination Acid Red 249 (Peng et al. 2014). The photocatalytic degradation of bromocresol green, bromocresol blue, bromocresol purple, rhodamine B, neutral red, methylene blue, Sudan III, methyl orange and Congo red on poly(p-phenylenediamine)/magnetite proved high activity under both UV and visible light while magnetite alone was inactive (Yang et al. 2014).
The copolymers of aniline with phenylenediamine are represented by poly(aniline-co–o-phenylenediamine)/iron(III) oxide composite, which served as a photocatalyst in the decomposition of methylene blue (Ossoss et al. 2019). In another study, titanium dioxide was prepared in the presence of poly(aniline-co-pyrrole) and tested in the photocatalysis of rhodamine B and methyl orange (Gao et al. 2019).
In a rare study on carbonized polyaniline, carbonized polyaniline/titanium dioxide composite displayed higher photocatalytic activity in the degradation of rhodamine B and methylene blue compared with the parent composite (Radoičić et al. 2017). Finally, the sulfonated polyaniline in the combination with titanium dioxide was used in the photocatalytic depomposition of methylene blue and Acid Blue 90 (Ansari et al. 2015).
Polypyrrole
The number of reports on photocatalytic performance of polypyrrole alone is very limited. The photocatalytic activity of polypyrrole in the degradation of safranin has recently been reported (Mohamed et al. 2018). No degradation was observed with globular polypyrrole, while polypyrrole prepared in the presence of a surfactant, sodium dodecyl sulfate, was active (Yuan et al. 2019). This was explained by different nanostructure of conducting polymer but this might have been also due to the difference in the hydrophilicity of both types.
Polypyrrole composites
There were various reports on the photocatalytic performance of polypyrrole-based composites (Table 9), even though they are less numerous compared with polyaniline studies.
The survey of photocatalysts ordered according to the inorganic components is as follows:
Ag The composite polypyrrole/Ag/BiOBr was active in the degradation of malachite green or phenol (Liu and Cai 2018) and polypyrrole/Ag/zinc oxide in removal of rhodamine B (Podasca et al. 2019).
Bi A composite comprising polypyrrole, carbon nitride and bismuth tungstate performed also well in the photocatalytic decomposition of rhodamine B under visible light (Jiao et al. 2019).
C Polypyrrole-coated graphitized carbon nitride was tested in the photocatalytic decomposition of rhodamine B (Hayat et al. 2019).
Fe Polypyrrole-coated magnetite-based particles were used for the degradation of Congo red (Wei et al. 2015) and methylene blue (Amer et al. 2019b). Thanks to the presence of ferromagnetic component, the adsorbent was separable by magnetic field. The incorporation of iron nanoparticles in polypyrrole/attapulgite also reported (Chen et al. 2019a).
Sr Strontium carbonate composite with polypyrrole was tested for the adsorption of methylene blue under visible light (Marquez-Herrera et al. 2016). Attapulgite clay coated with polypyrrole served for the degradation of Naphthol Green G (Chen et al. 2019a).
Ti The coating of titanium dioxide with polypyrrole improved the photocatalytic degradation of methylene blue (Castillo-Reyes et al. 2015; Sangareswari and Sundaram 2017; Amer et al. 2019b), methyl orange (Li et al. 2018b) or Reactive Red 45 (Krehula et al. 2019) even under visible light. The titanium dioxide/silver oxide surface-modified with polypyrrole was similarly active (Kumar 2016).
W Copper-complex phosphotungstate compound of Keggin type with in-situ-deposited polypyrrole photocatalytically degraded methylene blue (Kong et al. 2019b).
Zn Polypyrrole/zinc oxide was used for the photocatalytic decomposition of Acid Blue 25 (Gilja et al. 2018), Acid Violet 7 (González-Casamachin et al. 2019), rhodamine B (Lakshmi and Rajagopalan 2016) and methylene blue (Ovando-Medina et al. 2015b; Lakshmi and Rajagopalan 2016) or in the combination with silver for photocatalytic degradation of rhodamine B (Podasca et al. 2019). Polypyrrole/Zn-Fe layered double-hydroxide was active in the photodegradation of safranin (Mohamed et al. 2018). The composites performed better than any of its components, i.e. the synergism between conducting polymers and the substrate was observed.
Natural polymers Cellulose coated with polypyrrole adsorbed Reactive Red 120 at the capacity 16–96 mg g−1 depending on pH (Ovando-Medina et al. 2015a).
Other methods of dye degradation
In addition to UV–visible light irradiation in photocatalytic degradation of organic dyes, the catalytic effect of conducting polymers was supplemented by other energy sources. The exposure to microwaves was used in a single case for the dye degradation (Riaz et al. 2014). Conducting polymers are good absorbers of microwaves and resulting local heating may promote the dye decomposition. The photocatalytic decomposition of polyaniline-coated electrode was also supplemented by the application of an electric potential (Pirkarami et al. 2013). Sonocatalytic degradation of organic dyes with the help of ultrasonic agitation was also reported (Salavati and Kohestani 2013; Wang et al. 2015b; Das et al. 2017).
Other approaches have oriented on promoting the catalytic effect of conducting polymers by incorporation of noble-metals in reductive degradation of dyes. Cotton fabrics coated with polypyrrole and decorated with silver nanoparticles catalysed the reduction of p-nitrophenol to p-aminophenol (Ayad et al. 2017, 2018b) with sodium borohydride. The process is mentioned here because p-nitrophenol is yellow and converts to a colourless compound, and it can, therefore, be followed by the methods used for the degradation of dyes (Mu and Wang 2015). Wool fabric coated with poly(o-anisidine) (= poly(o-methoxyaniline)) and decorated with silver particles catalysed the reduction of methylene blue (Erdogan et al. 2019). Polyaniline-supported palladium catalyst efficiently degraded methylene blue, rhodamine B or methyl orange also by sodium borohydride (Roy et al. 2019).
Some studies report the catalytic effect of conducting polymers even in the absence of noble metals. For example, polyaniline was used for the catalytic activation of peroxymonosulfate in the degradation of methyl orange (Sun et al. 2019b). Enzymatic degradation of Reactive Blue 4 by ginger peroxidase immobilized on polypyrrole/cellulose/graphene oxide was also tested (Ali et al. 2018).
Conductivity
The conducting polymers and organic dyes share the conjugated structure of alternating double and single bonds that is reflected in their colour. They have the ionic character, as they are typically salts that dissociate in the presence of water and afford the contribution of ionic conductivity. They differ by the presence of charge carriers, e.g., polarons, in conducting polymers, and their absence in dyes. Since conducting polymers and dyes have been observed to interact, as they manifest themselves in adsorption experiments, it is a relevant question how much such interaction will affect the electric properties of conducting polymers. For instance, we can speculate that molecules of organic dyes may assist the charge transport by providing conjugated bridges between conducting polymer chains supported additionally by π–π interactions of aromatic ring of both moieties. There are virtually no studies that would attempt to address this issue.
One-dimensional morphology of conducting moieties is of benefit when the conductivity is the key parameter in the desired application. The percolation limit, and thus the good conductivity level, is much easier reached with one-dimensional objects that are present in the composite due to the formation of conducting network. For that reason, attention has been paid to the preparation of conducting polymer nanotubes or nanofibres, and this applies especially to polypyrrole. In this direction, the organic dyes play a decisive role in the morphology and conductivity control. Another approach relies on the coating of nanotubular objects, e.g., carbon nanotubes, with conducting polymers (Konyushenko et al. 2006; Zeng et al. 2013; Minisy et al. 2019c).
Polyaniline
The conductivity of “standard” globular polyaniline is 4 S cm−1 (Stejskal and Gilbert 2002). There are very few reports of the conductivity for polyaniline prepared in the presence of the dyes, probably because there was no significant effect worth to mention. Exceptionally, the conductivity of polyaniline, 2.2 S cm−1, increased to 15 S cm−1 when Sunset Yellow FCF was present in the reaction mixture and dropped down at still higher dye concentrations (Shi et al. 2017).
It has been observed that polyaniline/cellulose composite decreased its conductivity when exposed to the aqueous solutions with increasing concentration of methylene blue (Sarkar et al. 2018). Such effect, however, might have been caused by the simple deprotonation of polyaniline in distilled water and insufficient pH control. The exposure of polyaniline salt to the solutions of Orange G led to the decrease in the conductivity from 14.5 to 2 S cm−1. This was explained be the incorporation of the dye (Mahanta et al. 2008) but again a partial deprotonation of polyaniline in dye solutions of low acidity would again be alternative explanation. In the photocatalytic decomposition of dyes, the best composite performance was linked to its highest conductivity (Alipour and Lakouarj 2019). Such correlation may exist but it would involve many other parameters, too.
Polypyrrole
The conductivity of globular polypyrrole is 1–2 S cm−1 (Sapurina et al. 2017; Acharya et al. 2018). There are some indications in the case of polypyrrole that the smaller conducting polymer particles (Yuan et al. 2019) or thinner one-dimensional structures (Stejskal and Trchová 2018) have higher conductivity. It is tempting to link the conductivity to specific surface area, but, in fact, this is probably due to the better organization of polymer chains in smaller objects (Li et al. 2017b; Stejskal and Trchová 2018). As dyes, similarly like surfactants (Omastová et al. 2003), affect the polymer morphology especially in the case of polypyrrole, the conductivity enhancement seems to be indeed caused mainly by improved polymer-chain ordering rather than by the direct interaction between conducting polymers and dyes at molecular level.
Generally, however, the presence of dyes affects the morphology and conductivity of pyrrole in unpredictable manner (Table 10). This is in the contrast to polyaniline where the dyes have limited influence on conductivity as the published data and preliminary experiments indicate.
Methyl orange, however, is still the best dye routinely used in the improvement of polypyrrole conductivity. Polypyrrole prepared in the absence of a dye had conductivity 1.5 S cm−1. In the presence methyl orange it increased at first as the dye concentration grew to 86 S cm−1 and then started to decrease at higher dye concentration (Li et al. 2017b). The same trend was confirmed in another study (Fig. 6) (Sapurina et al. 2017). This was explained by the improvement of polypyrrole-chain organization during the formation of polypyrrole nanotubes, and later the dye acted only as non-conducting filler (Sapurina et al. 2017). With other dyes, the conductivity was improved only in the case of ethyl orange (Li et al. 2017b; Sapurina et al. 2017), methylene blue (*Sapurina et al. 2017) or alizarin red S (Zang et al. 2018) compared with the synthesis without a dye.
The conductivity enhancement from 0.07 to 1.25–3.3 S cm−1 was reported after introduction of tryptan blue to the reaction mixture (Yang et al. 2019). Direct Blue 2 and Direct Violet 1 had the similar effect and the conductivities were 1.05 and 0.58 S cm−1, respectively. Indigo carmine had no significant influence, the conductivity being 0.62–1.91 S cm−1 (Li et al. 2016). Nevertheless, such variations in conductivity are regarded as small and close to the experimental error of conductivity determination.
The similar pattern applies also to composites. The conductivity of polypyrrole increased from 12.5 to 25.0 S cm−1 with increasing fraction of rhodamine B/attapulgite in the reaction mixture and then decreased at still higher dye concentration (Wang et al. 2013d). It was suggested that the pathways for charge carriers were improved by the presence of the dye. The same trend, however, was observed in the preparation of polypyrrole/molybdenum or tungsten disulfides in the absence of any dye (Acharya et al. 2018; Stejskal et al. 2019) and was explained by the improved polymer-chain organization.
Other applications
The conducting polymers were reported to be active in dye removal in water-pollution treatment, regardless of the dye adsorption or degradation mechanism (Zare et al. 2018a). It is generally agreed that the adsorption of cationic and anionic dyes is comparable (Amer et al. 2018; Kaushal et al. 2018; Boukoussa et al. 2018) and conducting polymers are thus universal dye adsorbents. Other uses involving conducting polymers and dyes and related compounds have ocasionally been proposed and these are briefly reviewed below.
Energy conversion and storage
The interaction between conducting polymer and organic dyes is likely to manifest itself also in electrical properties of composites, although this direction has not received attention so far. The supercapacitors are the most studied energy-storage devices. The introduction of redox-active components represented both by the conducting polymers and organic dyes can promote faradaic reactions and enhance the ionic conductivity (Xu et al. 2018). For example, the capacitance of polyaniline prepared in the presence of Acid Red 27 was reached 423 F g−1 (Shi et al. 2018) and with Sunset Yellow FCF 467 F g−1 (Shi et al. 2017). Polyaniline prepared in the presence of activated carbon and methyl orange had higher capacitance compared with the composite made in dye absence (Jia et al. 2012).
Polypyrrole nanotubes prepared in the presence of methyl orange always contained a significant fraction of this dye (Alekseeva et al. 2015; Sapurina et al. 2016). They have been used in supercapacitor electrodes after deposition on cotton fabric (Xu et al. 2015; Wang et al. 2019e; Zhang et al. 2019b) or alone (Hryniewicz et al. 2019). In the former case the chemical deposition yielded a material with specific capacitance 64–565 F g−1, in the latter electrochemical synthesis the specific capacitance was 423 F g−1. Polypyrrole nanotubes prepared in the presence of methyl orange and decorated with nickel/cobalt sulfide had specific capacitance as high as 1705 F g−1 (Wang et al. 2019f).
The capacitances of polypyrrole/attapulgite improved when polypyrrole was prepared in the presence of rhodamine B (Wang et al. 2013d). The authors proposed that the dye provided pathways to charge carriers. The capacitance of polypyrrole prepared in the presence of trypan blue reached 649 F g−1 (Yang et al. 2019). Polypyrrole produced by electropolymerization in the presence of alizarin red and carbon nanotubes had the specific capacitance 274 F g−1. In all cases, the dye present in the synthesis has affected the polymer morphology and the associated specific surface area, and its role is not quite understood. Polypyrrole prepared in the presence of indigo carmine was also used as an anode in all-polymer battery (Sultana et al. 2012a, b). Polypyrrole prepared electrochemically in the presence of cationic thiazine dye, methylene green, served as an electrode in ethanol enzynamic biofuel cell (Bonfin et al. 2019). In all these reports, the role of dyes is not quite obvious.
Removal of drugs and herbicides
Many drugs are coloured and have similar features of molecular structure as organic dyes. For that reason such molecules can be similarly adsorbed by conducting polymers. For example, the adsorption of ciprofloxacin antibiotic on polyaniline/silver/silver molybdate composite was reported (Mondal et al. 2019b). Polypyrrole deposited on plant fibres adsorbed 69–78 mg g−1 of three antibiotics from floxacin family from solutions containing initially 100 mg L−1 (Duan et al. 2019). Polypyrrole/magnetite/silica composites were used for the extraction of sulfonamides from water (Sukchuay et al. 2015), polypyrrole nanotubes for electrochemically controlled extraction of atrazine, caffeine and progesterone (de Lazzari et al. 2019) and polypyrrole-coated polyamide nanofibres for extraction oxacilin and cloxacilin antibiotics followed by determination by capillary electrophoresis (Li et al. 2019a).
Various composites have been used for the photocatalytic decomposition of drugs. e.g., polyaniline/titanium dioxide composite photocatalysed degradation of metronidazole antibiotic (Asgari et al. 2019) or sulfaquinoxaline sulfonamide (Sandikly et al. 2019). The photocatalytic removal of another antibiotic, tetracycline hydrochloride, on poly(o-phenylenediamine)/zinc wolframate/fly-ash cenospheres (Ye et al. 2019a), polypyrrole/fly-ash (Pen and Huang 2019) or polypyrrole/silver phosphate/carbon nanotubes (Lin et al. 2019) was reported. Polypyrrole/zinc oxide supported the photodegradation of diclofenac (Silvestri et al. 2019). On the other hand, drug delivery was modelled using droplets of fluorescent dye, Nile red, dissolved in organic solvent encapsulated in polypyrrole shell (Bartel et al. 2018).
Harmful organic pollutants, such as atrazine herbicide (Wang et al. 2018b) are not dyes but they often share their conjugated molecular structure and could be removed by using conducting polymer composites, such as polyaniline/attapulgite (Wang et al. 2018c). The nicosulfuron herbicide was adsorbed at 5.5–13 mg g−1 capacity by polyaniline/BEA zeolite (Jevremović et al. 2019). Various chlorinated pollutants also fall to this category (Hayat et al. 2019).
Sensors
Both polyaniline and polypyrrole are regarded as responsive polymers that change their properties, such as conductivity and colour, in the response to external stimuli, for example pH, temperature, humidity or in the presence of various gases and chemicals or their vapours. This is exploited in the design of sensors (Runsewe et al. 2019) and dyes are occasionally mentioned along with conducting polymers.
Optical pH sensor was based on bromothymol blue immobilized in polyaniline sol–gel (Othman et al. 2017). The blend of polyaniline with non-commercial orange azo dye made the basis of impedance humidity sensor (Chani et al. 2013). The cyclic voltammograms of polyaniline film grown on the electrode were altered after the exposure to the solutions of dyes (Mahanta et al. 2011a). Such effect was proposed to be exploited for the dye detection.
Polypyrrole both in globular and nanotubular forms was used for the sensing of ethanol or n-heptane vapours (Kopecká et al. 2016). Methyl orange was used in the preparation of polypyrrole nanotubes and was partly incorporated in them. Polypyrrole prepared in the presence of eriochrome cyanine R provided a sensor for ammonia gas (Tavoli and Alizadeh 2013), and the preparation using tartrazine (Acid Yellow 23) yielded the material for a simultaneous electrochemical sensing of ascorbic acid, uric acid and dopamine (Wang et al. 2019a). Electropolymerization of pyrrole in the presence of Alizarin Red S afforded a potentiometric sensor for silver ions (Rounaghi et al. 2015) and with methyl orange the pH-sensor based on Raman spectra (Czaja et al. 2019). Polypyrrole/bromophenol blue was used as ultrasensitive quartz-microbalance sensor in the ppb-range detection of explosive nitro compounds (Eslami and Alizadeh 2019). Electrospun nanofibres coated with polypyrrole were used for the micro solid phase extraction in the determination of auramine O, chrysoidine and rhodamine B in the industrial wastewater (Qi et al. 2016). Carbonized polypyrrole was applied for the electrochemical determination of Sunset Yellow FCF (Wang et al. 2019b).
Other polymers, methods and applications
The results of some papers that do not exactly fit to above scheme are briefly enumerated here. For instance, in biomedicine polypyrrole nanotubes prepared in the presence of methyl orange were tested as adsorbents of human influenza viruses (Ivanova et al. 2017). Polypyrrole/methylene blue colloids were employed in photo-induced cancer therapy, when polypyrrole efficiently adsorbed light in near-infrared region and methylene blue acted as a photosensitizer promoting the formation of reactive oxygen species (Phan et al. 2017).
In addition to current conducting polymers, polyaniline and polypyrrole, reviewed above, also polythiophenes appear to be emerging class applicable in water purification (Dutta and Rana 2019). The photocatalytic activity of poly(3-hexylthiophene)/titanium dioxide in decomposition of Orange G (Acid Orange 10) was demonstrated (Mukhta et al. 2007). In another direction, poly(3,4-ethylenedioxythiophene) was prepared by the chemical oxidation of corresponding monomer in the presence of Congo red and tested for electrochemical performance (Bai et al. 2018). The maximum specific capacitance was 206 F g−1. Finally, polysafranin/Triton X-100 was used in the electrochemical selective determination of dopamine (Lavanya et al. 2015).
Polypyrrole nanotubes convert to nitrogen-containing carbon nanotubes by the carbonization in inert atmosphere (Ćirić-Marjanović et al. 2014; Kopecká et al. 2016; Sapurina et al. 2016; Xin et al. 2017; Kang et al. 2019; Minisy et al. 2019c) and can be applied as a new type of nanostructured carbons. They offer some alternative to multiwall carbon nanotubes from the morphology point of view but their structure and properties are different. The preparation of polypyrrole nanotubes involves the presence of organic dye, methyl orange, during the synthesis and for that reason they are mentioned here. Such nanotubes have been used in energy conversion devices (Stejskal and Trchová 2018). Carbonized analogues have also been of interest because of high specific surface areas (Lin et al. 2018), viz. 205 m2g−1 (Xin et al. 2017) and 257 m2g−1 (Ćirić-Marjanović et al. 2014). They were tested in electrodes of lithium (Lin et al. 2018) and flow batteries (Wu et al. 2018), in electrochemical capacitors (Xin et al. 2017) or for electrocatalysis in oxygen reduction reaction (Ćirić-Marjanović et al. 2014; Minisy et al. 2019c). On the other hand, they are also promising dye adsorbents, photocatalysts (Radoicic et al. 2017; Xin et al. 2017) and sensors (Kang et al. 2019).
Concluding remarks on perspectives of conducting polymers
The conducting polymers are not just conducting and their role in environmental issues will probably increase in future. Their electroactivity, i.e. their ability to be oxidized or reduced chemically or electrochemically, opens the route to the intelligent adsorbents. It has recently been demonstrated that the hydrophobicity of polypyrrole and, consequently, the adsorption on such materials would be affected by the applied electrical potential and thus controlled (Ren et al. 2018). The electrochemically oxidized or reduced forms of polypyrrole generally differ in the adsorption ability and may be used for dye adsorption and release or in adsorbent recovery (Haque and Wong 2017). The same is expected for polyaniline.
The chemical reaction on polyaniline or polypyrrole may also be helpful in the water pollution treatment (Huang et al. 2014; Zare et al. 2018a). In addition to removal of organic dyes and related compounds, conducting polymers are able to reduce noble-metal ions to corresponding metals, viz. silver (Stejskal 2013; Bober et al. 2018b; Maráková et al. 2017; Mahlangu et al. 2019), gold (Mu et al. 2015), platinum and palladium (Sapurina et al. 2016) and thus to separate them from aqueous media, too. There is vast number of papers on the adsorption combined with reduction of harmful chromium (VI) to chromium (III) based on the same principle (Ansari et al. 2017; Ma et al. 2018; Shirmardi et al. 2018; Jing et al. 2019; Mitra et al. 2019). Free halogens, such as bromine (Stejskal et al. 2001) or iodine (Stejskal et al. 2008c) become covalently bound to polyaniline and enable, e.g., to remove radioactive isotopes of iodine (Harijan et al. 2018).
The removal of heavy-metal cations by conducting polymers has recently been reviewed (Zare et al. 2018a). Nitrogen atoms in polyaniline, polypyrrole or related polymers promote the complex formation with heavy-metal cations and thus the purification of water from inorganic cations (Dutta and De 2017, Zare et al. 2018a), such as arsenic (III) (Che et al. 2018; Trung et al. 2018), arsenic (V) (Bhaumik et al. 2014), cadmium (II) (Canoluk and Gursoy 2017), chromium (III) (Beyki et al. 2016; Salehi-Barbarsad et al. 2019), chromium (VI) (Bhaumik et al. 2014; Chigondo et al. 2019), copper (II) (Herrera et al. 2018; Soltani et al. 2019), gallium (III) (Saugo et al. 2018), iron (III) (Salehi-Barbarsad et al. 2019) lead (II) (Beyki et al. 2016; Zare et al. 2018b; Teklu et al. 2019), nickel (II), mercury (II) (Kim et al. 2018; Zhao et al. 2019c), silver (I) (Stejskal 2013; Bober et al. 2018b; Maráková et al. 2017; Mahlangu et al. 2019), strontium (II) (Lu et al. 2018), uranium (VI) (Lei et al. 2018; Saghatchi and Ansari 2018) and zinc (II) (Kumar et al. 2017). The removal of cobalt and europium radionuclides from environment also falls into this category (Metwally et al. 2019).
Various inorganic anions have also been reported to adsorb on conducting polymers, such as chromate (Ansari et al. 2017; Ma et al. 2018; Shirmardi et al. 2018), fluoride (Parashar et al. 2016; Chen et al. 2017b), dichromate (Liu et al. 2018b; Chigondo et al. 2019), nitrate (Garcia-Fernandez et al. 2017) or phosphate (Wang et al. 2017).
The application potential of conducting polymers and their composites in water-pollution treatment is thus much broader compared with classical systems. In addition, conducting polymers have been found to be applicable in wound care and skin tissue engineering (Talikowska et al. 2019). Such polymers with adsorbed organic dyes or structurally related pharmaceuticals can be used in the controlled delivery of disinfectants and antibiotics or to improve the antimicrobial performance. This aspect is also important for the water treatment that requires the removal of various microorganisms (Ivanova et al. 2017; Hussein et al. 2019) in addition to current organic and inorganic pollutants.
Conclusions
The progress in the studies of the interaction between conducting polymers and organic dyes has been reviewed. Three research directions can be identified: (1) the effect of dyes on the preparation of polyaniline and polypyrrole, (2) the adsorption of dyes on conducting polymers and (3) the photocatalytic removal of dyes assisted by conducting polymers. The reports appearing in the literature have been organized and discussed according these categories with respect to individual conducting polymers, composite components and dye types.
Organic dyes share some features with surfactants and, consequently, they have similar colloidal properties. They have relatively large hydrophobic part and a hydrophilic moiety, typically represented by an anionic or cationic group. It would probably help to the understanding of their behaviour if the dyes were regarded as “coloured surfactants”. The interaction of dyes with produced conducting polymers, however, is more intimate and includes specific π–π interactions of the conjugated molecular structure inherent to both moieties.
The influence of dyes used in the preparation of polyaniline has not been systematically investigated so far and it seems to be marginal when it comes to the morphology and conductivity of this conducting polymer. On the other hand, the preparation of polypyrrole is strongly affected by the presence of dyes. The most frequently used methyl orange converts the globular polypyrrole morphology to nanotubes with the simultaneous increase in conducitivity. The preparation of one-dimensional polypyrroles can be achieved with both anionic and cationic dyes.
Both polyaniline and polypyrrole are polycations in their conducting states. It would be logical to expect preferential interaction with oppositely charged anionic dyes. The most dye-adsorption experiments, however, indicate that both anionic and cationic dyes interact with conducting polymers in similar manner. This means that electrostatic ionic interactions are not decisive. The anionic dyes, such as methyl orange and Congo red, along with cationic methylene blue, have most often been used as model sorbates. The deprotonated conducting-polymer bases adsorbed dyes generally more efficiently than corresponding protonated forms. This is due to the higher hydrophobicity of polymer bases compared with polymer salts. Polyaniline and its composites have been used as adsorbents more frequently than polypyrrole analogues. The papers comparing adsorption performance of polyaniline and polypyrrole are so far missing.
Dye adsorption and photocatalytic dye degradation are usually treated as separate phenomena in the literature but they are clearly related. Efficient photocatalysis requires a dye adsorption to catalyst sites. On the other hand, adsorption studies are often blind to the associated photocatalytic processes. Also in these experiments, the use of methyl orange and methylene blue dominates with significant contribution of papers exploring rhodamine B photodegradation.
Conducting polymers, such as polyaniline and polypyrrole, alone or in the composites are efficient materials for the removal of organic dyes from the environment. They have good application potential because of economic production cost of conducting polymers. In addition to organic dyes they remove noble-metal ions, heavy-metal cations and various anions from the aqueous media. Their application role, e.g., in water-pollution treatment, thus may be much broader. The electrical properties, such as the conductivity and redox electroactivity, are expected to be used in future to control the adsorption/desorption phenomena.
In addition to polyaniline, the attention should be paid in future research to aniline-related materials, such as aniline oligomers, polymers of ring-substituted anilines, aniline copolymers, chemically modified polyanilines and various carbonized analogues. This expectation similarly holds in part also for polypyrrole.
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Stejskal, J. Interaction of conducting polymers, polyaniline and polypyrrole, with organic dyes: polymer morphology control, dye adsorption and photocatalytic decomposition. Chem. Pap. 74, 1–54 (2020). https://doi.org/10.1007/s11696-019-00982-9
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DOI: https://doi.org/10.1007/s11696-019-00982-9