Laccase-mediated in situ polymerization of pyrrole for simultaneous coloration and conduction of wool fabric

Abstract

Polypyrrole catalyzed by laccase appears as a black color in solution. Conductive fabric coated with polypyrrole could present a dark green color, but the colorimetric and dyeing properties have not been discussed. In this study, pyrrole was used for the dyeing of wool fabric through in situ polymerization using laccase. The absorbance of reaction solution increases as time goes on and a significant peak appears at 460 nm on the ultraviolet-visible spectrum. Scanning electron microscopy indicated polypyrrole catalyzed by laccase does fix onto the wool fiber comparing to the control sample. The structure and electrochemical activity of dyed wool fabric were characterized by Fourier transform infrared spectroscopy and cyclic voltammetry, respectively. The dyeing depth of the dyed wool fabrics increased gradually with the extension of time and increasing of concentration of laccase and pyrrole. The dyed wool fabric prepared presents good electrochemical activity in terms of in situ polymerization using laccase in solution.

Keywords

pyrrole laccase coatings wool conductivity coloration

In the last few years, conducting polymers have attracted much attention as functional materials. As one of the most promising conductive polymers, polypyrrole (PPy) was selected to be coated on textiles because of its higher conductivity, easy synthesis and good stability.¹ It has also been widely used as sensors,^2,3 actuators,⁴ batteries,⁵ anti-electrostatic coatings⁶ and various biomedical devices.⁷ The conductive PPy can be easily polymerized with pyrrole using chemical or electrochemical methods. Chemical polymerization methods that were often used are as follows: in situ polymerization,⁸ two-step polymerization,^9–12 emulsion polymerization¹³ and vapor phase polymerization.^14,15

PPy was applied in preparing conducting textiles through different polymerization methods. The textile materials involved cotton,^9,14 cellulose derivatives,¹⁵ nylon,^11,12 silk,¹⁶ wool and polyester.^10,17,18 Conductive wool fabrics were produced by Varesano et al.¹⁹ through coating loose fibers of wool with PPy by in situ chemical oxidative polymerization from an aqueous solution of pyrrole. Babu et al.²⁰ found that PPy film formed on cotton fiber using the electrochemical method was thicker and more uniform than that prepared by the oxidative chemical method. The electrochemically formed conducting polymer exhibits relatively high electrical conductivity. Kim et al.²¹ also demonstrated that polyethylene terephthalate (PET) fabric/PPy composite shielded electromagnetic interference (EMI) by absorption as well as reflection and the EMI shielding property through reflection was enhanced with the electrical conductivity. In addition, the chemically synthesized polymer, which has low solubility and a poor process ability, contains a large amount of oxidation by-products.²² Unlike chemical and electrochemical methods, few conductive polymers, such as PPy, polyaniline, poly (2, 5-diaminobenzenesulfonate) and polybenzidine, can also be produced through enzymatic catalysis.²³

Enzymatic catalysis has been regarded as an environmentally benign synthesis process, which is generally carried out under milder conditions (e.g. aqueous conditions, room temperature and atmospheric pressure) with fewer by-products. Laccases (EC 1.10.3.2) are multi-copper enzymes capable of catalyzing the phenolic hydroxyl group and various primary amines.^24,25 Song and Palmore²³ discovered that the color of laccase-catalyzed PPy films and 2,2′-azino-bis-(3-ethylthiazoline-6-sulfonate) (ABTS) changed from colorless to dark blue or black within 1 h, which means that PPy could also be used as a colorant to dye solid materials, such as textiles. Although PPy-coated fabrics have been investigated for many years, the reports have barely focused on the dyeing properties of conducting textiles coated with PPy.

In this study, PPy was prepared by laccase-mediated in situ polymerization, and PPy-colored wool fabric with conductive function was simultaneously obtained. The influence of different periods on the polymerization of pyrrole catalyzed by laccase was examined in terms of ultraviolet visible (UV-Vis) spectra. Moreover, the conditions for the dyeing of wool fabric treated with pyrrole/laccase were studied in detail. Furthermore, the electrochemical activity and surface morphology of dyed wool fabric were also investigated.

Experimental details

Materials

Laccase (EC 1.10.3.2) from Ascomycete M. thermophila was purchased from Novozymes (Shanghai, China). Wool fabric (100 % gabardine) with a density of 220 g/m² was used in the experiment, which was supplied by Wuxi Xiexin Wool Textile Co., Ltd. Pyrrole (purity 99.5%) and ABTS were supplied by Aldrich and stored at 4℃ in the dark before use. All other reagents used were analytical grade from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China).

Determination of laccase activity

The activity of laccase was measured spectrophotometrically by monitoring the enzymatic oxidation of ABTS (ɛ420 = 36,000 M⁻¹·cm⁻¹) at 420 nm in 50 mM acetate buffer (pH 5.0), as described by Niku-Paavola et al.,²⁶ with some modification. The 3-mL reaction mixture included 0.1 mL laccase (5 g/L), 2.9 mL ABTS (0.5 mM) and 50 mM acetate buffer (pH 5.0) as the solvent. Spectrophotometric measurements were performed by recording the time taken for 0.05 unit increment in absorbance. One unit of laccase (U) was defined as the amount of enzyme required to oxidize 1 µmol of ABTS per minute.

UV-Vis spectroscopy

The polymerization of pyrrole in different periods of incubation was monitored by UV-Vis spectroscopy. A UV-2808S UV-Vis spectrophotometer (Unicosh, China) was prepared for the measurement of the reaction solutions every few hours in the range of 200–800 nm. A 0.2M sodium acetate buffer solution was used first to set a baseline.

Dyeing process of wool fabrics

In the case of laccase-catalyzed dyeing of wool fabric, 0.05 mM Pyrrole and 0.5 U/mL laccase were prepared in 0.2 M sodium acetate buffer (pH 5.0). Wool fabric samples were incubated in a reaction bath with a liquor-to-fabric ratio of 100:1 for 24 h at 30℃ with an agitation speed of 30 rpm, except where indicated. After treatment, the dyed wool fabrics were thoroughly washed several times with deionized water in order to eliminate any residual laccase or non-reacted pyrrole, as well as any unfixed PPy, and dried at room temperature for further evaluation. Using this method, the effect of reaction time, treatment temperature, dosages of pyrrole and laccase were also investigated.

Evaluation of dyeing depth

The CIELab color coordinates (L*, a*, b*) and dyeing depth of the dyed wool fabrics were evaluated at 400 nm using a Gretag Macbeth Color-Eye 7000 A Spectrophotometer (Gretag Macbeth, USA) with the standard illuminant D65 using a 10° observer. The K/S values were calculated based on the Kubelka–Munk equation (1)

K / S = (1 - R)^{2} / 2 R

(1)

where K is the absorbance coefficient, S is the scattering coefficient and R is the reflectance ratio. Before measuring K/S, the samples were dried overnight at room temperature.

Color fastness tests

The results of the color fastness tests of dyed wool samples were evaluated using established test procedures. The wash-fastness of dyed wool samples was measured according to ISO 105-C06-B1S:2010. Dyed wool samples were conducted at 50℃ for 30 min, rinsed with cold water and air dried. Then samples were appraised for staining on white adjacent fabrics (wool and cotton) and fading. The dry and wet rubbing fastness of dyed wool fabrics were tested based upon ISO 105-X12:2016 by mounting the fabrics on a panel and applying 10 strokes for both dry and wet rubbing fastness tests.²⁷ Gray scales were using to evaluate the changes in shade for fastness to washing and rubbing tests. Light fastness of dyed wool fabrics were conducted on a Xentotest 150 S+ with an air cooled xenon lamp according to the ISO 105-B02:2014 method.²¹ Scrubbing fastness was measured by scrubbing the wet sample 50 times with a nylon brush, washing with distilled water and air drying.

Fourier transform infrared spectroscopy

Fourier transform infrared (FT-IR) spectra of the control and dyed wool fabrics were obtained with a Nicolet iS10 infrared spectrophotometer (Thermo Nicolet, USA). At least 32 scans were performed to achieve an adequate signal-to-noise ratio. The spectra were obtained in the region of 450–4000 cm⁻¹ with a resolution of 8 cm⁻¹ at room temperature.

Nuclear magnetic resonance measurements

PPys were isolated from the reaction solution via centrifugation three times and dried overnight under vacuum at 50℃. The proton nuclear magnetic resonance (¹H NMR) spectra of extracted PPy dissolved in deuterated dimethylsulfoxide (DMSO) were recorded by using tetramethylsilane (TMS) as an internal standard on a Bruker Avance III spectrometer (Bruker, Germany) at an operating frequency of 400 MHz.

Cyclic voltammetry

Electroanalytical experiments of dyed wool samples were performed using a CHI 660D electrochemical workstation (CH Instruments, Inc., Austin, USA) at room temperature. A small piece of dyed wool sample was fixed to the glassy-carbon electrode surface by drying Nafion emulsion (1.5 wt %) before the measurement of the electrochemical activity of the wool samples, which possesses good cation-exchange properties, biocompatibility and film-forming properties.²⁸ The cyclic voltammogram was recorded with a scan rate of 100 mVs⁻¹.²⁹

Measurement of fiber resistance

The resistance of dyed wool fibers was tested using a 4200-SCS semiconductor parameter analyzer (Keithley Instruments Inc., USA). Both ends of the wool fibers were tied using two wires, drawn from a fixture box.³⁰ The electrical durability of wool fibers was tested through evaluating the resistance of wool fibers before and after standard washing. Dyed wool fabrics (5 × 5cm²) were washed according to ISO 105-C06-B1S:2010. After 5, 10 or 25 washing cycles, the dyed wool samples were rinsed with deionized water and dried at room temperature.

Scanning electron microscopy

The surface morphology of the dyed wool samples was observed using a Quanta-200 scanning electron microscope (FEI Company, Netherlands) with 2.00 k × and 4.00 k × magnification. The surface of the sample was sputtered with a thin layer of gold to avoid electrical charging.

Results and discussion

UV–Vis spectrophotometry analysis of pyrrole

PPy, a black precipitate, slowly forms upon addition of laccase to a solution of pyrrole. To discover the effect of time on polymerization, the reaction system at different periods of incubation was monitored and interpreted by UV-Vis spectrophotometry (Figure 1). The pyrrole solutions without laccase were also monitored as controls. Song and Palmore²³ observed that black precipitates slowly formed upon laccase when added to a pyrrole solution, which is regarded as PPy. As shown in Figure 1, the absorbance curves of solution in the visible range are smooth at the beginning of enzyme catalysis. However, a significant peak appears at 460 nm after 5 h immersion and its intensity continues to increase with time. When the reaction was carried out for 24 h, the absorbance of solution at 460 nm was four times larger than that reacted for 1 h. The color of pyrrole solution changed from colorless to dark green during enzymatic oxidation, which corresponded to the generation of the conjugate structure. The results also showed high activity of laccase for the polymerization of pyrrole.

Figure 1.

Ultraviolet visible spectra of pyrrole polymerization in the presence of laccase monitored at different periods; monomer solutions without laccase were used as reference.

FT-IR spectrophotometric and ¹H NMR analysis

The molecular structures of the control and wool fabric samples dyed with PPy were characterized by FT-IR spectra (Figure 2). Two distinct absorption peaks at 1171 and 1033 cm⁻¹ for dyed wool fabrics were also observed, which belong to the characteristic peaks of PPy due to over oxidation and N–H deformation, respectively.^20,31,32 The peaks around 1200 cm⁻¹ of dyed wool fabric were broader than that of the untreated wool sample. The most likely reason was the formation of an H-bond, which led to telescopic vibration of N-H shifting toward the low frequency position and the spectral band becoming wider. Furthermore, absorption peaks at 1122, 1009, 816 and 681 cm⁻¹ confirmed the =C–H in-plane vibration and out-of-plane vibration of PPy, respectively. These results revealed the existence of PPy on the wool fabric.

Figure 2.

Fourier transform infrared spectra of the untreated wool sample (upper) and polypyrrole-dyed wool sample (below).

The pyrrole and extracted polymer PPy were characterized by NMR spectroscopy in DMSO solution. As can be seen from Figure 3, the spectrum of pyrrole contains three peaks, the -NH at 10.75 ppm and the ring protons at 6.02 and 6.72 ppm. The signals in the polymer spectrum are broader and the chemical shift of the ring protons of PPy decrease. It proved that PPy formed like the structure shown in Scheme 1. In addition, we could conclude that polymerized PPy connected by site 1 or 2 through the spectra.³³

Figure 3.

Proton nuclear magnetic resonance spectra of pyrrole and extracted polymer polypyrrole.

Scheme 1.

Dyeing processes of wool fabric dyed by polypyrrole (R: wool).

Pyrrole can be oxidized to its corresponding radical cation by laccase, which was the foundation of its polymerization. Many active sites of the wool fiber are able to form covalent bonds with pyrrole. Amino, hydroxyl groups and tyrosine could connect with pyrrole by covalent bonds through reacting with its radical cation. Moreover, hydrogen bonds between PPy and the hydroxyl groups of wool fiber were easy to form, which also enhanced the color fastness of dyed wool fabric. The structure of PPy and the typically dyed wool fabric are suggested in Scheme 1.

Scanning electron microscopy study

As shown in Figure 4, the morphology of untreated and dyed wool samples was investigated by means of scanning electron microscopy (SEM) analysis. It can be clearly seen that there is a significant difference on the surface between the untreated wool sample and the dyed wool sample. The surface of dyed wool fiber, which is covered with nano-sized granules, was rougher than that of the untreated wool fiber. These granules were PPy catalyzed by laccase and attached to the surfaces of the wool fabrics, which enhanced the dyeing and electrical properties of wool.

Figure 4.

Scanning electron microscopy images of the untreated wool sample (left) and dyed wool sample (right) under different magnifications.

Effects of different conditions on the dyeing of wool fabrics

In the previous experiment, we obtained a dark green solution after 24 h reaction in 200 mM sodium acetate buffer. Now, the coloration of wool fabric dyed with pyrrole/laccase was carried out to study the effect of different periods on the dyeing. The K/S values and resistance of wool fabric dyed with PPy at different reaction periods are investigated in Figure 5(a). A deeper coloration of the dyed wool samples could be obtained when the reaction time lasted longer. The influence of time on the dyeing of wool fabric treated with pyrrole/laccase was as follows: the longer the time of the reaction, the higher the K/S value of dyed fabric attained. The K/S value of dyed wool fabric at 24 h was about 5, which was double that of 4 h, even though 24 h was a long reaction period for dyeing wool fabric. It is the same with the conductivity of the wool fabric sample, so we chose 24 h as the optimum reaction time.

Figure 5.

K/S values and resistance of the wool fabrics treated with pyrrole /laccase at various conditions. The reaction conditions are as follows: (a) 0.05 mM pyrrole, 0.5 U/mL laccase, 30℃: (b) 0.5 U/mL laccase, 30℃, 24 h; (c) 0.05 mM pyrrole, 30℃, 24 h; (d) 0.05 mM pyrrole, 0.5 U/mL laccase, 24 h.

Pyrrole can be enzymatically catalyzed to colored compounds in the solution, and the amount of PPy synthesized is affected by the amount of monomer (pyrrole) added. Figure 5(b) shows the K/S value and resistance of wool fabric dyed with PPy at different concentrations. The dyeing depth of wool fabric increased rapidly with an increase in the concentration of pyrrole. When the dosage of pyrrole was 0.5 mM, the K/S value of the dyed wool fabric nearly reached 20, which corresponds to a very dark color. The resistance of dyed wool fabric decreased with the increasing concentration of pyrrole. It can be concluded that a low concentration of pyrrole could present a dark color and good conductivity after catalyzing by laccase and PPy can be absorbed onto wool fabric easily under milder conditions. Above all, 0.5 mM pyrrole should be chosen as the optimum dosage of pyrrole for dyeing wool fabric.

The dyeing depths and conduction of wool fabric treated with pyrrole at different concentrations of laccase are shown in Figure 5(c). A lower dyeing depth was presented when the dosage of laccase was 0.1 U/mL. It also can be seen that the K/S value of wool fabric increased and the resistance of wool fabric reduced with increasing laccase concentration. However, the dyeing depth of wool fabric cannot reach a very high level, even though the dosage of laccase is 2 U/mL. A low concentration of pyrrole was the main reason for the low dyeing depth of wool fabric at high dosages of laccase. Consequently, the dosage of 0.5 U/mL of laccase is enough to dye wool fabric green and give wool fabric good conductivity.

An enzyme has its optimal temperature when it was used in reaction. To evaluate the effect of temperature on this reaction, the dyeing and conductivity of wool fabric treated with pyrrole/laccase was carried out at various temperature levels (Figure 5(d)). The K/S value of dyed wool fabric increases with increasing of reaction temperature. The reaction solution with a dark green color could be seen at any temperature due to the high effectivity of laccase and pyrrole, while the dyeing depths of wool fabric are different. The possible reason is that higher temperatures could improve the swelling of wool fibers, resulting in better penetration of the color compound into fibers. Most PPy polymerized by laccase at low temperatures is just adsorbed onto wool fiber and it can be washed with deionized water easily. However, the K/S value of dyed wool fabric at 30℃ was about 5, which was a little lower than that at 60℃. The resistance of PPy-coated wool fabric observed at 60℃ was similar to the data at 30℃. In view of the energy cost, 30℃ was chosen as the optimum temperature.

The colorimetric results of the untreated sample and dyed sample treated with pyrrole/laccase are summarized in Table 1. The L*, a* and b* values demonstrated the coloration of wool fabric samples treated with pyrrole/laccase in terms of lightness (L*) values, redness (a*) and yellowness (b*) values. Comparing with the untreated sample, the wool fabrics dyed with pyrrole/laccase presented characteristic dark green colors because of the formation of colored compounds. In a previous study, it was confirmed that the colored compounds were PPy catalyzed by laccase in solution. These colorimetric parameters presented an intuitive expression of the best coloration of a dyed sample treated with pyrrole/laccase.

Table 1.

CIELab values of the wool samples dyed at the optimum condition (pH 5 and 30℃)

Samples	K/S	L*	a*	b*
Untreated wool sample	0.369	84.331	0.042	12.964
Dyed wool sample	5.073	40.956	0.398	7.004

Colorfastness test

The fastness of dyed wool fabric treated with pyrrole/laccase is shown in Table 2. Washing fastness, scrubbing fastness and rubbing fastness tests were carried out to study the durability of the colored polymers attached to wool fabrics. The dyed samples treated with pyrrole/laccase achieved better colorfastness (4–5 grade), with the exception of the scrubbing fastness and wet rubbing fastness. The staining fastness on cotton adjacent fabrics was higher than that on the wool adjacent fabric. It could be that PP_Y has low affinity and a bad combination on cotton fabric. Higher light fastness (4–5 grade) of dyed wool fabric was also achieved using this method. These results further demonstrate that PPys formed after oxidation via laccase, which have an affinity for wool fabric surfaces in acid solution, were acceptable for textile coloration.

Table 2.

Dyeing fastness of the dyed wool samples

Samples	Light fastness	Washing fastness			Scrubbing fastness	Rubbing fastness
Samples	Light fastness	Washing fastness			Scrubbing fastness	Dry			Wet
Pyrrole/laccase	4–5	Fading	Staining		Fading	Fading	Staining		Fading	Staining
		Fading	wool	cotton	Fading	Fading	wool	cotton	Fading	wool	cotton
		4–5	4–5	4–5	3–4	4–5	4	4–5	3–4	3	3–4

Electroactivity of polypyrrole coated wool fabric

The dyed wool sample in 0.2 M sodium acetate buffer solution (pH 5) was electrochemically active, as can be seen from Figure 6(a). Two oxidation peaks were observed in the anodic sweep at 548 and 772 mV and a reduction peak at 453 mV versus Ag/AgCl, respectively. The three peaks were very well defined and reversible, compared with the curve of the untreated wool sample, which confirmed a high degree of electrochemical activity of the dyed wool fabric and indicated the potential for using this type of polymer in functional textiles. The resistance of coated wool fabric is presented in Figure 6(b). The resistance of PPy-coated fabric was 1/10⁴ lower than the untreated wool fabric, which proved the high conductivity of the coated fabric. The electrical durability of dyed wool fabric is presented in Figure 6(c). The resistance of dyed wool fabric increased during the first 10 times of washing, but the resistance of dyed wool fabric could still retain 2.5 × 10⁹ Ω after 25 cycles of water washing, which suggested a good electrical durability of the PPy dyed wool fabric after repeated water washing. PPy-coated fabric can also be used as an electromagnetic shield for protection against high-frequency electromagnetic fields (EMFs).³³

Figure 6.

Cyclic voltammograms of the untreated and dyed wool samples (a), resistance of wool samples (b) and resistance of dyed wool samples after different washing times (c).

Conclusions

In this paper, pyrrole was successfully polymerized by laccase in solution and a dark green color was presented on the dyed wool fabric. Pyrrole, as a new monomer, was developed for the in situ enzymatic dyeing of wool fabrics under milder conditions. Three different colors could be obtained through adjusting the dyeing conditions as well, namely light green, dark green and black. A high coloration depth of wool fabric was achieved by increasing the concentration of the monomer and laccase. The coloration of dyed wool fabric could reach a certain extent after 4 h reaction, while the K/S value of dyed wool fabric after 24 h reaction was double that. The optimum temperature of reaction is 30℃, which only needs low energy and could achieve higher coloration of wool fabric. Electric resistance of dyed wool fabric under different conditions was also measured. The resistance of dyed wool fabric decreased with the increasing of dosage of laccase and pyrrole. It also varies according to the change of temperature and reaction periods. The dyed wool fabrics have good conductivity, which was also indicated from cyclic voltammetry. Better colorfastness of dyed wool fabric was also obtained using this method. Higher light fastness and washing fastness were the basic requirements for the dyed fabrics. The in situ enzymatically dyed wool fabrics with the conductive property could be utilized for technical textiles. This study also presented an environment-friendly method for preparing colored and conductive wool fabrics under milder conditions.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: National Natural Science Foundation of China (21274055, 51673087), Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R26), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Graduate student innovation project (KYLX16_0800).

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Laccase-mediated in situ polymerization of pyrrole for simultaneous coloration and conduction of wool fabric

Abstract

Keywords

Experimental details

Materials

Determination of laccase activity

UV-Vis spectroscopy

Dyeing process of wool fabrics

Evaluation of dyeing depth

Color fastness tests

Fourier transform infrared spectroscopy

Nuclear magnetic resonance measurements

Cyclic voltammetry

Measurement of fiber resistance

Scanning electron microscopy

Results and discussion

UV–Vis spectrophotometry analysis of pyrrole

FT-IR spectrophotometric and 1H NMR analysis

Scanning electron microscopy study

Effects of different conditions on the dyeing of wool fabrics

Colorfastness test

Electroactivity of polypyrrole coated wool fabric

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References

FT-IR spectrophotometric and ¹H NMR analysis