sensors Article

Cotton Fabric Coated with Conducting Polymers and its Application in Monitoring of Carnivorous Plant Response Václav Bajgar 1 , Marek Penhaker 1 , Lenka Martinková 2 , Andrej Pavloviˇc 3 , Patrycja Bober 4, *, Miroslava Trchová 4 and Jaroslav Stejskal 4 1

2 3

4

*

Department of Cybernetics and Biomedical Engineering, Faculty of Electrical Engineering and Computer Science, VSB-Technical University of Ostrava, 708 33 Ostrava, Czech Republic; [email protected] (V.B.); [email protected] (M.P.) Inotex Ltd, 544 01 Dvur Kralove nad Labem, Czech Republic; [email protected] Department of Biophysics, Centre of the Region Haná for Biotechnological and Agricultural Research, Faculty of Science, Palacky University in Olomouc, 783 71 Olomouc, Czech Republic; [email protected] Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic; [email protected] (M.T.); [email protected] (J.S.) Correspondence: [email protected]; Tel.: +420-296-809-443

Academic Editor: Ki-Hyun Kim Received: 25 February 2016; Accepted: 28 March 2016; Published: 8 April 2016

Abstract: The paper describes the electrical plant response to mechanical stimulation monitored with the help of conducting polymers deposited on cotton fabric. Cotton fabric was coated with conducting polymers, polyaniline or polypyrrole, in situ during the oxidation of respective monomers in aqueous medium. Thus, modified fabrics were again coated with polypyrrole or polyaniline, respectively, in order to investigate any synergetic effect between both polymers with respect to conductivity and its stability during repeated dry cleaning. The coating was confirmed by infrared spectroscopy. The resulting fabrics have been used as electrodes to collect the electrical response to the stimulation of a Venus flytrap plant. This is a paradigm of the use of conducting polymers in monitoring of plant neurobiology. Keywords: conducting polymers; plant neurobiology; polyaniline; polypyrrole; Venus flytrap

1. Introduction Conducting polymers, such as polyaniline (PANI) and polypyrrole (PPy) (Figure 1), have recently been studied due to the variety of nanostructures they produce [1–3] and their application potential in energy storage and energy conversion devices, namely batteries, fuel cells, and supercapacitors. They have recently been applied as adsorbents, conducting inks, heterogeneous catalysts, in corrosion protection, sensors, electromagnetic interference shielding, and many other directions [4–6]. The use of conducting polymers in life sciences for monitoring or stimulation of biological objects ranks among the most modern trends [7–9]. The attractiveness of conducting polymers has been associated with the ease of their preparation [10]. Conducting polymers, however, are intractable as a rule and their mechanical properties are poor. The deposition of conducting polymers on various supports offers a solution to this problem. Various textiles have recently become coated with conducting polymers [11–19]. This is done in situ, by immersion of the template fabrics in the aqueous reaction mixture used for the preparation of these polymers by the oxidation of respective monomers.

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Figure 1. Conducting polyanilineand and(b) (b) polypyrrole polypyrrole salts. anion. Figure 1. Conducting (a)(a) polyaniline salts.AAis´an is arbitrary an arbitrary anion. −

The use of conducting textiles in wearable electronics, in contact with biological objects, is the

The use target of conducting electronics, contact for with biological objects, obvious [20]. The textiles cleaninginofwearable such fabrics is usuallyinrequired such applications. Theis the obvious target [20]. Thehowever, cleaninglose of such fabrics is usuallyatrequired for such applications. conducting polymers, most of its conductivity physiological conditions or during The washingpolymers, [18,21–23], however, when conducting polymer convert to non-conducting bases [10]. The or level conducting lose most of itssalts conductivity at physiological conditions during of conductivity laundering was maintained onlytoafter the incorporation of [10]. graphene washing [18,21–23], after whenwater conducting polymer salts convert non-conducting bases The level oxide [24,25].after It haswater recently been reported the problem can be overcome by using dry cleaning of conductivity laundering wasthat maintained only after the incorporation of graphene instead [18,25]. oxide [24,25]. It has recently been reported that the problem can be overcome by using dry cleaning While the most common conducting polymers, PANI and PPy, have been investigated in instead [18,25]. numerous papers, the reports on composites comprising simultaneously both polymers are While the most common conducting polymers, PANI and PPy, have been investigated limited [26–33]. Cotton has commonly been used as a substrate for the deposition of conducting in numerous papers, the reports onpresent composites comprising simultaneously bothwith polymers polymers [13,17,18,24,34–39]. In the paper, we have studied cotton fabric coated PANI are limited [26–33]. Cotton has commonly been used as a substrate for the deposition of conducting and subsequently with PPy, or vice versa, to see if there is any synergism or improvement in the polymers [13,17,18,24,34–39]. In thewith present paper, we have studied cotton fabric coated with PANI properties of the resulting system respect to conductivity and repeated dry cleaning. The application the or modified textiles theifmonitoring of electrical response from the plantin the and subsequently with of PPy, vice versa, to in see there is any synergism or improvement stimulation subsequently Plants usually respond toand different environmental stimuli by properties of thewas resulting systemtested. with respect to conductivity repeated dry cleaning. generation of electrical signals in the form of action and variation potentials [40]. Action potentials The application of the modified textiles in the monitoring of electrical response from theinplant plants are generated in response to cold or touch. They usually have an all-or-nothing character; that stimulation was subsequently tested. Plants usually respond to different environmental stimuli by is, after a stimulus reaches a certain threshold (which leads to membrane depolarization), further generation of electrical signals in the form of action and variation potentials [40]. Action potentials increases in stimulus strength do not change its amplitude and shape. Variation potentials are in plants are generated in response to cold or touch. They usually have an all-or-nothing character; usually generated in response to damaging stimuli (e.g., cutting or burning). The main difference to that is, after a stimulus a certain threshold (whichand leads to membrane depolarization), further action potentials liesreaches in longer, delayed repolarizations a large range of variation. This signal increases stimulus strength do amplitude Variation are usually variesin with the intensity of not the change stimulusitsand appearsand to shape. be a local changepotentials to a hydraulic generated in response damaging stimuli (e.g., cutting signaling or burning). The main difference action pressure [40]. The to best known example of electrical in plants is described in to the carnivorous Venus flytrap (Dionaea muscipula) sensitive (MimosaThis pudica) [41,42]. For with potentials lies in plant longer, delayed repolarizations and aand large rangeplants of variation. signal varies testing ofof the conductive to transduce signals plants, we chose Venus flytrap the intensity the stimuluspolymers and appears to be a electrical local change to in a hydraulic pressure [40]. The best because we had previously worked successfully with this species [43]. known example of electrical signaling in plants is described in the carnivorous plant Venus flytrap (Dionaea muscipula) and sensitive plants (Mimosa pudica) [41,42]. For testing of the conductive polymers 2. Experimental to transduce electrical signals in plants, we chose Venus flytrap because we had previously worked successfully with this with species [43]. Polymers 2.1. Cotton Coating Conducting Bleached, plain weave 100% cotton fabric CARLTON (Mileta a.s., Hořice, Czech Republic), 2. Experimental

specific mass 120 g·m−2, sett: 51.2 n·cm−1 (warp), 28.0 n·cm−1 (weft), yarn count 7.4/2 tex (warp), and

2.1. Cotton Coating with Conducting Polymers Bleached, plain weave 100% cotton fabric CARLTON (Mileta a.s., Hoˇrice, Czech Republic), specific mass 120 g¨ m´2 , sett: 51.2 n¨ cm´1 (warp), 28.0 n¨ cm´1 (weft), yarn count 7.4/2 tex (warp), and 14.5 tex

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(weft) was used as received. The fabric was coated with PANI by immersion in the reaction mixture containing 0.2 M aniline hydrochloride and 0.25 M ammonium peroxydisulfate [10] or in 0.2 M pyrrole and 0.5 M iron(III) chloride mixture for PPy coating [44]. The coated fabrics were rinsed with 0.2 M hydrochloric acid to remove the adhering polymer precipitate, and dried in air. The polymer powder produced outside the fabrics was collected on filter, washed with acetone, and dried. Polyaniline-coated or polypyrrole-coated fabrics have been used as substrates for the second coating with PPy or PANI, respectively, and processed as above. Four samples have subsequently been investigated, the cotton being coated with: (1) polyaniline (C+PANI); (2) polypyrrole (C+PPy); (3) polyaniline and polypyrrole (C+PANI+PPy); and (4) polypyrrole and polyaniline (C+PPy+PANI). In a separate experiment, the PANI powder, instead of PANI-coated cotton, was suspended in the reaction mixture used for the preparation of PPy. The PANI particles were thus coated with PPy, and the resulting composite contained about 50 wt% of each polymer. The coating of PPy with PANI was carried out in a similar manner. 2.2. Characterization The surface morphology of cotton fabric before and after coating with conducting polymers was characterized with the scanning electron microscopy (SEM) using a JEOL 6400 microscope. Point-to-point surface resistance was measured at 20 ˝ C and 64% relative humidity according to IEC 61340-4-10 with a METRISO® 2000-ESD Test Instrument (Wolfgang Warmbier, High-Resistance Tester, Toledo, OH, USA) using two-point probe with a replaceable head 844 and conducting rubber tips for rigid materials testing. The room temperature conductivity of polymer powders was determined by a four-point method in van der Pauw arrangement using a Keithley 220 Programmable Current Source (Keithley Instruments, Cleveland, OH, USA), a Multimeter as a voltmeter and a scanner equipped with a matrix card. The composite powders were compressed at 70 kN with a manual hydraulic press to the pellets 13 mm in diameter and «1 mm thick. Chemical cleaning was conducted according to the standard EN ISO 105-D01 “Textiles: Tests for colour fastness, Part D01: Colour fastness to dry cleaning using perchloroethylene solvent”. Tested samples of 4 ˆ 10 cm2 size set in a closed 10 ˆ 10 cm2 cotton bag were placed into the closed rotating stainless vessel of 550 mL volume containing 200 mL of perchloroethylene thermostatted at 30 ˝ C and operated at rotation speed 40 rpm for 30 min. The fabrics were then dried in air at room temperature. 2.3. FTIR and Raman Spectra FTIR spectra have been obtained by ATR spectroscopic technique using Golden Gate™ Heated Diamond ATR Top-Plate (MKII Golden Gate single reflection ATR system) (Specac Ltd, Orpington, UK) with Thermo Nicolet NEXUS 870 FTIR Spectrometer (Thermo Scientific, Madison, WI, USA) in a moisture-purged environment equipped with DTGS detector in the wavelength range 400–4000 cm´1 . Typical parameters used were 256 of sample scans, resolution 4 cm´1 , Happ-Genzel apodization, potassium bromide beamsplitter. The spectra were corrected for the presence of moisture and carbon dioxide in the optical path. Raman spectra excited with HeNe 633 nm laser have been collected on a Renishaw inVia Reflex Raman microspectrometer. A research-grade Leica DM LM microscope with an objective magnification ˆ50 was used to focus the laser beam on the sample placed on an X–Y motorized sample stage. The scattered light was analyzed by the spectroscope with holographic gratings 1800 lines¨ mm´1 . A Peltier-cooled CCD detector (576 ˆ 384 pixels) registered the dispersed light. 2.4. Plant Material and Culture Condition Venus flytrap (Dionaea muscipula Ellis) plants were cultivated in well-drained peat moss in plastic pots 7 cm in diameter placed in container filled with distilled water to a depth 1–3 cm.

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Daily temperature fluctuated between 20–35 ˝ C, relative air humidity 50%–70%, and maximum Sensors 2016, 16, x 4 of 12 daily irradiance reached max. 1500 µmol¨ m´1 ¨ s´1 PAR (photosynthetically active radiation). temperature fluctuated between 20–35 °C, relative air humidity 50%–70%, and maximum daily irradiance reached max. 1500 μmol·m ·s PAR (photosynthetically active radiation).

2.5. Monitoring the Electrical Response from Plant −1 −1

The electrical signals were recorded by a non-invasive device inside a Faraday cage [45,46]. 2.5. Monitoring Electrical Plant into the clip coated with cotton fabric coated with The trap of Venus the flytrap wasResponse gently from enclosed Thepolymers. electrical signals by fabric a non-invasive device inside [45,46]. The to conducting The were striprecorded of cotton protruded from thea Faraday clip andcage was connected trap of Venus flytrap surface was gently enclosed into the clip coated withCzech cottonRepublic). fabric coated non-polarizable Ag-AgCl electrodes (Scanlab systems, Prague, The with reference conducting polymers. in The of water cotton in fabric from clip and waswere connected to electrode was submerged 1–2strip cm of dish protruded beneath the pot.the The electrodes connected to non-polarizable Ag-AgCl surface electrodes (Scanlab systems, Prague, Czech Republic). The 5 an amplifier made in-house (gain 1–1000, noise 2–3 mV, bandwidth ´3 dB at 10 Hz, response time reference electrode was submerged in 1–2 cm of water in dish beneath the pot. The electrodes were 10 µs, input impedance 1012 Ω). The signals from the amplifier were transferred to an analog-digital connected to an amplifier made in-house (gain 1–1000, noise 2–3 mV, bandwidth −3 dB at 105 Hz, PC data converter (eight analog inputs, 12-bit-converter, ˘10 V, PCA-7228AL, supplied by TEDIA, response time 10 μs, input impedance 1012 Ω). The signals from the amplifier were transferred to an Pilsen, Czech Republic), collected every 6 ms. The sensitivity of the device was The mechanical analog-digital PC data converter (eight analog inputs, 12-bit-converter, ±1013V,µV. PCA-7228AL, stimulation of trigger hairs in enclosed trap of Venus flytrap was performed by plastic stickwas and the supplied by TEDIA, Pilsen, Czech Republic), collected every 6 ms. The sensitivity of the device electrical response in the form of action potential was recorded. 13 μV. The mechanical stimulation of trigger hairs in enclosed trap of Venus flytrap was performed by plastic stick and the electrical response in the form of action potential was recorded.

3. Results and Discussion 3. Results and Discussion

3.1. The Coating of Cotton with Conducting Polymers 3.1. The Coating of Cotton with Conducting Polymers

The in situ coating of virtually any surfaces, including textiles, with conducting polymers is situ coating virtually anyinsurfaces, including textiles, with polymers This is based on The the in immersion of of the substrate the reaction mixture used forconducting their preparation. is based on the immersion of the substrate in the reaction mixture used for their preparation. This is for typically the aqueous mixture containing aniline hydrochloride and ammonium peroxydisulfate typically the aqueous mixture containing aniline hydrochloride and ammonium peroxydisulfate for PANI [47] or pyrrole and iron(III) chloride for PPy [18,44]. The naked eye immediately observes the PANI [47] or pyrrole and iron(III) chloride for PPy [18,44]. The naked eye immediately observes the difference in the color of textiles (Figure 2); originally white cotton becomes dark green after coating difference in the color of textiles (Figure 2); originally white cotton becomes dark green after coating with with polyaniline or black after thethe deposition polyaniline or black after depositionof ofPPy. PPy.

Figure 2. Cotton fabrics before and after the coating with polyaniline or polypyrrole (from left to right).

Figure 2. Cotton fabrics before and after the coating with polyaniline or polypyrrole (from left to right).

By using this deposition technique, the cotton fabrics (Figure 3) were coated with PANI (Figure 4a) or using PPy (Figure 4b) at first, and subsequently second polymer wascoated again with deposited top 4a) By this deposition technique, the cottonthe fabrics (Figure 3) were PANIon (Figure (Figure 4c,d).4b) Scanning microscopy reveals the uniform coating of again the individual fibers. or PPy (Figure at first,electron and subsequently the second polymer was deposited on top From the analogy with other substrates, the thickness of such coating is estimated to be close (Figure 4c,d). Scanning electron microscopy reveals the uniform coating of the individualtofibers. nm [47,48]. Some adhering polymer precipitate is observed on the top of fibers, and From100–200 the analogy with other substrates, the thickness of such coating is estimated to be close to especially PPy globules accompany the corresponding coating. 100–200 nm [47,48]. Some adhering polymer precipitate is observed on the top of fibers, and especially PPy globules accompany the corresponding coating.

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Figure 3. Scanning electron micrographs of cotton fabric (left) and its individual fiber (right).

Figure 3. Scanning electron micrographs of cotton fabric (left) and its individual fiber (right). Figure 3. Scanning electron micrographs of cotton fabric (left) and its individual fiber (right).

Figure 4. Scanning electron micrographs of cotton fibers coated with: (a) PANI; (b) PPy; (c) PANI+PPy; and (d) electron PPy+PANI. Figure 4. Scanning micrographs of cotton fibers coated with: (a) PANI; (b) PPy; Figure 4. Scanning electron micrographs of cotton fibers coated with: (a) PANI; (b) PPy; (c) PANI+PPy; (c) PANI+PPy; and (d) PPy+PANI.

and (d) PPy+PANI. 3.2. FTIR and Raman Spectra 3.2. FTIR and Raman Spectra The microscopy alone need not be convincing enough to demonstrate the uniformity of coating,

3.2. FTIR and Raman Spectra

and further evidencealone is provided bybespectroscopic techniques. The ATR the FTIR spectrumofofcoating, cotton The microscopy need not convincing enough to demonstrate uniformity

The alone not be to demonstrate the spectrum uniformity of coating, and microscopy further evidence is need provided by convincing spectroscopicenough techniques. The ATR FTIR of cotton and further evidence is provided by spectroscopic techniques. The ATR FTIR spectrum of cotton fabric coated with PANI and subsequently with PPy (spectrum C+PANI+PPy in Figure 5) exhibits the main

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fabric coated with PANI and subsequently with PPy (spectrum C+PANI+PPy in Figure 5) exhibits the main peaks of PPy [44,49,50] (cf. the spectrum C+PPy in Figure 5). This means that the coating peaks of PPy [44,49,50] (cf. the spectrum C+PPy in Figure 5). This means that the coating with PPy with PPy is thicker (estimated to be several hundreds of nanometers) than the effective penetration is thicker (estimated to be several hundreds of nanometers) than the effective penetration depth of depth of infrared radiation used. The underlying PANI coating is thus not detected in the spectrum. infrared radiation used. The underlying PANI coating is thus not detected in the spectrum.

Absorbance

ATR FTIR spectra

C+PPy+PANI C+PANI+PPy

C+PPy C+PANI

Cotton (C)

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Figure 5.5. ATR ATR FTIR FTIR spectra spectra of of cotton cotton fabrics fabrics coated coated with with polyaniline polyaniline (C+PANI) (C+PANI) or or polypyrrole polypyrrole Figure (C+PPy). Polyaniline-coated Polyaniline-coated cotton cotton was was again again coated coated with withpolypyrrole polypyrrole(C+PANI+PPy) (C+PANI+PPy) or or vice viceversa versa (C+PPy). (C+PPy+PANI). The spectrum of uncoated textile cotton is included. (C+PPy+PANI). The spectrum of uncoated textile cotton is included.

The spectrum of cotton fabric coated with PPy and then with PANI (spectrum C+PPy+PANI) The spectrum of cotton fabric coated with PPy and then with PANI (spectrum C+PPy+PANI) exhibits, in addition to the main peaks of PPy, also weak features of the spectrum of PANI (cf. the exhibits, in addition to the main peaks of PPy, also weak features of the spectrum of PANI (cf. the spectrum C+PANI in Figure 5) [51]. This corresponds to the fact that the PANI coating is much spectrum C+PANI in Figure 5) [51]. This corresponds to the fact that the PANI coating is much thinner (ca. 100 nm [48]), than the PPy coating and it is smaller than the effective penetration depth thinner (ca. 100 nm [48]), than the PPy coating and it is smaller than the effective penetration depth of used radiation. Thus, we also observe the bands of the bottom PPy layer in the spectrum. The of used radiation. Thus, we also observe the bands of the bottom PPy layer in the spectrum. The spectral features of neat cotton are not present in the samples because of strong absorption afforded spectral features of neat cotton are not present in the samples because of strong absorption afforded by by conducting polymers. conducting polymers. The situation is similar in the case of Raman spectra. The penetration depth for 633 nm The situation is similar in the case of Raman spectra. The penetration depth for 633 nm excitation excitation laser line is much lower than the thickness of the both polymer coatings. As a laser line is much lower than the thickness of the both polymer coatings. As a consequence, we observe consequence, we observe in the spectrum of cotton fabric coated with PANI and then with PPy (the in the spectrum of cotton fabric coated with PANI and then with PPy (the spectrum C+PANI+PPy in spectrum C+PANI+PPy in Figure 6) the main peaks of PPy [49,50] (cf. the spectrum PPy in Figure 6). Figure 6) the main peaks of PPy [49,50] (cf. the spectrum PPy in Figure 6). In analogy, we observe the In analogy, we observe the main bands of PANI (the spectrum PANI in Figure 6) [51] in the main bands of PANI (the spectrum PANI in Figure 6) [51] in the spectrum of cotton fabrics coated with spectrum of cotton fabrics coated with PPy and later with PANI (spectrum C+PPy+PANI in Figure 6). PPy and later with PANI (spectrum C+PPy+PANI in Figure 6). The spectrum of neat cotton displays a The spectrum of neat cotton displays a strong fluorescence with 633 nm excitation laser line, which is strong fluorescence with 633 nm excitation laser line, which is suppressed by polymer coating. suppressed by polymer coating. 3.3. Electrical Properties Sheet resistivity of PPy coating is considerably lower compared with PANI (Table 1). This is due to thicker coating of PPy, because the conductivity of both polymers measured on compressed pellets is of the same order of magnitude (Table 1). The same reasoning applies to the subsequent second coating where additional resistivity decrease is clearly associated with larger amount of conducting polymers deposited on cotton, rather than with more subtle details, such as the potential interaction between the two polymers. Repeated dry cleaning has led to the 2.0–12.1-fold decrease in sheet resistivity but good level of conductivity was maintained in all cases.

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Intensity

Raman spectra

C+PANI+PPy C+PPy+PANI

C+PPy C+PANI

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Figure 6. 6. Raman spectra of cotton fabrics coated with polyaniline (C+PANI) or polypyrrole (C+PPy). Figure Raman spectra of cotton fabrics coated with polyaniline (C+PANI) or polypyrrole Polyaniline-coated cotton was coated withwith polypyrrole (C+PANI+PPy) versa (C+PPy). Polyaniline-coated cottonagain was again coated polypyrrole (C+PANI+PPy)oror vice vice versa (C+PPy+PANI). Laser excitation wavelength 633 nm. (C+PPy+PANI). Laser excitation wavelength 633 nm.

3.3. Electrical Properties Table Sheet resistivity of coating cotton fabric coated with conducting polymers andPANI the conductivity Sheet1.resistivity of PPy is considerably lower compared with (Table 1). of This is such polymers. due to thicker coating of PPy, because the conductivity of both polymers measured on compressed

pellets is of the same order of magnitude (Table 1). ´1 The same reasoning applies to the subsequent Sheet Resistivity, Ω2 Conductivity Related second coating where additional resistivity decrease is clearly associatedof with larger amount of Cotton Powders, S¨ cm´1 as Prepared after Dry Cleaning conducting polymers deposited on cotton, rather than with more subtle details, such as the potential 5 +PANI 6.3 ˆ 104 Repeated 5.0dry ˆ 10cleaning interaction between the two polymers. has led to 2.2 the 2.0–12.1-fold decrease in 3 +PANI+PPy 630 7.2 7.7 ˆ 10 sheet resistivity but good level of conductivity was maintained in all cases. +PPy 4.2 1.7 ˆ 103 5.0 ˆ 103 +PPy+PANI 210 3.2 1.7 ˆ 103 Table 1. Sheet resistivity of cotton fabric coated with conducting polymers and the conductivity of such polymers.

Polyaniline and polypyrrole are regarded as mixed conductors displaying electronic and ionic −1 contribution to the overallSheet conduction [52–54]. polymers might be potentially suitable to operate Resistivity, Ω Such Cotton Conductivity of Related Powders, S·cm−1 as electrical transducers between ionic conductors, such as biological objects, and electronic conductors, as Prepared after Dry Cleaning such as metals. For that they have +PANI 6.3 ×reason, 104 5.0 × been 105 tested below in monitoring 2.2the electrical signals afforded by the plant stimulation. It should be noted that water is likely to play +PANI+PPy 630 7.7 × 103 7.2 an important role in these processes, as it supports +PPy 1.7 × 103 the ionic conduction. 5.0 × 103 4.2

□

+PPy+PANI 210 3.4. Response to Plant Stimulation

1.7 × 103

3.2

The modified leafpolypyrrole of Venus flytrap called trap catches prey by rapid movement of itsand bilobed Polyaniline and are regarded as mixed conductors displaying electronic ionic halves that shut when the trigger hairs protruding from the upper leaf be epidermis are stimulated contribution to the overall conduction [52–54]. Such polymers might potentially suitable to by touch.asThe stimulation of trigger hairs ionic activates mechanosensitive ion channels and generates a operate electrical transducers between conductors, such as biological objects, and electronic receptor potential, which induces an action potential (AP). Thetested struggling entrappedthe prey in the conductors, such as metals. For that reason, they have been belowofinthe monitoring electrical closed resultsby inthe generation of further APs, which tothat occur when prey moving [43]. signalstrap afforded plant stimulation. It should becease noted water is the likely to stops play an important We the AP inasclosed trap due to theconduction. stable geometric configuration of trap (in fact trap rolemeasured in these processes, it supports the ionic closure may results in loss of contact between trap tissue and electrode) (Figure 7). In Figure 8 you 3.4. see Response to Plant by Stimulation can AP recorded Ag/AgCl electrode (Figure 8A) and PANI-coated cotton fabric (Figure 8B). The most important thing to make a good contact betweenprey the fabric andmovement plant tissue The modified leaf of is Venus flytrap called trap catches by rapid ofto itsincrease bilobed the signal-to-noise ratiothe andtrigger signal hairs stability. The AP triggered by touch a great variability, mainly halves that shut when protruding from the upper leafhas epidermis are stimulated by

touch. The stimulation of trigger hairs activates mechanosensitive ion channels and generates a

receptor potential, which induces an action potential (AP). The struggling of the entrapped prey in the closed trap results in generation of further APs, which cease to occur when the prey stops moving [43]. We measured the AP in closed trap due to the stable geometric configuration of trap (in fact trap closure may results in loss of contact between trap tissue and electrode) (Figure 7). In Figure 8 you can see16,AP498recorded by Ag/AgCl electrode (Figure 8A) and PANI-coated cotton fabric (Figure8 8B). Sensors 2016, of 12 The most important thing is to make a good contact between the fabric and plant tissue to increase the signal-to-noise ratio and signal stability. The AP triggered by touch has a great variability, in the degree of hyperpolarization and amplitude (Figure 8). The (Figure amplitude mainly in the degree of hyperpolarization and amplitude 8). and Thehyperpolarization amplitude and usually decreased with the consecutive number of APs triggered by touch (Figure 9). Nevertheless, hyperpolarization usually decreased with the consecutive number of APs triggered by touch you can 9). see that the APs recorded using polymers are comparable with that recordedare by (Figure Nevertheless, you can see conductive that the APs recorded using conductive polymers Ag/AgCl electrode connected to the plant surface by means of a conductive aqueous gel of the type comparable with that recorded by Ag/AgCl electrode connected to the plant surface by means of a commonly aqueous used in electrocardiography. The measurement using gel has often been considered as conductive gel of the type commonly used in electrocardiography. The measurement using non-invasive but the usage of the osmotic conducting canof cause serious tissue damage gel has often been considered as non-invasive but the gel usage the osmotic conducting gelobservable can cause several hours/days after measurements. The present textile electrodes might avoid this drawback at serious tissue damage observable several hours/days after measurements. The present textile expanse of might the slightly noise. at However, canslightly be overcome by data averaging if they electrodes avoidincreased this drawback expansethis of the increased noise. However, thiswere can recorded with high sampling frequency. be overcome by data averaging if they were recorded with high sampling frequency.

Figure 7. The The layout layoutofofthe the electrodes used recording of action potentials: clip electrode with Figure 7. electrodes used for for recording of action potentials: clip electrode with cotton cotton fabric with coated with conducting polymer (A); non-polarizable Ag/AgCl surface electrode fabric coated conducting polymer (A); non-polarizable Ag/AgCl surface electrode connected to connected to the protruding strip of cotton fabrics coated with conducting polymer (B); and the protruding strip of cotton fabrics coated with conducting polymer (B); and reference electrode (C); reference electrode (C); A stick was used for stimulation of trigger inside closed A plastic stick was used forplastic stimulation of trigger hairs inside closed trap ofhairs Venus flytrap (D).trap of Venus flytrap (D).

In the preliminary testing, we have not observed any significant differences among fabrics coated with different conducting polymers: PANI, PPy, PANI+PPy, and PPy+PANI. All of them have been able to record the action potentials in similar quality of the plant to the repeated mechanical stimulation (data not shown) despite substantial differences in the sheet resistivity (Table 1). This is probably caused by the fact that the trap of Venus flytrap is a three-dimensional structure and contact area

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between the trap and coated fabric strongly varied during measurements from one another. The second possibility is that even the lowest conductivity is sufficiently high for recording of electrical signals in plants2016, and16, is xcomparable than those measured using conductive gels. Sensors 2016, 16, x of 12 12 Sensors 99 of

Figure 8. Action potentials potentials recorded recordedby byelectrode electrode(A) (A)and andPANI-coated PANI-coatedcotton cotton fabric (B) in response 8. Action potentials fabric (B) inin response to Figure recorded by electrode (A) and PANI-coated cotton fabric (B) response to touching of trigger hair in Venus flytrap. touching of trigger hair in Venus flytrap. to touching of trigger hair in Venus flytrap.

Figure 9. A A series of action potentials of Venus flytrap in response to 15 touches of trigger hairs hairs Figure 9. 9. Aseries seriesof of action actionpotentials potentialsof ofVenus Venus flytrap flytrap in in response response to to 15 15 touches touches of of trigger trigger hairs Figure recorded by cotton fabrics coated with PPy+PANI. recorded by by cotton cotton fabrics fabrics coated coated with with PPy+PANI. recorded

In the the preliminary preliminary testing, testing, we we have have not observed observed any any significant significant differences differences among among fabrics fabrics In The single coating of the textile with thenot conducting polymer is thus sufficient for the construction coated with different conducting polymers: PANI, PPy, PANI+PPy, and PPy+PANI. All of them coated with different conducting polymers: PANI, PPy,repeated PANI+PPy, andofPPy+PANI. of them of the sensor. The higher conductivity afforded by the coating textile withAll conducting have been able to record the action potentials in similar quality of the plant to the repeated have been however, able to record thetoaction similar quality of the plant to the repeated polymers, is likely be of potentials benefit forinthe improved collection and processing of the mechanical stimulation stimulation (data (data not not shown) shown) despite despite substantial substantial differences differences in in the the sheet sheet resistivity resistivity mechanical electrical response. (Table 1). 1). This This is is probably probably caused caused by by the the fact fact that that the the trap trap of of Venus Venus flytrap flytrap is is aa three-dimensional three-dimensional (Table structure and contact area between the trap and coated fabric strongly varied during measurements 4. Conclusions structure and contact area between the trap and coated fabric strongly varied during measurements from one one another. another. The The second second possibility possibility is is that even even the the lowest lowest conductivity conductivity is is sufficiently high high for for from The cotton was uniformly coated withthat conducting polymers, polyaniline sufficiently or polypyrrole, and recording of electrical signals in plants and is comparable than those measured using conductive gels. recording of electricalofsignals in plants and comparable thandecreased those measured using conductive by the combination both polymers. Theissheet resistivity after the second coatinggels. with The single single coating coating of of the the textile textile with with the the conducting conducting polymer polymer is is thus thus sufficient sufficient for for the the The polypyrrole or polyaniline, respectively, due to the increased total amount of conducting polymer construction of the sensor. The higher conductivity afforded by the repeated coating of textile with construction the sensor. higher bycoating the repeated coating of textile with ´1 for the deposited onof the fabrics, theThe best valueconductivity being 210 Ω2afforded with polypyrrole followed by conducting polymers, polymers, however, however, is is likely likely to to be be of of benefit benefit for for the the improved improved collection collection and and processing processing conducting of the the electrical electrical response. response. of 4. Conclusions Conclusions 4. The cotton cotton was was uniformly uniformly coated coated with with conducting conducting polymers, polymers, polyaniline polyaniline or or polypyrrole, polypyrrole, and and by by The

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polyaniline. The conductivity of corresponding conducting polymers was of the order of units S¨ cm´1 , both in parent polymers or their combined composites. No synergetic effect on the conductivity was observed. The repeated dry cleaning increased the sheet resistivity of fabrics by about one order of magnitude, but still maintained a good level of conduction, the best result being 1700 Ω´1 , again for the twin polypyrrole/polyaniline coating. The fabrics were tested as electrodes in monitoring the electrical response of Venus flytrap stimulation. All cotton fabrics coated with both conducting polymers and their combinations have been able to collect and transfer the electrical response from Venus flytrap plant. No differences in this ability were observed among these coated fabrics. The usage of cotton fabric coated with polyaniline or polypyrrole conductive polymers may avoid the use of conducting gels at expense of slightly lower signal to noise ratio compared with Ag/AgCl electrodes. The relatively simple and cost-effective preparation process enables obtaining textiles with unique properties suitable for new specific end-use applications and attractive niche markets products development. Acknowledgments: The authors wish to thank the Technology Agency of the Czech Republic (TE01020022), the Ministry of Education Youth and Sports of Czech Republic (LO1204, National Program of Sustainability I) and the budget of the Moravian-Silesian Region (RRC/07/2014) and grant Biomedical Engineering Systems XI (SP2015/179 ) for financial support. Author Contributions: Václav Bajgar and Marek Penhaker recorded and evaluated the electrical responses, Lenka Martinková provided the textiles and their characterization, Andrej Pavloviˇc cultivated carnivorous plants and measured electrical signals, Miroslava Trchová analyzed and interpreted FTIR spectra, and Patrycja Bober and Jaroslav Stejskal deposited conducting polymers on cotton textile. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14.

Sapurina, I.; Stejskal, J. Mechanism of the oxidative polymerization of aniline and the formation of supramolecular polyaniline structures. Polym. Int. 2008, 57, 1295–1325. [CrossRef] Stejskal, J.; Sapurina, I.; Trchová, M. Polyaniline nanostructures and the role of aniline oligomers in their formation. Prog. Polym. Sci. 2010, 35, 1420–1481. [CrossRef] Feng, J.; Jing, X.L.; Li, Y. Self-assembly of aniline oligomers and their induced polyaniline supramolecular structures. Chem. Pap. 2013, 67, 891–908. [CrossRef] Bhadra, S.; Khastgir, D.; Singha, N.K. Progress in preparation and applications of polyaniline. Prog. Polym. Sci. 2009, 34, 783–810. [CrossRef] ´ c-Marjanovi´c, G. Recent advances in polyaniline research: Polymerization mechanisms, structural Ciri´ aspects, properties and applications. Synth. Met. 2013, 177, 1–47. [CrossRef] Stejskal, J.; Trchová, M.; Bober, P.; Humpolíˇcek, P.; Kašpárková, V.; Sapurina, I.; Shishov, M.A.; Varga, M. Conducting Polymers: Polyaniline. In Encyclopedia of Polymer Science and Technology; Wiley Online Library, Wiley & Sons, Ltd: Hoboken, NJ, USA, 2015. Guimard, N.K.; Gomez, N.; Schmidt, C.E. Conducting polymers in biomedical engineering. Prog. Polym. Sci. 2007, 32, 876–921. [CrossRef] Qazi, T.H.; Rai, R.; Boccaccini, A.R. Tissue engineering of electrically responsive tissues using polyaniline based polymers: A review. Biomaterials 2014, 35, 9068–9086. [CrossRef] [PubMed] Khan, M.A.; Ansari, U.; Murtaza, N. Real-time wound management through integrated pH sensors: A review. Sens. Rev. 2015, 35, 183–189. [CrossRef] Stejskal, J.; Gilbert, R.G. Polyaniline. Preparation of a conducting polymer (IUPAC technical report). Pure Appl. Chem. 2002, 74, 857–867. [CrossRef] Hirase, R.; Shikata, T.; Shirai, M. Selective formation of polyaniline on wool by chemical polymerization, using potassium iodate. Synth. Met. 2004, 146, 73–77. [CrossRef] Hong, K.H.; Oh, K.W.; Kang, T.J. Preparation of conducting nylon-6 electrospun fiber webs by the in situ polymerization of polyaniline. J. Appl. Polym. Sci. 2005, 96, 983–991. [CrossRef] Patil, A.J.; Deogaonkar, S.C. A novel method of in situ chemical polymerization of polyaniline for synthesis of electrically conductive cotton fabrics. Text. Res. J. 2012, 82, 1517–1530. [CrossRef] He, X.P.; Gao, B.; Wang, B.B.; Wei, J.T.; Zhao, C. A new nanocomposite: Carbon cloth based polyaniline for an electrochemical supercapacitor. Electrochim. Acta 2013, 111, 210–215.

Sensors 2016, 16, 498

15. 16. 17.

18. 19.

20. 21. 22. 23.

24.

25. 26.

27.

28.

29.

30. 31. 32.

33. 34.

35.

11 of 12

Zhou, X.Y.; Zhang, Z.Z.; Xu, Z.H.; Men, X.H.; Zhu, X.T. Fabrication of super-repellent cotton fabrics with rapid reversible wettability switching of diverse liquids. Appl. Surf. Sci. 2013, 276, 571–577. [CrossRef] Engin, F.Z.; Usta, I. Electromagnetic shielding effectiveness of polyester fabrics with polyaniline deposition. Text. Res. J. 2014, 84, 903–912. [CrossRef] Yaghoubidoust, F.; Wicaksono, D.H.B.; Chandren, S.; Nur, H. Effect of graphene oxide on the structural and electrochemical behavior of polypyrrole deposited on cotton fabric. J. Mol. Struct. 2014, 1075, 486–493. [CrossRef] Bober, P.; Stejskal, J.; Šedˇenková, I.; Trchová, M.; Martinková, L.; Marek, J. The deposition of globular polypyrrole and polypyrrole nanotubes on cotton fabric. Appl. Surf. Sci. 2015, 156, 737–741. [CrossRef] Anand, A.; Rani, N.; Saxena, P.; Bhandari, H.; Dhawan, S.K. Development of polyaniline/zinc oxide nanocomposite impregnated fabric as an electrostatic charge dissipative material. Polym. Int. 2015, 64, 1096–1103. [CrossRef] Wallace, G.G.; Campbell, T.E.; Innis, P.C. Putting function into fashion: Organic conducting polymer fibres and textiles. Fibers Polym. 2007, 8, 135–142. Huang, H.H.; Liu, W.J. Polyaniline/poly(ethylene terephthalate) conducting composite fabric with improved fastness to washing. J. Appl. Polym. Sci. 2006, 102, 5775–5780. [CrossRef] Varesano, A.; Dall’Acqua, L.; Tonin, C. A study on the electrical conductivity decay of polypyrrole coated wool textiles. Polym. Degrad. Stabil. 2005, 89, 125–132. [CrossRef] Miˇcušík, M.; Nedelˇcev, T.; Omastová, M.; Krupa, I.; Olejníková, K.; Fedorko, P.; Chehimi, M.M. Conductive polymer-coated textiles: The role of fabric treatment by pyrrole-functionalized triethoxysilane. Synth. Met. 2007, 157, 914–923. [CrossRef] Tang, X.N.; Tian, M.W.; Qu, L.J.; Zhu, S.F.; Guo, X.Q.; Han, G.T.; Sun, K.K.; Hu, X.L.; Wang, Y.J.; Xu, X.Q. Functionalization of cotton fabric with graphene oxide nanosheet and polyaniline for conductive and UV blocking properties. Synth. Met. 2015, 202, 82–88. [CrossRef] Wu, B.T.; Zhang, B.W.; Wu, J.X.; Wang, Z.Q.; Ma, H.J.; Yu, M.; Li, L.F.; Li, J.Y. Electrical switchability and dry-wash durability of conductive textiles. Sci. Rep. 2015, 5, 11255-1–11255-9. [CrossRef] [PubMed] Mi, H.Y.; Zhang, X.G.; Ye, X.G.; Yang, S.D. Preparation and enhanced capacitance of core-shell polypyrrole/polyaniline composite electrode for supercapacitors. J. Power Sources 2008, 176, 403–409. [CrossRef] Hussain, S.T.; Abbas, F.; Kausar, A.; Khan, M.R. New polyaniline/polypyrrole/polythiophene and functionalized multiwalled carbion nanotube-based nanocomposites: Layer-by-layer in situ polymerization. High Perform. Polym. 2013, 25, 70–78. [CrossRef] Wilson, J.; Radhakrishnan, S.; Sumathi, C.; Dharuman, D. Polypyrrole-polyaniline-Au (PPY-PANi-Au) nano composite films for label-free electrochemical DNA sensing. Sens. Actuat. B Chem. 2012, 171–172, 216–222. [CrossRef] Liang, B.L.; Qin, Z.Y.; Zhao, J.Y.; Zhang, Y.; Zhou, Z.; Lu, Y.Q. Controlled synthesis, core-shell structures and electrochemical properties of polyaniline/polypyrrole composite nanofibers. J. Mater. Chem. A 2014, 2, 2129–2135. [CrossRef] Radhakrishnan, S.; Sumathi, C.; Dharuman, V.; Wilson, J. Polypyrrole nanotubes-polyaniline composite for DNA detection using methylene blue as intercalator. Anal. Meth. 2013, 5, 1010–1015. [CrossRef] Wang, Z.L.; He, X.J.; Ye, S.H.; Tong, Y.X.; Li, G.R. Design of polypyrrole/polyaniline double-walled nanotube arrays for electrochemical energy storage. ACS Appl. Mater. Interf. 2014, 6, 642–647. [CrossRef] [PubMed] Stejskal, J.; Sapurina, I.; Trchová, M.; Šedˇenková, I.; Kováˇrová, J.; Kopecká, J.; Prokeš, J. Coaxial conducting polymer nanotubes: Polypyrrole nanotubes coated with polyaniline or poly(p-phenylenediamine) and products of their carbonization. Chem. Pap. 2015, 69, 1341–1349. [CrossRef] Wang, Y.; Wang, R.G.; Xu, S.C.; Liu, Q.; Wang, J.X. Polypyrrole/polyaniline composites with enhanced performance for capacitive deionization. Desalin. Water Treat. 2015, 54, 3248–3256. [CrossRef] Liang, G.J.; Zhu, L.G.; Xu, J.; Fang, D.; Bai, Z.K.; Xu, W.L. Investigations of poly(pyrrole)-coated cotton fabrics prepared in blends of anionic and cationic surfactants as flexible electrode. Electrochim. Acta 2013, 103, 9–14. [CrossRef] Stempien, Z.; Rybicki, T.; Rybicki, E.; Kozanecki, M.; Szynkowska, M.I. In-situ deposition of polyaniline and polypyrrole electroconductive layers on textile surfaces by the reactive ink-jet printing technique. Synth. Met. 2015, 202, 49–62. [CrossRef]

Sensors 2016, 16, 498

36. 37. 38. 39. 40. 41. 42. 43.

44. 45.

46. 47. 48. 49.

50. 51. 52.

53.

54.

12 of 12

Zhu, L.G.; Zhang, L.X.; Sun, Y.Y.; Bai, Z.K.; Xu, J.; Liang, G.J.; Xu, W.L. Conductive cotton fabrics for heat generation prepared by mist polymerization. Fiber Polym. 2014, 15, 1804–1809. [CrossRef] Cetiner, S. Dielectric and morphological studies of nanostructured polypyrrole-coated cotton fabrics. Text. Res. J. 2014, 84, 1463–1475. [CrossRef] Yazhini, B.K.; Prabu, H.G. Study on flame retardant and UV-protection properties of cotton fabric functionalized with ppy-ZnO-CNT nanocomposite. RSC Adv. 2015, 5, 49062–49069. [CrossRef] Deogaonkar, S.C.; Bhat, N.V. Polymer based fabrics as transducers in ammonia & ethanol gas sensing. Fiber Polym. 2015, 16, 1803–1811. Fromm, J.; Lautner, S. Electrical signals and their physiological significance in plants. Plant Cell Environ. 2007, 30, 249–257. [CrossRef] [PubMed] Koziolek, C.; Grams, T.E.E.; Schreiber, U.; Matyssek, R.; Fromm, J. Transient knockout of photosynthesis mediated by electrical signals. New Phytol. 2004, 161, 715–722. [CrossRef] Krol, E.; Dziubinska, H.; Stolarz, M.; Trebacz, M. Effect of ion channel inhibitors on cold and electrically-induced action potentials in Dionaea muscipula. Biol. Plant. 2006, 50, 411–416. [CrossRef] Libiaková, M.; Floková, K.; Novák, O.; Slováková, L.; Pavloviˇc, A. Abundance of cysteine endopeptidase dionain in digestive fluid of Venus flytrap (Dionaea muscipula Ellis) is regulated by different stimuli from prey through jasmonates. PLos ONE 2014, 9, 104424. [CrossRef] [PubMed] Omastová, M.; Trchová, M.; Kováˇrová, J.; Stejskal, J. Synthesis and structural study of polypyrroles prepared in the presence of surfactants. Synth. Met. 2003, 138, 447–455. [CrossRef] Hlaváˇcková, V.; Krchnák, ˇ P.; Naus, J.; Novák, O.; Špundová, M.; Strnad, M. Electrical and chemical signals involved in short-term systemic photosynthetic responses of tobacco plants to local burning. Planta 2006, 225, 235–244. [CrossRef] [PubMed] Ilík, P.; Hlaváˇcková, V.; Krchnák, ˇ P.; Nauš, J. A low-noise multi-channel device for monitoring of systemic electrical signal propagation in plants. Biol. Plant. 2010, 54, 185–190. [CrossRef] Stejskal, J.; Sapurina, I. Polyaniline: thin films and colloidal dispersions (IUPAC technical report). Pure Appl. Chem. 2005, 77, 815–826. [CrossRef] Stejskal, J.; Sapurina, I.; Prokeš, J.; Zemek, J. In-situ polymerized polyaniline films. Synth. Met. 1999, 105, 195–202. [CrossRef] ´ c-Marjanovi´c, G.; Mentus, S.; Pašti, I.; Gavrilov, N.; Krsti´c, J.; Travas-Sejdic, J.; Strover, L.; Kopecká, J.; Ciri´ Morávková, Z.; Trchová, M.; et al. Synthesis, characterization, and electrochemistry of nanotubular polypyrrole and polypyrrole-derived carbon nanotubes. J. Phys. Chem. C 2014, 118, 14770–14784. [CrossRef] Kopecká, J.; Kopecký, D.; Vrnata, ˇ M.; Fitl, P.; Stejskal, J.; Trchová, M.; Bober, P.; Morávková, Z.; Prokeš, J.; Sapurina, I. Polypyrrole nanotubes: Mechanism of formation. RSC Adv. 2014, 4, 1551–1558. [CrossRef] Trchová, M.; Morávková, Z.; Bláha, M.; Stejskal, J. Raman spectroscopy of polyaniline and oligoaniline thin films. Electrochim. Acta 2014, 122, 28–38. [CrossRef] Kocherginsky, N.M.; Wang, Z. The role of ionic conductivity and interface in electrical resistance, ion transport and transmembrane redox reactions through polyaniline membranes. Synth. Met. 2006, 156, 1065–1072. [CrossRef] Stejskal, J.; Bogomolova, O.E.; Blinova, N.V.; Trchová, M.; Šedˇenková, I.; Prokeš, J.; Sapurina, I. Mixed electron and proton conductivity of polyaniline films in aqueous solutions of acids: beyond the 1000 S¨ cm´1 limit. Polym. Int. 2009, 58, 872–879. [CrossRef] Xiong, S.X.; Yang, F.; Ding, G.Q.; Mya, K.Y.; Ma, J.; Lu, X.H. Covalent bonding of polyaniline on fullerene: Enhanced electrical, ionic conductivities and electrochromic performances. Electrochim. Acta 2012, 67, 194–200. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).