The feasibility and application of PPy in cathodic polarization antifouling

Colloids and Surfaces B: Biointerfaces 164 (2018) 247–254

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The feasibility and application of PPy in cathodic polarization antifouling Meng-yang Jia a , Zhi-ming Zhang a,∗ , Liang-min Yu a,∗ , Jia Wang b,∗ , Tong-tong Zheng a a Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, PR China b Laboratory of Corrosion science and Engineering, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, PR China

a r t i c l e

i n f o

Article history: Received 8 July 2017 Received in revised form 5 January 2018 Accepted 27 January 2018 Keywords: Cathodic polarization antifouling PPy material Inertness Electro-activity

a b s t r a c t Cathodic polarization antifouling deserves attention because of its environmentally friendly nature and good sustainability. It has been proven that cathodic voltages applied on metal substrates exhibit outstanding antifouling effects. However, most metals immersed in marine environment are protected by insulated anticorrosive coatings, restricting the cathodic polarization applied on metals. This study developed a conducting polypyrrole (PPy)/acrylic resin coating (␴ = 0.18 Scm−1 ), which can be applied in cathodic polarization antifouling. The good stability and electro-activity of PPy in the negative polarity zone in alkalescent NaCl solution were verified by linear sweep voltammetry (LSV), chronoamperometry (CA), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), demonstrating the feasibility of PPy as cathodic polarization material. Furthermore, the antifouling effects of PPy/acrylicresin coating on 24-h old Escherichia coli bacteria (E. coli) which formed on PPy/acrylic resin-coated plastic plate were measured under different cathodic potentials and treatment time, characterized by fluorescent microscope. The results suggest that at cathodic potential around −0.5 V (vs. saturated calomel electrode (SCE)), there was little trace of attached bacteria on the substrate after 20 min of treatment. PPy/acrylicresin-coated substrates were also subjected to repeated cycles of biofilm formation and electrochemical removal, where high removal efficiencies were maintained throughout the total polarization process. Under these conditions, the generation of hydrogen peroxide is believed to be responsible for the antifouling effects because of causing oxidative damage to cells, suggesting the potential of the proposed technology for application on insulated surfaces in various industrial settings. © 2018 Published by Elsevier B.V.

1. Introduction Biofouling of marine infrastructures such as water-cooling pipes or ship hulls is economically undesirable. The accumulation of biomass on these surfaces causes energy inefficiencies which are due to increased fluid frictional resistance in the case of ship hulls and to decreased efficiency of heat transfer in the case of watercooling pipes. Biofouling may lead to a series of adverse effects such as blocked pipes, higher frictional resistance, corrosion of metal surfaces, increased frequency of dry-docking operation, and introduction of species such as heavy metal ions into the environment [2]. In the past, biofouling has been prevented by toxic chemical agents such as copper and organotin. However, this approach is not entirely successful because of the inherent heavy metal resistance of certain bacteria [3], and the leaching of the heavy metal

∗ Corresponding authors. E-mail addresses: [email protected] (Z.-m. Zhang), [email protected] (L.-m. Yu), [email protected] (J. Wang). https://doi.org/10.1016/j.colsurfb.2018.01.055 0927-7765/© 2018 Published by Elsevier B.V.

ions into the marine environment. To prevent the settlement of marine organisms, in addition to mechanical methods [4], various antifouling coatings have been developed such as tributyltin paints (TBT-SPC paints), copper oxide paints [5], zinc oxide paints [6]. However, these methods may bring pollution to the environment. Furthermore, chlorination has been widely used in the antifouling [7,8]. But owing to the increasing concern about toxicity of chlorine and its intermediates to aquatic life, several countries are making effort to develop other antifouling methods which are environmentally friendly [9]. In recent years, more and more people are paying attention to the non-toxic polymer coatings involving agarose [10], polyaniline [11], and siloxane modified resin coatings [12], etc. Besides, it is believed that breaking or destabilizing the initially adhering of biomolecules and organisms to a substrate is a more effective cleaning method. Since Gordon and coworkers reported that electrochemical polarization of metal surfaces can affect the attachment of marine bacteria, electrochemical methods have been demonstrated as a convenient means of biofilm prevention or removal recently [13–16], including applying anodic

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and cathodic voltages. There have been earlier reports of applying anodic potentials to remove the biofilm covered on substrate [17–19] due to the generation of chlorine [20]. However, it is generally accepted that cathodic and block potentials are more effective in removing a biofilm [21,22]. In addition, certain metal substrates can undergo corrosion at anodic potentials [4]. Actually, most studies on electrochemical antifouling focused on metal materials (e.g. stainless steel), which are often covered and protected by insulated anticorrosive coatings. In addition, there are numerous reports of change of pH [23] and production of reactive oxygen species as a result of electrolysis [24], which could induce corrosion of the substrate. So it is very promising to develop a new kind of antifouling coatings which can be used as cathodic polarization material to realize the electrochemical antifouling at the same time. Conducting polymers (CPs) have attracted great interest over the last 30 years [25–28]. Among these, polypyrrole (PPy) holds a unique position owing to its amazing properties, including its high electrical conductivity, good environmental stability, ease of preparation and ion exchange capacity, which makes it desirable for many different applications [29–33]. Especially, PPy has been applied in antifouling [34,35]. For example, during phenols electro-oxidation by electrochemical surface plasmon resonance technique, PPy/PSS film had an obvious antifouling effect for the fouling of phenols which are well known as a class of pollutants that have a fouling effect on the electrode during electrochemical detection [29]. Besides, the PPy-raw MWCNTs nanocomposite was used in nano-filtration membranes and showed improvement in antifouling properties [30]. However, limited studies have been conducted for cathodic polarization antifouling. Furthermore, most of the doped PPy is used in acidic media because it may otherwise be dedoped in the presence of OH− . Therefore, the goal of the study involves two parts: investigate the stability and electrochemical properties of PPy in alkaline solution and research the cathodic polarization antifouling effects of PPy. In this work, conducting PPy (␴ = 0.18 Scm−1 ) were prepared and used for cathodic polarization antifouling. The stability and electrochemical activities of PPy material in the negative polarity zone under alkalescent NaCl conditions were investigated to demonstrate its feasibility of being utilized in cathodic polarization antifouling. Besides, the antifouling effect of cathodic polarization applied on PPy/acrylic resin coatings was investigated. 2. Experimental 2.1. Preparation 2.1.1. Preparation of PPy composite coating HCl doped PPy (PPy-HCl), H3 PO4 doped PPy (PPy-H3 PO4 ), p-toluene sulfonic acid doped PPy (PPy-TSA) and dodecyl benzenesulfonic acid doped PPy (PPy-DBSA) were synthesized by a reported method [36]. Here, HCl, H3 PO4 , p-toluene sulfonic acid, and dodecyl benzenesulfonic acid were purchased from National Medicine Chemicals (China). PPy-DBSA and modified acrylic resin (made in our lab) were dispersed into CHCl3 to obtain 0.2 g/mL PPy-DBSA/CHCl3 and 1.6 g/mL resin/CHCl3 mixtures, respectively. Two parts were mixed with a volume ratio of 5:4 to obtain the PPy/acrylic resin mixed solution. The PPy/acrylic resin coating can be obtained by painting on insulated plastic plates, having the conductivity of 0.18 Scm−1 . 2.1.2. Preparation of the PPy electrode PPy tableting electrode was made to eliminate the influence of metal substrate, aiming to obtain the electrochemical properties of pure PPy. The doping level of PPy doped with four dopants were calculated from the N+ /N ratios via X-ray photoelectron spectroscopy

measurements (Thermo ESCALAB 250XI, US) [37,38], recorded as 13.5% (PPy-HCl), 19.3% (PPy-H3 PO4 ), 22.1% (PPy-TSA) and 25.8% (PPy-DBSA). To ensure the same effective mass of PPy, 0.215 g PPyHCl, 0.205 g PPy-H3 PO4 , 0.185 g PPy-TSA and 0.160 g PPy-DBSA powers were tableted with a convolute copper wire at the pressure of 20 MPa, the thickness of these tableting electrodes were estimated to be 1.32 mm, 1.33 mm, 1.16 mm and 1.28 mm. The electric contact was achieved by welding a straight copper wire with convolute copper wire. Then the PPy tableting electrode was sealed with the epoxy resin to form a work area (1.5 cm2 ). After that, the polished, ethanol-wiped electrode was submitted to electrochemical tests. 2.2. Characterizations and measurements of PPy’s electrochemical properties The surface morphology of PPy powders with different dopants were examined with electron microscope (SEM, Hitachi, S-4800, Japan). The phase compositions of PPy samples were investigated by X-ray diffraction (XRD, MAC SCIENCE, Japan) analysis with the use of a Rigaku Ultima IV diffractometer. And Fourier transform infrared spectroscopy (FTIR, IFS–113 V, Germany) spectrums were measured using a Nicolet IR 750 spectrometer with pure KBr as the background. All the electrochemical measurements were carried out in the three-electrode system using Autolab Potentiostat/Galvanostat, PGSTAT302N (NOVA Software, Switzerland). The PPy tableting electrode, platinum sheet and saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. Linear sweep voltammetry (LSV) of PPy doped with different dopants were done in aqueous NaCl electrolytes, at the scan rate of 1 mV s−1 from −1.5 V to 0 V. Chronoamperometry (CA) experiments were performed in aqueous NaCl electrolyte by applying a cathodic potential of −1 V (SCE) to investigate the background current of PPy electrode. CVs were also carried out in NaCl solution at the scan rate of 5 mV s−1 to study the electrochemical property of PPy in cathodic potential range. At the same time, the change of conductivity with different polarization voltage were recorded via the measurements of electrochemical impedance spectroscopy (EIS) at frequency from 100 kHz to 10 mHz with signal amplitude of 10 mV at open circuit potential (OCP), and electro-activity with different polarization voltage were recorded by cycling voltammetry (CV) from −1 V to 0 V at the scan rate of 5 mV s−1 . 2.3. Characterizations of antifouling effect 2.3.1. Biofilm development The E. coli obtained from the College of Marine Life Sciences (Ocean University of China, China) was used to form the biofilms. A single E. coli colony from a newly prepared agar plate was grown at 37 ◦ C overnight, E coli was transferred to nutrient broth to achieve E. coli liquid broth culture. This culture was diluted 1:2000 in sterile 19 g/L nutrient broth (National Medicine Chemicals, China) and cultivated in thermoshake (HZQ-F160, China) at 37 ◦ C, 120 rpm for 16 h to harvest. The PPy/acrylic resin coated plastic plates were sterilized under ultraviolet lamp for 1 h and subsequently immersed in the E. coli culture at 37 ◦ C for 24 h. At the same time, the blank and modified acrylic resin coated plastic plates were also immersed in E. coli solution under the same condition for comparison. 2.3.2. Electrochemical measurements of cathodic antifouling The effectiveness of PPy/resin composite coating for cathodic polarization antifouling was measured by a three-electrode system using Autolab Potentiostat/Galvanostat, PGSTAT302N (NOVA Software, Switzerland). The PPy/acrylic resin coating was painted on

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insulated plastic plates as the working electrode. Platinum sheet and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. 0.48 M NaCl solution (pH = 8) was the electrolyte in the experiments. Different cathodic voltages were applied on PPy/acrylic resin coating for 20 min, recorded every 5 min. Every experiment under different voltages for 5–20 min was repeated thrice. 2.3.3. Biofilm removal To investigate the effect of repeated electrochemical biofilm removal from the substrate, the PPy coated substrates were subjected to 8 cycles of biofilm formation and removal. In these experiments, the substrates covered with a 24 h old E. coli biofilm were polarized at various voltages (−0.1 V to −0.9 V) for 20 min at room temperature to remove the biofilm layer, after which they were subjected to another round of 24 h biofilm formation (at 37 ◦ C) under the same culturing conditions (without any sample pre-treatment). Then these substrates were retreated with various cathodic voltages for another 20 min (recorded as 40 min in total polarization time), and biofilm removal were characterized by fluorescence microscopy (10× objective). 2.3.4. Viability testing The viability test was performed to investigate the number of remained bacteria after cathodic polarization, thus the electrochemical biofilm removal effects of PPy composite coating can be obtained by comparing the bacteria number before and after polarization. Microbial cells attached to the substrates were observed with a fluorescence microscope (Leica CTR 5000, Germany), filter combination H3 (excitation filter BP 420–490, filter RKP 510, Long pass filter 515), exposition time: 2 s; the cells attached on the electrode were stained with acridine orange (0.1%) and propidium iodide (10 ␮g mL−1 ). Living bacteria appeared green or yellow-green stained by acridine orange when observed under a fluorescent microscope, while dead bacteria showed red stained by propidium iodide. All samples were observed with a 10 × objective, keeping a fixed working distance (∼12 mm). At least 30 images for each substrate were used for quantification of attached bacteria. The number of bacteria adhered in these sample was calculated using an image processing software (Image J, National Institutes of Health). Data and error bars represent averages and standard deviations of the attached bacteria on 30 images, which were noted in graphs. Besides, the results are expressed as percentages of adhered bacteria and are calculated as follows: Adhered bacteria = [(the number of bacteria remaining on the surface of the electrode after application of cathodic polarization)/(the number of bacteria that initially adhered to the surface of substrate prior to the application of cathodic polarization)] × 100%. 3. Results and discussion 3.1. Structure and morphology characterizations of PPy The morphology of the surface of PPy powders prepared by chemical synthesis were assessed by scanning electron microscopy (Fig. S1), including PPy/HCl (a), PPy-H3 PO4 (b), PPy-TSA (c), and PPyDBSA (d). The general appearances of PPy particles in Fig. S1 are closed to the well-known cauliflower-like structure. Additionally, the peaks at 2␪ = 20–27◦ in XRD patterns (Fig. S2) and characteristic absorption bands of PPy appeared at 1180 cm−1 in FTIR spectrums (Fig. S3) demonstrate that the PPy powders with different dopants had been successfully synthesized. Therefore, Fig. S1–3 indicate the successful synthesis of PPy, and PPy/DBSA exhibits the hugest specific surface area among these.

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3.2. Background current density of PPy A stable potential window in the cathodic area is essential for obtaining a cathodic polarization antifouling material. Fig. 1a shows the LSV curves of PPy doped with different dopants recorded from −1.5 V to 0 V. It is found that a wide cathodic potential window can be realized by changing the dopants. For PPy-HCl, PPy-H3 PO4 and PPy-TSA, an abrupt increase in the cathodic current emerged at −0.4 V to −0.6 V, related to the deprotonation of the cation radicals on the PPy chain and the nucleophilic attack of OH− on the cation radical sites in alkaline solution, giving rise to the unsuitability in the alkaline environment for most PPy materials. However, it is noted that the potential window of the PPy-DBSA material successfully expands to a wide range of −1.15 V to −0.15 V that is very similar to the ranges found for GC and Pt electrodes. The increase of the cathodic current at around −1.0 V is related to the hydrogen evolution reaction on the electrode, as reported previously [26,27], and the rapid anodic current increase around −0.2 V may be connected to the oxygen evolution reaction. The results indicate that the width of the potential window of PPy can be expanded by increasing the size of the dopant; this is associated with the different mobility of the counter-ions [39]. It is known that the dedoping process commonly occurs in an alkaline solution. But the low mobility of the large counter-ion increases the difficulty of separation between the counter-ion and the PPy main chain [40]. Therefore, the PPy-DBSA material exhibited stability between −1.15 V and −0.15 V in NaCl solution (pH = 8), providing the feasibility of cathodic polarization antifouling in the alkalescent condition by using PPy material. The PPy-DBSA material was therefore chosen as the subject of research in the following experiments. In addition to good stability in the cathodic area, a low PPyDBSA background current density is also necessary to reduce the cathodic polarization loss. To investigate the background current density of PPy material, chronoamperometry under cathodic polarization with a potential step of −1 V was performed in the aerated NaCl solution (0.48 mol L−1 , pH = 8), as shown in Fig. 1b. Here, the average concentration of Na+ in global seawater is reported as 11 g L−1 [41], which is mostly derived from NaCl, and the pH of seawater is around 8 [42], thus 0.48 mol L−1 NaCl solution is selected to simulate the seawater environment. As can be observed, a large Ic occurs at the very onset of the transient period, which declines to almost −100 ␮A cm−2 rapidly within 1000 s. This period of first 1000 s probably involves the reduction of PPy-DBSA and dissolved oxygen. The changing rate of Ic slows down gradually in the next period from 1000 s to 2000 s, where the Ic mainly derives from the reduction of dissolved oxygen, as reported previously [43]. In addition, the current density on the PPy-DBSA electrode remains constant (−39 ␮A cm−2 ) after 2000 s, indicating that the reduction of the dissolved oxygen turns extremely slow after a long-time cathodic polarization. The indelible Ic is due to the background current density on the PPy-DBSA electrode. For comparison, the steady Ic of a bare GC electrode is −23 ␮A cm−2 , slightly lower than that of the PPy-DBSA electrode. The background current density of a few tens of ␮A cm−2 are connected to the oxygen oxidative adsorption or reductive desorption at the electrode/solution interface [44,45]. As seen, the overall background current density of the PPy-DBSA electrode is larger than that of the GC electrode, which may be related to the higher porosity (Fig. S4) and rough texture of the PPy-DBSA electrode [46]. Therefore, the background current of the PPy-DBSA electrode can be reduced to a fairly low level by a period of cathodic polarization, confirming the feasibility of the use of PPy-DBSA as a cathodic polarization material.

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Fig. 1. (a) LSV curves for PPy electrodes doped with different dopants in aerated 0.48 M NaCl (pH = 8), at the scan rate of 1 mV s−1 ; (b) chronoamperometry for the PPy-DBSA electrode pressed at 20 MPa and GC electrode.

Fig. 2. Cyclic voltammograms at 5 mVs−1 in (a) aerated and (b) deaerated NaCl solutions (0.48 M, pH = 8).

3.3. Cyclic voltammetry of the PPy-DBSA in the cathodic area CV plots were obtained in the negative polarity zone to obtain more information about the PPy-DBSA material. Fig. 2 shows the CVs on bare GC and PPy electrodes in aerated NaCl solution and deaerated NaCl solution, where ultra-high purity N2 was fed to the electrochemical cells for 30 min to remove O2 . Fig. 2a shows the locations of peaks on both electrodes are parallel, indicating there is no redox current of PPy-DBSA material, agreed with its good stability in the cathodic area. Whereas, it is clear that two cathodic peaks appear on two electrodes in aerated solution around potentials of −0.4 V and −0.7 V. According to the previous reports [47,48], the first peak at −0.4 V may be attributed to a 2electron electrochemical reduction of O2 to H2 O2 , mediated by the active surface quinone-like groups with superoxide anion (O2 •− ) as the intermediate (reaction (I) and (II)). On the other hand, O2 also may be transformed into H2 O2 by a direct 2-electron reduction (reaction (III)) at the GC surface. And the H2 O2 produced by the reduction of O2 plays a main role in the electrochemical antifouling [49]. O2 + e− → O2 •−

(I)

O2 •− + 2H+ + e− → H2 O2

(II)

O2 + 2H+ + 2e− → H2 O2

(III)

The existences of two cathodic peaks on both electrodes are further verified when compared to the CVs measured in deaerated solution (Fig. 2b). The cathodic peaks corresponding to oxygen reduction reaction disappear on two electrodes as shown in Fig. 2b. Besides, the well shaped anodic peaks are observed on both electrodes, located around −0.5 V and −0.2 V, which may be related to the oxygen evolution reaction as reported before [50–52]. The results illustrate that the CVs of PPy-DBSA electrode and GC electrode are similar, demonstrating the absence of PPy’s redox peaks, thus verify the cathodic inertness of the PPy-DBSA material in alkalescent NaCl solution, further proving the feasibility of PPy as cathodic polarization material. 3.4. Changes of conductivity and electro-activity of PPy-DBSA material under continuous cathodic polarization A high conductivity is a crucial property for a cathodic polarization material. Previous studies reported that PPy doped with DBSA shows superior conductivity compared to other dopants [53]. The conductivities of PPy-HCl, PPy-H3 PO4 , PPy-TSA and PPy-DBSA were measured at ambient temperature using a conventional four-point probe method. The conductivities of the above samples were found to be 0.54 Scm−1 , 3.42 Scm−1 , 6.17 Scm−1 , and 10.68 Scm−1 , respectively. The superior conductivity of PPy-DBSA can be observed, which is sufficiently high for its use as an electrode material. The result is consistent with high conductivity of PPy doped with DBSA

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Fig. 3. The change of Z /Z0 at different polarization voltages. Potential E (V) vs. SCE. Data and error bars represent averages and standard deviations of samples measured thrice.

as reported previously [54]. On the other hand, the conductivity must be maintained at a relatively high value after cathodic  polarization. Fig. 3 presents the change of Z /Z0 at low frequency calculated from EIS, where Z0 is the module value of the PPy-DBSA  electrode before polarization, and Z is the module value of the PPy-DBSA electrode after polarization from −0.1 V to −1 V. Here, Z represents the low frequency impedance of the sample recorded in  Bode plots obtained by EIS measurement. It is found that the Z /Z0 value maintains at nearly 100% and even only slightly lower than 100% in the −0.1 V to −0.3 V potential range. This may be related to the marginal permeation of the solvent and solute ions into the  surface part of the PPy-DBSA electrode. The Z /Z0 values of PPyDBSA after the cathodic polarization of −0.5 V to −0.6 V increase to  110–140%. However, the Z /Z0 value exhibits an exceedingly obvious increase at −0.7 V to −0.8 V and shows the sharpest rise at −1 V. These results demonstrate that the PPy-DBSA electrode shows little change in terms of its conductivity under the polarization voltage range from −0.1 V to −0.6 V. Nevertheless, the conductivity will decrease clearly when the potential is more negative than −0.7 V, similar to the position of the second cathodic peak in Fig. 2a. The conductivity may decline due to the strong nucleophilic characteristic of OH− generated from the excessive reduction of O2 (reaction (III)). However, cathodic polarization of PPy material, particularly in neutral or alkaline solutions may lead to a decrease in its electro-

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activity and as a consequence to a diminishing of the total charge transferred by the PPy during cathodic process. This means the antifouling effects of PPy coating would be weaken if there is a severe loss in its electro-activity, which cause the decrease of a total charge density transferred by the PPy material. We therefore investigated the electro-activity of the PPy-DBSA electrode during cathodic reduction. The CV curves of the PPy-DBSA electrode were measured with an applied voltage between −0.1 V and −1.0 V for 3 h, and current capacity percentage (Qa /Q0 and Qc /Q0 ) values were calculated and shown in Fig. 4. Here, Q0 is the CV current capacity of the PPy-DBSA electrode before polarization, and Qa and Qc represent the anodic and cathodic current capacity after polarization, which were calculated by the integration of the fields corresponding to anodic and cathodic parts of CV curves respectively. At the beginning of polarization, the Qc /Q0 values maintains at nearly 100% from −0.1 V to −0.3 V, and over 95% at −0.5 to −0.6 V as shown in Fig. 4a. It is worth mentioning that the Qc /Q0 values decrease slightly with increasing polarization time, indicating that the cathodic electro-activity remains practically unchanged under the polarization voltage of −0.1 to −0.6 V. Qa /Q0 values remain at nearly 100% or even larger at the outset of polarization, with a minimum value that is also high, approximately 90% after polarizing at −0.7 V for 3 h. However, both the Qa /Q0 and Qc /Q0 values decrease rapidly at the potentials more negative than −0.7 V, and the cathodic electro-activity declines more severely. These results demonstrate that the overall electro-activity and conductivity of the PPy-DBSA electrode will be weakened when Ec is lower than −0.7 V. The diminishing of electro-activity and conductivity of samples under Ec < −0.7 V corresponds to the abundance of OH− ions generated from the reduction of dissolved oxygen (reaction (III)), which can attack the PPy chains [55], and damage the electro-activity. Fortunately, the majority of electro-activity on the PPy-DBSA electrode can be preserved under a small Ec (-0.1 V to −0.6 V), in agreement with the conductivity results. These measurements further prove that the OH− ions’ attack is weak at a relatively small Ec , and guarantee the stability of PPy-DBSA material during long-time polarization. In brief, the PPy-DBSA electrode exhibits good stability, a low background current density, high conductivity, and good electroactivity under cathodic polarization between −0.4 V and −0.6 V. This potential range may ensure the generation of H2 O2 to produce antifouling effects (reaction (I) and (II)) as well as avoiding the abundant generation of OH− (reaction (III)) which leads to the

Fig. 4. (a) Cathodic current capacity percentage (Qc /Q0 ) and (b) anodic current capacity percentage (Qa /Q0 ) of the PPy-DBSA electrode after polarization in aerated NaCl solution (0.48 M, pH = 8). Potential E (V) vs. SCE. Data and error bars represent averages and standard deviations of samples measured thrice.

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Fig. 5. Bacterial cells remaining on the PPy/acrylic resin-coated substrates after the application of various cathodic voltages for 20 min (recorded every 5 min): −0.1 V −0.3 V −0.5 V −0.7 V −0.9 V. Data and error bars represent averages and standard deviations of one sample measured by 3D fluorescence microscopy images.

excessive loss of conductivity and electro-activity, verifying its feasibility for using as cathodic polarization antifouling material. 3.5. The antifouling application of PPy composite coating 3.5.1. Kinetics of biofilm removal at different constant voltages As mentioned before, electrochemical antifouling effects may be different under various voltages. To investigate this in detail, a series of experiments were carried out in which the removal kinetic of E. coli biofilm was investigated as a function of the applied electrode potential. In this section, the PPy/acrylic resin-coated substrates covered by biofilm were polarized at different cathodic potential for 5–20 min. The number of bacterial cells remaining on the substrate was quantified via biofilm images collected from fluorescence microscopy and calculated as percentage of adhered bacteria. The percentage of adhered bacteria on coated substrates was shown in Fig. 5. It is clear that at −0.1 V and −0.3 V polarization voltages, the number of remaining bacteria on the PPy/acrylic resin-coated substrates decreases marginally than the untreated PPy/acrylic resin-coated substrate. More than 80% of E. coli attached on the substrate under polarization at these voltages for 20 min. The slight detachment of E. coli can be ascribed to electrostatic force generated between the attached bacteria and the substrate by polarizing the substrate surface [56,57]. However, the adhered bacteria declined

Fig. 7. Chronoamperometry of PPy/acrylic resin-coated substrates at various potentials for 20 min −0.1 V −0.3 V −0.5 V −0.7 V −0.9 V.

sharply under potentials more negative than −0.5 V (including −0.5 V), reaching a low level of ∼5% at 20 min. In particular, the rate of bacteria detachment decreases with increasing time, which means the most effective antifouling effect happens in the first 5 min. This may be related to the generation of H2 O2 from the reduction of O2 , as illustrated in reaction (I) and (II). Fig. 6a–c show the fluorescence microscopy images of E. coli cells on bare (a), acrylic resin-coated (b) and PPy/acrylic resin-coated plastic plate (c) respectively. Fig. 6d–h show the fluorescence microscopy images of PPy/acrylic resin-coated samples under different polarization potentials for 20 min. As shown in Fig. 6c, the quantity of adhered bacteria cluster on PPy/acrylic resin coated substrate deceased compared with that on blank plastic plate (Fig. 6a) and acrylic resin coated substrate (Fig. 6b), showing that PPy can prevent the bacteria from adhering. As expected, the adhered bacteria on PPy coated substrate decreased when cathodic potential was applied (Fig. 6d–h). We can see that the substrates treated at −0.1 V (Fig. 6d) and −0.3 V (Fig. 6e) still possessed a considerable amount of attached bacteria after 20 min’s treatment. However, when the polarization potential was increased to −0.5 V (Fig. 6f) or more negative voltages (Fig. 6g and Fig. 6h), there was little trace of attached bacteria on the surface after being treated for 20 min. The results show a cathodic current (<−0.5 V) is effective for the detachment of E. coli cells from the substrate surface, which is in agreement with that obtained from Fig. 2. The relationship between current density and the polarizing time under different voltages (−0.1 to −0.9 V) is illustrated in Fig. 7.

Fig. 6. Fluorescence microscopy images of E. coli cells remained on substrate after application of the cathodic voltages for 20 min (a) blank plastic plate (substrate) (b) acrylic resin-coated substrate (c) PPy/acrylic resin-coated substrate without polarization (d-h) PPy/acrylic resin-coated substrate under polarization of −0.1 V, −0.3 V, −0.5 V, −0.7 V, and −0.9 V respectively.

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the adhered bacteria remain less than 10% even after 8 rounds of electrochemical treatment (160 min in total). The biofilm removal can be attributed to on the generation of hydrogen peroxide at the surface of PPy coating, which may detach the bacteria biofilm effectively. The conductive PPy/acrylic resin coating can be coated on the surface of insulated anticorrosive coating to antifouling by employing the cathodic polarization, expanding the application of electrochemical antifouling methods. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 41476059); and the China Postdoctoral Science Foundation (No. 2016M600557). Fig. 8. Bacterial cells remaining on the PPy/acrylic resin-coated substrates after 8 rounds of electrochemical treatment (20 min every round): −0.1 V −0.3 V −0.5 V −0.7 V −0.9 V. Data and error bars represent averages and standard deviations of one sample measured by 3D fluorescence microscopy images.

Results show that the current density increases at larger cathodic potentials, but the changing trend of current density at a particular voltage differs dramatically. When a smaller polarization voltage (−0.1 V and −0.3 V) was applied, the current density increases slightly under longer treatment time, attributed to enlarged conductivity in the −0.1 V to −0.3 V potential range as demonstrated in Fig. 3. However, the current density drops with increasing time under more negative voltages (-0.5 V to −0.9 V), indicating the weakened electro-activity and conductivity. Therefore, the application of a cathodic voltage appears to result in the retention of live bacteria on the substrate surface while achieving the effect of detachment. The excellent antifouling effects can be achieved by a cathodic voltage more negative than −0.5 V for 20 min, agreed with our surmise above. 3.5.2. Repeated biofilm removal The influence of repetitive biofilm formation and removal on the efficiency of the removal process was investigated in Fig. 8, where the total polarization time means the sum of time of repeated rounds. Results show that the antifouling effect of smaller cathodic voltages (−0.1 V and −0.3 V) remain poor during the 160-min observation period. Additionally, the antifouling ability of PPy composite coating decreases when larger voltages were applied (−0.7 V and −0.9 V). This phenomenon is in accordance with the results in Figs. 3 and 4, where the conductivity and electro-activity decrease severely under voltages more negative than −0.7 V. Fortunately, the adhered bacteria reduces effectively when treated by −0.5 V, remaining the percentage below 10% during the whole process. As a consequence, −0.5 V polarization voltage applied on PPy/acrylic resin-coated plastic plate is suitable for long-time antifouling. 4. Conclusion The feasibility and application of PPy in cathodic polarization antifouling were investigated in this work. Results show that PPyDBSA has good stability in negative polarity zone (−1.15 V to −0.15 V), low background current of a few tens of ␮A cm−2 in alkalescent NaCl solution, and similar CVs to that of GC electrode. Besides, great conductivity and electro-activity can be guaranteed under polarization between −0.4 V and −0.6 V, where H2 O2 is produced to have an effect on antifouling, verifying the feasibility of PPy-DBSA material as cathodic polarization antifouling material. More important, the antifouling effect of PPy/acrylic resin coatings show excellent antifouling effects under polarization voltages (−0.5 V or more negative voltages). The adhered E. coli bacteria on PPy coatings decrease to ∼5% after polarizing for 20 min, and

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