Poly(4-vinylaniline)-Polyaniline Bilayer-Modified Stainless Steels for

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Poly(4-vinylaniline)-Polyaniline Bilayer-Modified Stainless Steels for the Mitigation of Biocorrosion by Sulfate-Reducing Bacteria (SRB) in Seawater Shaojun Yuan,*,† Shengwei Tang,† Li Lv,† Bin Liang,† Cleo Choong,*,‡ and Simo Olavi Pehkonen*,§ †

Multi-phases Mass Transfer & Reaction Engineering Lab, College of Chemical Engineering, Sichuan University, Chengdu 610065, China ‡ School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 § Chemical Engineering Program, Masdar Institute of Science and Technology, PO Box 54224, Abu Dhabi, United Arab Emirates S Supporting Information *

ABSTRACT: A novel strategy by combination of surface-initiated atom transfer radical polymerization (ATRP) and in situ chemical oxidative graft polymerization was employed to tether stainless steel (SS) with poly(4-vinylaniline)-polyaniline (PVAnPANI) bilayer coatings for mitigating biocorrosion by sulfate-reducing bacteria (SRB) in seawater. A trichlorosilane coupling agent was first immobilized on the SS surfaces to provide sulfonyl halide groups for surface-initiated ATRP of 4-VAn. A subsequent grafting of PANI onto the PVAn-grafted surface was accomplished by in situ chemical oxidative graft polymerization of aniline. The PVAn-PANI bilayer coatings were finally quaternized by hexylbromide to generate biocidal functionality. The sosynthesized SS surface was found to significantly reduce bacterial adhesion and biofilm formation. Electrochemical results revealed that the PVAn-PANI modified SS surface exhibited high resistance to biocorrosion by SRB. With the inherent anticorrosion capability and antibacterial properties of quaternized PVAn-PANI bilayers, the functionalized SS substrates are potentially useful to steel-based equipment under harsh marine environments.

1. INTRODUCTION Biocorrison by sulfate-reducing bacteria (SRB) in seawater has been extensively investigated.1−3 It has been widely recognized that SRB play a major role in the anaerobic biocorrosion of irons, low alloy steels, stainless steels, high nickel alloys, and copper alloys.4−9 A variety of mechanisms have therefore been proposed to interpret the enhanced corrosion by SRB,10 such as production of aggressive sulfide ions,11 precipitation of iron sulfide to catalyze proton reduction into hydrogen,12,13 cathodic depolarization by SRB hydrogenase,14 anodic depolarization resulting from the local acidification at the anode,15 and metal ion chelation by extracellular polymer substances (EPS).16 Among them, the corrosion products, such as sulfides and biofilm formation, have been widely proven to be the main factors in biocorrosion phenomenon of SRB.17,18 Microorganisms tend to form and grow biofilms (complex communities of microorganisms attached to the surfaces),19 in which the production of hydrogen sulfides by SRB is enhanced and intrinsic heterogeneity is established, thus modifying the physicochemical environment and favoring the electrochemical reactions that lead to critical localized corrosion processes.20−22 Thereby, it is of great significance to control biofilm formation and aggressive sulfide anions on contact with the metal substrates for the mitigation of biocorrosion by SRB. Different strategies have been developed over the past decades to address the growing need for combating biocorrosion of SRB, such as biocides,23,24 cathodic protection,25 protective coatings,26,27 and corrosion inhibitors,28,29 Biocide treatment, involving the use of oxidizing (i.e., halogen and ozone) and nonoxidizing agents (i.e., formaldehyde, © 2012 American Chemical Society

glutaradehyde, isothiazolones, and quaternary ammonium salts), is the most conventional and commonly used countermeasure to control biocorrosion in steel pipelines and closed systems.30 However, the practice is less than satisfactory owing to potential environmental pollution, high cost, and low efficiency against biofilms.31 Despite high effectiveness in resisting biofouling and biocorrosion in marine environments, protective coatings, such as tributyltin (TBT)-based paints, have been completely phased out since 2008 due to their toxicity to nontargeted marine organisms.32 On the other hand, the efficiency of cathodic protection has also been found to significantly decrease in an SRB-induced corrosion system,33 since the high applied positional has no effect on the adhesion of anerobic bacteria and is unable to prevent the initiation of localized corrosion by SRB.34 In view of the complex environmental, ecological, and economical impacts, an alternative effective approach to covalently immobilize antibacterial coatings on metal surface has been developed to inhibit biocorrosion induced by SRB in recent years.35 Because of the robust linkage of covalent bonds between the functional coatings and metal substrates, the problem of environmental contamination is minimized. This novel strategy is also designed to confer antibacterial and anticorrosive dual-functional properties on the metal surfaces to simultaneously resist the attack of aggressive sulfide anions and biofilm formation. Received: Revised: Accepted: Published: 14738

August 28, 2012 October 17, 2012 October 23, 2012 October 23, 2012 dx.doi.org/10.1021/ie302303x | Ind. Eng. Chem. Res. 2012, 51, 14738−14751

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Figure 1. Schematic diagram illustrating the process of silanization of the hydroxylated SS (SS−OH) by CTCS (the SS−Cl surface), surface-initiated ATRP of VAn from the SS−Cl surface (the SS-g-PVAn surface), chemical oxidative polymerization of aniline on the SS-g-PVAn surface to generate the PVAn-g-PANI bilayer-coated surface (the SS-g-PVAn-b-PANI surface), and subsequent functionalization via quaternization of the SS-g-PVAn-bPANI surface into the antibacterial SS-g-PVAn-b-QPANI surface.

documented to use conductive polymer coatings for combating biocorrosion.42,43 Polyaniline (PANI), the century-old aniline polymer, is the most widely used conductive polymer for anticorrosion coatings, owing to its environmental stability, controllable electrical conductivity, low cost, good processability, and stable intrinsic redox state associated with the chain nitrogen.44 Electrochemical synthesis of PANI on metals has often been used to coat substrates for corrosion protection.45,46 However, an inherent problem of electropolymerized PANI coatings is their poor adhesion on various substrates.47 To circumvent the

Thereby, various polymer coatings, which contain pendant or incorporated antibacterial moieties, such as poly(2dimethyamino)ethyl methacrylate) (PDMAEMA),35 chitosan,36 polyurethane,37 poly(4-vinylpyridine) (PVP),38 epoxy,39 and poly(N-methylaniline) (PNMA),40 have been tethered on the metal surfaces for the mitigation of biocorrosion. Among these materials, conductive polymers have been of particular interest, as they have been proposed to provide corrosion protection by serving as barriers and inhibitors, and via anodic protection and mediation of oxygen reduction.41 However, to the best of our knowledge, only few studies have been 14739

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adhesion problem of PANI coatings, different methods have been adopted (i) utilizing organic acid medium, such as oxalic acid and salicylic acid etc., instead of mineral acids,48 (ii) fabricating the adhesive primer layer for subsequent deposition of PANI coatings,49 (iii) doping PANI as composite additive in the epoxy coatings,50 and (iv) immobilizing the emeradine (EM) salt form of PANI on the surface-grafted poly(glycidyl methacrylate) (PGMA) via thermal curing.41,51 Recently, good adhesion of PANI coatings on the surfaces of silicon, cotton, and glass was achieved by formation of a covalently grafted PANI layer via in situ chemical oxidative graft polymerization.52−54 The covalently tethered PANI chains on substrate surface were found to preserve the intrinsic properties of the aniline homopolymers, such as unique oxidation states, protonation−deprotonation behavior, metal reduction ability, and electrical conductivity etc.54 Hence, in situ chemical oxidative graft polymerization of aniline is an alternative approach to prepare strongly adhered PANI coatings on metal surfaces for corrosion inhibition. Accordingly, the aim of this study is to tether poly(4vinylaniline)-polyaniline (PVAn-PANI) bilayer coatings with antibacterial functionality on stainless steel (SS) surface for combating biocorrosion by SRB in simulated seawater. The surface modification process is schematically shown in Figure 1. A sulfonyl-halide-containing trichlorosilane was immobilized on the active SS (SS−OH) surface to provide not only a passivation monolayer but also initiation sites for the grafting of PVAn brushes via surface-initiated atom transfer radical polymerization (ATRP) of 4-vinylaniline (4-Van). Subsequent grafting of PANI was accomplished via in situ chemical oxidative polymerization of aniline to produce the PVAn-gPANI surface. The so-synthesized PVAn-g-PANI bilayer coatings were further quaternized with hexyl bromide via Nalkylation of PANI to produce biocidal quaternary ammonium polycations. Success in each modification step was ascertained by X-ray photoelectron microscopy (XPS), scanning electron microscopy (SEM), and static water contact angle measurements. The antibacterial and anticorrosion behaviors of the surface-functionalized SS coupons in a Desulfovibrio desulfuricans (D. desulfuricans) inoculated simulated seawater-based modified Barr’s (SSMB) media were evaluated by viable cell assays and electrochemical analysis, respectively.

THF for 24 h to remove physically adsorbed PVAn homopolymer, if any. 2.2. In Situ Chemical Oxidative Graft Polymerization of Aniline and N-Alkylation of Surface-Grafted Polyaniline (PANI). The chemical oxidative graft polymerization of aniline on the free aniline groups (−NH2) of the covalently bonded VAn molecules was carried out in a 30 mL of 1 M HCl aqueous solution, containing the SS-g-PVAn coupons, 1 mL of aniline, and 2.45 g of ammonium persulfate ((NH4)2S2O8) . The reaction was allowed to proceed at 0 °C in an ice−water bath for 6 h. After being washed with deionized water and ethanol, the grafted PANI film in its emeraldine (EM) salt form was further converted to the neutral EM base form by immersing the SS substrate in 50 mL of 0.5 M NaOH solution for 2 h, followed by immersion in an excess volume of NMP for 12 h to ensure complete removal of the physically adsorbed aniline homopolymer and reactants. The resulting surface with PVAn-PANI bilayer is referred to as the SS-g-PVAn-b-PANI surface. The SS-g-PVAn-b-PANI surfaces were quaternized with a 20 vol % nitromethane solution of hexyl bromide at 70 °C for 24 h. Subsequently, the SS coupons were removed and washed with a copious amount of nitromethane to completely remove the unreacted hexyl bromide and solvents. After the Nalkylation reaction, the color of the EM base film changed from dark blue to dark green. The quaternized SS-g-PVAn-b-PANI surface, referred to as the SS-g-PVAn-b-QPANI surface, was dried under reduced pressure prior to being subjected to further characterization. 2.3. Surface Characterization. The composition of functionalized SS substrates was characterized by X-ray photoelectron spectroscopy (XPS). The XPS measurements were performed on a Kratos AXIS HSi spectrometer with an Al Kα X-ray source (1486.6 eV photons), using procedures similar to those described previously.56 Static water contact angles of various substrate surfaces were measured at 25 °C and 60% relative humidity using the sessile drop method with a 3 μL water droplet and a telescopic goniometer (model 100-00(230), Rame-Hart, Inc., Mountain Lake, NJ, USA). The telescope with a magnification power of 23× was equipped with a protractor of 1° graduation. The contact angles reported were the mean values from four substrates, with the value of each substrate obtained by averaging the contact angles from at least three surface locations. The film thickness on the SS substrates was determined by ellipsometry. The measurements were carried out on a variable angle spectroscopic ellipsometer (model VASE, J. A. Woollam, Inc., Lincoln, NE, USA) at incident angles of 70° and 75° in the wavelength range 250− 1000 nm. The refractive index of the dried films at all wavelengths was assumed to be 1.5 in the Cauchy film model used for the simulation of film thickness. All measurements were conducted in dry air at room temperature. For each sample, thickness measurements were made on at least three different surface locations. Data were recorded and processed using the WVASE32 software package. Surface morphology of the pristine and surface-functionalized SS coupons was characterized by scanning electron microscopy (JEOL JSM5600, Tokyo, Japan). 2.4. Antibacterial Assays of Surface-Functionalized SS Coupons. The cultivation and inoculation of D. desulfuricans were described in detail in Supporting Information S1. The antibacterial assay comprised two experiments to investigate whether the functionalized surfaces (a) reduced bacterial adhesion and (b) killed the bacteria. Bacterial adhesion

2. EXPERIMENTAL SECTION 2.1. Surface-Initiated ATRP of 4-VAn. A trichlorosilane agent was immobilized on the hydroxyl-enriched stainless steel (SS) substrates to provide sulfonyl chlorine groups as ATRP initiator. Details for materials used in surface functionalization and the preparation of the hydroxylated SS (SS−OH) and ATRP initiator-immobilized SS (SS−Cl) surfaces were described in Supporting Information S1. For the surfaceinitiated ATRP of 4-VAn from the SS−Cl surface, 4-VAn (2.6 mL, 20 mmol), CuCl (9.89 mg, 0.1 mmol), CuCl2 (1.34 mg, 0.01 mmol), and Me6TREN (26 μL, 0.11 mmol) were added to 10 mL of dried DMF in a round-bottom flask. After degassing with argon for 30 min, the SS−Cl substrate was introduced into the reaction mixture under argon atmosphere. The reaction flask was sealed and kept in a temperature-controlled oil bath at 90 °C for 4 h. After the reaction, the poly(4-vinylaniline)grafted SS (referred to as the SS-g-PVAn) surface was washed thoroughly with an excess amount of THF and deionized water. Finally, the SS-g-PVAn substrate was immersed in 20 mL of 14740

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Table 1. Static Water Contact Angles of Different SS Substrate Surfaces

a

Pristine SS refers to a newly polished stainless steel (SS) coupon. bSS−OH was obtained after the pristine SS coupon immersed in the piranha solution for 30 min. cSS−Cl was obtained after the SS−OH reacted with 2-(4-chlorosulfonylphenyl)ethyl trichlorosilane in anhydrous toluene solution for 6 h at room temperature. dReaction conditions: [VAn]:[CuCl]:[CuCl2]:[Me6TREN] = 100:1:0.2:2 in DMF at 90 °C for 4 h. eThe SS-gPVAn-b-PANI surfaces were obtained after SS-g-PVAn coupons immersed in 30 mL of 0.1 M HCl aqueous solution containing 1 mL of aniline and 2.45 g of (NH4)2S2O8 at 0 °C for 6 h. The EM salt form of PANI was converted to the neutral EM base after NaOH treatment. fThe SS-g-PVAn-bPANI surfaces were quaternized in a 10 mL of nitromethane containing 20 vol % hexyl bromide at 70 °C for 24 h. gSD denotes standard deviation, h WCA refers to water contact angles, determined by averaging the contact angles from at least three surface locations.

characteristics of the functionalized SS surfaces were assessed via scanning electron microscope imaging. The pristine and functionalized SS coupons were exposure to the D. desulfuricans (more than 106 MPN·mL−1) inoculated SSMB medium for 3, 14, and 30 days in the anaerobic chamber (Don Whitley, Model MASC MG 50, Maharashtra, India) under an atmosphere containing 5% H2, 5% CO2, and 90% N2. At the end of predetermined exposure time, the coupons were removed and washed with PBS solution to remove the dead and loosely attached bacteria and were then immersed in 3 vol % glutaradehyde (GA) of PBS solution at 4 °C for 6 h for fixation. After that, the substrates were washed with PBS solution twice, followed by step dehydration with 25, 50, 75, 90, and 100 vol % ethanol for 5 min each. Finally, the coupons were dried in a vacuum desciator under reduced pressure before being sputter-coated with a thin film of platinum for SEM imaging. The viability of the bacteria adhered on the substrates were investigated by fluorescence microscopy (FM) images. The Live/Dead Baclight bacterial viability kit (L131152), consisting of a mixture of SYTO 9 green fluorescent nucleic acid dye and propidium iodide (PI) red fluorescent nucleic acid dye, was used. The SYTO 9 is membrane permeable and therefore stains both viable and nonviable bacteria, whereas PI, which has a higher affinity for nucleic acids, is rejected from viable bacterial cells by membrane pumps.57 When both dyes are present, PI competes with SYTO 9 for nucleic acid binding sites. Thus, viable bacteria (which appear green) and dead bacteria (which appear red) can be distinguished under the fluorescence microscope. After the prescribed exposure time, the coupons were removed and then stained by 0.1 mL solution of the Live/ Dead Baclight kit on the substrate surface for 15 min. The stained coupons were imaged under a green filter (excitation/ emission wavelengths, 420−480/520−580 nm) or a red filter (excitation/emission wavelengths, 480−550/590−800 nm) with a Leica DMLM microscope equipped with a 100 W Hg lamp. At least three different surface locations on each substrate were randomly chosen for FM imaging. To assess the antibacterial property of the functionalized coupons in a

more quantitative manner, the viable cells adhered on each substrate surface was enumerated using the 3-tube most probable number (MPN) method as a function of exposure time. The experimental procedures have been described in detail previously.38 2.5. Anticorrosion Behavior of the Surface-Functionalized SS Coupons. Tafel polarization curves and electrochemical impedance spectra (EIS) were obtained to assess the anticorrosion behavior of the surface-functionalized coupons. After a predetermined exposure time, the coupon was removed from the culture medium and embedded in a PVDF-based holder, with a circular open area of 0.785 cm2, to serve as the working electrode. An Ag/AgCl electrode was used as the reference electrode, and a platinum rod as the counter electrode. Details on the procedures and parameters used for the electrochemical studies had been described previously.29 Briefly, the Tafel polarization curves were recorded at a scan rate of 2 mV·s−1 in the range of −250 to +250 mV vs the open circuit potential (OCP), to determine the corrosion current densities (jcorr) and the corrosion potentials (Ecorr). EIS measurements were performed at the OCP with 7 points per decade using a 10 mV amplitude sinusoidal signal in the frequency range of 100 000−0.005 Hz (a total of 51 points). The inhibition efficiency (IE) of the surface-functionalized coupons was calculated using the following equations:38,56 η% =

jo − jcorr jo

(1)

where jo and jcorr were the corrosion current densities of the pristine SS and the surface-functionalized coupons, respectively, as determined from the analysis of Tafel polarization curves.

3. RESULTS AND DISCUSSION 3.1. Immobilization of the ATRP Initiator on the Stainless Steel (SS) Substrate. As a strong oxidizer, piranha solution has been known to generate an oxidized and hydroxylenriched surface on most metals.56 The enrichment of hydroxyl groups (−OH) on the SS surface results in a more hydrophilic 14741

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Figure 2. (a) Wide scan, (b) Cl 2p, (c) C 1s, and (d) S 2p core-level spectra of the SS−Cl surface (the reaction conditions are given in Table 1).

spectrum consists of the S 2p3/2 and S 2p1/2 peak components at the BEs of 168.5 and 169.7 eV (Figure 2d), characteristics of the chlorosulfonyl species.58 The increase in static water contact angle of the substrates from about 24 ± 2° to about 63 ± 3° is consistent with the coverage of a silane monolayer with a thickness of 1.1 ± 0.7 nm on the SS substrates (Table 1). Thus, the ATRP initiator with reactive sulfonyl halide group has been successfully immobilized on the SS surface to cater for the subsequent ATRP process. 3.2. Surface-Initiated ATRP of 4-Vinylaniline (VAn). The SS-g-PVAn coupons were prepared via surface-initiated ATRP of 4-vinylaniline (VAn) from the SS−Cl surface by using CuCl, CuCl2, and Me6TREN as the catalyst system. The molar feed ratio of [4-VAn (monomer)]:[CuCl (catalyst)]:[CuCl2 (deactivator)]:[Me6TREN (ligand)] for the ATRP process was controlled at 200:1:0.2:1.1.59 Parts a and b of Figure 3 show the wide scan and N 1s core-level XPS spectra of the SS-g-PVAn surface. The appearance of strong C 1s and N 1s signals in the wide scan spectrum (Figure 3a), and the only N 1s component with BE at 399.4 eV (Figure 3b), characteristic of the −NH2 species,59 suggest the successful grafting of PVAn brushes on the SS substrates. Furthermore, the disappearance of the S 2p signal and the decrease in intensity of Cl 2p signals in the wide scan spectrum of the SS-g-PVAn surface, as compared to that of the SS−Cl surface, indicate that thickness of PVAn brushes is larger than the probing depth of the XPS techniques (about 8 nm in an organic matrix58) after 4 h of ATRP (Figure 3a). The thickness of PVAn brushes grafted on the SS substrates is about 24 ± 3 nm (Table 1). The static water contact angle of the SSg-PVAn surface is about 39 ± 3° due to the presence of rich hydrophilic amine moieties of PVAn chains (Table 1). Thus, the PVAn brushes containing pendant amine groups have been successfully grafted on the SS substrates for subsequent in situ chemical oxidative graft polymerization of aniline.

substrate surface, as indicated by the decrease in static water contact angle from about 55 ± 4° to about 24 ± 2° (Table 1). The chemical composition of the hydroxyl-enriched SS (SS− OH) surface was characterized by XPS. The dominant O 1s signal, together with Cr 2p and Fe 2p signals on the wide scan spectrum of the SS−OH surface, is consistent with the presence of hydroxyl-enriched layers (Supporting Information, Figure S1a). A predominant peak component at the binding energy (BE) of 531.7 eV in the O 1s core-level spectrum is attributed to the hydroxide species (Supporting Information, Figure S1b), accompanied by two minor peak components of oxide (with BE at 530.1 eV) and water (with BE at 533.2 eV),52 indicative of the formation of a thin oxide and hydroxide film on the SS substrate surface. A uniform layer of ATRP initiator immobilized on the substrate surface is essential for tethering polymer brushes on the substrates via surface-initiated ATRP.35 Figure 2 shows the respective wide scan, Cl 2p, C 1s, and S 2p core-level spectra of CTCS-coupled surface (SS−Cl surface). The successful anchoring of CTCS initiator can be deduced from the appearance of six additional signals at the BEs of about 99, 151, 165, 199, 228, and 271 eV in the wide scan XPS spectrum (Figure 2a), attributable to the Si 2p, Si 2s, S 2p, Cl 2p, S 2s, and Cl 2s species,58 respectively, as compared to that of the SS−OH substrate surface (Supporting Information Figure S1a). The Cl 2p3/2 and Cl 2p1/2 spin−orbit split doublet at the BEs of about 200 and 201.6 eV is attributed to the covalently bonded chlorine (Figure 2b). The curve-fitted C 1s core-level spectrum is composed of three peak components at the BEs of 283.9, 284.6, and 286.2 eV (Figure 2c), attributable to C−Si, C−H, and C−S species,58 respectively. The area ratio of [C−Si]:[C− S]:[C−H] is around 0.97:1.0:6.1, comparable to the theoretical value of 1:1:6 for CTCS. The π−π* shakeup satellite at the BE of about 291 eV is associated with the aromatic ring of CTCS (Figure 2c). Moreover, the curve-fitted S 2p core-level 14742

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Figure 3. (a and b) Wide scan and N 1s core-level spectra of the SS-g-PVAn surface, (c and d) wide scan and N 1s core-level spectra of the SS-gPVAn-b-PANI surface after base treatment, and (e and f) N1s and Br 3d core-level spectra of the SS-g-PVAn-b-QPANI surface (the reaction conditions are given in Table 1).

3.3. Chemical Oxidative Graft Polymerization of Aniline and N-alkylation of Surface-Grafted Polyaniline (PANI). Chemical oxidative graft polymerization was employed to graft PANI layers on the aniline moieties of the grafted PVAn brushes to give rise to the PVAn-PANI bilayer coatings on the SS substrates. The wide scan, C 1s, and N 1s core-level XPS spectra of as-synthesized SS-g-VAn-b-PANI surface in HCl solution are shown in Figure S2 (Supporting Information). As shown in Figure S2b, the N 1s core-level spectrum of assynthesized SS-g-VAn-b-PANI surface can be curve-fitted into four peak components with BEs at 398.2, 399.4, and >400 eV, attributable to the quinonoid imine (N), benzenoid amine (NH), and positively charged nitrogen (N+) species, respectively.44 The doped PANI layer is derived from the protonation of imine in HCl solution during the chemical oxidative polymerization process. The surface-grafted PANI has been deprotonated by equilibrating in copious amounts of 0.1 M NaOH solution to covert the EM salt form to the neutral EM base form, as protonation−deprotonation in PANI is an equilbrium.60 Parts c and d of Figure 3 show the wide scan and

N 1s core-level spectra of the SS-g-PVAn-b-PANI surfaces after base treatment. As shown in Figure 3d, the curve-fitted N 1s core-level spectrum of undoped (deprotonated) PVAn-PANI bilayers is mainly composed of quinonoid imine (N) and benzenoid amine (NH), located at the BEs of 398.2 and 399.4 eV, respectively. The presence of about equal proportions of the quinonoid imine (N) and benzenoid amine ( NH) is consistent with the 50% intrinsically oxidized EM base form of PANI.44 The residual high BE tail (>400 eV) in the N 1s core-level spectrum is probably associated with the persistence of minor positively charged nitrogen (N+) and the surface oxidation products.44 The thickness of the PVAn-PANI bilayer coatings is about 114 ± 3 nm after the base treatment (Table 1). The grafting of the neutral EM base PANI layer on the PVAn-grafted SS surface results in the increase in surface hydrophilicity, as water contact angle of the SS-g-PVAn-g-PANI surface (after base treatment) increases to about 52 ± 3°. Therefore, the PANI layers (in neutral EM base form) were successfully grafted on the PVAn-grafted SS surfaces for the 14743

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Figure 4. Surface morphology (SEM images) of the (a) pristine SS, (b) SS-g-PVAn, and (c and d) SS-g-PVAn-b-PANI surfaces (the reaction conditions are given in Table 1).

3.4. Surface Morphology. The changes in surface morphology of SS substrates after graft polymerization with PVAn and PANI were studied by SEM. The representative SEM images of the pristine SS, SS-g-PVAn, and SS-g-PVAn-bPANI surfaces are shown in Figure 4. The pristine SS coupon has a relatively uniform and smooth surface, albeit with the presence of some scratches introduced during polishing (Figure 4a). The coverage of a dense organic layer is clearly observed after grafting of PVAn brushes, accompanying with the increase in surface roughness and the disappearance of polishing scratches (Figure 4b). This result further confirms successful grafting of PVAn from the initiator-immobilized substrate surfaces via surface-initiated ATRP. Interestingly, the SS substrates become rather smooth and homogeneous with the coverage of dense and compact PANI layers (Figure 4c), indicative of a uniform deposition process of PANI by in situ chemical oxidative graft polymerization of aniline from the SSg-PVAn surfaces. The thick PANI layers on the SS substrates can be distinguished from the edge profile of the SEM images of the SS-g-PVAn-b-PANI surfaces (Figure 4d). 3.5. Adhesion and Viability Assay of Bacteria on the Surface-Functionalized SS Substrates. Figure 5 shows SEM images of the pristine and functionalized SS substrates after immersion in a D. desulfuricans inoculated SSMB medium of 106 cells/ml for 3 and 30 days. Numerous bacterial cells, either individually or in small-size clusters, are distributed on the pristine SS surface after 3 days of exposure (Figure 5a). Upon prolonging exposure periods to 30 days, dense and lumpy deposits (including biofilms and corrosion products) can be observed on the pristine SS surface (Figure 5b). The results confirm that the pristine SS substrate is prone to bacterial adhesion and biofilm formation will occur readily in contact with bacteria. A good number of bacterial cells of D. desulfuricans are also discernible on the SS-g-PVAn surfaces after 3 days of exposure (Figure 5c), although the density of bacterial cells appears to be lower than that of the pristine SS substrates. The phenomenon is probably associated with the

subsequent antibacterial assays and corrosion inhibition analyses. To produce biocidal functionality on the PVAn-PANI bilayer coatings, the PANI base film on the SS-g-PVAn-b-PANI surface was quaternized by N-alkylation with hexyl bromide. The quaternized PVAn-PANI bilayer-coated SS surface is referred to as the SS-g-PVAn-b-QPANI surface. Parts e and f of Figure 3 show the N 1s and Br 3d core-level XPS spectra of the SS-gPVAn-b-QPANI surface. The curve-fitted N 1s core-level spectrum shows a significant decrease in the proportion of imine (N) units and a sharp increase in the proportion of positive charged nitrogen (N+) after N-alkylation with hexyl bromide as compared to that of the SS-g-PVAn-b-PANI surface before N-alkylation reaction, while the proportion of amine ( NH) species only shows a slightly decrease (Figure 3e). This result implies that alkylation occurs preferentially on the quinonoid imine nitrogen (N), instead of the benzenoid amine units (NH), of the EM base PANI, which is in good agreement with previous findings that the imine nitrogen is more reactive as a nucleophile than the amine nitrogen in the alkylation reaction.61 It has also been reported that the degree of alkylation of the full reduced state (i.e., leucoemeraldine or LM) PANI is quite low (