Polyelectrolyte Multilayer Deposition on Nickel Modified with Self

Article pubs.acs.org/JPCC

Polyelectrolyte Multilayer Deposition on Nickel Modified with SelfAssembled Monolayers of Organophosphonic Acids for Biomaterials: Electrochemical and Spectroscopic Evaluation Sébastien Devillers, Jean-François Lemineur, V. S. Dilimon, Bastien Barthélémy, Joseph Delhalle, and Zineb Mekhalif* Laboratory of Chemistry and Electrochemistry of Surfaces (CES), University of Namur (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium ABSTRACT: Layer-by-layer assembly of polyelectrolytes is an efficient method for the formation of functional thin films with the prospect of biomedical applications. Polyethyleneimine (PEI) is commonly used as an adhesion promoter but has been shown to be potentially cytotoxic and therefore could not be suitable for biomedical applications besides providing no or little corrosion protection to the underlying substrate. In the present work, we report on the use of a self-assembled phosphonic acid monolayer as a corrosion inhibitor and adhesion promoter for polyelectrolyte multilayer (PEM) formation on nickel substrates. The interest in using a phosphonic acid monolayer as an adhesion promoter for the deposition of polyelectrolyte multilayers lies in the robust grafting of the PEM adhesion promoter to the modified surface and the corrosion inhibition provided by this monolayer. This protection is crucial when considering the biodegradability of many polyelectrolytes used for biomedical applications. Nickel surfaces have thus been modified with an 11-methylundecanoatephosphonic acid monolayer. This monolayer has been shown to significantly improve the nickel corrosion resistance. Hydrolysis of the ester terminal function of this monolayer allowed the formation of negative charges on the modified nickel surfaces and therefore the successive deposition of chitosan and alginate layers.

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INTRODUCTION

on the formation of such polyelectrolytes multilayers (PEM) on nitinol.21−29 Polyethyleneimine (PEI) is commonly used as first layer in order to obtain an important charge density at the surface that is necessary for ulterior polyelectrolyte deposition.21,30,31 However, a recent study stresses that PEI is potentially cytotoxic and therefore could not be suitable for biomedical applications.23 This is an important consideration as it has been shown that natural polyelectrolytes such as chitosan and alginate can be degraded, for instance, by enzymes,32−35 therefore exposing the underlying polyelectrolyte layers (including PEI) to the biological environment. Furthermore, polyelectrolyte multilayers are commonly used as drug delivery systems.36−39 However, this drug delivery behavior is often based on the degradation of the PEM system that can be triggered by external factors such as pH variations, chemical degradation, and so forth again leaving the underlying substrate unprotected. Another common way used to create a protective organic coating on oxide surfaces such as nitinol is the formation of a self-assembled monolayer (SAM). Several publications report on the modification of the nitinol oxide layer with organo-

Nitinol is a nickel−titanium (∼50/50) shape memory alloy that has become a material of great interest given its many possible applications especially in the biomedical field.1−4 However, one of the major problems of this alloy is its high nickel content (often more than 50%) and the cytotoxic nature of nickel(II).5 Therefore, in recent years, a great deal of attention has been paid to the improvement of nitinol corrosion resistance and the limitation of Ni2+ ions release from the material but also to the improvement of the interaction between the material and the biological environment. In this context, many works have been published on the modification of the nitinol surface with organic coatings. A large part of these studies reports on the coating of nitinol with a polymeric layer6−19 on which an antithrombogenic agent such as heparin is often immobilized. Since its introduction by Decher et al. in 1992,20 the formation of layer-by-layer assembly of polyanions and polycations into multilayers has become a very popular method for the formation of functional thin films. The popularity of this method lies in its simplicity. Polymers carrying charged functional groups on their structure when dissolved in adequate solvent are called polyelectrolytes. These polyelectrolytes can be deposited on charged surfaces by successive electrostatic adsorptions of polyanions and polycations. In the past few years, several publications report also © XXXX American Chemical Society

Received: March 30, 2012 Revised: August 21, 2012

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dx.doi.org/10.1021/jp303056e | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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silanes40 and organophosphonic acids41−46 and the use of such covalently grafted organic monolayers as adhesion promoters or polymerization initiators for the formation of a polymeric layer.47−49 The tendency of organophosphonic acids to form monolayers on metal oxide surfaces50−52 and their important resistance to homocondensation, hydrolysis,53 and thermal decomposition54 make them interesting candidates for nitinol surface functionalizations. The present work reports on the use of a self-assembled phosphonic acid monolayer as an adhesion promoter for PEM systems. The interest in using an SAM as an adhesion promoter for PEM systems lies in the covalent binding of this SAM on the surface (compared to electrostatic interactions obtained with PEI) and the ability of SAM to provide a corrosion protection to the substrate independently from the PEM system. Considering the fact that nickel is the problematic element in nitinol, this study has been carried out on pure nickel surfaces used as a model substrate for Ni containing alloys. The ability of organophosphonic acids to form protective SAMs on nickel surfaces has already been shown by Quiñones et al.55 In the frame of this study, three different phosphonic acids have been used to modify nickel surfaces: n-dodecylphosphonic acid (PC12), 11-phosphonoundecanoic acid (PC11COOH), and 11methylundecanoatephosphonic acid (PC11COOMe). PC12 has no functional end group but has been used as a comparative system regarding corrosion protection. PC11COOH has been used with the prospect of covalently binding phosphonic chains bearing terminal carboxylic functional groups on the nickel substrates in order to promote the electrostatic adhesion of a first polycation layer. The grafting of bifunctional phosphonic and carboxylic acid molecules is often used for metal oxide surface functionalization. For instance, Kruszewski and Gawalt recently reported on the use of such bifunctional molecule SAMs for surface-initiated polymerization.49,56 However, because of its ability to bind surface via both terminal functions and therefore because of the existing competition between these two functions, this molecule has been reported to form complex systems of simply/doubly bound molecules.57,58 Accordingly, PC11COOMe has been used in order to avoid this competition and to obtain more close-packed and protective SAMs. Negative charges have been formed on the surface of these PC11COOMe monolayers by hydrolysis of the ester terminal functions. In this study, chitosan and alginate (two biopolymers and polysaccharides) have been chosen as polyelectrolytes for their biocompatibility. Chitosan is obtained by deacetylation of chitin. As this deacetylation is never complete,59 chitosan is often considered as a copolymer of N-acetylglucosamine and glucosamine (Figure 1).60 This deacytylation reaction allows an increase of the chitin solubility and reactivity by the appearance of amino groups.61 Despite this deacetylation, chitosan remains poorly soluble except in aqueous acidic solution or in some particular salt organic mixtures.62 This polysaccharide has numerous interesting properties such as its biocompatibility and biodegradability leading to film forming and immunologic, antitumoral, anticoagulant, or wound healing applications.59−61,63 Alginate has almost the same structure as chitosan (Figure 1). Both compounds have been the subject of intensive research in recent years64−69 mainly because of their availability and low price. Furthermore, chitosan and alginate are used to remove metal ions from wastewater. Their complexing properties could thus act as a barrier to prevent leaching of

Figure 1. Schematic representation of (A) sodium alginate, (B) chitosan, and (C) the different grafted phosphonic acid structures.

ions which could be an additional benefit for biomedical application.68,70

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EXPERIMENTAL METHODS Chemicals. Absolute ethanol (AnalaR NORMAPUR, analytical reagent), sodium hydroxide (98.5%, Acros Organics), sodium chloride (99.5%, Acros Organics), n-dodecylphosphonic acid (Alfa Aesar, 95%, H26259), 11-phosphonoundecanoic acid (Aldrich, 96%), 11-methylundecanoatephosphonic acid (Sikémia, >95%, SIK7503-10), alginic acid sodium salt (Aldrich), low molecular weight chitosan (Aldrich), potassium ferricyanide (99+%, Acros Organics), potassium ferrocyanide (98.5%, Acros Organics), and lithium perchlorate (99+%, Acros Organics) were used without further purification. All aqueous solutions were prepared with ultrapure milli-Q water (18.2 mΩ). Substrate Preparation. The nickel substrates (nickel foil 99.99% temper annealed 10 × 10 × 0.1 cm3 plates, Advent Research Materials Ltd. Gi647) were cut in 2 × 1 cm2 coupons and were mechanically polished down to 1 μm on a Buehler Phoenix 4000 instrument using various grit silicon carbide papers and diamond pastes. After the polishing step, the substrates were copiously rinsed with milli-Q water, were cleaned by sonication 15 min in ethanol, were flushed dry under a nitrogen flow, and were stored until their modification. Film Preparation and Characterization. Phosphonic acid monolayers were prepared by immersion of the nickel substrate in a 10 mM aqueous modification solution for 17 h at 90 °C. At the end of the immersion time, nickel substrates were copiously rinsed with ethanol, were cleaned by sonication 15 min in ethanol, were flushed dry under a nitrogen flow, and were characterized directly or were stored until their next modification step. The hydrolysis of the ester terminal functions in PC11COOMe monolayers has been carried out by immersion of the modified nickel substrates for 2 h in an aqueous sodium hydroxide solution (pH 11) at 90 °C. After this hydrolysis step, the samples were copiously rinsed with milli-Q water, were flushed dry under a nitrogen flow, and were characterized directly or were used for the deposition of a chitosan layer. Chitosan and alginate layers have been formed by 30 min immersion in aqueous deposition solutions at room temperB



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XPS was used to investigate the elemental composition of the formed layers. The photoelectron spectra have been recorded on an SSX-100 spectrometer using a monochromatized X-ray Al Kα radiation (1486.6 eV) with the photoemitted electrons being collected at a 35° takeoff angle. Nominal resolution was measured as full width at half-maximum of 1.0 and 1.5 eV for core levels and survey spectra, respectively. The binding energy of core levels was calibrated against the C1s binding energy set at 285.0 eV, an energy characteristic of alkyl moieties. The peaks were analyzed using mixed Gaussian−Lorentzian curves (80% of Gaussian character). Quantitative XPS analyses have been carried out by calculation of relevant abundance ratios. These ratios were calculated on the basis of the XPS peaksʼ experimental intensities (peaks area) taking into account the corresponding Scofield sensitivity factors (SF). The SEM characterizations have been carried out on a JEOL 7550 FEG-SEM microscope using an acceleration voltage of 15 kV. Each surface modification has been repeated at least three times for each characterization method in order to ensure the reproducibility of the presented results.

ature. Chitosan deposition solution is prepared by adding 50 mg of chitosan to 50 mL milli-Q water (a 0.1% wt solution) and by adding drips of a 1 M aqueous acetic acid solution until the complete chitosan dissolution (pH ∼ 4). Alginate deposition solution is prepared simply by adding 50 mg of sodium alginate to 50 mL milli-Q water (also a 0.1% wt solution). After each polyelectrolyte layer deposition, the modified substrates were copiously rinsed with milli-Q water and were flushed dry under a nitrogen flow. The films were characterized by cyclic voltammetry (CV), polarization curve measurements (LSV), electrochemical impedance spectroscopy (EIS), water static contact angle measurements, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Electrochemical techniques provide information on the monolayer and multilayer systems quality through coverage and resistance to oxidation. Experiments were carried out with an EG&G Instruments potensiostat, model 263A, monitored by computer and M270 electrochemistry software. A three-electrode electrochemical cell was used with a saturated calomel electrode (SCE) as reference electrode and a platinum foil as counter electrode. The cell used enables analysis of a well-defined and reproducible spot (0.28 cm 2 ) on the sample. Cyclic voltammetry was carried out in a 0.1 M sodium hydroxide solution by sweeping a range of potentials from 0.2 to 0.6 V versus SCE at 20 mV/s. Coverage has been calculated by measuring the area of the nickel oxidation peak (around 475 mV vs SCE) for unmodified substrates (Au) and for modified ones (Am) and by applying the following formula C(%) = 100 × (Au − Am)/Au. Polarization curve experiments were carried out in a 0.5 M sodium chloride solution by sweeping a range of potentials from −1 to 1 V versus SCE at 1 mV/s. Electrochemical impedance spectroscopic studies were carried out using a potentiostat/galvanostat (EG&G model 273) and a Solartron impedance gain-phase analyzer (model SI 1260). A platinum foil auxiliary electrode and an SCE reference were used. Again, a spot cell has been used in order to analyze a reproducible area (0.28 cm2) of the nickel working electrode. The measurements were carried out in the frequency range from 100 kHz to 0.1 Hz at an AC amplitude of 5 mV peak to peak. The electrolyte used was 1 mM potassium ferricyanide (K4[Fe(CN)6]) + 1 mM potassium ferrocyanide (K3[Fe(CN)6]) + 0.1 M lithium perchlorate (LiClO4) aqueous solution. Cyclic voltammetry measurements have been carried out in the same medium previous to each EIS measurement. These CV measurements have been carried out by scanning a range of potentials from −100 to 650 mV versus SCE at a scan rate of 20 mV/s in order to determine the formal potential of the Fe2+/Fe3+ redox reaction. The charge-transfer resistance was determined by analyzing the EIS data using an equivalent circuit. The impedance data were fitted with a circuit consisting of a series combination of solution resistance with a parallel connection of constant-phase element and charge-transfer resistance, which was found to give good fitting for the curves, particularly with the modified electrodes. Static contact angle measurements were carried out using a DIGIDROP (GBX Surface Science Technology) contact angle goniometer at room temperature. A syringe was used to dispense 2 μL of probe droplets of milli-Q water on the sample surface. The presented values and error bars are the mean values and standard deviations calculated on at least 15 measurements for each surface state, respectively.

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RESULTS AND DISCUSSION Phosphonic Acid Monolayers. As mentioned in the Introduction, this work starts with a comparison of three different phosphonic acid monolayers: PC12, PC11COOH, and PC11COOMe. These monolayers have been formed as described in the Experimental Methods, and their blocking factor has been assessed with CV measurements carried out in NaOH medium. Representative voltammograms are presented in Figure 2. As mentioned earlier, PC12 has no functional end

Figure 2. Voltammograms of a bare mechanically polished nickel (solid line) and nickel substrates modified by immersing 17 h at 90 °C in a 10 mM aqueous solution of 11-phosphonoundecanoic acid (dashed line), 11-methylundecanoatephosphonic acid (dotted line), and n-dodecylphosphonic acid (dashed−dotted line).

group but has been used for comparative purposes. This molecule leads to the formation of a monolayer with a very high coverage of the surface (91%). PC11COOH monolayers bring carboxylic functional groups at the surface of the nickel substrates and are therefore expected to promote the electrostatic adhesion of the first polycation layer. As expected, its ability to bind surface via both terminal functions and therefore the existing competition between these two C



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Figure 3. Representative XPS survey spectra of (a) a bare nickel substrate, (b) a nickel substrate modified with a monolayer of n-dodecylphosphonic acid, (c) 11-phosphonoundecanoic acid, and (d) 11-methylundecanoatephosphonic acid. Insets: corresponding P2p core level spectra.

functions57,58 leads to very poorly covering monolayers (only about 6%). PC11COOMe was expected to lead to the formation of more close-packed and protective SAMs because of the absence of this competition. This is confirmed by the obtained coverage value for these monolayers (60%) by far better than the one obtained for PC 11 COOH monolayers. These monolayers have been systematically characterized by XPS. The corresponding survey and P2p core level spectra are presented in Figure 3. For each of the modified substrates, P2p and P2s peaks appear (around 134 and 191 eV, respectively), and the relative intensity of the C1s peak centered around 285 eV significantly increases confirming the presence of the grafted molecules on the nickel surface. The impact of these monolayers on the nickel corrosion resistance has also been assessed by polarization curve measurements. Representative curves are presented in Figure 4, and the corresponding numerical values are presented in Table 1. It clearly appears that the most efficient protection against corrosion is obtained with a PC12 monolayer leading to a decrease of the corrosion current density from 2.8 × 10−7 (measured for a bare nickel surface) down to 6.3 × 10−8 A/cm2 and an anodic shift of the corrosion potential from −665 (for a bare nickel surface) to −365 mV versus SCE indicative of a mainly anodic corrosion inhibition. The weakest corrosion protection is obtained with a PC11COOH monolayer. This monolayer only leads to a very small decrease of the corrosion

Figure 4. Polarization curves of a bare mechanically polished nickel (solid line) and nickel substrates modified by immersing 17 h at 90 °C in a 10 mM aqueous solution of 11-phosphonoundecanoic acid (dashed line), 11-methylundecanoatephosphonic acid (dotted line), and n-dodecylphosphonic acid (dashed−dotted line).

current density (from 2.8 × 10−7 to 1.4 × 10−7 A/cm2) and a small anodic shift of the corrosion potential (from −665 to −620 mV vs SCE). An intermediate corrosion protection is D



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Table 1. Corrosion Potential and Corrosion Current Density Values of Bare Mechanically Polished Nickel and Nickel Substrates Modified by Immersing 17 h at 90 °C in a 10 mM Aqueous Solution of n-Dodecylphosphonic Acid (PC12), 11Phosphonoundecanoic Acid (PC11COOH), and 11Methylundecanoatephosphonic Acid before (PC11COOMe) and after the Hydrolysis Treatment (PC11COO−) Ecor (mV vs SCE) bare PC12 PC11COOH PC11COOMe PC11COO−

−665 −365 −620 −360 −340

icor (A/cm2) 2.8 6.3 1.4 8.9 4.6

× × × × ×

10−7 10−8 10−7 10−8 10−8

obtained with the PC11COOMe monolayer, that is, a significant decrease of the current density (from 2.8 × 10−7 to 8.9 × 10−8 A/cm2) and an anodic shift of the corrosion potential (from −665 to −360 mV vs SCE). This confirms again the better coverage obtained with this PC11COOMe monolayer than with the PC11COOH one because of the absence of adsorption competition between the two terminal groups of the molecule. The hydrolysis of the terminal ester functions of this 11methylundecanoatephosphonic acid monolayer has been carried out as described in the Experimental Methods. This monolayer has been analyzed with XPS before and after this hydrolysis reaction in order to ensure that the grafted phosphonic acid molecules remain on the nickel surface. With this prospect, the P2p/Ni2p ratio has been calculated on the basis of these analyses. It appears that this ratio remains around 0.16 and thus is not essentially affected by the hydrolysis treatment. Furthermore, the water contact angle measurements carried out on these modified nickel substrates clearly indicate that the applied hydrolysis treatment significantly decreases the contact angle value (from 95° down to 60°) and thus increases the hydrophilicity of the surface. Therefore, it can be assumed that the hydrolysis method applied to the 11-methylundecanoatephosphonic acid monolayer efficiently changes the terminal ester functions into terminal carboxylic functions nearly without affecting the phosphonic grafting. The hydrolyzed 11-methylundecanoatephosphonic acid monolayers have been analyzed with CV in NaOH medium (see Figure 5). It appears that the hydrolysis treatment induces a significant increase of the blocking factor from 60% to 84%. Two hypotheses can be brought to explain this behavior. The first one is that the removal of the terminal methyl group decreases the steric hindrance of the grafted molecules end functions allowing a better close packing and organization of the monolayer during the 2 h immersion at 90 °C of the hydrolysis treatment. The second one is that this 2 h immersion at 90 °C in a pH 11 NaOH solution most probably induces a reinforcement of the nickel oxide layer in the monolayer defect areas similarly to a hydrothermal treatment commonly applied to nitinol.46 To check this latter hypothesis, CV analysis of nickel substrates immersed for 17 h in milli-Q water at 90 °C and 2 h in an aqueous sodium hydroxide solution (pH 11) at 90 °C has been carried out (Figure 6). It clearly appears that these two immersion steps induce a significant decrease of the nickel oxidation peak area (i.e., a blocking factor of 57%) confirming the supposed partial reinforcement of the oxide layer. However, this oxidation peak intensity decrease is much lower than the one obtained with the hydrolyzed 11-methylundecanoatephosphonic acid monolayer.

Figure 5. Voltammograms of a bare mechanically polished nickel (solid line) and nickel substrates modified by immersing 17 h at 90 °C in a 10 mM aqueous solution of 11-methylundecanoatephosphonic acid before (dashed line) and after (dotted line) the hydrolysis of the terminal ester functions by immersion of the modified nickel substrates in a sodium hydroxide aqueous solution (pH 11) for 2 h at 90 °C.

Figure 6. Voltammograms of a bare mechanically polished nickel (solid line) and nickel substrates immersed for 17 h at 90 °C in ultrapure water and in a sodium hydroxide aqueous solution (pH 11) for 2 h at 90 °C (dashed line).

The blocking factor increase after hydrolysis treatment can thus most probably be explained by a combination of both effects: on the one hand, a decrease of the terminal group steric hindrance allowing a better close-packing of the monolayer and, on the other hand, a reinforcement of the nickel oxide layer at the surface areas that may be less efficiently protected by the monolayer. Thus, the hydrolyzed PC11COOMe monolayer (noted PC11COO−) is expected to provide a better corrosion inhibition than the nonhydrolyzed one. This has been confirmed by polarization curve measurements (Figure 7 and Table 1). Indeed, it clearly appears that the hydrolysis treatment induces a further decrease of the corrosion current density. The anodic part of the polarization curves for the hydrolyzed samples shows interesting features: the anodic current is drastically decreased after hydrolysis reaction while E



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as described in the Experimental Methods. Each step of this polyelectrolyte multilayer (PEM) formation has been followed by contact angle measurements. For comparison, the same PEM system has been deposited on bare nickel substrates in order to compare the contact angle variations and therefore the polyelectrolyte deposition efficiency (Figure 8). First of all, it clearly appears that the water contact angle variations are very weak when polyelectrolytes are deposited on a bare nickel surface indicating poor deposition efficiency and therefore demonstrating the necessity of an adhesion promoter. On the other hand, when the deposition is carried out on a hydrolyzed PC11COOMe monolayer, the contact angle variations are really significant. This indicates variations of the surface state and thus the correct deposition of the different chitosan and alginate layers. XPS analysis of these surfaces has been carried out systematically. XPS survey spectra of surface states obtained after the three main steps of this surface modification procedure are presented in Figure 9. Only the nickel, oxygen, and carbon peaks are visible on the bare polished nickel surface spectrum (carbon peak being attributed to the presence of some physisorbed atmospheric contamination). On the XPS spectrum of the nickel surface after its modification with an 11-methylundecanoatephosphonic acid monolayer, the appearance of phosphorus peaks can be observed confirming again the presence of the monolayer at the surface. After hydrolysis, the phosphorus peaksʼ relative intensity remains constant indicating the resistance of the anchoring phosphonic acid groups to the hydrolysis treatment as shown previously. Finally, after the LbL deposition of a chitosan/alginate PEM system (seven layers), the peaks corresponding to the underlying surface (i.e., nickel) and SAM (i.e., phosphorus) completely disappear from the XPS survey spectrum. Actually, only the peaks corresponding to the deposited polyelectrolytes, namely, C1s, N1s, and O1s, are visible. On the one hand, this confirms the presence of the PEM system on the nickel surface and, on the other hand, indicates that the thickness of the deposited PEM is higher than

Figure 7. Polarization curves of a bare mechanically polished nickel (solid line) and nickel substrates modified by immersing 17 h at 90 °C in a 10 mM aqueous solution of 11-methylundecanoatephosphonic acid before (dashed line) and after (dotted line) the hydrolysis of the terminal ester functions by immersion of the modified nickel substrates in a sodium hydroxide aqueous solution (pH 11) for 2 h at 90 °C.

the pitting corrosion potential is shifted toward more anodic values (from 30 to 140 mV vs SCE for PC11COOMe modified nickel surfaces before and after hydrolysis, respectively). This electrochemical behavior reinforces our previous hypothesis, that is, that the immersion of modified nickel substrates in alkaline aqueous solution for 2 h at 90 °C induces a reinforcement of the nickel oxide layer at the surface areas that could be less efficiently protected by the monolayer. Layer-by-Layer (LbL) Deposition. After the hydrolysis of the ester terminal functions on the grafted PC11COOMe monolayer, LbL polyelectrolyte deposition has been carried out

Figure 8. Water contact angle evolution during the elaboration of chitosan/alginate multilayer system on (A) a bare polished nickel surface and (B) a nickel surface modified with a hydrolyzed 11-methylundecanoatephosphonic acid monolayer. Ni = bare polished nickel surface; S = PC11COOH SAM; hS = hydrolyzed PC11COOH SAM; L1 = first polyelectrolyte layer (chitosan); L2 = second polyelectrolyte layer (alginate); L3 = third polyelectrolyte layer (chitosan); L4 = fourth polyelectrolyte layer (alginate); L5 = fifth polyelectrolyte layer (chitosan); L6 = sixth polyelectrolyte layer (alginate); and L7 = seventh polyelectrolyte layer (chitosan). F



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Figure 9. XPS survey spectrum of (A) a bare polished nickel substrate, (B) a nickel surface modified with a hydrolyzed 11methylundecanoatephosphonic acid monolayer, and (C) a nickel surface modified with a hydrolyzed 11-methylundecanoatephosphonic acid monolayer covered with seven polyelectrolyte layers (chitosan/alginate).

Figure 10. N1s (left) and C1s (right) core level XPS spectrum of a nickel surface modified with a hydrolyzed 11-methylundecanoatephosphonic acid monolayer covered with seven polyelectrolyte layers (chitosan/alginate).

atmospheric contaminations. The assignment of the three PEM related contributions is presented in Table 2. The morphology of this PEM system has been characterized by SEM (Figure 11) using two different imaging modes, that is, lower secondary electron imaging (LEI, providing a nearly

the analysis depth of the spectrometer, that is, about 10 nm. The XPS spectra of the C1s and N1s core levels photoelectrons have been acquired for this PEM system (Figure 10). The N1s XPS spectrum can be analyzed with three components: a first one centered around 399.6 eV attributed to nitrogen from amine functions of chitosan, a second one at 400.6 eV that is attributed to nitrogen atoms from amide functions, and a last one centered at 402.0 eV attributed to protonated amine functions. The N1s peak attributed to amide functions is about 21% of the total nitrogen signal which is in good agreement with the degree of deacetylation indicated by the chitosan provider (i.e., between 75 and 85%). The C1s core level XPS spectrum has been analyzed with four components that can be attributed to the different carbon atoms present in the alginate and chitosan structures except for the one centered at 285 eV that is attributed to the presence of some physisorbed

Table 2. Attribution of the C1s XPS Spectrum Components (Figure 10) to the Different Carbon Atoms Present in the Structure of Chitosan and Alginate (Figure 1)

G

binding energy (eV)

corresponding carbon atom number

285 286.6 288.1 289.4

atmospheric contaminations C3 C2 C1 dx.doi.org/10.1021/jp303056e | J. Phys. Chem. C XXXX, XXX, XXX−XXX


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Figure 11. Characteristic SEM pictures of the PEM (chitosan/alginate) system deposited on a hydrolyzed C11COOMe monolayer obtained in LEI (lower secondary electron image, A and C) and in SEI (secondary electron imaging, B and D) modes (PEM means polyelectrolyte multilayer) (scale bars = 10 μm).

confirming our previous results. Furthermore, the hydrolysis of this PC11COOMe monolayer and its coverage with PEM system leads to a further improvement of this trend to a nearly complete blocking of these reactions. Besides the blocking of the Fe2+/Fe3+ redox reaction, the oxidation reaction of Ni to Ni2+ also appears to be blocked by the studied coatings. Figure 12 shows the electrochemical impedance analyses performed in the same solution at the formal potential of Fe2+/Fe3+ redox couple in this medium, that is, 210 mV versus SCE. First, the recorded spectra are depressed semicircles, and a constant phase element instead of a double layer capacitance in Randles circuit gave a good fitting. The depressed nature of the semicircles can be attributed to the important roughness of the electrode surface. Indeed, the mechanically polished polycrystalline nickel electrodes that were used as substrates for these coatings preparation have a mean roughness of 50 nm (measured with a DEKTAK contact profilometer). The bare Ni electrode shows a charge-transfer resistance of around 1740 Ω. Furthermore, the impedance spectrum for bare Ni shows a Warburg behavior at the low-frequency region. This observation indicates a diffusion-controlled process. The nickel substrates modified with a PC11COOH monolayer and with a PC11COOMe monolayer and the hydrolyzed PC11COOMe monolayer covered with a PEM (chitosan/alginate) do not show any Warburg behavior indicating a significant blocking of the redox reaction at the surface. The charge-transfer resistance measured for the nickel substrates modified with a PC11COOH

exclusively morphological contrast) and secondary electron imaging (SEI, providing a morphological and elemental contrast). It appears that the obtained surface is highly homogeneous and apparently smooth while presenting some structural imperfections. The observed samples have been scratched with a scalpel in order to obtain a very rough estimation of the PEM system thickness, that is, a few micrometers. Obviously, the number of seven polyelectrolyte layers has been chosen arbitrarily and can be changed in order to adapt the PEM thickness to the desired value. Finally, further electrochemical characterizations have been carried out on bare nickel substrates, nickel substrates modified with a PC11COOH monolayer, nickel substrates modified with a PC11COOMe monolayer, and nickel substrates modified with a hydrolyzed PC11COOMe monolayer covered with a PEM (chitosan/alginate). These nickel surface modifications are expected to lead to a higher charge transfer resistance. This was followed by voltammetry and impedance measurements. Figure 12 inset shows the voltammograms recorded with all these electrodes in the presence of 1 mM potassium ferricyanide + 1 mM potassium ferrocyanide with lithium perchlorate as supporting electrolyte. The anodic peak current (Ipa) and the cathodic peak current (Ipc) observed on modified nickel electrodes are significantly lower than that with bare nickel electrode. The blocking appears to be much more important when the nickel substrate is modified with a PC11COOMe monolayer than when modified with a PC11COOH monolayer H



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protection. PC11COOH monolayers bring carboxylic functional groups at the surface of the nickel substrates and are therefore expected to promote the electrostatic adhesion of the first polycation layer. However, because of the existing competition between the two terminal functions of this molecule, the resulting monolayers provide the lowest surface coverage and corrosion inhibition and have therefore been considered as unsuitable for the aimed purpose. On the contrary, PC11COOMe monolayers provide a much better surface coverage and corrosion resistance. These monolayers have been submitted to a hydrolysis treatment in order to form free carboxylate groups at its surface. This hydrolysis treatment has been shown to be efficient in forming negative charges at the monolayer surface and also to further improve the resulting corrosion resistance of the modified substrates. This has been explained by two different phenomena, that is, a reorganization of the monolayer (better close-packing because of the decreased steric hindrance of the terminal function) and a reinforcement of the underlying nickel oxide layer during the hydrolysis treatment. This hydrolyzed monolayer has then been shown to efficiently promote the adhesion of PEM system by studying the deposition of seven chitosan and alginate layers. The obtained polyelectrolyte covered surface is highly homogeneous and apparently smooth while presenting some structural imperfections. Nevertheless, EIS measurements have shown the efficiency of this PEM system to further increase the chargetransfer resistance of the resulting surface. Thus, the results of this study clearly show for the first time the double interest in using phosphonic acid SAMs as adhesion promoters for the deposition of polyelectrolyte multilayers, that is, the covalent grafting of the PEM adhesion promoter to the modified surface and the corrosion inhibition provided by this SAM that is crucial when considering the biodegradability of many polyelectrolytes used for biomedical applications.

Figure 12. Impedance spectra (Nyquist plot) of a bare nickel electrode (solid line), a nickel substrate modified with a PC11COOH monolayer (dashed line), a PC11COOMe monolayer (dotted line), and a hydrolyzed PC11COOMe monolayer covered with a PEM (dashed− dotted line) recorded in 1 mM potassium ferricyanide (K4[Fe(CN)6]) + 1 mM potassium ferrocyanide (K3[Fe(CN)6]) + 0.1 M lithium perchlorate (LiClO4) aqueous solution. The surface area of the electrode is 0.28 cm2. Inset: cyclic voltammograms recorded in the same medium for these four surfaces with a scan rate of 20 mV/s.

monolayer, with a PC11COOMe monolayer, and the hydrolyzed PC11COOMe monolayer covered with a PEM (chitosan/ alginate) was 4485, 7980, and 14 453 Ω, respectively. First, these results confirm the trend observed previously regarding the PC11COOH versus PC11COOMe monolayers, that is, the resulting protective properties are much higher when PC11COOMe is used because of the absence of any competition between the two terminal functions during selfassembly. The increase in the charge-transfer resistance that comes with the coverage of hydrolyzed PC11COOMe monolayer with PEM indicates that the final state of this surface modification method is expected to efficiently prevent electrochemical species to reach the nickel surface.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +32-(0)81-72 52 30. Fax: +32-(0)81-72 46 00. E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Since layer-by-layer assembly of polyanions and polycations into multilayers has been introduced, it has become a very popular method for the formation of functional thin films with the prospect of biomedical applications among others. Polyethyleneimine (PEI) is commonly used as an adhesion promoter but has been shown to be potentially cytotoxic and is therefore not suitable for biomedical applications besides providing no or little corrosion inhibition to the underlying substrate. In the present work, we report on the use of a selfassembled phosphonic acid monolayer as corrosion inhibitor and adhesion promoter for polyelectrolyte multilayer (PEM) formation on nickel substrates. First, three different phosphonic acid monolayers have been compared: n-dodecylphosphonic acid (PC12), 11-phosphonoundecanoic acid (PC11COOH), and 11-methylundecanoatephosphonic acid (PC11COOMe). PC12 monolayers have been shown to provide the highest surface coverage and corrosion inhibition. However, PC12 has no functional end group and has thus been used as a comparative system regarding corrosion

ACKNOWLEDGMENTS V.S.D. acknowledges FUNDP-CERUNA for the postdoctoral fellowship.

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Polyelectrolyte Multilayer Deposition on Nickel Modified with Self

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