Polyelectrolyte Multilayer Deposition on Nickel Modified with Self-Assembled Monolayers of Organophosphonic Acids for Biomaterials:Electrochemical and Spectroscopic EvaluationSebastien Devillers, Jean-Francois Lemineur, V. S. Dilimon, Bastien Barthelemy, 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 isan efficient method for the formation of functional thin filmswith the prospect of biomedical applications. Polyethylenei-mine (PEI) is commonly used as an adhesion promoter buthas been shown to be potentially cytotoxic and therefore couldnot be suitable for biomedical applications besides providingno or little corrosion protection to the underlying substrate. Inthe present work, we report on the use of a self-assembledphosphonic acid monolayer as a corrosion inhibitor andadhesion promoter for polyelectrolyte multilayer (PEM)formation on nickel substrates. The interest in using a phosphonic acid monolayer as an adhesion promoter for the depositionof polyelectrolyte multilayers lies in the robust grafting of the PEM adhesion promoter to the modified surface and the corrosioninhibition provided by this monolayer. This protection is crucial when considering the biodegradability of many polyelectrolytesused for biomedical applications. Nickel surfaces have thus been modified with an 11-methylundecanoatephosphonic acidmonolayer. This monolayer has been shown to significantly improve the nickel corrosion resistance. Hydrolysis of the esterterminal function of this monolayer allowed the formation of negative charges on the modified nickel surfaces and therefore thesuccessive deposition of chitosan and alginate layers.

■ INTRODUCTION

Nitinol is a nickel−titanium (∼50/50) shape memory alloy thathas become a material of great interest given its many possibleapplications especially in the biomedical field.1−4 However, oneof 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 beenpaid to the improvement of nitinol corrosion resistance and thelimitation of Ni2+ ions release from the material but also to theimprovement of the interaction between the material and thebiological environment.In this context, many works have been published on the

modification of the nitinol surface with organic coatings. A largepart of these studies reports on the coating of nitinol with apolymeric layer6−19 on which an antithrombogenic agent suchas heparin is often immobilized. Since its introduction byDecher et al. in 1992,20 the formation of layer-by-layerassembly of polyanions and polycations into multilayers hasbecome a very popular method for the formation of functionalthin films. The popularity of this method lies in its simplicity.Polymers carrying charged functional groups on their structurewhen dissolved in adequate solvent are called polyelectrolytes.These polyelectrolytes can be deposited on charged surfaces bysuccessive electrostatic adsorptions of polyanions and poly-cations. In the past few years, several publications report also

on the formation of such polyelectrolytes multilayers (PEM) onnitinol.21−29

Polyethyleneimine (PEI) is commonly used as first layer inorder to obtain an important charge density at the surface thatis necessary for ulterior polyelectrolyte deposition.21,30,31

However, a recent study stresses that PEI is potentiallycytotoxic and therefore could not be suitable for biomedicalapplications.23 This is an important consideration as it has beenshown that natural polyelectrolytes such as chitosan andalginate 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 deliverysystems.36−39 However, this drug delivery behavior is oftenbased on the degradation of the PEM system that can betriggered by external factors such as pH variations, chemicaldegradation, and so forth again leaving the underlying substrateunprotected.Another common way used to create a protective organic

coating on oxide surfaces such as nitinol is the formation of aself-assembled monolayer (SAM). Several publications reporton the modification of the nitinol oxide layer with organo-

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

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silanes40 and organophosphonic acids41−46 and the use of suchcovalently grafted organic monolayers as adhesion promoters orpolymerization initiators for the formation of a polymericlayer.47−49 The tendency of organophosphonic acids to formmonolayers on metal oxide surfaces50−52 and their importantresistance to homocondensation, hydrolysis,53 and thermaldecomposition54 make them interesting candidates for nitinolsurface functionalizations.The present work reports on the use of a self-assembled

phosphonic acid monolayer as an adhesion promoter for PEMsystems. The interest in using an SAM as an adhesion promoterfor PEM systems lies in the covalent binding of this SAM onthe surface (compared to electrostatic interactions obtainedwith PEI) and the ability of SAM to provide a corrosionprotection to the substrate independently from the PEMsystem.Considering the fact that nickel is the problematic element in

nitinol, this study has been carried out on pure nickel surfacesused as a model substrate for Ni containing alloys. The abilityof organophosphonic acids to form protective SAMs on nickelsurfaces has already been shown by Quinones et al.55 In theframe of this study, three different phosphonic acids have beenused to modify nickel surfaces: n-dodecylphosphonic acid(PC12), 11-phosphonoundecanoic acid (PC11COOH), and 11-methylundecanoatephosphonic acid (PC11COOMe). PC12 hasno functional end group but has been used as a comparativesystem regarding corrosion protection. PC11COOH has beenused with the prospect of covalently binding phosphonic chainsbearing terminal carboxylic functional groups on the nickelsubstrates in order to promote the electrostatic adhesion of afirst polycation layer. The grafting of bifunctional phosphonicand carboxylic acid molecules is often used for metal oxidesurface functionalization. For instance, Kruszewski and Gawaltrecently reported on the use of such bifunctional moleculeSAMs for surface-initiated polymerization.49,56 However,because of its ability to bind surface via both terminal functionsand therefore because of the existing competition betweenthese two functions, this molecule has been reported to formcomplex systems of simply/doubly bound molecules.57,58

Accordingly, PC11COOMe has been used in order to avoidthis competition and to obtain more close-packed andprotective SAMs. Negative charges have been formed on thesurface of these PC11COOMe monolayers by hydrolysis of theester terminal functions.In this study, chitosan and alginate (two biopolymers and

polysaccharides) have been chosen as polyelectrolytes for theirbiocompatibility. Chitosan is obtained by deacetylation ofchitin. As this deacetylation is never complete,59 chitosan isoften considered as a copolymer of N-acetylglucosamine andglucosamine (Figure 1).60 This deacytylation reaction allows anincrease of the chitin solubility and reactivity by the appearanceof amino groups.61 Despite this deacetylation, chitosan remainspoorly soluble except in aqueous acidic solution or in someparticular salt organic mixtures.62 This polysaccharide hasnumerous interesting properties such as its biocompatibilityand biodegradability leading to film forming and immunologic,antitumoral, anticoagulant, or wound healing applica-tions.59−61,63 Alginate has almost the same structure as chitosan(Figure 1). Both compounds have been the subject of intensiveresearch in recent years64−69 mainly because of their availabilityand low price. Furthermore, chitosan and alginate are used toremove metal ions from wastewater. Their complexingproperties could thus act as a barrier to prevent leaching of

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

■ EXPERIMENTAL METHODSChemicals. Absolute ethanol (AnalaR NORMAPUR,

analytical reagent), sodium hydroxide (98.5%, Acros Organics),sodium chloride (99.5%, Acros Organics), n-dodecylphos-phonic acid (Alfa Aesar, 95%, H26259), 11-phosphonounde-canoic acid (Aldrich, 96%), 11-methylundecanoatephosphonicacid (Sikemia, >95%, SIK7503-10), alginic acid sodium salt(Aldrich), low molecular weight chitosan (Aldrich), potassiumferricyanide (99+%, Acros Organics), potassium ferrocyanide(98.5%, Acros Organics), and lithium perchlorate (99+%, AcrosOrganics) were used without further purification. All aqueoussolutions were prepared with ultrapure milli-Q water (18.2mΩ).

Substrate Preparation. The nickel substrates (nickel foil99.99% temper annealed 10 × 10 × 0.1 cm3 plates, AdventResearch Materials Ltd. Gi647) were cut in 2 × 1 cm2 couponsand were mechanically polished down to 1 μm on a BuehlerPhoenix 4000 instrument using various grit silicon carbidepapers and diamond pastes. After the polishing step, thesubstrates were copiously rinsed with milli-Q water, werecleaned by sonication 15 min in ethanol, were flushed dryunder a nitrogen flow, and were stored until their modification.

Film Preparation and Characterization. Phosphonic acidmonolayers were prepared by immersion of the nickel substratein a 10 mM aqueous modification solution for 17 h at 90 °C. Atthe end of the immersion time, nickel substrates were copiouslyrinsed with ethanol, were cleaned by sonication 15 min inethanol, were flushed dry under a nitrogen flow, and werecharacterized directly or were stored until their nextmodification step.The hydrolysis of the ester terminal functions in

PC11COOMe monolayers has been carried out by immersionof the modified nickel substrates for 2 h in an aqueous sodiumhydroxide solution (pH 11) at 90 °C. After this hydrolysis step,the samples were copiously rinsed with milli-Q water, wereflushed dry under a nitrogen flow, and were characterizeddirectly 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 temper-

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

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ature. Chitosan deposition solution is prepared by adding 50mg 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 untilthe complete chitosan dissolution (pH ∼ 4). Alginatedeposition solution is prepared simply by adding 50 mg ofsodium alginate to 50 mL milli-Q water (also a 0.1% wtsolution). After each polyelectrolyte layer deposition, themodified substrates were copiously rinsed with milli-Q waterand were flushed dry under a nitrogen flow.The films were characterized by cyclic voltammetry (CV),

polarization curve measurements (LSV), electrochemicalimpedance spectroscopy (EIS), water static contact anglemeasurements, X-ray photoelectron spectroscopy (XPS), andscanning electron microscopy (SEM). Electrochemical techni-ques provide information on the monolayer and multilayersystems quality through coverage and resistance to oxidation.Experiments were carried out with an EG&G Instrumentspotensiostat, model 263A, monitored by computer and M270electrochemistry software. A three-electrode electrochemicalcell was used with a saturated calomel electrode (SCE) asreference electrode and a platinum foil as counter electrode.The cell used enables analysis of a well-defined andreproducible spot (0.28 cm2) on the sample. Cyclicvoltammetry was carried out in a 0.1 M sodium hydroxidesolution by sweeping a range of potentials from 0.2 to 0.6 Vversus SCE at 20 mV/s. Coverage has been calculated bymeasuring the area of the nickel oxidation peak (around 475mV vs SCE) for unmodified substrates (Au) and for modifiedones (Am) and by applying the following formula C(%) = 100× (Au − Am)/Au. Polarization curve experiments were carriedout in a 0.5 M sodium chloride solution by sweeping a range ofpotentials from −1 to 1 V versus SCE at 1 mV/s.Electrochemical impedance spectroscopic studies were

carried out using a potentiostat/galvanostat (EG&G model273) and a Solartron impedance gain-phase analyzer (model SI1260). A platinum foil auxiliary electrode and an SCE referencewere used. Again, a spot cell has been used in order to analyze areproducible area (0.28 cm2) of the nickel working electrode.The measurements were carried out in the frequency rangefrom 100 kHz to 0.1 Hz at an AC amplitude of 5 mV peak topeak. 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) aqueoussolution. Cyclic voltammetry measurements have been carriedout in the same medium previous to each EIS measurement.These CV measurements have been carried out by scanning arange of potentials from −100 to 650 mV versus SCE at a scanrate of 20 mV/s in order to determine the formal potential ofthe Fe2+/Fe3+ redox reaction. The charge-transfer resistancewas determined by analyzing the EIS data using an equivalentcircuit. The impedance data were fitted with a circuit consistingof a series combination of solution resistance with a parallelconnection of constant-phase element and charge-transferresistance, 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 anglegoniometer at room temperature. A syringe was used todispense 2 μL of probe droplets of milli-Q water on the samplesurface. The presented values and error bars are the meanvalues and standard deviations calculated on at least 15measurements for each surface state, respectively.

XPS was used to investigate the elemental composition of theformed layers. The photoelectron spectra have been recordedon an SSX-100 spectrometer using a monochromatized X-rayAl Kα radiation (1486.6 eV) with the photoemitted electronsbeing collected at a 35° takeoff angle. Nominal resolution wasmeasured as full width at half-maximum of 1.0 and 1.5 eV forcore levels and survey spectra, respectively. The binding energyof core levels was calibrated against the C1s binding energy setat 285.0 eV, an energy characteristic of alkyl moieties. Thepeaks were analyzed using mixed Gaussian−Lorentzian curves(80% of Gaussian character). Quantitative XPS analyses havebeen carried out by calculation of relevant abundance ratios.These ratios were calculated on the basis of the XPS peaksʼexperimental intensities (peaks area) taking into account thecorresponding Scofield sensitivity factors (SF).The SEM characterizations have been carried out on a JEOL

7550 FEG-SEM microscope using an acceleration voltage of 15kV.Each surface modification has been repeated at least three

times for each characterization method in order to ensure thereproducibility of the presented results.

■ RESULTS AND DISCUSSIONPhosphonic Acid Monolayers. As mentioned in the

Introduction, this work starts with a comparison of threedifferent phosphonic acid monolayers: PC12, PC11COOH, andPC11COOMe. These monolayers have been formed asdescribed in the Experimental Methods, and their blockingfactor has been assessed with CV measurements carried out inNaOH medium. Representative voltammograms are presentedin Figure 2. As mentioned earlier, PC12 has no functional end

group but has been used for comparative purposes. Thismolecule leads to the formation of a monolayer with a very highcoverage of the surface (91%). PC11COOH monolayers bringcarboxylic functional groups at the surface of the nickelsubstrates and are therefore expected to promote theelectrostatic adhesion of the first polycation layer. As expected,its ability to bind surface via both terminal functions andtherefore the existing competition between these two

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

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functions57,58 leads to very poorly covering monolayers (onlyabout 6%). PC11COOMe was expected to lead to the formationof more close-packed and protective SAMs because of theabsence of this competition. This is confirmed by the obtainedcoverage value for these monolayers (60%) by far better thanthe one obtained for PC11COOH monolayers. Thesemonolayers have been systematically characterized by XPS.The corresponding survey and P2p core level spectra arepresented in Figure 3. For each of the modified substrates, P2pand P2s peaks appear (around 134 and 191 eV, respectively),and the relative intensity of the C1s peak centered around 285eV significantly increases confirming the presence of the graftedmolecules on the nickel surface.The impact of these monolayers on the nickel corrosion

resistance has also been assessed by polarization curvemeasurements. Representative curves are presented in Figure4, and the corresponding numerical values are presented inTable 1. It clearly appears that the most efficient protectionagainst corrosion is obtained with a PC12 monolayer leading toa 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 abare nickel surface) to −365 mV versus SCE indicative of amainly anodic corrosion inhibition. The weakest corrosionprotection is obtained with a PC11COOH monolayer. Thismonolayer only leads to a very small decrease of the corrosion

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

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

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

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obtained with the PC11COOMe monolayer, that is, a significantdecrease 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 bettercoverage obtained with this PC11COOMe monolayer than withthe PC11COOH one because of the absence of adsorptioncompetition between the two terminal groups of the molecule.The hydrolysis of the terminal ester functions of this 11-

methylundecanoatephosphonic acid monolayer has beencarried out as described in the Experimental Methods. Thismonolayer has been analyzed with XPS before and after thishydrolysis reaction in order to ensure that the graftedphosphonic acid molecules remain on the nickel surface.With this prospect, the P2p/Ni2p ratio has been calculated onthe basis of these analyses. It appears that this ratio remainsaround 0.16 and thus is not essentially affected by thehydrolysis treatment. Furthermore, the water contact anglemeasurements carried out on these modified nickel substratesclearly indicate that the applied hydrolysis treatment signifi-cantly decreases the contact angle value (from 95° down to60°) and thus increases the hydrophilicity of the surface.Therefore, it can be assumed that the hydrolysis methodapplied to the 11-methylundecanoatephosphonic acid mono-layer efficiently changes the terminal ester functions intoterminal carboxylic functions nearly without affecting thephosphonic grafting. The hydrolyzed 11-methylundecanoate-phosphonic acid monolayers have been analyzed with CV inNaOH medium (see Figure 5). It appears that the hydrolysistreatment induces a significant increase of the blocking factorfrom 60% to 84%. Two hypotheses can be brought to explainthis behavior. The first one is that the removal of the terminalmethyl group decreases the steric hindrance of the graftedmolecules end functions allowing a better close packing andorganization of the monolayer during the 2 h immersion at 90°C of the hydrolysis treatment. The second one is that this 2 himmersion at 90 °C in a pH 11 NaOH solution most probablyinduces a reinforcement of the nickel oxide layer in themonolayer defect areas similarly to a hydrothermal treatmentcommonly applied to nitinol.46 To check this latter hypothesis,CV analysis of nickel substrates immersed for 17 h in milli-Qwater at 90 °C and 2 h in an aqueous sodium hydroxidesolution (pH 11) at 90 °C has been carried out (Figure 6). Itclearly appears that these two immersion steps induce asignificant decrease of the nickel oxidation peak area (i.e., ablocking factor of 57%) confirming the supposed partialreinforcement of the oxide layer. However, this oxidation peakintensity decrease is much lower than the one obtained with thehydrolyzed 11-methylundecanoatephosphonic acid monolayer.

The blocking factor increase after hydrolysis treatment can thusmost probably be explained by a combination of both effects:on the one hand, a decrease of the terminal group sterichindrance allowing a better close-packing of the monolayer and,on the other hand, a reinforcement of the nickel oxide layer atthe surface areas that may be less efficiently protected by themonolayer.Thus, the hydrolyzed PC11COOMe monolayer (noted

PC11COO−) is expected to provide a better corrosioninhibition than the nonhydrolyzed one. This has beenconfirmed by polarization curve measurements (Figure 7 andTable 1). Indeed, it clearly appears that the hydrolysistreatment induces a further decrease of the corrosion currentdensity. The anodic part of the polarization curves for thehydrolyzed samples shows interesting features: the anodiccurrent is drastically decreased after hydrolysis reaction while

Table 1. Corrosion Potential and Corrosion Current DensityValues of Bare Mechanically Polished Nickel and NickelSubstrates Modified by Immersing 17 h at 90 °C in a 10 mMAqueous Solution of n-Dodecylphosphonic Acid (PC12), 11-Phosphonoundecanoic Acid (PC11COOH), and 11-Methylundecanoatephosphonic Acid before (PC11COOMe)and after the Hydrolysis Treatment (PC11COO−)

Ecor (mV vs SCE) icor (A/cm2)

bare −665 2.8 × 10−7

PC12 −365 6.3 × 10−8

PC11COOH −620 1.4 × 10−7

PC11COOMe −360 8.9 × 10−8

PC11COO− −340 4.6 × 10−8

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

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

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the pitting corrosion potential is shifted toward more anodicvalues (from 30 to 140 mV vs SCE for PC11COOMe modifiednickel surfaces before and after hydrolysis, respectively). Thiselectrochemical behavior reinforces our previous hypothesis,that is, that the immersion of modified nickel substrates inalkaline aqueous solution for 2 h at 90 °C induces areinforcement of the nickel oxide layer at the surface areasthat could be less efficiently protected by the monolayer.Layer-by-Layer (LbL) Deposition. After the hydrolysis of

the ester terminal functions on the grafted PC11COOMemonolayer, LbL polyelectrolyte deposition has been carried out

as described in the Experimental Methods. Each step of thispolyelectrolyte multilayer (PEM) formation has been followedby contact angle measurements. For comparison, the samePEM system has been deposited on bare nickel substrates inorder to compare the contact angle variations and therefore thepolyelectrolyte deposition efficiency (Figure 8). First of all, itclearly appears that the water contact angle variations are veryweak when polyelectrolytes are deposited on a bare nickelsurface indicating poor deposition efficiency and thereforedemonstrating the necessity of an adhesion promoter. On theother hand, when the deposition is carried out on a hydrolyzedPC11COOMe monolayer, the contact angle variations are reallysignificant. This indicates variations of the surface state and thusthe correct deposition of the different chitosan and alginatelayers. XPS analysis of these surfaces has been carried outsystematically. XPS survey spectra of surface states obtainedafter the three main steps of this surface modification procedureare presented in Figure 9. Only the nickel, oxygen, and carbonpeaks are visible on the bare polished nickel surface spectrum(carbon peak being attributed to the presence of somephysisorbed atmospheric contamination). On the XPSspectrum of the nickel surface after its modification with an11-methylundecanoatephosphonic acid monolayer, the appear-ance of phosphorus peaks can be observed confirming again thepresence of the monolayer at the surface. After hydrolysis, thephosphorus peaksʼ relative intensity remains constant indicat-ing the resistance of the anchoring phosphonic acid groups tothe hydrolysis treatment as shown previously. Finally, after theLbL deposition of a chitosan/alginate PEM system (sevenlayers), the peaks corresponding to the underlying surface (i.e.,nickel) and SAM (i.e., phosphorus) completely disappear fromthe XPS survey spectrum. Actually, only the peaks correspond-ing to the deposited polyelectrolytes, namely, C1s, N1s, andO1s, are visible. On the one hand, this confirms the presence ofthe 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 °Cin a 10 mM aqueous solution of 11-methylundecanoatephosphonicacid before (dashed line) and after (dotted line) the hydrolysis of theterminal ester functions by immersion of the modified nickelsubstrates in a sodium hydroxide aqueous solution (pH 11) for 2 hat 90 °C.

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 = PC11COOHSAM; hS = hydrolyzed PC11COOH SAM; L1 = first polyelectrolyte layer (chitosan); L2 = second polyelectrolyte layer (alginate); L3 = thirdpolyelectrolyte layer (chitosan); L4 = fourth polyelectrolyte layer (alginate); L5 = fifth polyelectrolyte layer (chitosan); L6 = sixth polyelectrolytelayer (alginate); and L7 = seventh polyelectrolyte layer (chitosan).

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the analysis depth of the spectrometer, that is, about 10 nm.The XPS spectra of the C1s and N1s core levels photoelectronshave been acquired for this PEM system (Figure 10). The N1sXPS spectrum can be analyzed with three components: a firstone centered around 399.6 eV attributed to nitrogen fromamine functions of chitosan, a second one at 400.6 eV that isattributed to nitrogen atoms from amide functions, and a lastone centered at 402.0 eV attributed to protonated aminefunctions. The N1s peak attributed to amide functions is about21% of the total nitrogen signal which is in good agreementwith the degree of deacetylation indicated by the chitosanprovider (i.e., between 75 and 85%). The C1s core level XPSspectrum has been analyzed with four components that can beattributed to the different carbon atoms present in the alginateand chitosan structures except for the one centered at 285 eVthat is attributed to the presence of some physisorbed

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

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

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

binding energy (eV) corresponding carbon atom number

285 atmospheric contaminations286.6 C 3288.1 C 2289.4 C 1

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exclusively morphological contrast) and secondary electronimaging (SEI, providing a morphological and elementalcontrast). It appears that the obtained surface is highlyhomogeneous and apparently smooth while presenting somestructural imperfections. The observed samples have beenscratched with a scalpel in order to obtain a very roughestimation of the PEM system thickness, that is, a fewmicrometers. Obviously, the number of seven polyelectrolytelayers has been chosen arbitrarily and can be changed in orderto adapt the PEM thickness to the desired value.Finally, further electrochemical characterizations have been

carried out on bare nickel substrates, nickel substrates modifiedwith a PC11COOH monolayer, nickel substrates modified witha PC11COOMe monolayer, and nickel substrates modified witha hydrolyzed PC11COOMe monolayer covered with a PEM(chitosan/alginate). These nickel surface modifications areexpected to lead to a higher charge transfer resistance. This wasfollowed by voltammetry and impedance measurements. Figure12 inset shows the voltammograms recorded with all theseelectrodes in the presence of 1 mM potassium ferricyanide + 1mM potassium ferrocyanide with lithium perchlorate assupporting electrolyte. The anodic peak current (Ipa) and thecathodic peak current (Ipc) observed on modified nickelelectrodes are significantly lower than that with bare nickelelectrode. The blocking appears to be much more importantwhen the nickel substrate is modified with a PC11COOMemonolayer than when modified with a PC11COOH monolayer

confirming our previous results. Furthermore, the hydrolysis ofthis PC11COOMe monolayer and its coverage with PEMsystem leads to a further improvement of this trend to a nearlycomplete blocking of these reactions. Besides the blocking ofthe Fe2+/Fe3+ redox reaction, the oxidation reaction of Ni toNi2+ also appears to be blocked by the studied coatings. Figure12 shows the electrochemical impedance analyses performed inthe same solution at the formal potential of Fe2+/Fe3+ redoxcouple in this medium, that is, 210 mV versus SCE. First, therecorded spectra are depressed semicircles, and a constantphase element instead of a double layer capacitance in Randlescircuit gave a good fitting. The depressed nature of thesemicircles can be attributed to the important roughness of theelectrode surface. Indeed, the mechanically polished poly-crystalline nickel electrodes that were used as substrates forthese coatings preparation have a mean roughness of 50 nm(measured with a DEKTAK contact profilometer). The bare Nielectrode shows a charge-transfer resistance of around 1740 Ω.Furthermore, the impedance spectrum for bare Ni shows aWarburg behavior at the low-frequency region. This observa-tion indicates a diffusion-controlled process. The nickelsubstrates modified with a PC11COOH monolayer and with aPC11COOMe monolayer and the hydrolyzed PC11COOMemonolayer covered with a PEM (chitosan/alginate) do notshow any Warburg behavior indicating a significant blocking ofthe redox reaction at the surface. The charge-transfer resistancemeasured for the nickel substrates modified with a PC11COOH

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) (scalebars = 10 μm).

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monolayer, with a PC11COOMe monolayer, and the hydro-lyzed PC11COOMe monolayer covered with a PEM (chitosan/alginate) was 4485, 7980, and 14 453 Ω, respectively. First,these results confirm the trend observed previously regardingthe PC11COOH versus PC11COOMe monolayers, that is, theresulting protective properties are much higher whenPC11COOMe is used because of the absence of anycompetition between the two terminal functions during self-assembly. The increase in the charge-transfer resistance thatcomes with the coverage of hydrolyzed PC11COOMemonolayer with PEM indicates that the final state of thissurface modification method is expected to efficiently preventelectrochemical species to reach the nickel surface.

■ CONCLUSIONSSince layer-by-layer assembly of polyanions and polycationsinto multilayers has been introduced, it has become a verypopular method for the formation of functional thin films withthe prospect of biomedical applications among others.Polyethyleneimine (PEI) is commonly used as an adhesionpromoter but has been shown to be potentially cytotoxic and istherefore not suitable for biomedical applications besidesproviding no or little corrosion inhibition to the underlyingsubstrate. In the present work, we report on the use of a self-assembled phosphonic acid monolayer as corrosion inhibitorand adhesion promoter for polyelectrolyte multilayer (PEM)formation on nickel substrates.First, three different phosphonic acid monolayers have been

compared: n-dodecylphosphonic acid (PC12), 11-phospho-noundecanoic acid (PC11COOH), and 11-methylundecanoate-phosphonic acid (PC11COOMe). PC12 monolayers have beenshown to provide the highest surface coverage and corrosioninhibition. However, PC12 has no functional end group and hasthus been used as a comparative system regarding corrosion

protection. PC11COOH monolayers bring carboxylic functionalgroups at the surface of the nickel substrates and are thereforeexpected to promote the electrostatic adhesion of the firstpolycation layer. However, because of the existing competitionbetween the two terminal functions of this molecule, theresulting monolayers provide the lowest surface coverage andcorrosion inhibition and have therefore been considered asunsuitable for the aimed purpose. On the contrary,PC11COOMe monolayers provide a much better surfacecoverage and corrosion resistance. These monolayers havebeen submitted to a hydrolysis treatment in order to form freecarboxylate groups at its surface. This hydrolysis treatment hasbeen shown to be efficient in forming negative charges at themonolayer surface and also to further improve the resultingcorrosion resistance of the modified substrates. This has beenexplained by two different phenomena, that is, a reorganizationof the monolayer (better close-packing because of thedecreased steric hindrance of the terminal function) and areinforcement of the underlying nickel oxide layer during thehydrolysis treatment.This hydrolyzed monolayer has then been shown to

efficiently promote the adhesion of PEM system by studyingthe deposition of seven chitosan and alginate layers. Theobtained polyelectrolyte covered surface is highly homogeneousand apparently smooth while presenting some structuralimperfections. Nevertheless, EIS measurements have shownthe efficiency of this PEM system to further increase the charge-transfer 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 adhesionpromoters for the deposition of polyelectrolyte multilayers, thatis, the covalent grafting of the PEM adhesion promoter to themodified surface and the corrosion inhibition provided by thisSAM that is crucial when considering the biodegradability ofmany polyelectrolytes used for biomedical applications.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +32-(0)81-72 52 30. Fax: +32-(0)81-72 46 00. E-mail:zineb.mekhalif@fundp.ac.be.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSV.S.D. acknowledges FUNDP-CERUNA for the postdoctoralfellowship.

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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 ahydrolyzed 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 lithiumperchlorate (LiClO4) aqueous solution. The surface area of theelectrode is 0.28 cm2. Inset: cyclic voltammograms recorded in thesame medium for these four surfaces with a scan rate of 20 mV/s.

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