HAL Id: hal-02336451 https://hal.archives-ouvertes.fr/hal-02336451 Submitted on 28 Oct 2019 HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Electrografting of mixed organophosphonic monolayers for SI-ATRP of 2-methacryloyloxyethyl phosphorylcholine Bastien Arrotin, Jean-Marc Noël, Joseph Delhalle, Laetitia Mespouille, Zineb Mekhalif To cite this version: Bastien Arrotin, Jean-Marc Noël, Joseph Delhalle, Laetitia Mespouille, Zineb Mekhalif. Electrograft- ing of mixed organophosphonic monolayers for SI-ATRP of 2-methacryloyloxyethyl phosphorylcholine. Journal of Coatings Technology and Research, Springer Verlag (Germany), 2019, 16 (4), pp.1121-1132. 10.1007/s11998-019-00186-6. hal-02336451
Transcript
HAL Id: hal-02336451https://hal.archives-ouvertes.fr/hal-02336451
Submitted on 28 Oct 2019
HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, estdestinée au dépôt et à la diffusion de documentsscientifiques de niveau recherche, publiés ou non,émanant des établissements d’enseignement et derecherche français ou étrangers, des laboratoirespublics ou privés.
Electrografting of mixed organophosphonic monolayersfor SI-ATRP of 2-methacryloyloxyethyl
phosphorylcholineBastien Arrotin, Jean-Marc Noël, Joseph Delhalle, Laetitia Mespouille, Zineb
Mekhalif
To cite this version:Bastien Arrotin, Jean-Marc Noël, Joseph Delhalle, Laetitia Mespouille, Zineb Mekhalif. Electrograft-ing of mixed organophosphonic monolayers for SI-ATRP of 2-methacryloyloxyethyl phosphorylcholine.Journal of Coatings Technology and Research, Springer Verlag (Germany), 2019, 16 (4), pp.1121-1132.�10.1007/s11998-019-00186-6�. �hal-02336451�
Abstract Nitinol (NiTi), one of the most importantalloys for biomedical applications, is still hampered byits surface nickel inclusions, making it sensitive topitting corrosion and leading therefore to the release ofpotentially carcinogenic Ni2+ ions. In this work, weassess the impact of the combination of electrograftedmixed self-assembled monolayers (SAMs) on NiTifollowed by a polymer coating formed by surface-initiated atom transfer radical polymerization (SI-ATRP). The molecular ratio of 11-(2-bromoisobu-tyrate)-undecyl-1-phosphonic acid (BUPA) to 11-de-cylphosphonic acid (C10P) on the electroassisted
elaboration of the (BUPA/C10P)-NiTi-SAMs has beenoptimized. A small amount of BUPA (20%) appears tobe the most promising condition, as it providesan efficient corrosion resistance and promotes theSI-ATRP of 2-methacryloyloxyethyl methacrylate(MPC). This confers to the surface hydrophilic prop-erties and corrosion resistance close to those NiTi-SAMs when long polymerization times are used(‡ 6 h).
Graphical abstract
NiTi NiTi NiTiSI-ATRPElectrograftingRT / 1.2 V / 5’ 90°C
Nitinol, a nearly equiatomic nickel titanium alloy, has anumber of interesting intrinsic properties includingheat, impact and corrosion resistance, high fatiguestrength,1,2 super-elasticity close to that of bone3 andshape memory.4–6 This makes NiTi applicable ina wide range of biomedical materials such as self-expending stents, surgical endoscopic instruments,atrial septal occlusion devices, orthodontic wires, andorthopedic staples and plates.7–10 The major limitationof untreated NiTi is the risk of corrosion due to itsoxide breakdown. Although the TiO2 surface layer isspontaneously formed and highly contributes to itscorrosion protection,11 the remaining nickel inclusions
Electronic supplementary material The online version of thisarticle (https://doi.org/10.1007/s11998-019-00186-6) containssupplementary material, which is available to authorized users.
B. Arrotin, J. Delhalle, Z. Mekhalif (&)Laboratory of Chemistry and Electrochemistry of Surfaces(CES), University of Namur, rue de Bruxelles, 61, 5000Namur, Belgiume-mail: [email protected]
B. Arrotin, L. MespouilleLaboratory of Polymeric and Composite Materials (LPCM),Center of Innovation and Research in Materials andPolymers (CIRMAP), HEALTH and MATERIALSResearch Institutes, University of Mons, Place du Parc, 23,7000 Mons, Belgiume-mail: [email protected]
J.-M. NoelUniversite Paris Diderot, ITODYS, CNRS, UMR 7086,Universite Sorbonne Paris Cite, 15 rue J. de Baıf, 75013Paris, Francee-mail: [email protected]
in the superficial layer can induce NiTi pitting corro-sion12–14 and lead to the release of Ni2+ ions into bodilyfluids. This generates allergenic and inflammatoryreactions, breathing problems, or even various cancerssuch as lung, kidney, or liver.15,16 Various processeshave been developed for NiTi surface modification toenhance its corrosion resistance and confer otherinterfacial properties such as wettability, proteinabsorption prevention, biocompatibility, and/or bioac-tivity increase. Among them, plasma treatment,17–19
chemical vapor deposition,20,21 hydrothermal rein-forcement of the oxide layer,22,23 self-assembled mono-layers (SAMs),24–26 and surface initiation ofpolymers23,27 have been reported.
In the present work, we combine the last twoprocesses to modify NiTi surfaces. SAMs based onorganophosphonic acid derivatives are selected be-cause they are known to robustly graft on metal oxidesurfaces via M–O–P bonds which are very resistant tohydrolysis.28–32 They constitute a strong reinforcementof the metal surface in aggressive environments.33–37
To confer to the surface additional properties forapplications, e.g., antifouling,38,39 lubrication,40,41 cellculture42,43 or even cell differentiation,44 appropriatepolymer coatings are also used. One way to generatetheses polymers is the surface-initiated controlledradical polymerization (SI-ATRP) which leads to thegrowth of polymer brushes with a good level of controlover chain length, grafting density, and moleculararchitecture.39,45–47
ATRP mechanism is based on the reversible cat-alyzed homolytic rupture of the alkyl–halogen bond ofthe initiating group (e.g., bromoisobutyrate). The rela-tive weakness of the C–Br bond (Eb = 318.0 ± 8.4 kJmol�1),48 unstable in certain surface treatment condi-tions such as induction heating,49 decreases the effi-ciency of SI-ATRP. The electroassisted approach,recently reported for Ti–6Al–4V50 and NiTi,49,51 hasconsiderable advantages to lead to the formation ofhigh-quality SAMs within short modification times andat room temperature. Thus, it is an alternative way tograft SI-ATRP initiators based on phosphonic acidderivatives while preserving the sensitive chemicalfunctions, such as C–Br in the 11-(2-bromoisobu-tyrate)-undecyl-1-phosphonic acid (BUPA) used asATRP initiator.49 However, BUPA initiating functionsare quite bulky and may lead to disordered and lessprotective layers. The use ofmixedmonolayers resultingfrom the co-adsorption of two organophosphonic acidderivatives differing in length and/or terminal functionalgroups has been reported to improve the corrosionresistance as well as the layer organization.47,52–54
Among the promising and versatile surface charac-terization techniques, scanning electrochemicalmicroscopy (SECM), based on the use of an ultrami-croelectrode facing a substrate, allows the local prob-ing of its electrochemical property, i.e., its conducting/insulating behavior, and can provide high-resolutioninformation on the electrochemical processes occurringat the surface.55–58 Over the last years, SECM has been
widely applied for the study of self-assembled mono-layer properties and impact over corrosion resis-tance26,59–61 and on organic layer permeability.62 Butso far, only few SECM studies have been performed onNitinol,26 which thus remains of great interest.
The aim of this work is to improve the quality of theelectrografted SAMs on NiTi. To this end, mixedSAMs carrying alkyl-halogen initiating groups of 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid(BUPA) are formed on NiTi after its hydrothermaltreatment by incorporating 1-decylphosphonic acid(C10P) molecules with a shorter hydrophobic alkylchain. Various amounts of BUPA/C10P are studied tooptimize the quality of the SAM organization andconsequently the NiTi corrosion resistance whilekeeping sufficient amount of grafted BUPA for thepolymerization. The so-formed mixed SAMs are usedto initiate the surface polymerization of 2-methacry-loyloxyethyl phosphorylcholine (MPC), a biocompati-ble monomer (Scheme 1). The originality of this workresides in the combination of the electroassistedapproach for the grafting of mixed monolayer whichis in itself a relatively new promising method andis particularly suitable for temperature-sensitivegroups. In this context, the SI-ATRP can be consideredas an indirect evaluation way for the achievement ofthose surface modifications. The benefits of thismethod are highlighted over NiTi corrosion inhibition,surface functionalization and physicochemical proper-ties.
Experimental section
Chemicals
Disolol� (ethanol 99% denatured with isopropanol 2%and Butanone 2%, ChemLab), 11-(2-bromoisobu-tyrate)-undecyl-1-phosphonic acid (BUPA, Sikemia, ‡97%), 1-decylphosphonic acid (C10P, abcr, 98%),tris[2-(dimethylamino)ethyl]amine (Me6TREN,Aldrich, 97%), potassium nitrate (Merck, 98%), fer-rocenemethanol (FcMeOH, Aldrich, 97%), andsodium chloride (Fluka, ‡ 99.5%) are used withoutfurther purification. 2-Methacryloyloxyethyl phospho-rylcholine (MPC) (Sigma-Aldrich, 97%) is purifiedfrom the remaining stabilizing agent by mixing it withanhydrous diethyl ether and filtering.55 Copper(I)bromide (Aldrich, 99%) is purified by consecutivewashings with acetic acid and methanol and finallymaintained under N2 atmosphere. All aqueous solu-tions are prepared with ultrapure Milli-Q water(18.2 MX cm).
Nitinol substrates preparation
Nitinol (Ni 56%/Ti 44%) rectangular-shaped plates(20 mm 9 10 mm 9 0.3 mm), purchased from AMF,
are mechanically polished down to 0.1 lm roughnesson a Buehler Ecomet 300 instrument using siliconcarbide papers, diamond pastes (9 lm) from Struers,and a mixture of colloidal silica (Buehler MasterMetTM
2) and H2O2 (Acros Organics, 35 wt% solution inwater). These Nitinol substrates (subsequently denom-inated bare NiTi) are cleaned in denatured ethanolunder ultrasonication for 15 min before being blowndry with nitrogen and stored under nitrogen for furtheruse or analysis.
Prior to any modification, the substrates are system-atically cleaned by application of the following treat-ments: immersion in denatured ethanol in a sonicationbath for 15 min, drying under nitrogen, hydrothermaltreatment in boiling water for 1 h (HT-NiTi), andfinally cleaning in denatured ethanol for 15 min undersonication, before being blown dry under nitrogen.
Electroassisted grafting of mixed initiatingmonolayers on Nitinol
Solutions (20.0 mL) composed of 1 mM of a mixture of11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid(BUPA)/1-decylphosphonic acid (C10P) in variousratios and 10 mM KNO3 are prepared in ultrapureMilli-Q water.
The SAMs on HT-NiTi surface are obtained byimmersion of the substrate in the solution for 5 min,under a voltage of 1.2 V vs a saturated calomelelectrode (SCE) using a Princeton Applied ResearchVersaSTAT3 potentiostat/galvanostat. At the end, thesubstrates are ultrasonically cleaned in denaturedethanol for 15 min, before being blown dry undernitrogen and stored for characterizations.
SI-ATRP of MPC
To a four-neck round-bottomed flask are added 3.0 910�4 mol copper bromine (CuBr) and 6.0 9 10�4 moltris[2-(dimethylamino)ethyl]amine (Me6TREN). Thethree lateral necks are closed by septa into which a clipis inserted to hold two substrates. The flask is thenpurged by three repeated vacuum/nitrogen cycles. In asecond round-bottomed flask are introduced 3.4 9 10�3
mol of 2-methacryloyloxyethyl phosphorylcholine and19.8 mL of Milli-Q water to reach a concentration of0.17 M. The solution is then deaerated by nitrogenbubbling for a few minutes.
The content of the second flask is transferred intothe first one by means of a flamed dry capillary underN2 atmosphere. The polymerization is then carried outfor 1, 3, or 6 h at 90�C in an oil bath. At the end of thereaction, the flask is cooled down to room temperature.The substrates are then rinsed with denatured EtOHand sonicated in denatured EtOH for 15 min. They arefinally dried and stored under N2 atmosphere.
Substrate characterization
The modified substrates are characterized by X-rayphotoelectron spectroscopy (XPS), polarization mod-ulation-infrared reflection–absorption spectroscopy(PM-IRRAS), static water contact angle measure-ments (WCA), cyclic voltammetry (CV), linear sweepvoltammetry (LSV), and scanning electrochemicalmicroscopy (SECM). To assess the reproducibility ofthe grafting, all the analyses were performed intriplicate. In the case of XPS characterizations, eachsample is analyzed at three different spots.
NiTiBare NiTi
1 mM (total)H2O / RT / 1.2 V / 5’
NiTi NiTi
Br
OO
O
7 7
10
O
OO O
O
7 77
OOO
PO O
OOO
PP O
OO
OO
O
O
O R
O O O
RR
R = O-[PO3]−-C2H4-[N(CH3)3]
+
nnBr Br
7 77
OOO
PO O
OOO
PP
O
Br Br
OH
C P
HOPP
HO OH
BUPACuBr / Me6TREN
H2O / 90°C
N N N
N
Me6TREN
Scheme 1: Surface modification of Nitinol: self-assembly of BUPA/C10P mixed monolayers (left) and subsequent SI-ATRPof MPC in the presence of Me6TREN (right)
XPS spectra are recorded on a Thermo Scientific K-Alpha spectrometer using amonochromatizedX-rayKa
radiation (1486.6 eV), the photoelectrons being col-lected at 0� with respect to the surface normal anddetected with a hemispherical analyzer. The spot size ofthe X-ray source on the sample is 200 lm, and theanalyzer is operated with a pass energy of 200 eV forsurvey spectra and 50 eV for high-resolution core-levelspectra. The binding energy (BE) of core levels iscalibrated with respect to the C1s BE set at 285.0 eV.Spectra are analyzed using a linear combination ofGaussian and Lorentzian curves in 70–30 proportions.The different relative peak areas are measured on core-level spectra. Quantitative XPS analyses are carried outby calculating the relevant abundance ratios on the basisof the core-level spectra and taking into account thecorresponding Scofield sensitivity factors56: C1s 1.000,P2p 1.920, N1s 1.800, Ti2p 7.910, and Ni2p 22.180.Thickness of the formed coatings is estimated from thecollected XPS data. The thickness of a first material(i.e., SAM) deposited on a bulk material (i.e., NiTi) canbe obtained from the relative attenuation of this bulkmaterial (here limited to Ti), by the formula:
ITiITi;0
¼ exp � dSAM
kTi;SAMsin h
� �ð1Þ
where ITi/ITi,0 is the ratio of the bulk element peakintensities (modified/bare surface), dSAM the coatingthickness, kTi,SAM the bulk Ti photoelectrons meanfree path through the SAM layer, and h the takeoffangle (here h = 90�).57–59
PM-IRRAS data are collected to assess the presenceof the organic layer on nickel surfaces. They arerecorded on a Bruker PMA37 equipped with a liquidnitrogen-cooled mercury–cadmium–telluride (MCT)detector and a germanium crystal. All presentedspectra are the average of 1024 scans at a spectralresolution of 4 cm�1.
Static water contact angle measurements are carriedout using a DIGIDROP (GBX Surface Technology)goniometer at room temperature and ambient atmo-sphere, with a syringe to deliver 2 lL droplet of Milli-Qwater.
Voltammetry measurements (CV and LSV) arecarried out on a Princeton Applied Research, Poten-tiostat/Galvanostat Model VersaSTAT3-LC using athree-electrode electrochemical cell with a controlledanalysis spot surface (0.28 cm2) on the sample. Nitinolsubstrates (bare or modified) are used as workingelectrode, a platinum foil as counter electrode, and asaturated calomel electrode (SCE) as reference.
Cyclic voltammograms are recorded from � 0.10 to+ 0.65 V at a scan rate of 20 mV s�1 in 0.1 M NaOH.The blocking factor (BF) of the coatings on NiTi isdetermined from the formula:
BF ¼ aan;0 � aanaan;0
� 100 ð2Þ
where aan,0 and aan are the area of the anodic peaks forthe first cycle of bare and modified NiTi substrates,respectively.
Linear sweep voltammograms are recorded from200 mV below OCP (measured for 1 h) to + 1.0 V at ascan rate of 1 mV/s in 0.5 M NaCl. The corrosioninhibition efficiency (IE) is determined from theobtained corrosion current densities (jcorr) by theformula:
IE ¼ jcorr;0 � jcorrjcorr;0
� 100 ð3Þ
where jcorr,0 and jcorr are the corrosion current densitiesof bare and modified NiTi substrates, respectively.
SECM is operated in a feedback (FB) mode on aCHI 900B Bi-potentiostat (CH Instruments) using athree-electrode cell: a commercial Ag|AgCl referenceelectrode (CHI111, CH Instruments), a platinum foilas counter electrode, a platinum microdisc ultramicro-electrode (UME) (10 lm diameter with an estimatedRG ratio of 8). The sample is fixed at the bottom of thecell without polarization. FB mode analyses wereperformed in a 1 mM FcMeOH/0.1 M KNO3 aqueoussolution according to the following procedure. TheUME is polarized at 0.40 V vs Ag|AgCl to allow themass-transfer-controlled oxidation of FcMeOH toferricenium methanol (FcMeOH+) and moved downtoward the substrate at a rate of 1 lm s�1. Theapproach curve is recorded until a sharp change ofthe currents occurred at the tip. The UME is thenwithdrawn by 3 lm, and the imaging measurements istaken on a 500 9 500 lm2 area at a scan rate of25 lm s�1.
Results and discussion
The results are presented in two sections: the first onedeals with the NiTi modification with phosphonic acidderivatives based on C10P, BUPA, or mixtures ofBUPA/C10P molecules in different proportions toachieve monolayers in order to confer to NiTi multi-functional properties such as corrosion protection andthe ability to initiate SI-ATRP. The second part dealswith the benefit of using modified NiTi with mixedBUPA/C10P SAMs on the SI-ATRP of MPC, a modelof biocompatible monomer.
The electroassisted approach, based on the optimizedconditions determined in our previous work(E = 1.2 V vs SCE and t = 5 min),49 is transposed tothe grafting of C10P (C10P-NiTi), BUPA (BUPA-NiTi), and their mixture in different ratios (BUPA/
C10P-NiTi) on HT-NiTi. The resulting modified NiTisurfaces are characterized with contact angle measure-ments, PM-IRRAS, XPS, and electrochemical analy-ses.
Figure 1 highlights the wettability properties of bareNiTi, HT-NiTi, C10P-NiTi, BUPA-NiTi, and BUPA/C10P-NiTi. The water contact angle values (hw) rangingfrom 78� to 92� after NiTi modification point to asignificant increase in the contact angle compared toHT-NiTi (hw = 30�). The highest hw value, indicatingthe more hydrophobic surface, is obtained for C10P-NiTi (hw = 92�). Increasing the ratio of BUPA/C10Pdoes not drastically change the hw values which remainvery close (hw � 80�) to that of BUPA-NiTi. This isconsistent with the fact that C10P molecules are shorterthan BUPA and thus have less impact on the wetta-bility properties, even at high concentration in themixed monolayer.
Figure 2 displays PM-IRRAS spectra for HT-NiTiand BUPA/C10P-NiTi. HT-NiTi does not show anysignal of any chemical functions, while BUPA/C10P-NiTi show, whatever the molecular ratio, vibrationalbands typical of BUPA at 1276/1261 and 1170 cm�1,assigned, respectively, to the asymmetric and thesymmetric stretching of the C–O bond of the bro-moisobutyrate function, and to C–H deformation.60
Results of the X-ray photoelectron spectroscopy(XPS) analyses performed on HT-NiTi and modifiedNiTi are reported in Fig. 3 and Table 1. C1s core level(Fig. 3) displays peaks at similar energies but withhigher intensities for modified NiTi indicating there-fore an effective grafting of C10P and BUPA on HT-NiTi. These peaks at energies of 285 eV, 286.5 eV, and289.1 eV, correspond, respectively, to carbon atomsinvolved in C–H/C–C, C–O/C–P, and O–C=O bondspresent in the phosphonic acid derivatives (peak 1 forC10P) and (peaks 1–3 for BUPA) or in atmosphericcontaminations (peaks a-c for HT-NiTi). The one at
283.3 eV (*) is assigned to an artifact from thespectrometer.
In addition to the increase in C1s intensity, XPSanalyses reveal the presence of P2p for all BUPA-NiTi,C10P-NiTi and BUPA/C10P-NiTi and Br3 s for BUPA-NiTi and BUPA/C10P-NiTi. When adding C10P toBUPA, the C/NiTi and P/NiTi ratios increase (Ta-ble 1) indicating the formation of denser and moreorganized layers due to the insertion of C10P. Themaximum is achieved for 60% of BUPA. The C/NiTiratio then decreases due to the shorter chain of C10Preplacing the longer BUPA ones while P/NiTi remainsalmost stable (0.13–0.17). The Br/NiTi ratio is barelyaffected by the decrease in BUPA amount whichproves that pure BUPA layer is not very wellorganized and partially degraded.
Bare HT
Bare HT 0 10 20 30 40 50 60 70 80 90 100
100
100
90
80
70
60
50
40
30
20
10
090 80 70 60 50 40 30 20 10 0
BUPA amount (%)
Wat
er c
onta
ct a
ngle
(°)
C10P amount (%)
Fig. 1: Water contact angles for bare NiTi, HT-NiTi, BUPA-NiTi, C10P-NiTi, and BUPA/C10P-NiTi (in various ratios)
1400
HT
1276
0% BUPA10% BUPA
20% BUPA30% BUPA40% BUPA50% BUPA
60% BUPA
70% BUPA80% BUPA90% BUPA100% BUPA
1261
1170
1114
1300 1200
Wavenumber (cm−1)
Inte
nsity
(arb
. uni
ts)
1100 1000
Fig. 2: PM-IRRAS spectra for HT-NiTi, BUPA-NiTi, C10P-NiTi, and BUPA/C10P-NiTi (in various ratios)
291 289 287
Binding energy (eV)285 283 281
291 289a
bc
*
*
2
1
3
HOHOHOHO Br9
81
1
1
12 2
2
2
3PP
O O
O
O
287
HT
285 283 281
Fig. 3: Representative XPS C1 s core levels for HT-NiTi(bottom) and BUPA/C10P-NiTi in various ratios (top)
The approximated thickness of BUPA layer(1.93 ± 0.38 nm) is consistent with the simulated valueof 2.02 nm (Fig. 4a), whereas the estimated thicknessof C10P layer is underestimated from a theoreticalvalue of 1.41 nm to 0.60 ± 0.13 nm. This differencecould result from a tilted denser organization of theC10P layer. The coating thickness decreases with theincorporation of C10P (Table 1), which also attests tothe formation of a tilted denser layer as shown inFig. 4b.
Electrochemical evaluation of modified NiTi iscompared to HT-NiTi on the basis of three methods(LSV, CV, and SECM). LSV results (Fig. 5 andTable 2—Supporting Information Figure S1) indicatea strong improvement of the corrosion resistance ofBUPA/C10P-NiTi with the decrease in BUPA amount,e.g., corrosion current density (jcorr) decreases from
95.1 nA cm�2 to 9.4 nA cm�2 for 20% of BUPA. Thisis once again in accordance with the formation of adenser and more organized layer due to higher amountof C10P within the layer.
As expected, the use of high C10P amount leads tothe formation of more resistant layers. Among all theconditions, the layers made of 20 and 50% of BUPAexhibit the best inhibition efficiencies (IE) with respec-tive values of 98.7% and 98.9%, hence the bestresistance against corrosion.
Cyclic voltammetry (CV) (Fig. 6 and Table 2—Sup-porting information Figure S2) confirms the protectiveefficiency due to BUPA and C10P grafting, as well asthe reinforcement of the TiO2 external layer.
For bare NiTi substrate, the first cycle reveals twoanodic peaks at 200 and 500 mV vs SCE, respectively,assigned to Tin+ and Ni2+ oxidations, but only onecathodic peak assigned to Ni3+ reduction. On thesecond cycle, only nickel oxidation and reduction
Table 1: XPS atomic ratios for bare NiTi, HT-NiTi, and (BUPA/C10P)-NiTi (various ratios) and estimated SAMthickness
Fig. 4: Model structures of C10P and BUPA (a) andschematic representation of BUPA/C10P coating thickness(based on XPS results) (b)
Bare0.01
1E−3
1E−4
1E−5
1E−6
1E−7
1E−8
1E−9
1E−10
1E−11−0.50 −0.25 0.00 0.25 0.50 0.75 1.00
Potential (V vs SCE)
Cur
rent
den
sity
(A c
m−2
)
HT20% BUPA
Fig. 5: Representative polarization curves for bare NiTi,HT-NiTi, and representative BUPA/C10P-NiTi (20/80) in 0.5 MNaCl at a scan rate of 1 mV s21
peaks are still present due to the irreversible passiva-tion of the electrode via the titanium oxide layer. ForHT-NiTi, only the oxidation peak of Tin+ can beobserved on the first cycle and no more Ni reactions,even during the second cycle. This confirms theefficiency of the hydrothermal treatment on the rein-forcement of TiO2 layer and on the prevention of Nix+
release. Once electroassisted grafting of BUPA/C10P isperformed on HT-NiTi, their CV analysis does notshow any electrochemical activity, thus confirming theprotective efficiency of the grafted phosphonic deriva-tives. As expected, the increase in C10P concentrationwithin the layer leads to the formation of moreblocking layers. In line with the conclusions madefrom LSV results, BUPA/C10P ratios of 50/50 and 20/80 exhibit the best blocking factors and thus the bestprotection of the substrate.
SECM is used to assess the charge transfer processoccurring at bare NiTi, HT-NiTi, and BUPA/C10P-NiTi electrodes during the reduction of FcMeOH+.Figure 7 and Figure S3 (Supporting information) showthe normalized approach curves in FB mode. Thecurves are fitted on general analytical expressionsgiven for first-order heterogeneous finite kinetics61,62 atthe sample surface with respect to the FcMeOH+
generated at the tip surface.The apparent charge transfer rate constants (kct,
Table 2) attests to the regeneration rate of FcMeOHby the NiTi samples evolving through the modificationsteps. The obtained values of kct are in line with theresults presented for CV. Indeed, the hydrothermal
Table 2: Values of Ecorr, jcorr, IE, BF, and kct for bare NiTi, HT-NiTi, and BUPA/C10P-NiTi (in various ratios) (obtainedby LSV, CV, and SECM)
Sample Ecorr (mV vs SCE) jcorr (nA cm�2) IE (%) BF (%) kct (lm s�1)
Fig. 6: Representative first (main) and second (inset)cycles of the voltammograms for bare NiTi, HT-NiTi, andBUPA/C10P-NiTi (20/80) in 0.1 M NaOH at a scan rate of20 mV s21
ConductiveInsulating
Bare
0
2.0
1.2
0.8
0.4
0.0
1.6
1 2 3d/rT
i T/i T,
inf
4 5
HT20% BUPA
Fig. 7: Representative normalized SECM FB approachcurves for bare NiTi (kct = 15.3), HT-NiTi (kct = 11.2),representative BUPA/C10P-NiTi (20/80) (kct = 0.6) in 1 mMFeMeOH/0.1 M KNO3. The tip was set at 0.4 V vs Ag|AgCland moved at a scan rate of 1 lm s21. The symbolscorrespond to the experimental curves and the solid linesto the SECM theory61,62
passivation of NiTi leads to the formation of a slightlymore protective oxide layer, as attested by a kct downfrom 16.0 to 10.8 lm s�1. This phenomenon is ampli-fied by the additional presence of BUPA/C10P mono-layers, with kct ranging from 0.4 to 10.5 lm s�1.Among the assessed BUPA/C10P ratios, the bestresults are obtained for the coatings with 0, 60, 80,and 20% BUPA with respective kct of 0.4, 0.4, 0.5, or0.6 lm s�1.
From these results, the optimum BUPA/C10P layer(high P/NiTi, Br/NiTi, BF, IE, kct, and reproducibility)is prepared from 20% of BUPA and 80% of C10P. Thismolecular ratio will be used for all the upcomingfunctionalization.
Surface-initiated atom transfer radicalpolymerization of 2-methacryloyloxyethylphosphorylcholine on BUPA/C10P-NiTi
In this section, SI-ATRP of MPC is performed on theBUPA/C10P-NiTi (modified in the optimum conditionsdetermined above (i.e., E = 1.2 V vs SCE, t = 5mn,BUPA/C10P = 20/80) for 1, 3, and 6 h (PMPC-NiTi).
The modified surfaces are first characterized usingwater contact angle (Fig. 8). More hydrophilic surfaces(hw = 45�) are obtained for PMPC-NiTi compared toBUPA/C10P-NiTi (78.9�) which confirms, consideringthe hydrophilic nature of the MPC, the formation andthe grafting of the polymer.
Figure 9 shows the PM-IRRAS spectra. For PMPC-NiTi coatings, absorption bands in agreement with thestructure of the molecules appear at 1721, 1486, 1255–1279, 1170, 1125, 1092, and 1054 cm�1, respectively,assigned to C=Ostretch (ester), C–O–Cwag, C–Oasym
stretch (ester), C–Osym stretch (ester), C–Nstretch, C–Ostretch (alcohol), and P–Ostretch. An increased signalis noticed, especially after 6 h of polymerization, whichconfirms the presence of a PMPC film on NiTi surface.
XPS analyses confirm the formation of PMPC.Figure 10 presents C1s and N1s core-level spectra for
PMPC-NiTi and BUPA/C10P-NiTi. As expected, C1sand N1s peaks are more intense after SI-ATRP ofMPC and present different general shapes compared tothe initial BUPA/C10P-NiTi. The C1s core-level spec-tra (Fig. 10-left) present four components centered atenergies of 285.0, 285.7, 286.5, and 289.1 eV, respec-tively, attributed to carbon atoms involved in C–H/C–C, C–N, C–O/C–P, and O–C=O bonds present inBUPA, C10P, PMPC, and Me6TREN. The peak at283.3 eV (*) assigned previously to an artifact from thespectrometer is still visible.
The N1s core-level spectra (Fig. 10-right) presenttwo components centered at energies of 400.3 eV and403.5 eV, respectively, attributed to nitrogen atomscharacteristic of the amine function of the Me6TRENand of the ammonium present in PMPC. These confirmthe efficiency of SI-ATRP of MPC on BUPA/C10P (20/80) layer but also point out the presence of residues ofthe catalyst (complexed by Me6TREN) trapped in thecoating.
SEM images (Fig. 11) highlight the differences ofmodified NiTi morphologies as a function of thepolymerization time. Within short polymerization time(1 h), the surface is highly inhomogeneous and shows ahigh density of polymer aggregates. After 3 h, thecoating starts to be more homogenous but uncoveredNiTi domains are still visible. However, the formationof PMPC film for 6 h leads to a good coverage with aquite uniform distribution of small aggregation spotsprobably resulting from nonuniform PMPC growth.
The increase in the hydrophylicity of PMPC-NiTiand the time-dependent homogeneity of the surfaceinduced differences in the electrochemical propertieswhich are shown in the LSV curves (Fig. 12) andSECM images (Fig. 13).
For the substrate modified for 1 h, jcorr changes from9.4 nA cm�2 (BUPA/C10P-NiTi) to a value of 92.5 nAcm�2 as well as less negative Ecorr (from � 314 to� 82 mV vs SCE) due to the high hydrophilicity andinhomogeneity of the PMPC layer. For higher SI-ATRP times, jcorr values are 12.0 nA cm�2 and 17.8 nA
90
80
70
60
50
40
30
20
10
0SAM 1 h 3 h 6 h
Polymerization time
Wat
er c
onta
ct a
ngle
(°)
Fig. 8: Water contact angles for BUPA/C10P NiTi (20/80)and PMPC-NiTi prepared for 1, 3, or 6 h
1800
SAM
1 h
3 h
6 h
1721
1486
1279
1255
1170
1125
1092
1054
1600 1400
Wavenumber (cm−1)
Abs
orba
nce
(arb
. uni
ts)
1200 1000
Fig. 9: PM-IRRAS for BUPA/C10P NiTi (20/80) and PMPC-NiTi prepared for 1, 3, or 6 h
cm�2 for 3 h and 6 h, respectively. Ecorr values remainin the same range with values of � 110 and � 153 mVvs SCE.
Nevertheless, compared to HT-NiTi (jcorr � 103.6nA cm�2, IE � 88.1, and Ecorr � � 266 mV vs SCE),the corrosion current and inhibition efficiency stillindicate an improvements of the corrosion inhibitionproperties.
Representative SECM mapping images collected inFB mode are presented in Fig. 13. Those point out thesurface uniformity and the passivating behavior ofBUPA/C10P SAM (Fig. 13—upper left). However,once SI-ATRP of MPC occurred, the average mea-sured current increases up from � 0.2 to � 0.4 nA witha slightly less homogeneous behavior. These results arein line with the previous observations made from SEMpictures (Fig. 11) and LSV curves (Fig. 12). Theincreased PMPC thickness induces a heterogeneouselectroactivity, especially for the 6-h polymerization,where several high current spots are noticeable. Theincreased electroreactivity is in accordance with the
C1s N1s ab
O
O
OO 3 3
34
11
12
2 a
aa aN
N
NN
Me6TREN
2
222 2
22
1
1
1
1
1
3 34
9
13
O
7HO
HO
OP O
O
Br
OH
BUPA
OHP
C10P
2
22
2
2
22
bOO
Brn
N+
−
P
PMPC
b
b
N2
a
a
406 404 402 400 398 396
6 h 6 h4
4
4
4
3
3
3
3 2
2
2
1
1
1
1
*
*
*
*
3 h 3 h
1 h1 h
SAM SAM
291 289 287 285 283 281
Binding energy (eV) Binding energy (eV)
Fig. 10: XPS general shape of core spectra for C1s and N1s components obtained for BUPA/C10P NiTi (20/80) and PMPC-NiTi prepared for 1, 3, or 6 h
Fig. 11: SEM images of PMPC-NiTi prepared for 1, 3, or 6 h
SAM1 h
1E−4
1E−5
1E−6
1E−7
1E−8
1E−9
1E−10
1E−11−0.50 −0.25 0.00 0.25 0.50 0.75 1.00
Potential (V vs SCE)
Cur
rent
den
sity
(A c
m−2
)
3 h6 h
Fig. 12: Polarization curves for BUPA/C10P NiTi (20/80) andPMPC-NiTi prepared for 1, 3, or 6 h (obtained in 0.5 M NaClat a scan rate of 1 mV s21)
hydrophilic behavior of PMPC coatings and thus theinteractions between the coating and the redox probe,thus resulting in an intercalation of the probe inside theorganic layer. In 2010, Matrab et al. investigated totransport and reactivity within PGMA polymerbrushes made by SI-ATRP and proved the permeabil-ity of such methacrylate-based polymer once solvatedin aqueous media.
Conclusions
In this work, the electroassisted grafting of BUPA/C10P mixed monolayers in various ratios has beenachieved to improve NiTi corrosion resistance.
First, the successful surface modifications have beenattested by WCA, PM-IRRAS, and XPS results, and ithas been proven that the substitution of some BUPAmolecules by C10P (a smaller and less hinderedmolecule) improves the anticorrosion properties ofNitinol. Among all the tested conditions, the mostprotective and efficient layer (high blocking factor,high inhibition efficiency, low charge transfer constantkct) is obtained for a composition of 20% BUPA and80% C10P.
Second, the surface-initiated ATRP of MPC hasbeen achieved on the so-formed initiating layer for 1, 3,or 6 h. SEM images and SECM measurements evi-denced the nonuniformity of the PMPC coatings fortime lower than 6 h.
This work opens the prospect of biocompatiblepolymer growth and tuning on Nitinol substrate mod-ified by electrografted phosphonic-made ATRP initia-tors. Nevertheless, the structure of this system stillrequires some improvements with the prospect of afurther biomedical application, which is being evalu-ated in simulated body fluids.
Acknowledgments B. A. thanks UNamur andUMONS for the joint PhD grant. B. A., L. M., andP. D. acknowledge the Belgian Program onInteruniversity Attraction Poles initiated by theBelgian State, the Prime Minister’s office (P7/05).CIRMAP is grateful to the ‘‘Belgian FederalGovernment Office Policy of Science (SSTC)’’ forgeneral support in the frame of the PAI-7/05, theEuropean Commission and the Wallonia Region(FEDER Program), and OPTI2MAT program ofexcellence.
References
1. Duerig, T, Pelton, A, Stockel, D, ‘‘An Overview of NitinolMedical Applications.’’ Mater. Sci. Eng. A, 273–275 149–160(1999)
2. Elahinia, MH, Hashemi, M, Tabesh, M, Bhaduri, SB,‘‘Manufacturing and Processing of NiTi Implants: A Re-view.’’ Prog. Mater. Sci., 57 (5) 911–946 (2012)
3. Shabalovskaya, SA, ‘‘Surface, Corrosion and Biocompatibil-ity Aspects of Nitinol as an Implant Material.’’ Biomed.Mater. Eng., 12 (1) 69–109 (2002)
4. Centre d’Animation Regional en Materiaux Avances(C.A.R.M.A.). Alliages a Memoire de Forme (2001).
12. Shabalovskaya, SA, Tian, H, Anderegg, JW, Schryvers, DU,Carroll, WU, Van Humbeeck, J, ‘‘The Influence of SurfaceOxides on the Distribution and Release of Nickel fromNitinol Wires.’’ Biomaterials, 30 (4) 468–477 (2009)
00
0
0.0500
400
300
200
100
400
300
200
100
SAM 1 h
3 h 6 h
0.1 0.2 0.3 0.3 0.4 0.5 0.6 0.7
100 200 300 400 100 200 300 400 500
X distance (μm)
Current (nA)Y
dis
tanc
e (μ
m)
0
Fig. 13: Normalized SECM FB mapping for BUPA/C10P NiTi(20/80) and PMPC-NiTi prepared for 1, 3, or 6 h (obtained in1 mM FeMeOH/0.1 M KNO3)
13. Figueira, N, Silva, TM, Carmezim, MJ, Fernandes, JCS,‘‘Corrosion Behaviour of NiTi Alloy.’’ Electrochim. Acta, 54(3) 921–926 (2009)
14. Milosev, I, Kapun, B, ‘‘The Corrosion Resistance of NitinolAlloy in Simulated Physiological Solutions: Part 1: TheEffect of Surface Preparation.’’ Mater. Sci. Eng. C, 32 (5)1087–1096 (2012)
15. Lu, X, Bao, X, Huang, Y, Qu, Y, Lu, H, Lu, Z, ‘‘Mechanismsof Cytotoxicity of Nickel Ions Based on Gene ExpressionProfiles.’’ Biomaterials, 30 (2) 141–148 (2009)
16. Lee, J, Son, Y, Pratheeshkumar, P, Shi, X, ‘‘Oxidative Stressand Metal Carcinogenesis.’’ Free Radic. Biol. Med., 53 (4)742–757 (2012)
18. Shen, Y, Wang, G, Chen, L, Li, H, Yu, P, Bai, M, Zhang, Q,Lee, J, Yu, Q, ‘‘Investigation of Surface Endothelializationon Biomedical Nitinol (NiTi) Alloy: Effects of SurfaceMicropatterning Combined with Plasma Nanocoatings.’’Acta Biomater., 5 (9) 3593–3604 (2009)
19. Siu, HT, Man, HC, ‘‘Fabrication of Bioactive TitaniaCoating on Nitinol by Plasma Electrolytic Oxidation.’’ Appl.Surf. Sci., 274 181–187 (2013)
20. Lahann, J, Klee, D, Thelen, H, Bienert, H, Vorwerk, D,Hocker, H, ‘‘Improvement of Haemocompatibility of Metal-lic Stents by Polymer Coating.’’ J. Mater. Sci. Mater. Med., 10(7) 443–448 (1999)
21. Catledge, SA, Thomas, V, Vohra, YK, ‘‘Effect ofSurface Oxides and Intermetallics on Nanostructured Dia-mond Coating of Nitinol.’’ Curr. Nanosci., 2 (1) 9–12(2006)
22. Perez, LM, Gracia-Villa, L, Puertolas, JA, Arruebo, M,Irusta, S, Santamarıa, J, ‘‘Effect of Nitinol Surface Treat-ments on Its Physico-Chemical Properties.’’ J. Biomed.Mater. Res. B. Appl. Biomater., 91 (1) 337–347 (2009)
23. Devillers, S, Barthelemy, B, Delhalle, J, Mekhalif, Z,‘‘Induction Heating Vs Conventional Heating for theHydrothermal Treatment of Nitinol and Its Subsequent 2-(Methacryloyloxy)Ethyl 2-(Trimethylammonio)Ethyl Phos-phate Coating by Surface-Initiated Atom Transfer RadicalPolymerization.’’ ACS Appl. Mater. Interfaces, 3 (10) 4059–4066 (2011)
24. Quinones, R, Gawalt, ES, ‘‘Study of the Formation of Self-Assembled Monolayers on Nitinol.’’ Langmuir, 23 (20)10123–10130 (2007)
25. Petrovic, Z, Katic, J, Metikos-Hukovic, M, Dadafarin, H,Omanovic, S, ‘‘Modification of a Nitinol Surface by Phos-phonate Self-Assembled Monolayers.’’ J. Electrochem. Soc.,158 (10) F159–F165 (2011)
26. Maho, A, Kanoufi, F, Combellas, C, Delhalle, J, Mekhalif, Z,‘‘Electrochemical Investigation of Nitinol/Tantalum HybridSurfaces Modified by Alkylphosphonic Self-AssembledMonolayers.’’ Electrochim. Acta, 116 78–88 (2014)
28. Marcinko, S, Fadeev, AY, ‘‘Hydrolytic Stability of OrganicMonolayers Supported on TiO2 and ZrO2.’’ Langmuir, 20 (3)2270–2273 (2004)
29. Bhure, R, Mahapatro, A, Bonner, C, Abdel-Fattah, TM, ‘‘InVitro Stability Study of Organophosphonic Self AssembledMonolayers (SAMs) on Cobalt Chromium (Co-Cr) Alloy.’’Mater. Sci. Eng. C, 33 (4) 2050–2058 (2013)
30. Mahapatro, A, Matos Negron, TD, Nguyen, A, ‘‘Spectro-scopic Evaluations of Interfacial Oxidative Stability of
Phosphonic Nanocoatings on Magnesium.’’ J. Spectrosc.,2015 1–8 (2015)
31. Guerrero, G, Alauzun, JG, Granier, M, Laurencin, D, Mutin,PH, ‘‘Phosphonate Coupling Molecules for the Control ofSurface/Interface Properties and the Synthesis of Nanoma-terials.’’ Dalt. Trans., 42 (35) 12569 (2013)
32. Mani, G, Johnson, DM, Marton, D, Dougherty, VL, Feld-man, MD, Patel, D, Ayon, AA, Agrawal, CM, ‘‘Stability ofSelf-Assembled Monolayers on Titanium and Gold.’’ Lang-muir, 24 (13) 6774–6784 (2008)
33. Ishizaki, T, Okido, M, Masuda, Y, Saito, N, Sakamoto, M,‘‘Corrosion Resistant Performances of Alkanoic and Phos-phonic Acids Derived Self-Assembled Monolayers on Mag-nesium Alloy AZ31 by Vapor-Phase Method.’’ Langmuir, 27(10) 6009–6017 (2011)
34. Abohalkuma, T, Telegdi, J, ‘‘Corrosion Protection of CarbonSteel by Special Phosphonic Acid Nano-Layers.’’ Mater.Corros., 12 1382–1390 (2015)
36. Devillers, S, Lemineur, JF, Dilimon, VS, Barthelemy, B,Delhalle, J, Mekhalif, Z, ‘‘Polyelectrolyte Multilayer Depo-sition on Nickel Modified with Self-Assembled Monolayersof Organophosphonic Acids for Biomaterials: Electrochem-ical and Spectroscopic Evaluation.’’ J. Phys. Chem. C, 116(36) 19252–19261 (2012)
37. Guerrero, G, Alauzun, JG, Granier, M, Laurencin, D, Mutin,PH, ‘‘Phosphonate Coupling Molecules for the Control ofSurface/Interface Properties and the Synthesis of Nanoma-terials.’’ Dalt. Trans., 42 (35) 12569 (2013)
38. Liu, P, Huang, T, Liu, P, Shi, S, Chen, Q, Li, L, Shen, J,‘‘Zwitterionic Modification of Polyurethane Membranes forEnhancing the Anti-Fouling Property.’’ J. Colloid InterfaceSci., 480 91–101 (2016)
39. Rodriguez-Emmenegger, C, Hasan, E, Pop-Georgievski, O,Houska, M, Brynda, E, Alles, AB, ‘‘Controlled/LivingSurface-Initiated ATRP of Antifouling Polymer Brushesfrom Gold in PBS and Blood Sera as a Model Study forPolymer Modifications in Complex Biological Media.’’Macromol. Biosci., 12 (4) 525–532 (2012)
40. Shahsavan, H, Quinn, J, D’Eon, J, Zhao, B, ‘‘SurfaceModification of Polydimethylsiloxane Elastomer forStable Hydrophilicity, Optical Transparency and Film Lubri-cation.’’ Colloids Surfaces A Physicochem. Eng. Asp., 482267–275 (2015)
41. Zhang, J, Xiao, S, Shen, M, Sun, L, Chen, F, Fan, P, Zhong,M, Yang, J, ‘‘Aqueous Lubrication of Poly(N-HydroxyethylAcrylamide) Brushes: A Strategy for Their Enhanced LoadBearing Capacity and Wear Resistance.’’ RSC Adv., 6 (26)21961–21968 (2016)
42. Lilge, I, Schonherr, H, ‘‘Synthesis and Characterization ofWell-Defined Ligand-Terminated Block Copolymer Brushesfor Multifunctional Biointerfaces.’’ Polymer (Guildf), 98409–420 (2016)
43. Wu, T, Tan, L, Cheng, N, Yan, Q, Zhang, Y-F, Liu, C-J, Shi,B, ‘‘PNIPAAM Modified Mesoporous Hydroxyapatite forSustained Osteogenic Drug Release and Promoting CellAttachment.’’ Mater. Sci. Eng. C, 62 888–896 (2016)
44. Lilge, I, Jiang, S, Wesner, D, Schonherr, H, ‘‘The Effect ofSize and Geometry of Poly(Acrylamide) Brush-BasedMicropatterns on the Behavior of Cells.’’ ACS Appl. Mater.Interfaces, 8 (36) 23591–23603 (2016)
45. Ezzat, M, Huang, C-J, ‘‘Zwitterionic Polymer Brush Coat-ings with Excellent Anti-Fog and Anti-Frost Properties.’’RSC Adv., 6 (66) 61695–61702 (2016)
48. Luo, Y-R, Comprehensive Handbook of Chemical BondEnergies. CRC Press, Boca Raton (2007)
49. Arrotin, B, Delhalle, J, Dubois, P, Mespouille, L, Mekhalif,Z, ‘‘Electroassisted Functionalization of Nitinol Surface, aPowerful Strategy for Polymer Coating through ControlledRadical Surface Initiation.’’ Langmuir, 33 (12) 2977–2985(2017)
50. Metoki, N, Liu, L, Beilis, E, Eliaz, N, Mandler, D, ‘‘Prepa-ration and Characterization of Alkylphosphonic Acid Self-Assembled Monolayers on Titanium Alloy by Chemisorp-tion and Electrochemical Deposition.’’ Langmuir, 30 (23)6791–6799 (2014)
51. Vanhooland, A, Devillers, S, Issakova, T, Michaux, C,Delhalle, J, Mekhalif, Z, ‘‘Electroassisted Auto-Assemblyof Alkylphosphonic Acids Monolayers on Nitinol.’’ J. Elec-trochem. Soc., 163 (10) G173–G177 (2016)
52. Gupta, RK, Singh, RA, ‘‘Inhibition of Corrosion by Poly(N-Hexadecylaniline)/Docosanol Mixed Langmuir-BlodgettFilms on Copper in Sea Water.’’ Mater. Chem. Phys., 97(2–3) 226–229 (2006)
53. Li, C, Li, L, Wang, C, ‘‘Study of the Inhibitive Effect ofMixed Self-Assembled Monolayers on Copper with SECM.’’Electrochim. Acta, 115 531–536 (2014)
54. Barthelemy, B, Kanoufi, F, Combellas, C, Delhalle, J,Mekhalif, Z, ‘‘Phynox Improved Corrosion Resistance withMPC Initiated from Mixed Monolayers of PhosphonicAcids.’’ J. Electrochem. Soc., 161 (12) C1–C6 (2014)
57. Cumpson, PJ, Seah, MP, ‘‘Elastic Scattering Corrections inAES and XPS. II. Estimating Attenuation Lengths andConditions Required for Their Valid Use in Overlayer/Substrate Experiments.’’ Surf. Interface Anal., 25 (6) 430–446(1997)
58. Chamoulaud, G, Belanger, D, ‘‘Spontaneous Derivatizationof a Copper Electrode with in Situ Generated DiazoniumCations in Aprotic and Aqueous Media.’’ J. Phys. Chem. C,111 (20) 7501–7507 (2007)
59. Maho, A, Kanoufi, F, Combellas, C, Delhalle, J, Mekhalif, Z,‘‘Electrochemical Investigation of Nitinol/Tantalum HybridSurfaces Modified by Alkylphosphonic Self-AssembledMonolayers.’’ Electrochim. Acta, 116 78–88 (2014)
60. Socrates, G, Infrared and Raman Characteristic GroupFrequencies: Tables and Charts, Vol. Third. Wiley, London(2004)
61. Wei, C, Bard, AJ, Mirkin, MV, ‘‘Scanning ElectrochemicalMicroscopy. 31. Application of SECM to the Study ofCharge Transfer Processes at the Liquid/Liquid Interface.’’ J.Phys. Chem., 99 (43) 16033–16042 (1995)
62. Cannes, C, Kanoufi, F, Bard, AJ, ‘‘Cyclic Voltammetry andScanning Electrochemical Microscopy of Fer-rocenemethanol at Monolayer and Bilayer-Modified GoldElectrodes.’’ J. Electroanal. Chem., 547 (1) 83–91 (2003)
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