Thierry Thami
email: thierry.thami@umontpellier.fr

Lara Tauk
email: ltauk@ndu.edu.lb

Valérie Flaud
email: valerie.flaud@umontpellier.fr

Controlled structure and hydrophilic property of polymethylhydrosiloxane thin films attached on silicon support and modified with phosphorylcholine group

Keywords: Polymethylhydrosiloxane, Crosslinking, Chemical modification, Grafting, Hydrosilylation, Hydrophilic surface, Structure-property relation, X-ray photoelectron spectroscopy

come     Controlled structure and hydrophilic property of polymethylhydrosiloxane thin films attached on silicon support and modified with phosphorylcholine group

Highlights:

x Functionalization of hydroxylated substrate with a matrix surface rich in SiH group

x Surface of high density of phosphorylcholine group for biomedical applications

x Elaboration of functional film with cured polymethyldroxysiloxane in two steps

x Controlled structure and functionality of T-crosslinked polysiloxane network

x Evidence of 2:1 (Si:phospholipid) hydrosilylation adducts with linoleoyl chains

Introduction

The control of structure and composition of tailored interfaces between polymers and a surrounding medium are of great importance to improve interfacial properties in a number of applications. Examples are given by surfaces of biomaterials that control the wetting and the absorption of proteins which is required in implanted medical devices [START_REF] Cao | Effects of the morphology of sulfobetaine zwitterionic layers grafted onto a silicone surface on improving the hydrophilic stability, anti-bacterial adhesion properties, and biocompatibility[END_REF][START_REF] Yue | Bio-functionalisation of polydimethylsiloxane with hyaluronic acid and hyaluronic acid -Collagen conjugate for neural interfacing[END_REF], biosensors [START_REF] Akkahat | Development of a Novel Antifouling Platform for Biosensing Probe Immobilization from Methacryloyloxyethyl Phosphorylcholine-Containing Copolymer Brushes[END_REF][START_REF] Tanaka | Design and Fabrication of Biosensing Interface for Waveguide-Mode Sensor[END_REF], contact lenses [START_REF] Goda | Soft contact lens biomaterials from bioinspired phospholipid polymers[END_REF], lab-on-chips devices [START_REF] Xu | Phospholipid Polymer Biointerfaces for Lab-on-a-Chip Devices[END_REF], substrates for enzyme-linked immunosorbent assays (ELISA) [START_REF] Kumada | Development of a one-step ELISA method using an affinity peptide tag specific to a hydrophilic polystyrene surface[END_REF] and blood compatible materials [START_REF] Liu | Blood compatible materials: state of the art[END_REF][START_REF] Zhang | Blood compatibility of surfaces with superlow protein adsorption[END_REF]. Other applications in which functional polymer plays an active role at the interfaces include membrane separation [START_REF] Li | Making polymeric membranes antifouling via "grafting from" polymerization of zwitterions[END_REF][START_REF] Yang | Surface engineering of polymer membranes via mussel-inspired chemistry[END_REF], adhesion [START_REF] Murata | Polymeric coatings for biomedical devices[END_REF] and smart surfaces with sensing properties [START_REF] Luzinov | Adaptive and responsive surfaces through controlled reorganization of interfacial polymer layers[END_REF][START_REF] Zhai | Stimuli-responsive polymer films[END_REF]. However, it is still a challenge to fabricate polymer layers on solid surfaces with well-defined relationship between structures and properties, that are stable, uniform, continuously defect free, and that allow further chemical modification with molecules or various functionalities.

Our goal was to tailor polymer thin film grafted by covalent bonding with alkoxysilane reagents to afford a stable structure with strong adhesion to the substrate [START_REF] Pluedemann | Silane Coupling Agents[END_REF]. We developed recently a method for surface modification of solid substrates by chemical attachment of sol-gel cured polymethylhydrosiloxane (PMHS) polymer network containing SiH functionalities [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF].

Compared with the room temperature vulcanization process [START_REF] Brook | Silicon, in Organic, Organometallic and Polymer Chemistry[END_REF], this method offers the advantage of a one-step polymerization process that gives continuous and homogeneous thin films of controlled thicknesses in the micrometer range. Moreover, the mechanical properties of the PMHS thin films can be easily tuned by sol-gel formulation [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF]. This process results in a biocompatible and homogenous silicone platform containing reactive SiH groups with controlled flexibility. The introduction of siloxane building blocks with SiH bonds would be a suitable synthetic approach for the decoration of surfaces with functional groups. The reactive methyl hydrosiloxane Si-H polymers [START_REF] Saxena | A Systematic Approach To Decipher the Microstructure of Methyl Hydrosiloxane Copolymers and Its Impact on Their Reactivity Trends[END_REF] of structure [-OSi(H)Me-]/D H can serve as chemical links to covalently attach the desired functionality through well-known catalytic hydrosilylation reaction via an alkene precursor [START_REF] Brook | Silicon, in Organic, Organometallic and Polymer Chemistry[END_REF][START_REF] Saxena | A Systematic Approach To Decipher the Microstructure of Methyl Hydrosiloxane Copolymers and Its Impact on Their Reactivity Trends[END_REF][START_REF] Speier | Homogeneous Catalysis of Hydrosilation by Transition Metals[END_REF][START_REF] Lewis | Chemical Catalysis by Colloids and Clusters[END_REF][START_REF] Roy | A review of recent progress in catalyzed homogeneous hydrosilation (Hydrosilylation)[END_REF][START_REF] Lukevics | Latest Research on the Hydrosilylation Reaction[END_REF][START_REF] Marciniec | Comprehensive Handbook on Hydrosilylation[END_REF]. Hydrosiloxanes are useful for the preparation of many organic/inorganic hybrid materials [START_REF] Putzien | Functionalized polysilalkylene siloxanes (polycarbosiloxanes) by hydrosilylation-Catalysis and synthesis[END_REF][START_REF] Satoh | Highly Selective Synthesis of Hydrosiloxanes by Au-Catalyzed Dehydrogenative Cross-Coupling Reaction of Silanols with Hydrosilanes[END_REF][START_REF] Lukevics | Hydrosilylation. Recent Achievments[END_REF] and polymers with alkyl chains exhibiting selfassembly [START_REF] Kadib | Nanocomposites from natural templates based on fatty compound-functionalised siloxanes[END_REF][START_REF] Mukbaniani | Formation of polymethylsiloxanes with alkyl side groups[END_REF] using the reactivity of the Si-H bonds. We previously reported the fabrication of chemical sensors by surface modification of silica substrates with PMHS films for the detection of nitroaromatic compounds [START_REF] Amro | Supported thin flexible polymethylhydrosiloxane permeable films functionalised with silole groups: new approach for detection of nitroaromatics[END_REF] or metal ions [START_REF] Nasr | Functionalization of Polymethylhydrosiloxane Gels with an Allyl Ureido Benzocrown Ether Derivative: Complexation Properties[END_REF]. Similarly, the trend of reactivity of sol-gel cured PMHS films of various crosslinks densities was first studied by using terminal 1alkenes with Karstedt's catalyst [START_REF] Thami | Fonctionalization of Surface-Grafted Polymethylhydrosiloxane with Alkyl Side Chains[END_REF]. Hydrosylilation results in the formation of hydrolytically stable Si-C bonds. This is essential for polymer-based chemical sensors in order to avoid molecule leakage into the medium and inefficient analyses.

In the present work, we were interested in preparing smooth polysiloxane thin films on silicon wafer as a model to form surfaces with controllable structure and hydrophilic properties targeted for anti-biofouling applications. The main parameters for the design of protein resistance surfaces are related to the hydrophilic nature of the functionalized material, the density and accessibility of the hydrophilic groups [START_REF] Liu | Blood compatible materials: state of the art[END_REF]. The formation of electrically neutral surface [START_REF] Chen | Biocompatible polymer materials: Role of protein-surface interactions[END_REF] and the surface topography can also control the interactions with proteins [START_REF] Liascukiene | Lipid Layers on Nanoscale Surface Topography: Stability and Effect on Protein Adsorption[END_REF]. One of the most effective approach for preventing the adhesion of protein involves the functionalization of surfaces with polymer bearing very hydrophilic phosphorylcholine (PC) head groups [START_REF] Albrecht | Stable biomembrane surfaces formed by phospholipid polymers[END_REF][START_REF] Johnston | Phospholipid polymers -synthesis and spectral characteristics[END_REF][START_REF] Lewis | Phosphorylcholine-based polymers and their use in the prevention of biofouling[END_REF][START_REF] Nakaya | Recent progress of phospholipid polymers[END_REF]. In recent years, different strategies have been developed to allow the grafting of PC-based polymers or PC containing molecules on various substrates and to afford stable, homogenous and PC-rich interfaces with long term protein repellent behavior. Surface graft polymerization incorporating PC moieties in particular with 2-methacryloxyethyl phosphoryl choline (MPC) monomer has been used to covalently attach poly-(MPC) to silica [START_REF] Kim | Novel Brush Polymers with Phosphorylcholine Bristle Ends: Synthesis, Structure, Properties, and Biocompatibility[END_REF][START_REF] Feng | Adsorption of fibrinogen and lysozyme on silicon grafted with poly(2-methacryloyloxyethyl phosphorylcholine) via surface-initiated atom transfer radical polymerization[END_REF][START_REF] Kobayashi | Friction behavior of high-density poly(2-methacryloyloxyethyl phosphorylcholine) brush in aqueous media[END_REF] or polymers [START_REF] Kyomoto | Selfinitiated surface grafting with poly(2-methacryloyloxyethyl phosphorylcholine) on poly(ether-ether-ketone)[END_REF][START_REF] Kyomoto | Self-Initiated Surface Graft Polymerization of 2-Methacryloyloxyethyl Phosphorylcholine on Poly(ether ether ketone) by Photoirradiation[END_REF][START_REF] Hoshi | Protein adsorption resistant surface on polymer composite based on 2D-and 3D-controlled grafting of phospholipid moieties[END_REF][START_REF] Morimoto | Nano-scale surface modification of a segmented polyurethane with a phospholipid polymer[END_REF][START_REF] Yan | Graft copolymerization of 2-methacryloyloxyethyl phosphorylcholine to cellulose in homogeneous media using atom transfer radical polymerization for providing mew hemocompatible coating materials[END_REF][START_REF] Goda | Biomimetic phosphorylcholine polymer grafting from polydimethylsiloxane surface using photo-induced polymerization[END_REF][START_REF] Seo | Surface tethering of phosphorylcholine groups onto poly(dimethylsiloxane) through swelling-deswelling methods with phospholipids moiety containing ABA-type block copolymers[END_REF][START_REF] Ishihara | Phosphorylcholine Group-immobilized Surface Prepared on Polydimethylsiloxane Membrane by In Situ Reaction for Its Reduced Biofouling[END_REF] through conventional free-radical polymerization initiated by various methods. Self-assembled monolayers using alkane-thiol [START_REF] Holmlin | Zwitterionic SAMs that resist nonspecific adsorption of protein from aqueous buffer[END_REF][START_REF] Tegoulia | Surface properties, fibrinogen adsorption, and cellular interactions of a novel phosphorylcholine-containing self-assembled monolayer on gold[END_REF] or disulfide [START_REF] Chen | Strong resistance of phosphorylcholine selfassembled monolayers to protein adsorption: Insights into nonfouling properties of zwitterionic materials[END_REF][START_REF] Chung | Self-assembled biomimetic monolayers using phospholipid-containing disulfides[END_REF] molecules containing the PC head group were prepared on gold surfaces. Grafting PC on solid surfaces has also been achieved using siloxane chemistry through coupling of phospholipid derivatives with an aminosilane coupling agent on a silica surface such as glass or silicon wafers [START_REF] Kohler | Platelet adhesion to novel phospholipid materials: Modified phosphatidylcholine covalently immobilized to silica, polypropylene, and PTFE materials[END_REF][START_REF] Lu | Reduced protein adsorption on the surface of a chemically grafted phospholipid monolayer[END_REF]. PC derivatives with dimethylchlorosilane were synthezised to coat glass by covalent bonding [START_REF] Durrani | Biomembranes as models for polymer surfaces. 2. The syntheses of reactive species for covalent coupling of phosphorylcholine to polymer surfaces[END_REF][START_REF] Hayward | Biomembranes as models for polymer surfaces. 3. Characterization of a phosphorylcholine surface covalently bound to glass[END_REF]. Silanated zwiterrionic PC molecules were recently prepared by the hydrosilylation reaction of the MPC monomer with triethoxysilane before covalent fixation on titanium alloy [START_REF] Ye | Simple surface modification of a titanium alloy with silanated zwitterionic phosphorylcholine or sulfobetaine modifiers to reduce thrombogenicity[END_REF]. A crosslinkable coating from a PC-based polymer having pendent trimethoxysilane group has been prepared to improve the stability of the film and the possibility of anchoring the polymer to substrate [START_REF] Lewis | Crosslinkable coatings from phosphorylcholine-based polymers[END_REF].

However, chemical transformation of the phosphorycholine-containing molecule was necessary to obtain the polymerizable or functional group permitting covalent bonding to the surface. These methods have a great disadvantage: they present some difficulties in the reproductibility of the process of surface modification, in particular in the case of the polymerization process of the MPC monomer. Alternative methods have been recently developed by use of active surfaces with electrically switchable hydrophobic/hydrophilic wettability targeted to prevent bacterial attachment and biofilm formation [START_REF] Děkanovský | Dual-Action Flexible Antimicrobial Material: Switchable Self-Cleaning, Antifouling, and Smart Drug Release[END_REF][START_REF] Guselnikova | Smart, Piezo-Responsive Polyvinylidenefluoride/Polymethylmethacrylate Surface with Triggerable Water/Oil Wettability and Adhesion[END_REF].

Surfaces of PMHS modified with zwitterionic PC groups have also been investigated for protein resistance [START_REF] Ferez | Interface of Covalently Bonded Phospholipids with a Phosphorylicholine Head: Characterization, Protein Nonadsorption, and Further Functionalization[END_REF][START_REF] Tauk | Thin phosphatidylcholine films as background surfaces with further possibilities of functionalization for biomedical applications[END_REF]. The surfaces were prepared by grafting a natural phospholipid precursor bearing unsaturated fatty chains to PMHS films on glass or silicon. Additionally, very few studies have reported the hydrosilylation of unsaturated fatty acid derivatives [START_REF] Kadib | Nanocomposites from natural templates based on fatty compound-functionalised siloxanes[END_REF][START_REF] Delpech | Toward new biosilicones: hydrosilylation of fish oil unsaturated fatty acid esters[END_REF][START_REF] Lligadas | Novel organic-inorganic hybrid materials from renewable resources: Hydrosilylation of fatty acid derivatives[END_REF][START_REF] Saghian | Hydrosilylation of long chain unsaturated fatty acid esters[END_REF][START_REF] Behr | Catalytic processes for the technical use of natural fats and oils[END_REF][START_REF] Behr | The synthesis of silicon oleochemicals by hydrosilylation of unsaturated fatty acid derivatives[END_REF][START_REF] Behr | Platinum-catalysed hydrosilylation of unsaturated fatty acid esters[END_REF][START_REF] Kadib | Hydrosilylation of unsaturated fatty acid N-phenyl amides[END_REF][START_REF] Kadib | Silylation of triacylglycerol: an easy route to new biosiloxanes[END_REF]. The catalytic hydrosylilation process to PMHS films gave surfaces of high densities of phospholipid that are highly hydrophilic and strongly resistant to several proteins [START_REF] Ferez | Interface of Covalently Bonded Phospholipids with a Phosphorylicholine Head: Characterization, Protein Nonadsorption, and Further Functionalization[END_REF][START_REF] Tauk | Thin phosphatidylcholine films as background surfaces with further possibilities of functionalization for biomedical applications[END_REF] (fibrinogen adsorption < 5 ng/cm 2 ) in comparison with other zwitterionic materials (sulfobetaine, carboxybetaine) [START_REF] Yang | Film Thickness Dependence of Protein Adsorption from Blood Serum and Plasma onto Poly(sulfobetaine)-Grafted Surfaces[END_REF][START_REF] Zhang | Dual-Functional Biomimetic Materials: Nonfouling Poly(carboxybetaine) with Active Functional Groups for Protein Immobilization[END_REF]. However, there is no report to control hydrophilicity and consequently the adsorption of proteins for the PC-silicone surfaces prepared by hydrosilylation grafting to PMHS.

We hereby propose to analyze the relationships between the chemical structure and hydrophilic properties of the phospholipid functionalized PMHS polymer in relation with the hydrosilylation procedure used for its surface modification. The main goal was to get a description, as precise as possible, of the structure of these gels before and after reaction. The structural characteristics of the films were investigated by X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR). The surfaces properties were examined by water contact angle (WCA) measurements in air, atomic force microscopy (AFM) and scanning electron microscopy (SEM).

The surface wetting behavior and protein adsorption are finally discussed and correlated in terms of phospholipid grafting density in relation with the proposed chemical structure.

Experimental details

Chemicals

Both precursors methyldiethoxysilane HSi(CH 3 )(OCH 2 CH 3 ) 2 (DH) and triethoxysilane HSi(OCH 2 CH 3 ) 3 (TH), and the sol-gel catalyst trifluoromethanesulfonic acid CF 3 SO 3 H were purchased from Aldrich and used as received. Water for substrate cleaning was obtained from a Milli-Q water purification apparatus (Millipore). Absolute ethanol for solgel synthesis was of synthesis grade purity. Toluene for thin film hydrosilylation was dried and distilled with calcium hydride before use. The platinum complex [Pt 2 (sym-tetramethyldivinyldisiloxane) 3 ] in xylene (platinum concentration ca. 0.1 M assuming 2.4 % (w) Pt), also known as Karstedt's catalyst, was purchased from Aldrich. 1,2-dilinoleoyl-sn-glycero-3-phosphorylcholine (18:2 Cis) (PL) was purchased from AvantiPolarLipids.

Thin films preparation

Substrate cleaning and activation

Silicon wafers Si(100) (ACM, France) cut into square strips of 2 × 2 cm 2 were used as substrates for spin-coating deposition. To bond covalently the PMHS thin films to native oxide silica (thickness a 2 nm) of silicon wafer, the substrates were first cleaned and activated with "piranha" solution using the previously described procedure [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF]. Caution: 'piranha' solution must be handled extremely carefully.

Synthesis of crosslinked PMHS films

PMHS thin films of various crosslinking densities (30, 15 and 5 mol % TH) were prepared at room temperature by sol-gel polymerization of DH:TH sol mixtures deposited by spin-coating on silicon wafer (100) substrates freshly activated by "piranha" solution. The procedure is briefly summarized as follows [START_REF] Nasr | Functionalization of Polymethylhydrosiloxane Gels with an Allyl Ureido Benzocrown Ether Derivative: Complexation Properties[END_REF][START_REF] Thami | Fonctionalization of Surface-Grafted Polymethylhydrosiloxane with Alkyl Side Chains[END_REF]. The mixture DH/TH of monomers (4.0 M in EtOH; molar ratio

[EtOH]/[Si] = 1) was polymerized with hydrolysis ratio h = [H 2 O]/[SiOEt] = 0.5.
Trifluoromethanesulfonic acid CF 3 SO 3 H (1.0 M in absolute ethanol) was used as catalyst (0.5 mmol/mol of monomers). The freshly activated substrate was purged (2 min) in the spincoater (Spin150, SPS Europe) under a stream of nitrogen (2 L/min) to avoid air moisture. The resulting clear sol was allowed to age for about 30 minutes with magnetic stirring before spincoating deposition to give a fully crosslinked [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF] thin film (~ 0.5 µm). For all samples, the speed of rotation was 4000 rpm (spin acceleration 2000 rpm/s) and the rotation time 30 s. The coated samples were finally cured at 110 °C in an oven for ten minutes. This procedure gave PMHS spin-coated films of reproducible homogeneity and thickness as measured by IR spectroscopy (Section 2.3.2). The PMHS samples of various crosslinking densities were analyzed after curing.

Hydrosilylation of PMHS films with phospholipid 2.2.3.1. Casting

The hydrosilylation reaction between the dilinoleoyl phospholipid and the PMHS Si-H function was performed in air by casting a toluene solution of phospholipid in the presence of Karstedt's catalyst onto the substrate bearing 5 % lightly crosslinked PMHS film. 20 mg of the reactant 1,2dilinoleoyl-sn-glycero-3-phosphorylcholine (18:2 Cis) were dissolved in 1 mL of dry toluene under argon to avoid air moisture and 2 μL of the Pt-xylene solution were added. The samples were covered with 200 µL of phospholipid solution (25.6 mM) at room temperature and were kept for about one hour under a cap, and then left in open air to allow total evaporation of toluene. The phospholipid compound was in 1.5-fold excess with respect to available hydrogenosilane SiH group for a thin crosslinked PMHS film coated on silicon wafer or glass (0.5 µm thickness, 2 cm × 2 cm) assuming the density of the PMHS layer is about 1 g/mL. To finish the reaction, the sample was then heated for an additional curing time (1 to 15 hours) at 40 °C in a thermostated box (Relative Humidity 12 % ± 2 %) to protect the reaction from ambient moisture.

Immersion

The hydrosilylation reaction was also performed with 1.5-fold excess of PL over SiH group by placing 5 % crosslinked PMHS film anchored on wafer in 1.6 ml of phospholipid solution (3.2 mM) prepared as follows: 20 mg of PL were dissolved in 8 mL of dry toluene under argon with additional 7 µL of the Karstedt Pt-catalyst. Typically, various samples were immersed for 1 to 6 hours without stirring at room temperature.

Washing procedure

For both methods, the PL modified samples were denoted as PL-PMHS. They were rinsed after reaction successively in different toluene/chloroform mixtures from 100/0 to 0/100 (% Vol.) with a gap of 20 % and dried under a stream of argon. The samples were then rinsed with water (2 h) to remove any physisorbed material, and finally dried under a stream of argon. Freshly prepared PL-PMHS surfaces were analyzed to avoid pollutants contamination.

Characterization techniques

Infrared spectroscopy

The Fourier-transform infrared (FTIR) spectroscopy analyses were carried out on a Nicolet Nexus FTIR spectrometer. Transmission FTIR spectra were recorded in the 4000-400 cm -1 range with a resolution of 1 cm -1 and 32 scans to obtain qualitative and quantitative informations about yield of hydrosilylation reaction. A background spectrum was recorded using a piece of bare unmodified silicon wafer in air. Calculations for the films after reaction were based on CH 2 absorption with an accuracy of ± 5% neglecting SiH content (~ 0%) because 32 scans analysis was not enough to provide an accurate measurement.

Thickness measurements of virgin PMHS

The thickness (e) of virgin PMHS film (5% crosslinked) deposited on silicon wafer was measured by FTIR spectroscopy from the absorbance of the Si-H peak at 2169 cm -1 according to the Beer-Lambert law ܣ ଶଵଽ = α ୗ୧ୌ × ݁. The absorption coefficient α ୗ୧ୌ = ε ୗ୧ୌ × ]ܪ݅ܵ[ = 0.27 µm ିଵ was calibrated for thin film samples of 200 nm thick for which independent thickness was easily measured using ellipsometry (Accuracy of ± 5%). Ellipsometry was achieved on a Horiba Jobin Yvon PZ2000 ellipsometer equipped with a 632.8 nm He-Ne laser with a spot size of 2 mm. The angle of incidence was 70.0°. The thickness of the silicon oxide layer of about 2 nm was measured separately assuming the refractive index n = 1.457 for SiO 2 and ݊ = 3.871 -0.16i for silicon, and for PMHS coatings we used a fixed refractive index of n = 1.40 ± 0.02 [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF][START_REF] Thami | Fonctionalization of Surface-Grafted Polymethylhydrosiloxane with Alkyl Side Chains[END_REF].

X-ray photoelectron spectroscopy

XPS data were obtained on an ESCALAB 250 (Thermo Electron, UK) spectrophotometer using Al Kα (1486.6 eV) irradiation source (15 kV, 100 W). The photoelectrons were analyzed at normal incidence of the sample surface. The spot size was approximately 400 Pm 2 . The composition corresponds to depths of 5-10 nm. Survey scans (0-1350 eV) at low resolution were performed to identify the constitutive elements. The peaks were fitted with Gauss-Lorentz curves with the theoretical sensitivity factors reported by Scofield [START_REF] Scofield | Hartree-Slater subshell photoionization cross-sections at 1254 and 1487eV[END_REF]. Si 2p doublets were considered in the fitting process of the Si 2p core level [START_REF] O'hare | Development of a methodology for XPS curvefitting of the Si 2p core level of siloxane materials[END_REF][START_REF] Beamson | High resolution XPS of Organic Polymers[END_REF]. A charge correction was applied to set the binding energy scale. The spectra were calibrated using the hydrocarbon contaminant or alkane C 1s peak at 284.8 eV. In both cases, the peak of Si-CH 3 was found at a binding energy of 284.0 eV in agreement with PDMS polymer [START_REF] Alexander | A study of HMDSO/O2 plasma deposits using a high-sensitivity and -energy resolution XPS instrument: curve fitting of the Si 2p core level[END_REF] at 284.4 eV.

Atomic force microscopy

AFM experiments were performed using a Dimension 3100 microscope equipped with a Nanoscope IIIa controller system (Digital Instruments, Veeco Metrology Group). AFM images were obtained by scanning in tapping mode under air ambient conditions using silicon SPM probes (stiffness k ≈ 2 N/m, resonance frequency of 67 kHz, pointe probeplus, Nanosensors). All images were analyzed using data-processing software Gwyddion (Version 2.54).

Scanning electron microscopy

SEM pictures of cross section and thickness of the films were obtained with a Hitachi S-4800 instrument operating at an accelerating voltage of 2 kV, and with different magnifications of 18k u and 40k u.

Water contact angle measurement

Air captive bubble and sessile drop WCAs were measured (Contact angle meter, GBX, France) by applying to the surface respectively, an air bubble of about 8 µL in water, and a 3 µL water droplet in ambient air. The air captive bubble WCAs discussed are relative to the complement angle (T) to 180° through the liquid [START_REF] Maldonado-Codina | In vitro water wettability of silicone hydrogel contact lenses determined using the sessile drop and captive bubble techniques[END_REF]. The receding and advancing WCAs were measured using the injection and the withdrawn of a 5 μL water droplet by using a motor-driven syringe at constant speed of 0.5 µl/min. WCAs were calculated by an automated polynomial fitting method using computerized image analysis (Visiodrop, GBX).

Results and discussion

Synthesis of the PMHS and PL-PMHS films

Crosslinked PMHS films

A reactive polymethylhydrosiloxane PMHS polymer crosslinked by the sol-gel reaction of hydrogenosilane precursors was prepared in the first step (Scheme 1a). PMHS films were synthesized on silicon wafer at room temperature by acid-catalyzed polymerization of a sol-gel mixture of methyldiethoxysilane/triethoxysilane (DH/TH) using the spin-coating procedure giving fully crosslinked network. Neglecting a small amount of remaining cycles in the 3D network, the resulting surface-attached 3D polymer network can be characterized by the average number of monomer units [-OSi(H)CH 3 -] in PMHS segment between crosslink subunits SiHO 3/2 :

ܰ = 2ߙ/3(1 -ߙ) (1)
where D /(1-D) is the composition of DH/TH mixture. The soft PMHS material prepared with a small amount of TH additive (5 mol %) was used because polymer (3D-network) or multilayers of PMHS chains are created in such lightly crosslinked PMHS network with N ~ 13 from Eq. (1).

Phospholipid functionalized PL-PMHS films

The chemical grafting process is shown in Scheme 1b with the unsaturated phospholipid (PL) molecule bearing internal double bonds. The hydrosilylation reaction in the presence of Karstedt's platinum catalyst and air allows subsequent functionalization in PMHS layer by hydrosilylation of the SiH groups with one of the internal double bond of each linoleoyl chains.

The hydrosilylation reaction was carried out using an excess of alkenes with respect to available SiH on a spin-coated PMHS thin film (Experimental details). Thus, the linoleoyl chain of phospholipid molecule could be connected to SiH groups by hydrosilylation in different polymethylhydrosiloxane layers at the interface by bridging the polysiloxane chains. In addition, the reaction of many linoleoyl chains may also increase the crosslinking density of the PMHS gel.

By anticipating XPS results (Section 3.3), we postulated that hydrosylilation of PL molecule may lead preferentially to the formation of the 2:1 (Si:PL) alkene adduct resultant from onefold addition of methylhydrosilane on each linoleoyl chain of PL molecule as sketched in boxed Scheme 1b. This reaction scheme is different from that proposed in our previous work where we assumed that all double bonds of PL have reacted with SiH to form 4:1 (Si:PL) adduct [START_REF] Ferez | Interface of Covalently Bonded Phospholipids with a Phosphorylicholine Head: Characterization, Protein Nonadsorption, and Further Functionalization[END_REF].

Indeed, Behr et al. [START_REF] Behr | The synthesis of silicon oleochemicals by hydrosilylation of unsaturated fatty acid derivatives[END_REF][START_REF] Behr | Platinum-catalysed hydrosilylation of unsaturated fatty acid esters[END_REF] showed that the reactivity of internal double bonds of unsaturated fatty acid ester such as methyl linoleate is lower than that of terminal double bond. Such a low reactivity of internal double bonds was also reported in the case of fatty amide [START_REF] Kadib | Hydrosilylation of unsaturated fatty acid N-phenyl amides[END_REF] and triacylglycerol [START_REF] Kadib | Silylation of triacylglycerol: an easy route to new biosiloxanes[END_REF] derivatives. The proposed reaction scheme supports the assumption that there is a conjugation of the two internal double bonds before hydrosilylation and migration of the remaining double bond in the product after reaction [START_REF] Behr | The synthesis of silicon oleochemicals by hydrosilylation of unsaturated fatty acid derivatives[END_REF][START_REF] Behr | Platinum-catalysed hydrosilylation of unsaturated fatty acid esters[END_REF]. The formation of a mixture of cis/trans isomers is also postulated as side reaction.

Infrared analysis

Characterization of virgin PMHS and functionalized PMHS films

The chemical structure of the obtained PMHS film before and after hydrosilylation by casting method was characterized by FTIR spectroscopy (Fig. 1a andb). The peak assignments are summarized in Table 1. From spectrum of virgin PMHS (Fig. 1a) to spectrum PL-PMHS (Fig. 1b), the strong absorption peaks of Si-H groups (2169 and 838 cm -1 ) [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF][START_REF] Smith | The Analytical Chemistry of Silicones[END_REF] almost vanished while that of C=O (1736 cm -1 ), CH 2 (2856 and 2926 cm -1 ) and PO (1090 and 1250 cm -1 ) groups appeared. These observations seem to confirm the reaction of phospholipid molecules in PMHS film after hydrosilylation and solvent washing. The phosphate band Q(PO) in the phosphorylcholine subunit was found to overlap as shoulder peak at about 1090 cm -1 with the very strong SiOSi doublet of the PMHS matrix.

The grafting process of PL to the polysiloxane backbone is also evidenced by the modification of the single sharp band G s (SiCH 3 ) for PMHS which is split in PL-PMHS spectrum at 1271 cm -1 indicating clearly the appearance of new G s (SiCH) bands [START_REF] Thami | Fonctionalization of Surface-Grafted Polymethylhydrosiloxane with Alkyl Side Chains[END_REF]. As no IR spectral evidence for any remaining C=C bonds was found in PL-PMHS film by comparison with spectrum of PL molecule (Fig. 1b andc), we can conclude that hydrogenation of the remaining unreacted double bonds occurred as an undesired side pathway [START_REF] Behr | The synthesis of silicon oleochemicals by hydrosilylation of unsaturated fatty acid derivatives[END_REF][START_REF] Behr | Platinum-catalysed hydrosilylation of unsaturated fatty acid esters[END_REF][START_REF] Kadib | Silylation of triacylglycerol: an easy route to new biosiloxanes[END_REF] to give a saturated aliphatic grafted structure made of stearate-like chains. The remaining double bonds of grafted PL molecules can react with SiH silane groups present onto PMHS film. To conclude, these FTIR data are consistent with the chemical structure for the PL-PHMS film sketched in Scheme 1b.

Reaction of PMHS with water

The evolution of transmission IR absorption spectra of PMHS films (~ 0.5 μm) before (t = 0) and after hydrosilylation (at different times) is shown in Fig. 2 for casting conditions. In all cases, the reaction goes essentially to completion with a residual absorption intensity of the Q(SiH) peak at ~ 2170 cm -1 for PL-PMHS close to 0 % (Table 2). As shown in previous studies on hydrosilylation by PMHS for bulkiness allyl precursors [START_REF] Amro | Supported thin flexible polymethylhydrosiloxane permeable films functionalised with silole groups: new approach for detection of nitroaromatics[END_REF][START_REF] Nasr | Functionalization of Polymethylhydrosiloxane Gels with an Allyl Ureido Benzocrown Ether Derivative: Complexation Properties[END_REF], no SiH groups remained unreacted because the SiH underwent reaction with water (present from air moisture or bound to PC moiety). This led to the hydrolysis of part of the SiH groups into silanol Si-OH groups which may recombine to produce additional SiOSi siloxane crosslinks in the polysiloxane network by Pt-catalyzed dehydrogenative cross-coupling reaction [START_REF] Satoh | Highly Selective Synthesis of Hydrosiloxanes by Au-Catalyzed Dehydrogenative Cross-Coupling Reaction of Silanols with Hydrosilanes[END_REF][START_REF] Nasr | Functionalization of Polymethylhydrosiloxane Gels with an Allyl Ureido Benzocrown Ether Derivative: Complexation Properties[END_REF] of the newly formed silanols with SiH (Scheme 2). Ultimately, new T-crosslinks (T/SiCH 3 O 3/2 ) are formed into the polysiloxane network. This is confirmed by a significant increase (about 50 %) of the total area of the Q(SiOSi)

absorption band (Fig. 1a andb) and by the appearance of Q(SiO) and Q(OH) broad bands (Table 1) in PL-PMHS spectrum indicating the presence of SiOH groups.

Quantitative study of the PL grafting in PMHS films

Since the silane SiH groups may also undergo competitive reactions of hydrolysis, we preferred

XPS analysis

Characterization of virgin PMHS and functionalized PMHS interfaces

The chemical modification of the PMHS polymer film and the attachment of phospholipid molecule at the interface were confirmed by using XPS spectroscopy (Figures 35 and Tables 35). The XPS data of pristine PMHS of various crosslinking density (1 -D) obtained by increasing the amount of TH crosslinker are in good agreement with the elemental composition of the nominal PHMS structure (Fig. 3a and Table 3), summarized as (CSiO 2/2 ) D (SiO 3/2 ) (1-D) , for fully crosslinked polymer networks predominatly consisting of D H /SiHCH 3 O 2/2 and T H /SiHO 3/2 subunits [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF]. In addition, the Si 2p high resolution scan spectra of PMHS polymers were resolved in two components with increasing Si-O functionality attributed to D H and T H subunits (Fig. 4 and Table 4) in agreement with the Si 2p BE values reported for PDMS resins [START_REF] O'hare | Development of a methodology for XPS curvefitting of the Si 2p core level of siloxane materials[END_REF]. The fitting results for various D H /T H compositions (Table 3) match well the composition determined by quantitative 29 Si solid-state nuclear magnetic resonance [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF] or FTIR spectroscopy [START_REF] Thami | Fonctionalization of Surface-Grafted Polymethylhydrosiloxane with Alkyl Side Chains[END_REF] for DH/TH gels prepared in similar conditions. For a more detailed investigation, the C 1s high resolution XPS spectra of pristine PMHS (D = 0.95) is also shown in Fig. 4. The major C-Si peak at 284.0 eV is assigned to methyl carbons SiCH 3 in silicone in agreement with the value of 284.4 eV reported for bulk PDMS polymer [START_REF] Beamson | High resolution XPS of Organic Polymers[END_REF], along with a small amount of C-C contribution at 284.8 eV attributed to adventitious carbon.

After hydrosilylation of PMHS with PL, peaks attributed respectively to ammonium and phosphate group of the PC unit, nitrogen (N 1s , 403 eV) and phosphorus (P 2s , 134 eV), appeared clearly in the XPS spectra (Fig. 3b and Fig. 5). Furthermore, the C 1s peak increases dramatically upon grafting of the phospholipid by comparison with pristine PMHS, owing to the presence of the PC groups and the fatty acid ester with a long hydrocarbon chain. For a more detailed investigation, the C 1s high resolution XPS spectrum (Fig. 4) can be well fitted by four components at binding energies of 284.0, 284.8, 286.4 and 288.9 eV which were assigned to C-Si, C-C, C-O and C=O, respectively. The C-Si component can be attributed to both SiCH 3 carbon of the matrix and SiCH PL carbons in PL-PMHS resulting from hydrosilylation with the internal double bond of phospholipid (CH PL denotes hydrocarbon in SiCH bond formed with PL molecule). Indeed, we expected the C 1s binding energy of the Si-C bond in alkyl polysiloxanes [START_REF] Scofield | Hartree-Slater subshell photoionization cross-sections at 1254 and 1487eV[END_REF] to be relatively insensitive to alkyl substitution.

Reaction of water at the PL-PMHS interface

For the grafted more complex surface of PL-PMHS, the shift in the Si 2p binding energy to higher value after hydrosilylation (Fig. 4) indicates clearly that essentially oxygen has been incorporated into the material following SiH reaction with H 2 O in agreement with FTIR analysis (Section 3.2). Indeed, we expected the BE shift caused by replacement of SiH by SiCH PL to be small in comparison to the shift induced by oxygen [START_REF] O'hare | Development of a methodology for XPS curvefitting of the Si 2p core level of siloxane materials[END_REF][START_REF] Alexander | A study of HMDSO/O2 plasma deposits using a high-sensitivity and -energy resolution XPS instrument: curve fitting of the Si 2p core level[END_REF]. This result was confirmed by Si 2p curve-fitting of the high resolution spectrum by two different Si 2p environments (Tables 4 and5).

A shown in Fig. 4 for casting treatment, the minor Si 2p component SiO 5). To conclude, these observations show a strong increase in the crosslinking density of the polysiloxane network after hydrosilylation by creation of T-crosslinks in presence of trace of water.

Calculation of PL graft yield at the interface

The XPS results were rationalized by postulating a nominal composition for the PL-PMHS layer made from polysiloxane-bound 2:1 (Si:PL) adducts as sketched in Scheme 1b with additional Tcrosslinks (Scheme 2). So we applied the following composition summarized as: 4 and5). For simplicity, the T CH content (< 5 %) will be not considered in this calculation because for the Si 2p curve-fitting, the expected amounts were not enough to give reliable results. The calculated values are in good agreement with the experimental elemental composition as shown in Table 5 for all experiments.

[(C 44 NO 8 P) 1/2 (CSiO 2/2 )] x(1 -y) [CSiO 2/2 ] (1 -x)(1 -y) [
The PL graft yield (Z) at the interface is defined as a percentage of the surface grafted PL molecule over the total number of silicon atom [PL]/[Si] from XPS data. Because of the uncertainty as to the relative weight of phosphorus or nitrogen owing to weak signals, instead of estimating Z as P/Si or N/Si, we used the experimental ratio C/Si of carbon over silicon using the following expression derived from the above stoichiometric structure:

ܼ = ݅ܵ/ܥ( -1)/44 × 100 (3)
This expression is independent of the nature of the Si:PL adduct formed in the polymer network.

By assuming the exclusive formation of (2:1) Si:PL adduct, the graft yield can also be written as

Z = x(1 -y)/2 = U s /2 (4)
with an expression similar to Eq. ( 2) where U s is the SiH conversion hydrosilylation yield at the surface.

Effect of reaction conditions 3.3.4.1. Casting method

From Fig. 3b and Table 5, the greatest amount of grafted phospholipid (Z ~ 12%) was obtained in condition (1), by casting treatment at 40 °C/15 h. The grafting of hydrophilic PC groups is confirmed by captive air bubble contact angle of T q measured on these highly hydrophilic PC-functionalized surfaces (Section 3.4.2). The samples prepared by casting (curing times varying from 1 h to 15 h) are characterized in that the PL graft yield are closed to the upper value limit of T q These observations show that the hydrosilylation reaction of internal double bonds of PL molecule has been optimized by thermal activation of hydrosilylation at 40 °C, as expected using Karstedt's catalyst. By comparison of solvent washed layer with the final layer rinsed with water (Table 5), it is shown that washing of the PL-PMHS polymer surface after reaction with solvent (Toluene/CHCl 3 ) was generally sufficient to remove most of the unreacted phospholipid species.

Eq. ( 4) gives U s a 24 % for casting condition (1) in was about 3 times higher than in the bulk (z) as measured by IR absorption (Table 2), showing the preferential reaction of PL at the uppermost layer by the casting method. This method may be useful to those developing thin film gradients for application in material science [START_REF] Cui | Spatiotemporal Evolution of Fixed and Mobile Dopant Populations in Silica Thin-Film Gradients as Revealed by Single Molecule Tracking[END_REF] and surface of biocompatible polymers [START_REF] Meredith | Combinatorial materials science for Polymer Thin-Film Dewetting[END_REF].

Immersion method

The surfaces prepared by immersion at 20 °C, conditions ( 2) and (3) in Table 5, are characterized by lower PL graft yield (Z ~ 2.5 2.7 %) correlated with a contact angle of 40° higher than for casting method. The values (z) and (Z) are clearly of same order of magnitude (Table 2 and5).

The results suggest that a homogeneous reaction occurred in the bulk of the film and in the uppermost layer by immersion in swelling solvent. The XPS content of residual hydrophobic SiH group (D H ) at the interface was much higher (25 %) than estimated by casting method (0%).

Larger proportions of residual SiH groups are also confirmed by a small negative shift of the SiO 2/2 component of only 0.1 eV compared with virgin PMHS (Table 4). We believe that SiH groups remained mostly stable after rinsing the samples by immersion in neutral water for 2 h before XPS analysis.

Surfaces properties

Morphology

The PMHS films were grafted with the phospholipid molecule without any significant physical modification of the surface homogeneity in regard to its wetting and protein resistance ability as shown by AFM microscopy (Fig. 6). Briefly, the pristine PMHS films of thickness about 0.5 µm were flat with AFM roughness (R q ) of 3 nm (Fig. 6a). Homogeneous cured PMHS polymer layers were formed by spin-coating of the sol-gel mixture. Neither crack nor delamination was observed indicating that the layer is bound to the substrate by covalent SiOSi bonds. After hydrosilylation, the uniformity and flatness of initial PMHS were conserved showing the stability of the layer upon reaction and solvent rinsing. Reaction of PL induced always an increase in final roughness at the nanoscale level (R q ) of 13 and 24 nm varying with experimental conditions (Fig. 6b andc). The PL-PMHS film prepared by casting at 40 °C has a roughness higher than by immersing method which can be explained by difference in network reticulation of the PMHS films induced by side reaction with water. AFM (Fig. 6c) and cross-section SEM images (Fig. 7b) also revealed the presence of superficial grooves of less than 100 nm depth showing that there were no grooves descending to the silicon substrate. This peculiarity was not observed for immersion sample as shown in Fig. 6b. Moreover, reaction of PL induced always an increase in final thickness as shown by cross-section SEM images (Fig. 7a andb). This effect has also been observed by ellipsometry in thin PMHS films [START_REF] Tauk | Thin phosphatidylcholine films as background surfaces with further possibilities of functionalization for biomedical applications[END_REF] and for 1-alkene reactions [START_REF] Thami | Fonctionalization of Surface-Grafted Polymethylhydrosiloxane with Alkyl Side Chains[END_REF].

Wetting properties

The PMHS films show a high hydrophobic behavior, which is related to the quasi absence of residual SiOH in the gel. The static contact angle (T) with water for the as prepared 5 % crosslinked PMHS was measured after 20 s at 106 ± 1° (Fig. 8a) in ambient condition with an advancing WCA (T adv ) of 108 ± 1° close to the receding WCA (T rec ) of 103 ± 1° (Fig. 8b). The low hysteresis contact angle (T adv T rec ) of 5° for PMHS film is attributed to its low surface roughness as measured by AFM (Section 3.4.1). According to previous study [START_REF] Thami | Tailoring of elastomeric grafted coating via sol-gel chemistry of crosslinked polymethylhydrosiloxane[END_REF], by changing the DH/TH composition from 95/5 to 50/50, the static WCA decreased by less than about 5°

showing that the effect of the various content of Si-H, Si-CH 3 and Si-O is small on the surface energy. After conditioning the interface in pure water for different times, the initial WCA of 5% crosslinked PMHS measured by air captive bubble method (Fig. 8c) was decreased from 105° (after 30 s immersion in water) to 90° (5 minutes) and 47° (24 hours). Thus, the hydrophobic PMHS surface turns into hydrophilic indicating probably that hydrophilic surface SiOH groups are formed upon exposure to neutral water by hydrolysis of some hydrophobic SiH groups.

When the phospholipid surface comes into contact with water, the interfacial morphology may change because of the easy reorientation and migration of phosphoryl groups into the aqueous medium [START_REF] Futamura | Rapid development of hydrophilicity and protein adsorption resistance by polymer surfaces bearing phosphorylcholine and naphthalene groups[END_REF][START_REF] Yang | Group reorientation and migration of amphiphilic polymer bearing phosphorylcholine functionalities on surface of cellular membrane mimicking coating[END_REF]. We thus preferred to examine the wetting behavior by the captive air bubble method. By immersing in water the PL-PMHS samples, the WCA was stable after about five minutes (Fig. 8d) indicating a rapid rearrangement of phosphorylcholine groups upon exposure to water. Then, WCAs values did not change after conditioning for 24 h more or several days.

The PL-PMHS surfaces are very hydrophilic suggesting that all substrates tested were grafted with hydrophilic PC in agreement with XPS results (Table 5). In addition, the maximum N/Si and P/Si ratios and graft yield (Z ~ 12%) determined by XPS are well correlated with the lower WCA measured of about 0° (Fig. 8d) characteristic of a superhydrophilic behavior for PL-PMHS

(1) prepared by casting method. In this case, the air bubble could not adhere to the surface and a rolling bubble easily slips away from the surface following slight tilting. For the sample PL-PMHS (2) in Fig. 8d prepared by immersion method (Z ~ 3 %), the static WCA was increased to 40°, but was nevertheless lower than that of the pristine PMHS (after a few minutes of immersion in water).

Protein adsorption

We previously investigated protein adsorption towards single protein solutions in phosphate saline buffer pH 7.4. The results for bovine serum albumin (BSA) at concentration of 10 µg/ml are reviewed in Table 6 for comparison purpose with casting and immersion XPS data in Table 5.

The wettability behavior of the PL-PMHS surfaces was also indicative of a probable proteinrepellent character. Interestingly, BSA adsorption results for PL-PMHS are also in good agreement with the present XPS analysis in terms of PL grafting density for samples prepared with the same protocol (immersing or casting). Particularly, the phosphorylcholine modified PL-PMHS samples experienced almost "zero" protein adsorption of 0.05 ng/cm 2 for BSA in the optimized case by casting (Z ~ 12 %). By comparison, for samples prepared by immersion (Z ~ 3 %), the BSA adsorption is tenfold higher (Table 6). The presence of hydrophobic residual SiH surface groups found by XPS (Table 5) may also play a role in the protein-surface interactions. The low adsorption level on pristine PMHS seems not to be related to the hydrophobic character of the material but probably to hydrolysis of some SiH into hydrophilic SiOH surface groups. Nevertheless, the adsorption of BSA onto PL-PMHS prepared in optimized conditions by casting is 200 times lower than onto PMHS.

Conclusion

Polymethylhydrosiloxane films of low roughness and controlled crosslinking density have been 3 and5. where DD is the ratio of DH/TH mixture (mol %).

b The presence of trace of fluor (< 0.5 %) at the interface (Fig. 3a) was neglected in the calculation of atomic %.

  to measure the graft yield of PL molecule per silicon SiH monomer z = [PL]/[SiH] 0 based on intensity of ν as (CH 2 ) sharp and strong peak of CH 2 groups where [SiH] 0 is the initial total number of SiH groups at t = 0 in virgin PMHS for the same sample. With the assumption that the 2:1 (Si:PL) adducts are exclusively formed as explained in Section 3.1.2, (z) can also be written as:z = U (2)where U is the SiH conversion of hydrosylilation yield U = [SiCH]/[SiH] 0 with an upper boundaries PL-grafting ratio of z max = 50 % for U = 100 %. The value of z was calculated using the calibration formula obtained for hydrosilylation in 5% crosslinked PMHS with terminal 1alkene precursors up to n = 17 in length: I (alkyl-PMHS, 2926) /I (PMHS, 2169) = 0.284 n × U where (n) is the number of methylene group in the grafted hydrocarbon chain Si(CH 2 ) n CH 3[START_REF] Nasr | Functionalization of Polymethylhydrosiloxane Gels with an Allyl Ureido Benzocrown Ether Derivative: Complexation Properties[END_REF][START_REF] Thami | Fonctionalization of Surface-Grafted Polymethylhydrosiloxane with Alkyl Side Chains[END_REF]. In this equation, I (alkyl-PMHS, 2926) and I (PMHS, 2169) are the maximum intensities of PL-PMHS for CH 2 at 2926 cm -1 and of PMHS (t = 0) for SiH at 2169 cm -1 . The results are summarized in Table2for n = 17 which was the reference value for estimating the PL graft yield (z) in any experiment by considering the structure of the 2:1 (Si:PL) adduct and the hydrogenation of all remaining double bonds in PL-PMHS films.The best graft yield (z = 4 %) was obtained by casting and curing at 40 °C for 15 h. All results showed that the hydrosilylation grafting of PL catalyzed by Karstedt's catalyst was achieved in a few dozen minutes in air either at room temperature or slightly above at 40 °C by means of the two methods. Thus, the reaction proceeds under mild conditions in acceptable SiH conversion yield (U = 78 %). These observations show that PMHS polymer facilitates the hydrosilylation grafting process as compared with reaction of methyl linoleate which was effective only with more reactive chlorohydrosilanes[START_REF] Behr | The synthesis of silicon oleochemicals by hydrosilylation of unsaturated fatty acid derivatives[END_REF][START_REF] Behr | Platinum-catalysed hydrosilylation of unsaturated fatty acid esters[END_REF].To summarize, all SiH groups in PMHS films (~ 0.5µm) have reacted under optimized conditions (Residual SiH ~ 0 %) and about 8 % of the SiH group underwent hydrosilylation with C=C double bond. The remainder (~ 90 %) underwent quick reaction with water catalyzed by Karstedt's catalyst. The formation of Si-O-Si crosslinks hardens the polymer network and increases the film resistance.

2 / 2

 22 is attributed to the D CH /(CH 3 )(CH PL )SiO 2/2 silicon units resulting from hydrosilylation reaction (Scheme 1b), and the major SiO 3/2 component is attributed to T/SiCH 3 O 3/2 new crosslinks units formed by reaction of SiH with H 2 O (Scheme 2). The PL-PMHS network is characterized by a ratio D CH /T of about 25/75 (Table

  CSiO 3/2 ] y where the subscript ½ indicates that one PL molecule (C 44 NO 8 P) participates in the formation of two Si-CH bonds of D CH unit, x is the fraction of D CH silicon unit bound with phospholipid over the total amount of difunctionnal units including residual D H , and y = SiO 3/2 /100 is the percentage of SiO 3/2 (T-crosslinks) as measured by curve-fitting of Si 2p spectra (Tables

  prepared and anchored by use of sol-gel chemistry on oxidized silicon wafer by adjusting the molar ratio of methyldiethoxysilane and triethoxysilane reagents. Phosphorylcholine groups were then grafted onto the SiH functionalized PMHS surface by hydrosilylation of an unsaturated phospholipid molecule bearing linoleoyl chains in the presence of Karstedt's catalyst in air. XPS analysis and water contact angle measurements confirmed the presence of hydrophilic PC groups on the functionalized PMHS surface. FTIR showed that PL molecule chemically combines with the polysiloxane backbone by covalent silicon-carbon bonds. The reaction leads to the formation of the 2:1 (Si:PL) adduct resultant from one fold addition of SiH group per linoleoyl chain. We showed also that oxygen incorporation into the material had essentially occurred following SiH side reaction with water giving rise to the formation of more highly T-crosslinked polysiloxane film. The efficiency of the hydrosilylation reaction was examined by decreasing the temperature of reaction from 40 °C to 20° C. The correlated wettability changes of surfaces varying from 0° to 40° were determined and explained in terms of interfacial PL grafting density, owing to the partial hydrosilylation of SiH groups. The study shows also a correlation between protein adsorption and the structure of the PC-containing polymers in the case of BSA adsorption. The "grafting to" method we describe is a promising way of obtaining biomaterials permitting to reduce the nonspecific protein adsorption which could be useful in the biomedical field, e. g. biosensor, biomaterials, cell culture, substrates for ELISAs methods and biomedical devices.

Figure and Table captions Scheme 1 .Scheme 2 .

 12 Figure and Table captions

Figure 1 .

 1 Figure 1. (top) IR spectra of same PMHS sample before and after PL hydrosilylation by casting

Figure 2 .

 2 Figure 2. IR spectra of different PMHS samples (same thickness of 0.50 µm at t = 0) after

Figure 3 .

 3 Figure 3. XPS survey spectra of the surfaces of: (a) pristine PMHS of different crosslinking

Figure 4 .

 4 Figure 4. High resolution C 1s and Si 2p XPS spectra and curves fit (solid line) of: (top) pristine

Figure 5 .

 5 Figure 5. High resolution N 1s and P 2p XPS spectra of PL-PMHS after hydrosylilation by casting

Figure 6 .

 6 Figure 6. Representative AFM images of PMHS before and after modification: (a) pristine

Figure 7 .

 7 Figure 7. Cross-section SEM images of (a) pristine PMHS and (b) PL-PMHS prepared by

Figure 8 .

 8 Figure 8. Contact angles of PMHS surfaces (crosslinking density D = 0.95): (a) static WCA

da

  Out-of-plane CH bend attributed to of 1,2 disubtituted cis-alkene. Concentration of phospholipid at t = 0 in toluene.b Based on IR absorption (± SD over three samples); SiH conversion of hydrosilylation can be calculated by U z for 2:1 (Si:PL) adduct.

Sia= for 2 : 1 (

 21 The presence of trace of platinum (< 0.15 %) at the interfaces (Fig.3b) was neglected in the calculation of atomic %. b Calculated (Section 3.3.3). c Calculated from Eq. (3). SiH conversion yield of hydrosilylation at the surface can be calculated by U s Si:PL) adduct. d At least three samples were measured. Data precision is about ± 2.5° (Fig.8d).

  Fig. 1 Click here to download high resolution image

Table 5

 5 which is clearly in disagreement with the value found for %SiO 2/2 (24 %) in this study. To conclude, XPS elemental data and Si 2p curves fitting results are consistent by making the assumption that the 2:1 (Si:PL) adducts are preferentially formed. This supports the proposed mechanism of hydrosilylation of internal C=C double bonds of linoleoyl chains of PL molecule, in which only one double bond per chain reacts with SiH (Scheme 1b). The XPS graft yield (Z)

	, that is exactly the same value as

considering the formation of polysiloxane-bound 4:1 (Si:PL) adduct, this would give U s ~ 48 %

Table 1 .

 1 Infrared peak assignments of silicon wafer thin films grafted with PMHS (5% crosslinked), functionalized with PL molecule (PL-PMHS), and pure PL molecule (wafer).

	Table 2. Hydrosilylation of 5 % crosslinked PMHS thin films (~ 0.5 µm) with 1,2-dilinoleoyl-
	sn-glycero-3-phosphorylcholine (18:2 Cis) (PL) (1.5-fold excess of phospholipid over SiH, 4.2
	and 1.2 mol % Karstedt catalyst over SiH in exp. (1, 2) and (3, 4) respectively, solvent: toluene).

Table 3 .

 3 XPS analysis summarizing at the surface of various crosslinked PMHS thin films

	(~ 0.5 µm) prepared on silicon by sol-gel curing of methyldiethoxylsilane (DH) / triethoxylane
	(TH) mixtures.

Table 4 .

 4 Binding energies determined by curve-fitting of the Si 2p XPS high resolution spectra in two components, SiO 2/2 (D H , D CH ) and SiO 3/2 (T H , T, T CH ), indicating the structure of the various possible siloxane subunits encountered in as prepared crosslinked PMHS of various degree of crosslinking and in PL-PMHS derived from 5 % crosslinked PMHS after different treatments.

Table 5 .

 5 XPS analysis summarizing changes in elemental composition and SiO contents at the

	surface of phospholipid grafted PL-PMHS prepared in different reaction conditions by
	hydrosilylation grafting of 5 % crosslinked PMHS thin films (~ 0.5 µm) with 1,2-dilinoleoyl-sn-
	glycero-3-phosphorylcholine (18:2 Cis) (PL) and corresponding captive air bubble WCA (T).

Table 6 .

 6 Bovine serum albumin (BSA) adsorption on PL functionalized and virgin PMHS films

	[61,62].

Table 1 a

 1 Very broad band.

b Residual peak. c Broad envelop.

Table 2

 2 Element (atom % ) Calculated on the theoretical bulk composition D H /T H summarized as: (CSiO 2/2 ) D (SiO 3/2 ) 1-D

	Si(2p) (%)

a

Table 3

 3 Standard abbreviations for D H , T H and D, T are used, it also includes D CH /(CH 3 )(CH PL )SiO 2/2 and T CH /(CH PL )SiO 3/2 . PDMS resins [74]: D 102.0 eV; T 102.7 eV.

	Material	Treatment	Siloxane units a	Binding energy (eV)
				for Si 2p 3/2 doublet
	Crosslinked	As prepared	D H	101.5 ± 0.1
	PMHS b
			T H	102.4 ± 0.1
	Grafted PL-PMHS Casting 40 °C, 15h	D CH	101.2 ± 0.1
			T (major) + T CH	102.1 ± 0.1
	Grafted PL-PMHS Immersing 22 °C, 1h/6h	D CH + D H	101.4 ± 0.1
			T (major) + T CH	102.1 ± 0.1

a b Binding energies for 5, 15 and 30 % (mol %) TH-crosslinked PMHS are of same values (± 0.1 eV).

Table 4

 4 Element (atom %) a

Table 5 e

 5 SD ± 1 % for three samples. BSA concentration 10 µg/mL. All samples measured after conditioning overnight in phosphate saline buffer 10 mM (NaCl 150 mM) at pH 7.4.

	Film	Conditions	Adsorbed BSA protein
			(ng/cm 2 ) a
	Virgin PMHS [62]		10 ± 2
	PL functionalized PMHS [61]	Casting 15 h	0.05 ± 0.04
	PL functionalized PMHS [62]	Immersion 6 h 0.5 ± 0.3

a

Table 6 Scheme 1 Click here to download high resolution image Scheme 2 Click here to download high resolution image

 6

Acknowledgements

This work was financially supported by FP7 European grant NMP-214538 (BISNES project; Coord. D. Nicolau) and the CNRS. We thank Michel Ramonda (LMCP, UM2) for the AFM pictures, Didier Cot (IEM) for the SEM pictures and Eddy Petit (IEM) for his technical assistance concerning IR spectroscopy. We are also grateful to Philippe Déjardin, Jean-Marc Janot and Emmanuel Troneyl-Perroz for useful discussions about the physicochemical properties and surfaces characterization. The authors thank Damien Quemener for very valuable comments and suggestions on the manuscript.


Shuanglong Yuan
email: shuanglong@ecust.edu.cn

Yunxia Yang

François Cheviré

Franck Tessier

Xianghua Zhang

Guorong Chenz

Photoluminescence of Eu2+-Doped Strontium Cyanamide: A Novel Host Lattice for Eu2+

come

I. Introduction

The emission and absorption spectra of Eu 2+ usually consist of broad bands due to electronic transitions between the 8 S7/2 (4f 7 ) ground state and the crystal field components of the 4f 6 5d excited state configuration. Because the involved 5d orbitals are external, the position of these energy levels and consequently the wavelengths of excitation and emission bands strongly depend on the host matrix [1]. With the increasing covalency between Eu 2+ and ligands, the emission of Eu 2+ shifts to longer wavelength [2]. For example, the Eu 2+ emits from near violet to blue in halide hosts due to highly electronegative halogen ions, but at longer wavelengths in nitrides due to lower electronegative N 3- [2].

SrCN2 belongs to the hexagonal Pnma space group (no. 62) with a = 512.41 Å , b = 53.96 Å , and c = 55.39 Å. In the SrCN2 host crystal, [CN2] 2-anions in two orientations built up layers parallel to the a-c plane of the unit cell. The Sr 2+ ions also belong to these layers, forming distorted octahedra around the anions. Distorted nitrogen octahedra centered by Sr 2+ ions are connected via sharing corners and by NCN bridges [3].

As Sr 2+ ions connect to N 3-, it is reasonable to expect that SrCN2 would be used as host lattice for divalent lanthanides. However, reported work in this respect is rare. In the present work, Eu 2+ -doped SrCN2 were prepared and its optical properties were investigated in detail.

II. Experiments Procedures

Powder samples with general formula Sr(1-x)CN2:Eux (x = 0.01, 0.02, 0.03, 0.04, and 0.06) were prepared by using SrCO3, Eu2O3, and active carbon (CARBIO 12 SA-ref: C1220 G 90) as raw materials following a process inspired from the literature [4]. The mixture of europium oxide and the strontium carbonate was placed at the end of a graphite boat, while the active carbon was put in the upcoming flowing gas at the other end. The mixture was then fired at 1223 K under NH3 atmosphere for 6 h in a tubular furnace.

The synthesized powder samples were analyzed by X-ray diffraction (XRD) (Philips PW3710, PANalytical B.V., Almelo, the Netherlands) at room temperature using CuKα radiation (λ = l.54056 Å). A step size of 0.02° (2θ) was used with a scan speed of 2°/min. The nitrogen and oxygen contents were determined with an LECO TC-600 analyzer (LECO Corporation, St. Joseph,MI) using the inert gas fusion method. The measurements of photoluminescence (PL) and PL excitation spectra between 77 and 293 K were carried out by a Fluorolog-3 spectrofluorometer (Jobin Yvon Inc./specx, HORIBA Jobin Yvon Inc., Edison, NJ) equipped with a VNF-100 liquid nitrogen cryostat controlled by a cryogenic temperature controller model 32, and the excitation light resource is a 450 W Xe lamp. Diffuse reflectance spectra in the range of 200-700 nm were collected using a Varian Cary 100 UV-Vis spectrophotometer (Varian Inc., Fontaine, Grenoble, France) equipped with an integrating sphere Labsphere (DRA-CA-30) with 70 mm diameter.

III. Results and Discussion

(

1) Phase Characterization

The XRD patterns of the series of powder samples Sr(1-x)CN2:Eux (x = 0.01, 0.02, 0.03, 0.04, and 0.06) are displayed in Fig. 1. The results show that all the products are of a single phase of SrCN2, in good agreement with the JCPDS 82-0988 file. However, with the increasing Eu 2+ content, the diffraction peaks shift slightly to a lower d-value. This is most likely due to the partial substitution of Sr 2+ by Eu 2+ , which has the lower ionic radius (0.107 nm) than Sr 2+ (0.112 nm). (

2) The Reflection Spectra

The diffuse reflection spectra of SrCN2:Eu samples are gathered in Fig. 2. Both undoped and Eu 2+doped samples show a remarkable drop in reflection in the UV range around 280 nm, corresponding to the valence-to-conduction band transitions of the SrCN2 host lattice. The intense reflection in the visible spectral range is in agreement with the observed white daylight color for undoped SrCN2. In order to better localize the thresholds for host lattice absorption and the absorption by Eu 2+ , the absorption spectra F(R) were obtained (inset) from the diffuse reflection spectra by using the Kubelka-Munk function as follows [5]:

F(R) = (1-R) 2 /2R = K/S (1)
where R, K, and S are the reflectivity, the absorption coefficient, and the scattering coefficient, respectively. The value of the optical band gap is calculated to be about 4.56 eV. For the samples of high Eu 2+ concentration, two broad absorption bands can also be seen in the wavelength ranges of 320-500 and 280-320 nm, respectively. Both of them can be attributed to the 4f-5d absorption of Eu 2+ ions, compared with that of undoped SrCN2. As expected, their intensities increase with the increasing Eu 2+ concentrations. In contrast to the undoped samples with the white daylight color, Sr1-xCN2:Eux shows light yellow to bright yellow colors, varying with the Eu 2+ concentration (0 < x ≤ 0.06) due to a strong absorption in the visible range of 380-500 nm. (

3) The PL Characteristics at Low Temperature

The PL spectra of SrCN2:Eu0.02 at 77 K are presented in Fig. 3. With different excitation wavelengths at 396 and 491 nm, emission spectra show the similar broad orangish-red emission with a maximum at about 610 nm (16 393 cm -1 ) and full-width at half maximum about 82 nm (Γ = 2163 cm - 1 ). This broad emission band can be assigned to the 4f 6 5d 1 -4f 7 transition of Eu 2+ . We do not observe the characteristic sharp emission peaks due to Eu 3+ ions, which confirms existence of only Eu 2+ ions in the present phosphors. Because only an exclusive transition is observed, we can deduce that all Eu 2+ ions occupy identical sites. This is identical to the expectation because there is only one crystallographically distinct Sr 2+ ion in SrCN2 host. The unusual long-wavelength and broad emission bands are due to the effect of the nitrogen-rich surrounding on Eu 2+ ions in SrCN2, which caused the large ligand-field splitting of the Eu 2+ 5d levels, and then a larger nephelauxetic effect.

The excitation spectrum at 77 K monitored at an emission wavelength of 610 nm is also shown in Fig. 5. It consists of a broad band with several subbands, which are due to the parity allowed 4f 7 ( 8 S7/2)-4f 6 5d transitions of Eu 2+ ions. These subbands structures, peaked at 335, 396, 449, 456, and 491 nm, are assigned to the crystal field splitting of the 5d level of Eu 2+ ions. The excited 5d levels are not shielded from the ligand field, giving rise to a marked splitting of the excited levels. The symmetry of the anion coordination polyhedron around the emission centers determines the number of the splitting level. Dorenbos [6] has made a clear correlation between the splitting and the coordination polyhedron, revealing that the crystal field splitting tends to be the largest for octahedral coordination followed by cubic, dodecahedral, and it is the smallest for tricapped trigonal prism (3ctp) and cuboctahedron coordination. In the SrCN2:Eu 2+ crystal, Sr 2+ ions coordinating with six N 3-with different bond lengths form an asymmetric structure. Because the doped Eu 2+ ions occupy asymmetric Sr 2+ sites and are exposed to a strong ligand field due to the presence of highly covalent bonding of Eu-N, a large ligand-field splitting occurs to Eu 2+ ions, resulting in their excitation band extending into visible region. According to the excitation spectrum, the crystal field splitting is estimated to be about 12019 cm -1 , assuming no 5d subbands to be positioned in the conduction band of the host lattice. A rough estimate of the Stokes shift determined by the maxima of the emission and excitation bands is 8859 cm -1 , which is fairly large and attributed to the distortion of the excited state of Eu 2+ due to CN2 2- anion groups surrounding the Eu 2+ ions emission centers. The broad excitation band and orangish-red emission mean that the SrCN2 should be a good host lattice for doping Eu 2+ , like (Ca,Sr)S and Sr2Si5N8, which have been used as the red components of white light-emitting diodes illumination [7,8]. However, the emission of SrCN2:Eu 2+ is strongly quenched with increasing temperatures and not observable at room temperature. The emission intensity dependence on the temperature using the excitation wavelength of 396 and 491 nm is depicted in Fig. 4. When it is excited at 396 nm, the temperature at which the intensity is half of that at 77 K is about T1/2 = 90 K, and at 227 K the emission is completely quenched. These temperatures are slightly lower than those excited at 491 nm (T1/2 = 92 K). Besides, the emission peaks slightly broaden and shift to the longer wavelength with the increasing temperature. The luminescence behavior of SrCN2:Eu 2+ is similar to those of Ba(SCN)2:Eu 2+ and Sr(SCN)2:Eu 2+ . In Sr(SCN)2:Eu 2+ , it can be well explained by photoionization of the excited states in the conduction band of the host lattice [9,10], that is, the lowest 5d state is located at the level in the band gap of the host lattice slightly lower than the conduction band. After excitation, conventional d → f emission and/or nonradiative relaxation from an excitation state located in the conduction band may occur depending on the thermal population of the excitation state. However, this model seems unable to explain the thermal quenching behavior of SrCN2:Eu 2+ in the present case. This is because upon 491 nm excitation on SrCN2:Eu 2+ , the excited electrons of Eu 2+ ions most likely remained in the lower 5d levels rather than moved to the higher levels due to large crystal splitting energy (12019 cm -1 ). Besides, the low quenching temperature is not due to a large displacement between the ground and excited state in the configuration coordinate diagram [11]. Therefore, we present a configurational coordinate diagram as shown in Fig. 5 to explain the temperature dependence of luminescence, as well as the red shift and broadening of emission with the increasing temperature occurring to SrCN2:Eu 2+ phosphors. The lower and higher potential curves represent the total energy of the ground state of 4f 7 ( 8 S7/2) and the excited state of 4f 6 5d, respectively. The equilibrium positions of the two states are different from each other because of the spatial distribution of the electron orbitals. Generally, A → B1,B2 represents the absorption process and C1 → D1 the emission [12]. However, with the increasing temperature, the excited electrons would depart to nonequilibrium positions of the excited state (e.g., C2 or O, the crossing point between the excited state and the defect level). As a result, the emission shifts to the longer wavelength and its intensity decreases [START_REF] Shionoya | Phosphor Handbook[END_REF]. In general, the low quenching temperature originates from the crystal defect or the effect of activated ions with the host crystal. The best probability is from the crystal defect. In fact, in asprepared samples, almost 1% molar oxygen remained as measured by an LECO analyzer, and they were most likely introduced in the preparation process from the raw materials SrCO3 and Eu2O3. The oxygen could be incorporated on the N site in Eu 2+ -doped SrCN2, forming the defect of (ON) • . To realize the charge compensation of O substitution on N sites, some cation vacancies such as (VSr)′′ have to be formed. Upon UV excitation of the host lattice, these defects may serve as carrier traps by capturing electrons or holes. It is reasonable to assume that (ON) • will capture electrons, while (VSr)′′ will capture holes. Hence, when SrCN2:Eu 2+ is excited by UV light, the excited electrons could be captured by (ON) • , resulting in nonradiative transition. Therefore, it is essential to prepare oxygenfree samples, and thus the synthesis process should be investigated further.

Fig. 1 .

 1 Fig. 1. The X-ray diffraction spectra of Sr(1-x)CN2:Eux 2+ .

Fig. 2 .

 2 Fig. 2. The diffuse reflection and absorption spectra of Sr(1-x)CN2:Eux 2+ .

Fig. 3 .

 3 Fig. 3. The emission and excitation spectra of Sr0.98CN2:Eu0.02 at 77 K.

Fig. 4 .

 4 Fig. 4. (a) The emission spectra of Sr0.98CN2:Eu0.02 excited by 396 nm at different temperatures. (b) The emission spectra of Sr0.98CN2:Eu0.02 excited by 491 nm at different temperatures.

Fig. 5 .

 5 Fig. 5. Schematic illustration of a configurational coordinate model.

IV. Conclusion

Singe-phase SrCN2:Eu 2+ powder samples were prepared by a high-temperature solid-state reaction under NH3 atmosphere. SrCN2 has an energy gap about 4.56 eV obtained from its reflection spectrum. Eu 2+ -doped SrCN2 shows a broad band emission with a peak at 610 nm when it is excited by 350-520 nm at 77 K. With the temperature increasing, the emission intensity decreases remarkably. When it is excited by 396 nm, the temperature at which the intensity is half of that at 77 K is about T1/2 = 90 K, and at 227 K the emission is completely quenched; these temperatures are slightly lower than those excited by 491 nm (T1/2 = 92 K). Besides, the emission peaks experience a slight red shift and become broader with the increasing temperature. These phenomena can be explained by the configurational coordinate diagram and could be attributed to the defect levels originated from the oxygen impurity in the host.