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Applied Surface Science 254 (2008) 7143–7148

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APLE deposition of biomaterial multilayers

aleria Califano a,*, Francesco Bloisi a,b, Luciano R.M. Vicari a,b,aolo Colombi c, Elza Bontempi c, Laura E. Depero c

CNR-INFM Coherentia, Universita degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy

Dipartimento di Scienze Fisiche, Facolta di Ingegneria, Universita degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy

Laboratorio di Chimica per le Tecnologie, Universita degli Studi di Brescia, via Branze 38, 25123 Brescia, Italy

R T I C L E I N F O

rticle history:

eceived 29 February 2008

eceived in revised form 15 April 2008

ccepted 10 May 2008

vailable online 23 May 2008

ACS:

1.10.Eq

2.35.Gh

8.37.Ps

8.55.�a

1.15.Fg

eywords:

APLE

urface morphology

hin films

olyethylene glycol

A B S T R A C T

Double layers of polyethylene glycol (PEG) and 3-(3,4-dihydroxyphenyl)-2-methyl-L-alanine (m-DOPA)

thin films were obtained by matrix assisted pulsed laser evaporation (MAPLE) technique, by depositing a

first layer of m-DOPA on Si substrate and a second layer of PEG on top of it. The films were characterized

by low angle X-ray diffraction (LAXRD), X-ray reflectivity (XRR), atomic force microscopy (AFM), and

micro-Raman spectroscopy. From these analyses it resulted that PEG was deposited without any relevant

damage both in terms of chemical structure and molecular weight. Furthermore, PEG chains were mostly

in the extended conformation, although PEG micelles appeared.

� 2008 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Surface Science

journal homepage: www.elsevier .com/locate/apsusc

1. Introduction

Polyethylene glycol (PEG) is a polymer with biotechnologicallyimportant applications, such as tissue engineering [1], spatialpatterning of cells [2] and anti-biofouling and biocompatiblecoatings [3] (i.e. drug delivery coatings and orthopaedic implants).Biofouling is the cellular and proteinaceous adhesion on thesurface of biomaterials implanted in the body and is commonlyavoided through the immobilization of anti-fouling polymers, suchas PEG, on the surface to protect. For these applications, thin filmsof high quality are required. Moreover, the deposition of a film withthe same chemical and structural properties of bulk PEG isessential. Furthermore, for anti-biofouling applications, there is arequirement for a mechanism to anchor PEG on the surface toprotect. Several studies have shown that there exists a unique setof adhesive proteins capable of strong adhesion in wet environ-ment, which is responsible for mussel fouling on a variety of

* Corresponding author. Tel.: +39 0817 682585.

E-mail address: valeriacalifano@hotmail.com (V. Califano).

169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

oi:10.1016/j.apsusc.2008.05.295

surfaces [3–6]. The liquid protein adhesives secreted by theseorganisms rapidly harden to form a solid adhesive plaque capableof mediating firm attachment to a wide variety of wet surfaces.Each mussel species produces its specific set of proteins (forexample the ‘‘Mytilus edulis’’ produces five proteins: MeFP-1 toMeFP-5). The widely studied MeFP-1 (Mytilus edulis foot protein1) contains 15% of L-DOPA (3,4-dihydroxy-phenyl-L-alanine). Theadhesive and cohesive properties of mussel adhesive proteins(MAPs) have been linked to the presence of L-DOPA. DOPA is anunusual residue not found in many proteins outside of MAPs. Thecatechol side chain of DOPA is capable of many different types ofchemical interactions and, furthermore, is particularly susceptibleto oxidation under elevated pH conditions such as in the marineenvironment. The rich chemistry of catechols has made it difficultto clearly define the role of DOPA in MAPs. In spite of this, twodefined roles appear to be emerging: cohesive and adhesive. Thecohesive role primarily derives from reactions following theoxidation of DOPA to DOPA-quinone, while, at least on inorganicsurfaces, there appears to be an adhesive role for unoxidized DOPA.

The DOPA end-functionalized PEG was found to readily adsorbon metal surfaces [5].

Fig. 1. Chemical formula of (a) polyethylene glycol and (b) 3-(3,4-

dihydroxyphenyl)-2-methyl-L-alanine (m-DOPA).

Fig. 2. Schematic of the MAPLE deposition system.

Table 1Parameters use for MAPLE deposition

Deposition parameters

Wavelength (nm) 355

Pulse repetition rate (pulse/s) 10

Pulse duration (ns) 6

Fluence (J/cm2) 0.1

Beam spot size (mm2) �2

Total pulses �95000

V. Califano et al. / Applied Surface Science 254 (2008) 7143–71487144

Matrix assisted pulsed laser evaporation (MAPLE) techniquewas found to be suited for the deposition of both PEG [7–11] andMeFP-1 [6] films. MAPLE is a thin film deposition techniqueparticularly well suited for double layers deposition of organic/polymer thin films. MAPLE is a modification of the pulsed laserdeposition (PLD) or ablation technique. In the latter, a pulsed UVlaser is focused onto a bulk target and, above a certain powerdensity, ejection of material occurs in form of a forward-directedplume. The material will deposit on a substrate placed in the pathof the plume. PLD is largely applied for inorganic thin filmdeposition [12,13], although some addition polymers [14] weresuccessfully deposited. However, the ablation of addition polymersseems to proceed via chain scission, ablation of monomers and re-polymerization. This is clearly not possible for condensationpolymers or other organic molecules.

In MAPLE a frozen solution of the material to be deposited in theappropriate solvent is used as target. In this way, the solventabsorbs most of the laser energy, limiting the damage to themolecules of interest. When the laser pulse impacts the target, theformation of a forward-directed plume occurs, as in PLD. Themechanism of ablation is still under study, but it is thought that theenergy absorbed by the solvent is converted into thermal energy byphotochemical or photothermal processes [15,16] which lead tophase explosion [17,18]. As the solvent evaporates, the polymermolecules attain sufficient kinetic energy through collisions withthe evaporating solvent molecules to be transferred into the gasphase. During the target-to-substrate journey, the more volatilesolvent molecules are pumped away by a vacuum system, whilethe organic/polymer molecules are deposited onto the substrate.

This work deals with the deposition of PEG/m-DOPA (3-(3,4-dihydroxyphenyl)-2-methyl-L-alanine) double layers in order toimprove PEG adhesion for anti-biofouling applications. m-DOPA isan analogous of L-DOPA with a methyl group added in order toimprove its solubility in water (Fig. 1). Since the cathecol moiety,which is thought to be responsible for the adhesion properties of themolecule, is preserved, its functionality should be preserved as well.

MAPLE technique is potentially suited for this depositionbecause, being a non-contact technique, it allows multilayerdeposition and eliminates a major cause of contamination.

The chemical formulas of PEG and m-DOPA are reported inFig. 1.

2. Experimental

2.1. Materials

PEG target was a solution of PEG (10 wt%) in bi-distilled waterobtained starting from a polyethylene glycol 3000 mono-dispersesolution �50% in H2O (from Fluka). m-DOPA target was asuspension of m-DOPA (1.0 wt%) in bi-distilled water obtainedstarting from a m-DOPA hemihydrate powder (from Fluka). Alllayers were deposited onto 1 cm� 1 cm Si substrates.

2.2. MAPLE procedure

Our MAPLE deposition system (Fig. 2) consisted of a depositionchamber containing substrate and target holders. The target holderwas connected to a liquid nitrogen reservoir in order to allow insitu freezing in controlled atmosphere. A computer controlledmovement system allowed a full scan of the target surface, thusavoiding overheating or drilling. The Si substrate was mountedparallel to the frozen target surface.

A Q-switched YAG:Nd laser operated at 355 nm (laserthird harmonic wavelength). This wavelength radiation is notefficiently absorbed by the water–ice matrix but can minimize

the photochemical decomposition induced by shorter wavelengthradiations on polymers [19].

About 2 ml of target solution was placed into the target holderand the chamber was closed. A flux of dry helium passed in thechamber during the pre-deposition operations in order to push offatmospheric moisture. As soon as the target was frozen, the heliumflux was stopped and the vacuum pump started operating. Thefinal deposition parameters were: pressure about 10�5 Pa andtemperature 103 K. The substrate was placed 1 cm far from thetarget and the deposition started. During deposition, the pressureinside vacuum chamber rose to about 2 � 10�4 Pa. In order toobtain a double layer, the substrate was not removed from thevacuum chamber between m-DOPA and PEG depositions. Forcomparison, depositions of single layer of PEG and of m-DOPAwere carried out. The deposition parameters, which are the samefor all three samples (PEG, m-DOPA and PEG/m-DOPA bilayer), aresummarized in Table 1.

2.3. Characterization

Low angle X-ray diffraction (LAXRD) and X-ray reflectivity(XRR) measurements were performed with a Bruker ‘‘D8 Advance’’

V. Califano et al. / Applied Surface Science 254 (2008) 7143–7148 7145

diffractometer equipped with a Gobel mirror. The angularresolution was 0.0018. The Cu Ka line of a conventional X-raysource powered at 40 kV and 40 mA was used. XRR profiles wereanalysed with the REFSIM software (Bunker, 1998) in order toextract information on the layer density, thickness and roughness.

Atomic force microscopy (AFM) experiments were carried outby means of a Jeol JSPM 4210 (Tokyo, Japan) scanning probemicroscope (SPM) equipped with a wide-area scanner (maximumxy-movement = 50 mm; maximum z-movement = 8 mm) and afour-segment photodetector for cantilever deflection monitoring.

Rectangular silicon cantilevers NSG10 (NT-MDT, Russia) wereused for non-contact imaging and for nano-scratching experi-ments. The thickness of the polymeric layer was evaluated by AFM-tip scratch-test. The technique consists in scratching the coatingwith the AFM tip while the normal force applied is set to a valuehigh enough to penetrate the layer but low enough to avoidsignificant tip or substrate damaging. This value depends both onfilm and substrate mechanical properties and was empiricallydetermined by scratching at different loading forces. Afterscratching, the area is imaged in the non-contact mode usingthe same tip used for the scratching. Finally, the scratch depth, i.e.the polymer layer thickness, is evaluated by image analysis.

Micro-Raman spectra were collected by a LabRAM HR-Horiba-Jobin Yvon spectrograph. The exciting source was a He–Ne laser(632.8 nm) with power smaller than 10 mW at the sample surface.The microscope was coupled confocally to the spectrograph.Suppression of the exciting line was obtained with a holographicnotch filter. The spectra were measured at room temperature. Byemploying a 50� objective, a micro-Raman spectrum can beobtained from an area as small as few mm2.

3. Results and discussion

PEG is a linear polymer, available in a large range of molecularweights. Those polymers of average molecular weight over about1000 g/mol are in the solid-state at room temperature. PEG in thesolid-state is semi-crystalline and can form lamellar structures[20].

In this work PEG of average molecular weight of about 3000 g/mol was deposited onto Si and m-DOPA/Si substrates. Tocharacterize these films, several techniques were employed.

XRR is a well-established surface technique to study thin layers[21], and it was already employed to study PEG films depositedonto Si substrate [22].

Fig. 3 shows the experimental and simulated XRR pattern ofPEG sample deposited onto a Si substrate. By fitting by a singlelayer model we obtained a density value of 0.9 g/cm3, a roughnessvalue of about 1 nm and a thickness value of about 4 nm. These

Fig. 3. Experimental and simulated X-ray reflectivity spectra of PEG film on Si.

results are in good agreement with the values previously reportedfor films deposited onto SiO2 substrates [23] and also with valuesreported for similar films in literature [22,24]. XRR patternscollected on m-DOPA and PEG/m-DOPA samples (not shown) didnot allow any structure refinement. This is probably due to thelarge roughness of the film that prevents the observation of theXRR fringes of the layer. However, the small value of the criticalangle is consistent with the presence of a topmost organic layer.

Morphological characterization of all films was performed bymeans of AFM. Topographic images are shown in Fig. 4(a). PEG filmdeposited on Si substrate shows a heterogeneous morphology,consisting of a smooth layer and particles.

Fig. 4. NC-AFM images of (a) PEG (b) m-DOPA (c) PEG/m-DOPA. Greyscale is

different for each image and it is reported on the right side.

Fig. 5. Raman spectrum acquired on PEG (C), m-DOPA (D) and PEG/m-DOPA (E). For comparison the reference Raman spectra of PEG (A) and m-DOPA (B) are shown.

Fig. 6. Scratch-test experiment performed on PEG surface. In the graph at the

bottom the height profile is shown. AFM image after the scratch-test experiment

performed on PEG surface (a) and graph showing the height (b).

V. Califano et al. / Applied Surface Science 254 (2008) 7143–71487146

To verify the composition of the particles on the surface, micro-Raman experiments were performed. To this purpose, the beamspot (1 mm) was focused on the biggest particles for each sample,and the collected patterns are reported in Fig. 5. The Raman spectraof PEG from literature [25] and the pattern of m-DOPA sample arereported for comparison.

Considering the PEG sample [26], the band at 1153 cm�1

corresponds to the polymer chains (C–O–C) asymmetric stretch-ing; other bands corresponding to the polymer methylene groupvibrations are CH2 twisting at 1220–1270 cm�1 and CH2 scissoringat 1452–1510 cm�1. PEG/DOPA sample showed the same bands ofPEG, indicating that the particles over the surface are made of PEG.

The formation of PEG particles on top of a PEG layer was alreadyobtained by other deposition techniques [27–30]. Also otherpolymers show a similar behaviour [31]. These isolated domainshave been attributed to grafted PEG chains, as micellar aggregatesof different sizes. The particles were supposed to arise from netinter-chain attractive interactions that can lead to large, helicallyinterweaving structures [28].

The formation of PEG particles was already obtained by MAPLEdeposition as well [10,11]: particles of 20–150 mm of diameterwere observed on top of a PEG layer of 20 nm. In the present study,smaller particles were obtained and in small number, probablybecause of the smaller thickness of our films.

In Fig. 6, a representative nano-scratch image of the PEG layerand a depth profile (taken along the line) is shown. By evaluatingthe profile, the layer thickness is about 2 nm. The deposition of thisvery thin layer is attributed to densely packed PEG chains. Thesurface topography of this continuous PEG layer is rather smooth,with a surface roughness of 1 nm, in accordance with the resultsreported for similar samples [32]. The thickness value iscomparable with the value obtained by XRR.

AFM was used to examine m-DOPA and PEG grafted on m-DOPA/Si surfaces (Fig. 4). The morphology appears similar to thatof PEG sample, where isolated particles are present on the surface.However, these agglomerations are larger in dimensions producinghigher root mean square roughness (about 30–60 nm). AFM imageof the scratch-test performed on PEG/m-DOPA surface is shown inFig. 7(a). Analysis of the height profile (Fig. 7(b)) revealed aconsiderably thicker layer (about 100 nm). Since the size of theparticles and the thickness in PEG/m-DOPA films are closer to theone in m-DOPA film, we can assume that these features aredetermined by m-DOPA. The bigger thickness and roughness of the

m-DOPA and PEG/m-DOPA layers can be attributed to the fact thatm-DOPA starting solution is not a real solution but rather asuspension of eye-visible particles.

From studies of molecular dynamics [33] it was assumed that,following the laser pulse polymer molecules might be ejected onlyas a part of large polymer/matrix clusters. During the deposition,

Fig. 7. AFM image after the scratch-test experiment performed on PEG/m-DOPA

surface (a) and graph showing the height (b).

V. Califano et al. / Applied Surface Science 254 (2008) 7143–7148 7147

the evaporation of the volatile matrix molecules leads to anincrease in the polymer concentration in the surface region of thetarget. Consequently, both the evaporation of the solvent on thesubstrate and entanglement of polymer molecules in the polymer-rich clusters might contribute to the formation of particles.

LAXRD technique was applied to study the structure of PEG. Toperform this experiment, high intensity parallel beam and anappropriate collimation system are necessary [34]. LAXRDmeasurements are shown in Fig. 8. Only one peak clearly appears

Fig. 8. LAXRD measurements of the samples.

in the pattern of PEG sample at about 1.798 (2u) corresponding tod = 4.9 nm.

By means of small angle X-ray scattering (SAXS) measurementsthe lamellar structure of PEG has been reported [35]. In SAXSspectra, the shape and position of the peaks are found to dependupon the oxyethylene chain length [20], and the degree of foldingwas evaluated by modelling.

Considering the length of the oxyethylene unit (0.285 nm) [35],the theoretical length of the PEG chains can be calculated and inour case it resulted about 20 nm. Thus, the peak found in theLAXRD pattern, corresponding to 4.9 nm, is about one fourth of thePEG length. Since no other reflection was detected, the ordering ofthe chains cannot be discussed. However, we can say that most ofthe PEG chains preserved their length after the deposition.

The formation of a lamellar structure can be discussed in termsof formation of hydrogen bonds. PEG consists of hydrophilicoxygen and hydrophobic ethylene units. The distance betweenalternate oxygen atoms in the PEG chain is similar to the next-nearest-neighbour distance between oxygen atoms in water [36],then the water insertion into the already existing linear hydrogenbonding chain may form hydrogen bonds [37,38]. This kind ofstructure, where hydrogen bonding provides an important meansfor structural rigidity, is well described in liquid systems, but wasalready reported for solid-state phases [39]. Since the MAPLEdeposition of PEG started from ice matrix, it may be foreseen thatthe water molecules play a significant role in the formation of thelamellar structure. Indeed, for PEG films deposited by MAPLE, thepresence of OH groups belonging to the water matrix was alreadysupported by FTIR (Fourier transform infrared spectroscopy)results [10].

In Fig. 8, we can observe that the position of the Bragg peak inthe PEG and PEG/m-DOPA films is the same. This suggests that PEGchains in PEG/m-DOPA film have a structure similar to that in thePEG film on Si. Moreover, the high intensity of this reflection inPEG/m-DOPA film suggests that DOPA favours the PEG deposition.This is in agreement with the results obtained by AFM and scratch-test results.

4. Conclusions

Deposition of monolayers of PEG and m-DOPA and doublelayers of PEG/m-DOPA thin films was successfully performed byMAPLE. The characterization (XRR, AFM, LAXRD and micro-Raman)of the deposits allows concluding that PEG films grown onto Si andm-DOPA/Si substrates without any significant damage both interms of monomer chemical structure and of molecular weight.

In addition, the observed surface features in the PEG and PEG/m-DOPA samples appear consistent with PEG particle formation.

LAXRD measurements allowed hypothesising that PEG wasdeposited in the lamellar form.

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