Reactive & Functional Polymers 68 (2008) 1178–1184

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Reactive & Functional Polymers

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

Reactive polyelectrolyte multilayers onto silica particles

Frank Simon a, Ecaterina Stela Dragan b,*, Florin Bucatariu b

a Leibniz Institute of Polymer Research, D-01069 Dresden, Germanyb ‘‘Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41 A, RO-700487 Iasi, Romania

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 October 2007Received in revised form 17 April 2008Accepted 24 April 2008Available online 1 May 2008

Keywords:SilicaAdsorptionS-benzyl-L-cysteineElectrokinetic measurementsX-ray photoelectron spectroscopy

1381-5148/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.reactfunctpolym.2008.04.004

* Corresponding author. Tel.: +40 232 217454; faE-mail address: stela_dragan@yahoo.com (E.S. D

The two polyelectrolytes poly(vinylformamide-co-vinylamine) [P(VFA-co-VAm)] andpoly(acrylic acid) (PAA) have been alternately adsorbed from aqueous solution onto silicaparticles with sizes in the range 15–40 lm, and a mean pore radius of 60 Å. The growth ofthe alternately adsorbed P(VFA-co-VAm)/PAA film has been evidenced by the zeta-poten-tial-pH profiles as a function of the last layer adsorbed. After the multilayer formation,when P(VFA-co-VAm) was the last layer adsorbed, the hybrid materials were annealed at120 �C to stabilize the polymer layers by a heat-induced reaction forming amide groups.IR spectra of the hybrid material, before and after thermal treatment, showed the amidelinkages formation. The cross-linked hybrid materials were subsequently functionalizedwith S-benzyl-L-cysteine. X-ray photoelectron spectroscopy (XPS) was employed to obtaininformation about the amount of the amino acid S-benzyl-L-cysteine which was grafted onthe free amino groups on the hybrid particle surfaces.

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1. Introduction

The creation of specific surface sites on solid surfacesfor selective molecular attachment is considered a promis-ing approach for their applications in nano-fabrication,nano-patterning, self-assembly, nano-sensors, bioprobes,drug delivery, pigments, photocatalysis, LEDs, etc. [1]. Or-ganic/inorganic hybrids with well-defined morphologyand structure controlled at the nano-metric scale representa very interesting class of materials. In the early 1990s, anew preparation route to organize polymer films was re-ported by Decher et al. [2,3] which is based on the electro-static layer-by-layer (LbL) assembly of cationic and anioniccompounds on a solid substrate. Möhwald and coworkers[4,5] have used similar techniques for synthesis of novelpolymeric hollow spheres. The polyelectrolytes usuallyused have been poly(sodium 4-styrene sulfonate) and qua-ternary polyammonium salts, i.e., strong polyelectrolytes,which seem not suitable for further functionalization reac-tions under mild conditions. The presence of reactive

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x: +40 232 211299.ragan).

groups such as primary amino and carboxylic groups is aprerequisite for further functionalization reactions. Themultilayer formation of oppositely charged strong andrather weak polyelectrolytes was employed to equip sur-faces with exceptionally antibacterial properties [6]. Thebuild-up of such polyelectrolyte layers, mainly based onelectrostatic interactions, can be instable in the presenceof water.

Thus, Spange and coworkers [7–10] produced stablepoly(vinylformamide-co-vinylamine) [P(VFA-co-VAm)]/inorganic oxide hybrid particles. The authors used fuller-ene [7], (4,40-diisocyanate)diphenyl methane [8] and otherbifunctional cross-linkers to irreversibly fix the polyelec-trolyte layer onto the inorganic particle surfaces. It wasshown that P(VFA-co-VAm) is a highly interesting poly-electrolyte for surface functionalization of inorganic parti-cles because after coating the inorganic surface with thepolymer, a large number of reactive primary amino groupsremain available for subsequent functionalization reac-tions. Serizawa et al. [11,12] fabricated by LbL techniqueultrathin polymer films on a substrate using poly(acrylicacid) (PAA), as polyanion, and poly(vinylamine-co-N-vinyl-formamide)s and poly(vinylamine-co-N-isobutyramide)s,

F. Simon et al. / Reactive & Functional Polymers 68 (2008) 1178–1184 1179

as polycations. They studied the sequential amide forma-tion between polyanion and polycation using a water-soluble carbodiimide as a dehydration agent. In order toproduce stable super-hydrophobic coating systems wet-ting highly polar metal oxides, Höhne et al. used reactionsbetween pre-coated chitosan, which is polymer containinga high number of amino groups, and subsequently appliedmaleic anhydride copolymers [13]. Heat-induced reactionsinvolving two functional groups proved to be an interest-ing route in the formation of covalent bonds in the electro-statically or H-bonding stabilized polymer complexes[14,15] and polyelectrolyte multilayers [16–19]. In a previ-ous study, we showed the feasibility of the grafting of boc-S-benzyl-L-cysteine on the surface of poly[N-(b-aminoeth-ylene)acrylamide]/silica hybrid particles to build-up shortpeptide brushes [20].

The aim of this article was to investigate the formationof multilayers P(VFA-co-VAm)/PAA onto the silica surfaceand demonstrate that formed multilayer contains an ade-quate number of accessible primary amino groups for sub-sequent derivatization reactions with amino acids, e.g.boc-S-benzyl-L-cysteine to condense short peptide brusheson the surface.

2. Materials and methods

2.1. Materials

Kieselgel 60 (Merck, Darmstadt, Germany) a commer-cial available spherical silica, was used as inorganic sub-strate material. The main diameter of the microporoussilica particles ranged between 15 and 40 lm. The distribu-tion of the pore diameters has a maximum in the range of4–6 nm. The P(VFA-co-VAm) sample with a molar mass ofMw = 15.000 g mol�1 was provided by BASF (Ludwigshafen,Germany). The degree of hydrolysis was 96 mol.%. Thismeans 96 mol.% of the former formamide groups of thePVFA chains were converted into amino groups. PAA withMw = 68.000 g mol�1 (Polysciences Inc.), N,N’-dicyclohexylcarbodiimide (DCC, Merck, Darmstadt, Germany), andboc-S-benzyl-L-cysteine (Fluka, Germany) were used as re-ceived. The structures of all reagents are summarized inChart 1.

n n

NH

CH2 CH

HC O 0,04

NH2

CH2 CH

0,96

poly(vinylformamide-co-vinylamine)

[P(VFA-co-VAm)]

nCH2 CH

COOH

poly(acrylic acid)

(PAA)

CHNHC O C

CH 3

CH3

H3C

O

COOH

S CH2

CH2

boc-S-benzyl-L-cysteine

N C N

dicyclohexylcarbodiimide (DCC)

Chart 1. Structures of the polyelectrolytes P(VFA-co-VAm) and PAA, andthe amino acid boc-S-benzyl-L-cysteine used to produce peptide-func-tionalized hybrid particles.

2.2. Adsorption procedure and cross-linking

Sample of 2 g silica were suspended in 300 mL of a10�2 mol L�1 P(VFA-co-VAm) salt-free aqueous solution.During the adsorption process over 3 h, the suspensionwas gently shaken at room temperature. The modified par-ticles were washed three times with distilled water torinse the weakly adsorbed P(VFA-co-VAm) from the silicaparticles. A small amount of sample was removed anddried at 40 �C for electrokinetic measurements. For the sec-ond adsorption step the P(VFA-co-VAm) coated particleswere suspended in 300 mL of 10�2 mol L�1 PAA salt-freeaqueous solution, kept 3 h there, and then washed as de-scribed above. Similar adsorption and washing steps wereperformed until five layers of P(VFA-co-VAm) and PAAalternately adsorbed have been deposited. After the multi-layer formation, with P(VFA-co-VAm) as the last layeradsorbed, the hybrid materials were annealed at 120 �Cto stabilize polymer layers by a heat induced reactionforming amide groups.

2.3. Generation procedure of the short peptide brushes

The functionalization of the cross-linked hybrid parti-cles with S-benzyl-L-cysteine was carried out accordingto the procedure used by Merrifield [21,22]. For thecoupling reaction of the protected amino acid, 2 g of ther-mally cross-linked silica/(P(VFA-co-VAm)/PAA)1.5 (P(VFA-co-VAm) was the last layer adsorbed) was suspended intoa solvent mixture of 10 mL dichloromethane (DCM) and10 mL dimethylformamide (DMF). After 10 min, a solutioncontaining 0.200 g (0.64 mmol) of boc-S-benzyl-L-cysteinein 4 mL of DCM was added and shaken for 10 min. Then,a solution of 0.132 g of DCC (0.64 mmol) in 2 mL of DCMwas added. The suspension was shaken for further 3 h.The hybrid material was filtered off and washed threetimes with 10 mL of DMF and three times with 10 mL ofDCM to remove the excess of reagents and byproducts.For the deprotection reaction of the protected amino acidcoupled to the silica hybrides particles, the reaction with3 g of trifluoroacetic acid (TFA) in 10 mL of DCM was car-ried out. After deprotection, the hybrid particles were fil-tered and washed with 2 mL triethylamine in 10 mL DCMand three times with 10 mL DCM.

2.4. Characterization methods

Potentiometric titrations were performed with the par-ticle charge detector PCD 02 (Mütek, Germany) betweenpH 3.5 and 10, varied with 0.1 mol L�1 KOH and HCl,respectively.

The electrokinetic measurements were performed asstreaming potential experiments employing an Electroki-netic Analyzer (EKA, Anton Paar, Austria). In a specially de-signed powder-measuring cell [4], the hybrid materialswere packed as diaphragm, which was flown through byan aqueous KCl solution (c = 0.001 mol L�1). The pH valueswere varied during measurements by the addition of 0.1 MHCl or 0.1 M KOH employing an automatic titration unit.After recording the streaming potential values of the acidicpH range, the sample was exchanged for measuring the

1 2 3 4 5 6 7 8 9 10 11 12-1000

-800

-600

-400

-200

0

200

400

600

800

1000

1200Ψ

(m

V)

pH

pzcP(VFA-co-VAm)pzc

silica

pzcPAA

Fig. 1. Potentiometric titration of bare silica (circles), PAA (triangles), andP(VFA-co-VAm) (squares).

2 3 4 5 6 7 8 9 10 11-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

bare silica silica/P(VFA-co-VAm) silica/(P(VFA-co-VAm)/PAA) silica/(P(VFA-co-VAm)/PAA)

1.5

silica/(P(VFA-co-VAm)/PAA)2

silica/(P(VFA-co-VAm)/PAA)2.5

Zeta

Pot

entia

l (m

V)

pH

Fig. 3. Zeta-potential values determined from streaming potential mea-surements as a function of the pH of aqueous 0.001 mol L�1 KCl recordedto characterize the hybrid particles formed by alternated adsorption ofP(VFA-co-VAm) and PAA.

1180 F. Simon et al. / Reactive & Functional Polymers 68 (2008) 1178–1184

streaming potential values in the basic pH range. Theapparent zeta-potential (f) was calculated from the mea-sured streaming potentials (Us/Dp) according to the Smo-luchovski equation [23]:

f ¼ � g � L � Re � e0 � Q

� Us

Dp; ð1Þ

where g is the viscosity; e the relative dielectric constant, e0

the permittivity of the free space, Dp the pressure differ-ence, R the electrical resistance of the electrolyte-filled dia-phragm, L the actual capillary channel length and Q theactual cross-section channel area (L/Q is the cell constant,which can be separately determined by resistant measure-ments [24]).

XPS studies were carried out by means of an Axis UltraX-ray photoelectron spectrometer (Kratos Analytical, Man-chester, UK). The spectrometer was equipped with a mono-

2 3 4 5 6 7 8 9 10 11-40

-30

-20

-10

0

10

20

30

40

Zet

a p

ote

nti

al (

mV

)

pH

Fig. 2. Zeta-potential values determined from streaming potential mea-surements as a function of the pH of aqueous 0.001 mol L�1 KCl of baresilica (stars), silica/P(VFA-co-VAm) (circles), and silica/(P(VFA-co-VAm)/PAA) (squares): closed symbols – after each layer deposition the distilledwater at pH 6.6 was used; open symbols – washing with acidic water atpH 3.5 after the deposition of each polycation layer, and with basic waterat pH 9.5 after the deposition of each PAA layer.

chromatic Al Ka (hm = 1486.6 eV) X-ray source of 300 W at15 kV. The kinetic energy of the photoelectrons was deter-mined with a hemispheric analyzer set to pass energy of160 eV for wide-scan spectra. During all measurements,electrostatic charging of the sample was avoided by meansof a low-energy electron source working in combinationwith a magnetic immersion lens. Later, all recorded peakswere shifted by the same amount that was necessary toset the C 1s peak to 285.00 eV for saturated hydrocarbons.Quantitative elemental compositions were determinedfrom peak areas using experimentally determined sensitiv-ity factors and the spectrometer transmission function.

IR spectra were recorded with FTIR spectrometer Equi-nox 55 (Bruker Optik GmbH). Prior to analysis, dried sam-ples were mixed with KBr and pressed to form a tablet.

4000 3000 2000 1000 0

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Wavenumber (cm-1)

Abs

orba

nce

2925

2853

1622

1552

1451

Fig. 4. FTIR-ATR spectra of bare silica (dot line), silica/(P(VFA-co-VAm)/PAA)1.5 before (dash line) and after (solid line) thermal treatment at120 �C.

F. Simon et al. / Reactive & Functional Polymers 68 (2008) 1178–1184 1181

3. Results and discussion

3.1. Characterization of bare silica, PAA and P(VFA-co-VAm)

The surface charge of the silica particles suspended inwater is either the result of dissociation processes ofBrønsted-acidic silanol groups (Si–OH) forming negativelycharged silanolate ions (Si–O�) or proton adsorption yield-ing Si—OHþ2 species. In the presence of hydronium ions theprimary amino groups (–NH2) of P(VFA-co-VAm) can beprotonated ð—NHþ3 Þ and in presence of hydroxyl ionsthe carboxyl groups (–COOH) of PAA can be dissociated(–COO�). Hence, beside the entropy, an important compo-nent of the driving force of the polyelectrolyte multilayerformation onto silica surfaces is the Coulomb force be-tween the oppositely charged centers. Potentiometric titra-tions in water shows that the point of zero charge

SiO2

NH Boc

RO

HO

SiO2

O

HO

SiO2

NH Boc

RO

HO

SiO2

NH2 +

R :

Boc :

CF3COOH

-(CH3)2C=CH2

-CO2

S

O

O

DCC

NH NH2

RO

+

NH NH

RO

NH

Boc

RO

CF3C

-(CH3)2

-C

+ DCCNH

O

Fig. 5. Scheme of the step-by-step coupling of boc-S-benzyl-L-cysteine o

(pzc = pH|w = 0, where w is the potential) is reached at pH2.5, for bare silica, at pH 2.2, for PAA and at pH 10.2, forP(VFA-co-VAm) (Fig. 1).

3.2. Polyelectrolyte multilayer formation onto silica and thestability of the adsorbed layers

To form polyelectrolyte multilayers we carried out thepolyelectrolyte adsorption as described above. After eachadsorption step the samples were washed according totwo different procedures: (1) After each layer deposition,the samples were washed in distilled water at pH 6.6; (2)After polycation deposition the samples were washed withacidic water (pH 3.5), while the samples after the polyan-ion deposition were washed in basic water (pH 9.5) (Fig. 2).

The pH of the 10�2 mol L�1 P(VFA-co-VAm) salt-freeaqueous solution was approximately 9.5. At high pH val-

SiO2

NH Boc

R

SiO2

NH NH Boc

RO

DCC

OOH

C=CH2

O2

NH NH

RO

RO

NH2+

NH

R

NH

RO

NH

Boc

RO

nto the surface of silica/(P(VFA-co-VAm)/PAA)1.5 hybrid particles.

Fig. 6. Wide-scan XPS spectra of bare silica (a) silica/(P(VFA-co-VAm)/PAA)1.5, (b) and silica/(P(VFA-co-VAm)/PAA)1.5 after the first coupling ofboc-S-benzyl-L-cysteine (c).

1182 F. Simon et al. / Reactive & Functional Polymers 68 (2008) 1178–1184

ues, the macromolecules have a low charge density (Fig. 1)leading to coiled chains. Hence, the P(VFA-co-VAm)amount adsorbed onto silica surface is high. The zeta-po-tential of the silica/P(VFA-co-VAm) hybrid material as afunction of the pH value of a streaming aqueous KCl solu-tion can be seen in Fig. 2, closed circles. In the pH range4.5 < pH < 8, the positive zeta-potential remains nearlyconstant. Here, all amino groups that are able to form pos-itively charged ammonium salt species are protonated. AtpH > 8 the ammonium species are gradually deprotonatedby the excess of OH� ions in the aqueous solution and, as aconsequence, the zeta-potential value decreases. The iso-electric point (iep = pH|f = 0) of silica/P(VFA-co-VAm) hy-brid agrees with the pzc of the P(VFA-co-VAm) (Fig. 1).This indicates that the P(VFA-co-VAm) fully covers the sil-ica surface and determines the charging behaviour of thehybrid material. In the acidic range (pH < 4.5) the zeta po-tential showed a minimum (Fig. 2, curve with closed cir-cles). That minimum can be explained by the instabilityof the adsorbed P(VFA-co-VAm) layer in acidic environ-ment. The HCl stepwise added solvates and dissolves themacromolecules. The streaming liquid removes such dis-solved polymers. In order to proof the assumption men-tioned above, the silica/P(VFA-co-VAm) hybrid materialwas washed with acidic water at pH 3.5 before the electro-kinetic measurements were carried out. As can be seen inFig. 2 (curve with open circles), the zeta potential mini-mum in the acidic range was not found and the plateauphase is on a lower level showing that the weakly boundP(VFA-co-VAm) chains were already removed by the acidicwater.

The zeta-potential of the silica/(P(VFA-co-VAm)/PAA)hybrid material as a function of the pH value of a streamingaqueous KCl solution can be seen in Fig. 2, closed squares.In the pH range 5 < pH < 7.5, the negative zeta-potential re-mains nearly constant. Here, the carboxylic groups aredeprotonated. The iep of the silica/(P(VFA-co-VAm)/PAA)hybrid was 3.6. In the basic range (pH > 7.5) the zeta po-tential showed a minimum (Fig. 2, curve with closedsquares). That minimum also can be explained by theinstability of the adsorbed PAA layer, now in basic environ-ment. The stepwise addition of KOH forms carboxylateions, which can be easily solvated and dissolved by thestreaming liquid. The washing of the silica/(P(VFA-co-VAm)/PAA) sample with basic water at pH 9.5 gives morestable hybrid materials. As can be seen in Fig. 2 (curve withopen squares), the zeta-potential minimum in the basicrange was not found and the plateau phase indicates thatthe weakly bound PAA chains were removed during thesample’s washing procedure.

To build up stable multilayers onto silica particles, aftereach layer deposition, the hybrid particles were washedwith water having the same pH like of the polyelectrolytesolution from which the next layer was adsorbed. Electro-kinetic measurements have been used to follow the pro-cess formation of these hybrids. The shape of thefunction zeta-potential as a function of pH gives informa-tion on changes of the silica surface charge after the alter-nated adsorption of oppositely charged polyions. Typicalfunctions of zeta-potential versus pH for bare silica and sil-ica/(P(VFA-co-Vam)/PAA)n samples are presented in Fig. 3.

A positive zeta-potential can be observed in the range ofpH 4.0–7.5 when P(VFA-co-VAm) is adsorbed onto the sil-ica particle. After the adsorption of the first layer of PAAonto the silica/P(VFA-co-VAm) hybrid, the zeta potentialvalues remain negative over a wide range of pH, similarwith that of the bare silica having a plateau phase in therange of pH 4.0–9.0. The next P(VFA-co-VAm) layer shiftsthe iep into the basic range, while the following PAA layerre-establishes the Brønsted-acid surface properties. Obvi-ously, the kind of the last adsorbed polyelectrolyte layer,polycation or polyanion, strongly determines the surfaceBrønsted-acidity of the hybrid material.

The hybrid particles silica/(P(VFA-co-VAm)/PAA)1.5 havebeen stabilized by a subsequent thermal treatment, whereamide groups between the primary amino groups of theP(VFA-co-VAm) and the carboxylic groups of the PAA havebeen formed. The intermolecular reaction between the twoadsorbed polyelectrolytes cross-links the polymer layerand forms a self-stabilized polymer network wrappingthe silica kernel. Solvatation of the remaining polyions can-not remove parts of the cross-linked polymer shell.

The FTIR-ATR spectra of the silica/(P(VFA-co-VAm)/PAA)1.5 hybrid before and after the thermal treatment at120 �C were compared in Fig. 4.

Amide I (stretching vibration of the C@O bond) andamide II (deformation vibrations of the N–H bond) bandswere observed at approximately 1622 and 1552 cm�1,respectively, indicating the presence of amide groups ontosilica surface. The shifts of the peaks values with about 10–20 cm�1 in comparison with the free amide groups (non-Hbonded) could be explained by the intermolecular H-bond-ing of the amide groups attached onto silica surface. For

0 1 2 3 4 5 6 7 8 9 100

1000

2000

3000

4000

5000

CP

S

Steps

1, 3, 5, 7, 9 coupling2, 4, 6, 8 deprotection

Fig. 8. The intensity of the S 2s peaks (CPS = counts per second) after thecoupling of boc-S-benzyl-L-cysteine and its deprotection reaction. The i-ntensities of the S 2s peaks were related to the intensity of the C 1s peak,

F. Simon et al. / Reactive & Functional Polymers 68 (2008) 1178–1184 1183

example, the deconvoluted spectra of amide I peak ofpoly(N-isopropylacrylamide) in aqueous solution per-formed by Ramon et al. [25] evidenced an intramolecularH-bonded subband at 1630 cm�1, an intermolecular H-bonded subband at 1620 cm�1, and a free form of non-H-bonded subband at 1643 cm�1. In the case of the thermaltreated silica/(P(VFA-co-VAm)/PAA)1.5 hybrid, the H-bond-ing interactions between amide groups and silanols fromsilica surface seems to have also a contribution to the shiftsof the peak values with 10–20 cm�1 in comparison withfree amide groups.

3.3. Reaction of boc-S-benzyl-L-cysteine with the free aminogroups from the surface of silica hybrid particles

The functionalization of the thermally cross-linked hy-brid particles with S-benzil-L-cysteine was carried out asshown in Fig. 5, where DCC was used as the dehydratingagent.

Fig. 7. The S 2s XPS spectra of the silica/(P(VFA-co-VAm)/PAA)1.5 (a) be-fore the coupling of boc-S-benzyl-L-cysteine and (b) after the first, (c)after the third, and (d) after the fifth coupling of boc-S-benzyl-L-cysteine.The intensities of the S 2s peaks were related to the intensity of the C 1speak, which was kept constant.

which was kept constant.

The grafting reaction introduces sulfur in the samplesurface, which can be easily detected (as S 2s peaks) andquantified in the XPS spectra (Fig. 6). The amount of sulfurfrom the hybrid particles corresponds with the aminoacid’s grafting degree.

As expected, the bare silica surface shows intensivepeaks of silicon and oxygen (Fig. 6a). The alternatelyadsorption of P(VFA-co-VAm) and PAA introduces consid-erable amounts of carbon and nitrogen (Fig. 6b). After thegrafting of boc-S-benzyl-L-cysteine, the S 2s peak can beclearly seen in the XPS spectra (Fig. 6c). Its intensity in-creased step-by-step with stepwise grafting of the aminoacid (Fig. 7).

According to the reaction Scheme in Fig. 5, each cou-pling reaction was followed by a deprotection reaction toproduce new free amino groups of the covalently graftedamino acid to the particle surface. These free amino groupsare able to react with the boc-S-benzyl-L-cysteine offeredin a followed grafting step. After the first amino acid cou-pling, the raw area of the S 2s peak was 1750 counts persecond (CPS), the photoelectrons counted over a second(Fig. 7b). The raw area of S 2s decreased to 1100 CPS afterthe first deprotection reaction (Fig. 8), because the P(VFA-co-VAm) macromolecules from the cross-linked multilayer(togheter with bound aminoacid molecules), which are notstrongly fixed to the surface, were removed in trifluoroace-tic acid (TFA), used for deprotection.

After first deprotection reaction, the intensity of the S 2speak increased by the same amount, approximately 1000CPS. From this constant increase of the sulfur content, itwas concluded that the grafting reactions preferably tookplace on the pregrafted amino acids. In this way, oligopep-tides grow up on the hybrid surface (Fig. 8).

4. Conclusions

The alternated adsorption of P(VFA-co-VAm) and PAAonto silica particles was investigated. Subsequent cross-linking reaction between primary amino groups and car-

1184 F. Simon et al. / Reactive & Functional Polymers 68 (2008) 1178–1184

boxylic groups from multilayer built-up onto silica parti-cles offered a novel synthetic route to stabilize hybridmaterials consisting of weak polyelectrolyte multilayers.It was shown that the hybrid material can be equippedwith a reasonable number of reactive amino groups lo-cated on the outer surface, giving possibilities for furtherderivatization reactions such as the coupling of aminoacids to produce oligopeptides.

Acknowledgments

The authors thank Prof. Dr. Stefan Spange for providingthe P(VFA-co-VAm) and Dr. Cornelia Bellmann for her kindassistance in performing the electrokinetic measurementson silica particles. The financial support from the GrantMATNANTECH 50/2006 is gratefully acknowledged.

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