Reactive polyelectrolyte multilayers onto silica particles

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Author's personal copy Reactive & Functional Polymers 68 (2008) 1178–1184

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Reactive polyelectrolyte multilayers onto silica particles Frank Simon a, Ecaterina Stela Dragan b,*, Florin Bucatariu b a b

Leibniz Institute of Polymer Research, D-01069 Dresden, Germany ‘‘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

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

Keywords: Silica Adsorption S-benzyl-L-cysteine Electrokinetic measurements X-ray photoelectron spectroscopy

a b s t r a c t The two polyelectrolytes poly(vinylformamide-co-vinylamine) [P(VFA-co-VAm)] and poly(acrylic acid) (PAA) have been alternately adsorbed from aqueous solution onto silica particles with sizes in the range 15–40 lm, and a mean pore radius of 60 Å. The growth of the alternately adsorbed P(VFA-co-VAm)/PAA film has been evidenced by the zeta-potential-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 at 120 °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 amide linkages formation. The cross-linked hybrid materials were subsequently functionalized with S-benzyl-L-cysteine. X-ray photoelectron spectroscopy (XPS) was employed to obtain information about the amount of the amino acid S-benzyl-L-cysteine which was grafted on the free amino groups on the hybrid particle surfaces. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The creation of specific surface sites on solid surfaces for selective molecular attachment is considered a promising approach for their applications in nano-fabrication, nano-patterning, self-assembly, nano-sensors, bioprobes, drug delivery, pigments, photocatalysis, LEDs, etc. [1]. Organic/inorganic hybrids with well-defined morphology and structure controlled at the nano-metric scale represent a very interesting class of materials. In the early 1990s, a new preparation route to organize polymer films was reported by Decher et al. [2,3] which is based on the electrostatic layer-by-layer (LbL) assembly of cationic and anionic compounds on a solid substrate. Möhwald and coworkers [4,5] have used similar techniques for synthesis of novel polymeric hollow spheres. The polyelectrolytes usually used have been poly(sodium 4-styrene sulfonate) and quaternary polyammonium salts, i.e., strong polyelectrolytes, which seem not suitable for further functionalization reactions under mild conditions. The presence of reactive * Corresponding author. Tel.: +40 232 217454; fax: +40 232 211299. E-mail address: [email protected] (E.S. Dragan). 1381-5148/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2008.04.004

groups such as primary amino and carboxylic groups is a prerequisite for further functionalization reactions. The multilayer formation of oppositely charged strong and rather weak polyelectrolytes was employed to equip surfaces with exceptionally antibacterial properties [6]. The build-up of such polyelectrolyte layers, mainly based on electrostatic interactions, can be instable in the presence of water. Thus, Spange and coworkers [7–10] produced stable poly(vinylformamide-co-vinylamine) [P(VFA-co-VAm)]/ inorganic oxide hybrid particles. The authors used fullerene [7], (4,40 -diisocyanate)diphenyl methane [8] and other bifunctional cross-linkers to irreversibly fix the polyelectrolyte layer onto the inorganic particle surfaces. It was shown that P(VFA-co-VAm) is a highly interesting polyelectrolyte for surface functionalization of inorganic particles because after coating the inorganic surface with the polymer, a large number of reactive primary amino groups remain available for subsequent functionalization reactions. Serizawa et al. [11,12] fabricated by LbL technique ultrathin polymer films on a substrate using poly(acrylic acid) (PAA), as polyanion, and poly(vinylamine-co-N-vinylformamide)s and poly(vinylamine-co-N-isobutyramide)s,

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as polycations. They studied the sequential amide formation between polyanion and polycation using a watersoluble carbodiimide as a dehydration agent. In order to produce stable super-hydrophobic coating systems wetting highly polar metal oxides, Höhne et al. used reactions between pre-coated chitosan, which is polymer containing a high number of amino groups, and subsequently applied maleic anhydride copolymers [13]. Heat-induced reactions involving two functional groups proved to be an interesting route in the formation of covalent bonds in the electrostatically or H-bonding stabilized polymer complexes [14,15] and polyelectrolyte multilayers [16–19]. In a previous study, we showed the feasibility of the grafting of bocS-benzyl-L-cysteine on the surface of poly[N-(b-aminoethylene)acrylamide]/silica hybrid particles to build-up short peptide brushes [20]. The aim of this article was to investigate the formation of multilayers P(VFA-co-VAm)/PAA onto the silica surface and demonstrate that formed multilayer contains an adequate number of accessible primary amino groups for subsequent derivatization reactions with amino acids, e.g. boc-S-benzyl-L-cysteine to condense short peptide brushes on the surface. 2. Materials and methods 2.1. Materials Kieselgel 60 (Merck, Darmstadt, Germany) a commercial available spherical silica, was used as inorganic substrate material. The main diameter of the microporous silica particles ranged between 15 and 40 lm. The distribution of the pore diameters has a maximum in the range of 4–6 nm. The P(VFA-co-VAm) sample with a molar mass of Mw = 15.000 g mol1 was provided by BASF (Ludwigshafen, Germany). The degree of hydrolysis was 96 mol.%. This means 96 mol.% of the former formamide groups of the PVFA chains were converted into amino groups. PAA with Mw = 68.000 g mol1 (Polysciences Inc.), N,N’-dicyclohexyl carbodiimide (DCC, Merck, Darmstadt, Germany), and boc-S-benzyl-L-cysteine (Fluka, Germany) were used as received. The structures of all reagents are summarized in Chart 1.

CH2

CH2

CH

0,04 n

CH2

COOH n0,96

poly(acrylic acid) (PAA)

C

O O

CH 3

C

NH

CH

COOH

CH2 S

2.3. Generation procedure of the short peptide brushes The functionalization of the cross-linked hybrid particles with S-benzyl-L-cysteine was carried out according to the procedure used by Merrifield [21,22]. For the coupling reaction of the protected amino acid, 2 g of thermally cross-linked silica/(P(VFA-co-VAm)/PAA)1.5 (P(VFAco-VAm) was the last layer adsorbed) was suspended into a solvent mixture of 10 mL dichloromethane (DCM) and 10 mL dimethylformamide (DMF). After 10 min, a solution containing 0.200 g (0.64 mmol) of boc-S-benzyl-L-cysteine in 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 DCM was added. The suspension was shaken for further 3 h. The hybrid material was filtered off and washed three times with 10 mL of DMF and three times with 10 mL of DCM to remove the excess of reagents and byproducts. For the deprotection reaction of the protected amino acid coupled to the silica hybrides particles, the reaction with 3 g of trifluoroacetic acid (TFA) in 10 mL of DCM was carried out. After deprotection, the hybrid particles were filtered and washed with 2 mL triethylamine in 10 mL DCM and three times with 10 mL DCM.

n

[P(VFA-co-VAm)] CH 3

Sample of 2 g silica were suspended in 300 mL of a 102 mol L1 P(VFA-co-VAm) salt-free aqueous solution. During the adsorption process over 3 h, the suspension was gently shaken at room temperature. The modified particles were washed three times with distilled water to rinse the weakly adsorbed P(VFA-co-VAm) from the silica particles. A small amount of sample was removed and dried at 40 °C for electrokinetic measurements. For the second adsorption step the P(VFA-co-VAm) coated particles were suspended in 300 mL of 102 mol L1 PAA salt-free aqueous solution, kept 3 h there, and then washed as described above. Similar adsorption and washing steps were performed until five layers of P(VFA-co-VAm) and PAA alternately adsorbed have been deposited. After the multilayer formation, with P(VFA-co-VAm) as the last layer adsorbed, the hybrid materials were annealed at 120 °C to stabilize polymer layers by a heat induced reaction forming amide groups.

CH

poly(vinylformamide-co-vinylamine)

H3 C

2.2. Adsorption procedure and cross-linking

2.4. Characterization methods

CH NH2

NH HC O

1179

N

C

N

dicyclohexylcarbodiimide (DCC) CH2

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

Potentiometric titrations were performed with the particle charge detector PCD 02 (Mütek, Germany) between pH 3.5 and 10, varied with 0.1 mol L1 KOH and HCl, respectively. The electrokinetic measurements were performed as streaming potential experiments employing an Electrokinetic Analyzer (EKA, Anton Paar, Austria). In a specially designed powder-measuring cell [4], the hybrid materials were packed as diaphragm, which was flown through by an aqueous KCl solution (c = 0.001 mol L1). The pH values were varied during measurements by the addition of 0.1 M HCl or 0.1 M KOH employing an automatic titration unit. After recording the streaming potential values of the acidic pH range, the sample was exchanged for measuring the

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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

1200 1000

30

800

25 20

Ψ (mV)

400

pzcP(VFA-co-VAm)

pzcsilica

200 0 -200 pzcPAA

-400 -600

Zeta Potential (mV)

600

-800

10 5 0 -5 -10 -15 -20 -25

-1000 1

2

3

4

5

6

7

8

9

10

11

12

-30

pH

2

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

streaming potential values in the basic pH range. The apparent zeta-potential (f) was calculated from the measured streaming potentials (Us/Dp) according to the Smoluchovski equation [23]: f¼

15

g  L  R Us  ; e  e0  Q Dp

ð1Þ

where g is the viscosity; e the relative dielectric constant, e0 the permittivity of the free space, Dp the pressure difference, R the electrical resistance of the electrolyte-filled diaphragm, L the actual capillary channel length and Q the actual cross-section channel area (L/Q is the cell constant, which can be separately determined by resistant measurements [24]). XPS studies were carried out by means of an Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical, Manchester, UK). The spectrometer was equipped with a mono-

3

4

5

6

7

8

9

10

11

pH Fig. 3. Zeta-potential values determined from streaming potential measurements as a function of the pH of aqueous 0.001 mol L1 KCl recorded to characterize the hybrid particles formed by alternated adsorption of P(VFA-co-VAm) and PAA.

chromatic Al Ka (hm = 1486.6 eV) X-ray source of 300 W at 15 kV. The kinetic energy of the photoelectrons was determined with a hemispheric analyzer set to pass energy of 160 eV for wide-scan spectra. During all measurements, electrostatic charging of the sample was avoided by means of a low-energy electron source working in combination with a magnetic immersion lens. Later, all recorded peaks were shifted by the same amount that was necessary to set the C 1s peak to 285.00 eV for saturated hydrocarbons. Quantitative elemental compositions were determined from peak areas using experimentally determined sensitivity factors and the spectrometer transmission function. IR spectra were recorded with FTIR spectrometer Equinox 55 (Bruker Optik GmbH). Prior to analysis, dried samples were mixed with KBr and pressed to form a tablet.

40 30 0.35

10 0.30

-30

1622 1552

0.20 0.15

1451

-20

0.25 2925

-10

2853

0

Absorbance

Zeta potential (mV)

0.40

20

0.10

-40 2

3

4

5

6

7

8

9

10

11

pH Fig. 2. Zeta-potential values determined from streaming potential measurements as a function of the pH of aqueous 0.001 mol L1 KCl of bare silica (stars), silica/P(VFA-co-VAm) (circles), and silica/(P(VFA-co-VAm)/ PAA) (squares): closed symbols – after each layer deposition the distilled water at pH 6.6 was used; open symbols – washing with acidic water at pH 3.5 after the deposition of each polycation layer, and with basic water at pH 9.5 after the deposition of each PAA layer.

0.05 0.00

4000

3000

2000

Wavenumber

1000

0

(cm-1)

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 at 120 °C.

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(pzc = pH|w = 0, where w is the potential) is reached at pH 2.5, for bare silica, at pH 2.2, for PAA and at pH 10.2, for P(VFA-co-VAm) (Fig. 1).

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

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

The surface charge of the silica particles suspended in water is either the result of dissociation processes of Brønsted-acidic silanol groups (Si–OH) forming negatively charged silanolate ions (Si–O) or proton adsorption yieldþ ing Si—OH2 species. In the presence of hydronium ions the primary amino groups (–NH2) of P(VFA-co-VAm) can be protonated ð—NHþ 3 Þ and in presence of hydroxyl ions the carboxyl groups (–COOH) of PAA can be dissociated (–COO). Hence, beside the entropy, an important component of the driving force of the polyelectrolyte multilayer formation onto silica surfaces is the Coulomb force between the oppositely charged centers. Potentiometric titrations in water shows that the point of zero charge

To form polyelectrolyte multilayers we carried out the polyelectrolyte adsorption as described above. After each adsorption step the samples were washed according to two 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 with acidic water (pH 3.5), while the samples after the polyanion deposition were washed in basic water (pH 9.5) (Fig. 2). The pH of the 102 mol L1 P(VFA-co-VAm) salt-free aqueous solution was approximately 9.5. At high pH val-

R

O R

O

SiO2

DCC

+

NH2

NH

HO

CF3COOH

SiO2

-(CH3)2C=CH2

NH

NH

NH

Boc

Boc R

O

R

O

SiO2

+

NH2

NH

HO

DCC

Boc

-CO2

R

O

R

O Boc

SiO2

NH

NH

CF3COOH

NH

SiO2

-(CH3)2C=CH2

NH

NH

NH2

O

R

+

-CO2

O

R

O O

DCC

+ HO

R

O

R

NH

NH

NH

R

NH

SiO2

NH

Boc Boc O

R : Boc :

R

S

O O

Fig. 5. Scheme of the step-by-step coupling of boc-S-benzyl-L-cysteine onto the surface of silica/(P(VFA-co-VAm)/PAA)1.5 hybrid particles.

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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-potential of the silica/P(VFA-co-VAm) hybrid material as a function of the pH value of a streaming aqueous KCl solution can be seen in Fig. 2, closed circles. In the pH range 4.5 < pH < 8, the positive zeta-potential remains nearly constant. Here, all amino groups that are able to form positively charged ammonium salt species are protonated. At pH > 8 the ammonium species are gradually deprotonated by the excess of OH ions in the aqueous solution and, as a consequence, the zeta-potential value decreases. The isoelectric point (iep = pH|f = 0) of silica/P(VFA-co-VAm) hybrid agrees with the pzc of the P(VFA-co-VAm) (Fig. 1). This indicates that the P(VFA-co-VAm) fully covers the silica surface and determines the charging behaviour of the hybrid material. In the acidic range (pH < 4.5) the zeta potential showed a minimum (Fig. 2, curve with closed circles). That minimum can be explained by the instability of the adsorbed P(VFA-co-VAm) layer in acidic environment. The HCl stepwise added solvates and dissolves the macromolecules. The streaming liquid removes such dissolved polymers. In order to proof the assumption mentioned above, the silica/P(VFA-co-VAm) hybrid material was washed with acidic water at pH 3.5 before the electrokinetic measurements were carried out. As can be seen in Fig. 2 (curve with open circles), the zeta potential minimum in the acidic range was not found and the plateau phase is on a lower level showing that the weakly bound P(VFA-co-VAm) chains were already removed by the acidic water. The zeta-potential of the silica/(P(VFA-co-VAm)/PAA) hybrid material as a function of the pH value of a streaming aqueous KCl solution can be seen in Fig. 2, closed squares. In the pH range 5 < pH < 7.5, the negative zeta-potential remains nearly constant. Here, the carboxylic groups are deprotonated. The iep of the silica/(P(VFA-co-VAm)/PAA) hybrid was 3.6. In the basic range (pH > 7.5) the zeta potential showed a minimum (Fig. 2, curve with closed squares). That minimum also can be explained by the instability of the adsorbed PAA layer, now in basic environment. The stepwise addition of KOH forms carboxylate ions, which can be easily solvated and dissolved by the streaming liquid. The washing of the silica/(P(VFA-coVAm)/PAA) sample with basic water at pH 9.5 gives more stable hybrid materials. As can be seen in Fig. 2 (curve with open squares), the zeta-potential minimum in the basic range was not found and the plateau phase indicates that the weakly bound PAA chains were removed during the sample’s washing procedure. To build up stable multilayers onto silica particles, after each layer deposition, the hybrid particles were washed with water having the same pH like of the polyelectrolyte solution from which the next layer was adsorbed. Electrokinetic measurements have been used to follow the process formation of these hybrids. The shape of the function zeta-potential as a function of pH gives information on changes of the silica surface charge after the alternated adsorption of oppositely charged polyions. Typical functions of zeta-potential versus pH for bare silica and silica/(P(VFA-co-Vam)/PAA)n samples are presented in Fig. 3.

A positive zeta-potential can be observed in the range of pH 4.0–7.5 when P(VFA-co-VAm) is adsorbed onto the silica particle. After the adsorption of the first layer of PAA onto the silica/P(VFA-co-VAm) hybrid, the zeta potential values remain negative over a wide range of pH, similar with that of the bare silica having a plateau phase in the range of pH 4.0–9.0. The next P(VFA-co-VAm) layer shifts the iep into the basic range, while the following PAA layer re-establishes the Brønsted-acid surface properties. Obviously, the kind of the last adsorbed polyelectrolyte layer, polycation or polyanion, strongly determines the surface Brønsted-acidity of the hybrid material. The hybrid particles silica/(P(VFA-co-VAm)/PAA)1.5 have been stabilized by a subsequent thermal treatment, where amide groups between the primary amino groups of the P(VFA-co-VAm) and the carboxylic groups of the PAA have been formed. The intermolecular reaction between the two adsorbed polyelectrolytes cross-links the polymer layer and forms a self-stabilized polymer network wrapping the silica kernel. Solvatation of the remaining polyions cannot 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 at 120 °C were compared in Fig. 4. Amide I (stretching vibration of the C@O bond) and amide II (deformation vibrations of the N–H bond) bands were observed at approximately 1622 and 1552 cm1, respectively, indicating the presence of amide groups onto silica surface. The shifts of the peaks values with about 10– 20 cm1 in comparison with the free amide groups (non-H bonded) could be explained by the intermolecular H-bonding of the amide groups attached onto silica surface. For

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 of boc-S-benzyl-L-cysteine (c).

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

5000

4000

CPS

example, the deconvoluted spectra of amide I peak of poly(N-isopropylacrylamide) in aqueous solution performed by Ramon et al. [25] evidenced an intramolecular H-bonded subband at 1630 cm1, an intermolecular Hbonded subband at 1620 cm1, and a free form of non-Hbonded subband at 1643 cm1. In the case of the thermal treated silica/(P(VFA-co-VAm)/PAA)1.5 hybrid, the H-bonding interactions between amide groups and silanols from silica surface seems to have also a contribution to the shifts of the peak values with 10–20 cm1 in comparison with free amide groups.

3000

2000

1000

1, 3, 5, 7, 9 coupling 2, 4, 6, 8 deprotection

0 0

The functionalization of the thermally cross-linked hybrid particles with S-benzil-L-cysteine was carried out as shown in Fig. 5, where DCC was used as the dehydrating agent.

1

2

3

4

5

6

7

8

9

10

Steps Fig. 8. The intensity of the S 2s peaks (CPS = counts per second) after the coupling of boc-S-benzyl-L-cysteine and its deprotection reaction. The intensities of the S 2s peaks were related to the intensity of the C 1s peak, which was kept constant.

The grafting reaction introduces sulfur in the sample surface, which can be easily detected (as S 2s peaks) and quantified in the XPS spectra (Fig. 6). The amount of sulfur from the hybrid particles corresponds with the amino acid’s grafting degree. As expected, the bare silica surface shows intensive peaks of silicon and oxygen (Fig. 6a). The alternately adsorption of P(VFA-co-VAm) and PAA introduces considerable amounts of carbon and nitrogen (Fig. 6b). After the grafting of boc-S-benzyl-L-cysteine, the S 2s peak can be clearly seen in the XPS spectra (Fig. 6c). Its intensity increased step-by-step with stepwise grafting of the amino acid (Fig. 7). According to the reaction Scheme in Fig. 5, each coupling reaction was followed by a deprotection reaction to produce new free amino groups of the covalently grafted amino acid to the particle surface. These free amino groups are able to react with the boc-S-benzyl-L-cysteine offered in a followed grafting step. After the first amino acid coupling, the raw area of the S 2s peak was 1750 counts per second (CPS), the photoelectrons counted over a second (Fig. 7b). The raw area of S 2s decreased to 1100 CPS after the first deprotection reaction (Fig. 8), because the P(VFAco-VAm) macromolecules from the cross-linked multilayer (togheter with bound aminoacid molecules), which are not strongly fixed to the surface, were removed in trifluoroacetic acid (TFA), used for deprotection. After first deprotection reaction, the intensity of the S 2s peak increased by the same amount, approximately 1000 CPS. From this constant increase of the sulfur content, it was concluded that the grafting reactions preferably took place on the pregrafted amino acids. In this way, oligopeptides grow up on the hybrid surface (Fig. 8). 4. Conclusions Fig. 7. The S 2s XPS spectra of the silica/(P(VFA-co-VAm)/PAA)1.5 (a) before 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 1s peak, which was kept constant.

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

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boxylic groups from multilayer built-up onto silica particles offered a novel synthetic route to stabilize hybrid materials consisting of weak polyelectrolyte multilayers. It was shown that the hybrid material can be equipped with a reasonable number of reactive amino groups located on the outer surface, giving possibilities for further derivatization reactions such as the coupling of amino acids to produce oligopeptides. Acknowledgments The authors thank Prof. Dr. Stefan Spange for providing the P(VFA-co-VAm) and Dr. Cornelia Bellmann for her kind assistance in performing the electrokinetic measurements on silica particles. The financial support from the Grant MATNANTECH 50/2006 is gratefully acknowledged. References [1] H.S. Nalwa (Ed.), Handbook of Surfaces and Interfaces of Materials, vols. 1–5, Academic Press, 2001. [2] G. Decher, J.D. Hong, Ber. Bunsenges. Phys. Chem. 95 (1991) 1430. [3] G. Decher, Science 277 (1997) 1232. [4] G.B. Sukhorukov, E. Donath, H. Lichtenfels, H. Knippel, M. Knippel, A. Budde, H. Möhwald, Colloid Surf. A 137 (1998) 253. [5] E. Donath, G.B. Sukhorukov, F. Caruso, S.A. Davis, H. Möhwald, Angew. Chem. 110 (1998) 2324. [6] S. Bratskaya, D. Marinin, F. Simon, A. Synytska, S. Zschoche, H.J. Busscher, D. Jager, H.C. van der Mei, Biomacromolecules 8 (2007) 2960.

[7] I. Voigt, F. Simon, K. Esthel, S. Spange, M. Friedrich, Langmuir 17 (2001) 8355. [8] I. Voigt, F. Simon, H. Komber, H.J. Jacobasch, S. Spange, Colloid Polym. Sci. 278 (2000) 48. [9] S. Spange, T. Meyer, I. Voigt, M. Eschner, K. Estel, D. Pleul, F. Simon, Adv. Polym. Sci. 165 (2004) 43. [10] F. Bucatariu, F. Simon, S. Spange, S. Schwarz, S. Dragan, Macromol. Symp. 210 (2004) 219. [11] T. Serizawa, K. Nanameki, K. Yamamoto, M. Akashi, Macromolecules 35 (2002) 2184. [12] T. Serizawa, Y. Nakashima, M. Akashi, Macromolecules 36 (2003) 2072. [13] S. Höhne, R. Frenzel, A. Heppe, F. Simon, Biomacromolecules 8 (2007) 2051. [14] V.Yu. Baranovsky, L.A. Kazarin, A.A. Litmanovich, I.M. Papisov, Eur. Polym. J. 20 (1984) 191. [15] E.S. Dragan, M. Mihai, A. Airinei, J. Polym. Sci. Part. A 44 (2006) 5898. [16] C. Mengel, A.R. Esker, W.H. Meyer, G. Wegner, Langmuir 18 (2002) 6365. [17] J. Dai, A.W. Jensen, D.K. Mohanty, J. Erndt, M.L. Bruening, Langmuir 17 (2001) 931. [18] A.M. Balachandra, J. Dai, M.L. Bruening, Macromolecules 35 (2002) 3171. [19] E.S. Dragan, F. Bucatariu, in: E.S. Dragan (Ed.), New Trends in Ionic (Co)Polymers and Hybrids, Nova Science Publishers, New York, 2007. p. 165. [20] F. Bucatariu, E.S. Dragan, F. Simon, Biomacromolecules 8 (2007) 2954. [21] R.B. Merrifield, J. Am. Chem. Soc. 85 (1963) 2149. [22] J.M. Stewart, J.D. Young, Solid Phase Peptide Synthesis, W.H. Freeman and Company, San Francisco, 1969. [23] H.J. Jacobasch, F. Simon, C. Werner, C. Bellmann, Technisches Messen: Sensoren, Geräte, Systeme 63 (1996) 447. [24] F. Fairbrother, H. Mastin, J. Chem. Soc. 125 (1924) 2319. [25] O. Ramon, E. Kesselman, R. Berkovici, Y. Cohen, Y. Paz, J. Polym. Sci.: Part B: Polym. Phys. 39 (2001) 1665.