Macromol. Rapid Commun. 19,333-336 (1998) 333

An in-situ ATR-FTIR study on polyelectrolyte multilayer assemblies on solid surfaces and their susceptibility to fouling Dedicated to Prof. Dr. sc. nat. H.-J. Jacobasch in memoriam

Martin Muller*, Theresia Riesel; Klaus Lunkwitz, Siegfried Berwald, Jochen Meier-Haack, Dieter Jehnichen

Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany mamuller @ipfdd.de

(Received: February 18, 1998; revised: March 25, 1998)

SUMMARY In-situ attenuated total reflection (ATR)-FTIR spectroscopy was used to monitor the consecuti- vely alternating adsorption of polyethylenimine (PEI) and poly(acry1ic acid) (PAC) onto both Si crystals (SiO) and COz plasma-treated polypropylene (PP) films. The vibration bands v,(COO-) and v(C=O) of PAC are diagnostic for the polyelectrolyte layer build-up and sensitive to protonation changes. Human serum albu- mine (HSA) adsorption experiments revealed a strong decrease of fouling for the PP films, which were modi- fied with polyelectrolyte multilayers, in comparison to the unmodified ones.

Introduction The formation and behavior of polyelectrolyte complexes (PECs) in solution and on inorganic substrates is a well- studied topic in basic research with wide spanning appli- cations in the field of flocculation, waste water treatment, pharmacy and biomedicine (controlled drug release, encapsulation) 1 4 ) . Less attention is dedicated to technical applications concerning PECs at solid polymer surfaces. Recently, PECs were used for the modification of porous polypropylene (PP) membranes, which improved their non-fouling properties').

In order to have a more reproducible experimental access and to investigate this interfacial effect on a more molecular level, we have chosen PP films as a model sys- tem for a modifiable polymer surface. PP films, similar to PP membranes, are inherently hydrophobic and have a high tendency to protein adsorption. Increasing the hydrophilicity and/or the surface charge of the polymer surface helps to moderate this fouling effect, which is important for many applications*). The PEC formation on solid surfaces may be carried out by a modified technique of layer-by-layer-deposition of oppositely charged poly- electrolytes, whose diplrinse protocol was described by Decher et al.93'0)

Recently, several papers on the in-situ monitoring of polyelectrolyte multilayer formation using the highly thickness sensitive methods optical waveguide lightmode spectroscopy (OWLS)"), surface plasmon spectroscopy (SPS)'') and optical refle~tornetry'~) were published. We have chosen in-situ ATR-FTIR spectroscopy, whose sen- sitivity to phenomena at the solid/liquid interface is well

to monitor both layer build-up and further molecular processes (e.g. protonation changes) during the consecutively alternating adsorption of polyethylenimine (PEI) and polyacrylic acid (PAC). Beneath the COz

Macromol. Rapid Commun. 19, No. 7, July 1998

0 1998, Huthig & Wepf Verlag, Zug

plasma-treated polypropylene films also the bare SiO sur- face was used as a substrate for the multilayer assembly. Additionally, ATR-FTIR spectroscopy was used to inves- tigate the protein adsorption as a model for the fouling process onto modified surfaces.

Experimental part

Polypropylene films and plasma treatment Isotactic polypropylene (Hoechst) was dissolved in xylene and spin-coated on the Si internal reflection element (Si- IRE). Thin films with a thickness of about 30 nm could be obtained by this technique, which was confirmed by X-ray reflectometry .

COz plasma treatment was carried out with a Tepla 440G plasma reactor (Technics Plasma GmbH, Kirchheim) sup- plied with a microwave plasma source. The experimental conditions were kept constant at 70 W plasma power, 10 sccm (standard cubic centimeters) COz mass flow, 3 min treatment time and 10 Pa working pressure.

Polyelectrolytes and polyelectrolyte multilayer build-up Polyethyleneimine chloride (PEI, Aldrich, Mw = 750000) and poly(acry1ic acid) (PAC, SIGMA, Mw = 90000) were used without further purifications. Ultrapure water (Milli- pore, 18.2 MR) was used in all experiments. The polyelec- trolyte solutions (1 mg/ml) resulted in pH values of 3.8 for PAC and 9.8 for PEI and were used without buffer. Multi- layer assemblies of oppositely charged polyelectrolytes were generated by consecutive adsorption due to a standardized stream coating procedure upon a Si-IRE in the ATR-IR sorp- tion cell. Between every polyelectrolyte addition the cell was carefully rinsed with water.

Bulk films of PEI and PAC were fabricated by casting 50 pl of a 1 mg/ml polyelectrolyte solution on a Si ATR crys- tal and NZ drying.

CCC 1022- 1336/98/$10.00

334 M. Miiller, T. Rieser, K. Lunkwitz, S . Berwald, J. Meier-Haack, D. Jehnichen

.................................................................. -

("3) .................................. Adsor!!a!e.@d ....... :;

Fig. 1. Scheme of the ATR-FTIR principle

Protein adsorption

Human serum albumine (HSA) was provided by Hoechst and dissolved in a phosphate buffer solution (1 mg/ml, pH = 7.4), which was prepared from phosphate buffer tablets (Fluka). The protein adsorption measurements were per- formed directly after the generation of the specifically tai- lored multilayer systems in the same sorption cell.

In-situ ATR-FTIR measurements

The ATR-FTIR measurements were performed on the in-situ ATR-FTIR Attachment for Sorption Measurements (OPTIS- PEC, Zurich), consisting of a special mirror setup and a removable in-situ ATR sorption cell surrounding a trapezoi- dal silicon IRE. In Fig. 1 the ATR-FT'IR principle is shown schematically. A Fourier Transform Spectrometer (IFS 55, BRUKER) was used with a globar source and a liquid nitro- gen cooled mercurium cadmium telluride detector. ATR- FT'IR spectra of the adsorbed polyelectrolytes were recorded using the novel SBSR (single beam sample reference) tech- nique for sensitive adsorption measurements in aqueous environment^'^). Thereby, the ATR plate is divided into a lower reference and an upper sample half, sealed separately by O-rings forming a sample (S) and a reference (R) com- partment on the IRE surface. During the measurement inten- sity spectra (Is, IR) of the S- and R-compartment are alterna- tively recorded by the SBSR method and computationally 'ratioed' according to:

The resulting absorption spectra AsesR enable an accurate compensation of background absorptions due to SiO, the polymer film and the liquid water in the sample and in the reference chamber, respectively. Furthermore absorptions of the atmospheric water vapour in the spectrometer as well as water absorptions due to ice on the cooled detector window could be also sufficiently suppressed.

Generally, the sample compartment of the ATR liquid cell was filled with the aqueous polyelectrolyte solution, whereas the reference compartment was filled with ultrapure water. After 5 and 20 min of the adsorption process AsBsR-spectra were recorded. Then the sample compartment was rinsed

with water until the polyelectrolyte solution was completely displaced and a further AsBsR-spectrum was recorded. There- upon a solution of oppositely charged polyelectrolyte (with respect to the adsorbed one) was filled in the sample com- partment and AsBsR-spectra were recorded according to the above mentioned protocol.

Results and discussion

Multilayer deposition In Fig. 2A ATR-FTIR spectra (AsBsR), recorded after each adsorption step of PEI (odd numbers) and PAC (even numbers) at a PP film on a S I R E , are presented with increasing adsorption step from bottom to top. The char- acteristic relevant IR bands are listed in Tab. 1. Espe- cially, evident intensity variations of the IR signals origi-

Tab. 1 . Assignment of the IR bands of a PACPEI multilayer assembly in contact to water

Wavenumber water PEI (+) PAC (-) cm-I

3 390 (br) v(OH) 3000-2800 W H ) W H )

1707 W=O)

1548 v,(COO-) 1 400 v,(COO-)

1570 6(NH+), G(NHt)

nating from adsorbed PAC were observed, i.e., the v(C=O) at 1707 cm-' due to the COOH groups and the v,,(COO-) at 1548 cm-' due to the deprotonation of PAC. PEI gives less intensive and marked IR bands compared to PAC due to their small absorption coefficients, which is illustrated by ATR-FTIR spectra of the bulk films of PEI and PAC in Fig. 2B.

Hence, the vibration bands of PAC, v,,(COO-) and v(C--U) of COOH, were used as diagnostic markers for the monitoring of the polyelectrolyte layer deposition process. In Fig. 3 the integrated areas of the above men- tioned IR bands are plotted against the adsorption step

An in-situ ATR-FTIR study on polyelectrolyte multilayer assemblies on solid surfaces and their susceptibility to fouling 335

Fig. 2. (A) In-situ ATR-FTIR spectra (AsBsR) for the consecutive adsorption of PEI and PAC onto S I R E ( S O surface). Odd numbers are due to the PEI adsorption step, even numbers are due to the PAC adsorption step. (B) ATR-FTIR spectra of bulk films of PEI and PAC prepared by solution casting from 50 p1 of a polyelectrolyte solution (1 mg/ml) on a Si-IRE

(odd: PEI, even: PAC). Thereby the v,,(COO-) band showed a continuous increase after each PAC adsorption step due to the subsequent layer build-up at the SiO sur- face (A) and the plasma-modified PP film (B). After about the tenth cycle a plateau region was reached for both surfaces, since the film thickness had exceeded the sampling depth of the evanescent field.

Interestingly, in the plateau region we still observed a correlated modulation feature of the v,,(COO-) and the v(C-U) band with every polyelectrolyte addition. Accordingly, the v(C--U) was slightly increased whereas the v,,(COO-) band was slightly diminished with every addition of PAC (protonation phase). On the other hand, with every addition of PEI the COOH band was dimin- ished whereas the v,,(COO-) band was increased (depro- tonation phase). For an explanation the pH value of the bulk polyelectrolyte solution has to be considered. Obviously, by adding a PAC solution with pH = 3.8 onto the multilayer assembly the deprotonated COO- groups of PAC in the underlying layers were reprotonated. Vice versa, adding a PEI solution with pH = 9.8 caused depro- tonation of the COOH groups of the last and of all under- lying PAC layers.

Water adsorption and desorption Generally, in the ATR-FTIR (AsBsR) spectra (Fig. 2A) we observed negative v(0H) signals, which reflects that the

A -A- -OH (3700-3000 cm-')

--.--COO- (1640-1480 cm-') - -o--COOH (1770-1655 cm")

-- 20

8 -20

-40 Adsorption Steps

-A- -OH (3700-3000 cm") B

3 --.--COO- (1640-1480 cm") ' 30 254 -o--COOH (1770-1655 cm-')

--.--.--. 6

fp -10 6 7 8 9 10 11 12 13 14 15

--.--.--. 6

fp -10 6 7 8 9 10 11 12 13 14 15

2 -20

4 = -40 -451

-25 -30

* -35 Adsorption Steps

Fig. 3. Consecutive adsorption of PEI and PAC at (A) the SiO surface and (B) the COz plasma-treated polypropylene film. The integrated band areas from the AsssR spectra of Fig. 2 are plotted against the adsorption step (PEI: odd step numbers, PAC: even step numbers)

water in the sample compartment gives rise to a slightly smaller IR absorption than the water in the reference compartment. From ATR theory slightly smaller IR absorptions due to v(0H) can be expected, if water moves to regions where the exponentially decaying electric field (in direction of the IRE normal) of the evanescent has smaller amplitudes. Obviously, in the sample compart- ment the water has been slightly displaced by the adsorbed PEL layer to outer spheres with respect to the IRE surface. Hence the further decrease of the v(0H) can be explained by the increasing amount of adsorbed poly- electrolytes at the SiO surface causing the water desorp- tion from the surface. A quantitative analysis of the mea- sured ATR data will be published elsewhere.

Furthermore, similarly to the v,,(COO-) and the v(C--U) bands we observed a slightly modulated inten- sity of the v(0H) of water in dependence of the adsorp- tion step. Thereby for every PAC addition we have a slight diminuishing of the v(0H) band area and for every PEI addition we have a slight elevation of the v(0H)

336 M. Muller, T. Rieser, K. Lunkwitz, S. Berwald, J. Meier-Haack, D. Jehnichen

0.00 , 1800 I700 1600 I500

Wavenumber [cm-l ]

Fig. 4. Human serum albumine adsorption (1 mg/ml, pH = 7.4) onto (a) untreated and (b) C02 plasma-treated PP films compared to (c) a plasma-treated PP film which was modified with four polyelectrolyte layers (PEIPACPEIPAC)

band. This gives evidence for the alternating hydratability of the outermost layer, whereby an outermost PAC layer with partially protonated COOH groups has a lesser hydratability compared to an outermost PEI layer.

Protein adsorption In Fig. 4. ATR-FTIR spectra for the HSA adsorption at the bare spin-coated PP film (a), at the plasma-modified PP film (b) and at the plasma-modified PP film with four consecutively adsorbed layers of PEI and PAC (c) are presented. Thereby, for the bare (a) and the plasma-modi- fied (b) PP films in the spectral region between 1700- 1600 cm-l the diagnostic amide I band of HSA appeared with a measurable integrated absorbance. However, sig- nificantly for the polypropylene films which were modi- fied with four polyelectrolyte layers, we observed practi- cally no measurable amide I intensity. Therefore we con- clude a strong non-fouling effect (protein inertness) of the polyelectrolyte layer assembly at the surface of the PP films compared to the untreated and plasma-treated PP films. Principally, the adsorbed amount can be deter- mined quantitatively, if the ATR measurements are per- formed with polarized light and provided knowledge of the polymer thickness by ellipsometry”).

Hypotheses For the explanation of this non-fouling effect of polyelec- trolyte multilayers among several hypotheses three may be cited:

The outermost polyelectrolyte layers of the multilayer assemblies are negatively charged (PAC) under the applied conditions, hence the electrostatic interactions towards the HSA, which has a negative net charge (iso- electric point = 4.7), are repulsive (i).

Competitive binding studies of KabanovI8) on two lin- ear oppositely charged polyelectrolytes and proteins in solution revealed the formation of an interpolyelectrolyte complex between the two oppositely charged polyelectro- lytes, whereas the protein remained unbound. Therefore, IPECs formed by the consecutive multilayer assembly of PEVPAC at solid surfaces should cause protein resistance in a similar way (ii).

As it was pointed out by Decher and coworker^^^^^^^^), the self assembly process of oppositely charged polyelec- trolyte layers leads to a relatively small surface roughness of the multilayer film on a nanoscopic scale. Therefore in addition to (i) and (ii) the low protein affinity might be also due to the reduction of physical entanglements be- tween the outermost multilayer surface and the HSA (iii).

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