Langmuir 1994,10, 1246-1250
1246
Self -Assembled Disulfide-Functionalized Amphiphilic Copolymers on Gold Christian Erdelen, Lukas Haussling, Renate Naumann, Helmut Ringsdorf,' Heiko Wolf, and Jianling Yang Institut fur Organische Chemie, Universitat Mainz, 0-55099 Mainz, Germany
Martha Liley, Jurgen Spinke, and Wolfgang Knoll Max Planck Institut fur Polymerforschung, 0-55021 Mainz, Germany Received July 19, 1993. In Final Form: December 13, 1 9 9 9
A series of methacrylic lipids was polymerized to form amphiphilic copolymers with various amounts of 2-hydroxyethylacrylate (HEA)and 2-(methacryloyloxy)ethylmethyl disulfidecomonomers. The behavior of these polymeric lipids was investigated in monolayers both at the air-water interface and in selfassembled monolayers on gold supports. The HEA acta as a hydrophilic spacer along the polymer backbone, and improves the ability of the polymer to self-organize into ordered monolayers, while the disulfide groups act as anchors chemisorbed onto the gold surface. A combination of contact angle measurements, surface plasmon spectroscopy, and cyclic voltammetry shows that the amphiphiliccopolymer monolayers are swellable in water and thus that a water layer between the support and the lipid membrane is preserved. The barrier properties of the polymer-modified electrode are improved by annealing in water as well as by increasing the spacer length.
Introduction The modification of solid surfaces by deposition of organicmolecules has gained increasing interest recently.lV2 Typically, there are two different approaches to oriented ultrathin films on solid supports: In Langmuir-Blodgett (LB) techniquessc films are assembled at gas-liquid interfaces and transferred onto a solid support, whereas in a spontaneous self-assembly (SA) organic molecules self-assemble into an ordered film simply by adsorption from solution onto a variety of solid surfaces.2 SA techniques have a number of advantages over LB films in preparing stable, highly durable, and perfect ultrathin films for membranes and surface modifications. The best documented SA systems are based on low molar mass organic compounds. These include organic disulf i d e ~ thiols,a11 ,~~ and sulfides12 on gold surfaces and carboxylic acids13J4 and silanesl6l8 on various oxide surfaces. Although the modification of surfaces by
* To whom correspondence should be addressed. Abstract published in Advance A C S Abstracts, March 1,1994. J. J.;Chandross,E.A.;Garoff, (1) Swalen,J.D.;Allara,D.L.;Andrade,
@
S.;Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J.
F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuiPBlodgett to Self-Assembly; Academic Press: Boston, 1991. (3) Mbhwald, H. Angew. Chem. Ado. Mater. 1988, 100,750; Angew. Chem., Int. Ed. Engl. 1988,27, 728. (4) Embs, F.; Funhoff, A,; Laschewsky, A.; Licht, U.; Ohst, H.; Prass, W.; Ringsdorf, H.; Wegner, G.; Wehrmann, R. Adv. Mater. 1991, 3, 25. (5) Nuzzo, R. G.; Allara, D. L. J . Am. Chem. SOC.1983, 105,4481. (6) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J . Am. Chem. SOC.
1987,109, 733. (7) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. SOC.1987, 109,2358. (8)Bain, C. D.; Whitesides, G. M. Science 1988,240,62; J.Am. Chem. SOC.1988,110,3665,5897,6560;Angew. Chem., Znt. Ed. Engl. 1989,28, 506; Adu. Mater. 1989, 4, 110. (9) Bain, C. D.; Troughton, E. B.; Tao, Y-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J.Am. Chem. SOC.1989,111, 321. (10) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. SOC.1990, 112, 558. (11) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987, 109, 3559. (12) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4, 365. (13) Allara, D.; Nuzzo, R. G. Langmuir 1986, 1, 45.
0743-7463/94/2410-1246$04.50/0
adsorption of polymer molecules has received considerable attention recently,lS2l the number of well-documented chemisorbed polymeric monolayers is still quite limited?2* Our interest is to combine the self-assembly strategy with the potential of specially designed copolymers to engineer self-organized,densely packed polymeric assemblies chemisorbed (as tethered films) onto solid s~rfaces.~4As schematically shown in Figure 1 the cooperative effect of many surface-reactive headgroups attached to one macromolecule should result in a monolayer being superior to the low molar mass alternative concerning the stability of the adsorbed layer. Controlling molecular architecture by the use of spacer groups in both polymeric amphiphiles and polymeric liquid crystals has been demonstrated e x t e n s i ~ e l y . The ~~~~~ polymers synthesized within the presented concept contain separated functional units, each one essential for the different,tasks of fixation, stabilization, and orientation. These moieties are separated from each other by flexible spacers. Spacer systems incorporated into the polymer backbone and the long side chains of these copolymers allow extra degrees of freedom critical for self-organization on the solid surfaces. Variations of the number of alkyl chains attached to the hydrophobic monomer change the (14) Ogawa, H.; Chihera, T.; Taya, K. J. Am. Chem. SOC. 1988,107, 1365. (15) Sagiv, J. J. Am. Chem. SOC.1980, 102, 92. (16) Maoz, R.; Sagiv, J. Langmuir 1987,3, 1034, 1045. (17) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. SOC.1988,110,6136. (18) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989,5,101. (19) Hadziioannou,G.;Patel,S.; Grannick, S.;TirreU,M. J. Am. Chem. SOC.1986,108, 2869. (20) Luckham, P. E.; Klein, J. Macromolecules 1986, 18, 721. (21) Kawaauchi. M.; Takahashi. A. Macromolecules 1984., 17.. 1666., 2063, 2066. (22) Stouffer, J. M.; McCarthy, T. J. Macromolecules 1988,21,1204. (23) Higashi, N.; Mori, T.; Niwa, M. J. Chem. SOC.,Chem. Commun. 1990,225. (24) Hlussling, L.; Knoll, W.; Ringsdorf, H.; Schmitt, F.-J.; Yang, J. Makromol. Chem., Macromol. Symp. 1991,46, 145. (25) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem. 1988,100, 117: Anaew. Chem.. Znt. Ed. E n d . 1988.27. 113. (26) hachewsky; A.; Ringsdoa, H.; Schmidt, G.; Schneider, J. J. Am. Chem. SOC.1987,109, 788.
0 1994 American Chemical Society
Disulfide-FunctionalizedAmphiphilic Copolymers on Gold
Langmuir, Vol. 10,No. 4,1994 1247 Scheme 1. Synthesis of Disulfide-Functionalized Amphiphilic Copolymers CHz=GCOQCHzCHg-R
seiforganizing surface units
organization
I
+
+ CHz=CH-CODCH,CH,-OH
CHz=C.COO.CHzCHz.S-S.CH,
I
CHa
CHa
1, 2 or 3 polymer chain macer
stability + mobility
5
4
+
A
reactive headgroups
fixation
65% low moiar mass alternative/
h-1
?HZ
H .GCOQCHzCHz-OH
AIBN DioxenetToluene
2
*
FHZ
Figure 1. Concept for tethered layers on solid supports.
CH,.S-S.CHzCHg-OOC-GCH,
%
hydrophobicity of the copolymers and the tendency to self-organize. A third variation may come along with the introduction of functional, e.g., photolabile, units, which eventually allows a photovariation of the monolayers. In this paper the synthesis of self-assembled disulfidefunctionalized amphiphilic copolymers and the monolayer formation of these copolymers on water and gold surfaces will be reported.
1.
I -NH-COO-CH~(CHz)i&H,
Experimental Section Materials. All chemicals and solvents were commercially available. Unless otherwise stated, all chemicals were used as received. The water used for the subphase was Milli-Q water. Monomer Synthesis. The synthesis of the lipid monomer benzylammonium methacrylate 3 was described elsewherean aminolethyl methThe monomer 2-[N-[~octadecyloxy)carbonyll acrylate (1)is synthesized from 1-octadecanol,24socyanatoethyl methacrylate, and triethylamine as a base in dichloromethane. The mixture was kept under reflux for 24 h. The crude product was recrystallizedfrom acetone two times, after which the product was obtained in 73% yield: mp 64.5 OC; 'H NMR (CDCh) 6 (ppm) 0.85 (t,3 H, CHa(CHz)is),1.1-1.7 (m, 32 H, CHdCHz)id, 1.9 ( ~ , H, 3 C(CHs)=CHz), 3.5 (dt, 2 H, NCH~CHZO), 4.0 (t, 2 H, OCH2(CHz)ls), 4.2 (t, 2 H, NCHzCHzO), 4.9 (1H, NH), 5.6-6.1 (m,2 H, C H p C ) . Anal. Calcdfor CBH~~NOI (425.65): C, 70.54; H, 11.13; N, 3.29. Found: C, 70.37; H, 10.98; N, 3.20. The monomeric lipid Nfl-dioctadecyl-N'- [2-(methacryloyloxy)ethyllurea (2) was prepared by reaction of dioctadecylamine with 2-isocyanatoethylmethacrylate in dichloromethane at room temperature in 50% yield 'H NMR (CDCld 6 (ppm) 0.85 (t,6 H, C H ~ ( C H Z ) 1.1-1.7 ~ ~ ) , (m, 64 H, C H ~ C H Z )1.9 ~ ~(8,, 3 H, C(CH+CHz), 3.1 (t,4 H, (CH2)l&H2N), 3.5 (dt, 2 H, NCHzCHzO), 4.3 (t, 2 H, NCHzCHzO), 4.6 (t, 1 H, NH), 5.6-6.1 (m, 2 H, C H y C ) . Anal. Calcd for CaHaNzO3 (677.29): C, 76.25; H, 12.53; N, 4.14. Found: C, 75.83; H, 12.27; N, 4.25. The disulfide monomer 2-(methacry1oyloxy)ethyl methyl disulfide (5) was synthesized by a two-step reaction. First, the 2-hydroxyethylmethyldisulfidewasprepared by refluxing methyl disulfide and 2-mercaptoethanol with NaOH as the catalyst in water. The product was extracted in a Kutscher-Steudel liquidliquid extraction apparatus with petroleum ether. The disulfide monomer 5 was prepared by esterification of 2-hydroxyethyl methyl disulfide with methacryloyl chloride: lH NMR (CDCla) 6 (ppm) 1.9 (8, 3 H, C(CHd=CH2), 2.38 (8, 3 H, CHaS), 2.9 (t, 2 H, SCHZ),4.4 (t, 2 H, CHZO),5.6-6.1 (m, 2 H, C H 4 ) . Copolymerization. The copolymerization of monomeric lipids with 2-hydroxyethyl acrylate (HEA) and 2-(methacryloyloxy)ethyl methyl disulfide was performed in a toluene/dioxane (1:l) mixture with 1 mol % AIBN as the radical initiator. Typically 250 mg of the monomers (e.g., for copolymer 2c, 51 mg of monomer 2,174 mg of monomer 4,and 29 mg of monomer 5) was dissolved in 5 mL of the solvent, and the initiator was added last. The mixtures were flushed with nitrogen for 10 min, and then the polymerizations were carried out at 65 OC for 12 h. The amphiphilic polymeric lipids were obtained by precipitation in acetonitrile and freeze dried in benzene (or dioxane for the higher (27) Amdt, Th.;HHuesling, L.; Ringedorf, H.; Wegner, G. Adu. Mater. 1991, 3,58.
Br'
Table 1. Composition and Molecular Weight of the Disulfide-Functionalized AmDhiDhilic CoDolumers composition" GPCb no. la
lb 2a 2b 2c
mc 1.5 2
3 2 1 2 2
ne 1 12
6 14 10
MU 13500 11800 16400 10600 11400
Mw 24000 18900 27000 15700 17200
MwIMn 1.78 1.60 1.65 1.48 1.51
3a 0 d 3b 7 12400 23700 1.91 a Estimated by lH NMR analysis. PMMA standards; solvent THF. See also Scheme 1. d Copolymer 3a is soluble in CHCb but aggregates in THF. HEA comonomer content). The yields were about 45% for all copolymers. The copolymers were characterized by TLC, 1H NMR, and gel permeation chromatography (GPC). The synthetic route is given in Scheme 1. The molar weight and the composition-estimated by lH NMR spectra-of the copolymers are given in Table 1. Monolayers at t h e Gas-Water Interface. The copolymer was dissolved in chloroform to a concentration of about 0.5 mg/ mL. A small amount of methanol wasaddedto enhance solubility, particularly of the copolymer containing the higher amounts of hydrophilic spacer. A computer-controlled film balance utilizing a Wilhelmy plate pressure pickup system was used to examine the behavior of the spread monolayerem at the &water interface. The isotherms were recorded at 20 OC on pure water. Gold Substrates. Gold (purity of at least 99.99%) was deposited onto clean glass slides (25 mm X 75 mm) by resistive evaporation using a Balzers (BAE 250) vapor depositionapparatus at room temperature. The evaporation chamber was kept at 10-8 Torr. Before deposition of gold, the glass slides were primed with 10 nm of chromium for better adhesion of gold onto the glass surface. The thickness of the gold film waa about 50 nm. The glass slides were precleaned by sonication in "Hellmanex" (Hellma GmbH & Co., Germany) detergent solution and washed with copious amounts of Milli-Q water. The glass slides were stored in 2-propanol (pa) and washed with chloroformjust before use. Monolayer Formation on Gold. Self-assembledmonolayers of disulfide-containing amphiphilic copolymers were spontaneously adsorbed by immersing the fresh gold-coated substrate (28) Albrecht, 0. Thin Solid Films 1983,99, 227.
1248 Langmuir, Vol. 10, No. 4, 1994 6o
Erdelen et al.
1 CY
l a
40
0,o
H.k€X-CH,W#
I
I
I
i
1,o
2-0
3,O
4,o
0,o
area [nmzirepeat unit]
Figure 2. Pressure-area diagrams of amphiphilic copolymers with single-chain hydrophobic units (la and lb) (2' = 20 "C). into a freshly prepared solution (1 mg/mL) of copolymer in chloroform (in the case of a copolymer with a higher HEA content a small amount of methanol was added) at ambient temperature (20-25 "C) for at least 60 h. The electrochemical experiments showed that for all the copolymers a 60-h soak was sufficient for maximum coverage. The substrate was then washed thoroughly with chloroform and dried in an argon stream. The film thickness was estimated by surface plasmon spectroscopy. Contact angles with water were measured with a Kriiss G1 system (Kriiss GmbH, Germany). Surface Plasmon Spectroscopy. The gold film and selfassembled monolayer coated glass slides were refractive index matched to a 90° glass prism used as a surface plasmon (SP) coupler in the Kretschmann configuration.29Resonant excitation of the surface mode which is very sensitive to the actual interfacial architectureN*3lwas then monitored by recording the total internally reflected light from a HeNe laser (A = 633 nm) as a function of the angle of incidence. First, the bare gold substrates were characterized in air and water. After the monolayer formation the sample was rinsed and dried and then remeasured again in air and water. Electrochemical Measurements. Cyclic voltammograms (CV) were performed using a PCA 72L and VSG 72 potential control system (G. BankElektronik, Gtittingen, Germany) which contained a conventional three-electrode cell with a Pt wire as a counter electrode and a AgIAgC1 (saturated KCl) reference electrode. A gold disk electrode (Metrohm AG, Switzerland; diameter 4 mm) was polished with alumina powder and cleaned by sonication in Milli-Q water and then with ethanol. Next it was immersed in the copolymer solution for approximately 60 h, and then rinsed with chloroform and dried in a nitrogen stream. All electrolyte solutions were freshly prepared by using Milli-Q water and reagent grade KC1 and KsFe(CN)B. Before electrochemicalmeasurements, the electrolyte solution was purged with nitrogen for 5 min. Typically, the CV was run in the potential range between -0.3 and +0.6 V and employing a scan rate of 0.1 VIS.
Results and Discussion Monolayer Experiments on Water. The copolymers were investigated for their aggregation and organizing behavior at the air-water interface using surface pressurearea measurements (isotherms). Figures 2-4 show the isotherms measured for the series of single-chain (la and lb) and double-chain (2a, 2b, 2c, 3a, and 3b) polymeric lipids. The areas were calculated on the basis of the monomer repeat unit for each copolymer, assuming that the repeat unit contains a lipid monomer and the corresponding comonomer ratio of HEA and disulfide comonomers. Except l b all other copolymers exhibited at room temperature a solid analog phase with a high collapse pressure (above 40 mN/m). The liquid expanded (29) Kretschmann, E. Opt. Commun. 1972, 6, 185.
(30)Raether, H. Surface Plasmons. Springer Tracts in Modern Physics; Springer: Berlin, 1988; Vol. 111. (31) Knoll, W. MRS Bull. 1991, 16, 29.
2,o
I
i
4,o
6,O
area [nm2/repeat unit]
Figure 3. Pressure-area diagrams of amphiphilic copolymers with double-chain hydrophobic units (2a, 2b, and 2c) (2' = 20 "C). 60
-
40
-
I
E
2 E
I
2
0 2
20
Q
a
3a 0 I
Q,O
1,o
2,o
I
i
3,o
40
area [nmzirepeat unit]
Figure 4. Pressure-area diagrams of amphiphilic copolymers with (dialkoxybenzy1)ammoniumunits (3a and 3b) (T=20 "C), Table 2. Advancing and Receding Contact Angles of Copolymer Monolayers on Gold with Water content of lipid analog comonomeP copolymer (mol %) 0. (deg) 0, (dea) la 43 97 25 lb 13 67 25 2a 30 93 42 2b 12 76 22 2c 8 75 26 3a 67 96 41 3b 20 91 25 See also Table 1 and Scheme 1.
phase and the areas of the solid analog phase increase with increasing HEA comonomer content. The results are comparable to those of the investigations of analogous amphiphilic copolymers with a main-chain ~pacer.~ts2 Contact Angle Measurements. Contact angles of solid surfaces with water provide a useful insight into wettability, roughness, and molecular motions a t inter-
(e,) and receding (e,) contact angles of the copolymer monolayers are given
faces.9~33The results for the advancing
in Table 2. The advancing contact angles of the Copolymer monolayers are in general lower than loOo which indicates that in no case a perfect packing of alkyl side chains was achieved. In addition, it was observed that the advancing contact angles (8,) decreased with time after a series of measurements at the same position. At a place where the water drop rested on the substrate for a few minutes, the advancing contact angles were considerably lower than (32) Biddle, M. B.; Lando, J. B.; Ringsdorf, H.; Schmidt, G.; Schneider,
J. Colloid Polym. Sci. 1988, 266, 806.
(33) Stenius, P.; Smm, G.; Fredriksson, M. J. Colloid Interface Sci.
1987,119, 352.
Langmuir, Vol. 10, No. 4,1994 1249
Disulfide-FunctionalizedAmphiphilic Copolymers on Gold
.
.
.
0 10 20 30 40 50 60 70 content of lipid monomer [mol%]
Figure 5. Influence of the copolymer composition on the advancing contact angles with water demonstrated for the two copolymers types 2 (open boxes) and 3 (filled boxes; see Table 2). The line connecting the data points implies no curve fit or analytical function. Table 3. Film Thickneee of Copolymer Monolayers on Gold content of lipid analog comonomeF AOh AO,, dfit copolymer (mol 9%) (deg) (deg) ntit (nm) 0.76 1.6 2.6 la 43 0.47 0.42 0.49 1.5 2.6 lb 13 1.55 1.9 0.30 0.46 2a 30 0.48 1.5 2.5 2b 12 0.42 0.51 1.5 2.6 2c 8 0.41 1.55 1.6 0.27 0.40 3a 67 0.36 1.5 1.9 3b 20 0.33 a
See also Table 1 and Scheme 1.
the values taken immediately after the first contact of the water with the surface. This could be attributed to the swelling of the amphiphilic monolayers as to be expected for a polymer with hydrophilic parts. All reported values are the average of at least five measurements taken from the first measurement cycle at different locations on the gold surface. The uncertainty in these data (sample to sample variation) is about A2O. In general the advancing contact angles decreased with decreasing content of the lipid analog comonomer, which is due to the increasing overall hydrophilicity of the copolymers. The wetting behavior of the double-chain copolymer (types 2 and 3) monolayersis similar as shown in Figure 5. The hysteresis of the contact angles appears to be unusually large. Although we do not fully understand the structural implications,we believethese findingsto be due to disorder in the alkyl side chains of the copolymer and the sensitivity of the probe liquid (water) to the underlying hydrophilic polymer backbone. Surface Plasmon Optical Film Characterization. Table 3 summarizes the angular shifts of the surface plasmon resonance, AO, measured for the various copolymer monolayers in air and in water. As a first approach to analyze the data, we assume that the index of refraction, nnt, and the geometrical thickness, dwt, of each respective monolayer have to be identical in both media, i.e., in air and in water. In this sense the two data sets are considered to be an optical contrast variation experiment. From a Fresnel fit to the reflectivity curves we obtain the film parameters given in Table 3. The range of refractive indices between n = 1.5 and n = 1.6 is reasonable given the chemical nature of the copolymericsystems with their highly polarizable groups. This would explain the difference between the polymers and simple alkanethiols where an index of refraction of n = 1.45 is typically assumed. The double-chain polymeric lipids 2 and 3 show an increasing thickness with decreasing content of the lipid analog comonomer and therefore increasing content of the hydrophilic main chain spacer (HEA). This can be
0.6
0.3
0
-0.3
E(V) vs Ag/AgCI (sat'd KCI)
0.6
0.3
0
-0.3
E W ) vs Ag/AgCI (sat'd KCI)
Figure 6. Cyclic voltammograms of a clean and polymermodified (2a, 2b, and 2c) gold electrode: non-water-annealed (bold line); water-annealed (dashed line).
understood if one assumes that the lateral density of the self-organized monolayer is given by a more or less tight packing of the long alkyl chains. Then the increasing HEA content per chain must show up as an increase in thickness. This interpretation, however, should be taken with some caution since the structural details of these self-assembled systems are the result of a rather delicate balance of the interaction potentials between the various hydrophobic and hydrophilic molecular units and the substrate. Most problematic, however, is the assumption of an identical structure of the layers in air and water. The uptake of water into the hydrophilic headgroup region by swelling under water would, of course, also change the optical thickness of this layer. Model calculations within the Fresnel theory show that any effective index of refraction smaller than nfit would result in a thickness of the self-assembled monolayer which is larger in water than in air. We believe this situation to be very likely, but have-at present-no optical means to determine the correct value of neff and hence the degree of the water uptake. Cyclic Voltammetry of the Copolymer-Modified Electrode. The contact angle measurements and surface plasmon spectroscopic results provide important information on the macroscopic properties of the adsorbed copolymers, e.g., wettability, thickness, and swelling behavior. In order to examine the nature and extent of structural defects, we have exploited electrochemical measurements of heterogeneous electron transfer, which turns out to be highly sensitive to film defects.34J0A freely diffusingelectrolytesolutionwill react exclusivelyat defect sites if electron transfer is blocked across the majority of the monolayer. Figure 6 shows cyclic voltammograms of copolymermodified and bare gold electrodes in a solution containing 1mM &Fe(CN)s in 1M KC1. Reversible CV peaks of the F e ( c N ) ~ ~ / F e ( c Nsystem ) 6 ~ on the bare gold electrode were suppressed largely by depositing the disulfidecontaining copolymers on the electrode. In the used potential range the voltammograms were stable for at least five scans. In general, the peak potential (E,) on the monolayer-covered electrode was shifted to a large overpotential relative to the E,' of the bare electrode. The (34)Amatore, C.; Saveant, J. M.;Tessier, D. J. Electroanal. Chem. 1983, 147, 39.
Erdelen et al.
1250 Langmuir, Vol. 10, No. 4, 1994 Table 4. Barrier Properties of Polymer-Modified Gold Electrodesa non-watar-annealed water-annealed
copolymer la lb 28
2b 2c 3a 3b
idipo 0.50 0.33 0.46 0.42 0.42 0.57 0.44
hEp(V) 0.40 0.72 0.40 0.61 0.62 0.17 0.52
idiP0 0.48 0.30 0.46 0.40 0.38 0.57 0.36
hEp(V) 0.54 0.79 0.40 0.72 0.80 0.17 0.65
0 iplip* = ratio of the maximum oxidation current for the polymermodified electrode to the maximum oxidation current for the clean electrode (the second scan was taken in order to have a closed loop curve). AE,,= separation between the cathodic and anodic peak currents (second scan).
ratio (idiPo) of oxidation currents for the polymer-modified electrode to the bare electrode and the peak-to-peak separation (M,)of the oxidation and reduction currents for the polymer-modifiedelectrode were used as markers of the barrier effect of the copolymer for the penetration of ferrocyanide ions. The ip/ipo and AEp values are summarized in Table 4. All CV data are indicative of a blocked electrode with defects in the blocking layer. The major sources of the defects are probably a combination of incomplete surface coverage and disorder of alkyl side chains in the copolymer monolayer as discussed before. The redox ions migrate through these defects in the monolayer to be reduced or oxidized at the metal surface. Regardless of single-chain polymeric lipid or double-chainpolymeric lipid, the longer the flexible main chain spacer (HEA), the better is the barrier effect. Annealing of a coated electrode in water at 65 "C for 1h improves the barrier effect significantly. The copolymers with a higher content of hydrophilic main
chain spacer (HEA) show markedly higher annealing effects than those with shorter spacers. These results indicate that the hydrophilic spacer in the polymer backbone improves the ability of the adsorbed polymeric lipid monolayer to self-organize into an ordered structure.
Conclusions The combination of the self-assembly strategy and the special design of polymeric lipids with main chain spacers was used to study the self-organization of these amphiphilic copolymers on water and on gold. The structure of these monolayers depends highly on the composition of the copolymer. The mobility of the hydrophobic alkyl side chains improves with increasing content of the hydrophilic main chain spacer (HEA). Defects in the structures exist naturally because of the complex coil conformation of the polymers in solution and their insufficient uncoiling during the attachment. Annealing of the adsorbed amphiphilic monolayers improves the barrier properties of the thus modified gold electrode against the penetration of ferrocyanide ions. The performed experiments (contact angles, surface plasmon spectroscopy, annealing effect in water) point to polymer monolayerswhich are swellable in water, preserving a water layer between the support and the lipid membrane. The amphiphilic monolayers may thus serve as a new approach to construct stable as well as mobile tethered supported bilayer membranes in order to incorporate proteins with large extra membranous domains. Utilization of this approach is presently under investigation in this laboratory. Acknowledgment. We gratefully acknowledge the financial support by the German Bundesministerium fiir Forschungund Technologie (BMFT) (UDSProjekt). M.L. wants to thank The Wellcome Trust for financial support.
