Characterization of Ultrathin Poly(ethylene glycol) Monolayers on Silicon Substrates Alexander Papra,* Nikolaj Gadegaard, and Niels B. Larsen Condensed Matter Physics and Chemistry Department, Ris�� National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark Received April 25, 2000. In Final Form: September 7, 2000 Low molecular weight poly(ethylene glycol) silanes (PEG silanes) have been grafted onto the surface of silicon wafers in a one-step procedure yielding ultrathin and stable PEG monolayers. Structural investigation by means of X-ray reflectivity provided data on the thickness of the PEG monolayers. The layer thickness varied between 10 and 17 �� depending on the PEG silane concentration applied. These results have been confirmed by X-ray photoelectron spectroscopy measurements. Atomic force microscopy data indicate very smooth and homogeneous coverages with roughnesses of less than 3 ��. The PEG layers are hydrophilic as determined with advancing water contact angles between 36 and 39��. Introduction Many applications in biotechnology require specific surface properties.1 One of the major prerequisites for these applications is the prevention of nonspecific protein adsorption. This is, for example, important to ensure specific recognition for biosensing. PEGs [or PEO, poly- (ethylene oxide)] are powerful reagents to provide protein repellent surfaces on various kinds of substrates.2 A lot of effort has been put into modifying surfaces with PEGs in order to make them protein repellent.3,4 Several possibilities exist to coat surfaces with PEGs. Successful approaches used self-assembled monolayers,5 covalent grafting,6-8 andgraftpolymerization9 aswellasadsorption methods.10,11 Some of the procedures reported to yield a densePEGlayeronagivenglassorsiliconsurfacerequire three or four steps of reaction and sometimes a consider- able synthetic effort. Moreover, physisorption is simple but has potential probability of long-term instability due to desorption. Thus, there is still a strong demand for alternative methods. The commercial availability of low molecular weight PEG silanes offers an easy way to yield ultrathin PEG layers. In this article, we describe a one-step reaction to graft low molecular weight PEG silanes onto silicon surfaces. The small sizes of the PEG silanes used (Mw ) 460-590) offer the possibility to cover structures with nanometer scale topology while retaining their morphology. This requireslayerthicknessesintherangeofafewnanometers at most. Thorough structural characterization of the resultingPEGcoatingisthuscrucialtosuchapplications. A wide range of techniques has been used to investigate PEG layers and their behavior on surfaces probing global and local coverage.12-14 We used X-ray reflectivity (XR) to probe the global (millimeter scale) average coating thick- ness combined with X-ray photoelectron spectroscopy (XPS) for comparison. Atomic force microscopy (AFM) provided information on the local (nanometer scale) surfaceroughness.Insummary,ourdepositionprocedure yieldedextremelysmoothPEGmonolayersofthicknesses between 10 and 17 ��. On the basis of the experimental data, a model for the monolayer structure is being suggested. Materials and Methods Grafting of PEG Silanes to Silicon. Silicon wafers (100, single side polished) were purchased from Topsil (Frederiksund, Denmark).2-[Methoxypoly(ethyleneoxy)propyl]trimethoxysilane (Mw ) 460-590, purity 90% actual chain length distribution asmeasuredwithGC/MS,silaneswith4-8PEGunitsconstitute 95% of the material) was a product of Gelest (Tullytown, PA). All other chemicals were products of Aldrich (Milwaukee, WI) and Fluka(Buchs,Switzerland)andusedwithoutfurtherpurification. Silicon wafers were cleaned by sonication in ethanol/water (1:1, v/v) for 5 min prior to use. Ultrapure water was supplied by a MilliQ-System (Millipore, Boston, MA). Oxidation of the silicon wafers was carried out by treatment with a mixture of hydrogen peroxide (30%, w/v) and sulfuric acid (96%) (20:80, v/v) for 10 min at 120 ��C. This mixture should be used with extreme caution due to its high oxidizing power and risk of explosion.15 The samples were washed 3 times in water, sonicated in water for 10 min, blown dry with compressed air, and immediately immersed in the respective silane solution. Grafting was done by a solution of the PEG silane in toluene (with 0.8 mL of HClconc/ L) for 18 h16 at room temperature. Afterward the wafers were washed once in toluene, twice in ethanol, and twice in water and * Corresponding author. Ph: ++45 4677 4795. Fax: ++45 4677 4790. E-mail: alexander.papra@risoe.dk. (1) Thom, V. H. Altankov, G. Groth, Th. Jankova, K. Jonsson, G. Ulbricht, M. Langmuir 2000, 16, 2756. (2) Harris, J. M. In Poly(ethylenglycol) chemistry. Biotechnical and biomedical applications Plenum Press: New York, 1992. (3) Holmberg, K. Tiberg, F. Malmsten, M. Brink, C. Colloids Surf., A 1997, 123, 297. (4) Zhang, M. Desai, T. Ferrari, M. Biomaterials 1998, 19, 953. (5) Prime, K. L. Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714. (6) Malmsten, M. Emoto, K. Van Alstine, J. M. J. Colloid.Interface Sci. 1998, 202, 507. (7) Sofia, S. J. Premnath, V. Merrill, E. W. Macromolecules 1998, 31, 5059. (8) Yang, Z. Galloway, J. A. Yu, H. Langmuir 1999, 15, 8405. (9) Ulbricht, M. Matuschewski., H. Oechel., A. Hicke, H.-G. J. Membr. Sci. 1996, 115, 31. (10) Holmberg, K. Bergstrom, K. Brink, C. Osterberg, E. Tiberg, F. Harris, J. M. J. Adhes. Sci. Technol. 1993, 7, 503. (11) Malmsten, M. Lassen, B. Holmberg, K. Thomas, V. Quash, G. J. Colloid Interface Sci. 1996, 177, 70. (12) Kingshott, P. Griesser, H. J. Curr. Opin. Solid State Mater. Sci. 1999, 4, 403. (13) Emoto, K. Van Alstine, J. M. Harris, J. M. Langmuir 1998, 14, 2722. (14) Sanderson, L. A. W. Emoto, K. Van Alstine, J. M. Weimer, J. J. J. Colloid Interface Sci. 1998, 207, 180. (15) Dobbs, D. A. Bergman, R. G. Theopold, K. H. Chem. Eng. News 1990, 68 (17), 2. 1457 Langmuir 2001, 17, 1457-1460 10.1021/la000609d CCC: $20.00 �� 2001 American Chemical Society Published on Web 02/10/2001

thensonicatedinwaterfor2mintoremovenongraftedmaterial. The wafers were blown dry with compressed air and stored dry under ambient conditions. X-ray Photoelectron Spectroscopy. A SPECS Sage 100 X-ray photoelectron spectrometer (Berlin, Germany) with a Mg source was used to measure the chemical composition of the surface (topmost ca. 50 ��). Survey scans were performed to scan the sample for relevant elements (pass energy ) 100 eV, step size ) 0.5 eV). The 1s elemental peak from carbon (285 eV), 2s fromoxygen(534eV),and2pfromsilicon(103eV)weremeasured with higher resolution (step size ) 0.2 eV, pass energy 22.5 eV). From the relative peak intensities the thickness of the film could be calculated. All of the samples were measured with a takeoff angle of 90��. X-ray Reflectivity. The measurements were performed on a Rigaku (Tokyo, Japan) rotating anode with a copper target operating at 50 kV and 300 mA using the X-ray reflectometry instrument at Ris�� National Laboratory. The beam was mono- chromated by a perfect Ge(111) crystal, selecting the Cu KR1 wavelength �� ) 1.540 51 ��.17 The instrumental resolution18 was ���q ��� 5 �� 10-3 ��-1. AtomicForceMicroscopy. AFMimageswereacquiredwith aNanoscopeIIIDimension3000(DigitalInstrumentsInc.,Santa Barbara,CA).SuperSharpSilicontappingmodecantileverswith a resonance frequency of ca. 330 kHz were used (NanoSensors, Wetzlar-Blankenfeld, Germany). The tips below the cantilevers have a nominal radius of curvature of ca. 2 nm. Scan rates of 0.5 Hz were used. Contact Angle Measurements. Water contact angles have been measured on a stage from Edmund Scientific Corp. (Barrington, NJ). A Pulnix TM 7CN CCD camera (Sunnyvale, CA) was used to record the drop shapes. Advancing and receding angles were assessed by increasing and decreasing the drop size and analyzing the shape with VCA 2500 software from AST Products (Los Angeles, CA). The tabulated results are averages of three measurements on different parts of each sample. Results and Discussion Grafting of PEG onto Silicon. The scheme for the grafting procedure is displayed in Figure 1. The grafting is a one-step reaction without the need for anhydrous solvents, inert conditions, heating, etc. The sample is immersed in the respective solution immediately after oxidation as described above. Toluene is used because of its water-extracting capability.19 X-ray Reflectivity. Surface-grafted PEG has been studied previously by neutron reflectivity.20 This is an idealprobeformeasuringswollen/hydratedPEGcoatings. The scattering length density contrast between the dry film and the substrate is much lower for neutrons than for X-rays. Thus, we have used XR to measure the dry thicknessandcomparedtheresultstoXPS.Thescattering length density profile of each of the samples measured by XR was modeled as a single layer PEG on top of a semi- infinite medium (silicon). Corresponding reflectivity curves resulted from application of the dynamic Parratt formal- ism.21 The gradients at the substrate/film and the film/air interfacesweredescribedbyerrorfunctions,usingawidth equal to the interfacial roughnesses. We interpreted the thickness of the layer as the separation of the midpoints of the error functions describing the interfaces. Fittings of the models to the experimental data were done with the aid of the Parratt program from the Hahn-Meitner Institut,Berlin,Germany.22 Thereflectivitycurvesaswell asthefittedcurvesforthedifferentconcentrationsapplied are displayed in Figure 2. The table in the insert presents all fitted model parameters. Figure3showsthethicknessofthePEGlayerdepending on the grafting solution concentration. The thickness increases with concentration, passes a maximum, and finally decreases slightly. The initial increase is likely duetoahighergraftingdensity.Sincetheaveragedensity of the layer does not increase, the molecules tend to be more extended perpendicular to the surface. The maxi- mum grafting density is reached around 3 mM, still with a dry layer thickness of 17 ��. This is smaller than the size of one molecule (one extended silane molecule of seven PEG units is 31 �� in length, as modeled with ChemDraw3D, CambridgeSoft Corp., Cambridge, MA). Above 3 mM a slight decrease of the layer thickness is being observed. We currently have no consistent explana- tion for this behavior. The thicknesses between 10 and 17 �� indicate a very thin and presumably monomolecular PEG layer. X-ray Photoelectron Spectroscopy. We have used XR as an absolute measure for the film thickness and comparedittothethicknessderivedfromtheXPSresults. (16) The PEG layer is actually formed within the first 30 min. The samples investigated here were exposed to the respective reagent solution for 18 h, however. (17) Warren, B. E. X-ray Diffraction Dover Publications: New York, 1990. (18) Bouwman, W. G. Pedersen, J. S. J. Appl. Crystallogr. 1996, 29, 152. (19) McGovern, M. E. Kallury, K. M. R. Thompson, M. Langmuir 1994, 10, 3607. (20) Irvine, D. J. Mayes, A. M. Satija, S. K. Barker, J. G. Sofia- Allgor, S. J. Griffith, L. G. J. Biomed. Mater. Res. 1998, 40, 298. (21) Parratt, L. G. Phys. Rev. 1954, 95, 359. (22) Parratt32 written by Christian Braun, HMI, Berlin, Germany. Theprogramdoesnotcalculatethestandarddeviationoftheparameters. The uncertainties are expected to be in the range of 5-10%. Figure 1. Scheme for grafting a hydrophilic PEG layer onto silicon. Figure 2. X-ray reflectivity curves for PEG-modified silicon substrates using decreasing reagent concentrations [from top to bottom: 100, 30, 10, 3, 1, 0.1, 0.01 mM, control (bare oxidized silicon wafer)]. The inserts show the corresponding scattering density profile of the 3 mM sample and the model parameters. 1458 Langmuir, Vol. 17, No. 5, 2001 Papra et al.