Langmuir 2002, 18, 3537-3548
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Self-Assembled Monolayers of Dodecyl and Hydroxy-dodecyl Phosphates on Both Smooth and Rough Titanium and Titanium Oxide Surfaces S. Tosatti, R. Michel, M. Textor,* and N. D. Spencer Laboratory for Surface Science and Technology, Department of Materials, Swiss Federal Institute of Technology (ETHZ), CH-8092 Zu¨ rich, Switzerland Received September 21, 2001. In Final Form: February 1, 2002 Dodecyl phosphate and hydroxy-terminated dodecyl phosphate are shown to spontaneously assemble on smooth titanium oxide and titanium metal coated glass and silicon substrates, as well as on rough titanium metal implant surfaces. The surfaces were dipped in aqueous solutions of the corresponding ammonium salts for 48 h. The molecules are shown by X-ray photoelectron spectroscopy (XPS) to form densely packed, self-assembled monolayers (SAMs) on all surfaces investigated. The phosphate headgroups are believed to attach to the titanium (oxide) surface with the terminal end group (either methyl or hydroxy) pointing toward the ambient environment (air, vacuum, or water). Mixed SAMs are shown to be formed from mixed aqueous solutions of the two amphiphiles, with the hydroxy-terminated dodecyl phosphate adsorbing more favorably than the methyl-terminated molecule. The advancing water contact angles can be easily tailored via the composition of the self-assembly solution in the range of 110° (pure methyl) to 55° (pure hydroxy) on flat, smooth titanium surfaces. Surface roughness strongly modifies the wetting properties, with advancing contact angles in the range of 150-100° being observed, as well as the degree of hysteresis (difference between advancing and receding angles). Model calculations based on XPS intensities have been successfully used to quantify the adlayer composition and molecular surface densities across the whole range of mixed adlayer chemistry. The organophosphate monolayers on titanium are believed to have a significant potential for precise control of the surface chemistry and interfacial tension on both smooth and rough titanium surfaces in application areas such as medical implants and other devices where independent control of surface chemistry and topography is essential to performance.
1. Introduction Self-assembled monolayers (SAMs) have attracted a lot of attention, thanks to their ability to form chemically well-controlled, structurally ordered surfaces. Thiols on gold have been particularly well studied and used as model systems for a variety of applications including biomaterial and biosensor surfaces.1,2 Recently, octadecyl phosphate has been shown to form adlayers spontaneously from heptane/2-propanol solutions on tantalum oxide (Ta2O5) surfaces, resulting in highly hydrophobic surfaces with structural and chemical properties resembling those of long-chain alkanethiols on gold.3,4 In particular, the average tilt angle of 30° (relative to the surface normal) determined by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and the detection by atomic force microscopy (AFM) of (local) two-dimensional, hexagonal patterns of the terminal methyl group with a characteristic intermolecular spacing of 0.49 nm (measured parallel to the surface) are similar to the corresponding characteristic values reported for gold (111)/alkanethiol SAMs.1,5 Tantalum oxide was chosen as a substrate in view of its use in transparent, high-refractive-index waveguiding layers in optical sensor applications.6,7 The production of bio* To whom correspondence should be addressed. (1) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (2) Nuzzo, R.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) Brovelli, D.; Ha¨hner, D.; Ruiz, L.; Hofer, R.; Kraus, G.; Waldner, A.; Schlo¨sser, J.; Oroszlan, P.; Ehrat, M.; Spencer, N. D. Langmuir 1999, 15, 4324. (4) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Ha¨hner, G.; Spencer, N. D. Langmuir 2000, 16, 3257. (5) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (6) Duveneck, G. L.; Pawlak, M.; Neuscha¨fer, D.; Ba¨r, E.; Budak, W.; Pieles, U.; Ehrat, M. Sens. Actuators, B 1997, 38-39, 88.
affinity sensor surfaces with controlled chemical properties is of great relevance in the areas of DNA/RNA and protein sensing, genomics, and proteomics.8 The use of organic solvents to produce surface layers has clear disadvantages if the aim is to use such techniques on an industrial scale (due to both environmental emission and disposal issues). Moreover, for applications in areas such as medical devices and implants, the presence of even minor organic solvent residues in the adlayers cannot be tolerated, in view of potential cell-toxicity effects and other biological risks. On the other hand, self-assembled monolayers, in particular of molecules with functional terminal groups, are of great interest for the modification and functionalization of biomaterials and medical devices. Therefore, a technique based on the deposition of SAMs from aqueous alkyl phosphate solutions has been developed and successfully applied to a variety of metal oxide substrates.9 This paper covers the deposition and characterization of monolayers of dodecyl phosphate, 12-hydroxy-dodecyl phosphate, and a mixture of the two from aqueous solutions of their ammonium salts onto three different substrates: titanium oxide and titanium metal films, deposited by physical vapor deposition onto glass and silicon substrates (serving as smooth, flat model surfaces), and a special titanium dental implant surface with a rough, highly corrugated surface. The aim of the present investigation is twofold: first, to describe the physicochemical and structural properties of the single-component and mixed SAMs on titanium (oxide), and second, to test the (7) Budach, W.; Abel, A. P.; Bruno, A. E.; Neuschafer, D. Anal. Chem. 1999, 71, 3347. (8) Duveneck, G. L.; Abel, A. P. Proc. SPIE (Prepr.) 1999, Vol. 3858. (9) Hofer, R.; Textor, M.; Spencer, N. D. Langmuir 2001, 17, 4014.
10.1021/la011459p CCC: $22.00 © 2002 American Chemical Society Published on Web 04/05/2002
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feasibility of using alkyl phosphate based SAMs to tailor the surface properties of titanium medical devices. 2. Materials and Methods 2.1. Substrates. Three different substrates were used. Thin films of titanium dioxide (TiO2) (20 nm thick) and of titanium (Ti(metal)) (100 nm thick) were deposited by physical vapor deposition (PVD, reactive magnetron sputtering, Paul Scherrer Institut, Villigen, Switzerland) onto silicon wafers and flat glass slides, respectively. Before the coating, the uncoated substrates were ultrasonically cleaned in toluene (Uvasol, Merck, Dietikon, Switzerland) for 15 min and dried under flowing nitrogen gas (5.0). Commercially pure titanium (CP Ti) disks, 15 mm in diameter and 1 mm in thickness, were grit-blasted with alumina beads (average particle size, 250 µm) under low-impact-energy, industrial particle-blasting conditions and subsequently etched in a hot solution of HCl/H2SO4 (Institut Straumann AG, Waldenburg, Switzerland). This surface, known commercially as “SLA”, is characterized by a duplex surface topography with roughness contributions in the range of typically 20-50 µm (originating from the blasting process) and 0.5-2 µm (from the chemical etching process). 2.2. Self-Assembled Monolayers. Ammonium salts of dodecyl (DDPO4) and 12-hydroxy dodecyl phosphate (OH-DDPO4) were precipitated from a 2-propanol (UVASOL, Merck) solution of the free acid molecules using ammonia (25% aq, p.a., Merck). Details are given in refs 9 and 10. Equimolar solutions (0.5 mM) of both amphiphiles were prepared by dissolving their corresponding ammonium salts in high-purity water (HPLC, Fluka, Buchs, Switzerland) at 50 °C and subsequently cooling to room temperature. The alkyl phosphate and hydroxyalkyl phosphate aqueous solutions were mixed in different ratios from 0:100 to 100:0 vol % in steps of either 10% or 25% with respect to the amount of OH-DDPO4. Cleaning and SAM formation for TiO2 and Ti(metal) samples were carried out according to details given in previous papers.9,10 The samples were immersed for 48 h in the amphiphile solution and then removed, each being rinsed with 10 mL of high-purity water (HPLC, Fluka) and finally blown dry with nitrogen (5.0 purity). 2.3. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded with a SAGE 100 system (Specs, Berlin, Germany) using nonmonochromatized Al KR radiation at 320 W (13 kV) and an electron-energy analyzer pass energy of 50 eV for lowresolution surveys and of 14 eV for high-resolution detailed scans. The analyzed area was 6 mm2, and the results therefore represent a laterally averaged chemical composition. To determine the quantitative surface composition from XPS data, the sensitivity factors of Scofield11 are used. All binding energies are referenced relative to the main hydrocarbon peak (from residual contamination in the case of the clean surfaces and the SAM hydrocarbon chain in the case of the SAM-coated surfaces), set at a binding energy of 285.0 eV. 2.4. Atomic Force Microscopy (AFM). AFM measurements were performed with a commercial scanning probe microscope (Nanoscope III, Digital Instruments, Santa Barbara, CA). Measurements of surface topography and lateral force were made simultaneously by operating the instrument in contact mode while scanning the cantilever laterally. All measurements were performed in ambient air on freshly plasma cleaned samples. 2.5. Contact-Angle Measurements. Surface wettability was investigated by measuring advancing and receding contact angles in a sessile water drop experiment (Contact Angle Measuring System, G2/G40 2.05-D, Kru¨ss GmbH, Hamburg, Germany). The measurements were performed in an automated way by increasing and decreasing the water drop size stepwise. Averaged data and error bars refer to six advancing-angle and three recedingangle measurements at different locations on each sample. 2.6. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Measurements. Secondary ion mass spectra were recorded on a PHI 7200 time-of-flight secondary ion mass spectrometer (Physical Electronics, Eden Prairie, MN) in the (10) Hofer, R. Surface Modification for Optical Biosensor Application. Ph.D. Thesis No. 13873, ETH Zu¨rich, Zu¨rich, Switzerland, 2000. (11) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129.
Tosatti et al. Table 1. Roughness Values Ra and Rq for the Different Substrates substrate
PVD coating
glass TiO2 Ti(metal) Si wafer TiO2 Ti(metal) Ti(metal)
blasted, acid-etched (SLA)
roughness values Rq Ra Rq Ra Rq Ra Rq Ra Rq Ra Rq Ra Rq Ra
5.65 ( 0.28 nma 3.78 ( 0.19 nma 3.63 ( 0.18 nma 2.05 ( 0.10 nma 3.85 ( 0.19 nma 2.48 ( 0.12 nma 0.11 ( 0.01 nma 0.07 ( 0.01 nma 0.60 ( 0.03 nma 0.44 ( 0.02 nma 1.67 ( 0.08 nma 1.32 ( 0.07 nma 2.01 ( 0.22 µmb 2.56 ( 0.28 µmb
a Measured with AFM over a 10 × 10 µm area and calculated from lines of different scan lengths. b Measured with the noncontact laser profilometer technique (ref 12) in a scan range of 10-50 µm.
Figure 1. AFM images of TiO2 (left) and Ti(metal) (right) on a silicon wafer. Scan area: 2 × 2 µm. mass range 0-1000 m/e. The total ion dose of the 8 kV Cs+ primary ion beam (100 µm diameter) was below the static limit (
