Self-Assembled Monolayers of Alkanoic Acids on the Native Oxide

Stainless steel 316L is a widely used biomaterial substrate whose biocompatibility could be improved by surface modification. As a first step in this ...
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Langmuir 2007, 23, 2284-2288

Self-Assembled Monolayers of Alkanoic Acids on the Native Oxide Surface of SS316L by Solution Deposition Aparna Raman and Ellen S. Gawalt* Department of Chemistry and Biochemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282 ReceiVed October 20, 2006. In Final Form: January 7, 2007 Stainless steel 316L is a widely used biomaterial substrate whose biocompatibility could be improved by surface modification. As a first step in this process, self-assembled monolayers of octanoic acid, octadecylcarboxylic acid, 16-hydroxyhexadecanoic acid, 12-aminododecanoic acid, and 1,12-dodecane dicarboxylic acid have been formed on the native oxide surface of stainless steel 316L by a simple, one-step solution deposition method. The ordering, close-packing, and coverage of the monolayers formed were characterized by diffuse reflectance infrared spectroscopy, contact angle measurements, and atomic force microscopy. The same procedure was applicable for all long alkyl chain carboxylic acids. This process formed chemically and mechanically stable monolayers. These carboxylic acids formed a bidentate bond with the stainless steel substrate. Robust chemical attachment of the acids to stainless steel through a simple process provides a stepping stone to improving the biocompatibility of stainless steel 316L.

Introduction Stainless steel has been extensively used in the manufacture of vascular stents, guide wires, and other orthopedic implants due to its oxidation and corrosion resistance, relative ease of fabrication, and good mechanical properties.1 However, there is still a need to develop an implant interface which does not interact with the cells and tissues surrounding the metal, thus improving the biocompatibility and effectiveness of the implant. The most common method of altering the interfacial region has been by ultrathin organic layer formation by self-assembly2-4 or Langmuir-Blodgett films.5,6 Self-assembled monolayers have advantages over Langmuir-Blodgett films, including their ease of manufacture without external aid and strong chemical bonding to the substrate.5 Although there has been a large body of work performed on model substrates such as gold5,7-22 and silicon,23,24 these substrates cannot be employed in biomedical applications due to their poor mechanical properties, and standard thiol chemistry has not been * [email protected]. (1) Hanawa, T. Metallic biomaterials. In Recent research and deVelopments in biomaterials; Ikada, Y., Ed.; Research Signpost: Kerala, India, 2002. (2) Allara, D. L.; Nuzzo, R. G Spontaneously Organized Molecular Assemblies. 1. Formation, Dynamics, and Physical Properties of n-Alkanoic Acids Adsorbed from Solution on an Oxidized Aluminum Surface. Langmuir 1985, 1 (1), 45-52. (3) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Self-Assembled Monolayer Growth of Octadecylphosphonic Acid on Mica. Langmuir 1996, 12 (15), 36263629. (4) Chidsey, C. E. D.; Loiacono, D. N. Chemical Functionality in Self Assembled Monolayers: Structural and Electrochemical Properties. Langmuir 1990, 6, 682691. (5) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmiur Blodgett to Self-Assembly; XXX, Ed.; Academic Press: San Diego, CA, 1991. (6) Zhao, J.; Wu, Z. Y.; Zhang, J.; Zhu, T.; Ulman, A.; Liu, Z. F. Fourier Transform Infrared Spectroscopy Evidence of a Two-Dimensional HydrogenBonded Structure in Langmuir-Blodgett Films of a Novel Azobenzene Compound. Langmuir 1997, 13 (8), 2359-2362. (7) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Orthogonal Self-Assembled Monolayers: Alkanethiols on Gold and Alkane Carboxylic Acids on Alumina. Science 1989, 245, 845. (8) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. Formation of Monolayer Films by the Spontaneous Assembly of Organic Thiols from Solution onto Gold. J. Am. Chem. Soc. 1989, 111, 321-335. (9) Barbosa, J. N.; Barbosa, M. A.; Aguas, A. P. Inflammatory Responses and Cell Adhesion to Self-Assembled Monolayers of Alkanethiolates on Gold. Biomaterials 2004, 25 (13), 2557-2563. (10) Mrksich, M. A Surface Chemistry Approach to Studying Cell Adhesion. Chem. Soc. ReV. 2000, 29 (4), 267-273. (11) Park, J. S.; Smith, A. C.; Lee, T. R. Loosely Packed Self-Assembled Monolayers on Gold Generated From 2-Alkyl-2-Methylpropane-1,3-Dithiols. Langmuir 2004, 20 (14), 5829-5836.

successfully employed on stainless steel 316L (SS316L) or other oxides.25 It is noteworthy that, although SS316L is widely used as a biomaterial, very few papers on SAMs on stainless steel have been reported in the literature,26-32 and in those cases, the surface has been pretreated to enhance or remove the oxide layer. (12) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. Spontaneously Organized Molecular Assemblies. 4. Structural Characterization of n-Alkyl Thiol Monolayers on Gold by Optical Ellipsometry, Infrared Spectroscopy, and Electrochemistry. J. Am. Chem. Soc. 1987, 109 (12), 3559-3568. (13) Barrena, E.; Ocal, C.; Salmeron, M. Evolution of the Structure and Mechanical Stability of Self-Assembled Alkanethiol Islands on Au(111) Due to Diffusion and Ripening. J. Chem. Phys. 1999, 111 (21), 9797-9802. (14) Brewer, S. H.; Allen, A. M.; Lappi, S. E.; Chasse, T. L.; Briggman, K. A.; Gorman, C. B.; Franzen, S. Infrared Detection of a Phenylboronic Acid Terminated Alkane Thiol Monolayer on Gold Surfaces. Langmuir 2004, 20 (13), 5512-5520. (15) Geissler, M.; McLellan, J. M.; Chen, J. Y.; Xia, Y. N. Side-by-Side Patterning of Multiple Alkanethiolate Monolayers on Gold by Edge-Spreading Lithography. Angew. Chem., Int. Ed. 2005, 44 (23), 3596-3600. (16) Godin, M.; Williams, P. J.; Tabard-Cossa, V.; Laroche, O.; Beaulieu, L. Y.; Lennox, R. B.; Grutter, P. Surface Stress, Kinetics, and Structure of Alkanethiol Self-Assembled Monolayers. Langmuir 2004, 20 (17), 7090-7096. (17) Hu, W. S.; Tao, Y. T.; Hsu, Y. J.; Wei, D. H.; Wu, Y. S. Molecular Orientation of Evaporated Pentancene Films on Gold: Alignment Effect of SelfAssembled Monolayer. Langmuir 2000, 21, 2260-2266. (18) Hutt, D. A.; Leggett, G. J. Functionalization of Hydroxyl and Carboxylic Acid Terminated Self-Assembled Monolayers. Langmuir 1997, 13 (10), 27402748. (19) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X. SelfAssembled Monolayers of Dendron Thiols for Electrodeposition of Gold Nanostructures: Toward Fabrication of Superhydrophobic/superhydrophilic Surfaces and pH-Responsive Surfaces. Langmuir 2005, 21 (5), 1986-1990. (20) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. Structure and Wetting Properties of Omega-Alkoxy-N-Alkanethiolate Monolayers on Gold and Silver. J. Phys. Chem. 1995, 99 (19), 7663-7676. (21) Park, J. S.; Vo, A. N.; Barriet, D.; Shon, Y. S.; Lee, T. R. Systematic Control of the Packing Density of Self-Assembled Monolayers Using Bidentate and Tridentate Chelating Alkanethiols. Langmuir 2005, 21 (7), 2902-2911. (22) Li, L. Y.; Chen, S. F.; Zheng, J.; Ratner, B. D.; Jiang, S. Y. Protein Adsorption on Oligo(ethylene glycol)-Terminated Alkanethiolate Self-Assembled Monolayers: The Molecular Basis for Nonfouling Behavior. J. Phys. Chem. B 2005, 109 (7), 2934-2941. (23) Lee, M. H.; Brass, D. A.; Morris, R.; Composto, R. J.; Ducheyne, P. The Effect of Non-specific Interactions On Cellular Adhesion Using Model Surfaces. Biomaterials 2005, 26 (14), 1721-1730. (24) Altankov, G.; Groth, T. Fibronectin Matrix Formation and the Biocompatibility of Materials. J. Mater. Sci.: Mater. Med. 1996, 7 (7), 425-429. (25) Van Alsten, J. G. Self-Assembled Monolayers on Engineering Metals: Structure, Derivatization, and Utility. Langmuir 1999, 15, 7605-7614. (26) Mahapatro, A.; Johnson, D. M.; Patel, D. N.; Feldman, M. D.; Ayon, A. A.; Agrawal, C. M. Surface Modification of Functional Self-Assembled Monolayers on 316L Stainless Steel via Lipase Catalysis. Langmuir 2006, 22 (3), 901-905. (27) Ruan, C. M.; Bayer, T.; Meth, S.; Sukenik, C. N. Creation and Characterization of n-Alkylthiol and n-Alkylamine Self-Assembled Monolayers on 316L Stainless Steel. Thin Solid Films 2002, 419 (1-2), 95-104.

10.1021/la063089g CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007

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Langmuir, Vol. 23, No. 5, 2007 2285 Table 1. IR Stretch Values and Contact Angle Values for Monolayers of Functionalized Alkanoic Acids on SS316L

organic acid deposited

νCH2 symm dep. (cm-1)

νCH2 asymm dep. (cm-1)

νCH2 symm rinsed (cm-1)

νCH2 asymm rinsed (cm-1)

contact angle (°)

std deviation (°)

control SS316L C18CH3 C16OH C12COOH C12NH2

2914 2917 2913 2918

2847 2848 2849 2848

2916 2916 2913 2918

2847 2849 2849 2849

53.6 104.1 42 48.6 38.4

3 0.5 5.7 2.7 7.5

For example, Shustak et al.29,30 formed SAMs on SS316L using an electrochemical method which enhanced the reactivity of the oxide surface. In an effort to control the SS316L surface, we have developed a new method of self-assembly of long-chain acids on SS316L by a simple solution-deposition route. Self-assembled monolayers of long-chain carboxylic acids with -CH3, -COOH, -OH, and -NH2 terminal groups were formed on the native oxide surface of medical-grade SS316L. The strong covalent attachment and uniformity of the film on the substrate were confirmed using diffuse reflectance infrared spectroscopy, atomic force microscopy, and contact angle analysis. Experimental Section Materials and Methods. SS316L foils (0.5 mm, 99.99% pure, wt %: Fe 66%, Cr 19%, Ni 10%, Mn 3%, and Mo 2%) were obtained from Goodfellow, Inc. THF, methanol, octadecylcarboxylic acid (98+%), 16-hydroxyhexadecanoic acid (98%), 12-aminododecanoic acid (95%), and 1,12-dodecanedicarboxylic acid (99%) were obtained from Aldrich Chemical Co. and were used without further purification. Substrate Preparation and Monolayer Formation. Stainless steel 316L (SS316L) substrates were cut into 1 cm × 1 cm coupons and polished using the Buehler Ecomet 4 mechanical polisher using 220, 400, 800, and 1200 grit silicon carbide paper followed by a 1 µm diamond suspension. The polished samples were cleaned by ultrasonication in methanol (15 min) followed by immersion in boiling methanol to remove traces or residues of organics and metallic dust. The cleaned substrates were stored in an oven at 120 °C. SAMs of long-chain carboxylic acids with varying functional groups at the tail (octadecylcarboxylic acid (C18CH3), 12-aminododecanecarboxylic acid (C12NH2), 16-hydroxyhexadecanoic acid (C16OH), and 1,12-dodecanedicarboxylic acid (C12COOH)) were prepared as follows: Clean substrates were placed in an ice bath for 1 h and then dipped in a warm (50 °C) 1 mM THF solution of the respective acid for 2 h and stored in an oven at 120 °C overnight before further analysis. The modified substrates were tested for chemical and mechanical stability by performing rinse and sonication tests using THF, ethanol, and water and by linear adhesion test with Scotch tape 237 with adhesion strength to steel of 37 oz/in, respectively. Analysis Techniques. SAMs were analyzed using a Nexus 470 FT-IR, and a diffuse reflectance infrared spectroscopic attachment (DRIFT); purchased from Thermo Electron Corporation. Static contact angles were measured using a VCA Optima goniometer to determine the wettability of the surface using the procedure described by Bain.8 A PicoSPM atomic force microscope (Molecular Imaging) (28) Raman, A.; Dubey, M.; Gouzman, I.; Gawalt, E. S. Formation of SelfAssembled Monolayers of Alkylphosphonic Acid on Native Oxide Surface of SS316L. Langmuir 2006, 22, 6469-6472. (29) Shustak, G.; Domb, A. J.; Mandler, D. Preparation and Characterization of n-Alkanoic Acid Self-Assembled Monolayers Adsorbed on 316L Stainless Steel. Langmuir 2004, 20 (18), 7499-7506. (30) Shustak, G.; Domb, A. J.; Mandler, D. n-Alkanoic Acid Monolayers on 316L Stainless Steel Promote the Adhesion of Electropolymerized Polypyrrole Films. Langmuir 2006, 22 (12), 5237-5240. (31) Sinapi, F.; Naji, A.; Delhalle, J.; Mekhalif, Z., Assessment by XPS and Electrochemical Techniques of Two Molecular Organosilane Films Prepared on Stainless-Steel Surfaces. Surf. Interface Anal. 2004, 36 (11), 1484-1490. (32) Kingshott, P.; Wei, J.; Bagge-Ravn, D.; Gadegaard, N.; Gram, L. Covalent Attachment of Poly(ethylene glycol) to Surfaces, Critical for Reducing Bacterial Adhesion. Langmuir 2003, 19 (17), 6912-6921.

operating in noncontact mode using a silicon cantilever with a resonance frequency of 160-170 kHz and a typical spring constant of 40 N/m was used to confirm the uniformity of the film on the surface.

Results and Discussion Self-assembled monolayers of long-chain carboxylic acids with different terminal groups were formed on stainless steel 316L (SS316L) substrates using the solution-deposition technique discussed earlier. In brief, clean SS316L substrates were analyzed by X-ray photoelectron spectroscopy for composition. Results were found to be in agreement with the nominal SS316L bulk elemental composition.33 XPS results on the control SS316L confirmed that the native oxide surface was not altered by our cleaning technique.28 The cleaned, cold substrates were dipped in a warm (50 °C) solution of the respective acid and placed in a 120 °C oven to form the monolayers. It was found that cooling the substrates was necessary for monolayer formation, because monolayers did not form on room temperature or heated substrates. Additionally, the monolayers were formed but easily removed without the overnight annealing step. Although dry THF was used for deposition because it had been observed that water competed with the alkanoic acid for binding sites in the electrochemical method,29 the water does not appear to play a major role in this method, since the monolayers formed under atmospheric conditions. The modified SS316L substrates were analyzed by diffuse reflectance infrared Fourier transform spectroscopy. On the basis of extensive study, it has been observed that an “ordered” film is characterized by alkyl chains in an all-trans configuration that are all tilted from the normal to the surface at the same angle. This results in νCH2 asymm e 2918 cm-1.3,7,12,34 All of the acids tested in this study formed a well-ordered monolayer as indicated by the νCH2 asymm values (Table 1). Sample IR spectra of octadecylcarboxylic acid SAMs on SS316L can be seen in Figure 1. The monolayers formed readily and were not removed after rinsing and sonication in THF. The value of νCH2 asymm was found to be less than 2918 cm-1 after deposition,

Figure 1. Sample DRIFT spectra of the C-H stretching region of octadecylcarboxylic acid monolayer on SS316L after deposition, rinsing, and tape tests. Monolayers and similar spectra can also be obtained by deposition, rinsing, and sonication in THF.

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Figure 2. A. DRIFT spectra of the C-O region of octadecylcarboxylic acid solid. B. DRIFT spectra of the C-O region of octadecylcarboxylic acid deposited on SS316L. C. Scheme for the surface bonding of octadecylcarboxylic acid on SS316L.

rinse, and tape tests28 or deposition, rinse, and sonication in THF, indicating that the monolayers formed were strongly bound to the surface. The monolayers were also stable to rinses in both water and ethanol. The νCH2 asymm values seen in Table 1 are consistent with those of ordered monolayers of carboxylic and phosphonic acids on metal oxides.25,28,29,34,38,41 The position changes of the C-O stretches in the IR spectra indicate that a chemical transformation of the head group has taken place upon monolayer formation. In previous reports, the interaction or bonding mode between the carboxylic acid head group and the surface has varied among surfaces, and the interaction has been reported as monodentate35-37 (unsymmetrical carboxyl configuration) or bidentate.29,38-41 In the spectra of solid octadecylcarboxylic acid (Figure 2A), there are peaks at 1690 cm-1 and 1465 cm-1 which correspond to the νCdO and νC-O IR stretches. After deposition (Figure 2B), there is evidence of carboxylate νCOO- asymm and νCOO- symm stretches at 1551 cm-1 and 1446 cm-1. The peak corresponding to νCdO centered near 1700 cm-1 is not present. An increasing separation between the symmetric and asymmetric carboxylate vibration mode implies that the bonding is an ester-like structure and not a monodentate ligand attachment.38 This indicates that the acid groups have become carboxylate groups and, thus, the monomers are bound to the surface in a bidentate fashion (Figure 2C). This bonding motif persists even after the rinse and adhesion tests described earlier. A bidentate bonding motif was also observed in the electrochemically induced dodecanoic acid SAMs adsorbed on SS316L.29,30 However, a peak corresponding to CdO was (33) Wang, X. Y.; Wu, Y. S.; Zhang, L.; Yu, Z. Y. Atomic Force Microscopy and X-Ray Photoelectron Spectroscopy Study on the Passive Film for Type 316L Stainless Steel. Corros. Sci. Section 2001, 57 (6), 540-546. (34) Gawalt, E. S.; Avaltroni, M. J.; Koch, N.; Schwartz, J. Self-Assembly and Bonding of Alkanephosphonic Acids on the Native Oxide Surface of Titanium. Langmuir 2001, 17 (19), 5736-5738. (35) Liu, Y. L.; Yu, Z. F.; Zhou, S. X.; Wu, L. M. Self-Assembled Monolayers on Magnesium Alloy Surfaces from Carboxylate Ions. Appl. Surf. Sci. 2006, 252 (10), 3818-3827. (36) Lin, S. Y.; Tsai, T. K.; Lin, C. M.; Chen, C. H.; Chan, Y. C.; Chen, H. W. Structures of Self-Assembled Monolayers of n-Alkanoic Acids on Gold Surfaces Modified by Underpotential Deposition of Silver and Copper: Odd-Even Effect. Langmuir 2002, 18 (14), 5473-5478. (37) Oberg, K.; Persson, P.; Shchukarev, A.; Eliasson, B. Comparison of Monolayer Films of Stearic Acid and Methyl Stearate on an Al2O3 Surface. Thin Solid Films 2001, 397 (1-2), 102-108. (38) Allara, D. L.; Nuzzo, R. G. Spontaneously Organized Molecular Assemblies. 2. Formation, Dynamics, and Physical Properties of n-Alkanoic Acids Adsorbed from Solution on an Oxidized Aluminum Surface. Langmuir 1985, 1 (1), 52-66. (39) Tao, Y. T.; Hietpas, G. D.; Allara, D. L. HCl Vapor-Induced Structural Rearrangements of n-Alkanoate Self-Assembled Monolayers on Ambient Silver, Copper, and Aluminum Surfaces. J. Am. Chem. Soc. 1996, 118 (28), 6724-6735. (40) Tao, Y. T.; Lin, W. L.; Hietpas, G. D.; Allara, D. L. Infrared Spectroscopic Study of Chemically Induced Dewetting in Liquid Crystalline Types of SelfAssembled Monolayers. J. Phys. Chem. B 1997, 101 (47), 9732-9740. (41) Tao, Y. T. Structural Comparison of Self-Assembled Monolayers of n-Alkanoic Acids on the Surfaces of Silver, Copper, and Aluminium. J. Am. Chem. Soc. 1993, 115, 4350-4358.

Figure 3. A. IR spectra of the C-O region of 1,12-dodecanedicarboxylic acid deposited on SS316L. B. IR spectra of 12aminododecanedicarboxylic acid deposited on SS316L.

observed, indicating that some head groups were entrapped in the interface as carboxylic acids,29,30 which could be detrimental to the long-term integrity of the surface. Contact angle data were collected to determine the relative wettability of the modified substrates, because surface wettability has been shown to be a useful marker for estimating the monolayer quality and may play an important role in facilitating cell adhesion. Densely packed and well-ordered SAMs predominantly expose methyl groups at the surface, decreasing the surface wettability. In contrast, loosely packed SAMs expose a substantial fraction of methylene groups in addition to methyl groups at the surface, increasing the wettability and decreasing the contact angle.11 On the other hand, if the tail terminus of the SAMs is hydrophilic, the resulting contact angle values will be relatively more hydrophilic than the control sample under consideration.

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Langmuir, Vol. 23, No. 5, 2007 2287

Figure 4. A. AFM topography image of control SS316L B. AFM topography image of SS316L modified with a monolayer of octadecylcarboylic acid. C. AFM 2-D topography image of a disordered film of octadecylcarboxylic acid on SS316L. D. AFM 3-D topography image of a disordered film of octadecylcarboxylic acid on SS316L.

Octadecylcarboxylic acid modified surfaces had an average contact angle value of 104° with water compared to a contact angle of 53.6° with the control stainless steel substrate. This is consistent with the values found in literature for hydrophobic methyl-terminated thiols on gold,5,9,42 silanes on silicon,5,43,44 and also stainless steel substrates modified by electrodeposition.29 SAMs with hydrophilic tail groups, C12COOH, C12NH2, and C16OH (Table 1), had lower contact angle values than the control SS316L substrate. This suggests that the hydrophilic-terminated organic acids are not forming a bridge on the surface and that the hydrophilic tail groups are available at the interface for further interactions with organics and biomolecules. The values observed are in reasonable agreement with literature.9 Further evidence of the interfacial presentation of the hydrophilic tail groups can be seen in regions of the infrared spectrum related to the respective tail groups (Figure 3). For example, the C-O region of bound 1,12-dodecandicarboxylic acid includes stretches at 1740, 1552, and 1440 cm-1. The peaks at 1552 and 1440 cm-1 are similar to those seen in the bound octadecylcarboxylic acid spectra (Figure 2B) indicating a bidentate bonding motif, but the peak at 1740 cm-1 is consistent with νCdO. This evidence, when paired with the contact angle values, is indicative (42) Ulman, A.; Eilers, J. E.; Tillman, N. Packing and Molecular Orientation of Alkanethiol Monolayers on Gold Surfaces. Langmuir 1989, 5 (5), 1147-52. (43) Faucheux, N.; Schweiss, R.; Lutzow, K.; Werner, C.; Groth, T. SelfAssembled Monolayers with Different Terminating Groups as Model Substrates for Cell Adhesion Studies. Biomaterials 2004, 25 (14), 2721-2730. (44) Schmohl, A.; Khan, A.; Hess, P. Functionalization of Oxidized Silicon Surfaces with Methyl Groups and Their Characterization. Superlattices Microstruct. 2004, 36 (1-3), 113-121.

of free carboxylic acid at the terminus. In the spectra for 12aminododecanoic acid νNH2 ) 3197 cm-1, indicating the presence of free amine at the terminus. Additionally, the functionalized monolayers were stable to the pH changes associated with organic cross-coupling conditions necessary for further functionalization of the surface. The coverage of the surface by the organic monolayer was determined to be complete and uniform by performing a topography scan of the substrate using AFM in noncontact mode using silicon tips. Figure 4 shows a 500 nm scan of bare polished SS316L substrate (A, control) and octadecylcarboxylic acid modified substrate (B). The roughness analysis on the 2D topography image was based on the calculation of standard deviation of all height values within the given imaged area (rms roughness). The rms roughness value of the control SS316L substrate before modification with octadecylcarboxylic acid was 11 Å, and after reaction with octadecylcarboxylic acid, it was 10 Å (Figure 4). Since the surface before and after modification has the same rms roughness value, this suggests that the organization of the molecules follows the contour of the polished surface. There was no evidence of micelle or island formation in the film, which indicates that a single monolayer of molecules covers the oxide surface. A disordered film with island formation can be seen in Figure 4C and D. This type of film results in an rms roughness much larger (46 Å) than that of the SS316L control.

Conclusion Ordered monolayers of functionally terminated alkanoic acids on stainless steel 316L were formed using a one-step solution-

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deposition method. The acid forms a bidentate bond with the substrate as evidenced by DRIFT spectroscopy. The water contact angle values and infrared spectra of the hydrophilic tail groups suggest that there is no bridging and that the groups are available for further molecular interaction with other organics and biomolecules.

Letters

Acknowledgment. We thank The Samuel and Emma Winters Foundation, Faculty Development Fund, and the Pennsylvania Department of Health CURE Program for generous funding of the project. LA063089G