Peptides on GaAs Surfaces: Comparison between Features

GaAs surfaces were patterned by microcontact printing (μCP) and dip-pen nanolithography (DPN) using a peptide sequence composed of 15 amino acids...
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Langmuir 2006, 22, 8670-8674

Peptides on GaAs Surfaces: Comparison between Features Generated by Microcontact Printing and Dip-Pen Nanolithography Youngnam Cho† and Albena Ivanisevic*,†,‡ Department of Chemistry and Weldon School of Biomedical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907 ReceiVed February 20, 2006. In Final Form: July 21, 2006 Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) were employed to understand the size, composition, and conformation of lithographic patterns composed of peptide molecules. GaAs surfaces were patterned by microcontact printing (µCP) and dip-pen nanolithography (DPN) using a peptide sequence composed of 15 amino acids. The detailed surface evaluation showed that the patterns have similar chemical compositions but differ in the bonding among the molecules anchored on the GaAs substrate. Both types of patterns were crystalline-like in nature. The features created by DPN exhibited interchain hydrogen bonding, while the ones generated by µCP displayed non-hydrogen bonding. The differences in the lithographic structures can be utilized in future biorecognition experiments that take advantage of the electronic properties of the GaAs substrate and the tunable behavior of the covalently anchored biomolecules on the surface.

Introduction Unconventional lithographic approaches such as microcontact printing (µCP)1,2 and scanning probe lithography3 have been studied and optimized for a variety of different applications. In particular, their use in the construction of patterns composed of various biomolecules shows promise for the fabrication of novel sensors and tissue engineering platforms.4,5 Each of these lithographic techniques has been shown to be advantageous for biological applications because of its flexibility and compatibility with aqueous environments. For a number of practical applications one must evaluate the following parameters with respect to biomimetic patterns generated by different lithographic techniques: pattern size, composition, and conformation of the molecules anchored on the surface. Knowledge of these properties is essential when one tries to rationalize the behavior of lithographic patterns in biomolecular and cellular recognition experiments.6-8 A number of molecular “inks” used in µCP and scanning probe lithography such as dip-pen nanolithography (DPN)9,10 can form self-assembled monolayers (SAMs) on different substrates. Understanding the quality and properties of the SAMs generated by different lithographic approaches is essential. Such an understanding satisfies the fundamental scientific curiosity of scientists and is also required for the * Corresponding author. E-mail: [email protected]. † Department of Chemistry. ‡ Weldon School of Biomedical Engineering.. (1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550-575. (2) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823-1848. (3) Kramer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 43674418. (4) Gleason, N. J.; Nodes, C. J.; Higham, E. M.; Guckert, N.; Aksay, I. A.; Schwarzbauer, J. E.; Carbeck, J. D. Langmuir 2003, 19, 513-518. (5) Hyun, J.; Ahn, S. J.; Lee, W. K.; Chilkoti, A.; Zauscher, S. Nano Lett. 2002, 2, 1203-1207. (6) Renault, J. P.; Bernard, A.; Bietsch, A.; Michel, B.; Bosshard, H. R.; Delamarche, E.; Kreiter, M.; Hecht, B.; Wild, U. P. J. Phys. Chem. B 2003, 107, 703-711. (7) Falconnet, D.; Pasqui, D.; Park, S.; Eckert, R.; Schift, H.; Gobrecht, J.; Barbucci, R.; Textor, M. Nano Lett. 2004, 4, 1909-1914. (8) Biasco, A.; Pisignano, D.; Krebs, B.; Pompa, P. P.; Persano, L.; Cingolani, R.; Rinaldi, R. Langmuir 2005, 21, 5154-5158. (9) Hong, S.; Zhu, J.; Mirkin, C. A. Science 1999, 286, 523-525. (10) Zhang, H.; Chung, S. W.; Mirkin, C. A. Nano Lett. 2003, 3, 43-45.

successful utilization of patterned surfaces in applications such as biosensors. Studies to date have investigated and compared the properties of SAMs generated on gold using alkylthiols and different lithographic techniques.11-13 Comprehensive comparative reports on other technologically important surfaces such as semiconductors have not been published. In this paper we compare SAMs generated by peptide molecules on GaAs surfaces using µCP and DPN. GaAs surfaces offer distinct advantages in the fabrication of different devices.14 Therefore, understanding how to functionalize them with biomolecules can lead to the fabrication of new types of label free detection platforms. This study contrasts the properties of SAMs generated by TAT peptides on GaAs. These peptides are derived from the protein TAT of the human immunodeficiency virus type 1 (HIV-1), which has been linked through a variety of studies to important functions in cell permeation and viral replication.15 TAT peptides have been shown to participate in recognition events with specific RNA sequences,16 and there is a need to develop alternative approaches to quantify such interactions.17 The patterns that are discussed and evaluated in this paper can become parts of new types of microfabricated sensors. We use atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) to understand differences in the composition and orientation of the peptides anchored on GaAs by two different lithographic approaches. The surface characterization we report suggests that the two types of lithographic features have similar chemical compositions but differ in the bonding among the molecules on the surface. (11) Cho, Y.; Ivanisevic, A. J. Phys. Chem. B 2005, 109, 6225-6232. (12) Salaita, K.; Amarnath, A.; Maspoch, D.; Higgins, T. B.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 11283-11287. (13) Hampton, J. R.; Dameron, A. A.; Weiss, P. S. J. Am. Chem. Soc. 2006, 128, 1648-1653. (14) Ye, S.; Li, G.; Noda, H.; Uosaki, K.; Osawa, M. Surf. Sci. 2003, 529, 163-170. (15) Long, K. S.; Crothers, D. M. Biochemistry 1995, 34, 8885-8895. (16) Lewin, M.; Carlesso, N.; Tung, C. H.; Tang, X. W.; Cory, D.; Scadden, D. T.; Weissleder, R. Nat. Biotechnol. 2000, 18, 410-414. (17) Tassew, N.; Thompson, M. Anal. Chem. 2002, 74, 5313-5320.

10.1021/la060492e CCC: $33.50 © 2006 American Chemical Society Published on Web 09/07/2006

Peptides on GaAs Surfaces

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Experimental Section

Results and Discussion

Materials and Surface Preparation. Silicon-doped n-type GaAs wafers with a dopant concentration of (0.5-2) × 1018 cm-3 were purchased from University Wafer, Inc. (Boston, MA). Pieces of the wafer (0.5 × 0.5 cm2) were cleaned with acetone, pure ethanol, and deionized water for 30 s at room temperature using a sonication bath. All samples were dried with N2 gas for 30 s. Concentrated HCl (Aldrich) was used to remove the native oxide layer from the surfaces by immersing each sample in acid solution for 1 min at room temperature. The TAT peptide sequence, CGISYGRKKRRQRRR, containing an arginine-rich RNA-binding domain, was synthesized by Bio-Synthesis, Lewisville, TX, where it was purified by HPLC and analyzed by MALDI-TOF. The peptide was dissolved in PBS buffer (pH ∼6.5) in order to prepare 1, 20, or 24 mM solutions. Lithography by µCP. The PDMS stamp was fabricated and inked by literature protocols.1 The patterned stamp was dipped into the peptide ink solution with a chosen concentration. The stamp was brought into contact with the surface for ∼5 s. After the stamping each patterned surface was washed with water multiple times and dried with a stream of N2 gas. Lithography by DPN and Surface Evaluation by AFM. All of the studies were performed using a Multi-Mode Nanoscope IIIa from Digital Instruments. The AFM was enclosed in a home-built chamber which was utilized to maintain specific temperature and humidity conditions. The AFM probes used in the lithography experiments were purchased from Veeco, Model No. MSCT-AUHW, and had a spring constant of 0.05 N/m. Prior to patterning the rms value for each substrate was evaluated. The rms is the root-meansquare average of the height deviation taken from the mean data

Understanding the nature of submicron biomimetic patterns has been identified as an essential step toward their adaptation in the fabrication of functional molecular structures.19 One can begin to map the properties of lithographic structures composed of biomolecules when appropriate characterization tools are employed. In this study we utilize three surface science tools in order to compare peptide lithographic patterns generated by µCP and DPN. AFM was used to measure the height of the generated features. We assessed relative differences in the frictional properties of the lithographic components based on LFM images. For this purpose we patterned a GaAs surface with the two types of lithographic structures next to one another. The chemical compositions of the surfaces modified by each method were studied by XPS. Another surface technique, FT-IRRAS, allowed us to determine the orientation and conformation of the peptides anchored on the surface by µCP and DPN. We initiated our investigation by patterning the TAT peptide on separate GaAs surfaces using µCP and DPN, Figure 1a-d. The rms surface roughness of the clean GaAs was less than 1 nm. One can distinguish the line features in the height images. In the case of the DPN structures it is difficult to measure the values that correspond to their heights and widths using Figure 1a. The heights of the structures generated by µCP were 2.49 ( 1.19 nm. The height measurements were obtained after the structures were washed with buffer. A line profile is included in the Supporting Information. The LFM images, Figure 1b,d, clearly display the placement and width of both types of patterns. Comparison between the LFM images in Figure 1b,d suggests that the peptide patterns generated by µCP and DPN might differ in their frictional properties with respect to the unpatterned GaAs surface. The contrast in the LFM images that is often used to discriminate between features of higher or lower frictional force can be influenced by a number of parameters such as applied force, tip shape, and surface variations.20 A way to eliminate such variable influences is to place both lithographic patterns next to one another and scan the surface with one tip, Figure 1e,f. The data in Figure 1f indicate that both types of patterns have similar brightnesses. The brighter areas in the LFM images correspond to features with higher frictional force and can be attributed to the presentation of hydrophilic groups away from the surface after the peptide was delivered to the substrate by µCP and DPN.13 The hydrophilic nature of the features on the surface was also verified by imaging the lithographic patterns with a hydrophobic tip (see Supporting Information). The hydrophobic tip reduces the capillary force and allowed us to obtain images that verified the hydrophilic nature of the patterns and displayed much sharper contrast. We also examined the concentration dependence on height and friction measurements; see Figure 2. The images in Figure 2 were obtained with two different concentrations of inks, and the samples were imaged immediately without washing after the lithographic procedure was finished. The difference in the heights of the structures generated by DPN with the two concentrations was ∼1 nm, whereas the difference observed when µCP was utilized was ∼12 nm. The variation is not surprising, because the µCP patterns are likely to have a lot of physisorbed molecules since the samples were not washed. It is important to note that the water meniscus is known to have a large influence over the transport of ink from the tip to the surface.21 The kinetics of ink transport from the tip

plane and is defined as Rq ) x(∑Zi2)/n. The tips were coated with the peptide by placing them in a solution with a chosen concentration for ∼60 s. Each tip was rinsed with PBS buffer, and the backside was cleaned by placing it on a filter paper. All DPN experiments with the TAT peptide ink were done at room temperature (22 °C) and 75% relative humidity. After the tips were mounted into the AFM holder, the following speeds were used in the DPN procedure: 30.5 µm/s for 5 µm long lines and 61.0 µm/s for 10 µm long lines. The tip was scanned over a line 5-10 times. All patterning results, including the µCP ones, were evaluated with clean tips. For this purpose lateral force microscopy (LFM) images were collected at a 90° scan direction using scan speeds of 2-4 Hz. Surface Characterization by XPS and FT-IRRAS. All surfaces used for the XPS and FT-IRRAS analysis contained a large, micronsize marker that was used to locate a specific patterned area. All features generated by either DPN or µCP covered a total area of ∼100 × 100 µm2 to accommodate the spatial resolution of both techniques. A detailed drawing of this procedure has been previously published.11 All XPS data were collected using a Kratos Axis ULTRA X-ray Photoelectron Spectrometer. The Kratos system was equipped with a monochromatic Al KR X-ray source, small area extraction optics, a spherical capacitor electron energy analyzer, and a dual channel plate position sensitive detector. Survey spectra with pass energy of 160 eV were acquired between 0 and 1100 eV. The survey scans are included in the Supporting Information. The high-resolution XPS scans were collected with a pass energy of 40 eV. Highresolution data were collected for the following regions: Ga 3d, As 3d, S 2p, C 1s, and N 1s. All the data were deconvoluted using a commercial XPS analysis software package and have been described before.11,18 FT-IRRAS spectroscopy in a single reflection mode was done using an instrument purchased from Thermo Nicolet, Madison, WI (Model No. 670 FTIR spectrometer equipped with a narrowband mercury cadmium telluride (MCT) detector). Spectra were collected from 1024 scans at a resolution of 4 cm-1 on bare GaAs and patterned surfaces. The spectrum of the bare GaAs was always subtracted from the spectrum of samples containing TAT peptides. All experiments were done with p-polarization incident at 80° from the surface normal. The results were analyzed by the OMNIC software provided by Thermo Nicolet. (18) Cho, Y.; Ivanisevic, A. J. Phys. Chem. B 2004, 108, 15223-15228.

(19) Takeda, S.; Nakamura, C.; Miyamoto, C.; Nakamura, N.; Kageshima, M.; Tokumoto, H.; Miyake, J. Nano Lett. 2003, 3, 1471-1474. (20) Raghavan, D.; Gu, X.; Nguyen, T.; VanLandingham, M.; Karim, A. Macromolecules 2000, 33, 2573-2583. (21) Jang, J. Y.; Schatz, G. C.; Ratner, M. A. Phys. ReV. Lett. 2004, 92, 85504.

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Figure 1. AFM height (a) and LFM (b) images of GaAs surface patterned with TAT peptide using DPN. The images were collected with a scan size of 15 × 15 µm2 and scan speed of 3.05 Hz. The height scale is 10 nm in (a), and the LFM data scale is 30 mV in (b). AFM height (c) and LFM (d) images of GaAs surface patterned with TAT peptides using µCP. The images were collected with a scan size of 15 × 15 µm2 and scan speed of 3.05 Hz. The height scale is 30 nm in (c), and the LFM data scale is 60 mV in (d). AFM height (e) and LFM (f) images of GaAs surface patterned with TAT peptides using µCP (horizontal lines) and DPN (vertical lines). The images were collected with a scan size of 15 × 15 µm2 and scan speed of 3.05 Hz. The height scale is 10 nm in (e), and the LFM data scale is 30 mV in (f).

to the surface play a key role in the final composition of the patterns on the surface.12 Therefore, to understand the exact composition and conformation of the molecules on the GaAs surface, we used additional characterization techniques. XPS is a technique widely used to understand the chemical composition of surfaces.22 We utilized a micron-size alignment marker to take spectra from a specific region on the surface where the lithographic patterns were located. High-resolution spectra were recorded for the main core-level peaks of Ga 3d, As 3d, S 2p, C 1s, and N 1s. Three types of surfaces were evaluated: freshly etched GaAs, GaAs containing DPN structures, and GaAs containing µCP patterns; see Figure 3. The highresolution C 1s spectrum of the freshly etched surface displayed a peak centered at 284.0 eV, which was indicative of the presence of hydrocarbon chains on the surface due to contamination. The surfaces modified by DPN and µCP displayed three distinct peaks, which we classified as hydrocarbon, -C-N-, and amide-C species (Figure 3A). The high-resolution N 1s spectra (Figure 3B) indicated the presence of amide species after the functionalization with the peptide by both lithographic methods. The presence of amide moieties confirms the presence of the peptides at the surface. The broad sulfur spectra (Figure 3C) show the presence of As plasmon loss peak, Ga 3s, and As-S species. The existence of thiolated species (As-S) indicates the formation of (22) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569.

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Figure 2. AFM height (a) and friction (b) images of structures drawn by DPN using an ink with a concentration of 1 mM. AFM height (c) and friction (d) images of structures drawn by DPN using an ink with a concentration of 20 mM. AFM height (e) and friction (f) images of structures generated by µCP using an ink with concentrations of 1 and 20 mM.

a covalent bond between the sulfur and the surface after µCP and DPN.23-28 During the time it takes to transfer the samples into the XPS chamber, some small amount of oxide re-forms on the surface29 as indicated by the data corresponding to Ga 3d and As 3d regions; see Figure 3D,E. In summary, the XPS data support the conclusion that the molecules delivered on the surface by DPN and µCP are attached by a covalent bond using the sulfur from the terminal amino acid group on the TAT peptide. Our findings do not exclude the possibility that other peptides with Lewis base character might also bind weakly to the GaAs. Interactions between amine groups and the surface can result in alternative binding motifs. The AFM data support the notion that hydrophilic groups are present at the interface. On the basis of the gathered AFM and XPS data, one can speculate that the self-assembly process is driven by the optimal packing of the alkyl chains in the monolayer and can be further stabilized by the hydrogen bonding induced by the amide groups. This hypothesis was tested by carrying out further spectroscopic studies. The more detailed evaluation of the structure of the SAMs generated by DPN and µCP was done by FT-IRRAS; see Figure (23) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (24) Li, H.-W.; Muir, B. V. O.; Fichet, G.; Huck, W. T. S. Langmuir 2003, 19, 1963-1965. (25) Petrovykh, D. Y.; Kimura-Suda, H.; Whitman, L. J.; Tarlov, M. J. J. Am. Chem. Soc. 2003, 125, 5219-5226. (26) Winter, R.; Nixon, P. G.; Gard, G. L.; Graham, D. J.; Castner, D. G.; Holcomb, N. R.; Grainger, D. W. Langmuir 2004, 20, 5776-5781. (27) Yam, C.-M.; Pradier, C.-M.; Salmain, M.; Marcus, P.; Jaouen, G. J. Colloid Interface Sci. 2001, 235, 183-189. (28) Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 2003, 125, 2597-2609. (29) McGuiness, C. L.; Shaporenko, A.; Mars, C. K.; Uppili, S.; Zharnikov, M.; Allara, D. L. J. Am. Chem. Soc. 2006, 128, 5231-5243.

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Figure 3. High-resolution XPS spectra for five regions: (a) C 1s, (b) N 1s, (c) S 2p, (d) As 3d, and (e) Ga 3d. The numbers next to each spectrum correspond to the following surfaces: (1) freshly etched GaAs, (2) GaAs surface modified with a TAT peptide by DPN, and (3) GaAs surface functionalized with a TAT peptide by µCP.

Figure 4. FT-IRRAS spectra of the high-frequency region (left) and the low-frequency region (right) of GaAs surfaces functionalized with the TAT peptide by (1) DPN and (2) µCP.

4. Each spectrum of the monolayers formed by the peptide was subtracted from the spectrum of the bare GaAs. We used the OMNIC software to carry out an automatic baseline correction in the two spectral regions of interest: 3100-2750 and 18501500 cm-1. We performed this experiment multiple times on samples taken from different wafers, and despite the fact that the peak intensity was quite low, each peak we report was reproducible and distinguishable from artifacts. Prior to our analysis of the FT-IRRAS data, we recorded bulk experiments using the peptide in buffer solution (see the Supporting Information). In the highfrequency region, the surfaces modified by DPN and µCP methods exhibited two distinct peaks corresponding to υas(CH2) and υs(CH2). The TAT peptide sequence is rich in arginines and lysines. These are the residues that are likely to contribute to the observed methylene stretching peaks in the IR spectra. In addition, tyrosine, serine, and isoleucine residues can give rise to the observed methylene peaks even though they are closer to the N-terminus and the semiconductor surface. The methylene groups due to the presence of arginines and lysines are removed from the surface since they are located on the C-terminus of the peptide. Attachment to the GaAs surface takes place on the N-terminus

of the peptide via the sulfur on the terminal cysteine residue. There are 26 methylene residues due to the presence of two lysine and six arginine amino acids, and they can form all-trans crystalline domains in a fashion similar to alkylthiol SAMs. The film thicknesses measured by AFM support the notion that a monolayer was formed after both types of lithographic procedures, and the observed intensity of the peaks is reasonable based on the abundance of alkyl chains in the peptide sequence. The υas(CH2) bands were located at 2916 and 2913 cm-1 on surfaces modified by DPN and µCP, respectively. The υs(CH2) bands were located at 2849 and 2846 cm-1 on surfaces modified by DPN and µCP, respectively. The position of the methylene peaks is used to understand the monolayer organization. Bands assigned to υas(CH2) and located below 2920 cm-1 are characteristic of well-organized and crystalline-like alkyl chain packing. An indication of a disordered state is when these bands shift to frequencies higher than 2920 cm-1. The υs(CH2) bands are another marker of the conformation ordering of the methylenes. Monolayers are well-ordered when υs(CH2) bands are located below 2850 cm-1and disordered when the υs(CH2) bands are positioned higher than 2850 cm-1. Our results indicate that both lithographic methods produced well-ordered monolayers. These monolayers exhibit crystalline-like alkyl chain packing in all-trans conformation.22,30-33 The analysis of the low-frequency region can explain the orientation of the amide moieties as well as the nature of the interchain hydrogen bonding in the peptide structure. This can be done by mapping the peak positions and shapes in the lowfrequency region. The amide I peak (primarily CdO stretch) of the monolayer fabricated by DPN was centered at 1669 and 1640 cm-1. The monolayer fabricated by µCP displayed a peak at 1667 cm-1. Our IR data show a slight shift in the amide I peak (30) Azzam, W.; Wehner, B. I.; Fischer, R. A.; Terfort, A.; Woll, C. Langmuir 2002, 18, 7766-7769. (31) Pardo, L.; Wilson, W. C.; Boland, T. Langmuir 2003, 19, 1462-1466. (32) Nakano, K.; Sato, T.; Tazaki, M.; Takagi, M. Langmuir 2000, 16, 22252229. (33) Cho, Y.; Ivanisevic, A. J. Phys. Chem. B 2005, 109, 12731-12737.

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to ∼1669 and 1667 cm-1 on both types of lithographically generated monolayers. This suggests an alteration in the secondary structure of the peptide and results in the non-hydrogen bonding.33 However, on surface patterned by DPN we observed another strong peak at 1640 cm-1, which indicated the presence of interchain hydrogen bonding in the peptide monolayer.34,35 Amide II peaks (C-N-H stretching and bending) were located at ∼1534 and ∼1560 cm-1 on surfaces modified by DPN. The GaAs functionalized by µCP displayed peaks at ∼1530 and ∼1565 cm-1. The observation of the amide II peak confirms the presence of hydrogen bonding in the monolayers composed of a TAT peptide and patterned by DPN. In the two types of monolayers strong peaks were observed between 1700 and 1800 cm-1 corresponding to the CdO stretching mode. The FT-IRRAS results suggest that both monolayers generated by DPN and µCP have well-ordered and tightly packed alkyl chains. The observation that the peptides anchored on the surface by DPN exhibit interchain hydrogen bonding is very important.36 Such types of peptide layers on surfaces can be used to study electron transfer through the peptides35 and have previously been studied only on gold surfaces.37 This paper demonstrates that DPN can be used to form hydrogen bonding peptide assemblies on GaAs. This capability opens the possibility of designing new devices38 that incorporate the electronic properties of the semiconductor surface and the monolayer stability of the peptide assemblies. The observed structural differences between features formed on GaAs using a peptide ink and the two different lithographic protocols can be due to the following mechanistic grounds. Studies in the literature have pointed out that the DPN process is highly dependent upon the nature of the surface and factors that influence the formation of the water meniscus.39,40 In addition, the presence of other species on the surface such as impurities can alter the rate and direction of the ink transport from the AFM tip.13 As part of the microcontact printing process, (34) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371-4382. (35) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239-5243. (36) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486-2487. (37) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319-5327. (38) Lewis, P. A.; Inman, C. E.; Maya, F.; Tour, J. M.; Hutchison, J. E.; Weiss, P. S. J. Am. Chem. Soc. 2005, 127, 17421-17426.

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the peptide is physisorbed on the stamp. During this process the molecules can form multilayers on the stamp. It is possible that during the printing the peptide adsorbed on the stamp can unfold and exhibit a structural denaturing41 before being pressed onto the substrate.

Conclusion Through a series of surface characterization techniques we have shown that µCP and DPN can be used to covalently anchor peptides on GaAs surfaces. XPS results showed no differences in the surface chemistry and homogeneity of the monolayers fabricated by each lithographic technique. FT-IRRAS provided evidence that the SAMs formed by the lithographic protocols are crystalline-like in nature and have well-ordered packing. The peptides anchored on the surface by DPN exhibit interchain hydrogen bonding and can be used for rational design of novel devices. Acknowledgment. This work was supported by NASA under Award No. NCC 2-1363 and NSF (CHE-0614132). Any opinions, findings, and conclusions expressed in this material are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration. The authors acknowledge experimental help from Dr. Richard Haasch (UIUC) to carry out the XPS characterization. All XPS experiments were performed at the Center for Microanalysis of Materials, UIUC, which is partially supported by the U. S. Department of Energy under Grant DEFG02-96-ER45439. Supporting Information Available: Additional data showing the transmission FT-IR of the TAT peptide in PBS solution, the FT-IRRAS of the TAT peptides immobilized on the GaAs surface by solution adsorption, XPS survey scans, a line profile across a structure generated by microcontact printing, and images obtained with a hydrophobic AFM tip. This material is available free of charge via the Internet at http://pubs.acs.org. LA060492E (39) Rozhok, S.; Piner, R.; Mirkin, C. A. J. Phys. Chem. B 2003, 107, 751757. (40) Rozhok, S.; Sun, P.; Piner, R.; Lieberman, M.; Mirkin, C. A. J. Phys. Chem. B 2004, 108, 7814-7819. (41) Runge, A. F.; Mendes, S. B.; Saavedra, S. S. J. Phys. Chem. B 2006, 110, 6732-6739.