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Adsorption Behavior of DNA-Wrapped Carbon Nanotubes on Self-Assembled Monolayer Surfaces Rebecca A. Zangmeister,* James E. Maslar, Aric Opdahl, and Michael J. Tarlov National Institute of Standards and Technology, 100 Bureau DriVe, MS 8362, Gaithersburg, Maryland 20899-8362 ReceiVed October 23, 2006. In Final Form: March 14, 2007 We have examined the adsorption of DNA-wrapped single-walled carbon nanotubes (DNA-SWNTs) on hydrophobic, hydrophilic, and charged surfaces of alkylthiol self-assembled monolayers (SAMs) on gold. Our goal is to understand how DNA-SWNTs interact with surfaces of varying chemical functionality. These samples were characterized using reflection absorption FTIR (RAIRS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. We have found that DNA-SWNTs preferentially adsorb to positively charged amine-terminated SAMs and to bare gold surfaces versus hydrophobic methyl-terminated or negatively charged carboxylic acid-terminated SAMs. Examination of the adsorption on gold of single-strand DNA (ssDNA) of the same sequence used to wrap the SWNTs suggests that the DNA wrapping plays a role in the adsorption behavior of DNA-SWNTs.
Introduction Carbon nanotubes show great promise for applications in nanoelectronics and nanosensing due to their unique structurebased electronic properties.1-3 Advances in these fields have been hindered by difficulties in obtaining high-purity, monodisperse carbon nanotube samples and reproducibly fabricating nanoscale devices.4 Improvements in the purification and controlled deposition of carbon nanotube materials are needed to compare results between laboratories and ultimately enable economic fabrication of reliable nanoscale devices.5 Separation and directed assembly of carbon nanotubes are hindered by the inherent polydispersity (size and electronic character) and poor solubility of these materials in both aqueous and nonaqueous solution. DNA has been shown to interact with carbon nanotube materials, enabling the dissolution and separation of single-walled carbon nanotubes (SWNTs).6-9 Zheng and coworkers reported a method for dispersing SWNTs in aqueous solution by sonication in the presence of single-strand DNA (ssDNA).6,7 They further demonstrated that a polydisperse SWNT sample can be separated by ion-exchange chromatography6-8 due to the negative charge that the DNA imparts to the DNASWNT hybrid material and that metallic versus semiconducting carbon nanotubes can be separated from one another. Hybrid materials of DNA and carbon nanotubes have been achieved as described above or through covalent chemical attachment.10,11 The discovery of these types of hybrid materials * To whom correspondence should be addressed. Email:
[email protected]. Phone: 301-975-4912. Fax: 301-975-2643. (1) Collins, P. G.; Avouris, P. Scientific American 2000, 283, 62-69. (2) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787-792. (3) Dresselhaus, M. S.; Dai, H. MRS Bull. 2004, 29, 237-243. (4) de Heer, W. A. MRS Bull. 2004, 29, 281-285. (5) Haddon, R. C.; Sippel, A. G.; Rinzler, A. G.; Papadimitrakopoulos, F. MRS Bull. 2004, 29, 252-259. (6) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338-342. (7) Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S. G.; Diner, B. A.; Dresselhaus, M. S.; Mclean, R. S.; Onoa, G. B.; Samsonidze, G. G.; Semke, E. D.; Usrey, M.; Walls, D. J. Science 2003, 302, 1545-1548. (8) Lustig, S. R.; Jagota, A.; Khripin, C.; Zheng, M. J. Phys. Chem. B 2005, 109, 2559-2566. (9) Huang, X. Y.; McLean, R. S.; Zheng, M. Anal. Chem. 2005, 77, 62256228. (10) Gao, H. J.; Kong, Y. Annu. ReV. Mater. Res. 2004, 34, 123-150.
have motivated others to investigate biomimetic, bottom-upbased methods for fabricating nanoscale structures.12-16 Bare carbon nanotubes show preferential adsorption to hydrophilic surfaces from organic solvents,17-20 and recently it has been shown that DNA-SWNTs align on SiO2 surfaces modified with hydrophobic organosilane films.21 Our interest is to determine whether DNA wrapping of SWNTs can enable controlled deposition of these hybrid materials on chemically modified surfaces, with the long-term goal of using controlled surface chemistry to direct the placement of SWNTs with nanometer resolution to fabricate sensing devices. Here we report studies of the adsorption behavior of DNA-SWNTs on model SAM surfaces of different terminal chemical functionality using the characterization techniques of reflection absorption FT-IR (RAIRS), X-ray photoelectron, and Raman spectroscopies. Materials and Methods Materials. The fractionated DNA-wrapped carbon nanotube sample (commercially available HiPCo SWNTs wrapped with ssDNA of 15 repeating guanine/thymine base units, d(GT)15) was obtained using procedures published by Zheng and co-workers.6,7 The d(GT)15 ssDNA sequence (HPLC purified), NaCl (99.9%), tris[hydroxymethyl]aminomethane (Tris, >99%), 1-dodecanethiol (DDT, 94.6%), mercaptoundecanoic acid (MUA, 95%), and 11-amino-1-undecanethiol, hydrochloride (AUT, >90%) were obtained from commercial vendors. All materials were used without further purification. (11) Xin, H. J.; Woolley, A. T. Nanotechnology 2005, 16, 2238-2241. (12) Hazani, M.; Hennrich, F.; Kappes, M.; Naaman, R.; Peled, D.; Sidorov, V.; Shvarts, D. Chem. Phys. Lett. 2004, 391, 389-392. (13) Li, S. N.; He, P. G.; Dong, J. H.; Guo, Z. X.; Dai, L. M. J. Am. Chem. Soc. 2005, 127, 14-15. (14) He, P. G.; Li, S. N.; Dai, L. M. Synth. Met. 2005, 154, 17-20. (15) Sarikaya, M.; Tamerler, C.; Jen, A. K. Y.; Schulten, K.; Baneyx, F. Nat. Mater. 2003, 2, 577-585. (16) Hobbie, E. K.; Bauer, B. J.; Stephens, J.; Becker, M. L.; McGuiggan, P.; Hudson, S. D.; Wang, H. Langmuir 2005, 21, 10284-10287. (17) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125-129. (18) Rao, S. G.; Huang, L.; Setyawan, W.; Hong, S. H. Nature 2003, 425, 36-37. (19) Hannon, J. B.; Afzali, A.; Klinke, C.; Avouris, P. Langmuir 2005, 21, 8569-8571. (20) Wang, Y.; Maspoch, D.; Zou, S.; Schatz, G. C.; Smalley, R. E.; Mirkin, C. A. PNAS 2006, 103, 2026-2031. (21) McLean, R. S.; Huang, X. Y.; Khripin, C.; Jagota, A.; Zheng, M. Nano Lett. 2006, 6, 55-60.
10.1021/la063109e CCC: $37.00 © 2007 American Chemical Society Published on Web 04/25/2007
Adsorption BehaVior of DNA-Wrapped Carbon Nanotubes All thiol solutions were made using 200 proof ethanol; all aqueous solutions were made using 18 MΩ‚cm water. Preparation of Model Surfaces. Gold films on single-crystal Si(100) wafers were used as substrates. Prior to the deposition of the films, the wafers were cleaned using a “piranha solution” consisting of 70% H2SO4 and 30% H2O2 (30% H2O2 in H2O). (Caution: Piranha solution must be handled with care. It is extremely oxidizing, reacts Violently with organics, and should only be stored in loosely tightened containers to aVoid pressure buildup.) After cleaning, a chromium adhesion layer (20 nm) was deposited by vapor deposition, followed by 200 nm of gold. Each substrate was again cleaned with piranha solution and rinsed thoroughly with deionized water (18 MΩ‚cm) immediately prior to immersion in thiol solution. SAMs were prepared by immersing clean gold substrates (∼2 cm2) in 1 mmol/L thiol solutions (5 mL) at room temperature for 20 h. SAM surfaces were rinsed with EtOH and dried in a stream of N2 prior to analysis or immersion in DNASWNT solution. SAM surfaces were immersed in DNA-SWNT solution (0.5 µg/mL in 1 mol/L NaCl, 0.4 mmol/L Tris, adjusted to pH 7) for 20 h. Samples were rinsed with 18 MΩ‚cm water and dried in a stream of N2 prior to analysis. A multilayer film of the DNA-SWNT material on gold was prepared by pipetting DNASWNT solution (50 µg/mL in 0.2 mol/L NaCl, 40 mmol/L Tris, adjusted to pH 7) onto a piranha-cleaned gold slide and allowing to dry in air. Excess buffer salt was removed by briefly rinsing with 18 MΩ‚cm water. The sample was dried in a stream of N2 prior to analysis. RAIRS Measurements. FTIR absorption spectra were measured with a commercially available spectrometer with a wire grid infrared polarizer (p-polarized), a variable-angle specular reflectance accessory (reflectance angle 75°) and cryogenic mercury cadmium telluride detector. Presented spectra (1800-1450 cm-1) are the result of averaging 1024 scans at 4 cm-1 resolution. All FTIR measurements were performed on freshly prepared samples. XPS Measurements. The X-ray photoelectron spectroscopy measurements were made on a commercially available spectrometer at a pressure of 6 × 10-10 Torr with non-monochromatic Mg KR radiation, and an X-ray power of 144 W. All measurements were done using both electrostatic and magnetic lenses, with a step size of 0.1 eV and sweep time of 60 s. High-resolution scans were acquired for Au 4f, N 1s, O 1s, and C 1s regions in the fixed analyzer transmission mode with pass energy of 40 eV and an average of 20 scans. Only N 1s high-resolution scans are shown. Reported binding energies were calibrated with respect to Au 4f peak at 84.0 eV; no charge neutralization was necessary. Elemental core-level spectra were fit using commercial analysis software. After subtraction of a linear background, all spectra were fit using a convolution of Gaussian and Lorentzian line shapes with a typical ratio of 60:40. The peak fitting procedure used a minimum number of peaks consistent with the best fit, with consideration of peak position, full width at half-maximum, intensity and the Gaussian fraction (which determines the fraction of the Gaussian component in the convoluted peak shape). Raman Measurements. Raman spectra were obtained using a custom-made Raman microprobe with an excitation wavelength of 514.5 nm (argon ion laser) and a theoretical diffraction-limited spot size of C g G > T.31 The peak positions and relative intensities of the DNA-SWNTs that are specifically associated with the guanine and thymine bases can be compared to reported RAIRS spectra of homo-oligonucleotides of guanine and thymine adsorbed on gold. Kimura-Suda et al. reported that a homo-oligonucleotide of guanine, (dG)5, adsorbed on gold exhibits a strong feature at 1640 cm-1 with shoulders at 1705 and 1600 cm-1. Homooligonucleotides of thymine, (dT)5, adsorbed on gold gave much weaker features at 1710, 1610, and 1580 cm-1;31 the feature at 1710 cm-1 was later assigned by Petrovykh et al. to carbonyl groups in free thymine rings specific to dT nucleotides.33 The absorbance bands observed for DNA-SWNTs in that same spectral region are consistent and certainly a combination of these mentioned for homo-oligonucleotides guanine and thymine with absorbances at 1697, 1630, and 1574 cm-1. These characteristic DNA absorbance bands were also observed after exposing both the AUT SAM and the DDT SAM to DNASWNTs but were not observed on the MUA SAM. The absorbance band at ca. 1720 cm-1 that appears on the DDT SAM is lower in intensity than on either the bare gold or the AUT surfaces. Assuming that the intensity can be related to the amount of (33) Petrovykh, D. Y.; Kimura-Suda, H.; Whitman, L. J.; Tarlov, M. J. J. Am. Chem. Soc. 2003, 125, 5219-5226.
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Figure 2. RAIRS spectra taken of SAM model surfaces before and after (offset by 0.5 × 10-3) exposure to a solution of d(GT)15: bare gold, AUT, and DDT.
adsorbed DNA-SWNTs on these surfaces, we can conclude that the interaction between the DNA-SWNT and the methylterminated SAM is based on relatively weak forces. The phosphate backbone of the DNA imparts an overall negative charge to the hybrid DNA-SWNT at pH 7, the pH used for the adsorption experiments. The AUT SAM surface is expected to carry a net positive charge, and the MUA surface a net negative charge at this pH.34,35 We therefore expected that the negatively charged DNA-SWNT would be attracted to the net positively charged AUT (-NH3+) SAM surface and repulsed from the net negatively charged MUA (-COO-) SAM surface. This prediction is supported by the data in Figure 1. Intense DNA absorbance bands at 1699, 1628, and 1574 cm-1 are observed on the positively charged AUT SAM surface after DNA-SWNT exposure. In contrast, no change is observed in the MUA spectrum after exposure to the DNA-SWNTs. Bare SWNTs are insoluble in aqueous solutions. Therefore, direct comparison of the adsorption behaviors of bare SWNTs and DNA-SWNTs in aqueous buffer solutions is not possible. It has been reported, however, that bare SWNTs adsorb on carboxylic acid-terminated SAMs, and this behavior has been used to direct the placement of SWNTs on patterned SAM surfaces from organic solvent solutions.20 Our observation that there is no adsorption of DNA-SWNTs on the MUA surface suggests that the adsorption behavior of the DNA-SWNTs on the negatively charged MUA surface is not dictated principally by the SWNT surface but is likely influenced by the DNA wrapping. Bare SWNTs have also been reported to adsorb more strongly from organic solutions to carboxylic acid-terminated SAMs relative to amine-terminated SAMs.17,20 This behavior is in contrast to the results reported here where the DNA-SWNTs adsorb preferentially from aqueous buffer to amine-terminated SAMs versus the carboxylic acid-terminated SAM. Although it is impossible to know the extent to which the individual DNA and SWNT components contribute to the adsorption behavior of DNA-SWNTs, the observation that DNA-SWNTs adsorb to the (34) Zhang, H.; He, H.-X.; Wang, J.; Mu, T.; Liu, Z.-F. Appl. Phys. A. 1998, 66, S269-S271. (35) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675-3683.
Adsorption BehaVior of DNA-Wrapped Carbon Nanotubes
AUT surface and do not adsorb to the MUA surface suggests that electrostatic interactions of the -NH3+-terminated SAM and the negatively charged DNA wrapping of the DNA-SWNT influence DNA-SWNT adsorption. The hydrophilicity of the surfaces under study appears to have little effect on the observed adsorption affinity of the DNA-SWNTs, as both the MUA and AUT surfaces are hydrophilic. A similar set of experiments was performed to examine the adsorption of d(GT)15 sequences on bare gold, DDT SAM, and AUT SAM surfaces. The MUA SAM model surface was not included because no adsorption of the DNA/SWNT hybrid material on that surface was observed. RAIRS spectra acquired from these three model surfaces before and after exposure to a solution of the d(GT)15 are shown in Figure 2. Similarities are observed in the spectra acquired for DNA-SWNTs (Figure 1) and the d(GT)15 oligos. Comparing the spectra on the AUT modified surfaces, one can see that the spectral signature observed for the d(GT)15 oligo at 1706, 1634, and 1568 cm-1 is similar to that observed for the DNA-SWNTs and is slightly shifted to higher wavenumbers, presumably due to differences in the local environment of free DNA bases versus DNA associated with the DNA-SWNT material. We also observe spectral evidence for slight adsorption of the d(GT)15 oligo on the DDT SAM surface as observed for the DNA-SWNTs. Interesting spectral differences were observed when comparing gold and AUT surfaces exposed to d(GT)15 oligos. Similarities in peak position, but not relative peak intensities, are observed. The peak observed at 1705 cm-1 is weaker than the peak at 1634 cm-1 for d(GT)15 oligos on gold; the opposite is observed for d(GT)15 oligos on the AUT modified surface. Petrovykh et al. has assigned changes in relative peak height in this spectral region to the extent of oligonucleotide chemisorption to gold in the case of formation of d(T)25SH monolayers.33 In that study, a peak observed at 1714 cm-1 was assigned to carbonyl groups in free thymine rings that increased over time during the formation of a monolayer of d(T)25SH. If a similar rationale is applied to the present study and the peak at 1705 cm-1 is attributed to free thymine bases, we can conclude that the d(GT)15 oligos chemisorb more strongly to the gold surface than the AUT surface, as evidenced by the reduction in peak height at 1705 cm-1 and the presence of a strong peak at 1629 cm-1 which is reported for chemisorbed d(G)5 on gold.31 If we revisit the spectrum of adsorbed DNA-SWNTs on gold (Figure 1) we see that the spectral signature of adsorbed DNASWNTs on gold is distinctly different from adsorbed free d(GT)15 oligos on gold. We also observed that the peak at 1697 cm-1 is larger than the peak at 1630 cm-1, suggesting that there is more contribution from free thymine species and therefore less chemisorption of the bases associated with the DNA-SWNTs to gold as compared to the behavior observed for free d(GT)15 oligos on gold. This analysis, taken together with the congruence of the adsorbed DNA-SWNT on gold and the multilayer DNASWNT film on gold spectra suggests that the DNA-SWNT material adsorbs largely intact to the gold surface. Overall, the similarity of spectra (Figures 1 and 2) acquired from SAM and bare gold samples exposed to DNA-SWNTs and the d(GT)15 oligo suggests that the adsorption of the DNA-SWNT to these surfaces is influenced by the DNA-wrapping. In the case of bare gold, we theorize that some population of the GT bases of the DNA-wrapping is available to chemisorb to the bare gold surface but do not chemisorb to as great an extent as free d(GT)15 oligos. XPS Analysis. The results of the XPS experiments also indicate that DNA-SWNTs adsorb readily to bare gold and positively
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Figure 3. XPS N 1s spectra taken of SAM model surfaces before and after (offset by 0.5 kCPS) exposure to a solution of DNASWNTs: gold, DDT, and AUT. Peak fits shown by solid lines.
charged AUT SAM surfaces and interact weakly with hydrophobic terminated substrates. Of the photoelectron lines originating from the DNA-SWNT (C 1s, O 1s, N 1s, and P 2p), the N 1s XPS peak was chosen for close examination because the N 1s line provides good signal-to-noise versus the P 2p line which is much less intense and has been shown to yield characteristic N 1s binding energies and peak shapes for the different DNA bases.33,36 The XPS N 1s data for the three model surfaces, bare gold, DDT SAM, and AUT SAM before and after exposure to DNA-SWNTs are shown in Figure 3. The MUA SAM model surface was not included because little adsorption of the DNA-SWNT hybrid material on that surface was observed using RAIRS analysis. Before DNA-SWNT exposure, the bare gold and DDT SAM surfaces show no N 1s signal, as expected. The AUT SAM exhibits an N 1s signal fitted with two peaks with binding energies at 401.9 and 400.1 eV. These peak energies correspond well with peak energies of 401.9 and 400.2 eV assigned to free protonated and free unprotonated primary amine species, respectively, for alkylamine silanes on metal oxide surfaces.37 Following DNA-SWNT exposure, N 1s signals were detected on all surfaces but were most intense on the bare gold and AUT SAM surfaces. A less-intense N 1s peak is observed on the DDT SAM surface. Assuming that the intensity of the N 1s peaks scales with the amount of adsorbed DNA-SWNTs on these surfaces, qualitatively the adsorption of DNA-SWNTs occurs to the greatest extent on bare gold and amine-terminated surfaces and minimally to the methyl-terminated surface. These data are consistent with the RAIRS data presented in the previous section, where adsorption appeared to be greatest on the bare gold and AUT surfaces. N 1s peaks at binding energies of 400.6-400.8 and 399.5399.7 eV were observed on all three surfaces. We would expect that the N 1s binding energies observed for the DNA-SWNTs (36) Herne, T. M.; Tarlov, M. J. J. Am. Chem. Soc. 1997, 119, 8916-8920. (37) Moses, P. R.; Wier, L. M.; Lennox, J. C.; Finklea, H. O.; Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978, 50, 576-585.
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Figure 4. Raman microscopy spectra taken of DNA-SWNTs in solution (Bulk, scaled by 0.5×), bare gold and SAM model surfaces after exposure to a solution of DNA-SWNTs: DDT, bare gold, and AUT.
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conclusively confirm the adsorption behavior. Raman spectra of DNA-SWNTs in solution (DNA-SWNT; 50 µg/mL in 0.2 mol/L NaCl, 40 mmol/L Tris), obtained as a reference, and of a clean bare gold surface, taken as an estimate of background scattering, are shown in Figure 4 along with spectra of DDT SAM and AUT SAM model surfaces exposed to DNA-SWNTs. Again, the MUA SAM model surface was not included because little adsorption of the DNA-SWNT hybrid material on that surface was observed using RAIRS analysis. The Raman radial breathing modes are clearly seen in the reference spectrum of the DNA-SWNTs in Tris buffer solution and are comparable to those observed for fractionated DNA-SWNT materials.7,43 Similar SWNT spectral signatures are observed on the bare gold, amine-terminated, and to a small extent on the methyl-terminated model surfaces. Assuming that the Raman intensities scale with the amount of adsorbed DNA-SWNTs on these surfaces, these data support the FTIR and XPS results that adsorption of DNA-SWNTs occurs to the greatest extent on bare gold and amine-terminated model surfaces.
Conclusions to correspond with N 1s binding energies measured for homooligonucleotides for guanine and thymine. N 1s binding energies have been reported for homo-oligonucleotides on gold33 and on silicon39 surfaces. Single-stranded 20mers of guanine and thymine covalently attached to an aminosilane layer on Si exhibited XPS N 1s binding energies at 400.6 and 399.0 eV that correspond with the N 1s binding energies reported here.39 In the case of the bare gold surface, there is a third N 1s peak with a binding energy of 397.8 eV, which has been attributed to chemisorption of DNA to gold which results in a shift of N 1s peaks to lower binding energies.31,33 These data support the theory presented in the previous section that some population of the GT bases of the DNA-wrapping are available to chemisorb to the bare gold surface. Raman Analysis. Both RAIRS and XPS contain features that can be assigned to the DNA component of the DNA-SWNTs; however, neither of the techniques exhibits spectral features that are uniquely attributable to the carbon nanotube component of the hybrid material. Raman spectroscopy has been used extensively to characterize metallic and semiconducting SWNTs.40-43 We therefore have used Raman spectroscopy to (38) Petrovykh, D. Y.; Kimura-Suda, H.; Opdahl, A. M.; Tarlov, M. J.; Whitman, L. J. Abstracts of Papers of the American Chemical Society 2004, 227, U355. (39) Saprigin, A. V.; Thomas, C. W.; Dulcey, C. S.; Patterson, C. H.; Spector, M. S. Surf. Interface Anal. 2005, 37, 24-32. (40) Rao, A. M.; Richter, E.; Bandow, S.; Chase, B.; Eklund, P. C.; Williams, K. A.; Fang, S.; Subbaswamy, K. R.; Menon, M.; Thess, A.; Smalley, R. E.; Dresselhaus, G.; Dresselhaus, M. S. Science 1997, 275, 187-191. (41) Sauvajol, J. L.; Anglaret, E.; Rols, S.; Alvarez, L. Carbon 2002, 40, 1697-1714. (42) Strano, M. S.; Doorn, S. K.; Haroz, E. H.; Kittrell, C.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2003, 3, 1091-1096. (43) Chou, S. G.; Ribeiro, H. B.; Barros, E. B.; Santos, A. P.; Nezich, D.; Samsonidze, G. G.; Fantini, C.; Pimenta, M. A.; Jorio, A.; Plentz, F.; Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Zheng, M.; Onoa, G. B.; Semke, E. D.; Swan, A. K.; Unlu, M. S.; Goldberg, B. B. Chem. Phys. Lett. 2004, 397, 296-301.
The adsorption properties of DNA-SWNTs on bare gold and SAM model surfaces were investigated using RAIRS, XPS, and Raman spectroscopy. Combined data from all three techniques indicate that DNA-SWNTs adsorb preferentially to positively charged amine-terminated SAMs and to bare gold surfaces versus hydrophobic methyl-terminated or negatively charged carboxylic acid-terminated SAMs. Although we cannot determine definitively the extent to which the DNA-wrapping versus the SWNT surface dictates the adsorption behavior, our data suggest that electrostatic interactions between AUT and MUA SAM surfaces and the negatively charged phosphate groups of the DNAwrapping influence adsorption of DNA-SWNTs. Adsorption of DNA-SWNTs on bare gold likely occurs through chemisorption of GT bases to the surface, as evidenced by the observance of a lower-binding-energy peak in the XPS N 1s data. Similarities in the RAIRS absorbance frequencies for DNA-SWNTs and d(GT)15 oligos adsorbed on the model surfaces suggest that a portion of the DNA component of the hybrid material can interact with the substrate surface much like the free d(GT)15 sequence. These findings have implications for gaining competence in controlled deposition of these hybrid materials onto device substrates of various architectures. Acknowledgment. The authors thank Dr. Ming Zheng of DuPont Central Research and Development for his generous advice and consultation on this project and Dr. Bindhu Varughese, manager of the Department of Chemistry XPS facility at the University of Maryland, College Park, MD for acquiring the XPS spectra presented in this manuscript. A.O. acknowledges the NRC postdoctoral research program for funding. LA063109E
