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Langmuir 2008, 24, 9784-9789
Noncovalent Functionalization of DNA-Wrapped Single-Walled Carbon Nanotubes with Platinum-Based DNA Cross-Linkers Gordana N. Ostojic, John R. Ireland, and Mark C. Hersam* Department of Materials Science and Engineering and Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3108 ReceiVed April 28, 2008. ReVised Manuscript ReceiVed June 17, 2008 A method for noncovalent functionalization of DNA-wrapped single-walled carbon nanotubes (SWNTs) using platinum-based DNA cross-linkers is investigated. In particular, cisplatin and potassium tetrachloroplatinate are shown to bind to DNA that encapsulates SWNTs in aqueous solution. The bound platinum salt can then be reduced to decorate the DNA-encapsulated SWNTs with platinum nanoparticles. The resulting SWNT/DNA/Pt hybrids are investigated by optical absorption spectroscopy, circular dichroism spectroscopy, Raman spectroscopy, X-ray diffraction, transmission electron microscopy, and atomic force microscopy. The unique combination of catalytic activity of nanoscale platinum, biological functionality of DNA, and optoelectronic properties of SWNTs suggests a myriad of applications including fuel cells, catalysts, biosensors, and electrochemical devices.
1. Introduction Carbon nanotubes possess numerous desirable properties including excellent mechanical strength,1,2 sensitivity to their surroundings,3,4 ability to carry high currents,5,6 and distinct absorption and luminescence peaks.7,8 These properties have been exploited in a wide range of applications such as transistors and interconnects in electrical circuits,9–11 mechanical oscillators,12,13 sensors,4,14,15 and solar cells.16,17 In an effort to enhance their properties and thus enable further applications, a variety of schemes have been developed for covalently and noncovalently functionalizing carbon nanotubes.18–23 In many cases, noncovalent * To whom correspondence should be addressed. E-mail: m-hersam@ northwestern.edu. (1) Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature (London) 2003, 423, 703–703. (2) Yu, M. F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Phys. ReV. Lett. 2000, 84, 5552–5555. (3) Varghese, O. K.; Kichambre, P. D.; Gong, D.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. Sens. Actuator B-Chem. 2001, 81, 32–41. (4) Modi, A.; Koratkar, N.; Lass, E.; Wei, B. Q.; Ajayan, P. M. Nature (London) 2003, 424, 171–174. (5) Minoux, E.; Groening, O.; Teo, K. B. K.; Dalal, S. H.; Gangloff, L.; Schnell, J. P.; Hudanski, L.; Bu, I. Y. Y.; Vincent, P.; Legagneux, P.; Amaratunga, G. A. J.; Milne, W. I. Nano Lett. 2005, 5, 2135–2138. (6) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. J. Nature (London) 2003, 424, 654–657. (7) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593–596. (8) Bachilo, S. M.; Strano, M. S.; Kittrell, C.; Hauge, R. H.; Smalley, R. E.; Weisman, R. B. Science 2002, 298, 2361–2366. (9) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Science 2001, 294, 1317–1320. (10) Fuhrer, M. S.; Kim, B. M.; Durkop, T.; Brintlinger, T. Nano Lett. 2002, 2, 755–759. (11) Huang, Y.; Duan, X. F.; Cui, Y.; Lauhon, L. J.; Kim, K. H.; Lieber, C. M. Science 2001, 294, 1313–1317. (12) Sazonova, V.; Yaish, Y.; Ustunel, H.; Roundy, D.; Arias, T. A.; McEuen, P. L. Nature (London) 2004, 431, 284–287. (13) Kang, J. W.; Song, K. O.; Kwon, O. K.; Hwang, H. J. Nanotechnology 2005, 16, 2670–2676. (14) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622–625. (15) Chopra, S.; McGuire, K.; Gothard, N.; Rao, A. M.; Pham, A. Appl. Phys. Lett. 2003, 83, 2280–2282. (16) Kongkanand, A.; Dominguez, R. M.; Kamat, P. V. Nano Lett. 2007, 7, 676–680. (17) Barnes, T. M.; Wu, X.; Zhou, J.; Duda, A.; van de Lagemaat, J.; Coutts, T. J.; Weeks, C. L.; Britz, D. A.; Glatkowski, P. Appl. Phys. Lett. 2007, 90.
functionalization is preferred since it retains the superlative properties of the underlying nanotube. A variety of metals (e.g., Au, Pt, Pd, Cu) have been successfully affixed to carbon nanotubes.24–34 In particular, Pt nanoparticles have been bound to nanotubes via Pt evaporation or electrodeposition on nanotube mats,24,25,32 binding to functional groups on the nanotube surface28–30 or an encapsulating polymer,35 and physisorption.33 Deposition of Pt on carbon nanotubes has been shown to increase chemical reactivity compared to carbon black35 and holds promise for improved hydrogen storage devices and fuel cells.35–38 For these applications, Pt nanoparticle growth on dispersed, small-diameter carbon nanotubes is desirable since it enables enhanced surface coverage, reactant accessibility, and surface area to volume ratio. In this article, we report a noncovalent single-walled carbon nanotube (SWNT) functionalization strategy where platinum(18) Kam, N. W. S.; O’Connell, M.; Wisdom, J. A.; Dai, H. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 11600–11605. (19) 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. (20) Barone, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Nat. Mater. 2005, 4, 86–U16. (21) Besteman, K.; Lee, J. O.; Wiertz, F. G. M.; Heering, H. A.; Dekker, C. Nano Lett. 2003, 3, 727–730. (22) Arnold, M. S.; Guler, M. O.; Hersam, M. C.; Stupp, S. I. Langmuir 2005, 21, 4705–4709. (23) Pantarotto, D.; Partidos, C. D.; Graff, R.; Hoebeke, J.; Briand, J. P.; Prato, M.; Bianco, A. J. Am. Chem. Soc. 2003, 125, 6160–6164. (24) Day, T. M.; Unwin, P. R.; Macpherson, J. V. Nano Lett. 2007, 7, 51–57. (25) Qu, L. T.; Dai, L. M.; Osawa, E. J. Am. Chem. Soc. 2006, 128, 5523– 5532. (26) Huang, W. J.; Chen, H.; Zuo, J. M. Small 2006, 2, 1418–1421. (27) Sun, Y.; Wang, H. H. Appl. Phys. Lett. 2007, 90. (28) Xie, J. N.; Zhang, N. Y.; Varadan, V. K. Smart Mater. Struct. 2006, 15, S5-S8. (29) Govindaraj, A.; Satishkumar, B. C.; Nath, M.; Rao, C. N. R. Chem. Mater. 2000, 12, 202–205. (30) Guo, D. J.; Li, H. L. J. Colloid Interface Sci. 2005, 286, 274–279. (31) Cheng, J. P.; Zhang, X. B.; Ye, Y. J. Solid State Chem. 2006, 179, 91–95. (32) Ren, G. Q.; Xing, Y. C. Nanotechnology 2006, 17, 5596–5601. (33) Kong, J.; Chapline, M. G.; Dai, H. J. AdV. Mater. 2001, 13, 1384–1386. (34) Ou, Y. Y.; Huang, M. H. J. Phys. Chem. B 2006, 110, 2031–2036. (35) Kongkanand, A.; Vinodgopal, K.; Kuwabata, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 16185–16188. (36) Girishkumar, G.; Rettker, M.; Underhile, R.; Binz, D.; Vinodgopal, K.; McGinn, P.; Kamat, P. Langmuir 2005, 21, 8487–8494. (37) Reddy, A. L. M.; Ramaprabhu, S. J. Phys. Chem. C 2007, 111, 16138– 16146. (38) Narayanamoorthy, J.; Durairaj, S.; Song, Y.; Xu, Y.; Choi, J. Appl. Phys. Lett. 2007, 90.
10.1021/la801311j CCC: $40.75 2008 American Chemical Society Published on Web 07/23/2008
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Figure 2. Optical absorbance of SWNT/DNA and SWNT/DNA/cisplatin solutions after incubation for 7 days. The curves are offset for clarity. The near-infrared peaks that belong to the lowest energy transitions of semiconducting SWNTs are retained and slightly red-shifted (3 nm). The small magnitude of the red-shift reveals that cisplatin only induces subtle changes to the dielectric environment surrounding the SWNTs.
Figure 1. (a) Schematic of cisplatin binding to DNA (top) and SWNT/ DNA (bottom). (b) Photographs of SWNT/DNA (left) and SWNT/DNA/ cisplatin (right) solutions after seven days of incubation. The absence of visible aggregation suggests that cisplatin does not compromise the dispersion of the SWNTs.
based DNA cross-linkers (e.g., potassium tetrachloroplatinate and cisplatin) bind to oligonucleotide-encapsulated SWNTs. Following cross-linking, the platinum salt adduct can be reduced, thus decorating the DNA-encapsulated SWNTs with Pt nanoparticles. In this procedure, the DNA serves as both a noncovalent SWNT dispersant in aqueous solution and a binding layer for Pt compounds and nanoparticles. The advantage of this approach is that the functionalization occurs by covalent attachment to the encapsulating oligonucleotide and not to the SWNT itself. In this manner, the Pt binding is substantially stronger than simple physisorption, yet the SWNT surface is left largely unperturbed thus preserving the inherent and desirable properties of the underlying nanotube. The DNA also serves as a capping molecule for Pt growth, which yields high uniformity of the resulting Pt nanoparticles. To confirm the structure and properties of the SWNT/DNA/Pt hybrids, several analytical techniques have been employed including optical absorption spectroscopy, circular dichroism spectroscopy, Raman spectroscopy, X-ray diffraction, transmission electron microscopy, and atomic force microscopy. This thorough characterization verifies the structure and properties of the SWNT/DNA/Pt hybrids and suggests their utility in applications that require the catalytic activity of nanoscale Pt, the biological functionality and water solubility of DNA, and/or the optoelectronic properties of SWNTs.
2. Materials and Methods DNA Encapsulation of SWNTs. Single-walled carbon nanotubes (purified CoMoCAT, SWeNT) are encapsulated with (GT)10 oligonucleotide (AlphaDNA, Canada) in an aqueous solution of 0.1 M sodium chloride using a reported procedure.19 To remove the excess DNA, the dispersed nanotube solution is dialyzed for four days in 0.1 M NaCl using a 100 kDa dialysis membrane (FloatA-Lyzer, Spectrum Laboratories). Finally, to remove NaCl, an additional dialysis cycle (12 h) is performed in deionized water. Cisplatin Solution Preparation. A freshly prepared solution of 2 mM cisplatin (Sigma Aldrich) is mixed with DNA-encapsulated SWNTs to obtain a 1 mM Pt concentration and SWNT loading of 5.8 mg/L in a 20 mM NaCl aqueous solution. Similarly, (GT)10 (AlphaDNA, Canada) dissolved in deionized water is mixed with cisplatin to obtain the same Pt molarity (1 mM) and 12 µM DNA
Figure 3. Circular dichroism spectra of (a) free DNA and (b) SWNT/ DNA after three days of incubation with cisplatin (∆A corresponds to the difference in optical absorbance of left and right circularly polarized light). The CD spectrum of SWNT/DNA is blue-shifted by ∼24 nm compared to free DNA. Following incubation with cisplatin, a red-shift and decrease of CD signal is observed for both the control DNA and the SWNT/DNA solutions. These observations are consistent with the expected change in DNA conformation following binding with cisplatin.
concentration in 20 mM NaCl buffer. These samples are labeled SWNT/DNA/cisplatin and DNA/cisplatin respectively. In addition, control samples that contain the same concentration of DNA (12 µΜ) and DNA-encapsulated SWNTs (5.8 mg/L) but no cisplatin are prepared and labeled DNA and SWNT/DNA respectively. Potassium Tetrachloroplatinate Solution Preparation. A freshly prepared solution of 10 mM potassium tetrachloroplatinate (Sigma Aldrich) is mixed with DNA-encapsulated SWNTs to obtain a 1 mM Pt concentration and SWNT loading of 5.8 mg/L. As a control solution, (GT)10 dissolved in deionized water is mixed with K2PtCl4 to obtain the same Pt molarity (1 mM) and 12 µM DNA concentration. These samples are labeled SWNT/DNA/K2PtCl4 and DNA/K2PtCl4 respectively. After one week of incubation, both of these solutions were further diluted 2-fold. Then, heated 50 mM (dimethylamino)borane DMAB (Fluka) aqueous solution was added dropwise to each of these solutions to match the molarity of potassium tetrachloroplatinate. The solutions are stirred with an overhanging stirrer inside the water bath at a controlled temperature (45-50 °C) during the reduction process. This procedure was followed by dialysis
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Figure 4. Schematic showing tetrachloroplatinate bound to (a) DNA and (b) SWNT/DNA followed by subsequent Pt reduction and growth induced by DMAB (top row). Photographs of the solutions taken before and after reduction show increased optical absorption (bottom row). No precipitates are observed after Pt nanoparticle formation, which suggests that the Pt nanoparticles remain bound to the DNA.
Figure 5. Optical absorbance of the (a) control DNA/K2PtCl4 and (b) SWNT/DNA/K2PtCl4 samples before and after Pt reduction with DMAB. In both cases, the observed increase of absorbance and the appearance of an infrared tail are in accordance with the creation of metallic nanoparticles. For the SWNT/DNA sample, introduction of K2PtCl4 leads to a blue-shift and broadening of the first order SWNT optical transitions due to the solution pH change from 6 to 4. Following Pt reduction with DMAB, the broadening and diminishing of both the first and second order SWNT optical transitions suggest strong coupling between the Pt nanoparticles and the SWNTs.
of all samples in 10 kDa membranes (Pierce) in deionized water for 24 h. The reduced solutions are labeled DNA/Pt for the control and SWNT/DNA/Pt for the SWNT sample. The final Pt concentration is 0.6 mM. Optical Absorbance Measurements. Optical absorbance is measured in 1 or 0.5 cm optical path plastic cuvettes using a Cary 500 UV-vis-NIR spectrophotometer (Keck Biophysics Facility, Northwestern University). Suitable corrections were made to account for the known optical absorbance of water. Circular Dichroism Measurements. Circular dichroism is measured with a Jasco circular dichroism spectrometer (Keck Biophysics Facility, Northwestern University) at room temperature with a 0.5 nm step and a bandwidth of 1 nm in 0.5 cm quartz cuvettes.
Figure 6. Circular dichroism spectra of (a) control DNA/K2PtCl4 and (b) SWNT/DNA/K2PtCl4 samples after an incubation period of three days with potassium tetrachloroplatinate. A red-shift and decrease of the positive band is observed in both cases, thus suggesting that the DNA conformation is being perturbed by K2PtCl4 in a similar fashion to cisplatin.
Transmission Electron Microscopy. Transmission electron microscopy (TEM) samples are prepared by depositing 5 µL of the SWNT/DNA/Pt sample onto 400 mesh size copper grids coated with an ultrathin carbon film (Ted Pella). After 60 s, the excess solution is drained through the grid using filter paper, and the grid is allowed to dry for 1 h. The TEM samples are examined using a Hitachi 8100 transmission electron microscope at an energy of 200 keV. Atomic Force Microscopy. Atomic force microscopy (AFM) samples are prepared using the following procedure: A 10 µL volume of 10 mM magnesium chloride solution is deposited on freshly cleaved mica. The excess liquid is blown off with nitrogen after 10 s. Immediately after this procedure, 8 µL of sample solution (e.g., SWNT/DNA/K2PtCl4, SWNT/DNA/Pt, or DNA/Pt) is deposited for 2.5 min followed by a rinse with deionized water and blow dry with nitrogen gas. AFM measurements are performed with a Thermomicroscopes CP Research Atomic Force Microscope with Si tips
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Figure 7. X-ray diffraction spectra of thin films made from the reduced platinum SWNT/DNA/Pt samples (a) before and (b) after annealing at 300 °C for 1 h (CPS ) counts per second). For comparison purposes, the scaled scattering intensity of platinum from the ICDD database (PDF # 00-004-0802) is shown in (b). Annealing decreases the peak width, which implies an increase of crystal size. The most intense peak (1 1 1) is fitted by a Lorentzian function to obtain the crystal size from the Scherrer equation. The Pt nanocrystal diameter is 3.8 nm before and 6.1 nm after annealing.
(NSC36/Cr-AuBS, µMasch, Estonia) with a force constant 0.6 N/m and resonant frequency of 75 kHz. Topography images are recorded in intermittent contact mode. Thin Film Preparation. Thin films are prepared by vacuum filtration on anodized alumina membranes with a 20 nm pore size (Whatman). A total of 0.75 mL of SWNT/DNA/Pt solution diluted 7.6-fold with deionized water is deposited uniformly on the 13 mm diameter membrane. X-ray Diffraction. X-ray diffraction (XRD) is measured on films made by vacuum filtration with a Scintag diffractometer (J. B. Cohen X-Ray Diffraction Facility, Northwestern University) using the KR copper line. After taking initial XRD measurements, the same film is annealed in air at 300 °C for 1 h. After annealing, XRD is performed again for comparison. In both cases, the background obtained from a pristine anodized alumina disk was subtracted from the data. Raman Spectroscopy. Raman spectroscopy measurements are performed on a TriVista Raman System (Acton) with laser excitation of 568.2 nm from an Ar-Kr gas laser (Stabilite, Spectra-Physics). The solutions are measured in a 135° geometry with 20 mW laser power.
3. Results and Discussion 3.1. Cisplatin. A schematic of cisplatin binding to a DNAwrapped SWNT is depicted in Figure 1. Following the procedure outlined in the Materials and Methods section, SWNT/DNA/ cisplatin solutions were prepared and allowed to incubate for seven days in the dark. Photographs of the solution taken after the incubation period show that the sample remains optically transparent without visible SWNT aggregation (Figure 1b). To better quantify the optical properties of the SWNT/DNA/cisplatin solutions, optical absorbance spectra were gathered following incubation. A comparison of the SWNT/DNA and SWNT/DNA/ cisplatin optical absorbance spectra reveals that the SWNT spectral features remain intact (Figure 2,) thus demonstrating that SWNT solubilization is retained. Furthermore, the relatively
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small red-shift (3 nm) of the semiconducting SWNT optical transitions suggests that the dielectric environment surrounding the SWNT is only weakly perturbed following cisplatin binding. Due to its applications in anticancer treatments, the interaction between cisplatin and DNA has been thoroughly investigated. In particular, circular dichroism (CD) is commonly used since binding of cisplatin to DNA results in a bathochromic shift of the positive band and an overall decrease of the CD signal.39,40 These observations from CD are attributed to a DNA conformational change upon cisplatin binding to bases (preferentially guanine) and has also been confirmed for short oligonucleotides.39 To determine if similar conformational changes occur upon binding of cisplatin to SWNT/DNA, CD experiments were performed for SWNT/DNA and DNA control samples before and after cisplatin binding. It is important to emphasize that rotational asymmetry in CD measurements is not inherent for bare or surfactant encapsulated SWNTs. However, SWNT/DNA hybrids possess a CD signal that peaks at 258 nm and is shifted from the free DNA CD spectrum which peaks at ∼280 nm. Previous work has established that this wavelength shift and the appearance of nonzero CD at longer wavelengths can be attributed to electronic interactions between DNA and SWNTs.41 CD spectra of both DNA/cisplatin and SWNT/DNA/cisplatin following incubation for three days are shown in Figure 3. Both the control DNA/cisplatin and the SWNT/DNA/cisplatin sample exhibit qualitatively similar behavior: red-shift and decrease of CD signal following incubation in agreement with previous reports for cisplatin binding to DNA.40 Due to the separated maxima positions of DNA only and DNA-wrapped SWNTs, the red-shift of the 258 nm CD peak can be definitively attributed to cisplatin bound to the SWNT/DNA complex. From the CD results, the conformational change appears to be larger for the free DNA than for DNA encapsulating the SWNTs, which is consistent with the more restricted geometry in the latter case. 3.2. Potassium Tetrachloroplatinate. A schematic of potassium tetrachloroplatinate (K2PtCl4) binding to a DNA-wrapped SWNT is shown in Figure 4. Following the procedure outlined in the Materials and Methods section, SWNT/DNA/K2PtCl4 and control DNA/K2PtCl4 solutions were prepared and allowed to incubate for 7 days. Subsequently, DMAB was added to each sample, which is expected to yield the formation of Pt nanoparticles. Photographs in Figure 4 show the two solutions before and after the DMAB-induced reduction process. The solutions visibly darken after DMAB addition, which suggests the formation of Pt nanoparticles. Furthermore, since both the DNA/Pt and SWNT/DNA/Pt solutions are free of precipitates, it is expected that the reduced Pt remains bound to the DNA. The growth mechanism is shown in Figure 4. Initially, the tetrachloroplatinate cross-links with the DNA. Then, upon addition of DMAB, the bound platinum is reduced, thus enabling nanoparticle growth from the excess K2PtCl4 in solution.42 Optical absorbance spectra of the control DNA/K2PtCl4 and SWNT/DNA/K2PtCl4 samples before and after Pt reduction are compared in Figure 5. The control DNA/K2PtCl4 sample shows increased absorbance and a characteristic metallic infrared tail after adding DMAB. On the other hand, before reduction, the SWNT/DNA/K2PtCl4 sample shows a superposition of SWNT and Pt salt peaks in the visible spectrum. In addition, there is a (39) Vangarderen, C. J.; Altona, C.; Reedijk, J. Inorg. Chem. 1990, 29, 1481– 1487. (40) Macquet, J. P.; Butour, J. L. Eur. J. Biochem. 1978, 83, 375–387. (41) Dukovic, G.; Balaz, M.; Doak, P.; Berova, N. D.; Zheng, M.; McLean, R. S.; Brus, L. E. J. Am. Chem. Soc. 2006, 128, 9004–9005. (42) Seidel, R.; Ciacchi, L. C.; Weigel, M.; Pompe, W.; Mertig, M. J. Phys. Chem. B 2004, 108, 10801–10811.
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Figure 8. Intermittent-contact AFM topography images of the reduced platinum SWNT/DNA/Pt samples on mica: (a) 4.5 µm × 4.5 µm image; (b) 1.5 µm × 1.5 µm image. The SWNTs have a height of 1-2 nm and are decorated with Pt nanoparticles. The average nanoparticle height obtained from line profiles taken along the SWNTs is 4.2 nm, which agrees well with the nanocrystal diameter obtained from analysis of the XRD data.
Figure 9. Raman spectra of DNA-encapsulated SWNTs (SWNT/DNA) and SWNT/DNA/Pt hybrids in solution excited by a 568.2 nm laser. (a) Expanded view from 200 cm-1 to 1700 cm-1. (b) Focused view from 1500 cm-1 to 1620 cm-1. Traces are scaled to match the intensity of the G mode peak. The D mode peak at ∼1308 cm-1 indicates that the amount of SWNT disorder does not increase upon Pt formation, while the radial breathing mode (RBM) associated with diameter vibrations (
