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Chemoselective Immobilization of Gold Nanoparticles onto Self-Assembled Monolayers Eugene W. L. Chan and Luping Yu* Department of Chemistry and The James Franck Institute, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637 Received October 3, 2001 This Letter describes a general method for the immobilization of gold nanoparticles onto solid supports. Ketone-decorated gold colloids are covalently attached to self-assembled monolayers (SAMs) presenting aminooxy groups on a gold surface. The oxime formed by the chemoselective ligation is stable on the film as shown by the cyclic voltammetry of ferrocenecarboxaldehyde immobilized onto aminooxy SAMs. The colloids are characterized by infrared spectroscopy, transmission electron microscopy, and 1H NMR. Tapping mode atomic force microscopy shows that the gold nanoparticles are uniformly distributed on the surface with an average particle diameter of 5.5 nm.
Nanotechnology has attracted the attention of scientists from different disciplines in search of new functional materials.1 Numerous nanometer-scaled structures including semiconductors, metallic nanoclusters, and wires have been shown to exhibit interesting physical properties.2 These materials present us with a great opportunity for applications in the areas of electro-optic devices, sensors, catalysis, and molecular electronics.3 However, to harness the envisioned opportunity, approaches must be developed to manipulate nanostructured materials and assemble them into desired structural formssone-, two-, or three-dimensional structures. Supramolecular assembly of nanostructured materials could be the key to the success. In this Letter, we describe the exploration of the chemoselective ligation for the immobilization of nanoparticles, gold nanoparticles in this case, onto solid support. The immobilization of gold nanoparticles on the surface has been achieved by several different methods. While some methods take advantage of the ionic nature of the gold as well as its affinity toward thiol, other ones require the use of linker molecules.4 Ideally, it would be desirable to immobilize a variety of nanoparticles that are specific for the ligands presented on the surface with spatial control. The use of chemoselective ligation requires mutually exclusive reactivity of pairs of functional groups. Even among a number of potentially reactive functional groups, two chemoselective ligation partners will only react with each other. Because the reaction can tolerate * To whom correspondence should be addressed. (1) (a) Gross, M. Travel to the Nanoworld, Miniature Machinery in Nature and Technology; Plenum Trade: New York and London, 1999. (b) Nalwa, H. S. Handbook of Nanostructured Materials and Nanotechnology; Academic Press: San Diego, 2000; Vol. 1-4. (c) Jortner, J.; Ratner, M. Molecular Electronic Devices, IUPAC, Chemistry for the 21st Century; Blackwell Science: Malden, MA, 1997. (2) (a) Chen, L.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (b) Markovich, G.; Collier, C. P.; Henrichs, S. E.; Remacle, F.; Levine, R. D.; Heath, A. J. R. Acc. Chem Res. 1999, 32, 415. (3) (a) Rueckes, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C. L.; Lieber, C. M. Science 2000, 289, 94. (b) Park, H.; Park, J.; Lim, A. K. L.; Anderson, E. H.; Alivisatos, A. P.; McEuen, P. L. Nature 2000, 407, 57. (4) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (b) Harnisch, J. A.; Pris, A. D.; Porter, M. D. J. Am. Chem. Soc. 2001, 123, 5829. (c) Kumar, A.; Mandale, A. B.; Sastry, M. Langmuir 2000, 16, 6921. (d) Templeton, A. C.; Zamborini, F. P.; Wuelfing, W. P.; Murray, R. W. Langmuir 2000, 16, 6682. (e) Taton, T. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 2000, 122, 6305.
the presence of a diverse functionality that protectinggroup manipulations are typically unnecessary, it has been used extensively to conjugate peptides and proteins onto surfaces.5 Another advantage of the approach is that the chemoselective ligation reactions require mild conditions and possess fast reaction rates. For this work, we use self-assembled monolayers (SAMs) on gold as our model substrate because these structurally ordered films provide excellent flexibility in modifying ligands and the long-term stability for most physical characterization. Furthermore, the nature of the gold film permits the use of a variety of analytical techniques to characterize the interface, including ellipsometry, grazing angle infrared spectroscopy, and electrochemistry.6 Finally, these films are compatible with micro-contact printing, photopatterning, and other current fabrication techniques that are important in further exploring the potential application of the system.7 Among a number of known chemoselective coupling partners, we are particularly interested in the coupling between ketones and aminooxy groups for several reasons.8 These two functional groups react selectively to form a chemically stable oxime.9 Under mild conditions, the rate at which the chemoselective ligation occurs is rapid and, therefore, permits the interfacial reaction to take place within a reasonable time frame. Most importantly, both the ketone and aminooxy groups are unreactive toward thiols, which is essential for the preparation of gold nanoparticles as well as the monolayer film. Figure 1 shows a representation of ketone-functionalized gold colloids chemoselectively immobilized onto SAMs presenting aminooxy groups. The colloid was modified with a mixed monolayer of 11-mercapto-2-undecanone and dodecanethiol. The ketone-terminated alkanethiol reacts selectively with aminooxy groups on the surface to form a stable oxime. We used hexanethiol as the background on the planar gold film to provide the necessary spacing between the aminooxy groups on the surface. (5) (a) Schnolzer, M.; Kent, S. B. H. Science 1992, 256, 221. (b) Scheibler, L.; Dumy, P.; Boncheva, M.; Leufgen, K.; Mathieu, H. J.; Mutter, M.; Vogel, H. Angew. Chem. 1999, 38, 696. (6) Yousaf, M. N.; Chan, E. W. L.; Mrksich, M. Angew. Chem. 2000, 11, 1943. (7) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 7, 1823. (8) Lemieux, G. A.; Bertozzi, C. R. Trends Biotechnol. 1998, 16, 506. (9) Jencks, W. P. J. Am. Chem. Soc. 1959, 119, 475.
10.1021/la011512+ CCC: $22.00 © 2002 American Chemical Society Published on Web 12/18/2001
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Figure 1. Immobilization of a colloid decorated with 11mercapto-2-undecanone and dodecanethiol onto a mixed monolayer presenting aminooxy and methyl groups. The aminooxy and ketone groups form a stable oxime linkage at the interface.
Figure 3. (A) A TEM image of the gold nanoparticles after place exchange with 11-mercapto-2-undecanone. (B) An IR spectrum of the ketone-functionalized colloidal gold. The peaks at 2918 and 2850 cm-1 correspond to the C-H stretches of the methylene groups, while the peak at 1717 cm-1 corresponds to the C-O stretch of the ketone groups, respectively.
Figure 2. A cyclic voltammogram of ferrocenecarboxaldehyde (FcCHO) immobilized onto a mixed monolayer presenting aminooxy groups (χ ) 0.5) and methyl groups in 1 M HClO4 (s). Immersion of the monolayer in acetone (40 mM in DMF) for 3 h prior to the immobilization of FcCHO completely eliminated any electrochemical signal (- - -). A cyclic voltammogram of the mixed monolayer presenting aminooxy and methyl groups shows that the monolayer is not redox active within the range of applied potentials used during the experiment (- - -).
We first established that the aminooxy SAMs could undergo chemoselective ligation to form the corresponding oxime in high yield. We used an electroactive molecule, ferrocenecarboxaldehyde (FcCHO), to characterize the reaction on the surface by electrochemistry. Figure 2 shows a cyclic voltammogram of FcCHO immobilized onto aminooxy SAMs in 1 M HClO4. The peaks at 425 mV correspond to the one electron reversible oxidationreduction of ferrocene immobilized on the surface.10 The voltammograms can be cycled consecutively over 50 times
without any loss of the signal, showing that the oxime formed by the chemoselective ligation is stable on the film. Immersion of the monolayer film in acetone for 3 h prior to FcCHO completely eliminated any electrochemical signal, demonstrating that the aminooxy SAMs react efficiently with soluble molecules functionalized with ketone groups in the chemoselective ligation. We next prepared dodecanethiol decorated gold colloids according to Lin et al.11 The colloids were then purified by repeated precipitation from toluene with ethanol before 11-mercapto-2-undecanone was introduced by the placeexchange methodology described by Murray and coworkers.12 The resulting particles were characterized by transmission electron microscopy and IR (Figure 3). The ratio of ketone/dodecanethiol was 3:1 as determined by 1H NMR.13 (10) Chidsey and co-workers had reported that the peak potential for ferrocene alkanethiol on gold was 510 mV in 1 M HClO4 (J. Am. Chem. Soc. 1990, 11, 4301). The value we obtained from the electrochemical experiment (425 mV) is slightly different from the value previously reported. We believe that the direct attachment of the oxime resulted from the chemoselective ligation to the ferrocene influences the redox coupling of the ferrocene immobilized on the surface. (11) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J. J. Nanopart. Res. 2000, 2, 157. (12) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 2, 42. (13) The ratio of 3:1 was determined by comparing the integrated areas for the protons of 11-mercapto-2-undecanone and the terminal methylene groups of the dodecaethiolate.
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were first dissolved in toluene (1 mg/mL) and then added to aminooxy SAMs on mica for 12 h. The substrate was then rinsed with methylene chloride and ethanol and dried with a stream of nitrogen. Figure 4A shows an AFM image of gold nanoparticles immobilized onto aminooxy SAMs. An image at higher magnification shows that the immobilized colloids have average diameter of 5.5 nm (Figure 4B). However, when the substrate was first immersed in acetone for 3 h before exposing to the colloidal gold solution, no colloid was detected on the surface from the AFM images, proving that the colloids only attached to the surface chemoselectively with the aminooxy SAMs (data not shown). In conclusion, the chemoselective ligation described here provides an attractive and flexible method for the immobilization of gold nanoparticles onto solid support. Because the reaction is highly specific, it is possible to attach a variety of nanoparticles by simply altering the ligands presented on the surface. For example, we have also immobilized gold colloids decorated with aminooxyterminated alkanethiol onto aldehyde-derivatized glass substrate. Furthermore, this reaction can be applied to immobilize other materials of choice. We are currently exploring the chemoselective ligation of functional organic polymers on a solid support. We believe this methodology will provide an exciting range of tailored substrates for studies in harnessing the properties of nanoparticles and other materials on novel devices and applications in nanotechnology. Acknowledgment. We gratefully acknowledge the financial support of the National Science Foundation and the NSF MRSEC program at the University of Chicago. We also gratefully acknowledge The Petroleum Research Fund, administered by the ACS, for partial support of this research. Figure 4. (A) A tapping mode AFM image of the monolayerprotected gold colloids chemoselectively immobilized onto aminooxy SAMs on mica gold substrate. (B) An AFM image at higher magnification shows that the immobilized colloids have average diameter of 5.5 nm.
Immobilization of the gold colloids onto aminooxyterminated SAMs was characterized by tapping mode atomic force microscopy. Functionalized gold nanoparticles
Supporting Information Available: Experimental details including syntheses of the alkanethiolates and preparation and 1H NMR characterization of the monolayer protected colloids. This material is available free of charge via Internet at http://pubs.acs.org. LA011512+
