Langmuir 2000, 16, 5949-5954
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Monolayer-Protected Clusters with Fluorescent Dansyl Ligands Ailette Aguila† and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received February 2, 2000. In Final Form: April 14, 2000 Monolayer-protected Au cluster (MPC) molecules with mixed monolayers of alkanethiolate and ω-carboxylalkanethiolate ligands were functionalized with the fluorescent label dansyl cadaverine. The emission intensity per MPC-attached dansyl group varies with (a) the number of dansyl sites per MPC (i.e., loading, 4 to 24), (b) with the linker chainlength between Au core and dansyl site, and (c) with chainlengths of alkanethiolates in the surrounding MPC monolayer coating. Emission intensity per MPCattached dansyl group increases with loading and with the length of core-dansyl linker chains when the surrounding alkanethiolate monolayers have chain lengths equal to or greater than the linker. These results are interpreted in terms of distance-dependent energy transfer from the excited dansyl label to the metal-like MPC core and various steric constraints on thermal fluctuations of linker chains that influence the distance of approach of the label to the core. Correction for self-absorption by the highly colored MPC solution was necessary. Emission intensities of MPC-attached dansyl labels are lower than those from equivalent concentrations of dansyl units and MPCs as unattached cosolutes.
Introduction Nanoscopic particles have captured the interest of a wide range of scientists and comprise a broad research agenda, including size-tunable chemical reactivity and physical properties,1 ordered arrays in surface films,2 molecular electronic devices,3 analytical measurements,4 and new kinds of polyfunctional molecules.5 With regard to the latter topic, starting with the Brust6 synthesis of monolayer-protected gold clusters (MPCs), we have mapped out5,7 synthetic routes to a variety of polyfunctionalized nanoparticles. The Brust synthesis6 leads to nanoparticles composed of a several-nanometer diameter Au core coated with a relatively dense monolayer of alkanethiolate ligands. A * Corresponding author. † Present address: Glaxo Wellcome Inc., Five Moore Drive, P. O. Box 13398, Research Triangle Park, NC 27709-3398. (1) (a) Meldrum, F. C. Adv. Mater. 1995, 7, 607-632. (b) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (c) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (d) Pileni, M. P.; Tanori, J.; Filankembo, A.; Dedieu, J. C.; Gulik-Krzywicki, T. Langmuir 1998, 14, 7359-7363. (2) (a) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397-406. (b) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922-926. (3) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (4) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (b) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849-1862. (5) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 1, 26, and references therein. (6) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, N7, 801-802. (7) (a) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175-9178. (b) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. J.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 48454849. (c) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911. (d) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66-76. (e) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 70817089.
crucial and defining characteristic of these nanoparticles, or MPCs, is that they can be dried of solvent and redissolved without changesspecifically without core aggregation. This important property enables the chemist to manipulate and elaborate the chemical functionalities of MPCs through subsequent derivatization reactions, in the same sense that a polymer chemist would treat a functionalized polymer. Using combinations of placeexchange7a and coupling7b reactions, we have elaborated the chemical diversity of MPCs based on alkanethiolate ligands and on much more polar thiolate ligands7d,e that lead to water-soluble MPCs. These synthetic efforts are followed by investigations of the properties of these new chemical materials, as reported here. Electronically excited groups on MPCs with metal-like cores are a potentially large, yet relatively unexplored topic. Relevant literature8 includes fluorescence and Raman scattering observations from colloids. We have provided an introductory example7b of an MPC to which a fluorescein derivative was attached. This report describes MPCs polyfunctionalized with the fluorescent dansyl moiety. Dansyl cadaverine is widely used9 as a transglutaminase-mediated label for proteins and peptides and as markers for cell membranes. The amine coupling site is separated from the aromatic moiety by a 5-carbon spacer. A structural cartoon of a thusly derivatized MPC is shown in Figure 1. The MPC structure shown was assembled in a threestep reaction sequence consisting of (a) preparation and isolation of a dodecanethiolate-coated MPC with a gold core (C12 Au MPC) and average composition Au140(C12)53, (b) place-exchange of some of the C12 ligands with ω-carboxylundecanethiolate ligands (denoted in this report (8) (a) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466-9471. (b) Pileni, M. P. New J. Chem. 1998, 22, 693-702. (c) Tian, Y. C.; Fendler, J. H. Chem. Mater. 1996, 8, 969974. (d) Krug, J. T.; Wang, G. D.; Emory, S. R.; Nie, S. M. J. Am. Chem. Soc. 1999, 121, 9208-9214. (e) Ni, J.; Lipert, R. J.; Dawson, G. B.; Porter, M. D. Anal. Chem. 1999, 71, 4903-4908. (9) (a) Ariello, L., Biochemistry 1990, 29, 5103. (b) Jue, R. A.; Doolittle, R. F. Biochemistry 1985, 24, 162. (c) Pincus, J. H., Arch. Biochem. Biophys. 1975, 169, 724.
10.1021/la000145j CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000
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Aguila and Murray
Figure 1. Monolayer-protected cluster (MPC) representation after a place exchange and coupling reaction, e.g., C12 MPC exchanged with C10CO2- and coupled with the fluorophore dansyl cadaverine.
as C10CO2-), and (c) coupling of the latter to the amine functionality of dansyl cadaverine. This sample structure was varied in regards to the (average) number of dansyl labels attached to each MPC (from 4 to 24), to the chainlength of the ω-carboxylalkanethiolate ligands (from C2CO2- to C15CO2-) introduced in step (b), and to the chainlengths of the remaining, surrounding alkanethiolate units (from C4 to C12), that were used in step (a). The dansyl fluorophore’s emission undergoes substantial quenching upon its attachment to the MPC, as might be expected from excited-state energy transfer interaction with the metal-like Au nanoparticle core. The Au core is thought to be metal-like, for the core size used, based on the apparent absence of optical and electrochemical band gaps10 and from its double-layer charging properties.11 Energy transfer between point dipoles in solutions (Forster quenching) is known12 to vary with the sixth power of the separation distance, whereas energy transfer between a point dipole and a nearby metal surface varies as the third power of the intervening distance. In the present case, the MPC core is, strictly speaking, neither a point dipole nor a flat metal surface, so some intermediate distance (10) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science, 1998, 280, 2098. (11) (a) Chen, S.; Murray, R. W.; Feldberg, S. W. J. Phys. Chem. B. 1998, 102, 9898. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaff, T. G.; Khoury, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279. (12) (a) Chance, R. R.; Prock, A.; Silbey, R. Adv. in Chem. Phys. Rice, S. A.; Prigogine, I., Eds., Vol. 37, 1-65, Wiley-Interscience: NY, 1978. (b) Waldeck, D. H.; Alivasatos, A. P.; Harris, C. B. Surf. Sci. 1985, 158, 103-125.
power may be expected. Accurately identifying this distance power in the present case is complicated by the thermal fluctuations and folding that is allowed to the dansyl groups by the small radius of curvature of the MPC core (see structure) and the attendant gradient of chain density, and unfortunately, the distance power could not be identified. Exploration of excited-state quenching by metal nanoparticles is of interest for practical as well as fundamental reasons. Specifically, we envision that the quenching effect may be useful as a detector of the extent of kinetic- or equilibrium- controlled binding interactions (ligation, host-guest, antibody-antigen, etc.) between an MPC and a target analyte. Since the collective surface area of the MPCs in a solution can be quite large, a substantial sensitivity may be possible relative to the binding interactions traditionally conducted on flat surfaces. Experimental Section Chemicals. The fluorophore 5-dimethylaminonaphthalene1-(N-(5-aminopentyl)) sulfonamide (i.e., dansyl cadaverine) and two others, 1-aminopyrene and 1-aminomethylpyrene, were obtained from Fluka, Aldrich, and Molecular Probes at 99%, 97%, and 99% purity, respectively. The fluorescence excitation and emission maxima for dansyl cadaverine, 1-aminopyrene, and 1-aminomethylpyrene are, respectively, λex ) 342 nm and λem ) 498 nm (in CHCl3), λex ) 404 nm and λem ) 424 nm (in THF), and λex ) 340 nm and λem ) 375 nm (in 60% THF/40% CH3OH). 1-Butanethiol, 1-hexanethiol, and dodecanethiol were obtained from Aldrich. 11-Mercaptoundecanoic acid and 3-mercaptopropionic acid were purchased from Aldrich and Fluka, respectively, whereas 6-mercaptohexanoic acid and 16-mercaptohexadecanoic
Monolayer-Protected Clusters with Dansyl Ligands acid were synthesized according to Bain.13 Tetrahydrofuran (J. F. Baker, water content
