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Mercaptoammonium-Monolayer-Protected, Water-Soluble Gold, Silver, and Palladium Clusters David E. Cliffel,† Francis P. Zamborini, Stephen M. Gross, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received June 30, 2000. In Final Form: October 4, 2000 This paper describes gold, silver, and palladium nanoparticles coated with monolayers of trimethyl(mercaptoundecyl)ammonium ligands. The monolayer-protected clusters (MPCs) are synthesized in aqueous medium and are water soluble. In dry films, their highly charged surfaces inhibit the interdigitation of monolayer chains known (transmission electron microscopy) in dry films of alkanethiolate-coated nanoparticles. UV-vis spectra exhibit surface plasmon bands for Au and Ag MPCs at the expected wavelengths but none for Pd clusters. Thermogravimetric and transmission electron microscopy results suggest that Au and Ag nanoparticles readily aggregate through ionic association of the terminal ammonium groups but without metallic core fusion between particles. Two-component multilayers of MPCs can be affixed to surfaces by alternating exposure to anionic, mixed-monolayer hexanethiolate-mercaptoundecanoic acid MPCs and cationic Au ammonium-MPCs.
Introduction Substantial interest in thiol-passivated metal nanoclusters1 was stirred by the synthetic advance made by Brust et al.2 in their toluene-phase preparation of alkanethiolate-monolayer-coated, 1-3 nm Au nanoparticles. These nanoparticles offered the arresting property of good stability and lack of aggregation in the solid state and during derivatization procedures, which has made possible their chemical elaboration as large, polyfunctional molecular entities.3 Passivation by alkanethiolate monolayers has been applied to generate monolayer-protected clusters (MPCs) of other metals, namely, Ag4,5 and alloys6 of Au, Ag, Cu, and Pd and, most recently,7,8 of Au, Pd, Ir, and Pt. Alkanethiolate-protected clusters are soluble in nonpolar organic solvents but not soluble or sparingly so in polar solvents. Aqueous and polar-solvent solubility will be helpful to uses of MPCsssuch as catalysts, sensors, molecular markers, and drug delivery materials. The solubility of MPCs is dominated9 by the monolayer and especially its peripheral functionalities. The need for more polar monolayers has led to water- and/or polar organicsoluble Au, Pd, Pt, and alloy clusters synthesized using stabilizers such as sulfonated triphenylphosphine,10-12 † Present address: Department of Chemistry, Vanderbilt University, Nashville, TN 37235-1822.
(1) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci. 1997, 2, 42. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whymnan, R. J. Chem. Soc., Chem. Commun 1994, 801. (3) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (4) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (5) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (6) Hostetler, M. J.; Zhong, C.-J.; Yen, B. K. H.; Anderegg, J.; Gross, S. M.; Evans, N. D.; Porter, M.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 9396. (7) Chanel, K. Y.; Rainer, J.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3486. (8) Chanel, Y.; Scotti, M.; Ulman, A.; White, H.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 4314. (9) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (10) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780.
phenanthroline,12,13 polyoxoethylene,14 and dendrimers,15-17 and to water-soluble Au MPCs made by Brust-like syntheses and having tiopronin,9,18 glutathione,19 and mercaptosuccinic acid20 thiolate ligands. Au clusters with OH-, COOH-, and NH2-functionalized arylthiolate ligands21-23 dissolve in polar solvents (alcohols, acetone, acetonitrile) but not in water. Stabilities of these materials vary widely. Phosphine, nitrogen heterocycle, and polymerstabilized metal clusters are often intractably aggregated after drying, and the metal core can be strongly screened by the polymer matrix. Like their alkanethiolate cousins, MPCs protected with polar thiolate ligands are resistant to core aggregation upon being dried,24 the dried materials typically freely redissolving without change. It is additionally possible to alter the ligand constituents in polar MPC monolayers using ligand place-exchange and coupling reactions such as reported9 for tiopronin-protected clusters. This research undertakes to extend polar thiolate ligand protection to water-soluble MPCs of metals other than Au, while preserving the stability attributes generally known for thiolate-protected Au MPCs. The study has (11) Schmid, G.; Klein, N.; Korste, L.; Kreibig, U.; Scho¨nauer, D. Polyhedron 1988, 7, 605. (12) Schmid, G. Chem. Rev. 1992, 92, 1709. (13) Vargaftik, M. N.; Zagorodnikov, V. P.; Stolyarov, I. P.; Moiseev, I. I.; Likholobov, V. A.; Kochubey, D. I.; Chuvilin, A. L.; Zaikovsky, V. I.; Zamaraev, K. I.; Timofeeva, G. I. J. Chem. Soc., Chem. Commun. 1985, 937. (14) Naka, K.; Yaguchi, M.; Chujo, Y. Chem. Mater. 1999, 11, 849. (15) Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 6364. (16) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217. (17) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 364. (18) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66. (19) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643. (20) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075. (21) Brust, M.; Fink, J.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (22) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639. (23) Chen, S.; Murray, R. W. Langmuir 1999, 15, 682. (24) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17.
10.1021/la000922f CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/2000
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Figure 1. TEM images and histograms with average diameters for Au, Ag, and Pd ammonium MPCs, for films drop-cast from aqueous solutions onto carbon-coated Formvar films on Cu grids.
the additional result of providing an example of a fully cationic, quaternary ammonium thiolate ligand for metal MPCs, which can be used to fabricate multilayered films of nanoparticles of alternating charge typessa Au ammonium MPC and an anionic MPC that were recently employed25 in devising multilayer MPC films. The alternating MPC multilayer is an example of three-dimensional nanoscale architecture.26,27
MΩ cm water. For Au and Pd, the solution was stirred for 45-60 min before a 10-fold molar excess of aqueous NaBH4 (0.38 g in 15 mL H2O) is added; for Ag, NaBH4 was added immediately after the mercaptoammonium. A vigorous, evanescent reaction ensues (caution!), and the solution turns dark as MPCs are produced. After the solution was stirred for an hour, the MPCs were purified by dialysis in water over a 7-day period.29
Experimental Procedures The aqueous synthesis is done on a scale affording 50-200 mg of MPC products in yields (based on the metal) of ca. 80%. An equimolar amount of N,N-trimethyl(undecylmercapto)ammonium ligand28 (as the chloride salt) is added to the metal salt (1 mmol of HAuCl4, AgNO3, or K2PdCl4) dissolved in 20 mL of 18
Average core dimensions and organic content were established (as previously24) by a combination of transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). With these data, we can create models of the average MPC compositions. TEM images and core size histograms (Figure 1) show that the average Au and
(25) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514-4515. (26) Antonietti, M.; Go¨ltner, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 910. (27) Schmid, G.; Hornyak, G. L. Curr. Opin. Colloid Interface Sci. 1997, 2, 204.
(28) For the synthetic preparation of this thiol see: Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. (29) Dialysis was conducted from aqueous cluster solutions in 8-in. segments of cellulose ester dialysis membrane (Spectra/Por CE, MWCO ) 10 000) in stirred 4-L beakers containing 18 ΜΩ cm water, changed daily.
Results
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Table 1. Thermogravimetric Analysis Results M:L TEM size metal org wt % ratio (nm) Pd Ag Au
32.0 52.3 30.2b
5.6:1 2.4:1 3.3:1
2.7 4.8 4.4
MPC compositions expt model Pd800L145 Pd807L163 (Ag4000L1670)a (Ag4033L453)a (Au3000L910)a (Au2951L371)a
a Apparent compositions, see text. b Elemental analysis gave 31% (Galbraith Laboratory, Knoxville, TN).
Ag ammonium MPC core diameters are substantially larger than those with Pd core metal and are also more disperse. The Au and Ag MPC images also reveal some ordering of and regularity of distances between coress similar to24 TEM images of alkanethiolate MPCs. In the latter, extensive chain interdigitation occurs, and corecore separations are little more than the length of a single alkanethiolate chain. Such interdigitation seems not to occur in the Au and Ag ammonium MPCs; in the Ag MPC image for example, the average core-core spacing is 3 nm, very close to twice the ligand length (ca.1.6 nm). This result must reflect the unfavorable energetics of the cationic headgroups penetrating into the hydrophobic interiors of adjacent MPCs; they instead remain separated by the MPC counterions, which serve both to screen the cationic MPC surfaces from one another and to (see below) ionically cross-link them together. TGA of the monomer thiol (temperature ramped from 30 to 600 °C) shows that thermal decomposition occurs in several weight-loss transitions (the two largest near 260 and 320 °C) and complete vaporization by 500 °C. The first transition is likely attributable to the loss of a alkanethiol chain (HS(CH2)10- ) 61 wt % compared to 60.2% experiment) with the trimethylammonium chloride end vaporizing at a higher temperature. The three MPCs display a similar pattern (Figure S-1) of thermal decomposition and were reproducible among different synthetic batches. This pattern includes large transitions at 225 °C (Au MPC), 217 °C (Ag MPC), and 267 °C (Pd MPC) followed by a more gradual decomposition above 300 °C. It is likely these temperatures reflect to the different thiol-metal bonding energies and environments and, possibly, energetics of aggregations discussed above. Table 1 gives the organic weight fractions, metal-toligand mole ratios, the mean TEM core sizes, and the experimental average MPC compositions. Table 1 also shows compositions predicted from TEM data assuming a closed-shell, truncated octahedral (TO) core geometry model that has been applied to explain compositions of alkanethiolate MPCs.24 The Pd ammonium MPCs fit the compositional model reasonably well, but the Au and Ag MPCs are very ligand rich and the data are obviously only apparent compositions. The organic fractions of MPCs of the experimental core sizes require, respectively, very unlikely 1:1 and 1.5:1 ligand to surface metal ratios. The ligand content of the Au and Ag MPCs is in fact more consistent with much smaller, 2 nm diameter MPCs of composition ca. M201L85. One possible explanation for the Table 1 results is that during the drying of TEM samples, strong ionic association has occurred between terminal trimethylammonium groups and counteranions such that the MPCs clump together. This clumping hinders imaging of the core sizes of individual MPCs in the TEM data, thereby giving apparent TEM sizes that are larger than the actual individual MPC composition. Experimental behavior of the Au MPCs in fact reveals a propensity to aggregate when dried, and there is some graininess in the larger apparent-core images in Figure 1, which might be taken as substructure of large, multi-MPC particles. Also,
Figure 2. UV-vis spectra of Au, Ag, and Pd MPCs in aqueous solutions in ca. 0.3 mg/mL. SP bands for Au and Ag lie at ∼520 and ∼420 nm, respectively.
water-soluble tiopronin18 Au MPCs synthesized under similar conditions give smaller core TEM images and an Au201L85 composition. There is an implication, then, that much smaller Au and Ag ammonium MPCs might be seen in the TEM if means to avoid such ionic aggregation could be discovered. Electronic spectra of aqueous solutions of the metal ammonium MPCs (Figure 2) exhibit surface plasmon bands at ∼520 and ∼420 nm for Au and Ag, respectively, but none for Pd. The Au result is consistent with prior data24 for (nonaggregated) clusters with diameters >1.7 nm, and a small degree of ionic aggregation in solution may enhance the peak. In 1 M NaNO3 electrolyte, the magnitude of the Au surface plasmon band is reduced slightly, indicating a possible reduction in ionic aggregation. In acetone, the Au MPC spectrum includes in fact a broad collective band30 at ca. 700 nm as an indicator of ionic aggregation of the MPCs ammoniums in the less polar solvent. The absence of a band for the Pd MPCs (2.7 nm diameter) is in accord with theoretical predictions31 and experimental observations for 4.6 nm Pd particles,32 but not with the report of a 302 nm surface plasmon band for 2.2 nm octadecanethiolate-protected palladium clusters.7 The controlled assembly of nanometer scale structures has significant current interest and importance. Our laboratory recently demonstrated25 that multilayers of mixed monolayer Au145[S(CH2)10COO-]17[S(CH2)5CH3]33 MPCs (I) can be formed by coordinative carboxylate/Cu2+/ carboxylate linker bridges between the MPCs. The spectra in Figure 3 show that this architecture can be extended to alternating layers of cationic and anionic monolayerprotected clusters (MPCs). Figure 3 starts with a multilayer of I (assembled by Cu2+ coordination) and is followed by successive, alternating layers of the Au ammonium MPC and I. Figure 4 shows a cartoon of the alternating layer assembly, showing (for simplicity) only a single monolayer per attachment cycle. The changes in absorbance at 520 nm in Figure 3 indicate34 that ca. 9 × 10-11 mol/cm2 of I (ca. 5 monolayers) and 1.6 × 10-11 mol/ cm2 of ammonium Au MPC (ca. 1 monolayer) are attached, on average, to the surface per exposure cycle. The reasons for the alternating efficiencies of surface capture of MPCs, (30) Quinten, M.; Kreibig, U. Surf. Sci. 1986, 172, 557-577. (31) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881-3891. (32) Michaelis, M.; Henglein, A.; Mulvaney, P. J. Phys. Chem. 1994, 98, 6212-6215. (33) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85. (34) Hicks, J. F.; Patel, N.; Murray, R. W. Unpublished results.
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Figure 3. UV-vis spectra showing alternating layer assembly of anionic Au145[S(CH2)10COO-]17[S(CH2)5CH3]33 (I) and cationic Au ammonium MPCs. The substrate is a glass slide derivatized with (3-mercaptopropyl)trimethoxysilane33 in ethanol (“Cu2+ slide”, the blank for the figure) that was exposed to 5 mM Cu(ClO4)2 in ethanol overnight and rinsed and then exposed to I (0.1 mM in ethanol) for 30 min (gray line, “1st”). The figure shows four cycles of subsequent, alternating 30-min exposures to an ethanol solution (0.1 mM) of the Au ammonium MPC (dark lines) and to I (gray lines).
the level of alternating layer uniformity, and their electrical behavior await further study. It should be obvious25 that the chemistry in Figure 4 could be effected on other surfaces, such as Au electrodes. Last, proton NMR spectra (Figure S-2) of the MPCs, routinely used3 to confirm absence of impurities especially unattached ligands, are similar to that of the trimethylammonium thiolate ligand. Consistent with previous24 spectra of alkanethiolate MPC monolayers, the resonances are broadened and R-methylene peaks are not seen, which for unattached ligand would not be the case. The terminal methyl peaks are however sharper than those of terminal methylene units on alkanethiolate MPC spectra.24 The latter result suggests that the quaternary ammonium terminus enjoys a freely tumbling environment due to a combination of Coulombic self-avoidance of those sites and their core-radius-of-curvature spatial dispersion. As this paper was finished, a newly published paper35 gives similar TEM and UV-vis surface plasmon data for a Ag ammonium MPC based on the ligand 11-(trimethylammonium)decanoylaminoethyl disulfide. The reported
Figure 4. Schematic of the assembly of alternating layers of anionic and cationic MPCs onto a metalated, carboxylated glass surface. Although depicted as uniform alternating monolayers, the degree of layer uniformity is unproven, and the anionic MPCs actually form multilayers as shown by the results of Figure 3.
pH effect on cluster stability and solubility was not observed in our work. Acknowledgment. This work was supported by grants from the National Science Foundation and the Office of Naval Research. Supporting Information Available: Thermogravimetric analysis traces for the three metal ammonium MPCs and for trimethyl(mercaptoundecyl)ammonium chloride and NMR spectra for Pd, Au, and Ag MPCs. This material is available free of charge via the Internet at http://pubs.acs.org. LA000922F (35) Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2000, 16, 5218-5200.
