DOI: 10.1021/la901847s 13855 Langmuir 2009, 25(24), 13855–13860 Published on Web 07/28/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society Monolayer-Protected Gold Nanoparticles Prepared Using Long-Chain Alkanethioacetates † Shishan Zhang, Gyu Leem, and T. Randall Lee* Departments of Chemistry and Chemical Engineering, University of Houston, 4800 Calhoun Road, Houston, Texas 77204-5003 Received May 23, 2009. Revised Manuscript Received June 25, 2009 This letter describes the preparation of monolayer-protected nanoparticle clusters (MPCs) from the adsorption of n-tetradecanethioacetate onto colloidal gold nanoparticles using the Brust-Schiffrin two-phase synthesis method. The MPCs were characterized by transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) spectroscopy, 1 H nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. These studies found that the monolayer coatings on the gold nanoparticles were nearly indistinct with regard to chemical composition, monolayer structure, and Au-S ligation when compared to those prepared from the analogous adsorption of n-tetradecanethiol (i.e., the thioacetate headgroup adsorbs to gold as a thiolate, with concurrent loss of the acetyl group). Under equivalent conditions of formation, however, the size of the gold nanoparticles formed was larger when using the alkanethioacetate adsorbate (e.g., 4.9 ( 1.2 nm) compared to the alkanethiol adsorbate (e.g., 1.6 ( 0.3 nm). The observed difference in size is rationalized on the basis of the stronger ligating ability of the thiol compared to that of the thioacetate during gold nanoparticle nucleation and/or growth. The use of alkanethioacetates affords significant control of particle size and allows the formation of MPCs with thiol-sensitive ω-functional groups. Introduction Self-assembled monolayers (SAMs) generated by the spon- taneous assembly of organic molecules on two-dimensional (2-D) substrates are widely used in a variety of technological applications. 1,2 The process of self-assembly is associated with the spontaneous adsorption and organization of an active surfactant-like species on a solid surface and therefore in- cludes a variety of adsorbates and substrates, such as phos- phines on platinum or palladium, 3-5 silanes on silica, 6,7 and thiols on gold, silver, and copper. 8 Research involving SAMs on flat substrates has been adapted to form three-dimensional (3-D) SAM-coated structures, specifically, monolayer-pro- tected nanoparticle clusters (MPCs), which can be handled as isolable species and further functionalized with a variety of reagents. 9 Since the initial description of Au 55 (PPh 3 ) 12 Cl 6 by Schmid, 10 researchers have prepared MPCs using a var- iety of ligands, including thiols, disulfides, dialkyl sulfides, thiosulfates, xanthates, carbamates, phosphines, phosphine oxides, amines, carboxylates, selenides, and isocyanides. 11 Furthermore, the use of the Brust-Schiffrin 12 two-phase synthesis method now allows facile tailoring of the surface properties of thiolate-functionalized nanoparticles by select- ing from structurally diverse alkanethiols or alkyl disulfides having various chain lengths and/or ω-functional groups. 9,13 Given that the interfacial properties of SAMs are critical in technological applications, 1,2,11 researchers who utilize SAMs have explored various protecting-group strategies 14 in cases where the targeted ω-terminal groups are reactive toward thiols 15 or some intermolecular reactions may occur (e.g., aromatic thiols can easily undergo oxidation to form disulfide in the presence of a small amount of oxygen). 16 Notably, several research groups 17-19 have successfully utilized the S-acetyl-protected species in the formation of 2-D SAMs on flat gold substrates. In these studies, the direct attachment of adsorbates via their terminal thioacetate group was also observed; like SAMs derived from alkanethiols, the adsorbed sulfur species were thiolates. For the alkanethioacetate system, however, SAM formation occurred less readily than with analo- gous thiol-terminated adsorbates. In light of these studies, we were surprised to find no reports of the use of thioacetate-terminated adsorbates to prepare 3-D SAMs on colloidal substrates. Alkanethioacetates can be readily prepared from mesylate- or halo-functionalized organic precursors. The preparation of † Part of the “Langmuir 25th Year: Nanoparticles synthesis, properties, and assemblies” special issue. *To whom correspondence should be addressed. E-mail: trlee@uh.edu. (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Mallat, T.; Broennimann, C.; Baiker, A. Appl. Catal. 1997, 149, 103. (4) Mitchell, G. E.; Henderson, M. A.; White, J. M. J. Phys. Chem. 1987, 91, 3808. (5) Ugo, R. Coord. Chem. Rev. 1968, 3, 319. (6) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201. (7) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (8) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (9) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (10) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; van der Velden, J. A. W. Chem. Ber. 1981, 114, 3634. (11) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (12) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (13) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (14) Greene, T. W. Protective Groups in Organic Synthesis, 3rd ed.; Wiley: New York, 1999. (15) Witt, D.; Klajn, R.; Barski, P.; Grzybowski, B. A. Curr. Org. Chem. 2004, 8, 1763. (16) Tarbell, D. S. In Organic Sulfur Compounds; Kharasch, N., Ed.; Pergamon Press: New York, 1961; Vol. 1, p 97. (17) Tour, J. M.; Jones, L. II.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. J. Am. Chem. Soc. 1995, 117, 9529. (18) Kang, Y.; Won, D.-J.; Kim, S. R.; Seo, K.; Choi, H.-S.; Lee, G.; Noh, Z.; Lee, T. S.; Lee, C. Mater. Sci. Eng., C 2004, C24, 43. (19) B ethencourt, M. I.; Srisombat, L.; Chinwangso, P.; Lee, T. R. Langmuir 2009, 25, 1265.