Nanoparticles: Scaffolds and Building Blocks ROY SHENHAR AND VINCENT M. ROTELLO* Department of Chemistry, University of Massachusetts, Am herst, Massachusetts 01003 Received October 23, 2002 ABSTRACT Nanoparticles provide key tools for bridging the gap between “bottom-up” synthetic methods and “top-down” fabrication. In this Account we describe some of the unique structural aspects of nanoparticles and the use of these attributes to the creation of devices with tunable specificity and environmental response. We also explore the use of nanoparticles as “building blocks” for the creation of nanocomposite materials that feature structural control from the molecular to the micron scale. I. Introduction Multiscale fabrication is a crucial goal in nanotechnology. Top-down fabrication methods such as photo- and elec- tron-beam lithography provide a tool for etching surfaces to provide structures on the nanometer scale. 1 Two- and three-dimensional structures can be formed using lithog- raphy, but these structures, however, are limited by the inherent two-dimensionality of each lithographic step. Bottom-up methods using the techniques of organic and inorganic synthesis furnish a means of fabricating mo- lecular systems such as devices and sensors that are on the 0.5-2.5 nm scale with complete control of three- dimensional structure. 2 Synthesis and lithography provide complementary tools for nanotechnology, but the integra- tion of these techniques remains a significant challenge. Nanoparticle systems, including monolayer-protected clusters (MPCs) and mixed monolayer-protected clusters (MMPCs), 3 bridge the gap between lithographic and synthetic methods (Figure 1). MPCs are core -shell type systems that feature a self-assembled monolayer (SAM)- functionalized metallic or semiconductor core. The mono- layer coverage provides two key functions: shielding the particles from agglomeration and furnishing a scaffold for the attachment of functional molecular entities. MMPCs greatly extend the versatility of core -shell systems, pro- viding multiple functionalities on the monolayer shell. The fabrication of MPC and MMPC systems has been greatly facilitated by the methods developed by Brust et al. 4 In their approach chemical reduction of metal salts (Pd, Au, Ag, Pt) is performed in the presence of capping ligands (Figure 2). The mild conditions and moderate reducing agents used in this process are compatible with a wide range of ligand functionality. The size of nanopar- ticles can be controlled through the stoichiometry of the metal salt to capping ligand, providing MPCs and MMPCs ranging in size from 1.5 to 8 nm and overall diameters of 2.5-11nm. 5 These nanoparticles can be further elaborated through ligand displacement (Figure 2). Repeating this step with different ligands leads to multifunctional MMPCs. This is a potent technique for obtaining structural diver- sity, providing rapid generation of a wide variety of MMPC systems. 6 Nanoparticles themselves also provide a pragmatic approach to the challenge of multiscale engineering, functioning as “building blocks” of regular shape and size for the fabrication of larger structures. Combination of synthetic design with directed assembly of nanoparticles into ensembles provides direct control of structure from the molecular to the macroscopic level. Roy Shenhar was born in 1971 in Rehovot, Israel. After receiving his B.Sc. in Chemistry and Computer Science from the Hebrew University of Jerusalem in 1995, he continued onto a Ph.D. in Chemistry, which he obtained with Mordecai Rabinovitz and Itamar Willner in 2002. Currently a Fulbright Postdoctoral Research Associate in Prof. Rotello’s Group at the University of Massachusetts, his interests span the areas of self-assembly, nanoparticle, and polymer chemistry. His current research is focused on the combination of molecular recognition with nanopar- ticles and block copolymers. Vincent Rotello received his B.S. from Illinois Institute of Technology in 1985. He obtained his Ph.D. in 1990 from Yale University with Harry Wasserman. From 1990 to 93, he was an NSF postdoctoral fellow with Julius Rebek Jr. at M.I.T. Since 1993, Professor Rotello has been at the University of Massachusetts at Amherst as an Assistant (1993-1998), Associate (1998-2001), and Professor (2001-) of Chemistry, with appointments in Polymer Science and Engineering and in the Program in M olecular and Cellular Biology. He has been the recipient of the NSFCAREER and the Cottrell Scholar Award, as well as the Camille Dreyfus Teacher-Scholar and the Sloan Fellowship. His research focuses on the application of molecular recognition to devices, polymers, nanotechnology, and biological systems. FIGURE 1. Nanoparticles as scaffolds for molecular-level control of properties and as building blocks in macroscopic assemblies. Acc. Chem. Res. 2003, 36, 549-561 10.1021/ar020083j CCC: $25.00  2003 American Chemical Society VOL. 36, NO. 7, 2003 / ACCOUNTS OF CHEM ICAL RESEARCH 549 Published on Web 05/23/2003