Chem. Rev. 2005, 105, 1103−1169 1103 Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology J. Christopher Love,† Lara A. Estroff,† Jennah K. Kriebel,† Ralph G. Nuzzo,*,‡ and George M. Whitesides*,† Department of Chemistry and the Fredrick Seitz Materials Research Laboratory, University of Illinois−Urbana−Champaign, Urbana, Illinois 61801 and Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138 Received July 19, 2004 Contents 3.4.1. Defects Caused by Variations in the 1121 Surface of the Substrate 1. Introduction 1104 3.4.2. Reconstruction of the Surface during 1121 1.1. What Is Nanoscience? 1104 Assembly 1.2. Surfaces and Interfaces in Nanoscience 1106 3.4.3. Composition of SAMs 1121 1.3. SAMs and Organic Surfaces 1106 3.4.4. Structural Dynamics of SAMs Induce 1121 1.4. SAMs as Components of Nanoscience and 1106 Defects Nanotechnology 4. Removing SAMs from Surfaces 1122 1.5. Scope and Organization of the Review 1106 4.1. Electrochemical Desorption of SAMs 1122 2. Preparation of SAMs 1108 4.2. Displacement of SAMs by Exchange 1122 2.1. Types of Substrates 1108 4.3. Photooxidation of SAMs. 1123 2.1.1. Preparation of Thin Metal Films as 1108 Substrates for SAMs 5. Tailoring the Composition and Structure of SAMs 1123 2.1.2. Other Substrates for SAMs 1110 5.1. Why Modify SAMs after Formation? 1123 2.1.3. Why Is Gold the Standard? 1111 5.2. Strategies for Covalent Coupling on SAMs 1124 2.2. Protocols for Preparing SAMs from 1111 5.2.1. Direct Reactions with Exposed Functional 1124 Organosulfur Precursors Groups 2.2.1. Adsorption of Alkanethiols from Solution 1111 5.2.2. Activation of Surfaces for Reactions 1125 2.2.2. Adsorption of Disulfides and Sulfides from 1113 5.2.3. Reactions that Break Covalent Bonds 1126 Solution 5.2.4. Surface-Initiated Polymerizations 1126 2.2.3. “Mixed” SAMs 1113 5.2.5. How Does the Structure of the SAM 1126 2.2.4. Adsorption from Gas Phase 1114 Influence Reactivity on Surfaces? 3. Characterization of SAMs: Structure, Assembly, 1114 5.3. Noncovalent Modifications 1127 and Defects 5.3.1. Nonspecific Adsorption of Molecules from 1127 3.1. Nature of the Metal−SAM Interface 1114 Solution onto SAMs 3.1.1. Thermodynamic Analysis of 1115 5.3.2. Fusion of Vesicles on SAMs 1127 Gold−Thiolate Bonds 5.3.3. Selective Deposition onto SAMs 1128 3.1.2. Surface Structure of Thiolates on Gold 1115 5.3.4. Modifications via Molecular Recognition 1128 3.1.3. Surface Structure of Thiolates on 1116 6. SAMs as Surface Layers on Nanoparticles 1128 Palladium 6.1. Formation of Monolayer-Protected Clusters 1128 3.1.4. Surface Structure of Thiolates on Silver 1116 (MPCs) 3.1.5. Surface Structure of Thiolates on Copper 1117 6.1.1. Thiols Are a Special Subclass of 1129 3.2. Organization of the Organic Layer 1117 Surfactants 3.2.1. Single-Chain Model for Describing the 1117 6.1.2. Thiols Can Influence the Size and Shape 1129 Average Organization of the Organic of Nanoparticles Layer in SAMs 6.2. Strategies for Functionalizing Nanoparticles 1130 3.2.2. “Odd−Even” Effect for SAMs on Gold 1118 with Ligands 3.2.3. Multichain Unit Cells 1119 6.2.1. Formation of Nanoparticles in the 1130 3.2.4. Effect of the Organic Component on the 1119 Presence of Thiols Stability of the SAM 6.2.2. Ligand-Exchange Methods 1130 3.3. Mechanisms of Assembly 1119 6.2.3. Covalent Modification 1131 3.3.1. Assembly of SAMs from the Gas Phase 1119 6.3. Structure of SAMs on Highly Curved 1131 3.3.2. Assembly of SAMs from Solution 1121 Surfaces 3.4. Defects in SAMs 1121 6.3.1. Spectroscopic Evidence for SAM 1132 Structure on Nanoparticles * To whom correspondence should be addressed. R.G.N.: phone, 6.3.2. Evidence for the Structure of SAMs on 1132 217-244-0809; fax, 217-244-2278; e-mail: [email protected]. Nanoparticles based on Chemical G.M.W.: phone, (617) 495-9430; fax, (617) 495-9857; e-mail: Reactivity [email protected]. † Harvard University. 6.4. SAMs and the Packing of Nanocrystals into 1132 ‡ University of IllinoissUrbana-Champaign. Superlattices 10.1021/cr0300789 CCC: $53.50 © 2005 American Chemical Society Published on Web 03/25/2005 1104 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al. 7. Patterning SAMs In Plane 1133 7.1. Microcontact Printing 1134 7.1.1. Composition of Topographically Patterned 1134 Stamps 7.1.2. Methods for Wetting Stamps with Thiols 1135 7.1.3. Mechanism for Forming SAMs by Printing 1135 7.1.4. Structure of SAMs Formed by µCP 1136 7.1.5. Transfer of PDMS to the Surface during 1136 Printing 7.1.6. Fabrication of Nanostructures by µCP 1136 7.2. Photolithography or Particle Beam 1137 Lithography 7.2.1. Photolithography 1137 7.2.2. E-Beam and X-ray Lithography 1137 7.2.3. Atomic Beam Lithography 1138 J. Christopher Love received his B.S. degree in Chemistry from the 7.3. Other Methods for Patterning SAMs 1138 University of Virginia in 1999 and Ph.D. degree from Harvard University in 2004. Under the direction of Professor George M. Whitesides, his 7.3.1. Formation of Gradients 1138 doctoral thesis included studies on the surface chemistry of thiols on 7.3.2. Ink-Jet Printing 1138 palladium and fabrication of magnetic micro- and nanostructures. He 7.3.3. Topographically Directed Assembly 1138 currently is a postdoctoral research fellow in Hidde L. Ploegh’s laboratory 7.3.4. Orthogonal Self-Assembly 1139 at Harvard Medical School. His present research interests include nanotechnology, surface chemistry, self-assembly, microfabrication, im- 8. Applications of SAMs on Thin Metal Films 1139 munology, and cell biology. 8.1. SAMs as Etch Resists 1139 8.2. SAMs as Barriers to Electron Transport 1139 8.2.1. SAMs for Electrochemistry 1140 8.2.2. SAMs in Organic/Molecular Electronics 1141 8.3. SAMs as Substrates for Crystallization 1143 8.3.1. Oriented Nucleation of Crystals 1143 8.3.2. Alignment of Liquid Crystals 1145 8.4. SAMs for Biochemistry and Biology 1145 8.4.1. Designing SAMs To Be Model Biological 1146 Surfaces 8.4.2. SAMs for Cell Biology 1147 8.4.3. Structure−Property Considerations for 1148 SAMs Used in Biology 9. Applications of SAMs on Nanostructures 1150 9.1. Electrodeposited Metal Rods 1150 Lara A. Estroff is currently an NIH postdoctoral fellow in Professor George 9.2. Gold Nanopores as Selective Channels 1151 M. Whitesides’ laboratory at Harvard University working on understanding 9.3. Arrays of Metallic Nanostructures 1151 multivalency in the immune system. In 2003 she received her Ph.D. degree 9.3.1. Arrays of Gold Dots 1151 from Yale University for work done in Professor Andrew D. Hamilton’s laboratory on the design and synthesis of organic superstructures to control 9.3.2. Silver Tetrahedrons for Localized Surface 1152 the growth of inorganic crystals. As part of her graduate work, Lara spent Plasmon Resonance (LSPR) time at the Weizmann Institute for Science (Rehovot, Israel) working in 9.4. Metallic Shells 1152 the labs of Professors Lia Addadi and Steve Weiner. Before that she 9.4.1. Metallic Half-Shells 1152 received her B.A. degree in Chemistry from Swarthmore College, where 9.4.2. Gold−Silica Core−Shell Particles 1153 she worked in Professor Robert S. Paley’s laboratory. 9.5. Metal Nanoparticles and Quantized 1153 1. Introduction Double-Layer Charging 9.6. Functional Surfaces on Nanoparticles 1154 1.1. What Is Nanoscience? 9.6.1. Biocompatible Surfaces on Quantum Dots 1154 Nanoscience includes the study of objects and 9.6.2. Functionalized Magnetic Nanoparticles 1154 systems in which at least one dimension is 9.6.3. Nanoparticles for the Polyvalent Display 1154 1-100 nm. The objects studied in this range of sizes of Ligands are larger than atoms and small molecules but 10. Challenges and Opportunities for SAMs 1155 smaller than the structures typically produced for use 10.1. Rules for “Designing” Surfaces 1156 in microtechnologies (e.g., microelectronics, photon- 10.2. New Methods for Characterizing SAMs 1156 ics, MEMS, and microfluidics) by fabrication methods 10.3. New Systems of SAMs 1156 such as photolithography. The dimensions of these 10.4. SAMs with Different Physical Properties 1156 systems are often equal to, or smaller than, the 10.5. In-Plane Patterning 1156 characteristic length scales that define the physical 11. Outlook and Conclusions 1157 properties of materials. At these sizes, nanosystems can exhibit interesting and useful physical behaviors 12. Acknowledgments 1157 based on quantum phenomena (electron confine- 13. References 1157 ment,1 near-field optical effects,2 quantum entangle- Self-Assembled Monolayers of Thiolates Chemical Reviews, 2005, Vol. 105, No. 4 1105 Jennah Kriebel was born in Hawaii in 1976. She attended the University George M. Whitesides received his A.B. degree from Harvard University of Washington as an undergraduate and completed a thesis on microfluidic in 1960 and his Ph.D. degree from the California Institute of Technology systems with Professor P. Yager. She spent one year with Professor G. in 1964. A Mallinckrodt Professor of Chemistry from 1982 to 2004, he is Ertl at the Fritz Haber Institute in Berlin, Germany, where she studied the now a Woodford L. and Ann A. Flowers University Professor. Prior to adsorption of gases onto carbon nanotubes. She is currently in her fifth joining the Harvard faculty in 1992, he was a member of the chemistry year as a graduate student in Chemical Physics with Professor G.