SELF-ASSEMBLED MONOLAYERS ON GOLD NANOPARTICLES

Self-assembled monolayers (SAMs) of n-alkanethiolates on gold, silver, and copper have been intensively studied both as model organic surfac es and as modulators of metal surface properties, Sensitivity restrict ions imposed by monolayer coverage and the low surface area of planar metal substrates, however, limit the characterization of these films i n molecualar terms to surface enhancement techniques. As a result, key aspects such as film dynamics and alkyl chain ordering remain ill-def ined. The characterization of the thermal behavior of SAMs is importan t not only for the design of stable, well-ordered organic superlattice s, but also for the fundamental understanding of the factors that driv e molecular interactions in two dimensions. Phase properties in SAMs h ave been addressed here through the synthesis of gold nanoparticles of 20-30 Angstrom in diameter and fully covered with alkylthiol chains. These thiol-modified gold nanoparticles with large surface areas have enabled the monolayer film structure to be uniquely characterized by t ransmission FT-IR spectroscopy, NMR spectroscopy, and differential sca nning calorimetry. Our studies reveal that for long-chain thiols (grea ter than or equal to C-16), the alkyl chains exist predominantly in an extended, all-trans ordered conformation at 25 degrees C. Furthermore , calorimetry, variable temperature transmission FT-IR spectroscopy, a nd solid-slate C-13 NMR studies have established that a cooperative ch ain melting process occurs in these alkylated metal colloids. How this arises is not immediately evident, given the relation between the ext ended chain conformation and the geometry of the spherical nanoparticl es. Transmission electron microscopy (TEM) reveals that adjacent gold particles are separated by approximately one chain length; this sugges ts that chain ordering arises from an interdigitation of chains on nei ghboring particles. The thermotropic behavior is sensitive to the alky l chain length and chain packing density. The alkylated nanoparticles can thus serve as a highly dispersed analogue to the much-studied plan ar SAMs.