STRUCTURE AND DYNAMICS IN ALKANETHIOLATE MONOLAYERS SELF-ASSEMBLED ONGOLD NANOPARTICLES - A DSC, FT-IR, AND DEUTERIUM NMR-STUDY

Gold nanoparticles, cn. 30 Angstrom in diameter, have been derivatized with specifically deuterated (position 1 and positions 10 to 13) and perdeuterated (positions 2 to 18) octadecanethiols (C18SH). The phase behavior of the octadecanethiolate monolayers chemisorbed onto the col loidal Sold surface was characterized by differential scanning calorim etry(DSC). The DSC thermograms show that the C18SH-derivatized Au nano particles undergo distinct phase transitions which can be associated w ith the reversible disordering of the alkyl chains. Despite the highly curved geometry of these Au particles, there is a remarkable degree o f conformational order in the alkanethiolate chains and the thermotrop ic behavior of the thiol-modified gold nanoparticles is very similar t o that of conventional, planar self-assembled monolayers. Both the pea k maximum temperature and the enthalpy associated with the DSC transit ion strongly parallel those of the gel-to-liquid crystalline transitio n of n-diacylphosphatidylcholine lipid bilayer membranes of equivalent chain length. Restricted chain mobility due to covalent bonding of th e sulfur head group to the gold surface does not affect the cooperativ ity of the transition in terms of the transition temperature and entha lpy. Local chain ordering and dynamics in the deuterated C18S/Au nanop articles have been probed using variable-temperature solid-state deute rium NMR spectroscopy and transmission FT-IR spectroscopy. The tempera ture dependence of the symmetric CD2 stretching frequency has confirme d that the DSC-detected phase transition involves a thermally-induced change from a predominantly all-trans conformation to a chain disorder ed state. A comparison of the thermal behavior of d(35)-C18S/Au and 10 ,10, II, II, IZ, 12, 13, 13-d(8)-C18S/Au shows that disordering origin ates in the chain terminus region and propagates toward the middle of the chain as the temperature increases, Studies of I,1-d(2)-C18S/Au sh ow that the disorder does not extend to the tethered sulfur head group . Deuterium NMR spectroscopy specifically establishes that chain melti ng arises from an increased frequency of gauche bonds in the alkanethi olate chains. The H-2 NMR line shapes further indicate that the tether ed alkanethiolate chains are undergoing rapid trans-gauche bond isomer ization and axial chain rotation.