Structure and dynamics in alkanethiolate monolayers self-assembled on gold 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 colloidal Sold surface was characterized by differential scanning calorimetry(DSC). The DSC thermograms show that the C18SH-derivatized Au nanoparticles undergo distinct phase transitions which can be associated with the reversible disordering of the alkyl chains. Despite the highly curved geometry of these Au particles, there is a remarkable degree of conformational order in the alkanethiolate chains and the thermotropic behavior of the thiol-modified gold nanoparticles is very similar to that of conventional, planar self-assembled monolayers. Both the peak maximum temperature and the enthalpy associated with the DSC transition strongly parallel those of the gel-to-liquid crystalline transition of n-diacylphosphatidylcholine lipid bilayer membranes of equivalent chain length. Restricted chain mobility due to covalent bonding of the sulfur head group to the gold surface does not affect the cooperativity of the transition in terms of the transition temperature and enthalpy. Local chain ordering and dynamics in the deuterated C18S/Au nanoparticles have been probed using variable-temperature solid-state deuterium NMR spectroscopy and transmission FT-IR spectroscopy. The temperature dependence of the symmetric CD2 stretching frequency has confirmed that the DSC-detected phase transition involves a thermally-induced change from a predominantly all-trans conformation to a chain disordered 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 originates 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 show that the disorder does not extend to the tethered sulfur head group. Deuterium NMR spectroscopy specifically establishes that chain melting arises from an increased frequency of gauche bonds in the alkanethiolate chains. The H-2 NMR line shapes further indicate that the tethered alkanethiolate chains are undergoing rapid trans-gauche bond isomerization and axial chain rotation.