Structure and Dynamics in Alkanethiolate Monolayers Self-Assembled on Gold Nanoparticles:  A DSC, FT-IR, and Deuterium NMR Study

Abstract

Gold nanoparticles, ca. 30 Å in diameter, have been derivatized with specifically deuterated (position 1 and positions 10 to 13) and perdeuterated (positions 2 to 18) octadecanethiols (C 18 SH). The phase behavior of the octadecanethiolate monolayers chemisorbed onto the colloidal gold surface was characterized by differential scanning calorimetry (DSC). The DSC thermograms show that the C 18 SH-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 C 18 S/Au nanoparticles have been probed using variable-temperature solid-state deuterium NMR spectroscopy and transmission FT-IR spectroscopy. The temperature dependence of the symmetric CD 2 stretching frequency has confirmed that the DSC-detected phase transition involves a thermallyinduced change from a predominantly all-trans conformation to a chain disordered state. A comparison of the thermal behavior of d 35 -C 18 S/Au and 10, 10,11,11,12,12,13,13-d 8 -C 18 S/Au shows that disordering originates in the chain terminus region and propagates toward the middle of the chain as the temperature increases. Studies of 1,1-d 2 -C 18 S/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 2 H NMR line shapes further indicate that the tethered alkanethiolate chains are undergoing rapid transgauche bond isomerization and axial chain rotation.

Copper Electrodeposition on Alkanethiolate Covered Gold Electrodes*

“…Above 200 the infrared probing of the méthylènes' scissors vibration suggests progressive unlocking of the alkyl twist (a rotation around either anS-CoraC-C bond) and development of gauche defects in the otherwise all-trans configured chains [39]. This behaviour compares well with that of self-assembled thiolates on gold nanoparticles [40] and that of melting bulk alkanes [41]. Heating above 300 as we used for this study can even affect the Au-S bond: boundaries between different ordered thiolate domains move or vanish, mass transport of the substrate occurs and finally leads to the healing of the typical substrate vacancies.…”

Explicit all-atom modeling of realistically sized ligand-capped nanocrystals

“…On the other hand, they make a more disputable point of the tendency of ligands to "bundle" together at low temperatures. Badia et al 15 claim to observe bundling for gold nanoparticles. Similar observations of bundling were made by Lane et al 16 when modeling spherical nanoparticles capped by alkanethiol ligands in water, decane, and in vacuum.…”

Dynamics of Alkyl Chains in Monolayer-Protected Au and Ag Clusters and Silver Thiolates: A Comprehensive Quasielastic Neutron Scattering Investigation

“…This result has been confirmed with FTIR and 2 H NMR studies. 44 The degree of conformational order depends on the particle size and the nature of the terminal group in addition to the chain length. Evolution of a peak at 31 ppm in the solid-state variabletemperature 13 C NMR spectra of C 14 and C 18 clusters shows a phase transition.…”