Laser Desorption Fourier Transform Mass Spectrometry Exchange Studies of Air-Oxidized Alkanethiol Self-Assembled Monolayers on Gold

Abstract

Exchange of self-assembled monolayers (SAMs) of dodecanethiol (C12H25SH) on gold with decanethiol (C10H21SH) was detected with laser-desorption Fourier transform mass spectrometry (LD-FTMS) in the negative ion mode. The amount of adsorption of C10H21SH is dependent on the extent of air oxidation of the C12H25SH SAM. In this study, a partially air oxidized C12H25SH SAM with a 2.2:1.5:1 ratio of dodecanethiolate (C12H25S-, m/z 201) to the presumed dodecanesulfinate (C12H25SO2-, m/z 233) to dodecanesulfonate (C12H25SO3-, m/z 249) on the surface is examined. After the sample is soaked in an ethanol solution of C10H21SH for 30 min, the LD-FTMS spectrum shows that both the C12H25SO2- and the C21H25SO3- are gone, but a decanethiolate peak appears (C10H21S-, m/z 173) in the same ratio to C12H25S- (1.19) as the ratio of the combined oxidation products (1.16) prior to soaking. Subsequent air exposure (9 h) of this SAM composed of mixed thiolates gives a similar ratio (1.06) of the fully oxidized sulfonates of both ...