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 species, which can be completely removed upon subsequent soaking in a solution of C12H25SH, resulting in a SAM of pure dodecanethiolate by FTMS analysis. Negligible exchange occurs when nonoxidized C12H25SH SAMs are soaked in decanethiol solution for 30 min. From these observations, we draw several conclusions. First, the results refute suspicions that sulfonates form in a laser-induced reaction of thiolates with O2 absorbed in the SAM during air exposure. Second, ionization efficiency of thiolates and sulfonates during LD-FTMS analysis is approximately equal. Third, previous literature reports of exchange experiments should be carefully scrutinized for possible evidence of the effects of air oxidation.

Comparative electrochemical study of self-assembled monolayers of 2-mercaptobenzoxazole, 2-mercaptobenzothiazole, and 2-mercaptobenzimidazole formed on polycrystalline gold electrode

“…Horn et al [50] performed a detailed FT-IR study on gold alkanthiolate monolayers as a function of storage time in air and showed that these SAMs were stable against oxidation at least for 20 days. Fritsch et al [51] used laser desorption mass spectrometry and reported possible oxidation of adsorbed alkanthiolate on Au to sulfonates, but this oxidation may have been triggered by laser irradiation used in their studies. Rieley et al [52] studied photo-oxidation of Au SAMs in the presence of O 2 by XPS, SIMS, and NEXAFS and recommended to keep studying monolayers either in dark or under anaerobic conditions.…”

Dendrimer-Encapsulated Metals and Semiconductors: Synthesis, Characterization, and Applications

“…For example, self-assembled monolayers (SAMs) prepared from small molecules are of widespread interest because of their potential applications to corrosion passivation [37,38], lithography [39,40], biochemical and chemical sensing [41,42], and adhesion [43,44]. However, for some applications SAMs prepared from molecules dominated by simple alkyl chains have significant disadvantages; for example, strictly two-dimensional surfaces and limited stability [45][46][47][48][49][50] arising from monopodal surface attachment. Clearly, SAMs prepared from dendrimers, which have a well-developed three-dimensional structure and a large number of potential surface attachment points per molecule, should exhibit improved substrate adhesion, stability, and other properties associated with their threedimensional structure.…”

Exchange versus intercalation of n-dodecanethiol monolayers on copper in the presence of n-dodecaneselenol and vice versa

“…Others have pointed out that thiols can displace other alkanethiolates SAMs from the surface of gold [58][59][60][61][62][63][64][65][66][67][68], silver [58] and platinum [66]. Some groups took advantage from these properties to make UV photopatterning of alkanethiolate SAMs [69][70][71][72].…”