Laser desorption Fourier transform mass spectrometry exchange studies of air-oxidized alkanethiol self-assembled monolayers on gold

by Scott, Jill R.; Baker, Laura S.; Everett, William Russell; Wilkins, Charles L.; Fritsch, Ingrid

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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 (C(12)H(2)5S(-), 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 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 O-2 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.

Journal
Analytical Chemistry
Volume
69
Issue
14
Year
1997
Start Page
2636
ISBN/ISSN
0003-2700
DOI
10.1021/ac9609642