Self-assembled monolayers (SAMs) of thiolates, disulfides (RSSR+), and sulfides were studied on Au by N2 laser desorption followed by vacuum ultraviolet (VUV) (118-nm) photoionization of secondary neutrals in a time-of-flight mass spectrometer. Dimers (RSSR+) dominated the photoionization mass spectrum from all chain lengths of alkanethiolates and disulfides studied. Nonmethyl-terminated alkanethiolates with X = (OH and COOH) were detected as dimers without loss of the terminal group. Phenyl-SAMs with X = (H, OH, OCH3, Cl, and NO2) were detected as both monomers and dimers. Thiocholesterol SAMs were detected solely as monomers. The data suggest that dimerization occurs as a result of the recombination of surface thiolates during desorption. The alkane sulfides were detected intact, but with additional monomer and dimer species present in the spectra. The appearance of dimers is not a strong function of adsorbate structure or ordering and therefore cannot be taken as evidence for or against the recently proposed model of thiolate dimers on Au surfaces. Two receptor adsorbates, resorcin[4]arene tetrasulfide and β-cyclodextrin sulfide were examined by two-laser mass spectrometry (L2MS), but only the former gave identifiable high mass peaks. Mixed thiolate and disulfide monolayers generated both pure and mixed dimers, providing information on nearest neighbor interactions. The mixed disulfide results indicate there is a common adsorption state for thiolates and disulfides. The laser desorption and VUV photoionization cross sections for these various organosulfur SAMs were found to be similar. L2MS with VUV photoionization was nonselective in its detection of these organosulfur species and produced mass spectra with little fragmentation.
Biotin and biotinylated self-assembled monolayers (SAMs) on gold have been investigated using time-of-flight secondary ion mass spectrometry, direct laser desorption, laser desorption with 193 nm photoionization of ion- and laser-desorbed species, and laser desorption with vacuum ultraviolet (VUV, 118 nm) photoionization. Our results indicate that direct laser desorption and laser desorption combined with 193 nm multiphoton ionization can detect a chromophoric molecule like biotin that is covalently bound to a SAM. However, secondary ion mass spectra were dominated by fragmentation, and ion desorption/193 nm photoionization detected no species related to biotin. The dominant features of the laser desorption/VUV mass spectra were neat and Au-complexed dimers of intact and fragmented biotinylated SAM molecules. Multiphoton and single-photon ionization of laser-desorbed neutrals from biotinylated SAMs both led to the production of ions useful for chemical analysis of the monolayer. Multiphoton ionization with ultraviolet radiation was experimentally less challenging but required a chromophore for ionization and resulted in significant fragmentation of the adsorbate. Single-photon ionization with VUV radiation was experimentally more challenging but did not require a chromophore and led to less fragmentation. X-ray photoelectron spectra indicated that the biotinylated SAM formed a disordered, 40-60 Å thick monolayer on Au. Additionally, projection photolithography with a Schwarzschild microscope was used to pattern the biotinylated SAM surface and laser desorption/photoionization was used to detect biotinylated adsorbates from the ∼10 μm sized pattern.
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