SURFACE MASS-SPECTROMETRY OF BIOTINYLATED SELF-ASSEMBLED MONOLAYERS

Biotin and biotinylated self-assembled monolayers (SAMs) on gold have been investigated using time-of-night 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 d irect laser desorption and laser desorption combined with 193 nn multi photon ionization can detect a chromophoric molecule like biotin that is covalently bound to a SAIM. However, secondary ion mass spectra wer e dominated by fragmentation, and ion desorption/193 mn photoionizatio n 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 sin gle-photon ionization of laser-desorbed neutrals from biotinylated SAM s both led to the production of ions useful for chemical analysis of t he monolayer, Multiphoton ionization with ultraviolet radiation was ex perimentally less challenging but required a chromophore for ionizatio n and resulted in significant fragmentation of the adsorbate, Single-p hoton ionization with VUV radiation was experimentally more challengin g but did not require a chromophore and led to less fragmentation. X-r ay photoelectron spectra indicated that the biotinylated SAM formed a disordered, 40-60 Angstrom thick monolayer on Au. Additionally, projec tion photolithography with a Schwanschild microscope was used to patte rn the biotinylated SAM surface and laser desorption/photoionization w as used to detect biotinylated adsorbates from the similar to 10 mu m sized pattern.