MODIFICATION OF QUARTZ SURFACES VIA THIOL DISULFIDE INTERCHANGE

This paper focuses on procedures for silica modification by (3-mercaptopropyl)trimethoxysilane (MPS) studied by attenuated total reflection Fourier transform infrared (ATR-FTIR) and X-ray photoelectron (XPS) spectroscopies, along with energy dispersive X-ray analysis (EDAX). Emphasis is given to the accessibility of the surface-attached thiol groups and means for enhancing this accessibility. From the ATR-FTIR studies, the surface coverage of MPS (silanization A) and protected-MPS (silanization B) appears to reach a steady state in about 17 min. The elemental composition given by XPS verifies both the presence of MPS on the surface and the stability of the conjugate in alkaline solutions. Narrow region (20 eV) scans of the sulfur 2p orbital of the surface-attached MPS reveal the existence of an oxidized form of this element for silanization A. The possibility of interactions by the thiol group with the surface or other parts of the silane molecule is suggested by XPS. An alternative way of producing thiol-containing quartz surfaces is proposed where the mercaptosilane is initially protected by reacting its thiol group with 2,2'-dithiodipyridine prior to silanization and subsequently reduced to yield the free thiol group. When modified in this way, the surface shows no evidence of sulfur in the high oxidation state observed for the unprotected silane. By silver-staining the product of silanization B, it is possible to use both the XPS and SEM/EDAX techniques to assess the fraction of thiol groups which is accessible to a hydrophilic probe. The two analyses exhibit sulfursilver ratios (S:Ag) which converge to unity for decreasing concentrations of silane. Correlating the different probing depths of the two techniques along with the observed S:Ag ratios indicates that monolayer coverage appears to form in the presence of low concentrations of MPS.