Selective immobilization of functional organic molecules onto a microtemplate fabricated using an amino-terminated self-assembled monolayer

摘要

Micropatterned amino-terminated self-assembled monolayers (SAMs) were applied as templates to promote selective chemical reactions in minute spatial scale. The surface charge properties of the SAMs were investigated through zeta-potential measurements conducted in solutions of various pH values. These SAMs were then utilized in order to selectively immobilize carboxylate-modified fluorescence spheres (CM-FluoSpheres) on the sample surface. Each SAM was uniformly formed onto a clean Si(100) substrate covered with a thin oxide layer (SiO_2) by chemical vapor deposition using n-(6-aminohexyl)aminopropyltrimethoxysilane [AHAPS:H_2N(CH_2)_6NH(CH_2)_3Si(OCH_3)_3] as a precursor. Through a photolithographic technique employing vacuum ultraviolet light at 172 nm, microstructures consisting of AHAPS-SAM and SiO_2 were fabricated. The micropatterned AHAPS-SAM/SiO_2 samples were then immersed in a solution dispersed with CM-FluoSpheres. The solution was adjusted to pH = 2, 6 or 8. At pH = 8, the CM-FluoSpheres did not adsorb on the micropatterned AHAPS-SAM/SiO_2. At pH = 2 and 6, the CM-FluoSpheres adsorbed specifically on the regions covered with AHAPS-SAM. In particular, the adsorption behavior at pH = 6 was highly selective. These results were predictable considering the isoelectric points of AHAPS-SAM and SiO_2, that is, pH = 7.5 and 2.0, respectively. At pH = 6, the SiO_2 surface was negatively charged while the AHAPS-SAM surface was positive. Since the CM-FluoSpheres, which are terminated with carboxyl groups, were negatively charged at this pH value, the SiO_2 and AHAPS-SAM surfaces had repulsive and attractive interactions, respectively, with the CM-FluoSpheres. These opposite electrostatic interactions were the origin of the complete selectivity achieved in the adsorption.