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

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(1 0 0) substrate covered with a thin oxide layer (SiO2) by chemical vapor deposition using n-(6-aminohexyl)aminopropyltrimethoxysilane [AHAPS:H2N(CH2)6NH(CH2)3Si(OCH3)3] as a precursor. Through a photolithographic technique employing vacuum ultraviolet light at 172 nm, microstructures consisting of AHAPS-SAM and SiO2 were fabricated. The micropatterned AHAPS-SAM/SiO2 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/SiO2. 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 SiO2, that is, pH=7.5 and 2.0, respectively. At pH=6, the SiO2 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 SiO2 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.