Stepwise formation approach to improve ellipsometric biosensor response.

In this article we report a technique for improved response of ellipsometric sensors by means of self-assembling molecules and gold nanoparticles (AuNPs). First, we have examined the effects of dipping time and solution concentration on formation of 3-aminopropyltriethoxysilane molecules on glass surfaces. All of the surfaces were characterized by atomic force microscopy, imaging ellipsometry, and contact angle goniometer. Studies performed with ellipsometer showed that monolayers with a thickness of about 1.1 nm were formed when the dipping time was about 2 hours, whereas some disordered aggregates were observed over longer time periods. After the amino-terminated surfaces were obtained, they were modified by AuNPs having a nominal size of 40 nm on which 11-mercapto-undecanoic acid (MUA) was immobilized via thiol groups. Immobilization conditions of MUA were optimized by localized plasmon resonance. A nulling/imaging ellipsometry system with flow cell arrangement was used to monitor the biointeractions on the sensor surface. Ellipsometric data in terms of relative change in Delta (Delta) was recorded as sensor signal during the interaction with bovine serum albumin. To compare the effect of the AuNPs on sensor response, another sensor surface (without AuNPs) was prepared with 3-mercaptopropyltrimethoxysilane and MUA via thiol/disulfide exchange reaction. A significantly enhanced response was observed for the sensor having AuNPs as compared with the other sensor.