Adhesion of polymer films to oxidized silicon and its effect on performance of a conductometric pH sensor

Abstract

The use of a silane coupling agent, γ-methacryloxypropyl trimethoxysilane (γ-MPS), to promote adhesion of photolithographically patterned polymer hydrogels to oxidized silicon wafers and conductometric sensors has been examined. Poly-(hydroxyethyl methacrylate-co-dimethylamino ethylmethacrylate) (HEMA/DMA) polymer hydrogels are photolithographically patterned on wafers treated with γ-MPS. The wafers are immersed in buffered potassium chloride solutions of differing pH and subjected to a peel test of adhesion. Hydrogels tested in a high-pH (9.0) medium show considerably poorer adhesion than ones tested in lower-pH media (6.0 and 7.4). Hydrogels tested at elevated temperature (40°C) show comparable adhesion to those tested at room temperature (23°C). Adhesion of hydrogels that are cycled between pH 9.0 and a dry environment exhibit better adhesion than ones cycled between pH 9.0 and 6.0. This indicates that there is reformation of siloxane bonds between the polymer and the substrate. Since loss of siloxane bonds is the major determinant of sensor failure, it is possible to regenerate the performance of failed sensors by drying in a vacuum oven. These findings have implications in the design and construction of microfabricated sensors that utilize polymer hydrogel sensing layers. In recognizing the pH sensitivity of adhesion, it is possible to prevent sensor failure by restricting the pH of the testing environment to values that favor the maintenance of the siloxane bond.