The use of a commercial, silver-coated, piezoelectric quartz crystal as a disposable, low cost and reliable immunosensor is presented. The protection of the silver electrode from undesirable oxidation was achieved by polystyrene or carboxy-poly(vinyl chloride) (PVC-COOH) modification. In addition to serving as protection for the electrode, polymer films provided a substrate for antibody immobilization by either physical adsorption or covalent linkage. Polystyrene modification showed an additional advantage of improvement of surface smoothness. The atomic force microscope (AFM) and scanning electron microscope (SEM) were used to evaluate the morphologies of polymer films obtained by dip or drop coating techniques. It was found that drop coating provided more significant improvement in surface smoothness than dip coating, and the resulting sensor surfaces were more suitable for in situ liquid phase assay. Although PVC-COOH-modified sensors were not suitable for liquid phase assay because of the high surface roughness, the covalent linkages (amide bonds) between antibodies and -COOH groups in the polymer film offered better sensor performance in ex situ assay in terms of a higher antibody binding capacity and better antigen detection sensitivity.