We demonstrate an integrated optical Mach–Zehnder interferometer for sensing purposes with a surface functionalized by thiols via an additional gold layer. Therefore, the oxidic silicon oxynitride waveguide surface was first covered by a thiol terminated silane self-assembled monolayer. An ultrathin gold layer was then deposited onto the “active” sulfur surface by OMCVD. This gold layer was finally functionalized by a biotinylated thiol in order to specifically bind streptavidin and a biotinylated antibody. Unspecific binding of streptavidin was tested also.