In bioanalytical applications, many coating strategies have been established for so-called "blocking" of the surfaces. However, most of the procedures developed so far require additional processing steps for surface blocking and small variations in the blocking efficiency result in increased background noise, which lowers the overall sensitivity of an assay. In this study, we demonstrate the preparation of a bioanalytical surface with a thin film of a photo-cross-linkable copolymer that is transformed photochemically into a surface-attached hydrogel network. The presented coating is directly applicable to various plastic substrates that are used for bioassays without the need for any prior surface modification. Such a strategy allows facile one-step immobilization of biomolecules for bioanalysis and protein-repellent properties for avoiding unspecific adsorption of analyte molecules during the assay. The protein adsorption behavior of the hydrogel-coated and blank surfaces is measured by SPR with human serum and physisorption of labeled detection antibodies. We show that the hydrogel surfaces used lower unspecific background signals and background noise and thus increase the sensitivities of the microarray immunoassays.