A dielectric-barrier discharge (DBD) in an argon–water mixture is applied to plasma pretreatment (PP) of amorphous silica for subsequent vapor-phase silanization (VS) in the same reactor. Comparison of amino-silanization of silica fiber-optic biosensor probes using a PP/VS sequence with strategies involving wet-chemical pretreatment or silanization shows a considerable improvement in reproducibility by the completely dry process. Practical applicability is demonstrated by an immunoassay with human immunoglobulin G as analyte. Thanks to the advantages of shorter processing time, avoidance of washing steps, and improved reproducibility, PP/VS is highly promising for industrial use.