This article reports on a concept for a novel, compact fiber-optic sensor for the detection of pH-dependent swelling in a thin hydrogel coating. The sensor deploys a long-period fiber grating (LPG) in order to achieve highly sensitive surface plasmon resonance (SPR) of a cladding mode which interacts evanescently with a thin gold layer and a hydrogel coating. The pH-induced volume-phase transition of the hydrogel changes the attenuation of the cladding mode, which is detected using a Radio-over-Fiber (RoF) setup. This wireless read-out uses a modulated optical signal in order to yield a transmittable signal directly after opto-electric conversion by a photodiode.