Refractive index sensitive plasmonic sensors with nanostructured gold surfaces offer many advantages for continuous on-site monitoring of complex process liquids. In combination with stimulus-responsive hydrogels, they show potential for the selective detection of individual parameters (here: ethanol concentration in water) in the presence of simultaneously changing concentrations of other components. As first step towards the exploitation of this selectivity is to distinguish the ethanol-induced swelling effect in the polyacrylamide hydrogel in use from the refractive index change of the liquid to be analyzed as a result of concentration changes. This was achieved by using a hydrogel-functionalized measurement area and a non-functionalized reference area. The sensor concept presented here allows the determination of the ethanol concentration in aqueous solution between 40 and 60vol%. It is shown that the near-surface behavior cannot be directly inferred from the deswelling of the hydrogel volume, but that an individual calibration of the optical signal is necessary.