We report here on a new potentiometric biosensing principle for the detection of antibody-antigen interactions at the sensing membrane surface without the need to add a label or a reporter ion to the sample solution. This is accomplished by establishing a steady-state outward flux of a marker ion from the membrane into the contacting solution. The immunobinding event at the sensing surface retards the marker ion, which results in its accumulation at the membrane surface and hence in a potential response. The ion-selective membranes were surface-modified with an antibody against respiratory syncytial virus using click chemistry between biotin molecules functionalized with a triple bond and an azide group on the modified poly (vinyl chloride) group of the membrane. The bioassay sensor was then built up with streptavidin and subsequent biotinylated antibody. A quaternary ammonium ion served as the marker ion. The observed potential was found to be modulated by the presence of respiratory syncytial virus bound on the membrane surface. The sensing architecture was confirmed with quartz crystal microbalance studies, and stir effects confirmed the kinetic nature of the marker release from the membrane. The sensitivity of the model sensor was compared to that of a commercially available point-of-care test, with promising results.