The novelty of these fabricated sensors was raised from that, MG was used for the first time as a cationic exchanger for an anionic drug such as SDM. The very high lipophilicity and remarkable stability of the proposed ion-pair (MG–SDM) suggest its use as a novel electroactive material in PVC membrane sensors. The developed membrane was used for dual determination of the cationic MG and anionic SDM, selectively. The electrodes were easily prepared with high sensitivity, wide dynamic range and remarkable long lifetime. High selectivity and rapid response make these electrodes suitable for measuring the concentration of MG and SDM in a wide variety of samples without the need for pretreatment steps and without significant interference from other anionic or cationic species present in the aquatic environment.