The novel sensitive sensors, based on plastic membranes doped with either activated charcoal (AC) or graphene oxide (GO) as the La(III) sensitive electroactive material, were introduced. The AC-based sensor had an optimum Nernstian slope of 19.4 mV/decade of activity over a wide linear concentration range of 5 × 10−2–5 × 10−6 M with a low detection limit of 3.2 × 10−6 M. GO-based sensor (S7) exhibited a Nernstian slope of 18.7 mV/decade with a working range of 5 × 10−2–1.0 × 10−4 M and detection limit 3 × 10−5 M. The Maxwell-Wanger-Silars interfacial polarization explained the high values of ε' at low frequencies of the different membranes. Adding activated charcoal to the plasticized PVC increased the electric modulus (M') value, whereas it decreased by increasing the graphene oxide addition. The optimum concentration of doped filler in the membrane of PVC/DOP to improve its ac conductivity was observed for PVC/DOP/activated charcoal 1/2/0.033 (w/w).