A light-addressable potentiometric sensor (LAPS) with ceramic samarium oxide (Sm2O3)-sensing membrane treated by nitrogen plasma immersion ion implantation (PIII) has been proposed for chloride ion detection. For the pure Sm2O3-LAPS, a potassium ion sensitivity of 39.21mV/pK is obtained. With the nitrogen PIII treatment on Sm2O3-sensing membrane, the N-O peak is observed by X-ray photoelectron spectroscopy, implying the formation of positive charges, (N-O)(+) and (N-O-N)(+), within the Sm2O3 film. The positive charges can attract the chloride ions to react with the surface sites of OH2+, achieving a superior chloride ion sensitivity of 36.17mV/pCl for the Sm2O3-LAPS with nitrogen PIII treatment. The LAPS structure with ceramic Sm2O3-sensing membrane can be used for future biosensing applications, especially for the potassium and chloride ions detection in serum.