The present invention concerns an ion sensor based on differential measurement, that by means of at least two ion-sensitive field-effect transistors, compares the concentration of certain ions in a solution to be measured with the concentration of certain ions in a reference solution contained in a micro-reservoir with a micro-channel. To do this, the micro-reservoir and the micro-channel cover at least the gate of one of the ion-sensitive field-effect transistors and make up a unit partially filled with a porous material that covers the entirety of the aforementioned gate and at least the base of the micro-channel.