Unfortunately, the pesticides pollute a large palette from environment: plants, organisms, soil and water. Using monitoring tools, like chemical biosensors, the use of pesticides can be put under control. Microelectronics offers the convenient transducers. Borrowing micro-technological processes, the enzymatic biosensors can be easily integrated onto the Silicon wafers. This paper proposes a biosensor for paraoxon pesticide detection, as a small piece from a decontamination soil technology plan. The paper reveals the pesticide detection principle, by paraoxon hydrolysis assisted by the Acetyl-cholinesterase enzyme, as key receptor. The enzyme is entrapped on a porous thin layer by adsorption. An advantageous method of the porous intermediate material anchored onto the Si-substrate, converting p-type Si in Si-porous by anodization, is described. Then, some technological steps, with tests and microscopy analysis are presented. Finally, the preliminary tests of the developed biosensor with the AcHE enzyme immobilized onto the Si-porous layer, are discussed.