**3. The work principle of the pesticide biosensor with nanoporous Si layer**

#### **3.1. Work principle**

The proposed pesticide biosensor works with acetylcholinesterase noted by AChE, with the code—EC 3.1.1.7. This receptor element is used to degrade agonists of the acetylcholine (AcH) neurohormone. In the living matter, AcH is present at neuromuscular junction and in the cholinergic nervous system, modulating the electrical pulse transmission at synaptic spaces, as other neurotransmitters [25]. The AChE has a very high catalytic affinity for acetylcholine and for its agonists as parasympathomimetic pesticides. This property opens the door of pesticide-selective detection by AChE-based enzyme biosensors [26].

The pesticides are intensively used in agriculture usually as organochlorines, carbamates, and organophosphate. Paraoxon belongs to the organophosphate class, being an oxon and the active metabolite of the parathion pesticide. Their working principle on the pests is based on the inhibition of AChE, allowing acetylcholine to transfer nerve impulses indefinitely and causing paralysis. Paraoxon is a novel generation of pesticide, which reacts as an inhibitor of AChE. Pesticides from this group act directly by stimulating the nicotinic receptors or indirectly by the inhibition of cholinesterase, as an acetylcholinesterase inhibitor, abbreviated as AChEI. Paraoxon is one of the most potent acetylcholinesterase inhibitor available in insecticide [27]. In water solvent, it stands for a high risk of poisoning for humans or animals, due to its simply absorption through teguments in contact with the contaminated water from environment. As pesticide, the parathion is dissolved in water and usually is applied by treatment. It is frequently sprayed to rice and fruits. The usual concentrations are 0.05 and 0.1% [28]. After the rain, the pesticide is accumulated in water and soil. This parathion degradation in time produces multiple water-soluble products.
