## 2 Nanomaterials as Electrode Modifier to Boost the Biosensor Analytical Features  
The exploitation of nanomaterials in the different disciplines has allowed for the development of different products with improved features, rending the nanodimension one of the key features in the research and industrial sectors [6]. Searching in Google Scholar, the articles with the word \"nanomaterial\" and \"electrochemical biosensor\" in December 2020, more than 11,000 documents have been reported, demonstrating that the use of nanomaterials in the development of electrochemical biosensors is one of the determining research topics in the sensing field at the worldwide level. In the sector of electrochemical biosensors for chemical warfare agents, the presence of nanomaterials has augmented the smart configurations of the devices with the development of analytical tools characterized by improved analytical features in terms of sensitivity, selectivity, and stability. One of the first biosensors based on the use of nanomaterials for the detection of nerve agents was reported by [PERSON] et al. in 2005 [7], in which carbon nanotubes were used as a working electrode nanomodifier for the sensitive detection of acetylcholinesterase-based by-product. This biosensor was constructed by modifying the working electrode surface via drop casting with a dispersion of multiwalled carbon nanotubes at a concentration of 2 mg/mL in N,N-dimethylformamide. In detail, 15 mL of this solution were dropped onto the electrode surface and then the modified electrode was kept in an oven at 80 \({}^{\circ}\)C for 30 min under vacuum to allow for the evaporation of the solvent. After that, the acetylcholinesterase-cholinesteraseerase was dropped on the carbon nanotubes-modified electrode to immobilize the enzyme by physical adsorption. Rinsing twice with phosphate buffer allowed for the removal of the loosely adsorbed acetylcholinesterase-cholinesterase molecules, leaving only the acetylcholinesterase-cholinesterase molecules strictly bonded to carbon nanotubes. Being an amperometric monoenzymatic biosensor, acetylcholinesterase-hiocholine was used as a substrate with the production of thiocholine as the enzymatic by-product. The amperometric detection of thiols is characterized by high voltage and the fouling problem, and indeed the detection of thiocholine at bare electrode happened at +600 mV vs. Ag pseudoreference, while the presence of carbon nanotubes allowed for detection at +200 mV with a stable signal, thus avoiding the fouling problem (Figure 2A). This biosensor was tested with paraoxon, used as model organophosphate nerve agents, with a detection limit of 0.5 nM (0.145 ppb). The presence of carbon nanotubes demonstrated that this nanomaterial was used to immobilize the enzyme as well as to detect the enzymatic product at a low applied potential, thus avoiding the fouling issue. Carbon black is a nanomaterial widely used in the detection of analyte in the liquid phase since 2020 [8], demonstrating its outstanding features, such as the improved electron transfer [9; 10; 11], resistance to the fouling [12; 13], easiness to prepare a stable dispersion [14], compatibility with the biocomponent [15; 16], and cost-effectiveness (ca. 1 euro/kg). Carbon black was used to develop a biosensor for the detection of the same model compound (i.e., paraoxon) reported above, using a printed sensor modified with carbon black by drop casting 6 mL of carbon black dispersion 1 mg/mL prepared in a solution of water/N,N-dimethylformamide in a ratio of 1:1 (v/v). The enzyme butyrylcholinesteraseerase was selected to be immobilized onto the working electrode surface using a chemical immobilization through glutaraldehyde and Nafion(Figure 2B). The choice of butyrylcholinesteraseerase is due to improve storage stability of this biosensor in respect to the acetylcholinesterase-based one, being the biosensor able to detect the organophosphates even when stored at RT in a dry condition for several weeks [17].