###### Abstract  
The smart, rapid, and customizable detection of chemical warfare agents is a huge issue for taking the proper countermeasures in a timely fashion. The printing techniques have established the main pillar to develop miniaturized electrochemical biosensors for onsite and fast detection of nerve and mustard agents, allowing for a lab on a chip in the chemical warfare agent sector. In the fast growth of novel technologies, the combination of miniaturized electrochemical biosensors with flexible electronics allowed for the delivery of useful wearable sensors capable of fast detection of chemical warfare agents. The wearable microneedle sensor array for minimally invasive continuous electrochemical detection of organophosphorus nerve agents, as well as the wearable paper-based origami functionalized with nanomaterials for mustard agents in the gas phase, represent two examples of the forefront devices developed in the chemical warfare agent detection field. This review will highlight the most promising electrochemical biosensors developed by exploiting nanomaterials and cross-cutting technologies for the fabrication of smart and sensitive electrochemical biosensors for the detection of chemical warfare agents. screen-printing; paper-based sensor; wearable sensors; nerve agents; mustard agents +
Footnote †: journal: 13 January 2021  
by the measurement of residual enzymatic activity. For the detection of nerve agents, two types of biosensors are mainly reported in the literature (Figure 1). The one most used relies on the use of cholinesterase as biocomponent, exploiting the capability of nerve agents to inhibit acetyl- and butyrylcholinesterase for a highly sensitive measurement. However, these biosensors are not specific because they generally detect organophosphorus compounds, working like a \"family doctor\" [3]. The cholinesterase biosensors are characterized by different degrees of sensitivity towards organophosphorus pesticides and nerve agents, in the function of inhibition properties. Furthermore, the inhibitive biosensors require an incubation step needed for a sensitive measure. This analytical tool can only be used once, unless a step with a reactivator compound is carried out after the nerve agent is measured [4]. The other approach is based on the use of organophosphate hydrolase enzyme able to hydrolyzed the organophosphorus compounds, thus detecting a nerve agent as the enzymatic substrate. This type of biosensor does not require a time of incubation, allowing for a faster measure than the one using a cholinesterase biosensor, but it is characterized by a lower sensitivity. For mustard agent detection, the most recent biosensors developed rely on the monitoring of inhibition using choline oxidase as the enzyme. For a reliable, cost-effective, easy, and on-site measurement, the electrochemical transducers are among the most suitable candidates, being characterized by high sensitivity and capability to work in complex matrices. Electrochemical detection in the last years has undergone a growing period due to the exploitation of cross-cutting technologies, as reported by Escarpa \"With the appearance of advanced approaches such as screen-printed technology, biosensors, microchips and nanotechnology, among others, electroanalysis is undergoing a true Renaissance\" [5]. Indeed, printed technologies, also on paper, combined with nanomaterials and flexible electronics have boosted the development of devices with unprecedented features, namely high sensitivity, wireless connection, reagent-free measurement, and foldable sensors aiming to develop smart devices in different fields, including the defence one. The defence sector requires devices that are able to detect the chemical warfare agents fast, on site, and by easy procedures, to take the correct countermeasures in a timely fashion; thus, the electrochemical biosensors are useful and highly attractive analytical tools, with all features requiring the defence field.