**5. Summary and outlook**

This chapter provides a general overview of the status and recent advances in developing chemical sensors based on 1D MOS nanostructures. The contents focus on the most frequent strategies and methods used to achieve higher sensing performances to cope with present and future applications, which ultimately drive innovation in chemical sensors. The applications usually impose requirements in terms of capabilities and efficiency of chemical detection, miniaturization, fabrication costs, power consumption, sensor stability, and lifetime. The combination of these needs poses difficult challenges to a diversity of enabling technologies encompassing materials synthesis, nanotechnology, nano- and micro-fabrication, and printing technologies, amongst others. In this context, the survey covers the state-of-the-art and advances of the main synthetic methods to produce 1D sensitive materials used in chemical sensors, particularly, nanowires and nanofibers based on MOS. It also tackles the effect of incorporating second-phase materials to bring improved and/ or new chemical sensing attributes to single-phase 1D MOS nanostructures and the routes to form this type of heteronanostructures. Finally, it discusses the most common chemical sensing architectures to enable the response of 1D nanostructures via resistive or FET measurements, as well as the techniques used to assemble the nanostructures onto these sensor platforms (of resistive and FET type).

The vast reports and prospects in chemical sensors indicate that the trends of these devices are directed towards further miniaturization and operation at the minimum power consumption. The last will be achieved mainly by reducing the operating temperature (at which sensitive MOS materials are stimulated) to levels close to room temperature. Improvements on traditional functional parameters such as the sensitivity, stability, speed of response, and selectivity to applicationdependent target analytes are also expected. All these improvements may ultimately come true by gaining a better understanding of the synergistic sensing effects in 1D heteronanostructures composed of MOS, metals and/or 2D nanomaterials such as graphene, TMDC, or MXenes. So that these combinations can be tailored more precisely shortening the try and error steps currently employed when tuning sensitive heteronanostructures. Certainly, this knowledge needs to be developed in parallel to robust and reproducible routes to synthesize 1D MOS heteronanostructures. The new synthetic routes must be optimized to enable the high dispersion, homogeneous distribution, and maximal interfacial area between the MOS and the second-phase constituents, with the minimum penalty in terms of sustainability, cost-effectiveness, and scalability. One-step routes that allow to grow, form, or assemble 1D MOS nanostructures directly onto the transducer platforms are pursued in the first place, followed by the techniques that allow the transfer of pre-formed 1D MOS nanostructures onto the transducer platforms with good control over the nanostructure orientation, alignment, and electrical contacts. When applied to the fabrication of chemical sensors, both types of techniques need to ensure the localized integration of 1D MOS nanostructures over the active area of the sensor device. This means that direct and transfer methods for the integration of 1D nanostructures must be adapted to operate efficiently either, in small sensitive areas, in agreement with the materials used as platforms, for instance, silicon-based MEMS or temperature-sensitive substrates (polymers, textiles, etc.). Therefore, the integration methods must also ensure control in the sub-millimeter range, optimal connectivity and conductivity between the nanostructures bridging the interdigitated electrodes, and strong adhesion of the nanostructures to the transducer platform.

Traditional synthetic methods based on nucleation and growth processes, such as hydrothermal synthesis and chemical vapor deposition, must be exploited to allow one-step integration of 1D MOS nanostructures with transducer platforms. These techniques are appropriate for direct integration, particularly with siliconbased platforms. However, their potential is not fully exploited as most of the works on chemical sensors fabrication turn to a removal and re-deposition approach for integrating 1D MOS nanostructures synthetized by these techniques.

The electrohydrodynamic techniques emerge as a promising alternative for integrating 1D MOS nanostructures onto chemiresistive and FET platforms due to its high-precision for printing areas of less than 0.1 mm<sup>2</sup> (e-jet printing) or its highresolution to write directly (on electrodes and substrate patterns) lines of less than 100 nm in width. The direct printing of 1D MOS nanostructures may improve the reproducibility of chemical sensors. The application of electrohydrodynamic techniques to manufacture chemical sensors using MOS is still at an early stage. Much effort needs to be done to optimize the composition and properties of the dispersion and polymer solutions (inks), nozzle geometry and dimensions, and process conditions, before these techniques, become a rapid and cost-effective tool for large-scale fabrication of chemical sensors based on MOS. However, their availability in the short term could push forward the application of 1D nanostructures and emerging flexible substrates for chemical sensors.
