**1. Introduction**

The chemical sensor market requires high performance on all 4S parameters: sensitivity, selectivity, speed, and stability. Moreover, the prevalence of the internet of things (IoT) and wireless sensor networks, as the technology of choice for pervasive real-time monitoring, demands miniaturized sensors with very low power and low cost [1]. Over the past few decades, metal oxides semiconductors (MOS) have been intensively used in chemical sensors for a wide variety of applications. On one hand, MOS can be produced by low-cost wet-chemical synthesis routes from earth-abundant low-cost precursors and are, in general, non-toxic. On the other hand, MOS exhibit unique electronic, chemical and physical properties that are often sensitive to changes in their environment, making them suitable for

chemical sensing [2–4]. Finally, the MOS sensors are relatively easy to be miniaturized and integrated into microfabrication processes. In this regard, the microelectro-mechanical system (MEMS) technology has enabled the miniaturization of chemical sensors using MOS by allowing the implementation of heating and sensing elements by thin-film fabrication [5, 6]. Although, the MOS microsensors fabricated with MEMS technology are already on the market due to their balance in performance and cost, they still suffer from high power consumption and lack of selectivity. The former is connected with the high working temperatures (>200°C), which are necessary to activate sensing mechanisms such as redox reactions, and the latter refers to the non-specificity of MOS surfaces to gaseous analytes. Several strategies have been devised to reduce the power and control the response of the MOS sensors towards specific gases by structural and/or chemical modification of the bulk and/or the surface of the sensing material [7]. Also, a most widespread solution known as the electronic nose uses a broadly responsive array of sensors to generate complex multi-dimensional measurement data in combination with pattern recognition software to interpret the resulting signal pattern [8]. Another strategy relies on the operation of sensors in dynamic mode; this implies the active variation of control parameters such as the working temperature or bias voltage by the sensor electronics, allowing application-specific optimization of the sensor performance or the target application [9, 10]. Finally, UV light-activation has been exploited for the room-temperature operation of the MOS sensors with improved detection capabilities such as sensitivity, selectivity, or response/recovery time [11].

In this context, nanotechnology has emerged as a very promising route to overcome the current drawbacks of the MOS chemical sensors. Enormous research efforts have been devoted to designing and developing MOS nanomaterials for chemical sensing with the ultimate goal of achieving the sensitivity and selectivity levels required for real-world applications while operating the sensors at low temperature, ideally at room temperature [12]. All this, to pave the way for a new generation of chemical sensors consuming very low or zero power using nanostructured MOS layers mounted on cheap, abundant, and easy-to-process substrate materials such as polymers, paper, or fabrics. Then, the fabrication costs of the MOS sensors could be reduced by using state-of-the-art technologies to pattern the electrodes and the MOS nanostructures on top of the substrate, making it possible to produce chemical sensors on a large scale [13]. This would facilitate the introduction of chemical sensors based on MOS in growing markets such as smartphones and wearable devices.

Nanomaterials of all dimensionalities (0D, 1D, 2D, and 3D) and diversity of MOS such as TiO2, ZnO, SnO2, CuO, NiO and many others are being investigated for high-performance detection of gases, chemicals and biomolecules [14–20], mainly for applications in the fields of environmental monitoring, healthcare and health diagnostics [21, 22]. Nanomaterials serve as building blocks for the assembly of nanostructured layers with high surface area and porosity, leading to more sensitive and faster chemical sensors. Furthermore, novel synthesis techniques allow fine-tuning of the composition, morphology, and structure of the nanomaterials' surface, which, together with possible alterations in the electronic and chemical properties at the nanoscale, could contribute to enhancing the chemical affinity of the MOS nanostructures for specific species [23, 24]. Recently one-dimensional (1D) MOS nanostructures have gained increased attention for chemical sensing because they are more applicable to nanoelectronics and nanodevices due to their high-surface-area-to-volume ratio 1D morphology [25, 26]; this means that a significant fraction of the atoms is surface atoms that participate actively in surface reactions. In 1D nanostructures the width and thickness are confined to the nanoscale, while the length spans from the micrometric to the millimetric range,

#### *One-Dimensional Metal Oxide Nanostructures for Chemical Sensors DOI: http://dx.doi.org/10.5772/intechopen.101749*

allowing 1D nanostructures to contact the microscopic and macroscopic world for many physical measurements.

This chapter surveys the latest achievements in the design and development of 1D MOS nanostructures for chemical sensing. The focus lies on conductometric sensors, specifically resistive and field-effect-transistor-based (FET) sensors, where MOS finds the broadest application. Also, this survey is limited to the 1D nanostructures that have demonstrated the greatest potential for use in conductometric sensors, namely nanowires and nanofibers. Other 1D nanostructures such as nanorods, nanotubes, nanobelts, nanoribbons, or nanoneedles have also been investigated, but to a lesser extent. The techniques for the synthesis of nanowires and nanofibers based on MOS are presented and discussed in terms of their complexity, the potential for integrating the 1D MOS nanostructures into sensor architectures and scalability. Finally, the chapter summarizes the challenges ahead and the prospects for progress in this field.
