Metal oxide nanostructures for sensor applications

Human health, environmental protection and safety are just a few examples of humankind's current main concerns, that drive the scientific community to develop sensors able to monitor precisely and provide alerts of possible harm in real time. Over the years, semiconductor metal oxide-based materials have been extensively employed as sensors in several applications. They are of particular interest at the nanometer scale, since it is widely known that a smaller crystallite size enhances a sensor's performance. Moreover, these materials are highly appealing as they can be produced by low-cost wet-chemical synthesis routes and are in general nontoxic, earth abundant and low cost. This manuscript extensively reviews the recent developments of nanostructured semiconductor metal oxide sensors ranging from gas to humidity sensors, including ultraviolet sensors and biosensors. Sensors based on zinc oxide (ZnO), titanium dioxide (TiO2), tungsten trioxide (WO3), copper oxide (CuO and Cu2O), tin oxide (SnO and SnO2) and vanadium oxide (VO2 and V2O5), either as nanoparticles or as continuous films or layers, are described. Their sensing properties are correlated to size, shape, presence of defects and doping elements, amongst other relevant parameters. Various techniques and methods of fabricating these materials are addressed. The review concludes with novel approaches for functionalization and future perspectives for sensor developments.