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 (TiO ₂ ), tungsten trioxide (WO ₃ ), copper oxide (CuO and Cu ₂ O), tin oxide (SnO and SnO ₂ ) and vanadium oxide (VO ₂ and V ₂ O ₅ ), 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.

Here we present for the first time a TiO2/Cu2O all-oxide heterojunction solar cell entirely produced by spray pyrolysis onto fluorine doped tin oxide (FTO) covered glass substrates, using silver as a back contact. A combinatorial approach was chosen to investigate the impact of the TiO2 window layer and the Cu2O light absorber thicknesses. We observe an open circuit voltage up to 350mV and a short circuit current density which is strongly dependent of the Cu2O thickness, reaching a maximum of ~0.4mA/cm2. Optical investigation reveals that a thickness of 300nm spray pyrolysis deposited Cu2O is sufficient to absorb most photons with an energy above the symmetry allowed optical transition of 2.5eV, indicating that the low current densities are caused by strong recombination in the absorber that consists of small Cu2O grains

A novel generation of flexible opto-electronic smart applications is now emerging, incorporating photovoltaic and sensing devices driven by the desire to extend and integrate such technologies into a broad range of low cost and disposable consumer products of our everyday life and as a tool to bring together the digital and physical worlds. Several flexible polymeric materials are now under investigation to be used as mechanical supports for such applications. Among them, cellulose, the most abundant organic polymer on the Earth, commonly used in the form of paper, has attracted much research interest due to the advantages of being recyclable, flexible, lightweight, biocompatible and extremely low-cost, when compared to other materials. Cellulose substrates can be found in many forms, from the traditional micro-cellulose paper used for writing, printing and food/beverage packaging (e.g. liquid packaging cardboard), to the nano-cellulose paper which has distinct structural, optical, thermal and mechanical properties that can be tailored to its end use. The present article reviews the state-of-the-art related to the integration and optimization of photonic structures and light harvesting technologies on paper-based platforms, for applications such as Surface Enhanced Raman Scattering (SERS), supporting remarkable 10⁷ signal enhancement, and photovoltaic solar cells reaching ∼5% efficiency, for power supply in standalone applications. Such paper-supported technologies are now possible due to innovative coatings that functionalize the paper surfaces, together with advanced light management solutions (e.g. wave-optical light trapping structures and NIR-to-visible up-converters). These breakthroughs open the way for an innovative class of disposable opto-electronic products that can find widespread use and bring important added value to existing commercial products. By making these devices ubiquitous, flexible and conformable to any object or surface, will also allow them to become part of the core of the Internet of Things (IoT) revolution, which demands systems’ mobility and self-powering functionalities to satisfy the requirements of comfort and healthcare of the users.