Flexible physical sensors made from Paper substrates integrated with zinc oxide nanostructures

Paper-based physical sensors represent an emerging research direction in the field of flexible sensors, which offers a low-cost alternative to current silicon-based sensors. The ultralow cost and excellent biodegradability of paper substrates contribute to the major advantages of paper-based sensors. To enhance the sensor performance, a variety of functional nanomaterials have been utilized for developing paper-based physical sensors, among which zinc oxide (ZnO) nanostructures (e.g. nanoparticles, nanowires, and nanotrees) are popular choices because of their multiple physical sensing modalities and ease of synthesis on paper. This article reviews the recent advances of paperbased physical sensors integrating zinc oxide nanostructures. First, we summarize the methods for synthesizing ZnO nanostructures on paper, with a focus on the low-cost facile hydrothermal approach. We then discuss the physical properties (e.g. piezoelectricity, piezotronics, and ultraviolet (UV) sensitivity) of ZnO nanostructures that have been used for physical sensing applications. We review the representative designs of paper-based ZnO physical sensors and their applications such as nanogenerators, strain sensors, touch pads, and UV sensors. Finally, we conclude the current progress, and envision the future trends and research opportunities.