Towards high-performance, low-cost quartz sensors with high-density, well-separated, vertically aligned ZnO nanowires...

Resonant sensors with nanostructured surfaces have long been considered as an emergent platform for high-sensitivity transduction because of the potentially very large sensing areas. Nevertheless, until now only complex, time-consuming, expensive and sub-optimal fabrication procedures have been described; in fact, especially with reference to in-liquid applications, very few devices have been reported. Here, we first demonstrate that, by immersing standard, ultra-low-cost quartz resonators with un-polished silver electrodes in a conventional zinc nitrate/HMTA equimolar nutrient solution, the gentle contamination from the metallic package allows direct growth on the electrodes of arrays of high-density (up to 10 μm⁻²) and well-separated (no fusion at the roots) ZnO nanowires without any seed layer or thermal annealing. The combination of high-density and good separation is ideal for increasing the sensing area; moreover, this uniquely simple, single-step process is suitable for conventional, ultra-low-cost and high-frequency quartzes, and results in devices that are already packaged and ready to use. As an additional advantage, the process parameters can be effectively optimized by measuring the quartz admittance before and after growth. As a preliminary test, we show that the sensitivity to the liquid properties of high-frequency (i.e. high sensitivity) quartzes can be further increased by nearly one order of magnitude and thus show the highest ever reported frequency shifts of an admittance resonance in response to immersion in both ethanol and water.