Bare Silica Opals for Real‐Time Humidity Sensing

The photonic properties of 3D colloidal crystals made of Stober silica spheres are shown to significantly depend on the relative humidity of the environment. The photonic bandgap of bare artificial opals formed by hydrophilic silica markedly varies in humid air along the entire range of water vapor concentration without the need for infiltration of functionalization. The optical changes are highly sensitive to humidity variation (especially in low-humidity range) and very fast response times of 60 ms, mainly ascribed to the absence of intermediate processes, the favorable bandgap characteristics (high intensity and sharp edges) and the high air accessibility to the open opal voids. Contrary to common approaches for photonic crystal sensors-seeking visual detection via large spectral shifts but having important shortcomings,-it is demonstrated that the well-defined photonic bandgap of bare silica opals allows, even for moderate shifts, outstanding sensing performance by proper monitoring with inexpensive equipment (no spectroscopic detection is needed). As a result, the rapid and reproducible photonic response enables accurate, real-time retrieval of the ambient humidity. The economical, one-step fabrication, and the efficient performance make silica artificial opals suitable for a new type of precise, low-cost, and real-time humidity sensors.