Natural Slab Photonic Crystals as Biogenic, Customizable Nanomaterial for Label-Free Detection.

Photonic band gap-based sensors can detect small variations in the refractive index of surrounding media, facilitating the precise detection of changes in their environment. In this proof-of-concept study, we demonstrate that biosilica produced by single-cell microalgae called diatoms can work as a photonic crystal slab sensor. We exploited the pseudo photonic bandgap (pseudo-PBG) produced by the highly periodic nanoscale morphology to detect the presence of relevant chemical elements in solutions. We demonstrate that the pseudo-PBG of the natural system can be calibrated for refractive index changes in the environment using standard liquids. Subsequently, we demonstrate that the platform enables precise detection of minute refractive index variations, accurate to the second decimal place, caused by concentration changes induced by analytes, such as magnesium chloride and d-glucose. This underscores the potential of nanostructured biosilica as an advanced platform for photonic sensing. In addition, we show that it is possible to customize the working spectral region by surface functionalization using titanium dioxide nanoparticles that modify the effective refractive index of the biosilica and therefore change the spectral properties of the pseudo-PBG. The results highlight the precision of this natural, biogenic nanomaterial and propose sustainable alternatives for developing photonic nanomaterials tailored for sensing applications and beyond.