Engineering Nanoporous Anodic Alumina Bilayered Interferometers for Liquid and Gas Sensing

Nanoporous anodic alumina bilayered Fabry-Perot interferometers (NAA-BFPIs) act as optical sensing platforms to allow the study of refractive index-based detection of a model organic in liquid and gas form. The architecture of NAA-BFPIs comprises top and bottom anodic oxide films that feature large and small nanopores, respectively. The reflectometric interference spectroscopy (RIfS) fingerprint of these structures, produced by a combination of anodization and chemical etching, displays a complex interference pattern that can be resolved by fast Fourier transform (FFT). The FFT signature of NAA-BFPIs has three distinct peaks, each of which is associated with the average effective optical thicknesses of the top, bottom, and overall anodic films forming their structure. Drifts in the effective optical thickness of each layer were measured for precise quantification of refractive index changes induced by the infiltration of liquid and gas molecules of ethanol. Our findings reveal that the top layer of NAA-BFPIs is most sensitive to liquid ethanol. Conversely, the bottom layer of NAA-BFPIs with its smaller nanopores is more sensitive to ethanol gas molecules. This gives rise to a maximum sensitivity of 1.21 +/- 0.01 nm (g m(-3))(-1). The differences in sensing performance observed between liquid and gas molecules suggest that, whereas liquid-based refractive index sensitivity is mostly driven by the change in the refractive index of the medium filling the nanopores, sensitivity to gas molecules is strongly influenced by the adsorption and molecule-to-surface interactions with the inner wall of nanopores. These model optical interferometers provide an ideal platform to expand our current understanding on how structural designs of optical platforms can be harnessed to maximize sensitivity toward liquid and gas molecules-with implications across multiple photonic technologies and applications.