Tracking Molecular Diffusion in One-Dimensional Photonic Crystals

The intuitive use, inexpensive fabrication, and easy readout of colorimetric sensors, including photonic crystal architectures and Fabry-Pérot interference sensors, have made these devices a successful commercial case, and yet, understanding how the diffusion of analytes occurs throughout the structure is a key ingredient for designing the response of these platforms on demand. Herein, the diffusion of amines in a periodic multilayer system composed of two-dimensional nanosheets and dielectric nanoparticles is tracked by a combination of spectroscopic measurements and theoretical modelling. It is demonstrated that diffusion is controlled by the molecular size, with larger molecules showing larger layer swelling and slower diffusion times, which translates into important sensor characteristics such as signal change and saturation time. Since the approach visualizes the analyte impregnation process in a time- and spatially resolved fashion, it directly relates the macroscopic color readout into microscopic processes occurring at the molecular level, thus opening the door to rational sensor design.