1D photonic defect structures based on colloidal porous frameworks: Reverse pore engineering and vapor sorption

Stimuli-responsive 1D photonic crystals, referred to as Bragg stacks, are capable of translating environmental changes into a color read-out through changes in the effective refractive index (RI) of the multilayer system upon infiltration with an analyte. The sensitivity and versatility of prototypic SiO2/TiO2 multilayers can be greatly enhanced by the use of nanoparticulate or mesoporous building blocks, or inherently microporous structures such as metal-organic frameworks (MOFs). Here, we introduce a stimuli-responsive ZIF-8 “defect” layer into SiO2/TiO2 multilayers in order to combine the high optical quality of a Bragg stack with the characteristic sorption properties of the MOF. The addition of a planar defect layer, either embedded in or deposited on top of the Bragg stack acts as a “dopant” and introduces a narrow band of allowed states in the photonic band gap, which can be utilized for the precise determination of the optical response of the Bragg stack. We demonstrate the impact of layer morphology, layer sequence and the position of the defect on the optical and vapor sorption properties of the photonic architectures. Moreover, a facile process is presented which allows for the clean inversion of the acid-sensitive ZIF-8 defect layer into a mesoporous layer in a one-step fashion, while the layer structure and optical quality of the stack architecture is preserved.