Improving analyte selectivity by post-assembly modification of metal–organic framework based photonic crystal sensors

The porous nature and structural diversity of metal-organic frameworks (MOFs) provide a versatile platform for specific and selective sorption behavior. When integrated as functional layers into photonic crystals (PCs), loading of the porous network with organic solvent vapors translates into an optical response, allowing analyte discrimination according to the specific host-guest interactions and, hence, framework affinity to the analytes. However, the optical response of PCs is critically influenced by the overall PC architecture, leading to batch-to-batch variations, thus rendering unequivocal analyte assignment challenging. To circumvent these problems, we have developed a straightforward and mild "post-assembly" modification strategy to impart differences in chemical selectivity to the MOF layers whilst keeping the overall PC backbone constant. To this end, one-dimensional photonic crystal (1D PC) sensors based on CAU-1 and TiO₂ layers were fabricated to obtain a generic platform for post-assembly modification, targeting either the secondary building unit (SBU) or the linker unit of the as-assembled MOF nanoparticle layers. The optical response to solvent vapor exposure was investigated with the pristine CAU-1 based sensor as well as its modifications, showing enhanced analyte selectivity for the post-modified systems.