Abstract
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.
Highlights
Conceptual insightsThe development of integrated nanoscale sensing systems imposes high demands on the materials employed, including chemical versatility, stability and flexibility
Scheme 2 (a) Schematic representation of a multilayered photonic crystal comprising CAU-1 and TiO2 nanoparticle layers; (b) crystal structure of CAU-1 with carbon atoms given in black, oxygen in red, aluminium-oxygen octahedra in turquois and pores indicated by yellow and green spheres; (c) the two post-synthetic modification strategies of the framework applied in this work using de-methoxylation of the secondary building unit (SBU) (CAU-1-SBU) and amidification with hexanoic anhydride of the organic linker (CAU-1-Hex)
PXRD measurements on the dried particles further establish the structural integrity of the CAU-1 particles after post-synthetic modification (Fig. S2, Electronic supplementary information (ESI)†)
Summary
The development of integrated nanoscale sensing systems imposes high demands on the materials employed, including chemical versatility, stability and flexibility. Modification of the as-assembled PCs allows for the direct comparison of the optical responses, marking the step in the development of custom-made optical MOF sensors
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