Layer-by-Layer Assembly of SAM-supported Porphyrin-based Metal Organic Frameworks for Molecular Recognition

The development of the sensing systems based on the ultrathin films of functional metal organic networks as efficient synthetic alternative to molecular biosensors is in the focus of intense research. Herein we report how to fabricate uniformly structured surface-attached metal organic frameworks (SURMOFs) from zinc complex of meso-carboxyphenyl-substituted porphyrin and zinc acetate as films on the surface of self-assembled monolayers of silanes and thiols. The layer-by-layer self-assembly with shortened time of the layer deposition at room temperature was applied to obtain 5-layer SURMOFs with high crystallinity, which was confirmed by X-ray diffraction. These SAM-supported SURMOF films were used as sensing layers for qualitative response to organic molecules in aqueous solutions. By using surface plasmon resonance spectroscopy, we showed that the SURMOFs do not bind 4-nitrophenol and uracil whereas they are capable of absorption of purine compounds such as adenine and adenosine 5-monophosphate. This binding is a size sensitive process in favor to adenine suggesting the effect of the SURMOF structure on the molecular recognition. The UV-vis spectroscopy study of the process indicated that the recognition of purines by the porphyrin-based SURMOF proceeds through the interactions of the amine groups with zinc ions in metal clusters rather than with the central metal ions in porphyrin cores. Our findings provide an insight into the sensing properties of the porphyrin-based SURMOFs that is useful for further advancements in fabrication of responsive thin films for biologically relevant analytes.