Resolving Molecular Size and Homologues with a Self-Assembled Metal-Organic Framework Photonic Crystal Detector

Traditional environmental photonic sensing based on thebalancedreflection of a photonic structure can hardly be applied to distinguishmolecular size and homologues, since the refractive index change introducedby these analytes can be too similar to be unambiguously distinguished.Here, a sensing method based on the pore size selectivity and organicsadsorption-desorption capacity of self-assembled ZIF-8 photonicscrystal is demonstrated, which can tell apart different molecularsizes, homologues, and organics with similar structures and physicalproperties. Specificity is improved by the inherent relationshipsbetween the pore size of ZIF-8 metal-organic frameworks andthe associated molecular diffusion rates in the pores, so that thecombination of the observed peak shift and recovery time are uniqueto each analyte. Using this method, selective detection of molecularsize is also demonstrated in the case of linear vs cyclic molecules,since only the former can penetrate inside the ZIF-8 pores to inducea large wavelength shift. The reflection peak shift caused by thelinear molecules was found to be about 30-40 nm, an order ofmagnitude larger compared to those for the cyclic molecules. A relationshipbetween the diffusion rate of linear molecules in ZIF-8 pore and therecovery time of photonic crystal reflective peak is further established,linking the recovery time and the linear molecular diffusion coefficient.Finally, different linear molecules are identified by their associatedrecovery time to detect homologues or organics with similar refractiveindex.