Encapsulating Gold Nanoparticles in Zeolitic Imidazolate Framework Crystal for Novel Optical Response

We investigated optical properties of Au nanoparticles (Au NPs) loaded into the crystal of a porous material ZIF-8, abbreviated as Au@ZIF-8, by a diffuse reflectance spectroscopy. Au@ZIF-8 was synthesized by a reaction of ZIF-8 precursor with PVP-encapsulated-Au-NPs in methanol solution. The concentration of Au NPs in an aqueous solution of the ZIF-8 precursor was controlled from 0.05 mL to 1.0 mL. The crystal structure of ZIF-8 and detailed formation of Au@ZIF-8 are fully characterized by powder X-ray diffraction (PXRD) analysis, Raman spectroscopy, SEM, and TEM images. The upper limit of concentration for Au NPs positioned inside ZIF-8 crystal is approximately 0.05 mL. At higher concentrations, the crystal size of ZIF-8 becomes smaller; and some Au NPs are observed to be located outside the framework. The optical spectra of Au@ZIF-8 exhibit two main absorption bands at ≈520 nm and ≈820 nm. The former of which is attributed to the surface plasmon resonance of Au NPs, while the band at ≈820 nm can be assigned to the longitudinal mode and/or collective plasmonic modes, as long as the particles are closely aggregated. Finite-Difference Time-Domain (FDTD) calculation for extinction cross-section spectrum for a BCC arrangement of Au NPs with the radius of 5 nm and the center-to-center distance of 13 nm was utilized to confirm the observation of new optical response of Au@ZIF-8. The calculation is in rather good agreement with the experimental data showing two main bands at ≈500 nm and ≈800 nm. This work suggests a way to create a magnetic material in optical frequency in case of locating Au NPs periodically in the pores of ZIF-8 or more generally in the metal–organic frameworks (MOFs).