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


T. T. Pham, N. L. Le, N. H. Nguyen, K. T. Luong Thi, N. T. Pham, L. H. Nguyen, and Truong Khang Nguyen

Source title: 
Polyhedron, 148: 171-177, 2018 (ISI)
Academic year of acceptance: 

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).