Synthesis of Metal Nanoparticles Encapsulated with Skewered Porphyrins Assembled by Siloxane Coupling

Abstract

A protocol for encapsulation of metal nanoparticles with organic shells of porphyrin molecules via silane coupling is described. A strategy with silicon tetrachloride was used to produce a skewered arrangement of porphyrins that are linked through a central silicon atom by siloxane, Si-O-Si bridges. The planar macrocycles align cofacially to surround the periphery of metal nanoparticles (e.g. gold, iron oxide).  Skewered ‘shish kebob’ assemblies of porphyrins form an encapsulating shell by attachment to metal cores with silicon-oxygen-metal bridges. Free-base porphyrins were skewered through siloxane coupling using SiCl₄, with the silicon atom inserted to the center of the macrocycles. The Si atom binds to the four nitrogens at the center of the macrocycles, and also links to adjacent macrocycles through siloxane bridges. Iron and gold nanoparticles were used as core materials, while the organic shells were prepared with tetraphenyl porphyrin or octaethyl porphyrin. The thickness of the shells can be tuned by synthetic parameters such as concentration and immersion intervals. Structural changes were tracked using UV/Vis spectroscopy to evaluate spectral shifts. Nanoparticle samples were examined with tapping-mode atomic force microscopy to directly view changes in the size and shapes of nanoparticles before and after encapsulation with porphyrins. Phase images enabled sensitive mapping of the nanoparticle composition, revealing a soft organic shell surrounding the hard metal core. The synthetic approach with skewering porphyrins to metal nanoparticles should be generic for preparing metal core-shell nanoparticles encapsulated with shells of macrocyclic porphyrinoid molecules.