Redox-Mediated Synthesis and Encapsulation of Inorganic Nanoparticles in Shell-Cross-Linked Cylindrical Polyferrocenylsilane Block Copolymer Micelles

Abstract

Detailed studies of a new approach to the synthesis and encapsulation of silver and silver halide nanoparticles inside shell-cross-linked cylindrical block copolymer polyisoprene-block-polyferrocenyldimethylsilane (PI-b-PFS) micelles (1) through in situ redox reactions are reported. The cylindrical nanostructures 1 were prepared by the solution self-assembly of the PI-b-PFS diblock copolymer in the PI-selective solvent hexane followed by Pt(0)-catalyzed PI shell-cross-linking hydrosilylation reactions. The partial preoxidation of the swollen PFS core using tris(4-bromophenyl)aminium hexachloroantimonate [p-BrC6H4)3N][SbCl6] (2, Magic Blue) followed by redox reaction between the remaining Fe(II) centers in the PFS core and Ag(+) cations led to the formation of silver nanoparticles. High-resolution scanning transmission electron microscopy images of the resulting peapod structures provided a clear indication that the nanoparticles were encapsulated inside the micelles. The composition of the nanoparticles was analyzed by energy-dispersive X-ray spectroscopy (EDX). By combining the evolution of the UV-vis spectra of the reaction mixture and EDX measurements, we surprisingly found that silver halide seed particles were formed through a precipitation reaction at an early stage of the encapsulation process. The size of the silver nanoparticles varied with different amounts of silver ions added to the micelle solution. When I2 was used as the preoxidant, AgI nanoparticles were formed and encapsulated inside the cylinders through the precipitation reaction between iodide anions and silver ions. The packing density of the resulting AgI nanoparticles was increased by an iterative addition method, which utilizes the reversible redox properties of PFS. The small encapsulated AgI nanoparticles were also shown to serve as seeds for the formation of larger Ag nanoparticles when a silver salt was subsequently added.