A simple, solution-based method using block copolymers enables the creation of a variety of nanoparticle shapes from rods and spheres to vesicles is reported in newly published work. The particle shape can be varied by tuning the block copolymer template, the size and size distribution of the particles is easily controlled with the experimental conditions, and functionality is easily introduced to the particles using the method.
Amir Fahmi from the University of Nottingham and co-workers use an approach that relies on the initial self-assembly of block copolymers in solution. “Our work is inspired by using self-assembly as a simple and cost-effective tool to fabricate functional hybrid nanostructures based on metallic nanoparticles within self-assembled block copolymers,” explains Fahmi.
Block copolymers are known to self-assemble in solution into well-defined aggregates such as micelles with one type of block in the centre of the sphere surrounded by the other polymer blocks on the outside. These shapes can be manipulated by adding solvents that are attracted to the central blocks. For instance, when water is added to Fahmi and colleagues’ micelles, it is attracted to the central blocks of the spheres, so swells the micelles and triggers a change from a sphere to a rod. In this way, the researchers control the morphology of their polymer ‘templates’.
The trick here is that they use block copolymers that already have added metal precursors: the precursor is attracted preferentially to one end of the polymer. When the desired shape is formed, for instance with the precursor-heavy part of the polymer all segregated to the centre of a micelle, mixing in a reducing agent triggers the reaction to produce the final metal nanoparticle. They show that the method can be used to make spheres, rods, rings and vesicles of a variety of different metals.
Fahmi and colleagues also show how their approach can be applied to make a functional surface. After preparing vesicles – a spherical hollow metal shell – they spin-coat them onto a surface to produce a close-packed honeycomb structure. Oxygen plasma treatment removes the polymer leaving a honeycomb pattern of metal, and the size of the original vesicles determines the water repellancy of the surface.
In the future, the researchers hope to incorporate the technique into methods to make larger two- and three-dimensional architectures consisting of the nanoparticles, and look to creating hybrid materials for functional purposes, such as combining conductive polymers with magnetic or semiconducting nanoparticles, says Fahmi.