In a versatile approach to functionalizing surfaces, researchers from the University of Victoria (UVic) in Canada and Shanghai Jiao Tong University (SJTU) in China have grown tailored micellar brushes on a silicon wafer. The proof-of-concept study demonstrates tuning and tailoring of the wettability, antibacterial, catalysis, and filtration properties of the silicon wafer and may be suitable for a variety of types of surfaces.

At the molecular level, polymer “brushes” have emerged as an effective way to control the interface properties of materials. Brushes are formed by end-anchoring polymer chains to a surface by covalent bonds. With sufficient density, the brushes stretch away from the surface and form a coating whose properties vary by polymer and brush configuration.

The new approach makes it possible to coat surfaces in a similar fashion, but on much longer length scales. “This allows for different types of applications,” says Ian Manners, a UVic researcher who co-led the project with Huibin Qiu at SJTU. “For example, the brush layers made from fiber-like micelles can be used to selectively filter larger objects, such as nanoparticles, while smaller species such as molecules and ions pass through the relatively large pores,” Manners says.

The brushes were grown by crystallization-driven self-assembly, a technique Manners and his collaborators have been developing over the last 10 years. The technique utilizes block copolymers that have repeating subunits of two different polymer chains.

A key component of the growth process is the polymer poly(ferrocenyldimethylsilane) (PFS). When immersed in select solvents, PFS-containing block copolymers aggregate and self-arrange into cylindrical micelles, structures in which PFS “tails” form a cylindrical core from which the coil-like “heads” of the other block emerge. The PFS cores remains active, so when they are exposed to additional PFS-containing block copolymers in a solvent, crystal growth initiates self-assembly. Researchers have previously grown fiber-like and platelet nanoparticles through this process.

In this new effort, the research team created small seeds from an existing copolymer of PFS and the functional polymer poly(2-vinylpyridine) (P2VP), denoted PFS-b-P2VP. The seeds were densely and evenly distributed on a silicon wafer and anchored by hydrogen bonds between P2VP and the surface.

The researchers immersed the wafer in a solvent solution and added a second copolymer, composed of PFS and polydimethylsiloxane (PDMS). Through crystallization-driven self-assembly, the seeds grew to form micellar brushes about 20 nm in diameter and 1 µm in height.

Experiments revealed that brush density varied with seed concentration, while brush height varied with copolymer concentration. This had direct implications for surface wettability. Contact angle measurements, a direct measure of wettability, revealed that the hydrophobicity of the surface increased with both brush density and height, probably because of the hydrophobic nature of the PDMS surrounding the PFS core.

The researchers also fabricated brushes in which the second block copolymer, PFS-b-PDMS, was replaced by PFS-b-P2VP. These brushes became more hydrophilic with increasing height and density, in accordance with the hydrophilic nature of P2VP; they were more rigid than the PDMS brushes and took on a slightly different, coral-like structure. The P2VP brushes were about 25 nm in diameter and 40 nm in average height. They were grown on wafer areas up to 1 cm × 1 cm.

As proof-of-concept, the researchers tuned additional surface properties by exploiting the chemistries of micellar brushes. They selectively attached gold nanoparticles to P2VP, through coordination interactions, and the decorated micellar brushes catalyzed the reduction of 4-nitrophenol to 4-aminophenol. The researchers added silver nanoparticles to the brushes via an in situ redox reaction and the silicon wafer became resistant to the bacteria E. coli. The researchers immersed brushes in a solution of trifluoroacetic acid to positively charge them by protonation and then measured an increase in surface hydrophilicity.

“The approach is a lot more versatile than what’s already been done,” says Christine Luscombe, an expert in polymer science and self-assembly at the University of Washington. She says that the process can be used to functionalize many different kinds of surfaces with different properties. One of the main advantages is that it facilitates the growth of structures with precisely controllable dimensions, including cylinder diameter, she says. “[I]t’s a feat in self-assembly,” Luscombe says.

The research team expects that the approach is suitable for any surface on which micelle seeds can be anchored. The team has already demonstrated that it extends to graphene oxide nanosheets. Several polymers are known to undergo crystallization-driven self-assembly, including polymers with possible applications in optoelectronics and nanomedicine. “[T]here are many opportunities for expansion,” Manners says. “I hope that research groups elsewhere will get involved in exploring the potential of this approach to surface functionalization.”

Read the abstract in Science.