Aligned supramolecular nanofibres over one metre long have been made by encapsulating them in a hydrogel coating
Scientists in Japan have made extremely long supramolecular fibres of a lipid-type compound by self-assembling it in microfluidic channels. Encapsulating the fibre in a seaweed gel jacket helps to support the structure and enabled the fibres to be drawn into long strands up to 1m long. The researchers used the fibres as templates to produce conductive polymers.
Conventional preparation of supramolecular gels is carried out in bulk and produces a tangle of nanofibres held together by hydrogen bonds, pi-pi interactions and van der Waals forces. The weak nature of these interactions inhibits the formation of macroscopic assemblies. A team led by Shoji Takeuchi from the University of Tokyo circumvented these constraints by preparing the gels in microfluidic channels and supporting them with a robust gel.
A solution of the monomer was fed into the microchannel, followed by the alginate gel solution as a sheath fluid. The combined flow was surrounded by a solution of calcium chloride, which initiated gelation. ’Rapid gelation is important and alginate sol (a colloid of very small particles) can be gelled with calcium ions immediately,’ says Takeuchi.
Using lipid dyes and fluorescent nanobeads to visualise the supramolecular strands and gel jacket, respectively, microscopy revealed that the laminar flow produced nanofibres that were aligned along the direction of the supramolecular strands. Aligned structures were also obtained in the absence of the gel jacket but were very fragile.
Strand length was adjusted by varying the microchannel injection time, allowing the formation of a strand 1m long. It was drawn from the sample tube by a pair of tweezers and maintained its alignment while it was vertical. A second strand was drawn into a free-standing star formation supported by glass pillars.
The strength of the aligned supramolecular fibres allowed them to be used as a template for the synthesis of polyaniline by oxidative polymerisation. The monomers were held on the fibre surfaces by weak interactions and polymerised in strands without aggregation. They were freed from the template by EDTA treatment and were also sufficiently strong to be manipulated with tweezers. The conductivity of polyaniline was several tens of nanoamperes, which is sufficient for use as a sensor.
Jiyu Fang from the University of Central Florida, US, says: ’Many of us have been working on self-assembled supramolecular nanostructures such as fibres, ribbons and tubes. The technique presented in the paper provides a valid way to integrate, shape and pattern them over a large scale in a controlled manner.’
D Kiriya et alAngew. Chem., Int. Ed., 2011, DOI: 10.1002/anie.201104043