“For the first time, we have created switchable-surface nanostructures with a characteristic dimension on a truly molecular scale, about 10 nm,” Vladimir Tsukruk told nanotechweb.org. “That is well below any spatial dimensions achieved before.”
Tsukruk, Eugene Zubarev and colleagues attached Y-shaped molecules to a silicon surface to create a layer about 2 nm thick. A hydrophobic polystyrene polymer chain made up one arm of each molecule, while the other arm was a hydrophilic poly(acrylic acid) (PAA) polymer chain. The volume ratio of polystyrene to PAA was roughly 60:40. The scientists used two types of molecule - one with a short stem and one with a long stem.
After treatment with toluene – an organic solvent – the polymer surface became covered with islands measuring about 10 nm across. For surfaces made from the short-stemmed molecules, these islands were 1.8 nm high, while the long-stemmed molecules produced islands that were 2.5 nm high. The researchers believe that the islands have a pinned micelle morphology, with the top layer mainly consisting of polystyrene arms - which dissolve well in toluene. The micelle cores, on the other hand, are likely to contain collapsed PAA arms, which do not dissolve well in toluene.
Treating the brushes with water - a selective solvent for PAA - caused a dramatic surface reorganization. The surface formed an array of crater-shaped features about 8 nm across and with elevated rims. Tsukruk and colleagues believe these structures consisted of a collapsed central core of polystyrene arms surrounded by swollen PAA chains.
“Switching of the surface-wetting properties on the nanoscale is of potential interest because the resulting crater-like nanoscale morphology can facilitate assembly of inorganic nanoparticles, proteins, nanotubes and charged chemical species,” said Tsukruk. “The approach is a promising way towards patterning solid substrates with adaptive nanowells, which can be used for controlled and selective trapping of adsorbing nanoscale objects.”
The switchable surfaces could also find a use as a coating on nanochannel walls to control the flow of liquids through microfluidic devices.
The researchers reported their work in Langmuir.