"We have demonstrated a nanoscopic equivalent to the common practice of adding surface active agents to improve the performance of paints, inks and other coating formulations," Pat Collier of the California Institute of Technology told nanotechweb.org. "The addition of surfactants to inks represents a new control variable that complements control of relative humidity, tip-substrate contact force, scan speed, temperature and other variables used for dip-pen nanolithography."
Collier and colleagues added the non-ionic surfactant Tween-20 to a maleimide-PEO2-biotin ink in concentrations as low as 50 ppm by volume. Without the surfactant, the team could not get consistent patterning of the ink, even at high relative humidity values.
But using Tween-20 meant that the team could pattern biotin with sub-100 nm resolution. For an ink containing 0.1% surfactant, the minimum relative humidity needed for patterning was 22%. This value increased as the surfactant concentration decreased.
The researchers reckon the effect was due to the Tween-20 increasing the wettability of the ink on the mercaptosilanized glass surface.
"Using dip-pen nanolithography for direct nanoscale patterning of biological materials such as DNA, peptides and proteins on oxidized silicon or glass substrates without loss of activity often requires the incorporation of an organic thin film as a specific binding template," said Collier. "However, such films are generally hydrophobic, which can impede ink transport. The inclusion of a surfactant as an additive in the ink increases the wettability of these substrates, thereby activating the writing process."
Once the scientists had made patterns of biotin, they were able to attach the protein streptavidin and avidin-linked horseradish peroxidase enzymes to the biotin molecules.
Now the researchers say they are interested in "developing orthogonal patterning strategies for immobilizing different biomolecular species at specific sites on insulating substrates such as oxidized silicon or glass".
The researchers reported their work in Nano Letters.