Dip-Pen Nanolithography (DPN) is a direct-write printing technique that uses chemically modified AFM tips to pattern materials on a variety of substrates at the nanoscale. Direct nanoscale patterning of maleimide-linked biotin on mercaptosilane-functionalized glass substrates using DPN is facilitated by the addition of a small amount of the biocompatible nonionic surfactant Tween-20 [Nano Lett. 2004, 4, 2171]. A correlation was found between activated ink transfer from the AFM tip when surfactant was included in the ink and an increase in the wettability of the partially hydrophobic silanized substrate. The image on the left is a friction force image taken with AFM of 70 nm wide lines of biotin, a biomolecule commonly used in biochemistry, written on glass with DPN. The image on the right is a fluorescent image of two regions of biotin conjugated to fluorescently labeled streptavidin proteins.

The fabrication of streptavidin or avidin nanostructures built upon DPN patterning of biotin represents a general route toward molecular recognition-mediated protein immobilization at the nanoscale due to the ubiquity of biotin-tagged biomolecules. This includes capture of monoclonal antibodies, which expands the molecular recognition capabilities further. We are also exploring the possibility of developing Ni2+/nitrilotriacetic acid (NTA)-maleimide based inks for DPN that will bind to mercaptosilane functionalized SiO2 surfaces for the reversible immobilization of biomolecules containing polyhistidine tags. It is important to note that the significance of including surfactant additives to DPN inks extends beyond biotin-streptavidin or Ni2+ /NTA-polyhistidine conjugation chemistry. Surfactant concentration represents a new control variable for DPN that complements relative humidity, tip-substrate contact force, scan speed, and temperature. Using surfactants systematically as ink additives expands the possible ink-substrate combinations that can be used for patterning biotin and other biomolecules, including proteins.