Thursday, October 24, 2013

The art of making wet nanopores

Until recently, molecule-sized holes in graphene used to clog up with DNA-strands. Thanks to a coating that makes the surface hydrophilic, long organic molecules now gently flow through.

The dream of using nanopores to draw DNA through and then read the nucleotide sequence as the strand passes through was seriously hampered by the stickiness of the life-giving molecule. In fact, the ionic current through a nanopore will stop only moments after DNA is added to the solution. Electron microscope inspection will show a white blob where the nanopore used to be with fibril-like structures around it, acting as an anchor.

This adverse behavior was all the more irritating since nanopores can now be produced with incredible precision and roundness with diametres ranging from 3 to 20 nanometres. The newest technique drills a hole in a graphene layer with an electron microscope beam at temperatures above 500 Celsius. This somehow preserves the graphene lattice structure until the very edge of the pore.

DNA’s nasty stickiness is a consequence of the hydrophobic attraction between the DNA-molecule and the carbon atoms in graphene. They cling to each other like long lost brothers in their watery surroundings.

Solubility comes mainly in two sorts, either stuff dissolves in water, or in oil. If the constituents molecules or ions have a strong polarity, they will mix easily with water and are called hydrophilic, meaning water loving. If, on the other hand, the molecules are long and fatty, they will dissolve more easily in an nonpolar medium such as oil and are called hydrophobic or water fearing. Soap consists of molecules that have both a polar and a non-polar side and can thus play the intermediate between the greasy dirt or your hands and the cleansing water from the tap.

Now Dr. Grégory Schneider (bionano science at Applied Sciences) and colleagues have developed a molecular coating that makes graphene hydrophilic and thus repellant to organic molecules. The reseachers note that the coating does not alter graphene’s special electrical properties.

The self-assembling monolayer of pyrene ethylene glycol, as the coating is called, enables DNA strands (either single- or doubled stranded) to be detected in graphene nanopores with excellent nanopore durability and reproducibility, the researchers report in Nature communications.

More generally, they expect their patented hydrophilic graphene to be applied in a wide range of applications where and whenever graphene will be used in watery surroundings.

--> Grégory F. Schneider, Henny Zandbergen and Cees Dekker: Tailoring the hydrophobicity of graphene for its use as nanopores for DNA translocation, Nature Communications, 15 October 2013.