In research that may pave the way for graphene to be used as an effective biosensor, scientists at Helmholtz-Zentrum Berlin for Material and Energy in Germany have developed a way to both measure and control the thickness of an organic compound bound to a graphene layer.
Graphene has presented an attractive option for developing biosensors because of its large surface-to-volume ratio and also because its electrical conductivity quickly decreases as soon as molecules begin to bind to it.
While this change in electrical conductivity should be a strength in biosensors, the problem has been that graphene’s conductivity changes like this for just about any molecule it comes in contact with, so it lacks the ability to differentiate between molecules—poor “selectivity” as it’s called in the biosensor business.
The German researchers have found a way to improve graphene’s selectivity. To achieve this, the team electrochemically treated the graphene with an organic solution that grafted itself to the surface of the graphene. The organic molecules of the solution essentially served as mounting brackets to which the selective detector molecules could attach themselves.
“Thanks to these molecules, the graphene can now be employed for detecting various substances similar to how a key fits a lock,” explained Marc Gluba, one of the researchers, in a press release.
The bracket molecules on the surface of the graphene are highly selective and will only absorb the molecules that are being targeted.
While other research teams have done something similar to this work, those previous approaches always depended on flakes of graphene that were so small that they resulted in edge effects that have a strong influence on the electronic and magnetic properties of the material and can overwhelm the device, reducing its effectiveness.
Instead of depending on flakes only a few microns in diameters, the German researchers were able to make graphene several square centimeters in size, significantly reducing the deleterious edge effects in the material.
Another key development was a first: The ability to accurately detect how many molecules were actually grafted to the surface of the graphene. The researchers were able to move the graphene layers over to a quartz crystal microbalance such that any increase in mass would change the oscillatory frequency of the quartz crystal. Even a small amount, down to the individual molecular layers, could be measured.
The same amount of precision in measurement could also be achieved in the production by adjusting an applied voltage, making it possible to control precisely how many molecules would bind to the graphene.
The researchers believe that this research should open the way for using graphene in lab-on-a-chip devices that could provide medical diagnosis from a single drop of blood.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.