Up, up and away: Detecting enzymes with a graphene-based biosensor
- Published: Oct 15, 2012
- Author: Jon Evans
- Channels: Detectors
As well as showing great promise for use in such advanced technologies as nanoscale transistors and space elevators, carbon nanomaterials are also being utilized in the next generation of biosensors. This biosensing application takes advantage of the impressive electrical conducting abilities of the one-atom thick sheets of carbon known as graphene and certain kinds of carbon nanotube (CNT).
The idea is to attach specific proteins or antibodies to the surface of the CNT or graphene sheet, and then record the tiny change in conductivity that occurs whenever the attached protein or antibody binds with a target biomolecule. Now, a team of US and Korean chemists and material scientists, led by Ki-Bum Lee at Rutgers, the State University of New Jersey, has extended this technique beyond protein binding, by using it to detect enzymes.
Their approach involves placing a graphene sheet between two gold electrodes and then covering it with a positively-charged polypeptide linker. Next, they expose these polypeptides to negatively-charged gold nanoparticles, around 15nm in diameter, which naturally bind to the linkers via electrostatic interactions.
Exposing the graphene sheet to an enzyme that can cleave the polypeptide linker releases the charged gold nanoparticles, like cutting the tether holding a hot-air balloon. As the gold nanoparticles float off, they cause a detectable change in the conductivity of the graphene sheet.
Using this approach, Lee and his colleagues showed that they could detect the peptide-cleaving enzyme carboxypeptidase B, which has been proposed as a biomarker for severe acute pancreatitis, at concentrations down to 1μM. Furthermore, the rate of release of the gold nanoparticles and the size of the associated change in conductivity depend on the enzyme concentration. Higher concentrations mean a faster rate of release and a larger change in conductivity.
Releasing all the gold nanoparticles from the graphene sheet took 50 hours at enzyme concentrations of 1μM, but only three hours when the concentration was increased to 10μM. Although the relationship between the enzyme concentration and the size of the conductivity change is not linear, it is still sufficient to allow the enzyme concentration to be determined accurately from the change in conductivity .
This enzyme cleaving approach to biosensing also allows much smaller conductivity changes to be detected than is possible with the more conventional protein binding approach. This is because proteins need to be attached to the graphene sheet via strong covalent bonds, which tend to interfere with the electrical properties of graphene. In contrast, the polypeptides can be attached to both the graphene and the gold nanoparticles via weaker interactions that don’t interfere with graphene’s excellent electrical properties, allowing smaller conductivity changes to be detected.
Linkers and nanoparticles
Lee thinks that the sensitivity of this system can be improved still further by using different polypeptide linkers and metal nanoparticles with different compositions and sizes, and that is what he and his colleagues are now investigating. The other obvious advantage of using different polypeptide linkers is that it will allow the detection of whole range of other enzymes.
What is more, it should be fairly straightforward to manufacture this graphene-based biosensor on a large scale, as it is constructed using standard microfabrication techniques. It may thus soon take off as a method for diagnosing the many diseases that have been linked with raised enzyme levels, such as HIV, cancer and diabetes.
Advanced Materials (Article in Press): "Label-free polypeptide-based enzyme detection using a graphene-nanoparticle hybrid sensor"
Article by Jon Evans
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.