Sensors made from nanomaterials are able to detect individual chemical or biological molecules. Such devices could have widespread applications in genetic research, drug development and the diagnosis of diseases.

Now, Yasuhide Ohno and co-workers at Osaka University in Japan1 have developed a new sensor containing graphene — a single honeycomb-like sheet of carbon atoms. The sensor, modeled on a field-effect transistor (FET), can detect changes in pH and proteins adsorbed onto the graphene surface.

The detection of specific substances is often performed using a microscope after labeling the species of interest with other chemicals. This can be a complex and time-consuming process, so in recent years researchers have been learning to exploit the unique electrical properties of nanomaterials to develop label-free sensors.

Since 2003, Ohno and his colleagues have investigated sensors based on carbon nanotubes. However, the properties of nanotubes depend strongly on their structure and size, which are difficult to control, so the team have been diversifying their approach.

“In 2008, Andre Geim from Manchester University, who made the world's first graphene, visited our laboratory,” says Ohno. “We noticed that graphene FETs and nanotube FETs can have similar structures, and the characteristics of both are sensitive to their environment.”

Fig. 1: Optical micrographs of a graphene field-effect transistor (a) before and (b) after adding bovine serum albumin (BSA). Adsorbed BSA proteins, which change the conductance of the graphene sheet, can be seen in (b).Copyright © Yasuhide Ohno 2009

The researchers built their graphene FET by suspending a graphene sheet between two gold electrodes on a silicon substrate immersed in an electrolyte (Fig. 1). Molecules adsorbed onto the graphene sheet will either donate or remove electrons, thus changing the sheet's conductance.

Upon adding an alkali solution, the researchers found that the graphene conductance increased with pH. The conductance also increased when they added a protein, bovine serum albumin (BSA), indicating that BSA molecules had adsorbed onto the graphene surface (Fig. 1).

The results suggest that graphene FETs could act as very fast, sensitive and inexpensive detectors of pH and biomolecules. The researchers hope that they will soon be able to functionalize the graphene surface so that it only responds to specific proteins of interest.

The only problem, according to Ohno, is that the process of building a working graphene FET still relies on a certain degree of luck. “I believe developments in graphene growth techniques will resolve this issue,” he says.