A new type of hydrogel could make for high-performance energy-storage electrodes and biosensors. So say researchers at Stanford University in the US who have used the conducting polymer polyaniline (PAni) to develop a porous nanostructured material that they say has excellent electronic and electrochemical properties.

"Our material combines the advantages of hydrogels (which have a large surface area) and organic conductors, with their high electronic conductivity and good electrochemical properties," team members Lijia Pan and Guihua Yu explain. "It could thus be used in high-performance electrochemical devices such as supercapacitors and ultrasensitive biosensors, like those used to detect glucose, for example."

Hydrogels are 3D polymer networks that can hold a large amount of water and have a structure similar to that of biological tissue. Most hydrogels are based on non-conducting polymer matrices, however, which limits their applications in electronics. The researchers, led by Zhenan Bao and Yi Cui, have now used phytic acid, which is a good ionic conductor, to dope and crosslink the PAni in an effort to overcome this shortcoming.

Fast-forming gel

The team began by mixing two solutions. The first initiates the polymerization reaction while the second contains the monomer aniline and the doping phytic acid. A hydrogel forms in as little as three minutes, explains Yu, thanks to the fact that each phytic acid molecule contains six phosphorus groups that can interact with several polymer chains at once.

"We showed that we can synthesize the conductive polymer hydrogel in large quantities and also pattern it by inkjet printing and spray techniques," says Pan. "This means that we might be able to fabricate electronic and electrochemical devices such as biosensor arrays and microsupercapacitors on a large scale fairly easily."

The PAni hydrogel was found to have a high specific capacitance of around 480 F/g and a high rate capability (it charges and discharges energy very fast), which means that it might be ideal in applications such as electric vehicles and grid-scale energy storage. To compare, commercial carbon only has a specific capacitance of around 100 F/g.

Sensing glucose quickly

Spurred on by these results, the researchers decided to fabricate a glucose sensor by immobilizing the enzyme glucose oxidase (GOx) in the hydrogel. Glucose reacts with the GOx and its concentration in solution is monitored via electrochemical measurements using the GOx-PAni hydrogel electrode. "The electrode acts as an excellent interface between the biological (the enzyme GOx) and the synthetic (the electrode)," says Pan, "and the 3D conducting nanostructured framework allows the hydrogel to effectively collect electrons during the enzyme-catalysed glucose redox reactions."

The hydrogel also reacts very quickly – in around just 0.3 s, compared with a response time of around 20 s for commercial glucose sensors, he adds.

The team says that it is now busy developing other novel hydrogels based on various conducting polymers. "We are also trying to push our new technology into areas such as high-performance lithium batteries, electro-chromic devices, neuronal electrodes and even electronic skin," reveals Yu.

The research is reported in PNAS 10.1073/pnas.1202636109.