Graphene electrodes are of interest for use in neural implants that can be placed directly on the surface of the brain to record neuronal activity. Compared to conventional metal electrodes, these devices are more flexible and conform better to brain tissue. Transparency enables recording and viewing neuron activity directly beneath the electrodes that would otherwise be blocked by opaque metal materials.
A major drawback has prevented such use: graphene electrodes have high impedance, which means 
Low-impedance, transparent graphene microelectrode array. The inset is a microscopic image of the 4 x 4 array. Source: Yichen Lu/Advanced Functional Materials/University of California San Diegoelectrical current has difficulty flowing through the material. Resulting data are ‘noisy,' and transparency is lost with the application of available techniques to reduce graphene’s impedance.
Now researchers at the University of California San Diego have cooked up a new recipe to realize graphene electrodes with high transparency and 100 times lower impedance. The ingredients list calls for the addition of platinum nanoparticles to achieve both goals. The nanoparticles create an alternate set of paths to channel electron flow, overcoming the quantum capacitance nature of graphene which limits travel routes available to electrons.
With the inclusion of platinum, electrodes retained about 70 percent of their original transparency, which is considered sufficient to secure high-quality readings using optical imaging.
The new electrodes recorded and imaged neuronal activity, including calcium ion spikes, at the macroscale and single cell levels during tests with transgenic mice. While recording total brain activity from the surface of the cortex, the researchers also used a two-photon microscope to shine laser light through the electrodes. The activity of individual brain cells at 50 and 250 micrometers below the brain surface were in this way directly imaged. By obtaining both recording and imaging data at the same time, researchers were able to identify which brain cells were responsible for the total brain activity.
The development brings graphene electrodes a step closer to use in next-generation brain imaging technologies and various basic neuroscience and medical applications.
