Transparent conducting films based on reduced graphene oxide multilayers for biocompatible neuronal interfaces.

Among the most critical components in neuronal interfaces is the implanted electrode which requires the long-term stability of its electrical performance and biocompatibility of electrode material in contact with live neuronal cells. Reduced graphene oxide (rGO) renowned for its high electrical conductivity and optical transparency has shown great potential for a variety of applications such as transparent conducting electrodes and biosensors, and might be a potential candidate material for the next-generation neuronal interfaces. However, there have been only few systematic studies on graphene-based neuronal interfaces in terms of electrical conductivity and biocompatibility. In this report, we maintained rat hippocampal neurons on top of the rGO multilayers and observed that the viability of neurons is minimally affected and comparable to those grown on a glass substrate up to 30 days in vitro. These results implicate that rGO multilayer can be utilized for excellent neuronal interfaces with its high electrical conductivity and biocompatibility.

Highly tunable charge transport in layer-by-layer assembled graphene transistors.

We demonstrate a controlled, systematic method to tune the charge transport in graphene field-effect transistors based on alternating layer-by-layer assembly of positively and negatively charged graphene oxide followed by thermal reduction. Surprisingly, tuning the number of bilayers of thermally reduced graphene oxide multilayer films allowed achieving either ambipolar or unipolar (both n- and p-type) transport in graphene transistors. On the basis of X-ray photoemission spectroscopy, Raman spectroscopy, time-of-flight secondary ion mass spectrometry, and temperature-dependent charge transport measurements, we found that nitrogen atoms from the functional groups of positively charged graphene oxide are incorporated into the reduced graphene oxide films and substitute carbon atoms during the thermal reduction. This nitrogen-doping process occurs in different degrees for graphene multilayers with varying numbers of bilayers and thereby results in the interesting transition in the electrical behavior in graphene multilayer transistors. We believe that such a versatile method to control the charge transport in graphene multilayers will further promote their applications in solution-processable electronic devices based on graphene.