Since graphene foam possesses a high porosity of close to 100%, this offers the opportunity to use it as a scaffold for other nanomaterials to generate synergistic effects. One example is the integration of zinc oxide (ZnO) nanowires on graphene foams to enhance the sensitivity of electrochemical biosensors. A research team, led by Professor Young Hee Lee, Director of the Center for Integrated Nanostructure Physics, Institute for Basic Science at Sungkyunkwan University in Korea, has fabricated vertically aligned ZnO nanowire arrays on 3D graphene foam and used this electrode to selectively detect uric acid (UA), dopamine (DA), and ascorbic acid (AA) by a differential pulse voltammetry (DPV) method.
Schematic of the ZnO NWA/GF electrode and detection of UA, DA, and AA. (Reprinted with permission from American Chemical Society)
Explaining the background for this research, Lee explains that abnormal levels of UA are symptomatic of several diseases, including gout, hyperuricemia, and Parkinson's disease. "Dopamine is an important neurotransmitter that is widely distributed within the mammalian central nervous system. Low levels of DA are related to neurological disorders such as PD and schizophrenia. Ascorbic acid is a vital vitamin in the human diet and is well-known for its antioxidant properties. It is also well-known that UA, DA, and AA coexist in the extracellular fluid of the central nervous system and serum. However, it is difficult to simultaneously detect each species in a mixture with high selectivity and sensitivity when using conventional solid electrodes because their oxidation potentials overlap, the surface area is insufficient, and/or the kinetic accessibility of each species is limited."
Currently, diagnosis of PD essentially relies on the assessment of clinical symptoms and a blood test for PD is undoubtedly a major goal for researchers. Being able to accurately test for biomarkers associated with the disease with simple blood tests would be a major breakthrough in diagnosing PD in the early stages when treatments are most likely to be effective.
"Our optimized ZnO nanowire/graphene foam electrode provided a high surface area and high selectivity with a detection limit of 1 nM for UA and DA," Lee tells Nanowerk. "The key features of our structural design are a large surface area with mesoporous 3D graphene structures to facilitate ion diffusion easily; high conductivity from 3D graphene foam; and active sites of ZnO surface for high selectivity."
In their experiments, the researchers oxidized UA, DA, and AA biomolecules found in serum extracted from human peripheral blood of healthy individuals as well as PD patients. This process involved proton and electron generation at the surface of the ZnO nanowire arrays whereby electrons are transferred to the electrode. Using DPV measurements with the ZnO nanowire/graphene foam electrode, the samples were analyzed for UA levels.
"The average UA concentrations for the healthy individuals and the Parkinson's disease patients were 355±30 and 265±20 µM, respectively," says Lee. "This clear reduction in UA levels in the serum of PD patients with reliable statistics (p <0.001) strongly implies that our approach is a significant step forward, which we believe will be beneficial for diagnosing PD and monitoring disease progression."
These results with a ZnO/graphene electrode design for high sensitivity and high selectivity biosensors are promising for a future where test results for serious diseases can be achieved simply with a drop of blood. Improving on their design, the team hopes to be able to selectively detect other disease biomarkers and biomolecules accurately with high sensitivity simultaneously. Since the oxidation potential of some biomolecules may overlap, there is no guarantee that they can always succeed. Therefore, they will proceed by testing individual diseases, including cancers, in order to improve their sensor design and make it as generally applicable as possible.
Lee points out that, in addition due to structural advantages, there is also the possibility that this electrode material may be used for many other applications such as other types of biosensors, gas sensors, methanol oxidation reaction in fuel cells, solar cells, and supercapacitors, etc.