A biosensor with a metal semiconductor field effect transistor (MESFET) structure, based on a highly sensitive carbon nanotube (CNT) film, has been developed by Oh et al., which has the potential to be used for the early detection of Alzheimer's disease. The CNT-MESFET biosensor has a gold top gate located in the center of the CNT channel, and is coated with probe antibodies that are attached to the gold top gate using a protein that binds to antibodies, such as protein G or Escherichia coli outer membrane (OM) with the autodisplayed Z-domains of protein A. The CNT-MESFET biosensor was able to achieve a higher level of sensitivity than the CNT-FET biosensor because the orientation of the immobilized antibodies was controlled by the antibody-binding proteins. The use of E. coli OM was also effective in preventing nonspecific binding, which helped to improve the accuracy of the biosensor. In contrast, the CNT-FET biosensor relied on probe antibodies immobilized using a chemical linker and did not have the same level of sensitivity. The researchers employed the CNT-MESFET biosensors along with E. coli OM featuring Z domain to identify amyloid-β (Aβ), one of the biomarkers for detecting Alzheimer's disease in human serum.