Field effect transistor (FET) architectures are another sensing architecture that can be conveniently produced by micro-nanofabrication. FETs consist of a current source, a current drain, a conductive path (sensing channel) between them, and a sensing gate to which a bias can be applied. Analyte binding to the sensing channel induces a charge transfer resulting in a dipole between the surface and the underlying depletion region of the semiconductor: current that passes between the source and drain of a semiconductor FET is quite sensitive to the charge state and potential of the surface in the connecting channel region. Moving a standard silicon FET from depletion to strong inversion (i. e., shifting the surface potential by >~ 0.5 eV) requires less than ~ 10-7C/cm2 or ~ 6x1012 charges/cm2, corresponding to transfer of 6.25 x 1011 e/cm2. With FETs of 2,000 square micrometers, detection of biological ana-lytes in sub-nanomolar concentrations is easily feasible. Specificity for binding of macromolecular analytes of interest can be provided by deployment of biological affinity reagents in the FET sensing channel. Submicron FETs are routinely manufactured; use of carbon nanotubes in FETS will offer still greater miniaturization [10.43,83].
Carbon nanotubes also have excellent mechanical properties and chemical stability in addition to potentially tunable electrical properties, making them highly desirable electrode/nanoelectrical materials for any number of nanoelectrical applications [10.84]. Biomolecules can be bound to carbon nanotubes, particularly in FET and nanoelectrode applications. Most biomolecules bound to carbon nanotubes are not cova-lently bound (as discussed above) and do not exhibit direct electrical communication with the nanotube, though redox enzymes bound to nanotubes and other conductive nanomaterials may [10.84, 85]. Flavin adenine dinucleotide (FAD) and flavoenyzme glucose oxidase (Gox) both display quasi-reversible one electron transfer when absorbed onto unannealed carbon nano-tubes in glassy carbon electrodes. Gox so immobilized retains its substrate-specific (glucose) oxidative activity, leading to applications in sensing circulating glucose for diabetes and, perhaps, to a strategy of harvesting electrical power from metabolic energy.
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