Enzyme-Coated Carbon Nanotubes Form Ultrasmall Biosensors
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RESEARCH/RESEARCHERS
Enzyme-Coated Carbon Nanotubes Form Ultrasmall Biosensors Single-walled carbon nanotubes (SWNTs) are an ideal material for nanosensor fabrication because they have high mobility and all atoms are located on the molecular surface. Previously, semiconducting MRS BULLETIN/AUGUST 2003
SWNTs were shown to perform as fieldeffect transistors, where, instead of a solid-state gate, neighboring molecules modulate the tube conductance. Based on this general nanotransistor design, researchers previously fabricated a SWNT chemical sensor for the detection of NO2 and NH 3 gases. Recently, a team of
researchers from the Department of NanoSciences and DIMES, Delft University of Technology, The Netherlands, have demonstrated a biosensor made from an individual SWNT. As reported in the June issue of Nano Letters, researcher K. Besteman and coworkers in the research group of C. Dekker 547
RESEARCH/RESEARCHERS
at Delft used a linking molecule for the controlled immobilization of the redox enzyme glucose oxidase (GOx), which catalyzes the oxidation of β-D-glucose to D-glucono-1,5-lactone, on the outer wall of a semiconducting carbon nanotube (CNT) to create a nanosensor that acts both as a reversible pH sensor and as a sensor capable of measuring GOx activity. CNTs, 600 nm in length on average, were grown using chemical vapor deposition on degeneratively doped silicon wafers that had a 200-nm thermally grown oxide layer. The researchers conclude from atomic force microscopy images that GOx molecules are immobilized specifically on the SWNTs with a density of about one GOx molecule every 12 nm. Electrodes, composed of a 30-nm gold layer and a 5-nm titanium adhesion layer, were deposited onto the SWNTs using electron-beam lithography. The liquid solution, in which all electrical measurements were made, acts as a very efficient gate for the immersed semiconducting SWNTs. The researchers show that a substantial decrease in the conductance of SWNTs results from the attachment of only about 50 GOx molecules, thereby demonstrating the potential for sensing the presence of GOx proteins. The researchers said that the decrease in conductance cannot
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simply be due to electrostatic gating by GOx; they propose that it results instead from the decrease in the tube capacitance. They cite numerical estimates of the decrease in conductance, which agree with the experimental measurements, in support of the group’s hypothesis. The conductance of GOx-coa
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