Biological Template Controls Organization of Au Nanoparticles
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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-coated SWNTs is also strongly dependent on pH. The researchers believe that their nanotube sensors are sensitive to and can measure pH changes with an accuracy of 0.1 pH. They attribute this sensitivity to charged groups on the GOx molecules, which become more negative with increasing pH. This results in a decrease in the electrostatic gate voltage and an increase in the SWNT conductance. In addition, the researchers report that GOxcoated SWNTs also appear to be sensitive to glucose levels. However, quantitative reproducibility of the effect was hampered by substantial low-frequency noise. The researchers said that higher-mobility devices prepared from short SWNTs (~20 nm) may provide a powerful tool for enzymatic studies with the ability to measure the activity of even a single redox enzyme. In addition, they look forward to new nanotube biosensors that detect in real time a variety of biomolecules without the need for
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