B I O Suspended Carbon Nanotube devices for Single Molecule Sensing CNF

We have fabricated suspended carbon nanotube (CNT) devices specifically designed for biological sensing applications. We use 6-inch-wafer processes to maximize yield. Our fabrication process produces suspended CNT with clean surfaces and minimal environ mental noise. The electrode layout is compatible with a microfluidic system for delivering liquids and biological molecules to the sensor. We recently showed that this geometry is the quietest existing platform for electric field sensing in biological systems. Summary of Research: The two-terminal resistance of a carbon nanotube (CNT) device is sensitive to both the binding of biomolecules onto the CNT surface, and the internal motions of single biomolecules. Our group, and others, have demonstrated these effects using CNTs lying on substrates [1, 2]. However, there are compelling reasons to extend these experiments to suspended CNT biosensor devices. Suspended CNTs are distant from surfaces that have fluctuating charge. In addition, suspended CNTs allow measurements of the interactions between biomolecules and the CNT without interference from the substrate. Figure 1 shows a scanning electron microscope (SEM) image of a completed device. The CNT is hanging about 1 μm above an oxide surface. The metal electrodes are made from Pt which can withstand exposure to high temperature (820°C) during the CNT growth process. We published fabrication details [3]. Because the CNT is grown last, we expect the CNT surface to be pristine (no photoresist residue). We have confirmed the quality of our CNT devices using Raman spectroscopy. The absence of a defect peak at 1350 cm-1 is indicative of a pristine CNT (see Figure 2). We submerge the suspended CNT devices in aqueous solutions and measure the electrostatic noise in the environment. Our measurements are made in the subthreshold regime (the CNT transistor is nearly turned off), where current fluctuations can be related to environmental noise [4]. Figure 3 and 4 show representative noise measurements made on two different devices, one CNT is surface-bound, the other is suspended. The environmental noise measured by the surface-bound CNT (~ 1.8 mVrms) is consistent with the lowest noise devices that have previously been reported [4]. The noise floor of the suspended CNT device is three times lower. We have begun writing a manuscript describing these findings. Experiments to detect single-molecule binding events are ongoing.