Nanotubes are a unique material with rich electrical and chemical properties and extreme mechanical strength, which makes them suitable for wide range of applications, including sensing molecules in gaseous and liquid analytes. Nanotube-based sensors hold great promise for such applications as environmental and industrial monitoring, transportation, medical devices, medical/clinical diagnostics, biotechnology for drug discovery, agricultural and consumer markets, national security, including both homeland defense and military operations. For general information regarding carbon nanotubes, their integration in sensing devices and their principles of work, reference may be made to the following U.S. Pat. Nos. 6,346,189; 6,232,706; 6,401,526; 6,528,020 and also Franklin, et al., Appl. Phys. Lett. 79, 4571 (2001) and Zhang et al., Appl. Phys. Lett. 79, 3155 (2001), all of which are incorporated herein for reference for all purposes.
Fluid sensors of which the inventors are aware, and which use carbon nanotubes and nanowires as sensing elements, take advantage of high surface-to-volume ratio of nanoelements (nanotubes and/or nanowires), which makes their electrical properties sensitive to surface-adsorbed molecules. The detection scheme in these sensors is based on chemical interactions between the surface atoms of the nanoelements, or materials attached to the surface atoms of the nanoelements, and the adsorbed molecules in gases and liquids. There are a number of drawbacks associated with the above-described sensors. One technique for making a hydrogen sensor includes coating a nanotube with individual nanoparticles of palladium (Pd). In this sensor, detection is based on charge transfer from a Pd nanoparticle, which adsorbs H2 to the nanotube and results in lowering the nanotube conductance. The approach, however, has several shortcomings. For example, Pd nanoparticles have been found to be unstable due to their oxidation after exposure to H2 This results in shorter life for the sensors.
Moreover, coating nanotubes or nanowires with a thin layer of Pd nanoparticles is inherently difficult to control and scale up, as it often leads to electrical short circuits in the device. This is particularly the case if the sensor design calls for an array of individual sensors for detection of different molecules placed in close proximity to each other.
Another disadvantage of existing nanotube/nanowire-based sensors relates to an inability to effectively, or at all, prevent non-specific interaction between the nanoelement and other molecules present in the surrounding environment. For example, a non-functionalized nanotube may be sensitive to NO2. A device for detection of H2 with nanotubes coated with particles of Pd reacts to both H2 and NO2 if molecules of both gases are simultaneously present, since parts of the nanotube are functionalized with Pd, and parts are not.