VI. Carbon Nanotube Functionalization Efforts

Although nanotubes have been shown here to be significantly sensitive to Ammonia, as well as NO ₂ and other toxic substances elsewhere, many substances that are of interest do not react significantly with the intrinsic nanotubes. Also, it is desirable to increase sensitivity, selectivity, increase response time, and decrease recovery time. Nanotubes have been functionalized by various means to address these issues for different applications ^{75 83}. Nanotubes doped with nitrogen ( CN _x tubes) have been used in conductivity sensors and have shown responses to relatively high concentrations (>1%) of ammonia, acetone, ethanol, gasoline, pyridine, and chloroform75. Nanotubes have been doped with poly-m-aminobenzene sulfonic acid for use in a conductivity sensor, resulting in a reported two-fold increase in sensitivity to ammonia77. Tin Oxide coatings of 1 6nm have also been reported for nanotubes ^{76 78}. Sensors have been constructed with these structures with responses for ppm concentrations of NO, NO ₂, ethanal, and C ₂H ₂. Nanotubes have been used in conjuction with a Pd thin film for hydrogen detection with some success79. Doping nanotubes with Pd has been demonstrated to be very effective, with sensitivities to .5% H ₂ in a nitrogen environment80. However, these tin oxide and palladium functionalized sensors do require temperatures in excess of 200 C ⁰. Amidoferrocenyl-functionalized nanotubes have been demonstrated to recognize the presence of mM concentrations of H ₂PO ₄- at room temperature83. Room temperature responses have been observed...