Recent advances in the spectroscopy of single walled carbon nanotubes have significantly enhanced our ability to understand and control their surface chemistry, both covalently and non-covalently. We have used this insight to study how this chemistry controls intermolecular interactions for several applications. Molecular detection using near-infrared light between 0.9 and 1.3 eV has important biomedical applications because of greater tissue penetration and reduced auto-fluorescent background in thick tissue or whole-blood media. We have pioneered the use of carbon nanotubes as tunable near-infrared fluorescent sensors that are highly photo-stable In one system1, the transition of DNA secondary structure from an analogous B to Z conformation modulates the dielectric environment of the single-walled carbon nanotube (SWNT) around which it is adsorbed. The SWNT band-gap fluorescence undergoes a red shift when an encapsulating 30-nucleotide oligomer is exposed to counter ions that screen the charged backbone. We demonstrate the detection of the mercuric ions in whole blood, tissue, and from within living mammalian cells using this technology. Similar results are obtained for DNA hybridization and the detection of single nucleotide polymorphism. We also report the synthesis and successful testing of near-infrared ?-D-glucose sensors2 that utilize a different mechanism: a photoluminescence modulation via charge transfer. The results demonstrate new opportunities for nanoparticle optical sensors that operate in strongly absorbing media of relevance to medicine or biology.

Another problem we have focused on is how the electronic structure of a carbon nanotube influences its chemical reactivity. For example, rate constants for electron transfer reactions involving single walled carbon nanotubes should vary with their chirality vector (n,m), a measure of the helical ‘twisting' of the graphene lattice in the nanotube. To date, the functional form of this relationship has proven elusive. We have performed completely automated reactions of single walled carbon nanotubes (SWNT) with 4-hydroxybenzene diazonium salt under various experimental conditions, and analyzed their influence on the reaction selectivity using UV-vis-nIR absorption spectroscopy and a previously published spectral deconvolution procedure. The selectivity of the reagent to metallic SWNT over semiconducting SWNT was greater at low salt concentrations (73%) and lower at high salt concentrations (54%). The activity of diazonium was increased upon illumination; however, similar rate constants for the SWNT (relative to the (11,5) nanotube) were computed for the light and dark reactions, indicating that the type of diazonium intermediate affects the extent of reaction and not the rate. The steady state data has been modeled using an adsorption-reaction scheme, and an electron transfer theory is developed to yield the first structure-reactivity relationship for SWNT.