  Single-molecule chemical reactions with individual single-walled carbon
nanotubes were observed through near-infrared photoluminescence microscopy. The
emission intensity within distinct submicrometer segments of single nanotubes
changes in discrete steps after exposure to acid, base, or diazonium reactants.
The steps are uncorrelated in space and time, and reflect the quenching of
mobile excitons at localized sites of reversible or irreversible chemical
attack. Analysis of step amplitudes reveals an exciton diffusional range of
about 90 nanometers, independent of nanotube structure. Each exciton visits
approximately 104 atomic sites during its lifetime, providing highly efficient
sensing of local chemical and physical perturbations.
