Detection of DNA hybridization using the near-infrared band-gap fluorescence of single-walled carbon nanotubes.

We demonstrate the optical detection of DNA hybridization on the surface of solution suspended single-walled carbon nanotubes (SWNTs) through a SWNT band gap fluorescence modulation. Hybridization of a 24-mer oligonucleotide sequence with its complement produces a hypsochromic shift of 2 meV, with a detection sensitivity of 6 nM. The energy shift is modeled by correlating the surface coverage of DNA on SWNT to the exciton binding energy, yielding an estimated initial fractional coverage of 0.25 and a final coverage of 0.5. Hybridization on the nanotube surface is confirmed using Forster resonance energy transfer of fluorophore-labeled DNA oligonucleotides. This detection is enabled through a new technique to suspend SWNTs using adsorption of single-stranded DNA and subsequent removal of free DNA from solution. While the kinetics of free DNA hybridization are relatively fast (<10 min), the kinetics of the process on SWNTs are slower under comparable conditions, reaching steady state after 13 h at 25 degrees C. A second-order kinetic model yields a rate constant of k = 4.33 x 10(5) (M h)(-1). This optical, selective detection of specific DNA sequences may have applications in the life sciences and medicine as in vitro or in vivo detectors of oligonucleotides.