Multimodal biomedical imaging with asymmetric single-walled carbon nanotube/iron oxide nanoparticle complexes.

Magnetic iron oxide nanoparticles and near-infrared (NIR) fluorescent single-walled carbon nanotubes (SWNT) form heterostructured complexes that can be utilized as multimodal bioimaging agents. Fe catalyst-grown SWNT were individually dispersed in aqueous solution via encapsulation by oligonucleotides with the sequence d(GT)15, and enriched using a 0.5 T magnetic array. The resulting nanotube complexes show distinct NIR fluorescence, Raman scattering, and visible/NIR absorbance features, corresponding to the various nanotube species. AFM and cryo-TEM images show DNA-encapsulated complexes composed of a approximately 3 nm particle attached to a carbon nanotube on one end. X-ray diffraction (XRD) and superconducting quantum interference device (SQUID) measurements reveal that the nanoparticles are primarily Fe2O3 and superparamagnetic. The Fe2O3 particle-enriched nanotube solution has a magnetic particle content of approximately 35 wt %, a magnetization saturation of approximately 56 emu/g, and a magnetic relaxation time scale ratio (T1/T2) of approximately 12. These complexes have a longer spin-spin relaxation time (T2 approximately 164 ms) than typical ferromagnetic particles due to the smaller size of their magnetic component while still retaining SWNT optical signatures. Macrophage cells that engulf the DNA-wrapped complexes were imaged using magnetic resonance imaging (MRI) and NIR mapping, demonstrating that these multifunctional nanostructures could potentially be useful in multimodal biomedical imaging.

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.

Optical detection of DNA conformational polymorphism on single-walled carbon nanotubes.

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. The transition is thermodynamically identical for DNA on and off the nanotube, except that the propagation length of the former is shorter by five-sixths. The magnitude of the energy shift is described by using an effective medium model and the DNA geometry on the nanotube sidewall. We demonstrate the detection of the B-Z change in whole blood, tissue, and from within living mammalian cells.