1. ШАПКА DNA-Single Walled Carbon Nanotube Hybrids:
Optical Recognition of the Wrap
Stacy E. Snyder and Slava V. Rotkin
Department of Physics, Lehigh University, Bethlehem, PA
Background
Methods
DNA is used to disperse single-walled carbon nanotubes (SWNTs) in solution.
The exact structure of DNA-SWNT complexes is not detectible by even the best microscopes, but there is evidence of helical wrapping of DNA around SWNTs.
We investigate the effects of changes in symmetry from the bare tube to the hybrid structure.
The ionized DNA wrap is simulated as an idealized helix of point
charges with density, helical angle, and position varied in the model.
We numerically solve the joint Schrödinger-Poisson equations to determine the modulation of optical properties resulting from the DNA. A real-space tight-binding Hamiltonian is used:
Unscrolled nanotube surface showing potential due to a charged helical wrap
Results and Discussion
A plot of calculated optical strength vs. perpendicular absorption energy and parallel emission energy (absorption-luminescence map) for (A) the bare (7,0) nanotube and (B) the DNA-wrapped (7,0) nanotube (left) shows the appearance of new transitions at a frequency near that of the prohibited E11 transitions for the bare tube. For the (5,0) DNA-SWNT hybrid (right), additional transitions are also seen (C and D).
Symmetry-breaking due to the helical potential of the charged DNA wrap lifts selection rules and results in new peaks in the cross-polarized optical absorption spectrum of the nanotube. C D
(7,0)
(5,0)
Symmetry changes also result in different absorption for left and right circularly polarized light. Below, the calculated perpendicular absorption of the bare (7,0) SWNT (solid green curve) is compared to left and right polarized light absorption in the DNA-wrapped tube (dashed blue and dotted red curves).
New transitions can clearly be seen in the cross-polarized absorption spectrum of the (7,0) nanotube.
The charged helical wrap leads to natural optical activity in the complex. Circular dichroism is shown for the (7,0) hybrid.
Summary and Conclusions
Acknowledgements
For DNA-wrapped SWNTs we predict that symmetry lowering due to the Coulomb potential of the regular helical DNA wrap results in new peaks in the in the cross- or circularly polarized absorption spectrum. These peaks can be used for experimental detection of the wrapping.
A similar effect of the symmetry-breaking is predicted to result in strong circular dichroism, which is due to the SWNT itself and not the DNA optical transitions.
DoD-ARL (W911NF ) under Lehigh-Army Research Laboratory Cooperative Agreement,
NSF (CMS , NIRT),
PA Infrastructure Technology Fund (PIT ).