  Based on a model of the optical biosensors (Science 311, 508 (2006)) by
wrapping a piece of double-stranded DNA around the surface of single-walled
carbon nanotubes (SWCNT), we propose a new design model of this sensor, in
which the SWCNT is replaced by a semiconductor graphene ribbon (SGR). Using a
simple theory of exciton in SGRs, we investigated transition of DNA secondary
structure from the native, right-handed B form to the alternate, left-handed Z
form. This structural phase transition of DNA is the working principle of this
optical biosensor at the sub cellular level from DNA and semiconductor graphene
ribbons.
