  We study theoretically the bio-sensing capabilities of metal nanowire surface
plasmons. As a specific example, we couple the nanowire to specific sites
(bacteriochlorophyll) of the Fenna-Matthews-Olson (FMO) photosynthetic pigment
protein complex. In this hybrid system, we find that when certain sites of the
FMO complex are subject to either the suppression of inter-site transitions or
are entirely disconnected from the complex, the resulting variations in the
excitation transfer rates through the complex can be monitored through the
corresponding changes in the scattering spectra of the incident nanowire
surface plasmons. We also find that these changes can be further enhanced by
changing the ratio of plasmon-site couplings. The change of the Fano lineshape
in the scattering spectra further reveals that "site 5" in the FMO complex
plays a distinct role from other sites. Our results provide a feasible way,
using single photons, to detect mutation-induced, or bleaching-induced, local
defects or modifications of the FMO complex, and allows access to both the
local and global properties of the excitation transfer in such systems.
