AVS 63rd International Symposium & Exhibition
    Nanometer-scale Science and Technology Thursday Sessions
       Session NS+BI-ThA

Paper NS+BI-ThA6
Strong Coupling of Localized Surface Plasmon Resonances to Light-Harvesting Complexes from Plants and Bacteria

Thursday, November 10, 2016, 4:00 pm, Room 101D

Session: Applied Nanoscale Microscopy Techniques/Biomaterial Interfaces – New Advances
Presenter: Graham Leggett, University of Sheffield, UK
Authors: A. Tsargorodska, University of Sheffield, UK
M. Cartron, University of Sheffield, UK
C. Vasilev, University of Sheffield, UK
G. Kodali, University of Pennsylvania
J. Baumberg, University of Cambridge, UK
PL. Dutton, University of Pennsylvania
CN. Hunter, University of Sheffield, UK
P. Torma, University of Aalto
G.J. Leggett, University of Sheffield, UK
Correspondent: Click to Email
Plants and bacteria harvest solar energy with extraordinary efficiency. In chloroplasts, the quantum efficiency, defined as the fraction of captured photons that goes on to cause charge separation, is estimated to be ca. 90%. The mechanisms by which such extraordinary efficiencies are realised have been the subject of intense interest. We have explored the potential offered by plasmonic techniques for the investigation of biological light harvesting complexes. Macroscopically extended arrays of gold nanostructures are fabricated by interferometric exposure of an alkylthiolate SAM on gold, enabling the fabrication of macroscopically extended arrays of gold nanostructures in a rapid, simple process. After annealing, these structures yield strong localized surface plasmon resonances (LSPRs). In contrast to the behaviour observed for most proteins, the LSPRs are split when light-harvesting membrane proteins from purple bacteria and plants are attached to the gold nanostructures, yielding pronounced changes in their extinction spectra. The splitting is large, and is different for mutant proteins containing different pigment molecules, indicating that it is sensitive to the electronic structures of the membrane proteins. The splitting is attributed to strong coupling between the LSPRs and excitons in the light-harvesting complexes. The splitting is suggestive of an asymmetric Fano-type resonance, and the plasmon-exciton coupling has been modelled with coupled harmonic oscillators. The model yields good fits to the experimental spectra. It indicates that in light harvesting complexes 1 and 2 (LH1/2) from purple bacteria, coupling to the carotenoid S2 state dominates, with a strength of ~ 0.2 eV. However, in a carotenoid-free mutant of LH1 the LSPR couples with a strength of ~ 0.1 eV to the bacteriochlorophyll Qx transition, which has a smaller transition dipole moment than do the carotenoids. The coupling varies with the square root of the surface coverage of the protein, consistent with strong coupling theory. Strong coupling was also observed for self-assembling polypeptide maquettes that contain only chlorins. However, it was not observed for monolayers of bacteriochlorophyll, indicating that strong plasmon-exciton coupling is sensitive to the specific presentation of the pigment molecules.